Coliquefaction of Lignite and Corn Stalk in Ethanol–Water Mixed

Feb 6, 2018 - Also, iron ore was an effective catalyst, for instance, adding 20 wt ... products characterization suggested that the ore catalyst promo...
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Article Cite This: Energy Fuels 2018, 32, 3022−3030

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Coliquefaction of Lignite and Corn Stalk in Ethanol−Water Mixed Solvent with Addition of Formic Acid and Iron Ore Catalyst Hengfu Shui,*,† Hanren Jiao,† Fang He,† Tao Shui,‡ Xiaoling Wang,† Hua Zhou,† Chunxiu Pan,† Zhicai Wang,† Zhiping Lei,† Shibiao Ren,† Shigang Kang,† and Charles Chunbao Xu*,†,‡ †

School of Chemistry & Chemical Engineering, Anhui Key laboratory of Coal Clean Conversion & Utilization, Anhui University of Technology, Ma’anshan 243002, Anhui Province, P. R. China ‡ Department of Chemical and Biochemical Engineering, Western University London, Ontario N6A 5B9, Canada ABSTRACT: The coliquefaction of Xilinguole lignite (XL) and corn stalk (CS) was carried out in water, ethanol, and ethanol− water mixed solvent at different mixing ratios with or without iron ore as catalyst, under initial N2 atmosphere. Results showed that ethanol improved the liquefaction and coliquefaction of XL and CS. The liquefaction of XL and CS separately resulted in 31.0 and 41.9 wt % of oil yield in ethanol and 7.3 and 20.2 wt % of oil yield in water, respectively, whereas the ethanol and water coliquefaction process resulted in 34.4 and 13.2 wt % of oil yield, respectively. The addition of formic acid increased the oil yield in the single liquefaction of XL and CS and their coliquefaction in ethanol, water, or ethanol−water (50:50, v/v). The liquefaction of XL and CS and coliquefaction of XL/CS in ethanol at N2 atmosphere with formic acid resulted in higher oil and conversion yields, suggesting that formic acid can supply active hydrogen. Also, iron ore was an effective catalyst, for instance, adding 20 wt % H2-reduced hematite improved the oil yield by 9.2 and 6.3 wt % for XL and CS, respectively, compared with those without catalyst. Besides, XL/CS coliquefaction in ethanol and formic acid at 400 °C resulted in a 10.9 wt % oil yield increase. A synergistic effect promoted by the H2-reduced hematite catalyst was clearly present in the coliquefaction of XL and CS at 400 °C. The liquefied products characterization suggested that the ore catalyst promoted XL converting into oil fraction and thus enhanced the liquefaction conversion and oil yield.

1. INTRODUCTION

during liquefaction; the addition of biomass in the liquefaction of coal promotes pyrolysis of coal at mild temperatures.13,14 Karaca et al. obtained the highest total conversion at 325 °C in the coliquefaction of a Turkish lignite with sawdust in tetralin.15−17 The coliquefaction of bituminous coal and lignin at 400 °C in tetralin achieved 30% yield of benzene soluble compared to about 10% for the individual liquefaction of coal and lignin.18 Further, they found far more of the benzenesoluble material was also pentane-soluble oil resulting from the coliquefaction.18 Coughlin et al. stated that phenoxy radicals formed in thermal depolymerization of lignin at relatively low temperatures could attack the coal macromolecules, resulting in the separation of aliphatic carbon−carbon bonds in the coal.19 Although tetralin and some other hydrogenated solvents are widely employed in liquefaction of coal and biomass,20,21 there are some issues in the use of these solvents. Such as high cost and poor recyclability because of its high-boiling point. To overcome the low recyclability, low boiling point organic solvents, such as alcohols and cyclic carbonates, were studied in low-temperature liquefaction of biomass.22,23 Cheng et al. found that 50 wt % cosolvent of alcohol and water presented synergistic effects in the liquefaction of white pine sawdust. Their liquefaction produced approximately 65 wt % of bio-oil yield and >95% of biomass conversion with 50 wt % aqueous alcohol at 300 °C for 15 min.24 Matsumura et al. found that coliquefaction of low-rank coals and cellulose in supercritical

