Industrial Coal Gasification Technologies - Covering Baseline and High-Ash Coal 3527336907, 9783527336906

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Martin Gräbner Industrial Coal Gasification Technologies Covering Baseline and High-Ash Coal

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Martin Gräbner

Industrial Coal Gasification Technologies Covering Baseline and High-Ash Coal

Author Dr.-Ing. Martin Gräbner

Holbeinstraße 1A D-63755 Alzenau-Wasserlos Germany

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 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-33690-6 ePDF ISBN: 978-3-527-33692-0 ePub ISBN: 978-3-527-33693-7 Mobi ISBN: 978-3-527-33695-1 oBook ISBN: 978-3-527-33691-3 Cover Design Formgeber, Mannheim, Germany Typesetting Thomson Digital, Noida, India Printing and Binding Markono Print Media Pte Ltd, Singapore

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to Isabel

VII

Contents Preface

1

XV

Introduction

1

References 2 2

Coal Gasification in a Global Context

2.1 2.2 2.2.1 2.2.2 2.2.3 2.3 2.3.1 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.2.4 2.3.2.5 2.3.2.6 2.3.2.7 2.4 2.5 2.6 2.6.1 2.6.1.1 2.6.1.2 2.6.2 2.6.3

Applications of Coal Gasification 3 The Three Generations of Coal Gasifiers 4 First Generation of Coal Gasifiers 5 Second Generation of Coal Gasifiers 5 Third Generation of Coal Gasifiers 6 Typical Feedstock and Products 9 Feedstock 9 Products 9 Ammonia 10 Methanol and Derivatives 11 Electricity (Integrated Gasification Combined Cycle) 12 Substitute Natural Gas (Synthetic Natural Gas) 12 Fischer–Tropsch Liquids 13 Hydrogen Production 14 Others 14 Main Markets for Coal Gasification 16 Challenges and Opportunities for Coal Gasification 16 Environmental Aspects 18 Air Emissions 18 Pollutants 18 Greenhouse Gases 19 Water Effluents 20 Solid Waste 20 References 21

3

VIII

Contents

3

Coal Characterization for Gasification

3.1 3.2 3.2.1 3.2.2 3.2.3 3.2.3.1 3.2.3.2 3.2.3.3 3.2.4 3.2.4.1 3.2.4.2 3.2.4.3 3.2.5 3.2.6 3.3 3.3.1 3.3.2 3.3.3

Coal as Gasification Feedstock 25 Petrographic Coal Analysis 26 Introduction to Macerals 26 Technological Background 26 Groups of Macerals 27 Huminite and Vitrinite 27 Liptinite 29 Inertinite 29 Blend Identification 30 Background 30 Terms and Definitions 30 Interpretation of a Reflectance Analysis 31 Temperature Estimation Using Optical Reflectance 33 Detection of Other Material 34 Coal Classification 35 Introduction 35 Reporting of Coal Analyses 36 Classification According to the American Society for Testing and Materials Standard 38 Classification According to the International Organization for Standardization 40 Other Nomenclatures Relevant to Gasification 40 Salty Coals 40 Ballast Coals 40 Low-Value or Low-grade Gasification Coals 42 Three-High Coals 42 Coal Sampling 43 Proximate Analysis 44 Moisture Content 44 Technological Background 44 Analysis of Moisture 45 Ash Content 46 Volatile Matter Content 47 Fixed Carbon 48 Alternative Method 48 Fischer Assay 48 Ultimate Analysis 49 Technological Background 49 Analysis Procedure 49 Carbon 50 Hydrogen 51 Nitrogen 51 Sulfur 52 Oxygen 53

3.3.4 3.3.5 3.3.5.1 3.3.5.2 3.3.5.3 3.3.5.4 3.4 3.5 3.5.1 3.5.1.1 3.5.1.2 3.5.2 3.5.3 3.5.4 3.5.5 3.6 3.7 3.7.1 3.7.2 3.7.3 3.7.4 3.7.5 3.7.6 3.7.7

25

Contents

3.7.8 3.8 3.8.1 3.8.2 3.8.3 3.8.4 3.9 3.9.1 3.9.2 3.9.3 3.9.4 3.10 3.10.1 3.10.2 3.10.2.1 3.10.2.2 3.10.2.3 3.10.2.4 3.10.2.5 3.10.3 3.11 3.11.1 3.11.2 3.11.2.1 3.11.2.2 3.11.2.3 3.11.2.4 3.11.3 3.11.4 3.11.5 3.11.5.1 3.11.5.2 3.11.5.3 3.11.5.4 3.11.6 3.11.6.1 3.11.6.2 3.11.7 3.11.7.1 3.11.7.2 3.11.8 3.12 3.12.1 3.12.1.1 3.12.1.2

Chlorine 53 Heating Values 54 Technological Background 54 Analysis Procedure 54 Estimation by Empirical Correlations 55 Enthalpy of Formation 55 Caking Properties 57 Gray–King Assay 57 Free-Swelling Index 58 Roga Index 58 Dilatation Test 59 Reactivity 59 Technological Background 59 Determination of Reactivity 60 General Considerations 60 Thermogravimetric Analysis 61 Fixed-Bed Reactors 65 Entrained Particle Reactors 66 Wire-Mesh Reactors 67 Spontaneous Ignition 67 Mineral Matter and Ash Analysis 68 Technological Background 68 Minerals in Coal 69 Origin of Coal Mineral Matter 69 Minerals in High-Rank Coals 70 Minerals in Low-Rank Coals 70 Analysis of Mineral Matter in Coal 71 Transformation of Mineral Matter to Ash 71 Ash Component Analysis 72 Ash Fusion Analysis 73 Ash Fusion Test 73 Ash Clinkering Test 75 Influence of Atmosphere 76 Influence of Ash Compositions 76 Slag Viscosity Analysis 79 High-Temperature Viscometer Test 79 Prediction of Slag Viscosity 81 Devolatilization of Mineral Compounds 84 Partitioning 84 Behavior of Alkali Metals 85 Utilization Properties of Ash and Slag 86 Relevant Physical Properties 86 Coal Density 87 True Density 87 Apparent Density 88

