412
Energy & Fuels 1989,3, 412-420
Comparative Study on the Compositional Characteristics of Pyrolysis Liquids Derived from Coal, Oil Shale, Tar Sand, and Heavy Residue M. Rashid Khan,* Kalkunte S. Seshadri,t and Tersea E. Kowalskit Morgantown Energy Technology Center, US.Department of Energy, P.O. Box 880, Morgantown, West Virginia 26507-0880 Received August 18, 1988. Revised Manuscript Received March 15, 1989
A range of fossil fuels (e.g., coal, oil shale, tar sand, and a heavy residue) were pyrolyzed at a relatively low temperature and their liquid products were characterized. Identification of key chemical components present in low-temperature pyrolysis liquids is important in the selection and subsequent upgrading of a feedstock (e.g., coal liquids, shale oils, or taisand liquids) for generation of highenergy-density fuels (HDF). Fractionated liquid samples, as opposed to the whole liquids, were analyzed to obtain more meaningful and detailed results. Sequential elution solvent chromatography (SESC) was used for this purpose in this work. Subsequent separation of the first fraction of SESC into saturates/alkenes and aromatics was achieved by using a semipreparative high-performance liquid chromatograph (HPLC). SESC and HPLC fractions were analyzed by using field ionization mass spectroscopy (FIMS). Separation and spectroscopic characterization results including infrared analysis of solid fuels and pyrolysis liquids, chromatographic profiles of aromatics in several pyrolysis liquids, and a comparison of abundance of acyclic and cyclic alkanes for tar sand liquid, Colorado shale oil, and Pittsburgh No. 8 coal liquid are presented. Using FIMS data in the 2-series type formates, we identified alkanes and cycloalkanes in the three samples. The tar sand pyrolysis liquid and the Colorado shale oil apparently contain almost identical structures. The coal pyrolysis liquid, however, shows a different product distribution. On the basis of the naphthenic carbon content as percent of total liquids, the fuels can be ranked in the following order as feedstocks for advanced fuels: Asphalt Ridge tar sand > western shale > Pittsburgh No. 8 coal > eastern shale. The aromaticities of the liquids decreases in the following order: Pittsburgh coal liquid > eastern shale oil > western shale oil > tar sand liquid. However, the ranking of fuels based on liquid analyses can be expected to change depending on the process conditions chosen for upgrading and heteroatom removal. The characterization data of these liquids will serve as a basis for identification of processes for upgrading a particular fuel or recovering valuable chemicals from it.
Introduction and Background Because of shortages of liquid fuels in the United States, there is an impetus for converting coal, oil shale, and tar sand into transport fuels to achieve our long-term liquid fuel needs. The liquid fuels produced from these sources, however, will need to be upgraded to meet the fuel specifications. Identification of key chemical components is important in the selection and subsequent chemical treatment of a feedstock. Some analytical aids commonly used in the characterization of fuels are infrared spectroscopy, nuclear magnetic resonance, mass spectrometry, and both gas and liquid chromatographies. More meaningful resulta are obtained when fractions of the liquid fuels rather than the total liquids are characterized by using these techniques. The differences in product composition to a large extent are expected due to variations in the geological origin, chemical structure of the parent resource, and pyrolysis conditions. Fossil energy resources originated from terrestrial or marine plant sources. However, there are differences in the original type of plant material and in biochemical and geological pathways followed during formation.' Thus, the geological origins of fossil energy resources have a bearing on the composition of liquids. EG&G Washington Analytical Services Center. t Undergraduate Student Trainee through the Oak Ridge Associated Universities (ORAU) Program.
Humic coals are derived from higher terrestrial plants, whereas the origin of kerogen (in oil shale) is either marine of fresh water algae. Mineral oil shales are commonly clay minerals, but in the Green River formation, carbonates are the primary mineral components with minor amounts of quartz and clay minerals. Devonian oil shales were deposited in shallow seas in continental platforms and predate the tertiary Green River Formation oil shale. Tar sand is found often in the same geographical location as the petroleum crude used, and tar sands are, in essence, sand deposits impregnated with dense viscous petroleum. Fossil fuels can be categorized according to Van Krevelen's H / C versus O/C diagram (Figure 1). Kerogen in the Green River formation belongs to type I with the following characteristics: high initial H/C ratio, low O/C ratio, and high oil yield. Type I1 kerogen, also with relatively high H/C and low O/C ratios, has been found in source rocks of petroleum. Kerogens low in H/C and O/C ratios are classified as type 111. They contain relatively high proportions of polyaromatic compounds and are (1) Tissot, B. P.; Wele, D. H. Petroleum Formation and Occurrence. Springer-Verlag, New York, 1978. Tissot, G.; Vandenbroucke, M. in Geochemistry and Chemistry of Oil Shale; Miknii, F., McKay, J., Eds.; ACS Symposium Series No. 230; American Chemical Society: Washington, DC, 1983. Vorres, K. S. In Kirk-Othmer Encyclopedia of Chemical Technology;John Wiley and Sons, Inc.: New York, 1979; Vol. 6,p 224. Towson, D. Zbid.; 1983; Vol. 22, p 601. Given, P. H. In Coal Sciences; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press, Inc.: New York, 1984; p 63.
