Basic Factors Controlling Coal Quality and Technological Behaviorstoa.usp.br/.../20408/factors+controlling+coal+quality.pdf · 2012-10-16 · Liptinite maceral group, petrographic

CHAPTER 2

Basic Factors ControllingCoal Quality andTechnological Behaviorof Coal

Isabel Suarez-RuizColin R. Ward

2.1 Introduction

Conventionally, coal is used in processes such as combustion, gasifi-cation, and liquefaction and in carbonization for the manufacture ofmetallurgical coke. Coal and its derivative products are also used asprecursors of other materials and in the production of chemicals.Thus, a coal must be characterized before it is used, whether as asingle or blended coal. Characterization is performed in order to findout the properties of a coal, to determine its quality, and to predictits technological behavior. Basically there are two characteristics thatinfluence the use of coal: its composition and its rank. Coal composi-tion is in turn represented by two essentially independent factors (Ward,1984): type (nature of the organic components) and grade (extent ofdilution by mineral matter).

2.2 Coal Composition: Organic Components

Coal is a heterogeneous material, and evaluation of coal type may beapproached on two different levels: the macroscopical and microscop-ical, both of which form a part of coal petrology. Macroscopically,coals can be classified into two broad categories based on coal type:(1) humic coals or banded coals, which are the more common innature and are derived from a heterogeneous mixture of a wide rangeof plant debris, and (2) sapropelic, nonbanded, or massive coals (ICCP

Applied Coal Petrology

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20 Applied Coal Petrology

1963), which are homogeneous in appearance and require specialconditions for accumulation and preservation of the original organicmatter (Stach et al., 1982).

Lithotypes are the macroscopically recognizable bands in humiccoals, and four lithotypes—vitrain, clarain, durain, and fusain—havebeen described by the ICCP (1963). These lithotypes are the result ofplant growth and the physicochemical conditions existing in the peatswamps in which the organic remains accumulated. The minimumthickness of bands described as lithotypes has been established between3 mm and 10 mm. Lithotypes can be distinguished from one another,particularly in high volatile bituminous coals, on the basis of their phys-ical properties, such as luster, fracture pattern, color, and streak (ICCP,1963; Stach et al., 1982; and Taylor et al., 1998). Stach et al. (1982)extended the definition of lithotypes to include cannel and bogheadcoals, both of which have been identified in sapropelic coals.

2.2.1 Organic Petrography: Macerals and Microlithotypes

Macerals

Microscopically coal is composed of various constituents (macerals),which occur together in different associations (microlithotypes). Min-eral matter is also present in different proportions. Thus, maceralsare the coalified remains of various plant tissues or plant-derived sub-stances existing at the time of peat formation. Due to variable andoften severe alteration during the peatification and coalification pro-cesses, it is not always possible to recognize the plant material fromwhichmanymacerals were originally derived (ICCP, 1971). The forma-tion of macerals from plant remains during the early stages of peataccumulation depends on the type of plant community, climatic andecological controls, and conditions of the depositional environment(Stach et al., 1982). When the processes of biochemical degradationcease and the organic material is buried at great depths in the sedimen-tary environment, geochemical coalification over a long period of timeand under conditions of high temperature and pressure takes over. Asa result, the sediment of the original peat swamp is transformedand passes through the progressive evolutionary stages of lignite, sub-bituminous, and bituminous coal to anthracite and meta-anthracite.Throughout these stages the physicochemical characteristics of thecoal as well as its technological properties are modified (Stach et al.,1982, and Taylor et al., 1998).

In polished sections under the microscope using incident light,macerals are identified on the basis of their optical properties. Universalacceptance is given to the ICCP classification and redefinition (ICCP,1963, 1971, 1975, 1998, 2001; Sykorova et al., 2005) of macerals intothree groups: liptinite, inertinite, and huminite/vitrinite (Figure 2.1).

(a) (b)

SfTl

Sp Dt

Sf

(c) (d)

FF

SpDt

(e) (f)

RMi

T

MiTl

(g) (h)

Ct VV

Dt

(Figure legend continues on next page)

Basic Factors Controlling Coal Quality and Technological Behavior 21

FIGURE 2.1. Photomicrographs of coal macerals in bituminous coals (mediumcoal rank) taken in reflected white light andwith a 32� oil immersion objective(long side of the pictures: 200 mm). (a) Coalwith a vitrinite random reflectance of0.97%. Liptinite (Sp: sporinite), vitrinite (Dt: detrovitrinite), and inertinite(Sf: semifusinite). (b) Coal with a vitrinite random reflectance of 0.90%. Inerti-nite (Sf: semifusinite) and vitrinite (Tl: telovitrinite). (c) Coal with a vitriniterandom reflectance of 0.63%. Inertinite (F: fusinite with broken cell walls).(d) Coal with a vitrinite random reflectance of 0.63%. Inertinite (F: fusinite), lip-tinite (Sp: sporinite), and vitrinite (Dt: detrovitrinite). (e) Coal with a vitriniterandom reflectance of 0.70%. Vitrinite (T: telinite), liptinite (R: resinite in cellcavities), and inertinite (Mi: micrinite generated from hydrogenate maceralssuch as the sporinite). (f) Coal with a vitrinite random reflectance of 0.97%.Vitrinite (Tl: telovitrinite) and inertinite (Mi:micrinite). (g) Coalwith a vitriniterandom reflectance of 0.65%. Liptinite (Ct: cutinite) and vitrinite (Dt: detrovi-trinite). (h) Blend of coals with vitrinite (V) random reflectances of 1.40 and0.88%, used in carbonization processes. (Photomicrographs: I. Suarez-Ruiz.)


These maceral groups are subdivided into a variety of macerals, sub-macerals, and maceral varieties on the basis of their reflectance, degreeof destruction/preservation of original material, presence of cellularstructure, gelification, and morphological features. The three maceralgroups differ in both chemical composition and optical properties, andtheir names conventionally end in –inite.

Macerals of the liptinite group (Table 2.1 and Figure 2.1a, d, e, g)include all the chemically distinct parts of plants such as spores, cuti-cles, suberine cell walls, resins and polymerized waxes, fats and oils ofvegetable origin, some degradation products, and products of second-ary generation during the coalification process (coal evolution). Thesemacerals have the highest hydrogen content and contain compoundsof mainly an aliphatic nature. Their color in reflected light is darkand so their reflectance is the lowest among the maceral groups. Mostof the macerals of this group display a fluorescence of variable inten-sity when excited with short wavelength radiation, although thisproperty disappears with increasing coal rank. During coalificationmost macerals of this group disappear due to thermal transformation,or they develop similar optical properties (reflectance) to those of thevitrinite group at the medium volatile bituminous coal rank stage.

The influence of this maceral group in the technological proper-ties of coal is related to the proportion in which it occurs. Because oftheir high hydrogen content, liptinite macerals yield high proportionsof tars and gases during the carbonization process. Members of the lip-tinite group also have a high calorific value. The sensitivity to oxida-tion of liptinite macerals is low, and the hydrogenation capacity of

TABLE 2.1Liptinite maceral group, petrographic characteristics (Data compiled from ICCP, 1971, 1975.)

Maceral Origin Petrographic Characteristics

Sporinite From pollen and spores Individual bodies, usually compressed, well preserved, distinct botanicalform, high relief, variable wall thickness and size. Most frequent maceralof this group in coals.

Cutinite From cuticles of leaves Elongated bodies, serrated edges, well preserved, high relief.Resinite Diverse origins: resins, waxes Individual ovoid, globular and irregular bodies, cell fillings, impregnations

on vitrinite, relief þ / nul, red internal reflections, different propertiesaccording to its nature.

Alginite Algal or bacterial Rare in humic coals, main component in sapropelic coals.Telalginite: Individual bodies (discs) or colonies, rounded, elongated,semicompressed morphologies, internal structure, intense fluorescence.

Lamalginite: lamelae, thickness <5 microns and variable length, nointernal structure, lower fluorescence intensity than telalginite.

Suberinite From suberous tissues Rare, with cell structure, anisotropic, intense fluorescence. Only found inlignites.

Chlorophyllinite From chlorophyllic pigments Rare, variable morphology, intense red fluorescence, only known in lignitestage preservation under strong anaerobic conditions.

Liptodetrinite Fragments from the otherliptinite macerals

Small detrital fragments, diverse properties.

Fluorinite From vegetable oils Intensefluorescence(yellow),brilliantcolor, fluorescencealteration:weak,evennegative, in lenses orwithout definite shape, black in normalwhite light.

Bituminite Degradation product from algae,bacterial, zooplankton

Amorphous, ground-mass for other macerals, orange-brown fluorescence,strong positive alteration.

Exudatinite From hydrogenated substancesand liptinite macerals

Secondary maceral, filling voids and cell cavities, strong fluorescenceintensities of varying colors, variable fluorescence alteration, black innormal white light.

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liptinite is excellent. In wider fields of geology some liptinites, such assporinite (Figure 2.1a, d), have been used in coal seam correlation andas facies indicators. Other liptinite macerals (chlorophyllinite) havebeen used to assess the anaerobic conditions and the weather existingat the time of peat formation.

The inertinite maceral group (Table 2.2 and Figure 2.1a–d, f) isderived from plant material that was strongly altered and degradedunder oxidizing conditions before deposition, or by redox, biochemi-cal, and chemical processes at the peat stage. One maceral of thisgroup (micrinite, Figure 2.1e, f) may also be generated by transfor-mation of more hydrogenated macerals (Figure 2.1e) during thecoalification process (ICCP, 2001). Inertinite macerals exhibit a highdegree of aromatization and condensation and are made up of struc-tures that are mainly of an aromatic character with a high level ofcross-linking. They have the highest carbon and lowest oxygen andhydrogen contents of the maceral groups (van Krevelen, 1993). Thecomponents of this group are more inert (less reactive in carboniza-tion) than the macerals of the other groups. Their color in reflectedwhite light is grey or greyish white to yellowish. Their reflectance ishigher than that of the other groups, but this also depends on thechemical composition of the various inertinite macerals (ICCP,2001). When blue-violet to green light excitation is used, low-reflectinginertinite macerals are fluorescent (Diessel, 1985). Subdivision of theinertinite macerals (ICCP, 2001) depends on the presence or absence ofvegetable structures or whether they represent fragmentary material.

High proportions of highly reflecting inertinite macerals such asfusinite (friable, Figure 2.1c, d) may lead to the formation of dustduringmining (ICCP, 2001). In coal-cleaning processes the organic frac-tion of the high-densitywashery fractionsmay be enriched in inertinite,mainly because of the frequent intergrowth of these macerals withminerals.Depending on the coal rank and the types of inertinite present,different chars are generated during combustion, some of which arehighly reactive (Thomas et al., 1993, and Borrego et al., 1997). In cokingprocesses the reactivity of the inertinite depends on the physicochemi-cal characteristics, grain size and heterogeneity of the individual mac-erals, the coal rank, and so on (ICCP, 2001). The inertinite maceralswith lower reflectance and stronger fluorescence tend to have higherreactivity and to be fusible during carbonization and coking. An opti-mum inertinite contentwith the appropriate grain size generates a cokeof maximum strength and stability. Some of the inertinite (finely dis-persed fusinite) also improves the coke strength.

