Effects of Sodium Carbonate Additive on the Hydroliquefaction of a

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Effects of Sodium Carbonate Additive on the Hydroliquefaction of a Sub-bituminous Coal with Fe-based Catalyst under Mild Conditions Sheng Huang, Shiyong Wu, Youqing Wu, and Jinsheng Gao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02560 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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Effects of Sodium Carbonate Additive on the Hydroliquefaction of a Sub-bituminous Coal with Fe-based Catalyst under Mild Conditions Sheng Huang,†,‡ Shiyong Wu,†,‡ Youqing Wu,*,†,‡, and Jinsheng Gao†,‡ †

Department of Chemical Engineering for Energy Resources, East China University of Science and

Technology, Shanghai 200237, China ‡

Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East

China University of Science and Technology, Shanghai 200237, China ABSTRACT: In this paper, the influence of Na2CO3 (NC) on the hydroliquefaction performances of Hongliulin (HLL) coal with a Fe-based catalyst, γ-FeOOH (FC), under mild conditions were investigated, and the properties of the resulting preasphaltene (PA) and asphaltene (AS) were examined. Results show that the addition of NC to FC-catalyzed liquefaction significantly promoting coal conversion and oil production and reducing water production under mild conditions, especially at high initial H2 pressure. The H2 consumption of mild liquefaction (0.53～1.93%) are much smaller than that of the traditional liquefaction processes, and the addition of NC to FC can enhance H2 consumption, especially the runs with weak hydrogen donor solvents. The addition order of NC and FC is an important factor which can effect the activity of FC and NC binary catalysts. There exists a prominent synergistic effect between NC and FC in HLL coal hydroliquefaction, and the synergistic effect could be ascribed to the combined promotion actions of NC for the hydrolyzation of abundant oxygen-containing moieties (such as ether bonds and carbonyl groups) in HLL coal and the depolymerization of macromolecular structures of HLL coal.

1. INTRODUCTION With the rapid development of China’s economy, the demand for oil has skyrocketed in the past two ACS Paragon Plus Environment

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decades. Due to the slow growth of oil production in China, oil importation was regarded as an effective measure to ensure the oil supply, and the dependence of imported oil has reached above 60% in 2015.1 This bring a severe challenge for the energy security of China. Direct coal liquefaction (DCL) is one of the feasible Coal-To-Liquid technologies that have been available since the first half of the last century.2-5 Therefore, DCL technology attracts more and more attention for alleviating the oil shortage. China has devoted to R&D the DCL technologies in the past few decades, and by the end of 2008, the first and the largest DCL plant since WWII were completed and operated in Inner Monglia, China, which will produce about 780000 t/y of diesel fuel, 230000 t/y of naphtha and 100000 t/y of LPG.2,3 The successfully running of the industrial DCL plant aroused the interest of many researchers about the DCL technologies.6-17 In order to obtain high oil yield, severe liquefaction conditions are generally required included high temperature (440～470 ℃), high pressure (17～30 MPa), good hydrogen donor solvent and effective hydrogenation catalyst for the traditional liquefaction technologies, such as NEDOL, IGOR and Shenhua technologies.2-5 However, the traditional DCL technologies possess the disadvantages of high requirement of device, high capital and operating cost, and high H2 consumption due to severe liquefaction conditions.2-4 Therefore, the severe liquefaction conditions make the liquid fuels from DCL are economically uncompetitive compared with crude oil.4-6 In order to enhance the economy of DCL technologies, the severity of the liquefaction conditions should be reduced. In recently years, researchers are devoted to moderate the liquefaction conditions and increase the liquid yield of DCL technologies. Wu et al.18 conducted the mild liquefaction of Xilinhaote lignite on a 6.6 t/d pilot plant at 400-430 ℃ and 6～8 MPa of synthesis gas, and the products included 33% oil, 37% residue with calorific value of 28 MJ/kg and 22% fuel gas, and the process efficiency is up to 72%. Yan et al.6 investigated the liquefaction performances of Yunnan lignite at 350～425 ℃ and initial H2 pressure of 1～4 MPa, the results showed that Yunnan lignite is readily liquefied under the above mild conditions,

