Effects of flame impingement on the stability and NO/CO emissions of laminar premixed methane-ammonia impinging flame

Abstract

This study investigates the stable combustion range and CO/NO emissions of laminar premixed CH4-NH3 impinging flames experimentally. Effects of flame impingement on the flame stability and CO/NO formations are analyzed quantitatively. The result shows that the flame impingement improves the flame stability considerably through decelerating the unburned gases velocity in the stagnation region, with smaller nozzle-to-wall distance (H) exerting stronger improvement on the flame stability. The NO emission is increased initially but dropped finally with the increased H. This trend is ascribed to the suppressed NO productions at both small and large H due to the intensive cooling effects of cold wall and ambient air entrainment respectively in consideration of the relatively inferior reactivity of NH3. Additionally, due to the insufficient oxidizer in the fuel-rich flame, the more NH3 molecule can also facilitate the NO destruction via the DeNOx pathways at small or large H. In contrast, the CO emissions show similar variation trends with H like the NO emission for the fuel-lean and stoichiometric flames, while the CO emission has a “N” type variation trend with H for the fuel-rich flames. Since the CH4 reactivity can be improved by the NH3 addition at low temperatures, a low-temperature environment at small H due to the strong cooling effect, as well as more available NH3 in the flame, can effectively highlight the improvement of NH3 on the CH4 reactivity in the fuel-rich flame. This makes for the CO production at small H combined with the insufficient oxidizer and eventually results in the higher CO emission of the fuel-rich flame at small H. Additionally, the decreased CO emission at large H results from the improved CO oxidation that is caused by the increased residence time and more oxidizer from ambient air.