Fast Pyrolysis of Oil Mallee Woody Biomass: Effect of Temperature on

A recent life-cycle energy balance4 indicates that mallee biomass production in WA achieves an ..... The thermal behavior of pine is not reported here...
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Fast Pyrolysis of Oil Mallee Woody Biomass: Effect of Temperature on the Yield and Quality of Pyrolysis Products Manuel Garcia-Perez,†,‡ Xiao Shan Wang,† Jun Shen,†,| Martin J. Rhodes,† Fujun Tian,† Woo-Jin Lee,† Hongwei Wu,§ and Chun-Zhu Li*,† Department of Chemical Engineering, P.O. Box 36, Monash UniVersity, Victoria 3800, Australia; Department of Biological Systems Engineering, Washington State UniVersity, Pullman, Washington 99164; Department of Chemical Engineering, GPO Box U1987, Curtin UniVersity of Technology, Western Australia 6845, Australia; and Department of Chemical Engineering, Taiyuan UniVersity of Technology, Taiyuan, Shanxi 030024, People’s Republic of China

This paper presents an investigation of the production of crude bio-oil, char, and pyrolytic gases from the fast pyrolysis of mallee woody biomass in Australia. The feedstock was ground, sieved to several narrow particle size ranges, and dried prior to pyrolysis in a novel laboratory-scale fluidized-bed reactor. The effects of pyrolysis temperature (350-600 °C), and biomass particle size (100-600 µm), on the yields and composition of bio-oil, gas, and char are reported. In agreement with previous reports, the pyrolysis temperature has an important impact on the yield and composition of bio-oil, char, and gases. Biomass particle size has a significant effect on the water content of bio-oil. It is interesting to note that the temperature for maximum bio-oil yield, between 450 and 475 °C, resulted in an oil with the highest content of oligomers and, consequently, with the highest viscosity. Such observations suggest that the conventional viewpoint of pyrolyzing biomass at temperatures over 400 °C to maximize bio-oil yield needs to be carefully reevaluated, considering the final use of the produced bio-oil. The increases in oil yield with increasing temperature from 350 to 500 °C were mainly due to the increases in the production of lignin-derived oligomers insoluble in water but soluble in CH2Cl2. The yield and some fuel properties of the pyrolysis products were compared with those herein obtained for pine as well as those reported in the literature for other lignocellulosic feedstocks but using similar reactors. 1. Introduction In Western Australia (WA), mallee eucalypts are being developed as woody crops for managing dry-land salinity in the low-to-medium rainfall (300-600 mm mean annual rainfall) “wheat-belt” agricultural area.1-3 Since the early 1990s, more than 11 000 ha (30 million trees) have been planted across the region.2,3 Mallee crops integrated into wheat-belt agriculture systems deliver significant environmental benefits such as salinity control, carbon sequestration, and biodiversity. Mallee is a dedicated crop of multibranched shrubs or short trees able to be harvested on a short cycle and able to rapidly regenerate as coppice for every 3-4 years. A recent life-cycle energy balance4 indicates that mallee biomass production in WA achieves an energy ratio of 41.7 and an energy productivity of 206.3 GJ/(ha year). Such energy performance is considerably better than those achieved by annual energy crops, e.g., canola, which have energy ratios typically less than 7.0 and energy productivities less than 40.0 GJ/(ha year). In WA alone, the extensive adoption of mallee could potentially supply ∼10 million (dry ton of biomass)/year.5 However, the economic viability of this program will depend on the development of fuels and commodity chemicals from this now-abundant and underutilized bioresource.2,6,7 The production of bio-oil and charcoal from the pyrolysis of mallee biomass is among the possible routes to establish a mallee-based industry. In the past 30 years, significant progress has been made in developing pyrolysis technologies for converting lignocellulosic materials into fuel and chemicals. The main advantage of these * Corresponding author. Tel.: +61 3 9905 9623. Fax: 61 3 9905 5686. E-mail: [email protected]. † Monash University. ‡ Washington State University. § Curtin University of Technology. | Taiyuan University of Technology.

