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In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor
FUPROC-04827; No of Pages 6 Fuel Processing Technology xxx (2015) xxx–xxx
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Research article
In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor Lijun Jin a,b, Xiaoyu Bai a, Yang Li a, Chan Dong a, Haoquan Hu a,⁎, Xian Li b a b
State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 25 July 2015 Received in revised form 15 December 2015 Accepted 25 December 2015 Available online xxxx Keywords: Coal pyrolysis Upgrading Coal tar Char Activated carbon
a b s t r a c t Coal pyrolysis tar was in-site upgraded in a fixed-bed reactor over char and activated carbon (AC) catalysts, and the effects of catalysts and temperature on tar upgrading were examined. The content of light tar with boiling point below 360 °C was improved over the catalysts despite the decrease of total tar yield. Compared with char catalyst, AC exhibited better upgrading performances, and the content of light tar increased with the temperature. The light tar yield over AC catalyst increased by 18% compared with that without catalysts at 650 °C. The variation in coal tar fractions suggested that upgrading is mainly from the conversion of the heavy components (pitch) in tar into light tar and gases. The role of carbon-based catalyst in upgrading process was also explored through the characterization by TG-DTG, FT-IR, N2 adsorption and Raman spectroscopy. Compared with thermal cracking, adsorption on the catalysts and the minerals in char, high specific surface areas and relative more defects in the carbon catalysts seem to be the primary factor for upgrading tar. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Coal pyrolysis, as one of the most promising technologies for energy sustainability, can produce liquid fuels, valuable chemicals and high heating value gas by mild conversion of coal. However, coal tar usually has high content of heavy fractions with boiling points above 360 °C up to more than 50% of total tar [1–3]. This heavy tar with high viscosity, easy pollution and low economic value usually leads to serious problems in the operation of industrial equipment and further processing of tar, which restricts the application of pyrolysis process in the industry. To upgrade the coal tar, various methods were investigated including thermal cracking, hydrogenation and catalytic cracking for the decomposition of heavy components in tar. However, thermal cracking tends to result in insufficient decrease of heavy fractions even at high temperature and pressure, along with obvious loss of overall tar and energy inefficiency [4–6]. Hydrogenation of coal tar received substantial attention owing to remarkable improvement of tar quality and smallmolecule gases yield, especially the content of BTX and their derivates [7], but some of heavy components (mainly pitch) are still difficult to be hydrocracked, and more hydrogen as an expensive gas is consumed and high pressure is utilized [8]. Catalytic upgrading presents a potential method for the conversion of heavy tar into light tar and pyrolysis gases by secondary reaction of tar precursors in the process of coal pyrolysis [9,10]. More studies are ⁎ Corresponding author. E-mail address: [email protected] (H. Hu).
focused on the improvement of BTX and PCX contents [11,12]. Takarada et al. [13,14] found that the content of BTX on supported metal catalysts such as Co–Mo/Al2O3 was 5.8 (wt.% daf), which improved 30 times comparing with that on SiO2 sample in a fluidized bed reactor for coal pyrolysis reaction. Deng et al. [15] investigated the catalytic upgrading over olivine and Co-olivine catalysts, and the heavy fraction (boiling points above 360 °C) was obviously reduced. Compared with the olivine as the catalyst, the pitch content over Co-olivine decreases by 17% at 550 °C. Sonoyama et al. [16] reported about 97 wt.% of initial heavy components in tar from pyrolyzing the Loy Yang coal at 500 °C were decomposed over iron-oxide catalyst with metal promoters. Öztaş et al. [17] believed that better catalytic effects were acquired by pyrolysis of Zonguldak bituminous coal mixed with ZnCl2, CoCl2 and NiCl2. However, the metal catalysts including Ni and Ca are always subjected to some problems including sulfur poisoning, coke deposition and high cost despite their high activity for tar conversion. Carbon-based catalyst such as char or activated carbon is considered to be a most potential for industrial application owning to low cost and easy availability [18–21]. Han et al. [18] studied the catalytic upgrading of coal pyrolysis tar over char-based catalysts, and found that light tar fraction (boiling points below 360 °C) increased by 25% with char compared with coal direct pyrolysis at 600 °C. Better upgrading effect was obtained at 500 °C when metal was supported over char. Gilbert et al. [22] investigated tar reduction of pyrolysis vapors from biomass over hot char in a fixed-bed reactor. The content of heavy fractions decreased to 18.4% at 500 °C and 8.0% at 800 °C. Although the properties of carbonbased catalyst, including specific surface area, pore volume, sulfur and carbon deposition resistance and containing metal oxides, will influence
http://dx.doi.org/10.1016/j.fuproc.2015.12.028 0378-3820/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
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2.3. Analysis of tar and gases
Table 1 Proximate and ultimate analyses of SM coal and carbon-based catalysts. Sample
Coal Char AC
Proximate analysis (wt.%)
Ultimate analysis (wt.% daf)
Mad
Ad
Vdaf
C
H
N
S
O⁎
1.76 0.76 1.22
8.16 14.62 2.10
35.77 3.40 3.98
71.40 83.08 94.16
4.42 1.04 0.64
0.65 1.28 0.29
0.25 0.27 0.00
23.38 14.33 4.91
⁎ By difference.
Tar plus water was separated according to ASTM D95-05e1 (2005) using toluene as solvent. The tar yield (Ytar) and light tar yield (Ylight tar) in dry ash-free base are calculated as follows: Y tar ¼
W tar 100% W o ð1−Aad −Mad Þ
Y light tar ¼ Y tar w1 %
tar upgrading, it is still unclear as to the upgrading mechanism. Zeng et al. [23] believed that tar could be absorbed on the surface of carbon-based catalyst when it existed down the pyrolysis coal, and large specific surface area promoted more tar to be dispersed on the surface, further leading to the enhancement of retention time for heavy tar upgrading. To study the catalytic role of carbon-based catalyst in catalytic upgrading tar and the decrease of heavy tar components, in this work, two carbon-based catalysts, char and activated carbon, were used to upgrade the coal tar from coal pyrolysis in a fixed-bed reactor containing upper coal layer and lower catalyst layer. The effect of catalyst kinds and the temperature on the quality of coal tar was examined and the role in catalytic upgrading was also explored based on the experimental results and several analysis technologies. 2. Experimental section 2.1. Coal sample and carbon catalysts
where, Wtar and W0 are the weight of tar and coal sample; Aad and Mad are the ash and moisture contents of coal; and w1% is the light tar content. The results presented are the mean values of at least 3 equivalent experiments, and the estimated error of tar yield is within ±0.2%, indicating good repeatability. The components analysis of coal tar was conducted by simulated distillation (ASTM D-2887) according to boiling points of fractions listed in Table 2. The light tar content (w1%) herein refers to the fractions with a boiling point below 360 °C including light oil, phenol oil, naphthalene oil, wash oil and anthracene oil. Before the analysis, the tar was first dissolved in carbon disulfide, and then Na2SO4 was added to adsorb the water in tar. After filtration to remove Na2SO4, the tar with carbon disulfide was concentrated by distillation and used for analysis. Pyrolysis gases were analyzed by an on-line gas chromatography (GC7890II) equipped with a thermal conductivity detector packed with 5A molecular sieve and a flame ionization detector with GDX502 packed column. 2.4. Characterization of catalysts
Shenmu (SM) subbituminous coal was grounded and sieved below 0.15 mm and dried for the pyrolysis experiment. Two carbon-based catalysts were chosen for tar upgrading. One is the char prepared by pyrolysis of Shenmu coal at 850 °C for 3 h in N2 on a fixed bed reactor; another is the commercial coconut shell activated carbon (AC). Their proximate and ultimate analyses of the coal and carbon catalysts were given in Table 1. 2.2. Apparatus and procedure The upgrading of coal tar was carried out in a vertical fixed-bed reactor with an inner diameter of 18 mm and a length of 150 mm. In each run, the reactor was charged with upper coal sample (5 g) and lower carbon-based catalyst (1 g), and some quartz wool was placed between them for separation. After the reactor was purged with high-purity N2 for approximately 10 min to remove the air before the reaction, it was heated to the desired temperature within less than 10 min by a moving preheated furnace, and maintained for 30 min for pyrolysis and upgrading in the N2 flow rate of 100 ml/min. The volatile vapors during the coal pyrolysis were carried through upgrading zone of the tar by high-purity nitrogen, upgraded and collected by a cool trap. Long isotherm zone of the furnace kept the coal pyrolysis and upgrading of the resultant pyrolysis tar in the same temperature. The noncondensable gases were collected and the solid char and catalyst were removed from the reactor for analysis after the experiment.
