AN AMERICAN CHEMICAL SOCIETY JOURNAL JULYIAUGUST 1990
VOLUME 4, NUMBER 4
0 Copyright 1990 by the American Chemical Society
Art i d e s Assessment of Carbonization Properties of Coals by Analysis of Trapped Hydrocarbons Wilhelm Puttmann* Lehrstuhl fur Geologie, Geochemie und Lagerstatten des Erdols und der Kohle, R WTH Aachen, Lochnerstrasse 4-20,D-51 Aachen, FRG
Rainer G. Schaefer Institut fur Erdol und Organische Geochemie, Forschungszentrum Julich GmbH (KFA), Postfach 1913,0-517 Jiilich, FRG Received August 18, 1989. Revised Manuscript Keceived March 26, 1990
The analysis of hydrocarbons in 29 coal samples from the Ruhr and Saar district (Federal Republic of Germany) by use of solvent extraction methods revealed a close relationship between the composition of the extracts and the carbonization properties of the samples investigated. The correlation of coking parameters (free swelling index, dilatation) with results obtained from the investigation of saturated and aromatic hydrocarbons in solvent extracts of coals by using gas chromatography (GC) has shown that the carbonization potential of the coals increases concurrently with a shift in distribution from higher molecular weight to lower molecular weight hydrocarbons. Moreover, in the "thermal" extracts of excellent coking coals, analyzed by combined thermodesorption-gas chromatography (TD-GC), high amounts of benzene, toluene, and xylenes have been detected. The abundance of these compounds appears to be an analytical indicator for a good coking behavior of these coals. Hence, the composition of thermal extracts from coals provides information on a molecular level, which allows prediction of technological properties.
Introduction The assessment of the carbonization properties of bituminous coals is generally achieved by proximate analysis, by ultimate analysis, and by crucible coking test methods. In order to characterize the caking potential of coals, either the free swelling index (FSI) or the Roga index is determined. The coking behavior is specified by the dilatation value or by the G value.1s2 Moreover, the amount of volatile matter present in coals has been used as an additional parameter for the assessment of technological proper tie^.^ These bulk parameters are part of the in(1) Brewer, R. E. In Chemistry of Coal Utilization; Lowry, H . H.,Ed.; Wiley: New York, 1945; Vol. 1, pp 160-336. (2) Jasienko, S. Fuel 1978,57,131-146. (3) Francis, W. Coal, 2nd ed.; Edward Arnold: London, 1961, pp 361-435.
0887-0624/90/2504-0339$02.50/0
ternational classification system of However, the assessment of these properties based on the analysis of individual molecular constituents of coals has not been achieved so far. For a long time it has been well-known that bituminous coals lose their caking and coking properties after solvent extraction of the trapped bitumen."'O This solvent-soluble bitumen (mobile phase) was mentioned to be responsible for the solvation of molecular units in coal." Coals of high (4) International Classification of Hard Coals by Type; United Nations: Geneva, 1956. (5) Clark, A. H.; Wheeler, R. V. J. Chem. SOC.1913,103,1704-1715. (6) Wheeler, R. V. J. Chem. SOC.1913, 103, 1715-1722. (7) Bone, W. A. R. SOC.h o c . 1924, A105,608-620. (8) Bone, W. A. R. SOC.h o c . 1928, A120, 523-528. (9) Fischer, F.; Broche, H.; Strauch, J. BrennsbChem. 1925,6,33-48. (10) Broche, H.; Bahr, T. Brennst.-Chem. 1925,6, 349-364.
