Upgrading Fast Pyrolysis Oil via Hydrodeoxygenation and Thermal

Jan 8, 2014 - Upgrading of fast pyrolysis oil via catalytic hydrodeoxygenation: Effects of type of solvents. Shima Ahmadi , Ehsan Reyhanitash , Zhongs...
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Upgrading Fast Pyrolysis Oil via Hydrodeoxygenation and Thermal Treatment: Effects of Catalytic Glycerol Pretreatment Ehsan Reyhanitash,† Matthew Tymchyshyn,† Zhongshun Yuan,† Katherine Albion,‡ Guus van Rossum,*,†,§ and Chunbao (Charles) Xu*,† †

Institute for Chemicals and Fuels from Alternative Resources, Faculty of Engineering, Western University, London, Ontario, Canada N6A 5B9 ‡ The Bowman Centre, Western Sarnia-Lambton Research Park, Sarnia, Ontario, Canada N7W 1B8 § Sustainable Process Technology, University of Twente, 7500 AE Enschede, The Netherlands ABSTRACT: The effects of stabilizing fast pyrolysis oil (PO) with glycerol via catalytic glycerol pretreatment on upgrading via hydrodeoxygenation (HDO) or thermal treatment (TT) were studied. Nonstabilized (original) fast pyrolysis oil was also upgraded via HDO or TT to obtain benchmarks. Generally, HDO decreases the molecular weight of PO. The major beneficial effect of stabilization with glycerol was reduction in molecular weight of the upgraded oil. However, it was observed that the molecular weight reduction was largely induced by the dilution effect of glycerol/glycerol-derived fragments. It should also be noted that glycerol-consuming reactions via decreasing the carboxylic acid and phenolic contents of PO (e.g., via esterification) may play a role in reducing self-polymerization during HDO or TT. Stabilization of PO with glycerol, however, led to an increase in the yield of aqueous fractions of HDO and TT due to the formation of hydrophilic fragments from glycerol and PO constituents. After HDO, the oil fractions of stabilized and nonstabilized PO exhibited similar H/C and O/C molar ratios, suggesting that oxygen removal from the oil fractions was not significantly affected by stabilization.

1. INTRODUCTION In order to meet the increasing energy demand of increasing population and economic development, lignocellulosic biomass can contribute to energy security via (partly) replacing petroleum.1−3 Furthermore, it can be a resource to produce chemicals that are currently derived from petroleum.4 However, because of its low volumetric energy content, lignocellulosic biomass needs to undergo an initial energy content densification. Energy content densification via pyrolysis converts the solid biomass into a liquid, thus improving the feasibility of further processing at the same time.5 To minimize transportation costs, pyrolysis plants can be built at points near the origin of biomass and pyrolysis oil can then be transported to processing facilities. Compared with petroleum, pyrolysis oil has a much higher oxygen content in the form of water and oxygen-containing functional groups that results in an elemental composition similar to that of the biomass from which it was derived.6,7 The high oxygen content leads to a lower heating value (LHV) of 14−18 MJ/kg, which is approximately equal to that of biomass and half that of hydrocarbon fuels.8,9 It also makes pyrolysis oil immiscible with conventional hydrocarbon fuels.10,11 The functional groups (mainly hydroxyaldehydes, hydroxyketones, carboxylic acids, sugars, and phenolics) cause pyrolysis oil to be chemically unstable.12,13 The lack of chemical stability predominantly expresses itself through self-polymerization during storage and processing.14,15 Self-polymerization of pyrolysis oil during processing is highly undesirable, as it can lead to reactor plugging and catalyst deactivation.16 These drawbacks prevent pyrolysis oil from being suitable for direct use as a transportation fuel, thereby emphasizing the necessity of further processing (upgrading). © 2014 American Chemical Society

