Energy & Fuels 1994,8, 1360-1369
1360
Characterization of Successive Extract Fractions Released from a Sample of Coal during Liquefaction in a FlowingSolvent Reactor Bin Xu, E. Sebnem Madrali, Fan Wu, Chun-Zhu Li, Alan A. Herod, and Rafael Kandiyoti* Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, Prince Consort Road, London, SW7 2BY, U.K Received May 24, 1994@
Product fractions released from coal during successive temperature and time intervals have been recovered in a flowing-solvent liquefaction reactor. These experiments have been carried out using tetralin, quinoline, and hexadecane as vehicles; the selection was based on the H-donor ability of tetralin, the high solvent power of the essentially nondonor solvent quinoline and the absence of both solvent power and donor ability of hexadecane. The successive extract fractions have been characterized using size exclusion and planar chromatography, FT-IR and Wfluorescence spectroscopy. Monitoring changes in structure between successive product fractions has required the use of analytical methods which allow the discrimination of effects due to products from the reactions of the liquefaction solvent (vehicle): size exclusion chromatography (SEC), the tandem use of SEC with UV-fluorescence spectroscopy (W-F)and planar chromatography have proved useful in this respect. Successive extract fractions released from Point of Ayr coal in tetralin and quinoline showed progressively increasing molecular mass distributions with increasing intensity of reaction conditions (temperature and time). Parallel increases have been observed in the densities of large aromatic fused-ring systems during the W - F spectroscopy of SEC-elution-time-resolved cuts from individual extract fractions. Planar chromatography has also served to establish an order of polarity between the set of successive extract fractions as a function of increasing reaction intensity. While our findings suggest therefore that fractions released from coal during earlier stages of the liquefaction process appear more amenable t o further processing, quantitative differences in the ease of hydrocracking of these fractions remain to be demonstrated.
Introduction When heated to between 420 and 450 “C in appropriate donor solvents, high conversions of most low-tomiddle rank bituminous coals can be achieved within several hundred ~ e c o n d s . l - ~Coal extracts include, however, significant fractions of large molecular mass species, much of it polar structures containing sulfur, nitrogen, and oxygen. Reduction in molecular mass and of heteroatom content of these materials normally requires severe reaction conditions, with attendant high costs for producing saleable fuels, and chemical feedstocks relative t o petroleum-derived products. Problems raised by the refractory nature of coal extracts has led to questions about whether products released from coal particles during earlier stages of the liquefaction process may be more amenable to upgrading than the whole e x t r a ~ t .At ~ bench-scale, it is possible to attempt such short duration experiments in microbomb reactors. Clearly, however, in this reactor
* Author to whom correspondence should be addressed. E-mail:
[email protected]. Abstract published in Advance ACS Abstracts, September 1,1994. (1)Gibbins, J. R. Kandiyoti, R. Fuel 1991 70, 909-915. (2) Gibbins, J. R.; Kimber, G.; Gaines, A. F.; Kandiyoti, R. Fuel 1991, 70, 380-385. (3)Gibbins, J. R.; Kandiyoti, R. Reu. Sei. Instrum. 1991,62(9), 2234-2242. (4) Moroni, E. C. Prepr. Pup.-Am. Chem. Soc., Diu.Fuel Chem. 1991,36(2), 433. @
0887-0624/94/2508-1360$04.50/0
configuration, only the cumulative extract mixture released between the beginning of the experiment and the endpoint can be recovered: it is not possible to recover extract fractions released between specific time or temperature intervals. Furthermore, the lengths of heat-up and cool-down periods in these reactors (order of minutes) inevitably introduces uncertainties in the time resolution of extracts released from coal and allows time for extraparticle secondaryreactions to take place.2 A “short time reaction system” has recently been described, with the reaction mixture being driven through a reactor at well-defined time intervals, although, at the time of writing, detailed coal conversion data were not a ~ a i l a b l e . ~ The “flowing-solvent”liquefaction reactor developed in this l a b ~ r a t o r y -consists ~ of a direct electrically heated tubular reactor where a fixed-bed of coal is continuously swept with solvent during heat-up and the holding period at peak temperature. This reactor configuration enables the rapid removal, quench, and recovery of extract fractions with relatively short (order of 6-10 s) residence times of released products in the reaction zone. One immediate consequence of the rapid removal of products from the reaction zone is the relative suppression of extraparticle secondary reactions of primary products. Comparingresults with those from (5)Huang, H.; Calkins, W. H.; Klein, M. T. Prepr. Pup.-Am. Chem. SOC.,Diu.Fuel Chem. 1993,38(3),1080.