technologies is that the liquid produced can be stored and transported to biorefineries where it can be most effectively converted into transportation fuels and chemicals. New pyrolysis technologies have been proposed and successfully tested at laboratory, pilot, and industrial scales.8-14 In spite of all this technological progress and the large amount of experimental data reported in the literature, there is still considerable debate over the reaction mechanisms controlling the distribution of fast pyrolysis products.15-17 The mechanisms proposed so far are unable to fully explain the drastic differences observed in the yields and properties of oils obtained by different pyrolysis technologies. For example, the secondary intraparticle heterogeneous reactions as well as the mechanisms leading to the formation of oligomers are still poorly understood. In the early 1980s, it was established that maximum yields of bio-oil could be obtained at low pressures, at pyrolysis temperatures between 450 and 550 °C, at very high heating rates, using small particles (diameters < 2 mm), and under conditions minimizing the contact between vapor and char while achieving the rapid cooling of volatiles.11,18-20 However, it is still unknown if these conditions also result in a product of better quality.18 It was initially thought that the high yield of oil observed with fast pyrolysis could be explained by the low extent of secondary reactions in the vapor phase when the apparent residence times of volatiles inside the reactor were 2 times the minimum fluidization velocity. The corresponding residence time of vapor in the reactor was estimated to be ∼1.4 s and ∼0.7 s in the cyclones. Pressure built up in the reactor never exceeded 0.15 atm. Temperatures in the bed, in the free board, and at the exit of the cyclones were continuously monitored. Excellent stability and reproducibility of these temperatures were achieved. The biomass was directly fed into the silica sand bed 5 min after the desired temperature was achieved. The biomass particles were heated up rapidly to bed temperature. Two cyclones in series were used to separate the char particles. The upper cylindrical portions of the first and second cyclones were 6 and 3 cm in diameter, respectively, and were designed to have a collection efficiency of 50% for particles of 6 µm diameter. While the temperatures in the reactor bed and in the free board were varied between 350 and 580 °C in different pyrolysis tests, the temperatures in the cyclones were maintained at 420 °C. This temperature was chosen to prevent condensation of vapors while minimizing the secondary reactions that could lead to the formation of extra gas. Some exploratory tests using different cyclone temperatures between 390 and 420 °C were performed to check whether there were changes in the yield of gases. Little change was observed. The char collectors under the cyclones were maintained at ambient temperature. Bio-oil condensation and collection was realized in three sequential steps. The first step occurred in a jacketed tubular condenser with a cooling water coil, which collected between 57 and 72 mass % of the total oil. The temperature of vapor exiting this condenser was typically ∼23 °C. Fractions of very low boiling points and aerosols, representing between 18 and 34 mass % of the total oil, were collected in the second condenser. This unit used dry ice as cooling medium and was equipped with a series of meshes (90 µm) to enhance aerosols coalescence. Gas exit temperature of ∼ -8 °C was achieved in this condenser. Most of the remaining aerosols, representing between 8 and 20 mass % of the total oil, were trapped by an aerosol filter. This filter contained a 240 mm Advantec Quantitative filter paper no. 1 with 65% collection efficiency for solids over 0.3 µm. The flow rate of the carrier gas was measured with an orifice plate. The flow rate of pyrolysis produced gases was determined by the difference between the gas flow rates with and without feeding biomass. Mass balances were performed through quantification of all the inputs and outputs. The second condenser, the aerosol filter, and the char collectors were weighed before and after every test. The oil collected in the first condenser was stored in 500 mL graduate bottles and weighed. The quantity of bio-oil remaining on the walls of the first condenser was estimated by evaporating the solution resulting from acetone washings. It typically accounted for 1.9 mass % of the biomass substrate. All the liquids collected were mixed before analysis. The yields of oil, char, and gases were determined as the ratios between the collected materials and the dry bio-mass fed to the hopper. 2.3. Gases Analyses. Gas samples were collected in sample bags downstream of the aerosol filter. The main products (CO, CO2, CH4, H2, CH4, C2H4, C2H6, C3H6, C3H8, C4H8, and C4H10) were analyzed by GC. All the hydrocarbons were analyzed with an HP 5890 gas analyzer equipped with a HeyeSep DB column

