Energy Fuels 2010, 24, 2699–2706 Published on Web 03/10/2010
: DOI:10.1021/ef901517k
Upgrading Bio-oil through Emulsification with Biodiesel: Thermal Stability Xiaoxiang Jiang† and Naoko Ellis*,‡ †
Thermoenergy Engineering Research Institute, Southeast University, Nanjing 210096, People’s Republic of China, and ‡Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada Received December 11, 2009. Revised Manuscript Received February 21, 2010
Storage properties and thermal stability of fuels are important background information when dealing with a new potential fuel. Following the first paper on the preparation and characterization of the fuel mixture produced through emulsification of bio-oil and biodiesel, the second part of this investigation reports on the storage and thermal stability of the bio-oil/biodiesel mixture. The physicochemical properties of the samples stored at different temperatures (60 and 80 °C) for up to 180 h are measured. Fuel properties, such as viscosity, water content, acid number, and average molecular weight of the bio-oil/biodiesel mixture, are measured before and after aging. In contrast to the aging properties of bio-oil alone, very little changes in water content and viscosity are shown for the mixtures aged at 80 °C for 180 h. Overall, a slight decrease in acid numbers is observed for the aged mixtures. Chemical changes are characterized using gel permeation chromatography (GPC), showing a slight increase in the molecular weight over time, possibly because of some polymerization and condensation reactions during storage. Further confirmation of the changes is shown through a Fourier transform infrared spectrometer (FTIR), thermal decomposition analysis using a thermogravimetric analyzer (TGA), and proton assignment using proton nuclear magnetic residence (1H NMR) spectroscopy. Finally, the study indicates that the bio-oil/biodiesel mixture is stable within the conditions tested as a fuel. ture. At a higher aging temperature of 80 °C, molecular weight increased after storing the bio-oil for 1 week, which was equivalent to that for 1 year at room temperature. The other drawbacks of bio-oil, such as relatively polar, high oxygen content between 33 and 45 wt %, high viscosity, and high acid number, also limit its usage as a potential fuel.7,8 Various upgrading techniques, such as emulsification,9 hydrotreatment,10-15 catalytic cracking,16,17 esterification,18 adding solvent,19-21 and steam reforming,22-24 have been reported.
Introduction The diminishing supply of fossil fuels and increasing environmental concerns have impelled researchers to change their focus on alternative renewable resources for fuel production. Biomass is considered to be a promising resource of renewable energy. Pyrolysis is a thermo-chemical process that converts biomass into liquid (bio-oil), charcoal (biochar), and noncondensable gases by heating biomass to about 750 K in the absence of oxygen.1,2 Owing to its ease of transportation compared to raw biomass, various aspects of bio-oil, i.e., production and composition, have received considerable attention. An extensive review on the production is given by Briens et al.3 Bio-oil is a chemically complex mixture containing various components that can react within themselves,4 resulting in a thermodynamically unstable product. The effect of storage conditions on the properties of bio-oils stored at elevated temperature over extended periods has been studied by several researchers. Czernik et al.5 reported the increase in viscosity and molecular weight of bio-oil with the temperature and time, showing that the increase in viscosity after 3 months at 37 °C was equivalent to that of 4 days at 60 °C and 6 h at 90 °C. Similar variations of the change in bio-oil properties were observed by Boucher et al.,6 where the viscosity increased 1.8fold during the first 65 days of storage at ambient tempera-
