Different Types of Coal
It is important to be aware that "coal" can cover a
variety of finished products of differing values and requiring different
treatment. The main division is between "coking" coal and "energy or steaming" coal. Coking coals, used to make coke for
use in blast furnaces, have particular properties which allow it to form a good
coke when put through a coking process which involves heating in an atmosphere
deficient in oxygen. There are a number of coal properties which are important
in this context, including the "volatile content", "caking" properties,
hardness, and strength of the coke formed, free swelling index (how much the
material swells during the process), etc. These properties do not affect the
way coal is mined, but they do affect the value of the product and there are
different grades of coking coal, from "hard" coking coals (the best grade),
through "medium" to "soft" coking coals, and even "semi-hard" and "semi-soft".
The individual property values for each classification is more a marketing
exercise than a precise definition.
Energy or steaming coal is not suitable for coke
making and, as the name implies is used for heating, the vast majority in power
stations generating electricity, with lesser amounts used in cement
manufacture, small boilers and other specialist markets. Coking coal can be
used for these purposes, but that would be wasteful.
The ability to blend different types of coal enables
some coals which do not coke well to be used with higher grade coals to form an
acceptable mix. Developments in blast furnace technology (pulverized coal
injection in particular) have also allowed lower quality coals to be used in
steelmaking in place of a portion of the coke.
The value and treatment of coal is also affected by
other properties:
- Ash content – all coal contains
material which will not burn and which forms the ash left after burning, a
waste product which presents a disposal problem; other factors being equal, the
higher the ash content the lower the value of the coal
- Moisture content – all coal contains
some moisture naturally, and the mining process may remove some but usually
adds more. The weather will also affect the moisture content during transport
and storage. Most sales contracts specify a maximum moisture content with
penalties if this is exceeded (customers do not want to pay coal prices for
water)
- Coals may contain a number of impurities (such
as sulphur or phosphorous) which, while they may be in very small quantities
become significant because they can result in atmospheric pollutants when burnt
or can affect the properties of steel produced.
Coal is frequently treated or "washed", in most cases to remove some of the ash
to make it saleable or to increase its value. There are several examples where coal
is untreated, mostly where it is used by a power station or cement works
adjacent to or close to the mine site. In most other cases, some degree of
treatment is carried out.
There are some special coals:
- Brown coal, which is part way between peat
and black coal, basically a poor quality coal, usually with a very high
moisture content, but which can be economically important if mined in large
quantities and fed into boilers adjacent to the mine site (i.e. minimal
transport costs).
- Anthracite, a high quality coal due to its
high fixed carbon content (not a coking coal), once a premium coal for steaming
purposes but now relatively rare and only utilized in specialist applications
such as a graphitizing material for electrode paste and as a reductant for
reducing metal oxide ores.
ACARP reports
– various reports on research work related to coal types &
properties, in "Technical market Support section"
