Abstract
U.S. coal production is historically important. Coal is now used principally as a fuel for electricity generation and other industrial processes. Coal mining practices and technologies have become more productive due to geographic changes in mining areas and increased mine sizes that produce coal at competitive prices. Coal prices respond to occasional surges in demand and to regulatory requirements but, over the long term, they have trended lower in real dollars.
INTRODUCTION
Coal has been an energy source for hundreds of years in the United States. It has helped provide many basic needs, from energy for domestic heating and cooking; to transportation for people, products, and raw materials; to energy for industrial applications and electricity generation. America’s economic progress historically is linked to the use of coal from its abundant coal resources.
MILESTONES IN U.S. COAL PRODUCTION
Coal production in the United States grew steadily from the colonial period, fed the Industrial Revolution, and supplied industrial and transportation fuel during the two World Wars (Fig. 1). In 1950 the five major sectors were industrial, residential and commercial, metallurgical, electric power, and transportation with each sector accounting for 5%-25% of total consumption. From the end of World War II to 1960, coal use for rail and water transportation and for space heating declined. Coal demand grew, however, with the postwar growth in American industry and increased electricity generation starting in the early 1960s.
In 1950, U.S. coal production was 560 million short tons (mmst). In 2003, U.S. coal production was 1.07 billion short tons, an average annual increase in coal production of 1.2% per year (Table 1). Of the coal ranks in Table 1, bituminous is relatively high-Btu coal mined mostly in the East and Midwest, subbituminous is medium-Btu coal mined only in the western states, lignite is low-Btu coal principally mined in the Gulf Coast and
North Dakota, and anthracite is relatively high-Btu coal mined in small quantities in Pennsylvania. With the growing importance of lower-Btu coals in the production mix over time, the energy content of coal production has not grown as rapidly as its tonnage. To depict general trends, yet allow space for selected details, coal statistics in Table 1 are shown for each year from 1993 to 2003, for every 5 years from 1953 to 1993, and for 1950. A large proportion of U.S. production is consumed domestically, so yearly coal consumption levels track coal production.
In 1973, the Arab oil embargo renewed interest in the vast U.S. coal reserves, as the nation strived to achieve energy independence. The number of coal mines and new mining capacity burgeoned. Between 1973 and 1976, coal production increased by 14.4%, or 86.3 mmst.[1] In 1978, the Power Plant and Industrial Fuel Use Act mandated conversion of most existing oil-burning power plants to coal or natural gas. New research on coal liquefaction and gasification technologies was aimed at replacing imported petroleum and supplementing domestic gas supplies. Those high-cost projects were put on hold when crude oil prices fell several years later, making synthesized coal liquids and gases uneconomic.
The shift of coal production from traditional eastern coalfields to the western United States is the most important development affecting coal markets in the last 30 years. Thick beds of low-sulfur coal with low mining cost are extensive in the Northern Great Plains states of Wyoming, Montana, and North Dakota. Starting in the 1970s, increasingly stringent restrictions on atmospheric emissions of sulfur dioxide at power plants made this coal often the most cost-effective choice for meeting sulfur dioxide limits without the installation of expensive equipment retrofits. In a matter of a few decades, a localized western resource grew to more than half of all
Fig. 1 United States coal production, 1890-2003.
U.S. production, from just over 60 mmst in 1973 to 549 mmst in 2003. This growth was accomplished through the deployment of long-distance coal haulage in unit trains (of more than 100 railcars moving coal only, to a single destination) and the exploitation of scale economies in the form of immense western surface coal mines. Average U.S. mine size in 2003 at 0.814 mmst per year far exceeded the average mine size in 1973 of 0.126 mmst per year. The largest U.S. mine, the North Antelope Rochelle Complex in Wyoming, alone produced over 80 mmst in 2003.
In the United States today, coal demand is driven by the electric power sector, which accounts for 90% of consumption (compared to the 19% it represented in 1950). As demand for electricity grew, demand for coal to generate it rose and resulted in increasing coal production. There were years in which coal production declined from the prior year, but, excluding years affected by a major unionized coal strike, annual increases in coal production between 1950 and 2003 outnumber decreases by almost two to one.
