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Increase in Extraction Yields of Coals by Water Treatment: Beulah-Zap Lignite Masashi Iino,*,† Toshimasa Takanohashi,† Takahiro Shishido,† Ikuo Saito,† and Haruo Kumagai‡ National Institute of AdVanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569, Japan, and Center for AdVanced Research of Energy Technology, Hokkaido UniVersity, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan ReceiVed March 6, 2006. ReVised Manuscript ReceiVed October 8, 2006
In a previous paper, we have reported that water pretreatments of Argonne premium coals, Pocahontas No. 3 (PO), Upper Freeport (UF), and Illinois No. 6 (IL) at 600 K increased greatly the room-temperature extraction yields with a 1:1 carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent. In this paper, the water treatment of Beulah-Zap (BZ) lignite has been carried out and the results obtained were compared with those for the three bituminous coals above. The extraction yields of BZ with CS2/NMP increased from 5.5% for the raw coal to 21.7% by the water treatment at 600 K. Similar to the other three coals, the water treatments at 500 K gave little increase in the yields. The larger decrease in oxygen content and hydrogen-bonded OH and the increase in the methanol swelling ratio by the water treatment suggest that the yield enhancements for BZ are attributed to the removal of oxygen functional groups and the breaking of hydrogen bonds to a greater extent than that for IL. From the characterizations of the treated coals and the extraction temperature dependency of their extraction yields, it is suggested that, for high-coal-rank coals, PO and UF, the breaking of noncovalent bonds such as π-π interactions between aromatic layers and hydrogen bonds is responsible for the extraction yield enhancements.
Introduction Various mechanisms for the liquefaction and extraction yield enhancements by water pretreatments have been proposed, including the breaking of covalent bonds, such as ether bonds,1-4 and removal of oxygen functional groups5,6 and the breaking of noncovalent bonds, such as hydrogen bonds,6,7 and the removal of minerals.6,8 The suppression of metal carboxylateinduced retrogressive reactions in liquefaction by the association of metal cation with water9,10 and the effect of dihydroxy aromatics formed by water treatment and their reactions with coal7,10 were also considered. The reason for the diversity of the mechanisms proposed is partly because different experimental conditions were used in the literature, such as the kind of coal, treatment temperature, heating rate, with or without drying the sample, and protection to air. The liquefaction and extraction conditions can also have an influence on the treatment * To whom correspondence should be addressed. E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Hokkaido University. (1) Graff, R. A.; Brandes, S. D. Energy Fuels 1987, 1, 84. (2) Brandes, S. D.; Graff, R. A.; Gorbaty, M. L.; Siskin, M. Energy Fuels 1989, 3, 494. (3) Ivanenko, O.; Graff, R. A.; Balogh-Nair, V.; Brathwaite, C. Energy Fuels 1997, 11, 206. (4) Mapstone, J. O. Energy Fuels 1991, 5, 695. (5) Serio, M. A.; Kroo, E.; Solomon, P. R. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 1992, 37, 432. (6) Shui, H.; Wang, Z.; Wang, G. Fuel 2006, 85, 1798. (7) Bienkowski, P. R.; Narayan, R.; Greenkorn, R. A.; Chao, K. Ind. Eng. Chem. Res. 1987, 26, 202. (8) Mochida, I.; Iwamoto, K.; Tahara, T.; Korai, Y.; Fujitsu, H.; Takeshita, K. Fuel 1982, 61, 603. (9) Serio, M. A.; Kroo, E.; Teng, H.; Solomon, P. R. Prepr. Symp.s Am. Chem. Soc., DiV. Fuel Chem. 1993, 38, 577. (10) Serio, M. A.; Kroo, E.; Chapeney, S.; Solomon, P. R. Prepr. Symp.s Am. Chem. Soc., DiV. Fuel Chem. 1993, 38, 1021.