0 1994 American Chemical Society
Characterization of Successive Extract Fractions
Energy & Fuels, Vol. 8, No. 6,1994 1361
Table 1. Elemental Analyses of Point of Ayr (UK)Coal and Its Vitrinite Concentratea
0 samples
PoA whole coal
Cb
ash
H b N b Sb (by dim (drybasis)
84.5 5.4 1.8 1.5 vitriniteconcentrate 84.8 5.0 1.9 2.3
6.1 5.7
9.6 2.3
a Point of Ayr coal (84% vitrinites,6% liptinites, 10%inertinites) softens on heating but does not coke. Dry ash-free.
a small bomb reactor, we have found that in the presence of tetralin, sample weight loss values appeared comparable, but molecular masses of extracts prepared in the "flowing-solvent" reactor were considerably larger than those obtained in the microbomb reactor.2 Furthermore, the design allows the separate recovery and characterization of successive product fractions released from coal during the heatup and holding period^.^^^ Previous work undertaken in this reactor has helped to underline the refractory nature of primary liquefaction e x t r a ~ t s . ~Size , ~ ,exclusion ~ chromatography (SEC) derived molecular mass distributions OP have shown peaks in the 800-1300 u range, with traces of high molecular mass (MM) materials extending to between 6000 and 10000 u. These MMs were based on a calibration developed using vapor pressure osmometry to determine MMs of a range of coal-derived product fractions prepared by using preparative scale SEC.g-ll Results from several mass spectroscopic techniques tend to confirm the ranges of molecular masses identified in coal liquefaction extracts by SEC. Experiments using laser-desorption mass spectroscopy (LD-MS) have indicated the presence of material clustered between 1000 and 3000 u with traces at up to 12 000 u.12J3 Recently, matrix assisted laser-desorption ionization (MALDI) based experiments have indicated the existence of a major peak in the 1000-6000 u range with traces of materials extending to much higher values.14J5 The present paper describes experiments where successive product fractions released from coal during liquefaction in the flowing-solvent reactor have been recovered and characterized separately. The liquefaction experiments have been carried out using tetralin, quinoline and hexadecane as vehicles; this selection was based on the H-donor ability of tetralin, the high solvent power of the essentially nondonor solvent quinoline and the absence of both solvent power and donor ability of hexadecane. Structural features of the extract fractions (6) Gaines, A. F.; Li, C-Z; Bartle, K. D.; Madrali, E. S.; Kandiyoti, R. Proc. Int. Conf. Coal Sci. Newcastle-upon-Tyne, U.K., Sept. 1991, 1991,830-833. ( 7 )Li, C-Z.; Madrali, E. S.; Wu, F.; Xu,B.; Cai, H-Y.; Giiell, A. J.; Kandiyoti, R. Fuel 1994,73, 851-865. (8)Li, C-Z.; Gaines, A. F.; Kandiyoti, R. Proc. Int. Conf. Coal Sci. Newcastle-upon-Tyne, U.K., Sept. 1991, 1991,508-511. ( 9 )Bartle, K.D.; Taylor, N.; Mulligan, M. J.; Mills, D.; Gibson, C. Fuel, 1983,62, 1181. (10)Bartle, K. D.; Mills, D. G.; Mulligan, J. M.; Amaechina, I. 0.; Taylor, N. Anal. Chem. 1986,58, 2404. (11)Bartle, K. D.; Mulligan,M. J.;Taylor, N.; Martin, T. G.; Snape, C.E. Fuel 1984,63,1556. (12) John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Herod, A. A,; Gaines, A. F.; Li, C-2.; Kandiyoti, R. Rapid Commum. Mass Spectrom. 1991,5,364-367. (13)Herod, A. A.; Kandiyoti, R.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Li, C-Z. Chem. Commun. 1993,No.9,767-769. (14)Herod, A. A.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Li, C-Z.; Kandiyoti, R. Proc. 7th Int. Conf Coal Sci. 1993,Sept. 12-17, 282-285. (15) John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Herod, A. A.; Li, C-Z.; Kandiyoti, R. Rapid Commun. in Mass Spectrom. 1993, 7,795-799.
r'
w
PRESSUREGALICE
SOLVEKT WWIVOP
Figure 1. Schematic diagram of the flowing-solventreactor liquefaction system.
