High temperature thermal extraction of xianfeng lignite and FT-IR characterization of its extracts and residues

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 39, Issue 6, June 2011 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2011, 39(6), 401406

RESEARCH PAPER

High temperature thermal extraction of xianfeng lignite and FT-IR characterization of its extracts and residues WANG Zhi-cai*, LI Liang, SHUI Heng-fu, LEI Zhi-ping, REN Shi-biao, KANG Shi-gang, PAN Chun-xiu School of Chemistry and Chemical Engineering, Anhui University of Technology, Anhui Key laboratory of Clean Coal Conversion & Utilization, Ma’anshan 243002, China

Abstract: High-temperature thermal extraction (TE) of Xianfeng lignite by different solvents was carried out, and FT-IR spectra of extracts and residues were determined. The results indicate that Xianfeng lignite shows the macro-molecular network structure by chemical bond cross-linking, in which some low molecular compounds are associated by non-covalent bond interaction. The TE can distinctly increase the extraction yields, which are up to 20.7% and 21.3% at 300oC in toluene and tetralin, respectively. The extracts result from the release of low molecular compounds by thermal rupture of non-covalent bonds at high temperature. Meanwhile, there is no obviously pyrolysis in the process of thermal extraction at 300oC. So the hydrogen donor solvent and hydrogen bond solvent can not increase the thermal extraction yield. The thermal extracts of Xianfeng lignite contain a great of aliphatic alkyl and carbonyl ester, a little of hydroxyl and aromatic structure. The solvent of thermal extraction shows distinctly influences on the constitution and structure of extracts. Keywords: lignite; thermal extraction; FT-IR characterization

Lignite resources are very rich in China, which is about 17% of total coal reserves. As a low rank coal with lots of water and oxygen, lignite has a low calorific value. Further, self-ignition takes place by readsorption when water is removed from lignite. These disadvantages result in a low efficiency and bring about serious environmental problems in the utilization of lignite. In present, the lignite is mainly utilized for electricity generation at power stations located in the proximity of coal mines in China. The effective utilization of lignite has become a very important research subject of cleaning coal conversion techniques. Lignite is usually thought as a suitable feed coal of direct hydro-liquefaction due to low metamorphic grade and high reactivity. However, high hydrogen consumption resulting from lots of oxygen, and high water and ash content in Chinese lignite also increase the operation cost of lignite direct liquefaction. Since Iino et al[1] found that carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent could give 40–65% extraction yield for many bituminous coals at room temperature, the solvent extraction, especially specific solvent extraction, in which a lot of medium and low molecular weight compounds associated in coal matrix were released by the rupture of non-covalent bond interactions, has

become an important technique of coal structure research and cleaning coal conversion[2–8]. In order to produce HyperCoal, Yoshida et al[5,6] studied the TE property of various bituminous coals with several organic solvents at less than 380oC and 1 MPa pressure. Ashless coals with less than 0.1% in ash content were successfully produced. For the high-temperature extraction of Illinois No. 6 coal at 360oC with light cycle oil as extraction solvent, 38% extraction yield was obtained under room filtration but 60% extraction yield under hot filtration at 360oC. Further, crude methylnaphthalene oil gave 80% extraction yield at the same conditions as those of the light cycle oil. Miura et al[7] suggested that the extraction in a flowing stream of tetralin, methylnaphthalene or derived coal liquids under 10 MPa at 200–400oC is effective to separate coal into different molecule size fractions. Although the high-temperature extraction property varied with bituminous coals, the ultimate analysis, the structure and the molecular weight distribution were little different between different extraction fractions from the same coal. Meanwhile, the extraction with a flowing non-polar solvent at less than 200oC could also effectively removal the water from coal and showed better effect of energy saving[8]. Ashida et al[9] found that water could enhance the decomposition of functional groups in

Received: 14-Oct-2010; Revised: 22-Jan-2011 * Corresponding author. Tel: 86-555-2311807; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20876001, 20936007, 21076001) and the Natural Scientific Found of Anhui Province (090414152) and the financial support of innovative group of Anhui University of Technology. Copyright ” 2011, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

WANG Zhi-cai et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 401406

the fractionation of brown coal by sequential high temperature solvent extraction. Therefore, the high-temperature extraction to removal inorganic materials, harmful trace elements, and water in coal can produce the ashless cleaning fuels and the raw materials for preparing high performance carbon materials[7,10,11]. In this paper, the TE property of Xianfeng lignite was investigated by different solvent, such as toluene, methanol etc. in a TE device with hot filtration function. The extracts and extraction residues were characterized by FT-IR spectrum analysis.

