Characterization of bio-oil and bio-char obtained from sweet sorghum bagasse fast pyrolysis with fractional condensers

Fuel 112 (2013) 96–104

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Characterization of bio-oil and bio-char obtained from sweet sorghum bagasse fast pyrolysis with fractional condensers Renzhan Yin a,b, Ronghou Liu a,b,⇑, Yuanfei Mei a,b, Wenting Fei a, Xingquan Sun a a b

Biomass Energy Engineering Research Centre, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China

h i g h l i g h t s  Properties of bio-oil and bio-char of sweet sorghum bagasse were investigated.  The Properties of bio-oil were found to vary across the fractional condensers.  Using fractional condensers gave bio-oils with different compositions.  Bio-char has a higher carbon content, pore structure and potassium content.

a r t i c l e

i n f o

Article history: Received 10 December 2012 Received in revised form 16 February 2013 Accepted 30 April 2013 Available online 21 May 2013 Keywords: Sweet sorghum bagasse Fluidized bed Bio-oil Bio-char Pyrolysis

a b s t r a c t Fast pyrolysis serves as an alternative and eco-friendly method to dispose of biomass waste and to get bio-oil, bio-char and syngas simultaneously. In this study, sweet sorghum bagasse was pyrolyzed in a fluidized bed reactor with biomass throughput of 1–5 kg/h using fractional condensers and an electrostatic precipitator. GC–MS analysis showed an annulation feature of bio-oil compositions and the fractional condensers proved to be an effective way to separate water and chemical compounds from bio-oil. Surface morphology of bio-char was conducted using scanning electron microscopy and energy dispersive Xray analysis (EDS). Bio-char is carbon rich with high K content and vesicular structure. Additionally, proximate, ultimate, elemental and the FTIR analysis were carried out on sweet sorghum bagasse and its products to investigate the chemical changes after the fast pyrolysis process. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, people pay much attention to renewable energy because of the shortage of fossil fuels and carbon emission problems. Among the different kinds of energy forms, biomass is widely considered to be a major potential energy source for the future. In the past, biomass was the main energy resource for human beings until the industrial revolution when petroleum begun to be widely used. Compared to conventional fossil fuels, biomass is abundant, easy to store and carbon neutral. Over the last two decades, many researches have been carried out on the conversion of residual biomass into bio-oil. Bio-oil has a high energy density, and can be easily stored, transported and utilized. In addition, the byproduct biochar is considered as a feed-stock for the production of activated carbon, a soil amendment to improve soil properties, and a material for soil carbon sequestration [1]. ⇑ Corresponding author at: Biomass Energy Engineering Research Centre, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China. Tel.: +86 21 34205744; fax: +86 21 34205744. E-mail address: [email protected] (R. Liu). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.04.090

As an energy crop, sweet sorghum has a high photosynthetic efficiency. Its grain and juice contain a large percentage of sugars which can be fermented to produce ethanol. The sweet sorghum bagasse is also a vital supplementary material for energy production, which is mainly composed of cellulose, hemicellulose and lignin. It requires pre-treatment prior to enzymatic hydrolysis for conversion to ethanol. In our previous research, five pre-treatment methods have been investigated to improve the enzymatic digestibility of sweet sorghum bagasse and bioethanol production [2]. However, the shortcoming of these pre-treatment methods is that they have a high energy requirement and are costly. Thus it is considered that converting sweet sorghum bagasse to bio-oil and biochar may be a good alternative way for sweet sorghum bagasse utilization. Many biomass materials have been tested to produce bio-oil and bio-char in different kinds of reactors [3–6]. However there are few researches on the pyrolysis of sweet sorghum bagasse and its bio-oil and bio-char properties [7,8]. The objectives of this study are to investigate the physicochemical properties of bio-oil and bio-char obtained from fast pyrolysis of sweet sorghum bagasse in a fluidized bed reactor. In addition, gas chromatograph

R. Yin et al. / Fuel 112 (2013) 96–104

and mass spectrometer analysis (GC–MS), Fourier transform infrared spectrometer (FTIR) were used to analyze the characteristics of the pyrolysis products of sweet sorghum bagasse. 2. Methods 2.1. Feed stock preparation Sweet sorghum (Chongming No. 1 variety) was harvested in Qibao campus, Shanghai Jiao Tong University, China. The stalks were squeezed by a three-roller mill to obtain the liquid phase and the bagasse was produced. The bagasse was dried in the air and then ground to pass through sieve with 40 meshes and stored in plastic bag at room temperature. Before the pyrolysis experiment, sweet sorghum bagasse powders were put into an oven to dry the water at 105 °C for 12 h to make the water content consistently in each pyrolysis work.

