Coal how is it mined

Most lignite coal is used in power stations very close to where it was mined. Lignite is mainly combusted and used to generate electricity. In the U. Sub-Bituminous Coal Sub-bituminous coal is about million years old. Like lignite, sub-bituminous coal is mainly used as fuel for generating electricity. Most sub-bituminous coal in the U.

Outside the U. Bituminous Coal Bituminous coal is formed under more heat and pressure, and is million to million years old. It is named after the sticky, tar-like substance called bitumen that is also found in petroleum. Bituminous coal is divided into three major types: smithing coal, cannel coal, and coking coal.

Smithing coal has very low ash content, and is ideal for forges, where metals are heated and shaped. Cannel coal was extensively used as a source of coal oil in the 19th century. Coal oil is made by heating cannel coal with a controlled amount of oxygen, a process called pyrolysis. Coal oil was used primarily as fuel for streetlights and other illumination.

The widespread use of kerosene reduced the use of coal oil in the 20th century. Coking coal is used in large-scale industrial processes. The coal is coke d, a process of heating the rock in the absense of oxygen. This reduces the moisture content and makes it a more stable product. The steel industry relies on coking coal. Bituminous coal accounts for almost half of all the coal that is used for energy in the United States.

It is mainly mined in Kentucky, Pennsylvania, and West Virginia. Anthracite Anthracite is the highest rank of coal. It is harder, more dense, and more lustrous than other types of coal. Almost all the water and carbon dioxide have been expelled, and it does not contain the soft or fibrous sections found in bituminous coal or lignite. Because anthracite is a high-quality coal, it burns cleanly, with very little soot. It is more expensive than other coals, and is rarely used in power plants.

Instead, anthracite is mainly used in stoves and furnaces. Anthracite is also used in water- filtration systems. It has tinier pores than sand, so more harmful particles are trapped. This makes water safer for drinking, sanitation , and industry. Anthracite can typically be found in geographical areas that have undergone particularly stressful geologic activity. For example, the coal reserves on the Allegheny Plateau in Kentucky and West Virginia stretch to the base of the Appalachian Mountains.

Here, the process of orogeny , or mountain formation, contributed to temperatures and pressures high enough to create anthracite. China dominates the mining of anthracite, accounting for almost three-quarters of anthracite coal production.

Graphite Graphite is an allotrope of carbon, meaning it is a substance made up only of carbon atoms. Diamond is another allotrope of carbon. Graphite is the final stage of the carbonization process. Graphite conducts electricity well, and is commonly used in lithium ion batteries. It can be used in products such as fire-resistant doors, and missile parts such as nose cones. Coal Mining Coal can be extracted from the earth either by surface mining or underground mining.

Once coal has been extracted, it can be used directly for heating and industrial processes or to fuel power plants for electricity. Surface Mining If coal is less than 61 meters feet underground, it can be extracted through surface mining. In surface mining, workers simply remove any overlying sediment, vegetation, and rock, called overburden. Economically, surface mining is a cheaper option for extracting coal than underground mining.

About two and a half times as much coal can be extracted per worker, per hour, than is possible with underground mining. The environmental impacts of surface mining are dramatic. The landscape is literally torn apart, destroying habitats and entire ecosystem s. Surface mining can also cause landslide s and subsidence when the ground begins to sink or cave in. Toxic substances leach ing into the air, aquifer s, and water table s may endanger the health of local residents. In the United States, the Surface Mining Control and Reclamation Act of regulates the process of coal mining, and is an effort to limit the harmful effects on the environment.

The act provides funds to help fix these problems and clean up abandoned mining sites. The three main types of surface coal mining are strip mining, open-pit mining, and mountaintop removal MTR mining. Surface Mining: Strip Mining Strip mining is used where coal seams are located very near the surface and can be removed in massive layers, or strips.

Overburden is usually removed with explosives and towed away with some of the largest vehicles ever made. Dump trucks used at strip mines often weigh more than tons and have more than 3, horsepower. Strip mining can be used in both flat and hilly landscapes. Strip mining in a mountainous area is called contour mining. Contour mining follows the ridges, or contours, around a hill. A pit, sometimes called a borrow, is dug in an area. This pit becomes the open-pit mine , sometimes called a quarry.

Open-pit mines can expand to huge dimensions, until the coal deposit has been mined or the cost of transporting the overburden is greater than the investment in the mine. Open-pit mining is usually restricted to flat landscapes. After the mine has been exhausted, the pit is sometimes converted into a landfill.

After the summit is cleared of vegetation, explosives are used to expose the coal seam. After the coal is extracted, the summit is sculpted with overburden from the next mountaintop to be mined. By law, valuable topsoil is supposed to be saved and replaced after mining is done.

