Six Phase
Soil Heating
Description
Six Phase
Soil Heating (SPSH) enhances the recovery of soils contaminated with volatile
and
semi-volatile organic
compounds (VOCs
and SVOCs).
SPSH
assists soil vapor extraction
(SVE) by heating the contaminants
in the soil. With SVE, a vacuum created in the soil pulls contaminated
vapor
into a well and then to the surface where the vapors
are treated
to remove the contaminants. However, its effectiveness is limited if
the
contaminants cannot be easily vaporized or if the soil is too tight for
air to
pass through readily. SPSH raises the vapor pressure of VOCs and SVOCs,
increasing volatilization
and removal via SVE. As SPSH dries the soil, it also creates a source
of steam
that strips contaminants from soils.
SPSH splits conventional
three-phase electricity into six separate
electrical phases, producing an improved subsurface heat distribution.
Each
phase is delivered to a single electrode, which is placed in a
hexagonal
pattern at up to 100 feet deep. Each electrode conducts electricity to
all the
others and to a central neutral electrode. At one demonstration,
temperatures within
the array were elevated to 100 degrees Celsius after 8 days of heating.
The
vapor extraction well is located in the center of the hexagon. An
applied
electrical field heats soils internally, and the soil itself acts as a
heat
source. Thus SPSH is more
effective than
other soil-heating techniques such as using heating elements, which has
a much
smaller radius of influence. Because conventional SVE relies on
the ability
to draw vapors out of the soil, SPSH is especially suited to sites
where
contaminants are tightly bound to clays and are thus difficult to
remove.
Limitations
and Concerns
Engineered barriers to prevent
worker exposure to high voltages are
recommended.
An off-gas
treatment system for contaminated vapors removed from the subsurface
needs to
be installed. The system should be sized to handle peak extraction
rates and
the mix of VOCs in extracted vapor and liquid streams.
The extraction well should be
screened both above and below the target zone
to ensure sufficient vacuum pressure in the subsurface. This extraction
well
design should also ensure total capture of contaminants released
as a
result of the heating.
The presence of buried metal
objects presents a safety hazard. The
subsurface should be mapped before the heating system is installed.
Questions remain about how
quickly soil should be heated. When heating is
sufficient to dry the soil, electrical conduction stops because dry
soil is
more resistive to electricity. Water can be added to maintain
conduction.
A potential explosive hazard
exists from concentrated fumes released from
the vacuum unit.
Applicability
SPSH has been demonstrated to
enhance removal of VOCs and SVOCs in
unsaturated clay-rich soils. Bench-scale tests have demonstrated that
SPSH is
effective on lower volatility compounds.
Technology
Development Status
SPSH is commercially available.
Web
Links
http://www.cesiweb.com/ws_20pdf.htm
Other
resources
See related description of Electrical
Resistance Heating. Also see Soil Vapor
Extraction, Thermally
Enhanced Vapor Extraction, and Dynamic
Underground
Stripping.
See http://www.frtr.gov/matrix2/section4/D01-4-9a.html
for a general diagram.
R.A. Gauglitz, J.S. Roberts,
T.M. Bergsman, R. Schalla, S.M.Caley, M.H.
Schlender, W.O. Heath, T.R. Jarosch, M.C. Miller, C.A. Eddy Dilek, R.W.
Moss,
B.B. Looney, Six-Phase Soil Heating for Enhanced Removal of
Contaminants:
Volatile Organic Compounds in Non-Arid Soils Integrated Demonstration,
Savannah
River Site, PNL-101 84,
Battelle
Pacific Northwest Laboratory, 1994.
See http://costperformance.org/remediation/pdf/Navy-ERH_Review.pdf for a description of demonstrations using electrical resistance heating.