Variation in elemental composition of macerals; an example of the application of electron microprobe to coal studies

International Journal of Coal Geology, 22 (1993) 83-99

83

Elsevier Science Publishers B.V., Amsterdam

Variation in elemental composition of macerals; an example of the application of electron microprobe to coal studies

M a r i a M a s t a l e r z a n d R. M a r c B u s t i n

University of British Columbia, Department of Geological Sciences, Vancouver, B.C. V6 T 1Z4, Canada (Received March 19, 1992; revised version accepted July 15, 1992 )

ABSTRACT Mastalerz, M. and Bustin, R.M., 1993. Variation in elemental composition of macerals; an example of the application of electron microprobe to coal studies. Int. J. Coal Geol., 22: 83-99. Elemental composition of macerals in lignite, sub-bituminous,high-volatile bituminous, mediumvolatile bituminous and anthracitic rank coals have been investigated using the electron microprobe technique. The concentration of C, O, N, S, Fe, Si, and AI have been measured directly and hydrogen was calculated by difference. The chemical composition was compared to reflectance values and whole coal chemistry. In lignite, the sequence: ulminite (66.27% C), porigelinite (67.32% C), micrinite (67.92% C) corpocollinite (68.21% C), macrinite (70.24% C) and fusinite (73.03% C) represents a series with increasing carbon content. Sulphur concentration is the highest in porigelinite (2.95%) and the lowest in fusinite (0.21%) whereas nitrogen is the highest in corpocollinite (3.12%) and the lowest in micrinite and porigelinite. In sub-bituminouscoal, the sequence of decreasing carbon content is as follows: macrinite (80.01% C), sporinite (73.00% C), semifusinite (72.76% C) and telocollinite (67.49% C). Sulphur content is the highest in telocoilinite (3.29%). In high-volatile bituminous coal, carbon content is highest in fusinite (93.68%) and lowest in cutinite (82.82). Vitrinite macerals have the highest sulphur content and fusinite has the lowest. In medium-volatilebituminous coal, a sequence of decreasing carbon and increasing oxygen content: fusinite (90.02% C), semifusinite (89.73% C) and vitrinite ( 88.56% C) is detectable but differences in carbon and oxygen content between macerals are small. In anthracite, no statistical differences in carbon and oxygen content between vitrinite and inertinite macerals exist (about 94% C and 1% O for both macerals). Comparison of reflectance values to carbon and oxygen contents for individual macerals demonstrates that vitrinite and sporinite enter a common coalification path at 88.5% C and a reflectance of 1.25%. Semifusinite joins the path at about 89% C and a reflectance of 1.8-2.0%. More data are needed to determine the relationsip between reflectance and elemental composition for other macerals.

Correspondence to: M. Mastalerz, The University of British Columbia, Department of Geological Sciences, 6339 Stores Rd., Vancouver, B.C. V6T 1Z4, Canada.

0166-5162/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

g4

M.M,GSI,'~LERZANI) R M BUSIIN

1NTRODUCTION

The heterogenous nature of coal is discernible on both the megascopic and microscopic scale. The heterogeneity on a megascopic scale is manifested by the presence of distinct bright and dull bands and on a microscopic scale by the discrimination of variation in maceral assemblages. Macerals, which are microscopically defined components of coal and dispersed organic matter in rocks (International Comittee for Coal Petrology, 1963; 1971 ) vary with respect to morphology, physical properties and chemical composition. In addition, these properties evolve with increasing rank. In low rank coals, the distinction between macerals is marked, whereas at higher rank the properties of macerals merge. The variation in chemical composition of macerals has been the object of many studies (Stach, 1982 ). The main difficulty encountered in previous work was to obtain pure macerals and concentrate them in sufficient quantities for chemical analyses. Two methods of maceral concentration have been used: ( 1 ) hand picking; and (2) separation of macerals using the variation in density between maceral groups. Hand picking is most successful for vitrinite and fusinite because these two macerals occur as bands and inclusions of sufficient size to be recognized, separated by hand and then analyzed chemically. Therefore, the chemical composition of vitrinite has received much attention and, as a result, is understood best of all the macerals. There have been also some attempts to obtain liptinite concentrates in quantities sufficient to determine their properties (Zetsche, 1931 ). In density separation methods macerals are fractionated by the float-sink technique using heavy liquids (Dormans et al., 1957; Kr6ger et al., 1957 ). These methods yield results of varying quality, although it has been possible to get rough separation into vitrinite, inertinite and exinite fractions (Kr6ger and Bade, 1960). An important step in the study of coal macerals was application of density gradient centrifugation (DGC) for maceral separation (Dyrkacz and Horwitz, 1982). Using this method it is possible to separate coals into narrow density fractions and, thus, to obtain vitrinite, inertinite, liptinite and alginite as almost pure concentrates and in sufficient quantities to perform chemical analyses. The main drawback of the DGC method is that, in order to get good maceral isolation, coal has to be ground to a particle size of 2-3 p.m, which makes it difficult to classify macerals microscopically and evaluate the quality of maceral separation. Thus, optimal liberation of macerals has to be compromised with reliable maceral identification. In addition, grinding to such a fine particle size may enhance oxidation and, as a result, coal chemistry may be altered. Moreover, in this method the original texture of coal is destroyed, making interpretation of variation in chemical composition more difficult than in methods that permit analyses of macerals in situ. One of the methods which can be used to analyse macerals in situ is the