Barren land can be replanted with trees and other vegetation. Mountaintop removal began in the s as a cheap alternative to underground mining. It is now used for extracting coal mainly in the Appalachian Mountains of the U. MTR is probaby the most controversial coal mining technique. The environmental consequences are radical and severe. Waterways are cut off or contaminated by valley fill. Habitats are destroyed. Toxic byproduct s of the mining and explosive processes can drain into local waterways and pollute the air.

Miners travel by elevator down a mine shaft to reach the depths of the mine, and operate heavy machinery that extracts the coal and moves it above ground. The immediate environmental impact of underground mining appears less dramatic than surface mining. There is little overburden, but underground mining operations leave significant tailings. Tailings are the often-toxic residue left over from the process of separating coal from gangue , or economically unimportant minerals.

Toxic coal tailings can pollute local water supplies. To miners, the dangers of underground mining are serious. Underground explosions, suffocation from lack of oxygen, or exposure to toxic gases are very real threats.

To prevent the buildup of gases, methane must be constantly ventilated out of underground mines to keep miners safe.

There are three major types of underground coal mining: longwall mining, room-and-pillar mining, and retreat mining. Underground Mining: Longwall Mining During longwall mining , miners slice off enormous panels of coal that are about 1 meter 3 feet thick, kilometers The panels are moved by conveyor belt back to the surface. The roof of the mine is maintained by hydraulic supports known as chock s. As the mine advances, the chocks also advance. The area behind the chocks collapses.

Longwall mining is one of the oldest methods of mining coal. Before the widespread use of conveyor belts, ponies would descend to the deep, narrow channels and haul the coal back to the surface. Today, almost a third of American coal mines use longwall mining. Columns pillars of coal support the ceiling and overburden.

The rooms are about 9 meters 30 feet wide, and the support pillars can be 30 meters feet wide. There are two types of room-and-pillar mining: conventional and continuous. In conventional mining, explosives and cutting tools are used.

In continuous mining, a sophisticated machine called a continuous miner extracts the coal. In developing countries, room-and-pillar coal mines use the conventional method. Underground Mining: Retreat Mining Retreat mining is a variation of room-and-pillar.

When all available coal has been extracted from a room, miners abandon the room, carefully destroy the pillars, and let the ceiling cave in. Remains of the giant pillars supply even more coal. Retreat mining may be the most dangerous method of mining. A great amount of stress is put on the remaining pillars, and if they are not pulled out in a precise order, they can collapse and trap miners underground.

How We Use Coal People all over the world have been using coal to heat their homes and cook their food for thousands of years. 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. 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 feet apart, and the cross-cuts are usually about 50 to feet apart. The pillars formed by the entries and cross-cuts may be extracted or left standing depending on mining conditions.

In the. 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 , 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.

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 feet wide coal face width and more than 8, 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, b. 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 s, to about 50 percent of underground production more than 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. The composition of coals mined in different areas can vary widely Table 4. 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.

The Chance washer utilized sand and water as a medium. 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.

The U. During this time, coal production has doubled, while the number of active miners has been halved and the number of mines has dropped by a factor of three Figure 4. This has resulted in the concentration of production in a smaller number of larger mines.

The largest mines in the country produced million tons of coal in The remaining approximately 1, mines produced Nearly 70 percent of U. Since the s, there has been a continuous increase in the proportion of coal produced by the western states. At present, states west of the Mississippi account for more than 55 percent of total tons produced Figure 4.

Wyoming alone accounts for almost 36 percent of national coal production tonnage Table 4. Considerable data are compiled on the basis of the union or non-union status of mines throughout the coal industry by the Energy Information Administration EIA.

At present, some Note that the left axis scale represents two parameters, production tonnage and number of mines. The average number of workers at a union mine is A modern coal mine is a highly mechanized industrial plant that has to meet strict standards of engineering design and operation.

The size, power, strength, monitoring and control features, and automation of mining equipment dwarf. The overall coal mining process consists of several sequential stages: 1 exploration of a potentially economic coal seam to assess minable reserves, environmental issues, marketable reserves, potential markets, and permitting risks; 2 analysis and selection of a mining plan; 3 securing the markets; 4 developing the mine; 5 extracting the coal; 6 processing the coal if necessary; and 7 decommissioning the mine and releasing the property for post-mining use.

The two essential requirements that must be fulfilled before a prospective coal mine can enter the development stage are confirmation that there are sufficient minable reserves of adequate quality with no unacceptable environmental or permitting risks, and confirmation of an assured or contracted market for a substantial fraction of the coal that will be mined. Coal seams can be mined by surface or underground methods Figure 4.

