Differences in molecular composition of soluble organic species in two Chinese sub-bituminous coals with different reducibility

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), 769777

RESEARCH PAPER

Differences in molecular composition of soluble organic species in two Chinese sub-bituminous coals with different reducibility WU Fa-peng1, LU Hao1, YAN Jie1, WANG Rui-yu3, ZHAO Yun-peng1,2,*, WEI Xian-yong1 1

Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116,

China; 2

State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and Ministry Education, Taiyuan

University of Technology, Taiyuan 030024, China; 3

Low Carbon Energy Institute, China University of Mining & Technology, Xuzhou 221008, China

Abstract:

Naomaohu sub-bituminous (NS) with weak reducibility and Buliangou sub-bituminous (BS) with strong reducibility were

extracted in isometric carbon disulfide/acetone mixed solvent to get extracts and extraction residues (ERs). The ERs were thermal dissolved in cyclohexane and methanol to get soluble portions (SPs). The yields of the extracts from NS (E NS) and BS (EBS) are 10.6% and 8.0%, respectively, and the total yields of the SPs from NS and BS at 300°C are 36.3% and 11.5%, respectively, indicating the solubility of the organic species in NS are better than that in BS. Arenes are the dominated compounds both in E NS and EBS. The relative contents of aliphatic hydrocarbons and phenols in the SPs of NS are obvious higher than those of BS. The molecular weight distribution of the compounds in ENS is wider than that in EBS, while the molecular weight distribution of the compounds in the SPs from NS is narrower than that from BS. Key words:

sub-bituminous coals; soluble organic species; molecular composition; thermal dissolution

Thermal dissolution (TD), i.e. thermal extraction, has been widely used to isolate the organic species from low-rank coals, and it is also considered to be a potential process for converting low-rank coals to the feedstock of value-added chemicals and liquid fuel[1–4]. To obtain high extraction yield, low-rank coals were often thermally dissolved with high-boiling point organic solvents, such as 1-methylnaphtealene (1-MN)[5], light cycle oil[6], N-methyl-2-pyryrolidone (NMP)[7], and tetralin[8,9]. Nevertheless, the utilization of high-boiling point organic solvents brought severe difficulties to separate the soluble portions (SPs) of low-rank coals from these solvents for further identifying their component and utilizing them efficiently. Recently, it is found that the sequential thermal dissolution (STD) of low-rank coals in low-boiling point solvents below 320°C can not only efficiently extract the small organic

species of coal, but also bring tremendous convenience to recycle the solvents and investigate the properties of the SPs[10,11]. During STD, inert solvents, such as benzene and cyclohexane, mainly dissolve the organic species in coals connecting to the coal macromolecular network with non-covalent bonds, whereas nucleophilic reagents, such as methanol, easily attack the oxygen-bridged bonds, resulting in more organic species dissolved out from coals [12]. It is reported that the reducibility is also an important effect factor on coal properties as coal rank and maceral[13–15]. Higher O/C atomic ratio in weak reductive coals implies the higher oxygen-bridged bonds in them, which is beneficial to the dissolution of organic species during TD. However, little information about the differences in the molecular composition of low rank coals with different reducibility is reported.

Received: 29-Mar-2018; Revised: 22-Jun-2018. Foundation items: Supported by the National Natural Science Foundation of China (21206188), Open Foundation from State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and Ministry Education (MKX201502), and National Undergraduate Training Programs for Innovation of China University of Mining and Technology (201610290036). Corresponding author. Tel: +86 516 83885951, E-mail: [email protected]. Copyright  2018, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

WU Fa-peng et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 769777 Table 1 Sample

Proximate and ultimate analyses of coal samples

Proximate analysis w/% Mad

ultimate analysis wdaf/%

Ad

Vdaf

C

H

N

S

O

GR,I

Romax/%

Qgr, maf/(MJkg1)

NS

6.36

5.37

44.21

68.51

3.66

1.08

0.57

26.18

0

0.36

25.59

BS

2.01

17.66

35.95

77.50

4.62

1.27

0.84

15.77

0

0.79

23.32



Mad: moisture (air dried base); Ad: ash (dry base, i.e., moisture-free base); Vdaf: volatile matter (dry and ash-free base); : by difference; GR,I: caking index; Romax: average vitrinite reflectance; Qgr, maf: gross calorific value (moist ash-free base)

Fig. 1

SPs yields of ERNS (a) and ERBS (b) during STD in cyclohexane and methanol

Fig. 2

FT-IR spectra of the extracts and the SPs

In this work, two Chinese sub-bituminous with different reducibility were extracted in isometric carbon disulfide/acetone mixed solvent (ICAMS). The extraction residues were sequentially thermal dissolved in cyclohexane and methanol. The composition and structural characteristics of the soluble organic species were characterized with Fourier transform infrared spectroscopy (FT-IR), gas chromatograph/mass spectrometer (GC/MS) and direct analysis in real time/time of flight mass spectrometer (DART/TOF-MS).

