Impact of Mineral Impurities in Solid Fuel Combustion

Impact of Mineral Impurities in Solid Fuel Combustion

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Impact of Mineral Impurities in Solid Fuel Combustion Edited by

R. P. Gupta CRC for Black Coal Utilisation University of Newcastle NSW, Australia

T. F. Wall CRC for Black Coal Utilisation University of Newcastle NSW, Australia

and

L. Baxter Sandia National Laboratories Livermore, California

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PREFACE This book contains papers presented at the Engineering Foundation Conference on mineral matter in fuels held on November 2-7, 1997 in Kona, Hawaii. The conference is one of a continuing series that was initiated by the CEGB Marchwood Engineering Laboratories in 1963. The conference was to be eventually organised by the Engineering Foundation as the need for multi-disciplinary work related to controlling ash effects in combustors became apparent. The conference covers both the science and the applications. The papers also present case histories, particularly for

current fuel technologies, developments in advanced technologies for power generation and mathematical modelling of these processes. Developments since 1963 have been slow, but steady, due to the complexity of the chemical and physical processes involved. However, the research presented here displays great improvement in our understanding of the mechanisms by which mineral matter will influence fuel use. Steve Benson from EERC presented a review and current status of issues related to ash deposition in coal combustion and gasification.

The application of new analytical tools, which have been detailed in the previous conferences, is presented. These include CCSEM, as well as new techniques for characterising sintering of ash, such as TMA, image analysis, X-ray diffraction crystallography

and thermal analysis. The new analytical techniques were extended to encompass widely differing fuels such as biomass. Ole H Larsen from ELSAM Denmark presented a review of these advanced techniques. Thermodynamic equilibrium calculations and knowledge of mineralogy has helped in understanding ash reaction mechanisms and evaporation of alkali species during combustion and gasification. Together with the analytical tools, and mathematical models based on these mechanisms, the leap to practical predictions is now possible. Several examples of this approach are detailed. For example, Jouni Pyykonen from VTT modelled ash deposition in a boiler using the mineral distribution from CCSEM and FLUENT, a computational fluid dynamics code. John Harb and his group from BYU extended the models to predict the changes in heat transfer in boilers with deposit growth. There has not been enough emphasis in the past on the estimates of thermal properties

of ash. A review article on thermal conductivity of ash like materials presented by Raj Gupta from CRC Black Coal is a significant step in the direction to rectify this problem. Thermal properties of deposits are shown to be strongly influenced by the deposit structure. A greater integration of data from advanced analytical techniques, mechanistic models and sophisticated CFD codes is expected in the future in order to predict the thermal performance of boilers. The conference is truly an international one, with participants representing all major laboratories working in this technical area. Many papers outline collaborative efforts, which have become a feature of modern research. This requires a special mention

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Preface

of the ELSAM project involving collaboration between Danish, US and Japanese researchers. The fuels of interest are somewhat continent-specific. In Europe, there is major interest in the use of biomass and its blending with other fuels. Larry Baxter from Sandia Laboratories reviewed the issues related to ash deposition in co-firing of biomass with coal. In Australia, the major interest is in coal. In Japan and the USA, all fuels are of interest. There were several studies on large power station boilers and pilot scale plants specially related to advanced technologies. The technologies considered include pulverised fuel, co-firing aid advanced power systems, such as PFBC and IGCC. FBC technology is highly suitable for low rank and low grade coals. The economic is pushing the usage of such fuels. A complete session was devoted to the ash problems and their solutions in FBC such as agglomeration and additives. There were a number of reviews and keynote papers in the field of FBC and other advanced technologies (Basu from BHEL India, Zhang from CRC Low Rank Coal and Ohman from ETC Sweden). The conference presents research in traditional atmospheric pressure systems but also considers newer systems where higher pressures are used and where the mineral matter in the fuel might be extracted as slag rather than the ash, which has been the traditional combustion product. Mr Sadayuki Shinozaki from CCUJ presented an extensive review of the advanced clean coal technologies. The Bryers Award for the best paper was presented to Lone Hansen of the Technical University of Denmark for her paper on “Ash Fusion Quantification by Use of

Thermal Analysis”, and Raj Gupta of The CRC for Black Coal Utilisation on his paper on “The Thermal Conductivity of Ash Deposits: Particulate and Slag Structures”. We also noted two excellent papers presented by Tim Heinzel of the University of Stuttgart and Bengt-Johan Skrifvars of Abo Akademi University, Finland.

It was particularly gratifying to have Richard Bryers at the conference to present the award named after him and also to give a keynote paper. We would like to thank a number of sponsors who provided support to allow the attendance of delegates who would otherwise not have been able to attend. The sponsors were: Cooperative Research Centre for Black Coal Utilisation, Australia; Cooperative Research Centre for Power Generation from Low-Rank Coal, Australia; Centre for Coal Utilisation, Japan; Idemitsu Kosan Co. Ltd., Japan; EBARA Corporation, Japan; IHI Co. Ltd., Japan; Kawasaki Heavy Industries, Japan; Electric Power Development Company, Japan and Japan Cement Association, Japan. We would also like to thank the Engineering Foundation for their assistance, particularly Richard Fein and Rosa Landinez who provided on-site support during the conference. We are grateful to all the session chairpersons for organising their sessions and reviewing the papers. In addition, Hongwei Wu and Chris Bailey, two PhD students from the CRC for Black Coal Utilisation, helped with arrangements. We would also like to thank Susan Safren of Plenum Publishing for her care, help and forbearance in the preparation of this book. R. P. Gupta

T. F. Wall L. Baxter

CONTENTS

Keynote Papers Ash-Related Issues During Combustion and Gasification . . . . . . . . . . . . . . . . . . . Steven A. Benson and Everett A. Sondreal Mineral Characterization for Combustion: The Contribution from the Geological Sciences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Colin R. Ward The Development of Power Technologies for Low-Grade Coal . . . . . . . . . . . . . . K. Basu

33

Low-Rank Coal and Advanced Technologies for Power Generation . . . . . . . . . . . Dong-ke Zhang, Peter J. Jackson, and Hari B. Vuthaluru

45

The Thermal Conductivity of Ash Deposits: Particulate and Slag Structures . . . R. P. Gupta, T. F. Wall, and L. Baxter

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The Development of Advanced Clean Coal Technology in Japan: Mineral Matter Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sadayuki Shinozaki

Factors Critically Affecting Fireside Deposits in Steam Generators Richard W. Bryers

..........

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SECTION I Mineral Matter, Ash and Slag Characterisation Advanced Analytical Characterization of Coal Ashes—An Idemitsu Kosan—Elsam Cooperation Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ole Hede Larsen, Flemming J. Frandsen, Lone A. Hansen, Signe Vargas, Kim Dam-Johansen, Karin Laursen, Takeo Yamada, and

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Tsuyoshi Teramae vii

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A Novel Application of CCSEM for Studying Agglomeration in Fluidised Bed Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mika E. Virtanen, Ritva E. A. Heikkinen, H. Tapio Patrikainen, and

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Risto S. Laitinen

Thermomechanical Analysis and Alternative Ash Fusibility Temperatures . . . . . S. K. Gupta, R. P. Gupta, G. W. Bryant, L. Juniper, and T. F. Wall

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Ash Fusibility Detection Using Image Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . Klaus Hjuler

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Ash Fusion Quantification by Means of Thermal Analysis . . . . . . . . . . . . . . . . . Lone A. Hansen, Flemming J. Frandsen, and Kim Dam-Johansen

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Sticking Mechanisms in Hot-Gas Filter Ashes . . . . . . . . . . . . . . . . . . . . . . . . . . . John P. Hurley, Bruce A. Dockter, Troy A. Roling, and Jan W. Nowok

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Classification System for Ash Deposits Based on SEM Analysis . . . . . . . . . . . . Karin Laursen and Flemming J. Frandsen

205

Determination of Amorphous Material in Peat Ash by X-Ray Diffraction . . . . Minna S. Tiainen, Juha S. Ryynänen, Juha T. Rantala, H. Tapio Patrikainen, and Risto S. Laitinen

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System Accuracy for CCSEM Analysis of Minerals in Coal . . . . . . . . . . . . . . . . R. P. Gupta, L. Yan, E. M. Kennedy, T. F. Wall, M. Masson, and K. Kerrison

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The Microstructure and Mineral Content of Pulverised Coal Chars . . . . . . . . . F. Wigley and J. Williamson

237

SECTION II The Use of Low-rank and Low-grade Coals and Cofiring

Fireside Considerations when Cofiring Biomass with Coal in PC Boilers . . . . . Allen L. Robinson, Larry L. Baxter, Gian Sclippa, Helle Junker, Karl E. Widell, Dave C. Dayton, Deirdre Belle-Oudry, Mark Freeman, Gary Walbert, and Philip Goldberg

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Summary of Recent Results Obtained from Using the Controlled Fluidised Bed Agglomeration Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marcus Öhman and Anders Nordin

Deposition and Corrosion in Straw- and Coal-straw Co-fired Utility Boilers: Danish Experiences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flemming J. Frandsen, Hanne P. Nielsen, Peter A. Jensen, Lone A. Hansen, Hans Livbjerg, Kim Dam-Johansen, Peter F. B. Hansen (1) and Karin H. Andersen (2), Henning S. Sørensen, Ole H. Larsen, Bo Sander, Niels Henriksen, and Peter Simonsen

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Development of Blast-Furnace Gas Firing Burner for Cofiring Boilers with Pulverized Coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takashi Kiga, Takehiko Ito, Motoya Nakamura and Shinji Watanabe

285

Changes in Slagging Behaviour with Composition for Blended Coals . . . . . . . . Nicholas J. Manton, Jim Williamson, and Gerry S. Riley

297

Role of Inorganics During Fluidised-Bed Combustion of Low-Rank Coals . . . Hari Babu Vuthaluru and Dong-ke Zhang

309

Role of Additives in Controlling Agglomeration and Defluidization During Fluidised Bed Combustion of High-Sodium, High-Sulphur Low-Rank Coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Temi M. Linjewile and Alan R. Manzoori The Agglomeration in the Fluidized Bed Boiler During the Co-Combustion of Biomass with Peat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ritva E. A. Heikkinen, Mika E. Virtanen, H. Tapio Patrikainen, and Risto S. Laitinen Ash Fusion and Deposit Formation at Straw Fired Boilers . . . . . . . . . . . . . . . . .

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Lone A. Hansen, Flemming J. Frandsen, Henning S. Sørensen, Per Rosenberg, Klaus Hjuler, and Kim Dam-Johansen SECTION III

Case Studies in Conventional and Advanced Plant

Influence of Metal Surface Temperature and Coal Quality on Ash Deposition in PC-Fired Boilers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Karin Laursen, Flemming J. Frandsen, and Ole Hede Larsen Full Scale Deposition Trials at 150MW E PF-Boiler CO-Firing Coal and Straw: Summary of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Karin H. Andersen, Flemming J. Frandsen, Peter F. B. Hansen, and Kim Dam-Johansen Slagging Tests on the Suitability of Alternative Coals in a 325 MW E PC Boiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Timm Heinzel, Jörg Maier, Hartmut Spliethoff, Klaus R. G. Hein. and Werner Cleve

Predicting Superheater Deposit Formation in Boilers Burning Biomasses . . . . . . Rainer Backman, Mikko Hupa, and Bengt-Johan Skrifvars

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Deposit Formation in the Convective Path of a Danish 80 MW TH CFB-Boiler CO-Firing Straw and Coal for Power Generation . . . . . . . . . . Peter F. Binderup Hansen

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Research on the Melting Points of Some Chinese Coal Ashes . . . . . . . . . . . . . .

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Shen Xianglin, Chen Ying, and Liu Haibin

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Computer Controlled Scanning Electron Microscopy (CCSEM) Analysis of Straw Ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Henning Sund Sørensen

Prognosis of Slagging and Fouling Properties of Coals Based on Widely Available Data and Results of Additional Measurements . . . . . . . . . . . . . . Alexander N. Alekhnovich, Natalja V. Artemjeva, Vladimir V. Bogomolov, Vyacheslav I. Shchelokov, and Vasilij G. Petukhov The Slagging Behaviour of Coal Blends in the Pilot-Scale Combustion Test Facility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alexander N. Alekhnovich, Vladimir V. Bogomolov, Natalja V. Artemjeva, and Vladimir E. Gladkov

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SECTION IV Studies at Rig Scale (Including Corrosion) In Situ Measurements of the Thermal Conductivity of Ash Deposits Formed in a Pilot-Scale Combustor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Allen L. Robinson, Steven G. Buckley, Gian Sclippa, and Larry L. Baxter

485

Low Corrosivity of Coal Chlorine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elliott P. Doane and Murray F. Abbott

497

Laboratory Studies on the Influence of Gaseous HCl on Superheater Corrosion Keijo Salmenoja, Mikko Hupa, and Rainer Backman

513

The Role of Alkali Sulfates and Chlorides in Post Cyclone Deposits from Circulating Fluidized Bed Boilers Firing Biomass and Coal . . . . . . . . . . . . Bengt-Johan Skrifvars, Tor Laurén, Rainer Backman, and Mikko Hupa

Fly Ash Deposition onto the Convective Heat Exchangers during Combustion of Willow in a Circulating Fluidized Bed Boiler . . . . . . . . . . . . . . . . . . . . . . Terttaliisa Lind, Esko I. Kauppinen, George Sfiris, Kristina Nilsson, and Willy Maenhaut

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Ash Behaviour in Biomass Fluidised-Bed Gasification . . . . . . . . . . . . . . . . . . . . . Antero Moilanen, Esa Kurkela, and Jaana Laatikainen-Luntama

555

Bench-Scale Biomass/Coal Cofiring Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deirdre Belle-Oudry and David C. Dayton

569

Iron in Coal and Slagging: The Significance of the High Temperature Behaviour of Siderite Grains During Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gary Bryant, Christopher Bailey, Hongwei Wu, Angus McLennan, Brian Stanmore, and Terry Wall

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SECTION V Develpments in Advanced Coal Technologies

Fractionated Heavy Metal Separation in Biomass Combustion Plants—Possibilities, Technological Approach, Experiences . . . . . . . . . . . . . Ingwald Obernberger and Friedrich Biedermann

595

Distributions of Major and Trace Elements in Entrained Slagging Coal Gasification Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kenichi Fujii, Masamitsu Suda, Tadayoshi Muramatsu, and Masahiro Hara

609

Behavior of Inorganic Materials During Pulverized Coal Combustion . . . . . . . . Tsuyoshi Teramae, Toru Yamashita, and Takashi Ando Energy Production from Contaminated Biomass: Progress of On-Going Collaboration Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alexandre Grebenkov, Anatoli Iakoushev, Larry Baxter, Dave Allen, Helle Junker, and Jørn Roed

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Triboelectrostatic Coal Cleaning: Mineral Matter Rejection In-Line Between

Pulverizers and Burners at a Utility

...............................

645

John M. Stencel, John L. Schaefer, Heng Ban, TianXiang Li, and James K. Neathery Development of Advanced PFBC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Makoto Takai and Masahiro Hara

653

Operating Experiences of 71 MW PFBC Demonstration Plant . . . . . . . . . . . . . . . Hideki Goto and Syoichi Okutani

663

Development of an Innovative Fluidized Bed Cement Kiln System . . . . . . . . . . . Sadayuki Shinozaki, Isao Hashimoto, Katsuji Mukai, and Kunio Yoshida

675

The Influence of Pressure on the Behaviour of Fuel Carbonates . . . . . . . . . . . . . Arvo Ots, Tõnu Pihu, and Aleksander Hlebnikov

685

SECTION VI Modeling of Ash Behaviour and Ash Deposition

Modeling of Ash Deposit Growth and Sintering in PC-Fired Boilers . . . . . . . . . Huafeng Wang and John N. Harb Predicting Ash Behavior in Conventional Power Systems: Putting Models to Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Christopher J. Zygarlicke

Thermodynamic Modelling of the System to Characterise Coal Ash Slags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evgueni Jak, Sergei Degterov, Arthur D. Pelton, Jim Happ, and Peter C. Hayes

697

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Development of a Prediction Scheme for Pulverised Coal-Fired Boiler Slagging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jouni Pyykönen, Jorma Jokiniemi, and Tommy Jacobson Modelling the Initial Structure of Ash Deposits and Structure Changes Due to Sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hamid R. Rezaei, Rajender P. Gupta, Terry F. Wall, S. Miyamae, and K. Makino

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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ASH-RELATED ISSUES DURING COMBUSTION AND GASIFICATION

Steven A. Benson and Everett A. Sondreal Energy & Environmental Research Center University of North Dakota PO Box 9018, Grand Forks, North Dakota 58202-9018 USA

phone [701] 777-5177, e-mail [email protected]

1. INTRODUCTION The effects of ash on the performance of combustion and gasification systems depends on the inorganic composition of the fuel and on operating conditions. Ash is known to be a major problem that results in decreased efficiency, unscheduled outages,

equipment failures, and high cleaning costs. The detrimental effects of fuel-associated inorganic components on combustor or gasifier process performance include slag-tapping problems, fireside ash deposition, corrosion and erosion of system parts, production of fine particulates that are difficult to collect, blinding of filtering media, formation of hazardous air pollutants, and production of precursors to the formation of secondary fine particulate. The amount of literature on ash-related issues is immense. Overviews of ashrelated issues and compilations of work by many investigators can be found by referring to the work of Baxter and DeSollar [1996], Couch [1994], Williamson and Wigley [1994],

Benson and others [1993b], Benson [1992], Bryers and Vorres [1990], Raask [1985, 1988], and Watt [1969]. Variability in chemical and physical properties is a major problem in burning and gasifying fuels in an environmentally acceptable manner. For coal, most utilization problems are related to the complex associations of the inorganic components. The association and abundance of major, minor, and trace elements in coal is dependent upon coal rank and depositional environment. The abundance and association of minerals in coal have been reviewed [Benson and others, 1993b; Raask, 1988]. The sulfur oxide emissions in power plants are derived from organic sulfur and mineral forms such as pyrite, gypsum, barite, and others. Trace elements that can cause pollution and operational problems are found associated with coal and waste-derived fuels. Many considered to be air toxics— Hg, Cd, Pb, As, and others—are associated with sulfides. The association, fate, and Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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behavior of air toxic metals have been reviewed and published in a Special Issue of Fuel

Processing Technology [Benson and others, 1994]. Additionally, the primary precursors to the formation of respirable particles (particles less than 2.5 micrometers) are partly derived from sulfur dioxide and nitrogen oxide emissions produced as a result of combustion [U.S. Environmental Protection Agency, 1996]. The prediction of ash behavior relative to the performance of conversion, hotgas cleanup, and air pollution control systems based on conventional ASTM (American Society for Testing and Materials) methods of analysis is severely limited because of the inadequacy of such methods to determine the distributed characteristics of the inorganic components as they exist in the coal. Computer-controlled scanning electron microscopy (CCSEM) [Jones and others, 1992] is used to determine the size, composition, and abundance of minerals in coals. In lower-ranked coals, chemical fractionation is used to determine the abundance of organically associated cations that are important to predict ash deposition behavior [Benson and Holm, 1985]. The effects of these minerals on system performance and emissions can be predicted with a much higher degree of certainty using CCSEM and other advanced methods of analysis [Benson and others, 1993a]. This paper, based mainly on work conducted over the last three decades at the Energy & Environmental Research Center (EERC), focuses on the influence of ash con-

stituents on the fireside performance of conversion systems. The assessment of fuel con-

stituents will primarily involve coal; however, the effects of biomass and wastes will also be contrasted and compared.

2. RESULTS AND DISCUSSION 2.1. Fuel Quality Currently, a wide range of materials is combusted or gasified for production of chemical feedstocks and energy, destruction of hazardous materials, and reduction in the volume of waste. Table 1 provides a summary of the bulk chemical composition

of selected fuels and waste materials that are currently being fired. The components of the fuel that have been considered most significant with respect to the fireside performance of fuels in combustion and gasification systems include alkali and alkalineearth elements and iron, which when combined with silicates can produce wall slagging and high-temperature convective pass fouling problems. At lower temperatures, the alkali and alkaline-earth elements combine with sulfur, halogen species, and sometimes carbonates, producing bonded deposits on heat-transfer surfaces. The alkali and alkaline-

earth elements can also contribute to the blinding of hot-gas filters and corrosion of system parts.

The abundance and mode or form of occurrence of major, minor, and trace inorganic components have an influence on their transformations upon conversion and ultimate fate in the conversion and environmental control systems upon gas cooling. Figure 1 illustrates the modes of occurrence of inorganic components in coals. Table 2 lists the modes of occurrence of vapor, minor, and trace elements. The modes of occurrence of the major elements in fuels are known and can be determined by a variety of methods, including CCSEM [Kong and others, 1996] and chemical fractionation [Benson and Holm, 1995]. The modes of occurrence for trace elements are not as well known because of the uncertainty in determining the association. Finkelman [1994] reviewed the modes

Ash-Related Issues During Combustion and Gasification

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S. A. Benson and E. A. Sondreal

of occurrence of potentially hazardous elements in coal and levels of confidence in the

modes. The abundance in the various modes changes dramatically with coal range and fuel type. Biomass typically contains high levels of organically associated calcium, potas-

sium, and phosphorus in addition to silicates and aluminosilicate contaminants. When combined in the conversion process, low-melting-point silicates, sulfates, phosphates, and carbonates produce high- and low-temperature deposits.

The most significant problem in the effective utilization of the fuels is the high variability in the inorganic composition of the fuels. For example, coals vary considerably from mine to mine and within seams. In order to effectively use coals with high variability, planning and assessing the effects of fuel handling, preparation, and distribution within the coal-fired power plant are necessary.

2.2. Transformations/Growth/Sintering The inorganic constituents in fuels and waste undergo complex chemical and physical transformations during the conversion process and gas cooling. The types of transformations depend on the modes of occurrence of the inorganic material in the fuel or waste and on conversion conditions. The transformations include processes illustrated in Fig. 2. The inorganic components that undergo volatilization during conversion usually condense within the conversion and environmental control system. The components that have the potential to remain in the vapor state through the air pollution control system include various forms of vapor-phase Hg and Se and HF, and HCl. Most of the volatilized components will condense within the overall system. The condensation and/or reaction of the volatilized species usually produce liquid phases that result in the formation of sticky particles and deposits. Condensed phases that are known to cause significant problems upon condensation include sulfides, sulfates, phosphates, chlorides, fluorides, carbonates, and some oxides. Figure 3 illustrates the distribution of trace

Ash-Related Issues During Combustion and Gasification

5

elements as a function of particle size produced from gasification of a bituminous coal [Benson and others, 1994a]. Some trace elements are known to accumulate in various

areas within the conversion system. Examples of these phases include calcium arsenate, selenium compounds, barium sulfate, strontium sulfate, lead sulfate/sulfides, and antimony compounds [Benson and others, 1994a].

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The nonvolatile components such as silicates, aluminosilicates, and iron-rich phases that pass through a liquid state are responsible for producing the bulk of the ash deposits. Significant experimental and modeling efforts have been conducted using laboratory-,

pilot-, and full-scale combustion testing to elucidate the mechanisms of formation of intermediate ash species. The intermediate species are in the form of gases, liquids, and solids. Computer models [Erickson and others, 1993; Wilemski, 1992] are able to predict the size, composition, and abundance of solid and liquid particles and the abundance and composition of gas-phase components upon conversion and gas cooling. Knowledge of the chemical and physical properties of the intermediate materials is a prerequisite to predicting transport and deposit growth.

Ash-Related Issues During Combustion and Gasification

7

Ash particle transport, deposit growth, and sintering mechanisms depend upon the velocity, temperature, and composition of the gas phase; the temperature, size, and composition of the particles; the composition and temperature of the deposition surface; and the physical characteristics of the particles and deposition surface. Ash transport mechanisms include both small-particle (less than 5 and large-particle (greater than Small-particle transport is dominated by thermophoresis, electrophoresis, and diffusion. Large-particle transport mechanisms are dominated by inertial impaction. These mechanisms have been described in detail by Rosner [1986]. Deposit growth and sintering are dominated by the abundance and viscosity of the liquid-phase components [Raask, 1988]. The initial sticking of particles for ash deposit growth is not completely quantified. Viscosity of the surface of the particles has been found useful in predicting the stickiness of ash. Sticking coefficients for deposit growth are defined as the ratio of ash sticking rate to ash firing rate. The critical value for sticking is in the range of to poise [Wilemski and others, 1992]. Strength development in materials is the result of a combination of sintering processes. According to Watt [1969], there are four primary mechanisms: vapor transport, surface diffusion, volume diffusion, and viscous or plastic flow. These processes are all distinct, but likely occur simultaneously. The mechanism of viscous flow sintering [Frenke, 1945] appears to be

the dominant mechanism in creating the high-temperature silicate-based melts in ash deposits. The other mechanisms likely play a more significant, if not dominant, role at lower temperatures. The process involved in the viscous flow sintering of particles is illustrated in Fig. 4. On the basis of Raask [1988], the onset of sintering occurs at to poise, moderate sintering at to rapid sintering at to formation of nonflowing slag at to slow movement of slag at to and good slag flow at 250 poise. Currently, methods used to predict slag flow behavior are based on empirical relationships derived from bulk chemical analysis of the slag and viscosities measured as a function of temperature with a rotating-bob viscometer. The computational methods

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have been reviewed and revised by a number of investigators, including Watt [1969], Schobert and others [1985], Kalmanovitch and Frank [1990], and Folkedahl [1997]. Many of the methods are limited in their range of application for predicting the viscosity of slags. Only a limited amount of work has been conducted in developing relationships for various chemical compositions at high viscosities above poise, which is the upper limit for the rotating-bob viscometer. Research is being conducted at the EERC to measure high viscosities at low temperatures using a heated-stage microscope. The method uses the Frenke [1945] relationship to calculate the viscosity of the melt based on the neck growth between particles. The technique has the potential to measure viscosity up to poise. Tests are currently being conducted on particles derived from glasses and homogeneous slags of known composition. Figure 5 illustrates the sintering of sodalite spheres. The physical parameter controlling slag flow is viscosity. For example, the maximum viscosity at which a slag can be tapped from a cyclone-fired boiler is 250 poise. The temperature at a viscosity of 250 poise is called T250. Generally, for a coal to be appropriate for cyclone firing, it must have a slag T250 of 2,600°F or lower. In addition, the slag must be of sufficient thickness such that it will retain the burning coal particles, thereby transferring the coal ash to the slag and completing the combustion in the cyclone. In order to maintain slag flow, a boiler has to be operated at a temperature higher than T250. Figure 6 illustrates the measured viscosity temperature curves for a Powder River Basin (PRB) coal slag under various atmospheres [Folkedahl, 1997]. Some PRB coals

exhibit extremely low viscosities, making it difficult to maintain a sufficiently thick slag layer. These low-viscosity slags also freeze rapidly, as shown in Fig. 6. Atmosphere affects the viscosity of the slag. The most pronounced effect on viscosity resulted from changing the test atmosphere from air or reducing gas to a simulated combustion gas (calculated based on coal composition and excess air), which caused an extremely high freezing temperature.

