Investigation on compositional and structural features of Xianfeng lignite through sequential thermal dissolution

Fuel Processing Technology 138 (2015) 125–132

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Investigation on compositional and structural features of Xianfeng lignite through sequential thermal dissolution Tie-Min Wang a, Zhi-Min Zong a,⁎, Fang-Jing Liu a, Chang Liu a, Jing-Hui Lv a, Jing Liu a, Dong-Dong Zhang a, Meng Qu a, Juan Gui a, Xiang-Xue Liu a, Xian-Yong Wei a, Zhe-Hao Wei b, Yan Li b a b

Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, China The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA

a r t i c l e

i n f o

Article history: Received 29 January 2015 Received in revised form 13 April 2015 Accepted 24 April 2015 Available online xxxx Keywords: Lignite Thermal dissolution Cyclohexane Hydrocarbons Intermolecular interactions

a b s t r a c t Xianfeng lignite (XL) was subjected to sequential thermal dissolution (TD) in cyclohexane at 200–320 °C. The yields of soluble portions (SPs) decreased with raising temperature from 200 to 260 °C, but then increased with further raising the temperature. The SPs were analyzed with a Fourier transform infrared spectrometer, gas chromatograph/mass spectrometer, and atmospheric pressure solid analysis probe/time-of-flight mass spectrometer (ASAP/TOF-MS). The results show that cyclohexane is effective for thermally extracting inherent hydrocarbons in XL without breaking the covalent bonds. Intermolecular interactions such as hydrogen bonds, π–hydrogen bonds, and π–π interactions can be destroyed at higher temperatures and thereby release arenes and arenols trapped in the capsule structure of XL. According to ASAP/TOF-MS analysis, the molecular mass of organic species in the SPs is smaller than 550 u and SPs from different TD temperatures have different molecular mass distributions. The organic species detected with ASAP/TOF-MS have double bond equivalent values ranging from 0 to 11 and carbon numbers from 3 to 35. A series of arenes and alkylarenols were also confirmed by ASAP/ TOF-MS analysis. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Low-rank coals (LRCs) account for 40% of total coal reserves in China [1]. However, the high moisture content, high ash yield, and low calorific value of LRCs make their conventional use inefficient [2]. Compared to high-rank coals, LRCs, especially lignites, retain the macromolecular structures from the coal-forming plants and contain large amounts of valuable oxygen-containing organic compounds (OCOCs) [3,4]. Thus, non-fuel utilization technologies, such as thermal dissolution (TD), could play an important role in efficiently utilizing lignites. TD is a burgeoning technique and has been widely used in coal chemistry already. Extensive investigations have been performed on TD behavior of different coals in different solvents at different temperatures. Meanwhile, the molecular composition of soluble portions (SPs) from TD should be paid great attention for both value-added utilization and insight into structural features of lignites. The SPs are good raw chemical materials and fuels. Yoshida et al. [5,6] found that the SPs from TD of coals with ash yield less than 200 ppm showed good potential as clean fuel and precursor for producing carbon materials. Compared to traditional coal direct liquefaction technologies under severe conditions, such as CT-5 process in the USSR and H-Coal process in the USA [7], TD can be carried out ⁎ Corresponding author. Tel.: +86 516 83885951; fax: +86 516 83884399. E-mail address: [email protected] (Z.-M. Zong).

http://dx.doi.org/10.1016/j.fuproc.2015.04.029 0378-3820/© 2015 Elsevier B.V. All rights reserved.

