Energy & Fuels 1988,2, 657-662
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Separation and Preliminary Characterization of High-Purity Maceral Group Fractions from an Australian Bituminous Coal A. G. Pandolfo and R. B. Johns* Department of Organic Chemistry, The University of Melbourne, Parkuille 3052, Australia
G. R. Dyrkacz Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439
A. S . Buchanan CRA Ltd., 55 Collins Street, Melbourne 3001, Australia Received October 14, 1987. Revised Manuscript Received March 21, 1988
An Australian bituminous steaming coal (Blair Athol) was reproducibly fractionated into the maceral groups liptinite, vitrinite, and inertinite by using the density gradient centrifugation technique. Representative high-purity (98.5% maceral purity or better) maceral group fractions were analyzed by DRIFT spectroscopy and by Py-GC and Py-GC/MS techniques. The thermal volatility at 600 "C for the macerals was found to be in the order liptinite >> vitrinite > inertinite, with greater than 95% of the liptinite material volatilized at 600 OC. This volatility reflects the lower molecular weight and the strongly aliphatic nature of the molecular components of the liptinite fractions. Py-GC analyses reveal the relative paucity of aromatic material in the liptinite fractions although the aromatic contribution to the liptinite fraction increased with density. The vitrinite was found to have strong contributions from phenolic and alkyl aromatic material. The inertinite fractions, although highly aromatic, do not have as strong a phenolic contribution as seen for the vitrinite fraction. DRIFT spectroscopy confirmed these findings and Fourier self-deconvolution (FSD) of the aliphatic region (3000-2800 cm-') reveals the liptinite to be composed of long-chain aliphatic material incorporated into the macromolecule. The DGS profile of the liptinite fractions suggests chemical differences in this Gondwanaland coal from that found in a northern hemisphere coal. The inertinite has a low CH2/CH3ratio, which indicates shorter alkyl chain substitution on the macromolecule. The highly aromatic character and low volatility of the inertinite suggest that its predominant component (semifusinite) has a highly cross-linked structure with relatively short alkyl linkages and/or substituents. The behavior of the inertinite fraction is consistent with much structural degradation of the macromolecule during coalification.
Table I. Elemental and Proximate Analysis of Blair Athol Introduction Run-of-Mine Coal Coal maceral studies have been an important area of coal elemental anal. (dafl, research for many years. One of the main reasons for this proximate anal., w t % wt % interest is that a better understanding of the physical and chemical properties of coal macerals is invaluable to the carbon 81.2 moisture (inherent) 7.5 hydrogen 4.4 ash 8.0 understanding of the chemistry of the whole coal. The nitrogen 1.7 volatile matter 27.3 formation of coke, for example, has been shown to be sulfur 0.3 fixed carbon 57.3 influenced by the way in which the more reactive macerals oxygen (diff) 12.4 and inertinite macerals are associated with each other.'i2 thermal value 11697 Btu/lb (at 7.5% moisture) It has also been proposed that the outcome of hydrogenating a particular coal may be predicted, with some confidence, from a knowledge of its maceral c o m p o s i t i ~ n . ~ ~ ~technique to the routine separation and isolation of highpurity maceral group fractions from Blair Athol coal, a In addition to these commercially applied areas, maceral commercially important Australian bituminous coal. The studies can also be useful in establishing the geological DGC technique, developed by Drykacz and co-workers, has history of the coal by the examination of the maturation been successfully applied to the isolation of macerals from changes that have occurred from their precursor material^.^ northern hemisphere high-rank ~ o a l s . ~Blair * ~ Athol coal, The purpose of this present work was to determine the however, is a Permian Gondwanaland coal, and coals of suitability of the density gradient centrifugation (DGC) this geological age and location are thought to have had (1)Speight, J. G. The Chemistry and Technology of Coal; Marcel Dekker: New York, 1983;Chapter 3. (2) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1981. (3) Stach, E.; Mackowsky, Mm. T.;Teichmuller, M.; Taylor, G. H.; Chandra, D.; Teichmuller, R. Stach's Textbook of Coal Petrology; Gebruder Borntraeger: Berlin, 1982.
