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7 Alternatives for Future Coal Waste Disposal Coal system activities range from mining, to processing, to utilization, to disposal, and involve resources, transportation, and the environment (Figure 1.2~. Redesigning systems like this to eliminate or reduce substantially waste streams is the first priority of industrial ecology (Sidebar 7.1~. In the coal system, this can be accomplished by identifying the appropriate point in the process for reducing or removing noncombustible material. Currently, waste is created during the mining and processing stages and disposed of either in coarse refuse piles or fine waste slurry impoundments. Consideration of the entire coal system concept leads to the identification of alternatives to impoundment disposal of fine coal waste by reducing the amount of waste generated, utilizing waste, or disposing of waste elsewhere. These options attempt to minimize the waste stream, transfer it to another part of the coal system, or redirect it, respectively. Fine coal waste from the preparation plant can be reduced or eliminated during both the mining and the preparation phases. For example, selective mining reduces the amount of noncombustible material. However, many of the coal seams currently mined are of sufficiently low quality that they would not be mined if this were required. Options for preparation plant waste elimination are more numerous during coal preparation. Cleaning requirements are usually imposed on the mine operator, whose coal cleaning and waste disposal may be constrained by site and environmental considerations. One option is to transfer cleaning responsibilities to the power plants (the majority of end-users). Another is to add fine coal waste to the cleaned coarse coal product. Dewatering the slurry product solidifies (to a degree) and reduces the volume of the waste. Since 90 percent of the coal mined in the United States is used in power plants (Freme, 2000), significant benefits can be achieved if advances in power plant technologies are integrated into the coal system components. Emerging power plant technologies, developed around fluidized-bed combustion with appropriate flue gas desulfurization technology, allow 131
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132 COAL WASTEIMPOUNDMENTS burning of low value coals, i.e. coals with a large amount of noncombustible material (ash). Another technology is the use of coal water slurry as Mel for traditional combustors or gasifiers. There are several alternatives to disposing of fine coal waste in impoundments, such as disposing of it in surface fills and underground workings. However, these options are often limited by factors such as topography, cost, and safety. The ramifications of all potential options may be explored to create a Type III industrial ecology model for the coal/electric power industry (Sidebar 7.1~. Figure 7.1 shows that through selective mining and other means, coal preparation needs can be restricted to Level 3 (Sidebar 7.2) and below, thus producing only limited fine coal waste slurry. If coal is cleaned to Level 4, the options of waste utilization and alternative disposal locations remain. Finally, existing ponds can be reclaimed, and the fine waste can be used in a similar manner. SIDEBAR 7.1 Industrial Ecology In the past, many industries operated as individual entities; however, the individual operations can have widespread impacts. The philosophies and approaches of industrial ecology may be used to integrate an individual industry, such as coal mining, with natural ecosystems and other industries. Industrial ecology is defined by Graedel and Allenby (1995) as a rational way for humans to maintain their existence with changing economies, cultures, and technological capabilities. Individual systems must work with each other to optimize the total materials cycle, including resources, energy, and capital. In the biological world, metabolism is the key process for life, for ecological balance, and for providing an increased capacity for living things. When the ecological balance is disturbed, species perish or mutate until a new balance is established. Industrial metabolism adapts the concept to the industrial world. Related industries in the industrial system are designed to work together to imitate or mimic the metabolic process. This process does not exist in nature. Jelinski et al. (1992) present three models of industrial ecosystems based on this analogy (see figure next page). In Type 1, the flow process is unidirectional from resources through consumption to waste. With time, the system's resources will be depleted, and its wastes will overwhelm it. In Type 11, internal cycling loops are developed, leading to limited input of resources and limited waste. This system is also not sustainable because input flows to waste in only one direction. In the ideal Type 111 system, industrial processes are similar to the biological ecosystems model, and full cyclicity is achieved.
