Article pubs.acs.org/EF
Relationship between Extraction Yield of Coal by Polar Solvent and Oxygen Functionalities in Coals Naoto Sakimoto, Koji Koyano, and Toshimasa Takanohashi* National Institute of Advanced Industrial Science and Technology 16-1 Onogawa, Tsukuba 305-8569, Japan ABSTRACT: HyperCoal (HPC) is an ashless coal produced by thermal solvent extraction from a coal and is expected to be useful for several utilization technologies because of it is mineral matter free and high softening property. Although HPC can be produced from various ranks, HPC extraction yield from low-rank coals is low due to the influence of hydrogen bonds among functional groups. Extraction yield can be increased by using a polar solvent for breaking the hydrogen bonds leading to increased yield. In this study, the relationship between the extraction yield by a polar solvent and oxygen functional groups was investigated. The results showed that the effectiveness of polar solvent increased with increasing phenolic hydroxyl content and an excellent correlation between the effectiveness of polar solvent and the phenolic hydroxyl content in coals was found.
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INTRODUCTION HyperCoal (HPC) is produced by thermal solvent extraction of a coal, HPC shows high softening properties and does not contain inert organic compounds1 and is expected to be useful for several utilization technologies, for instance, fuel for lowtemperature catalytic gasification2 and alternative for coking coals which used in steel production.3 The extraction yield of coal is one of the most important factors in the HPC production process. Iino et al.4 reported that a carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent can dissolve a significant portion of bituminous coal (30−66 mass % daf) even at room temperature. This indicates that the cross-links in coal are consisted of both covalent and noncovalent bonds including Hydrogen bonds, van der Waals’ force, π−π aromatic interactions and cation bridges.5 The extraction yield of low-rank coal is generally low when a nonpolar solvent is used. By using a polar solvent, the yield can be enhanced. Masaki6 and Yoshida7 reported that the polar solvent, crude methyl naphthalene oil (CMNO) gave higher extraction yield than the nonpolar light cycle oil (LCO). Kashimura et al.8 reported that addition of polar component contained in CMNO, such as indole (IN) and quinoline derivatives to 1-methyl-naphthalene (1-MN), increased the extraction yield from 33 to 50 mass % daf. Acid pretreatment of low-rank coals also enhanced the effect of polar compounds on extract yield,6 because it breaks the cation-bridging cross-links which were not relaxed by polar solvent.9,10 Kashimura et al.11 attributed the enhancement of extraction yield with polar solvent to the relaxation of hydrogen bonds among phenolic hydroxyls. The relationship between the extraction yield and coal properties has been previously investigated. Iino et al.4 predicted the extraction yield by CS2/NMP for 56 coals (C% 66−94, daf) by utilizing the amount of volatile matter, oxygen, and sulfur in coals. Okuyama et al.1 and Yoshida et al.7 reported that softening temperature correlated with extraction yield by 1MN for 18 coals (C% 76−91, daf) and with extraction yield by LCO for 13 coals (C% 76−87, daf). Kashimura et al.12 reported that the amount of metal carboxylate correlated with extraction © 2013 American Chemical Society
yield by CMNO for 18 sub-bituminous and brown coals (C% 66−78, daf). Koyano et al.13 compared these solvents by investigating the correlation between extraction yield and coal properties such as ultimate and proximate analysis. When using 1-MN (nonpolar solvent), the extraction yield was influenced by the oxygen content as the solvent could not release hydrogen bonds. When O% daf was less than 8.2% daf, the extraction yield strongly correlated with C/H atomic ratio, but for O% daf higher than 8.2% daf, no correlation was observed. Also, hydrogen bonds were released by polar solvent, depending on the degree of its polarity. Therefore, it was found that as the polarity of solvent became higher, the extraction yield was less influenced by O% daf and was more correlated with C/H atomic ratio, which means the influence of aromaticity increased. Thus, the previous research suggests that the extraction yield of the coals rich in oxygen functional groups that contribute to hydrogen bonds, such as hydroxyl and carboxyl groups, would be improved by polar solvent. Here, the relationship between extraction yield by polar solvent and amount of oxygen functional groups was investigated using oxygen/carbon diagram from Blom’s14 study.
