Energy & Fuels 1992,6, 125-136
could have bioaccumulated methylated As in their tissues. Future work on the quantification of As in nonpyritized plant cell walls may lead to the identity of particular plant types that concentrated As. Cell-wall replacement pyrite is rare (less than 3% of the total pyriteg by volume) and As-bearing CWR pyrite is even rarer in facies samples of the Upper Freeport coal bed. This is to be expected as the toxicity of As precludes an organic association for all but a small amount of the substance. However, we do fiid it significant that some pyritized plant cell walls contain As. The Upper Freeport paleoswamp was probably similar to some other Pennsylvanian peat swamps, and because CWR pyrite is present in many Appalachian Basin Pennsylvanian age coals (W. Grady, 1990, personal communication) the postulated mechanism of As emplacement may apply to other
125
Pennsylvanian coal beds as well.
Conclusions Electron microprobe analyses of gravity separates from three facies of the Upper Freeport coal bed show that some CWR pyrite morphologies contain As. Arsenic values in CWR pyrite ranged from 0.06 to 1.87 w t %. Arsenicbearing CWR pyrite is most common in the uppermost facies of this coal bed. The As is postulated to have been present in the peat-forming environment where it may have been taken up by some of the plants and encapsulated in the cell walls through such processes as detoxification. Through chemical or biologic pathways, methylated organo-As compounds could have been released or volatilized and incorporated into cell walls during pyritization.
Chemical Structure of Bituminous Coal and Its Constituting Maceral Fractions As Revealed by Flash Pyrolysis+ Margriet Nip$ and Jan W. de Leeuw* Organic Geochemistry Unit, Faculty of Chemical Engineering and Materials Science, Delft University of Technology, De Vries van Heystplantsoen 2, 2628 RZ Delft, The Netherlands
John C. Crelling Coal Characterization Laboratory, Department of Geology, Southern Illinois University at Carbondale, Carbondale, Illinois 62901 Received December 10, 1990. Revised Manuscript Received November 25, 1991
To study the relationships between the chemical structures of coals, coal macerals, and their precursors (plant tissues), a high-volatile bituminous Upper Carboniferous coal and ita constituting maceral fractions, cutinite, resinite, sporinite, vitrinite, pseudovitrinite, semifusinite, and fusinite, were investigated by Curie point pyrolysis-gas chromatography and Curie point pyrolysis-gas chromatography-mass spectrometry. Single maceral fractions were isolated from the coal by density gradient centrifugation. In the pyrolysates of the density fractions similar types of pyrolysis products are observed; however, the relative contributions of these products vary considerably with the various density fractions. The variation in the distribution of the various pyrolysis products is sufficient to distinguish between the various maceral groups and to serve as a maceral “fingerprint”. The internal distribution patterns of the alkyl derivatives of some of these families of pyrolysis products are similar for all density fractions. This observation and the fact that all families of pyrolysis products occur in the pyrolysates of all density fractions probably indicate that on a chemical basis these fractions representing single macerals do not represent single chemical compounds. Despite this fact, it is still possible to relate the chemical nature of the most prominent types of pyrolysis products to structural elements present in the precursors of the macerals which are most dominantly present in the density fractions.
Introduction Coal is an extremely complex, heterogeneous material which consists mainly of a large variety of organic components derived from plant tissues. The biopolymers present in plant tissues and their chemical behavior upon coalification determine for the greater part the chemical structure of a coal. The organic components that constitute coal are called macerals. This term was originally ‘Delft Organic Geochemistry Unit, Contribution 139. Present address: Industrial Quimica del Nalon S. A., Apartado 8,33100 Trubia, Provincia de Oviedo, Spain.
*
introduced by Marie C. Stopes in 1935l in the following way: “I now propose the new word “Maceral ” (from the Latin macerare, to macerate) as a distinctive and comprehensive word tallying with the work “mineral”. Its derivation from the Latin word to “macerate” appears to make it peculiarly applicable to coal, for whatever the original nature of the coals, they now all consist of the macerated fragments of vegetation, accumulated under water. The concept behind the word “macerals”is that the complex of biological units (1)Stopes, M.C.h o c . R. SOC.,Ser. B lSlS,!W, 470.
0887-0624/92/2506-0125$03.00/00 1992 American Chemical Society
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126 Energy & Fuels, Vol. 6, No. 2, 1992
represented by a forest tree which crashed into a watery swamp and there partly decomposed and was macerated in the process of coal formation, did not in that process become uniform throughout but still retains delimited regions optically differing under the microscope, which may or may not have different chemical formulae and properties.” Implicit in this definition is the idea that macerals may differ from each other chemically and the idea that because they are derived from various altered plant tissues they need not be single chemical compounds. Indeed, because most plant tiasuea consist of a number of different chemical compounds, it should be expected that the same is true for macerals derived from them. This idea is supported at the most basic level by the general acceptance of the evidence that coal and coal macerals can be considered as two-phase systems consisting of at least a small portion of a largely aliphatic mobile extractable phase in a more aromatic host phase. Since the application of more advanced analytical techniques, such as FT-IR and solid-state 13C NMR spectroscopy, X-ray analysis, and analytical pyrolysis in combination with gas chromatography, maw spectrometry, and gas chromatography-mass spectrometry, a more detailed knowledge has been obtained of the chemical relationships between coals, coal macerals, and their precursors. Moreover, the development of more advanced isolation methods for the separation of macerals from a coal, such as density gradient centrifugation, has significantly enhanced the possibilities to investigate single maceral fractions. Studies in which FT-IR and 13C NMR spectroscopy and analytical pyrolysis are used for the analysis of maceral fractions isolated by density gradient centrifugation were reported by Brenner? Kuehn et al.? Pugmire et al.; and Nip et al.s In this paper, the above-mentioned approach was used to analyze an Upper Carboniferous coal and its constituting maceral fractions. Curie point pyrolysis in combination with gas chromatography and gas chromatography-mass spectrometry was used to analyze the coal and the maceral fractions. Curie point pyrolysis has proved to be a very useful technique for the chemical characterization of insoluble organic matter present in coals and sediments and its precursors.+” This article is divided into two parts. In the first part, attention is paid to the internal compition of the families of pyrolysis products which are most abundantly present
loo. 90-
80~
70-
a
9 600 M5c-
a
Iy
2
40.
ri a 20-
01;
1.1
1.1
1.1
1.5
DENSITY (gm/ml)
Figure 1. Density profile of coal SIU 6475. 1O O T
DENSITY (gm/ml) Figure 2. Density profile of the concentrated liptinite density fractions of coal SIU 6475.
in the pyrolysates of the coal and the maceral fractions. In the second part of the article, the differences between the chemical natures of the pyrolysates of the maceral fractions are discussed based on histograms, in which the relative contributions of these families of pyrolysis products to the “total pyrolysate” are shown. Moreover, the chemical relation between the pyrolysate of the coal on the one hand and the pyrolysates of the maceral fractions on the other hand is discussed.
