2 Chemical analysis and classification of coal

2.1 INTRODUCTIONCoal is a heterogeneous mixture of organic compounds, together with a certain amount of inorganic material in the form of moisture and mineral impurities. The nature of the organic constituents depends on the relative proportions of the various kinds of plant debris (wood, leaves, spores etc.) in the initial accumulation of peat (the coal type), and on the diagenetic or metamorphic changes that have occurred since the peat was originally laid down (the rank of the coal). Together with the nature and relative amount of any mineral matter that may be present, this assemblage of organic compounds is reflected by the physical appearance of the coal, and determines its behaviour when used.2.1.1 Classes of coal analysisAn account of the various organic compounds actually present in coal is given by Francis (1961), van Krevelan (1961) and Flaig (1968). This type of analysis is only attempted as part of a detailed research investigation. For most practical purposes, the general nature of these materials can be evaluated by a combination of two sets of analytical data:(a) Proximate analysisThis gives the relative amounts of light organic compounds (volatile matter), as opposed to non-volatile organic material (fixed carbon). This type of analysis also gives the amount of moisture in the coal, and a measure of the inorganic components, left as a residue or ash when the coal is burned.(b) Ultimate analysisThis determines the total amounts of each of the principal chemical elements in the coal: carbon; hydrogen; oxygen; nitrogen and sulphur. In some studies, the data may be recalculated to remove the percentage of these elements contained in the moisture and mineral impurities, in order to obtain the chemical composition of the organic fraction alone.In many cases, the results of proximate and ultimate analysis are sufficient to indicate how the coal will behave in industrial use. More specific tests may also be employed to give further information on various aspects of the coals properties. These tests may be regarded as miscellaneous analyses, and include determination of such properties as the amount of heat energy liberated from the coal on combustion, and the type of coke likely to be made from the coal in carbonization processes. Other mechanical properties, such as the coals relative density, and its amenability to upgrading in preparation plants may also be investigated.2.1.2 Errors in analysisThe methods of sample collection and laboratory testing for coal analysis have been refined by the experience of numerous workers in the field, and standardized procedures have been drawn up to minimize the likelihood of error. Most of the tests are covered by various national or international standards (Appendix 1), and, in commercial evaluations, strict adherence to these procedures is generally necessary. However, as many of the national standards for the same test may differ in points of detail, it is also necessary to indicate, when stating the results, which particular method was followed.Errors in analysis may arise from a number of sources including sample collection, sample storage, and the actual performance of the testing operation. In some cases, such as with errors introduced by biased sampling procedures or incorrectly calibrated laboratory instruments, they may not be immediately apparent, and the erroneous results incorporated, without question, into a far-reaching quality assessment.Many of the laboratory techniques in coal testing are based on arbitrarily chosen dimensions, materials, temperatures and heating rates, and errors may also be introduced if these procedures are not followed exactly. In other cases, however, the coal itself may be oxidized, or contain components that give rise to unusual test results. Rees (1966) gives a very complete discussion of the sources of error that may arise in a number of the tests commonly performed.2.2 SAMPLING FOR COAL ANALYSISEven the most precise analytical determination is useless for most practical purposes if the sample tested does not truly represent the mass of coal from which it was taken. Coal samples may be taken from outcrops, mine faces and other in situ exposures, from drill cores or cuttings, and from masses of broken coal such as in conveyor belts and stockpiles. The procedure for collection of these samples is also covered by many national standards, and these should be followed in detail if bias is to be minimized.2.2.1 Sampling a coal faceAn in situ exposure of a coal seam in a mine or outcrop is normally sampled along a line running perpendicular to the bedding planes. The sample may be taken by one of the following methods:(a) Pillar samplingThis involves taking a continuous rectangular block of coal, extending between the upper and lower bedding planes marking the limits of the interval in question. The block is normally 30-45 cm wide and around 450 cm2 in cross sectional area. Pillar samples are very time consuming (and therefore costly) to obtain, as well as difficult to handle, and are seldom used except for detailed research work.(b) Strip or channel samplingThis involves cutting a channel or groove into the coal face, and collecting all of the pieces removed on a clean plastic sheet at the base of the exposure. The channel should have a cross sectional area of at least 100 cm2, and sufficient material taken to give around 15 kg of coal per metre of seam thickness.For bulk sampling in well-known seams, the pillar or strip sample may extend, without break, from the roof to the floor of the interval in question. However, where significant partings, or bands of non-coal material occur (Section 5.2.1), the sample may be divided into sub-sections or plies between these natural breaks (Fig. 2.1).The amount of material in these strip or channel samples may be reduced, if necessary, by the cone and quarter technique to give a more suitable mass for transport. Samples from which it is intended to determine the total moisture content (i.e. before air-drying; Section 2.3.2) should be sealed in dry, airtight containers immediately after the sample is collected.2.2.2 Sampling coal by drillingThe methods used in exploratory drilling of coal seams are described more fully in Section 6.5. Due

