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Energy & Fuels 1998, 12, 570-573
Structural Characterization of Some North American Coals C. E. Burgess,† P. J. Redlich,† R. J. Sakurovs,‡ W. R. Jackson,*,† and M. Marshall† Department of Chemistry, Monash University, Clayton, Vic. 3168, Australia, and Division of Coal and Energy Technology, P.O. Box 136, North Ryde, NSW 2113, Australia Received September 10, 1997. Revised Manuscript Received February 13, 1998
The structure of five U.S. coals (three of low rank or sub-bituminous and two of high rank) has been investigated using reactivity criteria and spectroscopic methods. The results suggest that one of the high-rank coals PSOC 831 contains more cross-linking groups within its macromolecular structure than the other high-rank coal DECS 12. These cross-linkages help to explain some previously reported anomalous results resulting from reactions of PSOC 831 with hydrogen in the presence of MoS2. The DECS 9 and PSOC 1379 were shown to be typical low-rank coals and gave significant “guest”-derived materials (terpenoid and wax-derived hydrocarbons) from pyrolyses at 320 and 350 °C, and the sub-bituminous coal DECS 6 showed many structural similarities when compared with a range of other sub-bituminous coals from Central Queensland, Australia.
Introduction In experiments designed to explore the possibility of making a thermally stable jet fuel by coal liquefaction, the effect of coal structure on the conversion and product type was studied for five U.S. sub-bituminous and bituminous coals.1,2 Coal structure was investigated by solid-state 13C NMR, flash pyrolysis-gas chromatography/mass spectrometry (py-GC/MS), and hydroliquefaction in the presence of solvent and MoS2 catalyst. It was shown that the coal structure information obtained from these techniques was generally a good indicator of the type and amount of light product from the coals chosen. However, some questions remained unanswered. In the GC/MS solvent-derived peaks could have hidden some of the liquefaction product peaks, and asphaltenes were not determined or characterized. For one coal, PSOC 831, py-GC/MS did not predict the nature of the liquefaction products correctly and it was decided to carry out further work to try and explain the discrepancy. Redlich et al.3-11 have used a combination of reactions under prescribed conditions and a range of analytical †
Monash University. Division of Coal and Energy Technology. (1) Burgess, C. E.; Schobert, H. H. Energy Fuels 1996, 10, 718725. (2) Burgess, C. E. Measurement of Coal Characteristics by Pyrolysis Gas Chromatography/Mass Spectrometry and 13C Nuclear Magnetic Resonance and Comparison to Coal Liquefaction Product Composition. Ph.D. Dissertation, The Pennsylvania State University, May, 1994. (3) Redlich, P.; Jackson, W. R.; Larkins, F. P. Fuel 1985, 64, 13831390. (4) Redlich, P. J., Jackson, W. R.; Larkins, F. P.; Rash, D. Fuel 1989, 68, 222-230. (5) Redlich, P. J.; Jackson, W. R.; Larkins, F. P. Fuel 1989, 68, 231237. (6) Redlich, P. J.; Jackson, W. R.; Larkins, F. P.; Chaffee, A. L.; Liepa, I. Fuel 1989, 68, 1538-1543. (7) Redlich, P. J.; Jackson, W. R.; Larkins, F. P. Fuel 1989, 68, 15441548. (8) Redlich, P. J.; Jackson, W. R.; Larkins, F. P.; Chaffee, A. L.; Liepa, I. Fuel 1989, 68, 1549-1557. ‡