Lignite is low-rank coal and an abundant fossil fuel resource worldwide. It is typically used in combustion to generate electricity as well as gasification to produce synthetic natural gas. Another promising application for lignite is in direct coal liquefaction (DCL).1 The abundant presence of aliphatic C−O and C−C bonds, which are lower in bond dissociation energies, contributes to lignite’s high reactivity.2 For instance, the Solvent Refined Coal II method using North American lignites achieved a 60 wt % oil yield.1 However, the main disadvantage of DCL of lignite using hydrogen is the high consumption of hydrogen due to its high oxygen content in lignite, resulting in excessive costs.3 Another problem is its high moisture content, which requires large massive consumption.4 Water is considered a ‘‘green” solvent and is, therefore, vastly studied in conversion processes. Compared to ambient liquid water, hot-compressed water has a lower dielectric constant and weaker and fewer hydrogen-bonds; thus, it has an enhanced solubility for organic compounds.5 Xu et al. carried out the liquefaction of peat in supercritical water with an iron catalyst. They found that, at 400 °C for 2 h, near 40 wt % of heavy oil yield can be obtained with raw iron ore.6 Biomass has received great attention because it is renewable, carbon neutral, and low cost and it brings promising options for renewable energy generation. Biofuels (solid, liquid, and gaseous) produced from biomass can directly replace fossil fuels either fully or partially.7−10 Coliquefaction of coal with biomass, such as agricultural and forestry residues, has gained interest.11,12 There are synergistic effects of biomass and coal © 2018 American Chemical Society

Received: November 20, 2017 Revised: February 1, 2018 Published: February 6, 2018 3022

DOI: 10.1021/acs.energyfuels.7b03595 Energy Fuels 2018, 32, 3022−3030

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Energy & Fuels Table 1. Ultimate and Proximate Analysis Results of XL and CS proximate analysisb (wt %)

a

ultimate analysis (wt %), daf

sample

Mad

Ad

Vdaf

C

H

N

S

Ob

lignite corn stalk

15.77 8.80

11.01 3.49

40.30 72.22

62.67 45.17

4.83 5.93

0.98 0.44

0.44 0.21

31.08 48.25

ad: air-dry basis; d: dry basis; daf: dry and ash free basis. bBy difference. the CS was ground to 60 meshes and dried at 105 °C in air for 24 h. The proximate and ultimate analysis results of the XL and CS are given in Table 1. All solvents used were commercially purchased, chemical pure reagents and without further purification. The catalyst, a raw iron ore hematite, was supplied by a local mine in Ma’anshan, Anhui, China. It was ground into fine particles of less than 150 μm prior to use. Table 2 presents XRF data of the hematite

water produced superior coal conversion and increased yield and quality of the liquefaction products.13 Water and alcohol as solvents are broadly applied for liquefaction of biomass; yet, no publications were found on the use of iron ore as a catalyst for the coliquefaction of low-rank coal and biomass in a water−alcohol mixed solvent. Alkaline solutions, such as Na2CO3, K2CO3, CaOH, and BaOH, are extensively used as catalysts in the liquefaction of biomass to improve the yield of liquid products.25−28 For instance, yields of 21−36 wt % of heavy oil were obtained by Minowa et al. from the liquefaction of a variety of biomass residues in hot compressed water at 300 °C using Na2CO3 as the catalyst.25 A higher heavy oil yield (53.3%) using Na2CO3 catalyst was obtained at 380 °C from the liquefaction of woody biomass.26 Xu et al. concluded that for the direct liquefaction of woody biomass in sub/near-critical water Ba(OH)2 > Ca(OH)2 > FeSO4, in priority sequence, were active on enhancing the formation of heavy oil products at 280−340 °C. For example, the heavy oil yield in the experiment at 300 °C for 30 min increased from ∼30% without a catalyst to more than 45% with Ba(OH)2.27 Synthesized iron-based catalysts (FeOOH and Fe2O3) showed a lower activity, while some conventional biomass liquefaction catalysts, such as KOH, FeCl3, and FeSO4, presented negligible or even adverse effects on the heavy oil formation in the liquefaction of peat in supercritical water.6 Similarly, Xu et al. evaluated the impact of catalysts K2CO3, FeSO4, RuO2, and Ca(OH)2 on the upgrading of peat in supercritical water. They observed a higher performance for Ca(OH)2, intensifying the production of liquid oils, besides the expensive RuO2 proved to be very active promoting both oil and hydrogen-rich gas yields. A conversion of 91.5% was achieved by Grilc et al. using an imidazolium-based ionic liquid catalyst in solvolysis of beech wood in glycerol at 200 °C in 60 min.29 An efficient and low-cost catalyst for the direct liquefaction of biomass and lignite in water is of extreme importance to make this process feasible. An economic alternative is to use a raw iron ore. Besides being of low cost, it also contains a high concentration of goethite, magnetite, or hematite (Fe2O3). Fe2O3 was broadly used as a catalyst in biomass pyrolysis and gasification, demonstrating its potential in these methods.30,31 In this study, the coliquefactions of Xilinguole lignite (XL) and corn stalk (CS) were carried out in water, ethanol, or their mixed solvents with an iron ore inexpensive catalyst under initial N2 atmosphere. Further, the synergistic effects XL and CS liquefaction and coliquefaction were interpreted and discussed to evaluate the process proposed.

