Effect of pyrolysis temperature on the yield and properties of bio-oils obtained from the auger pyrolysis of Douglas Fir wood

Journal of Analytical and Applied Pyrolysis 93 (2012) 52–62

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Effect of pyrolysis temperature on the yield and properties of bio-oils obtained from the auger pyrolysis of Douglas Fir wood Shi-Shen Liaw a , Zhouhong Wang a , Pius Ndegwa a , Craig Frear a , Su Ha b , Chun-Zhu Li c , Manuel Garcia-Perez a,∗ a

Biological Systems Engineering, Washington State University, WA 99164, USA Voiland School of Chemical Engineering and Bioengineering, Washington State University, WA 99164, USA c Fuels and Energy Technology Institute, Curtin University, GPO Box U1987, Perth, WA 6845, Australia b

a r t i c l e

i n f o

Article history: Received 1 May 2011 Accepted 20 September 2011 Available online 29 September 2011 Keywords: Auger pyrolysis Bio-oil Pyrolysis Douglas Fir

a b s t r a c t This paper reports the effect of pyrolysis temperature on the yield and composition of bio-oils obtained from the auger pyrolysis of Douglas Fir wood. The tests were conducted at reactor wall temperatures between 200 and 600 ◦ C. Due to the relatively low heat transfer rates achieved between the reactor wall and the biomass particles, the temperature of the solid residue obtained was much lower (between 117 and 420 ◦ C). Bio-oil yields were close (maximum yield: 59 mass%) to those reported for fluidized bed reactors (more than 60 mass%). The maximum oil yield was obtained at a reactor wall temperature of 500 ◦ C (biomass residue heated up to 328 ◦ C). At this temperature, maximum yields of Douglas Fir primary degradation products (lignin oligomers, anhydrosugars, 2-furaldehyde, 2(5H)-furanone, 2furanmethanol, ␤-methoxy-(S)- and alkylated and methoxylated phenols) were observed. The yield of products from secondary thermochemical reactions (phenol; phenol, 4-ethyl, O-cresol; phenol, 3,4dimethyl; cresol; pyrotechol, and phenol 2,4-dimethyl, methanol and gases) increased with temperature. In all cases, the yield of the products from secondary reactions was higher than those reported for fluidized bed reactors at comparable temperatures. The water yield (11 mass%) obtained with the auger reactor was comparable to those reported for other materials processed in fluidized bed reactors. The results obtained confirm that the auger reactor is able to achieve good yields of both bio-oil and bio-char but that the overall composition of the oil obtained will be affected by the slower heating rates achieved and the intensification of secondary reactions in gas phase. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In 1850, ninety two percent of the energy consumed in US was derived from biomass. The gradual shift to a fossil fuel based economy started in the 18th century when steam engines fueled with coal were introduced to power the Industrial Revolution [1]. The world’s dependence on fossil fuels augmented in the 20th century with the invention and commercialization of the internal combustion engine [2]. In 2009, almost ninety two percent of the US energy consumption was derived from non-renewable resources [3]. Our over dependency on fossil fuels is the main cause of global warming and a powerful catalyst for political instability as the known petroleum resources are gradually depleted [4]. The imbalance between fuel supply and demand is likely to continue growing; increasing petroleum prices and the risk of international conflicts [5].

∗ Corresponding author. Tel.: +1 509 335 7758; fax: +1 509 335 2722. E-mail address: [email protected] (M. Garcia-Perez). 0165-2370/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2011.09.011