Ash-Related Issues During Combustion and Gasification

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As shown in Figure 6, another factor that influences the flow of slag is freezing, or crystallization. The temperature at which a sudden transition in viscosity occurs is called the temperature of critical viscosity Crystallization causes a rapid increase in viscosity on cooling below this temperature. Crystallization can cause sluggish slag flow and

must be considered in slags that exhibit such behavior. In estimating when a slag is completely molten, the following consideration should be used: if

then slag removal temperature =

if

then slag removal temperature =

2.3. Combustion 2.3.1. Pulverized Fuel. Extensive work has been carried out at the EERC for the past 30 years on ash fouling and slagging related to coal firing. Much of the early work was conducted on lower-rank western U.S. coals. More recently, bituminous and other fuels such as waste and biomass fuels have been tested. This work has been conducted using bench-, pilot-, and full-scale testing. To date in the ash-fouling unit (750,000 Btu/hr combustor), the EERC has conducted over 725 pulverized coal-fired tests and over 200 water-slurried fuel tests. Numerous runs have been made using a bench-scale drop-tube furnace system equipped with fly ash and ash deposition probes. The general mechanisms of ash deposit formation are illustrated in Fig. 7. Field testing has been conducted at more than 20 plants over the past 10 years in efforts to determine the mechanisms of ash formation and deposition aimed at developing ways to predict and mitigate ash-related problems. The key to better understanding ash behavior problems at the EERC over the last decade has been in developing advanced methods for sampling and aerodynamically classifying ash particulates by size, measuring deposit growth on cooled probes in pilot-plant

and full-scale systems, and analyzing inorganic species in coal and coal ash by a combination of 1) chemical fractionation for determining exchangeable cations and soluble

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S. A. Benson and E. A. Sondreal

minerals, 2) CCSEM for mineral grains, and 3) scanning electron microscopy point count (SEMPC) for analyzing phases in fly ash, deposits, and slags. Bulk composition and crystalline phases are determined by x-ray fluorescence (XRF) and x-ray diffraction (XRD), respectively. Ash transformation research at the EERC has, over time, delineated the hightemperature ash chemistry and particle-size distributions and mass transport mechanisms that control the rate of deposition and deposit strength development during combustion [Benson and others, 1996; McCollor and others, 1996; Zygarlicke and others, 1992]. The analysis of mineral grains and organically associated cations in low-rank coal can be used to predict patterns of fusion, vaporization, heterogeneous and homogeneous condensation, and coalescence or fragmentation. The products of these transformations are a wide combination of fused mineral fragments, inorganic vapor species, and condensed surface coatings and fine particulates. Ash deposition depends on the transport of these products to the cooler heat-transfer surface by a combination of vapor and smallparticle diffusion, thermophoresis, electrophoresis, and large-particle inertial impaction. Cohesion in deposit is provided by van der Waals, surface tension, electrostatic, and interlocking particle effects. Deposit growth and strength development depend most critically on the formation of low-viscosity melt phases at a microscopic level, which can be predicted by viscosity calculations based on the distributed CCSEM and SEMPC analysis

of precursors and products. Project Sodium, an industry-sponsored study [Benson and others, 1988] provided a detailed understanding of sodium-related ash-sintering mechanisms that are critical in high-temperature superheat sections of pf- and cyclone-fired boilers. The partitioning of ash species as a function of state and particle size was shown to be determined by their associations in the coal. Organically associated sodium was confirmed to be the most significant predictive factor of ash sintering and deposit growth. The sodium in hightemperature deposits was found to be concentrated in the amorphous melt phase and not

Ash-Related Issues During Combustion and Gasification

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in the crystalline phases. Most of the sodium-containing liquid phase identified in the high-temperature deposits (above l,900°F or 1,038 °C) was derived from aluminosilicates, but mixed alkali and alkaline-earth sulfates were found to be important at lower temperatures. Figure 8 illustrates the relationship between the surface tension and viscosity

and the relative deposition potential of the coal. At low surface tension-to-viscosity ratios, the deposition potential was high as determined in a pilot-scale combustion system. This relationship is consistent with the Frenke model, in which the sintering rate is related to surface tension/viscosity. Project Calcium, a multiclient program for high-calcium U.S. low-rank coals [Hurley and others, 1992], characterized several types of ash fouling deposits of operational significance below 1,900°F (1,038°C), where the thermodynamic equilibrium favors the formation of sulfate phases, including calcium sulfate crystals that impart strength to deposits. Deposit formation was studied both in utility boilers using cooled probes and in laboratory sintering tests. Entrained ash was aerodynamically sized, collected, and characterized to determine composition and size relationships. Deposits were collected from the same location as the entrained ash. Massive deposits forming on the upstream side of the leading tubes in the temperature zone below 1,900°F were rapidly sulfated and hardened unless removed at once by sootblowing. On the tubes behind the lead tubes, enamel-like deposits were formed by upstream deposition of small calciumenriched ash particles smaller than 3 the enamel hardened almost immediately and was most effectively removed by thermal shock using water blowers. A common type of powdery deposit formed on the downstream side of tubes from ash smaller than was easily removed by suitably angled sootblower nozzles; however, this deposit type had the greatest potential for reducing heat transfer if left unremoved because of its large surface coverage. Numerous field tests were conducted as part of Project Calcium to collect and characterize entrained ash and deposits. Figure 9 shows the partitioning of

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S. A. Benson and E. A. Sondreal

Ash-Related Issues During Combustion and Gasification

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the major inorganic ash components in the various size fractions of ash and the composition of the deposit formed on the upstream side of a reheater tube in a tangentially fired combustor. A predictive PC model of low-temperature deposition [LEADER] was developed that estimates deposition rates, deposit strength, and heat exchange loss on the basis of the coal analyses, including the ultimate analysis, the XRF ash analysis, and CCSEM coal mineral analysis. 2.3.2. Fluidized-Bed Combustion Systems. Agglomeration and deposition in fluidizedbed combustors (FBCs) [Hajicek and others, 1985; Henderson and others, 1995; Benson and others, 1982; Goblirsch and others, 1982, 1983] are determined by the same type of sulfate-based fouling that occurs in the lower-temperature zones of conventional boilers, except that the fluidized-bed material introduces another reactant and an entirely different fluid dynamic regime. Agglomeration of bed particles by sticky surface coatings

causes progressive accretion that leads to pressure and temperature fluctuations and ultimately the forced shutdown of the FBC. A high concentration of sodium in the coal ash is once again an important predictive factor. The EERC has investigated this problem in pilot-scale bubbling and circulating fluidized beds, in utility atmospheric fluidized-bed combustors (AFBCs) burning U.S. lignite and subbituminous coal and operated with sand and limestone beds, and in the Tidd clean coal demonstration of pressurized fluidized-bed combustion (PFBC). The mechanisms of ash formation in FBC systems are illustrated in Fig. 10. Much work has also been conducted on inorganic transformations and agglomerate formation during firing of Australian low-rank coals [Manzoori, 1990; Manzoori and others, 1992; Manzoori, 1992; Manzoori and Argarwal, 1993, 1994]. The primary contributor to increasing the sintering propensity of the bed particles was NaCl. More recently, Mann [1997] reviewed the literature related to the fate of alkali in FBC systems. Mann also examined sorbents that would remove alkali from PFBC flue gas to levels below that specified by turbine manufacturers. Methods have been established for determining the rate of agglomeration and deposition based on

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S. A. Benson and E. A. Sondreal

sieve analysis and SEM image analysis on the spent-bed material. The link between coal ash properties and coating/agglomerate formation is determined by the chemical fractionation and the CCSEM and SEMPC methods discussed above. The results of an ongoing investigation are being used to optimize the selection and replacement rate of bed material in relation to sulfur control requirements and the effect of coal ash constituents on agglomeration. Hot-gas filter ash characterization studies are being performed by the EERC in partnership with EPRI, the U.S. Department of Energy (DOE), and a consortium of companies to develop predictive models of ash-related problems in hot-gas filters for PFBC and integrated gasification combined-cycle (IGCC) systems. This program has been successful in identifying the causes of ash bridging between candle filters and related filter breakage that occurred during certain operating conditions at the Tidd PFBC Clean Coal demonstration project [Hurley and others, 1996]. Analysis of composition and size distributions down to 0.1 in diameter using the EERC’s SEMPC method established that ash bridging was caused by the smaller particles derived from the coal, cemented by a sticky potassium-enriched surface layer, rather than by larger particles derived from the dolomite sulfur sorbent. Thermochemical equilibrium modeling indicated that about 2% of the coal ash would exist in a liquid phase of potassium carbonate and sulfate at the conditions on the filter. The laboratory tensile strength of a simulated ash filter cake was shown to increase with temperature. Aged deposits from the Tidd filter were found to be

more sulfated than other filter cake material, which could account for the hardening and expansion of deposits that led to filter breakage.

2.4. Gasification 2.4.1. Fluidized Bed—Transport Reactor. The transport reactor demonstration unit (TRDU) (shown in Fig. 11) at the EERC is a 100-kg/hr pressurized circulating fluidizedbed gasifier that models the larger gasifier of similar design being tested by DOE at Wilsonville, Alabama, as a facility for evaluating hot-gas cleaning systems [Mann and others, 1995; Swanson and Ness, 1997]. The EERC aims to demonstrate acceptable filter performance at the pilot scale before long-term tests are performed at Wilsonville. The TRDU has also proven to be a valuable tool for evaluating ash deposition under the conditions of fluid dynamics and ash chemistry found in a circulating bed gasifier [Benson and Sondreal, 1996]. Deposits impeding operation of the unit formed in bends near the top of the gasifier loop, at the top of the riser, and at the cyclonic gas-bed disengager when a U.S. subbituminous coal was gasified that contained very finely divided clay and quartz in association with high levels of organically bound calcium. Analysis of these deposits by SEMPC found them to be enriched in low-melting-point calcium magnesium aluminosilicates derived from the coal ash and sulfided dolomite. A similar chemical composition was observed in the finer fraction of the circulating bed and in bridging deposits on the candle filters installed during the test. The bulk chemical composition of size-fractionated ash and the filter-bridging deposits are illustrated in Fig. 12. The results illustrated the important linkage between the operability of an advanced power system and the chemistry and morphology of the coal ash.

2.4.2. Entrained Flow Gasification. Coal ash behavior in reducing environments (CABRE) [Benson and others, 1994b] is a phased investigation being performed by the EERC under the support of different industrial consortia to develop analytical methods to characterize and classify reduced phase species found in species that condense to form

Ash-Related Issues During Combustion and Gasification

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low melting eutectics in gasification systems and to understand the high-temperature ash transformation mechanisms, develop predictive methodologies, and identify the properties of volatile sulfide species that condense to form low melting eutectics in gasification and hot-gas cleaning systems. Device-specific results are proprietary to the sponsors, but some general observations can be made. Corrosive alkali–iron sulfide eutectics are chemically stable in a gasification environment only below about 750 °C. CaS is stable at higher temperatures up to about 900 °C and may form CaS–FeS solid solutions at lower temperatures where FeS is stable. Eutectics formed from FeS, and are believed to be principal causes of ash sintering at temperatures below 700 °C. Gas transport processes that move volatile species from higher-temperature to lower-temperature zones serve to reduce the viscosity of eutectic melts and intensify sintering below 700 °C. Sulfide forming on the surface of an aluminosilicate particle is illustrated in Fig. 13.

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S. A. Benson and E. A. Sondreal

2.5. Fuel Quality Planning Predictive indices of ash fouling and slagging in conventional boilers developed for bituminous coals have been shown by experience to be largely unsuitable for low-rank coals. The EERC has recently compiled a series of indices for low-rank coals into a computer software package designated as the Predictive Coal Quality Effects Screening Tool or PCQUEST [Zygarlicke and others, 1996]. PCQUEST, as illustrated in Fig. 14, is

based primarily on the EERC’s ash transformation and transport research and related computer modeling, and the models used rely on detailed mineralogical and chemical fractionation analyses in addition to the conventional bulk measurements of coal moisture, principal elements, and ash oxides. Eight indices are calculated by PCQUEST to

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predict furnace wall slagging, high-temperature fouling, low-temperature fouling, slag tapping, opacity, tube erosion, coal grindability, and sootblower requirements. Utilities and coal companies have used the indices to map coal quality at the mine and follow performance at the power plant, and results have recently been validated [Zygarlicke and

McCollor, 1997]. Dynamic models to predict ash formation, transport, deposit growth, and removability have been developed [Erickson and others, 1995; Allan and others, 1996]. These models uses an ash transformation model to predict the particle-size composition distribution [Erickson and others, 1993; Wilemski and others, 1992] of the ash, which becomes input to the ash transport portion of the model. The ash transport model uses smallparticle (diffusion and thermophoresis) and large-particle (inertial impaction) mechanisms. Once the particles are transported to the surface, the deposited particles increase in temperature because of the insulating properties of the ash. The high temperatures increase the level of liquid in the deposit, resulting in increases in the ash-sticking coefficients. Deposit strength develops through the formation of low-viscosity liquid phases. The overall process is illustrated in Fig. 15 for prediction of fouling deposit formation.

3. SUMMARY AND CONCLUSIONS Improving the operability and performance of combustion/gasification systems and associated environmental systems hinges on our understanding the transformation processes involved in the formation of inorganic gas, liquid, and solid species that impede

heat transfer and gas flow, and promote degradation of materials. Understanding ash behavior allows for proper selection of fuels, operating conditions, and system modification and design. Variability in chemical and physical properties is a major problem in the burning and gasifying of fuels in an environmentally acceptable manner.

The prediction of ash behavior relative to the performance of conversion, hot-gas

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cleanup, and air pollution control systems on the basis of conventional ASTM methods

of analysis is severely limited because of the inadequacy of such methods to determine the distributed characteristics of the inorganic components as they occur in the coal. CCSEM can be used to determine the size, composition, and abundance of minerals in coals as a more valid basis for prediction. Significant progress has been made in understanding ash formation mechanisms and fate in conversion and environmental control systems. Assessing and predicting ash

behavior in pf-fired boilers have made significant advances over the years. The behavior of ash in FBC systems is becoming better understood. The reducing environment present in gasification systems results in the formation of a wide range of species that are not

well understood with regard to their effects on performance. Challenges remain in developing accurate methods for measuring the composition of highly variable fuels and feedstocks, assessing and monitoring the variability of fuels, developing methods to predict the impact of ash, meeting the environmental requirements related to ash components, and applying this information to a wide range of fuels and wastes.

4. REFERENCES Allan, S.E., Erickson, T.A., and McCollor, D.P. (1996). “Modeling of Ash Deposition in the Convective Pass of a Coal-Fired Boiler.” L. Baxter and R. DeSollar (Eds.), Applications of Advanced Technology to AshRelated Problems in Boilers, New York: Plenum Press, pp. 451–470. Baxter, L., and DeSollar, R. (Eds.) (1996). Applications of Advanced Technology to Ash-Related Problems in Boilers. New York: Plenum Press.

Benson, S.A. (Ed.) (1992). Inorganic Transformations and Ash Deposition During Combustion. New York: American Society of Mechanical Engineers for the Engineering Foundation.

Benson, S.A., and Holm, P.L. (1985). “Comparison of Inorganic Constituents in Three Low-Rank Coals.” Ind. Eng. Chem. Prod. Res. Dev., 24, 145. Benson, S.A., and Sondreal, E.A. (1996). “Impact of Low-Rank Coal Properties on Advanced Power Systems.” Proceedings of the Pittsburgh Coal Conference, September 1996, pp. 484–498.

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Benson, S.A., Karner, F.R., Goblirsch, G.M., and Brekke, D.W. (1982). “Bed Agglomerates Formed by Atmospheric Fluidized-Bed Combustion of a North Dakota Lignite.” Prepr. Pap.—Am. Chem. Soc., Div. Fuel Chem., 27(1), 174–181. Benson, S.A., Fegley, M.M., Hurley, J.P., Jones, M.L., Kalmanovitch, D.P., Miller, B.G., Miller, S.F., Steadman, E.N., Schobert, H.H., Weber, B.J., Weinmann, J.R., and Zobeck, B.J. (1988). “A Detailed Evaluation of

Sodium Effects in Low-Rank Coal Combustion Systems.” Project Sodium Final Technical Report, July. Benson, S.A., Hurley, J.P., Zygarlicke, C.J., Steadman, E.N., and Erickson, T.A. (1993a). “Predicting Ash Behavior in Utility Boilers.” Energy & Fuels, 7(6), 746–754. Benson, S.A., Jones, M.L., and Harb, J.N. (1993b). “Ash Formation and Deposition.” D.L. Smoot (Ed.), Fundamentals of Coal Combustion for Clean and Efficient Use. New York: Elsevier, Chapter 4, pp. 299–373. Benson, S.A., Erickson, T.A., Zygarlicke, C.J. (1994a). “Transformations of Trace Metals in Coal Gasification.”

Presented at the Joint AFRC/JFRC Pacific Rim International Conference on Environmental Control of Combustion Processes, Maui, HI, October 16–20, 1994. Benson, S.A., Erickson, T.A., Brekke, D.W., Folkedahl, B.C., Tibbetts, J.E., and Nowok, J.W. (1994b). “Coal

Ash Behavior in Reducing Environments.” DOE/METC-94, Vol. 1, pp. 322–331. Benson, S.A., Steadman, E.N., Mehta, A.K., and Schmidt, C.E. (Eds.) (1994). Trace Element Transformations in Coal-Fired Power Plants, Special Issue of Fuel Processing Technology, 39 (1–3). Benson, S.A., Steadman, E.N., Zygarlicke, C.J., and Erickson, T.A. (1996). “Ash Formation, Deposition, and Erosion in Conventional Boilers,” L. Baxter and R. DeSollar (Eds.), Applications of Advanced Technology to Ash-Related Problems in Boilers. New York: Plenum Press, pp. 1–15. Benson, S.A., Pavlish, J.H., Zygarlicke, C.J. (1998). “Trace Elements in Low-Rank Coals.” Proceedings of the Fifteenth Annual Pittsburgh Coal Conference, September 14–18, 1998. ISBN 1-890977-15-2. Brooker, D.D., and Oh, M.S. (1995). “Iron Sulfide Deposition During Coal Gasification.” S.A. Benson (Ed.), Special Issue of Fuel Processing Technology, Ash Chemistry in Fossil Fuel Processes, 44(1–3), 181–190. Bryers, R.W. (1995). “Utilization of Petroleum Coke and Petroleum Coke/Coal Blends as a Means of Raising Steam.” Fuel Processing Technology, 44, 121–144. Bryers, R.W., and Vorres, K.S. (1990). Proceedings of the Engineering Foundation Conference on Mineral Matter

and Ash Deposition from Coal. Santa Barbara, CA: United Engineering Trustees Inc. Couch, G. (1994). “Understanding Slagging and Fouling During Combustion.” J. Williamson and F. Wigley (Eds.), The Impact of Ash Deposition on Coal Fired Plants: Proceedings of the Engineering Foundation Conference. London: Taylor & Francis.

Erickson, T.A., Allan, S.E., McCollor, D.P., Hurley, J.P., Srinivasachar, S., Kang, S.G., Baker, J.E., Morgan, M.E., Johnson, S.A., and Borio, R. (1995). “Modeling Fouling and Slagging in Coal-fired Utility Boilers.” Special Issue of Fuel Processing Technology, Ash Chemistry in Fossil Fuel Processes, 44(1–3),

155–171. Erickson, T.A., O’Leary, E.M., Folkedahl, B.C., Ramanathan, M., Zygarlicke, C.J., Steadman, E.N., Hurley, J.P., and Benson, S.A. (1993). “Coal Ash Behavior and Management Tools,” J. Williamson and F. Wigley (Eds.), The Impact of Ash Deposition in Coal Fired Plants, London: Taylor & Francis, pp. 271–282.

Finkelman, R.B. (1994). “Modes of Occurrence of Potentially Hazardous Elements in Coal, Levels of Confidence.” Fuel Processing Technology, 39(1–3), 21–34. Folkedahl, B.C. (1997). “A Study of the Viscosity of Coal Ash and Slag,” Ph.D. Thesis, The Pennsylvania State

University. Frenke, J. (1945). “Viscous Flow of Crystalline Bodies Under the Action of Surface Tension.” Journal of Physics, 9(5), 385–390.

Goblirsch, G.M., Benson, S.A., Hajicek, D.R., and Cooper, J.L. (1982). “Sulfur Control and Bed Material Agglomeration Experience in Low-Rank Coal AFBC Testing.” Presented at the Seventh International Conference on Fluidized-Bed Combustion, Philadelphia, PA, October 1982. DOE/FC-1005, p. 15. Goblirsch, G.M., Benson, S.A., Karner, F.R., Rindt, D.K., and Hajicek, D.R. (1983). “AFBC Bed Material Performance with Low-Rank Coals.” Presented at the Twelfth Biennial Lignite Symposium, Grand Forks, ND, May 18–19, 1983, pp. 1–25. Gunderson, J.R., Anderson, C.M., Moe, T.A., Bolin, K., and Klosky, M. (1995). “Combustion Characteristics of RDF.” EERC 95-04-03, April. Hajicek, D.R., Zobeck, B.J., Mann, M.D., Miller, B.C., Ellman, R.C., Benson, S.A., Goblirsch, G.M., Cooper,

J.L., Guillory, J.L., and Eklund, A.G. (1985). “Performance of Low-Rank Coal in Atmospheric Fluidized Bed Combustion.” EERC Topical Report for the U.S.

Department of Energy.

DOE/FE/60181/1869, October. Henderson, A.K., Mann, M.D., Swanson, M.L., and Erickson, T.A. (1995). “Development of Methods to Predict Agglomeration and Deposition in FBCs.” Proceedings of the Advanced Power Systems 95 Review

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S. A. Benson and E. A. Sondreal Meeting. DOE/METC-95/1018, Vol. 1, June, pp. 440–447.

Hurley, J.P., Benson, S.A., Erickson, T.A., Allan, S.E., and Bieber, J.A. (1992). “Project Calcium Final Report,” KERC Publication, Grand Forks, ND: University of North Dakota. Hurley, J.P., Watne, T.M., O’Keefe, C.A., Katrinak, K.A., Nowok, J.W., Roling, T.A., and Dockter, B.A. (1996). “Chemical and Physical Analyses of Tidd Hot-Gas Filter Ash,” Proceedings of the Pittsburgh Coal Conference, September, 129–134.

Jones, M.L., Kaimanovitch, D.P., Steadman, E.N., Zygarlicke, C.J., and Benson, S.A. (1992). “Application of SEM Techniques to the Characterization of Coal and Coal Ash Products.” H.L.C. Meuzelaar (Ed.), Advances in Coal Spectroscopy. New York: Plenum Press. Kalmanovitch, D.P., and Frank, M. (1990). “An Effective Model of Viscosity for Ash Deposition Phenomena.” R.W. Bryers and K.S. Vorres (Eds.), Mineral Matter and Ash Deposition from Coal, United Engineering Trustees, Inc. Karner, F.R., Schobert, H.H., Falcone, S.K., Benson, S.A. (1986). “Elemental Distribution and Association with Inorganic and Organic Components in North Dakota Lignites,“ Mineral Matter and Ash in Coal. K.S. Vorres (Ed.), ACS Symposium Series 30: Washington, DC.

Kong, L., Zygarlicke, C.J., Benson, S.A. (1996). “Computer-Controlled Scanning Electron Microscopy Analysis of Minerals in Coal,” Proceeding of the Thirteenth Annual Pittsburgh Coal Conference, September 3–7, 1996. Vol. 1, p. 241.

Mann, M.D. (1997). “Capture of Alkali During Pressurized Fluidized-Bed Combustion Using In-Bed Sorbents.” Ph.D. Thesis, University of North Dakota, May. Mann, M.D., Swanson, M.L., Ness, R.O., and Haley, J.S. (1995). “Hot-Gas Filter Testing with the Transport Reactor Demonstration Unit.” Proceedings of the Advanced Power Systems 95 Review Meeting. U.S. Department of Energy, DOF/METC-95/1O18, Vol. 1, June, pp. 89–97.

Manzoori, A.R. (1990). “Role of the Inorganic Matter in Agglomeration and Defluidization During the Circulating Fluid Bed Combustion of Low Rank Coals.” Ph.D. Thesis, The University of Adelaide. Manzoori, A.R., Lindner, E.R., and Agarwal, P.K. (1992). “Inorganic Transformation During the Circulating Fluid Bed Combustion of Low-Rank Coals With High Content of Sodium and Sulphur.” Inorganic Transformations and Ash Deposition During Combustion. S.A. Benson (Ed.), New York: American

Society of Mechanical Engineers, pp. 735–762. Manzoori, A.R. (1992). “The Fate of Organically Bound Inorganic Elements and Sodium Chloride During Fluidized Bed Combustion of High Sodium, High Sulphur Low Rank Coals.” Fuel, 71,513–522. Manzoori, A.R., and Agarwal, P.K. (1993). “The Role of Inorganic Matter in Coal in the Formation of Agglomerates in Circulating Fluid Bed Combustors.” Fuel, 72(7), 1069–1075. Manzoori, A.R., and Agarwal, P.K. (1994). “Agglomeration and Defluidization Under Simulated Circulating Fluidized-Bed Combustion Conditions.” Fuel, 73(4), 563–568. McCollor, D.P., Zygarlicke, C.J., and Benson, S.A. (1996). “Mechanisms of Ash Fouling During Low-Rank Coal Combustion,” E. Baxter and R. DeSollar (Eds.), Applications of Advanced Technology to AshRelated Problems in Boilers, New York: Plenum Press, pp. 223–235. Raask, E. (1985). Mineral Impurities in Coal Combustion. Washington: Hemisphere Publication Corporation. Raask, E. (1988). Erosion Wear in Coal Utility Boilers. Washington: Hemisphere Publication Corporation. Rosner, D.E. (1986). Transport Processes in Chemically Reacting Flow Systems. Stoneham, MA: Butterworth, 539 p. Schobert, H.H. (1995). “Lignites of North America.” Coal Science and Technology, 23rd ed. New York: Elsevier.

Schobert, H.H., Streeter, R.C., and Diehl, E.L. (1985). “Flow Properties of Low-Rank Coal Ash Slags,” Fuel, 64,1611. Skrivfars, B.J., Hupa, M., Moilanin, A., and Lundqvist, R. (1996). “Characterization of Biomass,” E. Baxter and R. DeSollar (Eds.), Application of Advanced Technology to Ash-Related Problems in Boilers. New York: Plenum Press. Swaine, D.J. (1990). Trace Elements in Coal. Boston: Butterworths.

Swanson, M.L., Ness, R.O., Jr. (1997). “Hot-Gas Filter Testing with a Transport Reactor Development Unit.” Presented at the Advanced Coal-Based and Environmental Systems ’97 Conference, Pittsburgh, PA, July 22–24. U.S. Environmental Protection Agency. (1996). Review of National Ambient Air Quality Standards for

Particulate Matter: Policy Assessment of Scientific and Technical Information. EPA-452\R-96-013, July. Watt, J.D. (1969). “The Physical and Chemical Behavior of the Mineral Matter in Coal Under the Conditions Met in a Combustion Plant, Part I I . ” BCURA Industrial, Leather Head Laboratories. Wilemski, G., Srinivasachar, S., and Sarofim, A.F. (1992). “Modeling of Mineral Matter Redistribution and Ash Formation in Pulverized Coal Combustion.” S.A. Benson (Ed.), Inorganic Transformations and Ash Deposition During Combustion. Engineering Foundation Press.

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Williamson, J., and Wigley, F. (Eds.) (1994). The Impact of Ash Deposition on Coal Fired Plants: Proceedings

of the Engineering Foundation Conference. London: Taylor & Francis. Zygarlicke, C.J., Galbreath, K.C., McCollor, D.P., and Toman, D.L. (1996). “Development of Fireside Performance Indices tor Coal-Fired Utility Boilers,” L. Baxter and R. DeSollar (Eds.), Applications of Advanced Technology to Ash-Related Problems in Boilers. New York: Plenum Press, pp. 617–635. Zygarlicke, C.J., and McCollor, D.P. (1997). “Validation of Fireside Performance Indices.” EERC Report for the U.S. Department of Energy, July. Zygarlicke, C.J., Ramanathan, M., and Erickson, T.A. (1992). “Fly Ash Distribution and Composition: Experimental and Phenomenological Approach.” S.A. Benson (Ed.), Inorganic Transformations and Ash Deposition During Combustion. Engineering Foundation Press.

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MINERAL CHARACTERIZATION FOR COMBUSTION The Contribution from the Geological Sciences

Colin R. Ward School of Geology University of New South Wales Sydney 2052 Australia

1. INTRODUCTION The material referred to as “mineral matter” in coal has been to the combustion engineer a close to random association of chemical elements. These “random” elements are transformed in the combustion process to ash, react to form boiler deposits (slagging); abrade the internal parts of the boiler (erosion), and produce vapor phases that react with the metals in the boiler to cause metal loss (corrosion). Mineral matter in coal, however, is a far from random occurrence. The elements present in coal occur in predictable assemblages related to the processes that formed the coal. Knowledge of the way in which a coal formed, and of its post-depositional history, provides a guide to the forms and types of “mineral matter” that can be expected. This paper aims to develop such a theme, and to introduce the mineralogical tools that will allow the operator of combustion systems to characterize the non-organic component of coal, for it is this component that is the source of many problems in the combustion process. The knowledge base and techniques of the geological sciences have much to offer the combustion engineer in this particular field. Mineral characterization, often aided by the use of thermodynamic data to draw inferences as to the physical and chemical environment of formation, is the standard approach in the geological sciences. This applies not only to the relatively low-temperature processes that form the minerals actually found in coal, but also to the high-temperature processes associated with the formation and crystallization of magmas, which are similar in many ways to the behaviour of minerals in the combustion environment. When added to the generation of engineering indices, Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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there is the possibility to have predictive tools that are better based on the reality of the processes involved.

2. GEOLOGICAL ORIGIN OF MINERALS IN COAL Coal is made up of an organic and an inorganic component. The organic component is represented by the carbon-oxygen-hydrogen-nitrogen assemblages that are the coal macerals, the carbon analogues of the mineral material. The macerals in a coal reflect the parent plant material and the depositional and post-depositional histories of the coal deposit. There are many variations, which are reflected in the wide range of ranks and types that constitute the world’s coal resources. Mineral matter in coal encompasses three classes of material, namely:

• dissolved inorganic ions or compounds in the coal’s pore or surface water; • inorganic elements or compounds, with the exception of nitrogen or sulphur, incorporated in some way with the coal’s organic molecules; • crystalline mineral particles. Components of the first two types make up a large proportion of the mineral matter of lower-rank coals (brown coals and lignites). Changes in organic matter structure, however, reduce these to relative insignificance in higher-rank coals; crystalline mineral particles make up most if not all of the mineral matter in bituminous coals and anthracites. The minerals most commonly found in coals, including lower-rank coals, are listed

in Table 1. The relative abundance of these may vary considerably, depending on factors such as the depositional and hydrochemical environment in which the peat was formed, and the history of fluid migration, heating and alteration which the coal has suffered during and after burial. The minerals in coal (as opposed to the non-mineral inorganics) may represent particles of quartz, clay and other sediment washed or blown into the peat swamp. These are the detrital minerals. They may, however, also represent inorganic material produced by organisms, the biogenic minerals. Examples of the latter are shells of animals or diatoms that lived in the swamp and inorganic secretions (phytoliths) developed within the coal-forming plant tissues. A third possibility is that some of the minerals may represent compounds precipitated from solution. These are the authigenic minerals. They form in two ways, either precipitated in the pores associated with the plants that make up the peat in the early stages of coal formation, or deposited in cleats and other fractures during later stages of the coal’s geological history. The first, those associated with the early history, are the primary precipitates; the cleat fillers formed at later stages are called secondary precipitates. The processes of formation are important in combustion, because primary precipitates, particularly silica phytoliths, are normally fine grained and distributed throughout the coal in such a form that they are not unlocked when the coal is ground, even to the PF specification of 70% passing 75 microns. They remain in the PF particle and

subsequently reach temperatures beyond 2,000 °C, greater than the fuming temperature of silica.

