Combustion control for elimination of nitric oxide emissions from fossil-fuel power plants

Abstract

This paper describes the techniques currently available for the reduction of oxides of nitrogen in power-plant utility boilers. Combustion techniques of delayed mixing, premixed off-stoichiometric flames, off-stoichiometric diffusion flames (secondary combustion), and direct temperature control were examined theoretically by the use of a chemical kinetics digital computer program. The governing chemical reactions are presented, along with the kinetic rate constants utilized, and the results of the theoretical analysis. Tests were conducted on several full-size utility boilers to evaluate some of the possible operational methods. Using off-stoichiometric combustion, obtained by shutting off selected burner fuel flows and maintaining air flow, NO concentrations were decreased from 470 ppm to approximately 180 ppm. Direct temperature control tests were performed, by reducing combustion air preheat, at air preheats of 650°F and 580°F. A decrease in NO formation from 250 to 200 ppm was obtained. Another method of direct temperature control tested was combustion product (exhaust) gas recirculation. In this test, the NO concentration was reduced from 350 to 150 ppm for a recirculation ratio of approximately 20%. Combustion control often can be the most economical and practical method of reducing NO emissions. Current chemical kinetics technology allows quantitative estimates to be made of the NO reductions obtained by altered combustion. This permits more rapid improvement of existing plants, and provides an additional design guide for new boilers.