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60 L/min. At the elevated reactor temperature, sweet sorghum bagasse was pyrolyzed and decomposed into vapors and bio-char, which were rapidly removed from the reactor. The pyrolysis vapors first entered a cyclone to remove bio-char. Then, the pyrolysis vapors were passed to four fractional condensers and EP to trap the liquid oil. During experiment, the temperatures of the fractional condensations were monitored with time. The average temperature of the condensers I, II, III, IV were 310 K, 298 K, 296.5 K, 297 K, respectively, and the residence time of pyrolysis vapors in each condenser was 3.4 s. The bio-oil samples were collected from four condensers separately and labeled bio-oil 1, 2, 3 and 4 at the end of the experiment. The bio-oil from the EP was named as biooil 5. Throughout experiments, the yield of bio-oil can be determined from the condensed liquid and the feedstock used on dry bases. The solid char was removed and weighed. The non-condensable gas yield was then calculated by difference [10]. 2.3. Analysis methods

2.2. Pyrolysis reactor system A bench-scale fluidized bed reactor fast pyrolysis system with a feedstock throughput of 1–5 kg/h has been designed and constructed by our laboratory. The schematic diagram of the fluidized bed reactor fast pyrolysis system is shown in Fig. 1. The pyrolysis system comprises of the reactor section and associated auxiliary systems for biomass feeding and injection, pyrolysis products separation and vapor condensation. It has been previously described in detail in Chen et al. [9]. The reactor and the pre-heater are heated by heating jackets. The fluidized medium is silica sand and rests on a distributor plate. Fluidization is accomplished by N2 administered through a mass flow controller. In the separation section, a cyclone is used to separate the pyrolysis vapor and bio-char. The vapor condensation consists of four condensers in series using a water cooling system and an electrostatic precipitator (EP). The purpose of using fractional condensers is to preliminarily separate the components of bio-oil. The bio-oil from different condenser may have different properties so that it can be utilized accordingly. An EP is used to remove fine particulate matter, such as the smoke in the gases, using the force of an induced electrostatic charge. At the beginning of the pyrolysis work, sweet sorghum bagasse is put into the feeding hopper. It is conveyed from the hopper by a twin-screw feeder to keep the biomass injected to the fluidized bed reactor at the center of the sand. According to our previous work [8], the reactor temperature was set at 500 °C. The flow rate of carrier gas was

2.3.1. Water content The water content of the bio-oil is measured using Karl-Fischer titration (precision 0.01%) (KFT 870, Swiss Manthon Instrument Factory) according to ASTM E 203. 2.3.2. Acidity The pH value is tested using pH meter (PHS-3C, Shanghai Lei Ci Instrument plant) at room temperature. The instrument is calibrated with liquid calibration standards of pH 4 and 6.86 prior to the measurement. 2.3.3. Density The density of the bio-oil is analyzed using a density meter (precision 0.0001 g/cm3) (Anton Paar, DMA 4100 M, ASTM D4502). 2.3.4. Solids content The solids content of bio-oil was defined as ethanol insoluble and determined by Millipore filtration system. About 1–10 g of bio-oil was dissolved in 100 mL ethanol and filtered through a pre-dried and pre-weighed 1 lm pore size filter. The filter with the solids was then air-dried for 15 min and further dried in an oven at 105 °C for 30 min. Finally, the filter was cooled in a desiccator and weighed. The solids content was calculated by the original bio-oil sample. This method was recommended by Oasmaa and Peacocke et al. [11].

Fig. 1. The schematic diagram of the fluidized bed reactor fast pyrolysis system.

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2.3.5. Viscosity The viscosity of the bio-oil is measured according to the standard procedure (ASTM D445) using a capillary viscometer (SYD265C, Shanghai Changji Gealogical Instruments Co., Ltd.).

2.3.6. Proximate and ultimate analyses The proximate analysis determines the moisture, volatile matter, fixed carbon and ash contents of sweet sorghum bagasse and bio-char according to the ASTM standard methods (E1756-01, E872-82 and E1755-01). Ultimate analysis of the biomass particles was carried out for determination of carbon, hydrogen, nitrogen, sulfur, and oxygen using an Elemental Analyzer (Model Vario EL III). The higher heating value (HHV) was calculated from a correlation developed by Sheng and Azevedo [12] as shown by the following equation:

HHVðMJ=kgÞ ¼ 1:3675 þ 0:3137C þ 0:7009H þ 0:0318O where C and H are percentages on dry basis of carbon and hydrogen, respectively and O is 100-C-H. The lower heating value (LHV) was calculated from the HHV and the hydrogen content by the following equation [13]:

LHVðMJ=kgÞ ¼ HHV  2:442  8:936 ðH=100Þ

2.3.7. Elemental analysis K, Na, Ca, Mg, Fe, Zn, S, P and Si elemental analyses are performed by inductively coupled plasma–optical emission spectroscopy (ICP–OES) with reference to ICP general rule JY/T015-1996 method. The detection limit of ICP is 0.00 mg/L (1  107%).