techniques to correlate the atomic H/C ratio of coals with their structure and reactivity. In this study their techniques will be applied to the range of coals used in the previous work.1,2 Experimental Section All experimental procedures used in this study have been described previously.3-11 Briefly, hydrogenation experiments were carried out using coal that had been acid-washed and dried under nitrogen at 105 °C. A coal-tetralin (3 and 9 g, respectively) mixture was heated in an autoclave (70 mL) under 6 MPa initial hydrogen pressure for 1 h at 405 °C. The autoclave was then quenched in ice water and the products separated by a solvent-extraction scheme. Conversion was based on conversion to gases, water, and material soluble in dichloromethane. The dichloromethane-soluble material was fractionated into light petroleum (Shell X4) soluble material (oils) and light petroleum insoluble-dichloromethane soluble material (asphaltenes). The acid-washed coals were also reacted with hydrogen in the presence of SnO2 (1 mol SnO2 per kg dry coal) at 405 °C for 1 h under 10 MPa initial hydrogen pressure to isolate coal-derived liquids free from high-boiling reaction solvents. The coals were reacted in the absence of solvent or in Decalin (1:2 coal-decalin) under nitrogen at 320 or 350 °C in order to separate the guest material from the host.3,6 The elemental analysis and volatile matter of the coals and the elemental analysis of the residues, asphaltenes, and tetralin-free oils generated by the reactions were determined by standard methods.3,4 The coals and asphaltenes were also analyzed by solid-state CP/MAS 13C NMR following the procedure and using the parameters described previously7 and asphaltenes and oils by 1H NMR.6,8 The fa values were (9) Redlich, P. J.; Carr, R. M.; Jackson, W. R.; Larkins, F. P.; Chaffee, A. L. Fuel 1989, 69, 764-770. (10) Redlich, P. J.; Jackson, W. R.; Larkins, F. P.; Chaffee, A. L.; Krichko, A. A.; Grigoryeva, E. A.; Shatov, S. N. Energy Fuels 1990, 4, 28-33. (11) Lynch, L. J.; Sakurovs, R.; Webster, D. S.; Redlich, P. J. Fuel 1988, 67, 1036-1041.
S0887-0624(97)00181-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/16/1998
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Energy & Fuels, Vol. 12, No. 3, 1998 571
Table 1. Elemental Analysis, Aromaticity, and Petrographic Composition of the Five Coals Studied Penn State sample bank no. DECS 9
PSOC 1379
DECS 6
PSOC 831
DECS 12
seam state ASTM rank moisture (as received, wt %) mineral matter (dry, wt %)
Deitz Montana Sub B 23.7 6.1
Colorado F Colorado hvC b 12.0 5.1
Blind Canyon Utah hvA b 4.7 6.7
Brazil Block Indiana hvC b 13.0 4.2
Pittsburgh No. 8 Pennsylvania hvA b 2.4 11.9
carbon hydrogen nitrogen sulfur (organic) oxygen (by difference) hydrogen/carbon fa
76.6 5.20 1.0 0.51 16.8 0.81 0.57
Elemental Composition (wt % daf) 76.7 81.7 4.97 6.22 1.7 1.6 0.63 0.40 16.0 10.1 0.78 0.91 0.67 0.65
80.0 5.03 1.4 0.90 12.7 0.75 0.72
84.8 5.66 1.4 0.83 7.4 0.80 0.70
Vitrinite Exinite (Resinite) Inertinite
89 3.5(0) 8
Petrographic Composition (vol%) 92 69 0.6(0.1) 17(5) 7 14
88 5(0) 7
83 8(3) 9
Figure 1. Solid state
13C
NMR spectra of the coals.
obtained from the 13C NMR spectra. The tetralin-free oils were also analyzed by GC/MS.6,8 Variation in duplicate runs indicated that differences of g5 wt % in total conversion and oil + gas + water (OGW) and g2 wt % in asphaltene yield were significant. The hydrogen fractions in the 1H NMR results and the fa values from the 13C NMR results were reproducible to (0.02. The elemental analyses were reproducible to (0.2 wt % (C), (0.1 wt % (N), and (0.05 wt % (H, S).