0887-0624/89/2503-0412$01.50/00 1989 American Chemical Society
Energy & Fuels, Vol. 3, No. 3, 1989 413 Table I. Pyrolysis Yield in a Fixed-Bed Reactor at SO0 % yield (dry feedstock feedstock basis) Pittsburgh No. 8 coal 17.7 Mississippi lignite 21.3 Colorado oil shale 12.5 Sunbury oil shale 4.0 concentrated resinite sample 87.8 heavy resid (Kern River) 83.0 Asphalt Ridge tar sand 11.5 ~~~
I
I ' 0
1
5
10
15
20 25 30 ATOMIC RATIO O/C x 100
35
40
45
Figure 1. Fossil fuels utilized in the present study, presented in a Van Krevelen's diagram. Extensively modified and replotted from Tissot and Vandenbroucke.'
found in the Devonian oil shale. Structural differences among the fossil fuels above are indeed reflected in the chemical composition of derived liquids. For example, both the Colorado shale oil and Asphalt Ridge tar sand liquid, derived from type I kerogens, are somewhat similar in their composition. Shale oil derived from the Devonian deposits contains more polyaromatics and overall is more aromatic than the shale oil from the Green River formation. The objectives of this study are to investigate the compositional differences among the pyrolysis liquids derived from coal, oil shale, and tar sand and to assess their potential for a HDF feedstock. A subobjective of this study is to relate the differences among various liquids ta their geological origins. In studies of fossil fuel conversion and structures, there is a general lack of "cross-fertilization" in presenting the commonalities of structures and compositions in various fuels. In essence, researchers tend to concentrate exclusively on one fuel area or another. Consequently, fundamental information developed in one field does not necessarily transfer to another area. Although literature reports for various pyrolysis-derived liquids are innumerable, relatively little analytical data are available on the liquids derived at constant process conditions so that the products could be compared on a common basis. Information concerning the composition of liquids is important to determine upgrading requirements as well as evaluating what feedstock to use to produce a desired fuel such as the high-energy-density fuels (HDF). Furthermore, such comparison is expected to strengthen our knowledge on fundamental aspects of feedstock composition and product composition or yield relationships.
Experimental Section Feedstocks Origin a n d Pyrolysis Procedure. The experimental procedure for the production of pyrolysis liquids from solid fuels including reador system, experimental procedures, and reproducibility of results is described by Khan." A slow heating rate fiied-bed reactor waa used to generate the pyrolysis liquids at 500 OC. A range of feedstocks was pyrolyzed in this reactor. All sample preparation and handling procedures were performed under inert atmospheres. The tar and sample was procured from the Western Research Institute and Kern River heavy residue from the Stanford Research Institute. The following samples were characterized in this study: the Colorado shale oil, an Asphalt Ridge tar sand liquid, a Pittsburgh No. 8 coal liquid, concentrated resin liquid, and Sunbury shale oil. Because properties of coal (2) (a) Khan, M. R.Fuel Sci. Technoi. Znt. 1987,5(2),185. (b).Khan, M . R. Energy Fuels 1988,2,834.
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liquids vary widely as a function of rank, liquids from a low-rank (Misaiasippi lignite) coal were studied for comparisons. Additional details on the origin of these samples can be found elsewhere?b The resin sample, separated from a Utah coal, was processed at the University of Utah. Pyrolysis liquid yields are presented in Table I. Spectra. NMR spectra were recorded on a Varian 200-MHz spectrometer at the Energy Research Center of the University of North Dakota. The average molecular parameters were calculated from proton spectra by using the method of Clutter and co-workers? Infrared spectra were recorded on a Nicolet 7000 Fourier transform spectrometer. Each spectrum was obtained by the coaddition of 32 interferograms at 2-cm-' resolution. Spectra of liquid fuels were obtained as thin films between KBr windows and those of solid fuels as KBr pellets. Gravity-Flow Liquid Chromatography. The separation of fixed-bed liquids was carried out by using sequential elution solvent chromatography (SESC). The procedure is described by Farcasiu4 and Seshadri and Cronauer." In the fixed-bed unit, liquids produced are light and nearly 85 w t % of all liquids were recovered in the first six fractions. Therefore, after the column was eluted with the sixth solvent (i.e., methanol), the remaining tar was discarded along with the column packing. The first fraction, which contains saturates, alkenes, and aromatics, was studied by using high-performance liquid