Fig. 2.1 Channel sample collection from a coal seam on a ply-by-ply basis.

to the risk of contamination, samples of cuttings from non-cored holes are of limited value for coal analysis. However, because they are obtained easily and at low cost, they are sometimes used for the rapid appraisal of certain aspects of seam quality, such as the extent of oxidation in the outcrop zone. The rank of the coal may also be assessed by petrographic examination of material taken from the cuttings of non cored holes (Section 3.6).Cores samples of the coal seams are required for more comprehensive analytical programmes. Selection of the core size to be used depends partly on the anticipated drilling conditions, and partly on the amount of material needed to perform all the necessary tests on the thickness of coal involved. Small diameter (20 mm) cores tend to give less complete seam recoveries in banded or friable coals, and cores of around 45-60 mm diameter are more suitable for most studies. Large- diameter cores, up to 200 mm in diameter, are needed to provide samples for detailed washability tests (Section 8.2.7).A core sample of a coal seam is equivalent to a pillar sample with a small cross-sectional area, and can readily be divided into plies or sub-sections for analysis. Special attention may need to be given to cores where recovery of some segments of the seam is incomplete. Care should be taken in these cases to ensure that the mass of material from each segment included in the analysed sample is in proportion to its mass (length x relative density) in the total ply being studied. The need for these reconstructions in the sampling operation can be judged from details of the core recovery, measured either on a linear or, preferably, a volumetric basis.2.2.3 Sampling of broken coalIn a mass of broken coal, such as a stockpile, the particles tend to become segregated, with denser orfiner particles occupying different positions to the coarser or lighter components (Fig. 2.2). Sampling procedures in such cases must take this segregation into account in order to avoid the introduction of a bias to the subsequent analytical programme. The sampling operation may be covered by a national standard or other set of similar guidelines, and these should be followed rigorously to ensure good results. The mass of material taken depends partly on the requirements of the tests to be performed, and partly on the physical size of the coal fragments involved.2.2.4 Sample preparationOnce transported to the laboratory, the methods of sample preparation depend on the nature of the material and the analyses or tests to be carried out. Although the most effective procedures are best determined for each individual analytical programme, some typical requirements are:(a) Broken coalWhere the total moisture content is to be determined (Section 2.3.1), the sample must be transferred to the laboratory in an air-tight container. The mass of the

Coal from v. stackerFig. 2.2 Segregation of particles in a simple stockpile.

wet coal is determined on arrival, after which the coal is air-dried and reweighed prior to further testing. If necessary, the particle size distribution is also determined on the air-dry coal. Appropriate size fractions may be split off and subjected to float-sink or other washability tests if necessary (Section 2.6.3), prior to crushing for analysis (Fig. 2.3).(b) Exploration cores

Crush and analyserBulk sample from stockpile etc.\Transport in sealed containerDetermine total moisture or air-dry moisture loss\Determine particle size distribution1Crush and analyse1 \Float-sink testing Froth flotation testing1Crush and analyseCoarse fraction(s)1Fine fractionReserveEach density fractionReserveProduct and tailingsr) iCrush and analyse [ Combine as required to form TIReserveIReserve1 1 .Reconstituted clean-coal products Reconstituted refuse materialsFurther testing and analysis Further testing and analysisFig. 2.3 Possible laboratory preparation flowsheet for a broken coal sample.The core is generally transported to the laboratory with as little disturbance as possible. It may then be subjected to non-destructive tests such as X-radiography (Section 6.5.7), or ply-by-ply relative density determination (Section 2.6.1) as well as detailed

