Coal Dissolution by Heat Treatment at Temperatures up to 300 °C in N

Coals of different ranks were heat-treated at temperatures up to 300 °C in the aprotic dipolar solvent N-methyl-2-pyrrolidinone (NMP) with addition o...
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Energy & Fuels 2003, 17, 762-767

Coal Dissolution by Heat Treatment at Temperatures up to 300 °C in N-Methyl-2-pyrrolidinone with Addition of Lithium Halide. 1. Effects of Heat Treatment Conditions on the Dissolution Yield Chunqi Li, Toshimasa Takanohashi,* and Ikuo Saito Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 3058569, Japan

Masashi Iino Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 9808577, Japan Received November 19, 2002. Revised Manuscript Received April 1, 2003

Coals of different ranks were heat-treated at temperatures up to 300 °C in the aprotic dipolar solvent N-methyl-2-pyrrolidinone (NMP) with addition of lithium halides (LiCl, LiBr, and LiI). The treated products were extracted using a carbon disulfide/NMP (CS2/NMP) mixed solvent (1/1 by vol), and the dissolution yields were determined from the weight of the insoluble material. Addition of small amounts of lithium halides to CS2/NMP mixed solvent was reported to enhance the room-temperature extraction yields for some high-rank coals, while little effect was observed for low-rank coals (Energy Fuels 2001, 15, 141). On the other hand, in the present study, heat treatment in the presence of LiCl showed a greater effect on dissolution for low-rank than for high-rank coals. For low-rank Banko 97 coal (%C:70.0%), the dissolution yield was increased by 41.4% at 300 °C, from 50.4% in NMP alone to 91.8% with the addition of 2.4 mmol/g-coal LiCl, and the increased dissolution yield was attributed mainly to an increase in the amount of the heavy fraction; CS2/NMP-soluble and THF-insoluble (TIMS). Furthermore, the temperature of heat treatment (175-300 °C) and the quantity of LiCl added (0.6-3.6 mmol/g-coal) markedly affected the dissolution yield. When different lithium halides were used, the dissolution yield was increased in the order Cl- > Br- > I-; higher charge density of the anion of the additive was associated with higher dissolution yield. The mechanism of the observed effect of addition of LiCl on coal dissolution is discussed.

Introduction al.1,2

Iino et reported that a carbon disulfide/Nmethyl-2-pyrrolidinone (CS2/NMP) mixed solvent (1/1 by vol) gave high extraction yields (40-60%, daf) for some bituminous coals at room temperature. They also reported that addition of small amounts of electron acceptors, such as tetracyanoethylene (TCNE), to the mixed solvent markedly increased the extraction yield.3,4 Chen et al.5,6 reported that TCNE does not exist as a neutral molecule in NMP or the CS2/NMP mixed solvent, but forms a TCNE anion derivative, NMP 1,1,2,3-pentacyanopropene salt (NPCNP). They also confirmed that addition of a very small amount of NPCNP (0.2 mmol/g-coal) to the CS2/NMP mixed solvent * Corresponding author. E-mail: [email protected]. (1) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (2) Takanohashi, T.; Iino, M. Energy Fuels 1990, 4, 452. (3) Liu, H.; Ishizuka, T.; Takanohashi, T.; Iino, M. Energy Fuels 1993, 7, 1108. (4) Ishizuka, T.; Takanohashi, T.; Ito, O.; Iino, M. Fuel 1993, 72, 579. (5) Chen, C.; Kurose, H.; Iino, M. Energy Fuels 1999, 13, 1180. (6) Chen, C.; Iino, M. Energy Fuels 1999, 13, 1105.