effect. Slow heating of the treated coal in liquefaction was reported to lose the merit of the steam treatment.3 In a previous paper,11 we have studied the effect of water treatment on extraction yields of Argonne premium coals, Pocahontas No. 3 (PO), Upper Freeport (UF), and Illinois No. 6 (IL) coals. All of the coals used show that the water treatments at 600 K increased greatly the room-temperature extraction yields with a carbon disulfide/N-methyl-2-pyrrolidinone (CS2/ NMP, 1:1 volume ratio) mixed solvent, which has been found to give high extraction yields for bituminous coals at room temperature.12 The breaking of noncovalent bonds such as π-π interactions between aromatic layers and hydrogen bonds for PO and UF and the breaking of C-O covalent bonds and noncovalent hydrogen bonds for IL were proposed for the yield enhancement observed. In this paper, the water treatment of Beulah-Zap (BZ) lignite was carried out and compared to the results obtained with those for PO, UF, and IL and mechanisms of the water treatment are discussed. Experimental Section Coal Sample. Argonne premium BZ lignite with a particle size under 100 mesh were used after it was dried in a vacuum at 353 K overnight. Water Treatment and Extraction. Water treatment of BZ was carried out using a 50 mL autoclave at 500 and 600 K by the procedure described in the previous paper.11 The treated samples were dried in a vacuum at 353 K overnight. The weight loss at 600 K was 19.3 wt %, which is large as a result of the large gas evolution compared to those for PO, UF, and IL, i.e., 2.8, 3.0, and 5.4%, respectively. The extraction of raw and treated BZ with CS2/ (11) Iino, M.; Takanohashi, T.; Li, C.; Kumagai, H. Energy Fuels 2004, 18, 1414. (12) Iino, M.; Takanohashi, T.; Osuga, H.; Toda, K. Fuel 1988, 67, 1639.
10.1021/ef060099g CCC: $37.00 © 2007 American Chemical Society Published on Web 12/05/2006
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Figure 1. Extraction yields of raw and 600 K water-treated coals with NMP at room temperature, 473, 573, and 633 K. Table 1. CS2/NMP Mixed Solvent Extraction Yields of Raw, Water-Treated, and Heat-Treated BZ Lignite treatment raw coal water heat a
treatment temperature (K)
extraction yield (wt %)a
500 600 500 600
5.5 4.2 21.7 2.5 9.7
CS2/NMP (1:1) mixed solvent at room temperature.
NMP (1:1 by volume) mixed solvent at room temperature and with NMP or 1-methylnaphthalene (1-MN) from room temperature to 633 K were carried out by the procedure described in the previous paper.11 The extraction yield was calculated on a dry ash-free basis from the weight of the residue. Characterization of Water-Treated BZ. The Fourier transform infrared spectroscopy (FTIR) spectrum was measured by a diffuse reflectance method using a Nicolet MAGNA-IR spectrometer at a resolution of 4 cm-1. The difference spectrum between raw and treated BZ was obtained using the absorption of the aromatic Cd C bond stretching band at 1610 cm-1 as a standard peak. The swelling ratio, which is defined as the ratio of coal volume at bulk and in a solvent, was measured in methanol at room temperature by the volumetric method.11
Results and Discussion Extraction Yield. Table 1 shows the CS2/NMP extraction yields of raw, water-treated, and heat-treated BZ. The water treatment at 600 K increased the room-temperature extraction yield with CS2/NMP from 5.5 to 21.7%, while the water and heat treatments at 500 K showed no increase and the heat treatment at 600 K showed only a little increase in the extraction yield. Similar results were obtained for pyridine extraction at room temperature; i.e., the extraction yields of raw, 600 K watertreated, and 600 K heat-treated BZ are 3.3, 11.7, and 7.1%, respectively. Other premium Argonne coals also greatly increased the extraction yields by the water treatment at 600 K, from 2.2, 60.2, and 32.1% for the raw coals to 23.5, 82.3, and 70.2% for the treated PO, UF, and IL, respectively,11 irrespective of their different carbon content (Table 2). Figures 1 and 2 show the results of the extractions of BZ and the other three coals with polar NMP and nonpolar 1-MN
Table 2. Ultimate Analyses of Raw and Water-Treated Coals ultimate analysis (wt %, daf) PO UF IL BZ
a
treatment
C
H
N
raw coal water, 600 K raw coal water, 600 K raw coal water, 600 K raw coal water, 500 K water, 600 K
89.7 90.9 86.3 86.6 77.3 79.8 66.4 70.4 76.2
4.5 4.5 5.1 5.0 5.4 5.2 4.6 4.8 4.4
1.1 1.1 1.7 1.8 1.4 1.5 1.0 1.1 1.2
S
Oa
4.7 (S plus O) 3.5 (S plus O) 2.5 4.4 2.3 4.3 5.7 10.1 5.9 7.5 0.6 27.4 0.5 23.2 0.7 17.6
By difference.
