4
Coal
4.1 What is coal?
Coal is formed almost entirely from plants and parts of plants, woody material bark leaves etc. and is
the product of the effect of pressure, heat and hundreds and millions of years.
The most prolific period of coal formation occurred during the Carboniferous period, 300 to 400
million years ago. The black coal deposits mined in Australia were formed in the following period
known as the Permian period, 290 to 220 million years ago. During that time conditions were favorable
for coal formation over the eastern part of Australia and the coal now mined in the Sydney Basin in
NSW and Bowen Basin in Queensland was deposited. Coal was formed at other times in Australia but
apart form the Tertiary brown coal deposits of Victoria, such deposits are of much less importance
than the Permian Coal.
Australia is now the world’s largest exporter of coal, with 34% of the world’s seaborne trade,
but has less than 6% of global coal reserves. Total Australian coal exports are in the order of 170 million
tonnes per year.
Stages in coal formation
Coal requires special conditions for its formation and these only prevailed in certain areas and at
certain times throughout the long history of the earth.
Vast swampy forests made up of trees up to 30 metres in height and a dense undergrowth of
fern-like plants were flooded from time to time by subsidence, killing the plant life and burying it under
successive layers of mud and sand, creating an organic layer known as peat.
These sediments continued to pile up and caused this peat layer to become more compressed
under the increasing weight. This pressure together with heat from other geological processes was
partially responsible for driving gases and moisture out of the peat and causing it to undergo chemical
change. It also caused the peat to harden until eventually it became coal. The process by which plant
tissues are transformed into peat then coal is called coalification. Within the general description of
coal are four basic stages or rank:
•
brown coal
•
sub-bituminous coal
•
bituminous coal
•
anthracite
Figure 4.1 shows the changes in composition as peat progresses to bituminous coal.
4.2 Uses of coal
Thermal (also known as steaming) coal is used in power stations. When burnt in furnaces, it provides
the heat to boil water, which generates the steam to turn the turbines and generate electricity. In
principle, any coal can be used as thermal
The steel industry requires coal of particular quality for the production of coke, which is then
used in blast furnaces to produce iron. This is known (not surprisingly) as coking (or metallurgical)
coal, and only certain “special” coals are suitable for this purposes. As a consequence, coking coals are
more expensive than thermal coals, but there is a lower demand for it. Australia is far and away the
world’s biggest producer of coking coal.
Australia exports around 70% of its coal output, with coking coal being slightly higher than
thermal.
4. Coal
FIGURE 4.1 Chemical changes during coalification
4.3 Coal analysis
Coal is analysed for various reasons, using tests that are very general to any material (eg moisture
content) and those which are very specific (floats and sinks). The tests have three basic functions:
•
what is there other than the carbon (eg sulfur),
•
how it will perform during handling (eg moisture content, grindability)
•
how it will perform in its particular application (eg energy released in combustion)
Figure 4.2 summarises the basic components of coal in a simple but useful way.
Inorganic
Organic
Excess
Water
Inherent
Fixed C
Mineral
Ash
Matter
Volatiles
Volatiles
FIGURE 4.2 The components of coal (sizes of sections not related to % composition)
Industrial Products (Testing)
4.2
4. Coal
Moisture content
There are three moisture values that are of interest: total, excess and inherent. The are related as
shown in Equation 4.1
Total moisture = Excess + Inherent
Eqn 4.1
Too much water in the coal affects its combustion properties. Excess water is the free water in the
coal that is there because it rained (or whatever). It is readily removed in a 110°C oven within two
hours. Water in many solids can be bound quite strongly within the crystal structure and pores of the
material: this is the inherent moisture, and is dependent not on the atmospheric conditions, but the
chemical and physical structures of the coal. This means it is harder to remove, but will affect the
combustion properties of the coal in the same way that the excess.
The total moisture content is used to determine the price of the coal. If the total moisture was
10%, then the agreed price for the coal would be discounted 10%.