1 1.1

Experimental Coal sample

Xianfeng lignite from Yunnan province in China was used. It was ground to 200 mesh, dried at 80oC for 48 h under a vacuum and stored in a desiccator before use. Its proximate and ultimate analyses are shown in Table 1. All solvents used are analytical reagent without further purification. 1.2

High-temperature thermal extraction

Toluene, tetralin (THN), tetrahydrofuran (THF), methanol, and mixed solvent of toluene and methanol (T/M=n, n-volume ratio) were used as extraction solvent. The self-made TE extractor consisted of a 1 L autoclave with a stainless steel filter (0.5 Pm) and pressure storage tank. In a typical experiment, an approximately 30.0 g coal sample together with 300 mL (solvent:coal ca. 10 mL:1 g) were charged into autoclave. The extractor was purged with 99.99% nitrogen gas three times, and finally pressurized to 0.1 MPa at room temperature. Under continuous stirring, the extractor was heated to 300oC at 10oC/min heating rate. After the coal sample was extracted under autogenous pressure at 300oC constant temperature for 3 h, the high pressure valve at the autoclave bottom was promptly opened. The extracts were filtered into a pressure storage tank by the autogenous pressure. The extraction residues in autoclave were washed twice with 200 mL extraction solvent at each time, in which the procedure is same as that of extraction but no constant temperature to be used. The washing mixtures were also filtered into storage tank. Finally, the extraction residues were taken out of the autoclave, dried at 80oC for 48 h under a vacuum and weighted. The extracts were obtained from filtrate by rotary evaporation to remove the solvent, dried at 80oC for 48 h under a vacuum and weighted. The extraction residues and extracts were stored under nitrogen atmosphere before analysis. The extraction yield was calculated from the weight of residues on a dry ash free basis according to the following equation.

Table 1 Proximate and ultimate analysis of Xianfeng lignite Proximate analysis w / % Mad 33.56

Ad 18.45

Vdaf 60.60

Ultimate analysis wdaf / % C 63.07

H 6.01

O* 28.73

N 1.79

S 0.40

*: by difference

Thermal extraction yeild /%

(m1  m2 ) u 100 m1 u (1  Ad )

where Ad (%) is the ash content of the initial coal, and m1 (g) and m2 (g) are the masses of the coal sample and the residues on a dry basis, respectively. 1.3

Characterization of extracts and extraction residues

The FT-IR spectra of lignite, extracts and residues were determined at ambient temperature by Nicolet 6700 FT-IR spectrometer. An approximately 1 mg sample was mixed with 0.1 g KBr, and pressed into a pellet. The measurement conditions were as follows: scanning range, 400–4000 cm–1; number of scans, 32. Their element analyses were carried out at the mode of CHNS by the Vario EL III elementary analyzer. Thermogravimetric measurement of lignite and its residues was carried out using DTG-60 thermogravimetric apparatus. Approximately 10 mg of sample was used. The measurement was made at a heating rate of 10oC/min under a nitrogen flow (99.99%) of 50 mL/min. The equilibrium swelling ratio of lignite and its residues in toluene and NMP solvents were measured by the method of volumetric swelling ratio. A detailed description can be found elsewhere[12]. The equilibrium swelling ratio (Q) was obtained as Q=h2/h1, where h1 and h2 are the height of sample before and after swelling.

2 2.1

Results and discussion High-temperature thermal extraction of lignite

Modern models of coal structure including two-component mode[13], association mode[14], and complex mode[15], all suggest that there is a certain amount of solubles associated in coal matrix structure by non-covalent bonds such as hydrogen bond. The extractable ability of coal not only depends on solvent properties and extraction conditions but also on coal structure, especially the content of solubles associated by non-covalent bonds. Table 2 shows the results of thermal extraction of Xianfeng lignite. The Soxhlet extraction (SE) yield in THF solvent is 8.93%, and lower than 10.5% extraction yield of Shenhua sub-bituminous[12]. Its TE yields at 300oC are in range from 10.2% to 21.3%, varying with the solvents, and higher than the SE yield in THF solvent. Especially, the TE yield in Toluene solvent at 300oC increased 10% compared to the SE yield in THF solvent. It suggests that the rupture of non-covalent bonds at high temperature improved the extractable property of coal.

