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SHIPBOARD POLLUTION CONTROL: U.S. Navy Compliance With MARPOL Annex V 4 Long-Range Options: Competition for Incineration Alternative Destruction Technologies/Competitors for Incineration In Chapter 3, the committee discussed waste destruction by incineration and noted excellent results with large, heavy, expensive equipment. In the present section, the committee considers technologies that might replace incineration as the process of destruction. There are many possibilities (probably dozens) in this connection, and the committee will describe a few that illustrate alternative approaches; but the list of candidates included herein is far from exhaustive (NRC, 1993; U.S. EPA, 1993; Schwinkendorf et al., 1995). In general, the committee expects that development of any one of these technologies will take at least several years before commercial marine equipment is available and proven in use. It is too early to predict size, cost, and operating details. Even so, there is a clear-cut need for innovation in this area, and many groups are working actively toward this goal. Progress could be surprisingly rapid. In this connection, it should be kept in mind that incineration has the advantage of being the established leader at this time, and leaders tend to continue to improve and develop. Navy applications are demanding, however, and it may be difficult to predict which aspects of new technologies will produce winning entries. Pyrolytic Methods Pyrolytic methods are distinguished from oxidative methods even though the ultimate products of destruction are oxidized. Pyrolytic methods, as used for materials destruction, are two-stage processes in which the waste material is first pyrolyzed and then oxidized. This aspect makes it unlikely that any of the waste will escape destruction. Plasma Arc Thermal Conversion Technology Plasmas are highly ionized gases that can be brought to very high temperatures through coupling of electrical energy from a power supply. Whereas combustion temperatures rarely exceed 2,000ºF (1,100ºC), plasma temperatures range from 5,000ºF to 22,000ºF (3,000ºC to 12,000ºC) or higher. When chemical substances are subjected to temperatures in the plasma range, they are torn apart, i.e., reduced to atoms or fragments containing only a few atoms. This process is called pyrolysis. If waste is passed through a plasma arc, the materials are vaporized atomically and lose all memory of their former structure. As this “waste” passes out of the plasma region and cools, the metal and glass components form a slag or, alternatively, molten metal. The paper, cardboard, and plastic portion, consisting mainly of carbon, hydrogen, and oxygen, tends to form low molecular weight compounds such as the hydrocarbons—methane, ethane, and so on—and some related oxygenated species. This latter component is gaseous and may be used as low-grade fuel to recover some of the energy consumed in generating the plasma. The pyrolysis process differs from combustion. The temperature is much higher and oxygen does not participate in the reactions in a dominant way. The products of pyrolysis will be different from those of combustion, and the differences could be environmentally favorable. Oxidation must also be controlled to avoid formation of noxious compounds, e.g., dioxins. Pyrolysis products are usually burned in an afterburner. The vitreous slag resulting from plasma destruction of waste tends to occlude metals,
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SHIPBOARD POLLUTION CONTROL: U.S. Navy Compliance With MARPOL Annex V effectively removing them from the environment. Volatile metals from electrodes or feed stock will need remediation. A great deal remains to be done in characterizing plasma arc products, but there is hope of environmental advantage. The high temperatures of the plasma arc ensure that the reactions are very fast, and this allows for short residence times of materials being pyrolyzed. On this basis, the plasma arc processor might be made smaller than an incinerator with comparable throughput. The downside of the comparison with the incinerator is the required power source for the plasma arc machine. The Navy (Alig, 1995) has a demonstration program under way with the space and weight parameters shown in Table 4.1. Data for a large incinerator are included for comparison. Table 4.1 Plasma Arc and Incinerators for 1,000 lb/h Thermal Destruction NAVY DEMONSTRATION COMMERCIAL SYSTEM INCINERATOR Weight 63 tons (1) 93 tons (1) 70 tons (2) Volume 12,000 ft3 (2) 22,000 ft3 (1) 3,200 ft3 (2) Availability 2010 perhaps Now Now 1 From Navy briefings. 2 From vendor information. Another vendor offers 65 tons and 4,100 ft3. Thus, to match the size economy of current incinerators, plasma arc methods must achieve significant further size reductions. Note that the Diesel generator alone weighs 25 tons and consumes 1,700 ft3 of space. Diesel is a mature technology, and significant reduction of volume and weight is unlikely. Both incinerators and plasma arc machines need auxiliary apparatus, e.g., shredders, automatic feed, and ash or slag handling gear. Vitrification Vitrification is closely related to plasma arc. Waste is heated to about 3,000ºF by electrical current or by contacting an electrical discharge with the material to be destroyed. Organic materials are destroyed by pyrolysis and the products burned in an afterburner. A key feature of this technology is that inorganics are melted so that a liquid pool is formed at the bottom of the treatment chamber. When this melt is cooled, a vitreous solid mass is formed and elements contained therein are nonleachable by ground water. This is valuable when