Coal and coke petrographic investigations into the fusibility of Carboniferous and Permian coking coals

International Journal of Coal Geology, 9 (1987) 87-108

87

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Coal and coke petrographic investigations into the fusibility of Carboniferous and Permian' coking Coals CLAUS F.K. DIESSEL 1 and EVAMARIE WOLFF-FISCHER 2

1Department of Geology, The University of Newcastle, Newcastle, N.S. W. 2308, Australia 2Bergbau-Forschung GmbH, Franz-Fischer- Weg 61, 4300 Essen 13, F.R. Germany (Received February 16, 1987; accepted for publication March 23, 1987)

ABSTRACT Diessel, C.F.K. and Wolff-Fischer, E., 1987. Coal and coke petrographic investigations into the fusibility of Carboniferous and Permian coking coals. In: E. Wolff-Fischer (Editor), MarieTherese Mackowsky Memorial Issue. Int. J. Coal Geol., 9: 87-108. For the purpose of conducting coal/coke mass balance calculations ten Australian coals of Permian age and twenty Carboniferous coals from the Ruhr district of Germany have been carbonized, and both feed coal and coke samples have been subjected to petrographic and other laboratory analyses. The results demonstrate that inertinite dissociates thermally into four components: (1) gas and liquor; (2) fused coke matrix (FCM) ; (3) partly fused coke inclusions (PFCI) ; and (4) unfused coke inclusions (UFCI). Fluorescence intensity measurements offer the best means of identification of the boundaries between the above groups and, in doing so, divide coal components into fusible, partly fusible and infusible constituents without the need to refer to any maceral classification. Fluorescence intensity cut-offs for the three solid categories have been determined. They correspond to 3% 1 650 w for FCM/PFCI and 1.5% for PFCI/UFCI.

INTRODUCTION

The microscopic image of a metallurgical coke reveals a number of textural features, such as degassing pores, contraction fissures, and solid coke matter. The latter can be divided further into fused coke, as well as into coal- and mineral-derived inclusions. Unless oxidized or non-caking coal has been carbonized, the coal-derived coke inclusions belong mainly to the maceral group inertinite thus suggesting that vitrinite and liptinite macerals of coking coal rank fuse quite readily. However, on the basis of coking experiments, Diessel (1982, 1983) could demonstrate that a substantial proportion of inertinite is, in fact, not inert but softens too, and becomes integrated with the coke matrix during carbonization. In order to elucidate this problem and also, to investigate further the notion 0166-5162/87/$03.50

© 1987 Elsevier Science Publishers B.V.

88 of differences in fusibility between Carboniferous and Permian Gondwana coals (Diessel and Wolff-Fischer, 1986), two lines of microscopic investigations were pursued. One was coal-oriented and designed to characterize the inertinite content in coal, whereas the other was targeted on the residual inertinite left in the coke after carbonization. Both types of tests, in conjunction with other analyses, formed the basis for mass balance calculations. Inertinite in coal is traditionally characterized by its morphology and reflectance. The first property serves to make a distinction between structured (e.g. fusinite) and unstructured (e.g. macrinite) inertinite. Although variations in the fusibility of morphologically different inertinite macerals have been found ( Diessel, 1983 ), the differences are small and insignificant compared with the strong ties between fusibility and reflectance. However, as was pointed out previously ( Diessel et al., 1986; Diessel and McHugh, 1986), there is no direct numerical correlation between fusibility and reflectance, because the actual reflectance values which separate fusible and non-fusible inertinite also depend on the rank of the host coal. In order to overcome this problem fluorescence microscopy was successfully tested as an indicator of fusibility. Micro-fluorometric methods were introduced to coal microscopy by Jakob (1952, 1964 ) who developed them as an important tool in the study of liptinite macerals in low-rank coals and oil shales. As liptinite shows its highest fluorescence intensity in the green to yellow portion of the light spectrum, ultraviolet to very short wave blue light excitation has been traditionally used. When progress in analytical techniques (Ottenjann, 1980, 1982; Teichmfiller, 1982) made it possible to apply fluorometric methods to vitrinite, the optical system designed for liptinite was initially retained. Because vitrinite fluoresces more strongly in the red field of the spectrum, the stretching of the spectral gap (ideally 150 to 200 nm) between excitation radiation and optimum fluorescence to 300 and 350 nm resulted in a very weak signal. Even so, Ottenjann et al. (1982) and Wolf et al. (1983) could demonstrate the existence of a close link between vitrinite fluorescence and the carbonization properties of the host coal. The subsequent adaptation of fluorescence microscopy to the study of inertinite was achieved by increasing the wave length of the exciter beam and the changes in instrumentation which will be discussed below. The main part of the investigations is based on a petrographic comparison of 30 coals and cokes representing single seams or blends of closely related coals. Twenty of the coals used in the study are of Carboniferous age and come from the Ruhr Basin in West-Germany, the remaining ten being of Australian origin. Additional samples of freshly mined coal have been analyzed in order to improve further the fluorescence/reflectance relationship. ANALYTICALMETHODS -- COAL Standard methods have been used in sample preparation, white light microscopy, laboratory carbonization and chemical tests. Fluorometric analyses have

89 been conducted in a similar manner to those previously described by Diessel (1985), Brown et al. (1985a,b), and Diessel and McHugh (1986), although some important changes have been made in this study.

