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 (φG) = 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 φG = 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.