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Energy & Fuels 2002, 16, 18-22
Changes in Coal Aggregate Structure by Heat Treatment and Their Coal Rank Dependency Izumi Watanabe,† Kinya Sakanishi,*,‡ and Isao Mochida† Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-8580, Japan, and National Institute of Advanced Industrial Science and Technology(AIST), Tsukuba, Ibaraki 305-8569, Japan Received June 29, 2001
Several Argonne coal samples were heat-treated at variable heating rates of 10 to 1000 °C/ min in the temperature range of 250 to 450 °C, to clarify changes in secondary aggregated structure of the coals revealed by step scan XRD and solvent swelling measurements. Swelling ratio of the Wyodak(WY) coals with DMF once increased by the heat treatment at 250 °C, and then decreased above 300 °C, suggesting that the coal aggregate structure may be loosened by the removals of carboxylic and phenolic groups in coal macromolecules at around 250 °C. The step scan XRD profiles of the coal showed a slight enlargement of the peak at 20°, that is the so-called γ-band, reflecting from such loosening of coal aggregate structure revealed by the removal of oxygen functional groups through the heat treatment. In the case of Beulah Zap(BZ) coal, the XRD profile was not reflected in the solvent swelling, probably due to its higher content of ionexchangeable cations, which show some hardness against solvent swelling. For the higher ranking coals of Pocahontas(POC), the solvent swelling ratio did not change so much by the heat treatment, reflecting its insensibility to the low-temperature heat treatment. The swelling ratio of Upper Freeport(UF) coal was not able to be measured because of the high fusibility of the coal particles above 350 °C. The XRD profile of WY coal was in good agreement with the solvent swelling behavior. It is noted that the peak at 26° was intensified by the heating treatment of UF coal above 350 °C, suggesting its enhanced fusibility by rapid heating. The higher heating rate influenced more significantly such ordering of the coal aggregate structure, making the coals fusible at relatively lower temperatures below 400 °C. Such effects of the heat treatment are discussed in terms of the changes in the coal aggregate structures revealed by XRD and solvent swelling which are both strongly subjected to the coal rank.
Introduction Coal consists of primary macromolecules of polyaromatic-polynuclear structure with some heteroatom groups and their secondary networks, latter of which are derived from aromatic ring stacking, aliphatic side chain entanglement, and hydrogen bonds cation bridges, charge-transfer interactions through oxygen functional groups.1-5 The rank of coals has been believed to be correlated to coal aggregate structures governed by the noncovalent bonding interactions. Polar solvents such as pyridine, THF, and DMF have been reputed to be quite effective for the liberation of the noncovalent bonds interactions, e.g., hydrogen bonds, aromatic plane stacking, and electrostatic interactions, in the coal macromolecular network.6-10 The impregna* Corresponding author. Tel: 81-298-61-8437. Fax: 81-298-61-8408. E-mail:
[email protected]. † Kyushu University. ‡ National Institute of Advanced Industrial Science and Technology(AIST). (1) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (2) Cody, G. D.; Davis, A.; Hatcher, P. G. Energy Fuels 1993, 7, 455. (3) Carlson, G. A. Energy Fuels 1992, 6, 771. (4) Nakamura, K.; Takanohashi, T.; Iino, M.; Kumagai, H.; Sato, M.; Yokoyama, S.; Sanada, Y. Energy Fuels 1995, 9, 1003. (5) Larsen, J. W.; Gurevich, I. Energy Fuels 1996, 10, 1269.
tion of a small amount of pyridine was reported to enhance the coal fusibility and the production of volatiles in the following carbonization even after the extraction of the soluble fraction.11,12 Acidic treatment of lower ranked coals also rearranged such aggregate structure by removing ion-exchangeable bridging cations through oxygen functional groups.13,14 It was also reported that such bridging cations brought about some hardness against solvent swelling and thermal decomposition.14 Despite such importance of coal aggregate structure, its direct detection has not been assessed, although Hirsch pointed out the π-π stacking in his model. X-ray diffraction (XRD) has been applied to the characterization of carbonaceous materials including coals for better (6) Suuberg, E. M.; Otake, Y.; Yun, Y.; Deevi, S. G. Energy Fuels 1993, 7, 384. (7) Otake, Y.; Suuberg, E. M. Fuel 1998, 77, 901. (8) Otake, Y.; Suuberg, E. M. Energy Fuels 1997, 11, 1155. (9) Hall, P. J.; Larsen, J. W. Energy Fuels 1993, 7, 47. (10) Cody, G. D.; Eser, S.; Hatcher, P.; Davis, A.; Sobkowiak, M.; Shenoy, S.; Painter, P. C. Energy Fuels 1992, 6, 716. (11) Aida, T.; Nawa, Y.; Shiotani, Y.; Yoshihara, M.; Yonezawa, T. Proc. Int. Conf. Coal Sci. 1993, p 445. (12) Korai, Y.; Torinari, Y.; Mochida, I. Cokes Circular, Jpn. 1992, 41, 232. (13) Mochida, I.; Sakanishi, K. Fuel 2000, 79, 221. (14) Sakanishi, K.; Watanabe, I.; Nonaka, T.; Kishino, M.; Mochida, I. Fuel 2001, 80, 273.
