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

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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 development require some attention. It is imperative that the industry accept solidification temperature rather than melting temperature as a standard for evaluating fouling, slagging, and corrosion, as the process occurs under a quenching rather than a heating process. Correlations for such properties as viscosity should be reevaluated for ash whose composition is out of the

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range for which the correlation was developed. The role of Cl in high temperature corrosion must be resolved.

REFERENCES 1 ) Benson, S.A., Jones, M.L., and Bryers, R.W., (1993), “Practical Measures To Minimize Ash Deposition On Coal Fired Plants”, Proceedings of The Engineering Foundation Conference, England, 657–678. 2) Bryers, R.W., (1996), “Fireside Slagging, Fouling and High Temperature Corrosion of Heat Transfer Surface Due To Impurities in Steam Raising Fuels”, Prog. Energy Combustion Sci., 22, 29–120.

3) Hough, D.C., Sanyal, A. and Davis, D.G., (1990), ASME Ash Fusion Research Project, American Society of Mechanical Engineering Research Committee on Corrosion and Deposits, CRTD-18, New York.

4) Winegartner, E.C., (1974), “Coal Fouling and Slagging Parameter Book”, No. H86, ASME, New York. 5) Corey, R., (1964), “Measurement and Significance of the Flow Properties of Coal Ash Slag”, Bureau of Mines Bulletin, 618. 6) Reid, W.T., (1976), “Corrosion and Deposits in Combustion Systems”, Penn. State Fuels Seminar, College Park Pennsylvania. 7) Kalmanovitch, D.P., and Williamson, J., (1986), “Crystallization of Coal Melts”, ACS symposium Series

301, American Chemical Society, Washington, D.C., 234–255. 8) Huffman, G.P., Huggins, F.E., and Dunmyer, G.R., (1981), “Investigation of The High Temperature Behavior of Coal Ash In Reducing and Oxidizing Atmospheres”, Fuel, 60.

9) Moza, A.K., and Austin, L.G., (1981), “Studies On Slag Deposit Formation In Pulverized Coal

Combustion Part 1 - Results Of The Wetting and Adherence Of Synthetic Coal Ash Drops On Steel”, Fuel, 60. 10) Jackson, P., (1979), “From Mineral Matter to Deposits In P-F Fired Boilers; Part 1 The Behavior Of Minerals in The Flame; Part 11. The Basic Physics And Chemistry of Deposit Formation and High

Temperature Corrosion”, Pulverized Coal Firing—The Effect of Mineral Matter, T. Wall, (ed.), University of Newcastle, Australia, 136–139. 1 1 ) Thomas, W.H., (1938), “Inorganic Constituents Of Petroleum Part 11”, Science of Petroleum, Oxford University Press, London. 1 2 ) Bowden, A.T., Draper, P., and Rowlins, H., (1953), The Problem of Fuel Oil Ash Deposition In OpenCycle Gas Turbines”, Proc. Inst. Mech. Eng. (A) 167. 13) Shah, H., Huffinan, G.P., Higgins, E.E. and Shah, A., (1991), “Graphical Representation of CCSEM Data for Coal Minerals and Ash Particles”, Seminar on Fireside Fouling Problems, Brigham Young University Center for Advanced Combustion Engineering Research, Provo, Utah. 14) Miller, RN.. and, Given, PH., (1978), “A Geochemical Study of The Inorganic Constituents in Some Low Rank Coals”, U. S. Department Of Energy Report FE-2494 TRI. 15) Benson, S.A. and Holm, PL., (1985), “Comparison of Inorganic Constituents in Three Low Rank Coals”, Ind. Eng. Chem. Prod. Res. Dev., 24, 145. 16) Ulrich, G.D., Riehl, J.W., French, B. Rand Desrusiers, R, (1977), “The Mechanism of Submicron Fly Ash Formation in a Cyclone Coal Fired Boiler”, Engineering Foundation Conference On “Ash Deposition and Corrosion from Impurities in Combustion Gases”, R.W. Bryers (ed.). New England College, New Hamphire, 253–268. 17) Srinivasachar, S., Helble, J., Katz, C.B., Worench, J.R. and Boni, A. (1988), “A Transformation of Inorganic Coal Constituents in Combustion Systems”, PSI Technology Quarterly Report No.7, PSI1024/SR-386.

18) Lindner, E.R, Manzoori, A.R., and Wall, T.F., (1991), “A Theoretical Analysis of Sodium Silica Reaction During Pulverized Coal Combustion”, Proceedings of The Engineering Foundation Conference in Inorganic Transformations and Ash Composition During Combustion, S.A.Benson, (ed.), Shereton Palm Coast, Florida, 565–581.

19) Jackson, P., (1963), “The Physicochemical Behavior of Alkali Metal Compounds In Fireside Boiler Deposits”, “Proceedings Of The International Conference on The Mechanism of Corrosion by Fuel Impurities”, H . R . Johnson, and D.J. Littler (ed.), Marchwood England, 484–495. 20) Hedly, A.B., (1967), “Factors Affecting The Formation of Sulfur Trioxide in Flue Gases”, J. Inst. Fuel, 40, 142. 2 1 ) Levy, A. and Merriman, E.L., (1967), “Mechanisms of Formation of Sulfur Oxides In Combustion”, Trans. ASME J. Eng. Power, 89 Series A, 297.

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22) Gluskoter, H.J., and Rees, O.M., (1964), “Chlorine In Illinois Coal”, Geological Survey Circular 372, Urbana Illinois. 23) Chen, H.L., and Pagano, M., (1986), “The Removal of Chlorine From Illinois Coal by High Temperature Leaching”, Fuel Proc. Tech., 13, 261–269. 24) Muggins, F.E., and Huffman, G.P., (1991), “An XAFS Investigation of The Form of Occurrence of Chlorine in U. S. Coals", “Proceedings of CRSC-EPRI First International Conference on Chlorine in Coal”, J. Stringe (ed.), 11–26. 25) Chou, C.L., (1991), “Distribution and Forms of Chlorine in Illinois Basin Coal”, Proceedings of The CRSC-EPRI First Intentional Conference On “Chlorine In Coal", J. Stringer (ed.), 11–26. 26) Gibb, W.H. and Angus, J.G., (1983), “The Release of Potassium From Coal During Bomb Combustion”, Journal of The Institute of Energy, 149–157. 27) Engdahl, D.B., Dartoy, J. and Bebte, P., (1979), “European Refuse Fired Energy Systems; Evaluation of Design Practices”, U.S. Department of Commerce, PB 80-115322. 28) Mulcahy, M.F.R., Boow, J. and Goard, P.RC. (1966), “Fireside Deposits and their Effect on Heat Transfer

In a Pulverized Fuel Fired Boiler, Part 1; The Radiant Emittance and Effective Thermal Conductance of the Deposits, Part 11; The Effects of the Deposit On Heat Transfer from The Combustion Chamber Considered as a Well Stirred Reactor”, Journal of The Institute of Fuel, 39, 308, 385–394.

29) Benson, S.A., Steadman, F.H., and Kalmanovitch, D.P., “Studies of the Formation of Alkali and Alkaline Earth Aluminosilicates During Coal Combustion using a Laboratory Scale Furnace, Conference on “The Effects of Coal Slagging on Power Plants”, EPRI, Atlanta, Georgia. 30) Coats, A.W., Dear, .J.A., and Pendfold, D., (1968), “Phase Studies on the System and J. Inst. Fuel, 41, 129–132. 31) Nelson, W. and Cain, C, (1960), “Corrosion of Superheaters and Reheaters of Pulverized Coal Fired Boilers”, Trans. ASME, 82, 194–204. 32) Brown, M.H., Delong, W.B. and Auld, J.R, (1947), “Corrosion by Chlorine, Hydrogen, Chloride at High Temperature”, Ind. Eng. Chem., 39, 7, 834–844. 33) Brooks, S. and Meadowcroft, D.B. (19??), “Corrosion Resistant Materials For Coal Conversion Systems”, Applied Science Publishers, 105.

34) Lee, D.J. and Whitehead, M.E., (1983), “The Influence of Gas and Deposit Chemistry of The Fireside Corrosion of Furnace Wall Tubes In Coal Fired Boilers”, Proceedings of The Engineering Foundation

Conference on “Fouling Of Heat Exchangers”, R.W. Bryers (ed.), White Haven Pennsylvania, 69–104.

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ADVANCED ANALYTICAL CHARACTERIZATION OF COAL ASHES—AN IDEMITSU KOSAN— ELSAM COOPERATION PROJECT Ole Hede Larsen 1 , Flemming J. Frandsen2, Lone A. Hansen 2 , Signe Vargas2, Kim Dam-Johansen2, Karin Laursen 3, Takeo Yamada4, and Tsuyoshi Teramae5 1

Fælleskemikerne ELSAM, I/S Fynsværket DK-5000 Odense C Denmark 2 Department of Chemical Engineering, Technical University of Denmark DK-2800 Lyngby, Denmark 3 Geological Survey of Denmark and Greenland Thoravej 8, DK-2400 København NV Denmark 4 Energy Development Department Idemitsu Kosan Co., Ltd. No. 3-6, Kita-Aoyama 1-Chome Minato-ku, Tokyo 107, Japan 5 Coal Research Laboratories Idemitsu Kosan Co. Ltd. 3-1 Nakasode, Sodeguara Chiba, Japan

1. INTRODUCTION The fly ash from coal combustion can lead to operational disturbances and damage of boiler components due to deposition on the boiler heat transfer surfaces—slagging in the furnace and fouling on superheaters. Evaluations of the slagging and fouling propensity of a given coal or coal blend are based on different empirically indices calculated

from traditional coal analyses such as ash composition and ash fusion temperatures. However, these indices have a limited applicability (Couch, 1994). To improve the predictive capability, Elsam is running a R&D-project on slagging and fouling, targetting towards gaining a better understanding of the processes through Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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improved methods of characterization and modelling combined with full scale tests on boilers (Laursen 1997a; Laursen et al., 1997). At the same time, Idemitsu Kosan is working on improving methods for characterization of ash and ash melting behavoir, allthough following a different approach than Elsam. Idemitsu Kosan has a very well-equipped laboratory for coal analyses including comprehensive facilities for coal research (Teramae et al., 1997).

Due to these circumstances, and due to the fact that both in Japan and Denmark a large part of the electricity production is based on pf-boilers using coals of worldwide origins there was mutual interest to set up a collaborative project on advanced ash characterization. A parallel investigation scheme including Japanese and Danish analyses of 9 coals, 3 coal blends and 3 fly ash samples from actual boilers was carried out. The Japanese analyses were carried out by the Coal Research Laboratory at Idemitsu Kosan. The Danish analyses were carried out by the Geological Survey of Denmark and Greenland (GEUS), the Combustion and Harmfull Emission Control (CHEC) group at the Department of Chemical Engineering at the Technical University of Denmark and by the coal laboratories of the power plants within Elsam.

2. PRETREATMENT AND ANALYTICAL METHODS The investigation scheme is shown in Fig. 1. There are two types of samples, 12 pulverized coal (9 single coals and 3 blends) and 3 fly ash samples. The mineral contents of the coal samples are analyzed after three different pretreatment methods: no ashing, low temperature ashing and standard ashing.

2.1. Pretreatment The not-ashed pulverized coal samples were mounted in carnuaba wax and polished sections were prepared for Computer Controlled Scanning Electron Microscopy (CCSEM) analysis of the not-transformed mineral content at GEUS. Low temperature ashing (LTA-ash) of the coal samples were performed by Idemitsu Kosan in order to make an ashing with minimum transformation of the minerals. For

low temperature ashing, the pulverized coal was thinly placed on a vessel and put into a plasma reactor. The coal was ashed in plasma at a temperature of 150–200°C until the weight of the sample was constant. It took about 2–3 days for ashing the 2–3 g of coal. Standard laboratory ashing at 815°C (HTA-ash) to constant weight was performed by Idemitsu Kosan according to the Japanese Industrial Standard (JIS), which is very close to the ISO standard used by Elsam.

2.2. Analytical Methods The not-ashed pulverized coal samples were analyzed by CCSEM at GEUS (Laursen 1997b). The low temperature ashes (LTA) are analyzed by bulk ash composition (X-Ray Flourescence) by Idemitsu, X-Ray Diffraction (XRD) by Idemitsu and GEUS, High Temperature Light Microscopy (HTLM) by Idemitsu and Simultaneous Thermal

Advanced Analytical Characterization of Coal Ashes—An Idemitsu Kosan—Elsam Cooperation Project

Analysis (STA) by CHEC. The LTA-ashes and the corresponding ashes after STAanalyses were analyzed by Scanning Electron Microscopy (SEM) at GEUS. In HTLM a heating stage microscopy was used for measuring the linear shrinkage rate of a cylindrical ash pellet (5 mm in diameter, 1 mm in heigth) formed by hand press. During heating in atmosphere by 25°C/min, the pellet shrunk concentrically and the linear shrinkage rate was measured as the decrease in diameter in percentage of the original diameter. In the STA technique a sample and an inert reference are heated in atmosphere at a heating rate of 10°C/min. During heat up the sample weight and sample temperature are continously measured. The changes in weight in percentage give the Thermo Gravimetric Analyses (TGA) and differentiation of this curve by time provides the Dif-

ferential Thermo Gravimetric analyses (DTG) in %/min. The measurements of the sample temperature compared with the temperature of the inert reference sample give the Differential Thermal Analysis (DTA) or Differential Scanning Calorimetry (DSC).

In this case the DSC analysis is used and this shows the heat in W/g produced or consumed by chemical or physical processes during heat-up. A procedure for quantification of melt as a function of temperature based on the DSC-analysis was developed by Hansen (1997). From the onset of melting untill completion of melting, the melt fraction is quantified from integration of the DSC deviation from the base line. Further details are given in Hansen (1997), Hansen et al. (1997) and Frandsen et al. (1997). The high temperature ashes (HTA) were analyzed by bulk ash composition by Idemitsu (XRF) and by Elsam (Inductive Coupled Plasma-Atomic Emission Spectroscopy—ICP-AES), Ash Fusion Temperatures (AFT) by Idemitsu and Elsam and viscosity by Idemitsu. The ash fusion temperatures by Idemitsu were made according to JIS M8801 in oxidizing (air) and reducing atmospheres with a heat rate of 5°C/min.

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The AFT analyses by Elsam were made according to ASTM D1857 in reducing atmosphere with a heat rate of 8°C/min. Sample geometries are similar but sizes different for JIS and ASTM. Critical temperature points (Initial deformation Tempetature—IT, Hemispherical temperature—HT and Fluid Temperature—FT) are based on almost similar criteria, but ASTM includes an additional Softening Temperature— ST. The available maximum temperatures are 1,500°C and 1,650°C for the

Idemitsu and Elsam analyses respectively. Viscosity analyses were carried out in a Haake RV2 Rotovisco unit in a atmosphere, and decreasing temperature at 10–50°C intervals after the ash sample had been slowly heated to the desired temperature, typically about 1,500–1,700°C. The fly ash samples from power plants were analyzed by CCSEM at GEUS, ash composition by Idemitsu and Elsam and particle size distribution (PSD) by Idemitsu. The PSD was analyzed with a laser diffraction type CILAS Granulometer 715 after dispersion in water by use of ultrasonic waves.

3. RESULTS The results of all the analyses carried out on the 12 coal and 3 fly ash samples are

given in internal reports. The ranges of composition, mineralogy and ash fusion temperatures are given in Table 1 and some interesting results and findings are presented in more detail in the following section.

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4. DISCUSSION The evaluation of results will be discussed from the following characteristica: • Chemical and Mineralogical Composition of Coal Minerals and Fly Ash • Particle Size Distribution of Coal Minerals and Fly Ash • Sintering, Melting and Viscosity Characteristics of Coal Ash

4.1. Chemical and Mineralogical Composition of Coal Minerals and Fly Ash Analyses of bulk ash composition of HTA from coals and on fly ash were carried out by Idemitsu and by Elsam. A comparison of these two sets of data showed rather similar results on most oxides. The largest discrepancy on absolute basis is the content, where Elsam underestimates with appr. 4% (appr. 10% relative). This only lead to a slight overestimation of some of the traditionally used empirical slagging and fouling indices, such as those based on base to acid ratio. It has more effect on some types of modelling, primarily modelling of ash viscosities, where the content is an important figure. The largest discrepancy on relative basis was seen for with appr. 25%. This

has a larger effect on fouling indices. Idemitsu also determined the bulk ash composition of the LTA ashes and found almost identical composition of LTA and HTA ashes, with the exception of S. The sulfur content is higher in the LTA than in the HTA ash, probably due to that less

has evaporated. Based on the CCSEM analyses, bulk ash compositions were also calculated. All values are calculated on free basis. Comparison with standard ash analyses shows that the CCSEM analyses can reproduce the variations of chemical composition but with a general overestimation of and and underestimation of and

This is probably related to sectioning of samples, omission of ZAF-correction in the CCSEM analyses, small particle sizes not analyzed by CCSEM, organically associated elements and other factors. For many applications of CCSEM (i.e. comparing chemical/mineralogical composition of individual particles in coal, fly ash and deposits),

these deviations are of moderate importance, as they in general are systematic. In other applications, such as use for modelling of viscosity, these deviations can lead to erroneous interpretations and for such cases it should be considered to upgrade the CCSEM analyses, e.g. to include ZAF correction. The mineralogy of coal inorganics was determined in two ways: XRD analyses of LTA ashes and CCSEM analyses of the pulverized coal. In Table 2, the results for two coals and a blend of these are shown. XRD is semiquantitative and gives a ranging of minerals according to their dominant peak height. The CCSEM results will in this case be given the same way—in order of decreasing abundance. From the results, it is seen that the two laboratories range the minerals almost similarily based on XRD, but one discrepancy is, that GEUS never detects illite, whereas this clay mineral is generally found by Idemitsu. Comparison of XRD of LTA with CCSEM of pulverized coal for mineralogical analyses shows that the minerals detected by XRD, generally are also found as major phases with CCSEM, allthough the ranging is different. CCSEM generally ranges the clay minerals (kaolinite, illite, montmorillonite) higher and quartz lower than the XRD analyses. In addition to the minerals found by XRD, CCSEM find some minor phases

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of silicates and oxides. However, some of these might be two minerals placed very close to each other so that they are analyzed as one mineral. Thus, it seems that there are possibilities to be able to get quantitative mineralogical composition data from CCSEM analyses, but it probably requires, that the CCSEM is improved on element quantification (e.g. ZAF correction), data reduction and on differentiating individual particles.

4.2. Particle Size Distribution of Coal Minerals and Fly Ash The particle size distribution of the coal minerals was only analyzed by CCSEM, whereas the fly ash samples were analyzed with CCSEM and laser diffraction. In Fig. 2, a typical plot of the particle size distributions as analyzed by CCSEM and by laser diffraction is shown. The CCSEM can not analyze reliable in the few-micron size and below, which is mainly due to the spot size in the SEM. At higher particle sizes the curves cross

over, and the CCSEM allways measures a smaller particle mean size, than the laser diffraction. The most important reason for this is probably the cross-section effect for the CCSEM technique, where the minerals are generally not cut at their largest diameter. The difference found in particle mean size from the two techniques seems very systematic for the samples analyzed. This could indicate, that it might be possible to correct the CCSEM

results in order to give more reliable particle size distributions above appr. but the analyses are too few to verify this. These comparisons indicate, that it might also be possible to measure the coal mineral particle size distribution with the CCSEM analyses after proper correction of the values allthough it needs further verification. This is of great importance for mechanistic modelling of fly ash formation in the boiler, as the particle size distribution of coal minerals can not be analyzed by other methods.

4.3. Sintering, Melting and Viscosity Characteristics of Coal Ash Several analyses provide information on the sintering, melting and viscosity changes at heat-up or cooling of coal ashes. Mentioned in order of increasing temperature ranges,

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these analyses include: 1) linear shrinkage rate (LSR) by HTLM as a measure of sintering characteristics, 2) STA analyses as measure of weight and temperature changes due to reactions, evaporation and melting, 3) ash fusion temperatures as measure of fusion and melting and 4) viscosity measurements of the melted ash.

In Fig. 3, all these characteristics for one coal are presented on the same figure in order to get a total picture of these characteristics. Sintering characteristics of LTA ash were measured as linear shrinkage rate (LSR) by use of High Temperature Light Microscopy (HTLM). In figure 3 the LSR is shown

for coal A in the low temperature range. It was found for all ashes, that sintering started in the temperature range of 600–750°C. This onset of sintering is much lower than the ash fusion temperatures. The sintering characteristics differed much between the coal

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types. Sintering phenomena occur in ash deposits on boiler walls and tubes and affect build-up and removal of ash layers. Ash fusion temperatures of HTA ash for coal A are shown as AFT in Fig. 3. The 3 temperatures shown are Initial deformation Temperature (IT), Hemisphere Temperature (HT) and Fluid Temperature (FT). The ash fusion temperatures of all 12 HTA ashes measured by Idemitsu and by Elsam are shown in Fig. 4. The dotted lines are best fit through the data points and the solid lines represents same values from both laboratories. It shows, that there is some scatter of values between the laboratories as has also been shown by other workers (Coin et al., 1996). It is also seen, that allthough the correlation coefficient is very high for the IT and decreases a bit to HT and to FT, the latter two scatters around the solid line, whereas the initial deformation temperature (IT) shows a significant deviation in the way, that Elsam measures systematically higher IT-values than Idemitsu, and the deviation increases at lower temperatures. The reason for this is not clear, but it is important, as the IT-temperature is considered the most important of

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the ash fusion temperatures in relation to slagging and this temperature also is included in many boiler manufacturers design criteria. In Fig. 5, the result of STA analysis of LTA ash from coal A is shown. Supported by the XRD analyses, this STA analysis has been interpretated as follows: clay minerals dehydroxylate below 150°C. Kaolinite, pyrite and bassanite are transformed to metakaolinite, hematite and anhydrite, respectively, between 400°C and 650°C. Carbonate (calcite, dolomite and siderite) decomposition occurs around 400–700°C. Melting starts just below 1,000°C and is completed at 1,390°C. SEM analyses of the STA ash confirms complete melting at 1,390°C. SEM/EDX analyses of all ashes after STA analyses show, that most ashes are totally fused glass showing complete melting. Some ashes were fused glass with iron oxide or iron sulfate particles embedded indicating remnants of partially fused pyrite. One ash showed partly fused glass showing only partial melting. These features are important in

the interpretation of melting. From the DSC curve in the STA analysis, a melting curve has been calculated as described by Hansen (1997) and Hansen et al. (1997). The result is shown as Melt % for coal A in Fig. 3. For this coal, the IT temperature corresponds to 33% melt, HT corresponds to 74% melt and FT corresponds to 98% melt. Figure 6 show graphs of the melt % versus IT-temperature and HT-temperature for all 12 samples. The data seem to divide into two groups. One group with the major part of the samples shows low melt % at the IT-temperature (0–15%) and moderate melt % at the HT-temperature (10–40%) as would also be expected. The other group has high melt % at the IT-temperature (50–80%) and at the HT-temperature (70–100%). It is also clear, that the latter group has higher ITand HT-temperatures than the group with low melt %. The reasons for this picture is not clear but part of the explanation might be, that ash with high fusion temperatures devel-

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ops a more rigid skeleton by sintering before the melting occurs and hereby can contain more melt phase before deforming. Viscosity of the HTA ashes was measured by Idemitsu and the results for coal A are shown in Fig. 3. Six models designed to estimate viscosities of completely molten silica mixtures have been tested against the measurements by Idemitsu (Vargas et al. 1997). These models are categorized in 3 groups according to their temperature

relation: Tabulated values Weymann form: Arrhenius form:

Bottinga-Weill (1972) Urbain (1981), Kalmanovitch & Frank (1988), Streeter(1984) Watt (1968), Greenberg (1984)

The Bottinga-Weill model will not be treated herein, as surplus of alumina can not be accounted for in this model. The models are only evaluated for temperatures, where

the ash is completely molten. In Fig. 7 the results of the Weymann and Arrhenius form models are shown together with the measurements of Idemitsu for one coal. In this case, the Greenberg and the Urbain models give the best results, the Streeter model gives reasonable results, whereas the Kalmanovitch & Frank and the Watt models give poor results. When comparing results of all 12 coals, the Urbain model generally gives satisfactory estimates, the Greenberg model gives good estimates for ashes with more than 25 wt% the Kalmanovitch & Frank model gives good estimates for ashes with more than 50 wt% and the Streeter and the Watt models generally give poor estimates.

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4.4. Application of the Results Figures such as Fig. 3, where different analyses on ash sintering, melting and viscosity characteristica are collected might turn out to be useful in getting a more com-

plete picture of how different coal types behave with respect to slagging and fouling. However it still requires, that operational experiences with these coals in different boilers

are collected systematically and compared with the analyses. Also, more information might be gained upon a more detailled comparison of the individual analyses, e.g. rela-

tions between sintering and melting characteristica and the detailled chemical/miner-

alogical analysis by CCSEM. This kind of presentation also might enable better and faster evaluations of effects of coal blending. An example is shown in Fig. 8, where the sintering, melting and viscosity analyses are shown for two coal types and a blend of these. In this case the ash fusion temperatures of the blend is between those for the individual coal types. The same accounts for the viscosity, whereas the sintering and melt % analyses deviate from this

tendency. Other coal blends behave very differently from this picture.

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Some of the abovementioned analyses also can be of great value with respect to mechanistic modelling. CCSEM analyses of chemical and mineralogical composition and particle sizes of individual coal minerals are neccessary for modelling of fly ash formation. Measurements of sintering, melting and viscosity of coal ash are applied for the modelling of fly ash and deposit stickiness (fly ash capture), slag flow and strength build-up in deposits. In order to test the empirical predictions and mechanistic models, full scale tests and experiences are neccessary and should be included in further investigations along this line.

5. CONCLUSION A collaborative project between Idemitsu Kosan, Japan and Elsam, Denmark on advanced characterization of coal ash has been carried out. A worldwide suite of 9 coals, 3 coal blends and 3 fly ash samples were analyzed by traditional and advanced analyses. CCSEM is valuable as it can enable insight in chemical and mineralogical composition and grain size of individual coal minerals. These informations are important for gaining a better understanding of the processes and also in mechanistic modelling of fly

ash formation. Comparison with traditional bulk chemical analyses and with XRD analyses on low temperature ashed coal samples shows, that CCSEM is reasonably good for chemical and mineralogical analyses, but could probably be improved significantly by

upgrading to include ZAF correction. Comparison of fly ash particle size distribution measurements by CCSEM and by laser diffraction shows, that CCSEM can not analyze particle size distribution reliably below appr. 5 µm, but it is possible, that with proper correction enabled with laser diffraction analysis, the CCSEM can measure realistically above 5 µm, but this needs further verification. In case of succes it might also be possible to analyze particle size distribution of coal minerals, which is of great importance for mechanistic modelling of fly ash formation. Measurements of coal ash characteristics related to sintering, melting and viscosities were compared and further studies of these, together with comparison with the CCSEM analyses might enable a better understanding of processes in heating up and cooling of the coal ash in the boilers. Sintering characteristics were analyzed as linear shrinkage rate of pressed pellets by High Temperature Light Microscopy. The different coal types showed very different sintering characteristics, which is of importance for strength build-up in ash deposits and removal of deposits. STA analyses were carried out on all coal samples and a melt fraction as function of temperature was calculated. Melt quantification at the final high temperature of the analyses has been verified by SEM analyses of the STA ashes. Comparison with traditional ash fusion temperatures shows different relations between ash fusion temperature and melt % for low melting and high melting ashes respectively. The reasons for this are not yet clarified. Melt quantification is important in evaluations of fly ash deposition on heat transfer surfaces. Viscosities of the coal ashes has been analysed and compared with different viscosity models. The Urbain model generally gave satisfactory estimates, whereas the other

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models showed poorer performances. This has importance for modelling of fly ash deposition on heat transfer surfaces and slag flow characteristics. Coal blends respond very different to the applied analyses and it is possible, that

the combination of the traditional and advanced analyses can enable a better understanding and prediction of coal blend performance. In order to test the empirically predictions and mechanistic models, full scale tests and experiences are neccessary and should be included in further investigations along this line.

REFERENCES Bottinga, Y. & Weill, D.F. (1972). “The Viscosity of Magmatic Silicate Liquids: a Model for Calculation.” American Journal of Science, 1972, 272, 438–475. Coin, C. D.A., Kahraman, H. & Peifenstein, A.P., (1996). “An Improved Ash Fusion Test”. In: Baxter & deSollar (eds.) “Applications of Advanced Technology to ash-Related Problems in Boilers”, Proceed-

ings Engineering Foundation Conference, July 16–21, 1995, Waterville Valley New Hampshire, 187–200. Couch, G. (1994). “Understanding slagging and fouling in pf combustion”. IEA Coal Research/72, 118pp.

Frandsen, F.J., Hansen, L.A., Dam-Johansen, K. & Laursen, K. (1997). “Elsam-Idemitsu Kosan Cooperative Research project: STA- and SEM/EDX-Analyses of Low temperature Coal Ashes.” Department of Chemical Engineering, Technical University of Denmark, CHEC Report, August 1997.

Greenberg, S. (1984). “Viscosity of Synthetic and Natural Coal Slags.” Project Review METC 1984, Viscosity. Hansen, L.A. (1997). “Sintering of Ash Deposits”, Ph.D.-Thesis, Department of Chemical Engineering, Technical University of Denmark, To be finished November 1997. Hansen, L.A., Frandsen, F.J., & Dam-Johansen, K. (1997). Ash Fusion Quantification by Means of Thermal Analysis. Paper presented at the Engineering Foundation Conference “Impact of Mineral Impurities in

Solid Fuel Combustion”, Kona, Hawaii, November 2–7, 1997.

Kalmanovitch, D.P. & Frank, M. (1988) “An Effective Model of Viscosity for Ash Deposition Phenomena.”

in Bryers, R.W. & Vorres, K.S.: Mineral Matter and Ash Deposition from Coal, Engineering Foundation Conference, Santa Barbera, California, February 22–26, 1988, p.89. Laursen, K. (1997a). “Characterization of Minerals in Coal and Interpretations of Ash Formation and Deposition in Pulverized Coal Fired Boilers.” Ph.D.-Thesis, Geological Survey of Denmark and Greenland,

1997. ISBN-87-7871-022-7. Laursen, K. (1997b). “Advanced Scanning Electron Microscope Analyses at GEUS.” Geological Survey of

Denmark and Greenland Report 1997/1, 24pp. Laursen, K., Frandsen, F.J., & Larsen, O.H. (1997). “Ash Deposition Trials at Three Power Stations in Denmark.” Paper presented at the Engineering Foundation Conference “Impact of Mineral Impurities in Solid Fuel Combustion”, Kona, Hawaii, November 2–7, 1997. Streeter, R.C., Diehl, E.K. & Schobert, H.H. (1984) “Measurement and Prediction of Low-Rank Coal Slag

Viscosity.” in: Schobert, H.H.: The Chemistry of Low-Rank Coals. ACS Symposium series 264, p. 195. Teramae, T. (1997). “ELSAM/ldemitsu Coal Research Cooperation—Results and Interpretation of Analyses.”

Idemitsu Internal Report, April 1997. Teramae, T. & Yamashita, T. (1997). “Behavior of Inorganic Materials during Pulverized Coal Combustion.”

Paper presented at the Engineering Foundation Conference “Impact of Mineral Impurities in Solid Fuel Combustion”, Kona, Hawaii, November 2–7, 1997.

Urbain, G., Cambier, F, Deletter, M. & Anseau, M.R. (1981). “Viscosity of Silicate Melts.” Trans. J. Br. Ceram. Soc. 1981, 80, 139 141. Vargas, S., Frandsen, F. & Dam-Johansen, K. (1997). “Elsam-Idemitsu Kosan Cooperative Research project:

Performance of viscosity models for high-temperature coal ashes.” Department of Chemical Engineering, Technical University of Denmark, CHEC Report 9719, August 1997. Watt, J.D. & Fereday, F. (1968). “The flow properties of slags formed from the ashes of British coals: Part I . Viscosity of homogeneous liquid slags in relation to slag composition.” Journal of the Institute of Fuel

1968, 41, 99–103.

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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 Risto S. Laitinen Department of Chemistry, University of Oulu Linnanmaa FIN-90570 Oulu, Finland Bengt-Johan Skrifvars and Mikko Hupa Åbo Akademi University, Combustion Chemistry Research Group Lemminkäisenkatu 14-18 B, FIN-20520 Abo, Finland

1. INTRODUCTION Computer controlled scanning electron microscopy (CCSEM) combined with an energy dispersive X-ray spectrometer (EDS) has widely been used in examining coal and its mineral content [Huggins et al., 1982, Straszheim et al., 1986, 1988, Galbreath et al., 1995]. Automated image analysis (AIA) has many advantages over the manual point analysis or X-ray maps of the sample. This method of making a large number of individual point analyses which produces a vast tabular file demands a sophisticated way of representing the results. One way of doing this is to categorize the point analyses into certain classes in an analogous way to the classification of mineral matter in coal. When inspecting ashes derived from the original fuel, the mineral classification may be questioned, since the

classification may produce information related to the sampling artifacts rather than to a realistic problem. The problems involved in the classification have been circumvented by representing the data graphically, using binary and ternary diagrams [Shah et al., 1988]. During the recent years, fluidised bed combustion has become very popular. At the same time the environmental regulations have become more stringent. Although the combustion of lower rank coals and other low rank fuel like peat and biomass favors the use of fluidised-bed, the technique has its disadvantages. The coating and agglomeration of the bed material may lead to total defluidisation of the bed causing an unscheduled shut down of the boiler. Bed agglomeration and defluidisation have attracted substantial interest during recent years, since defluidisation seems to occur relatively often [Anthony et al., 1995, Nordin et al., 1995, Skrifvars, et al., 1992, Skrifvars, et al., 1997, Kauppinen et al., 1997]. Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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A key process is the formation of coatings on the bed particles. If the properties of the coatings can be identified, it is possible to obtain first hand indications about the bed agglomeration process itself. CCSEM has advantages over conventional analytical methods when investigating the nature of the coating and agglomerates in the bed. The use of image processing and extended ternary diagrams (so called quasi-ternary diagrams) [Heikkinen et al., 1997] provides the possibility of identifying different phases present in the coatings and the adhesive material between the agglomerated bed particles.

2. EXPERIMENTAL 2.1. Sampling Numerous samples were collected from a small pilot-scale (5kW) fluidised bed reactor at ETC Piteå Sweden during an extensive series of controlled agglomeration tests [Nordin et al., 1995]. To eliminate the influence of the particle temperature variation in the bed, every test was started by burning the fuel under examination in the reactor at 600°C u n t i l the bed/ash ratio was 6 wt-%. Subsequent to this “ashing procedure” the fuel

feeding was stopped and the bed temperature was externally raised by 3°C per minute. The onset of bed agglomeration was indicated by measuring differential pressures and temperatures of the bed as well as by video filming the process. The detection of initial bed particle agglomeration by considering all bed related variables simultaneously was performed by an online multivariant statistical process control (MSPC). In all the tests, pure quartz was used as the bed material and the fuel was varied.

During the tests, several different fuels were burned in a bed consisting of pure quartz sand (99.9%). From each test, one sample was taken at the beginning of the external heating, one or two samples during the external heating, and the final sample after

agglomeration occurred.

2.2. CCSEM All bed and agglomerate samples were mounted in epoxy resin, Struers Epofix ( 1 5 : 2 ) . Mounts were cross-sectioned by grinding with 200, 800 and 1,000 mesh SiC

powder in glycerol, and polished with 3 and

diamond pastes. Finally, the samples

were coated with a thin carbon layer to eliminate the electrostatic effects.

The measurements were carried out with a Jeol JSM-6400 scanning electron microscope combined with a Link ISIS energy dispersive x-ray analyzer. The analyzer included the Link ISIS software package. 1 5 k V acceleration voltage and a beam current of 120* A were used. The sample distance was 15mm. The SEM-analysis was started by searching for a suitable field from the sample and collecting a BE- (backscattered electron) image from it at 256*208 resolution. In the BEimage the epoxy is seen a as black background, the bed material as the darkest of the grey areas, and the coatings and adhering material are seen as brighter areas (see Figs. 1 and 2). The original image was smoothed by strengthening the contrast and by removing roughness in order to facilitate further treatment. One or two binary images were extracted from the smoothed BE-image, depending on the nature of the sample. In the case of bed material coatings, only one binary image was extracted, which included the bright areas on the surface of the bed particles. When examining bed

A Novel Application of CCSEM for Studying Agglomeration in Fluidised Bed Combustion

agglomerates with a possibility of obtaining different grey levels on the adhesive material, two binary images were extracted. Depending on the nature of BE-image, there is a possibility of variation in the elemental composition over large areas of a seemingly uniform grey level. Therefore, after producing the binary images, the large areas were divided into smaller domains; the

resulting images are shown in Figs. 1 and 2. This procedure provides information from over two hundred discrete domains over a large area of a coating or adhesive material in

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the agglomerate. For convenience, each of these domains could be considered as different particles, and their elemental composition could be determined separately. The actual determination was performed as a point analysis from the center of each domain. It is clearly seen from the Figs. 1 and 2, that the bed particles are easily excluded in all agglomerated samples. The ZAF-corrected relative amounts of sodium, magnesium, aluminum, silicon, phosphorus, sulfur, potassium, calcium, titanium, and iron were determined at each point. At least one thousand points from about ten fields were analyzed for each sample in order to gain statistical validity.

3. VISUALIZATION OF THE ANALYTICAL RESULTS Ternary diagrams have increasingly been used in the visualization of large number of point analyses [Laursen et al., 1995, Wigley et al., 1995]. The traditional ternary diagrams, however, are limited in their capability to visualize results, when ten elements are

present. To overcome this limitation, so called quasi-ternary diagrams have been used. They are a logical extension to the conventional ternary diagrams. In a corner of a conventional ternary diagram, the amount of an element is 100%,

and in the opposite side it is 0%. In a quasi-ternary diagram two or more elements can

be placed on one corner, and therefore, the sum of the contents of these elements in that corner is 100%. For a point analysis to appear in the diagram, the sum of the contents

of the chosen elements have to exceed a minimum limit, usually 80%. This means, that if the point analysis appears in a corner marked it contains at least 80% Si and Al. The relative amounts of silicon and aluminum, however, is not fixed as long as the

sum exceeds 80% and there can be up to 20% of other elements. If the point is exactly halfway towards the opposite side, the combined content of Si and Al is within a range of 46%–50%. Let us next consider a side of a quasi-ternary diagram, where

is marked on

the other end and a combination of is on the other (see diagram (b) in Fig. 3). Along the side, the sum of the contents of these six elements is 80-100%, and the contents of the elements on the third corner ( in diagram (b)) is 0%. The rest (up to 20%) consists of the analyzed elements that are not shown, e.g. P and S in the case of the diagram (b) of Fig. 3. A series of these diagrams is shown in Fig. 3. They show a distribution of 1,037 point analyses of a real sample. In the first diagram [Fig. 3(a)] all the ten analyzed elements are included in the corner definition and all point analyses are shown in the diagram. The corners are defined in order to deduce the presence of silicates or aluminosilicates, phosphates and sulfates. Because the distribution of point analyses does not spread towards the phosphorus and sulfur corner, it can be inferred, that the analyzed matter in the sample does not contain significant amounts of phosphates or sulfates. All the maxima in the diagram are along the side between the and corners, this implies the presence of silicates or aluminosilicates,

where there is none or a very small amount of phosphorus or sulfur present. There is also a maximum with a composition of 50–60% of and a smaller maximum with 75–90% of these will be discussed below. Since there are no phosphates or sulfates in the sample, phosphorus and sulfur are omitted in the next diagram. The corners in the diagram (b) of Fig. 3 are now defined in such a way as to reveal the presence of iron or titanium in the silicates and alumi-

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nosilicates. The majority of the particles still appear along the side between and corners, but in few cases these silicates or aluminosilicates contain small amounts of iron or titanium. Also there are a few point analyses exhibiting mainly iron or titanium. It should be noted that results from three of the analyzed points are not present in this diagram. These three points obviously contain at least 20%

In the next diagram [Fig. 3(c)] the corners are redefined again. The arrangement is now chosen to separate silicates and aluminosilicates. Referring to the previous diagram [Fig. 3(b)], the amount of iron and titanium was found to be negligible, and only six elements (Si, Al, Na, Mg, K, and Ca) are chosen for the diagram (c). The diagram still includes the majority of the analyzed points (1,015 of 1,037) according to the 80% rule. Now the diagram reveals the presence of a minor aluminosilicate phase, but the majority of points do not contain aluminum.

