Kinetics and mechanism of methane oxidation in supercritical water

We oxidized methane in supercritical water at 250 atm and at temperatures between 525 and 587°C. The methane conversions ranged from 3 to 70%. CO was the product present in the highest yields at low conversions (< 10%), but CO₂ became the most abundant product at higher conversions. These experimental results were used to test the predictions of a detailed chemical kinetics model, which is based on gas-phase oxidation mechanisms and kinetics and comprised 150 elementary reaction steps. The model predicted methane disappearance rates that were about 30-50% faster than those observed experimentally. This behavior led to consistently high predictions of the methane conversion and the CO₂ yield. However, the model accurately predicted the yields of CO and CO₂ as a function of the methane conversion. The predicted activation energy for the pseudo-first-order rate constants of 36±3 kcal/mol is similar to the experimental value of 44±6 kcal/mol. Overall, the ability of the model to predict several of the experimental observations demonstrates that the analogy between gas-phase oxidation and oxidation in supercritical water is a good one. A sensitivity analysis revealed that the calculated methane concentration is most sensitive to the kinetics of OH+H₂O₂=HO₂+H₂O, OH+HO₂=H₂O+O₂, H₂O₂=OH+OH and HO₂+HO2=O₂+H₂O₂. These reactions control the concentration of OH radical, which is the main oxidant under SCWO conditions.