Fluorescence microscopy A Carl Zeiss Universal Microscope equipped with an MPM 01 photometer head and a two-way mirror to which two HTV-R 928 photomultiplier assemblies are attached has been used for the petrographic work. Each photomultiplier is connected to its own signal processing and display unit. The revolving diaphragm holder of the photometer head is fitted with two measuring diaphragms and interference filters. The diaphragm through which fluorescence intensities are measured has a circular aperture of 1.0 mm and is covered by a 650 nm interference filter, whereas the diaphragm used for reflectance measurements has a likewise circular aperture of 0.4 mm in diameter and is covered by the usual 546 nm interference filter. By operating one lever for the revolving measuring diaphragm holder and another one for the rotating two-way mirror, it is possible to change from one measuring mode to the other. The same principle applies to the illumination system. White light is obtained from a 12 V/100 W halogen lamp through the back of the microscope and a modulator, whereas the HBO 100 W mercury lamp used for fluorescence work is attached to the side of the microscope by a short tube to which the necessary optical assembly is fitted which enables the operator to switch from one illumination mode to the other. Reflectance measurements are made in the usual way by calibrating against a set of reference standards. Both reflectance and fluorescence measurements are carried out through a Carl Zeiss Plan Neofluar 40/0.9 oil-glycerin-water objective by using water immersion. The latter has been found to be a reasonable compromise between the necessity of employing an immersion medium with a better light transmission than oil and, at the same time, achieving more contrast than is possible in air. Because of its lower refractive index, water produces an optical image of lower contrast than is obtained in oil immersion but it is satisfactory for the purpose, and since water is a natural component of coal, the samples do not require the careful drying that is necessary when reflectance measurements are carried out in oil. Based on 50 reflectance measurements of vitrinite made in oil and water on each of 30 samples ranging in rank from brown coal to semianthracite, a close linear relationship ( r = 0.992 ) between Rwr (random reflectance in water) and Ror (random reflectance in oil) was found: Rwr = 1.09 + 1.43 Ro,

(1)

Further correlations between random and maximum reflectance in oil have been quoted by Diessel and McHugh {1986):

90 Rom =1.07 Rot-0.01

(2)

In fluorescence mode an excitation beam ranging in wavelength from 450 to 490 nm isproduced by a combination of a long-wave pass band filter (LP 450) and a short-wave pass band filter ( KP 490), used in conjunction with a chromatic beam splitter ( F T 510) and a cut-off filter (LP 520) for blue light suppression. Both, zero and 100% calibration of the recording instrument are carried out with the aid of a non-fluorescing commercial SiC standard and an unmasked uranyl glass standard (GG 17-296). The considerable amount of stray light emitted from the uranyl glass standard is reduced by engaging during measurements a circular luminous field aperture, which is only 1.6 times larger than the measuring diaphragm. On each of the 30 coal samples for which the respective cokes were available, the following petrographic tests were carried out: (a) Maceral analyses in white light based on 1000 points; (b) Random reflectance determinations in oil on vitrinite, liptinite, and inertinite up to a total of 600 readings, depending on the spread of values; (c) Point-by-point determinations of reflectance and fluorescence intensities in water immersion on up to 120 inertinite macerals; (d) Maceral group analyses in fluorescent light based on 500 points; (e) Fluorescence intensity determinations either in water immersion or dry on telinite plus telocollinite, desmocollinite and the fluorescent portion (exceeding 0.5% relative intensity) of inertinite based on approximately 50 readings in each of the maceral groups. The petrographic analyses were supplemented by a variety of standard laboratory tests. ANALYTICALMETHODS - COKE Modal coke analyses were carried out in order to investigate the response of inertinite to carbonisation and to correlate the proportion of residual inertinite in the coke samples with the inertinite found in the coal charge.

Coke microscopy Each of the 30 cokes which were produced in pilot-type coke ovens, were divided into four lump samples spaced evenly between the cauliflower and the tar seam across one half of an oven width. In each sample an area of approximately 20 X 20 mm was polished and microscopically assessed by using reflected polarized white light in water immersion at a total magnification of 1000 X. In each of the four measuring areas 500 fabric elements were determined by pointcount method so that the composition of each coke used in the study is based on a total of 2000 points.

91 In the microscopic analyses the essential components of the coke fabric were divided into four groups referred to as: (a) Fused coke matrix (FCM), which was further subdivided into anisotropy domains according to Gray (1976); (b) Coke inclusions, consisting of partly fused anisotropic inertinite (PFCI), unfused isotropic inertinite (UFCI), and oxidized coal, none of which has been found in the analyzed cokes; (c) Mineral inclusions, which have not been further subdivided; (d) Pores and fissures, which were further subdivided into degassing pores in fused coke matrix, and pores in coke inclusions. The latter are often original cell lumens which have been made use of in the degassing process; MASS BALANCECALCULATIONS According to the method developed by Diessel and Wolff-Fischer (1986) the macerals analyzed in the coal are grouped into five categories based on similarities in density and carbonization behaviour: (a)Liptinite, having lowest density and highest volatile yield during carbonization; (b) Vitrinite, forming the bulk of the fused coke matrix; (c) Micrinite, which is added to the matrix-forming vitrinite and liptinite because it can neither be detected in fluorescent light in coal, nor in the groundmass of coke; (d) Semifusinite, macrinite and a proportion of inertodetrinite which is determined by the fusinite/semifusinite ratio; (e) Fusinite, having the highest density, plus an apportioned amount of inertodetrinite. The microscopically determined percentages (by volume) are converted into mass percentages by using the density values given by van Krevelen (1961) and Dyrkacz et al. (1984). The results are then normalized to 100 g of pure coal minus n grams of mineral matter, n being estimated as % ash (db) × 1.11, unless the microscopic analysis has revealed an unusual mineral composition, which has not been the case in the current investigation. Both coke yield and composition are estimated by calculating the reduction in weight due to loss of gas and fluids during carbonization. The estimation is based on the coking experiments carried out by KrSger and Pohl (1957) on maceral concentrations separated from coals of different rank. The latter was originally expressed as "whole coal" volatile matter (daf), which is not a good rank parameter when coals of varying maceral and mineral composition are compared. Inconsistencies in the results have been experienced when KrSger and Pohl's (1957) degassing figures were applied to some inertinite-rich Australian coals, because the original work had been carried out on inertinite-poor Carboniferous coals. As shown in Fig. 1, volatile matter decreases significantly

92 gO-

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.