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The elements in the corners are rearranged to examine the relation between alkali and alkaline-earth metals. In the diagram (d) silicon and aluminum are combined on one corner again, alkali metals are on the second corner, and alkaline-earth metals on the third. In this diagram three major phases are easily distinguished. In order to identify maxima 1 and 3, one more diagram [Fig.3(e)] is drawn, where the only difference to diagram (d) is the absence of aluminum. By comparing the diagrams (d) and (e), the maximum 1 can be identified as silicate and maximum 3 as aluminosilicate. In fact, the silicate phase can be presented with only three elements: silicon, sodium, and calcium. This is illustrated in the diagram (f) in Fig. 3, where 76.6% of analyzed particles are included (i.e. in 76.6% of analyzed particles The maximum 2 appears to be pure silicon derived from the bed material, as discussed below. It is also possible to estimate the amounts of different elements in a given phase. The diagram (f) in Fig. 3, shows the Na, Ca- silicate phase. The maximum in the diagram can be defined within the limits of and and when the possibility of having 20% of other elements (80% rule) is considered, the real limits for this phase can be expressed as: and

After a graphical inspection it is possible to go back to the original tabular file and extract the rows (point analyses) that belong to the major phases. By estimating the range

of different elements, criteria for extraction of information can be produced. Criteria would be defined by the ranges of different elements determined above and is analogous to the mineral classification. Closer examination of the extracts can reveal the role of minor elements like phosphorus or sulfur in the sample, which is not negligible in the formation of coatings and agglomerates.

4. DISCUSSION The CCSEM-technique is well suited for the characterization of bed material coatings and agglomerates. Using this method, more than a thousand points can be analyzed from the adhesive material of the agglomerates, as well as from coated bed particles. The quasi-ternary diagrams always show a peak of pure silicon, which implies quartz in the sample. This is a consequence of the use of the automated image processing. In some cases, the smoothing of the grey scale image can result in the inclusion of the smallest quartz particles in the adhesive material. Another reason is the relative roughness of the binary images, when the domain boundaries are not accurate enough. These problems can be solved by scanning the BE-image at a greater resolution (512*416 or 1024*832), but then the time required for image scanning and processing is radically extended. In all samples tested in this work, the bed material was easily excluded, and the division of the large areas of adhesive material into smaller domains was always successful. In the case of bed material coatings, however, the following considerations should be taken into account. The bed material coatings could only be analyzed, if they were thick enough. Then the bed particles could easily be separated and the determinations then focused on the coatings. A very thin layer of coating on the bed particles was hard to distinguish from its grey level in the BE image, resulting in difficulties in the image processing. Further,

the determinations were not focused solely on the coatings, but also on the bed particle itself, giving rise to the large amounts of silicon.

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The extension of traditional ternary diagrams to quasi-ternary diagrams is very informative. By defining the corners in terms of all elements, these diagrams can include all analyzed points. Furthermore, by systematically excluding elements from the corners, it may be possible to deduce the phases present in the sample. Some elements like iron and phosphorus can be important in the agglomeration process, even in small amounts. Thus, they have to be observed even in small quantities and this may require a more rigorous inspection of the original data files.

5. CONCLUSIONS CCSEM-technique is a valuable method when characterizing the bed material coatings and agglomerates. It is comparatively easy to carry out more than a thousand point

analyses and obtain valid statistical information by using automated image processing. The use of quasi-ternary diagrams provides a good insight into the nature of the sample and can even reveal the phases present in a coating or an agglomerate.

6. ACKNOWLEDGEMENTS This work was performed under LIEKKI 2-program, which was financed by the Ministry of Trade and Industry. We would also like to thank Dr Anders Nordin and Mr

Marcus Öhman ETC/Piteå for bed samples.

REFERENCES Anthony, E. J., Iribarne, A. P. and Iribarne, J. V. (1995). “A new mechanism for FBC agglomeration and fouling when firing 100% petroleum coke.” Proc. of the 13th ASME FBC Conference, Vol. 1, Orlando, Florida. Galbreath, K., Zygarlicke, C., Casuccio, G., Moore, T., Gottlieb, P., Agron-Olshina, N., Huffman, G., Shah, A., Yang, N., Vleeskens, J. and Hamburg, G. (1995). “Collaborative Study of Quantitative Coal Mineral Analysis using Computer Controlled Scanning Electron Microscopy.” In L. L. Baxter and R. DeSollar (Eds.), Application of Advanced Technology to Ash-Related Problems in Boilers, New York, Engineering

Foundation. Heikkinen, R., Laitinen, R. S., Patrikainen, T., Tiainen, M. and Virtanen, M. (in press). “Slagging Tendency of Peat Ash.” Fuel Processing Technology.

Huggins, F. E. Huffman, G. P. and Lee, R. J. (1982). “Scanning Electron Microscope-Based Analysis (SEMAIA) and Mössbauer Spectroscopy, Quantitative Characterization of Coal Minerals.” In E. L. Fuller, Jr (Eds.), Coal and Coal Products: Analytical Characterization Techniques, Washington DC, American

Chemical Society. Kauppinen, E. (1997). “Tuhkan muuntuminen Leijukerroskaasutuksessa ja -poltossa: Haitallisten hivenmetallien vapautuminen ja alkalien käyttäytyminen.” In M. Hupa, J. Matinlinna and M. Ljung (Eds.), LIEKKI 2 Year book 1997, Turku, Åbo Akademi Förbränningskemiska forskargruppen. Laursen, K., Frandsen, F. and Larsen, O.H. (1995). “Slogging and Fouling Propensity: Full-Scale Tests at Two Power Stations in Western Denmark.” In L. L. Baxter and R. DeSollar (Eds.), Application of Advanced Technology to Ash-Related Problems in Boilers, New York, Engineering Foundation. Nordin, A., Öhman, M., Skrifvars, B.-J. and Hupa, M. (1996). “Agglomeration and Defluidization in FBC of Biomass Fuels—Mechanisms and Measures for Prevention.” In L. Baxter and R. DeSollar (Eds.) Application of Advanced Technology to Ash-Related Problems in Boilers, New York, Engineering Foundation. Skrifvars, B-J., Backman, R. and Hupa, M., (in press). “Characterisation of the sintering tendency of ten biomass ashes in FBC conditions by a laboratory test and by phase equilibrium calculations.” Fuel Processing Technology.

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Skrifvars, B-J., Hupa, M. Anthony, E. J. (1997). “Mechanisms of bed agglomeration in the cyclone and the return leg of a petroleum coke fired circulating fluidised bed boiler.” Proc. of the 14th ASME FBC Conference, Vol. 2, Vancouver, Canada, May 1997. Skrifvars, B-J., Hupa, M. Hiltunen, M. (1992). “Sintering of Ash During Fluidized Bed Combustion.” Ind. Eng. Chem. Res 31(4), 1026–30. Shah, N., Huffman, G. P., Huggins, F. E. and Shah, A. (1988). “Graphical Representation of CCSEM Data for Coal Minerals and Ash Particles.” In S. A. Benson (Eds.), Inorganic Transformation and Ash Deposition During Combustion, New York, The American Society of Mechanical Engineers. Straszheim, W. E., Yousling, J. G. and Markuszewski, R. (1986). “Analysis of Ash-Forming Mineral Matter in Raw and Supercleaned Coals by Automated Image Analysis-Scanning Electron Microscopy.” In K. S.

Vorres (Eds.), Mineral Matter and Ash in Coal, Washington DC, American Chemical Society. Straszheim, W. E. and Markuszewski, R. (1988). “Characterization of Mineral Matter in Coal for Prediction of Ash Composition and Particle Size.” In S. A. Benson (Eds.), Inorganic Transformation and Ash Deposition During Combustion, New York, The American Society of Mechanical Engineers. Wigley, F. and Williamson, J. (1995). “Modeling Fly Ash Generation for UK Power Station Coals.” In L. L. Baxter and R. DeSollar (Eds.), Application of Advanced Technology to Ash-Related Problems in Boilers, New York, Engineering Foundation.

THERMOMECHANICAL ANALYSIS AND ALTERNATIVE ASH FUSIBILITY TEMPERATURES S. K. Gupta, R. P. Gupta, G. W. Bryant, L. Juniper 1 and T. F. Wall CRC for Black Coal Utilisation Department of Chemical Engineering University of Newcastle Callaghan, NSW, 2308 Australia 1 Ultra System Technology Indooroopilly QLD, 4068, Australia

1. INTRODUCTION Ash deposition on furnace walls in pf (pulverised fuel) fired boilers is termed slagging when it occurs in the high temperature areas of furnaces directly exposed to flame radiation, and fouling in other regions such as tubes in the convection section of the boiler. These deposits not only reduce the heat transfer efficiency, but they can lead to corrosion and significant damage to the boiler tubes. Therefore, the nature of slagging properties of ash derived from coal is very crucial for the boiler designer and consequently for the coal exporter. The ash fusibility test is the most widely used method of assessing whether a coal ash will deposit on the heat transfer surfaces of boilers. The ash fusibility test is based on observing the temperatures at which successive characteristic stages of fusion occur in a moulded ash specimen when heated in a laboratory furnace under specified conditions. For instance, the Australian Standard involves heating of a cone of laboratory ash (prepared at 815°C in a muffle furnace) at approximately 5°C to 10°C/minute from 1,000°C to 1,600 °C. Four temperatures that are recorded in the conventional ash fusibility test which are believed to indicate different stages of melting are given below and shown in Fig. 1. • The deformation temperature (DT) is noted when ash just begins to fuse or shows the first sign of deformation or rounding of the apex of the cone. • The sphere temperature (ST) is recorded when the height of the ash becomes equal to the width of sample. • The hemispherical temperature (HT) is noted when the height of fused ash becomes equal to half of its width. Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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• The final temperature, known as flow temperature (FT) is noted when the height

becomes a sixteenth of the width. I t may be noted that terminology, sample preparation and configuration for this test

differ from country to country. The deformation and sphere temperature are normally used in coal specifications in the belief that these temperatures may indicate ash behaviour in the furnace. For instance, deformation temperature has been accepted as the temperature where the ash first softens and therefore becomes sticky. The hemispherical and flow temperatures are considered invariably by most of the design engineers to avoid slagging,

whereas deformation, hemispherical and flow temperatures are believed to be more important to control fouling [Barret, 1987]. There are well-documented shortcomings of poor precision of ash fusibility temperatures, which is mainly related to the subjective nature of the test [Juniper, 1995]. Therefore, there is a strong need for an alternative method to char-

acterise the ash fusibility of coal, which would have greater reliability of the measurements. To achieve this objective, a new technique-thermo-mechanical analysis (TMA) has been

developed. Thermo-mechanical analysis is frequently used in the metallurgical and ceramic industries to investigate thermal behaviour of materials at high temperatures. Many studies have been reported in past, which used changes in electrical resistance and shrinkage properties to characterise the melting characteristics of coal ashes [Raask, 1979, Ellis. 1979, Doolan, 1984 and Sanyal, 1993]. However, none of these studies could provide a satisfactory alternative. The TMA technique utilised a similar approach but in a different manner. The technique measures the percentage shrinkage and first derivative of the shrinkage as

a function of temperature. The main aim of this paper is to propose alternative ash fusibility temperatures from the TMA shrinkage measurements so that various stages of ash melting can be reliably characterised.

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2. EXPERIMENTAL The ash fusibility was characterised using thermomechanical analysis of laboratory ashes of a large number of thermal coals from Australia, USA, UK and other countries. The sample selection was primarily based on ash chemistry. The new technique involved the measurement of the penetration of graphite ram into ash (~50 mg) placed in a cylindrical graphite crucible with a flat bottom, and referred as TMA shrinkage. As the assembly containing ash is heated under Argon from room temperature to 1,600°C at a rate of 5°C/minute, the penetrating ram sank into ash and eventually the slag flowed into the gap between the penetrating ram and the base of crucible as shown in Fig. 1 [Saxby et a/., 1996]. The rapid shrinkage events were obtained from the derivative of TMA shrinkage. Various “peaks” (representing the maximum rate of shrinkage with temperature), of various intensity and at progressively higher temperatures, were related to sintering and melting events in the ash sample. No significant shrinkage was observed in most of ashes up to 900°C. The temperatures corresponding to particular shrinkage levels were noted as TS%. Multiple measurements on the same ash indicated that major fusion events (rapid shrinkage) are reproducible to within [Saxby et al., 1996], The ash fusibility temperatures from the conventional test were also obtained under reducing conditions as per Australian Standards. The ash pellets were heated under simulated ash fusibility

test conditions and then quenched rapidly in air from various temperatures of interest. The extent of melting at various temperatures was quantified by visual examination of residual solid particles in the scanning electron microscope (SEM) images of quenched ash pellets.

3. SAMPLE GROUPING It is well known that ratio and different fluxing contents of ash are mainly responsible for differing melting characteristics [Huggins et al., 1981, Bryers, 1996], The extent and ratio of various oxides in ash may approximately relate to the proportion of the original minerals present in coal [Raask, 1986]. For instance, high ratio may indicate the presence of large amounts of as free quartz in ash, which may decrease the temperature range for the completion of melting. The from 1 to 3 wt% in ash may be related to 10 to 30% illite content of the total mineral matter in coal which is quite common in bituminous coals [Raask, 1986]. Therefore, a sample grouping scheme, based on silica/alumina ratio and various fluxing oxides, is designed to understand the TMA shrinkage measurements (Table 1). The melting and crystallisation behaviour of ashes is commonly explained by simplifying the ash compositions to three or four oxides where or [Huggins et al., 1981 and Kalmanovitch, 1986], Therefore, normalised ash composition of some of the ashes is also shown in Fig. 2 based on a ternary diagram (Levin et al., 1964).

3.1. Group A: Ashes with High Silica and Alumina Contents Group A ashes contain very high levels of refractory components and a limited amount of fluxing components. Quartz and clay are the major minerals expected to be present in the parent coals of Group A ashes. The ashes are further divided in to three series according to the difference in extent and nature of fluxing

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components. Series A1 and A2 contain low and high levels as the major fluxing oxide respectively, whereas Series A3 contain ~5 % iron oxide as an additional fluxing component. The TMA shrinkage and melting behaviour of these ashes will be primarily governed by the quartz and clay interactions in ash. Figure 2 shows that composition of these ashes lie in mullite primary phase field of the system and their liquidus temperatures are expected to be high. However, for a given series, liquidus temperature would fall as the ratio of ash increases. These ashes are likely to have large error in observing the deformation temperatures in the existing ash fusibility test [Wall, 1995].

3.2. Group B: Ashes with Moderate Iron and Calcium Oxide Contents Group B ashes are characterised by the presence of reasonable levels of fluxing components so that content is less than 90%. The ashes are further divided

in three series based on different

ratio and various fluxes. Series B1 and B2

contain iron oxide as the major fluxing component, while ratios are greater and less than 2.2 respectively. Series B3 contain both iron oxide and calcium oxide as the fluxing components. In addition to quartz and clay, the pyrite and calcite contents in the parent coal would also affect the melting behaviour of these ashes significantly. The ash composition of most of these samples lie in the mullite primary phase field in phase diagram except for AEC18 (anorthite), OPC03 (hercynite)

and OPC04 (tridymite). Consequently, the melting and hence the TMA shrinkage behaviour of ashes AEC18, OPC03 and OPC04 may be slightly different than the other ashes. The liquidus temperatures of Group B ashes are usually low, however these may be higher if ratios is lower (Fig. 2). The detailed ash composition and fusibility temperatures for all ashes are given in Table 2.

4. EXPERIMENTAL RESULTS AND DISCUSSION

4.1. Typical TMA Results of Ash Groups Figure 3 shows typical TMA shrinkage curves of selected ashes, which contain various levels of fluxing components and silica/alumina ratios. Ash AEC06 (Series A1: high ratio) did not indicate any significant shrinkage up to 1,500°C due to very low levels of fluxing oxides that is whereas, PN6 (Series A2) showed substantial shrinkage at a temperature less than 1,400°C due to high Thick curves show the TMA shrinkage of ashes which contain of iron oxide as the variable fluxing oxide for example AEC12, OPC01 and OPC03 ash. Figure 3 shows that the increasing fluxing levels moves the TMA shrinkage curves to a low temperature region. Figure 4 shows that low shrinkage occurs if melting is low (AEC06: 5% shrinkage and melting) and large shrinkage occurs if extent of melting is high (OPC01: 85% shrinkage and melting). Therefore, it is clear that the TMA technique appears to distinguish the effect of ash chemistry on various amounts of ash melting in the form of shrinkage curves.

4.2. Alternative Ash Fusibility Temperatures Ideally, the ash melting may begin at as low as 950°C due to the presence

which may form low temperature eutectics with

and

and continues up to

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greater than 1,600°C. Practically, the amount of liquid at low temperatures may be very low. SEM analysis of ash pellets indicated that greater TMA shrinkage levels were associated with greater levels of melting (Fig. 4). The melt phase composition of various ashes is quite different for various ashes, and also changes as the melting progresses. Therefore, it was difficult to specify a unique shrinkage level to a unique level of melting. Therefore,

four temperatures from the TMA technique were selected corresponding to shrinkage levels of 25%, 50%, 75% and 90%. These temperatures are proposed as alternative ash fusibility temperatures, which may be considered to represent initial, intermediate, completion of melting and slag flow characteristics for practical purposes. 4.2.1. Initial Melting Temperature: T25%. Figure 5 shows the relationship between extent of melting and various shrinkage levels. SEM examination of ash pellets quenched

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at the T25% temperature indicated the presence of 25% liquid at this stage of melting. In general, the amount of liquid required to cause 25% shrinkage is low for

refractory ashes (Group A) when compared to that required for samples containing reasonable fluxing components (Group B). It was observed that, T25% for Group A samples range from 1,100°C to 1,300°C and for Group B range from 1,050°C 1,150°C. It is

believed that this difference of temperature range, which changes the melt phase viscosity significantly, is mainly responsible for this deviation in predicting the extent of melting. In few ashes, which contain very high refractory oxides (series A1), indicated exceptionally low extent of melting which occurred by softening or sintering of particles as shown in Fig. 4B, and consistent with past observations [Vassilev et al., 1995]. Irrespective of mechanism that is either by sintering or reaction, substantial particle

agglomeration and melting was observed at T25%. The viscosity at the T25% would be sufficient to relate this stage of melting to sticky nature of particles which caused this shrinkage. Therefore, it is reasonable to propose T25% as the initial melting temperature of ash. No specific correlation between T25% and ratio is observed. However, a slight presence of fluxing component, or and CaO, was found to decrease the T25% values significantly. It appeared that at T25%, a small amount of liquid is formed by the low melting or decomposition point minerals (e.g. illite and anhydrite), and is detected in the TMA test due to the high sensitivity of measurement. The liquid formed at this stage is not sufficient to cause a visible physical change, and therefore can not be observed in a conventional ash fusibility test.

4.2.2. Intermediate Melting Temperature: T50%. T50%, defined as the temperature related to 50% shrinkage, corresponds to approximately 60% melting as shown in Fig. 5. Group A ashes indicated a high temperature range for T50%, being greatest for series A ashes that contain least amounts of fluxing oxides. For the Group B ashes, T50% occurred at low temperature and was associated with a higher extent of melting in most of the ashes. In general, ashes with higher ratio and higher fluxing contents would give lower values of intermediate melting temperature.

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4.2.3. Melting Temperature: T75%. For all practical purpose, 80% (mass fraction) or greater extent of melting may be considered as an approximation for the completion of melting. Therefore, T75% is proposed as the melting temperature of ash for practical purpose. The SEM examination of ash pellets quenched at T75% indicated the presence of 80% melt phase i.e. 80% mass fraction of the particles were melted (Fig. 5). In subsequent discussion, the melting temperature will refer to this state of melting. For Group A ashes (refractory ashes) T75% did not occur up to 1,600°C except for the ashes with high ratios (>3.5). Generally, the T75% for Group B ashes was low, ranging from 1,100°C to 1,600°C. Similarly to T50%, the T75% is also influenced by the ratios and amounts of fluxing components. Ashes with high ratios will result in low values of T75% for Group A ashes. For iron rich ashes from Group B, the significant fluxing effect of iron oxide appears to be responsible for very low values of T75%. The results will be further discussed in section 4.3. 4.2.4. Flow Temperature: T90%. T90% is proposed to represent the final stage of melting (melt phase >80%) and to indicate the slag flow characteristic of ash. Flow temperatures of Group A ashes were found to be usually greater than 1,600°C except for AEC10 and EN3 ashes, which contain significant amount of In general, the T90% of Group B ashes were less than 1,600°C. T90% decrease consistently with an increase in ratio and fluxing levels except for some ashes that contain very high concentration of fluxing components (especially CaO).

4.3. Comparison of Alternative TMA and Existing AFT Temperatures T25% and DT represent different events in the process of ash sintering and melting. T25% indicates the presence of small amounts of liquid phase, which can be detected by the sensitive penetrating ram. At the deformation temperature, an extensive melt phase is expected to cause a visible change in the shape of ash specimen. Therefore, the initial melting temperature from the TMA test did not find any equivalent temperature in the existing ash fusibility test. T25% was invariably less than the deformation temperature.

4.3.1. Intermediate Melting Temperature (T50%) and Deformation Temperature (DT). The Australian Standard [1995] allows a maximum acceptable difference of up to for repeatability and up to for reproducibility of deformation temperature. In practice, this difference was found to vary up to 300°C for some ashes [Wall, et al., 1995]. The deformation temperature for all the ashes vary from 1,100°C to 1,600°C, and is higher for group A ashes. In general, more than 60% melting was observed at the deformation temperature, however in exceptional cases, it might vary from 20% to 75% as shown in Fig. 4. Similar melting range has also been related to deformation temperature in past. [Huffman, 1981; Vassilev et al., 1995]. The T50% and deformation temperature were observed to represent a similar range of melting, which is approximately 60% as shown in Fig. 5. In addition to precision problem associated with the observation of deformation temperature, it does not represent the beginning of ash melting. Therefore, the common belief that deformation temperature is associated with surface stickiness may not be totally justified [Ely and Barnhart, 1963]. Figure 6 shows that apparently there is no direct correlation between deformation temperature and intermediate temperature from the TMA technique. However, in more than 60% of ashes, T50% was observed

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within

of the deformation temperature. T50% of some refractory ashes (CaO + < 10%) were very high compared to their respective deformation temperature and very low for a few Group B ashes which contain combined calcium and iron oxides greater

than 10%. The exact reason of this deviation is not clear at this moment. It appears that this deviation is not related to the normal inaccuracies associated with measurements of deformation temperature. However, the T50% from the TMA technique would still be a

better choice for measuring the intermediate melting due to a greater precision of repeatability 4.3.2. T75% and Hemispherical Temperature. The physical changes in ash specimen that occur at hemispherical are clearly visible, and are often used to indicate the completion of melting for practical purposes. SEM analysis of ash pellets in present study also indicated the presence of approximately 80% liquid at hemispherical temperature and T75%. Figure 7 compares T75% and hemispherical temperature. Ashes, OPC01 and AEC09, indicate very low values of T75%, which may be due to the high levels of fluxing components. Figure 8a and 8b show the effect of iron oxide on T75% and hemispherical temperatures for series Bl and B2 ashes, which have their ratios greater and less than 2.2 respectively. Both temperatures are found to decrease with an increase in iron oxide content provided iron oxide is less than 35%. The ash composition of samples, which contain iron oxide greater than >35%, is shifted to hercynite primary field that results in higher liquidus temperature. T75% of series Bl ashes is usually higher than series B2 ashes. In general, T75% and hemispherical temperatures followed the liquidus temperatures from phase diagrams (Fig. 8). Therefore, it is reasonable to define the T75% as the melting temperature of ash for a practical purpose.

4.3.3. T90% and Flow Temperature. Figure 9 shows that the correlation between T90% and the flow temperature is least satisfactory. For instance, T90% is very high for ashes

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that contain very low levels of fluxing oxides from Group B (e.g. AEC11 and AEC18) and very low for those containing high levels of fluxing contents when compared with

their respective flow temperatures. T90% is very low especially if CaO content in ash high (series B3). This large deviation may be related to the large error associated with measurements of flow temperature or due the amphoteric nature of iron oxide that may not affect the fluidity of the melt consistently, and hence the T90% values. The T90% may be used to measure the slag flow characteristics of the ash.

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5. CORRELATION OF MELTING WITH TMA AND AFT TEMPERATURES In the preceding section, it was shown that substantial melting occurred at the deformation temperature as the sample matrix did not deform due to the high viscosity of silicate slag. Therefore, any observer has to wait to record the first sign of deformation till an extensive melting is developed in the sample. Sphere, hemispherical and flow temperatures from conventional test are further believed to denote a similar degree of

melting, and differing only in the viscosity of the melt phase [Huffman, 1981 and Bryers, 1996]. The subjective nature of conventional test makes it further difficult to assign a specific level of melting with various ash fusibility temperatures.

The progressive shrinkage in the TMA technique basically measures the ease of relative movement of ash particles and the penetrating ram. At low temperatures, the ram movement into ash is primarily affected by various stages of ash melting i.e. sintering,

decomposition of clays and chemical reactions among various fluxing oxides. At higher temperatures, the TMA shrinkage is more dominated by the melt composition that changes by the continues dissolution of the refractory components. Therefore, TMA shrinkage depends on the proportion of melt, melt composition and temperature. The relationship between the TMA shrinkage and extent of melting can be expressed as

where

K is defined as the constant of proportionality, and is a function of ash chemistry and temperature. Evaluation of the extent of melting indicates that K is greater for refractory ashes compared to other ashes those contain reasonable amounts of fluxing oxides. Thus, we can see that the TMA technique has a potential for quantifying the various levels of melting with an improved accuracy, which is a crucial requirement for a reliable prediction of ash deposition tendencies of coal. For instance, in a pf combustion system, the proportion of melt phase from any ash fusion test would indicate the proportion of molten particles in the furnace at a particular temperature. Therefore, the T25%, which is significantly less than the existing deformation temperature, would be the temperature

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at which sufficient proportion of particles might be sticky and cause ash deposition

problem. It is worthwhile mentioning that slagging performance of some pf combustion systems indicated a good correlation with this level of shrinkage [Wall et al., 1995). T90%

is expected to indicate the slag flow characteristics of ash and hence provide a basis for another temperature of interest often required in other applications such as entrained flow slagging gasifier. For instance, T90% may be related to the temperature at which

ash will transform to a completely molten layer in a gasifier. However, the exact viscosity at which this molten layer may flow is not clear. The correlation of the extent of melting with the extent of sticky particles and slag viscosity in various combustion systems will enable the new temperatures to characterise slagging and fouling with improved reliability.

6. CONCLUSIONS The traditional measurement of the temperatures of fusibility of coal ash, which are called ash fusibility temperatures (AFT), are shown in correspond to the existence

of more than 60% of melt phase in the samples. These temperatures do not appear to correspond to the ash melting characteristics often associated with them. It was found that the deformation temperature is not the temperature at which initial melting begins as normally perceived and the hemisphere temperature is below the liquidus temperature. A new technique—Thermomechanical Analysis (TMA)—measures the progressive shrinkage of ash and is shown to be capable of characterising the sintering and melting behaviour at temperatures lower than the traditional technique. Shrinkage temperatures may be defined which corresponding to particular shrinkage levels are denoted as TS% (for S of 25%, 50%, 75% and 90%), and these are proposed as alternatives to the existing ash fusibility temperatures. The new temperatures are measured with much better accuracy than the traditional temperatures and are suggested as indicators of initial melting (25%), intermediate melting (60%), completion of melting (80%) and slag flow characteristics. Correlation between the alternative shrinkage temperatures, the traditional ash fusibility temperatures and the measured extents of melting indicated that the shrinkage temperatures provide an improved and objective method of quantifying the various stages of melting. The correlation of the extent of melting with the extent of sticky particles and slag viscosity will allow the application of the new shrinkage temperatures to characterise ash effects in combustion systems.

7. ACKNOWLEDGMENTS The authors wish to acknowledge the support of the CRC for Black Coal Utilisation which is funded in part by the CRC Program of the Commonwealth of Australia. We are most grateful to Mr. Dick Sanders of Quality Coal Consulting Pty Ltd for managing the project, selecting samples and providing insights in to the AFT procedures. The study is also supported by the Australian Coal Association Research Program with Mr Grant Quinn of BHP Coal Pty Ltd as industry monitor. The assistance of Dr John Saxby at CSIRO and Dr R. A. Creelman is greatly appreciated.

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8. REFERENCES Australian-Standard AS1038-15-1995. Coal and Coke Analysis and Testing, Part 15: Higher Rank Coal Ash and Coke Ash Fusibility. Barret, R. E. (1987). “Slagging and fouling in pulverised-coal-fired utility boilers—Volume 2: A survey of boiler design practices for avoiding slagging and fouling”, EPRI Report CS-5523. Bryers, R. W. (1996). “Fireside slagging, fouling and high temperature corrosion of heat transfer surface due to impurities in steam raising fuels”, Progress in Energy Combust. Science, Vol. 22, pp. 29-120. Ellis, G. C. and Ledger, R. C. (1989). “The Thermomechanical, electrical conductance and chemical characteristics of coal ash deposits”, In “SECV (R&D) Report SO/89/164, NERDDP Project No. 1181, SECV Project No. 2411, Australia. Ely, F. G. and Barnhart, D. H. (1963). In: H. H. Lowry (Ed.), Chemistry of coal utilisation, Supplementary volume, Wiley, New York, p. 820. Huggins, F. E., Kosmack, D. A. and Huffman, G. P. (1981). “Correlation between ash-fusion temperatures and ternary equilibrium phase diagrams”, Fuel, 60, 577–584. Huffman, G. P., Huggins, F. E. and Dunmyre, G. R. (1981). “Investigation of the high-temperature behaviour of coal ash in reducing and oxidizing atmospheres”, Fuel, 60, 585–597. Juniper, L. (1995). “Applicability of ash slagging “Indices” Revisited”, Australian Combustion Technology Centre, February, Combustion News p. 1–4. Kalmanovitch, D. P. and Williamson, J. (1986). “Crystallization of coal ash melts”, In Karl Vorres (Ed.), Mineral Matter and Ash in Coal, ACS Symposium series 301, August, 1984, pp. 234-255. Levin, E. M., McMurdie, H. F. and Hall, H. P. (1964), “Phase Diagrams for Ceramists” Am. Ceramic. Soc., Inc., Columbus, Ohio and references therein. Raask, E. (1979). “Sintering characteristics of coal ashes by simultaneous dilatometry-electrical conductance measurements”, Thermal Analysis, 16, p. 91. Raask, E. (1986). “Flame vitrification and sintering characteristics of silicate ash”, In “Mineral Matter and Ash in Coal”, ACS Symposium series 301, in (Ed. Karl Vorres), p. 139. Sanyal, A. and Mehta, A. K. (1994). “Development of an electrical resistance based ash fusion test”, In Williamson, J. and Wigley, F. (Ed.), The Impact of Ash Deposition in Coal Fired Furnaces, Taylor and

Francis, Washington, p. 445. Saxby, J. D. and Chatfield, S. P. (1996). “Fusion of coal ash by thermo-mechanical analysis”, Proceedings of the 7th Australian Coal Science Conference, Australian Institute of Energy, p. 391, Australia. Vassilev, S. V., Kitano, K., Takeda, S. and Tsure, Tl. (1995). “Influence of mineral and chemical composition of coal ashes on their fusibility”, Fuel Processing Technology, Vol. 45 pp. 27–51. Wall, T. F, Creelman R. A., Gupta, R. P., Gupta, S. K, Sanders, R. H. and Lowe, A. (1995), “Demonstration of the true ash fusion characteristics of Australian thermal coals”, ACARP Project Final Report C3093, Australia.

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ASH FUSIBILITY DETECTION USING IMAGE ANALYSIS Klaus Hjuler dk-TEKNIK Energy & Environment Gladsaxe Moellevej 15 DK-2860 Soeborg

Denmark

INTRODUCTION In the recent years the use of biofuels for power production has gained increasing

importance as a substitute for coal or by being co-fired with coal. This development is particularly forced by the international concern about antropogeneous carbon dioxide emissions. However, the use of biofuels for steam raising or in gas turbine cycles is so far restricted by the fact that biofuels generally have a higher content of potentially deposit

forming and corrosive elements than coal, especially on a heat value basis. The international standard method of estimating the deposit propensity of solid fuels, of which a number of variants exist (e.g. ISO, ASTM, AS, DIN), was originally proposed to estimate the suitability of a particular coal for grate firing. The result from the test is valuable for comparing new coal qualities with known coals that behaves satisfactorily in a specific plant. However, the standard fusion test has shown to be unsuitable for ashes from biomass fuels as the ash test body may “blow up” like a baloon or melt may flow out from it without overall changes in shape, leaving a “skeleton” composed of e.g. silicon and calcium. This complicates the interpretation and reporting of the test, especially for automatic equipments where the characteristic temperatures are determined only from the height and width of the test specimen. Moreover, the standard fusion test is more or less based on a subjective evaluation of the change in shape—i.e. the silhouette—of a test body (cone or cube) while this is being heated. Consequently the reproducibility as well as the repeatability is poor; even a skilled operator cannot obtain a repeatability better than about (ISO/TC, 1991). Another serious problem is that the appearance of the first melt is not detected because it takes place “inside” the test body and does not necessarily affect the shape of the body. The appearance of the first melt is important because the presence of a molten phase increases the probability of ash sticking significantly. Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al.

Kluwer Academic / Plenum Publishers, New York, 1999.

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The literature has been reviewed elsewhere [Coin et al., 1996; Hansen et al., 1997; Wall et al., 1996] and is therefore not discussed in this paper. However, it should be mentioned that it is well known that visual observation of an ash sample using light microscopy and a heating stage may reveal details in the process of ash shrinkage and melting during heating (e.g. [Vassilev et al. (1995]). Similar equipment is used in metallurgy for studying recrystallization and sintering processes, where the sample are observed using incident or transmitted light or a combination of both. The main task of the present work has been to develop a method to quantify the information.

EXPERIMENTAL

Apparatus The apparatus is commercially available: the microscope consists of an Olympus SZ 1,145 TR stereo microscope body with eyepieces for magnification, a base illuminator, and a fibre optic illuminator. The camera is a Sony RGB XC-711 CCD and the heating stage is a Leitz 1350, which is suitable for examinations in transmitted and incident light (Fig. 1). The heating stage is powered by a Heinzinger LNG 16–30

power unit. The image grabber is a Neotech colour video digitizer for personal computers. The software was programmed specifically for the image analysis and data acquisition, and for the operation of the heating stage.

Samples More than 80 samples have been subjected to the HTLM fusion test. As the main focus in the present work was posed on developing a method which is well suited for

ashes from biofuels, mainly fly ash, bottom ash, and deposits from straw firings have been investigated as well as laboratory prepared biofuel ash. Repetitions of runs with the above mentioned samples and tests with an inert (below 1,250°C) quartz fiber sample, a geological standard, and analysis quality salt mixtures have also been performed. All runs were performed at a rate of 10°C/min in a nitrogen atmosphere.

Analysis Procedure Before ashing, coal samples are milled to a maximum particle size of whereas biofuels are milled to a particle size of less than The milled fuel sample is ashed in a Pt-boat in air at 815°C for coal and at 550°C for biofuels. The residual

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carbon content of laboratory prepared ash is lower than 5%(w/w). Ash samples originating from a combustor are normally glown for determination of the residual carbon content, that is, the glown ash is used for fusion testing. Fly ash samples do normally not require milling, while most other ash types do. The maximum particle size should be less than about in order to observe a representative number of particles/agglomerates. Coals are milled to a maximum particle size of in an agate mortar as described in the standard ISO 540 method. Ashes from biofuels are milled in the same way as coal ashes. The quantity of sample needed is small, typically about (or about 1– ). In special cases information about the local melting behavior of eg. a deposit is needed. In this case a small piece, say may be broken off and analysed directly. The sample is placed randomly on a 7mm diameter sapphire specimen disc by means of e.g. a small spatula. The sapphire disc with sample is placed in the sample holder of the heating stage. When a glass cover is mounted the heating stage is gas tight and can be operated in a controlled atmosphere (normally nitrogen). The microscope is focused on a random part of the ash, the magnification is set at typically and the light conditions are optimized. The digital camera and the heating stage is operated from a personal computer. The temperature of the heating stage is ramped at typically 10°C/min from an initial temperature of 550°C for ashes from biofuels and 800°C for coal ashes. The ash sample is photographed initially and then every fifth second (0.2Hz). The image acquisition rate

can be adjusted according to the rate of heating/melting, but 0.2Hz has been found appropriate for 10°C/min. Each image is processed before the next image is acquired. The results are written to a text file. The power for the heating stage is automatically interrupted when the sample is completely melted or when the temperature exceeds 1,250°C (present maximum operation temperature).

Image analysis The image analysis is based on grey level images and binary images. In the binary images, the ash sample appears black and the background white. The resolution is 288 pixels, which gives a manageable file size of 1 1 1 kB. At a maximum magnification

of the corresponding specimen area is and the resolution is At the typical magnification of l00×, the corresponding figures are and respectively. Each grey level image that is acquired during heating of the specimen is converted to a binary image using either a constant threshold value or a subroutine, which calculates a proper threshold value. By using the last method one accounts for an eventual varying light intensity. In the binary image the solid part of the ash appears black and the melt and background appears white. This allows the total area covered by the solid part to be calculated. In addition a difference image is created, which contains information about areas in the actual image that have changed from black (B) to white (W). The

analysis result comes as the following two fractions: = Sample area in the actual image/Initial sample area, and = Total area of B-to-W changes/Initial sample area. Areas are measured in pixels and t is the actual temperature in °C. Ideally, the area fractions

are complementary, i.e.

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Examples of a grey scale image of a laboratory prepared straw ash and the corresponding binary images acquired during melting are shown in Figs. 2 and 3, respectively.

Calibration The heating stage is calibrated with materials, which do not form a protective oxide layer. Suitable materials are (394°C), (593°C), LiF (870°C), Ag (961°C) og Au (1,063°C), where the numbers indicate melting points. The heating stage

applied has a maximum operation temperature of 1,350°C measured at the specimen glass holder. Due to heat transfer primarily through the top glass seal it has been found that the maximum operation temperature corresponds to a specimen temperature of about 1,250°C. 5-point calibrations performed using the above mentioned materials show that the temperature measured at the specimen holder is comparable to the specimen temperature within up to about 830°C. Above 830°C the specimen temperature is corrected using a third order polynomial approximation.