The process of producing transportation fuels from pyrolysis oil can be integrated with the existing infrastructure by incorporating the upgrading step in a standard petroleum refinery. It may also be possible to add a stabilization step preceding the upgrading step, which can be incorporated in the pyrolysis plant. According to the above process, pyrolysis oil is stabilized immediately after production at the pyrolysis plant, and then the stabilized pyrolysis oil is transported to the standard petroleum refinery, in which the upgrading step is incorporated. Pyrolysis oil upgrading via hydrodeoxygenation (HDO) reduces the oxygen content of stabilized pyrolysis oil, thereby improving its miscibility with petroleum fractions.17 Upgraded pyrolysis oil can then be blended with conventional petroleum fractions (e.g., vacuum gas oil) and undergo corefining [e.g., via fluid catalytic cracking (FCC)] to produce transportation fuels. To summarize, the overall process is comprised of three steps; stabilization, upgrading, and corefining. The advantages of this process include the following: (1) Increased stability of pyrolysis oil to reduce selfpolymerization during storage and upgrading. Since there is a competition between self-polymerization and HDO during upgrading, stability increase can increase the selectivity of HDO reactions.18 (2) Decreased capital cost of producing transportation fuels from pyrolysis oil due to the use of the existing infrastructure for stabilization and upgrading. Received: November 11, 2013 Revised: January 8, 2014 Published: January 8, 2014 1132

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2. MATERIALS AND METHODS

(3) Reduction in hydrogen gas consumption during upgrading, as complete oxygen removal of pyrolysis oil is not necessary for co-refining of upgraded pyrolysis oil with a petroleum fraction.19 In the literature, stabilization with an alcohol (esterification) has been described as a promising route to reduce the reactivity of fast pyrolysis oil.20−22 This research examined the effects of stabilizing fast pyrolysis oil (PO) with an alcohol (i.e., catalytic alcohol pretreatment) on subsequent upgrading. Since self-polymerization reduction/ inhibition is the major expected benefit of stabilization, it was the main focus of this research. Thermal treatment (TT) was selected as a parallel upgrading route to HDO in order to obtain more insight into the effects of catalytic alcohol pretreatment on self-polymerization during upgrading. Moreover, to distinguish between the effects of stabilization and dilution (caused by added alcohol) on HDO, direct HDO of a PO/alcohol mixture (with the same alcohol:PO ratio used to stabilize PO) was performed, and the products were compared with those obtained via HDO of the stabilized PO. To produce stabilized PO via catalytic alcohol pretreatment, glycerol (a byproduct of biodiesel production) was used. However, other alcohols can also be effective to different extents. Then the glycerol-stabilized fast pyrolysis oil (GPO) was upgraded via HDO or TT, and the processed oils were compared with those obtained via HDO or TT of nonstabilized (original) PO. Figure 1 summarizes the experimental scheme.

PO was produced using the mechanically fluidized reactor (MFR) designed at the Institute for Chemicals and Fuels from Alternative Resources (ICFAR) at Western University, Canada. Biochar was the other product of this reactor. This unit was operated in continuous mode with a feed rate of 30 kg/h. Hardwood sawdust with an average particle size of 2 mm was used as the feed at a pyrolysis temperature of 500 °C with a vapor residence time of a few seconds. The resulting PO had a water content of 52 wt % and a higher heating value (HHV) of 14.6 MJ/kg. GPO was produced in a stirred autoclave supplied by Parr Instrument Company with a nominal internal volume of 500 mL. In each GPO production run, approximately 150 g of PO and 75 g of glycerol were loaded into the autoclave, and 3 wt % (on wet liquid feed basis) dry Amberlyst 35 (Rohm and Haas) was added to the liquid. Then the autoclave was sealed, and a leak test was performed at a nitrogen pressure of 20 barg for 20 min. If no leak was observed, the nitrogen was vented while removing the remaining air from inside the autoclave. The autoclave was purged twice with nitrogen at 20 barg to remove any residual air and filled with nitrogen at 2 barg as the initial pressure. The stirrer speed was set at 360 rpm, and the autoclave was heated at an approximate rate of 2 °C/min to a temperature of 120 °C and then maintained at this temperature. The retention time of the liquid inside the autoclave was 2 h including the heating time. After 2 h, the heating was stopped and a cooling water bath was used to cool the vessel to room temperature and quench the reactions. The gas was collected for analysis using a gas bag. The stirrer was kept on at 180 rpm until the entirety of gas was collected to remove any dissolved gas from the liquid. Since the amount of gas produced was negligible, its composition is not included in this article. Afterward, the autoclave was opened to collect the liquid product. No phase separation of the liquid was observed. The liquid product was collected, and the spent catalyst was separated by centrifuge. HDO experiments were performed using the same setup as previously used to produce GPO. The setup could be operated in batch or semibatch mode of hydrogen. It was operated in batch mode for all of the HDO experiments performed for this article. A schematic view of the setup is shown in Figure 2. In each HDO run, approximately 150 g of the feed [PO, GPO, or a mixture of PO and glycerol at a PO:glycerol ratio of 2:1 (denoted as PO+glycerol)] was loaded into the autoclave, and 3.33 wt % (on a wet feed basis) Ru/C catalyst (Sigma-Aldrich; Ru loading 5 wt %; used without any pretreatment) was added to the liquid. The autoclave was

