EFFECTS AND IMPLICATIONS OF DISSOLVED GAS EXSOLUTION ON IN-SITU THERMAL TREATMENT TECHNOLOGY APPLICATIONS

ABSTRACT: In-situ thermal technologies (ISTT) are effective methods for remediating non-aqueous phase liquid (NAPL) contaminants even in low permeability and heterogeneous soils, in which injection-based remediation methods may have limited effectiveness. However, convective heat losses from high groundwater flow can reduce the effectiveness of ISTT, resulting in increased cost or failure to reach target temperatures. Hydraulic and physical barriers or increasing energy input are typical options used to counteract convective heat losses. However, the costs of barriers increase substantially with depth, and additional energy input is not always available. A new potential method for convective heat loss control is the exsolution of carbon dioxide (CO2), which generates a gas phase at temperatures below the water boiling point and reduces aqueous relative permeability. In this study, bench-scale heating experiments were conducted at temperatures above and below the co-boiling temperature using sand-water-NAPL mixtures with and without dissolved CO2-saturated water. Results indicated that NAPL extraction was possible below the co-boiling temperature, although enhancement was limited when using only one pore volume of CO2-saturated-water. An estimate for using a continuous source of dissolved CO2 indicated more than 30 pore volumes were required to extract the initial emplaced NAPL. Another set of experiments was completed using electrical resistance heating (ERH) in a 2D flow cell supplied by water with and without dissolved CO2 to determine the extent of flow reduction. Results showed that water flow was reduced by three to four orders of magnitude and the flow regime changed from advective flow to natural convection dominated flow, and that temperature distributions converged along the flow direction. This proof-of-concept study showed that dissolved CO2 exsolution may be a new alternative for convective heat loss control, and further study is needed to assess impacts on energy consumption and to enhance NAPL extraction.