Effect of C–N Cross-Reactions between Volatiles and NH3 in NO Formation during Coal/Ammonia Co-Firing under Low-Oxygen Dilution

Coal/NH3 co-firing is one of the feasible technological pathways for the low-carbon transition of coal-fired plants, and reducing NOx emissions is key to promoting its application due to the extremely high nitrogen content in NH3 fuel. During co-firing, the gas-phase interaction between coal volatiles and NH3, which mainly involves the C–N cross-reactions between N-containing intermediates and hydrocarbon components, critically regulates NO formation and is strongly dependent on the local oxygen environment. The low-O2 dilution environment achieved in MILD (moderate or intense low-oxygen dilution) combustion favors this interaction and enhances its inhibitory effect on NO formation. However, the underlying mechanism of this influence remains unclear. This study combines experiments with chemical kinetics to elucidate how C–N cross-reactions suppress NO under low-oxygen dilution, in terms of free radical regulation, key reaction enhancement, and dominant pathway competition. Results indicate that C–N cross-reactions significantly inhibit the formation of the free radical pool, and their inhibitory effect is enhanced as the NH3 blending ratio increases. Besides, C–N cross-reactions inhibit the conversion of NH2 → HNO and NH → NO by weakening R95 (NH2 + O = HNO + H) and R112 (NH + O = NO + H), while promoting the conversions of NO → N2 and NNH → N2 by enhancing reactions R107 (NH2 + NO = N2 + H2O) and R121 (NNH = N2 + H). Furthermore, a two-step ROP analysis method was further developed to quantify the probability of occurrence for different fuel-N conversion pathways at the moment of maximum NO production rate, thereby evaluating changes in the contribution of main pathways to NO formation. The removal of C–N cross-reactions increases the probabilities of path a (NH3 → NH2 → HNO → NO) and path c (NH3 → NH2 → NH → NO) from 24.2 and 7.2% to 38.4 and 18.0%, respectively. The decoupling of C–N cross-reactions indicates that NH2 hydrogen abstraction (G1 subset) and N2Hx/CH3 interaction (G5 subset) play a key role in inhibiting NO formation.