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| Coal |
Coal is a general term encompassing a variety of combustible sedimentary and metamorphic rocks containing altered and fossilized terrestrial plant remains in excess of 50 percent by weight, and more than 70 percent by volume.
Categories of coal differ in relative amounts of moisture, volatile matter, fixed carbon, and degree of compaction of the original carbonaceous material. Coal is therefore commonly termed a fossil fuel. This key resource is the product of the carbon from ancient plants that have undergone sedimentary and metamorphic transformation over millions of years.
Formation
After dead land-plant matter has accumulated and slowly begun to compact, biochemical decomposition, rising temperature, and rising pressure all contribute to the lengthy process of altering the plant debris into coal.
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The more common coals are of vascular vegetable origin, formed from the compaction and induration of accumulated remains of plants that once grew in extensive swamp and coastal marsh areas.
These deposits are classed as humic coals consisting of organic matter that has passed through the peat, or earliest coal formation, stage. A variety of humic coals are known.
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| Formation |
Under such near-stagnant conditions plant remains are preserved, while the presence of hydrogen sulfide inhibits organisms that feed on dead vegetation. Analog environments under which coal is currently forming are found within the Atchafalaya swamp of coastal Louisiana and the many peat-producing regions of Ireland.
A layer of peat in excess of 2 meters in thickness and covering more than 5,000 square kilometers is present in the Great Dismal swamp of coastal North Carolina and Virginia.
The sapropelic class of coal, relatively uncommon in distribution and composed of fossil algae and spores, is formed through partial decomposition of organic matter by organisms within oxygen deficient lakes and ponds. Sapropelic coals are subdivided into boghead (algae origin) and cannel (spore origin) deposits.
The vegetable origin of coal has been accepted since 1825 and is convincingly evidenced by the identification of more than three thousand freshwater plant species in coal beds of Carboniferous (360 million to 286 million years ago) age. The common association of root structures and even upright stumps with layers of coal indicate that the parent plant material grew and accumulated in place.
Detailed geologic studies of rock sequences that lie immediately above and below coal deposits indicate that most coals were formed in coastal regions affected by long-term sea level cycles characterized by transgressing (advancing) and regressing (retreating) shorelines.
Such a sequence of rock deposited during a single advance and retreat of the shoreline, termed a cyclothem, typically contains non marine strata separated from overlying marine strata by a single layer of coal.
In sections of the Interior coal province, a minimum of fifty cyclothems have been recognized, some of which can be traced across thousands of square kilometers. Such repetition in a rock sequence is most advantageous to the economics of a coal region, creating a situation in which a vertical mine shaft could penetrate scores of layers of coal.
The formation of coal is a long-term geologic process. Coal cannot therefore be considered a renewable resource, even though it is formed from plant matter. Studies have suggested that 1 meter of low-rank coal requires approximately ten thousand years of plant growth, accumulation, biologic reduction, and compaction to develop.
Using these time lines, the 3-meter-thick Pittsburgh coal bed, underlying 39,000 square kilometers of Pennsylvania, developed over a period of thirty thousand years, while the 26-meter-thick bed of coal found at Adaville, Wyoming, required approximately a quarter of a million years to develop.
Coal formation favors sites where plant growth is abundant and conditions for organic preservation are favorable. Such climates range from subtropical to cold, with the ideal being classed as temperate. Tropical swamps produce an abundance of plant matter but have very high bacterial activity, resulting in low production of peat.
Modern peats are developing in temperate to cold climate regions, such as Canada and Ireland, where abundant precipitation ensures fast plant growth, while relatively low temperatures diminish the effectiveness of decay-promoting bacteria.
The first coal provinces began to form with the evolution of cellulose-rich land plants. One of the earliest known coal deposits, of Upper Devonian age (approximately 365million years ago), is found on Buren Island, Norway.
Between the Devonian period and today, every geologic period is represented by at least some coal somewhere in the world. Certain periods of time, however, are significant coal-forming ages.
During the Carboniferous and Permian periods (360 to 245 million years ago)widespread development of fern and scale tree growth set the stage for the formation of the Appalachian coal province and the coal districts of Great Britain, Russia, and Manchuria. Coal volumes formed during these periods of geologic time constitute approximately 65 percent of present world reserves.