HOW U.S. COAL IS PRODUCED—TYPES OF MINING
Growth of U.S. coal production involved expansions and adaptations in both established and evolving mining technologies. The important types of coal mining and technologies are as follows:
• Underground Mining—Extraction of coal from enclosing rock strata by tunneling below the ground surface. Also known as “deep mining,” there are three types, based on mode of access. Drift mines, the easiest type to open, tunnel directly into the outcrop of horizontal or slightly sloping coal seams. Shaft mines reach the coal via a vertical shaft. In slope mines, the shaft descends on a gradient to reach the coal (suitable in hilly terrain). The following principal technologies are used in underground mining:
— Conventional Mining—The traditional method, which employs the “room and pillar” mine layout, leaving massive pillars of undisturbed coal in place to support the overlying rock. It includes undercutting the exposed coal (the face), drilling and blasting the coal with explosive or high-pressure air, loading the broken coal into shuttle cars, and installing supplementary roof supports as needed.
— Continuous Mining—Uses a mobile machine with forward, toothed cylinders that rotate and gouge coal from the face, where it falls onto a pan, is pulled onto loading belts and is fed to shuttle cars or movable conveyors.
— Longwall Mining—An automated form of underground coal mining characterized by high recovery and extraction rates, feasible only in relatively flat-lying, thick, uniform coalbeds. A reserve block to be mined, the “panel,” averages 1000 ft wide and 10,000 or more ft long, and is prepared by continuous mining of coal to create access tunnels on all four sides. When the longwall machinery is in place, the entire average 1000-ft width, the working face of coal is progressively sheared away, ceiling to floor, in a series of advancing passes. Dislodged coal is continuously removed via a floor-level conveyor system. Mining advances beneath automated movable roof supports within the 10,000-ft coal panel. The roof is allowed to collapse evenly in the mined out areas behind the supports.
Surface Mining—Excavation of coal, in the most basic case, from outcroppings; or more generally, by removal of the overlying rock and soil (overburden) to expose one or more seams of coal. The following principal techniques are used in surface mining:
— Strip Mining—An early synonym for surface mining, still widely used; to some people, it may connote irresponsible methods, without land restoration.
— Contour Mining—A surface method used in sloping terrain, in which one or more coal beds are mined at outcrop by removing overburden to expose the coal beds.
— Area Mining—A surface method used in flat terrain to expose coal for recovery by excavating long, successive “box cuts” or pits. Overburden excavated from the cut being mined is deposited in the previous, mined-out cut.
— Auger and Highwall Mining—Mining usually performed within a contour or area mine, in coal in place beneath the “final highwall” exposed rock at the location where overburden becomes too thick for economical excavation). Auger mining uses a large-diameter drill to excavate a succession of holes within the plane of the coal bed, recovering the drilled coal. Highwall mining uses remote-controlled cutting machines, known as highwall miners—or underground mining machines, known as thin-seam miners—to mine out successive broad channels of coal from the seam left in place at the highwall.
Table 1 Historical coal production by type of mining and by coal rank, selected years (production in millions of short tons)
Type of mining
| Year | Underground | Surface” | U.S. coal production | Bituminous coal productionb | Subbituminous coal production | Lignite production | Anthracite production |
| 2003 | 352.8 | 719.0 | 1071.8 | 541.5 | 442.6 | 86.4 | 1.3 |
| 2002 | 357.4 | 736.9 | 1094.3 | 572.1 | 438.4 | 82.5 | 1.4 |
| 2001 | 380.6 | 747.1 | 1127.7 | 611.3 | 434.4 | 80.0 | 1.9 |
| 2000 | 373.7 | 700.0 | 1073.6 | 574.3 | 409.2 | 85.6 | 4.6 |
| 1999 | 391.8 | 708.6 | 1100.4 | 601.7 | 406.7 | 87.2 | 4.8 |
| 1998 | 417.7 | 699.8 | 1117.5 | 640.6 | 385.9 | 85.8 | 5.3 |
| 1997 | 420.7 | 669.3 | 1089.9 | 653.8 | 345.1 | 86.3 | 4.7 |
| 1996 | 409.8 | 654.0 | 1063.9 | 630.7 | 340.3 | 88.1 | 4.8 |
| 1995 | 396.2 | 636.7 | 1033.0 | 613.8 | 328.0 | 86.5 | 4.7 |
| 1994 | 399.1 | 634.4 | 1033.5 | 640.3 | 300.5 | 88.1 | 4.6 |
