Investigating repetitive reaction pathways for the formation of polycyclic aromatic hydrocarbons in combustion processes

Repetitive hydrogen-abstraction and methyl- and acetylene-addition reaction sequences that contribute to the formation and growth of polycyclic aromatic hydrocarbons (PAHs) during incomplete combustion processes have been analyzed in flame-sampled electron ionization mass spectra. Specifically, we analyzed the range from C6H6 to C16H10 in the mass spectra obtained from atmospheric-pressure opposed-flow flames fueled by n-butane, i-butane, and i-butene, with conditions identical to those chosen by Schenk et al. [PAH formation and soot morphology in flames of C4 fuels, Proc. Combust. Inst. 35 (2015)1761-1769]. To assist the interpretation of the complex flame-sampled mass spectral data, this work elucidates the possibility for providing mechanistic insights from a simple analysis approach that does not convert the mass spectral data into isomer-resolved mole fraction profiles but solely is based on signal strength and ratios. While such an approach has not been exploited before, it is shown in this work that the repetitive nature of the observed quantitative signal ratios in the methyl- and acetylene-addition reaction sequences provides interesting insights into the overall features of flame-sampled mass spectra and the growth chemistry of PAHs. For the flames studied here, the similarity between the spectra obtained from the three different flames suggests that the signal ratios in the covered range are not fuel-structure dependent and that it is possible to draw mechanistic conclusions without knowing the isomer-specific chemistry. For example, the chemical growth pathways supported by this work suggest that other isomers besides pyrene contribute to the measured signal at m/z = 202 u (C16H10), a result that adds concern regarding the general validity of the assumption of pyrene dimerization as the particle inception step.