Characterization of Products from Torrefaction of Sprucewood and

Oct 17, 2012 - Key Laboratory of Renewable Energy and Gas Hydrate, Chinese Academy of Science, Guangzhou 510640, People's Republic of. China. ABSTRACT...
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Characterization of Products from Torrefaction of Sprucewood and Bagasse in an Auger Reactor Sheng Chang,†,‡ Zengli Zhao,*,†,‡ Anqing Zheng,†,‡ Fang He,†,‡ Zhen Huang,†,‡ and Haibin Li†,‡ †

Guangzhou Institute of Energy Conversion, Chinese Academy of Science, Guangzhou 510640, People’s Republic of China Key Laboratory of Renewable Energy and Gas Hydrate, Chinese Academy of Science, Guangzhou 510640, People’s Republic of China

‡

ABSTRACT: Torrefaction experiments of sprucewood and bagasse were performed in an auger reactor at 260 °C, 280 °C, and 300 °C. The chemical composition and pyrolysis behavior of the resulting torrefied biomass were examined in detail. A number of water and lightweight organic compounds were removed from biomass through torrefaction treatment. Chemical component analysis showed that more acid insoluble fibers were formed in torrefied bagasses obtained at 280 and 300 °C, which suggested cross-linking and carbonization of carbohydrates probably took place in bagasse torrefaction. FTIR (Fourier transform infrared) analysis indicated that thermal decomposition of carbohydrate (mainly hemicellulose) predominated over lignin decomposition and the cross-linking of cellulose in torrefaction. XRD analysis revealed that the degradation of amorphous hemicellulose and amorphous regions of cellulose resulted in increases of crystallinity of torrefied biomass obtained below 300 °C. Thermogravimetric analysis revealed that the decomposition of cellulose in torrefied biomass was accelerated by torrefaction treatment. Py-GC/MS analysis exhibited that the yields of acetic acid and other lightweight compounds were lower in pyrolysis of torrefied sprucewood obtained at 300 °C and all torrefied bagasses than those in raw biomass pyrolysis, while the yields of levoglucosan in torrefied biomass pyrolysis were obviously higher. This implied that more stable pyrolysis oil with higher content of levoglucosan could be obtained from torrefied biomass.

1. INTRODUCTION As a result of the global growing energy demand, the depletion of fossil fuels, and increasing concern over environmental protection, it is urgent to develop advanced fuels based on renewable, abundant, and relatively clean resources. Biomass as the fourth largest energy source currently in the world, and is more and more recognized as an important source for renewable fuels and valuable chemicals.1 There are various biochemical and thermochemical conversion processes available to convert biomass to different forms of energy and chemicals, and the conversion efficiency for different processes is of the first magnitude.2−5 However, several problems and challenges are unavoidably encountered during the biomass utilization owing to its diversity of physical and chemical compositions, which depend on its origin. To enhance the utilization efficiency of bioenergy and the yields of selected products, pretreatment is usually required.6−10 When biomass is to be processed themochemically, torrefaction is considered as an effective pretreating method; it is the thermal treatment technology carried out at relatively mild temperature in an inert atmosphere.11 After torrefaction, the primary advantages of torrefied biomass including a higher volumetric energy density, improved grindability, hydrophobic properties, and uniformity in product quality are achieved.12−15 There are quite a few research studies on the torrefaction of wood, and the main application of torrefied wood is as a feed stock for combustion or gasification.16,17 The torrefied wood exhibits a relatively faster rate of combustion, releases significantly less smoke during combustion, and increases the quantity of syngas produced with the severity of torrefaction during gasfication compared to the parent wood.18,19 © 2012 American Chemical Society

The three main constituents of biomass, including cellulose, hemicellulose, and lignin, exhibit distinct thermal decomposition characteristics due to the discrepancy of their structures. Most of hemicellulose from biomass components can be removed during torrefaction, and the main condensable volatile products including water, organic acids, and lightweight compounds were produced,20 which implies that torrefaction can be also used as a pretreating method prior to fast pyrolysis to improve the quality of pyrolysis oil through lowering the contents of water and lightweight compounds.21,22 Besides, torrefaction-aided fast pyrolysis can be considered as a stepwise pyrolysis of biomass, through which the major biomass constituents will be selectively degraded into value-added chemicals and the staged products might be less complex and more stable than the product from direct fast pyrolysis where all biomass constituents are decomposed synchronously and at the same temperature.23,24 Torrefaction is influenced by many factors including the chemical properties of biomass and operating conditions such as the treatment temperature, reaction time, and the apparatus used. In previous work, the torrefaction experiments of different kinds of biomass have been performed mostly in the intermittent tube-type reactors,13,20 and changes of the physical properties of torrefied biomass have been well studied. To our knowledge, torrefaction of biomass as a pretreatment method prior to fast pyrolysis in an auger reactor is relatively less researched. Received: June 21, 2012 Revised: October 17, 2012 Published: October 17, 2012 7009