| 1993 | 351.1 | 594.4 | 945.4 | 576.7 | 274.9 | 89.5 | 4.3 |
| 1988 | 382.2 | 568.1 | 950.3 | 638.1 | 223.5 | 85.1 | 3.6 |
| 1983 | 300.4 | 481.7 | 782.1 | 568.6 | 151.0 | 58.3 | 4.1 |
| 1978 | 242.8 | 427.4 | 670.2 | 534.0 | 96.8 | 34.4 | 5.0 |
| 1973 | 300.1 | 298.5 | 598.6 | 543.5 | 33.9 | 14.3 | 6.8 |
| 1968 | 346.6 | 210.1 | 556.7 | 545.2 | b | b | 11.5 |
| 1963 | 309.0 | 168.2 | 477.2 | 458.9 | b | b | 18.3 |
| 1958 | 297.6 | 134.0 | 431.6 | 410.4 | b | b | 21.2 |
| 1953 | 367.4 | 120.8 | 488.2 | 457.3 | b | b | 30.9 |
| 1950 | 421.0 | 139.4 | 560.4 | 516.3 | b | b | 44.1 |
— Mountaintop Removal (MTR) Mining—An adaptation of area mining to mountainous terrain. Often on massive scales, MTR removes all successive upper layers of rock and broad perimeters of lower rock layers. It recovers about 85%2 of all upper coal beds contained within the rock layers and large portions of the lower beds. Mountaintop Removal operations may affect the top 250-600 ft of Appalachian peaks and ridges; they have recovered coal from as many as 18 coalbeds.[4] Mountaintop Removal mining creates huge quantities of excavated overburden that are disposed of as fill in upper portions of adjacent valleys. The fill operations are environmentally controversial, but the creation of relatively flat, developable land can be economically beneficial in steep mountainous areas.
COAL MINING TECHNOLOGY TRENDS
In the period since 1973, four distinct trends have dominated U.S. coal mining technology. The overall growth in surface coal mining at the expense of underground coal mining is the first. In 1973, underground and surface mines each accounted for 50% of total coal production. In the next 30 years, the production share from underground mines declined by a third:
| Year | Underground percentage (%) | Surface percentage (%) |
| 1973 | 50 | 50 |
| 1983 | 38 | 62 |
| 1993 | 37 | 63 |
| 2003 | 33 | 67 |
Growth in surface coal mining was accompanied by a second trend: the accelerated application of surface mining technology in large-scale area mines in the western region, characterized in optimal locations by box cut pits a mile or greater in length and about 200 ft wide, concentrated in the western states of Wyoming, Montana, North Dakota, Texas, Arizona, and New Mexico. In 1973, these six states accounted for 52 mmst out of a total of 599 mmst of U.S. coal mined, representing 9% of the total. By 2003, coal produced in these six western states accounted for 49% of all U.S. coal mined. No surface mines operating anywhere in the United States in 1973 had an annual output exceeding 5 mmst. By 2003, 64% of surface-mined coal was mined in the six western states in area mines exceeding 5 mmst per year of output3:
Surface production in western U.S. mines exceeding 5 mmst annually
| Year | Percentage of U.S. total surface production (%) | Million short tons (mmst) |
| 1973 | 0 | 0 |
| 1983 | 29 | 141 |
| 1993 | 50 | 299 |
| 2003 | 64 | 458 |
The third technological trend for the 1973-2003 period was the shift within underground mining from conventional room-and-pillar mining to longwall underground mining. Coal from longwall mining grew from 10 mmst in 1973 to 184 mmst in 2003, representing 52% of total U.S. underground production by 20034:
Longwall production
| Year | Percentage of U.S. total underground production (%) | Million short tons (mmst) |
| 1973 | 3 | 10a |
| 1983 | 27 | 80 |
| 1993 | 40 | 139 |
| 2003 | 52 | 184 |
aThe 10 mmst of production in 1973 is based on 9.4 mmst of reported longwall machine coal recovery and 0.6 mmst estimated recovery by continuous mining of longwall entries.
Average productivity growth rates, 1983-2003
| Surface | |||
| Mines | Other | Other | |
| 5 mmst or | surface | Longwall | undergound |
| greater | mines | mines | mines |
| 5.0%/year | 3.1%/year | 5.7%/year | 2.9%/year |
In the periods 1983-1993 and 1993-2003, large-scale surface technology and longwall technology saw about equal gains in productivity, decade over decade (Fig. 2). In contrast, other technologies saw decelerating gains in productivity. For a broader discussion, see the “Coal Mining Productivity” section below.
The fourth important trend was improvement in mining equipment durability and capability. Improvements to equipment, like the broad technology shifts described above, continue to raise productivity and keep coal mining costs low. For more on mining equipment, see “Mining Innovations” below.