RESULTS As mentioned previously tests were conducted using inert quartz glass fibers at a heating rate of 10°C in nitrogen. Figure 4 indicates that the parameter increases

significantly in the range 500–600°C and is not further increased above about 800°C. The cause of this error is movements of the specimen holder relative to the microscope

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objective caused by thermal stresses during heating. The area fraction is not as sensitive as in this respect due to the fact that the area of fibers moving into the image field

is approximately equal to the area moving out of the field as the fibers are placed randomly. In principle, does not detect sample movements, whereas detects any movements. Therefore, the melting curves discussed in the following are based on the area fraction by plotting versus the temperature. Also seen in Fig. 4 are the melting curves for 100% analysis quality KCl and (melting points 774°C and 1,069°C, respectively). Initially the melt fractions increase slowly to about 8–10%, which is believed

as being neither due to melting and particle shrinkage, nor particle movements. It is not yet clear why area reductions are detected initially with pure crystalline materials. Figure 5 shows the measured melting curves of a mixture (approx. 25%w/w KCl) and a mixture (approx. 79%w/w KCl). The mixtures were prepared from analysis quality reagents, which were milled in a mortar. Also shown are predictions obtained from the relevant phase diagrams. It is seen that the temperatures of significant melt formation are correctly identifyed. Initially the melt fractions increase slowly as discussed above. When more than about 60% melt has formed the measured results deviate from the predicted. These deviations can be reduced by optimizing the method of threshold value calculation. The threshold value has particularly importance when the melt is not totally transparent. Also shown in Fig. 5 for comparison is a melting curve for fly ash sampled from a straw fired combined heat and power plant. The ash contains about 58%(w/w) KCl, 18%(w/w) 9%(w/w) Si as and 5%(w/w) Ca as CaO. The initial part of the curve is quite similar to that of the salt mixture, but in this case visual inspection shows clearly that the ash shrinks before melting. The “tail” of the melting curve is caused by the presence of Si and probably also Ca. The sample was completely melted at 1,015°C (in nitrogen). This particular sample was also subjected to a Round Robin standard

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method test (ISO 540). As expected, the results (Table 1) confirmed some of the problems about using the standard test for ashes from biofuels. An important point is the repeatability of the method that may be influenced by the quantity and distribution of sample on the specimen disc, the magnification, and lighting conditions. Figure 6 shows results obtained when melting a laboratory straw ash,

a superheater deposit from straw firing, and a geological standard (BCR-l).Each run was repeated two, two, and three times, respectively. BCR-1 is considered as being very homogeneous, and with this material the magnification was varied from 50 to to enhance any magnification effects on the results. The resulting melting curves of BCR-1 are nearly identical. Generally, the deviations are greater, but still acceptable, with ashes as exemplified by the straw ash and the deposit. The decrease in the melt fractions in some

parts of the curves are caused by rearrangement of solid particles due to flow of melt.

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This is primarily observed when the quantity of melt produced initially is large as

observed with the deposit. Finally, an interesting feature of the method is that material evaporating from the sample may condense on the colder glass cover of the heating stage. This phenoma has frequently been observed when analyzing samples with relatively high salt contents. The resulting reduction in light intensity is tolerated by the method, except when the quantity of condensated material is very large. In this case the top glass seal can be rotated to a clear part. The condensing particles can be analysed using SEM-EDX directly on the top glass seal. Figure 7 shows an example of KC1 particles condensed in the temperature range 700–800°C during analysis of a fly ash from straw combustion.

DISCUSSION Two fundamental features of the ash fusion determination method presented in this paper is that the sample is placed randomly, and that the fusion characteristic is detected by changes in the area covered by the sample. It is implicitly assumed that changes in the area fraction are related to changes in the solid volume fraction or mass fraction. In the following this is shortly discussed. If A is the measured area, h m the mean height and e the porosity, then the solid volume of ash observed in the image field at a given time may be given by:

Solid volume at time t: Initially during heating a reduction in to about 0.95–0.90 due to ash shrinking is typically observed. However, the solid volume is unchanged, and that means that

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the porosity must decrease (the height does not increase). It is also observed that melting mainly takes place from below due to heat transfer from the specimen disc. In this case A is constant but h decreases, approaching zero as the material is melted. In reality shrinking and melting from below take place simultaneously, and consequently the effect

of height reduction may be cancelled by a reduction in the porosity. Based on these qualitative considerations it can be argued that changes in the volume fraction may be approx-

imated by changes in the area fraction. The volume fraction is again related to the mass fraction. These findings are supported by comparison of results obtained with the HTLM metod presented in this paper with results obtained using STA (combined TGA and DSC) [Hansen, 1997].

CONCLUSION The ash fusibility detection method using image analysis which has been presented in this work is very simple and is suitable for routine analysis. The equipment is robust and stable. The number of samples which can be run per day is high due to the uncomplicated sample preparation and a high cooling rate of the heating stage, which has a low heat capacity. Moreover, the resulting fusion characteristics are produced automatically directly from the measurements without using any expert knowledge or corrections. The method of image analysis can be refined depending on the application. If the sample is highly agglomerated as biomass ashes typically are, an analysis based on detection of changes in the area covered by the sample is suitable. If agglomeration is not predominant, single particles can be counted, measured, and observed when melting. In this way it is possible to evaluate the melting behavior as function of particle size. Normally details of ash morphology are not reported, but it is also possible to quantify such information. This work was primarily initiated due to the “problematic” melting behavior of biomass ashes with respect to the standard method. Consequently a maximum heating stage operation temperature of 1,250°C was not totally unacceptable. But for coal ashes it is, and work is in progress to extend the operation temperature to about 1,500°C. In addition, more samples will be analyzed in order to establish a data base of fusion characteristics and in order to obtain more knowledge about the relationship between mineral composition and melting behavior.

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ACKNOWLEDGMENTS This work was made possible by financial support from ELKRAFT, ELSAM, and the Danish Energy Research Programme. This support is gratefully acknowledged.

REFERENCES Coin C. D. A., Kahraman H., and Peifenstein A. P. (1996). “An Improved Ash Fusion Test”. Proceedings of the Enginering Foundation Conference on Applications of Advanced Technology to Ash-Related Problems in Boilers, July 16–21, Waterville Valley, New Hampshire. Eds. L. Baxter and R. DeSollar (1996). Hansen L., Frandsen F., and Dam-Johansen K. (1997).

This conference proceedings. Vassilev S. V., Kitano K., Takeda S., and Tsurue T. (1995). “Influence of Mineral and Chemical Composition of Coal Ashes on their Fusibility”. Fuel Processing Technology 45, 27 51. 1SO/TC 27/SC 5/WG 6 (1991). “International Round Robin for Determination of the Fusibility of Coal Ash”. ISO 540 (1981). “Coal and Coke—Determination of Fusibility of Ash”. International Standard Organisation. Wall T. F., Creelman R. A., Gupta R. P., Gupta S., Coin C., and Lowe A. (1996). “Coal Ash Fusion Temper-

atures—New Characterization Techniques, and Associations With Phase Equilibra”. Proceedings of the Enginering Foundation Conference on Applications of Advanced Technology to Ash-Related Problems in Boilers, July 16–21, Waterville Valley, New Hampshire. Eds. L. Baxter and R. DeSollar (1996).

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ASH FUSION QUANTIFICATION BY MEANS OF THERMAL ANALYSIS Lone A. Hansen, Flemming J. Frandsen, and Kim Dam-Johansen Department of Chemical Engineering Technical University of Denmark 2800 Lyngby, Denmark

1. INTRODUCTION The amount of melt present in an ash as a function of its temperature greatly influences the ash deposition propensity in thermal fuel conversion systems. The appearance of melt is believed to increase both the tendency for ash particles to stick to heat transfer surfaces [Srinivasachar et al., 1990; Walsh et al., 1990; Benson et al., 1993;

Richards et al., 1993] and the rate of strength build up in ash deposits [Skrifvars et al., 1996; Benson et al., 1993]. For years, laboratory tests have been carried out on fuel ashes to estimate their melting behaviour, and results have been used to estimate the slagging and fouling propensity of the ashes in full scale combustion systems. Laboratory tests used to estimate the melting behaviour of ashes include a variety of methods. Commonly used are the conventional ash fusion tests of which many variants appear [ISO 540, 1981; DIN 51730, 1984; ASTM D1857, 1987; AS1,03815, 1987]. These methods all imply the controlled heat up of an ash specimen of well defined shape,

and the simultaneous determination of temperatures corresponding to specified geometrical shapes. The main criticisms of these tests have been their low reproducibility and unreliability in the subsequent prediction of the ash behaviour in real boilers [Coin et al., 1996]. It has been emphasized that the initial deformation temperature is not the temperature at which the ash melting begins, and many coal ashes have been found to start melting at temperatures far below the initial deformation temperatures [Huffman et al., 1981; Huggins et al., 1981; Coin et al., 1996; Wall et al., 1996]. Alternatively, the ash melting behaviour has been estimated based on electrical resistivity [Raask, 1979; Sanyal and Cumming, 1981; Gibson and Livingston, 1992; Sanyal and Mehta, 1993] or conductance measurements of the ash [Cumming and Sanyal, 1981; Conn and Austin, 1984; Cumming et al., 1985] during heat up. These electrical quantities reflect the conduction path through the ash sample, and thereby the particle-particle contact and fusion. Both methods detect the onset of fusion in the ash as the temperature at which the electrical properties of the ash is drastically changed. The electrical conductance methods have higher repeatabilities than the standard ash fusion tests Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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and give better predictions of field slagging performance [Sanyal and Mehta, 1993]. However, these methods include some practical difficulties since satisfactory contact between ash and electrodes is hard to achieve and maintain [Wall et al., 1996]. Furthermore, the results contribute primarily with information on the onset of fusion and sintering, whereas the further melt quantity increase in the ash is harder to evaluate based on these methods. Recently, an improved ash fusion characterisation method based on dimensional changes of ash pellets during heating has been reported [Coin et al., 1996]. In this test four ash cylinders are used as pillars to separate two alumina disks. As the assembly is heated, the ash pellets shrink and the distance between the tiles is measured. Significant tile movement over a narrow temperature range is interpreted to correspond to melting of distinct chemical species, and the repeatability and reproducibility of the method is reported to be high, with reproducibilities below for significant tile movement Finally, ash melting behaviour can be estimated based on calculations. The fusion temperatures may be estimated by combining and weighting the effects of several compositional variables [Winegartner and Rhodes, 1975; Vorres, 1979; Gray, 1987; Lloyd et

al., 1989; Vassilev et al., 1995], or by use of chemical equilibrium calculations [Backman, 1989]. As indicated above, the estimation of melting behaviour of coal ashes, (and the subsequent prediction of ash behaviour in real boilers) is not a simple job. Still more problems arise, when trying to do the same job for biomass ashes. The chemical composition of biomass (i.e. in Denmark mainly straw) is very different from that of coal and thus the same kind of analyses that are useful for characterising coal ashes do not necessarily apply for biomass ashes. The standard AFT has shown to be unsuitable for ashes from biomass combustion and biomass laboratory ashes [Westborg, 1995]. Thus, the present work was initiated to generate a new method to quantify the melting behaviour of biomass ashes in order to improve the understanding and prediction of ash deposition propensities during firing of biomass. In a longer term, the aim was to apply the method also to coal ashes.

2. EXPERIMENTAL

2.1. Apparatus The new method for estimation of ash melting behaviour is based on Simultaneous Thermal Analysis, STA, and the results presented in this paper were obtained using a NETZSCH STA409. STA implies continuous measurement of sample weight (Thermogravimetric Analysis, TGA) and temperature (Differential Scanning Calorimetry, DSC) during heat treatment. The weight measurement reveals any mass changes taking place in the sample and by comparing the sample temperature to the temperature of an inert reference material, any heat producing or heat consuming (chemical or physical) processes occurring in the sample is detected, and the involved energy subsequently quantified.

2.2. Test Method STA was carried out on the ash samples, while heating them from 20 to typically atmosphere. On the resultant STA curves, melting is

1,390°C at 10°C/min in a

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detected as an endothermic process involving no change in mass. Melting of a pure substance would be seen as a single endothermic peak in the DSC signal, while for “real” ashes, the melting results in several endothermic peaks overlapping each other, corresponding to melting of the different chemical species in the ash, which melt at different temperatures. Conversion of the STA curves into a melting curve is done based on a DSC signal reflecting only melting energies, which implies that energies related to other processes than melting are first subtracted from the (raw) DSC signal. Evaporation is typically occurring simultaneous to part of the melting, and evaporation enthalpies thus typically need to be quantified and subtracted. Evaporation energies are quantified as the product of a reasonably estimated evaporation enthalpy and the derivative of the TG-curve. After subtraction of evaporation enthalpies, the melting curve calculation can be carried out in one of the following two ways.

The total area below the melting curve, i.e. the area below the DSC curve from the first point where melting is detected, [Backman, 1987], and to the temperature where the melting is completed, reflects the total energy consumed for melting of that ash (Fig. 1). Calculating the area below the DSC curve from any temperature to and dividing this area, AA–B by the total area below the melting curve, the fraction of “total energy used for ash melting” which has been used in the specific temperature interval, is obtained. This energy fraction is a simple quantitative estimate of the mass fraction of ash melted in the specified temperature interval. This estimate is only correct, if the melting enthalpy of all species in the ash are alike, which is not necessarily true. The presented method thus is a simple way of determining the melting behaviour of an ash and the result expresses the melting behaviour in what could be termed an “energy-percentage” of melt as a function of temperature. Alternatively, a method based on quantitative determinations of the peak areas can be used. Any given peak below the melting curve corresponds to an absolute quantity of energy used for melting. The position of the peak (onset and peak temperature) gives an indication of the identity of the melting substance(-s), i.e. a reasonable estimation of the relevant melting enthalpy can be made. Based on these two figures, the mass of material melted in the given temperature range can be calculated, and by relating this mass to the

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total mass of ash analysed, the mass fraction of ash melted in the given temperature range is obtained. The latter method gives the most correct estimates of “ mass fraction melted”, but this method implies that the substances melting at the different temperature intervals can be identified, so that a reasonable estimation of the involved melting enthalpies can be made, unless the species present in the ash have got equal melting enthalpies. The latter method thus typically implies an identification of the chemical species present in the ash (as provided by e.g. CCSEM) and detailed knowledge on the chemistry between the ash species.

3. RESULTS

3.1. Simple Systems First, the melting behaviour of two simple mixtures consisting each of only two chemical species will be presented. 3.1.1.

A sample of approximately 85 mole-% KC1 and 15 mole-% (

was prepared and analysed in the STA409. The resultant STA curves are shown in Fig. 2. The STA curves show evaporation (of crystal water from ) at 20–175°C. This is detected by 1) a decrease in mass (TG) and 2) a consumption of energy (the upward DSC peak). At 594°C, a new endothermic peak starts, peaking at 600°C, but also having a long “tail” behind it, so that the peak is not quite ended until the temperature has reached 696°C. The peak corresponds to the formation of a considerable

quantity of melt at the eutectic temperature, and the “tail” corresponds to the continuous increase in melt quantity as the temperature is raised from the eutectic temperature and to the liquidus temperature. Above 700°C, the DSC-signal is greatly increased due to the evaporation of KCl (seen as the decrease in the TG curve). This experiment was repeated three times. In Fig. 3, a comparison is made between the melting behaviour obtained by using the lever rule in the phase diagram [Levin et al., 1964] and the ones obtained when quantifying and comparing the areas below the DSC curve for each of the three experiments. It is seen that the “theoretical” curve predicts the melting to take place instantaneously, which is not happening in reality, but except for this, a very good correlation between

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“theoretically” and experimentally obtained melting curves is found. The deviation between the two types of curves is 10% at maximum (neglecting the temperature range, 594–608°C, just above the eutectic temperature). Melting onset and completion deviate less than respectively 3 and 15°C from the phase diagram values. The method repeatability data is given in Table 1. Repeatability is seen to be very good, deviances are 4°C for melting onset, 4% for the melt fraction obtained at the eutectic temperature (calculated at 608°C, which is the end of the large peak), and 11°C for melting completion. Concerning the completion temperatures, the phase diagram shows that increasing KC1 fractions increases the liquidus temperature, and this tendency is also found in the experimental results. Concerning the measured melting enthalpies, a very good correlation to theoretical calculations is seen: the measured energies correspond to between 99.0 and

99.7% of the theoretical value. The theoretical and experimental description of the melting behaviour are thus judged to correlate well in this case. 3.1.2. _ The same analysis and comparison was made for a mixture of KCl and the result of which is shown in Fig. 4. As can be seen, the experimental curve overestimates the quantity of melt formed at the eutectic temperature with approximately 4% compared to the “theoretical” prediction. At temperatures above 700°C, the deviation between experimental and the “theoretical” prediction varies between 4 and 7%, until the curves meet at 860°C. The temperature differences between the transition temperatures given in the phase diagram and the experimentally determined ones are judged to be acceptable. This example therefore confirms the above and supports the assumption that STA measurements are able to describe melting behaviour of simple systems.

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3.2. Ash Samples Several fly, bottom and deposit ash samples from 1) grate fired units firing pure straw and 2) PF-fired boilers co-fired with straw and coal have been investigated. In this paper, examples of the results will be given, representing melting behaviour results for a

fly ash and a bottom ash collected during a test run of different straws at a grate fired

boiler, and a fly ash collected during (pure) coal firing at a PF-fired boiler. The chemical composition of the ashes is given in Table 2, and a reduced data set of CCSEM results showing the dominant species in the ashes in Table 3. The STA curves for the fly ash collected at the straw fired boiler is shown in Fig. 5, where the increase in curve complexibility (compared to Fig. 2) is obvious. Referring to Fig. 5, the DSC curve is seen to show a distinct endothermic peak from 641°C to 712°C corresponding to an energy consumption of 176.1 J / g . As there is no simultaneous decrease in mass (TG curve), this peak corresponds to the onset of the ash melting. For increasing temperatures, a general increase in the DSC signal is seen. On top of this general increase, two distinct peaks are seen, one ranging from app. 920°C to 1,050°C and one starting at app. 1,150°C, which is not completely finished at 1,250°C. The first of these peaks is seen to

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occur simultaneously to a large decrease in mass, and since the shape of the DSC peak

and the d(TG) peak are quite—but not totally—alike, a large fraction—but not all—of the energy corresponding to this peak is used for evaporation of material rather than melting. As described earlier, the evaporation energies are estimated as the product of an estimated evaporation enthalpy and the d(TG) curve. In this case the evaporation enthalpy of KCl has been used, since KCl constitutes a large part of this ash (app. 40% (w/w)) and is assumed to evaporate at these temperatures. For the last DSC peak, the simultaneous mass decrease is very low, and thus the energy corresponding to the area

of this peak is predominantly used for melting of ash. The general increase of the DSC curve is caused by the fact that when great mass losses are occurring (as for this sample) the DSC baseline is shifted upwards [Netzsch, 1995]. This explanation is supported by the slope of the DSC curve, which is quite higher around the temperatures of rapid evaporation (800–1,050°C) compared to above 1,100°C, where the fast evaporation is ended, and the DSC baseline has found a new level at app. 1.5mW/mg. A typical set of STA curves for a silicate rich ash—e.g. a fly ash produced during coal combustion—is shown in Fig. 6. Nothing significant seems to occur until the DSC curve starts increasing at 1,180°C, which represents the melting onset. The DSC curve continuously increases until the termination of the experiment at 1,390°C, at which point

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the DSC peak is not ended; i.e. the ash is not completely melted at 1,390°C. Identification of chemical species melting in different temperature ranges above 1,180°C is not possible, why the melting curve is calculated based on area comparison (method no. 1), as is typical for silicate dominated ashes. The fraction of melt formed at 1,390°C is determined by studying the sample structure (after cooling) in the SEM. The material which has not been molten has maintained its original structure, and the melt fraction determination is made based on an area evaluation of material of original structure to that of fused material.

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In Fig. 7, the melting behaviour calculated on the basis of the STA curves for the three ashes is shown. For the straw derived fly ash, two curves are shown, representing respectively the comparison of areas under the DSC curve (method 1), and the calculation that includes the absolute energy represented by the first melting peak and the relevant melting enthalpy (method 2). The first melting peak for this fly ash is supposed to represent melting in a salt system containing large amounts of KCl and only minor quantities of “other” K- and Ca-salts, since the temperature agrees with the eutectic temperature for these systems and since these species have been found in the ash by means of CCSEM. As it is seen, the curve including the melting enthalpy corresponding to the first melting peak shows a larger fraction to melt at 653 °C than the curve based on area comparisons This reflects that the melting enthalpies for the potassium and calcium silicates melting at the higher temperatures are higher than the melting enthalpy for KCl. Comparing the three melting curves, it is seen that generally the fly ash from straw combustion is “lower melting” than the bottom ash from straw combustion, which is again “lower melting” than the fly ash from coal combustion. Comparing the melting curve for the straw-derived fly ash with that for the straw-derived bottom ash, it is seen that at temperatures between 600 and 1,100°C, the melt fraction is considerably lower for the bottom ash than for the fly ash. This is due to the high content of simple salts in the fly ash. For temperatures between 1,100 and 1,250°C, the bottom ash shows higher melt fractions than for the fly ash. This is probably due to the fact that the bottom ash is highly dominated by K- and Ca-silicates (i.e. K-, Ca-, and Si-rich compounds) whereas the silicate part of the fly ash contains larger fractions of the more refractory quartz [Sørensen, 1997]. The coal derived fly ash starts melting at the highest temperatures, with initial melting at app. 1,180°C, and only partly melting at 1,390°C. This is due to the ash consisting almost entirely of various alumino silicates and quartz [Sørensen, 1997] which melt at relatively high temperatures. In conclusion, the melting curves are thus seen to reflect the different chemical composition of the ashes.

3.3. Repeatability of Melting Curves Figure 8 shows melting curves for the two fly ashes as obtained during repeating experiments. Starting with the straw-derived fly ash, it is seen on the curves, that the reproducibility of the melting onset is very good, within 5°C, as well as is the slope of the first part of the curve. The first curve part corresponds to the very distinct peak occurring at low temperatures (641-712°C in Fig. 5), and since this peak is dependent on the salt chemistry of the sample, and this chemistry is quite simple (i.e. includes only a few possible reactions between the present species), the melting peak occur at precisely the same temperature every time. The melt fraction obtained at these first peaks do deviate slightly, though; in this case the level obtained is 50, 52 and 47% melt respectively. This deviation is caused both by method uncertainty but may also be influenced by the inhomogeneity of the sample. For the rest of the melting curve, the uncertainty is somewhat larger, but still within 10% melt. The larger uncertainty for the last part of the curve is due to the less distinct peaks corresponding to the melting of the silicate part of the ash. Since the peaks corresponding to silicate melting are less distinct, a precise characterisation is dependent on a very well known baseline. For higher temperatures, drift in the DSC baseline cannot be avoided [Netzsch, 1995]. This leads to larger uncertainties for the melting curve. As stated above, the uncertainty is still within 10% melt, though. For the coal-derived fly ash, which consist mostly of silicates, the problem with not very dis-

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tinct peaks may generally lead to a larger uncertainty for the melting onset, but as can be seen, the repeatability is still quite good: melting onset varies within 30°C, and melt fractions at given temperatures are within 10% melt. To reduce uncertainties from sample inhomogeneity, repeatability was also tested

by analysing a well-characterised and homogenized geological standard material (BCR1). Melting curves for these measurements are also shown in Fig. 8, and reveals that onset is determined with a deviation of 15°C, melting completion temperature with a deviation of 5°C, and the melt fractions at given temperatures deviate at maximum 14% (melt). Based on this, method repeatability is generally judged to be quite high.

4. DISCUSSION 4.1. Correlation to Standard AFT The results of the standard AFT (DS/ISO540) are marked on Fig. 7. For all ashes, melt is formed at temperatures considerably below the IDT, respectively 105, 42 and 65°C for each of the three ashes. This is consistent with previous criticism of the standard ash fusion tests [Huffman et al., 1981; Huggins et al., 1981; Coin et al., 1996, Wall et al., 1996]. For the straw fly ash, the STA predicts significant melt formation (51%) below the IDT. On the other hand, the melt fraction does not increase much for the next characteristic temperatures, since the hemispherical and the fluid temperature corresponds to a melt fractions of respectively 53 and 62% melt. For the bottom ash, the AFT temperatures seem to give a better description of the increasing melt fraction, with the three characteristic temperatures corresponding to respectively app. 3, 14 and 43 (energy) percent melt. For the coal-derived fly ash, the initial deformation temperature corresponds to 5% melt, whereas the hemispherical and the fluid temperature are both higher than the present maximum analysis temperature of 1,390°C and the only relation that can be given is that they correspond to more than 60% melt in the ash. Overall, the standard AFT and the STA melting behaviour curves thus seem to correlate qualitatively, as the three characteristic temperatures are all located in the temperature range corresponding to 3 to 65% melt in the ash. A further and more detailed comparison between the standard AFT and the STA melting behaviour results is presented else where [Hansen, 1997; Hansen et al., 1997].

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4.2. Correlation of Results with Ash Chemistry (Mineralogical Changes) The DSC signal is closely related to the chemical composition of the ash, since the melting peaks indicate the melting point of either single or mixtures of ash species. Identification of the species melting in the different temperature intervals can thus be made based on CCSEM and/or XRD analysis of the ash and relevant phase diagrams, supplied with STA of simple synthetic ashes, if necessary. For the less distinct “peaks”, the interpretation is correspondingly less certain, but typically wide melting (temperature) ranges can be correlated to the species melting. A detailed comparison between STA melting curves and CCSEM compositional data for ashes collected at straw fired boilers is provided elsewhere [Hansen et al., 1997].

4.3. Method Limitations At present, the described method works as an expert tool. The melting behaviour measurements are easy and simple to perform, the repeatability of the results is good, and the measurement procedure can easily be standardized. The interpretation of the STA signal, that is, the conversion from the STA curves to the melting curve, is on the other hand not simple and can not at the moment be standardized. The “certain” interpretation requires detailed knowledge on the chemical species in each ash sample (as provided either by CCSEM or XRD analysis) and the chemistry between these. This is necessary for expressing the melt fraction as a mass percent (method no. 2), but is also important to avoid that energies related to solid phase transitions occurring simultaneously to the melting will be wrongly detected as so (melting).

4.4. Result Applicability The new method provides an improved and more detailed characterisation of the melting process occurring in ashes during heating. The results reveal/provide the temperature for which the first melt is formed in the ash, and gives therefore important information for boiler designers. Furthermore, the melting curves can be used as input to mechanistic modeling of ash deposit formation by inertial impaction, which will hope-

fully improve the understanding of ash deposit formation mechanisms during biomass and/or coal combustion.

5. CONCLUSIONS A new experimental method for quantification of ash melting has been developed. Using the new method, a conventional STA apparatus is employed, and the melting is detected as endothermic reactions involving no change in mass. The DSC signal is transferred into a melting curve (showing the melt fraction in the ash as a function of temperature) either by simple comparison of the areas below the melting curve or by

accounting for the relevant melting enthalpies. The execution of the measurement is simple and the repeatability of the results is very good. The subsequent conversion of the STA curves to a melting curve requires knowledge of the identity of chemical species in the ash and the involved chemistry. The method has so far been tested on a number of simple salt mixtures, for which the measured melting behaviour agrees with the predictions from phase diagrams, and a

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number of ashes collected during combustion of pure straw, co-combustion of straw and

coal, and coal combustion, for which melt was detected between 40 °C and 110°C below the corresponding IDT. Characterising the fusion by STA provides a more detailed description of the ash fusion as compared to conventional methods, and the onset of ash fusion is precisely determined. Furthermore, the method typically enables identification of the chemical species melting in different temperature ranges. As ash melting has a major impact on the deposit formation tendency, the presented detailed ash fusion determination improves the prediction of ash deposition propensities.

ACKNOWLEDGEMENTS This work was carried out as part of the Combustion and Harmful Emission Control (CHEC) research program at the Department of Chemical Engineering, Technical University of Denmark. The CHEC research program is cofunded by ELSAM (The Jutland-Funen Electricity Consortium), ELKRAFT, the Danish Technical Research Council, the Danish and the Nordic Energy Research programs. Mrs. Gurli Mogensen (from Haldor Topsoe A/S) is acknowledged for her valuable contributions to the interpretation of the DSC curve behaviour during the long and troublesome running-in of the apparatus.

REFERENCES AS1038.15 (1987). “Methods for the analysis and testing of coal and coke—fusibility of higher rank coal ash and coke ash”. Australian Standards Association ASTM D1857-87 (1987). “Standard test method for fusibility of coal and coke ash”. American Society for

Testing and Materials Backman, R. (1989). Sodium and Sulfur Chemistry in Combustion Gases. Academic Dissertation, Åbo Academy University, Abo, Finland Backman, R., Hupa, M., and Uppstu, E. (1987). “Fouling and Corrosion Mechanisms in the Recovery Boiler Superheater Area”. Tappi Journal 70 (6) 123–127 Benson, S.A., Jones, M.L., and Harb, J.N. (1993). “Ash Formation and Deposition”. In L. D. Smoot (Eds.), Fundamentals of Coal Combustion for Clean and Efficient Use. New York: Elsevier Coin, C.D.A., Kahraman, H., and Peifenstein, A.P. (1996). “An Improved Ash Fusion Test”. In L.L. Baxter and R. DeSollar (Eds.), Proceedings of the Engineering Foundation Conference. New York and London:

Plenum Press Conn, R.E., and Austin, L.G. (1984). “Studies of Sintering of Coal Ash Relevant to Pulverised Coal Utility

Boilers. 1: Examination of the Raask Shrinkage-Electrical Resistance Method”. Fuel (63) 1664 Cumming, I.W., and Sanyal, A. (1981). “An Electrical Conductance Method for Predicting the Onset of Fusion in Coal Ash”. In R.W. Bryers (Eds.), US Engineering Foundation Conference on Slagging and Fouling from Combustion Gases. New York: Engineering Foundation Cumming, I.W., Joyce, W.I., and Kyle, J.H. (1985). “Advanced Techniques for the Assessment of Slagging and Fouling Propensity in Pulverised Coal Fired Boiler Plant”. In R.E. Barrett (Eds.), Proceedings of the Engineering Foundation Conference on Slagging and Fouling Due to Impurities in Combustion Gases. New York: Engineering Foundation DIN 51730, (1984) “Determination of Fusibility of Fuel Ash”. German Standard Gibson, J.R. and Livingston, W.R. (1992). “The Sintering and Fusion of Bituminous Coal Ashes”. In S.A. Benson (Eds.), Proceedings of the Engineering Foundation Conference on Inorganic Transformations and Ash Deposition During Combustion. New York: ASME Gray, V.R. (1987). “Prediction of Ash Fusion Temperatures from Ash Composition for Some New Zealand

Coals”. Fuel (66) 1230–1239 Hansen, L.A. (1997). Melting and Sintering of Ashes. Ph.D. Thesis; Department of Chemical Engineering, Technical University of Denmark

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Hansen, L.A., Frandsen, F.J., Sørensen, H.S., Rosenberg, P., Hjuler, K., and Dam-Johansen, K. (1997). “Ash Fusion and Deposit Formation at Straw Fired Boilers”; these conference proceedings Huffman, P.H., Huggins, F.E., and Dunmyre, G.R. (1981). “Investigation of the High-Temperature Behaviour of Coal Ash in Reducing and Oxidizing Atmospheres”. Fuel (60) 585 Huggins, F.E., Kosmack, D.A., and Huffman, G.P (1981). “Correlation between Ash-Fusion Temperatures and Ternary Equilibrium Phase Diagrams”. Fuel (60) 577–584

ISO 540 (1981). “Determination of Fusibility of Ash”. International Standard Organisation Levin, E.M., Robbins, C.R., and McMurdie, H.F. (1964). Phase Diagrams for Ceramists, Vol. I . Columbus,

Ohio: The American Ceramic Society Lloyd, W.G., Riley, J.T., Risen, M.A., Gilleland, S.R., and Tibbits, R.L. (1989). “Estimation of Ash Softening Temperatures Using Cross Terms and Partial Factor Analysis”. Energy and Fuels (4) 325 Netzsch, (1995). Personal Communication

Raask, E.J. (1979). “Sintering Characteristics of Coal Ashes by Simultaneous Dilatometry-EIectrical Conductance Measurements”. Journal of Thermal Analysis (16) 91 Richards, G.H., Slater, P.N., and Harb, J.N. (1993). “Simulation of Ash Deposit Growth in a Pulverised CoalFired Pilot Scale Reactor”. Energy & Fuels (7) 774–781 Sanyal, A., and Cumming, I.W. (1981). “An Electrical Resistivity Method for Detecting the Onset of Fusion in Coal Ash”. In R.W. Bryers (Eds.), US Engineering Foundation Conference on Slagging and Fouling from Combustion Gases. New York: Engineering Foundation Sanyal, A. and Mehta, A.K. (1994). “Development of an Electrical Resistance Method based on Ash Fusion Test”. In J. Williamson and F. Wigley (Eds.), Engineering Foundation Conference on Impact of Ash

Deposition on Coal Fired Plants. Washington: Taylor & Francis Skrifvars, B.-J., Backman, R., and Hupa, M. (1996). “Ash Chemistry and Sintering”. Preprints of Papers Presented at the 211th ACS National Meeting, New Orleans, LA, March 24–28, 1996 Srinivasachar, S., Helble, J.J., Katz, C.B., and Boni, A.A. (1990). “Transformations and Stickiness of minerals during pulverised coal combustion”. In R.W. Bryers and K. Vorres (Eds.), Proc. of the Engineering Foundation Conference on mineral matter and ash deposition from coal. New York: Engineering Foundation Sørensen, H.S.(1997). “Computer Controlled Scanning Electron Microscopy of Straw Ash”. These conference

proceedings Vassilev, S.V., Kitano, K., Takeda, S., and Tsurue, T. (1995). “Influence of mineral and chemical composition of coal ashes on their fusibility” . Fuel Processing Technology (45) 27–51 Vorres, K.W. (1979). “Effect of Composition on Melting Behaviour of Coal Ash”. Journal Eng. Power (101) 497 Wall, T.F., Gupta, R.P., Polychroniadis, P., Ellis, G.C., Ledger, R.C., and Lindner, E.R. (1989). “The Strength, Sintering, Electrical Conductance and Chemical Character of Coal Ash Deposits”. NERDDC Project No. 1181— Final Report, Vol. 1, Summary Report Wall, T.F., Creelman, R.A., Gupta, R.P., Gupta, S., Coin, C., and Lowe, A. (1996). “Coal Ash Fusion Temperatures: New Characterisation Techniques and Associations with Phase Equilibria”. In L.L. Baxter and R. DeSollar, Proceedings of the Engineering Foundation Conference on Applications of Advanced Technology to Ash-Related Problems in Boilers. New York: Plenum Press

Walsh, P.M., Sayre, A.N., Loehden, D.O., Monroe, L.S., Beér, J.M., and Sarofim, A.F. (1990). “Deposition of Bituminous Coal Ash on an Isolated Heat Exchanger Tube: Effects of Coal Properties on Deposit Growth”. Prog. Energy Comb. Sci. (16) 327–346 Westborg, S. (1995). Round Robin Test—Analysis of Straw and Straw Ash. Internal report (in Danish) Biomass

Ash Characterisation Project, dk-TEKNIK, Soeborg, Denmark Winegartner, E.C., and Rhodes, B.T. (1975). “An Empirical Study of the Relation of Chemical Properties to Ash Fusion Temperatures” Journal Eng. Power (97) 395

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STICKING MECHANISMS IN HOT-GAS FILTER ASHES John P. Hurley, Bruce A. Dockter, Troy A. Roling, and Jan W. Nowok University of North Dakota Energy & Environmental Research Center PO Box 9018 Grand Forks, ND 58202-9018 John Hurley: Phone (701) 777-5159, e-mail [email protected]

1. INTRODUCTION Large-scale hot-gas filter testing over the past 10 years has revealed numerous cases of cake buildup on filter elements that has been difficult, if not impossible, to remove.

At times, the cake can bridge between candle filters, leading to filter failure. Physical factors, including particle-size distribution, particle shape, the aerodynamics of deposition, and system temperature, contribute to the difficulty in removing the cake, but

chemical factors such as surface composition and gas–solid reactions also play roles in helping to bond the ash to the filter and to itself. In order to develop methods to predict the formation of sticky ash in hot-gas filtration systems, the University of North Dakota Energy & Environmental Research Center (EERC) worked with EPRI and a consortium of companies in partnership with the U.S. Department of Energy (DOE) to determine the factors causing hot-gas cleanup filters to develop deposits that can bridge the filters and cause them to fail. The primary deliverable was the Filter Bridging Index Code, a graphics-driven computer code to tie all of the knowledge together and make possible the prediction of rates of filter bridging based on coal, sorbent, filter, and system parameters. The objectives of this project were threefold: • Determine the mechanisms by which a difficult-to-clean ash forms and how it bridges hot-gas filters

• Develop a method to predict the rate of bridging based on analyses of the feed coal and sorbent, filter properties, and system operating conditions • Suggest and test ways to prevent filter bridging The research took place over the years 1994–1997 and comprised five tasks. Task 1 involved detailed sampling at large-scale, operating hot-gas filter test units and gatherImpact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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ing representative archived samples from completed programs, then subjecting the

samples to intensive physical and chemical analyses. Task 2 concentrated on thermochemical equilibrium modeling to determine possible chemical contributions to ash stickiness, along with laboratory measurements of the rates and mechanisms of tensile strength development in ash cakes. In Task 3, bench-scale testing was employed to determine factors affecting the formation of ash in pressurized fluidized-bed combustors (PFBCs) and the factors affecting the rates of residual cake development under both fluidized-bed combustion (FBC) and gasification conditions. Under Task 4, a graphical user interface computer code was created to tie all of the knowledge together and make possible the prediction of rates of filter bridging based on coal, sorbent, filter, and system parameters. Task 5 involved reporting. EPRI is the prime contractor to the other sponsors. All research was carried out by the EERC. The EERC, through its Cooperative Agreement with the DOE Federal Energy Technology Center (FETC), received funds to approximately match those contributed by the sponsors. The sponsors were EPRI, Lurgi Lentjes Babcock (LLB), a British consortium led by PowerGen plc, Schumacher America, Westinghouse, Electricité de France, the Netherlands Energy Research Foundation (ECN), a Swedish group led by ABB Carbon, and the Electric Power Development Company of Japan. In this paper, we report measurements of several ash- and system-related factors affecting the stickiness of the ash in hot-gas particulate filters used in PFBC systems. Data are presented on the relative effects of temperature, cake porosity, and the presence of surface liquids on the bridging propensities of ash cakes.

2. BACKGROUND Many factors control the adhesiveness of ash particles that leads to the formation of ash deposits or bridges between filter elements in hot-gas particulate filtration systems. The following is a short evaluation of some of the most significant factors affecting adhesion within particle cakes.

