Computational Modeling of Underground Coal Gasification [1 ed.] 9781138091597, 9781315107967, 9781351608640, 9781351608633, 9781351608657

Table of contents :

Chapter 1: Introduction


1.1. World energy scenario


1.2. Cleaner energy from coal


1.3. Underground coal gasification


1.4. Computational modelling of UCG


1.5. Organization of this book



Chapter 2: Underground Coal Gasification: State of the Art


2.1 Underground coal gasification


2.2 UCG technologies


2.3 UCG field trials


2.4 Mitigating environmental risks


2.5 Importance of coal properties in UCG process



Part I: Pre-requisites of Computational Modeling


Chapter 3: Physico-chemical Properties of Coal


3.1 Types of coal


3.2 Chemical properties of coal


3.3 Physical and mechanical properties


3.4 Summary



Chapter 4: Kinetics of Coal Gasification


4.1 Drying and pyrolysis


4.2 Chemical reactions


4.3 Kinetic studies – Experimental methods


4.4 Kinetic models for coal gasification


4.5 Catalytic effect of ash and char on different reactions


4.6 Summary



Chapter 5: Laboratory Studies on Underground Coal Gasification


5.1. Overview of laboratory-scale UCG experiments


5.2 Typical laboratory setup and procedure


5.3 Combustion cavity experiments


5.4 Gasification cavity experiments


5.5 Experiments to measure spalling rate


5.6 Determination of heat transfer coefficient in UCG cavity


5.7 Summary



Part II: Computational Modeling


Chapter 6: Approach towards computational modeling of UCG


6.1. Overall modelling approach


6.2. Role of thermodynamics and reaction kinetics


6.3. Role of flow patterns


6.4. Role of heat and mass transport


6.5. Role of spalling


6.6. Overview of available process models


6.7. Summary



Chapter 7: Thermodynamic & Reaction Engineering Models


7.1 Introduction


7.2 Thermodynamic models


7.3 Reaction engineering (CRE) models


7.4 Summary



Chapter 8: Multi-zonal and CFD Models


8.1. Introduction


8.2. Development of the compartment model


8.3. Results from compartment model


8.4. Model parametric studies


8.5. New integrated 3-D UCG simulator from LLNL


8.6. Computational fluid dynamics (CFD) based models


8.7. Well layout design for UCG


8.8. Summary and Conclusions



Part III: Summary


Chapter 9: Summary, Conclusions and Path Forward


9.1. UCG: Current status


9.2. Computational modelling for pushing frontiers & realizing potential of UCG


9.3. Path forward

Citation preview

Computational Modeling of Underground Coal Gasification

Computational Modeling of Underground Coal Gasification

Vivek Ranade, Sanjay Mahajani, and Ganesh Samdani

MATLAB® and Simulink® are trademarks of the MathWorks, Inc. and are used with permission. The MathWorks does not warrant the accuracy of the text or exercises in this book. This book’s use or discussion of MATLAB® and Simulink® software or related products does not constitute endorsement or sponsorship by the MathWorks of a particular pedagogical approach or particular use of the MATLAB® and Simulink® software.

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2020 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed on acid-free paper International Standard Book Number-13: 978-1-138-09159-7 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents Preface.......................................................................................................................xi Author Bios.............................................................................................................xv Notations.............................................................................................................. xvii 1 Introduction...................................................................................................... 1 1.1 World Energy Scenario .........................................................................2 1.2 Cleaner Energy from Coal.................................................................... 5 1.3 Underground Coal Gasification...........................................................6 1.3.1 UCG Process ............................................................................. 9 1.3.2 Advantages of the UCG Process........................................... 10 1.3.3 Environmental Impact of UCG............................................. 12 1.4 Computational Modeling of UCG .................................................... 13 1.5 Organization of This Book................................................................. 18 2 Underground Coal Gasification: State of the Art................................... 21 2.1 Underground Coal Gasification......................................................... 21 2.1.1 Drying and Pyrolysis.............................................................22 2.1.2 Chemical Reactions................................................................ 23 2.1.3 Cavity Growth ........................................................................ 25 2.2 UCG Technologies ............................................................................... 26 2.2.1 Early Methods of UCG .......................................................... 27 2.2.2 Linked Vertical Wells (LVW) Methods................................ 29 2.2.3 Steeply Dipping Coal Seam Method.................................... 30 2.2.4 Controlled Retracting Injection Point (CRIP) Method...... 32 2.3 UCG Field Trials................................................................................... 35 2.3.1 UCG Tests at Hanna, Wyoming............................................42 2.3.2 UCG Tests at Hoe Creek.........................................................43 2.3.3 The Centralia Series of Field Tests........................................ 45 2.3.4 Rocky Mountain I................................................................... 46 2.3.5 UCG Field Trials in Countries Other Than the United States and USSR......................................................... 49 2.4 Mitigating Environmental Risks....................................................... 51 2.5 Importance of Coal Properties in the UCG Process........................ 52

Part I  Pre-Requisites of Computational Modeling 3 Physicochemical Properties of Coal.......................................................... 59 3.1 Types of Coal........................................................................................ 59 3.2 Chemical Properties of Coal .............................................................. 62 v

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Contents

3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6

Proximate Analysis ................................................................ 62 Ultimate Analysis...................................................................64 Specific Energy .......................................................................65 Ash Fusion Temperature ......................................................65 Free Swelling Test................................................................... 66 Fourier Transform Infrared Spectroscopy (FTIR) Analysis.................................................................................... 66 3.2.7 X-Ray Diffraction (XRD) Analysis........................................ 67 3.2.8 Nuclear Magnetic Resonance (NMR) Analysis.................. 70 3.3 Physical and Mechanical Properties................................................. 72 3.3.1 Density..................................................................................... 72 3.3.2 Porosity..................................................................................... 72 3.3.3 Reflectivity............................................................................... 74 3.3.4 Scanning Electron Microscope Images............................... 75 3.4 Summary............................................................................................... 76 4 Kinetics of Coal Gasification...................................................................... 79 4.1 Drying and Pyrolysis..........................................................................80 4.2 Chemical Reactions.............................................................................. 81 4.3 Kinetic Studies – Experimental Methods.........................................83 4.3.1 Thermogravimetric Analyzer (TGA)................................... 85 4.3.1.1 Experimental Procedure........................................ 86 4.3.2 Drop Tube Furnace (DTF)...................................................... 88 4.3.3 Boat Reactor............................................................................. 91 4.3.4 Fixed Bed Reactor .................................................................. 94 4.4 Kinetic Models for Coal Gasification................................................ 95 4.4.1 Kinetic Modeling of CO2 Gasification of Char................. 100 4.4.1.1 Conservation Equations....................................... 100 4.4.2 Kinetic Modeling of Steam Gasification of Char.............. 101 4.4.3 Reaction–Diffusion Model for Particle Undergoing Gasification............................................................................ 106 4.4.4 Kinetic Modeling of Char Combustion............................. 109 4.4.5 Kinetic Modeling of Coal Pyrolysis................................... 110 4.5 Catalytic Effect of Ash and Char on Different Reactions............ 115 4.6 Summary............................................................................................. 119 5 Laboratory Studies on Underground Coal Gasification ..................... 121 5.1 Overview of Laboratory-Scale UCG Experiments........................ 121 5.2 Typical Laboratory Setup and Procedure ...................................... 128 5.3 Combustion Cavity Experiments.................................................... 130 5.3.1 Features of the Cavity ......................................................... 130 5.3.2 Cavity Growth Rate ............................................................. 132 5.3.3 Cavity Evolution.................................................................... 133 5.4 Gasification Cavity Experiments..................................................... 134 5.4.1 Features of Gasification Cavity and Spalling .................. 134

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5.4.2

Effect of Design and Operating Parameters on Cavity Evolution ................................................................... 136 5.4.3 Product Gas Quality ............................................................ 139 5.4.4 Effect of Type of Coal .......................................................... 140 5.5 Experiments to Measure Spalling Rate .......................................... 143 5.5.1 Spalling and Cavity Growth............................................... 144 5.5.2 Apparatus and Experimental Procedure.......................... 144 5.5.3 Identification of Spalling Events......................................... 145 5.6 Determination of Heat Transfer Coefficient in the UCG Cavity ........................................................................................ 147 5.7 Summary............................................................................................. 148

Part II  Computational Modeling 6 Approach to Computational Modeling of UCG.................................... 153 6.1 Overall Modeling Approach............................................................ 153 6.2 Role of Thermodynamics and Reaction Kinetics.......................... 155 6.3 Role of Flow Patterns......................................................................... 161 6.4 Role of Heat and Mass Transport.................................................... 169 6.5 Role of Spalling.................................................................................. 172 6.6 Overview of Available Process Models.......................................... 176 6.7 Summary............................................................................................. 185 7 Thermodynamic and Reaction Engineering Models........................... 187 7.1 Introduction ....................................................................................... 187 7.2 Thermodynamic Models................................................................... 188 7.3 Chemical Reaction Engineering (CRE) Models............................. 195 7.3.1 Packed Bed Models............................................................... 195 7.3.1.1 Comparison of Simulation Results with Daggupati et al. (2011a)......................................... 198 7.3.1.2 Comparison of Model Predictions with the Results of Nourozieh et al. (2010)................. 200 7.3.1.3 Comparison of Model Predications with Shirazi et al. (2013)................................................. 202 7.3.1.4 Packed Bed Models from Other Literature..................................................... 203 7.3.2 Channel Models.................................................................... 207 7.3.3 Coal Block Models................................................................ 215 7.4 Summary............................................................................................. 221 8 MultiZonal and CFD Models....................................................................223 8.1 Introduction........................................................................................ 223 8.2 Development of the Compartment Model.....................................225 8.2.1 Phase I of the UCG Process.................................................225

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Contents

8.2.2 8.2.3

8.3

8.4

8.5 8.6 8.7 8.8

Phase II of the UCG Process................................................ 229 Rubble Zone Model.............................................................. 233 8.2.3.1 Radial Plug-Flow Reactor Model for Rubble during Phase I����������������������������������������� 233 8.2.3.2 Mixed Flow Reactor Model for Rubble Zone during Phase II...................................................... 236 8.2.4 Back-Mixed Reactor Model for the Void Space................. 237 8.2.4.1 Gas-Species Balance.............................................. 238 8.2.4.2 Energy Balance...................................................... 238 8.2.5 Roof Model............................................................................. 239 8.2.6 Reaction Kinetics Used in the Model................................. 242 8.2.7 Treatment of Spalling in the Model................................... 244 8.2.8 Solution Procedure............................................................... 246 Results from Compartment Model.................................................. 250 8.3.1 Model Results for Phase I of UCG...................................... 251 8.3.1.1 Rubble Zone Results............................................. 251 8.3.1.2 Roof Zone Results................................................. 256 8.3.1.3 Void Zone Results................................................. 256 8.3.2 Comparison of Phase I Results with Laboratory Experiments........................................................................... 259 8.3.3 Model Results for Phase II of UCG..................................... 261 8.3.3.1 Rubble and Roof of Compartment 5................... 262 8.3.3.2 Exit Gas Quality.................................................... 264 8.3.4 Comparison of Phase II Results with Laboratory-Scale Experiments............................................ 265 8.3.5 Comparison between Phase I and Phase II of UCG for the Vastan Coal Reserve ............................................... 266 8.3.6 Comparison of Model Results with Field Trials .............. 269 Model Parametric Studies................................................................. 270 8.4.1 Key Performance Indicators for the UCG Process........... 271 8.4.2 Effect of Operating Conditions on UCG Performance................................................................. 272 8.4.3 Effect of Spalling Condition on UCG Performance......... 274 8.4.4 Effect of Kinetics on UCG Performance............................ 278 8.4.5 Effect of Outflow Channel Length on UCG Performance........................................................................... 279 New Integrated 3D UCG Simulator from LLNL........................... 280 Computational Fluid Dynamics (CFD)–Based Models................ 282 8.6.1 CFD Model Development.................................................... 283 8.6.2 Some Results from CFD Models......................................... 290 Well Layout Design for UCG............................................................ 293 8.7.1 Identification of Suitable UCG Site .................................... 296 8.7.2 Integrated Process and Subsurface Modeling ................. 296 Summary and Conclusions.............................................................. 297

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Part III Summary 9 Summary, Conclusions, and Path Forward............................................ 301 9.1 UCG: Current Status.......................................................................... 301 9.2 Computational Modeling for Pushing Frontiers and Realizing the Potential of UCG........................................................306 9.3 Path Forward...................................................................................... 313 References............................................................................................................ 317 Index...................................................................................................................... 331

Preface Coal remains abundant and broadly distributed around the world, unlike oil and gas. Economically recoverable reserves of coal are available in more than 70 countries worldwide. The recoverable coal is, however, only a small fraction of the total geological resources, and vast coal resources are otherwise unminable for a variety of reasons. Underground coal gasification (UCG) is a unique technology that can exploit such unminable coal reserves. The development of UCG started way back in the 1930s in the Soviet Union, and at present there are some commercial units that have been in operation for about 50 years (e.g., Angren, Uzbekistan). With the recent emphasis on ‘clean coal technologies’, UCG has started gaining importance again. UCG, which allows the reinjection of some of the CO2 on-site, will be a promising option particularly when combined with carbon capture and sequestration (CCS). The potential sequestration sites are coincidently found in the vicinity of most UCG sites worldwide. However, substantial research and development efforts are necessary to realize the full potential of UCG technology. UCG is the process of in-situ conversion of underground coal into combustible products. A UCG system consists of a pair of process wells – one injector and one producer – drilled from the surface into the coal seam, with a highly permeable channel linking the two wells. A gasifying agent (air, oxygen, steam) is forced into the coal seam via the injector well. Gasification reactions occur in the coal seam, and a cavity consisting of coal, char, ash, rubble, and void space is created underground. This consumes the bulk of the coal, producing a combustible gas mixture. The generated gas is brought to the surface via a producer well and is then cleaned, treated, and used for power generation or as a chemical feedstock. UCG offers a number of environmental and other benefits over conventional mining and is therefore one of the most promising coal utilization techniques of the future. It eliminates the need for mining and also eliminates the need for specialized coal-processing equipment and gasification reactors, thereby reducing capital investment significantly. Other benefits of UCG include safer operations, no surface disposal of ash and coal tailing, low dust and noise pollution, low water consumption, larger coal resource exploration, and low methane emission into the atmosphere. The UCG process is associated with much complexity arising from its chemical, thermal, and mechanical effects. It is inherently a transient process, and maintaining uniform product gas composition is the key requirement from the viewpoint of its downstream applications. The simultaneous occurrence of a number of exothermic and endothermic reactions, heat and masstransfer effects, nonideal flow patterns in the cavity, and thermomechanical failure of the coal leading to crack development and spalling make the xi

