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Coal Research and Development: to Support National Energy Policy Appendix E Coal Mining and Processing Methods This appendix presents additional details on the individual processes that are involved in extraction of coal from surface and underground mines, and the subsequent beneficiation of the coal in coal processing plants to produce a final product. COAL MINING METHODS Surface Mining In surface mining, the ground covering the coal seam (the overburden) is first removed to expose the coal seam for extraction. The elements of a surface mining operation are (1) topsoil removal and storage for later use, (2) drilling and blasting the strata overlying the coal seam, (3) loading and transporting this fragmented overburden material (called spoil), (4) drilling and blasting the coal seam, (5) loading and transporting the coal, (6) backfilling with spoil and grading, (7) spreading top soil over the graded area, (8) establishing vegetation and ensuring control of soil erosion and water quality, and (9) releasing the area for other purposes (Figure E.1). Steep topography, a steeply dipping seam, or multiple seams, all present challenging problems for designing stable slopes and productive operations in surface mining situations. Surface topography controls which of the surface mining methods—contour mining, area strip mining, or open-pit mining—is employed (see Figure 4.3). These differ principally in the methods employed for loading, transporting, and storing the spoil. Contour mines are common in the hilly Appalachian terrain of the eastern United States where the fragmented overburden has to be transported
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Coal Research and Development: to Support National Energy Policy outside the mining area for placement and storage. In the Midwest, where the surface topography and coal seams are generally flat, it is common to employ area strip mining in which the fragmented overburden is placed directly by large draglines in the space created where coal has been mined (Figure E.1). In some situations in the eastern United States, a coal seam occurring near the top of mountains is exposed by removing the top of the mountain (Figure 4.3) and transporting the fragmented overburden to a nearby valley. Underground Mining Underground mining is usually by the room-and-pillar mining or longwall mining method (Figure E.2). Even in mines where the longwall method is the principal extraction method, the development of the mine and the longwall panels is accomplished by room-and-pillar continuous mining. The thickness of the coal seam, the depth and inclination of the coal seam, the nature of roof and floor strata, and the amount of gas contained both in the coal seam and the roof and floor strata are all important for selection of the mining method. Mining difficulties are greatly increased if seams are extremely thick or thin or are steeply inclined. Longwall mining additionally requires large coal reserves to justify the capital cost of longwall equipment. As surface mining in the Powder River and Rocky Mountain Basins proceeds, it is likely that the stripping ratios (overburden to coal) will exceed an economic limit. If this coal is to be mined at reasonably high recovery rates, it FIGURE E.1 Schematic depiction of the unit operations in a surface coal mine. SOURCE: Royal Utilities.
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Coal Research and Development: to Support National Energy Policy FIGURE E.2 Schematic showing underground coal mine workings. The coal seam is accessed by both a slope and a shaft, shown on the right. The ventilation fan arrangement is shown adjacent to the surface opening of the shaft. The shaft has an elevator for lowering and raising miners and materials. Coal gathered from the workings by various conveyors is transported to the surface by the slope conveyor. The surface features shown are the raw coal storage silo fed by the slope conveyor, the coal preparation plant (the building on the left), the clean coal storage silos in the front, and the train load out. A longwall section and a room-and-pillar continuous miner section are shown. The room-and-pillar section is a five-entry development with rows of four pillars. The longwall face is between two three-entry developments. SOURCE: CONSOL Energy Inc. will require thick-seam underground mining methods such as large longwalls or multiple slice and/or caving techniques that have not been used in the United States. This will require improvements to mining equipment and practices that are likely to entail research and development (R&D) on mine design, ground control, mine automation, and new systems for protecting worker health and safety. Room-and-Pillar Mining. In the room-and-pillar method, a set of entries, usually between three and eight, are driven into a block of coal. These entries are connected by cross-cuts, which are usually at right angle to the entries. The entries are commonly spaced from 50 to 100 feet apart, and the cross-cuts are usually about 50 to 150 feet apart. The pillars formed by the entries and cross-cuts may be extracted or left standing depending on mining conditions. In the