WANG Zhi-cai et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 401406 Table 2 TE yields of Xianfeng lignite o

Temperature t / C

Solvent

Yield wdaf / %

THF extraction yield* wdaf /%

-

8.93(4.57 *)

Raw Coal (RC) 300

Methanol (M)

10.2

5.22 

300

Toluene/Methanol (T/M)=1

11.8

4.37 

300

Toluene/Methanol (T/M)=3

15.7

5.84 

300

Toluene/Methanol (T/M)=9

17.7

2.26 

300

Toluene

20.7

4.44  3.82

300

THN

21.3

300

THF

12.7

380

Toluene/Methanol (T/M) =3

28.5

4.66 

* SE yield of TE residues

Fig. 1 FT-IR spectra of original and residues of TE and SE RC: raw coal; M, T/M=1, T/M=3, T/M=9, T, THF, THN: TE residues by different solvents; 380: TE residues by T/M=3 at 380oC, THF-S: SE residues of original lignite by THF

Meanwhile, Table 2 shows that the TE yield in hydrogen donor solvent THN is only 21.3%, slight higher than that in toluene solvent and the extraction yield of Morwell coal in a flow solvent at 325oC (20%)[8], suggesting that no obvious hydrogen donor interaction occurred in the TE at 300oC. Since the thermal cracking of covalent bond in coal occurs generally above 350oC[16], the TE at 300oC and the SE in THF solvent only dissolve the compounds with low molecular weight, and do not change the macro-molecular network structure of coal. However, the TE in T/M=3 mixed solvent at 380oC shows 28.5% extraction yield, which increased 13% than that at 300oC. In addition, the TE residues all show 2.26–5.84% SE yield in THF solvent due to the residual of solvent and/or the extracts. Therefore, it is speculated that Xianfeng lignite consists mainly from the macro-molecular network structure cross-linked by covalent bond, in which a certain amount of low molecular weight compounds are associated by non-covalent bonds. There is only the dissolution of the low molecular weight compounds without obvious pyrolysis reactions in the TE at 300oC, but the disruption of covalent bond improved significantly the extraction yield along with further increasing temperature.

Further, Table 2 also shows that the TE yields vary distinctly with solvents. The TE yield in toluene is much higher than those in THF solvent and methanol solvents, and increases with increasing ratio of toluene in T/M mixed solvent, suggesting that the solubles consist of weak polar compounds, which show a low solubility in strong polar solvent, though Xianfeng lignite contains abundant function group containing oxygen. Meanwhile, the improvement of hydrogen bonding solvents such as methanol and THF can not be observed because the high-temperature can also disrupt the non-covalent bond interactions in coal[17]. Therefore, the TE below the pyrolysis temperature of coal proceeds mainly through the relaxation of coal structure to release the low molecular weight compounds, in which hydrogen donor solvent and hydrogen bond solvents did not display the obvious improvement. 2.2

Characterization of extracts

Figure 1 shows the FT-IR spectra of original and residues. There are some pronounced differences in the FT-IR spectra of the residues extracted under different conditions. These differences are embodied not only in the intensity of hydroxyl (3000–3750 cm–1), aliphatic C–H (2800–2950 cm–1), carbonyl (1720–1650 cm–1) and aromatic ring (near to 1600 cm–1) vibration peaks, but also in the shape of carbonyl absorption band. A carbonyl absorption band with multi-peak (1715 cm–1 and 1660 cm–1) is showed in the spectra of TE residues extracted by methanol, T/M=1 mad T/M=3 mixed solvents, but a shoulder peak at 1710 cm–1 can only be observed in the spectra of original lignite and other residues. According to Ref[18], the peaks near to 1710 cm–1 and 1715 cm–1 were attributed to the carbonyl of aromatic ester, and the peak at 1660 cm–1 was attributed to the carbonyl of aromatic ketone, suggesting that there is a selectivity of solvent on the TE of lignite. Figure 2 shows the area ratios of absorption peaks in FT-IR spectra of origin lignite and its extraction residues.