The remaining reserves, developed mainly over the past 200 million years, formed in swamps consisting of angiosperms (flowering plants). The reserves of the Rocky Mountain province and those of central Europe are representative of these younger coals.
Classification of Coal
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| Classification of Coal |
In 1937 a classification of coal rank using fixed carbon and Btu content was adopted by the American Standards Association. Adaptations of this scheme are still in use, listing the steps of progressive increase in coal rank as lignite (brown coal), subbituminous, bituminous (soft coal), subanthracite, and anthracite (hard coal).
Some classification schemes also list peat as the lowest rank of coal. Technically speaking, peat is not a coal; rather, it is a fuel and a precursor to coal.
Coalification is the geologic process whereby plant material is altered into differing ranks of coal by geochemical and diagenetic change. With an increase in rank, chemical changes involve an increase in carbon content accompanied by a decrease in hydrogen and oxygen.
Correspondingly, diagenesis involves an increase in density and calorific value and a progressive decrease in moisture. At all ranks, impurities include sulfur, silt and clay particles, and silica.
Peat, an unconsolidated accumulation of partly decomposed plant material, has an approximate carbon content of 20 percent. In many classification schemes, peat is listed as the initial stage of coal formation. Moisture content is quite high, at least at the 75-percent level.
When dry, peat has an oxygen content of about 30 percent, is flammable, and will freely but inefficiently burn slowly and steadily for months at a low heat-content value of 5,400 Btu’s per pound. (The British thermal unit, or Btu, is the quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit.)
Types of Coal
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| Types of Coal |
Deeper burial with even higher temperatures and pressures gradually alters lignite to bituminous coal, a dense, dusty, brittle, well-jointed, dark brown to black fuel that burns readily with a smoky yellow flame.
It is the most abundant form of coal in the United States. Calorific value ranges from 10,500 to 15,500 Btu’s per pound, and carbon content varies from 45 to 86 percent. Moisture content is as low as 5 percent, but heating value is high.
The subbituminous class of coal is intermediate between lignite and bituminous and has characteristics of both. Little woody matter is visible. It splits parallel to bedding but generally lacks the jointing of bituminous coal. It burns clean but with a relatively low heating value.
Anthracite is jet-black in color, has a high luster, is very hard and dust-free, and breaks with a conchoidal fracture. Carbon content ranges from 86 to 98 percent.
It is slow to ignite, burns with a short blue flame without smoke, and, with a calorific value in excess of 14,000 Btu’s per pound, is a high heating fuel. U.S. reserves are found mainly in eleven northeastern counties in Pennsylvania. Subanthracite coal has characteristics intermediate between bituminous and anthracite.
Bedded and compacted coal layers are geologically considered to be rocks. Lignite and bituminous ranks are classed as organic sedimentary rocks.
Anthracite, formed when bituminous beds of coal are subjected to the folding and regional deformation affiliated with mountain building processes, is listed as a metamorphic rock. Because peat is not consolidated or compacted, it is classed as an organic sediment.
Graphite, a naturally occurring crystalline form of almost pure carbon, is occasionally associated with anthracite. While it can occur as the result of high-temperature alteration of anthracite, its chemical purity and common association with crystalline rock causes it to be listed as a mineral.
Worldwide Distribution
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| Worldwide Distribution |
It is estimated that total world coal resources, defined as coal reserves plus other deposits that are not economically recoverable plus inferred future discoveries, are on the order of 10 trillion tons.
Of this amount, estimates of world coal reserves, defined as those deposits that have been measured, evaluated, and can be extracted profitably under existing technology and economic conditions, range up to a high of approximately one trillion tons.
World reserves can be divided into two categories, with 73 percent composed of anthracite and bituminous coals and 27 percent composed of lignite. Among nations, the United States possesses the greatest amount (approximately 500 billion tons) of total world reserves.
Modern Use
Historically, coal has been industry’s fuel of choice. Countries with large coal reserves have risen commercially, while those less endowed with this resource—or lacking it altogether—have turned to agriculture or stagnated in development. Different ranks of coal are employed for different purposes.