increased the extraction yield of Upper Freeport coal from 59.0 to 84.0%, which was similar to the increment observed with addition of the same amount of TCNE. These results indicated that the anion plays a key role in enhancement of the solubility of the coal. Takahashi et al.7 also reported that the CS2/NMP extraction yields of coals were markedly enhanced by the addition of a small amount of halogenide salt (0.2 mmol/g-coal) at room temperature. Acid-base interactions between acidic sites in the coal and anions were reported to be responsible for the increased coal extraction yields.7 However, for some low-rank coals (%C < 80.0%), the CS2/NMP extraction yields were still very low at room temperature, even with the addition of TCNE or halogenide anions.1,2,7,8 Our recent study9 indicated that the CS2/NMP extraction yields for some low-rank coals were increased by heat treatment in NMP at temperatures up to 300 °C. However, 40-50% of the organic contents (7) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M. Energy Fuels 2001, 15, 141. (8) Dyrkacz, G. R.; Bloomquist, C. A. A. Energy Fuels 2000, 14, 513. (9) Li, C.; Ashida, S.; Iino, M.; Takanohashi, T. Energy Fuels 2000, 14, 190.

10.1021/ef020280z CCC: $25.00 © 2003 American Chemical Society Published on Web 04/26/2003

Coal Dissolution in NMP and Heat Treatment

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Table 1. Ultimate Analysis and Ash Content of Coal Samples Used ultimate analysis wt% (daf) C H N S Oa

coal Pocahontas No. 3 Upper Freeport Illinois No. 6 Beulah-Zap Banko 97 a

89.7 86.2 76.9 71.6 70.0

4.5 5.1 5.5 4.8 5.3

1.1 1.9 1.9 1.0 1.3

0.7 2.2 5.6 0.9 0.3

4.0 4.6 10.1 21.7 23.1

ash (wt%, db) 4.8 13.1 15.0 9.6 2.4

By difference.

of the heat-treated coals could not be dissolved in CS2/ NMP mixed solvent. This suggested that the relatively strong acid-base and electrostatic interactions in lowerrank coals could not be dissociated under these conditions. In the present study, the effects of various lithium halides (LiCl, LiBr, LiI) on coal dissolution were investigated by heat treatment of coals at temperatures up to 300 °C in NMP with the addition of lithium halides. The treatment temperature, properties of additives, quantities of lithium halides, and the rank of coals were found to markedly affect the dissolution yields. Experimental Section Materials. Four Argonne Premium coals, Pocahontas No. 3, Upper Freeport, Illinois No. 6, Beulah-Zap ( I-, suggesting that anions with smaller ion radius or larger electronegativity (Cl, 3.0; Br, 2.8; I, 2.5) are more effective for enhancement of the dissolution yield; i.e., a higher dissolution yield was obtained with a higher anion charge density. Takahashi et al.7 reported that lithium halides increased the CS2/NMP extraction yields for several bituminous coals at room temperature, and the extraction yields were increased in the order Cl- > Br- > I-. As described above, our results showed the same tendency for the effects of the addition of lithium halides on coal dissolution. As described in the Experimental Section, most of the lithium halides added remained in free form. To clarify at which stage addition of lithium halides has the greatest effect on coal dissolution, i.e.,

Coal Dissolution in NMP and Heat Treatment

Figure 5. Fraction distribution of Banko 97 coal extracted using CS2/NMP mixed solvent (1/1 by vol) with the addition of different lithium halides at 2.4 mmol/g-coal at room temperature. Raw Coal; no heat treatment and extractions using CS2/NMP and THF at room temperature.

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Figure 7. Fraction distribution of Beulah-Zap coal heattreated in NMP alone and with the addition of LiCl at 2.4 mmol/g-coal at 300 °C for 1 h. Raw Coal; no heat treatment and extractions using CS2/NMP and THF at room temperature.

Figure 6. Fraction distribution of Banko 97 coal that was heat-treated in NMP at 300°C for 1 h, extracted using CS2/ NMP mixed solvent (1/1 by vol) with the addition of different lithium halides at 2.4 mmol/g-coal at room temperature.