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and its conversion exceeds 80% at 425 ℃ even without catalyst and no holding time. Except for reduce the severity of liquefaction conditions, the addition of active catalyst can enhance the oil yield and the economy of DCL technologies, especially the mild DCL technologies.4,12-15,19-23 Catalysts play an important role in accelerating the rates of the desirable reactions such as hydrogenation, hydrocracking and heteroatom removal in coal liquefaction. To date, Fe-based catalyst is regarded as the most practical for coal hydroliquefaction due to its relatively high activity, low cost and environmental benign for disposal.4,19-23 However, it is widely accepted that the catalytic activity is not high enough is the major problem of Fe-based catalyst, especially under mild conditions.4 In the past few decades, a large number of investigations are focus on how to enhance the activity of Fe-based catalyst in coal hydroliquefaction, and many methods are adopted to achieve this goal, such as reduce the particle size to nanometre scale and improve the dispersion of Fe-based catalyst.4,20,24,25 Therefore, this paper devoted to enhance the catalytic activity of Fe-based catalyst for the hydroliquefaction of low-rank coals under mild conditions. It is well-known that low-rank coals (lignite and sub-bituminous coal) are rich in oxygen, contain a variety of functional groups, and the liquefaction of low-rank coals can be effectively promoted by alkaline catalysts in CO (or synthesis gas)/water (or mixture of hydrogen donor solvent and water) system.26-29 Cassidy et al.26 reported that NaAlO2, NaSiOx and Mg(OAC)2 can significant promote the liquefaction of partially dried Victorian brown coal catalyzed by Cu, Ni and Co catalysts with synthesis gas in the presence of tetralin, and compared with 0.30 mol Cu(OAc)2/kg dry coal alone, the conversion and oil yield increased from 66% and 29% to 74～94% and 33～46% with the addition of 0.33～1.14 mol NaAlO2/kg dry coal. Hulston et al.27 reported that the treatment of Victorian coal with alkali solution (NaAlO2 or NaOH) before Ni-Mo impregnation led to a significant increase in oil yield without solvent or added sulfur. Fu et al.28 found that the addition of Na2CO3 to CoMo-catalyzed manure hydrotreating in manure oil and water mixed system at reaction


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temperature of 380 ℃ and 6.90 MPa synthesis gas, the oil yield increased from 42～44% to 46～48%. In summary, alkaline catalysts can promote the coal liquefaction in CO (or synthesis gas)/water (or mixture of hydrogen donor solvent and water) system. However, the catalytic action of alkali metal salt for coal hydroliquefaction under traditional liquefaction processes (hydrogen-donor solvent and H2 atmosphere) is scare. The aims of this study are firstly to identify the effect of NC on the hydroliquefaction behaviors of a sub-bituminous coal with FC and secondly to clarify the roles of NC in mild coal liquefaction (liquefaction temperature of 430 ℃, H2 pressure of less than 10 MPa). Besides, the effects of hydrogen donation capacity of solvent and the addition order of NC and FC on catalytic performances of NC and FC binary catalyst were also investigated. The purpose of this study is to prepare a low cost, environmental benign and disposable catalyst with high activity for low-rank coals hydroliquefaction under mild conditions. The results of this study can shed light on the efficient liquefaction of low-rank coals. 2. EXPERIMENTAL 2.1. Coal Sample and Regents. Table 1. Proximate and ultimate analysis of HLL coal. Proximate analysis (wt%) Mad

Ad

11.00 6.56

Ultimate analysis (wt%) O

a

VMdaf

Cdaf

Hdaf

Ndaf

37.11

84.34

5.17

1.04 >9.07 0.38

daf

St,d

H/Cb 0.74

Mad: moisture (air-dried basis); Ad: ash (dry basis); VMdaf: volatile matter (dry and ash free basis); daf: dry and ash-free basis; St, d: total sulfur (dried basis); a

: by difference; b: atomic ratio.

The used coal sample was from Hongliulin coal mine (a sub-bituminous coal, termed as HLL coal), located in Yulin City, Shaanxi Province. The coal sample was ground to below 74 µm and dried in


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vacuum overnight at 80 ℃ before use. Table 1 shows the proximate and ultimate analyses of HLL coal, and Figure 1 exhibits the thermogravimetric (TG) and differential thermogravimetric (DTG) curves of HLL coal pyrolysis in N2. As shown in Figure 1, the TG and DTG curves indicates that the pyrolysis of HLL coal proceed distinctly in the temperature range of 410～470℃. Therefore, in order to obtain high coal conversion and oil yield, the hydroliquefaction temperature for HL coal should be above 410℃. 100

1.0

95

0.5

-0.5

85

-1.0 80

-1.5

75

-2.0

70

-2

-1

DTG (×10 min )

0.0

90 TG (wt%)

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-2.5

65 0

-3.0 100 200 300 400 500 600 700 800 900 1000 Temperature (℃)

Figure 1. TG and DTG curves of HLL coal pyrolysis in N2. The recycle solvent used in this study was obtained from a pilot plant of Shanghai Research Institute, China Shenhua Coal to Liquids and Chemical Co. Ltd., and the GC/MS analysis of the recycle solvent in our previous article indicated that the recycle solvent used in this study is mainly composed of ca. 70% aromatics (mainly composed of dicyclic-, tricyclic- and tetracyclic-hydrogenated aromatics such as tetralin, hydrophenanthrene, hydroprene and their alkyl substituted compounds), ca. 25% saturates (such as decalin) and a small quantity of heteroatom-containing compounds.30 Other solvents and reagents, including tetrahydrofuran (THF), toluene, n-hexane, tetralin and 1-methylnaphthalene were commercially pure chemical compounds and used as received without further purification. 2.2. Catalysts Addition. A Fe-based catalyst, γ-FeOOH (termed as FC), used in this study was prepared according to the procedures reported in our previous article.15 Na2CO3 (termed as NC) was commercially pure chemical