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ALTERNATIVESFOR FUTURE COAL WASTE DISPOSAL Unlimited ~ Ecosystem resources ~ ~ component Ecosystem / / component`\ \ Energy and / ~ \> \ limited ~ Ecosystem Ecosystem resources \;ompor ent componenj/ Energy Limited resources — ·( / \ cosystem component / Ecosystem / component `\ Unlimited ) waste \ Ecosystem component / M—1~ ~ or grower j manufacturers processor ~ · Consumer - ~ Limited waste \ Limited · waste Reprinted with permission from Jelinski et al., 1992. Copyright 1992 by J.L. Jelinski. 133
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734 Seie~i~ . . mI 1ng Level O,1 Comae resect Non-sele~i~ ~ mining al 2~ ~ ~ Coal Surface disposal Advanced and con~n1ional pow> rants Coal combustion / by-p~ducts / utilizing / Haste Disposal Impoundment remining Underground disposal Fine coal cleaning o!Iy Fine waste / / FIGS 7.1 Coal system: Dining, prep ion, utHiz~ion, residues, Ed dispossL
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ALTERNATIVES FOR FUTURE COAL WASTE DISPOSAL 135 However, these options for impoundment alternatives raise other issues. For example, although the utilization of waste as a fuel eliminates the need for disposal of fine coal waste from the preparation plant, the burden of generating and disposing of coal combustion by-products is transferred to the power plant. Although this material has been utilized in a number of innovative ways (e.g., construction aggregates, synthetic soils, neutralizing agent), whether it is truly benign has been questioned. Another question is the safety and engineering aspects of alternative disposal locations, such as underground injection of slurry. Proven technologies can abet some of the issues with slurry disposal. To make this a reality, major institutional, organizational, environmental, and business issues must be addressed to encourage a shift from the traditional mining, processing, and utilization practices to an approach based on industrial ecology. Several alternatives are outlined in the sections that follow. REDUCING OR ELIMINATING SLURRY GENERATION Of the more than 1 billion tons of coal mined in the United States, only about 350400 million tons are cleaned in wet coal-processing circuits (C. Raleigh, CQ Inc., personal communication, 2001~. The opportunities for reducing slurry volume include mining and coal processing alternatives. Modern methods of surface and underground coal mining offer only a limited possibility for quality control during mining. Mining operations can be planned to extract coal from the best quality seams and minimize dilution with noncombustible material. This approach is commonly used in both surface and underground mining, especially for coal in the western United States. However, it is more difficult to apply in the eastern United States, where the highest quality seams have already been mined. Run-of-mine coal from both high and low quality sources can be blended to make a product of direct marketable quality. When coal is cleaned in wet-processing circuits, a fine waste stream containing water, fine coal, and noncombustible particles (ash) is produced in which the percentage of each depends upon the level and efficiency of the fine coal cleaning methods employed (Sidebar 7.2~. Slurry volume can be reduced by improving fine coal recovery and minimizing the mass of solids for disposal. The slurry volume can be further reduced by dewatering, which increases the proportion of solids to water. The ability to do either or both of these depends on the method of extraction, the amount of slurry dilution, the characteristics of the coal (e.g., the hardness of the coal, which affects the
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136 COAL WASTEIMPOUNDMENTS SIDEBAR 7.2 Levels of Coal Preparation There are five levels of coal preparation. Level The coal is not cleaned, and run-of-mine coal is shipped directly to the customer. Level 1 The run-of-mine coal is crushed to below a maximum size, and undesirable constituents are removed. The product of Level 1 preparation is commonly termed "raw coal." Level 2 The product from Level 1 is sized as coarse and fine coal. The coarse coal is cleaned to remove impurities; the fine material is added to the cleaned coarse coal or marketed as a separate product. Level 3- The fine product from Level 2 is sized into two products: intermediate and fine. The intermediate product is cleaned to remove impurities. The fine material is added to the cleaned product or marketed separately. Level ~CIeaning is extended to the fine material from Level 3. SOURCE: BTU Magazine, 1982. size and amount of fine particles produced), and the local geology (e.g., abundant clay in adjacent strata can produce a refuse stream that is more difficult to dewater). It has been nearly 20 years since dry coal preparation methods were used in the U.S. coal industry. According to Couch (1991), in the early 1960s, dry coal cleaning accounted for about 10 percent of all coal that was cleaned, but since then it has dropped to less than 1 percent. Dry cleaning is usually accomplished with "air jigs" or "air tables" using oscillating and fluidized bed principles. Most of the dry methods require closely sized and moisture-free feed. A combination of factors associated with the dry methods particle size, dust, transport, health and safety, noise, and the better performance of wet processes have contributed to the near abandonment of dry coal cleaning processes in recent years. The increased use of water for dust control in underground mines, and the increased efficiency of wet cleaning methods have continued the sharp decline in the use of dry cleaning methods at the mines. However, dry coal preparation methods, which do not create the same disposal challenges as slurry waste, can be effective in areas where the water supply is restricted. Currently, most coal preparation plants recover fine coal only from the size fraction greater than 100 mesh (150 micrometer). Typically, this is done with water-only cyclones, spirals, or both for 16 x 100 mesh (1.0 x 150 micrometer) material. Particles smaller than 100 mesh (150 micrometer) =