macroscopic logging (Section 3.2.3). The seam is divided into sub-sections for analysis, if necessary, after the volumetric recovery of each significant interval has been determined.Where washability testing of slim cores is required, the plies to be treated separately are crushed to a nominated top size (usually between 10 and 15 mm) in a way that produces a minimum proportion of fine particles. Large-diameter cores may be subjected to a special crushing and tumbling technique to simulate the effects of particle size reduction and degradation by the mining and handling process (Section 8.2.3). In both cases, splits can also be riffled off for analysis of in situ (i.e. raw) coal properties, but the major part of the material is subjected to float-sink or similar testing (Fig. 2.4), before being more finely crushed for detailed studies of the clean coal products.Almost all analysis requires the coal to be air-dried. After crushing, the sample is spread out and allowed to dry, without heating, in either the laboratory atmosphere or a humidity-controlled drying cabinet. The analytical results are initiallycalculated in relation to the air-dried coal, although they can also be re-expressed to any other basis (e.g. dry, ash free) if necessary (Section 2.10.1).2.3 PROXIMATE ANALYSIS

50Chapter 2

Chemical Analysis and Classification of Coal49As indicated in Section 2.1, proximate analysis gives a measure of the relative amount of volatile and non-volatile organic compounds in the coal as well as the percentage of water and non-combustible mineral materials. If corrections are applied to compensate for the non-organic impurities, proximate analysis enables the organic component of one coal to be compared with that of another, and hence provides much of the basic data for systematic coal classification. In commercial applications, however, it is normal for the coals properties to be assessed with all of the impurities included, as this represents the actual product that is to be used.

Bore coreIX-radiography Detailed loggingDensity and recovery determination

)Each non-coal ply*Coarse crushfEach coal plyTCoarse crushrrFine crush'and analyseReserveCombine with other coal plies to form working section coal compositeCombine with other non-coal plies to form working section non-coal compositeReserve1Fine crush and analysefFloat-sink testingIEach density fraction- Shale breakdown testingI

