Fact sheet: Hot Air Injection

From: Public Services and Procurement Canada

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Description

Hot air injection is an in situ remediation technology that applies to the soil vadose zone and increases the temperature within a contaminated area to facilitate the volatilization of organic contaminants. The injection of hot air allows the removal of volatile and semi-volatile organic compounds (solvents, certain pesticides and certain petroleum hydrocarbons) present in the residual and free phases. The organic compounds that are volatilized are recovered by a vapour extraction system installed around the hot air injection wells, and then conveyed to a treatment system. Hot air injection techniques are particularly suitable for homogenous granular soil with a high permeability and hydraulic conductivity.

The majority of volatile organic compounds require the injection of hot air at a temperature between 50 °C and 100 °C, but some special cases may require temperatures above 120 °C.

Sources

  • In situ thermal remediation of contaminated sites – A technique for the remediation of sources zones.
  • 4.10 Thermal Treatment - FRTR Remediation Technologies Screening Matrix and Reference Guide, Version 4.0
  • Analysis of Selected Enhancements for Soil Vapor Extraction (EPA-542-R-97-007 sept 1997) – Clu-in org - US EPA

Implementation of the technology

A hot air injection system includes the installation of a network of wells, trenches, permeable drains or other structures for injecting air into the vadose zone. The injection well network is designed so that the entire area to be treated is aerated and requires that the zone of influence of each well overlap. When air is injected, monitoring points are installed to ensure that there is no migration of harmful vapours. Blowers are used to inject air under pressure.

The air and vapours that are extracted are generally subjected to a treatment before being released into the atmosphere. Soil vapours are usually moist, and the extracted stream is often directed to a gas-liquid separator connected to the extraction system and the vapour treatment system. Treatment systems are generally composed of combustion units (thermal oxidation, catalytic oxidation) or filtration/adsorption units (activated carbon, biofiltration).

The implementation of this technology may include:

  • Mobilization, access to the site and setting up temporary facilities.
  • The installation of air injection points (wells, trenches, drains or other).
  • The installation of an air injection system consisting of individual blowers or a central blower and a set of distribution lines.
  • Installation of a heating system.
  • The installation of an extraction system consisting of vapour transport pipes, a suction system and, if necessary, atmospheric emission controls.
  • Treatment of air and/or vapours.
  • Dismantling of equipment and removal of injection and extraction points, if applicable.

Materials and Storage

  • This technology is implemented using traditional methods and equipment that are commonly available for well development and installation.
  • Treatment units can be built on-site or pre-assembled and transported in shipping containers, trailers or on pallets.
  • Equipment requires the establishment of an energy source.
  • Construction and landscaping generally have minimal impact and require little on-site storage.

Residues and Discharges

The implementation of the system could lead to the management of contaminated soils resulting from drilling or excavation activities. In this case, these soils must be removed off-site.

The waste generated is minimal and depends on the types of atmospheric emissions controls used. The treatment of the extracted vapours is generally required before they are released into the atmosphere. The most common air emission control systems use granular activated carbon or an oxidation process (with or without a catalyst). The air treatment sorbents used may need to be reclaimed or removed off-site periodically.

Recommended analyses for detailed characterization

Physical analysis

  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
  • Presence of non-aqueous phase liquids (NAPLs)

Recommended trials for detailed characterization

Physical trials

  • Gas permeability trials
  • Vapour survey
  • Evaluation of the radius of influence
  • Airflow rate
  • Evaluation of operating pressure/vacuum

Hydrogeological trials

  • Tracer tests

Notes:

Tests examining the effect of temperature change on hydraulic conductivity and establishing the zone of freezing with a pilot scale tubing system are recommended to properly design the full-scale containment system.

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
  • The type of surface covering

Phase III

  • Soil stratigraphy
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the seasonal fluctuations
    • the piezometric level

Notes:

Small-scale on-site studies are recommended to determine the feasibility of using the technology and to optimize parameters such as well design, injection well spacing, air injection temperature, hot air injection flow rate and soil extraction system design to best address site-specific characteristics of the contaminated area.

Applications

  • In situ treatment of soils in the vadose zone.
  • Suitable for volatile (chlorinated solvents and light petroleum hydrocarbons) and semi-volatile (certain pesticides and diesel) compounds in residual or free phase.
  • Efficient in homogenous, granular soils.