0 1990 American Chemical Society
340 Energy & Fuels, Vol. 4 , No. 4, 1990
Gieseler plasticity exhibited certain characteristic peaks in pyrolysis products generated at 450 OC.12 After solvent extraction of highly plastic coals these peaks were absent in the pyrolysis products of extraction residues. This supports the idea that substances present in the bitumen of these coals are involved in the development of the plastic state during thermal treatment.12 In a similar way Dryden and Pankhurst13 have shown that the material soluble in chloroform is associated with the development of plasticity in heated coals. A detailed discussion about the role of the mobile phase in the development of plasticity during thermal treatment of coal has been presented by Grint et a1.14 These authors also conclude that the mobile phase might act as solvent, enabling the molecular entities in coal to develop an initial degree of mobility during heating. As a consequence of all these experiments, the composition of the solvent extracts apparently governs the thermal softening and a theory for the mechanism of this process has been deve10ped.l~ Several analytical methods such as IR and NMR spectroscopy have been applied to characterize the extractable bitumen of coals by determination of statistical distributions of functional gr~ups.'~J' For a long time the analysis of individual hydrocarbons present in the extracts of bituminous coals appeared to be too ambitious because of the tremendous number of distinct constituents. However, advances in instrumental analytical techniques such as the development of high-performance liquid chromatography (HPLC), capillary gas chromatography (GC), and gas chromatography combined with mass spectrometry (GC/MS) opened the possibility of a more sophisticated analysis of coal e x t r a ~ t s . ' ~ J ~ The application of these methods has shown that the composition of the extractable hydrocarbons of bituminous coals is related to rank. Based on the relative concentrations of phenanthrene and methylphenanthrenes, the socalled methylphenanthrene index (MPI-l) has been developed and has been proven to be an excellent molecular coalification parameter for humic coal^.^^^^ Additionally, the analysis of the bituminous constituents provides information about the facies of coals. The concentration ratio of a distinct C4-naphthalene versus phenanthrene was revealed to be higher in coals from the Saar district compared to coals from the Ruhr district of a comparable vitrinite reflectance. This feature has been interpreted to be caused by major differences in the biological input and genetic conditions during formation of coals from both areas.22 The C4-naphthalene has later been identified to be 1,2,5,6-tetrameth~lnaphthalene.~~ In extension to the aspect of facies analysis, the investigation of the 29 Ruhr (11) Neavel, R. C. In Coal Science; Gorbaty, M., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1982; Vol. 1, pp 1-19. (12) Reasoner, J. W.; Hower, J. C.; Yates, L. P.; Lloyd, W. G. Fuel 1985.64. 1269-1273. (13) Dryden, I. G. C.; Pankhurst, K. S. Fuel. 1955,34, 363-374. (14) Grint, A.; Mehani, S.;Trewhella, M.; Crook, M. J. Fuel 1985,64, 1355-1361. (15) Brown, H. R.; Waters, P. L. Fuel 1966,45, 17-59. (16) Oelert, H. H. Brennst-Chem. 1967,48,362-369. (17) Oelert, H. H. Z . Anal. Chem. 1971,255,177-185. (18) White, C. M.; Lee, M. L. Geochim. Cosmochim. Acta 1980, 44, 1825-1832. (19) Radke, M.; Willsch, H.; Welte, D. H. Anal. Chem. 1980, 52, 406-4 11. (20) Radke, M.; Willsch, H.; Leythaeuser, D.; Teichmiiller, M. Geochem. Cosmochim. Acta 1982,46, 1831-1843. (21) Radke, M.; Leythaeuser, D.; Teichmiiller, M. Org. Geochem. 1984, 6,423-430. (22) Puttmann, W.; Wolf, M.; Wolff-Fischer, E. In Adoances in Organic Geochemistry 1985,Leythaeuser, D., Rullk6tter, J., Eds.; Pergamon Press: Oxford, 1986; Og. Geochem. 1986, 10, 625-632. (23) Puttmann, W.; Villar, H. Geochim. Cosmochim. Acta 1987, 51, 3023-3029.
Piittmann and Schaefer
and Saar coals provides the possibility to correlate the bitumen composition of coals with their technological properties. In the present study recent methods of hydrocarbon analysis in solid sample material are used to assess the coking capability of bituminous coals.
