Impact of flue gas recirculation methods on NO emissions and flame stability in a swirling methane burner

This study investigates NOx emissions and flame stability using three flue gas recirculation methods: air-side recirculation (FGR), fuel-side recirculation (FIR), and combined recirculation (FGR+FIR). Experiments are conducted with varying combustion air inlet temperatures, recirculation ratios, and equivalence ratios for methane-air flames in a 2 kW swirl burner. Nonintrusive in-situ measurements of O2 and NO are performed using tunable diode laser absorption spectroscopy (TDLAS), an advanced measurement technology. Among the three methods, FGR exhibits the widest extinction limit, while FGR+FIR reaches similar extinction limits to FGR with the supply of preheated combustion air. FGR+FIR reduces NO emissions by up to 65% at a lean equivalence ratio of 0.8. Numerical simulations conducted using Cantera offer valuable insights into the NOx formation mechanisms. The NO emission indices decrease by 50% for both FGR+FIR and FGR at a recirculation ratio of 20%. NO production primarily results from thermal NO and NNH mechanisms. Notably, reaction fluxes for the N2O-intermediate mechanism are the lowest in FGR+FIR. FGR+FIR demonstrates approximately 50% reduction of NO emissions at a 20% recirculation ratio under stable combustion conditions in both experiments and simulations. This study proposes FGR+FIR as the optimal recirculation method for low-NOx burners, offering effective NOx emission control and stable combustion performance.