(7) Boateng, A. A.; Mullen, C. A.; Goldberg, N.; Hicks, K. B.; Jung, H. G.; Lamb, J. F. S. Ind. Eng. Chem. Res. 2008, 12, 4115–4122. (8) Garcia-Perez, M.; Chaala, A.; Pakdel, H.; Kretschmer, D.; Rodrigue, D.; Roy, C. Energy Fuels 2006, 1, 364–375. (9) Ikura, M.; Stanciulescu, M.; Hogan, E. Biomass Bioenergy 2003, 3, 221–232. (10) Zhang, S.; Yan, Y.; Li, T.; Ren, Z. Bioresour. Technol. 2005, 5, 545–550. (11) Pindoria, R. V.; Megaritis, A.; Herod, A. A.; Kandiyoti, R. Fuel 1998, 15, 1715–1726. (12) Pindoria, R. V.; Lim, J.; Hawkes, J. E.; Lazaro, M.; Herod, A. A.; Kandiyoti, R. Fuel 1997, 11, 1013–1023. (13) Senol, O. I.; Viljava, T.-R.; Krause, A. O. I. Catal. Today 2005, 3-4, 331–335. (14) Elliott, D. C. Energy Fuels 2007, 3, 1792–1815. (15) Chiaramonti, D.; Oasmaa, A.; Solantausta, Y. Renewable Sustainable Energy Rev. 2007, 6, 1056–1086. (16) Vitolo, S.; Bresci, B.; Seggiani, M.; Gallo, M. G. Fuel 2001, 1, 17–26. (17) Vitolo, S.; Seggiani, M.; Frediani, P.; Ambrosini, G.; Politi, L. Fuel 1999, 10, 1147–1159. (18) Zhang, Q.; Chang, J.; Wang, T.; Xu, Y. Energy Fuels 2006, 6, 2717–2720. (19) Boucher, M. E.; Chaala, A.; Roy, C. Biomass Bioenergy 2000, 5, 337–350. (20) Scholze, B.; Hanser, C.; Meier, D. J. Anal. Appl. Pyrolysis 2001, 387–400. (21) Scholze, B.; Meier, D. J. Anal. Appl. Pyrolysis 2001, 1, 41–54. (22) Basagiannis, A. C.; Verykios, X. E. Catal. Today 2007, 1-4, 256– 264. (23) Rioche, C.; Kulkarni, S.; Meunier, F. C.; Breen, J. P.; Burch, R. Appl. Catal., B 2005, 1-2, 130–139. (24) Takanabe, K.; Aika, K.; Seshan, K.; Lefferts, L. J. Catal. 2004, 101–108.
*To whom correspondence should be addressed. Telephone: 1-604822-1243. Fax: 1-604-822-6003. E-mail:
[email protected]. (1) Ba, T.; Chaala, A.; Garcia-Perez, M.; Rodrigue, D.; Roy, C. Energy Fuels 2004, 3, 704–712. (2) Ba, T.; Chaala, A.; Garcia-Perez, M.; Roy, C. Energy Fuels 2004, 1, 188–201. (3) Briens, C. L.; Riskorz, J.; Berruti, F. Int. J. Chem. React. Eng. 2008, 6, Review R2. (4) Boateng, A. A.; Daugaard, D. E.; Goldberg, N. M.; Hicks, K. B. Ind. Eng. Chem. Res. 2007, 7, 1891–1897. (5) Czernik, S.; Johnson, D. K. Biomass Bioenergy 1994, 7, 187–192. (6) Boucher, M. E.; Chaala, A.; Pakdel, H.; Roy, C. Biomass Bioenergy 2000, 5, 351–361. r 2010 American Chemical Society
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: DOI:10.1021/ef901517k
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A promising method in using bio-oil as combustion fuel in transportation or boilers is to produce an emulsion with other fuel sources. Upgrading of bio-oil through emulsification with diesel oil decreases the viscosity and increases the calorific value and cetane number as a liquid fuel. Chiaramonti et al.25,26 studied the emulsion of bio-oil by the ratios of 25, 50, and 75 wt % with diesel fuel, resulting in more stable emulsions compared to the original bio-oil alone. The reductions in viscosity and corrosivity of the resulting emulsion have been reported.9 In the previous paper (10.1021/ef9010669), we have reported on a successful production of the bio-oil and biodiesel mixture.27 Biodiesel is comprised of monoalkyl esters of fatty acids derived from natural and renewable sources, such as animal fats and vegetable oils. One of the motivations for this process is to separate the mixture from the high-molecularweight compounds in the bio-oil, such as pyrolytic lignin, which are undesirable as fuel yet a promising source for highvalue chemicals.28 The resulting mixtures are characterized on the basis of the mixture stability and fuel properties. It is shown that the mixture characteristics, such as heating value, density, and viscosity, are comparable to #2 diesel from petroleum. Owing to the chemical instability of bio-oil during storage, the stability of the bio-oil/biodiesel mixture is of concern. In this paper, the aging of bio-oil/biodiesel mixtures with various initial volume percentages of bio-oil during typical storage conditions is discussed. This paper is the second part of a series of studies focusing on upgrading biooil through emulsification with biodiesel.
