5
Thermal Treatment and Preprocessing and Postprocessing Operations

Technology often means different things to different people. The Army's current baseline program uses what is often called incineration technology. When used in this general sense, technology refers to a system with component elements or steps that are referred to as processes or unit operations. The baseline incineration technology system includes such processes as combustion and flue gas cleanup and, as parts of these processes, such unit operations as liquid storage, liquid pumping, air compression, agent and fuel atomization, combustion, flue gas cooling by water quenching, flue gas scrubbing with decontaminating fluid (and its recycle or destruction), flue gas dewatering, and gas blowing to carry the effluent gas to the stack.

Thus, an evaluation of alternative technology systems must consider first the availability and capabilities of alternative processes and unit operations and then the effectiveness of combining them in an operating technology system. This kind of analysis is especially important in cases where available unit operations will only partially detoxify the chemical agent rather than fully destroy it and where they will convert chemical agent to other organic forms but will not convert all the carbon waste to acceptable wastes, such as CO2, sodium carbonate, or material roughly equivalent to sewage sludge.

Chapters 6 and 7 address a number of the principal alternative unit processes and technologies that might be used to destroy chemical agent and energetics and decontaminate metal parts and containers. Chapter 8 considers possible combinations of these elements as potential alternative technology systems. However, before a destruction technology is used, weapons must be prepared for subsequent destruction. For example, munitions must be disassembled to separate agent from propellants and explosives; ton containers must be drained of agent. In addition, the destruction processes result in gas, liquid, and solid waste that must be processed before being released to the environment.

This chapter covers some optional processes that might be used at the front end of the system to preprocess feed materials and at the back end of the system to treat, temporarily retain, or further prepare waste streams for



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions 5 Thermal Treatment and Preprocessing and Postprocessing Operations Technology often means different things to different people. The Army's current baseline program uses what is often called incineration technology. When used in this general sense, technology refers to a system with component elements or steps that are referred to as processes or unit operations. The baseline incineration technology system includes such processes as combustion and flue gas cleanup and, as parts of these processes, such unit operations as liquid storage, liquid pumping, air compression, agent and fuel atomization, combustion, flue gas cooling by water quenching, flue gas scrubbing with decontaminating fluid (and its recycle or destruction), flue gas dewatering, and gas blowing to carry the effluent gas to the stack. Thus, an evaluation of alternative technology systems must consider first the availability and capabilities of alternative processes and unit operations and then the effectiveness of combining them in an operating technology system. This kind of analysis is especially important in cases where available unit operations will only partially detoxify the chemical agent rather than fully destroy it and where they will convert chemical agent to other organic forms but will not convert all the carbon waste to acceptable wastes, such as CO2, sodium carbonate, or material roughly equivalent to sewage sludge. Chapters 6 and 7 address a number of the principal alternative unit processes and technologies that might be used to destroy chemical agent and energetics and decontaminate metal parts and containers. Chapter 8 considers possible combinations of these elements as potential alternative technology systems. However, before a destruction technology is used, weapons must be prepared for subsequent destruction. For example, munitions must be disassembled to separate agent from propellants and explosives; ton containers must be drained of agent. In addition, the destruction processes result in gas, liquid, and solid waste that must be processed before being released to the environment. This chapter covers some optional processes that might be used at the front end of the system to preprocess feed materials and at the back end of the system to treat, temporarily retain, or further prepare waste streams for

