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Novel crude glycerol pretreatment for selective saccharification of sugarcane bagasse via fast pyrolysis
Bioresource Technology 294 (2019) 122094
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Novel crude glycerol pretreatment for selective saccharification of sugarcane bagasse via fast pyrolysis
T
⁎
Yaxiang Wua,b, Liqun Jiangb, , Yan Linb, Le Qianb, Feixiang Xub, Xuemei Langa, Shuanshi Fana, Zengli Zhaob, Haibin Lib a Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Guangdong Key Laboratory of New and Renewable Energy Research and Development, Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Fast pyrolysis Levoglucosan Crude glycerol pretreatment Lignocellulose
Pretreatment is a vital process for efficient saccharification and utilization of lignocellulose. In this study, crude glycerol derived from biodiesel production was used for pretreatment to facilitate selective saccharification via fast pyrolysis. Due to the efficient removal of alkali and alkaline earth metals (> 95.0%) and lignin (79.4%) by crude glycerol pretreatment, the yield of levoglucosan was evaluated to 25.2% as compared to those from pure glycerol pretreated (14.4%) and untreated sugarcane bagasse (8.4%). Meanwhile, the production of inhibitors (e.g. acetic acid, phenol) to biocatalysts was also obviously inhibited from crude glycerol pretreated biomass. Consequently, this work provided a cost-effective and eco-friendly pretreatment mode, which could not only make full utilization of crude glycerol, but also improve the fermentability of lignocellulosic pyrolysate.
1. Introduction Concerning the energy and environmental security, it is crucial to search for more sustainable energy system instead of fossil fuels (Shen et al., 2019; Zolghadr et al., 2019). Lignocellulose, as an enormous, renewable and the only carbon-neutral energy resource, can be applied to produce bio-fuels and value-added chemicals (Fang and Smith Jr, 2016; Clomburg et al., 2017). Saccharification plays a considerable role in the chemical conversion and utilization of lignocellulose (Fang, ⁎
2013). Traditionally, this can be achieved by enzyme or acid hydrolysis, which has been extensively investigated (Clomburg et al., 2017). Nevertheless, there is several disadvantages for enzyme hydrolysis, such as a long reaction time (several days), high cost of enzymes, and low concentrations of glucose, drastically limiting its economic feasibility and industrial implementation (Binder and Raines, 2010). In terms of acid hydrolysis, the hazards of operating concentrated acids and the complexity of recycling acids have become a bottle-neck for its large-scale applications (Binder and Raines, 2010).
Corresponding author. E-mail address: [email protected] (L. Jiang).
https://doi.org/10.1016/j.biortech.2019.122094 Received 14 August 2019; Received in revised form 28 August 2019; Accepted 29 August 2019 Available online 04 September 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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it difficult to be used in the field of medicine, food and cosmetics as pure glycerol (Vivek et al., 2017). Purification of crude glycerol is getting less attractive due to the high equipment investment and low glycerol price, while direct applications of crude glycerol are becoming fascinating (Chen et al., 2018). The main component of cured glycerol is glycerol. While, the cheap and abundant features of crude glycerol enable it to be a potential candidate for chemical pretreatment. Herein, for achieving comprehensive utilization of crude glycerol and further reducing the cost of chemical pretreatment, crude glycerol was employed in biomass pretreatment to improve the efficiency of selective conversion in fast pyrolysis.
Despite most of attention has focused on hydrolysis for saccharification, fast pyrolysis is a burgeoning alternative route for biomass conversion. Fast pyrolysis is performed at a moderate temperature (500–750 °C) with a high heating rate (100–500 °C/s) and maintained within an ultrashort time (up to 2 s) (Bridgwater, 2012). The yield of bio-oil from fast pyrolysis is up to 75 wt.%, containing a significant amount of anhydrosugars, where 1,6-anhydro-β-D-glucopyranose (levoglucosan) is the major constituent present in the pyrolysate of lignocellulose. The maximum yield of levoglucosan from microcrystalline cellulose could reach up to 70.1 wt.% (Kwon et al., 2007). As a kind of anhydrosugar, levoglucosan can be metabolized as effectively as glucose by lots of prokaryotic and eukaryotic microorganisms. It has been demonstrated that levoglucosan could be fermented to plenty of chemicals (e.g. ethanol, itaconic acid and lipid, etc.) with considerable yields via microbial fermentation pathway (Jiang et al., 2019a). Besides, levoglucosan can be readily hydrolyzed to glucose by mild acid hydrolysis and then served as a favorable feedstock for fermentation (Abdilla et al., 2018). Economic analysis demonstrated that fast pyrolysis was comparable to enzyme/acid hydrolysis when coupled with a process of fermentation for ethanol production (So and Brown, 1999). Pyrolytic saccharification occurs much faster than hydrolysis, but it is hampered by the lack of specificity. The yield of levoglucosan from fast pyrolysis of lignocellulose is obviously lower than that from pure cellulose (Maduskar et al., 2018). There is enormous information in the literature indicating that the presence of ash and lignin in lignocellulose has negative influence on facilitating the formation of small molecule compounds at the expense of levoglucosan during fast pyrolysis (Hosoya et al., 2007). The low content of levoglucosan enlarges the cost of subsequent product separation and purification thereby weakening its economic viability. Improving the selectivity of levoglucosan from lignocellulose is the most important issue to promote the development of pyrolytic saccharification. Much efforts have been made to enhance the levoglucosan formation from lignocellulose. Pretreating lignocellulose into a suitable raw material prior to fast pyrolysis is regarded as an effective strategy for selective conversion. Considerable attention has been focused on chemical pretreatment due to its high efficiency, low energy consumption and fast reaction. Typically, chemical pretreatment is performed under high pressure, which requires a high equipment investment and leads operational complexity. Previous research has proposed that glycerol pretreatment favored levoglucosan production during fast pyrolysis of lignocellulose (Jiang et al., 2017). The high boiling-point of glycerol enables the pretreatment performed under atmospheric pressure, thereby saving the capital of equipment and showing economic advantages over common chemical pretreatment to some extent. However, chemical pretreatment needs lots of chemicals, and the utilization of pure glycerol for lignocellulosic pretreatment is still an expensive and unprofitable process. Thereby, from the point of economy, applying economical chemicals for this pretreatment is prerequisite. Crude glycerol is a major by-product of biodiesel, which takes up almost 10 wt.% of total end-products (Fig. 1) (Vivek et al., 2017). The dramatic increase in biodiesel production results in the surfeit of glycerol and the availability of cheap glycerol resource (Kim and Moon, 2019). The annual production of crude glycerol is around 600 million tons, and more than 65% of it comes from the transesterification of biodiesel, which is much more than the demand of global market (Anitha et al., 2016; Gao et al., 2016). Therefore, the surplus of crude glycerol, which should not be discharged without any treatment, has become a burden for biodiesel plants. At present, the main disposal approach of excessive crude glycerol is combustion, but it is easily polymerized or partially oxidized to highly toxic acrolein at high temperatures, which might cause serious pollution to environment and human health (Steinmetz et al., 2013). Crude glycerol is a mixture, including glycerol (70–98%), biodiesel, fatty acids, methanol, water, soap, ash and trace amounts of monoglycerides and diglycerides (Anitha et al., 2016). The complex components of crude glycerol make
2. Materials and methods 2.1. Materials Sugarcane bagasse was obtained from Guangzhou, China. The sugarcane bagasse was pulverized and screened to 60–80 mesh, and then dried to constant weight. Glycerol (purity > 99.7%) was purchased from Tianjin Yongda Reagent Co., Ltd. The crude glycerol A and B were obtained by transesterification reaction between soybean oil and methanol using sodium silicate (Na2SiO3·9H2O) and calcined sodium silicate (Na2SiO3) as catalysts, respectively (Zhang et al., 2016). Calcined sodium silicate was prepared by heating sodium silicate at 400 °C for 2 h and then triturating it to particles of 0.5 μm. The molar ratios of methanol to soybean oil and methanol to catalyst were 15:1 and 40:1, respectively. The reaction was held at 65 °C for 3 h. After this process, the solution was stratified after standing. The heavy layer was taken out for distillation. Most of methanol was separated for reuse, and the crude glycerol A and B were collected for analysis and pretreatment. 2.2. Crude glycerol componential analysis The component of crude glycerol was identified by GC–MS (TRACE 1300/ISQ QD). A TG-WAXms column was employed for the separation. The temperature of column was set at 60 °C for 2 min, then heated to 240 °C at a heating rate of 10 °C/min followed with a dwell time of 10 min. Helium was served as the carrier gas, and the flow rate was set at 1.2 mL/min. The split ratio was set at 50:1. MS was performed with the following operation conditions: transfer line, 260 °C; electron energy, 70 eV; ion source, 280 °C; scanning range (m/z), 15 ~ 350 amu. 2.3. Crude glycerol pretreatment The pretreatment of sugarcane bagasse was performed in a microwave reactor (MCR-3, Star Shuo Instrument Co., Ltd, Guangzhou, China), which consisted of a microwave generator system and a temperature control system. Crude glycerol and sugarcane bagasse were mixed in a flat bottom two-neck flask with a ratio of 10:1 (w/w). The mixture was heated to 220 °C by microwave irritation and maintained at 220 °C for 6 min. Then, the solid was filtrated and washed repeatedly with deionized water to remove residual chemicals. The solid residue was dried by a vacuum freeze dryer, which could dry the sample while maintain the inner structure. Pure glycerol pretreatment was used as a control group. 2.4. Elemental and compositional analysis The content of three main organic elements (C, H, N) in sugarcane bagasse was measured by an elemental analyzer (Vario EL cube, Elementar, Germany). The content of alkali and alkaline earth metals (K, Ca, Na, Mg) was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Optima 8000, PerkinElmer, USA). Firstly, the sugarcane bagasse (approximately 0.2 g) was digested in a test tube containing a mixed acid (1 mL HClO4 and 3 mL HNO3) solution for releasing metal elements in the form of ions. Then, the solution 2
Fig. 1. Schematic illustration of transesterification process.
Y. Wu, et al.
Bioresource Technology 294 (2019) 122094
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Fig. 2. Componential analysis of crude glycerol A and B.
was diluted to 20 mL and then filtered with 0.22 μm filter. Quantitative analysis was conducted in an external standard method. The content of cellulose, hemicellulose and lignin in sugarcane bagasse was determined by the method of NREL (Sluiter et al., 2008). In this process, the cellulose and hemicellulose were converted into monosaccharides by two-step acid hydrolysis. The content of released sugar was measured by HPLC system (Alliance 2695, Waters, America). An Aminex HPX-87P column (Bio-Rad) and a refractive index detector (Alliance 2414, Waters, America) were used for separation and detection, respectively. The temperatures of column and detector were 80 °C and 50 °C, respectively. Deionized water (0.3 mL/min) was employed as the mobile phase. The content of cellulose and hemicellulose was calculated according to the content of monosaccharide. Acid-soluble lignin was detected by ultraviolet-visible spectroscopy (λ = 320 nm). The solid residue was burned at 550 °C for 3 h, and its mass loss was defined as acid-insoluble lignin.
Ψ(E , T ) =
∫0
T
u′= E′ R′T ′
−E ⎞ dT exp ⎛ ⎝ RT ⎠
→
f (E ) =
(E − E0 ) − 1 e 2σ 2 2π σ
A pyroprobe 5200 CDS reactor (U.S.A.) coupled with Agilent GC (7890A, U.S.A.) and Agilent MS (5975C, U.S.A.) was used for fast pyrolysis of sugarcane bagasse and analysis of pyrolysis product. Fast pyrolysis of biomass occurred at 500 °C. The heating rate was set at 20 K ms−1. The volatile generated from fast pyrolysis was purged to GC–MS system by helium (20 mL/min) through a transfer line. Product separation was realized by a DB-1701 capillary column. The temperature program of GC oven started at 40 °C and remained for 3 min, then heated to 280 °C with a heating rate of 10 °C/min, and hold for 8 min. The mass spectrometer had an ion source of 230 °C, a quadrupole of 150 °C, an EI ion source of 70 eV, and a scan range (m/z) of 29–450 amu. Qualitative and quantitative analysis of pyrolysis products was carried out by NIST 14 library search and an external standard method. The yield of levoglucosan and relative content of other compounds were calculated as the following equation:
Relative content of compound (%) =
(1)
where P is the integrated pyrolysis coefficient; DTGmax is the maximum decomposition rate; Tmax is the temperature of the maximum decomposition rate; Ti and Tf are the temperature of initial and final decomposition, respectively. The kinetic parameters were obtained by the temperature integral of Distributed Activation Energy Model (DAEM). The standard form of DAEM was: T
−E ⎞ dT A exp ⎛ ⎝ RT ⎠
∫0
∞
A exp ⎡− 0 Ψ(E , T ) ⎤ f (E ) dE ⎢ ⎥ ⎣ β ⎦
Mass of levoglucosan × 100% Mass of cellulose
Area of a compound × 100% Area of all compunds
(6)
(7)
3. Results and discussion 3.1. Componential analysis of crude glycerol The proportion of each component in crude glycerol A and B is exhibited in Fig. 2. Glycerol was proved to occupy a considerable proportion in both crude glycerol A and B, which was 88.2% and 84.6%, respectively. The component of crude glycerol prepared in the paper was similar to the component of crude glycerol derived from biodiesel in the industry (Vivek et al., 2017). Besides glycerol, some residual alcohol and fatty acids, such as (Z, Z)-9,12-octadecadienoic acid and n-hexadecanoic acid were also contained in crude glycerol. The residual alcohol came from the excessive addition of alcohol for transesterification reaction. The content of (Z, Z)-9,12-octadecadienoic acid (6.5% vs 6.2%) and n-hexadecanoic acid (2.1% vs 1.4%) contained in crude glycerol B was higher than that of crude glycerol A. This phenomenon might attribute to the influence of water introduced into crude glycerol A along with sodium silicate in transesterification
(2)
where f(α) refers to the most probable mechanism function, E refers to the activation energy (kJ/mol), R refers to the universal gas constant, A refers to the pre-exponential factor, T and t refer to the reaction temperature (K) and time (min), respectively. The fundamental equation of DAEM was:
α =1 −
(4)
2.6. Fast pyrolysis
DTGmax P= Tmax × (Tf - T) i
∫0
exp(−u′) du′ u′ 2
(5)
Levoglucosan yield (wt.%) =
dα 1 = G (α ) = f (α ) β
∞
2
Thermal decomposition of biomass was compared by thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis using a simultaneous thermal analyzer (STA449 F3, Netzsh, Germany). In a typical thermogravimetric analysis, sample (around 10 mg) was added into an Al2O3 crucible and heated to 800 °C from 35 °C at a heating rate of 10 °C min−1. The integrated pyrolysis coefficient P was introduced to reflect the severity of pyrolysis process. P was defined as the following formula:
α
∫u
The density distribution f(E) was defined as:
2.5. Thermogravimetric analysis
∫0
E ⎛ ⎞ ⎝R⎠
(3)
where Ψ (E,T) was the temperature integral: 4
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Table 1 Elemental analysis of raw and pretreated sugarcane bagasse. Samples
Un-treated Pure glycerol pretreated Crude glycerol A pretreated Crude glycerol B pretreated
Organic elements
AAEMs
C (wt.%)
H (wt.%)
N (wt.%)
C/H
K (mg/kg)
Ca (mg/kg)
Na (mg/kg)
Mg (mg/kg)
Total (mg/kg)
48.2 46.6 45.3 43.6
5.9 6.2 6.3 6.4
0.1 0.0 0.0 0.0
8.2 7.5 7.2 6.8
876.0 53.3 63.6 84.2
1550.7 832.1 67.6 53.5
449.9 360.2 5.0 6.3
208.9 70.5 0.5 0.5
3085.5 1316.1 136.7 144.5
reaction, which could promote the saponification reaction between fatty acid and sodium silicate to form fatty acid sodium, thereby reducing the content of fatty acids in crude glycerol A.
Table 2 Componential analysis of raw and pretreated sugarcane bagasse.
3.2. Elemental analysis of raw and pretreated sugarcane bagasse The analysis of carbon, hydrogen, and nitrogen of biomass is given in Table 1. In original sample, the content of carbon, hydrogen and nitrogen was 48.2 wt.%, 5.9 wt.% and 0.1 wt.%, respectively. After pretreatment, carbon occupied a lower proportion in the pretreated sugarcane bagasse, and hydrogen had the opposite variation. The ratio of C/H was reduced from 8.2 to 6.8 after pretreatment. It was reported that the C/H ratio of lignin (11.1) was higher than that of cellulose (7.5) and hemicellulose (7.2) (Qu et al., 2011). Moreover, the C/H ratio of crude glycerol B pretreated biomass was lower than crude glycerol A pretreated sample, which might indicate that crude glycerol B had a better performance on lignin removal. Besides organic elements, lots of interests were lied on inorganic elements, especially the alkali and alkaline earth metals (AAEMs) due to their high abundance relative to other elements. The result of ICP-OES shown that there was high content of AAEMs (3085.5 mg/kg) present in original sugarcane bagasse (Table 1). After pure glycerol pretreatment, a notable reduction in the content of AAEMs was observed, and the total content of AAEMs declined to 1316.1 mg/kg at a 57.3% removal rate, while crude glycerol pretreatment benefited further demineralization. The total content of AAEMs declined to 136.7–144.5 mg/kg for crude glycerol pretreated sample. More than 95.0% of AAEMs were removed by crude glycerol pretreatments. Particularly, the removal of potassium was mostly obvious by both pure and crude glycerol pretreatment. The levoglucosan yield was sensitive to the purity of the initial cellulose, which was exponentially reduced by the presence of inorganic salts, especially AAEMs. It had been revealed that there was a competitive pathway between levoglucosan and low-molecular species during fast pyrolysis of biomass (Lindstrom et al., 2019). Levoglucosan was derived from the cleavage reaction of β-1,4-glycosidic bond, which preserved the structure of pyranose rings, whilst low-molecule species (e.g. acetaldehyde) was originated from homolytic fission of the pyranose rings (Patwardhan et al., 2010). It was reported that the AAEMs could form a coordinate bond with the oxygen atoms of the vicinal hydroxyl groups on pyranose rings, which lowered the stability of pyranose rings (Kuzhiyil et al., 2012). As a result, the homolytic fission of the pyranose rings was strengthened, which favored the accumulation of low molecular species. It was noteworthy that even AAEMs at a trace level could alter the primary pyrolytic pathway of cellulose, resulting in the formation of small molecule compounds at the expense of levoglucosan (Kuzhiyil et al., 2012). With respect to the reduction of levoglucosan production, order from the strongest to weakest catalytic influence was K+ > Na+ > Ca2+ > Mg2+ (Patwardhan et al., 2010). Consequently, the significant decrease of AAEMs content for pretreated biomass might favor the formation of levoglucosan in fast pyrolysis.
Samples
Cellulose (wt. %)
Hemicellulose (wt.%)
Lignin (wt. %)
Un-treated Pure glycerol pretreated Crude glycerol A pretreated Crude glycerol B pretreated
40.0 51.6 57.6
17.1 16.1 17.4
20.9 17.2 11.5
66.5
17.0
4.3
raw sugarcane bagasse consisted of 40.0 wt.% cellulose, 17.1 wt.% hemicellulose and 20.9 wt.% lignin. After pure glycerol pretreatment at 220 °C for 6 min, the pretreated biomass contained 51.6 wt.% cellulose, 16.1 wt.% hemicellulose, and 17.2 wt.% lignin, respectively. Pure glycerol pretreatment could dissolve some lignin and hemicellulose, resulting in the enrichment of cellulose fraction in the solid residue, which was consistent with previous results (Jiang et al., 2017). The crude glycerol pretreated sample was basically composed of 57.6–66.5 wt.% cellulose, 17.0–17.4 wt.% hemicellulose and 4.3–11.5 wt.% lignin. It was found that crude glycerol pretreatment could selectively remove lignin from biomass leading to a further accumulation of cellulose, while the content of hemicellulose exhibited negligible change before and after pretreatments. In terms of lignin removal rate, the following trends was observed: crude glycerol B > crude glycerol A > pure glycerol. The least lignin content (4.3 wt.%) was obtained in sample pretreated by crude glycerol B. This meant that 79.4% of lignin in the sugarcane bagasse could be removed under this condition. Residual alkaline present in the crude glycerol might favor the removal of lignin (Jiang et al., 2019b). Previous research had indicated that cellulosic sample with higher cellulose content could offer much higher yields of levoglucosan (Miura et al., 2001). During fast pyrolysis, there was some interaction amongst the lignocellulosic components. The interaction of cellulose-hemicellulose was weaker than that of cellulose-lignin. It had been established that an apparent interaction was observed in cellulose-lignin as the yield of levoglucosan diminished (Zhang et al., 2015). The presence of lignin suppressed the formation of levoglucosan, and the intensively boosted undesirable formation of other compounds (Hosoya et al., 2007). Fortunately, the lignin fraction was removed by crude glycerol pretreatment in this study, which might be beneficial to levoglucosan production.
3.4. Thermogravimetric analysis of biomass The TG and DTG curves acquired in thermogravimetric analysis are displayed in Fig. 3. The pyrolytic and kinetic parameters are tabulated in Tables 3 and 4. The native sample began to degrade at 235.3 °C. For crude glycerol pretreated A (257.7 °C) and B (268.8 °C) samples exhibited higher onset temperatures for decomposition. The maximum pyrolysis rate (DTGmax) of un-pretreated sugarcane bagasse was about 1.1%/°C, which was shifted to higher values after pure or crude glycerol pretreatments. The severity of pyrolysis was indicated by integrated pyrolysis coefficient P. It could be seen that the P value of the original
3.3. Compositional analysis of raw and pretreated sugarcane bagasse The chemical composition of biomass is exhibited in Table 2. The 5
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Fig. 3. TG (a) and DTG (b) curve of raw and pretreated sugarcane bagasse.
sample was only 2.6%/°C2, which was evaluated to higher values for the pure glycerol pretreated (4.2) and crude glycerol pretreated biomass (6.0–9.1%/°C2). Kinetic analysis indicated that pretreated samples exhibited higher E values (230.1–232.1 kJ/mol) compared to raw material (226.4 kJ/mol). Higher E meant higher thermal stability of the pretreated sample, and more energy input was required for initial degradation. The lower value of σ indicated that the decomposition of pretreated samples was concentrated and reacted explosively (Lin et al., 2019). It was also found that all adjusted coefficients (Radj2) fitted by the model were high (> 99%), which implied that the prediction matched the experimental data very well. Several factors might be responsible for those differences in pyrolytic and kinetic analysis. The presence of AAEMs, especially K, could form coordinate bonds with the oxygen atoms on the pyranose rings of lignocellulose, which lowered the stability. It was known that AAEMs could catalyze the pyrolysis reaction to lower initial pyrolysis temperature, decrease the decomposition temperature and the maximum pyrolysis rate, and reduce the activation energy of the reaction (Le Brech et al., 2016). The removal of AAEMs could reduce their catalytic effects. Therefore, as the content of AAEMs declined, the pyrolytic and kinetic parameters shifted to higher values. Furthermore, pretreatment made the pyrolysis process more intensive, which could also be explained by the alteration of lignocellulosic constituent. Lignin had the highest stability and widest temperature range of pyrolysis, while the pyrolysis of cellulose was easier and occurred in a narrower temperature range. The removal of lignin and accumulation of cellulose by pretreatment made the pyrolytic reaction more concentrated and intense.
Table 4 The kinetic parameters of raw and pretreated sugarcane bagasse. Samples
Log10(A) (min−1)
E (kJ/ mol)
σ (kJ/ mol)
Radj2 (%)
Un-treated pretreated Pure glycerol pretreated Crude glycerol A pretreated Crude glycerol B pretreated
20 20 20
226.4 232.1 230.1
15.7 11.3 8.6
99.8 99.6 99.6
20
231.3
5.7
99.7
Fig. 4. Products distribution from fast pyrolysis of biomass.
3.5. Fast pyrolysis products analysis of biomass pyrolysate was a complex mixture of various organic compounds, including acids, ketones, aldehydes, furans, phenols, esters and
The distribution of pyrolytic product from raw and pretreated sugarcane bagasse is summarized in Figs. 4 and 5. Lignocellulosic Table 3 The pyrolytic parameters of raw and pretreated sugarcane bagasse. Samples
Ti (°C)
Tmax (°C)
DTGmax (%/°C)
Tf (°C)
Residue (%)
P × 10−5 (%/°C2)
Un-treated pretreated Pure glycerol pretreated Crude glycerol A pretreated Crude glycerol B pretreated
235.3 253.5 257.7 268.8
337.6 340.9 331.0 329.7
1.1 1.6 1.9 2.4
358.3 364.0 352.8 348.4
12.7 13.0 12.0 11.1
2.6 4.2 6.0 9.1
Ti: The temperature of initial volatilization (5% loss in mass). Tmax: The temperature corresponding to the maximum decomposition rate. Tf: The temperature at the inflection point of about 350 °C in the DTG curve. DTGmax: The maximum decomposition rate. Residue: The proportion of pyrolytic residue. P: Integrated pyrolysis coefficient. 6
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used for pretreatment to promote the levoglucosan formation from sugarcane bagasse via fast pyrolysis. The yield of levoglucosan (25.2%) from crude glycerol pretreated sample was obviously improved as compared to those from pure glycerol pretreated (14.4%) and un-pretreated sugarcane bagasse (8.4%) due to the efficient removal of lignin and AAEMs. Meanwhile, crude glycerol pretreatment suppressed the formation of inhibitors (e.g. acid, furans, phenols) to the biocatalyst. Accordingly, this work provided an economically-viable and environmentally-benign strategy to make full utilization of crude glycerol and improve the fermentability of lignocellulosic pyrolysate. Funding This work was funded by the Science and Technology Planning Project of Guangzhou city and Guangdong province (No. 2017A020216007, 201707010236), the National Natural and Science Foundation of China (No. 51606204), and the Key Laboratory of Microbial Technology Open Projects (M2019-10).
Fig. 5. Relative content of main compounds from fast pyrolysis of biomass.
anhydrosugars, etc. The target product, levoglucosan, had a considerable proportion in the pyrolysate. For raw material, the yield of levoglucosan was 8.4%. After pure glycerol pretreatment, the yield of levoglucosan rose to 14.4%. Crude glycerol pretreatment further improved levoglucosan yield (21.0–25.2%). After pretreatment, the content of acids (from 9.3% to 5.4%), furans (from 13.6% to 9.5%) and phenolic compounds (from 11.9% to 2.3%) derived from fast pyrolysis of biomass exhibited an inverse trend and declined obviously (Fig. 4). Especially, the content of acetic acid diminished intensively to 0.6% from 7.2% (Fig. 5). Among the furan compounds, the content of 2,3dihydro-benzofuran had the most obvious diversification, and its content was reduced to 0.6% from 12.1% in the pyrolysate of crude glycerol A pretreated biomass. As regard to phenolic compounds, the content of phenol (from 0.6% to 0.1%) and 2-methoxy-4-vinylphenol (from 3.1% to 0.4%) was also decreased remarkably. The content of 2,3-butanedione, a type of ketone compound, was also reduced by about 54.4% after pretreatment. The alteration of the lignocellulosic component might cause the difference on distribution of pyrolytic products. It was believed that the presence of AAEMs limited the primary reaction leading to levoglucosan production, but promoted fragmentation, ring opening, and dehydration reactions to yield lower molecular weight oxygenates during cellulose pyrolysis (Le Brech et al., 2016). The remove of AAEMs might suppress the formation of low weight molecular compounds. Phenolic compound was derived from lignin and its reduction might be attributed to the removal of lignin fraction by crude glycerol pretreatment. The low yield of levoglucosan from lignocellulosic biomass, and the presence of inhibitors to biocatalysts hampered the fermentable utilization of the pyrolysate (Jiang et al., 2019c). It was reported that in the fermentation of pyrolysate, carboxylic acids and phenols were the most toxic inhibitors, while furans and alkanes were mildly inhibitory. In this paper, the yield of furfural increased to some extent (from 1.6 to 2.0%). While, the yields of acetic acid (from 7.2 to 0.6%) and phenols (from 0.6 to 0.1%) reduced significantly. Thereby, as a whole, the crude glycerol pretreatment was able to reduce inhibitors. Obviously, the enrichment of levoglucosan and the reduction of inhibitors in pyrolysate could enhance the feasibility of fermentation. After crude glycerol pretreatment, a great deal of glycerol existed in the liquid stream still caused a significant environmental pollution and resource waste. In previous research, it had been demonstrated that the recovered glycerol could be served as an attractive substrate for the fermentation of various value-added compounds (e.g. 2,3-butanediol, lactate) (Jiang et al., 2017, 2019d). In the next work, the fermentability of the pyrolysate from crude glycerol pretreated biomass will be evaluated.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122094. References Abdilla, R.M., Rasrendra, C.B., Heeres, H.J., 2018. Kinetic studies on the conversion of levoglucosan to glucose in water using bronsted acids as the catalysts. Ind. Eng. Chem. Res. 57 (9), 3204–3214. https://doi.org/10.1021/acs. iecr.8b00013. Anitha, M., Kamarudin, S.K., Kofli, N.T., 2016. The potential of glycerol as a value-added commodity. Chem. Eng. J. 295, 119–130. https://doi.org/10.1016/j.cej.2016.03. 012. Binder, J.B., Raines, R.T., 2010. Fermentable sugars by chemical hydrolysis of biomass. PNAS 107 (10), 4516–4521. https://doi.org/10.1073/pnas.0912073107. Bridgwater, A.V., 2012. Review of fast pyrolysis of biomass and product upgrading. Biomass Bioenergy 38, 68–94. https://doi.org/10.1016/j.biombioe. 2011.01.048. Chen, J.X., Yan, S., Zhang, X.L., Tyagi, R.D., Surampalli, R.Y., Valero, J.R., 2018. Chemical and biological conversion of crude glycerol derived from waste cooking oil to biodiesel. Waste Manage. 71, 164–175. https://doi.org/10.1016/j.wasman.2017. 10.044. Clomburg, J.M., Crumbley, A.M., Gonzalez, R., 2017. Industrial biomanufacturing: The future of chemical production. Science 355 (6320), aag0804. https://doi.org/10. 1126/science.aag0804. Fang, Z., 2013. Pretreatment Techniques for Biofuels and Biorefineries. Springer, Heidelberg https://doi.org/10.1007/978-3-642-32735-3. Fang, Z., Smith Jr., R.L., 2016. Production of biofuels and chemicals from lignin. Biofuels and Biorefineries. Springer https://doi.org/10.10 07/978-981-10-1965-4. Gao, C.J., Yang, X.F., Wang, H.M., Rivero, C.P., Li, C., Cui, Z.Y., Qi, Q.S., Lin, C.S.K., 2016. Robust succinic acid production from crude glycerol using engineered Yarrowia lipolytica. Biotechnol. Biofuels 9 (1), 179. https://doi.org/10.1186/s13068-0160597-8. Hosoya, T., Kawamoto, H., Saka, S., 2007. Cellulose-hemicellulose and cellulose-lignin interactions in wood pyrolysis at gasification temperature. J. Anal. Appl. Pyrolysis 80 (1), 118–125. https://doi.org/10.1016/j.jaap.2007.01.006. Jiang, L.Q., Wu, Y.X., Wang, X.B., Zheng, A.Q., Zhao, Z.L., Li, H.B., Xun, J.F., 2019d. Crude glycerol pretreatment for selective saccharification of lignocellulose via fast pyrolysis and enzyme hydrolysis. Energy Convers. Manage. 199, 111894. https://doi. org/10.1016/j.enconman.2019.111894. Jiang, L.Q., Fang, Z., Zhao, Z.L., Zheng, A.Q., Wang, X.B., Li, H.B., 2019a. Levoglucosan and its hydrolysates via fast pyrolysis of lignocellulose for microbial biofuels: A stateof-the-art review. Renewable Sustainable Energy Rev. 105, 215–229. https://doi.org/ 10.1016/j.rser.2019.01.055. Jiang, L.Q., Wu, Y.X., Zhao, Z.L., Li, H.B., Zhao, K., Zhang, F., 2019b. Selectively biorefining levoglucosan from NaOH pretreated corncobs via fast pyrolysis. Cellulose. https://doi.org/10.1007/s10570-019-02625-4. Jiang, L.Q., Wu, N.N., Zheng, A.Q., Wang, X.B., Liu, M., Zhao, Z.L., He, F., Li, H.B., Feng, X.J., 2017. Effect of glycerol pretreatment on levoglucosan production from corncobs by fast pyrolysis. Polymers 9 (11), 599. https://doi.org/10.3390/polym9110599. Jiang, L.Q., Zheng, A.Q., Meng, J.G., Wang, X.B., Zhao, Z.L., Li, H.B., 2019c. A comparative investigation of fast pyrolysis with enzymatic hydrolysis for fermentable sugars production from cellulose. Bioresour. Technol. 274, 281–286. https://doi.org/ 10.1016/j.biortech.2018.11.098. Kim, Y.C., Moon, D.J., 2019. Sustainable process for the synthesis of value-added products using glycerol as a useful raw material. Catal. Surv. Asia 23 (1), 10–22. https:// doi.org/10.1007/s10563-018-09263-z. Kuzhiyil, N., Dalluge, D., Bai, X.L., Kim, K.H., Brown, R.C., 2012. Pyrolytic sugars from cellulosic biomass. ChemSusChem 5 (11), 2228–2236. https://doi.org/10.1002/cssc. 201200341.
4. Conclusions In this work, crude glycerol derived from biodiesel production was 7
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pretreatment for cellulose enzymatic hydrolysis and lignin valorization. Green Chem. 21 (2), 275–283. https://doi.org/10.1039/c8gc03064b. Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeton, D., Crocker, D., 2008. Determination of Structural Carbohydrates and Lignin in Biomass. National Renewable Energy Laboratory. So, K.S., Brown, R.C., 1999. Economic analysis of selected lignocellulose- to-ethanol conversion technologies. Appl. Biochem. Biotechnol. 77–79, 634–640. https://doi. org/10.1007/978-1-4612-1604-9_57. Steinmetz, S.A., Herrington, J.S., Winterrowd, C.K., Roberts, W.L., Wendt, J.O.L., Linak, W.P., 2013. Crude glycerol combustion: Particulate, acrolein, and other volatile organic emissions. Proc. Combust. Inst. 34 (2), 2749–2757. https://doi.org/10.1016/j. proci.2012.07.050. Vivek, N., Sindhu, R., Madhavan, A., Anju, A.J., Castro, E., Faraco, V., Pandey, A., Binod, P., 2017. Recent advances in the production of value added chemicals and lipids utilizing biodiesel industry generated crude glycerol as a substrate- -metabolic aspects, challenges and possibilities: An overview. Bioresour. Technol. 239, 507–517. https://doi.org/10.1016/j.biortech.2017.05.056. Zhang, F., Wu, X.H., Yao, M., Fang, Z., Wang, Y.T., 2016. Production of biodiesel and hydrogen from plant oil catalyzed by magnetic carbon-supported nickel and sodium silicate. Green Chem. 18 (11), 3302–3314. https://doi.org/10.1039/c5gc02680f. Zhang, J., Choi, Y.S., Yoo, C.G., Kim, T.H., Brown, R.C., Shanks, B.H., 2015. Cellulosehemicellulose and cellulose-lignin interactions during fast pyrolysis. ACS Sustainable Chem. Eng. 3 (2), 293–301. https://doi.org/10.1021/sc500664h. Zolghadr, A., Kelley, M.D., Sokhansefat, G., Moradian, M., Sullins, B., Ley, T., Biernacki, J.J., 2019. Biomass microspheres - A new method for characterization of biomass pyrolysis and shrinkage. Bioresour. Technol. 273, 16–24. https://doi.org/10.1016/j. biortech.2018.09.137.
Kwon, G.J., Kim, D.Y., Kimura, S., Kuga, S., 2007. Rapid-cooling, continuous- feed pyrolyzer for biomass processing. J. Anal. Appl. Pyrolysis. 80 (1), 1–5. https://doi.org/ 10.1016/j.jaap.2006.12.012. Le Brech, Y., Ghislain, T., Leclerc, S., Bouroukba, M., Delmotte, L., Brosse, N., Snape, C., Chaimbault, P., Dufour, A., 2016. Effect of potassium on the mechanisms of biomass pyrolysis studied using complementary analytical techniques. ChemSusChem 9 (8), 863–872. https://doi.org/10.1002/cssc.20150 1560. Lin, Y., Tian, Y.L., Xia, Y.Q., Fang, S.W., Liao, Y.F., Yu, Z.S., Ma, X.Q., 2019. General distributed activation energy model 2019 (G-DAEM) on co-pyrolysis kinetics of bagasse and sewage sludge. Bioresour. Technol. 273, 545–555. https://doi.org/10. 1016/j.biortech.2018.11.051. Lindstrom, J.K., Proano-Aviles, J., Johnston, P.A., Peterson, C.A., Stansell, J.S., Brown, R.C., 2019. Competing reactions limit levoglucosan yield during fast pyrolysis of cellulose. Green Chem. 21 (1), 178–186. https://doi.org/10.1039/c8gc03461c. Maduskar, S., Maliekkal, V., Neurock, M., Dauenhauer, P.J., 2018. On the yield of levoglucosan from cellulose pyrolysis. ACS Sustainable Chem. Eng. 6 (5), 7017–7025. https://doi.org/10.1021/acssuschemeng.8b00853. Miura, M., Kaga, H., Yoshida, T., Ando, K., 2001. Microwave pyrolysis of cellulosic materials for the production of anhydrosugars. J. Wood Sci. 47 (6), 502–506. https:// doi.org/10.1007/BF00767906. Patwardhan, P.R., Satrio, J.A., Brown, R.C., Shanks, B.H., 2010. Influence of inorganic salts on the primary pyrolysis products of cellulose. Bioresour. Technol. 101 (12), 4646–4655. https://doi.org/10.1016/j.biortech.2010.01.112. Qu, T.T., Guo, W.J., Shen, L.H., Xiao, J., Zhao, K., 2011. Experimental study of biomass pyrolysis based on three major components: hemicellulose, cellulose, and lignin. Ind. Eng. Chem. Res. 50 (18), 10424–10433. https://doi.org/10.1021/ie1025453. Shen, X.J., Wen, J.L., Mei, Q.Q., Chen, X., Sun, D., Yuan, T.Q., Sun, R.C., 2019. Facile fractionation of lignocelluloses by biomass-derived deep eutectic solvent (DES)
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Novel crude glycerol pretreatment for selective saccharification of sugarcane bagasse via fast pyrolysis
T
⁎
Yaxiang Wua,b, Liqun Jiangb, , Yan Linb, Le Qianb, Feixiang Xub, Xuemei Langa, Shuanshi Fana, Zengli Zhaob, Haibin Lib a Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Guangdong Key Laboratory of New and Renewable Energy Research and Development, Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Fast pyrolysis Levoglucosan Crude glycerol pretreatment Lignocellulose
Pretreatment is a vital process for efficient saccharification and utilization of lignocellulose. In this study, crude glycerol derived from biodiesel production was used for pretreatment to facilitate selective saccharification via fast pyrolysis. Due to the efficient removal of alkali and alkaline earth metals (> 95.0%) and lignin (79.4%) by crude glycerol pretreatment, the yield of levoglucosan was evaluated to 25.2% as compared to those from pure glycerol pretreated (14.4%) and untreated sugarcane bagasse (8.4%). Meanwhile, the production of inhibitors (e.g. acetic acid, phenol) to biocatalysts was also obviously inhibited from crude glycerol pretreated biomass. Consequently, this work provided a cost-effective and eco-friendly pretreatment mode, which could not only make full utilization of crude glycerol, but also improve the fermentability of lignocellulosic pyrolysate.
1. Introduction Concerning the energy and environmental security, it is crucial to search for more sustainable energy system instead of fossil fuels (Shen et al., 2019; Zolghadr et al., 2019). Lignocellulose, as an enormous, renewable and the only carbon-neutral energy resource, can be applied to produce bio-fuels and value-added chemicals (Fang and Smith Jr, 2016; Clomburg et al., 2017). Saccharification plays a considerable role in the chemical conversion and utilization of lignocellulose (Fang, ⁎
2013). Traditionally, this can be achieved by enzyme or acid hydrolysis, which has been extensively investigated (Clomburg et al., 2017). Nevertheless, there is several disadvantages for enzyme hydrolysis, such as a long reaction time (several days), high cost of enzymes, and low concentrations of glucose, drastically limiting its economic feasibility and industrial implementation (Binder and Raines, 2010). In terms of acid hydrolysis, the hazards of operating concentrated acids and the complexity of recycling acids have become a bottle-neck for its large-scale applications (Binder and Raines, 2010).
Corresponding author. E-mail address: [email protected] (L. Jiang).
https://doi.org/10.1016/j.biortech.2019.122094 Received 14 August 2019; Received in revised form 28 August 2019; Accepted 29 August 2019 Available online 04 September 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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it difficult to be used in the field of medicine, food and cosmetics as pure glycerol (Vivek et al., 2017). Purification of crude glycerol is getting less attractive due to the high equipment investment and low glycerol price, while direct applications of crude glycerol are becoming fascinating (Chen et al., 2018). The main component of cured glycerol is glycerol. While, the cheap and abundant features of crude glycerol enable it to be a potential candidate for chemical pretreatment. Herein, for achieving comprehensive utilization of crude glycerol and further reducing the cost of chemical pretreatment, crude glycerol was employed in biomass pretreatment to improve the efficiency of selective conversion in fast pyrolysis.
Despite most of attention has focused on hydrolysis for saccharification, fast pyrolysis is a burgeoning alternative route for biomass conversion. Fast pyrolysis is performed at a moderate temperature (500–750 °C) with a high heating rate (100–500 °C/s) and maintained within an ultrashort time (up to 2 s) (Bridgwater, 2012). The yield of bio-oil from fast pyrolysis is up to 75 wt.%, containing a significant amount of anhydrosugars, where 1,6-anhydro-β-D-glucopyranose (levoglucosan) is the major constituent present in the pyrolysate of lignocellulose. The maximum yield of levoglucosan from microcrystalline cellulose could reach up to 70.1 wt.% (Kwon et al., 2007). As a kind of anhydrosugar, levoglucosan can be metabolized as effectively as glucose by lots of prokaryotic and eukaryotic microorganisms. It has been demonstrated that levoglucosan could be fermented to plenty of chemicals (e.g. ethanol, itaconic acid and lipid, etc.) with considerable yields via microbial fermentation pathway (Jiang et al., 2019a). Besides, levoglucosan can be readily hydrolyzed to glucose by mild acid hydrolysis and then served as a favorable feedstock for fermentation (Abdilla et al., 2018). Economic analysis demonstrated that fast pyrolysis was comparable to enzyme/acid hydrolysis when coupled with a process of fermentation for ethanol production (So and Brown, 1999). Pyrolytic saccharification occurs much faster than hydrolysis, but it is hampered by the lack of specificity. The yield of levoglucosan from fast pyrolysis of lignocellulose is obviously lower than that from pure cellulose (Maduskar et al., 2018). There is enormous information in the literature indicating that the presence of ash and lignin in lignocellulose has negative influence on facilitating the formation of small molecule compounds at the expense of levoglucosan during fast pyrolysis (Hosoya et al., 2007). The low content of levoglucosan enlarges the cost of subsequent product separation and purification thereby weakening its economic viability. Improving the selectivity of levoglucosan from lignocellulose is the most important issue to promote the development of pyrolytic saccharification. Much efforts have been made to enhance the levoglucosan formation from lignocellulose. Pretreating lignocellulose into a suitable raw material prior to fast pyrolysis is regarded as an effective strategy for selective conversion. Considerable attention has been focused on chemical pretreatment due to its high efficiency, low energy consumption and fast reaction. Typically, chemical pretreatment is performed under high pressure, which requires a high equipment investment and leads operational complexity. Previous research has proposed that glycerol pretreatment favored levoglucosan production during fast pyrolysis of lignocellulose (Jiang et al., 2017). The high boiling-point of glycerol enables the pretreatment performed under atmospheric pressure, thereby saving the capital of equipment and showing economic advantages over common chemical pretreatment to some extent. However, chemical pretreatment needs lots of chemicals, and the utilization of pure glycerol for lignocellulosic pretreatment is still an expensive and unprofitable process. Thereby, from the point of economy, applying economical chemicals for this pretreatment is prerequisite. Crude glycerol is a major by-product of biodiesel, which takes up almost 10 wt.% of total end-products (Fig. 1) (Vivek et al., 2017). The dramatic increase in biodiesel production results in the surfeit of glycerol and the availability of cheap glycerol resource (Kim and Moon, 2019). The annual production of crude glycerol is around 600 million tons, and more than 65% of it comes from the transesterification of biodiesel, which is much more than the demand of global market (Anitha et al., 2016; Gao et al., 2016). Therefore, the surplus of crude glycerol, which should not be discharged without any treatment, has become a burden for biodiesel plants. At present, the main disposal approach of excessive crude glycerol is combustion, but it is easily polymerized or partially oxidized to highly toxic acrolein at high temperatures, which might cause serious pollution to environment and human health (Steinmetz et al., 2013). Crude glycerol is a mixture, including glycerol (70–98%), biodiesel, fatty acids, methanol, water, soap, ash and trace amounts of monoglycerides and diglycerides (Anitha et al., 2016). The complex components of crude glycerol make
2. Materials and methods 2.1. Materials Sugarcane bagasse was obtained from Guangzhou, China. The sugarcane bagasse was pulverized and screened to 60–80 mesh, and then dried to constant weight. Glycerol (purity > 99.7%) was purchased from Tianjin Yongda Reagent Co., Ltd. The crude glycerol A and B were obtained by transesterification reaction between soybean oil and methanol using sodium silicate (Na2SiO3·9H2O) and calcined sodium silicate (Na2SiO3) as catalysts, respectively (Zhang et al., 2016). Calcined sodium silicate was prepared by heating sodium silicate at 400 °C for 2 h and then triturating it to particles of 0.5 μm. The molar ratios of methanol to soybean oil and methanol to catalyst were 15:1 and 40:1, respectively. The reaction was held at 65 °C for 3 h. After this process, the solution was stratified after standing. The heavy layer was taken out for distillation. Most of methanol was separated for reuse, and the crude glycerol A and B were collected for analysis and pretreatment. 2.2. Crude glycerol componential analysis The component of crude glycerol was identified by GC–MS (TRACE 1300/ISQ QD). A TG-WAXms column was employed for the separation. The temperature of column was set at 60 °C for 2 min, then heated to 240 °C at a heating rate of 10 °C/min followed with a dwell time of 10 min. Helium was served as the carrier gas, and the flow rate was set at 1.2 mL/min. The split ratio was set at 50:1. MS was performed with the following operation conditions: transfer line, 260 °C; electron energy, 70 eV; ion source, 280 °C; scanning range (m/z), 15 ~ 350 amu. 2.3. Crude glycerol pretreatment The pretreatment of sugarcane bagasse was performed in a microwave reactor (MCR-3, Star Shuo Instrument Co., Ltd, Guangzhou, China), which consisted of a microwave generator system and a temperature control system. Crude glycerol and sugarcane bagasse were mixed in a flat bottom two-neck flask with a ratio of 10:1 (w/w). The mixture was heated to 220 °C by microwave irritation and maintained at 220 °C for 6 min. Then, the solid was filtrated and washed repeatedly with deionized water to remove residual chemicals. The solid residue was dried by a vacuum freeze dryer, which could dry the sample while maintain the inner structure. Pure glycerol pretreatment was used as a control group. 2.4. Elemental and compositional analysis The content of three main organic elements (C, H, N) in sugarcane bagasse was measured by an elemental analyzer (Vario EL cube, Elementar, Germany). The content of alkali and alkaline earth metals (K, Ca, Na, Mg) was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Optima 8000, PerkinElmer, USA). Firstly, the sugarcane bagasse (approximately 0.2 g) was digested in a test tube containing a mixed acid (1 mL HClO4 and 3 mL HNO3) solution for releasing metal elements in the form of ions. Then, the solution 2
Fig. 1. Schematic illustration of transesterification process.
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Fig. 2. Componential analysis of crude glycerol A and B.
was diluted to 20 mL and then filtered with 0.22 μm filter. Quantitative analysis was conducted in an external standard method. The content of cellulose, hemicellulose and lignin in sugarcane bagasse was determined by the method of NREL (Sluiter et al., 2008). In this process, the cellulose and hemicellulose were converted into monosaccharides by two-step acid hydrolysis. The content of released sugar was measured by HPLC system (Alliance 2695, Waters, America). An Aminex HPX-87P column (Bio-Rad) and a refractive index detector (Alliance 2414, Waters, America) were used for separation and detection, respectively. The temperatures of column and detector were 80 °C and 50 °C, respectively. Deionized water (0.3 mL/min) was employed as the mobile phase. The content of cellulose and hemicellulose was calculated according to the content of monosaccharide. Acid-soluble lignin was detected by ultraviolet-visible spectroscopy (λ = 320 nm). The solid residue was burned at 550 °C for 3 h, and its mass loss was defined as acid-insoluble lignin.
