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Comparison of bio-chars formation derived from fast and slow pyrolysis of walnut shell
Fuel 261 (2020) 116450
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Full Length Article
Comparison of bio-chars formation derived from fast and slow pyrolysis of walnut shell
T
⁎
Tong Yuana, Wenjing Heb, Guojun Yina, , Shiai Xua a b
Shandong Provincial Key Lab of Chemical Engineering and Process, Yantai University, Yantai 264005, PR China Department of Chemical Engineering, Jiangsu Ocean University, Lianyungang 222002, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Pyrolysis Bio-char Free radical Functional groups Walnut shell
Fast and slow pyrolysis are good means of biomass transformation. Studying the formation of bio-char is not only helpful to intensify the transformation of biomass, but also to improve its performance. However, little work has been reported on the similarities and differences of bio-chars in fast and slow pyrolysis. We investigated the fast and slow pyrolysis of walnut shell at 400–800 °C to find the evolvement law of bio-char. It was found that the formation mechanism of bio-char in fast pyrolysis is almost the same as that in slow pyrolysis. There are two stages in the formation of bio-char: the generation of free radicals caused by the covalent bond breaking at temperature less than 600 °C; the reduce of free radicals through the condensation reactions from 600 to 800 °C. The contents of various oxygen-containing functional groups presented regularity with the increase of pyrolysis temperature, in which the quinone group increases with the pyrolysis temperature increasing, which phenomenon could be used as a signal to exhibit the stabilization of oxygen in bio-char. The bio-char yields, free radicals and quinone contents are slightly higher in slow pyrolysis. The graphite crystals in bio-char have the tendency of microcrystallization with the increase of temperature, in which progress the fast pyrolysis has superiority at the low temperature.
1. Introduction Biomass refers to various carbon-containing renewable resources formed by photosynthesis, including all animals, plants and microorganisms etc [1]. The utilization of biomass would address many ⁎
essential needs in energy and environment. Pyrolysis is commonly regarded as a simple and effective technology for biomass transformation, in which the biomass is conversed into bio-tar, pyrolysis gases (CO, CO2, CH4 and H2, etc.) and bio-char [2]. The main products of pyrolysis are bio-tar and pyrolysis gas which have been widely studied and
Corresponding author. E-mail address: [email protected] (G. Yin).
https://doi.org/10.1016/j.fuel.2019.116450 Received 31 August 2019; Received in revised form 14 October 2019; Accepted 17 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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utilized, while the bio-char is usually considered as a by-product. In fact, the bio-char has a broad spectrum of application, such as fertilizer in agricultural production, precursor of porous carbon material and fuel for burning etc [3,4]. Techno-economic studies suggested that the biomass pyrolysis is not very competitive without valuing the bio-char as an important product [5]. The physicochemical properties of bio-chars depend not only on the nature of the starting biomass [6–8], but also, to a very large extent, on the condition of preparation. According to the heating rate, there are two main pyrolysis technologies: slow pyrolysis (10 °C/min) and fast pyrolysis (102 °C/sec) [9]. There is an obvious difference that the biochar yield of fast pyrolysis is less than that of slow pyrolysis, indicating that the fast pyrolysis has a greater ablation degree of raw materials. This results in the pore structure of bio-char produced by fast pyrolysis being more developed than that by slow pyrolysis [6–8,10]. Apart from the pore structure, other physicochemical properties, such as surface functional group and crystal structure etc, can also affect the bio char performances. At present, many important physicochemical properties of bio-char during the fast and slow pyrolysis have not been detailedly reported in the literature. In sum, it is necessary to study the similarities and differences of bio-char formation in the fast and slow pyrolysis. To reveal the formation of bio-char, the following three aspects should be studied in detail. Firstly, the chemical mechanism of pyrolysis is generally considered as the free radical reactions caused by covalent bond breaking. Therefore, it is important to tackle these reactions. The group of Liu Zhenyu at Beijing University of Chemical Technology has conducted a series of studies on free radical reactions in coal pyrolysis [11–13]. These studies about free radical reaction mechanism of coal pyrolysis could be helpful to reveal the fundamental chemistry of biochar. The second is the composition of surface oxygen-containing functional groups which may affect on the formation of bio-char [14]. The performance of bio-char is often related to the oxygen-containing surface functional groups [15–17]. Therefore, the studies on the change of surface functional groups in pyrolysis process are helpful to both obtain the formation mechanism of bio-char and control the content of functional groups in the light of different applications. Besides, as a kind of carbon materials, the basic framework of bio-char is a cross-link structure of graphite crystallite. The graphite crystallite structure not only determines the hardness of bio-char, but also influences its performance [18,19]. Therefore, the change of graphite crystallite in pyrolysis needs to be studied. This work studied the fast and slow pyrolysis of walnut shell at 400–800 °C in an inert nitrogen atmosphere. The bio-chars were characterized by electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Through studying the bio-chars formation in the fast and slow pyrolysis, it could deepen the understanding of biomass pyrolysis reaction mechanism and is helpful for the directional preparation of bio-char according to different applications.
Table 1 Compositions analysis of walnut shell in this study. Lignocellulose content (%) Lignin 49.12
Cellulose 31.19
Hemicellulose 16.59
Ultimate analysis (wt.%, daf) H 5.64
S 0.25
N 0.16
Ca 0.08
K 0.05
Fe 0.02
Na 0.01
0.35
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o
Temperature ( C) Fig. 1. Yields and free radical concentrations of bio-chars. Square: F-P, Circle: S-P; Solid symbols: bio-char yields, Open symbols: free radical concentrations.
quickly pushed into the furnace constant temperature zone for 15 min. For slow pyrolysis (termed as S-P), the sample on the furnace constant temperature zone was heated from room temperature to the final temperature (400–800 °C) at a heating rate of 10 °C/min. The bio-char was collected after cooling to room temperature in the inert atmosphere. 2.3. Bio-char characterization The Electron Paramagnetic Resonance (EPR) used to determine the free radical concentration is EMXnano (Bruker, Germany) operated at 9.6 GHz and 0.3162 mW. The central field was 3431.20 G, the sweep width was 200.0 G, the sweep time was 30.16 s, the number of scans was 10 times, the modulation amplitude was 1.000 G, and the receiver gain was 20 dB. All the EPR tests were carried out at room temperature and the collected free radical signals were secondarily integrated using Xenon software to automatically calculate the number of spins. In addition, the capillary showed little influence on EPR measurement. The chemical state of elements was studied by using X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher, ESCALAB 250 xi), using monochromatized Al Kα irradiation. The crystal structure of the samples was tested using the X-ray diffraction (XRD, Rigaku SmartLab III, Japan) with Cu Kα radiation (0.154 nm) scan in the range of 10-80° and scanning speed of 5°/min.
2. Materials and methods 2.1. Raw material The walnut shell was ground and sieved to 20–40 mesh, then dried at 120 °C for 4 h. The lignocellulose contents and ultimate analyses were summarized in Table 1.
3. Results and discussion
2.2. Pyrolysis experiments
3.1. Bio-char yield
The pyrolysis experiments were carried out in a quartz tube furnace that includes a quartz crucible loaded with 5 g biomass sample. Before the experiment, the setup was purged by the purified nitrogen (99.999%) at a rate of 150 mL/min for 30 min. Fast pyrolysis (termed as F-P) was started by preheating the quartz tube to the specified temperature (400–800 °C), then quartz crucible loaded with sample was
Fig. 1 shows that the bio-char yields in the F-P and S-P decrease with the increase of pyrolysis temperature. When the pyrolysis temperatures are larger than 600 °C, the decline trend is slowing, indicating that the pyrolysis at less than 600 °C may be the main stage of weight loss [20,21]. Compared the F-P with S-P, it can be seen that the bio-char yields of F-P are lower than those of S-P at the same pyrolysis 2
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Fig. 2. A possible reaction model for generation of free radical fragments in biomass pyrolysis.
lignin has. Thus, the lignin selected as the model compound was used to study the free radical reaction during the bio-char formation, as shown in Fig. 2. Referring to the coal pyrolysis process, it can be found that the covalent bonds with energy less than 400 kJ/mol are broken at around 400 °C, and the covalent bonds with energy less than 500 kJ/mol are broken at around 500 °C [26]. This study proposed a possible reaction model for explaining the broken of covalent bonds to generate the free radical fragments shown in Fig. 2. By corresponding to the trend of biochar yields in Fig. 1, it could be concluded that the bio-chars obtained at the pyrolysis temperature between 400 and 500 °C are the residual structure of raw materials due to the release of free radical fragments. Liu et al. reported that the condensation reaction could enhance the aromatization of bio-char through decreasing the free radical concentrations in bio-char [27]. This study showed that the reduction of free radical concentrations in bio-chars obtained from 600 to 700 °C should be related to the condensation reaction. In addition, when the pyrolysis temperature is larger than 800 °C, the condensation reaction among the free radical fragments may be completed. It is noteworthy that the free radical concentrations of bio-chars from the S-P are greater than those from the F-P at 400–500 °C. The biomass pyrolysis volatile also contains free radical fragments, which could deposit on the surface of bio-chars or react with the bio-chars [28–30]. Because of the fast heating rate in the F-P, the rate of free radicals formation may be higher than that of free radicals’ deposition or reaction, which could lower the free radical concentrations of biochars. This may be the reason why the S-P has the higher bio-char yields in Fig. 1.
