Pulverized-Coal Combustion and Gasification: Theory and Applications for Continuous Flow Processes PDF
Preview Pulverized-Coal Combustion and Gasification: Theory and Applications for Continuous Flow Processes
Pulverized-Coal Combustion and Gasification Theory and Applications for Continuous Flow Processes Pulverized-Coal Combustion and Gasification Theory and Applications for Continuous Flow Processe.~ Edited by L. Douglas Smoot Brigham Young University Provo, Utah and David T. Pratt University of Utah Salt Lake City, Utah SPRINGER SCIENCE+BUSINESS MEDIA, LLC Library of Congress Cataloging in Publication Data Main entry under title: Pulverized-coal combustion and gasification. Includes bibliographical references and index. l. Coal, Pulverized. 2. Combustion engineering. 3. Coal gasification. I. Smoot, Leon Douglas. II. Pratt, David T. TP328.P84 662'.62 78-12564 ISBN 978-1-4757-1698-6 ISBN 978-1-4757-1696-2 (eBook) DOI 10.1007/978-1-4757-1696-2 © 1979 Springer Science+Business Media New York Originally published by Plenum Press, New York in 1979 Softcover reprint of the hardcover I st edition 1979 All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher Contributors Clayton T. Crowe, Department of Mechanical Engineering, Washington State University, Pullman, Washington M. Duane Horton, Department of Chemical Engineering, Brigham Young University, Provo, Utah Philip C. Malte, Department of Mechanical Engineering, Washington State University, Pullman, Washington David T Pratt, Department of Mechanical and Industrial Engineering, University of Utah, Salt Lake City, Utah Dee P. Rees, Department of Chemical Engineering, Brigham Young University, Provo, Utah F. Douglas Skinner, Department of Chemical Engineering, Brigham Young University, Provo, Utah Philip J. Smith, Department of Chemical Engineering, Brigham Young University, Provo, Utah L. Douglas Smoot, Department of Chemical Engineering, Brigham Young University, Provo, Utah J. Rand Thurgood, Department of Chemical Engineering, Brigham Young University, Provo, Utah Sneh Anjali Varma, Department of Mechanical and Industrial Engineering, University of Utah, Salt Lake City, Utah John J. Wormeck, Department of Mechanical and Industrial Engineering, University of Utah, Salt Lake City, Utah v Pre.face The past technology for describing and analyzing pulverized-coal furnaces, combustors, MHD generators, and other reactors has relied largely on empirical inputs for the complex flow and chemical reactions that occur while more formally treating the heat transfer effects. Growing concern over control of combustion-generated air pollutants has revealed a lack of understanding of the relevant fundamental physical and chemical mechan isms. Fortunately, recent technical advances in computer speed and storage capacity, and in numerical prediction of recirculating turbulent flows, two-phase flows, and flows with chemical reaction have opened new oppor tunities for describing and modeling such complex combustion systems in greater detail. We believe that most of the requisite component models to permit a more fundamental description of pulverized-coal combustion processes are available or can presently be developed. At the same time, with the rapidly increasing use of coal, and with the greatly increased emphasis being placed on development of new coal processes in the United States, interest in modeling of coal reaction processes has increased dramatically. Such interest was demonstrated at the ERDA Coal Conversion Modeling Conference held in late 1976, which was attended by over 200 representatives of industry, government, and universities. We have been working during the past six years on pulverized-coal combustion processes. This work has included both basic measurements and development of analytical models. In our modeling work, we have attempted to develop fundamental models based upon the general equations of conservation and to use the best literature information available as sources for model parameters. One of the major purposes of this book is to document our general modeling approach being used for pulverized-coal systems. This book emphasizes processes using finely pulverized coal which is entrained in a gaseous phase. Problems of particular interest include combustion in pulverized-coal furnaces and power generators, entrained coal gasifiers, coal-fired MHD power generators, and flame propagation in laminar and turbulent coal dust/gas mixtures. While many of the principles vii viii Preface and approaches could be adapted to other coal conversion and combustion problems, we have not considered combustion or gasification in fluidized or fixed beds or in situ processes. In addition, we have not considered other fossil-fuel combustion problems associated with oil shale, tar sands, etc., even though many aspects of pulverized-coal combustion would relate to these problems. For the case of pulverized-coal models, we have attempted to provide a detailed description of the model foundations. Parts I and II of this book emphasize general principles for describing reacting, turbulent or laminar, multiphase systems. General conservation equations are developed and summarized. The basis for computing thermochemical equilibrium in complex, heterogeneous mixtures is presented, together with techniques for rapid computation and reference to required input data. Rate processes are then discussed, including pertinent aspects of turbulence, chemical kinetics, radiative heat transfer, and gas-particle convective-diffusive interactions. Much of Part II deals with parameters and coefficients for describing these complex rate processes. This part of the book provides recommended values of coefficients and parameters for treating complex reacting flows. Parts I and II may well be suitable for use in an advanced course in reacting flows, and have been written partly with that in mind. Part III deals with more specific aspects of pulverized-coal characteristics and rate processes. Following a general description of coal structure and constitution, coal pyrolysis and char oxidation processes are considered. Gas-phase combustion of volatile matter and pollutant aspects of pulverized coal are next considered, and finally, a general modeling framework for pulverized-coal particle reactions completes this part of the book. Part III is based largely on a review and summary of published literature. Recom mendations are included, where appropriate, for describing important coal reaction rates. Part IV presents details of pulverized-coal models that have been or are being developed by the authors. A generalized model for predicting prop agation in premixed gaseous or multiphase