Analysis of an elementary reaction mechanism for benzene oxidation in supercritical water

A benzene supercritical water oxidation (SCWO) mechanism, based on published low-pressure benzene combustion mechanisms and submechanisms describing the oxidation of key intermediates, was developed and analyzed to determine the controlling reactions under SCWO conditions of 750-860 K, 139-278 bar, and equivalence ratios from 0.5 to 2.5. To adapt the combustion mechanisms to the lower temperature (< 975 K) and higher pressure (> 220 bar) conditions, new reaction pathways were added, and quantum Rice-Ramsperger-Kassel theory was used to calculate the rate coefficients and, hence, product selectivities for pressure-dependent reactions. The most important difference bt tween the benzene oxidation mechanism for supercritical water conditions and those for combustion conditions is reactions in supercritical water involving C6H5OO predicted to be formed by C6H5 reacting with O-2. Through die adjustment of the rate coefficients of two thermal decomposition pathways of C6H5OO, whose values are unknown, the model accurately predicts the measured benzene and phenol concentration profiles at 813 K, 246 bar, stoichiometric oxygen, and 3-7 s residence time and reproduces the finding that the carbon dioxide concentration exceeds that of carbon monoxide at all reaction conditions and levels of benzene conversion. Comparison of the model predictions to benzene SCWO data measured at several different conditions reveals that the model qualitatively explains the trends of the data and gives good quantitative agreement without further adjustment of rate coefficients.