Thermal Conduction Heating (TCH) is applicable in fractured rock and other complex formations such as:
- Tight and fractured clays
- Fractured igneous and metamorphic rock
- Permeable basalt
- Karst limestone
Cross-section showing ISTD of fractured rock for treatment of chlorinated
solvents above and below the water table
(Click to enlarge)
TCH, as incorporated within TerraTherm’s proprietary In Situ
Thermal Desorption (ISTD)
technology is ideally suited for complex sites such as DNAPL source
zones in fractured rock. While other technologies can effectively
heat the fractures, they are far less successful in heating the
rock matrix, since insufficient heat is delivered to the rock via
the fractures alone. For the vast majority of fractured rock types,
TCH achieves the desired target temperatures in both the rock fractures
and matrix. This offers distinct advantages in both consistency
of heating and reliability of extraction.
ISTD uses simple electrical heaters suspended inside steel casings installed in boreholes to deliver energy to the surrounding formation, coupled with vacuum extraction. The heat migrates away from the heater borings by a combination of thermal conduction (driven by a temperature gradient) and convection (migration of steam produced by boiling ground water). The fractures are major pathways for the generated vapor to escape and be captured by the vacuum extraction system. Since in a typical fractured rock application, every ISTD heater is supplied with a vapor recovery point, the entire treatment zone is kept under a vacuum, ensuring that even chlorinated volatile organic compound and oil vapors generated in dead-end fractures are captured.
The ISTD heaters are spaced between 10 and 20 ft apart in a triangular pattern. The extracted vapors are cooled and treated on-site using standard off-gas treatment equipment. Treatment durations vary from three months to one year, depending on the heater spacing and target contaminants.
Based on laboratory studies, field demonstrations and field projects completed to date, it appears that TCH is the only technology that can treat and remove contaminants such as DNAPL constituents that have diffused into the rock matrix.