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design and optimization of UCG tremendously challenging. Computational modeling offers an excellent way to develop the fundamental understanding required to address these challenges. Though notable contributions have been made in this area, we realized that adequate resources are not available to practicing engineers and researchers for harnessing state-of-the-art computational modeling to simulate complex UCG processes. Our groups at the Indian Institute of Technology Bombay and CSIR – National Chemical Laboratory – initiated work on UCG more than a decade ago and felt the need for such a resource. This book was therefore developed with the aim of addressing this need. The book attempts to develop and discuss an appropriate approach to model the complex processes occurring in UCG in a tractable way. The scope is restricted to combustion and gasification processes occurring in the UCG cavity (reactor). Processes occurring near and far field beyond the UCG reactor are outside the scope of this book. Similarly, system-level issues like site selection, site development, drilling, utilization of product gases, and end-of-life management of UCG sites are not discussed in this book. The focus is on providing adequate information and a framework for developing a multilayer modeling strategy to simulate UCG. Other important aspects such as identifying, estimating, or measuring the input data required for these models and designing laboratory-scale experiments for calibration and validation of the computational models are also discussed. We have written this book with the intention of describing the individual aspects of combustion and gasification processes occurring in UCG in a coherent fashion that may be useful to further improve design methodologies and optimize UCG performance. The intended users of this book are practicing engineers, engineering scientists, and researchers/students interested in understanding, designing, and optimizing UCG processes. Some prior background in reaction engineering and numerical techniques is assumed. The information in the book is organized into three parts. The first two chapters provide a general introduction and review the state of the art of UCG. The role of coal properties such as rank, ash content, and spalling behavior in the success of UCG is also highlighted. The next part, comprising three chapters, discusses the pre-requisites of computational modeling. Chapter 3 provides a brief review of coal characterization and data/ correlations for estimating the physicochemical properties of coal. Some of the inferential measurements and analyses that can be useful for the characterization of a given coal are also discussed. UCG involves multiple simultaneous reactions. Under the conditions that prevail, some of the reactions are controlled by reaction thermodynamics and others by kinetics and mass transfer rates. The reaction kinetics and the estimation of kinetic parameters for reactions relevant to UCG are discussed in Chapter 4. The deconvolution of mass transfer and diffusion from the kinetics is also discussed. Laboratory-scale experiments to quantitatively characterize features of the UCG cavity (shape and

Preface

xiii

size, growth rates), the composition of gases generated in UCG, and spalling are discussed in Chapter 5. The focus is on providing information and guidance on designing and operating such laboratory-scale UCG experiments. The third part focuses on computational modeling and also comprises three chapters. Chapter 6 presents an overall methodology for developing computational models of the UCG process based on the pre-requisites discussed in the earlier three chapters. The need to develop a multilayer modeling strategy is highlighted. Thermodynamic (zero-dimensional) and chemical reaction engineering (one-dimensional) models are discussed in Chapter 7. Key assumptions and their implications are discussed. The application, usefulness, and possible limitations of these models are discussed with the help of published comparison of simulated and experimental results. Some of the limitations of these simpler models can be overcome by developing detailed three-dimensional computational fluid dynamics (CFD) models and multizonal models. These models are discussed in Chapter 8. Here an attempt is made to provide specific comments to connect modeling with real-life applications. Key points are summarized in the last chapter (Chapter 9) along with some comments about the path forward. The book thus presents an end-to-end approach to the process modeling of UCG. It is written with the beginner in UCG in mind, who has sufficient background in process and reaction engineering. The purpose is to make the analysis of this complex but useful process more interesting and relatively simple. It goes a step further to serve as a template for approaching this problem systematically before large investments are made. Such a platform was nonexistent when we embarked on our journey about 10 years ago. We hope that the book meets its objectives. The material included in this book may be used in several ways. It may be used as a basic resource for methodologies or for making decisions about applications of computational modeling to UCG. The content could be useful as a study material for an in-house course on UCG design and optimization, or a companion book while solving practical problems associated with UCG design and optimization. The book will also be useful to researchers and research students embarking on unravelling the complexities of UCG. We hope that this book will encourage readers to exploit the power of computational models for realizing the true potential of UCG. We are grateful to many of our associates and collaborators with whom we learnt the intricacies of UCG. SM would like to acknowledge his coinvestigators and colleagues Profs. Preeti Aghalayam and Anuradda Ganesh. Many of SM’s and VVR’s students have contributed to this book in different ways. We would like to particularly acknowledge the contributions of Dr Daggupati, Dr Khadase, Dr Sminu Bhaskaran, Dr Ramesh, and Dr Singan. Mrs Nanda Ranade read the first draft and made suggestions to improve overall readability. We also wish to thank the editorial team at CRC Press, particularly Dr Gagandeep Singh, for their patience and help during the process of writing this book.

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We wish the reader an exciting exploration of these relatively untouched deep underground reserves! Vivek Ranade, Sanjay Mahajani, Ganesh Samdani December 2018 MATLAB® is a registered trademark of The MathWorks, Inc. For product information, please contact: The MathWorks, Inc. 3 Apple Hill Drive Natick, MA 01760-2098 USA Tel: 508 647 7000 Fax: 508-647-7001 E-mail: [email protected] Web: www.mathworks.com

Author Bios Vivek Ranade is a professor of chemical engineering at the School of Chemistry and Chemical Engineering of Queen’s University Belfast. His research focus is on developing insights, innovations, and intensified solutions for sustainable energy, water, and chemicals. He uses computational flow modeling, hydrodynamic cavitation, and MAGIC (modular, agile, intensified, and continuous) processes to achieve this. Before moving to Queen’s, he led Chemical Engineering at CSIR – National Chemical Laboratory, Pune, India. He has contributed significantly to chemical engineering science and practice; developed performance enhancement solutions, software products, and fluidic devices for a variety of commercialized applications; and developed new insights and methodologies for process intensification. He is an associate editor of Industrial & Engineering Chemistry Research and serves on the editorial boards of the Chemical Engineering Research & Design and Indian Chemical Engineer journals. He is a recipient of numerous awards, including the highest Indian scientific award, the Shanti Swarup Bhatnagar award for scientists under 45. He is a fellow of the Institute of Chemical Engineers, UK; the Indian National Academy Sciences; the Indian National Academy of Engineering; and the Indian Academy of Sciences. He has published more than 150 papers and 6 books (> 6500 citations, h index=45: from Google Scholar) and is the coinventor of more than 20 patents. He has also cofounded two technology companies: Tridiagonal Solutions and VIVIRA Process Technologies. More information may be found at http://go.qub. ac.uk/ranade and https​://en​.wiki​pedia​.org/​wiki/​Vivek​_Rana​de Sanjay Mahajani received his bachelor’s and PhD degrees in chemical engineering from the University Department of Chemical Technology (UDCT) (now known as Institute of Chemical Technology [ICT]), Mumbai in 1989 and 1996, respectively, and his Master’s degree from the Indian Institute of Technology (IIT) Bombay in 1992. After completing his Ph.D., he worked in Monash University, Australia (1996–2000) as postdoctoral research fellow. He joined IIT Bombay as assistant professor in 2000 and is presently a full professor (2008–date) in the Department of Chemical engineering. He also holds the position of professor-in-charge of the newly formed Tata Centre of Technology and Design at IIT Bombay. Prof. Mahajani’s research interests are reaction engineering and applied catalysis, process intensification, and gasification of coal and biomass. He has worked on several industry-sponsored projects that include underground coal gasification, catalyst development, and reactive distillation and its applications. He was part of the faculty team responsible for developing a laboratory facility and building a research group to conduct laboratory research on underground coal gasification for xv

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Indian coals. He has over 120 international research papers and notable awards in research and teaching to his credit. Ganesh Samdani is a chemical engineer by training, and he received his PhD in chemical engineering from the Indian Institute of Technology Bombay with a strong focus on computational modeling of underground coal gasification. He has published his work in peer-reviewed journals and has given several talks on the computational modeling of UCG as well as other reactive flow processes. For the last four years, he has been with the Honeywell India Technology center (HITC), Gurgaon, India, where he leads the kinetic and CFD modeling team. He has received a green belt in Six Sigma for its application in process improvement. His work at Honeywell is related to the application of multiphase and reactive flow modeling for process development and equipment design in the field of refinery and petrochemicals. His current research interests are related to refinery residue upgradation, novel reactors for petrochemicals, upscaling of industrial reactors, coal and biomass utilization, and renewable feedstocks for standard refinery units. He has also received various Honeywell internal awards for his technical excellence and process development efforts.

Notations A Ac Aroof BL C Cp C p–eff CV Deff Dw E ES FL Fg Fw G Gr H K Kper L M, MW M T N Nu P Pk Pr Q R Ra RC Re Rj R M RT

Frequency factor for rate constant (m3/s/mol OR 1/s) OR area available for heat transfer (m2) Cross-sectional area (m2) Area of roof surface (m2) Backward length (i.e., cavity growth in backward direction, cm Species concentration (mol/m3) Specific heat (kJ/kmol/K for gas and kJ/kg/K for solids) Effective specific heat (kJ/kg/K) Cavity volume (cc) Effective diffusivity through char and ash layer (m2/s) Distance between the two wells (cm) Activation energy (kJ/mol) Total energy of solid phase (J/kg) Forward length (i.e., cavity growth in forward direction, cm) Gas flow rate (kmol/s) Rate of water influx (kmol/s) Gibbs free energy (kJ/kmol) Grashof number Enthalpy (kJ/kmol) OR height of the cavity (i.e., cavity growth in vertical direction, cm) Adsorption equilibrium constant OR reaction equilibrium constant Permeability of rubble (m2) Sample length scale (m) Molecular weight (kg/kmol) Thiele modulus Order of reaction Nusselt number Pressure (kPa) Partial pressure of kth species (kPa) Prandtl number Flow rate (m3/s OR ml/min) Gas constant (kJ/kmol/K) Rayleigh number Lumped rate based on chemical reaction and internal resistance (kmol/m3/s) Reynolds number jth reaction (kmol/m3/s) Rate of mass transfer of limiting reactant (kmol/m3/s) Total rate (kmol/m3/s) xvii

xviii

S Sc Sh T V Vc Vcav W X Yi YO2 aij as,ij b bbl d dpore hT hTcav k k’ keff kg ky,cav k0 n rk T u vc vg vdf x y z α ∆H εr η Ψ ρ ρeff σ μ

Notations

Internal surface area per unit volume of the char particle (m2/m3) OR entropy (kJ/K) Schmidt number Sherwood number Temperature (K) Volume (m3) Volume of PFR (m3) Volume of cavity = volume of (rubble + void) (m3) Width of the cavity (i.e., cavity growth in transverse direction, cm) Local/total char conversion Mole fraction of species ‘i’ Mole fraction of oxygen Stoichiometric coefficient of ith gas species in jth reaction Stoichiometric coefficient of ith solid species in jth reaction Stoichiometric coefficient of gas reacting with char Barrel Diameter of particle (m) Diameter of pore (m) Heat transfer coefficient in between gas and bed of particles (kW/m3/K) Heat transfer coefficient from void to wall transfer (kW/m2/K) Rate constant (m3/s/mol.m3/m2) Rate constant (m3/s/mol) Effective thermal conductivity (kW/m/K) Mass transfer coefficient (m/s) Mass transfer coefficient from void to wall transfer (m/s) Specific rate constant (s-1 (kmole/m3)-NK-α) Order of reaction Rate of reaction for kth species (kmol/m3/s) Time (s or min or hr) Velocity (m/s) Stoichiometric factor for coal/char Stoichiometric factor for gas Velocity of drying front (m/s) Mole fraction/conversion Gas-phase mole fraction Z-direction coordinate (m) Temperature exponent in reaction kinetics Heat of reaction (J/mol) Radiation emissivity Effectiveness factor Structural parameter in random pore model Solid density (kg/m3) Effective density (kg/m3) Stefan Boltzmann constant (kW/m2/K4) Viscosity of gas mixture (Pa s) OR chemical potential

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Notations

τ υ Φ

Residence time in CSTR (s) OR time for complete conversion (s) OR tortuosity of coal Gas flow rate in CSTR (m3/s) Porosity

Subscripts 0 ash c cav char d df dry g i in j l MT r roof S s, S spall T tr vap void w, W wet

Inlet to the cavity OR initial values Ash property Cross-section OR coal property Cavity Char property Drying Drying front Dry zone Gas-phase Species index Inlet of CSTR Reaction index Liquid phase Mass transfer Values at roof Cavity roof Surface values Solid phase Conditions inside spalled rubble Volume of total coal seam (at the end of wet zone) Top of rubble Vaporization Void zone in channel Water influx or water Wet zone

