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

Abstract

In oxy-fuel combustion systems aimed at carbon capture, reducing the level of NO_x emissions is critical for corrosion mitigation and CO₂ utilization. Previous studies mostly focused on NO_x 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 (X_O2), and equivalence ratio (Φ) are thoroughly investigated. The experiments indicate that at Φ = 0.8 and X_O2 = 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 X_O2 (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.