temperature, which is consistent with the results reported in the literature [6–8]. It suggests that the F-P is more conducive to the release of volatile. However, the S-P is more favorable for the bio-char production [22]. This may lead to the significant differences in physicochemical properties of bio-char derived from the F-P and S-P. 3.2. Free radical reaction As mentioned in the introduction, the fundamental chemistry of pyrolysis is the free radical reactions caused by covalent bond breaking. The breaking of covalent bonds will inevitably result in the generation of free radical fragments in the bio-chars. Fig. 1 shows that the free radical concentrations of bio-chars in the F-P have the same tendency as that in the S-P. With the increase of pyrolysis temperature, the free radical concentrations increase between 400 and 500 °C, then decrease at around 600 °C. When the temperatures are larger than 700 °C, the free radical concentrations are close to zero (no paramagnetic signal detected by EPR). It is generally considered that the main components of biomass are hemicellulose, cellulose and lignin [23]. While the biomass is pyrolyzed, the free radicals of the bio-char may come from the decomposition of cellulose, hemicellulose and lignin. Considering that the detailed chemical structures of hemicellulose, cellulose and lignin in biomass are not clear, many kinds of model compounds were proposed to study their pyrolysis behavior [23–25]. The breakdown temperatures of hemicellulose, cellulose and lignin are 220–315 °C, 315–400 °C and 160–900 °C, respectively [23]. According to the decomposition temperatures, the cellulose and hemicellulose have been completely decomposed at the pyrolysis temperature of this work. In the light of the law of covalent bond breaking, the products of cellulose and hemicellulose decomposition should have the same bond energies as the 3
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the above results, it can be concluded that the temperature of 600 °C is a critical point of walnut shell in pyrolysis. At pyrolysis temperature less than the critical point, the main chemical reaction is the broken of covalent bonds to generate free radical fragments. At pyrolysis temperature larger than the critical point, whereas, the condensation reaction plays the major role, which is almost the same as the coal pyrolysis process [11,26]. The total contents of the other three peaks in C1s spectrum, reflecting the functional groups of C and O, have the opposite results of peak 1, as shown in Fig. 3(C). The oxygen-containing functional groups were further analyzed by the O1s spectra, as shown in Fig. 4 (A) and (B). The deconvolution of O1s spectra yielded four peaks [31,32]: peak 1 (531.0–531.9 eV), C]O in quinone; peak 2 (532.3–532.8 eV), C]O in ester or anhydrides, O–H in hydroxyl; peak 3 (533.1–533.8 eV), C–O in ester and anhydrides; peak 4 (534.3–535.4 eV), –COOH. It can be seen from Fig. 4(C) that the content of each peak has the same trend for the F-P and S-P, which suggests that the F-P and S-P follow the identical fundamental chemistry processes. However, each content of the four peaks in the F-P and S-P is different at the same pyrolysis temperature. The quinone contents (Peak 1) of bio-chars in the S-P are greater than those in the F-P. Through identifying the contents of peaks 2 and 3, it can be determined that the hydroxyl group content in the F-P is greater. However, the contents of ester and anhydride (Peak 3) in the S-P are higher. With the increase of temperature, the content of carboxyl group (Peak 4) does not change significantly and maintains at about 15%. The above results indicate that a certain functional group may be decomposed at a specific pyrolysis temperature, but it could be regenerated by the free radical reactions. In the four kinds of oxygen-containing functional groups, the quinone content increases with the increase of temperature, which indicates that the quinone could be the stable existing structure of oxygen element in bio-char.
3.3. Surface functional groups Mettler et al. reported that the formation of bio-char depends on the functional groups of each molecule in biomass [14]. As can be seen from Table 1, the main elements in the raw material are C and O, whose proportions are larger than 93%, suggesting that the functional groups in bio-chars are mainly composed of C and O. The surface functional groups of bio-chars in the F-P and S-P were analyzed by XPS (C1s and O1s spectra). The deconvolution of C1s spectra yielded four peaks [31,32]: peak 1 (284.6 eV), C–C and C]C; peak 2 (286–286.3 eV), C–O in phenolic, alcohol, ether; peak 3 (287.3–287.6 eV), C]O in carbonyl, quinine; peak 4 (288.8–289.1 eV), O–C]O in carboxyl, ester. For C1s spectra shown in Fig. 3(A) and (B), the peak 1 reflects the content of C–C and C]C. Fig. 3(C) shows that the change of peak 1 contents in both F-P and S-P both contains two stages: (1) the contents increase with the increase of pyrolysis temperature from 400 to 600 °C. (2) The contents decrease at temperature lager than 600 °C. Fig. 2 shows that the partial oxygen-containing structures are decomposed at low temperature, which leads to the increase of C content and decrease of oxygen-containing groups in bio-chars [33–35]. When the pyrolysis temperature exceeds 600 °C, the C–C bonds of lignin are broken, which leads to the C content of bio-chars decreasing [6–8]. It can be seen that the bio-chars in the S-P have the higher peak 1 contents than those in the F-P at 400–600 °C. However, the peak 1 contents of bio-chars in both F-P and S-P are almost the same at temperature larger than 600 °C. This trend is consistent with the change of free radicals in Fig. 1. But beyond this, the ratios of C and O in bio-chars at different temperatures are shown in Fig. 4(D). It is observed that the ratio increases rapidly within the temperature less than 600 °C, thereafter it becomes slow from 700 to 800 °C both in F-P and S-P. Based on 4
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3.4. Analysis of crystal structure
The bio-char has been widely applied in the liquid separation, such as heavy metal ions and dyes, etc. The adsorption of Pb2+ is related to the oxygen-containing group that could combine with Pb2+ by complexation reaction [15,16]. However, the oxygen-containing group could have a bad effect on the adsorption behavior of dyes. Qu et al. reported that the acid oxygen-containing groups could decrease the removal of malachite green [17]. This study showed the evolvement law of oxygen-containing groups, which could provide the suggestion to control the oxygen-containing groups for different adsorption processes.
The above studies show that when the pyrolysis temperature is larger than 600 °C, the condensation is taken place among the free radical fragments, which reactions could not only decrease the free radicals, but also modify the crystal structure of the bio-char. The evolution of graphite crystal structure in the bio-chars was analyzed by XRD, as shown in Fig. 5. Based on the XRD spectrum, the crystal parameters such as the interlayer spacing (d002), the stacking height (Lc) and the transverse size (La) could be calculated by the Bragg and Scherrer equations, respectively [36]. The calculation results of the three parameters are exhibited 5
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Acknowledgement
in Fig. 6. The interlayer spacing of graphite microcrystalline in biochars at different pyrolysis temperatures is almost unchanged (about 0.39 nm). The values of stacking height first increase at temperature less than 600 °C then remain at a certain level with temperature increasing. The trend of transverse size is reverse to that of stacking height. These indicate that the graphite crystals in bio-chars tend to microcrystalline with the increase of pyrolysis temperature. Although the change law of graphite crystal structures in the F-P is the same as that in the S-P, there is an obvious difference existing at the 400 °C between the two technologies. It suggests that the microcrystalline in FP is easier at the low pyrolysis temperature. The bio-char has good electrochemistry performances, which is related to the content of graphite crystallite [18]. The variation of graphite crystallite in bio-char has been correlated with the pyrolysis conditions in the above result, which indicates that the electrochemistry performances of bio-char could be well controlled.