chromatography (HPLC). High-Performance Liquid Chromatography. The chromatographic instrumentation consisted of a Perkin-Elmer Series 4 quarternary solvent delivery system, an IS-100 autosampler, a LC-85 UV detector, and a 3600 data station. Analytical chromatography was performed on a dual-column system (25 cm by 4.6 mm) with 5-pm silica gel bonded with cyano groups (Supelco LC-CN) in one column and 5-bm plain silica (Supelco LC-Si) in the other. Liquid mobile phases were HPLC grade (Fisher Scientific) solvents predried over 4-A molecular sieves for at least 12 h, filtered through a 0.2-pm membrane, degassed with helium, and pressurized to 5 psig. SESC fraction 1was dissolved in hexane (2 mg/mL), and 20 pL of the solution was injected onto the column. Then the columns were eluted with hexane at a flow rate of 1 mL/min, and eluant from the second column was monitored at 254 nm. This method provided fairly good resolution for aromatics. After the chromatogram of aromatics in the sample was recorded, the column was cleaned with methylene chloride and equilibrated with hexane prior to the analysis of the next sample. Semipreparative separation was performed on a 30 cm by 9 mm column packed with 5pm silica (ESIndustries Chromegabond S160 and Altech Econosphere). Hexane with a flow rate of 4 mL/min was employed in the separation of SESC fraction 1 into saturates/olefm and aromatics. Chromatographic conditions for the separation were established by injecting 10 pL of the sample in hexane at a concentration of -80 mg/mL. With the hexane mobile phase, UV response was fmt observed 5 min after injection. At that time, the flow was stopped, the Rheodyne valve in the system was switched to backflush the column, and the flow of hexane was started.e The UV response increased rapidly and returned to the base line in 10 min. The fraction that eluted for (3)Clutter, D.R.;Petrakis, L.; Stenger,R. L., Jr.; Jensen, R. K. Anal. Chem. 1972,44,1395. (4)Parcasiu, M.Fuel 1977,56,9. (5)Seshadri, K. S.;Cronauer, D. C. Fuel 1983,62, 1436. (6)Suatoni, J. C.; Swab, R. E. J. Chromatogr. Sci. 1976, 14, 535.
Khan et al.
Energy & Fuels, Vol. 3, No. 3, 1989
i
/
Table 11. Average Molecular Parameters for the Colorado Shale Oil, Asphalt Ridge Tar Sand Liquids, Sunbury Shale Oil, and Pittsburgh No. 8 Coal Liquids' I I1 I11 IV aromaticity (carbon) 0.55 0.28 0.38 0.23 aromaticity (hydrogen) 0.21 0.06 0.12 0.06
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64.1 28.1 7.8 77.0 3.7
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Aromatics were collected when the column was backflushed for 10 min. Analytes were recovered by removing the solvents by rotary evaporation at 40 O C under partial vacuum. Field Ionization Mass Spectroscopy (FIMS). FIMS data were obtained for saturates and olefins in select liquids at the Stanford Research Institute. The description of the instrument and data acquisition procedure are described by St. John et al.' Data were analyzed in order to identify individual components in the mixture.
Results and Discussion Infrared Spectroscopy. Infrared spectra of the liquid fuels (Figure 2) reveal several compositional differences between resin liquid and coal liquid and between eastern and western shale oils, which agree with the results of HPLC and NMR (vide infra). The resin liquids, although coal-derived, have a very low concentration of phenolic compounds as evidenced by weak absorption near 3300 cm-' and the low intensity of the 1600-cm-' band. LC results suggest that this tar contains predominantly neutral aromatic compounds (see Table 111). Similarly, the low concentration of phenolic compounds in the Colorado shale oil and tar sand liquids are reflected in the infrared spectrum. Spectra of Pittsburgh No. 8 coal liquids and the Kentucky Sunbury shale oil display several similarities. (7) St. John, G. A.; Buttrill, S. E. Jr.; Anbar, M. Field Ionization and Field Desorption Mass Spectroscopy Applied to Coal Research. In Organic Chemistry of Coal; Larsen, J. Ed.; ACS Symposium Series 71; American Chemical Society: Washington, DC, 1978; p 223.
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In the aromatic C-H wagging vibrational region (near 800 cm-'1, several well-resolved bands are observed, whereas the spectrum of the Colorado shale oil displays weak absorptions in the same region. This suggests that the Pittsburgh No. 8 coal liquid and the eastern shale oil have greater quantities of unsubstituted aromatic carbon atoms than the Colorado shale oil and tar sand liquid. Our results agree with 13C NMR data of Netzel and Miknis.* Spectra of solid fuels (Figure 3) are leea informative than those of liquid fuels with regard to their geological origin or compositional differences because absorptions due to (8) Netzel, D. A.; Miknis, F. P. Fuel 1982, 61, 1101.