water content, molecular-weight distribution (MWD), and acid number, and through thermogravimetric analysis (TGA) were measured over time. Structural changes in the mixture were also measured using Fourier transform infrared (FTIR) spectroscopy. The viscosity of the mixture samples was measured at 25 °C before and after storage and at 60 and 80 °C using the Advanced Rheometer 2000, with the shear rate at 100 s-1. The measurements were repeated 9 times to obtain the average values. The water content of the samples was determined by Karl Fischer titration. The mixture was dissolved in methanol and titrated using Metrohm 794 Basic Titrino and Karl Fischer volumetric reagent Aqualine Complete 5. The water content was repeated 3 times, and the average was reported. The acid number of the samples was determined by a Metrohm 728 stirrer, where 20 drops of sample were dissolved in 50 mL of methanol for each measurement. The titrant was 0.1 N potassium hydroxide solution. The MWD of the mixture samples were measured by gel permeation chromatography (GPC) using the Agilent 1100 series, with the refractive index RID model G1362A as a detector. Tetrahydrofuran (THF) was used as a solvent at a flow rate of 0.5 mL/min. The sample was dissolved in THF at a concentration of 8 g/L and filtered prior to being injected into the GPC column. Elution times were converted to apparent molecular weight by calibration with polystyrene standards. The average molecular weight was calculated automatically. The IR spectra were measured using the attenuated total reflectance (ATR), with a Varian 3100 FTIR spectrometer to record the spectra. The sample was applied as a film to the zinc selenide crystal. The resulting spectra were accumulated between 6000 and 650 cm-1. TGA was performed on a simultaneous TGA/DSC, SDT Q600, TA Instrument. Samples of 15-20 mg were heated from room temperature (RT) to 600 °C at a constant heating rate of 20 °C/min under nitrogen, with a flow rate of 30 mL/min. Proton nuclear magnetic residence (1H NMR) spectra of the bio-oil/biodiesel mixture before and after aging were measured using an AVANCE Bruker AV 400 spectrometer with a 5 mm BBO probe. Acetone-d6 was used as the solvent. All of the experiments were run at 40 °C. The spectra were obtained using a 90° pulse angle. The spectra width was 6793 Hz, and the acquisition time was 3.1 s. The frequency for 1H NMR was 400.19 MHz.
Experimental Section Sample Preparation. The bio-oil produced from fast pyrolysis of softwood residue was supplied from VTT, Finland. Soybeanbased biodiesel was supplied from World Energy, Worcester, MA. Octanol was used as the emulsifier. In the previous study (10.1021/ef9010669),27 the stability of the mixture was characterized by a parameter S defined as the volume of bio-oil dissolved in a unit volume of biodiesel. The optimal process conditions were emulsifier dosage of 4 vol %, initial ratio of biooil/biodiesel of 4:6 by volume, stirring intensity at 1200 rpm, mixing time of 15 min, and emulsifying temperature at 30 °C, resulting in the S value at 0.21. The desirable bio-oil/biodiesel ratio from the angle of preparing a stable mixture may be different with respect to its aging characteristics. Thus, in the present study, initial mixtures of bio-oil and biodiesel were prepared by adding 20, 30, 40, and 50% (by volume) bio-oil into biodiesel, following the previously reported procedure (10.1021/ef9010669).27 For all of the above mixtures, the quick separation can be observed and the stratification stopped in about 10 h. The upper layer (biodiesel-rich phase) was then placed in sealed vials and heated using an oil bath at 60 and 80 °C for 12, 60, and 180 h. After storage, the visual observation of the mixture indicated that the samples remained homogeneous throughout the experiment. The ultimate analysis was conducted on the initial samples of bio-oil, biodiesel, and the mixture using the Carlo Ebra elemental analyzer EA 1108. Thermal Stability Analysis. Various physicochemical properties of the upper layer (biodiesel-rich phase), such as viscosity,
