Thermal Blanket/Thermal Wells
An in-situ thermal desorption technology marketed by TerraTherm Environmental Services Inc. removes and destroys polychlorinated biphenyls (PCBs). The process is an integrated system with two key components: the “thermal blanket/thermal well,” and a vapor treatment system, called the flameless thermal oxidizer. The process uses electricity to heat soil to the boiling point of the contaminants. It covers the area of contamination with a “Thermal Blanket.” Thermal blankets have embedded heating elements that are heated from 800 to 1,000 degrees Celsius at the surface. Most of the contaminant destruction takes place underground near the heat source. As the heat moves through the soil, contaminants are vaporized and a vacuum system draws the vapors toward and through the blankets. Vapors that are not destroyed underground are processed through a flameless thermal oxidizer and an activated charcoal filter. The thermal blanket is effective for contaminants down to about 20 inches. For deeper contamination, “Thermal Wells”—applying the same thermal desorption technology—are used. This process is sometimes generically referred to as in-situ thermal desorption.
Limitations and Concerns
There has been some concern that as PCBs are heated, but before they vaporize, they will become mobile in the soil. The vapor extraction system should therefore be strong enough to create a negative pressure throughout the zone being treated so that any mobile contaminants are drawn toward the vacuum. In the demonstration in Glen Falls, New York, the PCBs did not migrate away from the thermal blankets during treatment.
In addition, there is some concern that dioxins and furans, which are products of incomplete combustion (PICs), will be created and retained in the soil matrix. Demonstrations of the technology did not indicate the presence of dioxins and furans at detection levels of 0.003 to 0.007 parts per billion (ppb). Some dioxins and furans are toxic in the parts per trillion (ppt) range.
There is a major concern that products of incomplete combustion (PICs) such as dioxins and furans could be emitted to the atmosphere. Flames vary in temperature, and they are only one culprit in the formation of PICs. The flameless oxidizer provides a controllable uniform heating zone, and stabilizing that variable. Stack emissions at demonstration sites for the thermal desorption technology that uses the flameless oxidizer for vapor control (i.e., Thermal Blanket) were in compliance with EPA air quality standards (see 40 CFR Part 266, Appendix V), but there are no specific standards for emissions of dioxins and furans, except for municipal and medical waste incinerators. Those incinerator standards are technology-based, not health-based.
The process boils off water in the soil before it boils off the contaminants. In locations where the control of soil moisture will be difficult, such as in sediments that are saturated by surface waters, this technology cannot be used effectively, unless the sediments are excavated and treated above ground.
This technology is primarily used for the removal and treatment of PCBs in soil. It may also be a solution for treating volatile and semi-volatile organic contaminants including chlorinated solvents, pesticides, and petroleum wastes, although it has not yet been tested for these contaminants.
Technology Development Status
There have been several demonstrations of this technology, and it is considered by TerraTherm to be commercial. However, treatability studies or pilot tests are still required by regulators.
Other Resources and Demonstrations
A pilot demonstration was conducted at Mare Island Naval Station in a joint collaboration among the U.S. Navy, the Bay Area Defense Conversion Action Team (BADCAT), TerraTherm (a subsidiary of Shell Technology Ventures, Inc.), and RT Environmental Services, which acted as general contractor. The U.S. Environmental Protection Agency, California EPA and the Bay Area Air Quality Management District also participated. At Mare Island, levels remaining in the soil were reduced below 0.033 parts per million (ppm).
This technology was also used successfully in its first field application at a Superfund site in Glen Falls, New York. PCB-contaminated soil near the surface was as high as 5,000 ppm. Levels were reduced to less than 2 ppm.
A demonstration of thermal wells was conducted at the Missouri Electric Works Site in Cape Girardeau, Missouri in 1997. PCBs were found above the action limit of 10 ppm from 0 to 4 feet deep and 100 ppm below 4-feet deep. 12 thermal wells were installed, achieving a cleanup goal of 2 ppm.
Another demonstration of thermal well technology was conducted at the Naval Facility in Ferndale, California in 1998. Contaminants included PCBs, ranging from 0.15 ppm to 860 ppm and dioxin, up to 3.2 parts per billion (ppb) 2,3,7,8-TCDD Toxicity Equivalents (TEQ). PCBs were found up to 15 feet deep. Cleanup goals (PCB concentration of 1 ppm or lower; and dioxins and furans total 2,3,7,8-TCDD TEQ less than 1.0 ppb) were achieved.