VARIATIONIN ELEMENTALCOMPOSITION OF MACERALS

85

electron microprobe technique. This is a technique in which analyses are obtained from areas as small as a few micrometres. This method enables direct determination of organic sulphur (Solomon and Manzione, 1977; Raymond and Gooley, 1978 and others) and other elements, such as Na, Mg, Si, Ca, Fe and Sr in coal (Dutcher et al., 1964; Karner et al., 1986). Younkin et al. (1987) applied this method to analyse oxygen in coal. Recently, Bustin et al. (1992) extended the application of the electron microprobe to analysing all major light elements (C, O, N) in coal. On selected samples they showed that this technique could provide accurate compositional data of coal macerals. The purpose of this paper is: ( 1 ) to study changes in chemical composition of coal macerals with rank as well as between and within macerals of a given rank; and (2) to demonstrate the utility of the electron microprobe in analysing the elemental composition of coal macerals on a microscopic scale not previously obtainable. METHODS

Samples of lignite, sub-bituminous, high-volatile bituminous, medium-volatile bituminous and anthracitic coals were selected for this study. Carbon and ash contents of these samples, together with their age and locations are listed in Table 1. Each sample was made into a polished block according to standard coal preparation techniques (Bustin et al., 1985 ) and afterwards a series of photomicrographs was taken using a Leitz MPV II microscope with 20× oil immersion objective. A photomosaic was made in order to select points for reflectance measurements and to make it easier to recognize macerals for analysis in the electron microprobe. The photomosaic facilitated analysing elemental composition precisely on the spots of previously measured reflectance and areas of known maceral composition. After reflectance measurements had been taken, the immersion oil was removed and the samples were covered with a carbon coat 0.023 ~tm thick, using a sputter coater (as described by Bustin et al., 1992 ). TABLE 1 Location, carbon t and ash content in the coals studied Rank

Location

Age

C Ash (wt%, daf) (wt%, dry)

Lignite Sub-bituminous High vol-bitumimous Medium vol-bituminous Anthracite

Popular River Mine, Saskatchewan Costa Rica Minto Mine, New Brunswick Vicary Mine, British Columbia Pennsylvania

Paleocene Tertiary Carboniferous L. Cretaceous Carboniferous

68.74 68.94 83.51 87.87 95.54

l ASTM method.

4.44 6.42 6.00 3.71 2.56

~6

M. M A S T A L E R Z A N D R M BUSI'IN

A Cameca SX 50 electron microprobe was used to analyse major and minor elements in coal. A PC2 (NiC) pseudocrystal was used for analysing light elements. The physical conditions were as follows: an accelerating voltage of 10 kV, a beam current of 10 nA, and a beam size of 5 ~tm. Anthracite, magnesite, boron nitride and barite were used as carbon, oxygen, nitrogen, and sulphur standards, respectively. In addition, Fe, Si, and A1 contents were determined, with siderite as a standard for iron, wollastonite as a standard for silica and anorthite as a standard for aluminum. Hydrogen content was calculated by difference. A full description of the analytical routine for analysing light elements in coal with the electron microprobe, together with its advantages and drawbacks are presented in Bustin et al. ( 1992 ). RESULTS

Lignite Lignite presents a special problem to microanalyses. Most analyses yielded a lower than expected number of counts per second, which resulted in suppressed carbon and, consequently, elevated hydrogen content. Such low counts are attributed to the high porosity of most lignite macerals (Fig. 1A). Only analyses on non-porous areas (Fig. 1B ) yielded carbon contents very close to that obtained following ASTM procedures and only these data are considered here. Due to the paucity of analyses the results summarized below must be considered preliminary. More data on non-porous areas are needed to determine elemental variations between and within macerals of lignite rank. Table 2 presents elemental composition and reflectance values of selected macerals in lignite. Reflectance of the macerals vary from 0.19% in porigelinire to 1.17% in fusinite. These data show that the sequence: ulminite, micrinite, porigelinite, corpocollinite, macrinite and fusinite represents a sequence of increasing C content. Sulphur content is highest in porigelinite and lowest in fusinite. There is a high variation in N content between macerals; the highest N content was found in corpocollinite, the lowest in micrinite and porigelinite.