\Fine crush and analyseH)Combine with appropriate other fractions to form ReserveIReconstituted clean-coal productReconstituted refuse materialFurther testing and analysisFurther testing and analysisFig. 2.4 Possible laboratory preparation flowsheet for an exploration bore core. Other procedures may be followed as well, depending on the sample and the data required. (Modified from Australian Standard 2519, 1981.)2.3.1 MoistureThe moisture in a coal may occur in four possible forms:(a) Surface moistureThis is extraneous water held as films on the surface of the coal particles.(b) Hygroscopic moistureThis is water held inside the capillaries of the coal substance.(c) Decomposition moistureThis is water incorporated in some of the coals organic compounds.(d) Mineral moistureThis is water which forms part of the crystal structure of clays and other minerals present in the coal.A coal sample collected from a mine or stockpile is usually wet with a variable amount of adventitious moisture from seepage, water sprays, rain and other sources. Such water is surface water, and generally evaporates readily in the laboratory atmosphere. The amount of mositure in other forms, often regarded as inherent moisture, is more or less constant for coals of a given rank. Much of it, especially the moisture in the pores of the coal, is driven off by heating to 100 C, but some of the mineral and decomposition moisture may not be liberated until temperatures exceed 500 C.A coal that is sold commerically usually contains a certain amount of surface water, and the mass of this water is naturally included in the mass of coal that is delivered. Knowledge of the total moisture content of the coal (i.e. adventitious + inherent moisture) is therefore essential to assess the value of any marketable consignment. However, because the amount of surface water is difficult to control, separate assessment of inherent or air-dried moisture is also necessary, as most other analyses are carried out on air-dried material.(a) Total moistureDetermination of total moisture is based on the loss in mass between the sealed sample as received in the laboratory (Section 2.2.4) and that of the same sample fully dried. The moisture is driven off by one of the following methods:(i) Distillation with toluene the coal is heated under reflux with toluene and the mass of water collected as a condensate is determined.(ii) Drying in a minimum free-space oven the coal is dried at 105-110C in an atmosphere of flowing nitrogen in a small, specially designed oven and the relative loss of mass is determined.(iii) Oven-drying in air the coal is dried at 105 110C in a laboratory air oven and the relative loss of mass is determined.Methods (i) and (ii) are suitable for total moisture determination in coal of all ranks, but because of the possibility of oxidation in low-rank coals, method (iii) is normally used only with high-rank materials. High-rank coals may also be air-dried before the final drying process, enabling the air-dry loss or free moisture content to be determined separately.(b) Inherent or air-dried moistureThe inherent or air-dried moisture content is represented by the relative loss of mass when the air-dried coal is heated, without oxidation, to a little over 100 C. It may be determined by one of the following techniques:(i) Drying in a minimum free-space oven the coal is dried at 105-110 C in a special small oven with a flowing nitrogen atmosphere, and the relative loss of mass determined.(ii) Direct gravimetric determination the coal is sealed in an atmosphere of dry nitrogen, heated to 105-110 C and the water collected in an absorption tube. The relative mass of water collected gives the moisture content of the coal.(iii) Drying in vacuo the relative loss of mass is determined when the coal is held at 105-110 C for a specified time under a vacuum of less than 5 mm Hg.Because of its capacity to treat a number of samples simultaneously, use of the minimum free-space oven is favoured by most commercial laboratories.American practice differs in that it involves drying the coal at 104-110 C in an atmosphere of air, dried by passage through sulphuric acid. Although the oven is required to have a minimum amount of air space, this method is probably only suitable for high-rank coals due to the risk of oxidation in the drying process.2.3.3 Volatile matterThe material referred to as volatile matter in a coal sample represents the components of the coal, except for the moisture content, that are liberated at high temperature in the absence of air. This material may be released from the organic compounds or from the mineral impurities, and the significance of the latter should not be overlooked. If the percentage of volatile matter is to be used to classify the coal, correction for that derived from inorganic sources (carbonates, clays etc.) must be made.The volatile matter content is determined by heating the coal under rigidly specified conditions. The percentage loss of mass, less the percentage air-dried moisture, gives the proportion of volatile matter present. British and Australian Standards require the coal to be heated in a cylindrical silica crucible in a muffle furnace of specified dimensions at 9005C for 7 min. American (A.S.T.M.) procedures, involve heating at 95025C using a platinum crucible in a vertical electric furnace. These differences in procedure give slightly different results for the same coal, and the method used must therefore be stated with the analytical results. A modification of the A.S.T.M. procedure, involving a two-stage heating process, is necessary for some low rank or sparking coals.2.3.4 AshThe ash of a coal is the non-combustible inorganic residue that remains when coal is burned. It represents the bulk of the mineral matter in the coal, after volatile components such as C02 (from carbonates), S02 (from sulphides) and H20 (from clays) have been driven off. As such, the ash makes up a lesser proportion of the coal than does the total mineral matter content.Coals with a high ash content are generally less suitable for utilization than low-ash coals. A greater mass of high-ash material must be used to provide a given amount of heat or other product due to the diluting