Coal
is classified into four main types, or ranks: anthracite, bituminous,
subbituminous, and lignite. The ranking depends on the types and amounts of
carbon the coal contains and on the amount of heat energy the coal can produce.
The rank of a coal deposit is determined by the amount of pressure and heat
that acted on the plants over time.
Anthracite:
It
contains 86%–97% carbon and generally has the highest heating value of all
ranks of coal. Anthracite accounted for less than 1% of the coal mined in the
United States in 2019. All of the anthracite mines in the United States are in
northeastern Pennsylvania. In the United States, anthracite is mainly used by
the metals industry.
Bituminous coal: (can be divided into two types: thermal and
metallurgical)
It
contains 45%–86% carbon. Bituminous coal in the United States is between 100
million and 300 million years old. Bituminous coal is the most abundant rank of
coal found in the United States, and it accounted for about 48% of total U.S.
coal production in 2019.
Bituminous
coal is used to generate electricity and is an important fuel and raw material
for making coking coal or use in the iron and steel industry.
Bituminous coal was produced in at least 19 states in 2019, but five states
accounted for about 75% of total bituminous production: West Virginia (27.5%),
Pennsylvania (14.0%), Illinois (13.5%), Kentucky (10.6%), and Indiana (9.3%).
Subbituminous coal:
It
typically contains 35%–45% carbon, and it has a lower heating value than
bituminous coal. Most subbituminous coal in the United States is at least 100
million years old. About 44% of total U.S. coal production in 2019 was
subbituminous and about 88% was produced in Wyoming and 9% in Montana. The
remainder was produced in Alaska, Colorado and New Mexico.
Lignite:
It
contains 25%–35% carbon and has the lowest energy content of all coal ranks.
Lignite coal deposits tend to be relatively young and were not subjected to
extreme heat or pressure. Lignite is crumbly and has high moisture content,
which contributes to its low heating value. Lignite accounted for 8% of total
U.S. coal production in 2019. About 51% was mined in North Dakota and about 41%
was mined in Texas. The other 9% was produced in Louisiana, Mississippi, and
Montana. Lignite is mostly used to generate electricity. A facility in North
Dakota also converts lignite to synthetic natural gas that is sent in
natural gas pipelines to consumers in the eastern United States.
Hard
vs. Soft: Coal falls into two main categories: hard (Anthracite and Bituminous)
and soft (Subbituminous coal and Lignite).
Soft
coal is also known as brown coal or lignite.
China produces more hard coal than any other country by a factor of about
three. The whopping 3,162 million metric tons of hard coal produced by China dwarfs
the output of the second and third-ranked producers—the U.S. at 932 million
metric tons and India at 538 million metric tons.
Germany
and Indonesia nearly tie for the honor of top honors in the production of
soft brown coal. These countries dug up 169 million and 163 million metric tons
respectively.
Coking
vs. Steam:
Coking
coal, also known as metallurgical coal, has low sulfur and phosphorus content
and can withstand high heat. Coking coal is fed into ovens and subjected to
oxygen-free pyrolysis, a process that heats the coal to approximately 1,100
degrees Celsius, melting it and driving off any volatile compounds and
impurities to leave pure carbon. The hot, purified, liquefied carbon solidifies
into lumps called "coke" that can be fed into a blast furnace along
with iron ore and limestone to produce steel.
Steam
coal, also known as thermal coal, is suitable for electric power production.
Steam coal is ground into a fine powder that burns quickly at high heat and is
used in power plants to heat water in boilers that run steam turbines. It also
may be used to provide space heating for homes and businesses.