Fact sheet: Electromagnetic heating

From: Public Services and Procurement Canada

On this page

Description

In situ electromagnetic heating of soils, also called radio-frequency heating, consists of the emission of electromagnetic waves at different frequencies in order to accelerate molecular movement and to increase the temperature within a soil matrix. Electromagnetic technologies add sufficient heat to the subsurface to exceed the boiling point of groundwater, a temperature which is typically sufficient to volatilize the target contaminants. The resultant vapour (water and volatile contaminants) is extracted through collection wells using a multiphase extraction system or soil vapour extraction system and treated at the ground surface. Electromagnetic heating of soils may be performed using radio frequencies or microwaves. The heat generated increases volatilization of volatile and semi-volatile organic compounds (VOCs and SVOCs, respectively) in the soil.

This technique also applies to pure products such as light non-aqueous phase liquid (LNAPL). The electromagnetic heating technique can increase the soil temperature up to 300 °C and is effective in low-permeability soils and bedrock.

Electromagnetic heating has been known for decades to be an effective technique for heating large volumes of subsurface soils. Applications to oil extraction and soil remediation are well studied; however, commercial development of this technique remains incomplete.

Internet links:

Implementation of the technology

Electromagnetic heating systems may include:

  • Physical and chemical characterization of the soil or sediment (including mapping of the subsurface);
  • Mobilization of equipment (Drilling equipment, electrodes material, vapour recovery well material, a steam and vapour collection system, a vapour treatment system, an electromagnetic heating power supply and control unit, etc.) and construction of temporary facilities and site access considerations;
  • Installation of electrodes;
  • Installation of MPE or SVE extraction wells;
  • Installation of insulated vapour conveyance pipelines;
  • Steam and vapour collection system (vacuum system) include cooling and phase separation equipment;
  • Equipment removal and extraction well decommissioning.

Materials and Storage

Extraction wells (MPE or SVE) contain significant water. The above-ground treatment system must include provisions for air-liquid separation, vapour condensation, vapour, condensate and liquid cooling, non-aqueous liquid phase separation, water treatment and vapour treatment. The water treatment system can consist of cooling towers and carbon adsorption units. Vapour treatment systems are commonly comprised of combustion (thermal oxidation, catalytic oxidation) or filtration/sorption (activated carbon, biofiltration) units.

Process equipment is generally provided by one of a limited number of specialized remediation subcontractors.

Sufficient storage space is required to house the electromagnetic heating unit as well as the water and vapour treatment units.

Waste and Discharges

  • Residuals depend on the water and vapour treatment processes;
  • The water treatment system will generate treated water requiring disposal and will potentially generate non-aqueous phase liquids;
  • The most common air emissions control systems use granular activated carbon or oxidation (with or without a catalyst). The spent granular activated carbon must be periodically transported off-site in order to be regenerated or disposed. In the case of catalytic oxidation units, oxidation of chlorinated organic produces acid vapour. This vapour is typically managed with a caustic scrubber. Scrubber water requires periodic neutralization and disposal.

Recommended analyses for detailed characterization

Biological analysis

  • Total heterotrophic and specific bacterial counts (according to the contaminants of interest)

Physical analysis

  • Soil water content
  • Soil granulometry
  • Soil thermal conductivity
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
  • Measurement of NAPL surface tension under site conditions
  • Presence of non-aqueous phase liquids (NAPLs)

Recommended trials for detailed characterization

Biological trials

  • Microcosm mineralization trial
  • In situ respirometry trials
  • Biodegradation trial

Hydrogeological trials

  • Pneumatic trials

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Applications

  • Applicable to residual VOC and SVOC contaminants in the vadose zone;
  • Allows for the volatilization of LNAPL;
  • Enhances the volatilization of compounds in low-permeability soils.

