Study on effects of ethylene or acetylene addition on the stability of ammonia laminar diffusion flame by optical diagnostics and chemical kinetics

Improvement of ammonia combustion characteristics by carbon-based combustion promoters is currently emphasized, but the impact of carbon‑carbon multi-bond structure on ammonia combustion characteristics and reaction mechanisms has not been sufficiently investigated. In order to investigate the mechanism of molecular structures (C=C and C≡C) of combustion promoters on the stability of ammonia flames, the flame morphology, flame structure, NH3-H2 cracking, and C-N cross-reaction of ammonia/ethylene and ammonia/acetylene laminar diffusion flame are investigated based on a Gu¨lder burner by utilizing high-speed photography, planar laser-induced fluorescence (PLIF) and laser spontaneous Raman scattering (SRS) techniques. The results show that acetylene addition is more effective than ethylene to improve the flame stability. Combined with the flame structure analysis, it is shown that C=C mainly enhances the low-temperature reaction stage intensity of laminar flame, while C≡C can both enhance the intensity of low-temperature and high-temperature reaction stages, thereby enhancing the flame combustion exothermic process. Besides, C=C mainly strengthens the exothermic reactions in the upstream of the flame whereas C≡C can strengthen the exothermic reactions in the upstream and downstream of the flame simultaneously. Investigation of the NH3-H2 cracking reactions reveals that C≡C promotes H2 generation mainly by increasing NH3→H2 cracking, whereas C=C promotes H2 generation mainly by increasing NH2→H2 and NH→H cracking, hence improving the flame stability. Further analysis through the reaction flows revels that C=C acts mainly on H-containing species HCN/HNCO whereas C≡C mainly acts on non-H-containing species CN/NCO to influence the reaction pathways, but C-N cross-reaction does not play an important role in the characteristics of ammonia composite combustion (ACC).