Changes

Coal is most commonly and usefully classified by rank, which essentially indicates the degree of coalification or stage of metamorphism that a coal reaches in its slow, progressive transformation from the oxygen-rich plant debris in peat deposits to [[lignite]], [[subbituminous coal|subbituminous]], and [[bituminous coal]]s, and finally to the carbon-rich, oxygen-poor [[anthracite coal]]s. As this transformation occurs, the moisture content and the volatile matter decrease, whereas the percentage of fixed carbon and the calorific value increase. The demarcation levels in this characterization of coal by rank, unfortunately, are not yet internationally uniform.

Coal is most commonly and usefully classified by rank, which essentially indicates the degree of coalification or stage of metamorphism that a coal reaches in its slow, progressive transformation from the oxygen-rich plant debris in peat deposits to [[lignite]], [[subbituminous coal|subbituminous]], and [[bituminous coal]]s, and finally to the carbon-rich, oxygen-poor [[anthracite coal]]s. As this transformation occurs, the moisture content and the volatile matter decrease, whereas the percentage of fixed carbon and the calorific value increase. The demarcation levels in this characterization of coal by rank, unfortunately, are not yet internationally uniform.

Coal deposits are known from all post-Silurian systems. Most prolific are the Permian (about 29% of the world's coal deposits) and the Carboniferous (20%), followed by the Tertiary (21%), the Jurassic (15%), and the Cretaceous (15%). Small deposits are known in the Devonian and the Triassic.

Coal deposits are known from all post-Silurian systems. Most prolific are the Permian (about 29% of the world's coal deposits) and the Carboniferous (20%), followed by the [[Tertiary]] (21%), the Jurassic (15%), and the Cretaceous (15%). Small deposits are known in the Devonian and the Triassic.

The use of coal has undergone profound changes in the last 70 or 80 years. In the early part of the 20th century, coal was used mainly as an industrial, transportation, and domestic fuel; the greater part (about 85%) of the coal mined today is used in the generation of electric power. As a percentage of the total commercial energy consumption, coal has declined from 90-95% at the beginning of the 20th century to about 25% in 2005.

The use of coal has undergone profound changes in the last 70 or 80 years. In the early part of the 20th century, coal was used mainly as an industrial, transportation, and domestic fuel; the greater part (about 85%) of the coal mined today is used in the generation of electric power. As a percentage of the total commercial energy consumption, coal has declined from 90-95% at the beginning of the 20th century to about 25% in 2005.