Table 2. XRF Analysis of Hematite Iron Ore compound

weight %

element

weight %

Fe2O3 SiO2 Al2O3 MnO K2O P2O5 MgO CaO TiO2 others

91.72 ± 0.14 3.66 ± 0.09 3.06 ± 0.09 0.42 ± 0.021 0.313 ± 0.016 0.217 ± 0.011 0.207 ± 0.01 0.154 ± 0.008 0.132 ± 0.007 0.1393 ± 0.0246

Fe Si Al Mn K Px Mg Ca Ti others

64.15 ± 0.1 1.71 ± 0.04 1.62 ± 0.05 0.325 ± 0.016 0.26 ± 0.013 0.0948 ± 0.0047 0.125 ± 0.006 0.11 ± 0.005 0.0793 ± 0.004 0.097 ± 0.0185

iron ore. In order to increase the activity of iron ore, the hematite was reduced by H2 reduction at 400 °C in situ for 4 h. Figure 1 shows the XRD spectra for the raw iron ore hematite and the reduced hematite iron ore.

Figure 1. XRD of the hematite and the in situ reduced hematite iron ore. 2.2. Liquefaction and Product Fractionation. The liquefaction experiments were performed in a 100 mL stirred reactor (Parr 4590 Micro Bench top reactor). In a typical run, 4.0 g of XL or CS (liquefaction) or 2.0 g of XL and 2.0 g of CS (coliquefaction) and 30 mL of water, ethanol, or water−ethanol (50:50, v/v) mixed solvent were charged into the reactor. The reactor was then sealed and the residual air inside removed by N2 purging-vacuuming at least 4 times. Then the reactor was pressurized to the initial pressure of 2 MPa with nitrogen, stirred, and heated to the desired temperature letting the reaction to occur at the desired time. Then, the reaction was stopped by quenching the reactor in a water/ice bath. For the liquefaction with ore catalyst, 0.8 g of hematite ore (or approximately 20 wt % of feedstock) and the required amount of

2. EXPERIMENTAL SECTION 2.1. Materials. The XL utilized in this study is a Chinese lignite from Inner Mongolia, China. It was ground to 200 meshes and dried under vacuum at 80 °C for 24 h beforehand. The CS was provided by the Institute for Chemicals and Fuels from Alternative Resources (ICFAR) of the University of Western Ontario, Canada. Prior to use, 3023

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Energy & Fuels solvent were loaded into the reactor with in situ reduced with H2 at 360 or 400 °C for 4 h before liquefaction. Then the reactor was cooled to room temperature, and the liquefaction feedstock (XL, CS, and XL/ CS) was loaded into the reactor to start the liquefaction. The liquefaction at 340 and 360 °C was carried out at ICFAR, and two replicate runs were conducted for the experiments. The liquefaction at 400 °C was carried out at Anhui University of Technology (China), with all of the experiments conducted for three replicate runs. The relative errors for all runs were mainly within ±3%, and the reported results are the mean values. After cooling the reactor to room temperature, the reactor was opened, the gas inside was collected with a vessel, and the solid/liquid products were rinsed from the reactor with acetone. The slurry and rinsing acetone were collected and filtered under vacuum through a filter paper. It was then subjected to acetone washing until the filtrate became colorless. The filter paper was then dried overnight at 105 °C, and the remaining residue obtained was defined as acetone insoluble fraction (AI). Next, the filtrate was evaporated under reduced pressure to remove acetone at 45 °C, and the dark color product was vacuumdried at 45 °C for 12 h. The resulted product was identified as the oil fraction. The fractionation procedure is shown in Figure 2.

