fundamentals Of AIR POLLUTION ENGINEERING

Transcription

1 fundamentals Of AIR POLLUTION ENGINEERING

2 fundamentals Of AIR POLLUTION ENGINEERING Richard C. Flagan John H. Seinfeld California Institute of Technology PRENTICE HALL Englewood Cliffs, New Jersey 07632

3 Library ofcongress Cataloging-in-Publication Data Flagan, Richard C. (date) Fundamentals of air pollution engineering. Includes bibliographies and index. \. Air-Pollution. 2. Environmental engineering. 1. Seinfeld, John H. II. Title. TD883.F ' ISBN Editorial/production supervision and interior design: WordCrafters Editorial Services, Inc. Cover design: Ben Santora Manufacturing buyer: Cindy Grant 1988 by Prentice-Hall, Inc. A Division of Simon & Schuster Englewood Cliffs, New Jersey All rights reserved. No part of this book may be reproduced, in any form or by any means, without permission in writing from the publisher. Printed in the United States of America Prentice-Hall International (UK) Limited, London Prentice-Hall of Australia Pty. Limited, Sydney Prentice-Hall Canada Inc., Toronto Prentice-Hall Hispanoamericana, S.A., Mexico Prentice-Hall of India Private Limited, New Delhi Prentice-Hall of Japan, Inc., Tokyo Simon & Schuster Asia Pte. Ltd., Singapore Editora Prentice-Hall do Brasil, Ltda., Rio de Janeiro

4 Contents Preface Chapter 7 AIR POLLUTION ENGINEERING 1.1 Air Pollutants Oxides of Nitrogen Sulfur Oxides Organic Compounds Particulate Matter 1.2 Air Pollution Legislation in the United States 1.3 Atmospheric Concentration Units 1.4 The Appendices to this Chapter A Chemical Kinetics A.1 Reaction Rates A.2 The Pseudo-Steady-State Approximation A.3 Hydrocarbon Pyrolysis Kinetics B Mass and Heat Transfer B.1 Basic Equations of Convective Diffusion B.2 Steady-State Mass Transfer to or from a Sphere in an Infinite Fluid B.3 Heat Transfer B.4 Characteristic Times C Elements of Probability Theory C.1 The Concept of a Random Variable C.2 Properties of Random Variables C.3 Common Probability Distributions xi v

5 vi Contents Chapter 2 D Turbulent Mixing D. 1 Scales of Turbulence D.2 Statistical Properties of Turbulence D.3 The Microscale D.4 Chemical Reactions E Units Problems References COMBUSTION FUNDAMENTALS 2.1 Fuels 2.2 Combustion Stoichiometry 2.3 Combustion Thermodynamics First Law of Thermodynamics Adiabatic Flame Temperature Chemical Equilibrium Combustion Equilibria 2.4 Combustion Kinetics Detailed Combustion Kinetics Simplified Combustion Kinetics 2.5 Flame Propagation and Structure Laminar Premixed Flames Turbulent Premixed Flames Laminar Diffusion Flames Turbulent Diffusion Flames 2.6 Turbulent Mixing 2.7 Combustion of Liquid Fuels 2.8 Combustion of Solid Fuels Devolatilization Char Oxidation Problems References Chapter 3 POLLUTANT FORMATION AND CONTROL IN COMBUSTION Nitrogen Oxides Thermal Fixation of Atmospheric Nitrogen Prompt NO Thermal-NO x Formation and Control in Combustors Fue1-NO x Fuel-NO x Control 191

6 Contents vii Postcombustion Destruction of NO x Nitrogen Dioxide 3.2 Carbon Monoxide Carbon Monoxide Oxidation Quenching 3.3 Hydrocarbons 3.4 Sulfur Oxides Problems References Chapter 4 INTERNAL COMBUSTION ENGINES Spark Ignition Engines Engine Cycle Operation Cycle Analysis Cylinder Turbulence and Combustion Rate Cylinder Pressure and Temperature Formation of Nitrogen Oxides Carbon Monoxide Unburned Hydrocarbons Combustion-Based Emission Controls Mixture Preparation Intake and Exhaust Processes Crankcase Emissions Evaporative Emissions Exhaust Gas Treatment Diesel Engine Diesel Engine Emissions and Emission Control Exhaust Gas Treatment Stratified Charge Engines Gas Turbines 280 Problems 286 References 287 Chapter 5 AEROSOLS The Drag on a Single Particle: Stokes' Law Noncontinuum Effects The Knudsen Number Slip Correction Factor Motion of an Aerosol Particle in an External Force Field Terminal Settling Velocity The Stokes Number 304

