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What is Thermal Conduction?
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 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.
TCH Installation
Electrically-powered heaters and vapor extraction points were installed 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.
Benefits of TCH
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), moderate (~100°C), and higher (>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:
Applicable to Both In Situ and Stockpiled Soils and Sediments
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. Steam injection was successfully used into the high-K zones to augment the ISTD process, thereby ensuring complete heat-up and treatment of both tight zones and permeable zones
- Electrical Resistance Heating (ERH)
ERH has been widely applied and proven effective for free product recovery and enhanced vapor extraction at sites with volatile contaminants such as VOCs, CVOCs, and NAPLs, and is applied at low and moderate temperatures.
What is Electrical Resistance Heating (ERH)?
When electric current is passed through the soil, the resistance it encounters causes the soil and fluids to heat up. The current flows from one electrode to another, primarily through the soil water. Once the water boils off, electrical conductivity becomes negligible and heating ceases; thus, water is added at each electrode to keep them from drying out. Heat-up with ERH is limited to the boiling point of water.
ERH Process
Electrodes are installed in wells throughout the contaminated soil and groundwater volume. The electrode array is connected to a Power Delivery System unit that uses standard, readily available three-phase power from the grid. The process begins by passing current between electrodes causing the soil temperature to rise. This increased temperature results in the volatilization of contaminant compounds into the vapor phase for removal with vapor extraction techniques.
Comprehensive computer controls are used to regulate and optimize the thermal response of the target formation.
ERH Advantages
- Superior energy delivery and heat transfer achieved through custom-built electrodes that provide higher output and improved control;
- More effective energy delivery and subsurface heating provided by real-time computerized power controls for each electrode;
- Convective energy delivery is available in permeable settings, making it possible to generate and inject steam for additional heating and contaminant flushing; and,
- Safety and process transparency are enhanced for both the project team and client with real-time Digital-at-the-source temperature and monitoring.
3.Steam Enhanced Extraction
Steam Enhanced Extraction (SEE), a highly effective technology used for the recovery of free product and the remediation of volatile organic compounds (VOCs) since the mid-1990s.
SEE achieves on-site separation and treatment through steam injection into wells and extraction of hot fluids. Steam propagation is a stable and predictable process, governed by heat transfer to the formation and has been studied intensively and utilized for oil recovery and remediation of a wide range of contaminants.
SEE is used at low and moderate temperatures. Injection and extraction wells was installed that are used to inject steam into the subsurface while simultaneously extracting steam, vapors, mobile non-aqueous phase liquid (NAPL) , and groundwater. The injected steam is used to heat the subsurface to target treatment temperatures, typically the boiling point of the contaminant of concern at the site.
SEE utilizes the following contaminant removal and destruction mechanisms:
- Displacement as a NAPL phase and extraction with the pumped groundwater
- Vaporization in the steam zone
- Accelerated vaporization and extraction in the vapor phase through pulsed pressurization and depressurization cycles
- Dissolution, destruction, and removal with the extracted water
Sites with Significant Groundwater Flow
SEE is a logical choice for large and deep sites with significant groundwater flow. The SEE technology allows for high net extraction of fluids and displaces large amounts of groundwater towards the extraction wells. As a result, less water has to be heated to allow the formation to reach target temperatures. In addition, this displacement facilitates hydraulic control of NAPL mobility. The steam sweep through the formation and the accompanying pressure gradient displace the mobile NAPL and vaporized components as an oil front, which is recovered when it reaches the extraction wells.
Pressure Cycling for Improved
Contaminant Removal Rates
Another significant benefit of SEE is the ability to conduct pressure cycling to improve contaminant removal rates. After the target zone has been heated and the majority of the NAPL extracted as a liquid, pressure cycling is induced by varying the injection pressure and the applied vacuum. This process has been shown to achieve very low concentrations in the original source zone.