2.1. Surface Area and Particle Attraction Predicting the ways in which a powder will agglomerate and form cakes is a matter of understanding not only all of the ways in which particles can attract one another, but also the way in which the particles fit together in the bulk. Any attempt to sum the microscopic contributions made by the particles themselves must take into account three factors: the interparticle forces, the three-dimensional shapes of the particles, and the way in which the particles interact geometrically to form a packed structure. Most powders are not easily characterized in terms of these properties. For example, most particles in PFBC ashes have complex shapes so that both the distributions of equivalent spherical diameters and the distributions of shape factors must be used in describing particle-size distributions. The situation is no less complicated with interparticle forces, which are

subject to change with environment and time. In hot-gas filter ashes, these forces can be of a variety of types, including the following from weakest to strongest:

• Mechanical forces caused by interlocking of irregular particles • Electrostatic forces, particularly for surfaces that easily become charged

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• Molecular (or van der Waals) forces, particularly significant for particles of small diameters, especially those less than 10µm in diameter • Surface tension forces caused by bridging between surface liquids • Solid bridge forces, where solidification at contact points causes joining of the particles Mechanical forces are those created as the particles pack together to form the powder cake. Even if the particles were all of the same size and shape and interparticle attractions were absent, several three-dimensional packing structures would still be available to the particles. Particle size and shape and interparticle attractive forces both modify this situation, resulting in a structure of fine, irregular particles with an extremely complex geometric configuration. Electrostatic surface forces also cause particles to stick. They can develop if the particles have been in contact with an ionic solid or if electrons transfer between the surfaces of particles by contact charging. These forces differ from van der Waals forces, which occur from random motion of the electrons in the surface molecules. Even if a particle were perfectly dry, clean, and smooth and showed no tendency toward solid bridge formation, forces on the surfaces would still cause interparticle attraction, the van der Waals, or molecular, forces [Parfitt and Sing, 1976; Israelachvili, 1991; Funk and Dinger, 1995]. Equations for van der Waals forces predict that larger particles are more strongly attracted to each other than are small particles, although the ratio of van der Waals to gravitational forces is much greater for smaller particles. Additionally, these equations are valid only when the particles are close to each other. When any two particles are separated by more than a micron, van der Waals forces are negligible. There-

fore, these forces are reduced in comparison to the force due to gravity for larger, highly irregular particles since relatively small parts of their total surface area are actually touching. If the surfaces are deformable, the real area of contact and hence the adhesion between the particles will increase with the force pressing them together. In addition to van der Waals forces, the surface energy of the material must be considered. This becomes an especially great factor if a liquid layer coats the particles, because if the liquid wets the particles a strong bond forms. Solid bridge formation can occur by either direct or secondary interaction. Solid bridges can form through chemical or physical vapor deposition of gases, chemical reaction between liquids, crystallization of dissolved substances, freezing of liquids, or viscous flow sintering. For particles that are already touching or are in close proximity, mainly liquid and solid bridges and van der Waals forces are responsible for adhesion. At temperatures below 150°C, van der Waals forces and the presence of liquid (usually aqueous) bridges play the main role in particle adhesion. At longer contact times and higher temperatures (above 150°C), the influence of interface reaction increases greatly. If the temperature rises above 600°C and the appropriate atmosphere is present, it is possible for glassy ceramics to crystallize. This would result in very strong particle adhesion. Also, the most dominant interparticle forces may change with time as the particles interact. The magnitudes of the forces described above ultimately depend on the temperature and size and composition distributions of the particles that make up the cake. Measuring the relative effects of these factors on cake tensile strengths allows us to understand their influence on the formation of ash bridges. With this knowledge, existing computer codes that predict the size and composition distributions of ash particles on the basis of analysis of the inorganic matter in a coal can be modified, creating a tool to predict the

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relative propensity for ash bridges to form for a given coal and system temperature. In

this way, fuels or conditions that will likely produce ash bridges can be avoided.

3. RESULTS AND DISCUSSION Tensile strengths of ash cakes at elevated temperatures in simulated combustion atmospheres were determined with the high-temperature tensile strength tester (HTTT).

A schematic of the HTTT is shown in Fig. 1. The tester consists of a split cylinder with a porous metal bottom. One side of the cylinder is fixed, and the other side is suspended like a pendulum. The force needed to swing the suspended side away from the fixed side divided by the cross-sectional area of the cake filling the cylinder gives the tensile strength of the cake. To determine the effects of gas composition and temperature on tensile strength, the tester is placed in a split oven and preheated gas is passed upward through the porous bottom and the ash cake. Because relative humidity may affect the cohesive properties of the filter ash, the samples are stored overnight at 200°C. In earlier tests, a weighed amount of ash was placed in the split cylinder and a weight was placed on top to compress the ash with a force similar to that experienced by a filter ash cake due to the pressure drop across the cake. However, the weight primarily compressed the top layers of the cake, leaving the bottom uncompressed so that the cake breaks were very uneven. In later tests, the ash was sifted onto the porous substrate while air was pulled through the cake at face velocities approximating those in a filter system. This preparation method resulted in much more realistic cake porosities (around 70%), although the porosity is still somewhat lower than that measured in actual cakes because the ash particles are less sticky at room temperature and so tend to settle into more compact cakes [Snyder and Pontius, 1996].

3.1. Critical Thickness Index In related EERC research on baghouse efficiency, it was determined that an ash cake tensile strength of between 0.5 and is recommended to ensure the cake

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does not disintegrate during backpulsing, allowing the ash particles to reattach to the filters (Miller et al., 1993). This lower strength range is not expected to change greatly at

pressurized hot-gas temperatures because the effects of higher pressure on the density of the gas that would reentrain the ash are largely canceled by the reduction in density caused by the higher temperature. In testing of ash collected from the Westinghouse filter vessel used at the American Electric Power Company (AEP) Tidd plant [Newby et al., 1995], it was determined that at strength levels of 2.5 to 3.0g/cm2, the Tidd ash begins to sinter slightly and the cake begins to slide along the porous frit bottom of the splitcylinder chamber. This means that the capabilities of the HTTT are being exceeded at this level of strength for this particular source of filter ash. It also indicates the strength necessary to form a bridge for this ash. In later tests with other ashes, higher strengths could be measured with some cakes, different cakes had different plasticities, and tensile strength did not always correlate directly with bridging propensity. Figure 2 shows the tensile strength of two ashes designated C-l and hot-gas ash provided by ABB Carbon which were collected from a PFBC system. The tensile strength measurements were made at 700° and 750°C. The graph shows that although strength generally increases with temperature, the error bars are broad enough that there is not a large difference in tensile strengths of the two ashes. ABB Carbon experience is that the hot-gas ash is much more likely to bridge between filters than is the C-l ash. Since many bridges must support themselves against gravity, tensile strength alone does not indicate the likelihood for a cake to form a bridge. What is more important is the specific strength of the cake, or the strength in relationship to the density of the cake. The following equation gives the definition of specific strength, a parameter that we prefer to call the cake critical thickness index (CTI): CTI =

tensile breaking force/cake cross - sectional area cake weight/cake volume

The CTI has units of length and indicates the relative thickness of a cake or deposit that can form before it will shed under its own weight. A high CTI indicates that the cake

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is relatively sticky for its weight and is more likely to form a bridge than a cake with a low CTI. Although by definition a CTI that is greater than half the distance between filters implies that an ash bridge can form, other forces caused by system vibration and friction may be involved in determining the likelihood of an ash bridge forming. Therefore, the CTI should not be viewed as an exact number, but as a relative number for comparing bridging propensities.

Measurements of the bulk densities of the cakes showed that the C-l ash is much more dense than the hot-gas ash. Figure 3 shows the measured CTIs for the ABB Carbon

samples. The data match the field observations of the relative bridging propensities of the ashes, showing that the hot-gas ash is more likely to bridge than the C-l ash and that between 700° and 750 °C, the relative bridging propensity of the hot-gas ash increases dramatically.

3.2. Effects of Cake Porosity and Temperature Figure 4 shows the CTIs measured at different temperatures and degrees of compaction for a filter hopper ash collected from the AEP Tidd plant during a period when filter bridging had been minimized [Newby et al., 1995]. The data show that as the temperature and level of compaction of the Tidd ash increase, the resulting tensile strength also increases. The effect of compaction is much more pronounced at higher temperatures, but generally causes a doubling in the CTI for every 6% decrease in void fraction (porosity) for temperatures above ambient. This increase in CTI indicates that strength increases relative to bulk density as void fraction decreases at higher temperatures, but much less at room temperature. However, for a given void fraction, the temperature has a much greater effect, increasing the CTI by an order of magnitude as temperature is increased from ambient to 700°C at a void fraction of 62%.

3.3. Effects of Surface Liquids To help determine the likelihood that liquids may form on the surfaces of ash par-

ticles, thereby increasing their stickiness, thermochemical equilibrium modeling of ash

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and gas compositions was performed with an ideal solution model called the Facility for the Analysis of Chemical Thermodynamics (FACT) code. Using the FACT code, the quantities of alkalies in the vapor phase, ash slag, and liquid salt that may occur in ash and cause its sticking problems under combustion conditions have been estimated. As shown in Fig. 5, the code predicts the formation of liquid alkali metal salts in Tidd ash at temperatures as low as 560°C, with a rapid increase in concentration as temperature rises to as much as 6% of the total ash-forming material. The code predicts that potassium and sodium sulfates, carbonates, and hydroxides make up this liquid material. However, the flatness of the curve above 820°C indicates that the code is extrapolating at higher temperatures, so the data are suspect above that point. Experimental verification of the phases predicted also has shown that a sulfate–carbonate blend with a composition similar to the predicted liquid salt is stable for only a short time (about 10 minutes) in the presence of (5,000 ppm–20 vol%– balance) atmosphere at 727°C. After cooling to room temperature, the liquid crystallizes and forms

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intermediate phases:

and

Over longer times at temperature

though, the material converts to complex sulfates, which may have somewhat higher, but

still troublesome, melting points. The transition to a higher melting point may also cause the material to solidify at a constant temperature, thereby making the deposit much harder. In order to determine the effects of surface liquid on the strength of an ash cake, tensile strength measurements were performed at room temperature on a bench-scale filter hopper ash by wetting it with ethylene glycol and glycerin. These liquids were chosen

because of their low vapor pressures and because they have viscosities approximating the lowest (ethylene glycol at 0.2 poise) and highest (glycerin at 10 poise) viscosities expected for the molten salts.

The filter hopper ash was sieved, and the fraction smaller than 270 mesh (53 microns) was used for the analysis. The liquid was then added to the ash and mixed thoroughly. Before the treated ash was placed in the split cell of the HTTT, it was passed through a sieve to break up any agglomerates. The ash was compacted in the split cell using the vacuum method. During the addition of the sample, very little change in pressure drop occurred. The sample was vacuumcompacted for a period of 10 minutes, and then the cake was broken. This was repeated four times to give an average and standard deviation. The CTIs for the different weight percentages of ethylene glycol are shown in Fig. 6. The graph shows that with the addition of 1% ethylene glycol to the filter ash, the CT1 increased by approximately 75% on average. This increase could turn a nonbridging ash into a bridging ash. The strength

variations in the l%-to-10% liquid range may be experimental error, although the literature suggests that some of these variations may be real, caused by changes in the mechanisms by which the particles are bonded [Kia, 1988]. The results obtained using the higher-viscosity glycerin were quite different, as shown in Fig. 7. The graph shows that with the addition of 2.8% glycerin to the filter ash, the cake strength increased by approximately 9% on average. This is a much smaller increase than with ethylene glycol, which has a much lower viscosity. However, the ethylene glycol appears to wet the ash much better than the glycerin, indicating that the ability of the liquid to wet the ash is much more important than the viscosity of the liquid

in increasing the strength of an ash cake.

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The question then becomes, Does a liquid alkali salt wet coal ash that is primarily silicate-based? To answer that question, a pressed pellet of sodium sulfate powder was placed on the surface of a button of fused Illinois No. 6 coal ash slag. The materials were

heated in a furnace in air to the melting point of the sodium sulfate and photographed. Since sodium sulfate is a pure compound, it completely converts to liquid at its melting point. Also, the liquid is of a very low viscosity. As shown in Fig. 8, the molten sodium

sulfate does not immediately wet the silicate slag. However, over periods of tens of

minutes, it does react with the silicate to form an intermediate phase at the surface of the slag, which is wetted. This means that alkali salts condensing on silicate-based ash will

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not immediately increase the strength of the ash cake, but that over tens of minutes, strength will continuously increase as the intermediate phase is formed.

4. SUMMARY AND CONCLUSIONS Tensile strength of an ash cake alone is not a good indicator of the propensity of an ash to form a bridge between filter elements in a hot-gas filter system. Since gravity is pulling down on the bridges, the tensile strength of the cake must be divided by the bulk density of the cake to give the critical thickness index, or CTI. The CTI has units of length and indicates the relative thickness of a cake or deposit that can form before

it will shed under its own weight. A high CTI indicates that the cake is relatively sticky for its weight and is more likely to form a bridge than a cake with a low CTI. CTI increases as cake porosity decreases, indicating that the strength of the cake increases relative to its weight as porosity decreases. The effect is much more prominent at higher temperatures. In addition, the strength of ash cakes is much higher at higher temperatures. Finally, the presence of wetting liquids on the surface of ash particles can dramatically increase the strength of a cake even at concentrations as low as 1%. However, the liquid must wet the ash in order to increase the strength and alkali salts do not wet

silicate ashes until they have had time to react with the silicate material, indicating that strength will increase significantly over periods of tens of minutes in situations where liquid alkali salts are depositing.

5. REFERENCES Funk, J. E., and Dinger D. R. (1995). “Flocculation by VDW Energy/Particle Size Distribution.” Journal American Ceramic Society, 74(1).

Israelachvili, J. N. (1991). Intermolecular and Surface Forces. London: Academic Press. Kia, S. F. (1988). “Modeling of the Retention of Organic Contaminants in Porous Media of Uniform Spherical Particles.” Water Research, 22(10), 1301–1309. Miller, S.J., Laudal, D.L., and Heidt, M.K. (1993). “Cohesive Properties of Fly Ash and How They Affect Particulate Control Optimization” presented at the 10th Particulate Control Symposium and 5th International Conference on Electrostatic Precipitation, April 5–8, 1993, Washington, DC, Volume 1: Session A l .

Newby, R. A., Lippert, T. E., and Mudd, M. J. (1995). “Tidd Experience Prepares Hot Gas Cleaning Technology for Commercialization.” Power Engineering, 99(09), 20–24. Parfitt, G. D., and Sing, K. S. W. (1976). Characterization of Powder Surfaces. London: Academic Press. Snyder, T. R., and Pontius, D. H. (1996). “Particle Characteristics and High Temperature Filtration.” Proceedings of the 13th Annual International Pittsburgh Coal Conference. University of Pittsburgh, Volume 1. Chiang, S.-H., Ed. pp. 116–121.

CLASSIFICATION SYSTEM FOR ASH DEPOSITS BASED ON SEM ANALYSES Karin Laursen 1 and Flemming J. Frandsen2 1

Geological Survey of Denmark and Greenland Thoravej 8, 2400 Copenhagen NW, Denmark Phone: +45 3814 2000 Fax: +45 3814 2050

2

Technical University of Denmark Building 229, 2700 Lyngby, Denmark

Phone: +45 4525 2883 Fax: +45 4588 2258

1. INTRODUCTION During the last 15 years scanning electron microscope (SEM) and energydispersive x-ray (EDX) has been used extensively for solving problems related to coal combustion, especially in relation to the ash forming components of the coal. SEM and EDX have been used for analyzing the inorganic components of the coal, the fly ash and ash deposits and various automatic techniques have been developed to analyze these materials (i.e. CCSEM and SEMPC) [Lee et al., 1978; Huggins et al., 1980; Straszheim et al., 1988; Zygarlicke and Steadman, 1990; Jones et al., 1992; Skorupska and Carpenter, 1993]. These automatic techniques are strong tools for analyzing ash deposits. However, a major limitation of these techniques is their lack of capability to provide information on the appearance (i.e., texture and morphology) of the deposits, which can have major influence on the physical properties of the material. Hatt (1990) suggested a classification system of slags based on their macroscopic appearance, but no such attempt has been made to classify deposits based on microscopic appearance and microanalyses. Thus, this paper includes a suggestion to a classification system for deposits based on their texture. Additionally, the appearance is combined with the information achieved from automatic SEM-EDX analyses (i.e., SEMPC) of the deposits.

2. EXPERIMENTAL As part of a Danish collaborative project on “Mineral transformation and ash deposition in pulverized coal fired boilers” six full-scale trials were conducted at three Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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power stations in Denmark [Larsen et al., 1996; Laursen, 1997, Laursen et al., 1997].

Four of the coals burned during these trials were fuels with low ash deposition propensities (e.g., an Indonesian, a Colombian, a South African and a Polish coal). One coal had medium ash deposition propensities (e.g. a US high-S coal) and the last coal was a blend of the US and the Polish coal [Laursen, 1997]. During the full-scale trials deposits were collected on air and water-air cooled

probes with adjustable metal surface temperatures. The probes consist of a 1.5m long stainless steel pipe with a diameter of 3.8cm. The deposits are collected on a 10cm exchangeable piece of tube (test element) located approximately 10cm from the tip of the probe. In addition, some deposits from and an un-cooled ceramic protection cap of a suction pyrometer were analyzed. The test elements from the cooled probes were generally covered with a loose fly ash deposit on the downstream side (shelter side) and to a lesser extend on the sides. These loose deposits were often blown off the probes when the probes were retracted from the boiler due to air pressure from the probe and the pressure from the boiler. In addition, some of the fly ash was lost during dismantling of the test tube. Thus, no test elements contain a complete, intact deposit that indicates the magnitude of the deposition rate on the downstream side. Hardly any deposits were visible on the upstream (windward side) of the test elements from the combustion of the fuels with low ash deposition propensities,

except for a thin, black scale and some small irregularities (e.g. islands). The upstream side of the test elements exposed during combustion and the US coal and the blend were normally covered with a hard-bonded, rough deposit. Video recordings of one of the probes taken shortly before the probe was removed from the boiler showed that most of the deposits on the upstream side of the probe were lost during retraction. These deposits were significantly thicker (approximately 3–4cm) than the deposits found on the test elements (maximum 2mm after 8 hours) especially during combustion of the US coal. Thus, the deposits collected on the probes only represented the hard-bonded deposits.

3. CLASSIFICATION OF ASH DEPOSITS BASED ON TEXTURE Based on the texture, the deposits collected on the probes can be classified into five main types 1) porous deposit; 2) powder deposit; 3) iron-rich deposit; 4) semi-fused slag; and 5) fused slag (Fig. 1).

3.1. Porous Deposit The porous deposits are dominated by deformed, large iron-rich particles (10–250 surrounding mainly smaller and some larger Al-silicate fly ash particles (Fig. la). The deformed shape of the iron-rich particles is caused by collision with surrounding particles indicating that the iron-rich particles had low viscosity during impact. Locally, the iron-rich particles and large Al-silicate fly ash particles are arranged in “finger-structures” perpendicular to the tube surface. In some areas, several large particles create a very compact deposit. Holes or “pockets” between the large particles are normally loosely packed with smaller Al-silicate fly ash particles. These Al-silicate particles are only loosely connected to each other but locally they are totally incorporated into iron-rich particles. The porosity of the porous deposits varies from approximately 20 to 30% (based on image analysis). However, locally the porosity is close to zero in islands of iron-rich particles and up

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to 60% in “pockets” filled with small Al-silicate particles. Porous deposits were only collected during combustion of the US coal and the coal blend of US coal and Polish coal. Porous deposits were mainly seen on the upstream side of the probes and partly on the sides. The thickness of the collected porous deposits varies from to a maximum observed thickness of 2mm (after 8 h). Walsh and others (1990) reported deposits of a similar appearance as the porous deposits on a probe inserted at the exit of a pilot-scale furnace. Walsh and others (1990) termed the deposits “hard bonded deposits”, and related their morphology and occurrence to preferential deposition of iron-rich fly ash particles.

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SEMPC analyses of the porous deposits reveal that two main phases are present: iron-rich particles (i.e. iron oxide) and Al-silicates (i.e. illite, kaolinite and montmorillonite derived). The two chemical phases, iron-rich and clay-derived, are also clearly distinct on the ternary diagrams illustrated in Fig. 2, represented by the peak in apex and the peak on the respectively.

3.2. Powder Deposit Powder deposits consist of loosely bonded fly ash particles (Fig. 1b). The fly ash particles are mainly Al-silicates but some Fe-rich and Ca-rich particles are also present. The fly ash particle sizes varies from to The individual fly ash particles are very loosely bonded and only few neck formations between adjacent particles are seen. Sub-micron particles are often located between larger particles and these small particles are probably important for the strength of the deposit. Spot analyses and x-ray mappings of neck-formations revealed that Ca-sulfate often is the bonding material between two particles. The individual particles within the deposit are clearly only loosely connected and no neck formations between large particles are seen. Some of the larger

fly ash particles have a rough surface that appears like “scales” Spot analyses of these “scales” reveal that they often are rich in Ca-sulfate but Al-silicate “scales” are also seen. The “scales” are not spherical as the small fly ash particles but they posses a more irregular shape. The powder deposits were observed on probes exposed during combustion of the four coals with low ash deposition propensities (e.g. the Indonesian, the Colombian, the South African and the Polish coal). Powder deposits were mainly present on the downstream side of the probes but locally also on the wings. The total thickness of the deposits varies from to (after 6.5h) but the thicker deposits are only seen

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locally on the probes. As previously discussed most of these powder deposits were lost as the probes were retracted from the boilers. Deposits of similar appearance as the powder deposits were reported by Hurley and others (1994; 1995) as “downstream powder deposits”. These deposit were collected on the downstream side of probes and their deposition could be caused by eddy deposition of small fly ash particles [Hurley et al., 1994; 1995]. As indicated by the SEM images, SEMPC analyses reveal that the majority of the fly ash particles in these deposits are Al-silicates but some Fe-Al silicates are also present

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(Table 1). The Al-silicates (represented by the peak located on the and Fe-Al silicates (represented by the numerous particles located on the tie line between the Al-silicate peak and the apex) are also apparent on the ternary diagrams (Fig. 2). SEMPC analyses of powder deposits showed very low concentrations of calcium sulfate (Table 1). Manual SEM investigations reveal that Ca-sulfate is present mainly where individual particles in the powder deposits stick together and it is believed that Ca-sulfate is responsible for some of the strength of the deposits. The Ca-sulfate is not present as single particles but as a coating on larger particles. This phenomenon explains why few points in the SEMPC analyses are classified as Ca-sulfate. If a phase is present as a thin coating on another particles, a spot analysis of the coating will be influenced by the chemical composition of adjacent and underlying particles. The small amount of Ca-sulfate and the few observations of particles bonded by Ca-sulfate indicate that this bonding mechanism cannot solely be responsible for the strength of the deposits. Often very small fly ash particles (sub-micron) are seen between the larger fly ash particles. It is possible that the bonding of the powder deposits is not only a chemical phenomena but also a physical phenomena controlled by surface forces including van der Waals, electrostatic and surface-tension forces [Raask, 1985].

3.3. Iron-Rich Deposit The iron-rich deposits consist of Al-silicate fly ash particles embedded in a dense iron-rich matrix (Fig. 1c). Iron-rich deposits are always located adjacent to the tube surface or the oxide layer and it is difficult to identify the boundary between the oxide and the deposit. In general, the presence of fly ash particles indicates that the layer is deposited. The Al-silicate fly ash particles are characteristically spherical but irregular particles are also seen (mainly quartz). On the upstream side of the tubes, the contact between the fly ash particles and the matrix is less distinct compared to the downstream side. These diffuse contact zones could be reaction rims indicating dissolution of the fly ash particles on the hotter upstream side. The fly ash particles are considerable larger on the downstream side of the probes compared to on the upstream side whereas the deposits are thicker on the upstream side compared to on the downstream side (exposure time 6 to 36h.). The fly ash particles in these deposits are significantly smaller than the mean diameter of the fly ashes from the electrostatic precipitator, which was measured to be in the range of 11 to (based on CCSEM). A noticeable difference between the upstream and the downstream side deposits is the texture of the matrix. On the downstream side, the matrix appears homogeneous, whereas on the upstream side it appears more non-homogeneous. Locally, in the matrix on the upstream side, remnants of fly ash particles can be seen. Iron-rich

deposits were, in addition to the powder deposits, the only type of deposit observed on the probes during combustion if the coals with low ash deposition propensities. During combustion of the US coal and the coal blend this type of deposit was only observed locally on the probes. Lee and Whitehead (1982) have reported deposits of similar appearance as the ironrich deposits on corroded steel tubes. Lee and Whitehead (1982) termed these deposits “iron oxide/sulfide scale” and related the deposits to a sudden increase in the steel corrosion rates. Due to the very limited thickness of the iron-rich deposits only very few SEMPC

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analyses have been conducted on these deposits. As expected, these deposits are dominated by iron-rich phases mainly categorized as iron-oxide (Table 1). Due to the very small particle size of the Al-silicate fly ash particles in the deposits no “pure” Al-silicate particles were detected by the SEMPC technique. When analyzing such small particles, the matrix (iron) will always be detected, too. However, spot analyses in the center of large particles indicate that the fly ash particles mainly are iron-free. The ternary diagram also illustrates the mixing of the iron-rich matrix and the Al-silicate fly ash particles (Fig. 2).

3.4. Semi-Fused Slag Semi-fused slags consist of fly ash particles which are sintered together forming a deposit with high porosity (50–70%) (Fig. 1d). In some parts of the deposits, the fly ash particles are fused into a compact mass whereas in other parts only a neck formation between the individual particles is seen. The visual appearance of the slag is like loosely bonded sand grains. Semi-fused slags were only collected on the un-cooled ceramic probes during the combustion of the Colombian, the South African and the Polish coal. The thickness of these semi-fused deposits was approximately 1.5 to 2cm after an exposure time of 10 minutes. Hurley and others (1995) have reported deposits of a similar appearance on the upstream side of steam tubes located in the secondary superheater region. Hurley and others (1995) termed the deposits “conventional high temperature fouling” and related the bonding of these deposits to viscous sintering of silicate material. In the classification system suggested by Hatt and Rimmer this type of deposit would be classified as a “sintered deposit” which normally is found in the upper furnace and convective pass [Hatt, 1990]. The majority of the phases present in these deposits are classified as clay-derived (e.g. illite, kaolinite, and montmorillonite derived) (Table 1). The deposits collected during combustion of the South African coal differs significantly from deposits collected during combustion of the Colombian and Polish coal. The deposits from South African coal has a higher content of Ca-Al silicates and a low content of Fe-Al compared to the other two deposits. The high content of Ca-Al silicates and the characteristic white color of the deposits from South African coal compared to the other two deposits supports that the Ca-Al silicates are the main reason why South African coals produce a light grey fly ash and deposits [Laursen, 1997]. In general, the phases present in the semi-fused slags are the same phases as found in the fly ashes. However, there is a higher content of particles classified as unknown in the semi-fused slag compared to the fly ashes. These unknowns probably represent the fused phases in the deposit. The ternary diagrams reveal the high content of clay-derived phases in the deposits indicated by the dominant peak on the -binary in both the and the ternary diagram (Fig. 2).

3.5. Fused Slag Fused slags are highly sintered and only remnants of single quartz fly ash particles can are seen (Fig. le). Large pores are common. Crystallization of various minerals with crystal textures indicating growth during super-cooling (hopper- and dendrite crystals) is common. Fused slags were never sampled directly on any of the test elements and the few samples of this type of deposits were collected on the un-cooled tip of a

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probe exposed in the furnace during combustion of the high-S US coal. Fused slags are similar in appearance to the dense slags collected as bottom ash. In the classification system suggested by Hatt and Rimmer fused slags are classified as “visicular-glassy slag” which is related to the furnace and high temperature regions of the convective pass [Hatt, 1990]. The textural analyses of the few samples of fused slags indicated that glass constitutes the main phase in these deposits. In addition, a diversified fraction of newly formed crystals and few remnants of fly ash particles (mainly quartz) are present. Table 1 lists the result of a SEMPC analysis of a slag collected during combustion of the high-S US coal on the tip of a probe exposed in the furnace. A larger fraction of the phases in the fused slag from the probe are classified as unknown compared to the phases in the bottom ashes (not shown). This lower content of unknowns in the bottom ash is due to a higher amount of new-formed crystals in the bottom ash. The ternary diagrams for the bottom ash also indicate the crystallization of new minerals. These new crystals are indicated by the chemical compositions located on tie lines from the peak around the bulk chemical composition (i.e. the glass) towards the binaries of the diagrams (Fig. 2). The fraction of points located at a distance from the bulk chemical composition is indicators of crystalline phases present in the slag. The fraction of crystals is important for the physical state of a deposit as increasing fraction of crystals will increase the resistance to fracture formation and thus increase the strength of the deposit [Wain et a/.,

1992]. Thus, a deposit containing a high fraction of points located at a distance from the bulk chemical composition would indicate a stronger deposit compared to a glassy deposit with the chemical compositions clustered around the bulk chemical composition (i.e. the glass).

4. ASH DEPOSIT MECHANISMS The build-up of the porous deposits on the upstream side of the probes is most likely controlled by preferential inertial deposition of iron-rich particles (Fig. 3). The deformed shape of the large iron-rich particles clearly indicates that the majority of these

particles were molten when they impacted the tube surface or other ash particles in the

deposit. The viscosity of iron-rich particles is largely controlled by the oxidation state of the iron [Bool and Helble, 1996]. In case the fly ash particles are in a partly reduced state upon impact on a heat transfer surface they are likely to stick to the surface, however, if the particles are totally oxidized before impact they will have a high viscosity and likely to rebound [Moza and Austin, 1981; Abbott and Austin, 1982; Bool and Helble, 1996]. The build up of a “finger-like” structure in a porous deposit is probably initiated

by deposition of relatively large iron-rich particles. The outer part of such large particles may penetrate a thermal boundary layer around the tube to an area where the tempera-

ture is considerably higher. The temperature might even exceed the melting temperature of depositing particles and larger Al-silicate particles are likely to stick upon impact on the molten surface.

Local eddies between the “finger-like” structures are probably important for the deposition of the “pockets” of small Al-silicate fly ash particles. These small particles can be caught by eddies between the “fingers”. The velocity of these particles will be reduced and they will loose all their kinetic energy. As the porous deposits build-up, the temperature on the outside of the deposit gradually increases due to the low thermal conductivity of the deposit. Finally, the deposit sinters into a semi-fused slag and eventually

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into a fused slag. Fluxing occurs at the surface of a deposit between 815°C and 1,300°C (lowest eutectic in the system) [Bryers, 1996]. The deposition of the semi-fused slag and the fused slag is controlled by inertial impaction. However, it is no longer a preferential deposition of iron-rich particles, but a deposition of any particle reaching the hot surface of the deposit. This type of deposition leads to a chemical composition in the slag which is closer to that of the fly ash compared to the initial deposited layers. Crystallization of phases within the fused slags occurs when the melt reaches the liquid us for the crystals. Quenching of the melt can either occur when the slag is still bonded to a heat transfer surface due to insulation from radiation by newly deposited

material, or after detachment from the surface when the slag falls into the ash hopper. The crystallization may also take place as a re-crystallization from the glass. The fraction of crystals is important for the physical state of a deposit as increasing amount of crystals will increase the resistance to fracture formation and thus increase the strength of the deposit [Wain et al., 1992]. The deposition of the powder deposits is probably controlled by eddy deposition. Particles can be caught by eddies behind the tubes, loose their kinetic energy, and be more likely to stick upon impact. The powder deposits are significantly finer grained than the majority of the fly ash because the medium and large fly ash particles were not caught by the eddies but followed the main gas flow lines. The chemical analyses show that Casulfate may be a contributor to the bonding of the powder deposits. Sulfates are recognized as important bonding phases for low-temperature fouling deposits [Walsh et al., 1992; Osborn, 1992]. However, the mechanism of sulfate formation has not yet been clarified, especially regarding whether the sulfation takes place before or after deposition. Recent studies have revealed that for high-calcium coals the sulfate is fixed in-situ after the deposition [Hurley and Benson, 1995; Richards et al., 1996]. Richards et al., 1996, reported that the sulfation only occurred on Ca-rich fly ash particles and not

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on Al-silicates. These results do not agree with observations in this study where Casulfates also are seen on Al-silicates. The small amount of Ca-sulfate and the few observations of particles bonded by Ca-sulfate indicate that this bonding mechanism cannot

solely be responsible for the build-up of the strength in the deposits. Often, very small fly ash particles (sub-micron) are seen between the larger fly ash particles. It is possible that the bonding of the powder deposits is not only a chemical phenomena but also a physical phenomena controlled by surface forces including van der Waals, electrostatic and surface-tension [Raask, 1985]. The matrix of the iron-rich deposits may have two origins: 1) deposition of ironrich fly ash particles; and 2) diffusion of iron from the tube due to oxidation (corrosion) [Cutler et al., 1975]. Remnants of iron-rich particles can be identified locally in the deposits, mainly on the upstream side, indicating that impacting iron-rich particles contributes to the matrix. However, this does not rule out that iron diffusion was an active mechanism. The Al-silicate fly ash particles embedded in the iron matrix of the iron-rich deposits on the upstream side of the probes are numerous and relatively smaller than on the downstream side, where fewer but relatively larger fly ash particles are seen. This differential particle size distribution is probably controlled by various mechanisms. The deposition of the small particles on the upstream side is probably controlled mainly by thermophoresis, whereas the larger particles on the downstream side are controlled

by eddy deposition. On the upstream side, larger particles impact and bounce off due to their higher kinetic energy. On the downstream side, these particles are caught by eddies behind the tubes. The particles will have lower velocity and are more likely to stick on impact. Larger fly ash particles which comprise the majority of the fly ash, will not be caught by the eddies but they will follow the main gas flow.

5. SUMMARY AND CONCLUSIONS Based on SEM analyses of deposits collected during full-scale trials at three power stations in Denmark it has been possible to classify the deposits into five textural: 1) porous deposit; 2) powder deposit; 3) iron-rich deposit; 4) semi-fused slag; and 5) fused slag. These five textural types not only have district textural characteristics, but they also posses characteristic micro-chemical features, which especially are apparent by the differences in the distribution of point analyses (SEMPC analyses) of the deposits in the two ternary diagrams: and In the development of a deposit the porous deposit and the iron-rich deposit represent initiation stages for coals with medium and low ash deposition propensities, respectively. The semi-fused slags and especially the fused slags represent consolidation or maturation stages of the build-up of an ash deposit. The powder deposits will develop on downstream sides of superheater tubes, either directly on the oxide layer or on an iron-rich deposit.

ACKNOWLEDGMENTS This project was funded by ELSAM (The Jutland-Funen Power Consortium, Denmark) and the Danish Research Academy. The three power stations: Ensted, Funen and Nordjylland-Vendsyssel are acknowledged for allowing and for helpful support during the full-scale trials. The Energy and Environmental Research Center, University

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of North Dakota, is acknowledged for support with development of SEM-EDX techniques at GEUS.

REFERENCES Abbott, M.F. and Austin, L.G. (1982) Studies on slag deposits formation in pulverized-coal combustors. 4. Comparison of sticking behavior of minerals and low-temperatures and ASTM high-temperature coal ash on medium carbon steel substrates. FUEL, 61 (8), 765–770. Bool I I I , L.E. and Helble, J.J. (1996) Iron oxidation state and its effect on ash particle stickiness. In L.L. Baxter

and R. DeSollar (Eds.), Applications of advanced technology to ash-related problems in boilers. Plenum Press. Bryers, R.W. (1996) Fireside slagging, fouling, and high-temperature corrosion of heat-transfer surfaces due to impurities in steam-raising fuels. Prog. Energy. Combust. Sci., 22, 29–120. Cutler, A.J.B. and Grant, C.J. (1975) Corrosion of iron and nickel base alloys in alkali sulfate melts. Conf.

Metal-Slag-Gas Reactions. Electroch. Soc. Princeton. N J. Hatt, R.M. (1990) Fireside deposits in coal-fired utility boilers. Prog. Energy Combust. 235–241.

Huggins, F.E., Kosmack, D.A., Huffman, G.P. and Lee, R.J. (1980) Coal mineralogy by SEM image analysis. Scanning Electron Microscopy. 1, 531–540. Hurley, J.P., Benson, S.A. and Mehta, A.K. (1994) Ash deposition at low temperatures in boilers burning high calcium coals. In J. Williamson and F. Wigley (Eds.), The Impact of ash deposition on coal fired plants. Ed. Taylor and Francis. Hurley, J.P., Benson, S.A., Erickson, T.A., Allan, S.E. and Bieber, J. (1995) Project calcium. Final report.

DOE/MC/10637-3292. Hurley, J.P. and Benson, S.A. (1995) Ash deposition at low temperatures in boilers burning high-calcium coals. 1) Problem definition. Energy and Fuels. 9, 775–781. Jones, M.L., Kalmanovitch, 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. In H.L.C. Meuzelaar (Eds.),

Advances in coal spectroscopy. Plenum Press. Larsen, O.H., Laursen, K. and Frandsen, F. (1996) Danish collaborative project on ash deposition in PF-fired boilers. In L.L. Baxter and R. DeSollar (Eds.), Applications of advanced technology to ash-related problems in boilers. Plenum Press. Laursen, K. (1997) Characterization of minerals in coal and interpretations of ash formation and deposition in pulverized coal tired boilers. Ph.D. Thesis. Geological Survey of Denmark and Greenland. Report 1997/65. ISBN 87-7881-022-7. Laursen, K., Frandsen, F. and Larsen, O.H. (1996) Slagging and fouling propensity: full-scale tests at two power stations in western Denmark. In L.L. Baxter and R. DeSollar (Eds.), Applications of advanced tech-

nology to ash-related problems in boilers. Plenum Press. Laursen, K., Frandsen, F. and Larsen, O.H. (1997) Ash deposition trials at three power stations in Denmark. Paper presented at the Engineering Foundation Conference in Kona, November, 1997. Lee, R.J., Huggins, F.E. and Huffman, G.P. (1978) Correlated Mössbauer-SEM studies of coal mineralogy. Scanning Electron Microscopy, 1 , 561–568. Lee, D.J. and Whitehead, M.E. (1982) Microanalysis of scales and deposits formed on corroding furnace tubes in coal-fired boilers. In D.B. Meadowcroft and M.I. Maning (Eds.), Corrosion resistant material for coal

conversion systems. Applied Science Publishers. London and New York. Moza, A.K. and Austin, L.G. (1981) Studies on slag deposits formation in pulverized coal combusters. 1 . Results on the wetting and adherence of synthetic coal ash drops on steel. FUEL, 60 ( 1 1 ) , 1057–1064. Osborn, G.A. (1992) Review of sulfur and chlorine retention in coal-fired boiler deposits. FUEL, 71 (2), 131–142.

Raask, E. (1985) Mineral impurities in coal combustion. Behavior, problems, and remedial measures. Hemisphere publishing corporation. Springer Verlag. Richards, G.H., Harb, J.N. and Baxter, L.L. (1996) Investigations of mechanisms for formation of deposits for

two Powder River Basin coals. In L.L. Baxter and R. DeSollar (Eds.), Applications of advanced technology to ash-related problems in boilers. Plenum Press. Skorupska, N.M. and Carpenter, A.M. (1993) Computer controlled scanning electron microscopy of minerals

in coal. IEA Coal Research. Perspectives, 1–21. Straszheim, W.E., Yousling, J.G., Younkin, K.A. and Markuszewski, R. (1988) Mineralogical characterization

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of lower rank coals by SEM-based automated image analysis and energy-dispersive X-ray spectrometry. FUEL, 67, 1042–1047. Wain, S.E., Livingston, W.R., Sanyal, A. and Williamson, J. (1992) Thermal and mechanical properties of boiler slags of relevance to sootblowing. In Proceedings of the Engineering Foundation conference on Inorganic transformations and ash deposition during combustion. Palm Coast, Florida. 10–14 March, 1991. New York. American Society of Mechanical Engineers.

Walsh, P.M., Sayre, A.N., Loehden, D.O., Monroe, L.S., Beer, J.M. and Sarofim, A.F. (1990) Deposition of bituminous coal ash on an isolated heat exchanger tube: effects of coal properties on deposition growth. Prog. Energy Combust. Sci., 16, 327–346. Zygarlicke, C.J. and Steadman, E.N. (1990) Advanced SEM techniques to characterize coal minerals. Scanning Microscopy, 4 (3), 579–590.