Ψ(E , T ) =
∫0
T
u′= E′ R′T ′
−E ⎞ dT exp ⎛ ⎝ RT ⎠
→
f (E ) =
(E − E0 ) − 1 e 2σ 2 2π σ
A pyroprobe 5200 CDS reactor (U.S.A.) coupled with Agilent GC (7890A, U.S.A.) and Agilent MS (5975C, U.S.A.) was used for fast pyrolysis of sugarcane bagasse and analysis of pyrolysis product. Fast pyrolysis of biomass occurred at 500 °C. The heating rate was set at 20 K ms−1. The volatile generated from fast pyrolysis was purged to GC–MS system by helium (20 mL/min) through a transfer line. Product separation was realized by a DB-1701 capillary column. The temperature program of GC oven started at 40 °C and remained for 3 min, then heated to 280 °C with a heating rate of 10 °C/min, and hold for 8 min. The mass spectrometer had an ion source of 230 °C, a quadrupole of 150 °C, an EI ion source of 70 eV, and a scan range (m/z) of 29–450 amu. Qualitative and quantitative analysis of pyrolysis products was carried out by NIST 14 library search and an external standard method. The yield of levoglucosan and relative content of other compounds were calculated as the following equation:
Relative content of compound (%) =
(1)
where P is the integrated pyrolysis coefficient; DTGmax is the maximum decomposition rate; Tmax is the temperature of the maximum decomposition rate; Ti and Tf are the temperature of initial and final decomposition, respectively. The kinetic parameters were obtained by the temperature integral of Distributed Activation Energy Model (DAEM). The standard form of DAEM was: T
−E ⎞ dT A exp ⎛ ⎝ RT ⎠
∫0
∞
A exp ⎡− 0 Ψ(E , T ) ⎤ f (E ) dE ⎢ ⎥ ⎣ β ⎦
Mass of levoglucosan × 100% Mass of cellulose
Area of a compound × 100% Area of all compunds
(6)
(7)
3. Results and discussion 3.1. Componential analysis of crude glycerol The proportion of each component in crude glycerol A and B is exhibited in Fig. 2. Glycerol was proved to occupy a considerable proportion in both crude glycerol A and B, which was 88.2% and 84.6%, respectively. The component of crude glycerol prepared in the paper was similar to the component of crude glycerol derived from biodiesel in the industry (Vivek et al., 2017). Besides glycerol, some residual alcohol and fatty acids, such as (Z, Z)-9,12-octadecadienoic acid and n-hexadecanoic acid were also contained in crude glycerol. The residual alcohol came from the excessive addition of alcohol for transesterification reaction. The content of (Z, Z)-9,12-octadecadienoic acid (6.5% vs 6.2%) and n-hexadecanoic acid (2.1% vs 1.4%) contained in crude glycerol B was higher than that of crude glycerol A. This phenomenon might attribute to the influence of water introduced into crude glycerol A along with sodium silicate in transesterification
(2)
where f(α) refers to the most probable mechanism function, E refers to the activation energy (kJ/mol), R refers to the universal gas constant, A refers to the pre-exponential factor, T and t refer to the reaction temperature (K) and time (min), respectively. The fundamental equation of DAEM was:
α =1 −
(4)
2.6. Fast pyrolysis
DTGmax P= Tmax × (Tf - T) i
∫0
exp(−u′) du′ u′ 2
(5)
Levoglucosan yield (wt.%) =
dα 1 = G (α ) = f (α ) β
∞
2
Thermal decomposition of biomass was compared by thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis using a simultaneous thermal analyzer (STA449 F3, Netzsh, Germany). In a typical thermogravimetric analysis, sample (around 10 mg) was added into an Al2O3 crucible and heated to 800 °C from 35 °C at a heating rate of 10 °C min−1. The integrated pyrolysis coefficient P was introduced to reflect the severity of pyrolysis process. P was defined as the following formula:
α
∫u
The density distribution f(E) was defined as:
2.5. Thermogravimetric analysis
∫0
E ⎛ ⎞ ⎝R⎠
(3)
where Ψ (E,T) was the temperature integral: 4
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Table 1 Elemental analysis of raw and pretreated sugarcane bagasse. Samples
Un-treated Pure glycerol pretreated Crude glycerol A pretreated Crude glycerol B pretreated
Organic elements
AAEMs
C (wt.%)
H (wt.%)
N (wt.%)
C/H
K (mg/kg)
Ca (mg/kg)
Na (mg/kg)
Mg (mg/kg)
Total (mg/kg)
48.2 46.6 45.3 43.6
5.9 6.2 6.3 6.4
0.1 0.0 0.0 0.0
8.2 7.5 7.2 6.8
876.0 53.3 63.6 84.2
1550.7 832.1 67.6 53.5
449.9 360.2 5.0 6.3
208.9 70.5 0.5 0.5
3085.5 1316.1 136.7 144.5
reaction, which could promote the saponification reaction between fatty acid and sodium silicate to form fatty acid sodium, thereby reducing the content of fatty acids in crude glycerol A.
Table 2 Componential analysis of raw and pretreated sugarcane bagasse.
3.2. Elemental analysis of raw and pretreated sugarcane bagasse The analysis of carbon, hydrogen, and nitrogen of biomass is given in Table 1. In original sample, the content of carbon, hydrogen and nitrogen was 48.2 wt.%, 5.9 wt.% and 0.1 wt.%, respectively. After pretreatment, carbon occupied a lower proportion in the pretreated sugarcane bagasse, and hydrogen had the opposite variation. The ratio of C/H was reduced from 8.2 to 6.8 after pretreatment. It was reported that the C/H ratio of lignin (11.1) was higher than that of cellulose (7.5) and hemicellulose (7.2) (Qu et al., 2011). Moreover, the C/H ratio of crude glycerol B pretreated biomass was lower than crude glycerol A pretreated sample, which might indicate that crude glycerol B had a better performance on lignin removal. Besides organic elements, lots of interests were lied on inorganic elements, especially the alkali and alkaline earth metals (AAEMs) due to their high abundance relative to other elements. The result of ICP-OES shown that there was high content of AAEMs (3085.5 mg/kg) present in original sugarcane bagasse (Table 1). After pure glycerol pretreatment, a notable reduction in the content of AAEMs was observed, and the total content of AAEMs declined to 1316.1 mg/kg at a 57.3% removal rate, while crude glycerol pretreatment benefited further demineralization. The total content of AAEMs declined to 136.7–144.5 mg/kg for crude glycerol pretreated sample. More than 95.0% of AAEMs were removed by crude glycerol pretreatments. Particularly, the removal of potassium was mostly obvious by both pure and crude glycerol pretreatment. The levoglucosan yield was sensitive to the purity of the initial cellulose, which was exponentially reduced by the presence of inorganic salts, especially AAEMs. It had been revealed that there was a competitive pathway between levoglucosan and low-molecular species during fast pyrolysis of biomass (Lindstrom et al., 2019). Levoglucosan was derived from the cleavage reaction of β-1,4-glycosidic bond, which preserved the structure of pyranose rings, whilst low-molecule species (e.g. acetaldehyde) was originated from homolytic fission of the pyranose rings (Patwardhan et al., 2010). It was reported that the AAEMs could form a coordinate bond with the oxygen atoms of the vicinal hydroxyl groups on pyranose rings, which lowered the stability of pyranose rings (Kuzhiyil et al., 2012). As a result, the homolytic fission of the pyranose rings was strengthened, which favored the accumulation of low molecular species. It was noteworthy that even AAEMs at a trace level could alter the primary pyrolytic pathway of cellulose, resulting in the formation of small molecule compounds at the expense of levoglucosan (Kuzhiyil et al., 2012). With respect to the reduction of levoglucosan production, order from the strongest to weakest catalytic influence was K+ > Na+ > Ca2+ > Mg2+ (Patwardhan et al., 2010). Consequently, the significant decrease of AAEMs content for pretreated biomass might favor the formation of levoglucosan in fast pyrolysis.
Samples
Cellulose (wt. %)
Hemicellulose (wt.%)
Lignin (wt. %)
Un-treated Pure glycerol pretreated Crude glycerol A pretreated Crude glycerol B pretreated
40.0 51.6 57.6
17.1 16.1 17.4
20.9 17.2 11.5
66.5
17.0
4.3
raw sugarcane bagasse consisted of 40.0 wt.% cellulose, 17.1 wt.% hemicellulose and 20.9 wt.% lignin. After pure glycerol pretreatment at 220 °C for 6 min, the pretreated biomass contained 51.6 wt.% cellulose, 16.1 wt.% hemicellulose, and 17.2 wt.% lignin, respectively. Pure glycerol pretreatment could dissolve some lignin and hemicellulose, resulting in the enrichment of cellulose fraction in the solid residue, which was consistent with previous results (Jiang et al., 2017). The crude glycerol pretreated sample was basically composed of 57.6–66.5 wt.% cellulose, 17.0–17.4 wt.% hemicellulose and 4.3–11.5 wt.% lignin. It was found that crude glycerol pretreatment could selectively remove lignin from biomass leading to a further accumulation of cellulose, while the content of hemicellulose exhibited negligible change before and after pretreatments. In terms of lignin removal rate, the following trends was observed: crude glycerol B > crude glycerol A > pure glycerol. The least lignin content (4.3 wt.%) was obtained in sample pretreated by crude glycerol B. This meant that 79.4% of lignin in the sugarcane bagasse could be removed under this condition. Residual alkaline present in the crude glycerol might favor the removal of lignin (Jiang et al., 2019b). Previous research had indicated that cellulosic sample with higher cellulose content could offer much higher yields of levoglucosan (Miura et al., 2001). During fast pyrolysis, there was some interaction amongst the lignocellulosic components. The interaction of cellulose-hemicellulose was weaker than that of cellulose-lignin. It had been established that an apparent interaction was observed in cellulose-lignin as the yield of levoglucosan diminished (Zhang et al., 2015). The presence of lignin suppressed the formation of levoglucosan, and the intensively boosted undesirable formation of other compounds (Hosoya et al., 2007). Fortunately, the lignin fraction was removed by crude glycerol pretreatment in this study, which might be beneficial to levoglucosan production.
3.4. Thermogravimetric analysis of biomass The TG and DTG curves acquired in thermogravimetric analysis are displayed in Fig. 3. The pyrolytic and kinetic parameters are tabulated in Tables 3 and 4. The native sample began to degrade at 235.3 °C. For crude glycerol pretreated A (257.7 °C) and B (268.8 °C) samples exhibited higher onset temperatures for decomposition. The maximum pyrolysis rate (DTGmax) of un-pretreated sugarcane bagasse was about 1.1%/°C, which was shifted to higher values after pure or crude glycerol pretreatments. The severity of pyrolysis was indicated by integrated pyrolysis coefficient P. It could be seen that the P value of the original
3.3. Compositional analysis of raw and pretreated sugarcane bagasse The chemical composition of biomass is exhibited in Table 2. The 5
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Fig. 3. TG (a) and DTG (b) curve of raw and pretreated sugarcane bagasse.
sample was only 2.6%/°C2, which was evaluated to higher values for the pure glycerol pretreated (4.2) and crude glycerol pretreated biomass (6.0–9.1%/°C2). Kinetic analysis indicated that pretreated samples exhibited higher E values (230.1–232.1 kJ/mol) compared to raw material (226.4 kJ/mol). Higher E meant higher thermal stability of the pretreated sample, and more energy input was required for initial degradation. The lower value of σ indicated that the decomposition of pretreated samples was concentrated and reacted explosively (Lin et al., 2019). It was also found that all adjusted coefficients (Radj2) fitted by the model were high (> 99%), which implied that the prediction matched the experimental data very well. Several factors might be responsible for those differences in pyrolytic and kinetic analysis. The presence of AAEMs, especially K, could form coordinate bonds with the oxygen atoms on the pyranose rings of lignocellulose, which lowered the stability. It was known that AAEMs could catalyze the pyrolysis reaction to lower initial pyrolysis temperature, decrease the decomposition temperature and the maximum pyrolysis rate, and reduce the activation energy of the reaction (Le Brech et al., 2016). The removal of AAEMs could reduce their catalytic effects. Therefore, as the content of AAEMs declined, the pyrolytic and kinetic parameters shifted to higher values. Furthermore, pretreatment made the pyrolysis process more intensive, which could also be explained by the alteration of lignocellulosic constituent. Lignin had the highest stability and widest temperature range of pyrolysis, while the pyrolysis of cellulose was easier and occurred in a narrower temperature range. The removal of lignin and accumulation of cellulose by pretreatment made the pyrolytic reaction more concentrated and intense.
Table 4 The kinetic parameters of raw and pretreated sugarcane bagasse. Samples
Log10(A) (min−1)
E (kJ/ mol)
σ (kJ/ mol)
Radj2 (%)
Un-treated pretreated Pure glycerol pretreated Crude glycerol A pretreated Crude glycerol B pretreated
20 20 20
226.4 232.1 230.1
15.7 11.3 8.6
99.8 99.6 99.6
20
231.3
5.7
99.7
Fig. 4. Products distribution from fast pyrolysis of biomass.
3.5. Fast pyrolysis products analysis of biomass pyrolysate was a complex mixture of various organic compounds, including acids, ketones, aldehydes, furans, phenols, esters and
The distribution of pyrolytic product from raw and pretreated sugarcane bagasse is summarized in Figs. 4 and 5. Lignocellulosic Table 3 The pyrolytic parameters of raw and pretreated sugarcane bagasse. Samples
Ti (°C)
Tmax (°C)
DTGmax (%/°C)
Tf (°C)
Residue (%)
P × 10−5 (%/°C2)
Un-treated pretreated Pure glycerol pretreated Crude glycerol A pretreated Crude glycerol B pretreated
235.3 253.5 257.7 268.8
337.6 340.9 331.0 329.7
1.1 1.6 1.9 2.4
358.3 364.0 352.8 348.4
12.7 13.0 12.0 11.1
2.6 4.2 6.0 9.1
Ti: The temperature of initial volatilization (5% loss in mass). Tmax: The temperature corresponding to the maximum decomposition rate. Tf: The temperature at the inflection point of about 350 °C in the DTG curve. DTGmax: The maximum decomposition rate. Residue: The proportion of pyrolytic residue. P: Integrated pyrolysis coefficient. 6
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used for pretreatment to promote the levoglucosan formation from sugarcane bagasse via fast pyrolysis. The yield of levoglucosan (25.2%) from crude glycerol pretreated sample was obviously improved as compared to those from pure glycerol pretreated (14.4%) and un-pretreated sugarcane bagasse (8.4%) due to the efficient removal of lignin and AAEMs. Meanwhile, crude glycerol pretreatment suppressed the formation of inhibitors (e.g. acid, furans, phenols) to the biocatalyst. Accordingly, this work provided an economically-viable and environmentally-benign strategy to make full utilization of crude glycerol and improve the fermentability of lignocellulosic pyrolysate. Funding This work was funded by the Science and Technology Planning Project of Guangzhou city and Guangdong province (No. 2017A020216007, 201707010236), the National Natural and Science Foundation of China (No. 51606204), and the Key Laboratory of Microbial Technology Open Projects (M2019-10).
Fig. 5. Relative content of main compounds from fast pyrolysis of biomass.
anhydrosugars, etc. The target product, levoglucosan, had a considerable proportion in the pyrolysate. For raw material, the yield of levoglucosan was 8.4%. After pure glycerol pretreatment, the yield of levoglucosan rose to 14.4%. Crude glycerol pretreatment further improved levoglucosan yield (21.0–25.2%). After pretreatment, the content of acids (from 9.3% to 5.4%), furans (from 13.6% to 9.5%) and phenolic compounds (from 11.9% to 2.3%) derived from fast pyrolysis of biomass exhibited an inverse trend and declined obviously (Fig. 4). Especially, the content of acetic acid diminished intensively to 0.6% from 7.2% (Fig. 5). Among the furan compounds, the content of 2,3dihydro-benzofuran had the most obvious diversification, and its content was reduced to 0.6% from 12.1% in the pyrolysate of crude glycerol A pretreated biomass. As regard to phenolic compounds, the content of phenol (from 0.6% to 0.1%) and 2-methoxy-4-vinylphenol (from 3.1% to 0.4%) was also decreased remarkably. The content of 2,3-butanedione, a type of ketone compound, was also reduced by about 54.4% after pretreatment. The alteration of the lignocellulosic component might cause the difference on distribution of pyrolytic products. It was believed that the presence of AAEMs limited the primary reaction leading to levoglucosan production, but promoted fragmentation, ring opening, and dehydration reactions to yield lower molecular weight oxygenates during cellulose pyrolysis (Le Brech et al., 2016). The remove of AAEMs might suppress the formation of low weight molecular compounds. Phenolic compound was derived from lignin and its reduction might be attributed to the removal of lignin fraction by crude glycerol pretreatment. The low yield of levoglucosan from lignocellulosic biomass, and the presence of inhibitors to biocatalysts hampered the fermentable utilization of the pyrolysate (Jiang et al., 2019c). It was reported that in the fermentation of pyrolysate, carboxylic acids and phenols were the most toxic inhibitors, while furans and alkanes were mildly inhibitory. In this paper, the yield of furfural increased to some extent (from 1.6 to 2.0%). While, the yields of acetic acid (from 7.2 to 0.6%) and phenols (from 0.6 to 0.1%) reduced significantly. Thereby, as a whole, the crude glycerol pretreatment was able to reduce inhibitors. Obviously, the enrichment of levoglucosan and the reduction of inhibitors in pyrolysate could enhance the feasibility of fermentation. After crude glycerol pretreatment, a great deal of glycerol existed in the liquid stream still caused a significant environmental pollution and resource waste. In previous research, it had been demonstrated that the recovered glycerol could be served as an attractive substrate for the fermentation of various value-added compounds (e.g. 2,3-butanediol, lactate) (Jiang et al., 2017, 2019d). In the next work, the fermentability of the pyrolysate from crude glycerol pretreated biomass will be evaluated.
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