The most important technical factors are the thickness of the coal seam, the depth of the coal seam, the inclination of the seam, and the surface topography. Each of these technical factors can set limiting conditions when considering the economic recoverability for a given coal, mining method, and market. The important economic parameters are the relative costs of mining coal by surface and underground methods including costs associated with any site-specific land use constraints, the cost of removing the material above the coal seam in the surface method, and the price of coal.

The price for any particular coal is related directly to coal quality Box 4. Because more than 90 percent of the coal mined in the United States is used in power plants to generate electricity using steam turbines, the price for steam coal is dependent primarily on its heating value and sulfur content.

Relatively shallow coal deposits are generally extracted by surface mining, and deeper deposits are extracted by underground mining more detailed descriptions of surface and underground mining processes can be found in Appendix E. There are also situations in which a seam is mined by surface methods first, and then if adequate reserves are still available, the mine is developed for underground extraction.

Where remaining reserves are limited, other methods of mining—such as auger mining or highwall mining—may be used. Surface mining has many advantages compared to underground mining. The productivity of surface mines is generally higher than that of underground mines Figure 4.

Surface-mined coal from the Powder River Basin is usually simply sized and screened in preparation for market, whereas underground-mined coal and surface-mined coal from the Interior and Appalachian basins often requires a greater amount of processing see below to improve its marketability. The cost per ton of mining coal by surface methods is generally lower than that by underground methods.

In the United States, in addition to a continuous growth in coal production since the s, there has been a dramatic shift in production from underground mining to surface mining Figure 1. In the Powder River Basin PRB , where deposits of coal more than feet thick occur close to the surface, individual surface mines can produce more than 90 million tons each year. Underground coal mining is more common east of the Mississippi River, particularly in Appalachia.

Some of the largest underground coal mines, each producing around 10 million tons annually, are located in Pennsylvania and West Virginia. The largest underground mining complex in the United States produces about 20 million tons per year.

Bituminous coals in the eastern and central United States are mined by both surface and underground mining methods. Anthracite coal is mined exclusively in northeastern Pennsylvania, also by both underground and surface mining methods. Lignite and subbituminous coal production is centered in a small number of large mines Table 4.

Subbituminous coal and lignite comprise about The distinction between tonnage and energy content is particularly important when considering possible shifts in coal supply and demand by producing and consuming regions. Although the term coal refers to any readily combustible rock containing more than 50 percent by weight of organic matter, coals differ considerably in their physical and chemical characteristics Table 4. These differing characteristics impact coal utilization in both the electricity generation and metallurgical markets.

The term coal quality is used to distinguish the range of different commercial steam coals that are produced directly by mining or are produced by coal cleaning. While not as obvious as the impact of sulfur content on environmental emissions, differences in the moisture content and heating values among different coal types affect CO 2 emissions upon combustion, with higher-rank bituminous coals producing 7 to 14 percent lower emissions than subbituminous coals on a net calorific value basis Winschel, Standards that are broadly similar, but differ in detail, are used by the international coal trade and some coal mining countries.

The individual importance of these factors varies according to the intended use of the coal. TABLE 4. Chapter 2 rely heavily on increased production from this region for consumption east of the Mississippi River. Because it takes about 50 percent more subbituminous coal on a tonnage basis to replace a ton of bituminous coal in electricity generation, 1 this has significant implications for transportation infrastructure and power plant design and capacity. When used for electricity generation, coal from the PRB generally produces more CO 2 per kilowatt-hour than the bituminous coal mined in the east.

Combustion of subbituminous coal from the PRB produces about pounds of CO 2 for every million Btu British thermal units of heat generated on a net calorific basis , compared with about pounds for the bituminous coal mined in the East Winschel, An increasingly important by-product of U.

Captured methane may be used as a fuel source at the mine or, where feasible, distributed in natural gas pipelines. In recent years, coal bed methane production has increased and now comprises about 8 percent of the U. An ancillary benefit of recovering coal mine methane is reduced atmospheric methane emissions, because methane is a potent greenhouse gas.

In addition to the difference in heating value i. This can lead to differences in generating capacity when using different coals. Processing adds value in several ways:. Improved and consistent quality increases the efficiency and availability of steam boilers and is particularly important for the quality of metallurgical coke. Physical processing see Appendix E can, to some extent, reduce sulfur and trace element contents, particularly on a heating value basis.

However, generally coal cleaning is not practiced primarily for this purpose except for the metallurgical coal market. The decision whether or not to process a particular raw coal depends on the coal and its intended market.

Most coal preparation plants are in the eastern states, with more than 80 percent of the plants and almost 80 percent of capacity located in West Virginia, Virginia, Pennsylvania, Ohio, and Kentucky Fiscor, ; see Appendix E. The 11 coal preparation plants in the western states are located at bituminous. Ash content is determined by combusting the coal in air and converting the inorganic elements to their oxides.

