Combustion and Pollution Control in Heating Systems

Combustion and Pollution Control in Heating Systems

v. I. Hanby Combustion and Pollution Control in Heating Systems With 44 Figures Springer-Verlag London Berlin Heidelberg New York Paris Tokyo Hong Kong Barcelona Budapest

Victor Ian Hanby, BSc, PhD, CEng, MInstE, MCmSE Department of Civil Engineering, University of Technology, Loughborough, Leicestershire LEII 3TU, UK ISBN-13:978-3-S4G-19849-9 DOl: 10.l007/978-3-4471-2071-1 e-isbn-13:978-3-4471-2071-1 British Library Cataloguing in Publication Data Hanby, V.I. Combustion and Pollution Control in Heating Systems I. Title 621.402 ISBN-13:978-3-540-19849-9 Library of Congress Cataloging-in-Publication Data Hanby, V. I. (Victor Ian), 1942- Combustion and pollution control in heating systems 1 V.I. Hanby. p. em. Includes bibliographical references and index. ISBN-13:978-3-540-19849-9 I. Heating-Equipment and supplies. 2. Hydrocarbons-Combustion. 3. Flue gases-purification. I. Title. TH7345.H36 1993 93-27560 697-dc20 CIP Apart from any fair dealing for the putposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. Springer-Verlag London Limited 1994 The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Typeset by Richard Powell Editorial and Production Services, Basingstoke, Hants, RG224TX. 69/3830-543210 Printed on acid-free paper

Preface Combustion is very much an interdisciplinary topic, drawing together elements of chemistry, fluid mechanics and heat transfer. It is an ingredient in many undergraduate degree programmes, ranging from a pivotal role in fuel science through to a component part of courses in chemical, process and building services engineering. For many students in those disciplines where combustion in heating plant is an important part of their studies, there are often problems in coming to grips with the basic principles underlying the combustion of hydrocarbon fuels. In particular, the concepts of chemical and related thermodynamic changes can prove difficult to assimilate. The scientific literature dealing with combustion tends to be rather polarised, with a wealth of literature aimed at the specialist reader, but at a basic level the fundamentals of this important process are often treated rather tersely in textbooks on thermodynamics. The objective of this book is to provide an introduction to the basic principles of the combustion of hydrocarbon fuels in heating plant for buildings and industrial processes. In those chapters where practice in problem solving can make a positive contribution to understanding, some numerical problems have been included. Acknowledging the ever-widening use of computers in technical education, a number of algorithms which can be easily coded up for solving numerical problems have been incorporated in the text. These can prove particularly useful in, for example, the calculation of certain fluid properties, either for use in hand calculation or for incorporation into larger programs. It would be wholly inappropriate to leave out some discussion of the properties of hydrocarbon fuels and the equipment in which they are burned, but this has been deliberately confmed to an outline treatment in Chapters 7, 8 and 9. There are many sources of information dealing with the practical aspects of combustion equipment, and reference is made in the text to a selection of appropriate books and guides to current practice. Barrow-upon-Soar, 1994 V.l. Hanby

Contents 1 The Combustion of Hydrocarbon Fuels 1 1.1 Introduction 1 1.2 The Combustion Process 2 1.3 The Complete Combustion Reaction 2 1.4 Quantification of the Combustion Reaction 4 :1 Stoichiometric Calculations 7 2.1 Applications of the Combustion Equation 7 2.2 Combustion Air Requirements - Gaseous Fuels 7 2.3 Flue Gas Composition - Gaseous Fuels 9 2.4 Combustion Air Requirements - Solid and Liquid Fuels 11 2.5 Combustion Products - Solid and Liquid Fuels 13 2.6 Practical Significance of the Flue Gas Composition 15 2.7 Sub-stoichiometric Combustion 19 2.8 Problems 20 3 Heat Release in Combustion 23 3.1 Introduction 23 3.2 Constant-pressure Combustion 23 3.3 Enthalpy of a Mixture of Gases 24 3.4 Enthalpy of Combustion 25 3.5 Constant-volume Combustion 27 3.6 Internal Energy of Combustion 27 3.7 Relationship between Ml 2S and ~ U 2 S 28 3.8 Calorific Values 30 3.9 List of Symbols 34 3.10 Problems 34