Compositional Characteristics of Pyrolysis Liquids
Energy & Fuels, Vol. 3, No. 3, 1989 415
Table 111. Sequential Elution Solvent Chromatographic Fractions of Pyrolysis Liquids' wt % of liauid fraction no.b lignite Pittsburgh Sunbury Colorado tar sand 1 2 3 4 5 6
residue
65.8 3.5 13.7 2.2 2.3 3.3 9.2
37.0 2.5 20.7 9.7 11.2 2.5 16.4
48.5 3.8 17.8 3.9 6.3 2.2 17.5
74.6 8.6 7.9 0.5 3.9 3.2 1.3
71.1 3.2 6.4 2.2 3.4 0.7 13.0
Utah 60.2 12.4 15.0 4.4 4.7 3.3 0.0
Key: Pittsburgh, Pittsburgh No. 8 coal liquid; Colorado, Colorado shale oil; tar sand, Asphalt Ridge tar sand liquid; lignite, Mississippi lignite liquid; Utah, Utah shale oil; Sunbury, Kentucky Sunbury shale oil. bSee Table V for detailed description of various fractions.
organic matter are obscured by mineral material. The strong 0-H absorption noticed in the spectrum of coal resin is probably due to moisture. The spectrum of coal displays a strong band at 1605 cm-' due to the aromatic ring mode that is practically absent in the spectrum of coal resin. The band near 1600 cm-' appears with enhanced intensity when the aromatic structures are substituted with 0-H groups.6 Average Structural Parameters of Fuels As Determined by NMR. The computer program designed for the analysis of aromatic fraction of fuels was used here to derive average structural parameters of total oil, and the results were reasonable. Typical average molecular parameters for some representative pyrolysis liquids are presented in Table 11. They appear to be in agreement with the trend observed in the separation results and in the liquid chromatographic profiles of aromatics, which are discussed in subsequent sections. As shown in Table 11, the carbon aromaticity of pyrolysis liquids are in the following order: Pittsburgh No. 8 coal > Sunbury shale oil > Colorado shale oil > tar sand liquid. The aromaticity data presented in this study are consistent with data available in the literature, even though different pyrolysis conditions were used. For example, Collins et al.9 reported that the carbon aromaticity of the flash pyrolysis tars derived from Australian coal range from 0.5 to 0.6. Gavalas and Oka'O measured the carbon aromaticity of coal tar generated from a high-volatile bituminous and subbituminous coal and reported the values to be 0.48 and 0.66, respectively. Netzel and Miknis* have reported carbon aromaticity for different shale oils. The aromaticities of western shale oils produced in fixed-bed (Fischer assay) and by hydropyrolysis (IGT Hytort) were 0.24 and 0.27, respectively. Corresponding values for the Kentucky Sunbury oil were 0.38 and 0.45,respectively. Table I1 shows that the Sunbury shale oil contains a higher concentration of multiring aromatic.compounds than the Colorado shale oil. The distribution of mono- and diaromatics in the tar sand liquid is comparable to that in the coal liquid, but its aromaticity is about 40% of that of the coal tar. These results suggest that, in tar sand liquid, the aromatic structures have relatively long alkyl substituents compared to the aromatic structures in coal liquid. Colorado shale oil has the largest concentration of monoaromatics, which include hydroaromatics, but the tar sand liquid is richer in cycloalkanes (to be shown later). If the quality of the HDF is determined solely on the basis of total naphthenic carbon, then both shale oil and tar sand liquid are good feedstocks for high-density fuel. Chromatography. The relative quantities of the SESC fractions of liquids of the Colorado oil shale, Asphalt Ridge tar sand, Mississippi lignite, Utah oil shale, the Kentucky Sunbury oil shale, and Pittsburgh No. 8 coal are given in (9) Collin, P. J.; Tyler, R. J.; Wilson, M. A. Fuel 1980,59, 479. (10) Gavalas, G. P.; Oka, M. Fuel 1978,57,285.
Table IV. Sequential Elution Solvent Chromatographic Fractions of Pyrolysis Liauids" wt % of liquid fraction no. Kern resin 1 2 3 4 5 6
residue
87.7 4.9 4.0 1.3 1.1 1.0 0.0
82.6 5.0 6.4 0.4 2.5 1.7 1.4
Key: Kern, Kern River heavy resid liquid; resin: concentrated resin liquid.
Table 111. The first two fractions, which make up a major portion (40-93 wt %) of these tars, contain mainly saturated hydrocarbons/alkenes and neutral aromatics, based on infrared and 13CNMR spectra of these factors.6 The third fraction is of moderate yield (44%) in some liquids and high yield (13-28%) in others contains chiefly hydroxylated aromatic compounds. The fourth and subsequent fractions are of relatively low yield. Compounds in these fractions are high molecular weight phenolics and basic nitrogen compounds with 0-H groups. These liquids are categorized more precisely by further separating SESC fraction 1 into saturates/alkenes and aromatics using semipreparative HPLC. The moderate yield of fraction 3 in the Colorado shale oil and tar sand liquid and the high yield of the same fraction in the coal liquid are reflected in the relative intensity of the 1600-cm-' band in the infrared spectra of these liquids. In addition to coals and oil shales, Kern River resid and concentrated resin from a Utah coal were pyrolyzed in the fixed-bed reactor in order to assess other possible feedstocks for HDF. Separation results for these two liquids are given in Table IV. Several important differences among HDF feedstocks are revealed by the separation results. Although fraction 1constitutes nearly 80% of the resin liquid, further separation of this fraction into saturates, olefins, and aromatics indicated that the resin liquid is highly aromatic. However, the concentration of polars in this liquid is much less than in coal liquids. Thus, upgrading this liquid may be more economical than upgrading the coal liquids. On the other hand, fraction 1of the Kern River resid liquid, the Colorado shale oil, and the Asphalt Ridge tar sand liquid constitutes 70-90% of the total liquid. However, the carbon aromaticity of fraction 1is much less than that of resin and coal liquid, implying that they have larger concentrations of saturates relative to those in coal liquid. Saturates have been separated from this fraction and further classified. Mississippi lignite liquid and the Utah shale oil fall into a category, despite the fact that the fossil fuels from which they are derived have different geological origins. The Kentucky Sunbury shale oil is rather similar to the coal liquid in composition and may require similar refinery treatment.