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions off-site shipment. It also addresses thermal treatment that could reduce waste gas volumes. The following types of operations are reviewed in this chapter: preprocessing unit operations: cryoprocessing and mechanical removal of energetics; heat treating of contaminated parts: pyrolysis and oxidation; and postprocessing and pollution control unit operations: drying, activated-carbon adsorption beds, and stack gas holdup. The postprocessing and pollution control operations examined here are more demanding than normal industrial applications because chemical weapons destruction must consider the potential extreme toxicity of any residual chemical agent in the waste stream. Commercial preprocessing and postprocessing liquid and solid handling, holdup, and pollution control processes do not. need special evaluation to be considered for use. (The pollution abatement system that removes acid gases and particulates was examined at a pollution abatement workshop held by the National Research Council's Committee on Review and Evaluation of the Army Chemical Stockpile Disposal Program on May 15-17, 1991.) PREPROCESSING OPERATIONS Although the development of alternative approaches has generally assumed the use of a front-end reverse assembly facility similar to that now being demonstrated at the Johnston Atoll Chemical Agent Disposal System (JACADS), certain variations of this front-end operation are possible. Cryoprocessing There are several possible cryogenic processes that would use liquified nitrogen to cool the still-assembled weapon below its brittle fracture temperature, which would be followed by the use of a mechanical method to break up the weapon. The resulting mixture of frozen agent and fractured metal parts and energetics would then be fed into a single rotary kiln incinerator. The NRC recently completed a separate study on this technology (NRC, 1991). One conclusion was that ''...although there exists a reasonably good chance that the cryofracture process can eventually be made to operate satisfactorily, the start up time for the proposed full-scale facility at Tooele might be extended over several years, and major modifications or even a

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions complete replacement of the currently designed incinerator and pollution abatement system might well be required.'' Mechanical Disassembly of Explosives, Propellants, and Solidified Agent The baseline reverse assembly process punches and drains chemical agent from the M55 rockets but does not separate the explosives and propellants from the contaminated metal parts. If explosives or propellants are left in place, any subsequent process must be able to withstand potential explosions of these components. In artillery projectiles and mines, explosives are removed and the agent is then drained. Ton containers are punched and drained; however, mustard gels and solids sometimes adhere inside the containers. Similar problems are presented in many other weapon demilitarization programs in which used explosives and propellants must be removed to avoid explosive conditions. The methods used in these programs to remove explosives and propellants from missile casings do add a data base that could be further developed if needed. For example, one such method uses a high-velocity water jet to break the explosives and propellants into small pieces that are then flushed from the system by the returning water stream (Fossey et al., 1991). This method is of interest because some of the alternative technologies can process slurried energetics (a watery mixture) and because it might be used to remove undrained agent. In this method, a high-pressure pump, operating near 15,000 psi, feeds a jet nozzle located on the end of a moveable lance. The lance is inserted into the open end of a rocket casing and is rotated to direct the high-velocity water jet to the surfaces of the propellant, causing it to break into small pieces, which are flushed from the system. Considerable work has been done on critical water velocities (high-velocity water jets have sometimes added too much energy, inadvertently igniting the explosive), the best water jet angles, the resultant forces acting on the lance, and the degrees of freedom in aiming the jet with existing equipment. Such an application of a water jet would result, in most cases, in water contaminated with agent that would need to be handled accordingly. In some circumstances, use of decontamination solution instead of water might be used to achieve decontamination of the remaining metal parts and containers and would reduce the problems of dealing with water contaminated with agent.

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions THERMAL TREATMENT Many processes require heating or cooling steps before or after the destruction operation. Conventional heat exchange methods are assumed to be available for most of these needs. Chemical agents can be destroyed by simple thermal decomposition through indirect heating without their reaction with oxygen in combustion or with other chemicals. Simple thermal decomposition results in the release of less exhaust gas, representing only the products of agent decomposition without the larger volume of additional fuel combustion products, excess oxygen, and nitrogen present in the baseline processes. However, the products of thermal decomposition would still require oxidation (in an afterburner) before release. This alternative approach is dearly applicable to the treatment of energetics and contaminated metal parts and containers. For bulk agent, the use of combustion heat provides a self-sustaining flame, and there appears to be less advantage to the two-step process of thermal decomposition followed by oxidation. Several indirect heating methods are available for heating contaminated metal parts to the required time and temperature conditions for achieving the 5X level of decontamination (i.e., 1000°F for 15 minutes; see Chapter 4). These methods include indirect gas firing and electrical resistive heating. (Plasma arc and molten-metal heating are discussed as principal alternative processes in Chapter 7.) Gas burners, electric heating elements, and heating chamber equipment are commercially available, although special designs would be required for chemical demilitarization. Special factors that would require consideration for the use of indirect heating include the following: remote operation: required for solids handling because of the presence of both agent and explosives; uniform heating: distribution must be ensured to treat all material to 5X criterion time and temperature; molten aluminum: provisions would be required to handle molten aluminum when decontaminating M55 rockets; and decomposition products: available data on the thermal decomposition of agents do not describe the quantities or nature of by-products. However, if oxygen is not present, the decomposition products must include a variety of only partially oxidized hydrocarbons (only two atoms of oxygen are present in the molecule of the agent GB). A subsequent oxidation process (afterburner) will still be required to transform the decomposition products, which can include hazardous organic chemicals, into wastes that are suitable for disposal.

