Coal originates through the collection of plant material under anaerobic conditions that prevents decay and oxidation. In this manner the plant material is retained.Coal is a non-renewable fossil fuel formed in ecosystems where plant remains were preserved by water and mud from oxidization and biodegradation, thus sequestering atmospheric carbon. Coal is a readily combustible black or brownish-black rock. It is a sedimentary rock, but the harder forms, such as anthracite coal, can be regarded as metamorphic rocks because of later exposure to elevated temperature and pressure.
It is composed primarily of carbon and hydrogen along with small quantities of other elements, notably sulfur. It is the largest source of fuel for generation of electricity world-wide, as well as the largest world-wide source of carbon dioxide emissions, which according to the IPCC, contribute to climate change and global warming.
In terms of carbon dioxide emissions, coal is slightly ahead of petroleum and about double that of natural gas. Coal is extracted from the ground by coal mining, either underground mining or open pit mining (surface mining).As geological processes apply pressure to dead biotic matter over time, under suitable conditions it is transformed successively into Peat, considered to be a precursor of coal. It has industrial importance as a fuel in some countries, for example, Ireland and Finland.
Lignite, also referred to as brown coal, is the lowest rank of coal and used almost exclusively as fuel for steam-electric power generation. Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Iron Age.
Sub-bituminous coal, whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation. Additionally, it is an important source of light aromatic hydrocarbons for the chemical synthesis industry.
Bituminous coal, a dense mineral, black but sometimes dark brown, often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities also used for heat and power applications in manufacturing and to make coke.
Anthracite, the highest rank; a harder, glossy, black coal used primarily for residential and commercial space heating. It may be divided further in to metamorphically altered bituminous coal and petrified oil.
Graphite, technically the highest rank, but difficult to ignite and is not so commonly used as fuel: it is mostly used in pencils and, when powdered, as a lubricant.