Experimental and kinetic study of the effects of CO2 and H2O addition on PAH formation in laminar premixed C2H4/O2/Ar flames

This paper aims to investigate the chemical effects of CO2 and H2O addition on PAH formation in laminar premixed C2H4/O2/Ar flames using laser-induced fluorescence (LIF) technique. For this purpose, the equivalence ratio, dilution ratio and maximum flame temperature were kept constant when the blending ratios of CO2 and H2O varied. The experiment results indicated that the fluorescence signal intensities of different PAHs monotonically decreased with higher blending ratios of both CO2 and H2O, and the suppression effect on PAH formation caused by H2O addition was stronger than that caused by CO2 addition. The kinetic analysis was carried out to clarify the experimental phenomenon using a chemical kinetic model in Chemkin-Pro Software coupled with a PAH formation mechanism proposed by Appel et al. (ABF). The simulation results reproduced the tendencies observed in LIF experiments. The suppression on PAHs was likely resulted from the decreased concentrations of propargyl and phenylacetylene, which were influenced by the variation of H and OH radical concentrations. The CO2 addition increased the reverse reaction rate of CO + OH⇌CO2 + H reaction, resulting in the decrease of H radical concentration and the increase of OH radical concentration. On the other hand, the H2O addition significantly increased the reverse reaction rates of OH + OH⇌O + H2O and CH3 + OH⇌CH2* + H2O reactions, promoting the formation of OH radical. In further steps, PAH growth pathway from benzene to large PAHs still need to be completed in more accurate PAH mechanism, which needs more useful data from LIF technique with better selectivity.