Effect of dimethyl ether, NO x, and ethane on CH 4 oxidation: High pressure, intermediate-temperature experiments and modeling

Abstract

The effects of small amounts of dimethyl ether (DME), NO, and NO 2 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% C 2 H 6 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 CH 4 autoignition is found to be CH 3 +HO 2 =CH 3 O+OH. Good agreement, with and without NO x perturbations can be obtained by modifying the rate constants of three reactions (CH 3 +HO 2 =CH 3 O+OH; CH 3 +HO 2 =CH 4 +O 2 : CH 2 O+HO 2 =HCO+H 2 O 2 ) and by adding the reaction CH 3 +NO 2 =CH 3 O+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.