laminar flows is documented. Model predictions are compared with experimental data for gaseous and pulverized-coal flames, and mechanisms of propagation are discussed. Propagation in turbulent flames is also briefly treated. A generalized, one-dimensional treatment of pulverized-coal combustion and gasification is then formulated. Model predictions for coal furnaces and entrained gasifiers are shown, and compared with experimental results. This model has potential use in systems analysis and data interpre tation, but does not predict multidimensional variation of properties inside a coal reactor. Finally, a generalized, multidimensional code for describing pulverized coal combustion and gasification processes is outlined and a solution Preface ix technique is described. Steady-state, two-dimensional solutions have been emphasized. The model considers turbulent, recirculating, multiphase, reacting flows. The authors recognize the several uncertainties in constructing and applying models of this nature. Coal is a very complex heterogeneous substance whose structure and behavior are highly variable and are not well known. Pyrolysis and oxidation of coal are dependent upon coal type, size, size distribution, temperature history, etc., thus making generalization difficult. In addition, the complexities of turbulent recirculating flows, turbulent reacting flows, and turbulent two-phase flows are not fully resolved. Only recently has formal computation of recirculating flows been possible and the extent of comparison of measurements with predictions is limited. Suitable generalized parameters for the turbulence models are not well established. The interaction of turbulence with chemical reaction is even less well developed, and general computational techniques for reliably treating this complexity are simply not available. Significant uncertainties also pertain to the effect of gas turbulence on the motion of the particles. It is known that the random gas fluctuations can be a major force in dispersing particles, but treatments of this effect are in the formative stages. Finally, there has been little previous attempt to develop models which consider several of these aspects jointly. For this reason, models of pulverized-coal systems must be validated by comparison with experimental measurements. Measurements of outlet gas or char composition or temperature alone are not suitable for this model validation. Profiles of at least time-mean, local properties from within the reactor must be the basis for the model eval uation. As a part of the research program being conducted by the authors, such measurements are being obtained, some of which are presented in this book and compared with model predictions. These measurements include data for laminar, premixed coal-air and methane-air flames, as well as spatially resolved measurements of gas and char composition from inside entrained gasifiers and furnaces. Further, profile measurements have been and are being made for particle-laden, recirculating jet flows without chem ical reaction. Comparison of these measurements with model predictions will permit evaluation of particle- and gas-flow effects in the absence of chemical reaction complications. Because of the high level of interest in pulverized-coal combustion, and because research work is expanding in this area, it is anticipated that considerable additional results pertinent to the contents of this book will be published during the coming years. We can foresee the necessity of updating or altering the material included in this book. However, we hope that it has been a useful effort to provide this publication at a time when modeling of pulverized-coal processes, while perhaps not well developed, has a high level of technical interest. X Preface Much of the work upon which this book has been based is being supported by federal and industrial agencies. We especially express our appreciation to the United States Bureau of Mines (Pittsburgh Mining and Safety Research Center), The United States Department of Energy (Fossil Energy Division), the Electric Power Research Institute (Fossil Fuels Division), and the National Science Foundation for contract or grant sup port related to measurements and model development for pulverized-coal systems. We further acknowledge the Chemical Engineering Department and Research Division of Brigham Young University for assistance in preparation of the document. We are also grateful to Elaine Alger, Michael King, and Scott Foister for typing of the manuscript and preparation of the illustrations. Finally, we thank the several participating authors, both professors and Ph.D. candidates, who conducted literature searches and analyses and who prepared book materials when there were many other demands upon their time. The participating authors are primarily those who are involved in this pulverized-coal research program at Brigham Young University and at the University of Utah. Professor M. Duane Horton is a senior investigator for the U.S. Bureau of Mines study on mine explosions. Dee P. Rees and J. Rand Thurgood are Ph.D. candidates in Chemical Engineering, working on the EPRI pulverized-coal combustion study, while F. Douglas Skinner is a Ph.D. candidate working on the DOE entrained coal gasification study. Professors Clayton T. Crowe and Philip C. Malte of Washington State University (Mechanical Engineering) are consultants to the project. Sneh Anjali Varma is a Ph.D. candidate in Mechanical Engineer ing at the University of Utah, working on the EPRI study, while Dr. John Wormeck is a Senior Research Engineer at the University of Utah conduct ing modeling studies on the DOE contract. L. Douglas Smoot David T. Pratt Contents PART I. FUNDAMENTALS OF HETEROGENEOUS FLOW SYSTEMS Chapter 1 M ulticomponent Equilibrium David T. Pratt 1. Homogeneous Gas-Phase Equilibrium 3 1.1. Gibbs Function Minimization . 4 1.2. Newton-Raphson Solution of the Extremum Equations 6 2. Condensed Phases 9 3. Thermochemical Properties 10 4. Computational Techniques 11 5. Recommended Approach 13 6. Notation 13 7. References . . . . . . . 14 Chapter 2 Multicomponent Conservation Equations Clayton T. Crowe and L. Douglas Smoot 1. Reynolds Transport Theorem 15 2. Continuity Equation 18 2.1. Gas-Phase Species Continuity 19 2.2. Gas-Phase Continuity in a Gas-Particle Mixture 21 2.3. Discrete-Phase Continuity 22 2.4. Overall Continuity 23 3. Momentum Equation 23 3.1. Gas-Phase Momentum 23 3.2. Gas-Phase Momentum in a Gas-Particle Mixture . 25 3.3. Discrete-Phase Momentum 26 3.4. Overall Momentum Equation 28 4. Energy Equation 28 4.1. Gas-Phase Energy Equation 28 xi
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