TG/DTG experiments of carbon-based catalysts were performed using a Mettler Toledo TGA/SDTA851e thermogravimetry analyzer. About 20 mg sample was heated from 25 °C to 850 °C with a heating rate of 10 °C/min using argon as the carrier gas at a constant flow rate of 60 ml/min. The surface analysis of carbon-based catalyst was measured by FT-IR on an EQUINOX55 spectrometer using KBr pellet technique. N2 adsorption of the fresh and spent catalysts was measured on the physical adsorption apparatus (ASAP 2420) at −196 °C. The samples were outgassed at 300 °C for 4 h prior to adsorption. The parameters on the pore structure were obtained by using Brunauer–Emmett– Teller (BET) and Barrett–Joyner–Halenda methods. Raman spectra of carbon catalysts were recorded on a Thermo Scientific DXR Microscope using the 532 nm line of solid state laser. The laser was focused to about 150 μm in diameter at a power of 1 mW in order to prevent thermal degradation. Raman spectral from 800 to 1800 cm−1 were measured with a spectral resolution of 2 cm−1. 3. Results and discussion 3.1. Effect of carbon catalysts on the upgrading of coal tar Two kinds of carbon catalysts were used to in-situ upgrade the coal tar from pyrolysis at different temperatures. To acquire the distribution of tar fractions, simulated distillation was used to analyze the tar compositions from pyrolysis at 650 °C, and the curves in Fig. 1 referred to
Table 2 Simulated distillation fractions of tar from pyrolysis at 650 °C. Catalyst
– Char AC
Fraction (wt.%) Light oil (b170 °C)
Phenol oil (170–210 °C)
Naphthalene oil (210–230 °C)
Wash oil (230–280 °C)
Anthracene oil (280–360 °C)
Pitch (N360 °C)
2.8 4.3 8.9
5.6 7.3 15.5
5.4 5.6 8.8
15.6 16.5 23.8
15.0 16.4 18.5
55.6 49.9 24.5
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
L. Jin et al. / Fuel Processing Technology xxx (2015) xxx–xxx
Fig. 1. Simulated distillation curves of tar from pyrolysis at 650 °C.
the percentage of tar fractions with boiling points below the heating temperatures. Lower boiling points suggest lighter tar components. As illustrated in Fig. 1, both char and AC as the upgrading catalysts led to higher percentage of tar fractions at the same boiling point temperatures, indicating higher light tar contents and the upgrading quality of tar. Additionally, the initial boiling point of coal tar also shifts from 130.9 °C without upgrading down to 128.5 °C over char and 111.1 °C over AC catalyst, and the final boiling points are at 563.6 °C, 559.2 °C and 541.2 °C, respectively. All these results indicate that carbon catalysts can upgrade the pyrolysis tar and decrease the contents of heavy components. Compared with char catalyst, the AC exhibits better upgrading effect. Table 2 shows that all the fractions except pitch increased when carbon catalysts were used to upgrade the pyrolysis tar. In comparison, light oil and phenol oil with low boiling points increased by 6.1 wt.% and 9.9 wt.% over AC, rather than 1.5 wt.% and 1.7 wt.% over char catalyst, respectively. The decrease of pitch fraction means that the increase of light fractions over char and AC catalysts is from the conversion of pitch. The variation in light tar contents and yields at different temperatures are presented in Fig. 2. The lowest content of light tar was obtained and the temperature has slight effect on the light tar content when no catalyst was used. However, light tar content obviously increased over the carbon-based catalysts. For example, the content of light fractions increased from 46.2 wt.% of coal pyrolysis tar at 650 °C to 51.7 wt.% over char catalyst, and further increased to 76.6 wt.% over the AC catalyst. Additionally, light tar contents increased at relatively high temperature (ca. 650–750 °C) on AC catalyst, different from that on char catalyst or without catalyst, which indicates high catalytic activity of AC catalyst in upgrading tar at high temperature. Fig. 2b showed that tar yield had a decrease on carbon catalyst, especially the AC compared to coal pyrolysis without catalyst at 650 °C.
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Lower tar yield on char or AC catalyst than that without catalyst is attributed to tar upgrading catalyzed by the carbon catalysts, which resulted in the conversion of heavy fraction (pitch) into more light tar components and gases by carbon catalyst (see Fig. 3). Nevertheless, compared with the char catalyst, the AC exhibited better performance in upgrading tar, and the yield of light tar over AC catalyst is always higher than those over char catalyst or without the catalyst, shown in Fig. 3c, indicating that AC is a good catalyst for upgrading coal tar. At 650 °C, the yield of light tar yield increased by 18% compared to that without catalyst. Pyrolysis gas volumes under different temperatures were shown in Fig. 3a. Obviously, pyrolysis gas considerably increased with increasing temperature, and more gases were produced over AC catalyst than char catalyst or without catalyst, indicating higher catalytic activities of AC catalyst. The component variety of gas products at 650 °C in Fig. 3b showed that all the gas volumes increased after tar cracking with carbon catalysts. Low temperature was easy for the decomposition of carboxyl, methoxyl and ether groups, while high temperature was beneficial to the decomposition of phenolic hydroxyl, carbonyl and epoxy groups [24]. Compared with other gas components, more H2 and CH4 were produced, especially over AC catalyst. It is usually thought that H2 is mainly from the condensation of aromatic and hydroaromatic structures or decomposition of heterocycle compounds [25]. CH4 mainly comes from the pyrolysis of aliphatic hydrocarbons, aliphatic side chains of aromatic hydrocarbons and –OCH3 [26]. That is, carbonbased catalyst facilitated heavy tar upgrading into small-molecule gases besides light tar. But it is still unknown what's the role of carbon catalysts in the upgrading the tar.
3.2. Role of carbon-based catalyst in upgrading the tar In the tar upgrading, many factors including thermal decomposition, surface properties and inherent minerals will influence the catalytic upgrading performances. The tar upgrading is usually carried out at high temperature above 500 °C, which will result in its thermal polymerization and/or cracking of coal tar. Therefore, to examine the influence of thermal cracking on the upgrading and explore the role of carbon catalysts in the upgrading process, the char catalyst was replaced by inactive SiO2 with the same height in the reactor so as to keep the similar resident time. As shown in Fig. 4a, the light tar content had a slight increase when SiO2 was used, which was ascribed to long residence time for polymerization and/or cracking at 650 °C caused by the adsorption of tar on SiO2, but obviously lower than 51% over char catalyst. To further increase the residence time of coal tar in the heat zone, we tripled the SiO2 height in the reactor and found that almost the same content (48.5%) of light tar was obtained, which indicates that heat is beneficial for tar upgrading, but not the dominant factor for upgrading over char and activated carbon. Additionally, the gas compositions in Fig. 4b further ascertained that the catalytic role was mainly from the char, not from the thermal
Fig. 2. Light tar content (a), tar yield (b) and light tar yield (c) at different pyrolysis temperatures.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
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L. Jin et al. / Fuel Processing Technology xxx (2015) xxx–xxx
Fig. 3. Total gas yield (a) and gas component yield (b) over different catalysts in pyrolysis process of SM coal.