Results and Discussion Coal Characterization. The elemental analyses, coal characteristics (Table 1), and the solid-state 13C NMR spectra (Figure 1) of the five coals are repeated from the earlier publications.1,2 The two lower-rank coals DECS 9 and PSOC 1379 (oxygen content of g15%) are similar in elemental composition, but PSOC 1379 had a higher fa value, and dipolar dephasing data suggested that it contained slightly larger rings on
average or was more cross-linked than DECS 9.2 Both coals had a higher oxygen content than the other three coals in the suite. Two of the higher-rank coals, DECS 12 and PSOC 831, were similar in maceral content, atomic H/C ratio, and fa value, although there was a significant rank difference. DECS 6 is the most unusual coal in the suite, with a high atomic H/C ratio and high concentrations of resinite and liptinite giving a high py-GC/MS yield of long-chain alkanes and alkylnaphthalenes. In these ways this coal resembles the Australian Surat Basin coals studied by Redlich et al.4,5,8 Relation of Coal Conversion to Atomic H/C Ratio. Redlich et al.3,5,9,10 showed, for a wide range of Australian and international coals, that the conversion (CH2Cl2 basis, CO2-free) for reactions in tetralin/H2 fell on a single trend-curve when plotted against the atomic H/C ratio. A single trend-curve was also found for reactions in SnO2/H2. The results shown in Table 2 indicate that for the five coals investigated here the corresponding results fell on the trend-curves established by the earlier work. (For coals of sub-bituminous or higher rank, CO2 yields under these reaction conditions are small.)5 It should be noted that three of the coals have similar H/C ratios and coal conversions, independent of oxygen content. The 10% lower conversion noted for reactions of PSOC 831 with H2/MoS2/ tetralin compared with that for the other four coals1,2 did not carry over to the catalyst/gas systems used here. The analytical data for the 405 °C asphaltenes and the PMRTA results for this coal (see below) suggest a possible explanation for this apparent anomaly (see Conclusion). Analysis of Tetralin-free Oils. Previous work has shown that the character of light products changes little with reaction conditions (350- 425 °C)1-3,5,6,9,10 and is similar to that of the products of py-GC/MS.1,2 This section presents the GC/MS data for the oils; solution 1H NMR supports the GC/MS data but does not give further insight (as found by Redlich et al.6,8) and so will not be discussed further. Figure 2 shows the total ion chromatograms (TIC) of the oils from all five coals for reaction with SnO2 at 405 °C for 1 h. For DECS 6, the TIC was dominated by longchain alkanes and two-ring aromatic compounds, as for Surat Basin coal oil.8 For the other four oils, the
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Burgess et al.
Table 2. Conversion Data for All Five Coals under Various Reaction Conditions reaction conditions
d
solvent or catalyst
405 °C, 1 h, 10 MPa H2
SnO2
405 °C, 1 h, 6 MPa H2
tetralind
350 °C, 1 h, 6 MPa N2
none
320 °C, 1 h, 6 MPa N2
none
coal
H/C ratio
total conva
asph yieldb
OGW yieldc
DECS 9 PSOC 1379 DECS 6 PSOC 831 DECS 12 DECS 9 PSOC 1379 DECS 6 PSOC 831 DECS 12 DECS 9 PSOC 1379 DECS 6 DECS 9 PSOC 1379
0.81 0.78 0.91 0.75 0.80 0.81 0.78 0.91 0.75 0.80 0.81 0.78 0.91 0.81 0.78
45 45 46 41 40 64 62 80 61 56 22 16 19 11 5
6.5 4.1 5.1 7.7 8.2 21.3 23.7 24.0 30.2 31.7 0.0 0.0 1.0 0.0 0.0
39 41 41 33 32 43 38 56 31 24 22 16 18 11 5
a Total conversion based on dichloromethane insolubles. b Asphaltene yield. c OGW: oil + gas + water yield calculated by difference. Tetralin used as solvent 9:3 g/g tetralin/coal used in the reaction.
