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Effect of solvent extraction pretreatments on the variation of macromolecular structure of low rank coals
JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 46, Issue 7, July 2018 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2018, 46(7), 778786
RESEARCH PAPER
Effect of solvent extraction pretreatments on the variation of macromolecular structure of low rank coals HU Ru-nan, WANG Zhi-cai *, LI Lei, WANG Xiao-ling, PAN Chun-xiu, KANG Shi-gang, REN Shi-biao, LEI Zhi-ping, SHUI Heng-fu School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Clean Coal Conversion & Utilization, Anhui University of Technology, Ma’anshan 243002, China
Abstract:
In order to understand the effects of solvent pretreatment on the inherent macromolecular structure of low rank coal,
Xilinguole lignite (XLL) and Shenfu sub-bituminous coal (SFC) were extracted by tetrahydrofuran (THF) Soxhlet extraction, carbon disulfide/N-methyl-2-pyrrolidone (CS2/NMP) mixed solvent extraction and thermal dissolution, respectively. The extracted coals were characterized by diffuse reflection FT-IR spectroscopy (DRIFT), thermogravimetric analysis (TGA), mercury intrusion method (MI) and swelling ratio determination. The results indicated that the extraction resulted in the arrangement and reassociation of coal inherent macromolecules. THF Soxhlet extraction and CS2/NMP mixed solvent extraction can relax the macromolecular structure of coal to varying degrees by changing the non-covalent bond cross-linking, especially the distribution of hydrogen bond interactions. However, thermal dissolutions at high temperature mainly increased the covalent cross linking of coal macromolecules, especially for XLL. Swelling of all extracted coals was limited by Fickian diffusion, and the extracted coal showed lower swelling activation energy than the corresponding raw coals. Key words:
pretreatment; extraction; macromolecular structure; swelling; diffusion
Coal is an important fossil resource and considered to be complicated heterogeneous mixture[1]. According to the modern coal model, low-rank coal, such as lignite and sub-bituminous coal, is of macromolecular structure cross-linking by covalent and non-covalent bonds[2–5]. The network structure of coal significantly affects its conversions such as liquefaction, pyrolysis, thermal dissolution, etc [1, 6–9]. Swelling and extraction by solvent are not only important approaches to investigate coal structure, but also are used to upgrade coal[10–12]. Therefore, it is very significant in attempting to understand the macromolecular structure of coal by different solvent extractions. Since extraction and swelling can disrupt non-covalent bonds by the interaction of coal-solvent to a certain degree[13], the macromolecular structure and reactivity of coal could be changed. For example, Shin et al[14] reported that the swelling increased the combustion rate of swelling coal. Xie et al[1] confirmed that the swelling by NMP obviously increased the yield of volatile matter of pre-swelling coal pyrolysis. Hu et
al[15] found that the pre-swelling in THF and pyridine solvent could improve the liquefaction reactivity of coal. In our previous work[11,16], it was also found that the pre-swelling of SFC in toluene, NMP and THF increased its liquefaction conversion to varying degrees, and the swelling temperature and solvent had great efforts on the liquefaction behaviors. Further, it was also found that the swelling of coal in solvent could form macropores[10] and enhance the porosity of swelling coal[17], which would reduce the diffusive limitations. Thus, the relaxation of macromolecular networks should have significant impact on the structures and reactivities of swelling coal. Compared to swelling of coal, the solvent extraction was always accompanied with swelling of network[18], and further separated the small molecular extracts, which might further improve the diffusion in coal. Swelling and extraction are always important methods to investigate the structure of coal. Based on the extraction and swelling of solvent, hydrogen bonding in coal was accounted[19,20], the molecular weight between crosslinks was
Received: 25-Apr-2018; Revised: 10-Jun-2018. Foundation items: Supported by the Natural Scientific Foundation of China (21476004, 21476003, 21476002, 51174254). The Natural Science Foundation of Anhui Provincial Education Department (KJ2016A808), the Provincial Innovative Group for Processing & Clean Utilization of Coal Resource and the Innovative Group of Anhui University of Technology. Corresponding author. Tel: + 86 13955530691, Fax: + 86 555 2311552, E-mail: [email protected]. Copyright 2018, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
determined[21], and the concept of associated structure model of coal was proposed[22]. Swelling was also a feasible method to understand the diffusion of solvent in coal particle. By solvent swelling, Otake et al[23] suggested that the diffusion was controlled by relaxation in the coal network structure. However, the effects of swelling on the cross-link structure of coal are very complex due to its heterogeneous constituent. To separate the small molecular mobile phase in coal by extraction is help to investigate the macromolecular structure. Although many methods were used to extract coal, low-temperature extraction was believed to be an effective approach to investigate the macromolecular structure of coal because it can avoid cleavage of covalent bonds. For example, the isometric carbon disulfide/N-methyl-2-pyrrolidone (CS2/NMP) mixed solvent found by Iino et al[13] proved to be a very effective solvent for extraction of bituminous coal at room temperature, but it gave much lower extract yields of lignite because it cannot disrupt the larger portion macromolecular moiety in lignite[24]. In order to obtain more organic matter and produce HyperCoals, a thermal dissolution processes were developed in the past decade[25–27]. The relaxation of coal aggregates induced by solvent and thermal was responsible for high extract yield of thermal dissolution[27]. In our previous work[28, 29], 15.8% of extraction yield was obtained in the thermal dissolution of Xianfeng lignite in toluene/methanol mixed solvent at 300°C, in which there was no obvious pyrolysis to be observed. Therefore, it is necessary for understanding the macromolecular structure of coal by different extractions to obtain varying molecular skeleton structures of coals. In this study, Xilinguole lignite (XLL) and Shenfu sub-bituminous coal (SFC) were treated by tetrahydrofuran (THF) Soxhlet extraction (SE), the extracted coal by CS2/NMP (1:1 by volume) mixed solvent at room temperature (ME), and the thermal dissolved coal by toluene/methanol (3:1 by volume) at 300°C (TE). The swelling properties of treated Table 1
coals and raw coals were characterized by diffuse reflection FT-IR spectroscopy (DRIFT), thermogravimetric analysis (TG), mercury intrusion method (MI) and swelling ratio determination. Further, the effects of pretreatment on the diffusion property of coal were also investigated by the outward diffusion method of probe molecule. Influences of solvent extraction on the macromolecular structure of coal and its performance of swelling and diffusion were discussed.
