Heating the Subsurface

The Basics

Heating the subsurface accomplishes three goals:

• Volatilize liquid contaminants – the vapors are captured in the vadose zone and treated at the surface.
• Lower the viscosity of long-chain hydrocarbons – liquid phase contaminants are removed by standard pumping systems.
• Increase chemical reaction rates – biotic and abiotic processes convert the contaminants into favorable end products.

There are four basic ways to heat the subsurface, two of which are commonly used.

Steam injection

Generating steam at the surface is well understood and reasonably inexpensive. The problem is that when you inject fluids, they tend to go into permeable strata. In complex systems, injected steam tends to bypass tight units or break through to the surface. As other heating technologies generate steam in situ in virtually all strata, practitioners use those far more frequently.

Radio frequency heating

You are probably familiar with this heating technology, as microwave ovens use energy in the radio frequency spectrum to heat up food. Because water is such a good absorber of radio waves, it is difficult to heat the subsurface any appreciable distance from the RF antenna. This technology is rarely used.

Thermal conduction heating

Shell Oil developed this technology and gave the patent rights to the University of Texas, which licenses the technology to a single company. Electricity is used to raise the temperature of heater wells, much like electricity heats wires in a toaster oven. The heat is transferred to the surrounding formation via thermal conduction. You must have a temperature gradient to conduct heat. The heater wells and adjacent soil can reach temperatures in excess of 500 degrees Celsius. The advantage of this technology is that you can heat the subsurface to very high temperatures. That said, the maximum temperature of water is its boiling point. Thus, to heat the subsurface beyond the boiling point of water, you have to desiccate the soil, which may cause subsidence and structural issues at the surface. Furthermore, because the desiccated region is so hot, the system will continue to generate steam even if you shut the system down for maintenance or you lose power. Fugitive emissions could be quite problematic.

Electrical resistance heating

Instead of heater wells, you place electrodes in the contaminated zone. An electrode is a well that directs the flow of electricity through the formation. A power control unit delivers alternating current to the electrodes and the formation completes the electrical circuit. The resistance to the flow of electricity raises the temperature of the formation. Because it is necessary to have water in the matrix to conduct electricity, it is virtually impossible to desiccate the soil. Similarly, the maximum temperature you can achieve is the boiling point of water. ERH is easy to control and works equally as well in the vadose zone as in the saturated zone. Further, it works just as well in clay as in sand.

Boiling occurs when the vapor pressure of the liquid equals the ambient pressure. The vapor pressure of water is one atmosphere when its temperature is 100°C. According to Dalton’s Law of Partial Pressures, vapor pressures are additive if the two liquids are not mutually soluble. That means all common VOCs in the presence of water will boil at a temperature less than the boiling point of water. This is not intuitive. While PCE in air boils at about 121°C, when PCE is in the presence of water, its eutectic boiling point is 88°C. This phenomenon has great utility. Literally, you can use electrical resistance heating to volatilize any liquid; however, it may not be cost effective for low volatility compounds. The reason is that the percentage of the contaminant in the generated gas bubble is a function of the contaminant volatility. For example, there is about twice as much TCE in a TCE/steam bubble as there is PCE in a PCE/steam bubble. Thus, it is more expensive to remediate PCE than TCE using heat.

The Company Dajak Represents

TRS Group provides electrical resistance heating services to remediate contaminated soils and groundwater.

Typically, ERH involves heating the subsurface to convert liquid phase contaminants to a gas phase, extracting the gases from the vadose zone and treating them with conventional technologies at the surface. TRS has also applied ERH at sites impacted by semi-volatile hydrocarbons, such as diesel fuel, tars and grease.

Some important points follow:

• TRS Group frequently guarantees results.
• TRS routinely removes more than 99% of contaminant mass within 4 to 9 months.
• Electrical resistance heating is unaffected by heterogeneities or low permeability strata. Although it is not intuitive, DNAPL in clay or sedimentary rock is generally easy to remediate.
• ERH works equally well in the vadose zone as in the saturated zone.
• ERH is quite flexible and simple to control.
• ERH does not desiccate the vadose zone or saturated zone.
• Heat enhances bioremediation and abiotic reaction rates.
• TRS has a perfect safety record with over 300,000 hours of field labor.
• Total project cost (electricity, contaminant recovery and treatment, permitting, well design and installation, etc.) for 99% mass removal of TCE generally ranges from $50 to $200 a cubic yard. Remediating less volatile compounds usually costs more; for PCE, about 30% more.

Links

www.thermalrs.com
http://en.wikipedia.org/wiki/Electrical_resistance_heating