What is Coal?

Coal is a readily combustible rock containing more than 50 percent by weight of carbonaceous material, formed from compaction and induration of variously altered plant remains similar to those in peat [adapted from AGI's "Glossary of Geology"]. Most coal is fossil peat. Peat is an unconsolidated deposit of plant remains from a water-saturated environment such as a bog or mire; structures of the vegetal matter can be seen, and, when dried, peat burns freely [adapted from AGI's "Glossary of Geology"].

How is Coal Formed?

Coal is formed when peat is altered physically and chemically. This process is called "coalification." During coalification, peat undergoes several changes as a result of bacterial decay, compaction, heat, and time. Peat deposits are quite varied and contain everything from pristine plant parts (roots, bark, spores, etc.) to decayed plants, decay products, and even charcoal if the peat caught fire during accumulation. Peat deposits typically form in a waterlogged environment where plant debris accumulated; peat bogs and peat swamps are examples. In such an environment, the accumulation of plant debris exceeds the rate of bacterial decay of the debris. The bacterial decay rate is reduced because the available oxygen in organic-rich water is completely used up by the decaying process. Anaerobic (without oxygen) decay is much slower than aerobic decay.

For the peat to become coal, it must be buried by sediment. Burial compacts the peat and, consequently, much water is squeezed out during the first stages of burial. Continued burial and the addition of heat and time cause the complex hydrocarbon compounds in the peat to break down and alter in a variety of ways. The gaseous alteration products (methane is one) are typically expelled from the deposit, and the deposit becomes more and more carbon-rich as the other elements disperse. The stages of this trend proceed from plant debris through peat, lignite, sub-bituminous coal, bituminous coal, anthracite coal, to graphite (a pure carbon mineral).

Because of the amount of squeezing and water loss that accompanies the compaction of peat after burial, it is estimated that it took 10 vertical feet of original peat material to produce 1 vertical foot of bituminous coal in eastern and western Kentucky. The peat to coal ratio is variable and dependent on the original type of peat the coal came from and the rank of the coal.

Identification of Coal Components

Coal can be examined on both macroscopic (observable with the unaided eye or a hand lens) and microscopic levels. When attempting to describe a coal specimen, starting out on the macroscopic level is the best option. At this level, coal will appear banded or nonbanded. Nonbanded massive coals include cannel and boghead coal, both of which are dull and blocky. Cannel coal is composed of spores, whereas boghead coal consists mainly of algal matter. Cannel is derived from the word "candle," because pencil-shaped pieces were used as candles in the past.

Banded coals grade from dull banded ("splint coal") to bright banded, depending on whether dull bands or bright bands are dominant. The bands are divided into four major lithotypes: durain (dull, grainy texture, tough), fusain (dull black, charcoal texture, hands get dirty), clarain (bright, satiny texture, brittle), and vitrain (bright, black, glassy, brittle).

Bright coals have lots of vitrain and clarain; dull coals are rich in durain bands. Fusain generally occurs only in thin and sporadic bands. Splint coals are durain-rich and can be massive (nonbanded) or banded. Most vitrain-and clarain-rich banded coals break into small blocky pieces along joints, called cleats. Vitrain and clarain are brittle and break easily. "Block coals" are dull coals that break into large blocks because they have fewer vitrain and clarain bands, but have a composition higher in liptinite macerals, which are tough.

"Bone" and "bone coals" have a high ash content in the form of clays and silts; they form part of a continuum between dark shale and dull (banded or nonbanded) coal in the following sequence: dark shale, bone (greater than 50 percent ash), boney coal (less than 50 percent ash), dull coal (cannel, boghead, or splint).

On the microscopic level, coal is made up of organic grains called macerals. Coal petrographers (people who study coal under the microscope) separate the macerals into three maceral groups, each of which includes several maceral types. The groups are liptinite, vitrinite, and inertinite. Macerals are defined according to their grayness in reflected light: liptinites are dark gray, vitrinites are medium to light gray, and inertinites are white and can be very bright. Liptinites are composed of hydrogen-rich hydrocarbons derived from spores, pollens, cuticles, and resins in the original plant material. Vitrinites are composed of "gelified" wood, bark, and roots, and contain less hydrogen than liptinites. Inertinites are mainly oxidation products of other macerals and are consequently richer in carbon than liptinites or vitrinites. The inertinite group includes fusinite, most of which is fossil charcoal, derived from ancient peat fires.

Classification and Rank of Coal

The kinds of coal, in increasing order of alteration, are lignite (brown coal--immature), sub-bituminous, bituminous, and anthracite (mature). Coal starts off as peat. After a considerable amount of time, heat, and burial pressure, it is metamorphosed from peat to lignite. Lignite is considered to be "immature" coal at this stage of development because it is still somewhat light in color and it remains soft. As time passes, lignite increases in maturity by becoming darker and harder and is then classified as sub-bituminous coal. As this process of burial and alteration continues, more chemical and physical changes occur and a the coal is classified as bituminous. At this point the coal is dark and hard. Anthracite is the last of the classifications, and this terminology is used when the coal has reached ultimate maturation. Anthracite coal is very hard and shiny.