Applications to sites in northern regions

  • Thermal remediation systems may not be appropriate for remote sites without access to an adequate electrical supply;
  • Extreme cold can hamper volatilization in shallow material. However, because deep soil temperatures are relatively constant over the course of the year, electromagnetic heating may still be applied effectively in northern environments, particularly for sites in which the targeted contamination is located well below the ground surface;
  • Northern systems require climate-appropriate design, including consideration of deep frost, permafrost, seasonal changes in saturation and air permeability, fuel supply or sorbent removal capabilities. ;Even with an electrical supply, electromagnetic heating can be expensive or not feasible due to the high power requirement, high electricity consumption and long heating times which be required due to difficult geology and heat loss.;

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Applies
Chemical
Does not exist
Control
Does not exist
Dissolved contamination
Does not exist
Free Phase
Applies
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Applies

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Exist
Commercialization
Does not exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
Applies
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
Applies
Phenolic compounds
Does not apply
Policyclic aromatic hydrocarbons
Applies
Polychlorinated biphenyls
Does not apply

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Applies
3 to 5 years
Does not apply
More than 5 years
Does not apply

Notes:

Electromagnetic heating is a short-term technology. Treatment time frames should vary between 6 and 18 months but the number of application examples is limited. 

Long-term considerations (following remediation work)

ERH has not been observed to adversely affect geotechnical properties. However, care should be taken when treating soils around or under buildings or infrastructure.

Secondary by-products and/or metabolites

  • There is limited potential for in situ by-products.
  • Biodegradation of certain contaminants such as aliphatic chlorinated hydrocarbons, can generate toxic metabolites (such as the biological transformation of dichloroethene can produce vinyl chloride).

Limitations and Undesirable Effects of the Technology

  • Requires high voltage on site;
  • Soils enriched with organic matter are difficult to treat because of their high sorption capacity of contaminants;
  • Not suitable for inorganic contamination;
  • Electromagnetic heating technology is expensive;
  • In high permeability aquifers, groundwater inflow may inhibit the achievement of an adequate temperature increase;
  • Preferential pathways or soil matrices with layers of varying permeability may affect the release of vapours;
  • When applied at DNAPL sites, DNAPL may be mobilized due to the heat-induced decrease in viscosity and interfacial tension. The potential for downward movement of steam condensate may also be associated with this technology;
  • May have undesirable impacts on nearby buildings, utilities or other enclosed spaces (such as elevated temperatures, high soil vapour concentrations, and potential for soil vapour migration and intrusion into buildings or enclosed spaces);
  • Handling, for example, fuel vapours at levels near the lower explosive limit (LEL) could pose a potential fire/explosion risk;
  • Buried metal objects constitute a safety hazard. The subsurface should be mapped before the heating system is installed;
  • Risks associated with the exposure to high voltages require implementation of engineering controls.

Complementary technologies that improve treatment effectiveness

In situ biological technologies (biostimulation, bioaugmentation, and bioventilation) can be used when temperatures are less than 40 °C of after thermal treatment application. Elevated temperatures result in more rapid biodegradation; there is some evidence that microbial recolonization of aquifers occurs rapidly following thermal treatment. 

Required secondary treatments

  • Vapour extraction system
  • Treatment of vapours

Application examples

Application examples are available at these addresses:

Performance

The performance of a combined vapour extraction and electromagnetic treatment system varies according to the nature of the soil matrix and the chemical properties of the contaminant(s).

During a pilot study at Savannah River, U.S.A., electromagnetic heating was responsible for the extraction of over 170 kilograms of chlorinated solvents from contaminated sediments. The system destroyed the contaminants with an efficiency ranging from 80 to 95%. This result is an example of the performance of the electromagnetic heating technology for TCE and PCE contamination.

Measures to improve sustainability or promote ecological remediation

  • Optimization of heating array placement to reduce energy requirements.
  • Optimization of process equipment related to soil vapour extraction (e.g., pumps, air treatment systems).
  • Schedule optimization for resource sharing and fewer days of mobilization. 
  • Wastewater process optimization to reduce wastes and consumables (activated carbon).
  • Use of heat model for evaluating technology efficiency, particularly in areas with faster moving and cooler groundwater, where heat loss is of concern.

References

Author and update

Composed by : Mahaut Ricciardi-Rigault, M.Sc., MCEBR

Updated by : Josée Thibodeau, M.Sc, National Research Council

Updated Date : March 1, 2008

Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée

Updated Date : March 31, 2018

Version:
1.2.1