from Tech Trends June 1995

Thermal Desorption at Gas Plants

By Ronald F. Lewis, EPA National Risk Management Research Laboratory

There are an estimated 3,000 former manufactured gas plants (MGP) in the United States. The EPA SITE Program (Superfund Innovative Technology Program) evaluated a Thermal Desorption System (TDS) designed to remove organic compounds and total petroleum hydrocarbons from soil, including contaminants typically found at MGPs from: (1) coke plant residuals; (2) purifier bed wastes; (3) sediments; and (4) water gas plant residuals.

The TDS, developed by Clean Berkshires, Inc. (now renamed Maxymillian Technologies), was evaluated at the Niagara Mohawk Power Corporation's Remediation Technologies Demonstration Facility at Harbor Point in Utica, New York. Harbor Point is a former MGP and has been contaminated with coal coking by-products.

Contaminant levels were: benzene, toluene, ethylbenzene and xylene (BTEX) at concentrations ranging from 13 milligrams per kilogram (mg/kg) in coke plant residuals to 320 mg/kg in water gas plant residuals; polyaromatic hydrocarbon concentrations ranging from 320 mg/kg in coke plant residuals to 4,420 mg/kg in water gas plant residuals; organo-metallic ferricyanide complexes' (cyanide) concentrations ranging from 730 mg/kg in coke plant residuals to 1,120 mg/kg in purifier bed wastes. Estimated average removal efficiencies were: 99.7% total BTEX and total PAHs and 97.5% total cyanides. Estimated average concentrations for critical pollutants in processed solids were (estimated) 0.066 mg/kg total BTEX, 12.4 mg/kg total PAHs, and 5.4 mg/kg total cyanide. Spikes of the volatile compound xylene at 21.8 to 27.2 mg/kg and the semi-volatile compound naphthalene at 232 to 318 mg/kg were added to assess air emissions of Clean Air Act principle organic hazardous contaminants (POHCs); destruction removal efficiencies were 99.99% or greater for both compounds.

The Clean Berkshires TDS is a direct-fired co-current thermal desorber based on standard rotary kiln technology. The kiln is made of an innovative metal alloy which allows the soil to be quickly heated for short passage time (eight minutes), thereby producing purified soil rather than ash. It is a process which is composed of three different operations: feed preparation, contaminant volatilization and gas treatment. Contaminant volatilization begins after the prepared feed material enters the kiln. The soil temperature is increased up to 800 degrees F through contact with an air stream heated by a natural gas burner located at the kiln's entrance. The kiln has special design features that lift and toss the soil within the kiln, exposing greater surface area to the hot gases, thereby improving volatilization. Treated soil exits the kiln and enters a pug mill which combines the material with solid residuals from the gas treatment sequence to form a consolidated processed solids stream. Water recycled from the quench tower is added at this time to cool the processed solids and control fugitive dust emissions. The solids are deposited onto a discharge conveyor and stockpiled.

Gas treatment begins when the kiln offgas, now filled with volatilized contaminants and entrained particulate, enters a multi-stage treatment sequence. Kiln offgases are first drawn through a cyclone to remove coarse particulate matter. The gases then enter a high-efficiency, natural gas-fired afterburner which combusts organic constituents at temperatures up to 1,800 degrees F. A quench tower cools the combustion gases by passing them through a highly atomized water mist. The cooled gas stream then enters a baghouse to remove fine-sized filterable particulate. If any acid levels are high enough to impact air quality standards, a scrubber could be added at this point in the treatment sequence. Treated gases exit the system through a 75-ft. high stack. Solid residuals from gas treatment are transferred by a screw auger to the pug mill and are combined with the treated soil from the kiln. The TDS layout is flexible and facilitates the rearrangement or addition of process equipment, as required. This permits Maxymillian Technologies to customize operations based on site-specific combinations of media and pollutants. It is trans- portable and is monitored and controlled by a computer-based data acquisition system. The TDS is capable of handling a variety of solid waste types including, soil, sediment and sludge.

Maxymillian Technologies treated approximately 1,500 tons of waste during the SITE demonstration and an additional 6,600 tons during other tests at Harbor Point outside the scope of this SITE project. All 8,100 tons of treated materials have met special site-specific New York State Department of Environmental Conservation treatment standards and are currently stockpiled onsite. The process does generate some residuals that require further treatment; as such, the technology should not be considered entirely stand-alone.

For more information, call Ron Lewis at EPA's National Risk Management Research Laboratory at 513-569-7856. A SITE Technology Capsule, (Document No. EPA/540/R-94/507a) describing the demonstration in more detail can be ordered by calling 513-569-7562.