Five-stage sequential extraction of Hefeng coal and direct liquefaction performance of the extraction residue

Five-stage sequential extraction of Hefeng coal and direct liquefaction performance of the extraction residue

Fuel 266 (2020) 117039 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Five-stag...

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Fuel 266 (2020) 117039

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Five-stage sequential extraction of Hefeng coal and direct liquefaction performance of the extraction residue ⁎

T



Ya-ya Ma, Feng-yun Ma , Wen-long Mo , Qiang Wang Key Laboratory of Coal Clean Conversion & Chemical Engineering Process (Xinjiang Uyghur Autonomous Region), College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, Xinjiang 830046, China

G R A P H I C A L A B S T R A C T

Yaya Ma’s paper “Five-stage sequential extraction of Hefeng coal and direct liquefaction performance of the extraction residue” focused on the effect of sequentially ultrasonic-assisted extraction (UAE) on the structure and performance of the acid-washed Hefeng coal (HBCAW). Petroleum ether (PE), carbon disulfide (CDS), methanol (MeOH), acetone and isometric CDS/acetone mixture (IMCDSAM) were used as solvent to sequentially extract HBCAW via UAE to obtain extracts (E1–E5). The yields of E1–E5 were 0.17%, 0.64%, 5.24%, 1.89% and 3.21%, respectively. Gas chromatograph/mass spectrometer (GC–MS) analysis showed that most of the free molecules were extracted in the first two stages, and PE extract (E1) and CDS extract (E2) were mainly consisted of aliphatics and alkanes. PE presented a strong extraction ability for the low polar compounds, and the average molecular mass of E2 was higher than E1. Methanol extract (E3) containing more complex molecules was more diverse than the E1 and the E2. IMCDSAM was more favorable for the diffusion and dissolution of alcohol compounds.

A R T I C LE I N FO

A B S T R A C T

Keywords: Hefeng bituminous coal Sequential extraction GC–MS Direct liquefaction

Ultrasonic-assisted extraction (UAE) and direct liquefaction of the corresponding residue were conducted on an acid-washed ashless Hefeng bituminous coal (HBCAW). Petroleum ether (PE), carbon disulfide (CDS), methanol (MeOH), acetone and isometric CDS/acetone mixture (IMCDSAM) were used to sequentially extract HBCAW via UAE to obtain extracts (E1–E5). Gas chromatograph/mass spectrometer (GC–MS) analysis showed that most of the free molecules were extracted in the first two stages, and E1 and E2 were mainly consisted of aliphatics and alkanes. PE presented a strong extraction ability for the low polar compounds, and the average molecular weight of E2 was higher than E1. While E3 is more varied containing more complex molecules than E1 and E2. IMCDSAM was more favorable for the diffusion and dissolution of alcohols. Direct liquefaction performance suggested that the oil yields of Hefeng bituminous coal (HBC), HBCAW and the fifth stage extraction raffinate (ER5) were 66.00%, 63.57% and 44.11%, respectively, and that the conversion of ER5 was only 69.67%, much lower than HBC and HBCAW (82.75% and 82.68%). Higher yield of gaseous product for ER5 might be resulted from the looser coal structure by UAE process, resulting in more small molecules during the direct coal liquefaction.



Corresponding authors. E-mail addresses: [email protected] (F.-y. Ma), [email protected] (W.-l. Mo).

https://doi.org/10.1016/j.fuel.2020.117039 Received 7 November 2019; Received in revised form 20 December 2019; Accepted 6 January 2020 Available online 22 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

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1. Introduction

derived from sequential extraction of HBC by petroleum ether (PE), carbon disulfide (CDS), methanol (MeOH), acetone and isometric CDS/ acetone mixture (IMCDSAM) were characterized by TG-DTG, FTIR, SEM and GC–MS methods to acquire detailed molecular composition information and functional group character in the coal. And the direct hydroliquefaction experiments for HBC, HBCAW and ER5 were also carried out to provide basic knowledge for downstream process of the extract residue.