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions A subsequent oxidation process could probably be easily incorporated into a combined thermal decomposition and oxidation system, because decomposition products of concern are likely to be gases under 5X treatment conditions. Such a system would have the advantage over an incinerator of a substantially lower rate of gas production. Equipment required for temporary gas storage and certification would be correspondingly smaller. POSTPROCESSING OPERATIONS Because any process can only transform agents from one chemical form to another, all processes have waste streams. These waste streams must be suitable for disposal to the environment or for reuse. Although many conventional processes are available for use in waste treatment, the nature of chemical agents invokes some special considerations and requires the use of protective measures that go beyond those of normal industrial practice. Chapter 4 addressed the need for effluent retention systems for dynamic processing units to certify the suitability of all effluents for disposal. Other postprocessing operations that could be used in a system for chemical demilitarization are described below. Solid Wastes The two major solid waste streams in chemical demilitarization axe decontaminated metal and salts formed from neutralization of the acidic products of oxidation (HF, HCl, H2PO4, H2SO4, and CO2). There are proposals to solidify organic wastes from chemical processes for subsequent disposal, thus avoiding the necessity for complete oxidation (Kalyon, 1992). Metal wastes. Chapter 4 discussed the need to use high temperatures to achieve full decontamination of metals (i.e., to the 5X level). The baseline process is designed to meet the 5X requirement of treatment at 1000°F for 15 minutes. The ability of alternative technologies to carry out this step is discussed in Chapters 7 and 8. Chemical decontamination to the 3X level would allow transportation and storage, but it is not currently believed capable of reaching the 5X level required for release of materials to the general public. If materials decontaminated to the 3X level can be disposed of in central hazardous waste facilities, the high-temperature 5X treating requirements could be modified, making construction and operation of the baseline traveling-grate metal deactivation furnace for large metal parts or any similar alternative unnecessary. Small metal parts would still require, along with energetics, a

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions high temperature for destruction and deactivation to meet the 5X criterion as currently carried out in the rotary kiln in the baseline technology. Salt wastes. The quantities of waste salts produced will vary with the agent feed rate and the processes used. As a theoretical minimum, if all carbon atoms are converted to CO2, the only salts produced would be those resulting from oxidation of fluorine, phosphorus, sulfur, chlorine, and nitrogen to acids, which could be neutralized by NaOH or Ca(OH)2. Table 5-1 presents the weight of salts formed from Ca(OH)2 neutralization of GB. If CO2 is discarded as a gas, the minimum produced would be 2.3 pounds of salts per pound of GB, with calcium phosphate representing the largest component (65 percent). For most systems, some CO2 will also be captured. If all the CO2 generated by agent oxidation was captured, calcium carbonate would be the major component; the total salt weight would approximately double to 5.1 pounds of salts per pound of GB. A process to capture all the CO2 would probably require excess base, and any CO2 from fuel combustion would also increase the total amount of salts. Decontamination fluid, when oxidized, would also add to total salt loading, as would the use of organic solvents such as alcohol or organic reagents such as ethanolamine. Total dry waste salts for GB, then, would probably be from 5.1 to perhaps 20 pounds per pound of agent. The character of the waste salts will influence their ultimate disposition. Notably, their solubility will be a major determinant of their acceptability for land disposal. If NaOH is used as the reactant to destroy agent, then the fully oxidized reaction products would be sodium carbonate, sodium fluoride, sodium phosphate, sodium sulfate, sodium chloride, and sodium nitrate. Although most of these sodium salts are relatively innocuous, all are quite soluble, requiring that land disposal methods take into account potential groundwater contamination. (There would be a greater potential for contamination by the more soluble chlorides if a destructive reagent containing chlorine atoms was used.) Sodium fluoride is considered a toxic waste and might present a special problem. Calcium fluoride, which has low solubility, would probably be a more acceptable waste product. Metals used as catalysts or in small metal parts, which might be mixed with the salts, could also require special handling. An alternative approach would be to produce the generally less soluble calcium salts, by substituting Ca(OH)2 for NaOH as the reagent when possible. The same result could be achieved by a later reaction step in which calcium ions would be substituted for the sodium ions in the waste salts. Such a method would convert the salts to generally less soluble forms and also regenerate the sodium ions for reuse in agent destruction. However, it would not solve the problem of generating soluble calcium chloride when mustard is destroyed.