Biomass pyrolysis has been used for centuries to produce charcoal. Fast pyrolysis, a relatively new concept, uses biomass particles with diameters below 2 mm to obtain high bio-oil yields [6]. It is typically performed at temperature between 400 and 600 ◦ C in the absence of oxygen. In these conditions the biomass particles are heated very fast and the vapors produced can easily escape the particle resulting in high bio-oil yields (between 60 and 75 mass%) and low bio-char yields (between 15 and 20 mass%) as well as gas by product (between 10 and 25 mass%). The gas produced is commonly used as an energy source to heat the pyrolysis reactor. The oils can be easily stored and transported to centralized or rural bio-refineries where second generation transportation fuels and chemicals can be produced with economies of scale [7]. Although fluidized bed pyrolysis is a relatively mature technology that results in high bio-oil yields [8], the use of large volumes of inert carrier gases dilutes the pyrolytic gases, which reduces the thermal efficiency of the process. Furthermore, the attrition of sand bed particles results in bio-chars with high sand content. The presence of sand in the char reduces the quality of these materials and may prevent their use in many applications. Intense erosion

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Fig. 1. Scheme of auger pyrolysis reactor without sand as heat carrier.

has been observed on heat exchange surfaces in contact with the fluidized bed. Thus, several new designs for pyrolysis reactors are being studied to overcome the drawbacks of fluidized bed systems [9]. Auger pyrolysis reactors, with and without the use of sand as a heat carrier, are among the most popular reactors being evaluated today. These reactors are robust, do not require large volumes of carrier gases, can use a wide range of biomass particles and appear to be promising for processing capacities between 50 and 100 tons/day. Several companies: Renewable Oil International (http://www.renewableoil.com/), Biogreen Advanced (http://www.biogreen-energy.com/biogreen.html), Bio-refinery (http://www.advbiorefineryinc.ca/products/), Forschungszentrum Karlsruhe (FZK) (http://www.bioliq.com/ schnell.html), International Tech Corporation (http://www. internationaltechcorp.org/IT-info.htm), eGenesis (http://www. egenindustries.com/) and Agri-tech producers (http://www.agritechproducers.com/), are currently commercializing auger pyrolysis reactors to produce bio-char, bio-oil or heat. In spite of the growing interest in developing auger pyrolysis reactors, there are very few parametric studies describing the effect of pyrolysis conditions on the yield and composition of the products obtained [10–12]. This contrasts with the large number of papers published on the effect of pyrolysis conditions on the yield and properties of bio-oils obtained in fluidized bed reactors [13–16].

Thus, the main goal of this paper is to study the effect of pyrolysis temperature on the yield and composition of bio-oils produced in an auger reactor (that does not use sand as a heat carrier) and to compare the results obtained with those reported for fluidized bed reactors. 2. Experimental methods 2.1. Biomass collection and characterization The Douglas Fir wood used as feedstock was grown in Port Angeles, Washington and was donated by Herman Brothers Logging & Construction, Incorporated (Port Angeles, WA). The samples were stored in a refrigerator at 4 ◦ C. A pioneer mill (Model number 400 HD, serial number 2404, Bliss Industries, Inc.) was used to grind the received samples to a 2 mm fraction or less. The contents of ash, extractives, cellulose, lignin, and hemicelluloses in the Douglas Fir wood were measured following ASTM methods [17–20]. 2.2. Reduction of alkali and alkaline earth metal (AAEM) content by hot water washing Alkali and alkaline earth metallic (AAEM) species (potassium, sodium, calcium and magnesium salts) are major constituents of woody biomass ash. The AAEM species are known to be strong catalysts of charcoal formation reactions during pyrolysis [21]. These species are responsible for a drastic reduction in bio-oil yield. Thus, it is desirable to remove as much AAEM species as possible before pyrolysis. The Douglas Fir wood samples were soaked with deionized water (250 g sample/3000 ml DI water) and autoclaved (Consolidated Stills & Sterilizers- P26 SteroMaster MK II) at 121 ◦ C and 15 psi for 30 min. The autoclaved samples were washed with deionized water and the liquid and solids were separated in a 10 mesh sieve. Solid samples were dried in an air oven at 105 ◦ C for 24 h to remove all the remaining water (including bone water). The content of alkalines in the sample was quantified by atomic absorption (Varian SpectrAA 220). It is important to point out that this procedure was used to ensure low alkaline content in the biomass pyrolysed but that this method is not practical in industrial conditions. 2.3. Pyrolysis

Fig. 2. Reactor wall temperature vs. temperature of solid residue produced (biochar).