Commercial coal characteristics, such as heating value, ash, moisture, sulfur, etc. Wyoming—the largest coal-producing state in the country—has no coal preparation plants, largely because the subbituminous Powder River Basin coal is low in ash. The conditions that will be encountered in future coal mines will undoubtedly be different from those of today—the more easily mined coal has already been extracted.

As shallower coal is depleted and seams with greater amounts of overburden 6 are mined, surface mining will become more expensive because stripping ratios 7 will increase and multiple benches 8 will be needed. This will increase the number of unit operations and the associated cost. In underground mining, the mines of the future will have to access seams that are deeper, thinner, or thicker, often with higher gas content and potentially with greater ground-control issues.

Overlying or underlying seams may have been mined, or it may be necessary to mine multiple seams simultaneously to meet increased production requirements. These more difficult mining situations will have an effect on economic decisions related to mine profitability, on the health and safety of mine workers as they encounter more challenging or different mining requirements, on the technical ability to mine, and on the management of waste materials generated by mining.

Small mines i. They also play a critical role in fully utilizing national coal resources, particularly those resources that may not be mined by larger operations. However, small mines will continue to exist, and the technical and societal issues they will face in the future should be considered in agency plans. Although statistics show substantially improved health and safety conditions in mines in recent years—with continuous decreases in both the incidence and.

Overburden is the overlying rock and soil that must be removed to gain access to a coal seam to be mined. The ratio of overburden thickness to coal thickness; may also be measured by weight or volume. If overburden exceeds a certain thickness, equipment size requires that the overburden is removed in more than one pass leaving a flat bench for equipment access between each pass. Larger mining companies will only mine when sufficient coal reserves are available to sustain the considerable capital investment required for advanced mining equipment and technologies.

These coals are rarely processed before shipment or use. However, the oxygen and moisture contents of low-rank coals are greater than those of bituminous coals.

This reduces the heating value of the coal as mined, which increases the transportation cost on a heating value basis and reduces the thermal efficiency of the steam boilers that use these coals. Most lignite mined in the United States is used in minemouth plants. Subbituminous coals, however, are generally transported considerable distances, so their high moisture content and low heating value add to the effective transportation cost and environmental impact.

One way to offset these disadvantages is to dry the coal before transportation or utilization. Numerous processes for drying low-rank coals to upgrade them have been proposed, demonstrated e.

The characteristics of dried low-rank coal—it is friable, has a tendency to spontaneously heat, and readily reabsorbs moisture—constitute major obstacles that must be overcome to produce a saleable, transportable, dry coal product. As the coal mine disasters in early demonstrated MSTTC, , the safe operation of mines remains a major challenge—there needs to be constant monitoring and control of health and safety threats as well as continuous safety training and improvements in operating practices.

Past experience has shown that changes in mining operations or practices e. Similarly, experience has shown that adequate engineering controls and a knowledgeable workforce are the prerequisites for a safer work environment. Continued health and safety research is needed to identify new hazards and hazard sources as well as to improve the engineering controls for existing hazards, particularly through the development of reliable monitoring and intelligent control systems.

The likelihood of deeper mines in the future means that there has to be increased attention to methane control including methane capture before, during, or after mining , dust control, ignition sources, fires, and explosions. Recent disasters have shown that there are major knowledge gaps and technology needs in the areas of escape and survival, and emergency preparedness and rescue, emphasizing the need for research to develop systematic and comprehensive.

In addition, since powered haulage and machinery now have the dubious honor of surpassing ground-control incidents e. Mining of deeper seams, multiple seams, thicker seams, and seams underlying or overlying mined-out seams will all require a greater understanding of strata control aspects. The risks associated with mining coal seams adjacent to previously mined-out seams, with their actual or potential void spaces, emphasize the need for accurate, comprehensive, and readily available mine maps showing the distribution of older mine workings.

Adequate information on the nature and characteristics of a coal seam prior to mining is vitally important for safe and efficient mine operations. Any unexpected anomalies in a coal seam and associated strata, such as sand channels, washouts, faults, and roof instability, can substantially impair mining productivity and create safety hazards. Research to devise improved techniques for imaging coal seams and associated strata, including coal quality parameters, prior to and during mining is needed for mine planning, permitting, and operations NRC, b.

The Martin County coal impoundment failure NRC, a and the Quecreek inundation PDEP, illustrated the need for increased accuracy and reliability of the geological information used for mine and mine facilities planning. At present, the drilling and blasting required for overburden fragmentation is the major limitation on increased surface mining productivity, and the development of improved rock fragmentation practices is an essential requirement for increased surface mine production.

For underground mining, the increased use of longwall mining see Appendix E offers the greatest potential for higher productivity.