Coal is complex organic mixture composed of macromolecular network and small molecules embedded in the network [1–6]. Investigations on coal chemical structures have long been of great interest as a core issue for coal chemistry [7–12]. Studies on supercritical fluid extraction of a long-flame coal and a coking coal showed that there were many sulfur-containing organic compounds in the extract, such as alkyl or aryl mercaptan, sulfur ether, sulfur oxide and disulfide [13]. The compounds could be decomposed at high temperature during the coking process. Small molecules embedded in the macromolecular network might be separated by solvent extraction under mild conditions. Analytical works for the extracted components provided an effective path to study the composition and structure of coal [14,15]. Ultrasonic extracts and thermal dissolution (TD) products from Shengli lignite were analyzed using Orbitrap mass spectrometer (MS). Two cluster analysis methods, hierarchical cluster analysis and expectation maximum algorithm based on Gaussian mixture model (EMGM), were introduced to obtain in-depth statistical results for compounds in both extracts and TD products. Seven types of heteroatomic compounds in the extracts were clustered with EMGM, and possible structures of the related models might be inferred by analyzing the relationship between carbon number and double bond equivalent. Extraction performance of coal was significantly correlated with its characteristics [16,17], extraction conditions [18,19] and types of solvent [20,21]. Iino et al. [22] used carbon disulfide (CDS)/N-methyl-2pyrrolidinone (NMP) mixture to extract various metamorphic coals from lignite to anthracite. Results showed that the mixed solvent presented well extraction effect on bituminous coal, and the extract yields of 29 bituminous coals distributed in the range of 30–66% (daf), while the 5 lignites gave much lower extract yields from 6.4% to 8.6%. Such high yields might be derived from that the mixed solvent could improve molecular diffusion performance in the coal. Recent studies [23–27] showed that PE, CDS, MeOH, acetone and IMCDSAM could selectively destroy the intermolecular interactions – the winding effect of alkyls, aromatic π-π interaction, weak hydrogen bond, strong hydrogen bond and hydrogen bond/π-π combination effect. The two-phase model of coal structure indicates that there are a large number of small molecules (molecular weight less than 500 Da) embedded in the main structure of coal. The model, also known as the host – guest model [28], consists of a covalently bonded macromolecular network (host) and a mobile phase (guest), i.e., small molecular compounds trapped in the network. The macromolecular network should be insoluble in organic solvents, while the mobile phase can be released through extraction. It is known that the relative content of the total amount of small molecules in some lignites is higher than 30%. Therefore, solvent extraction of coal is essentially a process in which low-molecule substances dissolve in solvent from the main structure. At room temperature, the extraction of coal by polar solvents such as PE, acetone, CDS, etc., mainly relied on the interactions between solvent molecules and compounds in coal, inducing separation of small molecules from the macromolecule network [29]. In order to increase the extract yield of coal and obtain more structural information, lots of efforts have been taken to enhance the solubility of coal in organic solvent. Ultrasonic vibration, microwave radiation and reverse micelle extraction have been introduced as assistant approaches. And many chemical pretreatments, such as acid washing, hydrolysis and oxidation, etc., were used to break non-covalent bond in the coal structure, and therefore the solvent extract yield of coal could be improved [17–22]. Wyodak low-rank coal was deashed with methoxyethoxyacetic acid (MEAA) and hydrochloric acid, and extracted with N-methyl-2-pyrrolidone (NMP) at 200–360 °C. The extract yield of the coal increased from 58.4% without acid washing to 82.9% with MEAA washing [30]. However, experiment on the extract raffinate has not been well studied. In the present investigation, extracts (E) and extract raffinates (ER)