Between 500 and 600 g of biomass sample with a reduced content of AAEM was dried and pyrolysed using an auger pyrolysis reactor as shown in Fig. 1. The dried biomass was introduced into the hopper of a volumetric feeder (Barbender Technologies) and fed into the auger reactor at a feeding rate of 10–12 g/min.

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Table 1 Douglas Fir composition (mass %). Ash

Extractives

Acid insoluble lignin

Acid soluble lignin

Arabinose

Galactose

Glucose

Mann/xylose

Total

0.3

1.6

23.4

0.5

1.0

3.0

46.3

21.4

97.5

Table 2 Reduction of selected metals after hot water extraction (mass %).

Douglas Fir wood Douglas Fir wood (hot water extracted) Decrease %

Ca

Mg

Na

K

Total

0.081 0.039 51.9

0.012 0.004 68.9

0.030 0.001 97.0

0.031 0.006 81.6

0.15 0.05 66.7

Nitrogen at 20 l/min was used as the carrier gas. The estimated residence time of the vapors inside the pyrolysis reactor was around 8 s. The biomass was pushed through the hot zone of the reactor with an auger screw driven by a 1 hp variable speed motor. All the tests were conducted at auger speeds of 13 rpm which corresponds to a biomass residence time inside the reactor of 1 min. A stainless-steel tube with a length of 58.5 cm and diameter of 10 cm was heated by a Lindberg/Blue M (model HTF55322A) furnace to be the hot zone. The temperature on the external wall of the reactor was recorded and maintained at set temperatures. Because of the relatively low heat transfer coefficient between the wall and the biomass moving bed, a significant temperature gradient was established between the biomass bed and the wall of the reactor. The temperature of the solid residue produced was also quantified. Fig. 2 shows the relationship between temperature at the wall of the reactor and the temperature of the solid residue before leaving the auger reactor. The charred particles were collected, left to cool for 2 h and weighed in a char pot. The pyrolysis vapors were condensed in three condensation units. The first unit was a vertical tube with cooling water coils where pyrolysis vapors were cooled to approximately 24 ◦ C. The second condensation unit consisted of four traps in series immersed in ice. The third unit, a bubbling trap with water cooled by ice, was used to precipitate the aerosols not collected in the two previous condensers. The pressure inside the reactor was kept close to atmospheric pressure (under a very slight vacuum −2 mm H2 O) by applying suction at the final condenser. A vacuum pump with a valve to regulate the suction had a dual purpose: (1) helping to suck the pyrolysis vapors from the pyrolysis reactor, and (2) the centrifugal force of the pump impeller helped to remove the aerosols that did not condense. The yield of liquid was determined by weighing the traps, the vacuum pump and the liquid collected in the first condenser. The liquid condensed in the first condenser was collected in a bottle of known weight. The liquid left on the wall of the condenser was recovered by washing it with acetone. The weight of the residues left on the wall of the first condenser was quantified after acetone removal at the rotary evaporator. The non-condensable gases were calculated by difference. Pyrolysis tests in a fluidized bed reactor at 500 ◦ C using Douglas Fir with a reduced content of alkalines was conducted at the Curtin University of Technology (Australia) for comparison purposes. The method used is described elsewhere [13,14].

2.4.2. GC/FID and GC/MS The content of methanol in the pyrolytic oil was determined with a GC/FID (Shimadzu GC-2014 with AOC-5000 auto injector)

2.4. Bio-oil analysis The chemical composition of the oils was determined using analytical techniques to quantify individual species (KF titration, GC–FID, GC/MS and IEC). 2.4.1. Water content Bio-oil water content was measured by Karl-Fisher Titration (Schott Titroline KF) using Hydranal Composite 5 K as reagent (ASTM E203-08). All the tests were conducted in triplicates.

Fig. 3. Effect of the pyrolysis temperature on the yield of products from auger pyrolysis.

