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In-depth comparison of the physicochemical characteristics of bio-char derived from biomass pseudo components: Hemicellulose, cellulose, and lignin
Journal of Analytical and Applied Pyrolysis xxx (xxxx) xxx–xxx
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In-depth comparison of the physicochemical characteristics of bio-char derived from biomass pseudo components: Hemicellulose, cellulose, and lignin Zhongqing Maa,b, Youyou Yangb, Youlong Wub, Jiajia Xub, Hehuan Pengb, Xiaohuan Liub, ⁎ Wenbiao Zhangb, Shurong Wanga, a
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, Zhejiang, 310027, China School of Engineering, Zhejiang Provincial Collaborative Innovation Center for Bamboo Resources and High-Efficiency Utilization, Zhejiang A & F University, Hangzhou, Zhejiang, 311300, China
b
ARTICLE INFO
ABSTRACT
Keywords: Pyrolysis Bio-char Cellulose Hemicellulose Lignin Physicochemical characteristics
Bio-char is a porous, recalcitrant, and highly aromatic carbon-rich material which can be widely used in energy, agriculture, environment, and material industry. In this study, a systematic comparison of the physicochemical characteristics of bio-char derived from three pseudo components of biomass (cellulose char (CC), hemicellulose char (HC), lignin char (LC)) was carried out at different pyrolysis temperatures (250, 350, 450, 550, 650, 750, and 850 °C). The results indicated wide variation in the physicochemical properties and quality of bio-char depending on different pyrolysis temperatures and biomass feedstock composition. Mass and energy yields of CC, HC, and LC decreased with the increase in the pyrolysis temperature, however, LC exhibited higher mass and energy yield than CC and HC because of its better thermal stability. With the increase in the pyrolysis temperature, the carbon content in CC, HC, and LC increased, while the contents of hydrogen and oxygen decreased, because the oxygen and hydrogen containing groups were easily fallen off at higher pyrolysis temperature based on the FTIR analysis. XRD analysis showed that the crystalline structure (Iα-triclinic and Iβ-monoclinic) of cellulose in CC and HC disappeared with the increase in the pyrolysis temperature above 350 °C. However, the graphite structure (002) and (101) in CC, HC, and LC increased. 13C NMR analysis indicated that the carbon structure of alkyl-C, O-alkyl-C, and carboxylic-C gradually decreased in CC, HC and LC as the pyrolysis temperature increased, while the aryl-C increased, indicating the formation of a more polyaromatic graphite-like structure at higher pyrolysis temperature. SEM results revealed that the volume of CC and LC reduced because of the significant particle agglomeration during pyrolysis process. However, the volume of HC sharply increased because of the transformation of hemicellulose into foam-like material at higher temperature. The lower values of mean absolute error strongly suggests that it is feasible to predict the properties (the contents of C, H, O, mass and energy yield) of real biomass derived bio-char based on the properties of three pseudo components derived bio-char.
1. Introduction Bio-char is a porous, recalcitrant, and highly aromatic carbon-rich material, mainly arising from thermochemical conversion process of biomass in inert or very low stoichiometric oxygen atmospheres [1,2]. Bio-char has several fascinating properties including strong pore structure, high energy density, low electrical resistivity, high pH (alkalinity), and it is also rich in plant nutrients [3,4]. Based on its versatile physicochemical properties, bio-char has been widely used in energy, agriculture, environment, and material industry. Bio-char with
⁎
high calorific value is preferred as high-quality bio-fuels for heat supply [5]. Bio-char with high pH value and being rich in plant nutrients (Ka, Ca, Na, Mg, etc.) is preferred for acid soil remediation [6]. This can ameliorate the soil quality, reduce fertilizer consumption, and sequestrate carbon. Strong pore structure and high specific surface area in biochar make it a suitable adsorbent for contaminant reduction in water or air [7,8], or a primary starting material for the production of supercapacitors [9]. Moreover, bio-char is also reported to be an excellent precursor for production of activated carbon [10], and as a direct catalyst or catalyst support material in tar reduction technology during
Corresponding author at: State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang, 310027, China. E-mail address: [email protected] (S. Wang).
https://doi.org/10.1016/j.jaap.2019.03.015 Received 14 December 2018; Received in revised form 6 March 2019; Accepted 19 March 2019 0165-2370/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhongqing Ma, et al., Journal of Analytical and Applied Pyrolysis, https://doi.org/10.1016/j.jaap.2019.03.015
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Table 1 Ultimate and proximate analysis of cellulose, hemicellulose, and lignin. Feedstock
Cellulose Hemicellulose Lignin a
Ultimate analysis (mass%, db)
LHV (MJ kg–1)
Proximate analysis (mass%, db)
C
H
Oa
N
S
Volatiles
Fixed carbon
Ash
41.46 40.69 58.67
5.97 5.75 5.43
52.48 53.49 34.51
0.07 0.04 1.13
0.02 0.03 0.26
94.25 83.15 64.66
5.75 16.85 32.47
0 0 2.87
15.36 14.57 24.55
Oxygen content was obtained using differential method.
biomass gasification process [11]. Owing to the wide range of applications of bio-char in the multidisciplinary area, the bio-char production technology has gained significant interest of researchers. Bio-char can be produced by several thermochemical conversion technologies, including conventional pyrolysis (CP), gasification, hydrothermal carbonization, and microwave assisted pyrolysis [12]. CP is the most important method to produce bio-char from lignocellulosic biomass [13]. Depending on different heating rate and solid residence time, pyrolysis can be divided into three typical types, namely, slow pyrolysis (carbonization), fast pyrolysis, and flash pyrolysis [14]. Bio-char is the dominant product from slow pyrolysis technology along with part of bio-gas and bio-oil. Properties of bio-char are remarkably influenced by the pyrolysis temperature [2,15–19], biomass species and their chemical composition [4,20,21], heating rate [5,22,23], and holding time [24–26], etc. Chen et al. [20] reported that bio-char derived from woody biomass had higher calorific value than that from herbaceous biomass because of higher content of carbon element and lower content of ash. Ma et al. [17] reported that higher pyrolysis temperature resulted in an increase of the content of carbon element and fixed carbon, while a decrease of the content of oxygen element and volatiles. Angin et al. [23] claimed that higher heating rate resulted in higher yield of bio-char because of the shortened heating time. Lei et al. [24] investigated the effect of holding time (1, 2, 4, and 8 h) on the properties of bio-char, and found that higher holding time led to a decrease of bio-char yield. Lignocellulosic biomass is mainly composed of three pseudo components, namely hemicellulose, cellulose, and lignin [27–30]. The process of formation of bio-char derived from lignocellulosic biomass is known as a synergistic pyrolysis process of three pseudo components. Therefore, it is essential to investigate the formation mechanism of biochar derived from three pseudo components. However, with respect to the study of pyrolysis of three pseudo components, majority of literature studies focused on the determination of thermal degradation behaviors (weight loss characteristics and kinetics) and identification of the components of volatiles (noncondensable gas and bio-oil) by thermogravimetric analysis-fourier transform infrared spectroscopy (TGAFTIR) and pyrolyzer-gas chromatography/mass spectrometry (Py-GC/ MS). First, some researchers [31–35] investigated the difference of thermal degradation behaviors among the three pseudo components. They found that the highest thermal stability was observed for lignin because lignin exhibited the widest temperature range of weight loss (100–800 °C), followed by cellulose (270–400 °C) and hemicellulose (100–365 °C). Second, other researchers investigated the pyrolysis mechanism of three pseudo components (hemicellulose [36,37], cellulose [38,39], and lignin [40–42]) based on their typical pyrolytic products (noncondensable gas and bio-oil). However, the formation mechanism of bio-char produced from three pseudo components has rarely been investigated, and the evolution of basic physicochemical properties of bio-char derived from three pseudo components is still overlooked. In this study, the pyrolysis experiment of three pseudo components (hemicellulose, cellulose, and lignin) of biomass was carried out for biochar production at different pyrolysis temperatures (250, 350, 450, 550, 650, 750, and 850 °C). Then, the effect of pyrolysis temperature on the physicochemical properties of bio-char derived from three pseudo