Sub-bituminous coal Table 3 presents elemental composition and reflectance data of a sub-bituminous coal from Costa Rica (Fig. 1C). The maceral reflectance varies from 0.06% in sporinite to 0.59% in semifusinite. The data show a decreasing carbon and increasing oxygen content as follows: macrinite, sporinite, semifusinite and telocoUinite. With respect to C and O content, telocollinite and macrinite are far more uniform than sporinite and semifusinite. Semifusinite, especially, is inhomogenous, showing standard deviations, both for O and C,

Fig. 1. Photomicrographs of selected coals, reflected light, oil immersion. A. Plobaphinite (p) and porigelinite (g) in texto-ulminite, Paleocene lignite. Note high porosity through almost the entire coal. B. Plobaphinite (p) in a porous textinite tissue, Paleocene lignite. C. Thin suberinite cell walls (arrows) with phlobaphinite cell fillings surrounded by vitrinite macerals (v), Tertiary sub-bituminous coal from Costa Rica. D. Alternating vitrite and trimacerite layers in Carboniferous high-volatile bituminous coal from New Brunswick. Note spots of microprobe analysis in the upper vitrite layer (arrow).

OO

t~

x ©

"0

©

> e"

K Z

g~ t" m

88

M MASTALERZAN[)RM B[ISTIN

TABLE 2 Chemical composition of selected macerals of lignite (wt%)

Ro C O N S Fe Si AI H H/C O/C n

Ulminite

Corpoc.

Porigel.

Mic.

Mac.

Fus.

0.29 66.27 26.41 0.30 0.87 0.14 0.02 0.34 5.70 1.03 0.30 15

0.40 68.21 18.90 3.12 2.34 0.25 0.11 0.75 6.32 1.11 0.21 8

0.19 67.32 21.87 0.00 2.95 0.06 0.30 1.04 6.46 1.15 0.24 3

0.57 67.92 22.36 0.00 1.91 0.27 0.29 1.13 6.12 1.08 0.25 2

1.1 70.24 25.59 0.91 0.76 0.03 0.08 0.29 2.10 0.35 0.27 5

1.17 73.03 21.14 0.31 0.21 0.33 0.17 0.69 4.22 0.69 0.25 3

(0.8) (1.1) (0.3) (0.2) (0.1) (0.1) (0.1) (0.9)

Corpoc. = corpocollinite; Porigel. = porigelinite; Mic. = micrinite; Mac. = macrinite; Fus. = fusinite: n = number of analyses; parentheses = standard deviation. TABLE 3 Chemical composition of selected macerals from sub-bituminous coal (wt%) Telocollinite

Macrinite

Sporinite

Semifusinite

Ro C

0.47 67.49 (0.85) 25.16 (0.63)

N S

0.47 (0.43) 3.29 (0.78)

Fe

0.69 (0.22)

Si

0.05 (0.01)

AI

0.59 (0.02)

H

2.25 (0.74)

0.06 73.00 (4.12) 14.54 (4.77) 0.00 2.05 (0.29) 0.12 (0.03) 0.14 (0.11) 0.72 (0.38) 9.40 (0.54) 1.55 0.149 10

0.59 72.76 (9.65)

O

0.78 80.01 (0.13) 13.00 (1.62) 0.00 2.47 (0.87) 0.16 (0.03) 0.04 (0.02) 0.22 (0.17) 4.30 (0.68) 0.64 0.120 11

H/C O/C n

0.40 0.279 30

20.68 (9.32) 0.15 (0.30) 2.79 (0.49) 0.22 (0.16) 0.06 (0.03) 0.34 (0.29) 2.99 (0.33) 0.49 0.210 18

n = number of analyses; parentheses = standard deviation.

o f more than 9%. Telocollinite has the highest S content (3.29%). Macrinite and semifusinite have similar S values, whereas in sporinite the sulphur content is the lowest o f all the macerals analysed (2.05%). The Fe content is the

VARIATIONIN ELEMENTALCOMPOSITION OF MACERALS

89

highest in telocollinite, which suggests that part of the sulphur in this maceral is pyritic. The highest H values occur in sporinite, which means that this maceral has the highest H/C ratio.

High-volatile bituminous coal Elemental compositions, together with reflectance data on macerals from high-volatile bituminous coal, are presented in Table 4. Reflectance of the macerals varies from 0.21% in sporinite to 3.36% in fusinite. Fusinite, macrinite, resinite, sclerotinite, sporinite, semifusinite, desmocollinite, telocolTABLE 4

Chemical composition of macerals of high-volatile bituminous coals (wt%) Tel.

Des.

Spor.

Cut.