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 University of Oulu, Department of Chemistry

Linnanmaa FIN-90571 Oulu, Finland

1. INTRODUCTION The combustion of peat in power plant boilers has increased in recent years in Finland. While boilers involving pulverized fuel are still common, the utilization of flu-

idized bed boilers is rapidly increasing. FCB boilers are best suited for fuel with a low energy value because the increased efficiency is beneficial for the community heat

distribution. Peat has a low energy value and high water content and therefore resembles low rank lignitic coal [Moilanen et al., 1993]. The sulfur content of Finnish peat, however, is very low. The inorganic material inherent in peat is typical to that of the plants in the

bog [Spedding, 1988] and therefore the slagging tendency of peat ash can be expected to be dependent on the bog from which the peat is originating. Generally the operation of power plants utilising peat is unproblematic, but in some cases severe slagging may occur leading to a plant shutdown thus causing significant economical losses to the entire chain of energy production. Therefore the slagging tendency of peat ash needs to be predicted from the peat fuel prior to its combustion. The slagging of peat ash is connected with the high iron content leading to the formation of low melting point iron aluminosilicates [Heikkinen et al., 1997]. The partial melting of ash particles might lead to agglomerate formation in the fluidized bed. While SEM-EDS connected with an automated image analysis provides a convenient method to investigate the formation of the coating on the bed particle as well as the nature of the adhesive material binding the bed particles [Virtanen et al., 1997], The ease of agglomerate formation and slagging can also be tested by inspecting the compression strength of ash that can be considered as a measure for the degree of sintering of the ash particles [Hupa et al., 1989]. Since the melting and sintering—and ultimately slagging—all involve the formation of amorphous material in ash, it is important to devise an independent method to determine its content. The X-ray powder diffraction (XRD) technique is commonly used to identify the crystalline phases in solid samples. However, it also offers the option to determine the Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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content of the amorphous material, since the presence of non-crystalline phases creates a broad hump in the diffraction pattern, called the amorphous halo. The position and the width of the halo indicate the distribution of interatomic distances in the structure. The area under the halo depends on the amount of amorphous material in the sample and can therefore be used in its semiquantitative determination [Nakamura et al., 1989]. In this work we report a systematic study of the formation of amorphous material as a function of peat ash composition and its thermal history. The work is mainly carried out for synthetic oxide mixtures, but comparison is made to actual samples of peat ash obtained in standard laboratory conditions, pilot reactors, and in the power plant boilers.

2. EXPERIMENTAL A Siemens D5000 diffractometer with a goniometer was used for recording of the powder diffraction diagrams using radiation and 40mA) and a zirconium filter. Step size was 0.02° and the counting time was 0.3 seconds for each step. Diffraction diagrams were recorded at a range 5–60°.

The amorphous material induces a broad hump in the diffraction diagram, called a halo. The area of the halo is related to the content of the amorphous material in the sample.

The calculation was made by recording the background at three values of the angle: 52°, 57° and 59°. The integration over the whole range of 5–60° yielded the total area over the background (see Fig. 1). By use of profile fitting the area of peaks could be subtracted thus yielding the area of the halo.

The calibration standards were made by mixing silica gel and synthetic ash at different proportions (0, 10, 30, and 50 w-% of silica gel). They were ground in a

mortar for one minute and homogenized manually in an agate mortar. The powder diffraction diagrams of the calibration standards are shown in Fig. 2 and the calibration curve relating the area of the amorphous halo to the content of silica gel in the standard is shown in Fig. 3. Synthetic ash was made by mixing suitable metal oxides and sulfates in order to mimic the composition of the actual peat ash. A test series involving nine different samples of synthetic ash was carried out by subjecting the samples to heat treatment at different temperatures (300, 500, 600, 700, 800, 900, 1,000, 1,100, 1,200°C) for 1.5h. In addition, the tenth sample was included in the test series without heat treatment. After cooling to room temperature all samples were ground and homogenized as described for the calibration standards.

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The slagging Finnish peat was ashed using the ASTM (D 3174–89) standard ash procedure [ASTM, 1989a]. The composition of the peat ash was determined by mixing

the ash with (ASTM, D 3682–97) [ASTM, 1989b] and analyzed with a Philips PU-7000 ICP-AES and SpectraSpan IIIB DCP-AES spectrometers. The slagging peat ash contains over 75% of which is thought to be the main reason for its slagging properties [Heikkinen et al., 1997]. A Jeol JSM 6400 scanning electron microscope, Link X-ray spectrometer, and a Link ISIS-image processing program were used to study compositional distribution for the discrete particles in the samples. The acceleration voltage was and the current The sample distance was 15mm and the magnification 130. The sample treatment and the automated image analysis is described elsewhere [Virtanen et al., 1997].

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3. RESULTS

3.1. Calibration Direct calibration seems to be possible for the ash samples. The silica gel was selected as an appropriate candidate tor the amorphous material in the standards because the majority of the amorphous material formed in ash during combustion can be thought to be different silicates. Silica gel is known to form a substantial amorphous halo at the range 10–15° (see Fig. 2). Silica gel was mixed with the synthetic ash without heat treatment. It can be seen from Fig. 2 that the area of the halo increases as the content of silica gel increases. It can also be seen that the intensities of the diffraction peaks due to crystalline material decreases as the content of silica gel increases. This is exemplified by the main peak due to quartz at 12°. The calibration curve indicating the halo area as a function of the silica gel content is shown in Fig. 3. The calibration curve is approximately linear with a correlation coefficient of 0.955. This facilitates a semiquantitative determination of the content of the amorphous material in actual ash samples. The formation of the halo is also effected by grinding of the solid material. Long grinding times seem to induce the formation of microcrystalline particle in the powder that may explain the growth of the halo in the samples [Altree-Williams et al., 1981]. It is therefore important to standardise the grinding procedures as fully as possible.

3.2. Peat Ash The actual peat ash that has been selected for this study is known to be problematic in pf-boilers. It easily forms deposits during the combustion. This is believed to be due to the high iron oxide content in the ash [Heikkinen et al., 1997]. The composition of this peat ash is presented in Table 1. The X-ray diffraction diagram for the peat ash is shown in Fig. 4. Two amorphous halos can be seen. One halo located at 20 range 10–13° is caused by silicates and the other that is located at 15–20°, is probably caused by iron silicate phases. The quartz peak at 12° is clearly visible in the diagram. The other possible quartz peaks overlap with those due to iron-containing compounds. The quasi-ternary diagram [Virtanen et al., 1997] for the standard laboratory ash of this peat is shown in Fig. 5. It can be inferred from this figure that iron can form a wide range of aluminosilicates in ash that may explain its melting at low temperatures

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[Hutchings et al., 1995]. Whereas it is well-known that the composition of the inherent inorganic material in peat is typical to that of the plants from which the peat has been formed [Spedding, 1988], it is also probable that external mineral grains (like quartz) may be introduced into the peat during the production processes.

3.3. Synthetic Ash In this work the synthetic ash was used to model the thermal behaviour of the real peat ash that has been described above. The composition of synthetic ash was made as near to this peat ash as possible. The diffraction patterns of synthetic ash mixtures treated

at different temperatures seem to be very similar (see Fig. 6). This can be explained by the dominating presence of iron oxide in the synthetic ash. The large iron oxide content may well disguise the changes in diffraction patterns. The total area under the two halos

observed at ranges 7–13° and 15–20° is dependent on the temperature during the heat treatment (see Fig. 7). It is obvious that the content of amorphous material in synthetic ash depends on the temperature. The sample that has not been heat treated does not contain amorphous material. Upon heating the synthetic ash at 300°C for 1.5h the content of amorphous material is increased to about 8%. A sample that has been heated at 800–900°C shows

approximately 17% amorphous material. At higher temperature the content of amorphous material decreases. At 1,200°C only 5% of the material seems to be amorphous in the synthetic ash mixture. Possibly new phases are formed at high temperatures that

crystallize during the cooling of the sample to room temperature. It was not possible to identify these phases by XRD because of the dominance of iron oxide. SEM-EDS was

used to elucidate these new phases formed during the heat treatment processes.

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The composition of the synthetic ash has also been inspected by SEM-EDS. The quasi-ternary diagrams indicating the compositional distribution of discrete ash particles are shown in Fig. 8. It can be seen that with no heat treatment [Fig. 8(a)] the main components were expectedly pure aluminum oxide, quartz and iron oxide. Upon heating the material at 800°C the formation of aluminosilicates can be inferred [Fig. 8(b)]. At higher temperatures (1,200°C) a second aluminosilicate phase is formed [Fig. 8(c)]. The formation of this second silicate phase may explain the increase of crystallinity in the ash that was deduced by XRD (see Fig. 7). It is interesting to note that the synthetic ash, when heated at 1,200°C, and stan-

dard peat ash exhibited virtually identical contents of amorphous material (see Fig. 7). Consequently, their diffraction patterns are virtually equal (see Fig. 9). However, the SEM-EDS results indicate that the composition of the particles formed during standard ashing of actual peat covers a significantly wider range than those from synthetic ash as

clearly demonstrated by their quasi-ternary diagrams shown in Fig. 5 and Fig. 8(c). The standard peat ash has a more complicated mineral composition than the idealized synthetic ash. The inspection of the compositional distribution of the particles in terms of five elements provides only a rough qualitative approximation. The melting behaviour, however, that is seen as the formation of the amorphous halos may depend on the main phases present in ash. It seems that both the standard peat ash and synthetic ash behave similarly upon heating, and therefore this simple system of synthetic ash may be used to predict the factors affecting the slagging properties of real peat ash.

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4. CONCLUSIONS X-ray powder diffraction has turned out to be a potential technique to inspect the

slagging properties of ash. In this work we have reported the semi-quantitative determination of the content of amorphous material both in peat ash that is known to be slagging, as well as in synthetic ash the composition of which has been adjusted to mimic

the actual peat ash. It was established that the content of amorphous material in the synthetic ash depended on the temperature the material was heated to. A sample that had not been heated contained virtually no amorphous material. However, already at 300°C about 8% of amorphous material was found in the sample. The content of amorphous material reached a maximum value of 17% at 800°C. Upon further heating to 1,200 °C the content of amorphous material decreased to about 5%. This is probably due to the crystallisation of new phases upon cooling of the sample to room temperature prior to the recording of the XRD diagram. The synthetic ash was found to be a good model for the actual peat ash. Diffraction patterns of the standard peat ash and the synthetic ash that had been heated to

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1,200°C were virtually identical. Due to the high content of iron oxide phases in ash XRD could not be used to identify other phases formed during the heat treatments. The quasi-ternary diagrams that were obtained from the SEM-EDS analysis of about 1,000 ash particles provided additional information that can be used to discuss the composition of ash.

5. ACKNOWLEDGEMENTS Financial support from Academy of Finland and Liekki-2 Research Program is gratefully acknowledged.

REFERENCES Altree-Williams, S., Byrnes, J. G. and Jordan, B. (1981). “Amorphous Surface and Quantitative X-ray Powder Diffractometry.” Analyst, 106, 69–75. Nakamura, T., Samcshima, K., Okunaga, K., Sugiura, Y. and Sato, J. (1989). “Determination of Amorphous Phase in Quartz Powder by X-Ray Powder Diffractometry.” Powder Diffraction, 4(1), 9–13. Heikkinen, R., Laitinen, R. S. and Patrikainen, T., Tiainen, M. and Virtanen, M. (In press). “Slagging Tendency of Peal Ash.” Fuel Processing Technology.

Hutchings, I. S., West, S. S., Williamson, J. (1995). “An Assessment of Coal-Ash Slagging Propensity Using an Entrained Flow Reactor.” In L. L. Baxter and R. DeSollar (Eds.), Applications of Advanced Technology to Ash-Related Problems in Boilers, New York, Engineering Foundation.

Hupa, M., Skrifvars, B.-J. and Moilanen, A. (1989). “Measuring the Sintering Tendency of Ash by a Laboratory Method.” J. Inst. Energy, 131–137. Moilanen, A. (1993). “Studies of Peat Properties for Fluidized-Bed gasification.” VTT Publications 149, Espoo, 69. Spedding, P. J. (1988). “Peat” Fuel, 67, 883–900. Standard Test Method for Ash in the Sample of Coal and Coke From Coal, Annual book of ASTM standards,

05.05 (1989a), D 3174–89, 302. Standard Test Method for Ash in the Sample of Coal and Coke From Coal, Annual book of ASTM standards,

05.05 (1989b), D 1857–87, 222. Virtanen, M., Heikkinen, R., Patrikainen, T., Laitinen, R. S., Skrifvars, B.-J. and Hupa, M. (1997). “A Novel Application of CCSEM for Studying Agglomeration in Fluidized Bed Combustion”, Engineering Foundation Coference on The Impact of Mineral Impurities in Solid Fuel Combustion, November 2–7, 1997, Kona, Hawaii.

SYSTEM ACCURACY FOR CCSEM ANALYSIS OF MINERALS IN COAL R. P. Gupta, L. Yan, E. M. Kennedy, T. F. Wall1, M. Masson, and K. Kerrison 2 1

CRC for Black Coal Utilisation Department of Chemical Engineering The University of Newcastle 2 Pacific Power Advanced Technology Centre The University of Newcastle

1. INTRODUCTION The advanced techniques such as CCSEM or QEM*SEM (Skorupska and Carpenter, 1993, and Gottileb et al., 1991) are now able to provide the detailed analysis related to the minerals present in coal. The CCSEM technique uses an automated scanning electron microscope (SEM) and is programmed to scan pre-selected areas of a polished sample to capture the back scattered emission (BSE) images. The mineral particles are automatically detected by an increase in the BSE signal above a pre-set value of signal, termed as threshold intensity, corresponding to that of the coal matter. The electron

micro-beam detects the centre of the mineral grain by an iterative bisection of chords. The area of each mineral grain is also determined.

An energy dispersive X-ray (EDX) spectrum is acquired (0–20 keV) for five seconds from the centre of each particle detected. X-ray intensity data and size and shape parameters for a statistically significant number of particles are then collected at different

magnifications. The relative intensity of elements is related to a mineral type according to some heuristic rules (Steadman et al., 1991). CCSEM and QEM*SEM are the successful automated versions of this technique. The CCSEM technique determines the particular mineral types from the elemental analysis provided by the SEM, its size from measured dimensions and can establish if the mineral grain is included within a coal particle or if it is excluded from coal. This latter analysis requires mounting the coal sample in wax to differentiate the X-ray signal from coal matter and the mounting medium. An analysis of many thousands of mineral grains is then assembled into the final analysis in terms of mineral types, their size distribution and included/excluded Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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nature. This paper details a study in which two pulverised coals were analysed by three CCSEM systems: the ATC system at Pacific Power, Australia, the BYU system at the Chemical Engineering Department of Brigham Young University in Provo, Utah and MTI system at Microbeam Technologies Incorporated in Grand Forks, North Dakota. The coals were mounted prior to analysis in each laboratory in order to compare sample preparation techniques, and the same prepared sample (stub) was also analysed at ATC and BYU in order to compare the analysis system alone excluding the sample preparation differences. The major aspects leading to differences in analysis results are identified as the sample preparation technique and the number of mineral grains analysed. The ATC technique mixes crushed coal and wax into a pressed pellet, thereby guaranteeing a representative distribution, while the other laboratories disperse coal in molten wax where differential settling effects are possible. If a pulverised coal contains coarse minerals, then a greater number of grains comprising large size minerals will need to be analysed. The estimation of the progressive average of the analysis as the number of mineral grains increase may be used to establish the optimum number. This will depend on the coal characteristics and the application.

2. DIFFERENCES IN CCSEM TECHNIQUES The study has revealed that there are differences in sample preparation and operating conditions of the SEM used by different laboratories. This comparison is detailed in Table 1. Yang and Baxter (1992) have discussed in detail the effect of sample preparation and operating conditions on the results obtained from CCSEM. There are differences in the elemental analysis due to application of ZAF correction when the intensity spectrum is converted to elemental composition. The ZAF correction accounts for the changes in the relative intensity due its atomic number (Z), absorption (A) of X-rays by the sample and the fluorescence (F). A ZAF correction procedure (Holt, 1974) is usually applied to improve the overall elemental composition analysis. BYU applies the ZAF correction to their data, however, the other systems do not apply this correction. The ZAF correction is based on a calibration using standard mineral samples of known

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composition. The correction also accounts for the machine dependent parameters. For Si and Al species, the correction effect is small For the minor components, the correction is significant. The correction procedure should be developed by ATC. The number of mineral grains analysed by ATC was substantially less than that examined by the other laboratories. There were differences in the heuristic rules for the conversion of the elemental analysis to the mineral phases. BYU used a different set of mineral phases and different heuristic rules (Yu et al., 1993). The threshold intensities, defining mineral matter, coal matter and wax, are established by an operator during each analysis. Therefore, there are inherent problems in determining the total mineral content in coal due to assigning of the two threshold intensities required. ATC had problems in setting a threshold intensity to differentiate between coal matter and wax, and consequently, in the estimation of the total coal area. The total mineral matter content (as a percentage of coal) can not, therefore, be determined accurately. It should be noted that MTI determines their total coal area on the basis of the proportion of coal and wax. Setting up of the threshold intensity between the coal matter and the mineral matter appears to be, comparatively, simpler. Although it might be possible to establish a standard technique of sample preparation, and standard operating conditions for the SEM and a set value for threshold intensity, no such standards exist. Comparison of analysis from different laboratories for the same sample is, therefore, the best way of evaluating this technique.

3. RESULTS AND DISCUSSIONS Comparisons are made for the CCSEM elemental analysis presented as equivalent oxides to compare with the XRF analysis of ash, the major minerals, the size distributions of particular minerals and the total minerals as well as coal performance indices. Galbreath et al. (1996) have also investigated the differences in the CCSEM techniques of six different laboratories. There are some inherent differences among these laboratories due to significant differences in operation of respective systems. The present paper also investigates the effect of these differences in CCSEM analyses on the performance indices derived from these analyses.

3.1 Equivalent Oxide Composition The elemental composition can be converted to the equivalent oxide composition and is compared with the standard ash analysis in Table 2, Figs. 1 and 2. It must be noted here that the XRF analysis accounts for all the inorganic material present in the coal,

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whereas the CCSEM technique cannot detect any inorganic matter that is organically

associated or in grains smaller than one micron in size. The oxide composition determined from CCSEM analysis is close to that determined by XRF except for aluminum and calcium oxides. The CCSEM data from BYU also gave similar values for . It is not been possible to explain the low estimation of alumina by ATC.

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3.2 Comparison of Major Minerals The mineral grains are classified into a number of mineral types based on the elemental composition and heuristic rules. The heuristic rules used by ATC are the same as

those used by MTI (Steadman, 1991). The heuristic rules used by BYU are slightly different (Yu et al., 1993). The mineral compositions for major mineral phases thus determined are compared in Figs 3 and 4 for Coal A and Coal B respectively. Hereafter, BYU(P) is the analysis by BYU on the same coal sample (stub) as that used by ATC. It can be seen that the BYU(P) results of mineral contents are closer to the ATC results than the MTI and BYU results, except that there is a 9% deviation of clays content between the BYU(P) and ATC data. This suggests that the different sample preparation techniques add to the differences in the analyses. Similar differences in major minerals from different laboratories have been observed by Galbeath et al. (1996).

3.3 Comparison of Size Information The Particle Size Distribution (PSD) of mineral matter from ATC is compared here with that determined from BYU. The comparison could not be made with the results from MTI, as that laboratory had crushed the samples before analysis. The differences are quite significant for stubs prepared by different laboratories. However, approximate results are obtained when the same stubs are analysed by ATC and BYU(P). This suggests the sample preparation technique used by BYU is different. The particle size distributions (PSD) from these laboratories are based on 2Dimensional images from SEM and need stereological correction to obtain the true PSD. A stereological correction for the particle size distribution of minerals PSDs was also developed to transform data from two dimensional information (as measured in CCSEM) to three dimensional results, which is required for estimation of particle volumes. The

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correction does influence the absolute PSD values, making the PSD curve shift to coarser grain sizes. However, it was not found to influence the ranking of coals according to the indices proposed here. The correction has not been used for any of the above results, as other laboratories do not apply this correction. However, the correction is expected to be very important in determining the mineral-mineral and mineral-coal associations. Figures 5 and 6, and Table 3 show the comparison of mineral PSD determined at ATC and BYU.

4. ACCURACY OF RESULTS The accuracy of results is a function of number of particles analysed and the uniformity of coal distribution in the stub being examined. The uniformity of distribution

is improved by refining the sample preparation technique. The effect of number of grains analysed on the accuracy of results is discussed in the following sub-section. The

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usefulness of the CCSEM data lies in deriving advanced indices for the performance of coal. The performance indices derived from the CCSEM data from different laboratories for two coals are also compared in this section.

4.1 Number of Mineral Grains The fraction of mineral grains less than size determined by ATC for both the coals matches that from BYU(P). However, there are significant differences in the coarse size fractions for Coal B. One large grain of can result in a difference of about 2% by mass. This can influence the mineral percentage or composition significantly. Table 4 presents the number of grains larger than from the ATC system. It is observed that 1–2 mineral grains (0.1–0.2% by number) of siderite or pyrite larger than 50µm, analyzed in the total area, comprise about 3–4% of these minerals by mass. Rolling averages of individual components of mineral grains shown in Figs 7 and 8 suggest the analysis of additional mineral grains for better estimates of compositions of coal A is needed. Analyzing 400 mineral grains for coal A at 50X magnification is not adequate. For coal B, it appears, an adequate number of mineral grains may have been examined. However, if a coal performance index is based on the presence of large pyrite grains, then additional grains would need to be analysed.

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4.2 Coal Performance Indices There have been several attempts in developing advanced indices for ash deposition and abrasion based on CCSEM data (Gupta and Wall, 1995, Kalmanovich, 1992, Wigley and Williamson, 1995, and Zygarlicke et al., 1992). First order estimates of indices for ash deposition, and are defined as the mass percentage of particles with basic oxides greater than particular levels (40% and 80% respectively). , the mass percentage of equivalent ash particles having viscosity less than particular value (l00 Pa.s or 1,000 poise at 1,250°C) is also defined as one of the ash related index. The first order estimates for indices of abrasion, and are derived from the mass of quartz particles greater than a certain size (10 µm and 20 µm, respectively), expressed as a percent-

age of total minerals. The indices for Coal A and Coal B are compared in Figs. 9 and 10. The indices determined from the same plug, related to ash deposition, are similar. However, the indices from ATC and BYU(P) for abrasion differ for Coal B. This may again be related to the number of mineral grains analyzed by ATC. Fine quartz particles below in size may easily be entrained in PFBC and cause erosion of turbine blades. Another index, is, therefore, defined as mass fraction of quartz particles less than in size (expressed as a percentage of total minerals) for coal erosion performance in PFBC. The indices derived from the two laboratories are seen to be similar. The first order estimates for the indices can be improved with an analysis of the included/excluded nature of the mineral grains. For example, included quartz is reactive and would either react with other mineral grains present in the same coal particle or would soften in a highly reducing environment. On the other hand, excluded quartz particles would not react in this way due to the oxidising environment in the combustion stream. Thus, a second-order estimate for abrasion index would be based on the mass fraction of excluded quartz particles bigger than a critical size. The comparisons for included and excluded minerals could not be made in this study. MTI and BYU did not determine the included/excluded mineral distribution. A semi-automatic and interactive computer program for identifying included/excluded

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minerals from CCSEM data files and images has been developed in this project. This technique has been validated by comparison with results from the Energy & Environment Research Center (EERC) at BYU. A fully automated system being developed at ATC awaits proving.

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5. CONCLUSIONS It can be concluded from this international cooperative study that the main factor affecting the accuracy of CCSEM analysis is the coal sample preparation technique. The differences of the CCSEM systems including their operating conditions only play a minor

role to account for the discrepancy of CCSEM measurements, as different laboratories gave similar CCSEM results on the same coal sample stub. The performance indices related to ash deposition determined from the CCSEM analyses from BYU and ATC were found to be similar. This study indicates that the examined number of mineral grains which is necessary for a correct analysis will depend on the character of the minerals in a coal, in that the presence of a few large grains can influence the composition reported on a mass basis. The need for this information depends on the application, for abrasion issues it will clearly be important. An analysis system may also be configured to preferentially detect such grains. The reporting of results as the mass of particular minerals related to the mass of coal matter, rather than related to the total minerals, would also give results which are satisfactory for the fine minerals. The analysis procedure can therefore be optimized once the application of the analysis is specified.

6. ACKNOWLEDGEMENTS The authors are grateful to John Harb and Peter Slater of Brigham Young University and Steve Benson of Microbeam Technology Inc. The authors also acknowledge the financial support for the project from Cooperative Research Centre for Black Coal Utilisation.

7. REFERENCES Dehoff, R.T. and Rhines, F.N. (1968), Quantitative Microscopy, McGraw-Hill, New York. Holt, D.B. et al. (1974), Quantitative Scanning Electron Microscopy, Academic Press.

Galbreath, K. et al. (1996), “Collaborative Study of Quantitative Coal Mineral Analysis Using ComputerControlled Scanning Electron Microscopy”, Fuel, Vol. 75 No. 4. Gottlieb, P. et al. (1991), The Characterisation of Mineral Matter in Coal and Fly Ash, Eng. Foundation Conf., Palm Coast., FL., USA. Gupta, R.P. and Wall, T.F. et al. (1995), Inorganic Transformations and Ash Deposition During Combustion,

Eng. Foundation Conf., New Hampshire. Kalmanovitch, D.P. (1991), ibid. Skorupska, N.M. and Carpenter A.M., Computer Controlled Electron Microscopy of Minerals in Coal, IEA

Report (1993). Steadman, E.N. et al. (1991), Inorganic Transformations and Ash Deposition During Combustion, Eng. Foun-

dation Conf., Palm Coast. Williamson, J. and Wigley, F. (1995), ibid. Yang, N.Y. and Baxter, L.L. (1991), The Characterisation of Mineral Matter in Coal and Fly Ash, Eng. Foun-

dation Conf., Palm Coast., FL., USA. Yu, et al. (1993), The Impact of Ash Deposition on Coal Fired Plants, Engg. Found. Conf., Solihull. Zygarlicke, et al. (1991), Inorganic Transformations and Ash Deposition During Combustion, Eng. Foundation

Conf., Palm Coast.

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THE MICROSTRUCTURE AND MINERAL CONTENT OF PULVERISED COAL CHARS

F. Wigley and J. Williamson Imperial College Department of Materials London SW7 2BP, UK

1. INTRODUCTION The nitrogen release and burnout behaviour of a suite of eight world-wide coals has been studied in a UK collaborative research programme. As part of this programme,

samples of pyrolysis char were prepared in a drop-tube furnace at International Combustion Ltd and analysed by CCSEM at Imperial College. The eight coals used were Asfordby, Betts Lane, Hunter Valley, Kaltim Prima, Koornfontein, La Jagua, Pittsburgh and Thoresby. The objective of this CCSEM study was to characterise the particles from the suite of coal char samples, in respect of the size, porosity, microstructure and ash content of individual char particles. The aim of this work was to provide a description of the char samples that would assist in a comparison between the behaviour of the eight coals in the drop-tube furnace, and would provide basic data for the modelling of char combustion reactions.

2. ANALYTICAL TECHNIQUE Each coal sample was mounted in epoxy resin and prepared as a polished crosssection perpendicular to the settling direction, to eliminate the effects of density segregation. Digital back-scattered electron images were collected at magnification at forty random points across the sample cross-section (e.g. Figs. 1–4). Clusters of pixels with intensities corresponding to the average atomic number range of the carbonaceous component of char were located. In addition, clusters of pixels with intensities corresponding to the average atomic number range of ash derived from coal mineral matter were located. The carbon and ash pixel clusters were combined, to identify the char particle cross-sections. Each char particle cross-section was analysed for size, shape

and composition (carbon, ash and included pores). In addition to the parameters that Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwcr Academic / Plenum Publishers, New York, 1999.

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have been measured for each cross-section, an estimate of particle wall thickness was calculated. Although some attempt has been made to automatically rejoin breaks in the crosssections of porous particles, the extent of this process has been limited by the desire not to connect adjacent cross-sections into a single particle. No correction was made for the difference between the apparent size, in cross-section, and the true dimensions of the mineral occurrences. No account has been taken of pores that were not fully enclosed. To minimise the effect of noise within the image, particle cross-sections smaller than in size were ignored. Between 1,200 and 5,000 char particles were analysed for each sample.

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3. RESULTS

3.1. Particle Size There are a number of parameters that can be used to measure the sizes of particles in cross-section. For this analysis, a commonly used approach has been adopted, in which size has been defined as the diameter of a circle with the same area as the carbon, ash and enclosed pores in the particle cross-section. The average particle sizes for the eight samples are listed in Table 1, and the particle size distributions for each sample are shown in Fig. 5. The eight char samples can be grouped by average particle size in the following way: La Jagua, Asfordby, Kaltim Prima Thoresby, Hunter Valley, Koornfontein Pittsburgh, Betts Lane.

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The char particle size distributions were all similar. Figure 5 shows a typical size distribution, in which there are very few particles that are smaller than the lower size limit of the char analysis procedure However, the shape of the size distributions at the

upper end of the size range indicates that the very largest particles in the char samples have not been analysed.

3.2. Particle Porosity Particle porosity has been measured as a pore fraction, using the ratio between the area of the enclosed pores and the total area of the particle cross-section (carbon + ash + enclosed pores). The eight char samples can be ranked and grouped by average particle porosity (Table 2—Pore fraction) in the following way: La Jagua, Asfordby, Kaltim Prima Koornfontein, Hunter Valley, Thoresby Betts Lane, Pittsburgh. Char porosity results from the interaction between plastic carbonaceous material and evolving gasses; included mineral matter probably plays little part in this process. The “pore/carbon ratio”, calculated as the ratio between the enclosed pore area and the carbon area for a particle, has been calculated in order to eliminate the effect of the ash component from the estimation of porosity. Average pore/carbon ratios (Table 2) rank the char samples in the same way as average porosity (above), but this ranking, which is

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independent of the widely varying ash contents of the chars, shows a clearer distinction between the three char groups. From Table 3, it can be seen that char particles below about diameter were fully solid for all eight samples. For char particles larger than about the area fraction of enclosed pores increased with increasing particle size. The epoxy resin used to mount the char samples contains pores, which may occasionally appear as solid circular char particle cross-sections. In addition, the inability of the mounting medium to penetrate large enclosed pores may result in these pores being identified as carbon, rather than pore. As a result, the measured porosity of the larger char particles Table 3) is less precise than for smaller char particles, and probably underestimates the true porosity.

3.3. Particle Ash Content Particle ash content has been measured using the ratio between the area of the inherent ash and the total area of the particle cross-section (carbon + ash + enclosed pores). The average ash contents vary between 5 and 19% (Table 2), and correlate well with the proximate ash contents of the chars with the exception of the Kaltim Prima sample. Because the ash content has been determined on an area basis, rather than by mass, the measured ash contents are lower than the actual values. The way in which ash content varies with particle size is not consistent between samples (Table 4).

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3.4. Particle Composition Each char particle cross-section has been analysed in terms of three components: carbon, ash and enclosed pores. In every char sample, the particles have a wide range of compositions; there are no clear distinctions between porous and non-porous p or between particles with and without ash. Many cross-sections contained carbon, ash and enclosed pores.

3.5. Particle Microstructure Char particle microstructures are conventionally divided into “solid”, “network” and “cenosphere” categories. Both network and cenosphere char particles are porous; cenospheres contain one large pore, while network chars contain several pores. Further microstructural categories have been proposed for particles that contain a significant ash component, for porous particles with thick and thin walls, and for particles that contain features of more than one “ideal” microstructure, as reviewed by Cloke and Lester [1994]. These frameworks for classifying char particle cross-sections are based on optical microscopy, which is unable to reliably identify ash inclusions and usually classifies them as pores. In addition, char classification is frequently performed manually (rather than automatically), with a resultant emphasis on larger char particles that display more “ideal” microstructures. The char classification is frequently described, rather than defined. The analytical procedure used for the char samples described in this paper is readily able to distinguish carbon, ash and enclosed pores, and has measured the same parameters for a large number of randomly-chosen char particle cross-sections of widely varying size and shape. However, this analytical procedure is currently unable to re-create the microstructure of large fractured particles in the same way as the subjective analysis of a skilled microscopist. Given the continuous distribution of particle compositions described above, a simple, numerically based classification scheme for the microstructure of char particle cross-sections has been devised. The particles analysed in this study have been classified as “solid” if they have a pore fraction below one tenth, and as “porous” if the have a greater pore fraction. Between a quarter and one half of the char particles in each sample were solid (Table 5), and the fraction of solid particles increased in the following way: Pittsburgh Betts Lane, Koornfontein, Hunter Valley, Thoresby Asfordby, La Jagua, Kaltim Prima.

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Almost all the char particles smaller than 8µ m are solid (Table 6), and the fraction of solid particles decreased with increasing particle size. Between 15% and 35% of the larger char particles were solid, but these values were not consistent for the two size bands (Table 6) and probably reflect the analytical errors described above. A value for “wall thickness” has been measured for each particle analysed, based on the average distance between each pixel in the cross-section of the particle (combined carbon and ash) and the nearest pixel on the internal or external perimeter of the particle cross-section. The porous particles have been classified as “thin-walled” if their thickness is less than one fifth of their diameter, or as “thick-walled” if they have a greater thickness. The proportion of thick-walled porous particles in each char sample was relatively constant at about 10% (Table 5), although the fraction for the Pittsburgh sample was higher. The proportion of thin-walled porous particles in each char sample showed the opposite variation to the proportion of solid particles, and ranks char samples in the reverse of the ranking shown above. The fraction of porous thin-walled particles in each size band is listed in Table 7. There are very few thin-walled porous particles below in size, and the proportion of this particle type increased rapidly with increasing particle size. For the larger particles, the analytical errors described above made measurements of thickness slightly less reliable.

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4. DISCUSSION From the results described above, it is clear that none of the char samples could be distinguished by the presence or absence of particles of a particular size, porosity or microstructural type. However, there is a high degree of consistency between the ranking and grouping of the eight coal char samples, based on average particle size, average particle porosity, average pore/carbon ratio, average fraction of solid particles, and average fraction of thin-walled porous particles (Table 8). Comparisons between samples based on the properties of individual particle size bands frequently produced the same ranking and grouping. The ranking of the eight char samples by char microstructure did not correlate with the proximate analyses of either the chars or their parent coals (Table 9). Comparison

with data from other experiments within the collaborative project indicates that the char particle properties do not correlate with either initial rates of volatile loss or final levels of burnout during combustion trials of these eight coals.

5. CONCLUSIONS Based on CCSEM analysis of eight char samples prepared from coals of different rank and ash contents, the following conclusions can be drawn: 1. The carbon, ash and pore components of char particle cross-sections can be distinguished in back-scattered electron images; image processing of these images has been used to provided quantitative descriptions of char samples.

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2. There was no significant difference between the eight char samples in terms of the presence or absence of char particles of a particular size, porosity or microstructure. 3. There was a consistent ranking of the char samples with respect to size, porosity, pore/carbon ratio, fraction of solid particles, and fraction of thin-walled porous particles. This ranking separated the coal char samples into three groups: La Jagua, Asfordby, Kaltim Prima; Koornfontein, Hunter Valley, Thoresby; Betts Lane, Pittsburgh. For all eight samples, char particles smaller than about contained very little porosity. The level of porosity increased rapidly in the larger particles, with the most porous particles having walls that were thin with respect to their size.

6. ACKNOWLEDGEMENT The authors gratefully acknowledge the funding and samples provided by Dr A. Thompson of International Combustion Ltd.

7. REFERENCE Cloke, M. and Lester, E. (1994). “Characterisation of coals for combustion using petrographic analysis: a review.” Fuel, 73, 315–320.

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FIRESIDE CONSIDERATIONS WHEN COFIRING BIOMASS WITH COAL IN PC BOILERS# Allen L. Robinson, Larry L. Baxter, Gian Sclippa¶, Helle Junker § , Karl E. Widell†1, Dave C. Dayton, Deirdre Belle-Oudry 2 , Mark Freeman, Gary Walbert‡, and Philip Goldberg3 1

Combustion Research Facility Sandia National Laboratories Livermore, CA 94551-0969 2 National Renewable Energy Laboratory Golden, CO 80401-3393 3 Federal Energy Technology Center Pittsburgh, PA 15236-0940

INTRODUCTION This paper discusses fireside issues associated with cofiring biomass and coal in pulverized-coal-fired (pc-fired) boilers. The primary motivation for such cofiring is effective reduction of emissions, as sustainably managed biomass feedstocks are essentially neutral. Sustainable management in this context means that biomass resources are consumed at the same rate as they are produced with no adverse environmental effects. Under such management, generated during biomass combustion is reincorporated in plants via photosynthesis, helping to close the carbon cycle and resulting in virtually no atmospheric accumulation. In many cases, cofiring offers several additional environmental benefits, including: (1) reduced production of criteria pollutants such as oxides of sulfur and nitrogen (2) reduced open-field burns and relief from the environmental costs associated with them; and (3) decreased quantities of landfill or other waste materials. The realization of these benefits requires optimization of power systems to make best use of the blended fuels. The US has considerable experience in operating biomass-fired power stations, #

Presented at the Engineering Foundation Conference on the Impact of Mineral Impuriteis in Solid Fuel Combustion, Nov. 2–7, 1997, Keauhou Beach Hotel, Kona, Hawaii ¶ Onsite Engineering, Livermore, CA § lsamprojekt/Aalborg University, Denmark † Professor, Mechanical Engineering, Aalborg University, Denmark ‡ Parsons Power Group, Inc., Library, PA Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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ranging from reliable operation of wood-based systems to troublesome operation of systems using agricultural residues (Baxter, Miles, Jr., Jenkins, Milne, Dayton, et al., 1997a). Overall efficiencies and availabilities of biomass power plants are low compared to traditional coal-fired plants. For example, net plant conversion efficiencies of biomass systems are roughly half that of coal (16–20% for biomass compared with 33–38% for coal, HHV basis), and availability of biomass stations is considerably lower than that of coal (Baxter, Miles, Thomas R. Miles, Jenkins, Dayton, Milne, et al., 1996). At the same time, the demand for renewable energy is growing, with biomass offering some of the greatest potential for significant market penetration.

Biomass-coal cofiring takes advantage of the best aspects of both coal-based and biomass-based systems while addressing and other environmental and economic issues. Power production from coal exceeds that from biomass by at least two orders of

magnitude. Therefore, even a relatively minor incorporation of biomass as a cofiring fuel with coal would significantly increase the potential unsubsidized market for renewable energy (biomass). Biomass cofired with coal achieves an effective reduction in emissions proportional to the thermal input from biomass. The efficiency of power production from biomass is nearly doubled in a cofired system compared to use of the same fuel in stand-alone biomass facilities. Biomass cofiring can reduce emissions of criteria pollutants compared to use that produced from pure coal systems. In addition, capital and operating costs decrease as a fraction of total power cost in a cofired system compared to existing biomass facilities.

This document presents selected results from a multi-year, multi-laboratory, interagency effort to evaluate the use of biomass as a cofiring fuel during power production from coal-fired power plants. The essential objectives of this project are to assess the potential fireside impacts of firing biomass in coal-based systems and to demonstrate the potential emission reductions and other environmental benefits from cofiring. The scope of the project is limited to fireside issues. Other issues of major importance, such as ash disposal, fuel handling and preparation, regulations and other institutional issues, and resource assessment/economic analyses of cofiring options at specific plants are not included except as they impact the fireside behavior. In this document, we use results of one of our most recent tests using switchgrass (SG) and Pittsburgh (P8) coal to illustrate cofiring combustion behavior. In several cases, we introduce results from our previous experiments (Robinson, Junker, and Baxter, 1997b) to help place the SG/P8 results in context.