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In mined coals the minerals may also include components derived from intraseam bands or layers of non-coal material within the seam, or from dilution of the coal by associated roof and floor rocks. Igneous intrusions may also contribute to the mineral matter in mined coals, either directly if the intrusions themselves are extracted or indirectly through formation of mineral-impregnated heat-altered coal or natural coke phases. Much of the mineral matter introduced to mined products from intra-seam or inter-seam materials may be removed by washing. The minerals that can be washed from the coal are sometimes referred to as “extraneous” mineral matter. Such minerals are only significant if the coal is used as a run-of-mine (ROM) or un-washed product. The

mineral matter intimately associated with the coal itself, the “inherent” mineral matter, can possibly be removed only by chemical demineralization techniques. There is, however, a set of elements associated with the organic component of the coal that cannot be removed by washing or chemical demineralization. These elements, mainly locked in the organic compounds, are an unavoidable part of the coal when it is used in combustion processes.

3. METHODS OF MINERAL MATTER ANALYSIS Effective use of coal by modern technology requires definitive information on the nature and distribution of the minerals present, as opposed simply to the chemical composition of the ash. It also increasingly requires mineralogical information to be

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supplied in quantitative form. This produces a need to assess the relative proportions of the different minerals or mineral groups present, together with the size of the mineral grains, the nature of any mineral associations, and possibly a range of mineral shape parameters. Quantitative mineralogical analysis, however, is an inherently difficult process, due to factors such as variability of composition and variations in crystallographic characteristics. The indices that have been used to assess coals for specific purposes have relied to date on elemental analysis, or ratios of elemental analyses. However, it is the minerals that react in the furnace, not the elements. Many of the minerals and other inorganics in coal undergo changes or interactions at high temperature, and as a result some of the mineral matter components may be lost from the solid phase during carbonization or combustion processes. The ash of the coal therefore usually represents only the nonvolatile or non-combustible residue of the mineral matter. By implication the elemental ash analysis is not representative of even the original inorganic elements in the coal, much less the mineralogy. The relation between mineral matter and ash is complex, particularly in low-rank coals. Kiss (1982), for example, indicates that up to one-third of the ash produced by combustion of Australian brown coals may be derived from fixation of organic sulphur by calcium, sodium or magnesium originally present in non-crystalline form, rather than as direct residues of the coal’s crystalline mineral particles.

A number of different techniques have been used for determination of the mineral matter (as opposed to ash) percentage in a coal sample. These include acid digestion of the mineral components (Radmacher and Mohrhauer, 1955), heating for a protracted period at around 370 °C, and calculations based on ash yield and other chemical data (King et al., 1936). Phong-anant et al. (1991), in their study of mineral matter in coal, recognized that no single analytical method was adequate for characterization of coal minerals and their transformation in boiler deposits, but noted that a combination of techniques was effective and desirable.

3.1. The Mineralogical Tools Mineralogical tools are diverse. They range from the human eye to sophisticated computer-controlled instruments. Each has a specific application, and the data delivered are limited or enhanced by the instrument used. In a typical investigation the tools can be thought of as qualitative or quantitative. Qualitative instruments can deliver descriptive data, and in some instances semiquantitative information. Such instruments are used to gain an overview; they are for reconnaissance. The optical microscope and the scanning electron microscope (SEM) are

two examples. Quantitative instruments deliver numbers. Quantitative systems are of two varieties, those that deliver precise determinations from single points and those that provide determinations, in some cases less precise, from many points on a specimen. 3.1.1. Optical Microscopy. Minerals in coal can be seen under the microscope in coal petrology studies (Stach et al., 1982), and volume percentages of the main mineral groups are typically included along with maceral percentages in the output from petrographic analyses. Many of the minerals are difficult to identify, however, partly because of a lack of distinguishing characteristics in reflected light and partly because they are fine-grained and often intimately associated with the organic components. Although useful in identifying the mode of mineral occurrence, petrographic analysis

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based on point counting under the optical microscope gives only an approximate guide to the total mineral matter content. 3.1.2. Scanning Electron Microscopy. Many of the limitations on mineral matter identification using optical microscopy can be overcome if polished sections or other samples of the coal are examined using a scanning electron microscope (Stanton and Finkelman, 1979; Corcoran, 1989). SEM technology enables higher magnification to be used, and also allows the chemical composition of individual particles to be investigated by an accessory energy-dispersive X-ray spectrometer (EDS) unit. The EDS capability of the SEM gives the instrument a semi-quantitative capacity that can be a useful adjunct to its imaging and characterization role.

3.1.3. X-ray Diffraction Analysis. X-ray diffraction (XRD) analysis is a technique that has traditionally been considered to be qualitative, but with recent developments has become more quantitative. Identification of minerals by XRD is based on the crystallographic characteristics of the different components. Variations in crystallographic characteristics, however, coupled in some cases with preferred orientation or differential X-ray absorption effects, traditionally limit the technique to a qualitative or at best semi-quantitative role (Renton, 1986). Several techniques have been developed to improve the quantitative determination of mineral percentages by X-ray diffraction. One of these, based on principles detailed by Rietveld (1969), is the computer-based S IROQUANT system developed by Taylor (1991). Use of this technique on minerals isolated from coals of the Callide Basin, Australia, in comparison to estimates of mineral percentages based on normative evaluation of chemical analysis data from the high-temperature (815°C) ash (see below), has been investigated by Ward and Taylor (1996). Direct application of SIROQUANT to coal and similar organic-rich materials, without a preliminary low-temperature ashing step, has also been used by Mandile and Hutton (1995) to indicate both the overall mineral matter content and the relative proportions of the different minerals present. XRD techniques are particularly suited to identification of the clay minerals, which in many cases make up the bulk of the total mineral matter (Gluskoter, 1967; Rao and Gluskoter, 1973; Ward, 1977, 1978) and are difficult to identify positively by other methods.

3.1.4. Electron Microprobe Analyser. The electron microprobe analyser (EPMA) is a prime example of an instrument able to give precise measurements from a single data point. EPMA technology is well advanced (Reed, 1996), and is a mainstay for materials science in general. The data from the EM PA for minerals can be plotted on phase diagrams, and inferences drawn from these plots. It is a specialist tool that in certain cases can yield unique results, and has the advantage of being direct and non-destructive. Ramsden and Shibaoka (1982) have demonstrated well the application of the technique in a benchmark study where ash types were characterized in an exhaustive appraisal that related composition to morphology and erected a general classification system for fly ash particles. Electron microprobe analysis is most widely used for study of relatively heavy (high atomic number) elements, including components such as sulfur and chlorine in the organic matter of coal samples. Development of special measuring techniques in recent years, however, has also allowed it to be applied also to determination of light elements, including the carbon and oxygen in the organic matter of individual coal macerals (Bustin

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et al., 1993; Ward and Gurba, 1997). Such a capacity can usefully complement investigations of mineral particles. 3.1.5. Computer-controlled SEM and Image Analysis Systems. Computer-controlled scanning electron microscopy (CCSEM) has been under development for the coal industry over some years, and incorporates many techniques common to other instruments that use electron beams. QEM*SEM (quantitative evaluation of materials by scanning electron microscopy) is perhaps the best example. The application of this

technology is the subject of several other papers in this volume, and so details are not listed here. QEM*SEM has been under development for the minerals industry and has become a prime tool in mineral processing. The object of QEM*SEM is to obtain statistically reliable information on the materials that are passing through the process. It also has image analysis capabilities, and can generate statistical information on textural features such as associations, rimming and simple shape functions, although the latter are still under development. The instrument’s application to coal and the products of coal combustion (Creelman et al., 1993) has been mainly in the field of erosion. It has been used in this capacity for erosion studies in fluidized-bed combustors. Creelman and Ward (1996) have recently applied (QEM*SEM) to the quantitative

evaluation of mineral matter in bituminous coal samples. The technique determines the association of chemical elements at individual points on a coal polished section from the output of several X-ray analysers directed at each point under the SEM. The element association at each point is then processed to identify the mineral species or group involved. Data from numerous such points in a scan of the sample are integrated using the image analysis unit, enabling volumetric assessment of the relative proportions of the different minerals or element-associations present. The system also has the capacity to map the individual mineral particles, and to provide a statistical evaluation of the size and shape of the different mineral occurrences in the coal sample.

3.2. Integrated Techniques for Quantitative Evaluation of Coal Mineral Matter 3.2.1. Low-temperature Oxygen-plasma Ashing. Oxidizing the organic matter and isolating the minerals without major alteration can be achieved by ashing coal at low temperature (120–150°C) in an electronically-excited oxygen plasma (Gluskoter, 1965). This probably represents the most reliable method for determining the percentage of total mineral matter (Standards Australia, 1990). The use of hot, concentrated hydrogen

peroxide to remove the organic matter and isolate an unaltered mineral fraction (Ward, 1974) may represent a useful substitute in some circumstances, but has more limited overall application. The nature of the minerals isolated by either technique can then be investigated by X-ray diffraction and similar methods. Precise determination of the mineral matter percentage in coal by low-temperature ashing also involves correction of the oxygen-plasma ash yield for un-oxidised organic carbon residues and for sulfur fixed in the LTA from the organic sulfur component (Standards Australia, 1990). These are often relatively small corrections, however, and for many purposes the proportion of low-temperature ash (LTA) gives an adequate indication of the percentage of mineral matter present.

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3.2.2. Normative Interpretation of Ash Analysis Data. Estimates of the relative proportions of the different minerals in coal can also be made by interpretation of the chemical composition of the coal’s conventional (high-temperature) ash, as determined in routine analytical programs. Such techniques are based on the assumption that the various elements in the ash were originally partitioned between particular minerals, and hence represent an attempt to derive a “theoretical” mineralogy from the chemical analysis data. They are most effective if the actual minerals present are known from independent evidence, such as X-ray diffraction, and if the element proportions in minerals that may be of variable composition, such as interstratified clay minerals, can also be determined for that particular sample. An interactive computer-based procedure for normative analysis of sedimentary materials (SEDNORM), developed by Cohen and Ward (1991), has been applied to ash analysis data for a range of Australian and other coals, in the light of XRD data from the same coals’ LTA residues. The results were compared against quantitative mineralogical evaluations obtained from other techniques, such as XRD of the LTA using SIROQUANT (Ward and Taylor, 1996) and analysis of mineral matter in coal polished sections using the QEM*SEM technique (Creelman and Ward, 1996). Each method has its advantages and limitations, but relatively good agreement was achieved between SEDNORM and the other techniques for the main mineral groups involved. 3.2.3. Application to Low-rank Coals. Plasma ashing of lower-rank coals typically results in the formation of abundant calcium, ammonium and other sulfates, due to interaction of the principal non-mineral inorganics with the coal’s organic sulfur during the low-temperature oxidation process. The non-mineral inorganics that typically dominate

the mineral matter of such coals may therefore need to be removed and investigated separately by a process of selective leaching (Miller and Given, 1979; Benson and Holm, 1985; Ward, 1991; 1992) before low-temperature ashing can take place. One such procedure (Ward, 1992) involves: • Soaking the coal in distilled water, with analysis of the extract to identify the material dissolved, or at least potentially soluble, in the coal’s pore water; • Treatment of the water-washed coal with a concentrated ammonium acetate solution, with analysis of the extract to identify water-insoluble but exchangeable elements, such as those associated with carboxylate groups;

• Leaching of the -exchanged coal with hydrochloric acid, followed by analysis of the extract to identify the inorganics occurring in any acid-soluble organometallic complexes and in any otherwise-insoluble carbonate minerals. The acid-treated coal can then be subjected to oxygen-plasma ashing, to remove the organic matter (which has in the exchangeable-ion positions to prevent formation of mineral artifacts by organic sulfur fixation) and isolate the acid-insoluble mineral components.

3.3. Ancillary Techniques to Evaluate Coal Mineral Matter Ancillary techniques provide information that although strongly linked to mineralogical data does not necessarily in itself represent mineralogical data. Oxide determinations, the old standby, can be produced not only by conventional chemistry but also by newer and more rapid techniques such as inductively-coupled plasma (ICP) and X-

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ray fluorescence (XRF) spectrometry. Chemical analyses are useful but they are in themselves imperfect as predictors. Elemental oxides are combined in minerals, singularly or with other oxides. It is the mineral that has the specific physical and chemical properties, not the component chemical part. Infra-red (IR) and Fourier Transform Infra-red (FTIR) spectrometry are related techniques that yield specific information on crystal structure and hence on mineral matter transformations that occur as minerals are heated. Application to mineralogy has been successfully attempted for a wide range of processes (Fredericks et al., 1985; Nyquist et al., 1990). FTIR has the potential to be an on-line method to monitor the development of certain glass/crystalline phases as they form in the furnace. Nuclear magnetic resonance (NMR) spectroscopy is also a specific technique for investigating mineral structure, able to quantify minerals with specific structures involving Al and Si (Phong-anant et al., 1991). This method is similar in concept and application to FTIR.

4. CONCLUSIONS The nature and distribution of the mineral matter have a fundamental effect on the behaviour of coals when used for various purposes. Quartz particles have long been recognised as a cause of erosion in grinding mills and on exposed furnace surfaces, and sulphur in various forms can give rise to furnace corrosion and stack-gas pollution. Mineral groups such as the iron sulfides and siderite or calcite, as well as inorganically-associated calcium, also appear to be major contributors to slagging in furnace operation. Interaction of sulphur trioxide, water vapor and coal ash can produce fouling deposits on the lower-temperature convective parts of a furnace system. Phosphate and sulphur-bearing minerals can also transfer P and S to coke when coal is heated in coke ovens. These elements are then transferred to the iron when the coke or when S and P-bearing pulverized coal is injected with iron ore into a blast furnace, producing a need for costly removal at a later stage in the metallurgical process. Evaluation of minerals and other inorganics in coal, based on techniques such as oxygen-plasma ashing and X-ray diffraction, provides a range of fundamental data essential to modern coal characterization, extending the information from traditional sources such as chemical ash analysis. Given the growing sophistication of mineral analysis technology, the relevant data can also increasingly be provided in a quantitative form. The behaviour of minerals and other inorganics during combustion depends to a significant extent on how the various components occur within the coal, the form in which they exist at combustion temperatures, and the opportunities that they have to interact with each other in the course of the combustion operation. Calcium occurring in pore water or in an organic association, for example, is typically released as independent Ca ions or atoms on combustion. These are relatively free to react with other ash-forming components. Calcium occurring as the carbonate mineral calcite, on the other hand, forms discrete CaO particles that are different aerodynamically, less reactive and therefore less likely to be associated with slagging or similar problems. Use of mineralogical analysis opens up a number of paths to engineers and others concerned with coal combustion, providing a better basis for assessment of potential problems with materials handling (through the presence of smectite and similar clay minerals), abrasion (due to liberated quartz particles), slagging, fouling, sulphur capture and ash handling characteristics. It is also relevant to the applicability of on-line quality

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assessment systems. Understanding of the distribution of mineral matter in coal at the source (i.e. within the deposit being mined) can sometimes be used for better control of coal product quality through mining and preparation, and hence provide a basis for optimising the total resource utilisation process.

5. ACKNOWLEDGEMENTS The author would like to thank the Engineering Foundation, and Bob Creelman

as Session Convenor, for assistance in the presentation of this paper.

6. REFERENCES Benson, S.A. and Holm, P.L. (1985). ”Composition of Inorganic Constituents in Three Low-rank Coals”. Industrial and Engineering Chemistry, Product Research and Development, 24, 145–149. Bustin, R.M., Mastalerz, M. and Wilks, K.R. (1993). “Direct Determination of Carbon, Oxygen and Nitrogen Content in Coal using the Electron Microprobe”. Fuel, 72, 181–185. Cohen, D.R. and Ward, C.R. (1991). “SEDNORM—a Program to Calculate a Normative Mineralogy for

Sedimentary Rocks based on Chemical Analyses”. Computers and Geosciences, 17(9), 1235–1253. Corcoran, J.F. (1989). “The Use of SEM/EDS Techniques in Studies of Coal and Associated Mineral Matter”. Proceedings of Mineralogy-Petrology Symposium, MINPET 89, Australasian Institute of Mining and Metallurgy, Sydney, 85–87. Creelman, R.A., Agron-Olshina, N. and Gottlieb, P. (1993). “The Characterization of Coal and the Products of Coal Combustion using QEM*SEM”. Final Report, Project 1467, National Energy Research, Development and Demonstration Program, Australian Department of Primary Industries and Energy, Canberra. Creelman, R.A. and Ward, C.R. (1996). “A Scanning Electron Microscope Method for Automated, Quantitative Analysis of Mineral Matter in Coal”. International Journal of Coal Geology, 30, 249–269. Fredericks, P.M., Osborn, P.R. and Swinkels, D.A.J. (1985). “Rapid Characterization of Iron Ore by Fourier Transform Infra-red Spectrometry”. Analytical Chemistry, 57, 1947–1950. Gluskoter, H.J. (1965). “Electronic Low Temperature Ashing of Bituminous Coal”. Fuel, 44, 285–291. Gluskoter, H.J. (1967). “Clay Minerals in Illinois Coal”. Journal of Sedimentary Petrology, 37(1),

205–214. King, J.G., Maries, M.B. and Crossley, H.E. (1936). “Formulas for the Calculation of Coal Analyses to a Basis of Coal Substance free from Mineral Matter”, Journal of the Society of Chemical Industry, 55, 277–281. Kiss, L.T. (1982). “Chemistry of Victorian Brown Coals”. Australian Coal Geology, 4(1) , 153–168. Mandile, A.J. and Hutton, A.C. (1995). “Quantitative X-ray Diffraction Analysis of Mineral and Organic Phases in Organic-rich Rocks”. International Journal of Coal Geology, 28, 51–69. Miller, P.N. and Given, PH. (1978). “A Geochemical Study of the Inorganic Constituents of some Low-rank Coals”. Report, Contract EX-76-C-01-2494, U.S. Department of Energy, Coal Research Section, Pennsylvania State University (unpublished): 314 pp. Nyquist, R.A., Leugers, M.A., McKelvy, M.L., P-yen-uss, R.R., Putzig, C.L. and Yurga, L. (1990). “Infra-red Spectrometry”, Analytical Chemistry, 62, 223R–255R. Phong-anant, D., Pang, L.S.K., Vassalo, A.M. and Wilson, M.A. (1991). “Mineral Matter in Coal—the Characterization, Transformation and Effects on Boiler Deposit Formation and Boiler Erosion”. Final Report, Project 1227, National Energy Research, Development and Demonstration Program, Australian Department of Primary Industries and Energy, Canberra. Radmacher, W. and Mohrhauer, P. (1955). “The Direct Determination of the Mineral Matter Content of Coal”. Brennstoff-Chemie, 36, 236. Rao, C.P. and Gluskoter, H.J. (1973). “Occurrence and Distribution of Minerals in Illinois Coals”. Illinois State Geological Survey, Circular 476, 56 pp. Reed, S.J.B. (1996). “Electron Microprobe Analysis and Scanning Electron Microscopy in Geology”. Cambridge University Press, Cambridge, 201 pp. Renton, J.J. (1986). “Semiquantitative Determination of Coal Minerals by X-ray Diffractometry”. In: Vorres, K.S. (ed), “Mineral Matter and Ash in Coal”. American Chemical Society Symposium Series, 301.

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Ramsden, A.R. and Shibaoka, M. (1982). “Characterization and Analysis of Individual Fly-ash Particles from Coal-fired Power Stations by a Combination of Optical Microscopy, Electron Microscopy and Quantitative Electron Analysis”. Atmospheric Environment, 16(9), 2191–2206.

Rietveld, H.M. (1969). “A Profile Refinement Method for Nuclear and Magnetic Structures”. Journal of Applied Crystallography, 2, 65–71. Stach, E., Mackowsky, M.Th., Teichmuller, M., Taylor, G.H., Chandra, D. and Teichmuller, R. (1982). “Stach’s Textbook of Coal Petrology”. Gebruder Borntrager, Berlin, 535 pp. Standards Australia (1992). “Higher Rank Coal—Mineral Matter and Water of Constitution”. Australian Standard 1038.22, 16 pp.

Stanton, R.W. and Finkelman, R.B. (1979). “Petrographic Analysis of Bituminous Coal: Optical and SEM Identification of Constituents”. Scanning Electron Microscopy, 1, 465–471.

Taylor, J.C. (1991). “Computer Programs for Standardless Quantitative Analysis of Minerals using the Full Powder Diffraction Profile”. Powder Diffraction, 6, 2–9. Ward, C.R. (1974). “Isolation of Mineral Matter from Australian Bituminous Coal using Hydrogen Peroxide”,

Fuel, 53, 220–221. Ward, C.R. (1977). “Mineral Matter in the Harrisburg-Springfield (No. 5) Coal Member of the Carbondale Formation, Illinois Basin”. Illinois State Geological Survey, Circular 498, 35 pp. Ward, C.R. (1978). “Mineral Matter in Australian Bituminous Coals”. Australasian Institute of Mining and Metallurgy Proceedings, 267, 7–25.

Ward, C.R. (1991). “Mineral Matter in Low-rank Coals and Associated Strata of the Mae Moh Basin, Northern Thailand”. International Journal of Coal Geology, 17, 69–93. Ward, C.R. (1992). “Mineral Matter in Triassic and Tertiary Low-rank Coals from South Australia”. International Journal of Coal Geology, 20, 185–208.

Ward, C.R. and Gurba, L.W. (1997). “Use of the Electron Microprobe in Chemical Analysis of Coal Macerals, with Special Reference to the Direct Determination of Organic Sulphur”. In: Boyd, R.L. and Allen, K. (eds), Proceedings of the 31st Symposium, Advances in the Study of the Sydney Basin, Department of

Geology, University of Newcastle, New South Wales, 115–122. Ward, C.R. and Taylor, J.C. (1996). “Quantitative Mineralogical Analysis of Coals from the Callide Basin, Queensland, Australia using X-ray Diffractometry and Normative Interpretation”. International Journal of Coal Geology, 30, 211–229.

THE DEVELOPMENT OF POWER TECHNOLOGIES FOR LOW-GRADE COAL

K. Basu Bharat Heavy Electricals Limited Corporate Research & Development Division

Vikas Nagar, Hyderabad-500093. INDIA

1. INTRODUCTION The need for Clean Coal Technology, CCT, is well understood for the sustainable development and growth of a country. It can be defined as “technology designed to enhance both the efficiency and environmental acceptability of coal extraction, preparation and use”. Various clean coal technologies are available and under development to enhance the acceptability of coal as a clean fuel. For conventional technology, pre and post combustion cleaning are generally retrofit systems to meet the pollution regulation. The new technologies promise to increase efficiency and to reduce pollutant generation, beyond the process capability of conventional power plants. In spite of Human Development Index, HDI, proposed by UNDP to measure the wellbeing of a society, supply of clean commercial electricity to the consumer at an affordable price still remains the vital link to socio-economic development of a country. It is particularly relevant for India, since its per capita electricity consumption is still about l/30th fraction of developed countries. The country’s development is further hampered due to chronic shortage of electricity supply. A GDP loss of Rs. 180 billion was attributed to shortage of electricity during 1996–97. Coal has been India’s mainstay fuel for power generation. At present, 65 per cent of total electricity of the country is generated by using non-coking coal in sub- critical

pulverized fuel (PF) fired boilers. The coal supplied to power stations contains about 40 per cent ash. Highly abrasive nature of ash and non-standard quality of coal supplied to power plants reduces thermal efficiency and increases forced outage. Average plant load factor (PLF) and operating availability of Indian plants are 64 per cent and 80.7 per cent respectively. To achieve projected power growth rate, improvement in performance of existing power plants and installation of 56,700 MW new capacity is proposed. Repowering and Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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refurbishing of old power plants and supply of beneficiated coal to distant power stations, are expected to improve their performance and PLF. Improvement of one per cent PLF today is equivalent to addition of 750 MW new capacity. Relaxation for import of coal also allows performance improvement by switching to or blending with low ash and high heat value imported coal. However, domestic fund constraint and limitation in infrastructural facilities for port handling and inland transportation can restrict bulk import of coal. India’s ambitious plan for addition of new capacity offers a large potential market for both conventional and emerging clean coal technologies. Other than domestic fund constraint, technical reliability and economic viability of emerging technologies are presently the major concerns for commercial investment. Consequently, proven sub critical PF boilers will continue to have the major share of electricity generation in the near future. India has already imposed a strict emission regulation for power plants. However, less efficient sub-critical technology cannot meet the expected emission lelvel of and other pollutants in the distant future. Also, at present, Indian power plants are not equipped with FGD and threfore, cannot use high sulphur Indian or imported coal. To address the future market need, BHARAT HEAVY ELECTRICALS LIMITED, BHEL, the major engineering and supplier of power plant equipment in India, has been

assessing and developing clean coal technologies for Indian coal.

2. PRESENT OPTION As cited earlier, majority of India’s new power plants will be based on conventional technologies and will use Indian coal. Due to presence of about 85 per cent silica and alumina in ash and medium volatile content in coal, the major challenges for this technology are erosion of, coal and ash paths and combustion stability of boiler. Oil support is required even at as high as 50 per cent of the design load. Beneficiation of Indian coal and use of low sulphur and high heat value coal offer best possibility for efficiency

improvement and emission reduction for the conventional technology.

2.1. Coal Other than poor characteristics of Indian coal, present mining practice further reduces the quality of coal. The total coal reserve of India is estimated to be 200 billion tonnes out of which 68.6 billion tonnes are proven reserves. The reserves are distributed in central and eastern sectors of the country. Some lignite deposits are also found in the southern sector. There are 10 coal bearing river basins with 58 coal and lignite fields consisting of about 450 coal seams and 60 per cent of coal deposits are within the depth of

300 meters. This shallow deposits encourage surface mining of coal. Indian coal is said to have 25 per cent inherent ash content similar to other Gondwanan coal. However, the quality is progressively deteriorating over the years due to emphasis on open cast mining. The share of coal produced from open cast mines

increased from 26 per cent in 1975 to 73 per cent in 1993. The non coking coal for power sector is graded based on their useful heating value, UHV between 26.0 MJ to 10.0 MJ per Kg. into seven categories, “A” to “G”. “E” “F” and “G” are graded as inferior quality which have UHV between 14.0 MJ to 10.0 MJ per Kg. While no inferior grade was produced in 1947, its present share is 70 per cent and

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it is expected to reach 78 per cent by the turn of the century under “Business as usual” scenario.

2.2. Beneficiation At present, about 33 MMT of coking and semi coking coal is beneficiated for industrial use. So far, beneficiation of non coking coal has not been taken up for regular use in power plants. High proportion of near gravity material in coal makes the separation difficult and expensive. Separation is further aggravated by the dispersion of minerals throughout the coal matrix. Hard and abrasive nature of alumina and silica which constitute more than 85 per cent of ash particles, increase the capital and running cost of the plant. The experts claim that to reduce ash from 45 per cent to 19.7 per cent, the generation of middlings and tailings will be around 85 per cent with high carbon content. Further use of these rejects in fluidized bed boiler will be necessary to fully utilise the fuel content of coal. To evaluate economic level of cleaning of mineral, number of technologies had been evaluated in India. Also, tests had been conducted with beneficiated coal to evaluate improvement in performance of a power plant. Contra Flow Separation technololgy could reduce 8 to 10 per cent ash from coal. The cost of a 100 TPD plant was estimated to be between Rs.5 million to Rs.55 million. The ROM Jig technology is expected to reduce ash from 45 to 29 per cent in deshaled portion and 31.6 per cent in total mix. The yield is expected to be 67 per cent. Tests at the Satpura power station recorded an improvement of plant utilisation factor from 73

per cent to 93 per cent and energy generation from 3.71 MU to 4.83 MU per day with coal, beneficiated from 44 per cent to 31 per cent ash content. A study indicates that the reduction of ash from 45 per cent to 34 per cent will be economically viable against transportation cost of coal to power plants situated beyond 1,000 Km. A cost benefit analysis at a macro level including all socio economic cost of each element can resolve the debate ensuing in India on coal beneficiation. However, the Ministry of Environment & Forests has given a directive to supply coal with less than 30 per cent ash to all power plants beyond 1,000 KM. from fuel source. At present, 13 per cent of total power coal is transported to power plants beyond 1,000 Km. and its share is expected to go up to 18.4 per cent by 2002.

2.3. Switching and Blending Since slagging and fouling is a combined effect of boiler design and coal property, some operators are concerned to use imported low ash, high heat value coal in existing boilers. High heat flux in burner area and slagging and fouling of boilers are their main

concerns even with characteristically low slagging coal.

3. EMISSION The major pollutants from Indian power plants are the generation of particulates and emission of Emission of is generically similar to other power plants. Low incidence of sulphur in coal reduces the burden of post combustion cleaning of gaseous

sulphur emission.

where required is cleaned using Ammonia.

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Under “Business as usual scenario”, we will be generating 128 MMT of ash, 4.3 MMT of and 570MMT of to meet the energy demand of 475 billion Units in 2002 from coal based power plants. In 1996–97, the respective pollutant generations were 85 MMT of ash, 2.8 MMT of and 380 MMT of based on average specific coal consumption of 0.75 Kg/KWH. Hypothetically, if we use only imported coal with ash content of 15 per cent, sulphur content of 3 per cent and also improve specific coal consumption from 0.75 Kg/KWH to 0.35kg/KWH, then we will be generating 20MMT of ash and 10MMT of in 2002. This will require installation of FGD plants at an additional cost of about 25 per cent of generation cost. India’s total emission to global environment is about 6 per cent with over 16 per cent population. Consequently, our per capita emission is insignificant as compared to the developed countries. Also, the role of emission for global warming and its subsequent fallout as predicted by Inter Governmental Panel on Climate Change has been debated by World Energy Council and by other agencies. However, other than emission, the strongest compulsion for India to improve the efficiency of power plants is to conserve its precious fuel.