the waste is hazardous (specifically, radioactive), but the advantage for shipboard waste destruction is not so clear. Battelle Pacific Northwest Laboratories (Chapman, 1994; Surma, 1995) has extensive experience in research, development, and application of this technique to management of radioactive and other hazardous wastes. The technology has been successfully tested on medical wastes at a nominal throughput of 25 tons/day. Shipboard waste on the largest ships in the Navy amounts to about 10 tons/day. Commercial interests are planning a demonstration plant to treat municipal wastes, which are similar in composition to shipboard wastes. A Navy contract is being negotiated for a demonstration plant to handle Navy waste including hazardous materials. With suitable modifications, vitrification can probably be employed to destroy black water sludge. This technology is viewed as sufficiently advanced that major research is not required. Normal engineering and testing work remains for shipboard waste destruction applications. Flux addition may be necessary to obtain a stable glass. Proponents of the method see no major hurdles in applying vitrification to shipboard solid wastes.
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SHIPBOARD POLLUTION CONTROL: U.S. Navy Compliance With MARPOL Annex V Molten Metal Pyrolysis This technology is somewhat similar to the above pyrolytic methods. The waste stream is pumped into the bottom of a molten metal (iron, copper, or cobalt have been used) bath at 1,650°C (3,000ºF). The bath is made molten by passing an electric current through the metal. Organic materials are decomposed under reducing conditions and pass out of the melt as gases. They are subsequently oxidized and cleaned. The inorganics form a slag which floats on the metal pool and is skimmed off. Two commercial vendors have developed this process: the Elkem Multipurpose Furnace, and the Molten Metal Technology catalytic extraction process. The processes are very similar to techniques used in the steel industry. A number of substantial plants are under construction for the management of industrial and Department of Defense and Department of Energy (DOE) wastes. Application to waste streams similar to Annex V materials has not been undertaken, but a variety of feeds have been processed that are relevant to the Annex V problem. Oxidative Methods With oxidative methods, the waste stream is decomposed under oxidizing conditions, as with incineration. The products are fully oxidized and no afterburner is required. Temperatures involved are considerably lower than for the pyrolytic methods discussed above. Supercritical Water Oxidation Water above its critical point, i.e., above 374ºC and 220 atmospheres pressure, behaves very differently from water as we normally encounter the substance. Supercritical water is more like a gas than a liquid and is miscible with other gases, but not salts. Under supercritical conditions organic compounds and oxygen are both soluble in water and can react readily. Exposure time within the reactor is less than 2 minutes for very complete (> 99 percent) conversion. Shorter residence times, which might mean smaller reactors, may be possible by manipulating temperature and oxidant concentration. The supercritical water oxidation (SWCO) process (Modell, 1989; Tester et al., 1993; Peters et al., 1994; Shanableh and Gloyna, 1991), as applied to Annex V waste, would entrain shredded waste in water at concentrations of 1 to 20 percent. This mixture would be pressurized and preheated and then introduced into the reaction chamber for exposure to the oxidant (oxygen, air, or hydrogen peroxide). The temperature-time history of exposure is rigorously controlled. Organic materials present in shipboard waste should be largely converted to carbon dioxide and water. To the extent that sulfur, chlorine, and phosphorus are present, acids will be formed: sulfuric, hydrochloric, and phosphoric acids. In SCWO, these acids are converted to salts through the injection of sodium hydroxide, and processes to remove the salts must be carried out. Since the concentration of acid-forming elements is low in shipboard wastes, this problem can probably be solved. The process can be made self-sustaining by employing reaction heat to prepare the incoming waste stream. SCWO has been successfully employed at the bench and pilot plant levels to destroy a large number of organic compounds with high levels of destruction. Bench-scale destruction has been demonstrated for various mixed waste including paper, protein, and human waste. The committee is unaware of studies to determine SCWO parameters for the destruction of Annex V waste, but high destruction efficiencies for paper and related materials (Tester et al., 1993) suggest possible suitability for this application. Off gases are said to be free of noxious compounds found in incineration processes, e.g., dioxin. Up to this time, the only commercial application of SCWO is in the destruction of alcohols, glycols, and amines in aqueous wastes from a chemical plant. Development of plants to destroy army munitions and solid rocket propellants is being actively pursued. A pilot plant in Germany has successfully destroyed sewage sludges, cutting oils, and other waste products at a rate of a few hundred
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SHIPBOARD POLLUTION CONTROL: U.S. Navy Compliance With MARPOL Annex V pounds per day. ARPA has recently announced a program aimed at achieving a full-scale demonstration of a relatively compact land-based SCWO system for eventual shipboard deployment. This unit is designed to destroy hazardous waste. The 8 ft × 9 ft × 10 ft unit is expected to weigh less than 4 tons, and it is designed to destroy 100 lb of hazardous waste per hour. Assuming success of the ARPA program, it can be calculated that a facility to handle the shipboard waste of an aircraft carrier might require 1,200 to 3,200 ft2 of deck space. SCWO should be effective in destroying Annex V waste. Food waste, black water sludge, and other non-Annex V materials are well matched to the destructive capabilities of SCWO. The relatively large high-pressure reactor may or may not be acceptable on Navy ships. Other Oxidation Methods Ambient temperature partial oxidation of dissolved organics can be accomplished through the addition of ozone or hydrogen peroxide and ultraviolet (UV) light. This method has been used to treat lightly contaminated water in the United States, and ozonation is used in Europe as an alternative to chlorination for drinking water. The committee sees no application of this technology to Annex V waste streams. Wet air oxidation is another technology that is useful in special situations but shows no promise for Annex V waste. Molten Salt Oxidation Oxidation with air can be carried out in molten sodium carbonate above 900ºC. Acidic products react to form salts which dissolve in the molten bath. It can be applied to combustible solids, organic liquids, solutions, and slurries. The gaseous products may contain unoxidized waste that has passed through the bath, and an afterburner may be needed. The method has been employed on a small scale for over 40 years for destruction of military waste materials. The relatively low temperature of operation, as compared with incineration, minimizes the formation of nitrogen oxides. Application to Annex V waste has not been investigated. Disposal of the spent salt bath is a problem. Shipboard Compatibility It is important to recognize the many impediments that must be overcome if a practice is to be adopted by the U.S. Navy. It is unlikely that any other area of marine service makes comparable demands on candidate technologies. Table 4.2 summarizes the committee's assessment of shipboard compatibility attributes for the long-range waste disposal technologies discussed in this chapter. The attributes may be described as follows: Status of Technology: The developmental status of the technology: Has it been demonstrated? Laboratory, full scale, or production? If further development is required, has it been funded? Process Versatility: Identification of which shipboard wastes can be processed with this technology. Preprocessing requirements must also be considered. Process Density: Lb/h/ft2—waste-processing rate in pounds per hour normalized by the projected footprint of machinery required to apply the technology. Ship System Demands: Support system requirements for the machinery required to apply the technology, i.e., ventilation, cooling water, special containments, noise and vibration isolation, electromagnetic sensitivity or emissions, and external supply requirements. Installation Flexibility: Based on constraints on noise and vibration, electromagnetic emissions, space, weight, system demands, or other attributes noted, determination of which Navy ship classes the machinery required to apply the technology can reasonably be installed on? Ship Motion Effects: Equipment installed on naval ships is expected to operate with platform motion up to and including 45º roll and 10º pitch and permanent list of 15º in accordance with General
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SHIPBOARD POLLUTION CONTROL: U.S. Navy Compliance With MARPOL Annex V Specifications for Ships of the United States Navy (NAVSEA S9AAO-AA-SPN-010/(Gen-Spec). Evaluation of waste-processing equipment and technologies for their suitability in this environment. Process Sensitivity: Susceptibility of the process to contaminants in the waste stream or other outside influences such as temperature fluctuations and power loss. End Products: Identification of the process effluent and processed end product and their impact on the ship and the environment. Process Safety: Significant hazards to operating personnel or the ship in both normal and upset operations and anticipated casualties, i.e., high temperatures, explosions, toxic chemicals, biological contaminants, and electrical shocks. Projected Reliability: Evaluation to determine if the machinery required to apply the technology and the technology itself employ known and consistent concepts and equipment. Based on this evaluation, are the technology and supporting equipment expected to perform repeatedly and reliably for suitable periods of time? Projected Maintainability: Evaluation to determine if the equipment and technology maintenance requirements are compatible with the capabilities of the ship's crew in the at-sea environment. Projected Controllability: Evaluation to determine if the equipment and technology can be started up quickly and easily. Can it be shut down quickly and safely in both normal and casualty conditions? Can it be operated continuously with minimal adjustments? Is it user friendly? Does it lend itself to automation? Discussion It is clear that a number of candidate techniques are available for the destruction of Annex V wastes. Products of the various methods are similar. Control of noxious compounds can be handled in each method. Details of installation costs and size of the equipment were not available to the committee. Present models do not appear to be competitive with incineration in the area of shipboard waste disposal, but further development and demonstration projects are under way that could change the picture. The committee suggests that it will be years before a clear-cut replacement for incineration of Annex V wastes is established.