IX

X

Contents

3.12.1.3 3.12.1.4 3.12.2 3.12.2.1 3.12.2.2 3.12.3 3.12.3.1 3.12.3.2 3.12.3.3 3.12.3.4 3.12.3.5 3.12.3.6 3.12.4 3.12.4.1 3.12.4.2 3.12.4.3 3.12.4.4 3.12.4.5 3.12.4.6

Bulk Density 88 Washability Test 89 Thermal Properties 90 Heat Capacity 90 Thermal Conductivity 91 Granulometric Properties 91 Technological Background 91 Representative Diameters 92 Rosin-Rammler-Sperling-Bennett Particle Size Distribution 92 Fragmentation 93 Hardgrove Grindability Index 94 Abrasion Index 95 Fluid-Dynamic Properties 95 Technological Background 95 Coal Bed Pressure Drop 95 Minimum Fluidization Velocity 96 Fluid Bed Pressure Drop 96 Terminal Entrainment Velocity 97 Visualization in the Reh Diagram 98 References 100

4

Fundamentals

4.1 4.2 4.3 4.4 4.5 4.5.1 4.5.1.1 4.5.1.2 4.5.1.3 4.5.2 4.5.3 4.5.4 4.5.4.1 4.5.4.2 4.5.5 4.5.6 4.5.7 4.5.8 4.5.9 4.5.9.1 4.5.9.2 4.5.9.3

Terms and Definitions 107 Gasification Reactions 108 Pyrolysis Reactions 109 Gasification Parameters 110 Classifying Gasification Methods 112 Bed Type (Particle Size) 112 Moving-Bed Gasifiers 112 Fluid-Bed Gasifiers 113 Entrained-Flow Gasifiers 114 Temperature Range 114 Pressure Level 116 Feeding Method 116 Dry Feed Systems 116 Hydraulic Feed Systems 118 Wall Type 120 Syngas Cooling 121 Oxidant 122 Solid Residue Removal 123 Addition of Catalysts 124 General Considerations 124 Groups of Catalysts 125 Application of Catalytic Coal Gasification 126 References 126

107

Contents

5

Coal Gasification Modeling

5.1 5.2 5.3 5.3.1 5.3.2 5.4 5.4.1 5.4.2 5.4.3 5.4.3.1 5.4.3.2 5.4.4 5.5 5.5.1 5.5.2 5.5.2.1 5.5.2.2 5.5.3 5.5.4 5.5.5 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.6.4.1 5.6.4.2 5.6.4.3

Introduction 129 Balancing of Gasification Systems 130 Thermodynamic Modeling 131 Equilibrium Constant-Based Calculations 131 Minimization of Gibbs Free Energy 134 Kinetic Modeling 135 Conversion Processes 135 Fundamentals 135 Heterogeneous Reaction Kinetics 138 Analysis of Kinetic Data from Literature 138 Selection of Kinetic Data for Modeling 139 Homogeneous Reaction Kinetics 144 Computational Fluid Dynamics Modeling of Coal Gasifiers 145 Background 145 Typical Case Setup 145 Definition of the Calculation Domain 145 Solver Settings and Numerical Submodels 147 Convergence Strategies 148 Results for the Internal Circulation Gasifier Case 148 Conclusions of the Computational Fluid Dynamics Study 150 Generic Models for Case Studies 152 Temperature Approach Concept 152 Modeling Approach 153 Limitations of the Approach Temperature Concept 154 Boundary Conditions 155 Coal Selection 155 Reference Case Definition 156 Sensitivity Analysis 157 References 164

6

Coal Gasification Technology Survey 169

6.1 6.1.1 6.1.2 6.1.2.1 6.1.2.2 6.1.2.3 6.1.2.4 6.1.2.5 6.1.3 6.1.3.1 6.1.3.2 6.1.3.3 6.1.3.4

Entrained-Flow Gasifiers 169 Introduction 169 Shell and Uhde Coal Gasification Technology 170 Historical Background 170 Process Description 170 Enhancements 176 Verification Case for Model Setup 177 Modeling Results 180 Siemens Fuel Gasification Technology 181 Historical Background 181 Process Description 182 Enhancements 187 Verification Case for Model Setup 188

129

XI

XII

Contents

6.1.3.5 6.1.3.6 6.1.4 6.1.4.1 6.1.4.2 6.1.4.3 6.1.4.4 6.1.4.5 6.1.4.6 6.1.5 6.1.5.1 6.1.5.2 6.1.5.3 6.1.5.4 6.1.5.5 6.1.6 6.1.6.1 6.1.6.2 6.1.6.3 6.1.6.4 6.2 6.2.1 6.2.2 6.2.2.1 6.2.2.2 6.2.2.3 6.2.2.4 6.2.2.5 6.2.3 6.2.3.1 6.2.3.2 6.2.3.3 6.2.3.4 6.2.3.5 6.2.3.6 6.2.3.7 6.3 6.3.1 6.3.2 6.3.2.1 6.3.2.2 6.3.2.3 6.3.3 6.3.3.1 6.3.3.2

Modeling Results 191 Other Similar Technologies 192 GE Energy Technology 193 Historical Background 193 Process Description 194 Enhancements 202 Verification Case for Model Setup 203 Modeling Results 206 Other Similar Technologies 208 E-Gas Technology 210 Historical Background 210 Process Description 211 Enhancements 215 Verification Case for Model Setup 217 Modeling Results 219 Other Entrained-Flow Technologies 220 East China University of Science and Technology Gasifiers 220 Mitsubishi Heavy Industries Gasifier 221 Thermal Power Research Institute Gasifier 222 Pratt & Whitney Rocketdyne Gasifier 224 Fluid-Bed Gasifiers 225 Introduction 225 High-Temperature Winkler Technology 226 Historical Background 226 Process Description 227 Enhancements 233 Verification Case for Model Setup 234 Modeling Results 237 Other Fluid-Bed Technologies 237 Utility-Gas Gasifier 237 Agglomerating Fluidized-Bed Gasifier 239 Kellogg Brown & Root Transport Reactor 239 Kellogg Rust Westinghouse Gasifier 242 Bharat Heavy Electrical Limited Technology 243 HRL Integrated Drying Gasification Combined Cycle Process 243 Circulating Fluidized-Bed Technology 244 Moving-Bed Gasifiers 244 Introduction 244 Lurgi Fixed-Bed Dry Bottom Technology 245 Historical Background 245 Process Description 247 Enhancements 261 Other Similar Technologies 263 SEDIN Dry Bottom Gasification 263 Sasol Dry Bottom Gasification 263