TECHNICAL APPROACH

Experimental Facilities This investigation utilizes the experimental facilities and expertise of three institutes. The molecular beam mass spectrometer (MBMS) (Dayton, French, & Milne, 1995) used at the National Renewable Energy Laboratory (NREL) to characterize the composition of fuel off gases provides fundamental chemical data as a function of extent of combustion. The Captive Particle Imaging (CPI) system at Sandia National Laboratories (SNL) is a small flow reactor used to monitor the combustion history of an individual particle over its lifetime (Hurt and Davis, 1994). Sandia’s Multifuel Combustor (MFC) is a small (0.03–0.1 MMBtu/hr, depending on fuel type), pilot-scale, down-fired, turbulent flow facility that is capable of firing a wide variety of fuels and of reproducing the temperature and gas composition histories experienced by particles in commercial-scale pc systems systems (Baxter, 1992). The Combustion and Environmen-

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tal Research Facility (CERF) at the Federal Energy Technology Center (FETC) is a larger-scale pilot facility (0.5 MMBtu/hr) that allows commercial-scale engineering issues to be addressed (Freeman, Chitester, James, Ekmann, & Walbert, 1997). The combination of the facilities, together with observations from commercial-scale trials, allows more definitive data interpretation than any of the facilities could accomplish alone.

Fuel Selection and Characterization This project has examined a wide range of commercially viable coal and biomass fuels. This document focuses on the combustion behavior of a Wisconsin switchgrass

(SG) and a high-volatile A bituminous Pittsburgh coal (P8), properties of which are indicated in Table 1. Results from the previous tests are used to place the P8/SG results in context. Several replicate analyses were performed on both SG and P8. The coal properties exhibited little variation from analysis to anlaysis, but the biomass properties exhibited much greater variation. Such heterogeneity is a common characteristic of biomass. The Pittsburgh coal was fired as a utility grind (70% through 200mesh) fuel. The switchgrass sample was prepared using a tub grinder followed by a micropulverizer. resulting in a final product that passes through a 1 mm screen. Unlike equant coal particles, many SG particles exhibit aspect ratios of 3 or higher; consequently, the largest particles were approximately cylinders. Specifically, 98% of the sample was less than 20mesh (0.84mm) and 55% was less than 100 mesh (0.15 mm). The biomass and coal were blended prior to injection in all of the experiments reported here. All laboratories conducted comparative experiments with pure coal. SNL and NREL also conducted comparative experiments with pure biomass.

RESULTS AND DISCUSSION

Ash Deposition Mechanisms of ash deposition include inertial and eddy impaction, thermophoresis, condensation, and chemical reaction (Baxter, 1993). The contribution of each of these

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mechanisms depends on local chemistry, aerodynamics, and operating conditions, but alkali materials influence both the rates of deposition and the properties of the deposits much more than their concentration sometimes suggests. The mechanisms of alkali

release, deposition, and reaction are at least qualitatively understood. Alkali chlorides represent the most volatile of the common forms of alkali and are expected to be the most easily released from the particle. Chlorine is volatile in essentially all of its forms

in fuels and typically is released from the fuel early in the combustion process. Alkali, on the other hand, is dominantly released from the fuel during the high-temperature, char oxidation stage. Once vaporized, the alkali condenses on heat transfer surfaces, where it often contributes to fouling, slagging, and corrosion. Ash deposition in pc boilers is commonly most severe on the first bank of superheater/reheater tubes in the convection pass but can also affect the furnace and backpass regions significantly. Furthermore, reflective deposits that may not present a maintenance problem directly can alter heat absorption patterns in the boiler and raise furnace exit gas temperatures. All of these issues have been noted in previous experiments with a variety of coals, biomass fuels, and blends of

coal and biomass. Investigation and interpretation of the P8/SG ash deposition data are made in the context of these conceptual mechanisms. Figure 1 illustrates MBMS data indicating the extent of alkali vaporization during combustion of P8, SG, and blends of 5, 15 and 25% SG. Blend ratios are based on the high heating values of the fuels. Time-resolved results from the MBMS are integrated during the char oxidation phase for several fuel blends to produce a semi-quantitative

indication of the vapor concentration, as shown in Figure 1 for NO, HCl, K, and The detection limit is approximately 10ppm, which corresponds to an ion signal of about 1 in the units of the figure. The error bars represent one standard deviation as determined

from three replicate experiments. The most significant conclusion from the figure is that the K and HCl signals are both below detection limits, indicating essentially no vapor-

ization of K or evolution of Cl. This contrasts sharply with similar experiments conducted on higher-chlorine biomass fuels. The relatively low chlorine concentrations and high sulfur concentrations of these fuels inhibit the formation of high concentrations of alkali-containing vapors; KCl is the alkali species with the highest vapor pressure. Other than potassium- and chlorinecontaining vapor, the amount of material released from these fuels scales with the concentration of the material in the fuel within the precision of the measurement. The values

indicated for HCl and K are near the detection limits of MBMS in all cases and their trends with respect to fuel type should not be overinterpreted.

The implication of these MBMS results for ash deposition (and corrosion) is that

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alkali vaporization will play a fairly minor role during the combustion of switchgrass and Pittsburgh #8 coal, which is confirmed by experiments in both the MFC and the CERF. Ash deposits are collected in the MFC by inserting an air-cooled stainless steel probe into the test section. The outside diameter of the sample probe is 1.6cm, to match the Stokes number of tubes used in heat exchangers found in commercial power plants. Two embedded type-K thermocouples monitor probe surface temperature, which is controlled by varying the cooling air flowrate. A probe temperature of 500 °C was selected based on typical operating temperatures of the convective pass in power generation systems. Deposits are collected for a 1-hour period. After each experiment, the deposition probe

is removed from the combustor, the deposits are weighed and photographed, and a removable section of the probe with an undisturbed sample of the deposit is potted in epoxy for SEM analysis. To characterize the ash deposition rate, we calculate the particle capture efficiency, defined as the ratio of the mass of ash deposited on the probe to the mass of ash in the swept area of the probe. The mass of ash in the swept area of the probe depends on crosssectional area of the combustor occupied by the probe and the ash flow. At constant firing rate, the particle collection efficiency is effectively the ash deposition rate normalized the ash content of the fuel and the size of the probe.

Measured particle capture efficiencies indicate that Pittsburgh #8 fuel collects with a much higher particle capture efficiency than switchgrass. Considering the high potassium content of the switchgrass sample (Table 1), this is a potentially surprising result. Alkali metals such as potassium generally cause fouling problems. However, the switchgrass sample examined in this study has a very low fuel chlorine content (Table 1) which, as shown by the MBMS results, significantly inhibits the vaporization of alklai. Similar experiments indicating more significant levels of alkali establish the capacity of the equipment to measure alkali if it is present (Belle-Oudry and Dayton, 1997; Dayton and BelleOudry, 1997). In the MFC, ash deposition characteristics can be characterized for arbitrary blend ratios, ranging from pure biomass to pure coal. An instructive analysis is to compare the observed characteristics of the blends with an anticipated behavior, the latter being a linear interpolation of the measured characteristics of the two pure fuels. Figure 2 presents such a comparison for a variety of fuels tested in the MFC (Robinson, et al., 1997b), including the switchgrass/P8 blend. Each point in the figure requires three measurements:

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the behavior for the two pure fuels and the blend. When plotted on a parity diagram, points above the parity line indicate the interpolated values are too high and points below indicate it is too low. As illustrated, measured capture efficiencies are commonly less than the interpolated values, indicating that ash accumulation on surfaces proceeds at rates

slower than anticipated based on the behavior of pure fuels. This behavior we attribute

to the combination of alkali from the biomass and sulfur from the coal forming solid alkali sulfates on surfaces. These sulfates are less prone to capture particles than molten salts such as chlorides and hydroxides. However, there is insufficient sulfur in many biomass fuels to completely convert the alkalis to sulfates. The effects of cofiring SG and P8 on heat transfer rate and deposit removability are illustrated by data from the CERF (Table 2). These data indicate the changes in heat transfer rate in convection pass fouling probes and simulated slag panels/waterwalls as measured by the percent of the clean-surface value. Results are reported based on a 4hour sootblower cycle. The deposit cleanability is measured on a relative scale of 0–3, based on the peak impact pressures required to remove the deposits using an incremental sootblowing procedure. This cleanability indicates the ease with which deposits are removed from the surface, with cleanabilities of 2 or less regarded as manageable by conventional sootblowing. The data indicate that the switchgrass blend reduces heat transfer rates slightly more and requires slightly more sootblower pressure to clean from the surface than deposits formed from Pittsburgh #8 coal alone. The heat transer rate during the P8/SG blend tests always returns to its clean surface value after sootblowing at pressures regarded typical of coal-fired boilers. Also, the differences in heat transfer rate are not very large and in all cases the cleanability is within established limits of manageability. The data from the MFC generally indicate that there is a potential for severe ash deposition problems when cofiring some biomass fuels, especially herbaceous fuels, with coal. However, this particular sample of switchgrass exhibited only minor ash-related problematic behavior in both the CERF or the MFC. These conclusions are all based on relatively short deposition tests at modest deposit temperatures. Reactions that lead to sintered and unmanageable deposits include reactions of alkali with silica to form alkali silicates (Baxter et al., 1996). There is ample alkali and silica in these fuels for such reactions to occur. Such reactions are generally slow, perhaps too slow to significantly affect the measured results over the time of the tests. Also, increases in deposit temperature would increase the reaction rate significantly

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and could lead to considerably different results. Finally, many agricultural practices could lead to significantly higher concentrations of potassium and chlorine in the fuel, resulting in significantly different behavior.

emission is a complicated issue made potentially more complicated when cofiring biomass and coal. The MFC has a capability for measuring while maintaining gas, wall, and flame temperatures essentially constant. The results reported here indicate concentrations in the absence of any reduction technology such as fuel/air staging, reburn, lowburners, etc. Experiments over a wide range of oxidizing stoichiometric ratios conducted in the MFC indicate that concentrations at typical coal-combustion conditions (3% in flue gas) for pure biomass combustion are about 35% lower for the blend than for the pure coal (Fig. 3). At lower exit oxygen concentrations, emissions from biomass are lower still relative to coal, with a maximum of about 45% less from biomass at exit concentrations of 0.5%. However, at higher levels (6% and above), concentrations produced by pure biomass exceed those of the coal. This behavior is believed to be related to combustion of the residual biomass char, which requires higher oxygen content or longer residence time than that of coal due to its size. Qualitatively, the produced during combustion of the blend approximates a linear combination of the measured values for the blend behavior. Quantitatively, an interpolation of the behavior of the pure fuels generally overpredicts the observed behavior by –2 to 10 percent (average of 5 percent, see right ordinate of Fig. 3). Therefore, the blend produces slightly less in these experiments than the interpolation suggests. The absolute concentrations and its dependence on local stoichiometry, temperature, and extent of char combustion is under further investigation. These results are similar to our previous results (interpolation = measured value ±

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10%) (Robinson, Junker, and Baxter, 1997a; Robinson, et al., 1997b) and those from commercial experience (Aerts and Raglunc, 1997), all of which suggest that cofiring coal with low nitrogen-content biomass can significantly reduce emission under pc conditions. The relatively high production from SG at exit oxygen contents greater than 6%

despite SG’s low fuel nitrogen content compared to P8 illustrates that conversion efficiencies of fuel nitrogen to strongly depend on operating conditions and fuel type. Past experience with both coal and biomass suggests that concentrations are only modestly correlated with fuel N content. Several observations should be considered before extrapolating these results to new conditions. Highly fertilized plants commonly have very high fuel nitrogen contents and can produce concentrations higher than those produced from coal, even under pc conditions, as is observed in MFC results using high-nitrogen biomass. Cofiring biomass with coal can cause flame instabilities, change flame stoichiometry, and produce other

changes that may significantly alter

emissions. Such behavior was observed in the

CERF, where detached flames generated high concentrations. Also, few commercial coal-fired facilities are capable of firing pure biomass for a point of reference. For these reasons, emissions from coal-biomass blends are often difficult to anticipate.

Burnout Biomass is more difficult to comminute than coal and, consequently, is typically fired at large sizes compared to pulverized coal. Furthermore, biomass fuel commonly exhibits high aspect ratios compared to the commonly equant coal particles. The consequences of these size and shape considerations are that biomass fuels most commonly oxidize under diffusion control (because of their size), often at rates that scale with minimum size to the first power (because of the aspect ratios). We have attempted to identify the maximum size biomass particle that can be used without excessive loss of

fuel or increase in residual ash carbon content of either bottom or fly ash. These results depend on fuel moisture content, particle properties (size, density, shape, volatile yield, ash content, etc.), boiler design, and boiler operating conditions. Figure 4 illustrates typical modeled results of particle burning times under the

assumptions of a boiler that measures 40m (131 ft) from bottom to furnace exit, with biomass injected at a height of 9.1 m (30ft) from the bottom. The boiler is assumed to

be operating at full load, with an average furnace gas velocity of about 20m/s (65ft/s).

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The biomass is assumed to yield 80% of its mass through devolatilization. The cylindri-

cally shaped char is assumed to have a specific gravity of 0.2 and the ash of 2.5. The fuel is assumed to contain 7% ash on a dry basis. Moisture vaporization and char reaction are assumed to be heat transfer and mass transfer limited, respectively. Devolatilization is assumed to occur rapidly relative to both moisture vaporization and char oxidation. All of these assumptions and values are reasonable, but they can vary depending on the fuel, boiler design, and operating conditions. Figure 4 illustrates both the available residence time and combustion time for complete burnout for particles as a function of size. Particle sizes at which the combustion time is less than the residence time are predicted to completely burn. Particles with initial diameters less than about 5mm are predicted to exit the top of the furnace with residence times increasing with size from 1.5s at small sizes. A particle of about 5mm in diameter has a terminal velocity approximately equal to the assumed boiler gas velocity under these conditions. At larger sizes, the particles flow opposite the direction of the gases and exit the bottom of the boiler. Particles larger than about 5.5mm are predicted to require longer to burn than is available in the boiler and are predicted to exit the boiler through the bottom ash. The prediction also indicates that particles between about 1.5 and 3.5 mm are also predicted to leave the top of the boiler with a small amount of residual carbon. The linear relationship between particle size and burnout time indicated in Fig. 4 assumes a cylindrical particle shape and is different than coal behavior, the latter typically exhibiting a burnout time proportional to initial sphere diameter squared when burning under diffusion control. Experiments in Sandia’s CPI facility indicate that the linear relationship between burnout time and size is valid. A statistical analysis indicates that the assumed linear relationship is a significantly better model than a or relationship, but only marginally better. Several practical conclusions can be drawn from Fig. 4. First, the overall residence time of biomass fuel particles can be much longer than that of pulverized coal because of the non-negligible terminal velocities of such particles. The pulverized coal residence times in the boiler modeled for Fig. 4 is about 1.5s. Since many pilot- and research-scale facilities are down fired, the biomass particles have shorter rather than longer residence times, and erroneous conclusions with respect to burnout could easily be drawn based on experiments in such reactors if proper account for this effect is not made. Second,

biomass particles larger than about 1/4 inch in their minor dimension are likely to be found as partly burned char in the furnace bottom ash. While this conclusion strictly applies only to the simulation summarized in Figure 4, most perturbations on the simulation in terms of changes in operating conditions or fuel properties would decrease or have little effect on the top size at which biomass can be burned. Some smaller particles are also predicted to be found incompletely burned in the fly ash, that is, those with initial diameters ranging from about 1.5 to 2.8mm. Third, biomass should not be injected in or under the bottom row of burners, as most of it would relatively rapidly arrive in the bottom ash unless it is extremely small. Similarly, it should not be injected very high on the furnace wall or large fractions of it will enter the convection pass incompletely burned out.

Chlorine and Corrosion One proposed mechanism for chlorine-related corrosion in boilers suggests that chlorine plays an important role in concentrating alkali-metal containing salts on heat transfer surfaces. The conceptual mechanisms involve the combination of chlorine with

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alkali to form alkali chlorides. Alkali chlorides are the most volatile forms of alkali and

the most stable form of chlorine under a wide range of oxidizing conditions relevant to combustion. Condensation of these alkali vapors selectively deposits both chlorine and alkali on the surface. At surface temperatures, the most stable form of alkali is sulfates. The chlorides react to form sulfates, releasing chlorine-containing gases (typically HCl under moist, oxidizing conditions). However, if there is insufficient sulfur to completely react with the chlorides, the chlorides remain on the surface, potentially leading to rapid corrosion (Baxter and Nielsen, 1997). This scenario suggests that there should be a relationship between deposit chlorine content and the potential for forming sulfates from alkali chlorides. Experimental indication of this relationship is indicated in Fig. 5, where deposit chlorine concentration is plotted versus the ratio of fuel sulfur to twice the maximum fuel alkali content for a variety of fuels tested in the MFC (Baxter, Robinson, Buckley, Shaddix, Lunden, & Hardesty, 1997b). This scaling parameter is chosen because, at values greater than unity, there is sufficient sulfur to convert all of the alkali chloride to sulfate. As is seen, the chlorine content of the ash deposit is very low when this scaling parameter exceeds unity, but increases to high values when the parameter is less than unity. The

highest two chlorine concentrations in the figure represent deposits formed from highchlorine biomass, but the next highest represent blends of coals with high-chlorine biomass. These data illustrate how the combination of biomass and coal properties can influence the potential for corrosion in boilers. The high-sulfur, low-chlorine, and moderate-alkali contents of the SG/P8 blend investigated here place it in the low-depositchlorine region of the graph. The absence of chlorine in the deposits does not necessarily imply that corrosion is not an issue, as it still leaves corrosion by sulfates as a potential problem. In particular, alkali tri-sulfates are known to be aggressive on heat transfer surfaces. The potential for formation of sulfates on surfaces is generally greater for biomass-coal blends than for pure coal because of the often high alkali content of the biomass. Corrosion of surfaces by alkali sulfates can usually be managed by maintaining low surface temperatures, preventing occurrences of locally reducing conditions, and proper sootblowing.

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CONCLUSIONS Biomass-coal cofiring is an important future technology for power generation that introduces complexities into the fireside behaviors of such fuels. The potential consequences for ash deposition, formation, carbon burnout, and corrosion are all sensitive to biomass and coal fuel properties. The overall result is that judicious combinations of fuels for cofiring and boiler operation can avoid major problems and may reduce pollutant emissions from pc boilers. However, each of these issues has the potential of developing into a significant problem if they are not specifically addressed in the

cofiring plan. The switchgrass and Pittsburgh #8 coals investigated here produced an ash accumulation rate greater for the coal than for the switchgrass in both absolute terms and when normalized by the amount of total ash. Ash deposit formation is higher for the blend than for switchgrass alone, but not as much higher as is predicted by interpolating between the behaviors of pure switchgrass and pure Pittsburg #8 coal. This is consistent with previous observations for a variety of biomass-coal blends. We postulate that sulfur from the coal and the alkali from the switchgrass combine to form sulfates on heat transfer surfaces to which particles are less prone to adhere than chlorides or hydroxides. Deposits from the switchgrass-Pittsburgh #8 coal blend form a higher barriers to heat transfer and are somewhat more difficult to remove than those from the coal alone, but they remained in a manageable range within the duration of these tests. The inorganic composition of the fuels suggests that longer-term behavior and behavior at higher temperatures may result in more severe ash management problems. Many proposed energy crop proposals involve aggressive agricultural practices with regard to fertilization and harvesting and may have significantly higher chlorine and alkali contents than our sample of switchgrass. This, too, would lead to less manageable and more rapidly accumulating deposits. Qualitatively, formation for blends interpolates between that of the pure biomass and coal if combustion conditions are otherwise unchanged. Quantitatively, the interpolated values are about 5% higher than the measured values. The trends of formation with overall stoichiometry are fairly complex, with switchgrass producing lower than coal at about 6% oxygen content in the gases or less but higher at higher exit concentrations. In most cases of practical interest, it may not be possible to keep combustion conditions constant as biomass is introduced and establishing the amount of produced from pure biomass combustion may not be possible. In general, biomass fuels with low nitrogen contents have been observed to decrease emissions significantly under most pulverized-coal-relevant conditions. Most biomass fuel particles are large and have high aspect ratios compared to pulverized coal particles. Burnout time for such particles scales approximately with the smallest dimension. Under typical, full-load, utility-boiler conditions, particles as large as 5.5mm are predicted to burnout before exiting the furnace. This result depends on

operating conditions, point of injection, boiler design, and fuel properties. Larger particles are predicted to exit the furnace bottom while smaller particles exit with the fly ash. The size at which the transition is made is 5mm in the particular simulation here and also depends on operating conditions, etc. Deposit chlorine content is anticipated to be minimal in many biomass-coal cofiring scenarios due largely to the relatively low fuel chlorine content and the high overall sulfur content relative to alkali. This reflects the tendency of alkali chlorides to react to

form sulfates if there is available sulfur. The absence of chlorine in deposits indicates that

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corrosion may not be extremely severe. However, sulfates, in particular alkali tri-sulfates, can be very corrosive and biomass coal combustion will enhance the formation of sulfates on heat transfer surfaces. Highly fertilized or aggressively managed energy crops may have greater potential for increased corrosion due to increased alkali and chlorine contents. All of these results find support in commercial-scale data but there are not sufficient long-term commercial-scale tests to provide definitive evidence of many of these trends. These laboratory results will be used to help design commercial-scale tests and indicate sampling and analysis needed in future tests to provide better supporting information or indicate weaknesses in our current understanding.

ACKNOWLEDGMENTS Funding for this project was provided by DOE's Office of Fossil Energy through the Advanced Research and Technology Development (FETC & Sandia) and Office of Energy Efficiency and Renewable Energy through the Biomass Power Program (NREL and Sandia).

REFERENCES Aerts, D., and Raglunc, K. Co-firing Switchgrass and Coal in a 50MW Pulverized Coal Utility Boiler (Final Report No. University of Wisconsin-Madison. (1997). Baxter, L. L., Combustion and Flame, 90, 174–184. (1992). Baxter, L. L., Biomass and Bioenergy, 4(2), 85–102. (1993). Baxter, L. L., Miles, T. R., Jr., T. R. M., Jenkins, B. M., Milne, T., Dayton, D., Bryers, R. W., and Oden. L. L. The Behavior of Inorganic Material in Biomass-Fired Power Boilers: An Overview of the Alkali

Deposits Project:. In A. V. Bridgwater & D. G. B. Boocock (Eds.), Developments in Thermocheimcal Biomass Conversion (pp. 1424–1444). London: Blackie Academic & Professional. (1997a).

Baxter, L. L., Miles, T. R., Thomas R. Miles, J., Jenkins, B. M., Dayton, D., Milne, T., Bryers, R. W., and Oden, L. L. The Behavior or Inorganic Material in Biomass-Fired Power Boilers—Field and Laboratory Experiences: Volume II of Alkali Deposits Found in Biomass Power Plants No. SAND96-8225, NREL/TP433-8142). Sandia National Laboratories; National Renewable Energy Laboratory. (1996). Baxter, L. L., and Nielsen, H. P. The Effects of Fuel-bound Chlorine and Alkali on Corrosion Initiation. In 214th American Chemical Society National Meeting, (pp. 1089–1095). Las Vegas, NV: (1997). Baxter, L. L., Robinson, A. L., Buckley, S. G., Shaddix, C. R., Lunden, M., and Hardesty, D. R. Task 4. Development of Guidelines for Cofiring Biomass and Pulverized Coal (Quarterly Progress Report No. Sandia National Laboratories. (1997b).

Belle-Oudry, D. A., and Dayton, D. C. Analysis of Combustion Products from the Cofiring of Coal with Biomass Fuels. In 214th American Chemical Society National Meeting, (pp. 10-96-1100). Las Vegas, NV: (1997).

Dayton, D. C., and Belle-Oudry, D. A. Bench-Scale Biomass/Coal Cofiring Studies. In Engineering Foundation Conference on the Impact of Mineral Impurities on Solid Fuel Combustion, Kona, HI: (1997).

Dayton, D. C., French, R. J., and Milne, T. A., Energy and Fuels, ( to appear ). (1995). Freeman, M. C., Chitester, D. C., James, R. A., Ekmann, J. M., and Walbert, G. F. Results of Pilot-Scale Biomass Co-Firing for PC. Combustors. In USDOEIFETC Advanced Coal-Based Power Systems and Environmental Control ’97 Conference, Pittsburgh, PA: (1997). H u r t , R. H., and Davis, K. A. Near-Extinction and Final Burnout in Coal Combustion. In 25th Symposium (International) on Combustion Pittsburgh, PA: The Combustion Institute. (1994).

Robinson, A., Junker, H ., and Baxter, L. Pollutant Formation, Ash Deposition, and Fly Ash Properties When Cofiring Biomass and Coal. In Engineering Foundation Conference on the Economic and Environmental Aspects of Coal Utilization, Santa Barbara, CA: (1997a). Robinson, A. C., Junker, H., and Baxter, L. L. Ash Deposition and Pollutant Formation when Corfiring Biomass with Coal in PC Boilers. In EPRI Coal Quality Conference, Kansas City, MO: (1997b).

SUMMARY OF RECENT RESULTS OBTAINED FROM USING THE CONTROLLED FLUIDISED BED AGGLOMERATION METHOD Marcus Öhman and Anders Nordin Energy Technology Center, Department of Inorganic Chemistry, University of Umeå, S-901 87 Umeå, Sweden

1. INTRODUCTION The most promising energy conversion technologies for solid fuels, and biomass in particular, are based on fluidized bed combustion (FBC) or gasification (FBG). These

processes enable higher electrical and total efficiency as well as greater fuel flexibility. Due

to the relatively low temperatures in FBC and FBG, sulfur emissions and the extent of deposit formation can also be kept to a minimum. However, bed agglomeration could be a potential problem which can decrease both the heat transfer in the bed and the fluidization quality, resulting in poor conversion efficiencies and loss of control of bed operational parameters. In the most severe cases, bed agglomeration can lead to total defluidization, resulting in unscheduled plant shut down. The state-of-the-art concern-

ing methods to determine fluidized bed agglomeration tendencies were recently compiled by Öhman (1997).

1.1. Review of Previous Work It is generally agreed that agglomeration may proceed through several different sintering mechanisms [Skrifvars 1994, Manzoori 1994; Öhman 1997]; viscous flow sintering of liquid silicates; reactive liquid sintering of molten salt systems; chemical reaction sintering by formation of new compounds; solid state sintering; and vaporization preceded by re-condensation. It can further be assumed that, although they all may contribute to the initial mechanisms of the ash transformations, the melting behavior of the ash or coating material probably is decisive for the final agglomeration in the turbulent fluidized beds.

1.1.1. Prediction of Ash Melting Behavior. Several techniques are currently used in the fuel industry to predict ash fusion temperatures from elemental composition. The most Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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common is the use of phase diagrams to determine liquid temperatures that parallel the fusion temperatures of the formed ash [Huggins et al. 1981; Hastie and Bonell 1985]. The accuracy of this technique declines with an increase of primary elements in the ash and wood ashes, for example, contain up to seven major elements. However, phase diagrams are very useful as a first screening method for a better understanding of the general ash melting behavior. The ideal technique would be to utilize chemical equilibrium model calculations, incorporating all intermediate stoichiometric phases, non-ideal solid and liquid solutions required for the system of interest. Several investigations using equilibrium calculations have been performed during the last few years

and many of these have included non-ideal solid and liquid solutions, see Nordin et al. [1997] for a review. In addition, Blander et al. [1997] recently performed a round robin of different programs and databases. The melting behavior can presently be accurately predicted for the salt system the most important binary and some ternary silicate systems but several ternary and higher ordersilicate systems still remain to be evaluated/optimized. Further, data for the interactions between the salt system and and silicates are needed for a general equilibrium model of the melting behaviors. New evaluations of solution data are continuously made available in the literature, and through for example the FACT database.

There are also many empirical slagging and fouling indexes proposed, that are based on ash composition [Nichols and Reid 1940; Sage and Mcllroy 1960; Reid and Cohen 1944]. However, these indexes have been shown to be of very little general value for different fuel types. Empirical models using multiple regression techniques have also been used as predictors of ash fusion temperature [Wintergarten and Rhodes 1975; Sonderal and Ellman 1975; Vorres 1979]. Where successful, the empirical models have been limited to coals from a certain field.

1.1.2. Available Laboratory Methods. The standard ash fusion tests (AFT; ISO 5401981; BS 1016.15-1970; GOST 2057-1982; SABS 932; ASTM D1857, 1987; DIN 517 30, 1984; AS 1038.15, 1987; GE 219-74) are the most commonly used laboratory methods and also the only standardized technique to predict the behaviors of ashes in different processes. These tests are based on following the external shape (deformation, shrinkage and flow) of a pyramidal or cylindrical pellet of ash during heating in a laboratory furnace in either an oxidizing or a reducing atmosphere. Of particular concern is the estimation of the initial deformation temperature (IDT). The standard AFT have been extensively criticized in the literature. One general criticism is the relevance of the ash sample which is subjected to the test. The ashing temperature used to generate the ash sample, is much lower and the history and atmosphere of the ash is quite different than that experienced in combustion situations. Another criticism is that changes in the shape of the sample during heating, due to phase transitions and chemical reaction, can also be interpreted as initial melting. Further, strict control and observance of the test conditions are necessary to obtain reproducible results. Both repeatability and reproducibility have recently been shown to be poor [Wall et al. 1995; Coin et al. 1995]. In one investigation the reproducibility has been demonstrated to be as poor as 140°C [Slegeir and Singletary 1988]. Gerald et al. [1981] have revealed that significant melting of the ashes occurs far below (200–400 K) the IDT. In addition, Huffman & Huggins [1983] and Huggins et al. [1981] showed that most ashes are completely melted at temperatures far below the IDT and that the progression from the IDT to the fluid temperature can be accomplished by holding the IDT constant for a period

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of 45 minutes. They suggested that this behavior was due to decreasing of the slag viscosity rather than increasing the amount of melt. Compression strength tests of heat-treated cylindrical pellets have been used by several authors, to study the ash melting behaviors of different types of coals [Barnhart and Williams 1956; Conn and Jones 1984; Nowok et al. 1990] and biomass fuels [Skrifvars et al. 1995]. The sample, which can be ashes collected from industrial plants, laboratory ashes, or a mixture of chemical compounds, is first crushed and screened. Then the sample is pelletized to a cylindrical pellet and heated in a controlled gas atmosphere. After cooling, the pellets are crushed in standard compression-testing equipment, and the crushing strength is taken as a measure of the sintering degree. The method has proven to give results with a relatively high reproducibility [Skrifvars 1994], but no thorough evaluation of the accuracy concerning bed agglomeration seemed to be available. Smith [1956] used a dilatometric shrinkage technique to study sintering characteristics of pulverized fuel ash. This technique is based on shrinkage measurements of an ash sample, i.e. if an ash sample shrinks, it indicates sintering. An intercept on the temperature axis is taken to define the initial sinter point. Similar techniques have been used by several authors [Raask 1979; Smith 1956; Manzoori 1990; Coin et al. 1995; Wall et al. 1995] and the method was shown to be superior to the ash fusion tests. However, with some coal ashes, results have been obtained where the shrinkage measurements showed no change although a significant degree of sintering had taken place [Raask 1985]. Raask therefore suggested that the dilatometry method should be supplemented with continuous measurements of the conductance. Before sintering, an ash would show low thermal- and electrical conductance because of the lack of particle to particle contact. As the cross sectional area of sinter bonds between the particles grows, the conductance path (both thermal and electrical) is increased. Simultaneous measurements of thermal conductance and dilatometric shrinkage for detection of the onset of sintering of coal ashes have been used by several authors [Cumming et al. 1985; Conn and Austin 1984]. A similar method based on the electrical conductance of ash has also been proposed [Sanyal and Metha 1993; Sanyal and Cumming 1981; Gibson and Livingston 1991]. A disadvantage of these methods is that satisfactory contact between the ash and the electrodes is hard to achieve and maintain [Wall et al. 1989]. An interesting complement would be the use of capacitive measurements for potential information about initial melt formation, as recently proposed by Nordin and Leven [1997]. To determine the effect of temperature on ash agglomeration, a simple method was developed and used by Stallmann and Neavel [1980], Samples of coal less than 100 mesh were ashed at low temperature. The ash produced was screened to ensure that its top size remained below 100 mesh. These samples were then heated at different temperatures in a platinum crucible. After cooling, the samples were screened to determine the weight percent retained on a 100 mesh screen. The effect of the straw ash characteristics on agglomeration of silica sand [Ghaly et al. 1994] and alumina sand [Ghaly et al. 1993] at various temperatures was investigated using a high temperature muffle furnace. SEM/EDS was then used for the analysis. Padban et al. [1995] used a similar approach to study the effect of two biomass ashes on agglomeration of silica sand.

The viscosity is a critical property when assessing the sintering, slagging and fouling tendencies of different inorganic materials including fuel ash. Previous experimental work have shown that there is a critical viscosity for adhesion of ash particles [Srinivasachar et al. 1988; Wibberly and Wall 1982; Srinivasachar et al. 1990]. At temperatures and velocities typically found in coal fired boilers, this critical viscosity has been shown to be

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and

[Senior and Srinivasachar 1995]. The measurable viscosity of

glassy materials can extend over a wide range, more than 10 orders of magnitude [Raask 1985]. There is no single method capable of measuring the viscosity of fuel ash slags over the entire range. Most laboratory viscosity measurements at high temperatures are carried out using a rotating crucible viscometer. The viscosity of less fluid slags in the range of to can be measured by a rod penetration viscometer. Several empirical methods have also been proposed for the estimation of the viscosity’s for coal ashes [Nowok 1994; Urbain and Boiret 1990). In a thermogravimetric analysis (TGA), the mass of a sample is continuously recorded as a function of temperature or time. The information provided by thermogravimetric methods is limited because a temperature variation must bring about a change in mass of the analyte. Thus, thermogravimetric methods are mainly limited to decomposition and oxidation reactions and to such physical processes as vaporization, sublimation, and desorption. Differential thermal analysis (DTA) is a technique in which the difference in temperature between a substance and a reference material is measured as a function of temperature while the substance and reference material are subjected to a controlled temperature program. Usually, the temperature program involves heating the

sample and reference material in such a way that the temperature of the sample increases linearly with time. The difference between the sample temperature and the reference temperature is then monitored and plotted versus sample temperature to give a differential

thermogram. DTA has been used to study thermal behavior of inorganic materials such as silicates, ferrites, clays, oxides, ceramics, catalysts and glasses. Information about different processes such as fusion, dehydration, oxidation, reduction, adsorption and solid state reactions is provided. DTA can preferably be used to accurately obtain eutectic,

solidus and liquidus temperatures in different systems [c.f. Vassilev et al. 1995]. High temperature microscopy (HTM) may be used in combination with DTA to obtain the liquidus temperature of a fuel ash. In this method the solidification of a sample is followed from melt to solid phase. Hereby, the initial crystallization temperature may be obtained with a high accuracy. 1.1.3. Previous Bench Scale Agglomeration Studies. Several detailed agglomeration studies on coal in bench scale have been conducted, under both combustion and gasification conditions. The effect of temperature on the extent of agglomeration has been studied by Sandstrom et al. [1979] and Goblirsch et al. [1983]. An increase in temperature was shown to increase both the agglomeration tendencies and the size of the agglomerates. The different effects of bed materials and additives on agglomeration tendencies was also studied. The effect of limestone addition on agglomeration, was determined

both in combustion [Benson et al. 1982; Dawson and Brown 1992] and gasification [Kline et al. 1990; West et al. 1993]. Depending on the ash composition in the fuel, the limestone addition could both increase and decrease the bed agglomeration tendencies. In addition, experiments with gabbro, alumina sand and dolomite as bed material have been performed in combustion [Goblirsh et al. 1980]. These experiments showed that gabbro was the most effective additive regarding bed agglomeration. Parameters such as fluidization velocity [Atakul and Ekinci 1989; Basu and Sarkar 1983; Sandstrom et al. 1979], bed particle size, bed height [Atakul and Ekinci 1989], bed particle surface and contact area [Siegell 1976; Huang 1985] have also affected the agglomeration/defluidization tendencies. In most of the above bench scale studies, at least one unknown variable was introduced by the burning particles in the bed. This could lead to a incomplete mapping and poor understanding of the relations between the studied process parameters and the

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agglomeration tendencies. In addition, the operational parameters in many of these studies have been chosen outside of practical ranges in full scale plants. Only a few studies on agglomeration in biomass combustion [Salour et al. 1993] and gasification [Ergundler and Ghaly 1993; Bruce and Bitoft 1988, Soltes et al. 1982; Le Pori et al. 1980] have been performed. In contrast to the results obtained from studies with coal, Ergudenler and Ghaly [1993] showed that the operational velocity had no effect on the agglomeration tendency when wheat was used as fuel. In a biomass study [Bruce and Bitoft 1988], none of the studied operational parameters had an effect on the bed agglomeration. In addition, results have shown that the agglomeration tendencies could be greatly affected by fuel mixing. By blending wood into rice straw, Salour et al. [1993] showed that the bed agglomeration could be controlled and prevented. The conclusion form reviewing the literature was that in spite of a relatively frequent reporting, a precise and quantitative knowledge of bed agglomeration processes has not yet been presented. In addition, no reliable and realistic methods were found to be available to determine bed agglomeration tendencies of different fuels, fuel combinations or fuels with additives. As none of the existing laboratory methods comprises the ash transformations in a fluidized bed, a more relevant method would be to use a fluidized bed for actual and controlled bed agglomeration studies. Several attempts were made prior to 1993 to use bench or pilot scale fluidized bed reactors for this purpose but they all suffered from either bed temperature inhomogeneity, caused by the burning particles in the bed, or inadequate evaluation and documentation. Although the publications by Gluckman et al. [1976], West et al. [1993] and Basu and Sarkar [1993] reported commendable work in the right direction, no bench scale studies had previously been performed in such way that the fuel specific bed agglomeration temperature could be determined in a realistically and accurately optimal way. Since the presentation of the controlled fluidized bed agglomeration (CFBA) method [Nordin et al. 1995] at the last EF ash conference, our continued work have resulted in a licentiate thesis [Öhman 1997] including four papers, two subsequent submitted papers and some still not published results, all so far with limited availability. The objectives of the present report was therefore to review all these CFBA studies performed and present our gathered conclusions.

2. THE CONTROLLED FLUIDIZED BED AGGLOMERATION METHOD

2.1. Summary of the CFBA Results Obtained 2.1.1. Particle Temperature Studies in Fluidized Bed Combustion. As discussed in previous sections, the most important individual parameter influencing the bed agglomeration is the actual process temperature. It is well known that the temperature of the burning particles significantly exceeds the bed temperature. It is also clear that the increased temperature of the burning particles is crucial for the ash transformations and bed agglomeration process in fluidized beds. The effects of different bed- and particle variables on the particle temperature have been extensively studied during the last 30 years. However, to our knowledge, no simultaneous evaluation of all the studied variables was previously performed. The objectives of the work presented by Öhman and Nordin [1996] were therefore; to review the literature of existing results from particle temperature studies in

fluidized beds; and based on literature data to determine an empirical PLS (partial least

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squares projections to latent structures) model describing the effects of the different variables on the particle temperature.

The results from the literature survey and from the determined PLS-model, showed that; i) the difference between the burning particle temperature and the bed temperature may be considerable (40-600 K); ii) many different fuel and process variables have a large influence on the temperature difference; and iii) the total oxygen concentration, bed temperature, fluidization velocity and bed particle diameter are the most influential variables.

