Model TCH/ISTD & IPTD - Thermal Conduction Heating System
Thermal conduction is the process of heat flowing from the hot end of a solid object (like an iron rod) to the cold end. In soil or rock, heat flows from TerraTherm’s heater wells out into the formation by grain-to-grain contact (in soil) and across solid objects (rocks). The fluids (water, air, NAPL) in contact with the solids also heat up at the same time. The heat moves out radially from each thermal well until the heat fronts overlap.
TerraTherm installs a series of patented electrically-powered heaters and vapor extraction points in situ, to heat contaminated soil to target treatment temperatures. Target treatment temperatures are typically 100°C for volatile contaminants and between 150°C and 325°C for semi-volatile contaminants.
Thermal conductivity values for the entire range of known soils vary by a factor of less than plus or minus three, while fluid conductivity of soils may vary by a factor of a million or more. Compared to fluid injection processes, the conductive heating process is uniform in its vertical and horizontal sweep. Transport of the vaporized contaminants is further improved by the creation of permeability, which results from drying (and, if clay is present, shrinking) of the soil close to the heaters. Preferential flow paths are created even in tight silt and clay layers, allowing flow and capture of the vaporized contaminants. TCH produces uniform heat transfer through thermal conduction and convection in the bulk of the soil volume. This allows the achievement of very high contaminant removal efficiency with a nearly 100% sweep efficiency, leaving no area untreated.
TCH can be applied at low (100°C) temperature levels to accomplish the remediation of a wide variety of contaminants, both above and below the water table.
TCH is the only major in situ thermal remediation technology capable of achieving target treatment temperatures above the boiling point of water. TCH is effective at virtually any depth in almost any media.
TCH works in tight soils, clay layers, and soils with wide heterogeneity in permeability or moisture content that are impacted by a broad range of volatile and semi-volatile contaminants, such as:
- Explosives Residue
- Chlorinated Solvents
- Heavy Hydrocarbons
The TCH technology can be utilized to heat in situ soils and stockpiled soils and sediments. The design of the treatment system for in situ soils (ISTD) typically includes vertically installed heaters whereas the design of the treatment system for the stockpiled soils (In-Pile Thermal Desorption, or IPTD) typically incorporates horizontally installed heaters. Examples of the elements of each system are shown below:
The TCH technology can operate inside, beneath, and near buildings and infrastructure. This capability has been field proven at numerous projects.
The TCH technology can be applied to contaminants in soils both above and below the water table (see also Permeability and Geology) where the soils can be heated up to target treatment temperatures. Contaminants such as TCE, PCE, and other VOCs that have boiling points similar to water can be treated simply by steam distillation. Contaminants such as PAHs, dioxins, PCBs, and other SVOCs that have higher boiling points than water are treated by boiling off the water within the treatment zone, and then by heating the soil to the designated treatment temperatures. Where significant groundwater flow is present, additional measures such as groundwater management or a hydraulic barrier may be required. TerraTherm has successfully used steam injection into the high-K zones to augment the ISTD process, thereby ensuring complete heat-up and treatment of both tight zones and permeable zones.
TCH is used for the thermal desorption and destruction of Semi-Volatile Organic Compounds (SVOCs) such as PAHs, PCBs, pesticides, and dioxins. TCH is the only major in situ thermal remediation technology capable of achieving target treatment temperatures above the boiling point of water. With the application of higher temperatures for (>100°C) for treatment of SVOCs, most contaminants are destroyed in the soil before reaching the surface. Contaminants that have not been destroyed in situ are removed from the produced vapor stream at the surface with an Air Quality Control system.
Typical target treatment temperatures for SVOCS (measured at centroid locations between the heaters) are above 480°F (250°C). In Situ Thermal Desorption (ISTD) utilizes TCH to effectively raise the soil temperature by installing an array of heater wells while simultaneously applying a vacuum to subsurface soils through the installation of vapor collectors to capture the volatilized contaminants and steam. The volatilized contaminant vapors are then extracted through vapor collection points, collected, and treated in a site-specifically designed Air Quality Control system. TerraTherm prevents fugitive emissions by applying a vacuum to the treatment zone beneath a surface vapor barrier.
At moderate temperatures (~100°C), TCH is applied to the recovery of Volatile Organic Compounds (VOCs) such as TCE, PCE, and BTEX. A non-desiccation approach is used for sites impacted with VOCs since it is not necessary to boil off much of the soil water to successfully recover the contaminants. Using the moderate temperature application of TCH, the temperature of the soil within most of the treatment volume is raised to the boiling point of water, generating steam in situ. This results in very effective steam-stripping/distillation of the contaminants. The contaminant vapors and steam are then collected and captured for treatment in an Air Quality Control system.
The moderate temperature application of TCH can also be used to achieve In Situ Thermochemical Solidification (ISTS) of coal tar at sites such as former Manufactured Gas Plants (MGP). During this moderate temperature application of TCH, coal tar is removed and organic compounds such as benzene and naphthalene are volatilized, permanently stabilizing and solidifying any remaining MGP coal tar constituents in place. The remaining constituents are immobilized within an asphaltic matrix, similar to asphalt pavement. They are non-leachable by both Toxicity Characteristic Leaching Procedure and Synthetic Precipitation Leaching Procedure (Hayes, 2002).
At lower temperatures (<100°C), TCH is applied to the thermal enhancement of free product recovery and the thermal enhancement of existing Soil Vapor Extraction (SVE) systems.
Thermally Enhanced Free Product Recovery
Many non-aqueous phase liquids (NAPLs) are difficult to recover at ambient temperatures. The application of TCH at temperatures below 100°C is used to facilitate the recovery of NAPLs. This application of TCH uses an array of heaters at a spacing of 20 to 40 ft. Horizontal trenches are used for shallow applications while vertical wells are used for deeper applications. Existing or new free product recovery wells are located between the heaters. By increasing subsurface temperatures to below the boiling point of water, LNAPL viscosities are reduced ten- to one hundred-fold, greatly enhancing free product recovery.
TerraTherm applied the TCH technology to recover >16,000 gallons of coal tar. Previous attempts at cold temperatures had recovered only a few gallons.
Thermally Enhanced Soil Vapor Extraction
Contaminants trapped in low-permeability or high-saturation layers are not efficiently removed using typical SVE systems.
The primary SVE induced vapor flow bypasses those layers and contaminant removal stalls. This can lead to years of continued SVE operations, without reaching soil cleanup goals.
The application of TCH to these “stalled” sites can accelerate mass removal from such layers by increasing the vapor pressure of the contaminants, partially drying the recalcitrant layers, and accelerating biological and chemical degradation reactions. This application of TCH uses an array of heaters placed between existing SVE wells without interruption to present SVE operations. The site is heated slowly by applying power to the TCH heaters, while maintaining SVE operations.