Fact sheet: High temperature thermal desorption—ex situ

From: Public Services and Procurement Canada

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Description

High temperature thermal desorption (HTTD) is a process where excavated contaminated materials are heated from 315 0C to 540 0C (600 to 1,000 0F) in a chamber to volatilize water, organic contaminants and certain metals. The higher temperatures used in this process facilitate removal of semi-volatile organic compounds (SVOCs) (<0.01 mm Hg vapor pressure). A gas or vacuum system transports the vaporized water and contaminants to an air emission treatment system, where particulates and contaminants are removed. In contrast to incinerating, the operating temperatures and residence times in a thermal desorption system are designed to volatilize selected contaminants, without oxidizing them. Thermal desorption treatment is not designed to destroy organics.

Treatment of a contaminated soil using a thermal desorption system requires excavation and transportation of the soil to the reactor, were it is heated to a pre-determined 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.. In thermal screw units, screw conveyors or hollow augers are used to transport the soil through an enclosed trough. Hot oil or steam circulates through the auger to indirectly heat the soil. These units may be transported to the contaminated site.

Sources :

Recommended analyses for detailed characterization

Chemical analysis

  • Contaminant concentrations:
    • identification and concentration of all contaminants (sorbed, dissolved, and free phase)

Physical analysis

  • Soil water content
  • Soil granulometry
  • Contaminant physical characteristics includes:
    • viscosity
    • density
    • solubility
    • vapour pressure
    • etc.

Notes:

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

Applications

  • The target contaminants for HTTD are SVOCs, PAHs, PCBs and pesticides
  • VOCs and fuels may also be treated, but this may be less cost-effective
  • Volatile metals may be removed by HTTD systems
  • Compared to in situ soil vapor extraction treatments, this ex situ treatment system separates a wider range of organic contaminants
  • This process is applicable for the removal of organics from refinery waste, coal tar waste, wood-treatment waste, creosote contaminated soils, hydrocarbon-contaminated soils, synthetic rubber processing waste, pesticides and paint wastes
  • This ex situ process is mobile and can be transported to the site

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
Does not exist
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Applies

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
With restrictions
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Does not apply
Pesticides
Applies
Petroleum hydrocarbons
Applies
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
Applies
Polychlorinated biphenyls
Applies

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

Secondary by-products and/or metabolites

Control and treatment of air emissions from thermal desorption operations is an extremely important consideration. There should be no emission of metals, polycyclic aromatic hydrocarbons (PAHs) or dioxins/furans from the system.

Limitations of the technology

  • Highly abrasive material can damage the processor unit
  • Clayey, silty and high humic content soils require increased residence times as a result of contaminant binding
  • Dust and organic matter in the soil increases the difficulty of treating off-gases
  • The presence of chlorine can affect the volatilization of some metals such as lead
  • Soil storage piles need to be covered to protect from rain (to minimize water infiltration) and from wind
  • Leaching of mercury from stockpiled soil into groundwater is of concern
  • Thermal desorption of mercury-contaminated waste is not recommended
  • Treated soil may no longer support microbiological activity If the soil is returned to a previously or partially contaminated site, this may be of concern

Complementary technologies that improve treatment effectiveness

  • HTTD is frequently used in combination with incineration, solidification/stabilization or dechlorination, depending upon site-specific conditions;

Required secondary treatments

  • Control and treatment of air emissions
  • Dust control system
  • Treatments such as dewatering, sizing, crushing, blending with sand or removing debris may be necessary prior to thermal desorption
  • Heavy metals in the contaminated material may produce a treated solid residue that requires stabilization

Application examples

Application examples are available at these addresses:

Performance

This technology is capable of reducing final contaminant concentrations to below 5 mg/kg for target compounds (CPEO , 2002). HTTD costs in Canada are competitive with landfills or biological treatment (CSMWG, 2005).

References

Author and update

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

Latest update provided by:Martin Désilets, B.Sc. National Research Council

Updated Date: March-07-19

Version:
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