Coal Waste Impoundments Risks, Responses, and Alternatives (2002) / Chapter Skim
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7 Alternatives for Future Coal Waste Disposal
7 Alternatives for Future Coal Waste Disposal
Pages 131-164
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From page 131... ...
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.
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From page 132... ...
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.
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From page 133... ...
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.
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From page 134... ...
. 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!
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From page 135... ...
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~.
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From page 136... ...
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.
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From page 137... ...
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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From page 138... ...
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.
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From page 139... ...
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.
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From page 141... ...
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.
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From page 142... ...
Another alternative is to create a slurry fuel from fine coal waste. In 1973, the oil embargo prompted the development of coal-water slurry technology, which consists of a pulverized, fluidized coal feed that can be transported by pipeline and used as a fuel for utility boilers or gasifiers.
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From page 143... ...
The committee concludes that technologies for utilization offne coal waste for electricity generation in conventional coal-fired" power plants are available. These technologies offer near-term opportunities for the reduction of fine coal waste disposed of in impoundments.
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From page 144... ...
The first-generation pressurized fluidized-bed combustor, which has been demonstrated by a joint DOEAmerican Electric plant in Ohio, the Tidd Plant, uses a bubbling-bed technology. A relatively stationary fluidized bed is established in the boiler using low air velocity to fluidize the material, and a heat exchanger (boiler tube bundle)
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From page 145... ...
Similarly, fluidized-bed plants in Utah and Montana burn coal from mine refuse piles consisting of low-grade surface coal layers discarded during the beginning of surface mining operations (Couch, 1998;
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From page 146... ...
Fluidized-bed power plants can eliminate waste generation at the preparation plant, but they increase the waste produced during the utilization phase (Figure 1.2~. In general, fluidized-bed power plants produce more coal combustion waste than conventional coal-fired boilers.
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From page 147... ...
The 140-megawatt Redbank Power Project in New South Wales uses two fluidized-bed boilers fired with 100 percent waste coal, both from an impoundment and from the mine's preparation plant. The coal waste is fed from a preparation plant using Jameson cell technology followed by centrifugation to reduce the moisture content to approximately 30 percent.
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From page 148... ...
The conventional boiler is fed by an 8 percent moisture vacuum-filtered material from the preparation plant, while the fluidized-bed boiler is fed by a fine coal slurry from the preparation plant and old slurry impoundments (Couch, 1998~. The slurry burned by the fluidized-bed boiler contains 33 percent water, 30 percent or more ash, and a high clay content, which makes it difficult to dewater further.
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From page 150... ...
The committee concludes that the combustion of fine coal waste in advanced combustion technologies, such as fluidized-bed combustion and gasification, is an alternative that also shows considerable long-term promise. Atmospheric fluidized-bed units are already in use for combustion of fine coal waste slurries from both preparation plants and old slurry impoundments, but they have not gained wide usage.
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From page 151... ...
DOE is already promoting advanced combustion technologies through research and demonstration projects. Further research is needed on the use of fine coal waste slurries as feeds, and incentives may be needed if these technologies are to be utilized widely for fine coal waste combustion.
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From page 153... ...
For stability, benches must be constructed every 50 feet to reduce the overall slope angle. The advantage of this method over conventional cross-valley slurry impoundments is that each cell is small and self-contained and can be designed according to the strength properties of the coarse refuse.
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From page 154... ...
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.
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From page 155... ...
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 communication, 2001~. Deep cone paste thickeners produce a homogeneous, nonsegregating paste with a solids content of approximately 60 percent.
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From page 156... ...
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.
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From page 157... ...
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.
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From page 158... ...
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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From page 159... ...
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~.
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From page 160... ...
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.
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From page 161... ...
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.
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From page 162... ...
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.
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From page 163... ...
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.
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From page 164... ...
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.
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