Applied Coal Petrology: The Role of Petrology in Coal Utilization

About the Editors Dr. Isabel Sua´rez-Ruiz is a scientific researcher at the Instituto Nacional del Carbon (INCAR-CSIC, Spain) working in the field of applied organic petrology. She received her Ph.D. in 1988 from the University of Oviedo (Spain) for her doctoral thesis on oil shales and source rocks. She has spent extensive periods of time carrying out research in petrology and organic geochemistry in well-known laboratories in France (Orle´ans) and the United States (Carbondale in Illinois and Lexington in Kentucky). Dr. Sua´rez-Ruiz has published extensively on topics of coal and organic petrology related to fundamental and applied aspects of this science, receiving the 2006 Organic Petrology Award from the International Committee for Coal and Organic Petrology (ICCP). Dr. John C. Crelling earned his B.A. in geology at the University of Delaware (1964) and his M.S. (1967) and Ph.D. (1973) degrees at the Pennsylvania State University. He started his professional career in 1972 at the Homer Research Laboratories of the Bethlehem Steel Corporation and joined the faculty at Southern Illinois University (U.S.) in 1977. As the leader of the Coal Characterization Laboratory, he established a research program in coal petrology and started the Maceral Separation Laboratory for the separation and characterization of pure coal macerals. He recently created an Internet-based petrographic atlas of coals, cokes, chars, carbons, and graphites. Professor Crelling has received numerous awards, including the Gilbert H. Cady Award from the GSA, the Joseph Becker Award from the Iron and Steel Society, and the Reinhardt Thiessen Medal from the ICCP.

Contributing Authors John C. Crelling (Chapters 7, 8, 11) Department of Geology, Southern Illinois University, Carbondale, USA Joan S. Esterle (Chapter 3) Mining Geoscience Group, CSIRO Exploration and Mining, Kenmore, Australia Robert B. Finkelman (Chapter 10) University of Texas at Dallas, Richardson, USA Stephen F. Greb (Chapter 10) Kentucky Geological Survey, University of Kentucky, Lexington, USA Gareth D. Mitchell (Chapter 6) Coal & Organic Petrology Labs, EMS Energy Institute, Pennsylvania State University, University Park, USA Jack C. Pashin (Chapter 9) Geological Survey of Alabama, Tuscaloosa, USA Isabel Sua´rez-Ruiz (Chapters 1, 2, 4, 8, 11) Instituto Nacional del Carbo´n (INCAR-CSIC), Oviedo, Spain Nicola J. Wagner (Chapter 5)* Coal & Carbon Research Group, School Chemical & Metallurgical Engineering, University of Witwatersrand, Wits, South Africa Colin R. Ward (Chapters 1, 2, 4) School of Biological, Earth & Environmental Sciences, University of New South Wales, Sydney, Australia

xiv Contributing Authors

*Assisting Dr. Wagner with Chapter 5 M. Coertzen Research and Development, Sasol Technology, R&D, Sasolburg, South Africa R. H. Matjie Research and Development, Sasol Technology, R&D, Sasolburg, South Africa J. C. van Dyk Research and Development, Sasol Technology, R&D, Sasolburg, South Africa

Acknowledgments The material covered in this book represents the contributions of highly qualified and well-known specialists in the coal field, particularly in the branch of coal petrology. We, the editors, are extremely grateful to the authors for their individual chapters and to the contributors of Chapter 5, M. Coertzen, R. H. Matjie, and J. C. van Dyk from SASOL (South Africa), for the great effort they made in preparing their contributions that have made this book possible. Thanks are also due to those colleagues who helped by reviewing chapters and sections of the original manuscript. Their opinions, remarks, and comments contributed greatly to the improvement of the final version of this book. Special thanks are given to J. M. Diez Tasco´n and to M. Granda (INCAR-CSIC, Spain) for their revision of Chapter 8; and to M. Granda, C. Blanco, and B. Ruiz (INCAR-CSIC, Spain); R. A. Creelman (Australia); R. H. Matjie (SASOL, South Africa); R. Gray (United States); Bill Huggett (SIU-C, Illinois, United States); CAER (UKY, United States), and Z. Baoshan (China) for providing us with the pictures included in Chapters 4, 5, 7, 8, 10, and 11. We would also like to thank A. Go´mez (INCAR-CSIC, Spain) for offering his support in the bibliographic section. Permissions to reproduce copyrighted material relating to figures, tables, and microphotographs were given by Elsevier, Blackwell, the American Chemical Society (ACS), Gebru¨der-Borntraeger (Germany), CSIRO (Australia), the Pittsburgh Coal Conference (University of Pittsburgh, United States), Taylor & Francis, the Energy Institute (London), Steel Publications, Association for Iron & Steel Technology (AIST), the Royal Microscopical Society (Wiley Blackwell Publishing), and the Society for Mining, Metallurgy, and Exploration, Inc. Finally, we want to acknowledge the cooperation of Elsevier for giving us the opportunity to publish this book, particularly for the help of Kenneth P. McCombs, Kristi Green, Diana Spencer, Jane Macdonald, Anne McGee and Mageswaran BabuSivakumar for providing the facilities and their support and help in preparing the manuscript.

Preface This book is intended as a reference book, providing extensive information on the applications of coal petrology and the importance of that discipline for characterization of coals in terms of composition and rank, for the assessment of coal quality, and for the prediction of coal properties and behavior during utilization. The book is especially recommended for coal producers, coal marketers, and others associated with the coal industry, as well as for coal researchers and teachers. It provides extensive background information not only on different coal characteristics and properties but also on the way those characteristics determine technological properties. The compilation begins with an introduction to coal petrology and an outline of the fundamental concepts, familiarizing the reader with the general nature of the discipline. This discussion is followed by a synthesis of the current geographical distribution of coal resources and coal reserves. The most recent estimates for world coal mining, coal production, and consumption are outlined, as are the likely trends for the near future. Coal mining and some of the environmental issues associated with coal are only briefly described here; these topics are dealt with in other chapters. The role that applied coal petrology has played in understanding coal and the history of coal petrology from the very beginning are also briefly described. Chapter 2 is devoted to a description of the basic factors, especially those related to coal composition and rank, which influence coal properties, coal quality, and technological behavior in various processes. Current coal petrographic nomenclature and the terminology of coal components, as well as the main coal classifications, are also discussed. Coal composition in terms of its organic/inorganic components, including trace elements, coal evolution during the coalification process, and the role of coal rank and its determination, are emphasized. The importance of blending coals to obtain the quality required by the final consumer is also stressed, since coal petrography as a tool used in coal petrology is the only means by which the individual coals that make up blends can be identified. Chapter 3 covers the coal mining issues and coal beneficiation processes prior to coal utilization. In this section the influence of

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Preface

coal composition during mining and beneficiation is discussed in depth, together with the influence of mining and beneficiation on the physical properties of the coal products. Chapters 4 to 7 deal with the main uses of coal in combustion as a source of electrical energy, and in gasification, liquefaction, and carbonization processes. Particular stress is placed on the specific properties of coal for each process and the role of petrology in the characterization and prediction of properties and coal behavior, as well as in the characterization of certain derived coal products or byproducts such as fly ash, gasification residues, and cokes. Because coals are also precursors of carbon materials, coal petrology, particularly petrography, has been recently applied to the characterization of synthetic carbon products. This is an issue that is discussed in Chapter 8, where examples from applied petrology are used in the study of well-known carbon materials such as carbon/carbon composites, graphites, and activated carbons. Two other significant aspects of coal—its importance as a petroleum source rock and its function as a reservoir rock due to its physical properties—are dealt with in Chapter 9. Environmental and health matters in relation to coal as a material resource and in coal utilization are covered in Chapter 10. This chapter also includes some well-known examples of the impact of coal on health and the environment in relation to coal mining and coal utilization in various parts of the world. Chapter 11 covers the role and potential role of organic petrology applied to wider fields, perhaps only partially related to coal, such as archaeology or forensic science. These topics are illustrated and highlighted with examples. Each of the subjects discussed in these chapters is a major topic in itself, and this book is able to offer only a short review of each, highlighting in particular the successful applications of coal petrology to coal utilization. The bibliographic material at the end of the book can be used as a further guide to the topics discussed in the chapters. We have attempted to provide comprehensive information that may serve as a reference for the reader and a basis for further understanding of this wide-ranging field. We hope that the book will provide an accurate and meaningful source of data and information for many years to come.

CHAPTER 1

Introduction to Applied Coal Petrology Colin R. Ward Isabel Sua´rez-Ruiz

1.1 Fundamental Concepts Coal is a combustible sedimentary rock, composed essentially of lithified plant debris. The plant debris was originally deposited in a swampy depositional environment to form a soft, spongy sediment called peat. However, physical and chemical processes brought about by compaction and elevated temperatures with prolonged burial at depths of up to several kilometers and over periods of up to several hundred million years then changed the peat into coal through a process referred to as coalification or rank advance. The properties of a given coal can be related to three independent geological parameters, each of which is determined by some aspect of the coal’s origin. As discussed more fully by authors such as Ward (1984), Diessel (1992a), Taylor et al. (1998), and Thomas (2002), these parameters are briefly defined as follows: l

l

Rank. Coal rank reflects the degree of metamorphism (or coalification) to which the original mass of plant debris (peat) has been subjected during its burial history. This depends in turn on the maximum temperature to which it has been exposed and the time it has been held at that temperature and for most coals reflects the depth of burial and geothermal gradient prevailing at the time of coalification in the basin concerned. Heat flow from nearby igneous intrusions, however, may also play a part. Type. Coal type reflects the nature of the plant debris from which the original peat was derived, including the mixture of plant components (wood, leaves, algae, etc.) involved and the degree of degradation to which they were exposed before burial.

Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

2 Applied Coal Petrology

l

The individual plant components occurring in coal, and in some cases fragments or other materials derived from them, are referred to as macerals (see Chapter 2); these form the fundamental starting point for many different coal petrology studies. Grade. The grade of a coal reflects the extent to which the accumulation of plant debris has been kept free of contamination by inorganic material (mineral matter), including the periods before burial (i.e., during peat accumulation), after burial, and during rank advance. A high-grade coal is therefore a coal, regardless of its rank or type, with a low overall proportion of mineral matter, and hence a high organic matter content.

Although organic matter derived from marine algae occurs in very old (Precambrian) sedimentary rocks, land plants capable of forming coal did not appear until the Silurian and Devonian periods. Major coal deposits occur in the Carboniferous strata (354–290 My) of Europe and North America, and in the Permian (290–248 My) sequences of Australia, India, South America, and the other land masses that made up the former continent of Gondwanaland. Coals of Carboniferous and Permian age also occur in China. Mesozoic coal occurs in a number of areas, notably the Jurassic (205–142 My) of Australia and China and the Cretaceous (142–65 My) of North America. There are also significant resources of Palaeogene and Neogene age (65–1.8 My) in various continents, including Europe, North America, Asia, and Australia.

1.2 Coal Resources, Mining, and Utilization Coal is a versatile fossil fuel that has long been used for a variety of domestic and industrial purposes. It currently provides around 25% of the world’s total primary energy (International Energy Agency, 2007) and, although subject to some possible variation with different policy developments, is expected to provide a similar share in future years (e.g., 23–26% in 2030; International Energy Agency, 2007). Most of the world’s coal is used for the production of electric power (see Chapter 4). The other main use is for production of coke as a reducing agent in the iron and steel industry (see Chapter 7). Coal is also used as fuel for a range of manufacturing processes, such as the production of heat in cement kilns and other industrial plants, gasification and petrochemical production (see Chapter 5), and heating domestic and commercial buildings. In addition, it is used as a raw material in a range of nonenergy applications (see Chapter 8), such as the production of carbon electrodes for the aluminum industry or as a precursor for a number of other carbon-based industrial materials.

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The availability of coal resources has been a major contributor to the economic growth of many countries, either directly through their own resources or indirectly through access to the international coal trade. In the late 19th and early 20th centuries, coal was mainly used as a transport fuel (such as for ships and railway locomotives) or as a source of heat and power for industrial and domestic applications. In the middle of the 20th century the use of coal decreased in some areas because of low oil prices, but the oil supply crisis of the 1970s reversed this trend and led to an increase in coal consumption. Another consequence of the oil supply crisis was a significant increase in coal liquefaction research and development (see Chapter 6), although much of this work was subsequently put on hold when oil prices stabilized. Though coal usage has continued to increase, environmental concerns and changes in the political climate have again begun to give coal an unfavorable public image. Increasing concerns about coal utilization as a contributor to greenhouse gas emissions, particularly CO2, have led to more intense questioning of the role of coal and a renewed search for alternative energy sources. According to International Energy Agency data (International Energy Agency, 2007), coal became the world’s principal source of anthropogenic CO2 emissions in 2004, moving ahead of emissions derived from oil and natural gas sources. This has led to another change in the focus of coal research, with the emphasis shifting toward increasing the efficiency of coal utilization and to integrating coal utilization with CO2 sequestration/ storage processes.

1.2.1 Coal Resources and Production According to data reported by the World Energy Council (2007), the total proven recoverable reserves of coal worldwide (all ranks) are currently estimated at 847 Mt, made up of 431 Mt of bituminous coal and anthracite, 267 Mt of subbituminous coal, and 150 Mt of lignite. The reserves are located on every continent and in over 70 countries, with major proportions identified in the United States, the Russian Federation, China, India, Australia, South Africa, Ukraine, and Kazakhstan. World coal production in 2006 was 6,284 Mt (International Energy Agency, 2007), represented by 5,370 Mt of hard coal (bituminous coal and anthracite) and 914 Mt of subbituminous coal and lignite (brown coal; Table 1.1). This continues the trend previously reported by the World Coal Institute (2005), indicating an overall increase of about 40% in coal production during the past 20 years. China is now the largest single producer, with 2,841 Mt of hard coal in 2006. Other major producers, especially of hard coal, include the United States, India, Australia, South Africa, Russia, and Indonesia.

Coal Production People’s Republic of China United States of America India Australia South Africa Russia Indonesia Poland Kazakhstan Colombia Rest of world Total

Hard Coal (Mt)

Brown Coal (Mt)

2,481

(a)

Australia

231

Japan

178

990

76

Indonesia

129

Korea

80

427 309 244

30 71 0

92 69 63

Taiwan United Kingdom Germany

64 51 41

233 169

76 0

60 45

61

92 64 266 5,370

5 0 595 914

India People’s Republic of China United States of America Russia Italy Rest of world Total

41 37

95

Russia South Africa People’s Republic of China Colombia United States of America Canada

(a) Included with hard coal production.

Coal Exports

Kazakhstan Vietnam Rest of world Total

Hard Coal (Mt)

27 26 22 51 815

Coal Imports

Hard Coal (Mt)

33 26 25 243 819

4 Applied Coal Petrology

TABLE 1.1 Coal production, exports, and imports by country, 2006 (Data compiled from International Energy Agency, 2007.)

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5

The size and extent of the world’s coal reserves suggest that there is enough available to meet demands for the next 150–190 years at current production rates. The life of the reserves could be extended still further through discovery of new reserves and through upgrading less well-known deposits as a result of further exploration activities. Reserves may also be effectively increased by advances in mining technology that would allow previously inaccessible resources to be reached. On the other hand, limitations on coal mining and use due to higher costs, increased regulatory restrictions, carbon penalties, and land-use conflicts may reduce the recoverable reserves below levels that would otherwise be available for economic use. The great magnitude, widespread distribution and relatively long projected life of the world’s coal resources, compared to those of oil and gas, and the fact that many economies still depend on coal for a significant part of their energy needs (Thomas, 2002; Mills, 2004) mean that coal is expected to continue as a major energy resource for the next few decades at least. This does not, however, remove the need to establish new technologies or to improve those already in existence, to control and where possible reduce the emissions from coal utilization so that they are in accord with emerging environmental regulations and trading agreements.

1.2.2 Coal Mining and Utilization Coal is mined via two basic methods, surface and underground mining, with the choice being mainly determined by the geology of the deposits involved. Coal seams close to the ground surface, where the overlying strata as well as the coal itself can be safely and economically removed, may be won by open-cut mining techniques (see Figure 1.1). Although such operations may have a more significant environmental impact than coal extraction by underground methods, a higher proportion of the in situ coal (usually more than 90%) is recovered for use, including in many cases seams that are either too thin or too thick for effective recovery in underground operations. With some possible exceptions, open-cut methods usually provide coal at a lower overall cost than underground mines; they also avoid some of the safety hazards, such as roof falls, gas outbursts, and coal-dust explosions, that can occur in the underground mining environment. Most of the world’s coal resources, however, occur at depths where only underground mining is likely to be possible. Underground mining currently accounts for about 60% of world coal production (World Coal Institute, 2005), and open-cut mining the remaining 40%. The coal extracted from an open-cut or underground mine, referred to as run-of-mine (ROM) coal, often contains impurities such as rock from the roof and floor or layers of noncoal material occurring

6 Applied Coal Petrology

FIGURE 1.1. Aerial view of open-cut coal mine, Queensland, Australia, showing overburden removal to expose the coal seam. (Photo: C.R. Ward.)

within the seam. A coal preparation process (see Chapter 3) is commonly used to reduce the proportion of this material, ensuring a consistent standard of coal quality and enhancing the suitability of the mine product for specific end uses. Coal currently supplies fuel for 39% of the world’s electricity generation, a proportion that is expected to remain at a similar level for at least the next 30 years (International Energy Agency, 2007). The demand for coal in the iron and steel industry is expected to increase by almost 1% per year over the same period of time. The biggest market for coal, particularly for steam-raising (power generation) and coking purposes, is found in the developing Asian countries, which account for 54% of total coal consumption. China is one of the most significant coal users, and the United States, India, Japan, and Russia are large coal consumers. Coal is also traded all over the world. Australia is the world’s largest coal exporter (Table 1.1), with almost 75% of the country’s coal exports going to the Asian market. Other major exporters include Indonesia, Russia, South Africa, China, and Colombia. Certain characteristics of coal ensure its place as an efficient and competitive energy source and that it contributes to stabilizing energy prices. As reported by the World Coal Institute (2005), key factors include (1) the very large reserves without associated geopolitical or safety issues, (2) the availability of coal from a wide variety of sources, (3) the facility with which coal can be stored in normal conditions, and (4) the nonspecial and relatively inexpensive protection required for the main coal supply routes.

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7

1.3 Environmental Issues and Options Both coal mining and coal consumption have a significant impact on the natural environment. Thus, although coal is an important contributor to the economic and social development of many countries, there is a strong need to minimize and where possible reduce the negative impacts associated with its mining and use. Some of these impacts and their implications, especially to human health, are further discussed in Chapter 10.

1.3.1 Impacts Associated with Coal Mining Coal mining, particularly in the case of open-cut mines, requires that large areas of land are temporarily disturbed, and perhaps withdrawn from other productive uses such as agriculture, while the process of coal extraction is under way. The land may, however, be returned to productive use after mining or even while mining in other parts of the area is still in progress (Figure 1.2). With underground operations, subsidence may result in a lowering of the ground surface above and around the mined-out coal bed. Other environmental risks associated with coal mining may include increased generation of noise and dust around the mine installations, soil erosion, water pollution (including in some cases acid mine drainage), and potential impacts on local biodiversity (wildlife and vegetation). An associated problem may be the release of methane (referred to as coal mine methane, or CMM), which, as well as being potentially explosive, is also a significant greenhouse gas. If present in the subsurface coal seam, the methane has to be drained before underground

FIGURE 1.2. A golf course developed on rehabilitated overburden dumps at an open-cut coal mine in northern Thailand. (Photo: C.R. Ward.)

8 Applied Coal Petrology

mining operations, to maintain safe working conditions. Ideally, however, the methane should be used rather than simply released into the atmosphere, even though, if mixed with the ventilation exhausts from underground mines, it may only be at very low concentrations. Methane accounts for 18% of the overall global-warming effect associated with human activities (World Coal Institute, 2005), although methane from coal accounts for only 8% of the world’s major methane emissions. Utilization of methane produced during coal mining would clearly reduce the environmental impact its release might otherwise generate and could also have a commercial benefit through uses such as additional power generation. Before a coal mine is allowed to commence operations, thorough studies must be carried out to identify all the potential risks to the surrounding environment and to minimize any negative impacts. Such a study, especially for an open-cut mine, should include a final land rehabilitation plan aimed at returning the land to other acceptable uses once the mining operations are completed.

1.3.2 Impacts of Coal Combustion Coal consumption for power generation and heat production is of growing environmental concern, due mainly to emissions of CO2 associated with the combustion process. The release of CO2 into the atmosphere as a consequence of human activities, especially those related to fossil fuel combustion, has been reported to be linked to increased global warming and associated climate change. According to the World Coal Institute (2005), CO2 emissions from all sources, including coal, account for around 50% of the overall global-warming effect associated with human activities. Although coal is only one of many sources represented by this anthropogenic CO2, the coal industry is searching for and developing technological options to mitigate its contribution to the problem. As part of this process, new technologies and improvements to existing technologies have been developed to increase the efficiency of combustion and power generation and reduce the CO2 and other emissions per unit of electrical energy produced. As well as CO2, the emissions released during coal combustion (described more fully in Chapters 4 and 10) may include oxides of sulphur and nitrogen (SOx and NOx), fine solid particulates, and possibly a range of trace elements, among which mercury is of special interest in some areas (U.S. Environmental Protection Agency, 2005a; Commission of the European Communities, 2005). Clean coal technologies offer a series of technological options for improving the environmental performance of coal, reducing emissions and at the same time increasing the amount of useable energy derived

Introduction to Applied Coal Petrology

9

from each ton of coal. These can include more efficient coal preparation (see Chapter 3), which serves not only to increase the heating value of the coal and therefore the efficiency of the combustion process but may also reduce the levels of sulphur and ash-forming mineral matter. This may in turn help to reduce the amount of waste as well as, perhaps, SOx and potentially harmful trace elements associated with the combustion process. Sulphur and nitrogen oxides released into the atmosphere from coal combustion (as well as from other noncoal sources) may chemically react with water vapor and other substances to form acids that are finally deposited as acid rain. Use of feed coals with a low sulphur content is in many cases the most economical way of reducing SOx emissions. An alternative, however, is the incorporation of flue gas desulphurization (FGD) systems in power plants (see Chapter 4), which can remove as much as 99% of the SOx emissions otherwise released and therefore help considerably in the prevention of acid rain problems. The technologies developed to reduce SOx emissions from coal combustion in power plants are also effective in some cases for reducing emissions of sulphur-related trace elements such as mercury. Emissions of nitrogen oxides contribute not only to the formation of acid rain but also to the development of photochemical smog. In the case of coal combustion these emissions may be controlled by the use of improved burner designs (low NOx burners) and possibly by using selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) technologies to treat the flue gas stream. Although such an approach may reduce NOx emissions by up to 80–90% (World Coal Institute, 2005), there may be a trade-off due to less efficient combustion of the coal, resulting in higher levels of unburnt carbon in the ash from the power plant. This may in turn impact the usefulness of the ash for processes such as cement and concrete manufacture. Another alternative is to use fluidized-bed combustion (FBC), an advanced, high-efficiency technology that may reduce both nitrogen and sulphur oxide emissions by 90% or more (World Coal Institute, 2005). The release into the atmosphere of fine particulates from coal combustion can be reduced, if not totally eliminated, by incorporation of electrostatic precipitators (ESP) or baghouses with fabric filters (FF) in the combustion stream. These may recover the suspended coal ash (fly ash) and other fine particulates with an efficiency of up to at least 99.5%.

1.3.3 Amelioration of Combustion Impacts Because of CO2’s association with global warming, the reduction of CO2 emissions from coal utilization is one of the biggest present-day challenges faced by the world coal industry. In the case of coal

10

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combustion a significant step has been taken with the development of supercritical and ultra-supercritical steam cycle technologies, which can achieve thermal efficiency levels of 43–45% (supercritical) to 50% in the case of ultra-supercritical power plants (World Coal Institute, 2005). Integrated gasification combined cycle (IGCC) technology (see Chapters 4 and 5) is another possible option, producing gas from coal for use in a gas turbine rather than using direct coal combustion. IGCC-based power plants may reach high efficiency levels (50%) and can also be designed to capture CO2 emissions more effectively for input to subsurface storage systems. Carbon capture and storage technologies are perhaps the most promising options for substantially reducing CO2 emissions from coal utilization, and a great deal of research is being carried out in this field (see Chapter 9). This research is directed toward both the procedure for CO2 capture from the gases produced in different utilization plants and the identification of appropriate geological environments and sites where CO2 can be permanently stored in a way that prevents its escape back into the atmosphere. Although there are other sources of CO2 emissions from coal, such as the iron and steel industry, cement plants, and domestic usage, and there are also numerous sources of CO2 unrelated to coal utilization (e.g., motor vehicles, air transport, and charcoal production), the initial focus of such activities is mainly on coal combustion in power plants. This is partly because they represent large but stationary sources of CO2 and partly because the organizations involved in their operation might be expected to have a capacity for implementing any remediation measures that may be developed.

1.4 The Role of Applied Coal Petrology For as long as coal has been used in industry it has been important to assess the quality of the coal and determine the chemical and physical properties that influence its suitability for the purpose in question. Some coals can be sold and used in the as-mined state (i.e., as ROM coal), whereas others may require quality improvement through coal preparation processes. Coals from different sources can also be blended to obtain a product that has particular quality characteristics. Although some of the tests and analyses applied to coal have changed in response to new technological developments and improvements in analytical techniques, coal science still has a strongly traditional basis. Many of the tests that are used in coal characterization, such as proximate and ultimate analysis and heating value determination (see Chapter 2), are little changed from those applied to coal testing over 100 years ago.

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1.4.1 History and Scope of Coal Petrology Coal petrology is also a branch of coal science that dates back to the beginning of the 20th century, and it was in 1913 that White and Thiessen laid down some of the fundamentals that underpin coal petrology today. The titles of Thiessen’s works in the 1920s, Under the Microscope Coal Has Already Lost Some of Its Former Mystery, reflected new discoveries in this emerging field (Thiessen, 1920a–c, 1921, 1926). However, the thin-section techniques Thiessen used and the resulting nomenclature developed at the U.S. Bureau of Mines (the Thiessen–Bureau of Mines system) are not used in modern coal characterization. At about the same time, Stach (1935) was developing the discipline in Europe, in this case using reflected-light microscopy techniques. These techniques are still used in modern coal petrography. Stopes (1919, 1935), who was also interested in the nature of coal, coined the term maceral and established the concept of lithotypes that is also used today (see Chapter 2). Cady (1939) introduced the lithotype concept to the North American coal community, although the concept of macerals was not adopted until later. The widespread use of coal petrology in the steel industry and the founding of the International Committee for Coal Petrology (now the International Committee for Coal and Organic Petrology, ICCP) in 1953 and later the North American Coal Petrographers group (succeeded by The Society for Organic Petrology, TSOP, in 1984) served to emphasize the use of reflected-light techniques in the study of coal, to unify maceral nomenclature, and to establish a classification of the coal components that can be identified using microscopic techniques. The currently accepted classification of coal components is a result of the work of the ICCP, expressed through the production of a number of editions of the International Handbook of Coal Petrology (ICCP, 1963, 1971, 1975, 1993). These have been replaced in part by more recent publications (ICCP, 1998, 2001; Sy´korova´ et al., 2005), detailing revised and expanded nomenclature and a new reclassification. Because coal is a complex rock, coal petrology is a broader subject than merely the simple study of its organic constituents, the macerals. In the present work, coal petrology is broken down into three fundamental components: (1) organic petrology, (2) inorganic petrology and geochemistry, and (3) coal rank, or the metamorphic transformation of the macerals and minerals in coal. Investigating a coal for the purpose of utilization involves knowing something about all these characteristics, none of which should be separated from the others. Coal quality is a function of these factors and their interactions, and coal petrology is the fundamental

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discipline that contributes to the knowledge of coal quality. The petrology of a coal may be expressed by a number of fundamental parameters, including (1) the nature of the organic constituents in terms of macerals or maceral groups (an indicator of coal type), (2) the mineral matter, including the major elements in the coal or oxides in the ash, the minerals in the coal, the forms of sulfur, and the trace elements that may also be present (indicators of coal grade), and (3) the vitrinite reflectance (which is usually taken as an indicator of coal rank). These parameters reflect the composition and rank of the coal and are the primary factors that contribute to the coal’s specific physical and chemical properties. The physical and chemical properties in turn determine the overall quality of the coal and its suitability for specific purposes. The analytical procedures used to determine the petrographic, physical, and chemical properties have been standardized in a number of international norms (ISO, ASTM, etc.) and additional discussion is given in compilations such as those of Peters et al. (1962), Karr (1978a,b; 1979), Ward (1984), van Krevelen (1993), and Thomas (2002).

1.4.2 Coal Characteristics for Utilization The basic chemical parameters of a coal are determined by proximate analysis (moisture, ash, volatile matter, and fixed carbon percentages) and ultimate analysis (carbon, hydrogen, nitrogen, sulphur, and oxygen contents). Other analyses that may be carried out include determining the forms of sulphur in the coal (pyritic, sulphate, organic) and the carbon (or CO2) content derived from the carbonate mineral fraction. The chlorine content, which is mainly associated with inorganic salts (relatively high proportions of chlorine may give rise to corrosion in coal utilization), and the phosphorous content (an undesirable element in coals to be used in the steel industry) may also be determined. The ash of the coal may be analyzed to determine the major and minor metal oxides (these influence coal and ash behavior during usage), and the proportions of a number of different trace elements, some of which could be potentially hazardous, may also be evaluated. In addition to the chemical properties, effective use of coal also requires knowledge of particular physical properties, such as the coal’s density (which is dependent on a combination of rank and mineral matter content), hardness, and grindability (both related to coal composition and rank). Other properties include the coal’s abrasion index (derived mainly from coarse-grained quartz) and the particle size distribution. Float-sink testing may also be integrated with the analysis process, separating the (crushed) coal into different density fractions as a basis for assessing its response to coal preparation processes. Float-sink techniques may also be used to provide a coal sample that

Introduction to Applied Coal Petrology

13

represents the expected product of a preparation plant, to assess the quality of the coal that will actually be sold or used rather than the in situ or run-of-mine material represented by an untreated (raw) coal sample. As well as proximate and ultimate analysis data, the coal quality parameters that need to be taken into account in coal combustion, such as in coal-fired power plants, include information from a number of specific tests, such as: l

l

l

l

The Hardgrove grindability index (HGI). This indicates the ease with which the coal can be ground to fine powder and is important for gauging the coal’s compatibility with the precombustion pulverization system of the plant concerned. This HGI is most directly related to the maceral and maceral group composition (see Chapter 3) but is also dependent on rank and mineral content. The heating value, calorific value, or specific energy. This indicates the amount of heat liberated per unit of mass of combusted coal and is of fundamental importance in setting the price of particular coals for combustion applications. Although generally regarded as a rank-related parameter, the calorific value is also dependent on the macerals in the coal and the mineral composition. The total sulfur content. This may be derived from a combination of the organic constituents and the mineral matter. As well as the overall percentage, it may be expressed in some cases as the sulfur dioxide emissions expected in relation to the heating (or calorific) value—for example, as kg SO2/GJ. The ash fusion temperatures. These indicate the behavior of the ash residues from the coal at high temperatures and are mainly related to the chemical composition of the ash and the nature of the coal’s mineral matter (see Chapter 4). They are used to indicate whether the ash will remain as a fine powder within the furnace system after the coal is burned or whether it might partly melt and form a slag on the boiler’s heat exchange surfaces.

Other tests that provide information about the potential behavior of coals during carbonization and coking processes (see Chapter 7) include: l

The free-swelling index (FSI) or crucible swelling number (CSN). This is a measure of the increase in volume of the coal when it is heated in a small crucible in the absence of air. This test is also used to characterize coals for combustion, especially

14

Applied Coal Petrology

l

l

l

where the coal is burned in beds of coarse-crushed material in stoker-based systems. The Roga index. This test provides information on the caking properties of the coal, in a similar way to the free-swelling index. The index itself is derived from the strength or cohesion of the coke produced in the crucible, as evaluated by a subsequent tumbler test. The Gray-King and Fischer assays. These determine the proportions of coke or char (carbonaceous solids), tar (organic liquids), liquor (ammonia-rich solutions), and gas produced when the coal is carbonized (heated in the absence of air) under particular laboratory conditions and hence provide a basis for estimating the yields of coke and coke byproducts obtained from the coal in an industrial coke oven or oil-shale processing plant. Giseler plastometer and Audibert-Arnu dilatomer tests. These monitor how the coal behaves as the different macerals melt, devolatilize, and resolidify at different temperatures during the carbonization process (see Chapter 7). The Gieseler plastometer evaluates the coal’s behavior by measuring the fluidity of a packed coal powder as it is heated, whereas the AudibertArnu dilatometer measures the contraction and expansion of a powdered sample pressed into a cylindrical coal “pencil.” Such properties are significant when different coals are blended for coke production, to ensure compatibility of the different blend components. Indeed, coal-blending strategies for coke production are generally decided from a combination of rheologic and petrographic parameters for individual coal samples, which are used to select coals to make up a blend with specific coking properties.

1.4.3 Petrological Controls on Coal Characteristics Although various combinations of these tests are used to evaluate the suitability of particular coals for a variety of industrial processes, the properties determined by the various tests are ultimately related to the coal composition (organic and inorganic components) and the coal rank (degree of metamorphism). Organic petrology therefore plays a fundamental role in determining coal behavior, and evaluation of petrographic properties (e.g., maceral percentages, vitrinite reflectance, mineral matter composition) should be an essential part of any coal analysis and testing program. Another factor that must be taken into account in determining coal quality is the degree of coal oxidation. Coal oxidation may result from exposure to weathering processes during handling and transport or when the coal is stockpiled under different environmental conditions.

Introduction to Applied Coal Petrology

15

Oxidation may affect both the organic and inorganic components and can give rise to deterioration in the coal properties, especially those relevant to coking applications (see Chapter 7). Petrographic examination may help to identify coals that have been oxidized and perhaps explain any anomalous behavior associated with the oxidation process. A different consequence of coal oxidation is the development of spontaneous combustion (see Chapters 10 and 11), when the heat generated by in-situ oxidation causes the coal to smolder and ultimately burn without any external heat source. The liability to oxidation is mainly determined by the coal’s rank, in conjunction perhaps with the maceral and mineral (e.g., pyrite) content (see Chapters 3 and 11). Low-rank coals are particularly prone to spontaneous combustion; other factors, such as access of air to coal stockpiles, may need to be controlled to reduce spontaneous combustion risk. Role of Organic Constituents The organic constituents of coal, including both the maceral groups (liptinite, inertinite, and huminite/vitrinite) and the individual macerals in those groups (see Chapter 2), are, singly and in combination (as natural associations or microlithotypes), fundamental to many coal properties. Vitrinite is the most common maceral group in many coals, especially the Carboniferous coals of the Northern Hemisphere, and it is the properties of the vitrinite in such coals, together with the variations in those properties with rank, that to a large extent determine the properties of the coal concerned. There are, however, major exceptions to this principle, such as with the Permian Gondwana coals of India, Australia, Southern Africa, South America, and Antarctica. These coals are commonly rich in inertinite-group macerals, with vitrinite in some cases forming only a relatively minor component. The different maceral assemblage reflects deposition in a cooler and drier climate and a more terrestrial environment than the Carboniferous coals of Europe or North America. The Gondwana coals are also more variable, and vitrinite-rich and inertinite-rich coals may occur in close proximity, and even as different parts of the same coal seam. Environmentally Significant Inorganic Components The inorganic constituents of coal are often expressed on the basis of simple parameters such as ash yield and sulfur content. It is, however, often convenient to express an inorganic constituent relative to another parameter, such as the expression of sulfur in terms of SO2 emissions per unit energy, with the latter being derived from the heating value.

16

Applied Coal Petrology

Knowledge of the inorganic constituents in coal may take on a more complex role if emissions of the so-called hazardous air pollutants (abbreviated as HAPs) are regulated. HAPs generally include Sb, As, Be, Cd, Ni, Pb, Se, Hg, Co, Cr, and Mn, with Cl and the radionuclides, Th and U, also included in some assessments. In the United States, the focus has been on Hg, with the U.S. Environmental Protection Agency (2005a) mandating a reduction in Hg emissions to 15 short tons/year by 2018 (from early 21st-century levels of about 45 short tons/year). The European Union is also taking measures on this matter, as described in Chapter 4. Many of the elements that may be of concern are trapped with the fly ash after coal combustion (discussed in Chapter 4) and, in the case of power plants using flue-gas desulphurization (FGD) systems, with the FGD (or scrubber) byproducts. Coal beneficiation processes prior to utilization may also serve as a means of reducing the levels of at least some trace elements (e.g., Hower et al., 1998). Elements of concern that occur at significant levels in the processing or utilization residues may give rise to waste disposal or control problems that are different from air pollution, through processes such as leaching into the natural environment following ground or surface water infiltration (e.g., Jankowski et al., 2006) and the potential for such issues may also need to be investigated. The lower heating value of lower-rank coals means that more coal must be burned to produce one unit of electricity compared to higher-rank (e.g., bituminous) coals. Just as the amount of sulfur in a coal can be translated into kg of SO2 per GJ of energy produced, on the basis of the heating value, the abundance of HAPs and similar elements may also need to be expressed in energy terms. Evaluation and Significance of Coal Rank Coal rank is commonly expressed in terms of vitrinite reflectance (see Chapter 2). Because it is measured by optical microscopy and takes into account only one coal component, this parameter has the capacity to provide an indicator that is independent of other factors (e.g., coal type or grade). Unlike other indicators, such as total (organic) carbon, volatile matter, or calorific value, it is not dependent on the overall coal composition (e.g., relative proportions of different macerals); the only requirement for the determination is that vitrinite is present in the coal. Although vitrinite reflectance is widely used as a measure of coal rank, it is not always a truly independent rank indicator. As discussed further in Chapter 2, some vitrinites may have anomalously low reflectance ( due, for example, to the original depositional environment), a phenomenon known as reflectance suppression (Barker, 1991), which

Introduction to Applied Coal Petrology

17

may give misleading results if other indicators are not taken into account. Despite the advantages and simplicity of vitrinite reflectance, it is very difficult to find an indicator of coal rank that is totally independent of the organic and inorganic composition or of other influencing factors such as the depositional environment of the original peat deposit. Despite the difficulties in identifying a robust rank indicator, a number of coal properties progressively change with rank advance (Figure 1.3), and the rank of a coal is thus a major factor influencing its potential usage. For example, the heating value determines how much coal is required to produce a given amount of steam and hence to generate a given amount of electricity, and the rank thus represents the fundamental basis for assessing the values of coals, per tonne, on the steaming coal market. The free-swelling index (FSI), which is important for both metallurgical and steaming coals, is also at least in part a rank-dependent parameter, increasing with rank through the high-volatile bituminous range but decreasing again above the medium volatile bituminous range. The free-swelling index also depends on the maceral composition of the coal, with the vitrinite maceral group being the main contributor to swelling properties. Some of the inertinite group macerals,

FIGURE 1.3. Variation in some key coal properties with rank advance. (Source: Coal Geology and Coal Technology, by C. R. Ward (Ed.), “Blackwell Scientific Publications, Melbourne,” 345 pp., copyright 1984, with permission from Blackwell.)

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Applied Coal Petrology

and also the mineral matter, act as diluents, reducing the swelling properties that would otherwise apply to vitrinite-rich coals at the relevant rank level.

1.4.4 Integration of Coal Petrology in the Evaluation Process This book considers coal as the sum of all its components, organic and inorganic, together with the metamorphic changes they have undergone through the rank advance process. It is the integration of these fundamental factors that is important, rather than the features or percentages of any individual component. The interplay of coal characteristics and coal utilization has been important as long as coal has been used and will continue to be important (as discussed in Chapters 8 and 9) until coal is no longer a viable economic resource. Since the beginnings of the science, coal petrology has been found to be a powerful tool in the characterization of coals for both geological and industrial applications. This book focuses on the applications of coal petrology to coal mining, preparation, and (especially) utilization as well as to other related areas, such as archaeological studies (see Chapter 11). Each of these subjects is a major topic in itself, and the book is able to present only a brief review of each. Bibliographic references are provided, however, for additional information on specific aspects and applications.

CHAPTER 2

Basic Factors Controlling Coal Quality and Technological Behavior of Coal Isabel Sua´rez-Ruiz Colin R. Ward

2.1 Introduction Conventionally, coal is used in processes such as combustion, gasification, and liquefaction and in carbonization for the manufacture of metallurgical coke. Coal and its derivative products are also used as precursors of other materials and in the production of chemicals. Thus, a coal must be characterized before it is used, whether as a single or blended coal. Characterization is performed in order to find out the properties of a coal, to determine its quality, and to predict its technological behavior. Basically there are two characteristics that influence the use of coal: its composition and its rank. Coal composition is in turn represented by two essentially independent factors (Ward, 1984): type (nature of the organic components) and grade (extent of dilution by mineral matter).

2.2 Coal Composition: Organic Components Coal is a heterogeneous material, and evaluation of coal type may be approached on two different levels: the macroscopical and microscopical, both of which form a part of coal petrology. Macroscopically, coals can be classified into two broad categories based on coal type: (1) humic coals or banded coals, which are the more common in nature and are derived from a heterogeneous mixture of a wide range of plant debris, and (2) sapropelic, nonbanded, or massive coals (ICCP Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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Applied Coal Petrology

1963), which are homogeneous in appearance and require special conditions for accumulation and preservation of the original organic matter (Stach et al., 1982). Lithotypes are the macroscopically recognizable bands in humic coals, and four lithotypes—vitrain, clarain, durain, and fusain—have been described by the ICCP (1963). These lithotypes are the result of plant growth and the physicochemical conditions existing in the peat swamps in which the organic remains accumulated. The minimum thickness of bands described as lithotypes has been established between 3 mm and 10 mm. Lithotypes can be distinguished from one another, particularly in high volatile bituminous coals, on the basis of their physical properties, such as luster, fracture pattern, color, and streak (ICCP, 1963; Stach et al., 1982; and Taylor et al., 1998). Stach et al. (1982) extended the definition of lithotypes to include cannel and boghead coals, both of which have been identified in sapropelic coals.

2.2.1 Organic Petrography: Macerals and Microlithotypes Macerals Microscopically coal is composed of various constituents (macerals), which occur together in different associations (microlithotypes). Mineral matter is also present in different proportions. Thus, macerals are the coalified remains of various plant tissues or plant-derived substances existing at the time of peat formation. Due to variable and often severe alteration during the peatification and coalification processes, it is not always possible to recognize the plant material from which many macerals were originally derived (ICCP, 1971). The formation of macerals from plant remains during the early stages of peat accumulation depends on the type of plant community, climatic and ecological controls, and conditions of the depositional environment (Stach et al., 1982). When the processes of biochemical degradation cease and the organic material is buried at great depths in the sedimentary environment, geochemical coalification over a long period of time and under conditions of high temperature and pressure takes over. As a result, the sediment of the original peat swamp is transformed and passes through the progressive evolutionary stages of lignite, subbituminous, and bituminous coal to anthracite and meta-anthracite. Throughout these stages the physicochemical characteristics of the coal as well as its technological properties are modified (Stach et al., 1982, and Taylor et al., 1998). In polished sections under the microscope using incident light, macerals are identified on the basis of their optical properties. Universal acceptance is given to the ICCP classification and redefinition (ICCP, 1963, 1971, 1975, 1998, 2001; Sy´korova´ et al., 2005) of macerals into three groups: liptinite, inertinite, and huminite/vitrinite (Figure 2.1).

Basic Factors Controlling Coal Quality and Technological Behavior 21

Sf Tl Sp

Dt Sf

(a)

(b)

F F Dt Sp (c)

(d)

R

Mi T

Tl Mi

(e)

(f)

Ct

V V

(g)

Dt

(h) (Figure legend continues on next page)

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Applied Coal Petrology

FIGURE 2.1. Photomicrographs of coal macerals in bituminous coals (medium coal rank) taken in reflected white light and with a 32
oil immersion objective (long side of the pictures: 200 mm). (a) Coal with a vitrinite random reflectance of 0.97%. Liptinite (Sp: sporinite), vitrinite (Dt: detrovitrinite), and inertinite (Sf: semifusinite). (b) Coal with a vitrinite random reflectance of 0.90%. Inertinite (Sf: semifusinite) and vitrinite (Tl: telovitrinite). (c) Coal with a vitrinite random reflectance of 0.63%. Inertinite (F: fusinite with broken cell walls). (d) Coal with a vitrinite random reflectance of 0.63%. Inertinite (F: fusinite), liptinite (Sp: sporinite), and vitrinite (Dt: detrovitrinite). (e) Coal with a vitrinite random reflectance of 0.70%. Vitrinite (T: telinite), liptinite (R: resinite in cell cavities), and inertinite (Mi: micrinite generated from hydrogenate macerals such as the sporinite). (f) Coal with a vitrinite random reflectance of 0.97%. Vitrinite (Tl: telovitrinite) and inertinite (Mi: micrinite). (g) Coal with a vitrinite random reflectance of 0.65%. Liptinite (Ct: cutinite) and vitrinite (Dt: detrovitrinite). (h) Blend of coals with vitrinite (V) random reflectances of 1.40 and 0.88%, used in carbonization processes. (Photomicrographs: I. Sua´rez-Ruiz.)

These maceral groups are subdivided into a variety of macerals, submacerals, and maceral varieties on the basis of their reflectance, degree of destruction/preservation of original material, presence of cellular structure, gelification, and morphological features. The three maceral groups differ in both chemical composition and optical properties, and their names conventionally end in –inite. Macerals of the liptinite group (Table 2.1 and Figure 2.1a, d, e, g) include all the chemically distinct parts of plants such as spores, cuticles, suberine cell walls, resins and polymerized waxes, fats and oils of vegetable origin, some degradation products, and products of secondary generation during the coalification process (coal evolution). These macerals have the highest hydrogen content and contain compounds of mainly an aliphatic nature. Their color in reflected light is dark and so their reflectance is the lowest among the maceral groups. Most of the macerals of this group display a fluorescence of variable intensity when excited with short wavelength radiation, although this property disappears with increasing coal rank. During coalification most macerals of this group disappear due to thermal transformation, or they develop similar optical properties (reflectance) to those of the vitrinite group at the medium volatile bituminous coal rank stage. The influence of this maceral group in the technological properties of coal is related to the proportion in which it occurs. Because of their high hydrogen content, liptinite macerals yield high proportions of tars and gases during the carbonization process. Members of the liptinite group also have a high calorific value. The sensitivity to oxidation of liptinite macerals is low, and the hydrogenation capacity of

Maceral

Origin

Petrographic Characteristics

Sporinite

From pollen and spores

Cutinite Resinite

From cuticles of leaves Diverse origins: resins, waxes

Alginite

Algal or bacterial

Suberinite

From suberous tissues

Chlorophyllinite

From chlorophyllic pigments

Liptodetrinite

Fragments from the other liptinite macerals From vegetable oils

Individual bodies, usually compressed, well preserved, distinct botanical form, high relief, variable wall thickness and size. Most frequent maceral of this group in coals. Elongated bodies, serrated edges, well preserved, high relief. Individual ovoid, globular and irregular bodies, cell fillings, impregnations on vitrinite, relief þ / nul, red internal reflections, different properties according to its nature. Rare in humic coals, main component in sapropelic coals. Telalginite: Individual bodies (discs) or colonies, rounded, elongated, semicompressed morphologies, internal structure, intense fluorescence. Lamalginite: lamelae, thickness <5 microns and variable length, no internal structure, lower fluorescence intensity than telalginite. Rare, with cell structure, anisotropic, intense fluorescence. Only found in lignites. Rare, variable morphology, intense red fluorescence, only known in lignite stage preservation under strong anaerobic conditions. Small detrital fragments, diverse properties.

Fluorinite Bituminite Exudatinite

Degradation product from algae, bacterial, zooplankton From hydrogenated substances and liptinite macerals

Intense fluorescence (yellow), brilliant color, fluorescence alteration: weak, even negative, in lenses or without definite shape, black in normal white light. Amorphous, ground-mass for other macerals, orange-brown fluorescence, strong positive alteration. Secondary maceral, filling voids and cell cavities, strong fluorescence intensities of varying colors, variable fluorescence alteration, black in normal white light.

Basic Factors Controlling Coal Quality and Technological Behavior 23

TABLE 2.1 Liptinite maceral group, petrographic characteristics (Data compiled from ICCP, 1971, 1975.)

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Applied Coal Petrology

liptinite is excellent. In wider fields of geology some liptinites, such as sporinite (Figure 2.1a, d), have been used in coal seam correlation and as facies indicators. Other liptinite macerals (chlorophyllinite) have been used to assess the anaerobic conditions and the weather existing at the time of peat formation. The inertinite maceral group (Table 2.2 and Figure 2.1a–d, f) is derived from plant material that was strongly altered and degraded under oxidizing conditions before deposition, or by redox, biochemical, and chemical processes at the peat stage. One maceral of this group (micrinite, Figure 2.1e, f) may also be generated by transformation of more hydrogenated macerals (Figure 2.1e) during the coalification process (ICCP, 2001). Inertinite macerals exhibit a high degree of aromatization and condensation and are made up of structures that are mainly of an aromatic character with a high level of cross-linking. They have the highest carbon and lowest oxygen and hydrogen contents of the maceral groups (van Krevelen, 1993). The components of this group are more inert (less reactive in carbonization) than the macerals of the other groups. Their color in reflected white light is grey or greyish white to yellowish. Their reflectance is higher than that of the other groups, but this also depends on the chemical composition of the various inertinite macerals (ICCP, 2001). When blue-violet to green light excitation is used, low-reflecting inertinite macerals are fluorescent (Diessel, 1985). Subdivision of the inertinite macerals (ICCP, 2001) depends on the presence or absence of vegetable structures or whether they represent fragmentary material. High proportions of highly reflecting inertinite macerals such as fusinite (friable, Figure 2.1c, d) may lead to the formation of dust during mining (ICCP, 2001). In coal-cleaning processes the organic fraction of the high-density washery fractions may be enriched in inertinite, mainly because of the frequent intergrowth of these macerals with minerals. Depending on the coal rank and the types of inertinite present, different chars are generated during combustion, some of which are highly reactive (Thomas et al., 1993, and Borrego et al., 1997). In coking processes the reactivity of the inertinite depends on the physicochemical characteristics, grain size and heterogeneity of the individual macerals, the coal rank, and so on (ICCP, 2001). The inertinite macerals with lower reflectance and stronger fluorescence tend to have higher reactivity and to be fusible during carbonization and coking. An optimum inertinite content with the appropriate grain size generates a coke of maximum strength and stability. Some of the inertinite (finely dispersed fusinite) also improves the coke strength. Macerals of the huminite/vitrinite groups (see Tables 2.3 and 2.4) originated mainly from lignin and cellulose and partly from tannins and colloidal humic gels. Proteins and lipid substances may also have played a part in the formation of the macerals of this group

Maceral

Origin

Petrographic Characteristics

Fusinite

From ligno-cellulosic cell walls

Semifusinite

From parenchymatous and xylem tissues of stems, herbaceous plants and leaves

Funginite

From fungal spores and tissues, sclerotia, mycelia No totally clear, oxidation product of resins, humic gels From alteration of humic substances, metabolic product of fungi and bacteria, from coprolites, etc. Coalification product, residues of former lipoid substances, strong fragmentation of other inertinites From phytogenetic material subjected to fusinization

Well-preserved cell walls, open cell lumina, bogen or star structure, high reflectance, white to yellowish color Well to semi-preserved cell walls, smaller cell lumina often closed, color and reflectance between those of vitrinite/ fusinite. Deformed cell cavities. Anisotropic Ovoid bodies of fungal remains, sclerotia, hyphae, mycelia, with cell structure high reflectance Round, ovoid, polygonal, vesicular bodies, without cellular structure, at times vesiculated, high reflectance Amorphous bodies of irregular shape, structureless rounded fragments (>10 microns), compact appearance, high reflectance Very fine-grained material. Fine particles of small size (2 microns), high reflectance

Secretinite Macrinite

Micrinite

Inertodetrinite

Without structure, fragments of size <10 microns

Basic Factors Controlling Coal Quality and Technological Behavior 25

TABLE 2.2 Inertinite maceral group, petrographic characteristics (Data compiled from ICCP, 2001.)

Telohuminite

Detrohuminite

Gelohuminite

Origin

Maceral

Petrographic Characteristics

Cell walls of parenchymatous and woody tissues composed of cellulose and lignin

Textinite

Precursor of telovitrinite

Ulminite

From strong decay of parenchymatous and woody tissues of stem and leaves.

Attrinite

Precursor of detrovitrinite

Densinite

Diverse origins: intensely gelified plant tissues and humic detritus, from precipitated humic colloids and from primary phlobaphenic cell fillings

Corpohuminite

Precursor of most of gelovitrinite

Gelinite

Primary cell wall structure still distinguishable, cell lumina open, isotropic, variable fluorescence High degree of humification; cell wall structure still visible, or not visible, cell lumina closed, variable fluorescence intensity decreasing with increasing coal rank Structural degradation product, particle size <10 microns, spongy texture, low fluorescence intensity in the high wave lengths but it depends on its composition. More packed than attrinite, more or less homogeneous huminitic groundmass, nonor very weak dark fluorescence. Globular to tabular morphologies, without structure, compact or cavernous variable size, without fluorescence. Two maceral types are distinguished: phlobaphinite and pseudo-phlobaphinite. Homogeneous structureless or porous, it can fill cavities of other macerals, without fluorescence. Two maceral types are distinguished: levigelinite and porigelinite.

Applied Coal Petrology

Maceral Subgroup

26

TABLE 2.3 Huminite maceral group (low rank coals), petrographic characteristics (Data compiled from Sy´korova´ et al., 2005.)

Maceral Subgroup Telovitrinite

Origin

Maceral

Petrographic Characteristics

From parenchymatous and woody tissues composed of cellulose and lignin

Telinite

Well-preserved cell walls, size, shape and openness of cell lumens are variable, empty or filled with other macerals or minerals. Similar fluorescence to the collotelinite in the same coal. Relatively homogenized, more or less structureless appearance. Its reflectance value is an index of the coal rank. It fluoresces over a wide rank range (high volatile bituminous to semi-anthracite). Small vitrinitic fragments (<10 microns), variable shape. Difficult to distinguish from other macerals when increasing rank. Variable fluorescence.

Collotelinite

Detrovitrinite

From strong decay of parenchymatous and woody tissues of stem and leaves originally composed of cellulose and lignin

Vitrodetrinite

Collodetrinite

Gelovitrinite

From jelling humic solutions and not corresponding to specific plant tissues. From contents of plant cells or humic fluids.

Corpogelinite

Gelinite

Mottled and compact vitrinitic groundmass, binds other coal components. Variable color and fluorescence intensity. Structureless bodies, homogeneous, variable shape, humic cell fillings in situ or isolated. Higher reflectance than the other vitrinitic macerals. Weak fluorescence or no fluorescence. Pure colloidal gel, homogeneous aspect, without structure, filling cracks and cavities, variable shape and size, higher reflectance. Weak fluorescence or not fluorescence. Least common maceral.

Basic Factors Controlling Coal Quality and Technological Behavior 27

TABLE 2.4 Vitrinite maceral group (medium and high rank coals), petrographic characteristics (Data compiled from ICCP, 1998.)

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Applied Coal Petrology

(ICCP, 1971, 1998, and Sy´korova´ et al., 2005). Their chemical structure is represented by aromatic compounds and hydroaromatics in lower-rank coals, but with increasing coal rank the aromaticity, condensation, and order of the polyaromatic units increase. The successive sets of processes by which the vegetable tissues are transformed into huminite and later into vitrinite are known as humification, gelification, and vitrinization (Stach et al., 1982, and Teichmu¨ller, 1989). Huminite macerals are identified in low rank coals (Sy´korova´, 2005), and these are regarded as the precursors of vitrinite macerals (see Figure 2.1a–b, d–h) in higher rank coals (ICCP, 1971, 1998, and Sy´korova´ et al., 2005). The members of the huminite/vitrinite group are organized in each case into three subgroups (ICCP, 1971, 1975, 1998, and Sy´korova´, 2005) and six macerals (see Tables 2.3 and 2.4). The chemical composition and most of the properties of the macerals of these groups are rank dependent. The color of huminite/vitrinite macerals in polished section is medium grey and the macerals have reflectances (see Figure 2.1a, d) generally between those of the associated darker liptinites and the lighter inertinites over the rank interval in which the three maceral groups are identified. Huminite is generally isotropic, but anisotropy (bireflectance) occurs if remnants of cellulose are present. In the case of the vitrinite group, bireflectance normally increases with increasing coal rank (ICCP, 1998). The color and intensity of fluorescence in huminite macerals are variable, depending on rank, level of degradation, and humification (Sy´korova´ et al., 2005). For vitrinite macerals the fluorescence depends on the maceral type, the coal rank, and the degree of bituminization (ICCP, 1998). Chemically the huminite and vitrinite groups have relatively high oxygen contents compared with the other two maceral groups. The technological properties of the low rank coals in which the huminite macerals are identified are linked to those of the predominant huminite maceral, and it is the degree of humification and gelification of the huminite that influences most of the technological properties of these coals in industrial processes (Sy´korova´ et al., 2005, and references therein). In higher rank coals, where the vitrinite group is the major component, the properties of the vitrinite also influence those of the coal. Vitrinite in medium rank coals fuses in carbonization (coking) processes (see Chapter 7), and this property is also important in combustion and hydrogenation (ICCP, 1998). The technological properties of vitrinite macerals are in some cases related to their fluorescence properties, as has been demonstrated for collotelinite (Taylor et al., 1998). The reflectance measured in this maceral is universally used as an index of coal rank and as an indicator of the maturation of dispersed organic matter. Depending on its composition and rank, vitrinite may be oxidized during prolonged storage and shipping because of exposure to air and moisture, significantly reducing some of its otherwise desirable properties.

Basic Factors Controlling Coal Quality and Technological Behavior 29

Maceral Analysis For geological research on coal basins and for an evaluation of coal seam quality it is important to know the quantitative composition of a coal in terms of the macerals (and minerals in some cases) or maceral groups. This is because differences in maceral composition may indicate differences in chemical composition and consequently differences in the technological properties of a coal. Maceral analysis is standardized in the ISO 7404/3 (1994a) and ASTM D2799-05 (2005a) norms and is used to determine on a volumetric basis the relative proportions of the coal components in a representative coal sample. Coal pellets should be particulate and prepared for petrographic analysis according to ISO 7404/2 (1985), ASTM D2797-04 (2004) or equivalent procedures. The petrographic microscope for maceral analysis should be equipped with incident white light, oil immersion objectives (25
-60
magnification), and 8
to 12.5
oculars, one of which must contain an adjustable eyepiece with a micrometer or cross-hair. Maceral analysis can be performed using a manual or an automatic point counter coupled to the microscope stage. Although maceral analysis must be carried out in white light, supplementary observations in fluorescence mode are recommended so that some components of the liptinite group will not be undercounted, especially in low rank coal analysis. To ensure the required precision, a total of 500 points should be counted in a maceral analysis. The results are reported on a volume percentage basis for each category under consideration. The equations used to calculate the probable margin of error and repeatability are described in ISO 7404/3 (1994a). Microlithotypes Microlithotypes (Table 2.5) are the natural assemblages of macerals at microscopic level. They are defined as having a minimum bandwidth (perpendicular to the stratification) of 50 mm and as containing at least 5% of a maceral group (ICCP, 1963, 1971, and Taylor et al., 1998). In addition to the maceral content, 20–60% (vol.) of silicate or carbonate minerals or 5–20% (vol.) sulfide minerals redefines the microlithotype as a carbominerite. Microlithotypes may be mono-, bi-, or trimaceralic, and their names conventionally end in –ite. The chemical properties of microlithotypes are very similar to those of the predominating macerals (Stach et al., 1982, and Falcon and Snyman, 1986). Their physical properties, however, are related not only to those of the macerals but also to the combined effect of the association. The microhardness of bi- and trimaceralic microlithotypes is always higher than that of monomaceralic associations. The density of the microlithotypes varies with rank, maceral composition, and size, as well as the form

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TABLE 2.5 Microlithotypes of hard coals (Data from Taylor et al., 1998.) Microlithotype

Maceral Composition

Vitrite Liptite Inertite Clarite Durite Vitrinertite Duroclarite Vitrinertoliptite Clarodurite Carbargilite Carbopyrite Carbankerite Carbosilicate Carbopolyminerite

vitrinite (V) >95% liptinite (L) >95% inertinite (I) >95% V þ L >95% I þ L > 95% V þ I > 95% V > L, I (each >5%) L > V, I (each >5%) I > V, L (each >5%) Coal þ 20-60% (vol.) clays Coal þ 5-20% (vol.) sulfides Coal þ 20-60% (vol.) carbonates Coal þ 20-60% (vol.) quartz Coal þ 20*-60% (vol.) various minerals *5% if high pyrite

Group Monomaceralic

Bimaceralic

Trimaceralic

Carbominerite

Source: Organic Petrology, G. H. Taylor, M. Teichmu¨ller, A. Davis, C. F. K. Diessel, R. Littke, and P. Robert, 704 pp., copyright 1998, with permission from Gebru¨derBorntraeger (www.borntraeger-cramer.de).

and quantity of associated minerals (Stach et al., 1982). The degree of heterogeneity in a microlithotype is also important in its technological behavior, particularly in carbonization (see Chapter 7), combustion (see Chapter 4), and gasification (see Chapter 5) processes. The maceral, mineral, and microlithotype composition of a coal seam may change over short distances both vertically and laterally, in response to the conditions existing during the formation of the original peat swamps (Stach et al., 1982). These changes can be quantified by petrographic assessment of the microlithotypes in relevant coal samples. Microlithotype Analysis Microlithotype analysis is used to determine the relative proportions of the various microlithotypes and coal-mineral associations (carbominerites) present in a coal sample (ICCP, 1963). The procedure is standardized as indicated in the ISO 7404/4 (1988) norm. Although microlithotype analysis is carried out in a similar manner to maceral analysis, a suitable 20-point reticule must be placed in one of the oculars of the microscope as a substitute for the micrometer or cross-hairs. Two conventions (ICCP, 1963) must be observed: (1) the minimum bandwidth of the association to be measured must be 50 microns, and (2) macerals present in the association in amounts

Basic Factors Controlling Coal Quality and Technological Behavior 31

smaller than 5% should be disregarded (the 5% rule). Each observation on a 20-intersection reticule is regarded as one point in the analysis, and each intersection on the reticule represents 5% of the total number of intersections (20), providing guidance in use of the 5% rule. For a complete microlithotype analysis, at least 500 points should be measured, and the results should be expressed as volume percentages. Microlithotype analysis is less accurate than maceral analysis. The calculation of repeatability and reproducibility (ISO 7404/4, 1988) is made in the same way as for maceral analysis.

2.2.2 Elemental Composition of Coal Macerals Chemical analysis of coal provides data gathered from “whole-coal” materials, embracing moisture and mineral matter as well as the organic constituents. The data from ultimate analysis (C, H, O, N, and S percentages) may be corrected to a moist, mineral matter-free (mmmf); dry, mineral matter-free (dmmf); or dry, ash-free (daf) basis to assess composition of the organic matter alone, but even so the composition of the organic matter determined in this way inherently represents an aggregation of the composition of the different maceral components. Variations in chemical composition indicated by ultimate analysis data derived from whole-coal samples therefore reflect variations in the coal type (i.e., the mixture of macerals present) as well as the rank of the coals concerned. As indicated previously coal is a heterogeneous solid, and it is the individual macerals within the coal that react, both independently and with each other, when the coal is used. In addition to providing further insights into the coalification process, knowledge of maceral chemistry may therefore be of value to understand the processes associated with factors such as burning rate, emission release, CO2 generation, fouling and slagging, as well as reactions during gasification and coking associated with the different coal components. Although some success has been achieved in maceral separation through density gradient centrifugation (see below), it is inherently difficult to cleanly isolate the individual macerals in a coal for separate chemical analysis without contamination by minerals or other organic components. The development of special techniques for light-element analysis using the electron microprobe (e.g., Bustin et al., 1993, 1996, and Mastalerz and Gurba, 2001) provides a mechanism for directly determining the elemental composition of the individual macerals in coal-polished sections by analyzing selected areas only a few micrometers in size, without the need for a prior maceral separation process. Electron microprobe techniques have been used to evaluate the elemental composition of the individual macerals in a number of North American and Australian coals (Mastalerz and

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Bustin, 1993, 1997; Ward and Gurba, 1998; Gurba and Ward, 2000; and Ward et al., 2005, 2007), helping to explain more fully the variations in coal composition indicated by whole-coal analysis as well as some of the geological processes associated with coal formation. Details of the procedures used for microprobe analysis of coal macerals are given by Bustin et al. (1993, 1996), Mastalerz and Gurba (2001), and Ward et al. (2005). Samples for electron microprobe study are prepared as polished sections in the same way as for optical microscopy, although specific-sized mounts may be needed to suit the microprobe’s sample handling facilities. The sections, and also the relevant calibration standards, are coated with a thin film of carbon, to provide a conductive surface (Bustin et al., 1993) prior to the analysis process. An accelerating voltage of 10 kV is used for the electron beam in most studies, with a filament current 20 nA. An overall magnification of 20,000x gives a spot size for the electron beam of around 5 to 10 mm in diameter on the sample for the actual measurement process. As discussed by Bustin et al. (1993), an independently analyzed anthracite sample provides a more effective calibration standard than graphite for carbon in microprobe analysis of coal macerals. Separately analyzed mineral samples are generally used as standards for other elements (Ward et al., 2005), such as O, N, S, Ca, Al, Si, and Fe. Care should be taken to avoid analyzing areas of the coal where visible minerals are also present. Points that include mineral contaminants may, for example, be indicated by high Si or unexpectedly high Fe and S percentages. Points that include some of the mounting epoxy resin may be indicated by unusual oxygen and high nitrogen contents. Figure 2.2 indicates the changes in elemental composition of the vitrinite and inertinite macerals in coals from the Bowen Basin in Australia (Ward et al., 2005) with variation in rank over a range, as indicated by vitrinite reflectance, from subbituminous coal to semianthracite. For a given rank level the vitrinite in these coals, especially the collotelinite, has the lowest carbon and highest oxygen contents, while the inertinite, especially the inertodetrinite, has the highest carbon and lowest oxygen contents. The difference in composition between the vitrinites and the inertinites decreases steadily as the rank (vitrinite reflectance) increases; the maceral groups have quite different compositions in lower rank coals, but only very small differences at the upper end of the rank range. Because it is applied directly to the organic material, electron microprobe analysis provides a mean of directly measuring the organic sulphur content of coal macerals, a parameter that is only determined indirectly for coals by conventional analysis techniques. As indicated in Figure 2.2 and also in other studies (e.g., Ward and Gurba, 1998), the vitrinite macerals have significantly higher organic

Carbon

Oxygen 30

Collotelinite Collodetrinite Semifusinite Fusinite Inertodetrinite

80

70

Oxygen in maceral %

90

Collotelinite

20

Collodetrinite Semifusinite Fusinite Inertodetrinite

10

60

0 0.0

0.5

(a)

1.0

1.5

2.0

2.5

3.0

3.5

0.0

4.0

0.5

(b)

Rv max (collotelinite) %

1.0

2.5

3.0

3.5

4.0

3.0

0.75

Collotelinite Collodetrinite Semifusinite Fusinite Inertodetrinite

0.50

0.25

0.00

Nitrogen in maceral %

Sulphur in maceral - %

2.0

Organic Nitrogen

Organic Sulphur 1.00

Collotelinite

2.0

Collodetrinite Semifusinite Fusinite Inertodetrinite

1.0

0.0 0.0

(c)

1.5

Rv max (collotelinite) %

0.5

1.0

1.5

2.0

2.5

3.0

Rv max (collotelinite) %

3.5

0.0

4.0

(d)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Rv max (collotelinite) %

FIGURE 2.2. Plots showing percentages of (a) carbon, (b) oxygen, (c) organic sulphur, and (d) organic nitrogen in different macerals of Australian (Bowen Basin) coals in relation to the coal rank as measured by vitrinite reflectance. (Source: International Journal of Coal Geology 63, by C. R. Ward, Z. Li, and L. W. Gurba, “Variations in coal maceral chemistry with rank advance in the German Creek and Moranbah Coal Measures of the Bowen Basin, Australia, using electron microprobe techniques,” 117–129, copyright 2005, with permission from Elsevier.)

Basic Factors Controlling Coal Quality and Technological Behavior 33

Carbon in maceral %

100

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sulphur contents than the fusinite and inertodetrinite, with semifusinite typically having intermediate organic sulphur contents. Electron microprobe studies typically show that the vitrinite macerals have around twice the organic sulphur of the fusinite and inertodetrinite in the same coal samples. There is also evidence, especially in some high-sulphur coals, that the sulphur replaces oxygen in the vitrinite’s chemical structure (Ward et al., 2007). Figure 2.2 also indicates that the nitrogen content of the vitrinite macerals is generally higher than that of the inertinite components, especially fusinite and inertodetrinite, in the same coal samples. This is further supported by the work of Mastalerz and Gurba (2001). For the coals on which Figure 2.2 is based, however, the difference in nitrogen content between these maceral groups appears to decrease at high rank levels (semi-anthracite and above), possibly due to redistribution of the organic nitrogen into some of the clay minerals (Ward et al., 2005).

2.2.3 Organic Geochemistry Two significant works on the organic geochemistry of coal published in the last 25 years are the papers by Given (1984) and Hatcher and Clifford (1997). Because vitrinite is the dominant maceral in most coals, the present review is primarily confined, following the works of those authors, to the transformation of wood to vitrinite. Of the original components of wood, cellulose is preferentially lost, whereas lignin is retained. The preservation of plant structures in brown coals indicates that at least some of the chemical transformation is not accompanied by maceration of the wood structure. The transformation of lignin to lignite involves processes such as demethylation, dehydroxylation, and the cleavage of b-O-4 aryl ethers (Figure 2.3). More significant structural alteration of plant structures occurs at subbituminous rank, with some annealing of the plant cells. The chemical transformations involve the sidechain dehydroxylation and dehydroxylation of catechols. The latter involves condensation to hydroxylated diaryl ethers and the loss of water from the structure. With further coalification, the ether bond is cleaved, resulting in the formation of a catechol-like structure and a phenolic structure. The pathway to high volatile bituminous coal involves the condensation of phenols to aryl ethers or dibenzofuran-like structures. Overall, the aromaticity of vitrinite increases with increasing rank, implying the condensation of benzene-like structures with aliphatic functional groups to polycyclic aromatic structures. Density gradient centrifugation (DGC), a tool for obtaining narrow-density fractions from a coal (Dyrkacz et al., 1981), can be used

Reactions of lignin to form brown coal and lignite dehydroxylation CH2OH

CH2 CH

CH

alkylation

HCOH

CH2OH HC

OCH3 O

HCOH

CH2OH

β-O-4 ether cleavage

HC

OCH2 OH

HCOH

HCOH

demethylation

OCH3

OH OH

OH

Reactions of lignite to form subbituminous coal CH2

CH2

side-chain dehydroxylation

CH

CH CH2

HCOH

CH3

CH2OH HC

OH

HC

OH

OH

CH2

HCOH

dehydroxylation of catechols OH OH

OH

Reactions leading to bituminous coal CH2

CH2

CH

CH

CH2 CH3 HC CH2

CH2 CH3

OH

HC CH2

OH

FIGURE 2.3. Transformation of lignin to lignite to subbituminous coal to bituminous coal. (Source: Organic Geochemistry 27, by P. G. Hatcher and D. J. Clifford, “The organic geochemistry of coal: from plant materials to coal,” 251–274, copyright 1997, with permission from Elsevier.)

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to study chemical variation between macerals. Various studies have been conducted, including an investigation of a number of British and U.S. bituminous coals using Curie-point pyrolysis mass spectroscopy (Meuzelaar et al., 1984); a study of a high volatile C bituminous Pennsylvanian Indiana coal using Curie-point pyrolysis mass spectroscopy and the latter technique coupled with gas chromatography (Nip et al., 1992); and a study of a medium volatile bituminous Permian Australian coal using electron spin resonance (ESR), crosspolarization 13C nuclear magnetic resonance (NMR), and single-pulse 13 C NMR excitation (Maroto-Valer et al., 1998). In general, all these results demonstrated that macerals increased in aromaticity from liptinite to vitrinite to inertinite. Hower et al. (1994a) examined both the organic and inorganic geochemistry of a DGC sample taken from a 0.4%-ash high volatile A bituminous lithotype from a Pennsylvanian coal bed of Kentucky (U.S.). By means of Fourier-transform infrared (FTIR) spectroscopy, they demonstrated that the indicators of aliphatic bonds were strongest in the density concentrates dominated by liptinite macerals (no liptinite macerals were found in near-monomaceral concentrates, unlike the vitrinite and inertinite group macerals) and that the indicators of aromatic bonds were strongest in the >95% vitrinite concentrates but not as strong in the inertinite-rich fractions (Figure 2.4). Coincident with an increase in aromaticity, the Blue Gem (Rimmer et al., 2006) and the Australian (Maroto-Valer et al., 1998) maceral concentrates showed a decrease in atomic H/C with an increase in density (liptinite to vitrinite to inertinite for the Blue Gem coal; vitrinite to semifusinite for the Australian coal). Mastalerz and Bustin (1993, 1996), Walker and Mastalerz (2004), and Li et al. (2006) have used FTIR, sometimes in conjunction with other techniques (Mastalerz et al., 1998), to study macerals on a microscopic scale. They noted an increased aromatic character in the vitrinite and inertinites with an increase in rank and a shift toward greater aromaticity from liptinite to vitrinite to inertinite.

2.3 Coal Composition: Inorganic Components 2.3.1 Minerals and Mineral Matter As discussed by Ward (2002), the material classed as “mineral matter” embraces all the minerals and other inorganic elements occurring in coal, including (1) dissolved salts and other inorganic substances in the pore water of the coal, (2) inorganic elements incorporated within the organic compounds of the coal macerals, and (3) discrete inorganic particles (crystalline or noncrystalline) representing the actual mineral components.

Basic Factors Controlling Coal Quality and Technological Behavior 37

FIGURE 2.4. Maceral concentration versus aliphatic C-H and aromatic C=C for maceral density concentrates of the Blue Gem coal bed, Kentucky. Note that the liptinite concentrate contains little more than 80% litpinite, with much of the remainder being vitrinite. The vitrinite and inertinite concentrates exceed 95% purity. Mineral matter is negligible in most of the concentrates. (Source: Energy and Fuels 8, by J. C. Hower, D. N. Taulbee, S. M. Rimmer, and L. G. Morrell, “Petrographic and geochemical anatomy of lithotypes from the Blue Gem coal bed, southeastern Kentucky,” 719–728, copyright 1994, with permission from American Chemical Society [ACS].)

