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Characteristics and mechanism study of analytical fast pyrolysis of poplar wood
Energy Conversion and Management 57 (2012) 49–59
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Characteristics and mechanism study of analytical fast pyrolysis of poplar wood Chang-qing Dong ⇑, Zhi-fei Zhang, Qiang Lu ⇑, Yong-ping Yang National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing 102206, China
a r t i c l e
i n f o
Article history: Received 20 August 2011 Received in revised form 16 November 2011 Accepted 20 December 2011 Available online 13 January 2012 Keywords: Fast pyrolysis Mechanism Py–GC/MS Poplar wood
a b s t r a c t Analytical pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) was applied to achieve fast pyrolysis of poplar wood and on-line analysis of the pyrolysis vapors. Experiments were conducted to reveal the distribution of pyrolytic products under different pyrolysis temperatures (300–1000 °C) and times (5–30 s). During the fast pyrolysis process, the poplar wood started decomposition to form organic volatile products at the set temperature of 300 °C, and reached the maximum volatile product yield at around 550 °C. The products included various anhydrosugars, furans, phenolic compounds, linear carbonyls, linear acids, hydrocarbons, and so on. They exhibited different formation characteristics. Based on the experimental results, we discussed the possible pyrolytic pathways for the generation of the major products. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Fast pyrolysis of biomass is a thermal decomposition process that occurs in the absence of oxygen, to convert solid biomass mainly into a liquid product known as bio-oil [1–3]. Liquid biooil covers many potential application fields and is regarded as the potential substitute of petroleum fuels [4,5]. However, bio-oil is totally different from petroleum fuels, as it is a complex mixture of water and hundreds of organic compounds that belong to acids, aldehydes, ketones, alcohols, esters, anhydrosugars, furans, phenols, as well as large molecular oligomers. The complex chemical composition makes the bio-oil difficult for commercial application. To solve this problem, various upgrading methods have been proposed on either crude bio-oil or fast pyrolysis process [6,7]. Hence, it is very essential to understand the biomass fast pyrolysis mechanism and product formation characteristics. Till now, most of the pyrolysis studies were performed under the slow or conventional pyrolysis conditions, the product distribution differed considerably from that of the fast pyrolysis conditions. In regard to the fast pyrolysis studies, attentions were mainly paid to the yields and overall properties of bio-oil, char and gas products. Very limited studies are available to report the detailed characteristics of biomass fast pyrolysis, based on the changes of individual products in the biooil [8–11]. Lignocellulosic biomass mainly consists of cellulose, hemicellulose and lignin, together with a small portion of extractives and ash. Cellulose is a linear homopolysaccharide of b-D-glucopyranose units linked together by (1 ? 4)-glycosidic bonds. It starts pyrolysis ⇑ Corresponding authors. Tel./fax: +86 10 61772031. E-mail addresses: [email protected] (C.-q. Dong), [email protected] (Q. Lu). 0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2011.12.012
at as low as 150 °C [12]. At temperatures above 300 °C, pyrolysis of cellulose will undergo depolymerization and fragmentation (ring scission) reactions to mainly form a liquid product, including anhydro-oligosaccharides, monomeric anhydrosugars (dominated by the levoglucosan (LG)), furans, linear carbonyls and other products [13–15]. Till now, various reaction schemes have been proposed for the formation of liquid products from pyrolysis of cellulose under different conditions [16–18]. Hemicelluloses are amorphous polysaccharides with building units belong either to hexoses or to pentoses. The primary hemicellulose components are galactoglucomannans (glucomannans) and arabinoglucuronoxylan (xylan). Hemicelluloses are less thermally stable than cellulose, presumably due to their lack of crystallinity. Their pyrolysis is generally thought to be analogous to cellulose in the reaction mechanisms. However, due to the structure difference, fast pyrolysis of hemicellulose would obtain more char and less oil than that of cellulose, and the composition of the oils from them differs greatly from each other [8,19–22]. Till now, very limited studies are available to report the pyrolytic reaction schemes of hemicellulose [23,24], especially under the fast pyrolysis conditions. Lignin is a complex, heterogeneous polymer formed by the polymerization of three phenyl propane monomers, i.e. guaiacyl (4-hydroxy-3-methoxyphenyl), syringyl (3,5-dimethoxy-4-hydroxyphenyl) and p-hydroxyphenyl units. Primary pyrolysis of lignin begins with thermal softening at temperatures around 200 °C, while most lignin pyrolysis occurs at higher temperatures. Fast pyrolysis of lignin will obtain higher char yield than the holocellulose, and the liquid product mainly includes the large molecular oligomers (known as pyrolytic lignins), the monomeric phenolic compounds, as well as light linear compounds [25–28]. Due to the un-defined lignin structure, its pyrolytic reaction schemes could only be proposed based on the monomeric lignin units or model compounds [29,30].
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In this study, fast pyrolysis of biomass was achieved using the analytical Py–GC/MS technique which allowed the on-line analysis of the pyrolysis vapors. Poplar wood was selected as the biomass feedstock, because it is very abundant in China. The experiments were carried out from 300 °C to 1000 °C under the pyrolysis times of 5–30 s and heating rate of 20 °C/ms, to reveal the effects of pyrolysis temperature and time on the product distribution, especially the important single products. Based on the experimental results, we modified previous reaction schemes and then proposed the pyrolytic pathways for the major products. We also discussed the product formation characteristics relating to their generation pathways.
2. Experimental The biomass used in this study was poplar wood. Prior to experiments, it was firstly ground in a high speed rotary cutting mill and then sieved. The particles with the size of 0.2–0.3 mm were selected, dried in an oven at 110 °C and stored in a vacuum desiccator for experiments. The component composition of the poplar wood was cellulose 49.70%, hemicellulose 24.10%, lignin 23.55%, extractive 2.22% and ash 0.43%. Its elemental composition on the dry basis was C 49.6%, H 6.31%, N 0.08% and S 0.09%. It is to note that the poplar wood is a kind of hard wood, its major hemicellulose component was the xylan. The analytical Py–GC/MS experiments were performed using the CDS Pyroprobe 5200HP pyrolyser (Chemical Data Systems) connected with the Perkin Elmer GC/MS (Clarus 560). Fast pyrolysis was achieved by the analytical pyrolyser, the pyrolysis vapors were directly transferred to the GC/MS and analyzed by it. During the preparation of the experimental samples, the pyrolysis tube was successively filled with a quartz rod, some quartz wool, 0.50 mg poplar wood and some quartz wool. Quartz wool was placed at both sides of the poplar wood, to prevent the escape of the solid particles. The details of the sample preparation could be found in our previous study [31]. An analytical balance with the readability of 0.01 mg was used for weighing, to ensure the poplar wood quantity of strictly 0.50 mg. The pyrolysis was carried out at the set temperature range of 300–1000 °C and the time range of 5–30 s. The heating rate was 20 °C/ms. The pyrolysis vapors were analyzed by GC/MS. The injector temperature was kept at 300 °C. The chromatographic separation was performed using an Elite-35MS capillary column (30 m 0.25 mm i.d., 0.25 lm film thickness). Helium (99.999%) was used as the carrier gas with a constant flow rate of 1 mL/min and a 1:80 split ratio. The oven temperature was programmed from 40 °C (3 min) to 180 °C with the heating rate of 4 °C/min, and then to 280 °C (4 min) with the heating rate of 10 °C/min. The temperature of the GC/MS interface was held at 280 °C, and the mass spectrometer was operated in EI mode at 70 eV. The mass spectra were obtained from m/z 20 to 400. The chromatographic peaks were identified according to the NIST library, Wiley library and the literature data of previous studies. For each sample, the experiments were conducted at least three times to confirm the reproducibility of the reported procedures. For each identified product, the average values of the peak area and peak area% were calculated and used for discussion. The GC/ MS technique could not give the direct quantitative analysis of the compounds, due to the complex pyrolytic products and the lack of commercially available standards for them. However, the chromatographic peak area of a compound is considered linear with its quantity, and the peak area% is linear with its content. Therefore, for each product, its average peak area value obtained under different reaction conditions can be compared to reveal the changing of its yields, and the peak area% value can be compared to show the changing of its relative content among the detected products.