1 1.1
Experimental Coal and solvents
XLL and SFC were used in this study. These coal as received was ground to particles of a size less than 74 μm, stored under nitrogen atmosphere and dried under vacuum at 80°C overnight before used. Their ultimate and proximate analyses are shown in Table 1. All solvents were commercial pure chemical reagents without further purification. 1.2
Extraction of coal
In Soxhlet extractor, coal was extracted by THF solvent to obtain THF Soxhlet extracted coal. Extraction of coal with the CS2/NMP mixed solvent (1:1 by volume) was carried out at the room temperature as described elsewhere[30]. Thermal dissolution of coal with the toluene/methanol mixed solvent (3:1 by volume) at 300°C was carried out in an autoclave extractor with a stainless steel filter (0.5 m). A detailed description can be found in elsewhere[28]. All extraction yields were determined from the weight of extracted coal on base of dry and free ash. In present work, above extracted coals were abbreviated as SEC, MEC and TEC, respectively. 1.3
Structural characterization
Ultimate and proximate analyses of XLL and SFC
Proximate analysis w/%
Sample
Ultimate analysis wdaf/%
Mad
Ad
Vdaf
C
H
N
S
O*
XLL
15.77
11.01
40.34
62.67
4.83
0.98
0.44
31.08
SFC
7.40
5.58
32.44
75.95
5.18
1.05
0.33
17.49
*: by difference Table 2
Extraction yields of XLL and SFC Yield w/%
Sample SE
ME
TE
XLL
4.4
6.6
11.4
SFC
10.3
14.0
16.9
DRIFT spectra (DRIFTS) of all coals were determined using a DRIFTS chamber (Spectra-Tech collector II) coupled to a Nicolet 6700 FT-IR spectrometer and a DLaTGS detector (Thermo Nicolet Instrument Corporation, USA). The spectrum was collected at a resolution of 4 cm–1 and converted to absorbance form directly by OMNIC 8.0 series software.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
The multi-peak resolution of DRIFTS ranging from 2100 cm–1 to 3750 cm–1 was carried through a curve-fitting method provided by OMNIC 8.0 series software. TG curve of coal was recorded with a Shimadzu TG/DTA/DSC thermogravimetry in 30 mL/min nitrogen at a heating rate of 10°C/min, and the final pyrolysis temperature was 800°C. Porosity and surface areas of coals were determined by mercury intrusion porosimetry (MIP) using Micromeritics AutoPore IV 9500. A stem volume of 1.836 mL was used for this analysis, and samples can be pressurized from 0.51 psi to 33,000 psi. A detailed procedure can be available in elsewhere[31]. 1.4
Determination of swelling and diffusion
The volumetric method as described elsewhere[20] was used to determine the swelling ratio of coal. The swelling ratio (Q) is defined as h1/h0. Here, h0 and h1 are the height of coal column before swelling and after swelling, respectively. Effective diffusivity of solvent in coal particle was roughly determined by the dissolution-out of immersed anthracene as a probe. In detail, the coal firstly was impregnated in the toluene solution of anthracene for 65 h, subsequently washed by hexane and dried under vacuum at 80°C to remove the anthracene-adsorbed on the surface of coal and hexane, respectively. Then obtained coal was immersed in pure toluene to carry out the dissolving-out of probe under vigorous stirring at specified constant temperature. The concentration of anthracene was determined to obtain the dissolving-out curve of probe.
2 2.1
Results and discussion Extraction of XL lignite and SF sub-bituminous coal
Network mode of coal generally includes covalent and non-covalent network structure. According to the viewpoint of Table 3 Sample
Masashi Iino[3], the amount of solvent-soluble components existed in coal, which was the maximum extraction yield without the breaking of covalent bonds, was one of the key points to clarify a kind of bonds forming network. Up to now, the CS2/NMP mixed solvent may be the strongest extraction solvent reported due to its strong dissociation on the non-covalent bonds such as hydrogen bonds, π–π, charge transfer, electrostatic and chain entanglement interactions [3]. The results in Table 2 display that XLL has significantly less extraction yield than SFC regardless of extraction method. For same coal, the extraction yield increases in turn as following order SE, ME and TE. It can be concluded that SFC consists of more extractable components than XLL because of no significant breaking of covalent bonds occurred in the SE and ME process[32]. It may be the rupture of weak covalent and the dehydration of OH by condensation at 300°C, resulting that TE generates higher extraction yield than SE and ME. Compared with bituminous coal, two types of coal show very low ME extraction yield, which is slightly higher than SE extraction yield. Therefore, XLL and SFC mainly consist of covalent bonding macromolecular structure. In order to investigate the composition of macromolecular structure of XLL and SFC, the elemental analysis results of two coals and their extracted coals are listed in Table 3. Due to the lowest extraction yield, the elemental compositions of SEC are approximate to those of raw coal. Decreasing H/C of SEC suggests that the extracts should consist of more aliphatic structure. However, the content of groups containing oxygen of extracts may be lower than that of SEC because of slightly increasing O/C of SEC. Except of increasing heteroatom contents, the elemental compositions of MEC show similar changes to SEC. The increase of nitrogen and sulphur contents indicates that there are obviously remained NMP and CS2 in MEC. Significant decreasing H/C and O/C can be found in two TEC, suggesting that substantial aliphatic structure and groups containing oxygen were removed as extracts by TE.