I
550
-
F 5
450 350
0
k
P
250
I50 50
0 0
50
100
200
300
400
500
600
Reaction time (scc.)
Figure 2. Temperature intervals corresponding to time-
resolved products fractions. were compared using size exclusion and planar chromatography, FT-IR and W-fluorescence spectroscopy.
Experimental Section Samples. Table 1presents elemental analysis of the sample of Point of Ayr (UK) coal and that of its vitrinite concentrate. The coal sample was ground and sieved to separate the 106-150 pm fraction in a nitrogen-filled glovebox. The vitrinite concentrate was prepared by British Gas plc.16 Samples were dried in a vacuum oven at 35 "C for 16 h and stored under nitrogen in a freezer until required. Preparation of Liquefaction Extract Fractions. Details of the flowing-solvent reactor system (Figure 1) have been presented el~ewhere.l9~ The reactor consists of a direct electrically heated tubular vessel; the temperatures of the preheater (lower) and reaction (upper) sections are driven and controlled independently. A fixed bed composed of about 200 mg of coal mixed with acid-washed sand is positioned in the upper (reactor) part by means of wire-mesh stoppers. The continuous stream of solvent is preheated to the reactor-section temperature in the lower section and flows through the fixed-bed of sample. The temperature is measured by two thermocouples, placed above and below the sample bed, and their average is used as the control signal: Figure 2 presents a typical time-temperature trace. In this study, unless otherwise stated, samples were heated at 5 K s-l t o the peak temperature and held for 400 s. Reactor solvent flow rate was maintained at 0.9 mL s-l at a pressure of 70 bar. The residence time of (16)White, A.; Davis, M. R.; Jones, S. D. Fuel 1989,68, 511.
1362 Energy & Fuels, Vol. 8, No. 6, 1994 Table 2. Temperature Intervals Corresponding to Time-Resolved Product Fractionsa fraction no.
time interval (5)
1 2 3 4 5 6 7 8
0-70 70-80 80-90 90-100 100-140 140-190 190-490 490-
temp interval ("C) ambient-350 350-400 400-450 450 450 450 450 450-ambient
a Samples were heated a t 5 K s-l to 450 "C and held for 400 s interval; a tetralin flow of 0.9 mL ssl was maintained a t 70 bar.
products in the reaction zone was within 6-10 s, following release from parent coal particles. Two different product collection procedures have been used: (i) cumulative collection of the reaction mixture (vehicle plus extracts) exiting from the reactor during startup, the heatup stage and during the holding period a t peak temperature (usually 400 SI; (ii) separate collection of successive product fractions released from coal during successive time-temperature intervals (Table 2 and Figure 21, while carrying out experiments to a peak temperature of 450 "C with 400 s holding. After a run, the solid residues of liquefaction left in the reactor were washed with (4:lv/v) chloroform-methanol solution, and any extract removed was added to the reaction mixture. Characterizations by SEC (and SEC coupled to WF) have been carried out using the total reaction mixture as recovered from the reactor, i.e., vehicle plus extracts. The usual product preparation procedure involved concentration to a slurry in a rotary vacuum evaporator; the slurry was washed out of the vessel with a 4:l(v/v) chloroform-methanol solution and prepared for planar chromatography; samples submitted to IR and solidstate 13CNMR were hard-dried to remove the maximum possible amount of solvent until no further sample weight loss could be observed: products were transferred to a watch glass and successively dried at 80 "C (1day), 100 "C (4days), 125 "C (3 days), and 138 "C (4 days) under vacuum. Little coal-derived material could be detected in the boiling range (up to 450 "C), near the top of which tetralin-derived dimers and adducts have been identified. Similar procedures applied to coal extracts prepared in a microbomb reactor would probably have given a different result: products prepared in the flowing solvent have been shown to have larger molecular masses and appear to have undergone little extraparticle cracking of primary extracts.2 Apart small amounts barely undetectable by GC-MS, little coal derived material appears to be removed by the hard drying procedure. In experiments where successive product fractions released from the coal were collected separately, excess liquefaction solvent was distilled off from the reaction mixture (between 10 and 100 mL) in smaller stills (20 or 100 mL). Tetralin was removed under vacuum at room temperature and quinoline under vacuum at 40 "C. Hexadecane remaining in the samples could not be removed in this way. The pentane-insoluble fractions of hexadecane extracts were used for characterization by IR. Size Exclusion Chromatography. Retention time distributions of liquefaction extract fractions were determined by size exclusion chromatography (SEC) using polystyrene/poly(divinylbenzene)packed columns (PLgel; 3 ,um mixed-E; Polymer Laboratories Ltd.). The
