Variations in coal maceral chemistry with rank advance in the German Creek and Moranbah Coal Measures of the Bowen Basin, Australia, using electron microprobe techniques

International Journal of Coal Geology 63 (2005) 117 – 129 www.elsevier.com/locate/ijcoalgeo

Variations in coal maceral chemistry with rank advance in the German Creek and Moranbah Coal Measures of the Bowen Basin, Australia, using electron microprobe techniquesB Colin R. WardT, Zhongsheng Li, Lila W. Gurba1 School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, 2052, Australia Received 1 January 2004; received in revised form 1 January 2005; accepted 6 February 2005 Available online 19 April 2005

Abstract Variations in the elemental composition of individual macerals in seams from the Permian German Creek and Moranbah Coal Measures in the Bowen Basin of Queensland have been studied over a wide range of coal ranks, using light-element electron microprobe techniques, to establish the coalification tracks of key macerals in a single coal-bearing interval from subbituminous through bituminous coal to anthracite. Vitrinite reflectance (Rvmax) in the seams studied increases from 0.39% in the western part of the basin to over 3.5% in the east, apparently due to increases in burial depth. The study extends significantly the rank range covered by previous work on elemental analysis of individual macerals in the Gunnedah Basin, and provides a more useful basis than whole-coal analysis to evaluate the performance of coals in different utilisation processes. The microprobe results show that the carbon content of the telocollinite increases dramatically from 66% to 90% as the vitrinite reflectance of the coals (Rvmax) increases from 0.39% to around 1.75%, but increases only slightly, from 90% to 91%, as Rvmax increases from 1.75% to 3.52%. Oxygen decreases from around 26% to approximately 5% as Rvmax increases from 0.39% to around 1.75%, and then decreases only very slightly into the anthracite range. The nitrogen content of the telocollinite in these coals also appears to decrease slightly with rank advance, and appears moreover to display a relatively abrupt drop at around 2% Rvmax. This may be associated with the development of ammonium illite in the mineral matter. Organic sulphur in the telocollinite, on the other hand, seems to remain essentially constant with rank advance, at least in this particular succession. In contrast to vitrinite, fusinite and inertodetrinite have significantly higher but somewhat more constant carbon contents, varying only from around 81% to 93% C over the rank range studied. Oxygen in these macerals decreases from around 12% to a little over 2% with the same degree of rank advance. Sulphur and possibly nitrogen also appear to be significantly lower in fusinite and inertodetrinite than in the vitrinite of the same coal samples. Semifusinite is somewhat more variable in

B

Paper presented at the 20th Annual Meeting of the Society for Organic Petrology, Washington, DC, September 2003. T Corresponding author. Fax: +61 2 9385 1558. E-mail address: [email protected] (C.R. Ward). 1 Present address: Cooperative Research Centre for Coal in Sustainable Development, Kenmore, 4069, Australia.

0166-5162/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2005.02.009

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composition, with characteristics intermediate between those of the fusinite/inertodetrinite and those of the vitrinite in the same coal over the rank range studied. D 2005 Elsevier B.V. All rights reserved. Keywords: Maceral chemistry; Coalification; Electron microprobe; Carbon; Nitrogen; Sulphur; Permian; Queensland

1. Introduction Ultimate analysis has long been one of the fundamental types of chemical analysis carried out on coal samples, providing data on the proportions of carbon, hydrogen, oxygen, nitrogen and sulphur in the coal for a range of classification and utilisation purposes. Such data, however, are gathered by analysis of bwhole-coalQ samples, embracing moisture and mineral matter as well as the organic constituents, and corrections need to be made to those data if the composition of the organic matter alone needs to be evaluated. The composition of the organic matter, obtained from ultimate analysis data corrected to a moist, mineral matter free (mmmf), dry, mineral matter free (dmmf) or dry, ash free (daf) basis, is often taken as an indicator of coal rank (e.g. ASTM, 1997). However, the composition of the organic matter determined in this way inherently represents an aggregation of the composition of the different maceral components, and thus variations in chemical composition from ultimate analysis data reflect variations in the coal type (i.e. the mixture of macerals present) as well as the rank of the coals concerned. Although a significant amount of data are available on the chemical composition of Australian coals based on ultimate analysis data (e.g. Joint Coal Board and Queensland Coal Board, 1987; Maher et al., 1995), very little information is available on the chemical composition of the individual macerals within those coals. The difference, however, may be significant in better understanding the chemical mechanisms and dynamics of carbonisation and combustion processes; coal is not a homogeneous solid, and it is the individual macerals within the coal that react, both independently and with each other, during processes such as coking and electric power generation. A better knowledge of maceral chemistry may therefore provide important insights into the coalification process, allowing the changes in the individual constituents of the coal to be studied separately. It

