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Investigate the interactions between biomass components during pyrolysis using in-situ DRIFTS and TGA
Accepted Manuscript Investigate the interactions between biomass components during pyrolysis using in-situ DRIFTS and TGA Jianqiao Wang, Boxiong Shen, Dongrui Kang, Peng Yuan, Chunfei Wu PII: DOI: Reference:
S0009-2509(18)30736-X https://doi.org/10.1016/j.ces.2018.10.023 CES 14555
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
10 July 2018 11 October 2018 16 October 2018
Please cite this article as: J. Wang, B. Shen, D. Kang, P. Yuan, C. Wu, Investigate the interactions between biomass components during pyrolysis using in-situ DRIFTS and TGA, Chemical Engineering Science (2018), doi: https:// doi.org/10.1016/j.ces.2018.10.023
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Investigate the interactions between biomass components during pyrolysis using in-situ DRIFTS and TGA Jianqiao Wanga, Boxiong Shena*, Dongrui Kanga, Peng Yuana, Chunfei Wua,b* a b
School of Energy & Environmental Engineering, Hebei University of Technology, Tianjin, China
School of Chemistry and Chemical Engineering, Queens University Belfast, Belfast, Northern Ireland, BT7 1NN * Corresponding authors: E-mail: [email protected] (C. Wu); [email protected] (B. Shen). co-first author: Jianqiao Wang and Boxiong Shen
Abstract Biomass pyrolysis is a promising thermal chemical processing technology to produce renewable and sustainable energy materials. However, the interaction between biomass components (cellulose, hemicellulose and lignin) during pyrolysis process needed further investigation. In this work, an insitu Reflectance Infrared Fourier Transform Spectroscopy (in-situ DRIFTS) reactor was introduced to investigate the interactions assisted with Thermogravimetric Analysis (TGA). The experiments demonstrate that there exist complicated interactions between the main components of biomass during pyrolysis. The results from TGA/DTG indicate that the interactions between cellulose and xylan introduce higher pyrolysis temperature, while the interactions between cellulose and lignin, xylan and lignin, resulte less weight loss. The increase of compression pressure for the mixture of main components changes the weight loss of the mixture. The results from in-situ DRIFTS show that the decomposition and aromatization of xylan are promoted when the cellulose is added. Cellulose will also hinder the accumulation of the benzene rings of the lignin. The in-situ DRIFTS experiment results also suggest that the contact pressure between the components conduct more residue solid, mainly because the dehydration reaction of the mixture samples was inhibited and the charring process was promoted, especially the reaction between cellulose and lignin. Key words: In-situ DRIFTS; biomass pyrolysis; cellulose; xylan; lignin 1
1. Introduction Biomass is a renewable and carbon neutral resource. The utilization of biomass has obtained extensive attention due to climate change derived from using fossil fuels. Through thermochemical conversion technologies, biomass can be processed into fuels and chemicals. These technologies include pyrolysis, gasification, combustion, hydrothermal liquefaction and hydrothermal carbonization (Wang et al., 2017). The products of biomass pyrolysis contain bio-oil, biochar, and non-condensable gas (Anca-Couce, 2016; Kan et al., 2016). Biochar produced from biomass pyrolysis can be used as fertilizer, carbon capture, secondary fuel and etc. Bio-oil can be applied as heavy fuel oil in diesel engines and as sources of valuable chemicals. The produced gas from biomass pyrolysis can be combusted to provide heat for the process of pyrolysis. Biomass pyrolysis is usually taken place at temperature around 500 ˚C within an inert atmosphere. Biomass contained three main components including cellulose (40-50 wt. %), hemicellulose (20-40 wt.%) and lignin (10-40 wt.%) (McKendry, 2002). The ratio of these components depend on the type of the biomass, such as the hardwoods contain higher cellulose and hemicellulose content than softwoods on average. Li et al.(Li et al., 2004) pyrolysis legume straw and apricot stone in the temperature range 500-800 ˚C and conclude that more cellulose and hemicellulose are better sources for the hydrogen rich gases production than lignin. However, the distribution of the pyrolysis products not only depend upon the types of biomass, but the react temperatures, residence time and the heating rate. Fast pyrolysis of biomass is conducted with the high heating rate up to 1000 ˚C s-1 at temperature below 650 ˚C for the liquid products and slow pyrolysis has been used for the production of char(Williams and Besler, 1996). High temperature and short residence times
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facilitate the formation of gases, while low temperatures and long residence times promote the formation of the char. Bio-oils are usually produced with medium temperatures and short residence times(Velden et al., 2010). As the pyrolysis temperature is around 500 ˚C and heating rate is 10-100 ˚C (an intermediate pyrolysis process), the decomposition of cellulose primarily occurs between 325 ˚C and 400 ˚C, higher than hemicellulose pyrolysis, which happens between 250 ˚C and 350 ˚C. Lignin is the most stable component which can be decomposed at a higher temperature range of 300-550 ˚C (Stefanidis et al., 2014). It was found that the pyrolysis of cellulose was characterized by a high yield of liquid and a low yield of char during the intermediate pyrolysis. In contrast, lignin pyrolysis produces the highest amount of residue char (Wang et al., 2011; Xin et al., 2013). Some previous studies have focused on understanding the pyrolysis mechanisms of individual component. However, the interaction of cellulose, hemicellulose and lignin during biomass pyrolysis is a very interesting topic because cellulose is always connected with hemicellulose and lignin through hydrogen bonds, while hemicellulose is linked to lignin with covalent and hydrogen bonds (Lee et al., 2014; Vorwerk et al., 2004). The interaction of cellulose, hemicellulose and lignin during pyrolysis is complicated and not very clear. Some literatures investigated the mechanism of the interactions in the pyrolysis of biomass components through kinds of various methods and apparatuses. For example Wu et al. (Wu et al., 2016a, b) investigated cellulose-lignin and cellulosehemicellulose interactions with various conditions by Py-GC-MS through detecting the gas/liquid products. Raveendran et al. (McKendry, 2002) and Yang et al. (Yang et al., 2006) used a thermogravimetric analyzer (TGA) to study the pyrolysis characteristics of biomass components and found negligible interactions among the components during pyrolysis. However, Yu et al. (Yu et al., 2017) indicated that except the mixture of xylan (representation of hemicellulose) and lignin, 3
there were evident interactions between synthetic mixture of cellulose and xylan, and cellulose and lignin. Liu et al. (Liu et al., 2011) coupled a thermogravimetric analyzer with Fourier transform infrared spectrometer to detect the intermediate gas products during biomass pyrolysis process. The addition of lignin to the lignin and hemicellulose mixture resulted in the decrease of the yield of 2furaldehyde in the volatiles released from pyrolysis of hemicellulose. Meanwhile, the existence of hemicellulose intensively decreased the yield of levoglucosan. In-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (In-situ DRIFTS) technology had been utilized by detecting the chemical groups on the surface of solid during biomass pyrolysis reaction. For instance, due to the nature of the technology, Shao et al. (Shao et al., 2017) and Uchimiya et al. (Uchimiya et al., 2013) used this technology to explore the change of functional groups on the surfaces of biomass/biochar during catalytic reaction. So it is valuable to introduce In-situ DRIFTS technology to investigate the solid surface interaction of cellulose, hemicellulose and lignin during pyrolysis, though there are no reports about this topic. In this work, to investigate the interaction between the biomass components, the surfaces properties of individual component (cellulose, xylan, and lignin) and their mixtures will be investigated using in-situ DRIFTS analysis and TGA. Considering that the biomass components do not exist independently in nature, the three substances are prepared with different compressing pressures to simulate their intimacy in reality.
