Coal | Properties, Formation, Occurrence and Uses (2022)

Coal is a non-clastic sedimentary rock. They are the fossilized remains of plants and are in flammable black and brownish-black tones. Its main element is carbon, but it can also contain different elements such as hydrogen, sulfur and oxygen. Unlike coal minerals, it does not have a fixed chemical composition and crystal structure. Depending on the type of plant material, varying degrees of carbonization and the presence of impurities, different types of coal are formed. There are 4 recognized varieties. Lignite is the lowest grade and is the softest and least charred. Sub-bituminous coal is dark brown to black. Bituminous coal is the most abundant and is often burned for heat generation. Anthracite is the highest grade and most metamorphosed form of coal. It contains the highest percentage of low-emission carbon and would be an ideal fuel if it weren’t for comparatively less.

Coal is mainly used as a fuel. Coal has been used for thousands of years, but its real use began with the invention of steam engines after the industrial revolution. Coal provides two-fifths of electricity production worldwide and coal is used as the main fuel in iron and steel production facilities.

Name origin: The word originally took the Old English form col from the Proto-Germanic *kula(n), which is supposed to derive from the Proto-Indo-European root *g(e)u-lo- “live coal”.

Color: Black and Brownish black

Hardness: Changeable

Grain size: Fine grained

Group: Non-Clastic Sedimentary Rock

Coal Classification

As geological processes put pressure on dead biotic material over time under favorable conditions, the degree or order of metamorphic successively increases as follows:

Lignite, the lowest level of coal, the most harmful to health, is used almost exclusively as a fuel for electric power generation

Jet, a compact form of lignite, sometimes polished; Upper Paleolithic Lower-bituminous coal, whose properties range from those of lignite to bituminous coal, was primarily used as an ornamental stone as it was used as a fuel for steam-electric power generation.

Bituminous coal, a dense sedimentary rock, usually black, but sometimes dark brown, often with well-defined bands of shiny and dull material. It is primarily used as a fuel in the production of steam-electric power and in the production of coke. In the UK it is known as steam coal and has historically been used to raise steam in steam locomotives and ships.

Anthracite, the highest grade of coal, is a harder, glossy black coal used primarily for residential and commercial space heating.

Graphite is difficult to ignite and is not commonly used as a fuel; it is most commonly used in pencils or powdered for lubrication.

Channel coal (sometimes called “candle coal”) is a variety of fine-grained, high-grade coal composed primarily of liptinite with significant hydrogen content.

There are several international standards for coal. The classification of coal is generally based on the content of volatile substances. But the most important distinction is thermal coal (also known as steam coal), which is burned to generate electricity through steam; and metallurgical coal (also known as coking coal), which is burned at high temperature to make steel.

Coal Composition

The composition of coal can be analyzed in two ways. The first is reported as a close analysis (moisture, volatile matter, fixed carbon and ash) or a final analysis (ash, carbon, hydrogen, nitrogen, oxygen and sulfur). A typical bituminous coal may have a final analysis on a dry, ash-free basis of 84.4% carbon, 5.4% hydrogen, 6


Coal Formation

The process of turning dead vegetation into coal is called coalification. In the geological past there were low wetlands and dense forests in various regions. The dead vegetation in these areas has generally started to biodegrade and transform with mud and acidic water.

This trapped the carbon in huge peat bogs that were eventually buried deep by sediments. Then, over millions of years, the heat and pressure of the deep burial caused a loss of water, methane, and carbon dioxide and increased carbon content.

The grade of coal produced depended on the maximum pressure and temperature reached; Lignite (also called “brown coal”) and sub-bituminous coal, bituminous coal or anthracite (also called “hard coal” or “hard coal”) produced under relatively mild conditions is produced with increasing temperature and pressure.

Of the factors involved in charring, temperature is much more important than pressure or burial time. Sub-bituminous coal can form at temperatures as low as 35 to 80 °C (95 to 176 °F), while anthracite requires a temperature of at least 180 to 245 °C (356 to 473 °F).

Although coal is known from most geological periods, 90% of all coal deposits were deposited during the Carboniferous and Permian periods, which represent only 2% of Earth’s geological history.


Coal is a common energy and chemical source. Terrestrial plants necessary for the development of coal were not abundant until the Carboniferous period (358.9 million to 298.9 million years ago), large sedimentary basins containing rocks of Carboniferous age and younger are known on almost every continent, including Antarctica. The presence of large coal deposits in regions with currently arctic or subarctic climates (such as Alaska and Siberia) is due to climate changes and tectonic movement of crustal plates that have moved older continental masses over the Earth’s surface, sometimes through the subtropical and even tropics. regions. Some areas (like Greenland and most of northern Canada) lack coal because the rocks found there predate the Carboniferous Period, and these regions, known as continental shields, lack the abundant terrestrial plant life needed for the formation of large coal deposits.

