Thermal desorption is an environmental remediation technology that utilizes heat to increase the volatility of contaminants such that they can be removed (separated) from the solid matrix (typically soil, sludge or filter cake sediment). The volatilized contaminants are then either collected or thermally destroyed. A thermal desorption system therefore has two major components; the desorber itself and the offgas treatment system. Thermal desorption is not incineration.
History Thermal desorption first appeared as an environmental treatment technology in 1985 when it was specified in the Record of Decision for the McKin Company Superfund site within the Royal River watershed in Maine. It is frequently referred to as "low temp" thermal desorption to differentiate it from high temperature incineration. An early direct fired thermal desorption project was the treatment of 8000 tons of toxaphene (a chlorinated pesticide) contaminated sandy soil at the S&S Flying Services site in Marianna Florida in 1990, with later projects exceeding 170,000 tons at the Cape Fear coal tar site in 1999. A status report from the United States Environmental Protection Agency shows that thermal desorption has been used at 69 Superfund sites through FY2000. In addition, hundreds of remediation projects have been completed using thermal desorption at non-Superfund sites. For in-situ on-site treatment options, only incineration and stabilization have been used at more Superfund sites. Incineration suffers from poor public acceptance. Stabilization does not provide a permanent remedy, since the contaminants are still on site. Thermal desorption is a widely accepted technology that provides a permanent solution at an economically competitive cost. The world’s first large-scale thermal desorption for treatment of mercury-containing wastes was erected in Wölsau, for the remediation of the Chemical Factory Marktredwitz (founded in 1788) was considered to be the oldest in Germany. Operation commenced in October 1993 including the first optimising phase. 50,000 tons of mercury-contaminated solid wastes were treated successfully between August 1993 and June 1996. 25 metric tons of mercury had been recovered from soil and rubble. Unfortunately the Marktredwitz plant is often misunderstood in the literature as a pilot-scale plant only.
Desorbers Numerous desorber types are available today. Some of the more common types are listed below.
Indirect fired rotary Direct fired rotary Heated screw (hot oil, molten salt, electric) Infrared Microwave Most indirect fired rotary systems use an inclined rotating metallic cylinder to heat the feed material. The heat transfer mechanism is usually conduction through the cylinder wall. In this type of system neither the flame nor the products of combustion can contact the feed solids or the offgas. Think of it as a rotating pipe inside a furnace with both ends sticking outside of the furnace. The cylinder for full-scale transportable systems is typically five to eight feet in diameter with heated lengths ranging from twenty to fifty feet. With a carbon steel shell, the maximum solids temperature is around 1,000 °F, while temperatures of 1,800 °F with special alloy cylinders are attainable. Total residence time in this type of desorber normally ranges from 30 to 120 minutes. Treatment capacities can range from 2 to 30 tons per hour for transportable units. Direct-fired rotary desorbers have been used extensively over the years for petroleum contaminated soils and soils contaminated with Resource Conservation and Recovery Act hazardous wastes as defined by the United States Environmental Protection Agency. A 1992 paper on treating petroleum contaminated soils estimated that between 20 and 30 contractors have 40 to 60 rotary dryer systems available. Today, it is probably closer to 6 to 10 contractors with 15 to 20 portable systems commercially available. The majority of these systems utilize a secondary combustion chamber (afterburner) or catalytic oxidizer to thermally destroy the volatilized organics. A few of these systems also have a quench and scrubber after the oxidizer which allows them to treat soils containing chlorinated organics such as solvents and pesticides. The desorbing cylinder for full-scale transportable systems is typically four to ten feet in diameter with heated lengths ranging from twenty to fifty feet. The maximum practical solids temperature for these systems is around 750 to 900 °F depending on the material of construction of the cylinder. Total residence time in this type of desorber normally ranges from 3 to 15 minutes. Treatment capacities can range from 6 to over 100 tons per hour for transportable units. Heated screw systems are also an indirect heated system. Typically they use a jacketed trough with a double auger that intermeshes. The augers themselves frequently contain passages for the heating medium to increase the heat transfer surface area. Some systems use electric resistance heaters instead of a heat transfer media and may employ a single auger in each housing. The augers can range from 12 to 36 inches in diameter for full-scale systems, with lengths up to 20 feet. The auger/trough assemblies can be connected in parallel and/or series to increase throughput. Full scale capabilities up to 4 tons per hour have been demonstrated. This type of system has been most successful treating refinery wastes. In the early days, there was a continuous infrared system that is no longer in common use. In theory, microwaves would be an excellent technical choice since uniform and accurately controlled heating can be achieved with no heat transfer surface fouling problems. One can only guess that capital and/or energy costs have prevented the development of a microwave thermal desorber at the commercial scale.
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