Thus, it was clear that a modified bench scale method had to be developed without the burning particles, if a fuel specific agglomeration temperature was to be accurately determined. 2.1.2. The Controlled Fluidized Bed Agglomeration Method. A bench scale fluidized bed combustor was therefore constructed, enabling realistic and highly controlled bed agglomeration tests with a homogeneous bed temperature (Fig. 1). The bench scale reactor is made of stainless steel (SS 2,343), being 2m high, 100mm and 200mm in diameter in the bed and freeboard sections, respectively. A perforated stainless steel distributor plate with 1% open area and a total of 90 holes is used. The maximum temperature of the equipment is 1,020°C. To allow for a constant increase of the bed temperature, with a homogeneous temperature profile, much effort was focused on the

bed section of the reactor as well as the controlling system. To keep the walls at the same temperature as the bed, the reactor is equipped with controlled electrical wall heating elements. An air-pre-heater allowing primary air temperatures up to 1,050°C was also constructed to control the bed temperature. Forced convection is utilized in a cyclonelike stainless steel cylinder equipped with Kanthal electrical wall heating elements. All temperatures are manipulated by Eurotherm temperature controllers and the maximum temperature deviation within the bed was determined to be less than The reactor is utilized in one of two operational modes. During the first normal combustion operational mode, a relatively accurate simulation of a full-scale process is accomplished, and the bed is loaded with ash with appropriate characteristics. The subsequent mode of operation is based on controlled increase of the bed temperature by applying external heat to the primary air and to the bed section walls. In addition, temperature homogeneity is secured by switching from normal fuel feeding to a propane precombustor. Bed temperatures at four locations in the bed are measured by shielded type S thermocouples and differential bed pressures at four other positions are determined by differential pressure transducers. The initial agglomeration temperature is determined by on- or off-line principal component analysis of the small variations in measured bed temperatures and differential pressures, preceding the definite defluidization. Samples of ash

and bed material for evaluation of agglomeration mechanisms may be collected throughout the operation. The method is more closely described in the publication by Nordin et al. [1995], which also presents a measure for prevention of agglomeration of biomass fuels. Several repeated combustion tests with two biomass fuels alone (Lucerne and olive flesh), all resulted in agglomeration and defluidization of the bed within less than 30 minutes. By controlled agglomeration experiments, the initial cohesion temperatures for the two fuels were determined to be as low as 670 °C and 940 °C, respectively. However, by cocombustion with coal, the initial agglomeration temperatures increased to 950 °C and more than 1,050°C, respectively. When co-fired with coal during ten hour extended runs, no agglomeration was observed for either of the two fuel mixtures. Samples of bed materials, collected throughout the experimental runs, as well as the produced agglomerated

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beds, were analyzed using SEM/EDS and X-ray diffraction. The results showed that loss of fluidization resulted from formation of molten phases coating the bed materials; a salt

melt in the case of Lucerne and a silicate melt in the case of the olive fuel. By fuel mixing, the in-bed ash composition is altered, conferring higher melting temperatures, and thereby agglomeration and defluidization can be prevented. 2.1.3. Quantification of Fluidized Bed Agglomeration Tendencies—Sensitivity Analysis. The objectives of the work described by Öhman and Nordin [1997] were to; i) determine the inaccuracy and reproducibility of the new method; and ii) determine potential effects of all the process related variables on the determined agglomeration temperature.

An extensive sensitivity analysis was performed according to a statistical experimental design to evaluate the effects of eight different process analytical variables on the determined agglomeration temperature of a biomass fuel. The results showed that amount of bed material, heating rate, fluidization velocity and air to fuel ratio during both ashing and heating did not influence the determined agglomeration temperature. Only ash to bed material ratio, the ashing temperature and the bed material size had significant effects on the agglomeration temperature, but still the effects were relatively small. The agglomeration temperature of the fuel could be determined to 899°C with a repeatability of

(STD). Based on the results, the inaccuracy was determined to be

(STD), considering the normal variations in all operating variables. A corresponding, still

unpublished, study using another type of fuel (wheat straw) confirmed the above results. The only significant operating parameter was found to be the ashing temperature.

To further evaluate the accuracy of the method, another study has been initiated, where “synthetic ashes” with known melting behaviors are used for comparison. So far, different compositions (50–99% of the binary system was prepared by heat treatment and crushing and then fed to the bed. The rich part of the system has an eutectic temperature of 690°C [Bergman et al. 1995] and the liquidus temperature increases almost linearly up to 1,076°C for the pure The results from

the CFBA method although used at somewhat lower temperatures than normal, were found to be in good agreement with the more accurately determined eutectic temperatures. Further, the composition did not seem to influence the temperature of onset of agglomeration, indicating that only small amounts of melts are needed for salt systems. The continued sub-project will include also silicate systems to qualitatively determine also the effect of the higher viscosity melts.

2.1.4. Mechanisms of Bed Agglomeration in Combustion of Biomass Fuels. In a recent unpublished collaborative study with Abo Akademi University, the mechanisms of the chemical processes during bed agglomeration of ten different biomass reference fuels

[Nordin 1993] were determined by extensive SEM/EDS analysis of bed material sampled continuously throughout the agglomeration process as described above. The biomass fuels were chosen by principal component analysis (PCA) to represent all biomass fuels, with respect to variations in content of ash forming elements. The results from the study show that the agglomeration mechanism of most types of biomass fuels are governed by the formation and stickiness of high Ca- and K-silicate melts (sintering by viscous flow) but also high elemental inhomogeneity, even within ash from specific fuels. S and Cl was in general found not to participate in the final agglomeration mechanism. This was further supported by the careful study of ash formation during waste sludge and bark incineration performed by Latva-Somppi et al. [1997]. The significant vaporization and transport of K, Na, Cl and S was determined by impactor sampling and elemental analy-

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sis both for full scale FBC and corresponding runs in the bench scale reactor. The vaporization in the bench scale unit was also found to increase linearly by increased bed temperature. In addition, no vaporization was found during external heating phase of CFBA runs. 2.1.5. Predicting Agglomeration Tendencies—A Comparison of Three Techniques. A comparison between three different techniques to predict bed agglomeration tendencies during FBC was performed by Skrifvars et al. [1997]. The standard ASTM ash fusion test, a compression strength based sintering test and actual controlled bed agglomeration in a bench scale FBC reactor were used to determine the critical temperature of ten biomass reference fuels (Bark, Bagasse, Cane Trash, Lucerne, Olive Flesh, Peat, RDF, Reed Canary Grass, Wheat Straw, Wood Residues). The fuels, once again, were chosen

based on a PCA of a compilation of about 300 different samples of Nordic biomass fuels. The results from the comparison showed significant differences in the critical temperatures obtained, depending on which technique was used. The limited applicability of the ASTM standard ash fusion test was clearly illustrated as the resulting initial deformation temperatures were found to be 100–900 °C higher than those from the compression strength and bench scale FBC tests. The general tendency, as determined by the compression strength tests, seemed to agree with the results from the actual agglomeration tests, although a large spread was obtained. For five of the fuels, the compression strength test gave sintering temperatures some 20–40 °C lower than those determined as the bed agglomeration temperature with the bench scale FBC reactor. For Reed Canary grass, a significantly lower sintering temperature was obtained and for three fuels significantly higher values were obtained. The results indicated that the reason for this disagreement could be that the critical temperature previously has been taken at the onset of sintering, i.e. at the intercept between the zero pressure line and the line of increased sintering strength. If a higher specific strength is used instead, much better agreement between the two methods was obtained. In addition, Natarajan et al. [1997] used the CFBA method to determine the initial agglomeration temperature, during FBC and FBG in both SiO2 and CaO, for rice husk, bagasse, cane trash and olive flesh. The results were further compared with the results from corresponding ASTM fusion tests. The results showed a significantly higher initial deformation temperature than the actual agglomeration temperature for all fuels considered. For rice husk, the discrepancy was found to be as high as 600°C, due to its high Si rigid cage-like ash structure. The use of lime instead of quartz increased the agglomeration temperature somewhat for combustion conditions. Cane trash and olive flesh resulted in agglomeration of the quartz bed at somewhat lower temperatures during FBG than FBC, while a small increase in agglomeration temperature was obtained for rice husk, going from FBC to FBG.

3. CONCLUSION AND FUTURE WORK The conclusions from the different studies are:

• the new method seems to be accurate in determining fuel specific agglomeration tendencies, with high reproducibility and most importantly closely related to actual full scale behavior. • for fuel analysis, the laboratory methods may suffer significantly from the origin

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of the analyzed samples, and the standard ash fusion tests have been shown to produce erroneous results due to several different reasons. • the identification of low temperature liquids as the reason for the agglomeration can be used to propose measures for prevention, either by sorption of fluxing elements such as K and Na or alteration of the ash composition conferring higher melting temperatures. This can simply be accomplished by co-combustion with coal. • the CFBA method can preferably be used both for FBC and FBG conditions.

In addition to the study of different synthetic ashes for evaluation purposes, our future work will include more of the corresponding studies in gasification environments and with other bed materials. Careful studies of the effect of potential additives for prevention of bed agglomeration, and some related fundamental studies of alkali behavior in fluidized beds have also been initiated.

4. ACKNOWLEDGEMENTS The authors thank Dr Bengt-Johan Skrifvars, Dr Rainer Backman and Professor

Mikko Hupa, Åbo Akademi University for the stimulating multi-year collaboration, as well as our shorter term visiting scientists. The financial support from the Swedish

National Board for Industrial and Technical Development (NUTEK) is gratefully acknowledged.

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a sensitivity analysis.” Energy & Fuels, 12, 90–94.

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, and Kirn Dam-Johansen1, Peter F. B. Hansen (1) and Karin H. Andersen (2)2, Henning S. Sørensen 3 Ole H. Larsen4, Bo Sander and Niels Henriksen 5 , and Peter Simonsen6 1

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2

E-mail: ff/hpn/paj/lah/hl/[email protected]

Midtkraft I/S Power Company, Studstrup Power Station, DK-8541 Skødstrup, Denmark. Phone: +45 86 99 17 00, Fax: +45 86 99 37 20 E-mail: [email protected],[email protected] (1) Currently with Rockwool International A/S, DK-2640 Hedehusene, Denmark. (2) Currently with SunChemical A/S, DK-4600 Køge, Denmark. 3 Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmark. Phone: +45 38 14 20 00, Fax: +45 33 63 39 89 E-mail: [email protected] Currently with Danfoss A/S, DK-6430 Nordborg, Denmark. 4 Faelleskemikerne, I/S Fynsvaesrket, Havnegade 120, DK-5000 Odense C, Denmark. Phone: +45 65 90 44 44, Fax: +45 65 90 38 12 5 Faelleskemikerne, ElsamProjekt A/S, Kraftværksvej 53, DK-7000 Fredericia, Denmark Phone: +45 79 23 33 33, Fax: +45 75 56 44 77 E-mail: bos/[email protected] 6 Elkraft A.m.b.a., Lautruphøj 5–7, DK-2750 Ballerup, Denmark Phone: +45 44 66 00 22, Fax: +45 44 65 61 04

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INTRODUCTION Ash-forming elements, ie. Al, Ca, Fe, K, Mg, Na, and Si, occur in fossil or biofuels as internal or external mineral grains, simple salts such as or KCl or associated with the organic matrix of the fuel. Coals utilized in Danish power stations contain 5–15% (w/w) ash, usually rich in Si, Al, Fe, and/or Ca, while Danish straw has 2–7% (w/w) ash, usually rich in Si, K, Ca, and Cl (Sander (1997)). In pc-firing, approximately 1% (w/w) of the inorganic metals is vaporized, while the rest occurs as ash droplets (Flagan and Friedlander (1978)). Depending on the gas/particle temperature and local stoichiometry during coal particle heat-up, devolatilization and char burnout, these mineral inclusions will undergo phase transformations and approach each other to form residual fly ash, ie. fly ash particles with (Fig. 1). The vaporized metal species may undergo several transformations: nucleation, subsequent coagulation, scavenging, heterogeneous condensation and/or interactions with mineral inclusions in the burning char or residual fly ash particles. These transformations depend on the total specific surface area of the residual fly ash particles, the rate of cooling of the flue gas, the local stoichiometry, and the mixing in the gas phase. Local supersaturation with respect to certain chemical species such as and may lead to the formation of submicron ash particles by homogeneous nucleation (Flagan and Friedlander

(1978); Christensen (1995)) (Fig. 1). Vapors and fly ash particles may be deposited on heat transfer surfaces in the boiler through a number of mechanisms, e.g. inertial impaction, thermophoresis, and diffusion. Ash deposits may cause several operational problems, e.g. changes in the heat uptake of the boiler, corrosion of heat transfer metal surfaces and/or in extreme cases plugging of the convective pass of the boiler. This may cause unscheduled outages of the boiler with significant financial loss as a consequence.

DANISH EXPERIENCES WITH STRAW-UTILIZATION Through the years, Danish utilities have gained significant knowledge about how to minimize/avoid ash deposition problems in utility boilers firing worldwide high-volatile

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bituminous coals (Laursen (1997)). Recently, the Danish Government has decided on a 20% reduction in the carbon dioxide emissions before year 2005 with reference to 1988. Biomass is considered (CO2)-neutral due to its short time of regeneration compared to fossil fuels. Thus, the Danish power producers are enjoined to burn 1.0 Mtons of straw,

0.2Mtons of wood chips and 0.2 Mtons of straw/wood chips (free choice) every year beyond year 2004. In addition, the Danish Government has recently decided not to allow new power stations based on coal thermal conversion to be build. As a consequence, over the past few years, a number of full-scale investigations related to power generation from straw combustion has been carried out at Danish power stations. This paper provides a review of Danish experiences with deposit formation and corrosion in utility boilers fired with straw or co-fired with straw and coal. The boilers include various types of grate-fired boilers designed for straw combustion (Larsen (1996); Stenholm et al. (1996); Michelsen et al. (1996); Jensen et al. (1997)), a CFB-boiler cofiring coal and straw with up to 50% straw on an energy base (Henriksen and Hansen (1995); Henriksen et al. (1995); Hansen et al. (1996a)) and PF-boilers co-firing coal and straw with up to 30% straw on an energy base (Hansen (1994); Larsen and Inselmann (1994); Henriksen et al. (1995); Hansen et al. (1996b); Andersen et al. (1996,1997)). Further details on ash chemistry aspects of straw utilization will be provided in a subsequent number of publications (Andersen (1998), Nielsen (1998) and Frandsen et al. (1998)).

THE STRAW-FIRED SLAGELSE AND HASLEV GRATE-BOILERS Stenholm et al. (1996) and Jensen et al. (1997) have investigated the combustion of twelve well-defined batches of different straws (wheat, barley, rape) at two CHP boilers in Slagelse and Haslev, Denmark. In the Slagelse CHP, bales of straw are shredded before entering the combustion chamber through a screw-feeder. The straw burns out on a sloping grate. Boiler data are provided in Table 1. Deposition measurements were carried out at two locations using air-cooled probes, in the upper half of the furnace and in the third pass, the latter probe

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being located between the primary and the secondary superheater. The mean gas temperature during the wheat and barley experiments was 873 °C for the furnace location and 647 °C for the superheater location (Fig. 2). In the Haslev CHP, bales of straw are fed to the boiler, which is equipped with four cigar burners implying that the straw bales burn on the front side, from one end to the other as the bales are fed into the combustion chamber. The final burnout takes place on a sloping grate. Boiler data are provided in Table 1. Deposition measurements were carried out at two locations using air-cooled probes: in the top of the furnace and at the entrance to the third pass, just in front of the superheaters. The mean gas temperatures at the two locations were approximately 835 °C and 650°C, respectively (Fig. 2). Each experiment at the Slagelse and Haslev CHPs lasted approximately eight hours, and the parameters measured included: local gas temperatures, exit flue gas composition and aerosol particles in the flue gas. In addition, detailed analyses of the straw, fly and bottom ashes and deposit samples were conducted (Jensen et al. (1997)). For wheat and barley straw, the extent of deposit formation on the inserted probes could be correlated to the content of potassium in the straw fuel. In all experiments firing wheat and barley straw, a faster build-up of deposit was seen at the furnace probes compared to the superheater probes. Although they must be interpreted with great care, the measured total deposition fluxes from Slagelse and Haslev CHPs are provided in Table 2 (Jensen et al. (1997)). From Environmental Scanning Electron Microscopy (ESEM) combined with Energy Dispersive X-Ray (EDX) analysis, the inner and outer layer of the deposits were found to be alike, both consisting primarily of K, S and Cl which correlates with the aerosol findings in the experiments (Christensen (1995)). Detailed Computer Controlled

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Scanning Electron Microscopy (CCSEM) analyses of fly and bottom ashes and deposits provided from Slagelse and Haslev CHPs are by Hansen et al. (1997) and Sørensen (1997). The aerosol measurements have revealed high concentrations of submicron aerosols, during wheat-firing, and during barleyfiring. In the rape experiment at Slagelse CHP, a submicron aerosol concentration of more than was measured (Christensen (1995), Christensen et al. (1997)). The aerosols consisted almost solely of K, Cl, and S.

THE STRAW-FIRED RUDKØBING GRATE-BOILER Combined corrosion and deposition studies were carried out at the wheat strawfired Rudkøbing CHP boiler (Michelsen et al. (1996), Larsen and Henriksen (1996), Henriksen and Larsen (1996), Michelsen et al. (1998)). The boiler is a grate-fired unit with both a stationary and a moving grate. Boiler data are shown in Table 1. The experiments were performed mainly in order to evaluate the effect of increasing the steam temperatures on deposit formation and corrosion. The existing superheater tubes experienced only negligible corrosion at a steam temperature of 450 °C, but the corrosion probe experiments have shown that at a steam temperature above 520 °C, a highly temperature-dependent corrosion takes place, showing severe internal corrosion. The internal corrosion is believed to be caused by selective Cl corrosion, where gaseous chlorine reacts with primarily chromium and iron at the metal/scale interface to form volatile metal chlorides which diffuse out through the oxide layer forming a loose non-protective oxide layer and leaves behind a degrated metal phase enriched in nickel (Nielsen (1998)). The deposits collected on an air-cooled probe were larger, darker and more dense with no direct signs of molten phases on the upstream side, and powdery on the downstream side, of the probe. The deposits were rich in K and Cl (K and Cl makes up about 40–80% (w/w)) and to a smaller extent in Si, Ca and S. No significant difference in deposit composition was found as a function of the metal temperature (460 °C and 550 °C) or with increasing sampling periods (2, 4, 14–16h). SEM-analyses have revealed that the upstream deposits mainly consisted of fly ash particles and large amounts of condensed KG. Close to the metal surface, a layer of almost pure KC1 material was found, which was very dense and approximately 20–30µm thick on the (550°C)-probes. The corresponding layer was less dense and only 5–l0µ m thick on the (450°C)-probes. Threads of iron through the molten layer were seen on the (550°C)-probe, but not on the (450 °C)probe. The differences in structure of the inner layer of condensed material could be one of the reasons for the difference in the corrosion behaviour observed at the different temperatures (Nielsen (1998), Michelsen et al. (1998)).

THE STRAW-FIRED KYNDBY PF-BOILER A 2-week test run co-firing straw and oil at the Kyndby Power Station, Unit 11, was performed in September 1995. Boiler data are provided in Table 3. During the test period, the boiler was fired with oil (10–30% on an energy base) and pelletised straw,

which was pulverised in the coal mills (Michelsen (1996)). Deposit measurements were performed at the furnace outlet, and in three positions in the convective pass, at flue gas temperatures between 1,050°C and 600 °C, using air-

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and water-air-cooled probes. The probe metal temperature was 530 °C, and sampling periods of 3–5 and approximately 24 hours were used. The deposits collected were light grey, with most deposit forming on the upstream side of the probes. The up-stream deposits from the convective pass had an inner uniform layer with islands of deposited material on top. This was also the appearence of the mature deposits collected from boiler tubes. The deposits collected at the furnace outlet were more loose in appearence and contained more unburned material than the deposits collected in the convective pass. Deposits were rich in K, Si, Ca, S, and Cl, with a maximum content of KCl and K2SO4 of 27–60% (w/w) of the deposit. SEM-analyses revealed, that the deposits consisted of large amounts of condensed KCl and K2 SO4 as well as fly ash particles. No mixing between the salts were observed (Michelsen (1996)).

THE COAL-STRAW CO-FIRED GRENÅ CFB-BOILER Co-firing of biomass and coal have been performed with reasonable succes at the Grenaa CHP, a circulating fluid bed combustor with a maximum biomass share of 60% (energy base). Limestone is added for in-situ capture of S and quartz sand and fuel ash constituted the bed material. Boiler data for the Grenaa CFB are provided in Table 3. During the initial 8 months of operation, the combustor load never has exceeded 80% of full load and no problems with formation of fouling deposits were reported. However, increasing the load to 100%, the cyclone temperatures have increased from app. 850 °C to 900 °C–1,000°C causing serious fouling in the cyclones and on the superheaters in the convective pass (Hansen et al. (1996)). After 1½ year of operation, the final

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superheater was severely damaged by selective Cl corrosion and had to be replaced. The subsequent decrease of the furnace temperature by adding extra heat transfer surfaces, changing to a low-S coal, and replacing the limestone by a better quality, has reduced fouling significantly and corrosion to a somewhat lesser extent (Hansen et al. (1996a,b)).

A measuring campaign was carried out, co-firing Danish wheat straw with two different coal types (Hansen (1997a,b)). Deposit measurements were made in front of the screen tubes (just outside the cyclone) using water-air-cooled probes. The mean gas temperature at that position was 875°C–890°C and the probe metal temperature was kept at 525 °C. A significant increase in the content of water-soluble K and Cl was seen when comparing upstream probe deposits with the fly ash (from co-firing of both coals). This indicates that a major mechanism for build-up of upstream deposits (on probes) could be the condensation of KCl and/or thermoforetic transport of KCl aerosols (Hansen (1997a,b)). The effect of coal type was significant: co-firing coal A gave a probe deposit with a CCSEM-based porosity of 30%, while the corresponding coal B deposit was almost solid (CCSEM based porosity: 2.5%). Mature deposits were collected from five different positions in the boiler and analysed. It was observed that the deposits were comprised of numerous distinct layers with K present as K2SO4 or K-Al-Si-species (Hansen (1997a,b)). Deposits removed from

the final superheater tubes situated in the convective pass contained mainly K (30–35% and and some Si, Ca, and Al. Chlorine accounted for less that 0.5%, (w/w) (Hansen (1997a,b)).

Thus, comparing probe deposits with mature in-boiler superheater deposits have shown that a deposit sampling period of 3 hours may not be sufficient to obtain a complete understanding of deposit formation mechanisms in a coal-straw co-fired CFB-boiler (Hansen (1997b)). Test tubes built into the final superheater and corrosion probes inserted into the convective pass for 400–2,800 hours and operated at ultra super critical conditions, showed corrosion rates 10–25 times higher during co-firing of coal and straw as compared to 100% coal combustion. Selective Cl corrosion was found to be the predominant corrosion form (Henriksen et al. (1995)), possibly indicating a significant degree of solid phase sulfation of KC1, according to reaction (1), which will release the HC1 necessary

for the observed selective Cl corrosion to occur:

THE COAL-STRAW CO-FIRED AMAGER PF-BOILER In October 1994, a coal-straw co-combustion test was performed at the Amager Power Station, Unit 3. Part of the test programme was deposition inspections inside the furnace (Pedersen et al. (1995)). The unit is boxer-fired with oil or pulverised

coal, producing 778 tons steam per hour at 250 bar and 545 °C. During the experiments, the straw was added as pellets, and grounded with the coal in the coal mills. The coal used through most of the test period was a Canadian high-S coal. The in-boiler deposit inspections revealed, that the boiler was relatively clean after a test period of coal-firing, whereas the amount of deposit around the burners had increased after a week of coal-straw co-firing (10, 20% straw on an energy basis).

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Fig. 3 show pictures of the pendant tertiary superheater placed in the top of the furnace, after a period of coal-firing and one week of coal-straw co-firing, respectively.

THE COAL-STRAW CO-FIRED VESTKRAFT PF-BOILER A full-scale coal-straw co-firing test was performed at the Vestkraft Power Station, Unit 1, from October 1993 to March 1994. Boiler data are provided in Table 4. The unit is wall-fired with three burner levels, each containing four burners. Two of the mid-level burners were replaced with straw burners during the test period, see Fig. 4. Significantly lower temperatures were measured in the straw burners (800°C–900°C) compared to the coal burners (1,200°C–1,300°C). A part of the straw, introduced to the furnace in pieces of 10–12cm, was not burned, but hit the rear wall and fell directly into the bottom ash hopper. The test programme included corrosion tests at 10% straw share (energy base). At a position with a gas temperature of 1,140°C, the results from the corrosion tests showed

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no difference in the appearence of the corrosion test material samples at any of the metal temperatures used (560°C–630°C), when comparing coal-firing to coal-straw co-firing (Larsen and Inselmann (1994); Henriksen et al. (1995)). Most of the potassium in the fly ash was found to be non-soluble in water, and thus assumed to be present as K-Alsilicates. The chlorine is assumed to be released as HCl and Cl 2 in concentrations lower than those needed for selective Cl corrosion to occur (Inselmann and Larsen (1995)). An investigation at the Vestkraft power station by Baldacci et al. (1994) have revealed low deposition rates, <12mg/m2/s, independent of the straw-fraction fired (0, 10, 20% straw on an energy base) and sampling periods (20min.–2 hours). Based on SEM analyses, coal-firing produced sticky deposits, while coal-straw co-firing (10, 20% straw on an energy basis) gave non-sticky deposits, probably due to a lower gas temperature in the top of the furnace during co-firing (Baldacci et al. (1994)).

THE COAL-STRAW CO-FIRED MIDTKRAFT PF-BOILER Based on the experiences from the Vestkraft Power Station, a 2-year test programme of coal-straw co-firing was carried out at the Midtkraft Power Station, Unit 1. Boiler data are provided in Table 4. The unit is wall-fired with three burner rows, each of four burners, see Fig.4. The middle burner row has been rebuilt for coal-straw co-firing. Straw pieces with a max. length of 5cm, was introduced through the centre of the burner, forming a core, surrounded by a swirling annulus coal-flame. The test programme was initiated in 1996, and comprised in-situ sampling of deposits, fly ash particles, flue gas temperatures and composition, gas phase alkali metal compounds and aerosol measurements, as well as mass balance closures and examination of the fly ash with regard to

utilization in concrete and cement production. Included in the test programme were also long-term corrosion tests at 10 and 20% straw share (energy base), the effect of co-firing on de-NO x catalysts and boiler performance, including an evaluation of the straw han-

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dling, transport and combustion. Experiments have been performed with 0, 10 and 20% (energy base) straw share, and at 50, 75 and 100% boiler load (Andersen et al. (1996, 1997); Andersen (1998); Hansen et al. (1996)). Deposits were collected at probes with metal temperatures from 400°C to 620°C, in five positions ranging from the top of the furnace through the convective pass to the economiser section, see Fig. 4. The upstream deposits collected at flue gas temperatures from 1,300°C to 750°C during coal-straw co-firing were in all cases larger and more tenacious than the deposits collected during coal combustion (Andersen et al. (1997); Andersen (1998)). No visual effect of probe metal temperature on the upstream deposits was observed. All deposits collected at flue gas temperatures below 750°C were powdery on the up- as well as the downstream side of the probes (Andersen et al. (1997); Andersen (1998)). Increased deposit formation was seen when the load was increased from part load (50, 75%) to full load. In the long-term corrosion test at 10% straw share (energy base), the corrosion was generally found to be low, and no significant increases compared to coal-firing was observed (Larsen (1997)). The aerosol measurements revealed shown low submicron particle loadings, 30–110 compared to the measurements in the straw-fired CHPs, ie. The submicron particles from coal-straw co-firing consist of small spherical fly ash particles and even smaller, dendritic clusters, each consisting of numerous very small primary particles. The morphology and chemical composition of the submicron particles were unaffected by the straw share except for a significant enrichment in water soluble which was observed only when straw is co-fired (Nielsen et al. (1996)). The chlorine content in the particles was negligible in contrast to the high content of chlorine in the submicron aerosol particles from straw-fired CHPs (Christensen (1995); Christensen et al. (1997)).

SUMMARY AND CONCLUSION Straw-fired grate boilers produce fly ash and deposits with a very high content of K and Cl (40–80% (w/w)). Ash deposition and corrosion may constitute a significant problem in these boilers, particularly if the steam temperature is increased above 520 °C (Michelsen et al. (1996), Stenholm et al. (1996), Jensen et al. (1997)). During co-firing of straw and coal there seems to be a significant capture of potassium released from the straw, in the coal ash. This phenomenon has been observed both in the Grenaa CFB-boiler (Hansen (1997a,b)) and in the Vest- and Midtkraft pf-fired boilers. Due to the capture of K in the coal ash, only low concentrations of were observed in the fly ash and deposits from these plants. CCSEM compositional data for bottom and fly ashes and probe deposits from the Slagelse and Haslev CHPs, the Grenaa CFB and the Midtkraft Power Station, Unit 1, may be illustrated in a triangular diagram of: 1) quartz, Al silicates, and illite, 2) K + Ca silicate and 3) KCl (Hansen et al. (1997)). The three end-members in the diagram are chosen to represent: 1) relatively refractory quartz and Al silicates, 2) more easily fusible potassium and calcium dominated silicates (formed eg. by reaction between evaporated potassium and Ca silicates), and 3) low melting KCl. Figure 5 summarizes all the compositional data in such a diagram. The top triangle provides composition of fly ashes (FA) and probe deposits (D) and the bottom triangle contain the same information for bottom ashes (BA). In Fig. 5, it is seen that the fly ash and probe deposits in the straw-fired grate-boilers

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are rich in KC1, while in the pf-boilers the fly ash and probe deposits are rich in alumi-

nosilicates (often mixed with small amounts of Ca and K). Thus, KCl may constitute a deposition and/or corrosion problem in the grate boilers, but not in the pf-boilers when

utilizing up to 20% straw on an energy base. Recent CCSEM-analyses of probe deposits from the coal-straw co-fired Midtkraft Power Station, Unit 1, has revealed in the deposits from the convective pass, indicating condensation of during cooling of the flue gas. According to Andersen et al. (1997), ash deposition and/or corrosion will not be the major problems in coal-straw co-firing in pf-boilers. Instead, other problems

like fly ash quality aspects and poisoning of Selective Catalytic Reduction (SCR ) catalysts for reduction, may be more pronounced. The Grenaa CFB-boiler has two distinct areas marking fly ash and probe deposit composition in Fig. 5. Only little Cl is present in the fly ash, but Cl is highly enriched in

the (upstream) probe deposits. However, less than 0.5% (w/w) Cl is present in mature superheater deposits from the CFB-boiler, indicating a transformation of KC1 to with time, in the mature (boiler) deposit. This may cause a release of HCl(g) close to the metal surface and could thereby explain the 10–25 times increase in the selective Cl corrosion rate observed when co-firing coal and straw as compared to coal combustion (Henriksen et al. (1995)).

ACKNOWLEDGEMENTS This work has been financially supported by the CHEC Research Programme, ELKRAFT, ELSAM, the Danish Energy Research Programme and the Midtkraft Studstrup Demoprogramme.

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Conf., Copenhagen, Denmark, June 24–27. Andersen, K. H., Frandsen, F. J., Hansen, P. F. B., Dam-Johansen, K. (1997) “Full-Scale Deposition Trials at a 150 MWe PF-Boiler Co-Firing Coal and Straw: Summary of Results”. Proc. Eng. Found.

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‘Corrosion and Corrosion Protectioning in Power Plant Technology’. Henriksen, N., Larsen, O. H. (1996) “Fouling and Corrosion in Straw and Coal-Straw Fired USC Plants”, Proc. 9th Eur. Bioenergy Conf. on Biomass for Energy and the Environment, Copenhagen, Denmark, June 24–27, 1996. Inselmann, S., Larsen, O. H. (1995) “The Corrosivity of Coal Types on Superheaters”, Fælleskemiker Report No. 4.19 (In Danish). Jensen, P. A., Stenholm, M., Hald, P. (1997) “Deposition Investigation in Straw-Fired Boilers”, Energy and Fuels, 11(5), 1048–1055. Larsen, O. H., Inselmann, S. (1994) “Superheater Corrosion during Co-Firing of Straw and Coal—Investiga-

tions at Vestkraft Unit 1”, ELSAM R&D Project Internal Report. Larsen, O. H. (1996) “Investigation on High Temperature Corrosion during Straw Combustion at Super High Steam Data”, ELSAM R&D Project 268. Larsen, O. H., Henriksen, N. (1996) “Ash Deposition and High Temperature Corrosion at Combustion of Aggressive Fuels”, Proc. Int. Conf. on Power Plant Chemical Technology, Kolding, Denmark, September 4–6, 1996. Larsen, O. H. (1997) “ MKS1 Demoprogramme: Superheater Corrosion at 10% Straw Share”, Fælleskemiker Report No. 4.06 (In Danish). Laursen, K. (1997) “Characterization of Minerals in Coal and Interpretations of Ash Formation and Depo-

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sition in Pulverized Coal Fired Boilers”, Geological Survey of Denmark and Greenland, Ph.D.-Thesis, ISBN-87-7871-022-7. Michelsen, H. P. (1996) “Results of Deposit Measurements during Straw Firing at the Kyndby Power Station”, CHEC Report No. 9604, Dept. Chem. Eng, Techn. Univ. of Denmark. Michelsen, H. P., Larsen, O. H., Frandsen, F. J., Dam-Johansen, K. (1996) “Deposition and High Temperature Corrosion in a 10 MW Straw Fired Boiler”, Proc. Eng. Found. Conf. "Biomass Usage for Utility and Industrial Power", Snowbird, Utah, April 28–May 3. Michelsen, H. P., Frandsen, F. J., Dam-Johansen, K., Larsen, O. H. (1998) “Deposition and High Temperature Corrosion in a 10 MW Straw Fired Boiler”, Fuel Processing and Technology, 54 , 95–108. Nielsen, H. P. (1998) “The Influence of Alkali Metals and Chlorine on Deposition and High-Temperature Corrosion in Biomass-Fired Boilers”, Ph.D.-Thesis, Department of Chemical Engineering, Technical University of Denmark. Nielsen, L. B., Pedersen, C, Røkke, M., Livbjerg, H. (1996) “Aerosol Measurements at MKSI—Final Report”, The Aerosol Laboratory, Dept. Chem. Eng. Techn. Univ. of Denmark (CONFIDENTIAL). Pedersen, L. S., Michelsen, H. P., Hansen, L. A., Kiil, S. (1995) “Full-Scale Co-Firing of Straw and Coal”, CHEC Report No. 9503, Dept. Chem. Eng, Techn. Univ. of Denmark. Sander, B. (1997) “Properties of Danish Biofuels and the Requirements for Power Production”, Biomass and Bioenergy, 12(3), 177–183. Stenholm, M., Jensen, P. A., Hald, P. (1996) “The Fuel and Firing Characteristics of Biomass—Combustion Trials”, Final Report, EFP Project No. 1323/93-0015 (In Danish). Sørensen, H. S. (1997) “Computer Controlled Scanning Electron Microscopy (CCSEM) Analysis of Straw Ash”, Proc. Eng. Found. Conf. 'Impact of Mineral Impurities in Solid Fuel Combustion', Kona, Hawaii, November 2–7, 1997.

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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 Combustion Engineering Dept. Ishikawajima-Harima Heavy Industries Co., Ltd. 3-2-16, Toyosu, Koto-ku, Tokyo 135 JAPAN

1. INTRODUCTION To utilize surplus gases such as blast-furnace gas (BFG), Linz Donawitz gas (LDG) and coke-oven gas (COG) as fuels in power plants is one of the effective energy-saving measures in iron and steal companies. For the purpose, a 145 M We, wall firing boiler was designed and constructed by Ishikawajima-Harima Heavy Industries Co., Ltd. ( I H I ) to fire three kinds of surplus gas, BFG, LDG and COG other than pulverized coal and heavy oil. Among these fuels, BFG assisted by COG or heavy oil and pulverized coal were main fuels with whichever the full load could be obtained. Commissioning tests have started with pulverized-coal firing, and satisfactory performances have been obtained with the coal alone. However, BFG burners and LDG burners were found to be subjected to a serious ash trouble on their burner mouth and gas nozzle after 2 weeks’ coal firing. A lot of amount of ash was deposited on the burner mouth and it was plugged the gas nozzle partially or entirely as shown in Fig. 1. In addition, as shown in Fig. 2, which shows the status of the gas nozzle of the BFG burner after removing ash, it was deformed. In order to investigate the cause of the trouble and to propose the method for burner modifications, experimental and analytical studies were carried out.

2. SPECIFICATIONS OF THE BOILER The subject boiler was constructed in the iron and steal company as a steam generator of a 145 MWe thermal power plant and it started the commercial operation in 1997. The unit is equipped with flue gas treatment system such as selective catalytic reactor (SCR), flue gas desulphurization (FGD) and electrostatic precipitator (ESP). Figures 3 and 4 show the sectional side view of the boiler and the burner arrangement, respectively. Twenty four (24) burners are arranged in three (3) stages with four (4) Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York, 1999.

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rows for each opposed wall. Upper two (2) stages of front wall are for BFG, an upper stage of rear wall is for LDG, and lower three (3) stages are for pulverized coal from three (3) roller mills. COG nozzles and heavy fuel oil burner are also installed in the BFG and LDG burners. So they are called as “combination burners”. The IHI NOx Preventing Advanced Combustion Technology (INPACT) [Kiga, 1989] is applied so that a very

low NO x operation can be realized; that is, over fire air ports (two-stage combustion air ports) are settled up in consideration of the residence time in the furnace. The main specifications of this boiler are shown in Table 1. Typical properties of surplus gases are tabulated in Table 2. Since the BFG has a very low heating value of around its flame is stabilized by a pilot flame of COG or heavy fuel oil. On the other hand, properties of coal used in the commissioning test are presented in Table 3. The coal was Australian bituminous coal and it was pulverized by the mills into over 90% through 200 mesh sieve [Tamura, 1996].

3. EXPERIMENTAL Several kinds of half-scale size burner model were manufactured and they were utilized both for flow pattern testing using cold air and for combustion testing.

3.1. Test Facilities These tests were conducted in the 12 MW(thermal) pilot-scale combustion test facilities in IHI’s Aioi Works. The specifications of the test facilities are listed in Table 4 and the general arrangement of its furnace is schematically shown in Fig. 5. The furnace comprises of a multi-burner furnace and a single-burner one. The single burner furnace was used in the test.

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3.2. Test Burners The tests were conducted for three (3) kinds of BFG burner model, Type I (original burner model), Type II, and Type III, where Type II was a model of a temporally modified burner and Type III was that based on the final modification design. The photographs of these burners tested are shown in Fig. 6.

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The BFG is provided from an annulus nozzle, and multi-spud type COG nozzles are arranged in the inside of the inner barrel of the BFG nozzle. The heavy fuel oil burner is

mounted in the center of the burner and a swirler is attached near an atomizer for flame stabilizing. The primary air which is divided from the combustion air at the inlet of the burner is supplied in the center of the burner, and it is minimized when the BFG is not provided. The secondary air is supplied from multiple rectangular nozzles arranged in the BFG nozzle for Type I and Type II, and it is supplied from around the BFG nozzle for Type III. Type II and Type III models are equipped with multiple openings on the outer barrel of the BFG nozzle to lead secondary air into BFG nozzle. In addition, Type II model has a baffleplate at the outlet of the secondary air nozzle so as to avoid the reverse flow toward the secondary air nozzle. Type III model has an baffleplate at downstream of the openings to change the direction of the air flow from the openings into a suitable way to avoid the reverse flow toward BFG nozzle when the burner is out of service.