The first two forms, sometimes described as nonmineral inorganics, are typically most abundant in the mineral matter of lower rank coals (Kiss and King, 1977, 1979; Given and Spackman, 1978; Benson and Holm, 1985; Miller and Given, 1986; Given and Miller, 1987a,b; and Ward, 1991, 1992). Although there are some exceptions (e.g., Ward et al., 2007), the nonmineral inorganics usually disappear from the coal with an increase in rank. Discrete mineral particles, however, may occur in coal of any rank and are usually the dominant component of

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the mineral matter in higher rank materials (Rao and Gluskoter, 1973; Ward, 1977, 1978; Renton, 1986; and Ward et al., 2001). Coals produced from mines may also contain minerals derived from intra-seam noncoal bands or admixed roof or floor strata. This material (extraneous mineral matter) may be at least partly removed by cleaning processes in coal preparation plants. Mineral matter closely associated with the macerals (inherent mineral matter), however—including both intimately admixed minerals and nonmineral inorganics in the maceral components—remains a part of the clean coal product and must be taken into account when assessing the behavior of a coal with handling, storage, and use. Determination of Mineral Matter Content Many of the minerals occurring in coal undergo major chemical changes at the high temperatures associated with combustion and ash formation, including the loss of CO2 from carbonates, the loss of structural water from clay minerals, and the loss of sulfur from sulfides (Rees, 1966; Raask, 1985a; Vassilev et al., 1995; and Reifenstein et al., 1999). The nonmineral inorganics in the macerals may also react with some of the other coal components to form mineral artifacts, such as sulfates, in the ash residue. Because of such changes, the percentage of ash determined in routine coal analysis is usually less than the percentage of mineral matter contained in the original coal sample. The chemical composition and crystal structure of the ash may also be somewhat different from the chemical composition and structure of the original mineral matter. One of the most widely used methods for determining the percentage of mineral matter (as opposed to ash) involves removing the organic matter at low temperature (around 120 C) by exposing the coal to a reactive oxygen plasma produced by a radio-frequency electromagnetic field (Gluskoter, 1965; Frazer and Belcher, 1973; Miller, 1984; and Standards Australia, 2000). The residue remaining after oxidation of the organic matter consists of the essentially unaltered mineral components of the original coal, together in some cases with additional artifacts produced from the nonmineral inorganic components. Exposing the coal to air at around 370 C (Brown et al., 1959, and Ward et al., 2001) or treating the coal with hot concentrated hydrogen peroxide to oxidize the organic components (Nawalk and Friedel, 1972, and Ward, 1974) may also serve to isolate a mineral residue, although these techniques may irreversibly alter some of the mineral components. Calculations based on the ash percentage and ash composition, combined with other chemical data, may also be used to provide an estimate of the mineral matter content (e.g., King et al., 1936; Rees, 1966; Given and Yarzab, 1978; Pollack, 1979; and Scholz, 1980).

Basic Factors Controlling Coal Quality and Technological Behavior 39

Mineral Analysis in Coal and LTA Samples The identity of the crystalline minerals in coal or LTA residues can be evaluated by X-ray diffraction techniques (Rekus and Haberkorn, 1966; O’Gorman and Walker, 1971; Rao and Gluskoter, 1973; Ward, 1977, 1978; Russell and Rimmer, 1979; Renton, 1986; and Harvey and Ruch, 1986). Semiquantitative methods were used in many early studies, based on comparing key peak intensities with intensities associated with known proportions of added-in crystalline spike components. More recent XRD analyses, however, are based on the fullprofile analysis methods developed by Rietveld (1969). The Rietveld approach allows a calculated XRD profile of a sample to be generated from the structural parameters of each mineral present and to be adjusted iteratively by least-squares techniques to fit the observed XRD profile of the analysis sample (Taylor, 1991). Rietveld-based XRD techniques have been applied to the analysis of the minerals in both LTA and whole-coal samples (e.g., Mandile and Hutton, 1995; Ward and Taylor, 1996; Ward et al., 1999, 2001; and Ruan and Ward, 2002), with independent checks against ash analysis and other data confirming the consistency of the mineralogical evaluations. French et al. (2001a) used a Rietveld-based technique to determine the overall percentage of crystalline mineral matter in the coal, as well as the relative proportions of each mineral, by performing a direct XRD analysis of the whole-coal samples without a lowtemperature ashing step to concentrate the mineral components. Structure models were developed separately for the organic matter of coals at different rank levels on the basis of XRD traces derived from chemically demineralized coals, and these were incorporated into the Rietveld analysis to allow the organic matter to be quantified as if it was another “mineral” phase. The proportions of microscopically visible minerals in a coal sample may also be determined at the same time as the percentages of the different maceral components by means of the microscopic point-count analysis (Davis, 1984, and Taylor et al., 1998). Some mineral occurrences intimately associated with the macerals may, however, be inadvertently overlooked by the point-counting process. The mineral percentages determined by point counting are also volumetric percentages, whereas the mineral matter evaluated by low-temperature ashing and similar methods is expressed as a mass percentage. Conversion of volumetric percentages to mass percentages for silicate materials involves approximately doubling the volumetric percentage values (Davis, 1984) and multiplying by even higher factors for sulfides and other dense mineral materials. Mineral particles in coal can be investigated by using a scanning electron microscope and similar techniques (e.g., Stanton and Finkelman, 1979; Russell and Rimmer, 1979; Allen and Vander Sande, 1984;

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Martinez-Tarazona et al., 1992; Hower et al., 1994a, 2000a; and Ward et al., 1996). The automatic collection of element data by means of computer-controlled scanning electron microscopy (CCSEM) techniques (Straszheim and Markuszewski, 1990; Galbreath et al., 1996; and Gupta et al., 1998, 1999a), including element associations, serves to evaluate more fully the nature and distribution of minerals in coal samples. More advanced CCSEM techniques, including the QEM*SEM and QEMScan systems (Gottlieb et al., 1992; Creelman et al., 1993; and Creelman and Ward, 1996), allow the integration of SEM data with image analysis methods. In such operations the electron beam passes over relevant parts of a polished section, stopping at predetermined intervals to collect X-ray spectra. As each X-ray spectrum is collected it is processed through a species identification protocol (SIP) and the mineral represented at that point is identified from its chemical characteristics. Individual mineralogical identifications are thus made at each point (or pixel) in the area scanned by the electron beam, and these can be used to generate a digital map showing how the various minerals occur within the coal sample. Data from such systems can be used to provide a variety of mineral matter information, including the relative abundance of the different minerals in the coal; the particle size, shape, and mode of occurrence of particular minerals; the textural associations between minerals; and the degree of liberation of minerals and mineral aggregates from organic matter associated with coal-crushing and pulverization processes. More precise determination of the composition of particular minerals may also be obtained from electron microprobe analysis of polished coal sections, using methods outlined by Reed (1996). Electron microprobe and similar techniques have been applied to minerals in coal by authors such as Minkin et al. (1979), Raymond and Gooley (1979), Kolker and Chou (1994), Patterson et al. (1994), and Zodrow and Cleal (1999). They have also been used to investigate the elemental composition of individual coal macerals (Bustin et al., 1993, 1996; Gurba and Ward, 2000; and Ward et al., 2005), including the occurrence of nonmineral inorganic elements. The nature and relative abundance of iron-bearing phases in coal may be evaluated by Mo¨ssbauer spectroscopy (Gracia et al., 1999). A wide range of other methods have also been used to identify the minerals present in coal samples, including thermal analysis (Warne, 1964), Fourier-transform infrared (FTIR) spectrometry (Painter et al., 1978), and a range of other instrumental techniques (Ward, 2002; Huggins, 2002; and Vassilev and Tascon, 2002). Minerals in Coal and LTA Residue A list of the minerals that may be found in coals or LTA residues is given in Table 2.6. The most abundant of these are usually clay minerals, although quartz, pyrite, siderite, calcite, and dolomite or

Basic Factors Controlling Coal Quality and Technological Behavior 41

TABLE 2.6 Principal minerals found in coal and LTA (Data from Ward, 2002.) Silicates Quartz Chalcedony Clay minerals: Kaolinite Illite Smectite Chlorite Interstratified clay minerals Feldspar

SiO2 SiO2 Al2Si2O5(OH)4 K1.5Al4(Si6.5Al1.5)O20(OH)4 Na0.33(Al1.67Mg0.33)Si4O10(OH)2 (MgFeAl)6(AlSi)4O10(OH)8

Tourmaline Analcime Clinoptilolite Heulandite

KAlSi3O8 NaAlSi3O8 CaAl2Si2O8 Na(MgFeMn)3Al6B3Si6O27(OH)4 NaAlSi2O6H2O (NaK)6(SiAl)36O7220H2O CaAl2Si7O186H2O

Sulfides Pyrite Marcasite Pyrrhotite Sphalerite Galena Stibnite Millerite

FeS2 FeS2 Fe(1x)S ZnS PbS SbS NiS

Phosphates Apatite Crandallite Gorceixite Goyazite Monazite Xenotime

Ca5F(PO4)3 CaAl3(PO4)2(OH)5H2O BaAl3(PO4)2(OH)5H2O SrAl3(PO4)2(OH)5H2O (Ce,La,Th,Nd)PO4 (Y,Er)PO4

Carbonates Calcite Aragonite Dolomite Ankerite Siderite Dawsonite Strontianite Witherite Alstonite

CaCO3 CaCO3 CaMg(CO3)2 (Fe,Ca,Mg)CO3 FeCO3 NaAlCO3(OH)2 SrCO3 BaCO3 BaCa(CO3)2

Sulfates Gypsum Bassanite Anhydrite Barite Coquimbite Rozenite Szomolnokite Natrojarosite Thenardite Glauberite Hexahydrite Tschermigite

CaSO42H2O CaSO4½H2O CaSO4 BaSO4 Fe2(SO4)39H2O FeSO44H2O FeSO4H2O NaFe3(SO4)2(OH)6 Na2SO4 Na2Ca(SO4)2 MgSO46H2O NH4Al(SO4)212H2O

Others Anatase Rutile Boehmite Goethite Crocoite Chromite Clausthalite Zircon

TiO2 TiO2 AlOOH Fe(OH)3 PbCrO4 (Fe,Mg)Cr2O4 PbSe ZrSiO4

Source: International Journal of Coal Geology 50, by C. R. Ward, “Analysis and significance of mineral matter in coal seams,” 135–168, copyright 2002, with permission from Elsevier.

ankerite, together in some cases with phosphate minerals such as apatite, may also be found as significant components of the mineral matter in many coal seams (O’Gorman and Walker, 1971; Rao and Gluskoter, 1973; Ward, 1977, 1978, 2002; Davis et al., 1984; Vorres, 1986; Ward et al., 2001; Vassilev and Tascon, 2002; and Pinetown et al., 2007). Iron-bearing sulfate minerals, such as jarosite and coquimbite, may be formed by the oxidation of pyrite on the exposure of the coal to the atmosphere, a process which also liberates

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sulfuric acid when there is associated runoff water. Many other sulfates in oxidation residues, such as bassanite, glauberite, and hexahydrite, usually represent mineral artifacts formed by the interaction of nonmineral inorganics during the destruction of the organic matter. The crystalline mineral matter in coal may occur as bands, lenticles, fracture fillings, plant impregnations, mineral-rich nodules, and other masses visible at a macroscopic scale. Some of the nonmineral inorganics may also be precipitated when coal pore water evaporates on exposed outcrops, mine faces, or drill cores. Microscopically visible mineral matter in coal includes material intimately admixed within the macerals as well as discrete mineral fragments or crystals and a range of nodules, lenticles, veins, pore infillings, and cell replacement structures (Kemezys and Taylor, 1964; Taylor et al., 1998; and Ward, 2002). The minerals in coal may represent transformed accumulations of biogenic constituents such as phytoliths and skeletal fragments (Raymond and Andrejeko, 1983), or they may be of detrital origin, introduced as epiclastic or pyroclastic particles into the peat bed (Davis et al., 1984; Ruppert et al., 1991; and Bohor and Triplehorn, 1993). Other minerals are produced by authigenic precipitation, either syngenetically with peat accumulation or at a later stage in cleats and other pore spaces through epigenetic processes (Rao and Gluskoter, 1973; Cobb, 1985; Spears, 1987; Querol et al., 1989; Sykes and Lindqvist, 1993; Kortenski and Kostova, 1996; Faraj et al., 1996; Ward et al., 1996; and Rao and Walsh, 1997). The syngenetic minerals may represent solution and reprecipitation products of biogenic and detrital material, or they may be derived from solutions or decaying organic matter within the peat deposit.

2.3.2 Nonmineral Inorganic Components The nonmineral inorganics in coal occur either as dissolved constituents in the pore waters or as an inherent, though sometimes exchangeable, part of the maceral components. They may represent exchangeable ions attached to carboxylic, phenolic, or hydroxyl groups (Durie, 1991) as well as metalloporphyrins and other organometallic compounds (Kiss, 1982; Bunnett et al., 1987; Durie, 1991; and Saxby, 2000). Selective leaching with water, ammonium acetate, and hydrochloric acid may be used to determine the abundance and mode of occurrence of the principal nonmineral inorganic elements in lower rank coals (Miller and Given, 1986; Benson and Holm, 1985; and Ward, 1991, 1992). As an example, Figure 2.5 shows the percentage of various elements released by each process in a sequential leaching study.

Basic Factors Controlling Coal Quality and Technological Behavior 43

Water Washing

Acetate

Acid

Leachable % of Element

100 80 60 40 20 0 Al

Ca

Fe

K

Mg

Mn

Na

P

S

Si

Ti

FIGURE 2.5. Percentage of selected elements leached from a low rank coal by sequential treatment with water, ammonium acetate, and hydrochloric acid. (Source: International Journal of Coal Geology 50, by C. R. Ward, “Analysis and significance of mineral matter in coal seams,” 135–168, copyright 2002, with permission from Elsevier.)

Elements released by soaking in water are commonly taken as representing ions originally in solution in the coal’s pore water, whereas elements released by treatment with ammonium acetate are usually regarded as representing exchangeable ions attached to carboxylates and other functional groups in the maceral components. However, some of the carbonate minerals in the coal, if present, may also dissolve in ammonium acetate solutions (e.g., Matsuoka et al., 2002) and contribute to the elements liberated in this way. Elements released by hydrochloric acid treatment may include those elements incorporated as organometallic complexes into the maceral components as well as any calcite or dolomite not affected by the acetate treatment. Siderite has only limited solubility in cold acids, but iron occurring as oxide or hydroxide material—associated, for example, with iron staining—may be readily dissolved by acid treatment. The most abundant elements associated with the organic matter are usually Na, Ca, Mg, and, in some cases, Al and Fe (Figure 2.5). An electron microprobe analysis may also show measurable concentrations of such inorganic elements in the maceral components (Ward et al., 2003). More detailed element mapping by Li et al. (2007) has identified up to 1.5% Ca, 0.5% Al, and 0.7% Fe as consistent components of supposedly “clean” macerals, especially vitrinite, in several lower rank coals. These elements show a uniform distribution pattern within the macerals similar to that of the organic sulfur component. Huggins (2002) discusses a number of other techniques that have been used to investigate the inorganic elements in coal and ash. These include the proton microprobe using proton-induced X-ray emission (PIXE) for elemental analysis (Minkin et al., 1982; Hickmott and

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Baldridge, 1991; and Caridi et al., 1993), the ion microprobe mass analyzer (IMMA) (Finkelman et al., 1984), the laser microprobe mass analyzer (LAMMA) (Lyons et al., 1987, and Morelli et al., 1988), laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAMP-ICP-MS) (Chenery et al., 1995), and the sensitive high-resolution ion microprobe (SHRIMP) (Kolker et al., 2000a). Another technique that has been used to investigate the forms of occurrence of trace elements in coal and ash is X-ray absorption fine structure (XAFS) spectroscopy (Huggins and Huffman, 1996). Many of the inorganic elements in coal were probably inherited from the original plant tissues and fixed by processes such as carboxylation, metallation, and chelation during peat accumulation (Filby and van Berkel, 1987, and Given and Miller, 1987a, b). Subsurface waters may have subsequently redistributed the inorganic elements and possibly introduced additional material, leading in some cases to the distribution of element concentrations within individual coal beds that were controlled mainly by post-depositional ion migration processes (Brockway and Borsaru, 1985). The inorganic elements are usually expelled from the organic matter as the maceral structures change with rank advance (Filby and van Berkel, 1987). However, the expulsion may be inhibited in some cases. For example, microprobe studies by Ward et al. (2007) found significant concentrations of Ca and Al in the macerals of several bituminous coals together with perhydrous vitrinites and anomalously low reflectance properties. Elements occurring in nonmineral inorganic form are generally more reactive than the same elements occurring in crystalline mineral phases when the coal is used. The nonmineral inorganics may also interact with the mineral components in the coal to form new minerals if the coal is heated by igneous intrusions while still at low rank levels (Susilawati and Ward, 2006).

2.3.3 Trace Elements in Coal The total concentrations of the individual inorganic elements in coal, including both major and trace components, are usually determined by chemical analysis of the coal or coal ash material. Direct analysis of the coal is preferred for elements that may be partly volatilized at elevated temperatures, but for most elements ashing increases their concentration in the analysis sample, thereby helping the analysis process. Even if the element concentration is measured by ash analysis, the result is usually expressed as a fraction of the original coal sample. A number of techniques have been used to determine the concentration of individual inorganic elements in coal at major and trace levels (Karr, 1978a, b, 1979; Davidson and Clarke, 1996; Huggins,

Basic Factors Controlling Coal Quality and Technological Behavior 45

2002; and Vassilev and Tascon, 2002). These include X-ray fluorescence (XRF) spectrometry, neutron activation analysis (NAA), atomic absorption spectrometry (AAS), optical emission spectrometry (OES), and inductively coupled plasma vaporization, combined with atomic emission spectrometry (ICP-AES) or mass spectrometry (ICP-MS). Some techniques are more suitable than others for particular elements, depending in part on the concentration of that element and the matrix within which it occurs. Almost every element in the periodic table has been identified in coal (Swaine, 1990; Finkelman, 1994a; Swaine and Goodarzi, 1995; and Ren et al., 1999), and the extent of knowledge on trace elements is increasing as more sensitive analytical methods are developed. With the possible exception of selenium, boron, arsenic, and antimony, which appear to be more abundant in coal, most of the trace elements in coal occur at comparable to lower concentrations than the same elements in other rock and soil materials. Sixteen of the elements occurring in coal have been included in a list of potentially hazardous air pollutants (HAPs) under the U.S. Clean Air Act (Demir et al., 1997), namely As, Be, Cd, Cl, Cr, Co, F, Hg, Mn, Ni, P, Pb, Sb, Se, Th, and U. Some of these may have other impacts on coal utilization, such as the adverse effect of phosphorus on iron and steel production. Web-based summaries for a number of individual elements, including range of abundance, modes of occurrence, analytical methods, and behavior during combustion and environmental effects, are given by CSIRO Energy Technology (2005). Like the major elements, the trace elements in coal may be associated with either the organic components (macerals) or with the crystalline mineral materials. An analysis of different density fractions prepared from finely ground coal may be used to indicate the “organic affinity” of particular elements (Zubovic, 1966; Gluskoter et al., 1977; and Querol et al., 2001), i.e., the extent to which they are associated with the relatively clean, low-density macerals (organic affinity) or the denser, mineral-rich fraction (inorganic affinity). Elements with an organic affinity (such as boron) may be intimately bound to the organic structure (i.e., nonmineral inorganics), but they may also represent fine particles of minerals occurring within the maceral components. Most trace elements in coal tend to have a relatively strong inorganic affinity (Davidson and Clarke, 1996, and Kolker and Finkelman, 1998), representing either low concentrations of minerals with that element as a major constituent or higher concentrations of minerals containing minor proportions of the element in question. The sequential digestion of the coal in a series of solutions, including HNO3 (to dissolve pyrite) and HF (to dissolve silicates), may also be used to evaluate the association of the trace elements in coal with different mineral matter components (Finkelman et al., 1990;

46

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Dale et al., 1993; Palmer et al., 1993; Laban and Atkin, 1999; and Davidson, 2000). The effectiveness of such techniques, however, may be reduced if fine mineral particles are encapsulated by organic matter, limiting access for the relevant reagents. In other cases, because of the low solubility of siderite and dolomite in acid, the minerals present may be less soluble than expected in the reagent concerned. Direct association between element concentration and the abundance of particular minerals (Ward et al., 1999) in the coal may also help identify the mode of occurrence for some trace elements. The concentration of some mineral-related elements (elements with a high inorganic affinity) may be reduced by coal preparation, but the effectiveness of any such reduction depends at least in part on the particle size of the host minerals and the extent to which they are liberated from the organic matter when the coal is crushed. Elements with an organic affinity are also likely to be concentrated, rather than reduced, by coal-cleaning processes. Moreover, the removal of elements from the cleaned coal has the effect of concentrating them in the waste fraction, from which they may be released by a different route when the wastes are dumped at disposal sites. Knowledge of trace element occurrence in minerals and rocks, together with the help of a series of coal-specific studies, provides a basis for assessing the mineral associations of particular trace elements in coal samples (e.g., Finkelman, 1982, 1994a,b; Rimmer, 1991; Belkin et al., 1997a; Ward et al., 1999; Spears and Zheng, 1999; and Finkelman et al., 2002). Elements such as As, Cd, Se, Tl, Hg, Pb, Sb, and Zn, for example, sometimes referred to as chalcophile elements, are generally thought to be associated with sulfide minerals such as pyrite, either as solid-solution constituents or as discrete sulfide phases. Elements such as Rb, Ti, Cr, Zr, and Hf, often referred to as being among the lithophile elements, are more probably associated with aluminosilicates such as micas, feldspars, and the clay minerals. An indication of the possible mode of occurrence for some of the elements identified as hazardous air pollutants is given in Table 2.7. Further discussion of the associations of a number of elements is provided by Swaine (1990) and Finkelman (1994a).

2.4 Coal Metamorphism: Rank Determination Coal metamorphism involves the physical and chemical transformation from peat through bituminous coal through anthracite and meta-anthracite to graphite (albeit not necessarily a pure graphite). In general, coal metamorphism (or coalification), denoted as the coal rank, is marked by a progressive decrease in moisture and volatile

Basic Factors Controlling Coal Quality and Technological Behavior 47

TABLE 2.7 Indicative mode of occurrence of some potentially hazardous trace elements in coal. Element

Common Mode of Occurrence

Antimony Arsenic Beryllium Cadmium Chromium Cobalt Lead Manganese Mercury Nickel

Pyrite and accessory sulfides Pyrite and accessory sulfides Organic association Solid solution in sphalerite Organic and/or clay association Pyrite; some in accessory sulfides Galena Carbonates, especially siderite and ankerite Pyrite Unclear; perhaps sulfides, organics, or clay minerals Organic association; pyrite and accessory sulfides; selenides

Selenium

Level of Confidence* 4 8 4 8 2 4 8 8 6 2 8

*Level of confidence: A number between 1 (low) and 10 (high) expressing the consistency and predictability of the element’s indicated common mode of occurrence in coal. Source: Fuel Processing Technology 39, by R. B. Finkelman, “Modes of occurrence of potentially hazardous elements in coal: levels of confidence,” 21–34, copyright 1994, with permission from Elsevier.

functional groups with a consequent increase in the carbon content of the coal (see Table 2.8). Many of the fundamental properties of coal that are important for industrial use are rank dependent. Coal metamorphism is a function of heat and pressure acting over a period of time. Taylor et al. (1998) and Hower and Gayer (2002), among others, have reviewed the mechanisms of coal metamorphism. Among the three primary factors, heat is generally considered to be the most important. Traditionally, increased heat at greater depths of burial has been considered the primary factor (Hilt’s Law, after Hilt, 1873). Though this continues to be the primary argument in Taylor et al. (1998), it has been recognized that influences from tectonically driven geothermal fluids have also played an important role in coalification (Hower and Gayer, 2002, and Harrison et al., 2004). There is little doubt that time does play a role in coalification, with the amount of time necessary to achieve the coal rank varying from less than a year in contact metamorphism to 106–107 years for regional metamorphism. The role of pressure is now acknowledged as a hindrance to metamorphism in closed systems (Dalla Torre et al., 1997, and Carr, 1999), but it has always been considered a primary influence in the progression of coalification. Pressure causes physicostructural coalification, which influences the physical properties of coals.

Rank Stage

% Carbon % Volatile (daf) Matter (daf)

Gross Specific Energy % in situ (MJ/kg) Moisture

% Vitrinite Reflectance

(oil, 546 nm)

(Diessel, 1992a)

(oil, 546 nm)

(Teichmu¨ller, 1982)*

Rrandom

Rmax

Rank Subclass

Rrandom

Wood Peat Lignite Subbituminous

50 60 71 80

>65 >60 52 40

14.7 23 33.5

75 30 5

0.2 0.4 0.6

0.2 0.42 0.63

High volatile Bituminous

86

31

35.6

3

0.97

1.03

Medium volatile Bituminous Low volatile Bituminous Semianthracite Anthracite

90

22

36

<1

1.47

1.58

0.26 0.38 0.42 0.49 0.65 0.65 0.79 1.11 1.5

91

14

36.4

1

1.85

1.97

1.92

92 95

8 2

36 35.2

1 2

2.65 6.55

2.83 7

2.58 5

*in Stach et al. (1982)

C B A C B A

Applied Coal Petrology

% Vitrinite Reflectance

48

TABLE 2.8 Coal metamorphism nomogram showing general relationship along chemical rank parameters, heating value, and vitrinite reflectance (Data compiled from Taylor et al., 1998; Diessel, 1992a.)

Basic Factors Controlling Coal Quality and Technological Behavior 49

A more detailed description of the process of the coal evolution is provided in Chapter 9, where the coal is seen as a hydrocarbon source rock.

2.4.1 Bulk Chemical Measurements of Rank Whole-coal measurements of coal rank were used long before maceralspecific indicators such as vitrinite reflectance. The three traditional measurements are proximate analysis, ultimate analysis, and calorific or heating value. Each of these is used in various international standards (such as in ISO and ASTM norms), but each one is flawed in its inability to determine the contributions of different macerals which, especially at lower ranks, have markedly different chemical properties. Proximate analysis—the measurement of moisture, ash, volatile matter, and fixed carbon—has been widely used for over 160 years. Lea (1841) used proximate analysis to quantify rank changes along the SWNE length of the Southern Anthracite Field (Pennsylvania). However, proximate analysis is flawed as a rank indicator because at low ranks the contributions of the maceral groups are divergent, with the relatively aliphatic liptinite macerals contributing more to the volatile matter than the more aromatic vitrinite and inertinite macerals. This can be seen in Table 2.9, which compares a humic coal and a torbanite taken from nearby mines almost equal in rank. ASTM D388-99 (1999a) does note that the ASTM rank classification is to be used only in the case of vitrinite-rich coals. Volatile matter is used as a rank parameter only above the high volatile A/medium volatile bituminous boundary, for 31% volatile matter (dry, mineral-matter free basis; dmmf), the point where the chemistry of liptinites converges with the chemistry1 of vitrinite. Although not a measurement of coal rank, the definition of exactly what constitutes coal is tied to the basic chemical analysis. ASTM and other national standards generally define coal as having <50% ash yield. Ash yield is not equal to mineral matter, and the simplest and most widely used conversion is Parr’s (1932) formula: Mineral matter ¼ 1:08  Ash þ 0:55  Total S Given and Yarzab (1978) review the nuances of various formulas. At the same time, it must be realized that the ASTM standard applies to coal at the mine face, sampled according to accepted procedures (elimination of rock partings greater than a certain thickness, for

1 The ASTM classification uses fixed carbon, 100% minus volatile matter, on a dry, ash-free basis (Given and Yarzab, 1978).

50

Seam

Moisture

Ash

Vol

FC

CV (MJ/kg; mmmf)

Breckenridge Hawesville

0.90 7.15

5.75 23.38

72.80 35.60

20.55 33.87

39.27 30.98

Seam

Stot

Spyr

Ssulf

Sorg

C

H

N

O

Breckenridge Hawesville

2.26 4.11

1.10 3.22

0.03 0.08

1.13 0.81

75.59 55.25

8.71 4.47

1.88 1.15

5.81 11.64

Seam Breckenridge Hawesville

Vitrinite

Inertinite

Liptinite (total)

Telalginite

Lamalginite

8.6 78.6

0.6 11.1

90.8 10.4

4.9

85.9

Applied Coal Petrology

TABLE 2.9 Comparison of the basic petrology, chemistry, and heating value of Breckenridge torbanite, western Kentucky, and a humic coal from the same area. Key: Moisture, ash, VM (volatile matter), FC (fixed carbon), S forms, C, H, N, and O on as-determined basis; CV–calorific value; telalginite and lamalginite are part of the total liptinite value. (Data provided by the CAER, University of Kentucky, USA.)

Basic Factors Controlling Coal Quality and Technological Behavior 51

example), and not mined coal in which a considerable amount of rock (floor, roof, partings) and other noncoal debris (continuous miner bits, scrap metal, etc.) may be included. Presumably, magnetic removal of metal at the mine and beneficiation at a preparation plant would eliminate much of the noncoal material from the marketed product. Neither does the definition apply to waste piles of coal and rock that could be reprocessed as a fuel. Moisture content has been used as a rank indicator in lignites and subbituminous coals. For low rank coals, moisture is an important factor because the coal needs to be transported, handled, and stored, and the presence of moisture in large amounts will impede these operations and lead to greater costs. Moisture also replaces an equal amount of combustible material and thus decreases the heating value, thereby complicating the combustion process. For coal classification by rank, equilibrium moisture (the prescribed analysis) is considered equal to bed or inherent moisture (ASTM D388-99, 1999a). Luppens (1988), however, has noted that bed and equilibrium moisture diverge at low ranks and, as a further complication, equilibrium moisture analyses may vary significantly between laboratories and between different operators in the same laboratory. Standard moisture determinations do not include the water from the decomposition of organic constituents in coal, the water inherent in clays, or the water losses that occur at a higher temperature than the test temperature (Carpenter, 2002). Ultimate analysis, which measures the C, H, N, S, and O on the basis of ash and moisture content, is another common test. Of the parameters, C, for a broad rank range, and H, for anthracites, are common rank parameters. C analysis at lower ranks suffers from the same limitations as volatile matter or fixed carbon from proximate analysis in that it is a function of the variation in maceral chemistry in addition to being rank dependent. Nevertheless, many plots of rankdependent parameters (Hardgrove grindability index and free-swelling index, among others) are often plotted against C but are, in turn, influenced by the vagaries of the analysis. Apart from the drawback of maceral variation, mineral matter can make a positive contribution to the (apparent) organic analysis (Given and Yarzab, 1978). Clays will lose -OH groups and H2O; carbonates will lose CO2 but they may fix S as a Ca-sulfate. Pyrite will also burn to Fe2O3 and SO2 in air and decompose to FeS in the volatile matter test. All these reactions may influence the chemical tests. In addition, under ASTM standards, the total S is used in the calculation. A proper expression of the ultimate analysis should include corrections for the inorganic contributions of C and H and should include the organic sulfur, not the total sulfur (Given and Yarzab, 1978; Given, 1984). Organic sulfur is generally determined indirectly from the total sulfur by subtracting the pyritic

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and sulfate sulfur. Oxygen in the ultimate analysis is determined indirectly by subtracting C, H, N, and S from 100%. Thus, both the organic sulfur and O values incorporate the errors involved in the determination of the other components. The calorific value, expressed as MJ/kg, on a moist, mineralmatter-free basis, is used as a rank parameter for ranks lower than medium volatile bituminous coal. Since the moist basis implies equilibrium moisture, the calorific value is really the combination of two rank parameters: the equilibrium moisture, with the caveats noted above, and the final calorific value. As with proximate and ultimate analysis, measurement of the calorific value on the whole coal means that the properties of all the macerals are averaged together. This influence is obvious in the case of cannel coals or torbanites, where the high liptinite content yields heating values >8 MJ/kg more than in the case of equivalent-rank vitrinite-rich coal (Hower et al., 1986a). Furthermore, there are complications arising from the combustion heat of pyrite, the “organic” calorific value being 0.126(Spy) MJ/kg less than the determined mmmf calorific value (Given and Yarzab, 1978). Although the gross calorific value, or high heating value (HHV), is the determined parameter, the low heating value (LHV; HHV minus the heat required to vaporize the water) is a better estimate of the available heat in a combusted coal. This is particularly true for high moisture coals (Carpenter, 2002).

2.4.2 Vitrinite Reflectance Because vitrinite is the most abundant maceral group in most coals (certain Permian Gondwana coals are among the major exceptions), the maceral group plays a large role in defining the properties of the whole coal. The physical and chemical properties of vitrinite change through the course of coalification, and its reflectance has been calibrated against a number of other rank parameters (Table 2.8). Thus, the determination of vitrinite reflectance, the percentage of incident light reflected from a polished surface, is a fundamental tool in coal petrology. The change in reflectance of vitrinite with coalification is related to the increase in aromatization. More detailed explanations can be found in Stach et al. (1982), van Krevelen (1993), and Taylor et al. (1998). Measurements are standardized (ISO 7404/5, 1994b, and ASTM D2798-05, 2005b), and they must be achieved in monochromatic green light (546 nm) by means of a photomultiplier (digital cameras are used as an alternative in some cases) coupled to a microscope using incident light, in oil immersion (with a refractive index of 1.518 at 23 C), objectives with magnifications between 25
and 60
, and a cross-hair must be incorporated into one of the oculars as a reference point. Due to

Basic Factors Controlling Coal Quality and Technological Behavior 53

the natural scattering of reflectance values of vitrinite particles in a coal sample, the number of readings on different grains of vitrinite must be 100. At the end of the analysis, the statistical mean reflectance (in %) should be calculated. Usually the reflectance distribution is reported as V-types or 1/2 V-types (ICCP, 1971; Stach et al., 1982, and Davis, 1984), which represent ranges of 0.1% and 0.05%, respectively. The standard deviation of the mean for 100 readings in a single coal should be about 0.01% to 0.02% (ICCP, 1971, and ISO 7404/5, 1994b), but this may vary slightly as a function of the coal rank. The repeatability and the reproducibility of the analyses are about 0.06% and 0.08%, respectively (ISO 7404/5, 1994b). Particles representing all the various vitrinite macerals and submacerals may be included among those measured for vitrinite reflectance determination. However, for precise measurements of the degree of metamorphism, it may be better to restrict measurement to the thick, homogeneous bands of the vitrinite maceral collotelinite (also referred to in some national standards as telocollinite). The difference may be significant because the thinner bands of vitrinite matrix material commonly have slightly lower reflectance values, and measurements based on all vitrinite may have a wider degree of scatter. Whether the measurements cover all vitrinite or only the collotelinite in the sample should also be indicated in the report detailing the measurement results. Two types of reflectance measurements can be made to quantify the reflectance of a vitrinite grain from a particulate coal sample: the random reflectance of vitrinite and the maximum reflectance of vitrinite. Random reflectance is the reflectance of a grain in the orientation in which it is encountered, measured using nonpolarized light. For measurements of maximum reflectance on a particulate sample, the polarizer needs to be in the 45 position into the incident light beam. If the microscope stage is rotated 360 , the maximum reflectance can then be taken. In low rank coals, random and maximum reflectances are the same because coal is optically isotropic. As an alternative, random reflectance can be estimated as the average of any two orthogonal readings in the determination of maximum reflectance (Hower et al., 1994b). Although vitrinite reflectance is one of the most widely used parameters, it is not the ideal rank measure in all circumstances. Apart from inherent variations between vitrinites from different plants within the same coal, it has been recognized that perhydrous, or marine-influenced, coals have anomalously low (suppressed) vitrinite reflectance properties (Barker, 1991; Mukhopadhyay, 1992,1994; Sua´rez-Ruiz et al., 1994a,b; Price and Barker, 1985; Iglesias et al., 2002; and Wilkins and George, 2002). Some of the causes of the reflectance suppression (natural and artificial) have been synthesized by

54

Applied Coal Petrology

Barker (1991) and Mukhopadhyay (1992, 1994). Alternative optical methods (Wilkins et al., 1992, 1995, 2002; Veld et al., 1997; and Kalkreuth et al., 2004) and nonoptical alternatives (Rimmer et al., 1993, and Wang and Hu, 2002) have been used for rank determination and are especially useful in such circumstances. Measurement of the carbon (and oxygen) content of vitrinite macerals using light-element electron microprobe techniques, for example, has been shown by Gurba and Ward (2000) and Ward et al. (2007) to provide a better rank indicator than vitrinite reflectance in cases where anomalously low reflectance is developed due to the original depositional conditions of the coal seam. Optical Anisotropy When the coal rank increases and the structure of the carbonaceous material is reorganized, almost all the coal’s physical properties vary according to which part of the coal section is being considered. Thus vitrinite develops an anisotropic behavior and exhibits bireflectance. Minimum reflectance is usually observed in the direction perpendicular to the bedding plane and maximum reflectance in sections parallel to this plane. In sections with an intermediate orientation, the reflectance is intermediate between the maximum and minimum values. Bireflectance can be determined using polarized light and the rotating stage of the microscope. By measuring the true maximum and minimum reflectances, the anisotropy can be calculated from the difference (Rmax – Rmin). Methods for determining these parameters were developed by Ting and Lo (1978) and Ting (1978) and later modified by Kilby (1988, 1991) and Duber et al. (2000). The anisotropy in a coal is linked more to the overlying pressure and generally rises with increasing coal rank, but no strict relationship exists between rank and the degree of anisotropy (Davis, 1984). Tectonic stress in directions other than vertical may also produce reflectance maxima with different orientations (e.g., Hower and Davis, 1981, and Levine and Davis, 1989).

2.4.3 Fluorescence This type of petrographic analysis is based on the properties of some macerals that autofluoresce when irradiated with blue or ultraviolet light and may be of assistance in rank assessment. Fluorescence microscopy is currently employed in coal petrology and in kerogen studies for characterization of liptinite macerals, kerogen composition, rank/maturation studies (as an essential criterion for oil and gas formation), and hydrocarbon detection as well as in the correlation of the technological properties of vitrinites and coals (thermoplastic, coking and oxidation features) to vitrinite fluorescence characteristics.

Basic Factors Controlling Coal Quality and Technological Behavior 55

Fluorescence analyses should be carried out using a reflected light microscope coupled to a photomultiplier (ICCP, 1975, 1993). The microscope must be equipped with a high-pressure mercury or xenon lamp for illumination with the corresponding excitation filters to select the UV or blue light, barrier filters, and a variable interference filter covering the range of 400 to 700 nm. Dry or water immersion objectives (25
to 50
) are preferred to those for oil immersion. Changes in fluorescence intensity and in fluorescence colors are dependent on the type of organic substance and on the coal rank. These changes can be measured by the so-called monochromatic fluorescence microscopy and spectral fluorescence. The former includes quantitative measurements of fluorescence intensity at a specific wavelength (546 nm) which are recorded in relation to a standard (Jacob, 1980). Spectral fluorescence measurements determine changes in fluorescence color by recording the emission of the spectrum (Ottenjann, et al., 1975, and Ottenjann 1980, 1982) between 400 nm and 700 nm. They also measure spectral alteration or changes in fluorescence properties after 30 minutes of irradiation of organic substances (Teichmu¨ller and Ottenjann, 1977, and Ottenjann, 1980, 1982). Classical parameters commonly used in quantitative fluorescence analysis are: I = fluorescence intensity at 546 nm; lmax = spectral maximum; Q = spectral quotient (red/green ratio); AI = alteration of fluorescence intensity at 546 nm, and AS = spectral alteration, in each case for a period of 30 minutes’ irradiation. Other fluorescence parameters are described (e.g.) by Martinez et al. (1987).

2.5 Coal Classification The purpose of any classification scheme is to provide a convenient means for the preliminary evaluation of a coal product and for relating a particular coal to others on the basis of the appropriate accepted criteria. The ASTM D388-99 (1999a) and international coal classification systems were somewhat restricted because they did not take into account a sufficiently broad range of age and rank. The International Classification of Hard Coals by Type (United Nations Economic Commission for Europe, 1956) employed a three-digit code based on four parameters: either volatile matter (daf) or gross calorific value (maf), caking properties (free-swelling index or Roga index), and coking properties (Gray-King index or dilatometer maximum dilation). The system was most successful when used for classifying Carboniferous bituminous coals mined and sold in Europe. The subsequent expansion of the global coal trade toward the end of the 20th century made it necessary to broaden the system. The main objections to the 1956 International classification, (Uribe and Perez, 1985) were: (1) the classification was best suited to coals of a homogenous maceral

56

Applied Coal Petrology

composition with a low inertinite content; (2) the system was intended to be used for traditional purposes (combustion and coking) and so did not apply to new utilization processes such as gasification or liquefaction; (3) the rank parameters (volatile matter and calorific value) are dependent on coal type or maceral content (it was thought that both the maceral content and a rank parameter that is independent of coal type, such as vitrinite reflectance, should also be included); and (4) chemical or technological parameters cannot be ignored for a comprehensive characterization is to be achieved. On the other hand, the 1956 classification did not take into account the distinction between single coal beds and coal blends. For these reasons the United Nations Economic Commission for Europe (United Nations Economic Commission for Europe, 1988) replaced the 1956 coal classification system with a new International Codification System for Medium and High Rank Coals (>0.6% R or >24 MJ/kg) in international trade. This new system was applicable to all coals of different origin and geological age from different type of deposits as well as to single seams and multiseam blends of runof-mine coals and washed coals. Medium and high rank coals were characterized by means of a 14-digit code number comprising eight coal-quality parameters: (1) mean random vitrinite reflectance, either measured directly or estimated as maximum vitrinite reflectance divided by 1.06; (2) the character of the reflectrogram; (3) maceral composition expressed as (a) the percentage of inertinite and as (b) the percentage of liptinite, which provides a means of distinguishing, in part, between Gondwana and Carboniferous coals; (4) the free-swelling index; (5) volatile matter (dry, ash-free); (6) ash percentage (dry basis); (7) the total sulfur (dry basis); and (8) gross calorific value (dry, ashfree) in megajoules per kilogram (MJ/kg). Other parameters may be appended to provide a more thorough description of coal quality, such as ash composition, ash fusion characteristics, and Hardgrove grindability index for steam coals or AudibertArnu dilation properties and phosphorous content for metallurgical coals. In 1998 the United Nations Economic Commission for Europe proposed an international classification of in-seams coals. This system was intended to serve as a means of classifying coals and ensuring a better characterization of coal deposits. Unlike the previous United Nations Economic Commission for Europe (1988) classification system, which was intended for commercial purposes, this new system was clearly described as not intended for use in commerce or trade. The 1998 coal classification is based on three fundamental coal characteristics to be used in combination: coal rank (or degree of coalification), petrographic composition, and grade or amount of impurities (ash yield). Figure 2.6 shows a scheme for the classification of in-seam

C ar bo na c

A

D

C

B

HIGH-RANK

A

C

B

A

50

L% 50

100

V% (L >I)

100

GRADE

m

ic

)

l

30

B

N on -b co an al de d

50

MEDIUM-RANK

LOW-RANK C

0

Ve

H Me Low ry ig lo g h diu w gr m rad gr ad e g B ad r (m an c e e co ade oa ai de l co nl d a c y l oa al H coa u l

80 Ash (HT) mass %, (db)

Coal ↔ non-coal rock washability test

eo us

ro ck

S co peli apr tic oal

R

oc k

PE TR

sh Oil al e

O G R d AP ox ry HI ae id ↔ C ro ati w CO bi on et M c ↔ PO ↔ SI an red TI ac uc O ro tio N bi n c

Basic Factors Controlling Coal Quality and Technological Behavior 57

20 10

50 50

o

et a M

a

rth O

pe r

Pa r

o

et a M

a

rth

Pa r

V% (I > L) 100

I%

Rr %

BITUMINOUS

SU

LIGNITE

RANK

B

B

O

IT U

M

et a M

rth O

Paleo B-time

o

0

0.6

1.0

1.4

2.0

ANTHRACITE

3.0

4.0

PETROGRAPHIC COMPOSITION

Maceral analysis (m/nf) vol.%

Not to be included in the classification

GCV (MJ/kg, m, af)

15

20

24

Rr% - Vitrinite mean Random Reflectance, per cent (ISO 7404-5 standard) GCV (MJ/kg, m, af) - Gross Calorific value in MJ/kg, recalculated to moist, ash-free basis (ISO 1928 and 1170 standards) Ash (HT) mass % db - Ash content (High temperature), mass per cent, recalculated to dry basis (ISO 1171, 331 and 1170 standards) V%. L%. I% - Vitrinite, Liptinite and Intertinite contents respectively, volume per cent, recalculated to mineral-matter-free basis (ISO 7404-3 standard)

FIGURE 2.6. Coal classification. (Source: United Nations Economic Commission for Europe, 1998; reprinted from International Journal of Coal Geology 50, by B. Alpern and M. J. Lemos de Sousa, “Documented international enquiry on solid sedimentary fossil fuels; coal: definitions, classifications, reserves-resources, and energy potential,” 3–41, copyright 2002, with permission from Elsevier.)

coals. Although not very frequent, this system has sometimes been referred to in the literature (e.g., Alpern and Lemos de Sousa, 2002). A recently devised classification method is that of the “classification of coals” by the International Organization for Standardization (ISO-11760, 2005). Its development has been guided by the classification system of the United Nations Economic Commission for Europe (1998) and, as in the previous case, it is not intended to be used for commercial purposes. It is also based on the three fundamental coal properties: vitrinite reflectance (mean random reflectance), vitrinite content in percent per volume on a mineral-free basis, and ash yield. The ISO-11760 (2005) system provides a simple classification method of descriptive categorization that can be applied to coals of all ranks,

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a method of comparison of coals taking into account certain key characteristics, and guidance in selecting the appropriate ISO standard procedures for coal analyses.

2.6 Coal Blends Many of the installations that use coal, whether for carbonization (coking) or combustion purposes, use a feedstock blended from a number of different coals to obtain the appropriate quality specifications rather than coal of uniform rank from a single seam or deposit. Usually the blended coals originate from different sources, each having a different composition and/or coal rank (Figure 2.1h). In some cases the feedstock from a single source of supply (which may be a mine or a preparation plant) also encompasses coals having more than one rank level, possibly due to the incorporation of coal affected by igneous intrusions or other local heat sources or to the mixing of coals from different zones of the one deposit or region. Coal petrography (Figure 2.1h) is the only effective way to identify coal blends (as it is summarized in Sua´rez-Ruiz, 2004). This is particularly significant for blends that show similar chemical parameters (e.g., moisture, ash content, volatile matter), but they may differ in some of their technological properties or display a quite different technological behavior in processes such as coke production or even in coal combustion. Figure 2.7 shows the distribution of vitrinite reflectance values in five different coals in the form of a histogram illustrating the proportion of particles falling in the different reflectance intervals (V-steps). The coals have similar overall proportions of volatile matter and similar mean maximum vitrinite reflectances. At the same time each coal has quite different coking properties, as expressed by the respective free-swelling index values. The sample at the top of the diagram, for example, represents a single medium volatile bituminous coal with a vitrinite reflectance of 1.3%, whereas the sample in the lowest plot represents a mixture of two quite different coals, one with a vitrinite reflectance of around 0.9% and one with a vitrinite reflectance of around 1.9%. This figure shows that, although coals having particular overall properties can be prepared by blending two or more coals of different rank (and reflectance) characteristics, the resulting coal blends display a substantially different technological behavior in processes such as coke production. Careful petrographic analysis is required to analyze coal blends, with particular attention being paid to the distribution of vitrinite reflectance values as well as to the range of optical characteristics of other macerals. Both the histogram plots and the standard deviation (scattering) of the individual reflectance values (identified as S in Figure 2.7) may be relevant to the evaluation process.

Basic Factors Controlling Coal Quality and Technological Behavior 59

Volatile Matter

Swelling Index

24,5

9

25,2

9

24,2

S

1,31

0,063

20 15 10 5

1,26

0,117

7 1/2

10 5

1,31

0,234

24,2

4 1/2

10 5

1,26

(0,372)

25,0

1

15 10 5

1,41

(0,553)

Vol.-% 1/2 V-Step

Rm

40 35 30 25 20 15 10 5

0,5 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 % Reflectance =10 Vol.-%

40

35

30

25

20

15

10 % Vol. Matter Vitrinite

FIGURE 2.7. Reflectograms, mean reflectance and scatter, volatile matter, and swelling index of five different coals and coal blends. (Source: Stach’s Textbook of Coal Petrology by E. Stach, M.-Th. Mackowsky, M. Teichmu¨ller, G. H. Taylor, D. Chandra, and R. Teichmu¨ller (editors), 535 pp., copyright 1982, with permission from Gebru¨der-Borntraeger (www.borntraeger-cramer.de).

Petrographic analysis, vitrinite reflectance measurements, and the maceral analysis of coal blends may be used to obtain information on (1) the number of different component coals in the blend, (2) the proportion of each coal in the blend, (3) the overall mean (random or maximum) vitrinite reflectance of the coal blend, (4) the mean (random or maximum) vitrinite reflectance of each individual coal in the blend, (5) the overall maceral composition of the blend, and (6) the maceral composition of each individual coal in the blend. The identification and analysis of coal blends is especially relevant to metallurgical coke production. Authors such as Schapiro et al. (1961), Benedict et al. (1968a), Gray et al. (1979), and Taylor et al. (1998) have described methods for predicting coke strength and other properties from maceral composition and vitrinite reflectance (including the V-step) data that are applicable to coal blends. The evaluation of coal blends may also be significant to combustion processes as a means of identifying the sources of different types of unburned carbons.

CHAPTER 3

Mining and Beneficiation Joan S. Esterle

3.1 Introduction Similar to utilization behavior, the rank and composition of coal directly influence its material behavior during mining and beneficiation (van Aubel, 1928; Raub, 1937; Mackowsky and Abramski, 1943; Donahue and Leonard, 1967; Jansen, 1987; among others). Mining is designed to first liberate the coal from the host clastic rock and beneficiation to liberate higher grade coal and discard stone and poorer quality coal through a process of comminution, density separation, and flotation that is controlled by particle size and surface chemistry. Within a seam, trends in coal quality can often be predicted by understanding the distribution of coal lithotypes relative to the thickness and splitting characteristics of the coal seam that reflects original depositional controls (Moore, 1995; Staub, 2002; Greb et al., 2002; Moore et al., 2006; among others). This assists in optimizing coal quality through either selective mining of different areas or seam plies or processing and blending from different pits (for examples, see Gomez and Donaven, 1971; Clarkson, 1992; and Swanson and Mackinnon, 2003). Changes in maceral composition can also assist in predicting splitting in advance of mining (Esterle and Ferm, 1986, and Moore, 1991) and abrupt changes in coal rank or trace element content may forecast intrusions (Bostick and Pawlewicz, 1984; Stewart et al., 2005; Susilawati and Ward, 2006; and Golab et al., 2007) and in some cases faults by a change in vitrinite reflectance anisotropy that records changes in paleo stress (Stone and Cook, 1985; Levine and Davis, 1989; Cmiel and Idziak, 2003; and Langenberg and Kalkreuth, 1991). For mines producing thermal coals, keeping rank and grade to specification is the prime task, along with tracking deleterious trace elements in product and reject. For mines producing metallurgical coal products, coal type or vitrinite content is an added factor as are element distributions such as phosphorus and sulphur. The same Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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factors that make a good coking coal, such as bituminous rank up to low volatile stages and high vitrinite contents, also make a coal friable or easy to cut, crush, or grind (Hardgrove, 1931; Evans and Pomeroy, 1966; Gomez and Hazen, 1970; MacGregor, 1983; and Hower, 1998). The difference in intact strength between lithotypes, along with their density, will control their fragmentation behavior and the resulting size and composition of daughter particles. Hard, dull coals tend to concentrate in the coarser fractions, whereas more friable, vitrain-rich coals tend to break down into finer particles. This allows different products—for example, a thermal and coking coal—to be produced from the same seam by crushing, screening, and density separation. For coking coals, the weak nature of vitrain-rich lithotypes can create mining problems such as weak pillars in underground mines, excessive dust generation during cutting, or fines generation during blasting that will increase the load on the flotation circuit downstream in the preparation plant. Understanding the distribution of coal lithotypes, in addition to rank and grade, can assist in predicting the behavior of a coal seam at all stages of the mining and beneficiation chain.

3.2 Coal Strength There are different ways of estimating coal strength and hardness— compressive strength, fracture toughness, or grindability—but all will show a trend relative to rank, type, and grade of the coal. The measurement of coal strength is affected by the size of the test specimen, the orientation of stress relative to banding, and the confining pressure of the test (Hobbs, 1964; Zipf and Bieniawski, 1990; Mark and Barton, 1996; and Medhurst and Brown, 1998). By its nature coal is a banded material which makes it weak by comparison to most other rocks. Intact rock strength is commonly defined as the strength of the rock material that occurs between discontinuities, which in coal are closely spaced and related to lithotype banding and cleat. Cleat spacing will increase as a function of rank (Law, 1993, and Laubach et al., 1998) and also proportion of vitrain banding (Smyth and Buckley, 1993, and Pattison et al., 1996). For a given rank, individual lithotypes can have large compressive strength differences owing to wide ranges in maceral composition, banding texture, and cleat density (Medhurst and Brown, 1998), as shown in Figure 3.1. The larger the test sample, the greater the presence of inherent flaws due to cleats and banding, which significantly reduces the strength of the coal (Bienawski, 1968). Mark and Barton (1996) found size effects more pronounced in “blocky” (dull) coals than in friable (more well-banded, vitrain-rich) coals which fail more readily. For a given rank, vitrain-rich coals will also fail more readily when stress

Mining and Beneficiation 63

Peak Strength (MPa at 0.2MPa confining pressure)

35 30 25 20 15 10 5 BB

IB DB DM Coal Lithotypes

D

FIGURE 3.1. Cross-plots of peak compressive strength at 0.2 MPa confining stress against coal lithotype for a single seam of Rvmax ¼ 0.8%. BB: bright banded coal >60–90% vitrain; IB: interbanded coal 40–60% vitrain; DB: dull banded coal 10–40% vitrain; DM: dull coal 1–10% vitrain; D: dull coal <1% vitrain. High value for BB due to smaller core size (59 mm) compared to other tests at 61 mm core diameters. (Source: International Journal of Rock Mechanics and Mining Sciences 35, by T. Medhurst and E. T. Brown, a study of the mechanical behavior of coal for pillar design, 1087–1105, copyright 1998, with permission from Elsevier.)

is applied parallel rather than perpendicular to banding, which results in coal strength being referred to as anisotropic (Hobbs, 1964; Cook et al., 1978; and Medhurst and Brown, 1998). Strength anisotropy decreases with increasing rank and increased vitrain band proportion due to the increased density and connectivity of inherent flaws. Coal strength will also increase with increased confining pressure (Hobbs, 1964), as shown in Figure 3.2. At atmospheric pressure or unconfined conditions, dull coals can be two to three times stronger than brighter-banded lithotypes from the same seam with the same rank (Medhurst and Brown, 1998, and Lawrence, 1978), and low rank coals can be up to 14 times stronger than higher rank coals up to low volatile bituminous coals (Hobbs, 1964), thereafter becoming stronger again toward anthracite rank. At very high confining pressures of 5000 PSI (34.5 MPa), Hobbs (1964) found little variation between coals of different rank, with the exception of very strong anthracite. However, these confining pressures are extreme relative to present-day underground mines that, with many exceptions, generally operate at depths less than 1 km, equating to less than 10 MPa of overburden pressure. Therefore, coal type and rank should be tightly linked to pillar strength, but Mark (2006), and Mark and Barton

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90

Peak Strength (MPa)

80 70 60 50 40 30

IB to BB D to DB Linear (IB to BB) Linear (D to DB)

20 10 0 0

1

2 3 4 Confining Pressure (MPa)

5

6

FIGURE 3.2. Cross-plot of compressive strength of coal for differing confining pressures. Data for 60 mm core samples of Australian Permian age bituminous coal seam with rank of Rvo ¼ 0.8%. D to DB is dull to dull-banded lithotypes; IB to BB is interbanded to bright-banded lithotypes. Linear trend lines are meant to demonstrate that type differences hold for this sample set. 1MPa ¼ 145 PSI. (Source: International Journal of Rock Mechanics and Mining Sciences 35, by T. Medhurst and E. T. Brown, a study of the mechanical behavior of coal for pillar design, 1087–1105, copyright 1998, with permission from Elsevier.)

(1996) found poor correlations between laboratory-derived strength and in situ coal pillar strength for more than 100 case studies of in mine pillar performance in the United States, mostly in Appalachian Carboniferous age coals. They suggest that larger-scale discontinuities, such as roof and floor interface, have more effect on pillar strength. Since pillar strength is also determined by pillar size and shape (Bieniawski, 1968) for a given overburden pressure, variability in lithotype may play a more significant role in thicker seams—for example, those in the Bowen Basin Permian age coals, particularly for rib support (Collwell and Mark, 2005).

3.3 Coal Permeability, Premining Gas Drainage, and Outbursts Gas drainage behavior is a function of the measured gas content, desorption rate, and coal permeability. Permeability decreases with increasing effective stress, which generally occurs with increasing depth of cover. The same textural discontinuities that reduce coal strength (primarily cleat and vitrain banding) increase coal permeability for premining and commercial gas drainage (Smyth and Buckley, 1993;

Mining and Beneficiation 65

Pattison et al., 1996; Clarkson and Bustin, 1997; Gamson et al., 1993; Crosdale et al., 1998; Bustin, 1997; and Walker et al., 2001) and thereby help to reduce potential outburst situations (Evans and Brown, 1973; Beamish and Crosdale, 1998; Walker et al., 2001; Cao et al., 2003; and Wu, 1987). An example of the magnitude difference in permeability for bright-banded coals over dull coals for iso-rank samples from the Australian Permian age Bulli seam is indicated in Figure 3.3 (Wold and Esterle, 2000). For a given pressure or depth, cleat permeability can be reduced by mineral infilling or if horizontal stress is perpendicular to the face cleat direction (Gamson et al., 1993, and Li et al., 2004). In underground mines, stress magnitude and orientation is constantly shifting relative to the inherent macrostructure, which makes the prediction of areas that are low in permeability or outburst prone a multidimensional problem (Dumpleton, 1990, and Xu et al., 2006). The issue is when gas builds up and cannot escape, which is common around tectonically deformed or brecciated coal occurring in fault zones. In a study of Permian age coal seams in the Sydney and Bowen Basins, Australia, Gurba et al. (2003) noted the presence of specific petrographic “micro-markers” related to areas of difficult drainage. Areas of low permeability were related to the development of the

0

Estimated Depth (m) 200 400 600

Permeability (uD)

1000

800 1000

100

100

10

10 Bright coal Dull coal

1

1 0

2 4 6 Effective Stress (MPa)

8

FIGURE 3.3. Graph of permeability versus effective stress for bright-banded and dull coals from a Permian age coal from Sydney Basin, Australia. Depth estimated from effective stress. Bright coal here is defined as bright banded >60% vitrain; dull coal is <10% vitrain. (Source: CSIRO Petroleum Restricted Report No. 00–004, CSIRO Australia, M. B. Wold and J. Esterle, “Measurements of permeability, fracture density, and coal petrology on Bulli seam core, in the context of gas outburst risk assessment,” 16 pp., copyright 2000, with permission from CSIRO Australia.)

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micro-cleat system and its subsequent mineralization, micro-mylonitization of the coal, the mode of occurrence of particular minerals within coal macerals, the presence of oil and solid bitumen as fracture infill and in coal macerals, the presence of heat-affected coals, and the presence of pyrolytic carbon, possibly derived from more distant intrusion effects. Gray (1982) noted that microbreccia was more common but rarely assessed in U.S. Carboniferous age bituminous and higher rank coals that fail by brittle fracture in response to increasing or differential pressure. Identification of these micro-markers in petrographic samples of coal well ahead of mining, such as in exploration and/or development drilling, can assist in the design and timing of the underground drainage systems to mitigate potential outburst conditions (Gurba et al., 2003). Gas drainage drilling, either for mine site drainage or commercial production, in weak coal can result in fines generation that reduces reservoir permeability and/or results in borehole collapse (Palmer et al., 2005). In weak coal the borehole wall cannot withstand the pressure differential that occurs between the formation and the annulus of the borehole during drainage, resulting in borehole wall collapse. Strong coal provides good borehole stability and easy cutting but tends to have lower permeability. For thick seams with variable vertical composition, a simple brightness or lithotype profile can assist in designing both the well path and the pressure conditions of the well for improved drainage and borehole stability.

3.4 Self-Heating and Spontaneous Combustion Self-heating of coal leading to spontaneous combustion is a significant hazard during mining, transport, and storage of coal (e.g., Brooks and Glasser, 1986, and Falcon, 1986). Spontaneous combustion in coal occurs when the quantity of heat generated by oxidation of the organic matter is greater than the quantity of heat dissipated (Misra and Singh, 1994). Although lower rank coals are generally more prone to self-heating than higher rank coals, the relationship is not linear and is also affected by coal type (Chandra and Prasad, 1990; Clemens et al., 1991; Panigrahi and Sahu, 2004; Beamish, 2005; Cao et al., 2005; Singh et al., 2007; among others) as well as particle size and climate (Misra and Singh, 1994). Coal richer in reactive macerals, in particular liptinites and vitrinites, are more prone to self-heating than inertinite-rich macerals. It then also follows that for some coals, particularly Eocene age coals in New Zealand (Shaw, 1997) and Indonesia, that detrovitrinite-rich coals are more reactive than are telovitrinite-rich coals because they can contain abundant liptinite macerals. For Australian coals, Beamish (2005) also demonstrated

Mining and Beneficiation 67

the importance of mineral type, in addition to content, for selfheating. Coals with abundant clay tend to be nonresponsive versus those rich in pyrite or siderite. Similar trends have been observed elsewhere (Cao et al., 2005).

3.5 Breakage During Mining Breakage of coal in the mining operation is the initial step in the liberation of mineral matter from the organic matrix in coal beneficiation. Coal breakage is related not only to the inherent friability of a coal seam but also to the delivery of energy or power (usually measured in watts). Energy is delivered through cutting or shearing, blasting, ripping and dozing, and dropping the coal onto hard surfaces or coal piles during transport. The result is usually measured by the size distribution of the daughter particles after an energy event. As coal preparation plants are designed with circuits to handle and treat different size fractions and the expense increases with increasing amounts of fine coal, it is important to know how size distributions will vary based on coal seam properties and mining methods. Drop-shatter testing of exploration bore cores is a common method for estimating the breakage that will result from the mining process (ASTM D440-86, 1986, and Swanson et al., 1993). Within a single (iso-rank) seam, different plies or benches that have different lithotype compositions will yield different size distributions for a given amount of energy resulting from the number of drops from a known height (Esterle et al., 2002). For open-cut mining methods where the total seam is extracted, the size distribution is a composite of all plies. If this same seam is mined by selective auger or underground mining methods, different size distributions would be generated from different plies. For example, finer coal size distributions would be generated from weaker plies with abundant vitrain banding (e.g., Ply 4 in Figure 3.4). An unexpected change in size distribution resulting from a change in seam character or mining method often results in an extra and unwanted expense to upgrade the coal preparation plant circuits. The finer the coal, the more expensive it is to beneficiate (Nicol, 1992). Fine coal also holds more moisture than coarse coal, resulting in added transport costs to the port, handling and unloading problems, and sometimes customer penalties. Although each mining method delivers energy in a slightly different way, increased force or energy will increase the amount of damage, resulting in fragmentation of a material. The energy required for size reduction is proportional to the new surface area created; hence, more energy is required for a smaller parent to achieve an

Applied Coal Petrology

DM

IB

B

100

Depth in core (mm)

200 300 400 500 600

Cumulative Percent Passing

S

Ply GL4

0

Ply 1&2&3

68

1 Ply 4

700 800

1000 RAMP 27 LD CORE

10 Size (mm)

100

Ply GL4 (top) Plies 1+2+3 (middle)

900

(a)

100 90 80 70 60 50 40 30 20 10 0

Ply 4 (bottom)

(b)

FIGURE 3.4. (a) Lithotype or brightness profile of a midvolatile rank Permian age seam from the Bowen Basin and (b) the resulting size distributions from the different plies after 20 drops during drop-shatter testing. (Data from Esterle et al., 2000b.)

equivalent amount of breakage compared to a larger parent (Rittinger, 1867) (Figure 3.5). During the breakage process, a me´lange of coal particles with different (and changing) parent size and composition behave as interacting, but independent, entities as they undergo damage and size reduction (Esterle et al., 2002). In open-cut mines, some coals are blasted rather than free dug. During blasting, excessive damage occurs in the high-energy crushing zone around the charge in the drill hole, resulting in fine coal generation (Djordjevic et al., 1998). Fines can be reduced during blasting by using lower velocity explosives and/or placing the charge in duller plies of the seam (Esterle et al., 2000a). Fragmentation through the rest of the block occurs through the propagation of gases and pressure through the inherent flaws (joints, lithotype bedding, and cleat) within the coal mass structure. Hence, the majority of the fragmentation, after dislodging the coal during blasting, will take place during the excavation and loading, and these are lower energy events. The selection of mechanical mining equipment such as rippers, dozers, and shovels in open-cut mines and continuous miners and

Mining and Beneficiation 69

Size Distribution Parameter, t (%)

75 75345mm 45331.5mm 31.5316mm 1638mm

60

45

30

15

0 0

0.05 0.1 0.15 Specific Breakage Energy, Ecs (kWh/t)

0.2

FIGURE 3.5. Cross-plot of size distribution of daughter particles of different parent sizes for a given energy; t is approximately 1/10th of the averaged parent particle size. Data from midvolatile Permian age coal seam shown in Figure 3.4 from Esterle et al. (2000b). Methods for single particle breakage tests available in Narayanan and Whiten (1988).

longwall shearers used in underground and highwall mining operations requires knowledge of the power draw required to cut and break the coal (Dumbleton et al., 1958, and Doktan and Scott, 1998). Among lithotypes, McCabe (1942) determined that fusain required the least power for breakage, vitrain required twice as much power, clarain three times as much, and durain 7.5 times as much as fusain. Hence, for a given coal rank, the amount of power required to run a continuous miner is greater, as much as 40% more, for durain-rich versus vitrain rich portions of coal seams (Schapiro et al., 1961, and Peters et al., 1962). Conversely, for the same power input, vitrain-rich coals will fragment more easily than durain-rich coals, resulting in finer size distributions entering the downstream preparation plant (MacGregor and Baker, 1985) (Figure 3.6). In addition to coal rank and type, the sharpness, geometry, and attack angle of cutting bits will also affect the size and shape of the crush zone volume and resulting particle size (Zipf and Bienawski, 1988), which has implications for both coal fines and dust generation in underground mines (Zipf and Bienawski, 1989, and Page and Organiscak, 2002). The propensity of a coal to generate dust during mining is related to the comminution energy of the mining process as well as the inherent friability of the coal due to rank and type (Baafi and Ramani, 1979, and Gagarin, 2005).

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Mass percent passing after breakage at 0.075kWh/t (45x75mm parent)

100 90

Bright banded Interbanded

80

Dull with minor bright

70

Dull

60 50 40 30 20 10 0 0.1

1

10

100

Size (mm)

FIGURE 3.6. Daughter particle size distributions from single particle breakage tests of different lithotypes at 0.075 kWh/t comminution energy. Top size held constant. Samples from Permian age midvolatile rank coal from Bowen Basin, Australia. (Data from Esterle et al., 2000b.)

Hower and Lineberry (1988) and Hower et al. (1987) investigated the role of continuous miner bit sharpness on the daughter particle size distribution of mined high volatile A bituminous (0.75–0.78% Rvmax) Kentucky coals. They found that the use of sharp bits resulted in an increase in fusinite and semifusinite in the <53 mm (270 mesh) fraction, whereas the prolonged use of dull bits tended to pulverize lithotypes, placing more vitrinite in the <53 mm fines. The more durain-rich eastern Kentucky coal, with a higher proportion of inertinite- and liptinite-rich microlithotypes within the relatively low-HGI dull lithotypes, did not show as great a shift toward inertinite concentration in the fines as did the brighter western Kentucky coal.

3.6 Breakage During Preparation The objective of coal preparation is to upgrade the quality of the coal product(s). Coal is often broken or crushed to help liberate the coal from stone prior to sizing, screening, and density separation. The density of different coal lithotypes is linked to their ash yield and maceral composition (Figure 3.7), which, along with their breakage behavior, allows different products to be generated from a single seam or blend of seams.

Measured relative density (g/cc)

Mining and Beneficiation 71 1.70 1.65 1.60 1.55 1.50

LEGEND Ash% lithotype 28.1 D 18.0 DM 14.2 DB 10.2 IB

1.45 1.40 1.35 1.30 1.25 1.20

8.3 BB 3.4 B

0

10

20 30 Percent ash yield

40

FIGURE 3.7. Cross-plot of relative density versus ash yield for a Permian age coal of medium volatile bituminous rank from the Bowen Basin, Australia. N ¼ 247 lithotypes; n varies between 26 and 30 particles except for bright; B ¼ bright, BB ¼ bright banded; IB ¼ interbanded, DB ¼ dull banded, DM ¼ dull with minor bright, and D ¼ dull coal lithotypes. (Data from Kolatschek, 2000 in Esterle et al., 2000b.)

3.6.1 Measurement of Coal Breakage Properties Breakage occurs when the energy or work applied to a coal exceeds its strength. For a given rank, strength is reduced in coals with abundant vitrain bands from both closely spaced banding discontinuities and cleating. This difference in friability is the reason that duller coal lithotypes concentrate in coarser size fractions and vitrain breaks down into finer sizes after mining and/or crushing, as will fusain often to the ultra fines (Hacquebard and Lahiri, 1954). Direct methods for measuring breakage include dropping and shattering coal or coal core from a known height (Broadbent et al., 1957; Berenbaum, 1962; Barnard and Bull, 1985; ASTM D440-86, 1986; Teo et al., 1990; and Sahoo and Roach, 2005) or dropping a weight of known mass onto a piece of coal (Narayanan and Whiten, 1988). Other methods include fracture toughness and grindability (Bhagat, 1985; Zipf and Bienawski, 1990; and Hardgrove, 1931). All methods demonstrate that breakage, resulting in finer daughter particle size distributions, increases for increasing energy (see Figures 3.5 and 3.8). For a given energy, coals become easier to break as they increase in rank from sub- to midvolatile bituminous, thus generating finer particles for the same input energy. Scatter among vitrain-rich coal is greater than that for dull coal lithotypes. At higher ranks, low volatile to anthracite, the trend reverses and coals become harder to break (see Figure 3.8), requiring more power if a finer size distribution is

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80 Rock Breakage index (T10 at 0.075kWh/t)

70

D to DM

Bright banded coals

DB to IB 60

BB

50

Average all data

40 30 Dull coals

20 10 0

2.20

2.10

2.00

1.90

1.80

1.60

1.70

1.50

1.40

1.30

1.20

1.10

1.00

0.90

0.80

0.70

0.50

0.60

0.40

0.30

Rank (Rvo)

FIGURE 3.8. Relationship between low energy breakage, rank, and coal lithotype. Breakage index represented by the mass percentage of daughter particles passing T10, or 1/10th of parent particle top size for 45 mm to 75 mm blocks of coal. (From Esterle et al., 1994.)

required from the breaker or crushing plant. The exact rank at which the trend reverses will vary; low energy breakage tests seem to turn over around Rvmax ¼ 1.3%, whereas higher energy grindability tests turn over around Rvmax ¼ 1.6%. Data are too sparse to say with confidence. There is an energy at which coals begin to condition and the rate of breakage declines. This has been referred to as the end point or fatal size distribution (Swanson et. al, 1993). The amount of energy required to reduce coal to a set proportion of its top size will also increase as particles become smaller and contain fewer inherent flaws, as described earlier by Rittinger’s Law. Esterle et al. (2002) demonstrated that, for low energy events of breakage during mining and crushing, the fatal size could be related to the inherent size or thickness of vitrain, durain, and stone bands that make up the composite lithotypes. For a selected set of Permian age coals in the Bowen Basin, 32 mm is the size mode for pure dull bands and 10 mm is the size mode for vitrain bands at the macroscopic level. Similar size modes were observed by Wang et al. (1996) for Carboniferous age coals in the United States. Phyterals at the microscopic level commonly show a size mode around 0.3 mm (Moore and Ferm, 1992). The inherent influence of bands and phyterals on the

Mining and Beneficiation 73

P80 (mm) size at which 80% passes

100 Dull lithotypes Vitrain bands

10

1 Phyteral banding 0.1 P80-M dull banded coal P80-M bright banded coal 0.01 0.001

0.01

0.1

1

10

100

Comminution Energy (kWh/t)

FIGURE 3.9. Cross-plot of decreasing P80 size index with increasing comminution energy for bright-banded and dull coal lithotypes from a Permian age bituminous coal seam, Rvmax ¼ 0.8. Low energy test data (<1 kWh/t) from Esterle et al. (2000b); high energy test data from Bailey and Esterle (1996).

daughter particle size distributions can be observed from low energy events (cutting, handling, and crushing) to higher energy events of grinding or milling (see Figure 3.9). P80, or the size at which 80% of particles pass after crushing or grinding, is a size index used in the minerals processing field. P80 was calculated for a suite of bright- and dull-banded coal lithotypes from the same high volatile bituminous coal seam for increasing energy from single particle breakage (Esterle et al., 2000b) and grinding tests (Bailey et al., 1996). The relationship between size reduction and increasing comminution for these samples was not linear, and step changes were observed as at sizes equivalent to dull and vitrain band thickness modes at low energies and phyterals at grinding energies.

3.6.2 Grindability As in breakage, the grindability of a coal is a function of coal hardness, strength, tenacity, and inherent fracture (Yancey and Geer, 1945), which are linked (again) to coal rank, type, and grade (summarized in Hower and Wild, 1994, and Hower, 1998). Coal is commonly pulverized prior to utilization to increase its surface area and reactivity

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in a given process, whether combustion or metallurgical coke making. Because size reduction is related to energy and smaller particles require greater energies to achieve a given size, milling from a nominal 30 mm to 50 mm top size to pass micron sizes, say, for combustion requires greater mill power and therefore greater costs (Sligar, 1998). Because reactivity is related to maceral composition as well as particle size, coals rich in reactive macerals—for example, vitrinite or liptinite—may not have to be ground to the same fineness as coals with less reactive inertinite macerals and mineral matter (Sligar, 1998, and Spero, 1997). Abundant mineral matter will increase abrasion and wear in the milling process as well as reduce its efficiency. Increased moisture contents also reduce coal grindability for coals of a given rank. The Hardgrove Grindability test is commonly used to characterize the milling behavior of coals. The test, first conceived by Hardgrove (1931) and later modified to the current constant-weight test in 1951 (ASTM D409-71, 1971; ISO 5074, 1994c), attempts to mimic the operation of a continuous coal pulverizer using a batch process. As noted by Hardgrove (1931) and others (Roth et al., 1992; Hower, 1998) the test is empirical and has limitations, but it is widely used and even included as a specification in contracts for the supply of coal. In the Hardgrove Grindability test, 50 g of a limited particle size range (16  30 mesh, or 1.18 mm  600 mm) of coal is ground in a ball mill for 60 revolutions. The resulting coal is sized and the weight of the (<200 mesh) product is recorded and, by calibration to a reference coal, used in the calculation of the Hardgrove Grindability Index (HGI). The test results in a value for HGI generally between 30 and 100. The lower the HGI number, the harder it is to grind the coal. The test is highly nonlinear such that a change in HGI from 90 to 80 results in a small decrease in mill capacity, whereas a change from 50 to 40 leads to a considerably greater decrease in mill capacity (Sligar, 1998). The preparation of the 50 g quantity of 1.18 mm  600 mm (16  30 mesh) coal from the larger original size, introduces a bias into the test. A number of studies (Gray and Patalsky, 1990; Hower and Wild, 1994; Bailey et al., 1996, and Hower, 1998) have demonstrated that the elimination of the <600 mm (30 mesh) fraction from the test material creates a sample differing in petrographic composition from the original sample due to the partitioning of brittle vitrinite-rich and (unmineralized) inertinite-rich microlithotypes into the fine fraction, the fraction not tested. This is less likely to be a problem in the analysis of single-lithotype samples, particularly lithotypes dominated by either bright (vitrain, very bright clarain) or dull (durain) lithologies. Coals delivered to utilities are not single lithotypes, however, so the bias in the test is a problem.

