Fact sheet: Low temperature thermal desorption—ex situ

From: Public Services and Procurement Canada

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Description

Low temperature thermal desorption (LTTD) is a process in which excavated contaminated material is heated in order to volatilize contaminants. The target contaminants for LTTD are non-halogenated volatile organic compounds (VOCs) and fuels. This technology can also be used, at reduced effectiveness, to treat semi-volatile organic compounds (SVOCs).

LTTD is distinct from both incinerator systems and high temperature thermal desorption. LTTD involves heating soils to temperatures ranging from 93 °C to 315 °C (200 °F to 600 °F) compared to 315 °C to 538 °C (600 °F to 1,000 °F) for high temperature thermal desorption. In contrast to incinerator systems, the operating temperatures and residence times in thermal desorption systems are designed to volatilize target contaminants without oxidizing them. Because of lower temperatures used during LTTD treatment, the decontaminated soil may retain its physical properties and organic matter needed to support future biological activity.

LTTD typically involves heating excavated, pre-screened soil in a reactor. LTTD units are typically transportable directly to the contaminated site; common designs include the rotary dryer and the thermal screw. Throughput rates can vary from less than 10 to approximately 50 metric tons per hour depending on the type of soil and treatment unit. During treatment, gas emissions are extracted from the reactor and treated separately to remove particulates and volatilized contaminants. The treated soil can be used on-site as backfill if it meets regulatory requirements or disposed off-site.

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Implementation of the technology

LTTD systems may include:

  • physical and chemical characterization of the soil or sediment;
  • site preparations (clearing/grubbing/demolition, topsoil stripping and temporary stockpiling);
  • mobilization of equipment (including installation of thermal desorption system and construction of temporary facilities and site access considerations);
  • excavation:
    • excavation dewatering
    • slope stability controls
    • foundation protection for retained structures (shoring, underpinning, etc.)
  • soil screening to separate and remove (or crush) oversize materials (> 5 cm), if present, before placement in heating units referred to as the primary treatment chamber;
  • soil drying (required only if the moisture content of the excavated soil exceeds 20% to 25%);
  • soil heating in the primary treatment chamber to a predetermined temperature. Two common thermal desorption reactor designs are the rotary dryer and the thermal screw. rotary dryers are horizontal or inclined cylinders that can be indirectly or directly fired. The thermal screw unit consists of screw conveyors or hollow augers that are used to transport the soil through an enclosed chamber. hot oil or steam circulates through the auger to indirectly heat the soil;
  • gas collection and treatment through thermal oxidation, condensation, or adsorption (including removal of dust particles);
  • compliance sampling of treated soil stockpiles and on-site disposal of soil (backfilling excavations or spreading on-site) or off-site disposal;
  • decommissioning and removal of thermal desorption system and gas collection and treatment systems;
  • surface restoration (planting, paving, etc.).

Captured vapours including water vapour and VOCs vapours (referred to as off-gas) require treatment for the removal of particulates and contaminants. Particulate removal equipment may be wet scrubbers or fabric filters, while condensation or adsorption (e.g. through GAC) equipment may be used for contaminant removal. Alternatively, the contaminants in the off-gas may be destroyed in a thermal oxidation system, which can be operated flameless, with a direct flame or with oxidizers catalytic environment. A carrier gas or vacuum is used to transport water and VOC vapours to the gas treatment system.

Materials and Storage

The method relies on traditional/commonly available civil/earthworks construction equipment and methods for the excavation component. Commercial and transportable units are available for the treatment component. Depending on the soil throughput rates, units may be mounted on one to five trailers.

  • Sufficient storage space is required to house the thermal treatment system.
  • On-site stores typically limited to small amounts of fuel and lubricant (daily fuelling of excavators is often from a mobile tank) as well as miscellaneous construction site supplies.
  • Contractors may create temporary stockpiles of contaminated materials pending treatment.
  • To protect from rain and minimize soil moisture content, the soil stockpiles and feed equipment need to be covered.

Waste and Discharges

  • Residuals from thermal desorption include treated off-gas, particulates, filters, catalysts, non-contact combustion gases. Generation of off-gases containing dioxins and furans, and/or halogenated acids can occur during thermal desorption treatment of soil with halogenated. While there are processes that have been designed to minimize generation of dioxin and furans and/or remove these compounds from off-gas through incineration, treatment of halogenated compounds should be approached with caution.
  • Windblown dust can come, for example, from floor loading, vehicle movement or stacking and can also be deposited directly on leeward surfaces; Solid discharges from thermal desorption include particulates.
  • Spent activated carbon could require periodic off-site transport and regeneration or disposal. If a wet gas scrubber is used, the resulting sludge from particulates in the wastewater stream requires proper disposal.
  • Diverted and collected/treated stormwaters are typically passed into the local stormwater system.
  • Thermal desorption produces off-gas that requires monitoring and potentially treatment prior to release to the atmosphere. 
  • In specific cases, naturally occurring radon may also be mobilized.