at various extraction temperatures. For NMP extractions, the yield enhancement by the water treatment was observed at room temperature and 473 K for all of the coals, but the extractions at 573 K gave similar or lower yields compared to that of the raw coals, except for IL. IL showed the increase by 34.7% in the extraction at 573 K. Figure 2 shows that 1-MN gave a lower extraction yield than NMP and the large yield enhancement by the water treatments was observed only at 473 K (no extraction data at room temperature). The NMP and 1-MN extractions of BZ at and above 573 K include a large weight loss for raw BZ during the extraction, which leads to low yields of the residue, resulting in high extraction yields compared to the treated one, which had already experienced the weight loss during the water treatment at 600 K. Characterization of Water-Treated Coals. Table 2 shows the results of ultimate analyses of raw and 500 and 600 K watertreated BZ, including the results for PO, UF, and IL. The oxygen content decreased by the treatment, i.e., from 27.4% for raw BZ to 23.2 and 17.6% for 500 and 600 K water-treated BZ, respectively. The 9.8% decrease in the oxygen content at 600 K is much greater than that for IL, 2.6%, while the elementary compositions of 600 K water-treated PO and UF were not much different from those for the raw coals. Figure 3 shows IR spectra of raw and 600 K water-treated BZ and their difference spectrum. The difference spectrum shows the large decrease in the absorption of hydrogen-bonded OH (2600-3600 cm-1), in agreement to the large decease in the oxygen content. IL also shows a decrease in OH11 but to a
Increase in Extraction Yields of Coals by Water Treatment
Energy & Fuels, Vol. 21, No. 1, 2007 207
Figure 2. Extraction yields of raw and 600 K water-treated coals with 1-MN at 473, 573, and 633 K.
lesser extent than BZ. Miura et al. reported that weak hydrogen bonds, such as the OH-π bond, have a much smaller molar absorbency than strong hydrogen bonds, such as COOHCOOH.13 This kind of change in hydrogen-bond distribution seems very likely to occur by this treatment, because π electrons of aromatic rings and phenolic OH exist abundantly in BZ and IL; therefore, they are preferentially used when new hydrogen bonds form during the sample drying, that is, when hydrogenbonded water molecules with coal oxygen functional groups are removed by drying of the treated coal. This may be another reason for the decrease in OH absorption by the treatment, while UF and PO show little decrease in OH.11 The equilibrium swelling ratio in methanol increased from 1.02 for raw BZ to 1.18 for 600 K water-treated BZ. Similar increases in the ratio by the water treatment were observed for UF and IL.11 Mechanisms of the Water Treatment. An increase in the equilibrium swelling degree by the water treatment suggests that the cross-linking network becomes loosened by the treatment; i.e., the breaking of covalent and/or noncovalent bonds in coal
occurs. As indicated by the decrease in the oxygen content, the water treatments of IL and BZ were accompanied with a loss of oxygen functional groups; i.e., the breaking of C-O bonds occurs. The increase in extractable low-molecular-weight substances by the breaking of C-O bonds is responsible for the yield enhancement observed. A larger decrease in the oxygen content for BZ indicates the more extensive removal of oxygen functional groups than that for IL, although the CS2/NMP yield increment by the treatment, 16.2%, was not larger than 38.1% for IL. Noncovalent bond breaking, i.e., hydrogen-bond breaking, and the hydrogen-bond distribution change from stronger to weaker ones are also considered to be responsible for the yield enhancements for the both coals. For high-rank