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.

.

2/+

,

.

25

.

.

26

.

.

27

.

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V M . % (daf)

Fig. 1. The relationship between volatile matter (daf) content and inertinite percentage (total macerals = 100% ) in Australian isometamorphic coals of 1.22 _+0.1% Ro~t.

in isometamorphic coals with increasing inertinite content, which necessitated the conversion of volatile matter content to vitrinite reflectance on the basis of similar coals to those used by KrSger and Pohl (1957). Linear regression analysis covering the range from 10 to 40% volatile matter (daf) of Teichmiiller's (1971) correlation of mean random vitrinite reflectance and volatile matter gives the following equations:

V.M. (daf) = 53.62 - 19.75 Ro~

(3 )

Rorv = 53.62 - V.M. (daf)/19.75

(4)

r= -0.998

(5)

When applied to KrSger and Pohl's (1957) degassing figures (quoted by Diessel and Wolff-Fischer, 1986) the following weight reductions are obtained: % Loss in vitrinite = ( 5 3 . 1 - 19.6 Rorv) - 3 . 8

(6)

% Loss in liptinite = (149.1 - 54.9 Ror~) - 50.0

(7)

% Loss in inertinite (excl. fusinite) = (21.5-7.9 Ro~) +5.4

(8)

% Loss in fusinite= (15.0-5.5 Rorv) +3.2

(9)

By using the above equations for the calculation of weight loss during carbonization the detrimental influence of compositional variations is largely suppressed, because the reflectance values of the investigated coals have been converted to the volatile matter equivalents KrSger and Pohl (1957) used in their carbonization experiments. The validity of this procedure is demonstrated in Fig. 2 by the very close correlation between predicted and actually determined coke yields. The composition of the coke is estimated from the

93 B2.5 BO.O -

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Fig. 2. Correlationbetweendeterminedand estimated cokeyield (both dmmf). degassed residues of vitrinite, liptinite and micrinite, which are combined to give the amount of fused coke matrix, whereas, for calculation purposes, the remaining inertinite is regarded as producing unfused coke inclusions only. The results of the microscopic analyses carried out on the coke samples, as well as their ash values are adjusted to the actual coke yields and, after grouping into the categories fused coke matrix (FCM), partly fused coke inclusions (PFCI), and unfused coke inclusions (UFCI), the mass differences to the estimated coke composition are determined. Finally, the carbonization products of inertinite are determined from the combined masses of: (a) FCM (in coke) - F C M (estimated from coal) (b) PFCI ( in coke ) (c) UFCI ( in coke ) Invariably a small mass deficiency is left, which is assigned to the amount of gas and liquids expelled during carbonization plus some error. DISCUSSION OF THE RESULTS - COAL The correlation between mean fluorescence intensity and mean random vitrinite reflectance Following the first attempts of Teichmiiller (1982), Ottenjann et al. (1982), and Wolf et al. (1983) to establish a correlation between fluorescence and reflectance in vitrinites of German Carboniferous coals, Diessel and McHugh (1986) published a respective reference curve for Australian Permian coals. In the course of this project the data base for the fluorescence/reflectance relationship has been improved by restricting it to absolutely fresh coal sampled in deep mines and by conducting the analyses within hours or only a few days of sampling. Figure 3 gives the fluorescence/reflectance correlation for telinite plus telo-

94 1/*•

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Fig. 4. Fluorescence/reflectance reference distribution for Carboniferous telovitrinite.

collinite (in the following referred to as telovitrinite according to Australian Standard 2856-1986). The fluorescence intensities follow a normal distribution curve forming a broad peak of around 13% 1 650 wt at approximately 1.05% Ror t. In the more heterogeneous desmocollinite, the degree of scattering is greater, the intensity peak is broader and centred at 0.9% Rord. The general level of fluorescence is higher, which is probably due to the greater capacity of desmocollinite to absorb and generate bitumens. A comparison with Carboniferous coals shows both similarities and differences. Figure 4 displays the fluorescence/reflectance distribution for Carboniferous telovitrinite which has been supplemented by analysis results obtained from US coals. The general trend is similar to the one displayed by the Permian telovitrinite, but with maximum values of only 9.3% 1 650 wt, the Carboniferous fluorescence intensity levels are considerably lower. Since the analyses were carried out in Australia, this may be due, to some extent, to the difficulty of obtaining fresh samples from overseas. However, even in cases where fresh underground coals

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for P e r m i a n i n e r t i n i t e .

