NH₃/CH₄ and NH₃/Biogas Swirl Combustion: Turbulent Flame Propagation and Sensitivity Analysis of NO Emissions

The present work examined the impact of increasing the NH₃ and CO₂ mass fraction on NH₃/CH₄ and NH₃/biogas swirl combustion using a validated numerical model. Increased normalized strain rate nullified the lower reactivity of ammonia and promoted the turbulent flame propagation. Furthermore, elevated CO₂ mass fraction in NH₃/CH₄ swirl combustion also resulted in higher turbulent flame propagation, owing to the increased normalized strain rate. The reactions that principally governed NO formation in NH₃/CH₄ and NH₃/biogas swirl combustion were HNO + H ↔ NO + H₂, NH + O ↔ NO + H, and N + OH ↔ NO + H. At the global equivalence ratio (φ_G) = 0.8, OH and H radicals dominated NO formation, while the addition of CO₂ promotes NO consumption by increasing the concentrations of N, C, and NH radicals. At φ_G = 1.1, the reduction in oxygen (O₂) lowers the concentrations of the OH and O radicals, thereby suppressing the NH₃ decomposition and reducing the NO formation rate. However, the addition of CO₂ raises OH radical formation, which accelerates the N + OH → NO + H reaction, leading to an increase in NO emissions. Overall, NH₃/biogas swirl combustion presented improved turbulent flame propagation and reduced NO emissions compared to NH₃/CH₄. Increased NH₃ and CO₂ mass fractions enhanced NH₃/CH₄ combustion by increasing the normalized strain rate. These combined effects promoted turbulent flame propagation. CO₂ promotes NO consumption by increasing N, C, and NH radicals. The present findings demonstrated that NH₃/biogas combustion offers better operational stability and environmental benefits compared to NH₃/CH₄ combustion, reinforcing its viability for renewable energy applications and clean fuel transitions.