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Waste biomass to liquids: Low temperature conversion of sugarcane bagasse to bio-oil. The effect of combined hydrolysis treatments
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 2 1 0 6 e2 1 1 6
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Waste biomass to liquids: Low temperature conversion of sugarcane bagasse to bio-oil. The effect of combined hydrolysis treatments Josilaine A. Cunha a, Marcelo M. Pereira a, Ligia M.M. Valente a, Pilar Ramı´rez de la Piscina b, Narcı´s Homs b,c,*, Margareth Rose L. Santos a,** a
Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Instituto de Quı´mica, Departamento de Quı´mica Inorgaˆnica, Cidade Universita´ria, Rio de Janeiro, 21949-900 RJ, Brazil b Departament de Quı´mica Inorga`nica and Institut de Nanocie`ncia i Nanotecnologia, Universitat de Barcelona, C/Martı´ i Franque´s 1-11, 08028 Barcelona, Spain c Catalonia Institute for Energy Research (IREC), C/Jardins de les Dones de Negre 1, 08930 Barcelona, Spain
article info
abstract
Article history:
This article describes the influence of different sugarcane bagasse hydrolysis pretreatments
Received 10 March 2010
on modifications to biomass feedstock and the characteristics of the resultant pyrolysis
Received in revised form
products. Sugarcane bagasse was pretreated with acid, alkaline or sequential acid/alkaline
9 February 2011
solutions and pretreated samples were then subjected to a low temperature conversion (LTC)
Accepted 10 February 2011
process under He or O2/He atmospheres at 350e450
C. Both pretreated samples and
sugarcane bagasse in natura were analyzed by determination of their chemical composition and by thermogravimetric, FTIR and SEM analyses. The gases yielded during LTC were Keywords:
monitored on-line by quadrupole mass spectrometry, and the liquid fractions obtained were
Biomass
characterized by FTIR and 1H and
Sugarcane bagasse
ment applied, the main bio-oil component obtained was levoglucosan. However, the LTC
Pyrolysis
yield of bio-oil depended on the hydrolysis treatment of the biomass and decreased in the
Bio-oil
presence of O2. The acid hydrolysis pretreatment increased the LTC bio-oil yield notably.
LTC-pyrolysis
13
C NMR. Irrespective of the sugarcane bagasse pretreat-
ª 2011 Elsevier Ltd. All rights reserved.
Biofuels
1.
Introduction
The demand for energy is growing at a rapid rate due to an increase in both world population and industrialization. There is a recognized need, therefore, to move toward sustainable energy production in order to reduce greenhouse gas emissions and fossil fuel dependence [1e3]. In this context, biomass has been shown to be a potential source of renewable energy. Thus, both developing and
industrialized countries are now seeking new technologies which can efficiently transform biomass resources into alternative fuels [4]. The use of agricultural waste or agricultural residues for these purposes, in principle, does not add carbon dioxide to the atmosphere, in contrast to the use of fossil fuels [5e7]. Moreover, second-generation biofuels will not compete with food crops, since the raw material they use is the crop wastes that would otherwise be discarded [8e10].
* Corresponding author. Departament de Quı´mica Inorga`nica and Institut de Nanocie`ncia i Nanotecnologia, Universitat de Barcelona, C/Martı´ i Franque´s 1-11, 08028 Barcelona, Spain. ** Corresponding author. E-mail addresses: [email protected] (N. Homs), [email protected] (M.R.L. Santos). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.02.019
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Biomass can be transformed using biochemical methods (such as alcoholic fermentation) and thermochemical methods (such as direct combustion, pyrolysis or gasification) [11,12]. Pyrolysis of biomass has been employed to produce organic intermediates (methane and acetic acid), charcoal and fuel gas [13]. Many studies devoted to the thermal decomposition of lignocellulosic materials have been reported in the last two decades, due to the production of activated carbons from solid char derived from agricultural waste pyrolysis [14,15]. Nowadays, pyrolytic processes can be refined to obtain char, oil and/or gas, depending on the temperature and reaction time [16e18]. In this context, the Low Temperature Conversion (LTC) process, initially developed by Bayer et al. [19], involves pyrolysis at around 400 C. The aim of this process is to maximize the yield of liquid products having high-heating power (bio-oil) [20]. The pyrolysis treatment at temperature higher than 400 C can modify the bio-oil composition leading to undesirable products as polycyclic aromatic hydrocarbons [21]. On the other hand, the distribution of pyrolysis products can be modified by the presence of catalysts and/or by the chemical pretreatment of the biomass [21,22]. It is now well recognized that the chemical pretreatment of lignocellulosic materials can remove extractives, hemicellulose, and lignin, reduce the crystallinity of cellulose, and increase the porosity of material [23,24]. Thus, the yield and composition of ulterior bio-oil obtained by pyrolysis could be modified [25]. Sugarcane has historically played an important role in the Brazilian economy [26,27]. In recent years, special Brazilian governmental programs have led to a significant increase in the crop areas devoted to sugarcane and to an improvement in the sugarcane yield per hectare in order to meet the demand for ethanol as fuel. According to the Brazilian Institute of Geography and Statistic, this rise in sugarcane production generated about 160 million tons of bagasse in 2008 [28]. In this respect, sugarcane bagasse constitutes a typical example of an agricultural byproduct that is abundantly available worldwide [29e31]. Sugarcane bagasse typically has a cellulose:hemicellulose:lignin ratio of around 40:35:15. This lignocellulosic content can be hydrolyzed to liberate the lignin and depolymerize the polysaccharides [32,33]. Polysaccharides and lignin are bound through reactive ether and ester links [34,35] which may be hydrolyzed at mild temperatures in the presence of acids or bases. All these processes involve complex carbohydrate and lignin reactions [36]. We report here the influence of the acidic or alkaline nature of the sugarcane bagasse hydrolysis pretreatment on solid residue composition and on that of the bio-oil yielded after the LTC-pyrolysis process carried out at 350e450 C. We focused on the characteristics of the bio-oil liquid fraction due to its potential future interest for second-generation bio-fuel production.
2.
Materials and experimental procedures
Sugarcane bagasse in natura was sundried, ground and sieved. Only particles in the 20e80 mesh range were retained for analysis and experiments; the corresponding samples of bagasse in natura used were labeled BC. For comparative
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purposes, a sample of BC was washed with water at room temperature and then dried at 105 C until constant weight; the resulting sample was labeled BCW. Chemical composition of samples (C, H, N, %wt/wt) was determined using a Perkin Elmer 2400 CHN model and the oxygen content (%wt/wt) was then calculated by subtraction. Thermogravimetric and derivative thermogravimetric analysis (TG/DTG) data were recorded using a Universal TA 2060 apparatus. A heating rate of 20 C min1 and a N2 flow of 100 mL min1 were used. The infrared spectra were obtained with a Nicolet Magna 760 spectrometer at 4 cm1 of resolution. A Hitachi H-2300BSE microscope was used for the scanning electron microscopy study of the materials. 1 H NMR and 13C NMR were recorded using a Bruker 300 spectrometer (300 MHz for 1H and 75 MHz for 13C) and a Bruker 200 (200 MHz for 1H). Spectra were recorded at 25 C using acetone-d6, dH ¼ 2.04 ppm and dC ¼ 206.58 ppm as solvent signals references.