Results and Discussion Ultimate Analysis. Elemental analysis of carbon, hydrogen, and nitrogen contained in bio-oil, biodiesel, and mixture (40 vol % bio-oil) is shown in Table 1. The oxygen content of the different samples is calculated by difference. The bio-oil contains a high content of oxygen, which, in general, contributes to the instability over time. The resulting initial and aged mixtures are shown to contain less oxygen than the original bio-oil. The heating values of samples depend upon the elemental composition. In this work, the higher heating values were calculated from elemental data using the equation " # %O HHV ðMJ=kgÞ ¼ 338:2 %C þ 1442:8 %H 8 0:001
(25) Chiaramonti, D.; Bonini, M.; Fratini, E.; Tondi, G.; Gartner, K.; Bridgwater, A. V.; Grimm, H. P.; Soldaini, I.; Webster, A.; Baglioni, P. Biomass Bioenergy 2003, 1, 85–99. (26) Chiaramonti, D.; Bonini, M.; Fratini, E.; Tondi, G.; Gartner, K.; Bridgwater, A. V.; Grimm, H. P.; Soldaini, I.; Webster, A.; Baglioni, P. Biomass Bioenergy 2003, 1, 101–111. (27) Jiang, X.; Ellis, N. Energy Fuels 2010, 24 (2), 1358–1364. (28) Sukhbaatar, B.; Steele, P. H.; Kim, M. G. Bioresour. Technol. 2009, 4 (2), 789–804.
ð1Þ
The aged sample is shown to have a higher heating value compared to the initial mixture, as indicated in Table 1. Properties of Bio-oil, Biodiesel, and Bio-oil/Biodiesel Mixture Before Aging. Table 2 shows the properties of different samples, indicating that the viscosity and density of the 2700
Energy Fuels 2010, 24, 2699–2706
: DOI:10.1021/ef901517k
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Table 1. Composition and Energy Content of the Bio-oil, Biodiesel, and Mixture (Initial 40 vol % Bio-oil) elemental analysis (wt %)
bio-oil biodiesel mixture (40 vol % bio-oil) before aging mixture (40 vol % bio-oil) after aging at 80 °C for 180 h
C
H
O
N
HHV (MJ/kg)
39.96 77.54 65.92 66.48
7.74 11.75 11.33 11.89
52.19 9.71 22.59 21.50
0.11 1.00 0.16 0.13
15.28 41.43 34.57 35.76
Table 2. Fuel Properties of the Original Bio-oil, Biodiesel, and Bio-oil/Biodiesel Mixture mixture (initial vol % of bio-oil) raw materials 20 vol % 30 vol % 40 vol % 50 vol %
products
viscosity (T = 25 °C) (10-3 Pa s)
density (g/cm3)
acid number (mg of KOH/g)
bio-oil biodiesel upper layer upper layer upper layer upper layer
67.39 6.590 4.217 4.514 4.665 4.917
1.200 0.881 0.891 0.892 0.895 0.897
79.23 0.55 9.31 11.24 14.01 13.08
volume ratio of upper and bottom layers
average molecular weight (Mw)
water content (wt %)
90:10 72:28 73:27 60:40
421 280 303 307 311 312
28.05 0.1607 0.3785 0.3857 0.4558 0.4634
Figure 1. Water content of the bio-oil/biodiesel mixture with time.
upper layer (biodiesel-rich phase) are more desirable as a fuel compared to the original bio-oil. This is due to the mixture showing stronger characteristics of the biodiesel while having the high-molecular-weight components from bio-oil removed during emulsification. Further details on the mixture preparation are given elsewhere (10.1021/ ef9010669).27 The acid number of the mixture is higher than the original biodiesel, most likely because of acid molecules of bio-oil dissolving in the biodiesel-rich phase. The more bio-oil added, the higher the acid number of the mixture. The reduction in the molecular weight coincides with the reduction in the viscosity of the mixtures. The water content of the mixture remains lower than 0.5%, which is the limit set for the American Society for Testing and Materials (ASTM) standard for biodiesel.29 Through the emulsification process, the water and high-molecular-weight components remained mostly in the bottom, pyrolytic-lignin-rich phase, thus being removed from the upper, bio-oil/biodiesel mixture.
