Chemical kinetic study of the effect of air-staged injection position on NO emission in the CH₄/NH₃ swirling flame

The air-staged combustion strategy provides a feasible and straightforward method to reduce the NO_x in ammonia-methane combustion. In this work, the chemical reaction network method was utilized to explore the fundamental mechanisms of NO formation and C/N interaction at different secondary injection heights in the air-staged combustion. The simulation results indicated that the HNO pathway is critical for NO formation. In the air-staged combustion, the fuel-rich primary zone produces a lower concentration of O and OH radicals (5.0 × 10⁻⁷ kmol/m³) than that in premixed conditions (7.6 × 10⁻⁶ kmol/m³), thus suppressing the NO formation through the HNO pathway. Meanwhile, the reburn zone enhances the NO consumption via the NH_i and HCN pathways with a proportion above 54.8%. The C/N interaction is explored under the air-staged conditions. The cross reactions contribute 7.5% to NO formation via the HNO pathway in the primary zone, while the C/N interactions play significant roles in NO formation and consumption via the HCN pathway in the reburn zone, with the maximum contribution reaching 41.8%.