2.1.
The hydrolysis experiments
BC hydrolysis was carried out under acidic, basic or sequential acid/base treatments at 25 C or 122 C at ambient pressure for 1 h. For the hydrolysis treatments, BC samples (2.0 g) were treated with 2 M HCl (25 mL) or 0.5 M NaOH (25 mL) solutions at 25 C (samples BCA1 and BCB1 respectively), 122 C (BCA2 and BCB2 samples respectively) or under ultrasonic irradiation (BCA3 and BCB3 samples). In the sequential acid/base treatment, BC samples were first hydrolyzed under acidic conditions and then treated with the alkaline solution; the corresponding samples treated at 25 C, 122 C or under ultrasonic irradiation at 25 C were BCS1, BCS2, and BCS3, respectively. In all cases, the resulting samples were filtered off and washed with distilled water until the wash-water remained neutral. The samples were oven dried at 105 C until constant weight, their chemical composition was then determined and finally, they were analyzed by FTIR, TG/DTG and SEM.
2.2.
The pyrolysis experiments
The LTC-pyrolysis experiments were performed in an apparatus designed for this purpose using about 0.1 g of BC or hydrolyzed samples. The main element of this device was a tubular reactor inserted vertically into an electrically heated tubular furnace; the temperature was controlled inside the sample bed by a NieCr thermocouple. In all cases, gaseous, liquid and solid fractions were formed. The gases produced were analyzed on-line by a mass spectrometer (MS) with a quadrupole analyzer MKS model e-vision. The liquid fraction was condensed at the reactor outlet, extracted with acetone and then the solvent was removed at low pressure and the residue analyzed by FTIR, 1H and 13C NMR. The LTC-pyrolysis experiments were carried out in three series; in all cases the samples were heated at a rate of 10 C min1 and the final pyrolysis temperature was maintained for 15 min. All experiments were performed in duplicate. The first series of experiments was carried out to
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determine the effect of the final pyrolysis temperature on the yield of gas, liquid and solid fractions. In this series, LTCpyrolysis of BC samples was carried out under an inert atmosphere (He, 150 mL min1) with pyrolysis temperatures of 350, 400 or 450 C. The second group of experiments was performed to establish the effect of the bagasse hydrolysis pretreatment on LTC-pyrolysis yields, and this series of experiments was carried out under inert atmosphere (He, 150 mL min1) at 350 C. The last group of experiments was performed to establish the effect of the use of an oxidant atmosphere (5% O2/He, 150 mL min1) on LTC-pyrolysis yields; a final temperature of 350 C was used.
3.
Fig. 1 e FTIR spectra of a) sugarcane bagasse in natura (BC) and b) water washed sugarcane bagasse (BCW).
Results and discussion
3.1. Effect of different treatments on sugarcane bagasse characteristics As stated above, BC samples were hydrolyzed with acidic, alkaline or sequential acidic/alkaline treatments at 25 C, 122 C or under ultrasonic irradiation. All treated samples were studied by chemical analysis, FTIR, DT/DTG and SEM. For each treatment, acidic, alkaline or sequential acidic/alkaline, the sample treated at 122 C showed the highest degree of modification when compared with the original bagasse in natura; consequently, throughout the rest of the paper, the results of characterization of pretreated samples will refer exclusively to those samples treated at 122 C. The subsequent LTC experiments were carried out using BCA2, BCB2 and BCS2 samples. Table 1 shows the chemical composition of the BC and the modified sugarcane bagasse following the different hydrolysis treatments at 122 C. Although no significant differences in the chemical composition were observed after the hydrolysis treatments, a slight reduction in oxygen content was noted following acidic treatment, which could be related to the removal of oxygen-rich compounds by the acid treatment. As stated in the experimental section, for comparative purposes we prepared a blank sample (BCW) by washing in water and subsequently drying (105 C) a BC sample. Fig. 1 shows FTIR spectra of both BC and BCW samples. No significant differences can be distinguished related to the simple removal of extractives when BC was treated with water to give
Table 1 e Chemical composition of the sugarcane bagasse in natura and that of the samples obtained after hydrolysis treatments at 122 C. (%wt/wt) C H N Oa
BC
BCA2
BCB2
BCS2
43.89 7.05 0.32 48.74
48.62 6.90 0.08 44.40
42.48 7.37 0.15 50.00
44.69 7.43 0.16 47.72
a Calculated by subtraction.
BCW. The spectra are complex but the presence of different functional groups corresponding to the expected composition based on cellulose, hemicellulose and lignin structures can be deduced [37]. A broad band above 3000 cm1 was observed, corresponding to the y(OeH) of hydroxyl groups, such as those of alcoholic and phenolic components. y(CeH) absorptions in the 2800e3000 cm1 region were also clearly observed. Below 1800 cm1, the spectra fingerprint enabled the presence of C]O, CeOeC and C]C linkages, among others, to be determined. In the following paragraphs, we will analyze the more relevant features observed in the infrared spectra of the BCW and those of the different pretreated samples at 122 C (Fig. 2). The bands in the region 1700e1760 cm1 are characteristic of y(C]O) (Fig. 2A). The spectrum corresponding to the BCW sample showed a broad band with maximum at 1732 cm1. Absorptions at ca. 1735e1740 cm1 are associated with y(C]O) of carbohydrate structures; and a band at 1740 cm1 has been assigned to the acetyl, uronic, and feluric ester groups of hemicellulose [38]. On the other hand, a component of the carbonyl band appearing at a lower wavenumber is related to the conjunction of a carbonyl group with an aromatic ring, as occurs with carbohydrate linked with lignin components [39]. Following acidic hydrolysis, it was mainly the component at ca. 1702 cm1 which remained, although a shoulder at ca. 1735 cm1 was still present. No bands in the 1700e1735 cm1 region were observed for the samples BCB2 and BCS2. y(C]O) bands usually disappear with high-temperature treatments of cellulose, hemicellulose and lignin structures due to formation of CeOeC bonds between rings [40]. However, in our study, the intensity of the bands at ca. 1250 cm1 and 1050 cm1 characteristic of CeOeC and CeO bonds respectively, diminished following acid or basic treatment. On the other hand, when the FTIR spectra of BCB2 and BCS2 samples were compared with those of BCW and BCA2 (Fig. 2A), a high reduction in the intensity of the bands located ca. 1515 cm1, 1605 cm1 and 834 cm1 in the spectra of the samples BCB2 and BCS2 with respect to those of BCW and BCA2 was clearly noted. These bands are associated with the presence of lignin; 1515 cm1 and 1605 cm1 (aromatic ring vibrations) and 834 cm1 (CeH in plane bending) [38,41].
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Fig. 2 e FTIR spectra, A) 1800-600 cmL1 region and B) 4000-2500 cmL1 region, of several treated sugarcane bagasse samples; a) BCW, b) BCA2, c) BCB2 and d) BCS2.