may also be of value to understanding the processes associated with coal utilisation, including factors such as burning rate, emission release, CO2 generation, fouling and slagging, as well as reactions during gasification and coking associated with the different coal components. It is inherently difficult to isolate cleanly the individual macerals in a coal for separate chemical analysis, without contamination by minerals or other organic components. However, the recent development of special techniques for light-element analysis using the electron microprobe (e.g. Bustin et al., 1993, 1996; Mastalerz and Gurba, 2001) provides an opportunity for directly determining the elemental composition of the individual macerals in coal polished sections, by analysing areas only a few micrometres in size. These techniques have been applied to maceral studies in several Australian and North American coals, including high volatile bituminous coals from the Gunnedah Basin (Ward and Gurba, 1999; Gurba and Ward, 2000) and a series of coals from Canada (Mastalerz and Bustin, 1993; 1997), but only in a limited way to other coal basins. They have not, moreover, been applied to sequences in which coal of essentially the same age and depositional environment ranges from subbituminous to anthracite in rank. The coals of the Bowen Basin in central Queensland cover a wide rank range, with vitrinite reflectance (Rvmax) ranging from 0.35% to over 3.5% due mainly to variations in burial depth (Beeston, 1995). This represents a wider range of rank variation than that found in many other coal basins, and as such provides an opportunity to evaluate the variations in maceral chemistry with rank for a series of coals with a similar geological age and depositional setting. Since the Bowen Basin provides a large part of Australia’s black coal production, better knowledge of the constitution of these coals at the maceral scale is also of significance to different aspects of their marketing and use.

C.R. Ward et al. / International Journal of Coal Geology 63 (2005) 117–129

2. Sampling and methodology A series of coal samples from the German Creek and Moranbah Coal Measures, two laterally equivalent Late Permian successions occurring respectively in the central and northern parts of the Bowen Basin (Mallett et al., 1995), was made available for the study by the Queensland Department of Natural Resources and Mines (Table 1), from boreholes drilled by the Department as part of its regional exploration programs. The samples, taken from sites across the basin (Fig. 1), have vitrinite (telocollinite; equivalent to colotellinite in the classification by ICCP, 1995) reflectance values ranging from 0.39% to 3.52% (Table 1). Petrographic data for these samples, based on optical microscopy, have been published separately by Beeston (1978, 1981, 1995). Several other samples of Bowen Basin coal, including some from the overlying Late Permian Rangal Coal Measures (Mallett et al., 1995), were also included for comparison in the sample set. Coarsely crushed (b 5 mm) samples of each coal were prepared as polished sections in the same way as grain mount samples for optical microscopy, and coated with carbon for electron microprobe analysis as described by Bustin et al. (1993). Individual points on the various macerals in each coal were analysed using a Cameca SX-50 electron microprobe equipped

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with the Windows-based SAMx operating system and interface software. The accelerating voltage for the electron beam was 10 kV and the filament current was 20 nA, with a magnification of 20,000 giving a beam spot size on the sample of around 5 to 10 Am in diameter. As discussed by Bustin et al. (1993), an independently analysed anthracite sample was used as the standard for carbon in the analysis process. Mineral samples supplied with the instrument were used as standards for the other elements. Calibration data for the various elements, including spectral lines and analysing crystals, are given in Table 2. Other details of microprobe procedures for coal macerals are given by Bustin et al. (1993, 1996) and Mastalerz and Gurba (2001). The percentages of carbon, oxygen, nitrogen, sulphur, silicon, aluminium, calcium and iron were measured for each point, with a note on the type of maceral represented in each case. The results of the individual analyses were tabulated in spreadsheet format. Although care was taken to analyse only bcleanQ macerals and avoid areas where visible minerals were also present, the area analysed for some points unavoidably included mineral components (e.g. quartz, clay, pyrite) as well as the organic matter. Points that apparently included mineral contaminants (e.g. points with high Si or unexpectedly high Fe and S percentages) were

Table 1 List of coal samples studied Sample no.

Borehole

Location (Fig. 1)

Depth interval (m)

Sample details

Rvmax (%)

Borehole samples PS 2302 PS 1035 PS 1210 PS 1204 PS 1128 PS 1115 PS 1134 PS 6657 PS 6674 PS 6861

Consuelo NS 3 Emerald NS 53 Emerald NS 85 Talbot NS 75/76 Talbot NS 40 Talbot NS 30 Cairns County NS 33 Wodehouse NS 1 Wodehouse NS 1 Killarney NS 1

1 4 6 8 10 12 14 17 17 18

17.18–18.39 334.32–334.65 176.61–180.05 94.13–97.36 45.97–49.85 221.69–224.21 265.44–267.43 259.49–260.59 665.34–666.56 306.59–307.72

Bandanna Formation Ply, German Creek Seam German Creek Seam F1.60 German Creek Seam F1.60 German Creek Seam F1.60 German Creek Seam F1.50 German Creek Seam F1.60 Moranbah Coal Measures Moranbah Coal Measures Moranbah Coal Measures

0.39 0.70 0.90 1.10 1.31 1.52 1.75 2.14 2.70 3.52

German Creek Coal Measures Rangal Coal Measures Rangal Coal Measures Rangal Coal Measures Rangal Coal Measures

1.40 0.81 2.10 2.20 2.15

Other coal samples Central Colliery Ensham Nebo West Yarrabbee Baralaba (Coolum seam)

