Detailed chemical kinetic modeling of the supercritical water oxidation of simple hydrocarbons.

Abstract

A detailed chemical kinetic model (DCKM) was compiled to investigate the chemistry of the supercritical water oxidation (SCWO) of CH$\sb4$, CH$\sb3$OH, CO, and H$\sb2$. The model comprises 150 reversible elementary reactions and 22 species. Model parameters were chosen based on the hypothesis that SCWO is fundamentally similar to combustion and atmospheric chemistry. The kinetics of the elementary reactions, determined under combustion and atmospheric conditions, can be adapted to SCWO conditions by accounting for the elevated pressures and high water densities found above the critical point of water (374$\sp\circ$C and 218 atm). Experimental results, gathered for the sole purpose of validating the model, were obtained for CH$\sb4$, CH$\sb3$OH, and CH$\sb4$-CH$\sb3$OH mixtures. In addition, experimental results from the literature were also used for model validation. The model correctly predicted the reaction rate for H$\sb2$, although the model and experimental induction times differed. Model predictions for CH$\sb3$OH were quantitatively consistent with experimental results. The model also accurately predicted that the addition of CH$\sb3$OH would accelerate methane oxidation. In some cases, the differences between model and experimental results appear to be due to the difficulties associated with obtaining experimental data that solely reflect the chemical kinetics. Complicating effects include slow mixing at the reactor entrance and oxidation before the reactor entrance. No attempt was made to adjust the model kinetics to account for any differences between model predictions and experimental results. A sensitivity analysis revealed that many of the same reactions are important for all of the compounds studied. Of these reactions, the rate of decomposition of H$\sb2$O$\sb2$ into two OH radicals strongly affected reaction rates. The OH radicals would then react with stable molecules to form organic radicals. H$\sb2$O$\sb2$ was formed from two HO$\sb2$ radicals reacting together. In addition, CO concentrations were affected by the rate of reaction of HOCO with O$\sb2$. Methane concentrations were affected by HO$\sb2$ attack on CH$\sb3$O$\sb2$ among other reactions. Overall, the model and experimental results show a level of agreement that supports the hypothesis that a DCKM based on combustion and atmospheric chemistry can account for the kinetics of SCWO.