3. RESULTS AND DISCUSSION 3.1. Catalysis Effect of in Situ H2-Reduced Iron Ore. Raw iron ore mainly consists of goethite and hematite which can significantly improve heavy oil yield in the liquefaction in supercritical water.6 In this study, we observed the effect of in situ H2-reduced iron ore hematite effects on the liquefaction of XL, CS, and XL/CS. This could occur because highly dispersed nanoparticles of metallic iron are formed by in situ H2 reduction in the course of the supercritical water peat liquefaction process.6 The liquefaction was carried out at 400 °C, 30 min, ethanol + 10 vol. % formic acid as solvent, 2 MPa N2 initial pressure, with hematite catalyst at 20 wt % (against feedstock) after in situ H2reduced. Figures 3 and 4 show the liquefaction conversion and

Figure 3. Effects of iron of iron ore catalyst on the conversion in liquefaction of XL, CS, and XL/CS. Figure 2. Fractionation procedure of the liquefaction products. The liquefaction conversion was calculated using eq 1 based on the amount of AI. The oil yield was calculated using eq 2 based on the amount of oil fraction. m1 − m2 conversion, % = × 100 m1(1 − Ad ) (1) oil yield, % =

m3 × 100 m1(1 − Ad )

(2)

where m1 (g), m2 (g), m3 (g), and Ad (wt %, db) are the initial masses of the raw material (XL, CS, or the mixture of XL/CS), AI, oil fraction, and the ash content of the raw material, respectively. 2.3. Characterization of Liquefied Products. The elemental analysis was performed in a Vario EL III elementary analyzer at the mode of CHNS. The oil molecular weight distribution was measured by Shimadzu LC-20AT high-performance liquid chromatography equipped with a UV detector (λ = 254 nm). A Shim-pack GPC802.5 (30 cm length; 0.8 cm i.d.) separation column was operated isothermally at 25 °C using 1 mL/min tetrahydrofuran as the mobile phase. Fourier-transform infrared (FT-IR) spectra were recorded using a Nicolet 6700 FT-IR spectrometer at a resolution of 4 cm−1. Finally, samples were prepared by mixing 1 mg of sample with 100 mg of KBr and then pressed to form a pellet. The scan time was 32 in the scanning range of 4000−400 cm−1. X-ray fluorescence (XRF) measurement was accomplished with an ARL Advant’X Intellipower 3600. The voltage and current for the measurement are 60 kV and 120 mA, respectively. X-ray diffraction (XRD) data were obtained with a D8 Advance (Bruker AXS) X-ray diffractometer, using Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA.

Figure 4. Effects of iron ore catalyst on the oil yield in liquefactions of XL, CS, and XL/CS.

oil yield with and without the catalyst of XL, CS, and XL/CS, respectively. Clearly Figure 3 shows that the in situ H2-reduced iron ore presented a high activity in the liquefaction. The liquefaction conversions of XL, CS, and XL/CS increased from 40.8, 88.3, and 69.6%, without the catalyst, to 51.6, 93.5, and 80.4% under in situ H2-reduced iron ore catalyst; therefore, the percentage of improvement in the conversion was 10.8, 5.2, and 10.8%, sequentially. A similar behavior was identified in the oil yield shown in Figure 4. The oil yields also increased from 29.9, 40.2, and 37.4%, without the catalyst, to 39.1, 46.5 , and 48.3% with the 3024

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liquefaction results of XL and CS, respectively, in the individual liquefaction; and AX and AC are the ash contents of XL and CS, respectively. Hereafter the experimental results of coliquefaction of XL and CS were expressed by Exp. For example, if no synergistic effect is found in the coliquefaction of XL and CS, the Cal values should be similar to the corresponding Exp values; i.e., the Exp − Cal values of liquefaction conversion and oil yield should be close to zero. The Exp, Cal, and Exp − Cal values of the XL and CS coliquefaction with and without in situ H2reduced iron ore catalyst at 400 °C are presented in Table 3.

