Rank - Macerals - Chemical Composition -  Chemical Reactions - Physical Properties - Coal Bed Methane

Coal Rank

The second thing everyone learns about coal is that it comes as anthracite, bituminous, sub-bituminous and lignite. (The first thing is that coal is a dirty black solid, of course.) These terms denote the property called rank. Rank goes along with energy content, with anthracite the highest and lignite the lowest; other properties vary along the series as well. These variations are all related to the chemical structure of the organic materials, and they are very important in guiding the usage of the coal.

Coal Formation

Coals all started as plant materials that accumulated in swampy areas. Dead leaves, branches, and whole tree trunks fell into the water, where they were decomposed by microorganisms (initially aerobic, but later anaerobic). Lignin in the wood is difficult to digest, so most of what disappeared was the cellulosic and starchy materials. This left lignin and various resinous materials. Slowly over millenia the layers of undigested materials grew to tens or hundreds of feet in thickness. The layers also included sand and clay from soil washed into the swamp, as well as charcoal-like particles left over from forest fires.

Coal deposits typically got started 50 to 100 million years ago. Where the organic material became buried, the forces of pressure and heat transformed it. Water, carbon dioxide, and methane were produced, leaving behind solids relatively richer in carbon. Where the organic material was not buried, it remained as peat, which is harvested, dried, and burned in many areas of the world.

The combination of biological and physical processes is called maturation. The farther maturation proceeds, the higher the rank of the coal.

This picture of coal genesis explains why coals are found in layers, called seams. It explains why high rank coals have higher carbon contents and provide more energy per pound when they are burned. It explains why lignite deposits tend to be thicker on average than bituminous coal deposits, etc. It also goes far to explaining many of the differences in chemical behavior among coals of different ranks, differences that can be critically important in deciding what to do with a coal resource.

It does not, however, tell us a thing about sulfur levels or mineral (ash) content. Those two depend much more on where the coal formed and the environment of the original swamp.