The role of methylene in prompt NO formation

We address the plausibility of singlet methylene (1CH2) 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 CH2 with molecular nitrogen to form diazomethane, CH2NN, early in the flame. Although the majority of the diazomethane is predicted to react with flame radicals to regenerate N2, a small percentage (approximately 10%) is predicted to react via cleavage of the NN 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 CH2, the large underprediction of NO under lean conditions can be rectified by the inclusion of the 1CH2 prompt NO pathway in the kinetic mechanism.