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PROCESSES AT MEDIUM AND HIGH TEMPERATURES 152 ⢠The high reaction rate in SCWO is its major advantage over WAO. Reactor residence times are much lower-1 minute rather than 1 hour-and the oxidation reaction goes to completion. The process also has most of WAO's advantages. Major disadvantages of SCWO are the following: Major disadvantages of SCWO are the following: ⢠Pressures are very high. ⢠Corrosion will be severe, although it can probably be handled with suitable construction materials. The fluoride ion (resulting from GB) is a major concern, being highly corrosive. ⢠Salts will precipitate and may cause plugging problems in the high-pressure vessels and the pressure letdown system. Development needs. SCWO oxidation of slurries of propellants and explosives requires demonstration. (In principle, these materials need very little additional oxygen to complete their oxidation.) The behavior of such materials under SCWO conditions needs evaluation. For example, will the oxidizer in the solids themselves (nitro groups) be reactive? Will reaction rates be extremely fast, possibly explosive? The products of oxidation of these materials should be relatively easy to handle. The chemical warfare agents present problems of corrosion and solids handling because of the heteroatoms they contain. One question is whether suitable materials are available to withstand attack by F and Cl ions under either mildly acidic conditions or strongly alkaline conditions. The formation and precipitation of solid salts caused plugging problems in early SCWO experimental work. Proposals for handling the problem included a dual-temperature reactor with supercritical and subcritical conditions in separate parts of the reactor. Thus, a solution to the problems of solids precipitation may be available but demonstration is needed. The choice of feed concentration, reactor and process conditions, and reactor design will need study and ultimately some test work with actual agents. Experimental work on agents is underway. HIGH-TEMPERATURE, LOW-PRESSURE PYROLYSIS Molten Metal Pyrolysis Technology description. In the proposed molten metal processes, metals, such as copper, iron, or cobalt, are used at 1650°C (3000°F) to decompose organic compounds thermally and dissolve inorganic materials to form a slag. The process vessel is a steel converter as developed by Molten Metal
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 153 Technology (MMT) and Elkem Technology (Nagel, 1992; Aune, 1991; see Figures 7-3 and 7-4). The metal bath is heated by passing a current between electrodes in the bath. The material to be destroyed (gas, liquid, sludge, or solid) is pumped into the bottom of the vessel containing the molten metal. The waste material dissociates into small molecules or atoms and is distributed throughout the bath. The gases formed are emitted to an air pollution control system in which oxidation is completed and solids are removed. A molten inorganic slag insoluble in the liquid metal rises to the top and is skimmed off. The final gaseous products are the same as those of incineration. Development status. The Elkem Multipurpose Furnace and the MMT catalytic extraction process are essentially developed technologies. They are very similar to those used in steel production. However, the use of these technologies in the destruction of chemical agents, munitions, or propellants has not been tested or evaluated. Application to chemical weapons destruction. The Elkem furnace has been used for recovery of zinc and lead from electric arc furnace dust. MMT technology has been applied experimentally to many organic materials, such as alkanes (methane to pentane), aromatics (benzene to anthracene), alcohols (methyl and ethyl), olefins (ethylene to pentane), and mixtures (No. 2 fuel oil, polychlorinated biphenyls [PCBs], and others). No detailed analytical work has been presented. The Elkem furnace accepts only solids in the form of briquettes. The MMT process accepts feeds in any form. Both are potentially capable of destroying chemical agents, propellants, and explosives. They can melt metal parts while destroying any agent residues on them. Special considerations. The specific products from agent destruction are unknown and questions of operability have yet to be answered. Some polymeric material and soot will probably require subsequent processing. Products from the heteroatoms F, Cl, S, and P are not known. They may cause problems with refractory linings. Liquid feeds, particularly aqueous solutions, increase the possibility of superheated vapor explosions. By-products and waste streams. The pyrolysis products from agent destruction would be oxidized in an afterburner. Product gases would contain the usual acidic components, such as HF, which would have to be scrubbed and converted to their salts. Very high gas temperature and accompanying high NOx formation can result when air is added to the hot gases leaving the molten metal bed. Control of gas phase temperature and time will be important.
PROCESSES AT MEDIUM AND HIGH TEMPERATURES FIGURE 7-3 The Molten Metal Technology. Source: Nagel (1992). 154
PROCESSES AT MEDIUM AND HIGH TEMPERATURES FIGURE 7-4 Process flow sheet of the Elkem Multipurpose Furnace and associated equipment. Source: Aune (1992). 155