catalyst, resulting in an additional 9.2, 6.3, and 10.9% in oil yield for the liquefaction of XL, CS, and XL/CS, respectively. Comparing Figures 3 and 4, it can be established that the in situ H2-reduced iron ore promoted the liquefaction conversion reflected by the enhancement of oil yields. Hence, the in situ H2-reduced iron ore promoted the formation of oil in the liquefactions of XL, CS, and XL/CS, demonstrating similar outcomes as the findings discussed by Xu et al.6 In our previous study, we found that, when a XL was liquefied in water at 380 °C under FeS + S, a conventional DCL catalyst, the oil + gas yield was 22.4 and 21.0% in H2 and N2 atmospheres, respectively.32 These results are lower than our findings in the present study (39.1%) using the in situ H2reduced iron ore catalyst. The yield of CS heavy oil was 46.5% at 400 °C under the in situ H2-reduced iron ore catalyst (Figure 4). Other studies have also achieved similar or lower values. For instance, Qian et al. reached 46.2% heavy oil yield for woody biomass at 400 °C using water as solvent and Na2CO3 as catalyst.26 A comparable yield was found by Xu et al. for woody biomass liquefaction yielding 45% heavy oil at 300 °C for 30 min by Ba(OH)2 as catalyst.27 In addition, the liquefaction wood materials in water were carried out at 360 °C with K2CO3 as catalyst obtaining less than 30% heavy oil yields.33 The coliquefaction of XL and CS with in situ H2-reduced iron ore catalyst under N2 in water at 400 °C resulted in 48.3% heavy oil yield (Figure 4). Ikenaga et al. experimented with the coliquefaction of microalgae with Yallourn coal using various S/ Fe ratios coal liquefaction catalysts under pressurized H2 in 1methylnaphthalene at 350 °C. They found that the hexanesoluble oil yield increased from 41.8% (S/Fe = 2) to 54.7% (S/ Fe = 4).34 The in situ H2-reduced iron ore catalyst displays high activity for the promotion of heavy oil formation in the liquefaction of XL, CS, and for the coliquefaction of XL and CL; additionally, H2-reduced iron ore catalyst has the advantage of being cheaper than conventional coal or woody biomass liquefaction catalysts. Figure 1 shows few reduced iron species, but some characteristic peaks of Fe3O4 were observed in the XRD patterns for the in situ H2-reduced iron ore. This is caused by the reoxidation of the H2-reduced catalyst being exposed in the air, and the iron oxide species from the reduced iron on the surface of catalyst continually react with the unreduced Fe2O3 to form Fe3O4 in the surface of ore. Our experiments showed that the raw hematite barely increased the liquefaction conversion and oil yield for the coliquefaction of XL/CS. The results suggested that favorably dispersed nanoparticles of metallic iron formed from the raw hematite by the in situ H2reduction before liquefaction would account for the exceptionally high activity in oil production.35,36 Several publications [e.g., refs 14−17 and 37] found a synergistic effect in the coliquefaction of coal and biomass. In order to investigate the impact of iron ore catalyst on the coliquefaction of XL and CS, the weighted mean values of the conversion and oil yield were calculated (expressed by Cal) following eq 3 based on the individual liquefaction result of XL and CS as shown in Figures 3 and 4. Cal % =

Table 3. Experimental and Calculated Results and Their Difference for Coliquefaction of XL and CS with and without Iron Ore Catalyst without catalyst, wt % Expa Cala Exp − Cala

ore catalyst, wt %

conversion

oil yield

conversion

oil yield

69.6 65.5 4.1

37.4 35.3 2.1

80.4 73.4 7.0

48.3 42.9 5.4

a Experimental (Exp) and calculated (Cal) results and their difference (Exp − Cal).