Experimental Section ~~
(2) Brenner, D. In Chemistry and Characterization of Coal Macerals; Winans, R. E., Crelling, J. C., Eds.; ACS Symp. Ser. 252; American Chemical Society: Washington, DC, 1984; pp 47-64. (3) Kuehn, D. W.; Davis, A,; Painter, P. C. In Chemistry and Char-
acterization of Coal Macerals; Winans, R. E., Crelling J. C., Eds.;ACS Symp. Ser. 252; American Chemical Society: Washington DC, 1984; pp 99-120. (4) Pugmire, R. J.; Woolfender W. R.; Mayne C. L.; Karas J; Grant D. M. In Chemistry and Characterization of Coal Macerals; Winans, R. E., Crelling J. C., E&.; American Chemical Society: ACS Symp. Ser. 252; Washington, DC, 1984; pp 79-98. (5) Nip, M.; De Leeuw; J. W.; Schenck; P. A.; Windig; W.; Meuzelaar, H. L. C.; Crelling; J. C. Geochim. Coamochim. Acta 1989,53,671-683. (6) Larter, S. R.; Solli, H.; Douglas, A. G.; De Lange, F.;De Leeuw, J. W. Nature 1979,279,405-408. (7) Van Greas, G.; De Leeuw, J. W.; Schenck, P. A. J. Anal. Appl. Pyrol. 1980,2, 265-276.
(8) Van Graas, G.; De Leeuw, J. W.; Schenck, P. A. In Advances in Organic Geochemistry--1979; Douglas, A. G., Maxwell, J. R., Eds.; Pergamon Press: Oxford, U.K., 1980; pp 486-494. (9) Meuzelaar, H. L. C.; Harper, A. M.; Pugmire, R. J.; Karaa, J. Int. J. Coal Geol. 1984,4, 143-171. (10) Meuzelaar, H. L. C.; Harper, A. M.; Hill,G. R.; Given, P. H. Fuel 1984,63,640-652.
(11) Saiz-Jimenez, C.; Boon, J. J.; Hedges, J. I.; Hessels, J. K. C.; De Leeuw, J. W. J. Anal. Appl. Pyrol. 1987, 11, 437-450.
Samples. The coal used (SIU 6475)originates from the Brazil Block seam of Upper Carboniferous age at the Roaring Creek Mine, Parke County, IN.12J3 Ita constituent maceral fractions isolated from a single coal sample (SIU6475)by density gradient centrifugation were cutinite, resinite, sporinite, vitrinite, two semifusinites, and fusinite. The maceral fraction pseudovitrinite was also studied. However, this maceral was isolated by density gradient centrifugation from hand-picked layers of vitrain from another sample of the same seam at an adjacent location in the same mine. Density Fractionation. The density fractions cutinite, resinite,sporinite, vitrinite, semifusinte, and fusinite were separated from a demineralized sample of the Brazil Block seam (SIU6475) by means of density gradient centrifugation. For a more detailed description of coal SIU 6475 and the density gradient centrifu(12) Eggert, D. L.; Phillips, T. L. Guidebook to Environmenta of Plantcoal Balls, Paper Coals, and Grey Shale Floras in Fountain and Parke Counties, Indiana. In Ninth Znt. Conf. Carb. Strat. Geol. 1979, 46-73. (13) Eggert, D. L.; Phillips, T. L. Indiana Geol. Suru. Spec. Rept. 1982, 30, 1-43.
Chemical Structure of Bituminous Coal
Energy & Fuels, VoE. 6, No. 2,1992 127
1001
40
20 0 1.3
I
1.4 DENSITY (gm/ml)
I
1
.s
Figure 3. Density profile of the inertinite density fractions of coal SIU 6475 (A = SFUSA, B = SFUSB, C = FUS). gation method, see Nip et ala5and references therein. Figure 1 shows the density profile of sample SIU 6475. The most dominant peak represents the maceral vitrinite. From this peak, a vitrinite concentrate sample was collected with a peak density of 1.273g/mL (VIT). The shoulder which represents the liptinite density fractions (with density values ranging from 1.0 to 1.27 g/mL) was concentrated and then reseparated into narrower density ranges (Figure 2). Fluorescence microscopy of the individualpeak fractions in this profile showed that the three main peaks were composed of separate liptinite macer&-cutinite (d = 1.089 g/mL, CUT), resinite (d = 1.110 g/mL, RES), and sporinite (d = 1.116 g/mL, SPOR). Specifically,the material in the peak fractions had the same fluorescence spectra as the respective macerals in the whole coal. The same fractionation procedure was followed for the inertinite density fractions (with density values ranging from 1.35 to 1.49 g/mL). The density profile of the inertinite density fractions of sample SIU 6475 shows a broad peak on the low density and a definite shoulder on the higher density side (Figure 3). The broad peak represents the semifusinite density fraction of which two subsamples were collected (semifusinite A, d = 1.362 g/mL SFUSA, and semifusinite B, d = 1.413 g/mL, SFUSB). The shoulder represents the fusinite density fraction a sample of which was collected with a density value of 1.471 g/mL (FUS). The density fraction pseudovitrinite (d = 1.306 g/mL, PSEUDOVIT) was separated by density gradient centrifugation from a hand-picked vitrain band from the same seam as coal SIU 6475. The pseudovitrinite occurring in the hand-picked vitrain bands is a different substance than the normal vitrinite. The pseudovitrinite has a higher density and a higher reflectance than the normal vitrinite. While both the vitrinite and inertinite group maceral fractions showed petrographic and reflectance properties consistent with their occurrence in the whole coal, these were not quantified. Figure 4 shows photomicrographs of the cutinite density fractions (upper left), the sporinite density fraction (upper right), the vitrinite density fraction (lower left), and fusinite density fraction (lower right). Each of the individual density fractions as illustrated appear petrographically to be internally homogeneous and morphologically distinct from the other fractions. Petrographic Analysis. Petrographic analysis data of sample SIU 6475, obtained by combination of white and blue light microscopy,14are given in Table I. Three points should be noted here. First, the areas under the various density profiles may not be proportional to the petrographic maceral analysis because not all of the particles are single-phase particles. Second, to ensure maxi" uniformity the samples actually used in this study were taken only from the uppermost portions of the respective maceral peaks. Third, because of the two restrictions mentioned above a proportionalized estimate of the bulk chemical composition of the whole coal based on the maceral composition of the coal may (14)Crelling, J. C.;Bensley, D. F. In Middle and Late Pennsyloanian Strata on Margin of Illinois Basin, Vermilion County, Illinois and Vermilion and Parke Counties, Indiana; Langenheim, R. L., Mann, C. J., E%.; SOC.&on. Pdeontol. Mineral.: Tenth. Annual Field Conference Guidebook, 1980; pp 93-104.
