Energy & Fuels 2004, 18, 1535-1542
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Colloidal Properties of Bio-oils Obtained by Vacuum Pyrolysis of Softwood Bark: Aging and Thermal Stability Abdelkader Chaala, Tuya Ba, Manuel Garcia-Perez, and Christian Roy* De´ partement de ge´ nie chimique, Universite´ Laval, Sainte-Foy, Que´ bec, Canada G1K 7P4 Received August 25, 2003. Revised Manuscript Received June 21, 2004
The objective of this study is to provide background information on pyrolysis oil for gas turbine applications. The bio-oil investigated consists of an upper layer and a bottom layer. It has been produced by vacuum pyrolysis of softwood bark. The storage stability and the thermal behavior of the whole bio-oil, the upper layer, and the bottom layer were evaluated. The samples were stored at different temperatures (40, 50, and 80 °C) for up to 168 h and at room temperature for up to 1 year. Morphology, rheology, thermogravimetry, and differential scanning calorimetry tests were performed on the bio-oil samples after their aging. The results indicated that the properties of the whole bio-oil are significantly altered when the bio-oil was heated at 80 °C, whatever the range of time investigated. The impact after heating to 50 °C was not critical for storage and handling purposes however, as shown by a series of rheological data. It was found that the aging effect is more pronounced for the bottom layer than for the upper layer. The presence of the upper layer reduced the aging rate of the bio-oil as a whole. The molecular weight increase after heating the whole bio-oil for 1 week at 80°C was equivalent to keeping the sample for 1 year at room temperature. This increase was mainly caused by polymerization of the bottomlayer-contained compounds. Aging of the raw bio-oil at room temperature resulted in a dramatic viscosity increase during the first 60 days, after which a plateau was reached.
Introduction Pyrolysis bio-oil tends to be unstable when stored for extended periods at ambient or elevated temperature. The extent to which the properties of the pyrolysis biooils will change upon heating over a certain period of time is very important for fuel applications.1 Bio-oil must be preheated prior to its combustion in order to lower its viscosity and improve its atomization. Liquid fuels are usually pumped through a preheater toward the gas turbine injector, where a portion only of the fuel is consumed. The remainder of the preheated fuel is recirculated back into the feed tank. This was found to be problematic for bio-oils that are fed in a gas turbine because particles grow in size in the recirculation loop. This particle growth is thought to be due to polymerization reactions occurring in the heated bio-oil.2 It can also be attributed to the colloidal nature of bio-oils because agglomeration of structures is encountered.2 Rossi3 reported that the 0.8-mm-diameter holes in the fuel injector were plugged with deposits after preheating bio-oil to 90 °C prior to atomization into the furnace. This might be due to the low thermal stability of the * To whom correspondence should be addressed. Tel.: (418) 6567406. Fax: (418) 656-2091. E-mail:
[email protected]. (1) Czernik, S.; Johnson, D. K.; Black, S. Biomass Bioenergy 1994, 7, 187-192. (2) Diebold, J. P. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A. V., Ed.; IEA Bioenergy, CPL Press: Newbury, U.K., 2002; Vol. 2, pp 243-292. (3) Rossi, C. Proceedings of Biomass Pyrolysis Oil Properties and Combustion Meeting, CO, Sept 26-28, 1994; pp 321-328.
bio-oil investigated. It has also been reported that injection system parts became totally jammed and stuck 24 h after having stopped the operation.4 Similar problems such as plugged filters and nozzles at low temperature were observed by Bridgwater and Peacocke5 and Rossi.3 These problems are believed to be due to the precipitation of waxy materials and lignin derivatives. The effect of storage conditions on the physical and chemical properties of biomass pyrolysis oils exposed to elevated temperatures over extended periods of time has been studied by Czernik et al.6 The viscosity and molecular weight of the bio-oil significantly increased with time and temperature. The increase in viscosity after 3 months at 37 °C was equivalent to that of approximately 4 days at 60 °C and 6 h at 90 °C.6 A similar variation of properties was observed by Boucher et al.7 during vacuum pyrolysis of softwood bark derived oil. According to the authors, the viscosity increased significantly during the first 65 days of storage at room temperature. The bio-oil properties were altered significantly at the higher heating temperature (80 °C). For example, the molecular weight increase after heating the bio-oil at 80 °C for 1 week was (4) Casanova, K. J. Proceedings of Biomass Pyrolysis Oil Properties and Combustion Meeting, CO, Sept 26-28, 1994; pp 343-354. (5) Bridgwater, A. V.; Peacocke, G. V. C. Proceedings of Biomass Pyrolysis Oil Properties and Combustion Meeting, CO, Sept 26-28, 1994; pp 110-127. (6) Czernik, S.; Johnson, D. K.; Black, S. Biomass Bioenergy 1994, 7, 187-192 (7) Boucher, M. E.; Chaala, A.; Roy, C. Biomass Bioenergy 2000, 19, 337-350.