4. NEW TECHNOLOGIES BHEL had to modify design and constructional features of technology acquired for boiler to achieve desired performance with Indian coal. During 1970s, the major failures experienced were fuel pipes, burners and above all, boiler tubes. At one time, more than 56 per cent of plant forced outage was attributed to boiler tube failures. To address these problems, design changes were incorporated by introducing wear resistance material in critical paths, optimizing flue gas velocity and redesigning the surface to minimize erosion. Subsequently, the tube failure has been reduced to between 3–5% of total forced outage. Also, India’s experience with entrained bed gasifiers imported for fertilizer sector was below expectations. Thus, the assessment of new technologies for compatibility with Indian coal became a prerequisite for commercial use. In mid 1970s, BHEL had started evaluation of emerging and future clean coal technologies and planned a comprehensive development

programme. It was decided that near term and most promising emerging technologies applicable to Indian scenario would be given priority for initial development. Accordingly, Atmospheric Fluidized-bed Boiler (AFB) and Integrated Gsification Combined Cycle (IGCC) with relevant gasification processes were given priority. It was also decided that other upcoming technologies would be developed atleast upto the pilot plant level to generate experimental data with Indian coal. Under this category, Pressurized Fluidized Bed Combustion (PFBC) and Hot Gas Clean-up System (HGCS) were identified as next priority and Magneto Hydro-dynamic (MHD), Slagging Combustor and Fuel Cell for long term development.

4.1. AFB During development stage in late 1970s, three prototypes, two 2 MT/H, followed by one 10 MT/H, were constructed to evaluate wide range of Indian coals and to establish

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scale up methodology. The larger prototype had octagonal, water cooled and compartmentalized furnace connected to a square shaped freeboard section. It was a balance draft design, with finned tube economiser and spiral type bed coil for steam generation. Over the years, other than coal and lignite, coal water slurry, washery rejects and bio mass

fuels were tested in AFB. More than 30 boilers have been supplied for various ratings

ranging from 12 to 100 MT/H steam supply.

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4.2. IGCC Priority was given to IGCC technology demonstraion for its potentiality to achieve process inherent cleanliness over other CCT, Fig. 1. and high thermal efficiency, Fig. 2. The selection and development of a gasification process in BHEL was an integral

and complementary part of the development of IGCC technology. It consisted of two phase gasifier development for integration to a 6.2 MW IGCC test plant. Techno-economic studies concluded that for Indian coal with 40 per cent ash content and for an ash melting temperrature of approx. 1,673K, use of oxygen-blown slagging or agglomerating type gasification process would not be economical. That left the choice between air blown moving bed or fludized bed gasifier with dry ash extraction system. The moving bed gasifier was the first choice in spite of its inherent process inadequacy and cost penalty for this application. The selection was primarily due to tech-

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nological maturity of this process and backup experience India had in operating and testing of Lurgi type gasifiers at Central Fuel Research Institute, CFRI, Dhanbad, and at Indian Institute of Chemical Technology, IICT, Hyderabad. Air-blown pressurised fluidized bed gasifier was identified as the most techno-economically attractive process for Indian coal and lignite. The first phase of IGCC programme consisted of concurrent development of a 150 TPD air blown moving bed gasifier integrated to a power block with 4 MW gas and 2.2 MW steam turbines, Fig. 3. Wet gas cleaning was adopted due to its technology readiness. The plant commissioned in the year 1987, was tested extensively for its operational capability and

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performance with Indian coal. Under a joint programme sponsored by Department of Coal, Government of India, North Karanpura coal with 40 per cent of ash content was also successfully gasified in BHEL’s 150 MTPD and in IICT’s 24 MTPD gasifiers to generate data for scale-up. A 18 MT per day PFBG pilot plant was developed in 1986 and tested over 2,700 hours. It is a two diameter reactor with dry ash removal system. The process operates in “bubbling bed” region at a temperature of 1,230K to 1,313 K. The same configuration was scaled up to a 168 MTPD gasifier and retrofitted in the existing 6.2 MW IGCC plant in 1996, Fig. 4. At present, 168 MTPD PFBG is undergoing performance testing. BHEL’s achievement in gasification and IGCC programme is summarized in Table 1. To study the performance and carbon conversion capability of fluidized bed reactor

operating in a different fluidization region, a 100 mm. ash agglomerating gasifier, Fig. 5, and a 200mm gasifier coupled to a 100 mm. fluidized bed combustor, Fig. 6, were developed. The first phase of performance testing of both the gasifiers has been completed. Partial agglomeration of sub bituminous coal with enriched air was achieved with unburnt carbon between 0.5–1.5 per cent in discharged ash from the bottom. The coupled gasifier was tested for wide range of input parameters for optimization. Carbon in discharged ash from the combustor could be reduced to 0.5 per cent.

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4.3. PFBC & HGCS The other promising clean coal technology identified for near term application is combined cycle based on PFBC. The preliminary development was first conducted in

a 200 mm pseudo adiabatic reactor. A 440 mm. test facility was then established and integrated with a Circulating Bed Granular Filter (CBGF), Fig. 7. Over the years, various coals with ash content upto 52 per cent, lignite and agro based fuels have been tested to generate operational and performance parameters. The CBGF concept was selected for development since it offers the possibility of simultaneous removal of particulate and

4.4. MHD A 5 MW (thermal) MHD pilot plant was set up under Indo-USSR Science & Technology programme. The plant has a MHD channel with 2 Tesla iron core magnet

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and seed recovery system. Tests were carried out with preheated Blue Water Gas and LPG. Out of 15 major trials conducted, 10 were for power generation.

4.5. Slagging Combustor A 3 MW (thermal) Slagging Combustor was set up in BHEL in order to evaluate this technology for Indian coal. Possible application perceived was MHD and direct firing

of coal in gas turbine. However, the flow of slag and heat reecovery from it were not satisfactory mainly due to high melting temperature of ash.

4.6. Fuel Cell Subsequent to development of 1 KW Phosphoric Acid Fuel Cell module and

scaling it up to 5 KW stack, BHEL is presently developing a 50 KW fuel cell. Under

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India’s Science & Technology Programme, demonstration of a 200 KW PAFC has also been taken up. Recently development of solid oxide fuel cell has been started for its potential use with coal gasifier.

5. FUTURE PROGRAMME Over the years, BHEL has established test facilities to evaluate wide ranging technologies for efficient utilization of Indian coal. These facilities can also be used for testing other types of coal.

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With the experience gained from IGCC development, BHEL has proposed to set up an IGCC demonstration plant either for a capacity of 60 MW or 120 MW in association with CSIR under the Clean Coal Technology Mission. Site specific feasibil-

ity report has been prepared and submitted to Government of India . Another proposal to retrofit an existing steam plant with PFBC topping cycle has also been submitted to Government for its consideration.

6. CONCLUSION Beneficiation of Indian coal and operation of power plants with imported coal will improve the efficiency of power generation to some extent but they will not satisfy overall future requirements of pollution control and conservation of energy. Therefore, there is a need to adopt new clean coal technologies. BHEL has commercialized “bubbling” AFB based on in-house development and Circulating Fluidized-bed Boiler through technology acquisition.

Extensive theoretical and experimental data on compatibility of various clean coal technologies with Indian coal have been generated from the pilot scale development of PFBC, CBGF, MHD and Slagging Combustor. These data can be used for technology assessment in future. BHEL’s experience in designing and operating IGCC plant with both moving-bed and fluidized bed gasifiers spans over a decade. IGCC appears to be the most suitable emerging technology for Indian high ash low grade coal.

LOW-RANK COAL AND ADVANCED TECHNOLOGIES FOR POWER GENERATION Dong-ke Zhang’, Peter J. Jackson2, and Hari B. Vuthaluru 1,2 1

Department of Chemical Engineering The University of Adelaide Adelaide, SA 5005, Australia 2 CRC for New Technologies for Power Generation from Low-rank Coal Unit 8, 677 Springvale Road Mulgrave, Victoria 3170, Australia

1. INTRODUCTION

Substantial deposits of low-rank coals exist in southern Australia and a number of other countries including Indonesia, Thailand, Turkey, China, USA and Germany. Properties that mitigate against their widespread use for power generation are high moisture content, low ash-melting temperature and often highly fouling nature of the ash. South Australia and Victoria have large reserves of readily accessible low-rank coals. Victorian deposits have been used for many years for power generation and generally have high water contents (60–70%), but are low in ash (l–3%db), and low in total sulphur (<0.5% db) [Durie, 1991]. Sodium in ash is usually low in the coals currently utilised in Victoria (generally <5%). While the South Australian coal (Leigh Creek) which is presently utilised for power generation has a moisture content of <38%, other deposits have higher moisture (50–61%), higher ash contents (generally <20%db), and higher total sulphur (gen-

erally <5%db). Sodium in ash in the S.A. coals in particular is often relatively high (3 to 16%). The relatively high sodium and sulphur contents have mitigated against their use in conventional pulverised fuel combustion systems because of the fouling and hightemperature corrosion problems associated with the low-melting-point ash. At the present time, one-third of the electricity generated from coal in Australia uses low-rank coal as the primary energy source [Brockway and Zhang, 1997]. The Cooperative Research Centre for New Technologies for Power Generation from Low-rank Coal (CRC–Power Generation) was formally established in June 1993 to conduct research and technology development into those new generation technologies and processes that have the best prospects of overcoming the principal challenges (cost and environmental impact) facing the use of low-rank coal as a competitive energy source for electricity Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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generation. New generation technologies are the key to the objectives of reducing both costs and emissions to the environment. Clearly, development of any new technologies (or new technology components) for efficient and effective utilisation of low-rank coals for power generation must address two major problems inherently associated with low-rank coal, viz, drying of the large amount of moisture and the solving of the ash-related problems. The present contribution focuses on the role of mineral matter in low-rank coals in the selection or development of new technologies for power generation from these coals. Recent developments in research on ash behaviour of low-rank coals in various reaction environments are reviewed. Several

technologies and processes (particularly those based on fluidised-bed technologies) that could potentially alleviate the ash-related problems associated with the use of low-rank coals are proposed and areas where further research and development are necessary are identified.

2. COAL MINERAL MATTER AND ADVANCED POWER TECHNOLOGIES

2.1. Background The role of mineral matter in coal during high temperature processing is governed by its nature and occurrence in the coal matrix, reaction temperature and reacting environment (oxidising or reducing). In conventional p.f. fired power plants, where peak flame temperatures in the neighbourhood of 1500°C are not uncommon, the mineral matter in the coal is directly associated with problems of slagging, fouling and ash deposition

as well as high temperature corrosion [Raask, 1985]. At these high temperatures, alkali and alkaline-earth metal constituents (eg. Na, Ca, etc.) vaporise into the gas [Raask, 1985, Wall, 1992]. When the gas containing inorganic vapour (referred to as fume) contacts relatively cool heat transfer surfaces (about 400 °C), a series of physical and chemical processes occur leading to the formation of an initial ash layer composed chiefly of alkali sulphates. The condensed sulphates form a “glue” to capture ash particles that are conveyed to the vicinity of the heat transfer tubes by the aerodynamics of the gas in the furnace and around the tubes. The particles will adhere to the tube or the deposit surface if either the particles or the surface are “sticky” enough to overcome the particle kinetic energy. As the ash deposits grow, a molten layer will eventually form as a result of reduced heat transfer flux and thus increased deposit surface temperature. When the proportion of liquid in the solid/liquid matrix is high enough to cause the matrix to collapse and the deposit surface becomes fully molten, and if the viscosity of the melt is low enough, the deposit will stop growing any further due to the liquid running off the surface and a dynamic steady-state of the deposit layer establishes. Sandwiched between the initial and

molten layers is a so-called sintered layer which is held together by partial melting of the arriving particles during the build-up of the deposit. This layer becomes more sintered as the deposit grows and its temperature increases. The sintering of the deposit depends

on the state of melting of the incoming ash particles and the proportion of molten phase in the sinter. The formation of the adhesive initial layer and subsequent growth of ash deposit can also be influenced to a certain extent by the local gas environment which determines the thermodynamic equilibrium of the alkali vapour and the chemical processes during the fume formation. Figure 1 illustrates the processes involved in formation of ash deposits on heat transfer tubes in conventional p.f. fired furnaces. These deposits reduce the heat transfer flux through the tubes and thus limit the steam tem-

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perature. Periodic shutdowns are therefore required for tube cleaning to restore boiler performance. High temperature corrosion of the steam super-heater tubes in particular, leading to tube failure, due to attack by the deposits is also a major problem with high alkali content low-rank coals. Based on the above brief overview, it is evident that three principal factors, viz, coal ash composition, process temperature and processing environment, govern the formation of ash deposits on the heat transfer surfaces and cause the operational problems in the p.f. fired power plants. For low-rank coals, the usually high levels of inorganic constituents are inherent problems in high temperature p.f. firing systems. Fluidised-beds provide an isothermal operating environment, are conducive to vigorous heat and mass transfer and thus enhanced chemical reaction rates, and are easy to control. Also,

fluidised-beds operating at much lower temperatures (about. 700–900 °C) offer better prospects in reducing ash-related problems associated with the utilisation of low-rank coals. Thus fluidised-bed combustion and/or gasification can be the bases of new technologies for power generation from low-rank coal to achieve a high overall process efficiency.

2.2. Fluidised-Bed Based Advanced Technologies Several emerging advanced power generation technologies that have better prospects for low-rank coals are at various stages of development. These include Integrated Gasification Combined Cycle (IGCC) incorporating a fluidised-bed gasifier and steam fluidised-bed drying (SFBD), pressurised fluidised-bed combustion (PFBC) combined cycle, pressurised fluidised-bed combustion combined with partial gasification (Advanced PFBC), and circulating fluidised-bed combustion (CFBC). McIntosh [1997] recently reported a large number of case studies based on these four principal advanced technologies using a thermodynamic analysis to evaluate various advanced cycles for power generation using low-rank coals, with an attention to drying of the coals. Figure 2 compares the efficiencies (power sent out based on HHV) for a typical low-rank coal and a typical bituminous coal when different technologies, including the conventional p.f.

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fired cycle, are used. It can be seen that significant improvements in the power generation efficiency for both low-rank and high-rank coals can be achieved by using different advanced power cycle technologies, in ascending order as follows conventional p.f. firing systems < CFBC < PFBC < IGCC < A-PFBC

It is worth noting that as a more advanced technology is applied, the difference in efficiencies between the low-rank and high-rank coals decreases. For example, for the conventional p.f. firing system, the efficiency is about 29% for the low-rank coal while 37% for the high-rank coal. For the IGCC process, the efficiencies are about 40 and 42%, respectively. However, with the A-PFBC technology, the low-rank coal efficiency can be

as high as 44%. This is very significant, because fuel costs for low-rank coals are generally substantially lower than higher rank coals. The advanced technologies can therefore place low-rank coals in a much more competitive position in the power generation market. Fluidised-beds are a key to the above advanced technologies. However, low-rank coals do not present trouble-free operation of fluidised-beds. Growth in the size of bed material (silica sand particles) due to ash deposition during fluidised-bed combustion has been reported [Williams, 1984; Manzoori, 1990] while burning Morwell (Victoria) and Lochiel (S.A.) coals, respectively. Bed material agglomeration and bed defluidisation leading to unexpected plant shut-down may also occur due to undesired ash behaviour in fluidised-bed combustion or gasification. Corrosion and erosion of heat transfer tubes immersed in a fluidised-bed burning Victorian low-rank coals have also been reported in the abovementioned literature. These problems are again inherently related to behaviour and transformation of the inorganic matter present in the low-rank coals. Although the behaviour of the inorganic matter in the p.f. firing systems has received considerable research attention, the knowledge is not directly transferable to fluidised-bed systems due to significant differences in operating conditions between the two systems. In order to realise the full potential of fluidised-bed based advanced tech-

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nologies for power generation from low-rank coal, there is a clear need for research leading to the understanding of inorganic matter transformations in fluidised-bed combustion and gasification environments.

3. FLUIDISED-BED AGGLOMERATION AND DEFLUIDISATION 3.1. General Behaviour of Ash in Fluidised-Bed Combustion and Gasification One of the key parameters with which fluidised-bed operation is generally characterised is the minimum fluidisation gas velocity below which the bed is not fluidised

and appears as a fixed-bed.

depends on a number of parameters including bed

particle size and density, fluidising gas viscosity and therefore the bed (operating) temperature and configuration of the bed. For a given fluidised-bed, is only slightly dependent on and is expected to decrease with increasing due to increased gas viscosity at higher temperatures. The superficial gas velocity defined as the ratio of the volumetric flow rate of fluidising gas to the cross-sectional area of the bed, is usually about 3 to 5 times of for a bubbling fluidised-bed and is up to 20 times for a circulating fluidised-bed. However, during fluidised-bed combustion and gasification of coal, the bed particles can grow in size due to deposition of coal ash on the particle

surface. With a low-rank coal with a low melting-point ash, the bed particles can become sticky so that particles may adhere together to form agglomerates, the latter phenome-

non is referred to as agglomeration. The increase in particle sizes due to ash deposition and agglomeration is then translated into an increase in the minimum fluidisation gas

velocity. As a result, the superficial gas velocity has to be increased in order to maintain the fluidisation state in the bed. If is not increased accordingly, or in the extreme

case of agglomeration where the whole bed material agglomerates, fluidisation can no longer be maintained and the bed is said to be defluidised. The above processes are apparently temperature-dependent and may be illustrated by a schematic diagram as shown in Fig. 3. The temperature corresponding to the turning point on this diagram is termed as the initial sintering temperature of the bed material [Manzoori, 1990], above which the fluidised-bed has to operate with a much higher fluidising gas velocity, or if this demand can not be met, defluidisation will occur. The on-set of initial sintering of the bed material can occur at much lower temperatures when firing low-rank coals due to the deposition of low melting-point ash on the particle surface. Defluidisation is one major challenge faced by fluidised-bed combustion and gasification based technologies for power generation. Understanding of physical and chemical mechanisms of the

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particle agglomeration and bed defluidisation processes is a key to any solutions of these problems.

3.2. Mechanisms of Agglomeration In either fluidised-bed combustion or gasification of coal, ash constituents evolve as coal organic matter is consumed. If the temperature of the coal particles is high enough, a molten phase of ash can form on the coal particle surface. The molten ash is then transferred onto the inert bed medium (usually sand) particle surface by collision between burning char particles and bed particles. It is highly probable that the ash layer coating on the bed particle surface will remain molten at typical fluidised-bed operating temperatures, particularly for low-rank coals with a low melting-point ash. Bed particles thus become sticky over prolonged periods of operation and often, in order to achieve

desired carbon conversion levels, fluidised-beds are operated at temperatures above the melting point of ash. As a consequence, agglomerates will form if the kinetic energy of particles cannot overcome the so-called sticky force between particles [Manzoori, 1990]. Figure 4 schematically illustrates this process. 3.2.1. Fluidised-bed Combustion.

In fluidised-bed combustion, the coal inventory in the bed is usually less than 5%, with the bulk of the bed being an inert material, typically

sand. It is this bed material which can agglomerate under certain conditions. Studies by Goldberger [1967] at bench and pilot scales indicated that ash could be retained in bed agglomerates, thus eliminating fly ash carryover into the combustion flue gases. Their tests showed that the sand particles were coated with molten ash and formed agglomerates. In another study, Goblirsch et al. [1983] performed tests on high-sodium coal and suggested that sulphated ash was responsible for the formation of agglomerates at lower bed temperatures. More recently, Manzoori [1990], Vuthaluru and Zhang [1997a] and Vuthaluru et al. [1997] studied the combustion of high-sodium, high-sulphur South Australian low-rank coals in a spouted bed using silica sand bed material at temperatures ranging from 700 to 850 °C. The bed material resulting from these tests consisted of coated sand particles as well as agglomerates of two or more bed particles. Examination of the agglomerates by SEM indicated that the characteristics of the ash coating and the material at the interface of agglomerated bed particles were identical. This sug-

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gests that agglomeration occurs by random collision between the coated bed particles. Spot analyses and X-ray maps of polished cross-sections of ash coated sand grains showed that the ash coating is rich in sulphated compounds of Na and Ca which act as a binder. Figure 5 shows typical SEM images of ash-coated sand particles, agglomerated sand particles, and cross sections of ash-coated sand particles, resulting from combustion of Bowmans coal in a spouted fluidised-bed. Based on extensive investigations, Manzoori [1990] proposed the following criteria for bed material agglomeration: 1. operating temperatures must be above the initial sintering temperature of the coating; and 2. minimum “critical thickness” of the coating must be deposited on the bed particles. Studies conducted so far suggest that agglomeration of bed materials in fluidisedbed combustion is a consequence of stickiness of particles resulting from the presence of a molten phase which acts as a binding agent. The molten phase is usually rich in sulphates of Na and Ca. This is particularly true when South Australian low-rank coals are used. In fluidised-bed combustion, the burning char particle temperature

can be 100–300 °C higher than the bed temperature, Although may be less than the ASTM ash melting-point of the coal, the formation of a binding matrix of lowmelting eutectics and subsequent transfer of this molten phase onto bed particles are inevitable. Thus, agglomeration of bed particles is an inherent operational problem for fluidised-bed combustion of low-rank coals, particularly with coals with high alkali and sulphur contents. 3.2.2. Fluidised-bed Gasification. In contrast to fluidised-bed combustion, fluidised-bed gasifiers using steam and oxygen or air as the gasifying agents usually operate with a bed of coal char, without any bed material. Thus agglomeration of char and ash particles, rather than bed materials is of concern. Owing to the relatively low reaction rates in gasification environments, fluidised-bed gasifiers require relatively high bed temperatures. In addition, the high coal/char inventory in the fluidised-bed coal gasifier means a relatively high ash content in the bed as well. Thus the formation of a molten ash phase and consequent agglomeration of char and ash particles are potential problems. Studies by Yerushalmi [1976] suggested that ash agglomeration in fluid bed gasifiers consists of several steps which include:

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1. Formation of smaller molten beads on the surface of char particles. 2. Growth of the molten beads (both in size and number) due to generation of additional molten ash and possibly due to collision with other char particles. 3. Separation of larger beads from the char surface due to high interfacial contacts between the molten ash and the char, thus forming sticky agglomerates. 4. Coalescence of ash agglomerates with small beads or other agglomerates. The ash formation chemistry in fluidised-bed gasification is presently less understood. Formation of sulphides and chlorides with low-melting points is most likely in the

gasification environment. Godel [1966] suggested that char agglomeration may be avoided in gasification systems by operating the gasifier at high fluidising velocities. However, the high flowrates of fluidising gases presents an economic penalty because the gasification agents (steam and oxygen) would be less efficiently utilised.

3.3. Mechanisms of Defluidisation In fluidised-bed combustion and gasification, defluidisation is believed to result from particle stickiness. Some researchers [Langston and Stephens, 1960] postulate that defluidisation is the extreme case of agglomeration and is governed by the same princi-

ples as agglomeration. However, studies by Basu [1982] indicate defluidisation may not necessarily have the same mechanisms as agglomeration. Gransden et al. [1970] proposed

that defluidisation can take place either by the formation of agglomerates or without the formation of agglomerates. The latter case is thought to be caused by interaction of sticky bed particles without the formation of agglomerates which results in the increase in the minimum fluidisation velocity [Manzoori, 1990]. The mechanisms of defluidisation are not yet fully understood. In a more recent study of fluidised-bed combustion of three different low-rank coals in a spouted bed combustor, He and Zhang [1997] confirmed that two different types of defluidisation can occur. These are so-called partial and com-

plete defluidisation. As shown in Fig. 6, at 700°C, the pressure drop across the bed decreased gradually with time over a period of 8 hours without defluidisation. As pres-

sure drop is a measure of the mass of particles being fluidised, Figure 6 suggests that the fraction of particles fluidised decreased with time as some particles became defluidised. This is termed partial defluidisation. Comparison of the ash coating thickness between the fluidised and stationary particles revealed that the defluidised particles had less coating than those which remained fluidised. This was expected since the fluidised particles were continuously coated during combustion, whereas the contact of the defluidised particles with the burning coal particles was significantly reduced. Consequently, ash

transfer from the burning coal particles to these bed particles became minimal. However, this observation contradicts the trend reported in the literature that particles with a

thicker coating had a higher tendency to agglomerate and defluidise [Manzoori, 1990; Manzoori and Agarwal, 1994; and Arastoopour, et al., 1988]. However, at 850°C, the whole fluidised-bed collapsed (defluidised) within 45 minutes (Fig. 6). Visual examination showed that there was no apparent particle agglomeration. SEM analyses also did not identify any significant differences in the composition of the ash coatings on particle samples collected from the experiments at 700 and 850°C. However, bed temperature seemed to play a major role, with the time for total defluidisation rapidly decreasing with increasing bed operating temperature, as shown in Fig. 7. When the surface of ash-coated particles become sticky, it is likely to form agglom-

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erates, resulting in defluidisation. In the case of coal combustion with sand particles as the bed media, the thickness of the ash coating on the sand particles increases with time. When the coating thickness reaches a certain value, defined as the critical coating thickness, the bed will defluidise [Manzoori and Agarwal, 1994]. It has been observed that the critical coating thickness decreases with increasing bed temperature [Vuthaluru and Zhang, 1997a; Vuthaluru et al., 1997; and Manzoori, 1990].

3.4. Mineral Matter Transformations Agglomeration, sintering, deposition and defluidisation have all been observed in a number of fluidised-bed combustion and gasification units ranging from laboratory scale to full industrial scale. Analyses of various samples from different studies indicate that the initial layers of ash coating on the bed materials, in the case of agglomeration and defluidisation, and on the heat transfer tubes, in the case of ash deposition, consist of sodiumand potassium-rich calcium sulphates. As the temperature of burning char particles in a fluidised-bed can be several hundred degrees higher than the temperature of the bed material particles, sodium, potassium, and calcium are readily volatilised and will condense on the cooler surfaces. These condensed inorganic species can also react with sulphur compounds evolved from the coal to form substances that act as a glue in binding bed

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particles, leading to agglomeration and defluidisation. In addition, sodium can also react with silica sand to form sodium silicate which sinters at relatively low temperatures. The formation of low melting point eutectics between sodium- potassium-, calcium-, and sulphate-rich components within coal readily occur during fluidised-bed combustion processes [Benson et al., 1995]. These eutectics have significantly lower

melting-points than their respective pure compounds. The melting points of typical inorganic compounds and eutectics found in combustion and gasification of low-rank coals are given in Table 1. In a study of fluidised-bed pyrolysis of Bowmans coal, Telfer and Zhang (1997) observed that transformation of organically bound Na and Ca into their respective sulphides occurred within the coal particles. It is possible that the sulphides are further converted into sulphates and the sulphate matrix may be molten at typical operating temperatures for fluidised-bed combustion. The molten phase can be subsequently transferred onto bed material surfaces and serve to glue particles together in the form of

agglomerates or to form thicker deposits. In the extreme case, defluidisation occurs. In fluidised-bed gasification, the behaviour of the inorganic matter in low-rank coal

is substantially different to that in combustion systems. It is reported from laboratory scale studies that sodium compounds are likely to vaporise under gasification conditions, with the extent of vaporisation increasing with increasing gasification temperatures [Mojtahedi and Backman, 1989; Bellin et al., 1987; and Kosminski and Manzoori, 1990]. Results of thermodynamic calculations also suggest that vaporisation of sodium chloride is more intense in gasification environments than in combustion conditions

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[Kosminski and Manzoori, 1990]. It is also reported that sodium in low-rank coals forms silicates, aluminosilicates, sulphates, and chlorides during fluidised-bed gasification [Kosminski and Manzoori, 1990; and Bellin et al., 1987]. Alkali and alkaline-earth chlorides can melt at fluidised-bed gasification temperatures. Table 1 also includes some of the pure chemical compounds formed during gasification. Calcium sulphide and other sulphides are the main sulphur containing compounds in ash from a fluidised-bed gasification process. In addition to forming sulphides, calcium also leads to the formation of oxides, silicates and aluminosilicates [Laughlin and Reed, 1991]. However, magnesium mainly forms magnesium oxide under gasification conditions. Major problems arise from the iron compounds during gasification as these are known to initiate the formation of agglomerates in fluidised-bed gasification systems [Marinov et al., 1992]. Pyrite in coal decomposes to iron sulphide and a part of the sulphur is liberated. The iron sulphide undergoes further decomposition to form pyrrhotite, which oxidises to FeO. Iron oxide combines with silica to form silicates, which are precursors to agglomeration in fluidised-beds. FeS-FeO eutectics and metallic iron also initiate agglomeration, as these eutectics melt below 900 °C. Their formation is accelerated by the presence of alkali. During gasification processes the reactions involving inorganic matter occur not only on the char surface, but also extensively inside the char. This may lead to coalescence or sintering of inorganic matter into larger and consolidated ash particles [Kosminski and Manzoori, 1990]. Low-rank coals are generally rich in alkali and alkaline-earth elements that can be librated during combustion and gasification. Organically bound elements have been shown to be the major precursors to bed particle agglomeration and in-bed tube ash deposition. However, competing reactions with other coal inorganic matter may reduce the alkali availability. The fate of these potential deposit- and agglomerate-forming minerals will ultimately influence the extent of deposition and agglomeration. Therefore, it is important to understand the nature of inorganic components in the original coal and their chemical and physical transformation during various fluidised-bed combustion and gasification processes so that improvements can be made in the prediction of reactor performance.