Table I. Petrographic Analysis Data of Coal SIU 647J0 vitrinite 51.7' 44.96 pseudovitrinite 14.1' 6.6b sporinite 6.3' 13.ab resinite 0.5' 1.16 cutinite 4.1" 7.36 fluorinite 0.26 bituminite 0.3' 3.0b 0.4' liptodetrinite 2.3b fusinite 3.1' 8.3b semifusinite 4.ab 16.00 0.1' macrinite semimacrinite 0.P micrinite 3.00 8.0b
'Data obtained by the standard petrographic method ASTM D-2799. bData obtained by combination of white and blue light analysis. not agree with the actual bulk chemical Composition. Curie Point Pyrolysis. Prior to Curie point pyrolysis, approximately 10 pg of a crushed coal or density fraction (particle size < 10 pm) was pressed onto the surface of the ferromagnetic wire according to a method described by Venema and Veurink.l6 Curie Point Pyrolysis-Gas Chromatography (Py-GC). Py-GC was carried out using an instrument as described by van de Meent et al.16 The Curie temperatures of the wirea used were 770 and 358 "C. The wires were kept at the final temperature for 10 s. The gas chromatograph (Packard Becker, Model 419), equipped with a cryogenic unit (Packard, Model 799), was programmed from 0 "C (5 min) to 300 OC (25 min) at a rate of 3 OC/min. Separation was achieved using a fused silica capillary column (25 m X 0.32 mm i.d.), coated with CP-Sil5 ( f hthickneas 0.42 pm, C.B.). Helium was used as the carrier gas. Curie Point Pyrolysis-Gas Chromatography-Mass Spectrometry (Py-GC-MS). The same type of pyrolysis unit and capillary column as mentioned before were also used in the Py-GC-MS mode. GC-MS was performed on a Varian 3700 gas chromatograph connected to a Varian MAT 44 quadrupole maea spectrometer. Electron impact maas spectra were obtained at 80 eV under the following conditions: cycle time, 2 s; mass range, m/z 20-450 up to scan 250 and mlz 50-450 after scan 250; mlz 28,32,40, and 44 were omitted from the reconstructed total ion currents, because an open atmospheric split was used as the interface.
Results and Discussion Py-GC and Py-GC-MS of the Coal and the Density Fractions. Figure 5 shows the Py-GC trace of coal sample SIU 6475. The identification of the pyrolysis products is based upon comparison of their mass spectra and relative retention times with those of standard compound^.^^-^^ The pyrolysateof sample SIU 6475 is mainly characterized by the presence of alkylbenzenes, alkylphenols, alkylnaphthalenes, alkylindenes, alkylnaphthols, alkylphenanthrenes, alkylanthracenes,alkylfluoranthenes,alkylpyrenes, and homologous series of n-alkanes and nalk-1-enes. Because the chemistry of coal is by the chemical nature of ita constituent macerals, it was decided to analyze the density fractions CUT, RES, SPOR,VIT, SFUSA, SFUSB, and FUS by Py-GC and Py-GC-MSto (15)Venema, A.; Veurink, J. J. Anal. Appl. Pyrol. 1986, 7,207-213. (16)Van de Meent, D.; Brown, S. C.; Philp, R. P.; Simoneit,B. R. T. Geochim. Cosmochim. Acta 1980,44,999-1013. (17)Svob, V.; Deur-Siftar, D. J. Chromatogr. 1974,91,677-689. (18)Lee, M. L.; Vassilaros, D. L.; White, C. M.; Novotny, M.Anal. Chem. 1979,51,768-774. (19)Schroeder, H. J. High Resolution Chromatogr. Chromotogr. Commun. 1980,3,38-44. (20)Radke, M.; Willsch, H.; Leythaeuser, D.; Teichmueller, M.Geochim. Cosmochim. Acta 1982,46,1831-1848. (21)Radke, M.; Welte, D. H.; Willsch, H. Org. Geochem. 1986, IO, 51-63. (22)Rowland, S.J.; Aareskjold, K.; Xuemin, G.; Douglas, A. G. Org. Geochem. 1986, IO, 1033-1040.
Nip et al.
128 Energy & Fuels, Vol. 6, No. 2, 1992
a
Figure 4. SEM photomicrographs of the cutinite density fraction (upper left), sporinite density fraction (upper right), vitrinite density fraction (lower left), and fusinite density fraction (lower right), isolated from coal SIU 6475.
define their chemical relationships with coal SIU 647J. Figure 6 shows the Py-GC traces of CUT, VIT, and FUS. The pyrolysates of these density fractions are presented here because they represent the three major maceral groups in bituminous coals.23 The pyrolysates of the three density fractions are mainly characterized by the same pyrolysis products as those encountered in the pyrolysate of coal SIU 6475. It is clear, however, that the relative contributions of these pyrolysis products to the pyrolysates of these density fractions vary significantly. CUT is mainly characterized by the homologous series of n-alkanes and n-alk-l-enes,VIT contains relatively more phenolic and other aromatic pyrolysis products, and in the pyrolysate of FUS, a relatively high contribution of polycyclic aromatic hydrocarbons is observed. The Internal Distribution Patterns of the Various Groups of Pyrolysis Products. The relative abundances ~
~
_
_
_
(23) Stach, E.; Taylor, G. H.; Mackowsky, M. Th.; Chandra, D.; Teichmueller, M.; Teichmueller, R. Stachs Textbook of Coal Petrology, 3rd ed.; Gebrueder Borntraeger: Berlin, Stuttgart, 1982.
Table 11. Characteristic m / z Values, Used for Mass Chromatography in Order To Calculate the Internal Distribution Patterns of Various Groups of Pyrolysis Products for the Density Fractions groups of pyrolysis products alkylbenzenes alkylphenols alkylindenes alkylnaphthalenes alkylnaphthols alkylphenanthrenes/alkylanthracenes alkylfluoranthenes/alkylanthracenes C7 to C12 n-alkanes C, to C12n-alk-l-enes
CO"
C1"
cza cg
78 94 116 128 144 178 202 57 and 71 55 and 69
92 108 130 142 158 192 216
106 122 144 156 172 206
120 136 170 220
Alkyl derivatives.
of the individual components within the classes of pyrolysis produds mentioned above were investigated to see whether systematic changes occur within the different density fractions, thus leading to a better understanding of the chemical nature and the diagenetic evolution of the higher plant precursors of the most dominantly present macerals.
Energy & Fuels, Vol. 6, No. 2, 1992 129
Chemical Structure of Bituminous Coal
t6
0
1
10
9
11
12
13
14
15
16
17 18 19 20 21 22 23 24 25 2 6 2 7 2 8
-
H O C )
Figure 5. Py-GC trace of coal SIU 6475. For analytical conditions, see Experimental Section. The numbers refer to the chain lengths of the n-alk-l-enes and n-alkanes. i = indene (* = contamination).
CUT
I
C6
I
8
9
10
11
12
13
1L
15
16 17
18 19 20 21 22 23 24 25 26 27 28 29
16 17
18 19 20 21 22 23 24 25 26 27 28
t
C7
8
9
10
11
12
13
14 15 t
I
FUS
C6
7
8
9
10
11
12
13
14
1s
-
1s 17 18 19 20 21 22 23 24 K 26 27 2 8 2 9 3 0 , H O C )
Figure 6. Py-GC traces of the density fractions CUT, VI",and FUS. For analytical conditions, see Experimental Section. The numbers refer to the chain lengths of the n-alk-l-enes and n-alkanes. i = indene (* = contamination; the peaka indicated mainly reflect weakeners such aa phthalates).