19 20 21 22 23

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148 E 21

147 E

149 E

1.5

MACKAY

150 E

2.0

CORAL SEA

17

N 22 S

3.5

22 S

18 2.0

1.5

Rv max for German Creek and Moranbah Coal Measures

3.5 3.0 2.5

19

CLERMONT

14 12 15, 16 10 13 8 11

23 S

0.5

6

100 km

2.0

7 9

4 5

23 S

22

1.5

1.0

20

EMERALD

BLACKWATER 24 S

24 S

BOWEN BASIN

23

2

MOURA

25 S

SURAT 148 E

1 3

BASIN 149 E

150 E

Fig. 1. Outline map of the Bowen Basin, showing the location of the boreholes sampled (see Table 1) and the lateral variations in vitrinite reflectance for the German Creek and Moranbah Coal Measures over the main area of their development.

excluded from consideration wherever possible. This, however, could not always be achieved. The sample from Central Colliery was found to contain small (b 1%) but roughly equal proportions of Si and Al at many points in apparently bcleanQ vitrinite and semifusinite bands (Table 3), suggesting the presence of clay minerals such as kaolinite intimately admixed with the maceral structures. Exclusion of all points with such contamination would have resulted in a very limited data set to characterise this particular sample. Points that appeared to

include some of the mounting epoxy resin, indicated by unusual oxygen and high nitrogen contents, were also excluded from consideration in the data analysis process. A summary of the elemental composition for the main maceral groups in each sample, after removal of points embracing significant mineral or epoxy contaminants, is given in Table 3. The values given represent the averages of the data from a number of different points on each maceral for each coal sample. Although a few points on liptinite macerals

C.R. Ward et al. / International Journal of Coal Geology 63 (2005) 117–129 Table 2 Elemental calibration parameters and standards used in electron microprobe analysis Element

Line

Crystal

Count time (s)

Standard

C N O Al Si S Ca Fe

Ka Ka Ka Ka Ka Ka Ka La

PC2 PC4 PC4 TAP TAP PET PET PC4

20 20 10 10 10 10 10 10

Anthracite BN Sanidine Sanidine Diopside FeS2 Diopside FeS2

(sporinite and cutinite) were also identified in some of the coals at the lower end of the rank range, these macerals could not be separately identified in the higher rank coal samples.

3. Elemental composition of maceral groups 3.1. Comparison to other chemical data Fig. 2 shows the variation in the organic carbon content of the telocollinite in these and a number of other coal samples, plotted against the respective mean maximum vitrinite (telocollinite) reflectance values. The result is similar to a plot given by Davis (1984) relating carbon content of vitrinite to vitrinite reflectance values, confirming the general consistency of the microprobe analysis results with more general correlations between carbon and vitrinite reflectance data. The samples in Fig. 2 include a series of vitrains separated from several different high-rank coal seams of the United States, provided by Dr. Maria Mastalerz of the Indiana Geological Survey. They also include a number of coals from the Gunnedah Basin of Australia, previously studied by Gurba and Ward (2000). Some of the Gunnedah Basin coals in Fig. 2 plot outside the trend, similar to that indicated by Davis (1984), followed by the other coal samples. As discussed by Gurba and Ward (1998, 2000), the vitrinites in these coals have anomalously low reflectance characteristics in relation to their carbon content, due to marine influence effects. Fig. 3 shows the carbon and oxygen contents of the macerals for each coal in the Bowen Basin series, plotted alongside the carbon and oxygen contents of a

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number of Australian coals (dry ash-free basis) determined by whole-coal ultimate analysis techniques. Although there is a slight departure at the highrank end, for reasons that at this stage are not fully clear, the C and O contents of the individual macerals plot on the same band as the whole-coal analysis values. This again seems to confirm the general consistency of the microprobe technique with independent chemical analysis data. As might be expected, the vitrinite macerals tend to plot in the upper left part of that band, and the inertinite macerals from the same coal in the lower right, indicating broadly the relative contributions of each of these two main maceral groups to the whole-coal carbon and oxygen contents. 3.2. Variations in Bowen Basin macerals with coal rank 3.2.1. Carbon and oxygen The carbon and oxygen contents of the individual macerals in the coals of the present study, based on the microprobe data, are plotted against the vitrinite reflectance of the host coal samples in Fig. 4a and b. As indicated in Table 3 and Fig. 4a, the carbon content of the telocollinite in the Bowen Basin coals increases dramatically from 66% to 90% as the vitrinite (telocollinite) reflectance (Rvmax) increases from 0.39% to 1.75%. In contrast, carbon in telocollinite increases only slightly, from 90% to 91%, as Rvmax increases over the rest of the rank range. Oxygen in the telocollinite decreases from around 26% to approximately 5% as Rvmax increases from 0.39% to 1.75% (Table 3 and Fig. 4b), and then decreases only slightly as rank increases into the anthracite range. Desmocollinite shows a similar variation in carbon and oxygen content to telocollinite, but appears to have a slightly higher proportion of carbon and slightly lower proportion of oxygen relative to the telocollinite in the same coal samples. The difference is very slight and in view of the standard deviations (Table 3) may not be statistically significant, but seems to persist to at least some extent throughout almost all of the rank range; the two vitrinite types only have similar C and O contents in the highest rank coal sample (Rvmax =3.5%). Such a finding is in contrast to earlier work by Ward and Gurba (1999), based on slightly different calibration procedures,

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Table 3 Elemental analysis of maceral groups in coal samples Sample and maceral

No. of points

C%

Al%

Si%

Ca%

Fe%

Mean

St. dev.