(15 ft × 1/8 in.) and a flame ionization detector (FID). Carbon monoxide, carbon dioxide, and hydrogen were quantified in a Perkin-Elmer GC with a molecular sieve column and a Porapack N column followed by a thermal conductivity detector (TCD). Argon was used as a carrier gas for both pieces of equipment. 2.4. Bio-oil Analyses. The oils were stored in a dark fridge at 4 °C before analyses. The water content was determined by Karl Fischer titration (KF DL31 from Mettler-Toledo) using 5-keto as a titrant. The elemental composition and the calorific values of these oils were determined at HRL. The calorific value was measured using a Leco AC350 calorimeter according to the Australian Standard (AS1038.5). The content of ligninderived oligomers was determined by precipitation in cold water. Briefly, 3 g of bio-oil was added dropwise, to 300 mL of icecooled water under strong agitation. The water-insoluble fraction was removed by filtration. The solid was washed with water for 5 min and further extracted with dichloromethane until the filtrate was colorless. The remaining solid in the filter was dried at 105 °C for 1 h. The CH2Cl2 in the CH2Cl2 solution was rotary evaporated at 40 °C, and the remaining solid residue was weighed and reported as the “water-insoluble-CH2Cl2-soluble fraction”. The viscosity of the bio-oils was measured under dynamic shearing conditions with a controlled strain Fluids spectrometer RFS II rheometer. Oil (10 mL) was first homogenized and placed for at least 5 min in a temperature-controlled measurement cell at the desired temperature (22 °C) prior to measurement. Two concentric cylinders formed the measuring cell. The inner cylinder with an external diameter of 32 mm and a length of 32 mm was driven at a frequency between 50 and 100 rad/s. The external cylinder (cup) had an internal diameter of 34 mm and a length of 44 mm, so the gap between the cylinders was 1 mm. Strain sweep experiments at a frequency of 50 rad/s were initially performed to identify the linear viscoelastic region of the oils. The viscosity values reported were all obtained in the linear viscoelastic region. 2.5. Char Analyses. The procedures for determining the proximate and elemental compositions of chars were very similar to those previously described for the biomass substrates. The char samples were ground to particle sizes below 200 µm prior to analyses. 3. Results 3.1. Biomass Analyses. The TG and differential thermogravimetry (DTG) curves of mallee biomass with particles between 180 and 450 µm performed at heating rates between 10 and 60 °C/min are shown in Figure 2. The thermal behavior of pine is not reported here since it has been described elsewhere.35 As in the case of other woody biomasses, the thermal degradation of mallee biomass is a process that shows separated regions. A shoulder, evident at ∼300 °C, is commonly attributed to the thermal degradation of hemicellulose. The peak observed between 350 and 390 °C (at 10 °C/min) corresponds to the thermal degradation of cellulose. This peak tends to shift to higher temperatures as the heating rate increases. The thermal degradation of the lignin spans over a wide range of temperature.36 The final solid residue obtained at 500 °C does not change much within the range of heating rates studied and was ∼18 mass %. The SEM images shown in Figure 3 confirm that the biomass particles have irregular shapes. The biomass particles are commonly nonstandardslarge in size and elongated in shape.37 The shape of the particles is very important since it affects the fluidization behavior as well as the performance of the feeding system. The terminal settling velocities that control the entrainment of particles by gases, and therefore the residence times of particles in the reactor, depend on the particle shape, density, and sizes, Reynolds number, roughness of the external

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Figure 2. Thermogravimetric analysis of mallee biomass.