3. Results 3.1. Product distribution from fast pyrolysis of poplar wood Biomass fast pyrolysis vapors consisted of permanent gases (CO, CO2, CH4, H2, etc.), volatile compounds and non-volatile oligomers. The condensation of the latter two classes of compounds would obtain the liquid bio-oil, while GC/MS was only able to determine the organic volatile compounds. During the analytical experiments, no pyrolytic products were detected by GC/MS when the set pyrolysis temperature was lower than 300 °C. It indicated that the poplar wood was not decomposed to form organic volatiles at such low temperatures. When the set temperature was higher than 300 °C, various pyrolytic products were detectable, with more than 100 peaks displayed on typical ion chromatograms. Most of the peaks were identified by the NIST library, and some were referred to the Wiley library and previous studies, but some peaks were unable to be determined. These identified compounds were similar to literature data of the chemical composition of bio-oils [32–36]. One of the drawbacks of the analytical Py–GC/MS experiments is that it did not allow product collection, and thus, the exact bio-oil yield could not be determined. However, it is able to have a primary estimate of the yield changes of the total detected compounds, through the comparison of the total chromatographic peak area values obtained under different pyrolysis conditions. The results from pyrolysis at 350–1000 °C are shown in Fig. 1. The products detected at 300 °C were very low, and thus, the results at this temperature are not shown in Fig. 1 and will not be discussed in this paper. According to Fig. 1, as the rising of the pyrolysis temperature, the total peak area values were firstly increased and then decreased, with the maximal value obtained at around 550 °C. The significant increase before 550 °C should be due to the promoted heat transfer and pyrolysis reactions to form the organic volatile products, while the decrease after 600 °C might be due to the enhanced reactions towards permanent gases. Furthermore, as the rising of pyrolysis time, the total peak area values were gradually increased from 5 s to 30 s at temperatures lower than 550 °C, and then only increased in the first 20 s, 10 s or 5 s at elevated temperatures. The results clearly indicated that the complete devolatilization of the poplar wood could be shortened from 30 s to only 5 s as the rising of pyrolysis temperature. In addition, all the detected compounds were classified into eight groups, including anhydrosugars, furans, linear aldehydes, linear ketones, linear acids, phenolic compounds, hydrocarbons, and others (linear alcohols, linear esters, cyclopentanones, etc.). Their peak area% results are given in Table 1. It is to note that the sum of the peak area% values was not equal to 100%, due to the unidentified chromatographic peaks. Based on the peak area% results, it is seen that the relative content of each product group was influenced considerably by the pyrolysis temperature, but not very remarkably by the pyrolysis time. Generally, the acid group and the anhydrosugar group had the maximal peak area% values at low and medium temperatures, respectively. The linear ketone group and the hydrocarbon group had the maximal peak area% values at high temperatures. The peak area% values of the other three groups (furans, linear aldehydes and phenolic compounds) were not greatly affected by the pyrolysis conditions. The detailed results of the important and typical products in each product group are shown in Figs. 2–5, and will be discussed in the following sections.
3.2. Effects of pyrolysis temperature and time on the anhydrosugars During the fast pyrolysis process, the depolymerization of the holocellulose produced various anhydrosugars and related deriva-
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Fig. 1. The total chromatographic peak areas from fast pyrolysis of poplar wood under different conditions.
Table 1 The composition of the pyrolytic products under different pyrolysis conditions (peak area%). Temperature
Time (s)
Furans
Aldehydes
Acids
Phenols
Hydro carbons
Others
350 °C
5 10 20 30
Anhydro sugars 2.9 3.5 3.6 3.2
6.9 6.4 7.4 8.6
14.4 12.9 12.6 11.7
2.0 2.5 2.9 2.8
18.0 17.3 17.6 20.6
29.3 31.6 31.3 28.1
1.3 1.2 1.2 1.3
3.0 3.2 3.2 3.2
400 °C
5 10 20 30
4.1 4.5 4.2 4.3
6.1 6.2 5.9 5.9
10.4 10.4 10.5 10.6
3.0 3.4 3.1 3.1
21.0 20.9 22.6 22.5
31.7 29.9 28.8 28.4
0.8 0.8 0.5 0.5
7.1 7.6 7.5 7.7
450 °C
5 10 20 30
5.1 5.6 6.0 6.1
6.3 5.7 5.8 6.2
11.9 11.3 13.8 14.5
3.3 4.5 5.1 5.2
19.8 19.9 18.2 17.7
29.1 29.4 27.5 25.2
0.9 0.7 0.6 0.6
7.4 6.5 7.1 7.4
500 °C
5 10 20 30
6.0 7.2 10.0 9.4
6.4 7.4 6.7 7.6
15.4 16.0 16.4 16.4
6.1 6.9 7.5 7.2
15.3 13.0 12.5 12.4
26.3 24.1 22.7 22.2
0.7 0.6 0.6 0.5
7.9 8.9 8.6 8.8
550 °C
5 10 20 30
9.3 9.8 10.2 10.0
7.2 7.4 7.1 7.3
13.2 13.3 12.9 13.0
7.4 7.6 7.9 7.5
12.4 12.6 12.5 12.4
25.0 25.5 25.3 24.4
0.6 0.6 0.6 0.6
9.6 9.4 9.3 9.2
600 °C
5 10 20 30
8.3 9.0 9.8 9.1
6.1 6.3 6.3 6.9
12.6 12.2 12.5 12.8
8.5 8.4 8.8 8.5
12.5 12.7 12.9 13.1
26.8 26.0 26.3 25.7
0.7 0.7 0.8 0.9
9.1 9.5 8.9 9.1
700 °C
5 10 20
9.4 8.7 8.5
5.9 6.0 6.2
13.0 13.0 13.8
9.8 9.7 10.8
12.6 13.9 13.4
26.3 25.6 26.9
1.2 1.2 1.2
8.8 8.6 9.1
800 °C
5 10 20
8.9 8.8 8.0
5.6 5.8 4.9
15.4 15.2 14.4
10.4 10.1 11.8
13.1 13.9 13.4
24.9 24.2 26.7
1.8 1.9 2.2
8.2 8.4 8.6
900 °C
5 10 20
7.6 7.2 6.7
5.2 5.3 4.8
16.1 15.7 14.9
11.1 10.7 12.3
12.9 14.0 14.0
24.3 24.6 25.5
3.3 3.6 3.7
8.4 8.5 8.7
1000 °C
5 10 20
7.4 7.0 6.3
5.0 5.3 4.6
16.3 15.5 15.0
10.9 10.8 12.1
12.9 13.3 13.9
25.0 24.6 25.8
4.3 5.0 4.8
8.2 8.7 7.5
tives, dominated by the LG. Other anhydrosugar products mainly included the levoglucosenone (LGO), 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one (LAC), 1,4:3,6-dianhydro-a-d-glucopyranose (DGP), 1,4-anhydro-a-d-xylopyranose (ADX), etc., which were all formed in very low yields. The LG was mainly derived from cellulose, its formation was influenced remarkably by both of pyrolysis temperature and time, with the results given in Fig. 2. According to the peak area results, the LG yield was firstly increased and then decreased as the rising of the pyrolysis temperature, with the maximal yield obtained at around 550 °C. The complete formation of the LG required 30 s at low temperatures, and gradually decreased to only 5 s at high temperatures. Furthermore, based on the peak area% results, the maximal relative content of the LG was also obtained at 550 °C for 20 s pyrolysis.
Ketones
The LG was the most important pyrolytic product of cellulose, and has appealed to detailed mechanism study. Ponder et al. proposed the possible pyrolytic pathway to produce the LG [37], as shown in Fig. 6 (path 1), which was generally accepted by other researches [15]. The scission of the glycosidic bond yielded a glucosyl cation with a free primary hydroxyl group at C-6, which could readily form a stable 1,6-anhydride. The subsequent cleavage of another glycosidic bond liberated the monomeric LG, and meanwhile generated another glucosyl cation which could undergo similar reactions. Temperature would play an important role on the LG production. According to our results, the rising of pyrolysis temperature before 550 °C would accelerate the cleavage of the glycosidic bond to increase the LG yield and relative content. However, the further rising of temperature after 600 °C would enhance other
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Fig. 2. The effects of pyrolysis temperature and time on the peak area and peak area% of the LG.
Fig. 3. The effects of pyrolysis temperature and time on the peak area and peak area% of the HMF, FF, MF and F.
competing pyrolysis reactions (mainly fragmentation reactions) to inhibit the LG formation, and would also promote the secondary cracking of the LG, resulted in the decreased LG yield and relative content. Among the other anhydrosugars, it is necessary to note the ADX which was the typical anhydrosugar derived from the xylan (the major hemicellulose component of the poplar wood) [23,38–40]. It was produced via the cleavage of the two glucosidic bonds on the polysaccharide chain and the formation of the 1,4-anhydride, as shown in Fig. 7 (path 1). The ADX exhibited similar formation
characteristics as the LG. Both of its peak area and peak area% values were firstly increased and then decreased along with pyrolysis temperature. However, the yield of the ADX was much lower than the LG, with the peak area% value not exceeding 1% in either pyrolysis condition. It should be attributed to the structure difference between the cellulose and xylan. According to Ponder and Nichards, during cellulose pyrolysis, the glucosyl cation from the scission of the common glucans could readily form a stable 1,6-anhydride with the free primary hydroxyl group at C-6, and finally yield the volatile LG. However, it was difficult for the intramolecular ‘‘stabilization’’
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Fig. 4. The effects of pyrolysis temperature and time on the peak area and peak area% of the HAA, HA, acetaldehyde and acetone.