Elemental analyses results of raw coals and extracted coals Element content wdaf/%
Atomic ratio
N
C
S
H
O*
H/C
O/C
XLL
0.98
62.67
0.44
4.83
31.08
0.93
0.37
SEC(XLL)
1.05
62.05
0.47
4.77
31.66
0.92
0.38
TEC(XLL)
0.95
64.82
0.51
4.65
29.07
0.86
0.34
MEC(XLL)
1.27
61.65
0.62
4.79
31.67
0.93
0.39
SFC
1.05
75.95
0.33
5.18
17.49
0.82
0.17
SEC(SFC)
1.02
76.18
0.32
5.16
17.32
0.81
0.17
TEC(SFC)
1.03
78.13
0.39
4.92
15.53
0.76
0.15
MEC(SFC)
1.18
74.53
0.54
5.01
18.74
0.81
0.19
*: by difference
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
Fig. 1
Fig. 2
DRIFT spectra of raw coals and extracted coals
Changes of intensity and peak position of OH band
2.2 Structural characterization of raw coals and extracted coals Taking the extraction yields in Table 2 into consideration, XLL contains a little of extractable small molecular compounds, but substantial amount of weak covalent bonds, which could be thermally decomposed to improve the extraction yield of TE. Compared with XLL, SFC has more extracts and less weak covalent bonds, so that TE only produces 2.9 % of increment extraction yield relative to ME. It can be concluded that the macromolecular structure of SFC is cross-linked by more non-covalent bonds and less weak covalent bonds compared to XLL. DRIFT spectra of raw coals and extracted coals shown in Figure 1 were determined in order to investigate the influence of extraction on the distribution hydrogen bonded OHs. Firstly, the O–H stretching vibration of all samples displays a strong and wide absorbance band in the range of 3730–2050 cm–1 with a center on 3428 cm–1 (XLL series coals) and 3357 cm–1 (SFC series coals), suggesting that the low rank coal contains a great deal of OHs in form of diverse hydrogen bonding interactions. Secondly, the peak of carbonyl appears at 1703 cm–1 as a shoulder peak of the peak of aromatic ring near to 1615 cm–1 (XLL series coals) or 1606 cm–1 (SFC series coals).
It can be presumed that XLL and SFC contain abundant carbonyl groups. In addition, the carbonyl of NMP can also be observed at 1663 cm–1 in the spectra of two MEC spectra. Further, Figure 2 roughly compared the changes of intensity and maximum of OH band after extraction. All extracted coals except MEC (SFC) have a lower AOH/Aaromatic ring (the area of OH peaks/ the area of aromatic ring peaks) than their raw coal. Meanwhile, OH peaks of all extracted coals appear red-shift relative to corresponding raw coal. It can be concluded that the extractable small molecular components contain more hydroxyl groups than coal macromolecules (extracted coal), and the extraction results in the rearrangement from weak hydrogen bond to strong hydrogen bond to various degrees. TG and DTG data of raw coals and its extracted coals are plotted in Figure 3. Except for two TECs, all extracted coals have very similar TG/DTG curves and approximate total weight loss to their raw coals, suggesting that SE and ME have not changed the macromolecular structure of two coals. In detailed, a weak DTG peak located at 235°C (XLL series coals) and 200°C (SFC series coals) should result from the dehydration between hydrogen bonded OHs. Although two TECs have not the peak of dehydration due to thermal dissolution at high temperature, both TEC (XLL) and TEC (SFC) have a strong DTG peak at 160°C and 180°C, respectively, speculating that it should originate from the volatilization of remained solvent and small molecular extracts. In the range of pyrolytic temperature, XLL and its extracted coals show a very strong DTG peak at about 430°C, but the SFC and its extracted coals except for TEC (SFC) have two DTG peaks near to 446 and 584°C, respectively. Extraction all slightly rises the pyrolytic temperature of extracted coals. Interestingly, TEC (SFC) only displays a pyrolysis peak at 446°C though the thermal dissolution was carried out at 300°C. It might be speculated that some thermal extracts in SFC could provide active hydrogen for the pyrolytic fragment to depress the condensation, but these extracts cannot be removed by SE and ME.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
Fig. 3
TG and DTG of raw coals and extracted coals : raw coal; : SEC; : MEC; : TEC
Table 4 Sample
Total pore area 2
MIP results of raw coals and extracted coals Average pore
1
S/(m g )
diameter d/nm
Bulk density /(gmL1)
Apparent density
Porosity/%
/(gmL1)
XLL
8.26
269
0.8119
1.4792
45.11
SEC(XLL)
9.06
497.5
0.5459
1.4182
61.51
MEC(XLL)
8.01
348.9
0.6336
1.1372
44.29
TEC(XLL)
10.40
404.3
0.5632
1.3807
59.21
SFC
10.77
261.3
0.659
1.228
46.35
SEC(SFC)
7.39
405.2
0.619
1.153
46.33
MEC(SFC)
6.89
447.7
0.638
1.256
49.21
TEC(SFC)
10.2
272.2
0.665
1.236
46.18
Table 5 Swelling solvent
Swelling ratios of raw coals and extracted coals
XLL
SFC
raw coal
MEC
SEC
TEC
raw coal
MEC
SEC
TEC
Toluene
0.90
1.04
0.86
0.92
1.03
1.24
1.06
1.07
Methanol
1.16
1.22
1.09
1.13
1.15
1.13
1.02
1.18
THF
1.21
1.29
1.00
1.30
1.60
1.42
1.31
1.69
Pyridine
1.48
1.32
1.28
1.35
1.70
1.71
1.47
1.58
Mercury intrusion porosimetry (MIP) is a standard method of determining the porosity and surface areas of coals. The results of MIP measurement summarized in Table 4 show that the average pore diameter of all extracted coals is higher than that of corresponding raw coal. It suggests that in the process of extraction, the dissolution of small molecular mobile phase
and the relaxation of macromolecular structure increase the diameter of internal pores. By comparison with XLL, all its extracted coals possess lower extracted bulk density, true density and higher porosity. It suggests that extraction not only expands the pore but also relaxes the molecular structure of XLL.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
Fig. 4
Functions of swelling ratios of raw coal and treated coals as swelling time at 20°C Table 6
Swelling kinetic parameters of raw coals and extracted coals
Sample
Q∞
n
K×100
Ea/(kJ·mol–1)
XLL
1.28–1.81
0.07–0.25
10.0–67.4
27.20
SEC(XLL)
1.41–1.62
0.13–0.16
26.5–48.4
8.04
MEC(XLL)
1.25–1.54
0.15–0.28
14.4–61.5
20.50
TEC(XLL)
1.35–1.50
0.10–0.15
33.9–63.4
8.80
SFC
1.67–1.80
0.04–0.09
55.9–60.9
5.31
SEC(SFC)
1.62–1.70
0.04–0.06
60.9–72.9
2.63
MEC(SFC)
1.66–1.80
0.03–0.06
68.2–87.8
3.54
TEC(SFC)
1.54–1.62
0.04–0.07