components was systematically investigated by elementary analysis, FTIR spectrometry, X-ray diffractometer (XRD), nuclear magnetic resonance spectrometer (13C NMR), thermogravimetric analyzer (TGA), and scanning electron microscope (SEM). Finally, in-depth comparison of the physicochemical properties of the three types of bio-char was performed. 2. Experimental 2.1. Materials Hemicellulose, cellulose, and lignin are the three typical pseudo components in the lignocellulosic biomass. Cellulose and hemicellulose were purchased from Sigma-Aldrich Co., Ltd. (USA). Microcrystalline cellulose was a white powder with a product number of 435,236. Xylan with the product number of X4252 was used to represent the hemicellulose component. Klason lignin was isolated from palm kernel shell based on the standard of NREL method [41]. In this method, sulfuric acid/water solution (72 wt.% of sulfuric acid) was used as extraction solvent, and the reaction was carried out for 2.5 h at 20 °C. Then, the Klason lignin was obtained from the precipitate after washing using deionized water. Table 1 lists the result of ultimate and proximate analysis, and heating values of cellulose, hemicellulose and lignin. 2.2. Pyrolysis experiment The pyrolysis experiment of the biomass pseudo component was carried out in a tube furnace (SG-GL1200, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science). The schematic illustration of the tube furnace is shown in Fig. 1. In each experiment, biomass pseudo component (˜3 g) was put in a ceramic boat. Then, the ceramic boat was placed in the center of a quartz tube. Before pyrolysis, the air in the quartz tube was exhausted using a vacuum pump, and then the high-purity nitrogen (99.999%) was injected into the quartz tube with a fixed flow rate (300 mL min−1). Finally, the furnace was heated to different pyrolysis temperatures of 250, 350, 450, 550, 650, 750, and 850 °C at a fixed heating rate of 10 °C min―1, and the terminal temperature was maintained for 1 h. Bio-char was obtained after the temperature reached the room temperature. The experiment at different pyrolysis temperatures was repeated for at least 3 times. Biochars obtained from biomass pseudo components were represented as CC-xxx (cellulose bio-char), HC-xxx (hemicellulose bio-char), LC-xxx (lignin bio-char), where the symbol “xxx” represents the pyrolysis temperature. The mass yield and energy yield were calculated using Eqs. (1) and (2), respectively as follows:
Mass yield (%) =
Mass of biochar (g) × 100% Mass of raw feedstock (g)
Energy yield (%) =
Mass of biochar (g) × Calorific value (MJ/kg) Mass of raw feedstock (g) × Calorific value (MJ/kg) × 100%
2
(1)
(2)
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Fig. 1. Schematic illustration of the biomass pyrolysis device.
2.3. Characteristics of bio-char
promoted the thermal decomposition of three pseudo components, and higher mass percentages were converted into noncondensable gaseous products (CO, CO2, H2O, and CH4, etc.) and liquid bio-oil products (acids, aldehydes, ketones, and phenols, etc.) [5,10]. Noteworthy, when the pyrolysis temperature was higher than 350 °C, the yield of LC was remarkably higher than that of HC and CC. Ma et al. [41,42] reported lignin to be a three-dimensional, highly cross-linked polymer with macromolecular nature. Moreover, based on the TG analysis, the thermal degradation temperature of cellulose and hemicellulose was mainly in the range of 100–400 °C, while the thermal degradation temperature of lignin was at 100–800 °C [27,31]. Therefore, the thermostability of lignin was much higher than that of hemicellulose and cellulose, which resulted in higher yield of LC at high pyrolysis temperature. Fig. 2(b) exhibits the effect of pyrolysis temperature on the energy yield of bio-char derived from three pseudo components. With the increase in the pyrolysis temperature from 250 to 850 °C, the energy yield of CC, HC, and LC gradually decreased from 92.48, 55.7, and 77.82% to 10.61, 12.91, and 31.09%, respectively. When the pyrolysis temperature was above 450 °C, only a little decrease of energy yields of CC and HC was observed, because of the volatilization of large mass percentage below 450 °C [43]. However, a clear decreasing trend of the energy yields of LC could still be observed because of a wider temperature range of weight loss of lignin [31].
2.3.1. Ultimate analysis, higher heating value, and morphology measurement The ultimate analysis and higher heating value (HHV) of bio-char were studied using an elemental analyzer (Vario EL III, Elementary, Germany) and an automatic calorimeter (ZDHW-8, Hebi Hengfeng Coal Analysis Equipment Co., Ltd., China), respectively. Morphological characteristics of different bio-chars were characterized by the cold field emission scanning electron microscopy (SEM, SU8010, Hitachi, Japan). 2.3.2. FTIR and TGA analysis FTIR spectrometry (Nicolet 6700, Thermo Fisher Scientific, USA) was used to test the chemical functional group of bio-char. The mass ratio of KBr to bio-char was 100. The resolution and spectral region of the recorded FTIR spectra were 4 cm―1 and 4000˜400 cm―1, respectively, and the spectrum scan time was 8 s at intervals. TG analyzer (TG209 F1, Netzsch Instruments, Germany) was used to test the thermostability of the bio-char. In each test, about 5 mg of biochar sample was used. The terminal temperature was settled as 800 °C with a heating rate of 10 °C min―1. The flow rate of the carrier gas (high-purity N2) and protective gas was 40 mL min―1 and 20 mL min―1, respectively. 2.3.3. Analysis of crystallographic structure and graphitization degree The crystallographic structure of bio-char was tested using an X-ray diffractometer (XRD 6000, Shimadzu, Japan) with Cu radiation at 40 kV and 30 mA. Scan was performed at the speed of 0.5°/min over the angle (θ) range of 5 to 50°. The graphitization degree of bio-char was performed using a confocal-micro Raman Spectrometer (Renishaw RM2000, UK). The wavelength of laser was 514 and 532 nm. The range of Raman shift was 50–4000 cm―1, and the resolution ratio of spectra was 1 cm―1. The carbon structure of bio-char was measured by the solid-state 13C nuclear magnetic resonance spectrometery (13C-NMR, Bruker ADVANCE III 400 NMR).
3.2. Ultimate analysis Fig. 3 shows the effect of pyrolysis temperature on the ultimate analysis of bio-char derived from three pseudo components. Similar evolution trend of carbon, hydrogen, and oxygen was obtained for the three types of bio-char. Carbon exhibited an increasing trend, however, hydrogen and oxygen showed a decreasing trend with the increase in the pyrolysis temperature from 250 to 850 °C. The decrease of hydrogen and oxygen in three pseudo components was first ascribed to the breaking of functional groups (−OH, −OCH3, −COOH) by the dehydroxylation, decarboxylation, and decarbonylation reactions. Some part of hydrogen and oxygen elements were first converted into several gas components (CO, CO2, H2O, and CH4) with significantly low molecular weights [44]. Second, remaining parts of hydrogen and oxygen were converted into liquid bio-oil components (acids, aldehydes, ketones, phenols, etc.) by the ring-opening reaction of glucan unit in cellulose and hemicellulose, and phenylpropane unit in lignin [36,38,41]. The increase of carbon was mainly attributed to the enhancement of graphitization degree, because of the formation of more polyaromatic graphite-like carbon structure at higher pyrolysis temperature [45,46].
