NOx formation in counter-flow opposed-jet diffusion CH4/air flames

The present study focuses on the effect of pressure and strain rate on the formation of NOx and NO to NO2 conversion processes. The theoretical investigation was carried out using a stretched counter-flow opposed-jet diffusion flame code. The CHEMKIN thermodynamic and transport database were used in the model and a detailed reaction mechanism from GRI was employed. The fuel used was CH4 with an initial inlet temperature of 298 K reacting with an opposed air flow with an initial temperature of 800 K. Axial profiles of temperature, velocity, species concentration and rate of formation of NO and NO2 were predicted. Theoretical results indicate that as the strain rate is increased and the pressure decreased, the maximum flame temperature decreased, leading to lower NOx emission. The dominant pathway towards NO formation was found to be the prompt mechanism, especially at low pressure (1-5 atm) and high strain rate (400-1000 s(-1)) where its contribution was over 90%. NO2 formed in very lean zones but also exhibited lower peaks in rich zones, indicating the importance of HO2, radicals. The NO to NO2 conversion process was found to be enhanced at high pressure and high strain rate with a maximum ratio of 11.5% at 30 atm and 1000 s(-1).