Fig. 4. Light tar content (a) and gas components (b) over char catalyst and SiO2 at 650 °C.
cracking despite more gases produced over the SiO2 than that without catalyst. Activated carbon, owing to its high surface area and unique pore structure, is widely used in the adsorption [27]. Because low total tar yield and low contents of heavy compositions were obtained when AC is used as the upgrading catalyst, it is confused whether the decrease of tar yield is caused by the adsorption of heavy compositions on the AC catalysts during the upgrading. Therefore, the fresh and spent AC catalysts were examined by TG-DTG, FT-IR, and N2 adsorption, respectively. As demonstrated in Fig. 5, obvious weight loss occurred before 100 °C for the fresh and spent AC samples, which is attributed to the water evolution. Compared with the fresh AC, the spent sample had lower weight loss below 100 °C, which may be related with the decreasing surface area and pore volume (Table 3). But there is no remarkable difference in the range of 150 to 850 °C, and the weight loss is 2.8% for fresh AC, slightly higher than 1.7% over spend catalyst, suggesting that
the used AC has little adsorption of the coal tar, and the decrease of tar yield on AC catalysts is not caused by the adsorption on the surface of AC catalyst. Table 4 summarized the total volatile per gram coal at different temperatures. Obviously, the total volatiles increased with the pyrolysis temperature no matter AC catalyst is used or not, which is caused by the decomposition of the organic matters in coal. However, the difference in total volatile at the same temperature was quite small and within the experimental error when AC was used to upgrade the coal tar, which is in accordance with the TG-DTG results in Fig. 5, further confirming that low tar yield is not caused by the adsorption, but mainly from the cracking of coal tar on the AC catalyst into the light tar and gases at high temperature. Table 3 gives the texture properties of fresh and spent carbon catalysts. Obviously, the fresh AC sample had higher specific surface area (803 m2/g) than char catalyst (2 m2/g). The difference in the specific surface area seems to be the reason for different upgrading performances of carbon catalysts. The specific surface areas of spent catalysts decreased, especially the AC sample from 803 to 499 m2/g. The pore volume also decreased from 0.38 to 0.24 cm3/g. It is thought that the decrease in specific surface area and pore volume is probably caused by pore collapse suffered from high temperature and coke deposit of from the decomposition of coal tar. Low specific surface area resulted in poor upgrading performances. Table 3 Textural properties of carbon-based catalysts.
Fig. 5. TG and DTG curves of fresh and spent AC catalysts.
Sample
SBET (m2 g−1)
Smic (m2 g−1)
Smic/SBET
Vtotal (cm3 g−1)
Vmic (cm3 g−1)
Vmic/Vtot
Fresh char Spent char⁎ Fresh AC Spent AC⁎
2.2 1.4 803 499
/ / 658 383
/ / 0.8 0.8
/ / 0.38 0.24
/ / 0.30 0.18
/ / 0.79 0.75
⁎ Pyrolysis temperature: 650 °C.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
L. Jin et al. / Fuel Processing Technology xxx (2015) xxx–xxx Table 4 Total volatile at different pyrolysis temperatures. Temperature (°C)
450 550 650 750
Total volatile⁎ (g/g coal) No catalyst
AC catalyst
Difference
0.164 0.236 0.276 0.312
0.158 0.232 0.274 0.308
0.006 0.004 0.002 0.004
⁎ Total volatiles include water, tar and gases.
Coal char contains some inherent metal minerals, such as alkaline and alkaline earth metals, especially that prepared from the high ash coal [29]. Zeng et al. [23] believed that these inherent metal species may serve as the active sites for upgrading coal tar. Nevertheless, poorer catalytic upgrading on char than that on AC catalyst was obtained in spite of higher ash content of 14.52% in char rather than 2.10% in AC sample (Table 1), which suggested that the inherent mineral in carbon catalysts has less effect on coal tar upgrading compared with the specific surface area of carbon catalysts. FT-IR spectra of fresh and spent carbon catalysts reacted at 650 °C are compared in Fig. 6. Obviously, bands at 2923 and 2852 cm−1 (C–H stretching vibrations) almost disappeared on the used char at 650 °C, indicating the evolution of some organic matters in the char during the upgrading process, which is the reason for the weight loss in Fig. 5 at high temperature. As to the absorption bands attributed to oxygencontaining functional groups at 3440, 1700 and 1100 cm−1, the intensity of stretching and vibration bands of hydroxyl O–H at 3440 cm−1 remarkably decreased owing to the decarboxylation over the spent char catalyst, which is in accordance with the results by Guo et al. [28]. And the carboxyl or carbonyl CO bands at 1700 cm−1 became weaker or disappeared after char or AC was used as the catalyst, meaning that decarboxylation occurred in upgrading tar. The ultimate analysis of the used char and AC catalysts at 650 °C also confirmed the decrease in oxygen contents, which is quite accordant with the FT-IR analysis. Additionally, no new absorption bands attributed to the aliphatic or aromatic structures were formed. These results indicate that the main role of carbon-based catalyst is to provide catalytic active sites on its surface, not just as an adsorbent for the tar adsorption and increasing the residence time. 3.3. Raman spectra Raman spectroscopy is widely used to characterize the structure features of a range of carbonaceous materials, including activated carbons and coal chars [30–33]. Usually, only one band at about 1580 cm− 1 (G band) exists for the perfect graphite in the first-order region, which corresponds to the stretching vibration mode with E2g symmetry in the aromatic layers of graphite crystalline. However, for highly disordered carbons, there are additional bands at 1150 cm−1 (D4),
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1350 cm−1 (D1), 1530 cm−1 (D3 band) and 1620 cm−1 (D2) induced by the defects in the microcrystalline lattice. The 1350 cm−1 band is commonly called the defect band, which is ascribed to the in-plane imperfections or defects [33]. Therefore, studying the Raman spectra of carbon catalysts can provide information for evaluating the degree of ordering and crystallinity in carbonaceous materials. Fig. 7 illustrated Raman spectra of AC and char samples in the range of 800–1800 cm− 1, and peak fitting of each spectrum according to Sheng's method [33] was also presented, indicating good agreement. Both samples showed the similar spectra, and the G band was weaker than the D1 band, indicating poor order of two carbon catalysts. The D3 and D4 bands are low in intensity but wide FWHM, implying considerable amounts of amorphous forms of carbon [33]. Because the total Raman intensity is affected by many factors such as the intrinsic Raman scattering ability, the light absorption of the carbon material, and the presence of electron-rich functional groups like those containing N, O and S [31], it's inappropriate to compare the total intensity between the AC and coal char samples. However, the band area ratios between D1 band and G band and the FWHM of the G band can be used as indicators of carbon structural order. Obviously, the AC sample had higher band area ratio of D1 band to G band (5.1) than coal char (3.9) and broader FWHW. In addition, the area ratios of other defects bands (D2, D3 and D4) in AC sample are also higher than those in char catalyst. All these indicated that the AC had more structural defects and imperfections of the carbon crystallites than char sample, which is in accordance with Serrano's results that carbon material with higher surface area usually corresponds with a more number of defects [34]. These structural defects or imperfections in the carbon materials could serve as the active sites for upgrading the tar. Large surface area of carbon catalyst is preferable to improve utilization efficiency of active sites and tar upgrading. Therefore, the AC sample with higher surface area and relative more defects than char catalyst exhibited better tar upgrading. 4. Conclusion Char and AC as the catalysts can upgrade the quality of coal tar, increase the content of light tar and the gases yield although total tar yield decreased. Compared with the char, AC catalyst exhibited better upgrading performances, which is mainly related to its larger surface area and pore volume. The content of light tar increased with the temperature over AC catalyst and higher light tar yield than that without upgrading can be obtained. All the fractions except pitch in coal tar increased when char or AC was used as the catalyst, which was mainly from the conversion of the heavy components in tar into light tar and gases. The analyses results of the fresh and spent carbon catalysts showed that high specific surface area and relative more defects or imperfections of carbon catalyst are preferable to convert heavy fraction into light tar and gases compared with the adsorption of coal tar on carbon catalysts, heat effect and inherent minerals during tar upgrading.
Fig. 6. FT-IR spectra of fresh and spent char (a) and AC (b) catalysts.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
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Fig. 7. Raman spectra of fresh AC (a) and char (b) samples.
Acknowledgments This work was performed with support of the National Natural Science Foundation of China (No. 21176042), the National Basic Research Program of China (973 Program, No. 2011CB201301) and the Key International Science & Technology Cooperation and Exchange Project, the Ministry of Science and Technology, China (No. 2013DFG60060), the Joint Fund of Coal-based Low hydrocarbons by NSFC and Shanxi Provincial Government (No. U1510101), the Funds of State Key Laboratory of Coal Combustion (No. FSKLCCA1506) and State Key Laboratory of Multiphase Complex Systems (No. MPCS2014-D-07). References [1] P. Liang, Z. Wang, J. Bi, Process characteristics investigation of simulated circulating fluidized bed combustion combined with coal pyrolysis, Fuel Process. Technol. 88 (2007) 23–28. [2] X. Qu, P. Liang, Z. Wang, R. Zhang, D. Sun, X. Gong, Z. Gan, J. Bi, Pilot development of polygeneration process of circulating fluidized bed combustion combined with coal pyrolysis, Chem. Eng. Technol. 34 (2011) 61–68. [3] J.G. Wang, X.S. Lu, J.Z. Yao, W.G. Lin, L.J. Cui, Experimental study of coal topping process in a downer reactor, Ind. Eng. Chem. Res. 44 (3) (2005) 463–470. [4] J. Hayashi, S. Amamoto, K. Kasukabe, S. Morooka, Evaluation of vapor-phase reactivity of primary tar produced by flash pyrolysis of coal, Energy Fuel 9 (1995) 290–294. [5] J. Hayashi, T. Kawakami, T. Taniguchi, K. Kusakabe, S. Morooka, M. Yumura, Control of molecular composition of tar by secondary reaction in fluidized-bed pyrolysis of a subbituminous coal, Energy Fuel 7 (1993) 57–66. [6] J. Han, H. Kim, The reduction and control technology of tar during biomass gasification/pyrolysis: an overview, Renew. Sust. Energ. Rev. 12 (2008) 397–416. [7] J. Kusy, L. Andel, M. Safarova, J. Vales, K. Ciahotny, Hydrogenation process of the tar obtained from the pyrolysis of brown coal, Fuel 101 (2012) 38–44. [8] T. Kan, X.Y. Sun, H.Y. Wang, C.S. Li, U. Muhammad, Production of gasoline and diesel from coal tar via its catalytic hydrogenation in serial fixed beds, Energy Fuel 26 (2012) 3604–3611. [9] Q. Zhou, T. Zou, M. Zhong, Y.M. Zhang, R.C. Wu, S.Q. Gao, G.W. Xu, Lignite upgrading by multi-stage fluidized bed pyrolysis, Fuel Process. Technol. 116 (2013) 35–43. [10] C. Zhang, R.C. Wu, E.F. Hu, S.Y. Liu, G.W. Xu, Coal pyrolysis for high-quality tar and gas in 100 kg fixed bed enhanced with internals, Energy Fuel 28 (2014) 7294–7302. [11] G.L. Li, L.J. Yan, R.F. Zhao, F. Li, Improving aromatic hydrocarbons yield from coal pyrolysis volatile products over HZSM-5 and Mo-modified HZSM-5, Fuel 130 (2014) 154–159. [12] M. Chareonpanich, T. Boonfueng, J. Limtrakul, Production of aromatic hydrocarbons from Mae–Moh lignite, Fuel Process. Technol. 79 (2002) 171–179. [13] T. Takarada, T. Tonishi, Y. Fusegawa, K. Morishita, N. Nakagawa, K. Kato, Hydropyrolysis of coal in a powder-particle fluidized bed, Fuel 72 (1993) 921–926.
[14] T. Takarada, Y. Onoyama, K. Takayama, T. Sakashita, Hydropyrolysis of coal in a pressurized powder-particle fluidized bed using several catalysts, Catal. Today 39 (1997) 127–136. [15] J. Deng, W.Y. Li, X.H. Li, C.L. Yu, J. Feng, X.F. Guo, Products distribution of lignite pyrolysis with olive-based solid heat carrier, J. Fuel Chem. Technol. 41 (2013) 937–942. [16] N. Sonoyama, K. Nobuta, T. Kimura, S. Hosokai, J. Hayashi, T. Tago, T. Masuda, Production of chemicals by cracking pyrolytic tar from Loy Yang coal over iron oxide catalysts in a steam atmosphere, Fuel Process. Technol. 92 (2011) 771–775. [17] N.A. Öztaş, Y. Yürüm, Effect of catalysts on the pyrolysis of Turkish Zonguldak bituminous coal, Energy Fuel 14 (2000) 820–827. [18] J.Z. Han, X.D. Wang, J.R. Yue, B.F. Xi, S.Q. Gao, G.W. Xu, Catalytic upgrading of coal pyrolysis tar over char-based catalysts, Fuel Process. Technol. 122 (2014) 4934–4941. [19] S. Hosokai, K. Norinaga, T. Kimura, M. Nasaki, C.Z. Li, J. Hayashi, Reforming of volatiles from the biomass pyrolysis over charcoal in a sequence of coke deposition and steam gasification of coke, Energy Fuel 25 (2011) 5387–5393. [20] Z.A. El-Rub, E.A. Bramer, G. Brem, Experimental comparison of biomass chars with other catalysts for tar reduction, Fuel 87 (2008) 2243–2252. [21] L.X. Zhang, T. Matsuhara, S. Kudo, J.I. Hayashi, K. Norinaga, Rapid pyrolysis of brown coal in a drop-tube reactor with co-feeding of char as a promoter of in situ tar reforming, Fuel 112 (2013) 681–686. [22] P. Gilbert, C. Ryu, V. Sharifi, J. Swithenbank, Tar reduction in pyrolysis vapors from biomass over a hot char bed, Bioresour. Technol. 100 (2009) 6045–6051. [23] X. Zeng, Y. Wang, J. Yu, S.S. Wu, J.Z. Han, S.P. Xu, G.W. Xu, Gas upgrading in a downdraft fixed-bed reactor downstream of a fluidized-bed coal pyrolyzer, Energy Fuel 25 (2011) 5242–5249. [24] J. Mráziková, S. Sindler, L. Veverka, J. Macák, Evolution of organic oxygen bonds during pyrolysis of coal, Fuel 65 (1986) 342–345. [25] A. Arenillas, F. Rubiera, J.J. Pis, Simultaneous thermogravimetric–mass spectrometric study on the pyrolysis behavior of different rank coals, J. Anal. Appl. Pyrolysis 50 (1999) 31–46. [26] K. Xie, Coal structure and its reactivity, Science Press, Beijing, 2002 382 (205). [27] I.B. Hariz, F. Al. Ayni, L. Monser, Removal of sulfur compounds from petroleum refinery wastewater through adsorption on modified activated carbon, Water Sci. Technol. 70 (2014) 1376–1382. [28] F.B. Guo, Y.F. Zhang, G.J. Zhang, H.X. Zhao, Syngas production by carbon dioxide reforming of methane over different semi-cokes, J. Power Sources 231 (2013) 82–90. [29] P. Brandt, E. Larsen, U. Henriksen, High tar reduction in a two-stage gasifier, Energy Fuel 14 (2000) 816–819. [30] M.J. Wang, G.G. Robert, M.A. Kochanek, D.J. Harris, L.P. Chang, C.Z. Li, Raman spectroscopic investigations into links between intrinsic reactivity and char chemical structure, Energy Fuel 28 (2014) 285–290. [31] X.J. Li, J.I. Hayashi, C.Z. Li, FT-Raman spectroscopic study of the evolution of char structure during the pyrolysis of a Victorian brown coal, Fuel 85 (2006) 1700–1707. [32] N. Shimodaira, A. Masui, Raman spectroscopic investigations of activated carbon materials, J. Appl. Phys. 92 (2002) 902–909. [33] C.D. Sheng, Char structure characterized by Raman spectroscopy and its correlations with combustion reactivity, Fuel 86 (2007) 2316–2324. [34] D.P. Serrano, J.A. Botas, J.L.G. Fierro, R. Guil-López, P. Pizarro, G. Gómez, Hydrogen production by methane decomposition: origin of the catalytic activity of carbon materials, Fuel 89 (2010) 1241–1248.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
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Research article
In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor Lijun Jin a,b, Xiaoyu Bai a, Yang Li a, Chan Dong a, Haoquan Hu a,⁎, Xian Li b a b
State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 25 July 2015 Received in revised form 15 December 2015 Accepted 25 December 2015 Available online xxxx Keywords: Coal pyrolysis Upgrading Coal tar Char Activated carbon
a b s t r a c t Coal pyrolysis tar was in-site upgraded in a fixed-bed reactor over char and activated carbon (AC) catalysts, and the effects of catalysts and temperature on tar upgrading were examined. The content of light tar with boiling point below 360 °C was improved over the catalysts despite the decrease of total tar yield. Compared with char catalyst, AC exhibited better upgrading performances, and the content of light tar increased with the temperature. The light tar yield over AC catalyst increased by 18% compared with that without catalysts at 650 °C. The variation in coal tar fractions suggested that upgrading is mainly from the conversion of the heavy components (pitch) in tar into light tar and gases. The role of carbon-based catalyst in upgrading process was also explored through the characterization by TG-DTG, FT-IR, N2 adsorption and Raman spectroscopy. Compared with thermal cracking, adsorption on the catalysts and the minerals in char, high specific surface areas and relative more defects in the carbon catalysts seem to be the primary factor for upgrading tar. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Coal pyrolysis, as one of the most promising technologies for energy sustainability, can produce liquid fuels, valuable chemicals and high heating value gas by mild conversion of coal. However, coal tar usually has high content of heavy fractions with boiling points above 360 °C up to more than 50% of total tar [1–3]. This heavy tar with high viscosity, easy pollution and low economic value usually leads to serious problems in the operation of industrial equipment and further processing of tar, which restricts the application of pyrolysis process in the industry. To upgrade the coal tar, various methods were investigated including thermal cracking, hydrogenation and catalytic cracking for the decomposition of heavy components in tar. However, thermal cracking tends to result in insufficient decrease of heavy fractions even at high temperature and pressure, along with obvious loss of overall tar and energy inefficiency [4–6]. Hydrogenation of coal tar received substantial attention owing to remarkable improvement of tar quality and smallmolecule gases yield, especially the content of BTX and their derivates [7], but some of heavy components (mainly pitch) are still difficult to be hydrocracked, and more hydrogen as an expensive gas is consumed and high pressure is utilized [8]. Catalytic upgrading presents a potential method for the conversion of heavy tar into light tar and pyrolysis gases by secondary reaction of tar precursors in the process of coal pyrolysis [9,10]. More studies are ⁎ Corresponding author. E-mail address: [email protected] (H. Hu).
focused on the improvement of BTX and PCX contents [11,12]. Takarada et al. [13,14] found that the content of BTX on supported metal catalysts such as Co–Mo/Al2O3 was 5.8 (wt.% daf), which improved 30 times comparing with that on SiO2 sample in a fluidized bed reactor for coal pyrolysis reaction. Deng et al. [15] investigated the catalytic upgrading over olivine and Co-olivine catalysts, and the heavy fraction (boiling points above 360 °C) was obviously reduced. Compared with the olivine as the catalyst, the pitch content over Co-olivine decreases by 17% at 550 °C. Sonoyama et al. [16] reported about 97 wt.% of initial heavy components in tar from pyrolyzing the Loy Yang coal at 500 °C were decomposed over iron-oxide catalyst with metal promoters. Öztaş et al. [17] believed that better catalytic effects were acquired by pyrolysis of Zonguldak bituminous coal mixed with ZnCl2, CoCl2 and NiCl2. However, the metal catalysts including Ni and Ca are always subjected to some problems including sulfur poisoning, coke deposition and high cost despite their high activity for tar conversion. Carbon-based catalyst such as char or activated carbon is considered to be a most potential for industrial application owning to low cost and easy availability [18–21]. Han et al. [18] studied the catalytic upgrading of coal pyrolysis tar over char-based catalysts, and found that light tar fraction (boiling points below 360 °C) increased by 25% with char compared with coal direct pyrolysis at 600 °C. Better upgrading effect was obtained at 500 °C when metal was supported over char. Gilbert et al. [22] investigated tar reduction of pyrolysis vapors from biomass over hot char in a fixed-bed reactor. The content of heavy fractions decreased to 18.4% at 500 °C and 8.0% at 800 °C. Although the properties of carbonbased catalyst, including specific surface area, pore volume, sulfur and carbon deposition resistance and containing metal oxides, will influence
http://dx.doi.org/10.1016/j.fuproc.2015.12.028 0378-3820/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
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L. Jin et al. / Fuel Processing Technology xxx (2015) xxx–xxx
2.3. Analysis of tar and gases
Table 1 Proximate and ultimate analyses of SM coal and carbon-based catalysts. Sample
Coal Char AC
Proximate analysis (wt.%)
Ultimate analysis (wt.% daf)
Mad
Ad
Vdaf
C
H
N
S
O⁎
1.76 0.76 1.22
8.16 14.62 2.10
35.77 3.40 3.98
71.40 83.08 94.16
4.42 1.04 0.64
0.65 1.28 0.29
0.25 0.27 0.00
23.38 14.33 4.91
⁎ By difference.
Tar plus water was separated according to ASTM D95-05e1 (2005) using toluene as solvent. The tar yield (Ytar) and light tar yield (Ylight tar) in dry ash-free base are calculated as follows: Y tar ¼
W tar 100% W o ð1−Aad −Mad Þ
Y light tar ¼ Y tar w1 %
tar upgrading, it is still unclear as to the upgrading mechanism. Zeng et al. [23] believed that tar could be absorbed on the surface of carbon-based catalyst when it existed down the pyrolysis coal, and large specific surface area promoted more tar to be dispersed on the surface, further leading to the enhancement of retention time for heavy tar upgrading. To study the catalytic role of carbon-based catalyst in catalytic upgrading tar and the decrease of heavy tar components, in this work, two carbon-based catalysts, char and activated carbon, were used to upgrade the coal tar from coal pyrolysis in a fixed-bed reactor containing upper coal layer and lower catalyst layer. The effect of catalyst kinds and the temperature on the quality of coal tar was examined and the role in catalytic upgrading was also explored based on the experimental results and several analysis technologies. 2. Experimental section 2.1. Coal sample and carbon catalysts
where, Wtar and W0 are the weight of tar and coal sample; Aad and Mad are the ash and moisture contents of coal; and w1% is the light tar content. The results presented are the mean values of at least 3 equivalent experiments, and the estimated error of tar yield is within ±0.2%, indicating good repeatability. The components analysis of coal tar was conducted by simulated distillation (ASTM D-2887) according to boiling points of fractions listed in Table 2. The light tar content (w1%) herein refers to the fractions with a boiling point below 360 °C including light oil, phenol oil, naphthalene oil, wash oil and anthracene oil. Before the analysis, the tar was first dissolved in carbon disulfide, and then Na2SO4 was added to adsorb the water in tar. After filtration to remove Na2SO4, the tar with carbon disulfide was concentrated by distillation and used for analysis. Pyrolysis gases were analyzed by an on-line gas chromatography (GC7890II) equipped with a thermal conductivity detector packed with 5A molecular sieve and a flame ionization detector with GDX502 packed column. 2.4. Characterization of catalysts
Shenmu (SM) subbituminous coal was grounded and sieved below 0.15 mm and dried for the pyrolysis experiment. Two carbon-based catalysts were chosen for tar upgrading. One is the char prepared by pyrolysis of Shenmu coal at 850 °C for 3 h in N2 on a fixed bed reactor; another is the commercial coconut shell activated carbon (AC). Their proximate and ultimate analyses of the coal and carbon catalysts were given in Table 1. 2.2. Apparatus and procedure The upgrading of coal tar was carried out in a vertical fixed-bed reactor with an inner diameter of 18 mm and a length of 150 mm. In each run, the reactor was charged with upper coal sample (5 g) and lower carbon-based catalyst (1 g), and some quartz wool was placed between them for separation. After the reactor was purged with high-purity N2 for approximately 10 min to remove the air before the reaction, it was heated to the desired temperature within less than 10 min by a moving preheated furnace, and maintained for 30 min for pyrolysis and upgrading in the N2 flow rate of 100 ml/min. The volatile vapors during the coal pyrolysis were carried through upgrading zone of the tar by high-purity nitrogen, upgraded and collected by a cool trap. Long isotherm zone of the furnace kept the coal pyrolysis and upgrading of the resultant pyrolysis tar in the same temperature. The noncondensable gases were collected and the solid char and catalyst were removed from the reactor for analysis after the experiment.
TG/DTG experiments of carbon-based catalysts were performed using a Mettler Toledo TGA/SDTA851e thermogravimetry analyzer. About 20 mg sample was heated from 25 °C to 850 °C with a heating rate of 10 °C/min using argon as the carrier gas at a constant flow rate of 60 ml/min. The surface analysis of carbon-based catalyst was measured by FT-IR on an EQUINOX55 spectrometer using KBr pellet technique. N2 adsorption of the fresh and spent catalysts was measured on the physical adsorption apparatus (ASAP 2420) at −196 °C. The samples were outgassed at 300 °C for 4 h prior to adsorption. The parameters on the pore structure were obtained by using Brunauer–Emmett– Teller (BET) and Barrett–Joyner–Halenda methods. Raman spectra of carbon catalysts were recorded on a Thermo Scientific DXR Microscope using the 532 nm line of solid state laser. The laser was focused to about 150 μm in diameter at a power of 1 mW in order to prevent thermal degradation. Raman spectral from 800 to 1800 cm−1 were measured with a spectral resolution of 2 cm−1. 3. Results and discussion 3.1. Effect of carbon catalysts on the upgrading of coal tar Two kinds of carbon catalysts were used to in-situ upgrade the coal tar from pyrolysis at different temperatures. To acquire the distribution of tar fractions, simulated distillation was used to analyze the tar compositions from pyrolysis at 650 °C, and the curves in Fig. 1 referred to
Table 2 Simulated distillation fractions of tar from pyrolysis at 650 °C. Catalyst
– Char AC
Fraction (wt.%) Light oil (b170 °C)
Phenol oil (170–210 °C)
Naphthalene oil (210–230 °C)
Wash oil (230–280 °C)
Anthracene oil (280–360 °C)
Pitch (N360 °C)
2.8 4.3 8.9
5.6 7.3 15.5
5.4 5.6 8.8
15.6 16.5 23.8
15.0 16.4 18.5
55.6 49.9 24.5
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
L. Jin et al. / Fuel Processing Technology xxx (2015) xxx–xxx
Fig. 1. Simulated distillation curves of tar from pyrolysis at 650 °C.
the percentage of tar fractions with boiling points below the heating temperatures. Lower boiling points suggest lighter tar components. As illustrated in Fig. 1, both char and AC as the upgrading catalysts led to higher percentage of tar fractions at the same boiling point temperatures, indicating higher light tar contents and the upgrading quality of tar. Additionally, the initial boiling point of coal tar also shifts from 130.9 °C without upgrading down to 128.5 °C over char and 111.1 °C over AC catalyst, and the final boiling points are at 563.6 °C, 559.2 °C and 541.2 °C, respectively. All these results indicate that carbon catalysts can upgrade the pyrolysis tar and decrease the contents of heavy components. Compared with char catalyst, the AC exhibits better upgrading effect. Table 2 shows that all the fractions except pitch increased when carbon catalysts were used to upgrade the pyrolysis tar. In comparison, light oil and phenol oil with low boiling points increased by 6.1 wt.% and 9.9 wt.% over AC, rather than 1.5 wt.% and 1.7 wt.% over char catalyst, respectively. The decrease of pitch fraction means that the increase of light fractions over char and AC catalysts is from the conversion of pitch. The variation in light tar contents and yields at different temperatures are presented in Fig. 2. The lowest content of light tar was obtained and the temperature has slight effect on the light tar content when no catalyst was used. However, light tar content obviously increased over the carbon-based catalysts. For example, the content of light fractions increased from 46.2 wt.% of coal pyrolysis tar at 650 °C to 51.7 wt.% over char catalyst, and further increased to 76.6 wt.% over the AC catalyst. Additionally, light tar contents increased at relatively high temperature (ca. 650–750 °C) on AC catalyst, different from that on char catalyst or without catalyst, which indicates high catalytic activity of AC catalyst in upgrading tar at high temperature. Fig. 2b showed that tar yield had a decrease on carbon catalyst, especially the AC compared to coal pyrolysis without catalyst at 650 °C.
3
Lower tar yield on char or AC catalyst than that without catalyst is attributed to tar upgrading catalyzed by the carbon catalysts, which resulted in the conversion of heavy fraction (pitch) into more light tar components and gases by carbon catalyst (see Fig. 3). Nevertheless, compared with the char catalyst, the AC exhibited better performance in upgrading tar, and the yield of light tar over AC catalyst is always higher than those over char catalyst or without the catalyst, shown in Fig. 3c, indicating that AC is a good catalyst for upgrading coal tar. At 650 °C, the yield of light tar yield increased by 18% compared to that without catalyst. Pyrolysis gas volumes under different temperatures were shown in Fig. 3a. Obviously, pyrolysis gas considerably increased with increasing temperature, and more gases were produced over AC catalyst than char catalyst or without catalyst, indicating higher catalytic activities of AC catalyst. The component variety of gas products at 650 °C in Fig. 3b showed that all the gas volumes increased after tar cracking with carbon catalysts. Low temperature was easy for the decomposition of carboxyl, methoxyl and ether groups, while high temperature was beneficial to the decomposition of phenolic hydroxyl, carbonyl and epoxy groups [24]. Compared with other gas components, more H2 and CH4 were produced, especially over AC catalyst. It is usually thought that H2 is mainly from the condensation of aromatic and hydroaromatic structures or decomposition of heterocycle compounds [25]. CH4 mainly comes from the pyrolysis of aliphatic hydrocarbons, aliphatic side chains of aromatic hydrocarbons and –OCH3 [26]. That is, carbonbased catalyst facilitated heavy tar upgrading into small-molecule gases besides light tar. But it is still unknown what's the role of carbon catalysts in the upgrading the tar.
3.2. Role of carbon-based catalyst in upgrading the tar In the tar upgrading, many factors including thermal decomposition, surface properties and inherent minerals will influence the catalytic upgrading performances. The tar upgrading is usually carried out at high temperature above 500 °C, which will result in its thermal polymerization and/or cracking of coal tar. Therefore, to examine the influence of thermal cracking on the upgrading and explore the role of carbon catalysts in the upgrading process, the char catalyst was replaced by inactive SiO2 with the same height in the reactor so as to keep the similar resident time. As shown in Fig. 4a, the light tar content had a slight increase when SiO2 was used, which was ascribed to long residence time for polymerization and/or cracking at 650 °C caused by the adsorption of tar on SiO2, but obviously lower than 51% over char catalyst. To further increase the residence time of coal tar in the heat zone, we tripled the SiO2 height in the reactor and found that almost the same content (48.5%) of light tar was obtained, which indicates that heat is beneficial for tar upgrading, but not the dominant factor for upgrading over char and activated carbon. Additionally, the gas compositions in Fig. 4b further ascertained that the catalytic role was mainly from the char, not from the thermal
Fig. 2. Light tar content (a), tar yield (b) and light tar yield (c) at different pyrolysis temperatures.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
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L. Jin et al. / Fuel Processing Technology xxx (2015) xxx–xxx
Fig. 3. Total gas yield (a) and gas component yield (b) over different catalysts in pyrolysis process of SM coal.
Fig. 4. Light tar content (a) and gas components (b) over char catalyst and SiO2 at 650 °C.
cracking despite more gases produced over the SiO2 than that without catalyst. Activated carbon, owing to its high surface area and unique pore structure, is widely used in the adsorption [27]. Because low total tar yield and low contents of heavy compositions were obtained when AC is used as the upgrading catalyst, it is confused whether the decrease of tar yield is caused by the adsorption of heavy compositions on the AC catalysts during the upgrading. Therefore, the fresh and spent AC catalysts were examined by TG-DTG, FT-IR, and N2 adsorption, respectively. As demonstrated in Fig. 5, obvious weight loss occurred before 100 °C for the fresh and spent AC samples, which is attributed to the water evolution. Compared with the fresh AC, the spent sample had lower weight loss below 100 °C, which may be related with the decreasing surface area and pore volume (Table 3). But there is no remarkable difference in the range of 150 to 850 °C, and the weight loss is 2.8% for fresh AC, slightly higher than 1.7% over spend catalyst, suggesting that
the used AC has little adsorption of the coal tar, and the decrease of tar yield on AC catalysts is not caused by the adsorption on the surface of AC catalyst. Table 4 summarized the total volatile per gram coal at different temperatures. Obviously, the total volatiles increased with the pyrolysis temperature no matter AC catalyst is used or not, which is caused by the decomposition of the organic matters in coal. However, the difference in total volatile at the same temperature was quite small and within the experimental error when AC was used to upgrade the coal tar, which is in accordance with the TG-DTG results in Fig. 5, further confirming that low tar yield is not caused by the adsorption, but mainly from the cracking of coal tar on the AC catalyst into the light tar and gases at high temperature. Table 3 gives the texture properties of fresh and spent carbon catalysts. Obviously, the fresh AC sample had higher specific surface area (803 m2/g) than char catalyst (2 m2/g). The difference in the specific surface area seems to be the reason for different upgrading performances of carbon catalysts. The specific surface areas of spent catalysts decreased, especially the AC sample from 803 to 499 m2/g. The pore volume also decreased from 0.38 to 0.24 cm3/g. It is thought that the decrease in specific surface area and pore volume is probably caused by pore collapse suffered from high temperature and coke deposit of from the decomposition of coal tar. Low specific surface area resulted in poor upgrading performances. Table 3 Textural properties of carbon-based catalysts.
Fig. 5. TG and DTG curves of fresh and spent AC catalysts.
Sample
SBET (m2 g−1)
Smic (m2 g−1)
Smic/SBET
Vtotal (cm3 g−1)
Vmic (cm3 g−1)
Vmic/Vtot
Fresh char Spent char⁎ Fresh AC Spent AC⁎
2.2 1.4 803 499
/ / 658 383
/ / 0.8 0.8
/ / 0.38 0.24
/ / 0.30 0.18
/ / 0.79 0.75
⁎ Pyrolysis temperature: 650 °C.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
L. Jin et al. / Fuel Processing Technology xxx (2015) xxx–xxx Table 4 Total volatile at different pyrolysis temperatures. Temperature (°C)
450 550 650 750
Total volatile⁎ (g/g coal) No catalyst
AC catalyst
Difference
0.164 0.236 0.276 0.312
0.158 0.232 0.274 0.308
0.006 0.004 0.002 0.004
⁎ Total volatiles include water, tar and gases.
Coal char contains some inherent metal minerals, such as alkaline and alkaline earth metals, especially that prepared from the high ash coal [29]. Zeng et al. [23] believed that these inherent metal species may serve as the active sites for upgrading coal tar. Nevertheless, poorer catalytic upgrading on char than that on AC catalyst was obtained in spite of higher ash content of 14.52% in char rather than 2.10% in AC sample (Table 1), which suggested that the inherent mineral in carbon catalysts has less effect on coal tar upgrading compared with the specific surface area of carbon catalysts. FT-IR spectra of fresh and spent carbon catalysts reacted at 650 °C are compared in Fig. 6. Obviously, bands at 2923 and 2852 cm−1 (C–H stretching vibrations) almost disappeared on the used char at 650 °C, indicating the evolution of some organic matters in the char during the upgrading process, which is the reason for the weight loss in Fig. 5 at high temperature. As to the absorption bands attributed to oxygencontaining functional groups at 3440, 1700 and 1100 cm−1, the intensity of stretching and vibration bands of hydroxyl O–H at 3440 cm−1 remarkably decreased owing to the decarboxylation over the spent char catalyst, which is in accordance with the results by Guo et al. [28]. And the carboxyl or carbonyl CO bands at 1700 cm−1 became weaker or disappeared after char or AC was used as the catalyst, meaning that decarboxylation occurred in upgrading tar. The ultimate analysis of the used char and AC catalysts at 650 °C also confirmed the decrease in oxygen contents, which is quite accordant with the FT-IR analysis. Additionally, no new absorption bands attributed to the aliphatic or aromatic structures were formed. These results indicate that the main role of carbon-based catalyst is to provide catalytic active sites on its surface, not just as an adsorbent for the tar adsorption and increasing the residence time. 3.3. Raman spectra Raman spectroscopy is widely used to characterize the structure features of a range of carbonaceous materials, including activated carbons and coal chars [30–33]. Usually, only one band at about 1580 cm− 1 (G band) exists for the perfect graphite in the first-order region, which corresponds to the stretching vibration mode with E2g symmetry in the aromatic layers of graphite crystalline. However, for highly disordered carbons, there are additional bands at 1150 cm−1 (D4),
5
1350 cm−1 (D1), 1530 cm−1 (D3 band) and 1620 cm−1 (D2) induced by the defects in the microcrystalline lattice. The 1350 cm−1 band is commonly called the defect band, which is ascribed to the in-plane imperfections or defects [33]. Therefore, studying the Raman spectra of carbon catalysts can provide information for evaluating the degree of ordering and crystallinity in carbonaceous materials. Fig. 7 illustrated Raman spectra of AC and char samples in the range of 800–1800 cm− 1, and peak fitting of each spectrum according to Sheng's method [33] was also presented, indicating good agreement. Both samples showed the similar spectra, and the G band was weaker than the D1 band, indicating poor order of two carbon catalysts. The D3 and D4 bands are low in intensity but wide FWHM, implying considerable amounts of amorphous forms of carbon [33]. Because the total Raman intensity is affected by many factors such as the intrinsic Raman scattering ability, the light absorption of the carbon material, and the presence of electron-rich functional groups like those containing N, O and S [31], it's inappropriate to compare the total intensity between the AC and coal char samples. However, the band area ratios between D1 band and G band and the FWHM of the G band can be used as indicators of carbon structural order. Obviously, the AC sample had higher band area ratio of D1 band to G band (5.1) than coal char (3.9) and broader FWHW. In addition, the area ratios of other defects bands (D2, D3 and D4) in AC sample are also higher than those in char catalyst. All these indicated that the AC had more structural defects and imperfections of the carbon crystallites than char sample, which is in accordance with Serrano's results that carbon material with higher surface area usually corresponds with a more number of defects [34]. These structural defects or imperfections in the carbon materials could serve as the active sites for upgrading the tar. Large surface area of carbon catalyst is preferable to improve utilization efficiency of active sites and tar upgrading. Therefore, the AC sample with higher surface area and relative more defects than char catalyst exhibited better tar upgrading. 4. Conclusion Char and AC as the catalysts can upgrade the quality of coal tar, increase the content of light tar and the gases yield although total tar yield decreased. Compared with the char, AC catalyst exhibited better upgrading performances, which is mainly related to its larger surface area and pore volume. The content of light tar increased with the temperature over AC catalyst and higher light tar yield than that without upgrading can be obtained. All the fractions except pitch in coal tar increased when char or AC was used as the catalyst, which was mainly from the conversion of the heavy components in tar into light tar and gases. The analyses results of the fresh and spent carbon catalysts showed that high specific surface area and relative more defects or imperfections of carbon catalyst are preferable to convert heavy fraction into light tar and gases compared with the adsorption of coal tar on carbon catalysts, heat effect and inherent minerals during tar upgrading.
Fig. 6. FT-IR spectra of fresh and spent char (a) and AC (b) catalysts.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028
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L. Jin et al. / Fuel Processing Technology xxx (2015) xxx–xxx
Fig. 7. Raman spectra of fresh AC (a) and char (b) samples.
Acknowledgments This work was performed with support of the National Natural Science Foundation of China (No. 21176042), the National Basic Research Program of China (973 Program, No. 2011CB201301) and the Key International Science & Technology Cooperation and Exchange Project, the Ministry of Science and Technology, China (No. 2013DFG60060), the Joint Fund of Coal-based Low hydrocarbons by NSFC and Shanxi Provincial Government (No. U1510101), the Funds of State Key Laboratory of Coal Combustion (No. FSKLCCA1506) and State Key Laboratory of Multiphase Complex Systems (No. MPCS2014-D-07). References [1] P. Liang, Z. Wang, J. Bi, Process characteristics investigation of simulated circulating fluidized bed combustion combined with coal pyrolysis, Fuel Process. Technol. 88 (2007) 23–28. [2] X. Qu, P. Liang, Z. Wang, R. Zhang, D. Sun, X. Gong, Z. Gan, J. Bi, Pilot development of polygeneration process of circulating fluidized bed combustion combined with coal pyrolysis, Chem. Eng. Technol. 34 (2011) 61–68. [3] J.G. Wang, X.S. Lu, J.Z. Yao, W.G. Lin, L.J. Cui, Experimental study of coal topping process in a downer reactor, Ind. Eng. Chem. Res. 44 (3) (2005) 463–470. [4] J. Hayashi, S. Amamoto, K. Kasukabe, S. Morooka, Evaluation of vapor-phase reactivity of primary tar produced by flash pyrolysis of coal, Energy Fuel 9 (1995) 290–294. [5] J. Hayashi, T. Kawakami, T. Taniguchi, K. Kusakabe, S. Morooka, M. Yumura, Control of molecular composition of tar by secondary reaction in fluidized-bed pyrolysis of a subbituminous coal, Energy Fuel 7 (1993) 57–66. [6] J. Han, H. Kim, The reduction and control technology of tar during biomass gasification/pyrolysis: an overview, Renew. Sust. Energ. Rev. 12 (2008) 397–416. [7] J. Kusy, L. Andel, M. Safarova, J. Vales, K. Ciahotny, Hydrogenation process of the tar obtained from the pyrolysis of brown coal, Fuel 101 (2012) 38–44. [8] T. Kan, X.Y. Sun, H.Y. Wang, C.S. Li, U. Muhammad, Production of gasoline and diesel from coal tar via its catalytic hydrogenation in serial fixed beds, Energy Fuel 26 (2012) 3604–3611. [9] Q. Zhou, T. Zou, M. Zhong, Y.M. Zhang, R.C. Wu, S.Q. Gao, G.W. Xu, Lignite upgrading by multi-stage fluidized bed pyrolysis, Fuel Process. Technol. 116 (2013) 35–43. [10] C. Zhang, R.C. Wu, E.F. Hu, S.Y. Liu, G.W. Xu, Coal pyrolysis for high-quality tar and gas in 100 kg fixed bed enhanced with internals, Energy Fuel 28 (2014) 7294–7302. [11] G.L. Li, L.J. Yan, R.F. Zhao, F. Li, Improving aromatic hydrocarbons yield from coal pyrolysis volatile products over HZSM-5 and Mo-modified HZSM-5, Fuel 130 (2014) 154–159. [12] M. Chareonpanich, T. Boonfueng, J. Limtrakul, Production of aromatic hydrocarbons from Mae–Moh lignite, Fuel Process. Technol. 79 (2002) 171–179. [13] T. Takarada, T. Tonishi, Y. Fusegawa, K. Morishita, N. Nakagawa, K. Kato, Hydropyrolysis of coal in a powder-particle fluidized bed, Fuel 72 (1993) 921–926.
[14] T. Takarada, Y. Onoyama, K. Takayama, T. Sakashita, Hydropyrolysis of coal in a pressurized powder-particle fluidized bed using several catalysts, Catal. Today 39 (1997) 127–136. [15] J. Deng, W.Y. Li, X.H. Li, C.L. Yu, J. Feng, X.F. Guo, Products distribution of lignite pyrolysis with olive-based solid heat carrier, J. Fuel Chem. Technol. 41 (2013) 937–942. [16] N. Sonoyama, K. Nobuta, T. Kimura, S. Hosokai, J. Hayashi, T. Tago, T. Masuda, Production of chemicals by cracking pyrolytic tar from Loy Yang coal over iron oxide catalysts in a steam atmosphere, Fuel Process. Technol. 92 (2011) 771–775. [17] N.A. Öztaş, Y. Yürüm, Effect of catalysts on the pyrolysis of Turkish Zonguldak bituminous coal, Energy Fuel 14 (2000) 820–827. [18] J.Z. Han, X.D. Wang, J.R. Yue, B.F. Xi, S.Q. Gao, G.W. Xu, Catalytic upgrading of coal pyrolysis tar over char-based catalysts, Fuel Process. Technol. 122 (2014) 4934–4941. [19] S. Hosokai, K. Norinaga, T. Kimura, M. Nasaki, C.Z. Li, J. Hayashi, Reforming of volatiles from the biomass pyrolysis over charcoal in a sequence of coke deposition and steam gasification of coke, Energy Fuel 25 (2011) 5387–5393. [20] Z.A. El-Rub, E.A. Bramer, G. Brem, Experimental comparison of biomass chars with other catalysts for tar reduction, Fuel 87 (2008) 2243–2252. [21] L.X. Zhang, T. Matsuhara, S. Kudo, J.I. Hayashi, K. Norinaga, Rapid pyrolysis of brown coal in a drop-tube reactor with co-feeding of char as a promoter of in situ tar reforming, Fuel 112 (2013) 681–686. [22] P. Gilbert, C. Ryu, V. Sharifi, J. Swithenbank, Tar reduction in pyrolysis vapors from biomass over a hot char bed, Bioresour. Technol. 100 (2009) 6045–6051. [23] X. Zeng, Y. Wang, J. Yu, S.S. Wu, J.Z. Han, S.P. Xu, G.W. Xu, Gas upgrading in a downdraft fixed-bed reactor downstream of a fluidized-bed coal pyrolyzer, Energy Fuel 25 (2011) 5242–5249. [24] J. Mráziková, S. Sindler, L. Veverka, J. Macák, Evolution of organic oxygen bonds during pyrolysis of coal, Fuel 65 (1986) 342–345. [25] A. Arenillas, F. Rubiera, J.J. Pis, Simultaneous thermogravimetric–mass spectrometric study on the pyrolysis behavior of different rank coals, J. Anal. Appl. Pyrolysis 50 (1999) 31–46. [26] K. Xie, Coal structure and its reactivity, Science Press, Beijing, 2002 382 (205). [27] I.B. Hariz, F. Al. Ayni, L. Monser, Removal of sulfur compounds from petroleum refinery wastewater through adsorption on modified activated carbon, Water Sci. Technol. 70 (2014) 1376–1382. [28] F.B. Guo, Y.F. Zhang, G.J. Zhang, H.X. Zhao, Syngas production by carbon dioxide reforming of methane over different semi-cokes, J. Power Sources 231 (2013) 82–90. [29] P. Brandt, E. Larsen, U. Henriksen, High tar reduction in a two-stage gasifier, Energy Fuel 14 (2000) 816–819. [30] M.J. Wang, G.G. Robert, M.A. Kochanek, D.J. Harris, L.P. Chang, C.Z. Li, Raman spectroscopic investigations into links between intrinsic reactivity and char chemical structure, Energy Fuel 28 (2014) 285–290. [31] X.J. Li, J.I. Hayashi, C.Z. Li, FT-Raman spectroscopic study of the evolution of char structure during the pyrolysis of a Victorian brown coal, Fuel 85 (2006) 1700–1707. [32] N. Shimodaira, A. Masui, Raman spectroscopic investigations of activated carbon materials, J. Appl. Phys. 92 (2002) 902–909. [33] C.D. Sheng, Char structure characterized by Raman spectroscopy and its correlations with combustion reactivity, Fuel 86 (2007) 2316–2324. [34] D.P. Serrano, J.A. Botas, J.L.G. Fierro, R. Guil-López, P. Pizarro, G. Gómez, Hydrogen production by methane decomposition: origin of the catalytic activity of carbon materials, Fuel 89 (2010) 1241–1248.
Please cite this article as: L. Jin, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.12.028