HOW
COAL WAS FORMED
Coal is the altered remains of vegetation that originally accumulated in
prehistoric swamps and peat bogs.
The energy we get from coal today comes from the energy that plants absorbed
from the sun millions of years ago. All living plants store solar energy
through a process known as photosynthesis. When plants die, this energy
is usually released as the plants decay. Under conditions favourable to
coal formation, the decaying process is interrupted, preventing the release
of the stored solar energy. The energy is locked into the coal.
Coal formation began during the Carboniferous Period - known as the first
coal age - which spanned 360 million to 290 million years ago. The build-up
of silt and other sediments, together with movements in the earth's crust
- known as tectonic movements - buried swamps and peat bogs, often to great
depths. With burial, the organic plant material was subjected to high temperatures
and pressures. This caused physical and chemical changes in the vegetation,
transforming it into peat and eventually into coal. |
Coalification
The quality of each coal deposit is determined by:
-
varying types of vegetation from which the coal originated
- depths
of burial
- temperatures
and pressures at those depths
- length
of time the coal has been forming in the deposit
|
The degree
of change undergone by a coal as it matures from peat to anthracite is known
as coalification. Coalification has an important bearing on coal's physical
and chemical properties and is referred to as the 'rank' of the coal. Ranking
is determined by the degree of transformation of the original plant material
to carbon.
The classifications of coals, from those with the least carbon to those
with the most carbon, are lignite, sub-bituminous, bituminous and anthracite
|
Types
of Coal
Initially the peat is converted into lignite or 'brown coal' - these are
coal-types with low organic maturity. In comparison to other coals, lignite
is quite soft and its colour can range from dark black to various shades
of brown.
Over many more millions of years, the continuing effects of temperature
and pressure produces further change in the lignite, progressively increasing
its organic maturity and transforming it into the range known as 'sub-bituminous'
coals.
Further chemical and physical changes occur until these coals became harder
and blacker, forming the 'bituminous' or 'hard coals'. Under the right conditions,
the progressive increase in the organic maturity can continue, finally forming
anthracite.
In addition to carbon, coals contain hydrogen, oxygen, nitrogen, ash and
varying amounts of sulphur. High-rank coals are high in carbon and therefore
heat value but low in hydrogen and oxygen. Low-rank coals are low in carbon
but high in hydrogen and oxygen content. |
*
Learning Tip
The mnemonic “NO CASH” can be used to help you remember the
constituents of coal
Nitrogen, Oxygen, Carbon, Ash, Sulphur, Hydrogen |
