From Tech Trends August 1994

PCBs Destroyed by Combining Thermal & Chemical Treatments

by Paul de Percin, Risk Reduction Engineering Laboratory

Anaerobic thermal processor (ATP) technology involves a physical separation process that thermally desorbs organics such as polychlorinated biphenyls (PCBs) from soil and sludge. The ATP process is being used in conjunction with dehalogenation to chemically destroy PCBs in the soil at the Wide Beach Development Site in Brant, New York. The technology can also be applied to many other types of organic contaminants. ATP was developed by Alberta Oil Sands Technology and Research Authority and is licensed by SoilTech, Inc.

At Wide Beach, the transportable ATP unit processes about 10 tons of contaminated soil per hour. The ATP system heats and mixes contaminated soils, sludges and liquids in a unit that uses indirect heat for processing. The processor contains four separate internal thermal zones: the preheat, retort, combustion and cooling zones. For this demonstration, the contaminated soils are sprayed with a diesel fuel and oil mixture containing alkaline polyethylene glycol (APEG) reagents before entering the preheat zone. The oil mixture acts as a carrier for the dehalogenation reagents.

In the preheat zone, water and volatile organic compounds (VOC) vaporize. At the same time, the reagents dehalogenate or chemically break down chlorinated compounds (including PCBs). The vaporized contaminants and water are removed via a vacuum to a preheat vapor cooling system consisting of a cyclone, condenser and a three-phase preheat separator. The noncondensed light organic vapors are then fed by a blower directly into the combustion chamber of the processor. The oil fraction is recycled to a reagent blending tank, and recovered water is sent to the onsite treatment system.

From the preheat zone, the hot, granular solids pass through a sand seal to the retort zone. Here, heavy oils vaporize and thermal cracking of hydrocarbons forms coke and low molecular weight gases. The vapor stream from the retort zone is removed via a vacuum and passes first through a two-stage pair of cyclones to remove entrained particles. These dusts and fines are blended with the treated soil. The vapor is then cooled by oil circulating in two packed columns, acting as a two-stage direct contact condenser for the higher boiling point compounds. The uncondensed vapors are then cooled in a water-cooled noncontact condenser and pass through a three-phase separator. The final noncondensable gases are returned to the combustion chamber. The oil phase is combined with the condensate from the packed columns. This oil condensate is then sent to the reagent blending unit to mix with the APEG reagents. The blend is pumped at a measured rate and is applied to the untreated soils in the feed chute of the processor. Condensed water is pumped directly to the onsite treatment system.

The coked soils pass through a second sand seal into the combustion zone. Here the coked soils are combusted and either recycled to the retort zone or sent to be cooled in the cooling zone. Flue gas from the combustion zone is treated in a system consisting of a cyclone and baghouse that removes particulates, a scrubber that removes acid gases and a carbon adsorption bed that removes trace organics. The treated flue gas is then discharged to the atmosphere through a stack. Treated soils exiting the cooling zone are quenched with scrubber water and are then transported by conveyor to an outside storage pile.

The ATP unit removed over 99% of the PCBs in the contaminated soil (original concentrations of 50 to 100 parts per million), resulting in PCB levels below the desired cleanup concentration of 2 ppm. The ATP does not appear to create dioxins or furans. Additionally, no volatile or semivolatile organic degradation products or leachates were detected in the treated soil.

For more information, call Paul de Percin at the Risk Reduction Engineering Laboratory 513-569-7797.