1 1.1

Experimental Materials

Naomaohu sub-bituminous coal (NS) was collected from Naomaohu mine in Xinjiang Uygur Autonomous Region,

which was formed in the peat marsh of upper lake delta plain ascribing to a terrestrial sedimentary environment. Its coal seams were frequently exposed to air, and subjected to strong oxidative but weakly reductive effects during coal-forming process, therefore it is a weak reductive coal. Buliangou sub-bituminous coal (BS) was collected from Buliangou mine in Inner Mongolia Autonomous Region, which was formed in the delta lagoon ascribing to a marine and terrestrial facies interaction sedimentary environment. Its coal seams were subjected to strong reductive effects during coal-forming process, therefore it is a strong reductive coal. The proximate and ultimate analyses of the coal samples are shown in Table 1. According to GB/T 57512009, NS and BS belong to long flame coal and non-caking coal, respectively. Based on ASTM D388-2015, NS and BS are classified as sub-bituminous coal A and B.

WU Fa-peng et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 769777 Table 2 Wavenumber /cm

Assignment of adsorption peaks in the FT-IR spectra of the extracts and the SPs[19–22]

–1

Assignment

3415

phenolic –OH groups stretching vibration

2925

asymmetric stretching vibration of aliphatic C–H

2852

symmetric stretching vibration of aliphatic C–H

1612

stretching vibration of aromatic nucleus C=C

1454

asymmetric bending vibration of aliphatic C–H

1373

symmetric bending vibration of aliphatic C–H

1213

asymmetric stretching vibration of C–O

1082

symmetric stretching vibration of C–O

815, 752

out-of-plane deformation vibration of aromatic C–H

dissolved at desired temperature in cyclohexane to get cyclohexane soluble portion (CSP) and cyclohexane insoluble portion (CISP), then the CISP was thermal dissolved at same desired temperature in methanol to get methanol soluble portion (MSP) and methanol insoluble portion (MISP). MISP was dried at 105°C for 24 h in a vacuum oven to produce the TD residue. The CSP and MSP of ERNS at a desired temperature were named as CSPNS,temperature, and MSPNS,temperature, respectively, and the CSP and MSP of ERBS at a desired temperature were named as CSPBS,temperature, and MSPBS,temperature, respectively. Fig. 3

Group component distribution of SPs

Coal samples were crushed to pass through a 200-mesh sieve (particle size < 74 m), followed by desiccation in a vacuum oven at 105°C for 24 h before use. All solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. and are analytical reagents (>99.8%), which were distilled with a Büchi R-134 rotary evaporator prior to use. 1.2

Extraction and thermal dissolution procedure

To dissolve the free organic species embedded in coal macromolecular network structure (CMNS), about 30 g of coal samples were extracted with ICAMS in an ultrasonic bath at room temperature to produce the extracts and the extraction residues. Each run of the extraction was carried out for 2 h in 300 mL of ICAMS, followed by filtration and distillation. Such operations were repeated with fresh ICAMS until few GC/MS-detectable compounds were detected in the filtrate, suggesting that the free organic species in coals were thoroughly dissolved. The extracts of NS and BS were denoted as ENS and EBS, respectively, and the extraction residues of NS and BS were denoted as ERNS and ERBS, respectively. About 2 g of the ERs were sequentially thermal dissolved with cyclohexane and methanol in a 100 mL stainless steel, magnetically stirred autoclave from 210 to 330°C at intervals of 30°C as Ref [10,11]. In brief, the ERs were thermal

