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PROCESSES AT MEDIUM AND HIGH TEMPERATURES 171 ⢠Granular material of the bed can lead to problems. Finely divided dust can be difficult to capture or could end up in the acid scrubbing solutions or salts formed by reaction with bed materials, which could cause particle agglomeration with poor fluidization. ⢠The very high destruction desired for chemical agents (99.9999 percent) is difficult to achieve in a fluidized bed; staging would be needed. Development needs. The technology must be demonstrated on the feed materials of interest. The presence of heteroatoms will affect the chemistry and possibly the operability of the process. A pilot plant and a demonstration plant would be required. Molten Salt Oxidation8 Technology description. Molten sodium carbonate at 900 to 1000°C (1650 to 1830°F) is used with air as a medium in which to oxidize mixtures of combustible solids, organic liquids, aqueous solutions, and/or slurries. Acidic products, such as HF, HCl, SO2, and P2O5, would react to form salts that would dissolve in the molten salt bath (Figure 7-9). The sodium salts of chlorine, fluorine, phosphorus, arsenic, sulfur, and silicon as well as iron oxide, silver, and copper all end up in the molten salt bath. The gaseous effluents need to be filtered to remove some of the fine particulates formed in the bath, and the process would probably require an afterburner. Spent salt is discarded or could be recycled. Alternatively, a molten salt countercurrent tower design could be used as an afterburner and for removal of acid gases in the waste gas streams from other processes. Status and database. The Atomic Energy Commission began research and development on molten salt oxidation in the early 1950s. It funded Rockwell International in the early 1960s to further develop the process, and other work followed (Taylor, 1992). The U.S. Department of Energy (DOE) recently sponsored work on the application of the process to mixed waste 8 Subsequent to the committee's data collection activities, a large R&D program on molten salt oxidation has been implemented.
PROCESSES AT MEDIUM AND HIGH TEMPERATURES FIGURE 7-9 Molten salt oxidation system. Source: Taylor (1992). 172
PROCESSES AT MEDIUM AND HIGH TEMPERATURES 173 treatment. It was also tested on chemical warfare agents (VX, GB, and mustard) at Edgewood Arsenal, Maryland, in 1976, achieving 99.9999 percent destruction of agent, and is was tested on a number of propellants and explosives. Molten salt baths are used commercially for heating and pyrolysis operations. The current application would use the high heat capacity of the bath for two purposes: to carry out oxidation at relatively low temperatures (much lower than those used for incineration), which would minimize some undesirable combustion products (NOx and dioxins), and to avoid large temperature changes that can occur in conventional incineration that can result from variations in feed rate or composition. Application to chemical weapons destruction. Molten salt baths could be used to destroy agents, hydrolysis products, propellant, and highly explosive materials. Finely ground explosive and propellant solids would be fed at a low enough rate to avoid explosion. The small dimensions of these materials would also accelerate oxidation in the bath. Special considerations. The suggested salt bath material would be Na2CO3, which has a melting point of 851°C (1564°F). It should react with acidic combustion products to form sodium salts and emit fixed gases free of undesirable components (after filtration for fine solids). The hazards of handling a molten carbonate bed are minimal. These beds have been considered for commercial applications in molten carbonate fuel cells (Kalfadelis, 1977). Reactants and products rise through the hot molten salt as bubbles. As a result, some reactants may bypass the system. Very high conversion levels (99.9999 percent) may require staging or further product treatment, such as the use of an afterburner. Oxygen rather than air might be used as the oxidant to minimize the volume of gas effluents. One hazard of the process is potential superheated-vapor explosions when liquid agent or hydrolysis products are introduced. The presence of propellants or explosives could also lead to explosions. If not well dispersed, energetic materials could muse explosions or puffs. Waste streams. Very fine (possibly smaller than 1 µm) salt particles may be formed and carried with the gaseous discharge. Filters would be needed to prevent these salt particles from being emitted with the otherwise dean flue gas components. No liquid would be emitted. A certain fraction of the molten salt bath must be removed and fresh sodium carbonate added to maintain desirable salt composition. The total salt requiring disposal would be several times the weight of the chemical agent destroyed.