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Fig. 4. Effect of the pyrolysis temperature on the yield of water in the original biomass from auger pyrolysis.

equipped with a head space analyzer (oven temperature 85 ◦ C). Briefly, the vapor in contact with the bio-oil (250 ␮l) was injected into a 30 m × 0.25 mm inner diameter column with a 0.25 ␮m film (HP-INNOW) using helium at 27.9 cm/s as the carrier gas. The GC/FID was operated in split mode (split ratio: 1:25), with an inlet temperature of 180 ◦ C, and a FID temperature of 210 ◦ C. The oven temperature of the GC was held at 45 ◦ C for 1 min followed by heating at a rate of 5 ◦ C/min until 70 ◦ C (holding time at the final temperature: 2 min). The column was then heated to 200 ◦ C (at a heating rate of 65 ◦ C/min) and held at this temperature for 5 min to ensure the removal of all heavy molecules from the column.

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The content of selected volatile organic compounds was measured with a gas chromatographer coupled to a mass spectrometer (Agilent 6890N). The instrument was calibrated with solutions of standard compounds at five different concentrations. Phenanthrene was the internal standard used to determine the response factor for each of the compounds analyzed in the conditions used in the GC/MS. Methanol solutions containing 5 mass% of oil sample and 0.2 mass% of phenanthrene were analyzed. All of the solutions were filtered (0.45 ␮m) to remove bio-char particles from the oil. Filtered solutions (1 ␮l) were injected into the inlet working at 200 ◦ C and a split ratio of 20:1. The vapors from the inlet were separated by capillary column (Agilent HP-5 MS, HP19091S433) using helium as a carrier gas (1 ml/min). The column was held at 40 ◦ C for 1 min, heated at 3 ◦ C/min till 280 ◦ C and held for another 10 min at the final temperature. The mass spectrometer was operated with a transfer line temperature of 150 ◦ C, an ion source temperature of 230 ◦ C, and an electron impact ionisation (EI) set at 70 eV. The mass of the fragments obtained were scanned from 28 to 400 (amu). Each of the peaks obtained were identified by comparing the mass spectra obtained with the ones in the mass spectra library (NIST/EPA/NIH Mass Spectral Library Version 2.0d, Fair Com Corporation).

2.4.3. Lignin-oligomers The content of lignin oligomers in pyrolytic oils was determined by cold water precipitation following the method described elsewhere [22,23]. Briefly, the oil (2.5 g) was added drop wise with a syringe into a tree-neck round-bottom flask containing DI water (100 ml) that was stirred vigorously. The flask was immersed in ice to keep the water temperature close to 0 ◦ C. The solid precipitated was separated by filtration with Whatman # 42 filter paper and

Fig. 5. Effect of the pyrolysis temperature on the yield of light compounds in the original biomass from auger pyrolysis.

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washed with DI water (50 ml) until the color of the liquid became a completely clear yellow. The filtrates were stored in the refrigerator for hydrolysable sugars analysis. The remainder on the filter paper was washed with dichloromethane until the color of the filtrate became completely clear. The water insoluble/CH2 Cl2 soluble fraction was recovered after the removal of the solvent with a rotary evaporator. This fraction is typically associated with the low molecular weight lignin oligomers [14,22–24]. The solid left on the filter will be referred to this paper as “water–CH2 Cl2 insoluble pyrolytic lignin” or as “heavy lignin oligomers” [22–24].

2.4.5. Thermogravimetric analyses All the bio-oils produced were analyzed with a Mettler Toledo (TGA/SDTA 851e ) analyzer following the procedure described elsewhere [14,24]. Briefly, 5 mg of bio-oil samples were dropped into an alumina crucible, closed by an alumina lid; and then heated at a rate of 10 ◦ C/min from 25 ◦ C to 600 ◦ C inside the furnace of the TGA. Nitrogen was used as the carrier gas and the flow rate was 20 ml/min. The solid left on the alumina crucible will be considered to solid residue of bio-oil. 3. Results and discussion