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ALTERNATIVESFOR FUTURE COAL WASTE DISPOSAL 137 account for 3 to 7 percent of the total plant feed, of which coal may comprise as much as 50 percent (R. Honaker, University of Kentucky, personal com- munication, 2001~. These fine coal particles may be untreated, partially treated, or fully treated. With no treatment, the fine material, usually smaller than 100 mesh (150 micrometer), is sent to a thickener and then pumped to a slurry impound- ment. Alternatively, the fine coal and refuse could be blended with the coarse coal product and sent to the power plant. This feed can be either burned directly or further cleaned in the power plant after pulverization; advances in magnetic and electrostatic separation hold promise for dry cleaning the coal at this stage. In fact, pulverization liberates more of the mineral matter in coal. Dry cleaning with magnetic and electrostatic separators has shown encouraging results (Oder et al., forthcoming). However, this process still generates a waste stream (albeit not a slurry) that requires disposal. Partial treatment utilizes classifying cyclones to remove coal particles to approximately 325 mesh (45-micrometer) size. In full treatment, which theoretically captures all of the fine coal particles, the fine waste from the cyclone is subjected to flotation. As air bubbles rise through the flotation tank, the coal particles attach themselves to the bubbles and are carried to the top of a column of water. Although the recovery of all or most of the coal fines will not eliminate the need for slurry impoundments, it will reduce the required disposal volume. At the same time, by increasing the clay content, a slurry that is more difficult to stabilize may be produced. In contrast, dewatering the refuse stream could eliminate the need for slurry impoundments by changing the strength properties of the waste material, although disposal of the resulting dewatered waste raises other issues. Dewatering employs either sedimentation or filtration or both. In sedimentation, the liquid is constrained, and the solid particles move freely. This results in clarification of the liquid and thickening of the remaining slurry. In filtration, a medium constrains the particles while the liquid flows through. This is accomplished by screening and centrifugation (Osborne, 1988). In coal preparation plants, wet refuse is usually sent to a thickener (Figure 7.2~. The underflow Mom the thickener (30 to 35 percent solids by weight), the fine waste stream, is sent to a slurry impoundment. Deep cone or other paste thickeners produce an underflow with a higher solids content than a conventional thickener. Their steep-sided deep cone construction takes advantage of the high differential pressure applied by the depth of solids to produce a paste (Steve Slottee, Eimco Process Equipment Company,
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138 Walkwav - COAL WASTEIMPOUNDMENTS Steel tank flat bottom in\ Water level · Feedwel/ 7~ Cone scraper ~ Discharge cone . · Feedwell _ _ ,Arm Bedded-in ~ Steel tank bottom and side Concrete tank sloping bottom T 1' _ Walkway ~ as\ Water level ,Arm Cone scraper ~ Discharge cone FIGURE 7.2 Thickener tank design. Reprinted with permission of the Society for Mining, Metallurgy, and Exploration, Inc., www.smenet.org. -
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ALTERNATIVESFORFUTURECOALWASTEDISPOSAL 139 personal communication, 2001~. Deep cone thickeners produce dewatered paste in the range of 65 to 75 percent solids by weight (Osborne, 1988~. While this material still requires disposal, its volume is less than that of unthickened slurry. Four types of filtration devices are used in coal preparation: gravity, vacuum, pressure, and centrifugal. Although gravity and centrifugal methods are used extensively, vacuum and pressure filtration methods offer the greatest potential for dewatering material smaller than 100 mesh (150 micrometer) that typically leaves the conventional thickener as underflow. Three basic types of vacuum filtration devices are rotary drum, rotary disc, and horizontal belt or disc, depending on their filter configuration type. With all of these devices, the feed is applied to the filter medium under vacuum. This draws the water through the filter medium while retaining the solids on the surface. The filter cake is then removed either by a burst of air pressure, mechanical scraping, or both. Vacuum filters produce a filter cake with a moisture content of 20 to 30 percent (Leonard, 1991~. Pressure filtration devices are classified as either batch or continuous press. Batch devices, such as the plate and frame filter press, have not been widely accepted in the U.S. coal industry because they operate discon- tinuously (Osborne, 1988~. However, solids recovery is high and effluent water is clear. Belt filter presses operate continuously and are more widely used than batch methods (Figure 7.3~. Belt presses produce a cake with a moisture content reportedly in the range of 20 to 30 percent (Osborne, 1988~; however, in practice, moisture content ranging from 35 to 40 percent is more common. Belt filter presses currently produce a dewatered product that still must be disposed of behind a retaining structure. Hyperbaric pressure filters, a fairly recent development, combine the filtration technology of disc or drum filters and the low moisture levels of discontinuous plate and frame presses. The specific solids throughput of a hyperbaric filter is several times that of a vacuum filter or other batch- operation pressure filters (B. K. Parekh, University of Kentucky, personal communication, 2001~. Chemical additives are almost always used in conjunction with any dewatering mechanism. Most of the mechanical methods (thickeners, filters, and presses) rely on chemical additives (usually flocculants) to expedite and enhance the separation process. The additives represent a significant operating cost, but dewatering would be largely ineffective without them. Thermal drying is an effective way of removing moisture, but it is very expensive and energy intensive and is currently used only for drying fine coal. In addition' it entails the environmental cost of air pollutants produced =
