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Suppressed char agglomeration by rotary kiln reactor with alumina ball during the pyrolysis of Kraft lignin
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Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx
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Short communication
Suppressed char agglomeration by rotary kiln reactor with alumina ball during the pyrolysis of Kraft lignin Young-Min Kima,1, Seyoung Parka,1, Bo Sung Kanga,1, Jungho Jaeb,c, Gwang Hoon Rheed, Sang-Chul Junge, Young-Kwon Parka,* a
School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Republic of Korea d Department of Mechanical and Information Engineering, University of Seoul, Seoul 02504, Republic of Korea e Department of Environmental Engineering, Sunchon national University, Suncheon 57922, Republic of Korea b c
A R T I C L E I N F O
Article history: Received 5 February 2018 Received in revised form 21 May 2018 Accepted 27 May 2018 Available online xxx Keywords: Pyrolysis Kraft lignin Char agglomeration Rotary kiln reactor Alumina ball
A B S T R A C T
In this study, the pyrolysis of Kraft lignin was conducted in a rotary kiln reactor using rotating alumina balls as a filler to achieve a continuous pyrolysis process by the suppression of char agglomeration. Temperature variation experiments showed that the gas yield increased and the char yield decreased with an increase in the pyrolysis temperature from 550 C to 650 C. The maximum oil yield was obtained at a reaction temperature of 600 C. Compared to a fixed bed reactor, a rotary kiln reactor using alumina balls produced the higher quality oil containing larger amount of organic phase oil and higher selectivity to aromatic hydrocarbons. At all temperatures, no lignin char agglomeration occurred inside the reactor due to the effective collision of lignin char intermediates and alumina balls, allowing a continuous pyrolysis process without reactor plugging. © 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction The worldwide demand for renewable energy has increased due to the energy crisis, environmental contamination, and climate change caused by intensive use of fossil fuels [1]. Biomass is an alternative energy source that can potentially replace fossil fuels. Among the various kinds of renewable energy sources, the importance of biomass has been emphasized because it is the only carbon source that can produce both energy and carbon-based chemical feedstock [2]. Thermal treatment of biomass is being investigated intensively because value-added chemicals can be produced using the appropriate thermal conversion technology [3–8]. Pyrolysis, a thermal decomposition technology performed under a non-oxygen atmosphere at temperatures between 400 and 600 C, is a proper method which can produce the large amount of bio-oil from biomass. Recently, different kinds of pyrolysis, such as microwave-assisted pyrolysis [9], was also reported as an effective method for the production of high-quality oil from biomass.
* Corresponding author. E-mail address: [email protected] (Y.-K. Park). Co-first authors.
1
Lignin, a main lignocellulosic component of wood biomass, is a cross-linked phenolic polymer forming the support tissues of wood and bark [10]. A large amount of lignin is released to the environment as waste mainly from the pulp manufacturing process. Lignin has high potential to become an alternative source of petroleum-derived aromatic chemicals. The market value of aromatic chemicals, such as phenol, is also very high. On the other hand, a considerable amount of lignin is classified as waste and is incinerated [11]. The pyrolysis of lignin is a promising technology for decomposing the polymeric structure of lignin and converting it to a liquid fuel, called bio-oil, which contains large quantities of valuable phenolic compounds, in a simple one-step process [12]. On the other hand, research on the pyrolysis process of lignin is still in its initial stages using laboratory-scale batch-type reactors [13]. Several technical barriers need to be overcome before the lignin pyrolysis process can be scaled up [14]. One of the most difficult problems is that the stable and continuous operation of the reactor is quite challenging during lignin pyrolysis due to the large amount of lignin char accumulating inside the reactor. Nowakowski et al. [15] recommended that less pure lignin samples, such as lignin isolated by acid hydrolysis, which contain some sugary components, are better than pure lignin when a conventional continuous
https://doi.org/10.1016/j.jiec.2018.05.041 1226-086X/© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Y.-M. Kim, et al., Suppressed char agglomeration by rotary kiln reactor with alumina ball during the pyrolysis of Kraft lignin, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.05.041
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lignin pyrolysis system is used. In the case of pure lignin, they recommended the use of a new reactor system, such as entrainedflow, high temperature, long residence time, etc. The large agglomerates and swelling of char produced during the pyrolysis of lignin can cause a blockage of the reactor, requiring additional reactor maintenance to repeat the pyrolysis. Recently, several studies reported a method to minimize char agglomeration and swelling by the mechanical modification of the reactor or the pretreatment of lignin [16–18]. For example, modification of the reactor feeding system was effective in reducing the level of char agglomeration but it could not prevent the reactor from plugging completely. The co-feeding of lignin with calcium hydroxide was reported to be quite effective in preventing agglomeration of char inside the reactor because calcium hydroxide pretreatment can decrease the amounts of phenolic hydroxyl, carboxylic acid, and aldehyde groups, which cause the severe agglomeration of lignin during pyrolysis [19]. They also indicated that this pretreatment process enables us to carry out the pyrolysis of lignin in a conventional fluidized bed reactor without reactor modification, even though additional processes, such as char burning and leaching ash with water, are needed to recycle calcium hydroxide. In another study, Zhou et al. [20] reported the continuous pyrolysis of lignin derived from a bioethanol pilot plant in a PCR (pyrolysis centrifuge reactor), which