4. TECHNIQUES FOR AGGLOMERATION AND DEFLUIDISATION CONTROL As discussed above, fluidised-bed combustion and gasification of low-rank coals suffer from agglomeration and defluidisation problems, in which sodium, potassium, calcium, and sulphur compounds, and their reactions, as well as the sodium—silica reaction, play a major role. Consequently, in order to control agglomeration and defluidisation, the formation of the low melting-point eutectics of these constituents and their reactions have to be limited. The following sections discuss several techniques that are currently under investigation.

4.1. Additives Mineral additives may be used to control ash related problems in fluidised-beds by limiting the formation of low melting point eutectics. An additive with specific characteristics can be employed in fluid bed combustors to suit a particular problem. The testing of four additives in a laboratory-scale spouted bed reactor to determine

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their effectiveness in controlling ash deposition under CFBC conditions has been reported [Linjewile et al., 1995]. The additives investigated were dolomite, CW clay (a kaolinite and sillimanite-rich clay), DV clay (a kaolinite and quartz-rich clay) and gibbsite (a hydrated alumina referred to as AH). In this study gibbsite was found to be most effective. Evidence obtained from analyses of the bed materials from the runs with the use of these additives suggests that depending on the type of additive used, interference in the process of ash deposition may be due to either a chemical reaction or physical dilution. These interactions apparently occur at the surface of burning char particles. Although previous investigations have identified clays to be effective in sodium capture, care must be taken in the selection of clay additives since the mineralogical composition of the clays appears to play a major role in the extent of deposit formation control. Gibbsite and the DV clay have demonstrated potential for in-situ sodium capture which may facilitate use of low-rank coals with high sodium contents in advanced FBC based power generation plants.

4.2. Alternative Bed Materials In recognition of the chemical reactions between silica and alkali metals (eg. sodium) that contribute to the formation of low-melting point eutectics, it has been proposed that aluminium-rich minerals may be used as alternative bed materials for control

of agglomeration and defluidisation. The technique works on similar principles to the use of additives. Bauxite and calcined sillimanite have been trialed in a laboratory spouted bed combustor burning Bowmans coal. The bauxite is rich in (60.7%) and (26.4%), while the sillimanite contains 53.3% and 43.7% SiO2. Combustion runs were carried out at bed temperatures of 800 °C and 850°C with the two alternative bed materials as well as silica sand until defluidisation occurred. Table 2 shows the observed times to defluidisation for each bed material. The results indicate that the operation can last for at least 7–10 times longer with calcined sillimanite and bauxite than with sand as the bed material. This is a significant improvement in terms of

combustion operation and could lead to longer run lengths and fewer outages when these alternative bed materials are employed in fluid bed combustors. The total bed mass recovered at the end of each combustion run (after defluidisation) was weighed. Irrespective of the bed material used, all the samples showed an ash coating build-up on the surface of the bed particles. The ash coating thickness on sillimanite and bauxite bed material particles was greater than for the sand. This is mainly due to the extended operation with these two bed materials. The composition of the inorganic constituents in the bed material coating was determined by chemical analysis. The analyses for the runs using sand as the bed material are reported on a silica-free basis. Figure 8 shows the average compositions of the

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bed material coating (bed ash), cyclone ash (fly ash) and the feed coal ash for the different different bed materials. The results show that the coating on particles of all the bed materials is enriched in Na and S compounds. It may be noted that in comparison to the feed coal ash, a significant proportion of the Ca and Mg compounds was not retained by the bed particles and escaped from the bed into the cyclone. It is also evident that the ash coatings are enriched in alumina for the sillimanite and bauxite runs. Electron microprobe spot analyses and X-ray maps of polished cross sections of coated bed particles showed the presence of a highly sulphated ash dissolved in an aluminium-rich ash. XRD analyses indicated that low melting point eutectics were not formed, at least not in any significant quantity. It is postulated that, in the presence of aluminium-rich phases, the eutectics are less sticky at typical fluidised-bed combustion temperatures. Hence the use of bauxite and calcined sillimanite as the bed materials may result in longer operating periods (Table 2). XRD analyses of the ash-coated sand particles indicated the prevalence of thenardite, Na-Ca-sulphate and anhydrite. Thenardite and Na-Ca-sulphate are lowmelting point compounds capable of forming low-temperature eutectics with other ash compounds and hence are considered to be the basis of ash binder in the coatings. However, the bed material coatings from the combustion runs with calcined sillimanite and bauxite showed the presence of aluminium rich phases (such as nepheline, mayen-

ite, nosean) and complex silicates with Fe, Mg and Ca (Augite). It appears that the increased proportion of aluminium rich phases with calcined sillimanite and bauxite have prolonged the combustion operation. XRD analyses of the fly ash samples showed several mineral phases, such as quartz, maghemite, hematite, thenardite, in different proportions. The presence of aluminiumrich phases in the fly ash can lead to less sticky ash in downstream heat transfer sections. Hence the ash characteristics of the deposits on heat transfer surfaces may also be modified using these alternative bed materials.

4.3. Pretreatment of Coal Pretreatment of coal is one of the other options for controlling ash related problems in fluid bed combustors. This approach involves either addition or removal of specified elements in the coal to alter the coal ash properties before firing into the com-

bustor. The pretreatment is aimed at producing coal ash rich in high melting-point ash and lean in low melting-point ash. Several pretreatments have been conducted on Bowmans coal, including: water-washing (WW) (with demineralised water until the washing shows no traces of chloride), acid-washing (AW) (with 3M HC1 solution, followed by water-washing until the washing shows no traces of chloride), and Al enrichment (Al treated) (soaking of acid-washed coal in 1 M solution, followed by water-washing until the washing shows no traces of chloride). Combustion experiments using the pretreated coal samples were carried out on a spouted bed combustor at temperatures ranging from 700 to 850 °C. Silica sand was used as the bed material [Vuthaluru and Zhang, 1997b,c]. The bed material recovered at the end of a combustion run was weighed and the rate of ash deposition on the bed particles was estimated from the increase in the mass of bed inventory. Figure 9 shows the ash build-up with operating time for the samples

collected from the raw and the pretreated Bowmans coals. For the raw and acid-washed Bowmans coal runs, the bed defluidised after 80 and 15 minutes, respectively. However, for the water-washed and Al treated coals, no defluidisation occurred during the operat-

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ing periods shown in Fig. 9. The level of ash build-up for the raw coal was greater than for the pretreated coals. This is attributed to the small amounts of low melting point species in the pretreated coals. Despite the lower proportions of low melting species in the acid-washed coal, defluidisation did occur after a few minutes of operation at 800°C. This is in contrast to earlier observations by Manzoori and Agarwal [1994] with Lochiel

coal (SA) in the same fluidised bed; the raw Lochiel coal caused defluidisation after 2hr combustion at 800 °C, while the acid-washed Lochiel coal did not result in defluidisation for much longer operating periods (>10hrs). SEM analyses of bed particles of acidwashed Bowmans coal runs showed significant proportions of sulphur in the ash coating layer (in combination with Na). It appears that the difference in the proportions of sulphur in AW Bowmans coal and the AW Lochiel coal used by Manzoori and Agarwal [1994] may be responsible for the different behaviour of the two coals. This requires further study. Figure 10 shows the rate of ash deposition versus sodium content in different tests.

Comparison of results for different coals indicate that the ash deposition increases with the sodium content of the coal. The results suggest that sodium is transformed into low melting-point compounds in the ash. These compounds appeared to have enhanced the

rate of deposition on bed particles. Figure 11 shows the distribution of the inorganic constituents in the bed particle coating, cyclone ash and coal ash for the tests with the raw coal and the pretreated coals. The results show that the coating on the bed particles is enriched in Na and S compounds. A large proportion of Ca is retained in the ash coatings in runs with WW and AW coal samples. Al retention in the ash coating and the cyclone ash for the Al treated coal

runs is quite substantial compared to other coal runs. SEM analyses of ash-coated bed particles and cyclone ash samples from the Al treated coal runs indicated an Al-rich fluffy ash, similar to that reported by Vuthaluru et al. [1996]. This suggests that the increased levels of Al in the coal would lead to the formation of Al-rich compounds in the ash, rendering them less sticky when deposited on bed material particles. In addition, the formation of Al-rich fluffy ash in the cyclone ash in the Al treated coal runs would lead to fewer ash related problems in the downstream heat transfer sections of fluid bed combustors.

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X R D analyses [Vuthaluru and Zhang, 1997b] of ash-coated bed particles obtained from the raw and pretreated coal runs indicated that several minerals (maghemite, thenardite and hematite) were present in various proportions in the bed particles for raw and acid-washed Bowmans coal runs. However, for the water-washed and Al treated coal runs, those mineral phases were only present in trace quantities. In the latter cases, it appears that the absence of thenardite which acts as a binder may have prolonged the operation without defluidisation. In addition, periclase, maghemite, halite, magnetite, forsterite, anhydrite, and thenardite were also present in ash coatings with all the raw, WW and AW coal runs. For the Al treated coal runs, beta-aluminium oxide was detected, as a result of the increased levels of Al in the coal. The three techniques for agglomeration and defluidisation control discussed in this paper are effective to different extents, however continuing research and development are still required to determine the most effective and cost-efficient technique. Of the three techniques described, the use of sillimanite as an alternative bed material appears to be the most promising. Coal pretreatment is unlikely to be economic.

5. ALTERNATIVE ADVANCED TECHNOLOGIES Three principal factors, viz, coal ash composition, process temperature and processing environment, govern the behaviour of coal ash in fluidised-bed combustion and gasification processes and consequently determine the limits on operation due to agglomeration and defluidisation. The process temperature and coal residence time in fluidisedbed gasification are generally selected to achieve the desired level of coal conversion. Low temperature operation, while offering the potential for alleviation of ash related problems, will lower the extent of coal (and char) conversion, and possibly result in the formation of tars. Hence processes based on low-temperature operation, such as low temperature pyrolysis, will require different process configurations to conventional gasification processes, and may include co-production. In a low-temperature pyrolysis process, low-rank coal undergoes thermal

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decomposition (pyrolysis) in an inert environment to produce volatile matter and char at a moderate temperature, say, at about 600 °C. The char can then be burnt in a combustor with sub-stoichiometric air, again at relatively low temperatures (say, 650 °C), to generate an inert gas (flue gas) and heat for the endothermic pyrolysis reactions. The excess heat from the combustor can be readily used for power generation. The combustible volatile matter may be utilised in two ways, viz, direct combustion for power generation, or separated into condensables (fuel oil) and non-condensables (a low-specific-energy gas mixture), the latter can be utilised for power generation. Utilisation of volatile matter in low-rank coals is feasible because of their high volatile matter contents (35–50%db.). A number of similar processes for low-rank coal utilisation can also be proposed. Partial gasification of the pyrolysis char with steam and air (or oxygen) may be used to replace the char combustion. Activated carbon production from the pyrolysis char is also an option, given the highly porous and reactive nature of the char. Tars from pyrolysis of low-rank coal may also be used to produce high-value organic chemicals. Processes for co-production of power, methanol, and ammonia and/or fertiliser from low-rank coal have also been proposed. Laboratory-scale experimental research and process simulation studies are being conducted at the University of Adelaide and the CRC—Power Generation to evaluate the full potential of these processes.

6. CONCLUSIONS Fluidised-bed based advanced power generation technologies offer higher efficiencies than conventional pulverised fuel fired power plants and better prospects in reducing ash-related problems associated with low-rank coal in such plants. However, bed

material agglomeration and bed defluidisation present significant operational difficulties for the utilisation of the low-rank coal in fluidised-bed processes. Alkali and alkalineearth elements and sulphur compounds, often found in low-rank coals, form low melting point eutectics at typical fluidised-bed combustion and gasification operating temperatures. These low melting-point materials are subsequently transferred onto the bed material particle surfaces, and the ash-coated particles then become adhesive and agglomerate. Defluidisation can occur either as an extension of agglomeration as a rate process gradually leading to defluidisation or as an instantaneous event without agglomeration. A critical thickness of the ash coating layer on the particle surface exists, above which defluidisation occurs. This critical thickness decreases with an increase in bed temperature. Several mineral additives, alternative bed materials and pretreatment of coal have been shown to suppress, to different extents, particle agglomeration and bed defluidisation when burning a high sodium, high sulphur low-rank coal in a spouted fluidised-bed combustor. Sillimanite as an alternative bed material is found to be most effective for defluidisation control. Alternative advanced technologies such as low-temperature pyrolysis and co-production are proposed for future investigation.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre for New Technologies for Power Generation from Low-Rank Coal, which is established and supported under the Australian Government’s Cooperative Research Centres program. Thanks also go to Dr. Yinghe He,

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Mr. Adam Kosminski, and Ms Marnie Telfer for their contributions to the work reported in this keynote paper.

REFERENCES Arastoopour, H., Huang, C.S. and Weil, S.A. (1988). “Fluidisation behaviour of particles under agglomerating conditions.” Chemical Engineering Science, 43, 3063–3075. Basu, P. (1982). “A study agglomeration of coal-ash in fluidised-beds.” The Canadian Journal of Chemical Engineering, 60, 791–795. Bellin, A., Bruck, U. and Schrader, L. (1992). “Report on the results of the HTW-pilot plant gasification tests with Bowmans coal.” Journal of the Institute of Energy, 65, 55062. Benson, S.A., Sondreal, E.A., and Hurley, J.P. (1995). “Status of coal ash behaviour research.” Fuel Processing Technology , 44, 1–12. Brockway, D.J. and Zhang, O.K. (1997). “Cooperative Research Centre for New Technologies for Power Generation from Low-rank Coal.” in World Coal Combustion Research Centres (Ed. L.D. Smoot), Progress in Energy and Combustion Science, in press . Dean, J.A. (1985). Lange’s Handbook of Chemistry, New York: McGraw Hill Book Co. Durie, R.A. (1991). The Science of Victorian Brown Coals: Structure, Properties and Consequences for Utilisation. London: Butterworths, Heinemann. Gluckman, M.J., Yerushalmi, J. and Squires, A.M. (1976). Fluidisation Technology, (Ed. D.L. Keairns), Washington: Hemishpere Publishing Corporation, Goblirsch, G.M., Benson, S.A., Karner, F.R., Rindt, D.K.. and Hajicek, D.R. (1983). “AFBC bed material performance with low-rank coals.” a paper presented at the 12th Biennial Lignite Symposium, N.D. USA. Goblirsch, G.M., Vander Molen, K.H., Wilson, K. and Hajicek, D.R. (1980). “Atmospheric fluidised-bed testing of North Dakota lignite.”, 6th I n t . Conf. of Fluidised-bed Combustion, 850. Godel, A.A. (1966). (in French) Rev Gen Therm , 5, 349. Goldberger, W.M. (1967). “Collection of flyash in a self-agglomerating fluidized-bed coal burner.” a paper presented at ASME Winter Annual Meeting and Energy systems Exposition. Gransden, J.F., Sheasby, J.S. and Bergougnou, M.A. (1970). “An investigation of detluidisation of iron or during reduction by hydrogen in a fluidised-bed.” Chemical Engineering Progress Symposium Series, 66 (105), 208–214. He, Y. and Zhang, D.K. (1997). “Types of detluidisation in a spouted bed” CRC Power Generation Internal Report. Kosminski, A. and Manzoori, A.R. (1990). “Inorganic matter behaviour in the gasification of South Australian coals for combined cycle power generation”, Technical Services Department, The Electricity Trust of

South Australia, Report submitted to SENRAC. Langston, B.G. and Stephens, F.M. (1960). “Self-agglomerating fluidised-bed reduction.”, J Metals, 312. Laughlin, K. and Reed, G. (1991). “Behaviour of coal ash in a pressurised fluidised-bed gasifiers”, 6th International Conference on Coal Science, Newcastle upon Tyne, 396–399. Levin, E.M., Robbins, C.R. and McMurdie, M.F. (1979). Phase diagrams for Ceramists. The American Chemical Society, Ohio, USA. Linjewile, T.M., Linard, D. and Manzoori, A.R. (1995). “Mechanistic evaluation of the role of additives in controlling bed material ash deposition and agglomeration during fluidised-bed combustion of low-rank coal.”, Proc. 5 th Japan-Australia Joint Technical Meeting on Coal, Adelaide. Manzoori, A.R. and Agarwal, P.K. (1994). “Agglomeration and defluidisation under simulated circulating fluidized-bed combustion conditions.” Fuel, 73, 563–568. Manzoori, A.R. (1990). Role of the inorganic matter in agglomeration and defluidisation during the circulating fluid bed combustion of low-rank coals. PhD. Thesis, The University of Adelaide. Marinov, V., Marinov, S.P., Lazarov, L. and Stefanova, M. (1992).” Ash agglomeration during fluidised-bed gasification of high sulphur content lignites.“ Fuel Processing Technology, 31, 181–191. McIntosh, M. (1997). ”Assessment of advanced power cycles with high moisture content low-rank coals.“ 5th Annual Technical Seminar of APEC Experts’ Group on Clean Fossil Energy, Nevada, October 27–30, 1997. Mojtahedi, W. and Backman, R. (1989). “Release of alkali metals in pressurised fluidised-bed combustion and gasification of peat.” Technical Research Centre of Finland (VTT) Publication No.53, 48.

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Raask, E. 1985. Mineral Impurities in Coal Combustion. New York: Hemisphere Publishing Corporation.

Souto, M.A., Rodriguez, J.C., Conde-Pumpido, R., Guitian, Gonzalez, J.F. and Perez, J. (1996). “Formation

of solid deposits in the gas circuit of a pressurised fluidised-bed combustion plant”, Fuel, 75, 675–680. Telfer, M. and Zhang D.K. (1997). “X-ray Mapping of Sulphur Constituents in Low-Rank Coal Char: The Influence of Pyrolysis Conditions.”, To be presented at Australian Symposium on Combustion and 5 th Australian Flame Days, Sydney, November, 1997. Vuthaluru, H.B. and Zhang, D.K. (1997a). “Ash characteristics of low-rank coals during fluidised-bed combustion.” CRC—Power Generation Internal Report. Vuthaluru, H.B. and Zhang, D.K. (I997b). “Role of inorganics during fluidised-bed combustion of low-rank coals.” A paper to be presented at the Engineering Foundation Conference, November 2–7, Hawaii.

Vuthaluru, H.B. and Zhang, D.K. (1997c). “Effect of pretreatments on ash characteristics of low-rank coals during fluidised-bed combustion.” CRC—Power Generation Internal Report.

Vuthaluru, H.B., Domazetis, G., Wall, T.F. and Vleeskens, J.M. (1996). “Reducing fly ash deposition by pretreatment of brown coal: effect of aluminium on ash character.'Fuel Processing Technology, 46,117–132. Vuthaluru, H.B., He, Y, Zhu, J.N., Zhang, D.K., Yurismo, H. and Sastrawinata, T. (1997). “Fate of inorganic

constituents during fluidised-bed combustion of low-rank coals: effect of temperature on ash characteristics”, Proceedings, 9th International Conference on Coal Science, 1171–1174. Wall, T.F. (1992). “Mineral matter transformations and ash deposition in pulverised coal combustion.” Proceedings, the 24th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1119–1126. Williams, J.E.. (1984). “Fluidised combustion of brown coal.” State Electricity Commission of Victoria,

Research and Development Dept., Report No. ND/84/006. Yerushalmi, J., Gluckman, M.J. and Graff, R.A. (1976). Fluidisation technology. (Ed D.L. Keairns),

Washington: Hemishpere Publishing Corporation, 437.

THE THERMAL CONDUCTIVITY OF ASH DEPOSITS Particulate and Slag Structures

R. P. Gupta, T. F. Wall1, and L. Baxter 2 1

The Department of Chemical Engineering The University of Newcastle, Australia 2 Sandia National Laboratories Livermore, CA

1. INTRODUCTION The thermal properties of the ash deposits have a significant influence on the heat absorption rate in the pulverised fuel fired furnaces. Heat transfer rates depend primarily on surface absorptivity, the effective thermal conductivity, and thickness of the deposit. The radiative properties and thermal conductivity of deposits have significant influence on the boiler performance deposit (Mulcahy et al., 1966). The radiative properties of ash deposits and similar materials were reviewed earlier (Wall et al., 1994). This review is limited to the thermal conductivity of ash deposits and materials having similar chemical and physical character. Thermal conductivity of glassy ash like materials is a function of it constituents, however, it is in the range of for most solid glasses. The reported low thermal conductivities of some particulate ash deposits (typically less than for porous materials) can produce temperature gradients of or larger in the deposit resulting in accelerated ash deposition. The deposit thickness, however, depends indirectly on the temperature distribution in the deposit, which in turn is governed by the effective thermal conductivity. The thickness of deposits can vary from zero for clean tube surfaces to a maximum value, where the slag starts flowing, determined by viscosity and thermal properties of the deposit (Mulcahy et al., 1966). In that investigation the authors noted that a deposit thickness of about 1 mm can be expected to decrease the heat transfer significantly. The thermal properties are strongly influenced by the physical structures of the deposits: particle size, porosity and sintered condition. Sintering is a function of Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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chemical composition and temperature, which has already been discussed and causes

irreversible changes in the structure. As sintering increases, contact increase and the structure of ash deposits vary from granular packed bed for loose deposits to porous slags with solid phase continuous as shown in Fig. 1. The particles in particulate phase are also connected at a very minimum level not shown in Fig. 1. The ash deposits can be characterised broadly into three types of structures and all the three types can exist in a given deposit as seen in Fig. 2. The proportion of different types of structure however depends on the mineral matter in coal and the environment surrounding the deposit. The initial layer adjacent to the wall or a heat transfer surface is usually particulate in nature with continuous gas phase and discrete particles embedded in the continuous phase. This is characterised by low thermal conductivity resulting in increased deposit temperature. The outermost layer, at very high temperatures can be completely fused slag and has very high thermal conductivity depending on its porosity. In slags, the continuous phase is the solid ash with voids resulting in a porous structure. In porous ash deposits, apart from conduction, heat transfer also takes place via radiation mechanism as well, whose contribution increases as temperature increases. At high temperatures encountered in pulverized fuel fired furnaces, radiation plays an important role in heat transfer through the deposit. Since conductive and radiative heat transfer in the ash deposit occur both in parallel and in series, the concept of an “effective” thermal conductivity is commonly used. This quantity is defined such that the following form of the heat transfer equation for one-dimensional, steady-state conduction is satisfied.

Since the heat flux, q, is constant, the local effective thermal conductivity can be defined as;

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Depending on the temperature, physical state, and surface condition at the wall and deposit surface, the ratio of conductive to radiative heat fluxes will change with position in the deposit. An overall effective thermal conductivity calculated for the entire ash

deposit thickness, can be defined by integrating the local value with respect to location (y) and dividing by the deposit thickness. This effective value will increase rapidly at the temperatures when radiative transfer through the deposit becomes significant.

2. MEASUREMENT OF THERMAL CONDUCTIVITY This section reviews the thermal properties of ashy materials, their measurement techniques. Tye (1969) has compiled several reviews on methods for determining thermal conductivity of solids. Pratt (1969) has discussed specifically the experimental techniques for low conductivity materials ie. glasses, slags and powdered materials. Most of the techniques have been used under steady state, however, some of the methods utilise transient analysis of unsteady state measurements. The unsteady state methods were designed to

save time in experiments, however, the analysis is slightly more complex. Laubitz (1969) has discussed the effect of design parameters for these techniques on the accuracy of measurements. He has also discussed the possible sources of error involved in terms of: • measuring heat flux from power of heater,

• temperature measurement, • contact resistance and • departure from steady state. The thermal conductivity measurement techniques are of two types: radial heat flow technique mostly for powders and axial heat flow technique for discs

2.1. Radial Heat Flow Technique for Powders Most of the radial flow techniques consist of two concentric ceramic cylinders. The test material, which is in the form of a powder is filled in the annular space between these cylinders. The inner ceramic tube is heated electrically by a carbon rod. Temperatures are measured at several radial locations through the powder. The thermal conductivity of the particle bed is determined from temperatures at two radial locations by the following equation:

The typical experimental apparatus is described in Fig. 3. A number of variations are discussed below: Diessler and Boegli (1958) determined thermal conductivity of magnesia powder in different gases. The effective thermal conductivity for the powder in helium was found to be 4–5 times that in the air. A reasonable good correlation was obtained for the ratio of with the ratio The thermal conductivity of gas, rather than that of solid, was found to have more influence over the effective thermal conductivity of powders.

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Laubitz (1959) measured thermal conductivity of powders. The experimental apparatus essentially consisted of a long cylindrical furnace of about 25mm diameter inside which was a centrally located heater of 4mm diameter and 30cm long. The furnace is filled with powder. Two thermocouples located within the powder at radial distances of 5.6mm and 10.1 mm give the temperatures at two radial positions. The thermal conductivity is then determined from equation (3). Thermal conductivity of several powders of uniform particle size such as alumina, magnesia and zirconia were determined at temperatures up to 1,200°C. Flynn (1963) also used an apparatus based on radial flow method for measurements on powders and granular materials over a wide range of temperatures. The inner ceramic tube contained a central heater (approximately 41 cm long). There were heaters at both ends of the main heater, whose temperature is adjusted to that of the central heater. The outer ceramic tube has 3.2cm diameter and about 60cm long. One of the thermocouples is inserted into the central ceramic tube and the other is inserted into a thermocouple well in the outer ceramic tube. The accuracy of these methods is influenced by two factors: thermal resistance at the interface and temperature measurement or thermocouple placement. The possible sources of error are thermal resistance between sample and thermocouple, uncertainties in the exact location of the thermocouple. The location of the thermocouple may alter if the samples shrink. If the thermocouples are bare, they may be contaminated by reaction with test material. A combination of all the uncertainties showed that a maximum error was no greater than 3% at 1,000°C in Flynn’s experiments. Godbee and Ziegler (1966) used radial heat flow in a hollow cylinder to measure thermal conductivity under steady and unsteady conditions. The powdered sample is held between two concentric cylinders. Heat produced by an axially located electrical resistance is conducted radially outward through the sample. Power to the central heater was determined from the measured voltage and current. The transient method was mostly used at temperatures close to the ambient. The steady state method was used up to temperatures of 1,000°C. The thermal conductivity was determined for well-characterised powders. The accuracy of k e by both the steady state and unsteady state techniques was estimated to be within 10%, where as the reproducibility was within 3%.

2.2. Axial Heat Transfer Technique for Slabs/Discs A number of methods for the measurement of thermal conductivity of slags or glasses utilise axial heat transfer technique. The test material is in the form of two identical circular or square slabs, which are placed one on each side of a flat heater. A steady state is established by adjusting the power to the heater and flow rate of the cooling fluids

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of the sink. The temperature of the hot guards is maintained the same as the central metering section to eliminate the radial heat losses. The thermal conductivity can then be determined from temperature difference, across the thickness of the sample, by the following equation:

Ratcliffe (1963) determined thermal conductivity of 22 glasses using an electrically heated disc method of determining thermal conductivity of solids. Each glass sample was in the form of a pair of identical three inch diameter discs (10mm thick) with lapped faces. He derived some additive correlations for thermal conductivity as a function of chemical composition of glasses by weight. Boow and Goard (1969) used the axial heat transfer technique to measure the thermal conductivity and emissivity of an ash layer. The ash layer of 3–5 mm thickness is kept on a flat-bottomed stainless steel crucible sitting on top of a furnace. The surface

temperature of the layer and its emissivity are measured by a surface pyrometer. The bottom temperature is determined by means of a thermocouple. The heat flux used in equation (4) can be determined from top surface temperature and emissivity. The measurements proved to be remarkably reproducible. The measurements demonstrated the effect of particle size, chemical composition. They also demonstrated the irreversible rapid increase in the thermal conductivity at higher temperatures due to sintering. For very fine ashes, the thermal conductivity was observed to be less than that of air at that temperature. Anderson et al. (1987) developed an experimental facility to study steady state heat

transfer through coal ash deposits with an objective to determine local thermal conductivity of fly ash, slagging deposits and fouling deposits. The facility consisted of a large cavity kiln capable of continuous operation at 1,500 °C. Six silicon carbide electrodes irradiate uniformly a thin ash deposit sample. The irradiation level was similar to those in coal fired furnaces. A calorimeter was located directly under the center of the sample. The temperature profile across the calorimeter surface was uniform. Botterill et al. (1989) used the unsteady state two-dimensional heat transfer technique to measure thermal conductivity of alumina powder. A large amount of ash sample was uniformly heated by fluidising the powder in a cylinder of 70cm diameter by 70cm length. After attaining the required uniform temperature, a carefully set up array of thermocouples was located in the bed and the bed defluidized. As it cooled over a period of days, the radial temperature profiles were logged. The data was analysed to determine the thermal conductivity of the powder at different temperatures. Nowok and Steadman (1992) measured thermal conductivity of coal ashes and slags using the axial one-dimensional heat flow method. The method uses two reference material of known thermal conductivities as a function of temperature. Similar technique has been used at the University of Newcastle (Gupta et al., 1997) to investigate the effect of porosity and sintering on the effective thermal conductivity of several coal ashes and synthetic slags. The axial heat flow is assumed constant across the three materials. The schematic diagram is shown in Fig. 4. The temperature difference across one reference material is used to determine the heat flux, whereas, the temperature difference across the other reference material is used to estimate the errors in the measurements. It must be noted that mostof the techniques described above may not be suitable for highly porous deposits. One of the in-situ technique is described by Robinson and Baxter (1997).

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3. THERMAL CONDUCTIVITY OF PURE MATERIALS (ZERO POROSITY) It is important to consider the thermal conductivity of each individual pure phase before one delves into modeling the effective thermal conductivity of complex structures. The fundamental mechanism of heat transfer in material like dense and pure refractory oxides, glasses and ashy material can either be understood as propagation of elastic waves due to lattice vibration or as the interaction between quanta of thermal energy known as phonons. The thermal conductivity resulting from phonon transfer is represented by a function of mean free path of phonon, which decreases with decreasing order in molecular structure. For this reason, the thermal conductivity of crystalline material (alumina and magnesia) is much higher than the amorphous material (silica) and glasses by one to three orders of magnitude (Kingery, 1976) as seen in Fig. 5. This was best illustrated by Berman (Kingery, 1976) where the micro-structure order at molecular level in quartz crystal is randomized by neutron irradiation. Increased irradiation resulted in a

decrease of the thermal conductivity by three orders of magnitude.

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The mean free path of phonon in glasses and dielectric is limited to the order of inter-atomic distances by random structure resulting in lower thermal conductivity. The effect of composition of glasses or ashy material, has very small effect on their thermal conductivity resulting from phonon transfer. The temperature dependence of this process is well understood. The phonon mean free path in crystalline materials and subsequently, the thermal conductivity decreases rapidly with increase in temperature, whereas the phonon conduction does not vary significantly with temperature in amorphous materials like ash and glasses. In addition to heat conduction by lattice vibrations in such solids, a much smaller energy transfer results from higher-frequency electromagnetic radiation. This component is termed as photon conductivity and becomes important at high temperatures as it is proportional to the fourth power of temperature. The photon conductivity becomes important in Alumina above 1,600 K, whereas it is important in glasses even above 800 K. The photon conductivity is dependent on the optical properties of the material and is shown to be inversely proportional to the absorption coefficient of the material. The photon conduction becomes important at temperature of 800–900 K, whereas for materials with high absorption coefficient, the photon conductivity becomes important only at very high temperatures. There are two ways, in which temperature increases the photon conductivity. Firstly, more radiant energy being concentrated in shorter wavelengths and secondly, most of the materials have lower absorption index at smaller wavelength resulting in rapid increase in photon conductivity at higher temperatures (Kingery, 1976). It must be noted that the effect of macro-structure (porosity and pore/particle size) can overshadow the effect of the microstructure discussed in this section. Even half a

percent of porosity can bring down the effective thermal conductivity of pure materials at high temperatures by an order of magnitude.

4. THERMAL CONDUCTIVITY OF ASH AND ALIKE MATERIALS This section discusses the experimental values for thermal conductivities of coal ashes and ash like materials ie oxides and glasses. These results are discussed in order to understand the effect of main parameters (such as temperature, structure, porosity, chemical composition and particle or pore size) influencing the effective thermal conductivity of ash deposits.

4.1. Effect of Temperature In an earlier review (Wall et al., 1979), the thermal conductivities of pure oxides

and mixtures were compared with those of coal ashes. The thermal conductivity of dense and MgO decreases with increasing temperature, however, for silica (SiO2), the thermal conductivity increases continuously with temperature (Fig. 5). The rate of increase is rapid at high temperatures due to the radiation contribution. The thermal conductivity of shows a minimum at about 1,600K. At higher temperatures the increase is due to the radiative contribution. The effect of radiation becomes appreciable above 500 °C for glasses. It is seen that the variation with temperature for ashes is less than that for pure oxides and pure phases. The thermal conductivity measurements are seen to be nearly independent of temperature usually below a temperature of 900–1,200 °C, at which sintering occurs. A slow

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and steady increase in thermal conductivity up to this temperature is attributed primarily to changes in the thermal conductivity of gas phase. A steep rise in thermal conductivity is either due to appreciable increase in the radiative component as mentioned earlier or due to sintering that softens the sample and improves the contact between particles. The steep rise due to radiation is reversible, whereas the sintering causes irreversible changes in the structure responsible for higher thermal conductivity as observed by Anderson et al. (1987) in Fig. 6. After fusion, the sample has a higher thermal conductivity than that of the original particulate sample as it was heated. The chemical and physical changes result in the thermal conductivity measurements being irreversible. The higher the temperature at which the cooling cycle starts, the higher the thermal conductivity on cooling. However, on cooling of particulate (unsintered), the thermal conductivity is reversible.

4.2. Effect of Chemical Composition It is of interest to note that for mixtures of oxides, the thermal conductivity is approximately weighted to the mass proportions of the component oxides. The refractories having less than 50% silica have a decreasing thermal conductivity with temperatures. The dense mixtures with more than 50% silica show a continuous increase in thermal conductivity with temperature (Wall et al., 1979). According to Anderson et al. (1987) silica ratio is important at high temperatures, where strong sintering and fusion can occur. Ratcliffe (1963) investigated the thermal conductivity of glasses with varying composition at temperatures between –150 °C and 100°C. Additive formulae were prescribed, which are claimed to estimate the thermal conductivity of most glasses within 5% accuracy in the given range. The effect of chemical composition in this manner does not influence the thermal conductivity of the material significantly. However, glasses of varying chemical composition are expected to have different optical constants resulting in significant differences in the radiative component of the effective thermal conductiv-

ity at high temperatures. The glasses containing high iron oxides are expected to have lower thermal conductivity. However, Boow and Goard (1966) also observed higher thermal conductivity for iron containing ash deposits, which is attributed to improved sintering of the deposits rich in iron. Further, the chemical composition determines the structure of the deposit at a given temperature. There is a vast difference in the thermal conductivity of particulate ash and a sintered deposit.

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4.3. Effect of Particle Size The experimental data from Laubitz (1959) shows an increase in thermal conductivity with increase in particle size. Nasr et al. (1994) also measured higher effective

thermal conductivity for larger particle sizes in the size range of 3mm to 12mm alumina, glass and silica particles in a packed bed. In both the cases the effect of particle size was observed to be more evident at higher temperatures suggesting that the particle size plays an important role in radiative component of the effective thermal conductivity. Godbee and Ziegler (1966), somehow failed to observe a significant effect of particle size on the effective particle size of magnesia. Boow and Goard (1969) clearly show an increase of thermal conductivity with an increase in mean particle size. The published data for a number of ashes of varying mean particle size at 700 °C is presented in Fig. 7. Least square regression of their experimental data yielded a logarithmic equation:

The ash samples with low median diameters such as present in primary layers of boiler deposits or the laboratory ash, have very low thermal conductivity, even less than that of air (Boow and Goard, 1969). The thermal conductivity of a

gas is due to molecular motion, which decreases significantly when the inter-particle space is comparable to the mean free path of the gases. The same can happen at low gas pressures, where the mean free path of the gas increases and becomes comparable to interparticle space resulting in very low thermal conductivities. This behaviour was also observed by Diessler and Boegli (1958). The thermal conductivity of such layers has been observed to be less than that of the air.

4.4. Effect of Porosity The thermal conductivity of particulate and slag structures increases with a decrease in void fraction or porosity. This effect has been observed by several researchers. Krupiczka (1967) analysed the effective thermal conductivity of granular material for cylindrical and spherical grains embedded in a continuous gas medium available in literature. The resulting correlations predicted the effective thermal conductivity of particulate beds within 30% of the experimental data from several researchers for a number of powders in different gases. The correlation is given by the following equation:

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In more recent measurements, Nowok and Steadman (1992) measured thermal conductivity of coal ashes and slags of different porosity, sintered at different tempera-

tures. The thermal conductivity of gehlenite sintered at 1,300°C increased from about at room temperature to

at 450°C.

4.5. Effect of Deposit Structure The deposit structure is characterised by porosity, pore/particle size distribution, and state of sintering. Sintering is a function of chemical composition and temperature, which has already been discussed and causes irreversible changes in the structure. As sintering increases, contact increase and the structure changes from particulate nature to partially sintered to that of completely fused slag or foam type structure with solid phase continuous as shown in Fig. 1. The data in Fig. 6 shows that the thermal conductivity of powdery ash is quite low but will increase by up to a factor of ten for sintered ash. This agrees with data for pure oxide powders, bricks and cast refractory materials reported by Wall et al. (1979). This emphasises that the physical structure of the ash deposit has a strong influence on thermal conductivity.

5. MODELS OF THERMAL CONDUCTIVITY (MACRO-STRUCTURE) There have been a number of reviews on thermal conductivity of granular materials (Krupiczka, 1967, Hadley, 1986, Botterelli, 1989 and Bauer, 1993). This section discusses several approaches towards estimation of thermal conductivity of porous or particulate materials as shown in Fig. 1 at low temperatures where radiative heat transfer can be neglected. Treatment of radiation becomes essential in estimating the effective

thermal conductivity of the deposits at high temperatures and is discussed in the following section. The slagged deposits correspond to the porous material similar to the structure of foamed material with discrete bubbles or pores of entrapped air. The loose deposits formed from high melting point ash are similar to particulate structure with discrete solid particles embedded in continuous gas phase. Thermal conductivity of particulate or slag type structures depends on the following: • thermal conductivity of solid phase ks • thermal conductivity of gas phase kg • porosity and • the size distribution of pores or of the particles • connectedness of particles The thermal conductivity of solid and gas phase is influenced by its chemical composition. There does not appear to be any model that considers particle size in the estimation of the effective thermal conductivity. The effect of particle size is significant at high temperatures and is considered in the next section. Hence this section essentially considers the effective thermal conductivity models as a function of porosity. The porosity of ash deposits can vary from 0.1 for molten slags to 0.95 for particulate deposits

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depending on conditions of sintering and time. There has been an attempt to obtain the initial estimate of deposit by using a stochastic model. The initial estimate can be modified by using some sintering model (Rezaei, 1997). The simplest approach is to approximate the porous deposit structure consisting of parallel layers of solid and gas. The effective thermal conductivity is given by Equation (7) if the direction of heat transfer is parallel to these layers as shown Fig. 8a. The correlation is given by Equation (8), when the heat transfer is in a direction normal to these layers as in Fig. 8b.

where e is the porosity of the material. The heat transfer is higher when the heat trans-

fer is in the direction parallel to the layers. The equations (7) and (8) correspond to upper and lower limits on thermal conductivity for a given porosity in practice. Rayleigh (1892) derived an exact theoretical equation for the effective thermal con-

ductivity for a cubic array of uniform spheres (limits porosity greater than crete spheres are embedded in a continuous phase as in Fig. 9a.

The dis-

where

= thermal conductivity of continuous phase, = thermal conductivity of discrete phase, r = p = is the volume fraction of discrete phase. The simplified model assumes the temperature isotherms normal to the direction of heat transfer. The equation can be used for slags by considering air bubbles as discrete phase and for particlulate by considering air as continuous phase. Russell (1935) calculated the thermal conductivity for an array of cubes in a cubical array (Fig. 9b), assuming again the isotherms being planes normal to the direction of flow. This model has a definite advantage over Rayleigh’s model, as it can accommodate

the complete range of porosity.

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Diessler and Boegli (1958) solved the Laplace equation for a cubic lattice of spheres and presented results in the form of thermal conductivities normalised to the thermal

conductivity of gas. They approximated the heat transfer in the direction normal to heat flow direction is extremely small. Woodside (1958) also considered a cubic lattice of spheres and obtained an expression for the case, where isotherms are planes normal to the direction of flow. Leach (1993) considered cubic cells stacked together in an attempt to determine the thermal conductivity of foam like materials. He considered two ways of stacking the cubic cells of air. Air is considered as discrete phase. The cubic parallel series (CPS) model considers the layers gas and of the composite material stacked normal the direction of heat transfer (Fig. 10a). This model is the same as that of Russell’s model for a cubicle array of uniform cubes, except in this case the discrete phase consists of air bubbles. The cubic series parallel (CSP) model (equation 11) first considers solid and gas in series across which the heat transfer takes place, and then these series are considered as a composite material stacked in parallel with gas layers as shown in Fig. lOb.

Rayleigh’s model for an array of spheres was also used to determine the thermal conductivity of foams. It can be shown that equation for low density foams or slags all these three relations can be reduced to one single correlation given by Equation (12).

N being 1 , 2 , 3 for CSP, CPS and spherical models. For all the models the first term is sufficient to fit the experimental data on foams. These models are valid for slags, however, these can be used for particulate deposits by interchanging the discrete and continuous phases. Figure 11 compares these three models CPS (also similar to Russell’s model), CSP based on array of cubical cells, and array of spheres (Rayleigh’s Model) for particulate and slag type deposits. It should be noted that Rayleigh model can predict the thermal conductivity over a limited range of porosity as discussed earlier. The suffixes -S and -P in Fig. 11 relate to slags and particulate type structures, respectively. The thermal conductivity for slag structures is considerably higher than that for the particulate structures,

especially in the porosity range of 0.2–0.8. All these models lie within the two extreme limits given by Equations (7 and 8).

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The CPS model, considering layers of composite materials stacked in series with layers of fluid (isotherms normal the heat flow, or infinite conductivity in direction normal to the heat flow), predicts higher thermal conductivity compared to the other models. The figure also suggests that the predictions from Rayleigh’s model based on the array of spherical particles lie between the two models: CSP and CPS based on arrays of cubes. Sintering produces a solid continuous phase resulting in higher thermal conductivity. The reduction in density during sintering also results in increased thermal conductivity. The solid thick line in Fig. 11 shows how the porosity and effective thermal conductivity is expected to increase for a maturing ash deposit. Confirmation of the trends awaits future experiments. This increased thermal conductivity of the deposit due to sintering is very well observed in the experimental data of Mulcahy et al. (1966), as shown in Fig. 6.

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5.1. Thermal Conductivity of Complex Structures These models can not be used to predict the thermal conductivity for complex materials, where both the fluid continuous (Fig. 1a) and solid continuous (Figure 1b) regions co-exist as shown in Fig. 12. The next three models are related to the thermal conductivities of such materials, where both phases are continuous in some parts of the total

structure. The structure is therefore defined by some structural parameteres in all these models; the parameters were determined by best fit to their experimental data. Brailsford and Major (1964) considered sandstone as a random mixture of solidcontinuous-phase and fluid-continuous-phase for determining its thermal conductivity. The two-phase assembly can be regarded as having regions of the above mentioned phases embedded in a random mixture of the same two phases. The concept does not favour the spatial continuity of one single phase. The resulting relation is given by the following equation:

where ;

and and being the volume fraction of the two phases. The thermal conductivities and in equation correspond to the thermal conductivities of fluid-continuous phase and solid-continuous phase, respectively and can be determined using Rayleigh’s correlation for an array of spheres. It was assumed that the porosity in the two phases is similar. The equation (13) needs the volume fractions for the two phases for determining and for the solid and fluid continuous phases, respectively using equation (8). The assumption that the fraction of fluid-continuous phase is same as that of the overall volume fraction of the fluid phase resulted in an overestimate of of sandstone. Figure 13 compares the random distribution of the two phases with two continuous phases. The random model gives results closer to the solid phase continuous for low porosity and results closer to the fluid phase continuous for high porosity. They also suggested an empirical correlation to estimate the volume fraction of fluid continuous phase from fraction of fluid phase ‘p’ given by the following equation:

where factor is related to the structure of the deposit. The results for sandstone agreed with predictions for The proportion of solid phase continuous, a

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measure of sintering, can be an input parameter for the model. An increase in sintering results in a higher proportion of solid continuous phase and the from random phase distribution model would move closer to the slag model as shown by the arrow in Fig. 13. Hadley (1986) introduced an extra parameter and modified Rayleigh’s correlation for fluid phase being continuous, as the previous correlation was with the assumption of particles being too far apart. He accounted for increase in conductivity due to establishments of contacts between the particles by assuming a small fraction a (a measure of consolidation) of the matrix with solid phase continuous and by modifying the porosity

of fluid phase. The two phases exist in parallel to the flow of heat.

Nimick and Leith (1992) also used a mixture of fluid and solid continuous phases to determine of different plagioclase (fused silica). They considered discrete spheres

of fluid-continuous-phase embedded in a medium characterised as solid-continuous phase. They determined two empirical correlations for the porosity in the two phases and the fraction of fluid-continuous-phase. Another model is suggested by Nozad et al. (1985), where both the phases are continuous to account for contacts. Such structure can result, once the particulate phase starts sintering, establishing contacts between the particles resulting in solid phase

continuous. At the same time, the continuity of the fluid phase is not destroyed. This model can be used to determine the effective thermal or electrical conductivity, as the contact radius (neck growth) grows. The porosity of such a structure is given by equation (16).

where, a and c are ratios of particle size and neck size (contact) to the unit cell size. A simple equation for derived from electrical resistance analog for cubicle particles and square contacts (Fig. 14), is given below:

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All these correlations (equations 7–17) do not consider the particle size distribution of the grains that influences the thermal conductivity of granular beds at high temperatures due to radiation. It was assumed that the thermal conductivity component is not influenced by the particle size. The effect of sintering can be accommodated only by the last three models. Research is needed to evaluate such structural parameters as function of sintering.

5.2. Contribution of Radiation at high Temperature The radiation contrbutes to the effective thermal conductivity in two ways: • radiation across the solid transparent medium • radiation across voids

The contribution of first type known as photon conductivity is usually lumped into the thermal conductivity of solid medium with zero porosity and can be determined by solving radiation and conduction simultaneously (Viskanta, 1965). This contribution is always included while determining the same experimentally for glasses and pure oxides with zero density. The contribution of second type is dependent on porosity and the pore size in case of slags and of particle size in particulate materials. The following review corresponds to the contribution of the second type. The effect of radiation is additive to the effective thermal conductivity. McAdam (1954) suggested such term as given by the following correlations:

This term does not account for the emissivity of the particles. Yagi and Kunii (1957) obtained theoretical formula for estimating the effective thermal conductivity of packed beds with different size packing. Their model considered the heat transfer by conduction across solids, across gases in the voids, conduction across contacts, by convection, and by radiation. Their correlations expressed heat transfer in packed beds with motionless gases, especially at high temperatures of upto 800 °C.

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Some models (Godbee and Ziegler, 1958, Laubitz, 1959) calculate the overall effective thermal conductivity by adding the radiative conductivity to the conduction component calculated from the unit cell. Other models (Zehner, 1970; Kunii and Smith, 1960) calculate radiative conductivity as a resistance in the network of resistances representing the unit cell. Diessler and Boegli (1958) investigated effective thermal conductivity of powders in various gases at room temperatures. Accordingly, the free convection does not influence the effective thermal conductivity of powders of The effective thermal conductivity is strongly dependent of Larkin and Churchill (1959) determined the radiative heat transfer through porous insulations. The equations account for the porosity and pore size and temperature on the radiative heat transfer. Experimental results were successfully interpreted by a simple theoretical model. Laubitz (1959) measured for several powders from 100°C to 1,000°C and was able to estimate it from the correlations He modifies (doubles) the effective thermal conductivity determined by Russell’s Equation (9) by the radiative conductivity which is a function of particle size (d), its emissivity and the porosity (p) (Equation 19).

Kunii and Smith (1960) derived equations for predicting the effective thermal conductivity of beds of unconsolidated particles containing stagnant fluid. This is a function of porosity. At high temperatures, it is also a function of emissivity, mean temperature and the particle size. The equations correctly predict the for sandstone and sintered metal systems. A new relationship for radiation contribution to the thermal conductivity was modified by Schotte (1958) as given below:

Predicted thermal conductivities resulted in good comparisons with experimental

values. The above equation assumes that the composite material of solid and void is stacked in parallel to layers of voids. Botterelli et al. (1989) tested a number of published models and compared the predictions. They reviewed the radiative correction for thermal conductivity from several researchers and presented the correction in the following form:

and the parameter c varied from one model to another model. This parameter, also known as exchange factor, includes the effect of particle size and emissivity of the pores or particles. The average pore size or particle size is very important parameter, as the radiative component is directly proportional to it. According to Botterill the models of Godbee and Ziegler (1960) and Kunii and Smith (1960) gave reasonable predictions. The

radiative models of Zehner (1970) and of Kunii and Smith gave good estimates for the thermal conductivity for packed bed of particles at high temperatures with some modifications (Nasr and Viskanta, 1994). The steep increase in the thermal conductivity in Fig. 6 in temperature range of

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900–1,250°C is expected due to contribution from radiation. Figure 15 compares the thermal conductivity of ash deposits at 100°C and 450 °C using slag model for structure and radiative heat transfer model with exchange factor, The comparison is quite encouraging.

6. ESTIMATION OF THERMAL CONDUCTIVITY OF ASH DEPOSITS The predicted trends of thermal conductivity with structure and temperature in Fig. 11 and with porosity in Figure 15 can be interpreted on the basis of fundamental components ie conduction across solid and gas phase radiation across solid phase (if it is transparent like glass) and across voids. The estimation of thermal conductivity of ash deposits in practical situations would then involve following steps: Stepl: Estimate the thermal conductivity of zero density material at room temperature. Ratcliffe (1963) has discussed several additive formulae for estimating the thermal conductivity of glasses. As most of the deposits contain silica, alumina, calcium and iron oxide, determine the thermal conductivity by using the additive formula on the basis of volume or weight fraction. Step 2: Estimate the conduction part of thermal conductivity of zero density material at the given temperature. This information may not be available easily. However, as a first estimate, one can use values for alumino-silicate from Kingery (1976) for zero density as a function of temperature. The experimental determination of such thermal conductivity is expected to be more reproducible. This includes the radiation across solid phase in case of slag type structure. Step 3: Estimate the effective thermal conductivity of the slag or particulate material using a model for accommodating the effect of porosity (Russell’s model, equation 10) for particulate or slag structure. The effective thermal conductivity of partially sintered layer is more complex as it requires more structural parameters for the deposit. In absence of these parameters, the Brailford and Major’s model (equation 13) of random dispersion of the solid and gas continuous phases can be used. It is important to develop structural parameters such as the proportion of solid phase continuous in the partially sintered deposit as a function of sintering.

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Step 4: Take into account the effect of radiation through voids using equation (21). This effect takes into account the particle or pore size distribution.

7. CONCLUSIONS The thermal conductivity of granular or slag type deposits can be determined by correlations using two phase medium equations. At room temperature, the effective thermal conductivity appears to be independent of pore size or particle size. The influence of particle size or pore size is significant at higher temperatures when radiative transfer becomes important. Most of the equations for determining the radiative contribution indicate this transfer to be directly proportional to particle size and to the cube of the temperature of the deposit. The estimates of the thermal conductivity for a partially sintered deposit require

the knowledge of extra structural information describing pore size distribution and the connectedness of the particles. This information can be reduced to structural parameters, such as proportions of purely particulate and slag phases and their respective porosity. These parameters can then be used in the hybrid models described for complex structures. The structural parameters, would also help in providing the anisotropic nature of thermal conductivity of deposits (Ramer, 1996). There is little information available in literature on structural parameters of coal ash deposits and particularly on anisotropic nature of ash deposits. Hence it is proposed to develop the physical structural parameters as function of chemical character of ash and sintering history.

8. BIBLIOGRAPHY Anderson, D. W. et al., J. of Engg. for Gas Turbines, 109, 215, (1987). Bauer, R., Int. J. Heat Mass Transfer, 17, p4181, (1993). Boow, J. and Goard, P. D., J. Inst. of Fuel, 42, pp412, (1969). Botterill, J. S. M. et al.., Int. J. Heat Mass Transfer, 32, pp585, (1989).

Brailsford, A. D. and Major, K. G., Brit. J. Appl. Phys., 15, 313, (1964). Diessler, R. G. and Boegli, J. S., Trans. ASME, 1417, (1958).

Flynn, D. R., J. of Research of NBS, 67C(2), pp129–137 (1963). Godbee, H. W. and Ziegler, W. T., J. Appl. Phys., 37, pp40, (1966). Gupta, R. P., et al., The Heat Transfer Properties of Ash Deposits in PF Fired Furnaces, Project Report, Inst.

of Coal Res., Uni. Of Newcastle, (1997). Hadley, G. R., Int. J. Heat Mass Transfer, 29, pp909–920, (1986).

Kingery, W. D., Introduction to Ceramics (1968). Krupiczka, R., Int. Chem. Engg., 7, 122, (1967). K u n i i , D. and Smith, I. M., AICH.E.J., 6, pp71, (1960). Larkin, B. K. and Churchill, S. W., AICH.E.J., 5, pp467, (1959). Laubitz, M. J., Cah. J. Phys., 37, pp798–808, (1959). Leach, A. G., J. Phys. D: Appl. Phys., 26, 733, (1993). McAdams, W. H., Heat Transmission, 3rd Ed. McGraw Hill, New York, (1954). Mulcahy, M. F. R., J. Inst. Fuel, 39, pp385,394, (1966). Nasr, K., Viskanta, F. and Ramdhyani, S., J Heat Transfer. 116, pp829, (1994). Nowok, J. W. and Steadman, E. N., Proceedings Of the 12th Annual Gasification And Gas Stream Cleanup Systems Contractors Reveiw Meeting, R. A. Johnson and S. G. Jain Ed, Morgantown, West Virginia,

pp154–164, Sept (1992). Nozad, I., et al., Chem. Engg. Sci., 40(5), pp843–855, (1985). Ramer, F. R. and Martello, D. V., Applications of Advanced Technology to Ash Related Problems in Boilers, Ed.

L. Baxter and R. DeSollar, Plenum Press, (1996). Ratcliffe, E. H., Glass Tech., 4, 116, (1963).

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Rezaei, H. R., et al., Proceedings of EF Conf., Impact of Mineral Impurities in Solid Fuel Combustion, Kona, (1997). Russell, J. Am. Ceramic Soc., 18(1), (1935). Schotte, W., AICH.EJ., 6, pp63, (1958). Tye, R. P., Thermal Conductivity, Vol 1, Academic Press (1959). Viskanta, R., J. Heat Transfer, pp143–150, (1965). Wain, S. E. el al., Inorganic Transformations and Ash Deposition During Combustion, Ed. S. A. Benson, ASME, (1992). Wall, T. F., Mai-Viet T., Becker H. B. and Gupta R. P., Fireside deposits and their effect on heat transfer in p. f. boilers: The emmisivity and thermal conductivity of deposits and their components., Proceedings Pulverised Coal firing— The Effects Of Mineral Matter, University Of Newcastle, L8.1–L8.16, August (1979) Wall T. F., Bhattacharya S. P., Zhang D. K., Gupta R. P. and He X., The properties and thermal effects of ash deposits in coal-fired furnaces., Prog. Energy Combust. Sci., Vol. 19, pp487–504, (1993). Woodside, W., Can. J. Phys., 36, pp815, (1958) Yagi, S. and Kunii, D., AICH.EJ., 3, pp377, (1957). Zehner, P. and Schunder, E. U., Chemie. Ingr. Tech., 42, pp933, (1970).

THE DEVELOPMENT OF ADVANCED CLEAN COAL TECHNOLOGY IN JAPAN Mineral Matter Issues

Sadayuki Shinozaki Center for Coal Utilization, Japan

Tokyo, Japan

1. INTRODUCTION As shown in Table 1, Japan’s power facility plan in 1995 was a total of 20GW, and 42 GW of coal-fired thermal power is planned by 2010, an increase of about double the

1995 level. In 1995, Japan’s coal consumption was about 130 million tons, of which 45 million tons were used for power generation, 25 million tons for industry and 60 million tons for steel-making. The area whose consumption is expected to increase in the future is steaming coal for power generation.

On the other hand, however, because of the increased concern about global environmental issues, it is pointed out that coal generates higher than other fossil fuels. Therefore, in coal utilization, it is important to develop high efficient power generation technology and utilization technology and apply them as control measures for generation. Japan is a coal importing country and its import coal accounts for about 95% of the total consumption. Japan imports coal from a wide range of countries, including Australia, the United States, South Africa, Indonesia, China and Russia, and the quality of coal (steaming coal) that Japan uses ranges from sub-bituminous coal (low melting point and rather higher Ca content) to bituminous coal whose fuel ratio is around 2.5. For example, any power plant uses 20 to 30 types of coal. Therefore, the development of coal utilization technology faces difficulties because coal-fired boilers and gasified furnaces in Japan must be designed to fit coals having different quality. The thermal efficiency of coal-fired thermal power plants in Japan is high with an average of 38%, and the thermal efficiency of new coal-fired thermal power plants of 1,000 MW class use USC technology reaches 41%. Regarding environmental characteristics, on the other hand, less than 50ppm of SOx and NOx and less than 10mg/Nm3 of dust have been achieved by the introduction of the high performance flue gas desulfurization system Impact of Mineral Impurities in Solid fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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(wet method), low NOx combustion technology, and denitrification system (SCR). Therefore, coal-fired thermal power plants can be said to be high efficient and excellent in environmental characteristics even today, but the development of further clean and high efficient coal utilization technology is in progress to create better global environment.

2. DEVELOPMENT OF CLEAN COAL TECHNOLOGIES Because Japan promised to stabilize its generation at the 1990 level in “Frame Work Convention on Climate Change” it carries out “Higher Efficiency Power Generation Technology Development” under the CCT development target as shown in Table 2. The outline of the CCT development is introduced in this report.

3. OUTLINE OF CLEAN COAL TECHNOLOGIES DEVELOPMENT THEMES Table 3 shows the development system of advanced clean coal technology in Japan. Of the development themes of advanced clean coal technology, themes that are

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unique and typically affected by the behavior of mineral matters are outlined and introduced. As the concept of development themes indicate, in most themes, clarifying behavior of and measures for mineral matters are important factors for the continuous and stable operation as well as scale up of plants.

3.1. Pressurized Fluidized Bed Combustion Technology3 Establish a technology to commercialize a pressurized fluidized bed boiler with economic superiority by carrying out a series of demonstration test. As compared to conventional pulverized coal fired boilers, the boiler will have distinctive features as given below.