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ALTERNATIVES FOR FUTURE COAL WASTE DISPOSAL 141 during the drying processes. Other dewatering methods being developed include ceramic capillary filters, electro-acoustic dewatering, microwave dewatering, vacuum pressure hybrid filters, and pulsating vacuum filters (B. K. Parekh, University of Kentucky, personal communication, 2001~. Dewatering technologies represent one alternative to reduce the volume of waste deposited in coal slurry impoundments, but they do not eliminate the need for an impoundment. Many dewatering technologies are currently available for specific applications, though none is likely to be universally applicable. The committee believes that equipment vendors' current research and development will lead to improvements in these technologies and that operators of coal waste impoundments should monitor them carefully. DIRECT UTILIZATION OF SLURRY Slurry refuse can be utilized directly for power generation, either in conventional boilers or with advanced combustion and gasification technologies. These technologies have the advantage of turning a waste into a resource. Some of them can reduce cleaning requirements for coal in the preparation plant, but the use of low quality coal feed will increase the amount of waste generated at the power plant. Conventional Pulverized Coal-Fired Boilers Direct utilization of fine coal waste in conventional pulverized coal- fired boilers is an important alternative to its disposal in an impoundment. This does not require a significant change to the system of mining, cleaning, and burning coal. However, use of this material presents significant challenges to existing boilers, because the moisture content is high and the heating value and trace element quantities are inconsistent (Harrison and Akers, 1997~. If the fine waste material alone does not meet the required specifications for end-users, it can either be combined with a variety of other feeds, such as cleaned coal or biomass, to achieve the desired filet consistency or be agglomerated to improve handling. Fine coal is difficult to handle, even when dewatered, because it clogs equipment and is dusty and explosive. Agglomeration technologies can reconstitute the fine coal by briquetting, pelletizing, or extrusion and can solve the handling and transportation problems. The advantages and disadvantages of the various forms of agglomeration are summarized in Table 7.1. Which agglomeration technology is appropriate depends on the
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154 COAL WASTEIMPOUNDMENTS Even at this rate several cells must be permitted and in varying stages of operation at any one time. A variation of slurry cells is dewatered fine refuse cells. Mechanically dewatered fine refuse can be placed in bermed cells. However, the combined costs of dewatering and cell construction prevent this method from frequent use in the coal industry. Combined refuse disposal is another option. Combined refuse refers to fine refuse from the static thickener that has been mechanically dewatered and combined with coarse refuse for disposal on the surface. Depending on the percentage of coarse and fine refuse and the moisture content of each, the moisture content of combined refuse will range from 15 to 20 percent. This material must be transported mechanically, since it is too dry to be pumped. It is deposited in lifts approximately 2 feet thick, then graded with a bulldozer, and compacted to create a stable surface. Once the fill reaches the designed level, it is covered with 4 feet of soil and revegetated. Combined refuse fills require underdrains to capture any water that leaches from the fill, and perimeter drains to prevent runoff from entering it. Problems associated with this method of disposal include the frequent inability of mechanical dewatering to produce the moisture level required for disposal of a combined refuse material (David Carris, J. T. Boyd Company, personal communication, 2001~. Because the moisture and clay content are still fairly high, trucks or conveyors cannot handle the material easily, and compaction is difficult. Davies and others (1998) report using small amounts of cement to create a more stable refuse material. They found that the addition of 2 to 4 percent cement, by weight, results in a material that can be compacted using standard procedures. Small amounts of fly ash and lime have also been used to stabilize combined refuse. In addition, this method is fairly expensive because of the high cost of mechanical dewatering and the potential need for stabilizing chemical additives. The need for additives may be seasonal and depends on climatic conditions. Finally, the method is best suited to flat land. Co-disposal, developed and practiced primarily in Australia, involves the combination of fine refuse from the static thickener with coarse refuse (Williams et al., 1995~. Since the fine refuse is not mechanically dewatered, the combination has a fairly high moisture content that allows the combined material to be pumped to the disposal area. The main difference between this material and conventional slurry is the presence of coarse refuse in the mix. The advantages are that it requires less total storage volume than separate fine and coarse disposal methods, and the refuse stabilizes more quickly than typical slurry.