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Two (2) types of burner throat were prepared; one was made by styrene foam used in the flow pattern testing in order to measure the velocity in the burner by removing it‚ and the other was made by steal and refractory used in the combustion testing.

3.3. Test Conditions The ash troubles were occurred during pulverized-coal firing when the BFG burners were out of service. The flow pattern tests were therefore carried out in the condition that

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the air flow rate was settled so that the air velocity through the burner throat was simulated for actual conditions when the BFG burner was out of service. In the tests‚ the dis-

tributions of the air velocity were measured by a hot-wire anemometer at the section of the edge of burner tile and throat. On the other hand‚ in the combustion tests‚ an oil stabilizer was operated to simulate the condition in which the metal temperature measured on the BFG nozzle took the highest value in the actual boiler. The light oil was used in the tests.

4. STRESS AND DEFORMATION ANALYSIS The thermal stress and deformation analysis was performed for the real size burners of Type I (original) in order to examine the reason of the deformation of the gas nozzle

of the BFG burner. The analysis was carried out using the finite element program‚ MSC/NASTRAN code. Analytical conditions were given based on the metal temperature measured in the actual operation.

5. RESULTS AND DISCUSSIONS Figure 7 illustrates distributions of air velocity obtained from the flow pattern tests. A reverse flow coming from the outside into the inside on both the secondary air nozzle

and the BFG nozzle is found for the original burner (Type I). Such a reverse flow conveys

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hot flue gas and fly ash onto the burner throat and into the nozzles‚ and will cause ash depositions on them. On the other hand‚ Type II (temporally modified burner) and Type III (finally modified burner) have positive air velocity at the secondary air nozzle and BFG nozzles and near the burner throat. Especially for Type III burner‚ the strong cleanup and cooling effects on the burner throat are expected owning to the secondary air flow along the throat. The secondary air flow through the BFG nozzle also make the BFG nozzle and the burner tile clean and cool. The modifications of flow pattern enables

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to prevent these burners from ash troubles. Furthermore‚ Type III burner forms so-called a strong internal circulation flow‚ which will make the flame stable‚ although the test conditions are to simulate the status of burner’s out-of-service. The flames observed in the combustion tests are shown in Fig. 8. As is expected from the flow pattern test results‚ good flame stability is obtained about Type III burner. As for the metal temperature of the BFG nozzle‚ it was confirmed that that of modified burners (Type II and Type III ) was clearly lower than that of original burner as shown in Fig. 9.

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The result of the thermal deformation analysis is shown in Fig. 10. It is obviously found that the deformation pattern obtained in the analysis quite resembles to that observed in the actual boiler as previously shown in Fig. 2. From the analytical results‚

the rigid structure of the secondary air nozzle and the BFG nozzle regardless of thermal deformation are the cause of such damage. In the modification at the actual boiler‚ therefore‚ the swirl plates for secondary air and BFG were not fixed and the BFG nozzle was slit to prevent it from deformation by stretch‚ in addition to making the nozzle cool as mentioned above. Based on the above-mentioned studies‚ all of gas burners including LDG burners in the subject boiler were modified into new type ones. After the modification‚ the plant has been operated without such troubles.

CONCLUSIONS Against a serious ash trouble on BFG burners and LDG burners‚ experimental studies and thermal stress analysis were carried out and the followings were obtained. • Ash deposits on gas nozzles and burner mouth were caused by the reverse flow conveying hot flue gas and fly ash toward the gas nozzles and along the burner mouth. • The deformation of the BFG and secondary air nozzles was caused by its rigid and cooling-less structure.

The studies offered ash-free BFG and LDG burners for cofiring with pulverized coal and the subject boiler has been free from such troubles. Since to utilize surplus gases in power plants is one of the effective energy-saving and measures in iron and steal companies‚ we would like to proceed to spread our technologies widely.

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ACKNOWLEDGEMENTS We would like to acknowledge the assistance and suggestions of Mr. Yutaka Takamatsu of Volcano Co.‚ Ltd.‚ a manufacturer of the combination burners‚ in the tests and modifications.

REFERENCES Kiga‚ T‚ Miyamae‚ S.‚ Makino‚ K.‚ and Suzuki‚ K. (1989). “Low NOx Combustion Technologies for CoalFiring Boilers and Its Application Results to Commercial Boilers.” AIChE 1989 Summer National Metting‚ Philadelphia‚ PA. Tumura‚ M.‚ Kiga‚ T.‚ Endo‚ Y.‚ Murakami‚ H.‚ and Tanaka‚ T. (1996). “Practical Studies on Combustion Technologies of Micro-Pulverized Coal.” IHI Engineering Review‚ 29(3)‚ 81–86.

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CHANGES IN SLAGGING BEHAVIOUR WITH COMPOSITION FOR BLENDED COALS Nicholas J. Manton1‚ and Jim Williamson1‚ and Gerry S. Riley2 1

Department of Materials Imperial College of Science Technology and Medicine London SW7 2BP‚ UK 2 National Power Plc Windmill Hill Business Park Swindon SN5 6BP‚ UK

1. INTRODUCTION Coal blending has traditionally been used to provide fuels of a consistent quality‚ to reduce fuel costs and to improve the combustion behaviour of low volatile coals. In addition‚ blending may be used to enhance fuel flexibility‚ to extend the range of acceptable coal feeds and to control the sulphur content of the fuel to meet emission regulations. UK experience in the use of blended fuels for utility use is limited‚ although increasing amounts of overseas coals are now imported by the power generators. In 1994 British Coal was sold to private industry and the contracts which were in place at the time will expire early in 1998. Intensive bargaining between the coal users and the coal suppliers is now in progress to maintain a UK coal mining industry‚ which currently supplies approximately Tpa. This scenario has to be set against a background of an increasing use of natural gas for power generation and a market price for world traded coals which can be significantly less than indigenous coals. While the amount of imported coal used by utilities could be limited by the problems of transporting the coal from the ports to the power stations‚ the amount of imported coal could rise significantly above the amount currently used. Many imported coals would lie outside the normal range of boiler specifications; thus‚ blending with indigenous coals would be a prerequisite for efficient boiler operation. This study has been undertaken to provide a method of determining the change in slagging propensity when UK and overseas coals are blended for use in pulverised coalfired boilers. Impact of Mineral Impurities in Solid Fuel Combustion‚ edited by Gupta et al. Kluwer Academic / Plenum Publishers‚ New York‚ 1999.

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1.1. Properties of Blended Coals Certain properties of a coal blend can be calculated from the weighted average of the measured values of the single coals‚ as reviewed by Carpenter [1995]. These properties include:

• moisture content • volatile matter • ash content • fixed carbon • hydrogen‚ sulphur‚ nitrogen‚ oxygen‚ chlorine and proportions of macerals These properties can then be considered as additive properties of the individual coals. Coal properties which are non-additive include:

• swelling index • Hardgrove grindability • ash fusion temperatures

• slagging propensity

Thus the slagging propensity of a blend requires careful evaluation to avoid costly boiler operational problems.

1.2. Assessment of the Slagging Propensity of a Coal Fuel technologists have long sought reliable methods for predicting the slagging propensity of a coal ash‚ as described by Williamson and Wigley [1994]. Conventional methods of assessment based on ash fusion tests‚ viscosity of coal ash melts and empirical indices calculated from the chemical composition of the ash were reviewed by Couch [1994]. These techniques all fail to reflect the complex mineral matter transformations and reactions which occur at the combustion temperatures. In addition‚ the short residence times for ash particles within the radiant zone of the boiler (1–2 sec) frequently do not allow the decomposition of larger mineral particles to reach equilibrium. Wigley and Williamson [1996] reported the use of advanced microstructural analysis‚ combined with chemical analysis of individual ash particles obtained using computer controlled scanning electron microscopes (CCSEM). This demonstrated the diverse chemical nature of ash particles and the lack of chemical homogeneity. Thus‚ to obtain reliable data‚ fuel technologists have developed combustion test facilities which closely simulate the conditions in large pulverised coal boilers. The results obtained with such facilities still require

validation with full scale boiler trials‚ as reported by at a previous Engineering Foundation Conference by Gibb [1996]. The high cost of these trials‚ and the difficulty of obtaining an objective assessment of the slagging propensity of a coal ash stimulated the design and construction of a entrained flow reactor (EFR) at Imperial College to provide a

laboratory facility to study ash formation and deposition phenomena‚ as reported by Hutchins et al. [1996]. This facility has provided a means of preparing coal ash slags and deposits from both single and blended fuels‚ with preliminary findings being reported by Manton et al. [1996].

The EFR was designed to closely simulate the conditions which pf and ash parti-

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cles experience in large utility boilers. The reactor consists of a vertical multi-zoned furnace‚ approximately 5m in length‚ with an internal diameter of 100mm. Four independently controlled furnaces heat the reactor‚ providing a temperature gradient from 1‚650°C at the top to 1‚200°C at the bottom. A series of sample ports between each furnace allows ash and char samples to be withdrawn from the combustion atmosphere. Ash deposition probes may also be inserted into these ports. Pulverised coal is introduced at the top of the furnace at a rate of the chosen feed rate depending on the ash content of the coal or blend under investigation. Gas flow rates of approximately 70 (STP)‚ equivalent to at 1‚650°C‚ entrain all coal and mineral particles with a density of or less and a particle size of Under these conditions‚ particle residence times from the top to bottom of the reactor are approximately three seconds. The reactor is shown schematically in Fig. l(a)‚ with the position of the sample ports. Gas temperatures at the top sample port are approximately 1‚400°C and 1‚250°C at the second port. Both air-cooled metal deposition probes and uncooled ceramic probes have been used to collect ash deposits. Deposits collected at 1‚400°C are generally of a highly fused nature and give little indication of the likely slagging propensity of the coal. However‚ when deposits are collected at 1‚250°C on an uncooled ceramic probe‚ they range from a thin dusty covering of lightly sintered ash particles to well-bonded and coherent deposits‚ the type of the deposit depending on the nature and proportions of the mineral matter present in the coal. Deposits collected under these conditions closely resemble those which form on the platten superheaters of large utility boilers and are therefore suitable for characterisation.

2. EXPERIMENTAL TECHNIQUES The ceramic deposition probe used in this study was of simple construction‚ consisting of a mullite tube or coupon‚ 17mm OD and 12.5mm ID‚ and 150mm in length‚ see Fig. l(b). The coupon is held in place over a longer (approximately 1 m) mullite tube of 12mm OD‚ with a small amount of a high alumina cement mixed with a sodium silicate solution. The same cement was used to block the open end of the 12mm tube‚ thus preventing gas leakage into or out of the reactor. An aluminium holder with an “O” ring seal is used to hold the probe in position in the sample port.

2.1. Production of Deposits Each of the coals was ground separately to pf grade‚ i.e. 70% was less than and blends were prepared in a laboratory mixer. The blended coals were dried at 100 °C for an hour or more to remove adsorbed moisture which can cause problems with the coal feed system to the EFR. Over a l–2hr period‚ the deposits varied in thickness from 1–5mm depending on the amount of mineral matter in the coal and the feed rate.

2.2. Characterisation of Deposits and Determination of Slagging Propensity A small sample of each deposit was first removed for X-ray powder diffraction analysis to establish the crystalline phases present‚ while the remainder of the deposit was left undisturbed on the ceramic coupon. The deposit was then coated in a low viscosity epoxy resin‚ which penetrated the open porosity of the sample‚ but when set gave

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stability to even the most fragile deposits. Sections through the deposit were cut with a diamond saw, perpendicular to the surface, to give samples approximately 5mm in length. These samples were then set in resin blocks so that a cross-section through the deposit from the interface with the ceramic to the surface of the deposit could be examined. The samples were ground and polished to a diamond finish, and coated with carbon for SEM examination. At low magnification a series of backscattered images (BSI) were first obtained to give an overall view of the microstructure of the deposit. Typical microstructures are shown in Fig. 2. More detailed examination was then made of selected areas at higher magnifications, typically Quantitative chemical and image analysis data was then acquired using a Tracor Noran low element detector and Tracor Noran Voyager III software. Typically, a chemical analysis was acquired using a matrix of analysis points, giving 256 separate analyses for each area examined. Quantitative EDS analyses were obtained from this data using Proza corrections. The porosity of the microstructure, which gave a strong indication of the amount of viscous flow sintering which had occurred, could be obtained from the grey scale digital image which differentiated between the ash and the resin (i.e. the filled pores) or from the fraction of analysis points reporting oxide analyses, where the criteria used for this were that analyses with could reasonably be assumed to have originated from an ash particle. The chemical and microstructural data was normally collected from several areas within each section and from samples taken at intervals along the coupon length. Typically, the number of analysis points was 1,500 or so, thus giving sufficient data for reliable statistical variations to be established. An assessment of the slagging propensity of the blended coal ash was made by a consideration of the analysis and the chemical variations shown by the ash particles and the bonding phase(s). The four main oxides which make up the ash composition are and CaO and the chemical data may be displayed by normalising each ash composition to one of two ternary systems, i.e. and Only those analysis points where or exceeded 70wt% were taken, since the normalisation of compositions totaling less than this could lead to substantial errors. The data obtained may then be plotted on a ternary diagram, see for example Figs. 3 or 5. Each plot shows not only the range of chemical compositions shown by the structure, but also the degree of chemical inhomogeneity in

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the sample. This indicated the degree of interaction between the decomposed mineral residues‚ i.e. the clays (kaolinite and illites)‚ pyrite‚ calcite‚ quartz etc. An assessment of the slagging propensity was made by calculating the proportion of the analyses where the content lay between 10 and 50 wt%‚ or the CaO concentration was between 5 and 40 wt%. Within these compositional limits the aluminosilicate melts would be considered to contain sufficient fluxing oxides to reduce the viscosity of the slag to a value at which the sintering of the ash particles by a viscous flow process would proceed at a measurable rate.

4. RESULTS AND DISCUSSION The three UK coals (UK 1‚ 2 & 3) have been blended with either American coals (AM 1&2) or South African coals (SA 1 & 2). The ash contents and ash compositions of each coal are shown in Table 1. The UK coal ashes are typical of the range of and CaO contents to be found in boiler ashes in UK utility coals. UK 1 has the highest and CaO contents‚ and this is reflected in the B/A ratio. This coal would be considered a moderate to high slagging coal‚ whereas the other coals with much lower B/A ratios would not be expected to cause any real problems.

4.1. CCSEM Mineral Matter Characterisation A CCSEM analysis of the UK 1 pf indicated a coal with much free pyrite and some

pyrite intimately mixed with kaolin and illitic clays low in

By contrast‚ coals UK

2&3 contained much less pyrite but an illitic clay much richer in

The

CCSEM analysis of the American coal AM 1 is shown in Fig. 3. This coal contains relatively little free pyrite and only a small amount of pyrite mixed with the clays; the mineral matter consisting largely of quartz‚ kaolin and illites‚ some of which are intimately mixed. The South African coals are low in Fe‚ contain little free quartz‚ and show mostly kaolinite as opposed to illitic clays. Ca‚ Mg and Fe are found as calcite‚ dolomite and

siderite. Fig. 4 shows how the Fe and Ca is distributed amongst the various mineral types for UK 1 and AM 1 coals. The relatively high proportion of Fe present as free pyrite (Fe-rich) in the UK 1 is striking‚ whereas for coal AM 1 most of the Fe is as an intimate

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pyrite-clay mixture (Fe-Alsil). Almost all the Ca in UK 1 is found as carbonates (nonCFAS)‚ whereas for AM 1 the Ca is divided between carbonates‚ carbonate-clay mixtures (Ca-alsil) and illitic clays (Alsil). The differences in the distributions of the Fe and Ca for these two coals may play a crucial role in the initial stages of ash formation. The results of the CCSEM analysis of UK 1 are shown in Fig. 5. The differences compared to coal AM 1 (Fig. 3) could not be more marked. UK 1 shows significantly less association between the pyrite and the clays‚ with virtually no free quartz‚ and much less quartzclay inter-mixing.

3.2. Change in Slagging Propensity of UK Coals on Blending The results on blending the American coal AM 1 with UK 1 are shown in Fig. 6. The variation in slagging propensity‚ as determined from a number of cross-sections from

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each deposit‚ are shown in the Figure by the length of the error bars. Cross-sections closest to the centre of the deposit were in general slightly higher in than sections towards the end of the coupon. This is undoubtedly due to the heavier pyrite particles showing less inclination to be deflected around the ceramic probe by the gas stream than the lighter aluminosilicate ash particles. Thus‚ within the limits of error of the technique‚

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the slagging propensity of the UK 1—AM 1 blends would appear to be a linear function of the blend composition. The change in slagging propensity of UK 3 when blended with the South African coal SA 1 is shown in Fig. 7 by the solid line. An initial slight decrease in slagging propensity on adding SA 1 is followed by a sharp increase. The slagging propensity of pure SA 1 lies below the curve for the blends‚ suggesting that a small amount of UK 3 when blended with SA 1 leads to a rapid increase in the slagging propensity of this coal. The dotted curves in Fig. 7 show the proportion of points from either the or the systems which would be deemed to be of sufficiently low viscosity to allow viscous flow sintering. The increase in slagging propensity is thus seen to be entirely due to the Ca-aluminosilicate compositions‚ which for the pure SA 1 had insufficient time for all the CaO to flux and enter the melt; thus these compositions tended to be rich in CaO and to lie outside the arbitrary limits chosen to assess the slagging propensity.

Coal UK 1 and the American coal AM 2 were observed to have similar slagging

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propensities‚ see Fig. 8. Although the pyrite content of UK 1 is much higher than that of AM 2‚ much of the pyrite in UK 1 is present as free mineral matter and is therefore slow to react with the decomposing clays forming the aluminosilicate melts during the initial stages of ash and deposit formation. However‚ when these two coals were blended‚

pronounced deviations from linearity with respect to blend composition were observed. Similarly‚ the effect of blending the South African coal SA 2 with UK 2 showed

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positive deviations from linearity‚ as shown in Fig. 9‚ although the magnitude of the nonlinearity is less than with the other blends. CCSEM examination of the fly ash particles from a blended coal has revealed a similar chemical distribution to that from a mechanical mixture of the constituent ashes. Thus the origin of the observed non-linear behaviour must follow the deposition of the ash particles. At this stage the composition of individual ash particles is crucial in determining the rate at which undissolved iron oxide particles will dissolve in the melt. The

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evidence to date is that CaO species readily combine with decomposing clays‚ since the residues of high CaO minerals are seldom observed in fly ashes. As the basicity of an aluminosilicate melt increases‚ the viscosity decreases and its power to enhance the rate at which the iron oxides will dissolve increases. An understanding of the complex nature of the interactions between ash particles at high temperatures remains one of the key issues to improved slagging predictions‚ be it from a single coal or a blend.

4. CONCLUSIONS Blends of UK coals with either American or South African coals show non-linear behaviour‚ with enhanced slagging propensity frequently observed with relatively small additions of an overseas coal to a UK coal. The most pronounced increases were observed when a UK coal‚ high in was blended with a coal high in CaO. The ability of a Ca-aluminosilicate melt to increase the rate of dissolution of free pyrite from a UK coal would appear to be a significant feature in accounting for the observed phenomena.

5. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support provided by National Power plc and the UK Engineering and Physical Sciences Research Council (EPSRC) and the help and guidance given by Mr F. Wigley with the CCSEM analysis.

6. REFERENCES Carpenter‚ A. (1995)‚ “Coal Blending tor Power Stations”‚ IEA Coal Research London‚ IEACR/81. Couch‚ G. (1994)‚ “Understanding Slagging and Fouling in PF Combustion”‚ IEA Coal Research London‚ 1EACR/72. Gibh‚ W.H. (1996)‚ “The U K collaborative research programme on slagging in pulverised coal-fired boilers: summary of findings”‚ in “Applications of Advanced Technology to Ash-Related Problems in Boilers”‚ Eds. L. Baxter and R. DeSolar‚ pp 41–65‚ Plenum‚ New York. Mulchings‚ I.S.‚ West‚ S.S. and Williamson‚ J. (1996) “An assessment of coal-ash slagging propensity using an entrained flow reactor” in “Applications of Advanced Technology to Ash-Related Problems in Boilers”‚ pp 201–222‚ Plenum‚ New York. Manton‚ N.J.‚ Riley‚ G.S. and Williamson‚ J. (1996)‚ “A laboratory assessment of the slagging propensity of blended coals”‚ 212th American Chemical Society National Meeting‚ Fuels Division Vol. 41‚ No 3‚ pp 1113–1117. Wigley‚ F. and Williamson‚ J. (1996)‚ “Modelling fly ash generation for UK power station coals” in “Applications of Advanced Technology to Ash-Related Problems in Boilers”‚ Eds. L. Baxter and R. De Solar‚ pp 593–603‚ Plenum‚ New York. pp. 41–65‚ Plenum‚ New York. Williamson‚ J. and Wigley‚ F. (1994)‚ “Mineral Impurities in Coal-Fired Plants”‚ published by Taylor and Francis‚ Washington.

ROLE OF INORGANICS DURING FLUIDISEDBED COMBUSTION OF LOW-RANK COALS

Hari Babu Vuthaluru and Dong-ke Zhang

CRC for New Technologies for Power Generation from Low-Rank Coal C/- Department of Chemical Engineering‚ The University of Adelaide‚ Australia 5005

1. INTRODUCTION Inorganic matter in coal continues to be one of the major sources of operational problems during coal combustion. The role of inorganic matter during coal combustion is governed by its nature and occurrence in the coal matrix [Raask‚ 1985]. In pulverised coal combustion processes‚ slagging‚ fouling and corrosion are the main problems which

arise from the coal mineral matter. These problems can be even more severe when firing low-rank coals with high alkaline and transition metal ions. In contrast‚ fluidised bed combustion processes operating at much lower tempearures ( 800–900 °C) offer better prospects for reducing ash-related problems associated with the firing of low-rank coals. However‚ low-rank coals do not present trouble-free operation in fluidised beds. A major problem is presented by particle agglomeration‚ bed defluidization and ash deposition on heat exchanger surfaces. The inorganic constituents in low-rank coals (mainly sodium‚ calcium and organically bound sulphur) transform to low-melting eutectics at fluid bed operating temperatures [Manzoori and Agarwal‚ 1993; Hodges and Richards‚ 1989; Manzoori and Agarwal‚ 1992; Souto et al.‚ 1996]. These eutectics are transferred to the surface of inert bed particles leading to agglomeration and defluidization of the bed. This phenomenon is dependant to a great extent on the operating temperature of the fluid bed combustor. The present work is aimed at understanding the effects of bed temperature on ash characteristics during fluid bed combustion of an Australian and an Indonesian low-rank coal. Data is presented to demonstrate the effect of bed temperature on the distribution of inorganic matter into variuos streams such as bed ash and cyclone ash. Comparisons of the characteristics of ash deposition on bed material and bed defluidization for Australian and Indonesian low-rank coals are presented. In addition‚

the chemical nature of the ash deposits is discussed‚ along with the chemical interactions that might have occurred during the combustion experiments to cause ash-related problems. Impact of Mineral Impurities in Solid Fuel Combustion‚ edited by Gupta at al. Kluwer Academic / Plenum Publishers‚ New York‚ 1999.

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2. EXPERIMENTAL Experiments were carried out in a spouted bed combustor as described by Man-

zoori and Agarwal (1993). It consisted of a cylindrical furnace‚ 77mm i.d.‚ with a conical distributor (Fig. 1). The combustion zone was confined to the lower part of the furnace‚ where the bed particles and char recirculates internally. Combustion air was introduced at the base of the furnace after passing through an electric heater. The air flow was controlled to create an average fluidisation velocity in the combustion zone of A 200 g batch of bed material (silica sand) was fed into the fluidisation vessel from an opening at the top of the reactor. Air dried coal‚ ground and sieved to 1.0–3.35 mm‚ was fed into the combustion zone via a water-cooled screw feeder. Low-rank coal samples of South Australia (Coal A) and Indonesia (Coal B) were used in the present work. The

chemical analyses of coal samples are given in Table 1. At the end of a combustion run‚ the ash coated bed material was discharged into the ash can. Ash coated sand particles were collected on defluidization with Coal A runs whereas with Coal B the samples were collected at a given time (typically 6 h duration). Ash coated bed material and fly ash samples retrieved from the combustor and cyclone were weighed. Samples of ash coated bed material and cyclone ash collected at the end of each run were analysed using several analytical techniques which included: wet chemical analysis‚ electron microprobe (EMP)‚ scanning electron microscopy (SEM) and X-ray diffraction (XRD). Chemical analysis was done to determine the composition of bed material coating and cyclone ash. Elemental analyses were performed on polished cross-sections of bed material grains using SEM equipped an energy dispersive X-ray detection system (EDAX). Ash coating thickness was estimated by examining the polished cross-section of bed particles with an electron microprobe (EMP) operated in a back scattered-electron imaging mode. XRD was also used to identify the several mineral phases present in various ash

samples.

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3. RESULTS AND DISCUSSION

3.1. Ash Coating The ash samples showed an ash coating on the surface of the bed particles. Comparison of the samples from the two coals revealed that only a very small amount of Coal B ash deposited on bed material particles. The cause for the difference in ash coating was attributed to the small amount of low melting point species in Coal B ash as compared to Coal A (in particular‚ 0.25% in Coal B as compared to 15% in Coal A).

3.2. Composition of Ash Coating The composition of the ash coating on the bed material was calculated from the chemical analysis on a silica-free basis in order to eliminate the contribution of the quartz bed particles. Fig. 2 shows a comparison of the average composition of the bed material coating‚ cyclone ash and raw coal ash for the tested coals at a bed temperature of 800 °C. The results show that the coating on the quartz particles was enriched in Na and S compounds especially with Coal A runs. Electron microprobe spot analyses (Fig. 3) and X-ray maps of polished cross-sections of coated sand grains showed the presence of a highly sulphated ash in the coating‚ confirming previous observations [Manzoori and Agarwal‚ 1992]. On account of the lower interfacial tension between the ash and silica bed particles compared to that between the ash and char surface‚ previous researchers [Manzoori and Agarwal‚ 1993; Manzoori and Agarwal‚ 1992] suggested that the ash transfers

to the surface of bed particles by collision between sand grains and the burning char particles. It may be noted that in comparison to the composition of feed coal ash‚ a large proportion of Ca and Mg compounds was retained in the bed material coating and cyclone ash. For Coal B‚ however‚ no sulphur was seen in the ash coating (from electron microprobe spot analyses) and a significant proportion of Al has escaped into the cyclone.

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Figure 4 shows the effect of temperature on the distribution of inorganics in bed material coating and cyclone ash for the tested coals. The effect of bed temperature was more pronounced on Coal A than Coal B. The proportions of inorganic constituents (namely Fe‚ Ca‚ Mg and Al) in bed ash and cyclone ash showed an increase with increasing bed temperatures for Coal A runs. At a bed temperature of 850 °C‚ the levels of Ca‚ Mg and Al decreased in both ash coating and cyclone ash streams. For Coal B‚ similar trends (for all the inorganic constituents in the ash coating except Fe) were observed with increasing bed temperatures. At higher bed temperatures‚ the level of Al decreased in the ash coating. However‚ the bed temperature showed little influence on the inorganic constituents in the cyclone ash. The results indicated that Coal A is likely to cause more ash-related problems than Coal B at typical fluid bed temperatures. Owing to increased levels of sodium in fly ash with coal A (Fig. 5)‚ deposition of Na-rich compounds on heat transfer surfaces [Souto et al.‚ 1996; Dawson and Brown‚ 1992] can be significant‚ which presents another potential problem‚ fouling and high-temperature corrosion‚ in fluidised bed combustion of lowrank coals‚ in addition to agglomeration and defluidization.

3.3. Ash Coating Thickness Figure 6 shows the effect of temperature on coating thickness on the bed particles for the tested coals. It should be noted that the ash coated samples for Coal A were collected on defluidization whereas with Coal B the samples were collected for the same operating time (typically 6 h duration). The coating layer thickness decreased with increasing bed temperatures for Coal A runs. The increased proportion of low-melting point compounds (in the ash layer) in this coal decreases the critical thickness (ash coating with sufficient amounts of sticky ash material‚ Manzoori and Agarwal‚ 1994) required for agglomeration and/or defluidization. Hence the bed defluidized after 0.5hrs of operation at 850 °C‚ with only a relatively

thin coating of ash. For Coal B‚ however‚ the coating layer increased with increasing bed temperatures. This may be due to decreased proportions of low-melting species in the ash coating [Manzoori and Agarwal‚ 1994] in Coal B. The results suggest that the combustion of Coal A in fluid bed combustors for long operating periods would not be feasible as the coal is likely to defluidize after few hours of operation (even with only a thin ash coating). On the other hand‚ utilisation of Coal B is less likely to cause such prob-

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lems when employed in fluid bed combustors mainly due to the lower proportions of liquid phases in the ash coating.

3.4. Ash Characteristics 3.4.1. Ash Coated Sand Grains. XRD analysis of sand grains obtained from Coal B showed the presence of several minerals (quartz‚ maghemite and hematite) in various proportions (Table 2). In addition‚ several minerals were present in trace quantities. Analyses of sand grains obtained from Coal A also revealed quartz as the dominant phase in all the samples. However‚ all the other minerals were observed in minor and trace proportions. With increasing bed temperatures‚ the distribution of mineral phases in various proportions changed significantly for Coal A runs. This effect was not so pronounced with Coal B runs except at bed temperatures of 700 °C. In addition to quartz‚ maghemite and hematite were seen as the dominant phases in the ash coating at 700 °C. Thenardite was not present in the ash samples of Coal B at all temperatures. This indicates that the higher ash build-up with Coal A may have been due to the presence of sulfur bearing mineral phases (anhydrite and thenardite) in the ash coating. 3.4.2. Fly ash samples. XRD analysis of fly ash samples from Coal B showed that the most abundant minerals in fly ash are quartz‚ maghemite and hematite (Table 3). Other minerals present in trace quantities include mullite‚ anhydrite‚ hematite and nosean. However‚ fly ash samples from Coal A indicated the presence of periclase and maghemite in addition to quartz‚ with periclase becoming more dominant phase with increasing bed temperatures.

4. CONCLUSIONS 1. Significant differences were seen in the characteristics of ash coating layers on

bed material used in fluid bed combustion of two coals tested. The amount of

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inorganic matter in these coals were found to influence the nature of low-melting eutectics and their proportion in the ash coating. These in turn determined the agglomerating propensity of bed material.

2. Bed temperature was found to have a significant influence on the distribution of inorganic constituents into various streams viz. bed ash and cyclone ash. The distribution was found to depend on the proportions of inorganic constituents in the coal samples. Especially with Coal A‚ the bed temperature was found to influence the proportion of sodium in bed ash and cyclone ash. This was

expected to result in the formation sodium-rich deposits on heat exchanging

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surfaces with this coal‚ causing additional ash-related problems during fluidised

bed combustion. 3. The ash coating on bed material from Coal A showed the presence of low melting point compounds‚ and at higher bed temperatures‚ the proportions of low melting compounds increased‚ which led to decreased critical coating thickness at higher temperatures. Hence the bed defluidised after a short period of combustion of Coal A. The ash coating from Coal B‚ however‚ showed the presence of solid material‚ leading to reduced agglomerating propensity of the bed particles. This is expected to prolong the combustion operation with such coals with less ash-related problems.

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

REFERENCES Dawson‚ M.R. and Brown‚ R.C. (1992). “Bed material cohesion and loss of fluidization during fluidized bed combustion of midwestern coal.” Fuel‚ 71 585–592. Hodges‚ N.J. and Richards‚ D.G. (1989).“The fate of chlorine‚ sulphur‚ sodium‚ potassium‚ calcium and magnesium during the fluidized bed combustion of coal.” Fuel‚ 68 440–445.

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Manzoori‚ A.R. and Agarwal‚ P.K.. (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. Raask‚ E. (1985). Mineral Impurities in Coal Combustion. Washington: Hemisphere Publishing Corporation.

Souto‚ M.A.‚ Rodriguez‚ J.C.‚ Conde-Pumpido‚ R.‚ Guitian‚ F.‚ Gonzalez‚ J.F. and Perez‚ J. (1996).“Formation of solid deposits in the gas circuit of a pressurized fluidized bed combustion plant.” Fuel‚ 75 (6) 675–680.

ROLE OF ADDITIVES IN CONTROLLING AGGLOMERATION AND DEFLUIDIZATION DURING FLUIDIZED BED COMBUSTION OF HIGH-SODIUM‚ HIGH-SULPHUR LOW-RANK COAL

Temi M. Linjewile* and Alan R. Manzoori** CRC New Technologies for Power Generation from Low-Rank Coal Thebarton Commerce and Research Precinct Adelaide‚ SA 5031‚ AUSTRALIA

1. INTRODUCTION South Australian lignites contain high levels of sodium and sulphur which combine during combustion to form sodium sulphate. Due to the low melting temperature of sodium sulphate and its formation of low-temperature eutectics with other ash species‚ the use of these coals in fluidized bed combustors causes fouling of cyclone and heat exchange surfaces‚ and deposition of ash on bed material particles leading to agglomeration and defluidization. The problem of growth in the size of bed material particles during fluidized bed combustion while burning Lochiel coal‚ a South Australian lignite has been reported by Manzoori [1990]. The presence of sodium‚ calcium and organically bound sulphur in this coal caused the formation of mostly sulphates of sodium and calcium. These formed low-melting eutectics which caused the deposition of ash on the

surface of inert bed particles. The molten phase present in the coating of bed particles facilitated sintering and formation of agglomerates eventually leading to defluidization of the bed. Dawson and Brown [1992] also reported problems of bed material ash deposition‚ agglomeration and defluidization occurring during circulating fluidized bed combustion of a high-sulphur Iowa coal. In pulverized coal fired plants‚ by comparison‚ problems of slagging and fouling spurred development of effective countermeasures for ash deposition involving mechanPresent address: * Department of Chemical and Petroleum Engineering‚ University of Wyoming P.O. Box 3295‚ Laramie‚ WY 82071-3295‚ USA‚ [email protected] ** CSIRO Minerals‚ Box 312‚ Clayton Sth Vic 3169‚ AUSTRALIA [email protected]

Impact of Mineral Impurities in Solid Fuel Combustion‚ edited by Gupta et al. Kluwer Academic / Plenum Publishers‚ New York‚ 1999.

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ical procedures such as soot blowing with air‚ steam or water [Gorbaty et al.‚ 1983]‚ treatment of coal for addition or removal of specified elements in the coal structure [Vuthaluru et al.‚ 1990] and the use of additives [Raask‚ 1985; Barrett‚ 1989]. The introduction of additives‚ either with the coal or separately‚ offers the opportunity for controlling deposit formation without interruption of operations. As for control of bed material ash deposition‚ agglomeration and defluidization in fluidized bed combustion (FBC) systems‚ the use of additives does not feature prominently‚ presumably because lower operating temperature and the use of higher rank coal ruled out occurrence of bed material deposition and sintering. Clearly the situation changes dramatically when lowrank coals are considered. In recent times‚ developments in advanced coal-based power generation technologies such as pressurized fluidized bed combustion (PFBC)‚ integrated gasification combined cycle (IGCC) and direct coal fired turbine (CFT) present the need for clean-up of the hot flue gases for removal of alkali salts which may cause hot corrosion and ash deposition in gas turbines. This has increased research interest in the use of additives which may be classified into two general categories. The first approach involves the use of a bed of sorbent for removal of gaseous alkali salts [Lee and Johnson‚ 1980; Luthra and Blanc‚ 1984; Uberoi et al.‚ 1990]. The second approach entails in-situ use of additives for removal of alkali compounds [Logan et al.‚ 1990; Spiro et al.‚ 1990]. In each case kaolin

and alumina were used. Although additives have been shown to be effective in pulverized coal fired systems for deposit control and in advanced coal utilization systems for alkali vapour removal‚

mechanisms behind their effectiveness are not fully understood. This study is a report of the control of bed-material ash deposition—a primary step to agglomeration and defluidization—by the use of additives in a simulated circulating fluidized bed combustion (CFBC) unit.

2. EXPERIMENTAL The fluidization vessel‚ Fig. 1‚ consisted of a 77mm ID stainless steel tube with a conical section at its base connected to a 25mm ID gas inlet and solids discharge tube.

The vessel was fitted with an air preheater‚ coal screw feeder‚ additives feeder and a cyclone separator for collection of fines. The reactor is also fitted with thermocouples and differential pressure probes along its height. These were interfaced to a chart recorder for continuous monitoring. Below the conical section of the reactor a sealed bed material receptacle was provided for collection of spent bed particles. The bed material consisted of silica sand particles in the size range 0.85–1 .0mm. Coal samples obtained from the Lochiel coal deposit of South Australia were air-dried‚ crushed and sieved to a size range of 1.4–3.25 mm. Typical analysis of low mineral Lochiel coal is given in Table 1.

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The additives investigated included 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). The additives were screened prior to utilization to obtain particles of size less than 0.125mm. The chemical and mineralogical composition of the additives is presented in Table 2.

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Fluidization air‚ at 140 lit/min at room temperature‚ from a compressor was allowed to flow through a honeycomb-design air preheater where its temperature was raised to 873 K prior to entry into the reactor. A l00g batch of bed material was fed into the fluidization vessel from an opening at the top end of the reactor. A known amount of coal was loaded into the coal feed hopper and the coal was fed continuously into the combustion zone by means of a water-cooled screw feeder at an average rate of 240g/h. Upon sighting of the first spark via the viewing mirror‚ the additives screw feeder was also started. An additive was introduced into the combustion zone by preheater air blowing past the discharge end of the additive screw feeder. For the purpose of comparison‚ some experiments were conducted without the introduction of additives‚ referred to as the no additive run. During combustion‚ the bed temperature was maintained at 1‚073K by adjusting the preheated air temperature. At this temperature‚ the average fluidization velocity in the combustion zone was 8 m/s. The fast fluidization velocity and the internal recirculation of the bed material particles provided conditions similar to CFBC systems. After three hours of combustion‚ the bed particles were discharged into an ash can at the bottom of the fluidization vessel. Coated bed particles retrieved from the combustor were weighed and some were subjected to wet chemical analysis‚ examination with a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDAX)‚

electron microprobe analysis (EMPA) and X-ray diffraction analysis.

3. RESULTS AND DISCUSSION

3.1. Ash Deposition on Bed Material Particles The bed material recovered at the end of a combustion run without additives was weighed and the size distribution determined. The samples showed an ash coating buildup on the surface of the bed particles as shown in Fig. 2. Some agglomerates were also formed. In some combustion runs of long duration the formation of agglomerates led to defluidization of the bed. During combustion‚ inorganic matter introduced by the coal appears as an ash coating on bed material and fly ash in the cyclone. The partition of coal inorganic matter between the ash coating and cyclone ash was determined from the chemical analysis of the coal‚ its feed rate and the quantities of ash collected on bed

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material and in the cyclone separator. It was found that 21–32 percent of the inorganic matter in the coal feed deposited on the surface of sand bed particles.

The composition of the inorganic constituents in the bed material coating was calculated on a silica—free basis in order to eliminate the contribution of the quartz bed particles. Figure 3 shows a comparison of the average composition of the bed material coating (bottom ash)‚ cyclone ash and coal ASTM ash. The results show that the coating on quartz particles is enriched in Na and S compounds. On account of the lower interfacial tension between the ash and silica bed particles compared to that between the ash and char surface‚ Manzoori [1990] suggested that the ash transfers to the surface of bed particles by collision between sand grains and the burning char particles. It may be noted that in comparison to the composition of feed coal ash‚ a large proportion of Ca and Mg compounds wa