Xu et al. column temperature was maintained at 45 "C. An eluent flow rate of 1 mL s-l has been used throughout the set of experiments; retention times and retention volumes therefore have similar numerical values. A W-absorption cell and an evaporative analyzer were used in tandem as mass detectors; no corrections were applied to detector responses. The calibration of the SEC column was based on polystyrene standards. Before injection into the column, extract fractions were dissolved in the elution solvent, unstabilized tetrahydrofuran. FT-IR Spectroscopy. The coal sample was dried under vacuum at 35 "C for 15 h, ground to less than 45 pm; coal- or hard-dried (cumulative) extract samples were mixed with dried (105 "C for 15 h) KBr at a constant samp1e:KBr ratio of 1:300;13 mm diameter pellets were pressed in an evacuated die at 9 tons and dried for 48 h a t 35 "C under vacuum. Repeatabilities of the spectra were within 4%. In order to mount timetemperature resolved extract fractions on KBr disks, about 3 mL concentrated sample solution (in 4:l v/v ch1oroform:methanol) was injected by microsyringe onto a dried and weighed pellet; solvent was continuously evaporated from the pellet under preheated (32-35 "C) oxygen-free nitrogen. Sample-laden pellets were dried in vacuum at 35,40, or 60 "C (for hexadecane, tetralinor quinoline-derived samples, respectively) for 15 h and weighed to determine sample loading. In order to obtain a homogeneous mixture of sample and KBr, loaded and dried pellets were crushed, mixed, repressed and dried for 48 h under vacuum a t 35 "C. FT-IR spectra were acquired with a Perkin-Elmer Model 1760x FT infrared spectrometer using a TGS detector. Good quality spectra were recorded by co-adding 128 scans at 2 cm-l resolution. Spectra were normalized to the equivalent of 1 mg (dry basis) sample; the KBr spectrum was subtracted to give the final spectrum. W-Fluorescence Spectroscopy. Procedures for acquiring on-line W-fluorescence spectra of SEC-retention-time-resolved fractions have been given elsewhere.17J8 Briefly, the eluent from the SEC column first passed through the W-absorption detector and then through the HPLC flow cell accessory of a PerkinElmer LS 50 luminescence spectrometer. During SEC experiments, the spectrometer was set to scan at 1500 nm min-l. A 270 nm wide on-line spectrum can be recorded within 0.18 min; changes in solute concentration and properties during this interval could, in most cases, be neglected. The spectrometer features automatic correction for changes in source intensity as a function of wavelength. Emission spectra reported here were not further corrected for other factors, e.g., changes in emission photomultiplier's response as a function of wavelength. Planar Chromatography. Several methods were used. A set of fractions from each extraction solvent were compared on one thin layer analytical plate (20 x 20 cm) coated with silica gel, with multiple solvent development, successively using tetrahydrofuran, (4:1 v/v) chlorofordmethanol, toluene, and pentane. Before use, plates were washed with tetrahydrofuran and chlorofordmethanol, giving a band of yellow material along the upper edge of the plates. Developed plates (17)Li, C.-Z. Ph.D. Thesis, University of London, UK, 1993. (18)Li, C.-Z.; Wu, F.; Cai, H.-Y.; Kandiyoti, R. Energy Fuels submitted for publication.
Characterization of Successive Extract Fractions
Energy &Fuels, Vol. 8, No. 6, 1994 1363
Table 3. Conversions (Weight Loss) from the Liquefaction of Point of Ayr Coal in the Flowing-Solvent Reactor, and the Characterization of the Liquefaction Extractsa weight loss (%, w/w, daf)
samples
PoA whole coal vitrinite concentrate whole coal extracts 300 "C 350 "C 400 "C 450 "C
17 25 47 83
aromaticity (13C NMR)
WC
0.73
0.77 0.71
0.76 0.77 0.74 0.74
0.76 0.77 0.76 0.80
The extracts were obtained in the flowing solvent reactor by heating a t heating rate of 5 K s-l to the target temperatures and hold for 400 s Reactor pressure: 70 bar(g); nominal solvent (tetralin) flow rate: 0.9 mL s-l.
