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Coal Assay
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. Moisture may occur in four possible forms within
coal:
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;
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 is the components of coal,
except for moisture, which is liberated at high temperature in the absence of
air. This is usually a mixture of short and long chain hydrocarbons, aromatic
hydrocarbons and some sulphur. 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 for 7 minutes
in a cylindrical silica crucible in a muffle furnace. American Standard
procedures involve heating to 950 ± 25 °C 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, sulphur 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 sulphur. 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. This data is 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
utilized in furnaces and coking ovens at a certain size, so the crushability of
the coal must be determined and its behavior quantified. It is necessary to
know this data before coal is mined, so that suitable crushing machinery can be
designed to optimize 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 a 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
1600 °C. The following temperatures are recorded;
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 standardized crucible to around 800 degrees celsius. 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 standardized
profiles determines the Free Swelling Index. References
Ward,
C., 1984. Coal geology and Coal Technology Blackwell
Scientific Press, 1984. *Sections taken from From Wikipedia, the free encyclopedia |
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