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 found in Northern America falls
in the bituminous and sub bituminous categories, which rank below
anthracite and, for the most part, contain less energy per unit
of weight. 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 coal market. There are 7.3 billion
tons of anthracite reserves in the United States, found mostly
in 11 northeastern counties in Pennsylvania.
Lituminous
The most plentiful form of coal in the United States and Mexico,
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. 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.
**This information is provided for educational purposes only at this time.
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