under milder conditions with simpler process without the consumption of catalyst and hydrogen. Takanohashi et al. [5,6,8–10] studied TD of coals for producing “HyperCoal” in high boiling-point and high-viscosity solvents, such as tetralin, 1-methylnaphthalene, dimethylnaphthalenes, N-methyl-2pyrrolidinone, light cycle oil, crude methylnaphthalene oil, and coalderived oil. However, the difficulty in recovering the solvents retarded the industrial application of the TD processes. In order to overcome the difficulty, TD of coals using solvents with low-boiling point and low viscosity should be taken into account. Recently, low-boiling point solvents, such as benzene [11,12], methanol [11–13], ethanol [11–13], isopropanol [11], and ethyl acetate [14] were used for TD, providing a wealth of information on the molecular composition of SPs from coals. Benzene has good selectivity for aromatics, but it is a highly poisonous and carcinogenic reagent. The oxygen atom in methanol, ethanol, and isopropanol is nucleophilic, leading to alkanolysis and a lot of OCOCs were produced from the reaction. Ethyl acetate is also a nucleophilic reagent, so esterification and transesterification may occur during TD. The extract yields from coal extraction at room temperature are usually very low [15]. Isometric carbon disulfide/N-methyl-2-pyrrolidinone mixed solvent proved to be powerful for extracting some bituminous coals [16]. However, the strong association of the mixed solvent with organic species in coals leads to difficulty in identifying the organic species [17]. In the present study, we used cyclohexane as the solvent and investigated sequential TD of Xianfeng lignite (XL) at different

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2.3. Sample analyses

Table 1 Proximate and ultimate analyses (wt.%) of XL and XLR. Sample

XL XLR a

Proximate analysis

Ultimate analysis (daf)

Mad

Ad

Vdaf

C

H

N

Oa

33.56

18.45

60.60

63.07 85.96

6.01 3.44

1.79 1.34

N28.73 N8.61

St,d

H/C

0.40 0.65

1.1356 0.4769

By difference.

temperatures. It is worth pointing out that cyclohexane has low boiling, low viscosity, and stable chemical properties and could not react with XL during TD. Therefore, the SPs characterized with different spectrometers, could reflect the compositional and structural features of XL relatively authentically.

2. Experimental 2.1. Materials XL was collected from Xianfeng Coal Mine, Yunnan Province, China and pulverized to pass through a 200-mesh sieve (particle size b 74 μm) followed by desiccation in a vacuum at 80 °C for 24 h before use [18]. Table 1 lists the proximate and ultimate analyses of XL. Cyclohexane is an analytical reagent and was purified by distillation with a Büchi R-134 rotary evaporator prior to use.

2.2. TD of XL As Fig. 1 shows, 4 g XL and 20 mL cyclohexane were put into a 100 mL stainless steel, magnetically stirred autoclave. The air inside the reactor was replaced by nitrogen. The autoclave was heated to 200 °C within 10 min and kept at the temperature for 2 h, and then cooled to room temperature in an ice-water bath. The reaction mixture was taken out from the autoclave followed by ultrasonic extraction for 15 min and separated into filtrate and filter cake by filtration. The filter cake was exhaustively extracted with cyclohexane under ultrasonication until the filtrate was colorless and the concentrated filtrate was named as SP200–1. The extraction was repeated several times to extract organic species from XL as exhaustively as possible and afforded SP200 (including SP 200–1 to SP200 -n ) and XL residue (XLR). Similarly, the sequential TD of XLR was carried out at 240, 280, and 320 °C to afford SP240, SP280, and SP320, respectively. The yield (Y) of SPs was calculated as the mass ratio of the sample (mSP) to XL on a dry and ash-free basis (mXL, daf); i.e., Y = mSPs / mXL, daf.