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ALTERNATIVES FOR FUTURE COAL WASTE DISPOSAL 155 However, co-disposal does not eliminate the need for a starter impoundment structure. After an initial dam is built, the mixed beach sediments, settling out of the co-disposal material can be used to construct the embankment for above-ground storage. Ideally, the fine particles fill the voids between the coarse refuse fragments. As the mixture is discharged, it forms a steep beach sloped at approximately 10 percent. This slope decreases to approximately 1 percent at the end of the mixed beach. At this point, the sediment consists mainly of fine coal suspended in ponded water in a small impoundment. The success of this method is closely linked to the ratio of coarse to fine particles. It also depends on gradation of the refuse (large gaps in particle size are not acceptable) and proper particle shape (angular or "platy" particles cause problems). Since the impounding structure is raised by deposition of mostly coarse material, it does not compact as the structure increases in elevation. This method has been used primarily in sparsely populated areas with low annual rainfall. Questions remain about its suitability for steep hills with high annual rainfall. Unlike a conventional slurry impoundment, which contains only fine refuse, the co-disposal system places all refuse (both coarse and fine) in a slurry and deposits it behind an impounding structure. Therefore, even though the refuse dewaters more quickly and forms a stable bench, it requires more impoundment storage volume than an impoundment designed only for fine refuse. So, for steep terrains, this factor negates the advantage of less total storage area by actually requiring more material (both coarse and fine) to be placed in an impoundment. Its use would hinge on whether increased stability of the refuse outweighs the additional volume of the impoundment. If an effective dewatering approach, such as paste thickening, is used, the resulting waste can be disposed of by thickened high-density residue stacking (tech Brzezinski, LSB Consulting Services, personal communi- cation, 2001~. Deep cone paste thickeners produce a homogeneous, non- segregating paste with a solids content of approximately 60 percent. The degree of dewatering is determined by the pumping capabilities. Under controlled conditions, the paste can be deposited in thin layers over the disposal site at uniform slopes of 2 to 5 percent arid does not require an impoundment structure. This method is most suitable for homogeneous residues of fine gradation, where the thickening process prevents segregation of the coarse and fine particles during transportation. Thickened high-density residue stacking was developed more than 20 years ago to handle red mud tailings generated by alumina plants. It has been used for approximately 10 years for disposal of gold and base metal tailings; =
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156 COAL WASTEIMPOUNDMENTS has recently found application at power plants for disposal of ash; and has seen limited use in the coal industry for the disposal of fine coal refuse. Although rainfall does not reconstitute a properly formed paste, and erosion is not excessive on typical stacks with 2 to 5 percent slopes, this method is best suited to areas of low rainfall and high evaporation. Under these conditions, the surface of the stack can be traversed by bulldozers within days of final placement. Regardless of the amount of precipitation, perimeter drains are necessary to catch runoff and divert it around the stacks. Underdrains are also needed for stability to prevent an increase in the phreatic surface. Three considerations land availability, steep terrain, and cost hamper applying unsupported thickened high-density residue stacking to fine coal refuse disposal. This method is best suited to areas where the slope of the land is less than 5 percent. Secondly, although the technology is based on a significant modification of the standard thickener, these thickeners are much deeper and require a longer residence time. Therefore' the lower throughput rate of deep cone thickeners compared with that of standard thickeners may significantly affect the economic feasibility of the method. Underground Disposal In 1975, the National Research Council conducted a study entitled Underground Disposal of Coal Mine Wastes, which evaluated the technical and economic feasibility of several underground disposal methods; namely, pneumatic backfilling, hydraulic backfilling, mechanical backfilling, hand packing, controlled flushing, blind flushing, and pneumatic flushing. Much of the study drew heavily on European experience, where backfilling has long been used to control subsidence. The backfilling methods emphasized the use of coarse refuse and did not specifically address the issue of fine refuse or slurry; only the hydraulic methods dealt specifically with fine refuse. The report did not discuss pneumatic flushing of fine coal waste, but it did indicate that this method had been used to inject fly ash into underground mines. The two primary methods for injecting fine coal refuse into underground mines are controlled flushing, where the underground workings are accessible, and blind or uncontrolled flushing, where the underground workings are abandoned or have caved in. The 1975 NRC report found that a number of underground disposal methods were technologically feasible at that time; however, none was universally economically feasible. The optimal solution varies from site to site. In addition, the question of workers' health and safety was raised when
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ALTERNATIVESFOR FUTURE: COAL WASTE DISPOSAL 157 considering injection into active mines. The report recognized that safe and nonpolluting disposal is the operator's responsibility. If regulations are imposed that exceed this obligation, society must recognize its responsibility to avoid inequitable distribution of added costs to some operators. Under the right set of conditions, underground disposal of slurry is an attractive alternative to impoundments and is currently being used where suitable opportunities exist (Table 7.2~. Slurry injection creates additional factors that must be considered. Many issues related to underground injection of slurry are independent of the method of slurry injection. For example, it is essential to have an adequate supply of water. This is especially true when water is not being recaptured from the underground workings. Also, it is important to keep the solids content below 20 percent, and preferably in the range of 10 to 12 percent. Additional common issues include surface ownership, permits, surface layout, and surface drainage. In most cases, the right to mine coal is obtained after leasing the mineral rights from the owner, but often, the rights to the surface have been severed. Therefore, a lease to mine the coal does not automatically entitle the mine operator to inject coal waste back into the voids created by the mining process. Unless the mining company owns both the surface and the mineral rights, the operator must obtain the landowner's permission to inject coal waste back into the mine. In Kentucky, the state regulatory authority has determined that a company cannot inject coal refuse into an underground mine for which it has the mineral rights unless all of the surface owners above the underground mine approve of the plan. In areas where multiple ownerships are common, this policy seriously limits the areas available for underground injection. Under SMCRA, each state's regulatory authority has responsibility for issuing permits to inject coal waste into underground mines. The permit application must address such issues as: Source and quantity of waste, Area to be backfilled and the method to be used, Approximate percentage of the voids that will be filled, Design of underground bulkheads, Potential influence on any active underground mines, Source of the water to be used. Methods of dewatering the backfill, Amount of water that will remain underground, and Water treatment system that will be used.