Macerals of the huminite/vitrinite groups (see Tables 2.3 and 2.4)originated mainly from lignin and cellulose and partly from tanninsand colloidal humic gels. Proteins and lipid substances may alsohave played a part in the formation of the macerals of this group

TABLE 2.2Inertinite maceral group, petrographic characteristics (Data compiled from ICCP, 2001.)

Maceral Origin Petrographic Characteristics

Fusinite From ligno-cellulosic cell walls Well-preserved cell walls, open cell lumina, bogen or starstructure, high reflectance, white to yellowish color

Semifusinite From parenchymatous and xylem tissues ofstems, herbaceous plants and leaves

Well to semi-preserved cell walls, smaller cell lumina oftenclosed, color and reflectance between those of vitrinite/fusinite. Deformed cell cavities. Anisotropic

Funginite From fungal spores and tissues, sclerotia,mycelia

Ovoid bodies of fungal remains, sclerotia, hyphae, mycelia,with cell structure high reflectance

Secretinite No totally clear, oxidation product of resins,humic gels

Round, ovoid, polygonal, vesicular bodies, without cellularstructure, at times vesiculated, high reflectance

Macrinite From alteration of humic substances, metabolicproduct of fungi and bacteria, from coprolites,etc.

Amorphous bodies of irregular shape, structureless roundedfragments (>10 microns), compact appearance, highreflectance

Micrinite Coalification product, residues of former lipoidsubstances, strong fragmentation of otherinertinites

Very fine-grained material. Fine particles of small size(2 microns), high reflectance

Inertodetrinite From phytogenetic material subjected tofusinization

Without structure, fragments of size <10 microns

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TABLE 2.3Huminite maceral group (low rank coals), petrographic characteristics (Data compiled from Sykorova et al., 2005.)

MaceralSubgroup Origin Maceral Petrographic Characteristics

Telohuminite Cell walls of parenchymatous and woodytissues composed of cellulose and lignin

Textinite Primary cell wall structure still distinguishable,cell lumina open, isotropic, variablefluorescence

Precursor of telovitrinite Ulminite High degree of humification; cell wallstructure still visible, or not visible, celllumina closed, variable fluorescenceintensity decreasing with increasing coalrank

Detrohuminite From strong decay of parenchymatous andwoody tissues of stem and leaves.

Attrinite Structural degradation product, particle size<10 microns, spongy texture, lowfluorescence intensity in the high wavelengths but it depends on its composition.

Precursor of detrovitrinite Densinite More packed than attrinite, more or lesshomogeneous huminitic groundmass, non-or very weak dark fluorescence.

Gelohuminite Diverse origins: intensely gelified planttissues and humic detritus, fromprecipitated humic colloids and fromprimary phlobaphenic cell fillings

Corpohuminite Globular to tabular morphologies, withoutstructure, compact or cavernous variablesize, without fluorescence. Two maceraltypes are distinguished: phlobaphinite andpseudo-phlobaphinite.

Precursor of most of gelovitrinite Gelinite Homogeneous structureless or porous, it canfill cavities of other macerals, withoutfluorescence. Two maceral types aredistinguished: levigelinite and porigelinite.

26

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TABLE 2.4Vitrinite maceral group (medium and high rank coals), petrographic characteristics (Data compiled from ICCP, 1998.)

MaceralSubgroup Origin Maceral Petrographic Characteristics

Telovitrinite From parenchymatous and woodytissues composed of cellulose andlignin

Telinite Well-preserved cell walls, size, shape and opennessof cell lumens are variable, empty or filled withother macerals or minerals. Similar fluorescence tothe collotelinite in the same coal.

Collotelinite Relatively homogenized, more or less structurelessappearance. Its reflectance value is an index of thecoal rank. It fluoresces over a wide rank range (highvolatile bituminous to semi-anthracite).

Detrovitrinite From strong decay of parenchymatousand woody tissues of stem and leavesoriginally composed of cellulose andlignin

Vitrodetrinite Small vitrinitic fragments (<10 microns), variableshape. Difficult to distinguish from other maceralswhen increasing rank. Variable fluorescence.

Collodetrinite Mottled and compact vitrinitic groundmass, bindsother coal components. Variable color andfluorescence intensity.

Gelovitrinite From jelling humic solutions and notcorresponding to specific planttissues. From contents of plant cellsor humic fluids.

Corpogelinite Structureless bodies, homogeneous, variable shape,humic cell fillings in situ or isolated. Higherreflectance than the other vitrinitic macerals.Weak fluorescence or no fluorescence.

Gelinite Pure colloidal gel, homogeneous aspect, withoutstructure, filling cracks and cavities, variable shapeand size, higher reflectance. Weak fluorescence ornot fluorescence. Least common maceral.

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(ICCP, 1971, 1998, and Sykorova et al., 2005). Their chemical struc-ture is represented by aromatic compounds and hydroaromatics inlower-rank coals, but with increasing coal rank the aromaticity, con-densation, and order of the polyaromatic units increase. The succes-sive sets of processes by which the vegetable tissues are transformedinto huminite and later into vitrinite are known as humification,gelification, and vitrinization (Stach et al., 1982, and Teichmuller,1989). Huminite macerals are identified in low rank coals (Sykorova,2005), and these are regarded as the precursors of vitrinite macerals(see Figure 2.1a –b, d–h) in highe r rank coals (ICCP, 1971, 1998, andSykorova et al., 2005). The members of the huminite/vitrinite groupare organized in each case into three subgroups (ICCP, 1971, 1975,1998, and Sykorova, 2005) and six macerals (see Tables 2.3 and 2.4).The chemical composition and most of the properties of the mac-erals of these groups are rank dependent. The color of huminite/vitri-nite macerals in polished section is medium grey and the maceralshave reflectances (see Figure 2.1a, d) generally between those of theassociated darker liptinites and the lighter inertinites over the rankinterval in which the three maceral groups are identified.

Huminite is generally isotropic, but anisotropy (bireflectance)occurs if remnants of cellulose are present. In the case of the vitrinitegroup, bireflectance normally increases with increasing coal rank (ICCP,1998). The color and intensity of fluorescence in huminite macerals arevariable, depending on rank, level of degradation, and humification(Sykorova et al., 2005). For vitrinite macerals the fluorescence dependson the maceral type, the coal rank, and the degree of bituminization(ICCP, 1998). Chemically the huminite and vitrinite groups have rela-tivelyhighoxygen contents comparedwith theother twomaceral groups.

The technological properties of the low rank coals in which thehuminite macerals are identified are linked to those of the predomi-nant huminite maceral, and it is the degree of humification and gelifi-cation of the huminite that influences most of the technologicalproperties of these coals in industrial processes (Sykorova et al.,2005, and references therein). In higher rank coals, where the vitrinitegroup is the major component, the properties of the vitrinite alsoinfluence those of the coal. Vitrinite in medium rank coals fuses incarbonization (coking) processes (see Chapter 7), and this property isalso important in combustion and hydrogenation (ICCP, 1998). Thetechnological properties of vitrinite macerals are in some cases relatedto their fluorescence properties, as has been demonstrated for col-lotelinite (Taylor et al., 1998). The reflectance measured in this mac-eral is universally used as an index of coal rank and as an indicatorof the maturation of dispersed organic matter. Depending on its com-position and rank, vitrinite may be oxidized during prolonged storageand shipping because of exposure to air and moisture, significantlyreducing some of its otherwise desirable properties.


Maceral Analysis

For geological research on coal basins and for an evaluation of coalseam quality it is important to know the quantitative compositionof a coal in terms of the macerals (and minerals in some cases) ormaceral groups. This is because differences in maceral compositionmay indicate differences in chemical composition and consequentlydifferences in the technological properties of a coal. Maceral analysisis standardized in the ISO 7404/3 (1994a) and ASTM D2799-05(2005a) norms and is used to determine on a volumetric basis the rel-ative proportions of the coal components in a representative coalsample. Coal pellets should be particulate and prepared for petro-graphic analysis according to ISO 7404/2 (1985), ASTM D2797-04(2004) or equivalent procedures. The petrographic microscope formaceral analysis should be equipped with incident white light, oilimmersion objectives (25�-60� magnification), and 8� to 12.5� ocu-lars, one of which must contain an adjustable eyepiece with amicrometer or cross-hair. Maceral analysis can be performed usinga manual or an automatic point counter coupled to the microscopestage. Although maceral analysis must be carried out in white light,supplementary observations in fluorescence mode are recommendedso that some components of the liptinite group will not be under-counted, especially in low rank coal analysis. To ensure the requiredprecision, a total of 500 points should be counted in a maceral anal-ysis. The results are reported on a volume percentage basis for eachcategory under consideration. The equations used to calculate theprobable margin of error and repeatability are described in ISO7404/3 (1994a).

Microlithotypes

Microlithotypes (Table 2.5) are the natural assemblages of maceralsat microscopic level. They are defined as having a minimum band-width (perpendicular to the stratification) of 50 mm and as containingat least 5% of a maceral group (ICCP, 1963, 1971, and Taylor et al.,1998). In addition to the maceral content, 20–60% (vol.) of silicate orcarbonate minerals or 5–20% (vol.) sulfide minerals redefines themicrolithotype as a carbominerite.

Microlithotypes may be mono-, bi-, or trimaceralic, and theirnames conventionally end in –ite. The chemical properties of micro-lithotypes are very similar to those of the predominating macerals(Stach et al., 1982, and Falcon and Snyman, 1986). Their physical prop-erties, however, are related not only to those of the macerals butalso to the combined effect of the association. The microhardness ofbi- and trimaceralic microlithotypes is always higher than that ofmonomaceralic associations. The density of the microlithotypes var-ies with rank, maceral composition, and size, as well as the form

TABLE 2.5Microlithotypes of hard coals (Data from Taylor et al., 1998.)

Microlithotype Maceral Composition Group

Vitrite vitrinite (V) >95%Liptite liptinite (L) >95% MonomaceralicInertite inertinite (I) >95%Clarite V þ L >95%Durite I þ L > 95% BimaceralicVitrinertite V þ I > 95%Duroclarite V > L, I (each >5%)Vitrinertoliptite L > V, I (each >5%) TrimaceralicClarodurite I > V, L (each >5%)Carbargilite Coal þ 20-60% (vol.) claysCarbopyrite Coal þ 5-20% (vol.) sulfidesCarbankerite Coal þ 20-60% (vol.) carbonates CarbomineriteCarbosilicate Coal þ 20-60% (vol.) quartzCarbopolyminerite Coal þ 20*-60% (vol.) various minerals

*5% if high pyrite

Source: Organic Petrology, G. H. Taylor, M. Teichmuller, A. Davis, C. F. K. Diessel,R. Littke, and P. Robert, 704 pp., copyright 1998, with permission from Gebruder-Borntraeger (www.borntraeger-cramer.de).


and quantity of associated minerals (Stach et al., 1982). The degree ofheterogeneity in a microlithotype is also important in its technologi-cal behavior, particularly in carbonization (see Chapter 7), combustion(see Chapter 4), and gasification (see Chapter 5) processes. The mac-eral, mineral, and microlithotype composition of a coal seam maychange over short distances both vertically and laterally, in responseto the conditions existing during the formation of the original peatswamps (Stach et al., 1982). These changes can be quantified by petro-graphic assessment of the microlithotypes in relevant coal samples.