Figure 2. Total ion chromatograms of the oils from reactions of the coals with SnO2 (1 mol SnO2 per kg dry coal) under 10 MPa hydrogen (cold) at 405 °C for 1 h: (a) DECS 6; (b) DECS 9; (c) PSOC 1379; (d) PSOC 831; (e) DECS 12.
chromatograms were similar in peak distribution up to 35 min retention but significant differences were found at higher retention times. Even though the parent coals DECS 9 and PSOC 1379 had similar elemental analyses and atomic H/C ratios, the n-alkane distributions were significantly different. The oil from PSOC 1379 contained a higher concentration of shorter-chain aliphatics than the oil from DECS 9. This is consistent with PSOC 1379 being of higher ASTM rank. It has been shown that higher rank leads
to a reduction in average chain length of aliphatic material in oil or pyrolysis products.12,13 Oils from DECS 12 and PSOC 831 showed the classical features of a high-rank coal oil with very little long-chain (>C15) aliphatic material,8 despite the difference in carbon content and ASTM rank. To improve our understanding of these coals, further analyses were carried out. Low-temperature reactions, analysis of asphaltenes from tetralin reactions, proton magnetic resonance thermal analysis (PMRTA) data, and geological information were used to further characterize each coal. Oils from Low-Temperature Reactions. Reactions of the coals were carried out with nitrogen in the absence of solvent at 320 and 350 °C (see Table 2). The two low-rank coals DECS 9 and PSOC 1379 gave low yields of oil + gas + water (OGW) at 320 °C (11 and 5%) and higher yields (22 and 16%) at 350 °C. The oils were typical of those from low-rank coals in that they were dominated by guest-derived material (terpenoid and wax-derived hydrocarbons)6 together with some one- and two-ring aromatic compounds. At 350 °C the proportion of aromatic material including phenols increased, consistent with a greater degree of breakdown of the macromolecular structure. The other three coals showed no reactivity at 320 °C. They gave significant amounts of OGW (15-20%) at 350 °C (e.g., DECS 6 result in Table 2), but the GC traces of the oils from the 350 °C reactions (not shown) indicated that they had constituents very similar to those of the oils from 405 °C reactions, though the longchain aliphatics were more prominent in the former. This is consistent with the aliphatic components of the coal being cleaved from the structure at 350 °C (but not at 320 °C) but with little breakdown of the overall macromolecular structure of the coal at this temperature. Asphaltenes from 405 °C Reactions. Reactions of the two low-rank coals, DECS 9 and PSOC 1379, in tetralin at 405 °C gave substantial amounts (ca. 20%) of asphaltenes (Table 2) whose1H NMR spectra were very similar not only to each other but to spectra of other asphaltenes from low-rank coals.7 Two of the three higher-rank coals gave higher yields of asphalt(12) Brooks, J. D.; Smith, J. W. Geochim. Cosmochim. Acta 1967, 31, 2389-2397. (13) Radke, M.; Schaefer, R. G.; Leythaeuser, D.; Teichmu¨ller, M. Geochim. Cosmochim. Acta 1980, 44, 1787-1800.
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Energy & Fuels, Vol. 12, No. 3, 1998 573
Table 3. Solution 1H NMR Hydrogen Distribution for Asphaltenes Generated from Reaction at 405 °C, 1 h, with Tetralin as Solvent coals
Hara
HRb
Hβc
Hγd
DECS 9 PSOC 1379 DECS 6 PSOC 831 DECS 12
0.30 0.29 0.39 0.33 0.34
0.39 0.41 0.32 0.32 0.34
0.25 0.25 0.22 0.27 0.24
0.06 0.05 0.07 0.07 0.08
a Aromatic protons, 9.5-5.8 ppm. b Aliphatic protons R to ring, 4.5-1.96 ppm. c Aliphatic protons β to ring, 1.96-1.0 ppm. d Aliphatic protons γ to ring, 1.0-0.4 ppm.
