Energy Fuels 2010, 24, 1063–1068 Published on Web 10/20/2009
: DOI:10.1021/ef9008482
Effect of Solvent Pretreatments on the Aggregation Behavior of Upper Freeport Coal Hengfu Shui* and Zhicai Wang School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Coal Clean Conversion and Utilization, Anhui University of Technology, Maanshan 243002, Anhui Province, China Received August 5, 2009. Revised Manuscript Received September 28, 2009
Upper Freeport (UF) coal was pretreated with various solvents with or without additive tetracyanoethylene (TCNE), and the residues were further extracted and characterized by Fourier transform infrared (FTIR) and differential scanning calorimetry (DSC) measurements in this study. It was found that pretreatments of UF coal by N-methyl-2-pyrrolidinone (NMP) with TCNE and CS2/NMP mixed solvent (4:1 by volume) gave almost the same extraction yields. However, the two soluble residues RNT and RCN, which were obtained from the extraction of UF coal with NMP þ TCNE and the CS2/NMP mixed solvent (4:1 by volume), respectively, demonstrated quite different extraction yields in the CS2/NMP mixed solvent (1:1 by volume). FTIR indicated that a large amount of the -OH groups was extracted by NMP þ TCNE solvent extraction. An endothermic peak at 350 °C in the difference DSC thermogram of RNT had been observed as that seen in UF raw coal. However, this endothermic peak at 350 °C in the difference DSC thermogram of RCN disappeared. The results show that the aggregation states of the two residues RNT and RCN are different. RNT with a less aggregated state gave a much higher extraction yield in CS2/NMP mixed solvent (1:1 by volume) than that of RCN. The mechanism of the effect of solvent pretreatments on the aggregation behavior of coal was discussed. interactions are believed to affect greatly the physical and chemical properties of coal. In our previous study,3,4 we made three kinds of pyridine insolubles (PIs) from Upper Freeport (UF) coal extracts in the CS2/NMP mixed solvent (1:1 by volume), and the three PIs have different solubility in the mixed solvent. The difference of the three PIs in solubility was attributed to their different aggregation states. The PI with less aggregated structure had larger heat capacity and expansion. Differential scanning calorimetry (DSC) has previously been used for the determination of glass transition temperatures of coal.8,9 Because of its sensitive nature, DSC is an ideal tool for investigating the thermal changes associated with softening as well as glass transition and thus for identifying the temperatures of significant structural changes. It is also one of the powerful tools to characterize the aggregation state of coal molecules at the solid state. Yun et al.10 found that UF coal exhibited a structural relaxation just before the major thermal decomposition process. Takanohashi et al.11 examined the extraction residues associated with different extraction yields from UF coal by using DSC. They found that, for the residues from UF coal with extraction yields lower than 30 wt % (daf), a similar endothermic peak was observed around 350 °C, as seen in the raw coal. While in the case of the high extraction yields, the DSC peak disappeared. They suggested that the
Introduction Coal structural analysis is attracting the interest of coal scientists more and more because it is the basis for coal efficiency and cleaning uses. Coal extraction in organic solvents is one of traditional and useful methods to investigate the structure of coals. However, most of the solvents used gave low extraction yield, although they were mostly Soxhlet extractions carried out at temperatures near the boiling point of the solvent, and chemical reactions of coals, such as decomposition and oxidation, may occur in certain cases. In the 1980s, Iino and co-workers1,2 found that a carbon disulfide-N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent (1:1 by volume) gave more than 60% extraction yields at room temperature for some bituminous coals. It means that the main structure for these bituminous coals is an associated model. This causes the attention of coal scientists about