Computational Fluid Dynamics Modeling of a Two-Step Ethanol Oxidation Mechanism in a Supercritical Water Oxidation Reactor

Abstract

Supercritical water oxidation (SCWO) reactors are used to destroy recalcitrant chemicals, including perfluoroalkyl substances (PFAS). Addition of fuel enables autogenic operation, where heat is generated to achieve the reaction temperature necessary for PFAS destruction in a SCWO reactor. This study utilizes a Computational Fluid Dynamics (CFD) model of a two-step ethanol oxidation mechanism to gain insight into the operation of the University of Washington (UW) continuous-flow SCWO reactor. The reactor uses ethanol as a fuel and compressed air as an oxidant. Compared to the single-step mechanism, the global two-step reaction mechanism is expected to yield a more accurate temperature distribution and reaction zone characterization and can be validated against the experimental data that includes O2, CO, and CO2 measurements at the reactor exit. However, the use of a two-step ethanol oxidation mechanism in continuous-flow reactors under a wide range of reaction temperatures (475- 650 °C) has not been reported in the literature. The development of the two-step ethanol oxidation mechanism and its validation in a laboratory-scale system are useful for optimizing SCWO for the destruction of recalcitrant compounds and reactor scale-up. The CFD model of ethanol oxidation in a continuous-flow UW SCWO reactor is based on the two-step mechanism published by Helling et al. [1]. The model simulates ethanol oxidation under four different fuel loadings. The baseline case (100% loading) uses the injection of a 5% molar ethanol/ water mixture, which corresponds to a fluid temperature of 650 °C. Experimentally, the fuel injection rate was varied in the 49-100% range, resulting in fluid temperatures of 475- 650°C. These conditions were modeled using perfectly stirred reactor and in a 2D axisymmetric CFD simulations to study the fluid flow, species, and temperature distribution inside the reactor. The reaction temperature and incomplete products of decomposition from the PSR and CFD simulations were compared with the experimental data. The predicted reaction temperatures from the CFD model agreed within ~9% with the temperature measurements from the experiment, and the CO concentrations in the exhaust -- within ~28% across all test cases. The two-step mechanism is as an improvement of the single-step global kinetic allowing to gain insight into oxidation. The approach can be more validated based on the temperature measurements, but also on the CO and O2 data over the wide range of operational conditions.