The Exp − Cal values of liquefaction conversion and oil yields were 4.1 and 2.1%, respectively, without the catalyst, and then increased to 7.0 and 5.8%, in the same order, under the iron ore catalyst. The results strongly demonstrated a synergistic effect in the coliquefaction of XL and CS without the catalyst. This effect was further promoted by the iron ore catalyst at 400 °C. Furthermore, the iron ore catalyst mostly reflected in the increase of oil yield. Coughlin et al. also found an interdependent influence in the coliquefaction of lignin and bituminous coal. An explanation is that lignin could be thermally depolymerized at relatively low temperatures to form phenoxy radicals, and then the phenoxy radicals attacked the coal causing scission of aliphatic carbon− carbon bonds in the coal.38 In our recent publication on cothermolysis of biomass-related model compounds and coal, we demonstrated that thermolysis of benzyl phenyl ether could form benzyl and phenoxy radical intermediates, and these intermediates promoted the depolymerization of coal.39 From our findings, it can be inferred that in the coliquefaction of XL and CS, CS pyrolyze at low temperatures to form some intermediates, such as phenoxy and benzyloxy radicals. These radicals further attack XL molecules breaking weak bonds in XL, thus promoting the liquefaction of XL. The in-site H2-reduced iron ore can promote the formation of active H, therefore making the fragments formed from the broken, weak bonds in XL to be stable and further promoting the synergistic effects in the coliquefaction of XL and CS. 3.2. Effect of Solvent. Figures 5 and 6 display the conversion and oil yields of the XL, CS, and XL/CS (1:1 in weight) liquefaction at 360 °C, 60 min, with 2 MPa of N2 initial pressures in different solvents. The CS showed similar conversion yields in water (W) and ethanol (ET) solvents. On the other hand, the XL yielded much higher conversion in ethanol (40.4 wt %) than in water (29.0 wt %), thus resulting in the superior coliquefaction conversion of XL/CS in ethanol (60.0 wt %) than in water (55.5 wt %). Ethanol is expected to readily dissolve relatively high molecular weight products because of its lower dielectric constants compared to water;40

[WX(1 − AX )YX + WC(1 − A C)YC] × 100% WX(1 − AX ) + WC(1 − A C) (3)

where WX and WC are the weights of feedstock XL and CS, respectively, in the coliquefaction; YX and YC are the 3025

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for the formation of oil in the liquefaction of XL, CS, and XL/ CS although they were still lower than the corresponding values in ethanol. Therefore, to explore the influence of FA in the liquefaction, the oil yields obtained in ethanol solvent at 360 °C with different hydrogen resources are compared in Figure 7. At

Figure 5. Effects of type of solvent on the conversion in liquefaction of XL, CS, and XL/CS.

Figure 7. Effects of hydrogen resource on the oil yield in liquefaction of XL, CS, and XL/CS.

H 2 atmosphere, the oil yields were higher than the corresponding values in N2 atmosphere. Adding 10 vol. % of FA in N2 atmosphere resulted in slightly higher oil yields than the corresponding values at H2 atmosphere. This may suggest that FA can act as hydrogen resource in the liquefaction of XL, CS, and XL/CS. In fact, FA can easily pyrolyze to produce H2 and the in situ formed H is more active than H2; hence, promoting the liquefaction and formation of a larger oil fraction. 3.3. Effects of Temperature and Time. In order to mild the liquefaction condition, the effects of temperature and time on the liquefaction were accessed. The liquefaction was carried out in a water−ethanol (50:50, v/v) mixed solvent (W + ET) with 10 vol. % of FA. This condition was chosen because it resulted in the maximal liquefaction conversion among the solvents examined in the previous section at an initial pressure of 2 MPa N2. Due to the limitation of reactor pressure, only two temperature points, 340 and 360 °C, were tested. Figures 8 and 9 show the conversion and oil yields of the liquefaction of XL, CS, and XL/CS for 60 min at different temperatures, respectively. With the temperature rising from 340 to 360 °C, the liquefaction conversions and oil yields of XL, CS, and XL/CS slightly increased. This demonstrates that this temperature variation on the liquefaction in water−ethanol (50:50, v/v) mixed solvent does not significantly affect the liquefaction; thus, it can be carried out in at a mild temperature. In the conventional liquefaction of coal, for instance, coal is pyrolyzed to form macromolecular preasphaltene and asphaltene, and then it is converted into oil and gas.41,42 In this method, the liquefaction conversion is driven by the acetone insoluble fraction; therefore, a temperature change from 340 to 360 °C results in more macromolecular preasphaltene and asphaltene (acetone insoluble) and gas formed, thus slightly increasing the liquefaction conversion and oil yield. The effect of time in the conversion and oil yield is demonstrated in Figures 10 and 11. Prolonging the time from 30 to 90 min in the liquefaction at 360 °C, the conversion and oil yield for XL and CS slightly increased, and for XL/CS it

Figure 6. Effects of type of solvent on the oil yield in liquefaction of XL, CS, and XL/CS.