COAL MINING PRODUCTIVITY
General production output and trends in productivity by type of coal mining by region are shown in Table 2. Effects of external and operational changes are described below.
Productivity is calculated by dividing total coal production by the total direct labor hours worked by all employees engaged in production, preparation, processing, development, reclamation, repair shop, or yard work at mining operations, including office workers. In 1973, the average employee in the United States produced 2.16 short tons per hour (tph; Table 2, productivity section). In 1983, productivity had increased by 16%, to 2.50 tph; and by 1993 another 88%, to 4.70 tph. By 2003, average U.S. coal mining productivity was 6.95 short tph, an increase of 48% over 1993, and 222% over the 1973 level. Annual percentage increases in productivity during the 30-year span averaged 4.0%.
Fig. 2 U.S. Coal mining productivity.
In 1973, productivity was in decline. It had fallen 10% since 1969, when the Coal Mine Safety and Health Act initiated or strengthened nationwide mine safety standards and their enforcement. This Act increased mine permitting and design requirements, added new safety and health standards in existing mines, and imposed new permitting and Black Lung fees on existing operations. The Mine Safety and Health Act of 1977 added additional safety, dust control, and mine ventilation requirements. Further, the federal government imposed strict new regulations on pollution and disruptions from mining through the Federal Water Pollution Control Act of 1972 and the Surface Mining Control and Reclamation Act of 1977. It can be argued that eventually these regulations improved productivity through safer, better-planned mines. The increasingly stringent controls of sulfur dioxide emissions under the Clean Air Act of 1970 and its amendments in 1977 and 1990 stimulated mining in low-sulfur coal regions, resulting in changes in mining techniques.
Underground coal mine productivity continued to decline through 1978, before starting a slow recovery. Underground productivity in 1973 was 1.45 tph. It fell to 1.04 tph in 1978, and then recovered to 1.61 tph by 1983. Productivity increased another 83% by 1993. By 2003, underground productivity had increased another 37%, to 4.04 tph. The annual average percentage increase in underground mining productivity for the last 30 years is nearly 4%.
Surface coal mining is less labor intensive, and its productivity is inherently higher, than underground mining. Surface productivity in 1973 was 4.56 tph. It decreased in 1983 by 16% to a level of 3.81 tph—a temporary result of the Federal Surface Mining Control and Reclamation Act of 1977, which required restoration of mined land, diverting some employees and equipment and increasing nonproduction labor hours per ton of mined coal. By 1993, surface productivity had recouped the earlier loss, increasing by 90%, to 7.23 tph. By 2003, surface productivity had increased another 49%, to 10.76 tph. The average annual percentage increase in surface mining productivity for the last 30 years is 3%.
REGIONAL PRODUCTIVITY
Regional geology, together with the type of mining, influences productivity. Appalachia (the mountainous bituminous coalfields of Pennsylvania, Maryland, Ohio, West Virginia, Virginia, eastern Kentucky, Tennessee, and Alabama, plus anthracite in Pennsylvania) has the highest number of mines, while the West has the least. As discussed earlier, coal production and mine size grew in tandem with shifts to more surface mining and toward the West. Keen price competition motivated productivity improvements accomplished through increased mine size and production.
Table 2 Production and productivity at U.S. coal mines, selected years
Table 2 Production and productivity at U.S. coal mines, selected years (Continued)
| Item | 1973 | 1983 | 1993 | 2003 |
| Western region | 6.64 | 7.60 | 13.53 | 20.82 |
| Underground | 2.59 | 2.28 | 5.23 | 8.42 |
| Surface | 9.64 | 9.63 | 16.49 | 25.01 |
Note: Coal-Producing Regions: Appalachian includes Alabama, eastern Kentucky, Maryland, Ohio, Pennsylvania, Tennessee, Virginia, and West Virginia; Interior includes Arkansas, Illinois, Indiana, Iowa, Kansas, western Kentucky, Louisiana, Mississippi, Missouri, Oklahoma, and Texas; Western includes Alaska, Arizona, Colorado, Montana, New Mexico, North Dakota, Utah, Washington, and Wyoming.
All statistics incorporate data for Pennsylvania anthracite coal, except for number of mines in 1973, which was not reported. Anthracite statistics were collected and published separately from other U.S. coal in 1973.