Mining and Beneficiation 75

On the petrographic scale, HGI is a function of the rank, maceral composition, and mineral content of a test coal. The most fundamental overall relationship is the peak of HGI in the medium to low volatile bituminous rank range (see Figure 3.10). The actual rank at which this peak occurs will vary based on the data used, and the large scatter in the data, particularly in the peak range, is most probably due to differences in composition. The lower energy breakage results shown in Figure 3.8 also show greater scatter among vitrain-rich coal lithotypes in this range compared to the more massive dull coals. Low-rank and high-rank coals can both have low HGI numbers, although for very different reasons. Hower and Wild (1988) demonstrated a strong correlation between an increase in liptinite content and decreased HGI (harder to grind) for coals of narrow vitrinite reflectance ranges for a suite of Carboniferous age coals. Indeed, they noted a 39 HGI difference between a vitriniterich lithotype and a durain within the same, iso-rank seam section, indicative of the maceral/microlithotype versus HGI relationship.

140 USGS DATA 120

Poly. (USGS DATA) ----- Hypothetical trend

Highly banded or telovitrinite-rich coals

100

HGI

80

60 Massive to poorly banded or telovitrinite-rich coals

40

20

0 0.2

0.6

1.0

1.4

1.8

2.2

2.6

3.0

3.4

Rvmax (calculated)

FIGURE 3.10. Cross-plot of Hardgrove Grindablity Index against coal rank. Data from the USGS Coal Quality Database (from Bragg et al., 1997); Rvmax calculated from volatile matter as Rvmax ¼ 4.89  1.129*LN (volatile matter daf). Hypothetical trends for vitrain-rich and vitrain-poor coals are shown.

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For any given liptinite content in their sample set, HGI increased with an increase in rank within the high volatile bituminous rank range investigated (Hower and Wild, 1988). They suggested that liptinite from high volatile bituminous coal is resistant to breakage while vitrinite, fusinite, and semifusinite tend to be brittle. Trimble and Hower (2003) corroborated this relationship between an increased concentration of liptinite-rich microlithotypes and a decrease in HGI for eastern Kentucky coals from narrow vitrinite reflectance ranges. For example, for 66 coals in the 0.85–0.90% Rmax range, they found that liptinite, durite, duroclarite, and clarodurite all decreased HGI. For coals with inherently low liptinite contents, the general rule of thumb that duller or finer grained or nonbanded lithotypes will be harder to grind than more well-banded lithotypes can be applied. Working with Eocene age coals from Indonesia, Moore et al. (1990) demonstrated a direct relationship between HGI and megascopic coal lithotypes and in particular the texture and proportion of vitrain bands in hand specimens and phyterals in polished blocks. The link between megascopic coal type and HGI or any other breakage behavior allows the trend to be tracked through coal seam mapping in combination with bore cores. Just as there are regional differences in coal composition and rank between coals of difference ages and basins, there will be differences in the grindability linked to a coal’s rank, texture, and composition. Minerals and moisture content will also have an impact on coal grindability (Urala and Akyildiz, 2004). Hsieh (1976) placed minerals into four groups based on their grinding behavior: clays and sulfates; quartz, oxides, and silicates; pyrite and other sulfides; and carbonates. Quartz and other nonclay silicates and oxides tend to be the hardest minerals and clays and sulfates are the softest minerals. Shales can have a wide range of grindability but generally are softer than the associated coal (Agus and Waters, 1971). Intercalated shale with coal will tend to lower the HGI (Kanjilal et al., 1979) and intergrown carbonates, as found in some Permian Gondwana coals, tend to decrease HGI (Falcon and Falcon, 1987).

3.7 Maceral and Mineral Partitioning During Beneficiation Coal beneficiation’s ultimate objective is to produce a (relatively) uniform product that will meet the specifications of the end user. In the steam coal industry (see Chapter 4), those specifications have traditionally included ash yield, sulfur content, and heating value, with some users specifying other parameters such as HGI or Cl. Maceral composition has been important for end users in the metallurgical coal industry (see Chapter 7) and may ultimately prove to be useful in coal liquefaction (see Chapter 6).

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The beneficiation process operates by being able to separate grade on the basis of size, density, and surface chemistry. The size and composition of the daughter particles after a breakage event will reflect both the starting size and composition of the parent particles and the energy of the event. Not only will maceral composition differ between coarse and fine particles, but the distribution of mineral matter will differ based on its occurrence within the clastic stone partings or as bands, lenses, or disseminated particles associated with the organic fraction of the coal. Coarse coal cleaning relies on simple size and density separation, whereas fine coal cleaning has the added complexities of surface chemistry and the interaction with surfactants. For an overview of the methods of coal cleaning, the reader is referred to Leonard (1991), which covers the equipment and the overall operations of coal preparation plants and to Aplan (1988) that covers the influence of coal character on washability. The objective here is to review the coal quality aspects or coal preparation (also known as coal cleaning or coal beneficiation) and the role of the coal petrographic composition (Falcon and Falcon, 1983).

3.7.1 Maceral Partitioning During Beneficiation The initial separations are dependent on the hardness, size, and density of the individual particles. As noted previously, the maceral and microlithotype composition of the lithotypes, the fundamental petrographic unit in coarse coal cleaning, is important in determining the breakage properties. Macerals will be segregated into different size fractions, with the proportion of macerals and mineral dependent on the mineral/maceral association and the degree of liberation. A number of studies (Mackowsky and Abramski, 1943; Bayer, 1960; Harrison, 1963; Maitra et al., 1979; Beck, 1981; Hower et al., 1986b; Hower and Wild, 1991; and W. Wang et al., 2006) have demonstrated the partitioning of macerals through the unit operations in a coal preparation plant. Fundamentally, Mackowsky and Abramski’s (1943) observation that durain will concentrate in the coarse sizes and vitrain and unmineralized fusain will concentrate in the fine and ultrafine sizes underlies the later studies on high volatile bituminous Illinois coal (Harrison, 1963), high volatile bituminous western Kentucky coals (Hower et al., 1986b), high volatile A bituminous Pennsylvania coal (Bayer, 1960), high volatile A bituminous eastern Kentucky coals (Hower and Wild, 1991), and high volatile A and medium volatile bituminous Pennsylvania and West Virginia coals (Beck, 1981). By understanding the distribution of particle composition and density by size, one can high grade some coals to produce optimum coking coal products through breakage and screening in

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the preparation plant (see Burstlein (1954) for a description of the Longwy-Burstlein or SOVACO method; Spackman et al. (1976a) and Hower and Wild (1991)).

3.7.2 Mineral and Trace Element Partitioning As noted, one of the fundamental reasons for coal beneficiation is the reduction of ash yield and deleterious minerals and elements with an inorganic affinity. The partitioning of major, minor, and trace elements depends on the degree of liberation of the minerals, their inorganic versus organic association, and the specific gravity of the separation. Mineral matter occurring as discrete bands and lenses within the coal can often be removed easily, but that disseminated within the coal matrix or within the organic compounds of the macerals will be more difficult to remove by simple density separation and may require extensive (and expensive) grinding to beneficiate. In low rank coals, dissolved salts or inorganic elements incorporated within the organic compounds of the macerals are common. An overview of analytical methods used to determine inorganics in coal is given by Huggins (2002), and mineral matter in coal is presented in Chapter 2 of this book. A common method for determining whether a mineral or element will partition during beneficiation is through analysis of the float/sink fractions for different size fractions (Querol et al., 2001). As stated in Huggins (2002), “the higher the organic affinity, the more the element reports to light-specific gravity fractions, and hence, the more it is associated with the organic fraction of the coal.” One would assume that these lighter fractions would be dominated by vitrain, but that is not always the case. Various studies (Zubovic, 1966; Gluskoter et al., 1977; Cavallaro et al., 1978; Fiene et al., 1978; and Kuhn et al., 1980) suggest that the organic affinity of many elements varies significantly from coal to coal. More direct methods of analyzing maceral separates or scanning electron microscopy will assist in characterizing this variability for specific macerals and minerals. Mitchell and McCabe (1937), Helfinstine et al. (1971, 1974), Cavallaro et al. (1976), and, more recently, Mastalerz and Padgett (1999) studied the ash and sulfur partitioning of (generally) high-S Pennsylvanian Illinois Basin coals. Because of the fine nature of much of the pyrite and an organic association of about half of the total S, the S in the clean product was generally above 2%. Finkelman (1994b) discussed the associations of the hazardous trace elements. His work was based both on his own research (Finkelman, 1981) and on comprehensive works by others (e.g., Gluskoter et al., 1977; Raask, 1985b; Eskenazy, 1989; and Swaine, 1990). Akers and Dospoy (1994) demonstrated the magnitude of element reduction

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through a number of coal beneficiation schemes and DeVito et al. (1994) examined the trends in a large collection of coal company data from the Illinois Basin and the Northern Appalachians. Summaries of the associations of elements and the estimated ease of removal by conventional coal cleaning are shown in Table 3.1. As shown in the table, because of the varying modes of occurrence and the fine mineral associations, removal of trace elements by coal beneficiation can be quite inefficient. Further studies of the association of trace elements in coals have been conducted by Senior et al. (2000a) and

TABLE 3.1 Hazardous trace elements, their most common mode of occurrence, and the estimated percentage of removal by conventional coal cleaning for suite of coals from the United States

Element

Mode of Ocurrence

Arsenic Beryllium

Pyrite Organic association; substitution for Al in silicates Sphalerite, pyrite Pyrite and some in accessory sulfides; chelates Organic or clay association; chromite and Cr-spinels Pyrite; organic association Carbonates, especially siderite and ankerite; binding by carboxylic acid in low-rank coals Unclear; sulfide association; accessory in chromite and Cr-spinel Galena, clausthalite Pyrite and accessory sulfides (stibnite); organic association possible Organic association, pyrite and some in accessory sulfides, selenides

Cadmium Cobalt Chromium Mercury Manganese

Nickel

Lead Antimony

Selenium

% Removal by Coal Cleaning (Range)

% Removal by Coal Cleaning (Average)

22.4 to 77.3 25 to 57

54 41

14.3 to 75.8 28.2 to 70.8

49 52

41.9 to 74.7

65

20 to 42.9 57 to 92.4

14 75

39.1 to 69.1

54

35.2 to 72.3 33.7 to 73.7

58 33

0 to 45.1

25

Source: Fuel Processing Technology 39, M. S. DeVito, L. W. Rosendale, and V. B. Conrad, “Comparison of trace element contents of raw and clean commercial coals,” 87–106, copyright 1994, with permission from Elsevier.

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Palmer et al. (2004) and their partitioning by size and gravity separation by W. Wang et al. (2006). Specific studies (and reviews of other studies) have been conducted for As (Kolker et al., 2000b; Yudovich and Ketris, 2005a), Hg (Yudovich and Ketris, 2005b,c; Brownfield et al., 2005; Wang M. et al., 2006), and Se (Yudovich and Ketris, 2006a). It should also be noted that trace elements are not uniformly distributed in minerals, such as As in pyrite (Ruppert et al., 2005) and Hg in pyrite (Hower and Robertson, 2003, citing unpublished work from 2000 by same authors). Not all element concentrations will be reduced by beneficiation. Organic sulfur is an obvious example of an element that will not easily be eliminated in beneficiation. Hower et al. (1998) noted an increase in total S from run-of-mine to clean Eastern Kentucky coals, since organic sulfur will go with the product rather than high density reject coal. Similarly, chlorine (associations reviewed by Spears, 2005; Yudovich and Ketris, 2006b) is generally associated with the organic fraction; therefore, removal of the diluent mineral matter increases relative Cl concentration.

3.7.3 Froth Flotation/Column Flotation of Fine Particles Froth flotation is a long established technique in mineral processing. Extensive discussion of flotation can be found in the book by Leja (1982); see also recent work by Fecko et al. (2005) and Melo and Laskowski (2006). Basically, particles are either hydrophilic or hydrophobic (Gutierrez-Rodriguez et al., 1984). Surface properties of particles are modified with a surfactant (collector). Bubbles, produced by mechanical (froth flotation) or passive (column flotation) aeration and stabilized by a frother (a surfactant), are used to bring hydrophobic particles to the surface. Macerals, with some variation as will be discussed, and pyrite are hydrophobic and will adhere to the bubbles while clays and other silicates are hydrophilic and will sink. Whether pyrite is truly hydrophobic or entrained by the bubbles or interlocked with floatable particles is a matter of research, but one necessary to develop mechanisms that suppress pyrite from being recovered with clean coal product (Kawatra and Eisele, 1997). Since froth flotation (and oil agglomeration) relies on the surface properties of coal in separating particles, coal petrology plays an important role in the process. Arnold and Aplan (1989) reported a number of studies which examined maceral partitioning or lithotype partitioning, generally without consideration of the maceral association. In order of decreasing floatability, they noted that liptinite > vitrinite > fusinite and vitrain > clarain > durain > fusain. For an eastern Kentucky coal, Hirt and Aplan (1991) noted the relationship, in order of decreasing floatability: pseudovitrinite (high Rmax) > pseudovitrinite (low Rmax) >

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vitrinite (high Rmax) > vitrinite (low Rmax) ¼ micrinite ¼ exinite (liptinite) ¼ semifusinite > resinite > fusinite. Note, this differs from the ICCP (1998) maceral definitions, but pseudovitrinite does have a usage precedent in the coal petrology literature (Benedict et al., 1968b). Arnold and Aplan (1989) also examined microlithotype relationships, finding vitrite to be concentrated in the faster floating fractions and inertite in slower floating fractions. In order of decreasing floatability, microlithotypes follow the general order vitrite > inertite > vitrinertite > clarite > duroclarite. A rank relationship exists, with hydrophobicity increasing sharply through the high volatile bituminous rank range (Aplan, 1993). Honaker et al. (1996) investigated differences in maceral partitioning in column flotation related to pH. Since different researchers have used a variety of coals with varying petrographic composition and varying rank, exact comparisons between studies are difficult (Arnold and Aplan, 1989). Overall, the behavior of particles is best understood by understanding the composition of the entire particle (Sarkar et al., 1984; Ofori et al., 2006), although it is really only the outer surface that is important in processing. Flotation, generally conducted on <0.5 mm particles, is processing particles from the lithotype to microlithotype scale. The maceral and microlithotype composition of the particle can be used as a proxy for the surface interface with the surfactants. Ofori et al. (2006) and O’Brien et al. (2003, 2006) have developed a semiautomated image analysis system to diagnose coal grains by their grain size, composition, and density and relate this directly to flotation performance. Imaging is used to classify grains as liberated (single component) or composite and to derive the density of each grain from maceral and mineral type and abundance (O’Brien et al., 2006). This method provides a tool for tracking behavior in the flotation circuit and for optimizing processes in advance. Weathering has an influence on hydrophobicity. In a study of Spanish bituminous coals, Garcia et al. (1991) demonstrated that the formation of humic acid complexes and the oxidation of Fe led to poor flotation recovery at the same pH as unweathered coal. Adjustment of the pH towards the basic range did yield some improvement in recovery.

3.7.4 Oil Agglomeration Oil agglomeration, similar to froth flotation, could be used to recover the finest coal particles (<150 mm) passing through a preparation plant. Only the finest particles are processed because excessive coal crushing and pulverization, beyond the point necessary to optimize the efficiency of the typical array of gravity-based coal-cleaning procedures, would not normally be cost effective. In addition, the reagents used in oil agglomeration are an added expense. The costs of fine-coal

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cleaning cannot exceed the cost recovery in the added clean coal to be economically viable. Oil agglomeration has a long history of development dating to the Trent Process, developed in the United States (Perrot and Kinney, 1921, and Ralston, 1922), followed in the 1950s by the German Convertol Process (Brisse and McMorris, 1958). Both were abandoned due to poor economics. Spherical agglomeration, using a light oil, which assists in the control of the product size, was developed by the National Research Council of Canada (Capes et al., 1971, 1972). OTISCA’sTProcess utilizes pentane in a coal-water slurry to process sub-0.5-mm coal. Pilot operation of the T-Process produced clean coal with 0.8% ash at 90% yield at processing costs in the range of $1.40/GJ ($0.60/ 106 Btu) (Keller, 1987; Keller and Burry, 1990). Recent work has successfully employed various vegetable oils in agglomeration (Garcia et al., 1996, 1998; Hower et al., 1997a; Alonso et al., 1999, 2002; and Valde´s and Garcı´a, 2006a,b). Hower et al. (1997a) investigated the role of a variety of oils for three petrographically complex Eastern Kentucky (Pennsylvanian) coals spanning the high volatile A bituminous rank. Coal rank was noted to be a significant parameter in the agglomeration behavior of the coals, particularly for the hexane agglomeration, with the higher rank coal having greater clean-coal yield at lower oil concentrations than the other two coals. The maceral and microlithotype content proved to be a complicating factor, with the maceral composition of duroclarite, the most abundant microlithotype, partitioned between vitrinite-rich varieties in the concentrate and vitrinite-poor varieties in the tails at higher oil concentrations for all three coals. On the other hand, Garcı´a et al. (1996) demonstrated the feasibility of agglomerating Spanish anthracites with low concentrations of n-heptane.

3.7.5 Magnetic Separation High gradient magnetic separation (HGMS) (Trindale et al., 1974, and Liu, 1982) is based on coal being diamagnetic (repulsed by a magnet), whereas pyrite is paramagnetic (attracted to a magnet). The magnetic susceptibility of pyrite is 0.3  106 G/g (G ¼ gauss), compared to coal at –0.4  106 to – 0.8  104 G/g. If pyrite is not altered to the more magnetic pyrrhotite, perhaps via microwave heating, the separation can be enhanced through control of the particle size and/or through control of the field strength. With the primary purpose of HGMS being the removal of pyrite from fine coal, experiments on high-S Illinois Basin coals by Murray (1977), Hise et al. (1979), Harris and Hise (1981), and Hower et al. (1984) were a logical test of the concept. Harris and Hise (1981) noted an increase in inertinite in the magnetic fraction, perhaps a function of coarse pyrite with fusain, while Hower et al. (1984) did not see

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much maceral difference between the clean and the refuse. Hower et al. (1984) noted that most of the pyrite in the clean coal was very fine (98% <10 mm in one case).

3.7.6 Triboelectrostatic Separation Triboelectrostatic processing is a dry beneficiation technique involving imparting a charge onto fine particles and separating the particles toward electrodes (Ban et al., 1997). Commercial separation of fly ash has been accomplished, the systems described by Bittner and Gasiorowski (2005) and Cangialosi et al. (2005) being examples. Commercial separation of fine coal is not done, in part due to the inertia of wet processing of coal and also because of the low profit margin of coal processing in U.S. coals. Adding new equipment for fine coal processing has not been economical. Hower et al. (1997b) studied the behavior of three high volatile A bituminous Eastern Kentucky coals, a high volatile C bituminous Western Kentucky coal, and a high volatile B bituminous Illinois coal in a bench-scale triboelectrostatic processing unit. In tests of the 200– 325 mesh (75–45 mm) fractions, they found a partitioning of vitrinite to the clean product with the inertinites, liptinite, and mineral matter reporting to the tailings. Considering just the duroclarite, representing about a third of the eastern Kentucky coals, the partitioning of vitrinite to the clean product is observable with the microlithotype, indicating that it is not just partitioning of vitrite that accounts for the overall behavior of coals in the system. The lowest rank coal in the experiment, the Springfield coal (Western Kentucky; Rvmax ¼ 0.45%), did not show the same degree of maceral partitioning and did not have as much mineral matter in the tailings. The higher moisture content of the feed coal may have contributed to an attenuation of the charge on the particles, decreasing separation efficiency. In summary, the decisions on how to mine and beneficiate the coal are ultimately based on economics and safety, but the rank and composition of the coal will control coal behavior across the value chain, from site selection through extraction and beneficiation to utilization as a power or metallurgical coal. The composition of the coal can be characterized macroscopically by lithotype composition or microscopically by microlithotype and maceral composition. The manner of characterization isn’t as important as the deed itself, since understanding the variability in the response of different coal lithotypes across a range of ranks will assist in understanding how the coal will behave under stress, through either loading onto pillars or impact crushing and grinding. This assists one in optimizing the design of pillars, gas drainage programs, blasting, cutting and extraction, handling, crushing and milling—all from the simple concept of coal rank and type.

CHAPTER 4

Coal Combustion Isabel Sua´rez-Ruiz Colin R. Ward

4.1 Introduction Combustion to produce steam for electricity generation represents the principal use of coal at the present time and in the immediately foreseeable future. Coal provides 25% of global primary energy needs and currently generates 40% of the world’s electricity—more than twice the individual contributions from natural gas, nuclear, and hydroelectric sources (World Coal Institute, 2006a; International Energy Agency, 2007). In the United States, coal accounted for slightly less than 50% of electricity production in 2004 (1.977 PWh of 3.97 PWh from all sources) (Energy Information Administration [EIA], 2005). Worldwide, the United States is responsible for over 27% of the electric power generated by combustible fuel, with China having a 13% share (2003 data; Energy Information Administration, 2005). Coal markets are in flux because of (1) increased environmental controls, (2) new clean coal technologies, and (3) the variety of methods available to comply with environmental standards. An example of all three factors at work is the U.S. Environmental Protection Agency (EPA) Clean Air Interstate Rule (CAIR) (US-Environmental Protection Agency, 2005a), which obliges Eastern U.S. coal-fired utilities to substantially reduce SO2, NOx, and fine particulate emissions by 2015. SO2, for example, is to be reduced by 70% from 2003 levels. Under this plan, utilities must incorporate flue-gas desulphurization (FGD) or switch to a clean-coal technology. An added consequence of the CAIR is the reduction of mercury emissions, one of the goals of the Clean Air Mercury Rule, or CAMR (U.S. Environmental Protection Agency, 2005b), through a combination of FGD and selective catalytic reduction of NOx (Senior, 2006). The goal of the CAMR is to reduce Hg emissions in the United States to 38 short tons by 2010, largely though CAIR reductions of SO2 and NOx, and to 15 short tons by 2018. Mercury is also a topic of Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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environmental concern in Europe. In its interest in developing an EU mercury strategy, the Commission of the European Communities reported that coal combustion in power plants and residential heating were the major sources of mercury emissions in the European countries (Commission of the European Communities, 2004) despite significant reductions in emissions in recent decades. In 2005, the Commission of the European Communities (Commission of the European Communities, 2005) adopted a mercury strategy that envisages a series of actions to protect both citizens’ health and the environment. Evaluation of coal properties for combustion involves all aspects of coal petrology. Coal rank is fundamental in establishing the heating value of a coal as well as for its combustion characteristics. The maceral composition is also fundamental to the combustion properties, since different maceral groups combust at different temperatures and rates. The inorganic composition is also basic to the heating rate of the coal, to the yield and properties of the boiler and fly ashes, and to the emission of gaseous oxides and trace elements from the combustion process. Fundamental works on coal combustion and power plant engineering include Essenhigh (1981) and Stultz (2005).

4.2 Combustion Processes and Technology Coal may be burned by passing air through a bed of relatively large coal particles (solid coal combustion) or as a stream of finely ground (or pulverized) coal blown with air into the furnace system (pulverized coal combustion). Solid coal combustion may be used in different types of small-scale, stoker-based installations, such as industrial boilers (Cudmore, 1984, and Juniper, 2000). Pulverized coal combustion (PCC), however, is the method of choice for large-scale power generation, accounting for over 90% of the world’s current electric utility capacity (International Energy Agency, 2006). Other types of systems, such as cyclone burners, are also in widespread use. Yet another technology, fluidized-bed combustion (FBC), is becoming increasingly popular in some areas (Juniper, 2000) because of its ability to handle a wide variety of low-quality fuels (including coal preparation wastes) and its capacity to produce minimal SO2 emissions from combustion of coals with relatively high sulphur contents. Pulverized coal combustion (PCC) involves grinding the feed coal to about 70% <70 mm and injecting the powdered coal into the boiler from either wall-mounted burners or corner-mounted (tangential) burners. Combustion takes place within a few seconds at flame temperatures up to 1,500
C. Supercritical PCC is a variation that seeks to improve thermal efficiency, from the typical values of up to about 40% for PCC to 43–47% in supercritical systems through higher

Coal Combustion

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steam temperatures and pressures. For example, the Tachibanawan unit in Japan operates with steam at up to 600oC and 25.1 MPa. Higher temperatures, up to 700oC, are planned but depend on the development of new materials for pipes and turbines. Such steam temperatures will raise the thermal efficiency of PCC systems to 55%. The developments are limited by engineering problems rather than coal quality factors, and the same high quality coals used for PCC will be sought for supercritical and ultra-supercritical PCC installations (summaries after Berkowitz, 1979, and Henderson, 2003). Fluidized bed combustion (FBC) develops when a bed of fine particles of coal is subjected to an upward gas flow. The bed remains static, but the pressure drop across it increases in proportion to the gas flow rate. When the pressure drop across the bed particles equals the weight per unit area of the bed, the bed is suspended and it is at its minimum degree of fluidization. When the gas flow increases above the minimum, bubbles are produced. The movement provides intense agitation and mixing among the particles in the fluidized bed. This results in the bed particles conveying the heat produced from coal combustion at very high rates to the cooler surroundings. Coal is burned in the bubbling bed at temperatures of 800–900
C, significantly lower than the temperatures in pulverized fuel installations. Limestone may be added to the bed, both as a support for the coal during combustion and as a sorbent for the SO2 and other sulfur oxide (SOx) emissions. In addition to the primary air in the combustor, secondary air is introduced at several levels above the fluidized bed. Together with the relatively low combustion temperatures, this helps reduce the levels of nitrogen oxides (NOx) emitted from the system. The lower combustion temperatures also allow the use of coals with relatively low ash fusion temperatures. Because of the low SOx and NOx emissions, FBC is a technology that is able to burn a variety of otherwise poor quality fuels efficiently and in an environmentally friendly way. Two main processes are in use: bubbling FBC (BFBC) and circulating FBC (CFBC). Both may operate under atmospheric or pressurized conditions. Bubbling FBC works well with coals that have high moisture and ash percentages and with high rank coals having low proportions of volatile matter. In CFBC the velocity of the air is increased and the fuel particles are carried upward from the bed surface. The combustion chamber is filled with a turbulent cloud of particles that are not in close contact with each other. The burning particles are recovered from the air flow and brought back into the lower part of the combustion chamber. A circulating fluidized bed is able to sustain combustion, and the contact between coal particles stabilizes the overall temperature. CFBC involves using a coarser coal than PCC, typically 3–6 mm in diameter, in a fluidized bed of coal, ash, limestone, and air. Combustion is also

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at lower temperatures than for PCC (i.e., less than 1,000oC). Subcritical operation yields efficiencies of up to 40% for units up to 300 MW. Emissions are generally lower than in PCC, and thus fluidized bed units have lower levels of primary pollutants such as NOx, SO2, CO2, and particulates. PFBC, operating at 1–1.5 MPa with combustion temperatures of 800–900
C, has the advantage that the hot combustion gases are removed from the combustor under pressure. If these conditions are maintained and the gases are cleaned, they can be sent directly into a gas turbine (Thomas, 2002). PCBC has been used commercially to burn low quality coals (Department of Trade and Industry (DTI), 2000a) but is still at the pilot stage. Both CFBC and PFBC technologies hold the promise of increased thermal efficiency, with the ability to burn low quality and even waste coal materials (summaries after Henderson, 2003, and International Energy Agency, 2006). A further development is the use of integrated gasification combined cycle (IGCC) technology (Collot, 2006, and references therein). IGCC systems produce electricity by first gasifying coal with a controlled shortage of air or oxygen in a pressurized reactor (gasifier) to generate syngas (a mixture of hydrogen and carbon monoxide) that is then cooled and cleaned of impurities. The syngas is combusted with air or oxygen to drive a gas turbine. The exhaust gases are then heatexchanged with water/steam to produce a superheated steam that drives a steam turbine (World Coal Institute, 2006b). This system offers efficiencies up to 50%, with a potential of 56% in the future, improving the environmental performance of coal. Pollutant emissions are significantly reduced and up to 90% of mercury emissions can be captured (World Coal Institute, 2006b). In addition to the potential for greater thermal efficiencies, IGCC also produces a CO2 stream more amenable to geological sequestration. The gasification process is not in itself a coal combustion technology and therefore is not discussed further in this chapter. However, a more detailed discussion is provided in the coal gasification section (see Chapter 5).

4.3 Coal Behavior in Pulverization Although stoker systems and fluidized-bed combustion units use relatively coarse-crushed coal, most coal combustion at the present time takes place in pulverized-fuel boilers. One of the objectives of the pulverizer coal in such systems is to reduce the coal particle size so that 70–80% passes through a 200 mesh (75 mm) screen (summary after Scott, 1995). Pulverizing is accomplished in several different ways, including grinding with large steel balls in a bowl mill. The air flow through the pulverizer serves to segregate the coal by separating out

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the larger particles for further grinding or transfer to the reject stream and by transporting the fine particles from the mill to the boiler. Pyrite, hard rock, and harder coal lithotypes (see Chapter 2) are commonly found among the rejects from the pulverization process. Hower et al. (2005a) reported nearly 50% ash and over 20% pyritic sulfur, along with significant concentrations of Hg and As, in pulverizer rejects, indicating that the yield and quality of such material is an important factor to be taken into account when trying to reduce SO2 and trace element emissions from the combustion plant. Utilities sometimes make coal-purchasing decisions based on the ability of the pulverizer to handle the coal without slowing the output of pulverized fuel for the boiler installation. Coals with a Hardgrove grindability index (HGI) of less than 50 are sometimes overlooked for this reason. In other cases the supplier may be required to provide coal with a higher heating value to compensate for the loss in pulverizer efficiency. Pulverization, like grindability, is a function of the composition and rank of the coal supplied. In general, the more brittle microlithotypes are related to the finest fractions, and those more resistant are related to the coarser fractions. Figure 4.1 shows samples of vitrite and duroclarite, representing brittle and resistant microlithotypes, respectively, and their behavior when high volatile bituminous coals are pulverized. Vitrite increases toward the finest (500 mesh, 25 mm) fraction as duroclarite decreases in abundance in the same direction. The HGI provides an approximation of the behavior of coal in utility boilers, with the estimated pulverizer mill capacity based directly on HGI. According to Fitton et al. (1957) and that reported in Hower (2008), a 53 HGI coal requires 2.5 times the number of revolutions as a 110 HGI coal to produce a fineness such that 80% of the product can pass a 200 mesh (75 mm) screen. Similarly, Austin et al. (1995) found that the capacity of a roll grinder doubled as the HGI increased from 44 to 106, close to Lowe’s (1987) estimate of 1% loss of mill throughput per HGI unit decrease. A blend of different HGI coals in a pulverizer could lead to segregation, disrupting the intended fuel composition (McGraw, 1986, and Conroy, 1994). Cho and Luckie (1995a,b) investigated both bituminous-anthracite-quartz blends and blends of high volatile bituminous coals with varying HGI. They found that the presence of quartz enhanced the grindability of the bituminous coal, whereas quartz grinding was buffered by the presence of the coal. A three-component blend followed the behavior of the quartz-bituminous blend, with the anthracite behaving independently. A four-component blend of bituminous coals exhibited a nonlinear breakage pattern with prolonged grinding, perhaps due to suppression of grinding with the accumulation of fines (also noted

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FIGURE 4.1. Abundance of vitrite and duroclarite in size fractions of pulverized coal at power plants burning high volatile A bituminous Central Appalachian coals and high volatile C/B bituminous Illinois Basin coals. (Unpublished data provided by the University of Kentucky, CAER, USA.)

by Uesugi, 1989, and Trimble and Hower, 2000). Douglas et al. (1990), investigating blends of 106 HGI and 56 HGI coals, found that the harder coal segregated to the coarse fraction whereas the softer coal segregated to the fines. Davis and Orban (1995), in a study of low and high volatile bituminous coals, attributed the segregation to both rank (and inherent HGI) differences and to the grinding behavior of the maceral assemblages, independent of coal rank. Mill power consumption in blends tends to follow the power consumption for the harder coal (Conroy et al., 1989). Bailey et al. (1996) related grinding energy, HGI, and the mesh size at which 80% of the pulverized coal passes (P80) for lithotypes from high and medium volatile bituminous Australian Permian Gondwana coals. They found a point in the P80 versus grinding energy curves beyond which comminution (decreased P80) did not substantially improve. As particles are reduced in size, they reach a point where the crushing forces are less effective, the energy being expended in frictional heat losses through particle slip (Austin et al., 1959). Contrary to the findings of Hower and Wild (1994), Bailey et al. (1996)

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found that the duller lithotype had a lower P80 than the bright lithotype from the same coal. Dull lithotypes in Australian coals tend to have more inertinite and less liptinite than superficially similar lithotypes in Appalachian coals, and inertinite-rich trimacerites would tend to be less resistant to breakage than the liptinite-rich varieties, which might perhaps account for the differences in behavior. As noted earlier, provincialism in coal properties makes it difficult to project the behavior of one coal onto another. Proper pulverization is essential to control of the amount of carbon burnout in the boiler. Increased carbon burnout is important for both increased boiler efficiency and enhancing the value of the fly ash (Yu et al., 2005). The amount of coarse (>100 mesh, 150 mm) material in a pulverized coal has negative impacts on both the carbon burnout (efficiency of carbon combustion) and on the efficiency of NOx reduction (Maier et al., 1994). An illustration of the effectiveness of pulverizers in reducing fly ash carbon was seen in an East Tennessee (U.S.) power plant study (Hower et al., 1997c), which found that fly ash carbon was reduced following conversion to lowNOx combustion. This was contrary to experience with other power plants (Hower et al., 1996, 1997d, 1999a), where the lower combustion temperatures resulted in a reduced carbon burnout. In the Tennessee case, maintenance of the pulverizers resulted in a decreased particle size and, consequently, reduced fly ash carbon. Barranco et al. (2006) compared different pulverization degrees using a high volatile C bituminous Colombian coal and a high volatile A bituminous British coal. In the latter case, they found that unburned carbon in the fly ash decreased with increasing fineness of the coal feed. The Colombian coal, containing a higher percentage of vitrinite than the British coal, had a greater combustion efficiency but did not exhibit the same relationship between carbon burnout and feed coal fineness.

4.4 Combustion Properties of Coal Neavel (1981a) identified the coal characteristics that are important in combustion as the following: calorific value, grindability, combustibility, and ash properties. Combustibility is treated here as the combustion properties of the macerals, and ash properties are discussed in the ensuing sections. In the current political and environmental climate (John and Paddock, 2004, and Swisher and McAlpin, 2006), discussions of emissions, clean-coal technologies (fluidized-bed combustion, gasification), and the fly ash produced by coal combustion are fundamental areas in the field of combustion properties.

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4.4.1 Coal Characteristics for Combustion: Basic Combustion/ Maceral Relationships In 1986 Bengtsson described four parameters that influence the coal combustion phases: (1) vitrinite reflectance, (2) particle size, (3) temperature, and (4) petrographic composition. There is a discrepancy when these parameters are compared to the parameters listed by Neavel (1981a), but there are interrelationships. Calorific value and vitrinite reflectance are both rank parameters, although calorific value is also related to the maceral content. Grindability and particle size are also related, with particle size being a function of the grindability of the coal and the energy expended in pulverization. Combustibility is the ability of coal, expressed in the individual macerals, to ignite at certain temperatures. As has already been pointed out, the petrographic composition is inherent in several of the parameters outlined by Neavel (1981a). It also encompasses the mineral content of the coal. The four phases in the combustion process for bituminous coals are (1) ignition of gas, (2) gas combustion, (3) ignition of char, and (4) char combustion (Bengtsson, 1986; Wu, 2005; among others). Under some conditions, the operation of these phases may be controlled by (1) increasing coal rank, (2) transition from clarite to vitrite to durite to fusinite, (3) increasing particle size, and (4) decreasing temperature in the <1,200
K range (Bengtsson, 1986). Rank plays a significant role in coal combustion. Low rank, high volatile coals are more ignitable than lower volatile, higher rank materials. The firing of low volatile coals requires added attention to engineering parameters to achieve and sustain combustion (Hough and Sanyal, 1987). Bengtsson (1986), in drop-tube furnace studies of a wide range of coals, found that a low volatile coal had poor reactivity at 800oC and at 1,000oC was only slightly more reactive than the anthracite employed for that study. Combustion temperatures are also rank dependent. In an investigation of char structures, Bengtsson (1986) determined that the thermal swelling of vitrinite was rank dependent, being greater in high volatile (low rank) than in low volatile (higher rank) bituminous coals. It was also observed that there was no swelling or pore formation in anthracites. Bengtsson (1986) found that Rrandom <0.5% vitrinite initiated weight loss and heat release at 200oC, in contrast to >260oC for 1.4–1.5% Rrandom vitrinite. Similarly, the T1/2, the time after which half of the sample had reacted, increased with an increase in rank (and with increased inertinite content). Crelling et al. (1992) found that the thermal gravimetric analysis (TGA) combustion profile temperatures of vitrinite increased steadily with an increase in coal rank for a series of Pennsylvanian coals. Similarly, Barranco et al. (2003) found that subbituminous to high volatile B bituminous coals became less reactive with increasing rank.

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The ordering of combustibility (temperature of combustion) is generally considered to be liptinite < vitrinite < inertinite. This order would explain the clarite < vitrite < durite < fusite ordering in the transition of the combustion phases mentioned previously. Liptinite is present in both clarite and durite, but its association with vitrinite in clarite is more conducive to the overall combustion properties of the microlithotype than in the liptinite-inertinite association in durite. However, Bengtsson (1984) did note that, within the same particle, liptinite may remain unchanged while devolatilization pores are being developed in the associated vitrinite. Figure 4.2 shows a liptinite relatively unchanged in a partially combusted coal particle, whereas the surrounding vitrinite shows a distinct devolatilization structure. Crelling et al. (1992), in a study of macerals isolated by density-gradient methods from a rank series, also noted that liptinite was initially reactive, but at temperatures above the volatile ignition temperature it became less reactive than vitrinite and semifusinite. Inertinite, as a relatively poorly reacting maceral group, has attracted special attention. Yavorskii et al. (1968) determined that the presence of inertinite in the feed coal was an important contributor to carbon in the fly ash from pulverized coal systems. Nandi et al. (1977) demonstrated that combustion efficiencies were inversely related to the proportion of inertinite macerals in Western Canadian bituminous and subbituminous coals. The fly ashes from the high inertinite coals contained a higher percentage of unburned carbon than the ashes from the low inertinite coals, but semifusinites from

FIGURE 4.2. Photomicrograph taken in reflected white light (32 oil immersion objective). Liptinite and vitrinite, the latter showing evidence of devolatilization, for a partially combusted coal particle from a Kentucky power plant. (Photo credit: University of Kentucky, CAER, USA.)

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Gondwana coals were found to be more reactive (Nandi, 1984). Bengtsson (1986) determined that the differences in char burnout between high and low inertinite coals narrowed at temperatures around 1,000oC. Kruszewska (1989) combined maceral studies, reflectance, and experimental char analysis to investigate the reactive, inert ratio of a variety of coals and found that lower-reflectance inertinite macerals were really semireactive in the combustion process. Pregermain (1988) obtained distinct burning profiles in thermogravimetric analysis of vitrinite-derived and inertinite-derived chars from a series of high volatile bituminous coals. Thomas et al. (1993), Vleeskens et al. (1993), and Borrego et al. (1997) found that inertinite macerals are combustible under the proper burning conditions. Hower et al. (1999a-c, 2005a), while observing a certain percentage of inertinite-derived carbon in fly ashes, noticed that, in general, most fly ash carbons appeared to be derived from vitrinite (although low-reflectance semifusinite may also have been a source). Sua´rez-Ruiz et al. (2006a, 2007), investigating fly ashes from the combustion of anthracitic feed blends, also noted significant percentages of unburned carbons derived from anthracitic/ meta-anthracitic vitrinites, underlining the role of coal rank in the combusted fuels. Pseudovitrinite, though not an officially recognized maceral in the ICCP 1994 system (ICCP, 1998), is well established as a maceral in the coal utilization literature (Benedict et al., 1968b, and Kruszewska, 1998). Bengtsson (1987) investigated a U.S. coal with a high percentage of pseudovitrinite that had poor char burnout characteristics. Petrographic examination of char samples showed that pore formation proceeded at a slower rate in pseudovitrinite compared to the “normal” vitrinite in the same coal. The more compact structure led to a lesser tendency for the char to be combusted, giving the maceral a character more akin to semireactive inertinite macerals.

4.4.2 Mineral Matter Behavior During Combustion The minerals and other inorganic constituents in coal react in different ways when the coal is used in combustion, as well as in gasification, coke production, and iron and steel manufacture (e.g., Nankervis and Furlong, 1980; Raask, 1985a; Burchill et al., 1990; Vassilev et al., 1995, 2005a,b; Gupta et al., 1999b; Reifenstein et al., 1999, and French et al., 2001b). Each mineral may be expected to undergo its own series of reactions at different temperatures, depending in part on the atmosphere involved, to form a range of minerals not usually found in the raw or unburnt coal feedstocks (Table 4.1). The different phases may also interact with each other, although this may be limited in some combustion systems by a lack of opportunity for the particle-to-particle contact necessary so that reactions can occur.

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TABLE 4.1 Principal minerals identified in high-temperature phases associated with coal utilization Mineral

Composition

Quartz Cristobalite Tridymite Metakaolin Mullite Albite Anorthite Sanidine Corundum Pyrrhotite Oldhamite Anhydrite Aragonite Vaterite Portlandite Lime Periclase Wuestite Hematite Maghemite Magnetite Spinel Magnesioferrite Calcium ferrite Srebrodolskite Brownmillerite Wollastonite Gehlenite Merwinite Melilite Whitlockite

SiO2 SiO2 SiO2 Al2O3.2SiO2 Al6Si2O13 NaAlSi3O8 CaAl2Si2O8 KAlSi3O8 Al2O3 Fe(1-x)S CaS CaSO4 CaCO3 CaCO3 Ca(OH)2 CaO MgO FeO Fe2O3 Fe2O3 Fe3O4 MgAl2O4 MgFe2O4 CaFe2O4 Ca2Fe2O5 Ca4Al2Fe2O10 CaSiO3 Ca2Al2SiO7 Ca3Mg(SiO4)2 Ca4Al12MgSi3O14 Ca3(PO4)2

Source: International Journal of Coal Geology 50, C. R. Ward, “Analysis and significance of mineral matter in coal seams,” 135–168, copyright 2002, with permission from Elsevier.

High-temperature processes involving mineral matter include transformations in the production of fly ash and bottom (boiler or furnace) ash, fusion, and crystallization to form sinters and slag deposits, and vaporization and condensation associated with the fouling of furnace systems. High-temperature products of the mineral matter may also interact with internal boiler components to produce corrosion.

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The abrasion of metals and other exposed materials may occur in the combustion plant as a result of impact from minerals or mineralderived particles. Mineral Reactions Associated with Combustion Due to its high fusion temperature (around 1,800
C), quartz appears to be essentially nonreactive in combustion processes, especially if it occurs as relatively large monomineralic particles. High-temperature phases such as tridymite and cristobalite may be formed from quartz by solid-state reactions (Reifenstein et al., 1999), but the reactions often take place at a slow rate and much of the original quartz may persist as such through the entire combustion process. Kaolinite loses the OH units in its crystal structure at around 500
C, forming an essentially amorphous material called metakaolin. The metakaolin undergoes phase changes to form gamma-alumina, mullite, and cristobalite at 950–1,000
C, and these products appear to persist as solid phases to at least 1,600
C (Reifenstein et al., 1999, and French et al., 2001b). Illite and smectite form spinel and mullite at around 950–1,050
C and commonly fuse to form glassy components at around 1,200 to 1,350
C, giving rise to relatively low ash fusion temperatures. Calcite decomposes to form lime (CaO) at around 900
C, and dolomite decomposes in a two-stage process to form both lime and periclase (MgO). The lime may interact with atmospheric water on cooling to form portlandite (Ca(OH)2). Calcium may also interact with aluminosilicate materials at elevated temperatures to form minerals such as gehlenite (Ca2Al2SiO7) and anorthite (CaAl2Si2O8). Calcium occurring in any form within the coal may take up sulfur to form anhydrite (Filippidis et al., 1996). Indeed, limestone may be added to the combustion stream in some cases, such as in stack gas desulphurization units and fluidized-bed combustion systems, to reduce the release of sulfur to the atmosphere, allowing much of the sulfur to be retained in solid form. Depending on the nature of the furnace atmosphere, pyrite and siderite break down to form iron oxide minerals such as hematite, maghemite, and magnetite. Iron in the coal may also react to form a range of other minerals, including spinel, magnesioferrite, calcium ferrite, srebrodolskite, and brownmillerite (Huffman et al., 1981; Raask, 1985a; Reifenstein et al., 1999). The nonmineral inorganics in the coal macerals, along with the organic sulfur, may also take part in the ash-forming process. Some elements, such as Na, S, and Cl, may be partly or completely vaporized and pass through with the combustion gases into cooler sections of the boiler system. Elements released from inorganic combination in

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the macerals are usually more reactive than the same elements associated with crystalline mineral phases. Ca and Fe, for example, form lime (CaO) and magnetite (Fe3O4) when derived from calcite or pyrite, respectively. These compounds may then react with other components to form relevant aluminosilicates (Russell et al., 2002). However, any Ca and Fe incorporated into the macerals is released in a more reactive form, allowing an easier and more rapid incorporation into new phases than equivalent elements derived from crystalline mineral components (Falcone and Schobert, 1986). Figure 4.3, based on dynamic high-temperature X-ray diffraction data (French et al., 2001b), shows the relative abundance of the different mineral and amorphous phases developed as the mineral matter (low-temperature ash) isolated from a coal, consisting in this case of kaolinite and quartz, is heated progressively in an oxidizing atmosphere to more than 1,500
C. The kaolinite begins to disappear at around 600
C and is replaced by amorphous material (metakaolin). 80.0

Weight % Mineral Phase

70.0 Quartz Kaolinite Amorphous Cristobalite Mullite Maghemite

60.0 50.0 40.0 30.0 20.0 10.0 0.0 50

250

450

650

850

1050

1250

1450

1650

Temperature °C

FIGURE 4.3. Relative abundance of minerals and amorphous material in the LTA of an aluminosilicate-bearing coal sample, as indicated by dynamic hightemperature XRD analysis. (Source: Proceedings of 18th Pittsburgh International Coal Conference, Newcastle, Australia, December, D. French, L. Dale, C. Matulis, J. Saxby, P. Chatfield, and H. J. Hurst, Characterization of mineral transformations in pulverized fuel combustion by dynamic high-temperature X-ray diffraction analyzer; 7 pp. (CD-ROM), copyright 2001, with permission from Pittsburgh Coal Conference, University of Pittsburgh (www.engr.pitt .edu/pcc).

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Mullite and cristobalite are formed at around 1,000
C, and the proportion of metakaolin decreases accordingly. Although no crystalline iron-bearing phases appear to occur in the original low temperature ash (LTA), a small proportion of maghemite is also formed at about the same temperature. Quartz persists through to around 1,500
C, but then abruptly disappears, along with the cristobalite and maghemite, to be replaced by a second generation of amorphous material. Thermomechanical analysis data (French et al., 2001b) suggest that the coal’s ash begins to fuse at this temperature. Thus, although the mullite persists, the quartz and cristobalite appear to be incorporated into a glassy component. Figure 4.4 shows similar data for a calcium-bearing coal, tracing the phases present from 800 to 1,500
C. Quartz, lime, and hematite are present in the ash at 800
C, and gehlenite begins to form, possibly at the expense of quartz and lime, at around 1,000
C. Quartz, lime, and gehlenite disappear at around 1,200
C, and anorthite is formed along with a slightly increased proportion of amorphous material. 30

20

95

90

15

85

10

80

5

75

0 800

900

1000

1100

1200

1300

1400

Weight % Amorphoud material

Weight % Mineral Phase

25

100 Gehlenite Anorthite Lime Hematite Quartz Amorphous Material

70 1500

Temperature °C

FIGURE 4.4. Relative abundance of minerals and amorphous material in the LTA of a calcium-bearing coal sample, as indicated by dynamic high-temperature XRD analysis. (Source: Proceedings of 18th Pittsburgh International Coal Conference, Newcastle, Australia, December, D. French, L. Dale, C. Matulis, J. Saxby, P. Chatfield, and H. J. Hurst, Characterization of mineral transformations in pulverized fuel combustion by dynamic high-temperature X-ray diffraction analyzer; 7 pp. (CD-ROM), copyright 2001, with permission from Pittsburgh Coal Conference, University of Pittsburgh (www.engr.pitt.edu/ pcc).

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The anorthite disappears at around 1,450
C, a temperature at which thermomechanical data suggest that it is probably incorporated into a melt, and amorphous material (glass) makes up virtually the entire ash product. Small mineral particles or aggregates occurring as inclusions within the organic matter of individual pulverized coal fragments, sometimes referred to as included mineral matter, may be heated to a higher temperature than liberated or excluded mineral particles during the actual combustion process, due to their intimate relationship with the organic matter being burned (Gupta et al., 1998). They also react in a more strongly reducing environment, with the higher temperatures and different conditions producing different combustion products. As the coal particle burns, the included minerals may fuse or coalesce, with escaping gas giving rise to vesicular mixtures of glassy and crystalline components. Vaporized inorganics may condense to form very fine ash particles, 0.1–0.2 mm in diameter, which are difficult to remove from the flue gas stream and may escape as particulate emissions from the combustion plant. Slagging and Fouling Processes The minerals and nonmineral inorganics in the coal may react in a number of ways (see Figure 4.5). While some of the coarser and denser particles may accumulate below the combustion chamber as bottom ash, most of the ash in pulverized coal furnaces remains as suspended fly ash particles in the combustion stream. They are then carried through the system and removed by electrostatic precipitators or fabric filters before the flue gases are discharged from the furnace stack (see Figure 4.6). Some of the ash particles may impact on the boiler tubes within the furnace in the course of the combustion process. If these have a low viscosity so that they do not readily rebound, or if they have a sticky surface, they may adhere to the boiler tubes and resist dislodgement by the turbulence of the gas stream or by the system’s sootblowing facilities (Bryers, 1996; Carpenter et al., 2005). Such a buildup provides an insulating layer, which slows the heat transfer between the furnace and the steam inside the tubes concerned. This leads to the buildup of higher temperatures on the furnace side of the deposit, since the heat is not so readily conducted away. The higher temperatures developed at the interface further increase the stickiness of the built-up material, allowing additional impacting particles to adhere to the surface and progressively increase the thickness of the deposit. Eventually the furnace side of the ash buildup may fuse or melt, forming a slag made up of aluminosilicate glass with a wide range of embedded mineral and ash particles (Figure 4.7).

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FIGURE 4.5. Processes associated with reaction of mineral matter in pulverized coal combustion systems. (Source: Progress in Energy and Combustion Science 22, R. W. Bryers, “Fireside slagging, fouling, and high-temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels,” 29–120, copyright 1996, with permission from Elsevier.)

Individual slag buildups may be internally layered, with sintered accumulations of mineral particles near the water tubes and more fused glassy materials toward the furnace side (Wang et al., 1999). Crystallization of minerals such as feldspar (commonly having the Ca-rich composition of anorthite) may also take place within the slag accumulation (Creelman et al., 2003). Similar sintering and partial melting, with associated clinkering and slagging processes, may also take place in gasification systems (Matjie et al., 2006). As the coal is consumed and the ash gravitates toward the hotter part of the gasifier, the fluxing effect of Ca/Mg and Fe lowers the melting points of dehydrated quartz/clay residues, facilitating aluminosilicate melt formation. Sintering is initiated by the crystallisation of anorthite and mullite from the partly molten material. The ash load is then cooled by the steam and oxygen that are introduced at the base of the gasifier, and the molten material solidifies to form a solid aluminosilicate glass.

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FIGURE 4.6. Cross-section of a pulverized coal boiler showing mineral matter reaction zones and possible locations of slagging and fouling deposits. (Source: Progress in Energy and Combustion Science 22, R. W. Bryers, “Fireside slagging, fouling, and high-temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels,” 29–120, copyright 1996, with permission from Elsevier.)

In contrast to slagging, which develops from the accumulation, sintering, and melting of solid ash particles, fouling is a process caused by the condensation of material in the combustion gases from the vapor state. Such accumulations may be initiated by silicates in the high temperature parts of the system, such as the superheater (Carpenter et al., 2005), or by the deposition of alkali sulfates in cooler areas such as the economizer. As with slagging, an initial deposit builds up from condensed vapors, typically made up of sodium, potassium, or calcium sulfates. The slower heat transfer due to the insulating effect of the deposits increases their temperature and stickiness, allowing more particles to be captured and increasing the thickness of the accumulation. A strong sintered deposit may result, involving entrapped ash particles as well as condensed inorganic materials.

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FIGURE 4.7. SEM images of slag from a pulverized-fuel boiler installation. Top: General view showing open pores or vesicles (black) and essentially unreacted quartz grains (mid-grey) in a fine-grained partly crystalline light grey matrix. Bottom: Close-up of matrix, showing laths of feldspar (dark grey) and fine mullite crystals (light grey) in homogeneous aluminosilicate glass. (Source: Proceedings of 12th International Conference on Coal Science, Cairns, Australia, 2–6 November, R. A. Creelman, W. J. Bamberry, L. A. Juniper, C. R. Ward, “Alkali-based slagging: a case study from Leigh Creek,” 10 pp., copyright 2003, with permission from R. A. Creelman.)

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A number of different indices have been developed to identify coals that are particularly prone to slagging and fouling problems (Cudmore, 1984; Bryers, 1996; Lee and Lockwood, 1999; Creelman et al., 2003; and Su et al., 2003). These are usually based on the relative proportions of particular inorganic oxides in the coal ash. The form in which the different elements occur, however, as well as the atmosphere, temperature distribution, and other aspects of the boiler, may also have an effect on the individual indices and the behavior of specific coals in particular furnace installations. The introduction of Ca, K, P, Cl, and other elements from different forms of biomass (Thy et al., 2006) may also give rise to increased fouling tendencies (Bryers, 1996, and Pronobis, 2006) when biomass is burned together with coal in combustion operations.

4.4.3 Fate of Trace Elements in Combustion and Ash Formation The behavior of major and trace elements during coal combustion depends on their natural volatility as well as their concentration and mode(s) of occurrence in the coal and any chemical reactions that take place with sulfur or other volatile components in the combustion stream. The volatility of various elements also depends on the combustion technology itself, including the temperature, time of exposure, type of ash generated, and other factors (Clarke and Sloss, 1992; Clarke, 1993; and Querol et al., 1995). The trace elements in coal can be divided on the basis of volatility into three overlapping groups (see Figure 4.8). The least volatile

FIGURE 4.8. Partitioning of trace elements in combustion based on volatility. (Source: Fuel 72, L. B. Clarke, “The fate of trace elements during coal combustion and gasification: an overview,” 731–736, copyright 1993, with permission from Elsevier.)

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elements (Group 1) tend to be retained in the bottom ash or partitioned between the bottom ash and the fly ash particles. The more volatile elements, however, especially Group 3, may be vaporized in the furnace and then either incorporated with any fouling or slagging deposits, recondensed in cooler parts of the system on to the surfaces of the suspended fly ash particles, or emitted as vapors or fine particulates in the stack gas stream. The elements attached to the surfaces of fly ash particles are especially prone to subsequent mobilization, and may pass into groundwater and other aquatic systems when the ash is disposed of at emplacement sites (Jankowski et al., 2006, and references therein) or used in processes such as abatement of acid mine drainage conditions. Numerous workers have studied the partitioning of trace elements in coal combustion systems, including Querol et al. (1995), Vassilev and Vassileva (1997), Hower et al. (2000b), Clemens et al. (2000), Vassilev et al. (2001; 2005a), Karayigit et al. (2001), and Li et al. (2005). Elements showing enrichment in the fly ash from PCC plants are mainly those that become volatile and then partly condense in the flue gases of the combustion system. These include As, B, Bi, Cd, Ge, Hg, Mo, Pb, S, Se, Sb, Sn, Tl (Querol et al., 1995). Elements showing enrichment in bottom ash or slag, on the other hand, include Cu, Cr, Ni, Fe, and Mn, concentrated in part by density segregation effects. Several authors, including Hower et al. (1999d), Vassilev et al. (2005b), and Vassilev and Menendez (2005), have investigated variations in mineralogy and chemistry (including trace elements) for different fractions of individual fly ashes, based on particle size, density, magnetic properties, and, in some cases, water solubility. Such studies may provide a basis for separating more marketable products from particular ash streams, or help in assessing the environmental impact of different ash disposal processes (e.g., Sushil and Batra, 2006). The dense fraction of the ash may be enriched in elements such as As, Ca, Cr, Cu, Gd, Mg, Mn, Mo, Fe, Ni, Pb, S, Se, Ti, V, Zn, and Zr, occurring in the crystal lattices of Fe and other oxides, in silicate and glassy components, and as discrete accessory minerals. The magnetic fraction is usually dominated by magnetite, spinel, and related minerals and may be enriched in Cr, Mn, Fe, Co, Cu, Ni, Zn, and other elements. Spears (2004) has investigated the mode of occurrence of trace elements in fly ash using laser ablation inductively coupled plasmamass spectrometry. Analysis of transects covering many particles showed that the glass in the ash is an important location for V, Cu, and Zn, and that Cr and V appeared to be concentrated in both the glass and the magnetite component. Many of the more environmentally significant elements, however, including As, U, Pb, Tl, Mo, Se, and probably, to a lesser extent, Ge and Ga, showed a major association with the surfaces of the ash particles, consistent with

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condensation of volatile elements from the coal on to the ash particles in the cooling combustion gases. Several other studies (e.g., Querol et al., 1995) show increased concentrations of the latter group of elements in the finer fractions of coal ashes, an observation also consistent with their occurrence on the surfaces of the particles rather than inclusion within the glass and mineral phases. Mercury is a notable exception to the capture behavior of other trace elements. Unlike As and Pb, for example, which show a definitive relationship between increased concentration and decreasing flue gas temperature and fly ash particle size, Hg is captured by the fly ash carbons, with little apparent capture by the fly ash inorganics (Hower et al., 2005a; Lee et al., 2006; Sua´rez-Ruiz et al., 2006a, 2007). Hg capture by fly ash increases with the amount of fly ash carbon (Hower et al., 1999a, b, 2005a; Sua´rez-Ruiz et al., 2006a, 2007) and with a lowering of the ambient flue gas temperature (Hower et al., 2000b,c; Sua´rez-Ruiz et al., 2007). Further partitioning of Hg occurs between the different forms of unburned carbon (Hower et al., 2000b,c; Maroto-Valer et al., 2001). High-resolution transmission electron microscopy studies of fly ash captured by electrostatic precipitators have demonstrated the existence of carbons, below the resolution of optical microscopy, with <3 nm metal or mineral (possibly Fe spinel) inclusions associated with As, Pb, and Se, among other elements (Graham et al., 2005). Further complications for Hg capture arise because, in general, only Hg2þ is captured by emission control systems, elemental Hg passing through the system and into the atmosphere (Senior et al., 2000b). Once in the environment (see an enlarged discussion in Chapter 10), Hg is microbially converted to methylmercury, concentrated at the higher ends of the aquatic (both fresh and marine) food chain, and is a potential neurotoxicant when consumed by humans (Transande et al., 2006). There is a synergism between Hg and Cl, with Cl enhancing the oxidation of Hg0 to Hg2þ and increasing the possibility of Hg capture by fly ash carbons or by FGD gypsum (Kellie et al., 2005; Gerasimov, 2005). Fe may also enhance oxidation, whereas Ca inhibits this process (Senior et al., 2000b). Control of Hg emissions by the injection of powdered activated carbons (PAC) may be a necessity under phase 2 of the CAMR (see above) because Hg reductions by SCR and FGD will not be sufficient to reach the U.S. emissions limit. Though this is a seemingly simple, albeit expensive, solution, coal quality again plays a role because the SO3 in the flue gas will compete with the Hg2þ for the active sites on the PAC (Srivastava et al., 2006).

4.4.4 Emissions (Particulates <10 mm, Fly Ash, Dioxins, NOx, CO2, SO2) The problem of emissions from coal combustion is a serious matter for concern because of the environmental and human health implications

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such as those shown in Chapter 10. Here the emissions subject is discussed in relation to the coal properties and coal behavior during the combustion process. The term emissions in this context refers to particulates of a size less than 10 mm, as well as to fly ashes, dioxins, nitrogen oxides, carbon dioxide, and sulfur dioxide. Fly ashes and particulates of less than 10 mm in aerodynamic diameter (PM10) are the solids generated by combustion that may be emitted from coal-fired power stations. Fly ashes are recovered from the combustion stream by collection devices such as fabric filters or electrostatic precipitators. As discussed, they are predominantly composed of inorganic material, with a minor but significant organic phase made up of unburned carbon particles. Fly ashes may contain trace elements in different proportions, due in part to condensation of volatile elements on to particle surfaces from the combustion gases. The concentration of major and trace elements in a particular fly ash is a function, among other factors, of the particle size of the ash, the mineral matter and trace element content of the feed coal, and the temperature at the collection point within the combustion system. In the case of some elements such as Hg, the amount and type of unburned carbon also influences the level of retention. Fine Ash and Particulate Emissions The fine particulates formed during combustion (PM10 and finer particles) may pass through the collection devices that retain most of the fly ash and hence be emitted into the atmosphere, potentially polluting the natural environment. The finest fractions of these particulates may remain airborne for long periods of time. If inhaled they may have deleterious impacts on human health, as has been demonstrated by correlations existing between ambient fine particulate matter and human mortality rates (Dockery et al., 1993). Emissions of fine ash particles and some toxic elements from coal combustion may also be closely associated because the fine particles tend to be considerably enriched in trace elements (Lighty et al. 2000; Xie et al., 2006, among others). The need to control the emission of fine particulates with an aerodynamic diameter of less than 10 mm (PM10 emissions), together with their trace element content, is therefore a high priority in many countries (Buhre et al., 2005, 2006). Standards have been introduced to assist in reducing ambient fine particulate concentrations, as is the case in the United States and Australia for coarse particulate matter PM10 and in some cases for PM2.5 (particulates finer than 2.5 mm). Particulate matter included in the <1 mm fraction (PM1) is also the subject of legislative interest.

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Coal-fired power plants are required to monitor and characterize their emissions of fine particulate matter and optimize their air pollution control devices (APCDs) so as to be able to recover where possible at least 99% of the particulates (mainly fly ash) from the flue gas (Thomas, 2002). Because a further increase in APCD capture efficiency for reduction of fine emissions that pass through such systems may be a costly process, appropriate selection of the feed coal could provide a cost-effective method for reducing the proportion of fine particulates in the gas stream (Buhre et al., 2006). The mechanisms of ash formation have been extensively reported (e.g., Buhre et al., 2006) and they are the following: (1) coalescence of minerals included within the coal particles as the organic matter burns away; (2) fragmentation of unburnt carbon (char) particles; (3) fragmentation of independent (liberated or excluded) mineral particles in the furnace atmosphere; and (4) vaporization and subsequent condensation of inorganic elements occurring within the organic particles. The proportion and nature of fine particulates (PM10 and finer fractions) are affected by the coal characteristics, including the mode of occurrence of the mineral and inorganic matter and the combustion behavior of particles containing both organic and inorganic material. They are also affected by the combustion conditions, including the partial pressure of oxygen in the combustion chamber (Buhre et al., 2006; Liu et al., 2006). Coal properties such as particle size (Liu et al., 2005) further influence the formation of PM10 as well as the partitioning of metallic components in the PM10 and finer fractions (Xie et al., 2006). Other coal characteristics, including rank, petrographic composition, ash fusion temperatures, type of char, and related factors are also important (Buhre et al., 2005, 2006), and therefore selection of appropriate coals may help minimize the formation of fine ash from different combustion systems. As the organic matter in individual pulverized fuel particles burns away, any minerals included inside the burning particles may reach very high temperatures and, depending on the actual minerals involved, may coalesce into fused aggregates. Although the minerals intimately associated with the organic matter may be different from those in the coal overall, the potential for such coalescence may be indicated by the ash fusion temperatures of the coal concerned. In the absence of any coalescence, the fine minerals included with the organic particles may be expelled individually and give rise to increased proportions of fine particulates in the combustion stream. Experimental studies by Buhre et al. (2006), for example, have found a positive correlation between the proportion of PM10 particles and the ash deformation temperature, which could offer a basis for particulate control based on coal quality parameters.

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The extent of char fragmentation depends on the swelling behavior of the individual macerals during combustion. If the macerals swell extensively, the resulting char may fragment, and the minerals inside the char fragments will form fine ash particles. Chars that are highly porous and have low densities can give rise to extensive fragmentation during combustion (Yan et al., 2002), resulting in fine ash formation. The proportion of such char may be estimated from the vitrinite content of the coal and the pressure at which the char is likely to be formed (Benfell et al., 2000). Nonmineral inorganic elements occurring in the chemical structure of the coal macerals, especially alkalis, sulphur, and phosphorus, may be vaporized in the combustion process to an extent depending on the temperature inside the burning coal particle and the nature of its inorganic constituents (Quann and Sarofim, 1982; Quann et al., 1990; Buhre et al., 2005). Condensation of vapors formed in this way gives rise to very fine ash particles, commonly less than 1 mm (PM1) and typically around 0.3 mm in particle size. Experimental studies by Buhre et al. (2005) have shown coals with a high sulphur content tend to yield high proportions of submicron ash. Although there is significant variability depending on the coal concerned, the main constituents of these particles were found to be sulphur, silicon, phosphorus, and sodium, but many different trace elements may also be involved. Goodarzi (2006) has described the characteristics of the fine particulates from a Canadian power plant, using SEM/EDX and chemical analysis techniques. The particles included unburnt carbon, often with a cellular to spongy morphology; minerals from the feed coal, such as quartz; and by-products of mineral reactions in the coal, including angular gypsum crystals. The emitted particles were mostly spherical, with matrices composed of calcium-bearing aluminosilicate material. The PM>10 fraction contained small plerospheres, fragments of char, and angular quartz and feldspar particles. The PM10 fraction contained solid spheres and cenospheres, gypsum needles, and particles of char. The PM2.5 particle size fraction was mostly composed of solid spherical aluminosilicates with some surface enrichment of elements including Ba, Ca, and Fe. Cadmium, Cu, Mo, and Ti were also detected in the PM2.5 fraction. Dioxins and Gaseous Oxides The dioxins are a group of chemical compounds containing about 210 species (as summarized by Thomas, 2002), consisting of 75 polychlorinated dibenzo-para-dioxins (PCDDs) and 135 polychlorinated dibenzofurans (PCDFs). Although the majority of these species at the levels found is not harmful to human health, a small group of these compounds (17) are of great concern because of their toxicity/

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carcinogenicity. The incomplete combustion of fuels in the presence of chlorine might possibly result in the formation of traces of dioxins in the flue gases. The potential for formation of such compounds in large pulverized fuel plants is relatively low, because the combustion efficiency is usually fairly high. The plants also typically use fuel that contains more sulfur than chlorine, and this inhibits dioxin formation. Dioxin and related compounds, however, may be formed in fluidized-bed combustion, particularly if cofiring with biomass or plastics is involved (Sanchez-Hervas et al, 2005; Skodras et al., 2002). Nitrogen oxides (NOx) are produced by the reaction of the nitrogenous compounds contained in the coal and also by reaction of nitrogen in the air, with the oxygen used in the combustion process (Cudmore, 1984; Juniper, 2000). Both processes are favored by the high temperatures and rapid heating rates associated with conventional burners for pulverized coal. Coal devolatilizes at high temperatures and in air-rich conditions; the nitrogen in the coal is released during devolatilization and enters the gas phase as HCN or NH3. These products then react with the oxygen in the air to form a range of nitrogen oxide (NOx) compounds. Among the oxides so generated, nitrous oxide (N2O) has a significant effect on the atmosphere, absorbing infrared radiation and contributing to ozone depletion (see Chapter 10). NOx emissions may be reduced in power plants by special air control and combustion techniques (Cudmore, 1984; Juniper, 2000). In such cases the temperature is lowered and so, in turn, is the production of NOx. However, a negative effect of such processes is an increase in the proportion of unburned carbon in the fly ash. Selective catalytic reduction (SCR) is a post-combustion NOx control procedure in which ammonia is mixed with the flue gas in the presence of a catalyst (Juniper, 2000). The ammonia reacts with the NO and NO2 contained in the gas to form molecular nitrogen and water. SCR systems are being used in Japan and Germany (Thomas, 2002). Other procedures for removing NOx include combined SO2 and NOx removal systems such as are used in Denmark, Italy, and the United States (Thomas, 2002). Carbon dioxide (CO2) is an emission of major concern at the present time because of its effect on the global climate and ecology (see Chapter 10). CO2 emission control is therefore gaining increasing acceptance by industry and the community. One of the practical tools capable of reducing CO2 emissions from combustion in the short term, increasing the overall thermal efficiency of the generating system, has become a key concern when choosing appropriate technology for new plants or for upgrading existing plants. Efficiency is also important for longer-term solutions aimed at reducing CO2 emissions by carbon capture and sequestration (CCS). According to Bee´r, (2007),

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it is essential that the plants be highly efficient so as to mitigate the energy penalty for CCS technology application. The carbon dioxide emissions linked to a reduced level of efficiency during combustion, among other factors, are due to poor quality coals, small and aging combustion units, poor maintenance, or obsolete technologies (Thomas, 2002). Apart from increases in plant efficiency, reduction in CO2 emissions from pulverized coal power plants may be achieved by chemical capture (adsorption) and storage processes. Current research is seeking to improve these methods, but a full discussion is beyond the scope of this volume. During combustion the CO2 yield per unit energy is inherently related to the carbon content and the calorific value of the combusted fuel (Quick and Glick, 2000; Quick and Brill, 2002; Sakulpitakphon et al., 2003). The calorific value and carbon content are in turn related to the rank and type (maceral assemblage) of the coal concerned. Lower rank coals (e.g., lignites) give rise to greater emissions of CO2 per unit energy than higher rank (e.g., bituminous) coals, due to a combination of their higher moisture and lower calorific value. Inertinite macerals also have a higher carbon content than the vitrinite macerals in the same coal (see Chapter 2), and thus inertinite-rich coals may be expected to have higher yields of CO2 per unit energy than vitrinite-rich coals at equivalent rank levels. Quick and Brill (2002), for example, suggest a variation of 1.3 kg C/net GJ in carbon emissions due to inertinite abundance in some bituminous coals and indicate an additional variation of 0.9 kg C/net GJ due to differences in coalification across the bituminous rank range. Each percentage of sulfur in bituminous coal reduces carbon emissions by about 0.08 kg C/net GJ (Quick and Brill, 2002). Experimental work based on density-gradient centrifuge studies by Sakulpitakphon et al. (2003) also suggests that CO2 yields should increase from liptinite-rich through vitrinite-rich to inertinite-rich bituminous coal materials. The various sulfur oxides (SOx), particularly sulfur dioxide (SO2), are one of the main gaseous pollutants produced during combustion processes that in accordance with environmental regulations should be avoided as far as possible. SO2 emissions can be a hazard to human health and to the environment. Control of SO2 emissions can be achieved in several ways, including using coals with a lower sulfur content, removing the sulfur from the coal before it is used, or removing SO2 from the combustion stream by flue gas desulfurization (FGD) techniques before the stack gases are released to the atmosphere. Sulfur is found in coal in both organic and inorganic form, with the inorganic sulfur occurring as either sulfide minerals (pyrtitic sulfur) or a range of sulfate compounds (sulfate sulfur). The most

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common sulfur-bearing mineral in coal is usually pyrite (FeS2), and this can often be removed in an effective way by coal preparation and beneficiation processes. However, the organic sulfur and in some cases (e.g., low-rank coals) the sulfate forms are retained within the coal, and along with any remaining pyrite will contribute to SO2 production. In addition to the total sulfur of the coal, it is necessary to know the calorific value, the hydrogen and moisture contents (see Chapter 2), and the heating rate of the combustion system, to determine the amount of SO2 produced by a particular power plant. Flue Gas Desulfurization (FGD) Processes In many plants it is necessary to remove sulfur from the flue gases after the combustion process to meet environmental requirements. For this reason FGD is widely used to control emissions of sulfur dioxide (SO2) and sulfur trioxide (SO3) from plants burning coals with high sulfur contents. Various FGD technologies have been developed (as summarized in Department of Trade and Industry, 2000b, and Thomas, 2002), with selection made on an economic basis after taking into account technical considerations. Specific issues include the degree of desulfurization that the process can offer as well as its flexibility. Most FGD technologies use an alkali sorbent, such as limestone (calcium carbonate), quicklime (calcium oxide), hydrated lime (calcium hydroxide), or sometimes sodium and magnesium carbonate and ammonia, to capture the acidic sulfur compounds from the flue gas. In all cases the alkalis react with the SO2 in the presence of water (e.g., a spray of slurry containing the sorbent) to produce a mixture of sulfite and sulfate salts. This reaction may take place in the bulk solution or on the wetted surface of the solid alkali particles. The various types of processes are referred to as wet, dry, and semi-dry flue gas desulfurization techniques (Bigham et al., 2005). The most common FGD technology uses a limestone/gypsum wet-scrubbing process. Usually the plant is located downstream of the electrostatic precipitator (ESP) so that most of the fly ash generated from combustion is removed before the gas reaches the FGD plant. In the process of desulphurization the gas is scrubbed with a recirculating limestone slurry, which removes almost 95% of the SO2 from the flue gas. The process also removes almost all the HCl contained in the flue gas. The calcium carbonate in the limestone reacts with the SO2 and oxygen from the air to produce gypsum, which precipitates from the solution. The HCl is also dissolved in the water and neutralized, producing a solution of calcium chloride. The gypsum slurry is recovered from the absorber sump and stored or treated for further use (e.g., as plaster board), and fresh limestone

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is pumped into the absorber to maintain the pH conditions. The remaining gas is reheated. Other FGD technologies include seawater washing, ammonia scrubbing, and a method employing aqueous sodium sulphate solution (Wellman-Lord process). The semi-dry FGD processes employ a circulating fluidized bed, dry spray, or duct dry spray to produce powdered mixtures of calcium compounds. FGD technology using dry processes injects lime or sodium bicarbonate into the furnace of the boiler to absorb the SO2. The sorbent is then extracted, together with the fly ash, as a mixture of ash and calcium/sodium components. Wet FGD systems tend to utilize sorbent more efficiently than dry processes and typically can reduce SO2 emissions by more than 90% (Bigham et al., 2005). Dry FGD systems, however, are more easily retrofitted onto existing combustion facilities. In both cases, the removal of SO2 from flue gases results in a solid residue that must either be disposed of or utilized in a beneficial manner. Kost et al. (2005) and Bigham et al. (2005) provide detailed chemical and mineralogical data on a range of flue gas desulfurization products, including materials from fluidized-bed combustion systems as well as FGD units fitted to pulverized fuel plants. In addition to the mineralogy as indicated by X-ray diffraction and thermal analysis, the latter study also includes information on the swelling characteristics of the materials, brought about by interaction of anhydrite (CaSO4) in the materials with water to form gypsum, and data on the mobility of different trace elements when the materials are exposed to weathering conditions.

