From: Public Services and Procurement Canada
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.
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:
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.
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.
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.
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.
None
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.
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.
Hot air injection must be combined with a vapour extraction and treatment system.
The following sites provide application examples:
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.
Dust
Applies
Emissions monitoring at the source (choice of parameters, types of samples and type of intervention (source, risk or local requirements).
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