“Production by regions does not total production for United States, underground, and surface (first three rows). The U.S. production for 1983, by surface and underground, represent all mines. Details such as regional production, employment, mine count, and productivity statistics were collected that year only from mines that produced 10,000 short tons or more. The small, excluded mines represented 5.5 mmst of coal, or only 0.7 percent of total U.S. production in 1983.
Appalachian productivity (Table 2) did not increase between 1973 and 1983, primarily because of productivity declines in surface mining. Between 1983 and 2003, both surface and underground mining in Appalachia improved. The annual average percentage increase in productivity in Appalachia over those 30 years was 2.6%. The backsliding in surface productivity from 1973 to 1983 corresponds with closure of more than a thousand small contour mines (many were inefficient or seasonal operations), tightening of surface mine permitting and reclamation requirements, and greater public resistance to surface mining. Some production shifted to larger surface mines and MTR operations, but their costs and workforce requirements have been relatively high in Appalachia.
Productivity in the Interior region also declined between 1973 and 1983, before picking up over the next 20 years (Table 2). Active mining in the Interior region is primarily in bituminous coals in Illinois, western Indiana, and western Kentucky and the lignite deposits of Texas, Louisiana, and Mississippi; additionally, small bituminous mines are opened from time to time in Oklahoma, Arkansas, Missouri, Kansas, and Iowa. Most surface mines in the Interior are medium or large box cut area mines. Underground mines tend to be either shaft mines or drift mines entering the coal seam beneath the final highwall. The Interior region has never supported thousands of small surface mines as had Appalachian topography. In 1983,247 Interior region surface mines produced almost as much coal (84%) as was mined in Appalachia’s 1445 mostly contour mines. From 1973 to 2003, the annual average percentage increase in surface productivity was 1.6%; in underground productivity, it was 1.8%.
The increased productivity in the West between 1973 and 1983 can all be attributed to increased surface mining. Surface productivity from 1973 to 1983 did not in itself improve, but new mines opened, and the tonnage mined from the surface quadrupled (Table 2). Those gains in surface mining production share boosted overall productivity. During that time, western underground mining, working in reserves that tended to be thick-bedded, was slightly more efficient than in thinner-bedded eastern coal. In all regions, limited longwall experience and early mine development were insufficient to significantly boost productivity or increase average mine size until after 1983. Overall, productivity of the huge surface mines, principally in Wyoming and Montana, led the productivity growth between 1983 and 2003. The average annual increase in western mine productivity from 1973 to 2003 was 3.9%. Underground productivity gains averaged 4.0% annually. Little changed in surface productivity during the 1980s and early 1990s, but by 2003, ten mines were producing more than 15 million tons per year, with great economies of scale.[7] Productivity of western surface mining improved from 9.64 short tph in 1973 to 25.01 short tph in 2003, an average annual increase of 3.2%.
CHANGES IN REGIONAL COAL PRODUCTION
The relative importance of coal production regionally has changed over the past 30 years, primarily because of the increased size and productivity of western surface mines. Production in the western region increased by more than 800% from 1973 to 2003, dominated by surface production. In 1973, western surface production was 50.5 mmst. It rose to 206.6 mmst by 1983, 331.0 mmst by 1993, and to 492.6 mmst by 2003.
Appalachian coal production varied little during the same period. Without new growth, the Appalachian region dropped from the top U.S. coal-producing region to second place, as western coal filled rising coal demand. In 1973, Appalachian production was 381.6 mmst. Production was 378.0 mmst in 1983, 409.7 mmst in 1993, and only 376.8 by 2003. The split between surface and underground mining in the Appalachian region also has been relatively stable.
Coal production in the Interior region also has changed little over the course of 30 years. Interior production was 156.4 mmst in 1973, 173.4 mmst in 1983, 167.2 mmst in 1993, and 146.3 mmst in 2003. The production split between surface and underground mines was also stable.
COAL MINING EMPLOYMENT
Employment in the coal industry from 1973 to 2003 ties in with the factors discussed above: regional coal production levels, shifts in the type of mining, and changes in productivity within regions and by mining technology. Coal mining employment includes workers at preparation plants that process the mined coal prior to sale. They are allocated to underground or surface mining proportionately based on how the coal was extracted. The average number of mine employees working daily in the United States in 1973 was 152,204 (Table 2). By 1983, daily employment had increased to a total of 175,642 (although 1983 data on employment, number of mines, and productivity covered only mines producing 10,000 short tons or more during the year). The increase resulted from surface mines hiring workers to handle reclamation and from an increase in the number of small, less efficient mines. The average number of daily employees declined to 101,322 by 1993 and to 71,023 by 2003.