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compounds and used as received without further purification. For catalyst of NC or FC, ca. 20 g HLL coal was slurried with 200 ml deionized water and stirred at room temperature for 2 h to ensure complete wetting prior to catalyst addition, and then a predetermined weight of NC or FC was added into the coal-water mixture. The coal-water-catalyst solution/suspension was stirred at room temperature for 2 h to ensure good dispersion of catalyst. For FC and NC binary catalysts, after HLL coal was added to deionized water and stirred for 2 h, and then a predetermined weight of NC (or FC) was added into the coal-water mixture, the coal-water-catalyst mixture was stirred for 2 h and allowed to stand for 1 day. And then a predetermined weight of FC (or NC) was added to the mixture and stirred for 2 h. After the catalysts had been added, the coal-water-catalyst solution/suspension was dried at 50 ℃ under vacuum until the weight loss can be ignored, and the prepared catalyst termed as NCFC (or FCNC). In this study, FC, NC, NCFC and FCNC were added as follow. The added FC was 1 wt% Fe of HLL coal on dry and ash free basis and the molar ratio of added S to Fe was 1.2:1 in FC, NCFC and FCNC, the added NC was 1 wt% of HLL coal on dry and ash free basis in NC, NCFC and FCNC. 2.3. Liquefaction Experiments. Each experiment for HLL coal hydroliquefaction was carried out in a 150 mL stainless-steel, magnetically stirred autoclave. For each run, the reactor was charged with ca. 10 g HLL coal contained catalyst (or catalysts) and 15 mL solvent. Subsequently, the reactor was sealed and purged by H2 three times, followed by pressuring the system to the initial H2 pressure of 4.0 or 8.0 MPa, and then was heated to the desired liquefaction temperature of 430 ℃. After the liquefaction process was maintained at 430 ℃ for 60 min, the reactor was quenched to room temperature by cooling water. 2.4. Fractionation and Analyses. After the completion of the hydroliquefaction experiment, the gaseous products were collected with an aluminum foil bag. The gaseous products were analyzed using a gas chromatography (GC 9790-II,


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Zhejiang Fu Li Analytical instrument Co., Ltd.) for gas composition determinations and gas yield calculations. The liquid-solid mixtures were transferred from the autoclave with THF into a Soxhlet extractor and extracted as exhaustively as possible to afford residue (THF-inextractable portion) and THF-extractable portion. The water content in THF-extractable portion was determined using a Karl Fischer titration analyzer (ZTWS-8A, WeiFang ZhongTe Electronic Equipment Co., Ltd.) for the calculation of water yield. The THF-extractable fraction was sequentially extracted with n-hexane and toluene to afford oil (n-hexane soluble fraction), asphaltene (AS, n-hexane-insoluble but toluene-soluble fraction) and preasphaltene (PA, toluene-insoluble but THF-soluble fraction). The detailed fractionation procedures of liquefaction products are shown in Fig.2.

Figure 2. Fractionation procedures of liquefaction products. The HLL coal conversion was defined as the THF soluble fraction plus gas products, which was calculated from the THF insoluble residue. The oil yield was calculated as the difference between the HLL coal conversion and the sum of water, gas, AS and PA yields. All the above yields were calculated based on HLL coal on a dry and ash-free basis. The repeatability of the fractionation analyses is 2%. The H2 consumption (η H 2 ) on a dry and ash free basis were calculated by the following equations.

(η H 2 ,%)=

M H 2 − M H' 2 M daf.coal

× 100%


(1)

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where M H2 =

M H' 2 =

2.016V1 P1 +1.033 273.15 × × 22.41 1.033 T1 +273.15

2.016V2 P2 +1.033 273.15 × × × R2 22.41 1.033 T2 +273.15

(2)

(3)

Where Mdaf.coal is the weight of HLL coal on a dry and ash free basis, g; V1 is the H2 volume before liquefaction experiment, and V1 is equal to the volume of reactor minus the volume of solvent, L; V2 is the H2 volume after liquefaction experiment, and V1 is equal to V2, L; P1 and P2 are the pressures in the reactor before and after liquefaction experiment, respectively, MPa; T1 and T2 are the temperatures in the reactor before and after liquefaction experiment, respectively, ℃; R2 is H2 content in gaseous products after liquefaction experiment. The ultimate analyses of HLL coal and the resulting PAs and ASs were determined with an elemental analyzer (Vario Micro Cube, Elementar Trading Shanghai Co., Ltd.). FTIR measurement of HLL coal and the resulting PAs and ASs were performed by mixing 5 mg of samples with 200 mg of KBr, and the FTIR of samples were measured using a Perkin Elmer Spectrum One IR spectrometer at a resolution of 4 cm-1. Transmission electron microscope (TEM) images of NCFC and FCNC were taken on a JEOL JEM-1400 with an accelerating voltage of 100 kV. 3. RESULTS AND DISCUSSION 3.1. Effect of Initial H2 Pressure on HLL Coal Hydroliquefaction. Figure 3 displays the effect of NC addition on FC-catalyzed hydroliquefaction of HLL coal at reaction temperature of 430 ℃, initial H2 pressures of 4.0 MPa (reaction pressure of 8.3～8.5 MPa) and 8.0 MPa (reaction pressure of 15.4～15.8 MPa) and a tetralin to coal ratio of 1.5 : 1. It can be observed that the hydroliquefaction of HLL coal at initial H2 pressure of 4.0 MPa without catalyst presents 33.05%