Nip et al.
130 Energy &Fuels, Vol. 6, No. 2,1992
PH
B
E B M-/P-
T
0-
0
M+P
alkylbenzsnes
Figure 7. Distribution pattern of the alkylbenzenes in the pyrolysate of VIT. Peak intensities were calculated relative to the highest peak in the distribution pattern. B = benzene; T = toluene; EB = ethylbenzene; M-/P- = m- and p-xylene; 0- = o-xylene; C3 = C,-alkylbenzenes; (A) n-propylbenzene; (B) 1methyl-3-ethyl1-methyl-4-ethylbenzene; (C) 1,3,5-trimethylbenzene; (D) 1-methyl-2-ethylbenzene; (E) 1,2,4-trimethylbenzene;(F)1,2,3-trimethylbenzene(Puttmann, personal communication).
+
To this end, the relative abundance5 of the alkyl derivatives within these groups of pyrolysis products were calculated, based on data obtained by mass chromatog raphy. Characteristic m/z values were chosen for each group of pyrolysis products (Table II). The identifications of the peaks in the mass chromatograms were checked by inspection of the full mass spectra. The internal distribution patterns of the Co- to C3-alkylbenzenes,C3-alkylphenols, Co- to Cz-alkylindenes,Co- and C1-alkylfluoranthenes and alkylpyrenes, and C7-to ClZ-n-alkanesand C7to Clz-n-alk-l-enesare very similar for all density fractions. This similarity suggests a common origin of these groups of pyrolysis products in all density fractions. Because of this similarity, the distribution patterns of these groups of pyrolysis products will be discussed for only one density fraction (VIT). The internal distribution patterns of the Co- to C3-alkylnaphthalenes,Co-to C,-alkylnaphthols, and Co-to C3-alkylphenanthrenesand alkylanthracenes on the other hand are not similar for all density fractions. Hence, it is concluded that these pyrolysis products reflect the presence of characteristic structural elements of the macer& which are most dominantly present in the density fractions. Figure 7 shows the distribution pattern on the Co- to C3-alkylbenzenesin VIT (Table 11). Seven C3-alkylderivatives, represented by six GC peaks, are shown in the distribution pattern. Isopropylbenzene was recorded as well, but only in very low quantities. The origin of alkylbenzenes in the pyrolysates of coals and density fractions is not fully understood. They may originate from lignin, an important constituent of woody tissue. The phenolic moieties present in lignin may have been chemically altered due to the coalification and/or fusinization processes. It is thought that upon increasing influence of these diagenetic processes, all -OCH3 groups are finally removed from the lignin originally p r e ~ e n t .The ~ ~ re~~ maining structural elements may give rise to toluene and ethylbenzene upon pyrolysis (Figure 7). Probably only a selective part of the lignin biopolymer is diagenetically (24) Van Krevelen, D. W. Coal; Efeevier: Amsterdam, 1961. (25) Senftle, J. T.; Larter, S.R.; Bromley, B. W.; Brown, J. H. Org. Geochem. 1986,9, 345-350.
C2
c3
alkylphenols
c3
Figure 8. Distribution pattern of the alkylphenols in the pyrolysate of VI”. Peak intensities were calculated relative to the highest peak in the distribution pattern. PH = phenol; 0 = o-cresol; M + P = m- and p-cresol; Cz = Cz-alkylphenols,tentatively identified as (A) 2,4- + 2,bdimethylphenol and (B) 34-ethylphenol 3,5-dimethylphenol; C3 = C3-alkylphenols.
+
+
altered in such a way that it gives rise to alkylbenzenes upon pyrolysis, since phenolic pyrolysis products, which may be indicative also for the presence of diagenetically altered lignin, are encountered as well in the pyrolysates of the density fractions (vide infra). The alkylbenzenes may also originate from tannins,important constituents of barks and trees. Wilson and Hatche9 recently pointed out that tannins are selectively preaerved in brown coal deposits. Whether the preserved tannins will undergo loss of phenolic hydroxyl groups to form aromatic hydrocarbons upon further coalification is not yet known. In case the alkylbenzene pyrolysis products are, in part, derived from resins, a source from polysesquiterpenoids, recently encountered in Dammar resins and fossilized angiosperm resins, seems most appropriate, albeit that a relatively higher abundance of isopropylbenzene would have been expected (van Aarssen et al., manuscript in preparation). The alkylbenzene pyrolysis products can also be derived partly from sporopollenins. Recent publications have shown that sporopollenins do not consist of carotenoid esters as previously s u g g e ~ t e d ~but ~ -represent ~ macromolecular subetancea built up either from phenylpropanoid units30p31or from long n-alkyl chain moieties.32 It can be speculated at this moment that the polyphenylpropanoid type of sporopollenin upon coalificationundergoes changes similar to those in lignin ultimately resulting in polymeric substances yielding alkylbenzenes upon pyrolysis. Figure 8 shows the internal distribution patterns of the alkylphenols in VIT (Table 11). Five C,-alkylphenols are represented by only two GC peaks. They were tentatively identified aa 2,4-dimethylphenol and 2,Bdimethylphenol (peak A, Figure 8) and 3-ethylphenol,4-ethylphenol, and 3,5-dimethylphenol (peak B, Figure 8). 2,b-dimethyl(26) Wilson, M.A.; Hatcher,P. G. Org. Geochem. 1988,12,539-546. (27) Libert, P. PhD Diasertation, University of Bordeaux, France, 1974. (28) Marchand, A.; Libert, P.;Achard, M.-F.;Combaz, A. In Aduances in Organic Geochemistry-1973; Tissot, B.; Bienner, F., Eds.; Edition Technip: Par& 1974; pp 117-135. (29) Brooks, J.; Shaw,G. Grana 1978,17,91-97. H.;Grumbach, K. Wie Naturforsch., (30) Prahl, A. K.; Sp-tubbe, Sect. C., Biosci. 1985, 40,621-626. (31) Osthoff, K. S.;Wiermann, R. J. Plant Physiol. 1987,131, 5-15. (32) Guilford, W. J.; Schneider,D. M.;Labovitz, J.; Opela, S. J. Plant Physiol. 1988, 86, 134-136.
