The widespread occurrence and abundance of
coal makes it the world's principal source of energy for a wide
variety of
end
uses. Coal is a sedimentary rock of organic
composition consisting of carbon, hydrogen, oxygen and minor
proportions of nitrogen and sulphur. The formation of coal from various type
for vegetation making the deposit and varying degrees of deposition
history with respect to burial time and temperature leads to an
highly variable organic material which is a present in coal as a
mixture of solid components that are not necessarily chemically
bound. These solid components are classified by petrographic
analysis into the broad maceral groups of vitrinite, inertinite and
liptinite.The natural constituents of coal can be divided
into two groups: (i) the organic fraction, which can be further
subdivided into microscopically identifiable macerals; and (ii) the
inorganic fraction, which is commonly identified as ash subsequent
to combustion. The rank (thermal maturity) of a the organic
fraction of the coal is determined by the burial depth (pressure)
and temperature. The composition of organic fraction changes
with rank as shown below, with the main indicators of rank bring
the reflectance of the vitrinite, carbon and volatile matter
content on a dry ash free basis.
Coal is composed of microscopically recognizable
constituents, called macerals, which differ from one another in
form and reflectance. Three principal maceral groups are identified
and these are, in increasing order of carbon content, liptinite
(exinite), vitrinite and inertinite. In a single coal, vitrinite,
which is usually the commonest maceral, has a higher reflectance
than the associated liptinite, but a lower reflectance than
inertinite. There is, therefore, a correlation between carbon
content and reflectance and this is used to precisely determine
rank. The mean maximum reflectance of vitrinite in oil (Romax) as
the level of organic maturity, or rank, of a coal sample. Macerals
and maceral groups differ in their chemical composition and thus
their technical performance characteristics. The macerals of the
liptinite group contain more hydrogen and are more generally
reactive than the macerals of the inertinite group, while vitrinite
group macerals range between the two.
The rank and proportions of liptinite, vitrinite
and inertinite do dictate the behavior of a coal during heating
whether in a coke oven or a combustion flame. But other
properties of the coal also have a significant influence, such
as:
- the
distribution of the inertinite and liptinite within vitrinite of
the coal particle - a particle composed of only a pure
maceral or distinctive gains of macerals will have different
plastic properties (and therefore coke/ char morphology) than a
particle where the inertinite and/or liptinite is distributed
throughout the vitrinite.
- the
mineral matter can play a catalytic role which will influence the
decomposition of the coal and can also influence the reactivity of
the resulting char or coke.
Depending of the rank and plastic properties
coals are divided into the following types:
-
Hard coking coals are a necessary input in the
production of strong coke. They are
evaluated based on the strength
, yield of and size
distribution of coke produced which is dependent
on rank and plastic properties of the coal. Hard coking coals trade
at a premium to other coals due to their importance in producing
strong coke and as they are of limited resources.
-
Semi-soft coking coal (SSCC) or weak coking coal is
used in the coke blend, but results in a low coke quality with a
possible increase in impurities. There is scope for
interchangeability between thermal coal and SSCC and thus SSCC
prices have a high correlation with thermal prices.
- Coal
used for pulverised coal injection (PCI)
reduces the consumption of coke per ton of pig iron as it replaces
coke as a source of heat and, at high injection rates, as a
reductant. PCI coal tends to trade at a premium to thermal
coal depending on its ability to replace coke in the blast
furnace.
-
Thermal coals are mostly used for electricity
generation. All coals can be combusted to release
useful energy by the selection of suitable technology to match the
coal rank and ash content. The majority of thermal coal
traded internationally are fired as pulverised fuel
(PF) and vary in rank from sub- bituminous to
bituminous.
The influence of coal rank and plastic properties
on the potential end use for a coal is shown in the figure
below.
