Catalytic Fast Pyrolysis of Biomass Pretreated by Torrefaction with

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Catalytic Fast Pyrolysis of Biomass Pretreated by Torrefaction with Varying Severity Anqing Zheng,† 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 510640, People’s Republic of China ‡ Thermal & Environmental Engineering Institute, Tongji University, Shanghai 200092, People’s Republic of China S Supporting Information *

ABSTRACT: Pretreatment of corncobs using torrefaction was performed in a tubular reactor with varying reaction temperature (210, 240, 270, or 300 °C) and residence time (20, 40, or 60 min). The torrefied corncobs were subsequently catalytically fast pyrolyzed over nanosized HZSM-5 in a semibatch pyroprobe reactor. The torrefied corncobs were characterized by elemental analysis, thermogravimetric analyzer coupled with Fourier transform infrared spectroscopy (FTIR), and FTIR. The aromatic production was online analyzed by gas chromatography mass spectroscopy. The effect of torrefaction severity on product distribution and aromatic selectivity from catalytic fast pyrolysis of torrefied corncobs was investigated. The experimental results show that torrefaction can serve as an effective thermal pretreatment for improving the selectivity of BTX (benzene, toluene, and xylenes). Light and mild torrefaction (torrefaction at 210 and 240 °C) has little impact on the aromatic yield. However, severe torrefaction (torrefaction at 270 or 300 °C) can lead to the sharp increase of coke yield and reduction of aromatic yield. The results could be explained by the serious cross-linking and charring of corncob under severe torrefaction conditions. The optimal torrefaction condition is 210−240 °C with residence time of 40 min.

1. INTRODUCTION Catalytic fast pyrolysis is one of the most promising technologies for renewable aromatic production from solid biomass.1,2 In this process, solid biomass can be converted into high carbon yield of aromatic hydrocarbons over cheap ZSM-5 catalyst in a single reactor at intermediate temperature (400− 600 °C), short residence time (1000 k/s), and high catalyst to feedstock ratio.3 One of the crucial challenges facing catalytic fast pyrolysis is rapid deactivation of ZSM-5 catalyst caused by serious coking.4,5 Hence, the most suitable reactor for catalytic fast pyrolysis is circulating fluidized bed (CFB) combined with catalyst regenerator which is similar to a fluid catalytic cracking unit in the petroleum refining industry. Prior to injection of biomass into a CFB, biomass feedstock must be dried and ground to fine particles measuring a few millimeters for proper fluidization.[6] Furthermore, fine grinding of biomass is beneficial to increase reaction rates and reduce char/coke yield in catalytic fast pyrolysis. The reaction rate of biomass with small particle size ( lignin > hemicellulose in the temperature range of torrefaction (200−300 °C),13 indicating that torrefaction could selectively decompose the hemicellulose and lignin fractions of biomass to enhance aromatic production from catalytic fast pyrolysis of torrefied biomass,14,15 because cellulose can provide relatively higher aromatic yield compared to hemicellulose and lignin.16 Srinivasan et al. demonstrated that the total carbon yield of aromatics from catalytic pyrolysis of torrefied biomass was higher than that of untreated pine.17 The aromatic yield benefit is strongly dependent upon the torrefaction severity. Zheng et al. found that severe torrefaction severity could cause the cross-linking and charring of biomass, leading to significant increase of coke yield and reduction of liquid yield in subsequent fast pyrolysis of torrefied biomass.18,19 Consequently, the maximum aromatic yield benefit or minimum aromatic yield penalty can be achieved by controlling the torrefaction severity for maximizing the decomposition of hemicellulose and lignin without the crosslinking and charring of biomass. However, to date, very little information regarding the effect of torrefaction severity on catalytic fast pyrolysis of biomass is publicly available, although a number of papers about fast pyrolysis of torrefied biomass have been published.18−26 Here, the torrefaction of corncobs was performed in a tubular reactor with varying reaction temperature (210, 240, 270, or 300 °C) and residence time (20, 40, or 60 min). Then the catalytic fast pyrolysis of torrefied corncobs was conducted in a semibatch Received: April 21, 2014 Revised: August 10, 2014 Published: August 11, 2014 5804

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Figure 1. Torrefaction system.