the aggregation behavior of coal. Coals are known to readily aggregate in the bulk or solution state.3-7 Associative *To whom correspondence should be addressed. E-mail: shhf@ ahut.edu.cn. (1) Iino, M.; Takanohashi, T.; Osuga, H.; Toda, K. Extraction of coals with CS2-N-methyl-2-pyrrolidinone mixed solvent at room temperature. Fuel 1988, 67, 1639–1647. (2) Iino, M.; Takanohashi, T.; Obara, S.; Tsueta, H.; Sanokawa, Y. Characterization of the extracts and residues from CS2-N-methyl-2pyrrolidinone mixed solvent. Fuel 1989, 68, 1588–1594. (3) Shui, H.; Norinaga, K.; Iino, M. Effect of tetrabutylammonium acetate addition on the aggregation of coal molecules at solution and solid states. Energy Fuels 2001, 15, 487–491. (4) Shui, H.; Norinaga, K.; Iino, M. Characterizations of aggregation states of coal soluble constituents at solid state. Energy Fuels 2002, 16, 69–73. (5) Shui, H.; Zhou, H. Viscosity and fractal dimension of coal soluble constituents in solution. Fuel Process. Technol. 2004, 85, 1529–1538. (6) Shui, H.; Zhou, H. Kinetic study on the aggregation of coal soluble constituents in solution. Fuel Process. Technol. 2005, 86, 661–671. (7) Shui, H.; Wang, Z. Viscoelastic properties and adsorption behaviors of two coal soluble constituents with different aggregated states. Energy Fuels 2007, 21, 2827–2830. r 2009 American Chemical Society
(8) Lucht, L. M.; Larson, J. M.; Preppas, N. A. Macromolecular structure of coals. 9. Molecular structure and glass transition temperature. Energy Fuels 1987, 1, 56–58. (9) Hall, P. J.; Larsen, J. W. Evidence for low-temperature secondorder phase transitions in coal/solvent systems using differential scanning calorimetry. Energy Fuels 1991, 5, 228–229. (10) Yun, Y.; Suuberg, E. M. New applications of differential scanning calorimetry and solvent swelling for studies of coal structure: Prepyrolysis structural relaxation. Fuel 1993, 72, 1245–1254. (11) Takanohashi, T.; Terao, Y.; Iino, M.; Yun, Y.; Suuberg, E. M. Irreversible structural changes in coals during heating. Energy Fuels 1999, 13, 506–512.
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: DOI:10.1021/ef9008482
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endothermic peak observed at 350 °C for UF coal was likely the result of the relaxation of the network structure (aggregates through the noncovalent bonds) by heating. In our previous study,12 two-step extraction of UF coal, i.e. exhaustive extraction with the NMP solvent and subsequent extraction with the CS2/NMP mixed solvent (1:1 by volume) with or without additive, was compared to the direct extraction of UF coal with the CS2/NMP mixed solvent (1:1 by volume) with or without additive. It was found that there was almost no difference in extraction yields between the two-step extraction and direct extraction with or without additive. The result showed that NMP only gave external extraction to extract the outside fractions of coal particles because of its large viscosity being difficult to enter the inside network structure of coal, and this would not cause the new aggregates formed in the coal molecules. However, no direct evidence was provided to support this conclusion. In this study, UF coal was pretreated with solvents with or without additive tetracyanoethylene (TCNE) and the soluble residues extracted were characterized by DSC measurements. It was found that for the soluble residues from UF coal pretreated using NMP with or without TCNE, a similar endothermic peak was observed around 350 °C, as seen in the raw coal. While for the soluble residue obtained from pretreatment using CS2/NMP mixed solvent (4:1 by volume), the DSC peak disappeared, although its extraction yield was similar to that pretreated using NMP with the addition of TCNE. This provides direct evidence that extraction of NMP for UF coal cannot cause the new aggregation formed in the coal molecules because of its external extraction.
Table 1. Ultimate and Proximate Analyses of UF Coal ultimate analysis (wt %, daf) proximate analysis (wt %, db) sample C UF coal 86.2 a
H 5.1
N 1.9
S 2.2
Oa 4.6
ash 13.1
VM 28.2
FC 58.7
By difference.
Figure 1. Pretreatment procedures of UF coal.