Microlithotype Analysis

Microlithotype analysis is used to determine the relative proportionsof the various microlithotypes and coal-mineral associations (carbo-minerites) present in a coal sample (ICCP, 1963). The procedure isstandardized as indicated in the ISO 7404/4 (1988) norm. Althoughmicrolithotype analysis is carried out in a similar manner to maceralanalysis, a suitable 20-point reticule must be placed in one of theoculars of the microscope as a substitute for the micrometer orcross-hairs. Two conventions (ICCP, 1963) must be observed: (1) theminimum bandwidth of the association to be measured must be 50microns, and (2) macerals present in the association in amounts

http://www.borntraeger-cramer.de


smaller than 5% should be disregarded (the 5% rule). Each observationon a 20-intersection reticule is regarded as one point in the analysis,and each intersection on the reticule represents 5% of the total num-ber of intersections (20), providing guidance in use of the 5% rule. Fora complete microlithotype analysis, at least 500 points should bemeasured, and the results should be expressed as volume percentages.Microlithotype analysis is less accurate than maceral analysis. Thecalculation of repeatability and reproducibility (ISO 7404/4, 1988) ismade in the same way as for maceral analysis.

2.2.2 Elemental Composition of Coal Macerals

Chemical analysis of coal provides data gathered from “whole-coal”materials, embracing moisture and mineral matter as well as theorganic constituents. The data from ultimate analysis (C, H, O, N,and S percentages) may be corrected to a moist, mineral matter-free(mmmf); dry, mineral matter-free (dmmf); or dry, ash-free (daf) basisto assess composition of the organic matter alone, but even so thecomposition of the organic matter determined in this way inherentlyrepresents an aggregation of the composition of the different maceralcomponents. Variations in chemical composition indicated by ulti-mate analysis data derived from whole-coal samples therefore reflectvariations in the coal type (i.e., the mixture of macerals present) aswell as the rank of the coals concerned.

As indicated previously coal is a heterogeneous solid, and it isthe individual macerals within the coal that react, both independentlyand with each other, when the coal is used. In addition to providingfurther insights into the coalification process, knowledge of maceralchemistry may therefore be of value to understand the processesassociated with factors such as burning rate, emission release, CO2

generation, fouling and slagging, as well as reactions during gasifica-tion and coking associated with the different coal components.

Although some success has been achieved in maceral separationthrough density gradient centrifugation (see below), it is inherentlydifficult to cleanly isolate the individual macerals in a coal for sepa-rate chemical analysis without contamination by minerals or otherorganic components. The development of special techniques forlight-element analysis using the electron microprobe (e.g., Bustinet al., 1993, 1996, and Mastalerz and Gurba, 2001) provides a mecha-nism for directly determining the elemental composition of the indi-vidual macerals in coal-polished sections by analyzing selected areasonly a few micrometers in size, without the need for a prior maceralseparation process. Electron microprobe techniques have been usedto evaluate the elemental composition of the individual macerals ina number of North American and Australian coals (Mastalerz and


Bustin, 1993, 1997; Ward and Gurba, 1998; Gurba and Ward, 2000; andWard et al., 2005, 2007), helping to explain more fully the variationsin coal composition indicated by whole-coal analysis as well as someof the geological processes associated with coal formation.

Details of the procedures used for microprobe analysis of coalmacerals are given by Bustin et al. (1993, 1996), Mastalerz and Gurba(2001), and Ward et al. (2005). Samples for electron microprobe studyare prepared as polished sections in the same way as for opticalmicroscopy, although specific-sized mounts may be needed to suitthe microprobe’s sample handling facilities. The sections, and alsothe relevant calibration standards, are coated with a thin film ofcarbon, to provide a conductive surface (Bustin et al., 1993) prior tothe analysis process. An accelerating voltage of 10 kV is used for theelectron beam in most studies, with a filament current 20 nA. Anoverall magnification of 20,000x gives a spot size for the electronbeam of around 5 to 10 mm in diameter on the sample for the actualmeasurement process.

As discussed by Bustin et al. (1993), an independently analyzedanthracite sample provides a more effective calibration standard thangraphite for carbon in microprobe analysis of coal macerals. Separatelyanalyzed mineral samples are generally used as standards for otherelements (Ward et al., 2005), such as O, N, S, Ca, Al, Si, and Fe. Careshould be taken to avoid analyzing areas of the coal where visibleminerals are also present. Points that include mineral contaminantsmay, for example, be indicated by high Si or unexpectedly high Feand S percentages. Points that include some of the mounting epoxyresin may be indicated by unusual oxygen and high nitrogen contents.

Figure 2.2 indicates the changes in elemental composition ofthe vitrinite and inertinite macerals in coals from the Bowen Basin inAustralia (Ward et al., 2005) with variation in rank over a range, asindicated by vitrinite reflectance, from subbituminous coal to semi-anthracite. For a given rank level the vitrinite in these coals, especiallythe collotelinite, has the lowest carbon and highest oxygen contents,while the inertinite, especially the inertodetrinite, has the highestcarbon and lowest oxygen contents. The difference in compositionbetween the vitrinites and the inertinites decreases steadily as the rank(vitrinite reflectance) increases; themaceral groups have quite differentcompositions in lower rank coals, but only very small differences at theupper end of the rank range.

Because it is applied directly to the organic material, electronmicroprobe analysis provides a mean of directly measuring theorganic sulphur content of coal macerals, a parameter that is onlydetermined indirectly for coals by conventional analysis techniques.As indicated in Figure 2.2 and also in other studies (e.g., Ward andGurba, 1998), the vitrinite macerals have significantly higher organic

Carbon

Organic Sulphur Organic Nitrogen

Oxygen100 30

20

10

0

90

80

70

60

1.00

0.75

0.50

0.25

0.00

0.0 0.5 1.0 1.5 2.0

Rv max (collotelinite) %

Car

bon

in m

acer

al %

Sul

phur

in m

acer

al -

%

Oxy

gen

in m

acer

al %

3.0

2.0

1.0

0.0

Nitr

ogen

in m

acer

al %

2.5 3.0 3.5 4.0

0.0 0.5 1.0 1.5 2.0


2.5 3.0 3.5 4.0 0.0 0.5 1.0 1.5 2.0


2.5 3.0 3.5 4.0

Collotelinite

Collodetrinite

Semifusinite

Fusinite

Inertodetrinite

Collotelinite

Collodetrinite

Semifusinite

Fusinite

Inertodetrinite

Collotelinite

Collodetrinite

Semifusinite

Fusinite

Inertodetrinite

Collotelinite

Collodetrinite

Semifusinite

Fusinite

Inertodetrinite

0.0 0.5 1.0 1.5 2.0


2.5 3.0 3.5 4.0

(b)(a)

(d)(c)

FIGURE 2.2. Plots showing percentages of (a) carbon, (b) oxygen, (c) organic sulphur, and (d) organic nitrogen in differentmacerals ofAustralian (Bowen Basin) coals in relation to the coal rank as measured by vitrinite reflectance. (Source: International Journal ofCoal Geology 63, by C. R. Ward, Z. Li, and L. W. Gurba, “Variations in coal maceral chemistry with rank advance in the GermanCreek and Moranbah Coal Measures of the Bowen Basin, Australia, using electron microprobe techniques,” 117–129, copyright2005, with permission from Elsevier.)

Basic

Factors

Contro

llingCoalQuality


33


sulphur contents than the fusinite and inertodetrinite, with semifu-sinite typically having intermediate organic sulphur contents. Elec-tron microprobe studies typically show that the vitrinite maceralshave around twice the organic sulphur of the fusinite and inertode-trinite in the same coal samples. There is also evidence, especiallyin some high-sulphur coals, that the sulphur replaces oxygen in thevitrinite’s chemical structure (Ward et al., 2007).

Figure 2.2 also indicates that the nitrogen content of the vitrinitemacerals is generally higher than that of the inertinite components,especially fusinite and inertodetrinite, in the same coal samples. Thisis further supported by the work of Mastalerz and Gurba (2001). Forthe co als on which Figure 2.2 is based, however , the difference innitrogen content between these maceral groups appears to decreaseat high rank levels (semi-anthracite and above), possibly due toredistribution of the organic nitrogen into some of the clay minerals(Ward et al., 2005).

2.2.3 Organic Geochemistry

Two significant works on the organic geochemistry of coal pub-lished in the last 25 years are the papers by Given (1984) andHatcher and Clifford (1997). Because vitrinite is the dominant mac-eral in most coals, the present review is primarily confined, follow-ing the works of those authors, to the transformation of wood tovitrinite. Of the original components of wood, cellulose is preferen-tially lost, whereas lignin is retained. The preservation of plantstructures in brown coals indicates that at least some of the chemi-cal transformation is not accompanied by maceration of the woodstructure. The transformation of lignin to lignite involves processessuch as demethylation, dehydroxylation, and the cleavage of b-O-4aryl ethers (Figure 2.3). More significant structural alteration ofplant structures occurs at subbituminous rank, with some annealingof the plant cells. The chemical transformations involve the side-chain dehydroxylation and dehydroxylation of catechols. The latterinvolves condensation to hydroxylated diaryl ethers and the loss ofwater from the structure. With further coalification, the ether bondis cleaved, resulting in the formation of a catechol-like structureand a phenolic structure. The pathway to high volatile bituminouscoal involves the condensation of phenols to aryl ethers or dibenzo-furan-like structures. Overall, the aromaticity of vitrinite increaseswith increasing rank, implying the condensation of benzene-likestructures with aliphatic functional groups to polycyclic aromaticstructures.

Density gradient centrifugation (DGC), a tool for obtaining nar-row-density fractions from a coal (Dyrkacz et al., 1981), can be used

CH2

CH3

CH2

HC

CH2

CH

CH2

CH3

CH2

OH

HC

CH2

OH

CH

Reactions of lignin to formbrown coal and lignite

Reactions of lignite to form subbituminous coal

Reactions leading to bituminous coal

CH2OH

CH2OH

CH

CH2

OCH2

CH2OH

CH

OH

OH

OH

HC

HCOH

HCOH

OCH3

O

OCH3

OH

CH

HC

CH2OH

HC

HCOH

dehydroxylation

demethylation

side-chaindehydroxylation

dehydroxylationof catechols

CH2

CH2

CH3

CH2

HC

OH

OH

CH

alkylation

β-O-4 ether cleavage

HCOH

CH2

HCOH

HCOH

OH

OH

OH

OH

FIGURE 2.3. Transformation of lignin to lignite to subbituminous coal tobituminous coal. (Source: Organic Geochemistry 27, by P. G. Hatcher andD. J. Clifford, “The organic geochemistry of coal: from plant materials tocoal,” 251–274, copyright 1997, with permission from Elsevier.)


to study chemical variation between macerals. Various studies havebeen conducted, including an investigation of a number of Britishand U.S. bituminous coals using Curie-point pyrolysis mass spectros-copy (Meuzelaar et al., 1984); a study of a high volatile C bituminousPennsylvanian Indiana coal using Curie-point pyrolysis mass spec-troscopy and the latter technique coupled with gas chromatography(Nip et al., 1992); and a study of a medium volatile bituminousPermian Australian coal using electron spin resonance (ESR), cross-polarization 13C nuclear magnetic resonance (NMR), and single-pulse13C NMR excitation (Maroto-Valer et al., 1998). In general, all theseresults demonstrated that macerals increased in aromaticity fromliptinite to vitrinite to inertinite.