Contents

6.3.4 6.3.4.1 6.3.4.2 6.3.4.3 6.3.4.4

British Gas/Lurgi Technology 263 Historical Background 263 Process Description 264 Operational Data 275 Enhancements 275 References 277

7

Thermodynamic Process Assessment

7.1 7.1.1 7.1.2 7.1.3 7.1.4 7.1.5 7.1.6 7.1.7 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5 7.4 7.5 7.6 7.7

Introduction of a Ternary Gasification Diagram 289 Basic Idea 289 Domain Overview and Pressure Sensitivity 290 Diagram Types 292 Domain Boundaries for Gasification Systems 293 Treatment of H2O Stream 295 Displaying Gasifiers with Multiple Inlets 296 Optimum User Diagrams 296 Diagrams for Pittsburgh No. 8 Coal 298 Temperature and Carbon Conversion Diagram 298 Cold Gas Efficiency and Methane Yield Diagram 300 Syngas Yield and H2/CO Diagram 300 Optimum User Diagram 301 Optimum Correlations 301 Diagrams for South African Coal 304 Temperature and Carbon Conversion Diagram 304 Cold Gas Efficiency and Methane Yield Diagram 306 Syngas Yield and H2/CO Diagram 306 Optimum User Diagram 307 Optimum Correlations 307 Technology Potential Analysis 310 Influence of the Ash Content 312 Other Gasification Systems 314 Conclusions 315 References 316

8

Exergetic Process Assessment

8.1 8.1.1 8.1.2 8.1.3 8.1.4 8.2 8.2.1 8.2.2 8.2.2.1 8.2.2.2

Exergy Calculations 319 Exergy and Reference Environment 319 Exergy of Gaseous and Liquid Streams 320 Exergy of Solid Streams 323 Definition of Efforts and Benefits 323 Exergetic Analysis 324 Impact of Gas Cooling Methods 324 Comparison of Gasification Systems 326 Exergy Flow Analysis 326 Exergetic Process Efficiency 328

289

319

XIII

XIV

Contents

8.3

Conclusions of Process Assessment 329 References 330

9

Concept Study: The Internal Circulation Gasifier

9.1 9.2 9.3 9.3.1 9.3.2 9.3.2.1 9.3.2.2 9.3.2.3 9.3.3 9.3.4 9.3.5 9.3.6 9.4 9.4.1 9.4.1.1 9.4.1.2 9.4.1.3 9.4.2 9.4.3 9.4.4 9.5

Introduction 331 Basic Principle 333 Detailed Process Description 334 Fuel Preparation and Feeding 334 Reaction Chamber 334 Fluid-Bed Zone 334 Moving-Bed Zone 336 Particle Behavior 337 Gasifying Agent Injection 338 Process Control 339 Gas Cooling 340 Ash Removal and Cooling 340 Thermodynamic Modeling of the Internal Circulation Gasifier 341 Model Description 341 Flow Sheet Setup 341 Property Method and Block Settings 342 Design Specifications 342 Expected Performance 344 Derived Reactor Design 345 Process Scale-up 345 Next Development Steps 348 References 348

10

Trends of Gasification Development

Reference 353 Index 355

351

331

XV

Preface This book provides a comprehensive overview on topics that are related to industrial coal gasification technologies, combining scientific with technological aspects. The main vision for the book is to provide the reader with an innovative, highly structured, and detailed view on coal gasification technologies. The novel ternary diagram of gasification provides the first order scheme in which all gasifiers can be compared at one glance. Special emphasis is placed on new gasification concept developments and increasing ash content of the coal. After an introduction (Chapter 1) explaining the background and the scope, the book starts from a global perspective in Chapter 2. Coal gasification is put into a global context by identification of recent applications. Once it is clear why coal is gasified, the course of coal gasification development (generations of gasifiers) and the recent role of coal gasification (typical feedstock and products of realized plants), as well as main markets, are discussed. Subsequently, the reader will be sensitized to the main challenges hampering a broad market introduction of coal gasification and also for potential opportunities that keep coal gasification in the discussion. Chapter 2 concludes with discussion of environmental aspects, such as emissions of coal gasification plants. To guide less experienced readers into the complex topic of coal conversion by means of gasification, a survey on coal characterization limited to gasificationrelevant parameters is provided in Chapter 3. From a practical point of view, this chapter tells what information can be extracted from a coal sample in order to judge which gasification process is suitable. Hence, the necessary knowledge about coal standard analyses (e.g., ultimate, proximate, calorific analyses) and more sophisticated procedures, such as reactivity or maceral analyses, are presented. The most emphasis will be placed on the discussion of the minerals in the coal because they are limiting to all gasification processes. Chapter 3 concludes with a summary of physical and fluid-dynamic properties of the coal. Chapter 4 introduces the fundamentals of technical gasification processes starting with basic reactions and chemistry. The knowledge from the previous chapter allows us to define the necessary gasification performance parameters that are needed to discuss the advantages and disadvantages of the different gasification methods. Consequently, the differences between the processes are presented in a highly structured way according to bed type (moving bed, fluidized