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oil yield and 73.78% carbon conversion under mild conditions. In order to obtain higher oil yield and coal conversion, the hydroliquefaction of HLL coal over FC was carried out under the same conditions. The oil yield and HLL coal conversion rose to 37.36% and 78.46%, suggesting that the catalytic activity of FC for HLL coal hydroliquefaction under mild conditions is limited. However, the addition of NC results in a slight increase of oil yield and HLL coal conversion. This implies that the catalytic activity of NC in traditional liquefaction processes with hydrogen-donor solvent and H2 atmosphere under mild conditions is not obvious. 100 Product yields (wt%, daf coal)

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80

1: without catalyst; 2: NC; 3: FC; 4: NCFC 1

2

3

4

1

2

3

4

PA AS

60 water 40 gas 20 oil 0

H2 pressure of 4.0MPa

H2 pressure of 8.0MPa

Figure 3. Effect of NC addition on HLL coal hydroliquefaction with FC at different initial H2 pressures. Previous investigations reported that alkaline catalysts can obviously promote the hydrothermal liquefaction of coal (especially the low-rank coals) and biomass with CO or synthesis gas.26-32 However, investigations on the role of alkaline catalysts in traditional liquefaction processes with hydrogen-donor solvent and H2 atmosphere is scare, especially under mild conditions. In order to clarify the role of alkaline catalysts in traditional liquefaction processes, HLL coal liquefaction over NCFC (NC and FC binary catalyst) was performed. The result shows in Figure 3 indicates that with the addition of NC to FC, HLL coal conversion increased slightly, while oil yield appreciably increased from 37.36% to 43.16% and AS yield decreased from 14.18% to 11.64%, confirming the promotion action of NC in FC-catalyzed


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HLL coal liquefaction with hydrogen donor solvent and H2. From the above results, it could be speculated that there exists a synergistic effect between NC and FC for promoting coal conversion and oil production in HLL coal liquefaction. In order to investigate the effect of initial H2 pressure on the catalytic performances of NCFC, the hydroliquefaction of HLL coal at the initial H2 pressure of 8.0MPa was performed. As Figure 3 exhibits, in the absence of catalyst, HLL coal conversion increased from 73.78% to 76.89%, while oil yield slightly increased from 33.05% to 33.52% when the initial H2 pressure rose from 4.0 MPa to 8.0 MPa, indicating that the oil yield of HLL coal thermal liquefaction is insensitive to the H2 pressure. This demonstrates that the effect of H2 pressure on HLL coal conversion and product distribution is tiny in the absence of catalyst, and this could be ascribed to the amount of activated hydrogen dissociated from H2 vary slightly in the absence of catalyst under different H2 pressures.10,11 Therefore, the addition of an active catalyst is highly imperative for obtaining high coal conversion and oil yield. Moreover, as the initial H2 pressure increased from 4.0 MPa to 8.0 MPa, HLL coal conversion rose from 78.46% to 82.56% over FC and from 81.52% to 85.79% over NCFC. Meanwhile, oil yield rose from 37.36% to 40.62% over FC and from 43.16% to 47.22% over NCFC, suggesting that higher H2 pressure is conducive to improve the hydroliqeufaction performances of HLL coal over FC and NCFC. With the addition of NC to FC at the initial H2 pressure of 8.0 MPa, HLL coal conversion and oil yield respectively increased by 3.56% and 6.60%, which is greater than those at the initial H2 pressure of 4.0MPa. This indicates that the addition of NC can obviously enhance the hydroliquefaction performances of HLL coal over FC, especially at relatively high H2 pressure. 3.2. Effect of Solvent Quality on HLL Coal Hydroliquefaction. Previous investigations10,11,19,33 reported that solvent quality is an important factor in coal liquefaction. Therefore, the catalytic liquefaction performances of HLL coal using three solvents with different ACS Paragon Plus Environment

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hydrogen donating capabilities were performed. As presented in Figure 4, the hydrogen donating capability of solvent effect HLL coal conversion and product distribution notably without or with FC and NCFC. It is well-known that activated hydrogen is mainly dissociated from hydrogen donor solvent and molecular hydrogen in the presence of active catalyst, and the dissociation of molecular hydrogen without catalyst is limited.8,10,11,20,21,34,35 Niu et al.10,11 reported that about 65～70% activated hydrogen dissociated from hydrogen donor solvent and 30 ～ 35% from molecular hydrogen in the absence of catalyst. Therefore, the activated hydrogen is mainly coming from hydrogen donor solvent in HLL coal liquefaction without catalyst. Figure 4 shows the oil yield and conversion of HLL coal thermal liquefaction with tetralin, recycle solvent and 1-methylnaphthalene followed the order of tetralin> recycle solvent>1-methylnaphthalene. It is well-accepted that tetralin is a very good hydrogen donating and transferring solvent for coal liquefaction, while substituted aromatic, 1-methylnaphthalene has a very small amount of activated hydrogen. Recycle solvent is mainly composed of aromatics, hydrogenated aromatics and saturates, and it could dissociate a large amount of activated hydrogen in HLL coal liquefaction.30 For HLL coal liquefaction without catalyst, it could be inferred that the oil yield and HLL coal conversion are closely related with the hydrogen donating ability of solvent. This indicates that hydrogen donating capability of solvent is an important factor which can determine coal conversion and product distribution.