Experimental Section The analyzed sample series originated each from fresh coal seams in order to avoid a variation in the compound composition as a result of weathering effects or due to evaporation losses. Experimental details concerning solvent extraction and hydrocarbon group separation were described elsewhere.22 Gas chromatography of the fractions of saturated hydrocarbons and aromatic hydrocarbons were carried out by use of a Carlo Erba 5160 gas chromatograph equipped with a 25 m X 0.25 mm i.d. fused-silica column coated with SE 54 silicone as stationary phase. The oven temperature was programmed from 80 to 300 O C at a rate of 4 "C/min followed by an isothermal period of 15 min at 300 "C. Hydrogen was used as carrier gas. The identification of individual compounds by GC/MS analysis was carried out on a Varian 3700 gas chromatograph coupled t o a Finnigan MAT 8200 mass spectrometer using the same type of column and temperature program as for GC analyses. Analytical details about the hydrocarbon analyses using thermodesorption-gas chromatography were described elsewhere.24 Organic carbon contents (COT)were analyzed by use of a Leco WR-12 carbon determinator. Prior treatment of the samples with concentrated hydrochloric acid was undertaken in order to remove carbonates. Vitrinite reflectance data are drawn from a previous publication.22 Standard procedures were applied for the measurement of the free swelling index (DIN 51741), the dilatation (DIN 51739),and the amount of volatile matter (DIN 51720). The values have previously been correlated with data obtained from fluorescence s p e c t r o ~ c o p y . ~ ~
Results and Discussion Bulk Parameters. The yields of solvent extraction (dichloromethane) carried out on 16 coals from the Ruhr district and 13 coals from the Saar district are shown in Table I. In the series of Ruhr coals a continuous decrease of the extract yield with a simultaneous increase in coalification, determined by vitrinite reflectance measurements, is observed. In contrast, in Saar coals such a significant dependence between both parameters is missing (Table I). The high amount of the maceral alginite in some Saar coals has previously been used to explain their unexpected behavior.26 The percentage of saturated and aromatic hydrocarbons in the total extracts, determined by hydrocarbon group separation, shows that the proportion of each fraction is independent of the rank of the coals. Moreover, the comparison of extract yields with results obtained from measurements characterizing the technological properties (free swelling index, dilatation) reveals no significant correlation (Table I). For example, coal samples no. 82228 and no. 82229 differ strongly in their free swelling index but show similar values of the extract yield and in the results of hydrocarbon group separation. In terms of the volatile matter content of both coal series (Table I) similar values in relation to rank are observed. Therefore, a detailed analysis of individual compounds in the hydrocarbon fractions has been undertaken in order to find explanations for the peculiar features observed mainly in Saar coals. Gas Chromatography. Three selected gas chromatograms of saturated hydrocarbon fractions obtained from (24) Schaefer, R. G.; Puttmann, W. J. Chromatogr. 1987,395,203-215. (25) Hagemann, H. W.; Ottenjann, K.; Piittmann, W.; Wolf, M.; Wolff-Fischer,E. Erdoel Kohle, Erdgas, Petrochem. 1989, 42, 99-110. (26) Wolf, M.; Wolff-Fischer, E. GZueckauf-Forschungsh. 1984, 45, 243-246.
Carbonization Properties of Coals
Energy & Fuels, Vol. 4, No. 4, 1990 341 a) FSI : 2
a)
i
FSI : 2
f
I :
25
I
b)
FSI : 4 1/2 c3
h
3
1 II
A
20
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4
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FSI : 8 1/2
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5
20
.
3il
40
50
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lo
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30
40
50
60
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Figure 1. Gas chromatograms of saturated hydrocarbon fractions obtained from three coals characterized by different free swelling indices (FSI). The samples analyzed are (a) no. 82229 (Ruhr), (b) no. 82225 (Saar), and (c) no. 84164 (Ruhr). The analytical data are listed in Table I. Abbreviations: pri = pristane; some
n-alkanes are marked by their carbon numbers.
coal samples no. 82229, no. 82225, and no. 84164 are shown in Figure 1. In two coals (no. 82229 and no. 82225) the acyclic isoprenoid pristane is present as the dominant compound. This indicates a lower rank of these coals compared to coal no. 841fXn n-Alkanes, marked by their carbon chain length, are present in high abundance. The maximum intensity of the n-alkane distribution shifts from greater chain length to smaller chain length from sample no. 82229 to no. 82225 to no. 84164. Such as continuous relative decrease of long-chain n-alkanes has previously been detected in a set of Saar coals28and in Australian coalsz7and was shown to be related to increasing rank of the coals investigated. In the samples analyzed in the present study, the change in the distribution toward (27) Brooks, J. D.; Smith, J. W. Ceochim. Cosmochim. Acta 1967,31, 2389-2397. (28) Leythaeuser, D.; Welte, D. H. In Advances in Organic Ceochemistry 1%& Schenck, P. A., Havenaar, I., Eds.; Pergamon Press: Oxford, 1969; pp 429-442.