7 viii Contents Motion of a Charged Particle in an Electric Field Motion of a Particle Using the Drag Coefficient Aerodynamic Diameter Brownian Motion of Aerosol Particles Mobility and Drift Velocity Solution of Diffusion Problems for Aerosol Particles Phoretic Effects Diffusion to Single Particles Continuum Regime Free Molecule Regime Transition Regime The Size Distribution Function Distributions Based on log D p Relating Size Distributions Based on Different Independent Variables The Log-Normal Distribution General Dynamic Equation for Aerosols Discrete General Dynamic Equation Continuous General Dynamic Equation Coagulation Coefficient Brownian Coagulation Effect of van der Waals and Viscous Forces on Brownian Coagulation Homogeneous Nucleation Sectional Representation of Aerosol Processes 347 Problems 349 References 356 Chapter 6 PARTICLE FORMATION IN COMBUSTION Ash Ash Formation from Coal Residual Ash Size Distribution Ash Vaporization Dynamics of the Submicron Ash Aerosol Char and Coke Soot Soot Formation Soot Oxidation Control of Soot Formation Motor Vehicle Exhaust Aerosols 385

8 Contents ix Problems 387 References 388 Chapter 7 REMOVAL OF PARTICLES FROM GAS STREAMS Collection Efficiency Settling Chambers Laminar Flow Settling Chamber Plug Flow Settling Chamber Turbulent Flow Settling Chamber Cyclone Separators Laminar Flow Cyclone Separators Turbulent Flow Cyclone Separators Cyclone Dimensions Practical Equation for Cyclone Efficiency Electrostatic Precipitation Overall Design Equation for the Electrostatic Precipitator Generation of the Corona Particle Charging Field Charging Diffusion Charging The Electric Field Filtration of Particles from Gas Streams Collection Efficiency of a Fibrous Filter Bed Mechanics of Collection by a Single Fiber Flow Field around a Cylinder Deposition of Particles on a Cylindrical Collector by Brownian Diffusion Deposition of Particles on a Cylindrical Collector by Interception Deposition of Particles on a Cylindrical Collector by Inertial Impaction and Interception Collection Efficiency ofa Cylindrical Collector Industrial Fabric Filters Filtration of Particles by Granular Beds Wet Collectors Spray Chamber Deposition of Particles on a Spherical Collector Venturi Scrubbers 467

9 x Contents 7.7 Summary of Particulate Emission Control Techniques 469 Problems 472 References 476 Chapter 8 REMOVAL OF GASEOUS POLLUTANTS FROM EFFLUENT STREAMS Interfacial Mass Transfer Absorption of Gases by Liquids Gas Absorption without Chemical Reaction Gas Absorption with Chemical Reaction Adsorption of Gases on Solids Removal of S02 from Effluent Streams Throwaway Processes: Lime and Limestone Scrubbing Regenerative Processes Removal of NO x from Effluent Streams Shell Flue Gas Treating System Wet Simultaneous NO)SOx Processes Selective Noncatalytic Reduction Selective Catalytic Reduction NOx and SOx Removal by Electron Beam 516 Problems 517 References 519 Chapter 9 OPTIMAL AIR POLLUTION CONTROL STRATEGIES Long-Term Air Pollution Control A Simple Example of Determining a Least-Cost Air Pollution Control Strategy General Statement of the Least-Cost Air Pollution Control Problem A Least-Cost Control Problem for Total Emissions 529 Problems 534 References 534 Index 537

10 Preface Analysis and abatement of air pollution involve a variety of technical disciplines. Formation of the most prevalent pollutants occurs during the combustion process, a tightly coupled system involving fluid flow, mass and energy transport, and chemical kinetics. Its complexity is exemplified by the fact that, in many respects, the simplest hydrocarbon combustion, the methane-oxygen flame, has been quantitatively modeled only within the last several years. Nonetheless, the development of combustion modifications aimed at minimizing the formation of the unwanted by-products of burning fuels requires an understanding of the combustion process. Fuel may be available in solid, liquid, or gaseous form; it may be mixed with the air ahead of time or only within the combustion chamber; the chamber itself may vary from the piston and cylinder arrangement in an automobile engine to a lo-story-high boiler in the largest power plant; the unwanted byproducts may remain as gases, or they may, upon cooling, form small particles. The only effective way to control air pollution is to prevent the release of pollutants at the source. Where pollutants are generated in combustion, modifications to the combustion process itself, for example in the manner in which the fuel and air are mixed, canbe quite effective in reducing theirformation. Most situations, whethera combustion or an industrial process, however, require some degree of treatment of the exhaust gases before they are released to the atmosphere. Such treatment can involve intimately contacting the effluent gases with liquids or solids capable of selectively removing gaseous pollutants or, in the case of particulate pollutants, directing the effluent flow through a device in which the particles are captured on surfaces. The study of the generation and control ofair pollutants can be termed airpollution engineering and is the subject of this book. Our goal here is to present a rigorous and fundamental analysis of the production of air pollutants and their control. The book is xi

