Effect of oxygenates on fuel-rich oxidation of CH4: Shock-tube analysis with extinction, CO-concentration, and temperature measurements

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For conversion of ultra-rich fuel/air mixtures into potentially useful chemicals in a polygeneration context, oxygenated compounds are of interest to increase fuel conversion while suppressing soot formation. Thus, the influence of various oxygenates (diethyl ether (DEE), methanol, or dimethoxymethane (DMM)) on soot formation was addressed by studying the oxidation ( ϕ = 5) of gas mixtures using CH 4 as base fuel and additives (10 mol% relative to the base fuel) in a shock tube at 1480–1900 K at around 5 bar. Laser extinction at 633 nm was used to measure soot optical densities and absorption spectroscopy with mid-IR quantum cascade lasers yielded temperature and CO concentration as a function of reaction time. Experimental data is compared to simulations based on five different mechanisms. Good agreement of experimental and simulation data was observed for most conditions. Measured soot optical densities were compared with the simulations using a modified CRECK mechanism, where the original mechanism was complemented with decomposition and H-abstraction reactions of DEE and DMM. For this modified mechanism, the maximum values and the influence of the additives agreed well with measured data. It was found that while DEE increases the soot yield, DMM reduces soot formation compared to reactions of pure CH 4 . The additives shift the ignition and correspondingly also the soot formation to lower temperatures which is necessary to operate an IC engine under ultra-rich conditions with homogeneous charge compression ignition. DMM was found to increase the reactivity of methane, while also reducing soot formation. Optimized conditions for the polygeneration process of fuel-rich methane/DMM mixtures in IC engines can now be calculated using the modified CRECK mechanism, which was validated in this work for combustion and soot formation at fuel-rich conditions