Figure 2. Gas chromatograms of aromatic hydrocarbon fractions obtained from the same set of coals represented by Figure 1, i.e., (a)no. 82229 (Ruhr),(b) no. 82225 (Saar),and (c) no. 84164 (Ruhr). Abbreviations: C1 = monomethylated naphthalenes; C2 = dimethylated naphthalenes, C3 = trimethylated naphthalenes; FSI = free swelling index.
shorter chain length is accompanied by an increase in the free swelling index (Figure 1; Table I). A similar trend is observed in the aromatic hydrocarbon fractions. In Figure 2 three gas chromatograms, recorded from the samples discussed above, are compared. Again, a significant distribution shift of individual aromatic hydrocarbons accompanied by a variation of the free swelling index and of vitrinite reflectance is detected (Figure 2; Table I). In the aromatic hydrocarbon fraction of coal sample no. 82229 (Figure 2a) two individual components dominate by far. On the basis of GC/MS analysis and of coelution experiments with standard material, the compounds were identified to be 192,5-trimethylnaphthalene (3) and l92,5,6-tetramethy1naphthalene(4). Both compounds are discussed to be degradation products of pentacyclic triterpenoidseB In the coal samples no. 82225 and no. 84164 (Figure 2b,c) monomethylated (CJ, dimethylated (C&, and trimethylated (CJ naphthalenes occur as dominating aromatic compounds, whereas the relative intensity of tetramethylnaphthalene (4) decreases due to dealkylation reactions. The great amount of high molecular weight
Puttmann and Schaefer
342 Energy & Fuels, Vol. 4, No. 4, 1990
Table I. Analytical Data Obtained from 29 Coal SamDles from Ruhr District and Saar District (FRGP
c,,
extr yield, mg/g Cow
sample no.
R,, %
82229 84485 82230 85028 84163 82231 82232 84165 84164 82233 82234 82235 84168 82236 84167 84166
0.78 0.83 0.86 0.90 0.91 0.97 1.06 1.08 1.09 1.17 1.20 1.30 1.38 1.41 1.42 1.44
75.5 72.7 80.5 77.1 75.2 82.3 85.3 82.2 78.0 82.9 83.6 86.9 81.9 86.8 81.9 82.7
14.3 17.9 18.1 15.2 10.4 13.4 9.2 6.6 5.6 6.9 6.5 5.9 3.4 5.7 3.5 2.7
82237 84170 84147 82225 84149 82224 82227 82226 84169 82256 82228 84544 84545
0.78 0.79 0.79 0.85 0.85 0.86 0.88 0.91 0.93 0.95 0.99 1.01 1.10
71.7 66.3 71.2 64.8 79.8 78.6 75.9 78.0 79.5 73.8 82.1 82.1 82.6
16.6 9.4 12.3 7.9 9.8 14.7 11.0 14.8 10.8 9.6 15.8 10.4 9.6
satd, % aro, % Ruhr Coals 7.8 8.4 11.1
5.7 9.9 12.1 15.1 18.2 12.0 9.4 12.1 5.2 10.0 5.8 13.5 7.3
FSI
22.6 31.6 28.1 31.2 16.1 30.2 27.9 28.9 33.5 23.7 24.8 23.3 28.5 20.7 32.5 53.2
2
24.2 32.0 23.1 20.7 20.9 22.9 19.7 31.6 25.4 20.9 26.7 44.2 44.2
5
dil, %
VM, %
3 59 94 109 23 165 295 180 262 183 73 128 86 63
39.2 37.9 39.4 36.9 36.3 34.6 28.2 29.6 29.9 27.9 28.3 24.8 22.6 21.9 21.6 21.4
0.17 0.15 0.42 0.30 0.13 0.22 0.67 1.06 0.91 0.92 1.20
46 34 6 13 105 29 30 92 168 60 170 272 464
41.8 39.0 39.0 39.7 37.7 40.5 38.3 38.9 35.9 37.1 36.6 31.9 29.3
0.07 0.18 0.15 0.08 0.40 0.20 0.10 0.25 0.56 0.32 0.30 0.78 1.47
1
5'Iz 7'l2 5'1 z 8 2lIz 6'12 8'12 8'12 8'12 8'12
9 9 8'12 9
BiphIPhen
0.19
Saar Coals 7.7 8.4 5.6 6.8 4.1 9.4 7.6 8.6 6.7 9.8 8.7 10.4 11.3
1'12
6 4'1, 7'12 7'Iz 3'1 z 8 8 8 8 8 8'12
"Abbreviations: R, = random vitrinite reflectance; C,, = organic carbon content; extr = extract; satd = saturated hydrocarbons in % of total extract; aro = aromatic hydrocarbons in % of total extract; FSI = free swelling index: dil = dilatation; VM = volatile matter (dmmf); Biph/Phen = concentration ratio of (biphenyl + 2-methylbiphenyl + 3-methylbiphenyl)/(phenanthrene + 3-methylphenanthrene + 2methylphenanthrene + 9-methylphenanthrene + l-methylphenanthrene).
