Combustion Chemistry. Edited by W. C. Gardiner, Jr. Springer-Verlag New York Berlin Heidelberg Tokyo

Transcription

1 Combustion Chemistry

2 Combustion Chemistry Edited by W. C. Gardiner, Jr. With Contributions by A. Burcat G. Dixon-Lewis M. Frenklach W. C. Gardiner, lr. R. K. Hanson S. Salimi an 1. Troe 1. Warnatz R. Zellner With 164 Figures Springer-Verlag New York Berlin Heidelberg Tokyo

3 William C. Gardiner, Jr. Department of Chemistry University of Texas at Austin Austin, Texas U.s.A. Library of Congress Cataloging in Publication Data Gardiner, William C. (William Cecil), Combustion chemistry. Bibliography: p. 1. Combustion. I. Title. QD516.G ' by Springer-Verlag New York Inc. Softcover reprint of the hardcover 1st edition 1984 All rights reserved. No part of this book may be translated or reproduced in any form without written permission from Springer-Verlag, 175 Fifth Avenue, New York. New York 10010, U.S.A. The use of general descriptive names, trade names, trademarks, etc., in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. Typeset by Polyglot Pte Ltd., Singapore ISBN-l3: e-isbn-13: : /

4 Preface Detailed study of the rates and mechanisms of combustion reactions has not been in the mainstream of combustion research until the recent recognition that further progress in optimizing burner performance and reducing pollutant emission can only be done with fundamental understanding of combustion chemistry. This has become apparent at a time when our understanding of the chemistry, at least of small-molecule combustion, and our ability to model combustion processes on large computers have developed to the point that real confidence can be placed in the results. This book is an introduction for outsiders or beginners as well as a reference work for people already active in the field. Because the spectrum of combustion scientists ranges from chemists with little computing experience to engineers who have had only one college chemistry course, everything needed to bring all kinds of beginners up to the level of current practice in detailed combustion modeling is included. It was a temptation to include critical discussions of modeling results and computer programs that would enable outsiders to start quickly into problem solving. We elected not to do either, because we feel that the former are better put into the primary research literature and that people who are going to do combustion modeling should either write their own programs or collaborate with experts. The only exception to this is in the thermochemical area, where programs have been included to do routine fitting operations. For reference purposes there are tables of thermochemical, transport-property, and ratecoefficient data. The material is organized as follows. In Chapters 1 and 2 the basic chemistry and physics of reactive flow are described, first simply and then in the sophisticated form required for dealing with steady flames. The theory of elementary reaction rate constants is presented in Chapters 3 and 4,

5 VI Preface subdivided into bimolecular and unimolecular types and with particular emphasis on the temperature dependence of rate coefficients and procedures for estimating rate coefficients when little or no experimental information is available. Surveys of elementary reaction rate coefficients are given in Chapters 5 and 6 for reactions of molecules containing the elements carbon, hydrogen and oxygen and nitrogen, hydrogen and oxygen respectively. In Chapter 7 the mathematical theory of modeling is reviewed, with the particular purpose of showing how the predictions and parameters of combustion models can be subjected to critical statistical and sensitivity analysis. The concluding Chapter 8 gives methods of thermochemical property estimation and polynomial fitting for combustion-related purposes. Our current knowledge of combustion chemistry can be rather well characterized by stating what cannot yet be presented in a book like this. First, the fuel molecules whose combustion can be described with confidence have at most only two carbon atoms; propane and butane combustion models are topics of exploratory research and the mechanism of octane combustion can only be surmised. Second, when hydrocarbon fuels burn with too little oxygen for complete combustion, solid carbon is formed by mechanisms that are still not understood in enough detail to permit models to be constructed. Similarly, the interaction of physical and chemical processes at surfaces of burning solids such as coal is known only in outline. In the area of pollutant-formation mechanisms, while we can explain why oxides of nitrogen form when combustion temperatures get very high, we cannot explain how they are formed from nitrogen-containing fuel molecules or from attack of carboncontaining radicals on molecular nitrogen, nor how nitrogen oxides produced early in flames get reduced back to molecular nitrogen within the main reaction zone. We know little about the chemistry of sulfur in flames. On the modeling side, it is still usually necessary to oversimplify either the fluid dynamics or the chemistry: For chemically detailed combustion modeling, one must treat the fluid dynamics with restrictive approximations that usually amount to an assumption of steady one-dimensional adiabatic flow, which can only be achieved in practice using special laboratory burners; when unsteady or two- or three-dimensional flow is to be modeled, the demands upon computer time are already so great that chemical details must be suppressed, the flow model must be simplified, or computational accuracy must be sacrificed. This list of shortcomings in our understanding of and ability to apply combustion chemistry is not a declaration that real-world combustion is too complex to understand at the level of elementary reactions and realistic fluid dynamics. On the contrary, it is a program for combustion research that we hope readers of this book will help us to carry out. Austin, Texas William C. Gardiner, Jr.