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EXPERIMENTAL SECTION
Samples and Reagents for Thermal Extraction. The coal samples used were six of the Argonne Premium Coal15 and Mulia (MU) coal from AIST Coal Bank.16 The elemental analysis and ash yield of raw coals are shown in Table 1. The particle size of all coals was under 150 μm. All coals were dried for 12 h at 80 °C in a vacuum oven before use. Wyodak (WY) coal was acid-treated with 2-(2methoxyethoxy) acetic acid17 to remove the ion-exchangable cations, and the acid-pretreated WY (WY-MEAA) coal was used for extraction experiment. Polarity of solvent was adjusted by adding IN (0, 10, 20 mass%) to 1-MN. The solvents were supplied by Tokyo Chemical Industry (1-MN) and Wako Pure Chemical Industries (IN). Thermal Extraction. A semibatch extractor was used for solvent extraction. A schematic of the experimental setup is shown in Figure 1. About 0.1 g of sample was put into the sample cell made of SUS316, Received: August 14, 2013 Revised: October 21, 2013 Published: October 21, 2013 6594
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Table 1. Ultimate Analysis and Ash Yield ultimate analysis [daf., mass %]
Pocahontas No. 3 (POC) Upper Freeport (UF) Pittsburgh No. 8 (PIT) Blind Canyon (UT) Illinois No. 6 (IL) Wyodak-Anderson (WY) Mulia (MU)
C
H
N
S
O (diff.)
ash [db., mass %]
91.1
4.4
1.3
0.7
2.5
4.8
85.5
4.7
1.6
2.7
7.5
13.2
83.2
5.3
1.6
2.4
8.8
9.3
80.7 77.7 75.0
5.8 5.0 5.4
1.6 1.4 1.1
0.7 5.7 0.7
11.6 13.5 18.0
4.7 15.5 8.8
68.0
5.7
0.7
0.2
25.4
1.7
Figure 2. Extraction yield (1-MN, 360 °C) versus the oxygen content of coals.13
Figure 1. Schematic of semibatch extractor. and the top and the bottom of the cell were closed with a sintered metallic filter (0.5 μm). The sample cell was purged with N2 and initial pressure and back pressure were set to 1.0 MPa and 1.5 MPa, respectively. After the pressure was set to 1.0 MPa using N2, solvent was injected into the sample cell. When the solvent reached the extract trap, the solvent injection was stopped and the cell was heated up to 360 °C at 18 °C/min. After the temperature reached 360 °C, the solvent injection was restarted at 0.1 mL/min and continued for 60 min. After 60 min, the temperature was cooled down to room temperature. The solid deposit in the extract collected in the extract trap was filtered by vacuum filtration and dried (12 h at 80 °C in a vacuum oven). The residue in the sample cell was ultrasonically cleaned first by toluene followed by acetone and was filtered by vacuum filtration and dried (12 h at 80 °C in a vacuum oven). Extraction yield was obtained by subtracting residue from raw coal using the following eq 1. Extraction Yield mass %, daf=
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(coal residue) × 100 (coal ash)
Figure 3. Extraction yield (1-MN, 360 °C) versus the oxygen content of coals and WY-MEAA (acid-treated WY).
When ionic cross-links were removed from WY coal by acid treatment, the extraction yield was close to MU lignite. Extraction results for 1-MN showed that for majority of coals except POC and WY, the extraction yield decreased with increasing O% daf. The low yield for POC is due to high C/H atomic ratio as explained below. While for WY, the low yield is due to the presence of ionic bonds as explained before. The extraction yield versus C/H atomic ratio of coals is shown in Figure 4. Koyano et al.13 reported a correlation between extraction yield and C/H atomic ratio for KOBE samples (O% daf ≤ 8.2% daf). The extraction yield of POC and UF (O% daf ≤ 8.2% daf) was plotted together with KOBE’s data (O% daf ≤ 8.2% daf). The results show that POC and UF followed the trend as reported by Koyano et al.13 This suggests that POC was strongly influenced by π−π interaction of aromatic rings. Relationship between the extraction yield (polar solvent) and O% daf is shown in Figure 5. Several studies13 have reported that use of polar solvent (IN/1-MN) weakens the effect of oxygen content on extraction yield. For all coals, the extraction yield increased with IN concentration up to 20 mass %. Figure 6 shows that the change in extraction yield as a function of IN concentration for all coals is linear, indicating that the extraction yield of coals is strongly dependent on the solvent polarity. The slope of the regression line (increase in extraction yield/increase in IN concentration) is a measure of the polar
(1)
RESULTS AND DISCUSSION In our previous work, it was reported that the extraction yield by 1-MN was influenced by O% daf. The relationship between O% daf and the extraction yield with 1-MN for several coals tested by Kobe steel, Ltd. (KOBE), is shown in Figure 2.13 There is considerable scattering of the extraction yield, as the solubility of coals in solvent depends on some factors; however, the extraction yield seems to tend to decrease with increasing O% daf. In the same way, measured data from this experiment is shown in Figure 3. In this figure, WY-MEAA is acid-treated WY. It is known that WY coal has large number of metal carboxylate bonds, and the ionic cross-links among metal carboxylates cannot be released by polar solvents such as CMNO.11 Also, for POC, UF, and IL, it was confirmed that there was no influence of ionic cross-links on extraction yield.9 6595
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solvent effectiveness, and the value suggests the degree of hydrogen bonds released by polarity of solvent. Figure 7 shows the relationship between the value of the effectiveness (the slope) and the oxygen content of raw coals.
