Plasma technology Use of high-temperature plasma shows promise for metal recovery and hazardous waste destruction
By Hans G.Herlirz Plasma technology, an innovative a p proach to the thermal destruction of hazardous wastes, uses a high-temperature gas or a mixture of gases that can include ambient air to bring about chemical changes. The plasma is produced by heating the gas or gas mixture to temperatures as high as 5000 OC by passing the gas through and along an electric arc between two electrodes in a plasma generator. The intense electric energy of the arc is converted to heat. The plasma itself is a mixture of free electrons, positive and negative ions, and neutral atoms. The gas or gases absorb and retain a great amount of energy. The type of gas chosen can make the plasma system’s atmosphere oxidizing, reducing, or inert and thereby change its function. For instance, an oxidizing atmosphere is usually needed to destroy organic hazardous wastes; a reducing atmosphere is used to extract metals from ore or to melt scrap. As the plasma gas cools, it
FIGURE 1
Pilot waste destnrctlon system Refuse
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1102 Environ. Sci. Technol..Vol. 20. NO.11. 1986
such as flame incineration and chemical and biological treatment. Another more general factor spurring these develop ments is the growing public demand for a cleaner environment. Plasma technology has been known for more than 80 years, but it was not applied to environmental pollution abatement and materials recovery until relatively recently. Since the 1940s. plasmas have been used by Chemische Werke Hiils (West Germany) to proH ~ C.YHcwIit; duce acetylene from petroleum f&releases enough energy to dismember stocks. In East Germany, plasmas are molecules of hazardous wastes or to being used to melt scrap metals. In the provide the thermal energy needed for Scandinavian countries and South Africa, plasmas are used for the producendothermic reactions. One important motivation for apply- tion of ferroalloys. Newer applications, such as metal reing plasma technology to hazardouswaste destruction is found in the restric- covery, have been developed during the tions on land disposal of hazardous past 15 years. These were stimulated wastes imposed by the 1984 amend- partly by the rapid rise in fossil fuel ments to the Resource Conservation costs, which made technologies based and Recovery Act of 1976 (RCRA). on electrical energy more attractive ecThese restrictions also have stimulated onomically. the development of alternative technol- mmemhring hazardousorga,,irs ogies for managing hazardous wastes, Desieners of Dlasma technology are tindingways to &e some difticiit environmental problems. For example, chlorinated organic compounds, such as polychlorinated biphenyls (PCBs), are broken down into carbon monoxide and carbon dioxide, monatomic chlorine, hydrogen, nitrogen, and oxygen. (The latter two are contributed by the process gas, in this case, air.) The hot plasma gases are conduaed from the plasma generator into a reaction chamber where they cleave the PCB molecules. The products formed consist of diatomic nitrogen and oxygen, hydroFuel gen chloride (HCI), and carbon monox!w ide. These materials pass through a Hot scrubber that removes particulate matewater Clean rial and HCI. Next, the off gases from water the scrubbers are flared, although it is expected that in the future off gases will Solid be recycled to the system for use as fuel waste or sold to adjacent municipalities. SKF Steel Engineering in Hofors,
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Sweden, has designed and built a pilot plant that decomposes 0.5 metric tons per hour (t/h) of unsorted municipal refuse. The plant, which has been undergoing tests during the past several years, uses air as its plasma gas. Process products consist of fuel gas and slag; acid gases and particulate matter are scmbbed out. Dioxin emissions have been determined at less than 0.6 ng/m’ at standard temperature and pressure (STP) (conventional municipal waste incinerators emit 5-10 ngim’ [STPI . .dioxins). Wastes, some of which may be hazardous, are put into the upper part of a vertical furnace shaft after large objects are removed (Figure 1). There they are preheated, and some of the water they contain is vaporized. Hot air is injected near the bottom of the furnace. As the waste moves down, carbon monoxide, carbon dioxide, hydrogen, methane, higher hydrocarbons, tars, and nitrogen form. Noncombustible material is melted and removed