2. Materials and methods 2.1 Biomass components 4
Cellulose (Research Chemicals Ltd.), xylan (main components of hemicellulose) (Sigma-Aldrich Co., Ltd) and lignin (Sigma-Aldrich Co., Ltd) are the raw samples used in this work. Xylan was produced from beechwood cell wall polysaccharide and all samples are powders with particle size around 200 um. The element and proximate analysis of three components were detected by our previous work (Wu et al., 2013). The lignin sample in this work contains the highest C content (61.33 wt. %) and fixed carbon (32.60 wt. %). Cellulose has the highest volatiles content (60.37 wt. %) and lowest ash content (0.07 wt. %) compared with lignin and hemicellulose sample.
2.2. Preparation of the mixture of biomass components To evaluate the interactions between the biomass components during the pyrolysis, biomass components were mixed physically with different ratios (Hilbers et al., 2015; Stefanidis et al., 2014; Wu et al., 2016a, b; Yu et al., 2017). In these reports, biomass components were directly compressed and agglomerated to prepare composite samples. Therefore, in this work, the mixture of biomass components was prepared by physically mixing method. The mixed samples with weight ratio 1:1 (for cellulose and xylan, cellulose and lignin, xylan and lignin) were further mechanically compressed at various pressures (5, 10 and 20 MPa) for 30 minutes. These biomass samples were used to study the interactions between components during pyrolysis process. CX, CL and XL indicated the mixture of cellulose and xylan, cellulose and lignin, xylan and lignin, respectively. The compressed pressures was denoted at the end of the symbol, for example, CX5 indicated the mixture of cellulose and xylan was prepared with the weight ratio 1:1 at 5 MPa pressure.
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2.3. TGA experiments In order to evaluate the thermal decomposition of the individual component and their mixture, TGA analysis was conducted by using a SDTQ600 equipment. Around 10 mg of sample was heated at a heating rate of 10 ˚C min-1 from 30 ˚C to 600 ˚C. The experiment was conducted within a N2 atmosphere and the flow rate of N2 was 100 mL min-1.
2.4. In-situ DRIFTS monitoring of biomass pyrolysis Pyrolysis of biomass components and their mixtures was carried out in a diffuse reflection accessory (Praying Mantis™, Harrick Scientific) using a temperature controller (ATC-024-3, Harrick Scientific). The reactor chamber was purged with nitrogen with a flow rate around 600 mL min-1. The functional groups of biomass were detected in real time with the IR spectrometer (TENSOR II, BRUKER). In addition, cooling water was used to control the reactor chamber. The temperature of the In-situ DRIFTS reactor was controlled by heating rate of 10 ˚C min-1.
3. Results and discussion 3.1. Thermogravimetric analysis (TGA) 3.1.1. TG analyses of individual biomass components The mass loss and derived mass loss rate of these three biomass components are presented in Fig. 1. Lignin was decomposed at the range from 200 ˚C to 600 ˚C with the highest mass loss rate at 327 ˚C. The wide decomposing temperature window is mainly attributed to the presence of benzene ring 6
with diverse branches (Collard and Blin, 2014b). The highest carbon content and highly crosslinked nature of lignin is the major factor resulting in the highest solid residues after pyrolysis (~57 wt. %) (Yu et al., 2017). The high thermal stability of cellulose is attributed to the crystalline structure within this pure linear polymer (Yang et al., 2007), and it explains the start depolymerization of cellulose is around 100 ˚C later than the two others. Cellulose has the similar highest depolymerization temperature (~330 ˚C) compared with lignin during TGA pyrolysis, however it has a much more narrow temperate range (300 ˚C to 400 ˚C). The least solid residues (~10 wt. %) is consistent with the high condensable levoglucosan and volatiles such as CO content of cellulose(Yu et al., 2017). The main temperature window of the degradation of xylan is 200 ˚C to 400 ˚C. The two peaks present in the DTG curves (indicated as “-D”) of xylan pyrolysis are at around 250 ˚C and 290 ˚C, respectively. The decomposition of xylan around 250 ˚C was due to the dissappearance of branches such as acetyl and 4-O-methylglucuronic acid. The weight loss around 290 ˚C of xylan is suggested due to the depolymerization of the main chain into furfural (Collard and Blin, 2014a).
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Figure 1. TG and DTG curves of the cellulose, xylan and lignin. The solid line is TGA curve. The dash line is DTG curve
3.1.2. TGA analysis of the mixture of biomass components The experimental results of the mixed samples are compared with the calculated values, which are obtained from the average weight sum of individual components (Stefanidis et al., 2014) by following equations: (1) (2) Where
is the weight loss of the mixture (two components), and
means the weight loss of the individual component during pyrolysis at the temperature T 8
respectively. In addition, the
,
,
, are the differential mass loss of the mixtuer,
and the individual component at tempreature T. ‘i’ and ‘j’ indicate any two components of cellulose, xylan and lignin. From Fig.2 (a), an apparent difference is observed between experimental and calculated TGA results. From DTG results of the mixture of cellulose and xylan, the maximum decomposition temperature around 350 ˚C was clearly moved to higher temperature when compared to the pyrolysis of individual components. However, for the mixture of cellulose and lignin, and xylan and lingin, the maximum decomposition temperatures for the calculated results are very similar to the experimental results. In Fig. 2 (b) and Fig. 2 (c), the intensity of weight loss for the experimental results is lower when compared to the calculated results. From the above results, it is suggested that the interactions between biomass components requires higher tempertaure for pyrolysis compared to individual components.
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Figure 2. Experimental and calculated TG and DTG curves for each mixture . (a) cellulose + xylan; (b) cellulose + lignin; (c) xylan + lignin
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3.2. In-situ DRIFTS analysis of biomass components and their mixtures
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Figure 3. In-situ DRIFTS analysis results of biomass components and mixtures, (a) cellulose; (b) xylan; (c) lignin; (d) cellulose+xylan; (e) cellulose+lignin; (f) xylan+lignin
Fig. 3 (a) shows the results of In-situ DRIFTS analysis of cellulose sample at different temperatures with a heating rate of 10 ˚C min-1. Weight loss of cellulose before 220 ˚C was suggested to be the loss of physical water (J. Scheirs, 2001). This is consistent with the fast reduce of the intensity at around 1650 cm-1, which is corresponding to the remove of moisture before 220 ˚C. The intensity of absorption at around 3500 cm-1, assigned to OH groups, is gradually reduced with the increase of reaction temperature. The intensity decreasing before 300 ˚C may be attributed to the formation of anhydrocellulose through intramolecular and intermolecular dehydration actions. The water elimination at the beginning of the cellulose pyrolysis stabilizes the cellulose-derived char matrix. The intermolecular dehydration takes place between two chains in the polymer and result in higher reticulation of the biochar (Scheirs et al., 2001). Beyond that, the intramolecular dehydration will facilitate the cyclizing of the benzene by forming the C=C double bonds (Scheirs et al., 2001). The continuous decrease at around 3500 cm-1 from 300 ˚C to 500 ˚C corresponds to the hydroxyl group disappearance on the monomer unit of cellulose. The breaking of β-1,4-glycosidic between the two β-glucopyranose units, which are the components of the cellulose, result in the decrease intensity of the 1050 cm-1 (C-O-C bonds vibration) along the temperature rising up. Meanwhile, the increasing absorption around 1310 cm-1 and 1120 cm-1 (vibration of ether bonds within aromatic ring) become clear with the temperature increase during the pyrolysis process, so the residue become more and 12
more aromatic on account of many furan rings appearing, for example levoglucosan (C 6H10O5). Methanol removal during β-glucopyranose unit becoming the furan rings is also the reason for the decreasing of OH at around 3500 cm-1 at the high temperature. The decrease of 1098 cm-1 absorption indicates some volatile compounds such as oxygenated five-membered heterocycle furfural and furfuryl alcohol (Lu et al., 2011; Tomas E. McGrath, 2003; Wang et al., 2013). The increase of the absorption at around 800 cm-1 and 900 cm-1 suggests the cyclizing of the benzene rings. For xylan sample, as shown in Fig. 3 (b), the clear intensity reduction of the absorption at around 2900 cm-1 and 1450 cm-1 is assigned to methyl groups which are presented in xylan pyrolysis residue at high temperature (Zhao et al., 2017). The rupture of xylan branch corresponds to the first peak on the DTG curve of the xylan pyrolysis (Fig. 1), and during which many formic acid, methanol and acetic acid were produced (Collard and Blin, 2014a). The changes around 1720 cm-1 and 1610 cm-1 indicate the vibration of the carboxyl within acetyl of the xylan side chain. Compare with that in the cellulose pyrolysis, much more aromatic rings in the residue of xylan are detected with the In-situ DRIFTS. The vibrations of the C=C double bonds and aromatic skeletal compounds (around 1600 cm-1) are much more intense than that of cellulose pyrolysis at high temperature. It corresponds to the less char left after the cellulose pyrolysis than xylan in the TGA experiment. Fig. 3 (c) shows the In-situ DRIFTS experimental results using lignin sample. The propyl chain conversion to the benzene rings in lignin includes the dehydration and cleavage of alkyl C-H bond. The remove of moisture corresponding to the absorption at around 3500 cm-1 is clearly observed when the pyrolysis temperature is increased from room temperature to 200 ˚C during lignin
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pyrolysis. The intensity of the absorption peaks between 2960 cm-1 and 2850 cm-1 (the vibration of the C-H bond of the alkane) indicated the decomposition of aliphatic compounds was promoted when the pyrolysis temperature was higher than 300 ˚C (Candelier et al., 2011; Sharma et al., 2004). The accumulation of aromatics at high pyrolysis temperature (>400 ˚C) is clearly shown in Fig. 3 (c) where the absorption of trisubstituted benzene ring ( between 700-850 cm-1) was enhanced (Uchimiya et al., 2013; Zhao et al., 2017). This is consistent with the literature that with increasing the pyrolysis temperature of milled wood lignin, condensation, depolymerization and carbonization started in this sequence at 250, 350 and 400 °C, respectively. (Nakamura et al., 2008) It is worth noting that the increased absorption intensity at 1450 cm-1 and 1375 cm-1 imply the substituents sites on the benzene ring are gradually occupied by the methyl group. The mixture of cellulose and xylan was tested in In-situ DRIFTS with increasing temperature. As shown in Fig. 3 (d), it looks like that the decomposition of compounds containing OH groups (at around 3500 cm-1) was enhanced (e.g. dehydration reaction) when xylan and cellulose was mixed during the pyrolysis of the mixture of cellulose and xylan. As the conversion is around 30% at temperature of 300 ˚C for cellulose alone in Fig. 3(a), the conversion is increased to around 50% for the mixture of xylan and cellulose. Compared to the absorption intensity of the pyrolysis of pure cellulose and xylan（Fig. 3 (a) and Fig. 3 (b), the absorption band at around 2950 cm-1 shows obviously changes with increasing temperature during the co-pyrolysis of xylan and cellulose. It is indicated that a strong absorption of CH groups was present for the mixture of xylan and cellulose (Uchimiya et al., 2013). The changes around 1720 cm-1 and 1600 cm-1 seem like the curves overlap of cellulose with xylan. It is shown that the formation of C=O and C=C groups corresponding to carbonyl and aromatic skeletal compounds in xylan sample were promoted by mixing it with 14
cellulose. However, the absorption band between 1000 and 1300 cm-1 indicates that the decomposition of xylan was dominant in this region during the co-pyrolysis of xylan and cellulose, as the absorption in this region is very similar to the pyrolysis of pure xylan. It is due to the presence of large amounts of acetyl compounds in xylan. Compared to the pyrolysis of xylan, the co-pyrolysis of the mixture of cellulose and xylan has a strong absorption between 1310 cm-1 and 1120 cm-1. This is related to the strong absorption of ether groups in this region during the pyrolysis of pure cellulose. In addition，there is an opposite trend of absorption in this region for the pyrolysis of cellulose alone and the mixture of cellulose and xylan. It suggested that with the pyrolysis temperature increasing to 500 ˚C, the decomposing of cellulose to levoglucosan has been prohibited by the releasing products of the pyrolysis of xylan. This might be the reason why the decomposition temperature of cellulose and xylan pyrolysis moves to a higher temperature value in the DTG curve (Fig. 2 (a)). However, interactions between the pyrolysis of cellulose and hemicellulose (xylan) have been reported to be minor by other researchers (Svenson et al., 2004; Zhang et al., 2015). Fig. 3 (e) shows the results of using the mixture of cellulose and lignin. The overall absorption pattern of the mixture of cellulose and lignin in the process of pyrolysis is similar to that from cellulose alone, indicating that cellulose has a stronger signal of IR absorption. For example, the absorption around 1600 cm-1, related to C=C aromatic stretching, disappeared when cellulose was mixed with lignin for pyrolysis (Fig. 3(c)). This might be due to the strong absorption at 1650 cm -1 (OH group) for the cellulose sample. In addition, the changes of the absorption between 700 and 850 cm-1 are weaken for the mixture of cellulose and lignin compared to that from lignin alone. The small change of the absorption of lignin sample during pyrolysis could also be due to the inhibition 15
of reactions when cellulose was added. The decomposition of lignin was reported to be hindered due to the interaction between cellulose and lignin during the process of pyrolysis (Fushimi et al., 2009). However, negligible interactions between cellulose and lignin during pyrolysis have been reported when cellulose and lignin was physically mixed in previous research, as no covalent bonds were present for the mixed biomass components (Svenson et al., 2004). As shown in Fig. 3 (f), the in-situ DRIFTS analysis of co-pyrolysis of xylan and lignin shows that the absorption patterns of the mixture is similar to that of lignin alone. This result is consistent with the TGA analysis (Fig. 2). The changes of absorption at 1720 cm-1 and 1600 cm-1 with the increase of temperature are not observed for pyrolysis of the mixture of xylan and lignin, while the formation of C=O and C=C groups is found for pyrolysis of xylan alone. It is suggested that the addition of lignin might prohibit the decomposition of xylan to carbonyl and aromatic compounds (charring process). The charring process of lignin (between 700-850 cm-1) at high temperature (>400 ˚C) is minor as shown in Fig. 3 (f), when xylan and lignin were mixed. It is suggested there might be interactions between the pyrolysis of xylan and lignin, resulting in a slow charring process during pyrolysis.
3.3 TGA analysis of the mixture of biomass components prepared at different compression pressures
The weight loss and derivative weight loss curves of the mixture produced using different compression pressure are shown in Fig. 4. With the increase of compression pressure, more solid 16
residue is remained for the pyrolysis of the mixture of cellulose and xylan (Fig.4 (a), (b)). For cellulose and lignin, when the compression pressure of the mixture increases, the weight loss decreases at first and then increases again (Fig. 4. (c), (d)). As shown in Fig. 4 (e) and Fig. 4 (f), the weight loss of the mixtures of xylan and lignin prepared at different compression pressure varies slightly throughout the pyrolysis process, which suggested that the effect of compression pressure on weight loss is not so great for the mixture of xylan and lignin. The disagreement of the effect of compression pressure on the weight loss indicates that the compression pressure introduces the interaction of biomass components and the interaction is complicated. 1.0 0.9 0 Mpa 5 Mpa 10 Mpa 20 Mpa
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Figure 4. TG and DTG curves of the mixture of biomass components prepared at different compression pressures (a)(b) cellulose and xylan (c)(d) cellulose and lignin (e)(f) xylan and lignin
3.4 In-situ DRIFTS analysis of the mixtures of biomass components prepared at different compression pressures
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(c) Figure 5. In-situ DRIFTS analysis of cellulose and xylan mixture compressed at (a) 5 MPa (CX5), (b) 10 MPa (CX10) and (c) 20 MPa (CX20). From Fig. 5, as the compression pressure increases, the dehydration reaction (3500 cm -1) at around 200 ˚C becomes very inconspicuous. For CX5 sample, the changes of the IR absorption around 1600 cm-1 relating to aromatic ring stretching is not clear or the intensity of this peak is slightly increasing with the increase of pyrolysis temperature from room temperature to 500 ˚C. This IR absorption peak is slightly increasing with the increase of pyrolysis temperature for samples CX 10 and CX 20. Compared to cellulose and xylan mixed without any pressure (Fig.3 (d)), the compressed samples show regular increase absorption at 800-900 cm-1. It indicates that char is continuously formed during the cracking process. This implies that intimacy (high pressure indication more intimacy) is the main reason for the increase of the residue in the TGA experiment (Figure 4(a)). However, in general the influence of the compression pressure of the preparation of
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cellulose and xylan on the interaction of biomass component is suggested to be minor during pyrolysis process.
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(c) Figure 6 DRIFTS analysis of xylan and lignin mixture compressed at (a) 5 MPa (XL5), (b) 10 MPa (XL10) and (c) 20 MPa (XL20)
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From Fig. 6, for the mixture of xylan and lignin, all samples (XL5, XL10 and XL20) show almost similar trends of IR absorption when the pyrolysis temperature increases from room temperature to 500 ˚C. These changes include the reduction of IR band at around 3500 cm-1 as the temperature increases which is related to dehydration reactions. Aromatic groups on the surface of the biomass mixture are slightly increased with the increase of pyrolysis temperature. For example, the spectra at 600-900 cm-1, indicating the changes of the substitution group on the benzene ring of samples, show significant differences for the samples prepared by different pressure compression. In the XL10, xylan is gradually aromatized so the benzene rings substituents (such as –CH3 and –OH) increased in the residue. However, with the temperature increases, these substituents slowly separated from the benzene ring. When the components mixed with 20 MPa (XL20), the substituents on the benzene rings within lignin gradually decreases as the temperature increases from room temperature to 500 ˚C, and xylan decomposition was suppressed during the formation of char. It suggested that the high compression pressure will promote the interaction between xylan and lignin and help the solid decompose to low weight molecule.
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(c) Figure 7 In-situ DRIFTS analysis of cellulose and lignin mixture compressed at (a) 5 MPa (CL5), (b) 10 MPa (CL10) and (c) 20 MPa (CL20) From Fig. 7, dehydration of biomass components of cellulose and lignin (around 2900 cm-1) is enhanced with the increase of the pressure during the preparation of the mixture of biomass sample. At around 3000 cm-1 (C-H bonds), the changes of IR absorption is not clear with the increase of temperature for the pyrolysis of single components (cellulose or lignin). However, when these two biomass components are mixed, clear changes of absorption intensity at around 3000 cm-1 is observed in particular for the samples prepared by compression method. In addition, it is clear that the compression pressure influences the interactions between cellulose and lignin during pyrolysis. The intensity at around 3000 cm-1 is slightly increased for the sample CL5 with the increase of reaction temperature. However, the intensity of this peak is gradually reduced for the CL10 and CL20 samples, when the pyrolysis temperature is increased from room temperature to 500 ˚C. It is therefore suggested that methyl groups are easily removed due to fragmentation reactions when
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cellulose and lignin are mixed at high compression pressure. It is known that methane can be formed from the pyrolysis of lignin and cellulose in previous research. IR absorption at round 1600 cm-1 is assigned to benzene stretching. From Fig. 7, the intensity of this peak in relation to the intensity of IR absorption between 1200 and 1500 cm-1 is gradually increased when the compression pressure increases from 5 MPa to 20 MPa. It indicates that charring process is promoted during the pyrolysis of the mixture of cellulose and lignin when these two biomass components are more intensively mixed. It is suggested that the interaction between cellulose and lignin is enhanced for the pyrolysis of CL20 when compared with CL5 and CL10. Couhert et al. (Couhert et al., 2009) reported that the increase of the intensity of mixing of biomass components (from simple mixing to intimate mixing) resulted in the increase of CO 2 production during the pyrolysis process. Additional secondary reactions (the organization of benzene rings) happened inside the sample particles due to close contact between cellulose and lignin. C=O functional groups on the surface of biomass sample are represented between 1100 and 1300 cm-1. These IR absorption peaks are similar during the pyrolysis of the mixtures of cellulose and lignin which are prepared at the compression pressure of 5, 10 and 20 MPa, respectively.
Conclusions
The pyrolysis of three biomass components and their mixtures were monitored by in-situ DRIFTS and TGA. The experiments demonstrate that there exists complicated interactions between the main components of biomass during pyrolysis. The results from TGA/DTG indicate that the interactions between cellulose and xylan introduce higher pyrolysis temperature, while the interactions between 23
cellulose and lignin, xylan and lignin, result less weight loss. The increase of compression pressure for the mixture of main components changes the weight loss of the mixture. The in-situ DRIFTS experimental results show that the addition of cellulose affects the pyrolysis performance of xylan and lignin. It promotes the aromatization (the increase at 1600 cm-1) of xylan when cellulose and xylan are pyrolyzed together. However, when the cellulose and lignin are co-pyrolyzed, cellulose inhibits the accumulation of the benzene ring (~1600 cm-1) of lignin, and assists in substituents (1450 cm-1 and 1375 cm-1) detaching from the benzene ring. During the mixture of the lignin and xylan pyrolysis, lignin will prohibit the formation of xylan-derived carbonyl and C=C bonds. When the samples are compressed at different pressures, the dehydration reaction (~3500 cm -1) shows in the IR absorption almost disappears, which suggests that contact pressure removes the moisture in the mixed samples. Except that, the increase of pressure will also conduct the mixture to more char as the temperature increases. When mixed xylan and lignin pyrolyzed, the substituents (600-900 cm-1) on the aromatic ring in the residue become very unstable as the temperature and the compression pressure increases. It is more easily to fall off from the benzene ring and evolve into low weight molecules for these substituents while the sample is prepared with pressures. Meanwhile, the decomposition of xylan is significantly hindered by lignin. The interactions within cellulose and lignin results that amount of methyl groups are easily removed when they mixed with different compression pressures. Especially the higher compression pressure (CL20) promotes mutually the charring process of each component. Besides the compression pressure, taking consider of the different ratios of the components in the mixture is a potential work to compare the thermal degradation of a three component sample against an original lignocellulosic material.
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Acknowledgement The project was supported by National Key Research and Development Program of China (2018YFB0605101), National Natural Science Foundation of China (21706050), Key Project Natural Science Foundation of Tianjin (18JCZDJC39800), and European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 643322 (FLEXI-PYROCAT).