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%rt:/o Fig. 6. Fluorescence/reflectance distribution for Carboniferous inertinite.

have been analyzed, such as those represented by the cluster of points near the peak at 1% Ro~t, the fluorescence intensities are significantly lower than the respective Permian values displayed in Fig. 3. The correlation between mean fluorescence intensity and mean random inertinite reflectance Mean fluorescence/reflectance distributions have also been established for Permian and Carboniferous inertinite. As discussed by Diessel and McHugh (1986), this maceral group represents a statistical problem because of the highly skewed fluorescence/reflectance distribution. Fluorescence values above 1.8 to 2% inertinite reflectance approach and remain at zero, although, with increasing rank, reflectance continues to climb. The mean inertinite fluorescence intensities illustrated in Fig. 5 for Permian and in Fig. 6 for Carboniferous coals are thus based on the fluorescent portion of inertinite (1 650 wri > 0.5% ) only. The trends shown by the vitrinite group are repeated by inertinite at lower intensity levels. Apart from one exception-

96 ally high value, Permian inertinite fluoresces more strongly than the Carboniferous inertinites studied in this project. Nevertheless, anomalously high values, as have been reported for Carboniferous vitrinites from the German Saar Basin (Wolf et al., 1985; Diessel and Wolff-Fischer, 1986) will probably be found when more such coals are studied in detail.

Correlations between individual measurements of fluorescence intensity and reflectance in inertinite Point-by-point correlations of fluorescence intensity and random reflectance were made of a large number of inertinites in all coal samples. The results, one of which is illustrated in Fig. 7, demonstrate the existence of an inverse relationship between fluorescence intensity and reflectance, as well as a dependence of the fluorescence values on the rank of the host coal. When plotted, the data distribution forms exponential curves in all cases, such that after logarithmic transformation linear regression can be applied. Both slope and intercept values display a systematic relationship to coal rank. The rank/intercept relationship, which is particularly close, is illustrated in Fig. 8. The intercept values increase quite strongly up to the rank of medium-volatile bituminous coal, after which the curve flattens and the relationship begins to deteriorate. As has been suggeste previously (Diessel and McHugh, 1986), the reason for this trend can be attributed to progressing coalification, whereby the thermal cracking of intermicellar polymerizates leads to the development of fluorescing bitumens, whilst the condensation of aromatic clusters results in an increase in reflectance. As long as the porportion of bitumen in the coal increases with coalification, both fluorescence and reflectance continue to rise in regular fashion resulting in a concomitant increase in intercept values. However, above the rank of medium-volatile bituminous coal the two trends diverge, reflectance continues to rise whereas fluorescence declines and extinguishes in semianthracite, due to the progressing thermal cracking and degassing of the bitumen. A marked reduction of the regression coefficients in the destruction phase is not possible because reflectance keeps on increasing. Since the data base for both inertinite reflectance and fluorescence values narrows with increasing rank, the correlation between the two deteriorates.

The distribution of inertinite reflectance in relation to coal rank The various macerals of the inertinite group cover a wide range of reflectance values which vary with the rank of the host coal. The reflectance distribution indicates that inertinite reflectance is highly skewed towards low values (Fig. 7), i.e. most inertinite macerals display a reflectance not much above that of the associated vitrinite. A systematic relationship between such parameters as

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98

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99 mean, median, or modal inertinite reflectance and coal rank has been found in all samples. Figure 9 is an example of the correlation between the modes of the inertinite reflectance distribution and mean random reflectance of telovitrinite. Permian and Carboniferous coals have been grouped together because no significant differences have been found in our samples. In Fig. 10 mean random telovitrinite reflectance has been correlated with mean random inertinite reflectance, which gives a similar distribution. The illustrations demonstrate that with increasing coal rank both mean inertinite reflectance and mode values increase likewise, but at a lower rate, such that the gap between mean vitrinite and mean inertinite reflectance narrows. An example of this relationship is illustrated in Fig. 11. Although the spread of data points is large, it is clear that during coalification the ranges of inertinite reflectance decrease because the high values remain fairly constant as the low values are being pushed forward. DISCUSSION OF THE RESULTS - COKE Coke composition

There is no u n a n i m i t y about the m a n n e r in which the microscopic composition of coke should be analyzed and represented, but it is generally agreed that coke textures can be broadly divided into: (a) fused matrix (continuous phase) ; (b) organic and inorganic inclusions (discontinuous phase) ; and ( c ) pores and fissures. It is also agreed that a close link exists between the size and shape distribution of anisotropy domains in the fused coke matrix and the rank of the coal charge. High-volatile bituminous coals produce a coke, in which the fused matrix is statistically isotropic on account of the submicroscopic sizes of pregraphitic crystallites, which are responsible for the anisotropy pattern. With increasing coal rank larger crystallites are formed during carbonization, resulting in likewise larger anisotropy domains. In the case of coal blends the individual ranks of the contributing coals are still recognizable by their specific optical properties, which suggests that little mixing appears to occur during the plastic stage. Coin and Hall (1986) defined a Coke Mosaic Size Index (CMSI) whilst Diessel and Wolff-Fischer (1986) used the Anisotropy Quotient (AQ) to quantify the relationship between coal rank and the microtexture of the fused coke matrix. Both characterize the same properties and, similarly to the latter, can be expressed as: CAQ= (a+2b+3c+4d+

... n m ) / ( a + b + c + d +

... m )

(10)

There is an excellent correlation between CAQ ( coke anisotropy quotient) and

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Fig. 12. The correlation between the anisotropy Quotients of the fused coke matrices and mean random telovitrinite reflectance of the feed coal.

coal rank as is shown in Fig. 12. Permian and Carboniferous coals have been treated together, because no significant differences between the two have been found. Considering that the coke samples used in the paper have been produced in different countries and by different means, the close correlation between CAQ and coal rank suggests that the method is a robust and powerful tool in coke characterization.

