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COAL:
Ancient Gift Serving Modern Man
by the American Coal Foundation [http://www.teachcoal.org]
Types
of Coal
We
use the term "coal" to describe a variety of fossilized plant
materials, but no two coals are exactly alike. Heating value, ash
melting temperature, sulfur and other impurities, mechanical strength,
and many other chemical and physical properties must be considered when
matching specific coals to a particular application.
Coal
is classified into four general categories, or "ranks." They
range from lignite through sub-bituminous and bituminous to anthracite,
reflecting the progressive response of individual deposits of coal to
increasing heat and pressure. The carbon content of coal supplies most
of its heating value, but other factors also influence the amount of
energy it contains per unit of weight. (The amount of energy in coal is
expressed in British thermal units per pound. A BTU is the amount of
heat required to raise the temperature of one pound of water one degree
Fahrenheit.)
About
90 percent of the coal in this country falls in the bituminous and
sub-bituminous categories, which rank below anthracite and, for the most
part, contain less energy per unit of weight. Bituminous coal
predominates in the Eastern and Mid-continent coal fields, while
sub-bituminous coal is generally found in the Western states and Alaska.
Lignite
ranks the lowest and is the youngest of the coals. Most lignite is mined
in Texas, but large deposits also are found in Montana, North Dakota,
and some Gulf Coast states.
Anthracite
Anthracite
is coal with the highest carbon content, between 86 and 98 percent, and
a heat value of nearly 15,000 BTUs-per-pound. Most frequently associated
with home heating, anthracite is a very small segment of the U.S. coal
market. There are 7.3 billion tons of anthracite reserves in the United
States, found mostly in 11 northeastern counties in Pennsylvania.
Bituminous
The
most plentiful form of coal in the United States, bituminous coal is
used primarily to generate electricity and make coke for the steel
industry. The fastest growing market for coal, though still a small one,
is supplying heat for industrial processes. Bituminous coal has a carbon
content ranging from 45 to 86 percent carbon and a heat value of 10,500
to 15,500 BTUs-per-pound.
Sub-bituminous
Ranking
below bituminous is sub-bituminous coal with 35-45 percent carbon
content and a heat value between 8,300 and 13,000 BTUs-per-pound.
Reserves are located mainly in a half-dozen Western states and Alaska.
Although its heat value is lower, this coal generally has a lower sulfur
content than other types, which makes it attractive for use because it
is cleaner burning.
Lignite
Lignite
is a geologically young coal which has the lowest carbon content, 25-35
percent, and a heat value ranging between 4,000 and 8,300
BTUs-per-pound. Sometimes called brown coal, it is mainly used for
electric power generation.
The
Foundation offers a variety of educational resources for the classroom. [http://www.teachcoal.org]
See
also: National Mining Association at [www.nma.org]
and the
MiningUSA website: [www.miningusa.com]
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