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158 COAL WASTEIMPOUNDMENTS Underground injection also requires MSHA's approval. Depending on the state, this can be handled independently or through a joint approval process. Finally, underground injection of slurry requires approval by the regional EPA office, whose primary concern is groundwater quality. States may interpret the requirements of the Safe Drinking Water Act differently for example, West Virginia limits the injection of any slurry from circuits that employ petroleum products, such as those used in froth flotation. In addition to obtaining permission from the surface owners to install injection wells, the operator must obtain the right-of-way for the pipeline to deliver the slurry to the boreholes (Marshall Hunt, Consol, personal communication, 2001~. Variable terrain can impose severe pumping problems if pipelines must be laid over hills and down into valleys to access injection sites. The accuracy of mine maps, with regard to underground workings and surface surveys, is important if the slurry boreholes are to intercept the underground workings in the desired location. Filling above-drainage mine workings with slurry may increase hydraulic head on the coal barriers and result in a blow-out, making evaluation of mine workings above a surrounding stream valley critical. The mine maps must be evaluated for accuracy, and the underground barriers for adequacy to contain the slurry. Mines below the surrounding natural drainage level offer more secure underground disposal sites. Blind flushing is used in mines where access has been obstructed, such as by roof falls. ~ this method of slurry injection, the underground cavity may be dry, partially filled with water, or completely filled with water. The volume of the voids is estimated from old mine maps and any other available, relevant data. Since it is nearly impossible to determine how much slurry a borehole will accept, a series of holes is drilled; when one borehole becomes clogged, injection moves to the next. The sluny is pumped to the borehole and injected into the mine at a relatively high velocity. Once the slurry leaves the turbulent area at the bottom of the borehole, the coarser material will settle. In flooded mines, the injected slurry will displace the water, which often results in a new or increased discharge elsewhere (Marshall Hunt, Consol, personal communication, 2001~. Depending on the quality of the displaced water, treatment facilities may have to be upgraded to handle the additional volume, or the mine pool may have to be pumped to avoid discharge at an undesired location. Paste backfilling has been demonstrated in other types of mines. Although it has not been used to dispose of coal waste, it may be possible to extend the technology to this application.
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ALTERNA TILES FOR FUTURE COAL WASTE DISPOSAL 159 Controlled placement is used where underground workings are accessible. This method has a number of inherent advantages over blind flushing. First, the engineer can verify the accuracy of the maps, inspect the condition of the openings, and better estimate the available space. Second, and possibly more important, bulkheads can be constructed to control the direction and extent of flow. Finally, collection and reuse of the water is more practicable. Once areas have been identified and properly confined, controlled flushing can proceed similarly to blind flushing. However, in controlled flushing, the slurry can be delivered to the underground opening by a pipeline that enters the mine through a borehole or other opening and then extends laterally through the mine to the desired location. This allows the operator to begin filling the most remote part of the mine and then to withdraw the pipeline in stages as the voids are filled and sealed. The advantages of this method are greater certainty of the location of the slurry, better utilization of available space, and better reuse of the water needed to transport the refuse. This method also protects against subsidence. One of the main concerns is the safety of workers in the mine receiving the slurry and in any adjacent, down-dip mines. The committee concludes that although there are alternatives to disposing of coal waste in impoundments, no specific alternative can be recommended in all cases. Acceptable alternatives are highly dependent upon regional and site-specific conditions. Also, the alternatives that have been identified are in varying stages of technological development and implementation. One of the factors limiting implementation to this point has been the cost associated with the various alternatives. Additional research is needed to develop these alternatives further and to evaluate the economics of these processes. The committee recommends that a screening study be conducted that (1) establishes ranges of costs applicable to alternative disposal options, (2) identifies best candidates for demonstration of alternative technologies for coal waste impoundments, and (3) identifies specific technologies for which research is warranted. Input from MSHA and OSM regarding regulatory issues will be valuable to such a study. REMINING SLURRY IMPOUNDMENTS While coal waste impoundments have generally been viewed as permanent disposal sites, there may be situations in which impoundments can be a resource (Sidebar 7.6~. In the past, recovery of fine coal was not as efficient, resulting in many older slurry impoundments containing significant _