viii CONTENTS 4 Flame Temperature 36 4.1 Energy Balance on a System 36 4.2 Adiabatic Flame Temperature 37 4.3 Specific Heats of Gases 38 4.4 Calculation Algorithm 41 4.5 Calculated Adiabatic Flame Temperatures 43 4.6 References 46 4.7 List of Symbols 46 4.8 Problems 46 5 Equilibrium Composition of Flames 48 5.1 Introduction 48 5.2 Chemical Equilibrium 49 5.3 Calculation of the Equilibrium Composition 52 5.4 Dissociated Flame Temperature 54 5.5 Dissociation with Hydrocarbon Fuels 56 5.6 References 59 5.7 List of Symbols 59 6 Efficiency of Combustion Plant 61 6.1 Plant Efficiency 61 6.2 Direct Detennination of Efficiency 64 6.3 Inferential Measurement of Efficiency 65 6.4 Efficiency and Flue Gas Temperature 69 6.5 Flue Gas Dew Point 70 6.6 Efficiency of a Condensing Boiler 72 6.7 References 75 6.8 List of Symbols 75 6.9 Problems 76 7 Gaseous Fuels 78 7.1 Introduction 78 7.2 Natural Gas 78 7.3 Town Gas (Coal Gas) 79 7.4 Liquefied Petroleum Gas (LPG) 80 7.5 Combustion of Gaseous Fuels 81 7.5.1 Flammability Limits 81 7.5.2 Burning Velocity 82 7.5.3 Wobbe Number 83 7.6 Gas Burners 86 7.6.1 Diffusion Burners 86 7.6.2 Premixed Burners 86 7.7 References 87

CONTENTS Ix 7.8 List of Symbols 8 Liquid Fuels 8.1 Occurrence and Processing 8.2 Properties of Oil Fuels 8.3 Combustion of Liquid Fuels 8.4 Pressure Jet Burners 8.5 Rotary Cup Burners 8.6 Twin-fluid Atomisers 8.7 Storage of Liquid Fuels 8.8 References 9 Solid Fuels 9.1 Introduction 9.2 Coal Classification 9.3 Coal Properties 9.4 Coal Combustion 9.5 Coal Storage and Handling 9.6 References 10 Emissions from Combustion Plant 10.1 Environmental Considerations 10.2 The Greenhouse Effect 10.3 Carbon Dioxide Emissions 10.4 Carbon Monoxide 10.5 Oxides of Nitrogen 10.5.1 Atmospheric Nitrogen 10.5.2 Fuel-bound Nitrogen 10.5.3 NO x Levels 10.5.4 Low-NO x Burner Design 10.6 Oxides of Sulphur 10.6.1 The SOx Problem 10.6.2 Sulphur Trioxide Formation 10.6.3 Flue Gas Desulphurisation 10.7 Particulate Emissions 10.7.1 Solid Formation in Combustion 10.7.2 Soot Formation 10.7.3 Cenosphere Formation 10.7.4 Acid Smut Formation 10.7.5 Ash 10.8 List of Symbols 10.9 References 11 Flues and Chimneys 88 89 89 90 92 93 94 94 95 95 96 96 97 98 99 102 102 103 103 103 104 106 106 107 108 109 110 112 112 112 114 115 115 115 116 116 116 117 117 118

x CONTENTS 11.1 Functions of the Flue System 11.2 Chimney Heat Transfer 11.2.1 Heat Transfer Mechanism 11.2.2 U-value ofa Chimney 11.2.3 Temperature Distribution in a Chimney 11.3 Pressure Loss 11.3.1 Chimney Draught 11.3.2 Flow Resistance 11.4 Chimney Location 11.5 List of Symbols 11.6 References Index 118 119 119 119 127 130 130 131 134 135 136 139