Recommended analyses for detailed characterization

Physical analysis

  • Soil water content
  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure

Recommended trials for detailed characterization

None.

Other information recommended for detailed characterization

None.


Notes:

Treatability tests are recommended to determine the efficiency of thermal desorption for removing various contaminants at various temperatures and residence times.

Applications

  • Target contaminants for LTTD are VOCs.
  • LTTD is less effective for SVOCs.
  • This ex situ process is transportable to the site.

Applications to sites in northern regions

Remote sites are prone to high mobilization and on-site monitoring costs, limited equipment availability and short work windows. Since it requires large and complex equipment and large energy consumption, thermal desorption is not well adapted for northern and remote environments.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Does not apply
Ex situ
Applies
Biological
Does not exist
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
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
With restrictions
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
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
Does not apply
3 to 5 years
Does not apply
More than 5 years
Does not apply

Notes:

Depending on the volume of the soil requiring treatment, the treatment plant may be in operation from weeks to months.

Long-term considerations (following remediation work)

None related to the thermal treatment system. If the treated soil is used as backfill, minor long-term considerations are related to geotechnical factors (structural changes) and decomposition of soil constituents (e.g. organic compounds content).

Secondary by-products and/or metabolites

Treatment and control of air emissions from thermal desorption systems are extremely important considerations. There should be no emission of metals, polycyclic aromatic hydrocarbons (PAHs) or dioxins/furans.

Limitations and Undesirable Effects of the Technology

  • Highly abrasive debris in soils can damage the processor unit.
  • Clayey, silty and high humic content soils require increased residence times as a result of binding of contaminants.
  • Soils with high water or organic carbon content can reduce the efficiency of ex situ thermal remediation systems through decreased desorption.
  • Dust and organic matter in the soil increase the difficulty of gas emission extraction.
  • Water content exceeding 20% in the contaminated material should be reduced prior to LTTD treatment in order to lower the cost of treatment.
  • If the treated soil is used as backfill, its geochemical properties could have changed, which in turn, could impact the in situ geochemical conditions.
  • The physical disruption of excavation is significant. cave-ins, slumping and related damage to nearby structures are possible if geotechnical & civil engineering works are inadequate. Dramatic, although short-lived, changes to site-scale hydrology and hydrogeology are common. Large excavations below the water table typically require either groundwater cut-off walls or extensive pumping, both of which alter flow paths at the site scale.
  • There is limited potential of vapour exposure to off-site receptors if the off-gas treatment system is properly designed and operating. However, care must be taken to match technology with type of contamination to prevent unintended emissions of toxic compounds. For example, depending on the thermal desorption process and temperature, dioxins and furans, and/or halogenated acids may be generated when treating halogenated contaminants.
  • Handling, for example, fuel vapours at levels near the lower explosive limit (LEL) and/or using supplemental fuel or reactive oxidation catalysts may pose a fire/explosion risk. Designers typically specify special “intrinsically safe” equipment in areas where flammable vapours are handled and incorporate ventilation, alarm, control interlock and fire suppression measures.
  • The limitations to soil excavation in general apply (slope stability considerations and protection, groundwater dewatering, protection of infrastructure, safety measures, etc.).

Complementary technologies that improve treatment effectiveness

LTTD is frequently used in combination with incineration, solidification/stabilization, or dehalogenation, depending upon the type of contaminants present.

Required secondary treatments

  • Treatment and control of air emissions
  • Dust control system
  • Treatment such as dewatering, sizing, crushing, blending with sand, or removing debris may be necessary prior to thermal desorption

Application examples

Application examples are available at these addresses:

Performance

Contaminant destruction efficiency in the afterburners of LTTD units is greater than 95% (FRTR, 2002). Many vendors offer LTTD units that can be transported directly to the contaminated site.

Measures to improve sustainability or promote ecological remediation

  • Use of renewable energy and energy-efficient machinery (e.g., geothermal, wind or solar energy).
  • Process optimization to reduce wastes and consumables.
  • Schedule optimization for resource sharing and fewer days of mobilization.
  • Assessment of pre-treatment options for feedstock to increase efficiency of thermal treatment systems (such as optimum soil moisture content). Refer to respective worksheets for excavation or dredging activities.
  • Adjust the treatment temperature to the contaminants in concern (lower effective)

Potential impacts of the application of the technology on human health

Not available for this fact sheet.

References

Author and update

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

Updated by : Martin Désilets, B.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