2.4.4. Sugars The content of hydrolysable sugars in the pyrolytic oils was determined by ion exchange chromatography (IEC) using a Dionex ICS-3000 with an AS 50 auto-sampler, GP 50 gradient pump, ED50 electrochemical detector, and a 30 m × 3 mm inner diameter column (CarboPac PA20). Briefly, the filtrate from the determination of pyrolytic lignin (10 ml) was hydrolyzed at 130 ◦ C with 2 ml sulfuric acid (H2 SO4 ) at 0.5 M in a sealed glass vial for 4 h. The hydrolyzed solution (0.1 ml) was diluted with E-pure water 50 times. The diluted solution was then neutralized with 1 ml sodium hydroxide (NaOH) at 0.1 M. 10 ␮l of the prepared samples were injected into the IEC. The mobile phase was deionized water and an aqueous sodium hydroxide solution at a flow of 0.5 ml/min. The column was maintained at 35 ◦ C and the detector at a pH of 10.4. The characterization and quantification of all the sugars were performed based on their residence time and on linear calibration curves of sugar standards (Fructose, Arbinose, Galactose, Glucose, Mannose and Xylose).

3.1. Biomass collection and characterization The chemical composition of the Douglas Fir wood studied is shown in Table 1. The content of each of the fractions obtained is comparable to the ones reported in the literature [25]. Almost half of the biomass is cellulose (reported as glucose), while lignin and hemicelluloses (mostly arabinose, galactose and xylose) are approximately one quarter each. The ash and extractive are minor components. 3.2. Ash composition The content of alkalines in Douglas Fir wood before and after hot water extraction is shown in Table 2. The biomass as received had 0.15 mass% of alkalines, mostly calcium. The hot water pretreatment reduced 66.7 mass% of the metals quantified. The hot water pretreatment method is more efficient at removing alkalines (Na

Fig. 6. Effect of the pyrolysis temperature on the yield of furanic compounds in the original biomass from auger pyrolysis.

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Fig. 7. Effect of the pyrolysis temperature on the yield of phenolic compounds (Group 1) in the original biomass from auger pyrolysis.

Fig. 8. Effect of the pyrolysis temperature on the yield of phenolic compounds (Group 2) in the original biomass from auger pyrolysis.

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and K) (extraction efficiencies between 81 and 97%) than alkaline earth metals (Ca and Mg) (extraction efficiencies between 51 and 69%). The extraction efficiencies obtained with hot water are similar to those reported by Mourant et al. [15]. The results obtained by Mourant et al. [15] clearly show that at the alkaline concentration obtained after hot water extraction (0.05 mass%), the alkalines have limited effect on the yield of products but are still very reactive and in quantities large enough to affect bio-oil composition. Oasmaa et al. [26] showed that at higher ash contents the yield of bio-oil decreases. 3.3. Mass balance The effect of pyrolysis temperature on the yield of products (liquid, char and gas) is shown in Fig. 3. For comparison purposes, the yield of products obtained with the same feedstock but processed in

a fluidized bed reactor at 500 ◦ C and values reported in the literature [13,26–28] for other feedstocks in other reactors are also presented. The maximum yield of liquid obtained with the auger pyrolysis reactor (59 mass%) was close to those reported for fast pyrolysis (over 60 mass%) [13,27–29]. The solid residue becomes constant (at around 13 mass%) when the pyrolysis temperature reaches 370 ◦ C. A similar yield was obtained for the same material in a fluidized bed reactor. The yield obtained follows the same trend than those reported in the literature for other feedstocks processed in fluidized bed reactors [26–29]. The yield of gas, for the auger reactor, reaches almost 40 mass% at 418 ◦ C (wall temperature 600 ◦ C). In fluidized bed reactors the yield of gases was lower (up to 30 mass% at 600 ◦ C). These results are in qualitative agreement with the effect of pyrolysis vapor residence time on the yield of pyrolysis products. While fast pyrolysis reactors operate with vapor residence times of approximately 2 s,

Fig. 9. Effect of the pyrolysis temperature on the yield of phenolic compounds (Group 3) in the original biomass from auger pyrolysis.