Acronyms/Abbreviations AD AR C CCS

Air-dried basis – without surface moisture As-received basis – based on total weight of coal sample as received from mine Carbon Carbon capture and sequestration

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Notations

Computational fluid dynamics Controlled retracting ignition point Continuous stirred-tank reactor Differential-algebraic equation Dry, ash-free basis – without any moisture and ash Dry basis – without any moisture Dry, mineral-matter-free basis – without any moisture and the mineral matter ELW Extended linked well FTIR Fourier transform infrared spectroscopy H Hydrogen LVW Linked vertical wells N Nitrogen O Oxygen ODE Ordinary differential equation PFR Plug-flow reactor RPM Random pore model RTD Residence time distribution S Sulfur SCADA Supervisory control and data acquisition TGA Thermogravimetric analyzer UCG Underground coal gasification WGS Water gas shift XRD X-ray diffraction CFD CRIP CSTR DAE DAF DB DMMF

1 Introduction Energy touches the lives of everyone, every day. Global demand for energy has increased enormously since the advent of rapid industrialization, and this is especially true for growing economies like India. Coal supplies almost a third of all the energy used worldwide and makes up 40% of electricity generation. Despite legitimate concerns about pollution and greenhouse-gas emissions caused by coal, it will continue to play a significant role in energy scenarios for a foreseeable future. Significant efforts are being made by governments and industry to develop and to implement more efficient technologies to ensure that coal becomes a cleaner source of energy in the decades to come. Underground coal gasification (UCG) is one such technology which has a potential not just to make coal as a cleaner energy resource but also to harness otherwise un-mineable coal. The gasification of coal offers a cleaner option than combustion, and therefore significant research is being done on coal gasification. Conventionally, coal gasification is carried out using mined coal and on-surface gasifiers. This surface gasification route is not very attractive for coals of low rank/low calorific value and for coals with high ash content. Most of the coal resources in India comprise such low-rank and high-ash coals. In some cases, such coal is found at a great depth, and therefore conventional mining and processing becomes unsuitable. UCG becomes an attractive option in such cases. UCG involves the in-situ conversion of the coal into syngas without bringing the coal to the surface. It involves injecting steam and an oxidant (either pure oxygen or air) into underground coal seam. The coal reacts with the steam and oxidant to produce a gas which is a mixture of carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and hydrogen (H2) in varying proportions depending on coal quality and operating conditions. UCG can be a good option for tapping into the coal reserves of high-ash Indian coals (lignite and sub-bituminous) identified at great depth (which are expensive to mine and require expensive pretreatment before they can be used in surface combustors and gasifiers). UCG is not a new technology, and several attempts have been made to develop and deploy it on a large scale. Key efforts were undertaken by publicly funded laboratories of the U.S. Department of Energy (DOE) and the former USSR. Recent efforts in UCG include trials in China, Australia, and European Union (EU) countries like Poland, Spain, and Belgium. Most of these trials and tests rarely ‘proved’ more than one or two aspects of the technology, and many had uncovered undesired side effects. The U.S. DOE program from 1973 to 1989 addressed engineering concerns and compared methods for enhancing coal permeability. The development of the controlled retracting injection 1

2

Computational Modeling of Underground Coal Gasification

point (CRIP) as well as directional drilling are major advances which significantly enhance the potential of UCG. Despite these efforts in UCG process development in various countries (discussed in Sections 2.2 and 2.3), the progress towards the realization of the true potential of UCG in practice is not significant. This is mainly because of key challenges, for example: • The reactions take place underground and out of sight. • Only a limited number of parameters can be either controlled or measured. • Site selection criteria have not yet been well defined. • There are environmental issues (escape of gas or water contamination). • It requires a unique multidisciplinary integration of knowledge from geology, hydrogeology with the thermodynamics, reaction engineering and multiphase flows relevant to in-situ coal gasification. Computational modeling can play an important role in addressing and overcoming these challenges. Advances in computing resources, understanding of reacting multiphase flows, and numerical methods offer an unprecedented opportunity to harness computational modeling for realizing the true potential of UCG. Computational models based on underlying physicochemical processes occurring in UCG will not only help in better understanding UCG operations but will also pave the way towards their better design and optimization. This book attempts to provide a comprehensive framework for developing computational models of UCG and using them for real-life applications. This first chapter briefly discusses the current world energy scenario and the importance of coal among different sources of energy. Different technologies for producing cleaner energy from coal are then briefly discussed to provide a context for further discussion on UCG. The UCG process is then described, highlighting its advantages and constraints. Key attributes of UCG are compared with surface gasification, emphasizing potential advantages of UCG apart from access to more energy resources. The role of computational modeling in the development of the UCG process and its commercial success is discussed. An overview of different modeling techniques, highlighting the limitations and the challenges in constructing a UCG model, is provided. Overall organization of the book is discussed at the end.

1.1 World Energy Scenario Fossil fuels account for more than 80% of the world’s primary energy supply, and they will remain the biggest source of energy for at least few more decades (World Energy Resources, 2013). Among these fossil fuels,

Introduction

3

coal produces the biggest fraction of the world’s electricity. Energy generated from hydroelectric and nuclear power is less than 10% and is not likely to change significantly in the next 10–20 years. Hydroelectric power is limited by the lack of suitable sites and their ecologies, whereas nuclear power is constrained by political and environmental factors. In the case of renewables, its estimate will supposedly be affected by supply-side issues. Figure 1.1 presents estimates of the total primary energy supply from various resources till 2020. Among the fossil fuel sources, crude oil and gas will be depleted in four to six decades, whereas coal is going to stay with us for more than a century (World Energy Resources, 2013). In addition to its abundance, coal is widely distributed across the globe, is affordable for developing countries, and is in the dominant position in the global energy mix. Though the renewable energy technologies are rapidly being deployed and there are debates about climate change, coal has shown the largest increase in its demand compared to other energy sources. The reasons for this significant growth in energy demand from coal is because it is reliable, readily available, inexpensive, and free from price volatility, unlike oil and natural gas. Events in 2008 led oil prices to reach $150/bbl. Current prices (2018) are again approaching $80/bbl after reaching a low of nearly $30/bbl in 2015. This volatility in oil/gas prices will remain for a long time because of the unstable political scenario in major oil/gas-producing countries and the increased rate of production of shale-gas. In addition to nonvolatility in coal prices, different studies by the World Energy Council (World Energy Resources: 2013 review) have shown that coal will be available for more than 100 years to come. Furthermore, these

FIGURE 1.1 Total primary energy supply by resource 1993, 2011, and 2020. (Adapted from World Energy Council, 2013.)

4

Computational Modeling of Underground Coal Gasification

TABLE 1.1 Indian Coal Reserves and Production Proved amount in place (hard coal only, million tonnes) Proved recoverable reserves (total coal, million tonnes) Production (total coal, million tonnes)

105 820 60 600 515.8

(Used with permission of the World Energy Council.)

estimates have been made without considering nonconventional ways of accessing coal resources such as underground coal gasification. India is an example of a country wherein coal plays an important role in energy security – India is dominantly powered by coal. In addition to its huge domestic coal reserves, India also has easy access to affordable imported coal through various ports across its coastline. This is important at an international level as well, since India is one of the fastest growing economies. Generally, economic growth also increases demand for electricity and materials important in infrastructure development, such as steel and cement. Coal is a key component in the generation of electricity and the production of these chemicals/materials as well. Therefore, India’s increasing exploration, import, and use of coal supports its significant economic growth and vice versa. In spite of this growth in the economy, it is estimated that around one fourth of the population of India still lives in energy poverty. The gap in demand and supply in the power sector in India has reduced from ~10% a decade ago to less than 1% in 2018, according to the Central Electricity Authority (CEA). However, the accessibility of power does not guaranty the affordability of power, and therefore ~17% of households in rural India are yet to be electrified. To resolve the power issue, India has planned a huge increase in power generation. Although every other energy sources will have an important role to play in increasing the power-generation rates, coal is expected to meet much of the future energy demand in India. Table 1.1 presents the amounts of coal resources, recoverable reserves, and rate of production in India. The resources quoted are the outcome of geological exploration down to a depth of 1200 m. It has been observed that the major coal deposits in India are in the eastern half of the country, that is, the states of Andhra Pradesh, Arunachal Pradesh, Chhattisgarh, Jharkhand, Orissa, and West Bengal (Khadse et al., 2007). Some of the problems with Indian coal are low rank (lignite and subbituminous) and high ash (as much as 40% in some coals), and a significant fraction is more than 300 m below ground. As the quality of most of the country’s coal reserves is relatively low, India still relies on substantial imports of high-quality coal, mainly from Australia, China, Indonesia, and South Africa. In addition, India also exports a small fraction of coal to neighboring countries, including Bangladesh, Bhutan, and Nepal.

Introduction

5

1.2 Cleaner Energy from Coal Climate change because of global warming has become an important issue which can no longer be overlooked. To mitigate adverse climate change, the world has accepted some targets on climate change issues, and others are still being debated. These targets have made low-emission coal technologies increasingly important in almost all countries. The importance of these clean technologies increases further due to the increasing demand for coal. Traditionally, we have considered two main methods to reduce carbon emissions from coal-based power plants: the use of highly efficient technologies; or carbon capture, use, and storage. In addition to these two methods, the world has also started looking at out-of-the-box energy solutions. While doing so, an array of unconventional clean coal technologies has been developed to address ecological concerns arising out of coal utilization. Some of the sophisticated technologies for clean coal usage are listed here. Supercritical and Ultra Supercritical Pulverized Coal Combustion: This power-generation technology works at higher steam pressure and temperature. It has shown high conversion efficiencies by applying supercritical steam conditions. Typically, supercritical and ultrasupercritical plants have around 3–4% and 6–7% higher efficiencies in comparison with the subcritical plants (Nicol, 2013). Integrated Gasification Combined Cycle (IGCC): This technology, in addition to applying coal gasification with combustion of coal gas in a gas turbine, also works towards recovery of waste heat in a boiler by using a steam turbine (Crook, 2006). Direct Coal Fired Combined Cycle: This technology is similar to that used in an IGCC plant. It uses a gas turbine and a steam turbine in a combined cycle to recover waste heat. However, instead of gasifying coal, the technology incorporates the direct burning of coal in the gas turbine (Buskies, 1996). Pressurized Pulverized Coal Combustion (PCC): This technology involves a coal-based combined cycle with combustion of coal at temperatures close to 1500oC and pressure of up to 20 bar. The best part of this technology is its ash removal at high process temperature and pressure. This allows the flue gas to be directly used as input for a gas turbine (Saastamoinen et al., 1996). Underground Coal Gasification: The technology involves gasifying coal in situ and can utilize coal reserves which are not accessible using current technologies. In addition, the technology also has several environmental benefits (Gregg and Edgar, 1978).

6

Computational Modeling of Underground Coal Gasification

1.3 Underground Coal Gasification Underground coal gasification is one of the important technologies for utilizing coal resources in an efficient way, which are otherwise not accessible by current mining technologies. UCG involves the in-situ gasification of coal to convert it into combustible gases. Similar gasification reactions are also carried out in conventional above-ground gasification plants. In UCG, however, the gasification reactions are realized within the seam, eliminating the need to bring the coal above ground. The UCG process involves drilling injection and production wells from the surface to the coal seam. A highly permeable channel is then created between them in order to establish a link between the two wells within the coal seam. The process requires injecting a gaseous mixture containing varying amounts of nitrogen, oxygen, steam, and carbon dioxide into the coal seam. Essentially, it is a partial oxidation of the coal deposit along with a water gas shift reaction, followed by recovery of a combustible gas (synthesis gas). As the reactions proceed, a cavity consisting of coal, char, ash, rubble, and void space is created underground, and gas is generated. The generated product gas is collected through the production well. The produced gas can be used for power generation (UCG-IGCC) or as a synthesis gas source for the manufacture of hydrogen, synthetic natural gas (SNG), or methanol. Through this process, UCG allows access to reserves over and above the physical global coal resource. As shown in Table 1.2, UCG has a potential to increase world coal reserves by nearly 600 billion tonnes. The earliest instance of exploiting the concept of UCG dates back to 1868, when Sir William Siemens sought to utilize empty shafts to house a gasifier to convert coal dust left over after years of mining into gas and use it for lighting and heating. Siemens, along with his brothers, is also credited with the design and development of the gas producer, that is, the gasifier. In 1912, TABLE 1.2 Estimated Available Coal Reserves and Gas Reserves Estimated Available Coal Reserves for UCG (billion tonnes) USA Europe Russian Federation China India South Africa Australia Total

Potential Gas Reserves from UCG (as NG) (Trillion m3)

Natural Gas Reserves End 2005 (Trillion m3)

41.4 21.8 26.3 19.2 15.5 8.2 13.2 145.6

5.9 5.7 47.8 2.4 1.1 N 0.8 63.7

138.1 130.1 87.9 64.1 51.8 48.7 44.0 564.7

(Used with permission of the World Energy Council.)