• Scope of plant 71 MW demonstration plant (1) Development issues 1) Demonstration assessment test • Grasping of environmental and combustion characteristics • Demonstration of high efficiency and economic advantages • Verification reliability • Demonstration of optimum operation • Solutions to problems of the behavior of mineral matters (2) Development targets 1) Improvement in thermal efficiency by combined cycle power generation Net thermal efficiency: 42 · 43% (Pcf: 38·40%) Combustion efficiency: 99% over

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2) Environmental characteristics SOx: 30ppm Max. NOx: 150ppm Max. (30ppm Max. at outlet of stack) Dust: Max. (3) Subjects related to Mineral behaviors • Development period: April 1992 to March 1998

3.2. Pressurized Internally Circulating Fluidized Bed Combustion Technology In order to cope with global environment, research will be carried out on pressurized fluidized bed boiler with high reliability and excelling in load responsibility by using the characteristics—corresponding to advanced low SOx and low NOx environ-

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ment type—of revolving floe type internally circulating fluidized bed that can burn many kind of coal grades at high thermal efficiency. • Scope of 4 MW Pilot plant (1) Development issues 1) Verification of higher efficiency by 4MW pilot plant The study of reliability and operatability of plant with high environmental characteristics 2) Technology coping with variety coals 3) Verification of load capability 4) Verification of environmental characteristics 5) Solutions to problems of the behavior of mineral matters 6) Scale up technology • Development targets 1) Thermal efficiency target at 150 MW Trial design Gross thermal eff.: 42% Net thermal eff: 40.74% 2) Environmental characteristics and system design NOx: 30 ppm Max. CO: 50 ppm Max. Desulfurizing rate: 95% (1% S coal: about 40 ppm) 3) Load capability 2.5·3.0%/min at MCR 100% • Subject related to Mineral behaviors • Development period: April 1992 to March 1998

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3.3. Advanced Pressurized Fluidized Bed Combustion Technology In order to reduce a global environmental problem, the development of highly efficient power generation technologies is a matter of utmost urgency. To that end, the fluidized bed gasification technology that were researched in the past in combination with the pressurized fluidized bed technology which is under research at present. The research is aimed at the development of highly efficient power generation technology by next generation technology.

• Scope of plant: 5 MW, PDU (1) Development issues 1) Establishment of hot desulfurization system 2) Turning CaS into harmless substance 3) Checking the controllability of system and stabilized operability 4) Multi-coat utilization technology (Possible to apply to coal grades with high melting point) 5) Solutions to problems of the behavior of mineral matters 6) Process control technology 7) Scale up technology (2) Power generation efficiency: (Trial calculation values by 409 MW trial design)

Gross thermal: 49.7% Net thermal: 46.2%

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(3) Environmental characteristics at desulfurizing furnace outlet) dust: (4) Carbon conversion Calorific value of gas Unburnt • Subject related Mineral behaviors. • Development period: April, 1996 through March, 2001

3.4. Pressurized CPC Combustor Technology Development of a coal partial combustion furnace to convert boilers for general industrial use to coal, while establishing an application system technology for boilers of coal partial combustion furnace, and also develop pressurized CPC combustion technology that can be applied to highly efficient conbined cycle power generation.

• Scope of plant: 25 t/d pilot plant ( 1 ) Development issues 1) Development of pressurized CPC furnace 2) Establishment of high temperature dedusting technology 3) Solutions to problems of the behavior of mineral matters 4) Demonstration of continuos operability—One month continuos and stable operation

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5) Expansion of applicable coal grades To make it possible to utilize a wide range of coal types from low to high fuel ratios. 6) Collection of scale up data: 25t/d . 200t/d 7) Scale up technology (2) Development Targets 1) Gasification performance under pressure (air blow) Generated gas Cold gas efficiency: 69% (pilot), 72% (actual plant system) 2) Collection of oxygen rich combustion data Generated Cold gas efficiency: 73% (pilot), 80% (actual plant system) 3) High temperature dedusting technology Outlet

4) Power generation efficiency: 45% (3) Subject related to Mineral behaviors • Development period: April, 1990 to March, 1999

3.5. Coal Gas Production Technology for Fuel Cell (IGFC) Aimed at the establishment of a technology to manufacture gas for supply to the production of fuel cells, conduct tests on a total system of oxygen blown coal gasification to develop an optimum coal gasification furnace for fuel cells together with a purifying system capable of generating gas of composition best suited for fuel cells (MCFC and SOFC) • Scope of plant: 150t/h pilot plant ( 1 ) Development issues 1) Development of an optimum gasification furnace

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2) Establishment of technology for purifying product gas of composition best suited for fuel cells Highly efficient dedusting technology High performance gas purifying technology 3) Demonstration of high efficiency 4) Solutions to problems of the behavior of mineral matters

(2) Development Targets Reflect at-site investigation and research results on the fundamentals and detailed design of pilot testing facilities (150 t/d ) 1) Determine the type of coal gasification furnace 2) Carbon conversion rate: 98%min Low gas efficiency: 78% Gas (3) Predicted technological targets of a commercial plant 1) Power generating efficiency (Gross thermal: 50%min) 2) Gas purity Sulfur compounds: 1 ppm max Halogen compounds: N • D (0.1 ppm max) Trace element: N • D (0.1 ppm max) • Development period: March, 1995 to March, 2005

3.6. Fluidized Bed Cement Production Technology A cement calcination technology by the fluidized bed system which is completely different from the present kiln system. The characteristics of fluidized bed technology

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is utilized to aim at the development of a highly efficient environmentally-friendly manufacturing technology, making it possible to lower energy consumption, reduction in the consumption of finishing mill pulverizing power, lower NOx, improvement of combustion efficiency, expanding the usage of coal grades, and making it possible to manufacture a greater variety of cement products. • Scope of plant: 200 t/d scale-up plant (1) Development issues 1) Establishment of stable operation and control technology 2) Cement calcination technology that can also address cement-related

products and diversify cement production 3) Development of coal combustion technology that can handle a large

variety of coal grades Calorie: 5,000kcal/kg, Fuel ratio: 8–10 4) Collection of scale-up data 5) Scale up technology (2) Development Targets 1) In order to address the global environment, the kiln type was compared to develop a cement calcination technology based on the following.

—Energy saving type: Heat consumption, 750–760 kcal/kg clinker (Reduction of approximately 10–12% as compared to the kiln type) • Heat efficiency: 54–55% • Low type: reduction of approximately 10% • Low NOx type: 200 ppm max

• Pulverizing power consumption: Improvement by 6–7% • Production cost: Development of technology being capable of reducing production cost by about 15–18% • Development period: From 1987 through March 1998

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3.7. Metal Melting System by Oxy-Coal Combustion The metal melting technology is different from the present electric furnaces. Energy obtained at high temperatures by burning pulverized coal directly with oxygen is used for melting scrap aluminum, copper, iron, and other metals. The development aims to enhance recovery by improving overall energy efficiency and raising concentration. The technology is useful in lowering the melting cost. Therefore, it will be completed as a system technology that is capable of protecting the global environment. • Scope of plant: 5t/charge pilot plant (1) Major development issues 1) Continuous manufacturing technology 2) Optimization and automation of overall process 3) Extended life of refractories 4) Collection of scale-up data (2) Performance targets — Heat efficiency: Final target (Commercial plant: 60%) — Yield: Fe: > 94% Cu, Al > 96% — NOx: < 180ppm —SOx: Less than emission standard of K value 3 region —Dust: < 0.2g/Nm3 (Approximately 1/10 of electric furnace —CO2 concentration of emission gas: 90% min, easy to recover • Development period: April, 1992 to March, 1998

3.8. Multipurpose coal conversion utilization technology— Coal Flash Pyrolysis With a view to assure the availability of energies in Japan in the area of expected petroleum shortage in the 21st century and beyond, and to address the global environmental problems, develop a technology to manufacture at high efficiency gas of intermediate calorie (approximately by flash pyrolysis of carbon at comparatively low temperature, and contribute to the construction of a foundation for coal chemical in the future. • Scope of plant: 100t/d pilot plant (1) Development issues 1) Development of optimum char gasification and pyrolysis furnaces 2) The grasping of flow characteristics of pulverulent body and therma equipment that are involved with the scale up of pyrolysis and char gasification furnaces. 3) Performance check of optimum gas production process 4) Establishment of stable operation technology (2) Development targets 1) Total heat efficiency: 85%min 2) Co-gas and liquid production yield: 70%min 3) Production gas calorie: 4) Product tar: Raise the recovery rate of acid tar. • Development period: March, 1996 to March, 2001

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3.9. Production Technology of Coal Advanced Conversion Cokes Due to obsolescence of existing coke ovens, development of a new coke manufacturing technology which excels in environmental characteristics such as smokeless, technology odorless, low NOx emission, and low dust production, high productivity, and allows the use of general coal. • Scope of pilot plant: 12t/d (1) Development issues

1) Coal pretreatment technology 2) Plug transportation technology of coal particles under high temperatures 3) Coke improving technology at low to medium temperatures—optimum design of coking chamber 4) Establishment of new coke manufacturing technology 5) Summary of effects on heat efficiency, economic advantages, and working coal grades (2) Development targets 1) General alternating rate: 50% 2) NOx emission: 150ppm max 3) SO2 emission: 10% reduction 4) 5) Energy saving: 20% reduction 6) Productivity: 3 times that of conventional coke ovens • Development period: April, 1994 to March, 2002

4. EXAMPLE OF HINDERED STABLE PLANT OPERATION AND DEVELOPMENT DUE TO MINERAL MATTERS IN COAL The major CCT development condition in Japan is described above, and the reaction behavior of combustible organic compounds has been clarified to a certain extent in the development and plant operation stages and has been coped with through optimum design of plants.

However, as shown in Table 4, a large portion of the reaction behavior of mineral matters in coal under high temperatures and high pressure is still unknown, causing difficulties with stable plant operation and scaling up in advanced CCT development. On the other hand, plants in Japan must effectively utilize the discharged coal ash, and because the chemical and physical properties of coal ash differ for each coal lot and for each boiler design and operating condition, the performance of coal ash varies, therefore forming one of the factors that significantly hinders effective utilization of coal ash when it is used as civil engineering material. Table 4 gives examples of difficulties for stable operation of coal utilizing power plants due to mineral matter in coal. Particularly, because Japan is a coal importing country and each plant must use many types of imported coal, unlike on-site plants near origins of production, trouble due to mineral matters seems to occur more frequently and clarifying the behavior of mineral matters tends to be more difficult. Figure 9 shows problems for studies on reaction behavior of coal ashes under high temperatures that are assumed from the examples of hindered stable operation due to ashes described above.

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5. IMPORTANCE OF STUDIES ON CLARIFYING BEHAVIOR OF MINERAL MATTERS IN COAL As explained, mineral matters show many patterns of unknown reaction behavior under high temperatures, pressurized and reduced conditions. As reaction behavior of mineral matters in coal is presently being clarified by the

Brain-C Program of Japan, conclusions have not been reached. But types of main mineral matters in coal (refer to Table 5). and reaction behavior under high temperatures are roughly expected as shown in Fig. 10. On the other hand, the difference in chemical and physical properties of fly ashes generated from coals containing ashes with high melting point (herein after referred to as coal with high melting point ash) and coal containing ashes with low melting point (herein after referred to as coal with low melting point ash) was surveyed in order to assume the reaction behavior of mineral matters in pulverized coal-fired boilers, and the results are shown in Table 6, and Fig. 11 shows examples of SEM photos. Of the SEM photos, J-05 of Table 5 presents domestic coals (low melting point), and it is assumed that the shapes and physical property of fly ashes generated from different types of mineral matters vary significantly. Table 6 shows that fly ashes discharged from coals with low melting point have

small average radius and higher content of glassy matters (Ca-Al-silicate) than that of coals with high melting point. Applying micro analysis to these ashes will useful for the clarification of reaction behavior. Fly ash from PCF boiler (Domestic Coal)

Fly ash from PCF boiler (Imported Coal)

Fly ash from FBC (Imported Coal)

In the aspect of chemical properties, fly ashes from coals with low melting point contain larger amount of CaO compared with that in fly ashes from other coals. This may be due to the difference in the content of Ca compounds that is originally contained in coal, and Ca compounds are responsible for the generation of glassy matters. As a result, the strength of mortar when adding ash with low melting point to cement is greater. However, since slagging may occur depending on the design conditions (heat load, fluidizing condition of particles, etc.) of a boiler when a melting point is low, melting point design for coals fitting for the design conditions of each boiler will be necessary in order to obtain fly ashes having high utilization value for the purpose of preventing slugging.

However, it is quite difficult to use only the optimum coal in the current condition for Japan which must use many types of imported coal. The reaction behavior of mineral matters will be clarified accurately by correlating the data on mineral matters in coal and the micro analysis data on fly ash, eventually contributing to the establishment of boiler design and optimum operating conditions for slagging prevention and obtaining fly ashes with high utilization values. It will also be useful to the design of boilers and gasifiers and scaling up technology for advanced power generation if research progresses.

6. CONCLUSION Although high efficient power generation technology and utilization technology of coal are important to control generation, design of boilers and furnaces will take high temperature into consideration for economy and high efficiency.

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Mineral matters contained in coal differ depending on their types, namely composition and origin of production. Therefore, they may cause ash troubles unless they fit the design and operation conditions of boilers and furnaces. This represents obstacles to the development of advanced coal utilization technology. In the case of countries like Japan where diversed types of coal must be used, the

design of boilers and furnaces that are capable of handling many coal types becomes important for the stable operation of plants. Since there are still many unknown factors in the reaction behavior of mineral matters in coal, clarifying reaction behavior of mineral matters under high temperatures

and pressure is quite important for the stable operation of plant as well as for the smooth scale up of technologies under development, and in Japan, both public and private sectors

have jointly worked to clarify the behavior and to create simulation models. However, we are still in the research stage and unable to report the conclusion in this report. As the clarification of reaction behavior of mineral matters is considered as a common subject to many countries, it seems effective to clarify the reaction behavior of mineral matters with the cooperation of participating countries. Therefore, participating countries have planned a research scheme and proposed this to the technical meeting of IEA-CCS.

We hope to clarify this matter soon as the clarification of reaction behavior of mineral matters will be useful in many areas and will help promote global environmental measures that are common interest of all people.

7. REFERENCES 1) “Outline of Power Development,” Public Project Department of the Ministry of International Trade and

Industries, Oct. 1996. 2) Unpublished material, EPDC technical material. 3) “Proceeding of the Rheology Symposium,” CCUJ, Feb. 1997. 4) “Coal Fundamental Property Section of 1996 Report on Coal Utilization Fundamental Technology Development and Research,” CCUJ/NEDO, unpublished data, Mar. 1997.

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FACTORS CRITICALLY AFFECTING FIRESIDE DEPOSITS IN STEAM GENERATORS Richard W. Bryers Retired Flemington, New Jersey 08822

INTRODUCTION Fireside slagging, fouling, and corrosion due to impurities in steam-raising fuels has been under investigation for over a hundred years. From the very beginning impurities in steam-raising fuels has been responsible for costly maintenance problems, a reduction in heat transfer, derating, costly alterations or the use of more expensive fuels. For

the first eighty years scientists and engineers have taken an empirical approach to understanding and dealing with these problems. Although complex models have been proposed for various phases or steps in the deposition process, it was not until the development of sophisticated analytical techniques for characterizing fuels and the expanded use of the computer in the late seventies and early eighties that comprehensive modeling was attempted. Consequently a large data base critically outlining the various steps in the complex process of deposition already existed awaiting development of more comprehensive economic models. Fouling and slagging are very complex phenomena which depend upon the release of inorganic constituents in fuels during combustion and their transformation into troublesome compounds during the post-combustion heating and cooling process. Fouling, slagging, and corrosion also depends upon the juxtaposition of individual inorganic ash-forming species during comminution, combustion, and quenching while recovering heat; the chemical reactions between gas, liquid, and solid phases in motion and at rest; the kinetics of transformation of minerals and fly ash; the existence of nonequilibrium conditions frequently associated with super-cooling; and the attachment of impurities to surfaces and the detachment or re-entrainment of deposited solids (1). Figure 1 is a flow diagram illustrating these changes prior to deposition or emission from the steam generator. The complexity of fouling and slagging is compounded by the fact that fireside problems cannot be simply represented by a single rate of deposition on a target by ash characterized by a single elemental analysis. The inorganic components—minerals and organically associated cations—in coal and the fly ash it generates are heterogeneous with Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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regard to size and composition. Therefore, individual species behave differently during combustion and their subsequent flight through the steam generator. The degrees of fouling and slagging vary throughout the steam generator depending on local gas temperatures, tube temperatures, temperature differentials, gas velocities, tube orientation, and local heat flux. Furthermore, fireside problems manifest themselves in a variety of ways, some of which include:

Factors Critically Affecting Fireside Deposits in Steam Generators

• • • •

plugging of slag taps formation of agglomerates and clinkers burner eyebrows loss in furnace absorption due to excessive accumulation—be it molten or sintered deposit or due to a change in thermal properties of the slag • damage due to excessive slag fall from a specific location or type of heat exchange surface and it’s orientation • undersize furnace hopper due to a weak slag-to-tube bond, causing excessive sloughing of sintered ash • freezing of slag on hopper slopes • slagging of the heat recovery zone due to unpredictable low melting phases in the ash • fouling of convective heat recovery surfaces by condensation of volatile fumes or the sulfation of vulnerable submicron species at the tube surface • tube wastage due to flame impingement, the presence of CO due to striation of flue gases as a result of poor mixing, or excessive concentration of corroding elements in the fuel • surface oxidation and exfoliation of inside tube surfaces due to high gas temperatures created by excessive fouling • slagging and/or fouling due to the size distribution of the fuel • slagging and/or corrosion due to imbalanced air distribution • pluggage of gas passes due to excessive accumulation of fly ash in zones of low gas velocity • formation of agglomerates in fluidized beds and clinkers on grated (1,2). Some of these problems may be resolved by altering operating conditions, modifying surface or improving fuel handling techniques while others require a more thorough understanding of the deposition process. The wide variation in type, form, and location in which a deposit may form indicates slagging and fouling are very much dependent upon design and operating parameters. Boilers of identical design firing the same fuel have been reported to encounter very different slagging and/or fouling conditions, suggesting the deposition process is sensitive to operating procedures. Consequently slagging, fouling, and corrosion are also dependent upon design and operating parameters. These include:

1. Design a) furnace exit temperature b) furnace absorption c) furnace configuration d) burner arrangement e) burner size f) tube size, spacing, orientation, and temperature g) air distribution—steam conditions 2. Operation a) coal size b) air distribution between burners c) burner operation d) excess air, level e) flame impingement

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f) soot blower operation g) boiler load Since economics dictate that steam generators must be designed to minimize capital cost, operating cost and maintenance costs, with maximum efficiency and fuel flexibility, the philosophy used to characterize fuels must change from one of predicting the slagging or corrosion potential of fuels based on elemental composition and fluid temperature of the ash to one permitting the selection of a matrix of engineering parameters that will allow proper location and orientation of heat transfer surface in the combustion and heat recovery process and the cost of doing so. The option of selecting an alternative fuel as a means of solving a fireside problem is rapidly disappearing. Increased trading activ-

ity with fuels will require increased flexibility and a need for rapid assessment of the cost of changes. Empirical indices already in place are limited to the conditions from which they were devised. Mechanistic models must be developed to expand or eliminate these restrictions of applicability to accommodate changes in loads and blending of fuels. The introduction of very sophisticated microanalytical techniques such as the scanning electron microscope complete with microanalytical capabilities and the computer, has made it possible to characterize fuels and their impurities in greater depth. Models may now be developed employing the analytical techniques along with engineering design parameters and configuration to reflect all aspects of ash deposition.

FUEL CHARACTERIZATION Although the impurities in steam-raising fuel include most of the known elements only 15 occur in sufficient quantities to contribute to slagging, fouling, and/or corrosion. Fortunately, these elements may be categorized by the role they play in fireside problems,

by the fuel type and the rank as shown in Table 1. This significantly reduces the number of elements one must deal with for any given problem. In addition to those listed in Table 1, barium and arsenic have been known to contribute to fouling problems on rare occasions. All but aluminum contribute directly to fireside slagging, fouling, and/or corrosion. Aluminum tends to inhibit the deposition process by raising the melting temperature of the ash. Aluminum may also simply be entrapped in deposits as innocent aggregate by virtue of its high concentration in the coal. Historically fuel impurities have been characterized by the elemental composition of a sample of coal completely ashed at 1,500°F (815°C). Traditionally the elemental composition is reported as the elements oxidized to their highest oxide state and in some cases, normalized to 100%. 100% closure may not always be achieved due to unaccounted for elements, excess residual carbon, elements in the ashed coal not completely oxidized to the highest state and loss of some elements during the ashing process. Based on a large data base of prior fireside problems, elemental analysis alone may provide a good starting point for predicting the potential for slagging, fouling or corrosion. In fact, fuels are purchased and used, and steam generators are bid, designed and constructed today on the basis of the elemental composition of high temperature ash fuel analysis and the estimated deviation or limits imposed on the individual elements alone. This simple evaluation, however, leaves room for substantial error due to unaccounted for behavior of the compounds these elements may form. Fireside problems with few exceptions are associated with a molten phase formed as the result of a low melting compound or condensation of a vapor phase. Consequently, the melting temperature of the high temperature ash from a fuel sample has been used

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as a second criteria for the slagging or fouling potential of the impurities in fuels. The ASTM ash fusion temperature procedure was accepted in 1924. Since then seven other procedures have been accepted worldwide. Details of the terms and procedures have been described in an ASME publication prepared by Hough, Sanyal and Davis (3). It would seem reasonable that a correlation exists between the melting temperature and the ash chemistry. Consequently, numerous investigators have tried to interpret the ash composition and its melting temperature by plotting the data on phase diagrams, or developing fusibility diagrams. Such correlations provide a good test for the validity of the individual analysis and provide a basis for what must be done to alter the melting temperature of the ash and thereby reduce its potential for fireside problems. Others have developed empirical indices using elemental composition or ash fusion temperatures as independent variables for establishing the limit to trouble free operation. Winegartner assembled a classic lexicon summarizing these indices which are still used commercially today (4). This empirical approach, unfortunately, assumes that coal ash is homogenous and remains intack during combustion. As we shall see, this is not an accurate assumption in many cases. For specific designs such as a wet bottom furnace, or cyclone where it is essential that molten conditions be maintained, viscosity of the ash was selected as the most appropriate property for characterizing the impurities in the fuel. Since viscosity measurements are tedious and expensive, numerous correlations were developed on the basis of temperature and elemental composition. Viscosity as a property of molten ash is not only important to fuel selection of specific steam generator types where molten ash is essential, but also plays a significant role in the growth and size of deposits that may be initiated by the sintering process. Viscosity is also being used as a property for predicting the

sticking potential of fly ash which is either molten or encapsulated by a wet surface. Consequently, it is essential to understand the viscous behavior of fuel ash.

Corey reported that completely liquid slag behaves as a Newtonian fluid and that the rate of change in viscosity with a change in temperature at a given viscosity was essentially constant (5). Consequently, the functional relation of viscosity vs. temperature in

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the Newtonian range is the same for all coal slags. By making an appropriate parametric correction for composition, the viscosity of all slags can be represented by a single curve. A typical curve for velocity vs. temperature appears in Fig. 2. Reid indicated slags may be categorized into three different types according to their behavior as they are cooled below the crystallization point, also illustrated in Fig. 2 (6). Reid shows that slags rich in silica appearing as quartz remain glassy over the entire viscous temperature range. No divitrification occurs and viscosity depends only on temperature, not the previous thermal history. In a second group of slags, a solid phase enriched with alumina calcium or iron precipitates out abruptly at some given temperature in the cooling process forming a pseudoplastic fluid. Upon reheating, the solid phase is gradually redissolved, but at a much higher temperature than that at which the crystal was formed. The temperature difference between crystal formation and crystal melt depends on cooling rate. At high rates of cooling the freezing temperature is considerably below the critical melting temperature. Corey indicates the transition from Newtonian to plastic flow is due to the formation of crystals in the slag and corresponds to the softening temperature of the ash. A third group includes those slags in which the crystal precipitation, upon cooling, occurs at almost the same as the temperature as the crystals dissolve upon heating. Reid’s observation of a potentially large difference between melting temperatures of ash upon heating and solidification temperatures upon cooling is very important as ash is characterized for

its slagging potential while being heated while fly ash is deposited under a rapid quenching process. Consequently ash may deposit at much lower temperatures than predicted from fuel ash analysis.

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Kalmanovitch and Williamson examined the divitrification of eastern and western North American coal-type ashes quenched over a period of several hours. Their results show that the crystallization is well represented by the quarternary. (7) Huffman and Moza, however, both using somewhat different techniques but much higher quenching rates, found that freezing or solidification temperatures were 200– 400°C below the initial deformation temperature occurring during the heating cycle (8,9). Huffman's tests were performed in a muffle furnace whereas Moza’s experiments were performed in a drop tube. The solidification temperature rather than melting temperature of ash is a critical factor influencing fireside problems. Presently there are at least five expressions for viscosity in terms of temperature and the five major constituents of coal ash. All are claimed to be accurate over a given

range of slag composition. None of these formula, however, include modest amounts of the alkalis believed to be influencing the surface fusibility of fly ash and the fusibility of furnace wall slag formed by high sodium-bearing lignites. A plot of viscosity using the

Urbain and Watt-Fereday equations applied to various gravity fractions of a bituminous coal in Fig. 3 indicates substantial disagreement between correlations as iron concentration increases. This suggests that correlations developed to reduce time and cost of determined viscosity should be verified for slag whose composition fall out of the range used to derive the correlations. The discussion on viscosity also suggests that the liquid phases may be present at temperatures much below those predicted by melting temperatures for fuels containing significant quantities of free quartz. Consequently, biomass fuels should be much more vulnerable to slagging and conventional techniques used to assess slagging potential of coal may not apply for biomass.

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MINERAL COMPOSITION The three fundamental properties used to characterize fuel ash are all based on a composite sample assuming homogenous distribution of impurities. In reality, however, impurities in steam-raising fuels exist as minerals and mineral matter each of which is composed of several elements which have been oxidized, chlorinated, sulfated, or organically bound. Minerals refer to the completely inorganic constituents whereas mineral matter refers to the organically bound inorganic elements. Anyone familiar with coal cleaning operations, for example, will clearly recognize that minerals are heterogeneously dispersed throughout coal by size and juxtaposition with regard to the coal and other mineral species. Each mineral or group of minerals contributing to a single ash particle has its own physicochemical properties and goes through its own peculiar transformation, thereby subjecting the steam generator to a wide variety of ash particles with

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different physicochemical properties. Further partitioning of the mineral species may occur during the communition process depending upon the fuel, its hardness, moisture, and particle size distribution. The final degree of variability in fly ash properties will depend on the initial size distribution of the minerals in the fuel and the size reduction operation. There are about 125 minerals found in coal. Twenty-five are reported in Table 2 (10). Nine have been recognized as occurring in significant amounts. The remaining 14 are listed as accessory minerals. During the process of coalification, the minerals have been altered by time, heat, moisture, and pressure producing a variation in mineral composition by coal rank already implied by the variation in elemental behavior in Table 1. An examination of the genesis of coal in Fig. 4 indicates the elemental composition of coal and the minerals within which they are contained is dependent on the original peat swamp and the age of the coal. As the coal ages pressure and temperature convert the

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organometalic compounds, sulfides, sulfates, and carbonates into oxides associated with the silicates. Consequently, each coal bed has its own characteristic mineral distribution by size and composition which is influenced by its age and the surrounding geology. Coal beds are formed over long periods of time representing various periods in the history of the peat swamp. Therefore one should expect to encounter a variation in mineral composition by layer or lateral location in a fuel bed. In extreme cases one might encounter a variation in coal rank. The largest difference in mineral composition occurs by rank. Coals of similar rank generally contain minerals of similar composition and fireside behavior. The combustion characteristics of the fuel are also altered by age. With increased coalification all macerals become enriched with carbon, eventually forming anthracite. As coals age they become harder, contain less volatile, contain less moisture, and are more difficult to ignite. These factors all influence the steam generator size and configuration and have a direct impact on the final mineral composition of the fuel with regard to quantity and composition. Variability in the mineral composition of coal due to variability in mineral composition at the mine, unexpected inclusions and mining procedures are a major source of slagging and fouling problems. Wood and biomass, which are precursors of the peat swamp, are the primary source of the organometallic and water soluble salts in coal. A typical analysis for wood and biomass appear in Table 3. Wood is low in ash which is enriched with calcium and potassium. Biomass, which includes grasses, shells, pits, and manure, etc., is also low in ash

but rich in silica, calcium, potassium, and occasionally, phosphorous. All but the silica

are ionically bound to the plant life. Moisture level is high in both fuels resulting in a low flame temperature. Both fuels are most frequently fired on spreader stokers and more recently in fluidized beds. Petroleum and petroleum coke derivatives originate from decayed animal life and

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are frequently associated with sea beds. Thomas identified as many as 25 elements intrinsic to the crude (11). The principal ash forming constituents were reduced to 9 and tabulated by Bowden in Table 4 (12). Both inorganic and organically-bound oil soluble forms have been observed for each element. The mineral matter is not destroyed during processing and thus appears in heavy oil in the same form. Sodium is not intrinsically found in crudes. Consequently occurrence in oil must be due to intrusion during shipment.