Khan et al.
416 Energy & Fuels, Vol. 3, No. 3, 1989 Table V. Class Types of Predominant Components of Various Fractions Separated in SESC fraction no. eluent major components hexane alkanes/alkenes, neutral aromatics nonbasic N-, 0-,and hexane/l5% toluene S-heterocyclics, neutral aromatics monophenols chloroform chloroform/4% diethyl ether polyphenols, carbonyls, diethyl ether diethyl ether/3% ethanol polyphenols, basic nitrogen heterocyclics highly functional methanol (polar) molecules (large heteroatom contents)
Table VI. Separation Results for SESC Fraction 1 of Pyrolysis Liauids i t % in SESC fraction 1 saturates tar sample and olefins aromatics recovery Kern River resid 64.4 31.9 96.3 Asphalt Ridge tar sand bitumen 56.7 33.6 97.1 Colorado oil shale 50.7 45.2 95.9 Kentucky Sunbury oil shale 16.3 78.8 95.1 Pittsburgh No. 8 coal 15.1 81.5 96.6 concentrated resin 9.5 87.6 97.1 ~~
Table VII. Retention Data for Model Aromatic Compounds compd retention time, min triethylbenzene 8.52 tetralin 9.27 naphthalene 10.04 10.72 2-methylnaphthalene phenanthrene 13.67 Pyrene 14.34 22.60 benz[a]pyrene i
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Retention Time (min) Figure 4. Characterization of aromatic fractions of various fossil fuels using HPLC.
The relative quantities of saturates/alkenes and aromatics in SESC fraction 1 of tars mentioned above are presented in Table VI. The results demonstrate that while Fraction-1 of Pittsburgh No, 8 coal and resin liquids are highly aromatic, Sunbury shale oil, Kern River resid, tar sand liquids, and Colorado shale oil are highly aliphatic. Cyclic alkanes present in saturates/alkene fractions of tar sand liquid, Colorado shale oil, and Pittsburgh No. 8 coal liquid have been identified and their distribution in the three liquids are compared from a 2-series type analysis of FIMS data discussed later. A qualitative comparison of naphthenic carbon content in the aromatic fraction was
108
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possible by an examination of chromatographic profiles of aromatics in these and in Utah shale oil. Fraction 1 from SESC separation, dissolved in hexane, of each of the above-mentioned liquids was chromatographed on the dual-column system, using hexane as the mobile phase. Chromatograms of neutral aromatics are shown in Figure 6. The profiles display two distinct valleys and, therefore, can be divided into three regions. The first region is from the time UV response is first ob-
Energy & Fuels, Vol. 3, No. 3, 1989 4 7
Compositional Characteristics of Pyrolysis Liquids
served to the time at which the first valley is reached. The second region is between the two valleys, and the third region is between the second valley and the end of chromatogram. On the basis of retention times of model compounds (see Table VII), the first region is assigned to monoaromatics including tetralins, octahydrophenanthrenes, and similar structures, the second region is assigned to di- and triaromatic compounds, and the last region is assigned to tetraaromatic and higher aromatic ring systems. This demarcation is not definitive; overlap of ring systems is to be expected. The integrated intensity in the monoaromatic region of chromatograms of shale oils, tar sand, resid, and coal liquids have provided, although qualitatively, an insight into compositional differences in these liquids. The relative area covered by monoaromatic components in chromatograms of the Colorado and Utah shale oils is larger than in the coal liquid and Sunbury shale oil. Since response factors of monoaromatics are 1 order of magnitude less than those of diaromatics and 2 orders of magnitudes less than those of triaromatics," the Colorado and Utah shale oils contain more monoaromatics and, therefore, possibly more naphtlenic carbons (ring saturated carbons in hydroaromatic structures) than coal liquids and Sunbury shale oil. The resid and tar sand liquids, in this respect, fall in between shale oils and coal liquids. Simulated Distillation of Pyrolysis Liquids. Highdensity fuel feedstocks included in this report were also analyzed by simulated distillation, and the results are presented in Figures 5 and 6. The boiling range for the purpose of discussion can be divided into two regimes: 200-370 and 425-480 "C. Fraction weights boiling between 200 and 370 "C are comparable in the Colorado and Kentucky Sunbury shale oils, Pittsburgh No. 8 coal liquid, and Asphalt Ridge tar sand liquid. However, differences are observed in the fraction weight boiling at higher temperatures: 10-14% of the Sunbury shale oil and coal liquid, 17-20% of the Colorado shale oil and tar sand liquid, and 23-30% of the Kern River resid liquid boil between 425 and 480 "C. Thus, simulated distillation studies indicate that the Sunbury shale oil and Pittsburgh No. 8 coal liquid on one hand and the Colorado shale oil and Asphalt Ridge tar liquid on the other hand are similar in composition. Separation results suggest the same similarity. The high-boiling fractions of shale oil and tar sand liquid probably contain aromatics and basic nitrogen compounds with long alkyl chains and Czoand higher alkanes. The corresponding fractions of coal liquid and Sunbury shale oil contain polar compounds with multifunctional groups. Although nearly 50% of the Kern River resid liquid and concentrated resin liquid boil in the 370-480 "C range, they are very much different in composition. The resin liquid contains mainly neutral aromatic compounds, with a low concentration of polar compounds. Therefore, the majority of aromatics in this liquid have condensed structures. The Kern River resid liquid, unlike the resin liquid, is highly aliphatic. Therefore, the fraction boiling between 370 and 480 OC contains to a large extent long-chain and cyclic alkanes. Distillation results to a large extent agree with the separation results. More importantly, they have supplemented separation results and revealed some new structural information, particularly regarding the resin and the resid liquids. Field Ionization Mass Spectroscopy. The mass spectra of saturated hydrocarbons/alkenes in Pittsburgh (11) Friedel, R. A.; Orchin, M. Ultraviolet Spectra of Aromatic Compounds; John Wiley and Sons, Inc.: New York, 1951.