Introduction

7

during the height of the London smog, British scientist Sir William Ramsay also suggested that the smoke menace could be defeated by utilizing the coal within the ground. The methodologies suggested by Sir Ramsay and Sir Siemens have been described in a recent review article (Klimenko, 2009). The concept of UCG was made popular in the erstwhile USSR by the soviet scientist Dmitri Mendeleev upon receiving encouragement and political support from the communist leader Vladimir Lenin, who felt that UCG would be a tool that would protect laborers from being exposed to the hazards of deep-ground mining. Large chambers were built underground to gasify fractured coal. In fact, a very clear idea on gasification struck Mendeleev when he visited the Ural region in the USSR, where there were instances of underground fires. Mendeleev suggested that it could be possible to control the fires within the ground and generate gas, which could then be brought to the surface and utilized. Initial studies were mostly based on intuition, and there were no solid design protocols for the placement of the injection and production wells (Klimenko, 2009). The early 1900s saw a steady rise in interest in pursuing UCG as an option to utilize coal. The contribution of American engineer and inventor A. G. Betts deserves special mention, as his original ideas, patented in as early as 1910, have been utilized in successful UCG experimental runs which are a cornerstone of modern UCG technology. Betts had also distinguished coal-bed methane recovery and UCG. Early drawings and concept papers indicate that the effects of gas leakage were also considered while designing the systems for UCG. The solution provided was that subsidence could be overcome by ensuring a negative pressure within the seam, using high suction at the production well. This would remove the gas immediately and allow for the gradual sealing of the potential leakage (Klimenko 2009). The idea that contact between coal and reactant could be enhanced through gradual subsidence was also taken into consideration. The idea of using multiple injection and production wells in the form of manifolds was also put forth in his patent application (Betts, 1910 – US Patent 947608). The large-scale UCG facilities in the USSR are a standing proof that many of Betts’ design ideas were implementable and realistic when it came to process design for UCG. The USSR has abundant deposits of coal and is utilizing coal for producing gas, which is then used for power generation. In fact, from 1933 until 1964, the USSR had a very active UCG program, with more than 11 successful UCG facilities producing syngas fuel of average calorific value of 900–1000 kcal/m3 (Upadhye et al., 2006). The discovery of vast reserves of oil and natural gas in Russia and countries of the erstwhile USSR led to the gradual atrophy of the UCG program, and the last functioning UCG plant in Kiselevsk, Russia, was shut down in 1996. The Angren UCG facility in Uzbekistan (FSU) is still functional. Over 15 Mt of coal has been gasified through UCG for producing 50 Gm3 of syngas during the last 5+ decades (Shafirovich & Varma, 2009). A picture of the current status and future possibilities of UCG are shown in Figure 1.2. This shows that interest in UCG is growing in all the major coal-mining locations.

FIGURE 1.2 Status of UCG around the globe. (Reprinted from Yang et al., 2016.)

8 Computational Modeling of Underground Coal Gasification

Introduction

9

1.3.1 UCG Process A step-by-step procedure for a typical UCG process is shown in Figure 1.3. The process starts with the drilling of boreholes into the coal seam. One of these boreholes is used as an injection well and other as a production well. After making the boreholes, a highly permeable underground linkage between these two wells is created. The link facilitates the achievement of the significant flow of gas required for economic gasification. In the initial phase of UCG, only oxygen/air is injected into the cavity because a sufficiently large underground cavity at high temperature is desired before the commencement of endothermic gasification reactions. Once the required size of cavity and stable temperature field is obtained, a mixture of oxygen/air, along with steam, is injected. These gases react with coal through combustion and gasification reactions to form products in the form of gases, liquids, and ash. The reactions take place at hydrostatic pressure which increases at ~1 MPa for every 100m depth. The product gases are a mixture of carbon monoxide, carbon dioxide, hydrogen, water vapors, and methane along with nitrogen. The composition of the product mix varies depending on the coal, the oxidant used, reactor temperature and pressure, and gas residence time. The time to start the injection of steam can be decided on the basis of the moisture content

FIGURE 1.3 Steps in the UCG process.

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Computational Modeling of Underground Coal Gasification

FIGURE 1.4 Schematic of UCG process and underground cavity.

in the coal. With time, the cavity grows in all three dimensions as a result of heterogeneous reactions and a phenomenon called spalling. Spalling is thermomechanical failure of coal at higher temperatures, which causes fall of coal/ char particles from the roof of the cavity. Therefore, the UCG process creates a cavity consisting of rubble of coal, char, and ash on the cavity floor and a void space between the cavity roof and its floor. A schematic of the UCG process with its cavity and internals is shown in Figure 1.4. In the process of UCG, a low (4–7 MJ/Nm3) and medium (8–13 MJ/Nm3) calorific-value gas is produced. The quality of the gas produced depends on several factors, including inlet gas composition. It has been observed that the injection of air results in low-calorific-value gas, whereas injecting steam and oxygen leads to an increase in gas calorific value (Daggupati et al., 2011a; Aghalayam, 2010). 1.3.2 Advantages of the UCG Process Some of the advantages of UCG over conventional coal utilization technologies are: • Lower capital and operating costs than conventional mining: UCG involves no transportation costs, no reactor maintenance, and a reduced steam requirement because of the available moisture in coal. The UCG process also requires less equipment, and therefore capital costs are lower. It also does not require coal transport and stocking.

11

Introduction

• Safety issues in traditional mining are not relevant: The use of UCG results in improved health and safety. • Exploitation of unminable coal deposits: Coal resources present deep underground can be exploited, which is otherwise not possible with current mining technologies. Therefore, a significant increase in exploitable coal resources is possible. • Environmental advantages: Gasification is inherently a cleaner process, and in UCG, particulate pollution is also minimized. UCG is expected to reduce dust, noise, and visual impact compared to a power plant, and it does not need facilities for ash and dirt handling. • Possible sequestration of carbon dioxide: There is the possibility of storing the carbon dioxide generated in the UCG process and the further usage of syngas in the UCG cavities. Table 1.3 provides a comparison between surface gasification and underground coal gasification. It shows that UCG, as a process, comes with some TABLE 1.3 Comparison between Surface Gasification and Underground Gasification of Coal Criteria

Surface Gasification

Mining

Needed and cost intensive; limits on depths Required; may involve long distance transport too Higher as control is better Relatively difficult

No need but involves drilling cost; targets unminable resources Not required

Highly controllable More data is available, technology is mature Up to depth of 600 m Converted to H2S and NH3 Huge experience in managing and controlling subsidence Air (dust), noise are relatively more polluted High (specialized coal processing equipment, gasification reactors) High (double) Low Very high

Lesser control Less modeling data is available

Surface transportation Efficiency Ash handling and disposal Control Modeling data Coal resource Sulfur Nitrogen Surface subsidence

Pollution Capital investment

Cost of syngas Worker safety Water consumption

Underground Gasification

Lower due to uncontrollable factors No surface disposal

Below 600 m depth (unminable) Converted to H2S and NH3 May be a big problem

Aquifer water may get affected Low (in-situ coal utilization; several pieces of equipment, including reactors, are needed) Low (half) High Low consumption (natural water influx)

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Computational Modeling of Underground Coal Gasification

challenges as well. The performance of UCG in comparison with surface gasifiers depends strongly on the type of coal seam selected for gasification and the methods used. We will discuss the importance of coal properties and UCG methods in Chapter 2. Here, we provide a comparison between surface gasifier performance and UCG performance at two different sites (Figure 1.5). It shows that UCG performance in terms of the gas produced can be on a par with surface gasifiers if the right type of coal is used with the right type of UCG method. Coals from these two resources are different in their spalling tendency. The coal at Rocky Mountain formed a rubble of spalled coal in the cavity, while the coal at Newman Spinney did not spall. The importance of coal properties also suggests that better models are needed to develop the understanding that would eventually lead to better UCG methods and predictions. 1.3.3 Environmental Impact of UCG The environmental benefits of UCG include production of clean gas, increased efficiency of power generation in gas turbines, and the fact that there is no need to handle coal and ash at the surface (Khadse et al., 2007). The cavity, however, can pose some significant issues regarding its effects on the environment. Some of the major environmental issues are described here: • Groundwater contamination: Gases produced in UCG can diffuse to nearby aquifers, which may lead to groundwater contamination. Therefore, sites should be selected appropriately, and regular inspection of the quality of water from nearby aquifers should also be carried out to avoid any damage (Zeirzer, 2004).

FIGURE 1.5 Comparison of gas produced from different types of surface gasifiers and UCG of two different types of coals. (Adapted from Perkins et al., 2005.)

Introduction

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• Surface subsidence: As the cavity widens, the probability of roof collapse increases, which may result in subsidence. The subsidence will depend on mechanical properties and thermal stresses induced because of high temperatures in the cavity (Zeirzer, 2004). Appropriate site selection, the right operating conditions, effective process control, timely monitoring and inspection of the nearby environment, and suitable shutdown procedures can minimize the environmental impacts of the UCG process (DTI review, 2004).

1.4 Computational Modeling of UCG Computational modeling is used to translate complex problems from the field into tractable mathematical formulations which may be solved using digital computers and numerical methods. The models require extensive computational resources to solve complex sets of mathematical equations in order to study the behavior of the system under consideration. These mathematical formulations in terms of multiple equations can provide different insights, responses, and guidelines after theoretical and numerical analysis. They enable a thorough understanding of the system under consideration and potentially help in terms of better design and control. Typically, mathematical models contain governing equations, constitutive equations, and some initial and boundary conditions, along with some constraints. UCG is a complex process involving various phenomena at different zones in the UCG cavity. UCG being an underground process, it is very difficult to make experimental measurements or direct observations. Therefore, a computational model is required to understand the critical aspects of UCG and to identify the factors which have greatest impact on the overall performance of a UCG reactor. These mathematical models are based on fundamental laws of conservation of energy, mass, and momentum – coupled with submodels for kinetics, spalling, flow patterns, and different material properties. The complex interaction between and within various zones in the UCG reactor must be included as an important part of these models to make them more robust and complete. To develop a modeling framework for simulating UCG, the specific objectives may be formulated as follows: • Develop mathematical models representing various physicochemical processes occurring in UCG. • Develop new insights using computational models along with possible experiments. • Develop useful guidelines for realizing UCG in practice and for further optimization.

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Computational Modeling of Underground Coal Gasification

Several researchers have developed models for the UCG process that are at different levels of robustness and computational speed. The initial modeling efforts made during the 1970s and 1980s were focused on developing simple ways to predict the composition of the product gas from a UCG operation. However, as the saying goes, ‘Make it simple, but not simpler’; these models based on equilibrium and correlations were not able to capture the complex interaction between different phenomena and the scale factors. Even the semi-empirical, partly theoretical lumped-parameter models (Thorsness and Creighton, 1983; Upadhye, 1986; Upadhye et al., 2013) required some user-set adjustable parameters for determining the exit-gas composition with some degree of accuracy. The composition of the product gas from the UCG cavity is a function of the spatial and temporal evolution of the reactive flow, its interaction with reactive and nonreactive solids, and the thermal profile that is created due to endo- and exothermic reactions, heat transport, and drying. Another reason for the failure of simple equilibrium models is that the different reactions do not take place at same temperature or the same location. Therefore, the time available and the temperature conditions for reaching equilibrium composition are different for different reactions. For example, pyrolysis can take place at a lower temperature, and therefore the volatiles evolved at relatively lower temperature in the outflow channel cannot affect the equilibria of reactions that happened earlier near the reaction front. The addition of steam in the product gas while it flows through the outflow channel affects the species composition due to a water–gas shift reaction and takes it away from the expected ratios calculated by the equilibrium composition at cavity temperature. In addition, the catalytic effects of ash change the relative rates of reactions and further affect the exit-gas compositions (Camp, 2017). The major modeling efforts from literature have been summarized and discussed in various review articles (Greg and Edger, 1978; Shafirovich and Varma, 2009; Bhutto et al., 2013; Khan et al., 2015; Aghalayam, 2018; Perkins 2018). The available models can be broadly classified as follows [based on the recent review by Perkins (2018)]: • Packed bed model – To estimate the amount of syngas generated and its composition – e.g., Thorsness et al. (1978). • Channel model – To estimate the amount of syngas generated and its composition – e.g., Dinsmoor et al. (1978). • Coal block model – To model physics of block-gasification (lab-scale validation) – e.g., Tsang et al. (1980). • Computational fluid dynamics (CFD) model – To obtain the flow, concentration and temperature profile resulting from transport and reactions inside the cavity and to estimate the syngas properties and cavity growth dynamics. Typically, these models attempt to model all the regions of UCG cavity simultaneously. Reactive-flow

Introduction

•

•

•

•

•

15

modeling for a commercial-size UCG cavity requires significant computational power due to its large size. – e.g., Kuyper et al. (1994), Shirazi et al. (2012). Multizonal model – To study the detailed interactions among and within different zones of the UCG cavity for determining flow patterns, transport, and reaction. These models use a ‘divide and conquer’ strategy to simulate the complete UCG cavity and then estimate product syngas properties and cavity growth dynamics. While dividing UCG cavity into multiple zones, these models use relatively simpler but effective strategies for resolving each of the zones. – e.g., Samdani et al. (2016). Resource recovery model – To estimate the amount of coal conversion and syngas generation along with its quality – e.g., Thorsness and Cena (1983). Reservoir model – To determine well configuration and understand the effect of field-scale phenomena related to geological features – e.g., Seifi et al. (2011). Geomechanical model – To study the impact of UCG on mechanical and geological properties of the strata in nearby fields to understand stability and surface subsidence issues – e.g., Trent and Langland (1983). Coupled reservoir and geomechanical model – To study the combined effects of field-scale phenomena for well design and stability analysis, and to design an integrated UCG process for optimal resource utilization at high efficiency.