has a continuous char removal function under the mechanical action of rotating vanes. Another possible method to minimize char agglomeration is to use alumina ball as a packing material of the reactor during the pyrolysis of lignin. Alumina ball has been used as an effective grinding media in ball milling due to its thermal resistance and mechanical strength. Therefore, these properties of alumina ball can reduce the char agglomeration when it is used for lignin pyrolysis by providing the mechanical grinding effect. In this study, the pyrolysis of lignin was conducted in a rotary kiln reactor with alumina ball as a rotating filler to prevent or minimize lignin char agglomeration, thereby achieving a continuous pyrolysis process. To accomplish this, a new rotary kiln reactor, in which the rotation speed and slope of the reactor can be controlled, was designed, and alumina balls were used as the filler inside the reactor. The yields of the liquid products produced from
the pyrolysis of Kraft lignin at different temperatures were evaluated, and the level of char formation after the reaction was monitored and compared with that obtained from a conventional fixed-bed reactor. Gas chromatography/mass spectrometry (GC/ MS) analysis of the liquid product was also performed to examine the chemical composition of the product oil. Materials and methods Lignin and alumina ball The powder form of Kraft lignin (Sigma-Aldrich) was used as a sample for the pyrolysis of lignin in this study. The lignin was sieved to make a particle size smaller than 300 mm and dried at 80 C for 6 h to eliminate the residual water. Proximate, ultimate, and gel permeation chromatography (GPC) analysis were performed to determine the physico-chemical properties of lignin [21]. Proximate and ultimate analysis were performed according to the standard methods, ASTM D 7582 (2015) and ASTM D 5373 (2014) using an elementary analyzer (Flash EA 1113, CE). The gel permeation chromatography (GPC) experiments were carried out to know the average molecular weight distribution of lignin using a Waters 2695 instrument. For this, 100 mL of lignin solution diluted in tetrahydrofuran was injected to GPC Column (Styragel HR2/Styragel HR4/ Styragel HR5) at 30 C. Thermogravimetric analysis (TGA Pyris Diamond; Perkin–Elmer) of Kraft lignin was also carried out. For TGA, 5 mg of Kraft lignin was heated non-isothermally from ambient temperature to 700 C at 10 C/min, 20 C/min and 30 C/min under 20 mL/min in a nitrogen atmosphere. Alumina ball (diameter: 5 mm), purchased from a local supplier, was used to provide the stable temperature and physical collision with lignin pyrolysis intermediates. X-ray diffraction (XRD) measurement of alumina ball was performed to identify its crystal phase on a X0 pert PRO X-ray diffractometer (Panalytical) with Cu Kα radiation (l = 0.15406 nm). Rotary kiln reactor Fig. 1 shows a schematic diagram of the rotary kiln type pyrolyzer designed for the rapid pyrolysis of lignin. The rotary kiln
Fig. 1. Schematic diagram of rotary kiln reactor system.
Please cite this article in press as: Y.-M. Kim, et al., Suppressed char agglomeration by rotary kiln reactor with alumina ball during the pyrolysis of Kraft lignin, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.05.041
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pyrolysis reactor consisted of a lignin feed, pyrolysis reactor, and oil recovery system. The lignin feeding system was designed to transfer a solid lignin sample to the pyrolysis reactor with a feeding lignin storage hopper (#1), rotation feeding screw (#2), and nitrogen spray feeder (#3). The pyrolysis reactor system was designed to provide heat to the sample together with the rotation of the reactor with a rotation motor (#4), electronic heater (#5), cylindrical stainless steel reactor (#6), filler (#7), metal frame (#8), reactor angle controller (#14), and metal filter (#9). The rotation of the filler in the cylindrical stainless steel reactor (57 cm length 10 cm inner diameter) was applied to prevent internal blockage of the reactor due to lignin char foaming. A metal filter was used to separate the solids from the gases emitted from the pyrolysis reactor. The oil recovery system consisted of low and ambient temperature oil condensers (#10, #12), electric precipitator (#13), and oil recovery round bottom flasks (#11). Before the lignin pyrolysis reaction, the reactor was pre-purged with 10 L/min of nitrogen to eliminate the residual air inside the reactor while increasing the furnace temperature to the target temperature (550, 600, or 650 C) and stabilized during 40 min. To provide the same temperature of the furnace, the reactor was also rotated at 80 rpm using the rotation motor (#4). Alumina balls were also loaded inside the reactor. After 40 min reactor stabilization time, the lignin powder (100 g) with a particle size <300 mm was injected continuously into the pyrolysis reactor for 40 min at a constant feeding rate (2.5 g/min) and the product vapor was transferred to the oil condensers and the condensed oil was collected in the round bottom flasks. After the reaction, the weights of the oil collected in the flasks were measured by a balance and analyzed by GC/MS. Acetone was used as a washing solvent to achieve better extraction of the product oil from the oil condensers. Table S1 (Supplementary information) lists the GC/MS conditions. The elemental composition and calorific value of the product oil were also measured using an elemental analyzer (Flash EA 1112, Thermo) and bomb calorimeter (AC-350; LECO). The formation of char and the change in the filler were monitored after the pyrolysis reaction. Results and discussion Rotary kiln reactor Table 1 lists the physico-chemical properties of Kraft lignin. Compared to other components of biomass, such as hemicellulose and cellulose [22], lignin contains a larger amount of fixed carbon (32.6%). This suggests that large amount of char can be produced by the pyrolysis of lignin. TGA curve of Kraft lignin (Fig. 2) indicated that lignin decomposed over a wide temperature region between 250 and 500 C [23], and a large amount of solid residue (51.3%), which consisted of char and ash, remained in the sample cup after Table 1 Physico-chemical characteristics of Kraft lignin. Proximate analysis (wt.%) [19]
Ultimate analysisa (wt. %) [19]
GPC analysis
a b
On a dry basis. By difference.