Besides the reduction in the y(C]O) absorption intensity, the extraction of carbohydrates would produce a higher reduction in y(OeH) absorption intensity (band centered at 3400e3450 cm1) with respect to that of y(CeH) (band centered at ca. 2900 cm1); this can be observed clearly in Fig. 2B, if the spectra of BCA2, BCB2 or BCS2 are compared with that of BCW. The spectral changes of the BCB2 and BCS2 with respect BCW agree with the extraction of hemicellulose and the partial release or degradation of lignin; the partial release of both hemicellulose and lignin has already been reported as taking place at 55 C under alkaline conditions [38]. The thermogravimetric behavior of samples was analyzed and TG (%wt/wt) and DTG (%wt/wt/ C) curves were obtained up to 1000 C. Fig. 3 shows the curves corresponding to the bagasse in natura (BC) and those corresponding to the sample washed with water (BCW) (Fig. 3A and B respectively). Fig. 4 displays the thermogravimetric curves of pretreated samples. The DTG curve of BC shows three well-defined peaks with maximum at 225 C, 320 C and 370 C (Fig. 3A), whilst that of BCW shows peaks at 320 C and 370 C. The peak at 225 C in the DTG curve of BC (Fig. 3A) was related to the extractive residues present in this sample, which were removed after washing with water (see the DTG curve of BCW in Fig. 3B). The two peaks in the DTG curve of BC and BCW, with maxima at 320 C and 370 C, can be assigned primarily to the decomposition of hemicellulose and cellulose respectively [42,43]. Hemicellulose is formed of short branched polymer chains of several C6 (mainly glucose, mannose and galactose) and C5 (xylose and arabinose) sugars. The side chains and the axial hydroxyl groups of the sugars prevent hemicellulose from forming semi crystalline domains; around 200 sugar units comprise the hemicellulose chain. Cellulose is a polymer of glucose units (ca. 10000) without any branches. The hydroxyl groups in equatorial position permit strong hydrogen bonds, giving semi crystalline polymer chains of glucose. Hemicellulose presents an easier hydrolysis and thermal decomposition than cellulose, due to its amorphous structure. Although the highest weight loss took place up to ca. 400 C, in all cases a continuous loss of weight above this temperature, and up to the final temperature of the experiment, was observed; this loss did not produce significant
peaks in the DTG curves. The degradation of lignin has been reported to occur preferably above 300 C, with a very low mass loss rate. Lignin is a tri-dimensional polymer which comprises phenyl-propane units highly cross-linked and consequently it is difficult to decompose [44e46]. Fig. 4 shows the thermogravimetric analysis of sugarcane bagasse after acid (Fig. 4A), alkaline (Fig. 4B) and sequential acid/alkaline treatments (Fig. 4C) at 122 C. Several differences can be observed if the DTG profiles of pretreated samples are compared with that of BCW (Fig. 3B). Samples pretreated with alkaline (BCB2) or sequential acid/alkaline (BCS2) solutions showed only one wide DTG peak, which started at 250 C and reached maximum at ca. 350 C (Fig. 4B and C); this is mainly attributed to the superimposed thermal decomposition of cellulose and lignin. Although the peak is asymmetric, the disappearance of the maximum at 320 C indicates degradation of hemicellulose produced by the alkaline hydrolysis, in agreement with FTIR analysis of these samples. On the other hand, the main DTG peak for BCB2 and BCS2 appeared at a lower temperature (350 C) than that of BCW, and this could be related to a partial modification of the cellulose and/or lignin structure, agreeing with FTIR results, which indicated that the basic treatment produced partial degradation of lignin structures. On the other hand, a wide peak at 300e400 C with two maxima at 320 C and 370 C was still noted in the DTG profile of the sample pretreated with acid (sample BCA2 Fig. 4A); in this case, the intensity of the first maximum was slightly lower than that of BCW (compare Figs. 4A and 3B), which would agree with the partial removal of hemicellulose by an acid treatment as was shown by FTIR. The easy hydrolysis of hemicellulose can once again be related to its amorphous random branched structure, whereas the crystalline structure of cellulose and the cross-linked phenolic units present in lignin structure are more resistant to hydrolysis. The thermogravimetric results indicate that hydrolysis treatment affects the composition of sugarcane bagasse, and are in good agreement with the observed infrared features of the pretreated samples previously discussed. Both TG/DTG and FTIR results enable us to conclude that a basic hydrolysis treatment produces a more effective extraction of hemicellulose.
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Fig. 3 e TG and DTG curves of A) BC and B) BCW samples.
Fig. 5 comprises micrographs corresponding to the sugarcane bagasse in natura (BC) and those corresponding to the samples treated at 122 C with acidic, alkaline or sequential acidic/alkaline treatments, BCA2, BCB2 and BCS2 respectively. Several differences can be observed when comparing the micrographs of the samples treated at 122 C with different hydrolysis media. The BC micrographs show the presence of sugar crystallites on the surface of the material, which correspond to the residual extractives in this sample. The micrographs of the pretreated samples do not show the sugar crystallites, due to the easy removal of extractives. In the BCA2 sample, substantial disorganization can be observed in the fibers, compared to the BC sample. As stated above, it was deduced that acid treatment led to partial removal of hemicellulose. Basic or sequential acid/basic hydrolysis treatments resulted in materials consisting of skeletal structures without apparent
amorphous surface morphology. This effect was more notable in the BCS2 sample. These results indicate that extraction of hemicellulose by an alkali or sequential treatment is more efficient than that achieved by acidic hydrolysis, and are in good agreement with FTIR and thermogravimetric analyses.
3.2.
LTC-pyrolysis results
The effect of the pyrolysis process temperature (350 C, 400 C, 450 C) was firstly investigated in sugarcane bagasse in natura. Table 2 shows the liquid (bio-oil) and solid fraction yield obtained as a function of the temperature used in the pyrolysis experiments under inert atmosphere. An increase in biooil yield was obtained when the pyrolysis temperature decreased from 450 C to 350 C, with a bio-oil fraction of 18 (% wt/wt) at 350 C. Since the highest bio-oil yield was observed at the lowest LTC temperature (350 C), this was the
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Fig. 4 e TG and DTG curves of several treated sugarcane bagasse samples; A) BCA2, B) BCB2 and C) BCS2.