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90 Product yields (wt%, daf coal)

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75

1

3

2

3 2

60

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1: without catalyst 2: FC 3: NCFC

PA AS

1

45

1

2

3

30

water gas

15 oil

0

tetralin

recycle solvent 1-methylnaphthalene

Figure 4. Effect of hydrogen donating capability of solvent on product distribution of HLL coal hydroliquefaction without or with FC and NCFC. As Figure 4 displays, the oil yield and HLL coal conversion increased slightly from 9.18% and 39.10% to 11.16% and 41.87% over FC and 12.56% and 46.66% over NCFC with 1-methylnaphthalene as solvent. An active catalyst can promote the dissociation of activated hydrogen from molecular hydrogen, while an active catalyst has little effect on the dissociation of activated hydrogen from solvent.10,11,20,21,34,35 Consequently, the activated hydrogen dissociated from molecular hydrogen and hydrogen donor solvent increased in the presence of an active catalyst.10,11,20 In the presence of FC and NCFC, the amount of produced activated hydrogen increased, while the transfer of activated hydrogen to coal was restricted due to the weak hydrogen transfer ability of 1-methylnaphthalene. Simultaneously, Figure 4 shows that FC promotes the increase of oil yield and coal conversion from 17.03% and 54.09% to 26.29% and 65.18% with recycle solvent as solvent, and with the addition of NC to FC, the oil yield and coal conversion further increase to 34.13% and 73.81%. In the presence of FC or NCFC, the activated hydrogen dissociated from molecular hydrogen increased substantially, and the produced activated hydrogen could be quickly transferred by recycle solvent and stabilized the free radicals produced from HLL coal pyrolysis. Comparing to HLL coal liquefaction without catalyst, it is surprisingly found that the oil yield and coal conversion increased by 4.31% and 4.68% over FC and 10.11% and 7.74% over NCFC with


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tetralin as solvent, which are much smaller than those of recycle solvent. Undoubtedly, tetralin is the best solvent as hydrogen donor and transfer agent among the three solvents.22,34 The possible reason is that a large amount of activated hydrogen dissociated from tetralin could stabilize most of free radicals produced from HLL coal pyrolysis, and with the addition of FC or NCFC, the activated hydrogen produced from molecular hydrogen is more than the hydrogen needed for radical fragments stabilization. Besides, Figure 4 exhibits that with the addition of NC to FC, the water yield diminishes with the three solvents. One possible reason is the promotion of NC to the hydrolyzation reactions of abundant oxygen-containing moieties, such as ether bonds and carbonyl groups, in HLL coal.15,37,38 Another possible reason is the promotion of NC to the water-gas shift reaction during the liquefaction process,26-29 and this can be verified by the decreased CO yield and increased CO2 yield in gaseous products of HLL coal liquefaction over NCFC, as shown in Figure 5. So far, no general consensus for hydrogen transfer mechanism in hydroliquefaciton has been reached.10-12,35,36 Many researchers believed that active catalyst promotes hydrogen transfer from molecular hydrogen to solvent and then from solvent to coal.12 But other researchers suggested that the hydrogen transfer occurs directly from molecular hydrogen to coal in the presence of active catalyst rather than via hydrogen donor solvent.35,36 As presented in Figure 4, the different increment of oil yields and coal conversions of HLL coal hydroliquefaction with three different solvents indicates that the possible hydrogen transfer mechanism is solvent acts as a medium for transferring H2 to coal rather than directly from H2 to coal. There is no doubt that the activated hydrogen dissociated from hydrgen donor solvent and molecular hydrogen both are important to realize the efficient liquefaction of coal, and high oil yield and conversion are the results of the mutual effect of catalyst, hydrogen donor solvent and H2.33-36 The characteristics of Shenhua DCL technology include the use of hydrogenated recycle solvent and a nano-size iron-based catalyst, which can ensure the effective dissociation of activated hydrogen from


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recycle solvent and molecular hydrogen to realize the efficient conversion of coal.2,3 As Figure 5 exhibits, C1-C4 is the predominant component in gaseous products with the three solvents, and the yields of C1-C4 over FC and NCFC are higher than that without catalyst, declaring that FC and NCFC promotes the hydrocracking reactions to produce more C1-C4, especially NCFC. Besides, C1-C4 yields of HLL coal liquefaction with recycle solvent and 1-methylnaphthalene are higher than that with tetralin without or with FC and NCFC, and this could be attributed to the cracking of recycle solvent and 1-methylnaphthalene in HLL coal liquefaction. This indicates that solvent would undergo side reactions such as isomerization, cracking, polymerization reactions in liquefaction process, and the resulting losses can lead to solvent imbalance.4 Therefore, the liquefaction parameters, such as liquefaction temperature, H2 pressure and catalyst should be properly select to avoid the excessive decomposition of solvent. CO CO2

12

C1-C4

1: without catalyst 2: FC 3: NCFC

2.5 2.0

9

1.5

6

1.0

3

0.5

0

1

2 3 tetralin

H2 consumption (wt%, daf coal)

15 Gas yields (wt%, daf coal)

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0.0 2 3 1 2 3 1 recycle solvent 1-methylnaphthalene