Mixture Properties after Aging. To evaluate the stability of mixtures, several parameters, such as water content, viscosity, acid number, and MWD, were measured before and after storage. Water Content and Acid Number. Changes in the water content during storage can be a result of competing mechanisms of hydration consuming water and esterification and/or polycondensation reactions producing water. The initial water content of the bio-oil/biodiesel mixture was much lower than the initial bio-oil (Table 2). However, as shown in Figure 1, the water content in the mixture increased by an average of 31-42% over time, especially when stored for 180 h at 80 °C. In comparison to other aging studies of bio-oil, where a ∼8% increase in water content has been shown for pine and brown forestry residue bio-oil, at 26-27 wt %,30 the water content of the mixture remained at ∼0.5 wt %. When the initial bio-oil concentrations in the mixture are compared, the higher the bio-oil concentration, the higher the increase in the water content observed.
(29) American Society for Testing and Materials (ASTM). ASTM D6751. Standard Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels; ASTM: West Conshohocken, PA, 2009.
(30) Oasmaa, A.; Kuoppala, E. Energy Fuels 2003, 17 (4), 1075–1084.
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Figure 2. Acid number of the bio-oil/biodiesel mixture with time.
Figure 3. Viscosity change of the bio-oil/biodiesel mixture over time.
Contrary to the trend shown by aging of bio-oil alone, where the acid numbers increased from 79.23 to 125.58 mg of KOH/g when aged at 80 °C for 180 h, acid numbers of the mixture decreased over time, as shown in Figure 2. This decrease coupled with an increase in the water content over time suggests an esterification reaction consuming acids and producing water. The mixture of initial 40 vol % bio-oil resulted in the highest acid number because of the largest fraction of bio-oil being contained in the mixture. Viscosity and Average Molecular Weight. As shown in Figure 3, the viscosity of the stored blends exhibited a slight increase at first, followed by an overall decrease over 180 h. The larger the initial fraction of bio-oil added to the mixture, the higher the viscosity before and after aging. Furthermore, using GPC, the MWD of the blends was measured. The retention time determined by GPC depends upon not only the molecular size but also the functional groups of the chemical compounds included. A proper standard compound is needed to obtain calibration curves for the GPC measurements. However, owing to its complexity of the mixture, the relative average molecular weight of the samples was compared, as shown in Figure 4, indicating an average of 15% increase in average molecular weight for all samples. The esterification of acidic compounds may have increased
the water content, which in turn promoted the homopolymerization reaction, producing higher molecular-weight oligomers or polymers.31 The initial increase in viscosity between 10 and 60 h may be attributed to the production of these higher molecular-weight compounds. However, the decrease in viscosity while observing an increase in the average molecular weight requires further investigation on the changes in chemical composition. Functional Groups. FTIR spectroscopy is used to observe the changes in functional groups of the mixtures over time, as shown in Figure 5. IR spectra of the blends exhibit absorption peaks in the region between 700 and 4000 cm-1, which appear to be influenced by the aging process. The broad peak of O-H stretching in hydroxyl groups between 3050 and 3600 cm-1 indicate the presence of water.32 The increasing trend in the peak is in agreement with the higher water content of the stored mixtures measured by the Karl Fischer titration method. The absorbance at 1050 cm-1 (stretching of C-O-H) and 1200 cm-1 (stretching of C-O-C) suggests (31) Diebold, J. P. A review of the chemical and physical mechanisms of the storage stability of fast pyrolysis bio-oils. National Renewable Energy Laboratory (NREL) Internal Report, Golden, CO, 1999; Vol. 26. (32) Yurgun, S.; Simsek, Y. E. Bioresour. Technol. 2008, 99, 8095– 8100.
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Figure 4. Average molecular weight of the bio-oil/biodiesel mixture with time.
Figure 6. (a) TG and (b) DTG curves of the initial 40 vol % bio-oil and aged samples. Figure 5. FTIR spectra of the mixture with (a) initial 20 vol % biooil and (b) initial 50 vol % bio-oil mixtures aged at various conditions.
in the broadband at 3300-3600 cm-1 may suggest the presence of ketone or aldehyde groups.34 Another route of water production is the acetalization reaction between aldehydes and alcohols.31 However, in some samples, the
the formation of ester and ether during aging.33 A change in carbonyl peaks at 1700 cm-1 (CdO) along with the increase
(34) Gomez-Serrano, V.; Pastor-Villegas, J.; Perez-Florindo, A.; Duran-Valle, C.; Valenzuela-Calahorro, C. J. Anal. Appl. Pyrolysis 1996, 36 (1), 71–80.