Table 1. Mass Yield and Elemental Analysis of Torrefied Corncob anal of torrefied corncob temp: 270 °C

residence time: 40 min mass yield/(wt %) elem anal/(wt %), dafa

C H Ob N S

O/C a

raw corncob

210 °C

240 °C

270 °C

300 °C

20 min

40 min

60 min

100 44.81 6.14 48.76 0.28 0.01 1.09

91.36 45.72 6.21 47.78 0.27 0.02 1.05

77.12 46.42 6.24 47.03 0.31 0.00 1.01

67.07 50.72 6.00 42.93 0.35 0.00 0.85

50.42 56.51 5.75 37.33 0.42 0.00 0.66

79.23 47.43 6.14 46.13 0.30 0.00 0.97

67.07 50.72 6.00 42.93 0.35 0.00 0.85

58.70 53.65 5.87 40.10 0.38 0.00 0.75

daf: Dry ash free basis. bThe oxygen content was calculated by difference. thermogravimetric experiment, the sample was heated from 40 to 800 °C at a constant heating rate of 20 K/min and then held at 800 °C for 20 min, and a nitrogen gas flow of 40 mL/min was used as the purge gas. 2.3. Catalytic Fast Pyrolysis of Torrefied Corncob. The catalytic fast pyrolysis experiments were carried out in a semibatch pyroprobe reactor (Pyroprobe 5200, CDS Analytical). The probe was a computer controlled, resistively heated element which held an open ended quartz tube. Powdered reactants filled in the quartz tube were prepared by physically mixing the torrefied corncob and HZSM-5 catalyst using an agate mortar. The HZSM-5 catalysts (Si/Al = 25) with mean crystal size of 200 nm used in this work were purchased from the Catalyst Plant of Nankai University. For a typical run 10−15 mg of mixture with a catalyst to feedstock weight ratio of 9 was used, and the catalytic pyrolysis temperature, residence time, and heating rate were fixed to 600 °C, 50 s, and 20000 K/s, respectively. Each experiment was performed at least twice under the same conditions to ensure its repeatability. The oxygenates and aromatics were online analyzed by an Agilent 7890 series gas chromatograph (GC) coupled with an Agilent 5975 series mass-selective detector (MSD). The GC column used was HP-5MS 30 m × 250 μm, and the film thickness was 0.25 μm. The oven was programmed to hold at 50 °C for 1 min, then increased at 10 °C/min to 260 °C, and held there for 15 min. Helium was used as carrier gas. The injector temperature was 260 °C. The injector split ratio was set at 50:1. The ion source temperature was 230 °C for the mass spectrometer detector. The mass spectrometer was set at an ionizing voltage of 70 ev with mass range (m/z) of 28−500 u. The compounds were identified by comparison with NIST mass spectral data library, and their response factors for GC quantification were determined using an external standard method. 2.4. Methods of Data Processing. The aromatic yield, oxygenates yield, coke yield, noncondensable gas yield, and aromatic selectivity were defined as follows:

pyroprobe reactor. The effect of torrefaction severity on aromatic yield and selectivity from catalytic fast pyrolysis of corncobs is investigated.

2. EXPERIMENTAL SECTION 2.1. Preparation of Torrefied Corncob. Corncob (Baodi feed mill, Tianjin, China) was ground and sieved to a particle size less than 0.125 mm and then dried at 105 °C for 4 h before torrefaction. The ultimate analysis of raw corncob is shown in Table 1. The elemental analysis of corncob was performed on a Vario EL (Elementar Analysensysteme, Hanau, Germany). Torrefied corncobs were prepared in a horizontal tubular reactor system. The system is shown in Figure 1. For each test about 200 mg of corncob was put into a porcelain boat, which was placed into the left of the quartz tube and kept 12 cm from the furnace in order to avoid premature heating of the corncob. Nitrogen flow was used to maintain an inert atmosphere and to remove volatile products from the reactor. The flow rate of nitrogen is 200 mL/min. When the furnace reached the desired torrefaction temperature (210, 240, 270, or 300 °C), the porcelain boat was pushed into the reaction zone (the center of the furnace) and held there for the desired residence time (20, 40, or 60 min). After corncob torrefaction, the porcelain boat was pulled out and cooled with a nitrogen flow for 5 min until it reached room temperature so that the torrefied biomass could be collected and weighed. 2.2. Characterization of Torrefied Corncob. The elemental analysis of torrefied corncobs was performed on a Vario EL (Elementar Analysensysteme). The Fourier transform infrared spectroscopy (FTIR) analysis of torrefied corncobs was carried out on a Bruker TENSOR27 in order to characterize the main functional groups of the torrefied corncobs. KBr discs were prepared by mixing about 2 mg of sample with 200 mg of KBr. Thermogravimetric analyzer (STA409PC, Netzsch, Selb, Germany) coupled with FTIR (TENSOR27, Bruker, Ettlingen, Germany) was used to investigate the weight loss (thermogravimetry, TG) and weight loss rate (differential thermogravimetry, DTG) of torrefied corncob and formation of typical products at the same time. In the

