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THERMAL TREATMENT AND PREPROCESSING AND POSTPROCESSING OPERATIONS 103 CO2, water vapor, and nitrogen. However, flue-gas recycle does provide a solution to problems that might occur if pure oxygen were used. A modification of the flue-gas recycle method was commercialized recently (Ho, 1992). In this method, a modified fuel-and-oxygen burner injector uses the kinetic energy of the oxygen feed stream to induce internal burner-gas recirculation by aspirating internal combustion gas through the burner nozzle head. This method is reported to avoid hot spots and could presumably be used to atomize liquid agent (atomization is not done currently by pressurizing the agent, to avoid various pressure-related agent contamination problems). Heat extraction to control flame temperature is necessary in this approach. This function is normally achieved in demilitarization systems through the design of furnace and boiler combustors. The lower gas flow in the alternative approach would provide less dilution to manage puffs, but capture of puffs of organic compounds in the waste stream on activated carbon or in gas storage would still be effective. An additional consideration is that although oxygen is produced and used in many commercial operations, the production, transport, storage, and feed introduction steps all involve a degree of additional hazard until the oxygen has been diluted. Pure oxygen significantly changes the combustion characteristics of many materials compared with their combustion in air, requiring special organic-grease-free valves and other precautions against combustion conditions. Additional worker hazards include oxygen being easily trapped in the void spaces of woven clothing and contributing to clothing fires that would not normally be expected. Standards for these requirements are well developed and available but must be rigorously implemented for safety. Waste Gas Storage Requirement Although the retention of liquid and solid waste streams until they are certified for disposal should not generally be a technical problem, the retention of large-volume gas waste streams is common practice. However, the extreme toxicity of the chemical agents and the proximity of some stockpile sites to highly populated areas prompts consideration of storing waste gas until analyses establish that agent and other toxic materials in air are suitable for discharge. All alternative technologies in which oxidation to CO2 occurs will produce some gas waste stream, so the need for storage and certification is universal. If all gas waste streams must meet the same requirements, the choice of storage technology will depend on the ability to meet the specifications for gas composition reliably and efficiently. In all cases, afterburners should be used to ensure that toxic materials in air are below permissible concentrations under normal operating conditions. However, gas volumes can vary over a wide range. The smallest volume results when oxygen
THERMAL TREATMENT AND PREPROCESSING AND POSTPROCESSING OPERATIONS 104 is used and CO2 is removed by lime and the greatest volume results with the baseline design, in which air is used to burn agent and to bum the fuel added to the process stream to heat the furnaces and afterburners. Minimizing gas volume, and thereby reducing storage volume requirements, results in higher concentrations of toxic material for a given fractional destruction (such as 99.9999 percent destruction of original material), which would provide some improvement in the reliability of detecting residual toxics. Atmospheric dispersion of the leaving gas stream would depend on use of a large gas jet, and mixing with additional air might be needed. Gas reheat may also be required to ensure that the effluent stream is buoyant and will rise to an effective dispersion altitude in the atmosphere. The volume of gas that must be stored depends on many factors, including the plant processing rate, the amount of gas produced per unit of agent processed, and the certification and plant shutdown time (see Chapter 4). A genetic unit gas storage volume has been developed based upon the following assumptions: ⢠consideration of the liquid agent stream only: separate calculations will be required for those processes that treat the metal parts or other components; ⢠a liquid GB agent destruction rate of 100 pounds per hour (see Chapter 4): greater or lesser rates of operation can be scaled directly from this number; ⢠full oxidation of all carbon to CO2 assuming 20 percent excess air and no supplemental fuel used internal to the process: the water formed by oxidation of the hydrogen in agent is condensed so that it is in equilibrium with an exit gas stream that has been cooled to 120°F; and ⢠a minimum 1-hour storage time for certification and plant shutdown, if needed: use of longer storage periods would proportionately increase the amount of gas to be stored. The above assumptions result in an estimated total gas volume for temporary storage of approximately 11,500 cubic feet Use of an 8-hour storage cycle, which would allow more time for detailed analysis and operating decisions, would increase this volume to 92,000 cubic feet. In the event that pure oxygen rather than air is used as the oxygen supply, the similarly derived estimate is about 1,150 cubic feet for a 1-hour cycle. For the JACADS liquid incinerator, the storage volume for a 1-hour cycle would be 350,000 cubic feet. This larger volume results from the high feed rate (of 750 rather than 100 pounds per hour), the use of almost 300 percent excess air, and the use of some air to burn fuel for additional heat (SRI, 1992).