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Fig. 5 e Scanning electron microscopy of BC and several treated sugarcane bagasse samples.
temperature selected for the subsequent pyrolysis experiments of modified sugarcane bagasse. As stated above, LTC-pyrolysis of BC, BCA2, BCB2 and BCS2 samples was carried out and the effect of inert (He) and oxidant (5% v/v O2/He) atmospheres was explored. As described in the experimental section, the formed gases were analyzed by MS; in all cases, CO2, CO, C2H6, CH4 and H2O, were detected. Fig. 6 shows the amount of liquid and solid fractions (%wt/wt) obtained from LTC experiments of BC and pretreated
Table 2 e Distribution of products obtained after LTC under inert atmosphere of sugarcane bagasse in natura at different temperatures. T ( C) 350 400 450
Oil (%wt/wt)
Solid (%wt/wt)
18.0 16.0 12.5
11.0 16.5 15.0
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Fig. 6 e Yield of liquid (>, A) and solid (,, -) fractions (% wt/wt) obtained after the LTC process of BC and several treated sugarcane bagasse samples. LTC process was carried out at 350 C under He (filled symbols) or 5% O2/He (empty symbols) atmosphere.
samples. The yields of liquid and solid fractions depended on both the hydrolysis pretreatment and the LTC atmosphere used. In all cases, an LTC oxidant atmosphere led to the formation of a higher amount of solid fraction, whereas the
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liquid fraction was lower. However, the LTC atmosphere had less influence on the liquid yield than on the solid yield. On the other hand, when results of LTC of the pretreated samples were compared to those of sugarcane bagasse in natura, several differences in the amount of liquid and solid fractions obtained could be appreciated. The amount of solid fraction obtained was always higher for pretreated samples, being in the order BC < BCA2 < BCB2 < BCS2. The LTC of BCB2 and BCS2 led to a lower amount of bio-oil than that of the original BC sample. However, when the LTC products of BCA2 and BC were compared, a higher amount of bio-oil in the case of BCA2 was observed. Under inert atmosphere, the LTC of BCA2 sample gave a bio-oil yield (31 %wt/wt) 72% higher than that obtained from the BC sample (18 %wt/wt). These results indicate that LTC of extractives does not significantly contribute to bio-oil formation. Moreover, as stated above, the acid treatment produced structural modification, fiber disorganization and the partial extraction of hemicellulose. Thus, the relative amount of cellulose in the acid-treated sample increased when it was compared with BC sample, and this may be related with the higher bio-oil yield produced by the LTC process of the former. On the other hand, alkaline or sequential treatments, besides a more effective extraction of
Fig. 7 e 1H NMR spectra, corresponding to the bio-oil obtained by LTC of BC sample at 350 C under different atmospheres; A) under He (spectrum registered at 200 MHz), B) under 5% O2/He (spectrum registered at 300 MHz).
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hemicellulose than acid treatment produced the partial degradation of lignin. Under LTC conditions unprotected cellulose may suffer dehydration and oxidation reactions producing the diminution of bio-oil yield [25]. The obtained bio-oil was analyzed by NMR and FTIR spectroscopy. Fig. 7 shows the 1H NMR spectra of the bio-oil obtained by LTC of sugarcane bagasse in natura in inert (He) (Fig. 7A) and in oxidant atmosphere (5% O2/He) (Fig. 7B). Both 1 H NMR spectra showed prominent signals at d 9.54, s; 7.32, d, J ¼ 3.3 Hz; 6.53, d, J ¼ 3.3 Hz and 4.60, s characteristic of 5-hydroxymethylfurfural. The singlet at d w5.2 is related to levoglucosan [47]; signals at d 3.30e4.50 are ascribed to carbohydrate moiety (carbinolic protons). The signal at ca. 1.2 ppm is due to water impurity. Interestingly, the NMR characterization of the bio-oil produced by LTC of modified sugarcane (BCA2, BCB2, BCS2) indicated in all cases that levoglucosan was the main component. As an example, Fig. 8 shows 1H (Fig. 8A) and 13C NMR (Fig. 8B) spectra corresponding to the bio-oil produced
after the LTC under He atmosphere of BCA2. Characteristic signals of levoglucosan can be distinguished; 1H NMR signals at d 5.24, s (H-1); 4.43, m (H-5); 4.09, bd, J ¼ 7.0 Hz (H-6a); 3.58, dd, J ¼ 5.9 and 7.0 Hz (H-6b); 3.51 bs (H-4) and 3.37, bs (H-2) and those from the 13C NMR spectrum at d 103.22, 77.57, 74.53, 72.54, 72.25 and 65.83. FTIR spectra of the bio-oil produced by LTC of different samples are presented in Fig. 9. The broad absorbance bands of y (OeH) stretching vibration between 3200 and 3600 cm1 indicated the presence of highly polymeric hydroxyl groups and water impurities in the oil. The absorptions at 2800e3000 cm1 region and those around 1460 cm1 are characteristic of y(CeH) and d(CeH) vibrations of eCH3 and/or eCH2-groups. Other absorptions between 1630 and 1760 cm1 were also observed. As stated above, this is a characteristic spectral region of y(C]O) and could indicate the presence of ketones, aldehydes and acids or esters. Specifically, 5-hydroxymethylfurfural shows a characteristic and intense absorption at 1666 cm1. The FTIR spectrum of the bio-oil obtained from sugarcane bagasse in natura
Fig. 8 e NMR spectra of the bio-oil obtained from the BCA2 sample after LTC at 350 C under He atmosphere. A) 1H NMR (300 MHz); B) 13C NMR (75 MHz).
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The degradation of lignin after the basic and sequential treatments may leave cellulose unprotected and favor dehydration and oxidation reactions under LTC conditions, thus leading to a decrease in bio-oil yield.
Acknowledgments The authors are grateful to ANP/CENPES-Brazil, and the Spanish and Catalan governments (Consolider Ingenio 2010, Multicat CSD2009-00050, MAT2008-02561 and 2009SGR-0674 projects) for the financial support and to CAPES-Brazil, for a scholarship.
Fig. 9 e FTIR spectra of the bio-oil obtained after LTCpyrolysis at 350 C under He atmosphere of several samples; a) BC, b) BCA2, c) BCB2, d) BCS2.
(BC) (Fig. 9, spectrum a) shows a band at 1668 cm1 which could be assigned to the presence of 5-hydroxymethylfurfural, in agreement with NMR results. The intensity of this band decreased strongly in the spectrum corresponding to bio-oil of BCA2 (Fig. 9 spectrum b), and it was no longer present in the spectra of the bio-oil obtained from BCB2 and BCS2 samples (Fig. 9, spectra c and d). The residual extractives of BC sample may produce 5-hydroxymethylfurfural under the LTC conditions applied in this study.
4.
Conclusions
Sugarcane bagasse was modified by acid, alkaline and sequential acid/alkaline hydrolysis. SEM, TG-DTG and FTIR analysis showed that the hydrolysis treatments determined the composition and fiber organization of the sugarcane bagasse. Acid treatment removed extractives and hemicellulose and led to amorphous, highly disordered fibers. Alkaline hydrolysis removed more effectively hemicellulose, degraded lignin and left highly ordered residual fibers. With the goal of producing bio-oil, low temperature pyrolysis of modified biomass was carried out under inert or oxidant atmospheres. Bio-oil yielded by LTC of modified samples did not differ qualitatively in composition, and levoglucosan comprised its main component. The acid pretreatment increased the bio-oil yield of the LTC process, whereas the basic treatment produced the opposite effect when compared to that of the in natura sugarcane bagasse. The yield of bio-oil increased by 72% when the LTC process of BCA2 was carried out under He, and by 53% when it was carried out under oxidant atmosphere. The LTC bio-oil yield correlated with the fiber organization and composition of pretreated samples. The higher bio-oil production in LTC process from BCA2 sample with respect to BC sample is related to the relative amount of cellulose, which is higher in the acid-treated material because this treatment mainly removes extractives and hemicellulose.