1.3

Characterization

FT-IR spectra of the extracts and the SPs were recorded from 4000 to 400 cm–1 at a resolution of 2 cm–1 by an EQUINOX55 spectrometer using the KBr pellet technique. The extracts and the SPs were analyzed with a Hewlett-Packard 7890/5975 GC/MS equipped with a capillary column coated with HP-5 (cross-link 5% PH ME siloxane, 60 m length, 0.25 mm inner diameter, 0.25 m film thickness) and a quadrupole analyzer and operated in electron impact (70 eV) mode. Carrier gas was high-purity helium (99.999%) and the injector mode was injection by hand using micro-syringe with 0.2 L in each run. GC column temperature was raised from 60 to 300°C at a rate of 5°C/min and held at 300°C for 10 min. The mass range scanned was from 30 to 500 amu and compounds were identified by comparing mass spectra to NIST11 library data. A DART (IonSense, Saugus, MA, USA) ion source coupled with a TOF MS (Model G6210, Agilent Technologies, USA) was also used to analyze the extracts and the SPs. The operating conditions of the DART ion source were as follows: positive ion mode; helium flow: 4.0 L/min; discharge needle voltage: 3.0 kV; perforated and grid electrode potentials: +150 and 250V, respectively. The operating conditions of TOF-MS were as follows: cone voltage: +20 V, monitored mass range: m/z 60–1000; acquisition rate: 5 spectra/min; resolving power: approx. 6000 FWHM (full width at half maximum).

WU Fa-peng et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 769777 Table 3

Arenes identified by GC/MS in CSPs and MSPs

Compound

RC/% CSPNS,300

CSPBS,300

MSPNS,300

1.70

12.71

2.94

4.77

m-xylene

5.44

27.46

4.44

2.35

Isopropyl-benzene

0.87

0.48

Propyl-benzene

0.43

Toluene

2.27

Ethyl-benzene

MSPBS,300 21.06

1.34

Ethylmethylbenzene

1.90

0.26

3.01

2.69

1,2,3-trimethyl-benzene

0.46

2.53

7.77

1.71

Indan

1.12

Butyl-benzene

0.18

1-ethyl-2,3-dimethyl-benzene

1.01

But-2-enyl-benzene

0.22 2.48 0.6

But-1-enyl-benzene

0.57

(2-methyl-propenyl)-benzene

0.78

1,2,4,5-tetramethyl-benzene

0.09

0.15

(1-methyl-allyl)-benzene

0.12

3-methyl-1H-indene

0.28

Azulene

0.26

(1-methyl-but-1-enyl)-benzene 0.48

Cyclopentyl-benzene

0.10

2,3-dimethyl-1H-indene

1.49

Methylnaphthalene

0.88

1,8-dimethyl-1,2,3,4-tetrahydro-naphthalene

0.39

Heptyl-benzene

0.44

1,2,3-trimethyl-1H-indene

0.23

1-ethyl-naphthalene

0.09

2,6-dimethyl-naphthalene

1.07

Diphenylmethane

1.49

2.96

2.81

1.61

29.39

41.18

0.58 0.86 0.17

3-methyl-biphenyl

0.22

7-isopropyl-1-methyl-naphthalene

0.17

Decyl-benzene

0.15

4-isopropyl-1,6-dimethyl-naphthalene

0.16

1,4,5,8-tetramethyl-naphthalene

0.4

Phenanthrene

0.65

0.11 0.15

0.15

1-methyl-anthracene

0.40

2-methyl-phenanthrene

The distance between the DART gun exit and mass spectrometer inlet was 10 mm. The sampling glass rod was immersed for 1 s into the samples, and then transferred to the

1.84

0.29

9H-fluorene

Total

1.31

0.14

2-methyl-9H-fluorene

9-methylene-9H-fluorene

1.38

0.80

1,1,4,5,6-pentamethyl-indan Nonyl-benzene

2.97

0.25

Naphthalene

Octyl-benzene

2.97

0.25 23.93

52.36

optimized position in front of the DART gun exit. The sample was then desorbed from the glass rod surface within 30 s, while the spectral data were recorded.

WU Fa-peng et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 769777

Fig. 4

DART/TOF-MS spectra of EBS and ENS

The data were processed using the software of Agilent MassHunter WorkStation Qualitative Analysis (Version B.06.00).