Figure 4. Extraction yield (1-MN, 360 °C) versus C/H atomic ratio of coals (○ KOBE data;13 ■ our result).
Figure 7. Δ extraction yield/IN concentration as a function of the oxygen content of coals.
The plot is approximated by two trends. Up to around 13% oxygen content (POC, UF, PIT, and UT), the effectiveness increased almost linearly with increasing oxygen content. Above this oxygen content (IL, WY-MEAA and MU), it became a constant of about 0.8. It is important to understand the reason for increase in the effectiveness of polar solvent with increasing oxygen content (except IL, WY-MEAA, and MU) as observed in Figure 7. One reason could be the breaking of hydrogen bonds by polar solvent. Main oxygen functional groups forming hydrogen bond are hydroxyl and carboxyl groups. Kashimura et al.11 carried out the thermal extraction of O-acetylated coal, in which the hydrogen in phenolic hydroxyl was replaced with an acetyl group, by polar solvent CMNO. The extraction yield of the coal was similar to that of O-acetylated coal suggesting that polar solvent released hydrogen bond of phenolic hydroxyl. We estimated the amount of oxygen functional groups (phenolic groups) by using oxygen/carbon diagram from Blom’s14 study. Figure 8 shows the relationship between the
Figure 5. Extraction yield (360 °C) versus the oxygen content of coals. (----) and (−−−) are approximate lines of ● 1-MN and ■ IN20%/1MN, respectively.
Figure 6. Change in the extracted yield (360 °C) versus IN concentration of the mixed solvent. Figure 8. Relationship between the amount of phenolic hydroxyl and oxygen content in coals. 6596
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amount of phenolic hydroxyl and the oxygen content in coals. The trend between the effectiveness of polar solvent and the oxygen content observed in Figure 7 was quite similar to the trend between phenolic hydroxyl and the oxygen content in Figure 8. This result shows the effectiveness of polar solvent and phenolic hydroxyl has some relationship. Figure 9 shows
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Figure 9. Relationship between Δ extraction yield/IN concentration and the content of phenolic hydroxyl in coals.
the relationship between the effectiveness and phenolic hydroxyl in the coals. An excellent correlation coefficient (R2 = 0.93) was obtained, indicating that the effectiveness was mainly dominated by the amount of phenolic hydroxyl. This result suggests that the amount of phenolic hydroxyl released by polar solvent determines the enhancement of extraction yield in this extraction condition.
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CONCLUSION The enhancement of extraction yield with increasing solvent polarity was investigated. The extraction yield increased linearly with IN concentration up to 20 mass % IN concentration. The value of Δ extraction yield/IN concentration (means the effectiveness of polar solvent) increased with increasing oxygen content for coals containing less than around 13% oxygen, while it was constant for coals with oxygen content more than around 13%. The effectiveness of polar solvent increased almost linearly with increasing phenolic group content and an excellent correlation between the effectiveness and the phenolic hydroxyl content in coals was found. Use of polar solvent is recommended to achieve high extraction yield for high oxygen low-rank coals by thermal extraction method.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the New Energy and Industrial Technology Development Organization (NEDO) for their financial support and the staff of the Coal & Energy Project Department of Kobe Steel Ltd. for supplying data and coal samples. 6597
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