XVI

Preface

bed, entrained bed), temperature range (ash fusibility and slag viscosity), pressure level, feeding method (dry feeding or coal-water-slurry), wall type (membrane wall, refractory lining, steam jacket), syngas cooling (water/gas/chemical quench, heat recovery), oxidant (O2 or air), solid residue removal (ash/slag, fly ash, granulate), and catalyst addition. Because modeling approaches and detailed technology survey results are presented in Chapter 6, it is instructive to include an overview on gasification modeling before that. Hence, Chapter 5 presents the typical starting place for modeling, which is the balancing of gasification systems. In further course, thermodynamic models, kinetic models, and computational fluid dynamics (CFD) approaches for gasification modeling are introduced. Chapter 5 provides a practical overview on these methods, discussing strengths and weaknesses, main fields of application, usable data sets, and related laboratory investigations. It will only touch basic equations and scientific background as far as necessary for understanding. Chapter 5 ends with a description of the generic modeling approach, which is used in Chapter 6, and discusses the sensitivity of selected models to the applied boundary conditions. Chapter 6, “Coal Gasification Technology Survey,” represents the core of the book. The intention is to provide the most recent and most comprehensive data collection on coal gasification processes comprising much information that has not yet been published in the English language. Chapter 6 is organized following the bed type of the gasifiers and placing special emphasis on the technologies from Shell, Uhde (i.e. high-temperature Winkler (HTW), Prenflow), GE, Siemens, CB&I (i.e., E-Gas), Lurgi (i.e., fixed-bed dry bottom (FBDB)), and Envirotherm/ Zemag (i.e., British Gas/Lurgi (BGL)), as there are much public domain data available. For these processes, a historical background, a detailed process description, proposed enhancements, and current projects are presented. For selected technologies, generic model setup and results are also presented. Besides performance data from operation, which is given in the sections of detailed process description, the modeling permits the comparison of the gasifiers on unified boundary conditions pointing out the effect of high-ash versus conventional coal and standard versus enhanced systems. All other industrial coal gasification technologies, such as the new Chinese processes or other fluid-bed technologies, are introduced depending on the availability of public domain data. In Chapter 7, the main innovation of this book is disclosed, which is the introduction of a ternary gasification diagram. This newly developed order scheme allows putting all gasifiers in one diagram according to their consumption figures based on thermodynamic calculations. This new idea is introduced to the reader step by step, ranging from the basic idea over pressure sensitivity, diagram types, domain boundaries for gasification systems, displaying gasifiers with multiple inlets to the development of derived optimum user diagrams and correlations. Subsequently, the diagram is provided for conventional (Pittsburgh No. 8) and ash-rich (South African) coal, which is discussed in detail. Chapter 7 concludes with a technology potential analysis and the assessment of the influence of the ash content, both being conclusions from the new type of diagram.

Preface

In addition, other gasification systems will be touched on and displayed in the diagram (e.g., carbon dioxide (CO2)-gasification, biomass). Because the thermodynamic performance parameters of gasification do not reveal the effect of the gas cooling method on the overall process, an exergetic analysis is provided in Chapter 8. As exergy is strongly dependent on the applied reference environment and the applied chemical system, Chapter 8 begins with an introduction to exergy calculations, the reference environment, and the exergy definition of the gaseous, liquid, and solid streams. The definition of efforts and benefits in the present investigation is also explained. Subsequently, the impact of gas cooling methods, and the final exergetic comparison of the gasification systems for both high-ash versus conventional coal and standard versus enhanced systems is discussed. Taking together all conclusions from the previous chapters, a theoretical concept study is carried out designing a gasifier that potentially can cope with highash fine coal. There is no gasification technology on the market for such coal at the moment. The structure of Chapter 9 is similar to the gasifier descriptions in Chapter 6, comprising process basic principle, detailed process description, layout of the reaction chamber, gasifying agent injection, gas cooling, ash removal, and expected performance and control. In this framework, the utilization of the Reh diagram and the setup of a thermodynamic model will be discussed. Conclusions for process scale-up and suggestions for further development steps complete Chapter 9. A brief overall conclusion summarizing the trends in gasification development is given in Chapter 10, finishing the book.

XVII

1

1 Introduction A tendency toward a decrease in coal quality is reported from various parts of the world, especially in the coking coal sector. In terms of steam coals, countries such as South Africa [1], India [2], Japan [3], or even China [4] report about utilization of coal with elevated ash content. According to MacDonald et al. [5] and the ISO 11760 classification [6], coals are referred to as “high ash” or “moderately high ash” if they have an ash yield greater than 20 wt% (wf). In terms of gasification, several disadvantages can be expected as the ash content increases: 1) The physical heating and cooling and melting of the ash material reduce process efficiency. 2) High ash content is detrimental to carbon conversion for reasons of carbon encapsulation. 3) Addition of fluxing agents to influence the ash behavior is limited. 4) Increasing amounts of vaporized ash compounds could increase fouling in downstream heat exchangers. 5) Coal preparation expenditures increase in terms of grinding, drying, or de-ashing. Mineral matter reactions can additionally hamper the process, that is, oxygen consumption by substances that are not fully oxidized, such as Fe3O4 or FeS2, or CO2 release from carbonates. Special solvents might be considered to de-ash the coal [7]. However, because of recovery and regeneration problems, operational and capital costs increase while availability decreases and this option is mostly abandoned. As soon as the ash contains certain constituents (quartz and pyrite in particular), wear and abrasion in milling systems lead to extensive maintenance programs [8]. Thus, crushing should be kept at the lowest possible level. The traditional approach to gasify such kind of feedstock is, of course, employing moving-bed systems featuring dry-ash removal, for example, Lurgi fixed-bed dry bottom (FBDB) gasification. But moving-bed technologies require a suitable grain size for bed percolation and can cope only with limited amounts of fine coal. In addition, modern mining technologies produce increasing quantities Industrial Coal Gasification Technologies Covering Baseline and High-Ash Coal, First Edition. Martin Gräbner.  2015 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2015 by Wiley-VCH Verlag GmbH & Co. KGaA.