The di[ferences between estimated and determined coke composition In the discussion of the analysis methods it has been mentioned that the predicted coke yields agree well with the actually obtained results. However, when the main coke types are compared ,systematic differences between the estimated and determined results occur. The correlation between the estimated and determined percentages of fused coke, FCM (E) and (D), displayed in Fig. 13, demonstrates that the amount of fused coke matrix has been consistently underestimated in the mass balance calculations. The systematic difference between predicted and actually determined results is even more obvious in Fig. 14, in which estimated and determined percentages for unfused coke inclusions, UFCI (E) and ( D ), have been correlated. The differences between the predicted and determined analysis results cannot be explained by either analytical and statistical errors, or the application of wrong conversion factors. The first two aspects are responsible for much of the spread of the results to which six different types of analyses have contributed. These are: ( a ) Maceral analyses of the feed coal; ( b ) Reflectance determinations ( > 500 points ) on all macerals in feed coal;

101

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(c) Proximate analyses of feed coals; ( d ) Modal ( micro-textural ) analyses of cokes; (e) Determination of coke yield; (f) Proximate analyses of cokes. It is unlikely that a combination of the above analyses could produce the kind of systematic error accounting for the observed differences. The conversion factors can also be excluded as a source of any major error, because tests have shown that a reduction of the differences by changing the conversion factors would lead to quite unrealistic assumptions about such properties as maceral densities and rates of degassing. THE THERMALDISSOCIATIONOF INERTINITE The conclusion from the above discussion is that in all cokes analyzed more fused matrix has been formed than could be expected on the basis of the proportion of vitrinite and liptinite in the feed coal. On the other hand, all coke

102 TABLE 1 The carbonizationproducts (in %) of 100% Carboniferous (C = 6 samples) and Permian (P = 6 samples ) inertinite on the basis of mass balance calculations Median

FCM PFCI UFCI Gas, Liquor

Mean

Std. dev.

Std. error

C

P

C

P

C

P

C

P

15.0 20.0 51.5 12.0

21.0 20.0 42.5 17.5

15.3 20.8 49.8 14.0

18.7 19.0 44.3 18.0

3.14 6.76 9.70 4.82

5.85 3.10 4.97 2.61

1.28 2.76 3.96 1.97

2.39 1.26 2.03 1.07

samples contain fewer unmolten inclusions of inertinite t h a n should have been present had all the inertinite of the feed coal been truly inert. The discrepancy between the excess in fused coke matrix and the deficiency in inertinite-derived coke inclusions can only be explained by a complete fusion of some inertinite. However, a comparison of Figs. 13 and 14 demonstrates that the difference between the porportion of estimated and determined unfused coke inclusions is greater t h a n the difference between estimated and determined fused coke matrix ( i.e. the amount of inertinite-derived fused coke ). The explanation is t h a t inertinite-derived coke inclusions have been divided into two groups, one which is unfused and optically isotropic, and another one which is partly fused and anisotropic. Based on present and previous results (Diessel and Wolff-Fischer, 1986) the dissociation of inertinite during carbonisation can be presented as in Table 1. In order to reduce the effects of analytical and statistical errors, only coals with an inertinite content exceding 30% have been used in the construction of the table. The results show quite clearly t h a t on average less t h a n one half of the inertinite contained in a coking coal is seemingly unaffected by carbonisation, and even t h a t portion is subjected to some degassing. There is a notable difference between the Carboniferous and Permian samples suggesting t h a t the Permian inertinite is more fusible t h a n its Carboniferous equivalent. However, the data sets are small and further work is required to elucidate this interesting question. The varying degrees of coalification of the feed coal have not been considered in Table 1 because both sets of coals cover similar rank ranges. THE QUANTITATIVE ASSESSMENT OF FUSIBLE AND PARTLYFUSIBLE INERTINITE BY FLUORESCENCEMICROSCOPY The term "inertinite" was introduced into coal petrography by Stach (1952) in order to group together all coal macerals which were t h o u g h t to be largely infusible during carbonization. This assumption was based on the observation of almost unaltered inclusions of fusinite, semifusinite and macrinite in oth-

103 erwise fused coke. However, subsequent work of many authors, among them Ammosov et al. (1957, 1959), Schapiro et al. (1961), Schapiro and Gray (1964), Harrison et al., 1964), Okuyama et al. (1970), Kimura and Miyazu (1972), Kojima (1976), Steyn and Smith (1977), Mackowsky (1977), Duguid (1980), Keirnan and Handley (1980), Pearson (1980), Coin (1980), Diessel (1982, 1983), Brown N. et al. (1982), Brown K. et al. (1982), Pearson and Price (1985), Kosina and Heppner (1985), Wolf et al. (1985), Coin and Hall (1986), Diessel and McHugh (1986), Diessel and Wolff-Fischer (1986), has either suggested or demonstrated that a substantial portion of the inertinite in a coal charge undergoes either complete or partial fusion. However, there is substantial disagreement in the literature about the actual levels of inertinite reactivity. It is suggested that variations in carbonization procedures and the employment of different analytical and computational techniques are only partly responsible for the disagreements. Regional differences in carbonisation behaviour of Carboniferous coals (e.g. Ruhr versus Saar Basin) have been reported by Wolf et al. (1983), even when vitrinite reflectance and petrographic composition (both macerals and microlithotypes) are very similar. The carbonization properties are also known to drop significantly under conditions of oxidation too mild to be detected by conventional petrographic techniques (Diessel et al. 1984; McHugh, 1986). The figures presented in Table 1 are therefore only average values from which significant deviations can occur in individual coals, which cannot be explained with the techniques of conventional coal petrology. In order to satisfy the considerable commercial need for some guidance concerning the fusibility of inertinite, not merely in general terms but also in specific cases, new analytical methods will have to be employed. The adoption of Coin and Hall's (1986) Coke Mosaic Size Index or Diessel and Wolff-Fischer's (1986) Coke Anisotropy Quotient as a quality parameter are two such possibilities. Another one is the use of fluorescence intensity measurements as a means of determining the proportion of fusible, partly fusible and infusible macerals in the coal. The possibility to use fluorescence intensity as an indicator of maceral fusibility is based on the former's relationship to coal rank. The fluorescence intensities illustrated in Figs. 3-6 mirror quite closely the distribution of caking properties in bituminous coals. This proportionality to the ability of the coal to form a high-quality coke renders fluorescence superior to vitrinite reflectance which continues to rise, even when caking capacity diminishes. Following the initial work by Diessel (1985), Diessel and McHugh (1986), Diessel and Wolff-Fischer (1986) the mass fractions of inertinite products obtained from mass balance calculations were correlated with inertinite reflectance distribution curves, as well as with fluorescence distribution curves (see example in Fig. 7) in order to obtain the fluorescence cut-offs for fusible and partly fusible, as well as for partly fusible and infusible inertinite. This was done by taking account of the existence of an inversely proportional