WANG Zhi-cai et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 401406

Fig. 2 Comparison of the intensities of IR characteristic absorption of original lignite and residues of TE and SE

According to the absorption peak area ratios of carbonyl vibration to aromatic ring vibration (ACO/AAr), it is found that the carbonyl group content in the TE residues and the THF SE residues are much lower than that in original lignite, and the carbonyl group content of TE residues is higher than that of SE residues while THF is used as solvent. For the TE residues at 300oC, the carbonyl group contents with methanol, T/M=1 and T/M=3 mixed solvents as extraction solvent are higher than those with other solvents, and the ester group content with methanol as extraction solvent is higher than those with T/M mixed solvents according to their peak area ratios (A1660/A1720). The carbonyl group content in the TE residues at 380oC is the lowest in all residues. According to the absorption peak area ratios of aliphatic C–H vibration to aromatic ring vibration (ACH/AAr), and hydroxyl vibration to aromatic ring vibration (AOH/AAr), the aliphatic structure and hydroxyl group contents in residues are higher than those of original lignite except for the T/M=1 extraction residues, and the aliphatic structure and hydroxyl group contents in the residues extracted by methanol, T/M=1 and T/M=2 solvents are obviously higher than those of other residues. Similarly, the aliphatic structure and hydroxyl group contents in the residues at 380oC are distinctly lower than those at 300oC, and the aliphatic structure content of the TE residues by THF is lower than that of the SE residues but the effect on the hydroxyl group content is inverse. For the residues extracted by T/M mixed solvents, the aliphatic structure and hydroxyl group contents decrease with increasing toluene content of mixed solvent. In addition, there is no significant characteristic functional group difference between the TE residues by THN solvent and other solvents. Above results indicate that the TE removes mainly the compounds containing carbonyl, hydroxyl and aliphatic structure from lignite. The weak polar solvent, such as toluene, and high temperature are favorable for the removal of these compounds, but the hydrogen donor solvent (THN) has no obviously promotion observed. The differences of carbonyl, hydroxyl and aliphatic C–H absorption peaks between the TE residues extracted by methanol, T/M=1 and T/M=3 mixed solvent may be related to the residues of methanol[19], suggesting that there

are the esterification and association of methanol in the TE process. Figure 3 shows the FT-IR spectra of extracts from TE and SE. These extracts display similar spectra, in which some characteristic absorption peaks, such as associated hydroxyl, aliphatic C–H, carbonyl, aromatic ring and methylene (CH2)n•4 (722 cm–1), can all be observed. Meanwhile, they also show the peaks of the bending vibration of methyl and the stretch vibration of C–O with different absorbance at 1376 cm–1 and 1170 cm–1, respectively. However, the TE extracts of Xianfeng lignite only display weak hydroxyl and Ar–H peaks compared with that of Illinois No. 6 coal[6,19]. Above results indicate that the TE extracts of Xianfeng lignite consist mainly of higher aliphatic acid ester containing lots of aliphatic structure and carbonyl functional group, but a minor of hydroxyl and aromatic structure. In addition, the spectra difference between SE extracts and TE extracts with THF as extraction solvent is that the absorption strength of carbonyl peak at 1718 cm–1 is more than that at 1738 cm–1 for former, but the latter is just on the contrary, suggesting that the TE promoted the dissolution of aliphatic acid ester from lignite.

Fig. 3 FT-IR spectra of TE extracts and SE extracts of Xianfeng lignite M, T/M=1, T/M=3, T/M=9, T, THF, THN: TE extracts by different solvents, respectively; 380: TE extracts by T/M=3 at 380oC, THF-S: SE extracts of original lignite by THF

