Characterizing the fuel-specific combustion chemistry of acetic acid and propanoic acid: Laminar flame propagation and kinetic modeling studies

In order to study the combustion chemistry of carboxyl functionality, the laminar burning velocity of acetic acid/air and propanoic acid/air mixtures was investigated in a high-pressure constant-volume cylindrical combustion vessel at 423 K, 1 atm and equivalence ratios of 0.7–1.4. Experimental results reveal that the flame propagation of propanoic acid flame is much faster than that of acetic acid flame, especially under rich conditions, and the laminar burning velocity of propanoic acid/air mixtures peaks at richer conditions than that of acetic acid. The present theoretical calculations for the isomerization and decomposition of propanoic acid radicals indicate that the primary radical products are HOCO, H and C₂H₅, while those in acetic acid flame are CH₃ and OH based on previous studies. A kinetic model of the two acids was developed mainly based on previous and the present theoretical calculation results. It could reasonably capture the measured laminar burning velocities of acetic acid/air and propanoic acid/air mixtures in this work, as well as the previous experimental data in literature. Based on the present model, CH₃- and ketene-related pathways play an important role in acetic acid flames. Under rich conditions, ketene is mostly converted to CH₃ via CH₂CO+HCH₃+CO, and the chain-termination reaction of CH₃+H(+M)=CH₄(+M) is enhanced, which strongly inhibits the propagation of rich acetic acid flames. In contrast, C₂H₅ and ethylene chemistry play an important role in propanoic acid flames. Rich conditions promote the decomposition of C₂H₅, yielding ethylene and H, which can facilitate the flame propagation. This can explain the shift of the peak laminar burning velocity of propanoic acid/air mixtures towards a slightly richer condition compared with that of acetic acid/air mixtures.