2.3.8. Composition of bio-oil Composition of bio-oil is measured by GC–MS (AutoSystem XL GC/TurboMass MS, Perkin Elmer) with a quadruple detector and a DB-5MS capillary column (30 m  0.25 mm inner diameter  0.25 lm thickness). Helium (99.9999%) is used as the carrier gas with a constant flow of 1.0 mL/min. The oven temperature is programmed from 333 (4 min) to 513 K at a heating rate of 4 K/ min, then to 573 K held at a heating rate of 20 K/min, and hold at 573 K for 13 min. The injector (Fison SSL 71) and the GC–MS interface are kept at a constant temperature of 523 and 508 K, respectively. A sample volume of 1 lL (10% of pyrolysis liquid in chloroform) is injected. The MS is operated in electron ionization mode, and a m/z range from 33 to 550 is scanned. Standard mass spectra with 70 eV ionization energy are recorded. The identification of the peaks is based on computer matching of the mass spectra with the NIST98 and WILEY7.0 library or on the retention times of known species injected in the chromatographic column.

2.3.9. Fourier transform infrared (FTIR) analysis The FTIR spectra of the sweet sorghum bagasse, bio-oil and biochar were recorded in the transmission mode between 4000 cm1 and 400 cm1 using a Bruker EQUINOX 55 Model Fourier Transform Infrared Spectrometer. Dried KBr was used to prepare pellets.

2.3.10. FE-SEM and EDS analysis of biomass sample and bio-char The samples were mounted in conductive tapes and examined using an FEI Sirion 200 field emission scanning electron microscope (FE-SEM) coupled with Oxford INCA X-Act energy-dispersive X-ray (EDS) analysis. The magnifications of the SEM were selected as 1500, 2500, 5000 and 15,000.

3. Results and discussion 3.1. Pyrolysis product distribution The pyrolysis products include 43.5% bio-oil, 23.8% bio-char, and 32.8% non-condensable gas (NCG). In addition, the yields of bio-oil 1, 2, 3, 4 and 5 were 28.5 wt%, 2.6 wt%, 0.4 wt%, 4.5 wt% and 7.4 wt%, respectively. In fact, there may have been some biooil and bio-char trapped in the pipe lines and reactor that were not calculated in the experiment. So it is always difficult to estimate by gravimetric analysis. The yield of pyrolysis bio-oil in the fractional condenser system decreased from condenser I–III and increased from condenser IV to the EP. A great mass of bio-oil was collected in condenser I and the EP. Boateng et al. [14] pyrolyzed switchgrass in a bench scale fluidized reactor and got about 42.4% of the total bio-oil in the EP. 3.2. Chemical analysis of sweet sorghum bagasse and bio-char Table 1 shows physicochemical properties of the sweet sorghum bagasse, bio-oil and bio-char. From Table 1, it can be seen that there are fairly big distinction between sweet sorghum bagasse and its bio-char product in terms of proximate analysis and ultimate analysis. In comparison with sweet sorghum bagasse, a decrease in volatile matter and an increase in fixed carbon and ash content were observed for bio-char samples. Bio-char has a lower volatile matter of 14.75%, a higher fixed carbon and ash content matter of 62.81% and 18.78% respectively. The presence of volatile matter in bio-char samples shows incomplete thermal degradation during fast pyrolysis. The ash content of bio-char samples is also found higher than the biomass samples due to the mineral matter which forms ash remains in bio-char after pyrolysis. Compared with sweet sorghum bagasse, there was a decrease in the hydrogen content of bio-char. This is probably because the volatile mater that released from the biomass has great hydrogen content [15]. Bio-char is carbon rich, has a high calorific value and has the potential to be solid biofuel. It can be utilized in various industrial applications as an alternative to solid fossil fuels. Due to its high carbon content, bio-char can be also used as a source of carbon and for creating a C sink by putting it into soil. The long persistence of bio-char in soil makes it a prime candidate for the mitigation of climate change as a potential sink for atmospheric carbon dioxide [16]. Previous studies have proved that the return of bio-char to the soil can enhance soil quality. These benefits included the enhanced pH value, greater nutrient and moistureholding capacity, higher levels of organic matter, improved aeration and increased crop yield [1]. 3.3. Characteristics of bio-oil The physicochemical properties of the bio-oil from different fractional condenser are shown in Table 1. In general, bio-oil contains about 15–30 wt% water. The water contained in the bio-oil has both a positive effect on reducing viscosity and negative effect on reducing its heating value, increasing the ignition delay and decreasing combustion rate [17]. Water in bio-oil comes from the original moisture of the feedstock and the result of the dehydration reactions occurred during the biomass fast pyrolysis [18]. As Table 1 shows, the water contents of bio-oil 2 and 3 are almost the same being 37.07 wt% and 44.48 wt%, respectively. And they are lower than bio-oil 1 which has the highest water content of 56.29 wt% among the fractional condensers. The bio-oil 5 has the lowest water content of 6.08 wt%. It can be concluded that most water steam generated in the biomass dehydration reaction was condensed in condenser I, resulting in water separation during

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R. Yin et al. / Fuel 112 (2013) 96–104 Table 1 Physicochemical properties of the sweet sorghum bagasse, bio-oil and bio-char. Physicochemical properties