aromatic hydrocarbons represented by the nonresolved hump in the gas chromatogram of sample no. 82229 (Figure 2a) has almost disappeared in the case of sample no. 84164 (Figure 2c). This result indicates that high-boiling aromatic hydrocarbons undergo major fragmentation reactions with increasing rank of coals leading to smaller molecules. Besides naphthalenes an additional set of compounds begins to appear in the aromatic hydrocarbon fractions. As shown in the gas chromatogram of sample no. 84164 (Figure 2 4 , such compounds are biphenyl (I), 3-methylbiphenyl, and 4-methylbiphenyl(2). The occurrence of these compounds in relatively high abundance is only observed in coking coals of FSI > 7. Presumably, the compounds are generated in coals by thermal treatment of burried lignin, which in turn is the dominant biological precursor of vitrinite. Vitrinite is known to be the primary carrier of the coking potential, and the thermal softening is reported to be a prerequisite for coke f0rmati0n.l~ Therefore, the appearance of biphenyls in coal extracts might be a molecular indicator for the onset of high coking quality of coals. Further constituents of bituminous coals are phenanthrene (5) and methylphenanthrenes 6 (see Figure 2c). However, phenanthrenes are generated at a much earlier stage of coalifcation (high volatile bituminous A coals) compared to biphenyls. Phenanthrenes undergo significant relative distribution shifts during increasing coalification.20 For this reason, a suitable method to measure the degree of generation of smaller molecules with low boiling points in the aromatic hydrocarbon fractions is the determination of relative intensities of biphenyls compared to phenanthrenes. Thus, the concentration ratio of the sum of biphenyl 3-methylbiphenyl + 4-methylbiphenyl versus the sum of phenanthrene + four me-
+
thylphenanthrene isomers (Biph/Phen) is calculated from the aromatic hydrocarbon fractions of the coals included in this study (Table I). In the Ruhr coals of low rank this ratio cannot be determined because biphenyls are not detectable herein. The plot of the dilatation measured in 25 samples versus the concentration ratio of Biph/Phen is shown in Figure 3. For Saar coals a continuous increase of the dilatation together with an increase of the relative amount of biphenyls is observed. For Ruhr coals of lower rank a similar trend is recognized. However, at higher rank a further increase of the Biph/Phen ratio is accompanied by a decrease of dilatation. The tendency observed is similar to a tendency previously observed, when the dilatation of coals was plotted versus C content (p 343 in ref 29). This again confirms that the generation of smaller molecules in coals is rank dependent and at the same time is associated with the carbonization potential of coals. The development of the dilatation is only in the beginning parallel to the FSI. For the three coals represented by the gas chromatograms shown in Figure 2, a constant increase of the FSI from a value of 2 (no. 82229) over 4l/, (no. 82225) to 8lI2 (84164) is observed. However, in Ruhr coals of higher rank the dilatation drops whereas the FSI remains high. This might be due to the difference in heating rates applied during determination of FSI compared to dilatation. The generation of distinct low molecular weight aromatic compounds is associated with the so-called first coalification jump29~30 representing the change from the subbitu(29) Stach, E.; Mackowsky, M. T.; Teichmuller, M.; Taylor, G. H.; Chandra, D.; Teichmiiller, R. Textbook of Coal Petrology, 3rd ed.; Borntraeger: Berlin, Stuttgart, 1982; p 242.
Carbonization Properties of Coals 400
I
400
t
Energy & Fuels, Vol. 4, No. 4, 1990 343 I
a) Saar
coals
300 m m
+
200
0
100
0 0.00
0.30
0.60
0.90
1.20
1.50
Biph/Phen
350 280
1
Auhr
1
coals
\ 0.00
0.30
0.60
0.90
1.20
1I 1.50
Qluh/Phen
Figure 3. Plots of dilatation (%) versus ratio of Biph/Phen determined from the aromatic hydrocarbon fractions of 13 Saar coals (a) and 12 Ruhr coals (h). The correlation coefficient (r) based on 13 data points from Saar coals is calculated as 0.97. Biph/Phen = concentrationratio of (biphenyl+ 3-methylbiphenyl + 4-methylbiphenyl)/(phenanthrene + 3-methylphenanthrene + 2-methylphenanthrene + 9-methylphenanthrene + 1methylphenanthrene).