10.1021/ef030156v CCC: $27.50 © 2004 American Chemical Society Published on Web 08/27/2004
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equivalent to that after storing the same oil at room temperature for 1 year. It has been reported earlier that bark-derived vacuum pyrolysis bio-oil consists of two layers: an upper layer (16.3 wt %) and a bottom layer (83.7 wt %).8 Because these layers are physicochemically different, their behavior during thermal treatment and during storage at room temperature is also suspected to be different. Aging of these two layers individually and determining the influence of each layer on the aged whole bio-oil were the main objectives of this paper. Softwood bark derived bio-oils produced by vacuum pyrolysis seem to represent a category of bio-oils that are distinct of white wood bio-oils produced by alternate pyrolysis processes and whose thermal behavior has already been reported in the literature. Bark-derived oils exhibit properties similar to those of forest waste derived oils, which contain bark.9 Experimental Section Phase Separation and Sample Characteristics. The origin of the bio-oils investigated and the pyrolysis process used have already been reported.9 If left still, this kind of biooil will split into two phases. The phase separation was performed by centrifugation of the whole bio-oil using methods developed in the authors’ laboratory.8 The bio-oil was separated into an upper layer (16.3 wt %) rich in waxylike materials and a bottom layer (83.7 wt %) rich in polar compounds.8 The bio-oil upper layer is a waxylike foamy material with a low water content (3.5 wt %), while the bottom layer has a high water content of 14.6 wt %. Water was determined by Karl Fisher titration. The authors found that the main compounds in the upper layer are more or less neutral substances and include fatty and resin acids, resin and fatty alcohols, sterols, and phenolic compounds. These compounds confer a hydrophobic character to the upper layer. The viscosities at 50 °C of the upper layer, the bottom layer, and the whole bio-oil are 88, 66, and 62 cSt, respectively. The higher viscosity of the upper layer may be due to the presence of high molecular weight waxylike materials and low water content. The whole bio-oil density is approximately 1200 kg/ m3 at room temperature. The upper layer has a relatively low density (1089 kg/m3). The whole bio-oil has an acidic character with a pH of 3 and an acid number of 4.77 g of NaOH/100 g of oil. The acidity is mainly due to the presence of formic, acetic, and propanoic acids. The solid content of the upper layer, the bottom layer, and the whole bio-oil are 0.28, 0.58, and 0.55 wt %, respectively. The bio-oil gross calorific value was found to be 24.3 MJ/kg on an as-received basis. The upper layer has the highest gross calorific value (33.1 MJ/kg). Sulfur is uniformly distributed in the bio-oil. The ash content is lower for the upper layer, which contains a lesser amount of charcoal particles. The upper layer can be qualified as a desirable fuel fraction for turbines regarding its high calorific value and low corrosiveness at high temperature. Thermal Stability Tests. For the thermal stability tests, about 60 g of sample was placed in tightly closed 60-mL bottles and heated in an oven at 60 and 80 °C for 12, 48, and 168 h. The thermal stability was evaluated by measuring the variation in viscosity, molecular weight distribution (MWD), and water content. Viscosity is an important parameter when handling bio-oils. The viscosity increase with storage time is related to the (8) Ba, T.; Chaala, A.; Garcia-Perez, M.; Roy, C. Energy Fuels 2004, 18, 188-201. (9) Oasmaa, A.; Kuoppala, E.; Gust, S.; Solantausta, Y. Energy Fuels 2003, 17, 1-12.
Chaala et al.
