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Influence of interactions between biomass components on physicochemical characteristics of char
Journal of Analytical and Applied Pyrolysis xxx (xxxx) xxxx
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Influence of interactions between biomass components on physicochemical characteristics of char Jun Hu , Bingxing Jiang, Jing Liu, Yahui Sun, Xiaoxiang Jiang ⁎
⁎
Engineering Laboratory for Energy System Process Conversion & Emission Control Technology of Jiangsu Province, School of Energy & Mechanical Engineering, Nanjing Normal University, Nanjing, 210042, China
ARTICLE INFO
ABSTRACT
Keywords: Biomass Pyrolysis Interaction Char Physicochemical characteristic
To reveal the influence of interactions between biomass components on physicochemical characteristics of char, biomass components (cellulose, xylan, and lignin), component mixtures and real biomass (maple) were pyrolyzed in a fixed-bed reactor. The morphology, elements contents, crystallinity, porosity characteristics, and functional groups distribution of the solid chars were systematically analyzed. Comparisons of the experimental physicochemical characteristics with the predicted values reveal apparent interactions within the components in the formation of char. The mixture of xylan-lignin (X-L) showed the most significant interactions. Interactions resulted in lower yields and better porosity development for all chars. Chars from the mixtures of cellulose-xylan (C-X) and cellulose-lignin (C-L) showed higher crystallinity and less hydroxyl groups than predicted, while char from the mixture of X-L presented lower crystallinity and less CeO bonds than predicted. Maple exhibited stronger interactions than the synthesized ternary mixture. A possible mechanism of interactions between biomass components have been provided focusing on the formation of char.
1. Introduction The increasing consumption of fossil fuels and the associated environmental issues have stimulated the efforts to develop alternative resources. As a carbon-neutral, naturally abundant, and renewable carbonaceous resource, biomass is a promising choice to replace fossil fuels for producing fuels, chemicals, and materials [1]. Biomass is mainly comprised of cellulose, hemicellulose and lignin, in addition to some inorganics and extractives [2]. Distribution of these building components varies significantly across the biomasses, leading to complex structural differences [3]. Pyrolysis is a promising approach to convert biomass into valuable chemicals. A fundamental understanding of pyrolysis mechanism can facilitate the optimization of pyrolysis process and the design of commercial-scale reactors [3,4]. However, due to the multiscale structural complexity of biomass, a thorough understanding of the pyrolysis mechanisms remains challenging. Assuming that the three main components in biomass (cellulose, hemicellulose and lignin) react independently, the pyrolysis behavior of biomass has been predicted from the behavior of the individual components and their proportions in the mixture using the additivity law. Indeed, some researchers reported that negligible interactions exist between the components during pyrolysis process [5–7]. ⁎
Recently, however, many researchers pointed out that the pyrolysis behavior of biomass could not be well explained by the simple superposition of individual components [8–11]. Hosoya et al. investigated the interaction of cellulose-lignin and cellulose-hemicellulose at 800 °C, reporting that the presence of cellulose enhanced the formation of lignin-derived products [12]. Zhao et al. studied the pyrolysis of component mixtures in thermogravimetric analyzer (TG) and Pyroprobe analyzer coupled with Gas chromatography/Mass spectroscopy (Py–GC/MS), also reporting that polysaccharides promoted the formation of phenolics from lignin, while lignin inhibited the formation of sugars from polysaccharides [13]. Couhert et al. studied the flash pyrolysis of mixtures at 950 °C, demonstrating that the gas yields of component mixtures can not be predicted satisfactorily from that of individual components using the additivity law [14]. Yu et al. studied the pyrolysis of component mixtures and real biomass in both TG and a wire mesh reactor, reporting that no interaction was observed between xylan and lignin, while significant interactions existed between cellulose and the other two components [15]. Wu et al. studied the fast pyrolysis of mixtures of cellulose-hemicellulose and cellulose-hemicellulose in a Pyroprobe analyzer, finding that the interactions inhibited the formation of anhydrosugars like levoglucosan [16–18]. These work has achieved remarkable advances, and suggest that significant interactions do exist between the components during pyrolysis. However,
Corresponding authors. E-mail addresses: [email protected] (J. Hu), [email protected] (X. Jiang).
https://doi.org/10.1016/j.jaap.2019.104704 Received 9 July 2019; Received in revised form 9 September 2019; Accepted 29 September 2019 0165-2370/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jun Hu, et al., Journal of Analytical and Applied Pyrolysis, https://doi.org/10.1016/j.jaap.2019.104704
Journal of Analytical and Applied Pyrolysis xxx (xxxx) xxxx
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the interactions results were significantly different across the reports, which could be ascribed to the various materials, mixing methods and pyrolysis conditions, etc. Furthermore, most of these work only focused on evolved products, giving little description of the solid char, another important product from pyrolysis. A thorough understanding of the char can help to establish the pyrolysis mechanism of whole biomass, and facilitate the commercial use of this solid product [19,20]. The structural characteristics of chars from the three individual components have been well elucidated [19,21,22]. Despite these contributions, fundamental knowledge about chars obtained from co-pyrolysis of the components is lacking. To reveal the influence of interactions between biomass components on physicochemical characteristics of char, three main components of biomass (cellulose, xylan, and lignin), component mixtures (cellulosexylan, cellulose-lignin, xylan-lignin and cellulose-xylan-lignin), and real biomass (maple) were pyrolyzed at 500 °C to obtain solid chars. The three components can be mostly pyrolyzed at 500 °C, producing chars retaining obvious features of their parental components [19]. Pyrolysis at higher temperature could result in chars with close properties, which makes it more difficult to analyze the data accurately. The morphology, elements contents, crystallinity, porosity characteristics, and functional groups distribution of the chars were studied by Scanning electron microscopy (SEM), Elemental analysis, X-Ray spectroscopy (XRD), Nitrogen absorption/ desorption, and Fourier transform infrared spectroscopy (FTIR), respectively. Predicted yields and predicted physicochemical characteristics were then calculated for the chars of mixtures and maple, based on a simple additivity theory. By comparing the experimental data with the predicted values, interactions within the components in formation of char have been revealed and a possible mechanism was given.
Table 2 Contents of major inorganic elements in biomass components and maple (mg/ kg, db).
2.1. Materials Commercial microcrystallinie cellulose (Sigma-Aldrich), xylan from beechwood (Sigma-Aldrich), and cellulolytic enzyme lignin from corn stalk (Yanghai Co., China) were used as the biomass components. Beechwood xylan was used to represent hemicellulose. Maple was obtained from a manufacturer in southern Anhui, China. Cellulose, xylan, lignin and maple were dried at 80 °C for 12 h, ground, sieved to a size of 75–180 μm, and then stored in a desiccator for use. Other solvents were purchased from Nanjing Reagent Co. and used as received. The chemical composition of the three components and maple is shown in Table 1. The elemental analysis (C, H, and N) was performed on a Vario ELIII CHN/O analyzer following standard Method JY/T 017–1996. Oxygen content was obtained by difference. Ash content was determined by ashing samples in a muffle furnace (KF 1200, Nanjing Boyuntong Instrument Technology Co. Ltd.) in air at 575 °C, following the procedures proposed by National Renewable Energy Laboratory (NREL) [23]. Notably, to reach a constant solid residue weight, the ashing of cellulose, lignin and maple required 4 h, while that of beechwood xylan required 8 h, indicating that the xylan has a rather high thermal stability. The solid residue amount of xylan was 13.6 wt% after 4 h, and decreased gradually to a constant weight of 3.7 wt% after
a
H
N
O
Cellulose Xylan Lignin Maple
42.3 40.4 60.7 50.2
6.0 6.0 6.1 6.0
0.0 0.0 1.9 0.2
51.7 30.7 29.6 42.1
a b
Ash
Na
K
Mg
Fe
Al
Xylan Lignin Maple
2338 40 29967
14429 2219 30
0 71 969
5 19 3156
22 294 58
15 132 43
2.2. Preparation of mixtures Four mixtures were prepared based on mass ratio, including cellulose-xylan (C-X, 1:1 wt ratio), cellulose-lignin (C-L, 1:1 wt ratio), xylanlignin (X-L, 1:1 wt ratio) and cellulose-xylan-lignin (C-X-L, 1:1:1 wt ratio). To prepare the mixtures, desired amount of the three components were physically mixed. The mixed samples were then mechanically agglomerated at 20 MPa using a hydraulic press machine. After that, the agglomerated mixtures were ground again to a size < 180 μm, and then agglomerated again under 20 MPa. Finally, the agglomerated mixtures were collected and ground to a size < 180 μm for further use. Compared with simple mixing, mixing with a press can offer mixtures with more intimate components contact, and therefore interactions inside the particles could be possible [14].