room-temperature extraction or heat treatment, raw coal and that heat-treated in NMP alone were extracted using CS2/NMP mixed solvent with the addition of different lithium halides at room temperature. As shown in Figure 5, the CS2/NMP extraction yields for the raw coal did not change with the addition of any lithium halides. In addition, as shown in Figure 6, the CS2/NMP room-temperature extraction yields with the addition of lithium halides after heat treatment in NMP alone were also similar to those without any additives. Therefore, we concluded that the lithium halides exerted their effects on the dissolution yields during the heat treatment stage, and not the subsequent extraction stage. Effects of Coal Rank. As shown in Figure 7, the dissolution yield of Beulah-Zap lignite was also increased from 38.2% in NMP alone to 51.8% with the addition of LiCl at 2.4 mmol/g-coal. However, the increase in dissolution yield (13.6%) was much lower than that of Banko 97 coal of similar rank (41.4%), as shown in Figure 1. As shown in Figure 8, for three bituminous coals ((a) Illinois No. 6, (b) Upper Freeport, (c) Pocahontas No. 3) the increases in the dissolution yields with LiCl at 2.4 mmol/g-coal were only 6.0%, 6.1%, and 6.2%, respec-

Figure 8. Fraction distribution of Illinois No. 6 (a), Upper Freeport (b), and Pocahontas No. 3 (c) coals heat-treated in NMP alone and with the addition of LiCl at 2.4 mmol/g-coal at 300 °C for 1 h. Raw Coal; no heat treatment and extractions using CS2/NMP and THF at room temperature.

tively. These results indicated that the effect of addition of LiCl on the coal dissolution was strongly dependent on the coal rank, with a higher effect for lower-rank coals. NMP, an aprotic dipolar solvent, is an effective

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Li et al.

Table 2. The Contents of Cl- and Li+ Remaining in Coal Fractions Obtained Under Various Conditions coal

LiCl added temperature Cl- Li+ sample (mmol/g-coal) (°C) wt % wt %

Banko 97

raw coal TIMS TIMS TIMS TIMS TIMS TIMS MI MI MI MI MI MI Beulah-Zap raw coal TIMS MI Illinois No. 6 raw coal TIMS MI Upper Freeport raw coal TIMS MI Pocahontas No. 3 raw coal TIMS MI

2.4 2.4 2.4 0.6 1.2 3.6 2.4 2.4 2.4 0.6 1.2 3.6

175 150 300 300 300 300 175 250 300 300 300 300

2.4 2.4

300 300

2.4 2.4

300 300

2.4 2.4

300 300

2.4 2.4

300 300

0.02 1.38 1.40 1.98 1.63 1.70 2.17 0.27 0.31 0.33 0.25 0.29 0.39 0.05 1.90 0.15 0.09 2.18 0.21 0.00 0.93 0.01 0.13 0.81 0.17

0.00 0.26 0.29 0.41 0.32 0.36 0.46 0.05 0.07 0.09 0.04 0.06 0.10 0.00 0.39 0.02 0.00 0.45 0.04 0.00 0.16 0.00 0.00 0.17 0.03

solvent to release some noncovalent bonds as a result of its strong interaction with polar sites in coal, such as hydroxyl groups.9 Due to the relaxation of coal aggregation by heat and the polarity of NMP,17 high dissolution yields were obtained for raw Illinois No. 6 and Upper Freeport coals, which were higher than the extraction yields obtained by CS2/NMP extraction with the addition of LiCl at room temperature reported by Takahashi et al.7 Therefore, the effect of the addition of LiCl on dissolution for these two coals may have been masked by the high effect of NMP alone. The low dissolution yields obtained by heat treatment in NMP alone and with the addition of LiCl for Pocahontas No. 3 coal may be due to its high aromaticity and small polar sites available for hydrogen bonding.18,19 The mechanism of coal dissolution by the addition of LiCl for lowerrank coals will be described later and discussed in detail in our companion paper.16 Residual LiCl Remaining in Coal Fractions. As described above, LiCl showed a marked effect on coal dissolution under various conditions. However, this raises the question of the fate of the LiCl after heat treatment. To answer this question, the quantity of LiCl remaining in each coal fraction was measured. As mentioned in the Experimental Section, most of the LiCl was present in the TS fraction, and it was difficult to separate because of its high solubility in THF. Therefore, the LiCl remaining in the TS fraction could not be measured directly. As MI and TIMS fractions were washed with a large amount of acetone, the “free” LiCl was considered to have been removed, and the small amount of LiCl that interacted with the MI or TIMS fraction could be measured by determining the contents of Li+ and Cl- in each fraction. As shown in Table 2, for low-rank Banko 97 coal, the contents of Cl- and Li+ (17) Li, C.; Yoshida, T.; Takanohashi, T.; Saito, I.; Iino, M. Proc. Jpn. Inst. Energy Conf. 2002, P, 42. (18) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (19) Muntean, J. V.; Stock, L. S. Energy Fuels 1991, 5, 768.