references
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Waste biomass to liquids: Low temperature conversion of sugarcane bagasse to bio-oil. The effect of combined hydrolysis treatments Josilaine A. Cunha a, Marcelo M. Pereira a, Ligia M.M. Valente a, Pilar Ramı´rez de la Piscina b, Narcı´s Homs b,c,*, Margareth Rose L. Santos a,** a
Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Instituto de Quı´mica, Departamento de Quı´mica Inorgaˆnica, Cidade Universita´ria, Rio de Janeiro, 21949-900 RJ, Brazil b Departament de Quı´mica Inorga`nica and Institut de Nanocie`ncia i Nanotecnologia, Universitat de Barcelona, C/Martı´ i Franque´s 1-11, 08028 Barcelona, Spain c Catalonia Institute for Energy Research (IREC), C/Jardins de les Dones de Negre 1, 08930 Barcelona, Spain
article info
abstract
Article history:
This article describes the influence of different sugarcane bagasse hydrolysis pretreatments
Received 10 March 2010
on modifications to biomass feedstock and the characteristics of the resultant pyrolysis
Received in revised form
products. Sugarcane bagasse was pretreated with acid, alkaline or sequential acid/alkaline
9 February 2011
solutions and pretreated samples were then subjected to a low temperature conversion (LTC)
Accepted 10 February 2011
process under He or O2/He atmospheres at 350e450
C. Both pretreated samples and
sugarcane bagasse in natura were analyzed by determination of their chemical composition and by thermogravimetric, FTIR and SEM analyses. The gases yielded during LTC were Keywords:
monitored on-line by quadrupole mass spectrometry, and the liquid fractions obtained were
Biomass
characterized by FTIR and 1H and
Sugarcane bagasse
ment applied, the main bio-oil component obtained was levoglucosan. However, the LTC
Pyrolysis
yield of bio-oil depended on the hydrolysis treatment of the biomass and decreased in the
Bio-oil
presence of O2. The acid hydrolysis pretreatment increased the LTC bio-oil yield notably.
LTC-pyrolysis
13
C NMR. Irrespective of the sugarcane bagasse pretreat-
ª 2011 Elsevier Ltd. All rights reserved.
Biofuels
1.
Introduction
The demand for energy is growing at a rapid rate due to an increase in both world population and industrialization. There is a recognized need, therefore, to move toward sustainable energy production in order to reduce greenhouse gas emissions and fossil fuel dependence [1e3]. In this context, biomass has been shown to be a potential source of renewable energy. Thus, both developing and
industrialized countries are now seeking new technologies which can efficiently transform biomass resources into alternative fuels [4]. The use of agricultural waste or agricultural residues for these purposes, in principle, does not add carbon dioxide to the atmosphere, in contrast to the use of fossil fuels [5e7]. Moreover, second-generation biofuels will not compete with food crops, since the raw material they use is the crop wastes that would otherwise be discarded [8e10].
* Corresponding author. Departament de Quı´mica Inorga`nica and Institut de Nanocie`ncia i Nanotecnologia, Universitat de Barcelona, C/Martı´ i Franque´s 1-11, 08028 Barcelona, Spain. ** Corresponding author. E-mail addresses: [email protected] (N. Homs), [email protected] (M.R.L. Santos). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.02.019
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 2 1 0 6 e2 1 1 6
Biomass can be transformed using biochemical methods (such as alcoholic fermentation) and thermochemical methods (such as direct combustion, pyrolysis or gasification) [11,12]. Pyrolysis of biomass has been employed to produce organic intermediates (methane and acetic acid), charcoal and fuel gas [13]. Many studies devoted to the thermal decomposition of lignocellulosic materials have been reported in the last two decades, due to the production of activated carbons from solid char derived from agricultural waste pyrolysis [14,15]. Nowadays, pyrolytic processes can be refined to obtain char, oil and/or gas, depending on the temperature and reaction time [16e18]. In this context, the Low Temperature Conversion (LTC) process, initially developed by Bayer et al. [19], involves pyrolysis at around 400 C. The aim of this process is to maximize the yield of liquid products having high-heating power (bio-oil) [20]. The pyrolysis treatment at temperature higher than 400 C can modify the bio-oil composition leading to undesirable products as polycyclic aromatic hydrocarbons [21]. On the other hand, the distribution of pyrolysis products can be modified by the presence of catalysts and/or by the chemical pretreatment of the biomass [21,22]. It is now well recognized that the chemical pretreatment of lignocellulosic materials can remove extractives, hemicellulose, and lignin, reduce the crystallinity of cellulose, and increase the porosity of material [23,24]. Thus, the yield and composition of ulterior bio-oil obtained by pyrolysis could be modified [25]. Sugarcane has historically played an important role in the Brazilian economy [26,27]. In recent years, special Brazilian governmental programs have led to a significant increase in the crop areas devoted to sugarcane and to an improvement in the sugarcane yield per hectare in order to meet the demand for ethanol as fuel. According to the Brazilian Institute of Geography and Statistic, this rise in sugarcane production generated about 160 million tons of bagasse in 2008 [28]. In this respect, sugarcane bagasse constitutes a typical example of an agricultural byproduct that is abundantly available worldwide [29e31]. Sugarcane bagasse typically has a cellulose:hemicellulose:lignin ratio of around 40:35:15. This lignocellulosic content can be hydrolyzed to liberate the lignin and depolymerize the polysaccharides [32,33]. Polysaccharides and lignin are bound through reactive ether and ester links [34,35] which may be hydrolyzed at mild temperatures in the presence of acids or bases. All these processes involve complex carbohydrate and lignin reactions [36]. We report here the influence of the acidic or alkaline nature of the sugarcane bagasse hydrolysis pretreatment on solid residue composition and on that of the bio-oil yielded after the LTC-pyrolysis process carried out at 350e450 C. We focused on the characteristics of the bio-oil liquid fraction due to its potential future interest for second-generation bio-fuel production.
2.
Materials and experimental procedures
Sugarcane bagasse in natura was sundried, ground and sieved. Only particles in the 20e80 mesh range were retained for analysis and experiments; the corresponding samples of bagasse in natura used were labeled BC. For comparative
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purposes, a sample of BC was washed with water at room temperature and then dried at 105 C until constant weight; the resulting sample was labeled BCW. Chemical composition of samples (C, H, N, %wt/wt) was determined using a Perkin Elmer 2400 CHN model and the oxygen content (%wt/wt) was then calculated by subtraction. Thermogravimetric and derivative thermogravimetric analysis (TG/DTG) data were recorded using a Universal TA 2060 apparatus. A heating rate of 20 C min1 and a N2 flow of 100 mL min1 were used. The infrared spectra were obtained with a Nicolet Magna 760 spectrometer at 4 cm1 of resolution. A Hitachi H-2300BSE microscope was used for the scanning electron microscopy study of the materials. 1 H NMR and 13C NMR were recorded using a Bruker 300 spectrometer (300 MHz for 1H and 75 MHz for 13C) and a Bruker 200 (200 MHz for 1H). Spectra were recorded at 25 C using acetone-d6, dH ¼ 2.04 ppm and dC ¼ 206.58 ppm as solvent signals references.