(4)Dyrkacz, G. R.;Bloomquist, C. A. A.; Solomon, P. R. Fuel 1984, 63, 536-542. (5) Pugmire, R. J.; Zilm, K. W.; Woolfenden, W. R.; Grant, D. M.; Dyrkacz, G. R.; Bloomquist, C. A. A.; Horwitz, E. P. Org. Geochem. 1982, 4, 79-84.
0 1988 American Chemical Society
Pandolfo et al.
658 Energy & Fuels, Vol. 2, No. 5, 1988 Table 11. Blair Athol Whole Coal Macreal Analysis" maceral group maceral 70by vol liptinite sporinite 4.8 resinite 0.2 0.0 cutinite alginite 0.0 vitrinite 52.6 22.6 inertinite semifusinite micrinite 0.6 13.4 inertodetrinite 5.8 fusinite 5.0 totals liptinite 52.6 vitrinite 42.4 inertinite
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" Analysis performed under white light, at 800 X magnification and 500 counts/sample on a hand-picked sample.
Figure 1. DGC separation profile of Blair Athol coal (0)and fraction density (0).
a strong input from Glossopteris flora, which is quite different from coal-forming flora of the northern hemi~phere.~,~ The high-purity maceral group isolates thus obtained in a reproducible manner would open up the potential for detailed investigations for the first time of the chemical composition of Australian maceral groups. Differences in chemical composition will reflect variations in the physical and chemical reactivity of each maceral group as well as environments of deposition. The data obtained should aid the understanding of why whole coals can often show considerable variation in their utilization potential. The Blair Athol coalfield is located in central Queensland, Australia, and is contained within an isolated subbasin on the western margin of the Bowen Basin. The coal has a n ASTM classificationof high-volatile bituminous C, and a typical elemental and proximate analysis of the coal is listed in Table I. The sample used was hand picked, by the mine geologist, from the No. 3 seam of the coalfield. The DGC technique has been well documented in the literature.'~* Briefly the procedure involves the fine grinding (98.5 98.5 99.5 99.5
% wt loss upon 600 O C pyrolysis >98 >95 35 30
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Figure 2. Petrographic analysis (vol W )of selected fractions from liptinite; (e)vitrinite; (0)inertinite. Figure 1: (0)
ventional microscopic analysis, the three regions were found to correspond to cutinite, resinite, and sporinite with density maxima at 1.08,1.11, and 1.17 g/mL, respectively. The position of the two liptinite peaks in the DGC profile of Blair Athol coal, at 1.147 and 1.184 g/mL, suggest resinite and sporinite, respectively, which is consistent with the whole coal analysis (Table 11). However, the significant variations in the position of the peak maxima observed between the Blair Athol and the Illinois Basin coals reflect the chemical differences between the liptinite macerals of the two coals. This is a significant observation in developing an understanding of differences between northern hemisphere and Gondwanaland coals. The vitrinite maceral group separates out at intermediate densities (-1.2-1.3 g/mL). Although a small amount of overlap is seen, viz., by liptinite (low-density side) and inertinite (high-density side), the vitrinite maximum at a density of 1.285 g/mL has a maceral group purity of >99% vitrinite (Figure 2). Different vitrinite macerals could not be clearly distinguished by the petrographic methods employed, but the range of densities observed for the vitrinite group indicates that it is not a uniform material but rather a heterogeneous mixture. This heterogeneity is, presumably, a result of the diversity of organic material deposited during the formation of the coal or variations in the biochemical and geological transformations during coalification. These variations give a complex mixture of materials with a range of densities but related optical properties under white light microscopy. The inertinite separates out at the highest densities ( 1.3-1.5 g/mL) with a maximum at 1.335 g/mL). The wide density range seen for the inertinite group again indicates the heterogeneity of this maceral group. Unlike northern hemisphere fusinite and semifusinites, the similarly named macerals in Gondwanaland coals are thought to be formed as a result of some kind of biochemical oxidation rather than by pyrolysis in forest firesa3s6 A small shoulder on the inertinite peak is seen at 1.244 g/mL (Figure 2), which is outside the region normally quoted for inertinites. A similar additional peak has been observed for northern hemisphere inertinites separated by DGC from other bituminous coals.8 Microscopic examination of those fractions revealed that the additional peak may be due to f i e micrinite-like material bound to exinite, and thus the peak is due to insufficient maceral liberation. Although 50 density fractions were collected, the results from the chemical analysis of only a selected number of fractions are reported here. Each fraction has a maceral group purity, by microscopic analysis, of 98.5% or better (Table 111). DRIFT Analysis of the Maceral Fractions. The use of DRIFT spectroscopy as a powerful and versatile tech-
