Homogeneous Fuel-NO Mitigation during Flameless Oxy-Combustion of CH₄/NH₃ Mixtures

In oxy-fuel combustion systems aimed at carbon capture, reducing the level of NOx emissions is critical for corrosion mitigation and CO2 utilization. Previous studies mostly focused on NOx mechanisms in traditional flame oxy-combustion, while experimental evidence regarding the homogeneous fuel-NO reduction benefits of flameless oxy-combustion is lacking. To fill this gap, this study provides the first systematic investigation into homogeneous fuel-NO formation in flameless oxy-combustion of CH4/NH3 mixtures, combining experimental data with validated numerical simulations. The impacts of combustion mode (flameless vs flame oxy-combustion), initial oxygen concentration (XO2), and equivalence ratio (Φ) are thoroughly investigated. The experiments indicate that at Φ = 0.8 and XO2 = 30%, flameless oxy-combustion reduces homogeneous fuel-NO by 56.8% in comparison with flame oxy-combustion and by 15.8% compared to flameless air combustion. Notably, across varying initial XO2 (25–40%) and Φ (0.6–0.9), the effectiveness of fuel-NO reduction under flameless oxy-combustion remains largely unchanged. Finite-rate combustion modeling with an optimized skeletal mechanism and reaction pathway analysis further reveal that under flameless oxy-combustion of CH4/NH3 mixtures, homogeneous fuel-NO formation through N2O → NO, CN → NO, and NCO → NO pathways is inhibited, whereas the primary NO destruction route, NH2 → N2, is promoted. This research delivers pioneering experimental evidence of the efficacy of flameless oxy-combustion in reducing fuel-NO emissions and offers critical insights into the underlying mechanisms.