Study of NO emission from CH4-air, oxygen-enriched, and oxy-CH4 combustion under HTC and MILD regimes: Impact of wall thermal condition in different oxidant temperature and dilution level

The present work investigates the effect of wall thermal conditions on the NO emission in the different inlet temperatures and oxidant dilution levels. The studies are performed for the CH4-air, oxygen-enriched, and oxy-CH4 combustion under the HTC and MILD regimes by numerical simulation of the non-premixed furnace and chemical calculations of the WSR reactor. It is found that higher heat loss from the wall and more replacement of CO2 with N2 facilitate the establishment of the MILD regime and significant reduction of NO production. The results show an optimum point for achieving the minimum heat loss and NO emission. Under the MILD regime, dominant mechanisms in the NO emission are prompt through reaction CH + N2⇌HCN + N and thermal routes for the CH4-air condition and N2O-intermediate by reactions N2O + O⇌2NO and N2O + H⇌N2+OH and thermal mechanisms for oxy-CH4 combustion. In both atmospheres, the thermal route plays the main role in the low heat loss from walls, while other routes are more active in the low maximum flame temperature. Reaction N + NO⇌N2+O by supplying O radical from reactions O + CO + M⇌CO2+M and H + O2+M⇌HO2+M is activated by increasing O2 level, while reactions N + NO⇌N2+O and N + OH⇌NO + H show the main contribution in the sensitivity analysis of NO.