Thermal Conductive Heating

TerraTherm’s patented In-Situ Thermal Desorption (ISTD) process, also termed In-Situ Thermal Destruction, utilizes both conductive heating and vacuum to remediate soils contaminated with a wide range of organic compounds. Heat and vacuum are applied simultaneously to the soil with an array of vertical or horizontal heaters, under imposed vacuum. Each heater contains a heating element with an operating temperature of approximately 1400 to 1500°F (~750-800°C). Heat flows through the soil from the heating elements primarily by thermal conduction. As the soil is heated, volatile, semi-volatile and non-volatile organic contaminants in the soil (whether dissolved, sorbed, or present as NAPL) are vaporized and/or destroyed by a number of mechanisms, including: (1) evaporation; (2) steam distillation; (3) boiling; (4) oxidation; and (5) pyrolysis. The vaporized water and contaminants are drawn counter-current to the heat flow into the vacuum extraction wells (termed “heater-vacuum” wells).

Unlike fluid injection and extraction processes, ISTD conductive heating is very uniform in its vertical and horizontal sweep. This is because the energy injected into the soil by the heaters is uniform over each heater’s length and thermal conductivity values vary little (e.g., ± a factor of 2) over a wide range of soil types, leading to a highly predictable rate of heat-front propagation into the formation around each heater. As the heat fronts overlap, the entire treatment zone inevitably achieves the target temperature (which is selected depending on the contaminants of concern [COCs]).

By contrast, fluid injection and extraction processes are prone to preferential flow and by-passing because soil permeabilities vary by many orders of magnitude. Furthermore, ISTD creates permeability as a result of drying and shrinking of the superheated soil (i.e., >100°C) that develops around each thermal well. Closely spaced vapor flow paths are created even in tight silt and clay layers.

The combined effectiveness of both heat and vapor flow yields 100% sweep efficiency, leaving no area untreated, and a destruction/removal efficiency approaching 100%. This occurs because the coolest locations within the treatment zone can be heated, if desired, to the boiling points of the compounds, and maintained at such temperatures for many days. Every one of the seven competed ISTD field projects has achieved the required concentrations of the COCs, usually below their detection limits, even though their initial soil concentrations were often elevated (e.g., >10,000 ppm).

For remediation of soils contaminated with chlorinated solvents, thermal conduction heating can be used to accomplish steam distillation in the cooler interwell locations or below the water table, while locations near the thermal wells in the vadose zone become superheated, enabling in-situ destruction of the COCs as they are collected. In practice, regardless of the type of COC, 95-99% of the COCs are destroyed as they pass through the superheated soil in proximity to the heater-vacuum wells. COCs that have not been destroyed within the soil are removed from the vapor stream with the aboveground Air Quality Control system.