6 Contents Chapter to Combustion Modeling William C. Gardiner, J r. 1. Terminology of reaction kinetics 2. Rate laws and reaction mechanisms 3. Physical constraints on gas-phase combustion reactions 4. Differential equations of homogeneous reaction without transport 4.1. Constant-density isothermal reaction 4.2. Constant-density adiabatic reaction 4.3. Constant-pressure adiabatic reaction 4.4. Reactive steady flow 5. Methods of numerical integration 6. Interpretation of combustion modeling profiles 7. References Chapter 2. Computer Modeling of Combustion Reactions in Flowing Systems with Transport Graham Dixon-Lewis 2. Conservation or continuity equations, and other useful relations 2.1. Conservation of total mass 2.2. Conservation of y-direction momentum 2.3. Species equations. Conservation of atoms 2.4. Conservation of energy 2.5. Auxiliary equations

7 viii Contents 2.6. Eulerian and Langrangian coordinate reference frames 2.7. Space integral rate 3. Formulation of transport fluxes 3.1. Transport processes in mixtures of nonpolar gases 3.2. Mixtures containing one polar component 3.3. Application of the extended Chapman-Enskog procedure to reactive flow systems 3.4. Approximate equations for transport fluxes in multicomponent mixtures 4. One-dimensional premixed laminar flame properties by solution of the time-dependent equations 4.1. Preliminary transformations 4.2. Finite-difference formulation 4.3. Solution of equations 4.4. The convection term. Lagrangian and Eulerian calculations 4.5. Gasdynamic effects 5. Premixed laminar flames and kinetic studies 6. Two further solution techniques 6.1. Newton-type iteration around stationary flame equations 6.2. Finite-element collocation method 7. Implicit methods and general reactive flow problems 7.1. Boundary layer flows 7.2. Counterflow flame geometries. Stretched one-dimensional flames 7.3. Multidimensional flows 8. Operator splitting techniques in multidimensional systems 9. Chemical quasi-steady-state and partial equilibrium assumptions in reactive flow modeling 10. Concluding remarks 11. Nomenclature 12. References Chapter 3. Bimolecular Reaction Rate Coefficients Reinhard Zellner 2. Fundamental concepts 2.1. The rate coefficient and the Arrhenius equation 2.2. Thermodynamic predictions 2.3. Macroscopic and microscopic kinetics 3. Theoretical predictions of bimolecular reaction rate coefficients 3.1. Collision theory 3.2. Transition-state theory 4. Comparison between experiment and theory for rate coefficients of selected bimolecular gas reactions H2 -+ OH + H

8 Contents 4.2. OH + H2 --> H20 + H CH 4 --> OH + CH OH + CO --> CO2 + H 5. Summary and conclusions 6. Acknowledgments 7. References Chapter 4. Rate Coefficients of Thermal Dissociation, Isomerization, and Recombination Reactions William C. Gardiner, Jr. and Jurgen Troe 2. General mechanism of thermal dissociation and recombination reactions 3. Low-pressure rate coefficients 4. High-pressure rate coefficients 5. Rate coefficients in the intermediate fall-off range 6. Conclusions 7. References Chapter 5. Rate Coefficients in the C/H/O/System Jurgen Warnatz 1.1. Principles 1.2. Organization 1.3. Earlier reviews of rate data on hydrocarbon combustion 2. General features of high-temperature hydrocarbon combustion 2.1. Radical-poor situation: ignition and induction periods 2.2. Radical-rich situation: flame propagation 3. Reactions in the H 2 /0 2 system 3.1 Reactions in the H2/0Z system not involving HOz or H20z 3.2. Formation and consumption of H Formation and consumption of H Reactions of CO and CO 2 5. Reactions of C I-hydrocarbons 5.1. Reactions of CH Reactions of CH Reactions of CH Reactions of CHO 5.5. Reactions of CH Reactions of CH 5.7. Reactions of CH 30H and CH 3 O/CH2OH 6. Reactions of Cz-hydrocarbons 6.1. Reactions of C2H 6 ix