Figure 1. Kinematic viscosity of the whole bio-oil vs storage time. molecular weight increase of the bio-oil.6,7 Thus, the viscosity measurement is an indirect evaluation of the molecular transformation into the bio-oil during its storage at different temperatures. The apparent viscosity of the bio-oil samples was measured before and after storage at 40, 60, and 80 °C using a Brookfield LVDV III+ digital viscometer with a No. 18 spindle. Other rheological properties were performed in the temperature range of 30-80 °C with the same equipment. The apparent viscosities were determined at 40 rpm (shear rate ) 52.8 s-1) for viscosities below 50 cP. For viscosities between 50 and 200 cP, the spindle speed was 20 rpm and the shear rate was 26.4 s-1. The spindle speed of 4 rpm (shear rate ) 5.28 s-1) was used for measuring viscosities between 200 and 1000 cP. For viscosities above 1000 cP, the spindle speed was lowered to 1.2 rpm to obtain a shear rate of 1.58 s-1 (only for two viscosity measurements). The MWD was measured by gel permeation chromatography (GPC) using a Waters 510 pump with a Waters 410 refractive index detector. The separation was performed using four columns in series: Shordex AD-802/S, KD-802, AD-802.5/ S, and KD-803 with exclusion limits of 5 × 103, 5 × 103, 2 × 104, and 7 × 104, respectively. The columns AD-802/S and AD802.5/S were 250 mm long, and the columns KD-802 and KD803 were 300 mm long. All of the columns had an 8 mm internal diameter (i.d.). The samples were dissolved in N,Ndimethylformamide (DMF) with 0.04 wt % lithium bromide and 0.5 wt % trichloroacetate at a concentration of 10 mg/mL. The samples were filtered [CHROMSPEC syringe filters; pore size, 0.45 µm; material, poly(tetrafluoroethylene)] to avoid plugging of the columns. The analysis was performed at room temperature, and a volume of 20 µL was injected for each sample. The same DMF solvent mentioned above was used as an eluent with a flow rate of 1 mL/min. Poly(ethylene glycol) at different molecular weights of 106, 600, 960, 1470, 4250, 7100, and 12 600 g/mol was used for calibration. The average molecular weights, such as Mn, Mw, Mz, and Mz+1, were calculated automatically using the GPC “Millennium” software version 3.05 from Waters.
Results and Discussion Storage Stability of the Whole Bio-oil at Room Temperature. For the evaluation of the bio-oil stability, the viscosity, solid content, water content, and MWD of the whole bio-oil after storage periods at room temperature were measured. The viscosity variation values for different storage periods are given in Figure 1. The results indicate that, after a storage period of 6 months at room temperature, the viscosity increase reached 43.8% (at 80 °C measurement) compared to the fresh whole bio-oil.
Colloidal Properties of Bio-oils
Energy & Fuels, Vol. 18, No. 5, 2004 1537
Table 1. Solid Content, Water Content, and MWD of the Whole Bio-oil vs Storage Time at Room Temperature storage time (months)
solid content (wt %)
water content (wt %)
Mn
Mw
Mz
Mz+1
0 1 3 6
0.55 0.56 0.63 0.84
13.0 13.37 13.45 13.23
375 351 391 397
454 421 479 491
598 569 642 662
839 875 908 970
The measured solid content (dichloromethane insolubles), the water content, and the MWD of the whole bio-oil versus storage periods are presented in Table 1. The solid content results indicate no significant change during the first 3 months (0.55 wt %). However, after a storage period of 6 months, the solid content reached 0.84 wt %, following the same tendency as that for viscosity. The water content measurements did not show a clear tendency. This may be due to the complex kinetics of various reactions that occur during storage. For a 1-month storage period, the water content significantly increased, showing that polymerization reactions between molecules with one, two, or three functional groups probably occurred. These reactions occur step by step, depending on the medium conditions. They can lead to the formation of relatively low molecular weight products and water (for example, by esterification and etherification) and/or carbonyls (for example, by hydrolysis decomposition of acetals). In terms of the solid