Table 1 Chemical composition of biomass components and maple (wt%, db). C
Ca
8 h. The ash content of xylan was significantly higher than that of cellulose, lignin and maple. Beechwood xylan from Sigma-Aldrich has been reported to have an extremely high content of ash (4.1–7.0 wt%), possibly due to the alkali-based extractive procedure [2,6,14,24]. The contents of major inorganic elements in biomass components and maple were tested on an inductively coupled plasma atomic emission spectrometry (ICP-AES) system (Leeman Labs Inc., USA). Prior to the tests, the ash samples were acid-digested following standard Method JY/T 015–1996 as follows: 0.1 g ash sample was digested with 8 mL mixture of HNO3/HCl (1/3, v/v). The mixture was stirred for 1 h at ambient temperature, and then heated at 80 °C for another 2 h. After that, the mixture was diluted to 100 mL with deionized water and filtered. The filtrate was then subjected to the ICP-AES system to determine the contents of six main inorganic species of Ca, Na, K, Mg, Fe, and Al. Each run was conducted in duplicates to ensure the accuracy of the results. Table 2 shows the contents of major inorganic elements in biomass components and maple. As can be found, the ash of xylan has extremely high contents of sodium and calcium, which might be attributed to the alkali-based extractive procedure [25,26]. The ash of maple contains high levels of calcium, magnesium, and potassium. The contents of cellulose, hemicellulose, lignin and extractives in maple were determined on a ANKOM200 Fiber Analyzer following the procedure modified from Van Soest method [27–31]. In brief, 1 g of maple was extracted with neutral detergent (solution of sodium dodecyl sulfate, ethylenediaminetetraacetic disodium salt, sodium borate, sodium phosphate dibasic, and triethylene glycol), digested with mild acid detergent reagent (1N sulfuric acid) and digested with 72% H2SO4 sequentially, to obtain the amount of neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL). Ash amount has been determined previously following the procedure of NREL. The contents of the components were calculated as: lignin = ADL-ash; cellulose = ADF-ADL; hemicellulose = ADF-NDF. The difference between the starting maple and NDF was considered to be the amount of extractives. Maple has 52.4 ± 1.2 wt% cellulose, 18.3 ± 1.6 wt% hemicellulose, 24.4 ± 0.0 wt% klason lignin and 3.5 ± 0.3 w% extractives, and these values are close to the literature data [32].
2. Materials and methods
Sample
Sample
b
0.0 3.7 1.6 1.4
2.3. Pyrolysis procedure
Calculated by difference. Cellulose, lignin and maple were ashed for 4 h. Xylan was ashed for 8 h.
Pyrolysis was conducted to obtain chars in a well-built horizontal 2
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tubular resistance reactor (TL1200, Boyuntong Instrument Technology Co. Nanjing, China) under inert atmosphere. The quartz tube with a 50 mm inner diameter and a 600 mm length is heated by a tube furnace with a temperature-controller device. The apparatus allows a stable heating rate of 10–100 °C/min and a target temperature up to 1150 °C. For a typical run, 1 g of prepared sample was loaded in a ceramic crucible holder and fed into the center of the quartz tube close to the Ktype thermocouple. Prior to the heating, air in the quartz tube was swept out with a nitrogen (99.999%) flow of 1.0 L/min for 20 min. Then, the nitrogen flow was lowered to 0.6 L/min, and the reactor was heated from ambient temperature to 500 °C by electricity at a heating rate of 20 °C/min, and held at 500 °C for another 2 h. After that, the reactor was cooled to ambient temperature with a 0.6 L/min nitrogen flow. Evolved products formed during the pyrolysis process were swept out by the nitrogen flow to inhibit possible interactions between evolved products and solid residue. Because char was mainly focused in this work, bio-oils and non-condensable gases were not collected. The solid product was weighed and ground into < 180 μm size for characterization. Char of cellulose, xylan, lignin, C-X mixture, C-L mixture, X-L mixture, C-X-L mixture, and maple were denoted as Char-C, Char-X, Char-L, Char-C-X, Char-C-L, Char-X-L, Char-C-X-L, and Char-M, respectively. The mass yields of chars (Yi, wt%) were calculated as below,
equation as,
2.5. Interaction analysis Assuming that the pyrolysis behavior of overall biomass is the weighted sum of the partial contributions of each component, the pyrolysis characteristics of the mixtures and maple can be predicted following the additivity law, Amix,
(4)
(5)
Pr
Where RA is the experimental/predicted ratio (R) of yield or physicochemical characteristic (A); Amix is the experimental yield or physicochemical characteristic (A) of the mixtures and maple. If no interaction exists within the components, RA should equal to 1.0. Due to the inevitable experimental error, R between 0.95 and 1.05 could be within the error limits. RA larger than 1.05 indicates an obvious enhancement in A by interaction, while RA smaller than 0.95 indicates an obvious weakening in A. Larger deviation of RA from 1.0 indicates stronger interaction.
Elemental analyses (C, H, and N) of the samples were performed on a Vario ELⅢ CHN analyzer following the JY/T 017–1996 Method. The oxygen content was determined by difference. Prior to SEM analysis, the chars were gold sputtered. SEM images were captured with a JEOL JSM-6300 SEM system. Nitrogen absorption/desorption was carried out on a fully automated volumetric sorption analyzer (Quantachrome Nova 2000e) using nitrogen as the adsorbate at −196 °C. Prior to the gas adsorption measurements, the samples were outgassed at 120 °C for 2 h. Brunauer-Emmett-Teller (BET) theory and Barrett-Joyner-Halenda (BJH) theory were used to determine the surface area and volume, respectively. XRD for the crystallinity was performed in a D/max 2500 V L/PC X-ray diffractometer using Cu Kα radiation (40 kV, 200 mA). Scan was performed over an angular 2θ range of 5-90° at 0.5°/ min. The interlayer spacing d002 (nm) was determined using the Bragg equation as [33],
3. Results and discussion 3.1. Char of individual biomass components Table 3 lists the char yields and physicochemical characteristics of individual biomass components. Lignin produced the most char, in agreement with the results reported in literature [2,34]. Different from cellulose and hemicellulose which are composed of polysaccharides, lignin consists of phenylpropane units cross-linked heavily [35]. This structure makes lignin highly thermal-stable. Usually, lignin is the part mainly responsible for char production in real biomass [5]. The yield of xylan was also rather high. The higher char yield for xylan than cellulose should be partially ascribed to its lower crystallinity, which could lead to more dehydration reactions at low temperature, resulting in a higher reticulated and stable matrix to form char [36]. Besides, the extremely high contents of inorganic species in xylan could also
(2)
002
=AC×fC+ AX×fX+ AL×fL
RA = Amix / Amix,
2.4. Characterization methods
2sin
Pr
Where Amix, Pr is the predicted yield or physicochemical characteristic (APr) for the mixtures and maple; AC, AX, AL are the experimental yields or physicochemical characteristics (A) for cellulose, xylan and lignin, respectively; and fC, fX, and fL are the mass fractions of cellulose, xylan and lignin present in the corresponding mixture and maple. The experimental/ predicted ratios of the mixtures and maple are calculated as [14],
Where Wi is the mass weight of the solid char i, and W0 is the mass weight of the initial sample. In the present study, all sample preparation and corresponding pyrolysis experiments were performed in at least duplicates. The difference in char yields for all runs was lower than 0.8 wt%, indicating that the experiments have a good repeatability. All mass yields are expressed as the average of the replicates.
d 002 =
(3)
002
Where B002 is the half-width of the (002) peak. In this work, the shape factor of 0.9 was used to calculate Lc [33]. The FTIR spectra were recorded on a FTIR spectrophotometer (Bruker Vector 22). About 1% sample was ground with KBr, and then the mixture was pressed into a transparent wafer. Sixteen scans were conducted for each sample in the range from 4000 cm−1 to 400 cm−1 with 4 cm−1 resolution.
(1)
Yi = W/W i 0 ×100%
0.9 B002 cos
Lc =
Where λ is the X-ray wavelength and θ002 is the scattering angle for the (002) peak. Stack height of the aromatic carbon sheets Lc (nm) was determined from the half-width of the (002) peak using the Scherrer Table 3 Char yields and physicochemical characteristics of biomass components a. Char
Char-C Char-X Char-L a
Yield (wt%)
18.1 36.4 51.6
Element content
Crystallinity characteristics
Porosity characteristics 2
3
Chemical characteristics
N (wt%)
C (wt%)
H (wt%)
d002 (nm)
Lc (nm)
S (m /g)
V (cm /g)
RIH
RIA
0.0 0.0 0.5
84.0 72.7 80.4
3.5 3.2 3.6
0.37 0.40 0.39
0.61 0.51 0.66
20.32 6.46 2.94
0.0213 0.0105 0.0026
0.79 0.96 0.77
0.95 1.44 1.67
S, BET surface Area; V, pore volume; RIH, relative intensity of hydroxyl groups; RIA, relative intensity of CeO bonds in aryl ethers. 3
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Fig. 1. SEM micrographs of starting materials and chars.
promote the formation of char [24]. Fig. 1 shows the SEM micrographs of the starting materials and the chars. The starting cellulose is of obvious parallel vascular bundles (C in Fig. 1). After pyrolysis, the vascular bundles were well retained, with some debris particles on the surface (Char-C in Fig. 1). The starting xylan is of large irregular polygonal blocks with smooth surface (X in Fig. 1). After pyrolysis, the large blocks were cracked into small pieces, doped with fluffy deposits possibly due to carbon deposition (Char-X in Fig. 1) [19]. The original lignin was of round particles with various sizes (L in Fig. 1). The char of lignin, however, was of large agglomerates with non porous and few cavities, indicating that the melting of lignin and agglomeration of lignin char took place during pyrolysis (Char-L in Fig. 1) [7]. Few macropore could be found on the surface of the three chars.