10.1021/ef010144e CCC: $22.00 © 2002 American Chemical Society Published on Web 12/12/2001
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Table 1. Elemental Analyses of Coals coal name
elemental analysis (wt % daf) C
H
Beulah-Zap[BZ] Wyodak[WY] Illinois[IL] Upper-Freeport[UF] Pocahontas[POC]
74.1 75.5 80.7 87.1 90.9
4.92 5.40 5.15 4.72 4.38
N
O (diff) ash
1.18 19.8 9.7 1.11 17.9 8.7 1.39 12.3 15.1 1.49 6.7 12.8 1.28 3.4 4.5
understanding of ordered packing of macromolecules.15-20 Slow step scan XRD analyses have been reputed to give the higher resolution of the diffractograms, classifying the carbon-related peaks around 20-26° basically into two categories; one is derived from aromatic ring stacking around 0.26° (so-called π-band), and the other is named γ-band around 20°. The latter band is believed to be derived from aliphatic chains, although details are not concerned until recently. The intensity ratio of the two peaks appears to reflect the coal rank.13 The preheating treatment has been reputed to improve the fusibility and coking properties of nonfusible coals,21,22 especially at the rapid heating rate above 100 °C/min. The rapid heating pretreatment combined with hot molding has been introduced to the development of the novel coking process in Japan as SCOPE21 (Super Coke Oven for Productivity and Environment toward the 21st Century) project.23 The heating rate has been reported to be very influential on the fusibility and coking reactivity of the nonfusible and lower rank coals.23,24 It was also reported that drying of lignite at lower temperatures around 80 °C may affect the regularity of the coal through the change in the physical structure.25 Such change in the physical structure of the lignite can be related to its solvent swelling behavior and thermal reactivity.13,14 In the present study, five Argonne Premium coals before and after heat treatment were examined by step scan XRD measurements and solvent swelling with DMF in order to clarify changes in secondary aggregated structure of the coals. The effects of heating rates and temperatures on the coal aggregate structure were principally concerned. Experimental Section Coals. Five coals (100 mesh under) of Beulah-Zap(BZ), Wyodak(WY), Illinois No.6(IL), Upper Freeport(UF), and Pocahontas(POC) in the Argonne Premium Coal Bank were used in the present study. Their elemental analyses are summarized in Table 1. The coals were dried at 60 °C for 5 h under vacuum prior to the following experiments. Heat Treatment. Coal grains of ca. 2.0 g were placed between walls of two quartz tubes, of which inner and outer (15) Cartz, L.; Diamond, R.; Hirsch, P. B. Nature 1956, 177, 500. (16) Shiraishi, M.; Kobayashi, K. Bull. Chem. Soc. Jpn. 1973, 46, 2575. (17) Wertz, D. L.; Bissell, M. Energy Fuels 1994, 8, 613. (18) Wertz, D. L.; Quin, J. L. Energy Fuels 1998, 12, 697. (19) Wertz, D. L. Energy Fuels 1999, 13, 513. (20) Wertz, D. L.; Quin, J. L. Fuel 2000, 79, 1981. (21) Mochida, I.; Shimohara, T.; Korai, Y.; Fujitsu, H. Fuel 1984, 63, 847. (22) Mochida, I.; Shimohara, T.; Korai, Y.; Fujitsu, H.; Takeshita, K. Fuel 1983, 62, 471. (23) Nishioka, K. Iron Steel. Jpn. 1996, 82 (5), 353. (24) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672. (25) Vorres, K. S.; Wertz, D. L.; Malhotra, V.; Dang, Y.; Joseph, J. T.; Fisher, R. Fuel 1992, 71, 1047.
