The role of methylene in prompt NO formation

Abstract We address the plausibility of singlet methylene ( 1 CH 2 ) in the prompt NO formation mechanism via examination of experimental species profiles and kinetic flame modeling of several low-pressure methane–oxygen–nitrogen flames. Existing kinetic models assuming CH as the only prompt NO precursor greatly underpredict NO formation under very fuel-lean conditions. We have constructed a kinetic pathway initiated by the recombination of singlet CH 2 with molecular nitrogen to form diazomethane, CH 2 N N, early in the flame. Although the majority of the diazomethane is predicted to react with flame radicals to regenerate N 2 , a small percentage (approximately 10%) is predicted to react via cleavage of the N N bond leading to NO formation. This leads to accurate prediction of the experimental measurements of NO formation in lean, low-pressure flames. Assuming reasonable kinetic parameters for the reactions of CH 2 , the large underprediction of NO under lean conditions can be rectified by the inclusion of the 1 CH 2 prompt NO pathway in the kinetic mechanism.