Applications to sites in northern regions

Hot air injection is not always appropriate in remote areas that do not have easy access to utilities or local labour to operate and maintain the system. Extreme cold can affect the volatilization of shallow compounds, but the temperature of deeper soils is relatively constant throughout the year. Nordic systems generally require climate-adapted techniques, including deep soil freeze-up, seasonal changes in soil conditions and long periods without system operator intervention, refuelling and removal or sorbents.

Treatment type

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

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
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
With restrictions
Phenolic compounds
Does not apply
Policyclic aromatic hydrocarbons
With restrictions
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

Long-term considerations (following remediation work)

None

Secondary by-products and/or metabolites

The injection of air is not a destructive technique and does not generate any secondary product, because the contaminants are transferred from the aqueous phase to the gas phase.

Limitations and Undesirable Effects of the Technology

  • Applies only for treatment of contaminated soil within the vadose zone.
  • Selection of the optimum operating temperature must take into consideration the presence of free products, to avoid explosions.
  • The presence of buildings or infrastructure near the contaminated site may require monitoring of the vapour intrusions into buildings.
  • Air injection can cause vapour infiltration in some preferential pathways such as foundation drains, utility trenches, etc.
  • Not suitable for inorganic contamination.
  • Not suitable for soils with a high moisture content.
  • High soil heterogeneity.
  • Hot air injection for soils may be expensive.

Complementary technologies that improve treatment effectiveness

Treatment efficiency can be increased by adding soil fracturing to increase airflow (hydraulic or pneumatic fracturing), or by sealing the soil surface to avoid “short-circuiting”.

The hot air injection technique is effective only when it is possible to ensure the flow of air into the soil. In order to extend the efficiency of the system below the water table, it is possible to reinforce the technique by means of dewatering or air sparging methods (injection of air under the water table to oxygenate groundwater or extract contaminants from the groundwater saturated zone to transfer them to the unsaturated zone in a stream of bubbles). Hot air injection techniques may be combined with in situ biological technologies, such as biostimulation, when the operating temperature is less than 40 °C.

Required secondary treatments

Hot air injection must be combined with a vapour extraction and treatment system.

Application examples

The following sites provide application examples:

Performance

There are several sources of data addressing the performance of hot air injection systems that are available online. Some data is available concerning the remediation of trichloroethene or jet fuel contamination using hot air injection technology in a document published by U.S.A. EPA (1997). According to this document, hot air injection technology combined with a vapour extraction system reduced jet fuel contamination from an initial concentration of 23,000 ppm to between non-detectable and 215 ppm in 90 days. This result comes from a site located in Ottawa, Canada, and the volume of soil treated was approximately 200 m3.

Measures to improve sustainability or promote ecological remediation

  • Optimization of equipment size.
  • Optimization of the calendar to promote the sharing of resources and reduce the number of days of mobilization.
  • Use of renewable energy and energy-efficient equipment.
  • Use of biofilters for air treatment reduces energy demand and waste generation.
  • Allow a longer treatment time to avoid winter operation, eliminating the need to winterize the system while reducing the amount of energy required.
  • On-site treatment of condensate and purged water from air lines.
  • Limit the number of field visits using telemetry for remote monitoring of site conditions.

Potential impacts of the application of the technology on human health

Main Exposure Mechanisms Applies or Does Not Apply Monitoring and Mitigation

Dust

Applies

Emissions monitoring at the source (choice of parameters, types of samples and type of intervention (source, risk or local requirements).

Atmospheric/Steam Emissions - Point Sources or Chimneys Applies Emissions monitoring at the source (choice of parameters, types of samples and type of intervention (source, risk or local requirements).
Atmospheric/Steam Emissions - Non-point Sources Applies Modelling the effects of air injection, validation of the model and monitoring the migration of soil
Air/steam – by-products Does not apply N/A
Runoff Does not apply N/A
Groundwater - displacement Does not apply N/A
Groundwater - chemical/ geochemical mobilization Does not apply N/A
Groundwater - by-product Does not apply N/A
Accident/Failure - damage to public services Applies File verification and licensing of pre-drilling or excavation work, development of special excavation or drilling procedures, and emergency response.
Accident/Failure - leak or spill Applies Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions.
Accident/Failure - fire or explosion Applies Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions.
Other - Handling contaminated soils or other Solids Applies Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions

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 : Nathalie Arel, P.Eng., M.Sc., Christian Gosselin, P.Eng., M.Eng. and Sylvain Hains, P.Eng., M.Sc., Golder Associés Ltée

Updated Date : March 22, 2019

Version:
1.2.1