Figure 3. SEM of biomasses used in this study: (a) mallee and (b) pine chips.

surface, permeability, and compressibility of the gas, among other factors.38,39 While the tests with oil mallee biomass were carried out mostly with particles between 180 and 450 µm, the tests with pine were performed with particles between 450 and 600 µm. This is because pine particles with diameters between 180 and 450 mm tended to bridge in the hopper, preventing smooth feeding. 3.2. Yield of Products. The yields of products and the water content of bio-oils obtained from the pyrolysis of mallee biomass between 350 and 580 °C, with particle sizes between 180 and 425 µm, are presented in Figure 4. Also plotted in Figure 4, for the purpose of comparison, are the results obtained at 500 °C with pine, 425-600 µm in size. Higher yields of oil were obtained with pine. All the yields are reported on the basis of dry feedstock. Bed temperature is undoubtedly an important parameter affecting the yields and properties of the pyrolysis products.40 The yield of bio-oil from mallee biomass ranged between 54 and 63 mass %, attaining a maximum at ∼475 °C. The yield of char decreased, while that of the gas increased, over the range of temperatures studied. The yield of char is 5 mass % smaller than that determined at low heating rates using TGA (Figure 2). The yields obtained are in agreement with those reported in the literature.12,21,41 Any small difference in the yields observed can be explained by the differences in feedstock, pyrolysis conditions, and experimental errors. While the mass closures obtained by Wang et al.21 and Piskorz et al.41 varied between 94 and 104%, the closure obtained in this work was between

86 and 96 mass %. Considering the mass closures, it can be stated that the differences in yields obtained are within the experimental errors. The lower mass balance closures obtained in this study using a large experimental set up can be attributed to the difficulties in weighing heavy components, to the char trapped in the sand bed, to the char retained in the cyclones, to accumulated errors in quantifying the gases, to the tars converted into coke retained inside cyclones and hot pipes, and to the evaporation of very volatile compounds between the end of a pyrolysis run and the moment when the weights of the condensers were taken. These factors are difficult to estimate in large experimental apparatuses such as the one used here. Generally, the closure observed in large experimental setups tends to be lower than in small installations.42 On the other hand, the water content of bio-oil herein obtained and that reported by Piskorz et al.41 were very different from those found by Wang et al.21 These differences are possibly due to the different particle sizes. Figure 5 shows a compilation of data reported in the literature8,21,32,34,43-45 correlating the variation of pyrolytic water yields with particle size for different pyrolysis reactors operating at ∼500 °C. The water yield increases drastically with particle diameter. The figure contains data obtained by vacuum, fast, and auger pyrolysis reactors, so this correlation seems to be general and independent of the type of pyrolysis reactor employed. The data reported in Figure 5 do not include fixedbed slow pyrolysis reactors to avoid introducing the impact of

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Figure 4. Effect of pyrolysis temperature on the yields of liquid, char, gas, and the water content of bio-oil.

Figure 5. Relationship between particle size and the yield of pyrolytic water; (1) Herein obtained; (2) Wang et al.;21 (3) Garcia-Perez et al.;34 (4) Lede et al.;47 (5) Kang et al.;48 (6) Scott et al.;8 (7) Garcia-Perez et al.;32 (8) Agblevor et al.54

secondary extraparticle reactions, which are known to affect char yield significantly.46 Figure 5 is a clear indication of the important effects of intraparticle dehydratation secondary reactions on the quality of the bio-oils. It can be stated that the particle size is one of the most important parameters controlling the yields of pyrolysis water for similar lignocellulosic materials pyrolyzed at a similar temperature. The progressive thermal scission of chemical bonds in cellulose, hemicellulose, lignin, and extractives occurs when the biomass is heated. These reactions are the so-called primary thermal decomposition (pyrolysis) reactions. The species formed by this initial depolymerization could undergo additional

Figure 6. Effect of pyrolysis temperature on the yields of water and organic liquid.

cracking to form volatiles or may undergo condensation/ polymerization reactions to form part of the char. The volatile species released have to travel through the shell of the char formed around the particle. So, the surface of the pyrolyzing biomass/char particle could catalyze the dehydratation of some primary pyrolysis products. It has been proved that the secondary heterogeneous reactions catalyzed by the fresh char could be responsible for an increase in the yield of char up to 10 mass %.46 The moderate impact of these reactions on the yield of liquid is an indication that at least a part of the reactants and products of these reactions are compounds both found in the liquid. Consequently, these reactions cannot be easily studied