SSa

BCb

Bio-oil 1

Proximate analysisc (wt%) Moisture Volatile matter Fixed carbond Ash Ligocellulosic compositionc (wt%) Cellulose Hemicellulose Lignin Water content (wt.%) pH Density (g/cm3) Solid content (wt.%)

a b c d

2 – – – –

– – – –

4

5

4.17 76.33 15.98 3.53

3.66 14.75 62.81 18.78

34.2 24.3 6.5 – – – –

– – –– – – – –

– – – 56.29 2.84 1.0879 0.11

– – – 37.07 3.20 1.1479 0.22

– – – 44.48 3.28 1.1229 0.23

– – – 12.92 3.30 1.1732 0.72

–– – – 6.08 3.36 1.192 0.83

Ultimate analysisc (wt.%) C H N Od H/C O/C

45.71 5.80 0.33 48.16 0.13 1.05

69.03 2.78 0.59 27.60 0.04 0.40

22.08 6.52 0.21 71.20 0.30 3.22

52.11 6.38 0.69 40.81 0.12 0.78

52.40 6.27 0.73 40.60 0.12 0.77

53.19 6.44 0.86 39.52 0.12 0.74

59.48 6.29 0.96 33.27 0.11 0.56

Heating valuec (MJ/kg) HHV LHV

18.57 17.30

23.11 22.51

12.39 10.97

20.75 19.36

20.76 19.39

21.08 19.68

22.76 21.39

Elemental analyses (mg/g) K Ca Na Mg Al Zn Fe S P

8.50 2.50 3.12 1.24 1.10 0.28 0.67 0.75 1.44

39.61 7.73 1.84 4.25 0.54 0.17 1.92 1.44 6.09

0.12 0.09 0.09 0.02 0.01 0.01 0.04 0.23 0.01

0.46 0.35 0.21 0.06 0.03 0.02 0.21 0.36 0.03

– – – – – – – – –

0.42 0.26 0.27 0.06 0.03 0.03 0.15 0.60 0.03

0.59 0.19 0.24 0.07 0.05 0.02 0.10 0.66 0.03

Viscosity at different temperature (mm2/s) 30 °C 40 °C 50 °C 60 °C

– – – –

2.48 1.89 1.48 1.23

124.35 41.39 23.46 11.25

– – – –

277.02 90.15 45.21 32.15

1093.03 445.86 224.57 91.68

– – – –

– – – –

3

– – – –

– – – –

SS – sweet sorghum bagasse. BC – bio-char. Weight percentage on dry basis. Calculated by difference.

the bio-oil collection. Thus it is an effective way to use fractional condensation to separate the water from bio-oil. The pH of bio-oils from different fractional condensers at room temperature was from 2.84 to 3.36. Low pH can cause corrosion problem to the carbon steel and aluminum materials, especially at the high temperature and with the increase in water content. Results in Table 1 show that the pH had an increasing trend from condenser I to the EP, however, the pH improvement was limited with the use of fractional condensation. The density of bio-oil has a great influence on the atomization quality of spray. The density of bio-oil typically ranged from 1.05 g/cm3 to 1.35 g/cm3 and densities of wood derived bio-oils are a function of water content [11]. The density of bio-oil in this study was from 1.0879 g/cm3 to 1.192 g/cm3. When considering the water content, there is a close relationship between water content and density. The solids content in the bio-oil varied with feedstock, process condition and product recovery. It is represented by charcoal particles. The solids content of bio-oil increased from condenser I to the EP sequentially. Bio-oil 5 from the EP has the highest solids content of 0.83 wt%, while bio-oil 1 from condenser I has the lowest solids content of 0.11 wt%. Ultimate analysis shows that bio-oil 1 has lower carbon and nitrogen content but higher oxygen con-

Fig. 2. The diagram of Van Krevelen for bio-oil, bio-char and sweet sorghum bagasse.

tent than the bio-oil obtained from the other condensers. High oxygen content means high polarity and results in a lower energy density as well as lower miscibility with hydrocarbon fuels.

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The atomic ratios of H/C and O/C are used to characterize fuels. The Van Krevelen diagram for bio-oil, bio-char and sweet sorghum bagasse is presented in Fig. 2. It shows that bio-oil 1 has the highest H/C and O/C ratios. Bio-oil 2, 3, 4, 5 and sweet sorghum bagasse have the similar H/C and O/C ratios which were situated in the same region of the diagram, while the chars have the lowest ratios among all the samples. In general, the lower heating value (LHV) of bio-oil is in the range of 14–18 MJ/kg, which is similar to that for biomass and is only 40–45% of that for hydrocarbon fuels [18]. The minimum LHV of bio-oil 1 from condenser I was 10.97 MJ/kg due to its higher water, oxygen content and the lower carbon content. Because of the very low water content, bio-oil from the EP had the highest calorific value of in this study. Bio-oil 2, 3 and 4 have a similar HHV and LHV because of nearly the same composition of carbon, hydrogen, and oxygen. The viscosity affects the pumping and the atomization of the bio-oil and is an important parameter to be considered for a gas turbine fuel [19]. Viscosity of bio-oil measured at different temperatures is shown in Table 1. There is a variation in the kinematic viscosity of the bio-oil in condenser I, II, IV and the EP versus temperature. An obvious decline in the kinematic viscosity of bio-oil is observed when the temperature gradually increases. Compared to bio-oil 2, 4 and 5, the kinematic viscosity of bio-oil 1 is much lower because of its higher water content and lower solid contents. The linear relationship between viscosity (l) and inverse temperature was measured and modeled using an Arrhenius type relation. The Arrhenius relation for viscosity with an effect of temperature (T) can be expressed as follows:

l ¼ AeE=RT

ð1Þ

where l represents the viscosity of bio-oil; A is a constant (unit is similar to viscosity unit, mm2/s); E is the flow activation energy and R is the universal gas constant (8.314 J (mol K)1). Eq. (1) can be expressed as follows:

ln l ¼ ln A þ E=RT

ð2Þ

Arrhenius relation for viscosity of bio-oil is illustrated in Fig. 3. (It is a plot for ln (l) versus 1000/T.). The values of E which may be related to the vaporization of the oil [20] were calculated from the equations presented in Fig. 3. The activation energy for bio-oil from condenser II, IV and the EP are nearly the same being 65.34, 60.35,

68.00 kJ/mol, respectively. The activation energy for bio-oil 1 is lower at 19.70 kJ/mol because of higher water content. Generally, temperature has a much stronger effect on the viscosities of biooil 2, 4 and 5 than bio-oil 1. 3.4. Elemental analysis Elemental analysis of sweet sorghum bagasse, bio-oil and biochar is also shown in Table 1. It can be concluded that bio-oil contains much lower K, Ca, Na, Mg, Al, Zn, Fe, S and P compared with sweet sorghum bagasse. Bio-oil from the EP had higher K, Mg, Al and S concentration than other bio-oils. In addition, via the pyrolysis process, some elements were concentrated in bio-char, such as K, Ca, Mg, Fe, S, and P. These elemental contents in bio-char were 4.8, 3.1, 3.5, 3.2, 2.0 and 4.3 times higher than in sweet sorghum bagasse, respectively. It may be concluded that the bio-char can accumulate some elements through the pyrolysis process. Thus it can be inferred that pyrolysis can be an effective way to dispose of plants polluted by heavy metal. Liu et al. [21] pyrolyzed Cu-polluted biomass derived from phytoremediation, and found that more than 91% of the total Cu in the Cu-polluted feedstock was enriched in the bio-char in the form of zerovalent Cu. In addition, the yield and HHV of bio-oil were also significantly improved due to Cu catalysis. 3.5. GC–MS analysis of bio-oil GC–MS was carried out to identify the compounds of bio-oil from different condenses. Components of the bio-oil from sweet sorghum bagasse detected by GC–MS are shown in Table 2. The composition of bio-oil is very complicated, and a mixture of many aromatics and oxygenated compounds [10]. In this study, the GC– MS identified hundreds of compounds in bio-oil. However, only the compounds with relative concentration higher than 0.3% are list in Table 2. Others were very low and not examined in this study. The methanol, acetic acid and some short chain hydrocarbons were not detected in this study, just because the solvent peak is so strong that it covers the peaks of these compounds. Five types of compounds are most abundant, with relative mass content equal or greater than 3%. They include 2-Cyclopenten-1-one, 1,2-Ethanediol, diacetate, 2-Oxepanone, 2-methyl-, 2-Oxepanone and 2Furancarboxaldehyde, 5-methyl-. The abundant products are almost annulations due to cyclization of olefin structures followed by dehydrogenation reaction, which take place during the pyrolysis process [22]. Oxygenated aromatic compounds such as phenols, benzenediols and furancarboxaldehyde are commonly seen in biooil. The presence of these compounds may be explained by the thermal degradation of oxygenated components of the sweet sorghum bagasse. Phenols may originate from lignin degradation [23] and Furancarboxaldehyde may be formed due to the presence of hemicellulose in the sweet sorghum bagasse. Some chemical contents of bio-oil from condenser I to the EP showed a decreasing trend in sequence, such as 2-Cyclopenten-1-one, 2-Hexanone, 3,4-dimethyl-, 1,2-Ethanediol, diacetate, 2-Cyclopenten-1-one, 2-Furancarboxaldehyde, 5-methyl- and 2-Oxepanone, 2-Cyclopenten-1-one, 2-hydroxy-3-methyl-; While some others showed an increasing trend, such as Acetic acid, 2-ethylbutyl ester, 4-Octyne and 5-Hydroxymethylfurfural. This suggests that by using fractional condensers, chemicals of bio-oil can be separated because the condensation process is mainly dependent upon the dew point of the components of the bio-oil [24]. 3.6. Comparison of the FTIR results

Fig. 3. The diagram of Arrhenius relation for viscosity of bio-oil (lines are fits to an Arrhenius-type equation).

