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PROCESSES AT MEDIUM AND HIGH TEMPERATURES 174 Advantages and disadvantages. The advantages for molten salt oxidation are as follows: ⢠The process allows gaseous oxidation to occur at relatively low temperatures, thus minimizing the formation of such emissions as NOx. ⢠Most acid gases formed would be retained in the bed. Wet scrubbing would be minimized, possibly eliminated. ⢠This technology can be used for secondary oxidation after a primary agent detoxification process, such as hydrolysis or pyrolysis. ⢠It is a versatile technology, able to destroy agents, explosives, and propellants and to decontaminate metal parts. Energetic materials would have to be fed carefully (probably in slurry form) to avoid disastrous explosions. Disadvantages of the process include the following: ⢠The means of disposing of the salt removed from the system (blow-down) is currently uncertain; the salt is soluble. ⢠A better fundamental understanding is needed to avoid the possibility of superheated-vapor explosions. Development needs. This technology has been used on a small scale since 1950. Molten carbonate is a fairly well-known material, and there is some experience in using it for agent destruction on a laboratory scale. Specific development problems have not been identified, but a pilot plant and demonstration effort would be required. Catalytic Oxidation Technology description. Catalytic oxidation uses an oxidation catalyst with natural gas to heat the catalyst to an operating temperature of about 500°C (930°F). (It is also possible to preheat the feed stream by using other direct or indirect methods.) This technology is normally applied only to very dilute gas streams for final cleanup. With some modification, possibly with electrical heating, it could be used to replace the baseline combustion- based afterburner. The required operating temperature would depend on catalyst activity. For a temperature of 500°C and a space velocity of about 7,500 V/h/V, the catalyst can destroy volatile organic compounds containing halogens, sulfur, and phosphorus when they consist of about 0.1 percent of the gaseous feed. The catalysts are generally available on a cordierite monolith to avoid problems of drops in pressure and often contain noble metals on a gamma-alumina washcoat (Shaw et al., 1993; Wang et al., 1992; Yu et al.,
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 175 1992). The technology is very similar to that used in the automobile industry to reduce emissions of unburned hydrocarbons and CO. Development status. Laboratory data available in the literature are adequate for commercial design (Shaw et al., 1993). Catalyst manufacturers such as Engelhard (Farrauto, 1992) and Allied (Lester, 1992) have announced the availability of commercial oxidation catalysts that are not degraded by elements such as chlorine, phosphorus and sulfur. However, few data are available in the technical literature to verify these claims. Proprietary catalysts (of unknown composition) were tested on chemical agents in air streams at high dilutions with successful results (Snow, 1992). A fun-scale test for the destruction of trichloroethylene will be conducted in early 1993 under the U.S. EPA Site Program (Shaw et al., 1992). Application to chemical weapons destruction. Laboratory results have demonstrated the destruction of trichloroethylene, methylene chloride, and hydrogen sulfide (to better than 99.9 percent) to the products hydrogen chloride, sulfur dioxide, and sulfur trioxide; little deactivation occurred after the initial drop in fresh catalyst activity of about 25 percent. This technology can be used to fully oxidize trace gases produced in well- mixed oxidizing systems that need an afterburner. A lab demonstration showed that oxidation can be promoted by using the support material (cordierite) without catalyst; the temperature required was much higher, 1000°C (1830°F) versus the 500°C (930°F) when using catalyst. The oxidation was not as complete as with catalysis; some chlorinated hydrocarbon species survived in the product. For use of highly active catalysts, the best application appears to be for treatment of gases from low-or medium-temperature processes, such as chemical and biological oxidation and WAO or SCWO when the concentration of catalyst poisons can be within acceptable limits and where heat must be added to bring the gases to reactive temperature. For high-temperature agent oxidation the heteroatom content can limit catalyst activity; however, the available high temperature allows use of a ragged low-activity catalyst. Waste streams. Liquid products of oxidation may need to be removed, possibly by using scrubbers or activated-carbon filters. Scrubber effluent liquids contain unreacted alkaline materials and salts of chlorine, fluorine, sulfur, and phosphorus. The spent sorbem will need conventional disposal. The solids will contain calcium chloride, fluoride, sulfate, and phosphate. Similarly, the spent catalyst will need to be discarded. Advantages and disadvantages. Catalytic oxidation is used commercially for the oxidation of trace hydrocarbons, because of its several advantages over