XL and XLR were characterized with an Ulvac-Riko TGD 7000 thermogravimeter (TG), Hitachi S-3700 N scanning electron microscope (SEM), Nicolet Magna IR-560 Fourier transform infrared (FTIR) spectrometer, and Leco-2000 elemental determinator. The SPs were analyzed with a Hewlett–Packard 6890/5973 gas chromatograph/mass spectrometer (GC/MS), Nicolet Magna IR-560 FTIR spectrometer, and IonSense/Agilent 6210 atmospheric pressure solid analysis probe/time-of-flight mass spectrometer (ASAP/TOFMS) to understand the molecular composition of SPs and to reveal the structural features of XL. FTIR spectra of XL, XLR, and SPs were recorded by collecting 64 scans at a resolution of 8 cm− 1 in reflectance mode with measuring regions of 4000–400 cm− 1. The GC/MS was equipped with a capillary column coated with HP-5MS (cross-link 5% PH ME siloxane, 60 m length, 0.25 mm inner diameter, 0.25 lm film thickness) and a quadrupole analyzer operated in electron impact (70 eV) mode. The capillary column was heated at a rate of 3 °C min− 1 from 60 to 300 °C and held at 300 °C for 10 min. Compounds in the SPs were identified by comparing mass spectra with NIST11 library data. Data acquired were processed using Chemstation software. ASAP/TOF-MS is equipped with an atmosphere pressure chemical ionization ion source. The corona discharge current and capillary voltage were set to 4.0 μA and 4000 V, respectively. The hot nitrogen stream and drying gas temperatures were set to 250 and 350 °C, respectively. The operation was run in positive mode, and the m/z range was set to be from 100 to 1000. Mass spectral data were processed using MassHunter software. Noteworthily, FTIR analysis provides functional group information regarding coals and their soluble species. GC/MS can only detect relatively volatile, thermally stable, and less polar compounds. Some less volatile, thermally labile, and/or polar species can be analyzed with ASAP/TOF-MS. 3. Results and discussion 3.1. TG, SEM, and ultimate analyses of XL and XLR As Fig. 2 exhibits, the pyrolysis rate of XL first increased slowly with raising temperature and then increased rapidly at temperatures higher than 350 °C, which is consistent with a previous report that lignites started decomposing apparently at 350 °C [19]. Then, the pyrolysis rate decreased at temperatures higher than 500 °C and 47.70% of XL remained at 900 °C. For XLR, the thermal decomposition was hardly observed below 400 °C and only 12.66% of XLR was decomposed even at 900 °C. A higher TD temperature (TDT) facilitated the cleavage of covalent bonds in lignites [20] and hence the residual weights of both XL and XLR are inversely related to the TDT.

4 g XL TD at 200 oC

SP200

XLR200

TD at 240 oC

GC/MS, FTIR & ASAP/TOF-MS analyses

SP240

XLR240

TD at 280 oC

SP280

TG, SEM, FTIR & ultimate analyses

XLR280

TD at 320 oC

SP320

XLR320

Fig. 1. Procedure for the TD of XL.

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Due to the thermal lability of oxygen functional groups in lignites, the higher oxygen content in XL than that in XLR (Table 1) makes XL easier to be decomposed than XLR. TD in this investigation aims at obtaining SPs from XL with minimal destruction of of covalent bonds in XL. Therefore, 320 °C was set to be the highest TDT for XL to avoid the thermal decomposition. In addition, macromolecular network structures and ash in XL are relatively thermally stable and thereby could be enriched in XLR. As Fig. 3 exhibits, compared to XL, the particle sizes of XLR are much smaller and looser with some cracks on the surface, implying that TD significantly destroyed XL particles and led to the extraction of SPs [21]. As Table 1 shows, the carbon content increased from 63.07% in XL to 85.96% in XLR, while the hydrogen content decreased from 6.01% to 3.44%, resulting in significant decrease in H/C ratio. Meanwhile, the oxygen content (28.73%) in XL is three times more than that (8.61%) in XLR. These results indicate that OCs riching in aliphatic moieties and oxygen functional groups were easily dissolved out from XL through sequential TD.

XL

50 µm

XLR

50 µm

3.2. Yields of the SPs As shown in Fig. 4, the once-through yields (OTYs) of SPs decreased with raising TDT from 200 to 260 °C and then remarkably increased with further raising TDT. TD of XL could predominantly proceed through dissociation of weak intermolecular interactions in XL at TDTs lower than 260 °C, while the significant dissociation of strong intermolecular interactions and the release of species trapped in capsule structures of XL at higher TDTs increased the OTYs of SPs. The total yield of SPs from sequential TD of XL is 11.02%, indicating that 11.02% of organic matter in XL was extracted into SPs.

Fig. 3. SEM analysis of XL and XLR.

