Study on the reaction mechanism of nitrogen migration in pressurized oxy-fuel combustion

Pressurized oxy-fuel combustion (POFC) garners attention for its remarkable NO_x reduction capacity. This study used experimental analysis and numerical simulation to reveal nitrogen migration mechanisms in POFC under different operating conditions. Firstly, the Hashemi mechanism was updated by incorporating char gasification reactions (C + CO₂→2CO and C + H₂O→CO + H₂) and the CO-mediated NO reduction (CO + NO→1/2N₂+CO₂). Subsequently, a chemical reactor network (CRN) model was constructed, with its POFC simulation reliability verified (minimum NO prediction error: 7.52 %). Thirdly, the effects of different conditions on nitrogen migration were systematically investigated. The primary NO generation pathways are VOL-N (volatile nitrogen)→HCN→NCO→NO and VOL-N→HCN→NH→NO, with NH and NCO as key intermediates during the POFC process. Elevated pressure diminishes the formation of these key intermediates and O/OH radicals, inhibiting the oxidative conversion of NH and NCO to NO, consequently decreasing the flux of the pathway NH→NO (34.2 %→25.6 %). Although increasing temperature inhibits the conversion of HCN to NCO and NH intermediates, NO generation is ultimately promoted by strengthening the pathways for converting other intermediates to NH and the pathways NCO→NO, NH→NO, and HNO→NO. Decreased CO₂ concentration promotes HNO formation (NH→HNO and NH₂→HNO) and oxidation (HNO→NO), reduces NO conversion to NO₂, and weakens the reductive effect of CO on NO and N₂O, consequently increasing NO emissions. Increased H₂O concentration inhibits O radical generation, thereby leading to a decrease in the fluxes of pathways NH→NO (28.5 %→24.4 %) and HNO→NO (69.6 %→50.2 %), and consequently reducing NO generation. This study updated the Hashemi mechanism and clarified nitrogen transformation in POFC, providing a theoretical basis for low-nitrogen combustion optimization.