Hower et al. (1994a) examined both the organic and inorganicgeochemistry of a DGC sample taken from a 0.4%-ash high volatileA bituminous lithotype from a Pennsylvanian coal bed of Kentucky(U.S.). By means of Fourier-transform infrared (FTIR) spectroscopy,they demonstrated that the indicators of aliphatic bonds were stron-gest in the density concentrates dominated by liptinite macerals (noliptinite macerals were found in near-monomaceral concentrates,unlike the vitrinite and inertinite group macerals) and that the indica-tors of aromatic bonds were strongest in the >95% vitrinite concen-trates but not as strong in the inertinite-rich fractions (Figure 2.4).Coincident with an increase in aromaticity, the Blue Gem (Rimmeret al., 2006) and the Australian (Maroto-Valer et al., 1998) maceral con-centrates showed a decrease in atomic H/C with an increase in density(liptinite to vitrinite to inertinite for the Blue Gem coal; vitrinite tosemifusinite for the Australian coal).

Mastalerz and Bustin (1993, 1996), Walker and Mastalerz (2004),and Li et al. (2006) have used FTIR, sometimes in conjunction withother techniques (Mastalerz et al., 1998), to study macerals on a micro-scopic scale. They noted an increased aromatic character in the vitriniteand inertinites with an increase in rank and a shift toward greateraromaticity from liptinite to vitrinite to inertinite.

2.3 Coal Composition: Inorganic Components

2.3.1 Minerals and Mineral Matter

As discussed by Ward (2002), the material classed as “mineral matter”embraces all the minerals and other inorganic elements occurring incoal, including (1) dissolved salts and other inorganic substances inthe pore water of the coal, (2) inorganic elements incorporated with-in the organic compounds of the coal macerals, and (3) discrete inor-ganic particles (crystalline or noncrystalline) representing the actualmineral components.

FIGURE 2.4. Maceral concentration versus aliphaticC-H and aromaticC=C formaceral density concentrates of the BlueGemcoal bed, Kentucky.Note that theliptinite concentrate contains little more than 80% litpinite, with much ofthe remainder being vitrinite. The vitrinite and inertinite concentrates exceed95% purity. Mineral matter is negligible in most of the concentrates. (Source:Energy and Fuels 8, by J. C. Hower, D. N. Taulbee, S. M. Rimmer, and L. G.Morrell, “Petrographic and geochemical anatomy of lithotypes from the BlueGem coal bed, southeastern Kentucky,” 719–728, copyright 1994, with permis-sion from American Chemical Society [ACS].)


The first two forms, sometimes described as nonmineral inorgan-ics, are typically most abundant in the mineral matter of lower rankcoals (Kiss and King, 1977, 1979; Given and Spackman, 1978; Bensonand Holm, 1985; Miller and Given, 1986; Given and Miller, 1987a,b;and Ward, 1991, 1992). Although there are some exceptions (e.g., Wardet al., 2007), the nonmineral inorganics usually disappear from the coalwith an increase in rank. Discrete mineral particles, however, mayoccur in coal of any rank and are usually the dominant component of


the mineral matter in higher rank materials (Rao and Gluskoter, 1973;Ward, 1977, 1978; Renton, 1986; and Ward et al., 2001).

Coals produced from mines may also contain minerals derivedfrom intra-seam noncoal bands or admixed roof or floor strata. Thismaterial (extraneous mineral matter) may be at least partly removedby cleaning processes in coal preparation plants. Mineral matterclosely associated with the macerals (inherent mineral matter), how-ever—including both intimately admixed minerals and nonmineralinorganics in the maceral components—remains a part of the cleancoal product and must be taken into account when assessing thebehavior of a coal with handling, storage, and use.

Determination of Mineral Matter Content

Many of the minerals occurring in coal undergo major chemicalchanges at the high temperatures associated with combustion andash formation, including the loss of CO2 from carbonates, the loss ofstructural water from clay minerals, and the loss of sulfur from sul-fides (Rees, 1966; Raask, 1985a; Vassilev et al., 1995; and Reifensteinet al., 1999). The nonmineral inorganics in the macerals may alsoreact with some of the other coal components to form mineral arti-facts, such as sulfates, in the ash residue. Because of such changes,the percentage of ash determined in routine coal analysis is usuallyless than the percentage of mineral matter contained in the originalcoal sample. The chemical composition and crystal structure of theash may also be somewhat different from the chemical compositionand structure of the original mineral matter.

One of the most widely used methods for determining the per-centage of mineral matter (as opposed to ash) involves removingthe organic matter at low temperature (around 120�C) by exposing thecoal to a reactive oxygen plasma produced by a radio-frequency electro-magnetic field (Gluskoter, 1965; Frazer and Belcher, 1973; Miller, 1984;and Standards Australia, 2000). The residue remaining after oxidationof the organicmatter consists of the essentially unalteredmineral com-ponents of the original coal, together in some cases with additionalartifacts produced from the nonmineral inorganic components.

Exposing the coal to air at around 370�C (Brown et al., 1959, andWard et al., 2001) or treating the coal with hot concentrated hydrogenperoxide to oxidize the organic components (Nawalk and Friedel,1972, and Ward, 1974) may also serve to isolate a mineral residue,although these techniques may irreversibly alter some of the mineralcomponents. Calculations based on the ash percentage and ash compo-sition, combined with other chemical data, may also be used to providean estimate of the mineral matter content (e.g., King et al., 1936; Rees,1966; Given and Yarzab, 1978; Pollack, 1979; and Scholz, 1980).


Mineral Analysis in Coal and LTA Samples

The identity of the crystalline minerals in coal or LTA residues can beevaluated by X-ray diffraction techniques (Rekus and Haberkorn,1966; O’Gorman and Walker, 1971; Rao and Gluskoter, 1973; Ward,1977, 1978; Russell and Rimmer, 1979; Renton, 1986; and Harvey andRuch, 1986). Semiquantitative methods were used in many earlystudies, based on comparing key peak intensities with intensitiesassociated with known proportions of added-in crystalline spike com-ponents. More recent XRD analyses, however, are based on the full-profile analysis methods developed by Rietveld (1969). The Rietveldapproach allows a calculated XRD profile of a sample to be generatedfrom the structural parameters of each mineral present and to beadjusted iteratively by least-squares techniques to fit the observedXRD profile of the analysis sample (Taylor, 1991). Rietveld-based XRDtechniques have been applied to the analysis of the minerals in bothLTA and whole-coal samples (e.g., Mandile and Hutton, 1995; Wardand Taylor, 1996; Ward et al., 1999, 2001; and Ruan and Ward, 2002),with independent checks against ash analysis and other data confirmingthe consistency of the mineralogical evaluations.

French et al. (2001a) used a Rietveld-based technique to deter-mine the overall percentage of crystalline mineral matter in the coal,as well as the relative proportions of each mineral, by performinga direct XRD analysis of the whole-coal samples without a low-temperature ashing step to concentrate the mineral components.Structure models were developed separately for the organic matter ofcoals at different rank levels on the basis of XRD traces derived fromchemically demineralized coals, and these were incorporated intothe Rietveld analysis to allow the organic matter to be quantified asif it was another “mineral” phase.

The proportions of microscopically visible minerals in a coal sam-ple may also be determined at the same time as the percentages of thedifferentmaceral components bymeans of themicroscopic point-countanalysis (Davis, 1984, and Taylor et al., 1998). Some mineral occur-rences intimately associated with the macerals may, however, be inad-vertently overlooked by the point-counting process. The mineralpercentages determined by point counting are also volumetric per-centages, whereas the mineral matter evaluated by low-temperatureashing and similar methods is expressed as a mass percentage. Conver-sion of volumetric percentages to mass percentages for silicate materi-als involves approximately doubling the volumetric percentage values(Davis, 1984) and multiplying by even higher factors for sulfides andother dense mineral materials.

Mineral particles in coal can be investigated by using a scanningelectronmicroscope and similar techniques (e.g., Stanton andFinkelman,1979; Russell and Rimmer, 1979; Allen and Vander Sande, 1984;


Martinez-Tarazona et al., 1992; Hower et al., 1994a, 2000a; and Wardet al., 1996). The automatic collection of element data by means of com-puter-controlled scanning electron microscopy (CCSEM) techniques(Straszheim and Markuszewski, 1990; Galbreath et al., 1996; and Guptaet al., 1998, 1999a), including element associations, serves to evaluatemore fully the nature and distribution of minerals in coal samples.

More advanced CCSEM techniques, including the QEM*SEMand QEMScan systems (Gottlieb et al., 1992; Creelman et al., 1993;and Creelman and Ward, 1996), allow the integration of SEM datawith image analysis methods. In such operations the electron beampasses over relevant parts of a polished section, stopping at predeter-mined intervals to collect X-ray spectra. As each X-ray spectrum iscollected it is processed through a species identification protocol(SIP) and the mineral represented at that point is identified from itschemical characteristics. Individual mineralogical identifications arethus made at each point (or pixel) in the area scanned by the electronbeam, and these can be used to generate a digital map showing howthe various minerals occur within the coal sample. Data from suchsystems can be used to provide a variety of mineral matter informa-tion, including the relative abundance of the different minerals inthe coal; the particle size, shape, and mode of occurrence of particularminerals; the textural associations between minerals; and the degreeof liberation of minerals and mineral aggregates from organic matterassociated with coal-crushing and pulverization processes.

More precise determination of the composition of particularminerals may also be obtained from electron microprobe analysis ofpolished coal sections, usingmethods outlined by Reed (1996). Electronmicroprobe and similar techniques have been applied to minerals incoal by authors such as Minkin et al. (1979), Raymond and Gooley(1979), Kolker and Chou (1994), Patterson et al. (1994), and Zodrowand Cleal (1999). They have also been used to investigate the elementalcomposition of individual coal macerals (Bustin et al., 1993, 1996;Gurba andWard, 2000; andWard et al., 2005), including the occurrenceof nonmineral inorganic elements. The nature and relative abundanceof iron-bearing phases in coal may be evaluated byMossbauer spectros-copy (Gracia et al., 1999). A wide range of other methods have also beenused to identify theminerals present in coal samples, including thermalanalysis (Warne, 1964), Fourier-transform infrared (FTIR) spectrometry(Painter et al., 1978), and a range of other instrumental techniques(Ward, 2002; Huggins, 2002; and Vassilev and Tascon, 2002).