2 2.1

of CSPNS and MSPNS have little change, and the yields of CSPBS and MSPBS sharply decrease with the further increase of temperature. It is reported that the light molecular fragment from CMNS are easily polymerized through various cross-linking reactions into high molecular fragments trapping in residue at high temperature[16]. For both NS and BS, the yields of CSPs are obvious lower than those of MSPs at a desired temperature although cyclohexane was firstly used during STD, which is ascribed to the stronger solubility of methanol to the organic species of coals and more dissociation of oxygen bridge bonds in CMNS during TD in methanol [10]. The SPs yields of NS are obvious higher than those of BS, and the total yield of CSPNS,300 and MSPNS,300 (36.3%) are more than 3 times of the total yield of CSPBS,300 and MSPBS,300 (11.5%), which is possibly ascribed to the higher amount of oxygen bridge bonds in the CMNS of NS than that of BS. 2.2

Results and discussion Yields of extracts and SPs

The yields of ENS and EBS are 10.6% and 8.0%, respectively, indicating both NS and BS contain a certain amount of free organic species embedded in their CMNS. As shown in Figure 1, the yields of the SPs from both NS and BS increase with the increase of TD temperature at below 300°C, while the yields

Fig. 5

FT-IR analyses of extracts and SPs

The FT-IR spectra of the extracts and the SPs are shown in Figure 2 and the corresponding functional groups are listed in Table 2. As shown in Figure 2, the strong absorption peaks assigned to phenolic –OH (3415 cm–1) and aromatic C=C (1612 cm–1) in both ENS and EBS indicate that there possibly exist abundant phenols and other aromatic compounds in the free organic species of NS and BS[17].

Mass spectra of associated molecules in EBS

WU Fa-peng et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 769777

Fig. 6

Mass spectra of associated molecules in ENS

Compared with those of the extracts, the FT-IR spectra of the SPs possess the stronger absorption peaks attributed to aliphatic C–H stretching and bending vibration (2925, 2852, 1454, and 1373 cm–1), indicating that there exist more aliphatic moieties in the SPs than those in the extracts [18]. Obviously, the intensities of absorption peaks assigned to phenolic –OH (3415 cm–1), and C–O (1300–1000 cm–1) from MSPNS,300 and MSPBS,300 are stronger than those from CSPNS,300 and CSPBS,300, suggesting more oxygen bridge bonds cracked during TD in methanol[19,20]. 2.3

GC/MS analyses of extracts and SPs

The compounds in the extracts are simply classified into alkanes, arenes, and oxygen-containing organic compounds (OCOCs). The relative contents (RCs) of alkanes, arenes, and OCOCs in ENS are 3.04%, 82.76%, and 14.20%, respectively, and the RCs of alkanes, arenes, and OCOCs in E BS are 1.75%, 62.36%, and 35.89%, respectively, indicating arenes are the dominated free organic species both in NS and BS. As shown in Figure 3, the compounds in the SPs are classified into alkanes, alkenes, arenes, phenols, alcohols, ketones, ethers, carboxylic acids (CAs), esters, organosulfur compounds (OSCs), and organonitrogen compounds (ONCs). The RCs of alkanes and alkenes in CSPNS,300 and MSPNS,300 are obviously higher than those in CSPBS,300 and MSPBS,300, respectively. The higher RCs of aliphatic hydrocarbons in the

SPs of NS imply that there exists more aliphatic moiety connecting to the CMNS of NS with alkyl ether bonds than those of BS. The RCs of arenes in CSPNS,300 and MSPNS,300 are lower than those in CSPBS,300 and MSPBS,300, respectively. Table 3 shows that the dominated arenes in the SPs are alkyl benzenes and alkyl naphthalenes. Only mononuclear and 2 ring arenes were detected in the SPs of NS, while a few of 3 ring arenes were dissolved in CSPBS,300. The dissolved phenols from coals are mainly ascribed to the dissociation of aryl ether bonds in coals rather than the dissolution of free phenols embedded in the CMNS [21]. The free phenols are easily dissolved out, and the weak aryl ether bonds are easily ruptured in inert solvents, while the strong aryl ether bonds can be dissociated in nucleophilic reagents. The RC of phenols in CSPNS,300 is lower than that in CSPBS,300, while the RC of phenols in MSPNS,300 is higher than that in MSPBS,300, indicating the free phenols or weak aryl ether bonds in NS are less, but strong aryl ether bonds in NS are more than those in BS. The RC of ketones in CSP NS,300 is obviously lower than that in CSPBS,300, while the RC of ketones in MSPNS,300 is higher than that in MSPBS,300. Considering the high oxygen content and the SPs yield of NS, the soluble ketones in NS are more than those in BS, but most of them are insoluble in inert cyclohexane. 2.4

