Operational Control of the Nitric Oxide Emissions from Stationary Boilers
Stationary boilers, such as those used in the utility industry, are a major combustor of fuel in any industrial nation. Besides being such an important part of industry, these boilers have a unique characteristic which is not available to other combustion devices; they are designed for heat removal or in more technical terms, they are inherently nonadiabatic within their combustion zone. This characteristic can be used to control the temperature history during combustion and thus to control pollutant emissions. In practice, such emission control involves understanding the combustion and pollutant formation mechanisms plus the ability to work on large scale heavy equipment. Plant operating changes have been accomplished with five major utility companies in California and New York and have led to 70–80% reductions in nitric oxide emission.
This paper will first develop an understanding of nitric oxide formation from the point of chemical kinetics, turbulent mixing, and the statistical nature of ignition/ combustion phenomena. Next, this understanding will lead to an exploration of the practical ways in which the combustion related temperature histories and resultant nitric oxide may be controlled, particularly by use of the nonadiabatic characteristic of boilers. The means of temperature and nitric oxide kinetic control which have been investigated include off-stoichiometric combustion, product gas recirculation, lowering air preheat, and water/steam injection.
Each method of combustion profile control will be then discussed from the point-of-view of application to large boiler systems, the limits of application, and the results which have been obtained. Within these results, a wide range of system parameters have been investigated and the effects of boiler size, firing pattern, excess air operation, oil characteristics, and gas injection characteristics are explained. Basically, boiler construction requires the design of a radiant section for the loss of heat so that gases are below approximately 2400°F before entering convective tube sections. Because of this basic fact, parameters may be adjusted over a wide range of different boiler and fuel configurations so that essentially the same minimum nitric oxide operation may be achieved boiler-by-boiler with any of the available control techniques. Each nitric oxide control technique must be demonstrated to be long-term operationally feasible, to produce reduced combustible emission and to not interfere with plant efficiency.
A large amount of our work has been directed toward the extension of current plant operating data to predict new plant nitric oxide control. Our predictions are discussed in light of current and proposed pollution control regulations. The effects of different regulations in terms of ppm, lb./hr., and lb./MW or lb./Btu are shown in specific applications and the control techniques required to meet particular situations are discussed. Specifically, the way in which the wide variety of different regulations apply to actual plants is demonstrated.
Finally, the atmospheric nonadiabatic combustor is demonstrated to provide low emission characteristics in actual large scale operation through techniques which have wide range application. Future analytical modeling of the combustion processes and nitric oxide formation will point out further reductions and the equipment design direction which the nonadiabatic combustor must take. Such analytical modeling will strongly depend on an understanding of turbulence and flame holding/ignition processes.
KeywordsNitric Oxide Equivalence Ratio Diffusion Flame Nitric Oxide Formation Utility Boiler
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