Methyl formate oxidation kinetics up to 100 atm

Methyl formate (MF, CH₃OCHO), the simplest ester, is a representative oxygenated fuel with high oxygen content, and low sooting tendency. However, its oxidation behavior under high-pressure and intermediate-temperature conditions remains insufficiently understood, especially where low-temperature peroxy radical chemistry, methanol chemistry, and pressure-dependent reaction pathways play a critical role. In this study, MF oxidation experiments were conducted in the Princeton supercritical-pressure jet-stirred reactor (SP-JSR) at 20 and 100 atm over the temperature range of 400–950 K under both fuel-lean and fuel-rich conditions. Based on the experimental results, an updated HP-Mech was developed by incorporating previous MF sub-mechanisms, expanded low-temperature peroxy pathways, and evaluated pressure-dependent decomposition kinetics. The newly updated HP-Mech shows greatly improved performance in predicting the onset temperature, the key intermediate species fractions, methanol formation, and the progression of MF oxidation across all the experimental conditions. Path flux analysis indicates that MF consumption at the onset stage is dominated by H-abstraction at the methyl site, forming CH₂OCHO radicals that lead to the formation and isomerization of O₂CH₂OCHO, driving low-temperature chain propagation. Moreover, H-abstraction at the formate site forms CH₃OCO radicals that preferentially decompose to CH₃, initiating the methanol formation pathway linked to CH₃O₂ and HO₂ chemistry. At the same time, HO₂ formation is strongly coupled to MF oxidation through multiple MF-derived radical pathways. HCO originates from MF oxidation and acts as a key coupling species linking fuel consumption to HO₂ buildup, especially under high-pressure and intermediate-temperature conditions. In addition to this dominant channel, supplementary HO₂ formation pathways involving CH₃, CH₃O, CH₂OH, and CH₃O₂ reacting with O₂ further connect methanol chemistry and oxygenated radical chemistry to the HO₂ pool, indicating the central role of HO₂ in governing MF oxidation. Sensitivity analysis identifies MF with OH/HO₂/CH₃O₂ reactions and the HO₂/H₂O₂/OH sequence as the key factors controlling reactivity in the high-pressure and intermediate-temperature regime. MF directly reacts with OH/HO₂/CH₃O₂ to consume the fuel and produce reactive radicals like CH₂OCHO and CH₃OCO that undergo subsequent oxidation pathways. Moreover, HO₂ recombination suppresses oxidation at lower temperatures, while thermal decomposition of H₂O₂ accelerates OH production and promotes fuel consumption as temperature increases. The direct formation of active OH from HO₂ radicals further completes the mechanism, improving its prediction especially during the oxidation onset stage.