4.5 Fly Ash When coal is burnt in a pulverized fuel (pf) furnace, a range of solid residues or ash particles is produced, made either from unmelted minerals and uncombusted coal or from neoformed glass and minerals and repolymerized carbon materials. The fine fraction, with mainly silt-sized particles, remains in suspension with the combustion gases and is recovered by fabric filters or electrostatic precipitators as fly ash. This fraction typically represents up to 90% of the total solid residues that are produced from modern pulverized fuel combustion systems. The coarser fraction is mainly represented by sand- or gravel-size aggregates that fall to the bottom of the combustion chamber and are collected as bottom ash. Modern pollution-control devices, generally fabric filters (baghouses) or electrostatic precipitators (ESPs), are very efficient in capturing fly ash, typically removing >99% of the fly ash from the combustion stream. Along with bottom ash and, in some plants, FGD residues, these materials, which are collectively referred to as

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coal combustion products (CCPs), represent useful raw materials for a wide range of industrial applications (Ward et al., 2006). Low-carbon fly ash is used as a partial replacement for Portland cement in concrete manufacture because it reduces permeability, increases long-term strength, decreases the amount of damage caused by the heat from hydration, and increases resistance to sulfate attack (American Coal Council/American Coal Ash Association, 2003). Cement manufacture is also a large source of CO2 emissions. The substitution of one tonne of fly ash for Portland cement reduces CO2 emissions for the overall process of concrete production by about one tonne (American Coal Council/American Coal Ash Association, 2003; Heidrich et al., 2005). Other uses or potential uses of fly ash include the production of coarse and fine aggregates by either sintering with a flux at high temperature or the formation of silicate geopolymers at low temperature (Swanepoel and Strydom, 2002) and the production of synthetic zeolites by reacting the ash with appropriate cations under hydrothermal conditions (Querol et al., 2002). Fly ash may be added to help fill the void spaces in soils for construction, increasing the overall density and helping the soil to support roads, buildings, bridges, and other manmade structures. The material may also be used as a soil conditioner in agricultural and horticultural applications, with the porous silt-size particles increasing the level of water infiltration and retention and in some cases modifying the soil pH or providing supplementary nutrients (Adriano et al., 1980; Carlson and Adriano, 1993; Mittra et al., 2005; Yunusa et al., 2006). Fly ash is also used as backfill in a number of mining operations (Ilgner, 2000; Vories, 2001; Murarka, 2001), especially in cases where alkaline components in the ash can assist in neutralizing acid surface or groundwater associated with sulfide oxidation. Utilization of all fly ash, bottom ash, and boiler slag in the United States was just over 40% in 2005 (American Coal Council/American Coal Ash Association, 2006). The European Union used around 50% of its fly ash production in 2004 (European Coal Combustion Products Association, 2005). Europe has been able to achieve greater levels of utilization due to regulation and is pushing for 100% utilization in some countries, a measure motivated by the lack of space for landfills.

4.5.1 Carbon in Fly Ash Though the inorganic fraction of fly ash is the desired product for most industrial uses, fly ash carbon is the primary factor that determines whether the fly ash can be used, especially in cement and concrete industries. As noted, fly ash is used as a replacement for Portland cement in concrete manufacture. Air entraining agents

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(AEA), surfactants added to concrete mixes to improve the final properties of the finished product, may be adsorbed by fly ash carbons, which is not a desirable result (Hill et al., 1997 among others). As a consequence, the ASTM C618-99 (1999b) and ASTM C618-05 (2006) norms recommend that not more than 6% loss-on-ignition (as a proxy for carbon analysis) be allowed in fly ash to be used for highway construction. Before discussing the properties of fly ash carbons (see Figure 4.9), it is necessary to consider some of the terminology used in petrographic analysis. Bailey et al. (1990) introduced a fly ash classification system based on the texture of the ash, with an emphasis on the carbon components. In this system, the carbons were classified on the basis of whether they had thick (crassi-) or thin (tenui-) walls and whether they formed spheres or networks; various mixed forms and inertinites were included in this system. Hower et al. (1995) introduced a genetic system for the bituminous-derived ashes they examined, classifying carbons according to their manner of transformation in the boiler, as with inertinite (essentially uncombusted) versus melted and repolymerized carbons, with the latter being subclassified according to optical properties (isotropic versus anisotropic). Vitrinite from coals of ranks lower or higher than bituminous does not pass through the same melting and repolymerization stages (see Figure 4.9a–e). The classification of Hower et al. (1995) was modified by Hower et al. (2005b) to incorporate a broader range of carbon derived ranks into the carbon classification. Sua´rez-Ruiz et al. (2007) also attempted to integrate the textural and genetic classifications into a common scheme in their study on fly ashes from combustion of anthracitic coal blends. Such an objective is currently being further pursued by the International Committee for Coal and Organic Petrology (ICCP News, 2005, 2006). Some preliminary results are reported in Sua´rez-Ruiz and Valentim (2007). Hill et al. (1997) noted the dominant role of carbon in the attenuation of air entraining properties, with isotropic carbon considered to be more of an AEA adsorbent. Detailed studies of single fly ashes suggest a different ordering of carbon adsorptive properties. Hower et al. (2000b) and Maroto-Valer et al. (2001) showed that BET surface area and Hg concentration increased from inertinite to isotropic carbon to anisotropic carbon for carbons isolated from a fly ash produced by the combustion of a bituminous coal blend. Sua´rez-Ruiz et al. (2006a, 2007) also demonstrated that fly ash carbons from the combustion of anthracitic/meta-anthracitic coal blends (which are mainly anisotropic carbons) are able to capture Hg even if their BET surface areas are extremely low (Sua´rez-Ruiz and Parra, 2007). Moreover, as these authors pointed out, there are some differences with respect to the amount Hg captured between the various types of anisotropic

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(b)

(c)

(d)

(e)

(f)

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FIGURE 4.9. Photomicrographs of fly ash carbons. Optical microscopy, pictures taken in oil immersion (50x objective), with polarized light and 1 lambda retarder plate (long side of the picture: about 200 microns). (a) Unburned carbons, unfused and anisotropic particle derived from the combustion high rank coals (anthracites). (b) Same particle rotated of about 90
showing the anisotropic character. (c, d) Fused, porous, and anisotropic carbons derived from medium rank coals (bituminous coals). (e, f) Unfused and isotropic carbons derived from low rank coals (lignite). (Photomicrographs: I. Sua´rez-Ruiz.)

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carbons produced from the combustion of the same coal blend. Hg capture by the fly ash inherent in an ESP or baghouse needs to be considered in efforts to reduce overall Hg emissions from coal combustion processes.

4.5.2 Glass and Minerals in Fly Ash From the mineralogical viewpoint, fly ash can be regarded as essentially consisting of three types of components: crystalline minerals, unburnt carbon particles, and noncrystalline aluminosilicate glass. Quartz, mullite, cristobalite, magnetite, and hematite are the most common crystalline phases in ashes from higher rank coals. Gypsum, lime (CaO), portlandite (Ca(OH)2), periclase (MgO), calcite, and a number of calcium aluminate minerals may also be present in some cases, especially in the ashes from calcium-rich, lower rank coal seams. However, data from optical and electron microscopy, X-ray diffraction, and geochemical analyses (Hower et al., 1995, 1999d, 2005b; Vassilev et al., 2003, 2005a, b; Liu et al., 2004; Mastalerz et al., 2004a; Moreno et al., 2005; Kutshcko and Kim, 2006) indicate that, though crystal fragments and crystalline aggregates may be more abundant in some ashes than others, glass usually represents the dominant component present in most fly ash samples. Ward and French (2006) describe methods for determining the proportion of different crystalline components and of amorphous or glassy material in fly ashes using Rietveld-based X-ray powder diffraction (XRD) techniques. Several different sample preparation and processing methods were investigated, including the XRD analysis of samples spiked with known masses of synthetic corundum or zinc oxide as well as techniques based on analyzing the raw or unspiked fly ash directly, using poorly crystallized silicate mineral patterns (metakaolin or an opaline tridymite) to represent the amorphous constituents. The results of the different methods were found to be mutually consistent and consistent with other data from other sources for an international reference fly ash (Winburn et al., 2000). The mineralogy of fly ashes, including the proportions of quartz, iron oxide, and glassy constituents, appears to be related to the nature of the mineral matter in the relevant feed coals (Mardon and Hower, 2004). Ward and French (2006) indicate that the relative proportions of quartz and iron oxide minerals in a series of Australian fly ashes (including the glass content) is similar to the proportion of quartz, pyrite, and siderite in the mineral matter of the relevant feed coals. Calculations based on subtracting the inferred chemistry of the crystalline minerals in the fly ashes from the total fly ash chemistry can also be used to estimate the overall chemical composition of the

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X-ray diffraction data on raw or spiked ash

Chemical analysis data on raw ash

Mineral and glass percentages in raw ash by Siroquant

Allow for unburnt carbon

Calculate chemical composition of mineral fraction

Subtract minerals from whole-ash chemistry

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Calculate chemical composition of glass fraction

FIGURE 4.10. Flowsheet for estimation of glass composition in fly ash based on quantitative X-ray diffractometry and chemical analysis data. (Source: Fuel 85, C. R. Ward, and D. French, “Determination of glass content and estimation of glass composition in fly ash using quantitative X-ray diffractometry,” 2268–2277, copyright 2006, with permission from Elsevier.)

glass fraction in a fly ash (see Figure 4.10). Based on such calculations, Ward and French (2006) suggest that ashes derived from lower rank coals may have different glass compositions from those derived from higher rank (bituminous) materials. The different glass compositions appear to be related to several other ash properties, including particle density and particle surface area. The evaluation of glass content and composition may be significant for different aspects of ash utilization, such as zeolite or geopolymer production, and for evaluating interactions with water at ash disposal sites (Jankowski et al., 2006).

CHAPTER 5

Coal Gasification Nicola J. Wagner With assistance from M. Coertzen R. H. Matjie J. C. van Dyk

5.1 Introduction Gasification is a process that converts carbonaceous materials, such as coal, petroleum, petroleum coke, or biomass, into carbon monoxide and hydrogen (Wikipedia, 2006). The gaseous products are further processed for use as an energy source or as a material for the production of a variety of chemicals and/or liquid fuels. Coal gasification as a power generation technology is gaining popularity due to the ready global availability of the raw material (coal), as well as positive environmental issues associated with this technology over other combustion technologies. To ensure optimal process efficiency, an in-depth understanding of the coal properties influencing gasification behavior is required. Gasification performance is dependent on coal type (organic and inorganic composition) and gasifier configuration, both of which are discussed in the following chapter, the former in more detail. Alternative gasification technologies are briefly discussed.

5.2 Processes and Methods for Coal Gasification Gasification is defined as the reaction of solid fuels with air, oxygen, steam, carbon dioxide, or a mixture of these gases at temperatures exceeding 800 C in a reducing environment where the air:oxygen ratio is controlled. Heat and pressure break apart the chemical bonds in the molecular structure of the coal, setting in motion chemical reactions with the steam and oxygen (Coal Utilization Research Council, 2006). The yield is a gaseous product suitable for use as: (1) an energy source, or (2) as a raw material (syngas) for the production of chemicals, liquid fuels, or other gaseous fuels (Collot, 2006), or a combination of these. Referred to as synthesis gas (or syngas), this primarily consists Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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of carbon monoxide and hydrogen. The chemical composition of the gas produced depends on coal composition and rank, coal preparation (specifically particle size distribution and ash content), gasification agents (oxygen:air:steam ratios), gasification conditions (heating rate, residence time, temperature, pressure), and plant configuration (flow geometry, feed form, mineral behavior, syngas cleaning). It is generally assumed that the gas phase in a gasifier is controlled predominantly by the equilibrium of the water-gas shift reaction, of which the amount of carbon present in the gas phase is the important driver. Thus, an understanding of the carbon conversion process is essential for calculating water-gas shift reactions and gas concentrations and is key in predictive gasification modeling (Harris et al., 2006). At higher temperatures under pressure, a greater degree of carbon conversion occurs, and the extra carbon in the gas phase affects the equilibrium in the reaction such that the CO and H2 levels are higher at the expense of CO2. The study of fundamental coal gasification reactions is gaining momentum as the popularity of gasification as a coal conversion process increases. There are numerous gasification processes, which differ in terms of technical design, scale, fuel handling, and so on. Typically gasifiers are classified according to the type of reactor used and the way in which fuels and oxidant flow (Department of Trade and Industry, 1999). Higman and van der Burgt (2003) and Berkowitz (1985) provide detailed discussions on gasifiers and gasification processes, and Collot (2006) recently published a paper eloquently summarizing the three types of generic gasifiers and their suitability for coal gasification. The choice of gasification technology is dependent on a variety of factors, but coal type is generally the least flexible factor and gasification technology selection is typically based on the coal to be processed. The Sasol process in South Africa is a good example, where the abundant low grade, high ash coal makes it extremely suitable for the Sasol-Lurgi Fixed Bed Dry Bottom technology (previously known as the Lurgi Dry Ash process) (Erasmus and Scholtz, 2002). The Great Plains Synfuels plant in North Dakota, in the United States, uses the same technology to produce substitute natural gas from lignites. The Department of Trade and Industry (1999) Technology Status Report on gasification provides a useful reference detailing eight different specific gasification types, namely British Gas Lurgi (BGL), ConocoPhillips E-gas (Destec), Winkler, Lurgi, Mitsui Babcock Energy Ltd., Prenflo, Shell, and General Electric (Texaco).

5.2.1 Various Types of Gasifiers and Gasification Processes There are more than 100 gasifier technologies in existence, ranging from laboratory scale up to demonstration and commercial scale.

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However, most gasifiers can be grouped in one of three groups according to the type of reactor bed in which the coal is gasified, namely: (1) fixed bed or moving bed, (2) fluidized bed, or (3) entrained flow gasifiers (King, 1981; Collot, 2002, 2006; Higman and van der Burgt, 2003). Other types of gasifiers have been developed but are not yet near commercialization. Examples include rotary kiln gasifiers and molten bath gasifiers (Collot, 2002, 2006). The design of the gasifier affects its ability to handle particular feedstocks and the condition in which the feedstock can be fed into the vessel, oxidant and steam requirements, and gas outlet temperature and composition, as summarized in Figure 5.1. To select the optimum gasification process for a specific application, important factors need to be considered, including feedstock characteristics, quality requirements for clean gas, the quality of waste products, operating characteristics, and environmental legislation. The chemical and physical properties of a coal are directly related to

Fixed / moving bed Depiction

B

B

Fluidized bed

P

B

Entrained bed P

P

B

P

Preferred feedstock

Lignite, reactive bituminous coal, wastes

Coal feed size (mm) Ash content

<50 No limitation slagging type <25% preferred Operating temperature Co-current 700–1200 (°C) Counter-current 700−900 Scale co-current <5 MWt counter-current <20 MWt Exit gas temperature 420–650 (°C) CO2 in dry raw gas 26−29 CH4 in dry raw gas 8−10 H2/CO ratio 1.7−2.0 Ash conditions Dry/slagging Tars produced co-current - low counter-current mod-high Key distinguishing Hydrocarbon liquids in characteristic the raw gas Key technical issue

Utilization of fines and hydrocarbon liquids

P

B

Lignite, bituminous coal, cokes, biomass, wastes <6 No limitation

Lignite, reactive bituminous coal, petcokes <0.1 <25% preferred

Dense <900 Circulating <900 Dense 10<MWt<100 Circulating 20<MWt
>100 MWt

18 6 0.7 Dry/agglomerating intermediate

6−16 <0.3 0.7−0.9 Slagging absent

Large char recycle

Large amount of sensible heat energy in the hot raw gas Raw gas cooling

Carbon conversion

1500

±1200

FIGURE 5.1. Operating parameters for generic types of gasifiers (compiled from SFA Pacific, 2000; Simbeck et al., 1983; Biomass Technology Group, 2006).

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gasifier behavior, and detailed analyses of coal are essential for predicting gasification performance when a specific coal source or mixtures of coal sources are to be gasified (refer to Section 5.3.1). Fixed Bed Gasifiers Fixed bed or moving bed gasifiers are characterized by a bed in which the coal moves slowly downward under gravity as it is gasified (Higman and van der Burgt, 2003), usually counter-current to the steam and oxidant feed. Fixed bed gasifiers are suitable for solid feedstocks, usually in the form of coarse coal particles (5–75 mm, for example), to ensure good permeability. Poor permeability of the bed will result in an excessive pressure drop and channel burning that can lead to unstable gas outlet temperatures and composition. The fact that no excessive grinding is required greatly simplifies the feedstock preparation requirements and costs. The residence time in the gasifier can be as long as to 60 minutes, thus allowing for the complete conversion of the large carbon particles. The gas and solid product streams exit at relatively low temperatures due to the high cold gas efficiency of the heat exchange achieved in the counter-current operation. Because of the high thermal efficiency, fixed bed gasifiers have relatively low oxidant requirements. Fixed bed gasifiers produce tars and oils as a byproduct present in the raw gas. These byproducts can be considered as advantageous and can be marketed, but they need to be separated from the raw gas to avoid deposition on downstream equipment. The disadvantage of traditional fixed bed technology is that it has a limited ability to handle fine coal feed or high caking coal and has difficulty in handling broad particle size distributions. There are two main types of commercial fixed bed gasifiers, both originally developed by Lurgi: (1) the Sasol-Lurgi fixed bed dry bottom gasifier, and (2) the British Gas Lurgi (BGL) slagging gasifier (Higman and van der Burgt, 2003; Erasmus and Scholtz, 2002). In the dry ash version, temperatures at the bottom of the ash bed are kept below the ash fusion temperature to allow coal ash to be removed as solid particles. For the BGL, bed bottom temperatures are high enough to allow the ash to melt and be removed as a molten slag. There are currently 119 Lurgi dry bottom gasifiers operational throughout the world, most notably 97 operated by Sasol in South Africa (of which 17 have recently been shut down), producing syngas which is converted to Fischer-Tropsch liquids (fuels and chemicals) (Figure 5.2) (van Dyk et al., 2006a, b). Another significant installation is the North Dakota Gasification Company in the United States, where 14 Lurgi gasifiers are used to produce gaseous fuels (Simbeck et al., 1983).

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Gasifier top Coal

Gas

Steam, oxygen or air Ash Gasifier bottom

0

300

600 900 1200 Temperature (°C)

1500

FIGURE 5.2. Typical zones in the Sasol-Lurgi Fixed Bed Dry Bottom gasification process. (Source: Minerals Engineering 19, J. C. van Dyk, S. Melzer, and A. Sobiecki, “Mineral matter transformation during Sasol-Lurgi fixed bed dry bottom gasification: utilization of HT-XRD and FactSage modeling,” 1126–1135, copyright 2006, with permission from Elsevier.)

Fluidized Bed Gasifiers Fluidized bed gasifiers consist of vertical, cylindrical, refractory-lined vessels with recycle cyclones and bottom ash cooling systems (SFA Pacific, 2000). Fine dry coal (<6 mm) is introduced at the bottom of the gasifier and fluidized using the oxidant and steam (Simbeck et al., 1983). Extremely good mixing between feed and oxidant promotes both heat and mass transfer (Higman and van der Burgt, 2003). The residence times in the gasifier is typically in the order of 10 to 100 seconds but can be longer, with the feed experiencing a high heating rate on entering the gasifier. Because of the even distribution of material in the bed, a certain amount of only partially reacted fuel is inevitably removed with the ash (high carbon loss), placing a limitation on the conversion of carbon. Unreacted particles that are entrained in the gas are recovered by cyclones for recycling. Highly varying ash characteristics are problematic in fluidized beds, so the coal feed should be well mixed so that the same characteristics are maintained as far as possible. Operational temperatures are restricted to below the ash softening point, since slagging will disturb fluidization, making these gasifiers more suited for the gasification of reactive feedstocks such as low rank coals and biomass. Fluidized bed gasifiers are tolerant to coals with a high sulphur content as up to 90% of the sulphur in the coal feed can be retained by adding a sorbent (such as lime).

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Fluidized bed gasifiers use a moderate amount of steam and oxygen during gasification, usually operate at near atmospheric pressure, and are therefore well suited for smaller capacities. Higher capacities can be achieved if the gasifier is operated at elevated pressures. The raw gas exiting the vessel is essentially free of hydrocarbons heavier than methane. The two main contenders for fluidized bed gasifier technology are the High Temperature Winkler (HTW) and the Kellogg Rust Westinghouse (KRW, now known as KRW Energy Systems) gasifiers. These technologies have not been applied extensively on a commercial scale for various reasons, including low capacity throughput, high operating cost, low carbon conversion, large char recycle, and ash agglomeration problems. From a recent survey conducted by the Gasification Technologies Council it is evident that traditional fluidized bed technology is not likely to be a major contender for future applications (SFA Pacific, 2000). At present, 21 fluidized bed gasifiers are operational, most notably eight GTI U-GAS gasifiers in China, which are employed in the production of fuel gas and town gas. Perhaps the most promising fluidized bed technology is the transport gasifier developed by Kellogg, Brown, and Root Inc., which is currently being tested at the Power Systems Development Facility in Wilsonville (Power System Development Facility, 2006). The transport reactor is an advanced-circulating fluidized-bed reactor designed to operate either as a combustor or as a gasifier. Entrained Flow Gasifiers Entrained flow gasifiers are cocurrent reactors. Finely ground feed, or liquid feed, is introduced into the gasifier along with the oxidant and steam or liquid water as the moderator. Due to the short gas residence times the coal is pulverized to ensure a high carbon conversion, and the coal and gases flow concurrently at high speed. As documented by Collot (2006), the following coal properties have an important effect on slurryability (besides the particle size): equilibrium moisture, fixed carbon, surface carbon/oxygen bonding, and free-swelling index. Bituminous coals with low inherent moisture contents and a hydrophobic nature are the coals of choice for commercial high solid content coal/ water fuels. Higher and lower rank coals can be used with pretreatment or an additive addition. Due to economic and technical reasons associated with milling and gasification efficiency, entrained flow gasifiers are usually recommended for coals with low ash contents. No reaction zones can be distinguished in the entrained flow gasifier. Gasification reactions typically occur at temperatures in excess

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of 1,200 C, and the raw gas exits at temperatures that are above 1,000 C (Simbeck et al., 1983). Reactor temperatures are above the ash fusion temperature, and the ash forms a slag. Maximum ash content is usually fixed for each type of entrained flow gasifier, since the tolerance to ash depends on economic and technical factors and an ash fusion temperature below 1,400 C is preferred. The gasifier’s tolerance to sulphur and halogens emitted from the coal depends on the composition and resistance of the material used for cooling, cleaning, and tapping systems but also on the operating conditions of the gasification process. Entrained flow gasifiers are regarded as the most versatile type of gasifiers, accepting both solid and liquid fuels and operating at high temperature (above ash slagging temperatures) to ensure a high carbon conversion and syngas free of tars and phenols (Collot, 2002). These gasifiers have the ability to handle a wide variety of coals. Tar and oil formation is eliminated (not necessarily positive in all scenarios), and a readily disposable water condensate and solid residue are produced (Perry and Green, 1984). Problems associated with the operation of these gasifiers include handling high operating temperatures, refractory life, and slag control. Flexible load operation is more difficult to handle compared to the other two main types of gasifiers (Collot, 2002), due to the fact that control of the fuel/oxidant ratio is more critical because they have a smaller heat capacity and no inventory of process feedstock. Most commercial gasifiers in the world are entrained flow gasifiers, with more than 240 in operation, and 50 more in the planning phase. Of the operational gasifiers, more than 50% of the gasifiers are Texaco gasifiers and 36% are Shell gasifiers. This is also the type of gasifier most favored for Integrated Gasification Combined Cycle (IGCC) applications at present, specifically Texaco gasifiers (used in seven IGCC projects up to 2001) (Simbeck et al., 1983). In conclusion, gasification as a coal-based power generation technology is increasing globally primarily due to its high efficiency and significantly reduced emissions compared to conventional coal-fired power systems. As much as 99% of sulphur and other pollutants can be removed and processed into commercial products such as chemicals and fertilizers (Coal Utilization Research Council, 2006). IGCC systems offer emission levels approaching those of natural gas combined-cycle plants, with the advantage of the low cost of coal. The next generation of coal-based power generation facilities is likely to be based on entrained-flow gasification technologies. An understanding of the coal properties influencing gasification behavior under relevant process conditions is vital to enhancing gasifier performance. Ideally, gasification technology should be linked to the coals available for utilization in the required locality.

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5.3 Main Characteristics and Properties of Coals for Gasification As is well known, coal is a heterogeneous mixture of organic and inorganic constituents (refer back to Chapter 2), and it is from the organic component that syngas is generated during gasification. The inorganic component has a significant impact on gasifier performance, where certain minerals can act as catalysts, others as inhibitors, depending on the gasification technology and the type of mineral. As discussed by Hanson et al. (2002) and Wall et al. (2002), gasification of coal occurs in two stages, namely: (1) rapid pyrolysis devolatilizing the coal and char formation, and (2) char reaction. If the heating rate is slow, devolatilization is followed by gasification; if the heating rate is fast, the two processes may occur simultaneously. All the coals will go through a pyrolysis stage during conversion, when gases and higher hydrocarbons are released. Refer to Gavala (1982) and Berkowitz (1985) for in-depth discussions on coal pyrolysis. Devolatilization occurs between 350 C and 800 C, and the rate is dependent on the heating rate, particle size, pressure, and reaction with the gasification agent. The water-gas shift, Boudouard, and hydrogenation reactions govern the overall conversion rates during the char reaction stage; refer to Higman and van der Burgt (2003) for models describing these reactions. The rate of the gasification reaction is influenced by the particle size, internal surface area, and morphology (structure) of the char, and char consumption is the controlling process in gasification. The subsequent rate of char combustion will significantly influence the release of the heat, temperature profiles, char burnout characteristics, and unburned carbon in the ash. Gasification performance is dependent on the type of the coal and gasifier configuration. Typically, the most relevant coal properties are elemental composition (organic and inorganic), surface characteristics and porosity, and intrinsic reactivity (van Heek and Muhlen, 1986), but the importance of coal properties will differ depending on the different gasifier designs. For example: l

l

l

l

Fixed bed gasifiers. Caking and agglomeration characteristics are of importance. Entrained-flow gasifiers. The Hardgrove Index is of significance for pulverization and particle size distribution for slurriability. Slagging gasifiers. Ideally suited for low ash content coals with low ash fusion temperatures to maintain the flow of slag. Fluidized-bed gasifiers. These require high reactivity of the coal-derived chars to minimize the unutilized carbon due to the relatively low temperatures of gasification.

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Coal characteristics and behaviors pertinent to gasification are discussed in terms of the different analytical techniques applied for characterization. Whereas the organic and inorganic components are discussed separately to a large extent, cognizance must be taken of the fact that the gasifier sees heterogeneous coal particles where the combined effects of the organic and inorganic responses to gasification occur. The characteristics of liquid and gaseous feedstocks are not considered here (refer to Higman and van der Burgt, 2003). Berkowitz (1985), Collot (2002, 2006), Higman and van der Burgt (2003), King (1981), Richter (2001), Solomon et al. (1987) provide further discussions on fundamental processes in gasification. Wall et al. (2002) and Liu and Niksa (2004) discuss the effect of pressure on coal properties and reaction kinetics during gasification.

5.3.1 Coal Characterization and Behavior During Gasification (Primarily the Organic Component) The following analyses must be performed for appropriate characterization of coal: proximate, ultimate and sulphur analysis, particle size distribution, thermal fragmentation (atmospheric pressure), Fischer Assay/Gray-King assay for tar or oil production, caking propensity (swelling index), reactivity, analysis of pore size and surface area of chars, and determination of petrographic properties (rank by vitrinite reflectance, maceral, microlithotype and mineral group analysis, degree of weathering, and char morphology). Proximate Analysis Proximate analyses (moisture, ash, volatile content, and fixed carbon by difference) are conducted at atmospheric pressure. Pressure, however, causes volatiles to stay in a particle for a longer time, which has implications when extrapolating proximate analysis data for gasification performance. Messenbock et al. (2000) demonstrated that certain coal types are more reactive to pressure increases than other coals tested. Gadiou et al. (2002) relate the decrease in volatile yield under pressure with an enhancement of secondary reactions of gaseous volatile matter within the char pores, increasing the mean H/C ratio of the volatile matter leading to the formation of a higher amount of light hydrocarbons. Ultimate and Sulphur Analysis Carbon-to-hydrogen ratios are determined from the ultimate analysis and hence are mandatory for gasification prediction. The total sulphur can be included in this suite of analyses. During gasification, the total convertible sulphur exits with the raw gas, mostly as hydrogen

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sulphide, and is subsequently removed during gas cleaning. Sulphur form analysis (determination of pyritic, inorganic, and organic sulphur) is useful when beneficiation or abrasiveness is a concern. Particle Size Distribution (PSD) PSD determination is required for gasifiers which are only able to tolerate certain size ranges. Moving bed gasifiers are suitable for coals with a particle size distribution of 5–80 mm. Bed permeability is crucial for efficient heat and mass transfer between solids and gases within the bed; fines (<6 mm) are especially disruptive, leading to potentially unstable operation due to pressure-drop effects. Van Dyk et al. (2001) and Keyser et al. (2006) discuss the best-known method of estimation for pressure drop: the Ergun equation, based on particle size distribution. Thermal Fragmentation It is known that when lump coal from certain origins (such as SouthAfrican low rank, inertinite rich coals) is exposed to high temperatures (700 C), it will tend to undergo fragmentation (primary and secondary fragmentation) (van Dyk, 1999). Primary fragmentation occurs during devolatilization, and secondary fragmentation occurs during the combustion of the char by the burnout of the carbon bridges that interconnect parts of the particle. The consequence of enhanced fragmentation is the generation of fine coal particles in the pyrolysis zone of the gasifier. Fine coal particles in moving bed gasification negatively affect the permeability of the packed-bed and syngas production values. Elutriation of these generated fines in the raw gas can cause blockages in downstream processes due to fine coal buildup. Thermal fragmentation is measured by placing a sample with a specific predetermined size distribution in a vessel, heating to gasification temperature, cooling down under an inert gas, and rescreening to determine the change in particle size distribution. The percentage decrease in size distribution due to thermal fragmentation is reported in the Sauter diameter; the smaller the decrease, the better the thermal stability (van Dyk et al., 2001). Mechanical fragmentation is determined by the Micum tumble test, in which a sample with a predetermined particle size is placed in a steel drum and turned at a set number of revolutions per minute for a predetermined period of time. The sample is rescreened and the results are calculated using the change in the Sauter Mean diameter (Ergun Index). Fischer Assay/Gray-King Assay for Tar or Oil Production During pyrolysis, liquid products are generated, the composition of which is dependent on coal rank, petrographic composition, devolatilization

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kinetics, and operating temperature (Berkowitz, 1985). Laboratoryscale quantities of light oils are typically determined using Fisher assay or Gray-King assay tests, both of which determine the proportion of char, liquid hydrocarbon, water, and gas (by difference), thus measuring the propensity of a coal to produce tar and light oils. Anthracite yields little or no tar due to low H/C ratios, whereas sapropelic coals, with a high H/C and low O/C ratio, deliver more tar than humic coals of the same rank (Berkowitz, 1985). Vitrinite is an important component for tar production. Maximum tar yields occur between 500 C and 600 C with high heating rates; above 550 C, secondary reactions (cracking) may occur to produce lighter hydrocarbon gases (Peters and Bertling, 1965). Caking Propensity and Swelling Index Caking is the softening or plasticity property of coal, causing particles to melt or sinter together to form larger particles (agglomerates) when heated (van Dyk et al., 2001). It is dependent on vitrinite content and coal rank and is inhibited by the stone content of the coal, devolatilization, and oxidation and changes the shape and surface properties of coal particles. Even mild caking will influence coal performance in terms of particle size distribution, permeability, and pressure drop over the bed and may result in unstable gasifier operation and uneven gas flow in moving bed gasifiers. Agglomeration formation can be prevented if the plastic stage is passed rapidly and by mixing nonswelling particles with swelling particles. Zhuo et al. (2000) demonstrated that vitrinite melts and swells, whereas inertinite does not melt and only loses a small proportion of its mass under pyrolysis. Liptinite melts but does not swell or agglomerate and loses a large proportion of its mass by pyrolysis. Oxidized coals have a reduced caking capability. The amount of swelling tolerated by a gasifier will vary depending on the configuration of the system and can be accommodated by specific design. The swelling index is determined by heating a coal sample for a set time and temperature and comparing the size and shape of the sample against a well-defined scale. Standard caking tests do not assess caking under pressure, and Sasol has developed proprietary caking tests under pressures up to 26 bar (van Dyk et al., 2001). Caking is expressed as the percentage of agglomerated char. Gadiou et al. (2002) demonstrated that coal particle swelling increases with increasing pressure. Caking propensity is discussed further by Wall et al. (2002). Reactivity Reactivity in this context is the rate at which char will react with steam to be converted to syngas and is controlled by the chemical

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structure of char, the physical structure (that is porosity), and catalysts (including minerals). Vitrinite has the highest gasification reactivity, and inertinite gasifies with increasing pressure and residence time above 15 bar; liptinite is pyrolysed before gasification. There is no internationally accepted standard method of determining the reactivity of a given coal char. For predicting reactivity in the Sasol-Lurgi Fixed Bed Dry Bottom gasifier, a thermogravimetric technique is applied, where the CO2 reactivity is determined to obtain an indication of the expected rate of the gasification reaction. Fluidized-bed gasifiers typically operate in temperature ranges of 800–1,050 C, and the reactivity of the coal-derived chars must be relatively high to minimize the unused carbon. For entrained flow gasifiers, reactive coals can generally be gasified at lower temperatures and hence at higher cold gas efficiency, whereas less reactive coals may need higher gasification temperatures to achieve adequate conversion efficiencies. Pore Size and Surface Area of Chars The gasification rate of a coal is governed by its structure, with reaction gases penetrating particles and attacking carbon atoms and the resultant gases diffusing through the solid structure (Feng and Bhatia, 2003). Reactions occur only on the reactive surface area, and not all micropores may be reactive. The degree of porosity (mesopore, micropore) is an important consideration in chars. Vitrinite generally swells during devolatilization, becoming highly porous with meso- and micropores. However, this maceral type frequently has a high caking propensity, and as a result, the reactivity decreases at high temperatures and pressures when the vitrinite-rich coal particles become sticky, reducing the number of reactive sites. Inertinite, typically a less reactive char than vitrinite, can be considered an attractive coal type when a microporous char is required. Czechowski and Kidawa (1991) determined that CO2 molecules react with the external char surface, whereas steam molecules preferentially react within the internal pore structure of chars. Different types of macerals give rise to different chars, and hence the reaction of the chars to different agents must be considered to obtain the greatest gasification reaction and burnout characteristics. Petrographic Properties Petrography enables the optical consideration of coal and carbon via which the evolution of char structures, and hence the changes during the carbon conversion process, can be determined. The maceral content significantly affects char formation, which is also affected by the role of temperature, pressure, residence time, and particle size

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(Zhuo et al., 2000). Refer to Chapter 2 for a more detailed discussion of petrography. l

l

Rank determination. As discussed in Chapter 2, vitrinite reflectance analysis is required to determine the rank of a coal (lignite, subbituminous, bituminous, coking, or anthracite). Coal rank affects hydrogen and oxygen ratios as well as gasifier performance. Char morphology formation is largely rank dependent. Liu and Niksa (2004) consider the impact of rank on gasification rates. Maceral, microlithotype, and mineral group analysis. Maceral, microlithotype, and mineral group analyses conducted on coals for gasification are based on the ISO standards for petrography (see Chapter 2). All three analyses are useful in comparing new coal feeds to current sources, determining blend composition, organic and inorganic associations, major mineral groups, and any anomalies that may arise from the chemical analyses or coal performance during gasification. Parent coal properties significantly influence the resultant char formation (Wagner and Joubert, 2005; Harris et al., 2006), and, as with combustion, char morphology can be predicted from the initial maceral and microlithotype composition. Char type has a significant impact on pyrolysis and gasification, and coals with a high volatile content generally have a lower char yield. Vitrinite and inertinite differ in pyrolysis behavior, affecting the char yield and morphology. Wall et al. (2002) demonstrated that the proportion of porous chars increased with the increase in vitrinite content in the parent coal at all pressures, and the proportion of vitrinite in a particle will influence the porosity of the char particle. Megaritis et al. (1999) concluded that the gasification of inertinite begins at a later stage than liptinite and vitrinite, and inertinite chars were found to have high gasification reactivity with a longer residence time. Liptinite and vitrinite convert rapidly during pyrolysis and exhibit plasticity, whereas inertinite retains its rigid, well-defined porous structure through the gasification zone. Wall et al. (2002) also demonstrated that inertinite is capable of displaying high fusibility similar to vitrinite under conditions of high pressure (15 atm). Similar results were reported for coal particles of 4–4.75 mm by Matsuoka et al. (2005). As determined by van Heek and Muhlen (1986), and Zhuo et al. (2000) fusain, due to its inert nature, does not exhibit any changes in size during pyrolysis, whereas vitrain exhibits considerable swelling, increasing the particle size two- to four-fold. At 90 bar the particles retained their maximum size, but

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l

at lower pressures, the particles shrank again. At high pressures and heating rates for larger particles, the mass transfer to the outer atmosphere is inhibited, the products formed during pyrolysis (tars and volatiles) partially remain, and the grain is no longer capable of shrinking. Durain, containing high amounts of liptinite, exhibited significant shrinkage as the particles rapidly decayed during pyrolysis with the gases and tars escaping before the solidification of the outer shell (van Heek and Muhlen, 1986). Degree of weathering. A petrographic based Weathering Index Analysis (WIA), as developed by Wagner (1998), can be used to quantify and qualify the degree of weathering (oxidation) in coals and coal blends. In a case study reported by Wagner (2002), a link was determined between inertinite-rich coals with unusually high inherent moisture content and an increase in fine particulate carryover into the raw syngas stream. An extensive network of fissures and cracks was determined within the inertinite particles (Figure 5.3), enhancing the available surface area and thus retaining higher than normal moisture. These moisture-rich particles are likely to fragment more rapidly than drier coals when exposed to high temperatures, thus increasing fine coal carryover into the raw gas stream. The increase in the rate of syngas production reported when gasifying the same weathered inertinite-rich coals could also be attributed to the availability of a larger surface area for reaction.

FIGURE 5.3. Fissures and cracks in inertinite particle (magnification x500, oil immersion, reflected light). (Photomicrograph: N. J. Wagner.)

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Char morphology analysis. Char morphology (shape, size, thickness, porosity, and reflectance) has significant effects on coal combustion and gasification. Parent coal properties are significant in determining final char morphology, with macerals behaving in relatively characteristic ways when exposed to increasing temperatures and oxygen-rich or oxygen-depleted environments. Gasification occurs at active sites on chars, which is related to surface texture. Primary and secondary reaction zones are likely to exist in a gasifier due to the different reactivities of plastic and dull chars. Harris et al. (2006), working under entrained-flow conditions, determined that less dense chars are produced from coals that pass through a distinct fluid phase compared to nonfluidizing coals that retain the angular, coal-like morphology. Matsuoka et al. (2005) concluded that the reactivity of the porous char was lower than that of the dense char due to the skeletal structure being less reactive and more graphitic, as well as being lower in calcium. In a work conducted by Wagner and Joubert (2005) on samples obtained from a commercial fixed bed gasifier, maceral type and macro- and micropores formation, followed by inherent or subsequent cracks and fissures within the coal, appeared to play pivotal roles in the changes experienced by carbon particles when exposed to increasing temperature. Whole vitrite particles and bands within particles devolatilized first, followed at higher temperatures by reactive inertite microlithotypes (Figure 5.4). Fusite chars remained basically unchanged in physical appearance from the parent coal, as also reported by Czechowski and Kidawa (1991). In terms of fixed bed gasifier operation and carbon conversion, char morphology highlighted definite zones (Wagner and Joubert, 2005). The carbon conversion process does not appear to be linear, rather occurring in stages. At the point of these “stages,” rapid changes occur. This essentially confirms the concept of defined temperature and reaction zones within the gasifier. Secondary char deposition may reduce the extent of gasification of a coal as the reactivity of the particles decreases with increasing pressure.

5.3.2 Mineral Characterization and Behavior During Gasification Mineral matter in coals consists primarily of clay minerals (kaolinite, illite), carbonates, sulfides (pyrite), sulphates, silicates, and quartz. Solomon et al. (1987) and Stach et al. (1982), among others, provide a detailed list of minerals in coal, their chemical formulae, and associations. The sintering and softening points of minerals during

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FIGURE 5.4. (a) Devolatilizing vitrinite band in an intermediate microlithotype particle. (b) An anisotropic char originating from an inertite particle. Limited degree of swelling, but enhanced surface area due to small pore formation (reflected light, magnification 500, oil immersion, [b] cross-polars and lambda plate in). (Photomicrographs: N. J. Wagner).

gasification in reducing environments is known to be lower than in combustion environments. Mineral matter transformation and slag formation are directly related to the properties of a coal and will impact the suitability of the coal as a feedstock for a specific type of gasification. Therefore, the mineral chemistry and mineral interaction

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have to be understood up front. Solomon et al. (1987) and Wall et al. (2002) provide further discussion on mineral matter behavior during gasification. The aim of characterizing the mineral matter in coal is to enable the prediction of the ash or slag properties (fluidity, size, composition), the conversion rate, the size of the ash bed in the reactor, and ash/slag handling facilities following conversion. The analyses that need to be performed are those related to ash content, ash composition as a function of ash oxide analysis, ash melting properties (ash fusion temperatures), and the determination of the Hardgrove Grindability Index. Ash Content Determined in the suite of proximate analyses, the ash content value represents the residue after combustion and is used to infer the total mineral composition of the coal. Minerals may have a catalytic influence on the reactivity of hard coals, but the effects are more pronounced for lignites (van Heek and Muhlen, 1986). Sekine et al. (2006) found that the reactivity of chars differed according to the ash content, and the gasification rate constant was determined by the ash morphology rather than by the difference in carbonaceous structural changes. Czechowski and Kidawa (1991) and Sekine et al. (2006) concluded that the elements Ca, K, and Na strongly enhanced char reaction rates (catalytic effect) and that minerals associated with macerals affected char porosity development. Sekine et al. (2006) also revealed that Si and Al compounds tended to suppress the gasification reaction on the char surface because the reactant gas was unable to make contact with the carbon. There are certain economic and practical implications regarding the ash content of a coal feedstock. For example, in the case of entrained flow gasifiers, higher ash content coals will result in a decrease in gasifier efficiency. This is due to an increase in oxygen demand and loss of heat in the slag that cannot be fully recovered, as well as a possible reduction in throughput due to the additional slag that needs to be removed via the slag tap. Ash Oxides Ash composition is determined by ash oxide (elemental) analysis. Some compounds present in coal slag (SiO2, CaO, iron oxides) may penetrate deeply into refractory materials (especially applicable for certain entrained flow gasifiers) and eventually result in cracks leading to material loss (Collot, 2006). It is important to determine the mineral composition and solubility status (leachability) of the residue ash/slag for disposal and environmental reasons. Slag from slagging or entrained-flow gasifiers is

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typically considered to be inert, but ash may contain leachable minerals and elements, and it is necessary to conduct TCLP (toxicity characterization leaching potential) tests or similar on the ash to determine the most appropriate disposal mechanism. Ash Fusion Temperatures The fusibility of coal minerals can be expressed as a function of the content of the principal oxides occurring in the ash, namely: SiO2, Al2O3, TiO2, Fe2O3, CaO, MgO, Na2O, and K2O (Seggiani, 1999; Alpern et al., 1984). The acid/base ratio is the most frequently used parameter for correlating ash fusibility with its composition. However, coal ash fusibility characteristics are difficult to determine precisely, partly because coal ash contains many components with different chemical behaviors (Slegeir et al., 1988) and they will vary from coal source to coal source. The ash composition, specifically the Ca and Fe content in the coal, gives a fair indication of the expected ash fusion behavior. A Ca and/or Fe rich coal source normally has a low ash fusion temperature due to the fluxing properties of the Ca and Fe minerals. Ash fusion temperatures (AFT) are widely used as a guide to ash agglomeration, clinkering, and slag behavior resulting from the mineral matter transformations of coal sources during gasification. Ash clinkering inside fixed bed gasifiers may cause channel burning, pressure drop problems, and potentially unstable gasifier operation (van Dyk et al., 2001). Slagging or entrained flow gasifiers require the maintenance of a molten slag within the gasifier and for slag removal. Fluidized bed gasifiers usually operate at temperatures well below the AFT of the coals to avoid ash melting, thus avoiding clinker formation and loss of bed fluidity; coals with a high ash-melting point are the most suitable. However, the residual carbon content is therefore relatively high, and the “ash” is usually reburned in a secondary combustion unit or recirculated back into the gasifier. Entrained-flow gasifiers operate above the AFT in order for the molten slag to flow down the walls and drain as a molten slag from the gasifier and so that the slag tap does not block; hence coals with a low ash melting point are required. The slag viscosity-temperature relationship is important in slagging gasifiers (Higman and van der Burgt, 2003). The AFT of a coal can be manipulated by adding fluxes, such as limestone, or by blending with low ash fusion coals, thus decreasing the melting properties of the coal minerals. The ideal operational condition for fixed bed gasifiers is at a temperature above the initial deformation temperature, to obtain enough agglomeration to improve bed permeability but to operate below the

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ash melting temperature to prevent excessive clinkering. Coal sources currently used by Sasol (South Africa) for fixed bed gasification have an ash melting temperature >1,350 C and an initial deformation temperature of >1,300 C (van Dyk et al., 2001). The operating experience acquired from the Sasol-Lurgi Fixed Bed Dry Bottom gasifiers (South Africa) has shown that even when the gasifiers are operated at temperatures below the AFT as predicted by an AFT analysis, a percentage of slag (clinker) is formed (van Dyk et al., 2006b). Van Dyk et al. (2006b) concluded that HT-XRD and FactSageW modeling supplied insight into specific mineral interactions and slag formation at temperatures that were not reflected by AFT analyses. For example, the onset of decomposition of kaolinite occurs above 500 C, whereas mullite has a flow temperature of 1,850 C, and in the temperature range >500 C to 900 C the carbonates (calcite and dolomite) start to decompose, forming lime and periclase. At 1,000 C anorthite (CaAl2Si2O8) and gehlenite (Ca2Al2SiO7) become stable, probably due to partial melting of the phase assemblage. Anorthite and gehlenite are formed as products from anhydrite, alumina, and silica at temperatures around 900 C to 1,100 C, but gehlenite may also form directly from lime. Feldspar was one of the mineral species to have the lowest AFT and caused greatest formation of liquid slag (refer to Figure 5.5).

160

Kaolinite

Intensity (HT-XRD counts)

Dolomite decomposition 140

Kaolinite decomposition Carbonates start to decompose Mullite starts to form

120 100

Calcite Dolomite Mullite Anorthite

Anorthite starts to form

80 60 40 20 0 500

600

700

800

900

1000 1100 1200

1300

1400

Temperature (⬚C)

FIGURE 5.5. Integral intensities of identified phases as a function of temperature. (Source: Minerals Engineering 19., J. C. van Dyk, S. Melzer, and A. Sobiecki, “Mineral matter transformation during Sasol-Lurgi fixed bed dry bottom gasification: Utilization of HT-XRD and FactSage modeling,” 1126–1135, copyright 2006, with permission from Elsevier.)

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The Hardgrove Grindability Index The Hardgrove Grindability Index (HGI) is used to determine the grindability of the coal. This index is important for gasifiers that require fine coal as feed, as with entrained flow gasifiers. For slurryfed entrained flow gasifiers there is a trade-off regarding the grindability of a coal; a higher solid concentration with a coarser coal slurry may be produced, but larger particles will not gasify as well as smaller particles (Collot, 2006). Apart from milling and grinding, the hardness of a coal is not very relevant for gasification.

5.3.3 Fate of Trace Elements During Gasification Almost all the elements in the periodic table may be found in coal to a greater or lesser extent. Trace elements are those elements that have concentrations below 1,000 micrograms per gram, reported as parts per million (ppm) or per billion (ppb). They may have organic or inorganic associations, referred to as the mode of occurrence. The determination of the mode of occurrence is important when we consider the behavior of the trace element during coal beneficiation and utilization as well as for modeling to predict trace element behavior and its release to the environment. Trace elements emitted from coal facilities are receiving much attention internationally due to concerns about health effects. Mercury, for example, has been demonstrated as highly toxic and has a tendency to bioaccumulate through the food chain (U.S. Environmental Protection Agency, 2006a). Significant anthropogenic mercury emissions (Chapters 4 and 10) emanate from coal combustion (U.S. Environmental Protection Agency, 2006a). Other elements of concern include Pb, As, Se, Cd, Ni, Co, Mn, Zn, and Cr, and these are commonly referred to as hazardous air pollutants (HAPs) (Wagner, 2001). Gasification is the cleanest of all coal-based electric power technologies. Although trace elements are also mobilized during gasification, no trace elements are released directly to the atmosphere following gasification, since the system is closed. Oxygen stoichiometry, the gas composition of the flame and subsequent flue gas temperatures, particle residence time, and cooling rates, all influence the fate of the trace elements in the coal feed. Crude syngas is cleaned via various processes prior to Fischer-Tropsch synthesis to remove impurities such as hydrocarbons, sulphur, and carbon dioxide, and trace elements may also be removed in the gas stream. Most trace element behavior during gasification has been inferred by analogy with combustion studies (Helbe et al., 1996; Sloss and Davidson, 2001; Clarke 1993) and combined cycle processes (Forte et al., 2002). Gasification occurs in a reducing environment, and hence the behavior of the volatile species may differ. Baker (1993),

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working on a Shell coal gasification combined-cycle power plant, concluded that generally, trace elements were tightly bound in the inert slag and fly ash, with low levels of HAPs in the syngas. Beishon and Hood (1989) showed that more than 95% of most elements ended up in the slag when samples from a BGL gasifier (fixed bed countercurrent slagging gasifier) were analyzed; the gas liquor contained very low levels of trace elements, mostly below the detection limits of the analytical equipment. Reed et al. (2001), working on a pilot plant gasifier, only detected Hg and Se in the fuel gas, whereas Cd and Pb were concentrated in the fine dust removed by the hot gas filter. Helbe et al. (1996), working on a laboratory-scale entrained flow gasifier, showed that considerable trace element volatilization occurs during coal gasification. Richaud et al. (2000) reported that Co, Cr, Ni, and V condense under gasifier conditions (940–980 C, 1.3 MPa); Cu, Mo, and Zn mostly condense on cooling the gas stream to 600 C; Mn, Pb, and Sn condense on cooling to 400 C; and As, Cd, Hg, and Se still exist as vapors at 400 C. It was found that all the Hg was discharged into the gas stream, and the major destination of the other trace elements was into the primary fines stream. As, Mo, Pb, Sn, and possibly Zn and Se were found in the residue, with significant losses of these elements to the gas stream (Richaud et al., 2000). If the trace elements in feed coal, slag and ash are determined, the differences may indicate those elements lost as volatiles to the gas stream. Using the FactSage modeling package, it was predicted that Hg, As, and Se would occur predominantly in the gas phase (Hlatshwayo and Wagner, 2005). Mass balances conducted during commercial scale tests on Sasol-Lurgi Fixed Bed Dry Bottom gasifiers revealed that Hg and Se report predominately to the crude raw gas, whereas 60% of As is retained in the ash, the remainder being in the gas phase (Hlatshwayo and Wagner, 2005). Approximately 99% of the output of the elements Mn, V, Zn, Cu, Ba, and Co was found in the gasifier ash, indicating their nonvolatility under the gasification conditions and confirming earlier studies by Reed et al. (2001). However, anomalous reports exist for Zn, with FactSage modeling predicting Zn to be present in the gas phase at 1,673 K (Thompson and Argent, 2002). Bushell and Williamson (1996) found significant amounts of Zn in the gas filter fines in a gasification pilot plant.

5.4 Characterization of Gasification Residues Coal ash is an inevitable coproduct of gasification; Sasol Synfuels in South Africa, for example, produces approximately 7 million tons of gasification ash annually (Matjie et al., 2005). Commercial gasifiers differ widely in the way in which they produce ash—dry ash,

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agglomerated ash, or a slag may result (Benson et al., 1995). Fluidized bed gasifiers produce dry ash or a fused agglomerated ash, depending on the AFT, gasifier design, and operating temperatures. Entrained flow gasifiers produce a vitreous slag, and fixed bed gasifiers will produce a dry ash (Sasol-Lurgi) or a slag (high temperature gasifiers). Ash deposition characteristics are known to be influenced by coal type (mineral constituents, melting temperature, distribution of mineral matter, etc.), reaction atmosphere, particle temperature, surface temperature of a heat exchanger, surface composition of the material, flow dynamics, and so forth. By understanding the mineral attributes (physical and chemical) of the gasification ash, it is possible to understand and prevent clinker formation and slagging and fouling incidents. Cogasification with waste (such as plastics, municipal sludge) and biomass (such as bagasse, nut shells) changes the properties of the residue and should be considered on a case-by-case scenario. Wall et al. (2002) studied ash and slag characteristics from an entrained flow gasifier and considered the included and excluded minerals using CCSEM and SEM analyses. They determined that the slag is preferentially formed by the ash from the excluded minerals, not the total coal ash. Included minerals tend to be entrained off the gasifier. Matjie et al. (2005, 2006) conducted a detailed characterization of gasification ashes produced by Sasol-Lurgi gasifiers using a variety of analytical techniques. The coarse gasification ash varies in size from fine particles to aggregates (4 to 75 mm) and comprises predominantly of assemblages of major oxides (quartz, mullite and anorthite) and minor oxides (diopside, hematite, cristobalite) and anhydrite (Ginster and Matjie, 2005) (Figure 5.6). Course gasification ash produced by the Sasol-Lurgi gasification process contains between 4% to 6% carbonaceous matter. Wagner et al. (2004) characterized the remnant carbon forms in the gasification ash using petrography. Four different types of unburned carbon were identified based on their morphological appearance and included porous particles, dense particles, intermediate/layered particles, and unreacted coal. It was concluded that some of the unburned carbon could be reused, and the reason for residual carbon could be attributed to the possibility that particles are captured in cooler zones vertically through the reactor (channels) and are not exposed to reagents and reactive conditions required for full conversion. Due to costs associated with ash handling and disposal, much research has been undertaken to find beneficial utilization opportunities for gasifier ash (Larrimore and Morton, 2003; Center for Applied Energy Research, 2006). Residue byproducts can be suitably placed in concrete, where the residues have some pozzolanic behaviors (Holt and Ravio, 2006) or used as a cement extender in cement production

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G1 Q

S1

G2 Ka

FIGURE 5.6. Typical gasification ash particles; backscattered SEM image. (Courtesy R. H. Matjie.) (Width of the image is approximately 900 mm). Particle description: Q: Large extraneous quartz grain. G1: “Anorthite” laths (grey) in matrix of glass. Ka: Predominately alumino-silicate. G2: Predominantly Ca-Mg-Fe-bearing alumino-silicate glass with quartz inclusions (dark grey). S1: Quartz particle (grey) partially surrounded by Ca-Mg-Fe bearing aluminosilicate glass.

(Kearsley and Matjie, 2006). Ginster and Matjie (2005) discuss two major ash utilization options, namely, (1) ash cement bricks and road fill, and (2) utilization of ash dumps to treat wastes.

5.5 Advanced Gasification (Polygeneration, Cogasification) Coal is a finite, low value raw material, but there is an increasing realization of the crucial long-term requirements of this fossil fuel as a primary energy source. Higher energy demands in many countries, combined with political instability in oil-producing countries, have resulted in a refocus on indigenous coal resources and available technologies for the conversion of coal into useful products (Richter, 2001). Gasification is a key technology for a more efficient power generation from coal. It is a cheaper power generation option to natural gas; it can be utilized to produce valuable products; and it has a better environmental performance than conventional power generation (Hutchinson, 2006). The versatility and flexibility of gasification plants enable them to offer a wide range of products, including FischerTropsch liquids, chemicals, power, steam, gaseous fuels, and various other products (Richter, 2001). This referred to as polygeneration. By the partial supplementation of coal with renewable and alternative feedstock (such as biomass, wastes, and refinery byproducts) in gasification systems, coal resources can be extended, higher plant

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efficiencies achieved and new gasification technology developments are under way. Cogasification is currently attracting great interest on a global scale (Higman and van der Burgt, 2003; Department of Trade and Industry, 1999). In addition, with the growing interest in hydrogen as fuel, gasification is in a good position to be able to provide this fuel economically. Underground, or in situ gasification, is another form of gasification deviating away from traditional reactors. A few of these advanced gasification technologies are highlighted in the following sections, but the reader is advised to refer to the references for more detailed information.

5.5.1 Integrated Gasification Combined Cycle (IGCC) IGCC systems produce electricity by generating clean syngas, which drives a high efficiency gas turbine, and the exhaust heat is recovered to produce steam to power traditional high efficiency steam turbines (Coal Utilization Research Council, 2006). IGCC systems offer emission levels approaching those of natural gas combined cycle plants, and superior environmental performance compared to conventional pulverized coal (PC) fired steam plants (American Electric Power, 2006; Ratafia-Brown et al., 2002), with the added advantage of high pressure concentrated CO2 streams. IGCCs can be retrofitted to repower older coal plants that cannot justify expensive environmental controls or repower existing combined cycle plants that cannot be dispatched due to high natural gas prices (Worley Parsons, 2006). IGCC technology has been proven for a variety of fuels, particularly heavy oils, heavy oil residues, pet cokes, and bituminous coals in different parts of the globe (Chemed, 2006). It has reached commercialization stage in the United States and the European Union with a number of plants already in demonstration of operational phase (Coal Utilization Research Council, 2006). Chevron Texaco is considered the world leader in IGCC (Chevron, 2007). Several installations utilize their gasification technology for IGCC, but only one plant uses coal as a feedstock, the Tampa Electric plant in the United States, which cofeeds coal and petroleum coke (Chevron, 2002). The main technological barriers to IGCC are capital and operating costs. Even though IGCC is at least 10% to 12% more efficient than a conventional coal-fire power plant, the capital cost could be approximately 20% more than for a PCC boiler plant (U.S. Environmental Protection Agency, 1994).

5.5.2 Hydrogen Production via Cogasification There is increasing interest in the use of hydrogen as fuel, the socalled “hydrogen economy,” due to the fact that its conversion to heat

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or power is simple and clean. As discussed by Higman and van der Burgt (2003), the only realistic route available, especially in the short to medium term, is the generation of hydrogen from fossil fuels. Collot (2006) reported that new concepts for hydrogen production via coal gasification are under development, based on the combination of coal gasification (steam or hydro gasification), the shift reaction, and carbon dioxide removal. Hydrogen, steam, and power can easily be produced in a single gasification plant. In Normandy, France, such a plant will be constructed at Gonfreville to supply a refinery with hydrogen, steam, and other products via a gasification cogeneration plant (Chevron, 2002). Another contender for cleaner fuels is methanol, which can also be coproduced in a gasification complex.

5.5.3 Air-Blown Gasification Cycle (ABGC) This is a viable technique for future power generation from coal (Hanson et al., 2002). Coal particles up to 6 mm are pyrolysed and then gasified in air or steam in a spouted bed reactor. The gas is burnt in a gas turbine and the char is burnt in a circulating fluidized bed reactor to raise steam. Particle size distribution is crucial because it will impact the particle residence time which could have a significant impact on particle reactivity, which is in turn related to the type of coal used.

5.5.4 Underground Coal Gasification (UCG) UCG, or in situ gasification, is able to tap resources otherwise not readily accessible either technologically and/or economically. Mining and ash disposal is avoided because the coal is gasified underground, where it lies; the ash simply remains in place after the gases have been removed; and the construction of complex gasification plants is avoided. UCG has been employed in Russia for decades, and other countries have also had demonstration units (The Coal Authority, 2006).

5.5.5 Biomass Gasification Biomass, by definition, is a fuel or raw material derived from recently living organisms, and includes bagasse (from sugar cane), wood, nut shells, and other agricultural and forestry wastes. Biomass can also include purposefully grown material, and indeed this is a rapidly growing industry. Higman and van der Burgt (2003) provide further information on biomass types, properties, and production for gasification. Biomass gasification is to the benefit of sustainable development.

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5.5.6 Plasma Gasification Plasma gasification is still in an early stage of development (Higman and van der Burgt, 2003), and is usually employed to gasify various waste streams (U.S. Plasma Inc., 2005). The nonincineration thermal process uses extremely high temperatures to decompose the gasifier feed into very simple molecules, i.e., H2 and CO (U.S. Plasma Inc., 2005). Since the energy required for gasification is supplied by an independent source (i.e., the plasma torch), the energy input to the process can be strictly controlled. Almost no harmful gases are emitted. Heavy components and inorganic materials are fixed in a nonleachable slag and very small quantities of waste are produced.

CHAPTER 6

Direct Coal Liquefaction Gareth D. Mitchell

6.1 Introduction The principal objective of coal liquefaction is the production of liquid hydrocarbon distillate products from a coal’s mostly aromatic structure to be used as transportation fuels or chemicals. In broad terms, to generate petroleum-like products finely crushed coal (
85% minus 0.074 mm) is prepared as a slurry with a process-derived vehicle oil, preheated and pumped into a heated (400–500 C) pressure vessel (6.9–71.0 MPa) in the presence of hydrogen gas with or without a catalyst for some duration (30–150 minutes). Under these conditions and depending on heating rate and maximum temperature, thermal ruptures of chemical bonds occur. In addition to acting as a carrier medium, the vehicle disperses molecular fragments and stabilizes free radicals by actively engaging in hydrogen transfer. This process may be enhanced in the presence of catalysts so that the coal-derived chemicals remain low in molecule mass (Derbyshire and Gray, 1986). If free radical stabilization is impeded, the coal-derived chemical components may recombine into high mass products leading to the formation of nondistillable liquids and solids (i.e., retrogressive reactions). Direct coal liquefaction has long been recognized as a competition between hydrogenation to liquid products and destructive distillation that forms solids (Bergius, 1926). The former reaction represents the goal, whereas the latter leads us to consider the application of petrologic methods to help explain diminished productivity.

6.2 Process and Methods for Coal Liquefaction Coal liquefaction technology began in Germany when Frederick Bergius showed that it was possible to add hydrogen to coal at 300– 500 C and 1.0 MPa to produce distillate products (Bergius, 1914). During the period of 1914–1925, the process was refined and I. G. Farbenindustrie Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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began commercialization of the process that culminated in the only significant industrialization of the technology (Skinner, 1931). By 1939, seven hydrogenation plants were operating to convert brown and bituminous coal or coal tar to 1.2 million tons of liquids/year, and by 1944 there were 12 plants operating with a reported maximum output of 5 million tons/year (Gordon, 1947; Derbyshire and Gray, 1986). Half of these plants hydrogenated tar derived from destructive distillation plants (coke ovens and town gas), and only six employed coal directly to produce high quality aviation and motor fuels, aromatic chemical feedstocks, and liquefied gas. Since these early beginnings there have been cycles of development to improve both the operability and economics of direct liquefaction, mostly brought about by potential shortages of crude oil (Whitehurst et al., 1980). Research and development from 1950–1990 gave rise to a variety of processing schemes and some were built into fairly large-scale pilot facilities. Detailed discussion of these technologies is beyond the scope of this chapter but can be found elsewhere (Donath, 1963; Howard-Smith and Werner, 1975; Shah, 1981; Alpert and Wolk, 1981; Schindler et al., 1983; Derbyshire and Gray, 1986). During this period advances were made in lowering reaction conditions, reducing hydrogen consumption and increasing distillate yields with the implementation of recycling heavy liquids, adoption of multistage processing, and improved catalyst selection (Schindler et al., 1983; Neuworth and Moroni, 1984; Derbyshire and Gray, 1986). Adoption of multistage catalytic-catalytic liquefaction resulted in total conversion to distillate fractions exceeding 78% (five barrels per ton of MAF coal), organic rejection with the ash was reduced to below 15%, there were few incidences of coking or solids buildup as the overall temperature, pressure, and reaction time of each unit were minimized compared with earlier single-stage thermal operations (Schindler et al., 1983; Schindler, 1989). During the developmental history of coal liquefaction laboratory-scale batch reactors (that is, autoclaves, bombs, tubing bombs, etc.), employing a wide variety of reaction conditions, organic solvents as vehicle, in the presence or absence of different catalysts and hydrogen have been used for resource evaluation. To obtain a measure of the effectiveness of liquefaction, continuous-flow processes perform a mass balance of reaction products (from separation and distillation) and residuals, whereas batch processes employ solvent fractionation to separate chemical groups. Once the gases are removed, liquid and nondistillable products are fractionated into oils (soluble in pentane or other light alkanes) and asphaltenes (benzene-soluble and pentane-insoluble). Sometimes a third class of product, called preasphaltenes, are separated as pyridine-soluble and benzene-insoluble (Gorin, 1981; Derbyshire and Gray, 1986). In laboratory experiments conversion is determined by total solubility in a single solvent plus

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FIGURE 6.1. Conversion of a 1.35 specific gravity fraction of a high volatile C bituminous coal at 400 C, 2:1 tetralin:coal ratio, 0.1 MPa of air for various reaction times. (Source: Fuel 55, R. C. Neavel, “Liquefaction of coal in hydrogen-donor and nondonor vehicles,” 237–242, copyright 1976, with permission from Elsevier.)

gases and calculated to a dry ash-free basis. Figure 6.1 shows the disparity in total conversion determined by using different solvents that make direct comparisons of results difficult. In this chapter, our concern is with the application of petrologic methods to the characterization of the solid residual materials acquired from various parts of continuous-flow coal liquefaction operations. As in most industrial applications, knowledge about the origin of residue components can provide feedback regarding operation variables, such as coal selection, vehicle oil quality, reaction conditions, process design limitations, or some combination of these factors. The following sections will discuss what kinds of coal (rank and type) are suitable for direct coal liquefaction (Section 6.3), how coal macerals disintegrate under specific reaction conditions (Sections 6.4–6.8), and application of residue petrographic analysis to understand the role of coal selection and process conditions (Section 6.9).

6.3 Main Characteristics and Properties of Coals for Liquefaction Coal rank and composition are primary factors that influence liquefaction behavior (Fisher et al., 1942) and, although some of that influence may be overcome by increasing process severity or the use of catalysts, these options may be counterproductive in reducing operating expenses (Alpert and Wolk, 1981).

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Bergius (1926) recognized initially that anthracite and coals with high fusain content could not be hydrogenated effectively, and details of early plant design suggested that solid materials had to be withdrawn from the reactor when low rank (brown) coals were employed (Gordon, 1947). Work by the U.S. Bureau of Mines (cited in Given et al., 1980a,b) showed that most ranks of coal below midrange medium volatile bituminous could be converted successfully to distillable products, although some coals were more problematic or gave less advantageous distribution of products under their operating conditions. Similar levels of total conversion could be achieved for brown (lignite and subbituminous) and high volatile bituminous coals, but there were clear differences in the product slate under the same reaction conditions. Low rank coals tended to be more sensitive to reaction temperature (Storch et al., 1941; Fisher et al., 1942) and more occurrence of “coking” and precipitation of “dissolved materials” were reported. In general, higher yields of oxygenated gases and water increased with decreasing rank and the aromatic nature of the liquids increased with increasing rank. Reviews of systematic studies of the influence of coal rank and composition on liquefaction behavior (Graham and Skinner, 1929; Fisher et al., 1942; Wu and Storch, 1968; Gorin, 1981) suggest that bright coals of less than 89% carbon content were more amenable to liquefaction. Storch (1937) offered that conversion of European coals to distillable liquids (boiling at 360 C) was found to be roughly inversely proportional to a coal’s carbon content. Given et al. (1975a,b) and Davis et al. (1976) found that coals of high volatile rank (Rmax ¼ 0.49 – 1.02%) of >70% reactive maceral content (vitrinite þ liptinite) were best for liquefaction. Using a statistic approach, Abdel-Baset et al. (1978) evaluated conversion using a data set from 68 lignite through high volatile A bituminous coals that included batch liquefaction results (4:1 tetralin [1,2,3,4-tetrahydronaphthalene]:coal ratio, nitrogen atmosphere, 400 C, three hours; extraction 48 hours in benzene). Step-wise regression showed the expected correspondence among conversion, carbon content, and petrographic composition suggested by earlier work. But a strong correlation of total sulfur content with conversion led the researchers to speculate that its association might result from (1) the reduction of pyrite to pyrrhotite, which in turn might act as a hydrogenation catalyst; (2) the removal of organic sulfur compounds might generate additional free radicals that may somehow promote conversion, or (3) that conditions of organic deposition (peat swamp) may form particularly labile organic matter. Yarzab et al. (1980) utilized 104 high volatile bituminous coals distributed among three geographic provinces (Eastern, Interior, and Rocky Mountain) for laboratory-scale batch liquefaction (2.2:1

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tetralin:coal ratio, air removed from the reactors, 400 C, one hour; extraction in ethyl acetate) for statistical analysis. Principal component analysis performed for 14 coal variables and conversion showed that the data set was heterogeneous. Cluster analysis was used to partition the sample set into three relatively homogeneous populations that were distinguished by coal province, sulfur content, and rank, as shown in Table 6.1. Because of the apparent correlation of total sulfur content with greater conversion for the high pyrite-containing Eastern and Interior TABLE 6.1 Dependence of coal liquefaction behavior on coal properties; characteristic of groups separated by cluster analysis for bituminous coals

General Character No. of coals from: Eastern province Interior province Rocky Mtn. province Mean values: Conversion, % dmmf Volatile matter, % dmmf Mean max. Reflectance, % Carbon, % dmmf H/C ratio Total sulfur, % dry Total reactive Macerals* % Regression parameters to predict conversion

Group 1, G1

Group 2, G2

Group 3, G3

High Rank, Med. Sulfur

Med. Rank, High Sulfur

Med. to Low Rank, Low Sulfur

25 3 2

11 25 0

0 1 37

52.9

70.6

64.0

38.4

44.1

43.7

0.82

0.56

0.56

85.0 0.777 1.49

82.2 0.817 4.76

79.4 0.854 0.69

82.4

87.7

89.7

% reflectance, H/C ratio, % vitrinite

*Vitrinites þ liptinites Discriminant Equations: G1 ¼ 11.97 St þ 29.36 C – 1256.09

Vol. matter, % reflectance, total sulfur

G2 ¼ 16.97 St þ 28.78 C – 1223.31

Vol. matter,total reactive maceral

G3 ¼ 10.05 St þ 27.36 C – 1089.74

Source: Fuel 59, R. F. Yarzab, P. H. Given, W. Spackman, and A. Davis, “Dependence of coal liquefaction behaviour on coal characteristics: 4 Cluster analysis for characteristics of 104 coals,” 81–92, copyright 1980, with permission from Elsevier.

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province coals, there was a tendency to assign credit to the catalytic properties of pyrrhotite. However, Yarzab et al. (1980) and Given et al. (1980b) suggested that other geochemical factors related to coal deposition may be equally responsible for the correlation and having little to do with the concentration of sulfur. Coals of high total sulfur content are generally derived from high saline, marine peat-forming environments in which bacteria were more active. Contribution of lipid-like organic materials by bacteria to the biomass (Diessel, 1992b; Rathbone and Davis, 1993; Taylor et al., 1998) may have as much impact on total conversion and oil production during liquefaction as sulfur. A subset of the samples used by Yarzab and Given was evaluated in a 1 kg/hour laboratory-scale continuous-flow reactor at Gulf Research and Development Company (Given et al., 1980a; Given and Sood, 1982). In their first experiment, a 2:1 slurry of partially hydrogenated anthracene oil and coal was fed into a well-stirred 500 cm3 reactor under 20.7 MPa hydrogen pressure with a nominal residence time of 30 minutes and heated at 440 C and 455 C. Conversion was determined by solubility in ethyl acetate. Forty of the Eastern and Interior province coals were tested and the results showed a remarkable correspondence to the earlier tubing-bomb experiments. In a second experiment, 19 Rocky Mountain province coals were evaluated under the same reaction conditions, except coal slurries were made with a recycle solvent derived from Gulf’s P-99 SCR-II pilot plant. Unfortunately, results from the principal components analysis were not upheld using the new vehicle and in fact there was evidence for retrogressive reactions; that is, higher molecular weight products and coke formation. They concluded that a better hydrogen donor vehicle may be required for processing Group 3 coals, but it also suggested the importance of vehicle quality in mitigating retrogressive reactions. The research discussed above provides some basic guidelines for coal selection for direct coal liquefaction with regard to chemical composition, rank, and petrologic composition. Evidence leads us to believe that lignite and subbituminous coals may require more severe reaction conditions than bituminous coals and because of their higher water and oxygen content may be less suitable for maximizing yields of desirable products. Attempts to predict the amenability of bituminous class coals to form liquids revealed that those of high volatile rank with high total reactive maceral and total sulfur content were better prospects under a standard set of conditions.