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160 COAL WASTEIMPOUNDMENTS SIDEBAR 7.6 Pond Recovery Facility Ginger Hill is a coal slurry recovery facility in western Pennsylvania that produces approximately 300,000 tons per year of synthetic fuel (Akers et al., 2001~. A dredge is used to extract the fines from the impoundment. The slurry is pumped to a surge tank and then into the cleaning plant. The coal is classified at 150 mesh (106 micrometer) using 15-inch classifying cyclones. Particles larger than 150 mesh (106 micrometer) are cleaned using water- only cyclones and spirals, with an additional classification of the fine coal product by a two-stage Van sieve. Material smaller than 150 mesh (106 micrometer) material is classified using a 6-inch classifying cyclone at 270 mesh (53 micrometers). The 150 x 270 mesh (106 x 53 micrometer) fraction is cleaned by column flotation. After cleaning, all clean coal size fractions are mixed and dewatered by two 40-inch decanter screen-bowl centrifuges. The clean fines are pelletized using COVOL binder. amounts of coal refuse larger than 28 mesh (600 micrometer) with recoverable energy value. As processing technologies and the capacity of dewatering equipment have improved, the proportion of particles smaller than 100 mesh (150 micrometer) in slurry refuse in impoundments has increased. In many instances, the finer slurry materials being disposed of today with less recoverable and marketable coal can close off access to the more amenable, profitable, and recoverable slurry (Sidebar 7.1~. If an impoundment contains at least 1 million tons of in-situ slurry, a recovery rate of at least 30 percent of a marketable fine coal product (300,000 tons) from the slurry could prove to be a profitable venture. Fine coal from is recovered from an impoundment through several stages: investigation (preliminary site investigation, sampling and analysis of the slurry and embankment materials, and engineering design and economic evaluation), excavation and transport, and fine coal recovery. The preliminary site investigation involves inspecting the impoundment and reviewing maps and any other available information. The sampling and analysis of slurry and embankment materials are the most critical and most expensive phase of investigation. The preliminary sampling program requires approximately three samples from each embankment and a detailed core sampling plan throughout the basin. Samples are analyzed for size distribu- tion, float and sink characteristics, froth flotation, and percentage of moisture, ash, sulfur, and energy value per pound. In addition to these standard analyses, it may be advantageous to determine whether other materials, such as magnetite, may be recovered from the slurry. During engineering design and economic evaluation the final phase of investigation- the economic =
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ALTERNA TI VES FOR FUTURE COAL WASTE DISPOSAL 161 feasibility is assessed for the project, and the marketability of the fine coal product (i.e., potential buyers, final destination for the fine coal product, means of transport) is investigated. The excavation of the slurry material from the impoundment and delivery to the fine coal recovery plant is the most critical consideration in the development of a fine coal recovery project. Regardless of the simplicity or sophistication in the design of the fine coal recovery plant, the slurry delivered to the system requires a consistent feed rate (tons per hour) and percentage of solids concentration (weight of slurry and water) to maintain efficiency. Three methods for coal slurry removal and delivery systems are used: conventional excavation methods; stationary pump and wash-down systems; and dredge operations. Conventional excavation and earth-moving equip- ment includes earth-mover pans, excavators, backhoes, drag-lines, front-end loaders, dump trucks, and farm tractors equipped with discs or harrows. The stationary pump and wash-down system agitates, washes, and keeps in suspension the slurry material within the area around and directed to the suction of the floating agitator-feed pump. Dredging uses a floating pump with a movable suction device. It excavates along a side-to-side arcing or stewing motion with a positive forward movement, cutting materials from beneath the surface of the water and creating a slurry to be delivered to the pump suction. During removal of the fine coal, the site is vulnerable to storm water runoff and to slumping of surrounding slopes, and adequate diversion structures must be used to route excess water from the facility. The design of a fine coal recovery plant is the same as that in conventional coal preparation plants and typically processes the slurry material that is smaller than 3/S inch. The recovery plant includes sumps, pumps, vibrating screens, sieve bends, cyclones, spirals, flotation cells or columns, centrifuges, conveyors, and thickeners each unit system being of sufficient size and capacity to handle a specific feed rate and the size characteristics of the slurry material. The refuse from a remining operation can be handled with a variety of methods that are site specific because each impoundment has its own design, site conditions, and slurry materials. Methods include those outlined in this chapter such as underground injection, thickening of slurry, multiple cell construction deposition within the same impounding structure, a combination of these listed, or simply disposing of the refuse in a separate part of the impoundment being remined. Recovery of fine coal from slurry impoundments is an established practice. The committee concludes that as advances are made in the use of low value coal or coal water slurry, remining of slurry impoundments can be an attractive source for fuel supply. ._
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162 COAL WASTEIMPOUNDMENTS IMPLEMENTATION Coal waste impoundments are part of the system for mining, preparing, and combusting coal for energy production (Figure 1.2~. In order to assess alternatives thoroughly, the whole system of mining, preparation, refuse disposal, transportation, and power generation should be explored through an in-depth life-cycle assessment, including cost assessment, with the goal of optimizing the system to generate less fine coal waste while maintaining the performance and economics of the system. Only through such modeling can the benefits of system integration be fully explored. Of course, even if there are such benefits, they may be difficult to realize because of differences in interests and perceptions between the mining industry and the utility industry, and the resistance to change embedded in these mature industries. Several types of policies can effectively promote the use of alternative technologies. One approach is to