Swelling Measurement. The volumetric swelling ratio of the sample was measured in pyridine according to Larsen et al.13 with a simplified apparatus. A 0.1 g sample of coal particles (under 150 mesh) was placed in a 3 mm inner diameter Pyrex tube and centrifuged for 30 min at 1500 rpm. The height of the coal layer was measured as h0. About 0.6 mL of solvent was added and mixed thoroughly with the sample using a spatula. The sample was again centrifuged, and the height of the coal layer h1 was measured. The mixing and centrifugation were repeated until the height attained a constant equilibrium value. The swelling ratio (Q) was calculated as Q=h1/h0. Fourier Transform Infrared (FTIR) Measurement. Diffuse reflectance FTIR spectra were measured by a JEOL JIR-100 spectrometer at a resolution of 4 cm-1 by co-adding 200 scans. Samples for FTIR measurement were prepared by diluting 5 mg of sample in 200 mg of KBr. The FTIR spectra of the various solvents were measured by a Perkin-Elmer spectrometer using a demountable KBr cell. DSC Measurement. DSC was performed on SHIMADZU DSC-50 equipment. About 8 mg of sample was heated at 8 °C/min to 400 °C. The measurements were carried out under a nitrogen flow of 50 mL/min to keep the cell free of oxygen during measurement. An aluminum sample pan with a cover was used in an unsealed mode.8 After the first scanning, the sample was cooled under the nitrogen flow to room temperature and then the second scanning was made at the same conditions.
Experimental Section Solvent Pretreatment and Extraction of Coal. UF coal, with particle sizes finer than 150 μm, was dried in a vacuum oven at 80 °C for 12 h. The properties of UF coal are shown in Table 1. Dried UF coal was pretreated in solvents with or without TCNE to isolate some light fractions, and the dried soluble residues were extracted in the CS2/NMP mixed solvent (1:1 by volume) with or without TCNE again. The solvent pretreatment and extraction include ultrasonic (38 kHz) irradiation for 30 min at room temperature, centrifugation under 14000 rpm for 50 min, filtration through a membrane paper with a pore size of 0.8 μm, and washing with acetone solvent. The residue was dried in a vacuum oven at 80 °C for 12 h. The amount of TCNE added was 0.1 g per 1 g of coal. The solvents used in this study are NMP, CS2/NMP (1:1 by volume), and CS2/NMP (4:1 by volume) mixed solvents. The extraction yield was calculated on the basis of the weight of the residue. The total extraction yield (Yt) after solvent pretreatments can be calculated as Yt ð%, dbÞ ¼ Y1 þ Y2 ð1 -Y1 Þ ð1Þ
Results and Discussion Effects of Coal Pretreated in Solvents on the Extraction Yield. UF coal was pretreated with three kinds of solvents, i.e., NMP, CS2/NMP (1:1 by volume), and CS2/NMP (4:1 by volume) mixed solvents with or without additive TCNE. The extraction yields (Y1) are shown in Table 2. Table 2 shows that, although the highest extraction yield of UF coal (60.1 wt %, daf) was obtained by the CS2/NMP mixed solvent (1:1 by volume), the extraction yield was only 15.2 wt % (daf) for NMP single solvent. TCNE can obviously increase the UF coal extraction yield. With a small amount of TCNE addition into the NMP solvent, the extraction yield increased to 34.6% (daf), which was similar to the extraction yield in CS2/ NMP mixed solvent (4:1 by volume). These are consistent with the results of Iino et al.14 To understand the effects of UF coal pretreated in the solvents on the extraction yield, the
where Y1 (%, db) is the extraction yield in solvent pretreatment and Y2 (%, db) is the extraction yield of the soluble residue obtained by the solvent pretreatment. The reproducibility of the extraction yield was within (3%. The extraction procedure was described in detail elsewhere.3-5 The solvent pretreatment procedure is shown in Figure 1. UF coal was extracted with CS2/NMP (1:1 by volume), NMP, NMP with additive TCNE, and CS2/NMP mixed solvent (4:1 by volume) to give soluble residues signed as RM, RN, RNT, and RCN, respectively, and the corresponding extraction yields were 47.8, 86.8, 69.9, and 69.2%, respectively, on a dry basis.
