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Energy & Fuels 1996, 10, 1083-1090
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Fate of Aromatic Ring Systems during Thermal Cracking of Tars in a Fluidized-Bed Reactor Chun-Zhu Li and Peter F. Nelson* Division of Coal and Energy Technology, Commonwealth Scientific and Industrial Research Organisation, P.O. Box 136, North Ryde, New South Wales 2113, Australia Received April 3, 1996. Revised Manuscript Received June 21, 1996X
A suite of brown and bituminous coals have been pyrolyzed in a fluidized-bed reactor between 400 and 1040 °C. The reactor featured a relatively long heated freeboard where the nascent volatiles were thermally cracked. Tars were characterized with size exclusion chromatography (SEC), and gases were analyzed with gas chromatography. The yield of dichloromethane-soluble tar was observed to reach a maximum at around 500 °C for the brown coals and at around 600 °C for the bituminous coals. SEC experiments, however, suggested that the release of aromatic tar components from the brown coals studied was not complete at temperatures lower than about 700 °C under the present experimental conditions. Tars from the pyrolysis of the bituminous coals showed higher thermal stability than those from the brown coals under the same conditions. The difference in the concentrations of larger (with at least three or more fused benzene rings) aromatic ring systems in coal substrates and tars is thought to be the main structural reason for the observed difference in pyrolytic behavior between the brown and bituminous coals. Larger aromatic ring systems in the coals and in the tars serve as cross-linking sites of higher coordination number than the smaller aromatic ring systems. During tar thermal cracking, larger aromatic ring systems in the bituminous coal tars (containing higher concentrations of larger aromatic ring systems) are more likely to disappear from the tars between 800 and 1040 °C than those in brown coal tars (containing lower concentrations of larger aromatic ring systems). One of the main fates of the aromatic ring systems lost from the tars during tar thermal cracking processes is the formation of soot. Concurrent with the release of simple (mainly C1 and C2) hydrocarbon gases, significant amounts of single-ring aromatic systems were also released from the more complex tar molecules as a consequence of the thermal cracking of the tar. In agreement with our previous studies, significant amounts of polymethylene groups are thought to be chemically connected to aromatic ring systems in low-rank coal and their low-temperature tars.
Introduction Aromatic and heteroaromatic ring systems constitute an important part of coal, in which they are interconnected together in the three-dimensional macromolecular networks.1-10 The extent of the interconnection, or cross-linkage, is often described with a parameter called the coordination number:6,9,11 the number of chemical attachments to a heteroaromatic or aromatic ring system. Macromolecular network concepts have been * Author to whom correspondence should be addressed (telephone +61 29 887 8660; fax +61 29 887 8909; e-mail P.Nelson@syd. dcet.csiro.au). X Abstract published in Advance ACS Abstracts, August 1, 1996. (1) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Science; Meyers, R. A., Ed.; Academic Press: New York, 1982. (2) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 1668. (3) Suuberg, E. M.; Unger, P. E.; Larsen, J. W. Energy Fuels 1987, 1, 305. (4) (a) Niksa, S.; Kerstein, A. R. Combust. Flame 1986, 66, 95. (b), Niksa, S. Combust. Flame 1986, 66, 111. (5) Solomon, P. R.; Hamblen, D. G.; Yu, Z.-Z.; Serio, M. A. Fuel 1990, 69, 754. (6) Solomon, P. R.; Hamblen, D. G.; Serio, M. A.; Yu, Z.-Z.; Charpenay, S. Fuel 1993, 72, 469. (7) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (8) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Solum, M. S.; Grant, D. M. Energy Fuels 1992, 6, 414. (9) Grant, D. M.; Pugmire, R. J.; Fletcher, T. H.; Kerstein, A. R. Energy Fuels 1989, 3, 175. (10) Niksa, S.; Kerstein, A. R. Fuel 1987, 66, 1389. (11) Solomon, P. R.; Fletcher, T. H.; Pugmire, R. J. Fuel 1993, 72, 587.
widely used in modeling studies of coal pyrolysis (e.g. see refs 5, 6, and 9-11). In these coal pyrolysis models, aromatic, heteroaromatic, and hydroaromatic ring systems were essentially assumed to be uniformly distributed in the coal macromolecular networks. The aromatic structure of coal was approximated by an average (or a distribution of) aromatic ring system(s) and a set of parameters, often based upon nuclear magnetic resonance (NMR) spectroscopic measurements (e.g. see ref 8). In these models, the depolymerization of the coal matrix has been assumed to be a random process and described via various statistical methods. In a recent study of the role of aromatic ring systems in the thermal decomposition of the macromolecular network during coal liquefaction, Li and co-workers12 have, however, pointed out that aromatic ring systems in coal are not distributed uniformly. Large aromatic ring systems serve as cross-linking points of high coordination number. Increased densities of larger aromatic ring systems near edges or external surfaces of potential molecular fragments would render the release of the fragments from coal more difficult. Primary (nascent) tars, the released molecular fragments from coal during pyrolysis, are very reactive. When the primary volatiles are further exposed to high temperature, further thermal breakdown and crosslinking (char/soot formation) reactions will take place, (12) Li, C.-Z.; Wu, F.; Xu, B.; Kandiyoti, R. Fuel 1995, 74, 37.