Ash
When coal is burnt, an ash residue comprising metal oxides is left behind. The amount of ash resulting
from coal combustion and its chemical composition can vary significantly. It is important for electricity
generators, steel manufacturers, and boiler operators to know the ash of the coal or coke being used
in their process. Three pieces of information about the ash content of the coal are key:
•
total content – the simple muffle furnace method (typically with the temperature ramped from
400 to 900°C over two hours)
•
composition – digestion of the residue followed by ICP analysis
•
thermal behaviour (known as fusion) – examined by eye (video camera) at increasing
temperatures; different stages, denoted by changes in the shape of the ash cone, are recorded
The amount and composition of the ash must be known to ensure that it can handled by the
combustion equipment and so the coal ash content is determined to ensure that capacities of furnace
slag and boiler ash handling equipment are not exceeded. The melting point of ash from different coals
can vary significantly. It is important that coal to be used in boilers produces ash of a sufficiently high
melting point that it does not melt or become plastic in the boiler as this can cause deposits to build
up on the equipment which can be difficult to remove.
Ash in a coking coal represents a severe production penalty. Not only is heat required for
conversion of the ash in the coke to molten slag, but the quantity of slag produced from the ash takes
up useful furnace volume and reduces productivity. Elements in the ash can also be carried through
the steel making process and adversely affect the final steel quality.
There is also the potential pollution hazard of certain components, such as the heavy metals,
mercury and lead.
Volatiles
Mostly hydrocarbons but also excess water. This is determined by the mass lost by a brief (7 minute)
exposure to 900°C.
Fixed carbon
A simple measure of the %C available for combustion or conversion to coke. It is determined indirectly
by subtracting the %inherent moisture, %volatiles and %ash from 100%.
Industrial Products (Testing)
4.3
4. Coal
Calorific Value/Specific Energy
The important use of thermal coal is to produce energy, so the amount of energy produced per unit
mass is the most important measure of its usefulness. Different coals produce different amounts of
heat and hence energy.
It is measured by the combustion bomb (calorimeter) method, where a known mass of coal is
burnt in an insulated vessel, and the heat liberated is measured.
Trace elements
This includes all the elements other than carbon, none of which are wanted, but will be there in
different concentration because of the origins of the coal. Of these, sulfur is the most important in
terms of level and environmental or product effect. Thermal coals produce SOx and coking coals carry
the sulfur through into the steel, changing its properties.
The levels of S, H, N and O, as well as C, are measured by a dedicated instrument, known as an
elemental analyzer, which uses combustion to release the elements in their gaseous form, and then
detects them as shown in Table 4.1.
TABLE 4.1 Elemental analysis
Element
Method of detection
S
Infrared detection of SO2
H
Measured as H2O, by the effect on thermal conductivity detector before
and after adsorption column which removes the water; or by IR
absorption
C
Measured as CO2, by thermal conductivity or IR absorption
N
NOx is reduced to N2, and measured by thermal conductivity
O
Can be determined by IR as CO, following catalysed reaction with C in
the absence of air, or can be calculated indirectly after everything else
is subtracted away
Metallic trace elements include the non-toxic Al, Si, Fe, Ca, Mg, Na and K, as well as low levels of heavy
metals, such as lead and mercury. They are determined from the ash produced, and can be determined
by flame AAS or ICP emission after fusion with borate (AS1038.14.1) or digestion with acid
(AS1038.14.2) or in the solid state by X-ray fluorescence (AS1038.14.3).
Relative density (floats and sinks)
A coal preparation plant and washery is intended to remove a significant proportion of the non-coal
material from the mined product. It does this partly by exploiting the higher density of the mineral
matter, compared to the carbonaceous material.
In the laboratory, or more commonly, “down in the backshed”, the floats and sinks test is
performed as a simulation of the washery, and can give an estimation of the proportions of the various
fractions. This is done in stainless steel drums full of different combinations of organic solvents, such
as tetrabromoethane, perchlorethylene and white spirit, which give liquids of different densities. By
the amount of the material that floats or sinks in the various liquids, a measure of the mineral content
can be determined.
Industrial Products (Testing)
4.4
4. Coal
Mechanical and physical tests
There are a number of tests performed on coal and coke that measure physical or mechanical
properties. These are outside these the scope of this course, but graduates of this Diploma working in
a coal testing laboratory may still be required to perform them. They include:
•
thermal expansion and contraction
•
microscopic examination of structure
•
abrasion
•
strength
•
particle size
•
grindability
What You Need To Be Able To Do
•
•
•
•
•
define important terminology
outline the origins of coal
list industrial uses of coal
describe common tests performed on coal
explain the significance of the test
Industrial Products (Testing)
4.5