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Figure 7. Strain sweep tests (frequency 50 rad/s) (temperature 22 °C).

from global mass balances of the resulting phases. A more detailed analysis of the composition of liquid fractions is needed. The heterogeneous secondary reactions in the vapor phase seem to be a very particular class of polycondensation and cracking reactions with limited impact on the yield of liquid fractions.18,21 This behavior is certainly contrasting with reactions catalyzed by inorganic salts, which tend to generate water and char at the expense of organic liquids.47 It seems that the residence times of pyrolysis vapors in the shell of char formed are not long enough to fully integrate these primary products into the char structure, so these molecules find their way to the condensed liquid. The relationship herein reported between the yield of water and the particle size is very similar to the one reported by Di Blasi47 between the yield of water and the concentration of (NH4)2HPO4. Slow pyrolysis oils produced using large particles have larger contents of oxygenated alkylated naphtalenic and phenathrenic compounds compared with fast pyrolysis oils, which are usually obtained from very small particles.34,28 These small polyaromatic hydrocarbons are known to be precursors of char. The logical reactant seems to be compounds derived from carbohydrates,2,13,48 since it is well-accepted that the water results from the dehydration of carbohydrates. An important reduction in the yield of all the compounds derived from carbohydrates (hydroxyacetaldehyde, hydroxypropanone, acetic acid, levoglucosan, and levoglucosanone) has been reported by Di Blasi.47 The authors47 did not report the evolution of anhydrosugars, but it is likely that this fraction also contributes to the formation of water. It is noteworthy that the polycondensation reactions47 seem to have little impact on the yield of gases. Indeed, the yields of gas are very similar in slow and fast pyrolysis technologies. Unfortunately, most of the models proposed so far do not take into account these intraparticle secondary reactions. These models, therefore, are not suitable for predicting the quality of the bio-oils. Figure 6 presents the yields of water and organic liquids obtained. The trend for the yield of organic liquid is broadly similar to those obtained by Wang et al.21 However, the peak of the organics in our case occurred at somewhat lower temperatures. In our study, the yields of water and organics obtained correspond well to those reported for other fast pyrolysis reactors (between 10 and 15 mass %) when using small particles.41 3.3. Bio-oil Quality. Strain sweep tests were conducted at a frequency of 50 rad/s at 22 °C to identify the linear viscoelastic region of the bio-oils produced at pyrolysis temperatures between 350 and 580 °C (see Figure 7). The storage modulus (G′) obtained was very small compared with the viscous modulus (G′′), suggesting that these oils behave very much like

Figure 8. Viscosity of bio-oil at 22 °C as a function of pyrolysis temperature.

Figure 9. Effect of pyrolysis temperature on the content of water-insoluble fraction in bio-oils.

a Newtonian liquid. Non-Newtonian features have been reported for bio-oils obtained from forest residues.49 The fact that the fast pyrolysis oil studied behaves like a Newtonian fluid and does not show the sol-gel properties previously reported for other oils suggests that the oligomers present in fresh fast pyrolysis oils are mostly soluble in the matrix and do not form the micelles responsible for the formation of network structures.49,50 The larger oligomers formed during aging could be responsible for the structures observed in other bio-oils. The experimental values for the dynamic viscosity of oils produced at different temperatures are presented in Figure 8. It is interesting to see that the maximum viscosity was reached in the same range of temperature at which maximum yield of biooil was obtained. A minimum in bio-oil water content (Figure 5) and an increased amount of water-insoluble compounds (as shown in Figure 9) could explain this maximum in the viscosity. The smaller lignin-derived oligomers insoluble in water but soluble in CH2Cl222 show maxima in the same range of temperature at which a maximum in viscosity was observed

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Figure 10. Calorific value of bio-oils as a function of pyrolysis temperature.

