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THERMAL TREATMENT AND PREPROCESSING AND POSTPROCESSING OPERATIONS 102 Thus, it should be possible to use systems that recycle all water not discharged with other gases. Reduction of Waste Gas Volume Reduction of the volume of gaseous waste streams has certain advantages, especially if storage and certification of such streams is desirable. Any process or group of processes that oxidizes all the carbon compounds in chemical agents to CO2 and water requires a supply of oxygen. If this oxygen is supplied as air, about four volumes of inert nitrogen will also be present for each volume of oxygen. This nitrogen and the extra fuel (and air) to heat the nitrogen will increase the volume of the gas waste stream. If the water that is formed by the oxidation is condensed, the gaseous waste stream consists of only CO2, nitrogen, and minor gas impurities; the volume of waste gas volume is reduced substantially. Alternatively, most processes could use air enriched with oxygen or pure oxygen. Oxygen-enriched air and (essentially) pure oxygen are both commercially available, are transportable or producible on site, and would represent relatively insignificant cost in the scheme of overall program costs. Thus, tradeoffs of using oxygen versus air are determined largely by the impacts on selected processes and by the gas storage volume required to certify the gas for release. Generally, the processes that benefit the most from the use of more concentrated oxygen are those that rely on more expensive equipment; with smaller gas volumes, less equipment is needed. For example, wet air oxidation might use a 50:50 mixture of air and oxygen, whereas supercritical water oxidation is designed to use pure oxygen. However, other processes would also benefit substantially if the volume of stored waste gas could be substantially reduced. Other technical factors must also be considered when substituting oxygen for air. For example, the nitrogen in air provides a dilution that usually aids in avoiding hot spots in oxidation equipment. It may provide additional mass and energy to assist in the initial atomization or dispersion of the agent and any supplemental fuels, and it provides a larger gas volume that acts as a cushion in the event of puffs caused by the nonuniform oxidation of explosives or propellants. One method tested at the pilot stage for the use of oxygen in combustion furnaces involved substituting recycled flue gas for the nitrogen eliminated from the oxygen feed stream (MRK, Incorporated, 1992). This approach requires additional equipment for temporary flue gas stream surge capacity, recycle blowers and ducting, and redesign of the burners and furnace to reflect the different characteristics of the recycled flue gas, which is primarily