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions TABLE 5-1 Salt Formation from GB Oxidation Products Management of CO2 Salt lb/lb GB CO2 release as gas CaF2 0.8   Ca3(PO4)2 1.5   Total 2.3 All CO2 captured CaF2 0.8   Ca3(PO4)2 1.5   CaCO3 2.8   Total 5.1 Drying of solids. Most chemical processes, such as the various forms of hydrolysis, will produce a wet, dilute slurry of salt that can then be converted to a wet cake by appropriate precipitation and/or filtering. The storage volume required to retain these more concentrated salts for certification for release is not usually excessive, but the residual moisture content is often sufficiently high so that it is economical to dry the material further before shipping it to its disposal destination. Standard commercial drying equipment (e.g., spray dryers) can be used but usually depend on mechanical techniques to bring the wet waste into contact with warm, dry, fresh air that is then released to the atmosphere. However, such a release is also an effluent stream, and if the material being dried has only been treated to the 3X level, the moist air discharge must also be treated as potentially contaminated. Thus, for present purposes, such systems might have to be modified to recycle the air through indirect heat exchangers, which would cool the air, condense the bulk of the water, and recycle the dried air stream to the dryer or send it to an afterburner. The condensed water stream would then also have to be treated as potentially contaminated and be recycled, preferably as part of the feed stream. The equipment and technology to perform these additional functions is readily available, but they would involve added complications to integrated system operations as well as added costs. Methods of shipping of by-product 3X salts. The standard method of shipping contaminated wastes involves packaging them in barrels or in larger

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions approved containers. The salts may be premixed in a matrix to further minimize the risk of leakage during and after transport. Various methods are possible, including mixing salts in concrete or plastic matrix material. As a further extension of this approach, it might be possible to use a chemical matrix that contains the reactive hydrolysis detoxification reagent. Control of Nitrogen Oxides Various oxides of nitrogen (NOx) are generated in high-temperature oxidation processes, both from the oxidation of nitrogen from the air and from nitrogen contained in the materials being oxidized. For agent disposal, the largest sources of nitrogen are VX and the propellants and explosives that are being destroyed. GB often contains small amounts of nitrogen compounds as stabilizers, but HD contains nitrogen only as minor impurities if at all. The operations at JACADS (MITRE Corporation, 1993) produced NOx concentrations of 200 to 500 ppm, which is within the applicable federal regulations but may exceed local or future limits. Several methods for controlling NOx are available and currently used with combustion systems. The most important is to reduce the temperature and residence time used for high-temperature oxidation process. However, doing so in equipment such as afterburners may also reduce the operability of the process and/or decrease the degree to which combustible air pollutants are destroyed, depending on the choice of destruction technogies. Thus, it may be necessary to add to the pollution control system one of several available processes for NOx destruction. Water Recycle Water is used in some alternative processes, as well as in the current pollution abatement systems; in any event, it is a by-product, along with CO2, of the final oxidation steps necessary to destroy all organic carbon. Thus, water must be discharged from the plant by some method. In current pollution abatement systems involving aqueous scrubbing, a significant quantity of water is usually discharged as water vapor along with any other effluent gas. If the gas is cooled for chemical adsorption or storage, the amount of water vapor that is discharged with gas emissions to the atmosphere is less than the water generated by the oxidation of the chemical agent. Thus, water discharge by some other method might also be required. Numerous commercial systems are available for the adequate cleanup of this kind of wastewater stream that allow the wastewater to be recycled to the feed streams for the destruction processes or to general use on the site.

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions 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

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions 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

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions 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).

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions 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

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions 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.

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions 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.

OCR for page 94
Alternative Technologies for the Destruction of Chemical Agents and Munitions 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.