Eight Misconceptions about Thermal Conduction Heating (TCH) and Electrical Resistance Heating (ERH)

The Internet can be a tricky place. Sometimes when researching, it can become difficult to weed through the piles of information. Through some research of my own, I’ve come to find that there exist a few common misconceptions about thermal technologies. Many companies only practice one method of thermal heating, so it is understandable that misconceptions about another technology arise. TerraTherm on the other hand practices all three of the most accepted and mainstream methods of heating the subsurface: Thermal Conductive Heating (TCH), Electrical Resistance Heating (ERH), and Steam Enhanced Extraction (SEE). This post is meant to clarify misconceptions between TCH and ERH specifically. Coming from the only thermal vendor in the industry to use both TCH and ERH, here are eight misconceptions that you should be aware of.

Misconception number one: ERH costs are typically 30% lower than that of TCH.
Truth: The cost of a thermal project depends on a variety of site factors, and all three technologies have their own sweet-spot range and challenges. The premise that ERH is always cheaper than TCH is misleading. Clients have chosen to use TCH over ERH via competitive bids many times, also based on lowest cost. TerraTherm has more than 10 recent examples of this.

Misconception number two: ERH works equally well above and below the water table. The implication here is that TCH is only applicable to treat sites with contaminants lying in the unsaturated zone above the water table.
Truth: TCH has been used at more than 25 sites to treat below the water table with 100% success. Most TCH projects address both the unsaturated and saturated zones.

Misconception number three: TCH is not well suited for treatment of contaminants within clay soils.
Truth: TCH is ideally suited for treatment in clays, as we create the permeability paths for vapor recovery adjacent to the heaters. In fact, TCH performs better than ERH in that respect – with ERH, boiling occurs in places where there may not be an available path for the steam to flow to an extraction well. Secondarily, to keep the electrodes moist, water must be added at the same locations where energy density is highest, which reduces gas permeability there, impeding rather than enhancing vapor recovery during ERH. This advantageous vapor removal mechanism, which is unique to TCH, was recently described and published in GWMR (Heron et al. 2013).

Misconception number four: More energy is used to treat a site with TCH than with ERH.
Truth: The difference in electrical usage, for identical sites, is minor compared to total project costs. With TCH, since no water is injected, you need to extract less water, so for some sites TCH actually uses less energy.

Misconception number five: ERH is the most flexible and adaptable thermal technology.
Truth: While ERH is very flexible, TCH and SEE provide much more flexibility:

  1. Permeable zones can be heated much more quickly using steam.
  2. ERH cannot treat dry soils, because their resistance is too high.
  3. Varying the spacing of wells across a site is challenging with ERH due to the use of 3-phase alternating current (AC). The phasing among electrodes must be balanced and electrodes must be positioned within a specific pattern. In contrast, TCH heaters can be positioned and operated independently.
  4. By using a combination of TCH and SEE, the range of sites that can be effectively treated is expanded. In contrast, ERH applies best to those sites with a geology and hydrology that allow for the delivery of electricity thorough the subsurface relatively uniformly.

Misconception number six: TCH always desiccates and overheats the soil, leading to subsidence.
Truth: When using TCH to heat soil to the boiling point of water (e.g., to treat CVOCs), less than 5% of the soil volume is desiccated, and there is minimal impact on geotechnical stability. Over 95% of the soil volume remains moist throughout heating. Even with use of TCH to heat soil to over 300°C to treat higher boiling compounds such as PCBs and Dioxins, subsidence is rare. It is the presence of peat or under-consolidated material that governs whether subsidence is significant, not the heating method. Please refer to Nielsen et al. (2010) for details.

Misconception number seven: TCH heaters are really close together. This implies that drilling and installation of TCH heating systems is more expensive than for ERH.
Truth: TCH heaters are placed between 12-22 ft apart for CVOC treatment. Heater casings are 3 1/2-inch diameter (much smaller than electrodes) and much easier to install. At one recent large TCH site, 1,250 TCH heaters were direct-pushed in only 18 days using two rigs, which was very inexpensive.

Misconception number eight: ERH achieves more uniform heating than TCH, implying that less energy is spent on ERH than TCH.
Truth: ERH electrodes depend on the resistivity of the subsurface material for effectively delivering heat. In contrast, TCH heaters depend on the thermal conductivity of the subsurface material, which varies much less (a factor of about 3) over a wide range of soil types, from clay to sand, than does the resistivity (a factor of about 200 over the same range of soil types). TCH heaters inject the same amount of energy over the length of the heater, independent of subsurface conditions. As a result, much more even heating is seen with TCH, and often the total energy usage is lower than would have been needed for ERH.

The point of this blog is not to say one technology is better than another. There are cases in which either technology would be more applicable than the other given the characteristics of the subsurface and the desired cleanup goals. At TerraTherm, because we have access to ERH, TCH and SEE, we objectively look at each site and select the best-suited technology or combination. This unbiased approach is extremely important because when a technology provider recommends a thermal heating method, you should feel confident the recommendation is based on factual evidence and that you have not been fed misinformation due to a vendor having only one tool available to them and wanting you to select that one tool.

I hope this blog post has helped you to clarify some misconceptions that we have seen in the market. Feel free to contact me at gheron@terratherm.com if you have any questions.

 

References:

Heron, G., J. LaChance, and R. Baker. 2013.  Removal of PCE DNAPL from Tight Clays using In Situ Thermal Desorption. Ground Water Monitoring and Remediation, 33(4): 31-43.

Nielsen, S.G., G. Heron, P.J. Jensen, C. Riis, T. Heron, P. Johansen, N. Ploug and J. Holm. 2010.  “Thermal Treatment of Thick Peat Layers – DNAPL Removal and Shrinkage.”  Paper E-001, in K.A. Fields and G.B. Wickramanayake (Chairs), Remediation of Chlorinated and Recalcitrant Compounds—2010.  Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2010).  Battelle Memorial Institute, Columbus, OH.

About Gorm Heron

Gorm Heron, Ph.D. is Senior Vice President and Chief Technology Officer at TerraTherm, Inc. Dr. Heron has 21 years of experience in the environmental engineering field, with 14 years in design and management of in-situ thermal remediation projects. Based in TerraTherm’s Bakersfield , CA office, Dr. Heron provides technical leadership and oversight in the design and application of In Situ Thermal Remediation (ISTR) and combined In Situ Thermal Desorption(ISTD)/Steam Enhanced Extraction (SEE), Electro-Thermal Dynamic Stripping Process™(ET-DSP™).
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