45.9–63.1
4.51
It may be remained of NMP and CS2 solvent that MEC (XLL) has the smallest total pore area. By compared to SFC, the bulk density, true density and porosity of its extracted coals have no significant change though all SFC extracted coals have a decreasing total pore area. Substantial non-covalent cross-linking bonds in macromolecular structure of SFC may be responsible for above characteristics because the dissociation of non-covalent bonds in the process of extraction is easy to rearrange the coal structure. 2.3 Swelling and diffusion of raw coals and extracted coals The swelling behavior of coal is considered to be a property of its cross-linked structure, so solvent swelling has been extensively used to investigate the macromolecular network properties of coal[33]. In present study, four types of swelling solvents including toluene, methanol, THF and pyridine were respectively used to determine the equilibrium swelling ratios of XLL, SFC and their extracted coals. The results in Table 5 display that the swelling ratios of XLL and its extracted coals are almost less than that of SFC and its corresponding extracted coals. It indicates that the macromolecule of XLL should be cross-linked by more and stronger bonds including non-covalent and covalent bonds compared to SFC. In toluene and methanol solvent, swelling of all coal samples is very difficult, especially swelling hardly occurs in toluene. It is
easy to understand that the solvent-coal interactions in toluene and methanol are weaker than the coal-coal interactions. As a strong hydrogen bond receptor solvent, THF solvent has a certain swelling ability, especially for SFC series coals. It is also revealed that the weak hydrogen bonds in SFC series coals are more than those in XLL series. Since pyridine, which is a good nucleophilic solvent, can dissociate strong non-covalent interactions in coal, it displays the highest swelling ratio for all coal samples. Under same swelling solvent, the extracted coals almost have lower swelling ratio than their raw coal, especially the swelling ratio of SECs is the lowest in all swelling solvent. It is disclosed that extraction results in the arrangement and reassociation of coal macromolecules in lower free energy conformation. Similar to annealing action, long enough extraction time under mild condition improves the arrangement of coal structure and the formation of strong cross-linking interactions in SE process. It is also unexpectedly found that the swelling ratio of TEC is the closest to raw coal though it was extracted at high temperature. it can be announced that no significant covalent cross-linking bond forms in the TE at 300°C. Further, the swelling kinetic of above coals in pyridine solvent was preliminarily investigated. The swelling ratios at 20°C as a function of swelling time are taken an example as shown in Figure 4, and the main swelling kinetic parameters calculated by the method of Ref.[23] are summarized in Table 6.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
Fig. 5 Table 7
Dissolution-out curves of raw coals and extracted coals
Effective diffusion coefficients of raw coals and extracted coals Sample
De/(m2h1)
Correlation coefficient
–8
0.96
SEC(XLL)
7.0×10–8
0.94
MEC(XLL)
4.2×10–8
0.96
TEC(XLL)
–8
0.98
SFC
5.4×10–8
0.91
SEC(SFC)
5.6×10–8
0.93
MEC(SFC)
7.4×10
–8
0.91
TEC(SFC)
5.7×10–8
0.97
XLL
6.1×10
4.4×10
It is unexpected that all coals except for XLL display very quick swelling, in which the time to achieve 50% swelling is less than 10 min. As a rough guide as to the nature of swelling process, all n values are also less than 0.43, which represents for Fickian diffusion[23]. Except for XLL and its MEC, other coals show very small swelling activation energy. Above obtained kinetic results are approached to the swelling of Illinois 6 coal and SFC in our previous study though these are abnormal by comparison with most of coals [34]. Therefore, it is assumed that the swelling of coals in present study is limited by Fickian diffusion. Due to more and stronger hydrogen bonds, the swelling of XLL shows higher activation energy than those of SFC series. Meanwhile, the swelling activation energy of all extracted coals is also lower than those of corresponding raw coals because the extraction and thermal dissolution can loosen the macromolecular structure of coal at various extents. Since the swelling process of coals studied was speculated as typical Fickian diffusion, a diffusion in-out method[35] was used to determine the effective diffusion coefficient (De). Figure 5 shows the dissolving-out curves of anthracene in all coals. De fitted by the method of Ref.[35] is listed in Table 7. Here, the dissolution method does not take into account the effect of external diffusion, and the coal particles are seemed as spherical. The results show that the dissolving-out curves of anthracene from coal particles can be fitted by the differential
equation of spherical particles. De of all coals studied is in range of (4.2–7.4)×10–8 m2/h, which agrees fairly well with data reported by Ref.[36]. Since the coal hardly swollen in toluene, the relaxation of macromolecular structure of coal can be neglected in the process of dissolving-in and dissolving-out of anthracene, which is mainly limited by the diffusion in the pore of coal particle. Therefore, there is no significant difference between the effective diffusion coefficients.
3
Conclusions
By structural characterization and swelling determination of different extracted coals, it can be concluded that XLL and SFC mainly consisted of covalent bonding macromolecular structure. SFC was cross-linked by more non-covalent bonds and less weak covalent bonds compared to XLL. The extraction resulted in the arrangement and reassociation of coal inherent macromolecules in lower free energy conformation to various degrees, but no significant covalent cross-linking bonds formed in the TE at 300°C. The swelling of coals in present study were limited by Fickian diffusion. In addition, it was also found that the thermal extracts in SFC could provide active hydrogen for the pyrolytic fragment to depress the condensation, but these extracts cannot be removed by SE and ME.