Mean

O% St. dev.

Mean

St. dev.

Mean

St. dev.

Mean

Mean

Mean

Mean

PS2302–TC PS2302–DSC PS2302–SF PS2302–FUS PS2302–IND PS1035–TC PS1035–DSC PS1035–SF PS1035–FUS PS1035–IND PS1210–TC PS2310–DSC PS1210–SF PS1210–FUS PS1210–IND PS1204–TC PS1204–DSC PS1204–SF PS1204–FUS PS1204–IND PS1128–TC PS1128–DSC PS1128–SF PS1128–FUS PS1128–IND PS1115–TC PS1115–DSC PS1115–SF PS1115–FUS PS1115–IND PS1134–TC PS1134–DSC PS1134–SF PS1134–FUS PS1134–IND RC6657–TC RC6657–DSC RC6657–SF RC6657–FUS RC6674–TC RC6674–DSC RC6674–SF RC6674–FUS RC6674–IND RC6861–TC RC6861–SF RC6861–FUS RC6861–IND Central–TC Central–DSC Central–SF Central–FUS

15 6 14 13 1 12 3 8 6 3 8 13 12 10 2 10 6 4 7 2 8 2 8 5 2 13 6 7 2 3 17 5 12 6 4 22 3 12 2 29 6 16 6 1 14 8 14 1 11 5 10 8

66.38 69.53 76.80 81.18 86.01 76.43 77.38 82.23 87.85 88.93 78.64 79.90 82.78 85.61 88.41 84.41 85.40 88.89 89.41 93.57 87.19 86.95 90.05 91.62 95.01 87.27 87.90 90.46 92.75 91.69 89.84 90.42 92.33 92.42 93.07 90.69 91.00 92.77 93.88 91.01 92.52 93.61 94.94 94.31 91.05 91.01 92.95 92.06 88.63 87.86 91.39 92.65

1.15 0.93 2.02 3.41

26.25 24.24 16.23 12.58 9.18 15.76 14.56 11.10 6.44 5.81 12.88 12.15 9.75 7.90 5.99 8.42 6.78 5.17 5.16 2.55 4.66 5.06 3.90 3.84 1.37 5.49 5.05 4.20 3.56 3.45 5.35 4.75 3.87 3.51 3.38 4.53 3.94 3.04 2.71 5.18 4.30 2.81 2.36 2.61 5.52 5.10 3.57 3.61 4.89 5.48 4.85 2.28

1.11 1.39 2.01 3.33

2.08 1.65 1.46 0.77 0.58 1.89 1.99 1.74 0.83 0.71 2.42 2.17 1.30 1.34 0.27 2.28 2.05 0.92 0.99 0.45 2.42 2.82 1.17 0.58 0.84 1.88 1.82 0.95 0.84 1.12 1.96 1.82 1.23 1.26 0.85 1.49 1.31 1.08 1.14 1.49 1.35 0.83 0.33 1.26 1.43 1.34 1.04 1.04 1.90 1.72 1.00 1.19

0.70 0.42 0.33 0.35

0.37 0.34 0.19 0.15 0.16 0.74 0.76 0.50 0.28 0.33 0.63 0.64 0.40 0.37 0.31 0.58 0.64 0.36 0.32 0.23 0.68 0.72 0.48 0.31 0.15 0.54 0.51 0.35 0.26 0.27 0.53 0.56 0.43 0.34 0.36 0.72 0.75 0.63 0.47 0.70 0.69 0.51 0.36 0.31 0.84 0.87 0.78 0.28 0.60 0.53 0.38 0.27

0.05 0.07 0.04 0.05

0.06 0.03 0.01 0.01 0.03 0.03 0.03 0.04 0.04 0.04 0.03 0.04 0.03 0.01 0.06 0.02 0.02 0.03 0.02 0.00 0.04 0.05 0.03 0.02 0.00 0.03 0.05 0.02 0.20 0.01 0.03 0.03 0.02 0.01 0.02 0.02 0.03 0.04 0.00 0.03 0.07 0.02 0.05 0.01 0.02 0.01 0.02 0.02 0.36 0.81 0.42 0.07

0.03 0.06 0.02 0.02 0.02 0.04 0.06 0.08 0.05 0.06 0.02 0.04 0.03 0.02 0.04 0.03 0.04 0.03 0.05 0.01 0.07 0.08 0.05 0.04 0.01 0.03 0.09 0.04 0.19 0.02 0.06 0.05 0.03 0.02 0.02 0.05 0.09 0.06 0.02 0.05 0.10 0.03 0.05 0.02 0.04 0.04 0.03 0.02 0.49 1.12 0.52 0.09