The degree of alteration (or metamorphism) that occurs as a coal matures from peat to anthracite is referred to as the "rank" of the coal. Low-rank coals include lignite and sub-bituminous coals. These coals have a lower energy content because they have a low carbon content. They are lighter (earthier) and have higher moisture levels. As time, heat, and burial pressure all increase, the rank does as well. High-rank coals, including bituminous and anthracite coals, contain more carbon than lower-rank coals which results in a much higher energy content. They have a more vitreous (shiny) appearance and lower moisture content then lower-rank coals.

Methods of Mining

According to the Kentucky Department of Mines and Minerals, 131.8 million tons of coal was mined in Kentucky in 2000; 62 percent (81 million tons) was from underground mines and 38 percent (50 million tons) was from surface mines. There were 264 active underground mines and 240 active surface mines in Kentucky in 2000.

Underground modes of access include drift, slope, and shaft mining, and actual mining methods include longwall and room and pillar mining. Drift mines enter horizontally into the side of a hill and mine the coal within the hill. Slope mines usually begin in a valley bottom, and a tunnel slopes down to the coal to be mined. Shaft mines are the deepest mines; a vertical shaft with an elevator is made from the surface down to the coal. In western Kentucky, one shaft mine reaches 1,200 feet below the surface.

In room and pillar mining, the most common type of underground coal mining, coal seams are mined by a "continuous miner" that cuts a network of "rooms" into the seam. As the rooms are cut, the continuous miner simultaneously loads the coal onto a shuttle or ram car where it will eventually be placed on a conveyor belt that will move it to the surface. "Pillars" composed of coal are left behind to support the roof of the mine. Each "room" alternates with a "pillar" of greater width for support. Using this mining method normally results in a reduction in recovery of as much as 60 percent because of coal being left in the ground as pillars. As mining continues, roof bolts are placed in the ceiling to avoid ceiling collapse. Under special circumstances, pillars may sometimes be removed or "pulled" toward the end of mining in a process called "retreat mining." Removing support during retreat mining can lead to roof falls, so the pillars are removed in the opposite direction from which the mine advanced: hence the term "retreat mining."

Longwall mining is another type of underground mining. Mechanized shearers are used to cut and remove the coal at the face of the mine. After the coal is removed, it drops onto a chain conveyor, which moves it to a second conveyor that will ultimately take the coal to the surface. Temporary hydraulic-powered roof supports hold up the roof as the extraction process proceeds. This method of mining has proven to be more efficient than room and pillar mining, with a recovery rate of nearly 75 percent, but the equipment is more expensive than conventional room and pillar equipment, and cannot be used in all geological circumstances. As mining continues, roof bolts are placed in the ceiling to avoid ceiling collapse. In longwall mining, only the main tunnels are bolted. Most of the longwall panel is allowed to collapse behind the shields (which hold the roof as coal is excavated).

Surface Mining

Surface-mining methods include area, contour, mountaintop removal, and auger mining. Area mines are surface mines that remove shallow coal over a broad area where the land is fairly flat. Huge dragline shovels commonly remove rocks overlying the coal (called overburden). After the coal has been removed, the rock is placed back into the pit. Contour mines are surface mines that mine coal in steep, hilly, or mountainous terrain. A wedge of overburden is removed along the coal outcrop on the side of a hill, forming a bench at the level of the coal. After the coal is removed, the overburden is placed back on the bench to return the hill to its natural slope. Mountaintop removal mines are special area mines used where several thick coal seams occur near the top of a mountain. Large quantities of overburden are removed from the top of the mountains, and this material is used to fill in valleys next to the mine. Augur mines are operated on surface-mine benches (before they are covered up); the coal in the side of the hill that can't be reached by contour mining is drilled (or augured) out. Drift, contour, mountaintop removal, and augur mining are more common in the Eastern Kentucky Coal Field, and area, slope, and shaft mining are more common in the Western Kentucky Coal Field.

Uses of Coal

At one time coal was predominantly used to heat homes, as well as power railroad locomotives and factories. Today, however, coal serves different purposes for society. The chief use of coal is now electricity generation. Other uses include coking coal for steel manufacturing and industrial process heating.

Electricity Generation

Eighty-two percent of Kentucky's coal is used to generate electricity. After coal is mined, it is transported to power plants by trains, barges, and trucks. A conveyor belt carries the coal to a pulverizer, where it is ground to the fineness of talcum powder. The powdered coal is then blown into a combustion chamber of a boiler, where it is burned at around 1,400ºC. Surrounding the walls of the boiler room are pipes filled with water. Because of the intense heat, the water vaporizes into superheated high-pressure steam. The steam passes through a turbine (which is similar to a large propeller) connected to a generator. The incoming steam causes the turbine to rotate at high speeds, creating a magnetic field inside wound wire coils in the generator. This pushes an electric current through the wire coils out of the power plant through transmission lines. After the steam passes through the turbine chamber, it is cooled down in cooling towers and it again becomes part of the water/steam cycle.