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RESEARCH PAPER
Effect of solvent extraction pretreatments on the variation of macromolecular structure of low rank coals HU Ru-nan, WANG Zhi-cai *, LI Lei, WANG Xiao-ling, PAN Chun-xiu, KANG Shi-gang, REN Shi-biao, LEI Zhi-ping, SHUI Heng-fu School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Clean Coal Conversion & Utilization, Anhui University of Technology, Ma’anshan 243002, China
Abstract:
In order to understand the effects of solvent pretreatment on the inherent macromolecular structure of low rank coal,
Xilinguole lignite (XLL) and Shenfu sub-bituminous coal (SFC) were extracted by tetrahydrofuran (THF) Soxhlet extraction, carbon disulfide/N-methyl-2-pyrrolidone (CS2/NMP) mixed solvent extraction and thermal dissolution, respectively. The extracted coals were characterized by diffuse reflection FT-IR spectroscopy (DRIFT), thermogravimetric analysis (TGA), mercury intrusion method (MI) and swelling ratio determination. The results indicated that the extraction resulted in the arrangement and reassociation of coal inherent macromolecules. THF Soxhlet extraction and CS2/NMP mixed solvent extraction can relax the macromolecular structure of coal to varying degrees by changing the non-covalent bond cross-linking, especially the distribution of hydrogen bond interactions. However, thermal dissolutions at high temperature mainly increased the covalent cross linking of coal macromolecules, especially for XLL. Swelling of all extracted coals was limited by Fickian diffusion, and the extracted coal showed lower swelling activation energy than the corresponding raw coals. Key words:
pretreatment; extraction; macromolecular structure; swelling; diffusion
Coal is an important fossil resource and considered to be complicated heterogeneous mixture[1]. According to the modern coal model, low-rank coal, such as lignite and sub-bituminous coal, is of macromolecular structure cross-linking by covalent and non-covalent bonds[2–5]. The network structure of coal significantly affects its conversions such as liquefaction, pyrolysis, thermal dissolution, etc [1, 6–9]. Swelling and extraction by solvent are not only important approaches to investigate coal structure, but also are used to upgrade coal[10–12]. Therefore, it is very significant in attempting to understand the macromolecular structure of coal by different solvent extractions. Since extraction and swelling can disrupt non-covalent bonds by the interaction of coal-solvent to a certain degree[13], the macromolecular structure and reactivity of coal could be changed. For example, Shin et al[14] reported that the swelling increased the combustion rate of swelling coal. Xie et al[1] confirmed that the swelling by NMP obviously increased the yield of volatile matter of pre-swelling coal pyrolysis. Hu et
al[15] found that the pre-swelling in THF and pyridine solvent could improve the liquefaction reactivity of coal. In our previous work[11,16], it was also found that the pre-swelling of SFC in toluene, NMP and THF increased its liquefaction conversion to varying degrees, and the swelling temperature and solvent had great efforts on the liquefaction behaviors. Further, it was also found that the swelling of coal in solvent could form macropores[10] and enhance the porosity of swelling coal[17], which would reduce the diffusive limitations. Thus, the relaxation of macromolecular networks should have significant impact on the structures and reactivities of swelling coal. Compared to swelling of coal, the solvent extraction was always accompanied with swelling of network[18], and further separated the small molecular extracts, which might further improve the diffusion in coal. Swelling and extraction are always important methods to investigate the structure of coal. Based on the extraction and swelling of solvent, hydrogen bonding in coal was accounted[19,20], the molecular weight between crosslinks was
Received: 25-Apr-2018; Revised: 10-Jun-2018. Foundation items: Supported by the Natural Scientific Foundation of China (21476004, 21476003, 21476002, 51174254). The Natural Science Foundation of Anhui Provincial Education Department (KJ2016A808), the Provincial Innovative Group for Processing & Clean Utilization of Coal Resource and the Innovative Group of Anhui University of Technology. Corresponding author. Tel: + 86 13955530691, Fax: + 86 555 2311552, E-mail: [email protected]. Copyright 2018, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
determined[21], and the concept of associated structure model of coal was proposed[22]. Swelling was also a feasible method to understand the diffusion of solvent in coal particle. By solvent swelling, Otake et al[23] suggested that the diffusion was controlled by relaxation in the coal network structure. However, the effects of swelling on the cross-link structure of coal are very complex due to its heterogeneous constituent. To separate the small molecular mobile phase in coal by extraction is help to investigate the macromolecular structure. Although many methods were used to extract coal, low-temperature extraction was believed to be an effective approach to investigate the macromolecular structure of coal because it can avoid cleavage of covalent bonds. For example, the isometric carbon disulfide/N-methyl-2-pyrrolidone (CS2/NMP) mixed solvent found by Iino et al[13] proved to be a very effective solvent for extraction of bituminous coal at room temperature, but it gave much lower extract yields of lignite because it cannot disrupt the larger portion macromolecular moiety in lignite[24]. In order to obtain more organic matter and produce HyperCoals, a thermal dissolution processes were developed in the past decade[25–27]. The relaxation of coal aggregates induced by solvent and thermal was responsible for high extract yield of thermal dissolution[27]. In our previous work[28, 29], 15.8% of extraction yield was obtained in the thermal dissolution of Xianfeng lignite in toluene/methanol mixed solvent at 300°C, in which there was no obvious pyrolysis to be observed. Therefore, it is necessary for understanding the macromolecular structure of coal by different extractions to obtain varying molecular skeleton structures of coals. In this study, Xilinguole lignite (XLL) and Shenfu sub-bituminous coal (SFC) were treated by tetrahydrofuran (THF) Soxhlet extraction (SE), the extracted coal by CS2/NMP (1:1 by volume) mixed solvent at room temperature (ME), and the thermal dissolved coal by toluene/methanol (3:1 by volume) at 300°C (TE). The swelling properties of treated Table 1
coals and raw coals were characterized by diffuse reflection FT-IR spectroscopy (DRIFT), thermogravimetric analysis (TG), mercury intrusion method (MI) and swelling ratio determination. Further, the effects of pretreatment on the diffusion property of coal were also investigated by the outward diffusion method of probe molecule. Influences of solvent extraction on the macromolecular structure of coal and its performance of swelling and diffusion were discussed.
