Coal, Production of
COAL, PRODUCTION OF
GEOLOGY OF COAL
Coal is a fossil fuel—an energy source whose beginnings can be traced to once-living organic materials. It is a combustible mineral, formed from the remains of trees, ferns, and other plants that existed and died in the tropical forests 400 million to 1 billion years ago. Over vast spans of time, heat and pressure from Earth's geological processes compressed and altered the many layers of trees and plants, slowly transforming these ancient vegetal materials into what we know as coal today. The several kinds of coal now mined are the result of different degrees of alteration of the original material.
It is estimated that approximately 0.9 to 2.1 m of reasonably compacted plant material was required to form 0.3 m of bituminous coal. Different ranks of coal require different amounts of time. It has been estimated that the time required for deposition of peat sufficient to provide 0.3 m of the various ranks of coal was: lignite, 160 years; bituminous coal, 260 years; and anthracite, 490 years. Another estimate indicates that a 2.4 m bed of Pittsburgh Seam (bituminous) coal required about 2,100 years for the deposition of necessary peat, while an anthracite bed with a thickness of 9.1 m required about 15,000 years.
Depending on the environment in which it was originally deposited, coal will have higher sulfur content when it was formed in swamps covered by sea-water; generally, low-sulfur coal was formed under freshwater conditions. Although coal is primarily carbon, it's complex chemical structure contains other elements as well—hydrogen, oxygen, nitrogen, and variable trace quantities of aluminum, zirconium, and other minerals.
Coal is a very complex and diverse energy resource that can vary greatly, even within the same deposit. The word "rank" is used to designate differences in coal that are due to progressive change from lignite to anthracite. Generally, a change is accomplished by increase in carbon, sulfur, and probably in ash. However, when one coal is distinguished from another by quantity of ash or sulfur, the difference is said to be of grade. Thus a higher-grade coal is one that is relatively pure, whereas a higher-rank coal is one that is relatively high on the scales of coals, or one that has undergone devolatization and contains less volatile matter, oxygen, and moisture than it did before the change occurred.
In general, there are four ranks of coal: lignite, subbituminous, bituminous, and anthracite. Lignite, a brownish-black coal with generally high moisture and ash content, as well as lower heating value, is the lowest-rank coal. It is an important form of energy for electricity generation. Some lignite, under still more pressure, will change into subbituminous, the next higher rank of coal. It is a dull black coal with a higher heating value than lignite and is used primarily for generating electricity and space heating. Even greater pressure will result in the creation of bituminous, or "soft" coal, which has higher heating value than subbituminous coal. Bituminous coals are primarily used for generating electricity. Anthracite is formed from bituminous coal when great pressure developed during the geological process, which occurred only in limited geographic areas. Sometimes referred to as "hard" coal, anthracite has the highest energy content of all coals and is used for space heating and generating electricity.
In addition to carbon, all coals contains many non-combustible mineral impurities. The residue from these minerals after coal has been burned is called ash. Average ash content of the entire thickness of a coal seam typically ranges from 2 to 3 percent, even for very pure bituminous coals, and 10 percent or more for many commercial mines. These materials, which vary widely in coal seams with respect to kind, abundance, and distribution, among from shale, kaolin, sulfide, and chloride groups.
WORLD COAL RESERVES AND PRODUCTION
Coal is the most abundant and most economical fossil fuel resource in the world. Proven coal reserves exceed 1 trillion tons, and indicated reserves are estimated at 24 trillion tons. Coal is found in every continent of the world, including Antarctica, although the largest quantities of coal are in the Northern Hemisphere. Coal is mined in some sixty countries in nineteen coal basins around the world, but more than 57 percent of the world's total recoverable reserves are estimated to be in the United States, and China, which together account for more than two-thirds of the world's coal production.
COAL MINING METHODS
Depending on the depth and location of the coalbed and the geology of the surrounding area, coal can be mined using either surface or underground methods. In the United States, coal is usually mined underground if the depth of the deposit exceeds 200 ft.
In surface mining, the covering layers of rock and soil (called "overburden") are first removed using either a power shovel, a dragline (for large surface mines), or bulldozers and front-end loaders (for small mines). Front-end loaders also can be used to load coal. In large mines, coal usually is loaded using power shovels and hydraulic shovels. Depending on the size of the mine, shovels and draglines ranging from 4 cu m to 50 cu m are usually used for loading and excavating. Large-capacity haul trucks, usually in the range of 170 to 240 mt but possibly as big as 320 mt, are then used to transport coal to loading stations for shipping and sold as raw coal, or to a preparation plant for further processing. For post mining reclamation, draglines are used.
Depending on geologic conditions and surrounding terrain, there could be several types of surface mining. If the coal seam is of the same depth in flat or gently rolling land, area mining is developed where the over-burden from one cut is used to fill the mined-out area of the preceding cut. Contour mining and mountain-top removal are methods that follow a coalbed along the hillsides. The overburden is cast (spoiled) down-hill from this first pit, exposing the coal for loading by trucks. The second pit could then be excavated by placing the overburden from it into the first pit. Digging starts where the coal and surface elevations are the same and precedes toward the center of a hill or mountain until the overburden becomes too thick to remove economically. An open pit combines the techniques of contour and area mining and is used where thick coalbeds are steeply inclined.
After coal is extracted, the pit is backfilled with earth and subsequently reclaimed or restored to its approximately original contour, vegetation, and appearance.