0.18 0.21 0.18 0.16 0.21 0.02 0.01 0.09 0.19 0.18 0.01 0.01 0.02 0.06 0.32 0.01 0.02 0.01 0.09 0.08 0.02 0.00 0.02 0.15 0.01 0.01 0.03 0.07 0.27 0.14 0.02 0.02 0.07 0.14 0.14 0.01 0.02 0.01 0.01 0.01 0.01 0.04 0.14 0.26 0.01 0.02 0.01 0.51 0.01 0.00 0.10 0.08

0.06 0.02 0.09 0.02 0.27 0.06 0.03 0.04 0.01 0.00 0.10 0.07 0.02 0.04 0.06 0.04 0.01 0.00 0.07 0.15 0.00 0.05 0.05 0.01 0.04 0.06 0.00 0.01 0.15 0.00 0.01 0.02 0.04 0.07 0.10 0.04 0.07 0.02 0.14 0.04 0.07 0.02 0.04 0.21 0.07 0.04 0.08 0.08 0.04 0.00 0.03 0.09

0.89 1.13 3.70 1.71 0.81 0.77 0.94 1.89 2.58 0.34 1.43 0.77 1.74 0.49 1.05 1.42 0.44 0.73 0.50 1.40 0.41 0.35 1.60 0.61 1.13 0.48 0.85 0.80 0.60 2.59 0.86 0.45 0.34 1.76 0.51 1.30 1.10 1.53 2.13

N%

1.11 1.47 3.56 0.91 0.70 0.78 0.73 1.59 1.79 0.34 0.81 0.51 1.34 0.47 0.93 1.32 0.46 0.35 0.26 1.20 0.32 0.28 0.71 0.17 0.55 0.51 0.55 0.35 1.00 2.21 0.31 0.18 0.40 1.16 0.54 1.09 0.47 1.98 0.36

S%

0.28 0.49 0.87 0.37 0.70 0.51 0.80 0.55 0.62 0.51 0.63 0.84 0.70 0.74 0.95 0.65 0.60 0.84 0.54 0.48 0.54 0.30 0.90 0.52 0.13 0.44 0.35 0.61 0.41 0.56 0.54 0.34 0.34 0.39 0.49 0.86 1.06 0.59 0.70

0.09 0.09 0.15 0.03 0.06 0.09 0.10 0.14 0.09 0.10 0.21 0.04 0.07 0.09 0.11 0.09 0.04 0.07 0.06 0.13 0.05 0.07 0.08 0.07 0.06 0.06 0.07 0.11 0.06 0.04 0.14 0.05 0.06 0.09 0.05 0.06 0.12 0.11 0.09

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Table 3 (continued) Sample and maceral

No. of points

C%

Al%

Si%

Ca%

Fe%

Mean

St. dev.

Mean

O% St. dev.

Mean

N% St. dev.

Mean

S% St. dev.

Mean

Mean

Mean

Mean

Central–IND Ensham–TC Ensham–DSC Ensham–SF Ensham–FUS Ensham–IND Nebo West–TC Nebo West–DSC Nebo West–SF Nebo West–IND Yarrabbee–TC Yarrabbee–DSC Yarrabbee–SF Yarrabbee–FUS Yarrabbee–IND Coolum–TC Coolum–DSC Coolum–SF Coolum–FUS Coolum–IND

3 15 3 13 5 4 20 5 11 4 19 3 6 8 3 17 4 16 4 7

93.98 80.69 81.48 85.50 87.24 92.85 90.51 91.28 92.65 94.72 89.81 91.95 91.72 93.14 93.27 89.84 90.22 90.05 91.86 93.90

0.23 0.92 1.09 1.24 1.11 1.47 0.46 0.32 1.39 0.45 1.14 0.39 1.78 0.51 0.25 1.05 0.22 2.25 1.60 0.86

1.71 12.32 10.66 8.05 6.52 2.61 6.17 5.07 3.49 2.52 4.97 3.11 3.29 2.71 2.73 4.99 4.39 4.05 3.38 2.94

0.09 1.20 1.66 1.17 0.29 0.86 0.86 0.46 0.83 0.26 0.85 0.28 0.74 0.25 0.25 0.88 0.38 0.55 0.22 0.24

0.24 2.57 2.28 1.90 1.33 0.90 1.49 1.42 0.97 0.54 1.57 1.63 1.05 0.73 1.05 1.97 1.68 1.47 0.54 0.51

0.23 0.58 0.23 0.84 0.72 0.70 0.52 0.36 0.80 0.17 0.53 0.57 0.82 0.44 1.01 0.59 0.51 0.42 0.62 0.39

0.15 0.30 0.43 0.22 0.23 0.10 0.64 0.63 0.48 0.21 0.54 0.68 0.51 0.39 0.43 0.44 0.47 0.38 0.41 0.25

0.01 0.14 0.11 0.03 0.02 0.02 0.05 0.08 0.10 0.08 0.25 0.04 0.09 0.04 0.10 0.05 0.04 0.12 0.03 0.05

0.01 0.01 0.06 0.03 0.04 0.01 0.02 0.07 0.05 0.01 0.01 0.02 0.03 0.03 0.01 0.01 0.05 0.03 0.01 0.02

0.03 0.03 0.06 0.04 0.05 0.02 0.03 0.09 0.05 0.00 0.01 0.00 0.04 0.04 0.02 0.04 0.07 0.04 0.01 0.01

0.13 0.02 0.00 0.24 0.05 0.02 0.01 0.01 0.06 0.18 0.02 0.01 0.02 0.04 0.04 0.02 0.02 0.02 0.03 0.15

0.06 0.08 0.06 0.04 0.04 0.00 0.05 0.05 0.02 0.05 0.02 0.02 0.02 0.02 0.00 0.03 0.08 0.05 0.01 0.02

TC = telocollinite; DSC = desmocollinite; SF = semifusinite; FUS = fusinite; IND = inertodetrnite. No. of points = number of points analysed for individual maceral in each sample.

which suggested that there was little if any difference between these two vitrinite components in a series of Gunnedah Basin coal samples.