However, it should not exceed 60 PPM if properly desalted at the refinery. Asphaltic base crudes are generally rich in vanadium, particularly Venezuelan oil, which often contains 900 PPM reported as Paraffinic base crudes are usually free of vanadium. Petroleum derivative fuels such as fluid coke, delayed coke, etc. contain the same mineral forms as the crude but in much higher concentrations. Huffman, using X-ray absorption nearedge structure analysis, has confirmed that the vanadium porphyrin are not destroyed in the coking process (2). Nickel concentrations may be somewhat higher in cokes than crude oils due to contamination by cataylst. Petroleum products are moisture free and burn with very high flame temperatures. The rates of combustion of the liquid and solid fuels are decidedly different as is their ignitability. Consequently, they are fired in very different furnace configurations. Oil is fired in a conventional front wall furnace with a very high heat release rate where as petroleum coke is fired in an arch-fired anthracite type furnace. Municipal solid waste (MSW) is a man made fuel composed of all types of waste products from human consumption including grasses, leaves, and food materials. The impurities in MSW fuels include all the elements found in coal plus modest amounts of zinc and lead. Vanadium fortunately is not found in significant amounts. The ash is highly basic and hence resembles the ash from lignite enriched with sulfur. Moisture levels are high and the fuel is most frequently fired on a grate in a furnace lined with refractory. Numerous techniques are now available for characterizing minerals in samples. Xray diffraction performed on low-temperature ash is a powerful tool and the most widely used technique for qualitatively identifying the presence of minerals in their crystalline form in concentrations of a few weight percent or greater. Thermal analytical techniques such as differential thermal analysis (DTA) and thermogravimetric analysis (TGA)

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have been used as a signature analysis based on changes in physical properties with temperature. Microanalytical techniques including scanning electron microscopy (SEM) equipped with energy dispersive x-ray (EDX), electron probe microanalysis equipped with EDX, and transmission electron microscopy (TEM) and scanning transmission microscopy have very good resolution to very small particle sizes. The techniques may be applied to low temperature ash as well as raw coal and provide visual images of the

morphology of the mineral structure. Extended x-ray analyses (XANE) is another signature analytical technique recently applied to coal and petroleum coke. Mossbauer spectroscopy has been widely used for characterizing the mineral forms of iron in coal as well as in slags and deposits. Computer automation of the scanning electron microscope (SEM) and energy dispersive x-ray (EDAX)–commonly referred to as a computer controlled scanning electron microscope (CCSEM)—has been developed to locate dispersed inorganic particles in coal and quantify their size, shape, and composition. When coupled with automated digital image analysis, morphological data can be stored, examined, and used to determine differences between the included and excluded mineral matter as well as species sharing partially common boundaries i.e. juxtaposition of minerals with coal. The two systems may be applied to raw coal, partially spent char and fly ash or deposited ash, in effect revealing changes in mineral chemistry at various stages of combustion. Figure 5 illustrates the distribution of minerals in a coal sample as determined by CCSEM (13). Presently these techniques, although commercially available, require the skills of a

highly trained scientist and are expensive. The services for determining analysis via these techniques are provided by only a few laboratories, limiting their availability. Widespread practical application requires more extensive demonstration of their utility in predicting and solving problems. Their availability, however, is a critical factor in the transformation of slagging fouling and corrosion from an empirical art to an applied science. Chemical fractionation, a technique developed by Miller and Given and put to practice by Benson and Holm, uses selective extraction of elements based on solubility which reflects their association in the coal or biomass (14,15). The process consists of three successive extractions, first by water to remove water soluble elements such as

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sodium in sodium sulfate which was most likely associated with ground water, second by ammonium acetate to remove elements such as sodium, calcium, and magnesium that

may be associated as salts of organic acids and finally with 1M HCl to remove acid-soluble species such as iron, magnesium, aluminum, and calcium in the from, of hydroxides, oxides, carbonates and organically coordinated species. Table 5 illustrates the application of the widely accepted technique to coal and biomass. The elemental concentration of the fuel ash is insufficient to predict fireside problems. The mineral form is critical in determining the type and extent of fireside ash deposition.

TRANSFORMATION OF MINERAL MATTER DURING COMBUSTION During combustion, the inorganic elements in coal are transformed into inorganic gases, vapors, molten particles and solids depending upon their association in the fuel as included or excluded minerals, their juxtaposition with other mineral grains or their occurrence as organically bound species such as sulfur, chlorine, and the alkalis, or salts such as the alkalis, heavy metals, and chlorine. The excluded minerals include mineral grains released during size reduction (including minerals found in small partings, fractures, and fissures) and dirt picked up during mining, storage, and fuel handling. The excluded minerals generally constitute 90% of the ash in high rank coals and contribute primarily to the coarse size fractions of the fly ash. In low rank coals the mineral grains

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may constitute 60% or less of the total inorganic content of the coal. These mineral grains will under go further size reduction and partitioning during combustion attributed to ash shedding, char fragmentation, mineral fragmentation, decomposition and volatilization. The sulfides and carbonates decompose while free quartz and calcite will partially vaporize under reducing conditions. The organically bound cations decompose and form gases or a vapor which subsequently are oxidized, sulfated, or chlorinated, depending upon the levels of concentration of sulfur, chlorine, and oxygen. These elements form noncondensable gases, condensable vapors, or sublimated super-fine particles which interact with entrained fly ash, the tube surface, deposit surface or are discharged from the steam generator. Each mineral specie undergoes a unique thermal transformation as illustrated in Figure. Melting temperatures for all but illite biotite and pyrite generally exceed 2,552°F. (1,400°C.) Ulrich et al. examined the transformation of volatile species generated from minerals during combustion under reducing conditions using JANAF tables to predict vapor equilibrium compositions at high temperatures under varying degrees of oxygen (16). They found that substantial amounts of silicon, aluminum, iron, calcium, magnesium, sodium, and potassium appear as pure metal cyanides depending upon oxygen levels. The volatile species are formed at the carbon flue gas interface. The degree to which they are released is diffusion controlled. Vapors are oxidized and precipitated during the

migration process forming submicron particles. Nucleation generally occurs rapidly

forming a cloud of active particles that coagulate at a decreasing rate. Brownian collision and coalescence appear to be a major growth phenomena. Growth ceases in the case of oxides and metals when temperature drops below the solidification temperature. Via juxtaposition, groups of minerals will form eutectics with melting temperatures below

that of the pure compound. Phase diagrams indicate that below 1,900 to 2,000°F (1.038 to 1,093°C) the oxidized mineral species are generally dry and not vulnerable to unassisted capture. These are equilibrium diagrams and do not take into account supercooling due to a rapid quench. Illite and biotite contain small concentrations of iron and/or potassium and form a glassy phase at 1,742°F (950°C) and 2,012°F (1,100°C) respectively (2). Depending upon its fluidity, this glassy phase could be responsible for surface deformation at a relatively low temperature and provide the necessary sticking potential to permit retention on hot surfaces. These minerals are stable to high temperatures; however Cl in the environment will release potassium as a volatile species. Pyrite is a combustible component of the fuel with a specific gravity of about 5, approximately twice that of other minerals, a heating value of 3,000 BTU/lb, and a combustion rate comparable to that of anthracite coal of similar size. The rate of decomposition is governed by reaction rates, oxygen level, pore diffusion, and bulk or stream diffusion as well as the adventitious impurities (2). Although the bulk of the particle does not shrink during combustion, some fragmentation occurs altering the mineral size distribution. The melting temperature of the skeleton ash composed of or is 1,540°C and is of little consequence in ash deposition except as or a fluxing agent to other mineral species. An intermediate phase comprised of FeO and FeS may form a melt as low as 1,724°F (950°C). under reducing conditions pyrrhotite is formed generating a melting phase with temperatures reported as low as 1,300°F (2), significantly reducing the rate of combustion. At least a portion of the extraneous particles of pyrites fracture and disintegrate when thermally shocked, yielding a fume of 0.1–1 mm diameter particles (17). The inherent pyrites were found to generate an iron fume. The extent to which the iron fume originates as inherent or extraneous ash is not entirely resolved. However,

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the fume is estimated to be 4–7% of the iron in the coal. Approximately 90% of the iron in North American coals occur as pyrites. The remaining iron usually occurs as the siderite, a carbonate which decomposes at 1,085°F (585°C) with the release of Mineral matter includes the organically bound sodium, calcium, and magnesium found in coal, the vanadium found in oil, and the potassium, calcium, and phosphorous found in biomass. One might also want to include the organically bound sulfur and chlorine found in coal in this group. Although strictly speaking they are minerals the salts of sodium, potassium, zinc, and lead found in the low rank fuels and MSW may also be included in this group on the basis of their thermal behavior. Linder investigated the release of organically bound sodium and included sodium chloride from coal which are the major contributors of fouling for most coals (18). He found the sodium is released under reducing conditions as metallic sodium. The rate of release was dependent on pore size, pore diffusivity, and bulk diffusivity. The organically bound sodium is evaporated as the volatiles are released, intimating that the char is depleted of sodium once the volatiles have escaped. Formation of Na(OH) is completed in the gas stream. Sodium chloride melts at 1,473°F (801°C) and begins to volatilize at about 1,382°F (750°C). Above 1,382°F (750°C) the rate increases significantly. Its rate of escape, as in the case of organically bound sodium, is dependent upon pore and bulk diff-usivity and instant properties of the char affecting either. Evaporation of crystals 0.1

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in diameter was bulk diffusion controlled whereas particles 0.3 were pore diffusion controlled. Evaporization rates were comparable to devolatilization. Once released the dominant sodium bearing gas species may be either NaCl, Na, or Na(OH) depending upon the oxygen level and the presence of chlorine. As the temperature levels drop below 802°F (1,477 °C), equilibrium favors the formation of silicates. Below 525°F (977°C) sul-

fates begin to form. Precise temperature levels depend on and levels. The reaction rates of the sodium with silica to form silicates changes as the process proceeds in five stages: • • • • •

Transportation of gaseous reactants and products to the silicate particle Diffusion of gaseous reactants and products through the boundary layer Gas Phase Reactions at the molten surface Sodium silicate forming reactions at the sodium/silica interface Diffusion through the silica melt (18)

Organically bound calcium occurs substantially in low rank coals. It is known to

form a fume of reactive submicron CaO. Its release from coal has not been examined, however, like sodium has. The fume initiates furnace wall deposits as well as low temperature convection bank deposits by forming calcium sulfate bonded deposits. Calcium silicate may perpetuate the growth of partially developed slag as semi-molten amorthite or gehlinite.

Potassium is most frequently found as a mineral rather than mineral matter in coal. In wood and biomass the concentration of ionically bound potassium is high. Potassium is found in regions where plant growth is most vigorous. Consequently, tree branches and forest wood slash, as well as straw, grasses, etc. are generally rich in potassium compared to core wood. During combustion the potassium is released as a vapor either as a hydroxide or chloride. Like calcium, the release mechanism has not been explored as thoroughly as sodium. Vanadium in petroleum is rapidly and completely oxidized to vanadium pentoxide at very high temperatures, probably as it is released at the droplet surface. Organically

bound vanadium in solid coke derivatives behaves quite differently as complete oxidation of the vanadium is suppressed to very low levels despite virtually complete burnout of the char. Solid fuels burn slower at lower temperatures. In addition, the vanadium must migrate through an atmosphere enriched with CO and counter current to the flow of oxygen. Exposure to the maximum level of oxygen occurs at a lower temperature in the flame front removed from the surface of the particle. The rate of cooling is faster than the rate of oxidation of vanadium to Like calcium and potassium, details of

the release of the vanadium have not been investigated. Unlike the other elements, the practical impact of the release of this element has the greatest significance on slagging, fouling, and corrosion as vanadium in solid fuels forms innocuous or rather than the highly corrosive low melting

formed from vanadium in oil.

Nickel, lead, and zinc all are found in the combustibles of manmade products in a variety of compounds. During combustion, they volatilize forming vapors which react with the HCl or present to form low melting chlorides, sulfides, or sulfates with very high vapor pressures.

Approximately 60–90% of the sulfur in coals occur as pyrite. The remaining portion is organically bound. The release mechanism and rate depends on the source of the sulfur, i.e. pyrite or coal. In the case of coal, the mechanism is influenced by a shrinking core

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whereas with pyrites the particle core size remains constant. Initially, it was felt was formed in the flame. Hedly showed that cooling rates were too fast for equilibrium to be achieved. Consequently, only a small portion of the was converted to (20) Levy and Merryman showed the level in the boundary layer could be many times higher than in the gas stream due to longer residence times and catalytic conversion by Recent aqueous leaching experiments by Gluskoter et al. and Chen et al. along with XANE analysis by Huggins and Huffman have led these investigators to change their original thinking about chlorine in North American coals from chlorine occurring predominantly as inorganic alkali chlorides to a mixture of soluble chlorides and unspecified organic chlorides (22,23,24). With the chlorine occurring as anions of moisture in fine

cracks and pores, it would appear that a high level of removal could be achieved by fine grinding and aqueous removal. NaCl will precipitate from the chlorine rich solutions upon drying (25). Gibbs reports rapid dechlorination of coal during the early stages of devolatilization releasing and HCl (26). Equilibrium favors formation of HCl at the low temperature oxidizing conditions to which heat transfer surface is exposed. Engdahl indicates Cl has never been detected in incinerating steam generators (27) Once in the gas stream, particle fragmentation and coalescence of particles may occur altering the composition and solidification temperatures of some of the particle groups. During the quenching operation in the temperature range of 1,900°F and 1,400°F (1,038°C and 760°C) absorption of alkalis by silicates can produce a low melting eutectic making the particle sticky and vulnerable to unassisted capture or adherence to a contacting surface. The precise temperature at which the surface becomes sticky is dependent on cooling rates, rates of diffusion of alkalis to the surface, reaction rates, and diffusion rates through a viscous substrate. The absorption is also dependent on the HCl and Na(OH) levels of the gas stream. The mineral occurrence of the elements and their juxtaposition with regard to carbon and other mineral species are critical factors in predicting their contribution to fireside problems.

TRANSPORT MECHANISMS The mode of transport of fly ash to the heat-transfer surface is primarily inertial impaction for particles over 10 microns and thermophoresis and diffusion for particles 10 microns and smaller. The mechanism of transport by which the impurities reach the tube surface is strongly influenced by tube orientation in the gas stream, point of contact on the tube, wall effects, and location in the bundle. The bulk stream profile has gross effects on local turbulence also influencing rates and degrees of accumulation. The process of transporting particles of various sized to the surface via inertia impaction, thermophoresis or diffusion under laminar or turbulent conditions is reasonably well understood. Adherence or re-entrainment of impacting particles, and all the factors influencing particle retention are not well understood and certainly not quantified in meaningful parameters, i.e. tube temperature, surface roughness, impact angle, impact velocity, particle temperature, physiochemical state, size or body force (mechanical, Van der Waals, electrostatic). These forces may change with time as slags of many tons may be supported by a very small surface bond. Knowledge of sticking and subsequent bonding or adherence has a strong influence on ash removal and subsequent tube cleanliness.

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SLAGGING Slagging is defined as the formation of fused or sintered deposits on heat transfer surface or refractory in the furnace cavity subject to radiant heat exchange. Agglomeration in fluid beds or clinkering on stokers are special cases of slagging in the absence of heat exchanger surface. Designers try to avoid slagging by selecting a furnace exit

temperature below the initial deformation temperature of the ash. The furnace is sized to accommodate the moisture level of the fuel and local absorption rates are empirically adjusted to account for expected fouling factors based on the elemental fuel composition. The nature, degree and composition of slags may vary significantly throughout the furnace depending upon tube surface temperature, flame temperature, instant particle temperature, local gas temperature, absorption rate, direction of gas flow, mineral composition (i.e. coal rank and fuel type), mineral and coal size and distribution, concentration of mineral matter, and oxygen level at the flue gas boundary. Therefore an understanding of slagging requires a complete profile of the furnace gas temperature and composition with particular emphasis on the boundary layer. One must also be

able to predict changes in this profile as the deposit develops and changes from a dry sinter state through the plastic range and to a running fluid. Knowledge of the change in conductance and emittance of slag as a function of its composition and physical state is necessary. A generic conceptual model for slag formation on refractory surface evaporating surface or superheater surface is proposed in Figure 7 based on the data for emittance and conductance by Mulcahy et al. (28). Mulcahy’s data indicates the greatest resistance to heat flow occurs at the inner layer and tube slag interface where the layers of ash may be dry or sintered and quite thin. He indicates the inner layers may have a thermal conductance one quarter that of the fused or molten deposit. Consequently, once the initial layer has been developed and the heat absorption rate significantly reduced, the deposit can grow to substantial thickness with little further loss in absorption.

Evaporating and superheating surfaces run between 750 and 1,100°F (400–594°C) while refractory surfaces run 1,700°F (927°C) or higher depending upon refractory thickness. Consequently, slagging would be expected to be most severe on refractory surface exposed to low melting fly ash. Slagging is initiated with the attachment of particles in

a dry, semi-molten or molten state depending upon the composition of the fly ash particle, its particle size, the local heat flux, local gas composition, degree of super cooling, gas phase reactions with the tube surface or fly ash particle, or condensation of volatile constituents. Small dry particles may adhere to the surface when the tube-particle body forces exceed gravitational and gas dynamic shear forces. Semi-molten and molten par-

ticles form as the result of fluxing of quartz by alkalis, partial decomposition of pyrites, the chlorination or sulfation of heavy metals and alkali earth, metals, or supercooling due to a fast quench below the melting temperature of the ash particle. These semi-molten particles may bond to the tube surface by surface tension prior to freezing. Submicron particles and products of condensation generally deposit uniformly over the furnace wall, whereas larger semi-molten or molten deposits develop in irregular patterns depending upon the fluid dynamics of the gas stream and variations in distribution of fly ash in the

gas stream. Partitioning and segregation of minerals does occur within the gas stream due to differences in size and gravity of individual mineral species. The segregation may

begin as far back as the milling operation. Table 6 illustrates the different types of initial slag layers which may form by fuel

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type and ash composition. A sharp increase in surface temperature of the slag is experienced with the formation of a minimal layer due to the thermal resistance between the ash and the tube and the low thermal conductivity of sintered ash. The surface becomes more vulnerable to capture of low melting mineral species in the fly ash, the fluxing of dissimilar mineral species resulting in the formation of a low melting eutectic, or mechanical entrapment of otherwise innocuous fly ash.

The rate of growth depends upon the rate of impact of troublesome constituents, their physical size and state and the change in surface temperature of the deposited slag.

As the ash deposit develops, the absorption rate decreases, substrates cool below the solidification temperature of individual species in the deposited ash and the surface temperature increases at a reduced rate. The surface temperature of the slag may eventually reach a level at which melting of the composite sample begins. A plastic state is achieved. The rate of entrapment increases. Depending on the composition of the ash, the semimolten phase may remain plastic over a large temperature range as crystals dissolve and the higher melting species go into solution. The slag surface will remain plastic until all the deposited ash goes into solution and the surface exceeds the softening or liquidous temperature of the deposited ash. At slightly higher temperatures, the viscosity of the ash decreases significantly and the slag begins to flow. Equilibrium is achieved and the slag is fully developed when the rate of deposition is equal to the rate of flow of the fireside layer. Rates of deposit growth change as the percent re-entrainment changes with surface conditions. Flame temperature, surface temperature, local absorption rates, and heat transfer coefficients of deposited ash are critical factors influencing slagging for a fuel with a given ash characterization. Diagnosing and predicting slagging problems requires knowledge of the furnace temperature profile and gas composition as well as the composition and temperature profile of the slag. The composition of the deposited ash may be quite different from that of the bulk fuel ash. The composition of the slag may also vary throughout the accumulated deposit as well as from location to location in the furnace or about the tube surface. The variation in composition depends on partitioning of mineral matter

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in the gas stream followed by further partitioning in the deposit process. An understanding of how and why the initial layers of deposit are formed is critical to the understanding of the slagging process. Knowledge of the thermal characteristics of the deposit is critical to predicting the impact of deposition on the steam generator performance.

FOULING Fouling is generally used as a term to define ash deposition in convective heat transfer zones downstream of the furnace cavity where radiant heat exchange is minimal. Selecting a furnace exit temperature below the melting temperature of the lowest melting

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fly ash specie is critical to avoid slagging or catastrophic fouling of convective heat transfer surface. This means that furnace exit temperatures for oil may be in-excess of

2,300°F (1,260°C), for coal 1,800°F (982°C)–2,300°F (1,260°C) and as low as 1,250°F (676°C) for low rank fuels. Fouling may be due to condensation of volatile species on fly ash particles resulting in a low melting phase on the particle surface thereby increasing its sticking potential, condensation of low melting species directly on the tube surface, or the reaction of gases such as CL, HO, or CO with fly ash particles at the tube surface to form low melting species such as chlorides, sulfates, and sulfides. At the high velocities and low gas temperatures encountered in convection banks, the re-entrainment rate is very high for the bulk of the fly ash composed of coarse refractory oxides. Superfine particles with a

high surface to volume ratio and vapors which migrate to the tube surface by means of diffusion, thermophoresis or Brownian motion are considered the primary source of fouling. The vapors are formed by species which have high vapor pressure and low melting

temperatures in the range of 910–1,600°F (510–871°C). These are low compared to the oxides of Si, A1, Fe, Ca or Mg, which melt at temperatures around 1,750°F (954°C) or higher. Deposition may begin at the point of separation of flow from the tube surface, as in the case of superfine particles released from oil or the stagnation point for larger

particles released by ash from solid fuels. The submicron particles are held in contact with the surface by mechanical, electrostatic or Van der Waal forces as shown in Fig. 8 (29). Vapors such as HCl, etc. diffuse to the—surface and react with the particulate to form a low melting phase. As demonstrated by Levy, the concentration of gases such as at the tube surface maybe many times that in the bulk gas stream due to the catalytic conversion of to which proceeds at the tube surface where residence times are high and gas temperatures low. Within the inner layers conversions from silicates or chlorides to sulfates may also occur. The precise compounds that form will depend upon the local gas composition and the preferred equilibrium. In any event, a sticky surface is created reducing the rate of re-entrainment of impacting solid particles and increasing the rate of fouling. As in the case of slag, the substrate are subcooled as the deposit develops. Sintering occurs as the gases within the deposit react with the deposite surface

forming surface melts which subsequently freeze hard. Depending on bulk gas stream temperature, composition and residence time, some condensation or gas-solid reactions may occur prior to contact with the tube surface making the fly ash more vulnerable to capture once in contact with a stationary surface. Deposition does occur between tube rows and in dead gas space, causing unique fouling problems. With some of the low grade fuels rich in heavy metals, sulfur and chlorine it has been necessary to go to parallel flow convective heat transfer surface at very low gas temperatures to avoid severe fouling. Fouling may be broken down into various types depending upon fuel type and the mineral matter inclusions. They generally include alkali-bonded deposits, (calcuim sulfate-bonded) deposits, phosphorus-bonded deposits, vanadium, pentoxide, and heavy metals, as described in Table 7. The critical factors affecting fouling include the levels of concentration of the troublesome mineral matter in the fuel, the tube metal temperature, gas temperature, oxygen level, tube orientation and spacing. As in the case of slagging a model defining deposit thermal properties, coefficients of heat exchange, and strength are essential for and engineering solution to the fouling problem rather than an empirical assessment of the fouling potential of the fuel. Unfortunately, not all the mechanisms of fouling are clearly understood.

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CORROSION Fireside corrosion in steam generators has been attributed to sodium, sulfur, chlorine, CO, and salts of heavy metals. Although material selection has helped reduce

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high oxidation rates at high steam temperatures, the solution to most of the severe corrosion rates involves limiting metal temperature, limiting concentration of the trouble-

some impurity in the fuel and control of the CO level at the tube surface. Alkali trisulfates and pyrosulfates formed on superheater surfaces and furnace walls respectively are corrosive when in contact with tube surface in a molten state. The pyrosulfates melt at 732°F (398°C) and require 150 PPM

to 2,000 PPM

for

the formation of potassium and sodium pyrosulfate respectively under reducing conditions (30). For pyrosulfates to exist at tube metal temperature of 899°F (432°C), there must be at least 1,000 PPM present for to form and 2% for to form. Consequently, pyrosulfates are confined to evaporating surfaces. The alkali trisulfates melt at 1,000°F (537°C) and decompose above 1,295°F (701°C) (31). The molten ash acts as a fluxing agent for the iron in the tube and provides the media for counter current flow of and corrosion product sustaining the process on a continuous basis. Although chlorine does not enter into the process, it may promote the release of potassium from otherwise stable illite, permitting the

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formation of the potassium trisulfate. The alkali trisulfates and pyrosulfates are controlled by limiting superheater temperature and avoiding flame impingement on furnace walls. For years it was believed corrosion due to chlorine could be expected to occur once the level of chlorine in the coal exceeded 0.3%. Recent experience with coal and municipal solid waste have shown that corrosion associated with chlorine will only occur in the presence of reducing or cycling reducing-oxidizing conditions. Under oxidizing conditions, chlorine occurs as HCl in the gas stream, which does not become seriously corrosive until temperatures exceeding the evaporation temperature corresponding to the steam generator pressure are exceeded. Corrosion of Cl becomes excessive at about 400°F (204°C) (32). Under reducing conditions, HCl decomposes releasing which can react with the iron in the tube to form which has a low melting temperature and high vapor pressure volatilizes and passes through the protective layer of iron oxide counter current with In the presence of it decomposes releasing fresh and forming or Since has a specific volume many times that of the reacted metal, its formation and subsequent decomposition will induce a nonprotective porous scale allowing the process to proceed linearly rather than parabolically. Brooks and Meadocraft have shown in the laboratory that the corrosion most frequently associated with Cl is indeed CO attack enhanced by the presence of Cl by virtue of the

formation of a porous scale (33). Lee clearly showed that “Cl” attack only occurred in the presence of CO at the boiler tube surface (34). Attack in the presence of Cl in the fuel appears to be an operating or design problem which permits exposure of portions of furnace wall to reducing conditions. It is quite possible that some of the corrosion problems associated with alkali pyrosulfates were indeed Cl enhanced CO attack as the tube surface was inadequately analyzed to detect the presence of Cl. Chlorides of heavy metals such as zinc and lead may also cause corrosion by forming low melting eutectics with alkali and iron chlorides acting as a fluxing agent of the protective surface of the tube. These problems are generally associated with low sulfur low grade fuels such as MSW. Blending of MSW with a high sulfur fuel has reduced the corrosion potential. Vanadium in oil, when completely oxidized at very high temperature, forms vanadium pentoxide which is catastrophically corrosive in the molten state. melts at 1,245°F (675°C). In the presence of alkalis such as sodium occasionally introduced

as NaCl in the oil during shipment, the melting temperature drops to 950° (51O°C). The problem begins to occur in steam generators once the vanadium in the oil exceeds 80 PPM and the sodium levels exceed 40 PPM. In the presence of less than 3% excess air, the lower oxide states of vanadium are formed, i.e. or which have melting temperatures in excess of 3,500°F (1,926°C) and hence are innocuous. Vanadium attack is controlled by controlling metal temperatures. Where that is not acceptable, additives such as magnesium are used to form intermediate compounds which raise melting temperatures of the ash. Tube supports are frequently fabricated from 60/40 chrome nickel alloy. At low temperature steam conditions, oils with vanadium concentration as high as 950 PPM can be fired providing there is a provision for removal of furnace slag.

Oil derived fuels such as fluid coke contain very high levels of vanadium, i.e. as high as 140,000 PPM V. They can be fired corrosion free as the lower flame temperature and slow rate of burning from a solid fuel inhibit the formation of

Factors Critically Affecting Fireside Deposits in Steam Generators

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SUMMARY AND CONCLUSIONS The overview of the critical factors influencing high temperature fireside problems while firing various grades of steam-raising fuels and the various mechanisms that have been proposed suggest that a good many of the problems do not require detailed modeling to arrive at a solution. Variability in coal impurities in the mine, inclusions picked up during mining, transportation or handling, purchasing coal on the spot market can all be the source of unexpected fireside problems. Selection of fuels on the basis of elemental composition of the coal ash without due regard to the mineral form, size, or juxtaposition with regard to other minerals present can be the source of a drastic change in fireside slagging or fouling and unexpected furnace deposition. Improper milling of coal and burner adjustment or selection of burner size can be the source of fireside deposits. Flame impingement and low levels can cause unnecessary corrosion, slagging, and fouling problems. All these problems can be resolved by an adjustment in procedure or operation. Most corrosion problems can be handled by restricting metal and or gas temperatures and controlling levels at the tube surface. MSW steam generators, despite high excess air, require a good deal of furnace turbulence and flue gas mixing as well as imposed temperature limits to avoid corrosive situations. Modeling of air distribution in plenums through grates and into the furnace may be helpful. By itself wood does not often present a problem. Mixing with other fuels containing sulfur will introduce severe slagging and fouling problems. Biomass, including a large group of fuel types, must be treated on a case by case basis due to a large variation in fuel ash chemistry. Because of the large concentration of volatile mineral matter, characterization of the fuel ash is difficult. Users of this fuel type may have to develop new procedures peculiar to the fuel. Furnace temperatures must be kept low and designs requiring an intermediate convective furnace void of suspended surface between the radiant furnace and convective heat recovery may have to be used. On the other hand when switching, blending, penetrating new coal reserves, or dealing with a fuel known to be difficult to fire, it would be highly desirable to have a model which could predict steam generator performance based on changes in fuel ash chemistry. This model would be particularly helpful in dealing with coals for which there are no empirical data. Such a model must be capable of predicting complete velocity, temperature, and composition of combustion gas profiles, deposition rates and the thermal and compositional profile of the deposits formed. With such a model, one can not only predict performance, but also make an economic assessment of the problem and examining alternative approaches. Evaluation of slagging and fouling requires an understanding of the economic tolerance for the degree of ash deposition from a given fuel. It is not simply a matter of the degree of ash deposited. Development of a successful model requires an understanding of each step in the process a mineral must go through from the storage pile to the tube surface. In addition each step must be formulated and quantified. The brief discussions on mechanisms suggest that furnace partitioning, tube bonding, re-entrainment, deposit hardness, and a comprehensive model for deposit develop