2. Experimental 2.1. Sample and reagents The coal sample was collected from Hefeng Coal Mine, Xinjiang, China and pulverized to pass through a 200-mesh sieve and dried at 105 °C for 12 h. HBC (60 g) and concentrated hydrochloric acid (100 mL) were taken into a 500 mL beaker, stirred for 4 h, standing for 2 h. The liquid part was removed, and the solid portion was washed several times with deionized water. With the same procedure, hydrofluoric acid (w = 15%) was used to wash the solid part, after which the part was dried at 105 °C for 12 h to obtain acid washing coal (HBCAW). PE, CDS, MeOH, acetone and IMCDSAM were commercial analytical reagents and purified by distillation using a rotary evaporator (IKA-RV10, Guangdong Yike laboratory technology co. LTD) prior to use. The polarity, solubility parameters (δ), boiling point (BP) and distillation temperature (DT) of the used organic solvents are listed in Table 1. 2.2. FTIR analysis The functional groups of coal sample, extracts and residues were measured using FTIR method with EQUINOX − 55 spectrometer (Bruker, Germany). The scan region was ranged from 400 cm−1 to 4000 cm−1, with resolution of 0.4 cm−1 and wave number accuracy of 0.01 cm−1. The samples were drying for 10 h under vacuum condition. Sample and KBr mixture with a mass ratio of 1:160 was used to detect the functional groups in each experiment. 2.3. GC–MS analysis of extracts All the extracts were analyzed with an Agilent 7890/5975 GC/MS, which is equipped with a capillary column coated with HP-5MS (crosslink 5% PH MEsiloxane, 60 m length, 0.25 mm inner diameter, and 0.25 m film thickness) and a quardrupole analyzer with m/z range from 33 to 500 and operated in electron impact (70 eV) mode. Data acquired were processed using ChemStation software. Compounds were identified by comparing mass spectra with NIST11 library data. 2.4. TG-DTG analysis TG-DTG experiments were carried out on a thermogravimetric analyzer SDT-Q600. The temperature range was from room temperature to 1000 °C with a heating rate of 5 °C/min in a N2 atmosphere. Table 1 Polarity, solubility parameters, boiling point and distillation temperature of the used organic solvents.

2

Solvent

Polarity

δ (cal1/2/cm3/2)

BP (°C)

DT (°C)

PE CDS MeOH Acetone IMCDSAM

0.01 0.2 6.6 5.4 –

– 10.0 14.5 9.8 –

60–90 46.5 64.7 56.5 –

85 50 66 62 –

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ηoil =

moil × 100% mcoal, daf

η(PA + A) =

m (PA + A) × 100% mcoal, daf

ηliq. = ηoil + η(PA + A) ηgas =

1 V ′t × × mcoal, daf 22.41



ci Mi × 100%

i ≠ H2

ηconv. = ηoil + η(PA + A) + ηgas moil: mass of oil from coal liquefaction, g; mcoal, daf: mass of coal (dry and ash-free base), g; m(A + PA): mass of asphaltene and preasphaltene, g; ci: mole fraction of the ith component in the gaseous product, %; Mi: molecular weight of the ith component in the gaseous product, i ≠ H2; Vt’: total volume of the gaseous product, L.

Fig. 1. Sequential extraction of HBCAW.

2.5. SEM analysis

3. Results and discussion

The surface morphology of the sample was observed using a SU8010 field emission scanning electron microscope (SEM, Hitachi, Japan). The acceleration voltage is ranged from 0.1 kV to 30 kV. And the sample was treated with metal spraying process before analysis.

3.1. Coal analysis The proximate and ultimate analyses for HBC and HBCAW are shown in Table 2. As presented in Table 2, the content of ash is reduced from 21.18% for raw coal to 1.84% for the acid-washed one, indicating that over 90% of the ash was removed with acid-washing process. It was reported that the main substances removed by acid washing were crystalline SiO2 and other inorganic minerals [31]. On the other hand, the removal of ash enhanced the BET pore surface area, pore volume and average pore diameter in coal sample [31], and the extraction resistance might be reduced, which is beneficial for solvent extraction and direct liquefaction of the coal. After acid washing, the molar ratio of H/ C was decreased from 0.91 to 0.88, presenting that the structure of the coal was destroyed by pickling treatment, and the branched chains in macromolecular network in coal might be broken. Therefore, comparing to HBC, a decrease in oil yield and an increase in hydrocarbon gas during direct liquefaction for HBCAW would be observed.

2.6. Ultrasonic-assisted extraction UAE were conducted in a KQ-3000GVDV ultrasonic extractor (Kunshan ultrasonic instrument Co. Ltd., China) with maximal ultrasonic power of 100 W at a frequency of 45 kHz, and the extraction time was set as 2 h. As shown in Fig. 1, 24 g of HBCAW was sequentially and exhaustively extracted with 300 mL of PE, CDS, MeOH, acetone and IMCDSAM at room temperature to obtain extracts E1-E5 and raffinates ER1-ER5, respectively. Extract yield (EY) was calculated as the mass ratio of the extract (mE) to that of HBCAW (mHBCAW, daf).