WANG Zhi-cai et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 401406

Fig. 4 Fitting results of multiple peaks of the absorption band 1530–1800 cm–1

In order to investigate the influences of TE conditions on the structure of extracts, the absorption band of carbonyl and aromatic ring skeletal vibration in the range of 1530–1800 cm–1 was fitted and calculated by the fitting technique of multiple peaks of spectrum based on Origin 8.0 software. Fig. 4 shows that the spectrum in the range of 1530–1800 cm–1 can be fitted into three fitting peaks. The weak broad peak near to 1621 cm–1 results from the aromatic ring skeletal vibration, and the carbonyl vibration of aromatic ketone and quinine. It is frequently observed in the FT-IR spectra of coal derivates[18]. The strong broad peak near to 1724 cm–1 can be attributed to the carbonyl absorption peak of aromatic acid ester, carboxylic acid and aliphatic ketone. The peak is wider than the peak of normal carbonyl, suggesting that it arises from above diversified carbonyl. The narrow peak with medium strength at 1739 cm–1 is attributed to the carbonyl of aliphatic acid ester. It can be found from the peak area ratios of TE extracts to THF SE extracts shown in Fig. 5 that the TE extracts by methanol, THN, and T/M mixed solvent have higher ACO/ACH (the ratio of area of carbonyl absorption peak to that of C-H absorption peak) than other extracts, but the TE extracts by toluene solvent shows the lowest ACO/ACH, suggesting that former contains more carbonyl structure, and the latter has less amount of carbonyl. Meanwhile, the carbonyl content of TE extracts with T/M=3 mixed solvent or THF at 380oC are respectively lower than that of the extracts with mixed solvent at 300oC or THF SE extracts. However, AAr/ACH (the absorption peak area ratio of aromatic ring to aliphatic C–H) of some TE extracts, such as the extracts by methanol, THN, T/M=1 and T/M=3 mixed solvents, and the extracts at 380oC, distinctly lower than those by toluene and THF, though the TE extracts by methanol, T/M=1 and T/M=3 mixed solvent, and the extracts at 380oC show obviously lower A1740/A1720 (the peak area ratio of 1740 cm–1 to 1720 cm–1) than other extracts. Especially, the TE extracts by THN solvent show much lower A1740/A1720 than those by toluene and THF. The content of high aliphatic hydrocarbon in TE extracts may be responsible for above results, i.e., the increasing content of aliphatic hydrocarbon results in obviously decrease of AAr/ACH in the TE extracts by toluene and THF. In addition, the AOH/ACH (peak

area ratio of hydroxyl to aliphatic C–H) is the highest for TE extracts by THF, secondary those by T/M=1 and 3 mixed solvents, and the TE extracts by T/M=9 mixed solvent is the lowest. It indicates that the TE extracts also contain certain high aliphatic hydrocarbons besides the carbonyl compounds. For the TE extracts by methanol, T/M=1 and 3 mixed solvent, the compounds containing carbonyl structure, such as carboxylic ester, are primary components but the aliphatic hydrocarbons in these extracts are lower than other extracts. For the TE extracts by toluene and T/M=9 mixed solvent, the aliphatic hydrocarbons are primary component, and the proportion of aromatic acid ester in all ester is higher compared with other TE extracts. The TE and SE extracts by THF show the highest content of aromatic acid ester compared with other extracts. The TE could promote the pyrolysis of aromatic acid ester into gas products, such as CO2[6,9], resulting that the carbonyl and hydroxyl content of residues significantly decrease. Due to the low content of aromatic acid ester in the TE extracts, to increase the extraction temperature hardly effects on the structure of extracts. Therefore, the FT-IR results of TE extracts are agreement with those of its residues. The TE below the pyrolysis temperature of lignite carried out by the dissolution of compounds with low molecular weight. The decreasing polarity of solvent and the increasing toluene content of extraction solvent could promote the dissolution of aromatic acid ester and high alkanes to increase the TE yield. There are obviously decomposition of carbonyl structure and the rupture of weak bonds, so that the TE yield distinctly increases in the extraction process above pyrolysis of lignite. 2.3

Mechanism of high temperature thermal extraction

In order to investigate the TE mechanism, Table 3, the element analysis of TE residues, extracts and original lignite, and Fig. 6, thermogravimetric analysis of TE extract (T/M=3), were carried out. Table 3 shows that the Odaf% of TE extracts markedly reduces, but its Cdaf% and Hdaf% obviously increases in comparison with original lignite. The Odaf% and Hdaf% of TE residues is lower but its Cdaf% is higher than those of original lignite. In addition, the nitrogen and sulphur in lignite exist mainly in the TE residues. It suggests that the compounds with full of hydrogen and less nitrogen and sulphur were removed from the lignite in the TE process. The H/C (1.64) of TE extracts is distinctly higher than that of supercritical extracts of Mequinenza lignite (C/H 0.66–0.84)[20] and the thermal extracts of Mulia lignite (C/H 0.93)[19]. Therefore, the associated components by non-covalent bonding interaction are mainly high aliphatic acid esters for Xianfeng lignite with low degree of coalification, and contain a little of aromatic compounds. Meanwhile, a significant deashing occurs in the TE process, so that the ash content of TE extract is only 0.48%.