Figure 5. Gaseous product yields and H2 consumption of HLL coal hydroliquefaction using three solvents with different hydrogen donating capabilities. Furthermore, the H2 consumption of HLL coal thermal liquefaction followed the order of 1-methylnaphthalene>recycle solvent>tetralin, and this could ascribe to the activated hydrogen dissociated from good hydrogen donor solvent can stabilize more radical fragments from HLL coal pyrolysis, leading to the lower H2 consumption of HLL coal liquefaction with good hydrogen donor solvent. With the addition of FC, the activated hydrogen dissociated from molecular hydrogen was


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promoted, consequently the H2 consumption of HLL coal hydroliquefaction increased remarkably, especially the run with 1-methylnaphthalene as solvent. With the addition of NCFC, the H2 consumption further increased. This is probably due to NC can promote the hydrolyzation of abundant oxygen-containing moieties (such as ether bonds and carbonyl groups) in HLL coal and the depolymerization of macromolecular structures of HLL coal.26,31,32 Therefore, more radical fragments were produced from HLL coal pyrolysis and these radical fragments could combine more activated hydrogen, leading to the higher H2 consumption of HLL coal liquefaction over NCFC. The above explanation can be verified in the sections 3.3 and 3.4 of this study. This indicates that the addition of NC to FC is beneficial for the hydrogenation of HLL coal, which is in accordance with the higher oil yield and coal conversion in the presence of NCFC. The above results also suggest that the dominant function of catalyst is to promote the activation of H2 to produce more activated hydrogen in DCL process. Moreover, Figure 5 shows that the H2 consumption of HLL coal liquefaction under mild conditions in this study is 0.53～1.93%, which are much smaller than that of the traditional liquefaction processes under high temperatures and high pressures (liquefaction temperature of 440～470 ℃ and reaction pressure of 17～30 MPa). The H2 consumptions of Shenhua coal hydroliquefaction in the NEDOL process (reaction temperature of 465 ℃ and reaction pressure of 18 MPa) and Xianfeng lignite hydroliquefaction in the IGOR process (reaction temperature of 470 ℃ and reaction pressure of 30 MPa) were 6.1% and 11.2%,3,23 which are much higher than the H2 consumption of HLL coal liquefaction under mild conditions. Furthermore, Shenhua coal conversion and oil yield in the NEDOL process were 89.7% and 52.8%, and Xianfeng lignite coal conversion and oil yield in the IGOR process were 97.5% and 58.6%, respectively.3,23 The above results indicate that the coal conversion and oil yield of the traditional liquefaction processes, such as the HTI, IGOR, NEDOL and Shenhua coal liquefaction processes, are higher than those of HLL coal hydroliquefaction under mild conditions.


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The DCL process operates under high temperatures and high pressures lead to high capital and operating costs, and it is reported that the capital costs accounts for about 50 ～ 80% of the total investment of the traditional coal liquefaction processes.4,23 However, the operating and capital costs will be substantially reduced under the mild liquefaction conditions. Besides, it requires continuous supply of expensive H2 at high temperatures (via coal gasification) for coal liquefaction process, and the capital costs of the H2 production unit accounts for about 15～20% of the total investment of the traditional coal liquefaction processes.4,23,37,38 The H2 consumption of coal liquefaction under mild conditions are much smaller than that of the traditional liquefaction processes, therefore, the capital costs of the H2 production unit will be reduced greatly. Hence, the mild liquefaction processes would more economically competitive with the traditional liquefaction processes. 3.3. Effect of Addition Order of NC and FC on HLL Coal Hydroliquefaction over FC and NC Binary Catalyst. 45 Product yields (wt%, daf coal)

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FC only FCNC NCFC

40 35 30 25 20 15 10 5 0

oil

gas

water

AS

PA

Figure 6. Effect of addition order of FC and NC on HLL coal hydroliquefaction performances over FC and NC binary catalyst. Fig.6 presents the effect of addition order of NC and FC on the product distribution of HLL coal hydroliquefaction catalyzed by NCFC (HLL coal impregnated first with NC and second with FC) and FCNC (HLL coal impregnated first with FC and second with NC) at 430 ℃, initial H2 pressure of 4.0 ACS Paragon Plus Environment