This list of modeling approaches along with their primary objectives provides an understanding of the different aspects of the UCG process. One may also observe that these models involve phenomena at different scales of time and length. The vastness of these scales also indicates that a given UCG model would be able to include only the phenomena that happen in a smaller subset of this range. Therefore, these models include assumptions for some of the subscale phenomena that are essential for model development. The earliest models of UCG in the literature include models that describe process as a packed bed reactor. These models would consider the coal seam as a highly permeable porous media in which bed properties change as reaction proceeds. Most of the packed bed reactor models do not include homogenous reactions, diffusion effects, wall transport, etc. On the other hand, the coal seam has also been considered to be similar to a wet slab in coal block models. These models describe the process by governing the movement of various defined regions in a coal slab. These sections include gas film, ash layer, char region, dried coal, and virgin coal. There is an influx of injection gases towards the cavity wall, while there is a counter flux of steam and pyrolysis products from the wall to the cavity. Tsang (1980) was the first to

16

Computational Modeling of Underground Coal Gasification

use this approach and developed a one-dimensional unsteady UCG model. However, the applicability of even the most recent coal block models is limited because of unphysical assumptions such as constant temperature and fixed gas composition at the boundary. The UCG process can also be modeled with a channel approach. In this approach, the coal seam is represented by an expanding channel and air or oxygen flows down the central channel and is transported by turbulent flow to the boundary layer along the channel wall. The oxygen diffuses through the boundary layer to the solid surface and reacts. Dinsmoor et al. (1978) developed a steady-state model to describe processes in a UCG channel. It was concluded that the operation of UCG in the channel regime is undesirable and would lead to a decrease in gas heating value due to the bypass of oxygen. Applicability of these models is limited to the nonspalling coals; however, significant spalling has been observed to be an important factor for the UCG process. Most of the modeling efforts reviewed here so far have the drawback of focusing on some aspects of the UCG process while neglecting others. During UCG trials in the United States, the Lawrence Livermore National Laboratories (LLNL) funded many modeling studies of the UCG process. Several one-dimensional models were developed and validated with certain series of data from field trials. These segregated models were later combined to form the CAVSIM process model, which represents 15 years of continuous UCG research and development in the United States (Thorsness and Britten 1989). The model can be used to predict the lateral cavity growth of thick and shrinking coal seams in which oxygen is injected at the bottom of the coal seam. The model is based on some major assumptions, such as that the cavity is axisymmetric around injection; thermal radiation is the main heat transfer mechanism in well-mixed void space; cavity growth is dominated by thermomechanical failure of wall; and a packed bed of char and rubble forms over a thin layer of ash. This description also shows that the most recent models do not provide a complete description of the UCG process. Recently, some integrated UCG models which describe most important aspects of the complete UCG process have been presented in the literature. Those include resource recovery models, reservoir models, and CFD and CFD-based multizonal models. These models aim to achieve good forecasting capabilities for coal conversion rates, transient cavity evolution in three-dimensional space, syngas quality and quantity, environmental impacts, and geomechanical impacts. In this book, we focus more on the development of resource recovery models that are based on chemical reaction engineering (CRE). The resource recovery models are crucial from the point of view of providing the confidence required for commercial deployment of the UCG technology. Currently, the application of UCG and interest in its development is rather low in comparison with the current growth of energy requirements. For the widespread commercialization of UCG, greater investments in research and development (R&D) activities focused on the development of computational

Introduction

17

predictive tools are required to overcome the challenges presented by the design of a commercial UCG reactor. Some of the potential roadblocks for commercialization of UCG that can be addressed by using appropriate models (along with experience from various UCG trials) are as follows: • UCG is a technically feasible process for several coal resources; however, the number of suitable coal seams and deposits are limited because of adverse geologic and hydrologic conditions. If UCG sites are not selected properly, UCG can result in lower rates of coal conversion of low-quality gas, aquifer contamination, and subsidence of overburden. Models that calculate the coal conversion rates and yield of syngas and consider the geological aspects would be useful for proper site selection. • The low controllability of UCG compared to surface gasifiers is a big limitation. Controlling the rate of water influx, the gas flow, and spalling rates is very difficult. In addition, there are very few adjustable design parameters when compared with conventional gasification, given that the product is the same syngas in both cases. Being an inherently unsteady-state process, both the quality and quantity of the exit gas vary with time. A robust transient model that includes all the dynamics effects of cavity size, flow patterns, spalling, and so on would be useful in finding decision variables and their effect on process performance. • Obtaining reliable data is a difficult and expensive process, which does not allow for an understanding of the fundamental processes that occur during UCG. The application of validated models to generate more data and insights is essential. • The unavailability of comprehensive modeling solutions for all the stages of UCG for all kinds of coals hinders reliable predictions for new coal mines. A lack of reliable field-scale data makes it difficult to validate models and strategies. Therefore, there is a need for better tools and modeling frameworks and comprehensive process models for UCG which can provide better insight and guidance for enhancing performance. • The other potentially important, albeit nontechnical, hindrance to UCG commercialization is public perception, as the technology is relatively little known to the general public. In view of these roadblocks to the commercialization of UCG, further research is necessary to reduce technical uncertainties and build confidence about this promising technology. Conducting comprehensive field trials is not possible in all cases, and there is a need to develop mathematical models which can best capture the various aspects of UCG. In the case of UCG, the

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Computational Modeling of Underground Coal Gasification

TABLE 1.4 Key Factors to Consider in Modeling of Underground Coal Gasification Design Parameters

Operating Parameters

Coal seam: depth, thickness, dipping angle – presence and absence of aquifers/ rock formations Coal properties: physical and chemical

Wells and reactor conditions: temperature and pressure within the wells and the seam Flow rates of reactant and product gas

Reactor and well configuration Hydrology and geology of site selected

Oxygen to steam ratio in reactant gases Gas and pollutant concentration in water bodies (aquifers) nearby

Performance Parameters Product gas: quality and quantity, calorific value of product gas Resource recovery factor: product gas recovered / kg of coal or /m3 injected gas Cold thermal efficiency of gasification Profit post operation/cost / benefit ratio

entire coal seam and its surroundings, which include layers of rocks, soil, and water in the form of aquifers, behave as the reactor. The complex design of this reactor, of which much less can be determined, in combination with several physical, chemical and mechanical phenomena, makes the modeling of UCG challenging. The models developed for this purpose need to incorporate all the necessary details of the various operating phenomena, with an overall outlook on the ability to be able to monitor, predict, and control the performance of the process in a timely manner. Any mathematical model studying the kinetics of coal gasification must make judicious calls on delimiting the surface and internal reactions and ensure separate handling of the combustion and reforming. The model must be able to explain the effects of at least some select parameters that are based on the inhomogeneities, such as mineral content, water, and other discontinuities in the coal seam, that might occur in an underground coal seam. Multiscale modeling is essential for realistic simulations of UCG. The developed model should consider some of the important factors that affect the overall process of UCG (Singan, 2018). Table 1.4 provides a list of these key factors, classified as design, operating, and performance parameters.

1.5 Organization of This Book In this chapter, an overview of the UCG process and its historical development are provided with a global energy perspective. We also discussed the role of computational modeling in the development of UCG as a commercial process. The computational modeling techniques and their limitations are also discussed with emphasis on the requirements for a complete UCG

Introduction

19

FIGURE 1.6 Scope of this book.

model. The scope of this book is shown schematically in Figure 1.6. The book primarily focuses on the development and application of computational models for simulating the behavior of a UCG reactor. Models describing the behavior of near and far-field UCG reactors are beyond the scope of this book. Similarly, system-level issues like site selection, site development, drilling, utilization of product gases, and end-of-life management of UCG sites are not discussed in this book. Almost all the computational models discussed in this book need to be solved numerically using digital computers. However, numerical techniques and computer implementation of these methods are not discussed in the book. The discussion is focused on the development of a multilayer modeling strategy to simulate UCG and other relevant aspects such as identifying, estimating, or measuring required input data for these models and designing laboratoryscale experiments for calibration and validation of the computational models. A brief description of the contents of the book is as follows. Chapter 2 of this book discusses the fine details of the UCG reactions and the physical processes involved, along with the role of important coal characteristics. The state of the art of UCG, including various UCG technologies, is explained, with a description of various field trials from different parts of

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Computational Modeling of Underground Coal Gasification

the world. After this introductory part, we discuss the pre-requisites of the computational modeling of UCG in Chapters 3, 4, and 5. Among these, Chapter 3 provides a brief review of coal characterization and cites relevant references to the important physicochemical properties related to coal, injected stream, and generated gas. Chapter 4 is about the kinetics of coal gasification reactions that includes pyrolysis, gasification, etc. This chapter discusses various experimental techniques, their results, and their application for the development of kinetic models to estimate the relevant kinetic parameters. The discussion of kinetics is useful for selecting appropriate models and the influence of bed and particle diffusion effects. Laboratoryscale UCG experiments are discussed in Chapter 5. These discussions involve features of cavity in postburn observations, the composition of the gas produced, factors affecting the performance at experimental scale, and the development of correlations for cavity size and growth rates. This experimental data may be used subsequently for the validation of developed models. In this chapter, we also include a discussion on experiments performed for characterization of spalling behavior in UCG-like conditions. In the next part of the book, we discuss the computational modeling details in Chapters 6, 7, and 8. Chapter 6 covers the overall approach to developing a computational model for UCG along with the role of various important factors, such as thermodynamics, reaction kinetics, flow patterns, transport, and spalling. At the end of this chapter, we provide a classification of the available models for UCG and discuss the features of the same. Chapter 7 takes a deep dive into the model classes and describes the details of thermodynamic model and reaction engineering models. The discussion on thermodynamic models involves formulation of the model, solution strategies used, and modeling results from different literature models. We also compare the model results with the observations from the pilot tests in order to understand the further application of these models. Later, we discuss the class of reaction engineering models like the packed bed model, channel model, and coal block model, along with their major assumptions, model equations, and results obtained. Chapter 8 discusses CFD modeling of UCG along with its application in the development of a multizonal UCG process model based on the compartment modeling approach. In this chapter, we present gas and solid-phase balance equations for each of the zones in the given multizonal model. This chapter also includes a brief discussion on the modeling of the UCG field, that is, integrated models for well layout and far-field simulations. The combination of reaction engineering–based resource recovery models and geomechanical models leads to some comments on future overall system optimization. The last chapter (Chapter 9) summarizes the discussions and provides some suggestions for future modeling efforts to help achieve the successful commercialization of UCG.

2 Underground Coal Gasification: State of the Art This chapter presents a review of the literature on underground coal gasification (UCG) field trials and fine details of various aspects including reactions and physical properties and their relevance in process modeling. The chapter starts with a summary of important aspects of UCG, like coal gasification reactions, spalling, and flow patterns in Section 2.1. Later, Section 2.2 discusses the status of UCG technology and different methods of UCG with their development history. Section 2.3 is a review of the performance of UCG observed during several field trials. We discuss the U.S. field trials in more detail, as the available literature on these trials has advanced the understanding of UCG in a major way. The status of UCG around the world is discussed briefly along with the importance of coal properties for UCG performance. The state of the art is summarized at the end.

2.1 Underground Coal Gasification In the UCG process, the coal seam is exposed to reactive gases and high temperatures. The coal gets consumed to generate gaseous products through a sequence of processes and heterogeneous reactions including heating, drying, pyrolysis, combustion and gasification. A distinctive characteristic of the UCG process is that every coal particle goes through this sequence of processes and reactions, albeit at different instances. Achieving successful UCG involves difficult challenges because of several less known or unknown factors that govern the process deep underground in the cavity. These factors include the coal seam uniformity, the effect of coal properties and surrounding strata on UCG, the desired parameters for optimum performance, and factors affecting these desired parameters. These unknowns can affect the process control and stability of the UCG process. Ongoing research in the field of determining the role of coal properties in the UCG process and capturing this via mathematical models and simulators is providing newer insights and tools for better design and engineering of UCG. In this section, we discuss the different processes that occur underground inside the UCG cavity. 21

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Computational Modeling of Underground Coal Gasification

2.1.1 Drying and Pyrolysis Figure 2.1 shows a typical UCG cavity and a zoomed-in section of cavity wall showing the different zones formed during coal gasification. Inside the coal seam, the dry zone which is next to the gas film experiences reactions like pyrolysis and gasification, while drying of the wet coal occurs at the drying front. Drying essentially occurs because of high temperatures and exposure to unsaturated gas, resulting in steam and dry coal. Evaporation of moisture from the coal matrix requires substantial energy, and it must be supplied by heat transfer from the reaction zone. This transport of heat and conversion of moisture to steam is a complex process and it depends on operating conditions as well as the characteristics of the given coal. The complexity of drying increases further if the process is accompanied by shrinking and/or cracking in the porous material. If coal is heated further to a higher temperature in the range of 200ºC– 900ºC, it results in the release of volatile gases from the porous coal matrix. The coal heating rate in UCG is much lower than other surface gasification technologies, and it typically ranges over 1–10ºC/min. The phenomenon of the release of gases from coal when it is heated in a reducing atmosphere is called pyrolysis. Coal pyrolysis is a complex process that includes chemical and physical changes in the coal matrix. During pyrolysis, coal gets converted into lighter products including volatile matter and solid char. Volatile matter from the coal decomposes into tar and other gases. Tars are the highmolecular-weight hydrocarbons which are vapors at UCG cavity temperature but get condensed if exposed to normal temperature and pressure. The low-rank coals have high moisture and high volatile content, and therefore

FIGURE 2.1 Schematic of UCG and different regions in coal seam.