Elemental analysis revealed that the C content in the three components increased greatly from 40.4–60.7 wt% to 72.7–84.0 wt% after pyrolysis. The high C content and low H content were mainly due to the devolatilization of aliphatic functional groups from the chars and the aromatization and carbonization of char [19,37]. The C content of cellulose was higher than that of xylan char and lignin char. Fig. 2 shows the XRD patterns of chars from biomass components, together with binary mixtures, ternary mixture and maple. XRD patterns of chars of the three components showed broad peaks at 24° of (0 0 2) band and 44° of (1 0 0) band, corresponding to the stacking and inplane of aromatics layers, respectively [38,39]. The interlayer spacing distance d002 for the chars ranged from 0.37 nm to 0.40 nm, which were greater than that for graphite (0.33 nm), indicating the existence of short substituents, such as hydroxyl groups and aryl ethers [33]. 4
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Fig. 2. XRD patterns of chars.
Fig. 3. FTIR spectra of chars.
Complete removal of these substituents requires temperatures higher than 500 °C [36]. Xylan char had the largest d002 value of 0.40 nm and the lowest Lc value of 0.51 nm, indicating a relatively poorly ordered structure, consistent with its abundant substituents to be revealed below. Xylan char also displayed three sharp peaks at 2θ = 29.4°, 39.4°, and 43.1° for (1 0 4), (1 1 3), and (2 0 2) corresponding to CaCO3 (JCPDS file: 05-0586), and two peaks at 2θ = 30.4° and 27.1° for (2 1 0) and (2 0 0) corresponding to NaClO3 (JCPDS file: 05-0610), in agreement with the results revealed by ICP-AES (see Table 2). NaClO3 has been used for the preparation of xylan from biomass. Nitrogen absorption/desorption analysis showed that the char of cellulose had the highest specific BET surface area of 20.32 m2/g together with the highest pore volume of 0.0213 cm3/g. The char of lignin possessed a rather low surface areas of 2.94 m2/g and a low pore volume of 0.0026 cm3/g, close to the literature data [22]. The surface areas of chars were lower than some reported biochars, which is primarily due to the incomplete removal of volatile matters at 500 °C [19,40]. This is in agreement with the high char yields as compared with the previous results at both lower and faster heating rate [19,37]. Besides, the melting of lignin during pyrolysis to form a compact surface after cooling could also contribute to its extremely low surface area and pore volume [22]. Fig. 3 shows the FTIR spectra of chars from biomass components, together with binary mixtures, ternary mixture and maple. FTIR spectra of individual components chars showed obvious peaks for OeH stretching in hydroxyl groups at 3424 cm−1, vibrations of aromatic skeletal at 1600 cm−1 and CeO stretching in aryl ethers at 1160 cm−1 [41,42]. Peaks for functional groups like methyl groups, carbonyl groups, and glycosidic groups disappeared after pyrolysis [19]. The absorbance intensity at a specific wavenumber is considered to be linear with the functionality groups concentration, when the absorbance is under 0.5 [38]. Herein, for semi-quantitative analysis, relative intensity for hydroxyl groups (RIH) and CeO bonds in aryl ethers (RIA) were calculated, using the intensity of aromatic rings at 1600 cm−1 as the reference. The char of xylan showed the highest RIH and a second highest RIA, in line with its relatively poorly crystallinity described above. The char of cellulose showed the low values for both RIH and RIA, consistent with its highly ordered crystallinity.
Values of RYield for the binary mixtures ranged from 0.55 to 0.89, indicating that the formation of char was inhibited during the co-pyrolysis process. The mixture of X-L had the lowest RYield of 0.55, showing that the interaction between xylan and lignin was the most significant in terms of char yield. Yang et al. investigated the pyrolysis of synthetic mixtures with different proportion of components, and they found almost no significant interaction among the three components [5]. Yu et al. reported that the pyrolysis of mixtures of C-X and C-L produced more char than predicted, while the mixture of X-L gave char yield matching well with the predicted value [15]. Hosoya et al. and Volpe et al. concluded that the mixture of C-L resulted in less char than predicted [12,43]. Focusing on slow steam pyrolysis at 600 °C, Giudicianni et al. also observed a lower yield than predicted for all binary mixtures [40]. The same group then expanded the study considering the influence of inorganic species, reporting that both inorganics species and lignin can significantly influence the devolatilization of cellulose [25]. As can be found, the results of interactions were significantly different across the reports, which could be ascribed to the difference in starting materials, mixture preparation methods, proportion of components in the mixture, sample amounts, and pyrolysis conditions. It seems that copyrolysis with low-condensed lignin, like milled wood lignin and cellulolytic enzyme lignin, could lead to less char yields than predicted, as compared with Kraft lignin and Alkali lignin. Besides, compared with simple mixing, mixing with a press could offer mixtures with more intimate components contact, and result in more intense interactions [14]. According to the SEM micrographs, obvious vascular bundles of cellulose-derived char, together with some fluffy deposits derived from xylan, can be found for the char of C-X mixture (Char-C-X in Fig. 1). The C-L mixture char also exhibited the vascular bundles which, however, was highly dispersed, likely due to the presence of lignin (Char-C-L in Fig. 1). Such structural change reveals a stronger interaction on morphology for C-L mixture than C-X mixture. On the surface of X-L mixture char, many macropores appeared, and some fragments and needlelike particles were adhered which might be the result of violent volatilechar interactions (Char-X-L in Fig. 1) [39]. Considering the slight differences in H and N contents for the mixtures and components, only C content was used to evaluate the effect of interactions on elemental distribution. The char of X-L mixture has the lowest RC value of 0.89, indicating that the interaction on C content between xylan and lignin was most significant. Apparently, co-pyrolysis with xylan led to more C removal than predicted. All chars of binary mixtures had d002 values close to the predicted
3.2. Char of mixtures and maple Table 4 lists the char yields and physicochemical characteristics of binary mixtures, together with the experimental/predicted ratios. 5
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Table 4 Char yields and physicochemical characteristics of binary mixtures. Char
Char-C-X Char-C-L Char-X-L
Yield (wt%)
21.9 30.9 24.3
RYield
0.80 0.89 0.55
Element content
Crystallinity characteristics
Porosity characteristics 2
Chemical characteristics 3
C (wt%)
RC
d002 (nm)
Rd002
Lc (nm)
RLc
S (m /g)
RS
V (cm /g)
RV
RIH
RRIH
RIA
RRIA
75.5 83.9 68.0
0.96 1.02 0.89
0.38 0.38 0.38
0.99 1.02 0.97
0.60 0.65 0.55
1.08 1.03 0.94
7.92 6.86 9.04
0.59 0.59 1.92
0.0123 0.0104 0.0098
0.77 0.88 1.51
0.50 0.65 0.88
0.57 0.84 1.02
1.24 1.29 1.29
1.04 0.99 0.83
values. The Lc for char of X-L was much lower than estimated, indicating a poor crystallinity. Co-pyrolysis of cellulose and xylan, however, delivered char with a higher Lc than estimated, indicating an enhanced crystallinity development. The char from the samples containing xylan also displayed obvious peaks at 2θ = 29.4°, 39.4°, and 43.1° for (1 0 4), (1 1 3), and (2 0 2) corresponding to CaCO3 (JCPDS file: 05-0586), and peaks at 2θ = 30.4° and 27.1° for (2 1 0) and (2 0 0) corresponding to NaClO3 (JCPDS file: 05-0610). Besides, obvious diffraction peaks at 2θ = 31.7°, 45.5°, and 56.5° for (2 0 0), (2 2 0), and (2 2 2) corresponding to NaCl (JCPDS file: 05-0628) could also be assigned, indicating that NaClO3 has been decomposed into NaCl during the co-pyrolysis. Chars of C-X and C-L binary mixtures offered BET surface areas (6.86–7.92 m2/g) and pore volumes (0.0104–0.0123 cm3/g) significantly lower than the predicted values, indicating that co-pyrolysis of cellulose with xylan or lignin inhibited the formation of released volatiles and gaseous products [17]. The mixture of X-L, however, produced char with higher surface areas and pore volumes than predicted. According to the values of RS and RV, the co-pyrolysis of xylan and lignin had the strongest interaction on porosity development, followed by the co-pyrolysis of xylan and lignin. FTIR revealed that binary mixtures with cellulose gave chars with less hydroxyl groups than predicted, while the mixture of X-L produced chars with less aryl ethers than predicted. According to the values of RRIH and RRIC, co-pyrolysis of cellulose and xylan had the strongest interaction on chemical characteristics. Table 5 lists the char yields and physicochemical characteristics of ternary mixture and maple, together with the experimental/predicted ratios. The RYield for the ternary mixture and maple were 0.85 and 0.68, respectively, indicating that interaction in real biomass was more significant, consistent with the literature results [6,15]. The low RYield for real biomass is due in part to the existence of extractives and lignin–carbohydrate complex (LCC). LCC structures linking lignin and carbohydrate moieties, such as phenol glycoside linkages and benzyl esters, is easily decomposed to release evolved products to interact with the three main components [44]. Besides, the lignin in real biomass is also more reactive with abundant linkages like α-O-4 and β-O-4. Generally, lignin with more ether linkages produces less char [45,46]. Morphology of the chars from the ternary mixture showed irregular complex structure with abundant debris particles and pores, indicating significant interactions within the components (Char-C-X-L in Fig. 1). Maple char also presented complex structure with rough surface and abundant pores (Char-M in Fig. 1). The predicted carbon contents for both ternary mixture char and maple char were close to the experimental results. For crystallinity characteristics, however, the char of the ternary mixture showed a lower Lc of 0.55 nm than maple char of 0.59 nm, likely due to its