Fig. 5. The effects of pyrolysis temperature and time on the peak area and peak area% of the AA.
of the xylosyl cation via anhydride formation (shown in Fig. 7), and thus, the xylosyl cation was more likely to enter the non-specific dehydration pathways to char formation rather volatiles. Moreover, it was reported that 1,4-anhydride on the xylopyranose was less stable than the 1,6-anhydride on the glucopyranose [20]. Our results showed that the formation of the ADX was not favored at high pyrolysis temperatures, which might be due to the fact that the depolymerization reaction to form the ADX was less competitive than the fragmentation reactions at elevated temperatures.
3.3. Effects of pyrolysis temperature and time on the furan compounds A lot of furan compounds were produced in the pyrolytic process, typically the 5-hydroxymethyl-furfural (HMF), furfural (FF), 2-methyl furan (MF) and furan (F). As shown in Table 1, the peak area% values of the furan group were not greatly changed before 550 °C and then slowly decreased after 550 °C. In regard to the specific compounds, as the rising of pyrolysis temperature, most furans were firstly increased and then decreased, while some light ones
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Fig. 6. The proposed pyrolytic pathways for fast pyrolysis of cellulose, after Shen and Gu [17].
(MF and F) were increased monotonically. The formation characteristics of the four typical furan compounds are given in Fig. 3. The HMF was a six-carbon furan compound, derived mainly from cellulose. According to Fig. 3, the maximal HMF yield (based on the peak area results) was obtained at around 550 °C, and its relative content (based on the peak area% results) was decreased after 700 °C. The formation of the HMF included the cleavage of the ring glucosidic bond (C1–O), the intramolecular dehydration, and the acetal reaction between the C2 and C5 [10,17,41–43], as shown in Fig. 6 (path 2). Based on our results, the rising of the pyrolysis temperature before 600 °C could promote this pyrolytic pathway. However, the HMF yield was decreased after 700 °C, which might be due to the temperature-enhanced competing path-
ways (fragmentation reactions) to inhibit the HMF formation, or due to the secondary cracking of the HMF (Fig. 6, path 8, 9). The FF was derived from both of cellulose and hemicellulose. As shown in Fig. 4, its yield was influenced remarkably by the pyrolysis temperature, with the maximal yield obtained at around 600 °C. While its relative content was not greatly affected by the pyrolysis conditions. According to Paine et al., during the D-glucose pyrolysis, the FF could be produced from either the C1–C5 or the C2–C6, with the furan ring formed between the C1 and C4, C2 and C5, or C3 and C6 [43]. Therefore, it is able to propose the possible pyrolytic pathways for the FF derived from cellulose, as shown in Fig. 6 (path 3, 4). In some literature, it is believed that the FF was derived from the cracking of the HMF, via the elimination of the hydroxymethyl
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55
Fig. 7. The proposed pyrolytic pathways for fast pyrolysis of xylan, after Shen et al. [23].
group [17,41,44]. However, this should not be true. Shin et al. pointed out that the weakest bonds of the HMF were located on the hydroxymethyl group, 69 kcal/mol of the C–OH and 76 kcal/ mol of the C–H. As a result, the secondary cracking of the HMF would mainly produce the 5-methyl furfural through the cracking
of the C–OH bond (Fig. 6, path 9), and the 2,5-furandicarboxaldehyde through the cracking of the C–H bond (Fig. 6, path 8) [45]. Therefore, it is clear that most of the FF should be produced concurrently with the HMF, not from the secondary cracking of the HMF [18]. During the hemicellulose pyrolysis, the FF was usually
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the most important ring-containing product, and also regarded as the typical depolymerization product of the hemicellulose. Its production was similar as the formation of HMF from cellulose, as shown in Fig. 7 (path 2). Since the FF could be derived from several pathways, it is unable to know how much FF was generated by each pathway. Hence, it is difficult to know how each pathway would be affected by the pyrolysis conditions based on our results. The other furan compounds might also be generated through diverse pathways, with the most essential step of the acetal reaction to form the furan ring. In regard to the MF and F, both of their yields and relative contents were increased monotonically along with temperature in the tested range, which might be due to the temperature-promoted secondary cracking of the other furan compounds (Figs. 6, path 10, 11, and 7, path 11). 3.4. Effects of pyrolysis temperature and time on the linear carbonyls The pyrolytic fragmentation (ring scission) of the holocellulose generated various linear carbonyl products (linear aldehydes and ketones), including the HAA, HA, acetaldehyde, acetone, etc. The cracking of the lignin side-chain would also generate some linear carbonyls. As shown in Table 1, the peak area% values of the total linear carbonyls were increased monotonically along with the temperature, indicating the fragmentation reactions were favored at elevated temperatures. Specially, as the rising of pyrolysis temperature, some linear carbonyls (mainly heavy ones) were firstly increased and then decreased, while the others (mainly light ones) were increased monotonically. The formation characteristics of the four typical linear carbonyls are given in Fig. 4. The HAA was the most abundant linear carbonyl product, derived from both of cellulose and hemicellulose. Fig. 4 shows that both of its peak area and peak area% values were firstly increased and then decreased, with the maximal values obtained at 500 °C. The production of HAA from cellulose has been well-established. According to Piskorz et al., the bond between the C2 and C3 was longer than the other bonds of the glucose ring [46]. Hence, the glucose ring was prone to break at C1–O and C2–C3, forming the two-carbon (C1/C2) and four-carbon (C3–C6) fragments. The twocarbon fragment would form the HAA, as shown in Fig. 6 (path 6). This pathway was believed to be mainly responsible for the HAA production from cellulose. In addition, the HAA was also reported to be generated from the C5/C6 [47,48], as shown in Fig. 6 (path 7). During the hemicellulose pyrolysis, it is reasonable to expect that the HAA could be yielded in similar pathways, as shown in Fig. 7 (path 5). However, it is to note that the hemicellulose pyrolysis would not generate as much HAA as the cellulose pyrolysis, due to the substituted C2 position on most of the xylan units. Based on our results, the formation of the HAA was not favored at high temperatures. It might be due to the poor thermal stability of the HAA (Fig. 6, path 16–18), or might be because the pathways to produce the HAA were less competitive than the other fragmentation reactions at elevated pyrolysis conditions. The HA was the second most abundant linear carbonyl product. Fig. 4 indicates that its yield was increased greatly before 500 °C and then decreased slightly after 700 °C. Its relative content was gradually increased before 600 °C and then almost kept constant. During the cellulose pyrolysis, Piskorz et al. proposed that the HA was formed concurrently with the HAA, resulted from four-carbon (C3–C6) fragment [46]. Whereas Paine et al. confirmed that the HA could also be derived from the C3/C2/C1 during D-glucose pyrolysis [49]. Hence, the possible pyrolytic pathways for the HA from cellulose can be proposed as shown in Fig. 6 (path 13, 15 and 20). During the hemicellulose pyrolysis, it is reasonable to expect that the HA could be yielded in similar pathways, as shown in Fig. 7 (path 14, 17). Compared with the HAA, the HA yield was lower, suggesting that the pathways to generate the HA were less competitive
than the pathways to form the HAA. Whereas, the HA yield was only slightly decreased at high temperatures, which suggested that its formation pathways would not be greatly inhibited at elevated pyrolysis temperatures. In regard to the acetaldehyde shown in Fig. 4, its yield was increased slowly at first and then very quickly as the rising of pyrolysis temperature. Its relative content was gradually lowered from 350 °C to 550 °C, and then increased after 600 °C, with the high values obtained at both of low (350 °C) and high (900 °C, 1000 °C) temperatures. The acetaldehyde could be derived from several pathways, the deacetylation of hemicellulose (Fig. 7, path 6), fragmentation of holocellulose (Figs. 6, path 14, 19, 21, and 7, path 15, 18), and the cracking of the lignin side-chain. The deacetylation of hemicellulose could occur at relatively low temperatures, and was one of the earliest pyrolytic reactions to form organic volatiles during the fast pyrolysis process. It should be responsible for the abundant acetaldehyde (high relative content) obtained at as low as 350 °C. Whereas, the fragmentation reactions were favored at elevated pyrolysis conditions, which explained the gradually increased relative content of the acetaldehyde after 600 °C. Finally, it is necessary to note the acetone. Fig. 4 indicates that both of its yield and relative content were increased along with the temperature. The acetone was mainly derived from the fragmentation reactions of other linear carbonyls (Fig. 7, path 13, 19), which was favorable at elevated temperatures. In the case of the other linear carbonyls, they were produced in lower yields than the above four compounds, and not discussed in details here. 3.5. Effects of pyrolysis temperature and time on the linear acids The linear acid products detected in this study mainly included the acetic acid (AA) and some long-chain fatty acids such as tetradecanoic acid, n-hexadecanoic acid. These long-chain fatty acids should be derived from the extractive pyrolysis. Their highest peak area% values were obtained at 350 °C. It was because the extractives were thermally unstable and could decompose at 350 °C, while the major biomass components (especially the cellulose and lignin) were only slightly decomposed at such low temperatures. The formation characteristics of the AA are shown in Fig. 5. According to the peak area results, the AA yield was increased slightly before 450 °C, almost kept constant between 450 °C and 600 °C, and then decreased slowly after 700 °C. The peak area% values of the AA were over 16% at 400 °C, and then did not show significant changes after 500 °C. Similar as the acetaldehyde, the
Fig. 8. The fundamental pyrolytic reactions of the lignin monomeric unit.
