TYPES OF COAL FOUND ON OUR LIST:
Hunter's Energy and Mining International has four major types of coal (or “ranks”). Rank refers to steps in a slow, natural process called “coalification,” during which buried plant matter changes into an ever denser, drier, more carbon-rich, and harder material. The four ranks are:
Anthracite: The highest rank of coal. It is a hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter.
Bituminous: Bituminous coal is a middle-rank coal between subbituminous and anthracite. Bituminous coal usually has a high heating (Btu) value and is used in electricity generation and steel making in the United States. Bituminous coal is blocky and appears shiny and smooth when you first see it, but look closer and you might see it has thin, alternating, shiny, and dull layers.
Subbituminous: Subbituminous coal is black in color and is mainly dull (not shiny). Subbituminous coal has low-to-moderate heating values and is mainly used in electricity generation.
Lignite: Lignite coal, aka brown coal, is the lowest grade coal with the least concentration of carbon. Lignite has a low heating value and a high moisture content and is mainly used in electricity generation. The precursor to coal is peat. Peat is a soft, organic material consisting of partly decayed plant and mineral matter. When peat is placed under high pressure and heat, it undergoes physical and chemical changes (coalification) to become coal.
Coal is organic. It is a mineral formed from the remains of land-based plants buried hundreds of millions of years ago and subjected to tremendous heat and pressure.
Coalification has an important bearing on coal's physical and chemical properties and is referred to as the “rank” of coal. In general, the longer the organic material is subjected to heat and pressure, the higher its rank and the more carbon content will be contained per unit of weight. The ranking is determined by the degree of transformation of the original plant material to carbon. The ranks of coals, from those with the least carbon to those with the most carbon, are lignite, sub-bituminous, bituminous, and anthracite.
Figure 1: How does one type of coal differ from another?
Illustration of coal ranking