Minerals in Coal and LTA Residue

A list of the minerals that may be found in coals or LTA residues isgiven in Table 2.6. The most abundant of these are usually clayminerals, although quartz, pyrite, siderite, calcite, and dolomite or

TABLE 2.6Principal minerals found in coal and LTA (Data from Ward, 2002.)

Silicates CarbonatesQuartz SiO2 Calcite CaCO3

Chalcedony SiO2 Aragonite CaCO3

Clay minerals: Dolomite CaMg(CO3)2Kaolinite Al2Si2O5(OH)4 Ankerite (Fe,Ca,Mg)CO3

Illite K1.5Al4(Si6.5Al1.5)O20(OH)4 Siderite FeCO3

Smectite Na0.33(Al1.67Mg0.33)Si4O10(OH)2 Dawsonite NaAlCO3(OH)2Chlorite (MgFeAl)6(AlSi)4O10(OH)8 Strontianite SrCO3

Interstratified Witherite BaCO3

clay minerals Alstonite BaCa(CO3)2Feldspar KAlSi3O8

NaAlSi3O8 SulfatesCaAl2Si2O8 Gypsum CaSO4�2H2O

Tourmaline Na(MgFeMn)3Al6B3Si6O27(OH)4 Bassanite CaSO4�½H2OAnalcime NaAlSi2O6�H2O Anhydrite CaSO4

Clinoptilolite (NaK)6(SiAl)36O72�20H2O Barite BaSO4

Heulandite CaAl2Si7O18�6H2O Coquimbite Fe2(SO4)3�9H2ORozenite FeSO4�4H2O

Sulfides Szomolnokite FeSO4�H2OPyrite FeS2 Natrojarosite NaFe3(SO4)2(OH)6Marcasite FeS2 Thenardite Na2SO4

Pyrrhotite Fe(1�x)S Glauberite Na2Ca(SO4)2Sphalerite ZnS Hexahydrite MgSO4�6H2OGalena PbS Tschermigite NH4Al(SO4)2�12H2OStibnite SbSMillerite NiS Others

Anatase TiO2

Phosphates Rutile TiO2

Apatite Ca5F(PO4)3 Boehmite Al�O�OHCrandallite CaAl3(PO4)2(OH)5�H2O Goethite Fe(OH)3Gorceixite BaAl3(PO4)2(OH)5�H2O Crocoite PbCrO4

Goyazite SrAl3(PO4)2(OH)5�H2O Chromite (Fe,Mg)Cr2O4

Monazite (Ce,La,Th,Nd)PO4 Clausthalite PbSeXenotime (Y,Er)PO4 Zircon ZrSiO4

Source: International Journal of Coal Geology 50, by C. R. Ward, “Analysis andsignificance of mineral matter in coal seams,” 135–168, copyright 2002, with permissionfrom Elsevier.


ankerite, together in some cases with phosphate minerals such asapatite, may also be found as significant components of the mineralmatter in many coal seams (O’Gorman and Walker, 1971; Rao andGluskoter, 1973; Ward, 1977, 1978, 2002; Davis et al., 1984; Vorres,1986; Ward et al., 2001; Vassilev and Tascon, 2002; and Pinetownet al., 2007). Iron-bearing sulfate minerals, such as jarosite andcoquimbite, may be formed by the oxidation of pyrite on the expo-sure of the coal to the atmosphere, a process which also liberates


sulfuric acid when there is associated runoff water. Many othersulfates in oxidation residues, such as bassanite, glauberite, and hex-ahydrite, usually represent mineral artifacts formed by the interac-tion of nonmineral inorganics during the destruction of the organicmatter.

The crystalline mineral matter in coal may occur as bands, lenti-cles, fracture fillings, plant impregnations, mineral-rich nodules, andother masses visible at a macroscopic scale. Some of the nonmineralinorganics may also be precipitated when coal pore water evaporateson exposed outcrops, mine faces, or drill cores. Microscopically visiblemineral matter in coal includes material intimately admixed withinthe macerals as well as discrete mineral fragments or crystals and arange of nodules, lenticles, veins, pore infillings, and cell replacementstructures (Kemezys and Taylor, 1964; Taylor et al., 1998; and Ward,2002).

The minerals in coal may represent transformed accumulationsof biogenic constituents such as phytoliths and skeletal fragments(Raymond and Andrejeko, 1983), or they may be of detrital origin,introduced as epiclastic or pyroclastic particles into the peat bed(Davis et al., 1984; Ruppert et al., 1991; and Bohor and Triplehorn,1993). Other minerals are produced by authigenic precipitation, eithersyngenetically with peat accumulation or at a later stage in cleats andother pore spaces through epigenetic processes (Rao and Gluskoter,1973; Cobb, 1985; Spears, 1987; Querol et al., 1989; Sykes and Lind-qvist, 1993; Kortenski and Kostova, 1996; Faraj et al., 1996; Wardet al., 1996; and Rao and Walsh, 1997). The syngenetic minerals mayrepresent solution and reprecipitation products of biogenic and detri-tal material, or they may be derived from solutions or decayingorganic matter within the peat deposit.

2.3.2 Nonmineral Inorganic Components

The nonmineral inorganics in coal occur either as dissolved constitu-ents in the pore waters or as an inherent, though sometimes ex-changeable, part of the maceral components. They may representexchangeable ions attached to carboxylic, phenolic, or hydroxylgroups (Durie, 1991) as well as metalloporphyrins and other organo-metallic compounds (Kiss, 1982; Bunnett et al., 1987; Durie, 1991;and Saxby, 2000). Selective leaching with water, ammonium acetate,and hydrochloric acid may be used to determine the abundance andmode of occurrence of the principal nonmineral inorganic elementsin lower rank coals (Miller and Given, 1986; Benson and Holm,1985; and Ward, 1991, 1992). As an example, Figure 2.5 shows the per-centage of various elements released by each process in a sequentialleaching study.

0

20

40

60

80

100

Al Ca Fe K Mg Mn Na P S Si Ti

Leac

habl

e %

of E

lem

ent

Water Washing Acetate Acid

FIGURE 2.5. Percentage of selected elements leached from a low rank coalby sequential treatment with water, ammonium acetate, and hydrochloricacid. (Source: International Journal of Coal Geology 50, by C. R. Ward, “Anal-ysis and significance of mineral matter in coal seams,” 135–168, copyright2002, with permission from Elsevier.)


Elements released by soaking in water are commonly taken asrepresenting ions originally in solution in the coal’s pore water,whereas elements released by treatment with ammonium acetateare usually regarded as representing exchangeable ions attached to car-boxylates and other functional groups in the maceral components.However, some of the carbonate minerals in the coal, if present,may also dissolve in ammonium acetate solutions (e.g., Matsuokaet al., 2002) and contribute to the elements liberated in this way.Elements released by hydrochloric acid treatment may include thoseelements incorporated as organometallic complexes into the maceralcomponents as well as any calcite or dolomite not affected by theacetate treatment. Siderite has only limited solubility in cold acids,but iron occurring as oxide or hydroxidematerial—associated, for exam-ple, with iron staining—may be readily dissolved by acid treatment.

The most abundant elements associated with the organic matterare usually Na, Ca, Mg, and, in some cases, Al and Fe (Figure 2.5). Anelectron microprobe analysis may also show measurable concentra-tions of such inorganic elements in the maceral components (Wardet al., 2003). More detailed element mapping by Li et al. (2007) hasidentified up to 1.5% Ca, 0.5% Al, and 0.7% Fe as consistent compo-nents of supposedly “clean” macerals, especially vitrinite, in severallower rank coals. These elements show a uniform distribution patternwithin the macerals similar to that of the organic sulfur component.

Huggins (2002) discusses a number of other techniques that havebeen used to investigate the inorganic elements in coal and ash. Theseinclude the proton microprobe using proton-induced X-ray emission(PIXE) for elemental analysis (Minkin et al., 1982; Hickmott and


Baldridge, 1991; and Caridi et al., 1993), the ion microprobe mass ana-lyzer (IMMA) (Finkelman et al., 1984), the laser microprobe mass ana-lyzer (LAMMA) (Lyons et al., 1987, and Morelli et al., 1988), laserablation microprobe-inductively coupled plasma-mass spectrometry(LAMP-ICP-MS) (Chenery et al., 1995), and the sensitive high-resolu-tion ion microprobe (SHRIMP) (Kolker et al., 2000a). Another tech-nique that has been used to investigate the forms of occurrence oftrace elements in coal and ash is X-ray absorption fine structure(XAFS) spectroscopy (Huggins and Huffman, 1996).

Many of the inorganic elements in coal were probably inheritedfrom the original plant tissues and fixed by processes such as carbox-ylation, metallation, and chelation during peat accumulation (Filbyand van Berkel, 1987, and Given and Miller, 1987a, b). Subsurfacewaters may have subsequently redistributed the inorganic elementsand possibly introduced additional material, leading in some cases tothe distribution of element concentrations within individual coal bedsthat were controlled mainly by post-depositional ion migration pro-cesses (Brockway and Borsaru, 1985). The inorganic elements are usu-ally expelled from the organic matter as the maceral structures changewith rank advance (Filby and van Berkel, 1987). However, the expul-sion may be inhibited in some cases. For example, microprobe studiesby Ward et al. (2007) found significant concentrations of Ca and Al inthe macerals of several bituminous coals together with perhydrousvitrinites and anomalously low reflectance properties.

Elements occurring in nonmineral inorganic form are generallymore reactive than the same elements occurring in crystalline min-eral phases when the coal is used. The nonmineral inorganics mayalso interact with the mineral components in the coal to form newminerals if the coal is heated by igneous intrusions while still atlow rank levels (Susilawati and Ward, 2006).

2.3.3 Trace Elements in Coal

The total concentrations of the individual inorganic elements in coal,including both major and trace components, are usually determinedby chemical analysis of the coal or coal ash material. Direct analysisof the coal is preferred for elements that may be partly volatilized atelevated temperatures, but for most elements ashing increases theirconcentration in the analysis sample, thereby helping the analysisprocess. Even if the element concentration is measured by ash analy-sis, the result is usually expressed as a fraction of the original coalsample.

A number of techniques have been used to determine the con-centration of individual inorganic elements in coal at major and tracelevels (Karr, 1978a, b, 1979; Davidson and Clarke, 1996; Huggins,


2002; and Vassilev and Tascon, 2002). These include X-ray fluores-cence (XRF) spectrometry, neutron activation analysis (NAA), atomicabsorption spectrometry (AAS), optical emission spectrometry (OES),and inductively coupled plasma vaporization, combined with atomicemission spectrometry (ICP-AES) or mass spectrometry (ICP-MS).Some techniques are more suitable than others for particular ele-ments, depending in part on the concentration of that element andthe matrix within which it occurs.