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No. 8 coal liquid, the Colorado shale oil, and the Asphalt Ridge tar sand liquid are shown in Figure 7. The spectrum of tar sand liquid is somewhat similar to the spectrum of parent bitumen,12 suggesting that several molecular species in the bitumen survive pyrolysis and, therefore, are also found in the liquid. The FIMS spectrum of saturates and alkanes in the Colorado shale oil bears resemblance to that of the tar sand liquid. Although the spectrum of the coal liquid appears to be different from the other two, there are certain similarities. 2 type analyses of the three spectra bring out both the similarities and the distinguishing features. In the tabular form of FIMS spectral data, ion intensities of molecular species are arranged in 14 columns as a function of increasing molecular weight. Therefore, data in subsequential rows differ by 14 amu, equivalent to an additional methylene group. Data in a given column normally represent more than one homologous series. Each column is assigned another number (21, which identifies the parent compound of each series. For example, column No. 8 is assigned 2 numbers 0, -14, -28, .... Certain aromatic compounds are represented by 2 = -14 and -18; monoolefins and monocycloalkanes are represented by 2 = 0. Since data here are for a saturated hydrocarbons/alkenes fraction of a liquid, Column 8 represents monoolefins and monocyclic alkanes. (12) Holmes, S. A.; Raska, K.A. Fuel 1986, 65, 1539.
418 Energy & Fuels, Vol. 3, No. 3, 1989
Khan et al. LO
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Figure 8. Abundance and 2-series type distribution of saturated hydrocarbons in various fossil fuels: (A)coal liquid; (0)tar sand liquid; (0) shale oil. To identify compounds constituting a given column, a plot of logarithm of ion intensity versus molecular weight for a given column is prepared,I3 which for Z = +2 to -8 for the tar sand liquid, the Colorado shale oil, and Pittsburgh No. 8 coal liquid are shown in Figure 8. These plots indicate differences in the composition of saturated hydrocarbons plus olefins in these three liquids. On the basis of FIMS data alone, it is not possible to unequivocally identify all compounds in a given sample. However, by combination of some form of liquid chro(13) Whitehurst, D. D.; Butrill, S.E., Jr.; Derbyshire, F. J.; Furcasiu, M.; Odoerfer, G. A,; Rudnick, L. R. Fuel 1982, 61, 994.
matography with FIMS, useful structural information can be obtained.14J5 In any case FIMS data presented in Figure 8 have contributed to the comparative study of the three pyrolysis liquids with respect to their saturated hydrocarbons content. The abundance distribution for normal and branched alkane display a sharp component at carbon no. 39 (mlz = 548) in shale oil and tar sand liquid, but not in the coal (14) Allen, T. W.; Hurtubise, R. J.; Silver, H. W. Anal. Chem. 1985, 57, 666.
(16)Bcduszynski, M. M.; Hurtubise, R. J.; Allen, T. W.; Silver, H. F. Anal. Chem. 1983,55, 232.