Chemical Structure of Bituminous Coal
I
c1
c2 alkylindenes
Figure 9. Distribution pattern of the alkylindenes in the pyrolysate of VIT. Peak intensities were calculated relative to the highest peak in the distribution pattern. I = indene; C1 = C1alkylindenes; C2 = C2-alkylindenes.
phenol, 2-ethylphenol, 2,3-dimethylphenolYand 3,4-dimethylphenol were present as well, but in very small amounts. Eight C3-akyl derivatives were recorded by maas chromatography (Figure 8). For the greater part the phenolic pyrolysis products probably are derived from diagenetically altered lignin present in woody tissue. Apart from lignin, modern woody tissue consists of a cellulose and hemicellulose f r a ~ t i o n . ~ ~After " ~ deposition, the cellulose and hemicellulose portions are decomposed by microorgani~ms,"J+~~ although lignin is known to decelerate the decay of associated p o l y ~ a c c h a r i d e s . ~The ~~ rate of lignin decomposition depends highly on the burial conditions, which can be either aerobic or anaerobic.39 Compared with its associated polysaccharide fraction however, lignin is only slightly (bio-) chemically modified, viz., by the demethylation of the 4 C H 3 groups originally present in the guiacyl and syringyl monomeric units and/or demethoxylation of the lignin b i ~ p o l y m e r . ~ ~ Another possible source of phenolic pyrolysis products may be diagenetically altered sporopollenin. Phenolic pyrolysis products are encountered in the pyrolysates of modem and thermally treated sporopollenin of L. clavatumSu Given et a.l.45also noted the presence of phenolic groups in sporinites. As indicated above, the observed pyrolysis products may originate from the phenylpropanoid type of sporopollenins as described by Prahl et and by Osthoff and Wiermann.3l The strong similarity (33)Timell, T.E. Tappi 1957,40,568-572. (34)Aspinall, G. 0.In The Carbohydrates: Pigman, W., Horton, D., Eds.; Academic Press: London, 1970; pp 515-536. (35)Hatcher, P.G.;Breger, I. A.; Earl, W. I. Org. Geochem. 1981,3, 49-55. (36)Hatcher, P. G.; Breger, I. A.; Szeverenyi, N. M.; Maciel, G. E. Org. Geochem. 1982.4. 9-18. (37)Hatcher, P. G.; Breger, I. A,; Maciel, G. E.; Szeverenyi, N. M. Proc. 1983 Int. Conf. Coal Sci. 1983,31lF313. (38)Hatcher, P. G.; Spiker, E. C.; Szeverenyi, N. M.; Maciel, G. E. Nature 1983,305,498-501. (39)Hedges, J. I.; Cowie, G. L.; Ertel, J. R.; Barbour, R. J.; Hatcher, P. G. Geochim. Cosmochim. Acta 1985,49,701-711. (40)Barghoorn, E. S. J. Sediment. Petrol. 1952,22,34-41. (41)Sen, J.; Basak, R. K. Geol. Foeren. Foehandl. Bd. 1957, 79, 137-758. (42)Nicholas, D. D. Wood Deterioration and Its Preuentation by Preservative Treatments; Syracuse University Press: Syracuse, NY,1990; VOl. I. (43)Chaffee, A. L.; Johns, R. B.; Baerken, M. J.; De Leeuw, J. W.; Schenck, P. A.; Boon, J. J. Org. Geochem. 1984,6,409-416. (44)Schenck, P.A.; De Leeuw, J. W.; Van Graas, G.; Haverkamp, J.; Bouman, M. In Organic Maturation Studies and Fossil Fuel Erploration; Brooks, J., Ed.; Academic Press: London, 1981;pp 225-237. (45)Given, P.H.; Peover, M.; Wyss, W. F. Fuel 1960,39, 323-340.
Energy & Fuels, Vol. 6, No. 2, 1992 131
F
P
c1
alkylfluaronthenes/pyrcnes
Figure 10. Distribution pattern of the alkylfluoranthenes and alkylpyrenes in the pyrolysate of VIT. Peak intensities were calculated relative to the highest peak in the distribution pattern. F = fluoranthene; P = pyrene; C1 = Cl-alkylfluoranthenes/pyrenes.
of the alkylphenol distribution patterns in all pyrolysates may point toward a common origin of these pyrolysis products in the density fractions. Given& suggests a possible contribution of condensed tannins which occur ubiquitously in modern plants. In view of their chemical structure, they may give rise to phenolic pyrolysis products. The recent finding of selectively preserved tannin components in brown coals supports this.26 It is worthwhile mentioning that the possibility of a source of alkylphenols other than lignin is subject of increasing debate.25 Figures 9 and 10 show the distribution patterns of the Co-to C2-alkylindenesand Co- and C1-alkylfluoranthenes and akylpyrenea in VIT, respectively (Table 11). Although more C1- and C2-isomersare known for indene, only two C1- and four C2-alkyl derivatives were found in the pyrolysates of the density fractions. The C3-alkylderivatives were not recorded by mass chromatography (Figure 9). With respect to the akylfluoranthenes and akylpyrenes, only foure C1-alkyl derivatives were recorded in the pyrolysates of the density fractions. The Cz- and C,-alkyl derivatives were not encountered at all (Figure 10). The origin of these two latter groups of pyrolysis products is not known at present. The similar distribution patterns of the families of pyrolysis products discussed above suggest that diagenetically altered biopolymers originating from different plant tissues are at least in part present in all density fractions. In summary, these fractions, although petrographically defined as single maceral concentrates, have a number of chemical similarities. The internal distribution patterns of the Co- to C3-alkylnaphthalenes, Co- to C3-alkylphenanthrenes and alkylanthracenes for all density fractions are shown in Figures 11,12, and 13, respectively (Table II). With respect to the alkylnaphthalenes, 10 Cz-alkylnaphthalenes were tentatively identified to be represented by six GC peaks (Figure 11). Although more C3-alkylderivatives are known for this type of compounds, only 10 were revealed upon mass chromatography (Figure 11). The origin of these pyrolysis products in the density fractions is virtually unknown. They may be formed upon coalification and/or charring from other, probably less aromatic, structural moieties originally present in the structures of the macerals. (46)Given, P. H. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, Il., Eds., Academic Press: London, 1 9 M pp 63-252.
132 Energy & Fuels, Val. 6, No. 2, 1992
Nip et al.
I w
PSEUDOMT
RES
SPOR
vrr
R
m
Ns
Figure 11. Distribution patterns of the alkylnaphthalenes in the pyrolysate of CUT (a), RES (b),SPOR (c), VIT (d), PSEUDOVIT (e), SFUSA (0, SFUSB (g), and FUS (h). Peak intensities were
calculated relative to the highest peak in each distribution pattern. N = naphthalene; 2 = 2-methylnaphthalene; 1 = l-methylnaphthalene; Cz = Cz-alkylnaphthalene, tentatively identified as (A) 2-ethyl- 1-ethylnaphthalene; (B) 2,6- + 2,7-dimethylnaphthalene; (C) 1,3- 1,7-dimethylnaphthalene;(D) 1,6-dimethylnaphthalene; (E) 2,3- + 1,5-dimethylnaphthalene;(F) 1,2-dimethylnaphthalene; C3 = C,-alkylnaphthalenes (Puttmann, personal communication).
+
+
In the case of the density fraction RES, these types of pyrolysis products may be partly derived from higher plant resins, which are the maturation products of bicyclic sesquiterpenoid h y d r o ~ a r b o n s . ~ ~ - ~ ~ The pyrolysates of all density fractions except PSEUDOVIT reveal seven C1- and C2-alkylnaphthols. Only four C2-alkyl derivatives were recorded in the pyrolysate of PSEUDOVIT (Figure 12). The origin of these pyrolysis products is as yet unknown. With respect to the internal distribution patterns of the alkylphenanthrenes and alkylanthracenes, five C,-alkylphenanthrenes and alkylanthracenes,tentatively identified as 3-methylphenanthrene (peak A), 2-methylphenanthrene (peak B), 2-methylanthracene (peak C), 9-methylphenanthrene and 1-methylanthracene (peak D), and 1methylphenanthrene (peak E, Figure 131, and eight C2-and four C3-alkylderivatives were recorded in the pyrolysates of the density fractions (Figure 13). Phenanthrenes and anthracenes are known to be maturation products of tricyclic diterpenoid hydrocarbons present in plant re(47) Philp, R. p.;Gilbert, T. D.; Friedrich, J. M. Ceochim. Cosmochim. Acta 1981,45, 1173-1180.