1 1.1
Experimental Coal and solvents
XLL and SFC were used in this study. These coal as received was ground to particles of a size less than 74 μm, stored under nitrogen atmosphere and dried under vacuum at 80°C overnight before used. Their ultimate and proximate analyses are shown in Table 1. All solvents were commercial pure chemical reagents without further purification. 1.2
Extraction of coal
In Soxhlet extractor, coal was extracted by THF solvent to obtain THF Soxhlet extracted coal. Extraction of coal with the CS2/NMP mixed solvent (1:1 by volume) was carried out at the room temperature as described elsewhere[30]. Thermal dissolution of coal with the toluene/methanol mixed solvent (3:1 by volume) at 300°C was carried out in an autoclave extractor with a stainless steel filter (0.5 m). A detailed description can be found in elsewhere[28]. All extraction yields were determined from the weight of extracted coal on base of dry and free ash. In present work, above extracted coals were abbreviated as SEC, MEC and TEC, respectively. 1.3
Structural characterization
Ultimate and proximate analyses of XLL and SFC
Proximate analysis w/%
Sample
Ultimate analysis wdaf/%
Mad
Ad
Vdaf
C
H
N
S
O*
XLL
15.77
11.01
40.34
62.67
4.83
0.98
0.44
31.08
SFC
7.40
5.58
32.44
75.95
5.18
1.05
0.33
17.49
*: by difference Table 2
Extraction yields of XLL and SFC Yield w/%
Sample SE
ME
TE
XLL
4.4
6.6
11.4
SFC
10.3
14.0
16.9
DRIFT spectra (DRIFTS) of all coals were determined using a DRIFTS chamber (Spectra-Tech collector II) coupled to a Nicolet 6700 FT-IR spectrometer and a DLaTGS detector (Thermo Nicolet Instrument Corporation, USA). The spectrum was collected at a resolution of 4 cm–1 and converted to absorbance form directly by OMNIC 8.0 series software.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
The multi-peak resolution of DRIFTS ranging from 2100 cm–1 to 3750 cm–1 was carried through a curve-fitting method provided by OMNIC 8.0 series software. TG curve of coal was recorded with a Shimadzu TG/DTA/DSC thermogravimetry in 30 mL/min nitrogen at a heating rate of 10°C/min, and the final pyrolysis temperature was 800°C. Porosity and surface areas of coals were determined by mercury intrusion porosimetry (MIP) using Micromeritics AutoPore IV 9500. A stem volume of 1.836 mL was used for this analysis, and samples can be pressurized from 0.51 psi to 33,000 psi. A detailed procedure can be available in elsewhere[31]. 1.4
Determination of swelling and diffusion
The volumetric method as described elsewhere[20] was used to determine the swelling ratio of coal. The swelling ratio (Q) is defined as h1/h0. Here, h0 and h1 are the height of coal column before swelling and after swelling, respectively. Effective diffusivity of solvent in coal particle was roughly determined by the dissolution-out of immersed anthracene as a probe. In detail, the coal firstly was impregnated in the toluene solution of anthracene for 65 h, subsequently washed by hexane and dried under vacuum at 80°C to remove the anthracene-adsorbed on the surface of coal and hexane, respectively. Then obtained coal was immersed in pure toluene to carry out the dissolving-out of probe under vigorous stirring at specified constant temperature. The concentration of anthracene was determined to obtain the dissolving-out curve of probe.
2 2.1
Results and discussion Extraction of XL lignite and SF sub-bituminous coal
Network mode of coal generally includes covalent and non-covalent network structure. According to the viewpoint of Table 3 Sample
Masashi Iino[3], the amount of solvent-soluble components existed in coal, which was the maximum extraction yield without the breaking of covalent bonds, was one of the key points to clarify a kind of bonds forming network. Up to now, the CS2/NMP mixed solvent may be the strongest extraction solvent reported due to its strong dissociation on the non-covalent bonds such as hydrogen bonds, π–π, charge transfer, electrostatic and chain entanglement interactions [3]. The results in Table 2 display that XLL has significantly less extraction yield than SFC regardless of extraction method. For same coal, the extraction yield increases in turn as following order SE, ME and TE. It can be concluded that SFC consists of more extractable components than XLL because of no significant breaking of covalent bonds occurred in the SE and ME process[32]. It may be the rupture of weak covalent and the dehydration of OH by condensation at 300°C, resulting that TE generates higher extraction yield than SE and ME. Compared with bituminous coal, two types of coal show very low ME extraction yield, which is slightly higher than SE extraction yield. Therefore, XLL and SFC mainly consist of covalent bonding macromolecular structure. In order to investigate the composition of macromolecular structure of XLL and SFC, the elemental analysis results of two coals and their extracted coals are listed in Table 3. Due to the lowest extraction yield, the elemental compositions of SEC are approximate to those of raw coal. Decreasing H/C of SEC suggests that the extracts should consist of more aliphatic structure. However, the content of groups containing oxygen of extracts may be lower than that of SEC because of slightly increasing O/C of SEC. Except of increasing heteroatom contents, the elemental compositions of MEC show similar changes to SEC. The increase of nitrogen and sulphur contents indicates that there are obviously remained NMP and CS2 in MEC. Significant decreasing H/C and O/C can be found in two TEC, suggesting that substantial aliphatic structure and groups containing oxygen were removed as extracts by TE.
