Effect of dimethyl ether, NOx, and ethane on CH4 oxidation: High pressure, intermediate-temperature experiments and modeling

The effects of small amounts of dimethyl ether (DME), NO, and NO₂ on the autoignition and oxidation chemistry of methane, with an without small amounts of ethane present, were experimentally studied in a flow reactor at pressures and temperatures similar to those found in spark- and compression-ignition engines (under autoignition conditions). The reactions were studied at pressures from 10 to 18 atm, temperatures from 800 to 1060 K, and equivalence ratios from 0.5 to 2.0. It is found that 1% DME addition is as effective in stimulating the autoignition and oxidative behavior of methane as 3% C₂H₆ addition, and that NO_x at even ppm levels is more effective than hydrocarbon additives. For the same reaction time and temperatures below 1200 K, addition of small amounts of NO_x lowered the temperature at which reaction becomes significant by more than 200 K. Chemical kinetic mechanisms in the literature for the interactions of methane, ethane, and NO_x do not predict the reported observations well. The most significant rate-controlling reactions for CH₄ autoignition is found to be CH₃+HO₂=CH₃O+OH. Good agreement, with and without NO_x perturbations can be obtained by modifying the rate constants of three reactions (CH₃+HO₂=CH₃O+OH; CH₃+HO₂=CH₄+O₂: CH₂O+HO₂=HCO+H₂O₂) and by adding the reaction CH₃+NO₂=CH₃O+NO to the GRI-Mech v2.11 mechanism. These modifications do not significantly affect predictions for shock tube, flame, and other results used in developing GRI-Mech v2.11. Results strongly suggest that exhaust gas residuals and/or exhaust gas recirculation can have as profound an effect as natural gas contaminants on the apparent octane and cetane behavior.