(48) Mukhopadhyay, P. K.; Gormly, J. R. Org. Geochem. 1984, 6, 439-454. (49) Simoneit, B. R. T.; Grimalt, J. 0.;Wang, T. G.; Cox, R. E.; Hatcher, P. G. Niasenbaum, A. Org. Geochem. 1986,10, 877-899.
w
1
rl
d I
,
Ns
h CI
Figure 12. Distribution patterns of the alkylnaphthols in the pyrolysate of CUT (a),RES (b), SPOR (c), VIT (d), PSEUDOVIT ( e ) .SFUSA (f).SFUSB (E). and FUS (h). Peak intensities were &dated regtive to the t i h e s t peak in each distribution pattern. 2 = 2-naphthol; 1 = 1-naphthol; C1 = C,-alkylnaphthols; C2 = C2-alkylnaphthols.
sins.47~49~50 The distribution pattern of these classes of compounds in the density fraction RES might reflect such an origin. The dissimilar distribution patterns observed in the other density fractions point, at least partly, to other sources of these pyrolysis products. Schematic Representation of the Pyrolysates of the Density Fractions. The internal distribution patterns of the alkyl derivatives of the various families of pyrolysis products in the pyrolysates of the density fractions did not clearly show differences that could be related to differences in chemical structures of the macerals, which are on a petrographic basis most dominantly present in the density fractions. Therefore, it was decided to investigate whether the distribution patterns of the groups of pyrolysis products were characteristic for the chemical nature of the density fractions. To this end, the complex pyrolysates of the density fractions were represented by normalized histograms of the summed peak intensities of the C7-t o C,,-n-alkanes, C7- to C,,-n-alk-1-enes, Co- to C3-alkylbenzenes, Co- to C3-alkylphenols, Co- to C3-alkylnaphthalenes, Co- to C2-alkylindenes, Co- to C2-alkylnaphthols, Co-to C3-alkylphenanthrenes/anthracenes, and Co- to C,-alkylfluoranthenes/pyrenes. Figure 14 shows these histograms for the various density fractions. It is immediately clear that large differences are observed between their pyrolysates. The histogram of CUT (Figure 14a) shows that the C7-to C12-n-alkanesand (50) Venkatesan, M. I.; Ruth, E.; Kaplan, I. R. Geochim. Cosmochim. Acta 1986, 50, 1133-1139.
Chemical Structure of Bituminous Coal
I
..
an
I
RES
I
.
C 51
Energy & Fuels, Vol. 6, No. 2, 1992 133 PSEUDOvn
P . , . e ”. I t .
c1
Ns
I d
,
.
c,
h
Figure 13. Distribution patterns of the alkylphenanthrenesand alkylanthracenesin the pyrolysate of CUT (a), RES (b), SPOR (c),VIT (d),PSEUDOVIT(e),SFUSA (0,SFUSB (g),and FUS (h). Peak intensities were calculated relative to the highest peak in each distribution pattern. P = phenanthrene; A = anthracene; C1 = C1-alkylphenanthrenes/anthracenestentatively identified as (A) 3-methylphenanthrene; (B) 2-methylphenanthrene; (C) 2-methylanthracene; (C) 9-methylphenanthrene + 1-methylphenanthrene; (E) 1-methylphenanthrene; C2 = C2-alkylphenanthrenesfanthracenes; C3 = C3-alkylphenanthrenes/ anthracenes.
n-alk-1-enes are the most prominent pyrolysis products. These pyrolysis products, together with the Clz-to Ca-nalkanes and n-alk-1-enes and the C7-to Ca-au-alkadienes (represented by GC peaks of much lower intensity which elute just before n-alk-1-enes and n-alkanes with corresponding chain lengths), are reported to represent a nonhydrolyzable polymethylenic biopolymer which occurs in some modem and fossil plant cuticles.5l This biopolymer is the plant cuticle consituent with the highest chemical resistance toward diagene~is.~~ The cutinite fraction consists almost exclusively of the nonhydrolyzable polymethylenic biopolymer and hence explains the highly aliphatic nature of this ma~eral.~ The presence of the other families of pyrolysis products may point to contributions of other macerals. The density fraction RES is also highly aliphatic in nature (Figure 14b). Figure 15 shows the Py-GC trace of RES using a Curie temperature of 770 “C. The same homologous series of n-alkanes,n-ak-1-enes,and qwalkadienes are observed as in the Py-GC trace of CUT (Figure 6a). The dominant compounds present in plant resins are thought to be bicyclic sesquiterpenoid and tricyclic diterpenoid hydrocarbons. As stated earlier, these (51) Nip, M.; Tegelaar, E. W.; De Leeuw, J. W.; Schenck, P. A.; Holloway, P. J. Naturwiasenschajten 1986, 73, 579-585. (52) Nip, M.; Tegelaar, E. W.; Brinkhuis, H.; De Leeuw, J. W.; Schenck, P. A.; Holloway, P. J. Org. Geochem. 1986 10, 769-778.
Figure 14. Schematic representation of the pyrolysate of CUT (a),RES (b),SPOR (c),VIT (d),PSEUDOVIT (e),SFUSA ( f ) , SFUSB (g), and FUS (h). For explanation,see text. ALKA = C, to Clz n-alkanes; ALKE = C,- to Clz n-alk-1-enes;BENZ = alkylbenzenes; PHEN = alkylphenols; NAPHT = alkylnaphthalenes; INDE = alkylindenes; NO = alkylnaphthols;P/A = alkylphenanthrenes/anthracenes; F/P = alkylfluoranthenes/ pyrenes.
compounds are chemically modified upon diagenesis into alkylnaphthalenes and alkylphenanthrenes and alkylanthracenes, respectively. The relative abundances of these pyrolysis products in RES are, however, very low (Figure 14b, Figure 15). Generally, these types of products are also partly released from resinite samples upon “evaporation”. They are, however, not observed or observed in only in very small amounts among the evaporation products of RES (Figure 15b). Surprisingly, RES yields upon evaporation mainly a homologous series of n-alkanes, pristane and phytane (Figure 15b). The origin of these aliphatic pyrolysis and evaporation products in RES is not fully understood. The relatively high abundances of alkylbenzenes observed in the pyrolysate of RES (Figure 14b, Figure 15) may originate from monocyclic monoterpenoid hydrocarbons (vide supra) or from polyditerpenoid hydrocarbons present in exudated and/or fossilized resinss (van A m e n et al., manuscript in preparation). The density fraction SPOR is mainly characterized by a relatively high contribution of alkylphenols in its pyrolysate (Figure 14c). Phenolic pyrolysis products are reported to occur in the pyrolysates of the maceral sporinite and modern untreated and thermally treated sporopollenin, its likely precursor (vide supra). The presence of polycyclic aromatic structures among the pyrolysis (53) Carman, P. M.; Cowley, D. E.; Marty, R. A. A u t . J. Chem. 1970, 23, 1655-1665.