effect of the non-carbonaceous components. Greater amounts of waste also need to be removed after utilization.Determination of ash content requires heating the coal in a ventilated furnace, and expressing the mass of the residue remaining as a percentage of the original coal. The heating rate is normally slow at first, to prevent fixation of sulphur or other compounds in the ash. In British and Australian practice, the sample is heated slowly from ambient temperatures to 815 C, where it is held until constant mass is attained. A two-stage heating cycle, raising the temperature first to 500 C, holding at that level for a specified period and then heating at 815 C in a second furnace may also be used.American procedures depend on the nature of the minerals present. In normal circumstances, the coal is heated slowly to between 700 and 750 C and held at that temperature until constant mass is developed. For coals that have a high proportion of calcite and pyrite, a two-stage process is required to prevent fixation of sulphur in the ash. The coal is heated to 500 C over 1 h and 750 C over 2 h, and held at 750 C until its mass becomes constant.2.3.5 Fixed carbonThe fixed carbon content of a coal is the carbon found in the material that remains after the volatile matter has been expelled. It represents the decomposition residue of the coals organic components, and carries with it small amounts of nitrogen, sulphur, hydrogen and possibly oxygen as absorbed or chemically combined material (Rees 1966).The fixed carbon content is used as an index of the yield of coke expected from a coal on carbonization, or as a measure of the solid combustible material that remains in coal-burning equipment after the volatile fraction has been liberated. If corrections are applied for ash or mineral matter, it may be used as an index of coal rank and a parameter in coal classification.Fixed carbon is not determined directly, but is simply the difference, in an air-dried coal, between the sum of the other components (moisture, volatile matter, ash) and 100%. Appropriate corrections are made if the analysis is to be reported to some other basis (Section 2.10.1).2.4 ULTIMATE ANALYSISThe organic component of a coal consists essentially of chemical compounds made up from carbon, hydrogen, nitrogen, oxygen and sulphur, and ultimate analysis involves the determination of the percentage of each of these in the sample. With the exception of nitrogen, these elements are also found in many of the mineral species which occur in coals, such as carbonates, sulphides, sulphates and hydrous clay minerals. Hydrogen and oxygen also make up the total or inherent moisture present in the coal being analysed.Carbon, hydrogen and oxygen are of great commercial significance in assessing the coking, gasification and liquefaction properties of the coal, while nitrogen and sulphur represent possible sources of pollution in a coal to be used for these or for combustion applications. The percentages of carbon, hydrogen and oxygen in the organic fraction of thecoal can also be used as indices of rank and as parameters in coal classification.2.4.1 Carbon and hydrogenCarbon and hydrogen occur mainly as complex hydrocarbon compounds. They are liberated as C02 and H20 when the coal is burned, and as a result are most readily determined together. However, the combustion process also liberates C02 from any carbonate minerals that may be present, and additional H20 is derived from water in clays or other hydrous minerals. Hydrogen is also found in the inherent moisture of the air-dried coal, and thus any measure of the total carbon and hydrogen includes a certain amount of material derived from inorganic sources. Corrections and separate analyses can be made to allow for the inorganic material.Most techniques for the determination of carbon and hydrogen are based on heating the air-dried coal in a stream of dry oxygen, and collecting the C02 and H20 produced in a series of absorption tubes. However, oxides of nitrogen and sulphur, as well as possibly small amounts of chlorine are also produced by the combustion process, and provision must be made in the apparatus to ensure they do not interfere with the selective absorption system.Two methods of determination are covered by standard procedures in Britain and Australia. These are the Liebig method {Fig. 2.5), where the coal is heated to 800 C, and the high temperature method, where it is heated to 1350 C. The high temperature method requires fewer additional processes to allow for sulphur, nitrogen etc. and is favoured by many commercial laboratories. American (A.S.T.M.) standards provide only for the Liebig method to be used.2.4.2 NitrogenThe nitrogen found in coals appears to be mainly confined to the organic compounds present. The nature of these compounds, and their origin, isdiscussed by Francis (1961). No nitrogen-bearing minerals are known in coals, but some nitrogen compounds may occur in pore waters, especially in brown coals.In the carbonization process used to manufacture coke, some of the nitrogen reacts to form ammonium compounds. These may be extracted as a by-product for use as fertilizer, or in the manufacture of nitric acid. Similar ammoniacal compounds are also formed in liquefaction or gasification operations, using hydrogen that would otherwise form the more valuable hydrocarbon products. On combustion of the coal, the nitrogen helps to form oxides which may become atmospheric pollutants when the flue gases are released, and as a result the use of coals with a low nitrogen content is often preferred.The most commonly-used technique for nitrogen determination is the Kjeldahl method, which converts the nitrogen in the coal to ammonium sulphate by catalytic digestion in sulphuric acid. The amount of ammonium sulphate formed is then determined by a series of titration processes.2.4.3 SulphurSulphur may occur in coal in a number of ways:(a) Organic sulphur, where it is incorporated into the hydrocarbon compounds of the coal substance.(b) As sulphide minerals, such as pyrite, in the inorganic fraction (pyritic sulphur).(c) As sulphate minerals, mostly hydrous iron or calcium sulphates, usually produced by atmospheric oxidation of these sulphides.

