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).