3. Results and discussion 3.1. Mass and energy yields Fig. 2(a) shows the effect of pyrolysis temperature on the mass yield of bio-char derived from three pseudo components. With the increase in the pyrolysis temperature from 250 to 850 °C, the mass yield of CC, HC, and LC gradually decreases from 94.23, 63.06, and 87.14% to 17.01, 20.67, and 41.40%, respectively. Higher pyrolysis temperature 3
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Fig. 2. Effect of pyrolysis temperature on the mass and energy yields of bio-char derived from three pseudo components.
3.3. FTIR analysis The evolution of surface functional groups of bio-char at different pyrolysis temperatures is shown in Fig. 4. Six characteristic absorbance bands are clearly observed in the IR spectra of bio-char. The first remarkable band is the stretching vibration of −OH at the wavenumber range of 3700–3100 cm−1 [4]. The second remarkable band corresponds to the stretching vibration of CeH at 3000–2700 cm−1 and 592 cm―1 which is mainly contributed by the aliphatic −CH2 and alkanes −CH3 [19]. The band at 1706 cm−1 is ascribed to the stretching vibration of CeO which is mainly derived from the carbonyl (–C = O) and carboxyl functional groups (−COOH) [2]. The characteristic absorbance peak between 1680–1440 cm―1 is the stretching vibration of benzene ring skeleton (C]C) from the aromatics [24]. The characteristic absorbance peak at 1190–950 cm−1 is attributed to the stretching vibration of CeO from phenols [23]. Fig. 4 displays the IR spectra, exhibiting that the intensity of the six characteristic absorbance bands in the three types of bio-char gradually decreases with the increase in the pyrolysis temperature. Noteworthy, the characteristic absorbance bands for the first three functional groups including −OH at 3700–3100 cm−1, CeH at 3000–2700 cm−1, and 592 cm−1 for the CC, HC, and LC, could still be clearly presented at the lower pyrolysis temperature of 250 and 350 °C. However, when the pyrolysis temperature reached above 450 °C, these three functional groups, which were weakly linked on the basic structural unit of hemicellulose, cellulose, and lignin, were gradually cut off by thermal cracking. When the temperature continued to increase to 750 and 850 °C, the characteristic absorbance bands of these three functional groups became almost flat. This result was also confirmed for other biomass materials, such as palm kernel shell [2], bamboo [4], pitch pine [18], pinewood [24], and cotton stalk [47]. The evolution of the characteristic absorbance band of benzene ring skeleton (C]C) at 1680–1440 cm−1 in LC was quite different from that of CC and HC. With the increase in the pyrolysis temperature, this band in the CC and HC gradually faded away. However, a strong absorbance intensity of this band still existed in LC at higher pyrolysis temperature. First, different from cellulose and hemicellulose, lignin is rich in phenylpropane structure unit [28]. Then, as the pyrolysis temperature increased, more graphite-like polyaromatic structure was formed in LC, which led to a strong characteristic absorbance band of benzene ring skeleton at high pyrolysis temperature [48,49].
Fig. 3. Effect of pyrolysis temperature on the ultimate analysis of bio-char derived from three pseudo components of biomass: (a) CC, (b) HC, (c) LC.
3.4. XRD analysis
Furthermore, the H/C and O/C ratios of three types of bio-char sharply decreased at the pyrolysis temperature of 250–450 °C, while a slight decrease of H/C and O/C was observed at higher pyrolysis temperature above 450 °C. At the same pyrolysis temperature, the H/C and O/C of CC and HC were higher than those of LC, because cellulose and hemicellulose contains higher content of oxygen and hydrogen, but less content of carbon as listed in Table 1.
Fig. 5 shows the XRD patterns of bio-char derived from three pseudo components. Four characteristic diffraction peaks at 2θ of 16°, 22°, 26.6°, and 44° are clearly present in the XRD patterns of three types of bio-char. The first two peaks at 16° and 22° are attributed to the typical crystalline structure of the cellulose Iα (triclinic) and cellulose Iβ (monoclinic), respectively [2,50]. The other two peaks at 26.6° and 44° 4
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Fig. 5. XRD analysis of biochar derived from three pseudo components of biomass: (a) CC, (b) HC, (c) LC.
Fig. 4. FTIR analysis of bio-char derived from three pseudo components of biomass at different pyrolysis temperatures: (a) CC, (b) HC, (c) LC.
temperature. However, the intensity of the diffraction peak of graphite (101) increased with the increase in the temperature, indicating the formation of more graphite-like structure at higher pyrolysis temperature [52]. Fig. 5(b) shows that patterns of raw hemicellulose and HC contain two diffraction peaks of cellulose Iα (triclinic) and graphite (101). The evolution of these two peaks was quite similar to that of raw cellulose and CC. Fig. 5(c) exhibits the presence of two tiny diffraction peaks of graphite (002) and graphite (101) for raw lignin and LC. Compared to the raw lignin, a stronger intensity of these two peaks for LC was observed, indicating that polyaromatic graphite-like structure was formed in LC [48].
are contributed by the structure of graphite (002) and graphite (101), respectively [4,51]. Fig. 5(a) demonstrates that raw cellulose and CC contain three diffraction peaks of cellulose Iα (triclinic), cellulose Iβ (monoclinic), and graphite (101). For the C-control and CC-250 samples, two sharp peaks representing cellulose Iα and cellulose Iβ could still be clearly observed. However, with the increase in the pyrolysis temperature above 350 °C, these two sharp peaks turned into one dispersion peak. This result indicated the disappearance of crystalline region of cellulose, because cellulose was degraded into gas or liquid product at higher pyrolysis 5
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Table 2 Effect of pyrolysis temperature on the content of carbon structure in bio-char derived from three pseudo components of biomass. Alkyl–C 045 ppm, (%)
O-alkyl–C 45–95 ppm, (%)
Aryl–C 95–165 ppm, (%)
Carboxylic–C 165–200 ppm, (%)
C-control CC-250 CC-450 CC-650 CC-850 H-control HC-250 HC-450 HC-650 HC-850 L-control LC-250 LC-450 LC-650 LC-850
1 3 16 9 5 0 17 14 8 4 18 24 28 17 6
80 75 5 4 3 82 42 9 6 3 43 24 17 15 12
16 20 77 86 91 11 35 72 82 90 36 39 42 64 79
3 2 2 1 1 7 6 5 4 3 12 13 15 4 3
For the C-control and CC-250 samples, aryl–C (95–165 ppm) and O–alkyl–C (45–90 ppm) were the dominant carbon structures. With the increase in the pyrolysis temperature, aryl–C became the dominant carbon structure with an increase of relative content from 16 to 91%. This result showed that more polyaromatic hydrocarbon structure was formed in CC at higher pyrolysis temperature. However, an opposite trend of the oxygen-containing O–alkyl–C was obtained, with the content decreasing from 80 to 3%. This result indicated that the majority of oxygen element in CC was removed at higher pyrolysis temperature. This result was also confirmed by the ultimate analysis of CC as shown in Fig. 3. For the H-control and HC-250 samples, the dominant carbon structure was also aryl–C and O–alkyl–C. As the pyrolysis temperature increased, the relative content of aryl–C increased from 11 to 90%, while that of oxygen-containing O–alkyl–C decreased from 82 to 3%. Therefore, the evolution of carbon structure depending on pyrolysis temperature was quite similar to that of CC because of the relatively similar molecular structures of cellulose and hemicellulose. Compared to the C-control, CC-250, H-control, and HC-250 samples, more uniform distribution of the four types of carbon structure (carboxylic–C, aryl–C, O–alkyl–C, and alkyl–C) was presented for lignin and LC-250 (as presented in Table 2). However, with the increase in the pyrolysis temperature, the alkyl-C gradually became the dominant carbon structure in LC, with its content increasing from 36 to 79%. The oxygen element mainly existed in the hydroxyl (−OH), carbonyl (–C = O), carboxyl (−COOH), and methoxyl (−OCH3) groups linked on the basic structural unit of three pseudo components. Therefore, the reduction of oxygen-containing carbon structure (carboxylic–C and O–alkyl–C) was mainly contributed by the breakage of these oxygen-containing functional groups by the dehydroxylation, decarboxylation, decarbonylation, and demethoxy reactions [36,38,41]. The alkyl–C was also decreased because of the fact that more aliphatic chain in the three pseudo components was converted into other shorter alkyl moieties or polycyclic aromatic structural units [25]. When the pyrolysis temperature was above 250 °C, entire carbon structure was dominated by aryl–C which was caused by the increased in condensed polycyclic aromatic hydrocarbon units [46,54].