EY = mE / mHBCAW,daf

2.7. Direct liquefaction experiment

3.2. Extraction analysis

To verify the hydrogenation performance of the residue, HBC, HBCAW and ER5 were subjected to direct hydroliquefaction experiment. The experiment was conducted in a 100 mL autoclave reactor. 7.0 g of dry and ash-free (daf) sample, 14 g of tetralin as solvent and 0.5 g catalyst (S + Fe3O2, with the S/Fe molar ratio of 2:1) were put into the reactor. Air in the autoclave was replaced three times with hydrogen, and the autoclave was pressurized to an initial pressure of 6.0 MPa at room temperature and maintained at 435 °C for 1 h. Subsequently, the gaseous product was collected in a gas bag and analyzed by gas chromatograph. The resulting mixture was separated by Soxhlet solvent extraction sequentially with hexane and tetrahydrofuran (THBC) to obtain hexane-soluble portion (oil), hexane-insoluble but THBC-soluble portion (HITSP) as asphaltene and preasphaltene (A + PA), and THBCinsoluble portion (THBCISP) as residue. The yield of each portion was calculated as following.

3.2.1. Yield Polarity and solubility of the five solvents were considered to elaborate the yield difference of each extraction stage. Solvent polarity follows the order of MeOH > IMCDSAM > acetone > CDS > PE. As illustrated in Fig. 2, the extract yields of E1 to E5 are 0.17%, 0.64%, 5.24%, 1.89% and 3.21%, respectively, indicating that the extract yield of HBCAW varies greatly depending on the polarity of solvent. The yield of E2 is 3.76 times of E1, and E3 is 8.19 times of E2, presenting the obvious polarity effect for the three solvents [14]. PE, CDS and MeOH can destroy the entanglement between alkane and alkyl groups, the π-π entanglement among aromatic rings and the weak hydrogen bond in the coal [24,25]. According to the extract yields of the three solvents, a stronger solvent polarity induces a higher extract yield. After sequential UAE with the three solvents, most of the free small molecules in the HBCAW have been separated from macromolecules in the coal, hence

Table 2 Proximate and ultimate analyses (wt.%) of HBC and HBCAW. Samples

HBC HBCAW

Proximate analysis

Ultimate analysis

H/C

Mad

Ad

Vdaf

FCdaf*

C

H

N

S

O*

5.88 1.52

21.18 1.84

42.81 44.37

57.19 55.63

74.91 73.05

5.65 5.33

1.50 1.50

0.37 0.38

17.57 19.74

3

0.91 0.88

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various organic compounds in E1 to E5 were calculated from the TICs results. Six kinds of compounds were detected in E1, in which the relative content of arenes was as high as 69.17%. Arenes and alkanes are the major organic compounds in E1, indicating that PE presented strong extraction ability for compounds with less polarity. Contents of alkanes, alkenes, alcohols, aldehydes, arenes and nitrogen-containing organic compounds (OCNCS) in E2 were 56.22%, 10.91%, 10.75%, 10.40%, 10.09% and 1.63%, respectively. The content and the number of alkane compounds are higher than those of other compounds. A total of 16 kinds of alkanes were detected, in which the relative content of docosane was 13.30%. It is worth noting that the extracted normal paraffins are continuously distributed from C14 to C35. Comparing with E1, E2 contains more kinds of compounds, and the compounds with large molecular weight could be observed. With the characterization and analysis of each extract, the composition and association state of the easily dissociated components in coal could be obtained. Esters, aldehydes, alkanes, phenols, ONC, ketones and alcohols were observed in E3, with the relative content of 55.14%, 18.00%, 7.71%, 6.93%, 5.93%, 4.76% and 1.54%, respectively. Obviously, the content of esters is more than half of the extracted compounds, which might be derived from the weak combination of eOH and eCOOH in the coal during the extraction process. The number of species of extracted molecules in E3 is much more than E1 and E2, and MeOH can extract more complex substances, indicating that the stronger the solvent polarity, the better the extraction effect for more complex molecules. In addition, only 5 species of alkanes were detected in E3 with a content of 0.5%, and the total relative content of alkanes was only 6.07%, meaning that the alkanes with small molecular weight have been substantially dissolved in the first two stages of the extraction process. A total of 56 components were detected in E4, with the relative content of esters of 50.16%, ONC of 13.00%, arenes of 17.95% alkenes of 10.73% and other compounds. It is apparent that the esters content is higher than others. 132 compounds were detected in E5. Relative contents of alcohols, alkenes, arenes, phenols, aldehyde, alkanes and ONC were 32.50%, 16.80%, 27.60, 8.55%, 6.05%, 4.38% and 4.33%, respectively. A high content of alcohols indicates that the mixed solvent of IMCDSAM is more favorable for the diffusion and dissolution of alcohols.