Res.

Sem.

Scler

Fus.

Mac.

Ro C s.d.

0.69 86.53 0.69

0.67 86.58 0.49

0.21 87.61 1.25

0.26 82.82 4.65

0.24 90.94 3.21

1.45 87.03 2.27

1.78 89.90 2.12

3.36 93.68 0.11

2.51 91.27 2.42

O s.d.

7.27 0.50

7.31 0.24

5.94 0.60

10.76 4.41

2.59 3.27

7.85 3.60

6.69 3.24

3.06 0.24

4.99 4.99

N

0.66 0.16

0.68 0.14

0.00

0.00

0.00

0.01 0.21

0.00

0.00

0.11

s.d. S s.d.

3.23 0.41

3.19 0.41

3.12 0.42

3.32 1.36

2.39 1.43

1.45 0.51

1.40 1.72

0.66 0.13

2.13 1.74

Fe s.d.

0.12 0.09

0.07 0.07

0.12 0.09

0.08 0.11

0.06 0.12

0.15 0.12

0.25 0.22

0.05 0.08

0.53 0.78

Si s.d.

0.08 0.07

0.11 0.12

0.04 0.19

0.11 0.06

0.02 0.08

0.93 1.03

0.58 0.11

0.02 0.01

0.77 1.46

AI s.d.

0.06 0.06

0.06 0.06

0.04 0.03

0.05 0.04

0.02 0.03

0.71 0.91

0.36 0.31

0.01 0.00

0.57 1.12

H s.d.

2.64 0.81

2.63 0.75

3.13 1.39

2.84 1.81

3.98 2.21

2.41 2.19

0.01 0.02

2.52 0.25

2.52

H/C O/C n

0.37 0.062 86

0.37 0.063 70

0.41 0.097 10

0.53 0.02

0.33 0.067 36

0.002 0.05 13

0.32 0.024 20

0.23 0.046 11

0.43 0.05 22

8

0.22

1.73

Des. = desmocollinite; Spor. = sporinite; Cut. = cutinite; Res. = resinite; Sem.=semifusinite; Scler.=sclerotinite; Fus.=fusinite; Mac.=macrinite; n=number of analyses; s.d. = standard deviation (%). Tel. = telocollinite;

90

M, MASTALERZ a,ND RM. B1JS]IN

TELOCOLLINITE 4O

Cmean - 86.52 sd - 1.04

30

40 3O

c

20

20

~

10

10

ii

n m o a n - 7.32 I - 0.50

84

85

86

87

88

6

7

c (%)

8

9

10

o (%)

DESMOCOLLINITE 40

80

t c

v

30

¢

20

O" ¢D

,,

1

Omean - 7.32

._

60 s

tI

I - 0.25

40

10

2O

84

85

86

87

88

6

C (%)

7

8

9

10

r~ lev~

SPORINITE

40

40 Ome

A

~3o

30

~20

2O

S

O"

10

I,I.

10

85 86

87

88

89

C (%)

90

20

20

m 10

10

5

6

Omean - 5.76

30

~30

4 O (%)

40

40

8"

3

SEMIFUSINITE

sd - 1.55

O"

85 86

87

88

c (%)

89

90

91

3

4

5

6

7

8

9

o (%)

Fig. 2. Histograms of C a n d O c o n c e n t r a t i o n s in macerals of high-volatile bituminous coal. sd = s t a n d a r d deviation.

linite and cutinite form a series with decreasing C content, similar to that described earlier for sub-bituminous coal. Carbon content in vitrinite of highvolatile bituminous coal approaches that of sporinite and semifusinite (difference is only about 1% ). Vitrinite macerals have the most uniform O and C

91

VARIATIONIN ELEMENTALCOMPOSITIONOF MACERALS

TELOCOLLINITE 40

4O

3O c

- 3.21

2.66

0.40

0.80

30

20

20

10

10

EL

3

5

S (%)

1

2

3

40

4

5

H (%)

DESMOCOLLINITE 40

.~.46

~mean - 3.23 30 = O EL

sd - 0.40

20

30

72

20

1o

lO - - r

3

4

5

S (%)

1 SPORINITE

40

,"

3O

sd - 0.42

4

5

H (%) 7

20

20

10

10 - - i

3

4

S (%)

Hmean - 3.05

30 |.

O" 14.