6.3.1 Influence of Coal Petrologic Composition Graham and Skinner (1929) performed preliminary work on vitrain and clarain lithotypes to find that each of these generated more oil than the original coal from which they were derived. Using a 1.2 L

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batch rotating autoclave reactor, Fisher et al. (1939a–d) investigated the hydrogenation of various constituents of coal (1:1 tetralin: coal ratio, 1.0 g stannous sulfide, 6.9 MPa hydrogen, 400–430 C, 3 hours). Using the Thiessen-Bureau of Mines petrographic classification system of characterizing the constituents of coal by thin section, concentrates of the four main constituents (anthraxylon, translucent and opaque attritus, and fusain, [ICCP, 1971]) were hydrogenated. In general, Fisher’s group found that anthraxylon, the bright and vitreous constituent of coal, was readily hydrogenated, particularly those from high volatile bituminous coals. For anthraxylon samples obtained from coals above about 89% carbon, yields of oil declined with increasing rank. Anthraxylon from brown coals generated more solid residue under the same reaction conditions, but they could be more completely converted to products at higher temperature and pressure. Attritus, the dull portion of coal composed in part of finely divided vegetal debris and disintegrated plant tissues, is subdivided into translucent and opaque varieties. The five components that make up translucent attritus (spores, cuticles, algae, waxes, and resins) and generally contained in finely divided humic matter were readily converted to liquids. However, the semitranslucent attritus, or “brown matter,” was suspected to be less reactive, and fluctuations in conversion of intermediate rank coals were attributed to the difficulty in quantifying its relative proportion. Opaque attritus generally gave lower conversion than the former components. Fisher and coworkers observed that the degree of opacity (in transmitted light) was related to conversion, i.e., the more opaque, the less conversion. On hydrogenation, fusain, the charcoal-like, sooty constituent of coal exhibiting the greatest opacity and highest carbon content compared with the other coal constituents, reacted unconstrained to a certain point and then diminished. Several fusain concentrates and their corresponding residues were observed under the optical microscope. Prior to hydrogenation they contained significant semitranslucent material, but following reaction only a trace remained. Fisher et al. (1939b), using a point-count procedure to determine the distribution of the various constituents of coal, assigned a relative reactivity to those components based on experimental data. Though anthraxylon and translucent attritus were considered reactive and fusain considered totally inert, the variable reactivity of brown matter caused them to observe that their petrographic method did not adequately separate coal constituents into unique groups. Given et al. (1975a,b) and Davis et al. (1976) revisited the work of Fisher and coworkers by utilizing a different system of coal characterization: the Stopes-Heerlen classification (ICCP, 1971). Using incident-light microscopy, the individual components of translucence and opaque attritus and fusain could be identified and classified based on reflectance (a measure of opacity) and morphology.

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FIGURE 6.2. Dependence on conversion to benzene soluble following dry, catalytic hydrogenation (8.7 MPa of hydrogen) at 400 C. (Source: Energy Sources 3, A. Davis, W. Spackman, P. H. Given, “The influence of the properties of coals on their conversion to clean fuels,” 55–81, copyright 1976, with permission from Taylor & Francis.)

These investigations employed a “reactive maceral” concept that improved on Fisher’s method. The volume percentage of vitrinite (anthraxylon and translucent humic matter) and liptinite (translucent attritus) group macerals constituted the reactive portion of the coals studied, whereas inertinite (brown matter, opaque attritus, and fusain) macerals were mostly inert. However, as found by Davis et al. (1976), when attempting to establish a relationship between coal conversion and reactive-maceral concentration for a range of coal rank (Figure 6.2) there was rarely a one-to-one correspondence when the reflected-light technique was applied. Coal conversion was over-estimated by the concentration of reactive macerals, and because of this, a considerable effort was made to evaluate coal dissolution by using optical microscopy to study solvent-insoluble residues of liquefaction.

6.3.2 Solid Liquefaction Residues Davis et al. (1976), and Neavel (1976a) both employed incident light microscopy to study solvent-insoluble residues from batch liquefaction and although the reaction conditions used were considerably different, their observations were surprisingly comparable. Davis et al.

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(1976) observed residues derived from high volatile bituminous coals containing >70% vitrinite at three reaction temperatures (316 C, 399 C, and 427 C) while the other process conditions were held constant (hydrogenated phenanthrene as vehicle, 20.7 MPa hydrogen pressure, 30 minutes). At the lowest temperature, vitrinite exhibited only minor evidence of reaction and liptinite (sporinite and resinite) appeared unaltered. Residues from a 399 C run were markedly different; vitrinite had been transformed into an isotropic pitch-like substance of low reflectivity and thin-walled cenospheres and liptinite had disappeared. At the maximum temperature, the residues consisted of remnants of apparently unreacted fusinite, semifusinite, and small (1–10 micron) fragments of an indiscernible nature. In a sequence of batch reactions (2:1 tetralin:coal ratio, 0.1 MPa pressure (air), 400 C, reaction time varied from 20 seconds to 50 minutes), Neavel (1976a) observed vacuole formation in bituminous rank vitrinite within the first 20 seconds and swelling and deformation features in excess of 40 seconds that became more prominent with increasing time. Liptinite macerals were evident as small fragments up to 5 minutes reaction time because they were usually protected within a matrix of other macerals, but then disappeared at longer reaction time. After 10 minutes the pyridine-insoluble residue consisted “of fusinite, mineral matter and a few high reflecting cenospheres derived from vitrinite or semifusinite.” Mitchell et al. (1977) sought to establish a relationship between residues obtained from batch and continuous-flow processes. By characterizing the benzene-insoluble residues from the batch reactions of bituminous coals over a range of temperatures (300–450 C at 25 C increments; 4:1 tetralin:coal ratio, under nitrogen, 3 hours), insight was gained into the formation of residue components from specific coal macerals. The optical characteristics of residue components observed in these experiments were used as a basis for understanding the origin of residue components found in continuous-flow laboratory and pilot-plant scale facilities. Mitchell et al.’s (1977) work provided a base for coal maceral disintegration during liquefaction which has since been amplified by other researchers (see below). In 1993 the International Committee for Coal and Organic Petrology (ICCP) published a Classification of Hydrogenation Residues to standardize the terminology (Table 6.2) and that organized residue components into organic and inorganic categories by their degree of reaction or mode of formation. As shown in Figure 6.1, the solvent used to extract liquefaction residues can have an impact on the amount of solid residue recovered and the total conversion reported. Because Mitchell et al. (1977) employed benzene-insoluble residues it is likely that some portion of the residue components observed would have been soluble in a

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TABLE 6.2 Classification of hydrogenation residues from the International Handbook of Coal Petrology, ICCP (1993) ICCP (1993) Residue Groups Unaltered coal macerals

Reacted coal macerals Partially reacted coal macerals

Vitroplast

Cenospheres

Granular residue

Carbonized residue Primary semi-coke

Brief Description

Synonyms & Analogous Terms

All maceral types found in feed coal which show no sign of softening and that maintain their optical character.

1. Unreacted vitrinite 2. Inertinite 3. Totally unaltered or unreacted coal

Softened organic matter retaining sufficient residual structure that maceral origin can be identified.

1. Slightly altered vitrinite 2. Slightly altered coal 3. Partially dissolved coal 4. Partially converted coal

A plastic or once plastic isotropic, pitch-like material derived from vitrinite. [Primary vitroplast derived from vitrinite, secondary vitroplast of questionable maceral origin.] Thin-walled, isotropic hollow spheres derived from vitrinite. Mixture of finely dispersed carbonaceous material and alumino-silicate particles.

1. 2. 3. 4. 5.

Carbonized material derived directly from coal. Formed from softened vitrinite (huminite) and inertinite group macerals and may be isotropic or anisotropic.

1. Anisotropic semi-coke 2. Isotropic semicoke

Carbonplast Coagulant Hydroplast Plasticoal Plastosphere

(Continued)

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TABLE 6.2 (Continued) ICCP (1993) Residue Groups

Brief Description

Secondary semi-coke

Anisotropic carbon derived from mesophase or partially hydrogenated inertinite. Concentrically structured carbonaceous material deposited from gas phase.

Pyrolytic carbon

Inorganic solids Unaltered minerals

Altered minerals

Neo-minerals

Synonyms & Analogous Terms 1. Mesophase 2. Intermediate semi-coke

1. Graphitoid sphaerolith 2. Graphite 3. Carbon black

Pre-existing minerals that pass through liquefaction with little or no chemical alteration. Inorganic species formed from the alteration of preexisting minerals. Inorganic species formed during liquefaction process.

stronger solvent. However, using a milder extraction solvent was fortuitous in allowing for better observation of how coal macerals dissolve, particularly those macerals that were believed to be wholly reactive (vitrinite and liptinite). The occurrence of residue components derived from reactive macerals may be responsible for the difficulty in predicting total conversion (Figure 6.2).

6.4 Fate of Vitrinite Group Macerals Dissolution of vitrinite group macerals is central to the success of liquefaction because they compose the matrix of humic coals, are generally found in high concentration, and at bituminous rank possess thermoplastic properties. Senftle (1981), and Senftle et al. (1984) observed a strong correlation between thermoplastic properties and conversion provided that the fluid temperature range of the coal was in coordination with the temperature used in the liquefaction procedure. Davis et al. (1976) and Mitchell et al. (1977) found evidence for the plastic deformation of vitrinite at low temperatures (325 C) in batch liquefaction and described a spherical residue component derived from vitrinite called vitroplast. Noting variations in the formation of vitroplast from the different vitrinite submacerals, a granular submicron-sized material “granular residue” also was

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identified. As temperature was increased, vitroplast spheres were observed to have swollen in each successively higher temperature residues. Formation of vacuoles within the vitroplast leads to the formation of a variety of cenospheres. At high temperatures vitroplast and cenospheres were absent in batch residues, but in continuous-flow residues they were found in varying concentrations. Shibaoka (1981a,b) used a different batch reaction procedure where coal was combined with tetralin and the temperature of each test was increased (335 C, 340 C, 350 C, 370 C, and 390 C) observed spherical vitroplast developed from all vitrinite submacerals except telinite, which formed a very thin lamellar or fibrous material. As temperature increased vitroplast spheres continued to develop from telocollinite1 (see Chapter 2 for new maceral nomenclature), but the other submacerals began to disintegrate into thin and/or fine size particles. By 390 C, dissolution of the coarse vitroplast spheres was observed. The significance of these findings was that the degree of gelification of the vitrinite may influence the early stage of hydrogenation in that poorly gelified telinite (plus associated corpocollinite) and desmocollinite1 expanded and appeared to be dissolved in tetralin at a lower temperature, whereas the highly gelified telocollinite1 expanded moderately and tended to form coarse vitroplast particles that were more slowly dissolved. Shibaoka also studied the influence of nondonor vehicles (naphthalene and decalin) under similar reaction conditions and observed that the presence of naphthalene caused some dissolution of desmocollinite and telinite, but they did not become plastic. Reactions with decalin caused little particle expansion at lower temperatures and then developed plasticity in a manner similar to carbonization in an inert atmosphere at higher temperatures. As found by Neavel (1976a) the influence of nonhydrogen donating vehicles like those used by Shibaoka allows for a minor amount of dissolution of coal early in the thermal phase of liquefaction but could not sustain the process before carbonization reactions dominated. Shibaoka (1981b) further classified vitroplast into those of spherical structure (plastospheres) and those of rather coarse, irregular areas as massive vitroplast. Mitchell et al. (1977) observed that vitroplast in continuous-flow residues was usually broad (10–100 mm), angular, or slightly rounded particles of variable reflectance and was therefore massive vitroplast. Even though these larger particles of vitroplast possessed similar optical characteristics to that material described from batch reactions, there was no evidence to suggest that they were

1 The correspondence with the new nomenclature for vitrinite macerals is shown in ICCP (1998) and www.iccop.org.

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formed directly from vitrinite. Indeed, they may result from retrogressive reactions from a liquid phase derived from any or all reacted coal macerals as well as the vehicle solvent (Shibaoka, 1981b). The distinction between “primary” vitroplast or that derived directly from vitrinite and “secondary” vitroplast could be important; the former suggests inefficient reaction conditions for thermal degradation or coal selection, whereas the latter may reveal inappropriate reaction conditions or poor hydrogen donating capability of the vehicle. Regardless of origin, the appearance of vitroplast within a liquefaction residue or deposit represents a loss of potential products. Carbonization of these vitrinite- and process-derived components was apparent from continuous-flow liquefaction. Mitchell et al. (1977) described two distinct types of semi-coke. A relatively highly reflecting (generally >1.5% Rmax), homogeneous, and isotropic material was formed after the plastic deformation of vitrinite. Later, Hower et al. (1992) observed that these components may also develop an optical anisotropy comparable to that which may be generated during destructive distillation (coking). The ICCP (1993) designated these types of semi-coke as “primary.” Another type of semi-coke that displayed optical anisotropy formed from the alignment of large planar molecules into a lamelliform structure and was described as “mesophase” because it resembled material discussed by Brooks and Taylor (1968). This type of semi-coke was designated as “secondary” by ICCP (1993) because the size of the anisotropic optical textures (isochromatic areas) was larger than would have been derived from destructive distillation of the coal alone. Mesophase-derived carbon often appeared as coatings on inert coal-derived materials as wall-scales and plugs in pipes leading a way from the primary reactor vessel in continuous-flow operations. Consequently, recognition of these types of semi-coke was important because “primary” would represent a potential loss of production and “secondary” may represent the onset of coking (Wakeley et al., 1979). Microscopic study of batch residues under increasing heat-treatment temperature showed that a great variety of residue components were potentially vitrinite-derived. Most of these components grade from one to another below 400 C and above only a few remain. However, during continuous-flow operations, all these residue components have been observed, demonstrating the inherent inefficiency of singlestage continuous-flow liquefaction. For nonthermoplastic coals (lignite and subbituminous), Walker Jr. et al. (1978) observed a much higher concentration of unaltered or partially altered coal particles and semi-coke (primary and secondary) and lesser amounts of granular residue in filtration residues from continuous-flow operations. Laboratory studies by Shibaoka (1982) using much less severe reaction temperatures showed that huminite

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macerals and submacerals reacted in a similar manner to bituminous coals, i.e., the greater the degree of biochemical gelification, the more plastic the material becomes in the early stages of liquefaction. Those macerals having remnant cell structure formed swollen and partially dissolved residue components, but often maintained particle integrity, whereas more gelified huminite macerals contributed to the formation of vitroplast.

6.5 Fate of Liptinite Group Macerals Generally it has been agreed that liptinite-group macerals (sporinite, resinite, cutinite, alginite, bituminite, fluorinite, and exsudatinite) are highly reactive under most conditions of liquefaction (Fisher et al., 1942; Given et al., 1975b; Davis et al., 1976) providing a relatively higher yield of oil owing to their greater aliphatic content and the fact that they are most similar to the starting substances of petroleum. Although they are considered reactive, small fragments of sporinite have been found in residues from batch reactions resulting from short reaction time (Neavel, 1976a) or reactions below 400 C (Hower et al., 1991), and Mitchell et al. (1977) observed sporinite as separate particles in continuous-flow residues. In addition, evidence has been found for the contribution of other residue components that may be attributed to liptinite-group macerals. Mitchell et al. (1977) and Spackman et al. (1976b) observed a micron-size granular material within and around liptinite macerals (sporinite, resinite, and bituminite) after batch liquefaction at low temperature. Separation of this granular material from disintegrating liptinite indicated a comparatively lower reactivity that if it remained unreactive could contribute to the granular residue. Hower et al. (1991) evaluated the liquefaction behavior of maceral concentrates obtained by density gradient centrifugation (DGC) from a durain containing about equal amounts of vitrinite, liptinite, and inertinite group macerals. Using their standard batch reaction conditions (1.5:1 tetralin: coal ratio, 5.5 MPa hydrogen pressure for 15 minutes at three different temperatures [385 C, 427 C, and 445 C]), the pyridine insoluble residues of the liptinite fraction (mainly sporinite) showed increasing concentrations of granular residue with increasing temperature, exceeding that produced by the vitrinite concentrate or the parent durain. Although Given et al. (1975b) found greater yields of oil from some high alginite-containing coals, others generated almost no oil and were problematic with their interaction with the vehicle employed. Spackman et al. (1976b) observed small fluorescing spheres of material of approximately the same intensity as the original alginite in 375–400 C residues, but these disappeared at higher temperature.

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Hower (1989) investigated batch liquefaction residues derived from alginite/bituminite DGC concentrates from a boghead coal under their standard conditions at three different temperatures. Results showed that both macerals remained virtually unaltered at 385 C, began to soften at 427 C, and maintained their fluorescence properties. At the higher temperature fluorescence was lost and the main contributions to the insoluble residue were softened remnants (referred to as algalplast for alginite and bituplast for bituminite), granular residue, and an anisotropic semi-coke that resembled petroleum coke. Though alginite and bituminite are important members of petroleum source rocks and would be looked on as being ideal for liquefaction, in coal they are contained in densely compact, blocky lithotypes composed of finely divided organic matter. For this reason, presumably reactive coal macerals trapped in the matrix can be isolated from hydrogen and vehicle and lead to variable conversion. Teichmu¨ller (1974) and Spackman et al. (1976b), using fluorescence microscopy, identified a number of secondary liptinite macerals or materials in subbituminous through high volatile A bituminous coals that are potentially important to liquefaction; exsudatinite and mobile bitumen. Their modes of occurrence in coal suggested they were either formerly or currently mobile, respectively, and both were presumably highly reactive during batch liquefaction. Mobile bitumen, in particular, has been observed exuding from weakly reflecting vitrinite (desmocollinite1), from finely granular attrital bands of coal, and from small crevices around included mineral matter in other coal macerals during fluorescence irradiation. This phenomenon occurred largely from coals whose rank coincided with the initial stage of petroleum generation (Hood and Castan˜o, 1974). Although it was not possible to observe the fate of such materials during liquefaction experiments, it is likely that they contribute to the oil-producing capabilities of a coal and may be part of the reason that coals of that rank range perform well.

6.6 Fate of Inertinite Group Macerals The inertinite-group macerals (fusinite, semifusinite, macrinite, micrinite, inertodetrinite, and funginite) have been shown to be much less reactive or even inert during liquefaction (Fisher et al., 1942; Given et al., 1975b; Davis et al., 1976). However, it was their partial reactivity that led Mitchell et al. (1977) to study batch liquefaction residues of coals containing significant amounts of macrinite and semifusinite. One coal containing 21% macrinite gave higher conversion than expected during batch liquefaction. Inspection of the residues showed that some macrinite particles developed large gas

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vacuoles as low as 350 C and by 425 C no residue component identifiable as macrinite was observed. Furthermore, an Australian subbituminous coal consisting of 73% semifusinite and giving unexpectedly high conversion at 400 C exhibited semifusinite showing signs of plasticity in the corresponding residue. The partial reactivity of inertinite also was found for a Permian period coal from South Africa by Gray et al. (1980). Heng and Shibaoka (1983), using concentrates and blends of inertinite from their Australian coal in batch reactions (2:1 tetralin:coal ratio, 9.8 MPa hydrogen pressure, one hour at 400 C and 450 C), concluded that there was a significant contribution to solvent soluble products from inertinite as well as a greater yield of oil than expected at the highest temperature (450 C), which was attributed to low-reflecting semifusinite. Similarly, Hower et al. (1993) reported increasing yields of oil from semifusinite DGC concentrates that were more aromatic in nature as reaction severity was maximized but also found an increase in the concentration of secondary semi-coke in the residue. Although fusinite was found as small, highly reflecting fragments in continuous-flow residues studied by Mitchell et al. (1977), other inertinite macerals (micrinite, inertodetrinite, and funginite) were difficult to recognize and distinguish from fragments of other residue components (vitroplast and primary semi-coke). Much as with coke making, inertinite macerals of higher reflectance should be considered inert during liquefaction, whereas those of lower reflectance may be semi-inert until further information is provided.

6.7 Fate of Mineral Matter Most of the major mineral groups found in coal are largely unaffected by liquefaction, with pyrite and hydrated iron oxides being the principal exceptions (see Walker Jr. et al., 1975, 1977, 1978, 1980). Clay and some sulfate minerals (gypsum and bassanite) lose water and clays dissociate into their micron size crystalline components, whereas carbonate, quartz, and other silicate minerals are seemingly unaffected. ICCP (1993) refers to these as “unaltered minerals” that, if present in the feed coal, should be found in the resulting residue. High concentrations of unaltered minerals are a concern in regard to abrasion they can cause and their propensity for being retained within the primary reactor. The alteration of pyrite and hydrated iron oxides to pyrrhotite has been well studied because pyrrhotite has been supposed to act as a mild hydrogenation catalyst (Given et al., 1980b). ICCP (1993) refers to minerals such as pyrrhotite as altered minerals. Lambert et al. (1980), Montano and Granoff (1980), Montano et al. (1981), Richey (1981), Lambert (1982), and Montano and Stenberg (1985) have studied the pyrite to

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pyrrhotite transformation under the conditions of coal liquefaction. Though Richey (1981) noted that the transformation was controlled by temperature (also see Lambert et al., 1980); by the fugacities of S2, H2S, H2, O2, and H2O; and by buffering of other minerals and organic sulfur, it was Lambert (1982) who suggested that H2S might play a major role in hydrogen transfer instead of pyrrhotite. The precise mechanisms involved in the interaction of metal sulfides and H2S during liquefaction are still in question, but empirical evidence shows that addition of pyrite, dimethyl disulfide, elemental sulfur, and H2S tends to increase total conversion and light-end product yields, whereas addition of sources of iron (pyrrhotite, iron oxide, iron sulfate, etc.) without additional sulfur has a minimal effect or may decrease conversion. ICCP (1993) describes minerals that form during liquefaction as neo-minerals. The most common of these is the formation of carbonates; however, another often overlooked neo-mineral is the iron sulfide that forms from the interaction of sulfur with the exposed steel surface of the system and other sources of iron. Hower (1989) observed what appeared to be pyrrhotite in the framboid habit in batch liquefaction residues from a coal that had no framboidal pyrite and concluded that the iron sulfide was a neo-formed mineral. Davis et al. (1993) studied the nature of a pipe plug between the first- and second-stage reactors from Wilsonville pilot plant running in the Integrated TwoStage Liquefaction (ITSL) mode and found uniform layers of micron-size crystals of pyrrhotite at the pipe-deposit interface; similar types of materials have been seen in residue components from tubing bomb liquefaction testing as well. Corrosion of the steel vessels during liquefaction may contribute iron sulfides (pyrrhotite) to the solid residues in excess of the amount of pyrite in the original coal. By far the most important neo-minerals generated during liquefaction are those carbonate minerals formed during the processing of low-rank coals from the reaction of carbon dioxide and organically bound calcium (Walker Jr. et al., 1977; Wakeley et al., 1979; Given et al., 1980b). However, it is possible that some of the calcium sulfate found within the primary reactor could have been formed during liquefaction. Formation of carbonates has long been recognized; German technology of the 1940s incorporated design elements to “prevent and remove the build-up of sand and formation of caviare, i.e., small spheres consisting of coke formed round a central sand particle” that occurred during the hydrogenation of brown coal (Gordon, 1947).

6.8 Reactor Solids Neo-minerals formed within the primary reactor during continuousflow liquefaction are retained and grow larger in diameter during

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extended operation. These minerals, along with other carbonaceous materials, have been called reactor solids (Walker Jr. et al., 1977; Wakeley et al., 1979). Their smaller counterparts can be found in the solid residues and the components from which they were derived can show up as deposits found in pipes and vessels downstream from the primary reactor. Harris et al. (1979) and Shibaoka et al. (1984) characterized mineral deposits found in preheater and feed-line leading into the primary reactor unit of a continuous-flow facility. Wakeley et al. (1979) described the characteristics of reactor solids removed from processing several bituminous and a subbituminous coal from the Wilsonville SRC-I process in terms of their carbonaceous and mineral content. Besides retention of coal-derived minerals, lower rank coals tended to form a mixture of neo-minerals and secondary semi-coke, whereas those from bituminous coals were dominated by secondary semi-coke that incorporates entities normally found in the solid residues. Operating data from Wilsonville showed that accumulation rates were different for different coals, ranging from 0.1–0.5% solids per ton moisture-free coal for bituminous and 0.7–1.4% for a subbituminous (Lewis et al., 1977). Shibaoka et al. (1984) offered plausible explanations for secondary semi-coke (mesophase) development in the preheater that may very well apply to the reactor unit. Heating coal and vehicle above 420 C in the absence of sufficient donor solvent or when the vehicle viscosity has increased may change the flow characteristics of the system leading to zones of stagnation. Because most of the mesophase-derived carbon observed in these deposits exhibit limited flow characteristics, it is reasonable to suggest that they may have formed under relative quiescence. This can happen in the preheater, along the reactor wall, and even in the outlet pipes leading to a secondary reactor (Davis et al., 1993). Dynamic viscosity in such a system has yet to be measured directly, but those processes employing turbulent systems, like the ebullating bed catalyst used in the H-Coal process and in the second-stage reactor of Wilsonville (ITSL mode) experience fewer episodes of severe coking that were common in single-stage reactors. Elimination of carbonate-bearing reactor solids formed during hydrogenation of low-rank coals appears to have no easy remedy other than removing the solids on a regular schedule. Neavel (1981b) suggested pretreatment of low-rank coals with sulfur dioxide before hydrogenation, which would result in a certain amount of the Ca2þ ions to form a stable calcium salt, like calcium sulfate. Wakeley et al. (1979) reported finding significant amounts of anhydrite in reactor solids from bituminous rank coals and minor amounts of bassanite and gypsum, which shows that calcium sulfate minerals are probably benign during liquefaction.

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6.9 Applied Petrology Knowledge of how coal macerals and minerals react and contribute to the solid residues of liquefaction provides a useful means of raw materials and process evaluation through petrographic analysis. The appearance of significant concentrations of primary and secondary vitroplast and semi-coke, neo-minerals, or fragments of reactor solids in the solid residue could signify unfavorable reaction conditions for the selected coal or process design employed or that the hydrogen-donating capability of the vehicle was not being met or maintained during operation. Standard sampling and petrographic techniques have yet to be developed in this field to provide statistical confidence. Furthermore, sample preparation techniques that attempt to preserve original particle size and component associations are difficult and time consuming (Walker Jr. et al., 1978). Consequently, delivery of process-related petrographic information would lag behind production and be less effective as a tool for process control. Still, monitoring of the components of solid residues may be essential when major changes in feed materials or operating conditions are implemented and when there are severe upsets to system operation. Generally, pressure filters, centrifuges, hydroclones, or solvent precipitation systems (Kerr McGee’s critical solvent deashing and Lummus’s antisolvent deashing) have been used to separate solid materials from the high boiling point fraction of coal-derived liquids (Alpert and Wolk, 1981; Derbyshire and Gray, 1986; Schindler, 1989). These separation techniques were tested for the SRC processes being developed during the 1960–1970s where a solid clean-burning fuel was the product of interest. Residue samples from some of these processes provided an introduction to the range of residue components that might be expected from continuous-flow liquefaction. A significant body of work that attempted to apply petrographic techniques to continuous-flow liquefaction residues was conducted by Walker Jr. et al. (1975, 1977, 1978, and 1980) for single-stage thermal and catalytic liquefaction facilities, including SRC-I (Catalytic, Inc., six ton/day Wilsonville Pilot Plant and Hydrocarbons Research Inc., 9.6 kg/day SRC unit) and the H-Coal (Hydrocarbons Research Inc., 11.3 kg/day process development unit) processes. Generally, the types of solids obtained from these plants ranged from particulate samples generated from pyridine extraction or fist-size lumps of filter cake that had been washed in hot anthracene oil. Residue materials were obtained from three processes for a variety of coals (Table 6.3) and process conditions (Table 6.4) and prepared for petrographic analysis. Techniques were developed for filter cake samples in which about 500 g of broken cake was split from a larger sample, particle size

164 Applied Coal Petrology TABLE 6.3 General properties of coals used by Wilsonville and HRI corresponding to liquefaction residues (data from Walker Jr. et al., 1975, Weber et al., 1978a,b, and Lewis et al., 1977) State Seam Mine

Wyoming Wyodak Belle Ayr

hvCb Pennsylvanian

Utah A and B John Henry, a.k.a. Kaiparowits hvCb Cretaceous

Proximate analysis, as received Moisture 6.0 Volatile matter 38.0 Fixed carbon 51.0 Ash 5.0

2.65 37.21 47.96 12.18

1.33 43.36 49.11 6.20

7.39 49.18 37.98 5.45

Ultimate analysis, wt. % Carbon 70.8 Hydrogen 5.1 Nitrogen 1.3 Chlorine 0.1 Sulfur 3.2 Ash 10.8 Oxygen (diff.) 8.7

68.74 4.89 1.45 0.07 3.79 12.51 8.55

72.53 5.10 1.00 0.36 6.28 14.83

63.57 3.52 0.90 <0.01 0.93 5.89 25.19

1.92 0.09 1.78

0.00 0.02 0.43

0.08 <0.01 0.85

ASTM rank Geologic period

Illinois Illinois #6 Burning Star

Kentucky #9 and #14 Colonial

hvCb Pennsylvanian

Sulfur forms, wt. % Pyritic sulfur Sulfate sulfur Organic sulfur

1.1 0.1 2.0

Oxides of major elements in ash, wt. % Silica oxide 43 43.4 Ferric oxide 17 32.5 Aluminum 18 19.6 oxide Calcium oxide 5 0.70 Magnesium 1 0.50 oxide Sodium oxide 1 0.20 Potassium 1 1.40 oxide Titanium 1 0.00 oxide Phosphorus <0.1 0.60 pentoxide Sulfur trioxide 7 0.90 Undetermined 6 0.20

subbit Tertiary

ng ng ng

30.7 5.0 15.2

ng ng

25.7 1.4

ng ng

2.5 0.3

ng

1.4

ng

1.2

ng ng

15.6 1.0 (Continued)

Direct Coal Liquefaction

165

TABLE 6.3 (Continued) Petrographic properties, vol. % or % Total vitrinite 85 (huminite) Total liptinite 3 Total 12 inertinite Semifusinite 5 0.50 Vitrinite reflectance (mean max.)

91

81

91

2 7

2 17

1 8

3 0.56

9 0.49

2 0.38

ng ¼ not given

was reduced and homogenized. From this, about 100 g of sample was Soxhlet extracted in THF for 72 hours and dried, then combined with acetone and placed in an ultrasonic bath overnight to reduce particle agglomerates. Acetone was stripped from the sample which was then gently milled by hand to obtain a uniform particulate specimen. Petrographic mounts for reflected-light microscopy were prepared as a 50/50 mixture of sample and a cold-setting epoxy resin in a centrifuge tube, thoroughly mixed and placed under vacuum to force epoxy into small porosity, and then spun in a centrifuge for 5 minutes. This procedure forced a gradation of residue particles based on particle size and density which was maintained when the epoxy hardened. The hardened specimen was cut in half longitudinally to expose the particle gradation, and the sample was mounted in additional epoxy into a 25 or 28 mm diameter specimen for polishing. The cut surfaces were ground using 400 and 600 grit silicon carbide papers and polished using 0.3 mm and 0.05 mm alumina slurries on a high-nap cloth and silk, respectively. This procedure resulted in a fairly uniform polish with minor negative relief as long as the hard agglomerate particles were minimized. Petrographic analyses were conducted using reflected white-light illumination at 800 magnification and oil immersion. Cross- or oblique polarized light (employing both polarizer and analyzer) and a retardation accessory to perceive optical anisotropy were essential. The point-counting procedure involved identification of residue components appearing beneath an eyepiece crosshair as the specimen was traversed on a 0.2  0.2 mm grid perpendicular to the particle gradation; 500 counts were made for each of two polished surfaces for a total of 1,000 counts. Table 6.5 provides a comparison of the distribution of residue components obtained by point-counting specimens from runs of the four coals and three liquefaction processes conducted at approximately the same temperature; coal feed rates and pressures were variable among the runs, and no operational information was available for

TABLE 6.4 Comparison of basic run conditions and results for different processes and coals (Data from Walker Jr. et al., 1975, 1977; Weber et al., 1978a,b, and Lewis et al., 1977, 1978) W. Ky. #9 and #14, Colonial

Temperature,  C Solvent:coal ratio Feed rate: Lbs/hr/cu ft. Lbs/hr Pressure, psig Wt. % coal conversion Hydrogen consumption Wt. % insoluble in the filter cake

HRISRC

HRISRC

1779647B

Ill. #6, Burning Star

Wilsonville -SRC

HRISRC

1779660B

426.7

Kaiparowits

Wilsonville -SRC

HRISRC

HRI H-Coal

#64

1779666A

#20

1771008B

440.6

454.4

440.6

445.6

ng

ng

ng

ng

3.2

49.9

25.0

25.0

Wyodak, Belle Ayr Wilsonville -SRC

HRISRC

HRISRC

HRISCR

HRISRC

HRI H-Coal

177101-4B

#101

1771144B

17711413B

17711414B

1771154B

13064-11B

440.6

P

457.2

441

454

466

454

P

ng

ng

ng

1.99

2.12

2.11

2.04

P

25.0

P 1.14 2060 72.5

1.14 2050 85.1

0.98 2500 92.8

1.14 2510 93.3

P P

P

2500 94.1

1000 94.4

450 1750 93.0

6.0

6.7

3.2

7.5

ng

9.2

12.9

2.8

5.8

10.2

14.1

13.5

P

46.8

56.2

3.8

55.0

10.1

47.2

47.2

32.0

27.5

12.4

5.9

5.1

P

ng ¼ not given; P ¼ proprietary.

1500 94.0

528 1079 ng

2000 83.0

P 86.1

464 2400 84.0

TABLE 6.5 Comparison of the distribution of residue components for various coals and processes (Data from Walker Jr. et al., 1977, 1978) Partially Reacted Coal Macerals

Semi-Coke Process

Run #

Ceno- VitroGranular Semi- Pyrrhotite Carbonate Quartz Other* Residue Primary Secondary sphere plast1 Vitrinite Fusinite Fusinite

Illinois #6 Seam, Burning Star, Illinois HRI-SRC 17765 96-66A Wilsonville 20 71 SRC HRI H-Coal 13058 69-6A

4

0

Tr

7

5

4

5

8

2

1

0

5

2

0

11

0.5

1

1

3

1

0.5

4

1

0

4

15

2

2

3

8

5

1

Tr

3

Tr

24

1

3

3

Tr

4

0.5

0

1

Tr

29

1

2

1

Tr

4

0.5

0

0

Tr

5

4

2

5

12

Tr

Tr

0

0

0

20

Tr

1

1

6

0

Tr

0

9

4

0

15

25

1

1

2

Tr

0

0

6

10

0

31

10

1

1

2

0

Tr

Tr

7

12

0

29

9

2

1

1

Tr

Tr

Tr

A & B Seams (Kaiparowits), John Henry Mine, Utah HRI-SRC 17751 10 100-8B HRI H-Coal 17756 5 101-4B West Kentucky #9 & #14, Colonial Mine, Kentucky HRI-SRC 17763 7 96-60B Wilsonville64 66 4 SRC Wyodak, Belle Ayr Mine, Wyoming HRI-SRC 17743 114-13B Wilsonville101 38 SRC HRI H-Coal 13039 64-11B Tr ¼ Trace <0.5%.

* ¼ The category generally describes contaminants, such as small particles of ceramic, filter materials, and, in the case of Wilsonville, petroleum and metallurgical coke from the crushing facilities employed to prepare their coals. 1 ¼ Originally, primary and secondary vitroplasts were not recorded separately, but nearly all the vitroplast observed was secondary.

168 Applied Coal Petrology

H-Coal. In general, this comparison shows that granular residue was the dominant component observed in all residues and that, with some variation, vitroplast and primary semi-coke were also important components that appeared to be variable between coals and processes. The fact that primary and secondary vitroplast and semi-coke may represent retrogressive reactions makes their occurrence in residues important. Recognizable macerals (fusinite and semifusinite) tended to be of variable concentration that did not necessarily agree with the original volume percentage observed in the coals, although the relative amounts of inertinite macerals in the residues of the Illinois #6 and Kaiparowits coals were greater compared with West Kentucky #9 & #14 and Wyodak (Table 6.3). Cenospheres were found only in significant concentration in one H-Coal residue, which could suggest that these delicate structures were destroyed during continuous-flow processing or sample preparation. As might be expected, concentration of the mineral-derived components (pyrrhotite, carbonate, and quartz) tended to follow the original composition of the coal; for example, both the Illinois #6 and West Kentucky #9 & #14 had fairly high concentrations of pyrite and a considerable amount of pyrrhotite was observed in their respective residues, whereas the Kaiparowits coal had a significant concentration of carbonate which was identified in its respective residues. The fact that coals having little or no pyrite (Wyodak and Kaiparowits) should have any pyrrhotite in their associated residues suggests that these are probably neo-formed minerals. Without regard to specific operating conditions, comparison of the residue categories among the four coals exhibited some potentially important differences. Of the three bituminous coals, the Kaiparowits generated considerably more vitroplast and primary semi-coke compared with the Illinois #6 and West Kentucky #9 & #14, although the latter showed more variability in the concentration of these components in different processes. In contrast the subbituminous Wyodak coal generated higher amounts of recognizable, but altered, coal particles, some of which had developed a fine anisotropic texture (primary semi-coke). Vitroplast and primary and secondary semi-coke were found in significant concentrations regardless of the process from which the residues were derived or the level of conversion reported (Table 6.4). Vitroplast and secondary semi-coke probably resulted from retrogressive reactions that were apparently unchecked by the catalytic hydrogenation employed in the H-Coal process. As scale of operation decreased it becomes easier to obtain reliable samples that match operating conditions. Smaller-scale apparatus, like HRI’s SRC process development unit, provided far more control from which residue petrography could be compared with operating conditions. Table 6.6 compares the distribution of residue

TABLE 6.6 Influence of processing conditions (HRI-SRC-PDU) on distribution of residue components (Data from Walker Jr. et al., 1977, 1978) Partially Reacted Coal Macerals

Semi-Coke Run #

Variable Conditions*

Granular Residue

Pri- Second- Ceno- VitroSemi- Pyrrhosphere plast1 Vitrinite Fusinite Fusinite tite Carbonate Quartz Other mary ary

West Kentucky #9 & #14, Colonial Mine, Kentucky 71 2 177-96-47B Feed rate, 177-96-60B temperature, 63 7 pressure Wyodak, Belle Ayr Mine, Wyoming 177-114-4B 177-114-13B Temperature and 177-114-14B pressure 177-115-4B

35 43 50 51

24 9 9 7

0 0

0 Tr

7 5

6 4

1 2

4 5

9 12

0.5 Tr

Tr Tr

0 0

1 4 12 4

Tr 0 0 0

18 15 9 10

18 25 14 21

1 1 1 2

1 1 1 1

1 2 4 3

Tr Tr Tr Tr

Tr 0 Tr 1

1 0 0 1

Tr ¼ Trace <0.5%. * ¼ Reaction conditions given in Table 6.8. 1 ¼ Originally, primary and secondary vitroplasts were not recorded separately, but nearly all the vitroplast observed was secondary.

170 Applied Coal Petrology

components under varied process conditions for operations with the West Kentucky #9 & #14 and the Wyodak coals. In the case of the bituminous coal, run 177-96-60B was reacted at a higher temperature and lower feed rate and pressure which had a negligible effect on conversion (Table 6.4) or the distribution of residue components. However, a similar observation was not found with the Wyodak coal. For this series, reaction temperature and pressure were increased with a positive impact on conversion (72.5–93.3%). These improvements were achieved by first increasing the temperature (177-114-4B and 13B) and then increasing both temperature and pressure (177-114-14B) before reducing temperature at the elevated pressure (177-115-4B). Generally, as the severity of run conditions was increased the volume percentage of granular residue and secondary semi-coke increased with a corresponding decrease in vitroplast and those components which appeared to be altered coal particles and macerals. This would suggest that noncatalytic liquefaction with this particular subbituminous coal has a tendency to result in retrogressive reactions that can be controlled to some degree by operating at higher hydrogen pressure and moderate temperatures. The difficulty in comparing reaction severity of operations with production and the nature of the insoluble organic matter (residues) is exacerbated by the magnitude of operations and the many variables that potentially influence them. Hower et al. (1992) studied the liquefaction characteristics of a bituminous coal from Western Kentucky by comparing product distribution and solid residues against a severity index defined as: SI ¼ y=yR exp ½ð30;000=1:986Þð1=TR  1=TÞ where y is the residence time in minutes, yR is the reference time of 5 minutes, 30,000 is an average activation energy (in calories), 1.986 is the gas constant, T is the reaction temperature (K), and TR is the reference temperature of 598 K. Conversion increased rapidly up to an SI ¼ 200, above which increases were small. Above 200, the preasphaltene content decreased, indicating conversion to other soluble products, but there was a smaller decrease in the amount of solid residues than in the early, less severe range, indicating that much of the conversion to oil þ gas was derived from the asphaltene þ preasphaltene fraction, not from further breakdown of the solids. Using a modification of the ICCP residue classification, Hower et al. (1992) determined by point count analysis that the solid residues exhibited a transition from recognizable macerals to a mixture of inertinite and semi-inertinite þ vitroplast þ granular residue, with the vitroplast to granular residue transition occurring at higher severities. As a summary for this chapter, petrographic analyses of liquefaction residues using the ICCP classification of residues or modifications

Direct Coal Liquefaction

171

thereof clearly show differences among coals and to a lesser degree reaction conditions. Of great importance, however, is the fact that residue components that result from retrogressive reactions can be identified and their occurrence appeared to be dependent upon both coal and process conditions, at least in single-stage reactions. It has also been determined that residue components identified in lower severity batch reactions, those that disappear at high temperatures, are observed in continuous-flow residues; vitroplast and unaltered or slightly altered coal particles and macerals being primary examples. The appearance of these residue components along with the occurrence of semi-coke demonstrates the inefficiency inherent in single-stage liquefaction technology and suggests a reason for the difficulty of past researchers to adequately and accurately predict coal conversion. Two-stage coal liquefaction processes designed to convert coal to a soluble form in a high severity initial stage (short contact time and high temperature) reduced hydrogen consumption and cracking reactions so that the intermediate liquid products could be hydrotreated in a lower severity second stage showed much promise (Schindler et al., 1983; Derbyshire and Gray, 1986). Recycling of solid residues with the hydrogenated vehicle has shown potential for increasing overall coal conversion. Unfortunately, little work was performed on the characterization of residual organic matter generated from these processes. At present, development of direct coal liquefaction technology is at an impasse in the United States and Europe where it seems unlikely that private industry will invest upwards to four billion dollars to build and operate such a plant and governmental energy policies suggest that it is more expedient to stockpile petroleum reserves, seek alternative fuels, adopt conservation practices, or attempt to use diplomacy to avert shortages. But until a significant demonstration plant is built and brought into continuous operation, the technology will remain an often talked-about option but not a reality. However, in 2002 the Shenhua Group Corporation of the Peoples Republic of China licensed the Hydrocarbon Technologies, Inc., direct coal liquefaction process (H-Coal technology with design modification) to be built in the Inner Mongolia Autonomous Region (Fletcher et al., 2004). The SH-I plant was to be operational in 2007 using 7,000 ton/day of Shenhua’s subbituminous coal to produce 20,000 barrels of diesel fuel and gasoline. It can only be hoped that during the startup and early operation of this endeavor that application of petrographic techniques will not be overlooked.

CHAPTER 7

Coal Carbonization John C. Crelling

7.1 Introduction Metallurgical coke, along with iron ore (iron oxides) and limestone, is layered into a blast furnace to convert the iron ore to metallic iron. The coke, which is mostly carbon, reacts with the blast air to produce carbon monoxide, which in turn reacts with the iron oxide to produce carbon dioxide and metallic iron. In the reaction zone of the blast furnace, where the iron oxide is converted to gas and liquid iron and the limestone fluxing agent is melted, the coke is the only solid material and is thus required to support the burden in the furnace and maintain permeability for the blast air. Metallurgical coke is usually manufactured in a coke oven that is typically 6 meters high, 15 meters deep, and a half meter wide. The ovens are arranged in batteries of up to 100 ovens stacked together. The oven walls are gas heated, and a normal coal charge is 15 to 30 tons. The coal charge is heated to about 1,000
C and converted to coke in about 18 hours. The volatile matter in the coal is driven off as gases and tars and collected to be used as byproduct chemicals. Petroleum coke is made from the residues left from refining. Delayed coke is made when the residue feedstocks are pumped into drums and held for 24 hours. Fluid coke is produced when petroleum residue feedstocks are sprayed into a fluidized bed reactor.

7.2 The Coal-to-Coke Transformation The most successful industrial applications of coal petrology have been in the carbonization industry. Coal petrology is used to evaluate the suitably of a given coal for coking, predict the strength of cokes made from both single coals and coal blends, predict peak coking Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

174 Applied Coal Petrology

pressures, predict coke reactivity, evaluate the nature of quinoline insolubles in coal tar pitches, detect weathering in coal, and evaluate petroleum cokes. A coking coal is quite simply a coal that, when heated in the absence of air, will melt vesiculate and harden into a spongelike mass of almost pure carbon. Not all coals are coking coals; lignites, subbituminous, semianthracites, anthracites, and meta-anthracites do not coke. As coal is heated in the coking process, it begins to lose its optical anisotropy at around 350
C. As the coal is heated between 350
C and 450
C, it progressively melts, devolatilizes, and vesiculates. In this range it usually reaches its maximum fluidity. In the 450
C and 550
C range it progressively loses its fluidity and hardens into coke. Even though it has hardened at this point it still contains some significant volatile matter and is considered green coke until it is calcined at about 1,000
C. Petroleum coke and even baked anodes exhibit the same behavior but are usually calcined at slightly higher temperatures, 1,200
C to 1,400
C. Even at these treatment temperatures none of these materials is graphitized (their carbon atoms reorganized into a graphite layered structure). Graphitization occurs only at much higher temperatures, around 3,000
C. Though the details of the coking process are not completely understood, Neavel (1976b) and Marsh and Neavel (1980) argued that process is similar to coal liquefaction, in which the bridging bonds between small aromatic units are broken by heating and the hydrogen released from the coal caps these units and thus liquefies the coal until the hydrogen supply is exhausted and the liquid resolidifies into coke. Under a microscope, coke shows a distinct mosaic texture; coke made from a blend of different rank coals, usually a high volatile A and a low volatile coal, shows mixed mosaic textures. Looking at this aspect of the coking process, Brooks and Taylor (1965) were the first to describe the mesophase units in coke formation as liquid crystals. This idea was taken up by Marsh and coworkers in a series of papers (Marsh 1973, 1980, 1982; Marsh and Clarke 1986, and Marsh and Mene´ndez 1988, 1989) arguing for the development of nematic liquid crystals in the melted or liquefied coal that grow at the expense of the liquid phase until the mass hardens into coke. The liquid crystals are essentially zones of order in the liquid. These liquid crystals have a parallel structure of the aromatic carbon layers that allows them to coalesce readily. An excellent review of the role of mesophase in coking can be found in Marsh (1992). Supporting this concept is the fact that in studies of heated pitch such liquid crystals are routinely observed to form and coalesce and that Taylor (1961) actually observed mesophase liquid crystals in an occurrence of natural coke where coal had been coked in situ by an

Coal Carbonization

175

igneous intrusion and with SEM observations by Friel et al. (1980). Mesophase spheres are more easily observed under the microscope in heated pitches and photomicrographs (Figure 7.1), and even videos of mesophase behavior are easily obtained.

FIGURE 7.1. Photomicrographs of mesophase. (a) Incipient mesophase spheres (liquid crystals) in heated pitch. (b) Single large mesophase sphere (liquid crystal) in heated pitch; note the presence of smaller spheres. (c) Numerous mesophase spheres (liquid crystals) in heated pitch. (d) Numerous mesophase spheres (liquid crystals) in heated pitch occupying a large portion of the area of the field; note the start of coalescence of the two large spheres at right. (e) Numerous mesophase spheres (liquid crystals) in heated pitch, most of which are coalescing. (f) Field in which mesophase spheres have coalesced to the point of phase inversion where the liquid pitch is now the minor phase. All images were taken in incident polarized light. Long side of the microphotographs: 215 micrometers. (Photo credits: Ralph Gray.)

176 Applied Coal Petrology

The concept of liquid crystal mesophase units growing in the liquid coal easily explains the development of the mosaic texture of coke by assuming that the liquid crystals become the individual mosaic units upon the hardening of the coke. With all other conditions being equal, the rank of the parent coal controls the chemistry of the liquid and the developing mesophase and, thus, the size and structure of the mosaic units. The rank of the parent coal also controls the viscosity of the mesophase liquid and therefore the shape of the mesophase unit going from circular to lenticular to ribbon-like with increasing viscosity and increasing rank of the parent coal.

7.3 Coke Petrology Classification The petrographic characteristics of metallurgical coke reveal much about its composition and structure and can be used to evaluate its behavior and formation. In industry, coke petrography is used for quality control and troubleshooting. Because coke is a hard, sponge-like carbon mass with pores and cell walls, these features are studied. The abundance, size, and shape of the pores and the abundance, thickness, and carbon forms of the cell walls have been investigated in an attempt to understand the behavior of coke. Much of the study on coke porosity involved image analysis techniques, and though such work did generate some correlations with coke properties (Patrick J. Q. et al., 1977; Patrick J. W. et al., 1977, 1979; Marsh, 1982), petrographic studies of the carbon forms of the coke cell walls have been more productive. All the early published work on carbon forms in coke cell walls found various types of mosaic structure in circular, lenticular, and ribbon forms in various sizes (Grint et al., 1979; Patrick et al., 1973; White and Price, 1974; Gray, 1976). Coin reviewed existing systems of classification in 1987. Gray (1976) published the system of coke petrography the U.S. Steel Corporation had been using for metallurgical coke since the late 1960s. In this classification the coke structure is divided into a binder phase, essentially the mesophase forming part of the coke and a filler phase consisting of organic and inorganic inerts incorporated into the coke cell walls as well as a few miscellaneous carbon forms. This system was published again in slightly modified form by Gray and DeVanney (1986) and Gray (1989, 1991) and is shown in Table 7.1. The binder phase classification consists of isotropic and incipient isotropic phases and then circular, lenticular, and ribbon phases in fine, medium, and coarse sizes. These are illustrated in Figures 7.2, 7.3, and 7.4. A particularly useful aspect of this classification is that each binder phase type is associated with a vitrinite precursor of a specific

Coal Carbonization

177

TABLE 7.1 System of coke microscopy Coke Binder-Phase Carbon Form Classification Width (in Microns)

Binder Phase Isotropic Incipient (anisotropic) Circular (anisotropic) Fine circular Medium circular Coarse circular Lenticular (anisotropic) Fine lenticular Medium lenticular Coarse lenticular Ribbon (anisotropic) Fine ribbon Medium ribbon Coarse ribbon

Length (L) to Width (W) Relation

Parent Coal Vitrinoid Type

0.0 0.5

None L¼W

6, 7 8

0.5–1.0 1.0–1.5 1.5–2.0

L¼W L¼W L < 2W

9 10 11

1.0–3.0 3.0–8.0 8.0–12.0

L  2W, L < 4W L > 2W, L < 4W L > 2W, L  4W

12 13 14

2.0–12.0 12.0–25.0 25.0þ

L > 4W L > 4W L > 4W

15 16 17, 18

Coke Filler-Phase Carbon Form Classification Filler Phase Organic inerts Fine Coarse Miscellaneous inerts Oxidized coal (coke) Brecciated coal (coke) Noncoking vitrinite (coke) Inorganic inerts Fine Coarse

Size (Microns) <50 >50

Precursors Micrinite, macrinite, inertodetrinite Semifusinite, fusinite durain Oxidized and brecciated coal Vitrinite too high or low in rank

<50 >50

Mineral matter and coal bone

*Miscellaneous categories, including carbon additives, depositional carbons, and green and burnt coke, may be quantified. Source: Organic Geochemistry 17, R. J. Gray, “Some petrographic applications to coal, coke, and carbons,” 535–555, copyright 1991, with permission from Elsevier.

178 Applied Coal Petrology

(a)

(b)

(c)

(d)

(e)

(f)

FIGURE 7.2. Photomicrographs of metallurgical coke. (a) Isotropic coke cell walls. (b) Incipient mosaic texture. (c) Fine-grained circular mosaic texture. (d) Medium-grained circular mosaic texture. (e) Coarse-grained circular mosaic texture. (f) Fine-grained lenticular mosaic texture. Long side of the microphotographs: 215 micrometers. All images were taken in incident polarized light with analyzer and a retarder plate. (Photo credits: Ralph Gray.)

rank. Thus it is possible to determine the nature of the coal or coal blend that was coked to produce a given coke. If, for example, a coal blend was designed to have 30% low volatile bituminous coal and only 15% of low volatile carbon forms (ribbon type) are found, a problem in making up the coal blend can be suspected. The filler phase is divided into fine and coarse organics derived respectively from

Coal Carbonization

(a)

(b)

(c)

(d)

(e)

(f)

179

FIGURE 7.3. Photomicrographs of metallurgical coke. (a) Medium-grained lenticular mosaic texture. (b) Coarse-grained lenticular mosaic texture. (c) Finegrained ribbon mosaic texture. (d) Medium-grained ribbon mosaic texture. (e) Coarse-grained ribbon mosaic texture. (f) Extra-coarse ribbon mosaic texture. All images were taken in incident polarized light with analyzer and a retarder plate. Long side of the microphotographs: 215 micrometers. (Photo credits: Ralph Gray.)

micrinite, macrinite, inertodetrinite, and fusinite and semifusinite. In the organic inerts miscellaneous particles of oxidized coal and brecciated coal are also included (Gray, 1982). The inorganic inerts are mineral matter and rock fragments. All these materials are well illustrated with color photomicrographs in the Gray and DeVanney (1986) paper.

180 Applied Coal Petrology

(a)

(b)

(c)

(d)

(e)

(f)

FIGURE 7.4. Photomicrographs of inclusions in metallurgical coke. (a) Fusinite inerts in coke cell wall. (b) Pyrite in coke cell wall. (c) Petroleum coke particle on left side of field. (d) Vapor deposited carbon (roof carbon) lining coke pores. (e) Anthracite particle on left side of field. (f) Particle of coke breeze (fine coke) on right side of field; note the thin layer of vapor deposited carbon on the edge of the breeze particle. All images were taken in incident polarized light with analyzer and a retarder plate. Long side of the microphotographs: 215 micrometers. (Photo credits: Ralph Gray.)

7.4 Coke Strength Prediction Metallurgical coke is put into the blast furnace along with iron ore (iron oxide, usually magnetite or hematite) and limestone. Hot air is blasted through the furnace and the iron ore is reduced to metallic iron liquid that drops to the bottom of the furnace, where it is

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181

periodically drawn off and cast into ingots. In the furnace the coke has four functions: It is a fuel to provide heat to drive the chemical reactions; it reacts with the blast air to form carbon monoxide, which reduces the iron oxide to metallic iron; it maintains permeability in the furnace; and as the only solid material in the reaction zone, it supports the burden in the furnace. To successfully fulfill its support function, the coke must have sufficient strength to avoid being crushed. Thus, the strength of metallurgical coke is its most important property. Perhaps the greatest success of applied coal petrology is its use to accurately predict the coke strength (ASTM stability) of single coals and coal blends. Though it was generally understood that not all coal macerals behaved the same way in the coking process, Spackman et al. (1960) and Spackman (2000), in cooperation with the U.S. Steel Corporation, studied this in detail with hot stage microscopy and even produced a film showing the different thermal behavior of various macerals. The basic model behind the prediction systems is that the vitrinite acts as a binder and the inertinite macerals act as aggregate reinforcing the coke cell walls; thus, for a vitrinite of any given rank there is an ideal level of inertinite needed to yield the strongest coke. Ammosov et al. (1957) published the first prediction method based on petrographic data, but the most widely adopted method was that of Schapiro et al. (1961), which was followed by other publications: Schapiro and Gray (1964), Gray and Champagne (1988), and Gray (1989). This system requires a maceral point count analysis, a measure of the volume percentage of mineral matter, and a reflectance analysis of the volume percentage of the vitrinite and semi-inertinite distribution (Chapter 2). To facilitate the latter item, Schapiro et al. (1961) introduced the concept of a V-type which is vitrinite in a 0.10% reflectance range. Thus a V-type 9 would be the volume percentage of vitrinite in the range from 0.90% to 0.99% reflectance. With the maceral and reflectance analysis in hand, they calculate a composition balance index (CBI), which is the ratio of the actual inerts to the ideal amount needed for maximum coke strength. Because the coke strength also varies with the rank of the vitrinite, they calculate a strength index (SI), also called the rank index (RI), for each V-type. The CBI and the RI require a calculation of the total volume percentage of reactive macerals, which include the liptinite and vitrinite macerals and one third of the semi-inert macerals and the total inert content, which includes the inertinite macerals and two thirds of the semi-inert macerals as well the mineral matter, which is calculated from the Parr formula (mineral matter ¼ 1.08 ash – 0.55 sulfur). After the parameters are calculated they are located on a CBI/RI cross-plot (shown as Figure 7.5) to predict the coke strength. When coal blends are used,

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8

Stability Factor

7

70

Rank Index

6 60 5 50 40

4

30 20 10 0

3

2

1 10.0

6.0

4.0 3.0

2.0 1.5

1.0

0.6

0.4 0.3

0.2 0.15

0.1

Composition Balance Index

FIGURE 7.5. Coke stability (strength) diagram. To predict the coke stability of a sample, the CBI and the RI are calculated from petrographic data and plotted on this diagram. (Source: Journal of Institute of Fuel 37, N. Schapiro and R. J. Gray, “The use of coal petrography in cokemaking,” 234–242, copyright 1964, with permission from the Energy Institute, London.)

the CBI and RI are calculated proportional for each component. This system of coke strength prediction is now over 40 years old but it is still in use in a variety of adaptations. A different coke strength prediction method was developed by researchers at the Bethlehem Steel Corporation: Benedict et al. (1968a), Thompson and Benedict (1976), and Benedict and Thompson (1980). They discovered that some kinds of vitrinite that they called pseudovitrinite actually behaved as a semi-inert maceral in the coking process. They described this pseudovitrinite (Benedict et al., 1968b; Thompson and Benedict, 1974; Kaegi, 1985) as the type of material occurring in vitrain layers in hand specimens. This is similar to the material called vitrinite A by Brown et al. (1964a), telocollinite in the old ICCP classification, and telovitrinite in the new ICCP vitrinite classification (ICCP, 1998; Chapter 2). In this method a maceral analysis including pseudovitrinite is needed as well as reflectance values on both vitrinite types. The total effective inerts are calculated by adding

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the inertinite macerals to two thirds of the semi-inert macerals, the mineral matter calculated from a modified Parr formula, and a portion of the pseudovitrinite. To predict coke strength, the intersection of the total effective inert content and the vitrinite reflectance are plotted on a correlation chart. The correlation charts for high and medium volatile coals are shown in Figure 7.6 and for low volatile coal in Figure 7.7. A glance at Figure 7.6 clearly shows that the original model of an ideal amount of inerts for a given rank of vitrinite is correct. In their 1968 papers Benedict et al. also showed the curious fact that as long as there was at least 23% or more of a low volatile coal in a coking blend, the strength of the blend would be the same as the strength of the low volatile coal alone. Thus, in such cases the minor phase in the blend dominated. It should be noted that in both of these prediction methods the mineral matter is calculated from chemical parameters. This is done because an accurate mineral matter content cannot be determined 70

ASTM Coke Tumbler Ability

60 1.30* 1.20 1.10 50

1.00

0.95

40

0.90

30

20

0.85 0.80

10

0 20

25

30

35

40

Effective Inert Content,% *These numbers refer to percent vitrinoid reflectance

FIGURE 7.6. Coke stability (strength) diagram. To predict the coke stability of a high or medium volatile coal sample the effective content index calculated from petrographic data is plotted with the “normal” vitrinite reflectance. Note the pronounced peak in the curves, indicating an optimum inert content, and that the strength increases with rank. (Source: Iron and Steel Maker 3, R. R. Thompson and L. G. Benedict., “Coal composition and its influence on cokemaking,” 21–31, copyright 1976, with permission from Association for Iron and Steel Technology [AIST].)

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Peak Coking Pressure, PSI

70

60 1.40* 1.50 1.60

50

1.70 1.80

40

30

20 20

25

30

35

40

Effective Inert Content, % *These numbers refer to percent vitrinoid reflectance

FIGURE 7.7. Coke stability (strength) diagram. To predict the coke stability of a low volatile coal sample the effective inert content calculated from petrographic data is plotted with the “normal” vitrinite reflectance. Note the high strength for coals of this rank. (Source: Iron and Steel Maker 3, R. R. Thompson and L. G. Benedict., “Coal composition and its influence on cokemaking,” 21–31, copyright 1976, with permission from Association for Iron and Steel Technology [AIST].)

petrographically because the minerals are difficult to recognize in reflected-light microscopy and many of the minerals occur in sizes below the optical resolution limit of the petrographic microscope. Both of these methods have been unjustly criticized for not being broadly applicable. They were designed to predict the coke strength of Appalachian coals and they are very successful at this task. The assumption of two thirds of the semi-inert macerals being nonreactive in coking is valid for Appalachian coals but certainly not for high inert Gondwanaland and Western Canadian coals. Other methods are more appropriate for them. For example, the Schapiro et al. (1961) method has been modified for Illinois coals by Harrison et al. (1964) and Kaegi and Osterman (1980) and for Australian coals by Brown et al. (1964b). The work involved in developing these original methods was a huge effort, for example each correlation chart was developed from hundreds of expensive coke oven test. The work also involved some of the first strong industry/university cooperation and set an example for times to come. Another parameter of concern in coke manufacture is peak coking pressure. If the pressure is too great it can cause coke to stick in the coke oven and in extreme cases it can seriously damage an oven.

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20*

185

25

80 70 60 50 30

40

Peak Coking Pressure, PSI

30

20

35

10 9 8 7

40

6 5 4 3 *Inert Content of Low-Volatile Coal 2

1.40

1.50

1.60

1.70

1.80

Reflectance of Vitrinite, %

FIGURE 7.8. Coke pressure diagram. To predict the peak coking pressure of a low volatile coal sample the effective inert content calculated from petrographic data is plotted with the “normal” vitrinite reflectance. Note the decrease in pressure with decreasing effective inert content. (Source: A.I.M.E. Ironmaking Proc. 35, L. G. Benedict and R. R. Thompson, “Selection of coals and coal mines to avoid excessive coking pressure,” 276–288, copyright 1976, with permission from Association for Iron and Steel Technology [AIST].)

Petrographic methods have been developed to predict peak coking pressure by Benedict and Thompson (1976). As shown in Figure 7.8 under standard conditions peak coking pressure is a function of both the inertinite composition and reflectance (rank) of a given coal. The reactivity of the coke to the blast air in the blast furnace is an important coke property. Some of the concerns are that the coke will be used up too quickly and require an increase in feed rate and that the coke strength after reactivity will be reduced to the point of causing permeability problems. Coke reactivity is measured as the weight loss of coke when it reacts with CO2 under given conditions reported as the CRI and the coke strength after reactivity (CSR), a mechanical strength test after reactivity with CO2. For the best coke the CRI should be low and the

186 Applied Coal Petrology

CSR high (Valia, 1989). Recent studies well summarized by Diez et al. (2002) have shown that besides the petrographic properties of maceral composition and rank, mineral composition, and rheological properties are also important. Thus, no solely petrographic method has been developed for CSR and CRI prediction. However, the carbon forms of the coke itself may be a controlling factor. Isotropic carbon forms are more reactive to CO2 than anisotropic forms (Schapiro and Gray, 1963; Marsh and Taylor, 1975; Gray, 1989).

7.5 Quinoline Insolubles One of the main byproducts from carbonization is coal tar (8–12 gallons per ton of coal). It is usually refined into coal tar pitch, which is used to make chemicals, roofing and road tar, pipe enamels, and binder pitch in the manufacture of baked anodes and graphite electrodes. The tar contains a variety of solid carbonaceous particles that are derived from the carbonization and refining process. When the tar is dissolved in quinoline these solid particles (quinoline insolubles – QI) are filtered out for study. They are important because they influence the quality of the pitch. The particles in the QI are related to both the byproduct process and the coal (Gray and Rhoades, 1984, and Gray and Krupinski, 1997). The byproduct-related QI consists of the isotropic tar solids, normal/ primary QI, pyrolytic carbon, pitch coke, and mesophase spheres. The normal/primary QI consists of very small (not larger than a few microns) particles derived from the decomposition of hydrocarbons. The primary QI in the pitch, ideally 13–16%, enhances its value when the pitch is used as a binder in baked anodes and graphite electrodes. The primary QI helps to coat or butter the grist particles and it controls the penetration of pitch into pores. When the anode or electrode paste is carbonized, the primary QI facilitates the escape of volatiles, increases coke formation, and creates isotropic pitch coke bridges that are stronger than anisotropic ones (Krupinski and Windfelder, 1992). The pyrolitic carbon is vapor-deposited carbon that occurs as strongly anisotropic spheres or botryoidal aggregates. In some cases the actual pitch can itself be overheated and transformed into a very anisotropic coke (pitch coke). The coal-related material consists of coal and coke particles and even mineral matter that is carried over from the coke oven, usually during the charging operations when coal is put into the coke oven. In addition, some fine coal particles are partially combusted in the process, and these are included in the QI as char and cenospheres. This carryover QI is always the most undesirable kind because of its deleterious effects on tar properties and chemistry. It increases the ash content of the pitch, adds contaminating

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metal ions, increases abrasion in product materials, changes electrical resistivity, and forms sludge in tar tanks (Krupinski and Windfelder, 1992). See Figure 7.1 for photomicrographs of mesophase spheres and Figure 7.9 for photomicrographs of primary and secondary QI.

FIGURE 7.9. Photomicrographs of quinoline insolubles. (a), (b) Primary QI in pitch occurring as spheres a few micrometers in diameter. (c) Mesophase spheres in pitch surrounded by QI. (d) Pitch coke derived from pitch that was overheated. (e) A QI concentrate showing primary QI as matrix with carryover particles of coal (top center) and coal char in lower part of field. (f) A QI concentrate showing primary QI as matrix with carryover particles of mineral matter (upper center) and coal char in lower part of field. Long side of the microphotographs (a), (b), (c): 60 micrometers, and long side of the microphotographs (d), (e), (f): 215 micrometers. All images were taken in incident polarized light. (Photo credits: Ralph Gray.)

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The application of coal petrology in the case of quinoline insolubles is to determine the kinds and proportion of QI in a sample for quality control and troubleshooting operations.

7.6 Petroleum Coke When petroleum is refined, there are residues left behind that are processed into coke to yield either delayed coke or fluid coke. In the delayed coking process, various petroleum feedstocks, including vacuum and pyrolysis residues and decant oils heated to about 450
C are pumped into a drum and held for 24 hours (Adams, 1997). During this process polymerization, rearrangement, and dehydrogenation occur and three types of delayed coke, shot, sponge/regular, and needle are produced (Gray and Champagne, 1988; Gray 1991), as discussed here: l

l

l

Shot coke is the least desirable product from the delayed coker. In hand specimens it consists of clusters of small shot-like coke spheres. Its main use is as a packing material and in TiO2 production. Under the microscope it contains ribbon and lenticular anisotropic domains arranged in concentric patterns. Sponge/regular coke is frothy with cell walls and pores of varying size. It shows anisotropic mosaic structure with both ribbon and granular forms that can be layered. It is used in the production of anodes for aluminum production and as a fuel. Needle coke is the most desirable product. It is a dense coke with a well-developed lineation that causes it to have a needle-like form. It always has a clear ribbon texture in which the various anisotropic domains are elongated and parallel. During its formation the mesophase units have time to grow and coalesce and are stretched out by convection and gas streaming in the delayed coker. Because of its highly ordered structure, needle coke has a low coefficient of thermal expansion and high electrical conductivity. Its main use is in the production of graphite, especially graphite electrodes for the steel industry (see Figure 7.10).

Fluid coke is made in a continuous process in which heated feedstocks are sprayed into a fluidized bed of hot (500
C) coke particles. The feedstock is vaporized and deposited on the coke particles. The particles grow by layers until they are removed and new seed coke particles are added. The fluid coke formed shows a concentric structure of anisotropic layers. It is common for the individual coke particles

Coal Carbonization

(a)

(b)

(c)

(d)

(e)

(f)

189

FIGURE 7.10. Photomicrographs of petroleum coke. (a) Sponge coke from a delayed coker. (b) Regular delayed coke with a coarse mosaic texture. (c) Needle coke from a delayed coker; note the parallel nature of the mosaic texture. (d) Large particles of needle coke in a graphite electrode mixture of coke and pitch that has been calcined but not yet graphitized. (e) Particles of agglomerated fluid coke with a clear concentric structure. (f) A particle of fluid coke showing both a concentric and radial structure. All images were taken in incident polarized light with analyzer and a retarder plate. Long side of the microphotographs: 215 micrometers. (Photo credits: Ralph Gray.)

to agglomerate. The concentric structure of fluid coke is unique and makes it readily recognizable. The applications of petrology to petroleum coke involve distinguishing and quantifying the types of petroleum coke present in a sample (Figure 7.11) and especially determining how well the

190 Applied Coal Petrology

petroleum coke particles are incorporated into the pitch/coke mixes that are calcined at 1,200–1,400
C to produce baked carbon anodes or graphitized at 2,500–3,000
C to produce industrial graphite. Petrography can also be used to distinguish petroleum coke from metallurgical coke by its lack of inert inclusions and from graphite by its lower order interference colors in polarized light.

7.7 Weathering The topic of weathering is included in this chapter because even slight weathering in coking coals can destroy their fluid properties and render them useless for coke production. Weathering is a normal geological process that can eventually break down all rocks to sediment. Because most weathering processes occur at the Earth’s surface, usually only surface-mined coal is susceptible to weathering. Weathering is manifested in coal properties in a variety of ways. In bulk chemistry the content of oxygen, moisture, and volatile matter increases, while the sulfur content decreases. The sulfur decrease is the result of pyrite changing into soluble sulfate forms that contribute to acid mine drainage. Weathering also generates humic acids in coal that lower the pH of coal-water slurries (Iskra and Laskowski, 1967; Gray et al., 1976) and darken the color of alkali extracts. This later effect has been used by Atkinson and Hyslop (1961) and Lowenhaupt and Gray (1980) to detect and quantify the degree of weathering. Of greater importance to carbonization are the changes in fluid properties caused by weathering. The Free Swelling Index and the Gieseler Fluidity drop rapidly as weathering increases (Schmidt, 1945; Loison et al., 1963; Benedict and Berry, 1964; Gray et al., 1976; Crelling et al., 1979; Maloney et al., 1982; Marchioni, 1983; Senftle and Davis, 1984; Pis et al., 1988; Wu et al., 1988; Clemens et al., 1989; Davidson, 1990; Valia, 1990; Casal et al., 2003). This loss of fluidity destroys the coking ability of coal. Weathering also has a negative impact on the technological properties of coking coals. It reduces the particle size and can cause dusting problems and it alters the surface properties of coal so that flotation recovery is reduced (Gray et al., 1976; Crelling et al., 1979; Wu et al., 1988). In charging coke ovens a uniform bulk density is desirable and it is common practice to add oil to allow the coal particles to slide around each other and increase bulk density. When the coal is weathered the oil is taken up by the increased fracturing in the coal and bulk density control becomes difficult (Crelling et al., 1979). The negative effects of weathering on coke properties include loss of coke strength expressed as ASTM stability (Schmidt, 1945; Gray et al., 1976; Crelling et al., 1979; Pis et al., 1988; Lowenhaupt and Gray, 1980; Valia, 1990), an increase in the amount of coke breeze

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FIGURE 7.11. Photomicrographs of weathered coal. (a) Vitrinite particles with weathering cracks around the edges of the particles. (b) Vitrinite particle with discoloration around cracks and edges. (c), (d) Vitrinite particle with discoloration around cracks and edges and abundant fracturing. (e) Weathered vitrinite particle showing intense fracturing to the point of granulation. (f) Coal particles with white oxidation rims. All images were taken in incident polarized light. Long side of the microphotographs: 215 micrometers. (Photo credits: Ralph Gray.)

(½ in coke) (Crelling et al., 1979; Gray and Lowenhaupt, 1989), an increase in coking time (Crelling et al., 1979), an increase in coke reactivity (Crelling et al., 1979; Pis et al., 1988), an increase in roof carbon deposition (vapor deposition of carbon from volatile matter on the coke oven roof; Gray and Lowenhaupt, 1989), and a decrease in reactivity (Goodarzi and Murchison, 1976) and coke strength after reactivity (CSR) (Valia, 1990).

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Weathering changes to petrographic features of coal particles in several ways (Chandra, 1962, 1982; Benedict and Berry, 1964; Gray et al., 1976; Crelling et al., 1979; Lowenhaupt and Gray, 1980; Marchioni, 1983; Ingram and Rimstidt, 1984; Mathews and Bustin, 1984; Lo and Cardott, 1995). In reflected light weathered coal particles, especially vitrinite, usually show one or more of the following features: a lower reflectance, giving the weathered particles a darkish tint; dark reaction rims; increased fractures that are unrelated to cleat direction (in advanced weathering the particles can be so fractured that they become particle aggregates); high relief and sometimes discoloration around particle borders and maceral boundaries; and an etched and pitted appearance. These features are illustrated in Figure 7.11. It should also be noted that bright reaction rims around particles are due to thermal alteration and not natural weathering (Gray et al., 1976; Bend and Kosloski, 1993). Weathering has also been reported to reduce the anisotropy of the cell walls in coke made from weathered coal (Grint et al., 1983; Qian and Marsh, 1984; Mochida et al., 1986; Casal et al., 2003). In regard to the petrography of cokes made from weathered coal, the coke developed less anisotropy and smaller anisotropic units that coke made from the corresponding fresh coal (Goodarzi and Murchison, 1976; Yokono et al., 1981; Qian and Marsh, 1984; Mochida et al., 1986). Coal petrology has been very successfully applied to general aspects of carbonization such as quality control and general troubleshooting and specific problems such as predicting coke strength from both single coals and coal blends, predicting peak coking pressures, and predicting coke reactivity. Coal petrologic techniques have also been used to evaluate the nature of quinoline insolubles in coal tar pitches, to detect weathering in coal, and to evaluate petroleum cokes.

CHAPTER 8

Coal-Derived Carbon Materials Isabel Sua´rez-Ruiz John C. Crelling

8.1 Introduction Coal as a raw material (Chapter 1) is mainly used for the generation of power and in the iron and steel industry (Chapters 4 and 7). However, coal and its byproducts are also used in alumina refineries, the chemical and pharmaceutical industry, and in other areas such as the manufacture of paper (World Coal Institute, 2005). The nonfuel applications of fossil fuel resources are mainly dominated by petroleum and its derivatives, but coal may also be employed as a source of organic chemicals and carbon-based materials (Song and Schobert, 1993, 1996; Schobert and Song, 2002). To obtain chemicals from coal, Schobert and Song (2002) have reported several strategies that include: (1) gasification followed by a sequence of carbon chemistry processes, (2) the conversion of coal via carbonization, pyrolysis, or liquefaction to liquids and tars followed by the conversion of the components of these derived products to higher value products, (3) the direct conversion of coals to chemicals or materials, and (4) the coproduction of chemicals or materials and fuels along with electricity. Carbon materials are described as materials that are composed of a high percentage of carbon. Since all coals are carbon-rich solids, they can be used as precursors for new carbon-based materials. In fact, a wide range of solid carbon materials can be obtained from coal and its byproducts, either from coal itself or in combination with precursors obtained from other, different sources. Given the wide variety of procedures and conditions available for the preparation of carbon materials, the applications of carbon-based products cover a wide field that is constantly expanding.

Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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This chapter is mainly devoted to solid carbon materials derived from coal and coal byproducts, in which the conventional methods of coal petrology play an important role. Other coal-derived materials such as those obtained from the combustion, gasification, liquefaction, and carbonization processes (Chapters 4 to 7) have already been described and therefore they are not included in this chapter. To conclude this section, we make a brief reference to carbon materials derived from precursors other than coal.

8.2 Raw Materials and Precursors of Carbon Materials Coal, petroleum, organic chemicals, and biomass are the main raw materials from which precursors of carbon materials are obtained. Depending on their rank and composition, raw coals may be used directly as precursors to obtain solid carbon materials. For example, blends of bituminous coals have traditionally been used for metallurgical coke production (Chapter 7); peat, lignite, bituminous, and anthracite/semianthracite coals have been investigated for producing activated carbons (Derbyshire et al., 1995; Derbyshire, 1998; Sych et al., 2006; Ruiz et al., 2006, among others), including molecular sieve materials (Lizzio and Rostam-Adabi, 1993; Gergova et al., 1995), and anthracites have been carbonized and graphitized at experimental scale to prepare synthetic graphites (Oberlin and Terriere, 1975; Bustin et al., 1995a; Duber et al., 1993; Atria et al., 1993, 2002; Gonza´lez et al., 2003a,b). Solid residues with a high carbon content from coal utilization, such as fly ash carbons, are at present being investigated as precursors of activated carbons in the manufacture of molded carbon artifacts (Schobert and Song, 2002) and to obtain synthetic graphite (e.g., Cabielles et al., 2006; Rouzaud et al., 2007). Moreover, potential applications exist for unburned carbons as a filler material for carbon composites in which the binder material is a coal-tar pitch (Andre´sen et al., 2000) as well as in automobile brakes (Chapter 11). In the case of coal byproducts, coal tar is produced during the coal conversion (carbonization process, Chapter 7) of bituminous coals and coal blends to obtain metallurgical coke. Coke is a highly macroporous and graphitizable carbon material (summaries in Diez et al., 2002) and is used in the iron and steel industry in blast furnaces for the production of iron. Pitches are obtained from coal tar following a well-known and fully described industrial process (e.g., summaries in Granda et al., 2003, and references therein). Coal-tar pitches are in turn used as precursors for chemicals and carbon materials. Mesophase pitches obtained from petroleum or coal-tar pitches (Figure 8.1) are also used as precursors of carbon materials, with very good results. Mesophase pitch (the anisotropic fraction of pitches)

Coal-Derived Carbon Materials

(a)

(c)

50 µm

195

(b)

(d)

FIGURE 8.1. Optical microscopy. Photomicrographs (a, b) taken in polarized light, with an oil immersion objective and one-wave retarder plate, and (c, d) in polarized light and a dry objective. Mesophase formation in petroleum (a) and coal-tar (b) pitches. Heterogeneous size distribution of the mesophase spheres in both pitches. Irregular edges in the spheres of the mesophase obtained from the coal-tar pitch (image b) due to the primary quinoline insoluble particles. (c, d) Dynamic evolution of the mesophase during pyrolysis of a petroleum pitch as seen by hot-stage microscopy (HSM). (Sources: Images (a): Carbon 41, E. Mora, C. Blanco, R. Santamarı´a, M. Granda, and R. Mene´ndez, “A novel method to obtain petroleum-derived mesophase pitch suitable as carbon fibre precursor,” 445–452, copyright 2003. Image (b): Carbon 41, M. Pe´rez, M. Granda, R. Santamarı´a, and R. Mene´ndez, “Preventing mesophase growth in petroleum pinches by the addition of coal-tar pitch,” 1851–1864, copyright 2003, with permission from Elsevier.) Images (c) and (d): photo credits: M. Granda.

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displays the properties of a liquid crystal made up of basic units similar to spheres. It exhibits a pre-graphitic structure that is completely developed after heating at 2,500 C. These pitches and the materials obtained from them have special properties. There are several procedures for obtaining mesophase pitches from petroleum and petroleum products (Park and Mochida, 1989; Mochida et al., 1990; Matsumoto, 1994; Hutchenson et al., 1991). In the case of coal-tar pitches, mesophase (Figure 8.1) is generated during heating at temperatures higher than 400 C. Granda et al. (2003) provide several good examples, monitored via optical microscopy, of the formation and evolution of mesophase in two different coal-tar pitches during thermal treatment. The separation of the isotropic and the anisotropic phases of coal-tar pitches (see procedures reported in Singer et al., 1987; Kodama et al., 1988; Gschwindt and Hu¨ttinger, 1994; Blanco et al., 1997, 2000a; Granda et al., 2003) enables these individualized fractions to be used as precursor materials for the preparation of a wide variety of highdensity carbon materials. Since the isotropic and anisotropic phases obtained from coal-tar pitches have different nature and pyrolysis behaviors, their cokes have different microstructures (Blanco et al., 2000a; Granda et al., 2003). The isotropic fraction generates a coke with needle-type microstructures, indicating a system of low viscosity. The anisotropic fraction yields a coke made up of small crystallites because this fraction generates high viscosity systems upon carbonization, whereby the coalescence and formation of large anisotropic regions are restricted. Indicative of the different properties exhibited by these two phases is the fact that the isotropic phase from thermally treated coal-tar pitch has been used as a precursor for the preparation of general-purpose carbon fibers (Prada et al., 1999a). By contrast, the anisotropic part has been demonstrated to be more appropriate (for example) for preparing high-density polygranular graphites (Fanjul et al., 2000, 2001). As an example of precursor material other than coal is petroleum coke (Chapter 7), which is a solid product used in the preparation of carbon materials. Green petroleum coke is a final pyrolysis product of heavy petroleum fractions that has a high carbon content and an anisotropic texture. To remove whatever residual volatiles after pyrolysis, it is calcined at 1,200–1,400 C to be used as a grist fraction in the production of baked anodes (aluminum industry) and graphite electrodes. Other precursors that are used for preparing carbon materials include pitches derived from the thermal treatment and distillation of biomass (e.g., eucalyptus wood) and synthetic pitches obtained from aromatic compounds (such as naphthalene) or polymers (through pyrolysis). Charcoal, which is the carbon residue obtained from the pyrolysis of biomass at temperatures of 400–700 C, is also employed as a

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precursor in the production of adsorbent materials due to its porosity properties. Other chemicals are also used as precursors of carbonbased materials.

8.3 Optical Microscopy Approach to the Characterization of Coal-Derived Carbon Materials Because of the various precursors and processes used in the preparation of carbon materials, these exhibit a wide variety of properties. For this reason, a broad range of methods and techniques must be employed to ensure an efficient characterization of their mechanical, physical, and chemical properties. For the characterization of the structure, texture, and other physico-optical properties of solid carbon materials and their precursors, the procedures typically used in organic petrography play a major role. The information provided by petrographic analysis has been demonstrated to be very useful. This is because the relevant parameters obtained from the microscopic analysis of a material or precursor can be correlated with other analytical information and help to clarify the properties and behavior of a particular carbon material as has been shown (for example) by Bensley et al. (1994a,b), Crelling and Gray (1998), and Blanco et al. (2003) in their work on fibers and composite materials. When petrographic analysis is applied to the study of coalderived carbon materials, it is important to characterize the coal as a raw material (Chapter 2), the precursors of the carbon materials and the steps followed for the production and treatment of these precursors (see for different precursors Ferna´ndez et al., 1995a,b; Mene´ndez et al., 1997a; Casal et al., 1999; Prada et al., 1999b; Granda et al., 2003; Me´ndez et al., 2003a; Perez et al., 2003; Gonza´lez et al., 2004; Rocha et al., 2005; Calvo et al., 2005; and Panaitescu and Predeanu, 2007). The evolution of these precursors, once they have been subjected to the various processes employed for preparing carbon materials, can be successfully monitored by means of optical microscopy, which will help optimize the processing of the precursor and contribute to a more accurate prediction of the properties of the prepared materials (Mene´ndez et al., 1997a). The structure and texture of some of the resultant carbon materials can be analyzed via optical microscopy (Mene´ndez et al., 1997a; Blanco et al., 2003) to complement the information obtained from other high resolution techniques (such as SEM, TEM, scanning probe microscopies, atomic force and scanning tunneling, X-ray diffraction, Raman spectroscopy, etc.). All this information in turn can be correlated with some properties (Figueiras et al., 1995; Mene´ndez et al., 1997a; Granda et al., 2003; Klett et al., 2000),

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physical structures (Figueiras et al., 1995, 1998; Mene´ndez et al., 1997a; Granda et al., 2003; Me´ndez et al., 2003b; Ruiz et al., 2006), and degree of graphitization achieved (Sua´rez-Ruiz and Garcı´a, 2007) in the prepared materials.

8.3.1 Petrographic Characteristics The procedures applied to the characterization of carbon materials and their precursors in optical microscopy are derived from those conventionally used for characterizing coals (Chapter 2). Microscopic examination is performed on pellets that can be prepared following the standard procedures (Chapter 2) or, as in most cases, using modified procedures adapted to the characteristics of each specific carbon material (e.g., those used by Casal et al., 2001a in their study on unidirectional carbon/carbon composites). The most relevant characteristics to be analyzed by the microscopic examination of carbon materials and their precursors are those related to structure, texture, and the degree of structural order, which can be summarized as follows: l

The optical texture (surface appearance), i.e., the isotropic or anisotropic character (Figures 8.1 and 8.2). For anisotropic materials or anisotropic components in a material, the type, size, shape, and distribution of the anisotropic textures are important aspects that must be taken into account. There are several classification systems for optical textures, e.g., that of Gray (1991) for coke and binder-phase carbon included in Chapter 7. A simpler system of classification for describing the optical textures of carbon materials (Table 8.1 and Figure 8.3) was reported by Edwards (1989). This author also proposed the optical texture index (OTI) to characterize these materials because the OTI provides a measure of the overall anisotropy of the sample that is being analyzed. Examples of the successful application of optical texture analysis in the preparation of carbon materials are provided by Blanco et al. (2003), who observed the generation of different anisotropic textures (mosaics and domains classes, Figure 8.3) in the matrices of carbon/carbon composites depending on the type of pitch used as precursor (coal-tar, petroleum, and liquefaction pitches). In turn these factors influenced the type of fiber/matrix interface and the properties (e.g., fracture behavior) of the resulting material. The different anisotropic textures (Figure 8.3) are also related to the thermal and electrical conductivities of the carbon materials. On the other hand, Ruiz et al. (2006) observed a relationship between the isotropic/

Coal-Derived Carbon Materials

(a)

40 µm

(b)

15 µm

(c)

15 µm

(d)

10 µm

199

FIGURE 8.2. Optical microscopy (a, b) and scanning electron microscopy (c, d) photomicrographs. Images (a) and (b) were taken in polarized light, with an oil immersion objective and one-wave retarder plate. (a) Isotropic (polymerized) and anisotropic phases in a pyrolyzed petroleum pitch (450 C). The anisotropic fraction is made up of mesophase spheres of various sizes and regions of coalesced mesophase. (b) Unidirectional carbon-carbon composite. The reinforcement is made up of isotropic polyacrylonitrile (PAN) carbon fibers and the matrix is based on a pitch exhibiting an anisotropic optical texture of mosaic and lamellar type. SEM images of (c) isotropic and nongraphitic carbon fiber, and (d) a graphitic fiber showing a partially oriented structure. (Sources: Images (a): Carbon 41, M. Pe´rez, M. Granda, R. Santamarı´a, and R. Mene´ndez, “Preventing mesophase growth in petroleum pinches by the addition of coal-tar pitch,” 1851–1864, copyright 2003. Image (b): Carbon 39, M. Granda, E. Casal, J. Bermejo, and R. Mene´ndez, “The influence of primary QI on the oxidation behavior of pitch-based C/C composites,” 483–492, copyright 2001, with permission from Elsevier.) Images (c) and (d): photo credits: M. Granda.

anisotropic ratio and the improvement in the textural properties (surface area and microporosity) of the materials obtained from an oxidized semianthracite when preparing activated carbons. The classes of anisotropic textures were also used by Klett et al. (2000) to assess the properties of carbon foams prepared from commercial pitches.

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TABLE 8.1 Optical textures and their classes that may be observed in carbon materials (modified from Edwards, 1989) Optical Texture Isotropic Anisotropic

Classes

Size

No optical activity Fine mosaics Medium mosaics Coarse mosaics Granular flow Coarse flow Lamellar

Diameter <0.8 micrometers 0.8 < diameter <2.0 micrometers 2.0 < diameter <10.0 micrometers Length >2; Width >1 micrometers Length >10; Width >2 micrometers Length >20; Width >10 micrometers

(a)

(b)

(c)

(d)

50 µm

FIGURE 8.3. Optical microscopy. Photomicrographs taken in polarized light, with an oil immersion objective and a one-wave retarder plate. Examples of isotropic and anisotropic optical textures developed in a coke obtained from a polymerized anthracene oil with variable percentages of sulfur acting as catalyst. (a) Isotropic optical texture without optical activity. (b) Anisotropic optical texture. Mosaics class with a size <10 mm. (c) Anisotropic optical texture. Flow class or domains with a size between 10 and 60 mm. (d) Anisotropic optical texture. Lamellar class with a size >60 mm. (Source: Energy and Fuels 12, A. Ferna´ndez, M. Granda, J. Bermejo, R. Mene´ndez, and P. Bernad, “Carbon precursors from anthracene oil. Insight into the reactions of anthracene oil with sulfur,” 949–957, copyright 1998, with permission from American Chemical Society [ACS].)

Coal-Derived Carbon Materials l

l

201

The development of specific structures and/or components when the precursor materials are thermally treated at high temperatures via carbonization (e.g., the generation of mesophase spheres upon pitch carbonization, Figures 8.1 and 8.2), or graphitization processes (e.g., development of specific graphitic particles such as microspheres and flakes upon the graphitization of anthracites, Figure 8.4). The generation and dynamic evolution of the mesophase during pitch pyrolysis (Figure 8.1) is a characteristic that can be also monitored by hot-stage microscopy (HSM). The temperature of pitch softening, initiation of mesophase formation, mesophase sphere coalescence, and temperature of hardening are all relevant parameters to be taken into account, e.g., when selecting the composite processing (pressing/molding) temperatures (Figueiras et al., 1998). The evolution (development or reduction) of voids; the type of voids (fissures, cracks, channels between pores, porosity, etc.); their size, shape, orientation, distribution, open/filled

(a)

(b)

(c)

(d)

30 µm

FIGURE 8.4. Optical microscopy. Photomicrographs taken in polarized light, with a one-wave retarder plate and an oil immersion objective. (a) Microstructure and anisotropic texture of a natural graphite. (b–d) Aggregate microstructures, flakes, and microspheres developed during the high temperature treatment (2,600 C) of a carbonized anthracite (1,000 C) used as precursor material for preparing synthetic graphites. (Photomicrographs: I. Sua´rez-Ruiz.)

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porosity, etc. (Figure 8.5). Blanco et al. (2003) established a relationship between the development of voids and the type of precursor material, the conditions used in the preparation processes, and the final properties of the resulting composite. Other examples are provided, e.g., by Casal et al. (1999), who studied the reduction of porosity during the carbonization of tow pregs after their oxidative stabilization. Me´ndez et al. (2003a) analyzed the interaction of pitches with different types of granular carbons (anthracite, foundry, green pet coke, and graphite) and the subsequent effects on the evolution of microstructure and porosity in the prepared carbon composites (Figure 8.6). Ruiz et al. (2001, 2006), Casal et al.

(a)

(c)

200 µm

(b)

30 µm µm 30

(d)

10 µm

15 µm

FIGURE 8.5. Optical microscopy. Photomicrographs taken in polarized light, with a one-wave retarder plate and an oil immersion objective. Examples of some different types and distribution of porosity developed in unidirectional carbon-carbon composites. (a) Development of pores during carbonization and processing of a composite. Irregular distribution of the porosity generated in the carbonization step. (b) Development of cracks mainly at the fiber/ matrix interfaces in a composite material. (c, d) Porosity and cracks (arrows) filled after densification by means of liquid impregnation with a pitch. (Source: Journal of Microscopy 185, R. Mene´ndez, M. Granda, J. J. Ferna´ndez, A. Figueiras, J. Bermejo, J. Bonhomme, and J. Belzunce, “Influence of pitch air: blowing and thermal treatment on the microstructure and mechanical properties of carbon/carbon composites,” 146–156, copyright 1997, with permission from The Royal Microscopical Society [Wiley-Blackwell Publishing].)

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l

203

(2001a,b), and Calvo et al. (2005) investigated the generation of porosity in the preparation of other carbon materials such as activated carbons, unidirectional C/C composites, and carbon foams. The distribution of components (Figure 8.6), e.g., in the matrix of a composite material, such as the alignment/ misalignment of carbon fibers (Casal et al., 2001a), mesophase alignments within carbon-fiber bundles (Zimmer and White,

150 µm

(a)

(b)

20 µm

(c)

50 µm

FIGURE 8.6. Optical microscopy. Photomicrographs taken in polarized light, with a one-wave retarder plate and an oil immersion objective. Distribution of components in different carbon-carbon composite materials. (a) Misalignment of carbon fibers, and pore development during the wet-winding step due to the fiber misalignment, in a unidirectional carbon-carbon composite. (b) Distribution of components in a composite material made up of a granular reinforcement of anthracite in a pitch-based matrix. (c) Components in a coal-tar pitch coke. Flow and mosaic anisotropic textural classes, and aggregate of primary quinoline insoluble (QI) particles. (Sources: Image (a): Journal of Microscopy 201, E. Casal, M. Granda, J. Bermejo, and R. Mene´ndez, “Monitoring unidirectional carbon-carbon composite processing by light microscopy,” 324–332, copyright 2001. Image (b): Journal of Microscopy 209, A. Me´ndez, R. Santamaria, M. Granda, and R. Mene´ndez, “The effect of the reinforcing carbon on the microstructure of pitch-based granular composites,” 81–93, copyright 2003. Image (c): Carbon 33, J. J. Ferna´ndez, A. Figueiras, M. Granda, J. Bermejo, J. B. Parra, and R. Mene´ndez, “Modification of coal-tar pitch by air-blowing–II. Influence of coke structure and properties,” 1235–1245, copyright 1995, with permissions from The Royal Microscopical Society [Wiley-Blackwell Publishing] and Elsevier.)

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l

1983), mesophase spheres distribution (Mora et al., 2003a,b), homogeneous/heterogeneous distributions due to, e.g., the presence of components such as quinoline insolubles (Figueiras et al., 1998; Blanco et al., 2003; Panaitescu and Predeanu, 2007), etc. Indeed the ability to control the distribution of the components is of great importance because the microstructure of the materials strongly influences their final properties (McEnaney and Mays, 1993). The interactions and quality of the interfaces between two materials (e.g., carbon fiber/carbon matrix interfaces in the case of composite materials that use carbon fibers as filler). It is also possible to estimate the degree of union between the components, the debonding characteristics (Figure 8.7) at

15 µm

(a)

(c)

5 µm

(b)

100 µm

FIGURE 8.7. Optical microscopy (a, c) and scanning electron microscopy (b) photomicrographs. Images (a) and (c) were taken in polarized light, with an oil immersion objective and a one-wave retarder plate. Interaction between material components in two different carbon-carbon composites. (a) Quality of carbon fiber/matrix interfaces, intramatrix cracks, and cracks at the fiber/ matrix interfaces due to the absence of QI in a composite material. (b) SEM image showing the intramatrix cracks and development of debonding between the fiber and the pitch-based matrix in the same composite. (c) Excellent wetting behavior of the coal-tar pitch on the calcined regular petroleum coke during the blending step (paste) for carbon anodes. (Source Image (b): Journal of the European Ceramic Society 23, C. Blanco, E. Casal, M. Granda, and R. Mene´ndez, “Influence of fibre-matrix interface on the fracture behavior of carbon-carbon composites,” 2857–2866, copyright 2003, with permission from Elsevier.) Images (a) and (c): photo credits: M. Granda.

Coal-Derived Carbon Materials

l

205

the fiber/matrix (Ahearn and Rand, 1996; Appleyard and Rand, 2002; Blanco et al., 2003), the wetting behavior (Figure 8.7) in pitch/coke systems (Gray, 1991; Rocha et al., 2005), the presence of flaws, artifacts, etc. The amount of the various components or textures that may be present in a carbon material and the ratios between the components and/or characteristics, e.g., the fiber/matrix ratio in a composite (Casal et al., 2001b), the ratio of isotropic/anisotropic components (Ruiz et al., 2006), or specific structures (graphitic structures in Sua´rez-Ruiz and Garcı´a, 2007), the amount and size of solids in tars and pitches (Gray, 1991). These characteristics are related to the type of precursor, the conditions used during their preparation, and to the properties of the prepared material.

Crelling et al. (1997) provided a good example of the suitability of petrographic methods when characterizing materials for which a polymer resin was used as a binder phase in carbon-carbon composites as in the case of aircraft brakes (Figure 8.8) and when determining, for example, the amount of voids, the presence of additive materials, and the anisotropy and sequence of vapor deposited carbon.

8.3.2 Quantitative Determinations For a qualitative and/or quantitative analysis of the structures and textures, optical microscopy analyses are carried out using oil immersion objectives of various magnifications, and polarized light. A retarded plate needs to be incorporated into the microscopic system to enhance the contrast between the components so that the interference colors can be observed, particularly when increasing the orientation and ordering of the graphitic lamellae in the examined material. For a quantitative analysis, a point counter should be used. The procedure to be followed in point-counting textures and/or structures or any other physical characteristic is similar and derived from that described for coal maceral analysis (Chapter 2). Thus, quantification should be performed by recording 500 individual determinations using an interpoint or interline distance of 0.5 mm. The number of recorded counts is then transformed to a percentage by volume of the total. Optical microscopic analysis can be implemented using scanning electron microscopy analysis, to study the orientation of lamellar structures, porosity, and for specific detailed observations (Figures 8.2, 8.7, and 8.9). Usually any increase in the structural order of carbon-based materials that have been subjected to high temperature treatments (e.g., graphitization process) is measured via X-ray diffraction, Raman

206 Applied Coal Petrology

(a)

20 mm

(b)

20 mm

(c)

20 mm

(d)

20 mm

FIGURE 8.8. Optical microscopy. Photomicrographs of carbon/carbon aircraft brakes taken in oil in incident polarized light with a retarder plate. (a) Aircraft brake showing multidirectional carbon fibers. (b) Aircraft brake, the bright white areas are silicon metal and the hexagonal crystals associated with the white area are silicon carbide. (c) Aircraft brake that is perpendicular to the long axis of the fibers; the fibers are surrounded by a ring of anisotropic carbon that was deposited as a vapor. (d) Aircraft brake that is perpendicular to the long axis of the fibers; the fibers are surrounded by alternating rings of isotropic and anisotropic carbon that were deposited as a vapor. Because isotropic and anisotropic carbons are deposited at different temperatures, these multiple rings indicate temperature fluctuations in the manufacturing process. (Photo credits: B. Huggett.)

spectroscopy, and transmission electron microscopy (TEM). However, by means of optical microscopy it is also possible to obtain a series of parameters derived from reflectance measurements (e.g., maximum, minimum, and bireflectance determinations) that are related to the increase in the textural anisotropy in carbon materials. These parameters also provide useful information on the degree of structural order (Bensley et al., 1994b; Duber et al., 2000; Pusz et al., 2002; Gonzalez et al., 2004; Sua´rez-Ruiz et al., 2006b; Sua´rez-Ruiz and Garcı´a, 2007), which can in turn be related to the development of certain physical properties in the carbon materials. The fundamentals of anisotropy and the development of bireflectance in organic materials are summarized in Stach et al. (1982), Taylor et al. (1998), and Kilby (1988, 1991), among others, and are also mentioned in Chapter 2.

Coal-Derived Carbon Materials

(a)

15 µm

(b)

15 µm

500

400 10 µm

300 200 100

Stress (MPa)

Stress (MPa)

500

207

Brittle behavior

400 300 200 100

Pseudo-plastic behavior

5 µm

0

0

2 4 6 8 Strain (x 10-3 mm mm-1)

10

0

0

2 4 6 8 Strain (x 10-3 mm mm-1)

10

FIGURE 8.9. Optical microscopy. Photomicrographs (a) and (b) were taken in polarized light, with a one-wave retarder plate and an oil immersion objective. Relationships between the microtexture of the matrices and the carbon fiber/ matrix interfaces in two carbon-carbon composites and their mechanical behavior (with data from Figueiras et al., 1998, and personal communication of M. Granda). (a) Carbon-carbon composite with laminar microstructures. Anisotropic flow textures or small domains in the matrix of the composite promote weaker interfaces. Therefore this material shows a pseudoplastic behavior and a lower strength caused by the debonding of fiber/matrix interfaces during the test. The “pull out” effect in the carbon fibers at the fracture surface (as shown in the SEM image) is a typical characteristic of these materials. (b) Carbon-carbon composite with granular microstructures. Mosaic optical textures in the matrix of the material induce a stronger fiber/matrix interface in the resulting composites. These composites display a higher strength but they have a brittle behavior with a catastrophic failure mode (SEM image for the fracture type). (Photo credits: M. Granda.)

The parameters obtained from reflectance measurements have been used with very good results in the characterization of carbonized (Pusz et al., 2002; Gonzalez et al., 2004) and graphitized anthracites (Sua´rez-Ruiz and Garcı´a, 2007), some of which have been employed as precursors to obtain synthetic graphites. In a similar way, Bensley et al. (1994a,b) used rotational polarization reflectance to quantitatively assess the orientation of carbon within the matrix of composite materials. They also studied the characteristics of isotropy/anisotropy to ascertain the structural relationships between carbon fibers and the

208 Applied Coal Petrology

matrix, and to assess the microstructural characteristics of specific fibers. In oriented carbon fibers these authors discovered a relationship between the fiber anisotropy and its tensile modulus (GPa). They established a relationship between the degree of anisotropy by using rotational polarization reflectance and the thermal and mechanical properties of the parent polyacrylonitrile (PAN) fibers. Optical anisotropy measurements in connection with reflectance determinations were also used to characterize pyrocarbons (summaries in Bourrat, 2000). Reflectance measurements to establish the degree of anisotropy in carbon-based materials are carried out under polarized light to determine the maximum and minimum reflectances (usually apparent values) of the material. It is then necessary to follow a methodology (e.g., that proposed by Kilby et al., 1988, 1991, and Duber et al., 2000) designed to calculate the real maximum and minimum reflectances, bireflectance and anisotropy, and other related parameters that serve to characterize the structural order of carbon materials subjected to high temperature treatments. Isotropic and disordered materials or materials with a low structural order can also be characterized by reflectance measurements, but the type of information obtained is different. Crelling and Bensley (1995) observed that for a similar degree of structural disorder, the variation in reflectance values is related to the density of the material. They found that the higher the density, the higher the reflectance, which is a significant characteristic that must be taken into account when preparing activated carbons. Other quantitative information, such as that related to the heterogeneity of the carbon materials, can be also obtained from reflectance determinations. Crelling and Bensley (1995) showed that mapping the reflectance distributions in some carbon-carbon composites was useful for quantitatively assessing the heterogeneity of the carbon material. Examples of petrographic applications are the characterization of baked anodes and graphite electrodes. Baked anodes (Figure 8.10) are blocks of carbon that are prepared by mixing ground petroleum coke and recycled pieces of used anode with coal-tar pitch and forming them into blocks that are pyrolyzed at 500–600 C to carbonize the pitch. The blocks are then calcined at 1,200–1,400 C before they are used in the aluminum smelting process. Petrographic examination of the anode samples is the only way to control the process and to determine whether insufficient pitch was used in the mix (underpitched) or if too much pitch was used (overpitched). Both of these conditions lead to poor structural integrity, which causes the anodes to fail through fracture. The other aspects of petrographic analysis listed previously, such as composition, void space, and the like, can also be applied to anodes.

Coal-Derived Carbon Materials

(a)

20 µm

209

20 µm

(b)

B

(c)

20 µm

(d)

20 µm

FIGURE 8.10. Optical microscopy. Photomicrographs of baked carbon anodes taken in oil in incident polarized light with a retarder plate. (a) Baked carbon anode. Large particle at left is calcined petroleum coke and the dark area is a pore. The more granular material at right is carbonized and calcined coal-tar pitch. (b) Baked carbon anode showing a large anthracite particle at left; the spots on the anthracite are vapor deposited carbon. (c) Ideal well-bonded baked anode. The larger particles are calcined petroleum coke. (d) Anode that has insufficient pitch (underpitched), causing excessive pore space and poor bonding. (Photo credits: R. Gray.)

Graphite electrodes used in the steel industry for melting scrap iron in an electric arc furnace are produced by mixing ground petroleum coke, in this case needle coke (Chapter 7), and coal-tar pitch and extruding the mix to align the coke particles, thereby improving electrical conductivity. The electrode is first pyrolyzed at 1,200– 1,400 C and then graphitized. The graphitization process typically takes up to 48 days at temperatures in the 2,500–3,000 C range. As with the baked anodes the graphite electrodes need great structural integrity to withstand the harsh conditions in an arc furnace. Petrographic analysis is able to detect overpitched and underpitched conditions as well as other properties. Other examples of the effective use of optical microscopy for some of the well-known coal-derived carbon materials are described in the following section.

210 Applied Coal Petrology

8.4 Carbon-Based Materials from Coal 8.4.1 Carbon Fibers and Carbon-Carbon Composites Carbon fibers and carbon-carbon composites, their precursors, preparation procedures, properties, and applications, have already been extensively investigated and described elsewhere (e.g., Lubin, 1982; Shinamura, 1984; Watt and Perov 1985; Bunsell, 1988; Pierson, 1993; Chungh, 1994; Peebles, 1994; Donnet et al., 1998; Inagaki, 2000, for carbon fibers and Buckley and Edie, 1992; Savage, 1992; Thomas, 1993; Delhae`s, 2003, for carbon-carbon composites). Carbon Fibers Carbon fibers are solid materials of a fibrous morphology that have a minimum carbon content of 92 wt percent and a nongraphitic or graphitic structure (Figure 8.2). They are used in many applications but are mainly used as a filler or reinforcement material to form several types of composites with inorganic or organic matrices. Carbon fibers can be classified according to their modulus and strength properties into high tensile strength and high modulus carbon fibers (Shinamura, 1984). Other classifications refer to them as high performance-type fibers and general-purpose carbon fibers (HP and GP, respectively) depending on their corresponding applications (Yoon et al., 2000). To obtain carbon fibers, PAN, petroleum and coal-tar pitches, and resins are used as precursor materials. Depending on the precursor and chemical and technological process employed, a wide variety of carbon fibers may be obtained that differ in their properties. Carbon fibers display a wide range of structures (Zimmer and White, 1983; Edie et al., 2000) and nanotextures in the sections parallel and perpendicular to the fiber axis, all of which may have an effect on their mechanical properties. The quality of the carbon fibers depends on the structure and assembly of the aromatic constituents and their alignment in the fiber, which in turn reflects the structure of the precursors and the chemistry involved in the preparation of the fibers. The preparation procedures of these materials using pitches as precursors have certain steps in common, which Yoon et al. (2000) summarized as (1) spinning into homogeneous thin fibers from a solution prepared from the precursor material, (2) stabilization of the fibers by oxidation or thermal procedures, and (3) carbonization over a different range of temperatures and times according to the precursor material used. For high performance and high conductivity carbon fibers and graphitic fibers, additional treatments such as graphitization between 2,000–3,000 C and surface treatments to improve wettability and adherence to the matrix are required.

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Carbon fibers from precursors other than pitches include vapor grown fibers that are obtained from the pyrolytic decomposition of a hydrocarbon. These present various sizes and structures and exhibit a high degree of crystallinity. Such carbon fibers display good electrical properties. Carbon fibers prepared from pitches are characterized by a higher structural order than those obtained from a polyacrylonitrile precursor; therefore, they show a higher modulus, which gives them a higher conductivity and density and a lower expansion coefficient. However, polyacrylonitrile-derived carbon fibers, due to their much more disordered structure, exhibit a higher tensile strength. Fibers prepared from isotropic pitch fractions display an isotropic texture in the sections horizontal and perpendicular to the fiber axis, and so these fibers have a low performance with poor mechanical properties (relatively low tensile properties). They are therefore classified within the group of general-purpose carbon fibers, which are used in the field of chemicals, petrochemicals, structural materials, and automobiles. Carbon fibers of very high performance are prepared from mesophase pitches, which in turn are obtained from QI-free coal-tar pitches, petroleum pitches, and even from polycyclic aromatic hydrocarbon by catalytic polymerization. These fibers show a high Young modulus, high electrical and thermal conductivities, and a strong oxidation resistivity, in addition to having moderate tensile and low compressive strengths because of their graphitizability (Yoon et al., 2000). The fibers obtained from mesophase pitch are anisotropic and therefore exhibit a wide variety of textures/structures such as radial, onion skin, random, flat-layer, oriented, etc. (Zimmer and White, 1983; Edie, 1989; Edie et al., 2000). The anisotropy of the graphitic texture of these fibers determines their properties to some extent. However, the shape, texture, morphology, and size of the fibers can be modified by changing the preparation steps of the precursor. For example, the carbonization and graphitization of the precursor material under tension may improve the degree of anisotropy and orientation in the direction of the fiber axis and in turn the mechanical properties of the resulting carbon fibers. The mechanical strength and the Young’s modulus are used to measure the mechanical properties of carbon fibers when they are to be employed as filler material. The Young modulus depends on the orientation of the graphitic crystals inside the fiber and on its tensile strength, which is influenced by flaws in the fiber such as inclusions, holes, fissures, and similar features that to some extent may be monitored by petrographic analysis. Optical microscopy, particularly in polarized light but also in reflectance mode, has been used to determine the size of carbon fibers, their degree of anisotropy, the length and width of anisotropic domains and textures, and defects in the fiber

212 Applied Coal Petrology

structures (Crelling and Gray, 1998). All these features are related to the mechanical and structural properties of the fibers and, in turn, to their quality (Bensley et al., 1994a). Depending on the final application of the carbon fibers, Yoon et al. (2000) summarized a series of characteristics that also need be taken into account, such as density, surface area, electrical resistivity, and the like. Carbon-Carbon Composites Carbon-carbon composites are types of carbon material that are made up of a reinforcement or filler (e.g., carbon fibers, granular carbons of a different nature) that may vary in their nature and geometry (e.g., unidirectional, bidirectional, and multidirectional, in the case of the carbon fibers) and a carbon matrix produced from polymers, resins, or pitches (Buckley and Edie, 1992; Savage, 1992; Thomas, 1993; Ahearn and Rand, 1996; Murdie, 1997; Casal et al., 2001a; Appleyard and Rand, 2001; Me´ndez et al., 2003a; Delha`es, 2003, and references therein). These composites are lightweight and dense materials with high mechanical performances, a high thermal resistance in a nonoxidant atmosphere, and inertia toward various chemical agents. In an oxidant atmosphere and at temperatures higher than 400–500 C these composites are reactive materials. Carbon-carbon composites have many fields of application. The two basic procedures for preparing carbon-carbon composites are carbon vapor deposition (CVD) and liquid impregnation. In the former procedure, the carbon fibers are subjected to a flow of hydrocarbon gas that decomposes at high temperature and is then deposited in the form of pyrolytic carbon on the surface, e.g., of the carbon fiber. In the case of liquid impregnation (summaries in Granda et al., 2003), the carbon fibers are impregnated with a treated organic precursor and then transformed into a carbon matrix by applying from one to several thermal cycles in an inert atmosphere (carbonization process). Depending on the final application of the prepared materials, they may be graphitized at temperatures as high as 2,500 C. These composites may also be subjected to additional densification processes when materials with a high density and good mechanical properties are required. The characteristics of the carbon matrix influence the final properties (Figure 8.9) and performance of the prepared carbon-carbon materials (Mene´ndez et al., 1997a). In the case of liquid impregnation, resins used as matrix precursors show good properties, but they develop a high porosity and in some cases they lack graphitizability, which has a negative effect on the characteristics of the composite. The use of pitches as precursor materials for carbon matrices instead of resins has several advantages, such as graphitizability, the

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formation of a wide variety of microstructures, low price and high carbon yield, and so on. Moreover, all these characteristics can be improved by using modified pitches. For example, by modifying the composition of coal-tar pitches before carbonization or by modifying their pyrolysis behavior (Mene´ndez et al., 1997a; Granda et al., 2003, among others), pitches with high thermal stability, high mechanical strength, and a wide variety of microstructures can be obtained for use in the preparation of composites. Both physical and chemical methods have been developed to modify and optimize pitch properties. The chemical methods modify the properties of the pitches by acting on the structure of the pitch components, resulting in an irreversible change to the pitch composition. Of the various chemical procedures used to polymerize coal-tar pitches, thermal treatment in an inert atmosphere and air blowing at moderate temperatures have proven to be the most effective methods (Granda et al., 2003, and references therein). Some properties of the thermally treated pitches (carbon yield, softening point, and solubility parameters) are improved and no negative effect on their binding and wetting capacity is observed. Moreover, there is a general improvement in the structure and properties of the resulting coke pitch (Bermejo et al., 1995; Mene´ndez et al., 1996; Mene´ndez et al., 1997a; Blanco et al., 1999). These modified pitches are particularly appropriate for the preparation of matrices for various types of carbon materials. Air blowing is the second chemical method employed to modify the structure of pitches (e.g., Mene´ndez et al., 1997a; Blanco et al., 2000b). In this procedure, the presence of oxygen induces the polymerization of pitch components at temperatures lower than 300 C without affecting pitch graphitizability (Ferna´ndez et al., 1995b). Modified coal-tar pitches have been successfully applied in the preparation of a wide range of high density materials (Granda et al., 2003). The use of thermally treated and air-blown pitches in the preparation of carbon-carbon composites reduces the release of volatiles, swelling, and the development of pores and cracks. The resultant materials exhibit an improved structure and superior mechanical properties as in the case of fine particulate carbon materials (Mene´ndez et al., 1998; Blanco et al., 2000c; Granda et al., 2003). Carbon-carbon composites are mainly used as structural materials due to their mechanical properties. The factors that may affect these properties during their preparation are (1) low volumetric contraction during the carbonization of the pitch, which may affect the strength of the matrix/fiber interface; the extent of the contraction is also highly dependent on the nature of the fiber and matrix; (2) the microstructure of the matrix, which controls the development and propagation of fissures, thereby influencing the failure mechanisms of the material; (3) the graphitic order and the level of adherence

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of the matrix/fiber; and (4) porosity development because areas with porosity are weak areas where failures in the material first begin to develop. In the case of porosity it is important to know the extent of this porosity and the size, orientation, and morphology of the pores. Because carbon materials and carbon-carbon composites are frequently porous in nature, it is essential to consider the effects of porosity upon the mechanical properties. As was mentioned in Section 8.3 of this chapter, all this can be assessed by petrographic analysis. Examples can be found in Mene´ndez et al. (1997a); Casal et al. (1999, 2001a); Blanco et al. (2003); and Me´ndez et al. (2003a). The treatment of pitch precursors and the various steps required to prepare the composites may be also monitored by optical microscopy. Petrographic analysis of carbon-carbon composites reveals the nature (Figure 8.1), composition, and behavior (Figure 8.9) of the carbon pitch matrix (see Ferna´ndez et al., 1995a,b; Mene´ndez et al., 1997b; Figueiras et al., 1998; Granda et al., 2003); its influence on the microstructure and mechanical properties of composite materials (Figueiras et al., 1995); the interactions and degree of bonding between the fibers and fiber/matrix interfaces (Ahearn and Rand, 1996; Figueiras et al., 1998; Casal et al., 2001a,b; Appleyard and Rand, 2002; Blanco et al., 2003); the interactions between the pitch matrix and the granular carbons (Figure 8.6) in the case of pitch-based granular composites (Me´ndez et al., 2003a,b); the microstructural performance of the composites and its relation to the alignment/ misalignment of carbon fibers (Casal et al., 2001a); the evolution of voids (pores or microcracks) and their shape, size, and orientation (Figure 8.5) in the various steps of composite preparation (Granda et al., 1998; Casal et al., 1999, 2001a,b; Blanco et al., 2003)—all of which is of fundamental importance for the optimization of the final product. Figueiras et al. (1998) and Blanco et al. (2003) have shown that the fracture behavior of carbon-carbon composites is determined by the texture of the matrix and the fiber-matrix interface developed inside the material. For example, mosaic-like textures in the carbon matrices (Table 8.1) give rise to a strong fiber/matrix interface, a denser microstructure, and a higher interlaminar shear strength in the resulting composite material. Figure 8.9 shows an example of the relationship between the different optical textures in the matrices of the carbon-carbon composites and their mechanical properties. The use of modified pitches in the preparation of carbon composite materials offers some other advantages. For example, the possible uses of the separated isotropic and anisotropic fractions include the preparation of pitch/graphite/copper systems for electric applications or the insertion of lithium in mesophase spheres for lithium-ion batteries as reported in Kim (2001) and Concheso et al. (2005). Other

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trends with carbon-carbon composites include the development of materials that have an improved thermal conductivity with no concomitant loss of mechanical properties. For this reason, carbon matrices are treated (doped) with metals such as Ti to facilitate the application of the obtained composites in extreme conditions.

8.4.2 Graphites Graphite, an allotropic form of carbon, which exhibits two crystalline structures, hexagonal and rhombohedral, is a highly ordered material (summaries in Bourrat, 2000). It is because of these structures that graphite displays the high degree of anisotropy that determines its properties. Graphite shows a high directional strength and is highly inert, although its reactivity increases as the temperature rises and therefore it reacts with oxygen at temperatures higher than 300– 400 C. In most cases it is a polycrystalline material, made up of crystalline aggregates, whose morphology, size, and orientation may vary from one graphite to another. Although the natural reserves of graphite are plentiful these graphites may contain impurities that limit their application. However, synthetic graphites in which the crystalline structure and degree of purity are totally controlled display a set of optimal properties that make them suitable for use in a wide range of applications—for example, as anode materials in most commercially available lithium-ion batteries, as electrodes in electric furnaces, as specialty graphites (molded graphite) for high technology uses in chemical vapor deposition, and as epitaxial deposition devices, and so on (Schobert and Song, 2002). Graphite can be obtained via the deposition of carbon in vapor phase or via high temperature treatment of carbon precursors. Current graphite technology uses petroleum cokes (the main precursor for polygranular graphite, Figure 8.11) to manufacture many major graphite products used in engineering applications. However, there is growing interest in finding alternative precursors, such as coal and coal-derived materials, to produce commercially valuable graphite products. This is the case of the high rank coals (anthracites) that, due to their high graphitizable carbon content (>90%), have been the object of intensive investigation as precursors for synthetic graphites (Evans et al., 1972; Oberlin and Terriere, 1975; Ivanov et al., 1985; Deurbergue et al., 1987; Duber et al., 1993; Zeng et al., 1996; Blanche et al., 1995; Pappano et al., 2000; Atria et al., 1993, 2002; Gonzalez et al., 2002, 2003a,b, 2004, 2005). Graphite formation from anthracites may occur at low temperatures (1,200–1,370 C) if graphitization catalysts are used (Evans et al., 1972), but it has also been demonstrated that some anthracites also graphitize with high temperature treatments above 2,000 C (e.g.,

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(a)

50 µm

(b)

50 µm

FIGURE 8.11. Optical microscopy. Photomicrographs taken in polarized light, with a one-wave retarder plate and an oil immersion objective. (a) Mesophase pitch obtained from a coal-tar pitch and concentrated by means of a hot filtration procedure. Notice the homogeneous size distribution of the mesophase spheres and their irregular edges due to the presence of primary quinoline insoluble particles (QIs). (b) Polygranular synthetic graphite obtained by the high temperature treatment of the mesophase of picture (a). (Photo credits: M. Granda.)

Franklin, 1951; Oberlin and Terriere, 1975; Duber et al., 1993; Bustin et al., 1995a,b; Beysacc et al., 2000; Atria et al., 2002; Gonzalez et al., 2003b). The high temperature treatment of some graphitizable anthracites previously carbonized at 1,000 C leads to the elimination of crosslinks. This together with the high orientation of their aromatic layers facilitates their growth whereas the aromatic units undergo reorganization, giving rise to structures of a graphite type (Gonzalez et al., 2002, 2003a,b, 2004, 2005). By means of optical microscopy (Sua´rez-Ruiz et al., 2006b; Sua´rez-Ruiz and Garcı´a, 2007) it was possible to study the graphitization of anthracites by following the evolution of the anisotropic parameters and the development of certain microscopical structures that are typical of graphites (similar to those described by Kwiecinska and Petersen, 2004), during their thermal treatment (Figure 8.4). In some works anthracites have been shown to display remarkable differences in graphitization behavior even when subjected to identical heat treatment regimes (Pappano et al., 2000; Gonzalez et al., 2003b, 2004; Sua´rez-Ruiz et al., 2006b). Petrographic analysis has demonstrated that carbonized anthracites used as precursors for synthetic graphites develop different amounts of microstructures (microspheres and flakes, Figure 8.4) depending on the temperature of the graphitization and the type of precursor anthracite. These microstructures were found to be present in a higher proportion in materials prepared from more graphitizable anthracites (Gonza´lez et al., 2004; Sua´rez-Ruiz and Garcı´a, 2007). This type of anthracite

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shows a larger degree of textural anisotropy (Figure 8.12) as indicated by the values of the anisotropy parameters obtained from microscopical analysis (Sua´rez-Ruiz and Garcı´a, 2007) and/or structural order as reflected by the XRD and Raman crystalline parameters (Gonzalez et al., 2003b). The utilization of meta-anthracites as a replacement for petroleum coke in synthetic graphite production may be another option to consider. According to Schobert and Song (2002) meta-anthracites may be a better filler for molded graphite in view of the fact that the noncatalytic graphitization of meta-anthracite at 2,400 C yields a product with crystalline parameters that are relatively close to those of graphite.

(a)

(c)

50 µm

50 µm

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FIGURE 8.12. Optical microscopy. Photomicrographs taken in polarized light, with a one-wave retarder plate and an oil immersion objective. (a, b) Highly ordered particles in addition to those shown in Figure 8.4, obtained from the high temperature treatment (2,600 C) of carbonized anthracites used as precursors for preparing graphitic materials (Rmax: 12.9%; bireflectance (Bw): 12.7%; Ram: 0.29; d002: 0.3373 nm; Lc: 17.3 nm; La: 48.4 nm; ID/It: 14.0. (Data from Sua´rez-Ruiz and Garcı´a, 2007c,d.) Graphitic structures, flakes, and microspheres, developed after the high temperature treatment (2,600 C) of previously concentrated fly-ash carbons (Rmax: 9.3%; bireflectance (Bw): 9.2%; Ram: 0.29 for values of the XRD and Raman crystalline parameters similar to those obtained for the graphitized anthracites. (Unpublished data.) (Photomicrographs: I. Sua´rez-Ruiz.)

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The potential use of other coal-derived precursors to obtain commercially valuable graphite is a topic that deserves to be more fully investigated. This is the case of fly-ash carbons due to their carbon content (>80%), their crystalline structure with its tridimensional ordering, and their content in metallic species. The metallic species are able to act as catalysts during the high temperature treatment of unburned carbons (Cabielles et al., 2006; Rouzaud et al., 2007). Thus, Rouzaud et al. (2007) obtained carbon materials with a high degree of structural order from fly-ash carbons as reflected by the evolution of the XRD and Raman crystalline parameters. These authors used concentrates of unburned carbons from fly ashes derived from the combustion of high rank Spanish coals that were treated at high temperatures, in the range of 1,800–2,700 C. HRTEM-EDS analysis also demonstrated that this high degree of structural order in the heated materials was mainly due to the presence of iron and silicon in the precursor fly-ash carbons. The evolution of the corresponding anisotropy parameters of these materials during heating is currently being investigated via optical microscopy (Figure 8.12). For the preparation of molded graphite articles pet coke is normally used as filler material. However, other coal-derived products such as coal-tar pitch coke can also be employed. Although the graphitizability of coal-tar pitch coke is lower, it can be used as a filler material (in addition to other materials such as natural graphite, carbon blacks, and recycled graphite), whereas coal-tar pitches can be used as binders (in addition to petroleum pitches, and phenolic and epoxy resins) to prepare graphitic products. Graphite has many applications. Since it is a pure carbon material, it is used in the preparation of other carbon materials such as diamonds. It is also characterized by its excellent lubricant, thermal, and overall electrical properties.

8.4.3 Activated Carbons Activated carbons are carbon materials with a highly developed porous structure and large surface area (e.g., Bansal et al., 1988; Derbyshire et al., 1995; Derbyshire, 1998; Yoon et al., 2000; Bansal and Goyal, 2005). They are used as sorbent materials for liquid and gas-phase applications. Other usages include medical and environmental applications, as catalyst supports, and so on. Activated carbons have also been investigated as a means to accumulate hydrogen (Parra et al., 2004). The properties of activated carbons that determine their final application are their pore size distribution (mainly micropores) and their high surface area (1,000–3,000 m2/g). Activated carbons need to have a large adsorption capacity and good surface chemical properties.

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Other characteristics that should be taken into account when preparing activated carbons are their thermal stability, chemical and mechanical strength, and easy regeneration. The precursor materials of activated carbons have relatively high carbon and low mineral matter contents. There are a wide variety of materials that can be used as precursors including coals, pet coke, charcoal, biomass, or chemical products such as polymers. The precursor materials need to be carbonized to remove any volatiles, leaving a solid residue that must later be subjected to processes of, e.g., physical activation. In this step the carbon of the precursor materials reacts with the activating agent, generating a material with a high porosity that may be used as an activated carbon. The agents used in the physical activation include: air, water vapor (which is the most frequently used agent), CO2, and oxygen (this is scarcely used at an industrial scale). Prior to physical activation, some precursors require several pre-treatments to ensure that the particle size of the precursor material is adequate. In the case of chemical activation, if this is the selected process of activation, the precursor material is impregnated with chemical agents and pyrolyzed, which means that carbonization and activation occur in only one step. In this process, the precursors are made to react with a chemical agent and the activation process develops between 450–900 C. Activating agents at industrial scale include ZnCl2, H3PO4, and KOH, but the use of the zinc compound is restricted because of environmental problems. With precursor materials such as biomass, H3PO4 was used for chemical activation (e.g., Jagtoyen et al., 1993). Chemical activation with KOH was developed in the 1970s to generate superactivated carbons with very high surface areas. As precursors, materials with very low volatile matter contents such as high rank coals, cokes, pet coke, and the like were used. Because of the wide range of precursor materials and the large number of activation agents that can be used, various types of activated carbons can be obtained. All the activated carbons, however, are classified into two main categories: granular activated and powdered activated carbons, each category being destined for different final applications. It is well known that coals are good precursors for the manufacture of activated carbons because they can develop a highly porous structure during the carbonization and activation processes. However, usually activated carbons are disordered and isotropic materials, and therefore, those precursors that may display a fluid or pseudofluid state during the carbonization/pyrolysis processes and then undergo subsequent resolidification and structural re-ordering cannot be used as precursors for preparing activated carbons. Bituminous coals (those classified as coking coals, Chapters 2 and 7) have thermoplastic

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properties and during their carbonization generate a highly anisotropic material with a poorly developed porous structure (Ruiz et al., 2001, 2006). Therefore, they are not appropriate for preparing activated carbons unless their coking properties are first eliminated or destroyed, e.g., via an oxidative process before the carbonization step to obtain a precursor with a good and appropriate porous development (Maloney et al., 1982; Pis et al., 1988, 1998; Pisupati et al., 1993; Ruiz et al., 2001, 2006). Ruiz et al. (2001, 2006) used a low volatile bituminoussemiathracite vitrinite-rich coal (with volatile matter and ash contents of 14.7% and 2.7% db, respectively) as precursor material for activated carbons. It exhibited a slight plastic behavior in the carbonization step. As the adsorbent properties of activated carbons derived from coals are determined by the texture of the chars generated during the carbonization/pyrolysis step (Parra et al., 1996; Pis et al., 1998; Ruiz et al., 2006) it is essential that the chars have an adequate pore structure so that the activating gas has ready access to the entire volume of the particle. This will allow the pore networks to develop during the gasification processes. In the case reported by Ruiz et al. (2001, 2006) the process of carbonization of the raw coal and the resulting oxidized material was monitored via microscopic analysis. Therefore, the amount of vesiculated/macroporous or dense chars, and anisotropic/isotropic particles (chars), and in general the quality of the resulting product was determined prior to the activation of this material. Nonoxidized coals favor the generation of macroporous and anisotropic chars after the carbonization process (Figure 8.13) while subjecting the raw coal to a long period of oxidation treatment prior to the carbonization step results in an isotropic and nonvesiculated precursor material with a microporous character (Figure 8.13) and greater surface areas. Thus, the highly increased micropore volume and the development of greater surface areas in the chars obtained from oxidized coal precursors are clearly related to the increase in the isotropy/anisotropy ratio of the chars obtained. This is because oxidation treatment prevents any rearrangement of the turbostratic structure of the coals and leads to a densification of the carbonaceous material (Ruiz et al., 2001, 2006). Anthracites have also been envisaged as precursor materials for activated carbons. Gergova et al. (1995) and Sych et al. (2006) showed that activated anthracites produced by steam-air treatments under the appropriate conditions lead to microporous materials with a large number of pores of molecular dimensions. Other materials that may serve as potential precursors for activated carbons are unburned carbons from fly ashes. Depending on the rank of the combusted coal, fly ash carbons display different types of porosity but after steam activation the volume of microporosity

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(b)

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FIGURE 8.13. Optical microscopy. Photomicrographs taken in polarized light, with a one-wave retarder plate and an oil immersion objective. Char particles from semi-anthracite coals investigated as precursor materials for activated carbons. (a, b) Highly macroporous, vesiculated, and anisotropic chars obtained after the pyrolysis of the raw semi-anthracite. (c) Dense char particles from the pyrolysis of the semi-anthracite previously oxidized at 200 C for 48 hours. The generated chars are mostly of isotropic character but some regions of the particles still show a light anisotropic texture (arrow) demonstrating that not all the thermoplastic properties of the oxidized semi-anthracite were removed. (d) Dense and totally isotropic char particles obtained after the pyrolysis of the semi-anthracite previously oxidized at the same temperature but for 120 hours. The oxidation process totally destroyed the coking properties of the coal, giving rise to a precursor material for activate carbon with very high isotropy/anisotropy ratio and high microporosity and surface area (data in Ruiz et al., 2006). (Photo credits: B. Ruiz.)

increases until it represents 60% of the total porosity (summaries in Schobert and Song, 2002), which is why these materials are currently being investigated as precursors for activated carbons. Carbon molecular sieves are a special type of sorbent materials, whose porosity is closely controlled and highly homogeneous. They are used in selective adsorption processes. Because of their specific characteristics, anthracites can be used to produce molecular sieve materials (Gergova et al., 1995), although other coals have been also investigated as precursors for preparing molecular sieves (Lizzio and Rostam-Adabi, 1993).

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8.4.4 Carbon Foams Carbon and graphitic foams are porous materials that are used mainly in thermal, mechanical, and electrical applications (e.g., Klett et al., 2000; Bruneton et al., 2002; Gallego and Klett, 2003; Klett et al., 2004; Calvo et al., 2005). As in the case of other carbon-based materials, their properties are varied depending on the precursor material as well as the procedures used in their preparation. As summarized by Klett et al. (2000, 2004), and Calvo et al. (2005), carbon foams display high thermal and electrical conductivities, a high mechanical strength and thermal stability, and a low density and thermal expansion coefficient. Carbon foams are usually obtained via the thermal treatment of polymeric substances. However, it has been demonstrated that coal and mesophase pitch may be used as alternative precursor materials. By using pitches as precursors, carbon foams with a high thermal conductivity were obtained because the preparation processes enabled the pitch precursor to develop the necessary graphitic structure for the resulting carbon foam to have a high conductivity. Optical microscopy analysis was also applied for the microstructural characterization of carbon foams derived from commercial pitches carbonized at 1,000 C and graphitized at 2,800 C (Klett et al., 2000) to study their thermal conductivity among other properties. These authors observed that the development of multicolor anisotropic optical domains of a small size and with a relatively random orientation was limited to the junctions of the walls surrounding the pores and that these characteristics served to reduce the overall thermal conductivity of the foams. This was due to an increased thermal resistance in these regions in contrast with the characteristics developed by the pore walls (ligaments). The ligaments were made up of graphene layer planes oriented along their axis as indicated by the presence of large monochromatic optical domains. Moreover, the thickness of the pore walls and their capacity to develop different types of microcracks that structurally weakened the foams were found to be related to changes in the density of the pitch-derived foams (Figure 8.14). As a precursor of carbon foams, Calvo et al. (2005) subjected a high volatile bituminous coal (volatile matter and ash contents of 34.1% and 5.7% db, respectively) with good plasticity (as determined by the swelling index and Gieseler assay values) to a carbonization process under strict conditions of controlled pressure and temperature. The resulting product displayed a macroporous texture with a characteristic porosity. Optical microscopy was used to characterize the resulting precursor of the carbon foams (Figure 8.14) in terms of pore (macroporosity) development (primary and secondary porosity),

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FIGURE 8.14. Optical microscopy. Photomicrographs taken in polarized light, with a one-wave retarder plate and an oil immersion objective. Materials investigated as precursors for preparing carbon foams. (a) Carbonized and graphitized foam derived from a commercial pitch showing a well-developed anisotropic flow optical texture mainly along the axis of the ligaments. This is related to the thermal conductivities of this type of material. (b) Close view of the anisotropic optical texture developed at the junctions in the same graphitized foam. (c) Carbonized (1,100 C) precursor material for carbon foams obtained from a pyrolyzed bituminous coal at 450 C for 120 min and under a pressure of 10 bar. Anisotropic optical textures of mosaics. Primary and secondary development of porosity with irregular distribution. Variable cell-wall thickness. (d) Close view of the junctions between pores in the same precursor material. Homogeneous anisotropic optical texture of mosaics. (Photomicrographs: I. Sua´rez-Ruiz.)

pore morphology and distribution, and optical texture. The development of an anisotropic texture of a mosaic type on the pore walls was also observed when petroleum pitch was added to modify the properties of the material. All these characteristics were associated to the mechanical properties (failure strength) of the prepared materials. The physical properties of these products obtained by using coals of different rank (from high to low volatile bituminous coals, 0.70–1.50% Rr) and of different petrographic composition (from rich to low vitrinite coals, 75-36 vol, %) as precursor materials is also a matter that is currently being investigated (Calvo et al., 2006, 2007).

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8.5 Carbon-Based Materials from Precursors Other Than Coal The carbon materials mentioned so far are mainly derived from coal and coal-derived precursors and were characterized by organic petrology techniques such as optical microscopy. However, other carbon materials are also obtained from other precursor products. In these cases the characterization is usually performed using high resolution analytical techniques. The following carbon materials are described here because of their innovative character. Carbon blacks (see summaries on the structure of carbon blacks in Bourrat, 2000) are colloidal carbon materials that are industrially obtained by means of the thermal decomposition or incomplete combustion of hydrocarbons in the shape of spheres that form aggregates of 10 to 100 nm. These materials have many applications as reinforcement, insulating and semiconductor materials, as a pigment for toners, in paintings, and in the tire industry because they are able to act as a reinforcement material in rubber to increase its mechanical strength. Although the submicron size of carbon blacks puts them beyond the resolution limit of the optical microscope, petrographic methods have been used to troubleshoot manufacturing problems such as clogged nozzles and the coked agglomerates of the precursor materials. Synthetic diamond is industrially obtained from graphite subjected to high temperatures and pressures. Its chemical and mechanical strength and its low compressibility make it suitable for use as binders in carbon-carbon composites. Although it also serves as insulating material, when it is doped it acts like a semiconductor. Diamond is also a biocompatible material which makes it suitable for use in diverse fields. Glassy carbons are a nongraphitizable type of carbon material that have high anisotropy in their physical and structural properties and a very low permeability for liquids and gases due to the almost total absence of open porosity. These materials display a high thermal and chemical strength. To obtain glassy carbons it is necessary to subject the precursor material (polymers) to very high temperatures (3,000 C). Although glassy carbons are used in many different applications, their electrical properties make them useful in the electrode and supercapacitor industry (Wikipedia, 2007a). They are also used as materials for prostheses and dental implants. Carbon nanotubes are materials that are made up exclusively of carbon in which the basic unit is the graphitic plane in the shape of cylindrical tubes several nm in diameter (Wikipedia, 2007b). Carbon nanofibers are nanofilaments with a different arrangement of graphene layers. Nanofibers are seen as intermediate materials between micrometric fibers produced by spinning and nanotubes. All these

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materials are produced via carbon vapor deposition. Nanotubes exist in a wide range of sizes, diameters, configurations, and layer arrangements. For this reason, the properties of nanotubes are varied. They are therefore used in many fields including those of composite materials, microelectronics, semiconductors, etc. In general, these carbon materials belong to the field of nanotechnology where carbon alloys (materials composed of carbon and a wide variety of elements such as B, Cu, Fe, Ni, Pt, etc.) are of great interest. Carbon gels are carbon materials synthesized from chemical products. They are colloidal substances composed of nanostructures in the form of chains or polymeric agglomerates of a colloidal type. This gives them special thermal, acoustic, optical, electrical, and mechanical properties. Carbon gels display a high textural development and their ultraporous structure induces a very low thermal conductivity (Aerogelcomposite, 2007). Since the properties of carbon gels are intimately linked to their structure, they depend on the precursor and the synthesis conditions employed to obtain the gels. Carbon nanofoam is a relatively recent carbon material (Marketech International Inc., 2007) made up of a network of carbon nanotubes with a foam morphology. It is a very light material with a macroscopic appearance similar to that of carbon blacks. It is a semiconductor material which displays ferromagnetic properties, a characteristic that is very unusual in carbon materials. The applications of carbon nanofoams are still in the experimental stage but one potential use is to be found in the field of biomedicine. Carbynes and fullerenes (Wikipedia, 2007c) are carbon materials that are mainly synthesized at laboratory scale. The discovery of fullerenes paved the way for nanotechnology. The science and technology of carbon materials are in the process of continuous development and are constantly finding ways to improve the properties of the already existing materials even at the nanometric scale, whereas at the same time searching for new and competitive applications. The wide variety of materials now available has turned the field of carbon materials into a multi- and interdisciplinary science. Carbon materials have many well-tried applications whereas many other potential uses are predicted for them, especially in the aeronautics, military, and automobile sectors and in the field of environmental science and energy.

CHAPTER 9

Coal as a Petroleum Source Rock and Reservoir Rock Jack C. Pashin

9.1 Introduction A consensus has existed for many years that oil and natural gas are derived principally from the thermal and biological decomposition of organic matter. Coal is defined as a rock composed of more than 50% organic matter by weight and is thus by definition the rock type that is richest in organic matter. For this reason, coal is considered an important petroleum source rock, and the ways in which petroleum compounds can be generated and expelled from coal is a subject of vigorous debate (e.g., Wilkins and George, 2002). Coal is also an important reservoir for natural gas, and gaseous hydrocarbons produced from coal are commonly referred to as coalbed methane (e.g., Rightmire et al., 1984; Kaiser et al., 1994; Ayers, 2002). Coalbed methane accounts for about 9.5% of the dry natural gas produced in the United States, and coalbed methane reservoirs are rapidly being commercialized around the globe. In addition to hydrocarbons, coal can also contain significant quantities of other gases, such as carbon dioxide and nitrogen (Scott, 1993). These gases occur in coal naturally, and potential exists to inject gases into coal for environmental and economic benefit. For example, the large gas capacity of coal makes it an attractive sink for carbon dioxide derived from anthropogenic sources, such as coal-fired power plants (e.g., Gentzis, 2000; van Bergen et al., 2006). Long-term sequestration of carbon dioxide in coal has the potential to significantly reduce greenhouse gas emissions, and coal is gaining acceptance as an important geologic carbon sink. Moreover, injection of carbon dioxide into coal can be used to enhance the recovery of coalbed methane, thereby providing economic incentive for sequestration operations. Coal is also a potential sink for industrial waste gases. Accordingly, a number of acid Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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gases may be disposed in coal, including hydrogen sulfide, which is a common byproduct of oil and gas production, and oxides of sulfur and nitrogen, which are among the most important pollutants in the flue gas from coal-fired power plants (Chickatamarla and Bustin, 2003). A spectrum of geologic factors, including stratigraphy, sedimentology, structural geology, hydrogeology, geochemistry, and coal petrology determine the properties of coal as a source rock and as a reservoir rock (e.g., Pashin et al., 1991; Levine, 1993; Ayers et al., 1994). For this reason, coal petrology must be considered in a broad geologic context when applied to petroleum source rocks, coalbed methane reservoirs, and sinks for anthropogenic gas. Research in this area is extremely active and is driven by mankind’s increasing need for energy resources coupled with the necessity of developing these resources in an environmentally responsible manner. This chapter emphasizes the fundamentals of understanding coal as a source rock and reservoir rock and reviews many of the key issues that confront researchers and industry. Although this chapter emphasizes coal, many of the ideas and approaches it discusses can also be applied to dispersed organic matter in siliciclastic and carbonate rocks. The literature on petroleum source rocks and organic-rich hydrocarbon reservoirs is vast and continues to grow rapidly, and the many citations given herein will lead the reader to the larger literature.

9.2 Coal as a Petroleum Source Rock Coal contains diverse forms of organic matter spanning a broad range of chemical composition, and this compositional variability combined with geologic history determine what types of hydrocarbons can be generated. This section begins with a discussion of the basic coal constituents that are precursors to gaseous and liquid hydrocarbons, specifically kerogen and macerals. The discussion continues by reviewing the mechanisms of hydrocarbon generation and geochemical relationships among organic matter, oil, and natural gas. This section concludes by considering the petroleum system’s concept, in which commercial oil and gas accumulations are thought of as the product of an integrated system of source rocks, migration pathways, reservoir rocks, and reservoir seals.

9.2.1 Kerogen and Macerals The diverse forms of organic matter that are considered to be the precursors of petroleum are referred to collectively as kerogen. Breger (1961) defined kerogen as the organic constituents of carbonaceous shale that are insoluble in aqueous alkaline solvents and common

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organic solvents. This definition has been applied broadly to the full range of petroleum source rocks, although some workers treat kerogen as a constituent of dispersed organic matter in shale and carbonate rather than as a constituent of coal (e.g., Hunt, 1972; Durand, 1980; Tissot and Welte, 1984). According to Tissot and Welte (1984) kerogen typically constitutes 80–99% of the total organic matter outside of conventional reservoir rocks. Bitumen, which constitutes the soluble fraction of the organic matter, makes up the remainder. Kerogen is typically classified as types I through IV on the basis of chemical composition (van Krevelen, 1961; Forsman, 1963; Cornelius, 1978; Robert, 1980; Stach et al., 1982; see Figure 9.1). Chemically, kerogen is classified on the basis of the proportions of carbon, hydrogen, and oxygen. Petrographically, kerogen can be related directly back to the original organic material deposited in sedimentary rocks and the products of their mechanical, biological, and thermal alteration. In this way, each type of kerogen can be characterized using the terminology of coal macerals.

FIGURE 9.1. A van Krevelen diagram showing different kerogen types and positions of coal and maceral groups relative to hydrogen-carbon and oxygencarbon atomic ratios (modified from Cornelius, 1978).

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Type I kerogen is rich in lipids and is characteristic of sapropelic organic matter; it has a hydrogen-carbon atomic ratio of about 1.10 or higher and an oxygen-carbon atomic ratio of about 0.15 or less (Figure 9.1). This composition reflects the presence of abundant aliphatic and alicyclic structures. Thus, type I kerogen is highly oil-prone, yielding high quantities of paraffinic hydrocarbons during pyrolysis. This type of kerogen can be brightly fluorescent and has a distinctive green to yellow color when observed in blue-light optical microscopy. Alginite is the signature maceral in type I kerogen and includes marine and freshwater forms, such as Tasmanites, Botryococcus, and Torbanites. Thus, boghead coal is composed principally of type I kerogen. Type I kerogen can also be expressed as a strongly fluorescent, amorphous groundmass derived from biological degradation of algal material as well as lipids derived from microbes, plants, and animals. Type II kerogen is also rich in lipids, is characteristic of sapropelic organic matter, and is widely recognized as the most abundant and important constituent of petroleum source rocks; it generally has a hydrogen-carbon ratio between 0.75 and 1.75 and an oxygen-carbon ratio between 0.05 and 0.20 (Figure 9.1). This kerogen is of intermediate composition, containing a mixture of aliphatic and aromatic structures as well as ketone and carboxylic acid groups. Type II kerogen is oil-prone, although it does not yield the volumes of oil that can be obtained by pyrolysis of type I kerogen. When viewed under a bluelight microscope, the fluorescence of type II kerogen ranges from bright yellow through orange, red, and dull brown, depending on the original composition and thermal maturity. This type of kerogen comprises a broad range of macerals belonging to the liptinite group. Macerals with recognizable biological structure include sporinite, cutinite, and suberinite. Thus, cannel coal is composed chiefly of type II kerogen. Numerous types of amorphous organic matter can also be recognized as type II kerogen, including resinite, suberinite, bituminite, and exsudatinite. Bituminite is particularly common in oil shale and is commonly called matrix bituminite where it forms a fluorescent groundmass in fine-grained sedimentary rocks. Type III kerogen is characteristic of humic organic matter: It has a hydrogen-carbon ratio lower than 1.00 and an oxygen-carbon ratio that ranges widely from 0.05 to 0.40 (Figure 9.1). This type of kerogen contrasts strongly with types I and II because it is dominated by aromatic structures, heteroatomic ketones, and carboxylic acid groups and contains a relatively minor proportion of aliphatic and alicyclic compounds. In contrast to types I and II, type III kerogen is recognized as highly gas-prone. Type III kerogen is typically nonfluorescent, although it can exhibit a weak brownish fluorescence and suppressed reflectance in samples where hydrogen content is

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elevated (Teichmu¨ller, 1974). Type III kerogen is the product of woody plant tissue and thus encompasses the full range of vitrinite macerals, including huminite, telinite, and collinite. Semifusinite, which includes the partially oxidized remains of woody tissue, can also be included under type III. Vitrinite has been subdivided into subhydrous, orthohydrous, and perhydrous types depending on the weight percent of hydrogen and carbon (Diessel, 1992a; see Figure 9.2). Vitrinite is generally considered to be a gas-prone maceral. However, perhydrous vitrinite, which generally contains more than 5.5% hydrogen by weight and is present chiefly in JurassicTertiary coal, has been suggested to be the progenitor for liquid hydrocarbons in some basins (Bertrand, 1984; Smith and Cook, 1984). Vitrinite constitutes more than 80% of most Euramerican coal beds; thus these coals can be classified largely as gas-prone source rocks. As described in Chapter 2, vitrinite reflectance is a valuable indicator of coal rank and thermal maturity, and despite some pitfalls, the reflectance of type III kerogen is the most widely used thermal maturity parameter in the petroleum industry. Type IV kerogen is not universally recognized because it is of questionable significance as a source material for oil and natural gas. Type IV kerogen generally has the lowest hydrogen-carbon ratio of all kerogen types, typically below 0.50, and the oxygen-carbon ratio is generally lower than 0.1 (Figure 9.1). Polyaromatic structures predominate in type IV kerogen, and aliphatic structures are absent.

FIGURE 9.2. Diagram showing major elemental composition of vitrinite and relationship between gas-prone orthohydrous vitrinite and oil-prone perhydrous vitrinite (modified from Diessel, 1992a).

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Microscopically, type IV kerogen is nonfluorescent and has extremely high reflectance when compared to most type III kerogen. Accordingly, type IV kerogen comprises most of the inertinite group macerals, which are the products of oxidation associated with exposure or combustion and include fungal sclera. The most common forms of inertinite in coal and fine-grained sedimentary rocks that can be included in type IV are fusinite, pyrofusinite, sclerotinite, macrinite, and micrinite. The oxidized and advanced polyaromatic state of type IV kerogen indicates that it is effectively spent with regard to hydrocarbon generation. Thus, inertinite dilutes the hydrocarbon source material in coal. Inertinite-rich coal is common in many Gondwanan coal basins, and although source potential is intuitively reduced compared to sapropelic and humic coal, the concentration of humic and sapropelic organic matter in coal is still much higher than in the shale and carbonate strata that are thought to have sourced the majority of the world’s petroleum reserves.

9.2.2 Hydrocarbon Generation Kerogen and coal undergo significant mechanical and chemical changes during burial, and these changes are driven by compaction, biological activity, and thermal kinetics. These three factors are complexly interrelated and can be effective throughout the full depositional and tectonic history of a sedimentary basin. Hydrocarbon generation can perhaps be considered most effectively within the framework of coalification. During this process, critical changes in chemical composition, specifically hydrogen-carbon and oxygencarbon ratios, can be characterized using the van Krevelen diagram (van Krevelen, 1961; see Figure 9.1). At the peat stage, volumetric and compositional changes are driven principally by biological activity and compaction. Biological activity can be intense throughout the peat profile, and carbon content increases rapidly downward as cellulosic compounds are metabolized. Aerobic bacteria and fungi are active in the uppermost part of the peat profile, and within about 0.5 m of the surface, these biota are supplanted by anaerobic bacteria, and the peat becomes alkaline. Metabolism of organic compounds by anaerobic bacteria can generate large quantities of methane (i.e., swamp gas) and humic acids, and digestion of cellulosic compounds enriches the peat in lignin. Deeper in the profile, bacterial activity commonly decreases as the peat becomes putrefied, and the depths at which humification and lignification are effective vary depending on temperature, nutrient flux, and the flow of groundwater. Peat compacts significantly with depth as the weight of overburden increases, water and other fluids are expelled, and the constituent organic matter decays.

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According to Stach et al. (1982), the transformation from peat to lignite (brown coal) is effectively complete when moisture content falls below 75%, carbon content (daf) is greater than about 60%, and free cellulose is absent. They pointed out that the definition of the peat-lignite boundary is scientifically imprecise and typically lies at a depth between 200 and 400 m. As coal evolves through the lignite and subbituminous ranks, lignin and the remaining cellulosic compounds are transformed into humic compounds, which progressively increase in molecular weight and lose acidity. Thermal-kinetic transformations become increasingly significant during the progression from lignite to bituminous coal, and devolatization begins driving volumetric changes more than mechanical compaction. The principal volatile components generated from lignite and subbituminous coal are water and carbon dioxide. At subbituminous rank, humic substances become increasingly gelified and vitreous as huminite begins changing to vitrinite. Hydrocarbon generation by thermogenic processes is limited below bituminous rank (Figure 9.3). However, Stach et al. (1982) indicated that minor amounts of methane can be generated by reduction of methoxyl groups in lignin. In addition, oil can

FIGURE 9.3. Generalized diagram showing proportions of thermogenic gases generated from sapropelic and humic organic matter in relationship to temperature and thermal maturity (modified from Hunt, 1979).