Fig. 2. Yields of sequential extracts.

the yields of E4 and E5 are lower than that of E3. However, due to the strong polarity of solvent, the extraction yields of E4 and E5 are still higher than those of E1 and E2. The order of extract yield for each stage was different from Liuhuanggou bituminous coal reported in the literature from Wei and co-workers [18], which might be resulted from the acid washing process. 3.2.2. FTIR analysis Fig. 3 shows FTIR spectra of the five extracts from HBCAW sample. All the extracts shows absorption peaks at 3400 cm−1, indicating the existence of eOH, which is easily combined with C]O (1600 cm−1) to form eCOOH, which is consistent with the previous work [12,31]. There is an obvious absorption peak at 1600 cm−1 for the extracts of E1E5, while a weak one is observed for E1, indicating that the E2–E5 contain more carboxylic acid species. The absorption peaks around 2920 cm−1 and 1373 cm−1 are ascribed to the asymmetric stretching vibration of CeH for aliphatic hydrocarbons and asymmetric stretching vibrations of CH3e and CH2e for aromatic hydrocarbons, respectively. In addition, the absorption peaks of CeH around 2920 cm−1 and 1373 cm−1 from E1 and E2 are stronger than those from the other three extracts, demonstrating that the hydrocarbons (including arenes and alkanes) in the coal are mainly enriched by PE and CDS.

3.3. Raffinate analysis 3.3.1. FTIR analysis As exhibited in Fig. 5, all the IR absorption peaks of the five extraction residues (ER1-ER5) appear at the same position, but the peak intensity presents a certain difference, indicating the existence of similar functional groups in ER1-ER5. UAE is a physical swelling-dissolution process that does not destroy the bulk structure of coal [26,27]. In addition, the absorption peak around 3400 cm−1 can be attributed to eOH from alcohols, phenols etc., and the peak at 1600 cm−1 may be ascribed to C]O stretching vibration from aldehydes, esters, etc, impling the existence of oxygen-containing groups in ER1-ER5.

3.2.3. GC–MS analysis The five extracts were analyzed by GC–MS to obtain an overview of the compositional distribution, and the corresponding total ion chromatograms (TICs) are also shown in Fig. 4. The relative contents of

3.3.2. TG-DTG analysis The five extraction residues were also analyzed by TG-DTG. As exhibited in Fig. 6(a), the weight loss of ER1-ER5 before 150 °C is about 3%, which is derived from the removal of adsorbed water and free molecules (guest) trapped in the network. As the pyrolysis temperature reaches to 1000 °C, the weight losses of ER1-ER5 are 54.97%, 55.02%, 42.33%, 44.92% and 42.53%, respectively. Thus the weight loss of each residue obeyed the following order, ER2 ≈ ER1 > ER4 > ER5 ≈ ER3, and ER3 and ER5 showed the lowest weight loss. While the order of the extract yield is E1 < E2 < E4 < E5 < E3, indicating that the yields of E3 and E5 are the highest. Therefore, the low weight loss of ER3 is related to the high MeOH extract yield. It is speculated that large

Fig. 3. FTIR analysis of the sequential extracts from HBCAW. 4

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Fig. 4. TICs of the five extracts and the corresponding compositional distribution.

In Fig. 6(b), the weight loss rates of ER1-ER5 mainly occurred in the range of 300–600 °C, and the maximum weight loss rate is 0.225%·min−1 for ER4, and the minimum one is −0.136%·min−1 for ER2. With the process of extraction, relative contents of the stable components in the five residues gradually increased, and the component could be rapidly decomposed at 445 °C. In addition, the temperature of maximum weight loss peak for ER1-ER5 is about 445 °C with the same as HBC, indicating that UAE is a physical swelling process and does not destroy the main structure of the coal. On the other hand, a weight loss peak in the range of 200 °C–250 °C could be observed for ER1-ER4, while no peak appeared in the above range for ER5. It can be speculated that almost all small molecules – the mobile phase were extracted through five-stage sequential extraction process [28]. While the weight loss rates of ER3, ER4 and ER5 at 445 °C are much higher than those of ER1 and ER2. Presumably, a portion of small molecules in the macromolecular network was extracted by PE and CDS, with more diffusion channels formed, reducing the diffusion resistance and improving the internal diffusion of solvent, which is beneficial for the escape of small molecules during pyrolysis.