3

40

ean- 3.1 1 A

2

5

Bt

d.a 1

2

SEMIFUSINITE

3O

sd.1.41

3

4

5

H (%)

3O

A

41

san - 2.09

~20

sd - 0.75

C

6

20

2

10 EL

1

2

3 S (%)

4

5

0

1

2

3

4

5

H (%)

Fig. 3. Histograms of S and H concentrations in macerals of high-volatile bituminous coal. sd = s t a n d a r d d e v i a t i o n .

contents, whereas cutinite, resinite, macrinite and semifusinite show wide range of C and O values. Vitrinite macerals and cutinite are richest in S and fusinite has the lowest S content. Liptinite macerals have the highest H content followed by vitrinite and inertinite. Such is also the sequence of H / C

92

M. MASTAI,ERZAN[)R M BUS31N

TABLE 5 Chemical composition of telocollinite layers (wt%), each value represents mean of 20 anal? scs R o(% )

(7

O

N

S

Fc

H

t 1/('

I)/(

[

0.69

0.064

0.67

3.25 (1.04) 2.31 (0.79) 1.09 (0.56/ 2.44 (0.64)

0.29

4

0.11 (0.07) 0.14 (0.11) 0.12 (0.17) 0.15 (0.08)

0.061

0.63

2.74 (0.23) 3.45 (0.42) 3.75 (0.03) 3.32 (0.18)

0.44

3

0.13 (0.08) 0.14 (0.31) 0.13 (0.05) 0.00 (0.00)

0.059

0.67

6.87 (0.27) 6.99 (0.35) 7.28 (0.03) 7.93 (0.37)

0.45

2

86.93 (0.63) 86.91 (0.45) 86.51 (0.47) 85.84 (0.71)

0.34

0.069

Standard deviation in parentheses.

values: the highest for liptinite (0.51-0.53 ), intermediate for vitrinite (0.37 ) and the lowest for inertinite (0.002-0.33). Due to the presence of numerous macerals of the three maceral groups (Fig. 1D), the high-volatile bituminous coal was chosen for detailed examination of differences in chemical composition between maceral groups and within particular macerals. The data obtained (Table 4) demonstrate that vitrinite macerals are most homogenous of all macerals with respect to elemental composition. Histograms of their elemental composition (Figs. 2 and 3) show unimodal distributions of O and S and bimodal distributions of H and C. Sporinite shows unimodal distribution of S and O, but it has very wide range of C and H values. Semifusinite is the most variable maceral with respect to chemical composition of the major elements. For this maceral there is a bimodal distribution of C, O and S, which indicates the presence of at least two chemically distinct varieties of semifusinite. The occurrence of two distinct types may reflect differing conditions of deposition and/or primary plant material. The data obtained (Table 4) document that the vitrinite macerals, telocollinite and desmocollinite, have very similar elemental chemistries with respect to C, O, S and H contents. Histograms of their elemental composition (Figs. 2 and 3 ) show unimodal distribution of O and S and bimodal distribution of H and C. Table 5, however, shows that there are considerable variations in C and H contents between individual layers oftelocollinite in a single sample. Comparison of reflectance of the macerals analysed with C content and H / C values shows that there are no obvious correlations between these variables within particular macerals (Fig. 4). Some positive correlation exists between elemental composition and reflectance for semifusinite and sporinite, but there is no correlation for telocollinite or desmocollinite. For other macerals, there are insufficient data to evaluate if any correlation exists between reflectance and chemistry.

93

VARIATION IN ELEMENTAL COMPOSITION OF MACERALS

Ro (%)

Ro (%)

0,78

0.B0

telocollinite

desmocollinite

0.76 0.74

0.75

0.72 0.70

0.70 0,68 0.66

0.65 •

°

i

,

0.64 0.60

,

85.6

,

I

85.8

86

i

,

86.2

86,4

J 86.6

,

i

i 86.8

,

67

87.2

0,62 86.2

L

86.4

I

r

I

I

;

86,6

86.8

87

87.2

87.4

87.6

C content (%)

C content (%) Ro (%)

Ro (%) 0.32

2.0

0.30

1,8

semifusinite

1.6 0.26

1.4

0.24

1.2

0,22

1.0

sporinite 0.20

0.8

0.18 87

I 87,5

i

88

L

88.5

i

89

i

89.5

;

90

80.5

C content (%)

0.6

86

; 87

i 88

, 89

90

C content (%)

Fig. 4. Relationship between reflectance and C content in macerals (spots analysed by electron microprobe) of high-volatile bituminous coal.

Medium-volatile bituminous coal Table 6 presents elemental composition and reflectance values of vitrinite, semifusinite and fusinite in medium-volatile bituminous coal. Only these three macerals were distinguishable in the samples of this rank. Reflectance varies from 1.3% in vitrinite to 2.98% in fusinite. A sequence of decreasing C and increasing O content: fusinite, semifusinite and vitrinite is apparent• Fusinite has the lowest sulphur content of all analysed macerals. Hydrogen content (and H / C ) is the highest for vitrinite. Semifusinite is the most inhomogenous maceral with respect to elemental composition showing standard deviations for C, O, and H greater than 2%.