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MPa and a tetralin to coal ratio of 1.5 : 1. Interestingly, the effect of addition order of FC and NC on HLL coal conversion and oil yield is obvious, and the oil yield of HLL coal hydroliquefaction over NCFC (43.16%) is ca. 2.50% higher than that over FCNC (40.63%). Accordingly, HLL coal hydroliquefaction over NCFC presents lower gas, water and AS yields compared with those over FCNC. This is probably due to the modification of HLL coal structure induced by NC treatment. With the impregnation of NC onto HLL coal, the cationic Na will replace hydrogen and/or other cations of carboxyl or phenolic groups in HLL coal, and this will alter the non-covalent bond energy and the physical structure of HLL coal. The change of coal structure will alter the natural reactivity of HLL coal, consequently the liquefaction behaviors of HLL coal. Roberts et al.39 investigated the influence of alkali carbonates on benzyl phenyl ether cleavage pathways in superheated water and found that the alkali cation interacts strongly with the oxygen of benzyl phenyl ether, the ether bond was strongly polarized and cleaves eventually heterolytically, leading to the enhanced yields of toluene, 2 and 4-benzyl phenol in benzyl phenyl ether conversion with superheated water. Eom et al.40 reported that during the hydrolysis of phenethyl phenyl ether (PPE) with Na2CO3, the Na+-PPE was a pivotal intermediate, which was dissociated to yield phenol, styrene and ethylbenzene, consequently the conversion of PPE was greatly enhanced with the addition of Na2CO3. Besides, the changes of HLL coal structure can affect the dispersion and catalytic activity of FC. Extensive literatures reported that the nature of the support has a significant influence on the catalytic activity of the carbon-supported hydrogenation catalysts.41,42 HLL coal impregnated first with NC and second with FC would result in the displacement of cationic Na by cationic FeO, leading to the better dispersion of FC than that obtained without preliminary NC treatment. As shown in the TEM microphotographs of NCFC and FCNC (Fig. 7), some FC (needle shaped particles43-46) in the NCFC is evenly dispersed around and/or on the surface of the coal particles (irregular grain), while most of FC in the FCNC is agglomerated to form a FC cluster. This indicates that the addition order of FC and NC could


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effect the dispersion extent of FC on the surface of HLL coal, and FC in the NCFC presents better dispersion extent than that of the FCNC. (a)

(b)

HLL coal FC

FC

HLL coal

200 nm

Figure 7. TEM images of NCFC (a) and FCNC (b) binary catalysts. However, the specific reasons for lower oil yield and conversion of HLL coal hydroliquefaction over FCNC is uncertain. This could be attributed to the cationic FeO bound to acidic functional groups, such as phenolic and carboxyl groups, in HLL coal, and thus NC cannot effectively promote the hydrolyzation of abundant oxygen-containing moieties in HLL coal and the depolymerization of macromolecular structures of HLL coal. This is a very interesting phenomenon which deserves further study. Therefore, in order to enhance the catalytic performances of NC and FC binary catalyst in industrial scale DCL plants, NC and FC could be added to the different locations of liquefaction system. For instance, NC and FC could be respectively added into the coal slurry tank and coal slurry preheater or NC and FC could be added to the different locations of the coal slurry tank, to optimize the catalytic activity of the NC and FC binary catalyst.


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3.4. Characterization of HLL Coal and the Resulting PAs and ASs. Table 2. Elemental compositions of the resulting PAs and ASs from HLL coal liquefaction. Catalysts Samples

Ultimate analysis (wt%, daf) C

H

N

S

O

a

H/Cb

None

PA

89.16

5.88

1.08

0.26

3.62

0.75

None

AS

88.92

5.96

1.08

0.24

3.80

0.80

FC

PA

88.58

6.05

1.17

0.28

3.92

0.83

FC

AS

87.70

6.12

1.09

0.20

4.89

0.84

NCFC

PA

88.56

6.29

1.04

0.25

3.86

0.85

NCFC

AS

88.27

6.72

1.04

0.26

3.71

0.91

daf: dry and ash-free basis; a: by difference; b: atomic ratio. Table 2 presents the elemental analysis of PAs and ASs produced from HLL coal hydroliquefaction at 430 ℃, initial H2 pressure of 4.0 MPa and a tetralin to coal ratio of 1.5: 1. It can be seen in Tables 1 and 2 that the oxygen contents of PAs and ASs without or with FC and NCFC are much lower than that of HLL coal, indicating that severe deoxygenation reactions occurred during the mild liquefaction of HLL coal. Besides, the sulfur and nitrogen contents of HLL coal and the resulting PAs and ASs remained almost unchanged, suggesting that the catalytic effect of FC and NCFC on hydrodesulfurization and hydrodenitrogenation reactions are not as significant as hydrodeoxygenation reactions. H/C atom ratios of PAs and ASs obtained without or with FC and NCFC are much higher than that of HLL coal, implying that the hydrogenation reactions proceeded during the mild liquefaction of HLL coal. Furthermore, the H/C atom ratios of PA and AS with NCFC are higher than that without or with FC, suggesting that the addition of NC could enhance the hydrogenation activity of FC, which is in accordance with the results shown in Figs 3 and 4.


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(1) (2)

Transmittance

(3) (4) (5) (6)

4000

3500

3000 2500 2000 1500 -1 Wavenumbers (cm )

915

1000

750

1140

1620 1450

2920 2855

(7) 3420

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500

Figure 8. FTIR spectra of HLL and the resulting PAs and ASs from hydroliquefaction without or with FC and NCFC. FTIR spectra of HLL coal (1), PA obtained without catalyst (2), over FC (3) and over NCFC (4), AS obtained without catalyst (5), over FC (6) and over NCFC (7). As presented in Figure 8, the main characteristic peaks of HLL coal and the resulting PAs and ASs included O-H (3420 cm-1), Car-H (3040 cm-1, 745-880 cm-1), Cal-H (2920 cm-1, 2850 cm-1, 1450 cm-1), aromatic ring (1620 cm-1) and C-O-C (1140 cm-1) groups. It can be noted that HLL coal and all PAs and ASs display a broad OH peak at the wavenumber of 3420 cm-1, suggesting that HLL coal and the resulting PAs and ASs contain plenty of hydrogen bonded OH group. This indicates that some OH groups are difficult to remove under the mild liquefaction conditions. However, a decreased intensities of OH group (at 3420 cm-1) for ASs and PAs obtained without catalyst and over FC and NCFC in comparison with that of HLL coal, implying that some hydrogen bonded OH groups were removed during the hydroliquefaction of HLL coal. As Figure 8 shows, the intensities of the bonds at 2920 and 2855 cm-1, which are assigned as the symmertrical and asymmetrical aliphatic C-H bonds stretching in methyl and methylene groups of PA and AS over NCFC are higher than those over FC. This indicates that NCFC present higher hydrogenation activity than FC, which is in line with the H2 consumption in Figure 5. The peak at 1140 cm-1 is reported to result from the stretching vibration of C-O-C bond, and it can be observed that its intensities of PA and AS decreased with the addition of NC to FC. The above results


