Energy &Fuels 1995,9, 269-276
269
Characterization of Tars from the Pyrolysis of a Coal Liquefaction Extract Fraction E. Sebnem Madrali, Fan Wu, Bin Xu, Alan A. Herod, and Rafael Kandiyoti" Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, London SW7 2BY, U.K. Received April 19, 1994. Revised Manuscript Received November 17, 1994@
Mechanisms dominating pyrolytic processes in coal were investigated by pyrolyzing a wellcharacterized liquefaction extract; this is likely to contain the metaplastic tar-precursor phase formed during pyrolysis of most coal mass soluble in liquefaction. Experiments were carried out in a wire-mesh pyrolysis reactor at two heating rates (1 and 1000 K s-l) a t 500 and 700 "C. Molecular mass distributions of the extract and of the tars produced from it were compared by size exclusion chromatography (SEC) with W and evaporative analyzers in series. Structural comparisons were carried out by planar chromatography and FT-IR spectroscopy. Our findings may be summarized as follows: (i) The original (Point of Ayr) coal sample was pyrolyzed for comparison. Tar yields from the dried extract were considerably greater than those from the coal: tetralin-derived products in the dried extract contributed to this result. (ii) A significant shift to smaller molecular masses was observed when the dried extract was pyrolyzed. (iii)The fraction of lighter material in the pyrolyzing substrate with MMs comparable t o those of evolved tars was sufficient to make up the whole of the tar product; we cannot, however, distinguish between direct distillation of light components and cracking of large MM material as the origin of material ending up in the tar. (iv) The presence of tetralin derivatives in the dried extract and its tars indicates that the distillation of light components from the extract is possible. (v) Comparison of product distributions from untreated coal and dried extract, obtained during slow (1 K s-l) and rapid (1000 K s-l) pyrolysis, provides support for the view that recombination reactions may be responsible for the loss of tar during slow heating compared t o fast heating. (vi) The tars were more aliphatic and contained fewer aromatic substituents than the extract itself. Tar release and capture took place under conditions minimizing extraparticle secondary reactions and this work pinpoints the progress of particular reactions taking place in the metaplast prior t o tar release. (vii) Planar chromatography showed the dried extract and its tars to consist of strongly polar mixtures.
Introduction Of the mass of middle rank bituminous coals, 7085%can be solubilized within several hundred seconds when heated to 420-450 "C in a strong solvent like quinoline or a donor-solvent like tetralin (e.g., see ref 1). Molecular mass (MM) distributions of these liquefaction extracts are significantly broader and extend to greater masses than pyrolysis tars prepared from the same coals.2 It may be expected that a large proportion of the coal mass solubilizing during liquefaction would be present in the metaplastic tar-precursor phase which forms during a dry pyrolysis run. Evidence for this comes from experiments involving the addition of pyridine extracts from rapidly cooled pyrolysis chars and evolved volatiles to coal, where, a t its most fluid state, as much as 75%of Pittsburgh seam bituminous coal was shown to consist of volatiles plus extrach3v4 Comparison of structural parameters between pyrolysis tars and liquefaction extracts is likely to provide useful insights into mechanisms dominating pyrolytic processes beAbstract published in Advance ACS Abstracts, January l, 1995. (1)Li, C-Z.;Madrali, E.S.;Wu, F.; Xu, B.;Cai, H-Y.; Giiell, A. J.; Kandiyoti, R. Fuel 1994,73, 851-865. (2)Li, C-Z.;Gaines, A. F.;Kandiyoti, R. Proc. Znt. Conf: Coal Sci., Newcnstle-upon-Tyne, U.K.1991,508-511. (3) Fong, W. S.; Khalil, Y. F.; Peters, W. A.;Howard, J. B.Fuel 1986, 65,195. (4)Fong, W.S.; Peters, W. A.; Howard, J. B.Fuel 1986,65, 251. @
cause of the partial correspondence between liquefaction extractable material and metaplastic tar precursors. Suuberg and c o - ~ o r k e rreported s ~ ~ ~ that tetrahydrofuran (THF) extracts of rapidly-quenched pyrolysis chars gave systematically broader MM distributions than those of the corresponding pyrolysis tars; in these experiments, the MM distribution of the THF extract would depend on the extent and speed of quenching of pyrolyzing particles. In one case, a dried tar sample was pyrolyzed: the evolved tar showed a much reduced MM distribution compared to the substrate which had probably lost most lighter components and may have undergone extensive radical recombination reactions during the drying process. Their conclusion was that, at lower temperatures, tar evolution was dominated by evaporation after diffusion through a stagnant film around the particle whereas at higher temperatures5 the process was reaction rate controlled. Physical entrainment from particle surfaces by rapidly evolving volatiles was also considered likely,6particularly in the case of larger tar molecules. It is possible t o examine aspects of mass transfer of volatiles through external stagnant films by increasing the velocity of gas flowing over pyrolyzing coal particles. However, this type of investigation would require an ~~~
~~
~
(5) Unger, P. E.; Suuberg, E. M. Fuel 1984 , 63, 606.
(6)Suuberg, E.M.; Unger, P. E.; Lilly, W.D. Fuel 1986,64, 956.
0 1995 American Chemical Society
Madrali et al.