(13) Larsen, J. W.; Green, T. K.; Kovac, J. The nature of macromolecular network structure of bituminous coals. J. Org. Chem. 1985, 50, 729–736. (14) Liu, H. T.; Ishizuka, T.; Takanohashi, T.; Iino, M. Effect of TCNE addition on the extraction of coals and solubility of coal extracts. Energy Fuels 1993, 7, 1108–1111.
(12) Shui, H. Effect of coal extracted with NMP on its aggregation. Fuel 2005, 84, 939–941.
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Table 2. Extraction Yields of UF Coal Pretreated in Various Solvents
Table 4. Total Extraction Yields after Various Solvent Pretreatments
extraction yield (wt %) pretreatment solvent CS2/NMP (1:1) NMP NMP with TCNE CS2/NMP (4:1)
solubility parameter (MPa1/2) 21.7 22.9 21.0
db
daf
residue symbol
52.2 13.2 30.1 30.8
60.1 15.2 34.6 35.4
RM RN RNT RCN
mixed solvent (wt %)
ash content in residue (wt %)
pretreatment solvent CS2/NMP (1:1) NMP NMP with TCNE CS2/NMP (4:1)
27.4 15.1 18.7 18.9
mixed solvent with TCNE (wt %)
db
daf
db
daf
52.2 51.4 55.0 41.4
60.1 59.1 63.3 45.3
52.6 73.1 67.5 64.6
60.5 84.1 77.7 74.4
Table 3. Extraction Yields of Residues from Solvent Pretreatments in the CS2/NMP Mixed Solvent (1:1 by Volume) mixed solvent (wt %) sample RM RN RNT RCN
db 44.0 35.7 15.3
mixed solvent with TCNE (wt %)
daf
db
daf
51.8 43.9 18.9
0.8 69.0 53.5 48.9
1.1 81.3 65.8 60.3
extracted soluble residues from the solvent pretreatments of coal were further extracted with the CS2/NMP mixed solvent (1:1 by volume) with or without TCNE, and the extraction yields (Y2) are shown in Table 3. It can be observed from Table 3 that the residue (RN), which was from NMP extraction gives extraction yields of 51.8 and 81.3 wt % (daf) in the CS2/NMP mixed solvent (1:1 by volume) and the CS2/NMP mixed solvent with TCNE, respectively. RNT is the soluble residue of UF coal extracted by the NMP solvent with TCNE, and RCN is the soluble residue of UF coal extracted by the CS2/NMP mixed solvent (4:1 by volume). Although RNT and RCN were the soluble residues extracted from UF coal with a similar extraction yield in different solvents, as shown in Table 2, they gave different extraction yields in the CS2/NMP mixed solvent (1:1 by volume). The lighter constituents influence the aggregation of heavy constituents of coal. It has proven that lighter constituents influence the solubility of heavy constituents of coal.15 When lighter constituents AS (acetone soluble) and PS (pyridine soluble) were removed from the extracts of coal with CS2/NMP mixed solvent, the heavy constituents PI become partially insoluble in CS2/NMP mixed solvent. The lower extraction yield of RCN in the CS2/NMP mixed solvent (1:1 by volume) suggested it had a stronger aggregation structure compared to RNT. This means that removal of lighter constituents by CS2/NMP mixed solvent (4:1 by volume) extraction will cause a stronger aggregation of heavier constituent residue (RCN) compared to that of RNT, which was obtained by extraction of NMP with TCNE, although the extraction yields of UF coal to give the two residues (RNT and RCN) are similar. The difference of the extraction yields between RNT and RCN in the CS2/ NMP mixed solvent (1:1 by volume) with TCNE decreased, as shown in Table 3, suggesting the strong interactions between TCNE and coal molecules.10 Table 4 shows the total extraction yields (Yt) of UF coal after solvent pretreatments. It can be observed that, after pretreatment in NMP solvent with or without TCNE, the total extraction yield of UF coal pretreated was similar to that of UF raw coal, suggesting that this solvent pretreatment did not cause
Figure 2. FTIR spectra of two extracts ENT and ECN, which were extracted by NMP with TCNE and CS2/NMP mixed solvent (4:1 by volume), respectively.
