Coal Origin and Properties
is generally accepted that coal originated from plant debris including
ferns, trees, bark, leaves, roots and seeds some of which accumulated and
settled in swamps.
This unconsolidated accumulation of plant remains is called peat. Peat is being formed today in marshes and bogs.
Layers of peat, covered by sediment receiving heat and pressure from the subsidence of the swamps, went through a metamorphic process called coalification to form coal.
The metamorphic process is thought to have occurred in several stages. The conditions of the metamorphic process and the swamps and bogs greatly affected the formation of the coal.
Several factors which greatly affected the content, makeup, quality, and rank of the coal were:
Coal first formed from peat has a high moisture content and a relatively low heating value.
Coal usually is divided into two main classes - anthracite (hard coal) and bituminous (soft coal). When anthracite was formed, it was squeezed under greater heat and pressure than was bituminous. As a result, anthracite contains the highest percentage of carbon and the lowest percentage of moisture. Anthracite makes up only a small part of the world's supply of coal. About half of the world's coal reserve is bituminous coal. (See U.S. Coal Reserves map.) Remaining coal reserves are even softer (lignite and sub-bituminous).
|Moisture decreases, rank
Rank increases, fixed carbon increases.
Rank increases, volatile matter decreases.
Rank increases, heating value increases (optimum Btu at low-volatile bituminous).