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Figure 3. 4000-800-cm-' DRIFT spectra of selected maceral fractions: liptinite (12), vitrinite (26), and inertinite (32). nique for chemical analysis of coals has been well documented.13J4 The technique looks at reflected radiation so that structural differences at the surface of these isolates may be examined. The nondestructive nature of the technique was particularly appealing because of the small amounts of sample available for analysis. Figure 3 compares the 4000-800-~m-~DRIFT spectra for the three maceral groups. The liptinite fraction is clearly distinguishable from the other macerals by very strong aliphatic C-H stretching modes (2850-3000 cm-I). The intensity of these peaks is a useful quantitative measure of methylene and methyl absorbances and reflects the strongly aliphatic nature of the liptinite maceral group. A strong carbonyl ( C 4 ) absorption centered at 1700 cm-' (13) Smyrl, N. R.; Fuller, E. L. Jr. In Coal and Coal Products: Analytical Characterization Techniques;Fuller, E. L., Ed., ACS Symposium Series 205;American Chemical Society: Washington, DC, 1982;Chapter 5. (14)Griffiths, P. R.; Wang, S.-H.;Hamadeh, I. M.; Yang, P. W.; Henry, D. E. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel. Chem. 1983,28,27-34.
Pandolfo et al.
660 Energy & Fuels, Vol. 2, No. 5, 1988 ORIGINAL
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Table IV. Relative FID Response (in Arbitrary Units) of the Major 600 O C Pyrolysis Products for the Macerals Investigated 26 32 8 12 vitri- inertiliptinite liptinite nite nite~ a b a b a b a b alkenes c&30 alkanes C&BO alkylphenols Co-C4 alkylbenzenes Co-C4 alkylnaphthalenes Cd3
460 282 19 53 trace
469 198 209 61 175 288 124 131 51 145 19 63 67 88 252 54 103 109 42 119 trace 10 11 7 20
13 16 24 20 4
43 52 79 67 12
Totals in this column are normalized to the same initial weight loading of coal taken for pyrolysis. bTotals in this column are normalized to the same weight of volatile material released on pyrolysis.
3200 3050 2900 2150 2600
3200 3050 2800 2750 2600
WAVENUMBER ( CM-1 1
Figure 4. Original and deconvoluted DRIFT spectra (3200-2600 cm-') of the maceral fractions from Figure 3. in the liptinite spectrum is much better resolved and more intense in this spectrum than in the spectra of the other two maceral groups. This in part is due to the weaker overlap of the 1600-cm-' band, which in comparison is much stronger in the vitrinite and inertinite macerals. A similar carbonyl peak in the IR spectra of liptinite maceral isolates has been observed by other workers in North America4J5and British16J7maceral concentrates in the IR spectra of naturally occurring liptinite (exinite) concentrates.18 The presence and the position of the carbonyl group absorption, the strong aliphatic peak, the presence of a broad C-0 bond (centered at -1300 cm-'), and an apparent absence of any significant carboxylic acid hydrogen bonding, all taken together, strongly indicate the presence of alkyl esters as significant components in the liptinite fraction accounting for the marked 1700-cm-' absorption. Carbonyls in other environments can also be expected to be present and contribute to the broadening of the C=O region seen in all three maceral groups. The vitrinite and inertinite fractions are much less aliphatic with the inertinite fraction the least aliphatic. Other spectral differences between the fractions are subtle, and therefore more sensitive techniques were employed to further characterize these fractions. Fourier self-deconvolution (FSD)-a technique applied to coal by Griffiths and his co-workers19~20-allowsspectral enhancement of IR peaks. Figure 4 shows the Fourier self-deconvolutedDRIFT spectra (2600-3200-cm~'region) of the isolated maceral fractions. At least four bands became clearly distinguishable after the application of (15) Brenner, D. Prep. Pap-Am. Chem. SOC., Diu. Fuel Chem. 1983, 28(1), 85-92. (16) Given, P. H.;Spackman, W.; Davis, A.; Zoeller, R. G.; Jenkins, R. G.; Khan, R. Fuel 1984,63, 1655-1659. (17) Bent, R.; Brown, J. K. Fuel 1961,40, 47-56. (18) Allan, J. Ph.D. Thesis, University of Newcastle upon Tyne, 1975. (19) Griffiths, P. R.; Pariente, G. L. TrAC, Trends Anal. Chem. (Pers. Ed.) 1986, 5, 209-215. (20) Wang, S.-H.; Griffiths, P. R. Fuel 1984, 64, 229-236.
