U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Dense Nonaqueous Phase Liquids (DNAPLs)

Treatment Technologies

Thermal Processes: In Situ

Conductive Heating

Conductive heating uses either an array of vertical heater/vacuum wells or, when the treatment area is within about six inches of the ground surface, surface heater blankets. While it is feasible to deploy all the wells in a heater/vacuum mode, the typical deployment is to place six heater-only wells in a hexagonal shape with a heater vacuum well occupying the center of each hexagon.

The wells can be installed using conventional drilling techniques or direct push. Heater wells are constructed of steel pipe with the base sealed. A resistive heating unit is lowered into the well and current is supplied. The heating element typically operates at temperatures between 540 and 815°C (Baker and Heron 2004). The steel pipe is heated by radiant energy and the soil surrounding it by thermal conductance.

The vacuum well contains the same steel pipe and heating element components as a standard heater well, but it is placed within a larger screened well to which a vacuum can be applied. Heat propagates in a cylindrical fashion from the well outward. The heating is fairly even through all dry textures of soil. The hottest soil (typically 590°C) is in the immediate vicinity of the wells, while the coolest soil is at the midpoint between wells. When the vacuum is applied to the center well, volatilized organics are pulled through the high-temperature soil, where some of the contaminants may be degraded (Baker and Heron 2004). The extracted vapors are transported to the surface for treatment.

Well spacing is chosen based on contaminant type and depth, soil moisture content, the minimum required temperature between wells, and the time desired to reach that temperature (U.S. EPA 2004). SVOCs, including high boiling components, such as PAHs or PCBs, generally need a soil temperature of 325°C for adequate desorption, while VOCs require less heat (usually 100°C) (Baker and Heron 2004). The ability to treat high-boiling contaminants at temperatures well below their boiling points is largely due to the significant increase in vapor pressures at the temperatures present and the relatively long residence time in a very hot subsurface (Biershenk et al. 2004).The temperature requirements typically lead to well placement distances of 6 to 7.5 feet for the SVOCs and 12 to 20 feet for the VOCs. As with electrical resistance heating, the closer the wells, the faster the desired temperatures are reached.

Conductive heating operates best in unsaturated soil; however, it does find application in saturated soil with low hydraulic conductivity. As the temperature around the heater wells increases, the water evaporates and a "dry" zone is created that expands outward. At the leading edge of this cylindrical zone, steam is created, which further expands the zone. In low permeability soil, any replacement water that attempts to flow into the dry zone is quickly boiled off. In soil with high hydraulic conductivities, the influx of water to replace that boiling off may be sufficient to prevent the soil from exceeding the boiling point of water, and target temperatures may not be met. If the treatment area contains saturated high-hydraulic conductivity soil, then a dewatering system should be considered, or Baker and Heron (2004) suggest using a steam system to control water influx, as well as sweeping the permeable areas. The drying of soils, especially fine-grained silt and clay, at high temperatures can result in shrinkage and cracking that will promote the removal of organics contained within them (U.S. EPA 2004).

If concentrated halogenated organics are the contaminants of concern, the system—both piping and treatment—must be designed to withstand highly corrosive conditions.

Thermal conductance systems also can consume large quantities of power. At a site in Alhambra, California, the remediation had to be carried out in phases to avoid exceeding the capacity of the local power supplier. Vendor cost estimates cited range from $100 to $250 per ton (NAVFAC 1999). TerraTherm has an exclusive license in the United States to offer this technology for remediation.

This discussion is taken from Adobe PDF LogoEngineering Forum Issue Paper: In Situ Treatment Technologies for Contaminated Soil, EPA 542-F-06-013, 2006.

General Resources

Adobe PDF LogoA Description of the Mechanisms of In-Situ Thermal Destruction (ISTD) Reactions
R. Baker and M. Kuhlman.
2nd International Conference on Oxidation and Reduction Technologies for Soil and Groundwater, ORTs-2, Toronto, Ontario, Canada, Nov. 17-21, 2002, 10 pp, 2002

Summarizes how the ISTD technology works, explains the underlying reaction mechanisms and rates, and demonstrates that most of the remediation is due to in situ destruction reactions.

Adobe PDF LogoIn-Situ Delivery of Heat by Thermal Conduction and Steam Injection for Improved DNAPL Remediation
R. Baker and G. Heron.
Proceedings of the 4th International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, May 24-27, 2004. Battelle, Columbus, OH.

Presents an overview of how in situ thermal conductance works and how steam can be used to improve its performance, particularly in the saturated zone.

-Return to "Thermal Processes: In Situ > Steam Injection and Extraction"