Water Volatiles Fixed carbon Ash C H Ob N S Mn Mw Mz
1.8 62.9 32.6 2.7 62.45 5.68 30.61 0.56 0.70 655 1319 2123
3
Fig. 2. TG curve of Kraft lignin at 20 C/min.
TGA. Therefore, the minimum temperature for the isothermal pyrolysis reaction for Kraft lignin using the rotary kiln reactor was set to 500 C. Lignin prolysis oil Table 2 lists the yields of gas, oil and char obtained from the pyrolysis of Kraft lignin at different temperatures using a rotary kiln reactor. The oil yield increased gradually from 32.2% to 35.5% with increasing reaction temperature from 550 C to 600 C, but it decreased to 30.4% at 650 C. The increased yield of oil can be explained by the additional decomposition of char intermediates. Shafaghat et al. [21] reported that the increased yield of oil and gas were accompanied by a decrease in solid residue, which was attributed to the additional cracking of char by the higher heat energy supplied to the system. On the other hand, the liquid yield was lower at higher temperature, despite the decrease in char yield because a higher temperature can also enhance the secondary cracking of pyrolysis oil, which would increase the gas yield [17]. The yield of solid char was decreased from 50.3% at 550 C to 39.9% at 650 C, indicating clearly that the additional cracking of char occurs with an increase in the temperature. Similarly, the gas yield was increased from 17.5 to 29.7% with increasing temperature from 550 C to 650 C due to the additional decomposition of lignin and secondary cracking of lignin pyrolyzates [24]. Table 3 shows the gas product distribution from lignin pyrolysis at different temperatures using a rotary kiln reactor. When the pyrolysis temperature increased from 550 C to 650 C, the yields of CO and light hydrocarbons (C1–C4) were increased from 6.5 to 15.0% and from 0.6 to 2.2%, respectively. This clearly indicates that the additional cracking reaction occurs at higher pyrolysis temperatures. Table 4 lists the yields of the aqueous and organic phase oil obtained from the pyrolysis of Kraft lignin using two different reactors: the rotary kiln and bench–scale fixed bed reactor. The pyrolysis of the Kraft lignin using a fixed bed reactor was performed at 600 C using the method reported by Shafaghat et al. [21]. In this study, the amount of aqueous phase oil (3.74%) from the rotary kiln reactor was much lower than that from the Table 2 Yields of products obtained from the pyrolysis of Kraft lignin using rotary kiln reactor. Temperature ( C) Yield (wt.%) Oil Gas Char
550
600
650
32.2 17.5 50.3
35.5 24.5 40.0
30.4 29.7 39.9
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Table 3 The yields of CO, CO2, and light hydrocarbons (C1–C4) obtained from the pyrolysis of Kraft lignin at different temperature using a rotary kiln reactor. Temperature ( C) Component CO CO2 C1–C4
550 Yield (wt.%) 6.5 10.4 0.6
600
650
10.7 12.5 1.3
15.0 12.5 2.2
Table 4 Yields of aqueous and organic phase of bio-oil obtained from the pyrolysis of Kraft lignin on two reactors.
Yield (wt%) Aqueous phase Organic phase Total oil
Rotary kiln (600 C)
Fixed bed reactor (600 C)
3.7 31.8 35.5
14.1 19.6 33.7 Fig. 3. XRD pattern of alumina ball used as filler in this study.
fixed bed reactor (14.05%). In addition, the organic phase oil obtained from the rotary kiln reactor was 31.77%, whereas that obtained from the fixed bed reactor was 19.63%. The total oil yield from the rotary kiln reactor was higher than that of the fixed bed reactor, and the rotary kiln reactor is expected to produce more valuable chemicals than the fixed bed reactor. The higher content of organic phase oil in the bio-oil is important because valuable chemical compounds are retained mainly in the organic phase oil. Table 5 compares the product distribution of the bio-oil obtained from the pyrolysis of Kraft lignin using a rotary kiln reactor and fixed bed reactor [21]. On the fixed bed reactor, phenols, such as alkyl phenols, guaiacols, and catechols, occupied the largest peak area % in the product oil, and the peak area % of aromatic hydrocarbons was less than 0.5%. In contrast, the peak area % of aromatics obtained using the rotary kiln reactor were 3.79% for mono aromatic hydrocarbons (MAHs), which was much higher than those obtained using the fixed bed reactor, 0.28% for MAHs. The major reaction pathway for the formation of aromatic hydrocarbons from the pyrolysis of lignin is the secondary cracking of phenols, such as dehydroxylation, dealkylation, and demethoxylation. This indicates that the higher cracking efficiency enhancing the secondary cracking of phenols can be achieved by applying the rotary kiln reactor with alumina ball. The alumina ball had the typical XRD peaks of α-alumina phase as shown in Fig. 3. The XRD pattern of the alumina ball used for our experiments matches well with ICDD Database(00-005-0712 for α-Al2O3) [25]. This suggests that the additional cracking of lignin pyrolyzates in the rotary kiln reactor is difficult to be achieved by the catalytic effect of alumina ball. In addition, the selectivity to phenols were also different between the experiments using the fixed bed reactor and the rotary kiln reactor. Kraft lignin pyrolysis using the rotary kiln reactor showed higher selectivity to alkyl phenols, but much lower
Table 5 Product distribution of the bio-oil obtained from the pyrolysis of Kraft lignin using rotary kiln reactor at 600 C. (Area %)
Rotary kiln (600 C)
Fixed bed (600 C)
Mono Aromatics Phenols
3.79 35.83 24.28 3.70 0 2.53 29.87
0.28 27.77 34.46 13.56 0 12.60 11.33
Acids Other oxygenatesa Othersb a b
Alkyl Phenols Guaiacols Catechols
Ketones, furans, esters, alcohols and aldehydes (except phenols). N S compounds, PAHs and other cyclic compounds.
selectivity to guaiacol, catechol, and other oxygenates than the fixed bed reactor. This suggests that the demethoxylation and demethylation reactions of guaiacols are promoted in a rotary kiln reactor. Kim et al. [24] reported that the lignin pyrolysis product distribution are altered largely by its pyrolysis temperature on the pyrolysis research of Pinus radiata. Methoxy phenols, such as guaiacols and eugenols, are the main pyrolyzates of lignin at temperatures less than 500 C. Between 500 C and 600 C, alkyl phenols and pyrocatechols were the dominant pyrolyzates due to the increased dealkoxylation and dealkylation of guaiacols and eugenols. At temperatures higher than 600 C, aromatic hydrocarbons were also produced by the additional dealkylation reaction. Char agglomeration property Table 6 shows the elemental composition of char obtained from the pyrolysis of lignin at 600 C using the rotary kiln reactor. Fig. 4 shows images of the fixed bed reactor and the char formed after the pyrolysis of Kraft lignin at 600 C. The detailed pyrolysis conditions for the pyrolysis of Kraft lignin using the fixed bed reactor are reported elsewhere [21]. The char in the fixed bed reactor (Fig. 4(b)) consisted of sticky and agglomerated solid materials, which become the main factor causing reactor blockage. This can be a major impediment to the implementation of a continuous lignin pyrolysis process [14]. Based on previous studies on the continuous pyrolysis of lignin, a new rotary kiln type reactor using alumina balls as a filler was developed and applied to the pyrolysis of lignin. Fig. 5 shows images of the rotary kiln reactor and alumina balls obtained after the pyrolysis of Kraft lignin at 600 C. After the reaction, only black and fine char powder was observed inside the reactor and on the filter, suggesting that char agglomeration did not occur during pyrolysis. This also suggests that the use of a rotating alumina ball can effectively prevent the agglomeration of lignin char and allow the continuous operation of the reactor. The prevention of char agglomeration in a rotary kiln can be explained by physical interactions, such as the ball milling effect of alumina balls on char intermediates. Alumina ball is a low cost filler and can be reused after the simple calcination process. This also indicates Table 6 The composition of char obtained from the pyrolysis of Kraft lignin using rotary kiln reactor at 600 C. Temperature ( C)
Composition (wt.%) C
H
O
N
S
600
71.58
2.96
24.74
0.12
0.60
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Fig. 4. Reactor and lignin char foaming obtained from the fixed bed reactor after the pyrolysis of Kraft lignin at 600 C.
Fig. 5. Reactor, char, alumina ball obtained from rotary kiln reactor after the pyrolysis of Kraft lignin at 600 C.
the high potential of the rotary kiln reactor applied in this study as a future technology for the lignin conversion. Conclusion The continuous pyrolysis of Kraft lignin was achieved using a rotary kiln reactor with alumina ball as a filler. Compared to the conventional fixed bed reactor pyrolysis, it improved the quality of oil product significantly and minimized the char agglomeration. The temperature variation experiments showed that 600 C is the optimum temperature to maximize the oil yield in a rotary kiln reactor. Meanwhile, the yield of gas progressively increased from 17.5 to 30% and the solid yield decreased continuously from 50 to 40% from 550 C to 650 C with increasing reaction temperature. The product oil obtained at 600 C contained larger amounts of valuable aromatic hydrocarbons and alkyl phenols than that at 600 C using the fixed bed reactor. Importantly, char agglomeration and the swelling did not occur during the pyrolysis in the rotary kiln reactor with alumina ball, allowing the continuous operation of the reactor without any plugging. Ongoing experiments are focused on the investigation of the detailed degradation mechanism of lignin in the rotary kiln reactor with alumina ball. The results obtained in this study would be contributed to the design of an improved pyrolysis reactor for lignin conversion.
Acknowledgements This work was supported by the New and Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20153030101580). Also, this research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (NRF-2017M1A2A2087674). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2018.05.041. References [1] P.K. Kanaujia, Y.K. Sharma, M.O. Garg, D. Tripathi, R. Singh, J. Anal. Appl. Pyrol. 105 (2014) 55. [2] P. Gallezot, Chem. Soc. Rev. 41 (2012) 1538. [3] E.H. Lee, R.-S. Park, H. Kim, S.H. Park, S.-C. Jung, J.-K. Jeon, S.C. Kim, Y.-K. Park, J. Ind. Eng. Chem. 37 (2016) 18. [4] J.S. Cha, S.H. Park, S.-C. Jung, C. Ryu, J.-K. Jeon, M.-C. Shin, Y.-K. Park, J. Ind. Eng. Chem. 40 (2016) 1. [5] Y. Lee, H. Shafaghat, J. Kim, J.K. Jeon, S.C. Jung, I.G. Lee, Y.K. Park, Korean J. Chem. Eng. 34 (2017) 2180.
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Short communication
Suppressed char agglomeration by rotary kiln reactor with alumina ball during the pyrolysis of Kraft lignin Young-Min Kima,1, Seyoung Parka,1, Bo Sung Kanga,1, Jungho Jaeb,c, Gwang Hoon Rheed, Sang-Chul Junge, Young-Kwon Parka,* a
School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Republic of Korea d Department of Mechanical and Information Engineering, University of Seoul, Seoul 02504, Republic of Korea e Department of Environmental Engineering, Sunchon national University, Suncheon 57922, Republic of Korea b c
A R T I C L E I N F O
Article history: Received 5 February 2018 Received in revised form 21 May 2018 Accepted 27 May 2018 Available online xxx Keywords: Pyrolysis Kraft lignin Char agglomeration Rotary kiln reactor Alumina ball
A B S T R A C T
In this study, the pyrolysis of Kraft lignin was conducted in a rotary kiln reactor using rotating alumina balls as a filler to achieve a continuous pyrolysis process by the suppression of char agglomeration. Temperature variation experiments showed that the gas yield increased and the char yield decreased with an increase in the pyrolysis temperature from 550 C to 650 C. The maximum oil yield was obtained at a reaction temperature of 600 C. Compared to a fixed bed reactor, a rotary kiln reactor using alumina balls produced the higher quality oil containing larger amount of organic phase oil and higher selectivity to aromatic hydrocarbons. At all temperatures, no lignin char agglomeration occurred inside the reactor due to the effective collision of lignin char intermediates and alumina balls, allowing a continuous pyrolysis process without reactor plugging. © 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction The worldwide demand for renewable energy has increased due to the energy crisis, environmental contamination, and climate change caused by intensive use of fossil fuels [1]. Biomass is an alternative energy source that can potentially replace fossil fuels. Among the various kinds of renewable energy sources, the importance of biomass has been emphasized because it is the only carbon source that can produce both energy and carbon-based chemical feedstock [2]. Thermal treatment of biomass is being investigated intensively because value-added chemicals can be produced using the appropriate thermal conversion technology [3–8]. Pyrolysis, a thermal decomposition technology performed under a non-oxygen atmosphere at temperatures between 400 and 600 C, is a proper method which can produce the large amount of bio-oil from biomass. Recently, different kinds of pyrolysis, such as microwave-assisted pyrolysis [9], was also reported as an effective method for the production of high-quality oil from biomass.
* Corresponding author. E-mail address: [email protected] (Y.-K. Park). Co-first authors.
1
Lignin, a main lignocellulosic component of wood biomass, is a cross-linked phenolic polymer forming the support tissues of wood and bark [10]. A large amount of lignin is released to the environment as waste mainly from the pulp manufacturing process. Lignin has high potential to become an alternative source of petroleum-derived aromatic chemicals. The market value of aromatic chemicals, such as phenol, is also very high. On the other hand, a considerable amount of lignin is classified as waste and is incinerated [11]. The pyrolysis of lignin is a promising technology for decomposing the polymeric structure of lignin and converting it to a liquid fuel, called bio-oil, which contains large quantities of valuable phenolic compounds, in a simple one-step process [12]. On the other hand, research on the pyrolysis process of lignin is still in its initial stages using laboratory-scale batch-type reactors [13]. Several technical barriers need to be overcome before the lignin pyrolysis process can be scaled up [14]. One of the most difficult problems is that the stable and continuous operation of the reactor is quite challenging during lignin pyrolysis due to the large amount of lignin char accumulating inside the reactor. Nowakowski et al. [15] recommended that less pure lignin samples, such as lignin isolated by acid hydrolysis, which contain some sugary components, are better than pure lignin when a conventional continuous
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Please cite this article in press as: Y.-M. Kim, et al., Suppressed char agglomeration by rotary kiln reactor with alumina ball during the pyrolysis of Kraft lignin, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.05.041
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lignin pyrolysis system is used. In the case of pure lignin, they recommended the use of a new reactor system, such as entrainedflow, high temperature, long residence time, etc. The large agglomerates and swelling of char produced during the pyrolysis of lignin can cause a blockage of the reactor, requiring additional reactor maintenance to repeat the pyrolysis. Recently, several studies reported a method to minimize char agglomeration and swelling by the mechanical modification of the reactor or the pretreatment of lignin [16–18]. For example, modification of the reactor feeding system was effective in reducing the level of char agglomeration but it could not prevent the reactor from plugging completely. The co-feeding of lignin with calcium hydroxide was reported to be quite effective in preventing agglomeration of char inside the reactor because calcium hydroxide pretreatment can decrease the amounts of phenolic hydroxyl, carboxylic acid, and aldehyde groups, which cause the severe agglomeration of lignin during pyrolysis [19]. They also indicated that this pretreatment process enables us to carry out the pyrolysis of lignin in a conventional fluidized bed reactor without reactor modification, even though additional processes, such as char burning and leaching ash with water, are needed to recycle calcium hydroxide. In another study, Zhou et al. [20] reported the continuous pyrolysis of lignin derived from a bioethanol pilot plant in a PCR (pyrolysis centrifuge reactor), which has a continuous char removal function under the mechanical action of rotating vanes. Another possible method to minimize char agglomeration is to use alumina ball as a packing material of the reactor during the pyrolysis of lignin. Alumina ball has been used as an effective grinding media in ball milling due to its thermal resistance and mechanical strength. Therefore, these properties of alumina ball can reduce the char agglomeration when it is used for lignin pyrolysis by providing the mechanical grinding effect. In this study, the pyrolysis of lignin was conducted in a rotary kiln reactor with alumina ball as a rotating filler to prevent or minimize lignin char agglomeration, thereby achieving a continuous pyrolysis process. To accomplish this, a new rotary kiln reactor, in which the rotation speed and slope of the reactor can be controlled, was designed, and alumina balls were used as the filler inside the reactor. The yields of the liquid products produced from
the pyrolysis of Kraft lignin at different temperatures were evaluated, and the level of char formation after the reaction was monitored and compared with that obtained from a conventional fixed-bed reactor. Gas chromatography/mass spectrometry (GC/ MS) analysis of the liquid product was also performed to examine the chemical composition of the product oil. Materials and methods Lignin and alumina ball The powder form of Kraft lignin (Sigma-Aldrich) was used as a sample for the pyrolysis of lignin in this study. The lignin was sieved to make a particle size smaller than 300 mm and dried at 80 C for 6 h to eliminate the residual water. Proximate, ultimate, and gel permeation chromatography (GPC) analysis were performed to determine the physico-chemical properties of lignin [21]. Proximate and ultimate analysis were performed according to the standard methods, ASTM D 7582 (2015) and ASTM D 5373 (2014) using an elementary analyzer (Flash EA 1113, CE). The gel permeation chromatography (GPC) experiments were carried out to know the average molecular weight distribution of lignin using a Waters 2695 instrument. For this, 100 mL of lignin solution diluted in tetrahydrofuran was injected to GPC Column (Styragel HR2/Styragel HR4/ Styragel HR5) at 30 C. Thermogravimetric analysis (TGA Pyris Diamond; Perkin–Elmer) of Kraft lignin was also carried out. For TGA, 5 mg of Kraft lignin was heated non-isothermally from ambient temperature to 700 C at 10 C/min, 20 C/min and 30 C/min under 20 mL/min in a nitrogen atmosphere. Alumina ball (diameter: 5 mm), purchased from a local supplier, was used to provide the stable temperature and physical collision with lignin pyrolysis intermediates. X-ray diffraction (XRD) measurement of alumina ball was performed to identify its crystal phase on a X0 pert PRO X-ray diffractometer (Panalytical) with Cu Kα radiation (l = 0.15406 nm). Rotary kiln reactor Fig. 1 shows a schematic diagram of the rotary kiln type pyrolyzer designed for the rapid pyrolysis of lignin. The rotary kiln
Fig. 1. Schematic diagram of rotary kiln reactor system.
Please cite this article in press as: Y.-M. Kim, et al., Suppressed char agglomeration by rotary kiln reactor with alumina ball during the pyrolysis of Kraft lignin, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.05.041
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pyrolysis reactor consisted of a lignin feed, pyrolysis reactor, and oil recovery system. The lignin feeding system was designed to transfer a solid lignin sample to the pyrolysis reactor with a feeding lignin storage hopper (#1), rotation feeding screw (#2), and nitrogen spray feeder (#3). The pyrolysis reactor system was designed to provide heat to the sample together with the rotation of the reactor with a rotation motor (#4), electronic heater (#5), cylindrical stainless steel reactor (#6), filler (#7), metal frame (#8), reactor angle controller (#14), and metal filter (#9). The rotation of the filler in the cylindrical stainless steel reactor (57 cm length 10 cm inner diameter) was applied to prevent internal blockage of the reactor due to lignin char foaming. A metal filter was used to separate the solids from the gases emitted from the pyrolysis reactor. The oil recovery system consisted of low and ambient temperature oil condensers (#10, #12), electric precipitator (#13), and oil recovery round bottom flasks (#11). Before the lignin pyrolysis reaction, the reactor was pre-purged with 10 L/min of nitrogen to eliminate the residual air inside the reactor while increasing the furnace temperature to the target temperature (550, 600, or 650 C) and stabilized during 40 min. To provide the same temperature of the furnace, the reactor was also rotated at 80 rpm using the rotation motor (#4). Alumina balls were also loaded inside the reactor. After 40 min reactor stabilization time, the lignin powder (100 g) with a particle size <300 mm was injected continuously into the pyrolysis reactor for 40 min at a constant feeding rate (2.5 g/min) and the product vapor was transferred to the oil condensers and the condensed oil was collected in the round bottom flasks. After the reaction, the weights of the oil collected in the flasks were measured by a balance and analyzed by GC/MS. Acetone was used as a washing solvent to achieve better extraction of the product oil from the oil condensers. Table S1 (Supplementary information) lists the GC/MS conditions. The elemental composition and calorific value of the product oil were also measured using an elemental analyzer (Flash EA 1112, Thermo) and bomb calorimeter (AC-350; LECO). The formation of char and the change in the filler were monitored after the pyrolysis reaction. Results and discussion Rotary kiln reactor Table 1 lists the physico-chemical properties of Kraft lignin. Compared to other components of biomass, such as hemicellulose and cellulose [22], lignin contains a larger amount of fixed carbon (32.6%). This suggests that large amount of char can be produced by the pyrolysis of lignin. TGA curve of Kraft lignin (Fig. 2) indicated that lignin decomposed over a wide temperature region between 250 and 500 C [23], and a large amount of solid residue (51.3%), which consisted of char and ash, remained in the sample cup after Table 1 Physico-chemical characteristics of Kraft lignin. Proximate analysis (wt.%) [19]
Ultimate analysisa (wt. %) [19]
GPC analysis
a b
On a dry basis. By difference.
Water Volatiles Fixed carbon Ash C H Ob N S Mn Mw Mz
1.8 62.9 32.6 2.7 62.45 5.68 30.61 0.56 0.70 655 1319 2123
3
Fig. 2. TG curve of Kraft lignin at 20 C/min.
TGA. Therefore, the minimum temperature for the isothermal pyrolysis reaction for Kraft lignin using the rotary kiln reactor was set to 500 C. Lignin prolysis oil Table 2 lists the yields of gas, oil and char obtained from the pyrolysis of Kraft lignin at different temperatures using a rotary kiln reactor. The oil yield increased gradually from 32.2% to 35.5% with increasing reaction temperature from 550 C to 600 C, but it decreased to 30.4% at 650 C. The increased yield of oil can be explained by the additional decomposition of char intermediates. Shafaghat et al. [21] reported that the increased yield of oil and gas were accompanied by a decrease in solid residue, which was attributed to the additional cracking of char by the higher heat energy supplied to the system. On the other hand, the liquid yield was lower at higher temperature, despite the decrease in char yield because a higher temperature can also enhance the secondary cracking of pyrolysis oil, which would increase the gas yield [17]. The yield of solid char was decreased from 50.3% at 550 C to 39.9% at 650 C, indicating clearly that the additional cracking of char occurs with an increase in the temperature. Similarly, the gas yield was increased from 17.5 to 29.7% with increasing temperature from 550 C to 650 C due to the additional decomposition of lignin and secondary cracking of lignin pyrolyzates [24]. Table 3 shows the gas product distribution from lignin pyrolysis at different temperatures using a rotary kiln reactor. When the pyrolysis temperature increased from 550 C to 650 C, the yields of CO and light hydrocarbons (C1–C4) were increased from 6.5 to 15.0% and from 0.6 to 2.2%, respectively. This clearly indicates that the additional cracking reaction occurs at higher pyrolysis temperatures. Table 4 lists the yields of the aqueous and organic phase oil obtained from the pyrolysis of Kraft lignin using two different reactors: the rotary kiln and bench–scale fixed bed reactor. The pyrolysis of the Kraft lignin using a fixed bed reactor was performed at 600 C using the method reported by Shafaghat et al. [21]. In this study, the amount of aqueous phase oil (3.74%) from the rotary kiln reactor was much lower than that from the Table 2 Yields of products obtained from the pyrolysis of Kraft lignin using rotary kiln reactor. Temperature ( C) Yield (wt.%) Oil Gas Char
550
600
650
32.2 17.5 50.3
35.5 24.5 40.0
30.4 29.7 39.9
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Table 3 The yields of CO, CO2, and light hydrocarbons (C1–C4) obtained from the pyrolysis of Kraft lignin at different temperature using a rotary kiln reactor. Temperature ( C) Component CO CO2 C1–C4
550 Yield (wt.%) 6.5 10.4 0.6
600
650
10.7 12.5 1.3
15.0 12.5 2.2
Table 4 Yields of aqueous and organic phase of bio-oil obtained from the pyrolysis of Kraft lignin on two reactors.
Yield (wt%) Aqueous phase Organic phase Total oil
Rotary kiln (600 C)
Fixed bed reactor (600 C)
3.7 31.8 35.5
14.1 19.6 33.7 Fig. 3. XRD pattern of alumina ball used as filler in this study.
fixed bed reactor (14.05%). In addition, the organic phase oil obtained from the rotary kiln reactor was 31.77%, whereas that obtained from the fixed bed reactor was 19.63%. The total oil yield from the rotary kiln reactor was higher than that of the fixed bed reactor, and the rotary kiln reactor is expected to produce more valuable chemicals than the fixed bed reactor. The higher content of organic phase oil in the bio-oil is important because valuable chemical compounds are retained mainly in the organic phase oil. Table 5 compares the product distribution of the bio-oil obtained from the pyrolysis of Kraft lignin using a rotary kiln reactor and fixed bed reactor [21]. On the fixed bed reactor, phenols, such as alkyl phenols, guaiacols, and catechols, occupied the largest peak area % in the product oil, and the peak area % of aromatic hydrocarbons was less than 0.5%. In contrast, the peak area % of aromatics obtained using the rotary kiln reactor were 3.79% for mono aromatic hydrocarbons (MAHs), which was much higher than those obtained using the fixed bed reactor, 0.28% for MAHs. The major reaction pathway for the formation of aromatic hydrocarbons from the pyrolysis of lignin is the secondary cracking of phenols, such as dehydroxylation, dealkylation, and demethoxylation. This indicates that the higher cracking efficiency enhancing the secondary cracking of phenols can be achieved by applying the rotary kiln reactor with alumina ball. The alumina ball had the typical XRD peaks of α-alumina phase as shown in Fig. 3. The XRD pattern of the alumina ball used for our experiments matches well with ICDD Database(00-005-0712 for α-Al2O3) [25]. This suggests that the additional cracking of lignin pyrolyzates in the rotary kiln reactor is difficult to be achieved by the catalytic effect of alumina ball. In addition, the selectivity to phenols were also different between the experiments using the fixed bed reactor and the rotary kiln reactor. Kraft lignin pyrolysis using the rotary kiln reactor showed higher selectivity to alkyl phenols, but much lower
Table 5 Product distribution of the bio-oil obtained from the pyrolysis of Kraft lignin using rotary kiln reactor at 600 C. (Area %)
Rotary kiln (600 C)
Fixed bed (600 C)
Mono Aromatics Phenols
3.79 35.83 24.28 3.70 0 2.53 29.87
0.28 27.77 34.46 13.56 0 12.60 11.33
Acids Other oxygenatesa Othersb a b
Alkyl Phenols Guaiacols Catechols
Ketones, furans, esters, alcohols and aldehydes (except phenols). N S compounds, PAHs and other cyclic compounds.