aromatic yield = 5805

moles of carbon in aromatic production × 100% moles of carbon in torrefied corncob dx.doi.org/10.1021/ef500892k | Energy Fuels 2014, 28, 5804−5811

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Figure 2. TG curves for raw and torrefied corncobs: (A) varying torrefaction temperature with residence time of 40 min; (B) varying residence time with torrefaction temperature of 270 °C.

oxygenates yield = coke yield =

moles of carbon in oxgenates × 100% moles of carbon in torrefied corncob

deconvolution signals. Biomass has a complex composition, typically comprised of moisture, extractives, hemicellulose, cellulose, lignin, and ash. The overlapping temperature range for the devolatilization of extractives, hemicellulose, cellulose, and lignin made it hard to obtain quantitative results from DTG curves. It is assumed that there is no interaction among the pyrolysis of extractives, hemicellulose, cellulose, and lignin. In order to separate the contributions of different components, each DTG profile was deconvolved into five peaks corresponding to the minimum number to obtain a good superposition of experimental and fitting profile using Gaussian fitting.27−29 As shown in Figure 3, the signals centered at 91−94, 263−289, 300−321, 333−337, and 371−390 °C were assigned to the mass loss rates of moisture, extractives, hemicellulose, cellulose, and lignin, respectively. The normalized integration values of these signals, indicating the weight loss fractions (the weight loss of a specific component divided by the total weight loss), have been tabulated in Table 2. The peak temperature of weight loss rates of different components and pyrolysis residue of raw corncob and torrefied corncobs have also ben given in Table 2. As shown in Table 2, the weight loss fractions of torrefied corncobs were apparently influenced by the torrefaction severity of corncobs. The weight loss fractions for moisture, extractives, hemicellulose, cellulose, and lignin during pyrolysis of raw corncob were 2.46, 5.82, 36.26, 31.53, and 23.93%, respectively. The pyrolysis residue of raw corncob was 20.34%. As the torrefaction temperature increased from 210 to 300 °C, the weight loss fraction of hemicellulose decreased from 36.33 to 2.96%, whereas the weight loss fraction of lignin increased from 22.34 to 53.36%. At the same time, the weight loss fraction of cellulose changed slightly. These results suggest the significant decomposition of hemicellulose during corncob torrefaction and the increasing relative content of lignin in torrefied corncob with elevated torrefaction severity. The increasing lignin content of torrefied corncob with elevated torrefaction temperature could be explained by two reasons. One reason is the more severe decomposition of hemicellulose and less severe decomposition of lignin resulting in the increasing relative content of lignin. Another reason is the cross-linking and charring of hemicellulose and cellulose to form aromatic structures which are similar to the structure of lignin. As the torrefaction temperature increased from 210 to 240 °C, the peak temperatures of weight loss rates of hemicellulose and lignin remained constant. However, as the torrefaction temperature further increased from 240 to 300 °C, the peak temperature of weight loss rates of hemicellulose

moles of carbon in spent catalyst × 100% moles of carbon in torrefied corncob

noncondensable gas yield = 100% − aromatic yield − oxygenates yield − coke yield aromatic selectivity =

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

where moles of carbon in spent catalysts and torrefied corncobs were quantified by elemental analysis.

3. RESULTS AND DISCUSSION 3.1. Characterization of Torrefied Corncobs. Table 1 shows the mass yields of torrefied corncobs with different torrefaction severity. The yields of torrefied corncobs were strongly dependent on torrefaction temperature and residence time. As the torrefaction temperature increased from 210 to 300 °C, the yields of torrefied corncobs decreased gradually from 91.36 to 50.42%. As the residence time of torrefaction increased from 20 to 60 min, the yields of torrefied corncobs dropped from 79.23 to 58.70%. The thermal stability of hemicellulose, cellulose, and lignin decreased in the order of cellulose > lignin > hemicellulose in the temperature range of torrefaction.13 Hence, the mass loss of corncobs during torrefaction was primarily because of the decomposition of hemicellulose and lignin and partly due to the depolymerization of cellulose, suggesting that torrefaction at temperature below approximately 240 °C can selectively decompose hemicellulose and lignin fractions of biomass without severe decomposition of cellulose. Table 1 also shows the elemental analysis of torrefied corncobs with varying severity. As the torrefaction temperature increased from 210 to 300 °C, the carbon content of torrefied corncobs increased from 45.72 to 56.51%, whereas the oxygen content dropped from 47.78 to 37.33%. Hence, the O/C ratio of torrefied corncobs decreased significantly from 1.05 to 0.66. The same variation trend was observed with increasing residence time. The decreasing O/C ratio indicates that deoxygenation of corncobs may be the primary reaction during torrefaction. The oxygen of corncobs was likely released mainly in the form of water, CO2, and CO. Figure 2 shows the TG curves of torrefied corncobs with varying torrefaction severity, and Figure 3 shows the DTG curves of torrefied corncobs and their corresponding 5806

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Figure 3. DTG curves and their corresponding deconvolution signals for raw and torrefied corncobs: (A) raw corncob; (B) 210 °C and 40 min; (C) 240 °C and 40 min; (D) 270 °C and 20 min; (E) 270 °C and 40 min; (F) 270 °C and 60 min; (G) 300 °C and 40 min.

shifted from 300 to 321 °C, while that of lignin shifted from 371 to 390 °C. Synchronously, the pyrolysis residue of torrefied corncobs increased from 24.98 to 43.05%. The shift of the peak

temperature of weight loss rates of hemicellulose and lignin toward higher temperature, combined with the increasing pyrolysis residue of torrefied corncobs, indicate that the cross5807

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Table 2. Weight Loss Fraction of Torrefied Corncobs with Varying Torrefaction Severity weight loss fraction (%) torrefaction conditions raw corncob residence time: 40 min

torrefaction temp: 270 °C

a

moisture 210 °C 240 °C 270 °C 300 °C 20 min 40 min 60 min

2.46(94) 2.56(91) 1.37(91) 1.39(92) 2.54(92) 1.99(91) 1.39(92) 2.37(92)

a

extractives

hemicellulose

cellulose

lignin

residue (wt %)

5.82(263) 4.27(264) 3.82(268) 4.13(283) 2.44(288) 3.49(268) 4.13(283) 2.87(289)

36.26(300) 36.33(300) 33.41(300) 23.57(313) 2.96(321) 32.65(299) 23.57(313) 10.56(315)

31.53(337) 33.97(337) 35.12(336) 33.49(336) 38.70(333) 35.76(336) 33.49(336) 39.02(333)

23.93(371) 22.34(371) 26.26(371) 37.43(376) 53.36(390) 26.11(371) 37.43(376) 45.18(381)

20.34 22.34 24.98 32.80 43.05 27.45 32.80 37.55

The value in parentheses represents the peak temperature (°C) of its corresponding deconvolution signal.

linking and charring of hemicellulose and lignin occurred when the torrefaction temperature of corncob was equal to or greater than 270 °C. It is worthy of note that the peak temperature of the weight loss rate of cellulose shifted toward lower temperature with increasing torrefaction temperature, suggesting that the reactivity of cellulose was slightly raised with increasing torrefaction temperature although a certain extent of the cross-linking and charring of cellulose could be proceeded during torrefaction. It could be explained by the depolymerization of cellulose to form active cellulose during torrefaction. The same variation trends were observed with increasing residence time of torrefaction. The structural changes of corncobs caused by torrefaction with varying severity can be characterized by FTIR. The FTIR spectra of torrefied corncobs are shown in Figure 4. Some of

had the same variation trend. These changes can be attributed to the cross-linking and charring of torrefied corncobs through cross-linking and condensation reactions. It should be noticed that the spectra of torrefied corncobs obtained at 270 °C for 40 min and above were obviously different from th ose obtained at 270 °C for 20 min and below, suggesting that severe charring of corncobs occurred at 270 °C for 40 min and above. These results were in line with those obtained from TG/DTG analysis. 3.2. Catalytic Fast Pyrolysis of Corncobs Pretreated by Torrefaction with Varying Severity. 3.2.1. Product Distribution from Catalytic Fast Pyrolysis of Torrefied Corncobs. The total ion chromatograms resulting from catalytic fast pyrolysis of torrefied corncobs are shown in Figure 5. The identified aromatic compounds were classified

Figure 4. FTIR spectra of torrefied corncobs obtained at different torrefaction temperatures and residence times: (A) raw corncob; (B) 210 °C and 40 min; (C) 240 °C and 40 min; (D) 270 °C and 20 min; (E) 270 °C and 40 min; (F) 270 °C and 60 min; (G) 300 °C and 40 min.

Figure 5. Total ion chromatograms resulting from catalytic fast pyrolysis of torrefied corncob with different torrefaction severity: (A) raw corncob; (B) 210 °C and 40 min; (C) 240 °C and 40 min; (D) 270 °C and 20 min; (E) 270 °C and 40 min; (F) 270 °C and 60 min; (G) 300 °C and 40 min.

the signals with noticeable differences in peak height and peak area are tagged and assigned to specific functional group as follows: (1) 1730, (2) 1630, and (3)1605 cm−1 for CC stretching of the aromatic ring and (4) 1458 and (5)1425 cm−1 for CC breathing of the aromatic ring and C−O stretching in lignin.30,31 As the torrefaction severity increased, the signal at 1730 cm−1 was heightened/broadened and shifted to 1704 cm−1. Simultaneously, the signals at 1630 and 1605 cm−1 were heightened/broadened and merged together to form anew broad signal at 1605 cm−1. The signals at 1458 and 1425 cm−1

into 10 groups, including benzene, toluene, xylenes, ethylbenzene, alkylbenzenes, phenols, indenes, naphthalenes, benzofurans, and the group of fluorenes, anthracenes, and phenanthrenes. And the identified oxygenates mainly included furan, 2-methylfuran, acetic acid, and cyclopentenone. The quantified results, containing aromatic yield, coke yield, oxygenates yield, and noncondensable gas yield, are shown in Figure 6. The aromatic yield and coke yield were strongly affected by the torrefaction severity of corncobs. The effect of torrefaction temperature on product distribution from catalytic 5808

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Figure 6. Production distributions from catalytic fast pyrolysis of torrefied corncobs with varying torrefaction severity: (A) varying torrefaction temperature with residence time of 40 min; (B) varying residence time with torrefaction temperature of 270 °C.

linking and charring of torrefied corncob at corresponding torrefaction temperatures. The results were in accordance with the TG/DTG and FTIR analysis of torrefied corncobs. Concerning the effect of residence time of torrefaction, as shown in Figure 6B, the aromatic yield decreased from 22.5 to 17.2%, whereas the coke yield increased from 53.7 to 79.0% as residence time increased from 20 to 60 min. The results can also be ascribed to the elevated severity of charring of corncobs with increasing residence time. 3.2.2. Aromatic Selectivity from Catalytic Fast Pyrolysis of Torrefied Corncobs. The aromatic selectivity from catalytic fast pyrolysis of torrefied corncobs has been summarized in Table 3. The aromatic selectivity was also a function of the torrefaction severity of corncobs. The selectivity of benzene, toluene, xylenes, and BTX (benzene, toluene, and xylenes) from catalytic fast pyrolysis of raw corncob were 11.1, 14.3, 14.5,

fast pyrolysis of torrefied corncobs was illustrated in Figure 6A. The aromatic yield from catalytic fast pyrolysis of raw corncob was 26.1%. The aromatic yield of 27.5% was obtained by torrefied corncob with torrefaction temperature of 210 °C. It was close to the aromatic yield from catalytic fast pyrolysis of raw corncob. As the torrefaction temperature increased from 210 to 240 °C, the aromatic yield decreased slightly from 27.5 to 25.8%. As the torrefaction temperature further increased from 240 to 270 °C, the aromatic yield rapidly declined from 25.8 to 20.6%, and it dropped sharply to 13.2% as torrefaction temperature further increased to 300 °C. The coke yield was raised from 51.9 to 83.8% with increasing torrefaction temperature from 210 to 300 °C. The sharp increase of coke yield was also observed at torrefaction temperature of 270 and 300 °C. The sharp decline of aromatic yield and increase of coke yield at 270 and 300 °C were caused by the severe cross5809

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Table 3. Aromatic Selectivity from Catalytic Fast Pyrolysis of Torrefied Corncobs with Varying Torrefaction Severity aromatic selectivity (%) temp: 270 °C

residence time: 40 min benzene toulene xylenes BTXa ethylbenzene alkylbenzenesb phenolsc benzofuransd indenese naphthalenesf fluorenes, anthracenes, and phenanthrenes polycyclic aromaticsg

raw corncob

210 °C

240 °C

270 °C

300 °C

20 min

40 min

60 min

11.1 14.3 14.5 39.9 3.1 7.4 2.8 1.5 12.6 25.2 5.7 30.8

11.1 14.1 14.7 40.0 2.7 9.1 2.7 1.5 9.8 25.6 7.0 32.7

13.3 15.6 15.0 43.9 3.0 6.9 2.4 1.7 10.7 24.0 6.0 30.1

11.5 18.0 16.3 45.7 2.5 6.5 1.9 1.0 9.8 27.4 3.9 31.4

11.9 20.4 18.8 51.1 2.4 7.0 1.2 -9.5 24.1 1. 9 26.0

12.0 18.8 17.1 47.8 2.6 7.1 1.7 0.7 10.3 24.3 4.0 28.2

11.5 18.0 16.3 45.7 2.5 6.5 1.9 1.0 9.8 27.4 3.9 31.4

12.4 18.7 17.9 49.0 3.0 4.8 3.1 -13.1 23.4 2.1 25.5

a

BTX: benzene, toulene ,and xylenes. bAlkylbenzenes: 1-ethyl-2-methylbenzene and trimethylbenzenes. cPhenols: phenol, methylphenols, ethylphenols, and dimethylphenols. dBenzofurans: benzofuran and methylbenzofurans. eIndenes: Indane, indene, methylindenes, and dimethylindenes. fNaphthalenes: naphthalene, methylnaphthalenes, ethylnaphthalenes, dimethylnaphthalenes, and trimethylnaphthalenes. g Polycyclic aromatics: naphthalenes, fluorenes, anthracenes, and phenanthrenes.

optimal torrefaction condition is 210−240 °C with residence time of 40 min.

and 39.9%, respectively. As the torrefaction temperature increased from 210 to 300 °C, the toluene selectivity increased from 14.1 to 20.4%, while the xylenes selectivity was raised from 14.7 to 18.8%. Synchronously, the BTX selectivity increased from 40.0 to 51.1%. On the contrary, the minimum selectivity of polycyclic aromatics, including naphthalenes, fluorenes, anthracenes, and phenanthrenes, was obtained by torrefied corncob with torrefaction temperature of 300 °C. The increase in selectivity of BTX could be due to the increasing lignin content of torrefied corncob. Our previous study showed that the BTX selectivities from catalytic fast pyrolysis of cellulose, hemicellulose, and lignin were 36.3, 48.4, and 65.0%, respectively. The increasing lignin content of torrefied corncob with elevated torrefaction temperature could be explained by two reasons. One reason is the more severe decomposition of hemicellulose resulting in the increasing relative content of lignin. Another reason is the cross-linking and charring of hemicellulose and cellulose to form aromatic structures which are similar to the structure of lignin. The alkylbezenes, containing 1-ethyl-2-methylbenzene and trimethylbenzenes, decreased gradually from 9.1 to 6.5% as the torrefaction temperature increased from 210 to 270 °C and then increased to 7.0% as the torrefaction temperature further increased to 300 °C. The phenols, such as phenol, methylphenols, ethylphenols, and dimethylphenols, declined gradually from 2.7 to 1.2% with increasing torrefaction temperature. The indenes, including indane, indene, methylindenes, and dimethylindenes, first increased from 9.8 to 10.7% as torrefaction temperature increased from 210 to 240 °C and then decreased to 9.5% as torrefaction temperature further increased to 300 °C. Regarding the effect of residence time of torrefaction, torrefied corncob with residence time of 60 min exhibited the maximum selectivity of BTX and minimum selectivity of polycyclic aromatics.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing TG-FTIR spectra of the release of CO2, CH4, CO, and carboxlylic acid during pyrolysis and total ion chromatograms from catalytic fast pyrolysis of torrefied corncob. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 02087057721. Fax: +86 02087057737. 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 (Grant No. 51376186) for financial support of this work.

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4. CONCLUSION Torrefaction can serve as an effective thermal pretreatment method for improving the BTX selectivity from catalytic fast pyrolysis of corncob. However, severe torrefaction can cause the sharp increase of coke yield and reduction of aromatic yield due to the cross-linking and charring of torrefied corncob. The 5810

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dx.doi.org/10.1021/ef500892k | Energy Fuels 2014, 28, 5804−5811