Coal assay techniques are specific analytical
methods designed to measure the particular physical and chemical
properties of coals. These methods are used primarily to determine
the suitability of coal for
coking,
power generation or for
iron ore smelting in the manufacture of
steel.
Chemical properties of coal
Coal comes in four main types or
ranks:
lignite or
brown coal,
bituminous coal or black coal,
anthracite and
graphite. Each type of coal has a certain set of
physical parameters which are mostly controlled by moisture,
volatile content (in terms of
aliphatic or
aromatic hydrocarbons) and
carbon content.
Moisture
Moisture is an important property of coal, as all coals are mined
wet. Groundwater and other extraneous moisture is known as
adventitious moisture and is readily evaporated. Moisture
held within the coal itself is known as
inherent moisture
and is analysed quantitatively. Moisture may occur in four possible
forms within coal:
- Surface moisture: water held on the surface of coal
particles or macerals
- Hydroscopic moisture: water held by capillary action within the microfractures of the
coal
- Decomposition moisture: water held within the coal's
decomposed organic compounds
- Mineral moisture: water which comprises part of the
crystal structure of hydrous silicates such as clays
Total moisture is analysed by loss of mass between an untreated
sample and the sample once analysed. This is achieved by any of the
following methods;
- Heating the coal with toluene
- Drying in a minimum free-space oven at within a nitrogen
atmosphere
- Drying in air at and relative loss of mass determined
Methods 1 and 2 are suitable with low-rank coals but method 3 is
only suitable for high-rank coals as free air drying low-rank coals
may promote oxidation.Inherent moisture is analysed similarly,
though it may be done in a vacuum.
Volatile matter
Volatile matter in coal
refers to the components of coal, except for moisture, which are
liberated at high temperature in the absence of air. This is
usually a mixture of short and long chain hydrocarbons, aromatic
hydrocarbons and some sulfur. The volatile matter of coal is
determined under rigidly controlled standards.
In Australian and British
laboratories
this involves heating the coal sample to 900 ± 5 °C (1650 ±10 °F)
for 7 minutes in a cylindrical silica crucible in a muffle
furnace. American Standard procedures involve heating to 950
± 25 °C (1740 ± 45 °F) in a vertical platinum crucible. These two
methods give different results and thus the method used must be
stated.
Ash
Ash content of coal is the non-combustible residue left after coal
is burnt. It represents the bulk mineral matter after carbon,
oxygen, sulfur and water (including from clays) has been driven off
during combustion. Analysis is fairly straightforward, with the
coal thoroughly burnt and the ash material expressed as a
percentage of the original weight.
Fixed carbon
The fixed carbon content of the coal is the carbon found in the
material which is left after volatile materials are driven off.
This differs from the ultimate carbon content of the coal because
some carbon is lost in hydrocarbons with the volatiles. Fixed
carbon is used as an estimate of the amount of coke that will be
yielded from a sample of coal. Fixed carbon is determined by
removing the mass of volatiles determined by the volatility test,
above, from the original mass of the coal sample.
Chemical analysis
Coal is also assayed for oxygen content, hydrogen content and
sulfur. Sulfur is also analysed to determine whether it is a
sulfide mineral or in a
sulfate form. Sulfide content is determined by
measurement of
iron content, as this will
determine the amount of sulfur present as iron
pyrite or dissolution of the sulfates in hydrochloric
acid with precipitation as barium sulfate.
Carbonate minerals are analysed similarly, by measurement of the
amount of carbon dioxide emitted when the coal is treated with
hydrochloric acid. Calcium is analysed. The carbonate content is
necessary to determine the combustible carbon content and
incombustible (carbonate carbon) content.
Chlorine, phosphorus and iron are also determined to characterise
the coal's suitability for steel manufacture.
An analysis of coal ash may also be carried out to determine not
only the composition of coal ash, but also to determine the levels
at which trace elements occur in ash. These data are useful for
environmental impact modelling, and may be obtained by
spectroscopic methods such as
ICP-OES or
AAS
Physical and mechanical properties
Relative density
Relative density or specific
gravity of the coal depends on the rank of the coal and degree of
mineral impurity. Knowledge of the density of each coal ply is
necessary to determine the properties of composites and blends. The
density of the coal seam is necessary for conversion of resources
into reserves.
Relative density is normally determined by the loss of a sample's
weight in water. This is best achieved using finely ground coal, as
bulk samples are quite porous.
Particle size distribution
The particle size distribution of milled coal depends partly on the
rank of the coal, which determines its brittleness, and on the
handling, crushing and milling it has undergone. Generally coal is
utilised in furnaces and coking ovens at a certain size, so the
crushability of the coal must be determined and its behaviour
quantified. It is necessary to know these data before coal is
mined, so that suitable crushing machinery can be designed to
optimise the particle size for transport and use.
Float-sink test
Coal plies and particles have different relative densities,
determined by
vitrinite content, rank, ash
and mineral content and porosity. Coal is usually
washed
by passing it over a bath of liquid of known density. This removes
high-ash content particles and increases the saleability of the
coal as well as its energy content per unit volume. Thus, coals
must be subjected to a float-sink test in the laboratory, which
will determine the optimum particle size for washing, the density
of the wash liquid required to remove the maximum ash content with
the minimum work.
Floatsink testing is achieved on crushed and pulverised coal in a
process similar to
metallurgical testing
on metallic
ore.
Abrasion testing
Abrasion is the property of the coal which describes its propensity
and ability to wear away machinery and undergo autonomous grinding.
While carbonaceous matter in coal is relatively soft, quartz and
other mineral constituents in coal are quite abrasive. This is
tested in a calibrated mill, containing four blades of known mass.
The coal is agitated in the mill for 12,000 revolutions at a rate
of 1,500 revolutions per minute. The
abrasion index is
determined by measuring the loss of mass of the four metal
blades.
Special combustion tests
Specific energy
Aside from physical or chemical analyses to determine the handling
and pollutant profile of a coal, the energy output of a coal is
determined using a
bomb calorimeter which measures the
specific energy output of a
coal during complete combustion. This is required particularly for
coals used in steam-raising.
Ash fusion test
The behaviour of the coal's ash residue at high temperature is a
critical factor in selecting coals for steam power generation. Most
furnaces are designed to remove ash as a powdery residue. Coal
which has ash that fuses into a hard glassy slag known as
clinker is usually unsatisfactory in furnaces as it
requires cleaning. However, furnaces can be designed to handle the
clinker, generally by removing it as a molten liquid.
Ash fusion temperatures are determined by viewing a moulded
specimen of the coal ash through an observation window in a
high-temperature furnace. The ash, in the form of a cone, pyramid
or cube, is heated steadily past 1000 °C to as high a temperature
as possible, preferably . The following temperatures are recorded;
- Deformation temperature: This is reached when the
corners of the mould first become rounded
- Softening (sphere) temperature: This is reached when
the top of the mould takes on a spherical shape.
- Hemisphere temperature: This is reached when the
entire mould takes on a hemisphere shape
- Flow (fluid) temperature: This is reached when the
molten ash collapses to a flattened button on the furnace
floor.
Crucible swelling index (free swelling index)
The simplest test to evaluate whether a coal is suitable for
production of
coke is the free swelling
index test. This involves heating a small sample of coal in a
standardised crucible to around 800 degrees Celsius (1500
°F).
After heating for a specified time, or until all volatiles are
driven off, a small coke button remains in the crucible. The cross
sectional profile of this coke button compared to a set of
standardised profiles determines the Free Swelling Index.
See also
References
Ward, C., 1984.
Coal Geology and Coal Technology Blackwell
Scientific Press, 1984.
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