Oxidation of butane-2,3-dione at high pressure: Implications for ketene chemistry

Ketene (CH₂CO) mechanism is a building block for developing combustion kinetic models of practical fuels. To revisit the combustion chemistry related to ketene, oxidation experiments of butane-2,3‑dione (diacetyl, CH₃COCOCH₃), considered as an effective precursor of CH₂CO, are conducted in a jet-stirred reactor (JSR) at 10 bar and temperatures ranging from 650 to 1160 K. Identification and quantification of intermediates are achieved by Fourier transform infrared spectrometry, gas chromatography, and mass spectrometry. A kinetic model of diacetyl is constructed based on recent theoretical and modeling studies on diacetyl and ketene, which has been validated against the present data and experimental data of diacetyl and CH₂CO in literature. Generally, the present model can adequately predict most of them, and better predict the methyl-related intermediates under wide pyrolysis and combustion conditions than previous models. Based on modeling analyses, the unimolecular decomposition reaction of diacetyl is the dominant reaction pathway for fuel consumption under different equivalence ratio conditions, especially at high temperatures. Under lean conditions, both the H-atom abstraction reactions by methyl (i.e. CH₃COCOCH₃ + CH₃ = CH₄ + CH₂CO + CH₃CO, R3) and by OH (i.e. CH₃COCOCH₃ + OH = H₂O + CH₂CO + CH₃CO, R5) are important for diacetyl consumption, while under rich conditions R5 becomes negligible. As the most important intermediates in diacetyl oxidation, the main consumption pathways of CH₂CO and CH₃ are dependent on the equivalence ratio conditions. Under lean conditions, CH₂CO mainly reacts with OH to produce CH₂OH and CO (i.e. CH₂CO + OH = CH₂OH + CO, R10), while methyl reacts with HO₂ to produce CH₃O and OH (i.e. CH₃ + HO₂ = CH₃O + OH, R20). In contrast, under rich conditions, the addition-elimination reaction between CH₂CO and H becomes competitive with R10, while the CH₃ self-combination producing C₂H₆ plays a more important role than the CH₃ oxidation pathway R20. Sensitivity analysis of CH₂CO shows that not only the reactions of CH₂CO, but also those of CH₃ are sensitive to CH₂CO formation. This is because CH₃ related reactions influence the distribution of radical pool, which determines the oxidation reactivity of the reaction system.