The use of underground mining methods requires integration of transportation, ventilation, ground control, and mining methods to form a system that provides the highest possible degree of safety, the lowest cost per ton of product, the most suitable quality of final product, the maximum possible recovery of coal, and the minimum disturbance of environment. Depending on the location of coal deposits, there can be three different types of underground mines: a drift mine is one in which a horizontal (or nearly horizontal) coal seam crops to the surface in the side of a mountain, and the opening of a mine can be made into the coal seam. Transportation of coal to the outside can be by track haulage, belt conveyor, or rubber-tired equipment.
A slope mine is one in which the coal is of moderate depth and where access is made through an inclined slope (maximum, 16°). This type of mining also may follow the coalbed if the coal seam itself is included and outcrops, or the slope may be driven in rock strata overlying the coal to reach the coal seam. Either a belt conveyor (no more than 30% grade), coal trucks (maximum grade, 18%), or electrical hoist if the slope is steep, can be used to transport coal out of the mine.
When the coal seam is deep, a shaft mine is used because the other two types of access are cost-prohibitive. Vertical shafts are drilled for both production and ventilation.
Production methods underground are generally classified according to the types of mining equipment used (conventional, continuous mining, or longwall) or by the method in which coal is being extracted (longwall or longwall caving). Both conventional and continuous mining are room-and-pillar systems; even the longwall method uses room-and-pillar during development.
In the conventional mining system, the coal face is first undercut, center cut or top cut using a cutting machine that most nearly resembles a large chain saw on wheels. The outlined coal blocks are drilled in a predetermined drill pattern using a mobile powered drill, with holes charged with explosives, and the coal is dislodged. The broken coal is gathered by a loading machine onto a shuttle car and dumped onto a nearby belt, to be transported out of the mine.
In the continuous mining method, a continuous mining machine (also referred to as a continuous miner) is employed in the extraction process. This machine combines several extracting functions into one continuous process: cutting, loading, and tramming, thereby tearing the coal from a seam and automatically removing it from the area by a machine-mounted conveyor onto a shuttle car, which is used to transport the mined coal to a dumping station, then transported out of the mine using a conveyor belt. Remote-controlled continuous miners allow an operator to control the machine from a distance, increasing safety. The mine roof is further secured using wooden timbers; steel crossbars on posts; or, most commonly, roof bolts.
Both conventional and continuous mining methods use a room-and-pillar system in which the coal is mined by extracting a series of "rooms" into the coalbed, and leaving "pillars," or columns, of coal to help support the mine roof (Figure 1). Depending on the location, the rooms are generally 20 to 30 ft. wide and the pillars 20 to 90 ft. wide, with the height determined by the thickness of the coal seam. In the not-too-distant future, robotic versions of these machines, now under development, will allow for enhanced automatic operations and even greater efficiencies than now possible. Although still utilized in stand-alone production operations, continuous miners also are employed for main entry and longwall panel developments.
As a rule of thumb, 50 to 55 percent of coal can be extracted using continuous mining. To improve this extraction ratio, a pillar-recovery process usually is applied when mining reaches the end of the panel and the direction of the mining is reversed. The continuous miner mines into the pillars, recovering as much coal as possible, as the roof is allowed to systematically collapse. Usually this can increase the extraction ratio by up to 5 percent.
Although the development of the continuous mining system in the 1950s consolidated several operations in one machine and have greatly improved coal production, it is still not fully "continuous," as the face haulage and roof support operations remain as major impediments to truly continuous production.
The introduction of the longwall system has provided not only continuous cutting and loading but also continuous haulage and roof support. In the longwall mining system, large blocks of coal, outlined in the development process, are completely extracted in a single, continuous operation.
The longwall consists of a panel of coal, usually 8,000 to 10,000 ft in length and 800 to 900 ft in width (Figure 2). In the face area, a rotating drum (or a plow) is dragged mechanically back and forth across a wide coal seam. The loosened coal falls onto a conveyor for removal from the mine. The system has its own hydraulic roof supports, which advance with the machine as mining proceeds. The supports provide not only high levels of production but also increased miner safety. Newer versions of the longwall system employ sensors to detect the amount of coal remaining in the seam being mined, as well as robotic controls to enhance efficiency. As the face advances after the coal is mined, the roof is systematically allowed to cave behind to form a gob. In general, longwall systems can provide an extraction ratio of up to 80 percent.
Longwall mining has helped revolutionize underground coal mine operations in the past two decades, with its share of total U.S. underground production increasing from 10 percent to 48 percent, surpassing continuous mining tonnage in 1994, and the trend has held true since then.
Several modified versions of longwall methods also are practiced in areas where the coal seam is either thick or steeply inclined. Since the maximum height a shearer can reach is about 14 ft, thicker coal seams have to be mined using either a multiple pass method, where the top seam is mined followed by a lower pass, or a longwall caving method, where the lowest seam is mined using the traditional longwall method and the upper portion of the seam is allowed to cave under gravity; coal is then collected behind the shield support and shipped out of the mine.
COAL MINING AND THE ENVIRONMENT
It has been said that mining is a temporary use of the land. While mining does disturb the land, modern technologies and increased application of environmentally safe mining methods in the United States and other major mining countries have enabled today's coal mining industry to provide the valuable energy resources modern society requires without destroying the environment in the process. In the United States, stringent environmental regulations mandate specific standards for reclamation, quality of water discharge, and other mining practices that may disturb the land.
While some problems still exist, there is no question that coal mining operations are more efficient and safer for workers and leave less of an environmental footprint than operations several generations ago. As society's demand for energy from coal continues to increase and as coal's price declines (between 1978 and 1996 U.S. mine mouth prices fell from $47.08 to $18.50 per ton in constant 1996 dollars), there is certain to be even greater efforts to limit the environmental impact of mining operations.
Jerry C. Tien
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