3.3. FTIR analysis of XL, XLR, and SPs As demonstrated in Fig. 5, the absorbances of aliphatic moieties from the SPs around 2924 and 2857 cm−1 are much stronger than those from

XLR

90 80 70 60

XL

Rate of weight loss (%/ oC, db)

50 40 0.00

8

XLR

-0.03

XL

OTY (wt%, daf)

Weigh (wt%, db)

100

XL and XLR, indicating that aliphatic moieties in XL are relatively easily extracted, which was also confirmed by ultimate analysis. The absorbance of aromatic rings around 1596 cm− 1 in SP320 is significantly stronger than that in other SPs, indicating that high TDT facilitates the dissolution of aromatics perhaps due to the thermal release of π–π interactions among the aromatic rings. The absorbances around 1693, 1321, 1266, and 1033 cm− 1 represent oxygen-containing moieties, such as \\C_O, C\\O, and C\\O\\C, indicating that the SPs from XL are rich in OCOCs. Apparent absorbances around 751 and 544 cm−1 in SPs suggest the existence of halogen-containing organic compounds (HCOCs).

-0.06 -0.09 -0.12

6

4

2

-0.15 -0.18 150

300

450 600 750 Temperature (oC)

Fig. 2. TG/DTG curves of XL and XLR.

900

0 180

220

260

300

TDT (oC) Fig. 4. OTYs of SPs at different TDTs.

340

128

T.-M. Wang et al. / Fuel Processing Technology 138 (2015) 125–132

3.4. GC/MS analysis of SPs As exhibited in Figs. S1–S4 and Tables S1–S4, in total, 347 OCs were detected with GC/MS in the SPs, and 149, 100, 152, and 175 OCs were detected in SP200, SP240, SP280, and SP320, respectively, which could be classified into alkanes, alkenes, arenes, OCOCs (including 44 arenols, 18 ketones, 13 esters, 8 alcohols, 8 ethers, 6 furans, 3 aldehydes, 2 alkoxybenzenes, and a tricos-22-enoic acid), nitrogen-containing organic compounds (including pyridines, imidazoles, oxazoles, quinolones, nitriles, amidogen-containing species, and nitrates), sulfurcontaining organic compounds (including thiols, thiophenes, and sulfanes), and two HCOCs (pentadecyl 2-bromoacetate and octadecyl 2-chloropropanoate). As Fig. 6 exhibits, the SPs mainly consist of hydrocarbons, suggesting that cyclohexane is effective for dissolving hydrocarbons. Among the hydrocarbons, arenes are the most abundant in all the SPs. In total, 275 arenes with 1–7 rings were detected in the SPs and the alkyl groups on the aromatic rings are methyl, ethyl, propyl, and isopropyl. The arenes are dominated by condensed arenes with rings ≤ 5 as shown in Fig. 7. Much attention has been paid to the release of condensed arenes during coal combustion and pyrolysis [22–24] because they are mutagenic and carcinogenic. The relative content (RC) of arenes is 58.78, 42.15, 50.95, and 63.14% in SP 200 , SP240, SP280, and SP320, respectively. In addition, 28 alkanes and 21 alkenes appear in the SPs. The alkanes include 20 n-alkanes with carbon numbers ranging from 13 to 22, 5 branched alkanes, and 3 cyclanes. Most of the alkenes are mainly long-chain n-alkenes and enriched into SP240 and SP280, but only one alkene was detected in SP200 and SP320, respectively. TD in cyclohexane proved to be an effective method for selectively extracting hydrocarbons from XL without breaking the covalent bonds and is helpful for understanding the composition of the inherent hydrocarbons in lignites. Noteworthily, arenols are predominant among OCOCs as demonstrated in Table S3. As Fig. 6 displays, the RCs of phenols in SP200 and SP280 are 1.47 and 1.84%, respectively, and no phenols were detected in SP240, while the RC of phenols in SP320 is 13.32%. The hydrogen bonds between arenols and networks in XL could be more easily destroyed at higher TDTs, leading to the release of much more arenols at 320 °C. As Table S3 and Fig. 6 illustrate, both the number and RC of alcohols are less than those of arenols, indicating that the hydroxy groups in XL mainly exist in arenols.

XL SP200

SP240 SP280

SP320

4000

3500

3000

2500 2000 1500 Wavenumbers (cm-1)

Fig. 5. FTIR spectra of the samples.

751

1000

544

1033

1693 1596 1451 1321 1266

2924 2857

XLR

500

Fig. 6. Distribution of group components in SPs at different TDTs.

