Coal Classification
There are two main ways for classifying coal
- by rank and by type.
Coal Rank
The degree of 'metamorphisrn' or coalification
undergone by a coal, as it matures from peat to
anthracite, has an important bearing on its physical
and chemical properties, and is referred to as
the 'rank' of the coal.
Source: Coal, Power for Progress - World
Coal Institute
Low rank coals, such as lignite
and sub-bituminous coals, are typically softer,
friable materials with a dull, earthy appearance;
they are characterised by high moisture levels
and a low carbon content, and hence a low energy
content.
Higher rank coals are typically harder and stronger
and often have a black vitreous lustre. Increasing
rank is accompanied by a rise in the carbon and
energy contents and a decrease in the moisture
content of the coal. Anthracite is at the top
of the rank scale and has a correspondingly higher
carbon and energy content and a lower level of
moisture.
Between anthracites and peat there
are three broad coal rankings.
 Bituminous
coals are dense black solids, frequently
containing bands with a brilliant lustre. The
carbon content of these coals ranges from 78 to
91 percent and the water content from 1.5 to 7
percent.
The major NSW and Queensland deposits are bituminous
and many are suited to the production of metallurgical
coke. Non-coking bituminous coals are used for
power generation, cement making and to provide
heat and steam in industry.
 Sub-bituminous
coals usually appear dull black and waxy.
They have a carbon content between 71 and 77 percent
and a moisture content of up to 10 percent and
are used for electricity generation or can be
converted to liquid and gaseous fuels.
Queensland, NSW, Tasmania, South Australia and
Western Australia have deposits of sub-bituminous
coals.
 The
lower ranked coals are browner and softer. Brown
coals or lignites have a high oxygen content
(up to 30 percent), a relatively low carbon content
(60-75 percent on a dry basis), and a high moisture
content (30-70 percent).
Brown coals, found in Australia in Victoria's
Latrobe Valley, are used for power generation
but generally are uneconomic to transport because
of their high moisture content. These coals are
also susceptible to spontaneous combustion.
Coal Types
Geologists also classify coal types
according to the organic debris, called macerals,
from which the coal is formed. Macerals are identified
(microscopically) by reflected light - the reflective
or translucent properties of the coal indicating
the individual component macerals and the way
they have combined to form the coal.
The purpose of classifying coal in this way is
to determine its best uses. There is a finite
supply of the resource and, therefore, type and
chemical composition must be matched to the most
suitable end use.
The mineral or inorganic content of coal is another
significant factor affecting end use. Mineral
content is assessed by burning coal and measuring
the amount of incombustible material remaining,
referred to as the ash content of coal.
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