content, the induction period for the polymerization reactions leading to the formation of a significant amount of dichloromethane-insoluble materials is greater than 3 months. The MWD is usually expressed in terms of Mn, Mw, Mz, and Mz+1. These parameters are calculated automatically by the software during GPC analysis. As can be seen from Table 1, the average molecular weight started to increase after 3 months. After 6 months of storage, the amount of compounds with a molecular weight lower than 400 decreased by 7.9 wt %, while the amount of compounds with a molecular weight higher than 500 increased by 7.8 wt %. Compounds with a molecular weight between 400 and 500 remained at the same concentration. This indicates that the amount of molecules formed with low molecular weight compounds (M < 400) is approximately equivalent to the amount of molecules that are transformed into heavier molecules (M > 500). One can conclude that most of the molecules contained in the light and medium bio-oil portions, in terms of molecular weight, do participate in the polymerization and condensation reactions during storage. A similar conclusion has been reported by Garcia-Perez et al.10 Thermal Stability. For fuel application, bio-oil should be heated in order to lower its viscosity for pumping purposes and to obtain a good atomization. Therefore, it is quite important to understand the reasons for any changes in the bio-oil properties during the whole process, with the subsequent consequences. Accelerated aging upon heating of the whole bio-oil and its fractions (upper and bottom layers) at elevated temperatures enhances secondary reactions that require high activation energy. Therefore, for the evaluation of (10) Garcia-Perez, M.; Chaala, A.; Yang, J.; Roy, C. Fuel 2001, 80, 1245-1258.
the thermal susceptibility of these materials, several parameters such as the viscosity, the MWD, and the water content were measured before and after thermal treatment. Two important reactions occur during heating: polycondensation and polymerization. The polycondensation reactions involve functional groups (OH, moving hydrogen, COOH, etc.) whose role is central to the formation of new substances. These functional groups are present in high concentrations in the whole bio-oil, particularly in the bottom layer.8 The polycondensation reactions are accompanied by a release of low molecular weight compounds such as water, alcohols, etc. When molecules contain more than two functional groups, higher molecular weight compounds can be formed. This is the case of the bottom layer compounds, which contain aromatic rings and methoxyl and hydroxyl groups. The polycondensation of such monomers leads to the formation of three-dimensional or ramified polymer molecules, which appear in the bottom layer networks. The formation of these networks is revealed by a phase separation (formation of a nonsoluble gel and a soluble sol). Polymerization reactions pass through the formation of instable organic peroxides, which can spontaneously decompose, generating radicals. Polymerization, for example, of furan derivatives, which are present in the bottom layer of the bio-oil, may occur when the pH, the temperature, and the residence time are favorable. The high content of unsaturated species contained in the bio-oil and its layers8 indicates that, during thermal treatment, polymerization reactions could take place. Oligomerization reactions may also occur between two functional acid and glycol groups to yield polyether, which is a macromolecule with only carboxylic hydroxide groups at the end of the chain. Because the bio-oil contains minerals (0.3 wt %),8 catalytic polymerization reactions can also occur. Polymers with peroxides at the end of their chain are obtained by polymerization in the presence of tertiary hydroperoxides and iron salts. These types of reactions generate molecules that particularly influence the physicochemical properties of the bottom layer and the bio-oil. Viscosity Variation. The viscosities of the upper layer, bottom layer, and whole bio-oil increased with the heating temperature and time (Table 2). The viscosity increase was significant