Coal forms when vegetation partially decays in low-oxygen environments, such as swamps. Burial heats and compacts the plant matter, squeezing out water and gases (like methane) while leaving behind carbon. Thus, as the coal increases in rank, its carbon content--and the amount of energy it holds--goes up while its water content goes down.
The coal rank order is: Peat → Lignite (brown coal) → Bituminous coal → Anthracite Read more