2.1.
The hydrolysis experiments
BC hydrolysis was carried out under acidic, basic or sequential acid/base treatments at 25 C or 122 C at ambient pressure for 1 h. For the hydrolysis treatments, BC samples (2.0 g) were treated with 2 M HCl (25 mL) or 0.5 M NaOH (25 mL) solutions at 25 C (samples BCA1 and BCB1 respectively), 122 C (BCA2 and BCB2 samples respectively) or under ultrasonic irradiation (BCA3 and BCB3 samples). In the sequential acid/base treatment, BC samples were first hydrolyzed under acidic conditions and then treated with the alkaline solution; the corresponding samples treated at 25 C, 122 C or under ultrasonic irradiation at 25 C were BCS1, BCS2, and BCS3, respectively. In all cases, the resulting samples were filtered off and washed with distilled water until the wash-water remained neutral. The samples were oven dried at 105 C until constant weight, their chemical composition was then determined and finally, they were analyzed by FTIR, TG/DTG and SEM.
2.2.
The pyrolysis experiments
The LTC-pyrolysis experiments were performed in an apparatus designed for this purpose using about 0.1 g of BC or hydrolyzed samples. The main element of this device was a tubular reactor inserted vertically into an electrically heated tubular furnace; the temperature was controlled inside the sample bed by a NieCr thermocouple. In all cases, gaseous, liquid and solid fractions were formed. The gases produced were analyzed on-line by a mass spectrometer (MS) with a quadrupole analyzer MKS model e-vision. The liquid fraction was condensed at the reactor outlet, extracted with acetone and then the solvent was removed at low pressure and the residue analyzed by FTIR, 1H and 13C NMR. The LTC-pyrolysis experiments were carried out in three series; in all cases the samples were heated at a rate of 10 C min1 and the final pyrolysis temperature was maintained for 15 min. All experiments were performed in duplicate. The first series of experiments was carried out to
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determine the effect of the final pyrolysis temperature on the yield of gas, liquid and solid fractions. In this series, LTCpyrolysis of BC samples was carried out under an inert atmosphere (He, 150 mL min1) with pyrolysis temperatures of 350, 400 or 450 C. The second group of experiments was performed to establish the effect of the bagasse hydrolysis pretreatment on LTC-pyrolysis yields, and this series of experiments was carried out under inert atmosphere (He, 150 mL min1) at 350 C. The last group of experiments was performed to establish the effect of the use of an oxidant atmosphere (5% O2/He, 150 mL min1) on LTC-pyrolysis yields; a final temperature of 350 C was used.
3.
Fig. 1 e FTIR spectra of a) sugarcane bagasse in natura (BC) and b) water washed sugarcane bagasse (BCW).
Results and discussion
3.1. Effect of different treatments on sugarcane bagasse characteristics As stated above, BC samples were hydrolyzed with acidic, alkaline or sequential acidic/alkaline treatments at 25 C, 122 C or under ultrasonic irradiation. All treated samples were studied by chemical analysis, FTIR, DT/DTG and SEM. For each treatment, acidic, alkaline or sequential acidic/alkaline, the sample treated at 122 C showed the highest degree of modification when compared with the original bagasse in natura; consequently, throughout the rest of the paper, the results of characterization of pretreated samples will refer exclusively to those samples treated at 122 C. The subsequent LTC experiments were carried out using BCA2, BCB2 and BCS2 samples. Table 1 shows the chemical composition of the BC and the modified sugarcane bagasse following the different hydrolysis treatments at 122 C. Although no significant differences in the chemical composition were observed after the hydrolysis treatments, a slight reduction in oxygen content was noted following acidic treatment, which could be related to the removal of oxygen-rich compounds by the acid treatment. As stated in the experimental section, for comparative purposes we prepared a blank sample (BCW) by washing in water and subsequently drying (105 C) a BC sample. Fig. 1 shows FTIR spectra of both BC and BCW samples. No significant differences can be distinguished related to the simple removal of extractives when BC was treated with water to give
Table 1 e Chemical composition of the sugarcane bagasse in natura and that of the samples obtained after hydrolysis treatments at 122 C. (%wt/wt) C H N Oa
BC
BCA2
BCB2
BCS2
43.89 7.05 0.32 48.74
48.62 6.90 0.08 44.40
42.48 7.37 0.15 50.00
44.69 7.43 0.16 47.72
a Calculated by subtraction.
BCW. The spectra are complex but the presence of different functional groups corresponding to the expected composition based on cellulose, hemicellulose and lignin structures can be deduced [37]. A broad band above 3000 cm1 was observed, corresponding to the y(OeH) of hydroxyl groups, such as those of alcoholic and phenolic components. y(CeH) absorptions in the 2800e3000 cm1 region were also clearly observed. Below 1800 cm1, the spectra fingerprint enabled the presence of C]O, CeOeC and C]C linkages, among others, to be determined. In the following paragraphs, we will analyze the more relevant features observed in the infrared spectra of the BCW and those of the different pretreated samples at 122 C (Fig. 2). The bands in the region 1700e1760 cm1 are characteristic of y(C]O) (Fig. 2A). The spectrum corresponding to the BCW sample showed a broad band with maximum at 1732 cm1. Absorptions at ca. 1735e1740 cm1 are associated with y(C]O) of carbohydrate structures; and a band at 1740 cm1 has been assigned to the acetyl, uronic, and feluric ester groups of hemicellulose [38]. On the other hand, a component of the carbonyl band appearing at a lower wavenumber is related to the conjunction of a carbonyl group with an aromatic ring, as occurs with carbohydrate linked with lignin components [39]. Following acidic hydrolysis, it was mainly the component at ca. 1702 cm1 which remained, although a shoulder at ca. 1735 cm1 was still present. No bands in the 1700e1735 cm1 region were observed for the samples BCB2 and BCS2. y(C]O) bands usually disappear with high-temperature treatments of cellulose, hemicellulose and lignin structures due to formation of CeOeC bonds between rings [40]. However, in our study, the intensity of the bands at ca. 1250 cm1 and 1050 cm1 characteristic of CeOeC and CeO bonds respectively, diminished following acid or basic treatment. On the other hand, when the FTIR spectra of BCB2 and BCS2 samples were compared with those of BCW and BCA2 (Fig. 2A), a high reduction in the intensity of the bands located ca. 1515 cm1, 1605 cm1 and 834 cm1 in the spectra of the samples BCB2 and BCS2 with respect to those of BCW and BCA2 was clearly noted. These bands are associated with the presence of lignin; 1515 cm1 and 1605 cm1 (aromatic ring vibrations) and 834 cm1 (CeH in plane bending) [38,41].
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Fig. 2 e FTIR spectra, A) 1800-600 cmL1 region and B) 4000-2500 cmL1 region, of several treated sugarcane bagasse samples; a) BCW, b) BCA2, c) BCB2 and d) BCS2.
