Six-Phase Soil Heating

Task Description

This task involves the enhance-
ment of soil vapor extraction tech-
niques by heating a clay-rich soil to increase the vapor pressure. This process is accomplished by placing six electrodes in a circle surrounding a central vent (Figure 5.1). Six-phase alternating current (AC) is then applied to the elec-
trodes, each electrode receiving a single phase. Each electrode is equipped with a separate trans-
former wired to provide each elec-
trode with a separate AC phase. This ensures a more uniform distri-
bution of electrical currents in the soil, so that additives that would normally be needed to make the soil electrically conductive are not required.

Because the key to resistive heat-
ing is to maintain a small amount of moisture in the zone to conduct the electricity, splitting the current into six phases rather than the nor-
mal three phases prevents over-
heating and excessive drying around the electrodes. Resistive heating dissipates the electrical energy in the contaminated zone and vapor is withdrawn from the central vent as in conventional soil vapor extraction.

The six-phase heating process is quite rapid, on the order of weeks or months for many sites, and is expected to requirefor many sites, and is expected to requireminimal costs for onsite labor. Energy re-
quirements are also low compared to incineration and other thermal treatment methods, as the need to heat soil past 100°C is avoided.