Fig. 5. FTIR analysis of extraction residues from HBCAW.

amounts of small molecules have been extracted by MeOH, resulting in a higher extract yield and a smaller weight loss of ER3. In addition, ER1 and ER2 present the highest weight loss, which might be resulted from the low values of EY1 and EY2 during the extraction process. 5

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Fig. 6. TG-DTG diagram of extraction residues from HBCAW.

Fig. 7. TG-DTG analysis of the three samples.

0.202%·min−1 and 0.239%·min−1, respectively. The rate of ER5 is less than that of HBCAW, which might be resulted from the removal of dissociable components via sequential extraction.

3.4. Direct liquefaction performance of HBC, HBCAW and ER5 3.4.1. TG-DTG analysis of the three samples Fig. 7 shows TG (a) and DTG (b) profiles of the three samples. In TG diagram, weight losses of HBC, HBCAW and ER5 at 1000 °C are 37.92%, 41.03% and 42.53%, respectively. The amount of small molecules in HBCAW increased greatly because of the acid washing process. ER5 was obtained by five-stage sequential extraction, resulting a loose pore structure, which is favorable for the removal of small molecules and for the breaking of chemical bonds in hard-dissociating portion in the coal, see Fig. 8. Therefore, the weight loss of ER5 is higher than those of HBC and HBCAW at 1000 °C. DTG profiles of the samples show that temperature of the maximum weight loss peak is around 445 °C, indicating that the macromolecular network structure of the coal was not destroyed in either the acidwashing process or the sequential extraction process. In addition, the maximum weight loss rate of the three samples follows the order of HBC < ER5 < HBCAW, with the value of 0.190%·min−1,

3.4.2. FTIR analysis Fig. 9 shows the infrared spectra of the three samples. The absorption peak at 1706 cm−1 is attributed to the vibration of aromatic ring. However, the peak intensity of the aromatic ring of HBCAW and ER5 is stronger than HBC, indicating that HBC was subjected to pickling treatment and sequential extraction with C]C-containing compounds produced. The peak intensity of HBCAW at 1151 cm−1, which is ascribed to CeO stretching vibration, is stronger than those of other two samples. It is supposed that phenols were generated by acid washing and extracted by sequential solvent. In addition, the peak intensity of HBC at 1025 cm−1 is stronger than ER5 and HBCAW, demonstrating that the relative content of aromatic and vinyl ]CeOeC in the coal sample is higher than those of the acid washed one and the sequentially extracted one.

Fig. 8. SEM images of HBC, HBCAW and ER5 samples. 6

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Fig. 10. IR spectra of the three liquefied residues. Fig. 9. IR profiles of the three samples.

ER5 after the five-stage sequential swelling-extraction process might decrease the diffusion resistance for the small molecules in the samples. On the other hand, the contents of N2 for HBCAW and ER5 are higher than HBC, indicating that the removal of N atoms from N-containing compounds through direct coal liquefaction could be realized by acidwashing and sequential extraction pretreatment.

Table 3 Liquefaction performance of the three samples (%). Samples

ηoil

η(PA+A)

ηliq

ηgas

ηConv.

