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SHIPBOARD POLLUTION CONTROL: U.S. Navy Compliance With MARPOL Annex V
Molten Metal Pyrolysis
This technology is somewhat similar to the above pyrolytic methods. The waste stream is pumped into the bottom of a molten metal (iron, copper, or cobalt have been used) bath at 1,650°C (3,000ºF). The bath is made molten by passing an electric current through the metal. Organic materials are decomposed under reducing conditions and pass out of the melt as gases. They are subsequently oxidized and cleaned. The inorganics form a slag which floats on the metal pool and is skimmed off.
Two commercial vendors have developed this process: the Elkem Multipurpose Furnace, and the Molten Metal Technology catalytic extraction process. The processes are very similar to techniques used in the steel industry. A number of substantial plants are under construction for the management of industrial and Department of Defense and Department of Energy (DOE) wastes. Application to waste streams similar to Annex V materials has not been undertaken, but a variety of feeds have been processed that are relevant to the Annex V problem.
With oxidative methods, the waste stream is decomposed under oxidizing conditions, as with incineration. The products are fully oxidized and no afterburner is required. Temperatures involved are considerably lower than for the pyrolytic methods discussed above.
Supercritical Water Oxidation
Water above its critical point, i.e., above 374ºC and 220 atmospheres pressure, behaves very differently from water as we normally encounter the substance. Supercritical water is more like a gas than a liquid and is miscible with other gases, but not salts. Under supercritical conditions organic compounds and oxygen are both soluble in water and can react readily. Exposure time within the reactor is less than 2 minutes for very complete (> 99 percent) conversion. Shorter residence times, which might mean smaller reactors, may be possible by manipulating temperature and oxidant concentration.
The supercritical water oxidation (SWCO) process (Modell, 1989; Tester et al., 1993; Peters et al., 1994; Shanableh and Gloyna, 1991), as applied to Annex V waste, would entrain shredded waste in water at concentrations of 1 to 20 percent. This mixture would be pressurized and preheated and then introduced into the reaction chamber for exposure to the oxidant (oxygen, air, or hydrogen peroxide). The temperature-time history of exposure is rigorously controlled. Organic materials present in shipboard waste should be largely converted to carbon dioxide and water. To the extent that sulfur, chlorine, and phosphorus are present, acids will be formed: sulfuric, hydrochloric, and phosphoric acids. In SCWO, these acids are converted to salts through the injection of sodium hydroxide, and processes to remove the salts must be carried out. Since the concentration of acid-forming elements is low in shipboard wastes, this problem can probably be solved. The process can be made self-sustaining by employing reaction heat to prepare the incoming waste stream.
SCWO has been successfully employed at the bench and pilot plant levels to destroy a large number of organic compounds with high levels of destruction. Bench-scale destruction has been demonstrated for various mixed waste including paper, protein, and human waste. The committee is unaware of studies to determine SCWO parameters for the destruction of Annex V waste, but high destruction efficiencies for paper and related materials (Tester et al., 1993) suggest possible suitability for this application. Off gases are said to be free of noxious compounds found in incineration processes, e.g., dioxin.
Up to this time, the only commercial application of SCWO is in the destruction of alcohols, glycols, and amines in aqueous wastes from a chemical plant. Development of plants to destroy army munitions and solid rocket propellants is being actively pursued. A pilot plant in Germany has successfully destroyed sewage sludges, cutting oils, and other waste products at a rate of a few hundred