Plasmas offer variety in treating wastes


MIT scientists report success in tests of a new technique that could turn hazardous, gaseous wastes like carbon tetrachloride into carbon dioxide and salt.

The technique, which could provide significant environmental and cost advantages relative to incineration, was described in a Boston Globe article last week as an especially promising new waste-treatment technology. This assessment was made by Steven L. Stein of Battelle Pacific Northwest Laboratories, who oversees the US Department of Energy program that funds the MIT work.

The technique is part of a broad-based program at the Plasma Fusion Center begun last year to devise new ways to treat hazardous wastes (see MIT Tech Talk March 18, 1992). Specifically, scientists at the PFC are using plasma--the same electrically charged gas found in lightning bolts and fluorescent lightbulbs--to break the chemical bonds of a variety of solid and gaseous wastes.

The program has three main thrusts: two new plasma techniques to decompose hazardous wastes (including the one described above), and diagnostics to measure and monitor byproducts from these techniques.

"A year ago these were kind of our dreams. Now we're getting results on all three fronts," said Daniel R. Cohn, a senior research scientist at the Plasma Fusion Center and director of the program.

PROCESSING DILUTE GASES

The technique that was written about by Scott Allen in the March 29 Globe is designed to destroy gaseous organic compounds that are quite toxic but present in dilute concentrations.

The plasma technique has one big advantage over other techniques to dispose of such wastes: it is selective. It destroys only hazardous gases, "so you don't have to treat all of the molecules in the gas stream," Dr. Cohn said.

This selectivity could provide substantial savings in energy and dollars, Dr. Stein noted for the Globe.

So far the PFC scientists have shown that the technique can selectively destroy dilute concentrations of carbon tetrachloride in a gas stream. The work was reported in a recent issue of Physics Letters A (the authors were Dr. Cohn, who is also acting assistant director of the PFC; Leslie Bromberg, the principal research scientist who originated the technique at the PFC; Mathias Koch, a graduate student in nuclear engineering; Richard M. Patrick, a research engineer at the PFC; and Paul Thomas, a PFC technical supervisor).

In general, the technique works when a beam of electrons is shot through aluminum foil into a sample of gases containing the hazardous gas of interest (for now the scientists are concentrating on chlorinated organics). In the process, which runs at room temperature and at atmospheric pressure, extremely reactive secondary electrons-the plasma-are created.

These in turn attack the molecules of carbon tetrachloride, breaking them up into mainly chlorine, carbon dioxide, and hydrochloric acid. These initial byproducts are then sent to a scrubber that converts them to the final byproducts of carbon dioxide and salt. (Preliminary results indicate that chloroform, too, can be decomposed into these products.)

The scientists believe the reactor can also be adjusted for other gases at different concentrations. These could include a variety of other volatile organic compounds in addition to carbon tetrachloride and chloroform. "It should have fairly wide applications," Dr. Patrick said.

Currently the PFC scientists are working on a full-scale electron-beam reactor that they will take to the DOE Hanford reservation in Washington state for field tests next year.

At the Hanford site large amounts of hazardous solvents, including carbon tetrachloride, were dumped on the ground over the years. These chemicals have slowly leached through the soil, until now "there's concern that they'll work their way into water supplies," Dr. Cohn said. The PFC group will test the reactor on these chemicals after pumping them out of the ground as gases.

In addition to being selective and adjustable, the full-scale electron-beam reactor will also be small. It will fit inside a trailer with dimensions of about 30 feet by eight feet-and that includes a control room for the operator. "All it will need is electric power to get the beam going," Dr. Patrick said.

The full-scale unit will also include a feedback system to detect any hazardous wastes that get past the plasma. That information will then be used to automatically adjust the controls of the process "to ensure desired performance," Dr. Cohn said.

The scientists stress that two critical parts of the system-the unit that produces the electron beam and the scrubber that converts initial byproducts into salt and carbon dioxide-are established technologies. Electron-beam units, for example, "have been in the field for over 30 years and are used to create high-quality paper and more," Dr. Patrick said.

"That's central to our approach," he continued. "We wanted to create new applications for plasma, but also wanted to build on existing technologies."

Work on the electron-beam technique is specifically funded by the DOE Volatile Organic Compounds Arid Site Integrated Demonstration Program.

MELTING SOILS AND ASH

The PFC scientists and collaborators have also had successful results with a second plasma technique. This technique-which Dr. Cohn describes as a "sledgehammer" while he calls the electron-beam approach a "scalpel"-melts solid hazardous wastes into glassy blobs while vaporizing and decomposing any volatile chemicals present.

Specifically, a plasma arc discharged between two graphite electrodes melts the material in question, which could include contaminated soils, incinerator ash, hospital wastes, or other materials with high melting temperatures that are difficult to process with other techniques. Temperatures in the plasma can run to about 10,000 degrees C.

Using a small furnace developed by Electro-Pyrolysis, Inc., one of the collaborators in the project, the scientists have melted 13 different kinds of solid waste (some of these were simulated to model very hazardous or radioactive wastes). And according to Dr. Cohn, they have found that the resulting glassy blobs are stable and won't leach into water supplies. "They're suitable for landfills or possibly as a material for construction," he said.

The scientists are currently working on a much larger furnace, also developed by Electro-Pyrolysis, that will be capable of processing up to 700 pounds of waste per hour (the current furnace handles about 40 pounds per hour). They expect that the new furnace will be completed by June of this year.

Dr. Cohn notes that the plasma arc project is a good example of a collaboration between a national lab (Battelle Pacific Northwest Laboratories), industry (Electro-Pyrolysis), and a university (MIT). Jeffrey E. Surma of Pacific Northwest Labs has overall responsibility for the project; the Electro-Pyrolysis effort is led by Charles H. Titus.

Work on the plasma arc technique is funded by the DOE Buried Waste Integrated Demonstration Program.

DETECTING BYPRODUCTS

The PFC scientists are also moving forward in diagnostics-specifically, they have had good results with a technique to detect byproducts from the plasma-arc process. The technique could become part of a continuous feedback system the scientists are developing to detect byproducts and automatically regulate the furnace.

Dr. Cohn said that byproducts might include "particulates with some metals associated with them," although he notes that these shouldbe in much smaller concentrations than for incinerators because "there's ten times less gas evolved [as compared to incinerators] to carry particulates out."

One diagnostic technique developed thus far is actually yet another application of plasma. In this case a special microwave-induced plasma interacts with any material leaving the furnace. As a result, that material emits a variety of identifying characteristics. By measuring the intensity of these characteristics, "we can detect very small concentrations of materials emitted," Dr. Cohn said.

For example, in recent experiments PFC Research Scientists Paul P. Woskov and Donna Smatlak put an iron filing only 100 millionths of a meter long in the gas stream that went through the microwave plasma. The technique turned out to be so sensitive that "they were able to measure not only the amount of iron in the filing, but the amount of manganese impurity in the iron-which was only one tenth of one percent of the iron," Dr. Cohn said.

Such sensitivity is particularly important, Dr. Cohn noted, if the furnace is eventually used to treat radioactive metals.

When could the two plasma technologies with their respective feedback systems be available commercially? Dr. Cohn believes that both could be a reality by the end of the decade.

A version of this
article appeared in the
April 7, 1993

issue of MIT Tech Talk (Volume
37, Number
28).


Topics: Environment and energy

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