DryoxygenFurnace no.C02 and H20 : to absorption tubesCopper CpalCopperLeadSilvergauze sampleoxidechromategauzel1-1I1 (80CCC)2 (800C)3 (600C)StartFinishFig. 2.5 Schematic diagram showing combustion tube for determination of carbon and hydrogen by the Liebig method. The copper oxide ensures complete conversion of carbon monoxide to carbon dioxide, the yield of which is measured in the absorption tubes. Oxides of sulphur are retained by the lead chromate, and chlorine by the silver gauze, preventing interference with the absorption process. The tube is 1.22 m long. (After BS 1016, part 6, 1977.)In ultimate analysis, the total sulphur content is determined, representing material occurring in all these possible forms. For many purposes, however, the relative amount of sulphur in each form is also needed, and this can be determined by a separate analytical procedure (Section 2.5.1). Even if the complete ultimate analysis is not performed, total sulphur determination is nearly always carried out, along with proximate analysis, on a coal being evaluated for commercial purposes.

Coals with a high sulphur content present numerous problems on utilization. In combustion applications, the sulphur gives rise to corrosion problems in the boiler, and to build-up of insulating deposits (fouling) on the boiler tubes. It also causes problems with atmospheric pollution as large amounts of S02 are released with the stack gases. Installation of limestone-based stack gas scrubbers may be necessary for the plant to conform to environmental regulations.Part of the sulphur is carried through to the products of liquefaction, gasification and coking operations. It must be removed at some stage of the liquefaction or gasification process, or similar atmospheric pollution and corrosion problems will arise when these products are used. An unacceptably high level of sulphur may also be passed on to the iron and steel produced using coke from high-sulphur coals, resulting in material which is brittle or difficult to weld. The sulphur may be removed in the blast furnace (at additional cost) by producing additional volumes of slag, or by oxidation in the subsequent steel-making process.The total sulphur content of a coal may be determined by one of several methods that convert it to sulphate for wet chemical analysis. One of these, the Eschka method, involves oxidation of the coal at 800 C in magnesium oxide and sodium carbonate, followed by addition of barium chloride to form insoluble barium sulphate. The mass of barium sulphate formed gives the total sulphur content of the coal. Another is the high temperature method where the coal is burned in oxygen at 1350 C, converting all the sulphur present to S02. The S02 is then converted to sulphuric acid for titrimetric determination (Fig. 2.6).Other methods that may also be used, although giving a lesser degree of precision in the determination are:(a) Analysis of washings from the bomb calorimeter, used in specific energy determinations (Section 2.7.1).

(b) X-ray fluorescence or similar analysis, under appropriate conditions of the whole coal sample.The analysis of washings from the bomb calorimeter is included as an alternative to the Eschka and high temperature methods in American (A.S.T.M.) standard procedures.2.4.4 OxygenOxygen is a component of many of the organic compounds in a coal. It is also found in the moisture, and in many of the mineral species, such as clays and carbonates. If the coal is not fresh, it may be found in iron oxides, hydroxides and various sulphate minerals, as well as oxidized organic compounds.When corrected for the component found in moisture and mineral matter, the oxygen in a coal is important as an indicator of rank. It is one of the main parameters used in some coal classifications (Section 2.10.2). Oxygen is also of great significance in gasification and liquefaction operations, as it takes up much of the hydrogen otherwise used to produce hydrocarbon compounds.The oxygen content of a coal is traditionally determined by subtracting the amount of other chemical components (C, H, N and S) from 100%. Techniques involving direct determination are also available, which avoid the effect of cumulative error in the analysis by difference. These methods involve heating the coal in an inert atmosphere at temperatures of at least 900 C (Rees 1966) or up to nearly 2000 C (Kinson & Belcher 1975), liberating the oxygen as carbon monoxide. The monoxide is then converted catalytically to C02, the amount of which is determined in an absorption train to give the coals oxygen content.2.5 MISCELLANEOUS CHEMICAL ANALYSES

/Furnace (1350C)Fig. 2.6 Determination of total sulphur by the high temperature method. (After Br.tish Standard 1016, Part 6, 1977.)As well as proximate and ultimate analysis, a range of other chemical tests is often carried out on coal