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begin to be generated from sapropelic kerogen in sediment with vitrinite reflectance (Ro) as low as 0.35%, which corresponds with lignite rank (Paterson et al., 1997). Devolatization continues to be a dominant process as coal progresses through the bituminous ranks, and as rank progresses toward anthracite, major changes occur in the proportions of moisture and hydrocarbons generated, and coal structure becomes increasingly aromatic as functional groups are shed. Moisture decreases from 15% at the subbituminous-high volatile C bituminous boundary (Ro ¼ 0.5%) to less than 1% at the high volatile A-medium volatile bituminous boundary (Ro ¼ 1.1%). The lower boundary of the thermogenic oil window as typically applied in basin analysis (e.g., Hunt, 1979) corresponds with the subbituminous-bituminous transition, and peak oil generation corresponds with high volatile B and A bituminous rank (Ro ¼ 0.7 to 1.0%). In general, most oil generation occurs at temperatures between 50 and 150 C, and large volumes of carbon dioxide can be generated in this temperature range (Hunt, 1979; see Figure 9.3). However, rock-eval pyrolysis indicates that some oil generation from coal conceivably continues into anthracite rank (Ro > 2.0%) and at temperatures above 500 C (Peterson, 2006). Small volumes of thermogenic gas may begin to be generated as coal reaches a high volatile C bituminous rank (Scott, 1993). Major thermogenic generation of gaseous hydrocarbons is thought to begin at the boundary between high volatile B and A bituminous rank (Ro ¼ 0.8%) and continues across the low volatile bituminous-semianthracite boundary (Ro ¼ 2.0%) (Ju¨ntgen and Klein, 1975). Most thermogenic generation of gaseous hydrocarbons is thought to occur between temperatures of 100 and 225 C, and significant volumes of nitrogen can be generated between temperatures of 100 and 150 C (Hunt, 1979). Thermal cracking is an important process in this rank range, and as thermal maturity increases and an increasing proportion of hydrogen is given off from the coal structure, long-chain hydrocarbons can be transformed into short-chain structures, such as methane. At semi-anthracite rank the rate of gas generation declines significantly as the remaining carbon is bound in the aromatic net, and hydrocarbon generation is thought to be effectively complete at anthracite rank (Ro > 3.0%). Chemical changes in kerogen and coal macerals that are associated with coalification and hydrocarbon generation are commonly expressed using the van Krevelen diagram (Figure 9.1). As organic matter matures thermally, the hydrogen-carbon and oxygen-carbon atomic ratios decrease markedly along distinct evolutionary pathways corresponding to the major kerogen and maceral types. From the early stages of maturation into oil generation, hydrogen-carbon and oxygen-carbon ratios decrease significantly, although the rates vary

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considerably among the various kerogen and maceral types. In type I kerogen (i.e., alginite), for example, the oxygen-carbon ratio can decrease from 0.10 to less than 0.05 as the hydrogen-carbon ratio decreases from 2.0 to 1.0. In type II kerogen (i.e., liptinite), by comparison, the oxygen-carbon ratio can decrease from values exceeding 0.30 to about 0.05 as the hydrogen-carbon ratio decreases across a narrower range from about 1.5 to 1.0. And in type III kerogen (i.e., vitrinite), the oxygen-carbon ratio decreases from about 0.4 at the peat stage to about 0.01 at anthracite rank while the hydrocarbon-carbon ratio decreases from about 1.0 to 0.5. Importantly, the van Krevelen diagram shows that as the oxygen-carbon ratio falls below 0.05, type I and II kerogen become indistinct at a hydrogen-carbon ratio approximating 1.0, and all three types of kerogen become indistinct as the hydrogen-carbon ratio approaches 0.5. For this reason, caution should be used when using atomic ratios to interpret the origin of thermally mature hydrocarbon source rocks. At elevated thermal maturity, optical petrographic analysis can be an effective method for distinguishing the different types of organic matter that may have generated hydrocarbons. Thermal maturation and hydrocarbon generation are commonly modeled using two major techniques. The time-temperature (TTI) method of Lopatin (1971) and Waples (1980) is the most widely used technique and provides a relatively simple, quick-look method to consider the thermal maturity of source rocks, reservoirs, and seals in the context of burial history and geothermal gradient (Figure 9.4). The chemical-kinetic model (Easy Ro) of Burnham and Sweeney (1989) and Sweeney and Burnham (1990) provides some advantages over the time-temperature method because it considers the geochemical evolution of source rocks and hydrocarbon generation with respect to activation energy and the amounts of carbon, hydrogen, and oxygen available in the source rock. However, Zhang et al. (in press) pointed out that the application of this model to coal-bearing strata is complicated by the variability of source-rock composition and that basin-specific pyrolysis data are needed to characterize hydrocarbon generation accurately. In addition to thermal methanogenesis, bacterial methanogenesis is thought to be an important mechanism of gas generation in coal. Early methanogenic methane that forms at lignite rank is probably not retained in coal because the coal structure that can retain economic quantities of gas is not developed and because it is expelled by compaction (Levine, 1993). However, late-stage methanogenesis that is synchronous with or, perhaps more commonly, post-dates thermal maturation is thought to be a major process in coal (e.g., Rice, 1993; Scott et al., 1994; Scott, 2002) and some gas-shale formations (Martini et al., 1998). Mechanisms and rates of late-stage bacterial methanogenesis in organic-rich formations are not fully understood, but a

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0

Jurassic

Cretaceous

Tertiary

Biogenesis (Ro = 0.2-0.5)

1000

Depth (m)

2000 Early oil (Ro = 0.5 - 0.8)

3000 Oil and gas (Ro = 0.8 - 1.3)

4000 Main Gas (Ro = 1.3 - 2.5)

Shale seal

5000

Conventional oil and gas reservoir Coal zone Oil-prone limestone

6000 150

100

50

0

Age (Ma)

FIGURE 9.4. Idealized Lopatin model simulating burial history and thermal maturation of strata in a well-containing multiple conventional and unconventional source rock and reservoir intervals.

strong correlation exists between late-stage biogenic methane and fresh-water plumes that are fed by meteoric recharge along basin margins (Pashin et al., 1991; Ayers et al., 1994; Martini et al., 1998). The precise source of nutrition for methanogenic bacteria in coal and organic-rich shale is a matter for debate, although the two most likely sources include the organic matter in the source rock and the dissolved organic compounds in the formation water. In low-rank coal, for example, weakly bound functional groups and light hydrocarbons may sustain microbial metabolysis. However, coal becomes increasingly polymerized at elevated rank and thus contains less material that can be easily metabolized. Accordingly, bacterial methanogenesis in bituminous and higher rank coal may be supported principally by nutrients in formation water. The primary evidence for late-stage methanogenic gases in coal and shale comes from carbon-isotopic analysis of gas (Rice, 1993; Scott et al., 1994). Scott et al. (1994) also pointed out that only migration of gas or late-stage bacterial methanogenesis can explain gassaturated coal in sedimentary basins that have been uplifted and

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cooled. This is because the gas capacity of coal increases as temperature decreases (Yang and Saunders, 1985), so undersaturated conditions are expected where coal has cooled to the point that no new hydrocarbons can be generated thermally. Thermogenic and biogenic gases are isotopically distinct, and carbon and hydrogen (deuterium) isotopic analysis can be used to identify different metabolic pathways for microbial methanogenesis (Whiticar, 1994, 1996; see Figure 9.5). Carbon from pure thermogenic methane has d13C values between –50% and –20%, and as thermal maturity increases, the methane becomes enriched in 13C. The dD values of thermogenic methane range widely from about 300% to about –100%. Pure biogenic methane, by contrast, typically has d13C values ranging from approximately –150% to –50% and thus has minimal overlap with thermogenic methane. Biogenic gas derived from methyl fermentation of organic matter generally has dD values of –300% to –400%, whereas that derived from carbonate reduction has dD values ranging from –150% to –250%. Analysis of coal rank and carbon isotopic data indicates that thermogenic and biogenic gases are common in coal and that mixtures of different gas types are common (Pashin et al., 1991; Rice, 1993; Scott et al., 1994; Su et al., 2005). Furthermore,

FIGURE 9.5. Diagram showing isotopic composition of thermogenic and biogenic gases. Coalbed methane has diverse origins, and some basins contain gases that plot in multiple fields. (Source: International Journal of Coal Geology 32, M. J. Whiticar, “Stable isotope geochemistry of coals, humic kerogens and related natural gases,” 191–215, copyright 1996, with permission from Elsevier.)

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biogenic methane in coal has been attributed to microbial fermentation and carbonate reduction, and both processes may have been active in different parts of a given sedimentary basin (Scott et al., 1994; Rice, 1993; Pitman et al., 2003; Warwick et al., 2006).

9.2.3 Coal-Bearing Petroleum Systems Coal-bearing strata can be characterized using the petroleum systems concept, which states that sedimentary basins contain integrated systems of source rocks, migration pathways, reservoirs, and seals (Magoon and Dow, 1994; Ayers, 2002; Riese et al., 2005; see Figure 9.6). Building on this concept, Warwick (2005) proposed the coal systems concept, which considers coal not only as part of a petroleum system, but as a resource to be mined and utilized in a broad range of commercial processes. In this section, coal-bearing strata are considered in terms of the petroleum- and coal-systems concepts, thus providing a vital bridge to a detailed discussion of coal as a hydrocarbon reservoir and a sink for greenhouse gases and waste gases. Major topics to be discussed include the viability of coal as an oil- and gas-prone source rock, controls on the retention and expulsion of hydrocarbons generated from coal, and the relationship of coal to a spectrum of petroleum reservoirs and seals.

FIGURE 9.6. Idealized petroleum system model showing interrelationship of source rocks, expulsion and migration pathways, reservoir seals, and conventional and unconventional hydrocarbon accumulations.

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Coal contains a significant quantity of oil-prone organic matter, but is coal an effective oil-prone source rock? In a comprehensive review of the potential of coal to generate oil, Wilkins and George (2002) considered the pros and cons. Boghead and cannel coal are extremely rich in oil-prone organic matter and have almost certainly generated and expelled oil (Taylor et al., 1998), but these types of coal are so uncommon that they are almost certainly not the progenitors of a major portion of the world’s petroleum reserves. Alternatively, oilprone organic matter can be abundant in humic coal, which constitutes the vast majority of the world’s coal resources, and is therefore worthy of consideration as the source rock for economically significant oil accumulations. Liptinite macerals equivalent to kerogen types I and II constitute less than 15% of most humic coal beds, whereas perhydrous vitrinite is the dominant constituent of many Jurassic-Cretaceous coal beds. The origin of perhydrous vitrinite is unclear and may be diverse. In some cases, it may be the product of woody plant tissue that has been impregnated with oily material derived from the water column or the surrounding sediment in the original depositional environment (Sua´rez-Ruiz et al., 1994a), and bacterial degradation can help elevate the hydrogen content of some woody organic matter (Powell, 1987). Perhydrous vitrinite also is strongly associated with coal and shale that is rich in other types of oil-prone organic matter (Kalkreuth, 1982; Price and Barker, 1985). Thus, the high hydrogen content and suppressed reflectance of perhydrous vitrinite may be explained by sorption of oily hydrocarbons generated from other maceral and kerogen types (e.g., Ritter and Grover, 2005). Despite the oil-prone character of many coal macerals, tracing the origin of economic oil accumulations to coal sources has proven very difficult (Wilkins and George, 2002). This uncertainty owes largely to the ability of coal to retain large volumes of hydrocarbons within its own aromatic net. One possibility is that a large fraction of the oil generated in coal is cracked to gaseous hydrocarbons prior to expulsion. However, fluorescent pore- and fracture-filling macerals like exsudatinite may be a residuum left by expulsion of lighter hydrocarbon liquids. Thus, although coal has the potential to generate large volumes of oil, the precise conditions that determine the rate and volume of expulsion remain elusive and probably depend on an interplay among coal geochemistry, burial history, thermal history, and basin hydrology. Whereas the effectiveness of coal as an oil-prone source rock is unclear, the effectiveness of coal as a gas-prone source rock is practically doubtless. The primary line of evidence that coal is an effective source rock for natural gas is the sheer predominance of gas-prone vitrinite equivalent to type III kerogen. In addition, the large volume of natural gas that provides a mining hazard as well as

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an economic resource in coal has led to the widespread interpretation that coal is principally a self-sourced natural gas reservoir (e.g., Levine, 1993; Rice, 1993). But can coal expel large enough quantities of gas to be an effective source rock for sandstone and carbonate reservoirs? Pyrolysis experiments indicate that coal can generate much more natural gas than can be stored in coal during thermal maturation, although the results of these experiments vary. For example, open-system pyrolysis experiments, which are conducted at high temperature and low pressure (e.g., Ju¨ntgen and Karweil, 1966; Higg, 1986), appear to overstate the amount of gas generated and do not adequately simulate the generation and cracking of heavy hydrocarbons or the geochemical evolution of organic matter as expressed by the van Krevelen diagram. More recent experiments employing closed-system pyrolysis, which reproduces the temperatures and pressures observed in sedimentary basins, provide more reasonable estimates of the volumes of hydrocarbons generated and the corresponding geochemical changes that occur in organic matter (Monthioux et al., 1985; Tang et al., 1996; Zhang et al., in press). The closed-system experiments indicate that the amount of hydrocarbon gas generated can vary widely between 100 and 300 cm3/g of coal, and that a large amount of CO2 is also generated with the hydrocarbons. The storage capacity of bituminous coal is typically between 15 and 30 g/cc, thus the results of closed-system pyrolysis indicate that about 3 to 20 times more gas can be generated than can be retained. Therefore, it follows that an extremely large volume of thermally generated natural gas has been expelled from coal and that coal can be considered as a major source rock for natural gas in a range of reservoir types. If thermal methanogenesis in coal can source external gas reservoirs, can bacterial methanogenesis generate enough gas from coal to source other types of reservoirs? The answer to this question is necessarily incomplete because our knowledge of bacterial methanogenesis in coal is fledgling at best. In the San Juan Basin of the western United States, however, natural gas is produced from coal and sandstone that are in hydrologic communication. In some areas of the basin, apparent coal-sourced gas, which commonly includes latestage biogenic gas, has been produced from sandstone, and gas apparently generated from marine shale has been produced from coal (Ayers et al., 1994). Once generated, hydrocarbons can migrate within coal beds, into other formations, or escape to the surface (Figure 9.6). The major mechanisms of flow in coal include Darcian flow through interconnected pores, including fractures, and diffusion through coal matrix (Olague and Smith, 1988; Gamson et al., 1993; Scott, 2002). Darcian flow is influenced primarily by pressure gradients, density gradients,

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and compaction, whereas diffusion occurs in response to concentration gradients. Darcian flow and diffusion are also important migration mechanisms in other rock types (Tissot and Welte, 1984). Darcian flow occurs through interconnected pore spaces in permeable carrier beds, such as sandstone and limestone. Migration by Darcian processes may also occur through fractures and along faults. Diffusion may occur in all rock types and is an especially important flow mechanism in the matrix of shale, carbonate, and siliceous rocks having low permeability. Although diffusion appears to be an extremely important mechanism for the expulsion and migration of natural gas, diffusion of oil appears to be of limited significance. This is because diffusion coefficients tend to be one to three orders of magnitude lower for oil than for gas (Leythaeuser et al., 1982). Hydrocarbons and other gases can also be transported by dissolution in formation water. In general, the solubility of hydrocarbons in water decreases as the carbon number of the hydrocarbon increases (Peake and Hodgson, 1966; McAuliffe, 1966, 1980; Price, 1976). For most hydrocarbons, solubility increases with increasing reservoir temperature but decreases with increasing salinity. Other gases generated during coalification, including CO2 and N2, are soluble in formation water and may play an important role in the expulsion and transport of hydrocarbons (Bray and Foster, 1980). For example, CO2 is miscible in a broad range of hydrocarbons and increases the mobility of oil by reducing viscosity. Although the importance of miscibility in the natural expulsion and transport of hydrocarbons is not fully known, miscible flooding of conventional oil reservoirs with CO2 is an effective method of enhanced oil recovery. Sealing beds play a critical role in the migration and trapping of hydrocarbons in porous rock types (Figure 9.6). Effective reservoir seals characteristically have permeability on the order of a microdarcy, and typical sealing rock types include shale, evaporites, and fine-grained carbonates. Sealing beds can serve as confining units that are critical for defining bed-parallel migration pathways in carrier strata. Fault zones can be migration pathways or reservoir seals, and fault properties depend on a number of factors, including lithologic juxtaposition, cataclasis, clay smearing, and cementation (e.g., Allan, 1989; Knipe, 1992, 1997). The final element of the petroleum system is the hydrocarbon trap. In conventional oil and gas reservoirs, hydrocarbons are stored principally as free phases within open pore space. The buoyancy of oil and gas relative to formation water concentrates economic hydrocarbons in structurally high areas, such as the crestal regions of anticlines. Ultimately, the classic array of structural, stratigraphic, and

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combination traps reflects a combination of reservoir and seal geometry (Figure 9.6). Hydrodynamic trapping also is an important geologic process in which changes in reservoir pressure give rise to inclined fluid contacts. Basin-centered hydrocarbon accumulations occur in the structurally deep parts of sedimentary basins and are thought to form where gas generation has displaced pore water toward basin margins (e.g., Law, 2002). Basin-centered gas accumulations in low-permeability sandstone are common in the Mesozoic strata of western North America, and many of these accumulations are thought to be at least partly coal-sourced (e.g., Masters, 1984; Spencer and Mast, 1986). Trapping mechanisms for natural gas in coal and shale reservoirs contrast sharply with those in conventional hydrocarbon reservoirs because a major fraction of the retained hydrocarbons is adsorbed on internal rock surfaces. Adsorbed hydrocarbons cling to surfaces chiefly by van der Waals forces, and storage capacity increases substantially with confining pressure. In hydrocarbon reservoirs dominated by adsorption, no seal is required for retention of large quantities of natural gas. However, some gas can be stored as a free phase in coal macropores, including cleats, and can form a significant gas resource (Kaiser et al., 1994; Ayers, 2002; Pashin and McIntyre, 2003). Boghead coal, which is rich in type I kerogen, can apparently store large volumes of gas as a solution phase within liptinite (Chalmers and Bustin, 2007). Minor amounts of oil have been produced from coal. However, this production is typically no more than a few barrels per well and is regarded by producers as more of a nuisance than an economic resource (Rice et al., 1989; Clayton et al., 1991; Pashin and Hinkle, 1997). The dominance of adsorption as a gas storage mechanism in coal gives rise to extremely complex reservoir dynamics, and these dynamics are discussed in detail in the following section on coal as a reservoir rock.

9.3 Coal as a Petroleum Reservoir Rock Coal is classified as a continuous-type, unconventional gas reservoir because coalbed methane plays typically span large areas of sedimentary basins and because gas is stored dominantly in an adsorbed rather than a free state. Although coal reservoirs tend to be continuous, they are extremely heterogeneous and have complex reservoir properties. Indeed, diverse geologic factors influence storage capacity, hydrocarbon content, and production performance (e.g., Pashin et al., 1991; Ayers et al., 1994; Pashin and Groshong, 1998; Pashin, 1998; Scott, 2002). Moreover, geologic factors vary considerably among the

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sedimentary basins where commercial hydrocarbon production has been established, and this variability necessitates that strategies for coalbed methane development and carbon sequestration be tailored specifically to each basin. In the following sections, the basic properties of coal as a reservoir rock are reviewed.

9.3.1 Adsorption and Gas Capacity Adsorption is a process in which gas or liquid molecules adhere to a surface, thereby forming a monolayer or multilayer film. For gases in coal, an adsorbate film can approach liquid density at a much lower pressure than is predicted by ideal gas law. A monolayer is a film with the thickness of one molecule, whereas a multilayer is a film with a thickness of two or more molecules. Adsorption is thought to be a response to a natural bonding deficiency that exists along surfaces, and development of a monolayer or multilayer effectively satisfies this deficiency. Adsorption takes place by the processes of physisorption and chemisorption. Physisorption is adsorption by van der Waals force, which is a weak intermolecular attraction that takes place below the critical temperature of the adsorbate and can result in the development of a monolayer or multilayer. Chemisorption, by contrast, involves a strong covalent bond between the surface and the adsorbate that can take place at supercritical temperature and always results in a monolayer. In general, physisorption is considered to be the dominant mode of adsorption for gases in coal, although most workers consider monolayer adsorption to predominate because of the limited space in coal nanopores. Gas capacity is expressed in terms of the adsorption isotherm, which determines how gas capacity varies with pressure at a constant temperature (Figure 9.7). A fundamental equation describing adsorption is the Langmuir equation: Gs ¼ ðVL PÞ=ðPL þ PÞ;

(1)

in which GS is the gas storage capacity, P is pressure, VL is Langmuir volume, and PL is Langmuir pressure. Langmuir volume is the total adsorption capacity of a substance, whereas Langmuir pressure is the pressure at which storage capacity equals one half of Langmuir volume. Adsorption curves in isotherm plots have a distinctive shape in which the slope of the curve is steepest near the origin and decreases as pressure and gas capacity increase (Figure 9.7). The isotherm curve approaches the ultimate adsorption capacity, or Langmuir volume, asymptotically. Several methods exist for determining adsorption isotherms for coal (e.g., Kim, 1977a; Mavor et al., 1990; Goodman et al., 2005). A common method is the extended Langmuir

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FIGURE 9.7. Example adsorption isotherm showing decreasing slope with increasing pressure and critical desorption pressure for saturated and undersaturated reservoir conditions.

method, and for a description of a commonly used procedure and apparatus used to measure adsorption isotherms, see Mavor et al. (1990). Coal has a nanoporous aromatic fabric with an extremely large internal surface area where gases can adsorb. The adsorption capacity of coal varies greatly depending on the composition of the gas being adsorbed and the composition of the coal. A series of isotherms showing the adsorption performance of different gases in a low volatile bituminous coal (Ro ¼ 1.95%) from western Canada were plotted by Chickatamarla and Bustin (2003) (Figure 9.8). The principal gases that occur naturally in coal are CH4, CO2, and N2 (Kim, 1977a; Scott, 1993; Rice, 1993), and SO2 and H2S are gases that have the potential to be sequestered in coal (Chickatamarla and Bustin, 2003). The isotherms indicate that coal can adsorb extremely large quantities of H2S and SO2 at extremely low pressure, and the liquid point determines the maximum pressure at which the isotherm is valid for each gas. Of the naturally occurring gases, coal can adsorb significantly more CO2 than CH4, and significantly more CH4 than N2, and this basic relationship has been observed consistently in all types of coal (e.g., Arri et al., 1992; Harpalani and Pariti, 1993; Hall et al., 1994). Several factors explain why the adsorption performance of different gases can vary so greatly. Large gas molecules by definition form a thicker monolayer than small gas molecules, and this accounts for

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FIGURE 9.8. Adsorption isotherms showing variable adsorption performance of different gases in a low volatile bituminous coal sample from western Canada (modified from Chickatamarla and Bustin, 2003).

most of the differences among gases. Other factors, such as the polarity and fugacity of the sorbate molecule, as well as the ability of the molecule to satisfy sorbent-surface bonding deficiency through van der Waals force or covalent bonding, influence the volume of gas that can adsorb onto a solid. Coal composition has a significant effect on adsorption capacity, although the high variability of experimental results indicates that any number of compositional parameters can be important (e.g., Ettinger et al., 1966; Levine, 1993; Crosdale et al., 1998). Determining the effects of maceral composition can be difficult, although gas capacity tends to be inversely correlated with liptinite and inertinite content and positively correlated with telovitrinite content (Lamberson and Bustin, 1993; Mastalerz et al., 2004b). However, caution needs to be applied when examining liptinite-rich coal, because some liptinite macerals can store solution gas in quantities that rival the storage capacity of vitrinite (Chalmers and Bustin, 2007). Mineral matter dilutes organic matter, thus mineral matter and ash content are inversely related to adsorption capacity (Gunter, 1965). Importantly, proximate analyses are reported in weight percent, thus critical parameters such as ash and moisture content must be converted to a volumetric basis to properly characterize their effect on adsorption capacity (Scott et al., 1995).

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FIGURE 9.9. Relationship between adsorption capacity and coal rank for different gases in coal from the Black Warrior Basin of the southeastern United States (modified from Carroll and Pashin, 2003).

Coal rank is a major aspect of coal composition that influences adsorption capacity. Moisture content is an important parameter, particularly at below bituminous rank, and has long been known to be inversely related to CH4 capacity in coal (Joubert et al., 1973, 1974). Isotherms are commonly run on moist or thermally dried coal (e.g., Kroos et al., 2001), and because moisture can have a strong effect on adsorption capacity, workers should note how coal samples were prepared for analysis and on what basis (e.g., dry, moist, ash-free, etc.) the results are reported. Other rank parameters, such as volatile matter content and vitrinite reflectance, can correlate with adsorption capacity in some sedimentary basins (e.g., Kim, 1977a; Carroll and Pashin, 2003; see Figure 9.9), although Clarkson and Bustin (1999) indicated that adsorption capacity and rank cannot be correlated on a global basis.

9.3.2 Gas Content Gas content in coal must be estimated to determine the quantity of gas in place and the level of gas saturation in a coalbed methane reservoir. A variety of direct volumetric and gravimetric methods can be

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used to estimate gas content (e.g., Bertrard et al., 1970; Kissel et al., 1973; Levine et al., 1993), and the method most commonly employed by the natural gas industry is the U.S. Bureau of Mines modified direct method (Schatzel et al., 1987). Volumetric methods involve the determination of gas content by measuring changes of pressure caused by desorption in a canister, and an excellent review of the apparatus and methodologies that are used for desorption testing is provided by Diamond and Schatzel (1998). In the coalbed methane industry, gas content is typically determined from cores, and results of desorption studies indicate that gas content can be extremely variable but typically increases with depth (Figure 9.10). While some areas contain coal that is at or near isothermal saturation, other areas contain coal that is nearly devoid of natural gas (e.g., Malone et al., 1987). Ultimately, the same factors that influence sorption capacity are those that influence gas content, and differences in the burial, thermal, and hydrologic history of a

FIGURE 9.10. Plot of gas content versus depth from a core in the Black Warrior Basin showing a general downward increase in gas content with variable gas saturation (modified from Levine and Telle, 1989).

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sedimentary basin can result in highly variable levels of gas saturation, even among closely spaced coal beds in the same well (Scott, 2002). When evaluating gas content and gas saturation in coal, it is important to know gas composition for a number of reasons. First of all, dilution of hydrocarbon gases by N2 and CO2 can reduce the heating value of the produced gas, thereby affecting marketability. In some areas, high levels of nonhydrocarbon gases necessitate the operation of processing plants to upgrade the produced gas to pipeline quality. Equally important, failure to consider gas composition can result in faulty estimates of gas saturation because coal can adsorb different amounts of different gases (Figure 9.8). If one assumes erroneously that desorbed gases consist entirely of CH4, for example, high CO2 content can give the illusion of coal that is oversaturated with methane, whereas high N2 content can make a gas-saturated coal appear undersaturated. Understanding gas saturation levels is important for reservoir management, and using adsorption isotherms in tandem with desorption data can be a powerful tool (Figure 9.7). The critical desorption pressure is the pressure at which a coal sample with a given gas content reaches equilibrium saturation. If reservoir pressure is higher than the critical desorption pressure, the reservoir is undersaturated, and pressure must be lowered for gas to flow. When reservoir pressure falls below the critical desorption pressure, the coal will become oversaturated, and gas will begin desorbing until equilibrium is reached. The slope of the adsorption isotherm decreases as pressure increases, and so the sensitivity of reservoir behavior to gas saturation changes with reservoir pressure. Using a dry, ash-free isotherm from a medium volatile bituminous coal as an example (Figure 9.7), if the reservoir is highly pressured at 15 MPa, a minor undersaturation of 2.5 cc/g indicates that pressure must be lowered by 6 MPa to reach the critical desorption pressure. If initial reservoir pressure is only about 2.5 MPa, the same amount of undersaturation requires that reservoir pressure be lowered only by 1 MPa to reach the critical desorption pressure. Even in an isothermally saturated coal, the shape of the isotherm dictates that relatively little gas is available for desorption in the flat part of the isotherm at high pressure and that the amount of gas that can desorb increases as reservoir pressure falls into the steep part of the isotherm.

9.3.3 Porosity and Permeability Coal contains a complex network of nanopores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) in which fluids can be stored and can flow. The principal source of nanoporosity in coal is apparently associated with the aromatic molecular structure of the biopolymers that are preserved in coal, and as mentioned earlier, this is where the vast majority of adsorbed compounds, including gases, is stored.

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Within this polymeric network, flow is dominated by diffusion rather than Darcy flow, and thus flow within the nanostructure is driven by fluid concentration gradients rather than by pressure gradients. Less is known about the structure and origin of mesoporosity in coal, although Clarkson and Bustin (1999) suggested that mesopores may provide space for multilayer adsorption. Darcian flow becomes increasingly important as pore size increases, and coal can contain substantial macroporosity that can exceed 5% at standard pressure (Gan et al., 1975). Some of this porosity comprises the void space in coal macerals, such as the open cell lumens that can be preserved in pyrofusinite. Most of the macropores associated with primary coal fabric are not well interconnected and thus appear to be of limited significance in coalbed methane production. However, fusain bands can have considerable interconnected porosity and may form significant conduits for fluid flow in coal. Natural fractures, particularly cleat systems, form the vast majority of the interconnected macropore space in coal, and are thus a primary determinant of reservoir properties. Cleat systems are closely spaced (cm- to mm-scale), orthogonal fracture systems in coal that are analogous to joint systems in other rock types (Figure 9.11). Face cleats are systematic fractures; that is,

FIGURE 9.11. Generalized block diagram showing basic properties of cleat systems in coal.

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they tend to be planar, exhibit a high degree of parallelism, and strike parallel to the maximum horizontal stress direction during formation. Butt cleats, by contrast, are cross-fractures; they can curve or have irregular surfaces, tend to strike perpendicular to face cleats, and tend to terminate at intersections with face cleats. Cleat height within a coal bed can be highly variable, and Laubach and Tremain (1991) recognized a distinct hierarchy of cleat types. Primary cleats are strata-bound fractures that extend through a complete bench or bed of coal (height ¼ bed or bench thickness). Many coal beds contain closely spaced primary cleats, which can give coal a columnar, or “matchstick,” appearance. Secondary cleats are developed within a bed or bench (height < than bed or bench thickness). Tertiary cleats are those that are restricted to a single coal band, particularly vitrinite. Using a uniform match-stick model of cleating, porosity and permeability in coal can be expressed as a function of cleat spacing and cleat aperture, such that f ¼ 100  2a=s;

(2)

where f is porosity, a is aperture, and s is cleat spacing, and: k ¼ a3 =12s;

(3)

where k is permeability (Harpalani and Chen, 1995). Thus, porosity is a simple percentage of cleat volume relative to coal volume, and permeability and porosity are related by cubic law. Cleat spacing tends to follow normal to log-normal statistical distributions (Mazumder et al., 2006) and is considered to be a function of the stress history and rheology of coal and is influenced by coal composition. For example, cleats tend to be spaced more closely in bright coal lithotypes, such as vitrain, than in dull lithotypes, such as clarain and durain (e.g., Kendall and Briggs, 1933), and spacing tends to increase with increasing ash content (e.g., Spears and Caswell, 1986). Cleat spacing decreases markedly as rank increases (Ammosov and Eremin, 1960; Ting, 1977). Law (1993) observed that decimeterscale cleat spacing is common in brown coal, whereas millimeterscale spacing is common in medium volatile bituminous and higher rank coal. The relationship between cleat spacing and rank suggests that internal stress associated with devolatization during coalification may be an important mechanism of fracturing in coal. Face cleats tend to be aligned over large portions of sedimentary basins and are thus considered good indicators of the orientation of the paleostress field at the time of fracturing (e.g., Nickelsen and Hough, 1967; Laubach et al., 1998; Engelder and Whitaker, 2006). However, Laubach et al. (1998) emphasized that the interrelationship of regional tectonic stress and devolatization stress during cleat formation remains unclear.

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Cleat apertures range from hairline fractures narrower than one micron to major fractures wider than a millimeter, but information on natural aperture distributions is limited. Recent evidence from image analysis indicates that cleat can follow normal statistical distributions, which contrast sharply with the exponential and power-law distributions that predominate in other rock types (Mazumder et al., 2006). The normal distribution of cleat apertures in coal suggests that permeability is distributed with relative uniformity, whereas in other rock types, exponential and power-law distributions indicate that flow is channeled into a few large-aperture fractures. Cleat aperture and spacing can be related, and Laubach et al. (1998) found that threedimensional frequency and aperture have a power-law relationship. Authigenic minerals can occlude cleat porosity and can affect the permeability of coal. Calcite, quartz, pyrite, kaolinite, fluorite, and a range of lead-zinc minerals are the most common cleat-filing minerals (e.g., Hatch et al., 1976; Spears, 1987; Faraj et al., 1996). Mineralization can span the complete history of a sedimentary basin, and precipitation of authigenic minerals can be associated with coalification, hydrothermal activity, and late-stage bacterial methanogenesis (Hatch et al., 1976; Scott et al., 1994). In the Black Warrior Basin of the southeastern United States, for example, Pitman et al. (2003) used trace element and isotope geochemistry to determine that sulfide mineralization began during peat deposition and continued during coalification and concomitant expulsion of orogenic fluids, whereas carbonate cementation occurred primarily in association with latestage bacterial methanogenesis that followed coalification and postorogenic unroofing. Although cleat-filling minerals tend to occlude porosity and thus reduce permeability, partial mineralization of cleats may prop cleats open, thereby preserving permeability where changing tectonic stresses may otherwise have closed fractures. Well testing demonstrates that the permeability of coal tends to decrease exponentially with depth, and this decrease is thought to be related to the stress-sensitivity of coal (McKee et al., 1988; see Figure 9.12). Coal with permeability higher than 1 Darcy is common near the surface and supports commercial gas production from shallow coalbed methane reservoirs in the Powder River Basin of the western United States (Ayers, 2002). Most coalbed methane reservoirs have permeability on the order of 10 to 100 mD, and in some areas coal as shallow as 700 m can have permeability lower than 1 mD. Importantly, permeability within a single coal bed can vary by more than an order of magnitude at a given depth, which indicates that significant reservoir heterogeneity exists in coalbed methane reservoirs. Moreover, different sedimentary basins may exhibit substantially different permeability-depth gradients depending on lithostatic stress gradients and the magnitude and orientation of horizontal tectonic stresses.

252 Applied Coal Petrology 1000

Permeability (mD)

100

10

1

0.1

n = 34 −0.005x y = 106 e r = -0.80

0.01

0

500

1000

1500

Depth (m)

FIGURE 9.12. Plot of permeability versus equivalent hydrostatic depth based on results of well testing in selected coal basins of the United States (modified from McKee et al., 1988).

Well testing also indicates that permeability in coal can be highly anisotropic. In the Black Warrior Basin, for example, permeability in shallow, well-cleated coal beds can be 25 times higher in the face cleat direction than in the butt cleat direction (Koenig, 1989). In the same well, however, permeability anisotropy in deeper, less permeable coal beds can be minimal or can reflect interference from fracture networks outside of coal. Permeability and production performance can be influenced by the abundance and openness of natural fractures, and characterizing the regional structural framework can be important for identifying productivity sweet spots. In Carboniferous coalbed methane reservoirs of the southeastern United States, for example, faults tend to compartmentalize reservoirs into structural panels with different production characteristics, and productivity sweet spots are commonly aligned with fold hinges and shear zones (Pashin and Groshong, 1998; McIntyre et al., 2003).

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Changes in coal volume associated with adsorption and desorption of gases can also have a strong impact on permeability. Coal matrix shrinks during desorption, thereby increasing cleat aperture and permeability (Harpalani and Schraufnagel, 1990; Levine, 1996). Adsorption swells coal and can thus decrease permeability, and the amount of volumetric strain depends strongly on gas composition. For example, Chickatamarla et al. (2004) found that, at a pressure of 0.6 MPa, swelling strain for N2 is less than 0.03%, for CH4 is less than 0.30%, for CO2 is less than 0.66%, and for H2S is between 1.40% and 9.40%. Accordingly, swelling of coal matrix is a critical concern for the sequestration of CO2 and acid gases. In general, volumetric strain is proportional to Langmuir volume for each gas. For individual gases, volumetric strain has a linear relationship to the quantity of gas adsorbed, and so strain values can be predicted by the Langmuir isotherm (Levine, 1996). Swelling of coal matrix can be offset by hydrostatic stress, and above pressures of 10 MPa, loss of permeability due to swelling can be reversed (Moffat and Weale, 1955; Pan and Connell, 2007). Thus, high-pressure injection of CO2 has the potential to mitigate injectivity loss caused by matrix swelling during sequestration operations. Under reservoir conditions, stress associated with adsorption and desorption is accompanied by hydrostatic and lithostatic stress, and interaction among these factors results in complex relationships among permeability and reservoir pressure, and accounting for these relationships is critical for the development of accurate reservoir models (Palmer and Mansoori, 1998; Pekot and Reeves, 2003; Shi and Durucan, 2004; see Figure 9.13).

9.3.4 Reservoir Pressure Reservoir pressure is a basic control on gas capacity and reservoir behavior and consists of two principal components: lithostatic pressure and hydrostatic pressure. Lithostatic pressure is a consequence of overburden stress, whereas hydrostatic pressure is the component of reservoir pressure caused by pore fluid. Lithostatic pressure is a function of rock density and is generally between 22.7 and 25.0 KPa/m (1.0 and 1.1 psi/ft) in coal-bearing successions (McKee et al., 1988). A normal hydrostatic gradient for fresh water is 9.77 KPa/m (0.43 psi/ft), whereas a normal hydrostatic gradient for sea water is slightly higher at 10.45 KPa/m (0.46 psi/ft). Lithostatic pressure is an important control on permeability, whereas hydrostatic pressure determines how much gas can be held in coal. Hydrostatic stress tends to offset lithostatic stress because of pore-pressure and buoyancy effects, and effective stress can be calculated by subtracting the hydrostatic pressure gradient from the

254 Applied Coal Petrology 10 Low initial porosity, Low Young's modulus

Permeability change (k/k0)

8

6

Generalized range of behavior

4 High initial porosity, High Young's modulus 2

0 0

5

10 Pressure (MPa)

15

FIGURE 9.13. Permeability change versus reservoir pressure based on geomechanical reservoir model incorporating changing effective stress and shrinkage of coal matrix during gas production (modified from Palmer and Mansoori, 1998; Pekot and Reeves, 2003).

lithostatic gradient. Because hydrostatic pressure is reduced as water is removed during coalbed methane production, effective stress increases and can approach lithostatic stress as a coalbed methane reservoir reaches maturity. Increasing effective stress is associated with decreased permeability, but permeability loss can be countered by matrix shrinkage as gas desorbs from the reservoir (McKee et al., 1988; Harpalani and Schraufnagel, 1990; see Figure 9.13). Therefore, careful attention needs to be paid to changes in reservoir pressure and matrix properties, and these changes are commonly characterized using equations that have been published by Palmer and Mansoori (1998), Pekot and Reeves (2003), and Shi and Durucan (2004). A wide range of hydrostatic pressure regimes have been identified in coalbed methane reservoirs (Figure 9.14). Normally pressured reservoirs are those having normal or near-normal hydrostatic pressure gradients for fresh water and sea water and have been identified in several producing basins, such as parts of the San Juan and Black Warrior basins (Kaiser, 1993; Pashin and McIntyre, 2003; Pashin et al., 2003). Normal reservoir pressure is typically associated with water-saturated reservoir conditions and indicates a connection to a meteoric recharge area along the margin of the sedimentary basin,

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FIGURE 9.14. Pressure-depth diagram showing common regimes of reservoir pressure in coalbed methane reservoirs.

which facilitates development of a potentiometric surface at or near the land surface (Figure 9.15). Abnormal reservoir pressure is also common in coalbed methane reservoirs and consists of underpressure where hydrostatic gradients are below 8.0 KPa/m and overpressure where hydrostatic gradients are above 11.0 KPa/m. Underpressure is common in the interiors of geologically old sedimentary basins, like the Carboniferous coal basins of Europe and North America, which are beyond the reach of meteoric recharge (Figures 9.14 and 9.15) and have cooled to the point that pore fluids have contracted and fracture networks have dilated (Bradley, 1975; Kreitler, 1989; Kaiser, 1993). Extreme underpressure, where hydrostatic pressure gradients have been lowered below 5 KPa/m, are known in areas of coalbed methane production that have been affected by dewatering associated with longwall coal mining (Pashin and McIntyre, 2003). Overpressuring is common in geologically young coal basins, such as the Mesozoic-Cenozoic coal basins of the western United States (Kaiser, 1993; see Figures 9.14 and 9.15). Artesian overpressure occurs where a recharge area along a basin margin is elevated above the land surface in the interior of a sedimentary basin, thereby giving

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FIGURE 9.15. Generalized hydrogeologic models showing relationships among reservoir pressure, subsurface flow, and water chemistry in coalbed methane reservoirs.

rise to an elevated potentiometric surface (Figure 9.15). Artesian overpressure is thought to be effective in the San Juan Basin, where recharge occurs along a highly elevated monoclinal fold limb along the northern margin of the basin, which supports exceptionally high reservoir pressure near a permeability barrier that traverses the interior of the basin (Ayers et al., 1994). Hydrocarbon generation is another cause of overpressure that has been identified in low-permeability coal beds in the interior of some coal basins in the western United States (Scott, 2002). Importantly, different pressure regimes are common in different parts of sedimentary basins, thus it is common for pressure-depth data obtained from pressure gauging or water levels to occupy multiple fields (Figure 9.14). In the Black Warrior Basin, for example, pressuredepth data are distributed bimodally between normal to near-normal pressure and extreme underpressure (Pashin and McIntyre, 2003).

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By comparison, pressure-depth data in the San Juan Basin exhibit bimodality between underpressure and overpressure (Kaiser, 1993; Ayers et al., 1994). Bimodal or polymodal pressure-depth distributions are characteristic of compartmentalized hydrologic systems (Bradley and Powley, 1994), and evidence from major coalbed methane reservoirs underscores the heterogeneous, compartmentalized nature of coal as a reservoir rock.

9.3.5 Reservoir Temperature and Gas-Phase Relationships Adsorption isotherms demonstrate the importance of pressure as a critical variable influencing the adsorption capacity and gas content of coal (Figures 9.7 and 9.8), but temperature is an equally important determinant of reservoir behavior that must be considered when evaluating coal as a reservoir rock. Gas molecules become increasingly excited and are thus less prone to remain in an adsorbed state as temperature increases, and isothermal experiments on bituminous coal of the Appalachian Basin establish that adsorption capacity for CH4 decreases exponentially as reservoir temperature increases (Yang and Saunders, 1985; see Figure 9.16). Reservoir temperature in coalbed methane reservoirs typically ranges from 21 C to 65 C, and over this range, the adsorption capacity of a given coal sample can vary by more than 30% (Figure 9.16). For this reason, one should take care to run desorption tests and adsorption isotherms at reservoir temperature to ensure that the results are representative of reservoir conditions. Alternatively, when determining the impact of coal-compositional variables on adsorption performance, it is critical to run adsorption experiments at the same temperature to enable proper comparison. For regional assessments of coalbed methane and carbon sequestration potential, temperature logs and bottom-hole temperature data are invaluable for quantifying and mapping the geothermal gradient and for estimating the effect of temperature variation on the distribution of coalbed methane resources and the carbon sequestration potential of coal (Pashin and McIntyre, 2003; Pashin et al., 2003). Increasing adsorption capacity with decreasing temperature has major implications for gas saturation in sedimentary basins and the prospectivity for coalbed methane (Scott et al., 1994; Scott, 2002). Thermogenic methane generation occurs at temperatures above 100 C, where coal is a relatively weak sorbent. As coal-bearing strata are uplifted to productive depths (150 to 2,500 m), where reservoir temperatures are typically well below the threshold for thermogenesis (Figure 9.16), increasing adsorption capacity in the absence of thermogenic gas generation can be expected to result in undersaturated coal. For this reason, late-stage bacterial methanogenesis and migrated gas

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FIGURE 9.16. Plots showing relationships between methane adsorption capacity and temperature in the Pittsburgh coal of the Appalachian Basin, eastern United States. (Source: Fuel 64, R. T. Yang and J. T. Saunders, “Adsorption of gases on coals and heat-treated coals at elevated temperature and pressure,” 616–620, copyright 1985, with permission from Elsevier.)

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are required to explain high levels of gas saturation in economically viable coalbed methane reservoirs. Some substances, like CH4 and N2, are gases that exhibit effectively ideal behavior under all conceivable pressure-temperature conditions in coal. Other gases, such as CO2, SO2, and H2S, can exhibit distinct phase changes or nonideal pressure-volume-temperature behavior within the range of common reservoir conditions. For example, Chickatamarla and Bustin (2003) found that condensation of SO2 and H2S at pressures below 0.5 MPa at a temperature of 25 C affects the sequestration potential of these compounds in extremely shallow coal beds (note low pressures at which isotherms are terminated in Figure 9.8). CO2 is a strongly nonideal gas that can exhibit multiple phase changes under common reservoir or operational conditions (Kroos et al., 2001; Pashin and McIntyre, 2003; see Figure 9.17). The critical point for CO2, which is the point above which a liquid phase ceases to exist, is at a temperature of 31.1 C and a pressure of 7.4 MPa.

FIGURE 9.17. Phase diagram for carbon dioxide showing the relationship of the critical point and supercritical isochore to common pressure-temperature conditions in coalbed methane reservoirs. (Source: International Journal of Coal Geology 54, J. C. Pashin and M. R. McIntyre, “Temperature-pressure conditions in coalbed methane reservoirs of the Black Warrior Basin, Alabama, U.S.A: Implications for carbon sequestration and enhanced coalbed methane recovery,” 167–183, copyright 2003, with permission from Elsevier.)

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Under normal hydrostatic reservoir pressure, CO2 is stored in coal in a supercritical state beyond a depth of 756 m. Nonideality in terms of gas volume and density is greatest along the supercritical isochore, which is in essence an extension of the gas-liquid boundary into the supercritical fluid field. The liquidus for CO2 lies just outside the realm of typical reservoir conditions for coal (Pashin and McIntyre, 2003). However, injection of CO2 at pressures on the order of 4 MPa in shallow coal beds that are cooler than 31.1 C can take place in a liquid state, thus pressure-temperature conditions must be monitored at the wellhead and in the reservoir to characterize the performance of carbon sequestration activities in coal.

9.3.6 Formation Water Chemistry and Basin Hydrology Water chemistry is a vital variable for understanding coal as a reservoir rock from a number of standpoints. First, because a large volume of produced water must be handled during coalbed methane production, water chemistry is a primary consideration when developing a water management strategy (Ortiz et al., 1993; Pashin and Hinkle, 1997; Van Voast, 2003; Riese et al., 2005). The chemical composition of formation water also provides important clues regarding the regional hydrogeologic framework, the diagenetic processes that take place within coal, and the likely sites of late-stage bacterial methanogenesis (e.g., Scott et al., 1994; Kaiser, 1993; Pitman et al., 2003). In addition, water chemistry is used to classify groundwater resources in the United States and other industrialized nations and therefore determines the legal pathways for underground injection and thus carbon sequestration and enhanced coalbed methane recovery. Surface waters in coal-bearing regions include fresh calcium bicarbonate and calcium sulfate waters, and water chemistry changes significantly in the shallow subsurface (Van Voast, 2003). Microbial activity in the subsurface anaerobic zone results in reduction of sulfate to sulfide and enrichment of water in bicarbonate. Bacterial methanogenesis through methyl fermentation of acetate and carbonate reduction is also associated with this process. Shallow subsurface waters are further enriched in sodium by and depleted in calcium and magnesium, apparently because of cation exchange with clay and other minerals coupled with carbonate precipitation, (Lee, 1981; Van Voast, 2003). For this reason, fresh subsurface waters in coal-bearing strata are almost universally sodium bicarbonate waters. Regional characterization of subsurface water chemistry indicates that coal beds can accept meteoric recharge where they are exposed at the surface and can conduct fresh, sodium bicarbonate waters with total dissolved solids (TDS) content lower than 3,000 mg/L to reservoir depth in the interiors of sedimentary basins,

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thus forming freshwater plumes that extend basinward from the outcrop (Pashin et al., 1991; Ayers et al., 1994; Bachu and Karsten, 2003; see Figure 9.15). Beyond the freshwater plumes, sodium chloride waters of marine affinity predominate, and the TDS content of these waters can exceed that of sea water (30,000 mg/L; Ellard et al., 1992). Freshwater plumes can have a strong impact on the coalbed methane potential and pressure regime of a sedimentary basin (Figure 9.15). Fresh, bicarbonate-rich waters apparently host the bacterial consortia that are responsible for late-stage bacterial methanogenesis and thus high gas content in many sedimentary basins, and the intensity of methanogenesis is reflected in the geochemistry of the produced gas and the petrology of mineral cements (Rice, 1993; Scott, et al., 1994; Pitman et al., 2003). Freshwater plumes further support normal hydrostatic pressure gradients or abnormally high artesian gradients adjacent to recharge areas, whereas underpressure or hydrocarbon-related overpressure tends to predominate in basin interiors beyond the reach of freshwater plumes (Kaiser, 1993; Scott, 2002; Pashin and McIntyre, 2003). Scott (2002) further pointed out that areas with potential for downward flow can be prone to flushing and removal of gases from coal by dissolution, whereas gas commonly accumulates in coal in areas with potential for upward flow. Ultimately, the regional hydrologic framework exerts a major control on gas saturation and reservoir pressure, which in turn determine where a reservoir sits on the adsorption isotherm and how effectively reservoir pressure can be reduced below the critical desorption pressure during coalbed methane production operations (Figure 9.7). Dewatering coal to produce gas can result in a large volume of formation water that must be handled and disposed of safely, and water chemistry plays a central role in deciding how produced waters can be managed. Fresh formation waters with TDS content lower than 10,000 mg/L can in many areas be disposed of at the surface or into streams, although some treatment is commonly required to remove iron, magnesium, and other compounds prior to disposal (O’Neill et al., 1993; Van Voast, 2003). Saline water rich in chlorides can pose significant environmental risk, and in some areas, produced waters with high salinity must be disposed of by underground injection (Ortiz et al., 1993). Water chemistry also determines where and how carbon sequestration technology can be applied legally in coal within existing regulatory frameworks. The following discussion focuses on the United States, and similar regulatory frameworks exist in other regions, such as Canada, the European Union, and Japan. A key challenge for carbon sequestration in coal in the United States is that many coalbed methane reservoirs contain formation water with total dissolved solids content lower than 10,000 mg/L. Formations with TDS content lower

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than 10,000 mg/L are classified as underground sources of drinking water and are thus protected from underground injection. Underground injection is prohibited without administrative hearings in formations containing water with TDS lower than 3,000 mg/L, thus sequestration is effectively infeasible in coalbed methane reservoirs containing very fresh formation water adjacent to recharge areas. Where formation water contains between 3,000 and 10,000 mg/L TDS, an exception is available for enhanced oil and gas recovery. Therefore, enhanced gas recovery is the most viable pathway for the implementation of carbon sequestration technology in a broad range of coalbed methane reservoirs. Where formation water has a TDS content above 10,000 mg/L, carbon dioxide can be injected into coal in the absence of enhanced coalbed methane recovery, although the potential economic benefits tend to favor enhanced recovery over pure sequestration.

CHAPTER 10

Environmental and Health Impacts Robert B. Finkelman Stephen F. Greb

10.1 Introduction Concerns about the impacts of coal on the environment and human health are not new; they may date from its first use as a fuel in China about 3,000 years ago. In the 13th century the concern about the “sulfurous air” in London attracted the attention of the British royalty who issued proclamations banning the use of coal in the city. Until industrialization, the amounts of coal being used were minuscule and the environmental and health problems were local. However, during the past 150 years, increasingly large amounts of coal have been required to satisfy the ever-growing demand for global energy. World coal production exceeded 6 billion tons in 2004 (U.S. Energy Information Agency, 2007) with production anticipated to reach nearly 8 billion tons by 2025 (Energy Watch Group, 2007). In the 1970s a legacy of abandoned mined areas and red-stained streams from acid mine drainage of mines and preparation areas in the United States spurred public concern about the environmental impacts of mining. These concerns led to federal regulations to guide reclamation and limit off-site impacts to the environment. Most industrialized countries regulate modern mining practices, but in those countries with a long mining history, it will take time to mitigate the legacy of past mining. This legacy includes physical disturbances to the landscape, subsidence and settlement above abandoned underground mines, flooding and increased sedimentation, polluted groundand surface-waters, unstable slopes, long-burning fires, miners’ safety, and public safety and land disturbance issues. In areas where such regulations do not exist, these issues are a continued concern.

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Air pollution is another environmental impact of coal combustion. In the past several decades, the association of sulfur emissions with acid rain, and epidemiological studies on the health impacts of particulates, gases, organic compounds, radionuclides, and elements emitted into the atmosphere by coal combustion resulted in federal and state regulations limiting emissions of toxic compounds from coal combustion; the most recent restrictions being implemented in the United States is on mercury emissions. The mining and utility industries have responded with changes in their practices and facilities to meet or even exceed the regulatory limits. Nevertheless, health problems caused by coal emissions still persist, especially in countries in which emissions are unregulated (Finkelman et al., 2002). Various technologies have been developed that capture potentially harmful elements and compounds before they can be emitted to the atmosphere. These technologies convert the particulates and vapor-phase compounds into solid wastes. The solids are then disposed of in sediment ponds and landfills. Although much of this material is inert, understanding the conditions under which they might leach harmful compounds is important. As scientific and public awareness of the potential environmental and health impacts from coal use have been realized, there has been accelerated research into the elemental make-up of coals and the fate of elements and other potentially deleterious compounds during and following combustion. In many countries, technologies are being implemented and research is being conducted to develop new methods for reducing deleterious coal combustion emissions (see, for example U.S. Department of Energy [2007], Clean Coal Technology program). Nevertheless, it may be many years before the benefits of these new technologies are realized worldwide, because many developing countries cannot afford or are unwilling to invest in these technologies. In addition, hundreds of millions of people in developing countries use coal in their homes and workplaces to heat and to cook. This coal is generally used with little or no efforts to reduce pollution, exposing the residents and workers to the full impact of the resultant emissions, which can be benign or significant depending on the coal used. In this chapter, some of the environmental and human health issues related to coal and coal use are examined. Emphasis will be placed on those issues that are related to the petrographic, chemical, and mineralogical composition of coal. We will first look at environmental and health issues associated with in-ground coal, then the environmental and health issues created by coal mining and processing. The next topic is the emission-related environmental and health issues from commercial and residential coal use. Finally, the potential impacts of coal combustion byproducts are discussed.

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It is important to note that because coal beds have variable composition, the potential health and environmental impacts from coal mining, processing, and utilization are also variable. In addition to coal quality, some of the factors that influence whether or not coal use may have an adverse health or environmental impact in a given area are the geology of the coal and surrounding strata, local hydrology, climate, topography, mining methods and combustion and pollution control technology used, reclamation methods (if any), type of regulation and oversight (if any), cultural practices, and the state of public health.

10.2 In-Ground Coal Environmental and Health Issues In-ground coal beds do not generally pose an environmental or health threat. Locally, however, coal mineralogy influences groundwater chemistry and in situ coal fires can lead to subsidence, degradation of groundwater, and health and safety issues related to heat and smoke from the fires (discussed in the section on spontaneous combustion). Some of the critical hydrogeochemical processes that influence groundwater chemistry in coal-bearing strata include CO2 production in soils, silicate hydrolysis, pyrite oxidation, carbonate mineral dissolution, cation exchange, sulfate reduction, and the precipitation and dissolution of secondary minerals such as gypsum, kaolinite, and goethite (Groenwold et al., 1981; Powell and Larson, 1985). These reactions can result in increasing groundwater mineralization. For example, in many coal basins iron and magnesium concentrations are elevated in coal-bearing strata (Banaszak, 1980; Groenewold et al., 1981; Powell and Larson, 1985). Although rare, there are a few places in the world where inground coal can have a serious impact on human health prior to mining, transportation, and combustion. In the early 1990s an association was noted between the distribution of Pliocene lignites and the occurrence of a debilitating kidney disease known as Balkan endemic nephropathy (BEN) in the former Yugoslavia (Feder et al., 1991; Finkelman et al., 1991). There are numerous other factors that may be causative agents (Tatu et al., 1998; Orem, 2007), but field and geological investigations revealed that all the endemic regions with one possible exception are in close proximity to known deposits of Pliocene lignites (Feder et al., 1991; Goldberg et al., 1994). This link between BEN and Pliocene lignites has also been documented in Romania (Tatu et al., 2000a). In all of the endemic villages a common factor is that the primary source of drinking water is wells completed in lignite aquifers or in sediments rich in lignitic material. Analysis of these well waters

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has revealed the presence of organic compounds such as polycyclic aromatic hydrocarbons and aromatic amines (Tatu et al., 1998). Similar compounds have been water-leached from the lignites in the laboratory (Orem et al., 1999; Tatu et al., 2000b). These compounds were not detected in the water from wells in villages where BEN is unknown. It was postulated by Feder et al. (1991) that the organic compounds, leached from the lignites by groundwater and transported to water supplies, are significant contributory factors in causing BEN. Although the concentrations of organic molecules in the well water may be low, long exposure and/or accumulation in body tissues over time may lead to kidney lesions, the development of urothelial carcinomas in some individuals, and ultimately to chronic BEN. The conditions resulting in BEN and related health problems may exist in locations outside of the Balkans. Recent studies have demonstrated a similar suite of organic compounds in water from wells underlain by Paleocene-Eocene lignites in Louisiana (Bunnell et al., 2006). This area is known to have one of the highest incidents of renal pelvis cancers in the United States (U.S. National Cancer Institute, 2007). Moreover, in Portugal, the region where the Rio Maior lignites were formerly mined coincides with the highest incidence of kidney disease in the country (D. Flores, personal communication, 2006). It is uncertain whether these relationships in either area are coincidental, contributing, or causative. It would seem prudent to check the incidence of kidney diseases and renal pelvis cancers in regions wherever lignites, or sediments rich in low-rank organic material, are the principal aquifers. One important area of research that should be pursued would be to determine if there is any relationship between the types and amounts of leachable organic moieties and the petrographic components of the low rank coals.

10.3 Coal Processing and Mining Coal mining has long been known to cause highly visible environmental impacts. Summaries of pertinent issues in coal basins of most industrialized nations can be found on Websites and in publications of the regulatory agencies that oversee mining in those areas. In the United States, a wealth of information is available through the Office of Surface Mining, and Environmental Protection Agency. Greb et al. (2006) summarized the following coal mining issues from mining in the United States: (1) physical disturbance of the landscape; (2) subsidence and settlement; (3) erosion, surface runoff, flooding, and sediment control; (4) degradation of surface water and groundwater quality; (5) coal mine fires; (6) fugitive methane; (7) public safety and land disturbance issues; and (8) miners’ health and safety.

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Many of these issues have more to do with surrounding rock geology, mining method, climate, hydrology, type of reclamation used (if any), and regulation and enforcement (if any), than the petrography or geochemistry of the coal or disturbed strata. Herein, we will concentrate on those environmental impacts in which the mineralogy and geochemistry of the coal and surrounding strata are a significant factor.

10.3.1 Miners’ Health Although coal mining is a mature endeavor, new information is still emerging on its health consequences. It has long been recognized that inhalation of dust generated during mining is associated with a lung disease unique to coal (Collis and Gilchrist, 1928). black lung disease (coal workers’ pneumoconiosis, or CWP) is characterized by dust-induced lesions in the gas exchange regions of the lung (Finkelman et al., 2002). CWP is a debilitating respiratory disease resulting in diminished ability to exchange oxygen. The Federal Coal Mine Health and Safety Act of 1969 legislatively defined “black lung disease” to include not only CWP but also obstructive lung diseases, such as chronic bronchitis and emphysema, as well as silicosis associated with an employment history in coal mines. Data indicate a direct relationship between the mass of respirable coal mine dust inhaled and the incidence and severity of CWP (Hurley et al., 1982). Hence, the disease is a function of the amount of dust in the air and the cumulative times a miner inhales respirable dust. CWP is also affected by coal rank. Normalizing for similar dust exposures, CWP is five times more prevalent in anthracite miners than for miners of lower rank coal (Bennett et al., 1979). It has been shown in the United States, Great Britain, France, and Germany that the prevalence and severity of CWP differed markedly among different coal mines despite comparable exposures to respirable dust (Huang et al., 2004). Based on a national study of coal workers’ pneumoconiosis and the U.S. Geological Survey’s coal quality database (Bragg et al., 1997), Huang et al. (2004) has shown that the prevalence of CWP in seven coal mine regions in the United States correlates with levels of bio-available iron (BAI) in the coals from that particular region (correlation coefficient r ¼ 0.94, p < 0.0015). CWP prevalence is also correlated with contents of pyritic sulfur (r ¼ 0.91, p < 0.0048) or total iron (r ¼ 0.85, p < 0.016), but not with coal rank (r ¼ 0.59, p < 0.16) or silica (r ¼ 0.28, p < 0.54). Using the linear fit of CWP prevalence and the calculated BAI in the seven coal mine regions, they derived and mapped 7,000 coal samples’ pneumoconiotic potencies (Figure 10.1). The study

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FIGURE 10.1. Correlation between prevalence of black lung disease and BAI (bioavailable iron, an estimator of pyrite content) in seven U.S. underground coal mine regions. The numbers of coal samples per state for which analytical data were available are shown in parentheses (from Huang et al., 2004).

indicates that levels of BAI in the coals may be used for the prediction of coal’s potential toxicity, even before large-scale mining.

10.3.2 Water Quality: Acidic Drainage Coal and coal-bearing strata contain disulfide minerals, such as pyrite (FeS2), which can lead to acidic drainage. The term acid mine drainage (AMD) or acid mine water is generally used to describe such waters. AMD may originate from abandoned surface or underground coal mines, processing facilities, or waste-rock piles. Acidic drainage is an environmental concern because it causes depleted oxygen levels, toxicity, and corrosion that degrades water quality, and damages aquatic habitats (Appalachian Regional Commission, 1969; Kimmel, 1983; Earle and Callaghan, 1998; Rose and Cravotta, 1998; Skousen et al., 1998). AMD is most prevalent where mining or processing breaks up sulfide mineral-bearing strata, and brings those minerals into contact with water and oxygen. Because the most common sulfide mineral is pyrite, areas in which strata with high pyrite and low carbonate concentrations are disturbed have greater AMD potential. Coals, dark gray to black marine to brackish-water shales, underclays, and sandstones are the most common strata that may contain high pyrite concentrations disturbed by coal mining (Carrucio and Geidel, 1980; Brady et al., 1998; Renton et al., 1989). Weathering of pyrite (and other disulfide minerals) occurs as a series of reactions, which are summarized in numerous publications

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about acid mine drainage (Singer and Stumm, 1970; Nordstrom, 1982; Stumm and Morgan, 1996; Rose and Cravotta, 1998; Costello, 2003). The primary reactions are: 2 þ 2FeS2 þ 7O2 þ H2 O ! 2Feþ 2 þ 4SO4 þ 4H

(1)

4Fe2þ þ O2 þ 4Hþ ! 4Fe3þ þ 2H2 O

(2)

4Fe3þ þ 12H2 O ! 4FeðOHÞ3 þ 12Hþ

(3)

þ FeS2 þ 14Fe3þ þ 8H2 O ! 15Fe2þ þ 2SO2 4 þ 16H

(4)

The precipitation of the iron solids (Fe(OH)3) characteristic of AMD results from hydrolysis (Equation 3). These solids (“yellow boy”) precipitate if the pH of the water is more than 3.5, and continue to precipitate until the iron in solution is exhausted (Rose and Cravotta, 1998). Because the weathering of pyrite (and other disulfide minerals) is well recognized as acid-producing, coal mines in the United States (and many other countries) are required to test rock units that will be disturbed by the mining process for their potential to produce acidity or alkalinity in their permits, prior to mining. Permits also must outline specific mining and reclamation plans that will limit acid production and off-site migration of impacted groundwater or surface water. Acid-base accounting measures are generally used to determine the potential acidity and alkalinity of the rock strata that will be disturbed by mining (e.g., Sobek et al., 2000). Acid-base accounting measures the acid potential of rock strata or layers that will be disturbed from its sulfur content (generally measured from pyritic sulfur content) against the neutralization potential from its acid-soluble carbonate mineral content. Those layers that have high pyritic sulfur contents are removed selectively and disposed of in a manner that limits further oxidation or surface runoff. Those layers that have high-neutralizing potential will be selectively mixed with the pyritic material to neutralize acid production (Cravotta et al., 1990; Perry, 1998; Skousen et al., 1998; Smith and Brady, 1998). Although regulations and modern mining practices have limited acid mine drainage in the coal basins of most industrialized nations, there is an unfortunate legacy of acidic, rust-colored streams in older mine areas from pre-regulation mining and in countries where mining is unregulated. Research into that legacy has led to a wide range of mitigation and abatement techniques including alkaline additions, water treatment, engineered structures such as alkaline barriers and drains, and wetland construction (Cravotta et al., 1990; Hedlin et al., 1994; Skousen et al., 1998; Zipper and Jage, 2001; U.S. Environmental Protection Agency, 2001; Costello, 2003).

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10.3.3 Water Quality: Other Issues Although acidity is a major environmental issue in many areas, increased mineralization, dissolved solids, and alkalinity can be problems in some coal basins. The U.S. Geological Survey and Office of Surface Mining Reclamation and Enforcement compiled a series of case studies of the hydrological impacts of mining and processing (Richards, 1985), many of which are related to the mineralogy of the disturbed strata. An example of the type of environmental issue that can result from alkaline mine drainage is increases in soluble salts in arid coal basins of the western United States. In these areas, the uppermost zone of weathered overburden may contain soluble salts. If this material is used as fill and comes in contact with groundwater or is eroded in seasonal discharges, the salts can be leached. For this reason, an environmental concern in dry climates is that surface mining can lead to increased total dissolved solids and increased sulfate, calcium, and magnesium concentrations of groundwater, as well as the potential for increased concentration of trace metals associated with the alkaline material (Woessner et al., 1979; Committee on Ground Water et al., 1990; Ferris et al., 1996). Selenium is one of the trace metals of particular concern in the western United States because it can be mobilized as selenate (SeO42) in alkaline spoils, which is the form of selenium available for plant uptake. Animals grazing on the plants can be poisoned by high selenium concentrations in the plants (Committee on Ground Water et al., 1990). In the Powder River Basin, selenium-rich Cretaceous bedrock can lead to elevated selenium in soils and groundwater, and also is the ultimate source of detrital selenium in the mined coals (Oman et al., 1988). Dreher and Finkelman (1992) noted multiple forms of selenium in a study of a small backfill area of a surface mine in the Powder River Basin that had high selenium concentrations. Selenium occurred in water soluble salts, in ion-exchange positions on clays, in fine-grained sulfides such as galena (PbS), in fine-grained selenides such as clausthalite (PbSe), in clays and silicates, in solid solution in pyrite, and in organic associations. Specialized reclamation techniques are often needed in dry climates to prevent off-site contamination of surface- and groundwater from mining areas (Ferris et al., 1996). Selective placement of weathered overburden, similar to specialized handling of pyritic material in areas of potential acidic drainage, can aid in controlling potentially deleterious mine drainage in arid climates.

10.3.4 Mine Fires (Spontaneous Combustion) Uncontrolled coal fires have been reported from the United States, Canada, China, Australia, India, Indonesia, South Africa, England,

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Germany, Poland, Czech Republic, Russia, Ukraine, Turkey, Thailand, and other countries (Walker, 1999). Such fires can be started naturally through spontaneous combustion, lightning strikes, and forest fires, as well as through human influences such as accidental forest fires, in-mine ignitions, and burning wastes at old-mine sites adjacent to in-ground coal (U.S. Office of Surface Mining, 1992; Kim and Chalken, 1993; Stracher and Taylor, 2004a). Most of these mechanisms are unrelated to the chemical or physical properties of the coal. However, coal particles can naturally adsorb oxygen, a reaction that gives off heat and can lead to spontaneous combustion. The surface area of the particles exposed to oxygen is a major control on its heating tendency. Although largely a function of storage time and grain segregation, there are rank influences on a coal’s potential to spontaneously combust. According to the U.S. Department of Energy (U.S. DOE), anthracite is much less combustible than subbituminous coal. Lower rank coals with a high percentage of reactive macerals (vitrinite and liptinite) may increase the heating tendencies of a coal (Lyman and Volkmer, 2001). Higher heating tendencies are correlated to higher inherent moisture, lower ash-free Btu, and higher oxygen contents in coals. Also pyrite oxidation is exothermic so that higher sulfur coals have the potential for increased heating. It was once thought that pyrite oxidation was a major factor in spontaneous combustion, but it is now considered less important than other oxidizing characteristics (U.S. Department of Energy, 1994). Pyrite swelling can cause coals to disintegrate, exposing greater surface area to oxidation. Pyrite contents of more than 2% are generally required to have any influence (Kim, 1977b). The U.S. DOE suggests that spontaneous combustion can be limited by proper handling, storage, and keeping the moisture contents of stored coal at less than 3% and sulfur contents at less than 1% (as mined basis) (U.S. Department of Energy, 1993). Fires in coal beds burn slowly (tens of meters/year) and can burn for decades. They are an environmental concern and health risk because coal fires can cause unsafe heat, noxious emissions, and surface subsidence (Figure 10.2). Despite the abundant literature (see Stracher, 2004) on the causes of these fires, their environmental impacts, and the efforts to extinguish them, relatively little has been written about the potential human health impacts of burning coal beds and waste banks. Stracher (2002), quoting Anupma Prakash, notes that pollution from burning coal beds from the Jharia coal field in India can cause people to suffer from asthma, chronic bronchitis, skin disease, and lung disease. In some cases, there may be potential health hazards from exposure to high concentrations of trace elements and/or gases emitted due to high concentrations of certain elements in the burning coal

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FIGURE 10.2. Burning coal in the Jharia (India) coal field. (Photograph courtesy of Anupma Prakash).

or surrounding rock strata. Finkelman (2004), for example, discussed the health problems that could be caused by volatilization of arsenic, selenium, and fluorine from the uncontrolled coal fires. A recent study (Pone et al., 2005) from a coal mine in South Africa demonstrated that coal-fire gases at the mine contained toxic levels of benzene, toluene, xylene, and ethylbenzene Lapham et al. (1980) expressed concerns about the possible health impacts of trace elements mobilized by the burning coal waste banks. They recommended (p. 77) that there be further study of the trace- and minor-element composition of the air, soil, and water in the vicinity of the burning waste piles. They go on to say that “Such studies, carried out in conjunction with a compilation of health statistics for the region, would be of use in determining whether . . .” there were any harmful effects from the burning coal waste piles.

10.4 Coal Use: Emissions Coal is combusted as a fuel (see Chapter 4) to generate electric power and for heating and powering foundries, cement plants, and other industrial and manufacturing facilities. During combustion, elements in the coal are converted to their respective oxides. Some of these oxides are gaseous, aerosols (liquid droplets), or may be adsorbed by particulates (fly ash) and continue through the flue-gas stream as emissions. Gaseous emissions include water vapor, sulfur oxides (SOx), nitrogen oxides (NOx), carbon dioxide (CO2), and may contain compounds and elements identified as hazardous air pollutants (HAPs) including mercury.

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10.4.1 Sulfur Oxides Understanding the mode of occurrence of sulfur in coal is a critical part of quality control and coal processing (see Chapter 3). For some coals, conventional cleaning effectively removes a large percentage of inorganic sulfur associated with sulfides (most commonly pyrite) from the coal. Even in coals processed to remove pyrite, however, some sulfur remains, especially where the sulfur is organically associated. When the coal is combusted, any remaining pyritic sulfur and organic sulfur is oxidized to sulfur dioxide (SO2). Emissions of sulfur dioxide and nitrogen oxides can react with water and water vapor in the atmosphere to form sulfurous acid (H2SO3), sulfuric acid (H2SO4), and nitric acid (HNO), the components of acid rain. Acid rain is an environmental concern because it can cause crop damage, forest degradation, impaired visibility, chemical weathering of building stones and monuments, and increased acidity of lakes and streams. Acid rain is also a human health risk for asthma and bronchitis (U.S. General Accounting Office, 2000; U.S. Environmental Protection Agency, 2004a). In the 1960s and 1970s recognition that acid rain was causing environmental damage and possibly impacting human health downwind from major coal-fired power plant areas in parts of Europe and eastern North America spurred multinational agreements to limit sulfur emissions. Agreements such as the Convention on Long Range Transboundary Air Pollution in Europe (Economic Commission for Europe, 2004) and national regulations such as the 1990 Clean Air Act Amendments in the United States have led to substantial decreases in sulfur deposition in industrialized nations, all while increasing energy production (Figure 10.3). This is not, however, the case in all countries. Currently, China and India both emit largevolume SO2 emissions with little regulation. In 2006, the New York Times reported that China produced one sixth of the world’s sulfur

FIGURE 10.3. U.S. sulfur concentration map. Sulfate ion concentrations in the United States showing decreasing concentrations as a result of sulfate regulations. (Maps modified from National Atmospheric Deposition Program, 2007, http://nadp.sws.uiuc.edu/amaps2/.)

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pollution, approximately 22.5 million tons in 2004. This was more than twice the amount released in the United States, and affects China as well as areas downwind, including Japan, Korea, Taiwan, and the Philippines (American University, 1997; BBC, 2006). In countries where sulfur emissions have been regulated, coalfired utilities switch to lower sulfur coals or increased processing of coals to decrease sulfur content before combustion. In the United States, this led to a dramatic increase in the production of low-sulfur subbituminous coals from the Powder River Basin and decreases in the production of higher sulfur Midwest and Northern Appalachian basin bituminous coals. Several technologies have also been implemented to reduce sulfur emissions, including flue-gas desulfurization (FGD) units (see Chapter 4), often referred to as scrubbers; fluidized bed combustion (FBC) (Chapter 4); and integrated gasification combined cycle (IGCC) (Chapter 5) (U.S. Energy Information Administration, 1997, 2001; U.S. Department of Energy, 1999).