relatively high amount of hemicellulose which would lead to a lower ordered structure. Besides, the char of maple displayed three diffraction peaks at 2θ = 26.6°, 20.8°, and 50.1° for (1 0 1), (1 0 0), and (1 1 2) corresponding to SiO2 (JCPDS file: 46–1045), two peaks at 2θ = 28.4°, and 40.5°for (2 0 0), and (2 2 0) corresponding to KCl (JCPDS file: 41–1476), and three peaks at 2θ = 29.4°, 39.4°, and 43.1° corresponding to CaCO3 (JCPDS file: 05-0586), corresponding well with the results of ICP-AES (see Table 2). Both the chars of ternary mixture and maple showed lower BET surface areas than predicted. According to the values of RS and RV, interactions on porosity development are stronger in maple than that in the ternary mixture. FTIR reveals much more aryl ethers for the char of ternary mixture than predicted, indicating that reactions like crosslinking may occur within the components during pyrolysis. For the char of maple, the relative intensity of hydroxyl groups was substantially higher than predicted, which could be attributed to its abundant LCCs and lignin rich in weak ether linkages [3]. According to above results, there are significant interactions within the three components. According to the experimental/predicted ratios, pyrolysis of X-L mixture showed the most significant interaction among the binary mixtures, which might be attributed to their melted intermediates and wide pyrolysis temperature range [36]. The melted intermediates of xylan and lignin can penetrate and diffuse into each other, causing intense interactions during the pyrolysis across the wide pyrolysis temperature range. The more intense interactions in the present work than some literatures could be partially attributed to the sample preparation method. Compared with simple mixing, mixing with a press in this work offered mixtures with more intimate components contact, and therefore interactions inside the particles could be possible [14]. Components analysis has revealed that maple contains 52.4% cellulose, 18.3% hemicellulose, and 24.4% lignin. Based on the contribution of each component, interactions within the components in maple should be less intense than that in the ternary mixture. According to the experimental/predicted ratios, however, pyrolysis mixture of maple showed more significant interactions than the mixture of ternary mixture. This might be attributed to the existence of extractives and LCC in real biomass, which need further research [14,44]. 3.3. Possible mechanisms of interactions between the components Generally, pyrolysis mechanism of biomass consists of primary mechanisms and secondary mechanisms. Primary mechanisms include depolymerization and fragmentation to produce released products, and rearrangement within the residue to form char matrix. Secondary mechanisms include cracking to form lower molecular weight compounds and recombination to form higher molecular weight compounds, like
Table 5 Char yields and physicochemical characteristics of ternary mixture and maplea. Char
Char-C-X-L Char-M
Yield (wt%)
30.1 19.6
RYield
0.85 0.68
Element content
Crystallinity characteristics
C (wt%)
RC
d002 (nm)
Rd002
Lc (nm)
RLc
S (m2/g)
RS
V (cm3/g)
RV
RIH
RRIH
RIC
RRIC
78.9 73.2
1.00 0.95
0.39 0.39
1.03 1.10
0.55 0.59
0.94 1.03
8.16 7.47
0.82 0.60
0.0121 0.0093
1.06 0.68
0.76 1.06
0.90 1.36
2.21 0.77
1.64 0.66
6
Porosity characteristics
Chemical characteristics
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Fig. 4. Main reactions occurring in biomass pyrolysis against the temperature.
carbon deposits or PAHs [3]. Main reactions occurring in biomass pyrolysis have been summarized by Collard et al. as presented in Fig. 4 [36]. Based on these existing mechanisms and the above experiment results, possible mechanisms of interactions during biomass pyrolysis are given below. As the temperature increases to 200 °C, the surface part of hemicellulose and lignin particles begin to melt [36]. The melted intermediates of hemicellulose and lignin will dissolve each other, and cover the surface of cellulose. The decomposition of xylan and lignin is exothermic, while that of cellulose is endothermic [47]. Therefore, the heat produced by the decomposition of xylan and lignin can promote the decomposition of cellulose. As a result, dehydration of cellulose, the competitive reaction path of decomposition, is weakened. The weakening of dehydration contributes partially to the lower char yields of mixtures with cellulose than predicted, because dehydration is mainly responsible for forming highly reticulated char precursors at low temperature [48]. To be released, cellulose derived products, such as levoglucosan and 5-hydroxymethyl furfural, must diffuse across the melted hemicellulose and lignin. During the diffusing process, some reactive fragments, such as phenolic radicals from lignin, could be stabilized into evolved products by the hydrogen-rich products from cellulose and hemicellulose. This stabilization leads to reduced formation of char from the mixtures with lignin [12,43,49]. Releasing of the stabilized evolved products results in a enhanced development of porous structure as identified by SEM (Fig. 1) and Nitrogen absorption/ desorption (Tables 3–5). As the temperature increases to 450 °C, the aromatization degree of benzene rings-rich residue becomes higher due to the removal of remaining short substituents, such as methyl groups, hydroxyl groups and C–O aryl linkages, as revealed by the FTIR (Tables 3–5). Rearrangement reactions will also promote the ordering of the aromatic rings in the char [43].
crystallinity, and less CeO bonds than predicted. Char of the ternary mixture and maple showed similar physicochemical characteristics to the mixture of X-L, and maple showed stronger interactions than the ternary mixture. Possible mechanisms of interactions including the formation of char was provided. This work demonstrates the existence of component interactions and enhances the understanding of biomass pyrolysis mechanisms. Acknowledgements The authors greatly acknowledge the funding support from the projects supported by the National Natural Science Foundation of China (Grant 51706110) and Jiangsu Key Lab of Biomass Energy and Materials (Grant JSBEM201918). References [1] Y. Li, S. Huang, Q. Wang, H. Li, Q. Zhang, H. Wang, Y. Wu, S. Wu, J. Gao, Hydrogen transfer route and interaction mechanism during co-pyrolysis of Xilinhot lignite and rice husk, Fuel Process. Technol. 192 (2019) 13–20. [2] S.D. Stefanidis, K.G. Kalogiannis, E.F. Iliopoulou, C.M. Michailof, P.A. Pilavachi, A.A. Lappas, A study of lignocellulosic biomass pyrolysis via the pyrolysis of cellulose, hemicellulose and lignin, J. Anal. Appl. Pyrolysis 105 (2014) 143–150. [3] S. Wang, G. Dai, H. Yang, Z. Luo, Lignocellulosic biomass pyrolysis mechanism: a state-of-the-art review, Prog. Energy Combust. Sci. 62 (2017) 33–86. [4] Z. Gao, N. Li, M. Chen, W. Yi, Comparative study on the pyrolysis of cellulose and its model compounds, Fuel Process. Technol. 193 (2019) 131–140. [5] H. Yang, R. Yan, H. Chen, C. Zheng, D.H. Lee, D.T. Liang, In-depth investigation of biomass pyrolysis based on three major components: hemicellulose, cellulose and lignin, Energy Fuels 20 (2006) 388–393. [6] K. Raveendran, A. Ganesh, K.C. Khilar, Pyrolysis characteristics of biomass and biomass components, Fuel 75 (1996) 987–998. [7] T.J. Hilbers, Z. Wang, B. Pecha, R.J.M. Westerhof, S.R.A. Kersten, M.R. PelaezSamaniego, M. Garcia-Perez, Cellulose-Lignin interactions during slow and fast pyrolysis, J. Anal. Appl. Pyrolysis 114 (2015) 197–207. [8] S. Wang, X. Guo, K. Wang, Z. Luo, Influence of the interaction of components on the pyrolysis behavior of biomass, J. Anal. Appl. Pyrolysis 91 (2011) 183–189. [9] Y. Long, H. Zhou, A. Meng, Q. Li, Y. Zhang, Interactions among biomass components during co-pyrolysis in (macro)thermogravimetric analyzers, Korean J. Chem. Eng. 33 (2016) 2638–2643. [10] F. Chen, B. Hou, S. Chen, H. Zhang, P. Gong, A. Zhou, Biochemicals distribution and the collaborative pyrolysis study from three main components of Helianthus annuus stems based on PY-GC/MS, Renew. Energy 114 (2017) 960–967. [11] J. Wang, B. Shen, D. Kang, P. Yuan, C. Wu, Investigate the interactions between
4. Conclusion Significant interactions exist within the components during the formation of char. Among all mixtures, the mixture of X-L showed the most significant interaction, giving the least char with physicochemical characteristics of higher development of porous structure, lower 7
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Influence of interactions between biomass components on physicochemical characteristics of char Jun Hu , Bingxing Jiang, Jing Liu, Yahui Sun, Xiaoxiang Jiang ⁎
⁎
Engineering Laboratory for Energy System Process Conversion & Emission Control Technology of Jiangsu Province, School of Energy & Mechanical Engineering, Nanjing Normal University, Nanjing, 210042, China
ARTICLE INFO
ABSTRACT
Keywords: Biomass Pyrolysis Interaction Char Physicochemical characteristic
To reveal the influence of interactions between biomass components on physicochemical characteristics of char, biomass components (cellulose, xylan, and lignin), component mixtures and real biomass (maple) were pyrolyzed in a fixed-bed reactor. The morphology, elements contents, crystallinity, porosity characteristics, and functional groups distribution of the solid chars were systematically analyzed. Comparisons of the experimental physicochemical characteristics with the predicted values reveal apparent interactions within the components in the formation of char. The mixture of xylan-lignin (X-L) showed the most significant interactions. Interactions resulted in lower yields and better porosity development for all chars. Chars from the mixtures of cellulose-xylan (C-X) and cellulose-lignin (C-L) showed higher crystallinity and less hydroxyl groups than predicted, while char from the mixture of X-L presented lower crystallinity and less CeO bonds than predicted. Maple exhibited stronger interactions than the synthesized ternary mixture. A possible mechanism of interactions between biomass components have been provided focusing on the formation of char.
1. Introduction The increasing consumption of fossil fuels and the associated environmental issues have stimulated the efforts to develop alternative resources. As a carbon-neutral, naturally abundant, and renewable carbonaceous resource, biomass is a promising choice to replace fossil fuels for producing fuels, chemicals, and materials [1]. Biomass is mainly comprised of cellulose, hemicellulose and lignin, in addition to some inorganics and extractives [2]. Distribution of these building components varies significantly across the biomasses, leading to complex structural differences [3]. Pyrolysis is a promising approach to convert biomass into valuable chemicals. A fundamental understanding of pyrolysis mechanism can facilitate the optimization of pyrolysis process and the design of commercial-scale reactors [3,4]. However, due to the multiscale structural complexity of biomass, a thorough understanding of the pyrolysis mechanisms remains challenging. Assuming that the three main components in biomass (cellulose, hemicellulose and lignin) react independently, the pyrolysis behavior of biomass has been predicted from the behavior of the individual components and their proportions in the mixture using the additivity law. Indeed, some researchers reported that negligible interactions exist between the components during pyrolysis process [5–7]. ⁎
Recently, however, many researchers pointed out that the pyrolysis behavior of biomass could not be well explained by the simple superposition of individual components [8–11]. Hosoya et al. investigated the interaction of cellulose-lignin and cellulose-hemicellulose at 800 °C, reporting that the presence of cellulose enhanced the formation of lignin-derived products [12]. Zhao et al. studied the pyrolysis of component mixtures in thermogravimetric analyzer (TG) and Pyroprobe analyzer coupled with Gas chromatography/Mass spectroscopy (Py–GC/MS), also reporting that polysaccharides promoted the formation of phenolics from lignin, while lignin inhibited the formation of sugars from polysaccharides [13]. Couhert et al. studied the flash pyrolysis of mixtures at 950 °C, demonstrating that the gas yields of component mixtures can not be predicted satisfactorily from that of individual components using the additivity law [14]. Yu et al. studied the pyrolysis of component mixtures and real biomass in both TG and a wire mesh reactor, reporting that no interaction was observed between xylan and lignin, while significant interactions existed between cellulose and the other two components [15]. Wu et al. studied the fast pyrolysis of mixtures of cellulose-hemicellulose and cellulose-hemicellulose in a Pyroprobe analyzer, finding that the interactions inhibited the formation of anhydrosugars like levoglucosan [16–18]. These work has achieved remarkable advances, and suggest that significant interactions do exist between the components during pyrolysis. However,
Corresponding authors. E-mail addresses: [email protected] (J. Hu), [email protected] (X. Jiang).
https://doi.org/10.1016/j.jaap.2019.104704 Received 9 July 2019; Received in revised form 9 September 2019; Accepted 29 September 2019 0165-2370/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jun Hu, et al., Journal of Analytical and Applied Pyrolysis, https://doi.org/10.1016/j.jaap.2019.104704
Journal of Analytical and Applied Pyrolysis xxx (xxxx) xxxx
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the interactions results were significantly different across the reports, which could be ascribed to the various materials, mixing methods and pyrolysis conditions, etc. Furthermore, most of these work only focused on evolved products, giving little description of the solid char, another important product from pyrolysis. A thorough understanding of the char can help to establish the pyrolysis mechanism of whole biomass, and facilitate the commercial use of this solid product [19,20]. The structural characteristics of chars from the three individual components have been well elucidated [19,21,22]. Despite these contributions, fundamental knowledge about chars obtained from co-pyrolysis of the components is lacking. To reveal the influence of interactions between biomass components on physicochemical characteristics of char, three main components of biomass (cellulose, xylan, and lignin), component mixtures (cellulosexylan, cellulose-lignin, xylan-lignin and cellulose-xylan-lignin), and real biomass (maple) were pyrolyzed at 500 °C to obtain solid chars. The three components can be mostly pyrolyzed at 500 °C, producing chars retaining obvious features of their parental components [19]. Pyrolysis at higher temperature could result in chars with close properties, which makes it more difficult to analyze the data accurately. The morphology, elements contents, crystallinity, porosity characteristics, and functional groups distribution of the chars were studied by Scanning electron microscopy (SEM), Elemental analysis, X-Ray spectroscopy (XRD), Nitrogen absorption/ desorption, and Fourier transform infrared spectroscopy (FTIR), respectively. Predicted yields and predicted physicochemical characteristics were then calculated for the chars of mixtures and maple, based on a simple additivity theory. By comparing the experimental data with the predicted values, interactions within the components in formation of char have been revealed and a possible mechanism was given.
Table 2 Contents of major inorganic elements in biomass components and maple (mg/ kg, db).
2.1. Materials Commercial microcrystallinie cellulose (Sigma-Aldrich), xylan from beechwood (Sigma-Aldrich), and cellulolytic enzyme lignin from corn stalk (Yanghai Co., China) were used as the biomass components. Beechwood xylan was used to represent hemicellulose. Maple was obtained from a manufacturer in southern Anhui, China. Cellulose, xylan, lignin and maple were dried at 80 °C for 12 h, ground, sieved to a size of 75–180 μm, and then stored in a desiccator for use. Other solvents were purchased from Nanjing Reagent Co. and used as received. The chemical composition of the three components and maple is shown in Table 1. The elemental analysis (C, H, and N) was performed on a Vario ELIII CHN/O analyzer following standard Method JY/T 017–1996. Oxygen content was obtained by difference. Ash content was determined by ashing samples in a muffle furnace (KF 1200, Nanjing Boyuntong Instrument Technology Co. Ltd.) in air at 575 °C, following the procedures proposed by National Renewable Energy Laboratory (NREL) [23]. Notably, to reach a constant solid residue weight, the ashing of cellulose, lignin and maple required 4 h, while that of beechwood xylan required 8 h, indicating that the xylan has a rather high thermal stability. The solid residue amount of xylan was 13.6 wt% after 4 h, and decreased gradually to a constant weight of 3.7 wt% after
a
H
N
O
Cellulose Xylan Lignin Maple
42.3 40.4 60.7 50.2
6.0 6.0 6.1 6.0
0.0 0.0 1.9 0.2
51.7 30.7 29.6 42.1
a b
Ash
Na
K
Mg
Fe
Al
Xylan Lignin Maple
2338 40 29967
14429 2219 30
0 71 969
5 19 3156
22 294 58
15 132 43
2.2. Preparation of mixtures Four mixtures were prepared based on mass ratio, including cellulose-xylan (C-X, 1:1 wt ratio), cellulose-lignin (C-L, 1:1 wt ratio), xylanlignin (X-L, 1:1 wt ratio) and cellulose-xylan-lignin (C-X-L, 1:1:1 wt ratio). To prepare the mixtures, desired amount of the three components were physically mixed. The mixed samples were then mechanically agglomerated at 20 MPa using a hydraulic press machine. After that, the agglomerated mixtures were ground again to a size < 180 μm, and then agglomerated again under 20 MPa. Finally, the agglomerated mixtures were collected and ground to a size < 180 μm for further use. Compared with simple mixing, mixing with a press can offer mixtures with more intimate components contact, and therefore interactions inside the particles could be possible [14].