after heating the sample at 80 °C for 168 h. The viscosity of the upper layer at 40 °C increased about 1.2 times, while the viscosities of the whole bio-oil and bottom layer increased 6.6 and 8 times, respectively. This well illustrates the contribution of the bottom layer to the whole bio-oil viscosity. The viscosity increase rate of the upper layer, bottom layer, and whole bio-oil heated for 168 h as a function of the heating temperature and viscosity measurement temperature is shown in Table 3. To help understand Table 3, one should know that calculation of the viscosity increase rate was performed by assuming a linear relationship between the viscosity and heating time. In reality, this function is not linear: for example, the viscosity increase rate (at 40 °C) of the upper layer treated at 60 °C for 12 h was nil; it increased for the following 36-h treatment time to reach 0.48 cP/h and
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Table 2. Viscosity (cP) of Samples Treated at 60 and 80 °C vs Heating Time heating time (h)
40 °C
upper layer 60 °C
80 °C
0 12 48 168
235.4 235.4 252.7 287.9
53.1 54.4 58.9 63.6
20.2 21.1 22.0 23.5
0 12 48 168
235.4 243.7 285.7 429.7
53.1 56.1 63.4 81.9
20.2 21.4 23.9 28.7
80 °C
40 °C
whole bio-oil 60 °C
80 °C
Treatment at 60 °C 209.2 55.3 233.2 61.3 302.2 77.4 378.7 89.7
21.5 22.9 28.3 31.9
284.2 289.4 364.4 488.9
73.9 74.1 89.8 104.1
21.8 22.9 26.5 30.9
Treatment at 80 °C 209.2 55.3 260.9 68.4 505.4 110.1 1670 238.4
21.5 25.4 36.4 59.2
284.2 314.2 527.1 1875
73.9 78.0 115.5 458.9
21.8 24.1 35.5 100.1
40 °C
Table 3. Viscosity Increase Rate (cP/h) of Samples vs Heating Temperature and Viscosity Measurement Temperature
bottom layer 60 °C
Table 4. Average MWD of the Upper Layer, Bottom Layer, and Whole Bio-oil Treated at 80 °C vs Heating Time
heating temperature 60 °C 80 °C measurement temperature upper bottom whole upper bottom whole (°C) layer layer bio-oil layer layer bio-oil 40 60 80
0.31 0.06 0.02
1.01 0.20 0.06
1.22 0.18 0.05
1.16 0.17 0.05
8.70 1.09 0.22
9.47 2.29 0.47
then decreased during the following 120-h treatment period to reach 0.25 cP/h. The results presented in Table 3 indicated that the viscosity increase rate at high treatment temperature (>60 °C) is fast and it is also dependent on the viscosity measurement temperature. At low measurement temperatures, the viscosity increase rate is high. This was due to the presence of the waxylike materials in a gel state. At low treatment temperatures, because of the van der Waals forces and hydrogen bonding, the waxylike materials agglomerate and gelify, causing the reduction of the sample fluidity. At high treatment temperatures, these forces become weaker, the waxylike materials melt, and consequently the viscosity of the sample decreases. This behavior was also observed by Diebold and Czernik11 and Boucher et al.12 The fact that the viscosity increase rate of the upper layer is slower than that of the bottom layer may be explained by the differences in chemical compositions of the samples. As determined before, the upper layer contains a large amount of toluene-soluble compounds (85 wt %), such as high molecular weight hydrocarbons, fatty acids, and resin acids, and a small amount of compounds found in the bottom layer (15 wt %). The slight percentage of viscosity increase for the upper layer is indeed believed to be caused by those bottom layer related compounds. The toluene-soluble fraction is more or less thermally stable. It exhibits a high induction period for thermal conversion reactions. The bottom layer contains compounds with various functional groups that are very sensitive to the temperature. These compounds interact between themselves by polymerization and condensation reactions, resulting in the formation of high molecular weight molecules. Because the whole bio-oil is a mixture of the upper and bottom layers, it exhibits the highest viscosity at low temperature. This is due to the fact that, at this temperature, the bio-oil forms a dispersive complex (11) Diebold, J. P.; Czernik, S. Energy Fuels 1997, 11, 1081-1091. (12) Boucher, M. E.; Chaala, A.; Roy, C. Biomass Bioenergy 2000, 19, 351-36.