Figure 1. Experimental procedure.

Figure 2. Schematic diagram of the experimental setup. 1133

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Table 1. Properties of PO, GPO, PO+Glycerol, Glycerol, and Dehydrated Glycerol PO water (wt %) C (wt %) H (wt %) O (wt %)a C (glycerol- and water-free wt %) H (glycerol- and water-free wt %) O (glycerol- and water-free wt %)a H/C molar ratio (glycerol- and water-free) O/C molar ratio (glycerol- and water-free) initial glycerol (wt %) residual glycerol (wt %) a

GPO

PO+glycerol

Elemental Composition and Water Content 51.8 35.3 34.4 23.1 29.7 28.5 9.0 8.9 8.9 67.9 61.4 62.6 47.9 48.1 47.9 6.7 7.5 6.7 45.4 44.4 45.4 1.67 1.87 1.68 0.71 0.69 0.71 Glycerol Content − 33.7 − − 15.8 −

glycerol

dehydrated glycerol

− 39.1 8.7 52.2 − − − 2.67 1.00

− 48.6 8.1 43.2 − − − 2.00 0.67

Calculated by difference.

sealed, and a leak test was performed at a hydrogen pressure of 1500 psig (103 barg) for 20 min. If no leak was detected, the gas was vented and the residual air was removed from inside the autoclave with hydrogen. Afterward, the autoclave was purged twice with hydrogen at 20 barg to remove any remaining air and then filled with hydrogen at 1000 psig (69 barg) as the initial pressure. The stirrer speed was set at 360 rpm, and the autoclave was heated at approximately 12 °C/min to 300 °C and maintained at this temperature. The retention time of the liquid and the catalyst inside the autoclave was 3 h including the heating time. After 3 h, the heating was stopped and the cooling loop inside the autoclave was used to quench the contents of the autoclave with ice-cooled water for 30 min. During internal cooling, the stirrer was kept at 360 rpm. After 30 min, the temperature of autoclave reached ∼75 °C, and for further cooling to room temperature, the reactor was immersed in an ice−water bath. Also, the stirrer speed was reduced to 180 rpm. Once at ambient temperature, the gas was collected for analysis using a gas bag. The stirrer was kept on at 180 rpm until all of the gas was collected to remove any dissolved gas from the liquid. The autoclave was then opened to collect the liquid product. Phase separation was observed in the liquid product. To improve phase separation, the liquid product was poured into centrifuge vials and centrifuged for 30 min at 4500 rpm. Afterward, two phases were obtained; an organic phase (oil fraction) containing oxygen-lean (semihydrophobic) fragments and the spent catalyst and an aqueous phase or fraction containing water and hydrophilic fragments. These fractions were separated for analysis. For the TT experiments, the procedure was similar to that for the HDO experiments except that no catalyst was added to the feed (PO or GPO). To perform the leak test and purging of the autoclave, nitrogen was used at the same pressures as for hydrogen, and the autoclave was filled with nitrogen at 1000 psig (69 barg) as the initial pressure. The heating rate, temperature set point, and residence time were identical. The quenching procedure was also the same. After the autoclave was quenched to room temperature, the gas was collected. Since a clear phase separation was observed, the two phases were separated only by decantation. The obtained oil fractions were visually very different from those obtained via HDO. They were very viscous and contained char to some extent. A higher amount of char was observed in the oil fraction of PO compared with the oil fraction of GPO. Elemental analysis and water content analysis of the oil and aqueous fractions were performed with a CHNS-O analyzer (Thermo Electron Corporation, Flash-EA-1112 series) and a volumetric Karl Fischer titrator (Mettler Toledo V20), respectively. The glycerol content was measured with a GC-FID instrument (Gentech GC-2010). The molecular weight distribution was analyzed with a gel permeation chromatograph (Waters 1525 HPLC/GPC pumps, Waters 2414 RI detector, Waters 2487 UV detector). Gas composition analysis was performed with GC-TCD instrument (Agilent Micro-GC 3000).