selectivity to guaiacol, catechol, and other oxygenates than the fixed bed reactor. This suggests that the demethoxylation and demethylation reactions of guaiacols are promoted in a rotary kiln reactor. Kim et al. [24] reported that the lignin pyrolysis product distribution are altered largely by its pyrolysis temperature on the pyrolysis research of Pinus radiata. Methoxy phenols, such as guaiacols and eugenols, are the main pyrolyzates of lignin at temperatures less than 500 C. Between 500 C and 600 C, alkyl phenols and pyrocatechols were the dominant pyrolyzates due to the increased dealkoxylation and dealkylation of guaiacols and eugenols. At temperatures higher than 600 C, aromatic hydrocarbons were also produced by the additional dealkylation reaction. Char agglomeration property Table 6 shows the elemental composition of char obtained from the pyrolysis of lignin at 600 C using the rotary kiln reactor. Fig. 4 shows images of the fixed bed reactor and the char formed after the pyrolysis of Kraft lignin at 600 C. The detailed pyrolysis conditions for the pyrolysis of Kraft lignin using the fixed bed reactor are reported elsewhere [21]. The char in the fixed bed reactor (Fig. 4(b)) consisted of sticky and agglomerated solid materials, which become the main factor causing reactor blockage. This can be a major impediment to the implementation of a continuous lignin pyrolysis process [14]. Based on previous studies on the continuous pyrolysis of lignin, a new rotary kiln type reactor using alumina balls as a filler was developed and applied to the pyrolysis of lignin. Fig. 5 shows images of the rotary kiln reactor and alumina balls obtained after the pyrolysis of Kraft lignin at 600 C. After the reaction, only black and fine char powder was observed inside the reactor and on the filter, suggesting that char agglomeration did not occur during pyrolysis. This also suggests that the use of a rotating alumina ball can effectively prevent the agglomeration of lignin char and allow the continuous operation of the reactor. The prevention of char agglomeration in a rotary kiln can be explained by physical interactions, such as the ball milling effect of alumina balls on char intermediates. Alumina ball is a low cost filler and can be reused after the simple calcination process. This also indicates Table 6 The composition of char obtained from the pyrolysis of Kraft lignin using rotary kiln reactor at 600 C. Temperature ( C)
Composition (wt.%) C
H
O
N
S
600
71.58
2.96
24.74
0.12
0.60
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Fig. 4. Reactor and lignin char foaming obtained from the fixed bed reactor after the pyrolysis of Kraft lignin at 600 C.
Fig. 5. Reactor, char, alumina ball obtained from rotary kiln reactor after the pyrolysis of Kraft lignin at 600 C.
the high potential of the rotary kiln reactor applied in this study as a future technology for the lignin conversion. Conclusion The continuous pyrolysis of Kraft lignin was achieved using a rotary kiln reactor with alumina ball as a filler. Compared to the conventional fixed bed reactor pyrolysis, it improved the quality of oil product significantly and minimized the char agglomeration. The temperature variation experiments showed that 600 C is the optimum temperature to maximize the oil yield in a rotary kiln reactor. Meanwhile, the yield of gas progressively increased from 17.5 to 30% and the solid yield decreased continuously from 50 to 40% from 550 C to 650 C with increasing reaction temperature. The product oil obtained at 600 C contained larger amounts of valuable aromatic hydrocarbons and alkyl phenols than that at 600 C using the fixed bed reactor. Importantly, char agglomeration and the swelling did not occur during the pyrolysis in the rotary kiln reactor with alumina ball, allowing the continuous operation of the reactor without any plugging. Ongoing experiments are focused on the investigation of the detailed degradation mechanism of lignin in the rotary kiln reactor with alumina ball. The results obtained in this study would be contributed to the design of an improved pyrolysis reactor for lignin conversion.
Acknowledgements This work was supported by the New and Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20153030101580). Also, this research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (NRF-2017M1A2A2087674). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2018.05.041. References [1] P.K. Kanaujia, Y.K. Sharma, M.O. Garg, D. Tripathi, R. Singh, J. Anal. Appl. Pyrol. 105 (2014) 55. [2] P. Gallezot, Chem. Soc. Rev. 41 (2012) 1538. [3] E.H. Lee, R.-S. Park, H. Kim, S.H. Park, S.-C. Jung, J.-K. Jeon, S.C. Kim, Y.-K. Park, J. Ind. Eng. Chem. 37 (2016) 18. [4] J.S. Cha, S.H. Park, S.-C. Jung, C. Ryu, J.-K. Jeon, M.-C. Shin, Y.-K. Park, J. Ind. Eng. Chem. 40 (2016) 1. [5] Y. Lee, H. Shafaghat, J. Kim, J.K. Jeon, S.C. Jung, I.G. Lee, Y.K. Park, Korean J. Chem. Eng. 34 (2017) 2180.
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