Coals
Definition.
Ø These
may broadly be defined as metamorphosed sedimentary rocks of carbonaceous
character in which the raw material has mostly been supplied by plants of
various groups.
Ø
The original raw material passes through
many biomechanical and biochemical
processes
before it becomes a coal in technical terms;
Formation.
Ø In
most cases coals represent carbonized wood.
Ø The
process of coal formation involves a series of stages similar to formation of
sedimentary rocks such as wastage of forests and transport of the wood material
through different natural
agencies to places of deposition, accumulation of
the material in huge formations.
Ø Its
burial under clays and other matter and its compaction and consolidation under
superimposed load.
Ø Biochemical
transformation of the organic matter so accumulated starts and is completed
under the influence of aerobic and anaerobic bacteria available at the place of
deposition.
Ø The
degree of carbonification depends to a great extent on the time and type of
environment in which the above processes have operated on the source material
giving rise to different varieties
of coal.
Types
Peat.
Ø It is the lowest grade coal that
consists of only slightly altered vegetable matter. It may not be even
considered as a coal. It has very low calorific value, high percentage of
moisture and is rich in volatile matter.
Lignite.
Ø It
is also known as brown coal and forms the poorest grade of coal with calorific
value ranging between 6300-8300 B.th.V.
Ø
It is compact and massive in structure
with an upper specific gravity of 1.5 and hardness of 2.5 on
Mohs' Scale of Hardness.
Ø Some
varieties of lignite may still show to a good extent the traces of original
vegetable structure.
Bituminous
Coals
Ø These
form a broad group of common coals having essential properties varying within
wide limits.
Ø
The fixed carbon ranges between 69-78
per cent and the calorific value between 9,500 B.th.V to
14,000 B.th.V.
Ø Their
common character is that they contain enough volatile matter, which makes them
quite soft on heating, and they start agglomerating.
Some
of bituminous coals may contain volatile matter to such a high extent as 30 per
cent of their bulk.
Anthracite
Ø
It is considered the highest-grade coal
with fixed carbon ranging between 92-98 per cent.
Ø It
has highest calorific value in coals and burns almost without any smoke, as the
volatile
matter is negligible.
Occurrence
Ø Coals
of different varieties are found to occur almost in all countries of the world,
though in varying proportions.
Ø Coals form all-purpose fuels, some varieties being more suitable for specific industrial uses.
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