samples. Some of these tests are used to enable correction of proximate and ultimate analysis data to allow for the mineral matter constituents, while others are used to evaluate the coals suitability for specific purposes. A comprehensive summary of analytical procedures that may be used for industrial or research applications is given by Karr (1978).2.5.1 Forms of sulphurAlthough the total sulphur content provides sufficient data for most commercial applications, a knowledge of the relative amounts present in each of the three principal forms is useful for the following purposes:(a) To assess the level to which the total sulphur content might be reduced by coal preparation processes. It is possible that a preparation plant will remove much of the pyritic and sulphate sulphur, but such a plant is unlikely to reduce the organic sulphur content.(b) To assess, by normative calculation, the amount of mineral matter in the coal. This is based on formulae using the ash content of the coal and several other chemical properties (Section 2.9.2).Determination of the amount of sulphate sulphur in the coal is based on solution of the sulphates in hydrochloric acid, and their precipitation as barium sulphate for gravimetric determination. The total iron content of the sulphate fraction is also determined for use in the analysis of pyritic sulphur.The amount of pyritic sulphur in the coal is determined by dissolving both sulphates and sulphides in nitric acid, and determining the total iron content of the solution. The amount of iron (and hence sulphur) in the sulphide fraction is then determined by subtraction of the amount of iron liberated in the analysis of the sulphate sulphur content.The organic sulphur content of the coal is determined by subtraction of the sulphur present in these two forms from the total sulphur content determined in ultimate analysis.2.5.2 Carbonate carbon dioxideCarbonate minerals, principally calcite, dolomite, ankerite and siderite, are a common constituent of many coals. These minerals liberate C02 on combustion, and therefore contribute to the total carbon content of the coal as determined by ultimate analysis. However, this process is an endothermic reaction, and reduces the amount of energy available from a coal otherwise apparently having a high carbon content. Knowledge of the amount of this carbon is necessary for the following purposes:(a) Determination of the amount of combustible carbon from the total carbon content. This is required to give a detailed assessment of the coal in all areas of potential usage.(b) Estimation of the percentage of mineral matter by calculation (Section 2.9.2). Knowledge of the carbonate C02 content assists in calculation of the mineral matter content from the amount of ash residue by normative methods, and also enables adjustment of the volatile matter content to assess that fraction derived solely from the coals organic component (Section 2.10.1).The carbonate C02 content is determined by solution of the coal in hydrochloric acid and measuring the amount of C02 produced. This may be done either by a gravimetric method using an absorption train, or a titrimetric method using benzylamine and potassium methoxide. A manometric method based on measuring the pressure increase in a closed vessel may also be used for high rank (hard) coals.2.5.3 ChlorineSmall amounts of chlorine are commonly present in coals, either as inorganic salts (e.g. NaCl, KC1) in the mineral fraction, or as a component of the organic matter. In combustion applications, this chlorine may cause corrosion and fouling of boiler equipment and also possibly contribute to atmospheric pollution. Knowledge of the chlorine content is necessary to apply corrections in other analyses, such as specific energy and total sulphur determinations. It is included in the amount of oxygen in ultimate analysis as determined by difference, and must be allowed for in assessing the percentage of mineral matter from the ash content in some normative calculations.Chlorine may be determined by the Eschka method, where the coal is oxidized at 675 C in magnesium oxide and sodium carbonate. The chlorine is converted to chlorides which are then analysed by titration.As an alternative, American procedures allow for the combustion of the coal, together with the MgO and Na2C03, in a bomb calorimeter. The chlorine is determined by analysis of the bomb washing solutions.More commonly, chlorine is analysed in conjunction with the determination of total sulphur by the high temperature method. The coal is burned in oxygen at 1350 C, and the liberated chlorine absorbed in hydrogen peroxide to form hydrochloric acid. This can then be analysed titrimetrically at the same time as the sulphur (sulphuric acid) formed by the combustion process.

2.5.4. PhosphorusSmall amounts of phosphorus are also found in some coal samples. Most of this appears to be concentrated in the mineral matter, either as apatite (Ca5 (P04)3 (F, OH), or a member of the goyazite group (SrAl3 (P04)2(0H)5). Large quantities of phosphorus are undesirable in coking coals, as it is carried over and contaminates the final steel product.Chemical analysis for phosphorus is based on oxidation of the coal in strong acids, or on acid solution of the coal ash followed by spec- trophotometric determination with ammonium molybdate. Phosporus contained in the acid-insoluble goyazite minerals could be overlooked, particularly by the first technique, and methods such as emission spectroscopy or X-ray fluorescence may be necessary in some cases as alternatives.2.5.5 Ash analysisCoal ash consists almost entirely of the decomposed residues of silicates, carbonates, sulphides and other minerals, and at least 99% of its chemical composition can usually be expressed in terms of the same metal oxides as that of other crustal rocks. The composition of the ash gives a useful guide to the types of minerals present in the coal. Such data are also of commerical significance in coking coals, as the ash of coke plays a part in the formation of blast furnace slag, and infuel coals to indicate boiler fouling or ash viscosity characteristics.The ash may be dissolved by various means, treated with appropriate reagents and the resulting solutions analysed using either an optical absorption spectrophotometer or a flame photometer (Fig. 2.7). Alternatively, techniques such as atomic absorption spectrophotometry, emission spectroscopy or X-ray fluorescence may be used for the analysis process.2.5.6 Trace elementsApart from the major organic and inorganic constituents, coals contain a variable assemblage of elements in trace amounts. Some of these are more abundant in the less dense fractions of the coal, separated by float-sink methods (Section 2.6.3), and appear to be incorporated mainly with the organic components. Others are concentrated in the dense, mineral-rich material, and appear to have a dominant inorganic affinity (Gluskoter et a! 1977; Ward 1980). They may represent either metallo-organic compounds in the coal structure, minor amounts of a mineral with an abundance of the particular element (e.g. zinc in sphalerite) or major amounts of a mineral in which the element itself occurs in trace proportions (e.g. manganese in ankerite).