9 x 6.2. Reactions of C2HS 6.3. Reactions of C 2 H Reactions of C 2 H Reactions of C 2 H Reactions of C 2 H 6.7. Reactions of CH3CHO and CH3CO 6.8. Reactions of CH2CO and CHCO 7. Reactions of Cr and C 4 -hydrocarbons 7.1. Thermal decomposition and attack of H, 0, OH, and H02 on propane and butane 7.2. Thermal decomposition of C3H7 and C 4 H Reactions of propene and butene 7.4. Reactions of C 3 H4 and C4H 2 8. Mechanism of small hydrocarbon combustion 9. Acknowledgments 10. References Contents Chapter 6. Survey of Rate Constants in the N/H/O System Ronald K. Hanson and Siamak Salimian 2. Organization 3. N/O reaction survey N2 --> N + NO NO --> N + O NO + M --> N M 3.4. N 20 + M --> N M N20 --> NO + NO N20 --> N2 + O2 4. N/H reaction survey 4.1. NH3 + M --> NH2 + H + M 4.2. NH3 + M --> NH + H2 + M 4.3. H + NH3 --> NH2 + H H + NH2 --> NH + H2 5. N/H/O reaction survey 5.1. NH3 + OH --> NH2 + H NH > NH2 + OH 5.3. H02 + NO --> N02 + OH 5.4. H + NO --> N + OH 5.5. NH + NO --> N20 + H 5.6. H + N20 --> N2 + OH 5.7. NH2 + O2 --> products 5.8. NH2 + NO -+ products 6. N/H/O rate constant compilation 7. References

10 Contents Chapter 7. Modeling Michael Frenklach xi Basic concepts and definitions 3. Construction of models 3.1. The nature of a model 3.2. Empirical models 3.3. Physical models 4. Parameter estimation 4.1. Preliminary remarks 4.2. Linear models 4.3. Nonlinear models 5. Adequacy of fit 6. Design of experiments 7. Dynamic models in chemical kinetics 7.1. Preliminary remarks 7.2. Trial models 7.3. Sensitivity analysis 7.4. Optimization and interpretation of results 7.5. Further look at sensitivities 8. Closing remark 9. Acknowledgments 10. References Chapter 8. Thermochemical Data for Combustion Calculations Alexander Burcat The polynomial representation Extrapolation Thermochemical data sources Approximation methods Thermochemical polynomials in combustion chemistry Required accuracy of thermochemical information Acknowledgment References 470 Appendix A Program for finding coefficients of NASA Polynomials 475 Appendix B Program Written by A. Lifshitz and A. Burcat for Evaluating the Coefficients of the Wilhoit Polynomials 481 Appendix C Table of Coefficient Sets for NASA Polynomials 485

11 Contributors Alexander Burcat, Department of Aeronautical Engineering, Technion Israel Institute of Technology, Haifa, Israel. Graham Dixon-Lewis, Department of Fuel and Energy, The University, Leeds LS2 9JT, England. Michael Frenklach, Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A. William C. Gardiner, Jr., Department of Chemistry, University of Texas, Austin, Texas 78712, U.S.A. Ronald K. Hanson, Department of Mechanical Engineering, Stanford University, Stanford, California 94305, U.S.A. Siamak Salimi an, Department of Mechanical Engineering, Stanford University, Stanford, California 94305, U.S.A. Jiirgen Troe, Institute for Physical Chemistry, University of Gottingen, D-3400 Gottingen, Federal Republic of Germany. Jiirgen Warnatz, Institute for Physical Chemistry, University of Heidelberg, D-6900 Heidelberg, Federal Republic of Germany. Reinhard Zellner, Institute for Physical Chemistry, University of G6ttingen, D-3400 Gottingen, Federal Republic of Germany.