Electrical resistance heating remediation

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Electrical Resistance Heating (ERH) is an intensive in situ environmental remediation method that uses the flow of alternating current electricity to heat soil and groundwater and evaporate contaminants. Electric current is passed through a targeted soil volume between subsurface electrode elements. The resistance to electrical flow that exists in the soil causes the formation of heat; resulting in an increase in temperature until the boiling point of water at depth is reached. After reaching this temperature, further energy input causes a phase change, forming steam and removing volatile contaminants. ERH is typically more cost effective when used for treating contaminant source areas.

History[edit]

Three-phase heating (see Technology below) was originally created to enhance oil recovery. This design was patented in 1976 by Bill Pritchett of ARCO. The patent has expired and is now available for public use.

Six-phase heating (see Technology below) was created and patented for the US Department of Energy (DOE) in the 1980s for use on DOE sites as well as commercial applications.

Technology[edit]

Electrical resistance heating is used by the environmental restoration industry for remediation of contaminated soil and groundwater. ERH consists of constructing electrodes in the ground, applying alternating current (AC) electricity to the electrodes and heating the subsurface to temperatures that promote the evaporation of contaminants. Volatilized contaminants are captured by a subsurface vapor recovery system and conveyed to the surface along with recovered air and steam. Similar to Soil vapor extraction, the air, steam and volatilized contaminants are then treated at the surface to separate water, air and the contaminants. Treatment of the various streams depends on local regulations and the amount of contaminant.

Some low volatility organic contaminants have a short hydrolysis half life. For contaminants like these, i.e. 1,1,2,2-Tetrachloroethane and 1,1,1-trichloroethane, hydrolysis can be the primary form of remediation. As the subsurface is heated the hydrolysis half life of the contaminant will decrease as described by the Arrhenius equation. This results in a rapid degradation of the contaminant. The hydrolysis by-product may be remediated by conventional ERH, however the majority of the mass of the primary contaminant will not be recovered but rather will degrade to a by-product.

There are predominantly two electrical load arrangements for ERH: three-phase and six-phase. Three-phase heating consists of electrodes in a repeating triangular or delta pattern. Adjacent electrodes are of a different electrical phase so electricity conducts between them as shown in Figure 1. The contaminated area is depicted by the green shape while the electrodes are depicted by the numbered circles.

Fig 1. Typical three-phase ERH arrangement

Six-phase heating consists of six electrodes in a hexagonal pattern with a neutral electrode in the center of the array. The six-phase arrays are outlined in blue in Figure 2 below. Once again the contaminated area is depicted by the green shape while the electrodes are depicted by the numbered circles. In a six-phase heating pattern there can be hot spots and cold spots depending on the phases that are next to each other. For this reason, six-phase heating typically works best on small circular areas that are less than 65 feet in diameter.

Fig 2. Typical six-phase ERH arrangement

ERH is typically most effective on volatile organic compounds (VOCs). The chlorinated compounds perchloroethylene, trichloroethylene, and cis- or trans- 1,2-dichloroethylene are contaminants that are easily remediated with ERH. The table shows contaminants that can be remediated with ERH along with their respective boiling points. Less volatile contaminants like xylene or diesel can also be remediated with ERH but energy requirements increase as the volatility decreases.

List of compounds that can be remediated with ERH
Chemical Molecular Weight (g) Boiling Point (°C)
1,1,1-trichloroethane 133.4 74
1,1,2-trichloroethane 133.4 114
1,1-dichloroethane 99 57
1,1-dichloroethene 97 32
1,2-dichloroethane 99 84
1,2-dichloropropane 167.9 97
benzene 78.1 80
carbon tetrachloride 153.8 77
chlorobenzene 112.6 132
chloroform 119.4 62
cis-1,2-dichloroethyene 97 60
dibromoethane 187.9 132
ethylbenzene 106.2 136
1,1,2-Trichloro-1,2,2-trifluoroethane 187.4 48
gasoline 100 100
methylene chloride/dichloromethane 84.9 41
4-methyl-2-pentanone/methyl isobutyl ketone 100.2 117
2-methoxy-2-methylpropane/methyl tert-butyl ether 88.1 55
perchloroethylene 165.8 121
trichloroethene 131.5 87
tert-butyl alcohol 74.1 83
toluene 92.1 111
trans-1,2-dichloroethene 97 48
vinyl chloride 62.5 -14
xylene 106.2 140