S02Aro, Na20 K,0Fe20,TiO,Mn,0P,0,CaOMgOSO,Fig. 2.7 Flowsheet illustrating the analysis of coal ash using optical absorption spectrophotometer, flame photometer, titration and gravimetric techniques. (After British Standard 1016, Part 14, 1963.)In some areas, the trace elements may be useful as aids to seam correlation or as indicators of the coals depositional environment. Boron, in particular, appears

to be more abundant in coals influenced by marine conditions (Swaine 1962; Bohor & Gluskoter 1973).A number of the trace elements may be transferred to hydrocarbon liquids, coke and other products when the coal is used. They may act as catalysts or inhibitors in some of the complex processess of coal conversion.Trace elements may also be released to the environment, either by combustion of the coal or its products or by weathering of coal ash and colliery waste material. The elements involved include many that are essential to biosphere processess, as well as those that are toxic, in certain forms, to plants, animals and man. High concentrations of critical elements, such as lead, arsenic, cadmium and mercury, may limit the suitability of a coal for use in sensitive situations. In other cases, the trace elements may represent potentially valuable by-products from the materials

concerned.

AgAsBBaBeCdCoCrCuF.GaGeHgLaMnMoNiPPbScSeSnSrThUVWZnZrConcentration (p.p.m. air-dried coal) U.S.A.AustraliaIllinois Basin Range Mean*Appalachians Range Mean*Western coals Range Mean*N.S.W. and Qld Range Mean*0.02-0.080.030.01-0.060.020.01-0.070.033-6.5

IB>6.5-10

2>10-14

3A>14-16

3B>16-20

4>20-28

5>28-33

6. >33-41J .>32.45

' 7> 33-44J32.45-30.15

8>35-50f30.15-25.55

9> 42-50J25.55-23.87

Free swelling index

Value(crucible swelling no.)Roga index

2nd digit00-V20-5

(coal group)11-2>5-20

22V2-4>20-45

4>4>45

Dilatometer .

ValueGray-King coke typemax. dilation (%)

3rd digit0Anon-softening

(coal1B-Dcontraction only

sub-group)2E-G0-50

4G5-G8>50-140

5>g8. > 140

* at 30 C; 96% relative humidity, t For information only.

Table 2.11 International classification of sott coals (i.e. coals with a gross specific energy below 23.87 MJ kg * (After Williamson 1967).ValueTotal moisture content of freshly mined coal (ash free)(%)

1st and 2nd digits1033.82

733-44*32.02-33.82

835-50*28.43-32.02

942-50*27.08-28.42

ValueCrucible swelling no.

2nd digit0O-V2

(coal group)11-2

22V2-4

34V2-6

46V2-9

(continued on p. 2)

Table 2.12 Australian standard classification on hard coal. (From Australian Standard 2096 1977). (continued from p.71)ValueGray-King coke type

3rd digit0A

(coal sub-group)1B-D

2E-G

3GpG,

4g5-g8

5G,"

ValueAsh (dry basis) (%)

4th digit(0)^4.0

(ash number)(1)4.1-8.0

(2)8.1-12.0

(3)12.1-16.0

(4)16.1-20.0

(5)20.1-24.0 ,

(6)24.1-28.0

(7)28.1-32.0

(8)>32.0

* Values for information only.

third digit on the Gray-King coke type as illustrated in Table 2.13.Coals with less than 19.6% volatile matter (d.m.m.f.) are classified by this parameter alone (main classes 100 and 200), and the Gray-King values shown in the table are for information only. If the coal has high ash content (greater than 10%) it must be cleaned by a float-sink or similar process prior to analysis, giving the maximum yield of coal with an ash content of 10% or less. Coals that have been affected by igneous intrusions (heat-altered coals) are further designated by the suffix H after their numerical classification (e.g. 102H; 301aH), while those that are oxidized or weathered are designated by adding the letter W (e.g. 701W).Some examples of these different coal classification schemes, using a range of Australian bituminous coals, are given in Table 2.14.

Table 2.13 Coal classification system used by the National Coa Board. (After Williamson 1967.)Coal rank code Main class(es)ClassSub-classVolatile matter (d.m.m.f.)(%)Gray-King coke typeGeneral description .

100Under 9.1

101