Thermally enhanced SVE is a full-scale technology that uses electrical resistance/electromagnetic/fiber optic/radio frequency heating or hot-air/steam injection to increase the volatilization rate of semi-volatiles and facilitate extraction.
Thermally enhanced SVE is normally a short- to medium-term technology.
Electrical Resistance Heating
Electrical resistance heating uses an electrical current to heat less permeable soils such as clays and fine-grained sediments so that water and contaminants trapped in these relatively conductive regions are vaporized and ready for vacuum extraction. Electrodes are placed directly into the less permeable soil matrix and activated so that electrical current passes through the soil, creating a resistance which then heats the soil. The heat dries out the soil causing it to fracture. These fractures make the soil more permeable allowing the use of SVE to remove the contaminants. The heat created by electrical resistance heating also forces trapped liquids to vaporize and move to the steam zone for removal by SVE. Six-phase soil heating (SPSH) is a typical electrical resistance heating which uses low-frequency electricity delivered to six electrodes in a circular array to heat soils. With SPSH, the temperature of the soil and contaminant is increased, thereby increasing the contaminant's vapor pressure and its removal rate. SPSH also creates an in situ source of steam to strip contaminants from soil.
Radio Frequency/Electromagnetic Heating
Radio frequency heating (RFH) is an in situ process that uses electromagnetic energy to heat soil and enhance soil vapor extraction (SVE). RFH technique heats a discrete volume of soil using rows of vertical electrodes embedded in soil (or other media). Heated soil volumes are bounded by two rows of ground electrodes with energy applied to a third row midway between the ground rows. The three rows act as a buried triplate capacitor. When energy is applied to the electrode array, heating begins at the top center and proceeds vertically downward and laterally outward through the soil volume. The technique can heat soils to over 300 °C.
RFH enhances SVE in four ways: (1) contaminant vapor pressure and diffusivity are increased by heating, (2) the soil permeability is increased by drying, (3) an increase in the volatility of the contaminant from in situ steam stripping by the water vapor; and, (4) a decrease in the viscosity which improves mobility. The technology is self limiting; as the soil heats and dries, current will stop flowing. Extracted vapor can then be treated by a variety of existing technologies, such as granular activated carbon or incineration.
Hot Air Injection
Hot air or steam is injected below the contaminated zone to heat up contaminated soil. The heating enhances the release of contaminants form soil matrix. Some VOCs and SVOCs are stripped from contaminated zone and brought to the surface through soil vapor extraction.
High moisture content is a limitation of standard SVE that thermally enhancement may help overcome. Heating, especially radio frequency heating and electrical resistance heating can improve air flow in high moisture soils by evaporating water. The system is designed to treat SVOCs but will consequently treat VOCs. Thermally enhanced SVE technologies also are effective in treating some pesticides and fuels, depending on the temperatures achieved by the system. After application of this process, subsurface conditions are excellent for biodegradation of residual contaminants.
The following factors may limit the applicability and effectiveness of the process:
Debris or other large objects buried in the media can cause operating difficulties.
Performance in extracting certain contaminants varies depending upon the maximum temperature achieved in the process selected.
Soil that is tight or has high moisture content has a reduced permeability to air, hindering the operation of thermally enhanced SVE and requiring more energy input to increase vacuum and temperature.
Soil with highly variable permeabilities may result in uneven delivery of gas flow to the contaminated regions.
Soil that has a high organic content has a high sorption capacity of VOCs, which results in reduced removal rates.
Air emissions may need to be regulated to eliminate possible harm to the public and the environment. Air treatment and permitting will increase project costs.
Residual liquids and spent activated carbon may require further treatment.
Thermally enhanced SVE is not effective in the saturated zone; however, lowering the aquifer can expose more media to SVE (this may address concerns regarding LNAPLs).
Hot air injection has limitations due to low heat capacity of air.
Data requirements include the depth and areal extent of contamination, the concentration of the contaminants, depth to water table, and soil type and properties (e.g., structure, texture, permeability, and moisture content).
The thermally enhanced SVE processes are notably different and should be investigated individually for more detailed information. Because thermally enhanced SVE is an in situ remedy and all contaminants are under a vacuum during operation, the possibility of contaminant release is greatly reduced.
As with SVE, remediation projects using thermally enhanced SVE systems are highly dependent upon the specific soil and chemical properties of the contaminated media. The typical site consisting of 18,200 metric tons (20,000 tons) of contaminated media would require approximately 9 months.
DOE has developed and tested several thermally enhanced SVE processes. Dynamic underground stripping integrates steam injection and direct electric heating. Six phase soil heating is a pilot-scale technology that delivers six separate electric phases through electrodes placed in a circle around a soil vent. Thermally enhanced vapor extraction system combines conventional SVE with both powerline frequency and radio frequency soil heating.
Available data indicate the overall cost for thermally enhanced SVE systems is approximately $30 to $130 per cubic meter ($25 to $100 per cubic yard).