As pointed out by Ward and Gurba (1999), the microprobe analysis data refer only to the visibly homogenous matrix material in the desmocollinite.

5.5 Bowen Basin

5.0

US vitrains

4.5

Gunnedah non-marine

4.0

Gunnedah marine influenced

Rv max%

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 60

65

70

75

80

85

90

95

100

C% in Telocollinite - Microprobe Fig. 2. Relationship between carbon content of vitrinite (telocollinite) and vitrinite (telocollinite) reflectance in selected coal samples from Australia and the USA.

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Oxygen (wt %)

20

15

Telocollinite Desmocollinite Semifusinite

10

Fusinite Inertodetrinite

5

Ultimate (daf)

0 70

80

90

100

Carbon (wt%) Fig. 3. Carbon and oxygen contents of individual macerals in a range of Australian coals, as determined by electron microprobe, in relation to carbon and oxygen contents for a series of Australian whole-coal samples (daf basis) as determined by conventional ultimate analysis techniques.

Desmocollinite typically contains small inclusions of liptinite and inertinite components (Taylor et al., 1998), which if present would affect the elemental composition of larger desmocollinite masses, separated out by handpicking or density separation techniques. Fusinite and inertodetrinite have significantly higher but more constant carbon contents than the vitrinite macerals in the same coal samples, varying respectively from about 81% and 86% to around 93% C over the rank interval studied (Fig. 4a). Oxygen in these two inertinites decreases respectively from 12% and 9% to a little over 2% with the same degree of rank advance (Fig. 4b). Inertodetrinite appears to have higher proportions of carbon and lower proportions of oxygen than the fusinite in the same coals over the lower part of the rank range, although the difference becomes insignificant at higher rank levels. Semifusinite has carbon and oxygen contents that are intermediate between those of the fusinite/inertodetrinite and those of the vitrinite in the same coal samples. Carbon in semifusinite increases from a little below 77% to around 92%, and oxygen in the same maceral decreases from 16% to 4%, up to a vitrinite reflectance value of around 1.75%. In coals where the vitrinite reflectance is above 1.75%, the carbon content of the semifusinite is more or less constant at 93–94%, and the oxygen content of semifusinite remains fixed at around 3%. Indeed, Fig. 4 shows that the carbon and oxygen contents of all the maceral groups converge to the point where they show only

slight differences between each other in coals with a vitrinite reflectance above 1.75%, and with more than 90% carbon in the telocollinite component. The work of Mastalerz and Bustin (1993), based on a more diverse series of coal samples, showed a similar overall trend, with increasing carbon and decreasing oxygen contents of the different macerals with rank increase from subbituminous coal to anthracite. However, a somewhat different correlation was noted between the carbon and oxygen contents of the individual macerals and the corresponding vitrinite reflectance values in that particular study. As with the present study, Mastalerz and Bustin (1993) reported a relatively wide spread of C and O contents for the macerals at the lower end of the rank range (i.e. in subbituminous and high volatile bituminous coals), but a convergence in maceral composition in medium volatile bituminous coal (Ro = 1.3%) and no distinction at all between vitrinite and fusinite in a high-rank anthracite sample (Ro = 5.15%). 3.2.2. Nitrogen and sulphur Nitrogen and organic sulphur both appear to be significantly lower in the fusinite and inertodetrinite than in the vitrinite of the same coal samples (Table 3), confirming the findings from other coals by Ward and Gurba (1998) for sulphur and Gurba (2001) for nitrogen. Semifusinite generally has nitrogen and organic sulphur contents intermediate between those of the vitrinite and fusinite/inertodetrinite components.

A

C

Carbon

Desmocollinite

80

Semifusinite Fusinite Inertodetrinite

70

1.0

2.0

3.0

Nitrogen in maceral %

Telocollinite

2.0

Desmocollinite Semifusinite Fusinite

1.0

0.0 0.0

4.0

Rv max (telocollinite) %

B

1.0

2.0

D

Oxygen

3.0

4.0

Organic Sulphur 1.00

20

Telocollinite Desmocollinite Semifusinite Fusinite

10

Inertodetrinite

1.0

2.0

Rv max (telocollinite) %

3.0

4.0

Sulphur in maceral - %

Oxygen in maceral %

Inertodetrinite

Rv max (telocollinite) %

30

0 0.0

Telocollinite

0.75 Telocollinite Desmocollinite

0.50

Semifusinite Fusinite Inertodetrinite

0.25

0.00 0.0

1.0

2.0

3.0

C.R. Ward et al. / International Journal of Coal Geology 63 (2005) 117–129

Carbon in maceral %

90

60 0.0

Nitrogen 3.0

100

4.0

Rv max (telocollinite) %

Fig. 4. Variations in C, O, N and S contents (wt.% by microprobe) in the different macerals of the German Creek and Moranbah Coal Measures with rank advance. Each data point represents the average concentration of the respective element for a number of such macerals in each coal sample (Table 3). For ease of display only data from locations 1 to 18 (Table 1) are shown. 125