The FTIR spectra of sweet sorghum bagasse, bio-oil, and biochar samples are given in Fig. 4. The spectra were recorded in

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R. Yin et al. / Fuel 112 (2013) 96–104 Table 2 Components of the bio-oil from sweet sorghum bagasse detected by GC–MS. ID

Compound

Formula

Mw

RT (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70

2-Cyclopenten-1-one Acetic acid, 2-ethylbutyl ester 2-Furanmethanol 2-Hexanone, 3,4-dimethyl1,2-Ethanediol, diacetate 2-Heptynoic acid 4-Cyclopentene-1,3-dione 3H-2-benzopyran-3-one, 1,4-dihydro1,3-Propanediol, 2-(hydroxymethyl)-2-methylPropanoic acid, propyl ester 2-Cyclopenten-1-one, 2-methylEthanone, 1-(2-furanyl)2-Oxepanone Cyclopentanone, 2,4-dimethyl2-Cyclopenten-1-one, 2-hydroxy2,4-Pentanedione, 3-methyl2(5H)-furanone, 5-methylFurfuryl alcohol, tetrahydro-5-methyl-, cis2-Furancarboxaldehyde, 5-methylCyclohexanol, 2-methyl-, acetate, cis2-Cyclopenten-1-one, 3-methylBut-1-ene-3-yne, 1-ethoxy1H-imidazole-2-carboxaldehyde 4-Methyl-5H-furan-2-one 2-Ethyl-trans-2-butenal Phenol 2-Butanone, 4-(acetyloxy)1,4-Hexadiene, 2,3-dimethyl2-Cyclopenten-1-one, 2,3-dimethyl3-Hexen-2-one, 3-methylOxirane, butyl2-Cyclopenten-1-one, 2-hydroxy-3-methyl4-Octyne Crotonic acid, o-formylphenyl ester 2-Cyclopenten-1-one, 2-hydroxy-3,4-dimethylPhenol, 2-methylPhenol, 3-methylAcetophenone Phenol, 3-methylFuran, 2-(2-propenyl)Mequinol 4-Isobutoxy-2-butanone 1-Methylcyclooctene 4-Ethyl-2-hydroxycyclopent-2-en-1-one 2-Cyclopenten-1-one, 3-ethyl-2-hydroxyPhenol, 2,6-dimethylPhenol, 4-ethylCatechol Benzofuran, 4,7-dimethylN-Benzyl-2-phenethylamine 1,2-Benzenediol, diacetate 5-Hydroxymethylfurfural 3,4-Furandimethanol, diacetate Phenol, 2-ethyl-6-methyl1,2-Benzenediol, 3-methoxy1,2-Benzenediol, 4-methyl5-Isopropyl-3,3-dimethyl-2-methylene-2,3-dihydrofuran Phenol, p-(benzyloxy)-, acetate 1,2-Benzenediol, 4-methyl2-Methoxy-4-vinylphenol 1,3-Benzodioxole, 2-ethoxyPhenol, 2,6-dimethoxy1-Penten-3-one, 1,5-diphenylBenzaldehyde, 4-hydroxy4-Ethylcatechol 4-Hydroxymethylbenzaldehyde 3,5-Nonadien-7-yn-2-ol, (E,E)n-Propyl heptyl ether Acetic acid, 5-acetoxy-4-nitrotetrahydropyran-3-yl ester 1,2-Benzenedicarboxylic acid, mono(2-ethylhexyl) ester

C5H6O C8H16O2 C5H6O2 C8H16O C6H10O4 C7H10O2 C5H4O2 C9H8O2 C5H12O3 C6H12O2 C6H8O C6H6O2 C6H10O2 C7H12O C5H6O2 C6H10O2 C5H6O2 C6H12O2 C6H6O2 C9H16O2 C6H8O C6H8O C4H4N2O C5H6O2 C6H10O C6H6O C6H10O3 C8H14 C7H10O C7H12O C6H12O C6H8O2 C8H14 C11H10O3 C7H10O2 C7H8O C7H8O C8H8O C7H8O C7H8O C7H8O2 C8H16O2 C9H16 C7H10O2 C7H10O2 C8H10O C8H10O C6H6O2 C10H10O C15H17N C10H10O4 C6H6O3 C10H12O5 C9H12O C7H8O3 C7H8O2 C10H16O C15H14O3 C7H8O2 C9H10O2 C9H10O3 C8H10O3 C17H16O C7H6O2 C8H10O2 C8H8O2 C9H12O C10H22O C9H13NO7 C16H22O4

82 144 98 128 146 126 96 148 120 116 96 110 114 112 98 114 98 116 110 156 96 96 96 98 98 94 130 110 110 112 100 112 110 190 126 108 108 120 108 108 124 144 124 126 126 122 122 110 146 211 194 126 212 136 140 124 152 242 124 150 166 154 236 122 138 136 136 158 247 278

4.16 4.56 4.59 4.67 4.90 5.18 5.29 5.49 5.53 5.80 5.91 6.05 6.27 6.45 6.47 6.59 6.93 7.52 7.73 7.81 7.96 8.04 8.19 8.19 8.35 8.59 8.57 8.81 8.90 9.00 9.11 10.14 10.34 10.72 11.01 11.09 11.16 11.49 11.48 12.12 12.19 12.50 12.82 13.15 13.43 14.57 14.61 16.70 16.85 17.11 17.41 17.55 17.78 17.77 18.47 18.79 18.96 19.43 19.22 20.43 21.04 21.55 21.91 22.58 22.92 25.05 25.23 27.96 28.17 50.42

