Study of NO_x emission characteristics in CH₄/air non-premixed flames with exhaust gas recirculation

In this study, the characteristics of NO_x emissions for CH₄/air non-premixed flames using EGR (exhaust gas recirculation) methods were investigated using the AI-EGR (air-induced-EGR) and FI-EGR (fuel-induced-EGR) methods. For the fundamental experiment, coaxial non-premixed flames were verified using the non-premixed mode in a changeable EGR hybrid combustion system. For the numerical simulation, the 2-D commercial FLUENT program was used to verify the distributions of the flame temperature and mole fraction of the NO emissions. Additionally, the swirling non-premixed flames were tested to investigate a practical combustion system. Based on experimental results, the reduction rates of EINOX for the FI-EGR method and the AI-EGR were approximately 29% and 28% for an EGR ratio of 20% and 25%, respectively, which represented the maximum range needed to generate a stable flame. Based on numerical results, the FI-EGR method was determined to be more effective than the AI-EGR method in reducing NO_x emission because the high temperature region and the OH distribution region of the FI-EGR method were narrower. According to the results from the swirling flames, the reduction rates of EINOX for the FI-EGR method and the AI-EGR were approximately 49% and 45% for an EGR ratio of 15% and 25%, respectively.