€ (33) P€ ut€ un, A. E.; Ozcan, A.; P€ ut€ un, E. J. Anal. Appl. Pyrolysis 1999, 52, 33–49.
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Table 3. TGA Results of the Mixtures Made of Initial 40 vol % Bio-oil at Various Aging Conditionsa parameters
initial bio-oil / biodiesel mixture
aged at 12 h for 60 °C
aged at 12 h for 80 °C
aged at 60 h for 60 °C
aged at 60 h for 80 °C
aged at 180 h for 60 °C
aged at 180 h for 80 °C
T0 Tf Tp Rs
62 335 257 0.32
82 341 272 0.37
92 343 284 0.44
93 357 288 0.68
95 362 287 0.65
93 362 284 0.69
98 368 293 0.72
a T0, temperature (°C) corresponding to 1 wt % of sample weight loss; Tf, final temperature (°C) corresponding to 99 wt % of sample weight loss; Tp, peak temperature (°C) corresponding to a maximum weight loss; and Rs, final residue (wt %).
Figure 7. 1H NMR spectra of the bio-oil/biodiesel mixture (initial 40 vol % bio-oil) before aging: (A) 0.5-3.0 ppm, (B) 3.0-6.0 ppm, (C) 6.0-10.0 ppm, and (D) 0.0-10.0 ppm.
carbonyl frequency at 1700 cm-1 decreased slightly, indicating a possible further polymerization or degradation in the stored bio-oil/biodiesel mixture. In general, the changes in peaks are not substantial, indicating the stability of the mixture under aging conditions investigated. TGA. Typical thermogravimetry (TG) and differential TG (DTG) curves of the mixtures with an initial 40 vol % bio-oil mixture are presented in panels a and b of Figure 6, respectively. The aging conditions did not affect the trend in the thermal decomposition, where the decomposition always happened at higher temperature than the initial unaged
sample. Furthermore, Table 3 shows the TGA results of the initial 40 vol % bio-oil mixtures aged at various conditions. It indicates that the temperature corresponding to the initial 1 wt % loss slightly increased with an increasing storage temperature and time. This may be reflective of some polymerization and/or condensation reactions during storage. The peak temperatures corresponding to the maximum weight loss have slightly increased (up to 14%) through aging. The final residue (wt %) of all of the samples remained less than 0.8 wt %, indicating that very little high-molecularweight compounds have remained in the bio-oil/biodiesel 2704
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: DOI:10.1021/ef901517k
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Figure 8. 1H NMR spectra of the bio-oil/biodiesel mixture (initial 40 vol % bio-oil) after aging at 80 °C for 180 h: (A) 0.5-3.0 ppm, (B) 3.0-6.0 ppm, (C) 6.0-9.0 ppm, (D) 9.0-10.0 ppm, and (E) 0.0-10.0 ppm.