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The nitrogen content of both vitrinite macerals decreases slightly with rank (Fig. 4c), and in particular appears to display a relatively abrupt drop at around 2% Rvmax. This may be associated with the development of ammonium illite in the mineral matter, which has been identified on the basis of X-ray diffraction data (the d {001} crystallographic spacing of ammonium illite is 10.3 2, slightly greater than the 10.0 2 of potassium-bearing illite) in other Bowen Basin coals at a similar rank level (Ward and Christie, 1994). Nitrogen in the inertinite macerals, however, appears to remain relatively constant with rank advance, and may even show a slight increase with rank in the inertodetrinite component. Differences in nitrogen content between vitrinite and inertinite macerals are therefore most marked in the subbituminous and bituminous coals (up to Rvmax = 2%), and appear to be less significant above that rank level. As with findings reported from other coals (e.g. Ward and Gurba, 1998), the organic sulphur content of the inertinite macerals, with some exceptions, is typically around half that of the vitrinites in the same coal samples (Table 3). This is especially clear for those samples with less than 2% vitrinite reflectance. Apart from this contrast, however, the organic sulphur content of the different macerals seems to show no particular variation with coal rank over the range of samples examined. Although the macerals, especially the vitrinite macerals, in the lowest rank coal studied have the lowest (organic) sulphur contents, and those of the highest rank coal (with the exception of inertodetrinite) have the highest levels of organic sulphur (Fig. 4d), the proportion of organic sulphur in the individual maceral groups of the bulk of the samples does not appear to vary significantly with increasing rank levels. This is in contrast to some of the results from previous studies (e.g. Ghosh, 1971; Harrison, 1991), which suggest that organic sulphur in coal shows a decrease with rank advance.

4. Discussion and conclusions 4.1. Coalification paths of individual macerals The data in Fig. 4 provide a clear illustration of the coalification paths, in chemical terms, of the individual macerals in Bowen Basin coals over the rank

range from subbituminous coal to anthracite. Although not plotted in Fig. 4, the macerals in the samples studied from the Rangal Coal Measures (Table 3) have similar compositions to those in coals of equivalent rank from the German Creek and Moranbah Coal Measures, and thus would be expected to follow similar coalification paths. The results also show that, in addition to vitrinite reflectance, the carbon content of the vitrinite macerals, ideally separated into telocollinite and desmocollinite, is also a good rank indicator, at least for the subbituminous to low volatile bituminous range. As discussed by Gurba and Ward (2000), and illustrated to a certain extent in Fig. 2, this is significant in situations where the reflectance value may be anomalously low, due to factors such as marine influence or an abundance of associated liptinite macerals. The carbon content of all macerals in the coals studied increases along the individual coalification paths from subbituminous to low volatile bituminous rank. However, the coalification paths tend to converge to a more or less common band at a point where Rvmax is around 1.75% and the carbon content of the vitrinite is around 90% (Fig. 4a). A less pronounced change in carbon content occurs in the individual macerals as rank increases from low volatile bituminous to anthracite rank levels. Mastalerz and Bustin (1993), and also other authors such as Ward and Gurba (1999), have similarly shown that the coalification paths of liptinite and semifusinite join that of vitrinite at around 88.5% and 89.5% C respectively, corresponding in the present study to a vitrinite reflectance value of around 1.75% to 2.0%. Use of the electron microprobe allows direct determination of oxygen in the different coal macerals, a parameter only determined indirectly by difference techniques in conventional ultimate analysis techniques. The present study shows clearly that the oxygen content of the individual macerals decreases as the carbon content increases (Fig. 4b), and an inverted but otherwise similar trend in the coalification paths of the different maceral components. 4.2. Contrasts in elemental composition with rank advance In the samples examined for the present study, the contrast in carbon content between vitrinite and

C.R. Ward et al. / International Journal of Coal Geology 63 (2005) 117–129

inertinite macerals (Table 3 and Fig. 4a) is greatest at subbituminous rank, with an absolute difference of around 20% at 0.39% Rvmax. The contrast becomes less significant (around 4% absolute) in low volatile bituminous coals, and finally becomes more or less insignificant at higher levels, with only about 1% absolute difference when the coal reaches anthracite rank. Similar contrasts are seen in the oxygen content of the different macerals, although it is the inertinites that have the lower oxygen percentages at the different rank levels and not the vitrinite components. The dramatic chemical changes in the elemental composition of the vitrinite from Rvmax 0.39% to 1.75% probably reflect a series of coalification-related reconstruction processes among the organic compounds of the macerals, such as increasing aromaticity and the loss of aliphatic and oxygen-containing structures with increasing maturation (Ibarra et al., 1996; Bustin and Gou, 1999). These dramatic chemical changes may be also attributed to the breakdown of large-molecule organic compounds to smaller compounds, and the associated massive volatile loss with rank advance (cf. Taylor et al., 1998). Despite the contrasts in carbon and oxygen content between the vitrinite and inertinite macerals in the same coal samples, especially at lower rank levels, and despite the variation in elemental composition of the different macerals with rank, Fig. 3 suggests that the carbon and oxygen contents of each maceral group remain related to each other throughout the entire rank range studied. All of the data points in Fig. 3 plot on or close to the same linear trend regardless of the maceral group or rank level involved. The vitrinite macerals, however, especially for the lower rank coals, plot towards the upper left part of this trend, whereas the inertinite macerals are grouped towards the lower right-hand side. Recognition of the individual macerals in such a plot identifies more clearly the contribution of the different organic components to the total coal composition, and emphasises the point, long-recognised but often overlooked, that it is the relative proportions of the different macerals (i.e. the coal type), as well as the coal rank, that determines the overall chemical composition of a coal sample. It also provides a basis for more definitive assessment of the levels of CO2 generation associated with combustion of coals from

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different sources (cf. Quick and Brill, 2002; Sakulpitakphon et al., 2003), based on a combination of their rank and type characteristics. 4.3. Variations in nitrogen, sulphur and calcium As with similar studies on other Australian and North American coals (Ward and Gurba, 1998, 1999; Gurba, 2001; Walker and Mastalerz, 2004), the proportions of nitrogen and organic sulphur in the inertinite macerals were found in the present study to be approximately half those on the vitrinite macerals of the same coal samples. Combined with the common abundance of inertinites in the materials (Joint Coal Board and Queensland Coal Board, 1987), this helps to explain the relatively low concentrations of these elements in Australian coals generally. Despite some differences in overall sulphur values between the lowest and the highest rank coals studied, the proportion of organic sulphur in the individual macerals does not appear to vary systematically with rank advance. The microprobe data indicate that the nitrogen content of the vitrinites in the coals of the present study seems to decrease relatively suddenly at a rank level equivalent to a vitrinite reflectance above around 1.75%. This is similar to the rank at which the carbon and oxygen contents of the vitrinite come close to those of the other coal macerals, and also to the rank at which ammonium illites have been observed in some other Bowen Basin coal samples (Ward and Christie, 1994). As suggested by Juster et al. (1987) and Daniels and Altaner (1993), based on its occurrence in US anthracites, ammonium illite may be formed in such cases by interaction between nitrogen in the coal macerals and more normal potassium-bearing illite, or possibly between nitrogen and kaolinite, at the high temperatures associated with anthracite rank. The vitrinite and semifusinite macerals of the lowest rank coal among the samples studied (Rvmax = 0.39%) also have a small proportion of calcium in the organic matter. Ward (1991, 1992) and Li (2002), among others, have also noted significant proportions of organically bound calcium in lower rank coals, thought to occur as exchangeable ions attached to carboxylates, elements in organometallic complexes, and dissolved salts in the pore

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water of the coal concerned. As in the present study, the organically bound Ca in New Zealand coals is lost in the transition from subbituminous coal to bituminous material (Li, 2002). The absence of Ca from higher rank coals in the present study suggests that the element is liberated from the macerals as part of the molecular changes associated with the subbituminous to bituminous transition in the rank advance process (cf. Ward et al., 2003). Acknowledgements Financial support for the study was provided under the Large Grants Scheme of the Australian Research Council. Thanks are expressed to Rad Flossman for polished section preparation and Barry Searle for assistance with the electron microprobe analyses. Thanks are also expressed to Jim Beeston and Ray Smith, of the Queensland Department of Natural Resources and Mines, and also to various mining and exploration companies, for provision of the coal samples. Maria Mastalerz, of the Indiana Geological Survey, provided the reference anthracite standard used in the microprobe analysis program, and also other high-rank vitrains used for comparative purposes. Thanks are also expressed to Maria Mastalerz and an anonymous reviewer for constructive comments on the manuscript. References American Society for Testing and Materials, 1997. Annual Book of American Society for Testing and Materials Standards, Sec. 05.05 Gaseous Fuel, Coal and Coke. Philadelphia, PA. 536 pp. Beeston, J.W., 1978. Coal reflectivity, rank and carbonisation in Departmental borehole Wodehouse NS 1. Queensland Government Mining Journal 79, 159 – 164. Beeston, J.W., 1981. Coal rank variation in the Bowen Basin. Geological Survey of Queensland Record 1981/48 (unpublished). Beeston, J.W., 1995. Coal rank and vitrinite reflectivity. In: Ward, C.R., Harrington, H.J., Mallett, C.W., Beeston, J.W. (Eds.), Geology of Australian Coal Basins. Special PublicationGeological Society of Australia. Coal Geology Group, vol. 1, pp. 83 – 92. Bustin, R.M., Gou, Y., 1999. Abrupt changes (jumps) in reflectance values and chemical compositions of artificial charcoals and inertinite in coals. International Journal of Coal Geology 38, 237 – 260.

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