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4. Conclusions Although the bio-char yield of slow pyrolysis is higher than that of fast pyrolysis, the formation of bio-char in these two processes follows the same mechanism, which contains two steps with the increase of pyrolysis temperature: Generation of the free radical fragments by the covalent bond broken is the main chemical reaction at temperature less than 600 °C; while condensation of the free radical fragments plays the fundamental role from 600 to 800 °C. With the pyrolysis temperature increasing, the content of quinine group increases gradually; the contents of ester, anhydride and hydroxyl groups decrease; the content of carboxyl group is almost unchanged; the graphite crystals in bio-chars tend to microcrystallize. There are several differences existing for bio-char formation in fast pyrolysis and slow pyrolysis. The slow pyrolysis generates more free radical fragments than fast pyrolysis. The contents of quinine, ester and anhydride groups in bio-chars during slow pyrolysis are larger than those during fast pyrolysis at the same pyrolysis temperature. However, the content of hydroxyl group is opposite. From the change of crystal parameters, the fast pyrolysis facilitates the graphite microcrystalline of bio-char more easily at the low temperature.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Full Length Article
Comparison of bio-chars formation derived from fast and slow pyrolysis of walnut shell
T
⁎
Tong Yuana, Wenjing Heb, Guojun Yina, , Shiai Xua a b
Shandong Provincial Key Lab of Chemical Engineering and Process, Yantai University, Yantai 264005, PR China Department of Chemical Engineering, Jiangsu Ocean University, Lianyungang 222002, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Pyrolysis Bio-char Free radical Functional groups Walnut shell
Fast and slow pyrolysis are good means of biomass transformation. Studying the formation of bio-char is not only helpful to intensify the transformation of biomass, but also to improve its performance. However, little work has been reported on the similarities and differences of bio-chars in fast and slow pyrolysis. We investigated the fast and slow pyrolysis of walnut shell at 400–800 °C to find the evolvement law of bio-char. It was found that the formation mechanism of bio-char in fast pyrolysis is almost the same as that in slow pyrolysis. There are two stages in the formation of bio-char: the generation of free radicals caused by the covalent bond breaking at temperature less than 600 °C; the reduce of free radicals through the condensation reactions from 600 to 800 °C. The contents of various oxygen-containing functional groups presented regularity with the increase of pyrolysis temperature, in which the quinone group increases with the pyrolysis temperature increasing, which phenomenon could be used as a signal to exhibit the stabilization of oxygen in bio-char. The bio-char yields, free radicals and quinone contents are slightly higher in slow pyrolysis. The graphite crystals in bio-char have the tendency of microcrystallization with the increase of temperature, in which progress the fast pyrolysis has superiority at the low temperature.
1. Introduction Biomass refers to various carbon-containing renewable resources formed by photosynthesis, including all animals, plants and microorganisms etc [1]. The utilization of biomass would address many ⁎
essential needs in energy and environment. Pyrolysis is commonly regarded as a simple and effective technology for biomass transformation, in which the biomass is conversed into bio-tar, pyrolysis gases (CO, CO2, CH4 and H2, etc.) and bio-char [2]. The main products of pyrolysis are bio-tar and pyrolysis gas which have been widely studied and
Corresponding author. E-mail address: [email protected] (G. Yin).
https://doi.org/10.1016/j.fuel.2019.116450 Received 31 August 2019; Received in revised form 14 October 2019; Accepted 17 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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utilized, while the bio-char is usually considered as a by-product. In fact, the bio-char has a broad spectrum of application, such as fertilizer in agricultural production, precursor of porous carbon material and fuel for burning etc [3,4]. Techno-economic studies suggested that the biomass pyrolysis is not very competitive without valuing the bio-char as an important product [5]. The physicochemical properties of bio-chars depend not only on the nature of the starting biomass [6–8], but also, to a very large extent, on the condition of preparation. According to the heating rate, there are two main pyrolysis technologies: slow pyrolysis (10 °C/min) and fast pyrolysis (102 °C/sec) [9]. There is an obvious difference that the biochar yield of fast pyrolysis is less than that of slow pyrolysis, indicating that the fast pyrolysis has a greater ablation degree of raw materials. This results in the pore structure of bio-char produced by fast pyrolysis being more developed than that by slow pyrolysis [6–8,10]. Apart from the pore structure, other physicochemical properties, such as surface functional group and crystal structure etc, can also affect the bio char performances. At present, many important physicochemical properties of bio-char during the fast and slow pyrolysis have not been detailedly reported in the literature. In sum, it is necessary to study the similarities and differences of bio-char formation in the fast and slow pyrolysis. To reveal the formation of bio-char, the following three aspects should be studied in detail. Firstly, the chemical mechanism of pyrolysis is generally considered as the free radical reactions caused by covalent bond breaking. Therefore, it is important to tackle these reactions. The group of Liu Zhenyu at Beijing University of Chemical Technology has conducted a series of studies on free radical reactions in coal pyrolysis [11–13]. These studies about free radical reaction mechanism of coal pyrolysis could be helpful to reveal the fundamental chemistry of biochar. The second is the composition of surface oxygen-containing functional groups which may affect on the formation of bio-char [14]. The performance of bio-char is often related to the oxygen-containing surface functional groups [15–17]. Therefore, the studies on the change of surface functional groups in pyrolysis process are helpful to both obtain the formation mechanism of bio-char and control the content of functional groups in the light of different applications. Besides, as a kind of carbon materials, the basic framework of bio-char is a cross-link structure of graphite crystallite. The graphite crystallite structure not only determines the hardness of bio-char, but also influences its performance [18,19]. Therefore, the change of graphite crystallite in pyrolysis needs to be studied. This work studied the fast and slow pyrolysis of walnut shell at 400–800 °C in an inert nitrogen atmosphere. The bio-chars were characterized by electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Through studying the bio-chars formation in the fast and slow pyrolysis, it could deepen the understanding of biomass pyrolysis reaction mechanism and is helpful for the directional preparation of bio-char according to different applications.
