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New Plasma Program to Zap Wastes

A program begun by the Plasma Fusion Center recently is focusing on new ways to dispose of hazardous wastes ranging from hospital refuse to contaminants in soils. Central to the program are two techniques that could convert such wastes into harmless or less noxious compounds without many of the byproducts associated with incineration.

The two techniques could be available for practical application within the next three years, said Dr. Daniel Cohn, a senior research scientist and division head at the PFC who directs the new program.

Both techniques use plasmas, or high-temperature states of matter, to actually break the chemical bonds of a variety of solid and gaseous wastes at atmospheric pressure. According to Dr. Cohn, the techniques could offer a number of advantages over traditional methods of waste disposal. For example, they may emit 10 to 20 times less gaseous wastes than incinerators, and plasmas can be produced in relatively small units. (The PFC scientists envision plasma "disposals" at individual hospitals.)

One near-term application for the technology is the cleanup of hazardous wastes at Department of Energy and Department of Defense sites. The PFC hopes to run field tests on toxic gases at the Hanford DOE site in Washington state. The DOE has not yet committed funds for the work, "but we fully anticipate it will be supported," Dr. Cohn said.

Other potential applications for the plasma techniques include the treatment of gaseous waste streams emitted from factories and the destruction of chemical weapons.

Dr. Cohn stresses that in addition to the two plasma techniques, scientists in the PFC program are also developing monitoring and diagnostic techniques to characterize the gas byproducts of the plasma processes.

"Diagnostics are a natural for us because the PFC has already done a great deal of diagnostic work for high-temperature plasmas," he said. Although the plasmas used to treat hazardous wastes are low-temperature, or don't get any hotter than about 10,000 degrees C, the diagnostic work for their high-temperature cousins, which reach temperatures of one hundred million degrees C, is directly applicable.

The two plasma techniques in the program are based on different technologies and have different applications. In one, a plasma arc discharged between two electrodes thermally melts the hazardous material to be treated.

This technique is best suited for solid wastes. For example, it could be used to treat soils contaminated with hazardous chemicals by vaporizing and then breaking down those chemicals. Further, with the addition of glassifying material, it could turn incinerator ash or soils contaminated with metals into a stable material that can't leach into water supplies. Such a material, Dr. Cohn said, is more suitable for landfills. It might also be used as a building material, or in road beds.

Currently PFC scientists are working to develop the arc technique for practical use with Electro-Pyrolysis, Inc., which has developed one of the largest plasma arc furnaces for waste treatment to date. The researchers have an arc furnace on campus, though they are not running any experiments with hazardous wastes.

The second plasma technique is best suited for gaseous wastes. Here a beam of electrons is injected into a sample of gases containing the hazardous gas of interest. The electrons produce radicals, or very reactive molecules, which glom onto the hazardous gas molecules and change their composition.

"The reaction breaks down more complex molecules into simpler ones that are either easier to remove or nontoxic," said Leslie Bromberg, the principal research scientist who originated this approach at the PFC.

The electron-beam approach has an important advantage over other ways to dispose of hazardous gases: it is selective. The electrons will only attack the hazardous gases, or, more specifically, complex organic molecules. "You're creating electrons everywhere, but they're only doing their job on the organics," Dr. Cohn said.

As a result, he explained, "you don't have to treat all of the molecules in the gas stream in order to destroy the hazardous ones, which often make up only a very small fraction of the whole."

Dr. Cohn also notes that the electron-beam technique is tunable, or can be adjusted for different hazardous gases at different toxic concentrations. As a result, he said, "In principle it has much more flexibility than an incinerator, although we still have to prove it."

At the DOE Hanford site the scientists hope to use the technique to clean up large amounts of hazardous solvents, including carbon tetrachloride, that were dumped on the ground over the years. "There's concern that they'll work their way into water supplies and migrate to the Columbia river," Dr. Cohn explained. The PFC group hopes to treat the wastes with the electron-beam technique after pumping them out of the ground as gases.

In another application the PFC group is working on a small program with Ballena Systems and Children's Hospital to treat the toxic smoke emitted from laser surgery with an electron-beam system. That work is funded by the National Institutes of Health.

Such collaborations between the university, government and industry in the new program are very important, said Professor Ronald Parker, director of the PFC. "We're all working together and playing different roles in developing the technology," he said.

Of course, to be competitive the bottom line for both the arc and electron-beam techniques is cost. Dr. Cohn expects that the arc system "will have an overall cost similar to that of incineration," while the electron-beam system "could provide significant cost advantages [over existing systems] because of the ability to efficiently process dilute waste streams."

In addition, he concluded, "both techniques could be significantly more attractive from the environmental viewpoint."

A version of this article appeared in the March 18, 1992 issue of MIT Tech Talk (Volume 36, Number 24).

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