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imply that the addition of NC to FC can promote the cleave of ether bonds, which is probably due to the hydrolyzation of ether bonds. In summary, it can be concluded that the addition of NC to FC can obviously improve the hydroliquefaction performances of HLL coal. The promotion actions of NC for FC-catalyzed hydroliquefaction of HLL coal could be attributed to NC can interact with some oxygen-containing functional groups, such as ether bonds and carbonyl groups, in HLL coal, and subsequently the reacted oxygen-containing functional groups were strongly polarized.39 Consequently, the non-covalent bond energy of HLL coal was altered and the reacted oxygen-containing functional groups were readily cleaved due to the hydrolyzation reactions.15,39,40 Ultimately, the depolymerization of macromolecular structures of HLL coal was promoted in the presence of NC. Therefore, the addition of NC to FC is conducive to promote the HLL coal conversion and oil production. The FC and NC binary catalyst (especially NCFC) is effective for promoting hydroliquefaction of low-rank coals under mild conditions. 4. CONCLUSIONS (1) The catalytic activity of FC for HLL coal hydroliquefacton under mild conditions is limited, and the addition of NC to HLL coal hydroliquefaction over FC can significantly promote coal conversion and oil production and reduce water production, especially at relatively high H2 pressure. The oil yield of HLL coal liquefaction over NC and FC binary catalysts are higher than that of HLL coal thermal liquefaction by ca. 10% under mild conditions. The hydrogen donating capability of solvent effect HLL coal conversion and product distribution notably without or with FC and NCFC, and the possible hydrogen transfer mechanism is solvent acts as a medium for transferring molecular hydrogen to coal. (2) The H2 consumption of HLL coal mild liquefaction are much smaller than that of the traditional liquefaction processes, and the addition of NC to FC-catalyzed HLL coal liquefaction can enhance H2 consumption, especially the runs with weak hydrogen donor solvents. The effect of addition order of FC ACS Paragon Plus Environment

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and NC on HLL coal hydroliquefaction performances is obvious, and the oil yield of HLL coal liquefaction over NCFC is ca. 2.5% higher than that over FCNC. (3) There exists a prominent synergistic effect between NC and FC in HLL coal hydroliquefaction, and the synergistic effect could be ascribed to the combined promotion actions of NC for the hydrolyzation of abundant oxygen-containing moieties (such as ether bonds and carbonyl groups) in HLL coal and the depolymerization of macromolecular structures of HLL coal. The NC and FC binary catalyst (especially NCFC) is effective for promoting hydroliquefaction of low-rank coals under mild conditions. AUTHOR INFORMATION Corresponding Author *Tel.: +86-021-64252037. E-mail: [email protected]. ORCID Sheng Huang: 0000-0003-4328-8756.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21476079, 21476080). REFERENCES (1) Moerkerk, M. V.; Crijns-Graus, W. Energy Policy 2016, 88, 148-158. (2) Liu, Z.Y.; Shi, S. D.; Li, Y. W. Chemical Engineering Science 2010, 65 (1), 12-17. (3) Shui, H. F.; Cai, Z. Y.; Xu, C. B. Energies 2010, 3 (2), 155-170. (4) Vasireddy, S.; Morreale, B.; Cugini, A.; Song, C. S.; Spivey, J. J. Energy & Environmental Science 2010, 4 (2), 311-345.


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Figure Captions Figure 1. TG and DTG curves of HLL coal pyrolysis in N2. Figure 2. Fractionation procedures of liquefaction products. ACS Paragon Plus Environment

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Figure 3. Effect of NC addition on HLL coal hydroliquefaction with FC at different initial H2 pressures. Figure 4. Effect of hydrogen donating capability of solvent on product distribution of HLL coal hydroliquefaction without or with FC and NCFC. Figure 5. Gaseous product yields and H2 consumption of HLL coal hydroliquefaction using three solvents with different hydrogen donating capabilities. Figure 6. Effect of addition order of FC and NC on HLL coal hydroliquefaction performances over FC and NC binary catalyst. Figure 7. TEM images of NCFC (a) and FCNC (b) binary catalysts. Figure 8. FTIR spectra of HLL and the resulting PAs and ASs from hydroliquefaction without or with FC and NCFC. FTIR spectra of HLL (1), PA obtained without catalyst (2), over FC (3) and over NCFC (4), AS obtained without catalyst (5), over FC (6) and over NCFC (7).


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