2

1 Introduction

of fine coal and the high-ash content prevents acceptable agglomeration properties, for example, for briquetting. Consequently, vast amounts of high-ash coal fines are left over from moving-bed processes or other coal washing and beneficiation processes. These cannot be gasified efficiently using today’s standard technologies [9]. The task of the present book is to investigate the capability of existing technologies and the potential of new concepts for processing high-ash coal. A study is carried out using a high-ash coal from South Africa – especially fines for pulverized coal application – compared to a baseline standard coal, which is American Pittsburgh No. 8 bituminous coal. To compare the different approaches, thermodynamic modeling and exergy analysis will be applied. The evaluation of the results should lead to the identification of the most promising concept, which is intended to be investigated in a case study.

References 1 Everson, R.C., Neomagus, H.W., Kaitano,

2

3

4

5

R., van Falcon, R., Alphen, C., and du Cann, V.M. (2008) Properties of high ash char particles derived from inertinite-rich coal: 1. Chemical, structural and petrographic characteristics. Fuel, 87 (13–14), 3082– 3090. Iyengar, R. and Haque, R. (1991) Gasification of high-ash Indian coals for power generation. Fuel Processing Technology, 27 (3), 247–262. Kurose, R., Ikeda, M., and Makino, H. (2001) Combustion characteristics of high ash coal in a pulverized coal combustion. Fuel, 80 (10), 1447–1455. Liu, G., Zheng, L., Gao, L., Zhang, H., and Peng, Z. (2005) The characterization of coal quality from the jining coalfield. Energy, 30 (10), 1903–1914. MacDonald, M., Chadwick, M., and Aslanian, G. (1996) The Environmental

6

7

8

9

Management of Low-Grade Fuels, Earthscan Publications Limited, London. ISO (2005) 11760, Classification of Coals, International Standards Organization, Geneva, Switzerland. Okuyama, N., Komatsu, N., Shigehisa, T., Kaneko, T., and Tsuruya, S. (2004) Hypercoal process to produce the ash-free coal. Fuel Processing Technology, 85 (8–10), 947–967. Wells, J.J., Wigley, F., Foster, D.J., Gibb, W.H., and Williamson, J. (2004) The relationship between excluded mineral matter and the abrasion index of a coal. Fuel, 83 (3), 359–364. Govender, A. and van Dyk, J.C. (2003) Effect of wet screening on particle size distribution and coal properties. Fuel, 82 (18), 2231–2237.

3

2 Coal Gasification in a Global Context 2.1 Applications of Coal Gasification

Any carbonaceous feedstock, may it be gaseous, liquid, or solid, can undergo a partial oxidation. As soon as oxidation heat is released, high-temperature conditions evolve permitting other gases, such as steam or carbon dioxide, to react with the carbonaceous feedstock. The result is the autothermal breakdown of the feedstock to the smallest stable chemical units that can still carry some energy. These units are the gases hydrogen, carbon monoxide, and sometimes methane. The breakdown process is called gasification and the gaseous product is called synthesis gas or syngas. Although the term “syngas” traces to gases produced for the sole purpose of downstream syntheses, it established itself as a term for any product gas from gasification independent of application. (Further details and thermodynamic definitions are provided in Chapter 4.) The composition of the syngas varies and is essentially linked to the quality of the feedstock and the conditions of the gasification process, such as temperature and pressure. Furthermore, each kind of gasification process is specialized in a certain feedstock spectrum. Finally, the usage of the gas produced specifies varying parameters, such as heating value, pressure level, H2/CO ratio, and maximum concentration of sulfur compounds. Hence, the closing of the gap between carbonaceous feedstock and a selected final product, which is intended to be sold from the plant, is a technical and economical optimization problem with usually more than one solution. In this framework, the different gasification technologies are the basis for competition on the market. The conversion chain of gasification plants as shown in Figure 2.1 can be generalized in three steps moving from feedstock to product: gasification, gas treatment, and conversion to product. In Figure 2.1, the sum of installed and under-construction capacity in GW syngas is distinguished for feedstock and products. On the feedstock side, it can be seen that coal with 126.9 GW represents more than 75% of the global feed for gasification plants. And coal is expected to grow by another 74 GW Industrial Coal Gasification Technologies Covering Baseline and High-Ash Coal, First Edition. Martin Gräbner.  2015 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2015 by Wiley-VCH Verlag GmbH & Co. KGaA.

2 Coal Gasification in a Global Context

Feedstock Petroleum

16.5

Gas

16.5

Coal

Gasification Gas treatment Conversion

6.2

126.9

Combined cycle Approx. 800 gasifiers in 300 plants

SYNGAS

Petcoke

GW

cleaning & preparation

4

Product 14.0

Power

26.0

Gaseous fuels

44.6

Liquid fuels

81.5

Chemicals

Synthesis

GW

Figure 2.1 Gasification conversion chain from feedstock to product. (The numbers in gigawatt (GW) refer to global syngas capacity of currently operating units and plants under construction [1].)

until 2018 [1]. The gasification step is currently accomplished in nearly 300 plants employing approximately 800 gasifiers. The intermediate product of syngas undergoes a gas treatment step including usually a cleaning stage (e.g., particulate matter removal, acid gas removal) and a preparation stage (e.g., water gas shift). Subsequently, the syngas is subjected downstream to the final conversion step. If the syngas is combusted in a combined cycle, the generated product is electricity from a so-called integrated gasification combined cycle (IGCC) process. This is true for only 14 GW or 8.4% of the total syngas produced. Of more importance are the products that preserve the chemical energy in form of gaseous or liquid fuels (e.g., town gas, substitute natural gas, gasoline, and diesel fuel) as well as chemicals such as ammonia, methanol, or hydrogen. The chemicals are dominant, representing nearly 50% of the syngas capacity. Another aspect with regard to Figure 2.1 is that mixtures of different solid feedstock are frequently fed to gasifiers. They are called blends and consist of different coals or mixtures of coal and petroleum coke (petcoke). But also on the product side, single plants are not limited to one specific output. Syntheses in parallel or in conjunction with a combined cycle can be feasible and the plants produce several products (e.g., Schwarze Pumpe, Germany: methanol and power), which is referred to as “polygeneration.”

