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Coal assayFrom Wikipedia, the free encyclopedia

Coal Analysis 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 forcoking, power generation or foriron oresmelting in themanufacture ofsteel.

Contents

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1 Chemical properties of coal

o 1.1 Moistureo 1.2 Volatile mattero 1.3 Asho 1.4 Fixed carbono 1.5 Chemical analysis

2

Physical and mechanical propertieso 2.1 Relative densityo 2.2 Particle size distributiono 2.3 Float-sink testo 2.4 Abrasion testing

3 Special combustion tests

o 3.1 Specific energyo 3.2 Ash fusion testo 3.3 Crucible swelling index (free swelling index)

4 See also

5 References

6 External links

Chemical properties of coalCoal comes in four main types orranks:lignite orbrown coal, bituminous coal or blackcoal, anthracite and graphite. Each type of coal has a certain set of physical parameterswhich are mostly controlled by moisture, volatile content (in terms ofaliphatic oraromatichydrocarbons) and carbon content.

Moisture

Moisture is an important property of coal, as all coals are mined wet. Groundwater andother extraneous moisture is known as adventitious moisture and is readily evaporated.Moisture held within the coal itself is known as inherent moisture and is analysedquantitatively. Moisture may occur in four possible forms within coal:

Surface moisture: water held on the surface of coal particles ormacerals

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 silicatessuch as clays

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Total moisture is analysed by loss of mass between an untreated sample and the sampleonce analysed. This is achieved by any of the following methods;

1. Heating the coal with toluene

2. Drying in a minimum free-space oven at 150 C (302 F) within a nitrogen

atmosphere3. Drying in air at 100 to 105 C (212 to 221 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-rankcoals as free air drying low-rank coals may promote oxidation. Inherent moisture isanalysed 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 areliberated at high temperature in the absence of air. This is usually a mixture of short andlong chain hydrocarbons, aromatic hydrocarbons and some sulfur. The volatile matter ofcoal is determined under rigidly controlled standards. InAustralian and British laboratoriesthis involves heating the coal sample to 900 5 C (1650 10 F) for 10 min.

Ash

Ash content of coal is the non-combustible residue left after coal is burnt. It represents thebulk mineral matter after carbon, oxygen, sulfur and water (including from clays) has beendriven off during combustion. Analysis is fairly straightforward, with the coal thoroughlyburnt 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 aftervolatile materials are driven off. This differs from the ultimate carbon content of the coalbecause 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 isdetermined by removing the mass of volatiles determined by the volatility test, above, fromthe original mass of the coal sample.

Chemical analysis

Coal is also assayed for oxygen content, hydrogen content and sulfur. Sulfur is alsoanalysed to determine whether it is a sulfide mineral or in a sulfate form. Sulfide content isdetermined by measurement ofiron content, as this will determine the amount of sulfurpresent as iron pyrite or dissolution of the sulfates in hydrochloric acid with precipitation asbarium sulfate.

Carbonate minerals are analysed similarly, by measurement of the amount of carbondioxide emitted when the coal is treated with hydrochloric acid. Calcium is ana he carbonatecontent 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 forsteel manufacture.

An analysis of coal ash may also be carried out to determine not only the composition ofcoal ash, but also to determine the levels at which trace elements occur in ash.

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Physical and mechanical properties

Relative density

Relative density or specific gravity of the coal depends on the rank of the coal and degree ofmineral 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 forconversion of resources into reserves.

Relative density is normally determined by the loss of a sample's weight in water. This isbest achieved using finely ground coal, as bulk samples are quite porous. To determine in-place coal tonnages however, it is important to preserve the void space when measuringthe specific gravity.

Particle size distribution

The particle size distribution of milled coal depends partly on the rank of the coal, whichdetermines 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 thesedata before coal is mined, so that suitable crushing machinery can be designed to optimisethe 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 washedby passing it over abath of liquid of known density. This removes high-ash content particles and increases thesaleability of the coal as well as its energy content per unit volume. Thus, coals must besubjected to a float-sink test in the laboratory, which will determine the optimum particle sizefor washing, the density of the wash liquid required to remove the maximum ash contentwith the minimum work.

Floatsink testing is achieved on crushed and pulverised coal in a process similarto metallurgical testing on metallic ore.

Abrasion testing

Abrasion is the property of the coal which describes its propensity and ability to wear awaymachinery and undergo autonomous grinding. While carbonaceous matter in coal isrelatively soft, quartz and other mineral constituents in coal are quite abrasive. This is testedin a calibrated mill, containing four blades of known mass. The coal is agitated in the mill for12,000 revolutions at a rate of 1,500 revolutions per minute.(I.E 1500 revolution for 8 min.)The abrasion indexis determined by measuring the loss of mass of the four metal blades.

Special combustion testsSpecific energy

Aside from physical or chemical analyses to determine the handling and pollutant profile ofa coal, the energy output of a coal is determined using a bomb calorimeterwhich measures

the specific energy output of a coal during complete combustion. This is requiredparticularly for coals used in steam-raising.

Ash fusion test

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The behaviour of the coal's ash residue at high temperature is a critical factor in selectingcoals for steam power generation. Most furnaces are designed to remove ash as a powderyresidue. Coal which has ash that fuses into a hard glassy slag known as clinkeris usuallyunsatisfactory in furnaces as it requires cleaning. However, furnaces can be designed tohandle the clinker, generally by removing it as a molten liquid.

Ash fusion temperatures are determined by viewing a moulded specimen of the coal ashthrough an observation window in a high-temperature furnace. The ash, in the form of acone, pyramid or cube, is heated steadily past 1000 C to as high a temperature aspossible, preferably 1,600 C (2,910 F). The following temperatures are recorded;

Deformation temperature: This is reached when the corners of the mould first becomerounded

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 hemisphereshape

F

low

(fluid) temperature:

This is reached when the molten ash collapses to a flattenedbutton on the furnace floor.

Crucible swelling index (free swelling index)

The simplest test to evaluate whether a coal is suitable for production ofcoke is the freeswelling index test. This involves heating a small sample of coal in a standardized crucibleto around 800 degrees Celsius (1500 F).After heating for a specified time, or until allvolatiles are driven off, a small coke button remains in the crucible. The cross sectionalprofile of this coke button compared to a set of standardized profiles determines the FreeSwelling Index.