mixture. As indicated by the GPC analysis, the TG and DTG curves may also reflect the polymerization or condensation reactions that changed the volatility of the compounds. On the other hand, the minimal change in the thermal decomposition trend with various aging conditions further strengthens the general stability of the mixtures during aging. From Figure 6b, a two-stage decomposition under the nitrogen flow is emphasized. At first, because of the highvolatility composition, such as formaldehyde, acetone, methanol, and ethanol, the mixture went through a relatively fast weight-loss stage between the temperatures of 50 and 130 °C, and second, beyond 200 °C, the decomposition of nonvolatile compounds, such as oligomers, remaining in the mixture occurred. Owing to the very low water content (even after aging with a slight increase), a peak on the DTG curve as a result of the evaporation of water was not shown. 1 H NMR. The 1H NMR spectra of bio-oil/biodiesel mixture samples before and after aging are shown in Figures 7
and 8, respectively, for an initial 40 vol % bio-oil mixture. The selected regions of the spectra on a quantitative percentage basis are presented in Table 4. The spectral region between 0.5 and 1.5 ppm represents aliphatic protons that are attached to carbon atoms. Approximately 55 wt % of the bio-oil/biodiesel mixture indicates a high aliphatic content, resulting from the dominant fraction of the aliphatic chain of biodiesel in the mixture. The small change of the aliphatic proton composition in the aged mixture reflects the thermal stability of the mixture. The peaks of the region from 1.5 to 3.0 ppm, representing protons bonded to the carbon atoms of acetylenic, benzylic, allylic, ester, acid, and carbonyl compounds, have changed from 24.6 to 26.1% after aging, further supporting the condensation and/or esterification reaction during aging. The slightly decreasing trend of the proton in the 1H NMR spectra region between 3.0 and 4.5 ppm in the aged sample may indicate that some alcohol and/ or ester compounds reacted with other functional groups 2705
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Table 4. Distribution of H in the Bio-oil/Biodiesel Mixture before and after Aging Determined by 1H NMR distribution of H as a percentage of total H in chemical shifts (ppm)
type of H
0.5-1.5
RCH3, R2CH2, R3CH CC-H CdCCH3, H-C-COOR, Ar-C-H H-C-COOH, H-C-CdO H-C-OH, H-COR, RCOO-C-H CdC-H Ar-H R-(H-)CdO
1.5-3.0 3.0-4.5 4.5-6.0 6.0-8.5 9.5-10.0
bio-oil/biodiesel mixture before aging (%)
bio-oil/biodiesel mixture aged at 80 °C for 180 h (%)
54.7
55.6
24.6
26.1
9.21 8.90 1.46 0.235
during storage. In the region from 4.5 to 6.0 ppm, where phenolic or olefinic protons32 are detected, only weak peaks are present. This further confirms that the sources of ligninderived methoxy-phenols from the pyrolytic lignin35 are very small in the mixture and do not change over time. The minor peaks detected in the 6.0-8.5 and 9.5-10.0 ppm regions represent aromatic proton and aldehydic compouds, respectively. The slight changes in peaks may be attributed to reactions with other active proton and/or hydroxyl radicals and aldehydic compounds, respectively. However, any changes related to the higher chemical shifts are considered minimal.
8.61 8.57 0.589 0.003
slightly increased thermal decomposition temperature with the TGA analysis. The FTIR spectra have shown very little changes in the typical functional groups for bio-oil and biodiesel, expect for the increase because of the water content. 1 H NMR spectra have shown some reactions among the active protons bonded to different carbon atoms. Overall, the change in the aged bio-oil/biodiesel mixtures is considered to be minimal and, thus, suitable to consider as a stable fuel during storage. Acknowledgment. The authors acknowledge the financial support from the China Scholarship Council (CSC), Natural Science and Engineering Research Council of Canada (NSERC), Natural Resources Canada, BIOCAP Canada, Canadian Funding for Innovations (CFI), and National Basic Research Program of China (973 Program, 2007CB210208).
Conclusions Emulsifying bio-oil with biodiesel is shown to be a promising method in upgrading bio-oil as a fuel while extracting high-molecular-weight pyrolytic lignin. All of the mixtures tested remained in a single phase throughout the studied aging conditions of 60 or 80 °C of up to 180 h. Changes in certain properties and the chemical structure of mixtures as a function of the time and temperature of storage have been investigated using various techniques. The water content of the mixtures has shown a slight increase over time yet remained less than 0.5 wt %. The acid number and viscosity have indicated an overall decrease during storage. GPC analysis has indicated a slight increase in the average molecular weight, in line with a
Nomenclature HHV = higher heating value, calculated by eq 1 (MJ/kg) Rs = final residue (wt %) S = volume of bio-oil dissolved in unit volume of biodiesel T = temperature (°C) T0 = temperature corresponding to 1 wt % of sample weight loss (°C) Tf = temperature corresponding to 99 wt % of sample weight loss (°C) Tp = peak temperature corresponding to a maximum weight loss (°C)
(35) Mullen, C. A.; Strahan, G. D.; Boateng, A. A. Energy Fuels 2009, 23, 2707–2718.
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