104 5.0 ~+.5 /+.0 +5

3.5 3.0 - - n - - I - o

•

2.5

--I

~11 .1_11- -

~

•

__

--

~

J

•

•

•

l

•

.

.

.

-m m

+6~

.

mr_m --6~

mm

-6

2.0 J

1.5 1.00.50

Observations

Fig. 15. The distribution of fluorescenceintensity values obtained for the boundary between fusible and partly fusible inertinite (LF cut-off FCM=lower fluorescence cut-off for fused coke matrix) in Permian and Carboniferouscoals. 32 2.5 u_

--

1.5

+6

•

2.0

"G

•

__

•

__

• .i

• • ~. • 7 , - -. .

n

D

.-.-'-~- = - m - =

..

*SR

-6~ -5

1.O 0.5

Observations

Fig. 16. The distribution of fluorescenceintensity valuesobtained for the boundarybetweenpartly fusible and infusible intertinite (UF cut-off UFCI =upper fluorescencecut-off for unfused coke inclusions) for Permian and Carboniferouscoals. relationship between inertinite reflectance and fusibility. The results are illustrated in Figs. 15 and 16. Both Permian and Carboniferous coals have been treated together because, as shown by the statistical information given in Table 2, no significant differences could be found between the two sets of coals. The results support the notion t h a t fusibility a n d / o r infusibility cut-offs based on fluorescence intensity values can be applied without reference to coal rank which is not possible when cut-off values are based on reflectance. With the experimental conditions used in this project cut-off values of 3% relative fluorescence intensity for the F C M / P F C I boundary and of 1.5% for the P F C I / U F C I boundary appear to be reasonable assumptions when taking into account the various statistical averages listed in Table 2.

105 TABLE 2 The fluorescence cut-offs for fusible, partly fusible and infusible inertinite in 20 Carboniferous (C) and 10 Permian (P) coals Lower fluorescence Cut-Off for FCM

Mode Mean Std. deviation Std. error Count

Upper fluorescence Cut-Off for UFCI

C

P

C

P

3 3.14 0.801 0.179 20

3 2.99 0.367 0.116 10

1.5 1.70 0.469 0.105 20

1.5 1.56 0.306 0.097 10

PRACTICAL APPLICATIONS T h e f l u o r e s c e n c e - b a s e d c u t - o f f v a l u e s listed a b o v e do n o t m e r e l y divide inert i n i t e i n t o t h r e e c a t e g o r i e s o f v a r y i n g fusibility b u t t h e y also allow t h e fusibility o f all coal m a c e r a l s to be a s s e s s e d w i t h o u t a n y r e f e r e n c e to e s t a b l i s h e d coal p e t r o g r a p h i c n o m e n c l a t u r e . T h i s n o t i o n p r e s u p p o s e s t h a t coal c o m p o n e n t s of equal f l u o r e s c e n c e r e a c t s i m i l a r l y to c a r b o n i z a t i o n i n d e p e n d e n t o f t h e i r a c t u a l m a c e r a l grouping. D i f f e r e n t m a c e r a l s of equally high f l u o r e s c e n c e do n o t nece s s a r i l y f o r m i d e n t i c a l a n i s o t r o p y p a t t e r n s b u t t h e y will fuse j u s t t h e s a m e . T h i s c a n n o t be said o f m a c e r a l s s h a r i n g t h e s a m e r e f l e c t a n c e . F o r e x a m p l e , a s e m i f u s i n i t e w i t h a r e f l e c t a n c e of 1.5% c o n t a i n e d in a h i g h - v o l a t i l e coal posTABLE3 A comparison between estimated and determined coke stabilities in relation to coal rank and the proportion of fusible and partly fusible macerals

Rort

Romt

Est. fusibles

A.S.T.M. stab. (E)

A.S.T.M. stab. (D)