Nip et al.
134 Energy & Fuels, Vol. 6, No. 2, 1992
I
f
-
t('C1
RES Cu- t emp.770"C
I
t [6
7
0
9
10
11
12
13
14
15
16
17
18 19
-
20 21 22 23 24 25 26 272029
t("C1
Figure 15. GC traces of RES using Curie temperatures of 770 and 358 "C. For analytical conditions, see Experimental Section. The numbers refer to the chain lengths of the n-alk-1-enes and n-alkanes.
products of degraded sporopollenin as found here (Figure 14b) and as reported by Schenck et al." is in accordance with the finding that severe heat treatments do increase the aromaticity of sp~ropollenin.~~ The origin of the aliphatic pyrolysis products and the alkylbenzenes in the pyrohte of SPOR is not fully understood. These pyrolysis products are consistent with recent investigations that have shown that sporopollenins are built up either from phenylpropanoid units30t31or from n-alkyl moieties.32 Figure 14d shows the histogram of VIT. The most prominent pyrolysis products of VIT are alkylphenols, which may represent diagenetically altered lignin, present in woody tissue, although the substitution patterns of the alkyl derivatives of these pyrolysis products are difficult to relate with the structure of lignin. As noted earlier, an origin related to tannins and phenylpropanoid-type sporopollenins may have contributed. This may explain to some extent the observed substitution patterns. The alkylbenzenes may also be derived from woody tissue (vide supra), although the possibility of another source which is not related to the chemical structure of the most dominantly present maceral in VIT cannot be precluded. Once again, the aliphatic pyrolysis products do not seem to represent characteristic structural elements of this maceral in VIT. Coalified wood from very low rank coals, yields upon Py-GC exclusively phenolic and methoxyphenolic pyrolysis products.43 No aliphatic pyrolysis (54) Brooks, J. In Sporopollenin; Brooks, J.; Grant, P. R.; Muir, M. D.; Van Gyzel, P., Shaw, G.,Eds.;Academic Press: London, 1971; pp
351-407.
products are encountered in the pyrolysates of these materials. In the density fraction PSEUDOVIT, a higher relative contribution of phenolic pyrolysis products is observed than in VI" (Figure 14d,e). In view of the possible origin of these types of compounds, their higher contribution to the pyrolysate of PSEUDOVIT is in agreement with the microscopic observation that PSEUDOVIT is derived mainly from unmacerated woody tissue.65@ Generally, the pyrolysate of PSEUDOVIT is more aromatic in nature than VIT (Figure 14d,e), which is also consistent with the higher reflectance of pseudovitrinite. However, also in the pyrolysate of this density fraction the C7-to C12-n-alkanes and n-alk-1-enes are present. The macer& which are most abundantly present in the density fractions SFUSA, SFUSB, and FUS are probably formed from woody plant tissues, which have been chemically modified due to the increasing influence of the fusinization process, which is an accelerated thermal degradation process occurring at a very early stage of diagenesis.B The effect of fusinization is mainly reflected in the pyrolysates of these density fractions by an increase of condensed aromatic pyrolysis products (Figure 14f,g,h). The changes in the relative contributions of the selected pyrolysis products in the pyrolysates of VIT, SFUSA, SFUSB and FUS are more clearly revealed by the histograms of Figure 16. These histograms show the relative (55) Winans, R.E.; Crelling, J. C. In Chemistry and Characterization Coal Macerals; Winans, R. E., Crelling J. C., E&; ACS Symp. Ser. 252; American Chemical Society: Washington, DC, 1984; pp 1-20. (56) Crelling, John C. Zronmaking Proc.-MME 1988, 43, 351-356. of
Chemical Structure of Bituminous Coal
density (gmlml) E
u//1
v/1 v/1 / / A I
1273 1362 1413 1471
density (gmiml)
Figure 16. Schematic representation of the relative intensities of C7to Clz n-alkanes (a), C7to C12n-alk-1-enes (b), alkylphenola
(c), alkylindenes (d), alkylnaphthols (e), alkylnaphthalenes(0, alkylphenanthrenesand alkylanthracenes (g), alkylfuoranthenes and alkylpyrenes (h),and alkylbenzenes (i)to the total pyrolyaatea of the density fractions VIT (d = 1.273 g/mL), SFUSA (d = 1.362 g/mL), SFUSB (d = 1.413 g/mL), and FUS (d = 1.471 g/mL), as deduced from Figure 13.
abundance of each group of pyrolysis products, as deduced from the histagrams in Figure 14, to W,SFUSA,SFUSB, and FUS. PSEUDOVIT is not included in this comparison since it does not originate from sample SIU 6475. Figure 16a-d shows that the relative contributions of the Cr to C12-n-alkanesand n-alk-1-enea,the alkylphenols, and alkylindenes gradually decrease from VIT to FUS. The relative contribution of the alkylnaphthols shows a similar trend, although less smoothly (Figure 16e). The gradual increase in the aromatic nature of SFUSA, SFUSB, and FUS, when compared with VIT, is mainly caused by increasing contributions of alkylnaphthalenes, alkylphenanthrenes, and alkylanthracenes and, although less gradually, alkylfluoranthenes and alkylpyrenes (Figure 16f,g,h). The relative abundance of the alkylbenzenes shows a slightly different trend (Figure 16j). Whereas their relative contribution increases significantly in SFUSA, when compared with VIT, a small gradual decrease is observed from SFUSA to FUS. Obviously, upon increasing influence of the fusinization process, a signifcant reduction of phenolic and naphtholic moieties occurs in the coal matrix, probably by loss of hydroxyl groups, while polycyclic aromatic moieties are preferentially generated. In this respect, the impact of fusinization is very similar to that of coalification. Pyrolysates of density fractions which contain relatively purified vitrinite of increasing coalification stages reflect similar trends as those reported here for f u s i a t i o n . van Graaa et noted a decrease of alkylphenols and alkylindenes and a relative increase of alkylnaphthalenes whereas Senftle et al." reported a decrease of alkylphenols in pyrolysates of a series of humic coals upon increasing
Energy & Fuels, Vol. 6, No. 2,1992 135 coal rank. Likewise, it is reported that vitrinite concentrates of higher carbon content yield pyrolysates similar to those of inertinite concentrates of lower carbon content.9~5' The Chemical Homogeneity of the Density Fractions. The observation that all families of pyrolysis products occur in the pyrolysates of all density fractions and that the internal distribution pattern of some of these families are very similar for all fractions indicates that the macerals isolated from coal SIU 6475 are far from unique in a chemical sense. All density fractions seem to contain macromolecules that give rise to straight-chain aliphatic pyrolysis products upon pyrolysis, phenolic macromolecules and macromolecular structures similar to those derived from resinous materials. The latter possibility is supported by the microecopic observation that sometimes resinous-like materials are observed as cell filling of woody cells.58 It is well established that at defined temperature and pressure conditions during coalification certain liptinite macromolecular substances like the cutinite biopolymer and resinous materials become fluidized and start to migrate. Due to these physical changes, these