Pacific Northwest Laboratory
Description
Most Department of Energy (DOE) sites have been contaminated with volatile organic compounds (VOCs). Techniques for retrieving these VOCs from soils are being developed and demonstrated at various sites. These techniques generally remove VOCs as vapors (off-gas) from contaminated soil. These vapors must, in most cases, be treated prior to release to the environment. The High Energy Corona (HEC) technology is one of many approaches toward decontaminating soil off-gases prior to atmospheric release. The objective of the HEC technology is to provide a stand-alone, field-portable means of treating soil off-gases produced during soil treatment operations. Contaminants that can be treated include most or all volatile and semivolatile organic compounds. The potential also exists for treating inorganic compounds, such as oxides of nitrogen and oxides of sulfur.
The HEC process uses high-voltage electricity to destroy VOCs at room temperature. As shown in the figure below, the equipment consists of the following: an HEC reactor in which the VOCs are destroyed; inlet and outlet piping containing process instrumentation to measure humidity, temperature, pressure, contaminant concentration, and mass flow rate; means for controlling inlet flowrates and inlet humidity; and a secondary scrubber.
The HEC reactor is a glass tube filled with glass beads through which the prefiltered contaminated off-gas is passed. Each reactor is 2-in. in diameter, 4 ft long, and weighs less than 20 lb. A high voltage electrode is placed along the centerline of the reactor, and a grounded metal screen is attached to the outer glass surface of the reactor. A high-voltage power supply is connected across the electrodes to provide 0 to 50 mA of 60-Hz electricity at 30 kV. The electrode current and power depend upon the type and concentration of contaminant.
The technology is packaged in a self-contained mobile office that includes gas handling equipment and on-line analytical capabilities. Site power is not absolutely required. Installation consists of connecting inlet and outlet hoses to the HEC process trailer. Training in the use of the equipment can usually be accomplished well within one hour. Failure control is provided by a combination of automated and manually activated means, addressing electrical failure, loss of flow, and loss of VOC containment caused by breakage of the glass reactor vessel. The HEC process can be operated with little, if any, maintenance required. Neither catastrophic failure nor any diminishing in levels of performance have been observed through months of periodic operation in the laboratory. The on-line gas chromatograph and process instruments do require periodic recalibration to ensure data quality.
Technical Performance
The HEC technology appears to destroy more than 99.9% trichloroethylene (TCE). The technology destroys tetrachloroethylene (PCE) to a level of 90 to 95%. In preliminary tests with heptane, destruction levels appear to be extremely high, but have not been quantified. When chlorinated VOCs are treated, water containing either sodium hydroxide or baking soda is recirculated in a scrubber to remove acid gases from the reactor effluent (hydrochloric acid and bleach.) It should also be noted that further contaminant destruction appears likely in this wet scrubber. This is presumably because of strong gaseous oxidants that exit the HEC reactor. Typical outlet properties would be nondetectable concentrations of TCE, ozone, hydrochloric acid, phosgene, and chlorine, with up to 1 ppmv NOx (below regulatory limits.) Air exits the HEC process at temperatures of 100° C or lower or slightly above ambient temperature if a wet scrubber is used. A scrub solution (containing less than 10-wt% table salt in water) is produced when chlorinated VOCs are treated.
One reactor processes up to 5 scfm of soil off gas. The HEC field-scale process that will be demonstrated early in 1993 at Savannah River uses 21 HEC reactors in parallel to treat up to 105 scfm of contaminated soil off-gas. A typical application will involve an inlet stream containing 1800 ppmv of TCE in humid air at 10 to 20° C. Power input is typically 50 to 150 W/scfm being processed. For dry inlet streams, deionized water is added as steam to produce an inlet humidity (hr) of 60 to 80%. Less than 20 mL/min of water is required to humidify a bone-dry stream at a flow of 105 scfm. For water-saturated inlet streams, the stream is preheated (using electric heaters) to lower the inlet humidity (hr) from 100% to 80%. In many cases, the vapor-extraction blower associated with retrieving the VOCs from soil will sufficiently preheat the soil off gas to 80% hr or lower so that no further preheating is required.
Cost. Initial outlay for a 105 scfm process, the prototype field treatment system, is $50K. As with any other technology, large-scale production and customization would significantly reduce costs, perhaps to as little as $20K. Labor requirements are projected as ~0.25 fulltime equivalent (FTE). Energy requirements are $27/day, or roughly $0.35/lb of contaminant. Total cost is roughly $10.00/lb contaminant, including a 25% contingency to account for any unknown additional costs. Although maintenance costs are minimal, the total cost figure assumes 8% downtime and a capital payback period of 6 months.
Projected Performance
Continued research & development (R&D) is planned to accomplish the following: fully characterize the reactor emissions to complete mass balances; adapt the HEC process to complete real-time control; better understand the physical and chemical phenomena that make the HEC process work; develop larger reactors; and optimize the hardware and packaging associated with the technology for specific, as well as modular or generic, treatment applications.
Waste Applicability
This technique is specifically useful for destroying organics and chlorinated solvents such as TCE, PCE, carbon tetrachloride, chloroform, diesel fuel, and gasoline. Both gas and liquid phase contaminants are treatable.
Status
Discussions with manufacturers/licensees have been initiated with the belief that HEC is now ready for commercial availability. The 105-scfm field prototype is available now for commercial testing and evaluation. Pacific Northwest Laboratory (PNL) is continuing R&D to improve and scale the technology. Scaleup to 50 scfm per reactor seems feasible for extremely large applications.
Regulatory Considerations
Compliance with the Occupational Safety and Health Administration regulations is required for hazardous waste operations and protection of occupational workers from high-voltage electricity.
Potential Commercial Applications
Since this technology is applicable to treating process off-gases and liquid contaminants in government or industrial settings, the potential commercial applications are very broad. Any remediation or manufacturing process that produces off-gases and/or liquid contaminants that contain organic compounds could possibly be treated with this technology.
Baseline Technology
The most ubiquitous baseline technology is carbon sorption, in which off-gas contaminants are absorbed onto containerized activated carbon. Once ``spent,'' the carbon is shipped off-site and incinerated, which partially reactivates the carbon for reuse. In many or most cases, the spent carbon must be treated as Mixed Waste because of radon contamination, even if the soil being cleaned has not been contaminated with radioactive wastes. This further increases the cost of baseline carbon sorption use. Some other baseline technologies involve thermal treatment, such as incineration and high-temperature catalysis. Ionizing radiation sources, such as X-rays and electron beams, are also used.
Intellectual Property Rights
Pacific Northwest Laboratory has applied for a patent.
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References
Go to the Remediation Technology Profiles Index