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A study on catalytic depolymerization of a typical perhydrous coal for improving tar yield
Journal of Analytical and Applied Pyrolysis xxx (xxxx) xxx–xxx
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A study on catalytic depolymerization of a typical perhydrous coal for improving tar yield Zhan Xu, Litong Liang, Qian Zhang , Xiaodong Wang, Jianwei Liu, Hao Shen, Lingwei Kong, ⁎ Wei Huang ⁎
Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan, 030024, China
ARTICLE INFO
ABSTRACT
Keywords: Perhydrous coal Tar yield Catalytic depolymerization Char structure
Catalytic depolymerization proposed by our team was used on the thermal decomposition study of a typical perhydrous coal for improving the tar yield. The products from the coal catalytic depolymerization with or without a small amount of ZnCl2 or CoCl2 catalysts were investigated and compared. The composition of the tar and the structure of the coal char were characterized by a comprehensive two-dimensional gas chromatography coupled with mass spectrometric detection (GC × GCeMS), thermogravimetric analysis (TGA), Raman spectrum and Elemental analysis, respectively. Compared with 8.21% of tar yield generated from the depolymerization of the raw coal, the tar yield increased to 9.19% and 10.02% when ZnCl2 and CoCl2 catalysts were added, respectively. The light fractions of tar also increased from 81.71% to 84.02% and 84.51%, respectively. Meanwhile, the yield of the aromatic ring components (> 3 fused benzene rings) and the ratio of aromatic rings with six or more fused benzene rings in the tar decreased. Further analysis reflected that the addition of catalysts in the catalytic depolymerization process could facilitate the cracking of more Cal-O bonds and precipitate the reaction start at lower temperature, which finally increases the volatiles released during the depolymerization stage.
1. Introduction Coal as one of the most important fossil fuels is primarily composed of organic compounds, along with a small amount of inorganic minerals [1–4]. Pyrolysis is testified to be an effective and clean process to acquire the liquid tar, residue char and gases from thermal treatment of the organic components in coal [5–8]. The tar that contains aromatic compounds is more favorable as it could be used to upgrade high valueadded chemicals or transportation fuel further. However, the low H/C ratio of coal is not benefited for the production of tar. Besides, the heavy complex fractions in the tar will cause the equipment to be corroded or blocked. These elements strongly restricted the industrialization progress of the coal pyrolysis [9]. It is of vital importance to solve the problems discussed and to acquire high yield and quality tars. One of the methods is to employ the catalytic aided pyrolysis technics [10–14]. However, for the traditional catalytic pyrolysis process, extravagant amount of catalysts is demanded, which would cause various sensitive issues such as costly production investment, accelerated equipment depreciation caused by corrosion, and so on [15,16]. Another issue that cannot be dismissed for the existing
⁎
catalytic pyrolysis process is that the catalyst might primarily exert its effect on the volatiles reformation and consequently induce the increase content of lighter components in tar, in the meantime, the decreased yield of whole tar [17,18]. The faults of the catalytic pyrolysis should be firstly solved to propel the industrialization of the process. To fix the problems met in the catalytic pyrolysis process, our team proposed the concept of catalytic depolymerization. In this process, liquid reagent act as carrier to dissolve and disperse the catalyst in small share and then the dispersion system was uniformly sprayed into the coal [19]. By means of the form of dispersion system of assistant ingredient and catalyst, it’s believed that the catalyst can have a better contact with the coal and fully exert its role on depolymerize the coal selectively at low temperatures, thus the yield of the high value-added chemicals can be anticipated to increase. Applying the catalytic depolymerization process in the research of Liang et al. [20], the tar yield of increased from 3.6% sharply to 5.0% and 7.8% for Neimeng lignite and from 8.1% to 11.4% and 10.9% for Xinjiang lignite with the addition of Fe- and Mo-based catalysts, respectively. The experiments indicated a great potential that the catalytic depolymerization process have in enhancing both the yield and quality of the liquid tar. Liu et al. [21] also
Corresponding authors. E-mail addresses: [email protected] (Q. Zhang), [email protected] (W. Huang).
https://doi.org/10.1016/j.jaap.2019.01.001 Received 22 July 2018; Received in revised form 2 January 2019; Accepted 3 January 2019 0165-2370/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xu, Z., Journal of Analytical and Applied Pyrolysis, https://doi.org/10.1016/j.jaap.2019.01.001
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Table 1 Proximate and ultimate analyses of the coal sample. Proximate analysis (wt %, ad) M 3.16
A 38.36
H/Cb
Ultimate analysis (wt %, daf) V 26.94
FCa 31.54
C 65.74
Oa 24.25
H 5.05
N 1.20
S 3.76
0.92
ad, air dried base; daf, dry ash-free base. a By difference. b Atomic ratio.
studied the prospect of the catalytic depolymerization of a semi-char which generated from the low temperature pyrolysis process of a low rank coal on Fe-based catalyst in producing economic valued chemicals. The experiments witnessed the liquid yield improved sharply from 0.20% to 1.61% when a kind of iron-based catalyst was applied to the semi-char for the process of catalytic depolymerization, and they speculate that the catalyst accelerate the process of smaller molecule production through the chemical bond breaking and increased the content of liquid organic constituent in the final products. Based on the above experiments, catalytic depolymerization can be regarded as a promising method in increasing the yield of the tar from coal. In this paper, Zaozhuang coal was testified. The Zaozhuang coal, as a typical perhydrous subbituminous coal, was mined from Zaozhuang coal mine, Shandong province. The Zhaozhuang coal mine have a reserve estimated to more than 1 billion tons. However, the high ash and sulfur content in the coal (as shown in Table 1) made it incompetent for combustion or coking process. The Zaozhuang coal could be characterized as a perhydrous coal for the hydrogen content is higher than 5%. Moreover, the petrographic analysis showed that the Zaozhuang coal is rich in vitrinite content (85.5%, listed in Table 2). The high hydrogen content and the rich vitrinite content in the Zaozhuang coal make it a well substitute for tar extracting through pyrolysis [22]. It was assumed that if the proposed catalytic depolymerization could be successfully applied to the pyrolysis process of Zaozhuang coal, more high value-added products could be obtained, which would make this process more competitive. However, due to the limited application of catalytic depolymerization on the coal, it is still uncertain whether the catalyst depolymerization could be effectively applied to Zaozhuang perhydrous coal and improved the yield of tar consequently. The catalyst based on ZnCl2 and CoCl2 has been verified well suited for the process of catalytic pyrolysis of coal [23,24]. Therefore, in our experiments, the ZnCl2 and CoCl2 based catalysts were applied to the catalytic polymerization process of the Zaozhuang coal to testify whether it could improve the tar yield. In the process, we mainly concentrate on the analysis of the products of the tar and char. Besides, the catalytic effect of different catalysts was also compared and discussed.
2.2. Catalyst addition Analytical grade of ZnCl2 and CoCl2 chemicals (Kermel Chemical Reagent Co. Ltd., Tianjin, China) were used. Metal chloride of ZnCl2 or CoCl2 were separately dissolved in 2 mL accessory ingredient to prepare a uniform solution as catalyst [19]. The solution was sprayed into 30 g coal samples, blended mechanically and then stayed for 30 min [20]. The metal ion content of the catalyst was 0.1 wt% of the dry basis coal samples. Before the catalytic depolymerization experiment, the prepared samples of the coal with catalyst were dried for 2 h under the temperature of 110 °C. The coal samples that with the addition of accessory ingredient and ZnCl2 or CoCl2·catalysts were named as Coal-Zn and Coal-Co, respectively. 2.3. Catalytic depolymerization experiments The catalytic depolymerization experiments were carried out with a Gray-King equipment and operated according to China National Standard method (GB/T 1341-2007). Four quartz tubes with effective length of 300 mm and internal radius of 10 mm were used. For each quartz tube, one end was round and closed, and at the other end, a side tube was fused to serve as an exit for the volatiles released from the experiment [25]. 10 g coal sample was placed evenly in each tube. When the Gray-King reactor was heated to 300 °C, the tube was quickly inserted into the reactor. Then the reactor tube was heated at a heating rate of 5 °C/min to the set temperature of 600 °C and maintained at this temperature for 15 min. Each experiment had two comparative tests in order to ensure the experimental accuracy. The relative error tested was less than 0.5%. The liquid component of the products was gathered in a conical flask and kept in a low temperature environment created by icesalt bath. According to China National Standard method (GB/T 4802000), using toluene as solvent, the water was extracted from the liquid products. The yield of the tar and water can be calculated by this way. The yields of char, gas and tar were calculated on dry basis with the following equations [25]:
YTar =
2. Experimental
WTar × 100% W0 WChar × 100% W0
2.1. Coal sample preparation
YC har =
The perhydrous coal used were obtained from Shandong province, Zaozhuang City. The raw coal was mechanically crushed into a size range of 2.5–5.0 mm. Proximate and ultimate analyses of Zaozhuang coal were carried by China National Standard methods GB/T 212-2008 and GB/T 476-2001 and listed in Table 1. It could be seen that the coal was rich in hydrogen and it also contains of high amount of ash. Table 2 was the petrographic components of Zaozhuang coal performed by China National Standard method of GB/T 13773-2008.
YGas = 100
Vitrinite
Inertinite
Exinite
Minerals
Raw coal
85.50
8.80
–
5.70
YC har
YTar
(2)
YWater
(3)
YTar , YChar , YGas and YWater were the yield of tar, char, gas and water in dry base, respectively. WTar and WChar respectively represented the mass of tar and char. W0 was the initial mass of coal sample in dry base. 2.4. TG analysis The catalytic depolymerization experiments of the prepared samples were also performed using a Setaram SETSYS Evolution thermogravimetric analysis (TGA). About 5 mg of the prepared sample was uniform paved and kept at the same thickness in an alumina crucible and heated from room temperature to 600 °C at a heating rate of 5 °C/min under high pure argon (99.999%) atmosphere flow rate of 100 mL/min. TG and DTG curves were used to characterize the thermal decomposition
Table 2 Petrographic analysis of the coal sample (vol %). Coal sample
(1)
2
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parameters of the samples. T1 for which reflected the temperature at the mass loss reached to 10% of the total sample mass loss was defined as the temperature of the depolymerization process commenced [26]. Tp represented the temperature of the maximum mass loss rate peak appeared. T2 was defined as the temperature at which the depolymerization process was completed. It was calculated as the point of the mass loss reached to 90% of the total mass loss of the samples. 2.5. Product analysis The hydrocarbon compositions of the tar were performed on a comprehensive two-dimensional gas chromatography coupled with a mass spectrometric detection (GC × GC–MS). This system including an Agilent 7890B chromatograph equipped with an Agilent 5977 A MSD mass spectrometer and a ZX1 thermal modulator (Zoex). The two-dimensional capillary column system utilizes a combination of a nonpolar first column (DB-1, 15 m × 0.25 mm I.D. × 1.00 μm film thickness) with a medium polar second column (BPX-50, medium polar, 2.75m × 0.1 mm I.D. × 0.10 μm film thickness). High pure GC-grade Helium (99.999%) was served as the carrier gas. The primary GC oven temperature was set to increase from 50 °C to a final temperature of 300 °C at a heating rate of 5 °C/min. The modulator temperature was programmed to increase from 150 °C to 300 °C at a heating rate of 3 °C/ min. 1 μL sample was injected at 300 °C with a split ratio of 10:1, and the scanning mass range (m/z) was set from 30 to 500 amu. Data processing and display of the GC × GC–MS chromatograms were identified by matching the mass spectra database with the National Institute of Standards and Technology (NIST). The hydrocarbons were found automatically and further corrected manually when required. The fractions of the tar was analyzed by a TGA. In each experiment, about 5 mg of the tar sample was placed in an alumina crucible and heated from room temperature to 600 °C at a heating rates of 5 °C/min [27]. Then the fractions of the tar was divided into seven stages according to the evolution temperature, and named accordingly as light oil (< 170 °C), phenol oil (170–210 °C), naphthalene oil (210–230 °C), wash oil (230–300 °C), anthracene oil (300–360 °C), and pitch (> 360 °C) [9,28,29], respectively. The boiling point less than 360 °C was all defined as light fractions. Structural parameters of the coal and char samples were determined using a RENISHAW-in Via Raman Microscope equipped with a He: Cd laser at 514 nm as its light source. The scanning range was recorded from 800 to 1800 cm−1.
Fig. 1. Products distribution of coal depolymerization with and without catalyst.
3.2. Tar fractions analysis The data of the fractions of the tar produced from the catalytic depolymerization of the Zaozhuang coal with or without the catalysts was presented in Table 3. The percentage of the light fractions (< 360 °C) increased from 81.71% to 84.02% and 84.51% for Coal-Zn and Coal-Co, respectively. The catalysts on coal depolymerization decreased the relative content of pitch in the tar, while the content of anthracene oil increased sharply. While the percentage of the other light components of light oil, phenol oil, naphthalene oil and wash oil had no obvious change. The main components of pitch were polycyclic compounds, condensed aromatic hydrocarbons and their derivatives [31]. Thus the decrease of the pitch fractions might mainly be caused by the reduction of the generation of polycyclic aromatic hydrocarbons or inhibition condensing of the aromatic hydrocarbons in tar. According to Zou et al. [24], ZnCl2 and CoCl2 catalysts had certain acidity, which has advantage in the cracking of large aromatics and hydrogenating of the cracked tar fragments. 3.3. GC × GC–MS analysis of tar Fig. 2 illustrated the Total intensity chromatogram (TIC) images (3D) of the tar samples from the catalytic depolymerization of the Zaozhuang coal. The X-axis reflects the volatility-based retention time (min), the Y-axis reflects the polarity-based retention time (s), and the Z-axis reflects the MS detector response. The hydrocarbon compounds in the tar could be separated by their boiling point as well as their polarity through the two-dimensional chromatogram. Based on the library search using NIST and the volume of different peaks, the proportions of the detected components in the liquid products could be determined and divided into several species [20,32]: phenol, 1-ring aromatics (Ar), 2-ring Ar, 3-ring Ar, higher-ring Ar (> 3 fused benzene rings), aliphatic hydrocarbons and heteroatom compounds, as showed in Fig. 3. Compared with the raw coal tar, the content of phenols, 3-ring Ar and higher-ring aromatics (> 3 fused benzene rings) decreased obviously with the addition of catalysts. While the yield of 1-ring Ar and aliphatic hydrocarbons derived from the catalytic depolymerization with the addition of ZnCl2 and CoCl2 catalysts increased. These results indicated that the ZnCl2 and CoCl2 catalysts could decompose the larger aromatics ring systems (> 3 fused benzene rings) consisted in the coal structure and thus improved the content of smaller aromatic ring systems and aliphatic hydrocarbons in the tar.
3. Results and discussion 3.1. Product distribution and analysis The yield of the products from depolymerization of Zaozhuang coal by different catalysts were presented in Fig. 1. From Fig. 1, the tar yield of the Zaozhuang coal comes to 8.21%. As shown in Tables 1–2, Zaozhuang coal contains of high H/C ratio and high vitrinite content, which were both favourable for the formation of tar [30]. That’s the reason that Zaozhuang coal had a relative high tar yield though it contains of high ash content (38.36%). For the catalytic depolymerization experiment of Zaozhuang coal with the addition of ZnCl2 or CoCl2 catalysts, an obvious catalytic effect showed on increasing of the tar yield, which increased to 9.19% for Coal-Zn and 10.02% for CoalCo. On the other hand, the char yield decreased to 80.45% for Coal-Zn, and 80.44% for Coal-Co compared with the raw coal char yield of 82.72%. Moreover, the water yield increased from 3.01% to 3.74% and 3.20% with the addition of ZnCl2 and CoCl2 catalysts, respectively. The gases yield also slightly increased. All these results confirmed that the addition of ZnCl2 or CoCl2 catalysts increased the overall conversion of the Zaozhuang coal.
3
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Table 3 The amount of different fractions of tar from coal depolymerization with different catalysts. sample
Coal-Non Coal-Zn Coal-Co
Light oil %
Phenol oil %
Naphthalene oil %
anthracene oil %
210-230 °C
Wash oil % 230-300 °C
300-360 °C
Pitch % > 360 °C
< 170 °C
170-210 °C
21.74 20.60 21.55
15.42 14.66 15.71
8.10 7.60 8.01
27.06 27.27 27.41
9.39 13.89 11.83
18.29 15.98 15.49
to the aromatic quadrant ring-breathing mode. The D band at 1345 cm−1 were usually belonged to the disorder of fused benzene rings at least 6 fused aromatic rings but less than graphite. The three bands lied in the region of the G and D bands were named as GR, VR and VL bands. Among them, GR band at 1540 cm1 represented the carbon structure with 3–5 fused aromatic rings. VR band at 1465 cm−1 and VL band at 1385 cm−1 could be attributed to the methylene, methyl and semi-circle breathing of aromatic rings, respectively. Usually, the peak areas of GR, VR and VL bands were added together to represent the condensation degree of the smaller aromatic ring systems in the char structure. The intensities (peak areas) ratio ID/IG of D band and G band could be extensively used as a principal parameter to evaluate the char structure evolution during the catalytic depolymerization process of the Zaozhuang coal. Normally, the decrease of the ID/IG ratio indicated the relative decrease of the concentrations of large aromatic rings in the char structure, mainly reflect the decrease of the six or more fused benzene ring systems [34]. The intensities (peak areas) ratio ID/I(GR+ VL+ VR) of D band and the combined of GR + VL + VR band had been extensively used to compare the variation of the ratio of the large aromatic rings (> 6 rings) and the small aromatic ring systems (3–5 fused aromatic rings) in the coal thermal depolymerization process. Table 5 showed the changes of the bands ratios of different samples. When the raw coal converted into char through the thermal decomposition process, the ratio of ID/IG increased from 1.69 to 1.83 and the ID/I(GR+ VL+ VR) ratio increased from 0.83 to 1.01. The dehydrogenation of hydroaromatics and the further condensation of the aromatic rings during the coal thermal depolymerization process caused the growth of the ratios of the large aromatic rings. The ratio of ID/IG of the chars decreased from 1.83 for that of the raw coal to 1.71 and 1.73 for CoalZn and Coal-Co, respectively, reflected the content of the larger aromatic rings systems (six or more fused benzene rings) in char decreased
Fig. 2. Total intensity chromatogram (TIC) images (3D) of tar products from the depolymerization of coal with and no catalysts.
Fig. 3. Composition of the tar products from the depolymerization of coal with and no catalysts.
3.4. Raman spectra analysis on char characterization Raman spectrum of the coal and char samples in the 800–1800 cm−1 range were curve-fitted into ten Gaussian bands for which contained much structural information of the microcrystalline [33,34]. Fig. 4 was an example for which showed the curve fitting result of the Raman spectrum of the Coal-Co char and Table 4 were the summary of peak/band assignment. The G band at 1590 cm−1 belonged
Fig. 4. An example of Curve fitting of Raman spectrum for the Coal-Co char. 4
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Table 4 Characteristic Raman peak/band assignment of functional groups in coal [20,33]. Band name
Band position, cm−1
Description
GL G GR VL VR D SL S SR R
1700 1590 1540 1465 1385 1345 1230 1185 1060 800-960
Carbonyl group C]O Graphite E2g2; aromatic ring quadrant breathing Aromatics with 3–5 rings; amorphous carbon structures Methylene or methyl; semi-circle breathing of aromatic rings;amorphous carbon structures Methyl group; semi-circle breathing of aromatic rings; amorphous carbon structures D band on highly ordered carbonaceous materials; C–C between aromatic rings and aromatics with not less than 6 rings Aryl–alkyl ether; para-aromatics Car–Cal; aromatic (aliphatic) ethers; C–C on hydroaromatic rings; hexagonal diamond carbon sp3; C–H on aromatic rings C–H on aromatic rings; benzene (ortho-di-substituted) ring C–C on alkanes and cyclic alkanes; C–H on aromatic rings
aromatic ring systems in the catalytic depolymerized char caused the decrease of the ratios of ID/IG and ID/I(GR+ VL+ VR) .
Table 5 Fitting parameters obtained from Raman spectra of the different samples. sample
ID/IG
ID/I(GR + VL+ VR)
Raw Coal Coal-Non, Char Coal-Zn, Char Coal-Co, Char
1.69 1.83 1.71 1.73
0.83 1.01 0.83 1.01
3.5. Elemental analysis of char and tar Elemental analysis of the pyrolysis products were listed in Table 6. Compared with the raw coal char and tar, the content of the hydrogen and the molar ratio of the H/C increased obviously for the char and tar obtained from catalytic depolymerization of Coal-Zn and Coal-Co. According to the hydrogen balance of the products, combined with the variation of the water yield, it could be deduced that the hydrogen content in the gas products would be decreased. These results reflected that the addition of the catalysts obviously affected the depolymerization pathway of the Zaozhuang coal and impel more hydrogen migrated to the char and tar. Furthermore, compared with the raw coal depolymerization, the oxygen content of the tar obtained from depolymerization of the Coal-Zn and Coal-Co decreased, which indicated that the stability of the tar was improved. The variation of the hydrogen and oxygen content in the products reflected that the catalyst must have some kind of catalytic effect on the activation of the relative chemical bonds in the coal structure.
Table 6 Elemental analysis of char and tar. Sample
Char Tar
Ultimate analysis(wt%, daf)
Coal-Non Coal-Zn Coal-Co Coal-Non Coal-Zn Coal-Co
a
C
H
N
O
S
72.87 72.56 72.42 80.87 81.31 81.46
2.55 2.82 2.77 7.36 7.58 7.53
1.32 1.50 1.52 0.90 0.82 0.84
20.01 19.65 19.73 9.16 8.68 8.40
3.25 3.46 3.56 1.71 1.61 1.77
H/C molar ratio 0.42 0.47 0.46 1.09 1.12 1.11
daf, dry ash-free base. a By difference.
3.6. TG-DTG analysis of coal catalytic depolymerization
greatly with the adding of the Co or Zn catalysts. The ratio of the ID/I(GR+ VL+ VR) of the Coal-Zn char decreased from 1.01 for that of the raw coal to 0.83, but no obvious change was observed on that of Coal-Co char. Based on the above analysis, the increase content of the small
Thermogravimetric analysis was used to in-situ analysis the catalytic depolymerization behavior of the Zaozhuang coal. Fig. 5 showed TG and DTG curves obtained from the depolymerization process. The char yield decreased with catalysts and the variation tendency was well
Fig. 5. TG and DTG curves for coal with and without catalyst. 5
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Fig. 6. The enlarged partial view of DTG curves.
364 °C to 323 °C and 337 °C for Coal-Zn and Coal-Co, respectively. The decrease of the initial temperature indicated that the depolymerization reactions could commenced at a lower temperature. The maximum mass loss rate increased significantly after the adding of ZnCl2 and CoCl2 catalysts, as shown in Fig. 6b. The total volatile matters (VM) for coal depolymerization with the addition of catalysts were all higher than that of the raw coal (listed in Table 7). These results reflected that the addition of the catalysts could facilitate the cracking of more Cal-O bonds and precipitate the reaction starting at a lower temperature and finally increased the amount of the volatile matters evolved during the depolymerization stage [14].
Table 7 Characteristic temperatures in TG/DTG curves of coal samples. sample
T1 (°C)
Tp (°C)
T2 (°C)
-(dω/dt)max(%/min)
VM (%)
Coal-Non Coal-Zn Coal-Co
364 323 337
426 425 426
535 535 537
1.74 1.91 1.88
16.37 19.26 18.75
T1, initial temperature; Tp, maximum mass loss rate temperature; T2, terminal temperature; (dω/dt)max, maximum mass loss rate; VM, Total volatile matter evolved during the depolymerizapytion stage.
consistent with the results that obtained from the Gray-King assay. Fig. 6 was the enlarged partial view of DTG curves. Li et al. [35] stated that the DTG curve with temperature could reflect the cleavage of the covalent bonds during the coal pyrolysis, for which the major reactions were the cleavage of Cal-O, Cal-H, Cal-N, Cal-S and Cal-Cal bonds. Among them, Cal-Cal bond which had higher bond energy could break at a temperature of 450 °C, while the Cal-O, Cal-H, Cal-N and Cal-S bonds with lower band energy could break at the temperature less than 350 °C [36]. Fig. 6a showed the mass loss rate in the range of 210–330 °C enlarged for the coal with the addition of catalysts. The enlarged DTG curves at this temperature range might be ascribed to the cleavage of some more weak bonds, such as Cal-O, Cal-N and Cal-S [35–37]. Combined with the elemental analysis of char (listed in Table 6), the content of nitrogen, sulfur and hydrogen increased while the oxygen content decreased for the chars that obtained from catalytic depolymerization of the coal with the addition of catalysts. Thus it could concluded that catalysts might facilitate the cracking of Cal-O bonds and finally caused the lower content of the oxygen of the chars. Table 7 listed the characteristic parameters of coal samples calculated according to the results shown in Fig. 5. The initial temperature (T1) obviously decreased from
3.7. Possible role of catalysts Coal was comprised of polycyclic aromatic clusters cross-linked by bridge bonds, which would cleaved to forming volatiles during the thermal decomposition process [38]. From the results shown above, ZnCl2 and CoCl2 showed different catalytic effect for they may had different interaction with the coal structure. CoCl2 was better than ZnCl2 catalyst on increasing tar yield of thermal decomposition of the Zaozhuang coal. On the other hand, the two catalysts behaved roughly the same catalytic effect on products compositions, i.e. the tar yield increased sharply, the percentage of the light fractions of the tar increased, the large aromatic rings (> 6 rings) content in char reduced, the mass loss rate of the coal decomposition at the range of 210–330 °C enlarged and the content of oxygen in char decreased. So the possible role of the two catalysts on increasing the tar yield of Zaozhuang coal thermal decomposition could be concluded by the process listed below: The addition of the ZnCl2 or CoCl2 catalysts with the assistance of the accessory ingredient could have good contacts with the coal structure. During the catalytic depolymerizaiton process, the catalysts had an obvious catalytic effect on the Cal-O bridge bonds, for which linked with
Fig. 7. The role of catalyst during coal catalytic depolymerization process. 6
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the small aromatic rings to form the polynuclear aromatic clusters (the schematic diagram of the Cal-O bridge bonds had been shown in the blue circle in Fig. 7). The catalyst catalyzed the cleavage of Cal-O bonds and more precursors of tar were produced. As shown in Fig. 7, more dior tri-sextets polycyclic aromatics precursors of tar were generated (shown in route (1)). At the same time, the precursors of tar could be stabilized by radical such as H radical to form tar, as shown in the route (2) of Fig. 7, which finally increases the high value-added tar yield.
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4. Conclusions The catalytic depolymerization of a typical perhydrous coal over ZnCl2 and CoCl2 catalysts were studied. The results indicated that the tar yield increased from 8.21% to 9.19% and 10.02% with the addition of ZnCl2 and CoCl2 catalysts, respectively. Moreover, the presence of catalysts effectively increased the light fractions of tar, for which increased from 81.71% to 84.02% and 84.51% with the addition of ZnCl2 and CoCl2, respectively. The GC × GCeMS showed that catalysts reduced the yield of large aromatic ring systems (> 3 fused benzene rings) in tar. The analysis of the DTG curve confirmed that the mass loss rate of the coal depolymerization at 210–330 °C enlarged. All of these results indicated that, during catalytic depolymerization process, the addition of ZnCl2 and CoCl2 catalysts catalyze the cleavage of the Cal-O bonds, which finally increased the volatile matter released during the depolymerization stage. Acknowledgements This research is financially supported by National Natural Science Foundation of China (No.21776195, No. 21706175 and No. 21703151), and International Science & Technology Cooperation Program between China and Japan (2013DFG60060). References [1] B.K. Saikia, R.K. Boruah, P.K. Gogoi, A thermal investigation on coals from Assam (India), Fuel Process. Technol. 90 (2009) 196–203. [2] B.O. Oboirien, A.D. Engelbrecht, B.C. North, Study on the structure and gasification characteristics of selected South African bituminous coals in fluidised bed gasification, Fuel Process. Technol. 92 (2011) 735–742. [3] L. Zou, L. Jin, X. Wang, Pyrolysis of Huolinhe lignite extract by in-situ pyrolysistime of flight mass spectrometry, Fuel Process. Technol. 135 (2015) 52–59. [4] J. Yao, X. Wei, L. Xiao, Fractional extraction and biodepolymerization of Shengli Lignite, Energy Fuels 29 (2015) 2014–2021. [5] B. Tian, Y. Qiao, Y. Tian, Investigation on the effect of particle size and heating rate on pyrolysis characteristics of a bituminous coal by TG-FTIR, J. Anal. Appl. Pyrolysis 121 (2016) 376–386. [6] X. Zhao, Z. Liu, Q. Liu, The bond cleavage and radical coupling during pyrolysis of Huadian oil shale, Fuel 199 (2017) 169–175. [7] C. Ye, Z. Yang, W. Li, Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Process. Technol. 156 (2016) 415–420. [8] L. Ding, Z. Zhou, Q. Guo, Gas evolution characteristics during pyrolysis and catalytic pyrolysis of coals by TG-MS and in a high-frequency furnace, Fuel 154 (2015) 222–232. [9] Y. Li, M.N. Amin, X. Lu, Pyrolysis and catalytic upgrading of low-rank coal using a NiO/MgO-Al2O3 catalyst, Chem. Eng. Sci. 155 (2016) 194–200. [10] P. Zhu, Z. Yu, J. Zhang, Catalytic pyrolysis of bituminous coal under pyrolysis gas over a Ni/MgO catalyst, Chem. Eng. Technol. 40 (2017) 1605–1610.
7
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A study on catalytic depolymerization of a typical perhydrous coal for improving tar yield Zhan Xu, Litong Liang, Qian Zhang , Xiaodong Wang, Jianwei Liu, Hao Shen, Lingwei Kong, ⁎ Wei Huang ⁎
Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan, 030024, China
ARTICLE INFO
ABSTRACT
Keywords: Perhydrous coal Tar yield Catalytic depolymerization Char structure
Catalytic depolymerization proposed by our team was used on the thermal decomposition study of a typical perhydrous coal for improving the tar yield. The products from the coal catalytic depolymerization with or without a small amount of ZnCl2 or CoCl2 catalysts were investigated and compared. The composition of the tar and the structure of the coal char were characterized by a comprehensive two-dimensional gas chromatography coupled with mass spectrometric detection (GC × GCeMS), thermogravimetric analysis (TGA), Raman spectrum and Elemental analysis, respectively. Compared with 8.21% of tar yield generated from the depolymerization of the raw coal, the tar yield increased to 9.19% and 10.02% when ZnCl2 and CoCl2 catalysts were added, respectively. The light fractions of tar also increased from 81.71% to 84.02% and 84.51%, respectively. Meanwhile, the yield of the aromatic ring components (> 3 fused benzene rings) and the ratio of aromatic rings with six or more fused benzene rings in the tar decreased. Further analysis reflected that the addition of catalysts in the catalytic depolymerization process could facilitate the cracking of more Cal-O bonds and precipitate the reaction start at lower temperature, which finally increases the volatiles released during the depolymerization stage.
1. Introduction Coal as one of the most important fossil fuels is primarily composed of organic compounds, along with a small amount of inorganic minerals [1–4]. Pyrolysis is testified to be an effective and clean process to acquire the liquid tar, residue char and gases from thermal treatment of the organic components in coal [5–8]. The tar that contains aromatic compounds is more favorable as it could be used to upgrade high valueadded chemicals or transportation fuel further. However, the low H/C ratio of coal is not benefited for the production of tar. Besides, the heavy complex fractions in the tar will cause the equipment to be corroded or blocked. These elements strongly restricted the industrialization progress of the coal pyrolysis [9]. It is of vital importance to solve the problems discussed and to acquire high yield and quality tars. One of the methods is to employ the catalytic aided pyrolysis technics [10–14]. However, for the traditional catalytic pyrolysis process, extravagant amount of catalysts is demanded, which would cause various sensitive issues such as costly production investment, accelerated equipment depreciation caused by corrosion, and so on [15,16]. Another issue that cannot be dismissed for the existing
⁎
catalytic pyrolysis process is that the catalyst might primarily exert its effect on the volatiles reformation and consequently induce the increase content of lighter components in tar, in the meantime, the decreased yield of whole tar [17,18]. The faults of the catalytic pyrolysis should be firstly solved to propel the industrialization of the process. To fix the problems met in the catalytic pyrolysis process, our team proposed the concept of catalytic depolymerization. In this process, liquid reagent act as carrier to dissolve and disperse the catalyst in small share and then the dispersion system was uniformly sprayed into the coal [19]. By means of the form of dispersion system of assistant ingredient and catalyst, it’s believed that the catalyst can have a better contact with the coal and fully exert its role on depolymerize the coal selectively at low temperatures, thus the yield of the high value-added chemicals can be anticipated to increase. Applying the catalytic depolymerization process in the research of Liang et al. [20], the tar yield of increased from 3.6% sharply to 5.0% and 7.8% for Neimeng lignite and from 8.1% to 11.4% and 10.9% for Xinjiang lignite with the addition of Fe- and Mo-based catalysts, respectively. The experiments indicated a great potential that the catalytic depolymerization process have in enhancing both the yield and quality of the liquid tar. Liu et al. [21] also
Corresponding authors. E-mail addresses: [email protected] (Q. Zhang), [email protected] (W. Huang).
https://doi.org/10.1016/j.jaap.2019.01.001 Received 22 July 2018; Received in revised form 2 January 2019; Accepted 3 January 2019 0165-2370/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xu, Z., Journal of Analytical and Applied Pyrolysis, https://doi.org/10.1016/j.jaap.2019.01.001
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Table 1 Proximate and ultimate analyses of the coal sample. Proximate analysis (wt %, ad) M 3.16
A 38.36
H/Cb
Ultimate analysis (wt %, daf) V 26.94
FCa 31.54
C 65.74
Oa 24.25
H 5.05
N 1.20
S 3.76
0.92
ad, air dried base; daf, dry ash-free base. a By difference. b Atomic ratio.
studied the prospect of the catalytic depolymerization of a semi-char which generated from the low temperature pyrolysis process of a low rank coal on Fe-based catalyst in producing economic valued chemicals. The experiments witnessed the liquid yield improved sharply from 0.20% to 1.61% when a kind of iron-based catalyst was applied to the semi-char for the process of catalytic depolymerization, and they speculate that the catalyst accelerate the process of smaller molecule production through the chemical bond breaking and increased the content of liquid organic constituent in the final products. Based on the above experiments, catalytic depolymerization can be regarded as a promising method in increasing the yield of the tar from coal. In this paper, Zaozhuang coal was testified. The Zaozhuang coal, as a typical perhydrous subbituminous coal, was mined from Zaozhuang coal mine, Shandong province. The Zhaozhuang coal mine have a reserve estimated to more than 1 billion tons. However, the high ash and sulfur content in the coal (as shown in Table 1) made it incompetent for combustion or coking process. The Zaozhuang coal could be characterized as a perhydrous coal for the hydrogen content is higher than 5%. Moreover, the petrographic analysis showed that the Zaozhuang coal is rich in vitrinite content (85.5%, listed in Table 2). The high hydrogen content and the rich vitrinite content in the Zaozhuang coal make it a well substitute for tar extracting through pyrolysis [22]. It was assumed that if the proposed catalytic depolymerization could be successfully applied to the pyrolysis process of Zaozhuang coal, more high value-added products could be obtained, which would make this process more competitive. However, due to the limited application of catalytic depolymerization on the coal, it is still uncertain whether the catalyst depolymerization could be effectively applied to Zaozhuang perhydrous coal and improved the yield of tar consequently. The catalyst based on ZnCl2 and CoCl2 has been verified well suited for the process of catalytic pyrolysis of coal [23,24]. Therefore, in our experiments, the ZnCl2 and CoCl2 based catalysts were applied to the catalytic polymerization process of the Zaozhuang coal to testify whether it could improve the tar yield. In the process, we mainly concentrate on the analysis of the products of the tar and char. Besides, the catalytic effect of different catalysts was also compared and discussed.
2.2. Catalyst addition Analytical grade of ZnCl2 and CoCl2 chemicals (Kermel Chemical Reagent Co. Ltd., Tianjin, China) were used. Metal chloride of ZnCl2 or CoCl2 were separately dissolved in 2 mL accessory ingredient to prepare a uniform solution as catalyst [19]. The solution was sprayed into 30 g coal samples, blended mechanically and then stayed for 30 min [20]. The metal ion content of the catalyst was 0.1 wt% of the dry basis coal samples. Before the catalytic depolymerization experiment, the prepared samples of the coal with catalyst were dried for 2 h under the temperature of 110 °C. The coal samples that with the addition of accessory ingredient and ZnCl2 or CoCl2·catalysts were named as Coal-Zn and Coal-Co, respectively. 2.3. Catalytic depolymerization experiments The catalytic depolymerization experiments were carried out with a Gray-King equipment and operated according to China National Standard method (GB/T 1341-2007). Four quartz tubes with effective length of 300 mm and internal radius of 10 mm were used. For each quartz tube, one end was round and closed, and at the other end, a side tube was fused to serve as an exit for the volatiles released from the experiment [25]. 10 g coal sample was placed evenly in each tube. When the Gray-King reactor was heated to 300 °C, the tube was quickly inserted into the reactor. Then the reactor tube was heated at a heating rate of 5 °C/min to the set temperature of 600 °C and maintained at this temperature for 15 min. Each experiment had two comparative tests in order to ensure the experimental accuracy. The relative error tested was less than 0.5%. The liquid component of the products was gathered in a conical flask and kept in a low temperature environment created by icesalt bath. According to China National Standard method (GB/T 4802000), using toluene as solvent, the water was extracted from the liquid products. The yield of the tar and water can be calculated by this way. The yields of char, gas and tar were calculated on dry basis with the following equations [25]:
YTar =
2. Experimental
WTar × 100% W0 WChar × 100% W0
2.1. Coal sample preparation
YC har =
The perhydrous coal used were obtained from Shandong province, Zaozhuang City. The raw coal was mechanically crushed into a size range of 2.5–5.0 mm. Proximate and ultimate analyses of Zaozhuang coal were carried by China National Standard methods GB/T 212-2008 and GB/T 476-2001 and listed in Table 1. It could be seen that the coal was rich in hydrogen and it also contains of high amount of ash. Table 2 was the petrographic components of Zaozhuang coal performed by China National Standard method of GB/T 13773-2008.
YGas = 100
Vitrinite
Inertinite
Exinite
Minerals
Raw coal
85.50
8.80
–
5.70
YC har
YTar
(2)
YWater
(3)
YTar , YChar , YGas and YWater were the yield of tar, char, gas and water in dry base, respectively. WTar and WChar respectively represented the mass of tar and char. W0 was the initial mass of coal sample in dry base. 2.4. TG analysis The catalytic depolymerization experiments of the prepared samples were also performed using a Setaram SETSYS Evolution thermogravimetric analysis (TGA). About 5 mg of the prepared sample was uniform paved and kept at the same thickness in an alumina crucible and heated from room temperature to 600 °C at a heating rate of 5 °C/min under high pure argon (99.999%) atmosphere flow rate of 100 mL/min. TG and DTG curves were used to characterize the thermal decomposition
Table 2 Petrographic analysis of the coal sample (vol %). Coal sample
(1)
2
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parameters of the samples. T1 for which reflected the temperature at the mass loss reached to 10% of the total sample mass loss was defined as the temperature of the depolymerization process commenced [26]. Tp represented the temperature of the maximum mass loss rate peak appeared. T2 was defined as the temperature at which the depolymerization process was completed. It was calculated as the point of the mass loss reached to 90% of the total mass loss of the samples. 2.5. Product analysis The hydrocarbon compositions of the tar were performed on a comprehensive two-dimensional gas chromatography coupled with a mass spectrometric detection (GC × GC–MS). This system including an Agilent 7890B chromatograph equipped with an Agilent 5977 A MSD mass spectrometer and a ZX1 thermal modulator (Zoex). The two-dimensional capillary column system utilizes a combination of a nonpolar first column (DB-1, 15 m × 0.25 mm I.D. × 1.00 μm film thickness) with a medium polar second column (BPX-50, medium polar, 2.75m × 0.1 mm I.D. × 0.10 μm film thickness). High pure GC-grade Helium (99.999%) was served as the carrier gas. The primary GC oven temperature was set to increase from 50 °C to a final temperature of 300 °C at a heating rate of 5 °C/min. The modulator temperature was programmed to increase from 150 °C to 300 °C at a heating rate of 3 °C/ min. 1 μL sample was injected at 300 °C with a split ratio of 10:1, and the scanning mass range (m/z) was set from 30 to 500 amu. Data processing and display of the GC × GC–MS chromatograms were identified by matching the mass spectra database with the National Institute of Standards and Technology (NIST). The hydrocarbons were found automatically and further corrected manually when required. The fractions of the tar was analyzed by a TGA. In each experiment, about 5 mg of the tar sample was placed in an alumina crucible and heated from room temperature to 600 °C at a heating rates of 5 °C/min [27]. Then the fractions of the tar was divided into seven stages according to the evolution temperature, and named accordingly as light oil (< 170 °C), phenol oil (170–210 °C), naphthalene oil (210–230 °C), wash oil (230–300 °C), anthracene oil (300–360 °C), and pitch (> 360 °C) [9,28,29], respectively. The boiling point less than 360 °C was all defined as light fractions. Structural parameters of the coal and char samples were determined using a RENISHAW-in Via Raman Microscope equipped with a He: Cd laser at 514 nm as its light source. The scanning range was recorded from 800 to 1800 cm−1.
Fig. 1. Products distribution of coal depolymerization with and without catalyst.
3.2. Tar fractions analysis The data of the fractions of the tar produced from the catalytic depolymerization of the Zaozhuang coal with or without the catalysts was presented in Table 3. The percentage of the light fractions (< 360 °C) increased from 81.71% to 84.02% and 84.51% for Coal-Zn and Coal-Co, respectively. The catalysts on coal depolymerization decreased the relative content of pitch in the tar, while the content of anthracene oil increased sharply. While the percentage of the other light components of light oil, phenol oil, naphthalene oil and wash oil had no obvious change. The main components of pitch were polycyclic compounds, condensed aromatic hydrocarbons and their derivatives [31]. Thus the decrease of the pitch fractions might mainly be caused by the reduction of the generation of polycyclic aromatic hydrocarbons or inhibition condensing of the aromatic hydrocarbons in tar. According to Zou et al. [24], ZnCl2 and CoCl2 catalysts had certain acidity, which has advantage in the cracking of large aromatics and hydrogenating of the cracked tar fragments. 3.3. GC × GC–MS analysis of tar Fig. 2 illustrated the Total intensity chromatogram (TIC) images (3D) of the tar samples from the catalytic depolymerization of the Zaozhuang coal. The X-axis reflects the volatility-based retention time (min), the Y-axis reflects the polarity-based retention time (s), and the Z-axis reflects the MS detector response. The hydrocarbon compounds in the tar could be separated by their boiling point as well as their polarity through the two-dimensional chromatogram. Based on the library search using NIST and the volume of different peaks, the proportions of the detected components in the liquid products could be determined and divided into several species [20,32]: phenol, 1-ring aromatics (Ar), 2-ring Ar, 3-ring Ar, higher-ring Ar (> 3 fused benzene rings), aliphatic hydrocarbons and heteroatom compounds, as showed in Fig. 3. Compared with the raw coal tar, the content of phenols, 3-ring Ar and higher-ring aromatics (> 3 fused benzene rings) decreased obviously with the addition of catalysts. While the yield of 1-ring Ar and aliphatic hydrocarbons derived from the catalytic depolymerization with the addition of ZnCl2 and CoCl2 catalysts increased. These results indicated that the ZnCl2 and CoCl2 catalysts could decompose the larger aromatics ring systems (> 3 fused benzene rings) consisted in the coal structure and thus improved the content of smaller aromatic ring systems and aliphatic hydrocarbons in the tar.
3. Results and discussion 3.1. Product distribution and analysis The yield of the products from depolymerization of Zaozhuang coal by different catalysts were presented in Fig. 1. From Fig. 1, the tar yield of the Zaozhuang coal comes to 8.21%. As shown in Tables 1–2, Zaozhuang coal contains of high H/C ratio and high vitrinite content, which were both favourable for the formation of tar [30]. That’s the reason that Zaozhuang coal had a relative high tar yield though it contains of high ash content (38.36%). For the catalytic depolymerization experiment of Zaozhuang coal with the addition of ZnCl2 or CoCl2 catalysts, an obvious catalytic effect showed on increasing of the tar yield, which increased to 9.19% for Coal-Zn and 10.02% for CoalCo. On the other hand, the char yield decreased to 80.45% for Coal-Zn, and 80.44% for Coal-Co compared with the raw coal char yield of 82.72%. Moreover, the water yield increased from 3.01% to 3.74% and 3.20% with the addition of ZnCl2 and CoCl2 catalysts, respectively. The gases yield also slightly increased. All these results confirmed that the addition of ZnCl2 or CoCl2 catalysts increased the overall conversion of the Zaozhuang coal.
3
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Table 3 The amount of different fractions of tar from coal depolymerization with different catalysts. sample
Coal-Non Coal-Zn Coal-Co
Light oil %
Phenol oil %
Naphthalene oil %
anthracene oil %
210-230 °C
Wash oil % 230-300 °C
300-360 °C
Pitch % > 360 °C
< 170 °C
170-210 °C
21.74 20.60 21.55
15.42 14.66 15.71
8.10 7.60 8.01
27.06 27.27 27.41
9.39 13.89 11.83
18.29 15.98 15.49
to the aromatic quadrant ring-breathing mode. The D band at 1345 cm−1 were usually belonged to the disorder of fused benzene rings at least 6 fused aromatic rings but less than graphite. The three bands lied in the region of the G and D bands were named as GR, VR and VL bands. Among them, GR band at 1540 cm1 represented the carbon structure with 3–5 fused aromatic rings. VR band at 1465 cm−1 and VL band at 1385 cm−1 could be attributed to the methylene, methyl and semi-circle breathing of aromatic rings, respectively. Usually, the peak areas of GR, VR and VL bands were added together to represent the condensation degree of the smaller aromatic ring systems in the char structure. The intensities (peak areas) ratio ID/IG of D band and G band could be extensively used as a principal parameter to evaluate the char structure evolution during the catalytic depolymerization process of the Zaozhuang coal. Normally, the decrease of the ID/IG ratio indicated the relative decrease of the concentrations of large aromatic rings in the char structure, mainly reflect the decrease of the six or more fused benzene ring systems [34]. The intensities (peak areas) ratio ID/I(GR+ VL+ VR) of D band and the combined of GR + VL + VR band had been extensively used to compare the variation of the ratio of the large aromatic rings (> 6 rings) and the small aromatic ring systems (3–5 fused aromatic rings) in the coal thermal depolymerization process. Table 5 showed the changes of the bands ratios of different samples. When the raw coal converted into char through the thermal decomposition process, the ratio of ID/IG increased from 1.69 to 1.83 and the ID/I(GR+ VL+ VR) ratio increased from 0.83 to 1.01. The dehydrogenation of hydroaromatics and the further condensation of the aromatic rings during the coal thermal depolymerization process caused the growth of the ratios of the large aromatic rings. The ratio of ID/IG of the chars decreased from 1.83 for that of the raw coal to 1.71 and 1.73 for CoalZn and Coal-Co, respectively, reflected the content of the larger aromatic rings systems (six or more fused benzene rings) in char decreased
Fig. 2. Total intensity chromatogram (TIC) images (3D) of tar products from the depolymerization of coal with and no catalysts.
Fig. 3. Composition of the tar products from the depolymerization of coal with and no catalysts.
3.4. Raman spectra analysis on char characterization Raman spectrum of the coal and char samples in the 800–1800 cm−1 range were curve-fitted into ten Gaussian bands for which contained much structural information of the microcrystalline [33,34]. Fig. 4 was an example for which showed the curve fitting result of the Raman spectrum of the Coal-Co char and Table 4 were the summary of peak/band assignment. The G band at 1590 cm−1 belonged
Fig. 4. An example of Curve fitting of Raman spectrum for the Coal-Co char. 4
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Table 4 Characteristic Raman peak/band assignment of functional groups in coal [20,33]. Band name
Band position, cm−1
Description
GL G GR VL VR D SL S SR R
1700 1590 1540 1465 1385 1345 1230 1185 1060 800-960
Carbonyl group C]O Graphite E2g2; aromatic ring quadrant breathing Aromatics with 3–5 rings; amorphous carbon structures Methylene or methyl; semi-circle breathing of aromatic rings;amorphous carbon structures Methyl group; semi-circle breathing of aromatic rings; amorphous carbon structures D band on highly ordered carbonaceous materials; C–C between aromatic rings and aromatics with not less than 6 rings Aryl–alkyl ether; para-aromatics Car–Cal; aromatic (aliphatic) ethers; C–C on hydroaromatic rings; hexagonal diamond carbon sp3; C–H on aromatic rings C–H on aromatic rings; benzene (ortho-di-substituted) ring C–C on alkanes and cyclic alkanes; C–H on aromatic rings
aromatic ring systems in the catalytic depolymerized char caused the decrease of the ratios of ID/IG and ID/I(GR+ VL+ VR) .
Table 5 Fitting parameters obtained from Raman spectra of the different samples. sample
ID/IG
ID/I(GR + VL+ VR)
Raw Coal Coal-Non, Char Coal-Zn, Char Coal-Co, Char
1.69 1.83 1.71 1.73
0.83 1.01 0.83 1.01
3.5. Elemental analysis of char and tar Elemental analysis of the pyrolysis products were listed in Table 6. Compared with the raw coal char and tar, the content of the hydrogen and the molar ratio of the H/C increased obviously for the char and tar obtained from catalytic depolymerization of Coal-Zn and Coal-Co. According to the hydrogen balance of the products, combined with the variation of the water yield, it could be deduced that the hydrogen content in the gas products would be decreased. These results reflected that the addition of the catalysts obviously affected the depolymerization pathway of the Zaozhuang coal and impel more hydrogen migrated to the char and tar. Furthermore, compared with the raw coal depolymerization, the oxygen content of the tar obtained from depolymerization of the Coal-Zn and Coal-Co decreased, which indicated that the stability of the tar was improved. The variation of the hydrogen and oxygen content in the products reflected that the catalyst must have some kind of catalytic effect on the activation of the relative chemical bonds in the coal structure.
Table 6 Elemental analysis of char and tar. Sample
Char Tar
Ultimate analysis(wt%, daf)
Coal-Non Coal-Zn Coal-Co Coal-Non Coal-Zn Coal-Co
a
C
H
N
O
S
72.87 72.56 72.42 80.87 81.31 81.46
2.55 2.82 2.77 7.36 7.58 7.53
1.32 1.50 1.52 0.90 0.82 0.84
20.01 19.65 19.73 9.16 8.68 8.40
3.25 3.46 3.56 1.71 1.61 1.77
H/C molar ratio 0.42 0.47 0.46 1.09 1.12 1.11
daf, dry ash-free base. a By difference.
3.6. TG-DTG analysis of coal catalytic depolymerization
greatly with the adding of the Co or Zn catalysts. The ratio of the ID/I(GR+ VL+ VR) of the Coal-Zn char decreased from 1.01 for that of the raw coal to 0.83, but no obvious change was observed on that of Coal-Co char. Based on the above analysis, the increase content of the small
Thermogravimetric analysis was used to in-situ analysis the catalytic depolymerization behavior of the Zaozhuang coal. Fig. 5 showed TG and DTG curves obtained from the depolymerization process. The char yield decreased with catalysts and the variation tendency was well
Fig. 5. TG and DTG curves for coal with and without catalyst. 5
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Fig. 6. The enlarged partial view of DTG curves.
364 °C to 323 °C and 337 °C for Coal-Zn and Coal-Co, respectively. The decrease of the initial temperature indicated that the depolymerization reactions could commenced at a lower temperature. The maximum mass loss rate increased significantly after the adding of ZnCl2 and CoCl2 catalysts, as shown in Fig. 6b. The total volatile matters (VM) for coal depolymerization with the addition of catalysts were all higher than that of the raw coal (listed in Table 7). These results reflected that the addition of the catalysts could facilitate the cracking of more Cal-O bonds and precipitate the reaction starting at a lower temperature and finally increased the amount of the volatile matters evolved during the depolymerization stage [14].
Table 7 Characteristic temperatures in TG/DTG curves of coal samples. sample
T1 (°C)
Tp (°C)
T2 (°C)
-(dω/dt)max(%/min)
VM (%)
Coal-Non Coal-Zn Coal-Co
364 323 337
426 425 426
535 535 537
1.74 1.91 1.88
16.37 19.26 18.75
T1, initial temperature; Tp, maximum mass loss rate temperature; T2, terminal temperature; (dω/dt)max, maximum mass loss rate; VM, Total volatile matter evolved during the depolymerizapytion stage.
consistent with the results that obtained from the Gray-King assay. Fig. 6 was the enlarged partial view of DTG curves. Li et al. [35] stated that the DTG curve with temperature could reflect the cleavage of the covalent bonds during the coal pyrolysis, for which the major reactions were the cleavage of Cal-O, Cal-H, Cal-N, Cal-S and Cal-Cal bonds. Among them, Cal-Cal bond which had higher bond energy could break at a temperature of 450 °C, while the Cal-O, Cal-H, Cal-N and Cal-S bonds with lower band energy could break at the temperature less than 350 °C [36]. Fig. 6a showed the mass loss rate in the range of 210–330 °C enlarged for the coal with the addition of catalysts. The enlarged DTG curves at this temperature range might be ascribed to the cleavage of some more weak bonds, such as Cal-O, Cal-N and Cal-S [35–37]. Combined with the elemental analysis of char (listed in Table 6), the content of nitrogen, sulfur and hydrogen increased while the oxygen content decreased for the chars that obtained from catalytic depolymerization of the coal with the addition of catalysts. Thus it could concluded that catalysts might facilitate the cracking of Cal-O bonds and finally caused the lower content of the oxygen of the chars. Table 7 listed the characteristic parameters of coal samples calculated according to the results shown in Fig. 5. The initial temperature (T1) obviously decreased from
3.7. Possible role of catalysts Coal was comprised of polycyclic aromatic clusters cross-linked by bridge bonds, which would cleaved to forming volatiles during the thermal decomposition process [38]. From the results shown above, ZnCl2 and CoCl2 showed different catalytic effect for they may had different interaction with the coal structure. CoCl2 was better than ZnCl2 catalyst on increasing tar yield of thermal decomposition of the Zaozhuang coal. On the other hand, the two catalysts behaved roughly the same catalytic effect on products compositions, i.e. the tar yield increased sharply, the percentage of the light fractions of the tar increased, the large aromatic rings (> 6 rings) content in char reduced, the mass loss rate of the coal decomposition at the range of 210–330 °C enlarged and the content of oxygen in char decreased. So the possible role of the two catalysts on increasing the tar yield of Zaozhuang coal thermal decomposition could be concluded by the process listed below: The addition of the ZnCl2 or CoCl2 catalysts with the assistance of the accessory ingredient could have good contacts with the coal structure. During the catalytic depolymerizaiton process, the catalysts had an obvious catalytic effect on the Cal-O bridge bonds, for which linked with
Fig. 7. The role of catalyst during coal catalytic depolymerization process. 6
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the small aromatic rings to form the polynuclear aromatic clusters (the schematic diagram of the Cal-O bridge bonds had been shown in the blue circle in Fig. 7). The catalyst catalyzed the cleavage of Cal-O bonds and more precursors of tar were produced. As shown in Fig. 7, more dior tri-sextets polycyclic aromatics precursors of tar were generated (shown in route (1)). At the same time, the precursors of tar could be stabilized by radical such as H radical to form tar, as shown in the route (2) of Fig. 7, which finally increases the high value-added tar yield.
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4. Conclusions The catalytic depolymerization of a typical perhydrous coal over ZnCl2 and CoCl2 catalysts were studied. The results indicated that the tar yield increased from 8.21% to 9.19% and 10.02% with the addition of ZnCl2 and CoCl2 catalysts, respectively. Moreover, the presence of catalysts effectively increased the light fractions of tar, for which increased from 81.71% to 84.02% and 84.51% with the addition of ZnCl2 and CoCl2, respectively. The GC × GCeMS showed that catalysts reduced the yield of large aromatic ring systems (> 3 fused benzene rings) in tar. The analysis of the DTG curve confirmed that the mass loss rate of the coal depolymerization at 210–330 °C enlarged. All of these results indicated that, during catalytic depolymerization process, the addition of ZnCl2 and CoCl2 catalysts catalyze the cleavage of the Cal-O bonds, which finally increased the volatile matter released during the depolymerization stage. Acknowledgements This research is financially supported by National Natural Science Foundation of China (No.21776195, No. 21706175 and No. 21703151), and International Science & Technology Cooperation Program between China and Japan (2013DFG60060). References [1] B.K. Saikia, R.K. Boruah, P.K. Gogoi, A thermal investigation on coals from Assam (India), Fuel Process. Technol. 90 (2009) 196–203. [2] B.O. Oboirien, A.D. Engelbrecht, B.C. North, Study on the structure and gasification characteristics of selected South African bituminous coals in fluidised bed gasification, Fuel Process. Technol. 92 (2011) 735–742. [3] L. Zou, L. Jin, X. Wang, Pyrolysis of Huolinhe lignite extract by in-situ pyrolysistime of flight mass spectrometry, Fuel Process. Technol. 135 (2015) 52–59. [4] J. Yao, X. Wei, L. Xiao, Fractional extraction and biodepolymerization of Shengli Lignite, Energy Fuels 29 (2015) 2014–2021. [5] B. Tian, Y. Qiao, Y. Tian, Investigation on the effect of particle size and heating rate on pyrolysis characteristics of a bituminous coal by TG-FTIR, J. Anal. Appl. Pyrolysis 121 (2016) 376–386. [6] X. Zhao, Z. Liu, Q. Liu, The bond cleavage and radical coupling during pyrolysis of Huadian oil shale, Fuel 199 (2017) 169–175. [7] C. Ye, Z. Yang, W. Li, Effect of adjusting coal properties on HulunBuir lignite pyrolysis, Fuel Process. Technol. 156 (2016) 415–420. [8] L. Ding, Z. Zhou, Q. Guo, Gas evolution characteristics during pyrolysis and catalytic pyrolysis of coals by TG-MS and in a high-frequency furnace, Fuel 154 (2015) 222–232. [9] Y. Li, M.N. Amin, X. Lu, Pyrolysis and catalytic upgrading of low-rank coal using a NiO/MgO-Al2O3 catalyst, Chem. Eng. Sci. 155 (2016) 194–200. [10] P. Zhu, Z. Yu, J. Zhang, Catalytic pyrolysis of bituminous coal under pyrolysis gas over a Ni/MgO catalyst, Chem. Eng. Technol. 40 (2017) 1605–1610.
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