sample upper layer
bottom layer
whole bio-oil
heating time (h)
Mn
Mw
Mz
Mz+1
0 12 48 168 0 12 48 168 0 12 48 168
379 387 395 399 376 390 397 399 375 375 396 401
476 489 510 525 444 478 501 511 454 452 524 534
660 684 734 768 567 624 700 725 598 652 744 786
948 986 1070 1111 798 836 1010 1046 839 1049 1056 1131
system where the bottom layer, which contains a high content of charcoal particles and a high water content, plays the role of the pseudocontinuous medium while the upper layer rich in waxylike materials represents the dispersed phase. According to Einstein’s theory applied for diluted dispersive systems and assuming that the bottom layer is a continuous medium, the dispersive-structured waxylike materials must increase the viscosity of the whole bio-oil. At a measurement temperature higher than 70 °C, when the structured materials melt, the interstructure frictions and the friction between molecules of the pseudocontinuous medium and the structures are significantly reduced. Furthermore, the whole bio-oil exhibits a viscosity lower than that of the bottom layer. Molecular Weight Variation. The MWDs of the upper layer, bottom layer, and whole bio-oil treated at 80 °C were determined. In these complex mixtures, Mn, Mw, Mz, and Mz+1 concepts were applied to describe the MWD (Table 4). The results are presented in Figure 2. Despite the fact that the upper layer has a low density, it exhibits a higher average molecular weight than the whole bio-oil and bottom layer. The fatty and resin acids and the high molecular weight hydrocarbons form a tridimensional network, where the bottom layer heavy components are entrapped. The bottom layer is rich in low molecular weight water-soluble compounds and contains a low amount of upper layer components. Because the whole bio-oil is a mixture of the upper layer (low amount) and bottom layer (large amount), it possesses a MWD between both layers but closer to that of the bottom layer. The variations of the average molecular weight for the upper layer, the bottom layer, and the whole bio-oil exhibited the same tendency. The concentration in low molecular weight compounds (less than 500 Da) de-
Colloidal Properties of Bio-oils
Energy & Fuels, Vol. 18, No. 5, 2004 1539 Table 5. Water Content (wt %) of the Upper Layer, Bottom Layer, and Whole Bio-oil vs Heating Temperature and Heating Time heating at 60 °C
heating at 80 °C
heating time (h)
upper layer
bottom layer
whole bio-oil
upper layer
bottom layer
whole bio-oil
0 12 48 168
3.46 3.32 3.51 3.57
14.63 14.49 14.62 15.34
13.0 12.71 13.35 13.36
3.46 3.70 3.84 3.89
14.63 15.65 16.22 16.38
13.0 13.64 14.36 14.86
Table 6. TGA Results of the Fresh and Treated Upper Layer, Bottom Layer, and Whole Bio-oila upper layer
bottom layer
whole bio-oil
parameter
fresh
treated
fresh
treated
fresh
treated
To Tf Tp Rs
30 528 263 8.1
66 563 262 7.3
56 542 176 22.4
53 562 232 20.1
53 525 237 16.4
58 560 229 18.3
a T ) temperature (°C) corresponding to 1 wt % of dry material o weight loss; Tf ) final temperature (°C) corresponding to 99% of the final residue; Tp ) peak temperature (°C) corresponding to a maximum weight loss; Rs ) final residue (wt %).
Figure 2. MWD of the upper layer (a), bottom layer (b), and whole bio-oil (c) vs heating time.
creased with heating time, while the opposite was observed for the high molecular weight components (higher than 500 Da). For the upper layer heated for 168 h, the concentration of heavy compounds (M > 500) increased by 5.6%, while an increase of 6.6% was found for the bottom layer. The effect of the heating was more significant for the whole bio-oil; it reached 9.5%. The whole bio-oil does behave synergistically because the bio-oil composition change is higher than that of its fractions when they are treated separately. Water Content Variation. The water content of the samples increased with heating temperature and time (Table 5). The samples that contain more functional groups undergo more transformation. The bottom layer, as determined before, contains a relatively higher content of low molecular weight carboxylic acids and the polar methanol-soluble fraction than the upper layer.8 During heat treatment, there is competition between reactions that yield water (for example, esterification and polycondensation), while others consume water (for example, hydration). The net increase of the water content is greater for the bottom layer than for the