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Table 1. Properties of the Coals Studied Laubag Fortuna Gottelborn Ensdorf Prosper Yallourn ash, wt % (dry) volatile matter, wt %a C, wt %a H, wt %a N, wt %a S, wt %a O,b wt %a Cl, wt %a
6.5
Composition of Coals 12.9 7.1 7.6
8.3
1.1
54.4
43.2
39.2
37.9
34.8
53
67.4 5.03 0.71 0.86 26.0 n/dc
67.3 5.38 0.76 0.56 26.0 0.01
82.3 5.64 1.66 0.99 9.4 0.06
82.4 5.72 1.77 0.82 9.1 0.17
79.9 5.95 1.31 1.33 11.4 0.12
67.4 4.6 0.6 0.3 27.1 n/d
Main Metal Components in Ash on the Dry Basis of Coal Al, ppm 1960 1740 8150 8640 12000 1610 Ca, ppm 12600 14000 4260 5050 3090 770 Fe, ppm 13300 12100 6300 7120 5740 2390 Mg, ppm 2350 4980 1970 960 1200 1530 Na, ppm 50 810 650 220 540 510 Si, ppm 2060 30000 12600 14700 16200 50 a
daf. b By difference. c n/d, not determined.
leading to the observed decreases in tar yields at high temperatures.13-17 Previous studies18,19 have investigated the distribution of aromatic ring systems in tars from the pyrolysis of coals at high temperatures. These studies18-20 showed that the conversion of tar to soot was a major fate of the aromatic ring systems. However, little is known about the transition from the crosslinked aromatic ring systems in the tars produced at low temperatures to individual (either substituted or unsubstituted) aromatic ring systems in the tars produced at very high temperatures. In the present study, the fate of the aromatic ring systems during the thermal cracking of tar in a fluidized-bed coal pyrolysis reactor is investigated using size exclusion chromatography (SEC) and gas chromatography (GC). Experimental Section Pyrolysis of Coal. Proximate and ultimate (including the main inorganic elements) analyses of the coals used in the present study are given in Table 1. Coal samples of particle sizes between 75 and 106 µm were prepared from bulk coal samples, dried under vacuum for at least 24 h prior to pyrolysis experiments. Pyrolysis experiments were carried out in a fluidized-bed reactor, similar to that described by Tyler.13 Coal particles were injected directly into the fluidized bed of zircon sand (106-150 µm) and heated at a rate in excess of 104 K s-1 to high temperatures.13 The nitrogen gas used in pyrolysis experiments was of ultrahigh purity grade (99.999% minimum) and was purchased from CIG, Australia. All gas flows to the reactor were metered at room temperature. The reactor featured a relatively long (∼13 cm) freeboard heated to temperature levels similar to those of the fluidized-bed of sand within the same furnace. The residence time of volatiles in the heated freeboard was between 0.4 and 0.5 s when the reactor was operated between 800 and 1000 °C. The total (13) Tyler, R. J. Fuel 1979, 58, 680. (14) Tyler, R. J. Fuel 1980, 59, 218. (15) Stiles, H. N.; Kandiyoti, R. Fuel 1989, 68, 275. (16) Hayashi, J.-i.; Kawakami, T.; Taniguchi, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1993, 7, 57. (17) Hayashi, J.-i.; Amamoto, S.; Kusakabe, K.; Morooka, S. Energy Fuels 1995, 9, 290. (18) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1987, 1, 431. (19) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. 22nd Symposium (International) Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 135-143. (20) Nenniger, R. D.; Howard, J. B.; Sarofim, A. F. Proceedings of the International Conference on Coal Science: Pittsburgh Energy Technology Center: Pittsburgh, PA, 1983; pp 521-524.