Table 1 Chemical composition of biomass components and maple (wt%, db). C
Ca
8 h. The ash content of xylan was significantly higher than that of cellulose, lignin and maple. Beechwood xylan from Sigma-Aldrich has been reported to have an extremely high content of ash (4.1–7.0 wt%), possibly due to the alkali-based extractive procedure [2,6,14,24]. The contents of major inorganic elements in biomass components and maple were tested on an inductively coupled plasma atomic emission spectrometry (ICP-AES) system (Leeman Labs Inc., USA). Prior to the tests, the ash samples were acid-digested following standard Method JY/T 015–1996 as follows: 0.1 g ash sample was digested with 8 mL mixture of HNO3/HCl (1/3, v/v). The mixture was stirred for 1 h at ambient temperature, and then heated at 80 °C for another 2 h. After that, the mixture was diluted to 100 mL with deionized water and filtered. The filtrate was then subjected to the ICP-AES system to determine the contents of six main inorganic species of Ca, Na, K, Mg, Fe, and Al. Each run was conducted in duplicates to ensure the accuracy of the results. Table 2 shows the contents of major inorganic elements in biomass components and maple. As can be found, the ash of xylan has extremely high contents of sodium and calcium, which might be attributed to the alkali-based extractive procedure [25,26]. The ash of maple contains high levels of calcium, magnesium, and potassium. The contents of cellulose, hemicellulose, lignin and extractives in maple were determined on a ANKOM200 Fiber Analyzer following the procedure modified from Van Soest method [27–31]. In brief, 1 g of maple was extracted with neutral detergent (solution of sodium dodecyl sulfate, ethylenediaminetetraacetic disodium salt, sodium borate, sodium phosphate dibasic, and triethylene glycol), digested with mild acid detergent reagent (1N sulfuric acid) and digested with 72% H2SO4 sequentially, to obtain the amount of neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL). Ash amount has been determined previously following the procedure of NREL. The contents of the components were calculated as: lignin = ADL-ash; cellulose = ADF-ADL; hemicellulose = ADF-NDF. The difference between the starting maple and NDF was considered to be the amount of extractives. Maple has 52.4 ± 1.2 wt% cellulose, 18.3 ± 1.6 wt% hemicellulose, 24.4 ± 0.0 wt% klason lignin and 3.5 ± 0.3 w% extractives, and these values are close to the literature data [32].
2. Materials and methods
Sample
Sample
b
0.0 3.7 1.6 1.4
2.3. Pyrolysis procedure
Calculated by difference. Cellulose, lignin and maple were ashed for 4 h. Xylan was ashed for 8 h.
Pyrolysis was conducted to obtain chars in a well-built horizontal 2
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tubular resistance reactor (TL1200, Boyuntong Instrument Technology Co. Nanjing, China) under inert atmosphere. The quartz tube with a 50 mm inner diameter and a 600 mm length is heated by a tube furnace with a temperature-controller device. The apparatus allows a stable heating rate of 10–100 °C/min and a target temperature up to 1150 °C. For a typical run, 1 g of prepared sample was loaded in a ceramic crucible holder and fed into the center of the quartz tube close to the Ktype thermocouple. Prior to the heating, air in the quartz tube was swept out with a nitrogen (99.999%) flow of 1.0 L/min for 20 min. Then, the nitrogen flow was lowered to 0.6 L/min, and the reactor was heated from ambient temperature to 500 °C by electricity at a heating rate of 20 °C/min, and held at 500 °C for another 2 h. After that, the reactor was cooled to ambient temperature with a 0.6 L/min nitrogen flow. Evolved products formed during the pyrolysis process were swept out by the nitrogen flow to inhibit possible interactions between evolved products and solid residue. Because char was mainly focused in this work, bio-oils and non-condensable gases were not collected. The solid product was weighed and ground into < 180 μm size for characterization. Char of cellulose, xylan, lignin, C-X mixture, C-L mixture, X-L mixture, C-X-L mixture, and maple were denoted as Char-C, Char-X, Char-L, Char-C-X, Char-C-L, Char-X-L, Char-C-X-L, and Char-M, respectively. The mass yields of chars (Yi, wt%) were calculated as below,
equation as,
2.5. Interaction analysis Assuming that the pyrolysis behavior of overall biomass is the weighted sum of the partial contributions of each component, the pyrolysis characteristics of the mixtures and maple can be predicted following the additivity law, Amix,
(4)
(5)
Pr
Where RA is the experimental/predicted ratio (R) of yield or physicochemical characteristic (A); Amix is the experimental yield or physicochemical characteristic (A) of the mixtures and maple. If no interaction exists within the components, RA should equal to 1.0. Due to the inevitable experimental error, R between 0.95 and 1.05 could be within the error limits. RA larger than 1.05 indicates an obvious enhancement in A by interaction, while RA smaller than 0.95 indicates an obvious weakening in A. Larger deviation of RA from 1.0 indicates stronger interaction.
Elemental analyses (C, H, and N) of the samples were performed on a Vario ELⅢ CHN analyzer following the JY/T 017–1996 Method. The oxygen content was determined by difference. Prior to SEM analysis, the chars were gold sputtered. SEM images were captured with a JEOL JSM-6300 SEM system. Nitrogen absorption/desorption was carried out on a fully automated volumetric sorption analyzer (Quantachrome Nova 2000e) using nitrogen as the adsorbate at −196 °C. Prior to the gas adsorption measurements, the samples were outgassed at 120 °C for 2 h. Brunauer-Emmett-Teller (BET) theory and Barrett-Joyner-Halenda (BJH) theory were used to determine the surface area and volume, respectively. XRD for the crystallinity was performed in a D/max 2500 V L/PC X-ray diffractometer using Cu Kα radiation (40 kV, 200 mA). Scan was performed over an angular 2θ range of 5-90° at 0.5°/ min. The interlayer spacing d002 (nm) was determined using the Bragg equation as [33],
3. Results and discussion 3.1. Char of individual biomass components Table 3 lists the char yields and physicochemical characteristics of individual biomass components. Lignin produced the most char, in agreement with the results reported in literature [2,34]. Different from cellulose and hemicellulose which are composed of polysaccharides, lignin consists of phenylpropane units cross-linked heavily [35]. This structure makes lignin highly thermal-stable. Usually, lignin is the part mainly responsible for char production in real biomass [5]. The yield of xylan was also rather high. The higher char yield for xylan than cellulose should be partially ascribed to its lower crystallinity, which could lead to more dehydration reactions at low temperature, resulting in a higher reticulated and stable matrix to form char [36]. Besides, the extremely high contents of inorganic species in xylan could also
(2)
002
=AC×fC+ AX×fX+ AL×fL
RA = Amix / Amix,
2.4. Characterization methods
2sin
Pr
Where Amix, Pr is the predicted yield or physicochemical characteristic (APr) for the mixtures and maple; AC, AX, AL are the experimental yields or physicochemical characteristics (A) for cellulose, xylan and lignin, respectively; and fC, fX, and fL are the mass fractions of cellulose, xylan and lignin present in the corresponding mixture and maple. The experimental/ predicted ratios of the mixtures and maple are calculated as [14],
Where Wi is the mass weight of the solid char i, and W0 is the mass weight of the initial sample. In the present study, all sample preparation and corresponding pyrolysis experiments were performed in at least duplicates. The difference in char yields for all runs was lower than 0.8 wt%, indicating that the experiments have a good repeatability. All mass yields are expressed as the average of the replicates.
d 002 =
(3)
002
Where B002 is the half-width of the (002) peak. In this work, the shape factor of 0.9 was used to calculate Lc [33]. The FTIR spectra were recorded on a FTIR spectrophotometer (Bruker Vector 22). About 1% sample was ground with KBr, and then the mixture was pressed into a transparent wafer. Sixteen scans were conducted for each sample in the range from 4000 cm−1 to 400 cm−1 with 4 cm−1 resolution.
(1)
Yi = W/W i 0 ×100%
0.9 B002 cos
Lc =
Where λ is the X-ray wavelength and θ002 is the scattering angle for the (002) peak. Stack height of the aromatic carbon sheets Lc (nm) was determined from the half-width of the (002) peak using the Scherrer Table 3 Char yields and physicochemical characteristics of biomass components a. Char
Char-C Char-X Char-L a
Yield (wt%)
18.1 36.4 51.6
Element content
Crystallinity characteristics
Porosity characteristics 2
3
Chemical characteristics
N (wt%)
C (wt%)
H (wt%)
d002 (nm)
Lc (nm)
S (m /g)
V (cm /g)
RIH
RIA
0.0 0.0 0.5
84.0 72.7 80.4
3.5 3.2 3.6
0.37 0.40 0.39
0.61 0.51 0.66
20.32 6.46 2.94
0.0213 0.0105 0.0026
0.79 0.96 0.77
0.95 1.44 1.67
S, BET surface Area; V, pore volume; RIH, relative intensity of hydroxyl groups; RIA, relative intensity of CeO bonds in aryl ethers. 3
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Fig. 1. SEM micrographs of starting materials and chars.
promote the formation of char [24]. Fig. 1 shows the SEM micrographs of the starting materials and the chars. The starting cellulose is of obvious parallel vascular bundles (C in Fig. 1). After pyrolysis, the vascular bundles were well retained, with some debris particles on the surface (Char-C in Fig. 1). The starting xylan is of large irregular polygonal blocks with smooth surface (X in Fig. 1). After pyrolysis, the large blocks were cracked into small pieces, doped with fluffy deposits possibly due to carbon deposition (Char-X in Fig. 1) [19]. The original lignin was of round particles with various sizes (L in Fig. 1). The char of lignin, however, was of large agglomerates with non porous and few cavities, indicating that the melting of lignin and agglomeration of lignin char took place during pyrolysis (Char-L in Fig. 1) [7]. Few macropore could be found on the surface of the three chars.