In the middle of the twentieth century it was common to see separate listings of coking, gas, steam, fuel, and domestic coals. Each had its specific uses. Coal for home use could not yield excessive smoke, while coal for locomotives had to raise steam quickly and not produce too high an ash content.
Immediately after World War II fuel coal use, representing 78 percent of annual production, was divided into steam raising, railway transportation, domestic consumption, electric generation, and bunker coal. The remaining 22 percent was employed in the production of pig iron, steel, and gas.
Fifty years later, more than 80 percent of the approximately 1 billion tons of coal produced annually in the United States was used to generate electricity. Industrial consumption of coal, particularly in the production of coke for the steel and iron manufacturing industry, is the second most important use.
Other industrial uses of coal are food processing and the manufacture of paper, glass, cement, and stone. Coal produces more energy than any other known fuel, including natural gas, crude oil, nuclear, and renewable fuels.
While expensive to produce, the conversion of intermediate ranks of coal into liquid (coal oil) and gaseous (coal gas) forms of hydrocarbon fuels will become more economically viable, especially during times of increase in the value of crude oil and natural gas reserves. New uses of coal are constantly being explored and tested.
Two promising techniques are the mixing of water with powdered coal to make a slurry which can be burned as a liquid fuel and the underground extraction of coal-bed methane (firedamp). Interest in the latter by-product as an accessible and clean-burning fuel is especially high in Appalachian province localities distant from conventional gas resources.
Coal Mining
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| Coal Mining |
These openings are connected to the surface so the coal can be removed. Prior to the development of the gigantic machinery used in open-pit mining, deep mining was the industry norm.
This early period was characterized by labor intensive pick and shovel work in cramped mine passages. Constant dangers to miners included the collapse of ceilings, methane gas explosions, and pneumoconiosis, known as black lung disease.
Today augers and drilling machinery supplement human labor to a large extent. Mine safety and health regulations have greatly reduced a once high annual death toll. The common method of underground extraction involves initial removal of about 50 percent of the coal, leaving a series of pillars to support the mine roof.
As reserves are exhausted, the mine is gradually abandoned after removal of the pillars. Another modern underground mining technique, with a coal removal rate approaching 100 percent, involves the use of an integrated rotary cutting machine and conveyer belt.
Surface mining of coal, accounting for about 61 percent of U.S. production, is a multiple-step process. First the overburden material is removed, allowing exposure of the coal.
Coal is then mined using surface machinery ranging from bulldozers to gigantic power shovels. Finally, after removal of all the coal, the overburden is used to fill in the excavated trench, and the area is restored to its natural topography and vegetation.
Economics usually determine whether underground or open-pit techniques are preferable in a given situation. Generally, if the ratio of overburden to coal thickness does not exceed twenty to one, surface mining is more profitable.
With increased concern for the environment, and with federal passage of the Coal Mine Health and Safety Act (1969) and the Clean Air Act (1970), the mining of coal in the United States has undergone both geographic and extraction-technology changes.
Because the Rocky Mountain province coals,while lower grade than eastern coals, contain lower percentages of sulfur, the center of U.S. production has gradually shifted westward. The burning of high-sulfur coals releases sulfur dioxide into the atmosphere; it is a significant contributor to acid rain.
Surface mining of coal, accounting for about 61 percent of U.S. production, is a multiple-step process. First the overburden material is removed, allowing exposure of the coal.
Coal is then mined using surface machinery ranging from bulldozers to gigantic power shovels. Finally, after removal of all the coal, the overburden is used to fill in the excavated trench, and the area is restored to its natural topography and vegetation.
Economics usually determine whether underground or open-pit techniques are preferable in a given situation. Generally, if the ratio of overburden to coal thickness does not exceed twenty to one, surface mining is more profitable.
With increased concern for the environment, and with federal passage of the Coal Mine Health and Safety Act (1969) and the Clean Air Act (1970), the mining of coal in the United States has undergone both geographic and extraction-technology changes.
Because the Rocky Mountain province coals, while lower grade than eastern coals, contain lower percentages of sulfur, the center of U.S. production has gradually shifted westward. The burning of high-sulfur coals releases sulfur dioxide into the atmosphere; it is a significant contributor to acid rain.
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