obvious aggregation of coal molecules. However, after pretreatment in CS2/NMP mixed solvent (4:1 by volume), the total extraction yield decreased obviously compared to that of UF raw coal in the CS2/NMP mixed solvent (1:1 by volume), suggesting that extraction with CS2/NMP mixed solvent (4:1 by volume) caused strong aggregation of coal molecules in the residue. Characterization of Residues Extracted in Solvents. The difference of the aggregation structure between the two residues RNT and RCN is interesting. Figure 2 is the FTIR spectra of the two extracts ENT and ECN, which were obtained by extraction with NMP þ TCNE to give RNT and CS2/NMP mixed solvent (4:1 by volume) to give RCN, respectively. Figure 3 shows the FTIR spectra of the two soluble residues RNT and RCN. A broadband centered at 3300 cm-1 assigned as OH-ether-type hydrogen bonds16,17 can be observed from Figure 2 for the extract ENT, with a peak height ratio of aromatic/aliphatic 3030:2920 cm-1 (0.28), compared to the extract ECN, with the peak height ratio of 0.25. The peak at 2200 cm-1 is the -CN group of TCNE,14 suggesting that it was strongly associated with the extract and difficult to be removed by washing with acetone and water mixed solvent (1:4 by volume). This suggested that (16) Cai, M. F.; Smart, R. B. Comparison of seven west Virginia coals with their N-methyl-2-pyrrolidinone-soluble extracts and residues. 1. Diffuse reflectance infrared Fourier transform spectroscopy. Energy Fuels 1994, 8, 369–374. (17) Painter, P. D.; Sobkowiak, M.; Youtcheff, J. FT-i.r. study of hydrogen bonding in coal. Fuel 1987, 66, 973–978.
(15) Takanohaski, T.; Fengjuan, X.; Saito, I.; Sanokawa, Y.; Iino, M. Effect of lighter constituents on the solubility of heavy constituents of coals. Fuel 2000, 79, 955–960.
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Figure 5. DSC thermogram of UF coal residue RCN from CS2/ NMP mixed solvent (4:1 by volume).
Figure 3. FTIR spectra of two residues RNT and RCN, which were extracted by NMP with TCNE and CS2/NMP mixed solvent (4:1 by volume), respectively.
Figure 6. DSC thermogram of UF coal residue RN from NMP extraction.
Figure 6. In DSC thermograms, the second scan thermogram serves as a reference for the first scan thermogram of the same sample, and the resulting subtracted thermogram will be called the difference DSC thermogram (first - second) hereafter. The irreversible heat effect can be observed from the difference DSC thermogram.10 A broad peak was observed in the difference DSC thermogram at 230-280 °C in Figure 4 because of the NMP retained in RNT.11 It is noted that a distinct endothermic peak can be observed in the difference DSC thermogram of RNT in Figure 4. Takanohashi et al.11 found that the DSC thermogram of UF raw coal showed a distinct endothermic transition peak at 350 °C. In the case of low extraction yields, this endothermic peak at 350 °C still existed in the DSC thermograms of residues. They believed that the relaxation of the aggregates of coal molecules by heating or solvent treatment was most likely responsible for the peak. In Figure 6, this endothermic peak at 350 °C was also observed for RN with a lower extraction yield of 15.2 wt % (daf), as shown in Table 2. This indicates that pretreatments of UF coal with NMP single solvent with or without TCNE will cause a mild change of the coal structure. This is consistent with the extraction results mentioned above. After pretreatment of UF coal with NMP with or without TCNE, the total extraction yield in the CS2/NMP mixed solvent (1:1 by volume) was similar to that of UF raw coal. However, it is very interesting that the endothermic peak at 350 °C disappears for RCN, as shown in Figure 5, suggesting that pretreatments of UF coal with CS2/NMP mixed solvent (4:1 by volume) causes a distinct change of the aggregation structure of coal. Although pretreatments of UF
Figure 4. DSC thermogram of UF coal residue RNT from NMP extraction with TCNE.
