Quantitative analyses of NO formations from different reaction pathways in methane-air combustion at various pressures and temperatures

Abstract

Numerical calculations of the laminar counterflow diffusion flames of methane-air are conducted to quantitatively study NO formations from different reaction pathways (mechanisms) at various pressures and/or initial air temperatures. A modified method, which deactivates the initiating reactions in a specific reaction pathway only for NO production while maintaining interactions with other pathways, is developed to calculate NO formation in each mechanism. Results indicate that in the thermal, prompt, N2O, and NNH reaction mechanisms, the NO production rate largely increases as the pressure or the initial air temperature increases, and in particular, as these two parameters are increased together as in a practical combustion system. In the thermal pathway, as the pressure is increased to 15 atm, NO is produced almost entirely on the fuel-lean side. In the prompt pathway, NO production generally occurs in regions with the progress variable ranging from 0.8 to 0.95; the maximum NO production rate moves from the fuel-rich towards the stoichiometric region as the pressure is increased. NO formation from the N2O or NNH mechanism occurs mainly at high pressures, but the corresponding production rate is an order of magnitude smaller than that from the thermal or prompt mechanism, respectively. In all the reaction mechanisms, NO consumption occurs through the reburn mechanism on the fuel-rich side, and the consumption rate significantly increases as the pressure and initial air temperature increase. Results herein provide a quantitative understanding of the effects of pressure and temperature on NO formations from different reaction mechanisms.