Coal is a fossil fuel that, according to the World Coal Association (WCA), is made of "the altered remains of prehistoric vegetation." It originally began to form during the Carboniferous period, which took place between 360 and 290 million years ago. Put simply, plant matter accumulated in swamps and peat bogs, and after being buried and exposed to high heat and pressure — largely due to the shifting of tectonic plates — was transformed into coal. The WCA notes that the quality of coal that is created during this period of heat and pressure is largely determined by:
- the type of vegetation the coal originated from
- the coal's depth of burial
- temperatures and pressures at that depth
- how long it took the coal to form
- Ligniteais the youngest form of coal. It is soft and ranges in color from black to shades of brown, and as a result is sometimes called brown coal.aIt is mainly used for power generation and accounts for 17 percent of the world's coal reserves.
- After millions of years, continued pressure and temperature convert lignite into sub-bituminousacoal, whichaburns more cleanly than other types of coal due to its low sulfur content. It has applications in power generation and also in industrial processes. This type of coal makes up 30 percent of the world's coal reserves.
- Bituminousacoalais harder and blacker than lignite and sub-bituminous coal, and can be divided into two types: thermal and metallurgical. Together, they make up 52 percent of the world's coal reserves.Thermal coal is mainly used for power generation, cement manufacturing and other industrial purposes.
Metallurgical coal is used primarily for manufacturing iron and steel.
- Anthraciteais the most mature coal and thus has the highest carbon content of any type of coal. It is frequently used for home heating and, accounting for about 1 percent of the world's total coal reserves, represents a very small portion of the overall market. Anthracite coal can be used as a smokeless fuel in domestic and industrial contexts.