more -OH groups of UF coal molecules could be extracted into ENT by the NMP solvent with additive TCNE. Figure 3 shows similar FTIR spectra for RNT and RCN, suggesting their similar chemical structure. A little increased intensity of the peak around 3300 cm-1 can be observed for RCN compared to that of RNT, suggesting that more -OH groups were retained in RCN corresponding to less -OH groups to be extracted into ECN. This is also consistent with the solvent-swelling behavior of the two residues RCN and RNT. The swelling ratios in pyridine of RCN and RNT were 1.79 and 1.40, respectively, corresponding to that of the UF raw coal 1.34. The greater content of -OH groups in RCN led to its higher swelling ratio in polar solvent pyridine compared to that of RNT or UF raw coal.18 To further characterize the two residues RNT and RCN, DSC thermograms of RNT and RCN were measured, as shown in Figures 4 and 5, respectively. In addition, the DSC thermogram of RN, which was the soluble residue obtained by extraction of UF coal with NMP single solvent is shown in (18) Shui, H.; Wang, Z.; Cao, M. Effect of pre-swelling of coal on its solvent extraction and liquefaction properties. Fuel 2008, 87, 2908–2913.
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interactions among lighter and heavier constituents, the lighter constituents inside the network of coal are crosslinked as active sites in the coal molecules, similar to a “lock”. Removal of the lighter constituents will make the coal macromolecular chains have more mobility, and structural rearrangement (relaxation) may occur. In addition, some new stronger aggregations are formed because of the opening of the “lock”. For UF coal, -OH groups may mainly exist outside of the network of coal. The enhanced mobility of coal macromolecular chains for RCN because of the opening of the “lock” results in strong aggregates formed by hydrogen bonds because of the large amount of -OH groups retained, as indicated by FTIR spectra (Figure 3). This will result in the decreased extraction yield in the CS2/NMP mixed solvent (1:1 by volume) and disappeared endothermic peak at 350 °C in the difference DSC thermogram for RCN. However, in the case of pretreatment of UF coal using NMP with TCNE, although it gives almost the same extraction yields with CS2/NMP mixed solvent (4:1 by volume), NMP can only give external extraction to extract the outside fractions of coal particles. This will have little effect on the mobility of coal macromolecular chains for RNT and will not cause the new aggregates to form. That is why the total extraction yields for the pretreatments using NMP with or without TCNE and subsequent extraction with CS2/NMP mixed solvent (1:1 by volume) with or without TCNE have almost no change in comparison to that of the direct extraction of UF coal with the CS2/NMP mixed solvent (1:1 by volume) with or without TCNE. In addition, both difference DSC thermograms for RN and RNT appear as endothermic peaks at 350 °C as that observed in UF raw coal.
Figure 7. FTIR spectra of NMP and CS2/NMP mixed solvents with 1:1 and 4:1 by volume ratios.