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Coal Research and Development: to Support National Energy Policy conventional room and pillar method, several pieces of equipment are used in sequence at a working face to extract the coal. These unit operations include the undercutting, drilling, blasting, loading and roof bolting operations. In the continuous room and pillar method, the unit operations of undercutting, drilling, and blasting are eliminated and the cutting and loading functions are performed by a mechanical machine—the continuous miner. The room-and-pillar method accounts for 50 percent of the underground production in the United States, and continuous mining makes up more than 90 percent of this production. In both conventional and continuous methods, coal is loaded onto coal transport vehicles and then dumped onto a panel-belt conveyor for transport out of the mine. Once the coal has been cut, the strata above the excavated coal seam are supported by roof bolts. Under favorable conditions, the production from a continuous miner section can exceed 800,000 tons per year per continuous miner. Longwall Mining. Longwall mining is an automated form of underground coal mining characterized by high recovery and extraction rates, feasible only in relatively flat-lying, thick, and uniform coal beds. A high-powered cutting machine (the shearer) is passed across the exposed face of coal, shearing away broken coal, which is continuously hauled away by a floor-level conveyor system (Figure E.2). Longwall mining extracts all machine-minable coal between the floor and ceiling within a contiguous block of coal, known as a panel, leaving no support pillars within the panel area. Panel dimensions vary over time and with mining conditions but currently average about 900 feet wide (coal face width) and more than 8,000 feet long (the minable extent of the panel, measured in the direction of mining). Longwall mining is done under movable roof supports that are advanced as the bed is cut. The roof in the mined-out area is allowed to fall as the mining advances (EIA, 2007b). The use of longwall mining in underground production has been growing in terms of both amount and percentages, increasing from less than 10 percent of underground production (less than 10 million annual tons) in the late 1960s, to about 50 percent of underground production (more than 200 million annual tons) at present. The production from a longwall mine today (one longwall section and two or three continuous miner sections) can exceed 7 million tons per year. With a second longwall and the necessary complement of continuous miners, production from an underground longwall mine can be well over 10 millions tons per year. COAL PROCESSING METHODS The composition of coals mined in different areas can vary widely (Table 4.2). Since the very early days of mining, coal quality has been improved by removing unwanted mineral matter. Over this time, coal preparation plants have evolved considerably, from simple size segregation in the early twentieth century, into lump coal for domestic use and intermediate sizes for industrial use. The fines were rejected as unfit for use, leading to a substantial quantity of coal refuse
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Coal Research and Development: to Support National Energy Policy (“waste coal” or “gob” piles) particularly in the eastern states. The first washing methods were imported from Europe, The “Chance” washer, in which the density differences between coal and mineral matter was exploited to clean raw coal was introduced in 1918. The Chance washer utilized sand and water as a medium. Today, the “heavy-media” process using magnetite is standard for coarse coal cleaning. Attempts to recover middlings and fine coal have continued through the years, and near the middle of the twentieth century, processes to wash and recover fine coal resulted in the introduction of equipment such as centrifuges, froth flotation cells, disc filters, thickeners, cyclones, and thermal dryers. The unit processes in coal preparation plants vary, but the following sequence of steps is typical. Crushing and breaking. Run-of-mine coal must be crushed to an acceptable top size for treatment in the preparation plant. Typical crushing and breaking devices are feeder breakers, rotary breakers, hammer mills, and roll crushers. Sizing. Different cleaning processes are used on different sizes of coal. Therefore raw coal entering the plant will be screened (sieved) into three or four sizes. Clean