phase out the dominant technology, providing sufficient time for commercialization of alternatives. Such an approach can have a significant economic impact on the industry affected. A "safety valve" can be provided by extensions to the phase-out dates where alternative technologies are not available and by site-specific variances. This approach was used effectively in the Hazardous and Solid Waste Amendments of 1986 (42 U.S.C. § 6924 ~ to phase out the landf~lling of untreated hazardous waste and to give a clear signal to the market for alternative treatment technologies. This approach has the benefit of stimulating the market for alternatives without necessarily promoting specific technologies. To bring on the alternatives, this approach can be combined with other policies, such as financial incentives and research, development, and demonstration. A second type of policy for promoting alternative technologies is the use of financial incentives. These can take the form of tax credits or direct government subsidies. Such subsidies can be paid for by a fee on the activity being discouraged, such as a few cents per ton of fine coal waste disposed of in slurry impoundments, or through a general fee on coal mining, like the Abandoned Mine Land program under SMRCA. Both the alternatives for refuse disposal and increased use of finer coal products will aid in the reduction of slurry impoundment quantity and additional and future areas required for refuse disposal. These incentives may also be extended to companies that reprocess, reduce, or relocate the slurry materials within an existing impounding structure through the use of alternative technologies, techniques, or methods. If this type of operation is utilized in abandoned or "grandfathered" slurry impoundments, an additional incentive could be
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ALTERNATIVESFORFUTURECOALWASTEDISPOSAL 163 granting reduced bonding or regulatory obligations and moneys from Abandoned Mine Land funds for environmental cleanup. A third type of policy is research, development, and demonstration funded wholly by government or jointly with the industry. The federal government, and particularly the DOE, has long had research and development programs in this area, and many of the alternatives discussed above were first proven in these programs. There is always a gap between research and development and widespread implementation of technologies, but greater emphasis on joint large-scale demonstrations with industry can help bridge this gap. A fourth approach, particularly useful where alternative technologies are already commercially available, is to assist the industry in evaluating these alternatives in an integrated, objective, and thorough manner. Typically, information about alternatives comes to the industry piecemeal through vendors of new technologies. Rarely is it possible to perform an objective evaluation of the full range of alternatives on the same basis. If such an evaluation is performed by one company, it is rarely shared with others in the industry. The EPA's Design for the Environment Program has developed an approach for evaluating and comparing the relative costs, performance, and health and environmental risks of alternative technologies that provide the same Unctions. In this program multiple companies and vendors participate and supply information to an independent academic institution that evaluates the technologies. Costs are assessed using activity-based cost accounting methods that allocate the All costs of technologies, bringing costs out of overhead accounts that may have been overlooked in a traditional cost estimate. Other stakeholders, such as government agencies, labor organizations, and environmental groups, are involved in setting up the evaluation and advising the team throughout. The academic institution can keep proprietary data confidential and out of the hands of regulatory agencies. The results of the evaluation are then widely disseminated to members of the industry for their use in choosing technologies. EPA neither makes these decisions nor recommends technologies. The committee recommends that the total system of mining, preparation, transportation, and utilization of coal and the associated environmental and economic issues be studied in a comprehensive manner to identify the appropriate technologies for each component that will eliminate or reduce the need for slurry impoundments while optimizing the performance objectives of the system. The committee concludes that a similar analysis of the waste use and disposal technologies that make up the coal system would have value. The committee recommends incorporating life-cycle assessment of the costs and environmental impacts of the alternatives to evaluate =
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164 COAL WASTEIMPOUNDMENTS them on a more objective, comprehensive basis. In addition, a detailed analysis of the economic and environmental impact of the various policy alternatives should be performed. A combination of policies can encourage the use of alternatives to coal slurry impoundments. If there is no consensus on the need to phase out slurry impoundments, financial incentives for alternatives and research and development programs can expedite the switch to alternatives. In any event, a thorough, systematic understanding of the whole system through life-cycle assessment is needed for evaluating alternatives, as is the comparison of alternatives. One of the factors limiting implementation to this point has been the cost associated with the various alternatives. Additional research and development is needed to refine these alternatives and to demonstrate their economic implementation. The committee recommends that the use of economic incentives be explored as a way of encouraging the develop- ment and implementation of alternatives to slurry impoundments. The development of incentives should be based on the full range of the portfolio of technologies as well as the economics of the technologies. The incentives should be linked directly to the reduction in slurry production of the utilization of slurry. =
Representative terms from entire chapter: