Catalytic upgrading of volatiles from coal pyrolysis over sulfated carbon-based catalysts derived from waste red oil

Fuel Processing Technology 189 (2019) 98–109

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Research article

Catalytic upgrading of volatiles from coal pyrolysis over sulfated carbonbased catalysts derived from waste red oil

T

Deliang Wanga,b, Zhaohui Chena, , Zhimao Zhoua,c, Demin Wanga,b, Jian Yua, Shiqiu Gaoa, ⁎

⁎

a

State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100190, China c Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China b

ARTICLE INFO

ABSTRACT

Keywords: Catalytic pyrolysis Sulfated carbon-based catalysts Red oil Red oil-derived carbon materials Tar upgrading

Sulfated carbon-based catalysts, red oil-derived carbon materials (RDCMs), were prepared by the self-assembly process from industrial alkylation process waste red oil. The performance of catalysts on upgrading of coal pyrolysis volatiles was investigated in a two-stage fixed-bed reactor. Compared with activated carbon (AC) or char, RDCMs exhibited better performance in increasing the yield and fraction of light tar (boiling point < 360 °C). The higher specific surface areas and relatively more defects for RDCMs promoted the conversion of more heavy tar into light tar. The high-content doped sulfur in RDCMs contributed to more oxidation reaction in the secondary reaction for producing gaseous products. The pyrolysis water could be activated on the sulfur functional group to form ·H and ·OH radicals, which were able to stabilize the larger radical fragments from tar cracking to form more oxygenated organic compounds. The yield and fraction of light tar after catalytic upgrading using RDCMs (10 wt% of the tested coal) at 500 °C were 7.9 wt% and 80.0%, respectively, which increased by 19.6% and 33.3% in comparison with that from coal pyrolysis without catalyst.

1. Introduction Pyrolysis of low-rank coals can produce tar as an alternative to petroleum route. It is important to many countries with poor in oil but rich in coal [1–3]. The breakage of weak bonds and the rearrangement of free radical during coal pyrolysis are very complex, so it is hard to control the pyrolysis reactions to produce high-quality tar. As a result, heavy fraction in tar accounted for about 50–70 wt%, which could cause operational troubles and subsequent treatment problems [4,5]. Therefore, catalytic upgrading of volatiles from coal was considered as the most prospected technique to decrease the content of heavy tar in mild conditions [6,7]. Various kinds of catalysts including zeolites [8,9], metal modified zeolites [10–12], metal salts [13], metal oxide [14], red mud-based catalyst [15], and carbon-based catalysts (CBC) [4,16] were evaluated for upgrading of coal tar. Among them, CBC such as char from coal pyrolysis and metal-modified char, activated carbon, bio-char, et al. showed promise candidate for industrial application because of easy preparation and low cost [4,6,17]. But different CBC have various characteristics, thus showing different catalytic performances on tar upgrading. Generally, their catalytic activities were related with porous structure, alkali and alkaline earth metallic species over char, acids

⁎

distribution and functional groups. Han et al. [4] created L acids sites over char by loading NieCe metal to increase the catalytic activities of char and the results showed that the light tar fraction increased by 32%. Jin et al. [6] found that carbon catalyst with high specific surface area and relatively more defects benefited the conversion of heavy fraction into light fraction. Thus, the increase of light tar yield over AC catalyst was more than that over coal char. Fu et al. [18] investigated the catalytic effect of AAEM in bio-char and found that the presence of mineral contributed to the catalytic cracking of tar. Many studies also reported that O-containing functional groups in char could serve as acid sites for adsorption and cracking of tar [6,18,19]. Recently, sulfated carbon-based catalysts (SCBC) were successfully prepared from biomass and household waste [20,21]. The introduction of sulfur to carbon could alter the surface charge distribution and electrostatic potential of graphene remarkably, thus resulting in much higher catalytic activity in phenol oxidation reaction [22]. More importantly, it was evidenced that ·OH radicals was formed during the catalytic oxidation over the surface of the SCBC [22,23]. Duan et al. [23] proposed the radical-based mechanism through water oxidation. The HeO bond in H2O was lengthened to donate electrons to the carbon matrix, meanwhile carbon matrix served as bridge for charge transport to activate the sulfur functional group. That would induce electron by

Corresponding authors. E-mail addresses: [email protected] (Z. Chen), [email protected] (S. Gao).

https://doi.org/10.1016/j.fuproc.2019.03.003 Received 29 December 2018; Received in revised form 15 February 2019; Accepted 10 March 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. The schematic of the technical route for upgrading of volatiles using RDCMs.

Table 1 Proximate and ultimate analyses of Naomaohu Coal, Char, AC, PCMs, and RDCMs. Sample

Proximate analysis (wt%, dry base) Ash

Raw coal Char AC PCMs RDCMs a

7.16 16.62 13.68 4.11 5.35

V.M. 47.78 7.77 4.87 58.17 8.12

Ultimate analysis (wt%, dry and ash-free) F.C.

C

45.06 75.61 81.45 37.72 86.53

H 74.77 90.27 92.06 55.54 87.42

S 5.67 1.10 1.18 3.67 1.34

Oa

N 0.27 0.48 0.79 7.85 7.32

0.93 1.16 0.88 0.05 0.39

18.36 6.99 5.09 32.89 3.53

By difference.

subsequence electrons transport to enhance OeO breakup, exhibiting excellent route for organic oxidation [23]. It is well accepted that the reactions of volatiles from coal pyrolysis follows by free radicals mechanism [5]. How to directly stabilize free radicals in volatiles rather than repolymerization and condensation of larger free radical fragments is still a huge challenge. This is because in the normal conditions the larger free radicals were difficult to capture ·H due to the limit of hydrogenation reaction rate. The SCBC provided possibility that free radical fragments adsorbed on the catalyst surface and could react with the ·OH and ·H radicals via water activation to form tar component not coke. Waste material red oil (RO) largely produced from the alkylation process. RO was a kind of mixture solution and contained 5–7 wt% acid soluble oil and 90 wt% sulfuric acid, which is difficult to deal with [24–26]. In this study, we made full of RO to prepare a novel and natural sulfated carbon-based catalyst, red oil-derived carbon materials (RDCMs) for catalytic upgrading of coal pyrolysis volatiles. As is shown in Fig. 1, RO was treated with one-pot hydrothermal reaction to form sulfated polycarbon materials (PCMs), thus separating the acid soluble oil and sulfuric acid [27]. Under pyrolysis conditions, the PCMs was changed to RDCMs with doped sulfur and high specific surface area because of the release of volatiles. In this paper, the self-assembly

preparation of RDCMs was investigated based on several analysis methods and the catalytic effect of sulfur group in RDCMs on secondary reaction of volatiles was discussed. 2. Experimental 2.1. Material and Properties The Naomaohu subbituminous coal from Xinjiang province, China, was grounded, sieved to 0.4 to 1.0 mm, and dried at 105 °C for 2 h before experiments. RDCMs were made from red oil from Puyang Shengyuan Group, Henan province, China. The contents of sulfuric acid, water, and acid soluble oil were determined by titration, mixing a fuming sulfuric acid, and by difference, respectively [28]. The compositions of red oil were 90.0 wt% sulfuric acid, 3.7 wt% water, and 6.3 wt % acid soluble oil. RDCMs was prepared in the following procedure. 100 mL red oil was added to a 500 mL flask and heated to 200 °C for 4 h. Then, the obtained suspended solution was filtered. The filter cake, poly carbon materials (PCMs), was dried at 105 °C for 12 h. Finally, the RDCMs was prepared by pyrolyzing the PCMs in N2 at 750 °C for 1 h in a fixed bed reactor. Moreover, a commercial activated carbon (AC) and char

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valve 2 were closed and high purity N2 swept the whole system. When the upper and lower stages reached their designated pyrolysis (Tpy) and cracking (Tcr) temperatures, the coal was introduced to the upper stage of reactor quickly through N2 blowing by opening the valve 2 lower ball valve 5. Then the valve 2 and lower ball valve 5 were closed to start the experiment. The pyrolysis volatiles were cooled down by a pipe condenser at −13.5 °C. Several acetone-washing bottles immersed in an ice-water bath were used to obtain tar. A saturated sodium bicarbonatewashing bottle and an allotropic silica gel-washing bottle were used to absorb H2S and water, respectively. The gas was collected by gas bag and measured its composition in a micro GC. After the experiment, acetone was employed to wash pipe and condenser for collecting the tar. Thereafter, water was removed by adding MgSO4 into the tar. Solid particles were removed by filtration. After evaporation of the acetone at 25 °C and reduced pressure in a rotary vacuum evaporator, the obtained tar was weighted to calculate the tar yield. In order to maximize the yield of tar, a series of tests were conducted in the adopted reactor by changing the N2 flow rate, reaction time, and pyrolysis temperature. The results showed that the highest tar and light tar yields of about 11.1 wt% and 6.6 wt% (dry basis, db) was obtained at 600 °C and N2 flow of 100 mL/min in 45 min. Therefore, all the coal pyrolysis experiments herein were conducted at 600 °C to maintain the possibly highest tar yield. 2.3. Characterization of the carbon-based catalysts Fig. 2. Schematic diagram of experimental apparatus: (1) mass flow meter; (2) valve; (3) upper ball valve; (4) coal feeder; (5) lower ball valve; (6) upper thermocouple; (7) upper stage; (8) electric furnace; (9) lower stage; (10) lower thermocouple; (11) condenser; (12) acetone trap; (13) dry silica gel bottle; (14) valve.

Preliminary pyrolysis of the CBC was determined using a thermogravimetry analyzer (TG, Hitachi 7300). About 6 mg of catalyst was heated in high-purity N2 (200 mL/min) up to 105 °C, kept it there for 10 min, further heated to 800 °C at a rate of 5 °C/min. The TG-MS experiments were performed simultaneously using a thermogravimeter (TG, KEP Technologies LABSYS evo TGA 1600) and a mass spectrometer (MS, Tilon grp technology limited). The crystal patterns of the catalysts were obtained from an X-ray diffraction analyzer (XRD, Smartlab) with a scanning rate of 0.02°/s in 5–90° for 2θ at 40 kV and 200 mA. The functional groups on the catalysts were studied using Fourier transform infrared spectroscopy (FTIR, Bruker, Tensor 27). Approximately 10 mg of catalysts and 90 mg of potassium bromide (KBr) were grinded, tableted physically, and measured by FTIR spectroscopy. The valence state of sulfur in the catalysts were measured by X-ray photoelectron spectra (XPS, Thermo Scientific Escalab 250Xi). The porous structures of the catalysts were investigated on a micropore physisorption analyzer (ASAP 2020 HD88). Before the measurement, the samples were degassed at 300 °C for 5 h under vacuum. The specific surface area and micro surface area were calculated with the BrunauerEmmett-Teller (BET) and t-plot methods, respectively. The structure defects of the catalysts were studied by a Raman Spectrometer (Horiba Jobin Yvon, Labram HR800). The laser power at the sample surface was controlled at about 1 mW. The Raman spectra were recorded in the range of 800–1800 cm−1 with a spectral resolution of 2 cm−1.

prepared by pyrolyzing the above-mentioned raw coal in N2 at 750 °C for 1 h in a fixed bed reactor were employed as comparison catalysts. Table 1 shows the results of proximate and ultimate analyses of the tested coal, Char, AC, PCMs, and RDCMs. Compared to that of PCMs, the carbon content of RDCMs increased dramatically but the oxygen content and volatiles were decreased dramatically. The domain content in the CBC was carbon about 90 wt%, while the sulfur in RDCMs was much higher than that in AC and char. Moreover, the ashes of PCMs and RDCMs were characterized by an X-ray fluorescence spectrometer (XRF, PANalytical, AXIOS). The results showed that the content of SO3 in ashes reached to 48 wt% and 61 wt%, respectively. 2.2. Apparatus and Methods The experiments were conducted in the two-stage fixed-bed reactor used in our previous study [4,17]. As is shown in Fig. 2, the schematic diagram of experimental apparatus consisted of a gas supply section, a two-stage fixed-bed reactor, an electric furnace, and the products collection part. The reactor had an upper stage for coal pyrolysis and a lower stage for volatiles cracking. Moreover, the length of the upper tube was 400 mm with an inner diameter of 30 mm and the lower tube was 800 mm in length with an inner diameter of 38 mm. The distributors in the upper and lower stages located at 330 mm and 530 mm from the reactor top, respectively. High purity N2 was controlled with a mass flow meter as the sweeping gas. The catalysts were placed in the lower tube at the beginning of every experiment. 20 g of coal was added to a feeder 4 connected with the upper tube. The valve 2 and the upper ball valve 3 were opened to sweep the feeder using high purity N2. Then the upper ball valve 3 and

2.4. Analysis of gases and tar The composition of non-condensable pyrolysis gas was analyzed by a micro gas chromatograph (Agilent 3000A). The coal tar was analyzed in a simulated distillation GC (Agilent 7890B) to characterize its boiling point distribution. The coal tar components were divided into six groups: light oil, phenol oil, naphthalene oil, wash oil, anthracene oil and pitch with boiling point ranges of < 170 °C, 170–210 °C, 210–230 °C, 230–300 °C, 300–360 °C, and > 360 °C, respectively. The light tar and heavy tar refer to the total fraction with boiling point below and above 360 °C, respectively. The coal tar was also analyzed

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Fig. 3. TG/DTG diagrams of different carbon-based materials: (a) Char; (b) AC; (c) PCMs; (d) RDCMs.

using a gas chromatography-mass spectrometer (Agilent 6890A GC and Agilent 5375 MS). The tar yield Ytar (wt%), light tar yield Ylight (wt%), gas component yield Yi (mL/g), gas yield Ygas (wt%) and water yiled Ywater (wt%) were calculated with Eqs. (1) to (5) to evaluate the pyrolysis products.

Mtar Mcoal

(1)

Ylight = Ytar × flight

(2)

Ytar =

Yi =

Vi Mcoal

Ygas =

Mi V/1000V i m Mcoal

Ywater = 100

Ygas

Ytar

(4)

Ychar

(5)

where Mcoal and Mtar are respectively the mass of coal on dry basis (db), tar in grams. Vi (mL) and Mi refer to the gas volume and molar mass of individual non-condensable gas components i in pyrolysis gas, respectively. Vm is the standard molar volume (22.4 L/mol). The light tar fraction (flight) was analyzed via the simulated distillation GC.

(3)

Fig. 4. TG/MS curves of PCMs pyrolysis.

Fig. 5. XRD patterns of different CBC.

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Table 2 Specific surface area, pore volume and pore diameter of different CBC. Sample

Specific surface area (m2·g−1) SBETa

Char AC RDCMs a b c

73.9 229.5 417.2

Smicrob

Pore volume (cm3·g−1) Smesoc

38.1 150.0 372.4

35.9 79.5 44.8

0.017 0.065 0.068

BET methond. t-plot methond. Smeso = SBET - Smicro.

3. Results and discussion 3.1. The self-assembly process of RDCMs Fig. 3 shows the TG/DTG curves of the carbon-based materials measured by a temperature program. The detected total weight loss of RDCMs, AC or char was near to 5.0–8.0 wt%. The weight loss of about 1.0–2.0 wt% before 105 °C was due to the moisture emission. The total weight loss of char, AC and RDCMs were 5.26 wt%, 4.62 wt% and 7.73 wt%, respectively. Whereas the weight loss of PCMs was > 40 wt %, and relatively higher weight loss rate was found at 500–600 °C. Fig. 4 shows the evolution of gases species from PCMs decomposition with increasing temperature using TG-MS. The PCMs sample was heated to 750 °C at a rate of 10 °C/min and hold there for 1 h. Lots of gases were produced during the transformation from PCMs to RDCMs. The decomposition of carboxyl, hydroxyl and ether groups resulted in the release of CO2 and CO at lower temperature [6,29]. CH4 mainly came from the cracking of aliphatic hydrocarbons and aliphatic side chains of aromatic hydrocarbons during the PCMs pyrolysis [7,30]. With the increase of temperature, hydrogen was produced due to the intensification of aromatization degree of PCMs. Part of the sulfur species released in the form of SO2 at about 300–400 °C. Fig. 5 compares the XRD patterns of different CBC. All the samples revealed two broad peaks at 2θ of 10°–30° and 35°–50°, corresponding to plane graphene nanosheets in disorder (002) and plane of graphite (101), respectively [31,32]. These results indicated these carbon

Fig. 7. The XPS spectra for (a) different carbon-based materials and (b) RDCMs.

materials mainly consisted of disordered polycyclic aromatic carbon sheets [33,34]. Some metal oxides were detected in AC and char but not found in RDCMs. This suggested that RDCMs was a kind of CBC with less metallic species or metal-free. Table 2 gives the texture properties of the carbon-based materials. Obviously, the RDCMs had the larger specific surface area (417.2 m2·g−1) with more micropores compared with those of AC and char. The mesopores of RDCMs was less than that of AC. Consequently, the pore volume of RDCMs was near to that of AC. The relatively higher specific surface area of RDCMs was caused by the release of gases from the internal and external of PCMs. FT-IR spectra of different carbon-based materials are shown in Fig. 6. All the spectrum exhibits signals at 3421 cm−1, 3000–2800 cm−1, 1700–1400 cm−1, 1300–900 cm−1 and −1 900–700 cm , corresponding to OeH vibrations in hydroxyl groups, CeH vibrations, C]C and C]O vibrations, aromatic C]C bonds and oxygen groups, and aromatic substitution, respectively [31,32,35]. Moreover, the bands at 1400 cm−1, 1145 cm−1 and 1031 cm−1 of PCMs and RDCMs are connected with S]O and –SO3H groups [32]. Compared with that of PCMs, the relatively intensity of functional groups containing sulfur of RDCMs decreased due to the decomposition of sulfur species to form SO2 under treated conditions (Fig. 4). The decrease of RDCMs at 3000–2800 cm−1 and 1400 cm−1 indicated the loss of –CH3 and C]O through the production of CH4, CO2 and CO, respectively. The enhancement of RDCMs at 930 cm−1 attributed to the

Fig. 6. FT-IR spectra of different carbon-based materials.

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Fig. 8. Raman spectra of different carbon-based materials fitted into 10 bonds: (a) Char; (b) AC; (c) PCMs; (d) RDCMs.

increase of aromatic C]C. The S(2p) XPS spectra of the different carbon-based materials are shown in Fig. 7. Compared with that of char and AC, the peak intensities of PCMs and RDCMs were much higher, indicating the existence of much more sulfur group. The peak at 169 eV and 163–166 eV are attributed to the -SO3H group, sulfone group (165 eV), and -SH group (164 eV) [8,27,36]. It revealed that the -SH, S]O, and -SO3H groups were presence in PCMs and RDCMs. Moreover, pyrolysis of PCMs promoted the oxidization reaction to decrease the amount of –SO3H groups whereas higher the intensity of sulfone and -SH groups. Fig. 8 illustrates the Raman spectra of different carbon-based

Table 4 Ratio of major bands of Raman spectra of different carbon-based materials. Catalysts Char AC PCMs RDCMs

AD/AG

AD/A(GR + VL + VR)

0.98 1.34 0.54 1.42

0.77 1.21 0.52 0.90

Table 3 Typical Raman bands description between 1800 and 800 cm−1 [37,39]. Band name R SR S SL D VR VL GR G GL

Band position (cm−1) 960–800 1060 1185 1230 1345 1385 1465 1540 1590 1700

Description C-C on alkanes and cyclic alkanes; CeH on aromatic rings C-H on aromatic rings; benzene (ortho-di-substituted) ring Car–Cal; aromatic (aliphatic) ethers; CeC on hydroaromatic rings; hexagonal diamond carbon sp3; CeH on aromatic rings Aryl–alkyl ether; para-aromatics D band on highly ordered carbonaceous materials; CeC between aromatic rings and aromatics with not < 6 rings Methyl group; semi-circle breathing of aromatic rings; amorphous carbon structures Methylene or methyl; semi-circle breathing of aromatic rings; amorphous carbon structures Aromatics with 3–5 rings; amorphous carbon structures Graphite E2g2; aromatic ring quadrant breathing Carbonyl group C=O

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Fig. 9. Scheme of the self-assembly process of RDCMs.

Fig. 11. Simulated distillation curves (a) and fraction distribution (b) of tar obtained with different carbon-based catalysts (Tpy = 600 °C, Tcr = 500 °C, catalyst: 10 wt% of the tested coal).

materials. All the curves in the range of 800–1800 cm−1 were fitted into 10 Gaussian bonds (Table 3) according to semi-quantitative analysis method [37,38]. Moreover, the two main bands G band at 1590 cm−1 and D band at 1300 cm−1 mainly represent aromatic vibration and the degree of condensation for larger aromatic ring systems (< 6 fused aromatic rings), respectively. The other three bands GR, VL, and VR are assigned to smaller aromatic ring systems (< 6 fused aromatic rings) [37,39]. Therefore, the ratios of peak areas for these major bands can show some details of the degree of aromatic ring growth, defects in the graphite carbon structure and rough relative measure of the abundance ratio between the smaller and larger aromatic ring systems [38]. Table 4 summarizes the ratios of major bands of Raman spectra of all the carbon-based materials. As seen, the AD/AG was in the order of RDCMs > AC > Char > PCMs, which suggested that RDCMs had the most structural defects and imperfections of the carbon crystallites. The

Fig. 10. (a) Product yield, (b) tar quality, and (c) pyrolysis gas component yield obtained with different carbon-based catalysts (Tpy = 600 °C, Tcr = 500 °C, catalyst: 10 wt% of the tested coal).

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presence of electron–rich functional groups consisting of O and S and the higher specific surface area were usually corresponds with a more number of defects [6]. Whereas the order of AD/A(GR + VL + VR) was AC > RDCMs > Char > PCMs, suggesting that besides AC the aromatic systems in the other carbon-based materials were majority of smaller aromatic ring systems. This result was consistent with the H and C content (Table 2). Compared with that of PCMs, the AD/ A(GR + VL + VR) of RDCMs was much higher, indicating a relative increase in the concentration of aromatic rings through ring condensation reactions and aromatization of polycyclic paraffin. The relatively higher temperature treatment facilitated the enlargement of aromatic rings through further condensation reaction [32]. As an illustration, PCMs was a kind of conjunct polymer sulfates with various hydrophilic surface functional groups (such as, –COOH, –OH, and –SO3H) through the process of polymerization, oxidation and other reactions under the catalysis of sulfuric acid. The reactions could be described as (6)–(10). After pyrolysis of PCMs, the conjunct polymer sulfates became amorphous carbon materials consisting of polycyclic aromatic carbon sheets with attached –SO3H, sulfone, and –SH groups [32,40]. The above characterizations suggested that both RDCMs and AC had similar apparent cross-linking aromatic systems with more defects than Char [34,41]. It is likely the more defects in RDCMs due to the bigger specific surface area and sulfur doping. The RDCMs was assembled by polymerization, cyclization, oxidation, acidification and aromatization, as shown schematically in Fig. 9.

Fig. 12. AD/A(GR + VL + VR) ratio of Raman spectra of different RDCMs. Table 5 The ultimate analysis result of different RDCMs. Catalysts

C

H

S

N

Oa

650RDCMs 750RDCMs 850RDCMs WRDCMs

84.63 87.42 90.75 88.63

1.87 1.34 0.59 1.61

8.06 7.32 4.67 3.58

0.36 0.39 0.51 0.56

5.08 3.53 3.48 5.62

a

By difference.

(6)

Fig. 13. The S(2p) XPS spectra of different RDCMs: (a) 650RDCMs; (b) 750RDCMs; (c) 850RDCMs; (d) WRDCMs.

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(7)

(8)

(9) increased by 44.4%, 88.9% and 133.3%, respectively. It is inferred that CBC could stabilize the free radicals with high molecular weight through reacting with smaller radicals generated on the surface of the catalysts to prohibit the coke formation [22,23]. Overall, all these results demonstrated that RDCMs exhibited the best upgrading effect among these cabon-based catalysts. 3.3. Role of sulfur groups in RDCMs for upgrading the tar

(10)

RDCMs were prepared under different pyrolysis temperature, and noted as CRDCMs, in which C was the pyrolysis temperature. Moreover, 750RDCMs was washed by ammonia water to investigate the catalytic effect of doped sulfur and noted as WRDCMs. Fig. 12 compares the AD/ A(GR + VL + VR) ratio of Raman spectra of all the RDCMs. Obviously, with the increase of the treatment temperature, the AD/ A(GR + VL + VR) ratio of RDCMs increased that revealed higher temperature treatment facilitated the enlargement of aromatic rings through further condensation reaction [32]. Moreover, compared with that of 750RDCMs, the AD/A(GR + VL + VR) ratio of WRDCMs changed little, suggesting that ammonia water washing could not cause effect on the carbon framework of the carbon materials. Table 5 compares the results of ultimate analysis of the RDCMs prepared under different conditions. With the increase of the treated temperature, the content of C increased whereas the H decreased, indicating the degree of further condensation reaction. Obviously, the sulfur content could be removed partly through increasing temperature and ammonia washing. Fig. 13 shows the S(2p) XPS spectra of different RDCMs. The results showed that temperature and ammonia treatment could change the sulfur species. With the increasing pyrolysis temperature, the –SO3H at the peak of 169 eV decreased while the changes of sulfone group (165 eV) and –SH group (164 eV) were not obvious. Ammonia treatment decreased the sulfur species significantly. Fig. 14 summarizes the results of product yield, tar quality and pyrolysis gas component yield obtained with RDCMs treated under different temperature. With the increase of the treated temperature, the gas yield decreased from 18.4 wt% to 14.0 wt%, whereas the water yield increased from 11.0 wt% to 15.9 wt% (Fig. 14(a)). The light tar fraction decreased with the increase of the treatment temperature, which may mainly due to the decrease of doped sulfur in the RDCMs. Moreover, compared with that of 750RDCMs, the light tar fraction of

3.2. Tests with different carbon-based catalysts Pyrolysis with different CBC was conducted in the two-stage fixedbed reactor at the pyrolysis and cracking temperatures of 600 °C and 500 °C, respectively. The weight ratio of catalyst to coal was 10 wt%. Fig. 10 compares the product yield, tar quality and pyrolysis gas component yield obtained with different CBC. Compared with pyrolysis without catalyst, the tar yield decreased over all the CBC, whereas the fraction and yield of light tar all increased (Fig. 10(a)). In comparison with the yield of tar and the fraction of light tar without catalyst, the former from coal pyrolysis over Char, AC and RDCMs decreased by 15.3%, 17.1% and 11.7%, respectively, whereas the later increased by 20.0%, 25.0% and 33.3%, respectively. Consequently, compared with the light tar yield of 6.6 wt% without catalyst, the light tar yield over Char, AC and RDCMs were 6.8 wt%, 6.9 wt% and 7.9 wt%, which increased by 3.0%, 4.5% and 19.6%, respectively (Fig. 10(b)). It was found that the gas yields over CBC all increased by over 62.0%, indicating the release of gaseous products through catalytic cracking reaction [4,6,42]. Compared with that of Char or AC, the yields of H2, CH4 and CO in pyrolysis gas obtained over RDCMs were lower, whereas the yield of CO2 was much higher (Fig. 10(c)). The simulated distillation curves and fraction distribution of tar obtained by catalytic upgrading over CBC are shown in Fig. 11. Compared with that for coal pyrolysis without catalyst, the yields of pitch over all the CBC were lowered by over 20.0%. The content of naphthalene oil over Char, AC and RDCMs changed a little, whereas contents of light oil, wash oil and anthracene oil increased dramatically. In comparison with that for coal pyrolysis without catalyst, the content of wash oil increased by 58.8%, 64.7% and 52.9% by catalytic upgrading over Char, AC and RDCMs, while the content of anthracene oil

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WRDCMs decreased from 82.0% to 79.0%, which also demonstrated that the doped sulfur could facilitate the catalytic cracking of heavy tar (Fig. 14(b)). The yield of CO2 was lowered with the decrease of sulfur content in RDCMs, indicating less catalytic oxidation reaction during catalytic coal pyrolysis (Fig. 14(c)). According the above results, the doped sulfur in RDCMs was attribute to decrease the yield of water and heavy tar whereas higher the yield of gas. Literature suggested that doped sulfur in carbon contributed to the catalytic oxidation reaction that may decrease the water yield and increase the gas yield [43]. Table 6 listed the content of oxygenated organic compounds (OOCs) in the tars obtained over different RDCMs by GC–MS analysis. Here, the OOCs referred to the organic compounds contained oxygen element. The quantity of OOCs obtained over different RDCMs was the sum of relative amount of all the compounds containing oxygen element in the tars. The contents of OOCs in tar over RDCMs all increased compared with that without catalyst, but decreased when treated RDCMs with ammonia water washing. Compared with that of coal pyrolysis without catalyst, the contents of OOCs in the tar obtained over 750RDCMs increased by 19.2%, which indicated more ·OH radical was generated from H2O activated by sulfur functional groups and fixed in the tar during the secondary cracking reaction. This result was highly agreed with the result of products yields (Fig. 14(a)). Moreover, the sulfur functional groups in carbon materials could also activate the hydrocarbons containing O to form radicals to produce hydrocarbons containing O in the tars [31]. Doped sulfur in RDCMs was a main factor to improve the tar quality during catalytic reaction. According to above discussion, more doped sulfur in RDCMs led to more gas yield and heavy tar cracking through catalytic oxidation reaction. The water from coal pyrolysis would be activated on the surface of RDCMs to form ·OH radical and ·H. The electron would be transferred to the sulfur groups through the carbon framework. The ·OH and ·H radicals reacted with the radicals from coal pyrolysis products cracking to form tar not coke. The electron on the sulfur groups would attack the O-containing group to product more carbon oxide. Combing the utilization of waste red oil from alkylation process with coal pyrolysis exhibited the potential in industrial application for producing high-quality tar from coal pyrolysis process. 4. Conclusion Based on one-pot hydrothermal reaction, the acid soluble hydrocarbons and sulfuric acid in red oil were separated. Poly carbon materials (PCMs) were made as byproducts. Red oil-derived carbon materials (RDCMs) were prepared by PCMs pyrolysis and evaluated for catalytic upgrading of coal pyrolysis volatiles. Compared with activated carbon or char, RDCMs exhibited better performance in increasing the light tar yield and fraction. The characterization of RDCMs evidenced that RDCMs was an amorphous carbon material with higher specific surface areas and relatively more defects, resulting in more cracking of pitch to light oil, wash oil, and anthracene oil. Compared with that from coal pyrolysis without catalyst, the light tar and fraction increased by 19.6% and 33.3%, respectively, during catalytic coal pyrolysis over 10 wt% RDCMs at 500 °C. The sulfur groups in RDCMs was attributed to form more smaller radicals, for example, ·H and ·OH radicals, so as to capture free radical fragments to form tar not coke.

Fig. 14. (a) Product yield, (b) tar quality, and (c) pyrolysis gas component yield obtained with different RDCMs (Tpy = 600 °C, Tcr = 500 °C, catalyst: 10 wt% of the tested coal).

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Table 6 The content of oxygenated organic compounds (OOCs) in the tars obtained over different RDCMs (wt%). Name

Phenol Cyclohexanone 4-hydroxy-4-methyl-2-pentanone 2-methyl-phenol 3-methyl-phenol 2-methyl-1,3-benzenediol Benzofuran 2-ethyl-phenol 4-ethyl-phenol 2,6-dimethyl-phenol 2,4-dimethyl-phenol 2,3-dimethyl-phenol 3,4-dimethyl-phenol 2-methyl-benzofuran Cis-1,2-diol, 2,3-dihydro-1H-indene 2-(1-methylethyl)-phenol 4-(1-methylethyl)-phenol 1-ethyl-4-methoxy-benzene 3-ethyl-5-methyl-phenol 2,3,5-trimethyl-phenol Nonanoic acid 1-Naphthalenol 2-Decanone 4,7-dimethyl-benzofuran Oxygenated C11-C28 Sum of OOCs

Formula

C6H6O C6H10O C6H12O2 C7H8O C7H8O C7H8O2 C8H6O C8H10O C8H10O C8H10O C8H10O C8H10O C8H10O C9H8O C9H10O2 C9H12O C9H12O C9H12O C9H12O C9H12O C9H18O2 C10H8O C10H20O C10H10O

Content (wt%) Without catalyst

650RDCMs

750RDCMs

850RDCMs

WRDCMs

6.10

7.67 0.38 1.02 3.57 6.89 0.45 0.39 0.39 1.19 0.22 1.68 1.63 0.36 0.74 0.30 0.44 0.47 0.43 1.28 0.33

6.92 2.97 1.91 2.90 6.68

3.26

4.77

4.06 8.91 0.58

3.08 7.02 0.45 0.37 0.65 1.22 0.36 1.7 0.36 1.68 0.76 0.23 0.27 0.23 0.44 1.33 0.35 0.23 1.39 0.34 0.50 6.70 34.4

2.93 4.96 0.39 0.32 0.68 1.14 1.44 1.77 0.41 0.38 0.67 0.55 1.16 0.23 0.56 9.71 33.4

1.31 0.38 7.48 39.0

Acknowledgments

0.37 0.67 1.13 0.4 1.52 1.71 0.44 0.89 0.71 0.60 0.39 0.83 1.23 0.30 0.48 6.75 39.8

0.86 1.66 4.42 0.87 1.12 2.31 0.68 0.33 0.54 0.57 0.77 0.25 0.21 0.55 1.16 3.89 37.0

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