After millions of years of more heat and pressure
from within the Earth's surface, lignite changes into bituminous or hard
coal. Bituminous coal is an intermediate grade of coal that is the most
common and widely used in the United States. A grade referred to
sub-bituminous is a dull black coal with a higher heating value than
lignite, but lower than true bituminous. Bituminous coal is primarily
used for power generation, and the production of cement, iron and steel.
Bituminous coal in nature, transforms into a harder form called
anthracite coal. Anthracite is the hardest type, consisting of nearly
pure carbon. Anthracite coal has the highest heating value and lowest
moisture and ash content. It is used for domestic and industrial
purposes, including smokeless fuel.