The role of mixing in burner-generated carbon monoxide and nitric oxide
TL;DR: In this article, a simple one-dimensional flow model is coupled with the mixing model to predict the variation of CO and NO concentrations with atomizing pressure and distance along the burner.
About: This article is published in Combustion and Flame.The article was published on 1972-12-01. It has received 55 citations till now. The article focuses on the topics: Combustor & Mixing (process engineering).
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01 Jan 1988
TL;DR: In this paper, the authors present a rigorous and fundamental analysis of the production of air pollutants and their control, including the formation of gaseous pollutants in combustion processes, and a thorough treatment of the internal combustion engine, including its principles of operation and the mechanisms of formation of pollutants therein.
Abstract: 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 10-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, can be quite effective in reducing their formation. Most situations, whether a 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 of air pollutants can be termed air pollution 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 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 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 of Air 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
749 citations
TL;DR: In this article, the authors reviewed the progress achieved so far, discusses the various definitions of the flammeless combustion regime, and attempts to point the directions for future research, and analyzes the most promising approaches for the prediction of reaction zones and pollutant emissions.
146 citations
TL;DR: In this paper, the effect of temporal fuel concentration fluctuations and spatial non-uniformities of mean concentration profiles on NO V emissions from lean premixed combustion was investigated and the results showed that temporal unmixedness contributes significantly to higher NOX emissions.
Abstract: Unmixedness effects of lean fuel-air mixtures on emissions of nitrogen oxides (NOV) are experimentally studied. The objective is to study the relative influence of temporal fuel concentration fluctuations and spatial nonuniformities of mean concentration profiles on NO V emissions from lean premixed combustion. The NO2 laserinduced fluorescence (LIF) technique is used to quantify Unmixedness levels at the flameholder. Unmixedness data are related to NOr emissions measurements from downstream of the flames. The results show that temporal unmixedness contributes significantly to higher NOX emissions. It is not sufficient to eliminate only spatial nonuniformities of mean concentration in order to minimize NO_V emissions from lean premixed combustion. Nomenclature c = volumetric fuel concentration c' = rms fluctuation component of c c - time-mean fuel concentration Dc = combustor diameter Z)cf = coflow diameter Dj = jet diameter Recf = coflow Reynolds number Rej - jet Reynolds number U = unmixedness parameter Xj - distance from jet to flameholder z - vertical coordinate; origin at combustor centerline = fuel-air equivalence ratio
134 citations
TL;DR: In this article, the U.S. Environmental Protection Agency standards and Air Force goals for aircraft jet engine emissions of unburned hydrocarbons (HC), CO and NOx are reviewed in terms of the contribution of air transport to the overall pollution problem.
80 citations
TL;DR: In this paper, a stochastic model of turbulent mixing has been developed for a reactor in which mixing is represented by n-body fluid particle interactions, and rate-limited upper and lower bounds of the nitric oxide produced by thermal fixation of molecular nitrogen and oxidation of organically bound fuel nitrogen are compared with experimental measurements obtained using a laboratory burner operated over a wide range of test conditions.
68 citations
References
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01 Jan 1948
TL;DR: In this article, a simplified treatment for mixing of nozzle and ambient fluids in a vertical jet is presented, where the simplifying assumption of constant velocity and composition in a cross-section normal to the axis of flow is combined with a force-momentum balance, continuity and the perfect gas laws to obtain a relation between mean concentration and jet spread.
Abstract: Visible flame lengths and concentration patternshave been obtained in turbulent jets of flame formed by combustible gas issuing from circular nozzles into stagnant air. The nozzle velocities were above those which, in a previous paper, were found necessary to insure that the mixing should be turbulent. As a basis for analysis of the data a simplified treatment is presented for mixing of nozzle and ambient fluids in a vertical jet. The simplifying assumption of constant velocity and composition in a cross-section normal to the axis of flow is combined with a force-momentum balance, continuity, and the perfect gas laws to obtain a relation between mean concentration and jet spread. The relation allows for initial difference in density of nozzle and ambient streams, density variation due to combustion, and buoyancy. The qualitative agreement between the analysis and the experimental data on visible flame lengths and axial concentration patterns indicates plainly that the process of mixing resulting from the momentum and buoyancy of the jet is the controlling factor in determining progress of the combustion. For tree flames in which the effects of buoyancy are small (high nozzle velocity, small diameter) the analysis leads to the following simple relation for the length of free turbulent flame jets:
L/D=5.3CτTFαTTN[Cτ+(1−Cτ)MSMN]
where L=visible flame length D=nozzle diameter TF=adiabatic flame temperature, absolute TN=absolute temperature of nozzle fluid MS, MN=molecular weights of surrounding and nozzle fluids, respectively CT=mol fraction of nozzle fluid in the unreacted stoichiometric mixture αT=mols of reactants/mols products, for the stoichiometric mixture. It is to be noted that fuel gas flow rate is no factor, as long as it is great enough to prodace a turbulent jet. Although data for testing this relation covered the small range of port diameter of 0.12 to 0.30 inches, a wide variety of fuels was studied, including propane, acetylene, hydrogen, carbon monoxide, city gas, mixtures of carbon dioxide with city gas, and mixtures of hydrogen with propane. Turbulent flame lengths varying from 40 to 290 nozzle diameters are predicted with average and
240 citations
TL;DR: In this article, the rate of decrease in concentration fluctuations of a scalar contaminant is estimated in terms of the turbulence scale and the power input to the system with stationary isotropic turbulence.
Abstract: With stationary isotropic turbulence postulated, the rate of decrease in concentration fluctuations of a scalar contaminant is estimated in terms of the turbulence scale and the power input to the system.
85 citations
25 Jan 1971
60 citations