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 incineration.)
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 heat the soil indirectly.
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 ÁC (600 to 1,000 ÁF). 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 ÁC (200 to 600 ÁF). 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 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 should 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.
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.
Other Resources and Demonstrations
See https://portal.navfac.navy.mil/portal/page/portal/NAVFAC/NAVFAC_WW_PP/NAVFAC_NFESC_PP/ENVIRONMENTAL/ERB/INHTGLO and https://portal.navfac.navy.mil/portal/page/portal/NAVFAC/NAVFAC_WW_PP/NAVFAC_NFESC_PP/ENVIRONMENTAL/ERB/INHTGHI for descriptions of low and high temperature thermal techniques. See http://terratherm.com/blog/?p=795 for a description and cartoon of ex-situ thermal desorption that is intended to remove dioxin from contaminated soil.