Anthracite Table 7 depicts chemical composition of vitrinite and inertinite from Pennsylvania anthracite. In this anthracite the reflectance of vitrinite is the same as that of fusinite. The only difference in elemental composition between these two maceral groups is a higher H content (calculated by difference ) in vitrinite and higher N content in fusinite.

94

M. M,.K',;TALERZAND R,M. BI ~S'I'I~

TABLE 6 Chemical composition of macerals in medium-volatile bituminous coal (wt%)

R,, C O N S Fe Si AI H H/C O/C n

Vitrinite

Semifusinite

Fusinilc

1.3 88.56 (0.211 4.25 (0.30) 0.31 (0.82) 0.84 (0.09) 0.03 (O.05) 0.09 (0.03) 0.03 (0.02) 6.21 (0.32) 0.88 0.04 40

1.84 89.73 (2.22)

2.98 90.02 (1.74)

3.87 (2.44)

3.69 ( 1.51J

0.00

0.00

0.91 (0.25)

0.39 (0.06)

0.13 (0.05)

0.02 (0.04)

0.09 (0.27)

O.09 (O.03)

0.07 ( 0.12 )

0.03 (0.03)

5.20 (2.?4)

5.73 (0.37)

0.70 O.03 20

0.77 0.03 15

n = number of analyses: parentheses = standard deviation.

TABLE 7 Chemical composition of anthracite (wt%)

Ro

C O N S Fe Si AI H H/C O/C n

Vitrinite

Fusinite

5.15 93.89 1.05 0.00 0.95 0.07 0.02 0.06 3.96 0.51 0.01 20

5.20 93.88 1.00 0.54 1.00 0.07 0.06 0.03 3.39 0.43 0.01 20

(0.16) (0.48) (0.07) (0.05) (0.01) (0.02) (0.43)

n = number of analyses; parentheses = standard deviation.

(1.15) (0.52) (1.21) (0.12) (0.06) (0.07) (0.06) (1.34)

VARIATION

IN ELEMENTAL

95

COMPOSITION OF MACERALS

A

~h

,

h

m

[3

8i'- 3

O r-

m

"6

a)

gI I

m

,

s .Ig

oS

-" h

i

£] m

"'L

1

'

m--f" h A

,S . ........... i. . . . .

60

/~

70

~he

h

•

80 90 Carbon (%)

100

~

-,~ h ......

10

5

Oxygen - cutinite

o

- vitrinite

o

. sporinite

•

- resinite

= - semifusinite

o

- sclerotinite

•

- macrinite

s

~s 15

20

2;

(%)

Ig - lignite s - sub-bituminous coal h - high-volatile bituminous coal m - medium-vot, bituminous coal a - anthracite

Fig. 5. Relationship between reflectance and C and O content in macerals in coals of lignite to anthracite rank. * = Fusinite. VARIATION OF MACERAL CHEMISTRY WITH RANK

The comparison of elemental composition of coals from lignite to anthracite shows how the differences between and within maceral groups decrease with rank. In high-volatile bituminous coal, the C content in vitrinite approaches that of sporinite and semifusinite (difference is only about 1%), as opposed to subbituminous coal, where this difference is in a range of 5-6%. Further, macerals of high-volatile bituminous coal have more uniform C and O contents than the same macerals in sub-bituminous coal. For example, in sub-bituminous coal the standard deviation of the C content for telocollinite is 0.85%, for sporinite 4.12% and for semifusinite 9.65% (Table 3 ); whereas in high-volatile bituminous coals, the standard deviation values are: 0.69%, 1.25% and 2.27%, respectively (Table 4 ). Standard deviations for C, O, S and H contents of vitrinite in medium-volatile bituminous coal are much lower than in high-volatile bituminous coal. Fusinite in medium-volatile bituminous coal shows much wider dispersion of C and O than fusinite of highvolatile bituminous coal, which may reflect the occurrence of several populations of fusinite. In the anthracite, no differences between macerals were found. Figure 5 shows the variation in C and O content of sporinite, vitrinite, sere-

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ifusinite and fusinite with reflectance. This figure shows that, below about 88% C (the point where liptinite and vitrinite macerals obtain the same reflectance level), the macerals at a given level of C content have notably different reflectances. For example, with 83% C, reflectance of sporinite is 0.09%, cutinite 0.25%, vitrinite 0.5%, and semifusinite 0.8%. With 10% O, reflectance of sporinite is about 0.1%, cutinite 0.25%, vitrinite 0.5% and semifusinire about 1.2%. Consequently, at a given level of reflectance below Ro= 1.71.8%, the C and O content of particular macerals vary. For example, at Ro = 1%, the sequence of increasing C and decreasing O content is as follows: semifusinite, vitrinite and sporinite. Macerals such as eutinite, resinite, sclerotinite and macrinite were analysed in high-volatile bituminous coal only and, as a result, they represent single points on the diagrams in Fig. 5; more data are needed to trace the relationship between C and O contents for these macerals. DISCUSSION

