The degree of change undergone by a coal as it matures from peat to anthracite – known as coalification – has an important bearing on its physical and chemical properties and is referred to as the ‘rank’ of the coal.
Low rank coals, such as lignite and sub-bituminous coals are typically softer, friable materials with a dull, earthy appearance. They are characterized by high moisture levels and low carbon content, and therefore a low energy content.
Higher rank coals are generally harder and stronger and often have a black, vitreous luster. They contain more carbon, have lower moisture content, and produce more energy. Anthracite is at the top of the rank scale and has a correspondingly higher carbon and energy content and a lower level of moisture.