Impact of Torrefaction on the Chemical Structure and Catalytic Fast

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Impact of Torrefaction on the Chemical Structure and Catalytic Fast Pyrolysis Behavior of Hemicellulose, Lignin, and Cellulose Anqing Zheng, Liqun Jiang, Zengli Zhao,* Zhen Huang, Kun Zhao, Guoqiang Wei, Xiaobo Wang, Fang He, and Haibin Li Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, People’s Republic of China ABSTRACT: To understand the effect of torrefaction severity on structure changes of hemicellulose, cellulose, lignin and their subsequent catalytic fast pyrolysis (CFP) behavior, torrefaction of lignin, hemicellulose, and cellulose was performed in a tubular reactor with different reaction temperatures (210−300 °C) and residence times (20−60 min). The experimental results show that the rank order of thermal stability during torrefaction was cellulose > lignin > hemicellulose. The torrefied hemicelulose, cellulose, and lignin were subsequently catalytic-fast-pyrolyzed over HZSM-5 in a semi-batch pyroprobe reactor. The effects of the torrefaction temperature and residence time on aromatic yields and selectivity from CFP of torrefied hemicellulose, cellulose, and lignin were investigated. The experimental results showed that torrefaction can cause the reduction in the aromatic yield and increase in benzene, toluene, and xylenes (BTX) selectivity from CFP of torrefied hemicellulose and lignin. It has little impact on CFP of torrefied cellulose. The results can be explained by Fourier transform infrared (FTIR) spectroscopy and 13C crosspolarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) analysis of torrefied hemicellulose, cellulose, and lignin. The rank order of structure change during torrefaction was hemicellulose > lignin > cellulose. The devolatilization and polycondensation of hemicellulose and lignin during torrefaction could be mainly responsible for the yield penalties of aromatic production from CFP of torrefied hemicellulose and lignin.

1. INTRODUCTION Dwindling fossil fuel reserves and increasing emission of CO2 have stimulated the intense search for alternative renewable fuels that are capable of satisfying the fast growing energy demand.1,2 Biomass is the only renewable source of organic carbon that can be converted into liquid fuels and chemicals. In addition, biomass conversion have been considered as being environmentally friendly because it has negligible sulfur and nitrogen contents and is CO2-neutral.3 Among various biomass conversion processes, catalytic fast pyrolysis (CFP) has been considered as one of the most promising technologies for renewable aromatic production from lignocellulosic biomass.4−6 In this process, lignocellulosic biomass can be directly converted into aromatic hydrocarbons over a MFI-type zeolite catalyst.7−9 However, the inherent properties of raw biomass, such as structural heterogeneity, high moisture content, low bulk density, low energy density, hydrophilic, and tenacious fibrous nature pose enormous challenges for their collection, transportation, storage, grinding, feeding, and conversion processes.10,11 These challenging properties of raw biomass can be improved by means of pretreatment. Previous studies have demonstrated that torrefaction is an effective pretreatment method to simultaneously overcome the aforementioned drawbacks of raw biomass.12−14 Torrefaction is a low-temperature pyrolysis process carried out at 200−300 °C under an inert atmosphere for upgrading raw biomass by altering the composition and structure of biomass.15−18 Biomass consists of three major components (hemicellulose, cellulose, and lignin). In general, cellulose, lignin, and hemicellulose account for 30−50, 10−35, and 15−40 wt % of dry lignocellulosic biomass, respectively.19 Cellulose is a semi-crystalline © 2015 American Chemical Society

linear polymer composed of D-glucose subunits. Lignin is a three-dimensional, highly branched, polyphenolic substance that is composed of an irregular array of variously bonded “hydroxy-” and “methoxy-” substituted phenylpropane units.20,21 Hemicellulose is an amorphous, branched polymer made up mainly of five carbon sugar monomers (mostly xylose). The different structures of hemicellulose, cellulose, and lignin determine their different thermal decomposition behaviors during torrefaction. The chemical composition and structure of biomass are highly variable because of genetic and environmental influences and their interactions.19 Hence, the thermal decomposition behavior and structure change of biomass during torrefaction depend upon not only its operation conditions but also the original composition and structure of biomass. The experimental investigation into the thermal decomposition behavior and structure changes of basic constituents in biomass (hemicellulose, cellulose, and lignin) during torrefaction can provide fundamental insight into the torrefaction of biomass. Previous studies have shown that torrefaction can serve as an effective pretreatment method prior to CFP of biomass and its major constituents for enhancing the aromatic yield or aromatic selectivity.7,22−25 However, severe torrefaction can cause significant reduction in the aromatic yield from CFP of torrefied biomass.7 Up to date, the structure changes of hemicellulose, cellulose, and lignin and their contributions to the cross-linking and charring of biomass during torrefaction are still unclear. Moreover, the effects of torrefaction severity on CFP behavior of hemicellulose, cellulose, Received: August 2, 2015 Revised: November 5, 2015 Published: November 5, 2015 8027

