The inhibiting effect of NO addition on dimethyl ether high-pressure oxidation

The high-pressure dimethyl ether (DME, CH3OCH3) oxidation has been investigated in a plug flow reactor in the 450-1050 K temperature range. Different pressures (20, 40 and 60 bar), air excess ratios (λ = 0.7, 1 and 35), and the absence/presence of NO have been tested, for the first time under these conditions. An early reactivity of DME and a negative temperature coefficient (NTC) zone have been observed under the studied conditions, although under very oxidizing conditions (λ = 35), NTC zone is almost imperceptible because DME is completely consumed at lower temperatures. A chemical kinetic mechanism has been used to describe the DME high-pressure oxidation, with a good agreement with the experimental trends observed. In general, modeling calculations with the present mechanism have been successfully compared with experimental data from literature. The presence of NO has an inhibiting effect on DME high-pressure consumption at low-temperatures because of: (i) the competition between CH3OCH2+O2⇌CH3OCH2O2 and CH3OCH2+NO2⇌CH3OCH2O+NO reactions, and (ii) the participation of NO in CH3OCH2O2+NO⇌CH3OCH2O+NO2 reaction, preventing CH3OCH2O2 radicals continue reacting through a complex mechanism, which includes a second O2 addition and several isomerizations and decompositions, during which highly reactive OH radicals are generated. Consequently, NO and NO2 are interchanged in a cycle but never consumed.