Elemental analyses results of raw coals and extracted coals Element content wdaf/%
Atomic ratio
N
C
S
H
O*
H/C
O/C
XLL
0.98
62.67
0.44
4.83
31.08
0.93
0.37
SEC(XLL)
1.05
62.05
0.47
4.77
31.66
0.92
0.38
TEC(XLL)
0.95
64.82
0.51
4.65
29.07
0.86
0.34
MEC(XLL)
1.27
61.65
0.62
4.79
31.67
0.93
0.39
SFC
1.05
75.95
0.33
5.18
17.49
0.82
0.17
SEC(SFC)
1.02
76.18
0.32
5.16
17.32
0.81
0.17
TEC(SFC)
1.03
78.13
0.39
4.92
15.53
0.76
0.15
MEC(SFC)
1.18
74.53
0.54
5.01
18.74
0.81
0.19
*: by difference
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
Fig. 1
Fig. 2
DRIFT spectra of raw coals and extracted coals
Changes of intensity and peak position of OH band
2.2 Structural characterization of raw coals and extracted coals Taking the extraction yields in Table 2 into consideration, XLL contains a little of extractable small molecular compounds, but substantial amount of weak covalent bonds, which could be thermally decomposed to improve the extraction yield of TE. Compared with XLL, SFC has more extracts and less weak covalent bonds, so that TE only produces 2.9 % of increment extraction yield relative to ME. It can be concluded that the macromolecular structure of SFC is cross-linked by more non-covalent bonds and less weak covalent bonds compared to XLL. DRIFT spectra of raw coals and extracted coals shown in Figure 1 were determined in order to investigate the influence of extraction on the distribution hydrogen bonded OHs. Firstly, the O–H stretching vibration of all samples displays a strong and wide absorbance band in the range of 3730–2050 cm–1 with a center on 3428 cm–1 (XLL series coals) and 3357 cm–1 (SFC series coals), suggesting that the low rank coal contains a great deal of OHs in form of diverse hydrogen bonding interactions. Secondly, the peak of carbonyl appears at 1703 cm–1 as a shoulder peak of the peak of aromatic ring near to 1615 cm–1 (XLL series coals) or 1606 cm–1 (SFC series coals).
It can be presumed that XLL and SFC contain abundant carbonyl groups. In addition, the carbonyl of NMP can also be observed at 1663 cm–1 in the spectra of two MEC spectra. Further, Figure 2 roughly compared the changes of intensity and maximum of OH band after extraction. All extracted coals except MEC (SFC) have a lower AOH/Aaromatic ring (the area of OH peaks/ the area of aromatic ring peaks) than their raw coal. Meanwhile, OH peaks of all extracted coals appear red-shift relative to corresponding raw coal. It can be concluded that the extractable small molecular components contain more hydroxyl groups than coal macromolecules (extracted coal), and the extraction results in the rearrangement from weak hydrogen bond to strong hydrogen bond to various degrees. TG and DTG data of raw coals and its extracted coals are plotted in Figure 3. Except for two TECs, all extracted coals have very similar TG/DTG curves and approximate total weight loss to their raw coals, suggesting that SE and ME have not changed the macromolecular structure of two coals. In detailed, a weak DTG peak located at 235°C (XLL series coals) and 200°C (SFC series coals) should result from the dehydration between hydrogen bonded OHs. Although two TECs have not the peak of dehydration due to thermal dissolution at high temperature, both TEC (XLL) and TEC (SFC) have a strong DTG peak at 160°C and 180°C, respectively, speculating that it should originate from the volatilization of remained solvent and small molecular extracts. In the range of pyrolytic temperature, XLL and its extracted coals show a very strong DTG peak at about 430°C, but the SFC and its extracted coals except for TEC (SFC) have two DTG peaks near to 446 and 584°C, respectively. Extraction all slightly rises the pyrolytic temperature of extracted coals. Interestingly, TEC (SFC) only displays a pyrolysis peak at 446°C though the thermal dissolution was carried out at 300°C. It might be speculated that some thermal extracts in SFC could provide active hydrogen for the pyrolytic fragment to depress the condensation, but these extracts cannot be removed by SE and ME.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
Fig. 3
TG and DTG of raw coals and extracted coals : raw coal; : SEC; : MEC; : TEC
Table 4 Sample
Total pore area 2
MIP results of raw coals and extracted coals Average pore
1
S/(m g )
diameter d/nm
Bulk density /(gmL1)
Apparent density
Porosity/%
/(gmL1)
XLL
8.26
269
0.8119
1.4792
45.11
SEC(XLL)
9.06
497.5
0.5459
1.4182
61.51
MEC(XLL)
8.01
348.9
0.6336
1.1372
44.29
TEC(XLL)
10.40
404.3
0.5632
1.3807
59.21
SFC
10.77
261.3
0.659
1.228
46.35
SEC(SFC)
7.39
405.2
0.619
1.153
46.33
MEC(SFC)
6.89
447.7
0.638
1.256
49.21
TEC(SFC)
10.2
272.2
0.665
1.236
46.18
Table 5 Swelling solvent
Swelling ratios of raw coals and extracted coals
XLL
SFC
raw coal
MEC
SEC
TEC
raw coal
MEC
SEC
TEC
Toluene
0.90
1.04
0.86
0.92
1.03
1.24
1.06
1.07
Methanol
1.16
1.22
1.09
1.13
1.15
1.13
1.02
1.18
THF
1.21
1.29
1.00
1.30
1.60
1.42
1.31
1.69
Pyridine
1.48
1.32
1.28
1.35
1.70
1.71
1.47
1.58
Mercury intrusion porosimetry (MIP) is a standard method of determining the porosity and surface areas of coals. The results of MIP measurement summarized in Table 4 show that the average pore diameter of all extracted coals is higher than that of corresponding raw coal. It suggests that in the process of extraction, the dissolution of small molecular mobile phase
and the relaxation of macromolecular structure increase the diameter of internal pores. By comparison with XLL, all its extracted coals possess lower extracted bulk density, true density and higher porosity. It suggests that extraction not only expands the pore but also relaxes the molecular structure of XLL.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
Fig. 4
Functions of swelling ratios of raw coal and treated coals as swelling time at 20°C Table 6
Swelling kinetic parameters of raw coals and extracted coals
Sample
Q∞
n
K×100
Ea/(kJ·mol–1)
XLL
1.28–1.81
0.07–0.25
10.0–67.4
27.20
SEC(XLL)
1.41–1.62
0.13–0.16
26.5–48.4
8.04
MEC(XLL)
1.25–1.54
0.15–0.28
14.4–61.5
20.50
TEC(XLL)
1.35–1.50
0.10–0.15
33.9–63.4
8.80
SFC
1.67–1.80
0.04–0.09
55.9–60.9
5.31
SEC(SFC)
1.62–1.70
0.04–0.06
60.9–72.9
2.63
MEC(SFC)
1.66–1.80
0.03–0.06
68.2–87.8
3.54
TEC(SFC)
1.54–1.62
0.04–0.07
45.9–63.1
4.51
