Numerical evaluation of NOx mechanisms in methane-air counterflow premixed flames

The control of nitrogen oxides (NO_x) has been a major issue in designing combustion systems, since NO_x play a key role in ozone depletion and the generation of photochemical smog. The characteristics of NO_x emission can be essential information for the development of a clean combustor having suitable reduction methodologies. In the present study, NO_x emission characteristics were evaluated numerically, accounting for the effect of equivalence ratio, stretch rate, pressure, and initial temperature. In general, peak NO_x emission appeared near the equivalence ratio of unity case, and NO_x emission increased with pressure and initial temperature due to the temperature sensitivity in NO_x mechanism. NO_x decreased with stretch rate due to the decrease in residence time in high temperature region. Furthermore, the thermal and prompt mechanisms were evaluated with equivalence ratio for two calculation methods. The conventional methods ignore the interaction of coupled mechanism of thermal and prompt NO_x. The reaction path diagram was introduced to understand effective reaction pathways in various conditions.