were examined in visible and UV light (254and 366 nm) for measurement of retention factors relative to the pentane solvent front. Fractions were also examined using HPTLC plates (5 x 5 cm), using single solvents as a preliminary exercise; results were in agreement with those from multiple solvent development. Because the extract samples in tetralin and quinolinelphenanthrene left a substantial spot at the origin on the 20 x 20 cm plates after development, additional experiments were performed using 5 x 5 cm plates with a series of solvents more powerful than tetrahydrofuran to investigate the mobility of the residual material. Solvents used in this stage were pyridine, nitromethane, ethylene glycol dimethyl ether, NJV-dimethylformamide, and 1-methyl 2-pyrrolidinone. Fraction 6 of the tetralin extracts and fraction 6 of the quinolinelphenanthrene extracts were used in these experiments, since these fractions showed a high proportion of immobile material a t the origin when eluting with tetrahydrofuran, and were likely to be more polar and/or of high molar mass. The 20 x 20 cm analytical TLC plates (Whatman International Ltd.) were coated with silica gel K6 (1012.5 pm particle size, 60 A pore size); the silica gel coating of the 5 x 5 cm HPTLC plates was 4.5 pm particle size with 60 pore size. Solvents. Properties and sources of the solvents used were as follows. Tetralin: Aldrich 99+% purity, bp 207 "C, sg 0.973; hexadecane: Aldrich 99% purity, bp 287 "C, sg 0.773; quinoline: Aldrich 98% purity, bp 237.1 "C, sg 1.093; methanol: BDH (HPLC-grade) 99.8% purity, bp 64.6 "C, sg 0.791; chloroform: BDH (HPLCgrade) 99.8% purity, bp 60.5 "C, sg 1.492; pentane: BDH (AnalaR grade) 99.0% purity, bp 36.1 "C, sg 0.626; toluene: BDH (AnalaR grade) 99.5% purity, bp 110.6 "C, sg 0.867; tetrahydrofuran: BDH (unstabilized) 99.7% purity, bp 67 "C, sg 0.886; pyridine: Sigma Chemicals (HPLC grade), 99.9% purity, bp 115 "C, sg 0.978; nitromethane: Sigma Chemicals (HPLC grade) 96+% purity, bp 85 "C, sg 0.867; ethylene glycol dimethyl ether: Sigma Chemicals (HPLC grade) 99.9% purity, bp 101 "C, sg 1.127; Nfl-dimethylformamide: Sigma Chemicals (HPLC grade) 99.9% purity, bp 150 "C, sg 0.944; 1-methyl-2-pyrrolidinone: Sigma Chemicals (HPLC grade) 99+% purity, bp 81.5 "C, sg 1.033.
A
Results and Discussion Table 3 presents conversions (weight loss) from the liquefaction of Point of Ayr coal in the flowing-solvent reactor, together with the 13C-NMR derived carbon aromaticities and atomic WC ratios of the cumulatively
Table 4. Comparison of Sample Weight Loss from the Liquefaction of the Point of Ayr Whole Coal and Its Vitrinite Concentratea weight loss heating rate (K s-l)
holding time (s)
medium
PoA vitrinite 5 5 5 5 PoA whole coal
400 400 400 400
hexadecane &/p" quinoline tetralin
12.5 38.0 28.ad
27.3 (2Ib 73.8 (2) 72.7 (2) 77.6 (2)
400 400 400
tetralin quinoline hexadecane
24.6 39.5 -
82.5 (4) 74.7 (2) 24.0 (1)
5 5 5
(% w/w daf basis) 350 "C 450 "C!
Experiments were performed using a solvent flow rate of 0.9 mL s-l at 70 bar(g). Number of repeated runs used for calculating the average value. Q/p: quinolinelphenanthrene (2.5:l w/w) mixture. Hold time: 500 s. The weight loss from 100 s experiments under the same conditions was 29.2%,within experimental error.
