Plasmas Offer Hope of Improved Environmental Clean-Up Techniques
Plasma treatment is an attractive alternative because it is powered by electricity and therefore does not produce an additional gaseous waste stream, as is the case with combustion. Moreover, plasma technology can provide high temperatures with a high degree of controllability, and plasma chemistry effects can be used for selective processing, thus improving efficiency.
According to Daniel Cohn, a physicist at MIT's Plasma Fusion Center, thermal equilibrium, or "hot", plasmas are particularly appropriate for treatment of solid waste, and can also be employed for destruction of toxic molecules by thermal decomposition, making them well-suited for processing hazardous metals mixed with solvents. These man-made lightning bolts for breaking down and monitoring waste represent a potential near-term spin-off of long-term nuclear fusion research.
High-temperature plasmas can break down solids without the need for combustion, produce glass compounds that do not leach into groundwater or produce harmful by-products such as dioxins. In a pilot-scale research furnace at MIT, a 10,000-degree plasma arc, created by passing an electric current between a pair of graphite electrodes in a nitrogen-filled gas chamber, has been used to melt waste material (consisting of soil, metals, combustible materials, and sludges) into a lavalike liquid. The liquid solidifies into a stable black glass that can be safely stored or even used as a construction material. The process produces no toxic ash, virtually no dioxin, and less gas emission than traditional incineration techniques. Furthermore, it has the potential of being more economical than present techniques.
Cold plasmas can be used to destroy toxic molecules by selective reactions with plasma electrons and radicals, and thus are well suited for treatment of dilute concentration of volatile organic compounds (VOCs) in air streams. In recent months, MIT researchers have successfully used low-temperature plasmas, generated by electron beams, to seek out and destroy minute concentrations of chemically hazardous compounds found in waste sites at the Hanford nuclear facility in Washington State.
Room-temperature electron plasmas selectively acted upon minute concentrations of hazardous carbon tetrachloride molecules vacuum-pumped from waste deposits and split them into less stable compounds that were eventually broken down into carbon dioxide, table salt, water, and some carbon monoxide. Cohn's team is now exploring ways to control electron beam current, voltage, and gas flow rate, as well as the use of externally applied electric fields to vary electron energy, and adding substances to promote specific reactions.
Cost is an important factor in transferring plasma waste processing technology from the laboratory to the marketplace. Cohn estimates that the cost for hot plasma processing, including electricity and capital equipment costs, is between $200 and $300 per ton of hazardous waste, compared to between $200 and $800 per ton using current combustion methods. The cost of cold plasma processing of toxic materials is between 40 cents and $4 per pound, compared to about $10 per pound using current methods.
Plasmas can also be employed in diagnostic schemes to measure the hard-to-determine temperatures at the center of furnaces, and to monitor emissions of such hazardous metals as cadmium and arsenic. MIT's Paul Woskov described a system that determines the hard-to-measure temperatures at the center of a furnace by detecting the high-frequency microwaves that cut through the smoke in the furnace. The unique active probing capability of the millimeter wave radiometer can also provide information on melt surface turbulence, changing furnace wall emissivity and millimeter-wave optic losses inside the furnace.
David Rhee, also of MIT, discussed a real-time system that continuously monitors heavy metal emissions in a waste-burning process. The system uses microwaves to create a high-temperature plasma in the waste gas. The plasma excites the heavy metals and causes them to emit radiation that reveals their spectroscopic fingerprints; concentrations as low as 1 ppb can be detected. Other diagnostics are under consideration to continuously monitor feed composition, molecular composition of the off-gas, and gas velocity.
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