The possibility of analysing the elemental composition of in situ macerals enables, for the first time, tracking of differences between macerals through different ranks. Such data cannot be obtained using maceral density separation methods because differences in density between macerals decrease with rank. Furthermore,in coal with a C content higher than 93-94%, density differences between macerals are not apparent (Dyrkacz et al., 1984). In addition, by using a photomosaic of the samples, the problem of maceral identification during electron microprobe analysis is eliminated. Moreover, using electron microprobe analysis, elemental composition can be obtained on areas as small as a few micrometres and reflectance values can be obtained on the same areas. This study was carried out on a limited number of samples varying in rank from lignite to anthracite, so relations between elemental composition of macerals obtained here may not necessarily hold true for other coals, Nevertheless, our results give a consistent picture of how elemental composition of macerals change with rank and what kind of chemical variation can be expected within and between maceral groups of a particular rank. Macerals of sub-bituminous and high-volatile bituminous coal show considerable differences with respect to major element contents. For medium-volatile bituminous coal, chemical composition of the macerals is more homogenous, whereas for anthracite, differences in elemental concentrations within macerals are very low and of the same magnitude as between macerals. The data on high-volatile bituminous coal are particularly interesting with respect to the determination of the degree of chemical variation between the vitrinite macerals, telocollinite and desmocollinite. Very similar mean chemical composition of these two macerals and bimodal distribution of C and H

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in both macerals suggest that neither of these macerals are homogenous and each consists of at least two varieties of differing elemental composition, and probably different densities. This, in turn, implies that these two macerals cannot be separated based on differences in density. Separating teloconinite and desmocollinite into two fractions would give mixture of these two macerals in every fraction. In fact, Dyrkacz et al. ( 1991 ) encountered this problem during maceral separation using density gradient centrifugation. The reason for very weak (or no) correlation between reflectance and elemental composition within macerals of high-volatile bituminous coal is unclear. On one hand, an increase in vitrinite reflectance is associated, in general, with an increase in C content during coalification (M. Teichmuller, 1971 ). On the other hand, C content is a poor rank indicator in the range of bituminous coals (M. and R. Teichmuller, 1967). It is likely that, on a maceral scale and the same maturation level, these two rank-related parameters are not the function of the same factors. Determination of functional group distribution at places where both chemical composition and reflectance are known would undoubtedly give more insight into this problem. The present study confirms that there are major differences in the coalification path between particular macerals. Such differences have already been reported based on C and H contents (Dormans et al., 1957) and reflectance values (Smith and Cook, 1980). Our data, in general, agree with the previous results. The main difference is that, in our study, liptinite and inertinite enter the vitrinite coalification path at a lower C content than was obtained in previous studies. In the studies of Dormans et al. ( 1957 ), reflectance of liptinite reached the reflectance of vitrinite at about 91% C, whereas Ghosh (1971 ) reported 90% C. Our study shows that, at the maturation level corresponding to a 1.25% reflectance and 88.5% C, the properties of vitrinite and liptinite macerals become similar and they have a common coalification path at higher rank (Fig. 5). Inertinite, according to previous studies, joins the vitrinite path at about 92% C (Ghosh, 1971 ). According to the data obtained in this study, semifusinite joins the coalification path of vitrinite and liptinite at 89.5% C, which corresponds to a reflectance of 1.8-2.0% (Fig. 4). Because there are multiple populations of semifusinite in our samples (and probably in most coals), the point where this maceral enters the vitrinite-liptinite coalification path will vary. We have no data available for fusinite in sub-bituminous coal and, therefore, it is not possible to trace the coalification path of this maceral at ranks below high-volatile bituminous coal (Fig. 5 ). The fact that the reflectance of fusinite in medium-volatile bituminous coal is lower than in highvolatile bituminous coal shows that there are multiple populations of fusinite, similarly to semifusinite, and thus, the point at which fusinite joins the common path will vary. Previous data (Dormans et al., 1957; Ghosh, 1971 ) were obtained on maceral concentrates and were restricted to maceral groups only. They represent

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mean values for particular maceral groups and, thus, a wide scatter has been reported. In our study, chemical analyses were carried out at sites of known reflectance and undoubtful maceral identification. Therefore, it is believed that our data have a higher degree of precision. At this time, coalification paths for all macerals cannot be presented and more samples are required to obtain amore complete characterization of chemical changes with rank. The results obtained, however, clearly show that there are distinct differences in the relationship between reflectance and C and O contents for particular macerals (Fig. 5 ). Althought it has been known for years that such differences exist, precise data have not been provided because it was not possible either to obtain chemical analyses on individual macerals (spots as small as 5 I~m in size) or to measure reflectance on the same macerals. ACKNOWLEDGEMENTS Financial support for this study was provided from NSERC grant A-7337 to R.M. Bustin. Aknowledgment is made to the donors of the Petroleum Research Fund, administrated by ACS, for partial support o f the research.

