Optical Monitoring of the Oxidation of Methane in Supercritical Water

Experiments were conducted in a static, high-pressure reactor to investigate the oxidation of methane in supercritical water. Pressures ranged from 138 to 275 bar, temperatures from 380 to 440 C., and equivalence ratios from 0.2 to 2.0 for initial methane concentrations around 0.1 mole/I. In these experiments, Raman spectroscopy was used as an in-situ diagnostic to monitor the concentrations of methane, oxygen, and carbon dioxide. Over this pressure range the reaction rate of methane with oxygen is unexpectedly observed to decrease with increasing pressure. A non-linear least squares fit was performed to determine four global reaction rate parameters. In contrast to results from experiments at lower initial methane concentrations, the reaction order dependency on methane is found here to be greater than unity. This finding implies that the former results cannot safely extrapolate to concentrations around 0.1 mole/l.