Gas Dynamics during Thermal Remediation: Visualization, Quantification and Enhancement

Abstract:

In situ thermal treatment (ISTT) technologies, such as electrical resistance heating (ERH) and thermal conductive heating (TCH), rely on the in situ production of a gas phase composed of steam and vaporized volatile organic compounds (VOCs). This gas phase must be captured, extracted, and processed in an aboveground treatment system to meet remediation objectives. When used to treat volatile non-aqueous phase liquids (NAPLs), gases can be created at temperatures below the boiling points of both the groundwater and the NAPL, in a process commonly referred to as co-boiling, and vaporized VOCs can condense if gases are transported to colder regions or are not captured before thermal treatment has stopped. As such, an understanding of gas formation, connection, and flow is important for the design and operation of ISTT technologies. A recent series of laboratory experiments focused on the visualization and quantification of gas dynamics during water boiling and NAPL-water co-boiling, and the investigation of potential NAPL redistribution. Experiments were conducted in a sand-packed glass-walled chamber (40 cm tall × 20 cm wide × 1 cm thick) heated by electrical resistance. Temperatures and electric currents were measured, and digital images were captured throughout the experiments to quantify gas saturations using light transmission techniques. Additional experiments also investigated the exsolution of dissolved gas as a technique to enhance gas production at lower temperatures. Results showed the development of disconnected and connected gas flow regimes, with disconnected flow occurring at early times and during co-boiling. Results also showed the potential for NAPL redistribution due to displacement by gas formed within pools, and due to condensation in colder regions. These results highlight the need to carefully consider gases in the design of ISTT heating and gas extraction systems to ensure remediation performance.