Elemental analysis revealed that the C content in the three components increased greatly from 40.4–60.7 wt% to 72.7–84.0 wt% after pyrolysis. The high C content and low H content were mainly due to the devolatilization of aliphatic functional groups from the chars and the aromatization and carbonization of char [19,37]. The C content of cellulose was higher than that of xylan char and lignin char. Fig. 2 shows the XRD patterns of chars from biomass components, together with binary mixtures, ternary mixture and maple. XRD patterns of chars of the three components showed broad peaks at 24° of (0 0 2) band and 44° of (1 0 0) band, corresponding to the stacking and inplane of aromatics layers, respectively [38,39]. The interlayer spacing distance d002 for the chars ranged from 0.37 nm to 0.40 nm, which were greater than that for graphite (0.33 nm), indicating the existence of short substituents, such as hydroxyl groups and aryl ethers [33]. 4
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Fig. 2. XRD patterns of chars.
Fig. 3. FTIR spectra of chars.
Complete removal of these substituents requires temperatures higher than 500 °C [36]. Xylan char had the largest d002 value of 0.40 nm and the lowest Lc value of 0.51 nm, indicating a relatively poorly ordered structure, consistent with its abundant substituents to be revealed below. Xylan char also displayed three sharp peaks at 2θ = 29.4°, 39.4°, and 43.1° for (1 0 4), (1 1 3), and (2 0 2) corresponding to CaCO3 (JCPDS file: 05-0586), and two peaks at 2θ = 30.4° and 27.1° for (2 1 0) and (2 0 0) corresponding to NaClO3 (JCPDS file: 05-0610), in agreement with the results revealed by ICP-AES (see Table 2). NaClO3 has been used for the preparation of xylan from biomass. Nitrogen absorption/desorption analysis showed that the char of cellulose had the highest specific BET surface area of 20.32 m2/g together with the highest pore volume of 0.0213 cm3/g. The char of lignin possessed a rather low surface areas of 2.94 m2/g and a low pore volume of 0.0026 cm3/g, close to the literature data [22]. The surface areas of chars were lower than some reported biochars, which is primarily due to the incomplete removal of volatile matters at 500 °C [19,40]. This is in agreement with the high char yields as compared with the previous results at both lower and faster heating rate [19,37]. Besides, the melting of lignin during pyrolysis to form a compact surface after cooling could also contribute to its extremely low surface area and pore volume [22]. Fig. 3 shows the FTIR spectra of chars from biomass components, together with binary mixtures, ternary mixture and maple. FTIR spectra of individual components chars showed obvious peaks for OeH stretching in hydroxyl groups at 3424 cm−1, vibrations of aromatic skeletal at 1600 cm−1 and CeO stretching in aryl ethers at 1160 cm−1 [41,42]. Peaks for functional groups like methyl groups, carbonyl groups, and glycosidic groups disappeared after pyrolysis [19]. The absorbance intensity at a specific wavenumber is considered to be linear with the functionality groups concentration, when the absorbance is under 0.5 [38]. Herein, for semi-quantitative analysis, relative intensity for hydroxyl groups (RIH) and CeO bonds in aryl ethers (RIA) were calculated, using the intensity of aromatic rings at 1600 cm−1 as the reference. The char of xylan showed the highest RIH and a second highest RIA, in line with its relatively poorly crystallinity described above. The char of cellulose showed the low values for both RIH and RIA, consistent with its highly ordered crystallinity.
Values of RYield for the binary mixtures ranged from 0.55 to 0.89, indicating that the formation of char was inhibited during the co-pyrolysis process. The mixture of X-L had the lowest RYield of 0.55, showing that the interaction between xylan and lignin was the most significant in terms of char yield. Yang et al. investigated the pyrolysis of synthetic mixtures with different proportion of components, and they found almost no significant interaction among the three components [5]. Yu et al. reported that the pyrolysis of mixtures of C-X and C-L produced more char than predicted, while the mixture of X-L gave char yield matching well with the predicted value [15]. Hosoya et al. and Volpe et al. concluded that the mixture of C-L resulted in less char than predicted [12,43]. Focusing on slow steam pyrolysis at 600 °C, Giudicianni et al. also observed a lower yield than predicted for all binary mixtures [40]. The same group then expanded the study considering the influence of inorganic species, reporting that both inorganics species and lignin can significantly influence the devolatilization of cellulose [25]. As can be found, the results of interactions were significantly different across the reports, which could be ascribed to the difference in starting materials, mixture preparation methods, proportion of components in the mixture, sample amounts, and pyrolysis conditions. It seems that copyrolysis with low-condensed lignin, like milled wood lignin and cellulolytic enzyme lignin, could lead to less char yields than predicted, as compared with Kraft lignin and Alkali lignin. Besides, compared with simple mixing, mixing with a press could offer mixtures with more intimate components contact, and result in more intense interactions [14]. According to the SEM micrographs, obvious vascular bundles of cellulose-derived char, together with some fluffy deposits derived from xylan, can be found for the char of C-X mixture (Char-C-X in Fig. 1). The C-L mixture char also exhibited the vascular bundles which, however, was highly dispersed, likely due to the presence of lignin (Char-C-L in Fig. 1). Such structural change reveals a stronger interaction on morphology for C-L mixture than C-X mixture. On the surface of X-L mixture char, many macropores appeared, and some fragments and needlelike particles were adhered which might be the result of violent volatilechar interactions (Char-X-L in Fig. 1) [39]. Considering the slight differences in H and N contents for the mixtures and components, only C content was used to evaluate the effect of interactions on elemental distribution. The char of X-L mixture has the lowest RC value of 0.89, indicating that the interaction on C content between xylan and lignin was most significant. Apparently, co-pyrolysis with xylan led to more C removal than predicted. All chars of binary mixtures had d002 values close to the predicted
3.2. Char of mixtures and maple Table 4 lists the char yields and physicochemical characteristics of binary mixtures, together with the experimental/predicted ratios. 5
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Table 4 Char yields and physicochemical characteristics of binary mixtures. Char
Char-C-X Char-C-L Char-X-L
Yield (wt%)
21.9 30.9 24.3
RYield
0.80 0.89 0.55
Element content
Crystallinity characteristics
Porosity characteristics 2
Chemical characteristics 3
C (wt%)
RC
d002 (nm)
Rd002
Lc (nm)
RLc
S (m /g)
RS
V (cm /g)
RV
RIH
RRIH
RIA
RRIA
75.5 83.9 68.0
0.96 1.02 0.89
0.38 0.38 0.38
0.99 1.02 0.97
0.60 0.65 0.55
1.08 1.03 0.94
7.92 6.86 9.04
0.59 0.59 1.92
0.0123 0.0104 0.0098
0.77 0.88 1.51
0.50 0.65 0.88
0.57 0.84 1.02
1.24 1.29 1.29
1.04 0.99 0.83
values. The Lc for char of X-L was much lower than estimated, indicating a poor crystallinity. Co-pyrolysis of cellulose and xylan, however, delivered char with a higher Lc than estimated, indicating an enhanced crystallinity development. The char from the samples containing xylan also displayed obvious peaks at 2θ = 29.4°, 39.4°, and 43.1° for (1 0 4), (1 1 3), and (2 0 2) corresponding to CaCO3 (JCPDS file: 05-0586), and peaks at 2θ = 30.4° and 27.1° for (2 1 0) and (2 0 0) corresponding to NaClO3 (JCPDS file: 05-0610). Besides, obvious diffraction peaks at 2θ = 31.7°, 45.5°, and 56.5° for (2 0 0), (2 2 0), and (2 2 2) corresponding to NaCl (JCPDS file: 05-0628) could also be assigned, indicating that NaClO3 has been decomposed into NaCl during the co-pyrolysis. Chars of C-X and C-L binary mixtures offered BET surface areas (6.86–7.92 m2/g) and pore volumes (0.0104–0.0123 cm3/g) significantly lower than the predicted values, indicating that co-pyrolysis of cellulose with xylan or lignin inhibited the formation of released volatiles and gaseous products [17]. The mixture of X-L, however, produced char with higher surface areas and pore volumes than predicted. According to the values of RS and RV, the co-pyrolysis of xylan and lignin had the strongest interaction on porosity development, followed by the co-pyrolysis of xylan and lignin. FTIR revealed that binary mixtures with cellulose gave chars with less hydroxyl groups than predicted, while the mixture of X-L produced chars with less aryl ethers than predicted. According to the values of RRIH and RRIC, co-pyrolysis of cellulose and xylan had the strongest interaction on chemical characteristics. Table 5 lists the char yields and physicochemical characteristics of ternary mixture and maple, together with the experimental/predicted ratios. The RYield for the ternary mixture and maple were 0.85 and 0.68, respectively, indicating that interaction in real biomass was more significant, consistent with the literature results [6,15]. The low RYield for real biomass is due in part to the existence of extractives and lignin–carbohydrate complex (LCC). LCC structures linking lignin and carbohydrate moieties, such as phenol glycoside linkages and benzyl esters, is easily decomposed to release evolved products to interact with the three main components [44]. Besides, the lignin in real biomass is also more reactive with abundant linkages like α-O-4 and β-O-4. Generally, lignin with more ether linkages produces less char [45,46]. Morphology of the chars from the ternary mixture showed irregular complex structure with abundant debris particles and pores, indicating significant interactions within the components (Char-C-X-L in Fig. 1). Maple char also presented complex structure with rough surface and abundant pores (Char-M in Fig. 1). The predicted carbon contents for both ternary mixture char and maple char were close to the experimental results. For crystallinity characteristics, however, the char of the ternary mixture showed a lower Lc of 0.55 nm than maple char of 0.59 nm, likely due to its