coals, UF and PO, little decrease in the oxygen content was observed by the treatment, suggesting that the breaking of noncovalent bonds is responsible for the yield enhancement.11 The observed extraction temperature dependency of the extraction yields supports this mechanism. For UF, the increment in the NMP and 1-MN extraction yields by the water treatment decreased with increasing extraction temperatures and the benefit of the treatment was lost at and above 573 K. At the high-temperature extractions, the effect of noncovalent bond breaking for the treated UF should be lost because, for even raw UF, almost all of the noncovalent bonds are broken at such high temperatures, resulting in little difference between the treated and raw UF, while for IL, in which covalent bond breaking occurs during the water treatment, the benefit of the treatment was not lost in the extractions at 573 K. Considering a very small content of OH groups in PO, π-π interactions between aromatic layers for PO and both π-π interactions and hydrogen bonds for UF may be broken by the treatment. In regard to the interaction of water and aromatic layers, Mastral et al. reported that water molecules are adsorbed mainly in the molecular-size micropores (ca. 0.3 nm) of active carbon.14 It is also assumed that water adsorption on carbons occurs through hydrogen bonding to hydrophilic centers located on the edges of the aromatic sheets of carbons.14 It should be noted that we cannot exclude the possibility that coal swelling by water absorption on micropores of coal induces
(13) Miura, K.; Mae, K.; Li, W.; Kusakawa, T.; Morozumi, F.; Kumano, A. Energy Fuels 2001, 15, 599.
(14) Mastral, A. M.; Murillo, T. G. R.; Callen, M. S.; Lopez, J. M.; Navarro, M. V. Energy Fuels 2002, 16, 205.
Figure 3. IR spectra of raw and 600 K water-treated BZ and their difference spectrum, in which the spectrum of the raw BZ was subtracted from that of the treated BZ.
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Table 3. Physical Properties of Water at 500 and 600 K temperature (K)
vapor pressure (atm)
density (g/mL)
viscosity (×103, poise)
dielectric constant
500 600
27 124
0.83 0.65
1.17 0.76
30.4 16.5
some covalent bond breaking, contributing to the extraction yield enhancements. Recently, Shui et al. also reported12 that FTIR of a high-rank coal (88.2% C, daf) shows the removal of minerals by water treatment at 513 K, which is considered to decrease the interaction between π electron of aromatic rings and metal cations in coal, resulting in the extraction yield enhancement. However, in our case, the removal of minerals was not observed for PO and UF, although FTIR of IL shows the decrease in clay minerals by the water treatment at 600 K.11 The water treatment at 500 K was found to be ineffective. Table 3 shows physical properties of water at 500 and 600 K. Among them, vapor pressure has the most striking temperature
dependency, i.e., 27 and 124 atm at 500 and 600 K, respectively. It may be responsible for the effectiveness of the 600 K treatment, because high water vapor pressure facilitates water to diffuse into the micropores and bulk structure of coal, increasing the interactions of water with coal functional groups. For BZ and IL, it should also be considered that the elimination reactions of oxygen functional groups occur more readily at 600 K than at 500 K. Acknowledgment. This work was supported by the “Research for the Future” project of the Japan Society for the Promotion of Science (JSPS). The authors are grateful to the other members of the project for their collaboration in this work. The authors are also grateful to Dr. N. Kashimura, National Institute of Advanced Industrial Science and Technology, for his helpful discussion. EF060099G