Figure 4. Plot of M2T16 vs temperature (PMRTA) for the three higher-rank coals in the suite: (9) PSOC831; (0) DECS12; ([) DECS 6.
Conclusion
Figure 3. Plot of M2T16 vs temperature (PMRTA) for the two low-rank coals in the suite: (9) DECS 9; (0) PSOC 1379.
enes (ca. 30%), and in all three cases the 1H NMR spectra showed a decrease in the number of HR protons relative to Har in comparison with the asphaltenes from the low-rank coals (Table 3), consistent with the presence of fewer side chains in the higher-rank coal structures. Interestingly, the Hβ value for PSOC 831 was significantly higher than for the other coals, and this could be interpreted as evidence for methylenecontaining cross-linking structures in this coal. Further evidence to support this proposal is given below. Proton Magnetic Resonance Thermal Analysis (PMRTA). PMRTA is a technique that can give information about the molecular mobility of segments within the coal structure by measuring 1H NMR transverse relaxation signals during exposure to high-frequency radiation over a range of temperatures.14 Plots of M2T16 data versus temperature for the five coals are shown in Figures 3 and 4. The plots for DECS 9 and PSOC 1379 are similar to each other and to plots obtained from other low-rank coals of similar H/C atomic ratio.11 The plots for the higher-rank coals were consistent with results of earlier work.14 However, the profile for PSOC 831 showed low mobility at 400 °C relative to that for DECS 12, a coal of similar H/C atomic ratio, consistent with a higher degree of cross-linking in the former coal. Geology of PSOC 831 and DECS 12. The interesting differences between these two coals noted above could arise from their different geological histories. Geological data suggest that the sample of PSOC 831 was obtained from a portion of the seam close to an intense fusain band, and the inertinite in this coal was mainly fusinite. Such macerals frequently arise as a result of fires, and the consequent high temperatures could lead to significant cross-linking. In contrast, the DECS 12 coal apparently contained little fusain. (14) Sakurovs, R.; Lynch, L. J.; Barton, W. A. In Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; Advances in Chemistry Series 229; American Chemical Society: Washington, DC, 1993; pp 229-251.
The investigations described above demonstrate that this group of five U.S. coals in general fits into previously established classes of coals based on reactivity and structural measurements. DECS 9 and PSOC 1379 are typical low-rank coals, and DECS 6, although of high H/C atomic ratio for its rank, appears to be closely related to a range of sub-bituminous coal deposits of Central Queensland, Australia.4,5,8 The two high-rank, bituminous coals, DECS 12 and PSOC 831, showed some unexpected differences, especially in their PMRTA profiles and to some extent in the NMR spectra of their liquefaction products, which suggested that PSOC 831 contained more cross-linking groups within the macromolecular structure. This cross-linking could be associated with the presence of a significant quantity of fusain maceral in the sample. Interestingly, this structural difference apparently involved bonds that were readily cleaved under liquefaction conditions, since very similar yields of products were obtained from hydrogenation reactions either in tetralin or with tin catalysts. This structural difference of PSOC 831 from the other coals was apparently responsible for the anomalous results obtained in MoS2catalyzed reactions.1,2 The MoS2 catalyst apparently opens up reaction pathways completely different from those dominating pyrolysis or uncatalyzed tetralin reactions, and for this coal this leads to an unexpected product distribution, emphasizing the limitations of reactions with such good catalysts in studies of coal structure. Acknowledgment. We thank the National Science Foundation for a postdoctoral fellowship (to C.E.B.). This work is part of a collaborative program established by the New Energy and Industrial Technology Development Organization of Japan (NEDO) to promote efficient methods for the production of fluid fuels for power generation from coal, and we thank NEDO for financial support to W.R.J., P.J.R., and M.M. We also thank Mr David Glick, senior research assistant for the Coal and Organic Petrology Laboratories at the Pennsylvania State University, for his contribution to the geological information. EF970181Z