coal in NMP with TCNE and in CS2/NMP mixed solvent (4:1 by volume) gave almost the same extraction yields, as shown in Table 2, the two residues RNT and RCN obtained had different aggregation states. RCN seems to have a stronger aggregation state compared to RNT. This is reflected by its lower extraction yield in the CS2/NMP mixed solvent (1:1 by volume) (Table 3) compared to that of RNT and the disappeared endothermic peak at 350 °C in the difference DSC thermogram (Figure 4). The missing endothermic peak at 350 °C in Figure 4 suggests that UF coal structure has been relaxed by the pretreatment of the CS2/ NMP mixed solvent (4:1 by volume). Iino et al.1 believed that one of the reasons for the large synergistic effect of the mixed solvent of NMP with CS2 was a decrease in the viscosity from 1.69 (NMP solvent) to 0.615 mPa s (CS2/NMP mixed solvent, 1:1 by volume); thus, the ability of mixed solvent penetration into the network structure of coal increased. We19 also found that there was a strong interaction between CS2 and NMP in the CS2/NMP mixed solvent of the 1:1 by volume ratio. Figure 7 is the FTIR spectra of various solvents. It can be observed from Figure 7 that the absorbance peaks at 2156 and 1508 cm-1 are stronger for the CS2/NMP mixed solvent (1:1 by volume) than that of the CS2/NMP mixed solvent (4:1 by volume) or NMP solvent. This may strongly demonstrate that there is a strong interaction between CS2 and NMP in the mixed solvent of the 1:1 by volume ratio, which makes the strong absorbance at 2156 and 1508 cm-1 in the IR spectra for the CS2/NMP mixed solvent (1:1 by volume). This interaction may disrupt the dipole-based association of NMP, thus causing a lower viscosity of the CS2/NMP mixed solvent and penetrating more quickly into the network structure of coal, resulting in the larger solvent partner (NMP) entering and breaking the stronger coal-coal interactions. NMP cannot enter the inside of the network of coal, especially for UF coal that is believed to be strongly aggregated.4,6 It can only extract the outside of coal particles, giving external extraction, although the addition of TCNE can also increase its extraction yield to 34.6 wt % (daf) for UF coal. However, because the CS2/NMP mixed solvent (4:1 by volume) has lower viscosity, it can enter the inside of the network of coal, giving whole extraction, and much lighter constituents inside the network of coal can be extracted. Because of the strong
Conclusions UF coal pretreated by NMP with TCNE and CS2/NMP mixed solvent (4:1 by volume) gave almost the same extraction yields and produced two soluble residues RNT and RCN, respectively. However, the two soluble residues RNT and RCN showed quite different extraction yields in the CS2/ NMP mixed solvent (1:1 by volume). The different extraction yields for the two soluble residues RNT and RCN were attributed to their different aggregation structure. RNT is a less aggregated state and gave a much higher extraction yield than RCN. CS2/NMP mixed solvent (4:1 by volume) can enter the inside of the network of coal giving whole extraction. The removal of the lighter constituents to be extracted by the CS2/ NMP mixed solvent (4:1 by volume) makes increased mobility of the coal macromolecular chains and leads the structural rearrangement (relaxation) of heavier constituents (residue), resulting in the decreased extraction yield in the CS2/NMP mixed solvent (1:1 by volume) and disappeared endothermic peak at 350 °C in the difference DSC thermogram for RCN. NMP can only give external extraction. This will not cause the new aggregates to form because of the limited mobility of coal macromolecular chains, which was reflected by both difference DSC thermograms for RN and RNT appearing as endothermic peaks at 350 °C as that observed in UF raw coal. Therefore, the total extraction yields for the pretreatments using NMP with or without TCNE of UF coal in the CS2/NMP mixed solvent (1:1 by volume) were similar to that of the direct extraction of UF coal with the CS2/NMP mixed solvent (1:1 by volume) with or without TCNE, respectively.
(19) Shui, H.; Wang, Z.; Gao, J. Examination of the role of CS2 in the CS2/NMP mixed solvents to coal extraction. Fuel Process. Technol. 2006, 87, 185–190.
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Research and Development Program of China (863 Program 2007AA06Z113). Authors are also appreciative for the financial support from the Anhui Provincial Innovative Group for Processing and Clean Utilization of Coal Resource.
Acknowledgment. This work was supported by the Natural Scientific Foundation of China (20876001, 20776001, and 20936007), the International Cooperative Project of Anhui Province (07080703001), and the National High Technology
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