Thermal effects of CO₂ on the NO_x formation behavior in the CH₄ diffusion combustion system

The objective of this present study was to numerically investigate the quantitative thermal effects of CO₂ replacement of N₂ in the oxidizer side on NO_x formation for CH₄ diffusion combustion system established in counterflow configuration at atmospheric pressure. The mole fraction of O₂ in the oxidizer was kept constant at 21%. Calculations were conducted at different stretch rates and different percentage of CO₂ in the oxidizer. Calculations were also carried out with and without radiation to quantify the effects of radiation heat transfer. For this simulation, a modified CHEMKIN code and a radiation code developed by the Combustion Group of ICPET were used. Mainly due to its much higher specific heat, replacement of nitrogen in air by carbon dioxide significantly lowered the temperature of the flame, leading to much lower NO concentrations. For a specified stretch rate, there existed a maximum percentage of CO₂ in the oxidizer beyond which flame extinction occur. The value of the maximum percentage of CO₂ in the oxidizer decreased as the stretch rate increased. Effect of radiation heat transfer become more important as the stretch rate decreased. Replacement of N₂ by CO₂ in the oxidizer was an effective way to control NO_x formation in CH₄ diffusion flames mainly due to its thermal effect and secondly due to its chemical and radiative effects.