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Fig. 10. Effect of the pyrolysis temperature on the yield of lignin oligomers in the original biomass from auger pyrolysis.

the auger reactor used was operated at vapor residence times of 8 s and at wall temperatures considerably higher than the temperature of the solid. According to Calonaci et al. [29], the effect of vapor residence time on the yield of bio-oil is not very critical when the temperature is below 450 ◦ C. As the temperature increases any increase in vapor residence time will result in a significant reduction in the yield of oil and increase in gas production. This may explain why the increase in the yield of gases, at temperatures over 300 ◦ C (wall temperature 460 ◦ C) is more pronounced in the auger reactor (gas residence time 8 s) than in the fluidized bed reactors (gas residence time typically 2 s or less). 3.4. Karl-Fisher titration Water yield increases by increasing pyrolysis temperature, reaching a plateau of around 10–12 mass% at 270 ◦ C (wall temperature 420 ◦ C) (see Fig. 4). The water content was considerably lower than the 20 mass% typically reported for slow pyrolysis reactors (see yield reported for beech processed in the fixed bed reactor) [28] and comparable with the yields reported for some fast pyrolysis reactors for other feedstocks. The water content obtained in the auger pyrolysis reactor was higher to the yield measured when Douglas Fir with reduced alkaline content was pyrolysed in the fluidized bed reactor. The lower water yields obtained for the washed Douglas Fir processed in a fluidized bed reactor (around 6 mass%) compared with other values reported in the literature could be due to the removal of an important fraction of the AAEMs. Shen et al. [16] found that a linear correlation between the average particle size and water yield. The yield of water decreases when the size of particle decreases. This may explain the high water yield obtained for the fixed bed reactor (over 20 mass%). The formation of water during pyrolysis is associated with dehydratation, polycondensation and cross-linking reactions of cellulose [30–32]. Chaiwat et al. [31,32] proposes that one molecule of H2 O can be produced from two molecules of OH group structures and contributes to form cross-linked cellulose structures. Water could also be formed from the dehydration of hemicelluloses into furfural [33].

purposes. The yield of methanol increases as the pyrolysis temperature increases. The yields of glycolaldehyde, acetol and acetic acid reached a plateau at around 300 ◦ C (wall temperature 500 ◦ C). The results follow the same trend reported by Garcia-Perez et al. [13] and Branca et al. [28]. While methanol is mostly a product from the demethoxylation of lignin [29], acetic acid is a product derived mostly from acetylated groups in hemicelluloses. Glycolaldehyde and acetol are products from cellulose fragmentation reactions [34,35]. Acetic acid is mostly formed from the initial elimination of the acetyl groups linked to the xylan chain on the C-2 position (glucomannan) [31]. The mechanism by which these small molecules are formed seems to be very similar for all the reactors studied. The effect of pyrolysis temperature on the yield of furanic compounds is shown in Fig. 6. Furanic compounds are formed from carbohydrates (cellulose and hemicelluloses) by a dehydration reaction during fast pyrolysis [36]. The yield of furanic compounds reached a maximum when the biomass was heated between 250 and 325 ◦ C (wall temperatures between 420 and 500 ◦ C). The overall yield of furanic compounds obtained with the auger reactor was approximately 1.8 mass%. The yield of furanic compounds obtained was comparable to those reported by Garcia-Perez et al. [13] and Branca et al. [28]. 2-Furaldehyde is also a part of the products from the degradation of levoglucosan reported by Shafizadeh and Lai [37]. The yield of 2-furaldehyde is mostly in the range between 0.2 and 0.35 mass% for the fast pyrolysis of

3.5. GC/MS and GC–FID The yields of methanol, glycolaldehyde, acetic acid and acetol (in a biomass dry basis) with increasing pyrolysis temperature for the tests conducted in the auger pyrolysis reactors are presented in Fig. 5. The yields of these small molecules reported by GarciaPerez et al. [13] using a fluidized bed reactor and Branca et al. [28] using a fixed bed reactor were also plotted for comparison

Fig. 11. Effect of pyrolysis temperature on the yield of levoglucosan in the original biomass from auger pyrolysis.

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Douglas Fir, Mallee and Beech wood. The content of 2-furaldehyde, 2(5H)-furanone found was comparable with values reported by Garcia-Perez et al. [13] and Branca et al. [28]. The yield of furfuryl alcohol obtained seems higher than those reported by GarciaPerez et al. [13] and Branca et al. [28], but it follows the same behavior. Figs. 7 and 8 show how the yields of mono-phenols changes as a function of pyrolysis temperature and compares the values with those obtained by other authors Garcia-Perez et al. [13] and Branca et al. [28] for fluidized and fixed bed pyrolysis reactors. These small mono-phenols are produced by the primary and secondary depolymerization of lignin [38]. The yield of phenolic compounds from auger reactors tends to increase with increasing pyrolysis temperatures. The content of cresol and phenol, 3,4-dimethyl was higher than those reactors reported by Garcia-Perez et al. [13] and Branca et al. [28] particularly at temperatures greater than 328 ◦ C (wall

temperature over 500 ◦ C). The increase in the production of monophenols may be due to differences in the feedstock or due to the intensification of secondary reactions at higher residence times when wall temperature was over 500 ◦ C (solid residue at 328 ◦ C). The effect of pyrolysis temperature on the yields of alkylated and methoxylated phenol is shown in Fig. 9. The presence of alkylated and methoxylated structures makes these phenols more susceptible to secondary reactions at high temperatures. The mechanisms by which large phenolic compounds convert into small monophenols at high pyrolysis temperatures have been widely studied in the literature [39–43]. Clearly, the yields of large phenolic compounds reach a maximum at 328 ◦ C (wall temperature 500 ◦ C) and then decrease with increasing pyrolysis temperatures. The total yield of mono-phenols produced at 328 ◦ C (wall temperature 500 ◦ C) was close to 5 mass%. This represents a conversion of 21 mass% of the original lignin into mono-phenols. This result is

Fig. 12. Effect of the pyrolysis temperature on sugars in the original biomass from auger pyrolysis.

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very similar to the yield of mono-phenols reported in the literature for the fast pyrolysis of Mallee oil [13]. 3.6. Cold water precipitation The yield of pyrolytic lignin obtained with the auger pyrolysis reactor using Douglas Fir as well as the values reported in the literature for a fluidized bed reactor using Mallee oil [13] is presented in Fig. 10. The yield of both the water insoluble–CH2 Cl2 soluble fraction (low molecular weight pyrolytic lignin) and the water–CH2 Cl2 insoluble compounds (heavy pyrolytic lignin) show a maximum (6.8 mass%) at 328 ◦ C (wall temperature 500 ◦ C). The behavior observed was similar to the yield of pyrolytic lignin obtained in a fluidized bed reactor [13] with Mallee woody biomass in the same range of temperatures. However as the temperature increases, more intense secondary reactions occur along the auger pyrolysis reactor wall which may be responsible for the cracking of part of the light oligomeric lignin to form mono-phenols. The highest yield of lignin oligomers was obtained with the Douglas Fir sample with reduced content of alkalines (extracted with hot water) pyrolysed at 500 ◦ C in the fluidized bed reactor. This higher yield of lignin oligomers could be explained by the effect of alkaline removal [15]. The production of lignin derivatives is known to be very sensitive to the content of alkalines [15], the particle size used [16] and the condensation conditions employed [44]. Around 29 mass% of the lignin in the original biomass was collected as lignin oligomers. Overall 50 mass% of the lignin in the original biomass was converted into potential precursors of transportation fuels (monomers and oligomers), while the other half of the lignin was converted into other products (bio-char, gases, and light organic molecules). 3.7. Sugar analysis (IEC – ion exchange chromatography) and GC/MS The effect of pyrolysis temperature on the yield of sugars produced is shown in Figs. 11 and 12. The levoglucosan maximum yield using Douglas Fir was around 4 mass%. This yield agrees well with the results obtained by Garcia-Perez et al. [13] in a fluidized bed reactor using Mallee, but was higher than the results obtained by Branca et al. [28] in a fixed bed reactor using Beech. Levoglucosan is believed to be formed from the unzipping depolymerization of crystalline cellulose [37]. This anhydrosugar can be hydrolyzed and fermented to produce ethanol and lipids [45]. The impact of pyrolysis temperature on the yield of the hydrolysable sugars is shown in Fig. 12. A maximum glucose yield of 5 mass% was achieved at 328 ◦ C (wall temperature 500 ◦ C), which represents 10.8% of the original cellulose. This yield is very similar to the ones reported for other materials in the literature [45] and slightly lower than the yield obtained in the fluidized bed when the same material was processed (6 mass%). The reduction observed at higher temperatures is due to the secondary reactions in gas phase intensified by the high wall temperatures. These reactions lead to the formation of lighter molecules. 3.8. TGA of bio-oils Each of the bio-oils produced were subjected to thermogravimetric analyses (curves not shown). The solid residue obtained after heating the bio-oils produced (fixed carbon) at 500 ◦ C was measured and is shown in Fig. 13. The content of heavy fractions responsible for the production of solid residue during TGA studies gradually increased as the pyrolysis temperature increased. This indicates that heavy compounds were responsible for the formation of carbonaceous residues. These solid residues are known to