Fig. 6. 13C NMR analysis of biochar derived from three pseudo components of biomass: (a) CC, (b) HC, (c) LC.
3.5. Solid-state
Samples
13
C NMR analysis
NMR spectroscopy (13C NMR) is a good means to characterize the carbon structure in bio-char. Fig. 6 shows the 13C NMR spectra of three types of bio-char derived from three pseudo components. According to the literature [2,53], the carbon structure in bio-char could be mainly divided into four groups, namely, carboxylic–C (165–200 ppm), aryl–C (95–165 ppm), O–alkyl–C (45–90 ppm), and alkyl–C (0–45 ppm). The relative content of the four carbon structures in CC, HC, and CC at different pyrolysis temperatures is listed in Table 2.
3.6. TG analysis The thermal stability of bio-char derived from three pseudo components was investigated using a TG analyzer. Fig. 7 shows the differential thermogravimetric (DTG) curves of bio-char at a heating rate of
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exhibits the clear presence of two weight loss peaks for both H-control and HC-250 samples. This result indicated that H-control and HC-250 samples exhibited wider temperature range of weight loss than C-control and CC-250 samples. However, the weight loss rate at peaks of Hcontrol and HC was much lower than that of C-control and CC samples at their corresponding pyrolysis temperature. Fig. 7(c) demonstrates that LC shows the widest temperature range of weight loss among the three types of bio-char. The maximum weight loss at peaks of LC-350 to LC-850 was higher than that of HC-350 to HC-850 at their corresponding pyrolysis temperature, but was close to CC-350 to CC-850. This result indicated that the thermal stability of HC obtained at higher pyrolysis temperature was better than that of CC and LC. 3.7. Morphological analysis Fig. 8 shows the morphology of bio-char at different pyrolysis temperatures. Fig. 8 (a) shows that with the increase in the pyrolysis temperature, the color of bio-char derived from three pseudo components gradually becomes darker. Moreover, the volume of CC and LC samples reduced because of the significant particle agglomeration during pyrolysis process. However, the volume of HC sample sharply increased when the pyrolysis temperature reached above 350 °C, because of the transformation of hemicellulose into foam-like material at higher temperature. A clear melt phase was formed on the outer surface of HC. Fig. 8(b) shows long strip-shape of the raw cellulose and CC. With the increase in the pyrolysis temperature, the diameter of fiber decreases. The spherical raw hemicellulose was transformed into laminar structural HC by melting and foaming. Moreover, with the increase in the pyrolysis temperature, the pore structure of LC became stronger than that of raw lignin. Some salt crystal particles were also observed on the surface of LC which was probably obtained from the crystallization process of alkali and alkaline metals. 3.8. Prediction of the biomass properties based on the properties of three pseudo components According to the contents of three pseudo components in biomass, the physicochemical characteristics of bio-char derived from real biomass can be predicted based on the physicochemical characteristics of bio-char derived from three pseudo components. In this study, five parameters (C content, H content, O content, mass and energy yields) are selected to demonstrate the feasibility of prediction. They can be calculated by the Eq. (3).
Fig. 7. Thermogravimetric analysis of biochar derived from three pseudo components of biomass: (a) CC, (b) HC, (c) LC.
C content, H content, O content, Mass and Energy yield (%)
10 °C min−1. DTG curves demonstrate that as the carbonization temperature increases, the peak temperature of maximum weight loss rate in all three types of bio-char gradually moves toward the side of high temperature. Furthermore, the value of maximum weight loss rates at peaks gradually decreases. This result indicated that higher pyrolysis temperature promoted the thermostability of bio-char derived from three pseudo components [2]. Significant difference in weight loss characteristics was found among the three types of bio-char. Fig. 7(a) clearly shows two steep weight loss peaks for the C-control and CC-250 samples, with maximum weight loss rates of 67.73 and 47.72% min−1, respectively. However, for the CC-350 to CC-850 samples, the weight loss rate at peaks was in the range of 13.71–21.08% min−1 which is significantly lower than that of C-control and CC-250 samples. Ma et al. [27] reported that the thermal degradation of cellulose mainly occurred in a very narrow temperature range of 180 and 320 °C. Therefore, for the CC-350 to CC850 samples obtained at the pyrolysis temperature above 350 °C, majority of the mass was volatilized during pyrolysis process. Fig. 7(b)
= ax+ by+ cz
(3)
Where x, y, and z are the contents of cellulose, hemicellulose, and lignin in the biomass, respectively; a, b, and c are the contents of C, H, and O, mass yield, and energy yield in the bio-char derived from cellulose, hemicellulose, and lignin at different pyrolysis temperatures. Three species of biomass, namely, palm kernel shell (PKS) [2], Chinese chestnut (CNS) [55], and Jatropha curcas shells (JCS) [55], were selected to verify the accuracy of prediction according to the statistical parameter of mean absolute error (MAE) which was calculated by Eq. (4). Furthermore, lower MAE indicates higher accuracy of prediction.
MAE=
n Parameters predicted Parametersexperimental 1 n i= 1 Parametersexperimental
(4)
Fig. 9 exhibits the linear fitting curves and MAE of experimental and predicted data of C content, H content, O content, mass yield, and
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Fig. 8. Morphology analysis of bio-char derived from three pseudo components of biomass at different pyrolysis temperatures: (a) pictures of bio-char, (b) microstructure of bio-char.
energy yield in the bio-char derived from real biomass, and the detailed data is listed in Table S1. It can been seen that most of the linear fitting curves from predicted value is highly close to that from experimental value, indicating good accuracy of the correlations. The values of MAE obtained from C content, H content, O content, mass yield, and energy yield are 3.94%, 3.99%, 4.02%, 4.01%, and 4.01%, respectively. Therefore, the MAE obtained from five parameters are lower than 5%, indicating their good universal applicability. This result strongly suggests that it is feasible to predict the properties of real biomass based on the properties of three pseudo components.
4. Conclusions Wide variation in the physicochemical properties of bio-char depending on different pyrolysis temperatures and biomass feedstock composition was observed. LC exhibited higher mass and energy yield than CC and HC. Higher pyrolysis temperature led to an increase of carbon content, but a decrease of oxygen and hydrogen contents. Moreover, at higher pyrolysis temperature, the carbon structure of CC, HC, and LC was dominated by the aryl-C, indicating the formation of more polyaromatic graphite-like structure. The volume of CC and LC was reduced due to the particle agglomeration reaction, while the volume of HC sharply increased because of the formation of foam-like
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Fig. 9. Linear fitting curves and MAE of experimental and predicted data of real biomass: (a) C content, (b) H content, (c) O content, (d) mass yield, (e) energy yield, (f) MAE.
Appendix A. Supplementary data
structure. It is feasible to predict the properties of real biomass based on the properties of three pseudo components.
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jaap.2019.03.015.
Acknowledgements
References
This research was supported by Natural Science Foundation of Zhejiang Province (LQ19E060009, LQ17E060002), Key R & D Plan of Zhejiang Province (2018C02008), Natural Science Foundation of China (51706207), China Postdoctoral Science Foundation (2017M611987), Public Welfare Technology Research Fund of Zhejiang Province (LGN18B060001), the Young Elite Scientists Sponsorship Program by CAST (2018QNRC001).
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In-depth comparison of the physicochemical characteristics of bio-char derived from biomass pseudo components: Hemicellulose, cellulose, and lignin Zhongqing Maa,b, Youyou Yangb, Youlong Wub, Jiajia Xub, Hehuan Pengb, Xiaohuan Liub, ⁎ Wenbiao Zhangb, Shurong Wanga, a
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, Zhejiang, 310027, China School of Engineering, Zhejiang Provincial Collaborative Innovation Center for Bamboo Resources and High-Efficiency Utilization, Zhejiang A & F University, Hangzhou, Zhejiang, 311300, China
b
ARTICLE INFO
ABSTRACT
Keywords: Pyrolysis Bio-char Cellulose Hemicellulose Lignin Physicochemical characteristics
Bio-char is a porous, recalcitrant, and highly aromatic carbon-rich material which can be widely used in energy, agriculture, environment, and material industry. In this study, a systematic comparison of the physicochemical characteristics of bio-char derived from three pseudo components of biomass (cellulose char (CC), hemicellulose char (HC), lignin char (LC)) was carried out at different pyrolysis temperatures (250, 350, 450, 550, 650, 750, and 850 °C). The results indicated wide variation in the physicochemical properties and quality of bio-char depending on different pyrolysis temperatures and biomass feedstock composition. Mass and energy yields of CC, HC, and LC decreased with the increase in the pyrolysis temperature, however, LC exhibited higher mass and energy yield than CC and HC because of its better thermal stability. With the increase in the pyrolysis temperature, the carbon content in CC, HC, and LC increased, while the contents of hydrogen and oxygen decreased, because the oxygen and hydrogen containing groups were easily fallen off at higher pyrolysis temperature based on the FTIR analysis. XRD analysis showed that the crystalline structure (Iα-triclinic and Iβ-monoclinic) of cellulose in CC and HC disappeared with the increase in the pyrolysis temperature above 350 °C. However, the graphite structure (002) and (101) in CC, HC, and LC increased. 13C NMR analysis indicated that the carbon structure of alkyl-C, O-alkyl-C, and carboxylic-C gradually decreased in CC, HC and LC as the pyrolysis temperature increased, while the aryl-C increased, indicating the formation of a more polyaromatic graphite-like structure at higher pyrolysis temperature. SEM results revealed that the volume of CC and LC reduced because of the significant particle agglomeration during pyrolysis process. However, the volume of HC sharply increased because of the transformation of hemicellulose into foam-like material at higher temperature. The lower values of mean absolute error strongly suggests that it is feasible to predict the properties (the contents of C, H, O, mass and energy yield) of real biomass derived bio-char based on the properties of three pseudo components derived bio-char.
1. Introduction Bio-char is a porous, recalcitrant, and highly aromatic carbon-rich material, mainly arising from thermochemical conversion process of biomass in inert or very low stoichiometric oxygen atmospheres [1,2]. Bio-char has several fascinating properties including strong pore structure, high energy density, low electrical resistivity, high pH (alkalinity), and it is also rich in plant nutrients [3,4]. Based on its versatile physicochemical properties, bio-char has been widely used in energy, agriculture, environment, and material industry. Bio-char with
⁎
high calorific value is preferred as high-quality bio-fuels for heat supply [5]. Bio-char with high pH value and being rich in plant nutrients (Ka, Ca, Na, Mg, etc.) is preferred for acid soil remediation [6]. This can ameliorate the soil quality, reduce fertilizer consumption, and sequestrate carbon. Strong pore structure and high specific surface area in biochar make it a suitable adsorbent for contaminant reduction in water or air [7,8], or a primary starting material for the production of supercapacitors [9]. Moreover, bio-char is also reported to be an excellent precursor for production of activated carbon [10], and as a direct catalyst or catalyst support material in tar reduction technology during
Corresponding author at: State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang, 310027, China. E-mail address: [email protected] (S. Wang).
https://doi.org/10.1016/j.jaap.2019.03.015 Received 14 December 2018; Received in revised form 6 March 2019; Accepted 19 March 2019 0165-2370/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhongqing Ma, et al., Journal of Analytical and Applied Pyrolysis, https://doi.org/10.1016/j.jaap.2019.03.015
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Table 1 Ultimate and proximate analysis of cellulose, hemicellulose, and lignin. Feedstock
Cellulose Hemicellulose Lignin a
Ultimate analysis (mass%, db)
LHV (MJ kg–1)
Proximate analysis (mass%, db)
C
H
Oa
N
S
Volatiles
Fixed carbon
Ash
41.46 40.69 58.67
5.97 5.75 5.43
52.48 53.49 34.51
0.07 0.04 1.13
0.02 0.03 0.26
94.25 83.15 64.66
5.75 16.85 32.47
0 0 2.87
15.36 14.57 24.55
Oxygen content was obtained using differential method.
biomass gasification process [11]. Owing to the wide range of applications of bio-char in the multidisciplinary area, the bio-char production technology has gained significant interest of researchers. Bio-char can be produced by several thermochemical conversion technologies, including conventional pyrolysis (CP), gasification, hydrothermal carbonization, and microwave assisted pyrolysis [12]. CP is the most important method to produce bio-char from lignocellulosic biomass [13]. Depending on different heating rate and solid residence time, pyrolysis can be divided into three typical types, namely, slow pyrolysis (carbonization), fast pyrolysis, and flash pyrolysis [14]. Bio-char is the dominant product from slow pyrolysis technology along with part of bio-gas and bio-oil. Properties of bio-char are remarkably influenced by the pyrolysis temperature [2,15–19], biomass species and their chemical composition [4,20,21], heating rate [5,22,23], and holding time [24–26], etc. Chen et al. [20] reported that bio-char derived from woody biomass had higher calorific value than that from herbaceous biomass because of higher content of carbon element and lower content of ash. Ma et al. [17] reported that higher pyrolysis temperature resulted in an increase of the content of carbon element and fixed carbon, while a decrease of the content of oxygen element and volatiles. Angin et al. [23] claimed that higher heating rate resulted in higher yield of bio-char because of the shortened heating time. Lei et al. [24] investigated the effect of holding time (1, 2, 4, and 8 h) on the properties of bio-char, and found that higher holding time led to a decrease of bio-char yield. Lignocellulosic biomass is mainly composed of three pseudo components, namely hemicellulose, cellulose, and lignin [27–30]. The process of formation of bio-char derived from lignocellulosic biomass is known as a synergistic pyrolysis process of three pseudo components. Therefore, it is essential to investigate the formation mechanism of biochar derived from three pseudo components. However, with respect to the study of pyrolysis of three pseudo components, majority of literature studies focused on the determination of thermal degradation behaviors (weight loss characteristics and kinetics) and identification of the components of volatiles (noncondensable gas and bio-oil) by thermogravimetric analysis-fourier transform infrared spectroscopy (TGAFTIR) and pyrolyzer-gas chromatography/mass spectrometry (Py-GC/ MS). First, some researchers [31–35] investigated the difference of thermal degradation behaviors among the three pseudo components. They found that the highest thermal stability was observed for lignin because lignin exhibited the widest temperature range of weight loss (100–800 °C), followed by cellulose (270–400 °C) and hemicellulose (100–365 °C). Second, other researchers investigated the pyrolysis mechanism of three pseudo components (hemicellulose [36,37], cellulose [38,39], and lignin [40–42]) based on their typical pyrolytic products (noncondensable gas and bio-oil). However, the formation mechanism of bio-char produced from three pseudo components has rarely been investigated, and the evolution of basic physicochemical properties of bio-char derived from three pseudo components is still overlooked. In this study, the pyrolysis experiment of three pseudo components (hemicellulose, cellulose, and lignin) of biomass was carried out for biochar production at different pyrolysis temperatures (250, 350, 450, 550, 650, 750, and 850 °C). Then, the effect of pyrolysis temperature on the physicochemical properties of bio-char derived from three pseudo
components was systematically investigated by elementary analysis, FTIR spectrometry, X-ray diffractometer (XRD), nuclear magnetic resonance spectrometer (13C NMR), thermogravimetric analyzer (TGA), and scanning electron microscope (SEM). Finally, in-depth comparison of the physicochemical properties of the three types of bio-char was performed. 2. Experimental 2.1. Materials Hemicellulose, cellulose, and lignin are the three typical pseudo components in the lignocellulosic biomass. Cellulose and hemicellulose were purchased from Sigma-Aldrich Co., Ltd. (USA). Microcrystalline cellulose was a white powder with a product number of 435,236. Xylan with the product number of X4252 was used to represent the hemicellulose component. Klason lignin was isolated from palm kernel shell based on the standard of NREL method [41]. In this method, sulfuric acid/water solution (72 wt.% of sulfuric acid) was used as extraction solvent, and the reaction was carried out for 2.5 h at 20 °C. Then, the Klason lignin was obtained from the precipitate after washing using deionized water. Table 1 lists the result of ultimate and proximate analysis, and heating values of cellulose, hemicellulose and lignin. 2.2. Pyrolysis experiment The pyrolysis experiment of the biomass pseudo component was carried out in a tube furnace (SG-GL1200, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science). The schematic illustration of the tube furnace is shown in Fig. 1. In each experiment, biomass pseudo component (˜3 g) was put in a ceramic boat. Then, the ceramic boat was placed in the center of a quartz tube. Before pyrolysis, the air in the quartz tube was exhausted using a vacuum pump, and then the high-purity nitrogen (99.999%) was injected into the quartz tube with a fixed flow rate (300 mL min−1). Finally, the furnace was heated to different pyrolysis temperatures of 250, 350, 450, 550, 650, 750, and 850 °C at a fixed heating rate of 10 °C min―1, and the terminal temperature was maintained for 1 h. Bio-char was obtained after the temperature reached the room temperature. The experiment at different pyrolysis temperatures was repeated for at least 3 times. Biochars obtained from biomass pseudo components were represented as CC-xxx (cellulose bio-char), HC-xxx (hemicellulose bio-char), LC-xxx (lignin bio-char), where the symbol “xxx” represents the pyrolysis temperature. The mass yield and energy yield were calculated using Eqs. (1) and (2), respectively as follows:
Mass yield (%) =
Mass of biochar (g) × 100% Mass of raw feedstock (g)
Energy yield (%) =
Mass of biochar (g) × Calorific value (MJ/kg) Mass of raw feedstock (g) × Calorific value (MJ/kg) × 100%
2
(1)
(2)
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Fig. 1. Schematic illustration of the biomass pyrolysis device.
2.3. Characteristics of bio-char
promoted the thermal decomposition of three pseudo components, and higher mass percentages were converted into noncondensable gaseous products (CO, CO2, H2O, and CH4, etc.) and liquid bio-oil products (acids, aldehydes, ketones, and phenols, etc.) [5,10]. Noteworthy, when the pyrolysis temperature was higher than 350 °C, the yield of LC was remarkably higher than that of HC and CC. Ma et al. [41,42] reported lignin to be a three-dimensional, highly cross-linked polymer with macromolecular nature. Moreover, based on the TG analysis, the thermal degradation temperature of cellulose and hemicellulose was mainly in the range of 100–400 °C, while the thermal degradation temperature of lignin was at 100–800 °C [27,31]. Therefore, the thermostability of lignin was much higher than that of hemicellulose and cellulose, which resulted in higher yield of LC at high pyrolysis temperature. Fig. 2(b) exhibits the effect of pyrolysis temperature on the energy yield of bio-char derived from three pseudo components. With the increase in the pyrolysis temperature from 250 to 850 °C, the energy yield of CC, HC, and LC gradually decreased from 92.48, 55.7, and 77.82% to 10.61, 12.91, and 31.09%, respectively. When the pyrolysis temperature was above 450 °C, only a little decrease of energy yields of CC and HC was observed, because of the volatilization of large mass percentage below 450 °C [43]. However, a clear decreasing trend of the energy yields of LC could still be observed because of a wider temperature range of weight loss of lignin [31].
2.3.1. Ultimate analysis, higher heating value, and morphology measurement The ultimate analysis and higher heating value (HHV) of bio-char were studied using an elemental analyzer (Vario EL III, Elementary, Germany) and an automatic calorimeter (ZDHW-8, Hebi Hengfeng Coal Analysis Equipment Co., Ltd., China), respectively. Morphological characteristics of different bio-chars were characterized by the cold field emission scanning electron microscopy (SEM, SU8010, Hitachi, Japan). 2.3.2. FTIR and TGA analysis FTIR spectrometry (Nicolet 6700, Thermo Fisher Scientific, USA) was used to test the chemical functional group of bio-char. The mass ratio of KBr to bio-char was 100. The resolution and spectral region of the recorded FTIR spectra were 4 cm―1 and 4000˜400 cm―1, respectively, and the spectrum scan time was 8 s at intervals. TG analyzer (TG209 F1, Netzsch Instruments, Germany) was used to test the thermostability of the bio-char. In each test, about 5 mg of biochar sample was used. The terminal temperature was settled as 800 °C with a heating rate of 10 °C min―1. The flow rate of the carrier gas (high-purity N2) and protective gas was 40 mL min―1 and 20 mL min―1, respectively. 2.3.3. Analysis of crystallographic structure and graphitization degree The crystallographic structure of bio-char was tested using an X-ray diffractometer (XRD 6000, Shimadzu, Japan) with Cu radiation at 40 kV and 30 mA. Scan was performed at the speed of 0.5°/min over the angle (θ) range of 5 to 50°. The graphitization degree of bio-char was performed using a confocal-micro Raman Spectrometer (Renishaw RM2000, UK). The wavelength of laser was 514 and 532 nm. The range of Raman shift was 50–4000 cm―1, and the resolution ratio of spectra was 1 cm―1. The carbon structure of bio-char was measured by the solid-state 13C nuclear magnetic resonance spectrometery (13C-NMR, Bruker ADVANCE III 400 NMR).
3.2. Ultimate analysis Fig. 3 shows the effect of pyrolysis temperature on the ultimate analysis of bio-char derived from three pseudo components. Similar evolution trend of carbon, hydrogen, and oxygen was obtained for the three types of bio-char. Carbon exhibited an increasing trend, however, hydrogen and oxygen showed a decreasing trend with the increase in the pyrolysis temperature from 250 to 850 °C. The decrease of hydrogen and oxygen in three pseudo components was first ascribed to the breaking of functional groups (−OH, −OCH3, −COOH) by the dehydroxylation, decarboxylation, and decarbonylation reactions. Some part of hydrogen and oxygen elements were first converted into several gas components (CO, CO2, H2O, and CH4) with significantly low molecular weights [44]. Second, remaining parts of hydrogen and oxygen were converted into liquid bio-oil components (acids, aldehydes, ketones, phenols, etc.) by the ring-opening reaction of glucan unit in cellulose and hemicellulose, and phenylpropane unit in lignin [36,38,41]. The increase of carbon was mainly attributed to the enhancement of graphitization degree, because of the formation of more polyaromatic graphite-like carbon structure at higher pyrolysis temperature [45,46].
3. Results and discussion 3.1. Mass and energy yields Fig. 2(a) shows the effect of pyrolysis temperature on the mass yield of bio-char derived from three pseudo components. With the increase in the pyrolysis temperature from 250 to 850 °C, the mass yield of CC, HC, and LC gradually decreases from 94.23, 63.06, and 87.14% to 17.01, 20.67, and 41.40%, respectively. Higher pyrolysis temperature 3
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Fig. 2. Effect of pyrolysis temperature on the mass and energy yields of bio-char derived from three pseudo components.
3.3. FTIR analysis The evolution of surface functional groups of bio-char at different pyrolysis temperatures is shown in Fig. 4. Six characteristic absorbance bands are clearly observed in the IR spectra of bio-char. The first remarkable band is the stretching vibration of −OH at the wavenumber range of 3700–3100 cm−1 [4]. The second remarkable band corresponds to the stretching vibration of CeH at 3000–2700 cm−1 and 592 cm―1 which is mainly contributed by the aliphatic −CH2 and alkanes −CH3 [19]. The band at 1706 cm−1 is ascribed to the stretching vibration of CeO which is mainly derived from the carbonyl (–C = O) and carboxyl functional groups (−COOH) [2]. The characteristic absorbance peak between 1680–1440 cm―1 is the stretching vibration of benzene ring skeleton (C]C) from the aromatics [24]. The characteristic absorbance peak at 1190–950 cm−1 is attributed to the stretching vibration of CeO from phenols [23]. Fig. 4 displays the IR spectra, exhibiting that the intensity of the six characteristic absorbance bands in the three types of bio-char gradually decreases with the increase in the pyrolysis temperature. Noteworthy, the characteristic absorbance bands for the first three functional groups including −OH at 3700–3100 cm−1, CeH at 3000–2700 cm−1, and 592 cm−1 for the CC, HC, and LC, could still be clearly presented at the lower pyrolysis temperature of 250 and 350 °C. However, when the pyrolysis temperature reached above 450 °C, these three functional groups, which were weakly linked on the basic structural unit of hemicellulose, cellulose, and lignin, were gradually cut off by thermal cracking. When the temperature continued to increase to 750 and 850 °C, the characteristic absorbance bands of these three functional groups became almost flat. This result was also confirmed for other biomass materials, such as palm kernel shell [2], bamboo [4], pitch pine [18], pinewood [24], and cotton stalk [47]. The evolution of the characteristic absorbance band of benzene ring skeleton (C]C) at 1680–1440 cm−1 in LC was quite different from that of CC and HC. With the increase in the pyrolysis temperature, this band in the CC and HC gradually faded away. However, a strong absorbance intensity of this band still existed in LC at higher pyrolysis temperature. First, different from cellulose and hemicellulose, lignin is rich in phenylpropane structure unit [28]. Then, as the pyrolysis temperature increased, more graphite-like polyaromatic structure was formed in LC, which led to a strong characteristic absorbance band of benzene ring skeleton at high pyrolysis temperature [48,49].
Fig. 3. Effect of pyrolysis temperature on the ultimate analysis of bio-char derived from three pseudo components of biomass: (a) CC, (b) HC, (c) LC.
3.4. XRD analysis
Furthermore, the H/C and O/C ratios of three types of bio-char sharply decreased at the pyrolysis temperature of 250–450 °C, while a slight decrease of H/C and O/C was observed at higher pyrolysis temperature above 450 °C. At the same pyrolysis temperature, the H/C and O/C of CC and HC were higher than those of LC, because cellulose and hemicellulose contains higher content of oxygen and hydrogen, but less content of carbon as listed in Table 1.
Fig. 5 shows the XRD patterns of bio-char derived from three pseudo components. Four characteristic diffraction peaks at 2θ of 16°, 22°, 26.6°, and 44° are clearly present in the XRD patterns of three types of bio-char. The first two peaks at 16° and 22° are attributed to the typical crystalline structure of the cellulose Iα (triclinic) and cellulose Iβ (monoclinic), respectively [2,50]. The other two peaks at 26.6° and 44° 4
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Fig. 5. XRD analysis of biochar derived from three pseudo components of biomass: (a) CC, (b) HC, (c) LC.
Fig. 4. FTIR analysis of bio-char derived from three pseudo components of biomass at different pyrolysis temperatures: (a) CC, (b) HC, (c) LC.
temperature. However, the intensity of the diffraction peak of graphite (101) increased with the increase in the temperature, indicating the formation of more graphite-like structure at higher pyrolysis temperature [52]. Fig. 5(b) shows that patterns of raw hemicellulose and HC contain two diffraction peaks of cellulose Iα (triclinic) and graphite (101). The evolution of these two peaks was quite similar to that of raw cellulose and CC. Fig. 5(c) exhibits the presence of two tiny diffraction peaks of graphite (002) and graphite (101) for raw lignin and LC. Compared to the raw lignin, a stronger intensity of these two peaks for LC was observed, indicating that polyaromatic graphite-like structure was formed in LC [48].
are contributed by the structure of graphite (002) and graphite (101), respectively [4,51]. Fig. 5(a) demonstrates that raw cellulose and CC contain three diffraction peaks of cellulose Iα (triclinic), cellulose Iβ (monoclinic), and graphite (101). For the C-control and CC-250 samples, two sharp peaks representing cellulose Iα and cellulose Iβ could still be clearly observed. However, with the increase in the pyrolysis temperature above 350 °C, these two sharp peaks turned into one dispersion peak. This result indicated the disappearance of crystalline region of cellulose, because cellulose was degraded into gas or liquid product at higher pyrolysis 5
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Table 2 Effect of pyrolysis temperature on the content of carbon structure in bio-char derived from three pseudo components of biomass. Alkyl–C 045 ppm, (%)
O-alkyl–C 45–95 ppm, (%)
Aryl–C 95–165 ppm, (%)
Carboxylic–C 165–200 ppm, (%)
C-control CC-250 CC-450 CC-650 CC-850 H-control HC-250 HC-450 HC-650 HC-850 L-control LC-250 LC-450 LC-650 LC-850
1 3 16 9 5 0 17 14 8 4 18 24 28 17 6
80 75 5 4 3 82 42 9 6 3 43 24 17 15 12
16 20 77 86 91 11 35 72 82 90 36 39 42 64 79
3 2 2 1 1 7 6 5 4 3 12 13 15 4 3
For the C-control and CC-250 samples, aryl–C (95–165 ppm) and O–alkyl–C (45–90 ppm) were the dominant carbon structures. With the increase in the pyrolysis temperature, aryl–C became the dominant carbon structure with an increase of relative content from 16 to 91%. This result showed that more polyaromatic hydrocarbon structure was formed in CC at higher pyrolysis temperature. However, an opposite trend of the oxygen-containing O–alkyl–C was obtained, with the content decreasing from 80 to 3%. This result indicated that the majority of oxygen element in CC was removed at higher pyrolysis temperature. This result was also confirmed by the ultimate analysis of CC as shown in Fig. 3. For the H-control and HC-250 samples, the dominant carbon structure was also aryl–C and O–alkyl–C. As the pyrolysis temperature increased, the relative content of aryl–C increased from 11 to 90%, while that of oxygen-containing O–alkyl–C decreased from 82 to 3%. Therefore, the evolution of carbon structure depending on pyrolysis temperature was quite similar to that of CC because of the relatively similar molecular structures of cellulose and hemicellulose. Compared to the C-control, CC-250, H-control, and HC-250 samples, more uniform distribution of the four types of carbon structure (carboxylic–C, aryl–C, O–alkyl–C, and alkyl–C) was presented for lignin and LC-250 (as presented in Table 2). However, with the increase in the pyrolysis temperature, the alkyl-C gradually became the dominant carbon structure in LC, with its content increasing from 36 to 79%. The oxygen element mainly existed in the hydroxyl (−OH), carbonyl (–C = O), carboxyl (−COOH), and methoxyl (−OCH3) groups linked on the basic structural unit of three pseudo components. Therefore, the reduction of oxygen-containing carbon structure (carboxylic–C and O–alkyl–C) was mainly contributed by the breakage of these oxygen-containing functional groups by the dehydroxylation, decarboxylation, decarbonylation, and demethoxy reactions [36,38,41]. The alkyl–C was also decreased because of the fact that more aliphatic chain in the three pseudo components was converted into other shorter alkyl moieties or polycyclic aromatic structural units [25]. When the pyrolysis temperature was above 250 °C, entire carbon structure was dominated by aryl–C which was caused by the increased in condensed polycyclic aromatic hydrocarbon units [46,54].