The “capsule effect” was supposed to be present in coals [17] and solvent extraction could partly destroy the capsule structure to release more SPs from coals. In the same way, the capsule structure could be formed in XL through intermolecular interactions such as hydrogen bonds, π–hydrogen bonds, and π–π interactions. Simultaneously, some small OCs (e.g., arenes and arenols) could be trapped into the capsule, and these OCs can be released when the capsule dressing is significantly destroyed. The OTYs of SPs from XL decreased with raising TDT and then started to increase at 260 °C (Fig. 4), suggesting that 260 °C could be the initial temperature for destroying the capsule dressing. As listed in Table S5, there are lots of OCs, especially arenes and arenols, detected in SP200 or SP280 but not detected in SP240, further proving that the destruction of capsule dressing could play an important role in TD of XL.

3.5. ASAP/TOF-MS analysis of SPs ASAP/TOF-MS has been used for characterizing coal-related compounds and SPs from coals and coal liquefaction residue [25–27]. As displayed in Fig. 8, almost no species with molecular mass N 550 u were detected in all the SPs according to ASAP/TOF-MS analysis. The molecular mass distribution (MMD) of SP200 is mainly from 150 to 270 u, which is much narrower than that either in SP240 or in SP280. The MMD in SP320 shows a normal distribution from 100 to 350 u with a high peak around 200 u. As displayed in Fig. 9, the MMDs of OCs in SP200–SP320 mainly distribute from 150 to 400 u. The RCs of OCs with molecular mass N 250 u in SP240 and SP280 are remarkably higher than those in SP200 and SP320. In addition, no OCs over 400 u were detected in SP320. The base peaks of arenes could be M+ or [M + 1]+ in the positive mode from ASAP/TOF-MS analysis [27]. As illustrated in Fig. 8 and Table 2, some ions with high relative abundance could be assigned to arenes. For instance, the species with m/z 118, 168, 178, 202, 230, and 252 are most likely to be indane, diphenylmethane, anthracene, pyrene, dimethylpyrene, and benzo[pqr]tetraphene, respectively, which is consistent with GC/MS analysis that arenes are the predominant OCs in the SPs. The species with m/z 108 + 14n (n = 0–4) assigned to alkylphenols have high relative abundance in SP320 but are not observed in the mass spectra of other SPs, further indicating that phenols are mainly released at high TDTs. Double bond equivalent (DBE) could be used as a parameter to infer the aromaticity of OCs in heavy carbon resources [28]. As demonstrated in Fig. 10, the DBE of OCs detected by ASAP/TOF-MS ranges from 0 to 11, having carbon numbers (CNs) from 3 to 35. The CNs of OCs detected in SP200 and SP320 are mainly lower than 15, while most of

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129

Fig. 7. RCs of arenes with different aromatic rings.

the OCs with CN ranging from 15 to 35 were mainly concentrated into SP240 and SP280. 4. Conclusions

253.0981

SP200 229.0980

20000 10000

203.0832

168.0580

30000 119.0840

Intensity

40000

178.0759

The sequential TD in cyclohexane is an effective method for understanding the composition of inherent hydrocarbons and intermolecular interactions in XL. The structural and elemental compositions of XLR have a great change compared to XL. The yields of SPs decreased with

raising temperature from 200–260 °C, but then increased with further raising the temperature up to 320 °C. Intermolecular interactions and capsule dressing can be destroyed at higher TDTs to release the species trapped in the XL network. The most abundant SPs are hydrocarbons detected with GC/MS, and some OCs with higher relative abundances consistent with GC/MS analysis were detected with ASAP/TOF-MS in SP320, further proving that the destruction of capsule dressing could play an important role in TD of XL. The results show that TDT has a great effect on the yields and composition of SPs.

2000 1000

252.0931

202.0768

3000

119.0780

Intensity

4000

230.1117

0 SP240

252.0937

9000

SP280

119.0841

Intensity

12000

179.0825

168.0580

0

6000 3000 0

0 100

179.0824 151.1085

150

SP320

165.1145

5000

137.0936

10000

123.0783

15000 109.0634

Intensity

20000

200

250

300

350

400

m/z Fig. 8. Mass spectra of SPs from ASAP/TOF-MS analysis.

