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THERMAL TREATMENT AND PREPROCESSING AND POSTPROCESSING OPERATIONS 105 Storage and Retention Technologies Several such gas storage or retention technologies are possible, including large atmospheric gas holders, smaller pressurized tanks, and systems that either absorb or solidify the CO2 with separate CO2 storage in solution or as a solid. If the captured gas does not meet requirements for concentration of toxic materials in air and agent, it would be recycled to the afterburners or to a charcoal filter. If agent contamination occurs in the gas holding system, decontamination will be needed. Facilities for this purpose must also be provided. Care must also be taken to provide good mixing of the captured gas to ensure representative sampling; this mixing could be accomplished with fans. Thus, although such technologies are commercially available, they would need to be carefully analyzed for this storage application, especially regarding cleanup of the storage system, including its piping and valves, if it became contaminated through the storage of a contaminated gas waste stream. Near-atmospheric gas-holding tanks. Very large, near-atmospheric-pressure gas holders are proven technology. They were used by the natural gas and related industries for many years before becoming popular for use in other gas storage. These systems use concentric vertical cylinders that telescope to expand or contract the tank volume as needed. Because the bottom cylinder is stationary on a flat foundation, systems using only two concentric cylinders are called single-lift systems, and systems using three or more are called multiple-lift systems. The movable cylinders, or lifts, are usually partially supported by counterweights to adjust the internal pressure relative to atmospheric pressure (usually a few inches of water difference). Several types of seals have been used between these moving cylinders. Two positive seal systems are described here for possible use in chemical demilitarization, the water seal and Wiggins (bellows) seal types. The water seal type uses an inverted tub with sides extending down into the water-filled space in between the two walls of a double-walled bottom cylinder. The water makes the seal. The largest such single-lift unit has a usable capacity of 350,000 cubic feet (10,000 m3). One such unit is currently being constructed for a chemical manufacturer. Multiple-lift units have been built with usable capacities of up to 10 million cubic feet. These units are reportedly reliable and do not leak. However, they may result in contaminated water, requiting a water purification system, depending on their use (Bronson, 1992). The Wiggins (bellow) seal type uses two vertical concentric cylinders with a large connecting membrane of rubberized canvas that allows the inner cylinder to rise as gas is added. The top section is counterweighted to control