Relative mass content (%) 1

2

3

13.5

10.8

9.2 2.0

8.2 3.5

2.5 3.1 4.3

1.9 1.5 3.5

10.5 0.5 1.4 1.7 3.5

1.3 3.3

0.9

0.9 0.9

0.5 0.4

0.8 0.8

1.3 3.1 0.4 0.6 0.7

3.8 0.9 3.7

3.5 0.7 3.9

3.2 0.7 3.1 0.4

2.7 0.5 3.1

0.4 0.8 0.4

0.7 0.3

0.5

0.6

3.1 0.9 0.3 0.7

3.1 0.6 0.3 0.5

3.1 0.6 0.3

0.3

3.4 0.7 0.4 0.3

0.4

1.1

0.4

0.4

0.5

7.2 7.6 0.5

0.4 0.7 3.9 0.9 4.5 0.8 0.6 0.4 0.3 3.3 1.4

4

5

0.4

0.3 0.4 0.6 6.1 0.6 0.8 0.4 2.1

0.3 8.0 7.1 0.3

0.3

6.8 0.3 0.3

4.3 0.8 0.5

5.3 0.9 0.8

4.5 0.5 0.3

0.3 5.0

2.8

2.8

2.5

3.6

5.2

2.8

0.6 0.3

0.6

0.4

2.5 1.4 0.7

2.9 0.3 2.9 4.7 1.4

0.4

0.4 1.0 0.5 0.4 2.6

0.7 0.5 0.4 1.2

0.5 0.6 0.3 1.6

2.9 1.0 1.6

0.8 0.5 6.4 0.9 0.5 4.9 2.9 0.5

4.1

1.7

0.5

1.4

12.5 0.6

13.2 0.9

0.5 0.7

0.4 0.3 0.7

0.5 0.7

0.4 0.6

0.8 0.4

0.4 0.6 0.4

0.4 0.7 0.5 0.3 0.4 0.6 0.3 0.9 0.4

0.6 0.6 0.3

0.3 0.7 3.8 2.9

0.4 0.5 0.6

0.4 0.3 0.6

1.0 0.5 1.0 0.3 0.4 1.0 0.3 0.3 0.4 0.5 0.6 1.0 0.3

0.3 0.3 0.3 0.7 0.5

0.3 0.3 0.4

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R. Yin et al. / Fuel 112 (2013) 96–104

groups were completely detected in bio-oil. As can be seen from the FTIR results, bio-oil 1 and 2 have the same function groups, while the bio-oil 3, 4 and 5 have similar function groups. The presence of a large intense bond of OAH stretching vibration at 3355 cm1 corresponds to an OAH stretching vibrations that may be caused by acid and/or alcohol structures. The peaks near 2935 cm1 correspond to a CAH stretching vibration caused by cycloparaffin structure. The stretching vibration between 1735 and 1705 cm1 is an indication of ketonic functional group. C@C stretching vibration absorptions that cause the band at about 1645 cm1 in the spectrum represent the presence of the olefinic compounds. In addition, other bands also showed the presence of aromatic hydrocarbons, esters and phenol in the bio-oil. Table 3 lists the function groups that were identified from the FTIR spectra. The spectral absorption strength was labeled with different letters. Compared with the bio-oils, sweet sorghum bagasse and bio-char have less function groups. The peaks near 2075, 1445, 1110, 840, 760 cm1 which indicate the presence of alkyne, aliphatic, ketonic, esters, aromatic, phenolic and paraffinic compound were not seen in sweet sorghum bagasse and bio-char samples. However, they were detected in bio-oil samples. 3.7. SEM and EDS analysis of bio-char

Fig. 4. FTIR spectra of (a) bio-oil and (b) sweet sorghum bagasse and bio-char.

the transmission mode between 4000 cm1 and 400 cm1 for all samples. Biomass pyrolysis oils contain a very wide range of complex organic chemicals. Because secondary reaction was dramatically reduced during the pyrolysis reaction, most functional

Scanning electron microscopy (SEM) is an excellent technique for studying morphology of solid particles. From the SEM images, it is very easy to obtain accurate details about the pore structure of bio-chars. By comparing the SEM analysis of sweet sorghum bagasse and bio-char, interesting conclusions about morphological changes after the fast pyrolysis step can be drawn. Sweet sorghum bagasse and bio-char micrographs are shown in Fig. 5. From Fig. 5a–d, it can be seen that the surface morphology of sweet sorghum bagasse changed after pyrolysis. In a very short time, the pyrolysis of sweet sorghum bagasse leads to the formation of pores on the surface of the bio-char, while the pores of biomass samples were not found obviously. The presence of pores indicates that volatile components were formed and released and that the pores structure was formed through plastic deformation, a melt phase of cellular components [25]. For char materials, pores structures often relate to the specific surface area, and determine the adsorption capacity. The bio-char produced in this study has a number of regularly arranged pores with a diameter of 1–20 lm. It can be used as a feedstock for the production of activated carbon. Azargohar and Dalai [26] used bio-char produced from Spruce using fast pyrolysis, as a precursor to produce activated carbon. Furthermore, pores in bio-char particle are large enough to accommodate