2.2 The Three Generations of Coal Gasifiers

There have been many surveys summarizing the early history of gasification that should not be repeated here [2,3]. But it is, in general, reasonable to distinguish three main generations of gasifiers serving larger-scale industrial applications.

2.2 The Three Generations of Coal Gasifiers

2.2.1 First Generation of Coal Gasifiers

The first generation of industrial coal gasifiers arose from the idea of supplying a chemical synthesis with gas produced from coal. A typical example is the Winkler fluid-bed gasifier, which found its first commercial application in 1926 at the Leuna site close to Leipzig, Germany. From this framework emerged the term syngas. Because the process operated – as all gasifiers at that time – at atmospheric conditions, the advantages of a pressurized process quickly became clear. It was Professor Rudolf Drawe (1877–1967) who first saw the high potential in replacing the commonly used air with pressurized oxygen and steam mixtures as gasifying agents, which was possible after the invention of the Linde-Fränkel air separation process. In 1927, the German engineering company Lurgi patented the first pressurized oxygen-blown fixedbed gasifier, which was commercially applied in 1936 in Hirschfelde, close to Dresden, Germany. Besides the upcoming NH3 and methanol market, the fast development in Germany was mainly driven by the need to produce liquid fuels from domestic sources such as lignite, which was induced by both World Wars. And a technology for coal dust gasification – the Koppers-Totzek atmospheric entrained-flow process – was developed in Germany in the 1940s. Among this first generation of gasifiers, the Lurgi fixed-bed dry bottom (FBDB) technology remained the most successful one because it was the only pressurized technology available for years. However, the FBDB process left quite some leeway to improve single-unit capacity, gas quality (high CO2 and CH4 content, tar production), and steam consumption. 2.2.2 Second Generation of Coal Gasifiers

Besides minor individual factors, the oil crises relaunched interest in coal gasification again leading to the development of a second generation of coal gasification processes. The global targets of the development, which took place from the 1970s until early 1990s, can be summarized as follows [4–6]:

 For fluid-bed and entrained-flow processes, the gasification pressure should be raised from atmospheric to 20 to 60 bar to reach higher single-unit capacities (up to 500 MW) and lower capital investment.  For fluid-bed and fixed-bed processes, performance should be enhanced as regards carbon conversion rates, cold gas efficiencies, and consumables.  Another focus was on the integration of heat recovery from syngas by steam generation if gasification is employed for power generation.  The upcoming environmental concerns, especially the emission of sulfur species from coal gasification was reviewed in detail.

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 The development of higher single unit capacities requires high on-stream time (plant availability) and higher operational flexibility regarding feedstock and load variation compared to the former approach of running many gasifiers in parallel. These development efforts brought the second generation of coal gasifiers to the market comprising several technologies such as the British Gas/Lurgi (BGL) [7], high-temperature Winkler (HTW) [8], U-Gas [9], Kellogg Rust & Westinghouse (KRW) [10], Texaco [11], Gaskombinat Schwarze Pumpe (GSP) [12], E-Gas (Dow Chemical) [13], Shell [14], and Prenflo (Uhde) [15] processes. Most of them have become proven, mature technologies that have been successfully implemented in IGCC plants as well as for methanol, ammonia, and acetic acid anhydride syntheses. These are considered to be the traditional technologies sold from the shelf contributing to the substantial growth in China within the last 10 years. 2.2.3 Third Generation of Coal Gasifiers

However, after a decade of relative silence surrounding coal gasification, beginning around 2000, there have been four general trends that renewed interest in the technology:

 Substitution of crude oil by other energy carriers, such as biomass or coal, targeting supply security and local energy price stabilization (e.g., China)

 Increasing interest in the use of low-grade coals with high ash or moisture contents in emerging nations (e.g., India, Indonesia)

 Sustained efforts to reduce CO2 emissions because of the potential of CO2 separation from pressurized syngas (e.g., the United States)

 Stabilization potential of polygeneration plants for high electricity generation fluctuations caused by the increasing share of renewable energy sources (e.g., Europe) Numerous design variations of second-generation gasification processes were suggested for the state-of-the-art processes from Shell [16], Uhde [17], Siemens (formerly GSP) [18], GE (formerly Texaco) [19], Lurgi FBDB [20], and CB&I (E-Gas) [21]. But also a third generation of newly developed gasification processes emerged, such as those developed by Kellogg Brown & Root (KBR) [22], Pratt & Whitney Rocketdyne (PWR) [23], and Mitsubishi Heavy Industries (MHI) [24]. In parallel, five new Chinese processes were brought to commercial reality inside China: Hangtian Lu (HT-L) technology, the two-stage-oxygen gasifier developed at Tsinghua University Beijing, the gasifiers from the East China University of Science and Technology (ECUST), the two-stage-coal gasifier from the Thermal Power Research Institute (TPRI), and the multicomponent slurry gasification

2.2 The Three Generations of Coal Gasifiers

(MCSG) technology from the Northwest Research Institute of Chemical Industry. The Chinese developments mainly resulted from publicly supported production of chemicals (mainly ammonia, methanol) from domestically available coal resources in order to decrease dependencies from crude oil imports. Figure 2.2 shows the current commercially available gasification technologies as a result of the described development. Figure 2.2 distinguishes the gasifiers by bed type into entrained, fluid, and moving-bed processes. It can be seen that eleven different technologies evolved for entrained-flow-type gasification comprising systems of the second and third generation. This number is also an indicator for their present dominance on the market. The fluid-bed and moving-bed technologies are less in number and the dry-ash moving-bed systems are the only ones tracing back to the first generation of gasification, being for more than 80 years on the market. The general message from Figure 2.2 is that gasification technology achieved a remarkable level of diversification on the market and that only five countries are really involved in technological development (The Netherlands, Germany, China, the United States, and Japan).