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Highlights:
Interaction between biomass components during pyrolysis was studied
The mixtures of biomass components were prepared at different compressing pressure
Strong interactions between pyrolysis of cellulose and lignin were found
The interactions between xylan and lignin during pyrolysis are minor
30
S0009-2509(18)30736-X https://doi.org/10.1016/j.ces.2018.10.023 CES 14555
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
10 July 2018 11 October 2018 16 October 2018
Please cite this article as: J. Wang, B. Shen, D. Kang, P. Yuan, C. Wu, Investigate the interactions between biomass components during pyrolysis using in-situ DRIFTS and TGA, Chemical Engineering Science (2018), doi: https:// doi.org/10.1016/j.ces.2018.10.023
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Investigate the interactions between biomass components during pyrolysis using in-situ DRIFTS and TGA Jianqiao Wanga, Boxiong Shena*, Dongrui Kanga, Peng Yuana, Chunfei Wua,b* a b
School of Energy & Environmental Engineering, Hebei University of Technology, Tianjin, China
School of Chemistry and Chemical Engineering, Queens University Belfast, Belfast, Northern Ireland, BT7 1NN * Corresponding authors: E-mail: [email protected] (C. Wu); [email protected] (B. Shen). co-first author: Jianqiao Wang and Boxiong Shen
Abstract Biomass pyrolysis is a promising thermal chemical processing technology to produce renewable and sustainable energy materials. However, the interaction between biomass components (cellulose, hemicellulose and lignin) during pyrolysis process needed further investigation. In this work, an insitu Reflectance Infrared Fourier Transform Spectroscopy (in-situ DRIFTS) reactor was introduced to investigate the interactions assisted with Thermogravimetric Analysis (TGA). The experiments demonstrate that there exist complicated interactions between the main components of biomass during pyrolysis. The results from TGA/DTG indicate that the interactions between cellulose and xylan introduce higher pyrolysis temperature, while the interactions between cellulose and lignin, xylan and lignin, resulte less weight loss. The increase of compression pressure for the mixture of main components changes the weight loss of the mixture. The results from in-situ DRIFTS show that the decomposition and aromatization of xylan are promoted when the cellulose is added. Cellulose will also hinder the accumulation of the benzene rings of the lignin. The in-situ DRIFTS experiment results also suggest that the contact pressure between the components conduct more residue solid, mainly because the dehydration reaction of the mixture samples was inhibited and the charring process was promoted, especially the reaction between cellulose and lignin. Key words: In-situ DRIFTS; biomass pyrolysis; cellulose; xylan; lignin 1
1. Introduction Biomass is a renewable and carbon neutral resource. The utilization of biomass has obtained extensive attention due to climate change derived from using fossil fuels. Through thermochemical conversion technologies, biomass can be processed into fuels and chemicals. These technologies include pyrolysis, gasification, combustion, hydrothermal liquefaction and hydrothermal carbonization (Wang et al., 2017). The products of biomass pyrolysis contain bio-oil, biochar, and non-condensable gas (Anca-Couce, 2016; Kan et al., 2016). Biochar produced from biomass pyrolysis can be used as fertilizer, carbon capture, secondary fuel and etc. Bio-oil can be applied as heavy fuel oil in diesel engines and as sources of valuable chemicals. The produced gas from biomass pyrolysis can be combusted to provide heat for the process of pyrolysis. Biomass pyrolysis is usually taken place at temperature around 500 ˚C within an inert atmosphere. Biomass contained three main components including cellulose (40-50 wt. %), hemicellulose (20-40 wt.%) and lignin (10-40 wt.%) (McKendry, 2002). The ratio of these components depend on the type of the biomass, such as the hardwoods contain higher cellulose and hemicellulose content than softwoods on average. Li et al.(Li et al., 2004) pyrolysis legume straw and apricot stone in the temperature range 500-800 ˚C and conclude that more cellulose and hemicellulose are better sources for the hydrogen rich gases production than lignin. However, the distribution of the pyrolysis products not only depend upon the types of biomass, but the react temperatures, residence time and the heating rate. Fast pyrolysis of biomass is conducted with the high heating rate up to 1000 ˚C s-1 at temperature below 650 ˚C for the liquid products and slow pyrolysis has been used for the production of char(Williams and Besler, 1996). High temperature and short residence times
2
facilitate the formation of gases, while low temperatures and long residence times promote the formation of the char. Bio-oils are usually produced with medium temperatures and short residence times(Velden et al., 2010). As the pyrolysis temperature is around 500 ˚C and heating rate is 10-100 ˚C (an intermediate pyrolysis process), the decomposition of cellulose primarily occurs between 325 ˚C and 400 ˚C, higher than hemicellulose pyrolysis, which happens between 250 ˚C and 350 ˚C. Lignin is the most stable component which can be decomposed at a higher temperature range of 300-550 ˚C (Stefanidis et al., 2014). It was found that the pyrolysis of cellulose was characterized by a high yield of liquid and a low yield of char during the intermediate pyrolysis. In contrast, lignin pyrolysis produces the highest amount of residue char (Wang et al., 2011; Xin et al., 2013). Some previous studies have focused on understanding the pyrolysis mechanisms of individual component. However, the interaction of cellulose, hemicellulose and lignin during biomass pyrolysis is a very interesting topic because cellulose is always connected with hemicellulose and lignin through hydrogen bonds, while hemicellulose is linked to lignin with covalent and hydrogen bonds (Lee et al., 2014; Vorwerk et al., 2004). The interaction of cellulose, hemicellulose and lignin during pyrolysis is complicated and not very clear. Some literatures investigated the mechanism of the interactions in the pyrolysis of biomass components through kinds of various methods and apparatuses. For example Wu et al. (Wu et al., 2016a, b) investigated cellulose-lignin and cellulosehemicellulose interactions with various conditions by Py-GC-MS through detecting the gas/liquid products. Raveendran et al. (McKendry, 2002) and Yang et al. (Yang et al., 2006) used a thermogravimetric analyzer (TGA) to study the pyrolysis characteristics of biomass components and found negligible interactions among the components during pyrolysis. However, Yu et al. (Yu et al., 2017) indicated that except the mixture of xylan (representation of hemicellulose) and lignin, 3
there were evident interactions between synthetic mixture of cellulose and xylan, and cellulose and lignin. Liu et al. (Liu et al., 2011) coupled a thermogravimetric analyzer with Fourier transform infrared spectrometer to detect the intermediate gas products during biomass pyrolysis process. The addition of lignin to the lignin and hemicellulose mixture resulted in the decrease of the yield of 2furaldehyde in the volatiles released from pyrolysis of hemicellulose. Meanwhile, the existence of hemicellulose intensively decreased the yield of levoglucosan. In-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (In-situ DRIFTS) technology had been utilized by detecting the chemical groups on the surface of solid during biomass pyrolysis reaction. For instance, due to the nature of the technology, Shao et al. (Shao et al., 2017) and Uchimiya et al. (Uchimiya et al., 2013) used this technology to explore the change of functional groups on the surfaces of biomass/biochar during catalytic reaction. So it is valuable to introduce In-situ DRIFTS technology to investigate the solid surface interaction of cellulose, hemicellulose and lignin during pyrolysis, though there are no reports about this topic. In this work, to investigate the interaction between the biomass components, the surfaces properties of individual component (cellulose, xylan, and lignin) and their mixtures will be investigated using in-situ DRIFTS analysis and TGA. Considering that the biomass components do not exist independently in nature, the three substances are prepared with different compressing pressures to simulate their intimacy in reality.
2. Materials and methods 2.1 Biomass components 4
Cellulose (Research Chemicals Ltd.), xylan (main components of hemicellulose) (Sigma-Aldrich Co., Ltd) and lignin (Sigma-Aldrich Co., Ltd) are the raw samples used in this work. Xylan was produced from beechwood cell wall polysaccharide and all samples are powders with particle size around 200 um. The element and proximate analysis of three components were detected by our previous work (Wu et al., 2013). The lignin sample in this work contains the highest C content (61.33 wt. %) and fixed carbon (32.60 wt. %). Cellulose has the highest volatiles content (60.37 wt. %) and lowest ash content (0.07 wt. %) compared with lignin and hemicellulose sample.
2.2. Preparation of the mixture of biomass components To evaluate the interactions between the biomass components during the pyrolysis, biomass components were mixed physically with different ratios (Hilbers et al., 2015; Stefanidis et al., 2014; Wu et al., 2016a, b; Yu et al., 2017). In these reports, biomass components were directly compressed and agglomerated to prepare composite samples. Therefore, in this work, the mixture of biomass components was prepared by physically mixing method. The mixed samples with weight ratio 1:1 (for cellulose and xylan, cellulose and lignin, xylan and lignin) were further mechanically compressed at various pressures (5, 10 and 20 MPa) for 30 minutes. These biomass samples were used to study the interactions between components during pyrolysis process. CX, CL and XL indicated the mixture of cellulose and xylan, cellulose and lignin, xylan and lignin, respectively. The compressed pressures was denoted at the end of the symbol, for example, CX5 indicated the mixture of cellulose and xylan was prepared with the weight ratio 1:1 at 5 MPa pressure.
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2.3. TGA experiments In order to evaluate the thermal decomposition of the individual component and their mixture, TGA analysis was conducted by using a SDTQ600 equipment. Around 10 mg of sample was heated at a heating rate of 10 ˚C min-1 from 30 ˚C to 600 ˚C. The experiment was conducted within a N2 atmosphere and the flow rate of N2 was 100 mL min-1.
2.4. In-situ DRIFTS monitoring of biomass pyrolysis Pyrolysis of biomass components and their mixtures was carried out in a diffuse reflection accessory (Praying Mantis™, Harrick Scientific) using a temperature controller (ATC-024-3, Harrick Scientific). The reactor chamber was purged with nitrogen with a flow rate around 600 mL min-1. The functional groups of biomass were detected in real time with the IR spectrometer (TENSOR II, BRUKER). In addition, cooling water was used to control the reactor chamber. The temperature of the In-situ DRIFTS reactor was controlled by heating rate of 10 ˚C min-1.
3. Results and discussion 3.1. Thermogravimetric analysis (TGA) 3.1.1. TG analyses of individual biomass components The mass loss and derived mass loss rate of these three biomass components are presented in Fig. 1. Lignin was decomposed at the range from 200 ˚C to 600 ˚C with the highest mass loss rate at 327 ˚C. The wide decomposing temperature window is mainly attributed to the presence of benzene ring 6
with diverse branches (Collard and Blin, 2014b). The highest carbon content and highly crosslinked nature of lignin is the major factor resulting in the highest solid residues after pyrolysis (~57 wt. %) (Yu et al., 2017). The high thermal stability of cellulose is attributed to the crystalline structure within this pure linear polymer (Yang et al., 2007), and it explains the start depolymerization of cellulose is around 100 ˚C later than the two others. Cellulose has the similar highest depolymerization temperature (~330 ˚C) compared with lignin during TGA pyrolysis, however it has a much more narrow temperate range (300 ˚C to 400 ˚C). The least solid residues (~10 wt. %) is consistent with the high condensable levoglucosan and volatiles such as CO content of cellulose(Yu et al., 2017). The main temperature window of the degradation of xylan is 200 ˚C to 400 ˚C. The two peaks present in the DTG curves (indicated as “-D”) of xylan pyrolysis are at around 250 ˚C and 290 ˚C, respectively. The decomposition of xylan around 250 ˚C was due to the dissappearance of branches such as acetyl and 4-O-methylglucuronic acid. The weight loss around 290 ˚C of xylan is suggested due to the depolymerization of the main chain into furfural (Collard and Blin, 2014a).
7
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Figure 1. TG and DTG curves of the cellulose, xylan and lignin. The solid line is TGA curve. The dash line is DTG curve
3.1.2. TGA analysis of the mixture of biomass components The experimental results of the mixed samples are compared with the calculated values, which are obtained from the average weight sum of individual components (Stefanidis et al., 2014) by following equations: (1) (2) Where
is the weight loss of the mixture (two components), and
means the weight loss of the individual component during pyrolysis at the temperature T 8
respectively. In addition, the
,
,
, are the differential mass loss of the mixtuer,
and the individual component at tempreature T. ‘i’ and ‘j’ indicate any two components of cellulose, xylan and lignin. From Fig.2 (a), an apparent difference is observed between experimental and calculated TGA results. From DTG results of the mixture of cellulose and xylan, the maximum decomposition temperature around 350 ˚C was clearly moved to higher temperature when compared to the pyrolysis of individual components. However, for the mixture of cellulose and lignin, and xylan and lingin, the maximum decomposition temperatures for the calculated results are very similar to the experimental results. In Fig. 2 (b) and Fig. 2 (c), the intensity of weight loss for the experimental results is lower when compared to the calculated results. From the above results, it is suggested that the interactions between biomass components requires higher tempertaure for pyrolysis compared to individual components.
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1.0 Cellulose + Xylan TG (calc.) Cellulose + Xylan TG (exp.) Cellulose + Xylan DTG (calc.) Cellulose + Xylan DTG (exp.)
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Figure 2. Experimental and calculated TG and DTG curves for each mixture . (a) cellulose + xylan; (b) cellulose + lignin; (c) xylan + lignin
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3.2. In-situ DRIFTS analysis of biomass components and their mixtures
(a)
(b)
(c)
(d)
(e)
(f) 11
Figure 3. In-situ DRIFTS analysis results of biomass components and mixtures, (a) cellulose; (b) xylan; (c) lignin; (d) cellulose+xylan; (e) cellulose+lignin; (f) xylan+lignin
Fig. 3 (a) shows the results of In-situ DRIFTS analysis of cellulose sample at different temperatures with a heating rate of 10 ˚C min-1. Weight loss of cellulose before 220 ˚C was suggested to be the loss of physical water (J. Scheirs, 2001). This is consistent with the fast reduce of the intensity at around 1650 cm-1, which is corresponding to the remove of moisture before 220 ˚C. The intensity of absorption at around 3500 cm-1, assigned to OH groups, is gradually reduced with the increase of reaction temperature. The intensity decreasing before 300 ˚C may be attributed to the formation of anhydrocellulose through intramolecular and intermolecular dehydration actions. The water elimination at the beginning of the cellulose pyrolysis stabilizes the cellulose-derived char matrix. The intermolecular dehydration takes place between two chains in the polymer and result in higher reticulation of the biochar (Scheirs et al., 2001). Beyond that, the intramolecular dehydration will facilitate the cyclizing of the benzene by forming the C=C double bonds (Scheirs et al., 2001). The continuous decrease at around 3500 cm-1 from 300 ˚C to 500 ˚C corresponds to the hydroxyl group disappearance on the monomer unit of cellulose. The breaking of β-1,4-glycosidic between the two β-glucopyranose units, which are the components of the cellulose, result in the decrease intensity of the 1050 cm-1 (C-O-C bonds vibration) along the temperature rising up. Meanwhile, the increasing absorption around 1310 cm-1 and 1120 cm-1 (vibration of ether bonds within aromatic ring) become clear with the temperature increase during the pyrolysis process, so the residue become more and 12
more aromatic on account of many furan rings appearing, for example levoglucosan (C 6H10O5). Methanol removal during β-glucopyranose unit becoming the furan rings is also the reason for the decreasing of OH at around 3500 cm-1 at the high temperature. The decrease of 1098 cm-1 absorption indicates some volatile compounds such as oxygenated five-membered heterocycle furfural and furfuryl alcohol (Lu et al., 2011; Tomas E. McGrath, 2003; Wang et al., 2013). The increase of the absorption at around 800 cm-1 and 900 cm-1 suggests the cyclizing of the benzene rings. For xylan sample, as shown in Fig. 3 (b), the clear intensity reduction of the absorption at around 2900 cm-1 and 1450 cm-1 is assigned to methyl groups which are presented in xylan pyrolysis residue at high temperature (Zhao et al., 2017). The rupture of xylan branch corresponds to the first peak on the DTG curve of the xylan pyrolysis (Fig. 1), and during which many formic acid, methanol and acetic acid were produced (Collard and Blin, 2014a). The changes around 1720 cm-1 and 1610 cm-1 indicate the vibration of the carboxyl within acetyl of the xylan side chain. Compare with that in the cellulose pyrolysis, much more aromatic rings in the residue of xylan are detected with the In-situ DRIFTS. The vibrations of the C=C double bonds and aromatic skeletal compounds (around 1600 cm-1) are much more intense than that of cellulose pyrolysis at high temperature. It corresponds to the less char left after the cellulose pyrolysis than xylan in the TGA experiment. Fig. 3 (c) shows the In-situ DRIFTS experimental results using lignin sample. The propyl chain conversion to the benzene rings in lignin includes the dehydration and cleavage of alkyl C-H bond. The remove of moisture corresponding to the absorption at around 3500 cm-1 is clearly observed when the pyrolysis temperature is increased from room temperature to 200 ˚C during lignin
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pyrolysis. The intensity of the absorption peaks between 2960 cm-1 and 2850 cm-1 (the vibration of the C-H bond of the alkane) indicated the decomposition of aliphatic compounds was promoted when the pyrolysis temperature was higher than 300 ˚C (Candelier et al., 2011; Sharma et al., 2004). The accumulation of aromatics at high pyrolysis temperature (>400 ˚C) is clearly shown in Fig. 3 (c) where the absorption of trisubstituted benzene ring ( between 700-850 cm-1) was enhanced (Uchimiya et al., 2013; Zhao et al., 2017). This is consistent with the literature that with increasing the pyrolysis temperature of milled wood lignin, condensation, depolymerization and carbonization started in this sequence at 250, 350 and 400 °C, respectively. (Nakamura et al., 2008) It is worth noting that the increased absorption intensity at 1450 cm-1 and 1375 cm-1 imply the substituents sites on the benzene ring are gradually occupied by the methyl group. The mixture of cellulose and xylan was tested in In-situ DRIFTS with increasing temperature. As shown in Fig. 3 (d), it looks like that the decomposition of compounds containing OH groups (at around 3500 cm-1) was enhanced (e.g. dehydration reaction) when xylan and cellulose was mixed during the pyrolysis of the mixture of cellulose and xylan. As the conversion is around 30% at temperature of 300 ˚C for cellulose alone in Fig. 3(a), the conversion is increased to around 50% for the mixture of xylan and cellulose. Compared to the absorption intensity of the pyrolysis of pure cellulose and xylan（Fig. 3 (a) and Fig. 3 (b), the absorption band at around 2950 cm-1 shows obviously changes with increasing temperature during the co-pyrolysis of xylan and cellulose. It is indicated that a strong absorption of CH groups was present for the mixture of xylan and cellulose (Uchimiya et al., 2013). The changes around 1720 cm-1 and 1600 cm-1 seem like the curves overlap of cellulose with xylan. It is shown that the formation of C=O and C=C groups corresponding to carbonyl and aromatic skeletal compounds in xylan sample were promoted by mixing it with 14
cellulose. However, the absorption band between 1000 and 1300 cm-1 indicates that the decomposition of xylan was dominant in this region during the co-pyrolysis of xylan and cellulose, as the absorption in this region is very similar to the pyrolysis of pure xylan. It is due to the presence of large amounts of acetyl compounds in xylan. Compared to the pyrolysis of xylan, the co-pyrolysis of the mixture of cellulose and xylan has a strong absorption between 1310 cm-1 and 1120 cm-1. This is related to the strong absorption of ether groups in this region during the pyrolysis of pure cellulose. In addition，there is an opposite trend of absorption in this region for the pyrolysis of cellulose alone and the mixture of cellulose and xylan. It suggested that with the pyrolysis temperature increasing to 500 ˚C, the decomposing of cellulose to levoglucosan has been prohibited by the releasing products of the pyrolysis of xylan. This might be the reason why the decomposition temperature of cellulose and xylan pyrolysis moves to a higher temperature value in the DTG curve (Fig. 2 (a)). However, interactions between the pyrolysis of cellulose and hemicellulose (xylan) have been reported to be minor by other researchers (Svenson et al., 2004; Zhang et al., 2015). Fig. 3 (e) shows the results of using the mixture of cellulose and lignin. The overall absorption pattern of the mixture of cellulose and lignin in the process of pyrolysis is similar to that from cellulose alone, indicating that cellulose has a stronger signal of IR absorption. For example, the absorption around 1600 cm-1, related to C=C aromatic stretching, disappeared when cellulose was mixed with lignin for pyrolysis (Fig. 3(c)). This might be due to the strong absorption at 1650 cm -1 (OH group) for the cellulose sample. In addition, the changes of the absorption between 700 and 850 cm-1 are weaken for the mixture of cellulose and lignin compared to that from lignin alone. The small change of the absorption of lignin sample during pyrolysis could also be due to the inhibition 15
of reactions when cellulose was added. The decomposition of lignin was reported to be hindered due to the interaction between cellulose and lignin during the process of pyrolysis (Fushimi et al., 2009). However, negligible interactions between cellulose and lignin during pyrolysis have been reported when cellulose and lignin was physically mixed in previous research, as no covalent bonds were present for the mixed biomass components (Svenson et al., 2004). As shown in Fig. 3 (f), the in-situ DRIFTS analysis of co-pyrolysis of xylan and lignin shows that the absorption patterns of the mixture is similar to that of lignin alone. This result is consistent with the TGA analysis (Fig. 2). The changes of absorption at 1720 cm-1 and 1600 cm-1 with the increase of temperature are not observed for pyrolysis of the mixture of xylan and lignin, while the formation of C=O and C=C groups is found for pyrolysis of xylan alone. It is suggested that the addition of lignin might prohibit the decomposition of xylan to carbonyl and aromatic compounds (charring process). The charring process of lignin (between 700-850 cm-1) at high temperature (>400 ˚C) is minor as shown in Fig. 3 (f), when xylan and lignin were mixed. It is suggested there might be interactions between the pyrolysis of xylan and lignin, resulting in a slow charring process during pyrolysis.
3.3 TGA analysis of the mixture of biomass components prepared at different compression pressures
The weight loss and derivative weight loss curves of the mixture produced using different compression pressure are shown in Fig. 4. With the increase of compression pressure, more solid 16
residue is remained for the pyrolysis of the mixture of cellulose and xylan (Fig.4 (a), (b)). For cellulose and lignin, when the compression pressure of the mixture increases, the weight loss decreases at first and then increases again (Fig. 4. (c), (d)). As shown in Fig. 4 (e) and Fig. 4 (f), the weight loss of the mixtures of xylan and lignin prepared at different compression pressure varies slightly throughout the pyrolysis process, which suggested that the effect of compression pressure on weight loss is not so great for the mixture of xylan and lignin. The disagreement of the effect of compression pressure on the weight loss indicates that the compression pressure introduces the interaction of biomass components and the interaction is complicated. 1.0 0.9 0 Mpa 5 Mpa 10 Mpa 20 Mpa
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Figure 4. TG and DTG curves of the mixture of biomass components prepared at different compression pressures (a)(b) cellulose and xylan (c)(d) cellulose and lignin (e)(f) xylan and lignin
3.4 In-situ DRIFTS analysis of the mixtures of biomass components prepared at different compression pressures
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(a)
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(c) Figure 5. In-situ DRIFTS analysis of cellulose and xylan mixture compressed at (a) 5 MPa (CX5), (b) 10 MPa (CX10) and (c) 20 MPa (CX20). From Fig. 5, as the compression pressure increases, the dehydration reaction (3500 cm -1) at around 200 ˚C becomes very inconspicuous. For CX5 sample, the changes of the IR absorption around 1600 cm-1 relating to aromatic ring stretching is not clear or the intensity of this peak is slightly increasing with the increase of pyrolysis temperature from room temperature to 500 ˚C. This IR absorption peak is slightly increasing with the increase of pyrolysis temperature for samples CX 10 and CX 20. Compared to cellulose and xylan mixed without any pressure (Fig.3 (d)), the compressed samples show regular increase absorption at 800-900 cm-1. It indicates that char is continuously formed during the cracking process. This implies that intimacy (high pressure indication more intimacy) is the main reason for the increase of the residue in the TGA experiment (Figure 4(a)). However, in general the influence of the compression pressure of the preparation of
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cellulose and xylan on the interaction of biomass component is suggested to be minor during pyrolysis process.
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(c) Figure 6 DRIFTS analysis of xylan and lignin mixture compressed at (a) 5 MPa (XL5), (b) 10 MPa (XL10) and (c) 20 MPa (XL20)
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From Fig. 6, for the mixture of xylan and lignin, all samples (XL5, XL10 and XL20) show almost similar trends of IR absorption when the pyrolysis temperature increases from room temperature to 500 ˚C. These changes include the reduction of IR band at around 3500 cm-1 as the temperature increases which is related to dehydration reactions. Aromatic groups on the surface of the biomass mixture are slightly increased with the increase of pyrolysis temperature. For example, the spectra at 600-900 cm-1, indicating the changes of the substitution group on the benzene ring of samples, show significant differences for the samples prepared by different pressure compression. In the XL10, xylan is gradually aromatized so the benzene rings substituents (such as –CH3 and –OH) increased in the residue. However, with the temperature increases, these substituents slowly separated from the benzene ring. When the components mixed with 20 MPa (XL20), the substituents on the benzene rings within lignin gradually decreases as the temperature increases from room temperature to 500 ˚C, and xylan decomposition was suppressed during the formation of char. It suggested that the high compression pressure will promote the interaction between xylan and lignin and help the solid decompose to low weight molecule.
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(a)
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(c) Figure 7 In-situ DRIFTS analysis of cellulose and lignin mixture compressed at (a) 5 MPa (CL5), (b) 10 MPa (CL10) and (c) 20 MPa (CL20) From Fig. 7, dehydration of biomass components of cellulose and lignin (around 2900 cm-1) is enhanced with the increase of the pressure during the preparation of the mixture of biomass sample. At around 3000 cm-1 (C-H bonds), the changes of IR absorption is not clear with the increase of temperature for the pyrolysis of single components (cellulose or lignin). However, when these two biomass components are mixed, clear changes of absorption intensity at around 3000 cm-1 is observed in particular for the samples prepared by compression method. In addition, it is clear that the compression pressure influences the interactions between cellulose and lignin during pyrolysis. The intensity at around 3000 cm-1 is slightly increased for the sample CL5 with the increase of reaction temperature. However, the intensity of this peak is gradually reduced for the CL10 and CL20 samples, when the pyrolysis temperature is increased from room temperature to 500 ˚C. It is therefore suggested that methyl groups are easily removed due to fragmentation reactions when
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cellulose and lignin are mixed at high compression pressure. It is known that methane can be formed from the pyrolysis of lignin and cellulose in previous research. IR absorption at round 1600 cm-1 is assigned to benzene stretching. From Fig. 7, the intensity of this peak in relation to the intensity of IR absorption between 1200 and 1500 cm-1 is gradually increased when the compression pressure increases from 5 MPa to 20 MPa. It indicates that charring process is promoted during the pyrolysis of the mixture of cellulose and lignin when these two biomass components are more intensively mixed. It is suggested that the interaction between cellulose and lignin is enhanced for the pyrolysis of CL20 when compared with CL5 and CL10. Couhert et al. (Couhert et al., 2009) reported that the increase of the intensity of mixing of biomass components (from simple mixing to intimate mixing) resulted in the increase of CO 2 production during the pyrolysis process. Additional secondary reactions (the organization of benzene rings) happened inside the sample particles due to close contact between cellulose and lignin. C=O functional groups on the surface of biomass sample are represented between 1100 and 1300 cm-1. These IR absorption peaks are similar during the pyrolysis of the mixtures of cellulose and lignin which are prepared at the compression pressure of 5, 10 and 20 MPa, respectively.
Conclusions
The pyrolysis of three biomass components and their mixtures were monitored by in-situ DRIFTS and TGA. The experiments demonstrate that there exists complicated interactions between the main components of biomass during pyrolysis. The results from TGA/DTG indicate that the interactions between cellulose and xylan introduce higher pyrolysis temperature, while the interactions between 23
cellulose and lignin, xylan and lignin, result less weight loss. The increase of compression pressure for the mixture of main components changes the weight loss of the mixture. The in-situ DRIFTS experimental results show that the addition of cellulose affects the pyrolysis performance of xylan and lignin. It promotes the aromatization (the increase at 1600 cm-1) of xylan when cellulose and xylan are pyrolyzed together. However, when the cellulose and lignin are co-pyrolyzed, cellulose inhibits the accumulation of the benzene ring (~1600 cm-1) of lignin, and assists in substituents (1450 cm-1 and 1375 cm-1) detaching from the benzene ring. During the mixture of the lignin and xylan pyrolysis, lignin will prohibit the formation of xylan-derived carbonyl and C=C bonds. When the samples are compressed at different pressures, the dehydration reaction (~3500 cm -1) shows in the IR absorption almost disappears, which suggests that contact pressure removes the moisture in the mixed samples. Except that, the increase of pressure will also conduct the mixture to more char as the temperature increases. When mixed xylan and lignin pyrolyzed, the substituents (600-900 cm-1) on the aromatic ring in the residue become very unstable as the temperature and the compression pressure increases. It is more easily to fall off from the benzene ring and evolve into low weight molecules for these substituents while the sample is prepared with pressures. Meanwhile, the decomposition of xylan is significantly hindered by lignin. The interactions within cellulose and lignin results that amount of methyl groups are easily removed when they mixed with different compression pressures. Especially the higher compression pressure (CL20) promotes mutually the charring process of each component. Besides the compression pressure, taking consider of the different ratios of the components in the mixture is a potential work to compare the thermal degradation of a three component sample against an original lignocellulosic material.
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Acknowledgement The project was supported by National Key Research and Development Program of China (2018YFB0605101), National Natural Science Foundation of China (21706050), Key Project Natural Science Foundation of Tianjin (18JCZDJC39800), and European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 643322 (FLEXI-PYROCAT).
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Highlights:
Interaction between biomass components during pyrolysis was studied
The mixtures of biomass components were prepared at different compressing pressure
Strong interactions between pyrolysis of cellulose and lignin were found
The interactions between xylan and lignin during pyrolysis are minor
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