Table 1 Compositions analysis of walnut shell in this study. Lignocellulose content (%) Lignin 49.12
Cellulose 31.19
Hemicellulose 16.59
Ultimate analysis (wt.%, daf) H 5.64
S 0.25
N 0.16
Ca 0.08
K 0.05
Fe 0.02
Na 0.01
0.35
4
0.30
3
0.25
2
0.20
1
0.15
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500
600
700
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O 44.17
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o
Temperature ( C) Fig. 1. Yields and free radical concentrations of bio-chars. Square: F-P, Circle: S-P; Solid symbols: bio-char yields, Open symbols: free radical concentrations.
quickly pushed into the furnace constant temperature zone for 15 min. For slow pyrolysis (termed as S-P), the sample on the furnace constant temperature zone was heated from room temperature to the final temperature (400–800 °C) at a heating rate of 10 °C/min. The bio-char was collected after cooling to room temperature in the inert atmosphere. 2.3. Bio-char characterization The Electron Paramagnetic Resonance (EPR) used to determine the free radical concentration is EMXnano (Bruker, Germany) operated at 9.6 GHz and 0.3162 mW. The central field was 3431.20 G, the sweep width was 200.0 G, the sweep time was 30.16 s, the number of scans was 10 times, the modulation amplitude was 1.000 G, and the receiver gain was 20 dB. All the EPR tests were carried out at room temperature and the collected free radical signals were secondarily integrated using Xenon software to automatically calculate the number of spins. In addition, the capillary showed little influence on EPR measurement. The chemical state of elements was studied by using X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher, ESCALAB 250 xi), using monochromatized Al Kα irradiation. The crystal structure of the samples was tested using the X-ray diffraction (XRD, Rigaku SmartLab III, Japan) with Cu Kα radiation (0.154 nm) scan in the range of 10-80° and scanning speed of 5°/min.
2. Materials and methods 2.1. Raw material The walnut shell was ground and sieved to 20–40 mesh, then dried at 120 °C for 4 h. The lignocellulose contents and ultimate analyses were summarized in Table 1.
3. Results and discussion
2.2. Pyrolysis experiments
3.1. Bio-char yield
The pyrolysis experiments were carried out in a quartz tube furnace that includes a quartz crucible loaded with 5 g biomass sample. Before the experiment, the setup was purged by the purified nitrogen (99.999%) at a rate of 150 mL/min for 30 min. Fast pyrolysis (termed as F-P) was started by preheating the quartz tube to the specified temperature (400–800 °C), then quartz crucible loaded with sample was
Fig. 1 shows that the bio-char yields in the F-P and S-P decrease with the increase of pyrolysis temperature. When the pyrolysis temperatures are larger than 600 °C, the decline trend is slowing, indicating that the pyrolysis at less than 600 °C may be the main stage of weight loss [20,21]. Compared the F-P with S-P, it can be seen that the bio-char yields of F-P are lower than those of S-P at the same pyrolysis 2
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Fig. 2. A possible reaction model for generation of free radical fragments in biomass pyrolysis.
lignin has. Thus, the lignin selected as the model compound was used to study the free radical reaction during the bio-char formation, as shown in Fig. 2. Referring to the coal pyrolysis process, it can be found that the covalent bonds with energy less than 400 kJ/mol are broken at around 400 °C, and the covalent bonds with energy less than 500 kJ/mol are broken at around 500 °C [26]. This study proposed a possible reaction model for explaining the broken of covalent bonds to generate the free radical fragments shown in Fig. 2. By corresponding to the trend of biochar yields in Fig. 1, it could be concluded that the bio-chars obtained at the pyrolysis temperature between 400 and 500 °C are the residual structure of raw materials due to the release of free radical fragments. Liu et al. reported that the condensation reaction could enhance the aromatization of bio-char through decreasing the free radical concentrations in bio-char [27]. This study showed that the reduction of free radical concentrations in bio-chars obtained from 600 to 700 °C should be related to the condensation reaction. In addition, when the pyrolysis temperature is larger than 800 °C, the condensation reaction among the free radical fragments may be completed. It is noteworthy that the free radical concentrations of bio-chars from the S-P are greater than those from the F-P at 400–500 °C. The biomass pyrolysis volatile also contains free radical fragments, which could deposit on the surface of bio-chars or react with the bio-chars [28–30]. Because of the fast heating rate in the F-P, the rate of free radicals formation may be higher than that of free radicals’ deposition or reaction, which could lower the free radical concentrations of biochars. This may be the reason why the S-P has the higher bio-char yields in Fig. 1.