FSD, and these are the asymmetric -CH,- and -CH3 stretches (2920 and 2690 cm-l, respectively) and the symmetric -CH2- and -CH, stretches (2853 and 2870 cm-l, respectively). In the liptinite sample the -CH2- stretches are stronger than the -CH, stretches, indicative of a large number of long alkyl chains. The inertinite and vitrinite, however, have a greater methyl contribution to their aliphatic component suggesting fewer and/or shorter alkyl chains are associated with these two maceral groups. These C-H stretches have corresponding deformations at 1450 cm-', and the sharp absorption in this region of the liptinite is doubtless due to the methylene and methyl deformations. Its progressive reduction in intensity parallels the decrease of the methylene stretches in the order liptinite >> vitrinite > inertinite (Figures 3 and 4). The aromatic C-H stretch (centered at 3030-3050 cm-l) becomes more intense from liptinite through vitrinite to inertinite, where it strongly reflects the predominantly aromatic nature of the inertinite maceral. The intensification of the broad band at 1590-1630 cm-' (aromatic ring vibration) follows this same trend, consistent with the greater aromatic nature of the macromolecule in the inertinite maceral. The hydroxyl absorption extends over a broad range consistent with the many different structural environments of the hydroxyl groups. If the relative areas under the curve in the region 3100-3600 cm-'are compared for the three maceral groups, it becomes clear that the hydroxyl absorption maximizes in the vitrinite. This is in keeping with the high phenolic character of the vitrinite revealed by the pyrolysis data discussed below. Pyrolysis GC and Pyrolysis GC/MS. Figure 5 dtsplays the 600 OC pyrograms of selected maceral fractions and the relative abundance of selected components of the volatiles are listed in Table IV. The traces are normalized to the same initial weight loading for each sample and therefore are strictly comparable. The highly aliphatic nature of the liptinite is again evidenced by the strong homologous series of alkanes and alkenes (predominantly 1-alkenes) in the pyrograms. Alkenes are not normally found in the solvent extract of bituminous coals9and their occurrence as pyrolysis products is regarded as the result of cracking of n-alkyl groups or by the decomposition of alkyl carboxylic acids and alcohols joined to the coal macromolecule by an ester linkage.21 The pyrograms of the liptinite fractions 8 ( p = 1.102 g/mL) and 12 ( p = 1.142 g/mL) show a decrease in aliphatic content with increasing density. The pyrogram of
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(21) Charlesworth, J. M. Fuel Process. Technol. 1987, 16, 99-162. (22) Preliminary small-angle X-ray scattering (SAXS) data have shown that naturally occurring fusain concentrates with a high inertinite content are more than 2.5 times as porous as typical run of mine coal.
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Fractionation of a Bituminous Coal
Energy & Fuels, Vol. 2, No. 5, 1988 661
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RETENTION TIME Figure 5. 600 "C pyrolysis-GC chromatograms of selected maceral isolates normalized to the same initial weight loading: (a) benzene; (b) toluene; (c) C2-benzenes; (d) C3-benzenes; (e) phenol; (f) C1-phenols; (g) C2-phenols; (h) C3-phenols. Numbers refer to carbon chain length of alkene/alkane pairs.