Coal Characteristics and Properties

Many of the properties of coal vary with factors such as its composition and the presence of mineral matter. Different techniques have been developed to examine the properties of coal. These are X-ray diffraction, scanning and transmission electron microscopy, infrared spectrophotometry, mass spectroscopy, gas chromatography, thermal analysis, and electrical, thermal analysis, and electrical, optical and magnetic measurements.


Knowing the physical properties of coal is important in the preparation and use of coal. For example, coal density ranges from about 1.1 to about 1.5 megagrams per cubic metre, or grams per cubic centimeter. Coal is slightly denser than water and significantly less dense than most rocks and mineral matter. Density differences make it possible to improve the quality of a coal by removing most of the rock matter and sulfide-rich particles through heavy liquid separation.


Coal density is controlled in part by the presence of pores that persist throughout charring. Pore ​​sizes and pore distribution are difficult to measure; however, pores appear to have three size ranges:

(1) macropores (diameter greater than 50 nanometers),

(2) mesopores (2 to 50 nanometers in diameter), and

(3) micropores (diameter less than 2 nanometers).

(One nanometer equals 10−9 metres.) Most of a coal’s effective surface area—about 200 square meters per gram—is found in the pores of the coal, not on the outer surface of a piece of coal. The presence of pore space is important in coke production, gasification, liquefaction and high surface area carbon production to purify water and gases. For safety reasons, coal pores may contain significant amounts of adsorbed methane, which can be released during mining operations and form explosive mixtures with air. The risk of explosion can be reduced by adequate ventilation or prior removal of coalbed methane during mining.


An important property of coal is its reflectivity (or reflectivity), that is, its ability to reflect light. Reflectivity is measured by shining a monochromatic light beam (with a wavelength of 546 nanometers) onto a polished surface of vitrinite macerals in a charcoal sample and measuring the percentage of reflected light with a photometer. Vitrinite is used as its reflectivity gradually changes with increasing degree. Fusinite reflections are very high due to its coal origin and liptinites tend to disappear with increasing degrees. Although very little of the incident light is reflected (ranging from a few tenths of a percent to 12 percent), the value increases with degrees and can be used to grade most coals without measuring the percentage of volatile matter present.

Other features

Other properties such as hardness, grindability, ash fusion temperature, and free swelling index (a visual measurement of the amount of swelling that occurs when a coal sample is heated in a closed crucible) can affect coal mining and preparation. as well as the way a coal is used. Hardness and grindability determine the types of equipment used for mining, crushing and grinding, in addition to the amount of power consumed in their operations. Ash fusion temperature affects furnace design and operating conditions. The free swelling index provides preliminary information on the suitability of a coal for coke production.

Coal Uses

· Electricity Generation

The primary use of coal in the world is electricity generation. Thermal coal is burned to create steam that drives turbines and generators for electricity production.

· Metal Production

Metallurgical (coking) coal is an important component in steelmaking. Coal converted to coke is used to produce about 70% of the world’s steel. Coal is also widely used in the production of other metals such as aluminum and copper.

· Cement Production

Coal is used as the primary energy source in cement production. Coal combustion byproducts such as fly ash also play an important role in cement manufacture and the wider construction industry.

· Gasification and Liquefaction

It is heated and pressurized by steam to produce “city” gas for indoor lighting, heating and cooking. It is liquefied to make synthetic fuels similar to petroleum or diesel. The majority of coal-to-gas projects are in the US and China, with a few in Indonesia, India, Australia, Canada and South Africa.

· Chemical Production

Syngas from gasification can be further processed to produce chemical building blocks such as methanol, ammonia and urea.

· Other Industries

Other major users of coal are the paper, textile and glass industries. Charcoal is also used in the manufacture of specialty components such as carbon fiber and silicon metals used to produce ingredients for the home and personal care industries.

Facts about The Coal

  • It takes about 1 million years to form. Now you understand why it is classified as a non-renewable energy source.
  • There are 2 methods of coal extraction: surface and underground. The surface is cheaper and safer.
  • More than 90% of US coal is used for electricity.
  • It must be relatively dry in order to be successfully burned.
  • It was an integral part of trade during the Roman Empire.


  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Kopp, O. C. (2020, November 13). coal. Encyclopedia Britannica.
  • Wikipedia contributors. (2021, October 26). Coal. In Wikipedia, The Free Encyclopedia. Retrieved 09:57, November 1, 2021, from

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