Effects of NH3 addition on the combustion characteristics of laminar premixed CH4–NH3 flame in the wall vicinity

Abstract

This study numerically investigates the local combustion characteristics of stoichiometric laminar premixed CH₄–NH₃ flames with and without the influence of wall effects. Effects of flame-wall interaction (FWI) and ammonia addition on flame temperature, heat release rate (HRR), CH₄/NH₃ oxidation and CO/NO formation are analyzed comprehensively. The results show that the peak HRR and flame temperature are suppressed more significantly by wall cooling than by intrinsic flame dynamics. With the ammonia addition, the HRR decreases steadily owing to its lower chemical reactivity and reduced volumetric heating value, while the flame temperature shows only a moderate decline, mitigated by the reduced air requirement. In the wall vicinity, wall heat loss becomes the dominant factor, significantly reducing local reaction rates. Despite this, the specific variations in reaction pathways remain strongly governed by fuel-composition-driven chemical kinetics and the sensitivity of each pathway to thermal quenching. Specifically, CH₄ oxidation near the wall is hindered by the absence of HCO radical formation, while NH₃ oxidation in the wall vicinity exhibits an increasing tendency to proceed through the HNO radical pathway with higher NH₃ addition. Under the influence of FWI, the strong suppression of CO oxidation in the burned gases leads to enhanced CO emissions at higher NH₃ content. Furthermore, wall heat loss accelerates NO destruction and promotes the formation of both N₂O and NO₂ in the burned gases, with the improvement effect being particularly notable for NO₂. However, with higher NH₃ addition, the NO conversion pathway near the wall undergoes an evident shift towards N₂O formation owing to the substantial increase in NHi radical concentrations.