macromolecular substances may intermingle with each other and with other macerals possibly as a mobile/extractable phase that cannot be separated adequately by density gradient c0ntrifugation. Another explanation is that certain parts of macerals and/or microorganisms that are associated with them in the peat stage may also contribute. These components may also react with other maceral fragments at this coalification stage. In this way, products may be formed which occur dispersed throughout all macerals, without being recognized as such. These processes may also explain the observation that the density values of macerals do not all increase uniformly with increasing coal rank at certain coalification stages. Dulhunty and Penr0se,5~Zwietering and van Krevelen,@' and Frankline1reported that the density of vitrinite macerals decreases from more than 1.5 to 1.27 g/mL with an increase of carbon content from 50 to 87%. Such a decrease might point toward a "filling-up" process by which vitrinite components are replaced by other, relatively lighter, substances like either polymethylenic and resinous materials or reaction products of certain maceral components at these stages of coalification. AB a result of such a dispersion of a part of the macromolecules present, the density fractions obtained from coals with carbon contents over 50% must be considered asrmxtures of chemical compounds while, microscopically, these density fractions may still be defined as single macerals. The Pyrolysate of Coal SIU 6475 in Relation to the Pyrolysates of Its Density Fractions. The histogram of the pyrolysate of coal SIU 6475 (Figure 17), when compared with the histograms of pyrolysates of the individual density fractions, is very similar to that of VIT (Figure 14d). This is not surprising since VIT accounts for a majority of the volume and the weight of the organic fractions of coal SIU 6475 (Table I). It should be noted, however, that the occurrence of alkylphenola in coal pyrolysates such as that of SIU 6475 cannot be exclusively (57) Nip, M.; De b u w ,J. W.; Schenck, P. A. Geochim. Cosmochim. Acta 1988,52,637-648. (58) Nip, M.; De Leeuw,J. W.; Schenck, P. A.; Meuzelaar, H. L. C.; Stout,S. A.;Given, P. H.; Boon, J. J. J. A d . Appl. Pyrol. 1986, 8, 221-239. (69) Dulhunty, J. A.; Penrose, R. E. Fuel 1951,30, 104-113. (60)Zwietering, P.; Van Krevelen, D. W. Fuel 1954,33,331-337. (61) Franklin, R. E. Trans. Faraday SOC.1948,45, 274-286.
Energy & Fuels 1992,6, 136-142
136
be concluded also that the occurrence of each of the other families of pyrolysis product cannot be exclusively ascribed to the corresponding pyrolysis products present in only one individual density fraction.
34
32 30 28 26 24 22
20 18 16 14
12 10
8 6
4
2 0 ALKA
ALKE
BENZ
PHEN
NAPHT
INDE
NO
P/A
F/P
pyrolys~s products
Figure 17. Schematic representation of the pyrolysate of coal SIU 6475. ALKA = C, to C12n-alk-l-enes;BENZ = alkylbemnes; PHEN = alkylphenols; NAPHT = alkylnaphthalenes;INDE = alkylindenes; NO = alkylnaphthols; P/A = alkylphenanthrenes/anthracenes;F/P = alkylfluroanthenes/pyrenes. ascribed to the density fraction VIT alone. SPOR and SFUSA, which both contain relatively high abundances of alkylphenols in their pyrolysates (Figure 14c,f),account together for a significnat portion of the organic fraction of coal SIU 6475 (Table I). Therefore, the total amount of alkylphenols observed in the pyrolysate of coal SIU 6475 is not only represented by those of VIT. Likewise, it can
Conclusions The following conclusions are supported by the results of this study: 1. In the pyrolysates of the density fractions similar types of pyrolysis products are observed; however, the relative contributions of these products vary considerably with the various density fractions. 2. The variation in the distribution of the various pyrolysis products is sufficient to distinguish between the various maceral groups and to serve as a maceral "fingerprint". 3. The internal distribution patterns of the alkyl derivatives of some of these families of pyrolysis products are similar for all density fractions. This observation and the fact that all families of pyrolysis products occur in the pyrolysates of all density fractions probably indicate that on a chemical basis these fractions representing single macerals do not represent single chemical compounds. 4. Despite this fact, it is still possible to relate the chemical nature of the most prominent types of pyrolysis products to structural elements present in the precursors of the macerals which are most dominantly present in the density fractions.
Effects of Ion-Exchanged Calcium on Brown Coal Tar Composition As Determined by Fourier Transform Infrared Spectroscopy Mary J. Wornat and Peter F. Nelson* Division of Coal and Energy Technology, Commonwealth Scientific and Industrial Research Organization, P.O. Box 136, North Ryde, New South Wales 2113, Australia Received July 16, 1991. Revised Manuscript Received November 26, 1991
In order to investigate the effect of ion-exchanged metals on the yield and composition of coal tar, we have pyrolyzed raw and calcium-exchanged Yallourn brown coal in a fluidized bed reactor at temperatures of 600 to 1000 "C and have analyzed the product tars by Fourier transform infrared (FTIR) spectroscopy. At the lower temperatures (600-800"C),tar yields are lower for the calcium form coal than for the raw coal, indicating that the calcium either promotes tar conversion to char or tightens the coal structure and makes it difficult for larger tar molecules to escape. No measurable differences in tar yields are observed at higher temperatures. The yield of aromatic hydrogen in the tars varies only slightly with temperature but shows a marked reduction in the presence of calcium, suggestingthat the aromatic components of the tar are those that are affected most by the structural or catalytic influences of the calcium. In contrast, the aliphatic components exhibit little influence from the effects of calcium but prove to be most susceptible to the secondary pyrolytic reactions brought about by an increase in temperature. The yields of hydroxyl, etheric, and carbonyl functionalities in the tars of both coals decrease with increasing pyrolysis temperature as these groups form oxygen-containing gases. These functionalities are lower in the tar from the calcium form coal, partly as a result of the ion-exchange process itself and partly as a consequence of the decomposition mechanism of the calcium carboxylate structure. The presence of calcium also brings about a lower yield of tar with unsaturated hydrocarbon substituents-a result of the lower yields of aromatic and of some unsaturated hydrocarbons from the calcium form coal. Introduction The inorganic constituents of coal can have several influences on coal combustion: from radiation'T2 and particle
* To whom correspondence should be addressed.
temperature3effeds, to problems of fouling and corrosion of burner surface^.^^^ Since pyrolysis is such a funda(1) Sarofim, A. F. Symp. (Int.) Combust. [ R o c . ] ,21 1986,1-23. (2)Solomon, P.R.;Chen, P. L.; Carangelo, R. M.; Best, P. E.; Markham, J. R. Symp. (Int.) Combust. [Proc.],22 1988,211-221.
0887-0624/92/2506-Ol36$03.00/00 1992 American Chemical Society
