Methanol and hydrogen oxidation kinetics in water at supercritical states

Abstract

The oxidation kinetics of methanol and hydrogen in a supercritical water medium were investigated. Numerical analyses were performed using an isobaric, plug-flow reactor model coupled with the Chemkin Real-Gas software package to handle real-gas thermodynamic effects and elementary kinetics. In the present work, select unimolecular reactions that involve thermal decomposition have been corrected for the high-pressure conditions found in supercritical reactors. Predictions of fuel destruction rates obtained using an elementary reaction mechanism for hydrogen oxidation under isobaric, isothermal supercritical conditions ( P > 22.1 MPa, and T > 374 C) were verified by comparison with previous experimental results obtained in a laboratory-scale supercritical water reactor. The H 2O 2 model is a subset of a proposed elementary reaction mechanism for methanol oxidation which was also verified by comparing the present model predictions of the kinetic rate calculations with previous measurements. The mechanism was validated over a temperature range of 726 to 873 K, a fuel concentration range of 0.001 to 0.004 mole/L, and at a pressure of 246 bar. To facilitate future computational fluid dynamic (CFD) modeling efforts in SCWO, a two-step reduced reaction mechanism was constructed to simulate the oxidation process of methanol in supercritical water and H 2O. The reduced model reflects the sequential oxidation of methanol into carbon monoxide, and eventually into final products consisting primarily of carbon dioxide and H 2O. The calculations of the two-step reduced mechanism matched the elementary reaction model well with respect to major species concentration profiles.