Coal Rock Type, Composition, Formation, Rank, Types & Uses

Coal Rock
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Coal is one of the most commonly used rocks in the world. It is an organic sedimentary rock that forms in swamp conditions. The rock is primarily formed from the accumulation and preservation of plant materials.

It has numerous uses, but the most important is as a source of energy. It is used to produce electricity worldwide. Coal is a fossil fuel and a combustible rock. Its importance is similar to oil and natural gas. Countries with large coal reserves use it domestically and export it.

Coal is a brown-black rock and has many different types. The usage varies depending on the type. Coal is divided into different types based on the rock’s age and carbon content.

Also check: Rock Salt Type, Composition, Formation, Occurrence, Mining & Uses

Bituminous Coal
Bituminous Coal

Coal Composition

Coal’s composition is explained in two ways; proximate and ultimate analysis. The proximate analysis explains moisture, volatile matter, fixed carbon, and ash. The ultimate analysis is related to ash, carbon, hydrogen, nitrogen, oxygen, and sulfur.

The volatile matter in coal does not exist by itself but determines the volatile compounds produced when coal is heated. A typical bituminous coal’s ultimate analysis reveals the following composition.

  • Carbon: 84%
  • Hydrogen: 5.5%
  • Oxygen: 6.6%
  • Nitrogen: 2%
  • Sulfur: 1.9%.

Coal Formation

Coal forms in swamp environments as rock strata known as coal seams. Swamp is a low-lying and uncultivated ground where weather gathers. It is also known as a bog or marsh. It forms from the accumulation of plant debris. Plant debris is accumulated when a plant dies and falls into the swamp. Since swamp water lacks oxygen, the plants don’t decay. The plant debris persists due to a lack of oxygen and lack of insects and organisms that eat plant debris. Swamps don’t have these animals due to oxygen-deficient water.

Coal formation requires a continuous accumulation of plant debris. The accumulation rate must be greater than the decaying rate to produce a coal seam. The plant debris layer must be buried deep under the swamp for coal formation. Hence, it requires sediment and mud to put pressure on and compact plant debris. These are provided by nearby flooding rivers, lakes, and washouts.

Coal reserves take millions of years to form. The conditions that formed coal reserves began to develop around 300 years ago during the Carboniferous period. Moreover, around 50 feet of plant debris is required to form a 5-foot-thick coal seam. The water level in the swamp must remain constant during this time. Any disturbances in the water level will force the plant debris to decay due to changes in oxygen levels and coal will not form.

The primary conditions for a coal seam to form are highly unusual. Coal can form in one of the two conditions.

  1. A rising water level that keeps pace with rising plant debris accumulation, or
  2. A subsiding landscape that keeps pace with increasing plant debris over time.

Geologists believe that most of the coal we use today was formed under the second condition. The above discussion explains why coal-forming conditions have occurred a few times in history. Hence, the coal reserves are ending. Hence, it is estimated that the coal reserves would end in about 133 years.

Coal Rank and Types of Coal

Coal is made from plant debris. It is a fragile material compared to minerals from which most other rocks are composed. Coal rank is an important parameter when studying coal as a rock. It is the measure of how much coal has changed over time. Hilt’s law states that the deeper the coal seam, the higher its rank will be. The rock encounters greater pressure and temperature at deeper depths, converting more plant debris into carbon.

Coal is usually divided into 5 different types based on its composition (carbon content) and physical properties. These types are given below.

Peat: Peat is considered a precursor of coal. It is a mass of recently accumulated plant debris that has undergone carbonization up to some extent. The carbon content is usually below 60% in peat. Further burial, compaction, and coalification will convert peat into higher-grade coal.


Lignite: Lignite is a peat that has been converted into a rock. It is the lowest rank of coal. Lignite often contains recognizable plant structures and carbon content between 60-70%. Lignite has a heating value below 8300 BTU per pound.

Lignite stockpile
Lignite stockpile

Sub Bituminous: Lignite exposed to a higher level of organic metamorphism is called sub-bituminous. The external metamorphism has pushed away hydrogen and oxygen in the coal, resulting in higher carbon content (71-75%). This type of coal has a heating value from 8300 to 13000 BTU.

Bituminous: Bituminous coal forms when sub-bituminous coal undergoes more organic metamorphism. Bituminous coal is the most abundant type in the world. The rock has 77-87% carbon content and a heating value more than lignite and sub-bituminous. Bituminous coal is often called “soft coal” in layman’s terms. This definition doesn’t mean that this type of coal is soft.

Bituminous coal is subdivided into three categories based on the volatile content in the rock. These categories are known as low-volatile bituminous, medium-volatile bituminous, and high-volatile bituminous.


Anthracite: It is the highest rank of coal. When bituminous coal is subjected to metamorphism, it becomes anthracite. Anthracite is often considered a metamorphic rock.

Anthracite’s carbon content is higher than 87% percent and has the highest heating value than other coal types. Anthracite is divided into three categories (semi-anthracite, anthracite, and meta-anthracite) based on the carbon content in the rock. It is often termed “hard rock” in layman’s terms.


Where is Coal Found?

Coal is found worldwide. However, mining is a costly process. The largest coal producers in the world are China, India, the United States, Australia, Indonesia, and Russia. The largest consumers are China, India, and United States.

Coal Uses

Coal is used to produce electricity in different countries. A major portion of coal produced in the US is crushed and burned to produce steam. The steam runs generators that, in turn, produce electricity. Coal is also used as a heating source in kilns to produce bricks and cements. It is used as a power source in large industries.

<a href="">Coal-powered power plant, Utah, US</a>

Coal is converted into coke by heating it under controlled conditions and in the absence of air. It removes volatile material, leaving behind concentrated carbon content. It is used as a high-carbon fuel for metal processing.

The gases, tar, and residue produced after heating coal are used to manufacture synthetic rubber, plastics, roofing material, linoleum, paint products, medicines, insecticides, and synthetic fibers.

Coal is used for cooking food. It smokes the food and adds a certain flavor and aroma.

Coal Disadvantages

Coal’s use as a fuel has certain disadvantages. It causes ill health and even death in case of prolonged exposure. Coal-powered plants produce nitrogen oxides, sulfur dioxide, and heavy metals as waste materials. These materials are harmful to humans and the environment. Coal mining pollutes the air and water.

Coal smoke can cause asthma, strokes, artery blockage, coronary diseases, lung cancer, and mercury poisoning.

Coal usage leads to acid rain. It is extremely harmful to plants, animals, and humans. Acid rain also pollutes groundwater and open surface water resources. Coke production from coal produces ammonia, coal tar, and gaseous compounds. These elements pollute the environment if discharged untreated.

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