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PROCESSES AT MEDIUM AND HIGH TEMPERATURES 156 An inorganic slag would be produced that might contain some soluble salts requiring landfill disposal. Metal parts fed to the unit would be drawn off with the liquid metal of the furnace. Advantages and disadvantages. The process is broadly applicable to a variety of materials, including agents, metal parts, propellants, and explosives. The molten metal furnace could combine the functions of the three incinerators of the baseline technology. The metal furnace does not eliminate the need for a combustion process; the product gases would be oxidized in a separate unit. These gases would likely be very dirty, containing soot from the metal pyrolysis and possibly some slag particulate matter. Gas cleanup will be required before the gas is released. Development needs. To the extent that this technology is similar to that of steel production, it is well developed. However, it has not been applied to destruction of highly dangerous substances like agent. Whether there is much experience with remote operation of molten metal reactors is unclear. The exact gas and solid products that would be produced and their properties would need to be determined. Pilot plant work would probably need to precede construction of a demonstration unit. Plasma Arc Processes Technology description. Plasma arc torches generate ionized plasmas at temperatures of 3000 to 12,000 K7 At such high temperatures, waste materials can be completely pyrolyzed. Organic compounds in the presence of a small amount of oxygen are oxidized to a product gas consisting principally of hydrogen, CO, some CO2, and small hydrocarbons such as methane, ethylene, and acetylene. The source of oxygen may be steam or air used for the plasma. Some torches use nonoxidizing plasmas such as nitrogen or argon. Heteroatoms from the organic feed will eventually be bound in gaseous products such as HCL, HF, phosphorous oxides, H2S, or SO2, depending on temperature and oxygen availability. Salts or metallic elements will be melted or vaporized; such materials are usually recovered as slags and sometimes as molten metal. The gaseous product is a fuel with a low heating value, which would be burned with added air. Heteroatoms in the gas would be scrubbed out by aqueous alkaline scrubbers to form salts. Phosphorus or metal components in the vapor can be quenched rapidly to give a metallic 7 A number of plasma arc technology designs were presented at the committee's workshop (see Appendix F; Keairns et al., 1992; Schlienger, 1992; Titus, 1992).
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 157 powder (particles less than 1 µm) or, under oxidizing conditions, metallic and phosphorous oxides. Figures 7-5 and 7-6 portray some possible plasma arc furnace designs. Most furnaces use gases other than air for the plasma, for example, N2 or steam. An oxygen source is needed to prevent formation of carbon soot particles; steam is frequently chosen as the oxygen source, although power consumption increases because energy is required to dissociate the water. Development status. Plasma arc furnaces have been developed and used for recovery of waste metal in foundries. Both nontransferred arcs (two independent electrodes) and transferred arcs (molten metal product forming one of the electrodes) have been used. Large furnaces with very large melting capacity (about 45 tons/ hour) have been reported, with power inputs of 9 MW. More recently, the technology has been applied to mixed solid wastes and liquid wastes, such as PCBs. Destruction efficiencies of 99.9999 percent were reported for PCBs and CCL4 (Lee, 1989). Application to chemical weapons destruction. The plasma arc process could be used on chemical warfare agents for detoxification and destruction. It could also be used to destroy the products from a prior treatment, such as hydrolysis. Plasma arc furnaces can handle large solid-metal objects, such as rocket cases, but most of the energetic contents would need to be removed first to avoid possible destructive events in the furnace. Propellants and explosives could be destroyed but would obviously require careful control of feed rate to avoid destructive explosions or detonations. Air supply would represent only a small part of the stoichiometric requirement for liquid agent. The gas produced would consist predominantly of H2 and CO, with smaller amounts of CH4, acetylene, and other materials (e.g., HCL, H2S, or HF), depending on the composition of the liquid agent. The gas product is at a lower temperature than the plasma because of the endothermic heat of reaction and would be oxidized in a second burner. Combustion products would require treatment to remove acidic components such as HCL. By-products and waste streams. The waste streams from plasma arc destruction of agent would be essentially the same as those from incineration. The two-step operation of the plasma arc followed by a burner or catalytic oxidizer is itself a safeguard against transient breakthrough of agent. The following waste streams would be formed by plasma arc destruction of chemical agents: ⢠gases: carbon dioxide, nitrogen, oxygen, and steam; ⢠liquids: alkaline scrubbing liquid containing salts (e.g., NaF); and
PROCESSES AT MEDIUM AND HIGH TEMPERATURES FIGURE 7-5 The Westinghouse plasma system. Source: Keairns et al. (1992) 158
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 159 FIGURE 7-6 The Electro-Pyrolysis, Inc. (EPI) furnace design. Source: Titus (1992).
