Electrical Resistance Heating
Electrical resistance heating is an in-situ electrical heating technology that applies electricity into the ground through electrodes. The electrodes can be installed either vertically to about 100 feet or horizontally underneath buildings.
Electrical resistance heating enhances the recovery of soils contaminated with volatile and semi-volatile organic compounds (VOCs and SVOCs). Electrical resistance heating assists soil vapor extraction (SVE) by heating the contaminants in the soil. It raises the vapor pressure of VOCs and SVOCs, increasing volatilization and removal. As electrical resistance heating dries the soil, it also creates a source of steam that strips contaminants from soils.
Three-phase heating and six-phase heating (SPH) are varieties of electrical resistance heating. SPH splits conventional three-phase electricity into six separate electrical phases, with each phase delivered to a single electrode. The six electrodes are placed in a hexagonal pattern, with the vapor extraction well located in the center of the hexagon. SPH is very good for circular areas less than 65-ft diameter. In larger areas, there may be flaws that cause uneven heating. Three-phase is ideal for larger areas and irregular areas.
Limitations and Concerns
Engineered barriers are recommended to prevent worker exposure to high voltages.
An off-gas treatment system should be installed to prevent the release of contaminated vapors from the subsurface. The system should be sized to handle peak extraction rates and the mix of compounds in extracted vapor and liquid streams.
The extraction well should be screened both above and below the target zone to ensure sufficient vacuum pressure in the subsurface. This extraction well design should also ensure total capture of contaminants released as a result of the heating.
Buried metal objects constitute a safety hazard. The subsurface should be mapped before the heating system is installed.
Questions remain about how quickly soil should be heated. When heating is sufficient to dry the soil, electrical conduction stops because dry soil is more resistive to electricity. Water can be added to maintain conduction.
Concentrated fumes released from the vacuum unit pose a potential explosion hazard.
Electrical resistance heating has been demonstrated to enhance removal of VOCs and SVOCs in unsaturated clay-rich soils. It has also been used to enhance removal of petroleum compounds.
Recent field tests of this technology have shown success in removing 1,4-Dioxane from soil and groundwater. The resulting vapor can be captured with vapor-phase GAC.
Technology Development Status
This technology has been commercially deployed. Most of the applications have been underneath buildings for the remediation of chlorinated volatile organic compounds at concentrations indicative of dense non-aqueous phase liquids (DNAPLs) and fuel hydrocarbons as light non-aqueous phase liquids (LNAPL).
Other Resources and Demonstrations
R.A. Gauglitz, J.S. Roberts, T.M. Bergsman, R. Schalla, S.M.Caley, M.H. Schlender, W.O. Heath, T.R. Jarosch, M.C. Miller, C.A. Eddy Dilek, R.W. Moss, B.B. Looney, Six-Phase Soil Heating for Enhanced Removal of Contaminants: Volatile Organic Compounds in Non-Arid Soils Integrated Demonstration, Savannah River Site, PNL-101 84, Battelle Pacific Northwest Laboratory, 1994.
See http://www.afcee.af.mil/shared/media/document/AFD-071129-100.pdf for cost and performance evaluation of ERH at AFP 4, Dallas, and Ft. Worth.
See http://sti.srs.gov/fulltext/WSRC-STI-2007-00488.pdf for description of ERH to cleanup TCE at the Savannah River Site.
A treatability study conducted by the U.S. Army Corps of Engineers at Fort Wainwright, AK showed that radio frequency heating and six-phase heating could effectively enhance soil vapor extraction/air sparging (SVE/AS) in cold climates. By the study═s conclusion, more than 70,000 pounds of volatile organic compounds had been removed by the SVE systems. Six-phase heating was found to be more cost-effective than radio frequency heating in large areas due to its lower equipment costs and ability to treat a larger area at one time.
See http://www.epa.gov/nrmrl/pubs/540r08004/540r08004.pdf for a demonstration of SPH at Cape Canaveral.