Fig. 13. Solid residue of bio-oil (Rs) vs. pyrolysis temperature.

be derived from the dehydrated cross linked sugars or from the pyrolytic lignin. 4. Conclusion The effect of pyrolysis temperature on the yield and composition of bio-oils derived from the auger pyrolysis of Douglas Fir was studied. The yield of oil obtained was comparable to yields reported for fluidized bed reactors for the range of temperatures studied. The water yield was similar to those obtained with fluidized bed reactors indicating that the extent of cross-linking reactions and other dehydration reactions were comparable. The yield of light organic compounds (acetic acid, glycolaldehyde and methanol) was around 5 mass%. Mono-phenols and furanic compounds accounted for up to 7 mass% of the original biomass. Pyrolytic lignin yields were comparable to those reported for fluidized bed reactors (around 8 mass%). A sizable fraction of the biomass is converted into unknown water soluble compounds (likely to be cross-linked sugars). New analytical techniques need to be developed to quantify and characterize this fraction. Auger reactors are promising pyrolysis systems for the conversion of lignocellulosic materials to relatively high yields of oil and bio-char. Although bio-oil yields were lower than those obtained with fluidized bed reactors, auger reactors use much lower volumes of carrier gas and result in the production of a bio-char free of sand. Acknowledgement This project was financially supported by the Sun-Grant Initiative (Interagency Agreement: T0013G-A), the US National Science Foundation (CBET-0966419) and to the Washington State Agricultural Research Center. The authors are grateful to Mr. Shuai Zhou for biomass characterization and pyrolysis studies in a fluidized bed reactor and to Mr. Oisik Das for ash analysis. This project was also partially supported by the Commonwealth of Australia under the International Science Linkages program. The authors are very thankful for their support. References [1] K. Frenken, A. Nuvolari, The early development of the steam engine: an evolutionary interpretation using complexity theory, Industrial and Corporate Change 13 (2) (2004) 419–450. [2] F. Alizon, S. Shooter, T.W. Simpson, Henry Ford and the Model T: lessons for product platforming and mass customization, Design Studies 30 (5) (2009) 588–605. [3] EIA – Monthly Energy Review, April 2010, Table 1.3. http://www.eia.doe.gov/ emeu/mer/overview.html. [4] H.B. Goyal, Bio-fuels from thermochemical conversion of renewable resources: a review, Renewable and Sustainable Energy Reviews 12 (2008) 504–517.

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