Oxidation kinetics for methane/methanol mixtures in supercritical water

A detailed chemical kinetics model that had been previously validated for the oxidation of methane and methanol, individually, in supercritical water predicted that the presence of methanol in the reactor feed stream would accelerate the rate of methane disappearance. For example, the methane conversions at 540°C, 273 atm., and 1.0 s were predicted to be 5, 15, 37, and 53% when the methanol/methane molar ratio in the feed was 0, 1, 5, and 15 with constant methane concentration and constant excess oxygen in the feed. To test these and other predictions, we oxidized mixtures of methane and methanol in supercritical water at 540°C and 273 atm. The experimental results showed that the presence of methanol did indeed lead to higher methane conversions. For example, the methane conversions at a residence time of 1.3–1.4 s were 8, 40, and 50% when the methanol concentrations were roughly 0, 5, and 13 times that of the methane concentration in the feed. By qualitatively confirming model predictions for the effect of increasing amounts of methanol on the yields of methane, methanol, CO, and CO2 during the oxidation of methane/methanol mixtures, these experimental results provide additional evidence that gas-phase combustion chemistry and kinetics can be adapted to develop reliable detailed chemical kinetics models for supercritical water oxidation (SCWO). Moreover, these results show that a mechanism-based model can predict the results of kinetic interactions that occur during the oxidation of a mixture. Phenomenological kinetics models do not possess this predictive capability.