Shell, NL

Prenflo (Uhde), DE

GE Energy (Texaco), US

CB&I (E-Gas), US

Siemens (GSP), DE

HT-L, CN

Mitsubishi (MHI), JP

MCSG (NRICI), CN

Tsinghua twostageoxygen, CN

OMB (ECUST), CN

TPRI 2-stagecoal, CN

Entrained bed HTW (Uhde), DE

U-Gas (GTI), US

TRIG (KBR), US

Fluidized bed

Figure 2.2 Commercially available coal gasification technologies (BGL–British Gas/Lurgi, CN–China, DE–Germany, ECUST–East China University of Science and Technology, GSP– Gaskombinat Schwarze Pumpe, HT-L–Hangtian Lu, HTW–high-temperature Winkler, JP–Japan, KBR–Kellogg Brown & Root, MCSG–

Lurgi or SEDIN dry ash, DE, CN

BGL (Envirotherm), DE, CN

Moving bed

multicomponent slurry gasification, MHI– Mitsibushi Heavy Industries, NL–The Netherlands, TPRI–Thermal Power Research Institute, US–United States of America) (with permission from Ref. [25]. Copyright 2014 Wiley-VCH Verlag GmbH).

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Figure 2.3a exhibits the market share by thermal syngas capacity in GW of installed and under-construction units. Shell technology is the current market leader closely followed by GE technology and Lurgi FBDB technology. The upcoming Chinese technologies (SEDIN, MCSG, HT-L, and ECUST) have a recent share of 22%. The other group includes technologies such as BGL, MHI, Prenflo, Siemens, Choren, TPRI, KBR, Tsinghua, and U-Gas, which total to nearly a quarter of the installed capacity. Shell 33.0 GW 21%

(a)

Others (Siemens, MHI, U-Gas,…) 36.7 GW 23%

SEDIN 7.8 GW 5%

Lurgi FBDB 19.5 GW 12%

MCSG 5.3 GW 3% HT-L 5.5 GW 3%

GE 29.2 GW 18% E-Gas 7.0 GW 4%

ECUST 17.0 GW 11%

(b) Others (Siemens, MHI, U-Gas,…) 11.5 GW 28%  

Shell 4.5 GW 11% Lurgi FBDB 0.5 GW 1% GE 2.2 GW 5%

SEDIN 11.0 GW 26%

MCSG 0.8 GW 2%

ECUST 5.8 GW 14% E-Gas 3.7 GW 9% HT-L 1.5 GW 4%

Figure 2.3 Shares of the world wide gasification by technology: (a) operating in 2013 and currently under construction, (b) expected growth until 2018 [1] (ECUST–East China

University of Science and Technology, FBDB– fixed-bed dry bottom, HT-L–Hangtian Lu, MCSG–multicomponent slurry gasification, MHI–Mitsibushi Heavy Industries).

2.3 Typical Feedstock and Products

Looking at the expected growth until 2018 in Figure 2.3b, it can be seen that the Chinese version of the dry-ash moving-bed gasification from SEDIN is predicted to more than double its current capacity. The second largest increase is predicted for the Chinese ECUST technologies, which are offered in several variations. In contrast, the growth of the current market leaders (Shell, GE, Lurgi FBDB) is slowing down. The predicted growth of other technologies must not be neglected as there is a large project from Siemens envisaged, which aims at producing 8.5 GW syngas in order to generate Fischer-Tropsch liquids from coal processing technology [26].

2.3 Typical Feedstock and Products 2.3.1 Feedstock

As already shown in Figure 2.1, coal is the main feedstock in current gasification applications and the highest growth is expected for it. Hence, the following discussion will focus solely on coal. When selecting the appropriate coal for gasification, there are technical and economic aspects to be considered. Not all coals are equally suitable for gasification, and different gasification technologies have different fuel requirements. The most decisive properties are the ash content, moisture content, ash melting temperature, and available grain size [27]. Currently, a large spectrum of coals can be gasified and the main limitations exist for high ash content and small particle size. In general, the capital costs for coal-based syngas generation are approximately doubled compared to natural-gas-based syngas [27]. Because the coal gasification island represents a considerable part of the investment in the overall conversion chain, the feed coal should be as cheap as possible. Consequently, coals that are not suitable for other applications (e.g., coking, combustion) or residues from coal beneficiation processes end up in gasification units. Low-rank coals with high moisture content, high-ash, or high-sulfur coals are principal candidates for gasification. Of course, high-quality steam coals and caking coals are suitable for gasification processes as well, but the competition to other utilization paths may lead to a disadvantageous price level. 2.3.2 Products

There are many possibilities for syngas use, which poses different requirements to gas composition and gas purity. Relevant products and their gas specifications are described in the following section to provide a basic understanding of gas treatment and conversion units required between gasifier and gas usage. In the case of syntheses, often the pure stoichiometry of the foreseen reaction is a

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Table 2.1 Commercial attractiveness of products from coal gasification [26]. Product

Product yielda)

Market price

Specific revenuea)

Ammonia Gasoline Methanol Polypropylene SNG (LNG competitor) Power SNG (shale gas competitor)

0.98 t 360 l 0.72 t 0.19 t 420 m3 (STP) 2.4 MWh (electric) 420 m3 (STP)

333 EUR/t 0.685 EUR/l 333 EUR/t 1170 EUR/t 0.038 EUR/kWh 44.4 EUR/MWh 0.009 EUR/kWh

327 EUR 247 EUR 240 EUR 222 EUR 159 EUR 107 EUR 37 EUR

EUR: euro; LNG: liquified natural gas, SNG: synthetic natural gas. a) Referring to 1 tonne of subbituminous coal or 1600 m3 (STP) syngas (H2 + CO).