residence time of coal/char particles in the reactor depended on whether they were retained in the bed by agglomeration with the sand particles or were elutriated directly out of the reactor, but in all cases the residence times of the particles were no shorter than that of the volatiles. Tar was collected in a liquid-nitrogen-cooled trap fitted with a thimble.13 After the conclusion of a pyrolysis experiment, the tar was recovered by Soxhlet-extracting the thimble (and the char elutriated into the thimble) with dichloromethane (CH2Cl2, DCM). The recovered tar solution was accurately weighed. A known amount of the tar solution was then put into an aluminum dish to allow the DCM solvent to evaporate at room temperature. The weight uptake of the dish was used to calculate the tar concentration in the recovered tar solution and thus the tar yield. “Tar yield” in the present paper therefore refers to the yield of DCM-soluble tar. Gas samples were collected in a gas bag in a separate experiment in which the trap was immersed in hot water to avoid the condensation/ adsorption of the product gases. Both before and after a gas sample was taken, the total flow rate of the product gas was measured with a soap-film flowmeter connected to the exit of the trap. The collected gas sample was analyed with a Hewlett-Packard HP5830A gas chromatograph (GC) equipped with a flame ionization detector (FID). During the present study, benzene and toluene were collected both in tars and in gas samples. Size Exclusion Chromatography (SEC). A known volume (1.00 mL) of the tar solution recovered from the coal pyrolysis experiment was first dried by blowing nitrogen gas onto the solution. The resulting tar residue was then redissolved in a known volume (5.00 mL) of tetrahydrofuran (THF), which was used as the mobile phase. The injection volume was always 20 µL (nominal). Separation was carried out on a 30 cm long analytical column packed with 5 µm of 100 Å polystyrene-polydivinylbenzene PL gel. A Waters 420 UV fluorescence detector and a Jasco UVIDEC-100-V UV absorption detector were used in tandem. In the present study, the same tar samples were repeatedly injected into the column and the column eluates were monitored with the UV absorption detector set at different wavelengths. Settings for the fluorescence detector were fixed: the wavelength of maximum transmission of the excitation interference filter was 280 nm (bandpass 9.5 nm), and the cut-on wavelength of the emission filter was 425 nm. These settings allowed selective detection of larger (with three or more fused benzene rings) aromatic ring systems in the column eluates. Experiments showed that reproducibility in peak area from the UV fluorescence detector was usually better than 3% between injections of the same sample. Detector readings were multiplied by a factor
f)
total wt of tar solution after pyrolysis total wt of coal fed during pyrolysis
(1)
to express the detector responses on the basis of “per gram of daf coal”.
Results and Discussion Pyrolysis Tar Yields as a Function of Temperature. Figure 1 shows the changes in DCM-soluble tar yield with increasing temperature for the six coal samples studied. In agreement with previous studies (e.g. see refs 13-17), data in Figure 1 showed that tar yields first increased with increasing temperature and then decreased with further increases in temperature. The DCM-soluble tar yields from the brown coals (Figure 1A) were generally lower than those from the bituminous coals (Figure 1B). Although the three brown coals studied have almost the same carbon contents (Table 1), the DCM-soluble tar yield at e700 °C was observed to increase in the order Fortuna
600 °C). However, increases in total FID response (Figure 2) and hydrocarbon gas yields (see Figure 9 for Laubag coal) were only observed between 500 and 900 °C. The further losses of tars must be attributed to soot formation. The experimental data in the present study demonstrate a different propensity for the two types of tars to form soot. For the Gottelborn coal tars, soot formation took place above 800 °C, as can be seen from the decreases in peak heights and areas (Figures 6-8) above this temperature. As has been discussed above, the Gottelborn tar molecules are characterized by the presence of the high-coordination-number cross-linked aromatic ring systems, resulting in rather high thermal stability. Before the molecules can be thermally disintegrated, active free radicals generated by the high temperatures result in the formation of further chemical bonds between these tar fragments and, hence, to soot formation. During this process, some smaller aromatic ring systems would be broken off, and some of them would be still seen as tar components. It is also highly likely that some of the aromatic rings were cracked into gaseous molecules during this process, some of which in turn may form soot at temperatures above 900 °C (cf. Figure 2 and discussion given above). The net result of this tar thermal cracking process with increasing temperature, as is seen in Figures 6 and 7, is that a large portion of the larger aromatic ring systems disappeared from the tars. The majority of these larger aromatic ring systems were converted either directly or indirectly (e.g. through the formation of gases) to soot. This process has resulted in the decreases in the ratios between tar peak areas detected at 350 and 254 nm, as seen in Figure 8B. By contrast, in the Laubag brown coal tars, the concentrations of larger (with at least three or more fused rings) aromatic ring systems are much lower than in the Gottelborn tars: maximum peak areas detected by the UV detector at 350 nm or by UV fluorescence detector were only about one-sixth to one-eighth of those of Gottelborn tars. It is envisaged that these aromatic ring systems in the Laubag tars are effectively separated by various long bridges, e.g. -(CH2)n-. These bridges are easily broken at high temperatures; for