in TIMS were increased by increasing the heat treatment temperature from 175 °C to 300 °C, and the quantity of LiCl added from 0.6 to 3.6 mmol/g-coal. The contents of Cl- and Li+ were also increased in MI by these changes, but to a much lesser extent than in TIMS. Thus, the levels of residual LiCl (Li+ + Cl-) remaining in TIMS and MI obtained under various conditions were less than 3.0% and 0.5%, respectively. For other coals, the levels of residual LiCl remaining in TIMS and MI were also less than 3.0% and 0.3%, respectively. Therefore, it was not the weight of residual LiCl itself that increased the TIMS yield. Mechanism of Coal Dissolution by Addition of Lithium Halides. The results described above were significantly different from those of Takahashi et al.,7 who reported that the addition of lithium halides increased the extraction yields for some “high-rank coals,” and had little effect for low-rank coals.7 In contrast, the results of the present study indicated that the addition of lithium halides to NMP at 300 °C markedly increased the dissolution yield for “low-rank coals.” As low-rank coals such as Banko 97 coal have significant numbers of oxygen functional groups,18 various acid-base and electrostatic interactions are considered to occur between the molecules. At room temperature, these interactions may be too strong to be dissociated by the CS2/NMP mixed solvent, even with the addition of lithium halides. When low-rank coals were heat-treated in NMP at 300 °C, the dissolution yields were markedly increased. As mentioned above, the aprotic dipolar solvent NMP is an effective solvent to release some noncovalent bonds as a result of its strong interaction with polar sites in coals, such as hydroxyl groups. In addition, the associates newly formed between NMP and the polar sites of oxygen functional groups can prevent retrogressive reactions, such as coupling reactions between oxygen functional groups.9,20,21 However, 40-50% of the organic contents of the coal were still not dissolved in the CS2/NMP mixed solvent even after heat treatment in NMP, suggesting that the relatively strong acid-base and electrostatic interactions in low-rank coals would not be cleaved by heating in NMP alone. When lithium halide was added, these strong interactions might be cleaved more effectively by the cooperative effects of heating, NMP, and lithium halide. The detailed mechanism of low-rank coal dissolution by the addition of lithium halide during the heat treatment is reported in our companion paper.16 Conclusions The effects of the addition of lithium halides on coal dissolution after heat treatment at 300 °C in NMP were investigated, and the following conclusions were obtained. The high dissolution yields obtained in the present study were attributed to the cooperative effects of heating, NMP, and lithium halide. The aprotic dipolar (20) Murata, S.; Hosokawa, M.; Kidena, K.; Nomura, M. Fuel Process. Technol. 2000, 67, 231. (21) Suuberg, E. M.; Unger, P. E.; Larsen, J. W. Energy Fuels 1987, 1, 305. (22) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42.

Coal Dissolution in NMP and Heat Treatment

solvent NMP was an effective solvent for coal dissolution with the addition of lithium halides. The addition of lithium halides affected the dissolution more for lowerrank than for higher-rank coals. For low-rank Banko 97 coal, the dissolution yield at 300 °C increased from 50.4% in NMP alone to 91.8% with the addition of LiCl at 2.4 mmol/g-coal. The heat treatment temperature (175 to 300 °C) and the quantity of lithium halide added (0.6 to 2.4 mmol/g-coal) also markedly affected the

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dissolution yield. Higher dissolution yields were obtained with higher lithium halide anion charge density. Acknowledgment. This work was financially supported by the New Energy and Industrial Technology Development Organization (NEDO). The authors thank Mr. Shinsuke Okita of Shinnikka Environmental Engineering Co., Ltd. for ion chromatograph measurements. EF020280Z