consequently, it can stimulate more conversion of XL than water. Moreover, the conversion of CS slightly changed when using an ethanol−water mixed solvent (W + ET, 50:50, v/v) in the liquefaction. However, the coliquefaction conversion of XL/CS achieved 58.8 wt %, which is close the results of coliquefaction conversion of XL/CS in ethanol. This suggests that ethanol− water mixed solvent is successful in the coliquefaction of XL and CS. Furthermore, adding 10 vol. % of formic acid (FA) in the liquefaction solvent greatly increased the conversions, especially for XL as shown in Figure 5. For example, in the liquefaction solvent of W + ET + FA state, XL, CS, and XL/CS obtained maximal conversion as follows, 47.0, 90.0, and 69.8 wt % respectively, suggesting that FA can promote the liquefaction in the three cases studied. The goal is to obtain the highest amount of oil among the liquefied products. Figure 6 shows that ethanol is a beneficial solvent for oil formation in the liquefaction, once it promoted 30.9, 41.9, and 34.4% of oil yields in the liquefaction of XL, CS, and XL/CS, respectively. The addition of 10 vol. % FA in ethanol resulted in a great increase in the oil yield to 35.2, 51.8, and 44.0%, respectively. When the solvent changed to water, the oil yields decreased for all cases; similarly, lower yields were also obtained in the W + ET solvent. Similar as in ethanol, the addition of 10 vol. % FA in water promoted the oil yields, suggesting again that FA is beneficial 3026

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Figure 11. Effects of reaction time on the oil yield in liquefaction of XL, CS, and XL/CS.

Figure 8. Effects of temperature on the conversion in liquefaction of XL, CS, and XL/CS.

at 360 °C, 60 min, with 2 MPa of N2 initial pressures, in ethanol + 10 vol. % FA as a solvent, with and without H2reduced iron ore catalyst. The elemental compositions of Exp and Cal values of the residue obtained without catalyst were similar, although the H/C of Exp was slightly lower than that of Cal. This suggests that more constituents were liquefied into oil or gases in the coliquefaction of XL and CS. Besides, compared with the Cal values, the Exp values of CHNS under the iron ore catalyst are lower, except for O that is higher than the Exp values without catalyst (Table 4). This may imply that iron ore can promote the liquefaction of XL and CS, especially for XL, thus increasing the liquefaction conversion and oil yield. Moreover, under the iron ore catalyst, the Exp values of CHN are lower, especially for C, but O is higher than the Cal values, suggesting that it can further promote a synergistic effect in the coliquefaction of XL and CS. This synergistic effect is more beneficial for the liquefaction of XL, resulting in more XL being available to be further coliquefied. The FTIR spectra of oils from the individual liquefaction of XL and CS at 360 °C, 60 min, with 2 MPa of N2 initial pressures, in ethanol + 10 vol. % FA as a solvent, with and without ore catalyst can be observed in Figure 12. The intensification of the bands around 3400 cm−1 is attributed to −OH hydrogen bonding for the oils from the liquefaction without the catalyst. They are stronger than those of the oils from the liquefaction of XL and CS under ore catalyst, implying that ore catalyst could promote some −OH groups to convert into oil fraction. For the liquefaction of XL, ore catalyst can also promote CO groups to convert into oil fraction, which is reflected by the obviously increased peak intensity around 1710 cm−1. Figure 13 shows the comparison of the experimental (Exp) and calculated (Cal) FTIR spectra of oils from the coliquefaction of XL and CS with and without ore catalyst. A similar Exp and Cal FTIR spectra was identified for the oil from the coliquefaction of XL and CS without the catalyst. Nevertheless, the intensity of the Exp band near 1710 cm−1 was somewhat lower than the Cal possibly caused by the synergistic effect. Under ore catalyst, the intensity difference between Exp and Cal spectra of the band close to 1710 cm−1 increased, expressing the ore catalyst synergistic effect converting more XL into oil. The GPC molecular weight distributions of the oils from the single liquefaction of XL and CS at 360 °C, 60 min, with 2 MPa

Figure 9. Effects of temperature on the oil yield in liquefaction of XL, CS, and XL/CS.