HBC HBCAW ER5

66.00 63.57 44.11

10.43 11.57 17.43

76.43 75.14 61.54

6.32 7.54 8.43

82.75 82.68 69.97

3.4.4. Analysis of liquefaction residue Fig. 10 shows infrared spectra of the three liquefied residues (LRs). Both HBCAW-LR and ER5-LR show absorption peaks at 3051 cm−1, 2920 cm−1, 1702 cm−1 and 1430 cm−1, assigned to OH-N, asymmetric aliphatic eCH2 stretching vibration, aromatic ring C]C and aromatic ring stretching vibration, respectively. While HBC-LR presents very weak absorption at the above peaks, indicating that a large amount of aliphatics and alkenes were produced from HBC during the liquefaction process. In addition, the absorption peak of OH-π around 3656 cm−1 from HBC-LR is stronger than those from HBCAW-LR and ER5-LR, indicating that acid-washing process and sequential extraction can effectively destroy the OH-π bond. The remaining peak positions of the three liquefied residues are basically the same, and the intensity of each peak is slightly different. It can be inferred that the oil yield of HBC might be higher than other two samples, which is consistent with the result of the liquefaction experiment. It can also be observed in Fig. 10 that HBCAW-LR shows the highest peak intensity at 2920 cm−1 and 1594 cm−1, followed by ER5-LR. It indicates that the relative content of aliphatics, aldehydes and ketones produced from HBCAW during the liquefaction process are lower than those of the other two samples. However, the peak intensity of aromatic ring stretching vibration of the three liquefied residues at 1430 cm−1 follows the order of ER5-LR > HBCAW-LR > HBC-LR. It can be proposed that a large amount of complex organic substances still exist in the three samples without being converted into oil or gas via the hydroliquefaction process. Fig. 11 shows TG-DTG profiles of the three liquefied residues. The weight losses of HBC-LR, HBCAW-LR and ER5-LR at 1000 °C are 18.38%, 28.78% and 42.54%, respectively. The higher weight loss of ER5-LR is

3.4.3. Liquefaction products (1) Yield analysis Table 3 shows the results of direct liquefaction experiments for HBC, HBCAW and ER5. The oil yields of HBC, HBCAW and ER5 are 66.00%, 63.57% and 44.11%, respectively. The low oil yield of ER5 is responsible for its low conversion (69.97%), which might be resulted from the condensation reaction with the yield of PA + A higher than other two samples. Therefore, the gas and A + PA yields of the three samples follow the order of HBC < HBCAW < ER5, indicating that ER5 can produce a higher content of small molecules by prominent condensation process. The possible reason might also be that UAE is a physical swelling process resulting in a looser structure for coal and more small molecular fragments might escape from the loose channels in ER5 during the liquefaction process. (2) Gas product analysis Table 4 shows the composition and relative content of gaseous products from the direct liquefaction experiments of the three samples. Except for hydrogen, the content of CH4 is higher than those of others, followed by CO2 and C2H6. It could be concluded that three functional groups, eCH3, eCH2 and eCOOH, were easily detached in the three samples. The contents of CH4 and C2H6 obtained from ER5 are the highest among the three samples, indicating that the loose structure of

Table 4 Composition of the gaseous products (%). Components

H2 N2 CO CH4 CO2

Content

Components

HBC

HBCAW

ER5

71.13 1.615 1.303 6.145 3.405

71.56 2.762 0.6594 5.578 2.006

70.25 3.332 0.8712 8.882 2.741

C2H6 C2H4 C3H8 C3H6 C4H10

7

Content HBC

HBCAW

ER5

1.882 0.0261 0.8804 0.0516 0.1800

1.637 0.0239 0.8102 0.0442 0.1867

2.882 0.0344 1.699 0.0910 0.2828

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Fig. 11. TG-DTG analysis of the three liquefied residues.

and HBCAW, ER5 presented a lower oil yield, which was derived from the relative content of dissociable components in the residue. In addition, there are still a large number of complex organic substances in the three direct LR, which are more stable and might require a higher temperature to dissociate.

mostly related to the multiple swelling-extraction process, the formation of loose pores and the destruction of the macromolecular network during the liquefaction process. HBC-LR shows a constant weight loss of 18.38% at 760 °C, and there is almost no weight loss before 435 °C for the three samples, indicating that the liquefiable part in the samples is mainly composed of the portion that can be broken down by pyrolysis reaction at 435 °C, and the portion was almost converted into oil and gas during the liquefaction process. And thus the oil yield and conversion of HBC are 66.00% and 82.75%, higher than those of other two samples. In addition, the weight loss of 18.38% for HBC-LR indicates that the amount of heavy components in the sample is higher than other liquefaction residues. The weight loss difference between HBC-LR and ER5-LR means that there are still a large amount of organic components left in HBC-LR and not converted into oil or gas during the pyrolysis process. DTG profiles show that the weight loss rate peaks of the three LRs have changed greatly compared with Fig. 7(b). On the one hand, the number of weight loss peaks after 300 °C increased from only one before liquefaction to 2–4 after liquefaction. On the other hand, the peak temperature increased from 445 °C before liquefaction to 450–950 °C after liquefaction. It could be speculated that the direct catalytic liquefaction process could destroy the macromolecular network of the coal [32–34], and a large amount of organic substances still existed in the three samples without being converted into oil or gas via the hydroliquefaction reaction. Additionally, there are two weight loss peaks at 458 °C and 723 °C in the DTG profile for HBC-LR. It is also worth noting that there are four weight loss peaks for ER5-LR at 492 °C, 670 °C, 810 °C and 960 °C, respectively. Weak weight loss peaks at 490 °C, 610 °C and 810 °C were found for HBCAW-LR, and the weight loss rate increased with the pyrolysis temperature. It is proposed that the small molecules and dissociable components were gradually transformed into chemicals, oil or gas through solvent extraction, hydroliquefaction and pyrolysis, while the more stable substances might require a higher temperature to dissociate.