Figure 1. XRD profiles of low-rank coals before and after the heat treatment. diameters were 18 mm and 34 mm, respectively. The coal in the tubes was heated in an electronic furnace under nitrogen sealing at the heating rates of about 80 and 180 °C/min to the temperatures of 250-450 °C. The sample was held at the prescribed temperature for 3 min, and then was immediately cooled to room temperature by taking out the tube from the furnace. Thermal gravimetric analysis(TGA) of the coals were also performed at the heating rate of 10 °C/min for the measurement of weight loss of the coals during the heat treatment. Solvent Swelling. Coal (0.4 g) was mixed with a prescribed amount of solvent in a test tube, and settled at 40 °C under nitrogen flow for a few days to measure the swelling ratio by DMF. The swelling ratio(Q) was calculated as follows; Q ) ht/ ho; ht: height of swollen coal layer in the tube; ho: height of coal particle layer without solvent XRD Measurements. The slow step scanning XRD of coals before and after the heat treatments was measured using a Rigaku Geigerflex diffractometer(Cu KR1, 40 kV/30 mA) by the scanning speed of 5°/min in the scan range of angle 5 to 60 °C. The broad peak around 25° in the diffractograms was smoothed by the removal of peaks due to mineral matters, and then decomboluted into two Gaussian curves, which are called γ-band(20°) and π-band(26°), respectively.
Results and Discussion Change in XRD Profiles of Coals by Heat Treatment. Figure 1 illustrates the XRD profiles of BZ, WY, and IL coals before and after the heat treatment. The original BZ coal exhibited the broad peak around 20° with a broad shoulder at 26°. The BZ coal heat-treated at 300 to 350 °C gave the more intensified peak around 20° in comparison with that of nontreated coal. Relative
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Figure 2. XRD profiles of low-rank coals before and after the heat treatment.
intensity at 26° was certainly reduced by the heat treatment at this temperature range. A slight increase of d002 value was observed from 0.358 nm(original coal) to 0.365 nm(heat-treated at 350 °C), which were calculated based on the decombolution method described below. The coal heat-treated at higher temperatures above 400 °C exhibited intensified diffraction around 26° with the decrease of the diffraction at 20°. The similar tendency was observed with WY coal, although the change in the XRD took place at a lower temperature of 250 °C, reflecting its higher total contents of phenolic and carboxylic groups than those in BZ coal.26 The XRD profile of IL coal was not changed so much by the heat treatment up to 400 °C. Figure 2 illustrates the XRD profiles of relatively higher rank coals of UF and POC before and after the heat treatment. The heating of UF coal above 300 °C sharpened the peak at 26° to a small extent compared to those of the nontreated one due to the thermoplasticity. On the basis of the calculation of the decombolution method described below, the d002 values of UF coal slightly decreased from 0.357 nm to 0.354 nm by the heating from 200 to 400 °C. Effects of Heat Treatment on Solvent Swelling Behaviors. Figure 3 illustrates the swelling behaviors of WY, IL, and POC coals by DMF before and after the heat treatments. WY coal heat-treated at 250 °C gave the higher swelling ratio than the original and the heattreated ones at the higher temperatures. The swelling ratio of WY coal decreased above 300 °C due to the initiation of cross-linking reaction through the removal of oxygen functional groups. On the other hand, the swelling ratio of BZ coal did not change so much before and after the heat treatment up to 400 °C, probably reflecting its hardness against solvent swelling and thermal decomposition by the higher contents of ionexchangeable bridging cations.14 The control of balance (26) Aida, T.; Nishisu, A.; Yoneda, M.; Yoshinaga, T.; Tsutsumi, Y.; Yamanishi, I.; Yoshida, T. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2001, 46 (1), 325.
Figure 3. Swelling behaviors of coals by DMF before and after the heat treatment (180 °C/min).
Figure 4. Weight loss of heat-treated coals (10 °C/min).
between the liberation of hydrogen bonds and the crosslinking reaction is suggested very important for the heat treatment of the lower rank coals. The swelling ratio of IL coal decreased with increasing the heat-treatment temperature, although the swelling ratio of POC coal did not change so much regardless of the heat treatment. The swelling behavior of UF coalwas not able to be measured because of the fusibility of the coal particles by the heat treatment above 350 °C. It is suggested that the solvent swelling behaviors of the higher ranking coals may be influenced by the balance between the loosening of the noncovalent bonding interactions and the thermal softening of coal particles during the heat treatment. The latter effect
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Figure 5. Smoothing and decombolution of XRD of heat-treated BZ coal.