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C.-q. Dong et al. / Energy Conversion and Management 57 (2012) 49–59 Table 2 The composition of the phenolic compounds under different pyrolysis conditions (peak area%). Temperature
Time (s)
Guaiacol-type
Syringol-type
Phenol-type
Cresol-type
Catechol-type
Other-type
350 °C
5 10 20 30
26.0 27.8 28.7 27.7
32.3 29.0 27.0 25.9
14.4 15.0 16.1 15.7
1.8 1.9 2.1 2.7
5.5 5.5 5.6 6.1
20.0 20.8 20.5 21.9
400 °C
5 10 20 30
26.6 26.2 25.8 24.8
27.6 28.2 28.7 29.8
18.5 18.7 15.4 15.7
2.0 1.9 3.4 3.7
3.0 3.0 2.8 3.0
22.3 22.0 23.9 23.0
450 °C
5 10 20 30
24.9 23.8 23.6 23.6
30.6 32.9 33.0 32.8
13.7 14.6 13.1 12.5
4.2 4.0 5.2 5.2
2.7 2.5 2.8 2.9
23.9 22.2 22.3 23.0
500 °C
5 10 20 30
23.7 23.0 22.4 22.7
33.2 32.4 32.5 32.4
11.4 12.5 11.5 11.1
5.5 6.9 7.9 8.3
3.8 4.9 5.7 5.5
22.4 20.3 20.0 20.0
550 °C
5 10 20 30
24.1 24.0 23.9 24.0
33.0 31.3 31.2 31.5
11.1 12.0 11.6 11.3
6.2 7.0 7.9 8.1
5.5 5.9 6.3 6.6
20.1 19.8 19.1 18.5
600 °C
5 10 20 30
23.9 23.3 23.5 24.5
32.6 31.4 31.5 30.4
11.4 12.1 12.2 12.4
6.8 7.3 7.4 7.9
6.1 7.6 7.7 7.9
19.2 18.3 17.7 16.9
700 °C
5 10 20
24.6 24.1 23.1
28.9 27.0 24.5
13.1 14.4 15.1
7.7 8.7 9.1
7.9 9.1 13.0
17.8 16.7 15.2
800 °C
5 10 20
20.9 20.5 18.6
21.1 19.6 18.0
18.5 19.9 20.3
8.8 9.6 10.4
15.4 16.8 20.4
15.3 13.6 12.3
900 °C
5 10 20
16.9 14.7 13.3
16.5 14.4 12.7
23.6 27.3 27.5
10.2 11.2 11.4
18.2 19.2 23.0
14.6 13.2 12.1
1000 °C
5 10 20
15.9 13.6 11.9
15.3 13.1 10.7
24.4 28.4 29.9
11.3 12.3 12.5
19.4 20.4 24.1
13.7 12.2 10.9
AA could be derived from the deacetylation of hemicellulose (Fig. 7, path 7), the fragmentation of holocellulose (Fig. 6, path 18) or the cracking of the lignin side-chain. The deacetylation of hemicellulose is believed to be the major way to yield the AA. It could occur at low temperatures and was one of the earliest pyrolytic reactions to form organic volatiles during the fast pyrolysis process. Hence, the relative content of the AA was high at 400 °C.
2), and alkylation reaction (Fig. 8, path 3), respectively [50]. Furthermore, among these phenolic compounds, the relative peak area% values of the compounds containing carbonyl or carboxyl group were all decreased greatly along with temperature. It suggested that the rising of the pyrolysis temperature could enhance the decarbonylation and decarboxylation reactions of the side-chain [51].
3.6. Effects of pyrolysis temperature and time on the phenolic compounds
3.7. Effects of pyrolysis temperature and time on the other products
The phenolic compounds were mainly derived from the lignin pyrolysis, with the fundamental reactions shown in Fig. 8. According to Table 1, the peak area% values of the phenolic compounds only varied in a small range. More than 70 phenolic compounds were detected, without any dominant ones. In order to show how the pyrolysis conditions affected the distribution of the phenolic compounds, these compounds were classified into six fractions, including guaiacol-type, syringol-type, phenol-type, cresol-type, catechol-type and other-type compounds. The relative peak area% values of the six fractions are listed in Table 2. It is seen that as the temperature rising, the relative peak area% values of the guaiacol-type and syringol-type fractions were decreased. Meanwhile, the relative peak area% values of the phenol-type, catechol-type and cresol-type fractions were increased. The results suggested that the rising of pyrolysis temperature could promote the demethoxylation reaction (Fig. 8, path 1), demethylation reaction (Fig. 8, path
In addition to the above compounds, various other products were formed in the fast pyrolysis process but all in low yields, such as hydrocarbons, linear alcohols, linear esters and cyclopentanones. Attentions should be paid to the hydrocarbons due to their contribution to the heating values of bio-oils. The detected hydrocarbons mainly included the benzene, toluene, ethylbenzene, xylenes and naphthalene. They were mainly derived from the lignin pyrolysis, and also a small part from holocellulose pyrolysis. It is known that the cleavage of the phenolic hydroxyl group to produce aromatic hydrocarbons was difficult, due to the high bond energy (463.6 kJ/ mol). Hence, the yields of the hydrocarbons were very low, and would be increased as the rising of the pyrolysis temperature. 4. Conclusions In this study, analytical fast pyrolysis of poplar wood was carried out to reveal the product formation characteristics under
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different pyrolysis temperatures and times. Fast pyrolysis of poplar wood involved many pyrolytic pathways to produce various anhydrosugars, furans, phenolic compounds, linear carbonyls, linear acids, hydrocarbons, etc. The maximal total volatile product yield was obtained at around 550 °C. The anhydrosugars were derived from the depolymerization of holocellulose. The LG was the dominant product, resulted from the sequential cleavage of the two glycosidic bonds on the polysaccharide chain. Its production was favored at medium temperatures (around 550 °C). Various furan compounds were generated in the fast pyrolysis process, typically the HMF, FF, MF and F. The heavy furans (HMF, FF, etc.) were favorable to be produced at medium temperatures, while the light ones (MF, F, etc.) were favorable at high temperatures. The linear carbonyls were mainly derived from the fragmentation of the holocellulose. The HAA was the most abundant one, its maximal yield and relative content were obtained at around 500 °C. The HA, acetaldehyde and acetone were also important linear carbonyl products. The acetaldehyde was produced with high relative content at both of the low and high temperatures, while the HA and acetone had the maximal relative contents at high temperatures. The linear acid products mainly included the AA and some longchain fatty acids. The long-chain fatty acids were resulted from extractive fast pyrolysis. The AA was mainly derived from the deacetylation of the hemicellulose and the fragmentation of holocellulose. Fast pyrolysis at low temperatures was able to produce these acids with high relative contents. The phenolic compounds were resulted from lignin fast pyrolysis. The rising of temperature would promote the demethoxylation, demethylation and alkylation reactions to increase the phenoltype, catechol-type and cresol-type compounds, and meanwhile, decrease the guaiacol-type and syringol-type compounds. Moreover, the elevated pyrolysis conditions would also enhance the decarbonylation and decarboxylation reactions.
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Contents lists available at SciVerse ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Characteristics and mechanism study of analytical fast pyrolysis of poplar wood Chang-qing Dong ⇑, Zhi-fei Zhang, Qiang Lu ⇑, Yong-ping Yang National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing 102206, China
a r t i c l e
i n f o
Article history: Received 20 August 2011 Received in revised form 16 November 2011 Accepted 20 December 2011 Available online 13 January 2012 Keywords: Fast pyrolysis Mechanism Py–GC/MS Poplar wood
a b s t r a c t Analytical pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) was applied to achieve fast pyrolysis of poplar wood and on-line analysis of the pyrolysis vapors. Experiments were conducted to reveal the distribution of pyrolytic products under different pyrolysis temperatures (300–1000 °C) and times (5–30 s). During the fast pyrolysis process, the poplar wood started decomposition to form organic volatile products at the set temperature of 300 °C, and reached the maximum volatile product yield at around 550 °C. The products included various anhydrosugars, furans, phenolic compounds, linear carbonyls, linear acids, hydrocarbons, and so on. They exhibited different formation characteristics. Based on the experimental results, we discussed the possible pyrolytic pathways for the generation of the major products. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Fast pyrolysis of biomass is a thermal decomposition process that occurs in the absence of oxygen, to convert solid biomass mainly into a liquid product known as bio-oil [1–3]. Liquid biooil covers many potential application fields and is regarded as the potential substitute of petroleum fuels [4,5]. However, bio-oil is totally different from petroleum fuels, as it is a complex mixture of water and hundreds of organic compounds that belong to acids, aldehydes, ketones, alcohols, esters, anhydrosugars, furans, phenols, as well as large molecular oligomers. The complex chemical composition makes the bio-oil difficult for commercial application. To solve this problem, various upgrading methods have been proposed on either crude bio-oil or fast pyrolysis process [6,7]. Hence, it is very essential to understand the biomass fast pyrolysis mechanism and product formation characteristics. Till now, most of the pyrolysis studies were performed under the slow or conventional pyrolysis conditions, the product distribution differed considerably from that of the fast pyrolysis conditions. In regard to the fast pyrolysis studies, attentions were mainly paid to the yields and overall properties of bio-oil, char and gas products. Very limited studies are available to report the detailed characteristics of biomass fast pyrolysis, based on the changes of individual products in the biooil [8–11]. Lignocellulosic biomass mainly consists of cellulose, hemicellulose and lignin, together with a small portion of extractives and ash. Cellulose is a linear homopolysaccharide of b-D-glucopyranose units linked together by (1 ? 4)-glycosidic bonds. It starts pyrolysis ⇑ Corresponding authors. Tel./fax: +86 10 61772031. E-mail addresses: [email protected] (C.-q. Dong), [email protected] (Q. Lu). 0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2011.12.012
at as low as 150 °C [12]. At temperatures above 300 °C, pyrolysis of cellulose will undergo depolymerization and fragmentation (ring scission) reactions to mainly form a liquid product, including anhydro-oligosaccharides, monomeric anhydrosugars (dominated by the levoglucosan (LG)), furans, linear carbonyls and other products [13–15]. Till now, various reaction schemes have been proposed for the formation of liquid products from pyrolysis of cellulose under different conditions [16–18]. Hemicelluloses are amorphous polysaccharides with building units belong either to hexoses or to pentoses. The primary hemicellulose components are galactoglucomannans (glucomannans) and arabinoglucuronoxylan (xylan). Hemicelluloses are less thermally stable than cellulose, presumably due to their lack of crystallinity. Their pyrolysis is generally thought to be analogous to cellulose in the reaction mechanisms. However, due to the structure difference, fast pyrolysis of hemicellulose would obtain more char and less oil than that of cellulose, and the composition of the oils from them differs greatly from each other [8,19–22]. Till now, very limited studies are available to report the pyrolytic reaction schemes of hemicellulose [23,24], especially under the fast pyrolysis conditions. Lignin is a complex, heterogeneous polymer formed by the polymerization of three phenyl propane monomers, i.e. guaiacyl (4-hydroxy-3-methoxyphenyl), syringyl (3,5-dimethoxy-4-hydroxyphenyl) and p-hydroxyphenyl units. Primary pyrolysis of lignin begins with thermal softening at temperatures around 200 °C, while most lignin pyrolysis occurs at higher temperatures. Fast pyrolysis of lignin will obtain higher char yield than the holocellulose, and the liquid product mainly includes the large molecular oligomers (known as pyrolytic lignins), the monomeric phenolic compounds, as well as light linear compounds [25–28]. Due to the un-defined lignin structure, its pyrolytic reaction schemes could only be proposed based on the monomeric lignin units or model compounds [29,30].
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In this study, fast pyrolysis of biomass was achieved using the analytical Py–GC/MS technique which allowed the on-line analysis of the pyrolysis vapors. Poplar wood was selected as the biomass feedstock, because it is very abundant in China. The experiments were carried out from 300 °C to 1000 °C under the pyrolysis times of 5–30 s and heating rate of 20 °C/ms, to reveal the effects of pyrolysis temperature and time on the product distribution, especially the important single products. Based on the experimental results, we modified previous reaction schemes and then proposed the pyrolytic pathways for the major products. We also discussed the product formation characteristics relating to their generation pathways.
2. Experimental The biomass used in this study was poplar wood. Prior to experiments, it was firstly ground in a high speed rotary cutting mill and then sieved. The particles with the size of 0.2–0.3 mm were selected, dried in an oven at 110 °C and stored in a vacuum desiccator for experiments. The component composition of the poplar wood was cellulose 49.70%, hemicellulose 24.10%, lignin 23.55%, extractive 2.22% and ash 0.43%. Its elemental composition on the dry basis was C 49.6%, H 6.31%, N 0.08% and S 0.09%. It is to note that the poplar wood is a kind of hard wood, its major hemicellulose component was the xylan. The analytical Py–GC/MS experiments were performed using the CDS Pyroprobe 5200HP pyrolyser (Chemical Data Systems) connected with the Perkin Elmer GC/MS (Clarus 560). Fast pyrolysis was achieved by the analytical pyrolyser, the pyrolysis vapors were directly transferred to the GC/MS and analyzed by it. During the preparation of the experimental samples, the pyrolysis tube was successively filled with a quartz rod, some quartz wool, 0.50 mg poplar wood and some quartz wool. Quartz wool was placed at both sides of the poplar wood, to prevent the escape of the solid particles. The details of the sample preparation could be found in our previous study [31]. An analytical balance with the readability of 0.01 mg was used for weighing, to ensure the poplar wood quantity of strictly 0.50 mg. The pyrolysis was carried out at the set temperature range of 300–1000 °C and the time range of 5–30 s. The heating rate was 20 °C/ms. The pyrolysis vapors were analyzed by GC/MS. The injector temperature was kept at 300 °C. The chromatographic separation was performed using an Elite-35MS capillary column (30 m 0.25 mm i.d., 0.25 lm film thickness). Helium (99.999%) was used as the carrier gas with a constant flow rate of 1 mL/min and a 1:80 split ratio. The oven temperature was programmed from 40 °C (3 min) to 180 °C with the heating rate of 4 °C/min, and then to 280 °C (4 min) with the heating rate of 10 °C/min. The temperature of the GC/MS interface was held at 280 °C, and the mass spectrometer was operated in EI mode at 70 eV. The mass spectra were obtained from m/z 20 to 400. The chromatographic peaks were identified according to the NIST library, Wiley library and the literature data of previous studies. For each sample, the experiments were conducted at least three times to confirm the reproducibility of the reported procedures. For each identified product, the average values of the peak area and peak area% were calculated and used for discussion. The GC/ MS technique could not give the direct quantitative analysis of the compounds, due to the complex pyrolytic products and the lack of commercially available standards for them. However, the chromatographic peak area of a compound is considered linear with its quantity, and the peak area% is linear with its content. Therefore, for each product, its average peak area value obtained under different reaction conditions can be compared to reveal the changing of its yields, and the peak area% value can be compared to show the changing of its relative content among the detected products.
3. Results 3.1. Product distribution from fast pyrolysis of poplar wood Biomass fast pyrolysis vapors consisted of permanent gases (CO, CO2, CH4, H2, etc.), volatile compounds and non-volatile oligomers. The condensation of the latter two classes of compounds would obtain the liquid bio-oil, while GC/MS was only able to determine the organic volatile compounds. During the analytical experiments, no pyrolytic products were detected by GC/MS when the set pyrolysis temperature was lower than 300 °C. It indicated that the poplar wood was not decomposed to form organic volatiles at such low temperatures. When the set temperature was higher than 300 °C, various pyrolytic products were detectable, with more than 100 peaks displayed on typical ion chromatograms. Most of the peaks were identified by the NIST library, and some were referred to the Wiley library and previous studies, but some peaks were unable to be determined. These identified compounds were similar to literature data of the chemical composition of bio-oils [32–36]. One of the drawbacks of the analytical Py–GC/MS experiments is that it did not allow product collection, and thus, the exact bio-oil yield could not be determined. However, it is able to have a primary estimate of the yield changes of the total detected compounds, through the comparison of the total chromatographic peak area values obtained under different pyrolysis conditions. The results from pyrolysis at 350–1000 °C are shown in Fig. 1. The products detected at 300 °C were very low, and thus, the results at this temperature are not shown in Fig. 1 and will not be discussed in this paper. According to Fig. 1, as the rising of the pyrolysis temperature, the total peak area values were firstly increased and then decreased, with the maximal value obtained at around 550 °C. The significant increase before 550 °C should be due to the promoted heat transfer and pyrolysis reactions to form the organic volatile products, while the decrease after 600 °C might be due to the enhanced reactions towards permanent gases. Furthermore, as the rising of pyrolysis time, the total peak area values were gradually increased from 5 s to 30 s at temperatures lower than 550 °C, and then only increased in the first 20 s, 10 s or 5 s at elevated temperatures. The results clearly indicated that the complete devolatilization of the poplar wood could be shortened from 30 s to only 5 s as the rising of pyrolysis temperature. In addition, all the detected compounds were classified into eight groups, including anhydrosugars, furans, linear aldehydes, linear ketones, linear acids, phenolic compounds, hydrocarbons, and others (linear alcohols, linear esters, cyclopentanones, etc.). Their peak area% results are given in Table 1. It is to note that the sum of the peak area% values was not equal to 100%, due to the unidentified chromatographic peaks. Based on the peak area% results, it is seen that the relative content of each product group was influenced considerably by the pyrolysis temperature, but not very remarkably by the pyrolysis time. Generally, the acid group and the anhydrosugar group had the maximal peak area% values at low and medium temperatures, respectively. The linear ketone group and the hydrocarbon group had the maximal peak area% values at high temperatures. The peak area% values of the other three groups (furans, linear aldehydes and phenolic compounds) were not greatly affected by the pyrolysis conditions. The detailed results of the important and typical products in each product group are shown in Figs. 2–5, and will be discussed in the following sections.
3.2. Effects of pyrolysis temperature and time on the anhydrosugars During the fast pyrolysis process, the depolymerization of the holocellulose produced various anhydrosugars and related deriva-
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Fig. 1. The total chromatographic peak areas from fast pyrolysis of poplar wood under different conditions.
Table 1 The composition of the pyrolytic products under different pyrolysis conditions (peak area%). Temperature
Time (s)
Furans
Aldehydes
Acids
Phenols
Hydro carbons
Others
350 °C
5 10 20 30
Anhydro sugars 2.9 3.5 3.6 3.2
6.9 6.4 7.4 8.6
14.4 12.9 12.6 11.7
2.0 2.5 2.9 2.8
18.0 17.3 17.6 20.6
29.3 31.6 31.3 28.1
1.3 1.2 1.2 1.3
3.0 3.2 3.2 3.2
400 °C
5 10 20 30
4.1 4.5 4.2 4.3
6.1 6.2 5.9 5.9
10.4 10.4 10.5 10.6
3.0 3.4 3.1 3.1
21.0 20.9 22.6 22.5
31.7 29.9 28.8 28.4
0.8 0.8 0.5 0.5
7.1 7.6 7.5 7.7
450 °C
5 10 20 30
5.1 5.6 6.0 6.1
6.3 5.7 5.8 6.2
11.9 11.3 13.8 14.5
3.3 4.5 5.1 5.2
19.8 19.9 18.2 17.7
29.1 29.4 27.5 25.2
0.9 0.7 0.6 0.6
7.4 6.5 7.1 7.4
500 °C
5 10 20 30
6.0 7.2 10.0 9.4
6.4 7.4 6.7 7.6
15.4 16.0 16.4 16.4
6.1 6.9 7.5 7.2
15.3 13.0 12.5 12.4
26.3 24.1 22.7 22.2
0.7 0.6 0.6 0.5
7.9 8.9 8.6 8.8
550 °C
5 10 20 30
9.3 9.8 10.2 10.0
7.2 7.4 7.1 7.3
13.2 13.3 12.9 13.0
7.4 7.6 7.9 7.5
12.4 12.6 12.5 12.4
25.0 25.5 25.3 24.4
0.6 0.6 0.6 0.6
9.6 9.4 9.3 9.2
600 °C
5 10 20 30
8.3 9.0 9.8 9.1
6.1 6.3 6.3 6.9
12.6 12.2 12.5 12.8
8.5 8.4 8.8 8.5
12.5 12.7 12.9 13.1
26.8 26.0 26.3 25.7
0.7 0.7 0.8 0.9
9.1 9.5 8.9 9.1
700 °C
5 10 20
9.4 8.7 8.5
5.9 6.0 6.2
13.0 13.0 13.8
9.8 9.7 10.8
12.6 13.9 13.4
26.3 25.6 26.9
1.2 1.2 1.2
8.8 8.6 9.1
800 °C
5 10 20
8.9 8.8 8.0
5.6 5.8 4.9
15.4 15.2 14.4
10.4 10.1 11.8
13.1 13.9 13.4
24.9 24.2 26.7
1.8 1.9 2.2
8.2 8.4 8.6
900 °C
5 10 20
7.6 7.2 6.7
5.2 5.3 4.8
16.1 15.7 14.9
11.1 10.7 12.3
12.9 14.0 14.0
24.3 24.6 25.5
3.3 3.6 3.7
8.4 8.5 8.7
1000 °C
5 10 20
7.4 7.0 6.3
5.0 5.3 4.6
16.3 15.5 15.0
10.9 10.8 12.1
12.9 13.3 13.9
25.0 24.6 25.8
4.3 5.0 4.8
8.2 8.7 7.5
tives, dominated by the LG. Other anhydrosugar products mainly included the levoglucosenone (LGO), 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one (LAC), 1,4:3,6-dianhydro-a-d-glucopyranose (DGP), 1,4-anhydro-a-d-xylopyranose (ADX), etc., which were all formed in very low yields. The LG was mainly derived from cellulose, its formation was influenced remarkably by both of pyrolysis temperature and time, with the results given in Fig. 2. According to the peak area results, the LG yield was firstly increased and then decreased as the rising of the pyrolysis temperature, with the maximal yield obtained at around 550 °C. The complete formation of the LG required 30 s at low temperatures, and gradually decreased to only 5 s at high temperatures. Furthermore, based on the peak area% results, the maximal relative content of the LG was also obtained at 550 °C for 20 s pyrolysis.
Ketones
The LG was the most important pyrolytic product of cellulose, and has appealed to detailed mechanism study. Ponder et al. proposed the possible pyrolytic pathway to produce the LG [37], as shown in Fig. 6 (path 1), which was generally accepted by other researches [15]. The scission of the glycosidic bond yielded a glucosyl cation with a free primary hydroxyl group at C-6, which could readily form a stable 1,6-anhydride. The subsequent cleavage of another glycosidic bond liberated the monomeric LG, and meanwhile generated another glucosyl cation which could undergo similar reactions. Temperature would play an important role on the LG production. According to our results, the rising of pyrolysis temperature before 550 °C would accelerate the cleavage of the glycosidic bond to increase the LG yield and relative content. However, the further rising of temperature after 600 °C would enhance other
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Fig. 2. The effects of pyrolysis temperature and time on the peak area and peak area% of the LG.
Fig. 3. The effects of pyrolysis temperature and time on the peak area and peak area% of the HMF, FF, MF and F.
competing pyrolysis reactions (mainly fragmentation reactions) to inhibit the LG formation, and would also promote the secondary cracking of the LG, resulted in the decreased LG yield and relative content. Among the other anhydrosugars, it is necessary to note the ADX which was the typical anhydrosugar derived from the xylan (the major hemicellulose component of the poplar wood) [23,38–40]. It was produced via the cleavage of the two glucosidic bonds on the polysaccharide chain and the formation of the 1,4-anhydride, as shown in Fig. 7 (path 1). The ADX exhibited similar formation
characteristics as the LG. Both of its peak area and peak area% values were firstly increased and then decreased along with pyrolysis temperature. However, the yield of the ADX was much lower than the LG, with the peak area% value not exceeding 1% in either pyrolysis condition. It should be attributed to the structure difference between the cellulose and xylan. According to Ponder and Nichards, during cellulose pyrolysis, the glucosyl cation from the scission of the common glucans could readily form a stable 1,6-anhydride with the free primary hydroxyl group at C-6, and finally yield the volatile LG. However, it was difficult for the intramolecular ‘‘stabilization’’
C.-q. Dong et al. / Energy Conversion and Management 57 (2012) 49–59
53
Fig. 4. The effects of pyrolysis temperature and time on the peak area and peak area% of the HAA, HA, acetaldehyde and acetone.
Fig. 5. The effects of pyrolysis temperature and time on the peak area and peak area% of the AA.
of the xylosyl cation via anhydride formation (shown in Fig. 7), and thus, the xylosyl cation was more likely to enter the non-specific dehydration pathways to char formation rather volatiles. Moreover, it was reported that 1,4-anhydride on the xylopyranose was less stable than the 1,6-anhydride on the glucopyranose [20]. Our results showed that the formation of the ADX was not favored at high pyrolysis temperatures, which might be due to the fact that the depolymerization reaction to form the ADX was less competitive than the fragmentation reactions at elevated temperatures.
3.3. Effects of pyrolysis temperature and time on the furan compounds A lot of furan compounds were produced in the pyrolytic process, typically the 5-hydroxymethyl-furfural (HMF), furfural (FF), 2-methyl furan (MF) and furan (F). As shown in Table 1, the peak area% values of the furan group were not greatly changed before 550 °C and then slowly decreased after 550 °C. In regard to the specific compounds, as the rising of pyrolysis temperature, most furans were firstly increased and then decreased, while some light ones
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Fig. 6. The proposed pyrolytic pathways for fast pyrolysis of cellulose, after Shen and Gu [17].
(MF and F) were increased monotonically. The formation characteristics of the four typical furan compounds are given in Fig. 3. The HMF was a six-carbon furan compound, derived mainly from cellulose. According to Fig. 3, the maximal HMF yield (based on the peak area results) was obtained at around 550 °C, and its relative content (based on the peak area% results) was decreased after 700 °C. The formation of the HMF included the cleavage of the ring glucosidic bond (C1–O), the intramolecular dehydration, and the acetal reaction between the C2 and C5 [10,17,41–43], as shown in Fig. 6 (path 2). Based on our results, the rising of the pyrolysis temperature before 600 °C could promote this pyrolytic pathway. However, the HMF yield was decreased after 700 °C, which might be due to the temperature-enhanced competing path-
ways (fragmentation reactions) to inhibit the HMF formation, or due to the secondary cracking of the HMF (Fig. 6, path 8, 9). The FF was derived from both of cellulose and hemicellulose. As shown in Fig. 4, its yield was influenced remarkably by the pyrolysis temperature, with the maximal yield obtained at around 600 °C. While its relative content was not greatly affected by the pyrolysis conditions. According to Paine et al., during the D-glucose pyrolysis, the FF could be produced from either the C1–C5 or the C2–C6, with the furan ring formed between the C1 and C4, C2 and C5, or C3 and C6 [43]. Therefore, it is able to propose the possible pyrolytic pathways for the FF derived from cellulose, as shown in Fig. 6 (path 3, 4). In some literature, it is believed that the FF was derived from the cracking of the HMF, via the elimination of the hydroxymethyl
C.-q. Dong et al. / Energy Conversion and Management 57 (2012) 49–59
55
Fig. 7. The proposed pyrolytic pathways for fast pyrolysis of xylan, after Shen et al. [23].
group [17,41,44]. However, this should not be true. Shin et al. pointed out that the weakest bonds of the HMF were located on the hydroxymethyl group, 69 kcal/mol of the C–OH and 76 kcal/ mol of the C–H. As a result, the secondary cracking of the HMF would mainly produce the 5-methyl furfural through the cracking
of the C–OH bond (Fig. 6, path 9), and the 2,5-furandicarboxaldehyde through the cracking of the C–H bond (Fig. 6, path 8) [45]. Therefore, it is clear that most of the FF should be produced concurrently with the HMF, not from the secondary cracking of the HMF [18]. During the hemicellulose pyrolysis, the FF was usually
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the most important ring-containing product, and also regarded as the typical depolymerization product of the hemicellulose. Its production was similar as the formation of HMF from cellulose, as shown in Fig. 7 (path 2). Since the FF could be derived from several pathways, it is unable to know how much FF was generated by each pathway. Hence, it is difficult to know how each pathway would be affected by the pyrolysis conditions based on our results. The other furan compounds might also be generated through diverse pathways, with the most essential step of the acetal reaction to form the furan ring. In regard to the MF and F, both of their yields and relative contents were increased monotonically along with temperature in the tested range, which might be due to the temperature-promoted secondary cracking of the other furan compounds (Figs. 6, path 10, 11, and 7, path 11). 3.4. Effects of pyrolysis temperature and time on the linear carbonyls The pyrolytic fragmentation (ring scission) of the holocellulose generated various linear carbonyl products (linear aldehydes and ketones), including the HAA, HA, acetaldehyde, acetone, etc. The cracking of the lignin side-chain would also generate some linear carbonyls. As shown in Table 1, the peak area% values of the total linear carbonyls were increased monotonically along with the temperature, indicating the fragmentation reactions were favored at elevated temperatures. Specially, as the rising of pyrolysis temperature, some linear carbonyls (mainly heavy ones) were firstly increased and then decreased, while the others (mainly light ones) were increased monotonically. The formation characteristics of the four typical linear carbonyls are given in Fig. 4. The HAA was the most abundant linear carbonyl product, derived from both of cellulose and hemicellulose. Fig. 4 shows that both of its peak area and peak area% values were firstly increased and then decreased, with the maximal values obtained at 500 °C. The production of HAA from cellulose has been well-established. According to Piskorz et al., the bond between the C2 and C3 was longer than the other bonds of the glucose ring [46]. Hence, the glucose ring was prone to break at C1–O and C2–C3, forming the two-carbon (C1/C2) and four-carbon (C3–C6) fragments. The twocarbon fragment would form the HAA, as shown in Fig. 6 (path 6). This pathway was believed to be mainly responsible for the HAA production from cellulose. In addition, the HAA was also reported to be generated from the C5/C6 [47,48], as shown in Fig. 6 (path 7). During the hemicellulose pyrolysis, it is reasonable to expect that the HAA could be yielded in similar pathways, as shown in Fig. 7 (path 5). However, it is to note that the hemicellulose pyrolysis would not generate as much HAA as the cellulose pyrolysis, due to the substituted C2 position on most of the xylan units. Based on our results, the formation of the HAA was not favored at high temperatures. It might be due to the poor thermal stability of the HAA (Fig. 6, path 16–18), or might be because the pathways to produce the HAA were less competitive than the other fragmentation reactions at elevated pyrolysis conditions. The HA was the second most abundant linear carbonyl product. Fig. 4 indicates that its yield was increased greatly before 500 °C and then decreased slightly after 700 °C. Its relative content was gradually increased before 600 °C and then almost kept constant. During the cellulose pyrolysis, Piskorz et al. proposed that the HA was formed concurrently with the HAA, resulted from four-carbon (C3–C6) fragment [46]. Whereas Paine et al. confirmed that the HA could also be derived from the C3/C2/C1 during D-glucose pyrolysis [49]. Hence, the possible pyrolytic pathways for the HA from cellulose can be proposed as shown in Fig. 6 (path 13, 15 and 20). During the hemicellulose pyrolysis, it is reasonable to expect that the HA could be yielded in similar pathways, as shown in Fig. 7 (path 14, 17). Compared with the HAA, the HA yield was lower, suggesting that the pathways to generate the HA were less competitive
than the pathways to form the HAA. Whereas, the HA yield was only slightly decreased at high temperatures, which suggested that its formation pathways would not be greatly inhibited at elevated pyrolysis temperatures. In regard to the acetaldehyde shown in Fig. 4, its yield was increased slowly at first and then very quickly as the rising of pyrolysis temperature. Its relative content was gradually lowered from 350 °C to 550 °C, and then increased after 600 °C, with the high values obtained at both of low (350 °C) and high (900 °C, 1000 °C) temperatures. The acetaldehyde could be derived from several pathways, the deacetylation of hemicellulose (Fig. 7, path 6), fragmentation of holocellulose (Figs. 6, path 14, 19, 21, and 7, path 15, 18), and the cracking of the lignin side-chain. The deacetylation of hemicellulose could occur at relatively low temperatures, and was one of the earliest pyrolytic reactions to form organic volatiles during the fast pyrolysis process. It should be responsible for the abundant acetaldehyde (high relative content) obtained at as low as 350 °C. Whereas, the fragmentation reactions were favored at elevated pyrolysis conditions, which explained the gradually increased relative content of the acetaldehyde after 600 °C. Finally, it is necessary to note the acetone. Fig. 4 indicates that both of its yield and relative content were increased along with the temperature. The acetone was mainly derived from the fragmentation reactions of other linear carbonyls (Fig. 7, path 13, 19), which was favorable at elevated temperatures. In the case of the other linear carbonyls, they were produced in lower yields than the above four compounds, and not discussed in details here. 3.5. Effects of pyrolysis temperature and time on the linear acids The linear acid products detected in this study mainly included the acetic acid (AA) and some long-chain fatty acids such as tetradecanoic acid, n-hexadecanoic acid. These long-chain fatty acids should be derived from the extractive pyrolysis. Their highest peak area% values were obtained at 350 °C. It was because the extractives were thermally unstable and could decompose at 350 °C, while the major biomass components (especially the cellulose and lignin) were only slightly decomposed at such low temperatures. The formation characteristics of the AA are shown in Fig. 5. According to the peak area results, the AA yield was increased slightly before 450 °C, almost kept constant between 450 °C and 600 °C, and then decreased slowly after 700 °C. The peak area% values of the AA were over 16% at 400 °C, and then did not show significant changes after 500 °C. Similar as the acetaldehyde, the
Fig. 8. The fundamental pyrolytic reactions of the lignin monomeric unit.
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C.-q. Dong et al. / Energy Conversion and Management 57 (2012) 49–59 Table 2 The composition of the phenolic compounds under different pyrolysis conditions (peak area%). Temperature
Time (s)
Guaiacol-type
Syringol-type
Phenol-type
Cresol-type
Catechol-type
Other-type
350 °C
5 10 20 30
26.0 27.8 28.7 27.7
32.3 29.0 27.0 25.9
14.4 15.0 16.1 15.7
1.8 1.9 2.1 2.7
5.5 5.5 5.6 6.1
20.0 20.8 20.5 21.9
400 °C
5 10 20 30
26.6 26.2 25.8 24.8
27.6 28.2 28.7 29.8
18.5 18.7 15.4 15.7
2.0 1.9 3.4 3.7
3.0 3.0 2.8 3.0
22.3 22.0 23.9 23.0
450 °C
5 10 20 30
24.9 23.8 23.6 23.6
30.6 32.9 33.0 32.8
13.7 14.6 13.1 12.5
4.2 4.0 5.2 5.2
2.7 2.5 2.8 2.9
23.9 22.2 22.3 23.0
500 °C
5 10 20 30
23.7 23.0 22.4 22.7
33.2 32.4 32.5 32.4
11.4 12.5 11.5 11.1
5.5 6.9 7.9 8.3
3.8 4.9 5.7 5.5
22.4 20.3 20.0 20.0
550 °C
5 10 20 30
24.1 24.0 23.9 24.0
33.0 31.3 31.2 31.5
11.1 12.0 11.6 11.3
6.2 7.0 7.9 8.1
5.5 5.9 6.3 6.6
20.1 19.8 19.1 18.5
600 °C
5 10 20 30
23.9 23.3 23.5 24.5
32.6 31.4 31.5 30.4
11.4 12.1 12.2 12.4
6.8 7.3 7.4 7.9
6.1 7.6 7.7 7.9
19.2 18.3 17.7 16.9
700 °C
5 10 20
24.6 24.1 23.1
28.9 27.0 24.5
13.1 14.4 15.1
7.7 8.7 9.1
7.9 9.1 13.0
17.8 16.7 15.2
800 °C
5 10 20
20.9 20.5 18.6
21.1 19.6 18.0
18.5 19.9 20.3
8.8 9.6 10.4
15.4 16.8 20.4
15.3 13.6 12.3
900 °C
5 10 20
16.9 14.7 13.3
16.5 14.4 12.7
23.6 27.3 27.5
10.2 11.2 11.4
18.2 19.2 23.0
14.6 13.2 12.1
1000 °C
5 10 20
15.9 13.6 11.9
15.3 13.1 10.7
24.4 28.4 29.9
11.3 12.3 12.5
19.4 20.4 24.1
13.7 12.2 10.9
AA could be derived from the deacetylation of hemicellulose (Fig. 7, path 7), the fragmentation of holocellulose (Fig. 6, path 18) or the cracking of the lignin side-chain. The deacetylation of hemicellulose is believed to be the major way to yield the AA. It could occur at low temperatures and was one of the earliest pyrolytic reactions to form organic volatiles during the fast pyrolysis process. Hence, the relative content of the AA was high at 400 °C.
2), and alkylation reaction (Fig. 8, path 3), respectively [50]. Furthermore, among these phenolic compounds, the relative peak area% values of the compounds containing carbonyl or carboxyl group were all decreased greatly along with temperature. It suggested that the rising of the pyrolysis temperature could enhance the decarbonylation and decarboxylation reactions of the side-chain [51].
3.6. Effects of pyrolysis temperature and time on the phenolic compounds
3.7. Effects of pyrolysis temperature and time on the other products
The phenolic compounds were mainly derived from the lignin pyrolysis, with the fundamental reactions shown in Fig. 8. According to Table 1, the peak area% values of the phenolic compounds only varied in a small range. More than 70 phenolic compounds were detected, without any dominant ones. In order to show how the pyrolysis conditions affected the distribution of the phenolic compounds, these compounds were classified into six fractions, including guaiacol-type, syringol-type, phenol-type, cresol-type, catechol-type and other-type compounds. The relative peak area% values of the six fractions are listed in Table 2. It is seen that as the temperature rising, the relative peak area% values of the guaiacol-type and syringol-type fractions were decreased. Meanwhile, the relative peak area% values of the phenol-type, catechol-type and cresol-type fractions were increased. The results suggested that the rising of pyrolysis temperature could promote the demethoxylation reaction (Fig. 8, path 1), demethylation reaction (Fig. 8, path
In addition to the above compounds, various other products were formed in the fast pyrolysis process but all in low yields, such as hydrocarbons, linear alcohols, linear esters and cyclopentanones. Attentions should be paid to the hydrocarbons due to their contribution to the heating values of bio-oils. The detected hydrocarbons mainly included the benzene, toluene, ethylbenzene, xylenes and naphthalene. They were mainly derived from the lignin pyrolysis, and also a small part from holocellulose pyrolysis. It is known that the cleavage of the phenolic hydroxyl group to produce aromatic hydrocarbons was difficult, due to the high bond energy (463.6 kJ/ mol). Hence, the yields of the hydrocarbons were very low, and would be increased as the rising of the pyrolysis temperature. 4. Conclusions In this study, analytical fast pyrolysis of poplar wood was carried out to reveal the product formation characteristics under
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different pyrolysis temperatures and times. Fast pyrolysis of poplar wood involved many pyrolytic pathways to produce various anhydrosugars, furans, phenolic compounds, linear carbonyls, linear acids, hydrocarbons, etc. The maximal total volatile product yield was obtained at around 550 °C. The anhydrosugars were derived from the depolymerization of holocellulose. The LG was the dominant product, resulted from the sequential cleavage of the two glycosidic bonds on the polysaccharide chain. Its production was favored at medium temperatures (around 550 °C). Various furan compounds were generated in the fast pyrolysis process, typically the HMF, FF, MF and F. The heavy furans (HMF, FF, etc.) were favorable to be produced at medium temperatures, while the light ones (MF, F, etc.) were favorable at high temperatures. The linear carbonyls were mainly derived from the fragmentation of the holocellulose. The HAA was the most abundant one, its maximal yield and relative content were obtained at around 500 °C. The HA, acetaldehyde and acetone were also important linear carbonyl products. The acetaldehyde was produced with high relative content at both of the low and high temperatures, while the HA and acetone had the maximal relative contents at high temperatures. The linear acid products mainly included the AA and some longchain fatty acids. The long-chain fatty acids were resulted from extractive fast pyrolysis. The AA was mainly derived from the deacetylation of the hemicellulose and the fragmentation of holocellulose. Fast pyrolysis at low temperatures was able to produce these acids with high relative contents. The phenolic compounds were resulted from lignin fast pyrolysis. The rising of temperature would promote the demethoxylation, demethylation and alkylation reactions to increase the phenoltype, catechol-type and cresol-type compounds, and meanwhile, decrease the guaiacol-type and syringol-type compounds. Moreover, the elevated pyrolysis conditions would also enhance the decarbonylation and decarboxylation reactions.
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