Underground Coal Gasification

23

drying and pyrolysis may lead to the formation of a highly porous and permeable char matrix. Pyrolysis may also be accompanied by shrinking of coal, resulting in a further increase in porosity and weakening of the coal, which may lead to failure of the material. The increased porosity and shrinking of coal are the potential causes of widening of cracks in the coal bed/seam, and this can result in the mechanical failure of coal and its breakdown into small particles/pieces (Bhaskaran et al., 2015). In addition, the volatiles released during pyrolysis are combustible and they assist in burning of coal. Due to lower heating rates during the UCG process, the volatiles are also generated at a lower rate, which means that it takes a longer time for the tars to get cracked into lights. However, the lower temperature of the pyrolysis region in the underground cavity does not let the heavy tars become completely cracked. The uncracked heavy hydrocarbons in the tars may lead to environmental issues related to the pollution of nearby aquifers (Camp, 2018). 2.1.2 Chemical Reactions Once the pyrolysis is complete, what remains is the porous carbonaceous solid char, which contains carbon and mineral matter (ash) in it. Finally, this char reacts with the injected/pyrolysis gases to generate syngas. The reactivity of different gases, the internal surface area of the coal, the ash content, and its composition determine the rates of different gas–solid reactions for a given coal sample. In addition to combustion and gasification of coal/char, homogeneous reactions such as gas-phase oxidations of combustible gases and water– gas shift (WGS) reactions also occur inside the UCG cavity. Table 2.1 provides a list of main reactions occurring underground in the UCG cavity. In addition, tar cracking and reforming of other hydrocarbons may also take place. The overall outcome from UCG is a function of competing rates of intrinsic reactions and mass transport, and it also depends on the time available for the reactions. The ash content and composition of the ash has been observed to affect the rates of these heterogeneous reactions due to catalytic effects. As explained in Chapter 1, spalling is an important phenomenon that detaches coal particles from the seam. A more detailed discussion on spalling and its quantification is available in Sections 2.1.3 and 5.5. In UCG, whether spalling takes place or not, the conversion occurs in relatively large-sized particles/blocks of coal present on the floor of the cavity or at the roof. It is therefore necessary to investigate the kinetics of individual reactions separately on a relatively big block or a monolith under different conditions and to analyze the role of diffusion and conduction in the determination of overall reaction rates. There are only a few studies reported in literature which throw light on reactions in bigger monoliths under UCG-like conditions. Wellborn (1981) and Poon (1985) performed combustion experiments on small Texas lignite coal monoliths at atmospheric pressure. It was observed during these experiments that much of the oxygen consumption takes place in the gas-phase via homogeneous reactions with combustible volatiles leaving the pyrolyzing

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Computational Modeling of Underground Coal Gasification

TABLE 2.1 Main Reactions Occurring in UCG No.

Reaction Name

Reaction Equation heat

1.

Drying

H2 O( l ) Þ H2 O(v )

2.

Pyrolysis

CHa O b Þ a1C + a2 CH 4 + a3 H 2 O +a4 CO + a5 H 2 + a6 CO 2 + a7 C9 Hc

3.

Char combustion

C + O 2 Þ CO 2

4.

Steam gasification

C + H 2 O Þ CO + H 2

5.

Boudouard reaction

C + CO 2 Þ 2CO

6.

Methanation

C + 2H 2 Þ CH 4

7.

Water–gas shift reaction

CO + H 2 O Û CO 2 + H 2

kf ,p

k f ,1

k f ,2

k f ,3

k f ,4

k f ,5

H2 + 8.

Gas-phase oxidation

k f ,6 1 O 2 Þ H2 O 2

CO +

k f ,7 1 O 2 Þ CO 2 2 k f ,8

CH 4 + 2O 2 Þ CO 2 + 2H 2 O 9.

Tar reforming reaction

cö æ C9 Hc + 9H 2 O ® 9CO + ç 9 + ÷ H 2 2ø è

coal block, and not in the direct combustion of the char. Forrester (1979) performed devolatization and gasification experiments on larger cylindrical coal blocks and calculated the drying front velocity into the coal monolith. These experiments can be considered to be a better representation of the actual conditions in underground coal gasification. Experimental studies of relatively larger coal blocks have also been reported by some researchers (Mai et al., 1985; Yeary and Riggs, 1987; Thorsness and Hill, 1981; Hill and Thorsness, 1982). Massaquoi and Riggs (1981, 1983) have successfully developed a onedimensional (1-D) numerical model for drying and combustion of a wet coal slab. Under oxidizing conditions, the carbon conversion was proved to be controlled solely by gas-phase mass transfer from the bulk of the gas to the exposed coal surface. Tsang (1980) described a model for coal block/slab combustion with drying and devolatization. Perkins (2005) developed a 1-D mathematical model for a semi-infinite block of coal, and it was later used as input for simulation of the UCG process. The common feature of these experimental and numerical studies is that the experimental operating conditions do not represent the conditions inside the UCG cavity. The major difference between UCG conditions and other

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25

gasifiers is the coal/char particle size, which is much larger in the former case. Furthermore, every coal is different, and determining the kinetics of heterogeneous reaction becomes an important step in UCG process modeling. There have been some recent studies (Gomez and Mahinpey, 2015, Mandapati et al., 2012) on unification of kinetics for all coals; however, a lot remains to be done. Gomez and Mahinpey (2015) have shown that the internal surface area and the alkaline content in ash composition are the most significant variables influencing the gasification rate. Mandapati et al. (2012) also showed the presence of the catalytic effects of ash content. They found the same activation energies for CO2 gasification of demineralized chars obtained from different coals. In Chapter 4, all the important reactions and procedures to determine kinetic parameters are discussed in more detail. We also discuss the catalytic effect of ash and char on coal gasification and tar reforming reaction in Chapter 4. The reactions taking place in the UCG cavity cause the expansion of cavity, which is discussed in the following subsection. 2.1.3 Cavity Growth In UCG, the cavity grows mainly as a result of two mechanisms: chemical reactions and spalling (thermochemical and thermomechanical failure). The cavity may also grow due to sidewall/roof collapse; however, this is not desirable because of possible blockage of the gas flow. Spalling is the falling or detachment of coal blocks or particles from the heated coal surface. Many lignite and coal blocks, if heated, start showing a tendency of spalling, i.e., thermomechanical failure of the coal at high temperature. This tendency of coal blocks is also observed in cases where the coal seam is above the underground gasification cavity formed during UCG. At high temperature and with a high degree of pyrolysis, the coal particles from the coal seam above the cavity get affected in such a way that they start falling onto the floor of the cavity. These spalled particles react with the gases at high temperature to form combustible gases. The phenomenon of spalling is beneficial for the UCG process as it increases the resultant surface area for reaction and ensures better contact between the coal and reactant gases. These favorable effects lead to a faster consumption of coal and therefore the growth of the UCG cavity, which eventually increases the utilization factor for the given coal seam. However, determining the underground conditions required for spalling and the rate of spalling for a given coal is a difficult task which may require large-scale experimentation. In addition, the inclusion of spalling in UCG process models requires the model to be dynamic with discrete events, which is a challenging thing to achieve. Hettema et al. (1996) studied fundamental aspects of the thermal spalling of rock. Their study is of relevance to UCG as the analysis can be extended to thermomechanical failure of coal during UCG. Based on the analysis of the spalling of rocks, the effect of internal steam pressure on the spalling characteristics was determined. They performed controlled experiments with

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Computational Modeling of Underground Coal Gasification

temperatures up to 400–1200ºC, which are relatively high compared with those applicable to spalling. Further, they also showed that the failure of rock is caused by increased steam pressure in the internal pores and development of high thermal stresses. Similar factors may be responsible for the spalling of coal with low porosity, though the spalling tendencies of each type of coal may be different. Hence, further investigations in this direction are necessary. A two-parameter model for coal spalling, affected by the development of thermal stresses, has been used in cavity modeling for the UCG process (Britten, 1986). Another model, developed by Perkins and Sahajwalla (2006), has tested the effects of coal spalling on cavity growth dynamics. On the other hand, Park and Edgar (1987) have developed a 1-D model for initial growth of UCG cavity where they did not include spalling as a parameter for cavity growth. Around the same time, a two-dimensional extension that includes spalling and wall recession was developed by Thorsness and Kang (1986). Upadhye et al. (1986) performed experiments to study coal spalling by suspending a coal block in a hot stream of inert gas while monitoring the weight change due to spalling and pyrolysis. During these experiments, they did not observe a significant amount of spalling. Recently, several experiments have been performed by Bhaskaran et al. (2015), which show significant spalling of Indian lignite. Spalling rates in this study were too high to observe any effect of variation in spalling rates on the performance of laboratory-scale UCG experiments. The importance of spalling, heat and mass transport, flow patterns, and kinetics for UCG process modeling are discussed extensively in Chapter 6.

2.2 UCG Technologies UCG technology involves drilling two wells into the bottom of the coal seam and connecting the two by an underground permeable channel. The permeability of coal in its natural state is usually very low as the coal is formed from layers of carbonaceous material under great pressure. Gasification under the natural conditions of lower permeability is difficult and uneconomical. The necessity of linking the production and injection wells within the underground coal seam is one of the major challenges of UCG technology. There are several methods for performing UCG and linking the wells. Some of them are relatively inexpensive technologies which are used extensively. • Early methods of UCG • Chamber method • Stream method • Blind-borehole method • Small-bore method

Underground Coal Gasification

27

• Linked vertical well method • Reverse combustion linking method • Hydraulic fracturing method • Steeply dipping beds method • Controlled retracting injection point (CRIP) method • Long tunnel system method A review of UCG methods from early experiments is given by several authors including Olness and Gregg (1977) and Klimenko (2009). The methods used during U.S. UCG trials have recently been reviewed with their experimental field conditions, plans, and outcome by Camp (2017). 2.2.1 Early Methods of UCG An early Soviet program started in the 1930s, wherein large chambers were constructed underground to gasify waste and slack coal from three coal basins: Moscow, Donetsk, and Kuznetsk. This was a way to utilize coal in an economically efficient and safer way as it does not involve the labor costs associated with conventional mining and it does not put laborers at risk of fires in the mine. The first UCG trial in the USSR was started in 1933 by separating a 10m × 10m section of coal seam using brick walls (Figure 2.2). In this ‘chamber method’, air was supplied from one side of the chamber and the produced gas exited from the other. The method was improved with each experiment. The improvements included the use of significant instrumentation to monitor and control the process. During these early trials, the average calorific value obtained from a steady trial was 1300 kcal/m3. Based on these experiments using the chamber process, Klimenko (2009) argued that the instabilities during these experiments were due to the fragmented combustion zones separated by colder areas. The location of the combustion zones could not be predicted, and therefore the chamber process was not a repeatable experiment. The chamber experiments were judged to be unsatisfactory, and the next set of experiments was performed using a series of boreholes. Because of the number of boreholes, the permeability of coal increased significantly; however, air nozzles also needed to be connected to each of the boreholes. The borehole experiment was observed to be repeatable and produced stable combustible gases in a small amount of time. Another group at DUKhI (Donetsk Institute of Coal Chemistry) suggested a different method for UCG based on gasifying coal in a channel. Before suggesting the new method, they studied the steam gasification of coal blocks extensively in laboratory experiments. They observed that the temperatures attained in laboratory-scale experiments are much lower. On the other hand, the temperatures can be much higher in the UCG process due to lower heat losses, and it has been observed to be advantageous for producing higher heating value gas. The method they developed was called the

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Computational Modeling of Underground Coal Gasification

FIGURE 2.2 Chamber experiment at (a) Krutova mine and b) Lisichansk mine. (Reprinted from Olness and Gregg, 1977, with permission from LLNL.)

Stream method, and the process involved mining two shafts into the coal seam and a drift shaft between them. Boreholes in one of the shafts would be used for injecting oxidant and the syngas would come out from boreholes in the second shaft. Figure 2.3 shows a depiction of the DuKhI stream gasification method for a steeply dipping seam. The pilot scale operations under the leadership of DuKhI also found that the control of subsidence and locating

Underground Coal Gasification

29

FIGURE 2.3 The Stream method for gasifying coal in steeply dipping beds as proposed from DUKhI. (Reprinted from Olness and Gregg, 1977, with permission from LLNL.)

the fire-face was very difficult in large-scale operation. The blind borewell method and the small-bore method were also used in the USSR for some of the trials. The importance of trials and tests in the USSR is in their innovations in producing the first successful experiment in UCG at both laboratory and pilot scales, producing syngas with a calorific value in excess of 2000 kcal/m3. 2.2.2 Linked Vertical Wells (LVW) Methods This method was mainly developed on the basis of several years of UCG experience in the USSR. As discussed earlier, UCG experiments in the USSR were focused on linking the injection and production well in some way so that the coal permeability increased. Several methods were used to link the two vertical wells, including reverse combustion, electric resistive heating, hydraulic fracturing (hydrofracking), and directional drilling. Hydrofracking is the process of artificially fracturing the coal seam by injecting a liquid into the seam under very high pressures via a borehole, which is lined with tubing and cemented into the part lying outside the seam. As the liquid penetrates, it produces fractures in the seam. When this operation is complete, the liquid is withdrawn and most of it is recovered. The final phase consists of the addition of sand to the jelly, which is then deposited into the fractures. After the jelly is drawn off, the fracture remains open. In the reverse

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Computational Modeling of Underground Coal Gasification

FIGURE 2.4 A scheme of linked vertical wells methods as used during UCG at Shatskaya, USSR. (Reprinted from Olness, 1981 as adapted from Borovicka, 1959, with permission from LLNL.)

combustion method, the link between the process wells within the coal seam results from the reverse propagation of the combustion zone. This leads to the formation of a low permeability path between the injection and production wells. The burn recedes against the flow by conduction, producing channels with loose char and rubble. Figure 2.4 shows a typical array of wells and sequence of the operations as used at Shatskaya in the Moscow basin. This method has been used to produce a link at medium depths connecting wells up to 20–50 meters apart. However, reverse combustion does not give control over the path taken by the link and must be established at high to moderate depths. The lack of control produces undesirable gasification geometries. Other limitations of the LVW concept include the need for highly permeable coal seams and the slow process involved in linking the wells. Reverse combustion works well in nonswelling coals; in swelling coals, it works only when preceded by hydraulic fracturing (Edgar and Gregg, 1978). 2.2.3 Steeply Dipping Coal Seam Method The LVW method of UCG works well for coal seams that are almost horizontal. However, many coal seams are not horizontal, and some dip at significant angles. The application of the typical LVW method is difficult for these coal seams, and therefore USSR scientists developed an alternate method. In this method, the injection borehole is inclined and drilled from the surface