was more marked with the heat-treated coal at the higher heating rate of ca. 1000 °C/min by using a microimage furnace with a smaller amount of coal sample. These results will be reported in a separate paper. Structural Change of Coals by the Heat Treatment. Figure 4 illustrates the weight losses of the coals during the heat treatments. The weight loss of BZ coal reached 25% around 350 °C. Major products were CO2 and H2O due to the dehydration and decarboxylation reactions during the heat treatment. The weight loss due to H2O and CO2 evolution was more significant with the lower rank coals of BZ and IL because of their higher oxygen contents. Although some condensation reactions took place through the dehydration and decarboxylation, its contribution to the structural change was negligible compared to the liberation of hydrogen bonds during the heat treatment below 350 °C. Above 350 °C, the weight loss increased linearly with the temperature, being caused by the further evolution of CO and hydrocarbon gases such as methane and ethane, indicating that the thermal cracking of covalent bonds such as methylene and ether linkages may take place during the heat treatment in this temperature range. The weight loss of UF and POC became relatively larger above 350 °C, suggesting that gas formation as well as coking reactions may initiate around 400 °C. Quantitative Comparison of XRD Profiles of Coals. Figures 5 and 6 illustrate smoothing and decombolution procedures of XRD profiles of BZ and UF coals, respectively. The XRD patterns of the coals were smoothed by the removal of the peaks derived from mineral matters, and the residual peak was divided into the two Gaussian peaks at 20° (so-called γ-band) and at 26° (so-called π-band). The divided diffractograms of the coals were shown in the right side of the figures by the two different heating rates of ca. 80 and 180 °C/
min. This smoothing and decombolution closed up the difference of the two coals caused by the heat treatment. In the case of BZ coal, the peak intensity at 20° increased with heating temperatures, while the peak at 26° decreased with the heating temperatures in the temperature range of 250 to 400 °C, indicating that the loosening of noncovalent bonds may contribute to the disordering of the aggregate structure of the lower rank coal. In contrast, in the case of UF coal, the peak at 20° was weakened, while the peak at 26° was intensified by the heat treatment, especially around 400 °C, suggesting that the π-π stacking of aromatic sheets may be emphasized to the more stable alignment, liberating the alkyl entanglement through the partial fusibility and the coking reactions through the initial dealkylation reactions. Figure 7 shows the comparison of the relative intensity of I26/I20 of the two coals calculated on the basis of the processed XRD profiles as illustrated in Figures 6 and 7. The relative intensity of I26/I20 of BZ coal did not change so much in this temperature range, although a very slight increase was observed for the rapid-heated one. In the case of WY coal as illustrated in Figure 8, the relative intensity of I26/I20 once increased at around 250 °C, and then decreased above 300 °C. It is noted that the higher relative intensity was kept by the rapid heating treatment. Such effects of the heating rates on the coal aggregate structure were also reported in the modification of nonfusible coals.21-23 These results suggest that the rapid heating to around 300 °C may be essential to the liberation of the aggregate structure in the nonfusible coals through the rapid removal of oxygen functional groups without any cross-linking reactions. The relative intensity of UF coal started to increase around 300 °C from ca. 2.5 to ca. 3.5 by the rapid heating of 180 °C/min, while it stayed unchanged for
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Figure 6. Smoothing and decombolution of XRD of heat-treated UF coal.
were successfully evaluated. It can be said that the XRD profiles of the coals essentially represent the ordering extent of the aromatic plane stacking and the coking reactivity, naturally leading to the better sensitivity to the higher ranking coals. Conclusions
Figure 7. Effect of heat treatment of coals on XRD intensity ratio: I26/I20.
Figure 8. The relative intensity of Iγ/I002 of WY coal before and after the heat treatment.
the slow-heated one. This result shows that the change in the aggregate structure of BZ coal was not reflected in this semiquantitative XRD evaluation, although the partial fusibility and initial coking reaction of UF coal
Slow step scan XRD revealed that the heat treatment at 250-400 °C intensified the γ-band (the peak at 20°) by weakening the hydrogen bond interactions of WY coal and the π-bond (the peak at 26°) by ordering the aromatic plane stacking of the fusible UF coal, respectively. Such change in the aggregate structure of the lower rank coal was also reflected in their solvent swelling behavior by DMF. The higher heating rate of 180 °C/min during the heat treatment influenced more significantly the π-π bond ordering of the higher ranking coals, enhancing the fusibility of the coals at relatively lower temperatures below 400 °C. The control of the balance between the liberation of noncovalent bond interactions and the cross-linking reaction is suggested essential to the modification of coal aggregate structure by the heat treatment at relatively low temperatures and high heating rates. Acknowledgment. This work has been carried out as one of the “Research for the Future” projects of the Japan Society for the Promotion of Science (JSPS) through the 148 committee on coal utilization technology of JSPS. EF010144E