Table 3 Function groups of sweet sorghum, bio-oil and bio-char determined by the FTIR analysis. Wave numbers (cm)1

3355 2935 2075 1735–1705 1605 1515 1445 1370–1385 1225–1270 1110 1050 840–885 760 615

Functional groups

OAH stretching vibration CAH stretching vibration C„C stretching vibration Aromatic carbonyl/carboxyl C@O stretching C@C stretching vibration Aromatic C@C ring stretching Aliphatic CH2 deformation Aliphatic CH3 deformation Aromatic CAH stretching Ketone or ester bonding Aliphatic ether CAO and alcohol CAO stretching Aromatic CAH out of plane deformation Adjacent aromatic CAH deformation Phenol OAH out of plane deformation

Bio-oil 1

2

3

4

5

s – m s – m – m m – m w w w

s – m s – m – m m – m w w w

s vs – vs m m m m m m m – w w

s vs – vs m m m m m m m w w w

s vs – vs m m m m m m m w w w

SS – sweet sorghum bagasse; BC – bio-char; s-strong; vs – very strong; m – middle; w – weak.

SS

BC

s vs – vs m m – m m – m – – s

s – – – m – – m – – m – w

103

R. Yin et al. / Fuel 112 (2013) 96–104

(a) SEM of sweet sorghum bagasse 1500x

(b) SEM of bio-char 1500x

(c) SEM of sweet sorghum bagasse 5000x

(d) SEM of bio-char 5000x

(e) SEM of bio-char using lower magnification 2500x

(g) EDS analysis of point A

(f) SEM of bio-char using higher magnification 15000x

(h) EDS analysis of point B

Fig. 5. Sweet sorghum bagasse and its bio-char micrographs.

soil microorganisms when it is applied to the soil. This pore structure of bio-char serves as a refuge for colonizing fungi and bacteria resulting in improved microbial, especially mycorrhizal activity.

Fig. 5e shows that mineral crystals are observed on the bio-char surfaces. They were formed during the pyrolysis by crystallization of chlorine and potassium. This is further confirmed by a high res-

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R. Yin et al. / Fuel 112 (2013) 96–104

olution (15,000) in Fig. 5f. These crystals were mainly potassium minerals as evidenced in the EDS spectrum at the same location (Fig. 5g), which showed an extremely high peak of potassium and chlorine contents than point B (Fig. 5h). This result is consistent with the elemental analysis of bio-char. These mineral crystals were also found on the bio-char surface by other researchers. Yao et al. [27] found nano-sized magnesium crystals were presented on the surface of digested sugar beet tailing bio-char through slowpyrolysis at 600 °C. 3.8. Non-condensable gas analysis The composition of non-condensable gas which consists of CO, CH4, CO2, C2H2, C2H6, C3H6, and N2 was analyzed by GC. Usually, researchers often focus on bio-oil recovery during the pyrolysis study. However, if the combustible gas content is high, the gas may be useful. In this study, the gas collected from the outlet after the EP mainly consisted of N2 (the carrier gas) of 96.05% and a small part of gas of 3.95% generated during the biomass pyrolysis reaction. The gas generated during the biomass pyrolysis reaction consisted of 52.2% CO, 32.0% CO2, 10.4% CH4, 2.9% C2H2, 1.3% C2H6 and 1.1% C3H6. The caloric value of the non-condensable gas was very low due to the dilution by the carrier gas (nitrogen). If the non-condensable gases can be reused properly, they can be used as fuel gas. 4. Conclusion The characteristics of bio-oil, including viscosity and activate energy, were found to vary across the fractional condensers. Viscosity increases due to the decreasing water content. Activate energy is similar in all bio-oils, except bio-oil 1. All bio-oils, except bio-oil 1, were located in the same region of the Van Krevelen diagram, indicating they have similar calorific values. Most of the chemical components were found to be oxygenated aromatic compounds. Using fractional condensers gave bio-oils with different compositions. Finally, bio-char has a higher carbon content, better pore structure and higher potassium content. Acknowledgements Financial support from National Natural Science Foundation of China through contract (Grant No. 51176121) and financial support from The National Science and Technology Supporting Plan through contract (Grant No. 2011BAD22B07) are greatly acknowledged. In addition, Daniel Lycett-Brown from the University of Southampton, UK is greatly acknowledged for his valuable suggestion and correction of the manuscript. Also, the authors would like to express sincere appreciate to the analysts Mrs. Yu wenjuan of Instrumental Analysis Centre of Shanghai Jiao Tong University for the analysis work. References [1] Laird DA, Brown RC, Amonette JE, Lehmann J. Review of the pyrolysis platform for coproducing bio-oil and biochar. Biofuels, Bioprod Biorefin 2009;3:547–62.

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