CRediT authorship contribution statement Ya-Ya Ma: Investigation, Data curation, Writing - original draft. Feng-Yun Ma: Funding acquisition, Project administration, Resources. Wen-Long Mo: Conceptualization, Supervision, Writing - review & editing. Qiang Wang: Methodology, Software. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This workwas supported by the Key Project of Joint Fund from National Natural Science Foundation of China and the Government of Xinjian Uygur Autonomous Region(Grant U1503293) and the Natural Scientific Foundation of China (Grant 21276219). References [1] Kong J, Wei XY, Li Z-K, Yan H-L, Zong Z-M. Identification of organonitrogen and organooxygen compounds in the extraction residue from Buliangou subbituminous coal by FTICRMS. Fuel 2015;171:151–8. [2] Ashida R, Morimoto M, Makino Y, Umemoto S, Nakagawa H, Miura K, et al. Fractionation of brown coal by sequential high temperature solvent extraction. Fuel 2019;88(8):1485–90. [3] Tahmasebi A, Yu J, Han Y, Yin F, Bhattacharya S, Stokie D. Study of chemical structure changes of chinese lignite upon drying in superheated steam, microwave, and hot air. Energy Fuels 2012;26(6):3651–60. [4] Zheng A-L, Fan X, Wang S-Z, Liu F-J, Wei X-Y, Zhao Y-P, et al. Analysis of the products from the oxidation of geting bituminous coal by atmospheric pressure photoionization-mass spectrometry. Anal Lett 2014;47(6):958–69. [5] Ma Y-Y, Ma F-Y, He F, Sun Z-Q, Ma W-L, Zhang X-J, et al. Influence of microwave swelling with cavitated creosote oil on the direct liquefaction performance of Xigou coal from Xinjiang and its dynamics analysis. J China Coal Soc 2017;42(10):2732–40. [6] Sun Z-Q, Ma F-Y, Liu X-J, Wu H-H, Niu C-G, Su X-T, et al. Large-scale synthesis and catalysis of oleic acid-coated Fe2O3 for co-liquefaction of coal and petroleum vacuum residues. Fuel Process Technol 2015;139:173–7. [7] Liu F-J, Wei X-Y, Xie R-L, Wang Y-G, Li W-T, Li Z-K, et al. Characterization of oxygen containing species in methanolysis products of the extraction residue from Xianfeng lignite with negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2014;28(9):5596–605. [8] Rathsack P, Kroll MM, Otto M. Analysis of high molecular compounds in pyrolysis liquids from a german brown coal by FT-ICR-MS. Fuel 2014;115:461–8. [9] Wang S-Q, Tang Y-G, Schobert H-H, Guo Y-N, Gao W-C, Lu X-K. FTIR and

4. Conclusions Hefeng low rank coal treated with acid-washing process was subjected to a five-stage sequential extraction, and a direct liquefaction of the coal was also conducted. From the GC–MS data, E1 and E2 were mainly composed of aliphatics and alkanes, and the E3-E5 were derived from the relative stable molecules in coal. Petroleum ether presented a strong extraction ability for compounds with less polarity. CDS could extract more complex chemicals compared with petroleum ether. Methanol showed a high extraction yield of 5.24% and IMCDSAM was beneficial to the diffusion and dissolution of alcohol compounds. According to the IR spectra, functional groups in ER1-ER5 were almost the same, indicating the stability of coal structure. Compared with HBC 8

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