Electricity generation by conventional coal combustion.

Several by-products, including solids and gases, are created in the electricity generation process. A substance called "clinker" or bottom ash (glassy particles of melted coal ash) settles at the base of the furnace. This material is periodically removed and disposed of. Fly ash, the noncombustible minerals found in coal (including ash, dust, soot, and cinders) travels upward with gaseous by-products. Fly ash can be captured in an electrostatic precipitator and then transported by pipes to a holding pond, where it settles. Over 98 percent of all solids are captured in the plant. Gaseous by-products include carbon dioxide (CO₂), sulfur oxides (SO_x), and nitrogen oxides (NO_x). Sulfur oxides can be controlled by the installation of scrubbers at coal-fired power plants. Scrubbers allow high-sulfur coals to be used because they remove sulfur dioxides out of the gas stream in the stacks (a process called desulfurization). Scrubbers work by spraying limestone slurry directly in the path of the materials leaving the boiler chamber. The limestone reacts with the sulfur in the gases within the stacks. The combination of carbonate (limestone) and sulfur forms the mineral gypsum. Gypsum is a solid, which falls out of the gas to the bottom of the stacks, where it can be collected. The by-product gypsum created in this process can be used to make drywall and bowling balls. Nitrogen oxides are managed by careful control of the furnace temperature. There is current technology to control carbon dioxide; however, using high-efficiency coals (such as those found in Kentucky) helps reduce the output of CO₂.

Other Uses

When coal is heated in the absence of air, a porous, carbon-rich material called "coke" is formed. Bituminous coal (also called metallurgical coal or coking coal) is baked without air in an oven until most of its volatile matter is released. During this process, it softens, then liquefies and resolidifies into hard porous lumps. Coking coals are more expensive than coals used for heating or electricity. They must have a low sulfur and phosphorous content, which makes them less common than the types of coal used for heating and electricity. When iron and steel are made, coke is one of the constituents needed to properly heat the furnace (limestone and iron ore are two other constituents used). Gaseous by-products from coke ovens are also used. These include crude coal tar, light oils, and ammonia. Seventy percent of steel production comes from iron made in blast furnaces using coal and coke. In the recent past, however, the production of steel in the United States has declined because of, among other reasons, the use of plastics and imported steel. Therefore, the use of coal in the production of coke has declined over the years to 2 percent of Kentucky's annual mined coal.

In industrial process heating, coal is used to heat boilers and ovens. The cement (which represents the biggest worldwide industrial use of coal), glass, ceramic, and paper industries all use coal for this purpose. In Kentucky, industrial process heating accounts for 10 percent of Kentucky's annual mined coal.

Five percent of the coal mined in Kentucky is exported to other countries; Canada receives the most.

 Beneficiation of Coal

Coal is highly variable with respect to the physical and chemical properties that affect its use. Industries that use coal specify a range of properties that are required for their intended process. Coal suppliers try to find coals that most closely match those requirements. Coal is treated in processes called "beneficiation" to prepare a material that meets the customer's needs and is as homogenous as possible. Samples of coal from both cores and mines are taken to determine the treatment that must be performed. Preparation plants that perform specific beneficiation processes are constructed as near as possible to the location where the coal is mined.

Three kinds of processes may be performed at the plants: (1) sizing, controlled by a crushing and screening process, (2) increasing heating value, by removing noncombustible ash and rock by gravity separation, (3) removing or controlling undesirable mineral and chemical components (sulfur, sodium, and trace elements) by a combination of gravity separation and blending. Traditionally, most coal preparation was primarily concerned with sulfur and ash reduction. Today, however, much more sophisticated processes have more narrow and complex physical and chemical requirements for coal stock.

Coal and the Environment

Coal mining, preparation, and use, has environmental consequences. A long history of mining without regard to environmental consequences led to an unfortunate legacy of sediment-laden streams and rust-colored water in many parts of the United States. Public concern about industrial pollution (including coal mining, preparation, and use) led to a series of Federal Regulations in the 1970s including the Clean Air Act (1970; amended in 1977 and 1990), Clean Water Act (1972; amended in 1977), and the Surface Mine Control and Reclamation Act (1977). Since these regulations were enacted, a wide array of technologies and methods for reclaiming past environmental impacts have been developed as have methods for preventing or mitigating similar impacts from modern mining, preparation, and coal use. Research continues on many aspects of coal’s environmental impacts including carbon dioxide emissions.

A good generalized summary of the environmental concerns associated with coal mining, preparation, and use is Coal and the Environment. This 64-page color booklet was printed in the summer of 2006, and is part of the American Geological Institute’s Environmental Awareness Series. The booklet can be ordered from the Kentucky Geological Survey at http://kgsweb.uky.edu/PubsSearching/orderinginfo.asp, or the American Geological Institute. Supplementary material to the booklet is provided here to aid in your understanding of the environmental impacts associated with coal and how those impacts are prevented or mitigated today.