minous to the bituminous coal stage. Subbituminous coals are known to contain mainly polycyclic aromatic hydrocarbons such as chrysenes and picenesa31 Bicyclic and tricyclic aromatic hydrocarbons are generated mainly in the high volatile bituminous coal stagem and provide a pattern of compounds showing a structural relationship to natural precursor molecules. For example, phenanthrenes are expected to be derived from plant resins32and from steroids.= With increasing coalification the distribution of aromatic hydrocarbons becomes less complex due to rearrangement and fragmentation reactions. By that, primary products of a higher thermodynamic stability are generated. Consequently, the aromatic hydrocarbons in medium volatile bituminous coals (i.e., no. 84164, Figure 2c) have lost their precursor relationship and each shows very similar distribution patterns with thermodynamically controlled relative intensities of alkylated naphthalenes and phenanthrenes.m (30) Teichmirller, M.; Durand, B. Int. J. Coal Geol. 1983,2,197-230. (31) Chaffee, A. L.;Johns,R. B. Geochim. Cosmochim. Acta 1983,47, 2141-2155. (32) Hayatsu, R.; Winam, R. E.; Scott, R. G.; Moore, L. P.; Studier, H. M. Fuel 1978,57, 541-548. (33) Gaskell, S. J.; Eglinton, G. Geochim. Cosmochim. Acta 1976,40, 1221-1228.
However, deviations from the trends discussed above have been recognized in some samples investigated. According to the composition of the aromatic hydrocarbons, the free swelling index of coal no. 84170 (Table I) should be higher than measured. The organic geochemical results from this coal provide evidence for a severe bacterial activity during the early diagenesis of this coal. Within the gas chromatogram of the saturated hydrocarbons (not shown here) hopanes are detected as dominating compounds in the high-boiling range. These compounds are known to be degradation products of cell-wall material of bacteria.34 Probably, the presence of high amounts of hopanes in the bitumen prevents this coal from being a good coking coal. Further exceptions are represented by coal no. 85028 and coal no. 84163. According to the technological tests these coals are appropriate coking coals (Table I). However, in the gas chromatograms of saturated and aromatic hydrocarbons relatively high intensities of high molecular weight compounds are detected. Thus, regarding the extract composition of these coals, a good coking quality cannot be predicted. These exceptions allow speculation that in coals bituminous constituents might be present that are not detectable by extract analysis but show a major influence on the coking behavior. Thermodesorption-Gas Chromatography. In order to clarify this problem, the analytical method of thermodesorption combined with gas chromatography (TD-GC), previously applied for the analysis of trapped hydrocarbons in was employed for the analysis of coals. The method provides the possibility to analyze hydrocarbons in a much wider range of carbon numbers (C4-C33)than extract a n a l y ~ i s . * ~The . ~ ~particular advantage of using this method is in the registration and quantitation of the low molecular weight hydrocarbons desorbable from solid sample material. Thus, TD-GC analysis was applied to investigate selected coal samples. The gas chromatograms obtained from four samples characterized by a stepwise variation in the free swelling index are shown in Figure 4, revealing the hydrocarbons released from the coal matrix at 300 "C desorption temperature. The results of the quantitation of 45 individual compounds identified as main constituents are summarized in Table 11. The hydrocarbons present in the coal of the lowest free swelling index (value of 1) are mainly composed of long-chain saturated hydrocarbons marked by their carbon number (Figure 4a). Pristane 6) is present as the dominant component. Short-chain aliphatic hydrocarbons and individual aromatic constituents contribute to the total bitumen only to a minor extent. The Ruhr coal no. 84163 is characterized by a medium free swelling index of 5l/* The hydrocarbon composition (Figure 4b) differs significantly from the one observed in coal no. 84485. The absolute amount of long-chain n-alkanes is much lower in no. 84163 (Table 11). Instead, short-chain n-alkanes are enriched. Moreover, aromatic hydrocarbons such as benzene, toluene, and xylenes (BTX); alkylated naphthalenes; and phenanthrenes occur in high amounts (Figure 4b; Table 11). The most significant difference between both gas chromatograms (Figure 4a,b) is the high abundance of numerous low molecular weight hydrocarbons (