WANG Zhi-cai et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 401406

Fig. 5 Ratios of the characteristic absorption peaks area of TE extracts and THF SE extracts Table 3 Element analysis of TE residues, extracts and original lignite wdaf /%

H/C

Ad /%

1.14

18.54

H

O*

N

S

63.07

6.01

28.73

1.79

0.40

Extracts

77.66

10.60

10.04

0.93

0.77

1.64

0.48

Residues

67.59

5.43

23.46

2.45

1.06

0.96

22.00

C RC

*: by difference

Fig. 6 TG curves of the original lignite and the TE residues (T/M=3)

Further, the TG curves of original lignite and TE residues shown in Fig. 6 can divide into three steps clearly. The first step is the volatilization process of water, solvent and some small molecules in lignite or TE residues. For the original lignite, the first step finishes near to 175oC and shows a 10% loss of weight. For the TE residues, this step finishes about 150oC and has a 7% loss of weight. The weightless rate of the TE residues is quicker than that of the original lignite in the first step. It suggests that the removal of small molecular compounds in the TE process reduces the weight loss of residues in first step, which mainly arises from the volatilization of residues solvent. The second step results from the volatilization of compounds with medium molecular weight and the prolysis of coal matrix. Although the weightless rate of original lignite is quicker than that of

residues in this step, in which the finish temperatures of 10% weight loss are respectively 350oC and 375oC, the temperature of maximum weightless rate are all at 430oC. The weight losses of original lignite and TE residues are 37.6% and 30.5%, respectively. It indicates that the macro-molecular network structure of lignite has no change except for the removal of partial associated compounds in the TE process, because only different weight losses were observed in the prolysis process (the second step). The third step is the coking and carbonization of organic matter[21]. In this step, the original lignite and TE residues show very similar TG curves. The swelling property of coal is a reflection of the cross-linking degree of macro-molecular network structure of coal by covalent bond and non-covalent bonding interactions[22]. However, the experiment results show that Xianfeng lignite and its TE residues, including methanol, toluene and T/M mixed solvent, have same swelling ratios (1.2) in NMP solvent, which are lower than that of Shenhua subbitumenous coal (swelling ratio 1.95[12]), and hardly were swelled in toluene solvent (swelling ratio 1.0). It suggests that, on the one hand, there is very high cross-linking degree of Xianfeng lignite macro-molecular network structure, on the other hand the cross-linking degree of matrix had no significant change in the TE process. The removal of extracts not only disrupted the non-covalent bonding and weak covalent binding cross-linking interactions, but also promoted the formation of new non-covalent bonding cross-linking interaction along with the removal of small molecular compounds in the TE process.

3

Conclusions

The thermal extraction below the prolysis temperature of coal mainly dissolved the low molecular weight compounds associated into matrix by non-covalent bonding interactions. In this process, hydrogen donor solvent and hydrogen bonding solvent have no obvious promotion observed. The thermal extraction of Xianfeng lignite at high temperature removed mainly the compounds containing

WANG Zhi-cai et al. / Journal of Fuel Chemistry and Technology, 2011, 39(6): 401406

carbonyl and aliphatic structures. The TE extracts includes mainly aliphatic hydrocarbons and carboxylic acid esters, but the aromatic structures and hydroxyl contents are low in the TE extracts. Strong polar solvent such as methanol is favorable to dissolve aliphatic acid esters existed in lignite, and weak polar aromatic solvent such as toluene is beneficial to the dissolution of aliphatic hydrocarbon and aromatic acid esters. However, there is obvious prolysis of aromatic acid esters observed in the thermal extraction at 380oC. Xianfeng lignite is in formation of the macro-molecular network structure by chemical cross-linking interactions. The content of low molecular weight compounds is low. The TE at 300oC only disrupts the non-covalent bonding interactions to promote the dissolution of low molecular compounds and can not change the macro-molecular network structure.

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