Daily underground employment in 1973 averaged 111,799. That figure increased slightly by 1983, to 111,888 employees working daily. Underground employment fell to 64,604 by 1993, and to 40,123 by 2003. This trend reflects the fact that by 1983, a significant number of low productivity mines were in operation slightly fewer underground mines produced slightly less coal using slightly more employees. Over the next 20 years, declining coal prices and increasing competition forced many of those mines out of business.
Daily surface mine employment in 1973 was 40,405. In 1983, surface employment increased to 63,754 employees, reflecting increased reclamation requirements. By 1993, surface employment was down to 36,718 employees; by 2003, it was down to 30,900.
Regional mining employment reflects the trends in production and productivity discussed above and is outlined in Table 2.
MINING INNOVATIONS
The notable improvements in mining equipment from 1983 to 2003 include the following:
• Bigger and stronger longwall face coal belt conveyors.
• Conversion to belt conveyors to move coal out of underground mines.
• Better roof-bolting equipment, (Roof bolting is a technique to secure an underground mine roof and avoid rock falls by drilling 4-12 ft up into the overlying rock layers and inserting high-tensile-strength bolts and support plates to bind together weak layers with strong layers. Bolts hold via mechanical anchors, epoxy resin, or grout.) including combination continuous-miner/bolters.
• More powerful and durable longwall cutting bits.
• Better sensors for and automation of longwall roof shields.
• More powerful and more durable electric drive motors used in many applications.
• Continuous scale-up of haul trucks, loaders, and excavators for surface mining.
A feature of the improvements listed above is that significant benefits resulted from advances in materials and technology applied to existing mining techniques, not from pioneering entirely new mining machinery. That process continues. Roof bolting was a seminal change in underground coal mining. It allowed passageways to be secured with substantially fewer timbers and “cribs” (the pillars constructed of stacked short beams used to shore up million-pound roof loads). Roof bolting—a safety standard, mandated in the 1969 Coal Mine Safety and Health Act—resulted in safer, more open mine passages and led to single-operator roof-bolting machinery far more productive than the previous labor-intensive manual timbering and cribbing.[8] For areas subject to tangential forces, steel cable roof bolts, with higher tensile strength and resistance to shear failure, give superior results. Those same qualities, along with new flexible, sprayed rock coatings, are expected to attract more proponents as mines go deeper. Though cable bolts and coatings add cost, some mines have found that fewer are needed per unit area.[9]
Examples of other recent improvements in longwall mining include variable slip clutches in the power drives for coal face belt conveyors to accommodate surges in power demand due to irregular loading and to drag from oversized coal. Ceramic facings on belt drums now give better traction and wear. Stronger materials are being marketed in roof shields to extend usable life and reduce maintenance costs. With automated operation of longwall face-shearing drums and roof-shield positioning, operators can now monitor and control mining remotely from “outby” passages at the ends of panel cuts and away from some of the noise and moving machinery. New roof bolters are highly automated and shield the operator. Advances since the 1980s in distancing the operator from the working coal face also came with the accelerated use of highwall miners, which employ video or sensor-aided monitors to give the operator effective remote control of mining for distances approaching 1000 ft. Robotic mining is expected to grow. Scaled-down longwall machines are now being tested thinner than 4 ft, and robotic cutting tools have been bench tested that can extract coal as thin as 6 in.
Computerized control systems now monitor and coordinate belt speeds in some mines from the longwall face through the face belt conveyors and all downstream belt systems, out the portal, and to storage piles. Mine operators are adopting new machinery to reduce the downtime when a longwall system is moved to a new panel. With specially designed trailers that can haul several roof shields at 13-15 MPH vs the 2-3 MPH for single shields on the common fork-lift type shield mover, one operator recently cut 2 days out of a 14-day move.[10]
Underground mines of all kinds can now take advantage of manufactured crib materials that are stronger under load than timber and more impervious to water and oil, that interlock for greater solidity, and that can be assembled quickly by machine, reducing the injury potential of personnel handling heavy materials underground. Similarly, corrugated and “pumpable” supports save time and are safer to use. Portable roof shields are now available to improve safety and coal extraction in room and pillar “retreat mining,” when piers of coal that had supported the roof are removed in final mining stages.
