Isomer-specific speciation behaviors probed from premixed flames fueled by acetone and propanal

Chemical structures of low-pressure premixed flames respectively fueled by two C₃ carbonyl isomers, acetone and propanal, at different equivalence ratios (1.0 and 1.5) were experimentally investigated in this work. Detailed speciation information was obtained by employing molecular-beam mass spectrometry with tunable synchrotron photoionization. A detailed kinetic model including the chemistry of acetone and propanal was developed and tested with the current flame speciation measurements. By combining experimental observations and modeling interpretations, comparisons were made regarding fuel-specific reaction pathways and the resulting different species pools. Some fuel-specific intermediates were detected and quantified in this work, such as ketene in acetone flames and methylketene in propanal flames. Particularly, the quantitative speciation measurements of ketene, an important primary intermediate of acetone, were satisfactorily predicted by the current model, which included an updated ketene sub-mechanism. Major efforts in this work were devoted to gaining some insights into the effects of the carbonyl position in fuel molecules on the speciation behaviors under premixed flame conditions. Carbonyl functionalities in the two C₃ carbonyl compounds are tightly bonded and preferably preserved in CO. Due to the different position of the CO bond in the two isomers, the oxidation of propanal leads to abundant ethyl as a chain carrier, while the acetone consumption easily results in a significant amount of methyl, an inhibitor on the fuel reactivity. As a result, higher reactivity of propanal was observed. More importantly, the different fuel consumption patterns also influence the speciation behaviors. Specifically, the larger concentration of benzene precursors such as allyl, was observed in the propanal flames. Besides, typical oxygenated emissions formaldehyde and acetaldehyde had more remarkable concentrations in acetone and propanal flames, respectively.