Electrical Resistance Heating Primer
Electrical Resistance Heating
ERH uses electrodes placed in a grid pattern to pass electrical current through a volume of subsurface soil to be treated. The process is equally effective in vadose and saturated zones. As the subsurface resists this flow of electricity, it is heated to the boiling point of the water/contaminant mixture present, regardless if the water source is soil moisture in the vadose zone or a saturated zone aquifer.
Following Dalton’s and Raoult’s Laws, the temperature at which subsurface boiling occurs depends on contaminant types, contaminant concentration ratios, and depth below the groundwater table. The presence of less volatile contaminants produces higher boiling points and consumes more energy. Table 1 shows the boiling points of some common chlorinated hydrocarbons in air and when in contact with soil moisture or groundwater in our The Electrical Resistance Heating Primer.
|Substance||Boiling Point In Air||Boiling Point In Water|
The heat created by ERH boils soil moisture and groundwater to produce an in situ steam source. This steam source opens the soil matrix, strips away contaminants, and carries them to vapor recovery (VR) wells that are co-located with the electrodes.
ERH brings the subsurface to boiling in a smooth and controlled manner. Because soil moisture levels must be at least 5% to perform ERH, the technology cannot desiccate soil or cause subsidence. ERH electrodes do not become significantly hotter than surrounding soil and no excess energy is stored in the subsurface during ERH. If ERH is discontinued for any reason, in situ steam generation stops quickly. For this reason, backup generators for vapor recovery systems are not a mandatory requirement on ERH sites.
ERH preferentially targets the most impacted parts of the subsurface, which are typically tight silt and clay lenses. Once heating starts, pure phase contaminants boil first, then contaminated groundwater, and finally clean groundwater. This order is advantageous for remediation because contaminated water boils off before uncontaminated water, reducing the time and energy required to complete treatment.
At a site with a homogenous lithology, ERH will heat, and clean, clay and silt layers first before moving on to sandy zones as the tighter layers dry out. Because steam is formed inside the tight soil lenses, contaminant removal pathways are opened as the steam expands and forms micro fractures so that it can escape to the surface.
The type of contaminants and the cleanup goals affect the time and cost to clean a treatment volume. However, two key factors are the level of total organic carbon (TOC) in soil and the presence of semivolatile hydrocarbons such as heavy fuels. Both of these substances hold CVOCs in the subsurface, making them more difficult to remove regardless of the in situ remediation technology deployed.
During ERH, some CVOCs will be destroyed in situ by naturally occurring biological and abiotic reactions. Examples of these reactions include hydrolysis, biodegradation, and reductive dehalogenation. While quite slow at ambient temperatures, these reactions are capable of destroying measureable amounts of CVOCs at the temperatures produced by ERH. However, once the treatment volume starts boiling, the primary mechanism for COVC removal is volatilization and vapor extraction.
Electrical Resistance Heating Primer: Technology Advantages
ERH can treat vadose zones, saturated zones, or both zones simultaneously. It is capable of reaching stringent site closure goals, including MCLs, within months, without issues of contaminant rebound.
It is not hindered by soil type and is equally effective in gravels, sand, silt and tight clay as well as homogenous mixtures of soil type. It has been successfully deployed to depths of 100-feet below grade in glacial till, weathered sand stone, saprolite, bedrock without contaminant rebound.
The contaminant sweet-spot for ERH is chlorinated solvents, including PCE, even when found at DNAPL concentrations. However, it also outperforms other remediation technologies for petroleum hydrocarbons, creosote, and manufactured gas plant sites.
The technology has been applied at busy shopping malls, under operating manufacturing facilities, and in public streets. ERH sites can be operated with no restrictions to public access or commercial activity on top of the treatment area, issues with vapor intrusion, or with odor and noise.
ERH System Components
The components of typical ERH systems are shown in Figure 1 and listed below.
•Power Delivery System (PDS)
•Electrical cable between the PDS and the electrodes
•Wells for the collection and extraction of liquids, steam and contaminant vapors
•Conveyance pipe to transfer liquids, steam and vapors to treatment systems
•Steam condenser & vacuum blower
•Vapor and liquid treatment systems
•Temperature Monitoring Points (TMPs)
•Monitoring, control, data storage, site security, and communication systems
Power Delivery Systems
We manufacture our own PDSs in our Longview, WA UL 508A panel shop. This allows us to customize the size and specifications of each PDS. Based upon a silicon controlled rectifier (SCR) design, these PDSs weigh half as much as copper transformer PDSs and are at least 25% more efficient. This means that 25% more of the energy delivered by the utility reaches the electrodes.
Our SCR design allows truly automated adjustments of voltage and amps at the electrodes without the need for manually changing transformer taps. Because SCRs can adjust electrode voltages at any increment, our PDS are never “caught between taps” and forced to deliver either too little or too much power. The SCRs also ensure that electrical current cannot leave the electrode field and interfere with or “travel along” utility, security, or communication lines.
Typical ERH electrodes are constructed inside 8 to 12-inch diameter borings using sections of 3-inch diameter metal pipe, electrically conductive backfill, and Portland cement seals (Figure 2). Drilling may be performed by any method and the metal pipe can be slotted to allow the collection of steam and vapors from the subsurface.
Each electrode boring also contains a co-located vapor recovery (VR) or multi-phase extraction (MPE) well. As an option, injection points can be placed in the same boring to add air, chemical oxidants, or biological amendments and nutrients to the subsurface. The conductive backfill is a mix of steel shot and graphite used to increases the surface area of the electrode to maximize energy flux into the soil matrix.
