What Is Coal?

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Coal has been a major source of energy in the world since its discovery in the early 18th century. This discovery played a key role in kick-starting the industrial revolution in England during the early 1700s by enabling the development of steam engines and other modes of transportation.

Composition of Coal

Coal is one of the three major types of ‘fossil fuels,’ along with petroleum and natural gas. Fossil fuel is a term used to describe a type of rock, or rock-derived product, that can be burned for fuel and originates from old organic matter, such as plants. Coal is a fairly general term that can apply to a rock with more than 50% by weight and more than 70% by volume of carbonaceous matter.

Coal is a brittle and sedimentary rock that is composed of complex mixtures of both organic and inorganic compounds. While the organic components of coal originate from millions of different swamp plants, the inorganic components of coal originate from the decomposition of those same plants. Some of the most common minerals found in coal include carbon, oxygen, aluminum, silicon, iron, sulfur, and calcium. These minerals can arrange themselves as single crystals or clusters of crystalline structures within the coal rock.

Formation of Coal

All of the energy that comes from coal was initially derived from ancient sunlight. Like plants today, the plants during this time absorbed the sun’s energy through a process known as photosynthesis. When plants die, in most cases, the absorbed energy dissipates back into the atmosphere. However, if the plant is buried quickly, this energy is instead bottled up within the dead plant and stored in the ground. This burial subjects the dead plant material to high temperatures and pressures that ultimately induce physical and chemical changes to the material.

The initial deposition of plant debris into the ground forms peat, which is the mineral that all coal types originate from. The chemical and physical changes that peat undergoes, of which can include degradation and decomposition as a result of microbial populations within the swamp, as well as oxidation and other biogeochemical processes, plays an important role in determining the future properties that the final coal product will exhibit.

Origins of Coal

Coal began its journey to our collective fireplaces around 360 million years ago during the Carboniferous period, which translates from Latin to ‘coal-bearing’. During this time, the Earth’s tropical climate and humid temperatures were ideal for coal formation requirements. In fact, the tectonic movements and sediment build-ups during this period were perfect for quickly burying the plentiful swamps and bogs to great depths.

Different Forms of Coal

In general, coal is classified based on grade and, more importantly, its rank. Grade refers to the amount of ash that forms after the coal has burned.

Coal’s rank is determined by a natural process known as “coalification.” During coalification, buried plant matter within the coal rock slowly undergoes changes that make the mineral drier, denser, harder and even richer in its carbon concentration. The rank of coal, therefore, considers the percentage of fixed carbon, which refers to the combustible matter that remains in the rock following coalification, as well as the moisture content, calorific value, which is the amount of heat given off during burning, and its volatility.

The higher the coal rank indicates a greater depth of burial. In addition, higher coal ranks also indicate higher temperatures that the rock was subjected to during and following its burial. For example, older coal rocks tend to have been buried for extended periods of time, thereby causing their rank to be higher as compared to younger coals. The main forms of coal from lowest to highest rank include lignite, subbituminous coal, bituminous coal and anthracite.

Lignite is a soft and dusty material that can ignite spontaneously when certain environmental conditions are present. As the rock is buried deeper into the earth, organic maturity continues and causes the mineral to become harder and blacker. Subbituminous coal is a dark brown and crumbly substance that has a high moisture and oxygen content, whereas bituminous coal, which is often referred to as black coal, contains bitumen and exhibits higher calorific values. Anthracite, which is the hardest type of coal, is jet black in color and also exhibits a clear sheen to its surface.  These properties make anthracite difficult crush and burn, thereby preventing this type of coal from being used for steam coal.

How the formation of coal progresses

How the formation of coal progresses. Image Credits: www.wsgs.uwyo.edu

A photo of black, shiny anthracite.

A photo of black, shiny anthracite. Image Credits: Wikimedia Commons

Uses of Coal

Considering the many forms of coal, it is not surprising that this mineral can be used for a wide range of industrial purposes. As an important source of energy, it is estimated that coal-fired power plants currently fuel 37% of global electricity generation. To convert coal to electricity, coal is first made into a refined powder that increases the surface area and ultimately increases its rate of burning. The fine coal powder is created in a pulverized coal combustion (PCC) system. Within the PCC system, the coal powder is blown into the combustion chamber where it is burned at high temperatures. The burning of coal produces hot gases and heat energy that can then be converted into steam.  

Aside from energy production, coal is also used during the production of aluminum, paper, chemicals, and various pharmaceutical products. Additionally, the by-products of coal can also be used to create various chemical products. For example, refined coal tar can be used to manufacture chemicals including benzene, creosote oil, naphthalene, and phenol. Ammonia gas that arises following the use of coke ovens can be used during the production of agricultural fertilizers, nitric acid, and ammonia salts. The repurposing of coal by-products can also be found in soap, dyes, plastics and fibers.

Where is Coal Found?

In about 70 countries, approximately 1.1 trillion tons of coal reserves can be found around the world, most of which are located in the United States, Russia, China, and India. Other major deposits can be found in Germany, Poland, Australia, and South Africa. Additional developments, such as the discovery of new reserves during exploration activities and advancements in current mining techniques, have the potential to discover even more coal reserves that currently exist in the world.

The choice of mining technique depends on the geometry and position of the coal reserve. Underground mining accounts for more coal globally; however, some major coal-producing nations, such as the United States and Australia, produce the majority of their coal through surface mining of coal beds or seams. Surface mining, which is much cheaper than underground mining, requires the use of large machines that remove topsoil and several layers of rock in order to provide access to the coal seams. Once the coal has been removed, this area can be recovered with topsoil and repurposed for other planting purposes.

The Future of Coal

In 2015, it was estimated that approximately 1.1 trillion tons of recoverable coal reserves remain on the planet. The amount of recoverable coal reserves that remain within the United States alone is estimated to last about 325 more years. While this number may appear promising, the rapidly expanding global economy and consumption of coal, combined with the association between coal burning and global warming, has pushed international agencies to investigate the potential of alternative fuel sources.

Sources and Further Reading

This article was updated on 8th February, 2019.

G.P. Thomas

Written by

G.P. Thomas

Gary graduated from the University of Manchester with a first-class honours degree in Geochemistry and a Masters in Earth Sciences. After working in the Australian mining industry, Gary decided to hang up his geology boots and turn his hand to writing. When he isn't developing topical and informative content, Gary can usually be found playing his beloved guitar, or watching Aston Villa FC snatch defeat from the jaws of victory.

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