Coal – Definition, Types, Origin, Formation Processes, and More

Coal Definition

Coal is a fossil fuel, resulting from a series of transformations on plant remains accumulated in swampy places, lagoons and river deltas, mainly during the Carboniferous period of the Primary Era.

Due to various chemical actions and variations in pressure and temperature over long periods, these vegetables are transformed into carbon in carbonization.

In summary, it can say that after the plant deposition phase, the action of anaerobic bacteria begins (mainly on cellulose and lignin).

The changes that lead to the transformation of wood into charcoal are chemical and structural. In chemicals, hydrogen and oxygen are released as the proportion of carbon increases. In some cases (as in anthracite), it constitutes almost the entire resulting product.

Types of Coal

There are four different types of coals due to the different kinds of vegetables from which they come, especially to the duration and conditions (pressure and temperature of the carbonization process). These are:


It is hard coal, totally charred. Very compact and bright. With pearlescent shine and black colour.


It is hard coal, totally charred. Shiny black colourway. Pearly shine to glossy and matt bands.


Blackish. It is a lump of soft coal belonging (like peat) to post-carboniferous times, so it has not undergone the complete carbonization process. It looks like burnt wood and has a chunky shine.


It is the most recent of the coals. It is soft, brown in colour, matt, light in weight, and plants’ remains can still see.

There are important investigations for the sequestration of CO 2 emitted by pumping it to geological formations.

Origin of Coal

  • It generally accepts that most coals formed from plants that grew in and adjacent to swamps in warm, humid regions.
  • Material derived from these plants accumulated in low-lying areas that remained wet most of the time and converted to peat through microorganisms’ activity.
  • It should note that peat can occur in temperate regions [e.g., Ireland and the state of Michigan in the United States] and even in the subarctic areas [e.g., the Scandinavian countries].
  • Under certain conditions, this organic material continued to accumulate and later convert into coal.
  • Much of the plant matter accumulated on Earth’s surface never convert to peat or coal because it removes by fire or organic decomposition.
  • Hence, the vast coal deposits found in ancient rocks must represent periods during which several favourable biological and physical processes occurred simultaneously.
  • Asia has enormous coal reserves, amounting to nearly three-fifths of the world’s total, but they are unevenly distributed.
  • Evidence that coal was derived from plants comes from three principal sources. First, lignites, the lowest coal rank, often contain recognizable plant remains.
  • Second, sedimentary rock layers above, below, and adjacent to coal seams contain plant fossils in the form of impressions and carbonized films (e.g., leaves and stems) and casts of larger parts such as roots, branches, and trunks.
  • Third, even coals of advanced rank may reveal the presence of precursor plant material.
  • When examined microscopically in thin sections or polished blocks, cell walls, cuticles (the outer wall of leaves), spores. And other structures can still be recognized (see below Macerals).
  • Algal and fungal remains also may be present. (Algae are major components in bighead coal, a type of sapropelic coal.)

Formation Processes of Coal

1. Peat

  • Although peat used as a source of energy, it not usually consider coal.
  • It is the precursor material from which coals derive. The process by which peat form studies in existing swamps in many parts of the world (e.g., in the Okefenokee Swamp of Georgia, U.S., and along the southwestern coast of New Guinea).
  • The formation of peat control by several factors, including the evolutionary development of plant life, the climatic conditions (warm enough to sustain plant growth.
  • And wet enough to permit the partial decomposition of the plant material and preserve the peat).
  • And the physical conditions of the area (its geographic position relative to the sea or other bodies of water, rates of subsidence or uplift, and so forth).
  • Warm moist climates think to produce broad bands of bright coal, a type of bituminous coal characterized by its fine banding and high concentrations of nitrogen, sulfur, and moisture.
  • On the other hand, cooler temperate climates think to produce detrital coal (which belief to be the remains of preexisting coal beds) with relatively little bright coal.

2. Peat Bog

  • Initially, the area on which a future coal seam may develop must uplift to establish plant growth.
  • Areas near seacoasts or low-lying areas near streams stay moist enough for peat to form. Still, elevated swamps (some bogs and moors) can produce peat only if the annual precipitation exceeds annual evaporation and little percolation or drainage occurs.
  • Thick peat deposits necessary for coal formation develop at sites where the following conditions exist: slow, continuous subsidence; the presence of such natural structures as levees, beaches.
  • And bars that give protection from frequent inundation; and a restricted supply of incoming sediments that would interrupt peat formation.
  • The water may become quite stagnant (except for a few rivers traversing the swamp), and plant material can continue to accumulate.
  • Microorganisms attack the plant material and convert it to peat. The plant material’s decomposition produces mostly gaseous and liquid products very close to the surface where oxygen is still readily available (aerobic, or oxidizing, conditions).
  • T.
  • However, with increasing depth, the conditions become increasingly anaerobic (reducing), and moulds and peats develop.
  • The peat formation process—biochemical coalification—is most active in the upper few metres of a peat deposit.
  • Fungi did not find below about 0.5 metres (about 18 inches), and most forms of microbial life eliminate at depths below about 10 metres (about 30 feet).
  • Suppose either the rate of subsidence or the speed of influx of new sediment increases, the peat will be buried. And soon after that, the coalification process—geochemical coalification—begins.
  • The cycle may repeat many times, accounting for the numerous coal seams found in some sedimentary basins.

3. Coalification

  • The general sequence of coalification is from lignite to sub-bituminous to bituminous to anthracite.
  • Since microbial activity ceases within a few metres of Earth’s surface, the coalification process must control primarily by changes in physical conditions that take place with depth.
  • Some coal characteristics determined by events that occur during peat formation. E.g., charcoal-like material in coal attributes to fires during dry periods while peat was still forming.

4. Lignite Pit

  • Three major physical factors—duration, increasing temperature, and growing pressure—may influence the coalification process.
  • In laboratory experiments, artificially prepared coals influence by the experiment’s duration, but in nature, the length of time is substantially longer. And the overall effect of time remains undetermined.
  • Low-rank coal (i.e., brown coal) in the Moscow Basin deposit during the Carboniferous time but not buries deeply and never reached a higher rank.
  • The most widely accepted explanation is that coalification takes place in response to increasing temperature. In general, temperature increases with depth.
  • This geothermal gradient averages about 30 °C (about 85 °F) per kilometre. Still, the gradient ranges from less than ten °C (50 °F) per kilometre in regions undergoing very rapid subsidence to more than 100 °C (212 °F) per kilometre in areas of igneous activity.
  • Measurements of thicknesses of sedimentary cover and corresponding coal ranks suggest that temperatures lower than 200 °C (about 390 °F) are sufficient to produce coal of anthracite rank.
  • The effect of increasing pressure due to depth of burial does not consider causing coalification.
  • Increasing overburden pressure might have the opposite effect if volatile compounds such as methane that must escape during coalification retain. Stress may influence the porosity and moisture content of coal.


Coal, one of the most important primary fossil fuels, is a solid carbon-rich material, usually brown or black. And most often occurs in stratified sedimentary deposits.

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