(Figure 9). The small lignin-derived oligomers could be acceptable molecules in fuels for power generation and could also find applications for producing new materials such as binding agents, crystal growth modifiers, dispersing agents, and emulsion stabilizers, among other high-value applications. However, small lignin oligomers could be considered as a problem in transportation fuels. The relationship between the heating value of the bio-oil and the pyrolysis temperature at which it was produced is presented in Figure 10. The calorific value is expressed as a gross or high heating value (HHV) or a net or low heating value (LHV). The difference between HHV and LHV is the heat required to evaporate the water formed during combustion and is calculated with the knowledge of the hydrogen content of the bio-oil. An additional correction taking into account the water content in

Figure 11. Elemental analysis of bio-oils.

Figure 12. Effects of pyrolysis temperature on the yield of major gases.

the bio-oil was made in determining the heating value corresponding to the organic compounds (high heating value on a dry basis). The high heating value on a dry basis is expressed in MJ/(kg of organics). The high and low heating values of wet materials are expressed in MJ/(kg of bio-oil). The heating values obtained in our study are very close to those reported for other bio-oils.51 Variations of LHV and HHV (in MJ/(kg of wet oil)) are due to the changes in the water content. The increase in temperature actually leads to an increase in the heating value of organics. This monotonous increase suggests the intensification of the deoxygenation process with increasing temperature. The elemental composition of bio-oils, on as received and dry bases, is shown in Figure 11. The behavior of the oxygen content on wet basis follows the same tendency observed for the content of water. The elemental composition on dry basis confirms gradual deoxygenation of organic matter for the range of temperatures studied. So, the content of carbon in the organic matter increases gradually while the content of oxygen decreases with increasing temperature. 3.4. Gas Analyses. Noncondensable gases were also produced along with the bio-oil, as shown in Figure 4. Figure 12 shows variations of the yields of gases with pyrolysis temperature. The pyrolytic gases were rich in CO2, CO, methane, ethane, and propane. The yield of hydrogen was generally very low but increased with temperature. These results are in agreement with those reported by Di Blassi et al.52 and Piskorz et al.41 for fast and slow pyrolysis reactors. 3.5. Char Analyses. Figure 13 shows the changes in the yield of volatile matter of the char as temperature increased. The yield of volatiles matter was determined by heating the chars collected

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Literature Cited

Figure 13. Yield of volatile matter of produced char.

Figure 14. Elemental composition of char.

from cyclones to 900 °C in the absence of oxygen according to Australian Standard (AS1038.6.4). The yield of volatile matter appears to decrease with increasing temperature, presumably as a result of the intensification of polycondensation reactions to form a more stable char. These results are also supported by the elemental analyses of chars shown in Figure 14, where the contents of carbon and nitrogen also tended to increase at higher pyrolysis temperatures. These results agree with those reported by Wang et al.21 4. Conclusions Fast pyrolysis of mallee woody biomass from Western Australia was investigated. The yields of pyrolysis products were very similar to those reported for other biomasses under similar fast pyrolysis conditions. The maximum yield of bio-oil and minimum content of water in bio-oil were achieved at the same temperature range (450-475 °C). It was shown that the conditions for maximizing bio-oil yield also led to the formation of the largest amounts of small lignin-derived oligomers as a part of bio-oil, thus resulting in a bio-oil with the highest viscosity. The conventional viewpoint of pyrolyzing biomass at temperatures over 400 °C to maximize the yield of oil has to be reevaluated, taking into account the final use of the resulting bio-oil as a fuel or as a chemical feedstock. The yield of pyrolytic water appears to be significantly affected by the size of biomass particles, which may be a result of dehydration of primary products of the thermochemical degradation of cellulose and hemicellulose. Acknowledgment The financial support of this project provided by the Australian Research Council (ARC Discovery Project Grant DP0556098) is greatly appreciated. This project also received funding from the Australian Government as part of the AsiaPacific Partnership on Clean Development and Climate.

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ReceiVed for reView November 4, 2007 ReVised manuscript receiVed January 1, 2008 Accepted January 2, 2008 IE071497P