Lignite is the lowest rank of coal with the lowest energy content. Lignite coal deposits tend to be relatively young coal deposits that were not subjected to extreme heat or pressure. Lignite is crumbly and has high moisture content.
Sub-bituminous coal has a higher heating value than lignite. Sub-bituminous coal typically contains 35-45 percent carbon, compared to 25-35 percent for lignite. Most sub-bituminous coal is at least 100 million years old.
Bituminous coal contains 45-86 percent carbon, and has two to three times the heating value of lignite.
Bituminous coal was
formed under high heat
and pressure. Bituminous
coal is between 100 to 300
million years old.
Anthracite contains 86-97
percent carbon, and has a
heating value slightly
higher than bituminous coal.