• Microwaves can significantly reduce treatment costs and duration because of their
direct interaction with the contaminated matrix (if dielectric loss factor is high
enough); in this case, usual limitations to heat transfer no longer apply.
• Microwaves have selective heating characteristics.
• Radio frequency energy is transformed into heat at high levels (about 90%
conversion).
• In some cases, heat transport by microwaves is coupled with conduction heating.
• RFH can combine contaminant desorption and volatilization (via a temperature
rise) and in some cases, contaminant degradation (under the effect of molecular
vibrations).
The drawbacks of RFH are mainly as follows (Holzwarth et al. 1998; Jones et al.
2002; Lin et al. 2010; Liu and Yu 2006; Robinson et al. 2008; Roudier 2004; Yuan
et al. 2006):
• Microwaves are difficult to use with metals or gas.
• Increasing temperatures with RFH is limited with nonpolar compounds and soils
that have less favorable dielectric properties. In those situations one can:
– Add magnetic nanoparticles (Fe) or MnO 2 to increase the microwave absorption characteristics of the contaminant.
– Add granular activated carbon (GCA) to the soil, increasing temperatures up to
800
C (for a w/w GCA to soil ratio between 1 and 5).
• Occasional thermal instability due to nonlinear relationship between electromagnetic and thermal properties of the materials, and temperature.
3.4.4 Electrical Resistance Heating
Electrical resistance heating (ERH) was first implemented within the oil industry
(Wattenbarger and McDougal 1988). It was then studied for in situ vitrification, and
its application as an in situ thermal desorption method started in the late 1990s
(Beyke and Fleming 2005; Powell et al. 2007).
ERH heats the soil by passing a flow of alternating electric current through the
soil matrix. Electric current is passed through the soil matrix within a network of
electrodes (hexagonal or triangular arrays). The resistance provided by the porous
media generates an increase in temperature. The heat dissipates causing volatilization of moisture and volatile contaminants. In situ vapor generation facilitates
transport and removal of volatile pollutants; conventional methods are subsequently
used to treat the extracted VOC vapors. This technique can be used in saturated and
unsaturated zones (Beyke and Fleming 2005; U.S. Army Corps of Engineers 2014).
Figure 3.16 illustrates the principle of ERH.
The energy necessary for ERH can be estimated by Ohm’s law (Eq. 3.37):
3 In Situ Thermal Treatments and Enhancements: Theory and Case Study
185
direct interaction with the contaminated matrix (if dielectric loss factor is high
enough); in this case, usual limitations to heat transfer no longer apply.
• Microwaves have selective heating characteristics.
• Radio frequency energy is transformed into heat at high levels (about 90%
conversion).
• In some cases, heat transport by microwaves is coupled with conduction heating.
• RFH can combine contaminant desorption and volatilization (via a temperature
rise) and in some cases, contaminant degradation (under the effect of molecular
vibrations).
The drawbacks of RFH are mainly as follows (Holzwarth et al. 1998; Jones et al.
2002; Lin et al. 2010; Liu and Yu 2006; Robinson et al. 2008; Roudier 2004; Yuan
et al. 2006):
• Microwaves are difficult to use with metals or gas.
• Increasing temperatures with RFH is limited with nonpolar compounds and soils
that have less favorable dielectric properties. In those situations one can:
– Add magnetic nanoparticles (Fe) or MnO 2 to increase the microwave absorption characteristics of the contaminant.
– Add granular activated carbon (GCA) to the soil, increasing temperatures up to
800
C (for a w/w GCA to soil ratio between 1 and 5).
• Occasional thermal instability due to nonlinear relationship between electromagnetic and thermal properties of the materials, and temperature.
3.4.4 Electrical Resistance Heating
Electrical resistance heating (ERH) was first implemented within the oil industry
(Wattenbarger and McDougal 1988). It was then studied for in situ vitrification, and
its application as an in situ thermal desorption method started in the late 1990s
(Beyke and Fleming 2005; Powell et al. 2007).
ERH heats the soil by passing a flow of alternating electric current through the
soil matrix. Electric current is passed through the soil matrix within a network of
electrodes (hexagonal or triangular arrays). The resistance provided by the porous
media generates an increase in temperature. The heat dissipates causing volatilization of moisture and volatile contaminants. In situ vapor generation facilitates
transport and removal of volatile pollutants; conventional methods are subsequently
used to treat the extracted VOC vapors. This technique can be used in saturated and
unsaturated zones (Beyke and Fleming 2005; U.S. Army Corps of Engineers 2014).
Figure 3.16 illustrates the principle of ERH.
The energy necessary for ERH can be estimated by Ohm’s law (Eq. 3.37):
3 In Situ Thermal Treatments and Enhancements: Theory and Case Study
185