Besides the reduction in the y(C]O) absorption intensity, the extraction of carbohydrates would produce a higher reduction in y(OeH) absorption intensity (band centered at 3400e3450 cm1) with respect to that of y(CeH) (band centered at ca. 2900 cm1); this can be observed clearly in Fig. 2B, if the spectra of BCA2, BCB2 or BCS2 are compared with that of BCW. The spectral changes of the BCB2 and BCS2 with respect BCW agree with the extraction of hemicellulose and the partial release or degradation of lignin; the partial release of both hemicellulose and lignin has already been reported as taking place at 55 C under alkaline conditions [38]. The thermogravimetric behavior of samples was analyzed and TG (%wt/wt) and DTG (%wt/wt/ C) curves were obtained up to 1000 C. Fig. 3 shows the curves corresponding to the bagasse in natura (BC) and those corresponding to the sample washed with water (BCW) (Fig. 3A and B respectively). Fig. 4 displays the thermogravimetric curves of pretreated samples. The DTG curve of BC shows three well-defined peaks with maximum at 225 C, 320 C and 370 C (Fig. 3A), whilst that of BCW shows peaks at 320 C and 370 C. The peak at 225 C in the DTG curve of BC (Fig. 3A) was related to the extractive residues present in this sample, which were removed after washing with water (see the DTG curve of BCW in Fig. 3B). The two peaks in the DTG curve of BC and BCW, with maxima at 320 C and 370 C, can be assigned primarily to the decomposition of hemicellulose and cellulose respectively [42,43]. Hemicellulose is formed of short branched polymer chains of several C6 (mainly glucose, mannose and galactose) and C5 (xylose and arabinose) sugars. The side chains and the axial hydroxyl groups of the sugars prevent hemicellulose from forming semi crystalline domains; around 200 sugar units comprise the hemicellulose chain. Cellulose is a polymer of glucose units (ca. 10000) without any branches. The hydroxyl groups in equatorial position permit strong hydrogen bonds, giving semi crystalline polymer chains of glucose. Hemicellulose presents an easier hydrolysis and thermal decomposition than cellulose, due to its amorphous structure. Although the highest weight loss took place up to ca. 400 C, in all cases a continuous loss of weight above this temperature, and up to the final temperature of the experiment, was observed; this loss did not produce significant
peaks in the DTG curves. The degradation of lignin has been reported to occur preferably above 300 C, with a very low mass loss rate. Lignin is a tri-dimensional polymer which comprises phenyl-propane units highly cross-linked and consequently it is difficult to decompose [44e46]. Fig. 4 shows the thermogravimetric analysis of sugarcane bagasse after acid (Fig. 4A), alkaline (Fig. 4B) and sequential acid/alkaline treatments (Fig. 4C) at 122 C. Several differences can be observed if the DTG profiles of pretreated samples are compared with that of BCW (Fig. 3B). Samples pretreated with alkaline (BCB2) or sequential acid/alkaline (BCS2) solutions showed only one wide DTG peak, which started at 250 C and reached maximum at ca. 350 C (Fig. 4B and C); this is mainly attributed to the superimposed thermal decomposition of cellulose and lignin. Although the peak is asymmetric, the disappearance of the maximum at 320 C indicates degradation of hemicellulose produced by the alkaline hydrolysis, in agreement with FTIR analysis of these samples. On the other hand, the main DTG peak for BCB2 and BCS2 appeared at a lower temperature (350 C) than that of BCW, and this could be related to a partial modification of the cellulose and/or lignin structure, agreeing with FTIR results, which indicated that the basic treatment produced partial degradation of lignin structures. On the other hand, a wide peak at 300e400 C with two maxima at 320 C and 370 C was still noted in the DTG profile of the sample pretreated with acid (sample BCA2 Fig. 4A); in this case, the intensity of the first maximum was slightly lower than that of BCW (compare Figs. 4A and 3B), which would agree with the partial removal of hemicellulose by an acid treatment as was shown by FTIR. The easy hydrolysis of hemicellulose can once again be related to its amorphous random branched structure, whereas the crystalline structure of cellulose and the cross-linked phenolic units present in lignin structure are more resistant to hydrolysis. The thermogravimetric results indicate that hydrolysis treatment affects the composition of sugarcane bagasse, and are in good agreement with the observed infrared features of the pretreated samples previously discussed. Both TG/DTG and FTIR results enable us to conclude that a basic hydrolysis treatment produces a more effective extraction of hemicellulose.
2110
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Fig. 3 e TG and DTG curves of A) BC and B) BCW samples.
Fig. 5 comprises micrographs corresponding to the sugarcane bagasse in natura (BC) and those corresponding to the samples treated at 122 C with acidic, alkaline or sequential acidic/alkaline treatments, BCA2, BCB2 and BCS2 respectively. Several differences can be observed when comparing the micrographs of the samples treated at 122 C with different hydrolysis media. The BC micrographs show the presence of sugar crystallites on the surface of the material, which correspond to the residual extractives in this sample. The micrographs of the pretreated samples do not show the sugar crystallites, due to the easy removal of extractives. In the BCA2 sample, substantial disorganization can be observed in the fibers, compared to the BC sample. As stated above, it was deduced that acid treatment led to partial removal of hemicellulose. Basic or sequential acid/basic hydrolysis treatments resulted in materials consisting of skeletal structures without apparent
amorphous surface morphology. This effect was more notable in the BCS2 sample. These results indicate that extraction of hemicellulose by an alkali or sequential treatment is more efficient than that achieved by acidic hydrolysis, and are in good agreement with FTIR and thermogravimetric analyses.
3.2.
LTC-pyrolysis results
The effect of the pyrolysis process temperature (350 C, 400 C, 450 C) was firstly investigated in sugarcane bagasse in natura. Table 2 shows the liquid (bio-oil) and solid fraction yield obtained as a function of the temperature used in the pyrolysis experiments under inert atmosphere. An increase in biooil yield was obtained when the pyrolysis temperature decreased from 450 C to 350 C, with a bio-oil fraction of 18 (% wt/wt) at 350 C. Since the highest bio-oil yield was observed at the lowest LTC temperature (350 C), this was the
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 2 1 0 6 e2 1 1 6
Fig. 4 e TG and DTG curves of several treated sugarcane bagasse samples; A) BCA2, B) BCB2 and C) BCS2.