Fig. 6. 13C NMR analysis of biochar derived from three pseudo components of biomass: (a) CC, (b) HC, (c) LC.
3.5. Solid-state
Samples
13
C NMR analysis
NMR spectroscopy (13C NMR) is a good means to characterize the carbon structure in bio-char. Fig. 6 shows the 13C NMR spectra of three types of bio-char derived from three pseudo components. According to the literature [2,53], the carbon structure in bio-char could be mainly divided into four groups, namely, carboxylic–C (165–200 ppm), aryl–C (95–165 ppm), O–alkyl–C (45–90 ppm), and alkyl–C (0–45 ppm). The relative content of the four carbon structures in CC, HC, and CC at different pyrolysis temperatures is listed in Table 2.
3.6. TG analysis The thermal stability of bio-char derived from three pseudo components was investigated using a TG analyzer. Fig. 7 shows the differential thermogravimetric (DTG) curves of bio-char at a heating rate of
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exhibits the clear presence of two weight loss peaks for both H-control and HC-250 samples. This result indicated that H-control and HC-250 samples exhibited wider temperature range of weight loss than C-control and CC-250 samples. However, the weight loss rate at peaks of Hcontrol and HC was much lower than that of C-control and CC samples at their corresponding pyrolysis temperature. Fig. 7(c) demonstrates that LC shows the widest temperature range of weight loss among the three types of bio-char. The maximum weight loss at peaks of LC-350 to LC-850 was higher than that of HC-350 to HC-850 at their corresponding pyrolysis temperature, but was close to CC-350 to CC-850. This result indicated that the thermal stability of HC obtained at higher pyrolysis temperature was better than that of CC and LC. 3.7. Morphological analysis Fig. 8 shows the morphology of bio-char at different pyrolysis temperatures. Fig. 8 (a) shows that with the increase in the pyrolysis temperature, the color of bio-char derived from three pseudo components gradually becomes darker. Moreover, the volume of CC and LC samples reduced because of the significant particle agglomeration during pyrolysis process. However, the volume of HC sample sharply increased when the pyrolysis temperature reached above 350 °C, because of the transformation of hemicellulose into foam-like material at higher temperature. A clear melt phase was formed on the outer surface of HC. Fig. 8(b) shows long strip-shape of the raw cellulose and CC. With the increase in the pyrolysis temperature, the diameter of fiber decreases. The spherical raw hemicellulose was transformed into laminar structural HC by melting and foaming. Moreover, with the increase in the pyrolysis temperature, the pore structure of LC became stronger than that of raw lignin. Some salt crystal particles were also observed on the surface of LC which was probably obtained from the crystallization process of alkali and alkaline metals. 3.8. Prediction of the biomass properties based on the properties of three pseudo components According to the contents of three pseudo components in biomass, the physicochemical characteristics of bio-char derived from real biomass can be predicted based on the physicochemical characteristics of bio-char derived from three pseudo components. In this study, five parameters (C content, H content, O content, mass and energy yields) are selected to demonstrate the feasibility of prediction. They can be calculated by the Eq. (3).
Fig. 7. Thermogravimetric analysis of biochar derived from three pseudo components of biomass: (a) CC, (b) HC, (c) LC.
C content, H content, O content, Mass and Energy yield (%)
10 °C min−1. DTG curves demonstrate that as the carbonization temperature increases, the peak temperature of maximum weight loss rate in all three types of bio-char gradually moves toward the side of high temperature. Furthermore, the value of maximum weight loss rates at peaks gradually decreases. This result indicated that higher pyrolysis temperature promoted the thermostability of bio-char derived from three pseudo components [2]. Significant difference in weight loss characteristics was found among the three types of bio-char. Fig. 7(a) clearly shows two steep weight loss peaks for the C-control and CC-250 samples, with maximum weight loss rates of 67.73 and 47.72% min−1, respectively. However, for the CC-350 to CC-850 samples, the weight loss rate at peaks was in the range of 13.71–21.08% min−1 which is significantly lower than that of C-control and CC-250 samples. Ma et al. [27] reported that the thermal degradation of cellulose mainly occurred in a very narrow temperature range of 180 and 320 °C. Therefore, for the CC-350 to CC850 samples obtained at the pyrolysis temperature above 350 °C, majority of the mass was volatilized during pyrolysis process. Fig. 7(b)
= ax+ by+ cz
(3)
Where x, y, and z are the contents of cellulose, hemicellulose, and lignin in the biomass, respectively; a, b, and c are the contents of C, H, and O, mass yield, and energy yield in the bio-char derived from cellulose, hemicellulose, and lignin at different pyrolysis temperatures. Three species of biomass, namely, palm kernel shell (PKS) [2], Chinese chestnut (CNS) [55], and Jatropha curcas shells (JCS) [55], were selected to verify the accuracy of prediction according to the statistical parameter of mean absolute error (MAE) which was calculated by Eq. (4). Furthermore, lower MAE indicates higher accuracy of prediction.
MAE=
n Parameters predicted Parametersexperimental 1 n i= 1 Parametersexperimental
(4)
Fig. 9 exhibits the linear fitting curves and MAE of experimental and predicted data of C content, H content, O content, mass yield, and
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Fig. 8. Morphology analysis of bio-char derived from three pseudo components of biomass at different pyrolysis temperatures: (a) pictures of bio-char, (b) microstructure of bio-char.
energy yield in the bio-char derived from real biomass, and the detailed data is listed in Table S1. It can been seen that most of the linear fitting curves from predicted value is highly close to that from experimental value, indicating good accuracy of the correlations. The values of MAE obtained from C content, H content, O content, mass yield, and energy yield are 3.94%, 3.99%, 4.02%, 4.01%, and 4.01%, respectively. Therefore, the MAE obtained from five parameters are lower than 5%, indicating their good universal applicability. This result strongly suggests that it is feasible to predict the properties of real biomass based on the properties of three pseudo components.
4. Conclusions Wide variation in the physicochemical properties of bio-char depending on different pyrolysis temperatures and biomass feedstock composition was observed. LC exhibited higher mass and energy yield than CC and HC. Higher pyrolysis temperature led to an increase of carbon content, but a decrease of oxygen and hydrogen contents. Moreover, at higher pyrolysis temperature, the carbon structure of CC, HC, and LC was dominated by the aryl-C, indicating the formation of more polyaromatic graphite-like structure. The volume of CC and LC was reduced due to the particle agglomeration reaction, while the volume of HC sharply increased because of the formation of foam-like
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Fig. 9. Linear fitting curves and MAE of experimental and predicted data of real biomass: (a) C content, (b) H content, (c) O content, (d) mass yield, (e) energy yield, (f) MAE.
Appendix A. Supplementary data
structure. It is feasible to predict the properties of real biomass based on the properties of three pseudo components.
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jaap.2019.03.015.
Acknowledgements
References
This research was supported by Natural Science Foundation of Zhejiang Province (LQ19E060009, LQ17E060002), Key R & D Plan of Zhejiang Province (2018C02008), Natural Science Foundation of China (51706207), China Postdoctoral Science Foundation (2017M611987), Public Welfare Technology Research Fund of Zhejiang Province (LGN18B060001), the Young Elite Scientists Sponsorship Program by CAST (2018QNRC001).
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