REFERENCES Bustin, R.M., Cameron, A.E., Grieve, D.A. and Kalkreuth, W.D., 1985. Coal Petrology, Its Principles, Methods and Applications. Geol. Assoc. Canada, Short Course Notes 3, 2nd ed., Vancouver, B.C., 230 pp. Bustin, R.M., Mastalerz, M. and Wilks, K.R., 1992. Direct determination of carbon, oxygen and nitrogen content in coal using the electron microprobe. Fuel (in press). Dormans, H.N.M., Huntjens, F.J. and van Krevelen,D.W., 1957. Chemicalstructure and properties of coal--composition of the individual macerals (vitrinites, fusinites, micrinites and exinites). Fuel, 36: 321-339. Dutcher, R.R., White, E.W. and Spackman, W., 1964. Elemental ash distribution in coal components-use of the electron probe. In: D.H. Regelan and W.D. Gifford (Editors), Proc. "22nd Ironmaking Conf., Iron and Steel Division, Metal. Soc. AIME, New York, pp. 463483. Dyrkacz, G.R. and Horwitz, E.P., 1982. Separation of coal macerals. Fuel, 61: 3-12. Dyrkacz, G.R., Bloomquist,C.A.A. and Ruscic, L., 1984. Chemical variations in coal macerals separated by density gradient centrifugation. Fuel, 63: I 166-1173. Dyrkacz, G. R., Bloomquist,C.A.A. and Ruscic, L., 1991. An investigationof the vitrinite maceral group in microlithotypes using density gradient separation method. Energy Fuels, 5: 155-163. Ghosh, T.K., 1971. Change in coal macerals. Fuel 50: 218-221. International Committee for Coal Petrology (ICCP), 1963. International Handbook of Coal Petrography. Centre National de la Recherche Scientifique,Paris, 2nd ed. International Committee for Coal Petrology (ICCP), 1971. International Handbook of Coal Petrography. Centre National de la Recherche Scientifique,Paris, supplement to 2nd ed. Karner, F.R., Hoff, J.L., Huber, T.P. and Schobert, H.H., 1986. Electron microprobe technique

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for quantitative analysis of coal and coal macerals. Am. Chem. Soc. Div. Fuel Chem. Prepr., 31: 29-34. KrSger, C. and Bade, E., 1960. Anreicherung von Gefuegebestandteilen der Steinkohle durch Flotation. Gliickauf, 96: 741-747. KrSger, C., Pohl, A. and Kuthe, F., 1957. Comparative study of some physical and chemical properties of coal macerals. Gltickauf, 93: 122-127. Raymond R. and Gooley, R., 1978. A review of organic sulphur analysis in coal and a new procedure. Scanning Electron Microsc., 1: 93-104. Smith, G.S. and Cook, A.C., 1980. Coalification paths of exinite, vitrinite and inertinite. Fuel, 59: 641-646. Solomon, P.R. and Manzione, A.V., 1977. New method for sulphur concentration measurements in coal and char. Fuel, 56: 393-396. Stach, E., 1982. The microscopically recognizable constituents of coal. In: E. Stach, M.-Th. Mackowsky, M. Teichmtiller, G.H. Taylor, D. Chandra and R. Teichmiiller (Editors), Stach's Textbook of Coal Petrology. Borntraeger, Berlin, 3rd ed., pp. 87-218. Teichm/iller, M., 1971. Anwendung kohlepetrographischer Methoden bei der ErdSl- und Erdgasprospektion. ErdS1 Kohle, 24: 69-76. Teichmtiller, M. and Teichmtiller, R., 1967. The diagenesis of coal (coalification). In: G. Larsen and G.V. Chilingar (Editors), Diagenesis in Sediments. (Developments in Sedimentology, 8. ) Elsevier, Amsterdam, pp. 391-415. Winans, R.E., 1984. Chemistry and characterization of coal macerals:overview. In: R.E. Winans and J.C. Crelling (Editors), Chemistry and Characterization of Coal Macerals. ACS Symp. Ser., 252: 1-20. Younkin, K.A., Norton, G.A., Straszheim, W.E. and Markuszewski, R., 1987. Experimental electron beam microanalytical procedure for determining oxygen in coal. Fuel, 66: 10021007. Zetsche, F., 1931. Isolation of spore membranes, cuticles, etc. from coal. Helv. Chim. Acta, 14: 59-60.