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 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.
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 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.