Revisit to the oxidation of CH4 at elevated pressure

Abstract

The oxidation of CH4 was investigated in a jet-stirred reactor under equivalence ratios (Φ) of 0.5 and 1.0 within 875–1050 K at 12 atm. Mole fraction profiles of six species (CO, CO2, CH4, C2H4, C2H6 and propene (C3H6)) were quantified. Compared with previous oxidation studies of CH4, C3H6 was newly detected. A detailed chemical kinetic model consisting of 92 species and 616 reactions was developed to predict the oxidation of CH4 under a wide range of pressures. In general, the onset temperature of CH4 oxidation shifts higher with Φ increasing. Compared with previous models, the experimental data were reasonably reproduced by the present model. Rate-of-production (ROP) and sensitivity analyses reveal that the consumption of CH4 proceeds predominantly via H-abstraction by OH radicals under both conditions. The reaction sequences CH4→CH3→CH3O/CH2O→HCO→CO and CH3→C2H6→C2H5 are the two major channels governing CH4 conversion. C3H6 is mainly formed by the reaction C3H6+H=CH3+C2H4 under lean condition above 975 K. Particular attention was paid to the oxidation and negative temperature coefficient (NTC) behavior of CH4 at 1–90 atm and a wide range of Φ. It can be found that the increase of pressure is conducive to fuel consumption and the NTC behaviors are observed within temperatures of 810–900 K at pressures higher than 30 atm and Φ = 0.3–2.0. ROP analyses reveal that the competition between methyl peroxide and its subsequent chain-branching is responsible for the NTC behavior in CH4 oxidation. Moreover, the model also gives reasonable predictions against oxidation data (1–90 atm), ignition delay times (800–2000 K) and laminar burning velocities (1-10 atm) reported in the literature. This work will enrich the understanding of CH4 oxidation and promote experimental and theoretical studies on the oxidation of other larger hydrocarbons.