upper layer, particularly at 60 °C. At 80 °C, this increase is slightly higher for the upper layer, which also contains a small amount of bottom layer compounds. These compounds contribute to the formation of water. The aging properties, in terms of water content formation, indicate that the upper layer is more thermally stable than the bottom layer at temperatures lower than 80 °C. Thermogravimetric Analysis under Nitrogen. Thermogravimetry (TG) and differential TG (DTG) curves of the thermally treated (at 80 °C for 168 h) upper layer, bottom layer, and whole bio-oil are presented in Figures 3 and 4, respectively. Valuable information is revealed by the TG curves, as illustrated in Table 6. The temperature corresponding to 1 wt % of material weight loss increased by 36 °C for the upper layer, while it remained almost constant for the bottom layer and the whole bio-oil. This behavior of the upper layer during the heat treatment might be due to the fact that its main components, including the entrapped bottom layer molecules, polymerize without formation of any low molecular weight compounds. It is worth noticing that the upper layer contains some bottom layer components and vice versa. The final temperatures Tf of the thermally treated upper layer, bottom layer, and whole bio-oil increased by 35, 20, and 35 °C, respectively. These changes in temperature showed that the bottom layer undergoes more transformation during treatment than the whole bio-oil and upper layer. The latter remained insensitive to the heat treatment. The TG data are confirmed by the DTG curves, where the temperature of the peak that corresponds to the maximum weight loss rate is observed. In the DTG curves of the treated bottom layer and whole bio-oil (Figure 4), the main peak shifted to a higher temperature (for the bottom layer, about 56 °C higher, and for the whole bio-oil, about 8 °C lower). However, the thermally treated upper layer did not exhibit any temperature shift. This difference is attributed to the different thermal sensitivities of the upper and bottom layers. Microscopic Analysis. The morphology of the treated (at 80 °C for 168 h) upper layer, bottom layer, and whole
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Figure 3. TG curves (under nitrogen flow at 10 °C/min) of the fresh and treated upper layer, the fresh and treated bottom layer, and the fresh and treated whole bio-oil. Table 7. Microscopic Analysis of the Treated Upper Layer, Bottom Layer, and Whole Bio-oil temperature (°C) structure state
upper layer
bottom layer
whole bio-oil
structure dissolution structure precipitation droplets disappearance
42.1 36.0 56.5
no structure no structure 56.5
43.0 38.9 60.0
bio-oil at 25 °C was similar to that of the fresh samples. The treated products exhibited the same colloidal dispersion, with dark and dense microstructures (Figure 5). This may be attributed to the thermocyclization and polymerization of the bio-oil components. The analytical results are presented in Table 7. The measured structure dissolution temperature (SDT) of the treated samples is similar to that of the fresh samples. The temperature at which the droplets disappeared is higher for the treated samples than for the fresh samples. This may be explained by the presence of the high molecular weight compounds formed during the treatment, which increase the viscosity and decrease the continuousmedium solvent power. This decrease can be caused by the polymerization reactions, which principally occurred between the bottom layer heavy components. DSC Measurements. DSC measurements on the thermally treated upper layer, bottom layer, and whole bio-oil were performed at a constant heating rate of 10 °C/min in the temperature range of -30 to +100 °C. The curves presented in Figure 6 are similar to those of the fresh samples that are presented elsewhere.8 One
Figure 4. DTG curves (under nitrogen flow at 10 °C/min) of the fresh and treated upper layer (a), the fresh and treated bottom layer (b), and the fresh and treated whole bio-oil (c).
endothermic peak was observed in the thermally treated upper layer and whole bio-oil in the same temperature region of 30-60 °C. The DSC curve of the treated bottom layer did not show this effect. The endothermic effects observed for fresh and treated samples are identical. One can suggest that waxylike materials did not intensively participate in the aging reactions. The measured Tp (temperature of the endothermic peak), SDT range, and enthalpy of melting (∆H) are in good agreement with the results obtained for fresh samples. The Tp values of the treated upper layer and whole bio-oil were 42.9 ( 0.2 and 50.2 ( 0.1 °C, respectively. The SDT of the treated upper layer was between 30 and 57 °C, and the enthalpy of melting was 4.69 ( 0.03 J/g. For the treated whole bio-oil, the measured SDT and ∆H were 30-60 °C and 1.29 ( 0.09 J/g, respectively. Rheological Study. The influence of the measurement temperature on the viscosity (here represented by the limiting viscosity K) of the fresh and thermally treated (at 80 °C and different times) upper layer,
Colloidal Properties of Bio-oils
Energy & Fuels, Vol. 18, No. 5, 2004 1541
Figure 5. Microphotographs taken at 25 °C for the treated upper layer (a), the treated bottom layer (b), and the treated whole bio-oil (c).