Electrode spacing and operating time can be adjusted to balance the overall remediation cost with the desired cleanup time. A typical remediation may consist of electrodes spaced 15 to 20 feet apart with operating times usually less than a year. The design and cost of an ERH remediation system depends on a number of factors, primarily the volume of soil/groundwater to be treated, the type of contamination, and the treatment goals. The physical and chemical properties of the target compounds are governed by laws that make heated remediations advantageous over most conventional methods. The electrical energy usage required for heating the subsurface and volatilizing the contaminants can account for 5 to 40% of the overall remediation cost.

There are several laws that govern an ERH remediation. Dalton’s law governs the boiling point of a relatively insoluble contaminant. Raoult’s law governs the boiling point of mutually soluble co-contaminants and Henry’s law governs the ratio of the contaminant in the vapor phase to the contaminant in the liquid phase.

Dalton's Law

For mutually insoluble compounds Dalton’s Law states that the partial pressure of a non aqueous phase liquid (NAPL) is equal to its vapor pressure, and that the NAPL in contact with water will boil when the vapor pressure of water plus the vapor pressure of the VOC is equal to ambient pressure. When a VOC-steam bubble is formed the composition of the bubble is proportional to the composite’s respective vapor pressures.

Raoult's Law

For mutually soluble compounds, Raoult’s Law states that the partial pressure of a compound is equal to its vapor pressure times its mole fraction. This means that mutually soluble contaminants will volatilize slower than if there was only one compound present.

Henry's Law

Henry’s law describes the tendency of a compound to join air in the vapor phase or dissolve in water. The Henry’s Law constant, sometimes called coefficient, is specific to each compound, varies with temperature, and predicts the amount of contaminant that will stay in the vapor phase or transfer to the liquid phase when exiting the condenser.

Weaknesses[edit]

  • Weaknesses of ERH include heat losses on small sites. Treatment volumes that have a large surface area but are thin with respect to depth will have significant heat losses which makes ERH less efficient. The minimum treatment interval for efficient ERH remediation is approximately 10 vertical feet.
  • Co-contaminants like oil or grease make remediation more difficult. Oil and grease cause a Raoult’s Law effect which requires more energy to remove the contaminants.
  • Peat or high organic carbon in the subsurface will preferentially adsorb VOCs due to van der Waals forces. This preferential adsorption will increase the amount of energy required to remove the VOCs from the subsurface.
  • Fuel sites are less-commonly treated by ERH because other less-expensive remediation technologies are available and because fuel sites are usually thin (resulting in significant heat losses).
  • Sites within landfills are also challenging because metallic debris can distort the electric current paths. ERH is more uniform in natural soil or rock.

Strengths[edit]

  • ERH is adaptable to all soil types and sedimentary bedrock. ERH is also effective in both the vadose and saturated zones. Certain lithologies can limit traditional methods of remediation by preventing a reliable removal/destruction pathway for the contamination of concern. Because electricity can and does travel through any lithology that contains some water, ERH can be effective in any soil type. By forming buoyant steam bubbles during the heating process, ERH creates a carrier gas that transports the contamination of concern up and out of any soil type. ERH is not capable of desiccating the subsurface. In order for the subsurface to conduct electricity, there must be water present in the subsurface. Conductivity will cease before the subsurface is desiccated.
  • ERH is commonly applied under active buildings or manufacturing facilities. Electrodes can be installed above grade within a fenced area or below grade to allow for unrestricted surface access to the treatment area.
  • Although principally used for contaminant source areas, ERH can be used to achieve low remedial goals such as maximum contaminant levels, MCLs, for drinking water.
  • After ERH treatment, elevated subsurface temperatures will slowly cool over a period of months or years and return to ambient. This period with elevated temperatures is an important part of the remediation process. The elevated temperatures will enhance Bioremediation, hydrolysis and iron reductive dehalogenation.

External links[edit]