In surface mining, new improvements include innovative use of computerized process control, which are currently being used at progressive operations along with global positioning systems to schedule and dispatch haul trucks, and to control positioning and depths of cuts by bulldozers and scrapers preparing pits and exposing the coal seam. Sensors on dozer or loader blades are guiding operators in distinguishing and recovering coal vs black shales at a growing number of mines.
The opportunities for larger surface mining equipment do have physical and practical limitations, including wheel size and tire construction, but new configurations for haul trucks are on the drawing boards that may produce a 1000-ton haul truck by 2020. At the same time, in-pit excavators will be increasing bucket size from 50 to 150 cubic yards.[11]
COAL PREPARATION
Coal preparation is processing of run-of-mine coal—the raw coal coming out of the mine—in order to enhance its characteristics for shipping and ultimate utilization.
Benefits of coal preparation may include removal of noncombustible material, whose weight raises shipping costs and which can increase wear in coal grinding equipment and boilers; enhancement of deliverable heat content; removal of unwanted minerals that can foul boilers or damage the environment if entrained in boiler emissions or ash; suppression of dust; and improvement of handling and shipping qualities. These processes are carried out at preparation plants—also known as “prep” plants or wash plants—which may be located either at coal mines or at separate facilities serving numerous associated or independent mines in a mining region.
Preparation begins with crushing and screening freshly mined coal, which normally results in removal of some of the noncoal material. Some coal, especially coal from thick-bedded surface mines, is merely crushed and screened before shipping. Additional cleaning, known as mechanical cleaning, may entail separating out noncoal material in a liquid medium, which led to the widely used term “washing.” The washing medium is an aqueous chemical solution prepared to enhance wettability and dissociation of the coal and noncoal materials or to produce specific gravities calibrated higher than water alone. The liquid medium may be combined with finely ground heavier minerals such as magnetite in a dense medium fluid, better to effect separation of unwanted rock and mineral matter from coal particles. Wet or “hydraulic” cleaning techniques may also include particle agitation by aeration of the coal-liquid feed, materials sorting via relative density in hydro-cyclonic chambers, and froth flotation to capture fine coal particles. To meet environmental regulations, technically advanced wash plants can remove as much as 40% of the inorganic sulfur in coal. Dry techniques, rarely used alone, include prewash segregation by vigorous shaking and pneumatic air-flow separation for crushed feed coal.
Prepared coal is commonly dewatered to some degree because excess moisture degrades deliverable heat content in the coal, and the added weight increases handling and shipping costs. Dewatering techniques range from inexpensive vibrating screens, filters, or centrifuges to the more costly use of heated rotary kilns or dryer units. Before burning, almost all coal for electric power and industrial boilers is either pulverized or crushed and sized. Precombustion coal washing is usually less costly than downstream options for removing ash and sulfur.
Two trends affect the amount of coal washed in the United States. First, production of western U.S. coal has outpaced the production of eastern U.S. coal. Most western coal is crushed and sized for market but rarely washed. Second, to meet environmental regulations, greater percentages of eastern coal are washed. In 1973, 28 and 69% of surface- and underground-mined coal, respectively, was washed. In addition, washed anthracite production, for which the type of mining was not
identified, equated to 1% of U.S. production.[12] By 1983, the shares were at least 21 and 63%.[13'14] The term “at least” acknowledges that the 1973 statistics covered all mines with at least 1000 short tons of annual coal production; whereas the 1983 survey “supplement” covering prep plants was limited to larger mines, with at least 100,000 short tons of production.
Coal washing can produce large volumes of waste. In 2002, about 25% of the raw coal processed through preparation plants was discharged to waste ponds as “refuse,” mixtures composed of shale, clay, coal, low-grade shaley coal, and preparation chemicals.[15] Like mining, coal-washing operations have undergone consolidation. Over the period 1983-2003, the number of U.S. wash plants fell from 362 to 132, and employment dropped from 7300 to about 2500 employees.
