Coals are grouped according to rank. For the purposes of the
power-plant operator, there are several suitable ranks of coal:
Anthracite
Bituminous
Subbituminous
Lignite
The following description of coals by rank gives some of
their physical characteristics.
Anthracite.
Hard and very brittle, anthracite is dense, shiny black, and
homogeneous with no marks or layers. Unlike the lower-rank coals, it has a high
percentage of fixed carbon and a low percentage of
volatile matter.
Anthracites include a variety of slow-burning fuels merging
into graphite at one end and into bituminous coal at the other. They are the
hardest coals on the market, consisting almost entirely of fixed carbon, with
the little volatile matter present in them chiefly as methane, CH4.
Anthracite is usually graded into small sizes before being
burned on stokers. The “metaanthracites” burn so slowly as to require mixing
with other coals, while the “semianthracites,” which have more volatile matter,
are burned with relative ease if properly fired.
Most anthracites have a lower heating value than the
highestgrade bituminous coals. Anthracite is used principally for heating homes
and in gas production. Some semianthracites are dense, but softer than
anthracite, shiny gray, and somewhat granular in structure.
The grains have a tendency to break off in handling the
lump, and produce a coarse, sandlike slack. Other semianthracites are dark gray
and distinctly granular. The grains break off easily in handling and produce a coarse slack.
The granular structure has been produced by small vertical
cracks in horizontal layers of comparatively pure coal separated by very thin
partings. The cracks are the result of heavy downward pressure, and probably
shrinkage of the pure coal because of a drop in temperature.
Bituminous.
By far the largest group, bituminous coals derive their name
from the fact that on being heated they are often reduced to a cohesive,
binding, sticky mass. Their carbon content is less than that of anthracites,
but they have more volatile matter. The character of their volatile matter is
more complex than that of anthracites, and they are higher in calorific value.
They burn easily, especially in pulverized form, and their
high volatile content makes them good for producing gas. Their binding nature
enables them to be used in the manufacture of coke, while the nitrogen in them
is utilized in processing ammonia.
The low-volatile bituminous coals are grayish-black and
distinctly granular in structure. The grain breaks off very easily, and
handling reduces the coal to slack. Any lumps that remain are held
together by thin partings. Because the grains consist of
comparatively pure coal, the slack is usually lower in ash content than are the
lumps.
Medium-volatile bituminous coals are the transition from
high-volatile to low-volatile coal and, as such, have the characteristics of
both. Many have a granular structure, are soft, and crumble easily. Some are
homogeneous with very faint indications of grains or layers. Others are of more
distinct laminar structure, are hard, and stand handling well.
High-volatile A bituminous coals are mostly homogeneous with
no indication of grains, but some show distinct layers. They are hard and stand
handling with little breakage. The moisture, ash, and sulfur contents are low,
and the heating value is high.
High-volatile B bituminous coals are of distinct laminar
structure; the layers of black, shiny coal alternate with dull, charcoal-like
layers. They are hard and stand handling well. Breakage occurs generally at
right angles and parallel to the layers, so that the lumps generally have a
cubical shape.
High-volatile C bituminous coals are of distinct laminar
structure, are hard, and stand handling well. They generally have high
moisture, ash, and sulfur contents and are considered to be free-burning coals.
Subbituminous.
These coals are brownish black or black. Most are
homogeneous with smooth surfaces, and with no indication of layers. They have
high moisture content, as much as 15% to 30%, although appearing dry.
When exposed to air they lose part of the moisture and crack
with an audible noise. On long exposure to air, they disintegrate. They are
free-burning, entirely noncoking, coals.
Lignite.
Lignites are brown and of a laminar structure in which the
remnants of woody fibers may be quite apparent. The word lignite comes from the
Latin word lignum meaning wood. Their origin is mostly from plants rich in
resin, so they are high in volatile matter.
Freshly mined lignite is tough, although not hard, and it
requires a heavy blow with a hammer to break the large lumps. But on exposure
to air, it loses moisture rapidly and disintegrates. Even when it appears quite
dry, the moisture content may be as high as 30%.
Owing to the high moisture and low heating value, it is not
economical to transport it long distances.
Unconsolidated lignite is also known as “brown coal.” Brown
coals are generally found close to the surface, contain more than 45% moisture,
and are readily won by strip mining.