3. RESULTS AND DISCUSSION Table 1 compares the properties of PO, GPO, and PO+glycerol with those of glycerol and dehydrated glycerol (after removal of one molecule of water from one molecule of glycerol). Dehydrated glycerol was chosen for comparison because some connections were observed between its elemental composition and that of PO and GPO on a glycerol- and water-free basis. The glycerol- and water-free carbon and oxygen contents of PO and GPO were similar. However, GPO had a considerably higher hydrogen content, since hydrogen has a very low molecular weight. It appears that the carbon and oxygen contents of incorporated glycerol were similar to those of dry PO, as the carbon and oxygen contents of GPO (on a glyceroland water-free basis) did not significantly deviate from those of dry PO. Therefore, almost no excess oxygen was introduced into dry PO by the incorporation of glycerol. However, the incorporation of glycerol did introduce hydrogen into the dry PO. The carbon and oxygen contents of dehydrated glycerol were similar to those of dry PO, while its hydrogen content was higher than the hydrogen content of PO and GPO (on a glycerol- and water-free basis). Therefore, it seems that glycerol dehydration occurred (e.g., via esterification) and that the incorporated glycerol was partly in the form of dehydrated glycerol. Because of the possibility of considerable instrumental error during water content analysis, the measured values for water content were not used to judge the ongoing reactions on the basis of the change in water content. Glycerol content analysis indicated that approximately 50 wt % glycerol was incorporated into PO during catalytic glycerol pretreatment. Such a high glycerol conversion increased the H/ Ceff ratio, suggesting that the chemical structure of GPO likely deviated from that of PO. Figure 3 shows the molecular weight distributions of PO and GPO. The chromatograms are identical for M > 1000 g/mol. Thus, it seems that the incorporated glycerol did not contribute to a molecular weight increase of PO via polymerization. However, glycerol incorporation altered the molecular weight distribution of PO for M < 1000 g/mol (a decrease for M < 200 g/mol and an increase for 200 g/mol < M < 1000 g/mol). Figure 4 shows the water-free distribution of mass and carbon in the oil fractions, aqueous fractions, and gas fractions obtained via HDO or TT of PO, GPO, and PO+glycerol. The figure was obtained by calculating the water-free masses and carbon contents of the HDO and TT fractions on the basis of 1134

