Numerical investigation of NO generation reaction pathway with novel embedded flue-gas internal recirculation combustion loop consisting of flue-gas and CH4-air mixture

The novel embedded flue-gas internal recirculation (FIR) combustion technology has garnered increasing attention in gas burners due to its notable advantages in combustion efficiency and ultra-low NOx emissions. In this study, we design FIR channels for a gas burner and investigate the impact of recirculation ratio (R) on temperature, combustion efficiency, concentrations of free radicals (H, O, CH), and NO concentration via Computational Fluid Dynamics (CFD) method. In addition, this paper explores and elucidates the dynamics of four NO production pathways, namely, thermal NO, prompt NO, N₂O, and NNH, as well as the contributions of the four pathways to total NO. Observations indicate a constrained range of FIR recirculation ratios, specifically between 23.2 % and 35 %. The combustion efficiency of the FIR burner is 96 % and 99.8 % at R = 23.8 % and R = 35.0 %, which is superior to conventional burners. Moreover, as R increases, both pathways for NO generation from N₂O and NNH are intensified as R exceeds 23.8 %. In the embedded FIR combustion system, the contribution of N₂O to total NO increases from 0.23 % to 0.78 %, while the percentage of prompt NO decreases to 0.19 %. These findings provide a development direction and technical guidance for the practical implementation of the embedded FIR technology.