Compositional Characteristics of Pyrolysis Liquids
Energy & Fuels, Vol. 3, No. 3, 1989 419
/ASP
Tar Sand
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Figure 9. Gas chromatogram of saturate/olefinsin the pyrolysis liquids of Colorado shale oil.
liquid. FIMS data were obtained after separation of the pyrolysis liquid into a fraction containing exclusively saturated hydrocarbons plus olefins and aromatic hydrocarbons using SESC, followed by further separation of this fraction using HPLC into individual fractions. The data discussed here are for the former fraction and in particular for branched and n-alkanes. Therefore, this component has been assigned to a C39alkane. In the gas chromatogram of this fraction, a C39n-alkane was not observed, probably due to its very low concentration. Therefore, we have assigned this component to a C40 isoprenoid either derived from isoprene or perhydro-@-caroteneand subsequently degraded into the C39compound during pyrolysis. In the abundance distribution for monoolefins, cyclohexanes, and cyclopentanes (2 = 0), sharp maxima are observed at C19and CN,which are absent in the coal liquid. DiSanzo and co-workers,le based on their GC/MS data, have assigned the component between n-C17and n-Cl8 in the chromatogram of Tosco I1 shale oil to prist-1-ene. In the present work also, the gas chromatogram of the saturated hydrocarbon and olefins fraction of the Colorado shale oil (Figure 9) displays a strong component in the same region. On the basis of this circumstantial evidence, the C19 component in the FIMS spectrum has been assigned to prist-1-ene. Whether the phytyl side chain of the chlorophyll molecule or tocopherols are the natural source1' of pristane, olefins have not been observed in natural bitumens,18 suggesting olefins in shale oils are produced during pyrolysis. Possible assignments for the other major intensity at carbon no. 39 (mlz = 546) are an olefin, an alkylcyclohexane, or cyclopentane. However, in the mass spectrum of bitumen from the Green River shale,'* several molecular ions belonging to C,H2, series are observed of which the species at mlz = 560 is the most abundant and is assigned to the tetraalkylcyclohexane derivative of perhydro-@carotene. The bond adjacent to the quaternary carbon atom in perhydro-@-caroteneis relatively labile, and its clevage gives the molecular ion mlz = 560. Therefore, the C39component in the mass distribution by FIMS is assigned to the demethylated tetraalkylcyclohexane derivative of perhydro-@-carotene. The abundance distribution for dicyclic alkanes in the shale oil displays a major intensity at mlz = 558 (C,) and in the tar sand liquid at m/z = 544 (CSg). In the mass spectrum of natural bitumen from the Green River oil shale, the most abundant molecular ion is observed at mlz = 558 and has been assigned to perhydro-@-carotene. ~~~~
~~
(16) Disanzo, F.P.;Uden, P. C.; Siggia, S. A w l . Chem. 1980,52,906. (17) Goossens, H.; de Leeuw, J. W.; Schenck, P. A.; Brassell, S. C. Nature 1984, 312,440. (18) Anders, D.E.;Robinson, W. E. Geochim. Cosmochim. Acta 1971, 35, 661.
Figure 10. 2-Series type distribution of saturated compounds present in various fossil fuel pyrolysis liquids: Asp Tar Sand, Asphalt Ridge tar sand; Col Shale, Colorado shale; Pit 8, Pittsburgh No. 8 coal.
Based on this information, we have assigned the C, component in the shale oil to the same species and the C39 component in the tar sand liquid to its demethylated derivative. The abundance distribution for other cyclic alkanes (2 = -4, -6, and 4,in particular, for 2 = -6 and 2 = -8, reveal certain compositional differences among the three liquids. Major intensities are observed around C19, C2,, and C30 for both 2 = -6 and 2 = -8 series, suggesting that the coal liquid also, like shale oil and tar sand liquid, contains significant amounts of tetra- and pentacyclic alkanes. Thus, there are some similarities in the composition of saturated hydrocarbons/alkenes fraction of Pittsburgh No. 8 coal liquid, the Colorado oil shale, and the Asphalt Ridge tar sand liquid. However, coal liquid contains only about 5 wt % of saturates and olefins compared to nearly 30 wt 5% in the shale oil and the tar sand liquid. Therefore, extensive upgrading of coal liquid is needed before it can meet the requirements of HDF. On the basis of FIMS data, several cyclic alkanes have been identified in the Colorado shale oil. There are two separation steps before the FIMS data is gathered. One is liquid chromatographic separation of saturated hydrocarbons and olefins, and the second is separation by mass spectrometer into class types (i.e., alkanes, alkenes, and cyclic alkanes according to ring sizes). Therefore, the assignments are not purely speculative. Holmes and Raska12and Hurtubise and c ~ - w o r k e r s ~have ~ J ~obtained compositional data on tar sand bitumens and coal liquids, respectively, using a similar procedure. In any case, our assignments have shown that several biomarkers, after surviving the natural environment, can survive pyrolysis conditions also. In order to identify alkanes and cycloalkanes in the three tars, FIMS data were displayed in one format of 2-type distribution (Figure 8). In another format, percent ionization for series 2 = +2 to -8 is plotted as a function of 2 number, and the plot is presented in Figure 10. The second 2-series type distribution suggests that the saturateslalkene fraction of the coal liquid contains relatively higher concentrations of alkanes and monocyclic and dicyclic alkanes compared to those in the shale oil and the tar sand liquid. The total concentrations of dicyclic saturates, however, in the shale oil and the tar sand liquids appear to be comparable (see Figure 8). Also, in this series the abundance distribution from Cll to CZ3is similar, suggesting the same homologous series are present in these liquids. The second 2-series type distributions also highlight some subtle structural difference between the shale oil and the tar sand liquid. Shale oil is more pa-
420 Energy & Fuels, Vol. 3, No. 3, 1989 Table VIII. Distribution of (a) Alkanes, (b) Cycloalkanes and Olefins, (c) Aromatics, and (a) Polars in Three Feedstocks w t % of feedstock cycloalkanes feedstock alkanes and olefins aromatics DO^ 34.1 26.2 Colorado shale oil 4.5 34.6 25.6 31.0 tar sand bitumen tar 3.3 40.0 Pittsburgh No.8 coal tar 1.0 4.6 30.5 63.8
raffinic than the tar sand liquid and has a higher concentration of monocyclic compounds. But, multiring saturates are more prominent in the tar sand liquid than in the shale oil. FIMS data and separation results can actually be combined to map out relative distribution of polars, aromatics, cycloalkanes and olefins, and alkanes, which may provide guidelines in the selection and upgrading of feedstocks to satisfy the requirements of HDF. The concentration of polars were obtained by combining the weight percentages of fractions 2-6 of the SESC separation and the residue, and that of aromatics and saturates and olefins were calculated from the weight percentage of fraction 1 and the results of the HPLC separation. Then FIMS data were utilized to subdivide the saturates and olefins fraction into alkanes and olefins + cycloalkanes. In this subdivision, data were not corrected for sensitivity differences. Results are given in Table VIII.