It may be remained of NMP and CS2 solvent that MEC (XLL) has the smallest total pore area. By compared to SFC, the bulk density, true density and porosity of its extracted coals have no significant change though all SFC extracted coals have a decreasing total pore area. Substantial non-covalent cross-linking bonds in macromolecular structure of SFC may be responsible for above characteristics because the dissociation of non-covalent bonds in the process of extraction is easy to rearrange the coal structure. 2.3 Swelling and diffusion of raw coals and extracted coals The swelling behavior of coal is considered to be a property of its cross-linked structure, so solvent swelling has been extensively used to investigate the macromolecular network properties of coal[33]. In present study, four types of swelling solvents including toluene, methanol, THF and pyridine were respectively used to determine the equilibrium swelling ratios of XLL, SFC and their extracted coals. The results in Table 5 display that the swelling ratios of XLL and its extracted coals are almost less than that of SFC and its corresponding extracted coals. It indicates that the macromolecule of XLL should be cross-linked by more and stronger bonds including non-covalent and covalent bonds compared to SFC. In toluene and methanol solvent, swelling of all coal samples is very difficult, especially swelling hardly occurs in toluene. It is
easy to understand that the solvent-coal interactions in toluene and methanol are weaker than the coal-coal interactions. As a strong hydrogen bond receptor solvent, THF solvent has a certain swelling ability, especially for SFC series coals. It is also revealed that the weak hydrogen bonds in SFC series coals are more than those in XLL series. Since pyridine, which is a good nucleophilic solvent, can dissociate strong non-covalent interactions in coal, it displays the highest swelling ratio for all coal samples. Under same swelling solvent, the extracted coals almost have lower swelling ratio than their raw coal, especially the swelling ratio of SECs is the lowest in all swelling solvent. It is disclosed that extraction results in the arrangement and reassociation of coal macromolecules in lower free energy conformation. Similar to annealing action, long enough extraction time under mild condition improves the arrangement of coal structure and the formation of strong cross-linking interactions in SE process. It is also unexpectedly found that the swelling ratio of TEC is the closest to raw coal though it was extracted at high temperature. it can be announced that no significant covalent cross-linking bond forms in the TE at 300°C. Further, the swelling kinetic of above coals in pyridine solvent was preliminarily investigated. The swelling ratios at 20°C as a function of swelling time are taken an example as shown in Figure 4, and the main swelling kinetic parameters calculated by the method of Ref.[23] are summarized in Table 6.
HU Ru-nan et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 778786
Fig. 5 Table 7
Dissolution-out curves of raw coals and extracted coals
Effective diffusion coefficients of raw coals and extracted coals Sample
De/(m2h1)
Correlation coefficient
–8
0.96
SEC(XLL)
7.0×10–8
0.94
MEC(XLL)
4.2×10–8
0.96
TEC(XLL)
–8
0.98
SFC
5.4×10–8
0.91
SEC(SFC)
5.6×10–8
0.93
MEC(SFC)
7.4×10
–8
0.91
TEC(SFC)
5.7×10–8
0.97
XLL
6.1×10
4.4×10
It is unexpected that all coals except for XLL display very quick swelling, in which the time to achieve 50% swelling is less than 10 min. As a rough guide as to the nature of swelling process, all n values are also less than 0.43, which represents for Fickian diffusion[23]. Except for XLL and its MEC, other coals show very small swelling activation energy. Above obtained kinetic results are approached to the swelling of Illinois 6 coal and SFC in our previous study though these are abnormal by comparison with most of coals [34]. Therefore, it is assumed that the swelling of coals in present study is limited by Fickian diffusion. Due to more and stronger hydrogen bonds, the swelling of XLL shows higher activation energy than those of SFC series. Meanwhile, the swelling activation energy of all extracted coals is also lower than those of corresponding raw coals because the extraction and thermal dissolution can loosen the macromolecular structure of coal at various extents. Since the swelling process of coals studied was speculated as typical Fickian diffusion, a diffusion in-out method[35] was used to determine the effective diffusion coefficient (De). Figure 5 shows the dissolving-out curves of anthracene in all coals. De fitted by the method of Ref.[35] is listed in Table 7. Here, the dissolution method does not take into account the effect of external diffusion, and the coal particles are seemed as spherical. The results show that the dissolving-out curves of anthracene from coal particles can be fitted by the differential
equation of spherical particles. De of all coals studied is in range of (4.2–7.4)×10–8 m2/h, which agrees fairly well with data reported by Ref.[36]. Since the coal hardly swollen in toluene, the relaxation of macromolecular structure of coal can be neglected in the process of dissolving-in and dissolving-out of anthracene, which is mainly limited by the diffusion in the pore of coal particle. Therefore, there is no significant difference between the effective diffusion coefficients.
3
Conclusions
By structural characterization and swelling determination of different extracted coals, it can be concluded that XLL and SFC mainly consisted of covalent bonding macromolecular structure. SFC was cross-linked by more non-covalent bonds and less weak covalent bonds compared to XLL. The extraction resulted in the arrangement and reassociation of coal inherent macromolecules in lower free energy conformation to various degrees, but no significant covalent cross-linking bonds formed in the TE at 300°C. The swelling of coals in present study were limited by Fickian diffusion. In addition, it was also found that the thermal extracts in SFC could provide active hydrogen for the pyrolytic fragment to depress the condensation, but these extracts cannot be removed by SE and ME.
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