Simulating the Spatial Extent of Thermally Enhanced Reaction Zones for Low Temperature Thermal Treatment

Abstract

Low temperature thermal treatment (LTTT) is a technology that can enhance aqueous-phase degradation reactions for organic constituents in groundwater. Understanding heat transfer in groundwater is important for the design of LTTT applications. In this study, the effect of permeability heterogeneity on temperature distributions during and after the application of heat was investigated by numerical modeling. An enhanced reaction zone was determined for the hydrolysis of 1,1,1-trichloroethane (1,1,1-TCA) using an average half-life considering the temperature history during and after heating. For hydrolysis reactions, the average half-life could be reduced substantially by reaching a high temperature for a short period of time because their reaction rates increase exponentially with increased temperatures. Results showed that the enhanced reaction zone was shifted downstream of the heater well zone at high groundwater velocities. This suggests that heaters should be shifted upstream of the target treatment zone to fully utilize the applied heat. In addition, permeability heterogeneity leads to greater macroscopic dispersion at higher velocities. This resulted in higher spreading of heat and faster heat dissipation in the simulations with a heterogeneous permeability condition compared with a homogenous permeability condition. As a result, the enhanced reaction zone was smaller in simulations with higher levels of permeability heterogeneity at a mean velocity of 0.3 m/day.