DART/TOF-MS analyses of extracts and SPs

WU Fa-peng et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 769777

Fig. 7

DART/TOF-MS spectra of the SPs obtained at 300°C

DART ion source is a latest commercially ambient desorption ionization technique, which is shown to be efficient for soft ionization of a wide range of both polar and non-polar compounds[22]. DART/TOF-MS requires minimal or no sample treatment and no chromatography separation to obtain the mass spectrum of samples in real time, which was proved as an useful tool in the analysis of biomass oil, Chinese herbal medicine[23], pharmaceuticals[24], and pesticides[25], etc. As depicted in Figure 4, the molecular weight of ENS above 450 Da and the molecular weight of EBS above 350 Da are possibly ascribed to the dimers or multimers of the compound with high polarity in the extracts, which can be proved by the mass spectra (Figure 5 and Figure 6) of associated molecules in EBS and ENS detected using DART/TOF-MS. It was reported that dimers or multimers in the extracts of coals are easily formed through hydrogen bonds,  interactions between aromatic rings, and charge-transfer interactions instead of Waals interactions[26,27]. The wider molecular weight distribution of ENS and more obvious dimers or multimers distribution profile indicate that more free soluble organic species exist in NS, and these organic species possess higher polarity than those in BS. Figure 7 shows that the MSPs possess more number of compounds and wider molecular weight distribution than the CSPs corresponding to the higher yield of MSPs. The molecular weight of the SPs from BS distributes in wider range than that from NS, which in accord with the more ring

number of the aromatic compounds in the SPs of BS than those of NS.

3

Conclusions

There are obvious differences in the molecular composition of the soluble organics species between the two sub-bituminous coals with different reducibility. The total amount of soluble organic species in NS is significantly higher than that in BS. There are more free arenes while less free OCOCs in NS than those in BS. The amount of soluble aliphatic hydrocarbons of NS is obviously higher than that of BS, which is possibly ascribed to the higher amount of alkyl ether bonds in NS than that in BS. Moreover, there exist less weak aryl ether bonds but stronger aryl ether bonds in NS than those in BS. The molecular weight distribution of the free organic species of NS is wider than that of BS, while the molecular weight distribution of the organic species dissociated from the CMNS of NS are narrower than that of BS. These results are benefit for gaining in-depth understanding of the effect of reducibility on the macromolecular structure characteristics of coals.

References [1] Li X, Ashida, R. Miura K. Preparation of high-grade carbonaceous materials having similar chemical and physical

WU Fa-peng et al / Journal of Fuel Chemistry and Technology, 2018, 46(7): 769777 properties from various low-rank coals by degradative solvent extraction. Energy Fuels, 2012, 26(11): 6897–6904. [2] Shui H F, Xu H Y, Zhou Y, Shui T, Pan C X, Wang Z C, Lei Z

[16] Zou X W, Qin T F, Huang L H, Zhang X L, Yang Z, Wang Y. Mechanisms and main regularities of biomass liquefaction with alcoholic solvents. Energy Fuels, 2009, 23(10): 5213–5218.

P, Ren S B, Kang S G, Xu C B. Study on hydro-liquefaction

[17] Tian B, Qiao Y Y, Tian Y Y, Xie K C, Liu Q, Zhou H. FTIR

kinetics of thermal dissolution soluble fraction from Shenfu

study on structural changes of different-rank coals caused by

sub-bituminous coal. Fuel, 2017, 200: 576–582.

single/multiple extraction with cyclohexanone and NMP/CS2

[3] Jonathan P M, Caroline B C, Paul P. Interactions of Illinois No.

mixed solvent. Fuel Process Technol, 2016, 154: 210–218.

6 bituminous coal with solvents: A review of solvent swelling

[18] Canel M, Misirlioglu Z, Canel E, Bozkurt P A. Distribution and

and extraction literature. Energy Fuels, 2015; 29(3): 1279–1294.

comparing of volatile products during slow pyrolysis and

[4] Li X, Dedy E P, Ryuichi A, Kouichi M. Two-stage conversion of low-rank coal or biomass into liquid fuel under mild conditions. Energy Fuels, 2015, 29: 3127–3133. [5] Sönmez Ö, Gözmen B, Cevik T, Giray E S. Optimization of solvent extraction process of some Turkish coals using response surface methodology and production of ash-free coal. Asia-Pac J Chem Eng, 2016, 11(6): 1001–1011. [6] Griffith J M, Clifford C E B, Rudnick L R, Schobert H H. Solvent extraction of bituminous coals using light cycle oil: Characterization of diaromatic products in liquids. Energy Fuels, 2009; 23(9): 4553–4561.