1.21 1.31 1.31 1.33 1.51 0.98 1.12 0.99 0.78 1.22 1.18 1.15 0.84

1.28 1.39 1.39 1.41 1.60 1.04 1.19 1.05 0.82 1.29 1.25 1.22 0.89

69.4 83.7 78.9 83.2 83.1 74.8 71.2 74.6 88.2 70.0 58.0 64.5 84.2

62 67 66 68 62 56 60 57 20 61 60 54 35

63.5 68.5 68.3 67.9 55.4 56.3 55.0 55.0 21.3 60.2 60.1 53.7 35.2

106 sesses a lower fluorescence and fuses less readily than a similarly reflecting fusinite contained in a medium-volatile bituminous coal (Diessel and McHugh, 1986), and both behave differently to a vitrinite of the same reflectance. The good correlation between predicted and determined coke stabilities given in Table 3 supports the notion that the fluorescence-based estimation of the degree of fusibility of a coal yields realistic results. CONCLUSIONS Our results suggest that in coals of coking quality only approximately one half of the inertinite does not fuse when subjected to conventional carbonisation conditions. Inertinite fusibility appears to be slightly better in the Permian coals tested but a total of only 6 Permian and 6 Carboniferous coals of sufficiently high inertinite concentration might not be enough to yield a conclusive answer. However, the higher fluorescence intensities generally found in the Permian coals of Australia, plus the fact that these coals are capable of producing high-quality metallurgical coke, even when containing considerable proportions of inertinite, support the notion that their ability to cake is better than that found in carboniferous coking coals of similar rank and type. Because the latter commonly contain only small proportions of inertinite and are often generously endowed with vitrinite, their comparatively lower reactivity is not easily recognized. Conventional methods of coke stability estimates (e.g. the Schapiro-Gray System) assume that 30% of the semifusinite display fusibility in Carboniferous coals, which is less than has been found by our mass balance calculations. It should also be stressed that our findings cannot support the assumption of a simple and fixed demarcation between fusible and infusible inertinite. The results suggest that on carbonisation inertinite yields at least four products, only two of which can be readily recognized in the resulting coke. These are seemingly unfused and partly fused coke inclusions. The other two products consist of fused coke matrix, and a mixture of gas, tar and other fluids which balance the mass deficiency between inertinite in the feed coal and its solid residue in the coke. The most promising petrography-based quality test to be applied to coal is the determination of the proportions of the coal likely to be fusible, partly fusible and infusible by fluorescence microscopy. With the equipment used in this project the boundaries between the above fractions correspond to a relative fluorescence intensity of 3% and 1.5%, respectively. Because of the relativity of fluorescence intensity measurements, the use of different instrumentation is likely to yield different results. Standardization of analysis techniques is therefore urgently required. The main advantage over any reflectance-based cut-off values is the close correspondence of the fluorescence intensity distribution of coking coal ma-

107 cerals to t h e c a k i n g a b i l i t y o f t h e h o s t coal. B y u s i n g o u r c u t - o f f v a l u e s this m e a n s t h a t all m a c e r a l s w i t h a f l u o r e s c e n c e i n t e n s i t y a b o v e 3% 1 650 w are likely to fuse in t h e c o k e o v e n , b e t w e e n 3% a n d 1.5% t h e y will u n d e r g o l i m i t e d fusion, w h e r e a s t h e r e s t r e m a i n s infusible. N o r e f e r e n c e to a n y m a c e r a l t y p e is n e c e s s a r y a n d coal b l e n d s c a n be t r e a t e d as well as single s e a m coals. ACKNOWLEDGEMENTS F i n a n c i a l s u p p o r t for t h e w o r k r e p o r t e d h e r e h a s b e e n received f r o m t h e Commission of the Europian Community and the Australian National Energy R e s e a r c h D e v e l o p m e n t a n d D e m o n s t r a t i o n P r o g r a m , P r o j e c t No. 85/5141, w h i c h is g r a t e f u l l y a c k n o w l e d g e d . I n t h e p r e p a r a t i o n of t h e m a n u s c r i p t use has b e e n m a d e of m a t e r i a l originally i n c l u d e d in t h e c o m p l e t i o n r e p o r t of t h e a b o v e m e n t i o n e d N E R D D C p r o j e c t ( D i e s s e l et al., 1987).

REFERENCES Ammosov, I.I., Eremin, I.V., Sukhenko, S.I. and Oshurkova, I.S., 1957. Calculation of coking charges on the basis of petrographic characteristics of coals. Koks Khim., 12:9-12 (in Russian). Ammosov, I.I., Sukhenko, S.I., Eremin, I.V. and Oshurkova, I.S., 1959. The calculation of coke oven charges on the basis of petrographic composition of coals. Tr. Inst. Goryuch. Iskop. Moscow, 8:21-30 (in Russian). Brown, K., Diessel, C.F.K. and McHugh, E.A., 1985a. New developments in the fluorometry of coking coals. Adv. Stud. Sydney Basin, 18th Newcastle Symp., Proc., pp. 87-90. Brown, K., Diessel, C.F.K., McHugh, E.A., Wolf, M. and Wolff-Fischer, E., 1985b. The fluorescence properties of Carboniferous and Permian coking coals. Int. Conf. on Coal Science, Sydney, Proc., pp. 649-652. Brown, N.A., Coin, C.D.A. and Gill, W.W., 1982. Prediction of coke quality. BHP Tech. Bull., 26: 27-30. Coin, C.D.A. and Hall, K.N., 1986. Coke microtextures as a more objective means of assessing coking coals. BHP-CRL Report 10/86, 33 pp. (unpubl.). Diessel, C.F.K., 1982. On the reactivity of inertinite macerals in coal during carbonisation. Univ. of Newcastle, Dept. of Geology, Res. Rep. No. 1, 47 pp. Diessel, C.F.K., 1983. Carbonisation reactions of inertinite macerals in Australian coals. Fuel, 62: 883-892. Diessel, C.F.K., 1985. Fluorometric analysis of inertinite. Fuel, 64: 883-892. Diessel, C.F.K., 1986. Fusible and non-fusible components in coal. Aust. Coal Sci. 2, Proc., pp. 327-332. Diessel, C.F.K. and McHugh, E.A., 1986. Fluoreszenzintensit~it und ReflexionsvermSgen von Vitriniten und Inertiniten zur Kennzeichnung des Verkokungsverhaltens. Gltickauf-Forschungsh., 47: 60-70. Diessel, C.F.K. and Wolff-Fischer, E., 1986. Vergleichsuntersuchungen an Kohlen und Koksen zur Frage der Inertinitreaktivitiit. Gltickauf-Forschungsh., 47:203-211. Diessel, C.F.K., Brown, K. and McHugh, E.A., 1986. Coal characteristics by vitrinite and inertinite fluorescence. NERDDP Project No. 831, Completion Report. Diessel, C.F.K., Brown, K. and McHugh, E.A., 1987. Coal characterisation by vitrinite and inertinite fluorescence. NERDDP Project No. 85/5141, Completion Report.

