Environmental Engineering Reference
In-Depth Information
11.4.7 Thermal Extraction
In situ thermal treatment or thermal enhancement of SVE is used to enhance volatilization
or decompose contaminants and can be performed using a variety of techniques includ-
ing electrical resistance, conductive heating, electromagnetic/iber optic/radio frequency
heating, or hot-air, water, or steam injection to increase the volatilization rate of semivola-
tiles, DNAPLs, LNAPL, and facilitate extraction. These methods can be completed in the
short to medium term such as less than 40 days. Recent examples are three- and six-phase
soil heating (USEPA, 2013b). Large-scale in situ projects employ three-phase soil heating,
whereas six-phase soil heating is for the demonstration phase. Electrical resistance heating
uses electrodes to produce a current that heats the soil above 100°C for steam generation
and causes soil drying and fracturing. Vacuum extraction then removes the contaminants.
The radiofrequency technique can heat soils to over 300°C and enhance SVE by
(1) increasing the contaminant vapor pressure and diffusivity, (2) drying the soil, which
increases permeability, (3) increasing the volatility of the contaminant by stripping with
the water vapor, and (4) increasing the mobility by decreasing the viscosity. The current
stops once the soil is dried. Incineration or granular activated carbon is used to treat the
extracted vapor.
Steam injection or steam-enhanced extraction (SEE) heats the soil and groundwater and
can destroy contaminants. The steam drives the contaminants that can be removed by
groundwater and vapor extraction. Thermal conduction heating destroys contaminants
by electrical conductance or evaporates them for subsequent removal by a carrier gas or a
vacuum system. Costs are in the range of $40-80/m
3
(FRTR, 2007).
Another ex situ technique is a process that involves thermal desorption by heating the
soil to 90°C to 320°C for VOCs or 320°C to 540°C for SVOCs removal. The low temperature
process works well for oil contaminated soils. A major advantage is that the decontami-
nated soil retains its physical properties and organic matter in the soil is not damaged.
Therefore there is the potential for reinstating the past ability of the soil to support future
biological activity. The vapors subsequently obtained must be treated by thermal oxida-
tion for complete destruction (USEPA, 2012b).
With regard to heavy metals such as mercury, arsenic, and cadmium and their com-
pounds, these can be evaporated at 800°C with the appropriate air pollution control sys-
tem. Some of the metals remain in the solid residues and will have to be properly disposed.
Thermal extraction is applicable mainly for mercury since this metal is highly volatile.
Costs are in the order of $35 to 1000/tonne (Environment Canada, 1995).
There are several commercially available thermal chemical treatment processes for
soil, sediment and other hazardous wastes. The temperatures used differ according to
the process. Cement Lock (http://www.cement-lock.com), developed by the IGT, has been
used for dredged sediment in the New York/New Jersey harbor (Stern et al., 1997). The
sediment contained metal contamination (33 mg/kg As, 37 mg/kg Cd, 377 mg/kg Cr,
617 mg/kg Pb, 1.3 mg/kg Hg, 3.2 mg/kg Se, and 1.8 mg/kg Ag) and was fed with lime
into the rotary kiln reactor smelter at 1200°C-1600°C. The mixture was then melted and
quenched, forming micrometer-sized ibers. The mixture was then mixed with cement to
produce a suitable type I Portland cement construction material. The sediment passed the
Toxicity Characteristic Leaching Procedure (TCLP) for all metals. Volatilized heavy metals
and acid gases and other combustion products were treated in the offgas by iltration to
remove particulates, and activate carbon to remove heavy metals gas removal. A demon-
stration plant was completed in Bayonne, New Jersey, in July 2003 (Mensinger and Roberts,
2009). Two decontamination tests were performed between 2003 and 2007. Destruction and
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