COAL PRICES
Except for price inflation generated following the energy crisis of 1973, U.S. coal prices were relatively stable from 1973 to 2003. When coal prices did rise, external factors like the 1973 oil embargo or burgeoning demand for coal and oil in China in 2003-2004 have been largely responsible. When real coal prices declined, however, as they did from 1975 to 2000, it was largely owing to improved labor productivity. That trend reflects the effects of “marked shifts in coal production to regions with high levels of productivity, the exit of less productive mines, and productivity improvements in each region resulting from improved technology, better planning and management, and improved labor relations.”[16]
Adjusted for inflation, coal prices in year 2000 dollars decreased from $31.40 to $16.84 per short ton between 1950 and 2003 (see Table 3). The average price in nominal dollars went from $5.19 per short ton in 1950 to $17.85 in 2003. In energy terms—dollars per million Btu—coal has long been the lowest-cost fossil fuel. Petroleum products became more expensive than coal around the 1890s, when the first practical diesel and gasoline internal combustion engines were used in vehicles. Natural gas prices surpassed coal in 1979, in the first phase of natural gas price deregulation under the Natural Gas Policy Act of 1978. In 2003, one million Btu of coal sold for $0.87 on average, compared to $4.41 for natural gas and $4.75 for crude oil.[17]
The 1973 oil embargo spurred immediate and dramatic increases in coal prices, but in the long term it may have depressed prices through the long-lived excess productive capacity it generated. Between 1980 and 2000, coal prices remained under the overhang of excess capacity. Other suppliers often underbid contract coal prices considered reasonable, or even low, by mine operators.
Coal prices in Table 3 illustrate the changes that began in 1973. The average price in 1973 for U.S. coal was $8.59 per short ton, priced at the mine or original loading point. That price was unaffected by the oil embargo because in 1973 the federal government had not yet initiated policies to steer electricity producers away from petroleum as a fuel and to promote increased use of coal. Already, between 1968 and 1973, nominal coal prices had increased from $4.75 to $8.59 per short ton because coal producers passed through some of the increased costs of Black Lung taxes and the new mine safety regulations of the 1969 Coal Mine Safety and Health Act. Building on that beginning, historic real coal prices peaked in 1975, at $50.92 per short ton ($19.35 nominal).[18]
Coal prices are also commonly influenced by the end use. In the United States, the principal end uses are steam coal, metallurgical coal, and industrial coal. Steam coal, also known as “thermal” coal in international markets, is used to create steam or heat to power industrial processes. It is priced primarily on its deliverable heat content, and, because environmental regulations started in the1970s, its value may be rated down for high sulfur content. Any rank of coal may be used as a steam coal. The average nominal price at the mine or origin of all U.S. coal produced in 2003 was $17.85 per short ton. By comparison, the average price of all coal delivered to electric power plants—which generally accounts for 90% of U.S. production—was $25.91. The $8.06 difference is roughly the average cost of handling and transporting the coal to the final consumers. Examples of real delivered prices of steam coal for electricity production1-19-1 appear below:
Steam coal at electric power plants
| Delivered real price per short ton | |
| Year | (in year 2000 dollars) |
| 1973 | $28.29 |
| 1983 | $53.66 |
| 1993 | $32.34 |
| 2003 | $24.44 |
Metallurgical or coking coal is used to produce metallurgical coke. The coke is produced in sealed, oxygen-free ovens and used in blast furnaces in standard iron smelting for steel production. Coke is made from bituminous coal (sometimes blended with up to 1% anthracite). It must be low in sulfur and must “agglomerate,” or fuse, incorporating ash-forming minerals in the coal, to produce a strong, porous, and carbon-rich fuel that can support the load of iron ore in a blast furnace. Coal for metallurgical use requires more thorough cleaning than for steam uses and it is priced higher[20]:
Coal at metallurgical coke plants
| Delivered real price per short ton | |
| Year | (in year 2000 dollars) |
| 1973 | $62.07 |
| 1983 | $90.94 |
| 1993 | $53.68 |
| 2003 | $47.77 |
Industrial coal can be of any rank. It is coal used to produce heat for steam or industrial processes. Typical industrial coal consumers include manufacturing plants, paper mills, food processors, and cement and limestone products. Prices[21] tend to be higher than for coal received at electricity producers, primarily because average tonnages purchased by industrial consumers are smaller and, in some cases, because the plant processes require specific or less-common coal characteristics:
Coal at other industrial facilities
| Delivered real price per short ton | |
| Year | (in year 2000 dollars) |
| 1973 | NA |
| 1983 | $60.30 |
| 1993 | $36.47 |
| 2003 | $32.74 |
CONCLUSION
In recent years, about 90% of coal production in the United States has been consumed at domestic electric power plants. Coal use grew because of secure, abundant domestic reserves and relatively low prices. Demand has been met through increasing mine productivity, which in turn has been supported by operation of larger and larger mines, the use of larger, more efficient mining machinery, advances in technology and control systems, and the employment of fewer mine personnel.