108 Duguid, K., 1980. Reactive semifusinite in coking coals. World Coal, 6: 19. Dyrcacz, G.R., Bloomquist, C.A.A. and Rucuc, L., 1984. High resolution density variations of coal macerals. Fuel, 63: 1367-1373. Gray, R.J., 1976. A system of coke petrography. Ill. Mining Inst., Proc., pp. 20-47. Harrison, J.A., Jackman, H.W. and Simon, J.A., 1964. Predicting coke stability from petrographic analysis of Illinois coals. Ill. Geol. Surv. Circ., 366 pp. Jakob, H., 1952. Fortschritte auf dem Gebiet der Braunkohlen-Luminiszenz-Mikroskopie. Bergakademie, 4: 337-347. Jakob, H., 1964. Neue Erkenntnisse auf dem Gebiet der Luminiszenz-Mikroskopie fossiler Brennstoffe. Fortschr. Geol. Rheinld. Westfalen, 12: 569-588. Keirnan, P. and Handley, K., 1980. Prediction of coke strengths from coal petrographic properties. A.C.I.R.L.-P.R.80-6. Kimura, H. and Miyazu, T., 1972. Coal petrology and its applications. Tetsu To Hagane, 58: 158-179. Kojima, K., 1976. Application of coal petrology to coke making. Coke Circular, 25: 1-4. Kosina, M. and Heppner, P., 1985. Macerals in bituminous coals and the coking process. 2. Coal mass properties and coke mechanical properties. Fuel, 64: 53-58. Krevelen, D.W. van, 1961. Coal. Elsevier, Amsterdam - London - New York, 314 pp. KrSger, C. and Pohl, A., 1957. Die physikalischen und chemischen Eigenschaften der Steinkohlengefiigebestandteile (Macerale). III Das Entgasungsverhalten. Brennst.-Chem., 38: 102-107. Mackowsky, M.-Th., 1977. Prediction methods in coal and coke microscopy. J. Microsc., 109: 119-137. McHugh, E.A., 1986. The influence of oxidation on the fluorescence properties of coking coals. Adv. Stud. Sydney Basin, 20th Newcastle Symp., Proc., pp. 66-68. Okuyama, Y., Miyazu, T., Sugimura, H. and Komazai, M., 1970. Prediction of the coking property of coal by microscopic analysis. J. Fuel Soc. Jpn., 49: 736. Ottenjann, K., 1980. Spektrale Fluoreszenz-Mikrophotometrie von Kohlen und Olschiefern. Leitz Mitt. Wiss. Techn., 7: 262-272. Ottenjann, K., 1982. Verbesserungen bei der mikrophotometrischen Fluoreszenzmessung an Kohlenmazeralen. Zeiss Inform., 26: 173-179. Pearson, D.E., 1980. The quality of Western Canadian coking coal. CIM Bull., Jan.: 70-84. Pearson, D.E. and Price, J.T., 1985. Reactivity of inertinite (coal-typing) of Western Canadian coking coal. Int. Conf. on Coal Science, Sydney 1985, Proc., pp. 907-908. Schapiro, N. and Gray, R.J., 1964. The use of coal petrography in coke making. J. Inst. Fuel, 37: 234-242. Schapiro, N., Gray, R.R. and Eusner, G.R., 1961. Recent developments in coal petrography. Blast Furn., Coke Oven Raw Mater., Proc., 20: 89-112. Stach, E., 1952. Die Vitrinit-Durit Mischungen in der petrographischen Kohlenanalyse. Brennst.Chem., 33: 368. Steyn, J.G.D. and Smith, W.H., 1977. Coal petrology in the evaluation of South African coals. Coal, Gold and Base Minerals, pp. 1-25. Teichmiiller, M., 1982. Fluoreszenz von Liptiniten und Vitriniten in Beziehung zu Inkohlungsgrad und Verkokungsverhalten. Geol. Landesamt Nordrhein-Westfalen, pp. 1-119. Wolf, M., Wolff-Fischer, E., Ottenjann, K. and Hagemann, H.-W., 1983. Fluorescence properties of vitrinites of selected Saar and Ruhr coals. Prepr., 36th ICCP meeting, Comm. 3, Oviedo, 14 pp. Wolf, M., Hagemann, H.-W., Ottenjann, K., Piittmann, W. and Wolff-Fischer, E., 1985. Fluoreszenzeigenschaften von Vitriniten ausgewiihlter Saar- und Ruhrkohlen. 45th meeting Coal Petrol. Working Gr., Essen.