Almost every element in the periodic table has been identified incoal (Swaine, 1990; Finkelman, 1994a; Swaine and Goodarzi, 1995;and Ren et al., 1999), and the extent of knowledge on trace elementsis increasing as more sensitive analytical methods are developed.With the possible exception of selenium, boron, arsenic, and anti-mony, which appear to be more abundant in coal, most of the traceelements in coal occur at comparable to lower concentrations thanthe same elements in other rock and soil materials. Sixteen of the ele-ments occurring in coal have been included in a list of potentially haz-ardous air pollutants (HAPs) under the U.S. Clean Air Act (Demiret al., 1997), namely As, Be, Cd, Cl, Cr, Co, F, Hg, Mn, Ni, P, Pb, Sb,Se, Th, and U. Some of these may have other impacts on coal utiliza-tion, such as the adverse effect of phosphorus on iron and steel pro-duction. Web-based summaries for a number of individual elements,including range of abundance, modes of occurrence, analytical meth-ods, and behavior during combustion and environmental effects, aregiven by CSIRO Energy Technology (2005).

Like the major elements, the trace elements in coal may be asso-ciated with either the organic components (macerals) or with the crys-talline mineral materials. An analysis of different density fractionsprepared from finely ground coal may be used to indicate the “organicaffinity” of particular elements (Zubovic, 1966; Gluskoter et al., 1977;and Querol et al., 2001), i.e., the extent to which they are associatedwith the relatively clean, low-density macerals (organic affinity) orthe denser, mineral-rich fraction (inorganic affinity). Elements withan organic affinity (such as boron) may be intimately bound to theorganic structure (i.e., nonmineral inorganics), but they may also rep-resent fine particles of minerals occurring within the maceral compo-nents. Most trace elements in coal tend to have a relatively stronginorganic affinity (Davidson and Clarke, 1996, and Kolker and Finkel-man, 1998), representing either low concentrations of minerals withthat element as a major constituent or higher concentrations ofminerals containing minor proportions of the element in question.

The sequential digestion of the coal in a series of solutions,including HNO3 (to dissolve pyrite) and HF (to dissolve silicates),may also be used to evaluate the association of the trace elements in coalwith different mineral matter components (Finkelman et al., 1990;


Dale et al., 1993; Palmer et al., 1993; Laban and Atkin, 1999; andDavidson, 2000). The effectiveness of such techniques, however,may be reduced if fine mineral particles are encapsulated by organicmatter, limiting access for the relevant reagents. In other cases,because of the low solubility of siderite and dolomite in acid, theminerals present may be less soluble than expected in the reagentconcerned. Direct association between element concentration andthe abundance of particular minerals (Ward et al., 1999) in the coalmay also help identify the mode of occurrence for some traceelements.

The concentration of some mineral-related elements (elementswith a high inorganic affinity) may be reduced by coal preparation,but the effectiveness of any such reduction depends at least in parton the particle size of the host minerals and the extent to which theyare liberated from the organic matter when the coal is crushed. Ele-ments with an organic affinity are also likely to be concentrated,rather than reduced, by coal-cleaning processes. Moreover, theremoval of elements from the cleaned coal has the effect of concen-trating them in the waste fraction, from which they may be releasedby a different route when the wastes are dumped at disposal sites.

Knowledge of trace element occurrence in minerals and rocks,together with the help of a series of coal-specific studies, provides abasis for assessing the mineral associations of particular trace ele-ments in coal samples (e.g., Finkelman, 1982, 1994a,b; Rimmer,1991; Belkin et al., 1997a; Ward et al., 1999; Spears and Zheng, 1999;and Finkelman et al., 2002). Elements such as As, Cd, Se, Tl, Hg, Pb,Sb, and Zn, for example, sometimes referred to as chalcophile ele-ments, are generally thought to be associated with sulfide mineralssuch as pyrite, either as solid-solution constituents or as discretesulfide phases. Elements such as Rb, Ti, Cr, Zr, and Hf, often referredto as being among the lithophile elements, are more probably asso-ciated with aluminosilicates such as micas, feldspars, and the clayminerals. An indication of the possible mode of occurrence for someof the elements identified as hazardous air pollutants is given inTable 2.7. Further discussion of the associations of a number ofelements is provided by Swaine (1990) and Finkelman (1994a).

2.4 Coal Metamorphism: Rank Determination

Coal metamorphism involves the physical and chemical transforma-tion from peat through bituminous coal through anthracite andmeta-anthracite to graphite (albeit not necessarily a pure graphite).In general, coal metamorphism (or coalification), denoted as the coalrank, is marked by a progressive decrease in moisture and volatile

TABLE 2.7Indicative mode of occurrence of some potentially hazardous trace elementsin coal.

Element Common Mode of OccurrenceLevel of

Confidence*

Antimony Pyrite and accessory sulfides 4Arsenic Pyrite and accessory sulfides 8Beryllium Organic association 4Cadmium Solid solution in sphalerite 8Chromium Organic and/or clay association 2Cobalt Pyrite; some in accessory sulfides 4Lead Galena 8Manganese Carbonates, especially siderite and ankerite 8Mercury Pyrite 6Nickel Unclear; perhaps sulfides, organics, or clay

minerals2

Selenium Organic association; pyrite and accessorysulfides; selenides

8

*Level of confidence: A number between 1 (low) and 10 (high) expressing the consistencyand predictability of the element’s indicated common mode of occurrence in coal.Source: Fuel Processing Technology 39, by R. B. Finkelman, “Modes of occurrence ofpotentially hazardous elements in coal: levels of confidence,” 21–34, copyright 1994,with permission from Elsevier.


functional groups with a consequent increase in the carbon content ofthe coal (see Table 2.8). Many of the fundamental properties of coalthat are important for industrial use are rank dependent.

Coal metamorphism is a function of heat and pressure acting overa period of time. Taylor et al. (1998) and Hower and Gayer (2002),among others, have reviewed the mechanisms of coal metamorphism.Among the three primary factors, heat is generally considered to bethe most important. Traditionally, increased heat at greater depths ofburial has been considered the primary factor (Hilt’s Law, after Hilt,1873). Though this continues to be the primary argument in Tayloret al. (1998), it has been recognized that influences from tectonicallydriven geothermal fluids have also played an important role in coalifi-cation (Hower and Gayer, 2002, and Harrison et al., 2004). There is lit-tle doubt that time does play a role in coalification, with the amount oftime necessary to achieve the coal rank varying from less than a year incontact metamorphism to 106–107 years for regional metamorphism.The role of pressure is now acknowledged as a hindrance to metamor-phism in closed systems (Dalla Torre et al., 1997, and Carr, 1999), butit has always been considered a primary influence in the progressionof coalification. Pressure causes physicostructural coalification, whichinfluences the physical properties of coals.

TABLE 2.8Coal metamorphism nomogram showing general relationship along chemical rank parameters, heating value, and vitrinitereflectance (Data compiled from Taylor et al., 1998; Diessel, 1992a.)

Rank Stage% Carbon

(daf)% VolatileMatter (daf)

Gross SpecificEnergy(MJ/kg)

% in situMoisture

% Vitrinite Reflectance % Vitrinite Reflectance

(oil, 546 nm)(Diessel,1992a) (oil, 546 nm)

(Teichmuller,1982)*

Rrandom Rmax

RankSubclass Rrandom

Wood 50 >65Peat 60 >60 14.7 75 0.2 0.2 0.26Lignite 71 52 23 30 0.4 0.42 0.38Subbituminous 80 40 33.5 5 0.6 0.63 C 0.42

B 0.49A 0.65

High volatile 86 31 35.6 3 0.97 1.03 C 0.65Bituminous B 0.79

A 1.11MediumvolatileBituminous

90 22 36 <1 1.47 1.58 1.5

Low volatileBituminous

91 14 36.4 1 1.85 1.97 1.92

Semianthracite 92 8 36 1 2.65 2.83 2.58Anthracite 95 2 35.2 2 6.55 7 5

*in Stach et al. (1982)

48

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A more detailed description of the process of the coal evolutionis provided in Chapter 9, where the coal is seen as a hydrocarbonsource rock.

2.4.1 Bulk Chemical Measurements of Rank

Whole-coal measurements of coal rank were used long before maceral-specific indicators such as vitrinite reflectance. The three traditionalmeasurements are proximate analysis, ultimate analysis, and calorificor heating value. Each of these is used in various international stan-dards (such as in ISO and ASTM norms), but each one is flawed inits inability to determine the contributions of different maceralswhich, especially at lower ranks, have markedly different chemicalproperties.

Proximate analysis—the measurement of moisture, ash, volatilematter, and fixed carbon—has been widely used for over 160 years. Lea(1841) used proximate analysis to quantify rank changes along the SW-NE length of the Southern Anthracite Field (Pennsylvania). However,proximate analysis is flawed as a rank indicator because at low ranksthe contributions of the maceral groups are divergent, with the rela-tively aliphatic liptinite macerals contributing more to the volatilematter than the more aromatic vitrinite and inertinite macerals. Thiscan be seen in Table 2.9, which compares a humic coal and a torbanitetaken from nearby mines almost equal in rank. ASTM D388-99(1999a) does note that the ASTM rank classification is to be used onlyin the case of vitrinite-rich coals. Volatile matter is used as a rankparameter only above the high volatile A/medium volatile bitumi-nous boundary, for 31% volatile matter (dry, mineral-matter freebasis; dmmf), the point where the chemistry of liptinites convergeswith the chemistry1 of vitrinite.

Although not a measurement of coal rank, the definition ofexactly what constitutes coal is tied to the basic chemical analysis.ASTM and other national standards generally define coal as having<50% ash yield. Ash yield is not equal to mineral matter, and thesimplest and most widely used conversion is Parr’s (1932) formula:

Mineral matter ¼ 1:08 �Ashþ 0:55 � Total SGiven and Yarzab (1978) review the nuances of various formulas. Atthe same time, it must be realized that the ASTM standard appliesto coal at the mine face, sampled according to accepted procedures(elimination of rock partings greater than a certain thickness, for

1 The ASTM classification uses fixed carbon, 100% minus volatile matter, on adry, ash-free basis (Given and Yarzab, 1978).

TABLE 2.9Comparison of the basic petrology, chemistry, and heating value of Breckenridge torbanite, western Kentucky, and a humic coalfrom the same area. Key: Moisture, ash, VM (volatile matter), FC (fixed carbon), S forms, C, H, N, and O on as-determined basis;CV–calorific value; telalginite and lamalginite are part of the total liptinite value. (Data provided by the CAER, University ofKentucky, USA.)

Seam Moisture Ash Vol FC CV (MJ/kg; mmmf)

Breckenridge 0.90 5.75 72.80 20.55 39.27Hawesville 7.15 23.38 35.60 33.87 30.98

Seam Stot Spyr Ssulf Sorg C H N O

Breckenridge 2.26 1.10 0.03 1.13 75.59 8.71 1.88 5.81Hawesville 4.11 3.22 0.08 0.81 55.25 4.47 1.15 11.64

Seam Vitrinite Inertinite Liptinite (total) Telalginite Lamalginite

Breckenridge 8.6 0.6 90.8 4.9 85.9Hawesville 78.6 11.1 10.4

50

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example), and not mined coal in which a considerable amount of rock(floor, roof, partings) and other noncoal debris (continuous miner bits,scrap metal, etc.) may be included. Presumably, magnetic removal ofmetal at the mine and beneficiation at a preparation plant wouldeliminate much of the noncoal material from the marketed product.Neither does the definition apply to waste piles of coal and rock thatcould be reprocessed as a fuel.