relatively high amount of hemicellulose which would lead to a lower ordered structure. Besides, the char of maple displayed three diffraction peaks at 2θ = 26.6°, 20.8°, and 50.1° for (1 0 1), (1 0 0), and (1 1 2) corresponding to SiO2 (JCPDS file: 46–1045), two peaks at 2θ = 28.4°, and 40.5°for (2 0 0), and (2 2 0) corresponding to KCl (JCPDS file: 41–1476), and three peaks at 2θ = 29.4°, 39.4°, and 43.1° corresponding to CaCO3 (JCPDS file: 05-0586), corresponding well with the results of ICP-AES (see Table 2). Both the chars of ternary mixture and maple showed lower BET surface areas than predicted. According to the values of RS and RV, interactions on porosity development are stronger in maple than that in the ternary mixture. FTIR reveals much more aryl ethers for the char of ternary mixture than predicted, indicating that reactions like crosslinking may occur within the components during pyrolysis. For the char of maple, the relative intensity of hydroxyl groups was substantially higher than predicted, which could be attributed to its abundant LCCs and lignin rich in weak ether linkages [3]. According to above results, there are significant interactions within the three components. According to the experimental/predicted ratios, pyrolysis of X-L mixture showed the most significant interaction among the binary mixtures, which might be attributed to their melted intermediates and wide pyrolysis temperature range [36]. The melted intermediates of xylan and lignin can penetrate and diffuse into each other, causing intense interactions during the pyrolysis across the wide pyrolysis temperature range. The more intense interactions in the present work than some literatures could be partially attributed to the sample preparation method. Compared with simple mixing, mixing with a press in this work offered mixtures with more intimate components contact, and therefore interactions inside the particles could be possible [14]. Components analysis has revealed that maple contains 52.4% cellulose, 18.3% hemicellulose, and 24.4% lignin. Based on the contribution of each component, interactions within the components in maple should be less intense than that in the ternary mixture. According to the experimental/predicted ratios, however, pyrolysis mixture of maple showed more significant interactions than the mixture of ternary mixture. This might be attributed to the existence of extractives and LCC in real biomass, which need further research [14,44]. 3.3. Possible mechanisms of interactions between the components Generally, pyrolysis mechanism of biomass consists of primary mechanisms and secondary mechanisms. Primary mechanisms include depolymerization and fragmentation to produce released products, and rearrangement within the residue to form char matrix. Secondary mechanisms include cracking to form lower molecular weight compounds and recombination to form higher molecular weight compounds, like
Table 5 Char yields and physicochemical characteristics of ternary mixture and maplea. Char
Char-C-X-L Char-M
Yield (wt%)
30.1 19.6
RYield
0.85 0.68
Element content
Crystallinity characteristics
C (wt%)
RC
d002 (nm)
Rd002
Lc (nm)
RLc
S (m2/g)
RS
V (cm3/g)
RV
RIH
RRIH
RIC
RRIC
78.9 73.2
1.00 0.95
0.39 0.39
1.03 1.10
0.55 0.59
0.94 1.03
8.16 7.47
0.82 0.60
0.0121 0.0093
1.06 0.68
0.76 1.06
0.90 1.36
2.21 0.77
1.64 0.66
6
Porosity characteristics
Chemical characteristics
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Fig. 4. Main reactions occurring in biomass pyrolysis against the temperature.
carbon deposits or PAHs [3]. Main reactions occurring in biomass pyrolysis have been summarized by Collard et al. as presented in Fig. 4 [36]. Based on these existing mechanisms and the above experiment results, possible mechanisms of interactions during biomass pyrolysis are given below. As the temperature increases to 200 °C, the surface part of hemicellulose and lignin particles begin to melt [36]. The melted intermediates of hemicellulose and lignin will dissolve each other, and cover the surface of cellulose. The decomposition of xylan and lignin is exothermic, while that of cellulose is endothermic [47]. Therefore, the heat produced by the decomposition of xylan and lignin can promote the decomposition of cellulose. As a result, dehydration of cellulose, the competitive reaction path of decomposition, is weakened. The weakening of dehydration contributes partially to the lower char yields of mixtures with cellulose than predicted, because dehydration is mainly responsible for forming highly reticulated char precursors at low temperature [48]. To be released, cellulose derived products, such as levoglucosan and 5-hydroxymethyl furfural, must diffuse across the melted hemicellulose and lignin. During the diffusing process, some reactive fragments, such as phenolic radicals from lignin, could be stabilized into evolved products by the hydrogen-rich products from cellulose and hemicellulose. This stabilization leads to reduced formation of char from the mixtures with lignin [12,43,49]. Releasing of the stabilized evolved products results in a enhanced development of porous structure as identified by SEM (Fig. 1) and Nitrogen absorption/ desorption (Tables 3–5). As the temperature increases to 450 °C, the aromatization degree of benzene rings-rich residue becomes higher due to the removal of remaining short substituents, such as methyl groups, hydroxyl groups and C–O aryl linkages, as revealed by the FTIR (Tables 3–5). Rearrangement reactions will also promote the ordering of the aromatic rings in the char [43].
crystallinity, and less CeO bonds than predicted. Char of the ternary mixture and maple showed similar physicochemical characteristics to the mixture of X-L, and maple showed stronger interactions than the ternary mixture. Possible mechanisms of interactions including the formation of char was provided. This work demonstrates the existence of component interactions and enhances the understanding of biomass pyrolysis mechanisms. Acknowledgements The authors greatly acknowledge the funding support from the projects supported by the National Natural Science Foundation of China (Grant 51706110) and Jiangsu Key Lab of Biomass Energy and Materials (Grant JSBEM201918). References [1] Y. Li, S. Huang, Q. Wang, H. Li, Q. Zhang, H. Wang, Y. Wu, S. Wu, J. Gao, Hydrogen transfer route and interaction mechanism during co-pyrolysis of Xilinhot lignite and rice husk, Fuel Process. Technol. 192 (2019) 13–20. [2] S.D. Stefanidis, K.G. Kalogiannis, E.F. Iliopoulou, C.M. Michailof, P.A. Pilavachi, A.A. Lappas, A study of lignocellulosic biomass pyrolysis via the pyrolysis of cellulose, hemicellulose and lignin, J. Anal. Appl. Pyrolysis 105 (2014) 143–150. [3] S. Wang, G. Dai, H. Yang, Z. Luo, Lignocellulosic biomass pyrolysis mechanism: a state-of-the-art review, Prog. Energy Combust. Sci. 62 (2017) 33–86. [4] Z. Gao, N. Li, M. Chen, W. Yi, Comparative study on the pyrolysis of cellulose and its model compounds, Fuel Process. Technol. 193 (2019) 131–140. [5] H. Yang, R. Yan, H. Chen, C. Zheng, D.H. Lee, D.T. Liang, In-depth investigation of biomass pyrolysis based on three major components: hemicellulose, cellulose and lignin, Energy Fuels 20 (2006) 388–393. [6] K. Raveendran, A. Ganesh, K.C. Khilar, Pyrolysis characteristics of biomass and biomass components, Fuel 75 (1996) 987–998. [7] T.J. Hilbers, Z. Wang, B. Pecha, R.J.M. Westerhof, S.R.A. Kersten, M.R. PelaezSamaniego, M. Garcia-Perez, Cellulose-Lignin interactions during slow and fast pyrolysis, J. Anal. Appl. Pyrolysis 114 (2015) 197–207. [8] S. Wang, X. Guo, K. Wang, Z. Luo, Influence of the interaction of components on the pyrolysis behavior of biomass, J. Anal. Appl. Pyrolysis 91 (2011) 183–189. [9] Y. Long, H. Zhou, A. Meng, Q. Li, Y. Zhang, Interactions among biomass components during co-pyrolysis in (macro)thermogravimetric analyzers, Korean J. Chem. Eng. 33 (2016) 2638–2643. [10] F. Chen, B. Hou, S. Chen, H. Zhang, P. Gong, A. Zhou, Biochemicals distribution and the collaborative pyrolysis study from three main components of Helianthus annuus stems based on PY-GC/MS, Renew. Energy 114 (2017) 960–967. [11] J. Wang, B. Shen, D. Kang, P. Yuan, C. Wu, Investigate the interactions between
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