temperature, which is consistent with the results reported in the literature [6–8]. It suggests that the F-P is more conducive to the release of volatile. However, the S-P is more favorable for the bio-char production [22]. This may lead to the significant differences in physicochemical properties of bio-char derived from the F-P and S-P. 3.2. Free radical reaction As mentioned in the introduction, the fundamental chemistry of pyrolysis is the free radical reactions caused by covalent bond breaking. The breaking of covalent bonds will inevitably result in the generation of free radical fragments in the bio-chars. Fig. 1 shows that the free radical concentrations of bio-chars in the F-P have the same tendency as that in the S-P. With the increase of pyrolysis temperature, the free radical concentrations increase between 400 and 500 °C, then decrease at around 600 °C. When the temperatures are larger than 700 °C, the free radical concentrations are close to zero (no paramagnetic signal detected by EPR). It is generally considered that the main components of biomass are hemicellulose, cellulose and lignin [23]. While the biomass is pyrolyzed, the free radicals of the bio-char may come from the decomposition of cellulose, hemicellulose and lignin. Considering that the detailed chemical structures of hemicellulose, cellulose and lignin in biomass are not clear, many kinds of model compounds were proposed to study their pyrolysis behavior [23–25]. The breakdown temperatures of hemicellulose, cellulose and lignin are 220–315 °C, 315–400 °C and 160–900 °C, respectively [23]. According to the decomposition temperatures, the cellulose and hemicellulose have been completely decomposed at the pyrolysis temperature of this work. In the light of the law of covalent bond breaking, the products of cellulose and hemicellulose decomposition should have the same bond energies as the 3
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the above results, it can be concluded that the temperature of 600 °C is a critical point of walnut shell in pyrolysis. At pyrolysis temperature less than the critical point, the main chemical reaction is the broken of covalent bonds to generate free radical fragments. At pyrolysis temperature larger than the critical point, whereas, the condensation reaction plays the major role, which is almost the same as the coal pyrolysis process [11,26]. The total contents of the other three peaks in C1s spectrum, reflecting the functional groups of C and O, have the opposite results of peak 1, as shown in Fig. 3(C). The oxygen-containing functional groups were further analyzed by the O1s spectra, as shown in Fig. 4 (A) and (B). The deconvolution of O1s spectra yielded four peaks [31,32]: peak 1 (531.0–531.9 eV), C]O in quinone; peak 2 (532.3–532.8 eV), C]O in ester or anhydrides, O–H in hydroxyl; peak 3 (533.1–533.8 eV), C–O in ester and anhydrides; peak 4 (534.3–535.4 eV), –COOH. It can be seen from Fig. 4(C) that the content of each peak has the same trend for the F-P and S-P, which suggests that the F-P and S-P follow the identical fundamental chemistry processes. However, each content of the four peaks in the F-P and S-P is different at the same pyrolysis temperature. The quinone contents (Peak 1) of bio-chars in the S-P are greater than those in the F-P. Through identifying the contents of peaks 2 and 3, it can be determined that the hydroxyl group content in the F-P is greater. However, the contents of ester and anhydride (Peak 3) in the S-P are higher. With the increase of temperature, the content of carboxyl group (Peak 4) does not change significantly and maintains at about 15%. The above results indicate that a certain functional group may be decomposed at a specific pyrolysis temperature, but it could be regenerated by the free radical reactions. In the four kinds of oxygen-containing functional groups, the quinone content increases with the increase of temperature, which indicates that the quinone could be the stable existing structure of oxygen element in bio-char.