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Energy & Fuels, Vol. 10, No. 5, 1996 1089
example, these long polymethylene bridges would be broken at a rate similar to that of long-chain aliphatics. Data in Figure 1 indeed showed that the tar yield from the pyrolysis of Laubag coal decreased with increasing temperature almost to a plateau at about 800 °C. The tar destruction processes are dominated by the loss of functional groups and smaller aromatic ring systems (peak areas detected at 254 nm, Figure 5A). Due to steric hindrance, the coordination number of the larger aromatic ring systems is smaller than that of their counterparts in the bituminous coal tars. The majority of the larger aromatic ring systems (peak area detected at 350 nm, Figure 5A) in brown coal tars remained in tars. The DCM-soluble tar yields (Figure 1) from the pyrolysis of Laubag coal were seen to be higher than those from Fortuna coal, especially between 400 and 800 °C. Above 800 °C, the difference appeared to diminish. The SEC measurements have, however, given similar maximum peak areas (although they had somewhat different peak area ratios as can be seen in Figure 5B) for both sets of tars over the whole temperature range studied. As the single-ring aromatic systems (e.g. benzene and toluene) have much lower molar absorptivities than other larger aromatic ring systems even at 254 nm,26 the SEC measurements in the present study are thought to underestimate the contributions from the single-ring aromatic systems. GC analysis of the gas samples (Figure 9) shows that the total yield of benzene and toluene is much higher from the pyrolysis of the Laubag coal than from the Fortuna coal. The appearance of benzene and toluene was accompanied by increases in the yields of other hydrocarbon gases, e.g. CH4, C2H4, and C2H2 in Figure 9. Since these species are believed to originate in tar cracking reactions, the above observations suggest that a considerable proportion of the low-temperature tar was single-ring aromatic systems connected to other aromatic ring systems. Cracking reactions, probably proceeding at a rate similar to that of n-alkane cracking reactions, resulted in the release of hydrocarbon gases and single-ring aromatic systems. This is in agreement with our previous studies24,31 which have also shown that significant amount of polymethylene groups are attached to aromatic ring systems. The monotonic increases in the yields of benzene, toluene, and C2H2 (Figure 9) are also a good indication that the synthesis of larger aromatic ring systems from these species did not take place to a significant extent under the present experimental conditions. The majority of the larger aromatic ring systems seen in the high-temperature tars was present in the parent coal. Trends similar to those in Figure 9 were also seen with Gottelborn and other bituminous coals. The total yield of benzene and toluene from the pyrolysis of Gottelborn coal increased to 1.7 wt % (daf) of coal at 1000 °C. With the same reasoning as presented above, this also suggested a significant portion of the low-temperature tars was single-ring aromatic systems. However, these small aromatic ring systems in the Gottelborn coal tars are much less important than in the case of the brown coals, as they comprise a much smaller proportion compared to the larger aromatic ring systems. (31) Nelson, P. F. Fuel 1987, 66, 1264.
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Conclusions 1. In agreement with previous studies, intra- and extraparticle thermal cracking reactions of aliphatic materials were observed between 400 and 700 °C. 2. Experimentally measured tar yields alone may not be enough to judge the completion of volatile release. Under the present experimental conditions, more aromatic tar components seemed to have been released at higher temperatures after tar yields had reached their maxima at lower temperatures. 3. Tars from the pyrolysis of bituminous coals showed higher thermal stability than those from brown coals. The presence and concentrations of large aromatic ring systems in the coal and the tars are the main structural reasons for this difference. On the basis of the experimental data, it is believed that the synthesis of larger aromatic ring systems (seen in high-temperature tars) from the smaller ones and acetylene did not take place to a significant extent under the present experimental conditions. 4. Larger aromatic ring systems in the coals and in the tars serve as cross-linking sites of higher coordina-
Li and Nelson
tion number than the smaller aromatic systems. During tar thermal cracking, large aromatic ring systems in tars with higher concentrations of larger aromatic ring systems (e.g. Gottelborn coal tars) are more likely to disappear from the tars than those in tars with lower concentrations of larger aromatic ring systems (e.g. Laubag coal tars). Soot formation, either directly or indirectly, is one of the major fates for the aromatic ring systems lost from the tars. 5. Concurrent with the release of simple (mainly C1 and C2) hydrocarbon gases, significant amounts of single-ring aromatic systems were liberated from the tar molecules during thermal cracking of the tars. In agreement with our previous studies, significant amounts of polymethylene groups were found to be chemically connected to aromatic ring systems in low-rank coal. Acknowledgment. We thank Dr. Spliethoff (Universita¨t Stuttgart) for providing coal samples. A Rothmans Foundation Fellowship awarded to C.-Z.L. is gratefully acknowledged. EF960054N