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SHIPBOARD POLLUTION CONTROL: U.S. Navy Compliance With MARPOL Annex V Table 4.2 Shipboard Compatibility Attributes for Long-Range Waste Disposal Technologies Attribute Supercritical Water Oxidation Molten Metal Technology Plasma Arc Thermal Conversion Technology Vitrification Molten Salt Oxidation Status of technology One commercial plant operating ashore for a particular waste ARPA program starts mid-95 to develop hardware for eventual shipboard application studies (30month study) Commercial plant construction has begun for chemical and nuclear wastes Technology being marketed for commercial applications to waste disposal Naval application demos planned for waste Feed development needed Mature technology Pending municipal waste stream demo with industry Application development required Navy shoreside hazardous waste demo being funded Small-scale use since 1950s Large R&D program in 1990s DOD and DOE mixed waste Process versatility Solids pumpable as slurry Organics destruction Inorganics decontamination and concentration All shipboard solid waste can be processed Concentration of liquid waste streams required Shred solids All shipboard solid waste can be processed Concentration of liquid waste streams required Shred solids Concentration of liquid waste streams required Shred solids Large capacity possible Combustible solids Organic liquids Process density 0.5 to 1.4 lb/h/ft2 Not known 0.5 to 1 lb/h/ft2 100 to 300 lb/h/ft2 organics 15 to 30 lb/h/ft2 inorganics Not known Ship system demands Cooling water Electric power Discharge pump (brine) Ventilation Fresh water feed Cooling water Electric power Ventilation Stack gas treatment Cooling water Need inert gas Ventilation Stack gas treatment Electric power Cooling water Ventilation Need stack gas treatment Electric power Salt resupply Low-pressure air or oxygen Cooling water Ventilation Stack treatment Fuel/electricity for startup Installation flexibility Surface ship only Surface ship only Surface ship only Surface ship only Surface ships only Ship motion effects Manageable Need design development to minimize effects of ship motion on molten pool Need design development to minimize effects of ship motion on molten pool Need design development to minimize effects of ship motion on molten pool Need to minimize effects on molten salt pool Process sensitivity Streams that result in salt formation require additional in-reactor technology Must be maintained molten, or long startup required Unknown High temperature must be established but can be accomplished quickly Unknown Must be maintained molten, or long startup Keep molten salt or extended startup Salt disposal End products CO2 H2O N2 Solid salts possible Metal ions Combustible gas (fuel gas) HCl Slag Volatile metals Slag Stack gas Combustible pyrolysis gas Vitrified solid (glass) Stack gas Must burn off combustible gas (fuel gas) Combustible pyrolysis gas CO2, H2O, N2, O2 Salt particulates Spent salts Process safety High pressure water/steam May require injection of caustic High pressure O2 Very high temperatures Handling of hot slag Flammable product gases Very high temperature High voltage Molten slag Flammable product gas Very high temperature Handling of molten glass product Flammable product gas Hot molten salt Possible superheated vapor explosion Spent salts Projected reliability High pressure slurry feed pump Reactor materials (corrosion) Vessel cleaning of scale/salts Unknown Unknown Few moving parts High temperature risks Unknown Projected maintainability Mechanically able to service anticipated equipment Vessel life from corrosion standpoint unknown Unknown Unknown Unknown Unknown Projected controllability Auto control can be applied Control temperature pressure and effluents Unknown Development needed Shutdown not a problem Not complicated Unknown
Representative terms from entire chapter: