Kinetics of nitric oxide formation incombustion processes

Abstract

Recent investigations of nitric oxide formation in combustion are reviewed with the objectiveof obtaining mechanistic information on nitric oxide formation from atmospheric (molecular) nitrogen and from nitrogen-containing compounds in the fuel. Comparison of existing data on formation of nitric oxide from atmospheric nitrogen with results from detailed kinetics calculations shows that, for the most part, the experimental observations are consistent with a three-reaction mechanism for nitric oxide chemistry if radical concentrations and temperature during combustion are correctly evaluated. The principal nitric oxide formation reactions are O+N2”NO+N, (1) N+O2”NO+O; (2) however, N+OH”NO+H, (3) can be of importance in fuel-rich mixtures. Experimental studies of the formation of nitric oxide from nitrogen-containing compounds indicate that the formation rates are rapid, occurring on a time-scale comparable to that of the combustion reactions. A simplified mechanism for conversion of nitrogen-containing compounds to nitric oxide is proposed. In this mechanism, a partial equilibrium approximation is used to relate the maximum nitric oxide concentration in the combustion zone to the maximum radical concentrations. The proposed mechanism adequately models nitric oxide formation in the shock-induced combustion of lean hydrogen-oxygen mixtures containing small amounts of ammonia.