DOI: 10.1021/acs.energyfuels.5b01765 Energy Fuels 2015, 29, 8027−8034

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Energy & Fuels

Figure 1. Torrefaction system.

and lignin are currently not well understood. Here, the torrefaction of hemicellulose, cellulose, and lignin was carried out in a tubular reactor. Then, the CFP of torrefied hemicellulose, cellulose, and lignin was conducted in a semi-batch pyroprobe reactor. The effects of torrefaction severity on the structure and subsequent CFP of hemicellulose, lignin, and cellulose are investigated.

oxygenate yield =

coke yield =

moles of carbon in spent catalyst moles of carbon in torrefied biomass constituents × 100%

non‐condensable gas yield

2. EXPERIMENTAL SECTION

= 100% − aromatic yield − oxygenate yield − coke yield

2.1. Preparation of Torrefied Hemicellulose, Lignin, and Cellulose. Cellulose was obtained from Sinopharm Chemical Reagent Co., Ltd. The C, H, N, S, and O contents of cellulose were 42.06, 6.36, 0, 0, and 51.59% (dry and ash-free basis), respectively. Xylan (representative of hemicellulose) and alkali lignin were purchased from Sigma-Aldrich. The C, H, N, S, and O contents of hemicellulose were 35.37, 5.82, 0.01, 0, and 58.80% (dry and ash-free basis), respectively. The C, H, N, S, and O contents of lignin were 53.26, 6.07, 0.05, 2.88, and 37.73% (dry and ash-free basis), respectively. Torrefied hemicellulose, cellulose, and lignin were prepared through a horizontal tubular reactor system. The schematic diagram of this system is shown in Figure 1. The detailed experimental procedures are stated in our previous study.7 2.2. Characterization of Torrefied Hemicellulose, Cellulose, and Lignin. The Fourier transform infrared (FTIR) spectroscopy analysis of torrefied hemicellulose, lignin, and cellulose was carried out on a Bruker TENSOR27 to characterize the structural changes of hemicellulose, lignin, and cellulose during torrefaction. The resulting spectra were normalized to the highest peak (1023 cm−1) in the fingerprint region. The 13C cross-polarization magic angle spinning (CP/MAS) solid nuclear magnetic resonance (NMR) spectrometry analysis of raw and torrefied materials was conducted on a Bruker AV-300 solid NMR spectrometer. The weight loss [thermogravimetry (TG)] and weight loss rate [differential thermogravimetry (DTG)] of raw and pretreated materials were performed on a thermogravimetric analyzer (Q50 TGA, TA Instruments). The raw and torrefied materials were heated from 30 to 105 °C, held there for 10 min, and then increased at 20 K/min to 850 °C, and a nitrogen gas flow of 40 mL/min was used as the purge gas. 2.3. CFP of Torrefied Hemicellulose, Cellulose, and Lignin. The CFP experiments were carried out in a semi-batch pyroprobe reactor (Pyroprobe 5200, CDS Analytical). The experimental procedures are shown in our previous study.7 2.3. Methods of Data Processing. The aromatic yield, aromatic selectivity, coke yield, oxygenate yield, and non-condensable gas yield from CFP of raw and torrefied hemicellulose, cellulose, and lignin were defined as follows:

aromatic yield =

moles of carbon in oxgenates × 100% moles of carbon in torrefied biomass constituents

aromatic selectivity =

moles of carbon in specific aromatic species total moles of carbon in all aromatic species × 100%

where moles of carbon in torrefied hemicellulose, cellulose, lignin, and spent catalysts were measured by elemental analysis.

