Coal is a sedimentary deposit composed predominantly of carbon that is readily combustible. Coal is black or brownish-black, and has a composition that (including inherent moisture) consists of more than 50 percent by weight and more than 70 percent by volume of carbonaceous material. It is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time.

What are the types of coal?

Coal is very different from mineral rocks, which are made of inorganic material. Coal is made of fragile plant matter, and undergoes many changes before it becomes the familiar black and shiny substance burned as fuel. Coal goes through different phases of carbonization over millions of years, and can be found at all stages of development in different parts of the world. Coal is ranked according to how much it has changed over time. Hilt’s Law states that the deeper the coal seam, the higher its rank. At deeper depths, the material encounters greater temperatures and pressure, and more plant debris is transformed into carbon.

Peat

Peat is not coal, but can eventually transform into coal under the right circumstances. Peat is an accumulation of partly decayed vegetation that has gone through a small amount of carbonization. However, peat is still considered part of the coal “family” because it contains energy that its original plants contained. It also contains high amounts of volatile matter and gases such as methane and mercury, which are environmentally hazardous when burned. Peat retains enough moisture to be spongy. It can absorb water and expand the bog to form more peat. This makes it a valuable environmental defense against flooding. Peat can also be integrated into soil to help it retain and slowly release water and nutrients. For this reason, peat and so-called “peat moss” are valuable to gardeners. Peat is an important source of energy in many countries, including Ireland, Scotland, and Finland, where it is dehydrated and burned for heat.

Lignite

Lignite coal is the lowest rank of coal. It has carbonized past the point of being peat, but contains low amounts of energy. Its carbon content is about 25-35 percent. It comes from relatively young coal deposits, about 250 million years old. Lignite, a crumbly brown rock also called brown coal or rosebud coal, retains more moisture than other types of coal. This makes it expensive and dangerous to mine, store, and transport. It is susceptible to accidential combustion and has very high carbon emissions when burned. Most lignite coal is used in power stations very close to where it was mined. Lignite is mainly combusted and used to generate electricity. In Germany and Greece, lignite provides 25-50 percent of electricity generated by coal. In the U.S., lignite deposits generate electricity mostly in the states of North Dakota and Texas.

Sub-Bituminous Coal

Sub-bituminous coal is about 100 million years old. It contains more carbon than lignite, about 35-45 percent. In many parts of the world, sub-bituminous coal is considered “brown coal,” along with lignite. Like lignite, sub-bituminous coal is mainly used as fuel for generating electricity. Most sub-bituminous coal in the U.S. is mined in the state of Wyoming, and makes up about 47 percent of all of the coal produced in the United States. Outside the U.S., China is a leading producer of sub-bituminous coal.

Bituminous Coal

Bituminous coal is formed under more heat and pressure, and is 100 million to 300 million years old. It is named after the sticky, tar-like substance called bitumen that is also found in petroleum. It contains about 45-86 percent carbon. Coal is a sedimentary rock, and bituminous coal frequently contains “bands,” or strips, of different consistency that mark the layers of plant material that were compressed.

Three major types

• Bituminous coal is divided into three major types: smithing coal, cannel coal, and coking coal.
• Smithing coal: has very low ash content, and is ideal for forges, where metals are heated and shaped.
• Cannel coal: was extensively used as a source of coal oil in the 19th century. Coal oil is made by heating cannel coal with a controlled amount of oxygen, a process called pyrolysis. Coal oil was used primarily as fuel for streetlights and other illumination. The widespread use of kerosene reduced the use of coal oil in the 20th century.
• Coking coal: is used in large-scale industrial processes. The coal is coked, a process of heating the rock in the absense of oxygen. This reduces the moisture content and makes it a more stable product. The steel industry relies on coking coal. Bituminous coal accounts for almost half of all the coal that is used for energy in the United States. It is mainly mined in Kentucky, Pennsylvania, and West Virginia. Outside the U.S., nations such as Russia and Colombia rely on bituminous coal for energy and industrial fuel.

Anthracite

Anthracite is the highest rank of coal. It has the most amount of carbon, up to 97 percent, and therefore contains the most energy. It is harder, more dense, and more lustrous than other types of coal. Almost all the water and carbon dioxide have been expelled, and it does not contain the soft or fibrous sections found in bituminous coal or lignite. Because anthracite is a high-quality coal, it burns cleanly, with very little soot.

Features

• It is more expensive than other coals, and is rarely used in power plants. Instead, anthracite is mainly used in stoves and furnaces.
• Anthracite is also used in water-filtration systems.
• It has tinier pores than sand, so more harmful particles are trapped. This makes water safer for drinking, sanitation, and industry.
• Anthracite can typically be found in geographical areas that have undergone particularly stressful geologic activity. For example, the coal reserves on the Allegheny Plateau in Kentucky and West Virginia stretch to the base of the Appalachian Mountains. Here, the process of orogeny, or mountain formation, contributed to temperatures and pressures high enough to create anthracite.
• China dominates the mining of anthracite, accounting for almost three-quarters of anthracite coal production. Other anthracite-mining countries include Russia, Ukraine, Vietnam, and the United States (mostly Pennsylvania).

Graphite

Graphite is an allotrope of carbon, meaning it is a substance made up only of carbon atoms. (Diamond is another allotrope of carbon.) Graphite is the final stage of the carbonization process. Graphite conducts electricity well, and is commonly used in lithium ion batteries. Graphite can also resist temperatures of up to 3,000°C (5,400°F). It can be used in products such as fire-resistant doors, and missile parts such as nose cones. The most familiar use for graphite, however, is probably as pencil “leads.” China, India, and Brazil are the world’s leading producers of graphite.

What is coal used for?

Coal is primarily used as fuel to generate electric power. In coal-fired power plants, bituminous coal, subbituminous coal, or lignite is burned. The heat produced by the combustion of the coal is used to convert water into high-pressure steam, which drives a turbine, which produces electricity.

Which country has the most coal?

As of January 2020, the United States has the largest recoverable coal reserves with an estimated 252 billion short tons of coal remaining.

List of coal reserve countries in the world

List of coal reserve in Indian States

Major Coal Producing District in Uttar Pradesh