Figure 6. DSC curves of the treated upper layer, treated whole bio-oil, and treated bottom layer at 10 °C/min.
bottom layer, and whole bio-oil is presented in Figure 7a-c. This presentation shows the inflection point at which the material flow regime changes. It goes from a structured flow regime (pseudo-plastic-like behavior) to a nonstructured flow regime (Newtonian-like behavior). The ln K ) f(1/T) curves generally present two distinct
segments, forming an angle. Each segment is characterized by its own activation energy of flow. When the angle is high (approaching 180°), the sample passes smoothly from the structured state to the homogeneous state and consequently the inflection point is not clearly determined. For the bio-oil and its layers (Figure 7c), the angle decreased with the heating time because of the formation of high molecular weight compounds, which at low temperature formed gels. For the upper layer samples (Figure 7a), the angle formed by the segments is lower than that of the bottom layer and the whole bio-oil samples. The ln K ) f(1/T) curves exhibit a distinct inflection point of 46 °C for the upper layer samples, while for the bottom layer samples, a discrete inflection point of approximately 60 °C is observed (Figure 7b). The whole bio-oil segments form distinct inflection points for each heating time. The bottom layer seems to be a low dispersive-structured system; however, the upper layer is highly structured. This structuration has a substantial effect on the whole bio-oil behavior. The rheological curves of the upper layer and whole bio-oil samples present some anomalies at low measurement temperatures. These anomalies are pronounced for the 48- and 168-h treated samples. Because the whole bio-oil is formed from the upper and bottom
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emulsion that can cause the viscosity increase. The energy absorbed by the system is used to reduce the intermolecular van der Waals forces of the macropseudocontinuous medium (bottom layer), the interfragment attractions, the interdroplet interactions, the fragmentdroplet interactions, and the interactions between the droplets as well as the fragments and the continuous medium. Above 50 °C, the curves decreased normally. This confirms that the contribution of energy for the reduction of the intermolecular forces is predominant. With the decrease of the temperature, depending on the cooling rate, waxylike materials formed new gel structures. This phenomenon is responsible for plugging of the pipes when the flow is suddenly interrupted and the lines are not well-insulated. Conclusion
Figure 7. Rheological parameters of the fresh and treated upper layer samples (a), the fresh and treated bottom layer samples (b), and the fresh and whole bio-oil samples (c).
layers, it exhibits some particularities of these two fractions. The viscosity of the whole bio-oil sample treated at 80 °C for 168 h decreased in the same way as the other samples with the measurement temperature until 40 °C. At 45 °C, the viscosity was still constant, and at temperatures above 50 °C, it continuously decreased. This behavior may be explained by the fact that the structured materials plus the polymerized compounds formed started to be destroyed at 40 °C and the fragments obtained started to melt at the same time. Part of the energy provided to the system to reduce its viscosity has been absorbed by the endothermicity of the structure melting. The melting process of the dispersed fragment generates droplets in the system, forming an
Thermal stability tests indicated that the bio-oil obtained by vacuum pyrolysis of softwood bark residues can be stored at 50 °C for 1 week without any significant viscosity or solid content increases. However, the properties of the bio-oil are significantly affected when the heating temperature was raised to 80 °C. The viscosity, the solid content, and the average molecular weight of the bio-oil increased considerably when thermal treatment was performed at 80 °C. The thermal stability of the whole bio-oil is linked to the thermal susceptibility of the bottom layer compounds. The upper layer components are less thermally labile. Aging at room temperature of the raw bio-oil resulted in a dramatic increase of the viscosity during the first 65 days, after which a plateau was reached. The results showed that the presence of the upper layer compounds in the bio-oil presents advantages and disadvantages. The disadvantages are related to the biooil behavior during its handling and processing because there is formation of colloids, which must be destroyed by heat. The advantages provided by the upper layer are its high calorific value (33.1 MJ/kg) and its contribution to lowering the aging rate of the bio-oil. To take advantage of the total amount of bio-oil produced and on the basis of the compensation effect mentioned above, it is recommended to keep the upper layer in the biooil. Mixing of both layers is achieved by stirring or recirculating the oils at 45-50 °C.7 Acknowledgment. This project has been supported by NSERC. The authors thank the Pyrovac International research team for the pyrolysis tests as well as Micheline Gingras and Joanne Lagace´ for the laboratory analyses. Thanks are also due to Dr. X. Lu for the DSC analysis, to Dr. J. Yang for the TGA tests, and to Dr. H. Pakdel for the GPC analysis. EF030156V