10.4.2 Nitrogen Oxides Three types of nitrogen oxides (NOx) are produced by coal combustion. Nitrous oxide (NO) is the dominant form, with much less (a few volume %) nitrogen dioxide (NO2), and trace amounts of nitrous oxide (N2O). In coal combustion, the dominant source (as much as 80%) of NOx is the coal (fuel NOx), with secondary thermal NOx formed from the heating of atmospheric nitrogen in the combustion chamber. NOx is an environmental and health concern because it combines with atmospheric precipitation to form nitric acid, which is a component of acid rain. NOx can also contribute to ground-level ozone, which is a component of smog and a human respiratory irritant, and can lead to declines in agricultural crop and commercial forest yields (U.S. Environmental Protection Agency, 2004a,b). Wang et al. (1994) showed that coal N conversion to NO is rank dependent, with the fractional conversion of coal N increasing with rank. This was most noticeable in low to medium rank bituminous coals. They also noted that coal-char N conversion to NO during combustion is related to reactivity with the more reactive coals having lower NO-N ratios. Chars are material produced by incomplete combustion of carbon and represent a later stage source of N in the combustion process. Several studies have also noted a relationship between maceral content and nitrogen emissions. Hindmarsh et al. (1994) noted that nitrogen contents of vitrinite macerals were greater than semifusinite and fusinite macerals and their chars. However, fusinites produced more NO during the conversion of coal and char N to NO than semifusinite and vitrinite.

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As with SOx emissions, regulations on NOx emissions in the United States and many other countries have resulted in significantly lower emissions. However, rather than switching to low-nitrogen fuels, regulated NOx reductions at power utilities are mostly accomplished through the use of advanced burner technology (U.S. Energy Information Administration, 2003).

10.4.3 Particulate Matter Fly ash, NOx, and SO2, emitted from coal combustion, can react in the atmosphere to form fine particles and liquid droplets in the form of nitrates and sulfates called aerosols or particulate matter. Particulate matter comes from many sources, but when formed from coal combustion, the amount and type of particulate matter produced depend on the mineral content of the coal, combustion temperature and technology, emissions capture technology (if any), operating conditions, and regulations (if any). Particulate matter (PM) is an environmental concern because it contributes to haze in the atmosphere. These fine particles are a health concern as well, having been linked to respiratory and cardiovascular ailments as well as premature death (Wilson et al., 1980; U.S. Environmental Protection Agency, 1996, 2006b). Many industrialized nations have passed regulations to limit particulate emissions. There are two main technologies for reducing direct emissions of particulate matter in coal-fired power plants, called electrostatic precipitators and fabric filters. An electrostatic precipitator imparts an electrical charge to particles, and then attracts the particles to oppositely charged metal plates for collection. Fabric filters, or baghouses, collect particles by passing flue gases through a porous fabric material. The solid residue (fly ash) from both technologies is removed and disposed of in impoundments and landfills. Electrostatic precipitators commonly reduce direct particulate emissions by 99.9%, while baghouses can capture as much as 99.99% of particulates (U.S. Energy Information Administration, 1995; U.S. Department of Energy, 2004a). In addition, flue-gas desulfurization (“scrubbers”) have the added benefit of removing some particulates and trace elements. FGD units are often used in combination with fabric filters and/or electrostatic precipitators (Figure 10.4).

10.4.4 Trace Elements The fly ash produced by coal combustion contains trace elements from both the inorganic and organic components of the original coal, the concentrations of which are commonly enriched by about a factor of ten compared to the original coal. Nonvolatile elements tend to be

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FIGURE 10.4. Typical pollution controls on a conventional pulverized coalfired steam plant. There are many variations on these technologies. Future plants may add additional mercury and carbon dioxide controls. (Modified from U.S. Environmental Protection Agency, 2007, www.epa.gov/mercury/ control_emissions/tech_exist.htm.)

incorporated into boiler slag, but volatile elements such as As, Cd, Cu, Ga, Pb, Sb, Se, and Zn will condense or adsorb onto outlet and inlet fly ash as the flue gas cools (Klein et al., 1975a,b; Block and Dams, 1976; Van Hook, 1979). Elements such as As, Cd, Cr, Ni, Pb, S, Sb, Se, Ti, and Zn may be preferentially concentrated in fine particulate matter in emissions (Natusch and Wallace, 1974). Moreover, the finer particle sizes (those more readily inhaled and retained in the lung) have the highest concentrations of most trace elements (Natusch and Wallace, 1974). In its report (U.S. Environmental Protection Agency, 1998a,b) to Congress on the health impacts of coal-burning electric utilities, the U.S. EPA concluded that, with the exception of mercury, there is no compelling evidence to indicate that emissions of trace elements or organic compounds cause human health problems. This is not the case in all countries. Mercury The health problems attributed to mercury are not due to direct exposure to emissions from coal-burning power plants but are due to methylation of the mercury and resultant amplification of mercury concentration as it moves up the food chain. When mercury is deposited in water it can speciate into methylmercury, a form of mercury

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that can bioaccumulate in macroinvertebrates and fish. People who eat large amounts of contaminated fish can develop neurotoxicity. The greatest potential concern is for pregnant mothers who eat large amounts of methylmercury-accumulated fish during pregnancy, which could lead to neurological disorders in the fetus (U.S. Environmental Protection Agency, 1996, 1997, 1998b). Coal combustion is just one of many sources of mercury (Chapter 4) in our environment. According to Gray (2003) the atmosphere, volcanoes, land exhalation, mining, the oceans, and anthropogenic processes are all major contributors of mercury to the global environment. For example, in the United States, the EPA estimates that coal combustion accounts for only 1% of total mercury deposition (U.S. Environmental Protection Agency, 1996). Although accounting for only a small percentage of mercury emissions, coal-fired power plants are large point sources of anthropogenic mercury. Major reductions in mercury emissions have occurred through implementation of particulate, SOx, and NOx emissions controls. Newer technologies such as activated carbon injection are being evaluated for further mercury reductions (U.S. Department of Energy, 2004b). The situation in the United States and other developed countries that use sophisticated pollution control systems on their coal-burning power plants is not typical of many developing countries. Yudovich and Ketris (2005b) summarized examples of higher than background mercury concentrations resulting from coal combustion emissions in several countries, including the Ukraine, Russia, and China. In most cases, the reason for high mercury concentrations was high concentrations of mercury in the coals, coupled with a lack of emissions controls on power plants, coking ovens, or stoves where the high-Hg coals were used for home use. The lack of emissions controls and use of coal in homes in certain parts of China result in particularly acute environmental and health issues. Xie et al. (2006) found arsenic concentrations in particulate matter from coal-fired power plants in the Shanxi province of northern China that were 61 times the World Health Organization’s suggested guideline of 0.7 ng m3. Aside from the relative lack of emissions controls, there are large areas of China where coal is used for heating and cooking in homes. In parts of China, villagers are using coals containing from 20 to more than 50-ppm mercury (the average concentration of mercury in coals in China and elsewhere is less than 0.2 ppm). Residential use of coals with elevated trace elements can present serious human health problems because the coals are generally mined locally with little regard to their chemical composition and the coals are commonly burned in poorly vented or unvented stoves directly exposing residents to the emissions.

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The extraordinary high levels of mercury in these coals may be a contributing cause to the very high incidence of neurological conditions such as blindness in the elderly people in these villages. Arsenic Perhaps the most dramatic example of direct health impacts of coal use is the situation in Guizhou province, P.R. China (Zheng et al., 1996). In this region highly mineralized coal has been used in unvented residential environments resulting in severe health problems for an estimated 10 million people. Some of the coals in this region have arsenic contents as high as 35,000 ppm (Belkin et al., 1997a,b, 1998). For comparison, the average arsenic content in U.S. coals is more than 1,000 times less (Finkelman, 1993) and the average arsenic concentration in coals in China is about 5 ppm (Belkin et al., 2006). In coals, arsenic tends to occur in sulfides, especially pyrite, but can also be organically bound (Belkin et al., 1997a; Yudovich and Ketris, 2005c). Both organic and inorganic arsenic can exist as chemically bound forms and in sorbed (acid leachable) arsenate form. Relations to ash content and petrography are generally complex. Likewise, the source of the arsenic can be syngenetic or epigenetic. In most high-As coals, including those of the Guizhou province, epigenetic hydrothermal fluids are responsible for arsenic enrichment (Yudovich and Ketris, 2005c). In the Guizhou province, Belkin et al. (1997a) noted inorganic arsenic in pyrite (as framboids, idiomorphic crystals, and irregular grains), arsenopyrite grains and cell fillings, As-bearing phosphates, As-bearing jarosite, As-bearing Fe-oxides (likely hematite), and possibly scorodite. Dai et al. (2006) noted As, Hg, Sb, and Ti enrichment in getchellite (rather than pyrite and clay minerals) in a veined kaolinite. Zhang et al. (2002) found high concentrations of As, Mn, and Sr in calcites as well as submicron and micron pyrites in pyrite veins of the Guizhou coals. Varying arsenic concentrations were correlated to different forms of calcite (Zhang et al., 2002). Where arsenic is inorganically bound in pyrite, conventional coal cleaning can be efficient at removing significant arsenic, and where ESPs are used as part of an emissions control system, they capture 97–99% of any arsenic remaining in the coal that is volatilized during combustion. Scrubbers are also very good at removing arsenic from the gas stream (Yudovich and Ketris, 2005c). Aside from the inorganic associations, arsenic in the Guizhou coals is also associated with the organic components (Finkelman et al., 1999). This arsenic cannot be removed or recognized in hand specimens, as would be the case if the arsenic were limited to pyrite inclusions, as it commonly does in coals elsewhere. In the cool, damp fall, crops such as chili peppers and corn are brought into the houses

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and hung from the rafters to dry over the coal fires. The chili peppers dried in this manner absorb large contents of arsenic. Ingestion of the arsenic-bearing chili peppers and inhalation of the volatilized arsenic have resulted in thousands of cases of severe arsenic poisoning. Symptoms of arsenic poisoning including hyperpigmentation (flushed appearance, freckles), hyperkeratosis (scaly lesions on the skin, generally concentrated on the hands and feet), Bowen’s disease (dark, horny, precancerous lesions of the skin), and squamous cell carcinoma (Figure 10.5). Arsenic mobilized from coals being burned in power plants has also been cited as causing health problems in the former Czechoslovakia. An epidemiological study of a coal-burning power plant using lignite (low rank coal) containing high (about 800 ppm) arsenic found that children living near the plant had a marked loss of hearing that was attributed to the arsenic exposure (Bencko and Symon, 1977). Fluorine Fluorine is another element that is found in high concentrations in some Chinese coals and is associated with health problems in rural

FIGURE 10.5. Hyperkeratosis, the brown, scaly patches covering this man’s torso and Bowen’s disease, the black lesion on the left breast, are attributed to arsenic exposure from residential coal combustion. (Photograph by Zheng Baoshan.)

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FIGURE 10.6. Dental fluorosis attributed to fluorine exposure from residential coal briquette combustion. (Photograph by Zheng Baoshan.)

areas. In a broad geographic area millions of Chinese villagers are suffering from dental and skeletal fluorosis (Zheng and Huang, 1989; Ando et al., 1998). The fluorine, concentrated in the dried corn and chili peppers, is ingested as well as inhaled, is derived from both the coal and the fluorine-rich soil that the villagers use as a binder in their homemade briquettes. Typical signs of fluorosis include mottling of tooth enamel (dental fluorosis, Figure 10.6) and various forms of skeletal fluorosis including osteosclerosis, limited movement of the joints, and outward manifestations such as knock-knees, bowlegs, and spinal curvature. In the Guizhou province, high fluorine contents in late Permian coals are associated with hydrothermal fluids along tectonic faults (Zhang et al., 2002). Similarly, in the Daba area and southwest China, fluorine concentrations (as much as 3,000 mg/kg) are closely related to basinal structures, volcanic activity, and magmatic intrusions (Luo et al., 2004). Deep-seated structures and hydrothermal enrichment have also been inferred for anomalously high fluorine concentrations in lower Pennsylvanian coals of the Black Warrior Basin, USA (Goldhaber et al., 1997). Selenium Zheng et al. (1992) report nearly 500 cases of human selenosis in southwest China that are attributed to the use of selenium-rich carbonaceous shales known locally as “stone coal.” The stone coals have more than 8,000 ppm selenium. This selenosis is attributed to the practice of using the combustion ash rich in selenium (Zhu et al.,

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2004) as a soil amendment. This process introduced large amounts of selenium into the soil and resulted in uptake of this element by crops. Symptoms of selenium poisoning include discoloration of the skin and loss of hair and nails. Aside from the associations already discussed, selenium has been associated with: syngenetic pyrite in late Permian coals of China (Zhuang et al., 2003); epigenetic hydrothermal fluids in the southern Qinling and Daba regions of China (Luo et al., 2001); and weathering of coal and soils in the Shaanxi province of China (Fang et al., 2003). Mineral associations include pyrite, native selenium, clausthallite (PbSe), and ferroselite (FeSe2), which are absorbed in galena, possibly in clay or on coal, and in organic associations (Finkelman, 1981, 1995; Swaine, 1990; Mukherjee and Srivastava, 2005). PAHs Many studies have been carried out on the high incidence of esophageal and lung cancers in China (Lan et al., 2002), but the dominant causative agents of the cancer remain unclear. Polycyclic aromatic hydrocarbons (PAH) released during unvented coal combustion in homes in China have been cited as the primary cause for the highly elevated incidence of lung cancer (Mumford et al., 1987). Mumford et al. (1995) have linked the high lung cancer mortality rate (five times the national average of China) in Xuan Wei, China, to high PAH levels in homes burning “smoky” coal. It has yet to be determined if the PAH are causative factors, contributing factors, or just coincidental factors. Tian et al. (2002) suspect that the presence of abundant, extremely fine-grained acicular quartz grains in the coal may be a contributing factor to lung cancers in Xuan Wei county. As shown in the previous examples, the incidents of major health impacts from trace elements are largely a function of unusually high concentrations of hazardous trace elements in mineralized coals. Fortunately, such highly mineralized coals are quite rare. Chemical analysis by the U.S. Geological Survey of coals of several thousand mines from more than 50 coal-producing countries revealed only a small number of samples with extremely high contents of potentially toxic elements; few of the samples were from major coal mines. Nevertheless, it would be prudent for every country in which coal is mined to create a publicly available database containing information on the characteristics of the coals being mined, including the trace element concentrations. Identifying those areas having coals with anomalously high concentrations of potentially toxic trace elements may help to reduce health problems caused by these coals, yet would have a negligible effect on the total coal resource available.

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10.4.5 Carbon Dioxide Carbon dioxide (CO2) is a greenhouse gas. Greenhouse gases absorb infrared radiation as it is reflected from the earth back towards space, trapping heat in the atmosphere. Because atmospheric CO2 concentrations have increased 25% to 31% since large-scale industrialization began, there is concern that this increase is causing global climate change (Houghton et al., 1996, 2001; Keeling, 1997; U.S. Environmental Protection Agency, 2002). Because fossil fuels, especially coal, drove the industrial revolution in Europe and North America, and because there is increasing use of these fuels in many industrialized countries and in the new economies of China and India, there is growing concern about the potential anthropogenic impact of coal utilization on the global climate. During coal combustion, one atom of carbon and two atoms of oxygen combine to form one molecule of CO2. Based on the atomic weights of carbon and oxygen, if one pound of carbon is completely combusted, it should produce 3.667 pounds of CO2 (Hong and Slatick, 1994). With this relationship, the U.S. Energy Information Administration calculates potential CO2 emissions from coal as: %C  Btu=pound  36;670 ¼ Pounds ðlbsÞ CO2 per million ð106 Þ Btu The equation shows that CO2 emissions from coal combustion are partly a function of the fuel’s carbon content and, therefore, coal rank (Winschel, 1990; Hong and Slatick, 1994). The highest CO2 generation is found in lignites and anthracites and lowest in bituminous coals. On average, high-rank bituminous coals produce 5–10% less CO2 than lower rank coals (Winschel, 1990). Likewise, lower sulfur coals produce more CO2 than higher sulfur coals. This is because during combustion each dry weight percent sulfur provides approximately 0.1 MJ kg1 with no additional CO2 production (Quick and Glick, 2000). Also, carbonates in coals, which are common in many coal basins, produce CO2 when combusted. Quick and Glick (2000) noted that carbonates contribute to volatile matter upon pyrolysis, which reduces apparent fixed carbon. That means that a high carbonate percentage in the mineral matter of a coal will produce more carbon dioxide and lower the effective ASTM rank parameter (fixed carbon). Since rank is used to predict probable carbon dioxide formation, large carbonate contents would underestimate CO2 formation from high-rank coals by approximately 0.2 kg CO2 (net GJ)1 for each percent CaCO3 above 1 wt% (Quick and Glick, 2000). Hong and Slatick (1994) noted an interesting aspect of the relationship between rank and carbon emissions in the United States. To comply with the sulfur-emission limits of the 1990 Clean Air

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Act Amendments, many utilities decreased their sulfur emissions by switching from relatively high-sulfur bituminous coals to low-sulfur lignite and subbituminous coals. Although SO2 emissions decreased, the switch resulted in an increase in the national average CO2 emissions from 206.5 pounds/million Btu in 1980 to 207.6 pounds/million Btu in 1992 because the lower rank coals have higher CO2 emission factors. Because coal use is projected to increase in the future and there is growing concern about rising CO2 levels, many countries are looking at ways to reduce carbon emissions. One possibility being investigated is geologic sequestration. In geologic sequestration, CO2 would be captured from the flue-gas stream and injected deep underground in saline reservoirs, depleted oil and gas fields, unmineable coals, or organic-rich shales for permanent storage. Although there is no single definition of “unmineable” and any definition is likely to vary among basins and with respect to future coal economics, some coal beds may be attractive storage options because injected CO2 should adsorb onto the coal matrix and displace methane (Gale and Freund, 2001; Schroeder et al., 2001; Stanton et al., 2001; Reeves, 2003; see Chapter 9). Displaced coal bed methane could be collected, which would provide revenue to help offset the cost of carbon storage.

10.5 Coal Combustion Byproducts The combustion of coal in coal-fired power plants (see Chapter 4) produces several solid waste products referred to as coal-combustion products (CCPs), coal-combustion byproducts (CCBs), or pulverized fuel ash (PFA). Solid wastes include fly ash, bottom ash, boiler slag, flue-gas desulfurization (FGD), and fluidized bed combustion (FBC) wastes. In the United States alone, approximately 70 million tons of solid coal wastes are disposed of in impoundments and landfills annually (Gray et al., 1997; Kim, 2001). The mineralogy of these wastes is a function of the mineral content of the coal, method and condition of combustion, and environmental control technology utilized at the power plants. Fly ash (see Chapter 4) is composed mostly of silica and aluminum (as SiO2, Al2O3), with generally lesser iron (Fe2O3), calcium (CaO), magnesium (MgO), potassium (K2O), and sodium (Na2O), although silica and aluminum content varies inversely with calcium content (Pfughoeft-Hassett et al., 2000). Some of these compounds are insoluble solids (SiO2, Al2O3), some are water soluble (e.g., metal sulfates), and some are water-reactive metal oxides (e.g., CaO, MgO, K2O, Na2O) (Klein, 1975a,b). The potential environmental impacts are groundwater related. Because the CCBs can be enriched in trace elements there is a

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potential that metals might be mobilized from the CCBs to the groundwater (Theis and Wirth, 1977). Studies of U.S. impoundments, however, suggest that the potential is small; less than 1% of CCBs have the potential to leach harmful elements (Kim, 2001; Vories, 2004). Likewise, Sear et al. (2003) noted no significant contamination from fly ash after 50 years of fly-ash utilization in the United Kingdom. Nevertheless, it is important to understand and plan for potential contaminants. Evangelou (1996) noted that some CCB ponds and landfills contained locally high levels of selenium, chromium, boron, mercury, and barium. Also, although the U.S. Environmental Protection Agency decided not to classify coal ash as hazardous material, and found few cases of off-site impacts, they did determine that there is the potential for some combustion byproducts to pose hazards to human health and the environment when improperly disposed of in unlined landfills and mines. EPA has suggested that national regulations, especially concerning monitoring, be promulgated (U.S. Environmental Protection Agency, 2000a). The chemistry of the fly ash (partly a function of the original coal), the method of disposal, and the hydrological system are important to understanding the potential for leaching concentrations of trace elements into the environment. Preliminary data indicate that the oxidation of the chromium is not a common phenomenon in U.S. power plants, but Sheps-Pelleg et al. (2001) noted concentrations above drinking water standards in leaching tests of fly ash in Israel. Chromium in coal is generally present in the benign trivalent form. However, when coal is burned in an oxygen-rich environment as much as 50% of the chromium can be converted to hexavalent chromium, which is a potent carcinogen. In leaching experiments, this hexavalent chromium condensed on the surface of the fly-ash particles is predominantly water soluble, thus presenting a potential health problem where concentrations of chromium are elevated (Sheps-Pelleg et al., 2001). Similarly, increased boron concentrations are a potential concern in arid climates (similar to groundwater quality issues from mining), because boron solubility is high enough to possibly cause boron enrichment where fly ashes are used as soil amendments (Jones and Straughan, 1978). Because of the low incidence of recorded environmental problems and low potential for U.S. ash ponds and landfills to leach hazardous elements, the U.S. Environmental Protection Agency ruled that CCBs are not hazardous as defined by Subtitle C of the Resource Conservation Recovery Act (U.S. Environmental Protection Agency, 1993, 2000b). Not only did the EPA rule that CCBs did not pose an environmental hazard, they encouraged the use of CCBs in the making of cement and other products where this can be done without environmental impact (U.S. Environmental Protection Agency, 1993).

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10.6 Radionuclides and Radioactivity Finally, there has been speculation that radioactivity from coal and/or coal combustion products presents a health hazard (Gabbard, 1993), although there is little evidence to support this contention (U.S. Environmental Protection Agency, 1996). Coals may concentrate uranium (Swaine, 1990), but average uranium and thorium contents of coals are still very low: 0.5–10 ppm for world coals (Swaine, 1990) and 2.1 ppm for U.S. coals (Finkelman, 1993). Van Hook (1979) inferred that atmospheric releases of radionuclides do not represent a significant public health risk unless coal containing >5 ppm U becomes widely used. Data from nearly 10,000 U.S. coal samples (Bragg et al., 1997; Zielinski and Finkelman, 1997) indicate that the concentrations of radioactive elements (uranium, thorium, and potassium) are generally no higher in these coal samples than they are in other common rocks and soil. Furthermore, studies have shown that the maximum radiation dose to an individual living within 1 km of a modern coal-fired power plant in the United States is equivalent to a 1–5% increase from the natural environment; much less for the average citizen and so is not a health risk (Zielinski and Finkelman, 1997). The U.S. EPA has also determined that ambient radiation levels in areas using fly-ash fill were comparable to control areas with no fly ash and that radon and radioactivity were not a health concern from materials such as concrete blocks made from fly ash (U.S. Environmental Protection Agency, 2006c). Rare cases in other countries, however, show that higher concentrations of radionuclides may be associated with mined low-rank coals. Elevated concentrations of the radionuclides 238U and 226Ra were found in soils around a coal-fired power plant in Hungary, which had been burning a brown coal with unusually high uranium concentrations (Papp et al., 2002). Likewise, elevated 238U and high U-Ra activity in streams along the southern coast of Lake Issyk-Kul in the Kadji-Sai brown coal field of Kyrgystan in Central Asia has been attributed to mining a uraniferous brown coal, weathering of mine wastes, and from uranium extraction processing from the ash of a coal-fired power plant that used the uraniferous coal (Gavshin et al., 2005). Radon, a daughter product of uranium decay, may present a legitimate health concern to coal miners working in poorly ventilated mines (Jamil and Ali, 2001).

10.7 Final Comments Coal has a long history of environmental and health impacts that stem from decades in which there was a lack of scientific understanding concerning the potential impacts as well as a lack of regulation.

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As concerns were recognized, scientific research helped to clarify causes of various environmental and health issues, and methods for mitigating or abating many impacts. Regulations were passed and best practices developed to limit future impacts, and mitigate or abate past abuses. This research continues. It should be noted that not all the environmental and health concerns attributed to coal are valid. There have been many cases where concerns about an issue in one situation are raised in another area where they do not apply. Should we be concerned about the environmental and health impacts of coal (Finkelman et al., 2006)? Of course we should. Any prudent person should be concerned and should encourage and support efforts by the government and industry to reduce known and probable impacts until such time that a viable energy alternative is available. How concerned should we be? That would depend on your proximity to the source of the issue; the amount, rank, and chemistry of coal used; the mining method used; the type of processing prior to use (if any); the technology employed in the combustion process; local hydrology, local geology, regulations and oversight in an area; and other factors such as the state of your health. The direct health problems caused by coal and coal use are generally local and often associated with legacy issues (pre-modern regulation) or occur in developing countries that do not regulate their mining and utilization industries or use the advanced mining, reclamation, combustion, and emissions control technology and methods used by more industrialized nations. For people living in areas where high-quality coal is burned in modern boilers using the best available pollution control technology and sensible coal combustion byproduct disposal practices, the health threat is minimal. Nevertheless, as more coal is mined and used (especially in developing nations), new coal resources are utilized, new technologies are developed to use coal as a fuel (gasification, liquefaction), and coal byproducts are used for more industrial applications, scientific research will continue to define the variability and limits of potential environmental and health impacts from coal so that concerns can be addressed before they become a problem. These research efforts point out the critical necessity to acquire fundamental geological and geochemical data and that these data be applied beyond traditional uses to address global and/or local environmental coal issues. Coal geoscientists must be aware that the reliance on coal as a 21st-century energy source will also bring the burden of understanding and mitigating environmental issues related to human health. Although current information indicates that emissions of potentially hazardous air pollutants from utility coal combustion in the United States do not present a significant threat to human health, domestic coal use in developing countries has caused serious health

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problems. Coal scientists and technologists are ideally positioned to help medical and public health specialists improve public health in these countries. A better knowledge of coal quality parameters may help to minimize some of the health problems caused by domestic use of mineralized coals. Information on the concentrations and distributions of potentially toxic elements in coal may assist people dependent on local coal sources to avoid those areas of a coal deposit having undesirably high concentrations of toxic compounds. Information on the modes of occurrence of potentially toxic elements and the textural relations of the minerals and macerals in which they occur may help us to anticipate the behavior of the potentially toxic components during coal cleaning, combustion, weathering, and leaching. This situation offers coal scientists an opportunity to directly contribute to improved public health.

CHAPTER 11

Other Applications of Coal Petrology John C. Crelling Isabel Sua´rez-Ruiz

11.1 Introduction This chapter focuses on applications of the coal petrology other than those included in previous chapters. As has been shown, coal petrology and the techniques used in coal petrology, particularly optical microscopy, have important applications in a number of areas related to coal and its derivative products as well as in other areas not directly related to coal. In this chapter the application of organic petrology methods in archaeology in relation to the organic gems and artifacts, environmental studies, spontaneous combustion (in respects not mentioned in previous chapters), forensic geology, and auto brakes (as the commercial final product) is discussed. Other applications of coal petrography include those of a predictive character (already mentioned mainly in Chapters 3 and 4) which are used to predict the Hardgrove grindability of coal as reported by Hsieh (1976), Hower and Wild (1988), and Gray (1991); to evaluate the washability of pyrite in coal (Gray, 1991); and to evaluate pipe enamels made from coal tar pitch.

11.2 Archaeology Coal petrology and geochemistry have been used in archaeological investigation to determine the provenance of coal and other materials (e.g., Teichmu¨ller, 1992; Smith, 2005) such as those related to jet and occasionally amber, both of which are considered gemstones. Jet is a coal-derived gem that has been used in jewelry and amulets since prehistoric times. Muller (1987) provided a work on jet from different Applied Coal Petrology Copyright © 2008 by Elsevier, Ltd. All rights reserved.

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geographical areas that mainly focuses on its history with some insights on the different properties of this material. Although jet can be found all over the world, the main locations where it has been mined, because of its high-quality varieties as a gem, include areas in Europe (England, Spain, Portugal, France, Germany, Austria, Poland), Turkey, the United States (Utah), and India (Muller, 1987; Heflik et al., 2001). Most of the scientific research into the occurrences of jet, its nature, origin, physicochemical properties, and the quality of the material has been carried out in the last three decades. These investigations include jet from different geographical locations and mainly from the Jurassic and Cretaceous ages (e.g., Traverse and Kolvoord, 1968; Muller, 1980, 1987; Pollard et al., 1981; Petrova et al., 1985; Sales et al., 1987; Markova et al., 1988, 1989; Markova, 1991; Wert and Weller, 1991; Lambert et al., 1992; Weller and Wert, 1994; Watts et al., 1997; Sua´rez-Ruiz et al., 1994a,c; Sua´rez-Ruiz and Prado, 1995; Jime´nez et al., 1998a,b; Iglesias et al., 1995, 2000, 2001, 2002, 2003, 2006; Heflik et al., 2001; Laggoun-Defarge et al., 2003; Sua´rez-Ruiz and Iglesias, 2007). In Europe, a relatively large industry based on working jet was developed (e.g., Whitby in England, Galicia-Asturias in Spain) several centuries ago, at a time when this material was widely traded. The term jet is an old name (Muller, 1987) for this material. However, there are other names for this particular coal that are still in use such as, for example, gagate in some European countries, azabache in Spain, and azeviche in Portugal. The definition of jet reported by Stach et al. (1982) describes its particular characteristics. Most pieces of jet used in jewelry have several common physicochemical characteristics that are special and anomalous. They are determined via petrographic and geochemical analysis of jet allowing its differentiation from other coals (such as cannel coals, lignite) or other organic black materials (e.g., Pollard et al., 1981). The main anomalous properties of jet are related to their composition and rank. Usually jet is a huminite/vitrinite-rich coal with suppressed reflectance which is therefore lower than that of the coal or the associated organic matter in the surrounding rocks. Its hydrogen content is higher than expected for a normal coal of a given rank. It shows a high volatile matter content, H/C atomic ratio, and high oil yields. Usually, jet is a perhydrous coal, and consequently it displays significant discrepancies among its physicochemical rank parameters. This coal has an unusual chemistry for a huminite/vitrinite-rich coal, and usually those materials considered of high quality are quite resistant to weathering and long exposures to the air (Sua´rez-Ruiz et al., 1994a,c; Wert and Weller, 1991; Weller and Wert, 1994; Jime´nez et al., 1998a,b; Iglesias et al., 1995, 2000, 2001, 2002, 2003, 2006; Heflik et al., 2001; Laggoun-Defarge et al., 2003;

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(a)

100 µm

(b)

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100 µm

FIGURE 11.1. Optical microscopy. Photomicrographs taken in reflected white light with an oil immersion objective. Petrographic composition of the Spanish (a) and Whitby (b) jets. (Photomicrographs: I. Sua´rez-Ruiz.)

Sua´rez-Ruiz and Iglesias, 2007). Figure 11.1 shows the microscopic appearance of two jet samples from North Spain (Asturias) and England (Whitby), and Table 11.1 displays the conventional petrographic and geochemical parameters for some Cretaceous and Jurassic samples of jet. Other materials that from a macroscopic point of view may show a similar jet appearance and may be mistaken with jet or used as jet imitations are those summarized by Muller (1987) such as Kimmeridge shale (from England); some varieties of lignite (from Germany, Spain) and anthracite (from Britain, Wales, Pennsylvania, China, South Africa, and from areas of Russia and the ex-Sovietic Republics); and horn, bog oak, black onyx as naturally occurring materials; and French jet (black glass), vulcanite/ebonite, bakelite, epoxy resin, and so on as synthetic products. However, petrographic examination can easily discriminate jets from these materials. As an example, there is the classical work published by Teichmu¨ller (1992) in which organic petrology was applied to archaeological investigation. This work reported the results obtained from a study of Celtic-through-to-Roman ornaments of jet (22 of the 81 objects were investigated), bituminous coals, and oil shales, to determine their nature and geographical provenance. Based on the organic petrology combined with palynological, mineralogical, chemical, X-ray, and spectroscopic methods this author demonstrated that much of the material came from the Jurassic Kimmeridge “coal” of Dorset, England, with other material traceable to a sapropelic horizon on top of a Stephanian coal from the Czech Republic. The source of other investigated material was questionable, with Stephanian oil shale from Puertollano in Spain being one possibility among others. Kimmeridge shale was the source material for the Bronze Age Caergwrle Bowl (Smith, 2005).

Age Cretaceous Cretaceous Cretaceous Cretaceous Cretaceous Cretaceous Cretaceous Jurassic Jurassic Jurassic Jurassic Jurassic

Provenance Balkan Coal Basin. Bulgaria Balkan Coal Basin. Bulgaria Balkan Coal Basin. Bulgaria Balkan Coal Basin. Bulgaria Balkan Area. Bulgaria Balkan Area. Bulgaria Utah. USA Asturias. Spain Peniche. Portugal Whitby. England Whitby. England Soltykow. Poland

Samples

Ro (%)

VM (% daf)

Ash (% db)

H (% daf)

C (% daf)

H/C

Belnovrah

0.41

48.0

nd

6.25

80.8

0.93

Roneza

0.42

50.5

nd

6.23

82.2

0.91

Kamenarna

0.46

50.4

nd

6.44

82.5

0.94

Nikolaevo

0.21

55.9

1.8

5.7

72.3

0.95

Kaleitza

0.40

50.3

6.6

5.8

77.6

0.9

Beinovruh

0.42

53.2

1.8

5.9

80.1

0.88

UCV AJV PGJV WJVh WJVI Gagates

0.24 0.39/0.72 0.35 0.40 0.22 0.37–0.43

60.2 54.9 57.1 52.1 72.1 49.0

1.8 1.1 1.4 2.4 2.5 10.4

5.9 5.9 5.7 5.7 7.4 5.9

77.3 84.8 80.5 82.4 82.6 70.7

0.92 0.83 0.84 0.83 1.07 0.99

Ro: mean random huminite/vitrinite reflectance db: dry basis daf: dry ash-free basis nd: non determined

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TABLE 11.1 Petrographic and chemical conventional parameters for some Cretaceous and Jurassic jets. Data compiled from Petrova et al. (1985); Markova et al. (1988); Heflik et al. (2001), and Iglesias et al. (2002)

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Another particular interest regarding jets is the differentiation among them, finding specific characteristics linked to each of these materials that permit their discrimination and the assignment to a specific geographical area. As mentioned by Lambert et al. (1992) this is something very useful in determining (for example) the provenance of jet pieces found in North American sites and in general, in tracking the geographical origin of any piece of jet as was done by Teichmu¨ller (1992). Moreover, this type of information also provides many clues about the history and customs surrounding the exploitation and trading of jet. However, although for some jets this is possible, in other cases it is a very difficult task. For example, to differentiate between isolated pieces of jet from Portugal and north Spain, which are both of Jurassic age, requires a thorough chemical investigation (Iglesias et al., 2001, 2002, 2003, 2006) because its appearance under microscopic observation and the data from chemical conventional analysis are relatively similar (Table 11.1). However, the specific characteristics of each of these have now been identified and so they can be distinguished. According to Stach et al. (1982), jet “is formed from drift wood which has been secondarily impregnated with bitumen from the surrounding environments, leading to an abnormally low reflectance, strong fluorescence and its typically tough uniform physical properties.” The characteristics relating to low reflectance values and strong fluorescence were also mentioned by Teichmu¨ller (1992). The definition of jet given by Stach et al. (1982) is a relative adjusted description for some jets of Jurassic age that have been deeply investigated such as those found in Spain (Asturias), Whitby (Great Britain), Poland (Soltyko´w), and Portugal (Peniche area). However, this definition does not seem to suit, for example, the so-called Utah jet material (Cretaceous age), mainly because not all jets have the same origin. It was optical microscopic analysis (Traverse and Kolvoord, 1968; Iglesias et al., 2002), later implemented with geochemical data (e.g., Iglesias et al., 2002, 2003, 2006; Laggoun-Defarge et al., 2003; Arenillas et al., 2003) that permitted a full description of Utah jet, thereby allowing it to be differentiated from some of the Jurassic jets on the basis of their composition, properties, behavior, and nature. Utah jet shows good properties for being carved and polished (Muller, 1987) and like the other jets it is made up mainly of huminite macerals with a very low amount of liptinite (resinite) in this particular case. However, the huminitic tissues of the Cretaceous Utah jet contain terpene-type resins which were not found in any of the Jurassic jets mentioned here, such as those from Spain, Portugal, and England. This is a significant difference that shows that the composition of jets is clearly tied to the nature and source of the material. Such characteristics as these permit a clear differentiation among jets.

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Thus, organic petrology not only contributes to the identification of jets but may also help to discriminate between different jet materials. However, not always can a piece of jet be tracked to a specific geographical source, either because there is no complete data bank for all jets or there is a lack of information about geographical sources. The combination of rank, maceral content, and geochemistry has been demonstrated to be a critical determinant procedure but the addition of a palynological investigation along with the latter three mentioned parameters may also be highly definitive for describing and helping to understand this gemstone. Recently, Smith (2005) has summarized interesting data on the coal microscopy in the service of the archaeology mainly for identifying the provenance of coals and coaly materials recovered from archaeological excavations of different time periods and geographical sites. To determine coal provenance, Palmer et al. (2002, 2003) studied 20 pieces of coal recovered from the wreck of the Titanic. Coal geochemistry, including mineral matter analysis (major, minor, and trace elements) as well as palynologic determinations indicated that the coal found on board the Titanic had at least three, if not five, different sources and that most of the coal came from the United Kingdom, although the United States could also have been a source for some of the coal. Organic petrology has also been used to characterize amber, a fossil resin gem used in jewelry. Fossilized amber is found worldwide with two main commercial sources being the Baltic region and the Dominican Republic. Baltic amber is very well reputed due to its high quality and it is also called succinite due to its succinic acid content. Its composition and, in general, its characteristics and varieties have been intensively investigated (e.g., Gough and Mills, 1972; Mills et al., 1984; Gold et al., 1999; Kosmowska-Ceranowicz, 2006, among others). Because amber is a fossilized resin it often contains material trapped inside it, such as insects, leaves, pine cones and other seeds, spores and pollen, hairs, feathers, and even the occasional amphibian. All these inclusions are best studied by petrographic methods, examining, for example, the nature and geological age of included insects; the presence of associated dust, dirt, and trapped bubbles; and the presence of anthropomorphic artifacts such as hairs, pencil marks, drill borings, and glue (Grimaldi et al., 1994). A comprehensive chemical classification system of ambers has been developed by Anderson et al. (1992) which allows all varieties of amber to be distinguished. As in the case of jet, one of the principal concerns is to be able to differentiate it from other fossil resins of different origins and from good imitations. Petrographic techniques as well as chemistry can be used to detect the fake ambers as described above. In this way, scanning electron microscopy analysis carried out

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on Baltic amber have shown that it displays a foamy internal microstructure in which a large percentage of space is occupied by several generations of gas bubbles (Kosmowska-Ceranowicz, 2006). Gold et al. (1999) also used optical and scanning electron microscopy with a series of spectroscopic and geochemical techniques to study the nature of fossilized amber from various geographical locations as well as copal and modern tree resins. Coal petrology may also help to discriminate between modern soil organic matter and coal when dating soil samples by 14C methods. This is also important for a proper assessment of productive soil organic matter. Some studies of coal mixed with modern soil organic matter have been reported by Chitale (1986), Lichtfouse and Eglinton (1995), Lichtfouse et al. (1997a,b), Falloon and Smith (2000), and Copard et al. (2006). Another example of the potential use of coal-related analysis in archaeological investigations is that provided by Brooks (2004). This author reported the examination of an ash horizon in the Chan Chan area in northern Peru, an extensive site in South America prior to the arrival of Europeans. Using geochemical analysis and coal chemistry data, the ash chemistry was found to be consistent with the geochemical signature of coal ash. This is taken as evidence that coal was used as a fuel in Peru during the time of these ancient cultures.

11.3 Environmental Recovery Studies Both organic and inorganic petrology can be used to supplement other investigative tools in environmental studies. Stout and Wasielewski (2004) studied the abandoned site of a power station and manufactured gas plant (MGP) on a manmade island. MGP, or town gas, plants were common in the United States prior to the development of natural gas transmission lines, the last plants closing in the late 1960s when pipelines began to reach the northeastern United States (Hamper, 2006). Petrographic studies of the sediment confirmed the presence of coal and coke, bottom ash associated with combustion, and tars associated with a MGP. Coal and coke were associated with the MGP tars in the deeper soil horizons, this association confirming their common source as contaminants. Stout et al. (2002) cautioned that sediments suspected of containing coal must be carefully examined in order not to confuse coal-derived organic signals with those of hydrocarbons. The coal-fired power plant also contributed bottom ash and remains of stockpiled coal to the sediments (Stout and Wasielewski, 2004). Not all studies are quite as definitive. Ponz (2002) attempted to distinguish coal ash from other debris at the site of Thomas Edison’s laboratory in

296 Applied Coal Petrology

East Orange, New Jersey. Ultimately, he came to the conclusion it was difficult to distinguish coal-fired boiler slag deposits from metallurgical furnace slag. An atlas of anthropogenic particles has recently been jointly published by the International Committee for Coal and Organic Petrology and the Indiana Geological Survey (Crelling et al., 2006) which lists the various kinds of manmade organic particles that can be found in contaminated soils and sediments. These include particles of coal, char, metallurgical coke, petroleum coke, and fluid coke already illustrated in previous chapters as well as other materials including fly ash and atmospheric particles. In a practical way this atlas has grouped the anthropogenic particles following two different concepts: (1) anthropogenic particles grouped according to their source: combustion, carbonization, manufacture-derived, and other particles; and (2) anthropogenic particles grouped according to their site of occurrence: atmospheric, soil (peat), and water sediment particles.

11.4 Spontaneous Combustion Although this subject was already discussed in Chapters 3 and 10, here the spontaneous combustion of coal is included to complete some aspects of this topic related to the applied coal petrology. Spontaneous combustion of coal is a serious environmental problem (Chapter 10) because of the release of hot, toxic gases and because of the potential for loss of structural stability of the rock above the burning coal (Stracher and Taylor, 2004b). It is also an economic problem due to the loss of reserves and the cost of containing the fire. Chakravorty and Kolada (1988), and Morris and Atkinson (1988) reviewed the factors which contribute to the potential for spontaneous combustion of coal (Table 11.2). Combustion is a function of O2 absorption, which generates heat in the formation of peroxide complexes. The complexes are unstable and release CO and CO2 (De Faveri et al., 1989). Monitoring of the latter gases is a potential prevention tool, although, in practice, there are too many potential CO2 sources; therefore, monitoring CO from incipient combustion is more specific. CO detectors operating in the 0–1,000 ppm range are adequate (Chakravorty and Kolada, 1988). Other inherent heat sources are pyrite oxidation (Singh and Demirbilek, 1987) and the heat of wetting (Feng, 1985; Falcon, 1986). Feng (1985) examined several analytical methods designed to predict the potential for self-heating. The static isothermal test, measuring CO produced versus O2 adsorbed CO/DO2, was considered to be the best predictor. High CO/DO2 indicates high susceptibility to combustion. The cross-point temperature, a widely used test, determines

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TABLE 11.2 Critical factors contributing to spontaneous combustion Intrinsic Factors That Cannot Be Controlled

Intrinsic Factors That Cannot Be Controlled

Extrinsic Factors That Can Be Controlled

Coal Properties

Geologic Features

Mining Practices

High volatile matter High moisture

High pyrite

Thick seams Presence of inferior pyrite bands and carbonaceous shale Presence of faults

High liptinite

Weak and disturbed strata conditions

High friability

High strata temperature

Accumulation of fine coal in worked-out areas Leaving roof and floor coal during mining Poor maintenance of roadways and old works Inadequate measures to prevent air leakage through air crossings and doors Caving to surface under shallow overburden Multi-seams worked in close proximity Caving to surface under shallow overburden

Source: Mining Engineering 40, R. N. Chakravorty, R. J. Kolada, “Prevention and control of spontaneous combustion in coal mines,” 952–956, copyright 1988, with permission from The Society for Mining, Metallurgy, and Exploration, Inc.

the point at which self-heating exceeds the heat input. Gouws and Wade (1989a,b) were not particularly fond of the cross-point temperature, proposing a more-detailed combination of DTA (differential thermal analyzer)-derived parameters. Feng (1985), however, found cross-point temperature to be a reasonable substitute for the static isothermal method he favored. The ultimate oxygen correlates well with CO/DO2. Ultimate O2 is determined by difference, and, as such, should not be relied upon as the sole indicator. The liptinite content also correlates with combustion susceptibility. Liptinite and ultimate O2 can be used as checks on cross-point temperature or static isothermal tests. Beamish et al. (2001), Beamish and Blazak (2005), Beamish and Hamilton (2005), and Beamish (2005) correlated R70, the average rate of temperature rise from 40oC to 70oC in an adiabatic oven with O2 flow of 50 mL/min, with a number of properties for New Zealand and Australian coals. Beamish and Blazak (2005) and Beamish and Hamilton (2005) found R70 to decrease with an increase in mineral

298 Applied Coal Petrology

matter, with the mineral matter acting as a heat sink, and an increase in the moisture content. Subbituminous coals have the highest selfheating rate (Beamish et al., 2001; Beamish, 2005). Perhaps the best-known coal fire in the United States is the Centralia fire in the Western Middle Anthracite Field, Pennsylvania (Nolter and Vice, 2004). Not caused by spontaneous combustion, the fire was started by burning garbage in a dump abutting a coal face in 1962. With the fire still burning on four fronts, one advancing at a rate of about 20 m/year, the damage is a lesson of the destruction of all types of coal fires. Most of the town, with the exception of a few people who refused to move, has been abandoned and leveled due to the unstable ground and dangerous combustion gases and portions of a state highway had to be relocated because of collapse due to undermining by the fire.

11.5 Forensic Geology Forensic geology is the use of geological materials and methods to solve crime, and a number of cases involving coal and, hence, organic petrology have been reported. Murray (2004), in his book Evidence from the Earth, cites three cases. One was a case worked on by the first forensic scientist Georg Popp. It involved the presence of particles of coal, snuff, and mineral grains—especially hornblend—on a dirty handkerchief found at the scene of a murder. A snuff-using suspect who worked at a local coal-burning gas works and a hornblend-rich quarry was apprehended and later convicted of the murder. In another murder case, investigated by Georg Popp, a layer of soil on a suspect’s shoes containing coal, brick dust, and concrete fragments indicated that the suspect was at a site involved in the crime. The third example involves a rape case in which soil samples at the crime scene and in the trouser cuffs of a suspect contained a variety of particles including anthracite. This was puzzling because there were no occurrences of anthracite in the area of the crime. However, further investigation revealed that the site of the crime had been a laundry 60 years before and the coal found was from the former coal storage pile. Thus the presence of the anthracite was unique to the crime scene. Armstrong and Springs (1989) give three examples using coal petrology. In one a customer returned a coal shipment that was obtained from a third party to the purported source colliery. When it was examined, the palynology results were consistent with the colliery’s bituminous coal product, but the petrographic examination showed that the coal had been contaminated by about 50% with anthracite and smokeless fuel. In another case a local constabulary requested that three coal samples be analyzed to determine if one

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could have come from either of two existing coal piles. A palynological analysis showed that two of the coals were from the Middle Coal Measures and reflectance analysis showed that the same two samples had similar mean maximum reflectances and reflectance distributions. In another case, coal found at a Romano-British site in Lincolnshire, England, was examined to determine whether it was local or traded from more distant coal fields. Reflectance analysis showed that there were two groups of coal, one local one with a mean maximum reflectance in the 0.42–0.48% range and one with a higher reflectance range 0.84–0.94% from coal fields in Yorkshire or Northumberland supporting a coal trading hypothesis. Hower et al. (2000d) examined the remnants of a coal slurry spill at a coal mine in southwestern Virginia. The slurry, which had been disposed of in an underground mine, flowed out of a mine portal in October 1996, causing a fish kill and the pollution of streams in the Tennessee River system. The mine company originally attempted to blame slurry from an adjacent mine, separated by a 30-m barrier pillar. Petrographic examination, with emphasis on the microlithotype content, of coal slurries from both mines and comparison to the fines from the preparation plant at one of the potential sources confirmed that the slurry in the streams was different from the slurry and the preparation plant fines at the adjacent mine. Neither rank nor maceral content was a definitive discriminator between the sources. Further, although the difference was more subtle than with the microlithotype composition, the geochemistry of the slurry in the stream differed from the slurry and the preparation plant fines at the adjacent mine, particularly in the amount of rare earth elements. The latter products contain the fines from the processing of the Fire Clay coal bed, known to have zones of high rare earth content (Hower et al., 1999e; Mardon and Hower, 2004).

11.6 Automobile Brakes Modern automobile brakes are composite materials composed of various mixtures of up to thirty different compounds set in a phenolic resin matrix. These materials include particles of metal (iron, steel, copper and brass), various plastics, sulfur, barium sulfate, antimony trisulfate, potassium titanite, aramid polymer fibers, and a great variety of carbonaceous materials. In a study of over 75 commercially available automobile brakes, Crelling et al. (1997) identified all the compounds mentioned above plus a variety of carbonaceous phases including metallurgical coke, petroleum coke, coal, weathered coal, pyrolysis chars, graphite, and carbon fibers (see Figures 11.2 and 11.3). They report that the total amount of carbonaceous material present varied from a few

300 Applied Coal Petrology

(a)

(b)

(c)

(d)

(e)

(f)

20 µ

FIGURE 11.2. Optical microscopy. Photomicrographs taken in incident polarized light of various particles seen in automobile brakes. (a) Metallurgical coke on left and graphite on right (note the dark tone and brushed appearance of the soft graphite). (b) Particles of weathered coal (note the reaction rims). (c) Cross-sections of carbon fibers. (d) Coal particle with steel filings at lower left. (e) A particle of char (probably pyrolyzed nut shell). (f) Bright steel filings at left and metallurgical coke at right. (Photo credits: B. Huggett.)

percent to over 50%. Though X-ray diffraction and electron microscopy techniques are commonly used to identify the components in automobile brakes (Weiss et al., 2006), these techniques are not very helpful in discriminating the carbonaceous phases. However, these phases are easily identified with incident polarized light microscopy. Metallurgical and petroleum coke are distinguished by their porosity and well-developed mosaic texture. The presence of inertinite macerals in the coke cell walls indicate metallurgical coke and the flow mosaic texture and needle coke structure indicated petroleum coke. The graphite lacks porosity and has higher order interference colors in polarized light. The coal particles have their normal petrographic properties of a uniform gray reflectance and maceral composition. The weathered coal particles usually show a dissected

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(a)

(b)

(c)

(d)

(e)

(f)

301

20 µ

FIGURE 11.3. Optical microscopy. Photomicrographs (a–c, e–f) taken in incident polarized light with analyzer and retarder plate, and (d) in fluorescence mode, of various particles seen in automobile brakes. (a) Graphite at left and metallurgical coke at right. (b) Large particle of metallurgical coke (note the presence of a large isotropic fusinite particle in the coke). (c) A particle of metallurgical coke coated with pyrolytic carbon. (d) Yellow fluorescing aramid fibers with a particle of plastic in the center (this image was taken in incident ultraviolet excitation). (e) Large particle of petroleum coke. (f) Large particle of graphite (note the higher order interference colors). (Photo credits: B. Huggett.)

and etched appearance, reduced reflectance (especially around their edges), and increased fracturing. The chars show the familiar cenosphere, honey-comb cenosphere, or unmodified inertinite structure. In the case of the carbon fibers the PAN-derived fibers are isotropic while the pitch-derived fibers are anisotropic. Organic petrology is also useful in identifying the various polymer materials present. The commonly used aramid fibers are translucent in white light and can be difficult to identify but they can be easily identified by their bright fluorescence when illuminated with blue light. The polyester resin also fluoresces and is easier to distinguish in blue light as are a variety of “plastic” particles occurring in the brakes.

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Index A Acid, 42–43, 46, 79, 81, 111, 113, 136, 191, 230, 232–233, 268–270, 273–274, 278, 294 gas, 227, 253 mine drainage, 7, 104, 191, 263, 268–269 rain, 9, 264, 273–274 Acid-base accounting, 269 Activated carbons, 194, 199, 203, 208, 218–221, 277 Adsorption, 110, 219, 221, 242–246, 248–249, 253, 257–258, 261 Air-blown gasification, 143 Air pollution, 16, 107, 264, 273 Algalplast, 159 Alkaline mine drainage, 270 Amber, 289, 294–295 Anisotropic textural classes, 203 Anisotropy, 28, 54 optical, 54 permeability, 252 reflectance, 54, 206, 208 strength, 63 Anthracites, 3, 20, 27, 32, 46, 48–49, 51, 63, 71, 82, 89, 92, 115, 129, 131, 148, 174, 180, 194, 201–203, 207, 209, 215–218, 220, 222, 234–235, 267, 271, 282, 291, 298 Archaeology, 289–295 Aromaticity, 28, 34, 36 Arsenic, 45, 47, 79, 272, 277–279 Ash, 9, 12–13, 15, 31, 36, 38–39, 43–44, 49–51, 56–58, 70–71, 76–78, 82, 87, 89, 91, 96–101, 103–108, 112–114, 116–117, 120–127, 135–137, 139–141,

143, 146–147, 164, 181, 186, 220, 222, 245–246, 248, 250, 271, 278, 280, 283–285, 292, 295–296 analysis, 39, 44 fusion temperatures, 13, 87, 96, 107, 122, 125–126, 135–137 oxides, 135–136 ponds, 284 Audibert-Arnu dilatometer, 14 Automobile brake, 194, 299–301 B Bacteria, 23, 25, 150, 232, 235–236, 239–240, 251, 257, 260–261 Baked anodes, 174, 186, 196, 208–209 Balkan Endemic Nephropathy (BEN), 265–266 Beneficiation, 16, 51, 61–83, 111, 128, 138 Binder phase, 176–177, 198, 205 Biomass, 103, 109, 119, 121, 123, 140–141, 143, 150, 194, 196, 219 Bireflectance, 28, 54 Bituminous coals, 3, 16, 20, 22, 34–36, 44, 46, 52, 55, 58, 63–64, 73, 76, 81, 89–90, 92, 94, 110, 114–115, 121, 124, 142, 146–151, 153, 158–159, 162, 168, 170, 178, 194, 220, 222–224, 233, 240, 244–245, 248, 257, 274, 282–283, 291, 298 Bituplast, 159 Black Lung Disease, 267–268 Blasting, 62, 67–68, 83 Blending, 14, 58, 61, 136, 204 Breakage, 67–73, 75–77, 89, 91

382 Index C Caking, 14, 55, 122, 126–127, 129–130 Calcined coke, 174, 189–190, 196, 204, 208–209 Calorific value, 13, 16, 22, 50, 52, 55–56, 91–92, 110–111 Carbon, 2, 5, 9–10, 12, 16, 24, 32–33, 47–51, 54, 66, 88, 91, 106, 110, 110–117, 119–128, 130, 133, 135–136, 140, 148–149, 151, 162, 164, 173–174, 176–178, 180, 186, 190–191, 193–194, 196, 198, 206–207, 210, 212–213, 215, 218, 224–225, 227, 229–237, 272, 274, 282, 301 in ash, 9, 126 capture and storage, 10 fibers, 196, 199, 203–204, 206–208, 210–212, 214, 300–302 in fly ash, 91, 93, 109, 113–116 foams, 201, 203, 222–224 forms, 140, 176–178, 186 materials, 112, 193–194, 196–198, 200, 205–206, 208, 210, 212–214, 218, 224–225 Carbon-carbon composites, 199, 202–205, 207–208, 210, 212–215, 224 Carbon-carbon materials, 212 Carbon dioxide, 106, 109–110, 119, 138, 143, 161, 173, 228, 233–235, 260, 263, 273, 277, 282–283 Carbon dioxide emissions, 110 Carbonization, 13–14, 19, 22, 24, 28, 30, 58, 156–157, 173–194, 196, 201–202, 210–213, 219–220, 222, 296 Cenosphere, 108, 153–154, 156, 167–169, 186, 301 Char, 14, 92, 94, 107–108, 121, 124, 126–131, 133–135, 143, 186–187, 220–221, 274, 296, 299–300, 302 Charcoal, 10, 151, 196–197 Char morphology, 127, 131, 133 Chromium, 47, 79, 284

Clean Air Interstate Rule (CAIR), 85 Clean Air Mercury Rule (CAMR), 85, 105 Cleat, 42, 62, 64–66, 68, 71, 192, 242, 249–253 Coal ash formation, 38, 107–108 blends, 14, 19, 22, 56, 58–59, 89–90, 114, 116, 132, 173, 178, 181, 192, 194 blends in pulverization, 89–90 classification, 11, 49, 51, 55–58, 151 cleat, 62, 64, 68 combustion byproducts, 265, 283–284 combustion processes, 59, 86–88, 96, 110, 116 consumption, 3, 6–8, 90, 126 formation, 32 grade, 2, 12, 16, 19, 56, 61–62, 73, 77, 120 humic, 19–20, 23, 49–50, 129, 155, 232, 239 metamorphism, 46–55 mining, 5–8, 18, 255, 264–268 oxidation, 14–15, 81, 190, 220–221, 271 permeability, 64–66, 122, 128–129, 250–254, 256 petrology, 2, 10–11, 18–19, 52, 54, 80–81, 86, 173, 181, 188, 192, 228, 289, 295–296, 298 preparation, 6, 9–10, 12, 38, 46, 67, 70, 77, 86, 111, 120 processing, 61, 83, 161, 264–266, 268–269, 273–274, 285–286 production, 3–5, 263 pulverization, 13, 40, 81, 88–89, 91–92 quality, 11–14, 19, 29, 56, 58, 61, 70, 77, 87–88, 105, 107, 110, 265, 267, 286–287, 290 rank, 1, 11–12, 14, 16–17, 22, 24, 26–28, 33, 46–47, 49, 53–56, 58, 61, 69, 73, 75, 82–83, 86, 90, 92, 94, 128–129, 131, 147–148, 152, 231, 237, 246, 267, 282

Index reserves, 3, 5–6, 171, 296 resources, 2–3, 5, 18, 141, 143, 146, 238–240, 281, 286 sapropelic, 19–20, 23, 129, 230, 232–234, 291 strength, 62–64 tar, 146, 174, 186, 192, 194–196, 198–199, 203–204, 208–211, 213, 216, 218, 289 type, 1, 12, 16, 19, 31, 56, 61, 63, 66, 76, 119–120, 127, 130, 140 utilization, 3, 5, 9–10, 12, 18, 45, 94–95, 194, 282 Coalification, 1, 20, 22, 24–25, 31, 34, 46–47, 52, 110, 232, 234, 241, 250–251 Coal-based materials, 19, 31, 42, 88, 110, 125, 186, 193, 210–224 Coal-derived carbons, 193–225 Coal-tar pitch, 194–196, 199, 203–204, 208–211, 213, 216, 218 Coal Workers Pneumoconiosis, 267 Coalbed methane, 227–228, 237, 242–243, 246–249, 251–252, 254–257, 259–262 Coarse ash, 99, 106, 112–113, 140 Co-gasification, 140–142 Coke cell wall, 176, 178, 180–181, 300 petrology, 176–180 reactivity, 174, 185, 191–192 strength, 24, 59, 180–186, 191–192 Coking coal, 62, 77, 174, 191, 220 Coking pressure, 184–185, 192 Column flotation, 80–81 Combustion, 8–10, 13, 15–16, 19, 24, 28, 30, 38, 45, 51–52, 56, 58–59, 66, 74, 85–89, 91–94, 96, 99–101, 103–116, 119, 126, 128, 131, 133–136, 138, 194, 218, 224, 232, 264–266, 270–275, 277–287, 289, 295–298 Comminution, 61, 69–70, 73, 90 Compressive strength, 62–64, 211 Crucible swelling number, 13 Crushing, 40, 62, 68, 71–73, 81, 83, 90

383

D Darcian flow, 240–241, 249 Density gradient centrifugation (DGC), 31, 34, 36, 158–160 Diffusion, 240–241, 249 Dioxins, 105–106, 108–109 Drop shatter testing, 67–68 E Electron microprobe, 31–34, 40, 43, 54 Emissions, 3, 5, 8–10, 13, 15–16, 85–89, 91, 99, 105–107, 109–113, 116, 125, 138, 227, 264, 271–278, 282–283, 286 Enhanced recovery, 262 Entrained flow, 121, 124–126, 130, 133, 135–136, 138–140 Environmental impacts, 263, 265–267, 271, 283 F Fate of trace elements, 103, 138 Filler phase, 176–178 Fine coal, 67–68, 77, 81–83, 122, 128, 132, 138, 186, 297 Fires, 263, 265–266, 270–272, 279, 298 Fischer Assay, 14, 127–128 Fixed bed, 120–123, 126, 130, 133, 136–137, 139–140 Flue gas desulfurization (FGD), 9, 16, 85, 105, 110, 112, 274–275, 283 Fluid coke, 173, 188–189, 296 Fluidized bed, 9, 86–88, 91, 96, 109, 112, 121, 123–124, 126, 130, 136, 140, 143, 173, 188, 274, 283 Fluidized bed combustion, 9, 86–88, 91, 96, 109, 11, 274, 283 Fluorescence, 22–24, 26–29, 45, 54–55, 159, 230, 293, 301 Fluorine, 272, 279–280 Fly ash, 9, 16, 83, 86, 91, 93–95, 99, 104–107, 109, 111–117, 139, 194, 217–218, 221, 272, 275–276, 283–285, 296

384 Index Fly ash (continued) carbons, 91, 93–94, 105, 109, 113–116, 194, 217–218, 221, 272, 275, 283–285, 296 minerals, 116–117 trace elements, 86, 104, 275–276 Forensic geology, 289, 298 Fouling, 31, 95, 99–101, 103–104, 140 Fourier Transform Infra-Red (FTIR), 36, 40 Free swelling index, 13–14, 17, 51, 55–56, 58, 124, 191 Froth flotation, 80–81 G Gagate, 290, 292 Gas content, 246–248, 257, 261 drainage, 64, 66, 83 generation, 234–235, 242, 257 Gasification, 2, 10, 19, 30–31, 56, 88, 91, 94, 100, 103, 119–131, 133–144, 193–194, 220, 274, 286 Gasifiers, 120–126, 128–130, 135–140 Geochemistry, 11, 34–36, 117, 228, 237, 239, 251, 261, 267, 289, 294, 299 Geological age, 56, 294 Gieseler plastometer, 14 Granular Residue, 154–155, 157–159, 167–170 Graphite, 32, 46, 155, 174, 186, 188–190, 194, 196, 201–202, 207–209, 215–218, 224, 299–301 Graphitization, 174, 198, 201, 205, 209, 211, 216–218 Gray-King Assay, 127–129 Green pet coke, 202 Grindability (hardgrove grindability index, milling), 12–13, 51, 56, 62, 71–74, 76, 89, 91–92, 135, 138, 289 Groundwater impacts, 269, 283 H Hardgrove Grindablilty Index (HGI), 13, 70, 74–76, 89–90, 138

Hazardous air pollutants, 16, 45–46, 138, 272, 286 Health impacts, 263–265, 267, 269, 271–273, 275–279, 281, 283, 285–287 Heating value, 9–10, 13, 15–17, 48–52, 76, 86, 89, 248 High-temperature X-ray diffraction, 97–98 Hilt’s law, 47 Huminite, 15, 20, 24, 26, 28, 154, 157–158, 165, 231, 233, 290, 292–293 Hydrocarbons, 49, 54, 121, 124, 126–127, 129, 138, 145, 163, 171, 186, 211–212, 224, 227–228, 230–243, 248, 256, 261, 266, 281, 295 Hydrogenation, 22, 28, 126, 145–146, 148, 151–154, 156, 160–162, 168, 188 Hydrogen sulfide, 228 Hydrogeology, 228 Hydrology, 239, 260, 265, 267, 286 I Inertinite, 15, 17, 20, 22, 24–25, 28, 30, 32, 34, 36–37, 49–50, 56, 66, 70, 74, 82–83, 91–94, 110, 114, 128–132, 152–155, 158–160, 165, 168, 170, 181, 183, 185, 232, 245, 300–301 Inorganic elements (in coal), 36, 40, 42–44, 78, 107–108 Integrated gasification combined cycle (IGCC), 10, 88, 125, 142, 274 Intrusions, 1, 44, 58, 61, 280 Isotherm, 243–248, 253, 257, 259, 261, 296–297 Isotopes, 237, 251 Isotropic texture, 157, 198–200, 211 Isotropy, 207, 220–221 J Jet, 289–291, 293–294 K Kerogen, 54, 228–235, 237, 239, 242

Index L Langmuir equation, 243 Lignites, 3, 20, 23, 34–35, 48, 51, 110, 115, 120–121, 131, 135, 148, 150, 157, 174, 194, 233–235, 265–266, 279, 282–283, 290–291 and health impacts, 265–266, 279 Liptinite, 15, 20, 22–24, 28–30, 36–37, 49–50, 52, 54, 56, 66, 70, 74–76, 80–81, 83, 91, 93, 110, 129–132, 148–149, 152–153, 155, 158–159, 165, 181, 230, 235, 239, 242, 245, 271, 293, 297 Liquefaction, 3, 19, 56, 76, 145–171, 174, 193–194, 198, 286 Liquid crystals, 174–176, 196 Lithotypes, 11, 20, 36–37, 61–64, 66–77, 80–81, 83, 89–91, 150, 159, 250 Lithotypes in coal pulverization, 70, 73–74, 81, 89–91 Lopatin model, 236 M Macerals, 2, 11–17, 20, 22–24, 26–28, 36, 38, 49, 52–54, 58, 66, 74, 78, 80, 91, 228–231, 234, 239, 245, 249, 271, 274, 287, 293, 300 analysis, 29–31, 59, 131, 182, 205 behavior in carbonization, 181, 183, 184 behavior in combustion, 30, 38, 45, 58–59, 74, 86, 91–94, 96–97, 108, 110 behavior in gasification, 130, 133, 135 behavior in liquefaction, 147, 149, 152–160, 163, 167–171 classification, 11, 20, 55, 182 elemental composition, 17, 29–32, 40, 56, 59, 61–62, 70, 74–77, 82–83, 86, 186, 245, 300 partitioning, 77, 80–81, 83 reactivity (in coking), 24, 181, 185, 192 Magnetic separation, 82

385

Mercury, 9, 47, 55, 79, 85–86, 88, 105, 138, 264, 272, 276–278, 284 Mercury in combustion, 9, 86, 88, 138, 264, 277 Mesophase, 155, 157, 162, 174–176, 186–188, 194–196, 199, 201, 203–204, 211, 215–216, 222 Mesophase pitch, 174–175, 187, 194–196, 199, 201, 211, 215–216, 222 Methane, 7–8, 124, 227–228, 232–238, 242–243, 246–249, 251–252, 254–262, 266, 283 Methanogenesis, 235–237, 240, 251, 257, 260–261 Microlithotypes, 15, 20, 29–30, 70, 74, 76, 81, 127, 131, 133–134, 299 analysis, 30–31, 127, 131 in coal pulverization, 74, 89, 93 Micro-markers, 65–66 Microscopy, 11, 16, 32, 40, 54–55, 78, 105, 115–116, 151–152, 159, 165, 177, 181, 184, 195–207, 209–210, 212, 214, 216–218, 221–224, 230, 289, 291, 294–295, 300–301 Microstructure, 196, 201–204, 207, 213–214, 216–217, 295 Migration, 44, 228, 236, 238, 241, 269 Milling, 73–74, 83, 124, 138 Mineralization, 66, 251, 265, 270 Minerals behaviour in combustion, 94–100, 104, 107–108, 110, 112, 116–117 behaviour in gasification, 126, 130, 133, 135–137, 140 behaviour in liquefaction, 155, 160–163, 168 in fly ash, 116–117 matter, 2, 9, 12–14, 18–20, 31, 36–43, 45, 49, 51–52, 67, 74, 77–78, 80, 83, 94–95, 97, 99–101, 106, 116, 123, 133–137, 140, 153, 159–160, 177, 179, 181, 183, 186–187, 219, 245, 282, 294, 298

386 Index Minerals (continued) matter analysis, 14, 31, 38–39, 51, 181, 294 partitioning, 76, 78 Mining, 2, 5, 7–8, 18, 24, 61–64, 66–72, 113, 143, 239, 255, 263–271, 277, 284–286, 297 Mosaic texture, 174, 176, 178–179, 189, 199–200, 207, 214, 223, 300 Moving bed, 121–122, 128–129 N Needle coke, 188–189, 209, 300 Nitrogen, 12, 32–34, 109, 148, 153, 164, 227, 234, 274–275 Nitrogen oxides, 8–9, 87, 106, 109, 228, 272–274 Non-mineral inorganics, 37–38, 40, 42, 44–45, 96, 99, 108 O Oil agglomeration, 80–82 Oil generation, 234 Optical texture, 157, 198–200, 207, 214, 223 Organic geochemistry (of macerals), 34–36 Outbursts, 5, 64–66 Oxidation, 14–15, 22, 25, 38, 41–42, 54, 66, 81, 105, 113, 129, 132, 190, 199, 210–211, 220–221, 232, 265, 269, 271, 284, 296 P PAN carbon fibers, 199, 208, 210, 301 Particle size distribution, 12, 70–71, 73, 120, 122, 126–129, 143 Particulate emissions, 85, 99, 106–108, 275 Particulate matter, 106–107, 275–277 Perhydrous coal, 290 Permeability, 64–66, 113, 122, 128–129, 136, 173, 181, 185, 224, 242–243, 248, 250–254, 256 Petrographic analysis, 29, 54, 58–59, 114, 147, 163, 197, 208–209, 212, 214, 216, 235

Petrographic composition, 56, 74, 77, 81, 92, 107, 128, 148, 224, 291 Petrography, 11, 20, 58, 130–131, 140, 168, 176, 182, 190, 192, 197, 267, 278, 289 Petroleum coke, 119, 142, 159, 173–174, 180, 188–190, 192, 196, 204, 208–209, 215, 217, 296, 299–301 pitch, 195, 199, 211, 218, 223 system, 228, 238–242 Phase relationships, 257–260 Pillar design, 63–64, 83 Pitch, 153–154, 174–175, 186–187, 189–190, 192, 194–196, 201–205, 208–216, 218, 222–223, 289, 301 Plasma, 38, 45, 144 Plastosphere, 154, 156 Plies, 61, 67–68 Plumes, 236, 261 Point-counting analysis, 39, 165, 205 Polarized light, 54, 115, 165, 175, 178–180, 187, 189–190, 195, 199–209, 212, 216–217, 221, 223, 300–301 Polycyclic aromatic hydrocarbons, 266, 281 Polygeneration, 141 Pore size, 127, 130, 219, 249 Porosity, 126, 130–131, 133, 135, 165, 176, 197, 199, 201–203, 205, 213–214, 219, 221, 223–224, 248–251, 254, 300 Proximate analysis, 12, 49, 51, 127, 164 Pseudovitrinite, 80–81, 94, 182–183 Pulverized coal combustion, 86, 100 Pyrite,15, 30, 40–41, 45–47, 51–52, 67, 76, 78–80, 82–83, 89, 96–97, 111, 116, 133, 148–149, 160–161, 168, 180, 191, 251, 265, 268–271, 273, 278, 281, 289, 296–297 Pyrolytic carbon, 66, 155, 186, 212, 301

Index Q Quinoline insolubles, 174, 186–188, 192, 195, 203–204, 216 R Radioactivity, 285 Radionuclides, 16, 264 Rank advance, 1–2, 17–18, 33, 44 indicators, 12, 16–17, 28, 49, 51, 54, 231, 250 rank analysis, 46–55 Reactivity, 24, 73–74, 92, 126–127, 129–131, 133, 135, 143, 151, 158–160, 174, 185, 191–192, 215, 274 Reactor solid, 161–163 Reflectance, 22, 24–25, 27–28, 44, 94, 133, 149, 151, 156, 160, 181–182, 192, 206–208, 212, 230, 232, 239, 290, 293, 299–301 Reflectance suppression, 16, 53 Reservoir, 66, 227–229, 235–236, 238, 240, 242–246, 249, 251–253, 283 pressure, 242, 248, 253–257, 260–261 temperature, 241, 257–260 Residues, 13, 16, 25, 39–40, 42, 100, 112, 139–140, 142, 151–154, 156–164, 168, 170–171, 173, 188, 194 Retarder plate, 115, 178–180, 189, 195, 199–204, 206–207, 209, 216–217, 221, 223, 301 Retrogressive reaction, 145, 150, 157, 168, 170–171 Roga index, 14, 55 S Seal, 228, 235, 238, 241–242 Selective catalytic reduction (SCR), 9, 85, 105, 109 Selenium, 45, 47, 79, 270, 272, 280–281, 284 Self-heating. See Spontaneous combustion

387

Semi-anthracite, 27, 34, 221, 234 Semi-coke, 154–155, 157, 159–160, 162–163, 167–171 Sequestration, 3, 88, 109, 227, 243, 253, 257, 259–262, 283 Shrinkage, 132, 254 Slagging, 31, 99–104, 121–123, 125–126, 135–136, 139–140 Solution, 14, 27, 42–43, 45–47, 105, 109, 111–112, 209–210, 242, 245, 269–270 Source rock, 49, 227–228, 235–236, 238–240 Spontaneous combustion, 15, 66–67, 265, 270–272, 289, 296–298 Subbituminous coals, 3, 20, 32, 34–35, 48, 51, 92, 131, 148, 150, 157, 159–160, 162, 168, 170–171, 233–234, 271, 274, 283, 298 Sulphur forms, 12, 50, 128, 164 oxides, 110, 228, 272–274 Surface area, 67, 73, 114, 117, 126–127, 130, 132, 134, 199, 212, 218–221, 244, 271 Swelling, 13–14, 17–18, 51, 55–56, 58–59, 92, 108, 112, 124, 128–129, 131, 134, 153, 191, 213, 222, 252, 271 Swelling propensity, 127, 129 Synthetic graphite, 194, 201, 207, 215–217 T TDS, 260–262 Thermal fragmentation, 127–128 Thermogravimetric analysis (TGA), 92, 94 Trace elements, 8–9, 12, 16, 44–47, 61, 79–80, 275–277, 294 analysis, 44–45 and health impact, 272, 275–277, 281, 284 in combustion, 8–9, 103–106, 108, 112, 271, 275–277, 283–284

388 Index Trace elements (continued) in gasification, 138–139 in fly ash, 275–276 partitioning, 78–80, 103–104 Trap, 241 Triboelectrostatic separation, 83 U Ultimate analysis, 10, 12–13, 31, 49, 51–52, 127, 164 Underground gasification, 142–143 Uranium, 285 V Van Krevelen diagram, 229, 232, 234–235, 240 Vehicle oil, 145, 147 Vitrinite, 15–18, 20, 22–25, 27–28, 30, 32, 34, 36–37, 43, 49–50, 52–54, 57, 61–62, 70, 74, 76, 80–83, 91–94, 108, 110, 114, 129–131, 134, 148–149,

152–159, 165, 167, 169, 176–177, 181–183, 190, 192, 220, 224, 231, 233, 235, 239, 245, 250, 271, 274 Vitrinite reflectance, 12, 14, 16–17, 32–33, 48–49, 52–54, 56–59, 61, 75–76, 92, 127, 131, 165, 183–185, 231, 234, 246, 292 Vitroplast, 154–158, 160, 163, 167–171 W Water quality, 266, 268, 270, 284 Weathering, 14, 81, 112, 127, 132, 174, 190–192, 268–269, 273, 281, 285, 287, 290 X X-ray diffraction, 39, 97–98, 112, 116–117, 197, 205, 300 X-ray diffraction analysis of fly ash, 116–117