coal is rarely sized, except for some industrial markets. Storage and stockpiling. Coal is stored in silos or stockpiled before and after cleaning. Raw coal is stored between the mine and the preparation plant, and clean coal is stored between the preparation plant and product loadout. This is done to provide surge capacity at the interface between the mine and the plant, and between the plant and the loadout, to maintain workable product inventories, and in some cases to control the quality of coal going to a given customer by segregating different products. Density separation. Raw coal consists of organic and mineral matter components, with specific gravities ranging from 1.30 for the lighter organic material to 2.5 for rock. Coal is cleaned by separating the lower-density organic material from the higher-density refuse. In heavy-media separations, the specific gravity of the medium used for separation, usually a suspension of finely divided magnetite in water, is chosen to achieve a given degree of separation depending on the characteristics of the coal, the desired product quality, and the acceptable level of coal loss to the rejects. In water-only devices such as jigs, spirals, and water-only cyclones, separation is effected by the differential acceleration of coal and mineral particles in water. Froth flotation. Fine coal particles (i.e., smaller than 0.5 mm) are difficult to separate from mineral matter on a density basis and this fraction usually is cleaned by froth flotation. Froth flotation is a physiochemical process that exploits the selectivity of the attachment of air bubbles to organic coal particle surfaces and in the nonattachment to mineral constituents. Surfactants are used to create a hydrophobic surface on the coal particles to be floated, and a “collector,” typically fuel oil, is used to promote agglomeration of the floated particles to facilitate their removal.
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Coal Research and Development: to Support National Energy Policy Coal drying. Coal preparation plants that employ fine coal cleaning by froth flotation can produce an unacceptable amount of moisture in the product. Thermal drying, in which the wet coal is dried in the hot gas generated by a coal-or gas-fired burner, is used in some plants to reduce the moisture content. Refuse and tailings management. Waste management is an integral part of coal preparation. Coarse refuse is transported to the solids disposal area, where it can form a tailings impoundment or be placed in a suitable landfill. Tailings (fine solid waste in water) are usually transported by pipeline to an impoundment area where the tailings settle out; the clarified water is reused in the plant. Coal Preparation Plants Each year, Coal Age magazine conducts a census of coal preparation plants in the United States (Fiscor, 2005). The overall findings of the survey (summarized in Table E.1) are generally accepted within the industry as a reasonably accurate reflection of the condition of the coal preparation industry. According to the Coal Age article, “plants reported an average recovery rate of 57%.” Given the total raw coal capacity of the surveyed plants (158,187 tons per hour), this corresponds to a clean-coal capacity of 790 million tons per year, assuming 24/7 operation. The number of preparation plants increased by 53 since the 2000 survey, and at least 10 new plants were built and 25 were significantly upgraded in the TABLE E.1 Characteristics of Coal Preparation Plants in the United States in 2004, by State State Number of Plants Raw Coal Capacity (ton/hr) Average Age West Virginia 66 48,382 24 Kentucky 73 43,320 21 Pennsylvania (bituminous) 20 14,575 30 Virginia 25 10,700 21 Illinois 11 10,450 21 Indiana 19 8,950 17 Alabama 6 8,120 26 Ohio 15 5,360 24 Pennsylvania (anthracite) 15 1,980 35 Maryland 1 1,800 N/A Colorado 4 1,750 7 Washington 1 1,750 NA Utah 6 600 NA Tennessee 2 450 NA Total 264 158,187 23 SOURCE: Fiscor (2005).
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Coal Research and Development: to Support National Energy Policy five years to 2005 (Fiscor, 2005). The survey notes that “while they employ new equipment, technology, and circuitry, U.S. prep plants have in general remained essentially the same. The typical U.S. prep plant employs heavy media separation and was built in 1983. It has a raw capacity between 500 and 1,000 tons per hour. Although more plants are employing large diameter cyclones, the plants still rely mainly on heavy-media vessels for primary separation and heavy media cyclones for intermediate separation. For fine coal recovery, the plants prefer spirals. Centrifugal dryers are popular. On the technology side, the industry has not embraced online analysis on a widespread basis, but it has adopted the use of PLCs extensively” (Fiscor, 2005).1 1 A PLC is a Programmable Logic Controller.