Moisture content has been used as a rank indicator in lignitesand subbituminous coals. For low rank coals, moisture is an impor-tant factor because the coal needs to be transported, handled, andstored, and the presence of moisture in large amounts will impedethese operations and lead to greater costs. Moisture also replaces anequal amount of combustible material and thus decreases the heatingvalue, thereby complicating the combustion process.

For coal classification by rank, equilibrium moisture (the pre-scribed analysis) is considered equal to bed or inherent moisture(ASTM D388-99, 1999a). Luppens (1988), however, has noted thatbed and equilibrium moisture diverge at low ranks and, as a furthercomplication, equilibrium moisture analyses may vary significantlybetween laboratories and between different operators in the same lab-oratory. Standard moisture determinations do not include the waterfrom the decomposition of organic constituents in coal, the waterinherent in clays, or the water losses that occur at a higher tempera-ture than the test temperature (Carpenter, 2002).

Ultimate analysis, which measures the C, H, N, S, and O on thebasis of ash and moisture content, is another common test. Of theparameters, C, for a broad rank range, and H, for anthracites, are com-mon rank parameters. C analysis at lower ranks suffers from the samelimitations as volatile matter or fixed carbon from proximate analysisin that it is a function of the variation in maceral chemistry in addi-tion to being rank dependent. Nevertheless, many plots of rank-dependent parameters (Hardgrove grindability index and free-swellingindex, among others) are often plotted against C but are, in turn, influ-enced by the vagaries of the analysis. Apart from the drawback of mac-eral variation, mineral matter can make a positive contribution to the(apparent) organic analysis (Given and Yarzab, 1978). Clays will lose-OH groups and H2O; carbonates will lose CO2 but they may fix S asa Ca-sulfate. Pyrite will also burn to Fe2O3 and SO2 in air and decom-pose to FeS in the volatile matter test. All these reactions may influ-ence the chemical tests. In addition, under ASTM standards, thetotal S is used in the calculation. A proper expression of the ultimateanalysis should include corrections for the inorganic contributions ofC and H and should include the organic sulfur, not the total sulfur(Given and Yarzab, 1978; Given, 1984). Organic sulfur is generallydetermined indirectly from the total sulfur by subtracting the pyritic


and sulfate sulfur. Oxygen in the ultimate analysis is determined indi-rectly by subtracting C, H, N, and S from 100%. Thus, both theorganic sulfur and O values incorporate the errors involved in thedetermination of the other components.

The calorific value, expressed as MJ/kg, on a moist, mineral-matter-free basis, is used as a rank parameter for ranks lower thanmedium volatile bituminous coal. Since the moist basis implies equi-librium moisture, the calorific value is really the combination of tworank parameters: the equilibrium moisture, with the caveats notedabove, and the final calorific value. As with proximate and ultimateanalysis, measurement of the calorific value on the whole coal meansthat the properties of all the macerals are averaged together. Thisinfluence is obvious in the case of cannel coals or torbanites, wherethe high liptinite content yields heating values >8 MJ/kg more thanin the case of equivalent-rank vitrinite-rich coal (Hower et al.,1986a). Furthermore, there are complications arising from the com-bustion heat of pyrite, the “organic” calorific value being 0.126(Spy)MJ/kg less than the determined mmmf calorific value (Given andYarzab, 1978). Although the gross calorific value, or high heatingvalue (HHV), is the determined parameter, the low heating value(LHV; HHV minus the heat required to vaporize the water) is a betterestimate of the available heat in a combusted coal. This is particularlytrue for high moisture coals (Carpenter, 2002).

2.4.2 Vitrinite Reflectance

Because vitrinite is the most abundant maceral group in most coals(certain Permian Gondwana coals are among the major exceptions),the maceral group plays a large role in defining the properties of thewhole coal. The physical and chemical properties of vitrinite changethrough the course of coalification, and its reflectance has been cali-brated against a number of other rank parameters (Table 2.8). Thus,the determination of vitrinite reflectance, the percentage of incidentlight reflected from a polished surface, is a fundamental tool in coalpetrology.

The change in reflectance of vitrinite with coalification is relatedto the increase in aromatization. More detailed explanations can befound in Stach et al. (1982), van Krevelen (1993), and Taylor et al.(1998). Measurements are standardized (ISO 7404/5, 1994b, and ASTMD2798-05, 2005b), and they must be achieved in monochromatic greenlight (546 nm) by means of a photomultiplier (digital cameras are usedas an alternative in some cases) coupled to a microscope using incidentlight, in oil immersion (with a refractive index of 1.518 at 23�C), objec-tives with magnifications between 25� and 60�, and a cross-hair mustbe incorporated into one of the oculars as a reference point. Due to


the natural scattering of reflectance values of vitrinite particles in acoal sample, the number of readings on different grains of vitrinitemust be 100. At the end of the analysis, the statistical mean reflectance(in %) should be calculated. Usually the reflectance distribution isreported as V-types or 1/2 V-types (ICCP, 1971; Stach et al., 1982, andDavis, 1984), which represent ranges of 0.1% and 0.05%, respectively.The standard deviation of the mean for 100 readings in a single coalshould be about 0.01% to 0.02% (ICCP, 1971, and ISO 7404/5,1994b), but this may vary slightly as a function of the coal rank. Therepeatability and the reproducibility of the analyses are about 0.06%and 0.08%, respectively (ISO 7404/5, 1994b).

Particles representing all the various vitrinite macerals and sub-macerals may be included among those measured for vitrinite reflec-tance determination. However, for precise measurements of thedegree of metamorphism, it may be better to restrict measurementto the thick, homogeneous bands of the vitrinite maceral collotelinite(also referred to in some national standards as telocollinite). The dif-ference may be significant because the thinner bands of vitrinitematrix material commonly have slightly lower reflectance values,and measurements based on all vitrinite may have a wider degree ofscatter. Whether the measurements cover all vitrinite or only the col-lotelinite in the sample should also be indicated in the report detailingthe measurement results.

Two types of reflectance measurements can be made to quantifythe reflectance of a vitrinite grain from a particulate coal sample: therandom reflectance of vitrinite and the maximum reflectance of vitri-nite. Random reflectance is the reflectance of a grain in the orienta-tion in which it is encountered, measured using nonpolarized light.For measurements of maximum reflectance on a particulate sample,the polarizer needs to be in the 45� position into the incident lightbeam. If the microscope stage is rotated 360�, the maximum reflec-tance can then be taken. In low rank coals, random and maximumreflectances are the same because coal is optically isotropic. As analternative, random reflectance can be estimated as the average ofany two orthogonal readings in the determination of maximum reflec-tance (Hower et al., 1994b).

Although vitrinite reflectance is one of the most widely usedparameters, it is not the ideal rank measure in all circumstances.Apart from inherent variations between vitrinites from differentplants within the same coal, it has been recognized that perhydrous,or marine-influenced, coals have anomalously low (suppressed) vitri-nite reflectance properties (Barker, 1991; Mukhopadhyay, 1992,1994;Suarez-Ruiz et al., 1994a,b; Price and Barker, 1985; Iglesias et al.,2002; and Wilkins and George, 2002). Some of the causes of the reflec-tance suppression (natural and artificial) have been synthesized by


Barker (1991) and Mukhopadhyay (1992, 1994). Alternative opticalmethods (Wilkins et al., 1992, 1995, 2002; Veld et al., 1997; and Kalk-reuth et al., 2004) and nonoptical alternatives (Rimmer et al., 1993,and Wang and Hu, 2002) have been used for rank determination andare especially useful in such circumstances. Measurement of the car-bon (and oxygen) content of vitrinite macerals using light-elementelectron microprobe techniques, for example, has been shown byGurba and Ward (2000) and Ward et al. (2007) to provide a better rankindicator than vitrinite reflectance in cases where anomalously lowreflectance is developed due to the original depositional conditionsof the coal seam.

Optical Anisotropy

When the coal rank increases and the structure of the carbonaceousmaterial is reorganized, almost all the coal’s physical properties varyaccording to which part of the coal section is being considered. Thusvitrinite develops an anisotropic behavior and exhibits bireflectance.Minimum reflectance is usually observed in the direction perpendicu-lar to the bedding plane and maximum reflectance in sections parallelto this plane. In sections with an intermediate orientation, the reflec-tance is intermediate between the maximum and minimum values.

Bireflectance can be determined using polarized light and therotating stage of the microscope. By measuring the true maximum andminimumreflectances, the anisotropy can be calculated from the differ-ence (Rmax – Rmin). Methods for determining these parameters weredeveloped by Ting and Lo (1978) and Ting (1978) and later modified byKilby (1988, 1991) and Duber et al. (2000). The anisotropy in a coal islinked more to the overlying pressure and generally rises with increas-ing coal rank, but no strict relationship exists between rank and thedegree of anisotropy (Davis, 1984). Tectonic stress in directions otherthan verticalmay also produce reflectancemaximawith different orien-tations (e.g., Hower and Davis, 1981, and Levine and Davis, 1989).

2.4.3 Fluorescence

This type of petrographic analysis is based on the properties of somemacerals that autofluoresce when irradiated with blue or ultravioletlight and may be of assistance in rank assessment. Fluorescencemicroscopy is currently employed in coal petrology and in kerogenstudies for characterization of liptinite macerals, kerogen composition,rank/maturation studies (as an essential criterion for oil and gas forma-tion), and hydrocarbon detection as well as in the correlation of thetechnological properties of vitrinites and coals (thermoplastic, cokingand oxidation features) to vitrinite fluorescence characteristics.

Fluorescence analyses should be carried out using a reflectedlight microscope coupled to a photomultiplier (ICCP, 1975, 1993).The microscope must be equipped with a high-pressure mercury orxenon lamp for illumination with the corresponding excitation filtersto select the UV or blue light, barrier filters, and a variable interferencefilter covering the range of 400 to 700 nm. Dry or water immersionobjectives (25� to 50�) are preferred to those for oil immersion.

Changes in fluorescence intensity and in fluorescence colors aredependent on the type of organic substance and on the coal rank. Thesechanges can be measured by the so-calledmonochromatic fluorescencemicroscopy and spectral fluorescence. The former includes quantitativemeasurements of fluorescence intensity at a specific wavelength (546nm) which are recorded in relation to a standard (Jacob, 1980). Spectralfluorescence measurements determine changes in fluorescence colorby recording the emission of the spectrum (Ottenjann, et al., 1975, andOttenjann 1980, 1982) between 400 nm and 700 nm. They alsomeasurespectral alteration or changes in fluorescence properties after 30 min-utes of irradiation of organic substances (Teichmuller and Ottenjann,1977, andOttenjann, 1980, 1982). Classical parameters commonly usedin quantitative fluorescence analysis are: I = fluorescence intensity at546 nm; lmax = spectral maximum; Q = spectral quotient (red/greenratio); AI = alteration of fluorescence intensity at 546 nm, and AS =spectral alteration, in each case for a period of 30 minutes’ irradiation.Other fluorescence parameters are described (e.g.) by Martinez et al.(1987).