450

500

550

130

T.-M. Wang et al. / Fuel Processing Technology 138 (2015) 125–132

Fig. 9. MMD of the OCs identified by ASAP/TOF-MS in SP200–SP320.

Nomenclature ASAP/TOF-MS atmospheric pressure solid analysis probe/time-offlight mass spectrometer CNs carbon numbers DBE double bond equivalent FTIR Fourier transform infrared GC/MS gas chromatograph/mass spectrometer HCOCs halogen-containing organic compounds LRCs low-rank coals MMD molecular mass distribution NCOCs nitrogen-containing organic compounds OCOCs oxygen-containing organic compounds

OCs OOCs OTYs RC SCOCs SEM SPs TD TDT TG XL XLR

organic compounds other organic compounds once-through yields relative content sulfur-containing organic compounds scanning electron microscope soluble portions thermal dissolution thermal dissolution temperature thermogravimeter Xianfeng lignite Xianfeng lignite residue

Table 2 Molecular information of the compounds detected from TD of XL with ASAP/TOF-MS. Name

Molecular mass

Molecular formula

Arenes Indane

118.0783

C9H10

Diphenylmethane

168.0939

C13H12

Anthracene

178.0783

C14H10

Pyrene

202.0783

C16H10

Dimethylpyrene

230.1096

C18H14

Benzo[pqr]tetraphene

252.0939

C20H12

APs Cresols

108.0575

C7H8O

C2-phenol

122.0732

C8H10O

C3-phenol

136.0888

C9H12O

C4-phenol

150.1045

C10H14O

C5-phenol

164.1201

C11H16O

Molecular structure

T.-M. Wang et al. / Fuel Processing Technology 138 (2015) 125–132

131

12 SP200

SP240

SP280

SP320

DBE

9

6

3

0 12

DBE

9

6

3

0 0

5

10

15

20

25

30

CN

35

0

5

10

15

20

25

30

35

CN

Fig. 10. Distributions of DBE versus CN of the OCs identified in SP200–SP320.

Acknowledgments This work was subsidized by National Basic Research Program of China (Grant 2011CB201302), the Fund from Natural Science Foundation of China for Innovative Research Group (Grant 51221462), National Natural Science Foundation of China (Grants 51074153, 21206187, and 21206188), the Key Project of Coal Joint Fund from National Natural Science Foundation of China and Shenhua Group Corporation Limited (Grant 51134021), Strategic Chinese–Japanese Joint Research Program (Grant 2013DFG60060), the Fundamental Research Fund for the Central Universities (China University of Mining & Technology, Grant 2014QNA83), China Postdoctoral Science Foundation funded project (Grant 2014M561730), Jiangsu Postdoctoral Science Foundation funded project (Grant 1401098C), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.fuproc.2015.04.029. References [1] C.X. Pan, X.Y. Wei, H.F. Shui, Z.C. Wang, J. Gao, C. Wei, X.Z. Cao, Z.M. Zong, Investigation of the macromolecular network structure of Xianfeng lignite by a new two-step depolymerization, Fuel 109 (2013) 49–53. [2] P. Nolan, A. Shipman, H. Rui, Coal liquefaction, Shenhua Group, and China's energy security, Eur. Manag. J. 22 (2004) 150–164. [3] P.G. Hatcher, I.A. Breger, N. Szeverenyi, G.E. Maciel, Nuclear magnetic resonance studies of ancient buried wood-II. Observations on the origin of coal from lignite to bituminous coal, Org. Geochem. 4 (1982) 9–18. [4] J. Ralph, D. Catcheside, Action of aerobic microorganisms on the macromolecular fraction of lignite, Fuel 72 (1993) 1679–1686. [5] T. Yoshida, T. Takanohashi, K. Sakanishi, I. Saito, M. Fujita, K. Mashimo, The effect of extraction condition on “HyperCoal” production (1)—under room-temperature filtration, Fuel 81 (2002) 1463–1469. [6] T. Yoshida, C. Li, T. Takanohashi, A. Matsumura, S. Sato, I. Saito, Effect of extraction condition on “HyperCoal” production (2)—effect of polar solvents under hot filtration, Fuel Process. Technol. 86 (2004) 61–72.

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