11 xii Preface intended for use at the senior or first-year graduate level in chemical, civil, environmental, and mechanical engineering curricula. We assume that the student has had basic first courses in thermodynamics, fluid mechanics, and heat transfer. The material treated in the book can serve as the subject of either a full-year or a one-term course, depending on the choice of topics covered. In the first chapter we introduce the concept of air pollution engineering and summarize those species classified as air pollutants. Chapter 1 also contains four appendices that present certain basic material that will be called upon later in the book. This material includes chemical kinetics, the basic equations of heat and mass transfer, and some elementary ideas from probability and turbulence. Chapter 2 is'a basic treatment of combustion, including its chemistry and the role of mixing processes and flame structure. Building on the foundation laid in Chapter 2, we present in Chapter 3 a comprehensive analysis of the formation of gaseous pollutants in combustion. Continuing in this vein, Chapter 4 contains a thorough treatment of the internal combustion engine, including its principles of operation and the mechanisms of formation of pollutants therein. Control methods based on combustion modification are discussed in both Chapters 3 and 4. Particulate matter (aerosols) constitutes the second major category of air pollutants when classified on the basis of physical state. Chapter 5 is devoted to an introduction to aerosols and principles of aerosol behavior, including the mechanics of particles in flowing fluids, the migration of particles in external force fields, Brownian motion of small particles, size distributions, coagulation, and formation of new particles from the vapor by homogeneous nucleation. Chapter 6 then treats the formation of particles in combustion processes. Chapters 7 and 8 present the basic theories of the removal of particulate and gaseous pollutants, respectively, from effluent streams. We cover all the major air pollution control operations, such as gravitational and centrifugal deposition, electrostatic precipitation, filtration, wet scrubbing, gas absorption and adsorption, and chemical reaction methods. Our goal in these two chapters, above all, is to carefully derive the basic equations governing the design of the control methods. Limited attention is given to actual equipment specification, although with the material in Chapters 7 and 8 serving as a basis, one will be able to proceed to design handbooks for such specifications. Chapters 2 through 8 treat air pollution engineering from a process-by-process point of view. Chapter 9 views the air pollution control problem for an entire region or airshed. To comply with national ambient air quality standards that prescribe, on the basis of health effects, the maximum atmospheric concentration level to be attained in a region, it is necessary for the relevant governmental authority to specify the degree to which the emissions from each of the sources in the region must be controlled. Thus it is generally necessary to choose among many alternatives that may lead to the same total quantity of emission over the region. Chapter 9 establishes a framework by which an optimal air pollution control plan for an airshed may be determined. In short, we seek the least-cost combination of abatement measures that meets the necessary constraint that the total emissions not exceed those required to meet an ambient air quality standard. Once pollutants are released into the atmosphere, they are acted on by a variety of

12 Preface xiii chemical and physical phenomena. The atmospheric chemistry and physics of air pollution is indeed a rich arena, encompassing the disciplines of chemistry, meteorology, fluid mechanics, and aerosol science. As noted above, the subject matter of the present book ends at the stack (or the tailpipe); those readers desiring a treatment of the atmospheric behavior of air pollutants are referred to J. H. Seinfeld, Atmospheric Chemistry and Physics ofair Pollution (Wiley-Interscience, New York, 1986). We wish to gratefully acknowledge David Huang, Carol Jones, Sonya Kreidenweis, Ranajit Sahu, and Ken Wolfenbarger for their assistance with calculations in the book. Finally, to Christina Conti, our secretary and copy editor, who, more than anyone else, kept safe the beauty and precision of language as an effective means of communication, we owe an enormous debt of gratitude. She nurtured this book as her own; through those times when the task seemed unending, she was always there to make the road a little smoother. R. C. Flagan J. H. Seinfeld