COAL

Chemical Properties

Coal is an organic, combustible, rock-like natural substance that occurs in various forms from hard and brittle anthracite to soft and friable lignite. Coal is sometimes classified into two types: hard coal and soft coal. These terms do not, however, have a standardized meaning. One definition calls anthracite hard coal and places all other coal types in the soft coal category. A more common convention is that of Speight, in which anthracite and bituminous coals are termed hard coal and lignite and brown coal are classified as soft coal.

Production Methods

Coal exists as distinct seams of mineral laid down as sedimentary deposits within the earth. The seams are extracted by surface (strip) or underground mining. Surface mining generally involves the removal of rock and other strata to uncover the top or side of a coal seam. Using blasting and/or mechanical means, the coal is fractured and removed from the mine by train or truck. There are two main types of underground mining: room and pillar and longwall (each accounting for 50% of U.S. production in 2010). Room and pillar mining is the traditional method of mining, whereby pillars of coal are left to support the roof. These pillars may later be removed or “robbed,” a process that provides better reclamation of the coal but speeds up subsidence of the overlying strata. Room and pillar operations are commonly undertaken using conventional mining or continuous mining. In the former method, the bottom of the coal seam is undercut first, followed by blasting to bring the coal down. In continuous mining, a machine with a rotating cutting head removes the coal from the coal face. Longwall mining, of more recent origin, involves removing “slices” of coal from the edge of the seam by large coal cutting machines. All of the coal is extracted, and the roof is left to collapse as the coal face advances. A lesser-used method of underground mining is the shortwall system, which is a hybrid of the longwall and continuous mining methods and is used principally in Australia. After extraction, coal is typically crushed and cleaned. Cleaning can involve screening for size, removal of impurities by wet or dry methods, and drying.

Definition

coal: A brown or black carbonaceousdeposit derived from the accumulationand alteration of ancient vegetation,which originated largely inswamps or other moist environments.As the vegetation decomposedit formed layers of peat, whichwere subsequently buried (for example,by marine sediments following arise in sea level or subsidence of theland). Under the increased pressureand resulting higher temperaturesthe peat was transformed into coal.Two types of coal are recognized:humic (or woody) coals, derived fromplant remains; and sapropelic coals,which are derived from algae, spores,and finely divided plant material.
As the processes of coalification (i.e.the transformation resulting from the high temperatures and pressures)continue, there is a progressive transformationof the deposit: the proportionof carbon relative to oxygenrises and volatile substances andwater are driven out. The variousstages in this process are referred toas the ranks of the coal. In ascendingorder, the main ranks of coal are: lignite(or brown coal), which is soft,brown, and has a high moisture content;subbituminous coal, which isused chiefly by generating stations;bituminous coal, which is the mostabundant rank of coal; semibituminouscoal; semianthracite coal, whichhas a fixed carbon content of between86% and 92%; and anthracitecoal, which is hard and black with afixed carbon content of between 92%and 98%.
Most deposits of coal were formedduring the Carboniferous and Permianperiods. More recent periods ofcoal formation occurred during theearly Jurassic and Tertiary periods.Coal deposits occur in all the majorcontinents; the leading producers includethe USA, China, Ukraine,Poland, UK, South Africa, India, Australia,and Germany. Coal is used as afuel and in the chemical industry; byproductsinclude coke and coal tar.

Definition

A black mineral that consists mainly of carbon, used as a fuel and as a source of organic chemicals. It is the fossilized remains of plants that grew in the Carboniferous and Permian periods and were buried and subjected to high pressures underground. There are various types of coal, classified according to their increasing carbon content.

Agricultural Uses

Coal is a brown or black carbonaceous deposit, formed largely by the accumulation and alteration of ancient vegetation in swamps and other moist environments. The vegetation decomposes, forms layers of peat, and gets subsequently buried deep in the earth where, under increased pressure and temperatures, the peat gets transformed into coal. This, in short, is the process of coalification.
Two types of coals are known - humic coal or woody coal (derived from plant remains) and sapropelic coal (derived from algae, spores and finely divided plant materials).
As the process of coalification continues, the proportion of carbon rises relative to oxygen, and volatile substances and water are expelled. Various intermediates in the coalification process are referred to as the ranks of coal. Some of them are lignite or brown coal (soft and brown with a high moisture content), sub-bituminous coal (used mainly for power generation) and bituminous coal (the most abundant type of coal). Other varieties are semi-bituminous coal, semi-anthracite coal (with a fwed carbon content of 86 to 92%) and anthracite coal (hard and black, with a fwed carbon content of 92 to 98%).
Coal is used as a fuel and in the chemical industry. When coal is heated in the absence of air, its volatile components are expelled and it is converted into coke. When bituminous coal is subjected to destructive distillation, it is converted into a thick black liquid, called coal tar.
Coal has varying amounts of ash and clinker and impurities like clays and sulphur (between 0.5% and 3%). The calorific value of coal increases with its grade till it reaches the anthracite level. The design of coal burning (or gasifying) facilities is dictated by its calorific content and impurities. Based on its moisture percentage, coal is specified with values of volatile combustible matter, fixed carbon, ash and percentages of carbon, hydrogen, oxygen, nitrogen and sulphur.
Coal, used for the large-scale production of hydrogen, is probably the world's largest source for producing ammonia. Coal-based power generation units and fertilizer production units are located near coal mines.
Coal is also an important source of raw material for the chemical industry. Pyrolysis or distillation of coal yields coal tar and hydrocarbon gases which are upgraded by hydrogenation or methanation to synthetic crude oil and fuel gas, respectively. Catalytic hydrogenation yields hydrocarbon oils and gasoline. Gasification produces synthesis gas (CO+H) from which ammonia and other fertilizers are made.
However, coal creates environmental pollution and entails high plant costs. As a result, its use at the current scale may decline in the years to come.

Carcinogenicity

There is little epidemiologic evidence of a relationship between coal mine dust exposure and lung cancer development, although a link with silica dust exposure has been reported. Histological evaluation of lung tumors in coal miners reveals that these tumors vary little in cell type or pathological features from those associated with cigarette smoking.
Elevated stomach cancer rates have been reported in studies of U.S. underground coal miners compared to general population rates. A mechanistic explanation for coal dust-induced stomach cancer suggests that swallowed coal dust mixes with nitrates in food and under the acidic conditions in the stomach, nitrosation of organic material associated with this coal dust occurs, resulting in the production of carcinogenic products. Evidence indicates that nitrosation of coal dust extracts become mutagenic and can cause neoplastic transformations in mammalian cells. Some evidence of elevated stomach cancer mortality in coal miners has also been observed in more recent studies in the United Kingdom, Germany, and United States.