3. RESULTS AND DISCUSSION 3.1. Characterization of Torrefied Hemicellulose, Cellulose, and Lignin. 3.1.1. Mass Yield of Torrefied Hemicellulose, Cellulose, and Lignin. Table 1 shows the Table 1. Mass Yields of Torrefied Hemicellulose, Cellulose, and Lignin under Different Torrefaction Conditions feedstock hemicellose

residence time: 40 min

torrefaction temperature: 270 °C

cellulose



lignin

moles of carbon in aromatic production moles of carbon in torrefied biomass constituents



× 100% 8028

mass yield (%)

torrefaction condition 210 °C 240 °C 270 °C 300 °C 20 min 40 min 60 min 210 °C 240 °C 270 °C 300 °C 20 min 40 min 60 min 210 °C 240 °C 270 °C 300 °C 20 min 40 min 60 min

77.7 68.4 48.9 40.4 56.7 48.9 47.2 94.8 93.7 92.4 76.4 93.4 92.4 90.8 81.5 78.6 75.5 72.1 77.8 75.5 73.5


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temperature was less than or equal to 270 °C, the yield of torrefied cellulose changed slightly. With the torrefaction temperature further raised from 270 to 300 °C, the yield of torrefied cellulose dropped obviously from 92.4 to 76.4%. It can be concluded that the rank order of thermal stability of hemicellulose, cellulose, and lignin during torrefaction was cellulose > lignin > hemicellulose. The results were in line with the literature.26 3.1.2. Structural Characterization of Torrefied Hemicellulose, Cellulose, and Lignin by FTIR Spectroscopy and 13 C NMR. The structural changes of hemicellulose, lignin, and cellulose during torrefaction were characterized by FTIR spectroscopy and 13C CP/MAS NMR. The FTIR spectra of torrefied hemicellulose, lignin, and cellulose are shown in Figure 2. Some of the signals with evident differences in peak intensity tagged and assigned to specific functional groups are tabulated in Table 2.27−29 The spectra of torrefied hemicellulose with Table 2. Assignment of Main Bands in FTIR Spectra of Hemicellulose, Lignin, and Cellulose between the 2000 and 500 cm−1 Region27−29 peak

wavenumber (cm−1)

1 2 3 4

1630 1586, 1590 1506 1450, 1460

5 6

1376 1320, 1326

7 8 9 10 11

1254, 1260 1210, 1214 1159 1112 1023

12

880

band assignment aromatic skeletal vibration in lignin, CO stretch aromatic skeletal vibration in lignin, CO stretch aromatic skeletal vibration in lignin C−H deformation, asymmetry in −CH3 and −CH2− and the methoxy group (O−CH3) of the lignin structure C−H deformation in cellulose and hemicellulose C−H vibration in carbohydrate, condensed syringyl ring plus guaiacyl ring syringyl ring and C−O stretch in lignin and xylan C−C, C−O, and CO stretch C−O−C vibration in cellulose and hemicellulose aromatic skeletal and C−O stretch aromatic C−H in-plane deformation, C−O deformation in primary alcohols, and CO stretch (unconjugated) C−H deformation in cellulose and hemicellulose

varying severity are shown in Figure 2a. The signals at 1630 and 1586 cm−1 are predominately related to aromatic skeletal vibration in lignin and CO stretch. The higher intensity of the aromatic skeletal vibrations at 1630 and 1586 cm−1 and the shift to 1560 cm−1 can be attributed to the polycondensation and charring reactions of hemicellulose during torrefaction. The signal at 1320 cm−1 is assigned to C−H vibration in carbohydrate and condensed syringyl ring plus guaiacyl ring. Its intensity increased with elevated torrefaction severity. The results also proved that the polycondensation reactions and charring of hemicellulose occurred during torrefaction. It was in accordance with the decreased intensity of the signals at 800−1200 cm−1. The signals at 1159, 1023, and 880 cm−1 are mainly assigned to C−O−C vibration, C−O deformation, and C−H deformation in hemicellulose, respectively. Their intensity decreased with increasing torrefaction severity. These signals were almost entirely vanished in the spectra of hemicellulose torrefied at 300 °C for 40 min, indicating that the depolymerization, ring breakage, fragmentation, and serious polycondensation of hemicellulose were observed at this torrefaction temperature. The spectra of torrefied lignin with varying severity are shown in Figure 2b. With the increasing torrefaction severity, the higher intensity of signals at 1630, 1326, and 1590 cm−1 and the shift to 1560 cm−1 can be seen as a consequence of the splitting of aliphatic side