Underground Coal Gasification

31

to one of the coal seam strata to provide access to the overlying coal seam. The production borehole is drilled in the overlying seam, and that serves as a production well. The char produced from pyrolysis of coal near the injection point is converted into hot gases. These hot gases move upward through the coal seam and cause drying and pyrolysis of this coal. The coal spalling and these reactions together provide a continuous flow of hot gases for pyrolysis of overlying coal, and that results in a steady process. Therefore, the process of UCG in steeply dipping coal seams happens to have high thermal efficiency in comparison with the UCG of coal in flat beds. Due to the natural flow of hot gases and spalling, the cavity growth occurs mostly in the upward direction. The interesting part of this method is that the upward growth of the cavity produces least effects on the overburden. Therefore, this method shows high rates of coal consumption and low rates of thermal loss to overburden roof rock. This method was successfully used at the Rawlins site in the United States. Figure 2.5 shows a pictorial comparison between the LVW and steeply dipping coal seam methods. The gas quality obtained in the steeply dipping coal seams has been observed to be very high, possibly due to the availability of a large surface area for reactions. The high calorific values of the gas produced during the steeply dipping Rawlins I and Rawlins II tests demonstrate this. The steeply dipping coal seam method is limited to the steeply dipping type of coal beds, and multiple inclined injections may be necessary due to the creation of low permeability zones near injection zones. In addition, this method also has the potential drawback of gas losses, which makes the process highly challenging, if not impossible. The potential of gas loss from the underground cavity may lead to the transport of the same to the surface, eventually creating problems in the commercialization of the process. The reason for the high chance of gas escape in case of the steeply dipping coal seam method is related to the high-pressure gradient in the cavity. During the Rawlings test, the vertical distance between the injection and production wells was 18 m. This means that there was a very high pressure difference between these two points. Traditionally, a lower underground cavity pressure is recommended during the UCG process for safer operations. The cavity pressure at the highest elevation must be kept lower than the pressure of the water-saturated surroundings. This prevents the escape of gas pollutants as well as the escape of other contaminants into the groundwater. These pressure requirements are difficult to maintain for cavities with larger vertical extent or in the event of a very fast vertical cavity growth rate. If low pressure is maintained at the top of a vertically elongated cavity, it helps to stop gas escaping from the cavity; however, it also increases the water influx at the bottom of the cavity. Due to large water influx, the efficiency of the UCG process is adversely affected. The large water influx from the surrounding strata reduces the water pressure in the surrounding strata. This further reduces the pressure at which the cavity can be operated without getting flooded with water influx. If we operate the cavity at a higher pressure

32

Computational Modeling of Underground Coal Gasification

FIGURE 2.5 Schematic of (a) linked vertical wells and (b) steeply dipping mines, showing the difference between the two methods.

to avoid excessive water influx, the gases may start escaping from the overpressurized cavity top. Therefore, a clear strategy is required to solve this complex problem before going for the steeply dipping coal seam method. The complications can also be avoided by choosing an appropriate UCG site. 2.2.4 Controlled Retracting Injection Point (CRIP) Method After the USSR’s exploration of UCG methods, another impressive UCG program was conducted in the United States from 1973 to 1989. These efforts have significantly upgraded the methods of operating a UCG process and

Underground Coal Gasification

33

they have produced several successful UCG pilots. The initial trials at Hanna, Wyoming, have led to stable production of high heating value gas for several months. The success of UCG in the initial experiments made Hanna an attractive site for UCG, leading to several successful experiments, for example, Hanna I, II, III, and IV, and later Rocky Mountain I. These tests have used several new methods of performing UCG, including the CRIP technique. The CRIP technique is arguably the most important, vastly useful, and highly impactful outcome of the U.S. UCG studies. Figure 2.6 shows a typical schematic of the CRIP method and its working principle. The team at Lawrence Livermore National Laboratories (LLNL) in California, the United States, understood the importance of keeping the injection point lowest in the coal seam during the Hanna test and therefore wanted to develop a new method for UCG that ensures the same. The early thoughts were also influenced by the possibility of developing a method which can start creating successive new underground cavities in a controllable manner. A directionally drilled borehole in the coal seam with provision for injecting inlet gas at different points along its length solves both these issues. The importance of this method lies in the development of a controllable UCG process that does not require new injection wells for converting a large amount of coal resource. It practically extends the process from one location to another for converting more coals while using a single injection well. This new CRIP method promises to ensure a low injection well in the coal seam and the least interaction with the roof, thereby resulting in an efficient UCG process. As the name suggests, the CRIP method involves controlled retraction of the injection point that involves direction drilling.

FIGURE 2.6 Schematic (a) horizontal and (b) vertical cross-sections of three UCG reactors developed by the CRIP technology. (Modified from Nakaten et al., 2014.)

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The controlled retraction is achieved by carrying the injection gas through a liner, which can be inserted through the injection well till it reaches near the intersection point with the production well. As the cavity starts growing, it eventually touches the roof of the seam, and a considerable amount of heat is lost to the roof, which results in a decrease in the exit gas quality. This makes it very difficult to carry out an efficient long-term UCG operation. Therefore, the location of injection well has to be changed in order to take the injected gases in more-reactive zone to obtain higher efficiency. At this point in time, the injection point is retracted to an upstream point by the operators on the surface. The coal near the new injection point then starts reacting, resulting in the growth of a new cavity. A higher calorific value of exit gas is achieved with increase in rate of consumption of coal and reduced heat losses. CRIP can be considered a suitable option for UCG operation as the technology is robust and offers the possibility of having good control over the entire process (Zeirzer, 2004). LLNL used the new partial seam CRIP method during a field test in Centralia, Washington. This was the first demonstration of CRIP on a sizeable field test. The advantages of this method included the requirement for a single injection and production well and the separation of the vertical injection well from subsidence near the reaction front. The Rocky Mountain I field test used a silane torch to melt the steel liner of the horizontal part of the injection well at the required location to create a new injection point. This silane torch was retracted back, away from the active cavity and some distance up the injection well, to keep it safe. When the current cavity reaches its maximum size, the silane torch can be pushed forward to the desired position so that the next injection point can be created inside the coal seam (Thorsness et al., 1988). If one compares the CRIP method with the LVW or the extended linked well (ELW) method, there is no clear winner. The ELW method promises to be the most economic method for relatively shallow coal seams, the reason being that it is cheaper to drill to shallower depths, resulting in the affordable creation of multiple vertical wells. On the other hand, the CRIP method provides better economic potential for relatively deep coal seams, the reason being that it is costly to drill long wells to higher depths, and therefore a method with fewer deep wells becomes more cost effective. The long tunnel method is another method of operating a UCG process. It was developed at the Chinese University for Mining and Technology (CUMT). This method interestingly combines parts of the shaft and shaftless methods of UCG by involving boreholes, an airflow tunnel, and a gasification tunnel. It uses a two-stage approach with the injection of air in the first stage to attain higher temperatures and the injection of steam in the second stage to obtain a higher composition of CO and H2. This method can utilize additional coal resources from existing mining shafts.

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2.3 UCG Field Trials An overview of worldwide UCG field trials is provided in this section. We shall discuss the relevant laboratory-scale tests separately in Chapter 5. In this section, we describe the field-scale experimental studies that provide insight into the UCG process and a better understanding of the important fundamental aspects. Starting in the 1930s, several countries have attempted UCG field trials across the continents. Table 2.2 provides a list of major field trials and important details available in the open literature. As we discussed earlier, the first recorded UCG trials took place in Russia in the 1930s. The first such UCG trial in the Moscow coal basin was unsuccessful due to poor control preparations and issues with the chamber process. However, the understanding obtained about the UCG process from several trials using the chamber method led to the first successful UCG experiment in the Donetsk coal basin. Having learned more from the initial difficulties in the 1930s experiments, the USSR developed UCG technology further in the 1950s and 1960s and ran several improved trials. Most of the USSR’s UCG trials were performed on shallow (1200ºC).

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FIGURE 2.7 Cross-section along the axis of the Hanna II Phase 2 process wells based on postburn coring boreholes following Phase 3. (Reprinted from Youngberg et al., 1983.)

The achievements from the Hanna tests included the demonstration of UCG multiple times, showing its technical feasibility. The tests also demonstrated the flexibility of operation by switching the production and injection between multiple wells. The challenges were in restricting the gas leakage and groundwater monitoring. Sometimes the operation was also shut down due to high-temperature excursions at the production wellhead. Better instrumentation was recommended to measure and analyze these issues for better operation. 2.3.2 UCG Tests at Hoe Creek These tests were performed by LLNL on a sub-bituminous coal in the Powder river basin near Gillette, Wyoming. The importance of these tests lies in the fact that they included the first underground gasification of coal using oxygen-steam injection (Hoe Creek II – HC-II) and the first one with a horizontal borehole link (Hoe Creek III – HC-III). The oxygen-steam tests were successful in producing gas of very high quality, i.e., twice the product gas heating value in comparison with UCG carried out using air only. Coincidently, the performance of one of these tests suffered due to the injection point being at the top of the seam, the reason being a lack of control over the location of the reverse-burn link(s). Typically, cavity growth in an upward direction is faster than sideways growth. The HC II test also had

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some issues because of the harsh UCG environment surrounding the coal seam. The entire coal bed, at least its Felix seams, was highly porous and too permeable, resulting in huge gas losses and an unfavorably high amount of water influx. In case of the test with the upper injection point, the weak overburden continued to spall, and that allowed an excessive amount of wet overburden to enter the cavity. This resulted in a significant energy sink that further reduced the process’s efficiency and the gas quality. The comparison between UCG performance from the two different injection points, one at the top and one at the bottom of the seam, clearly showed the importance of injecting gas at the bottom. Figure 2.8 shows the amount of rubble that filled the cavity in the HC-II test. The cavity height is observed to extend way beyond the thickness of the coal seam due to the failure of overburden inside the cavity. It was also observed during these trials that the integrity of the injection point at the bottom of the seam is difficult to maintain with a vertical well in case of hotter oxygen-steam injection. Due to its being at the center of the cavity, it is subjected to more collapse events if they occur. Therefore, the team started considering the possibility of nonvertical wells. As a result, HC-III involved a horizontal production borehole and a new burn location to create new wells. This arrangement was also attempted to resolve the problem of declining gas quality due to excessive heat losses after the cavity reached the roof. One can clearly see the progress of UCG methods during HC-III that are pointing towards the CRIP method.

FIGURE 2.8 Hoe Creek II postburn cross-sections at different locations in the cavity, scale in meters. (Reprinted from Camp, 2017, adapted from Stephens, 1981, with permission from LLNL.)

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During the HC tests, serious efforts were put into monitoring water contaminants, and it showed an increase in the concentrations of phenols, organics, and inorganic species in groundwater. The observations of high contaminants in groundwater led to a more detailed investigation of this issue. Interestingly, the groundwater monitoring observations led to a method of performing cleaner UCG – the Clean Cavern Concept. This concept was used during the Rocky Mountain I trials to reduce contaminants and to remediate the surroundings. 2.3.3 The Centralia Series of Field Tests After the Hoe Creek tests, LLNL wanted to perform UCG at a location that is geologically more favorable, that is, with moderate permeability and stronger overburden. The objective of these tests included exploring the effect of the steam/oxygen ratio and flow rate on UCG performance and excavating the underground cavities to observe the nature of the cavities and thereby learn more about their development. The LLNL team also wanted to test their CRIP method. As a result, LLNL conducted large block tests (LBK) and a partial seam CRIP test at Centralia in southwestern Washington. The LBK tests involved five experiments using an exposed coal face on the side of a hill in the WIDCO coal mine. Each of these experiments lasted for 3–6 days, leading to consumption of 25–30 tons of coal in each test. The use of the exposed coal face for UCG allowed the postburn excavation of the cavities to observe it from inside. Figure 2.9 shows the pictures of excavated cavities from these tests. The cavity shapes indicated faster growth in the upward direction than horizontal growth. The cavities were filled with highly permeable dried coal, char, ash, and thermally altered overburden rubble.

FIGURE 2.9 Inside view of the underground cavity after excavation of LBK tests: (a) Slice-13 from LBK-1 and (b) Slice-3 from LBK-4. (Reprinted from Thorsness and Cena, 1983, with permission from LLNL.)

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FIGURE 2.10 Configuration of the partial seam CRIP test. The terraces on the left are real. The right face of the figure is a cut-away cross-section. (Reprinted from Hill et al., 1984, with permission from LLNL.)

