Characteristics of NOx emissions of counterflow nonpremixed water-laden methane/air flames

A computational investigation on the characteristics of nitrogen oxides (NOx) emissions for counterflow nonpremixed water (H2O)-laden methane (CH4)/air flames is conducted since a detailed observation of NOx formation for fuel having naturally high H2O vapor content is necessary. Using a detailed kinetic mechanism, NOx emissions are predicted for low and high flame strain rates (a), and the reaction paths are investigated. With H2O addition NOx emissions are reduced due to the chemical process as well as the thermal process such as diluting and cooling effects, the latter is more dominant than the former in NOx reduction, and the thermal and prompt NO mechanisms become less dominant for the chemical process. With increasing a, NOx emissions are also reduced, and it is mainly due to the thermal process. Reaction NH + O = NO + H in the prompt pathway is the major reaction step that results in reducing NO emissions via the chemical process due to H2O addition for both low- and high-stretched flames, though reaction N + OH = NO + H in the thermal pathway is also the major reaction step for the high-stretched flames. In addition, for the high-stretched flames the NO2 pathway in NO production becomes relatively more important with H2O addition.