Chemical kinetic study on the reduction of nitric oxide in highly preheated air combustion

Abstract

Characteristics of nitric oxide formation in methane/air counterflow flames were studied to obtain basic knowledge on highly preheated air combustion in practical industrial furnaces. It had been held that the preheating of combustion air causes a significant increase in nitric oxide emission due to high flame temperatures. However, our recent experiments have shown that the separated supply of fuel and preheated air into a furnace as well as high-momentum injection can reduce nitric oxide emission effectively. The present chemical kinetic calculations have proved that the suppression of nitric oxide emission occurs by combustion in the circumstances of low oxygen concentration. In addition, the calculations have shown that combustion reactions take place easily even in air excessively diluted with inert gas, as long as we use air preheated higher than the autoignition temperature of the fuel. As the fraction of inert diluent, such as nitrogen or carbon dioxide, in the diluted fuel or in the diluted air increases flame temperature, hence, the production of nitric oxides decreases, even though the formation mechanism of nitric oxides is retained. Further, the possibility of destruction of nitric oxide was shown if there are high initial concentrations of nitrix oxide contained in the flue gas used as diluent.