Figure 10. Effects of reaction time on the conversion in liquefaction of XL, CS, and XL/CS.

remained almost the same. A similar effect was observed for the temperature. The formation of macro-molecular substances and low molecular gaseous products became predominate.15 3.4. Characterization of Liquefied Products. The elemental composition measured in the experiment (Exp) and the weighted mean values calculated (Cal) are discussed in this section (Table 4). The Cal values are based on eq 1, considering the residues from the coliquefaction of XL and CS 3027

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Energy & Fuels Table 4. Ultimate Analysis Results of Residues from the Coliquefaction of XL and CS ultimate analysis, wdaf/% catalyst without iron ore a

Exp Cal Exp Cal

atomic ratio

C

H

N

S

Oa

H/C

O/C

74.83 74.32 43.76 55.18

3.63 3.90 2.25 2.79

0.80 0.75 0.46 0.59

0.37 0.46 0.28 0.29

20.37 20.56 53.25 41.15

0.58 0.63 0.62 0.61

0.20 0.21 0.91 0.56

By difference.

Figure 12. FTIR spectra of the oils from the liquefaction of individual feedstock of XL and CS with and without iron ore catalyst.

Figure 14. GPC molecular weight distribution of the oils from liquefaction of individual feedstock of XL and CS.

Figure 13. Comparison of the Exp and Cal FTIR spectra of the oils from coliquefaction of XL and CS with and without iron ore ore catalyst.

Figure 15. Comparison of the Exp and Cal GPC molecular weight distribution of the oils from coliquefaction of XL and CS with and without iron ore catalyst.

of N2 initial pressures, in ethanol + 10 vol. % FA as a solvent, with and without ore catalyst can be analyzed in Figure 14. The oil from XL liquefaction shows a larger molecular weight peak than the oil from the CS liquefaction. Thus, the average molecular weight of XL liquefaction oil is slightly higher than CS liquefaction oil. The molecular weight distributions of the oils from the liquefactions of XL with and without ore catalyst were similar. For the liquefaction of CS, the oil obtained with ore catalyst resulted in a slightly higher molecular weight than without catalyst; indicating that ore catalyst could promote the conversion of some heavy constituents of CS into oil. Finally, the Exp and Cal GPC molecular weight distributions from the coliquefaction are shown in Figure 15 with and without ore catalyst. Without the catalyst, the Exp molecular weight peak of XL/CS oil shifted to a lower level compared to

the Cal values, possibly because the synergistic effect mentioned above resulted in the formation of lighter oil. However, adding the ore catalyst changed the results. Compared with the Cal, the coliquefaction Exp oil of XL and CS contained more heavy constituents with larger molecular weight, suggesting that the catalyst promoted the conversion of more XL into the oil fraction.

4. CONCLUSIONS Ethanol was an effective solvent in the coliquefaction of XL and CS at N2 atmosphere; it achieved 60.0 wt % liquefaction conversion and 34.4 wt % of oil yield at 360 °C, 60 min. With the addition of 10 vol. % formic acid in ethanol, the liquefaction conversion and oil yield increased to 69.8 and 44.0 wt %, 3028

DOI: 10.1021/acs.energyfuels.7b03595 Energy Fuels 2018, 32, 3022−3030

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Energy & Fuels

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respectively. The formic acid forms in situ active H by pyrolysis at the liquefaction condition promoting the liquefaction and the formation of oil fraction. The ethanol−water (50:50, v/v) mixed solvent also promoted a high liquefaction conversion similar to the conversion obtained in ethanol only. However, the oil yield from ethanol−water (50:50, v/v) mixed solvent was much lower than that in ethanol. The in situ H2-reduced hematite ore was a highly active catalyst for the coliquefaction of XL and CS at N2 atmosphere. The liquefaction conversion and oil yield increased by 10.8 and 10.9 wt %, respectively, compared to those without the catalyst at 400 °C. A clear synergistic effect exists in the coliquefaction of XL and CS at 400 °C and in situ H2-reduced hematite ore catalyst. It dramatically promoted the conversion and oil production. The liquefied product characterization suggested that the ore catalyst enhanced the conversion of XL, thus increasing the oil yields.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hengfu Shui: 0000-0003-3458-5900 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Scientific Foundation of China (Grants 21476003, 21776001, 21476002, and 21476004). This work was also subsidized by Anhui Natural Science Foundation (Grant 1608085MB40). Authors are also appreciative for the financial support from the Provincial Innovative Group for Processing & Clean Utilization of Coal Resource.

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