National Academies Press: OpenBook

Alternative Technologies for the Destruction of Chemical Agents and Munitions (1993)

Chapter: Activated-Carbon (Charcoal) Adsorption Systems

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Suggested Citation:"Activated-Carbon (Charcoal) Adsorption Systems." National Research Council. 1993. Alternative Technologies for the Destruction of Chemical Agents and Munitions. Washington, DC: The National Academies Press. doi: 10.17226/2218.
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Page 106
Suggested Citation:"Activated-Carbon (Charcoal) Adsorption Systems." National Research Council. 1993. Alternative Technologies for the Destruction of Chemical Agents and Munitions. Washington, DC: The National Academies Press. doi: 10.17226/2218.
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Page 107
Suggested Citation:"Activated-Carbon (Charcoal) Adsorption Systems." National Research Council. 1993. Alternative Technologies for the Destruction of Chemical Agents and Munitions. Washington, DC: The National Academies Press. doi: 10.17226/2218.
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Page 108

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THERMAL TREATMENT AND PREPROCESSING AND POSTPROCESSING OPERATIONS 106 gas pressure. Patent rights are held by Brown Minneapolis, which sells units of up to 2 million cubic feet capacity (Liljegren, 1992). The maximum sizes mentioned above are more than adequate for use in demilitarization facilities. Gas homer safety. Gas-holding tanks were safely used for many years to temporarily store town gas (mostly CO and H2) for both residential and industrial uses. They were then used to store natural gas (CH4) until natural gas storage in pipelines or underground storage became more economical. Today, these tanks are still used to store industrial gases. Although there is the potential hazard of releasing the stored gas through an accident such as an airplane crash or catastrophic tank failure, the relative hazard from the sudden release of warm flue gas, with its large nitrogen content, is likely to be significantly less than for similar releases of town gas or methane. The hazard that gas containing chemical agent would be released is quite remote, being dependent on the simultaneous occurrence of a major external accident and an internal plant failure. Pressurized gas storage. Large tanks capable of holding five atmospheres of pressure (75 psig) are common in industry. Under these circumstances, the gas storage volume needed for chemical demilitarization would be reduced by 80 percent. Gas absorption and solidification. If oxygen is used instead of ordinary air (with essentially complete oxidation of carbon compounds to CO2) and all acid gases are removed from the flue gas with a water scrub that also condenses the newly formed water to the saturation level of a normal cooling tower system, then the flue gas should consist primarily of CO2 saturated with water vapor. The water vapor can be further reduced by refrigerant cooling, and the CO2 can be removed by conventional CO2 absorption or solidification processes used by the CO2 manufacturing industry. The CO2 can also be reacted with lime, Ca(OH)2, to form calcium carbonate, which would be suitable for landfill disposal. Activated-Carbon (Charcoal) Adsorption Systems The above-described methods store effluent gases from dynamic process systems to allow certification of the suitability of the gases for discharge to the environment. An alternative solution to ensure such suitability would be to use one or more static systems in a final gas cleanup step before gas discharge. One such system would be to pass all effluent gas through a charcoal adsorber (the method used in gas masks) before discharging the gas.

THERMAL TREATMENT AND PREPROCESSING AND POSTPROCESSING OPERATIONS 107 Charcoal filters are in commercial use and have been used at Army facilities to remove trace quantities of impurities, including chemical agent, from building ventilation air. This technique is sometimes used in inlet air systems to ensure a dean air supply to the control room and other critical working areas if an accident contaminates incoming ventilation air. Charcoal filters usually are box-like structures that hold beds of granulated charcoal (or other activated carbon) through which a gas waste stream is forced. Several sequential beds are generally used to provide backup for when the first beds are saturated with impurities and to allow replacement of beds without disturbing the backup beds. Flexible piping and valves may also allow bed switching. This sequence of beds provides adequate time for chemical analysis and certification of the leaving gas stream before saturation results in breakthrough of contaminants, a feature in common with the gas storage system discussed earlier. The charcoal or activated carbon selectively adsorbs certain types of molecules onto the internal surfaces of tiny pores and interstices of the granules, with the type and amount of such adsorption depending on the method of charcoal production and/or activation and on the concentration of the impurity being adsorbed. Typically, activated carbon will not strongly adsorb oxygen or nitrogen but will adsorb polar compounds, hydrocarbons, water vapor, and CO2. Although the presence of large quantities of water vapor and CO2 will tend to saturate the adsorption surfaces, they are displaced by larger molecules. (The gas must be dried to avoid the presence of liquid water or condensation.) The chemical agent will be strongly adsorbed in the first section until saturation is reached and breakthrough occurs. Replacement of sections is necessary when breakthrough is observed in the backup sections (Ward, 1992). The disposal of contaminated filter sections can be accomplished either by combustion or by direct burial in a landfill. The baseline metal parts and metal deactivation kilns might, with some modification, be used for this purpose. Entrainment of partly burned charcoal is expected to be a problem in the rotary kiln; however, severe heat treating and at least partial combustion of spent charcoal in trays passing through the traveling-grate metal parts kiln may be feasible. Any remaining charcoal would qualify for a 5X rating and landfill disposal. Disposal of the spent charcoal would involve the following (Ward, 1992): • The beds would be removed by methods that assume their contamination with chemical agent. • The beds would be tested for agent by bubbling air through the spent section to see if any agent could be detected in the air stream. Normally none is detected because the agent is tightly bound to the adsorption surfaces of the charcoal.

THERMAL TREATMENT AND PREPROCESSING AND POSTPROCESSING OPERATIONS 108 • If no agent was detected, the spent charcoal would meet the 3X criterion and be bagged, transported to, and buried in a commercial hazardous waste landfill. A hazardous waste landfill would be required because the charcoal would be expected to contain small amounts of chemical agent, even though it could not be detected by air sampling techniques. • Procedures would still need to be developed for handling any material that did not meet the 3X disposal criterion. These would include temporary storage with a slow flush of air (vented through the newly installed charcoal filters) to allow slow decomposition of the agent in the filters. This approach is reported to be effective for small quantifies of nerve agent but has not been tried for mustard. Charcoal filters could be used with both the baseline and alternative technologies. The Army and the NRC are now reviewing the possibilities and implications of using these filters on the gaseous waste streams generated by the currently used incineration technologies at the various chemical weapons storage sites.1 1 This activity is being carried out by the NRC Committee on Review and Evaluation of the Army Chemical Stockpile Disposal Program.

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The U.S. Army Chemical Stockpile Disposal Program was established with the goal of destroying the nation's stockpile of lethal unitary chemical weapons. Since 1990 the U.S. Army has been testing a baseline incineration technology on Johnston Island in the southern Pacific Ocean. Under the planned disposal program, this baseline technology will be imported in the mid to late 1990s to continental United States disposal facilities; construction will include eight stockpile storage sites.

In early 1992 the Committee on Alternative Chemical Demilitarization Technologies was formed by the National Research Council to investigate potential alternatives to the baseline technology. This book, the result of its investigation, addresses the use of alternative destruction technologies to replace, partly or wholly, or to be used in addition to the baseline technology. The book considers principal technologies that might be applied to the disposal program, strategies that might be used to manage the stockpile, and combinations of technologies that might be employed.

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