2.5 Coal Classification

The purpose of any classification scheme is to provide a convenientmeans for the preliminary evaluation of a coal product and for relatinga particular coal to others on the basis of the appropriate accepted cri-teria. The ASTM D388-99 (1999a) and international coal classificationsystems were somewhat restricted because they did not take intoaccount a sufficiently broad range of age and rank. The InternationalClassification of Hard Coals by Type (United Nations EconomicCommission for Europe, 1956) employed a three-digit code based onfour parameters: either volatile matter (daf) or gross calorific value(maf), caking properties (free-swelling index or Roga index), and cok-ing properties (Gray-King index or dilatometer maximum dilation).The system was most successful when used for classifying Carbonifer-ous bituminous coals mined and sold in Europe. The subsequentexpansion of the global coal trade toward the end of the 20th centurymade it necessary to broaden the system. The main objections to the1956 International classification, (Uribe and Perez, 1985) were: (1)the classification was best suited to coals of a homogenous maceral


composition with a low inertinite content; (2) the system was intendedto be used for traditional purposes (combustion and coking) and so didnot apply to new utilization processes such as gasification or liquefac-tion; (3) the rank parameters (volatile matter and calorific value) aredependent on coal type or maceral content (it was thought that boththe maceral content and a rank parameter that is independent of coaltype, such as vitrinite reflectance, should also be included); and (4)chemical or technological parameters cannot be ignored for a compre-hensive characterization is to be achieved.

On the other hand, the 1956 classification did not take intoaccount the distinction between single coal beds and coal blends.For these reasons the United Nations Economic Commission for Eur-ope (United Nations Economic Commission for Europe, 1988)replaced the 1956 coal classification system with a new InternationalCodification System for Medium and High Rank Coals (>0.6% R or>24 MJ/kg) in international trade. This new system was applicableto all coals of different origin and geological age from different typeof deposits as well as to single seams and multiseam blends of run-of-mine coals and washed coals. Medium and high rank coals werecharacterized by means of a 14-digit code number comprising eightcoal-quality parameters: (1) mean random vitrinite reflectance, eithermeasured directly or estimated as maximum vitrinite reflectancedivided by 1.06; (2) the character of the reflectrogram; (3) maceral com-position expressed as (a) the percentage of inertinite and as (b) the per-centage of liptinite, which provides a means of distinguishing, in part,between Gondwana and Carboniferous coals; (4) the free-swellingindex; (5) volatile matter (dry, ash-free); (6) ash percentage (dry basis);(7) the total sulfur (dry basis); and (8) gross calorific value (dry, ash-free) in megajoules per kilogram (MJ/kg).

Other parameters may be appended to provide a more thoroughdescription of coal quality, such as ash composition, ash fusion charac-teristics, and Hardgrove grindability index for steam coals or Audibert-Arnu dilation properties and phosphorous content for metallurgicalcoals.

In 1998 the United Nations Economic Commission for Europeproposed an international classification of in-seams coals. This sys-tem was intended to serve as a means of classifying coals and ensuringa better characterization of coal deposits. Unlike the previous UnitedNations Economic Commission for Europe (1988) classification sys-tem, which was intended for commercial purposes, this new systemwas clearly described as not intended for use in commerce or trade.The 1998 coal classification is based on three fundamental coal char-acteristics to be used in combination: coal rank (or degree of coalifica-tion), petrographic composition, and grade or amount of impurities(ash yield). Figure 2.6 shows a scheme for the classification of in-seam

LOW-RANK

C B A D C B A C B A

0

50

100

50

50 100

0 50 100

V% (L >I)

V% (I > L)

I%

15 20 24

0.6

10

Paleo B-time

20

30

50

80

Rr %

GCV (MJ/kg, m, af)

1.0 1.4 2.0 3.0 4.0Not to be included inthe classification

Rr% - Vitrinite mean Random Reflectance, per cent (ISO 7404-5 standard)GCV (MJ/kg, m, af) - Gross Calorific value in MJ/kg, recalculated to moist, ash-free basis(ISO 1928 and 1170 standards)Ash (HT) mass % db - Ash content (High temperature), mass per cent, recalculated to drybasis (ISO 1171, 331 and 1170 standards)V%. L%. I% - Vitrinite, Liptinite and Intertinite contents respectively, volume per cent,recalculated to mineral-matter-free basis (ISO 7404-3 standard)

Maceral analysis(m/nf) vol.%

Met

aOrth

oOrth

oOrth

oPar

aPar

a

SUBBITUM

per

Non-b

ande

d

coal

Bande

d co

al

(main

ly Hum

ic)

High g

rade

coal

Med

ium g

rade

coal

Low g

rade

coalVer

y low

gra

de co

al

Sapro

-

peliti

c

coal

Carbo

nace

ous r

ock

PETROGRAPHIC C

OMPOSITIO

N

dry ↔

wet

oxida

tion

↔ re

ducti

on

aero

bic ↔

ana

crob

ic

Rock

Oil

shale

Met

aM

eta

COMPOSITIONANTHRACITEBITUMINOUSLIGNITE

RANK

GR

AD

E

Ash

(H

T)

mas

s %

, (db

)

Coa

l ↔ n

on-c

oal r

ock

was

habi

lity

test

PETROGRAPHIC

L%

MEDIUM-RANK HIGH-RANK

FIGURE 2.6. Coal classification. (Source: United Nations Economic Com-mission for Europe, 1998; reprinted from International Journal of Coal Geol-ogy 50, by B. Alpern and M. J. Lemos de Sousa, “Documented internationalenquiry on solid sedimentary fossil fuels; coal: definitions, classifications,reserves-resources, and energy potential,” 3–41, copyright 2002, with permis-sion from Elsevier.)


coals. Although not very frequent, this system has sometimes beenreferred to in the literature (e.g., Alpern and Lemos de Sousa, 2002).

A recently devised classification method is that of the “classifi-cation of coals” by the International Organization for Standardization(ISO-11760, 2005). Its development has been guided by the classifica-tion system of the United Nations Economic Commission for Europe(1998) and, as in the previous case, it is not intended to be used forcommercial purposes. It is also based on the three fundamental coalproperties: vitrinite reflectance (mean random reflectance), vitrinitecontent in percent per volume on a mineral-free basis, and ash yield.The ISO-11760 (2005) system provides a simple classification methodof descriptive categorization that can be applied to coals of all ranks,


a method of comparison of coals taking into account certain key char-acteristics, and guidance in selecting the appropriate ISO standardprocedures for coal analyses.

2.6 Coal Blends

Many of the installations that use coal, whether for carbonization(coking) or combustion purposes, use a feedstock blended from a num-ber of different coals to obtain the appropriate quality specificationsrather than coal of uniform rank from a single seam or deposit. Usu-ally the blended coals originate from different sources, each having adifferent composition and/or coal rank (Figure 2.1h). In some casesthe feedstock from a single source of supply (which may be a mineor a preparation plant) also encompasses coals having more than onerank level, possibly due to the incorporation of coal affected by igne-ous intrusions or other local heat sources or to the mixing of coalsfrom different zones of the one deposit or region.

Coal petrography (Figure 2.1h) is the only effective way to iden-tify coal blends (as it is summarized in Suarez-Ruiz, 2004). This is par-ticularly significant for blends that show similar chemical parameters(e.g., moisture, ash content, volatile matter), but they may differ insome of their technological properties or display a quite different tech-nological behavior in processes such as coke production or even incoal combustion. Figure 2.7 shows the distribution of vitrinite reflec-tance values in five different coals in the form of a histogram illustrat-ing the proportion of particles falling in the different reflectanceintervals (V-steps). The coals have similar overall proportions of vola-tile matter and similar mean maximum vitrinite reflectances. At thesame time each coal has quite different coking properties, as expressedby the respective free-swelling index values. The sample at the top ofthe diagram, for example, represents a single medium volatile bitumi-nous coal with a vitrinite reflectance of 1.3%, whereas the sample inthe lowest plot represents a mixture of two quite different coals, onewith a vitrinite reflectance of around 0.9% and one with a vitrinitereflectance of around 1.9%. This figure shows that, although coalshaving particular overall properties can be prepared by blending twoor more coals of different rank (and reflectance) characteristics, theresulting coal blends display a substantially different technologicalbehavior in processes such as coke production. Careful petrographicanalysis is required to analyze coal blends, with particular attentionbeing paid to the distribution of vitrinite reflectance values as wellas to the range of optical characteristics of other macerals. Both thehistogram plots and the standard deviation (scattering) of the individ-ual reflectance values (identified as S in Figure 2.7) may be relevant tothe evaluation process.

VolatileMatter

24,5 9

40

1,31

S

0,063

0,234

(0,372)

(0,553)

0,1171,26

1,26

1,41

1,31

353025201510

5Vol

.-%

1/

2 V

-Ste

p

2015105

15105

105

105

25,2 9

24,2

24,2

25,0 1

0,5=10 Vol.-%

0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4

10152025303540

Reflectance

Vol. Matter Vitrinite

%

%

7 1/2

4 1/2

SwellingIndex

Rm

FIGURE 2.7. Reflectograms, mean reflectance and scatter, volatile matter,and swelling index of five different coals and coal blends. (Source: Stach’sTextbook of Coal Petrology by E. Stach, M.-Th. Mackowsky, M. Teichmuller,G. H. Taylor, D. Chandra, and R. Teichmuller (editors), 535 pp., copyright 1982,with permission from Gebruder-Borntraeger (www.borntraeger-cramer.de).


Petrographic analysis, vitrinite reflectance measurements, andthe maceral analysis of coal blends may be used to obtain informationon (1) the number of different component coals in the blend, (2) theproportion of each coal in the blend, (3) the overall mean (random ormaximum) vitrinite reflectance of the coal blend, (4) the mean (ran-dom or maximum) vitrinite reflectance of each individual coal in theblend, (5) the overall maceral composition of the blend, and (6) themaceral composition of each individual coal in the blend.

The identification and analysis of coal blends is especially relevanttometallurgical coke production. Authors such as Schapiro et al. (1961),Benedict et al. (1968a), Gray et al. (1979), and Taylor et al. (1998) havedescribed methods for predicting coke strength and other propertiesfrom maceral composition and vitrinite reflectance (including theV-step) data that are applicable to coal blends. The evaluation of coalblends may also be significant to combustion processes as a means ofidentifying the sources of different types of unburned carbons.

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Basic Factors Controlling Coal Quality and Technological Behaviorstoa.usp.br/.../20408/factors+controlling+coal+quality.pdf · 2012-10-16 · Liptinite maceral group, petrographic