Numerical investigation of NOx emission characteristics in air-staged combustion system fueled by premixed ammonia/methane

For the purpose of achieving global CO₂ reduction, decarbonization at the source of fuels is a practical approach. The transition phase of blending fossil fuels with carbon-free fuels for combustion is a hot topic in the current carbon emission reduction process. In order to achieve efficient and low-pollution combustion of NH₃/CH₄, the combustion and emission characteristics of NH₃/CH₄ under single-stage and air-staged combustion methods were numerically investigated in this work. The emissions were compared for different equivalence ratios and different ammonia content conditions. Rate of production (ROP) and sensitivity analysis were performed for NO_x, and the reaction path of NH₃/CH₄ was analyzed. The results indicate that the C-N interaction of the NH₃/CH₄ mixed combustion process is not significant and turns weaker in the lean flames. HNO intermediate is an important specie for NO generation, and HCN together with HCO intermediate, are essential species for CO generation. NH₂ and NH almost dominate the promotion and inhibition of NO generation. Given the contrasting NO_x and CO emission behavior of NH₃/CH₄ in rich and lean flames, the single-stage combustion approach is not suitable. Air-staged combustion achieves both, ensuring the complete burning of NH₃ and CH₄ while reducing NO_x and CO emissions. Moreover, the results suggest that Φ_pri = 1.2/Φ_total = 0.6 is the optimal NH₃/CH₄ combustion staging method for controlling NO_x emissions.