Thermal Desorption

Description

Thermal desorption separates contaminants from soil. Soil is heated in a chamber in which water, organic contaminants and certain metals are vaporized. A gas or vacuum system transports vaporized water and contaminants to an off-gas (i.e., air emission) treatment system. The design of a system aims to volatize contaminants, while attempting not to oxidize them. (Otherwise, thermal desorption would be another way of saying incinerator.) Two common thermal desorption designs are the rotary dryer and thermal screw. Rotary dryers are horizontal cylinders that can be indirect or direct-fired. The dryer is normally inclined and rotated. For the 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.

Based on the operating temperature of the desorber, thermal desorption processes can be categorized into two groups: high temperature thermal desorption (HTTD) and low temperature thermal desorption (LTTD). It is important to note that thermal desorption does not to destroy organics.

High Temperature Thermal Desorption (HTTD). In HTTD, wastes are heated to 320 to 560 °C (600 to 1,000 °F). HTTD is frequently used in combination with incineration, solidification/stabilization, or dechlorination, depending upon site-specific conditions.

Low Temperature Thermal Desorption (LTTD). In LTTD, wastes are heated to between 90 and 320 °C (200 to 600 °F). LTTD is most often used for remediating fuels in soil. Unless heated to the higher end of the LTTD temperature range, organic components in the soil are not damaged, which enables treated soil to retain the ability to support future biological activity.

Treatment of the off-gas must remove particulates and contaminants. Particulates are removed by conventional particulate removal equipment, such as fabric filters. Contaminants are removed through condensation followed by carbon adsorption, or they are destroyed in a secondary combustion chamber or a catalytic oxidizer.

Limitations and Concerns

Treatment and control of air emissions from thermal desorption operations is an extremely important consideration. There should be no emissions of metals, certain polycyclic aromatic hydrocarbons (PAHs) and dioxins/furans. Mercury emissions are very difficult to control, and using an afterburner is unacceptable.

Dust and organic matter in the soil increase the difficulty of treating off-gas.

Leaching mercury from stockpiled soil into the water is of concern, especially for communities that rely on fishing. Thermal desorption for mercury-contaminated waste is generally not appropriate.

Dewatering may be necessary to achieve acceptable soil moisture content levels.

Soil storage piles need to be covered to protect from rain (to minimize soil moisture and infiltration) and from wind.

Heavy metals in the feed may produce a treated solid residue that requires stabilization.

Clay and soils with high humic content need longer reaction time.

Treated soil may no longer be able to support microbiological activity that breaks down contaminants. If the soil is returned to a previously or partially contaminated site, this may be of concern.

Applicability

Thermal desorption systems remove volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), fuels, pesticides and some metals from soil. High temperature units are more effective removing volatile metals and SVOCs.

Technology Development Status

The technology is commercially available.

Web Links

http://enviro.nfesc.navy.mil/erb/restoration/technologies/remed/phys_chem/phc-36.asp

http://www.frtr.gov/matrix2/section4/4-26.html

Other Resources and Demonstrations

See http://www.environmental.usace.army.mil/library/success/itsucc/portmoll/portmoll.html for a description of thermal desorption at Port Moller, Alaska. All the equipment used during the project, including the thermal desorption unit, was transported to the site by barge. This approach eliminated soil shipment and the potential for off-site spills.

See the following documents produced by the Interstate Technology and Regulatory Council (ITRC): Technical Requirements for On-Site Thermal Desorption of Soil Media Contaminated with Hazardous Chlorinated Organics, September, 1997 (http://www.itrcweb.org/td-2.pdf); Technical Requirements for On-Site Low Temperature Thermal Treatment of Non-Hazardous Soils Contaminated with Petroleum/Coal Tar/Gas Plant Wastes, December, 1997 (http://www.itrcweb.org/td-1.pdf); and Technical Requirements for On-Site Low Temperature Thermal Desorption of Solid Media and Low Level Mixed Waste Contaminated with Mercury and/or Hazardous Chlorinated Organics, September, 1998 (http://www.itrcweb.org/td-3.pdf).

See http://enviro.nfesc.navy.mil/erb/erb_a/restoration/technologies/remed/phys_chem/tr-2090.pdf for several case studies of thermal desorption.