Study on methane oxidation affected by dimethyl ether oxidation at low temperatures using a micro flow reactor with a controlled temperature profile

The present study aims to identify cross-reactions of methane and dimethyl ether (DME) at low temperatures. A weak flame in a micro flow reactor with a controlled temperature profile (MFR) for a stoichiometric methane/DME/air mixture with 90% CH₄ and 10% DME (hereinafter expressed as 90%/10% CH₄/DME) was used and analyzed. The weak flame structure was investigated by chemiluminescence observation and laser-induced fluorescence (LIF) measurements of OH and CH₂O. Measured chemiluminescence and fluorescence intensity profiles were compared to computations with five detailed chemical kinetic mechanisms. Both experimentally and computationally obtained results showed the weak flame position of 90%/10% CH₄/DME shifted to lower temperature in MFR compared to that of 100% CH₄ reported earlier, indicating methane reactivity enhancement by the small DME addition. CH₂O-LIF showed extensive formation of CH₂O at low temperatures. Each mechanism predicted different levels of CH₂O formation. The four mechanisms among five which include intermediate-temperature oxidation chemistry for methane, CH₃ → CH₃O₂ → (CH₃O₂H) → CH₃O → CH₂O, predicted two-staged formation of CH₂O at 650 and 730 K, whereas another mechanism predicted single-stage formation of CH₂O at 650 K. Rate-of-production analysis revealed that OH radicals formed from low-temperature oxidation of DME initiate H-atom abstraction from methane; then intermediate-temperature oxidation chemistry for methane proceeds even at low temperatures. The contribution of intermediate-temperature oxidation chemistry for methane to CH₂O formation at 730 K is stronger than that at 650 K, causing two-stage formation of CH₂O.