270 Energy & Fuels, Vol. 9, No. 2, 1995
experimental design where tar yields would not be altered by other factors such as extraparticle secondary reactions. Many common pyrolysis reactor configurations do not allow quantitative capture of all tar released from pyrolyzing coal particles. For example, we have previously reported on changes in intensities of tar destruction reactions within the heated zones of fixed ("hot-rod")and fluidized-bed reactors, as a function of reactor temperature and volatile residence times in the heated This type of work clearly shows that increasing gas velocities over sample coal particles tends to cause increases in tar yields. However, only under very restricted reaction conditions has it been possible in a fixed or fluidized bed reactor to approach tar yields from a wire-mesh r e a ~ t o r ,where ~ a well-dispersed monolayer of coal is pyrolyzed within a shallow heated zone. Taken together, the observed improvements in yields with increasing carrier gas flow rates in fixed or fluidised beds could be explained in terms of improved tar survival resulting from reduction of tar residence times in the reaction zone. Nevertheless, on its own, the evidence from these experiments did not preclude the existence of film diffusion resistances around coal particles, particularly at low velocities. Wire-mesh cells used in the char-extraction studies cited above did not have facilities for forcing a flow of gas over pyrolyzing coal particles. Work performed in this laboratory using an atmospheric pressure wiremesh reactor with helium flows of up to 0.3 m s-l through the sample holder showed no experimentally significant variation in yields over the flow rate range, so long as a sufficient flow was maintained to prevent recirculation of tars in the vicinity of the heated mesh,1° and drive volatiles away from the reaction zone.11J2Gas flow rates up to 0.11 m s-l at 70 bar have been used in a recently designed high-pressure wire-mesh apparatus,13J4giving a mass flow rate many times greater than those of the atmospheric pressure experiments. As in the case of atmospheric pressure experiments, no experimentally significant changes could be observed in tar o r total volatile yields above a low velocity threshold (with Reynolds numbers around 100-too low to destroy stagnant films) just sufficient to prevent tar recirculation and remove volatiles from the reaction zone. It is unlikely that the use of high end temperatures (say 700 "C or above) would, in itself, mask the effect of a stagnant film at lower temperatures, since even in rapid heating experiments, over 80-90% of total tar release appears t o be achieved at temperatures up to 550-600 "C, i.e., during heatup and a t relatively low temperatures,ll this is probably why in the work of Suuberg et a1.5,6very little change was observed in MM distributions of tars produced at nominally higher temperatures. Existing high heating rate data (1000 K s-l) do not therefore confirm the control of tar evolution by diffusion through a stagnant film. It would, in any case, be difficult to envisage the survival of a stagnant film (7) Stiles, H. N.; Kandiyoti, R. Fuel 1989, 68, 275. (8) Gonenc, Z. S.; Fowler, T. G.; Kandiyoti, R.; Bartle, K. D. Fuel 1988, 67, 848. (9) Gonenc, 2. S.; Gibbins, J . R.; Katheklakis, I. E.; Kandiyoti, R. Fuel 1990, 69, 383. (10) Howard, J. B. Chemistry of Coal Utilization, Second Supplementary Volume; Elliott, M. A., Ed.; Wiley: New York, 1981; pp 665. (11)Gibbins-Matham,J. R.; Kandiyoti, R. Energy Fuels 1988,2,505. (12) Gibbins, J. R. Ph.D. Thesis, University of London, 1988. (13) Guell, A. J.; Kandiyoti, R. Energy Fuels 1993, 7, 943. (14) Guell, A. J . Ph.D. Thesis, University of London, 1993.
around particles pyrolyzing at high heating rate, particularly during the major volatile release phase (say between 450 and 600 "C), while anywhere up t o 4045% of the coal particle mass is released as volatiles within 100-150 ms. In the present study, mechanisms dominating pyrolytic processes in coal were investigated by pyrolyzing samples from a well-characterized liquefaction extract fraction. Experiments were carried out in an atmospheric pressure wire-mesh pyrolysis reactor at 500 and 700 "C and a t two heating rates (1and 1000 K s-l). Tar release was allowed t o proceed to completion in the present set of experiments, in contrast to earlier work where the effect of holding at peak temperature was minimized by rapid quenching. The liquefaction extract was selected as being close to a real coal sample, which could still be characterized by solution state methods (SEC and planar chromatography). Experiments were also conducted on the original Point of Ayr coal sample under similar reaction conditions, in order to compare yields and tar structures with those from the extract. Clearly, less information is readily available on molecular masses present in pyrolyzing coal particles. Recent coal molecular masses reported using matrix-assisted laser desorption mass spe~trometryl~ show the major part of detected molecules between 1000 and 6000 u, with traces at up to 270 000 u. Due to fine sample dispersion and rapid removal and quenching of evolved volatiles, the present wire-mesh reactor configuration allows the capture and recovery of tars in relative freedom from extraparticle secondary reactions. However, the small sample sizes (order of 2-5 mg) and even smaller quantities of tar product recovered (1mg or less) restrict the choice of analytical techniques for comparing structures of original samples and product tars to those requiring small amounts of sample. In work reported below some structural differences have been inferred by comparing responses from the two SEC detectors used in tandem (UV and evaporative analyzer [EA]).16,17Comparisons were also based on planar chromatography and the FT-IR spectra of the pyrolyzed substrate and product tars.
Experimental Section The liquefaction and pyrolysis reactors and experimental procedures used for preparing the coal extract and the tar samples have been described elsewhere; brief descriptions are given below. Preparation of the Liquefaction Extract Fraction. The liquefaction reactor consisted of a direct electrically heated tubular vessel.18-21 The fmed bed, composed of a mixture of about 200 mg of sample coal (particle size 106-150 pm) mixed with 3 g of acid-washed sand, was positioned in the upper part of the reactor by means of wire-mesh stoppers. A stream of solvent (normally tetralin) was forced out of the reservoir by high-pressure nitrogen and continuously fed to the reactor tube; the solvent crossed the lower "preheater" section and (15) John, P.; Johnson, C. A. F.; Parker, J . E.; Smith, G. P.; Herod, A. A,; Li, C-Z.; Kandiyoti, R. Rapid Comm. Mass Spectrom. 1993, 7, 795. (16) Li, C-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 3. (17) Li, C-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 1459. (18)Gibbins, J. R.; Kimber, G.; Gaines, A. F.; Kandiyoti, R. Fuel 1991, 70, 380. (19) Gibbins, J. R.; Kandiyoti, R. Fuel Process. Technol. 1990, 24, 237. (20) Gibbins, J . R.; Kandiyoti, R. Fuel 1991, 70, 909. (21) Gibbins, J. R.; Kandiyoti, R. Reu. Sci. Instrum. 1991, 62 (91, 2234.