recovered extracts. Samples were heated in tetraiin to peak temperatures of 300,350,400 and 450 "C with 400 s holding. While conversions were observed to increase considerably between 300 and 450 "C, differences in atomic WC ratios and solid-state 13C-NMR derived carbon aromaticities appear within or close t o experimental error. This apparent similarity in structures and compositions of extracts may be explained, at least in part, in terms of the cumulative nature of the product collection procedure: in practice, cumulative products collected at any indicated temperature would contain all product found in extracts collected at the lower experimental temperature, in addition to extracts released during the final temperature interval. Possible differences between the structures of extract fractions successively released from the coal, if any, would therefore be masked by this experimental procedure. As explained above, the problem can be eliminated in the flowing-solvent reactor by separately collecting successive product fractions released from the coal as a function of time and temperature, during a single run. Results from experiments carried out in this way, using three different solvents, will be described below. Another factor contributing to the apparent similarity of the extracts is thought to be the presence of tetralin dimers and related adducts; problems relating to the presence of these materials will be dealt with in more detail below. Sample Weight Loss in Tetralin, Quinoline, and Hexadecane. Table 4 presents sample weight loss (conversion) data from the liquefaction of Point of Ayr coal and its vitrinite concentrate in tetralin, in a mixture of phenanthrene and quinoline, in quinoline and in hexadecane. At 350 "C, where covalent bond scission is not thought to dominate the dissolution process, the greater solvent power of quinoline (and of the quinoline-phenanthrene mixture) for coal-derived materials was reflected in the greater sample weight loss compared to liquefaction in tetralin. When the peak experimental temperature was raised to 450 "C,however, somewhat greater conversions were obtained in tetralin than in quinoline (and in quinolinelphenanthrene), presumably due to the H-donor ability of tetralin. Differences in sample weight loss between liquefaction in tetralin and quinoline (and in quinolinelphenanthrene) nevertheless appear surprisingly small. In
Xu et al.
1364 Energy & Fuels, Vol. 8, No. 6, 1994 terms of hydrogen-transfer mechanism^,^^^^^ available hydrogen during extraction with quinoline would have been hydrogen originating from the coal mass itself-in addition t o minor amounts contributed from possible quinoline dimerization. However, a high concentration of free radicals in quinoline is a likely outcome: in the simultaneous absence of high solvent power and of H-donor ability, extractions in hexadecane gave results closer to those obtained duringpyrolysis in a wire-mesh r e a ~ t o r .[Heating ~ in helium at 1000 K s-l t o 350 "C with 150 s holding, the weight loss from Point of Ayr vitrinite concentrate was 3.3% (% wfw daf basis); at 450 "C, sample weight loss was 20.5%.1 Results presented in Table 4 may therefore be understood in terms of the coal (or vitrinite concentrate) sample depolymerizing quite substantially by heating to 450 "C. The outcome of the experiment then appears to depend on whether a large fraction of the depolymerized material can be moved out of the reaction zone by dissolving in a powerful solvent. In the absence of an H-donor solvent, the high levels of dilution (100-150 mg of coal extract in 600-1000 mL of solvent) and the short residence times of products in the heated zone of the reactor appear to limit (but not necessarily eliminate) recombination reactions leading to larger MMs and char formation. Supporting evidence for this statement was found in two very different types of experiments: 1. At 450 "C, the liquefaction of Point of Ayr coal in a microbomb reactor, using 1-methylnaphthalene as vehicle, gave increasing char yields for holding times above 100 s, while sample weight loss in the flowingsolvent reactor increased with increasing hold time.2 2. The high mass parts of MM distributions of hexadecane and tetralin liquefaction products (both prepared in the flowing-solvent reactor) appeared quite similar prior to concentrating the respective reaction mixtures; however, dried hexadecane extract redissolved in tetrahydrofuran showed a much larger increase in large-MM material compared to tetralin liquefaction products treated in the same manner (Figure 3). In the case of liquefaction in hexadecane, the occurrence of massive repolymerization was confirmed by the negligible amounts of extra material extracted from the solid residues, upon refluxing in a 4:l mixture of chloroform and methanol. FT-IR spectra of hexadecane extracts showed greater aliphatidhydroaromatic content than tetralin or quinoline extracts (see below), suggesting a degree of preferential extraction. Characterization of the Extract Fractions. In tetralin, it was estimated that about 30% of the products were released from coal during heatup to 450 "C (i.e., in combined fractions 1, 2 and 3; Table 2); about 4% of the products were estimated to be released from coal during the first 10 s of the holding period at 450 "C (fraction 41, about 10% during the 10-50 s interval of the holding period (fraction 5),15%during the 50-100 s interval (fraction 61, and finally about 24%during the 100-400 s interval of the holding period (fraction 7). These values are approximate and have been calculated as follows. Samples were heated at 5 K ssl to 450 "C and held for hold times of 76, 220, 280, 400, and 1100 s; the weight loss vs hold-time curve was then extrapo~~
~
~
(19) McMillen, D. F.; Malhotra, R.; Hum, G. P.; Chang, S-J.Energy Fuels 1987, 1 , 193. (20)McMillen, D. F.; Malhotra, R.; Nigenda, S. E. Fuel 1989, 68, 380.
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