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 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 ¡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 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.
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://www.frtr.gov/matrix2/section4/4-26.html
http://www.clu-in.org/download/citizens/citthermal.pdf
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
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/Documents/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/Documents/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/Documents/td-3.pdf).