Energy & Fuels, Vol. 9, No. 2, 1995 271
Characterization of Tars Table 1. Elemental Analysis of Point of Ayr Coal and Its Liquefaction Extracr C H N S+O (daf%) (daf%) (daf%) (bydim ash WC 8.06 9.6 0.72 5.13 1.71 Point ofAyrcoal 85.1 11.84 - 0.82 5.54 1.22 450°C extractb 81.4 a Liquefaction experiment conditions: 5 K s-l, 450 "C,400 s hold time at peak temperature. The extract was assumed ashfree.
Table 2. Wire-Mesh Pyrolysis Results of Point of Ayr Tetralin Liquefaction Extract and Original Point of Ayr Coal" heating rate (Ks-l)
tar yield (daf %)
total volatile yield (daf %)
500
1000
700
1000
38.8 40.5 38.3 39.2 (3Ib 50.9 50.3 50.6 (2) 35.8 34.9 35.4 (2)
45.2 46.8 51.W 46.0 (2) 59.3 56.2 57.8 (2) 55.6 55.3 55.5 (2)
temp ("C) Point of Ayr extract
swept through the fixed bed of sample in the upper "reactor" section. During the present set of experiments, the sample 700 1 was heated at 5 K s-l t o 450 "C with 400 s holding a t the peak experimental temperature. Reactor pressure was normally maintained at 70 bar. At the standard solvent flow rate of Point of Ayr coal 0.9 mL s-l, products are swept out of the heated zone with 700 1000 28.4 43.8 residence times varying between 6 and 10 s; the stream is then 26.1 44.1 quickly quenched. The solid residue of liquefaction was 27.4 (2) 44.0 (2) washed with about 100 mL of (4:l v/v) chlorofodmethanol 700 1 18.2 37.5 17.2 37.6 solution and residual extract material added to products in 17.7 (2) 37.6 (2) solution. Conversions (sample weight loss) were determined with a reproducibility better than f 2 % . Solvent was distilled Holding time is 30 s in all pyrolysis experiments. Data in off the product mixture in a rotary vacuum evaporator to near bold characters indicate the average value. Numbers inside dryness; products were transferred to a watch glass and parentheses indicate the number of experiments included in the average. Data not included in the average. successively dried a t 80 "C (1 day), 100 "C (4 days), 125 "C (3 days), and 138 "C (4 days) under vacuum. Some alteration in structure may be expected from the severity of the drying Polymer Laboratories Ltd.). A UV-absorption detector and an procedure; this was thought t o be acceptable since the aim of evaporative analyzer mass detector were used in tandem the study was to compare differences in structure between the during MM distribution determinations; no corrections were pyrolyzing substrate and product tars. Table 1 presents applied to detector responses. A calibration of the SEC column elemental and petrographic analyses of the Point of Ayr (UK was based on polystyrene standards has been shown on middle-rank bituminous) coal sample, and of the dried extract diagrams showing the SEC traces to facilitate identification fraction prepared using the procedure described above. of ranges of molecular masses. These standards overestimate Preparation of Pyrolysis Tars. Pyrolysis tars were MMs of coal-derived materials but are convenient due to their prepared in an atmospheric pressure wire-mesh r e a ~ t o r ~ ~ J ~wide , ~ ~availability ,~~ which facilitates interlaboratory comparison. consisting of a stainless steel mesh sample holder stretched Before injection into the column, the tar or extract in chloroform/ between two electrodes, the mesh also serving as the resistance methanol solution was concentrated to near-dryness by blowheater. A preset time-temperature ramp (in this study varied ing with oxygen-freenitrogen; concentrated samples were then between 1 and 1000 K s-l) was applied to the sample holder, redissolved in unstabilized tetrahydrofuran, the elution solvent followed by 30 s holding a t peak temperature. During operain SEC experiments. tion a t atmospheric pressure, a stream of gas (normally helium FT-IR Spectroscopy. The coal or "dried" extract sample at 0.1 m s-l) is used for sweeping volatiles from the reaction was kept under vacuum at 35 "C for 15 h, ground to less than zone into a liquid N2 cooled tar trap. Total volatile and tar 45 pm, and mixed with dried KBr (itself dried a t 105 "C for yields (Table 2) were determined from the weight loss of the 15 h) a t a constant coa1:KBr ratio of 1:300. Pellets of 13 mm sample holder and the weight uptake of the tar trap, respecdiameter were pressed in an evacuated die a t 9 tons and dried tively. Following a recent redesign16,the traps allow quantitafor 48 h a t 35 "C under vacuum. Repeatability of the spectra tive recovery of captured tars for subsequent characterization, was within 4%. In order to mount a tar sample, about 3 mL by washing with a 4:l v/v mixture of chloroform and methanol. of concentrated sample solution (in 4 : l v/v chloroform/ As dried extract melts extensively during high heating rate methanol) was injected by micro-syringe onto a previously experiments, smaller quantities of sample than normal were dried and weighed KBr pellet. Solvent was continuously used to prevent loss of sample from the mesh; if large amounts evaporated from the pellet under a stream of preheated (32of sample are used, sample drips from the mesh. Sample sizes 35 "C) oxygen-free nitrogen. Sample-laden pellets were dried were 2-3 mg for dried extract and around 5 mg for coal. under vacuum at 35 "C for 15 h and weighed to determine The design of the wire-mesh reactor used here minimizes sample loading. Loaded and weighed pellets were homogthe exposure of the pyrolysis tars to extraparticle secondary enized by crushing, mixing, and repressing; pellets were then reactions compared with tars prepared in previous studies; dried for 48 h under vacuum a t 35 "C. FT-IR spectra were where comparison with past work is possible, information acquired with a Perkin-Elmer Model 1760X FT infrared regarding structural features and changes in structural feaspectrometer using a TGS detector. Spectra were recorded by tures of the present set of tars is expected to be more accurate. co-adding 128 scans at 2 cm-l resolution, followed by normalSize-ExclusionChromatography. Molecular mass (MM) ization t o 1 mg (dry basis) sample equivalent; the KBr distributions of tars and extracts were determined by size spectrum was subtracted to give the final spectrum. exclusion c h r o m a t ~ g r a p h y ~(SEC) ~ - ~ ~using polystyrene/polyRelatively abbreviated procedures were used for evaluating (divinylbenzene) packed columns (PL-gel, 3 pm mixed-E; changes in aromaticity within the present set of samples. In Table 3, the &/Hm ratio is defined as the aliphatic CH (22) Gibbins, J. R.; King, R. A. V.; Wood, R. J.; Kandiyoti, R. Rev. stretching band intensity peak height (peak maximum at 2920 Sci. Instrum. 1989, 60, 1129. cm-l) divided by the aromatic CH stretching band peak height (23) Gibbins, J. R.; Kandiyoti, R. Fuel 1989, 68, 895. (peak maximum a t 3050 cm-l); these values are considerably (24) Bartle, K. D.; Taylor, N.; Mulligan, M. J.;Mills, D.; Gibson, C. Fuel 1983, 62, 1181. different from the ratio of areas under the curves. In order t o (25) Bartle, K. D.; Mills, D. G.; Mulligan, J. M.; Amaechina, I. 0.; calculate fa from FT-IR spectra, carbon aromaticities of a set Taylor, N. Anal. Chem. 1986, 58, 2404. of coals were calculated from C P - W I T O S S 13CNMR spectra; (26) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E.Fuel 1984, 63, 1556. these values were plotted against the HdH,, ratio as defined
272 Energy & Fuels, Vol. 9, No. 2, 1995
Madrali et al.
