KINETICS AND MECHANISM OF METHANE OXIDATION IN SUPERCRITICAL WATER

We oxidized methane in supercritical water at 250 atm and at temperatu res between 525 and 587 degrees C. The methane conversions ranged from 3 to 70%. CO was the product present in the highest yields at low con versions (< 10%), but CO2 became the most abundant product at higher c onversions. These experimental results were used to test the predictio ns of a detailed chemical kinetics model, which is based on gas-phase oxidation mechanisms and kinetics and comprised 150 elementary reactio n steps. The model predicted methane disappearance rates that were abo ut 30-50% faster than those observed experimentally. This behavior led to consistently high predictions of the methane conversion and the CO 2 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 ab ility of the model to predict several of the experimental observations demonstrates that the analogy between gas-phase oxidation and oxidati on in supercritical water is a good one. A sensitivity analysis reveal ed that the calculated methane concentration is most sensitive to the kinetics of OH+H2O2=HO2+H2O, OH+HO2=H2O+O-2, H2O2=OH+OH and HO2+HO2=O- 2+H2O2. These reactions control the concentration of OH radical, which is the main oxidant under SCWO conditions. (C) 1998 Elsevier Science B.V.