When you are new to the thermal remediation industry, choosing the best technology can be overwhelming and confusing. Each heating technology has its own strengths and weaknesses depending on the site conditions and contaminants of concern. If you have been reading our blogs, you have already seen an overview of thermal conduction heating from my colleague and will see a similar overview of steam enhanced extraction in an upcoming post. For this blog post, I’ll explain what electrical resistance heating is, how it works, and the sites and contaminants it is best suited to treat.
What Is ERH?
ERH works by passing current through the soil between an array of electrodes in a triangular grid pattern. Heat causes the contaminants in the soil to vaporize. Those vapors are extracted by vapor extraction wells (VEWs) or multi phase extraction wells (MPEs). The vapor is directed into an above ground vapor treatment system, treated and cleaned, then discharged back into to the atmosphere.
Heating the subsurface with ERH can mobilize large amounts of contaminant mass at a significantly faster rate than traditional pump and treat methods or cold soil vapor extraction. A typical ERH project only lasts six to eight months compared to pump and treat or soil vapor extraction methods which can take decades to achieve remediation goals.
How does ERH work?
Three-phase alternating current (AC) is used to power the ERH electrodes. A 3-phase power source consists of three separate phases, or electrical lines, with alternating sinusoidal voltage waveforms. Each line is offset by 120 degrees from the others. The electrodes in an array are on different phases. When one phase waveform is at maximum or minimum voltage, the other phase voltage waveforms are at different voltages, thus providing optimized potentials for performing work (e.g., turning motors). The difference in voltages between the phases at any point in time is like the pressure differential in a water pipe. The higher the pressure, the more flow the pipe will carry.
The voltage difference between the phases drives current flow through the soil. The amount of current conducted by the electrode and passed through the soil is a function of the electrical conductivity of the soil. The higher the electrical conductivity, the higher the current flow for a particular voltage.
The electrical resistance of the soil (which is the inverse of the electrical conductivity of the soil), will convert some of the current flow to heat. In this way, 3-phase ERH can be used to heat up the soil in between the electrodes to the desired temperature, which is typically 100°C.
A power distribution system (PDS) consisting of electrical breakers, silicon controlled rectifiers (SCRs), and a programmable logic controller (PLC) is used to control and regulate the voltage and/or current output to the electrodes. ERH systems also incorporate a drip system that injects water around the electrodes to keep the soil moist, allowing the current to pass through the electrode.
The electrodes are installed in a triangular-based hexagon grid pattern, usually spaced about 18 to 20 feet from one other depending on site conditions. Preservation of the relative timing of the individual waveforms (no shifting of A, B, and C phases) is critical to optimize performance. The illustration below shows how the current is passed through the electrodes.
At the centers of the triangular electrode grid, also referred to as the centroids, extraction points (MPEs or VEWs) or temperature monitoring points (TMPs) can be installed. The TMPs typically measure the temperature at depths every three or five feet in the treatment zone. This allows the thermal contractor and stakeholders to know when the subsurface has reached the desired temperature and monitor to ensure the temperature is maintained as expected.
ERH Design Considerations
Soil properties play a critical role in the safe and effective design of an ERH treatment system.
The electrical conductivity of soil is highly dependent on soil moisture content. If the soil dries out due to boil-off of the water during heating, electrical conduction and heating stops. This is why ERH is limited to achieving 100°C (or boiling temperatures at hydrostatic pressures).
Soil resistivity dictates how readily the soil will conduct electricity and how much heating will occur for a given current flow. The sweet spot for optimal ERH performance is generally soil resistivity between 10 and 500 ohm•m.
A site with very low resistivity runs the risk of requiring large amounts of electrical current to get efficient heating— beyond the capacity of typical electrical supply and distribution equipment and electrical cables.
When the soil resistivity is too high, this creates a situation where high voltages (>>480V) are needed to push sufficient current through the subsurface to get adequate heating. In this situation, special high voltage rated equipment must be used and significant efforts must be made to ensure the safety of nearby utilities, workers, building occupants, and equipment from stray currents.