hydropyrolysis of Turkish lignites. Fuel, 2016, 186: 504–517. [19] Xie X, Zhao Y, Qiu P H, Lin D, Qian J, Hou H M, Pei J T. Investigation of the relationship between infrared structured and pyrolysis reactivity of coals with different ranks. Fuel, 2018, 216: 521–530. [20] Song H J, Liu G R, Zhang J Z, Wu J H. Pyrolysis characteristics and kinetics of low rank coals by TG-FTIR method. Fuel Process Technol, 2017, 156: 454–460. [21] Michael S, Thomas A. Pyrolysis studies on the structure of ethers and phenols in coal. Fuel, 1983, 62: 1321–1326. [22] Vaclavik L, Cajka T, Hrbek V, Hajslova J. Ambient mass

[7] Shui H F, Wang Z C, Wang G Q. Effect of hydrothermal

spectrometry employing direct analysis in real time (DART) ion

treatment on the extraction of coal in the CS2/NMP mixed

source for olive oil quality and authenticity assessment. Anal

solvent. Fuel, 2006, 85: 1798–1802. [8] Sönmez Ö, Giraye S. Producing ashless coal extracts by microwave irradiation. Fuel, 2011, 90(6): 2125–2131. [9] Janusz P, Lukasz S. Effects of pressure on hydrogen transfer from tetralin to coal macerals. Energy Fuels, 2005, 19: 348–352.

Chim Acta, 2009, 645(1/2): 56–63. [23] Wang Y, Li C M, Huang L, Liu L, Guo Y L, Ma L, Liu S Y. Rapid identification of traditional Chinese herbal medicine by direct analysis in real time (DART) mass spectrometry. Anal Chim Acta, 2014, 845: 70–76.

[10] Zhao Y P, Xiao J, Ding M, Eddings E G, Wei X Y, Fan X, Zong

[24] Chernetsova E S, Bochkov P O, Ovcharov M V, Zhokhov S S,

Z M. Sequential extraction and thermal dissolution of Baiyinhua

Abramovich R A. DART mass spectrometry: A fast screening of

lignite in isometric CS2/acetone and toluene/methanol binary

solid pharmaceuticals for the presence of an active ingredient, as

solvents. Energy Fuels, 2016, 30(1): 47–53.

an alternative for IR spectroscopy. Drug Test Anal, 2010, 2(5/6):

[11] Ding M, Zhao Y P, Dou Y Q, Wei X Y, Fan X, Cao J P, Wang Y L, Zong Z M. Sequential extraction and thermal dissolution of Shengli lignite. Fuel Process Technol, 2015, 135: 20–24.

292–294. [25] Crawford E, Musselman B. Evaluating a direct swabbing method for screening pesticides on fruit and vegetable surfaces

[12] Lu H Y, Wei X Y, Yu R, Peng Y L, Qi X Z, Qie L M, Wei Q,

using direct analysis in real time (DART) coupled to an exactive

Lv J, Zong Z M, Zhao W, Zhao Y P, Ni Z H, Wu L. Sequential

benchtop orbitrap mass spectrometer. Anal Bioanal Chem, 2012,

thermal dissolution of Huolinguole lignite in methanol and ethanol. Energy Fuels, 2011, 25(6): 2741–2745.

403(10): 2807–2812. [26] Fan X, Wang C F, You C Y, Wei X Y, Chen L, Cao J P, Zhao Y

[13] Zhao Y P, Hu H Q, Jin L J, Wu B, Zhu S W. Pyrolysis behavior

P, Zhao W, Wang Y G, Lu J L. Characterization of a Chinese

of weakly reductive coals from Northwest China. Energy Fuels,

lignite and the corresponding derivatives using direct analysis in

2009, 23: 870–875.

real time quadrupole time-of-flight mass spectrometry. RSC

[14] Wu B Hu H Q, Huang S P, Fang Y M, Li X, Meng M. Extraction of weakly reductive and reductive coals with sub-

Adv, 2016, 6: 105780–105785. [27] Wang C F, Fan X, Zhang F, Wang S Z, Zhao Y P, Zhao X Y,

and supercritical water. Energy Fuels, 2008, 22: 3944–3948.

Zhao W, Zhu T G, Lu J L, Wei X Y. Characterization of humic

[15] Chang H Z, Wang C G, Zeng F G, Li J, Li W Y, Xie K C. XPS

acids extracted from a lignite and interpretation for the mass

comparative analysis of coal macerals with different reducibility. J Fuel Chem Technol, 2006, 34(4): 389–394.

spectra. RSC Adv, 2017, 7: 20677–20684.