Table 3. Infrared-Derived€€&Iar Ratio and Carbon Aromaticity Values for Point of Ayr Coal, Its Tars, the Extract Fraction, and the Extract Tars, and CP-MAS TOSS I3C NMR Derived fa Values, Available for the Point of Ayr Coal and the Liquefaction Extra& FT-IR sample Point of Ayr coal Point of Ayr extract (450 "C, 5 K s-l, 400 s ) extract tar (500 "C, 1000 K s-l, 30 s ) extract tar (700 "C, 1000 K s-l, 30 s ) extract tar (700 "C, 1 K s-l, 30 s ) coal tar (700 "C, 1000 K s-l, 30 s ) coal tar (700 "C, 1 K s-l, 30 s )
HadHar
13c fa
8.7 7.6
0.71 0.72
11.4 9.8 10.1 14.2 21.9
0.63 0.66 0.64 0.57 0.39
NMR fa
0.73 0.74
Table 4. Planar Chromatography Derived Retention (RO Values of Tars from Extract and coala High-Performance Plate solvent chloroform/ tetrapentane toluene methanol hydrofuran
sample extract (5 K s-l, 450 "C, extract tar (1000 K s - ] , 500 "C, 30 s) extract tar (1000 K s - ] , 700 "C, 30 s ) coal tar (1000 K s-l, 700 "C, 30 s ) 20
0.93-0
0.88
0.88
0
0
0.92
0.89
0
0
0.92
0.89
0
1.0-0
0.92
0.89-0
20 cm Thin Layer Chromatography Plate Rf values extractb ( 5 K s-l, 450 "C, 400 s ) 0.00-0.46-0.83b 0.35-0.28 extract tar (1000 K s-l, 500 "C, 30 s ) 0.36-0.28 extract tar(1000 K s-l, 700 "C, 30 s ) 0.71-0.35-0.27 coal tar (1000 K s-l, 700 "C, 30 s ) pentane 1.0, 1 . 0 b 0.75, 0.83b toluene ch1oroform:methanol 0.37, 0.47b 0.15, 0.32b tetrahydrofuran
H,iIHa, and fa have been defined in the Experimental Section.
above. This plot was used as a calibration curve for estimating carbon aromaticities of samples for which (due to small amounts available) only IR spectra could be obtained. and fa data calculated in this manner (cf. Table 3) are semiquantitative and can strictly be used for internal comparison only within the present set of samples. Reasonable agreement was obtained, however, between FT-IR derived aromaticities and those from CP-MAS TOSS 13CNMR, in the two cases where sufficient sample was available to perform 13C NMR spectroscopy. Planar Chromatography: High performance 5 x 5 cm thin layer plates from Whatman International Ltd. (coated with 4.5 pm diameter, 60 porosity silica gel particles) were spotted with sample and developed using either pentane, toluene, chlorofodmethanol (4:1 v/v), or tetrahydrofuran. Two 20 x 20 cm TL plates coated with silica gel of similar porosity and 10-12 pm particle size were also used; these plates were washed with THF before applying sample. The 20 x 20 cm plates were first developed in THF (the strongest solvent) to about 30 mm; the plates were removed and dried. They were then placed in (4:l) chlorofordmethanol; the solvent front was developed to approximately 70 mm, before removal and drying. The third solvent was toluene and the development was to about 140 mm. After drying, the plates were developed in pentane to nearly 190 mm from the sample spots. Plates were examined in daylight and W light to detect fluorescence; surfaces were marked t o indicate both visible sample and fluorescence. Results are presented as retention (Rf)values (Table 41, the ratio of distance travelled by the spot from the origin to the distance travelled by the solvent front-pentane in the case of the 20 x 20 plates. Boiling Points. Evaporation points determined in a TGA apparatus swept by inert gas are lower than boiling points because sweep gas reduces the partial pressure of evaporating material to virtually zero in the immediate vicinity of the sample holder. A calibration curve may be obtained between TGA-derived evaporation points of pure compounds and boiling points of these compounds, permitting the estimation of boiling point ranges. Solvents. Tetralin: Aldrich 99+% purity, bp 207 "C, sp gr 0.973. Methanol: BDH (HPLC-grade) 99.8% purity, bp 64.6 "C, sp gr 0.791. Chloroform: BDH (HPLC-grade) 99.8% purity, bp 60.5 "C, sp gr 1.492. Tetrahydrofuran: BDH (HPLC, unstabilized), 99.7% purity. Toluene: BDH (AnalaR), 99.8% purity n-Pentane: BDH (AnalaR), 99.0%.