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Table 1. Chemical Component and Ultimate Analyses of Samples (On Dry Basis) chemical component analysis (wt/%)

ultimate analysis (wt/%)

samples

cellulose

hemicellulose

acid insoluble fibers

extractives

C

H

O

N

S

ash

sprucewood bagasse

43.62 46.35

15.98 27.60

38.24 20.95

1.73 3.03

49.4 47.74

5.62 5.6

44.72 45.7

0.02 0.08

0.03 0.06

0.21 0.82

analysis. The noncondensable gas samples were collected in gasbags at intervals of 10 min, and the gas composition was analyzed by a gas chromatography. The gas yield could be calculated on the basis of gas composition and the known flow rate of nitrogen fed to the auger reactor. Each experiment was repeated three times under the same conditions, and the results were found to be reproducible. The overall mass balance of torrefaction experiments was generally in the range 96−99%. 2.3. Gaseous Products Analysis. The noncondensable gas products from torrefaction of biomass samples were analyzed by a gas chromatograph (GC-20B-1, Shmadzu, Japan) equipped with a thermal conductivity detector (TCD) and a GS-Carbon PLOT column. Argon (99.995%) was used as a carrier gas at a flow rate of 30 mL/min. The analysis of gas composition was carried out using mixed standard gas (H2 8.46%, O2 1.04%, N2 74.14%, CH4 2.14%, CO 10.30%, CO2 1.18%, C2H4 1.11%, C2H6 1.06%, C2H2 3.56%, mol %) as external standard. 2.4. Liquid Products Analysis. The condensable liquid products were analyzed by a gas chromatograph (HP 4890, U.S.A.) equipped with a flame ionization detector (FID). The GC instrument was fitted with capillary column (DB-1701, 60 m × 0.25 mm, 0.25 μm film thickness). The carrier gas was the helium of 99.99% with a flow rate of 1 mL/min. The injector temperature was 250 °C, and a split ratio of 1:30 was used. The GC oven was programmed to hold at 45 °C for 5 min, ramp at 5 °C/min to 280 °C, and hold at 280 °C for 5 min. The typical organic components in the liquid products were identified qualitatively using a pure compounds based on matching the retention time. The pure compounds corresponding to the verified peaks in the chromatograms were further used to calibrate the GC column. Calibration curves were established for the pure compounds relating the peak area to the mass, and then, the content of components in the liquid product was calculated using the calibration curves. GC analyses of each liquid product were made three times, and the reported data of quantitated compounds were the average of the three analyses. The water content of liquid products was determined by Karl Fischer titration (787KF Titrino, Metrohm, Switzerland). 2.5. Analysis of the Solid Products. 2.5.1. Chemical Component Analysis. Lignocellulosic composition of torrefied biomass samples was determined by the NREL method for “Determination of Structural Carbohydrates and Lignin in Biomass”.29 According to this method, the extractives in the sample were first removed, and then, the extractives-free sample of 0.15 g was treated with 72% H2SO4 for 1 h. The mixture was hydrolyzed by adding water of 42 mL in an autoclave at 121 °C for 1 h. The hydrolysis solution was then filtered using crucibles to separate the filtrate and residue. The filtrate was used to determine the content of cellulose and hemicellulose, while the residue was used to determine acid-insoluble fibers. The sugars in the filtrate were analyzed by high performance liquid chromatography (HPLC). Conversion factors were used to convert monose concentrations into the contents of carbohydrates in the sample. The residue was dried at 105 °C for 8 h, and the weight of the residue was comprised of ash and acid-insoluble lignin. Ash content of the residue was determined by NREL method for “Determination of Ash in Biomass”. The reported results of the composition analysis were the average of duplicate analysis. 2.5.2. Fourier Transform Infrared (FTIR) Analysis. The structural composition of the original biomass and torrefied bomass samples was investigated by FTIR spectroscopy. About 1 mg of the sample was carefully mixed with 300 mg of dry KBr and pressed into a tablet. The infrared spectra were measured by means of a FTIR spectrometer (TENSOR27, Bruker, Germany) between 4000 and 400 cm−1 with a resolution of 4 cm−1 and 32 scans. For each sample, five spectra were

Recently, there were some new research studies on torrefaction of biomass, which was considered as a pretreatment method of biomass prior to fast pyrolysis.25−27 In our previous work, two-staged biomass pyrolysis consisting of torrefaction and subsequent fast pyrolysis were performed to obtain bio-oil of higher quality. The effects of torrefaction temperature on the yield, chemical composition, and physical properties of bio-oil had been researched.28 However, there is more work about biomass torrefaction as a pretreating method prior to fast pyrolysis to do. The volatile products including gas products and liquid products from biomass torrefaction need to be analyzed, which would help to understand the process of biomass torrefaction. Particularly, the pyrolysis behavior of torrefied biomass and product distribution of fast pyrolysis of torrefied biomass need to be carefully examined. In our present work, the torrefaction of sprucewood and bagasse was conducted at the temperature range 260−300 °C in an auger reactor. The volatiles from torrefaction were analyzed. The main aim was to investigate the effect of torrefaction temperature on the pyrolysis behavior of torrefied biomass by thermogravimetric analysis and yields of important compounds from fast pyrolysis of torrefied biomass by pyrolysis gas chromatography mass spectroscopy (Py-GC/MS) analysis to provide fundamental data for fast pyrolysis of torrefied biomass.

2. METHODS 2.1. Material Preparation. The sprucewood used in this study was purchased from a local sawing mill in Guangzhou, China. The bagasse was collected from a sugar cane plant located in Jiangmen, China. The samples were ground to small particle size and subsequently sieved to a mean particle size of 0.18−0.38 mm. These particles were dried at 105 °C for 8 h in an oven to about 6% moisture content before the torrefaction experiment. The chemical components and ultimate composition of samples were analyzed, and the results are shown in Table 1. 2.2. Torrefaction Experiment. Torrefaction of sprucewood and bagasse was conducted in a stainless steel auger reactor, which was operated continuously. The schematic drawing of the auger reactor was presented in our previous paper. The torrefaction experiments were performed at 260 °C, 280 °C, and 300 °C. Biomass samples were stored in a hopper with a capacity of 6 kg. When the reactor reached the set temprature, the samples were transported to the reacting zone by means of a screw powered by an electric motor at a fixed speed to maintain the residence time of the sample in the reactor pipe for 10 min. The residence time is relatively short to prevent the severe decomposition of biomass samples. A nitrogen gas flow of 7 L min−1 was used as a carrier gas throughout the torrefaction experiment to eliminate the presence of oxygen and remove the volatiles out of the reactor tube, while another small nitrogen gas flow was fed to the hopper, which prevented volatiles and gases from flowing from the reactor to the hopper. The reactor was equipped with a collection bin at the end of the reactor tube to collect the solid product. After the torrefaction treatment, the solid products called as torrifed biomass were cooled and weighed to determine the solid yield. The solid samples were placed in the airtight sample bags and stored in a desiccator for further analysis. The volatile products from the reactor were cooled in two condensers. The condensable liquid products were collected in conical flasks and weighed to determine the liquid yield. The liquid samples were preserved in the fridge at 0 °C for further 7010

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accumulated and averaged. Peak area and height was measured using OPUS software version 6.5. First, a straight line was drawn between the peaks of the two frequency limits defined. The area above this baseline represented the peak area and was integrated automatically by the software. The peak intensity from the top of the peak to the baseline gave peak height value. Peak area and height values for bands assigned to aromatic skeletal of lignin were divided by the values of carbohydrate reference peaks to provide relative changes in the structural composition of sprucewood.30,31 2.5.3. X-ray Diffraction (XRD) Analysis. The overall crystalline of different biomass samples was examined by a X-ray diffractometer (X’Pert Pro MPD, PANalytical B.V., Netherlands) using Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 40 mA. The sample was scanned at a scanning rate of 2°/min from 2θ = 5° to 30° with a step of 0.02°. To investigate the effect of torrefaction on the crystal structure of biomass, crystallinity index (CrI) and crystallite size (L002) were calculated respectively by following formulas 1 and 2.

⎛ I − Iam ⎞ CrI(%) = ⎜ 002 ⎟ × 100 ⎝ I002 ⎠

5 standard solutions of each of these pure compounds with the concentrations of 400, 800, 1200, 2000, and 4000 μg/mL were prepared by dissolving them in the GC-grade acetone. Then, GC analyses of standard solutions with sample size of 1 μL were conducted, and the peak area of each calibration compound was obtained. To established the calibration curve for each standard compound, the peak area of the compound was plotted against the mass of the compound in the 1 μL solution and then linearly regressed (with R2 value ≥0.96). The weight of the compounds in the pyrolysis vapor, which were not calibrated, was estimated by the calibration curves of standard compounds with the nearest retention time on the GC column. The yields of main pyrolysis products that could be detected by GC were calculated based on the dry weight of the original biomass. Three parallel experiments were performed, and the yields of pyrolysis products were the average of three experiments.

3. RESULTS AND DISCUSSION 3.1. Product Distributions of Biomass Torrefaction. Three-phase products consisting of solid products known as torrefied biomass, condensable liquid products and permanent gaseous products were produced from torrefaction of sprucewood and bagasse in the auger reactor. The effect of torrefaction temperature on the yields of products is shown in Figure 1. Due to the increasing thermal decomposition of

(1)

where CrI is the crystallinity index, I002 is the maximum intensity of the diffraction of (002) plane at 2θ = 22.5◦, which represents both crystalline and amorphous intensity, while Iam is only the intensity of amorphous region at 2θ = 18.7°.32

L002 =

κλ β cos θ

(2)

where L002 is the crystallite size, κ is Scherrer constant (0.90), λ is Xray wavelength (λ = 0.15406 nm), β is the full width at halfmaximum(fwhm) of (002) peak, and θ is diffraction angle of (002) plane.33 2.5.4. Thermogravimetric Analysis. The thermogravimetric experiment of different biomass samples was performed by using a differential thermal analyzer (STA449F3-Jupiter, Netzsch, Germany). In all experiments, about 5 mg sample was heated from 35 to 800 °C at a constant heating rate of 10 °C/min, and a pure nitrogen gas flow of 20 L/min was used as the purge gas. 2.5.5. Py-GC/MS Analysis. The original biomass and torrefied biomass samples were pyrolyzed at 500 °C under a helium atmosphere using a single shot micropyrolyzer (PY-2020iS, Frontier laboratories Ltd., Japan) coupled to a gas chromatograph (6890N, Agilent Technologies, U.S.A.) with a mass spectrometer (5975C, Agilent Technologies, U.S.A.). The mircopyrolysis unit was composed of a sampler, a microquartz tube that can be preheated to a target temperature with a furnace, interface, and a deactiveated needle inserted into the GC injector. The temperature shown in the furnace was the centerline temperature of the quartz tube where the sample was pyrolyzed. In each experiment, a sample of approximately 0.2 mg was weighed using a microbalance with an accuracy of 0.001 mg (M2P, Sartorius, Germany) and placed in a stainless steel sample cup. Then, the loaded sample cup fell into the preheated furnace by gravity, and the sample was heated quickly to desired temperature at a ramp rate of 2000 °C/s with a dwell time of 10 s. The pyrolysis vapor produced was directly swept to the injection port of the GC instrument using helium as the carrier gas. A capillary column (DB-5MS, 30 m × 0.25 mm, 0.25 μm film thickness) with a carrier gas flow velocity 40 cm/s was used for the chromatographic separation of pyrolysis products. The injector temperature was 300 °C, and a split ratio 1:50 was used. The GC oven temperature program was 3 min at 50 °C, 10 °C/min to 250 °C, and then 5 °C/min to 300 °C, with a dwell time of 10 min. The mass spectrometer was operated in electron impact mode at 70 eV. The mass range from m/z 35 to 600 was scanned. The identification of the main peaks was made from NIST05 MS library and previously published literature. The 21 pure compounds corresponding to the proposed assignments for the peaks in the initial Py-GC/MS analysis were used to confirm the peak identification based on matching the retention time and mass spectrum. The pure compounds were purchased from Sigma-Aldrich and were further used to calibrate the GC column. The

Figure 1. Effect of torrefaction temperature on the product distributions of biomass torrefaction.

sprucewood and bagasse, the yields of solid products decrease, while the yields of condensable liquid products and permanent gaseous products increase with the increase in torrefaction temperature. However, in comparison to sprucewood, the bagasse give a relatively lower solid product yield in torrefaction at the same temperature. This indicates that the degree of devolatilization for bagasse is higher than that for sprucewood at the same treatment conditions of torrefaction. The discrepancy in weight loss for sprucewood and bagasse during torrefaction mainly results from the different chemical compositions. The hemicellulose, which is relatively more reactive than other components in the biomass, contributes to the main weight loss during the biomass torrefaction. The content of hemicellulose in bagasse sample is more higher than that in sprucewood sample as can be seen in Table 1. This is easy to explain the more weight loss for bagasse in torrefaction. Similar conclusions also were obtained in the torrefaction experiment in an intermittent tube-type reactor conducted by Chen et al.13 7011

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3.2. Composition of Gaseous Products. The effect of torrefaction temperature on the composition of gaseous products is depicted in Figure 2. Permanent gas products

Table 2. Quantitative Analysis of Main Compounds in the Liquid Product from Sprucewood and Bagasse Torrefaction contents (wt %) sprucewood compound water acetic acid propionic acid methanol 1-hydroxy-2propanone furfural furfuryl alcohol phenol guaiacol eugenol isoeugenol vanillin

bagasse

260 °C 280 °C 300 °C 260 °C 280 °C 300 °C 80 4.96 0.14 4.22 0.85

76 6.06 0.20 4.31 1.34

67 10.17 0.25 4.66 2.91

57 27.31 0.68 3.04 1.37

51 32.90 0.98 3.12 2.01

47 29.99 0.71 2.67 2.41

0.91 0.62 0.01 0.11 0.04 0.10 0.16

0.98 0.85 0.02 0.19 0.09 0.27 0.25

1.26 1.31 0.02 0.41 0.19 0.56 0.37

2.21 0.71 0.13 0.16 0.13 0.16 0.15

2.99 1.04 0.22 0.26 0.21 0.28 0.18

2.73 1.56 0.27 0.32 0.32 0.30 0.18

obviously seen from Table 2 that the contents of acetic acid in liquid products from bagasse torrefaction is higher than that in liquid products from sprucewood torrefaction. This indicates that there are more acetyl groups in the hemicellulose structure of bagasse. A small quantity of phenols can be observed in the liquid products. The cleavages of thermally unstable ether bonds including β−O−4 linkages in the lignin structure of sprucewood contribute to the formation of methoxyl phenols such as guaiacol, eugenol, isoeugenol, and vanillin.37,38 For the reason that bond cleavages are enhanced with rising temperature, the contents of methoxyl phenols in the liquid product increase. 3.4. Characterization of the Torrefied Biomass. 3.4.1. Chemical Component Analysis. The torrefied sprucewoods obtained from torrefaction at 260 °C, 280 °C, and 300 °C are respectively named TS-260, TS-280, and TS-300, and the torrefied bagasses obtained from torrefaction at 260 °C, 280 °C, and 300 °C are respectively named TB-260, TB-280, and TB-300. The results of the chemical component analysis of torrefied biomass are presented in Table 3. To investigate the degree of decomposition of chemical component in biomass, a recovery rate of chemical component in torrefied biomass is defined and calculated using eq 3.

Figure 2. Composition of gaseous products formed in biomass torrefaction at the different temperatures.

obtained from the torrefaction of sprucewood and bagasse mainly contain carbon dioxide and carbon monoxide, together with trace hydrogen and methane. The formation of carbon dioxide mainly is attributed to the decarboxylation reaction of unstable carboxyl group in the hemicellulose structure of sprucewood and bagasse,20,34 while the formation of carbon monoxide may be explained by the secondary reactions of carbon dioxide and steam with porous char and the decarbonylation of low molecular weight carbonyl compounds formed from torrefaction with increasing temperature.35,36 Owing to the secondary reactions, carbon monoxide is more aggressively formed with the increase in temperature, which results in the rise in the mole fraction of carbon monoxide, while the decline in mole fraction of carbon dioxide in the gaseous product as can be seen in Figure 2. 3.3. Composition of Liquid Products. The contents of main compounds in the liquid products derived from sprucewood and bagasse torrefaction at different temperatures are summarized in Table 2. Water is the main liquid product of biomass torrefaction. It implies that the dehydration reaction is prevalent during biomass torrefaction. However, more organic compounds are formed due to the higher degree of decomposition of biomass when raising the torrefaction temperature; thus, the water content in liquid products reduces with the increasing temperature, as shown in Table 2. The water content in the liquid products from sprucewood torrefaction is higher than that in the liquid products from bagasse torrefaction. This indicates that the larger proportion of water is removed from sprucewood in torrefaction. The organic components including acids, alcohols, ketones, and furans in liquid products are mainly originated from decomposition of the hemicellulose in sprucewood and bagasse. Acetic acid and methanol are the main organic compounds in the liquid products. The formation of acetic acid is mainly attributed to the acetyl groups in the hemicellulose structure, while methanol is formed from the elimination reaction of the methoxyl groups on the 4-O-methylglucuronic acid unit in the hemicellulose structure and the methoxyl groups of lignin.34 It can be

recovery rate = (chemical component content in the torrefied biomass/chemical component content in original biomass) × torrefied biomass yield

(3)

The decline of hemicellulose content and recovery rate in torrefied biomass with the increase in temperature can be seen in Table 3. This confirms that hemicellulose is significantly decomposed during biomass torrefaction. The hemicellulose recovery rate in torrefied bagasse is lower than that in torrefied sprucewood, which indicates that the hemicellulose in bagasse is more reactive and has a higher degree of degradation. The other components will be also decomposed more or less, and thus the cellulose recovery rate in torrefied biomass decreases when raising the torrefaction temperature, as shown in Table 3. However, the recovery rate of acid insoluble fibers (mainly lignin component) in torrefied bagasse increases with temperature and is above 100% in TB-280 and TB-300. This implies 7012

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Table 3. Chemical Component Analysis of Torrefied Biomass (On Dry Basis) component analysis (wt/%)a

a

chemical component

TR-260

TR-280

TR-300

TB-260

TB-280

TB-300

cellulose hemicellulose acid insoluble fibers extractives

43.67 (94.63) 13.37 (79.08) 39.48 (97.58) 1.23

43.40 (91.73) 11.86 (68.43) 40.21 (96.85) 0.75

43.19 (87.80) 8.86 (49.16) 40.93 (94.91) 0.13

46.79 (94.71) 23.17 (78.76) 21.85 (97.85) 2.16

50.77 (95.37) 18.58 (58.61) 25.58 (106.31) 1.17

46.64 (79.70) 8.72 (25.02) 36.90 (139.50) 0.53

Values in the parentheses are the recovery rate of chemical components in torrefied biomass.

Figure 3. FTIR spectra of original and torrefied bioimass.

spectra, 1249 cm−1 (6′) for syringyl ring and C−O stretch in lignin and xylan in bagasse spectra, 1160/1166 cm−1 (7) for C− O−C vibration in cellulose and hemicellulose, and 897/898 cm−1 (8) for C−H deformation in cellulose. The structural discrepancy in sprucewood and bagasse can be seen from Figure 3. There are lower relative intensity of band at 1738 cm−1 and higher relative intensity of band at 1511 cm−1 in sprucewood spectra owing to lower content of xylan containing acetyl groups and higher lignin content in sprucewood than that in bagasse. There are some structural changes in the torrefied biomass compared to the raw biomass due to torrefaction treatment. The intensity of peak at 1635/1633 cm−1 (2) reduces with torrefaction temperature as shown in Figure 3, which is indicative of the decrease of absorbed water content in torrefied biomass. The evident decline in relative intensity of band at 1249 cm−1 (6′) can be seen from the torrefied bagasse spectra, which probably is caused by the decomposition of xylan in bagasse during torrefaction. The ratios of the intensity of band at 1511/1514 cm−1 (I4) assigned to aromatic skeletal with peaks at 1738/1735 (I1), 1375/1378 (I5), 1160/1166 (I7), and 897/898 cm−1 (I8) associated to carbohydrate are presented in Table 4 to exemplify relative

that more acid insoluble fibers are formed with increasing temperature. The formation of acid insoluble fibers can be partly explained by the cross-linking and carbonization of carbohydrates in bagasse torrefaction.39 While the recovery rate of acid insoluble fibers in torrefied sprucewood decreases with temperature, it seems that the cross-linking and carbonization of carbohydrates are not obvious in sprucewood torrefaction. 3.4.2. Structural Composition Analysis. The changes of structural composition of biomass in torrefaction can be inspected by the FTIR analysis. The FTIR spectra of original and torrefied biomass are presented in Figure 3, and the spectra are normalized to the highest peak in the fingerprint region between 2000 and 400 cm−1. To describe some important structural alterations, some well-defined peaks are tagged in the spectra and assigned to structural components as follows:30,40 1738/1735 cm−1 (1) for unconjugated CO valence vibration of xylans (hemicellulose), 1635/1633 cm−1 (2) for absorbed water, 1602/1605 cm−1 (3) and 1511/1514 cm−1 (4) for aromatic skeletal vibration in lignin, 1375/1378 cm−1 (5) for C−H deformation in cellulose and hemicellulose, 1270 cm−1 (6) for guaiacyl ring breathing, C−O stretch in lignin and C−O linkage in guiacyl aromatic methoxyl groups in sprucewood 7013

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Table 4. Ratios of the Intensity of the Lignin-Associated Band with Carbohydrate Bands for Original and Torrefied Biomass relative intensity of aromatic skeletal band against typical band for carbohydratesa ratio I4/I1 I4/I5 I4/I7 I4/I8 a

sprucewood 1.638 1.825 1.688 4.046

(2.893) (1.884) (1.620) (6.231)

TS-260 1.955 2.178 1.708 4.512

(3.469) (2.265) (1.657) (7.400)

TS-280 2.353 (4.459) 2.437 (2.946) 1.729 (1.650) 5.69 (8.684)

TS-300 2.438 2.460 1.774 5.814

(4.475) (3.098) (1.629) (9.500)

bagasse 0.318 0.820 0.823 2.672

(0.952) (0.606) (0.976) (2.353)

PR-260

PR-280

0.403 (1.063) 1.280 (1.288) 0.89 (1.015) 2.971 (2.792)

0.412 1.532 0.924 4.361

(1.246) (1.775) (1.076) (4.437)

PR-300 0.484 1.758 1.059 5.804

(1.397) (2.212) (1.259) (9.125)

Relative intensity is calculated using peak areas (not in parentheses) and peak heights (in parentheses).

Table 5. Crystalline Parameters of the Original and Torrefied Biomass param.

sprucewood

TS-260

TS-280

TS-300

bagasse

TB-260

TB-280

TB-300

CrI (%) crystalline size (nm)

50.4 4.60

56.1 4.60

56.4 5.36

53.5 5.36

47.8 3.28

50.6 3.52

50.6 3.65

50.5 4.10

changes in the structural composition. As a result of removal of many acetyl groups in hemicellulose by torrefaction treatment, there is a increase of relative intensity of aromatic skeletal vibration against CO vibration as shown by the increase of values of I4/I1. The relative intensity of aromatic skeletal vibration against C−H deformation in cellulose and hemicellulose and the relative intensity of aromatic skeletal vibration against C−O−C vibration in cellulose and hemicellulose both increase as shown by the increase of values of I4/I5 and I4/I7, which suggests that the depolymerization and thermal decomposition of carbohydrate (mainly hemicellulose) predominate over lignin decomposition and the cross-linking of cellulose in sprucewood and bagasse torrefaction. In addition, the value of I4/I8 for relative intensity of aromatic skeletal against C−H deformation in cellulose increases, which probably means that intramolecular dehydration reaction takes place in cellulose during torrefaction. 3.4.3. Crystalline Analysis. Among biomass components, only cellulose is crystalline, while hemicellulose and lignin are both amorphous.41 Crystalline changes of biomass will occur when biomass undergo torrefaction treatment. As a useful crystalline analysis method, the X-ray diffraction is used to evaluate the crystalline of torrefied biomass. The CrI index and microcrystalline size of the original and torrefied biomass are listed in Table 5. The CrI index of sprucewood increases from 50.4% to 56.4% at 280 °C and then drops to 53.5% at 300 °C during torrefaction. Similarly, the CrI index of bagasse increases from 47.8% to 50.6% at 280 °C and then slightly drops to 50.5% at 300 °C. The increases of crystallinity of torrefied biomass obtained below 300 °C are attributed to the degradation of amorphous hemicellulose and amorphous regions of cellulose.31 However, the severe decomposition of crystalline cellulose in biomass may occur at 300 °C, which causes the decrease of crystallinity of torrefied biomass obtained at 300 °C. From Table 5, it can be seen that crystalline size of torrefied biomass increase with temperature. This can be explained by that some small and imperfect cellulose microcrystallite is decomposed and relatively large and perfect cellulose microcrystallite is left in torrefied biomass.33 3.4.4. Thermogravimetric Analysis. The pyrolysis behaviors of a wide temperature range of raw and torrefied biomass are investigated by thermogravimetric analysis, weight loss (thermogravimetry, TG), and weight loss rate (differential thermogravimetry, DTG) curves are presented in Figure 4, and the characteristic parameters of the devolatilization are summarized in Table 6. From the DTG curve for sprucewood (Figure 4a), two clear weight loss region can be seen. The first

Figure 4. TG and DTG curves for raw and torrefied biomass at 10 °C/ min in nitrogen atmosphere.

small peak below 100 °C is induced by the evaporation of moisture in samples. Subsequently, main thermal degradation takes place, and the onset temperature To corresponding to a weight loss of 3% respect to the sample weight is around 251 °C. A prominent peak corresponding to maximum weight loss rate (maximum DTG peak) appears around 365 °C owing to the main decomposition of cellulose.31,41 Different from sprucewood decomposition, main thermal degradation of 7014

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Table 6. Characteristics of Devolatilization for Raw and Original Biomass param.

sprucewood

TS-260

TS-280

TS-300

bagasse

TB-260

TB-280

TB-300

To (°C) Tmax (°C) DTGmax (%/min) residue (%)

251 365 10.02 19.08

260 367 10.83 19.24

274 365 10.67 23.09

283 366 10.32 24.52

241 346 11.45 17.80

250 344 12.25 22.37

253 341 12.25 24.42

253 343 11.27 31.98

Table 7. Quantitative Analysis of Main Compounds from Pyrolysis of Raw and Trorrefied Biomass yields (%) compound

spruce

TS-260

TS-280

TS-300

bagasse

TB-260

TB-280

TB-300

acetic acid 1,2-ethanediol, monoacetate propanoic acid, 2-oxo-, methyl ester total of esters hydroxyacetaldehyde 3-hydroxybutanal 2-methyl-2-butenal butanedial total of aldehydes 1-hydroxy-2-propanone 1,2-cyclopentanedione 2-hydroxy-3-methyl-2-cyclopenten-1-one 3-methyl-1,2-cyclopentanedione total of ketones furfural 5-methyl-2(5H)-furanone, 2(5H)-furanone 2-furanmethanol 3-methyl-2,4(3H,5H)-furandione 5-(hydroxymethyl)furfural total of furans phenol 4-methylphenol guaiacol 4-ethylphenol 4-methylguaiacol 2,3-dihydrobenzofuran 1,2-benzenediol 4-ethylguaiacol 4-vinylguaiacol syringol eugenol vanillin vanillic acid isoeugenol 4-propylguaiacol acetovanillone guaiacylacetone 3,5-dimethoxyacetophenone syringaldehyde 4-allylsyringol syringacetone coniferyl aldehyde coniferyl alcohol Total of phenols 1,4:3,6-dianhydro-α-D-glucopyranose levoglucosan total of anhydrosugars

1.69 0.85 0.83 1.68 0.49 0.13 0.13 0.60 1.34 0.94 0.41 0.15

1.44 0.76 0.93 1.69 0.49 0.14 0.07 0.63 1.33 1.11 0.51 0.16

1.36 0.81 1.11 1.92 0.62 0.09 0.15 0.55 1.41 0.89 0.37 0.15

1.03 0.55 0.70 1.25 0.30 0.08 0.04 0.43 0.85 0.72 0.34 0.13

1.77 0.34 0.18 0.35 0.11 0.13 0.10 1.21 0.11 0.12 0.76

1.42 0.29 0.20 0.31 0.13 0.14 0.20 1.28 0.10 0.09 0.71

1.19 0.23 0.14 0.24 0.08 0.08 0.12 0.89 0.11 0.13 0.69

0.43

0.62

0.60

0.78

0.11 0.11 0.87 0.01 0.13 0.39 0.29 0.45 0.23 0.44 0.23

0.18 0.13 0.87 0.01 0.15 0.44 0.29 0.51 0.19 0.34 0.31

0.22 0.12 0.77 0.02 0.12 0.33 0.31 0.37 0.20 0.25 0.28

0.30 0.17 0.53 0.05 0.06 0.17 0.16 0.20 0.13 0.12 0.19

6.05 1.84 1.33 3.17 0.60 0.13 0.14 0.87 1.74 1.32 0.62 0.08 0.17 2.19 0.62 0.06 0.48 0.21 0.23 0.09 1.69 0.22 0.10 0.26 0.06 0.17 1.30 0.07 0.04 0.56 0.46 0.02 0.06 0.17 0.08 0.03

4.77 1.47 1.13 2.60 0.57 0.12 0.12 0.88 1.69 1.13 0.56 0.08 0.15 1.93 0.49 0.05 0.49 0.18 0.17 0.10 1.49 0.21 0.12 0.25 0.06 0.15 0.85 0.11 0.05 0.43 0.36 0.02 0.05 0.15 0.07 0.05

2.88 1.12 1.02 2.14 0.41 0.11

1.50 0.34 0.18 0.31 0.11 0.10 0.16 1.19 0.10 0.07 0.72

7.95 2.32 1.44 3.76 0.77 0.17 0.16 1.04 2.14 1.50 0.66 0.16 0.17 2.49 0.66 0.09 0.50 0.21 0.23 0.22 1.91 0.15 0.03 0.26 0.03 0.16 1.93 0.10 0.04 0.77 0.45 0.03 0.10 0.12 0.12 0.05

0.29 0.11 0.30 0.05 0.10 0.09 5.24 0.13 3.16 3.29

0.30 0.14 0.26 0.06 0.09 0.04 4.46 0.28 5.31 5.59

0.21 0.09 0.18 0.04 0.05

0.19 0.10 0.15 0.04 0.02

3.50 0.25 5.00 5.24

3.15 0.29 4.80 5.10

0.62 1.41 6.61 0.13 1.46 1.59

0.59 1.11 6.71 0.19 2.67 2.86

0.50 1.01 5.98 0.23 2.63 2.86

0.12 0.13 4.05 0.21 3.41 3.62

0.75 1.27 1.04 0.54 0.05 0.19 1.82 0.34 0.05 0.42 0.16 0.14 0.07 1.18 0.24 0.10 0.29 0.05 0.20 0.55 0.18 0.08 0.30 0.39 0.02 0.02 0.16 0.07

formed in the DTG curve around 290 °C due to decomposition of hemicellulose in bagasse, and then, the main degradation of

bagasse takes place as a two-step process as shown in DTG curve for bagasse (Figure 4b). In the first step, a shoulder is 7015

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pyrolysis is lower than that in original biomass pyrolysis and obviously decreases with the increase in torrefaction temperature, as shown in Table 7. This will be favorable for obtaining pyrolysis oil with low acid content. It can be learned though analyzing the composition of torrefaction liquid products that water and a few lightweight organic compounds are formed in biomass torrefaction. As a result, the yields of lightweight compounds in pyrolysis of torrefied biomass are lower than raw biomass pyrolysis, as shown by the lower yields of most of esters, aldehydes, ketones, furans, and phenols in pyrolysis of TS-300 and all torrefied bagasses. Removing lightweight compounds during torrefaction prior to fast pyrolysis may lower the contents of lightweight compounds in pyrolysis oil and should produce a more stable, higher quality pyrolysis oil.43,44 However, in comparison to bagasse, the larger proportion of water is removed from sprucewood in torrefaction, as shown by higher water content in liquid product from sprucewood torrefaction; therefore, the yields of most of lightweight organic compounds including esters, aldehydes, ketones, furans, and phenols in pyrolysis of TS260 or T-280 are oppositely higher than raw sprucewood pyrolysis, as shown in Table 7. In previous studies, it is known that the main product in the pure cellulose pyrolysis is levoglucosan,31 but probably only a little or no levoglucosan is formed in biomass pyrolysis due to the complexity of biomass structure and the interactions of cellulose, hemicellulose and lignin.31,42,45 A positive influence of torrefaction treatment on the formation of anhydrosugars (mainly levoglucosan) can be seen from Table 7. Compared to raw biomass pyrolysis, the yields of levoglucosan in torrefied biomass pyrolysis are obviously higher. In biomass torrefaction, the integral biomass structure is broken; interactions of cellulose, hemicellulose and lignin in torrefied biomass may be weakened, as mentioned in thermogravimetric analysis. Thus, more cellulose may tend to form levgolucosan in pyrolysis of torrefied biomass although some structural changes of cellulose take place owing to the thermal decomposition and cross-linking of cellulose in biomass torrefaction.

cellulose occurs and maximum DTG peak appears around 346 °C. However, no shoulder appears in DTG curve for sprucewood, which can be explained by the low hemicellulose content in sprucewood. Compared to DTG curve for bagasse, the values of To and Tmax (temperature for the maximum weight loss rate) in DTG curve for sprucewood are higher, as shown in Table 6, which is indicative of higher thermal stability of sprucewood. The devolatilization characteristics of biomass have some slight changes through torrefaction treatment. First, the water loss peak in DTG curve of torrefied biomass is less pronounced due to the release of absorbed water in biomass torrefaction. It can be known that the some volatiles are removed from biomass by torrefaction treatment. As a result, the To values in DTG curve for torrefied biomass are enhanced, and the residual weight at 800 °C of torrefied samples increases with torrefaction temperature, as shown in Table 6. The DTGmax values (maximum weight loss rate) in DTG curve for all of torrefied sprucewood are higher than raw sprucewood, Similarly, the DTGmax values for TB-260 and TB-280 are higher than raw bagasse. The formation of maximum weight loss peak is mainly assigned to decomposition of cellulose, and the increase of DTGmax for torrefied biomass indicates that decomposition of cellulose in biomass is accelerated by torrefaction treatment in which the integrity of biomass is probably disturbed and linkages between main chemical components are weakened.42 However, significant degradation of cellulose in biomass torrefaction at more severe conditions can take place as shown by the decrease of DTGmax values for TB-300. Owing to the decomposition of hemicellulose in biomass torrefaction, the hemicelluose content in torrefied bagasse declines; thus, the shoulder attributed to hemicellulose degradation is less pronounced in the DTG curve for TB-260 and almost disappears in the DTG curves for TB-280 and TB300. 3.4.5. Analysis of Pyrolyis Products. The pyrolysis products (mainly lightweight compounds) of raw and torrefied biomass are analyzed by Py-GC/MS, and typical compounds with relatively high abundance are identified. The compound yield is calculated based on the sample amount used in the Py-GC/MS experiments. The quantitative analysis results are listed in Table 7. From Table 7, it can be seen that compounds detected by GC mainly contain acetic acid, esters, aldehydes, ketones, furans, phenols, and anhydrosugars. Because of disparities between chemical compositions of sprucewood and bagasse, there are some differences in pyrolysis product distributions of them. Owing to the existence of more acetyl groups in hemicellulose structure of bagasse, much more acetic acids are formed in bagasse pyrolyis as compared to sprucewood pyrolysis, and the yield reaches 7.47%. There is a higher content of lignin in sprucewood than that in bagasse; thus, the higher yields of phenols are obtained in pyrolysis of sprucewood, while higher yields of esters, aldehydes, ketones, and furans are attained in bagasse pyrolysis due to higher hemcellulose content in bagasse. The main structures for lignin in sprucewood as a typical softwood are guaiacyl units, and therefore, most of phenols from sprucewood pyrolysis are the guaiacyl phenols. However, some syringyl units may exist in lignin structure of bagasse, and thus, quite a few syringyl phenols are formed in bagasse pyrolysis. As in the previous analysis, a considerable number of acetyl groups in hemicellulose structure of biomass are removed in torrefaction; thus, the yield of acetic acid in torrefied biomass

4. CONCLUSION In biomass torrefaction, product distribution was significantly influenced by the treatment temperature and chemical composition of biomass. The torrefied biomass yields decreased with the increase in torrefaction temperature, and there were lower torrefied bagasse yields in bagasse torrefaction as compared to sprucewood torrefaction at same temperature due to higher content of hemicellulose in bagasse. A number of water and lightweight organic compounds were removed from biomass in torrefaction. From chemical component analysis, it was found that more acid insoluble fibers were formed in torrefied bagasses obtained at 280 and 300 °C, which suggested cross-linking and carbonization of carbohydrates probably took place in bagasse torrefaction. FTIR analysis indicated that thermal decomposition of carbohydrate (mainly hemicellulose) predominated over lignin decomposition and the cross-linking of cellulose in torrefaction as shown by the increases of relative intensity of aromatic skeletal band against typical band for carbohydrates. The degradation of amorphous hemicellulose and amorphous regions of cellulose resulted in increases of crystallinity of torrefied biomass obtained below 300 °C. It could be concluded from thermogravimetric analysis that the decomposition of cellulose in torrefied biomass can be 7016

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accelerated by torrefaction treatment, and the shoulder attributed to hemicellulose degradation is less pronounced or almost disappears in the DTG curve for torrefied bagasse. Compared to pyrolysis of raw biomass, the yields of acetic acid and other lightweight compounds were lower in pyrolysis of torrefied sprucewood obtained at 300 °C and all torrefied bagasses, while the yields of levoglucosan in torrefied biomass pyrolysis were obviously higher. Therefore, more stable pyrolysis oil with higher content of levoglucosan may be obtained from torrefied biomass.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support of the National High Technology Research and Development Program of China (863 Program) (No. 2007AA05Z456).

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