fraction 8 gave an average weight loss of >98% (Table 111) upon 600 "C pyrolysis and is predominantly composed of aliphatic material. Although hydrocarbons up to n-C3,, were detected, the majority of the products were below n-CI4,which must contribute to the high volatility observed for this fraction. As the density of the liptinite fraction increases, a decrease in volatility at 600 "C occurs. Fraction 17 ( p = 1.19 g/mL), which is 95.5% liptinite, occurs near the liptinite/vitrinite boundary and gave an average weight loss of -70% after 600 " C pyrolysis. Although the DRIFT spectra (Figures 3 and 4) show the presence of aromatic components in the liptinite, it is only at fraction 12 that a significant contribution from aromatic material becomes evident in the pyrogram. These components consist mainly of alkylated benzenes, phenol, and alkylated phenols (Table IV). The vitrinite fraction also contained a significant amount of aliphatic material releasable upon pyrolysis. Unlike the liptinite fractions, however, the carbon chain lengths of the aliphatics are more evenly distributed and do not show as strong a contribution from shorter chain (98% macera1 group purity) concentrates from Blair Athol coal. The high-purity isolates thus obtained allow a detailed chemical study of Gondwanaland coal macerals to be initiated. The liptinite-rich fractions were found to be highly volatile upon 600 OC pyrolysis, releasing large quantities of low molecular weight components. Within the liptinite maceral group the volatility of the fractions decreased with increasing density. This decrease in volatility is accompanied by an increase in aromatic character of the liptinite fractions. The evolution of pyroproducts became progressively less in the order liptinite >> vitrinite > inertinite. These data would link the volatility of the coal and the lower molecular weight nature of the volatiles, in the first instance, to the liptinite content. They also suggest that a significant contribution by this maceral group, and to a lesser degree the vitrinite group of Blair Athol coal, is
made toward the excellent free-burning properties of this coal. Preliminary chemical examination of the inertinite group suggests a cross-linked structure with a marked reduction in pyrolyzable volatiles. Although the inertinite contains some phenolic material, the amount released upon pyrolysis is less than that for vitrinite. Hence, although the friable and porous nature of inertinite is considered responsible for the high inherent water content associated with Blair Athol coal, it would also appear to be associated with its vitrinite content where the hydrophilic phenolic content is high. The structural analyses reported here suggest that the chemical differences between the maceral groups isolated reside in the very markedly different distribution patterns of the same or similar organic components. In addition all our data, including the increased density as revealed by DGC analyses, point toward a highly cross-linked and aromatized structure for the macromolecules of the inertinite maaceral group with releasable substituents limited in both quantity and carbon number. A subsequent publication will detail the components of each maceral group and investigate differences within a maceral group, particularly the vitrinite and inertinite groups.
Acknowledgment. We thank Pacific Coal Pty., Ltd., for financial support, John Sangster for valuable and informative discussions, and C. A. A. Bloomquist for guidance with the density gradient separation procedure.
Pore Structure Analysis of Coals via Low-Field Spin-Lattice Relaxation Measurements Christopher L. Glaves, Pamela J. Davis, David P. Gallegos, and Douglas M. Smith* UNM Powders and Granular Materials Laboratory, Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131 Received December 4, 1987. Revised Manuscript Received April 5, 1988 As a result of the complex nature of coal, it is difficult to probe its pore structure over the entire pore size range of interest. Multiple techniques such as gas adsorption (nitrogen and carbon dioxide), mercury porosimetry, SAXS, and density measurements are required. These techniques suffer from inherent problems such as a limited pore size range, errors due to network/percolation effects, the necessity of pore shape assumptions, and/or sample changes during analysis. In this work, the use of low-field NMR spin-lattice relaxation measurements as a pore structure analysis technique for coal is demonstrated. In principle, NMR pore structure analysis avoids many of the problems of the other methods, notably pore shape assumptions (for pores of radius greater than 5 nm), network/percolation effects, and sample compression. Spin-lattice relaxation measurements have been conducted at a proton frequency of 20 MHz and at 303 K on water contained in 10 different coals representing a range of rank and geographic origin and in a Spheron-6 carbon black sample. Pore size distributions were derived for these samples via deconvolution of the NMR relaxation measurements and application of the "two-fraction-fast-exchange" model of pore fluid behavior. For coals, a qualitative comparison of the NMR pore size distributions and surface areas (C02/N2)yielded good agreement. Monodisperse and bidisperse pore size distributions were noted with pore volume in the size range of