0
400 s)
x
a Extract and pyrolysis tars were obtained from Point of Ayr coal. Determined from separate experiment.
Results and Discussion
of Ayr coal. Considering 1000 K s-l results first, the tar yield from the extract fraction a t 700 "C was considerably greater than that from the untreated coal itself. The dried extract represents the heavy fraction of the organic matter in the coal sample (sample weight loss in this case was 82%); this high tar yield is not surprising, considering that the coal was liquefied in massive excess of the H-donor solvent, tetralin. Under these reaction conditions, most covalent bond scission reactions would be followed by quenching of free radicals by available hydrogen. The 50.6% tar yield from the extract corresponds to 41.5% of the original coal mass (wlw daf basis). A second factor possibly contributing to high tar yields is the presence of products from the thermal reactions of tetralin. Products from the Thermal Reactions of Tetralin. For present purposes, these products may be considered in two groups. (i) Naphthalene, 1-methylindane, a- and P-hydroxytetralin, a- and P-methyltetralin, and butylbenzene; the formation of these compounds at temperatures above 350 "C is w e l l - k n ~ w n . ~ ' These - ~ ~ materials have relatively low boiling points and would be expected to either distill off during the preparation of the dried extract (i.e. removal of tetralin from the extract mixture), or be driven off during the subsequent vacuum drying procedure (see Experimental Section). (ii) The second group of tetralin-derived products consists of dimers and related adducts of tetralin and of tetralin derivatives; many of these species with boiling points less than 450 "C have been identified by GC-MS.31 Among tetralin-derived dimers and related compounds, the formation of binaphthyls, chrysene, and
Tar Yields. In the wire-mesh reactor, negligible tar evolution is normally observed at atmospheric pressure at temperatures above 700 "C. This temperature was selected as the highest to be used in the present set of experiments. Table 2 presents product distributions from the pyrolysis of dried extract and untreated Point
(27) Benjamin, B. M.; Hagaman, E. W.; Raaen, V. F.; Collins, C. J. Fuel 1979,58, 386. (28) Curran, G. P.; Struck, R. T.; Gorin, E. Ind. Eng. Chem. Proc. Des. Deu. 1967,6,167. (29) Hooper, R. J.;Battaerd, H. A. J.;Evans, D. G. Fuel 1979,58, "" 13L. (30) McPherson, W. P.; Foster, N. R.; Hastings, D. W.; Kalman, J. R.; Gilbert, T. D. Fuel 1985,64, 457.
a
1
Characterization of Tars
Energy & Fuels, Vol. 9, No. 2, 1995 273
Oa4
g
g
0.3
.d
3 0
0
$
0.3
.d
3
L d
0.2
&
0.2
0.1
.Y 0
0.1
4
5
0.0
0.0
Retention time, min
Retention time, min
I
I
35075
856
72
35075
d
0.4
I
2
0.4
0
72
Estimated molecular mass, Da
Estimated molecular mass, Da 0.5 I
856
*+
I
I
0.3
3
.3
4
2
0.3
&
L
rcI
0.2
0.0
0.0
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Retention time, min
35075
I
856
1
72
Estimated molecular mass, Da Figure 1. SEC (a,top) UV-absorptiondetector and (b, bottom) evaporative analyzer derived retention time distributions of the liquefaction extract and pyrolysis tars prepared from this sample: curve 1,dried-liquefaction extract, 5 K s-l, 450 "C, 400 s; curve 2, extract tar, 1000 K s-l, 500 "C, 30 s; curve 3, extract tar, 1000 K s-l, 700 "C, 30 s; curve 4,extract tar, 1K s-l, 700 "C, 30 s. Polystyrene standard MM scale placed
underneath the graphs. tetralin-1-methylindane have previously been n0ted;27*30 more recently, a wide array of tetralin-derived products, principally of MM 258 and 262 u, apparently tetralintetralin dimers and tetralin-naphthalene adducts and their derivatives have been identified by GC-MS in tetralin heated and passed through the fixed bed of sand during blank runs and in mixtures of coal liquefaction products extracted in t e t ~ a l i n . ~ l These a~ larger molecular mass derivatives are less likely to be removed by the extract preparation procedure. Clearly, the occurrence of still higher-MM tetralin-derived products cannot be ruled out, but since they would not elute from chromatographic columns, they could not be observed by GC-MS. The presence of tetralin-derived dimers and adducts can be inferred in the SEC traces of the dried liquefaction extract and its tars, in peaks around 19.5 min (Figures 1and 2); these peaks are more pronounced in EA-derived retention time distributions (Figures l b (31) Brodzki, D.;Djega-Mariadassou, G.;Li, C-Z.; Kandiyoti, R. Fuel
1994, 73, 789-794.
(32) Brodzki, D.; Abou-Akar, A.; Djega-Mariadassou, G.; Li, C-Z.;
Xu,B.;Kandiyoti, R. Fuel 1994, 73, 1331-1337.