reasonable indicator for bulk gas composition, whereas parallel reactions and potential catalyst poisons may favor or restrict other components, respectively. Furthermore, one can already judge from the required gas quality which gasifier fits best to some specific gas utilization. Table 2.1 shows the commercial attractiveness of certain products from coal gasification according to Morehead [26]. The comparison is based on one tonne of subbituminous coal, which is equivalent to 1600 m3 (STP) syngas (H2 + CO) produced by entrained-flow gasification. The attainable yields of different products from that feed are shown, and the combination of the yield with the current price permits the calculation of the specific revenue for each product. It can be clearly seen that ammonia is by far the most attractive product followed by gasoline, methanol, and its derivate polypropylene. The price of synthetic natural gas (SNG) is strongly dependent on the competing source of natural gas. Of course, this is only an indication of product value, and the main question remains as to what cost the product can be produced through coal gasification. Therefore, specific boundary conditions (e.g., subsidies, regulations, and water availability), coal price, and capital costs of all required installations and infrastructure must be considered. This is subject to highly specific detailed economic analyses and is not discussed in this book. 2.3.2.1

Ammonia

Ammonia is the basis for industrial fertilizer production and, therefore, the most important bulk chemical in the world. About one-quarter comes from coal gasification [1]. It is produced by the Haber–Bosch synthesis, which takes place at 90–180 bar and 400–530 °C using the following reaction [28]: N2 ‡ 3H2 ƒ 2NH3

ΔR H° ˆ 92:2 kJ=mol

(2.1)

Consequently, the feed gas should feature a molar H2/N2 ratio of 3, while the sum of oxygen species (O2, CO, CO2, and H2O) should be below 30 ppmv, total sulfur below 0.1 ppmv and inerts including CH4 below 2 vol% [3,29]. Such conditions indicate that all the CO contained in the raw gas must be converted to H2 in a water–gas shift reactor (CO ‡ H2 O ƒ H2 ‡ CO2 , see also

2.3 Typical Feedstock and Products

Equation 4.9)) and all the resulting CO2 must be removed. Application of a Rectisol wash for CO2 and H2S removal, using chilled methanol as wash medium, can not only achieve the required gas purity, but may also show synergetic effects for cooling the ammonia product down to condensation temperature ( 33 °C). Because commercial coal gasification has so far not exceeded 65 bar of operating pressure, considerable syngas (and nitrogen) compression is required to approach synthesis pressure. The oxygen required for coal gasification may have a decreased purity around 95% since enrichment of usually inert N2 in the synthesis loop is essentially no problem. If a further conversion to urea (H2NCONH2) is intended, the separated carbon dioxide can also be used in the ratio NH3/CO2 = 2. Typical plants produce 1500–2000 t/d ammonia, which must be supplied by an appropriate gasification unit [3]. 2.3.2.2

Methanol and Derivatives

Hydrogen and carbon monoxide as well as minor quantities of carbon dioxide may react in the presence of a catalyst to form methanol: 2H2 ‡ CO ƒ CH3 OH

ΔR H° ˆ 90:1 kJ=mol

3H2 ‡ CO2 ƒ CH3 OH ‡ H2 O

ΔR H° ˆ 49:0 kJ=mol

(2.2) (2.3)

Depending on the employed catalyst, the reactor can be operated at 250– 280 °C and pressures between 50 and 100 bar. The feed gas should have less than 0.1 ppmv H2S and the catalysts are also sensitive to nitrogen compounds (NH3 and HCN), metal carbonyls, and unsaturated hydrocarbons [3,27,28]. Also any inert gas fraction hampers synthesis conversions rates. Consequently, the oxygen purity applied during the gasification should be as high as possible (99.5%) keeping nitrogen fractions small. The conversion has an optimum for 2.5 to 3.5 mol% of CO2 in the syngas and is operated at the so-called stoichiometric number SN ˆ 2:03, which is defined as SN ˆ

xH2 xCO2 ; xCO ‡ xCO2

(2.4)

where x is the molar fraction of the specific components in the syngas. Coal gasification syngas is usually too CO-rich to approach this optimum number. Hence, a water gas shift reactor is required to adjust the H2/CO ratio and a Rectisol wash, which uses methanol also as solvent, is favorable in terms of deep desulfurization and additional CO2 removal [3]. The typical size of methanol plants is between 2000 and 3000 t/d. More than one-third of the total world methanol production was based on coal gasification in 2013. Only 14.3% of total world production of methanol is used directly for blending into gasoline or combustion. The remaining part is converted to derivatives, such as formaldehyde (29.9%), methyl tert-butyl ether (MTBE, 13.2%), olefins (9.1%), acetic acid (8.8%), dimethyl ether (DME, 7.3%), and various other chemical products and gasoline totaling to 17.4% [1,30].

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2.3.2.3

Electricity (Integrated Gasification Combined Cycle)

Producing electricity from syngas on a larger scale means combustion in a gas turbine, which should preferably be attached to a bottoming steam cycle to increase efficiency. This arrangement is known as IGCC technology. The main advantages are the higher efficiency and the enhanced environmental performance compared to conventional coal-fired steam plants (less SOx, NOx, CO, and dust) [3]. The pressure of a gasifier fueling a combined cycle is usually in the range of 25–33 bar and heat recovery from the syngas may raise additional steam, which improves efficiency. The gas does not require a specific molecular composition as long as CO2 capture for sequestration is not targeted. In this specific case, any methane contained in the syngas cannot be separated and will cause inevitable CO2 emissions. In contrast, CO can be converted to CO2, yielding hydrogen by employing a water–gas shift step. Subsequently, nearly all CO2 can be efficiently separated from the pressurized syngas by conventional acid gas removal before the combustion. Therefore, it is referred to as “precombustion capture” technology [31]. As standard gas turbines are actually designed for natural gas fuel, the gas heating value and Wobbe index are important parameters that might be adjusted especially for H2-rich gases. Such gas is, for example, diluted by N2 from the air separation unit or water saturation achieving a lower heating value of 7.87 MJ/kg [32]. Consequently, high oxygen purities for gasification are not necessary. Other gas specifications, influencing mainly the lifetime of certain gas turbine parts, may vary for different turbine types. Some typical values are summarized in the following [32,33]:

     

Dust 96 wt%