After some time during UCG, the injected gas has to pass through the rubble and residual ash. Therefore, the amount and permeability of char, rock material, and ash in the rubble are supposedly very important in inlet gas distribution in a developing UCG cavity. After the successful LBK tests, LLNL started the first full-scale field test to try controlled retracting injection point (CRIP) concept – the PSC test. Unlike the LBK tests, the PSC test was similar in magnitude to most of the earlier tests. This test also included a horizontal injection well and two production wells. Figure 2.10 shows a sketch of the PSC test configuration. Overall, the PSC tests successfully demonstrated the CRIP method on a full-scale system. After the test, the cavity was excavated and inspected for its inside contents. It was observed that the cavity grew into an hourglass shape. The cavity contained ash around the injection point, unconverted char near the sidewalls, and partially fused overburden rocks spalled above the ash and char. 2.3.4 Rocky Mountain I Rocky Mountain I (RM-I) tests were conducted in the Hanna Basin as the same coal seam was earlier characterized and used during the Hanna test series. The main objective of this test was to demonstrate the application of CRIP and ELW for UCG of the same coal seam at a reasonable scale of operation. The well configuration of the modules for CRIP and ELW is shown in Figure 2.11. The oxygen-steam gasification of coal using the ELW method was operational from late November 1987 to mid-January 1988. On the other hand, the UCG operation based on the CRIP method ran from late November 1987 to late February 1988. The gasification cavities could not be excavated in the case of the RM-I tests as they were performed on a coal seam at a depth of 110 m. Therefore,

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FIGURE 2.11 Rocky Mountain I’s process well layout for its side-by-side ELW and CRIP modules. (Modified from Thorsness and Britten, 1989.)

a program with postburn drilling and coring was developed to identify the spalling extent, the nature of the cavity, and the cavity’s shape and size. The analysis of the cavity using this postburn coring was used, along with the thermal data obtained using the downhole thermocouple and material balance information obtained during the test, to determine the extent of the active cavity and to describe the cavity’s internal structure. Figure 2.12 shows the typical postburn coring analysis, showing different sections through the cavity near the injection well. The cavity was observed to consist of ash and slag at the bottom with char near sidewalls and bottom region, rubble-containing overburden rock material in the majority of the cavity, and relatively smaller void zones. The cavity extended from midpoint of the coal seam into the overburden. Seventy percent of energy content of the coal consumed using CRIP was available in the form of combustible gas and tar products with ~20% energy content left underground in the unconverted char. Another approximately 7% of coal energy was lost in heating the overburden during CRIP. On the other hand, ~60% of energy of coal burned was available in terms of combustible products from the ELW method. As can be seen from these numbers, the CRIP module using oxygen-steam injection produced energy efficiency as high as those of surface gasifiers. This final test of the U.S. UCG operations proved

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FIGURE 2.12 Rocky Mountain I’s ELW module final cavity geometry looking west towards the production well, cross-section B’-B. (Reprinted from Oliver et al., 1991.)

to be the largest and the most successful test, involving the most advanced methods and instrumentation. Several previous experiences have shown the adverse impact of UCG on groundwater contaminant levels. Reducing these impacts on the surroundings by using better operating methods and concepts was also one of the major objectives of the RM-I test. The location of these tests was in a clean area upside from the Hanna field test sites. In the Hanna field test, pyrolysis gases escaped during the reverse-burn as it involved higher gas pressures. The groundwater contaminant levels were being monitored by Wyoming

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Department of Environmental Quality (WDEQ) before, during, and after the RM-I tests to avoid potential adverse effects of UCG. As a result, a large-scale groundwater monitoring and sampling operation was set up in the environs of both modules. It was also the first application of the Clean Cavern method of shutdown, which helped reduce the spread of contaminants. The WDEQ released the project from further responsibilities in 2005 after it was satisfied with the remediation success. In addition to these tests, the United States also attempted UCG on swelling coals and steeply dipping coal seams. These were relatively less successful trials, and there were lessons learned from the same. The team at Morgantown Energy Technology Centre (METC, spun from the LETC/ LERC, which conducted the initial Hanna tests) conducted UCG tests at a Pittsburg seam with high-volatile swelling bituminous coal in 1979. This was the only U.S. test on swelling agglomerative coals. The researchers were able to create reverse burn links, but it was difficult due to high resistance to linking. The main conclusion from these tests was that the capabilities of UCG needed to be improved in multiple ways even before any attempts were made to gasify any swelling coals. During the same period in 1979, Gulf Research and Development Co. attempted to perform steeply dipping field tests at Rawlins. They experienced difficulties in ignition and reverse-burn linking for connecting the wells. However, once the link was established, this test produced a good quality product gas as the steeply dipping seams provided better thermal efficiency. There were some groundwater contamination and gas leakage problems (Camp, 2017). 2.3.5 UCG Field Trials in Countries Other Than the United States and USSR In addition to the USSR and the United States, several other countries also showed considerable interest in UCG and conducted several field trials as shown in Table 2.2. The Department of Trade and Industry (DTI) in the United Kingdom and other European countries have supported various trials in the period 1999–2005. The main difference between the European trials and the U.S./Russia trials is the depth of coal seam and its rank. The European trials were for deep mines like one of 860 m in Belgium (1979–1987) and one of 1200 m in France (1981–1986) (Dufaux et al., 1990). The UCG test at Thulin, Belgium, was performed on a low-volatile semi-anthracite coal seam with a thickness of 4–6 m and depth of 860 m. The tests struggled for two years with reverse combustion for linking the wells, and finally it was decided to use drilling for the linkage. As a result, the UCG operation started and produced a medium BTU gas with relatively high methane composition. The researchers also drilled postburn wells in order to understand the underground process. It was inferred that the underground cavity consisted of a rubble zone with affected coal, ash, slag and overburden rock material. Due to high pressures during dip underground operations, the

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gases can diffuse through the coal matrix and consume the coal while flowing. Another UCG test was also tried at 500–700 m in El Tremedal in Spain (1989–1998). Recently, the Central Mining Institute in Poland has developed UCG methods for abandoned and working underground coal mines to produce more hydrogen. A 6-day trial at Barbara coal mine in Poland resulted in the production of gas with heating values of around 10 MJ/Nm3. In the United Kingdom, there was a strong interest in UCG due to the availability of coal resources, the capability of using syngas in industry, and opportunities for carbon capture and sequestration. A large-scale UCG site has been identified in Scotland. However, the United Kingdom has very high quantity of natural gas as well. It has also experienced some large-scale objections from citizens against the deployment of UCG due to the perceived risks associated with safety and the environment. Australia, through the Commonwealth Scientific and Industrial Research Organization (CSIRO), has performed several UCG trials. CSIRO Australia coupled with GTL and the University of New South Wales in Australia has a major research interest in the UCG process. The most successful of the Australian trials have been performed at Chinchilla. This location has become the preferred choice for UCG, with five UCG demonstrations taking place from 1999 to 2013. The coal seam used in these trials was subbituminous coal at a depth of 130 m and thickness of 4–6 m. These tests started with an LVW method using reverse combustion and air and injected gas and evolved into parallel and linear CRIP methods with enriched air and oxy-steam as inlet gas. The linear CRIP gasifier was operated for more than 2 years, and it consumed more than 19,000 tons of coal (the largest test outside Russia). During the phases of highest activity, the Chinchilla UCG was operated for a 40 MW power plant (Walker et al., 2001). For these trials, technology was provided to Linc Energy by Ergo Exergy, Inc. (Canada), and their technology is based on those used in the former USSR (Walker et al., 2001). The final goal of the Chinchilla UCG–IGCC project is to scale up the initial pilot/meso-scale plant to the size optimal for commercial viability (DTI review, 2004). Recent and ongoing trials/pilots at Chinchilla were conducted by Linc Energy and Carbon Energy after approvals from Queensland government. The state government had also formulated an independent scientific panel (ISP) to assist in assessment of these trials. The panel recommended that the government should permit the companies to continue the pilot until the cavity had reached a commercially viable size. ISP also recommended that until the companies demonstrate their decommissioning capability on the reasonable sizes of cavity, no commercial facility should be commenced (Moran et al., 2013). In China, UCG trials have been conducted between 1990 and 1995 and from 2004 to the present. Several field trials were performed at the Xinhe mine, Xuzhou, in 1994 and the Liuzhang mine, Tangshan, in 1996 by the China University of Mining and Technology (CUMT), Beijing. These trials

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were conducted through the UCG engineering research center using the process developed in-house. The outcome of the process was the successful production of gas at a calorific value of 4.5 MJ/m3 (Yang, 2004). The depth of coal seams was around 200–300 m, and the UCG was either on deeply inclining seams or for reuse of abandoned coal mine shafts as gasifiers. The studies at CUMT have provided a strong push for UCG in China. In India, several coal mines are suitable for UCG process (Khadse et al., 2007), and there is significant interest in the application of UCG in that country (Sapru et al., 2007). Various coal companies, academic institutes, and technology companies like ONGC, GAIL, NCL, CIMFR, Skochinsky Institute of Mining (USSR), Ergo Exergy (Canada), GIPCL, GMDC and IIT Bombay have shown strong interest in UCG in India. India has lignite resources at depths of 600–1200 m which are good for UCG. The geological characterization of potential sites and blocks in Gujarat and Rajasthan was performed in 2006. A detailed hydrogeological assessment of every shortlisted site is required so that environmental and safety concerns are addressed properly.

2.4 Mitigating Environmental Risks Some of the important recommendations for keeping the UCG process clean are provided in this section. They are based on the reviews and reports of field trials published in the literature, notably those by Camp (2017) and Upadhye et al. (2008). These considerations will help in reducing pollution and mitigating environmental risks. We have divided these recommendations into before operation, during operation, and after operation as follows: Before operation: • Selection of the right site is key to minimizing adverse environmental effects. • The coal seam and the nearby strata (overburden and underburden) should all have lower permeability. • The coal seam should have minimum faultlines and a reasonable depth and thickness, and can be possibly horizontal. • Coal seams near potentially usable aquifers or groundwater and surface water use should be avoided. • Sites near human activities like towns, cities, business, and so on should be avoided. • A plan for avoiding land subsidence and water pollution should be ready even before starting the well-drilling.

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During operation: • Boreholes should be properly cemented to avoid gas leakage. • Seam fracturing should be highly localized so that it does not create bigger cracks. • The casing and liner of pipes in different wells carrying gas or in contact with the gas-filled cavity should be constructed to avoid gas leakage even with corrosive gases. • The effect on the overburden of cavity growth in the upward direction should be kept to a minimum. • Cavity pressure should be maintained below the hydrologic confining pressures at that depth to keep all the contaminants inside the cavity. • Advanced monitoring techniques and predictive models should be used for dynamic monitoring of the shape and size of the cavity and pressure field in the surroundings. • Best mining industry practices should be used and improved upon to avoid land subsidence, even with a less controllable technology. After operation: • Cavities should be flushed and vented postburn to remove pyrolysis vapors until the contaminant removal in flush has become negligible. Continuous water/steam removal can also be used to remove contaminants. • The cavity should be cooled down after the end of the operations as high temperatures can cause further pyrolysis of coal. • Methods like those based on LLNL’s Clean Cavern Concept for shutdown of a UCG cavity should be used so that the effect of contaminants is reduced. • Contaminant levels in surrounding groundwater, especially in/near aquifers, should be monitored before, during, and afterwards for several years to benchmark and ensure the same ecological conditions are restored in the wake of the process.

2.5 Importance of Coal Properties in the UCG Process Different countries have attempted pilot experiments and trials to understand the UCG process at a given site and thereby commercialize the process

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to improve the energy security of those individual countries. These tests have involved a wide range of coal types and different seam conditions like depth, thickness, water influx, overburden, and so on, as discussed in this chapter. For the commercialization of UCG at any site, we need a process with lower capital and operating costs, high thermal and chemical conversion efficiencies, high controllability, a steady flow of gas, and the least possible environmental impacts. In addition, better utilization of steam and oxygen, higher coal resource recovery, and reliable procedures for start-up and decommissioning are important from the UCG technology perspective. The overall UCG operation and its performance is significantly affected by the properties of the coal involved. From UCG trials at different locations, it is clear that low-rank coal is better for UCG due to its high reactivity and higher spalling tendency when heated. The high volatile fraction in low-rank coals help the process in two ways – by increasing porosity after pyrolysis and by adding more combustible gases to the UCG product gas. Bhaskaran et al. (2012) have shown through their laboratory experiments the significant difference between the performance of UCG for soft-lignite and that of hard-bituminous coal. Li et al. (2007) also performed a DoE (design of experiments) to determine the effect of coal ranks on the syngas composition. They observed that the sub-bituminous coal produces better gas at higher rate in comparison to the lignite. This indicates a more complex relationship between coal rank and UCG performance. In addition to the rank of coal and the spalling tendency, the ash content and its composition also influence UCG performance, and these two factors do not have a one-on-one relationship with coal rank. Generally, low-rank coals shrink after pyrolysis and spall upon heating. This helps increase the amount of coal getting exposed to hot reactive gases and thereby increases the rate of gasification. On the other hand, high-rank coals swell upon heating, and their spalling behavior has not yet been well understood. This decreases the amount of coal getting exposed to the reactive gases, while the diffusion of gases into the swollen coal on roof or sidewalls is also problematic. Therefore, most of the UCG trials in the literature are on low rank-coals. The first UCG trials during the Pricetown project were performed on bituminous coal, and researchers faced several challenges, including the swelling of coal and poor linkage between the injection and production wells (Camp et al., 2017). The UCG trials in Europe indicated that the operational challenges further increase as the rank of the coal increases. The Thulin UCG project on deep anthracite had unsatisfactory gasification conditions, resulting in the presence of oxygen in the exit gas (Defaux et al., 1990). Therefore, for UCG of high-rank coals, a good flow path must be established linking the injection and production wells while maximizing the coal exposed to the reactive gases. In addition to the coal properties discussed here, the amount of chlorine (Cl), sulfur (S), and other contaminants in the coal also affect the UCG process by requiring changes in the piping material and in gas-treatment and cleanup facilities.

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FIGURE 2.13 UCG projects classification according to the coal-seam depth and thickness with the relative importance of different parameters. (Reprinted from Perkins, 2018, with permission from Elsevier.)

An interesting analysis of the effect of coal-seam thickness and depth (Figure 2.13) is provided by Perkins (2018). In addition to surface interactions and drilling costs, the increased depth of coal seam also increases the operating pressure and thereby increases the composition of CO2 and CH4 in the produced gas. The thermodynamic analysis (discussed in Chapter 7) will explain this behavior in detail. The coal-seam thickness significantly affects the heating value of the produced gas due to various factors, including the amount of coal resource per unit length, the control of water influx, the loss of heat to the surroundings and so on. The heating value of the product gas increases with the coal-seam thickness as the heat losses reduce. However, the increase in heating value strongly depends on the water influx and reaches a constancy after some seam thickness (Perkins, 2018). Overall, it has been observed that for a highly successful UCG process, the coal seam should be relatively shallow (