8
12
10
14
16
18
20
22
24
Retention time, min 35075
856
72
Estimated molecular mass, D a Figure 2. SEC (a, top) LW-absorptiondetector and (b, bottom) evaporative analyzer derived retention time distributions of the liquefaction extract and pyrolysis tars prepared from liquefaction extract and original coal: curve 1, liquefaction extract, 5 K s-l, 450 "C, 400 s; curve 2, coal tar, 1000 K s-l, 700 "C, 30 s; curve 3, coal tar, 1 K s-l, 700 "C, 30 s; curve 4, extract tar, 1 K SKI, 700 "C, 30 s. Polystyrene standard MM scale placed underneath the graphs.
and 2b) and do not appear at all in the coal pyrolysis tars. As the aim of the present set of experiments was the comparison of structures in a pyrolyzing substrate and its tar, the presence of these materials was not seen as playing a role detrimental to the aims of the investigation. Considering next slow heating rate (1K s-l) experiments at 700 "C, the trend shown by the extract fraction was different from that observed in coal pyrolysis, where any loss in tar yield due to slower heating rates is usually matched (within several percent) by a drop in the total volatile yield. In coal pyrolysis, this parallel drop in volatile matter and tar yield during slow heating, compared to rapid heating, has been associated with increased repolymerization and charring of pyrolyzing tar precursors, with marginal extra gas format i ~ n By . ~ contrast, ~ the present slow heating of dried extract was found to give a sharp drop (about 15%)in tar yield but only a small decline in total volatiles. The morphology of dried-extract chars recovered after the experiments clearly show that as in the case of some Heinrich Robert (Germany)],the coals [e.g. Linby (UK),
274 Energy & Fuels, Vol. 9, No. 2, 1995
extract became fluid, almost runny, under rapid heating conditions but deformed only slightly during slow heat. absence of any pore structure, the ing (1K s ~ ~In) the increased surface area exposed by the flow of extract (mainly along the wires of the mesh) during rapid heating appears t o have facilitated tar release, compared to slow heating when particles retained their original morphology. The absence of a significant decline in total volatile yield during slow heating suggests that within the driedextract sample, radical recombination reactions do not intensify in the same way as during coal pyrolysis carried out at a slow heating rate, where they eventually produce more char. This is consistent with substantial quenching of free radicals in the original coal by the donor solvent and of radicals formed during the liquefaction process. The longer residence times at higher temperatures due to slower heating appear nevertheless t o have caused extensive cracking of tar precursors to lighter species. No ESR derived free-radical concentration data were available for the pyrolyzing extracts; however, the extent of quenching of free radicals in coal extracts prepared in tetralin could be inferred from the following experiments: while MM distributions of products extracted in tetralin have been observed to shift to somewhat larger values upon drying and immediate (apparently total) redissolution, the observed shift of MMs to higher values was found to be much larger in the case of a coal extract prepared in the nondonor (and weak) solvent hexadecane, where many more free radicals survive during liquefaction, presumably due to the vast excess of solvent. Molecular Mass Distributions. Figure 1presents SEC retention time distributions of the dried extract and of its pyrolysis tars. A large shift to smaller values was observed between the MM distributions of the dried extract and its tars. Integration of weight fractions of the dried-extract SEC trace gave 89% of the total sample eluting after 14 min, 65% after 15.8 min, and 55% after 16.4 min. The proportion of material in the dried extract with MMs comparable to those found in the tar was, therefore, sufficient to make up the whole of the tar produced during these experiments. However, the pyrolytic process involves the simultaneous evolution of tar and gas and formation of residual char. These complex competing reactions taking place during pyrolysis obscure the distinction between direct distillation of relatively light components of the extract and the cracking of larger MM components, as the origin of material ending up in the pyrolysis tar. The presence of tetralin-derived dimers and related adducts in the dried extract has helped to pinpoint that the direct distillation of some light components of the pyrolyzing substrate is possible: the peaks observed around 19.5 min appear in the SEC traces of both the dried extract and its tars but not in the coal tar. Comparison of SEC traces of the three tar samples (curves 2-4 in Figure 1) shows that with increasing pyrolysis temperature, the MM distributions of tars may shift in opposite directions according to the relative predominance of competing effects: (i) progressive cracking of tar precursors prior to escape (which increases with temperature) and (ii) release from the pyrolyzing substrate of larger MM components with increasing temperature.
Madrali et al. Comparing the rapid heating (1000 K s-l) 500 and 700 "C tar SEC traces, a small shift toward higher MM values was observed with increasing temperature. This trend differs from previous data on solid coals and macerals, showing small shifts of MM distributions to smaller values with increasing pyrolysis temperature at high heating rates. The difference appears related to the thermally more sensitive nature of the coal samples and the apparent prior loss of aliphatics from the dried extract during sample preparation. The difficulties of evaporation from the almost rigid pyrolyzing extract particles during slower heating experiments (1 K s-l; curve 41, however, appears t o have shifted tars to smaller molecular masses, in addition to producing a large gas yield as described above. Figure 2a shows little difference between molecular mass distributions of coal tars produced during fast and slow heating from the predominantly vitrinitic coal, unlike tars from the pyrolysis of dried extract; vitrinite tars have already been found16 to be considerably less sensitive to intraparticle cracking than those of liptinites. Significant differences were found, however, between the evaporative analyzer derived SEC traces of the coal tars (Figure 2b) and tars from dried extract (Figure lb). The high MM (short retention time 9-14 min) component in lines 2 and 3 of Figure 2b compared to the UV-absorption trace of the same tars suggests the presence of apparently high-MM aliphatidhydroaromatic material. It must be noted, however, that at similar MMs, aliphatics have shorter retention times (and therefore appear t o be of greater MM) than hydroaromatics, which themselves travel through the column more rapidly than aromatic compounds of similar MM.24-26 The presence of higher concentrations of aliphatic materials in coal tars compared to extract tars observed in the SEC's of Figure 2b was confirmed by the FT-IR spectra of these tars (see below and Table 3). Findings from the analyses of straight-chain aliphatic compounds in timekemperature resolved liquefaction extract fractions33suggest that straight-chain aliphatics are released from coal between 340 and 390 "C, probably by breaking off from larger molecules: n-alkanes up t o C35 were identified in a liptinite concentrate extract. Alkanes were likely to be present in the reaction mixture exiting from the liquefaction reactor in this work; they may have been removed during the extract preparation and drying procedure. Clearly, the extensive cracking of straight-chain aliphatics reported by Calkins and c o - ~ o r k e r does s ~ ~ not appear relevant in the present context: most tar cracking observed in that study is likely to have resulted from extraparticle secondary reactions taking place during the much longer residence times of volatiles within the heated fluidizedbed and the reactor freeboard (see ref 7). IR Characterization of the Samples: Figure 3 presents FT-IR spectra of the set of samples. (a) Original Coal and Dried Extract. Comparison of the coal spectrum with that of its dried liquefaction extract (spectra 1and 2, respectively) shows increased aromatic and aliphatic C-H stretching band intensities. Within the error of the determination, however, the HdI Ha, ratio was found to remain constant (Table 3). The (33) Brodzki, D.; Abou-Akar, A,; DjBga-Mariadassou, G.; Kandiyoti, R. Fuel, in press. (34) Calkins, W. H.; Tyler, R. J. Fuel 1984,63, 1119.