Summary and Conclusions Liquid feedstocks derived from pyrolysis of coal, oil shales, tar sands, and petroleum heavy resid were analyzed by using different analytical techniques to identify their compositional differences, which in turn can be used to identify procedures for upgrading these fuels. High-performance liquid chromatography (HPLC) (in particular, the chromatographic profile of aromatics) has provided an insight into the compositional differences between feedstocks. Field-ionization mass spectral data and separation results were used to map out classtype distribution in three feedstocks. Such a distribution is useful in the selection and upgrading of HDF feedstocks. FIMS data in 2 formats were used to identify cycloalkanes and their distribution in different feedstocks. The following conclusions have been drawn from this study: The Asphalt Ridge tar sand liquid and the Colorado shale oil are relatively low in polar compounds, which perhaps can be removed by using column absorptive techniques. The remaining oils, as such or with mild upgrading, should be a suitable feedstocks for HDF. In fact, adsorptive techniques appear to be more appropriate for the denitrification of these two fuels than catalytic hydrodenitrification (HDN). HDN converts pyridines to alkanes and possibly quinolines to cyclohexane and thus may reduce the relative concentration of naphthenic car-
Khan et al.
bons. Coal liquids and eastern shale oils, on the other hand, are relatively more aromatic and rich in phenols. A base wash or adsorptive technique to remove phenols followed by catalytic hydrogeneration is required to meet the requirements of HDF feedstock. The naphthenic content of the liquids can be ranked in the following order: Asphalt Ridge tar sand > western shale > Pittsburgh coal > eastern shale. However, the aromaticities of the liquids follow the order Pittsburgh No. 8 coal > eastern shale oil > western shale oil > tar sand liquid. Because hydrogenation of aromatics is expected to yield large amounts of cycloalkanes, the above distribution will be markedly altered when the liquids are upgraded. Average molecular parameters derived from proton NMR spectra of total oil suggest that aromatic structures in the tar sand liquid have long alkyl substituents compared to those in the coal pyrolysis liquid. Separation as well as NMR results suggests that eastern shale oil is intermediate in chemical composition to the Colorado shale oil and Pittsburgh No. 8 coal liquid. Therefore, any technique used to upgrade coal liquids should be suitable to refine eastern shale oils. Alkanes and cycloalkanes in the Colorado shale oil and the Asphalt Ridge tar sand liquid are about one-third of the total liquid and are structurally similar. Polar compounds constitute about 20% of the liquid and can be removed by using a large-scale chromatographic separation, provided the economics are favorable. The remaining portion, containing saturates, alkenes, neutral aromatics, and thiophenes and furans in minor amounts, upon mild hydrodesulfurization and hydrogenation should satisfy the requirements of HDF. Mass spectra data for the Green River shale oil and Asphalt Ridge tar sand liquid and for corresponding bitumens suggest that they contain similar or the same biomarkers, implying that these biomarkers possess appropriate chemical stability to survive throughout terrestrial diagneses and pyrolysis at elevated temperatures. Coal liquids, shale oil, and tar sand liquid contain similar higher cycloalkanes, such as tetracyclic and pentacyclic triterpanes, which appear to be common to many biosynthetic organisms.
Acknowledgment. Funding for this work was provided by the US. Department of Energy, Assistant Secretary for Fossil Energy, Office of Coal Utilization, Advanced Conversion and Gasification. We thank Dr. Knudsen of University of North Dakota Energy and Mineral Resource Institute for the NMR data, Dr. R. Malhotra of SRI for the FIMS data, Dr. Grace Wang of EG&G for the IR data, and Dr. H. Meuzelaar of University of Utah for the resin sample.