Figure 2. FTIR spectra of torrefied biomass major constituents: (a) hemicellulose, (b) lignin, and (c) cellulose. HC, LN, and CL are the abbreviations of hemicellulose, lignin, and cellulose, respectively.

mass yields of torrefied hemicellulose, cellulose, and lignin with varying torrefaction severity. Hemicellulose, cellulose, and lignin exhibited very different torrefaction behaviors. The yield of torrefied hemicellulose decreased remarkably from 77.7 to 40.4%, while the yield of torrefied lignin dropped from 81.7 to 72.1%, with an elevated torrefaction temperature. As the residence time of torrefaction increased, the yield of torrefied hemicellulose declined from 56.7 to 47.2%, while the yield of lignin decreased slightly from 77.8 to 73.5%. These results demonstrated that, in comparison to the residence time, the torrefaction temperature has greater influences on the mass yield of torrefied hemicellulose and lignin. When the torrefaction 8029


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Energy & Fuels chains and polycondensation reactions of lignin.30 Oppositely, the intensity of the signals at 1506 cm−1 for aromatic skeletal vibration in lignin, 1450 cm−1 for C−H and the methoxy group in the lignin structure, 1254 cm−1 for the syringyl ring and C−O stretch in lignin, 1210 cm−1 for C−C, C−O, and CO stretch in lignin, and 1023 cm−1 for aromatic C−H in-plane deformation decreased with increasing torrefaction severity. It could be due to the cleavage of β-O-4 bonds and demethoxylation and polycondensation of lignin via aromatic electrophilic substitutions of aromatic nuclei during torrefaction. The spectra of torrefied cellulose with varying severity are shown in Figure 2c. It is found that the structure of cellulose changed slightly during torrefaction. Hence, the rank order of structure change of biomass major constituents during torrefaction was hemicellulose > lignin > cellulose. The significant polycondensation of hemicellulose and lignin took place during torrefaction. These finding demonstrated that hemicellulose and lignin fractions were mainly responsible for the cross-linking and charring of biomass during torrefaction. The 13C CP/MAS NMR spectra of hemicellulose are illustrated in Figure 3a. The spectra of hemicellulose torrefied at 300 °C for 40 min were obviously different from those of untreated hemicellulose. The vanished signals at 50−110 ppm related to carbohydrate carbons and new broad signal at 106−160 ppm assigned to aromatic carbons demonstrated that severe polycondensation and charring of hemicellulose took place during torrefaction.31 The 13C CP/MAS NMR spectra of lignin are shown in Figure 3b. The signals at 141−160, 125−139, and 49−62 ppm are assigned to oxygenated aromatic carbons, aromatic CC structures, and methoxyl carbons, respectively.31 When torrefaction was applied, the intensity of oxygenated aromatic carbons and methoxyl carbons decreased, whereas the intensity of aromatic CC structures increased, indicating that the cleavage of β-O-4 bonds and demethoxylation and polycondensation of lignin occurred during torrefaction. The 13 C CP/MAS NMR spectra of cellulose are shown in Figure 3c. When torrefaction was performed, the spectra of cellulose were altered slightly. These results were in line with the FTIR spectroscopy analysis mentioned above. 3.1.3. TG/DTG Analysis of Torrefied Hemicellulose, Cellulose, and Lignin. The pyrolysis behaviors of pretreated hemicellulose, lignin, and cellulose were conducted in a thermogravimetric analyzer. The corresponding TG/DTG curves are shown in Figure 4. The characteristic parameters from thermal degradation of torrefied hemicellulose, lignin, and cellulose are tabulated in Table 3. As shown in Figure 4a, two peaks could be observed during the thermal decomposition of untreated hemicellulose. The first peak is predominately attributed to the cleavage of the glycosidic bonds and side chains, and the second peak should be ascribed to ring breakage and fragmentation of xylan units. When torrefaction was applied, the first peak disappeared, while the second peak decreased and shifted toward higher temperatures. As the torrefaction temperature increased, the onset temperature of devolatilization (Ti, corresponding to a weight loss of 5%) and the temperature for the maximum rate of devolatilization (Tmax) were enhanced, whereas the maximum weight loss rate (DTGmax) declined. The pyrolysis residues increased remarkably. These results implied that the main reaction during torrefaction of hemicellulose included depolymerization (cleavage of glycosidic bonds), sidechain splitting, fragmentation, and polycondensation. As shown in Figure 4b, when lignin was subjected to torrefaction, the first shoulder peak assigned to the depolymerization and side-chain

Figure 3. 13C CP/MAS NMR spectra of torrefied lignin, hemicellulose, and cellulose: (a) hemicellulose, (b) lignin, and (c) cellulose.

splitting of lignin decreased and it further decreased with an increasing torrefaction temperature. At the same time, Ti, Tmax, and pyrolysis residues for torrefied lignin also exhibited an increasing trend with enhanced torrefaction severity. It could be explained by the fact that the depolymerization, side-chain splitting, and polycondensation happened during lignin torrefaction. As shown in Figure 4c, Ti and Tmax for cellulose changed slightly when the torrefaction temperature was less than or equal to 270 °C. When torrefaction was applied, the pyrolysis residues decreased first and started to increase at the torrefaction temperature of 270 °C. The decrease in pyrolysis residues could be attributed to the formation of active cellulose with a low degree of polymerization (DP) during torrefaction at this temperature. 8030


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Figure 4. TG/DTG analysis of torrefied hemicellulose, lignin, and cellulose: (a and b) TG/DTG analysis of torrefied hemicellulose, (c and d) TG/DTG analysis of torrefied lignin, and (e and f) TG/DTG analysis of torrefied cellulose.

Figure 5. Production distributions from CFP of torrefied hemicellulose, lignin, and cellulose with varying torrefaction severity: (a) hemicellulose, (b) lignin, and (c) cellulose.

Table 3. Characteristic Parameters from Thermal Degradation of Torrefied Hemicellulose, Lignin, and Cellulose (Residence Time of Torrefaction: 40 min) feedstock hemicellulose

lignin

cellulose

untreated 210 °C 240 °C 270 °C 300 °C untreated 210 °C 240 °C 270 °C 300 °C untreated 210 °C 240 °C 270 °C 300 °C

Ti (°C)

Tmax (°C)

DTGmax (%/min)

residue (wt %)

205 224 242 262 271 229 236 250 261 258 297 302 299 297 279

276 275 276 294 417 332 334 335 338 342 333 340 336 335 327

0.73 0.80 0.73 0.26 0.13 0.15 0.15 0.16 0.15 0.15 2.66 2.69 2.59 2.46 2.02

23.9 26.4 32.2 44.3 48.0 46.2 46.9 47.9 48.6 49.7 4.6 2.6 2.6 5.4 6.9

the torrefaction temperature and residence time on product distributions from CFP of torrefied hemicellulose are illustrated in Figure 5a. The aromatic yield from CFP of untreated hemicellulose was 29.8%. It decreased obviously from 29.2 to 8.2% with the torrefaction temperature raised from 210 to 300 °C, whereas the yield of coke increased sharply from 29.4 to 69.2%. The same trend was found for increasing the residence time. The reduction in the aromatic yield and increase in the coke yield could be explained by the enhanced devolatilization and polycondensation of hemicellulose with increasing torrefaction severity. The effects of the torrefaction temperature and residence time on product distributions from CFP of lignin are plotted in Figure 5b. As shown in Figure 5b, the aromatic yield from untreated lignin was 10.2%. It declined from 10.2 to 5.7% as the torrefaction temperature was raised from 210 to 300 °C, whereas the yield of coke increased from 68.6 to 79.5%. With an increasing residence time, the aromatic yield dropped from 7.4 to 6.8%, while the yield of coke increased from 70.6 to 80.0%. It could also be ascribed to the increasing polycondensation of lignin with elevated torrefaction severity. The effects of torrefaction severity on product distributions from CFP of cellulose are shown in Figure 5c. It is observed that torrefaction had little impact on product distributions from CFP of cellulose. These results were in line with FTIR spectroscopy and 13C NMR analyses of torrefied cellulose. On the contrary, it is obvious that torrefaction exerted negative impacts on CFP of hemicellulose and lignin. During CFP, the first step

3.2. CFP of Torrefied Hemicellulose, Lignin, and Cellulose. 3.2.1. Effect of Torrefaction Severity on Product Distribution from CFP of Hemicellulose, Lignin, and Cellulose. The product distributions from CFP of torrefied biomass major constituents with varying severity are shown in Figure 5. The identified oxygenates mainly contained cyclopentenone, furan, 2-methylfuran, and acetic acid. The effects of 8031


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ketones, furans, etc.), char, and non-condensable gas can also proceed. The low-molecular-weight oxygenates subsequently diffuse into the zeolite channels and then form olefins through a series of dehydration, oligomerization, decarbonylation, and decarboxylation reactions. The olefins can further undergo an aromatization reaction to form aromatic production.32,33 During torrefaction, hemicellulose and lignin can undergo depolymerization, side-chain splitting, and ring-scission reactions to release low-molecular-weight oxygenates. In addition, the polycondensation and charring reactions also proceed. Hence, the torrefied hemicellulose and lignin produce less low-molecular-weight oxygenates and more char in subsequent CFP, resulting in a low yield of aromatics and a high yield of char/coke. In conclusion, the devolatilization and polycondensation of hemicellulose and lignin during torrefaction could be mainly responsible for the yield penalties of pyrolysis liquids from fast pyrolysis or CFP of torrefied biomass. 3.2.2. Aromatic Selectivity from CFP of Raw and Torrefied Hemicellulose and Lignin. The total ion chromatograms resulting from CFP of raw and torrefied hemicellulose and lignin are shown in Figures 6 and 7. It is obvious that the peak numbers and intensities decreased with increasing torrefaction severity. The aromatic selectivity from CFP of raw and torrefied hemicellulose and lignin is tabulated in Tables 4 and 5. The aromatic selectivity from CFP of torrefied cellulose was not presented because torrefaction has little impact on it. As shown in Table 4, the aromatic selectivity from CFP of torrefied hemicellulose was influenced significantly by torrefaction severity. The selectivity of benzene, toluene, and xylenes (BTX) was 13.4, 18.2, and 16.8%, respectively. It decreased first by applying torrefaction pretreatment at 210 °C. The selectivity of benzene increased gradually from 13.1 to 17.9% as the torrefaction temperature further increased, while the selectivity of toluene and xylenes increased from 16.9 and 15.0% to 32.5 and 29.3%, respectively. Hence, the selectivity of BTX increased sharply from 45.0 to 79.8%. The same trend was found for increasing the residence time. It can be concluded that torrefaction can promote the aromatic selectivity from CFP of hemicellulose. The results could be attributed to the polycondensation of hemicellulose during torrefaction to form an aromatic structure, which is similar to the lignin strucuture. Our previous study demonstrated that CFP of lignin exhibited maximum selectivity of BTX.33 The BTX selectivity from CFP of lignin, hemicellulose,

is the thermal depolymerizaiton of hemicellulose, lignin, and cellulose to form pyrans, phenols, and anhydrosugars. Simultaneously, the competing reactions, e.g., side-chain splitting, ringscission, rearrangement, and polycondensation reactions, to produce low-molecular-weight oxygenates (alcohols, aldehydes,

Figure 6. Total ion chromatograms resulting from CFP of raw and pretreated hemicellulose.

Figure 7. Total ion chromatograms resulting from CFP of raw and pretreated lignin.

Table 4. Effect of the Torrefaction Temperature and Residence Time on Aromatic Selectivity from CFP of Hemicellulose aromatic selectivity (%) temperature: 270 °C

residence time: 40 min benzene toulene xylenes BTX ethylbenzene alkylbenzenes phenols benzofurans indenes naphthalenes fluorenes, anthracenes, and phenanthrenes polycyclic aromaticsa a

hemicellulose

210 °C

240 °C

270 °C

300 °C

20 min

40 min

60 min

13.4 18.2 16.8 48.4 3.3 8.0

13.1 16.9 15.0 45.0 2.7 10.5 1.0 1.2 7.2 26.5 4.5 31.0

13.4 24.4 20.7 58.5 2.8 8.4

15.9 27.6 24.5 68.0 3.0 8.6

17.9 32.5 29.3 79.8 2.9 6.0

13.3 21.4 19.1 53.8 3.1 9.3

15.9 27.6 24.5 68.0 3.0 8.6

16.0 27.8 24.9 68.7 2.9 8.4

8.8 20.2 1.4 21.6

5.0 15.3

14.3

5.0 15.3

5.0 15.0

15.3

14.3

12.4 20.2 1.2 21.4

15.3

15.0

1.2 9.0 24.0 2.7 26.7

Polycyclic aromatics: naphthalenes, fluorenes, anthracenes, and phenanthrenes. 8032


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Energy & Fuels Table 5. Effects of the Torrefaction Temperature and Residence Time on Aromatic Selectivity from CFP of Lignin aromatic selectivity (%) temperature: 270 °C

residence time: 40 min benzene toulene xylenes BTX ethylbenzene alkylbenzenes phenols benzofurans indenes naphthalenes fluorenes, anthracenes, and phenanthrenes polycyclic aromatics

lignin

210 °C

240 °C

270 °C

300 °C

20 min

40 min

60 min

12.1 25.6 27.2 65.0 1.7 7.8 1.4

14.3 29.2 30.2 73.7

13.1 29.1 31.4 73.6

11.3 29.1 33.2 73.6

13.4 29.6 33.1 75.0

10.8 29.3 33.4 73.4

11.2 29.1 33.2 73.5

12.1 29.8 32.4 74.4

6.4

6.6

7.0

6.6

7.0

7.0

6.5

19.9

19.7

19.5

18.3

19.6

19.5

19.2

19.9

19.7

19.5

18.3

19.6

19.5

19.2

3.0 17.7 3.4 21.1

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and cellulose was 65.0, 36.3, and 48.4%, respectively. The other aromatics, such as alkylbenzene, indenes, and polycyclic aromatics (e.g., naphthalenes, fluorenes, anthracenes, and phenanthrenes), exhibited opposite variation trends. The yield of naphthalenes decreased from 26.5 to 14.3% with an increasing torrefaction temperature. At the same time, the yield of fluorenes, anthracenes, and phenanthrenes also dropped obviously. Hence, the yield of polycyclic aromatics declined rapidly from 31.0 to 14.3%. The polycyclic aromatics are generally considered as the product of the phenyl radical reacted with unsaturated hydrocarbon (e.g., olefins). In CFP, severe torrefied hemicellulose releases less low-molecular-weight oxygenates and subsequent unsaturated hydrocarbon than the light torrefied hemicellulose, leading to the low yield of polycyclic aromatics. The effects of the torrefaction temperature and residence time on aromatic selectivity from CFP of lignin are summarized in Table 5. The selectivity of BTX from untreated lignin was 12.1, 25.6, and 27.2%, respectively. When torrefaction of lignin was peformed at 210 °C, the selectivity of BTX increased obviously to 14.3, 29.2, and 30.2%, respectively. It changed slightly with the further increase of torrefaction severity. The slecitvity of BTX from CFP of torrefied lignin exhibited the same variation trend. The results could be due to the splitting of side chains and polycondensation of lignin during torrefaction. They were in accordance with the weight loss behaviors of lignin during torrefaction. The polycyclic aromatics decreased gradually with increasing torrefaction severity.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-2087057721. Fax: +86-2087057737. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 51376186 and 21406227), the Natural Science Foundation of Guangdong Province, China (Grant 2014A030313672), and the Science and Technology Planning Project of Guangdong Province, China (Grants 2014B020216004 and 2015A020215024) for financial support of this work.

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4. CONCLUSION The structure changes of hemicellulose, lignin, and cellulose during torrefaction and their influences on subsequent CFP were investigated in this study. The FTIR spectroscopy, 13C CP/MAS NMR, and TG/DTG analyses of torrefied hemicellulose, cellulose, and lignin showed that torrefaction had significant impacts on the chemical structure of hemicellulose and lignin. The severe polycondensation of hemicellulose and lignin was observed during torrefaction. Torrefaction has little impact on the chemical structure of cellulose. The CFP experiments demonstrated that torrefaction can cause the reduction in the aromatic yield and the increase in BTX selectivity from CFP of torrefied hemicellulose and lignin. It could be explained by the devolatilization and polycondensation of hemicellulose and lignin during torrefaction. 8033


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Impact of Torrefaction on the Chemical Structure and Catalytic Fast

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