Energy & Fuels, Vol. 9, No. 2, 1995 275
Characterization of Tars _-
I
I
1
CM"
Figure 3. Infrared spectra of original coal, its liquefaction extract, and extract tars: spectrum 1, original coal (Point of Ayr coal); spectrum 2, liquefaction extract, 5 K s-l, 450 "C, 400 s; spectrum 3, extract tar, 1000 K s-l, 500 "C,30 s; spectrum 4,extract tar, 1000 K s-l, 700 "C, 30 s; spectrum 5, extract tar, 1K s-l, 700 "C,30 s; spectrum 6, coal tar, 1000 K s-l, 700 "C,30 s; spectrum 7, coal tar, 1K s-l, 700 "C, 30 s.
spectra also indicated similarities between the intensities of the 860 and 810 cm-l bands (corresponding to single hydrogens and twolthree adjacent hydrogens on aromatic rings,35respectively) in the dried extract and the original coal; the 750 cm-' bands, however, showed considerable differences, as will be discussed below. As expected, oxygen-functional group concentrations (phenolic-OH, carbonyl and carboxylic groups) were observed to increase during the liquefaction process. This is in line with earlier work36-38indicating that near the softening temperature of coal, the number of phenolicOH, carbonyl, and carboxyl groups increases slightly: the increase has been ascribed to the cleavage of arylalkyl ethers, esters, and oxylates (ROOCCOOR) and conversion of heterocyclic groups. Preliminary indications from ongoing work in this laboratory suggest the presence of oxidation products derived from tetralin dimers and related compounds. We are at present investigating oxygen uptake by tetralin-derived products contained in the extracts. Briefly, these materials cannot be removed (evaporated) by the drying procedure: some of these materials have been found in the GC-MS molecular mass range. For purposes of the present study it was sufficient to characterize the extract before pyrolysis and to compare its characteristics with those of its pyrolysis tars. The 0 S in this study has been determined by difference.
+
(35) Painter, P. C.; Snyder, R. W.; Starsinic, M.; Coleman, M. M.; Kuehn, D. W.; Davis, A. Appl. Spectrosc. 1981,35 (5), 475. (36) Siskin, M.;Aczel, T. Proc. Coal Sci. Dusseldorf 1981, 651. (37) Mrazikova, J.; Sindler, S.; Veverka, L.; Macak, J. Fuel 1986, 65,342. (38)Whitehurst, D. D.; Mitchell, T. 0.; Farcasiu, M. Coal Liquefaction; Academic Press: New York, 1980; pp 152.
The presence of tetralin-derived dimers and related adducts appears to introduce a number of features into spectra of the dried extract and its tars, compared to the original coal. In order to clarify analysis of spectra presented in Figure 3, tetralin was heated, in the absence of coal, under conditions otherwise similar to those used for the preparation of the coal extract (5 K s-l to 450 "C with 400 s holding). On the basis of the IR spectrum of heated tetralin (not shown), tetralin dimers and related adducts are likely to have contributed, albeit to a limited extent, to the spectra of the extract and its tars (spectra 2-5 in Figure 3) in the following bands: (i) 3100-3000 cm-l, aromatic C-H stretching bands; (ii) 3000-2750 cm-l aliphatic C-H stretching bands (in the case of heated tetralin, this band would be due to hydroaromatic structures); (iii) near 1500 cm-', attributable t o structures containing single aromatic rings, and (iv) 750 cm-l band indicating the presence of four adjacent hydrogens on the aromatic ring. Based on its TGA-derived weight loss curve, the driedextract sample does not appear t o contain any tetralin; 13%of the sample was found to have a normal boiling point below 450 "C (corresponding to 246 "C in the TGA apparatus). As tetralin dimers and associated adducts identified by GC-MS were found to have boiling points below 450 "C, 13% constitutes the upper limit of the concentration of these materials in the dried extract. The coal and the dried-extract sample thus appear to have similar HdlHa, ratios with similar patterns of aromatic substitution; as outlined above, the similarities appear at least in part related to the presence of tetralin-derived materials in the extracts. Oxygen functional group (probably mainly phenolic-OH and carbonyl groups) concentrations were found to increase, possibly through cleavage of aryl-alkyl and aryl-aryl ether bonds during l i q u e f a c t i ~ n .The ~ ~previously ~~~~~~ reported greater concentration of aliphatickydroaromatic structures in liquefaction products compared to the original coal1 is not observed, probably due to loss of some of these materials through the rigours of the dried-extract preparation procedure. (b) Dried Extract and Its Tars. Comparison of spectrum 2 with spectra 3-5 in Figure 3 shows a more pronounced diphatichydroaromatic presence in the tars compared to the dried extract. This finding parallels observations made on products from the pyrolysis of coal1 (also see below);it is possible to visualize the more aromatic and more intractable parts of the pyrolyzing substrate remaining behind, forming the bulk of the char residue. IR-derived carbon aromaticities of the three tars (determined using a correlation between NMR-derived f a and IR-derived HallHa, for a rank ordered set of appear similar within the error of the determination. As expected from observations on coal pyrolysis, comparison of the 900-700 cm-l regions in spectra 2 and 3-5 suggests the presence of fewer aromatic substituents in extract tars compared to the dried extract itself. Likely mechanisms for the decrease of substituent groups include thermal cleavage of these groups and the aromatization of hydroaromatic struc(39) Youtcheff, J.; Painter, P.; Given, P. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1986,31 (l), 318. (40) Hodek, W.; Kirschstein,J.; van Heek, K. H. Fuel 1991,70,425. (41) Madrali, E. S. Ph.D. Thesis, University of London, 1994.
