Pressure effect on NO emission in methane/air lean-premixed flames

Nitrogen oxides (NOx) from combustion system are one of major pollutants which causes photochemical smog and ozone layer depletion. The interest in lean-premixed combustion has increased to reduce NOx emissions in the industrial gas-turbine as a suitable reduction strategy. The present study investigates methane/air premixed flames with a detailed chemical kinetic model to better understand the pressure effect on NOx formation. A detailed chemical kinetic model is developed by merging AramcoMech 3.0 and recently proposed nitrogen chemistry. The proposed mechanism is first validated against experimental data, including laminar flame speed, ignition delay times, and NO concentration in premixed flames at various pressures. Freely propagating methane/air lean-premixed flames are simulated over a pressure range 1-20 atm and equivalence of 0.5, 0.55 and 0.6. Prompt NO formation is dominant within a narrow heat release region and thermal NO production pathways lead to the total NO formation in the postflame zone. Prompt NO formation rate increases between 1 and 5 atm and then decreases with further increasing pressure due to pressure dependent NCN formation rates. On the other hand, the formation rate of NO in the postflame zone increases monotonically as pressure increases.