3.3. Surface functional groups Mettler et al. reported that the formation of bio-char depends on the functional groups of each molecule in biomass [14]. As can be seen from Table 1, the main elements in the raw material are C and O, whose proportions are larger than 93%, suggesting that the functional groups in bio-chars are mainly composed of C and O. The surface functional groups of bio-chars in the F-P and S-P were analyzed by XPS (C1s and O1s spectra). The deconvolution of C1s spectra yielded four peaks [31,32]: peak 1 (284.6 eV), C–C and C]C; peak 2 (286–286.3 eV), C–O in phenolic, alcohol, ether; peak 3 (287.3–287.6 eV), C]O in carbonyl, quinine; peak 4 (288.8–289.1 eV), O–C]O in carboxyl, ester. For C1s spectra shown in Fig. 3(A) and (B), the peak 1 reflects the content of C–C and C]C. Fig. 3(C) shows that the change of peak 1 contents in both F-P and S-P both contains two stages: (1) the contents increase with the increase of pyrolysis temperature from 400 to 600 °C. (2) The contents decrease at temperature lager than 600 °C. Fig. 2 shows that the partial oxygen-containing structures are decomposed at low temperature, which leads to the increase of C content and decrease of oxygen-containing groups in bio-chars [33–35]. When the pyrolysis temperature exceeds 600 °C, the C–C bonds of lignin are broken, which leads to the C content of bio-chars decreasing [6–8]. It can be seen that the bio-chars in the S-P have the higher peak 1 contents than those in the F-P at 400–600 °C. However, the peak 1 contents of bio-chars in both F-P and S-P are almost the same at temperature larger than 600 °C. This trend is consistent with the change of free radicals in Fig. 1. But beyond this, the ratios of C and O in bio-chars at different temperatures are shown in Fig. 4(D). It is observed that the ratio increases rapidly within the temperature less than 600 °C, thereafter it becomes slow from 700 to 800 °C both in F-P and S-P. Based on 4
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3.4. Analysis of crystal structure
The bio-char has been widely applied in the liquid separation, such as heavy metal ions and dyes, etc. The adsorption of Pb2+ is related to the oxygen-containing group that could combine with Pb2+ by complexation reaction [15,16]. However, the oxygen-containing group could have a bad effect on the adsorption behavior of dyes. Qu et al. reported that the acid oxygen-containing groups could decrease the removal of malachite green [17]. This study showed the evolvement law of oxygen-containing groups, which could provide the suggestion to control the oxygen-containing groups for different adsorption processes.
The above studies show that when the pyrolysis temperature is larger than 600 °C, the condensation is taken place among the free radical fragments, which reactions could not only decrease the free radicals, but also modify the crystal structure of the bio-char. The evolution of graphite crystal structure in the bio-chars was analyzed by XRD, as shown in Fig. 5. Based on the XRD spectrum, the crystal parameters such as the interlayer spacing (d002), the stacking height (Lc) and the transverse size (La) could be calculated by the Bragg and Scherrer equations, respectively [36]. The calculation results of the three parameters are exhibited 5
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Fig. 6. Crystal parameters of the bio-chars calculated by Bragg and Scherrer equations. (A): F-P; (B): S-P.
Acknowledgement
in Fig. 6. The interlayer spacing of graphite microcrystalline in biochars at different pyrolysis temperatures is almost unchanged (about 0.39 nm). The values of stacking height first increase at temperature less than 600 °C then remain at a certain level with temperature increasing. The trend of transverse size is reverse to that of stacking height. These indicate that the graphite crystals in bio-chars tend to microcrystalline with the increase of pyrolysis temperature. Although the change law of graphite crystal structures in the F-P is the same as that in the S-P, there is an obvious difference existing at the 400 °C between the two technologies. It suggests that the microcrystalline in FP is easier at the low pyrolysis temperature. The bio-char has good electrochemistry performances, which is related to the content of graphite crystallite [18]. The variation of graphite crystallite in bio-char has been correlated with the pyrolysis conditions in the above result, which indicates that the electrochemistry performances of bio-char could be well controlled.
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4. Conclusions Although the bio-char yield of slow pyrolysis is higher than that of fast pyrolysis, the formation of bio-char in these two processes follows the same mechanism, which contains two steps with the increase of pyrolysis temperature: Generation of the free radical fragments by the covalent bond broken is the main chemical reaction at temperature less than 600 °C; while condensation of the free radical fragments plays the fundamental role from 600 to 800 °C. With the pyrolysis temperature increasing, the content of quinine group increases gradually; the contents of ester, anhydride and hydroxyl groups decrease; the content of carboxyl group is almost unchanged; the graphite crystals in bio-chars tend to microcrystallize. There are several differences existing for bio-char formation in fast pyrolysis and slow pyrolysis. The slow pyrolysis generates more free radical fragments than fast pyrolysis. The contents of quinine, ester and anhydride groups in bio-chars during slow pyrolysis are larger than those during fast pyrolysis at the same pyrolysis temperature. However, the content of hydroxyl group is opposite. From the change of crystal parameters, the fast pyrolysis facilitates the graphite microcrystalline of bio-char more easily at the low temperature.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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