The backfill is very permeable and electrodes act as “steam chimneys” below the water table and large diameter VR wells in the vadose zone. Because the steel shot is a form of zero-valent iron it enhances reductive dehalogenation of CVOCs in the saturated zone.
Electrode heads are enclosed inside a secure polyvinyl chloride (PVC) pipe and fitting system that isolates them electrically and seals them from direct access. It is safe to touch an electrode enclosure when the electrodes are energized. Electrodes are connected to the PDS using flexible cables rated for prolonged exposure to foot traffic, severe outdoor temperatures, and rain.
For ERH to work, the treatment volume must remain moist. A saturation water system is used to keep the vadose zone moist during heating. This computer controlled system recycles treated condensate to the electrodes and keeps the soil/electrode interface electrically conductive. ERH systems remove steam at twice the rate they re-inject water to the electrodes, and are net removers of water from the subsurface.
Vapor Recovery and Treatment
A blower is used to apply the vacuum needed at each electrode to recover vapor and steam from the subsurface and transport them to the ERH Condenser. The vapor/steam conveyance system is constructed of chlorinated polyvinyl chloride (CPVC) piping sized for the expected airflow plus a 25% allowance for steam. In above-grade installations, conveyance pipe is run directly on grade and strategic knockouts are provided to address the possibility of fluids buildup in the pipe runs. Alternatively, piping can be run subsurface to allow public or commercial access on top of the treatment area.
Inside the condenser, vapors are cooled and steam is converted to condensate. The cooled vapors will be treated using vapor-phase granular activated carbon (GAC) vessels or thermal oxidation prior to discharge to the atmosphere. Condensate is treated using a liquid-phase GAC and either re-cycled as electrode saturation water or discharged to the local POTW. ERH condensers have been shown to closely follow Henry’s Law and over 99.5% of contaminants entering the condensers will exit with the vapor phase rather than the condensate phase. Therefore, significantly more vapor-phase GAC will be consumed during the project than liquid-phase GAC.
Temperature & Pressure Monitoring
Temperature monitoring points (TMPs) constructed of 1-inch diameter CPVC pipe and containing thermocouples at 5-foot intervals are used to track the progress of subsurface heating. A typical TMP schematic is provided on Figure 2. Data from these thermocouples is used to prepare a subsurface temperature profile for the treatment volume.
The vacuum at each electrode is measured by gauge. By closing off vacuum to surrounding electrodes, vacuum radius of influence can be measured at any electrode. Additionally, vacuum piezometers are placed to measure vadose zone pressure along the perimeter of the treatment volume. This field information is used to document that complete vapor and steam capture is being achieved within the treatment volume.
Protocols have been developed, and accepted by State and Federal regulatory agencies, for performing hot soil and groundwater sampling at ERH sites. These sampling techniques allow hot samples to be safely collected, and accurately analyzed, using slight modifications to standard sampling equipment and methods.
Site security can be simple a chain-link fence or a complex closed circuit television (CCTV) system. CCTV systems use self-powered wireless video cameras with internal motion detectors to monitor all access points to the treatment area. When a motion sensor is activated, equipment interlocks immediately stopping power application to the electrodes and an alarm is sent to a commercial monitoring provider who notifies Operations Staff and local law enforcement. Operation Staff can access live and recorded video at any time by computer, IPod, or cell phone to view the system, determine the cause of any alarm, and reset power application to the electrodes. A Seattle based company manufactures and provides support for these cutting-edge security/monitoring systems.
Biodegradation of chlorinated VOCs is most commonly observed as an anaerobic process. Elevated temperatures significantly increase biotic reaction rates. This mechanism is especially important at sites where relatively high levels of total organic carbon (TOC) or non-chlorinated hydrocarbons provide a carbon source that serves as an electron donor.
Hydrolysis is a substitution reaction in which the hydrogen ions in water react with organic molecules to replace chlorine atoms. An example hydrolysis pathway is 1,1,1-TCA → 1,1-DCE → ethane. Hydrolysis reactions require neither oxidizing conditions nor available oxygen. Hydrolysis can be a significant degrader of some CVOCs, particularly halogenated alkanes, at the temperatures attained by ERH.
Reductive Dehalogenation reactions that take place in the steel shot used for electrode backfill are the same reactions produced by iron-filing remediation walls. Reductive dehalogenation provides a significant polishing mechanism for dissolved phase CVOCs after ERH is complete and the treatment volume slowly cools.
In Situ Chemical Oxidization (ISCO) is being combined with ERH at solvent, fuel, creosote, and coal tart sites. A recent combination of heating and ISCO is the use of ERH to catalyse sodium persulphase in the treatment of creosote and coal tar in soil and groundwater.
Have a site or project in mind with general details on the treatment volume, soil type, depth to groundwater, and contaminants of concern?
Go to pnecorp.com/global-remedial-solutions/quote-request-form.html and follow the easy to use bid request process. Or Contact Michael Dodson at firstname.lastname@example.org , (360) 423-2245. All information provided through our bid request process will remain confidential. Project and site data are used only to perform a technology assessment and to develop accurate pricing.
Global Remediation Solutions (GRS) is an environmental remediation technology company focused on the design and construction of in situ thermal remediation (ISTR) systems. GRS is part of the PNE Corporation family of companies. The size and diversity of the PNE companies, coupled with our highly respected environmental industry professionals, allows GRS to pursue major remediation projects on a national basis.
To learn more about GRS and ERH, visit us at pnecorp.com/global-remedial-solutions/about.html.