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Fig. 5 e Scanning electron microscopy of BC and several treated sugarcane bagasse samples.
temperature selected for the subsequent pyrolysis experiments of modified sugarcane bagasse. As stated above, LTC-pyrolysis of BC, BCA2, BCB2 and BCS2 samples was carried out and the effect of inert (He) and oxidant (5% v/v O2/He) atmospheres was explored. As described in the experimental section, the formed gases were analyzed by MS; in all cases, CO2, CO, C2H6, CH4 and H2O, were detected. Fig. 6 shows the amount of liquid and solid fractions (%wt/wt) obtained from LTC experiments of BC and pretreated
Table 2 e Distribution of products obtained after LTC under inert atmosphere of sugarcane bagasse in natura at different temperatures. T ( C) 350 400 450
Oil (%wt/wt)
Solid (%wt/wt)
18.0 16.0 12.5
11.0 16.5 15.0
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 2 1 0 6 e2 1 1 6
Fig. 6 e Yield of liquid (>, A) and solid (,, -) fractions (% wt/wt) obtained after the LTC process of BC and several treated sugarcane bagasse samples. LTC process was carried out at 350 C under He (filled symbols) or 5% O2/He (empty symbols) atmosphere.
samples. The yields of liquid and solid fractions depended on both the hydrolysis pretreatment and the LTC atmosphere used. In all cases, an LTC oxidant atmosphere led to the formation of a higher amount of solid fraction, whereas the
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liquid fraction was lower. However, the LTC atmosphere had less influence on the liquid yield than on the solid yield. On the other hand, when results of LTC of the pretreated samples were compared to those of sugarcane bagasse in natura, several differences in the amount of liquid and solid fractions obtained could be appreciated. The amount of solid fraction obtained was always higher for pretreated samples, being in the order BC < BCA2 < BCB2 < BCS2. The LTC of BCB2 and BCS2 led to a lower amount of bio-oil than that of the original BC sample. However, when the LTC products of BCA2 and BC were compared, a higher amount of bio-oil in the case of BCA2 was observed. Under inert atmosphere, the LTC of BCA2 sample gave a bio-oil yield (31 %wt/wt) 72% higher than that obtained from the BC sample (18 %wt/wt). These results indicate that LTC of extractives does not significantly contribute to bio-oil formation. Moreover, as stated above, the acid treatment produced structural modification, fiber disorganization and the partial extraction of hemicellulose. Thus, the relative amount of cellulose in the acid-treated sample increased when it was compared with BC sample, and this may be related with the higher bio-oil yield produced by the LTC process of the former. On the other hand, alkaline or sequential treatments, besides a more effective extraction of
Fig. 7 e 1H NMR spectra, corresponding to the bio-oil obtained by LTC of BC sample at 350 C under different atmospheres; A) under He (spectrum registered at 200 MHz), B) under 5% O2/He (spectrum registered at 300 MHz).
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hemicellulose than acid treatment produced the partial degradation of lignin. Under LTC conditions unprotected cellulose may suffer dehydration and oxidation reactions producing the diminution of bio-oil yield [25]. The obtained bio-oil was analyzed by NMR and FTIR spectroscopy. Fig. 7 shows the 1H NMR spectra of the bio-oil obtained by LTC of sugarcane bagasse in natura in inert (He) (Fig. 7A) and in oxidant atmosphere (5% O2/He) (Fig. 7B). Both 1 H NMR spectra showed prominent signals at d 9.54, s; 7.32, d, J ¼ 3.3 Hz; 6.53, d, J ¼ 3.3 Hz and 4.60, s characteristic of 5-hydroxymethylfurfural. The singlet at d w5.2 is related to levoglucosan [47]; signals at d 3.30e4.50 are ascribed to carbohydrate moiety (carbinolic protons). The signal at ca. 1.2 ppm is due to water impurity. Interestingly, the NMR characterization of the bio-oil produced by LTC of modified sugarcane (BCA2, BCB2, BCS2) indicated in all cases that levoglucosan was the main component. As an example, Fig. 8 shows 1H (Fig. 8A) and 13C NMR (Fig. 8B) spectra corresponding to the bio-oil produced
after the LTC under He atmosphere of BCA2. Characteristic signals of levoglucosan can be distinguished; 1H NMR signals at d 5.24, s (H-1); 4.43, m (H-5); 4.09, bd, J ¼ 7.0 Hz (H-6a); 3.58, dd, J ¼ 5.9 and 7.0 Hz (H-6b); 3.51 bs (H-4) and 3.37, bs (H-2) and those from the 13C NMR spectrum at d 103.22, 77.57, 74.53, 72.54, 72.25 and 65.83. FTIR spectra of the bio-oil produced by LTC of different samples are presented in Fig. 9. The broad absorbance bands of y (OeH) stretching vibration between 3200 and 3600 cm1 indicated the presence of highly polymeric hydroxyl groups and water impurities in the oil. The absorptions at 2800e3000 cm1 region and those around 1460 cm1 are characteristic of y(CeH) and d(CeH) vibrations of eCH3 and/or eCH2-groups. Other absorptions between 1630 and 1760 cm1 were also observed. As stated above, this is a characteristic spectral region of y(C]O) and could indicate the presence of ketones, aldehydes and acids or esters. Specifically, 5-hydroxymethylfurfural shows a characteristic and intense absorption at 1666 cm1. The FTIR spectrum of the bio-oil obtained from sugarcane bagasse in natura
Fig. 8 e NMR spectra of the bio-oil obtained from the BCA2 sample after LTC at 350 C under He atmosphere. A) 1H NMR (300 MHz); B) 13C NMR (75 MHz).
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The degradation of lignin after the basic and sequential treatments may leave cellulose unprotected and favor dehydration and oxidation reactions under LTC conditions, thus leading to a decrease in bio-oil yield.
Acknowledgments The authors are grateful to ANP/CENPES-Brazil, and the Spanish and Catalan governments (Consolider Ingenio 2010, Multicat CSD2009-00050, MAT2008-02561 and 2009SGR-0674 projects) for the financial support and to CAPES-Brazil, for a scholarship.
Fig. 9 e FTIR spectra of the bio-oil obtained after LTCpyrolysis at 350 C under He atmosphere of several samples; a) BC, b) BCA2, c) BCB2, d) BCS2.
(BC) (Fig. 9, spectrum a) shows a band at 1668 cm1 which could be assigned to the presence of 5-hydroxymethylfurfural, in agreement with NMR results. The intensity of this band decreased strongly in the spectrum corresponding to bio-oil of BCA2 (Fig. 9 spectrum b), and it was no longer present in the spectra of the bio-oil obtained from BCB2 and BCS2 samples (Fig. 9, spectra c and d). The residual extractives of BC sample may produce 5-hydroxymethylfurfural under the LTC conditions applied in this study.
4.
Conclusions
Sugarcane bagasse was modified by acid, alkaline and sequential acid/alkaline hydrolysis. SEM, TG-DTG and FTIR analysis showed that the hydrolysis treatments determined the composition and fiber organization of the sugarcane bagasse. Acid treatment removed extractives and hemicellulose and led to amorphous, highly disordered fibers. Alkaline hydrolysis removed more effectively hemicellulose, degraded lignin and left highly ordered residual fibers. With the goal of producing bio-oil, low temperature pyrolysis of modified biomass was carried out under inert or oxidant atmospheres. Bio-oil yielded by LTC of modified samples did not differ qualitatively in composition, and levoglucosan comprised its main component. The acid pretreatment increased the bio-oil yield of the LTC process, whereas the basic treatment produced the opposite effect when compared to that of the in natura sugarcane bagasse. The yield of bio-oil increased by 72% when the LTC process of BCA2 was carried out under He, and by 53% when it was carried out under oxidant atmosphere. The LTC bio-oil yield correlated with the fiber organization and composition of pretreated samples. The higher bio-oil production in LTC process from BCA2 sample with respect to BC sample is related to the relative amount of cellulose, which is higher in the acid-treated material because this treatment mainly removes extractives and hemicellulose.
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