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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.

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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:

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 150 °C (300 °F) within a nitrogen atmosphere
Drying in air at 100 to 105 °C (210 to 220 °F) 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 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 (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, 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. Sulphur is also analysed to determine whether it is a sulfide mineral or in a sulfate form. This is achieved by dissolution of the sulfates in hydrochloric acid and precipitation as barium sulphate. Sulfide content is determined by measurement of iron content, as this will determine the amount of sulphur present as iron pyrite.

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 conent 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 (2900 °F). 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.

References

Ward, C., 1984. Coal Geology and Coal Technology Blackwell Scientific Press, 1984.

Categories: Analytical chemistry | Coal

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