from the bottom as slag, which is granulated to a glasslike product. The gases leave the furnace at 5IM-600 OC, and they are conducted to a cracking reactor where they are decomposed. Heat is supplied to the cracking reactor by plasma generators that ionize fuel gas and air. The ionized gases formed in the fierce heat of the plasma generators decompose hydrocarbons, chlorinated organics, and other hazardous substances. The steady heat from the plasma generators (> IO kWh/m3 [STP] or loo0 Btu/scf) keeps the temperature in the cracking reactor at 1300 OC. Carbon monoxide and carbon dioxide, hydrogen, nitrogen, and water vapor leave the reactor, which maintains a sufficiently reducing atmosphere to inhibit the formation of NO,. Gases are cooled to about 150 OC in a heat exchanger. Heat is recovered in the form of steam, hot water, or hot air. The hot air is recycled to the gasification furnace. The hot water or steam is available for use in the plant or for district heating. Gases pass through a filter that removes particulates at 150 OC. Heavy metals are precipitated at this stage and mixed with immobilizing materials. The gas then is cwled to 40 OC, and water, acid gases, and mercury are removed. The water is neutralized and cleaned. The cleaned gas is used for fuel, the main product of the plant. A small portion of this gas is recycled.
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Metal recovery Steel making generates large amounts of dust that contains metals such as chromium, iron, lead, molybdenum, nickel, and zinc. The chemical composition varies with the type of
Mrrnl Jrom .!red mill dust is recovered /or reuse
steel being made; chromium, molybdenum, and nickel are predominant in dusts from furnaces that produce stainless steel and specialty alloys. Dusts from electric arc furnaces (EAFs), on the other hand, normally are rich in zinc because galvanized and other zinccontaining scrap is used to charge the furnace. Ordinarily, I-2% of the charge to EAFs becomes dust and fume, which is captured in baghouses or scrubbers. In 1984, EAFs produced 33.9% of the 93 million t of steel made in the United States. What all dusts from EAF steel making in the United States have in common is their listing as hazardous wastes under RCRA because of the possibility that toxic constituents, such as cadmium, chromium, and lead, will leach out of landfills to contaminate soil and groundwater. Alternatives to land disposal of these wastes include pelletizing them and recharging the pellets to the EAF or upgrading the dust to a raw zinc oxide in a rotary kiln. Unless the dust has a very high zinc content, however, these alternatives are uneconomical. Thus, about 90%of the steel-making dust in the United States is stockpiled or transported to RCRA-certified landfills, and the cost of transportation and disposal can be as high as $150/t. About 500,000t/yr of EAF dust is generated in the United States; about 100,oOOt is used for recovery of metals. Plasma technology offers an option for addressing the problems of waste management, transportation, and disposal and for metal recovery from steel
making. One example is SKF Steel’s PLASMADUST process, which is a p plied on a commercial scale in the ScanDust plant in Landskrona, Sweden.
Expansion foreseen The application of plasma technology to hazardous waste management has passed initial development and is now approaching industrial acceptance. It is likely that plasma plants of 1-5 MW for the destruction of hazardous waste will be built during the next several years. A few 15-25-MW plants for recovering metals from steel-making wastes probably will be built, and larger municipal waste-to-fuel plants based on plasma may come later. Several factors, all of which are common to the introduction of technology, are delaying the application of plasma technology. First of all, development leads to new problems that take time to solve, and few companies are willing to be burdened with the cost and trouble of debugging new technologies. The design and construction of industrial plants can take several years. The need for environmental permits, especially for plants that will handle hazardous wastes, makes the process even more time consuming. Despite these inhibiting factors, the use of plasma technology to destroy hazardous and municipal wastes should continue to expand and should become fully accepted by the mid-1990s. Hans G. He& is engineering m n a g e r of SKF Steel Engineering (Avon. Conn.). Environ. Sci. Technol.. Vol. 20. NO. 11. 1986 1103