276 Energy & Fuels, Vol. 9, No. 2, 1995
tures during p y r ~ l y s i s . ~Smaller ~ - ~ ~ concentrations of phenolic and carbonyl groups were observed in the pyrolysis tars compared to the dried extract. This finding is in contrast with observations made on coaltar pairs. It is likely that etheric bridge scission reactions may have run to completion during the liquefaction stage, thus not leading to the formation of new phenolic, etc. groups upon pyrolysis of the driedextract. All four bands associated with tetralin dimers and associated adducts may be observed in the tar spectra, as well as in the spectrum of the dried extract (as in SEC results), suggesting a degree of distillation of lighter fractions during pyrolysis. Being totally absent in the untreated coal and its tar, the 1500 cm-l band is typical in this respect. (c) Untreated Coal and Its Tars. As expected, spectra of coal pyrolysis tars showed si&icantly more aliphatic/ hydroaromatic structure and fewer aromatic substituents than the original coal; the phenolic-OH concentration was greater in the tars compared to the original coal (Figure 3; curves 1 and 6, 7). In agreement with SEC-derived findings outlined above, the coal tar spectra showed both samples (particularly the 1 K s-l sample) to contain significantly more aliphatichydroaromatic structure than all other samples in the study. IR-derived carbon aromaticities and HalIHa, ratios allow semiquantitation of these observations (Table 3). Previous work in this laboratory16J7 has already indicated aliphatic components to be thermally more sensitive to intraparticle cracking reactions. Rapid heating pyrolysis experiments at temperatures up to 700 "C and above appear more likely to crack aliphatic components, probably by intraparticle or particlesurface reactions. The greater aliphatic presence in the slow heating rate tars (apparently signaling release of these more aliphatic tars before high temperatures were reached) compared to tars prepared under rapid heating parallel these earlier findings. Planar Chromatography. On high-performance plates, all samples moved with the solvent front in both tetrahydrofuran and in the chlorofordmethanol mixture; the samples appear therefore to be fairly polar, but not extremely so-compounds such as polyphenols and acids would not have moved significantly in these solvents. There are subtle differences too in polarity which distinguish movement between chlorofodmethano1 and tetrahydrofuran, but these were not entirely clear. Relative retention distances for the high-performance plates are listed in Table 4. No movement at all was observed in pentane: the method would not in any case assist in identifying the presence of straightchain alkanes, which have no color and do not fluoresce in UV light. Both the extract and the coal tar (1000 K s-l) displayed some movement in toluene, but quantities of the material that moved were too small for SEC characterization of the solute recovered from platescrapings. Components likely to be mobile in toluene include tetralin adducts and dimers as well as polynuclear aromatic hydrocarbons. The 20 x 20 cm plate was successivelywashed in tetrahydrofuran, chloroform/ methanol, toluene, and pentane: findings from these (42) Chen, P.; Yang, P. W-J.; Griffths, P. R. Fuel 1986,64, 307.
(43)Painter, P. C.; Yamada, Y.; Jenkins, R. G.; Coleman, M. M.; Walker Jr., P. L. Fuel 1979,58,293. (44) Painter, P. C.; Coleman, M. M. Fuel 1979,58,301.
Madrali et at?. experiments were substantially similar to those from the high performance plates (Table 4).
Conclusions 1. Tar yields from the dried extract were considerably greater than those from untreated coal. Some contribution to high tar yields is likely to have come from products of the thermal reactions of tetralin. At the slower heating rate (1K s-l) the tar yield from the dried extract dropped sharply (about 15%)compared to fast heating (1000 K s-l); only a small decline was observed in total volatiles. The greater char yields normally encountered in coal pyrolysis experiments at slow heating rate were not found, possibly because there were more quenched free radicals in the extract; instead, gas formation was much larger than expected. 2. A large shift was observed from the molecular mass distribution of the dried extract t o the smaller molecular masses of the pyrolysis tars. However, direct distillation of relatively lighter components in the pyrolyzing substrate and cracking of larger MM material cannot be distinguished as the preferred origin of material ending up in the pyrolysis tar. 3. The presence of tetralin-derived dimers and related adducts in both the dried extract and its tars indicates that the removal by distillation of some of the lighter components in the pyrolyzing substrate is likely. Evidence for the presence of tetralin-derived materials comes from SEC peaks observed around 19.5 min, and FT-IR peaks at 1500 cm-', which appear in distributions of both the dried extract and its tars but not in the coal tar or the untreated coal itself. 4. Comparison of product distributions from untreated coal and dried extract, obtained during slow (1 K s-l) and rapid (1000 K s-l) pyrolysis, has provided a measure of support for the view that recombination reactions may be primarily responsible for the loss of tar product during slow heating compared to fast heating. 5. IR spectroscopy derived findings showed the presence of significantly more aliphatidhydroaromatic structures in dried-extract pyrolysis tars and coal pyrolysis tars compared to the original substrates; in addition, the presence of fewer aromatic substituents were observed in extract tars compared to the dried extract, probably due both to thermal cleavage of these groups and the aromatization of hydroaromatic structures. The coal-tar spectra also showed fewer aromatic substituents than the original coal; the presence of phenolic-OH was found to be greater in tars compared to the original coal, apparently due to cleavage of ether bonds in the pyrolyzing coal. These findings serve to pinpoint the progress of particular reactions taking place in the metaplast prior t o tar release. 6. Results from planar chromatography indicated that the dried extract and its tars as well as the coal tar are very polar materials; marginally small amounts of sample moved in toluene. Acknowledgment. The authors thank the European Union for supporting the work under Research Contracts No. JOUF.0050.C(TT) and ECSC 7220-EC/ 862. The authors express their appreciation to Dr. Chun-Zhu Li for helpful discussions and to S-F. Zhang for performing the TGA experiment. EF940059P