If soil resistivity is known before design, certain modifications can be made to the ERH approach to ensure successful performance. Examples include tighter electrode spacing or increasing water injection in the electrodes. However, in the case of extremes in resistivity, it is very difficult to safely design and operate a system to achieve adequate and uniform heating. For these sites, alternative heating approaches should be considered, such as TCH, SEE or combined thermal technologies.
In this case study, ERH remediation of a gas station, TerraTherm successfully removed LNAPL and reduced dissolved phase concentrations by an average of 99%. ERH was selected to address both soil and groundwater impact within the source area of an active gas station. Underlying geologic units included sandy silt, fill, dense till, and a deep gravel outwash. Due to their varying electrical conductivities, different voltages and currents were applied to the distinct soil layers using a system of stacked electrodes. A multi-phase extraction (MPE) system was used to remove LNAPL from the top of the groundwater table and captured steam and contaminant vapors from the vadose zone.
How ERH Removes Contaminants
Heating a site to 100°C using ERH removes the contaminants from the impacted soils, through a variety of mechanisms: evaporation, steam stripping, boiling, removal as non-aqueous phase liquid (NAPL) or dissolved in extracted groundwater, and chemical reactions (hydrolysis).
- Evaporation: The temperature of the contaminated soil does not need to be at the contaminants boiling point to vaporize if water and air are present. This mechanism depends on the increase of vapor pressure of the contaminate in the vapor phase. If the vapor pressure increases, the contaminant of concern (COC) can be evaporated and extracted into the treatment system through the extraction wells.
- Steam Stripping: When water is present in the soil and the soil is heated to the boiling point of water (100 °C or 212 °F), steam is created. This steam transports the compounds into the extraction wells where they’re treated.
- Boiling: The temperature of the soil reaches the boiling point of the COCs and vaporization occurs. Once the COC is vaporized, it travels into the extraction wells where it’s treated. This method is limited to COCs with a boiling point no higher than 100°C.
- Removal as NAPL or in the dissolved phase: For some chemical mixes, heat substantially decreases the viscosity of a NAPL phase, making the contaminant mass easier to remove by pumping. Also, increasing temperatures often increases the chemical solubility, and therefore enhance the mass removal by liquid pumping. Substantial mass can potentially be removed by pumping from MPE wells at applicable sites. The pumped NAPL and dissolved mass are separated and treated in the treatment system.
Several methods are available to treat the contaminants within the above ground vapor treatment systems. Granular activated carbon (GAC), steam regenerative GAC, and thermal oxidation are commonly used. GAC is a porous media which absorbs the contaminates and thermal oxidation is the process of destroying the contaminants in the vapor stream with high temperatures, generally between 1,200 – 1,800°F.
What Contaminants can ERH Treat?
Since ERH depends on the ability to pass current through the subsurface ERH, it is only able to treat contaminants with boiling or co-boiling points near or below 100°C. If all the water is boiled off, the current flow through the subsurface with stop and so will heating. ERH can achieve very low soil and groundwater cleanup standards if designed and operated properly; for example, <10 µg/kg in soil and <5 µg/L in water for CVOCs. ERH works well for:
- Volatile organic compounds (VOCs)
- Benzene, toluene, ethylbenzene and xylene (BTEX)
- Lighter molecular weight semi volatile organic compounds (SVOCs)
- Physical removal of NAPL
- Total petroleum hydrocarbons (TPH)
- Coal tars
Get Started with ERH
ERH heating is a proven, predictable, reliable, and safe heating technology for thermally remediating contaminated sites. It’s been used all over the world for more than 25 years, and we’ve used it to successfully remediate 80+ contaminated sites.
If you would like to learn more about ERH and discuss if it’s the right thermal heating technology for your site, watch our on-demand webinars or contact us directly.
Kevin Crowder has 12 years of experience in working in the thermal remediation field. He specializes in the design, implementation and assessment of numerous ERH, TCH and SEE sites in the US, Canada, and internationally. As a Project Engineer, Kevin has been responsible for the design, oversight and implementation of over two dozen projects for…