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Higher carbon contents were obtained in the gas fractions from TT of PO and GPO in comparison with those from HDO of PO and GPO, respectively, indicating that gas formation was promoted during TT. Also, the carbon content of the gas produced by the reaction (TT or HDO) of GPO was lower than that of the reaction of PO, indicating that GPO was more resistant to gasification. Since the dominant component of the gas fractions was CO2, the above pattern was observed in CO2 production as shown in Table 2. The lower CO2 production via HDO or TT of GPO suggests that GPO underwent less decarboxylation than PO.23 This reduction in decarboxylation was likely due to a decrease in the carboxylic acid content of GPO via esterification reactions between carboxylic acids and glycerol. Table 2 shows that although residual glycerol was largely converted during HDO or TT of GPO, the converted amounts were very similar. It seems that the conversion of glycerol was mostly affected by the operational conditions (e.g., temperature) rather than the different reaction routes of HDO or TT. The table also indicates that the total glycerol conversion during catalytic glycerol pretreatment followed by HDO of GPO was similar to the glycerol conversion during HDO of PO +glycerol. This may suggest that the glycerol incorporated into GPO underwent similar reactions as the free glycerol. Furthermore, glycerol (incorporated plus free) conversion during HDO was very high. Table 3 shows the properties of the produced oil fractions. The oxygen contents of all oil fractions are similar. It shows that, although TT oil fractions are not likely the potential feeds for further processing due to a higher possibility of coke formation promoted by their high molecular weight fragments (see Figure 6), oxygen removal via TT was as efficient as that via HDO, as shown by the similar molar O/C ratios for all oil fractions (0.21 - 0.25) irrespective of the feed. However, the hydrogen content of HDO oil fractions (on mole basis) is considerably higher than that of TT oil fractions indicating that a part of the consumed H2 incorporated into the oil fractions as bonded hydrogen and was used to saturate double bonds in GPO, PO and PO+glycerol, as evidenced by higher molar H/C ratio of HDO oil fractions (∼1.3) in comparison to that of TT oil fractions (∼1.1). This effect will be further discussed via (∫ UVDdv)/(∫ RIDdv) (namely GPC area ratio) in terms of nonaromatic conjugated double bonds. Consumed H2 was also used to reject oxygen from oil fractions in the form of water. Figures 5 and 6 show the molecular weight distributions of the oil fractions obtained from PO and GPO via HDO and TT,

Figure 3. Molecular weight distributions of PO and GPO.

the data supplied by mass balance and elemental/water content analyses of the HDO and TT oil fractions, aqueous fractions, and gas fractions. Although TT resulted in oil fraction yields similar to those obtained via HDO, it produced a significant amount of char (solids collected together with the oil fractions). The amount of formed char was ∼20 wt % of the GPO TT oil fraction and ∼50 wt % of the PO TT oil fraction. Char was considered as a part of the TT oil fraction to obtain Figure 4. The TT oil fractions are not likely to be potential feeds for further processing (e.g., co-refining via FCC), as they were composed mostly of high-molecular-weight fragments (see Figure 6).23 After HDO or TT of GPO and HDO of PO+glycerol, higher carbon contents were recovered from the aqueous fractions in comparison with HDO or TT of PO (Figure 4), suggesting that the fragments produced via HDO or TT of catalytic glycerol pretreatment-derived compounds and/or residual glycerol are hydrophilic, leading to higher carbon content of the aqueous fractions. Table 2 shows that these hydrophilic fragments are not free glycerol. Glycerol can react with any carboxylic acid and phenol present to form highly water-soluble monoglyceride esters and with any carbonyl function present to form 2-alkyl-4hydroxymethyl-1,3-dioxolanes as well as 2-alkyl-5-hydroxy-1,3dioxanes,24−26 which might account for the higher carbon contents in the aqueous fractions (Figure 4). These compounds, particularly the primary hydroxyl functions, would be fairly resistant to further HDO reaction and accumulate in the aqueous phase, pulling the carbon content into the aqueous phase. The relatively high concentration of glycerol used for GPO production or directly loaded with PO (PO+glycerol) underwent HDO and resulted in the formation of significant amounts of glycerol-derived fragments, thus increasing the carbon contents of the aqueous fractions.

Figure 4. Water-free distributions of mass and carbon. 1135

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Table 2. CO2 Production and Glycerol Conversion CO2 production (g of CO2/kg of feed) residual glycerol (g)a,b

HDO (PO)

TT (PO)

HDO (GPO)

TT (GPO)

HDO (PO+glycerol)

48.3 −

68.7 −

24.2 15.0

42.9 14.0

32.9 12.6

a

Since the amounts of residual glycerol in the oil fractions were negligible in comparison with those in the aqueous fractions, the values reported here represent the residual glycerol contents of the aqueous fractions. bInitial glycerol contents used to get these values: GPO, 23.5 g; PO+glycerol, 52.2 g.

Table 3. Elemental Compositions of the Oil Fractions C (glycerol- and water-free wt %) H (glycerol- and water-free wt %) O (glycerol- and water-free wt %)a H/C molar ratio (glycerol- and water-free wt %) O/C molar ratio (glycerol- and water-free wt %) a

HDO (PO)

TT (PO)

HDO (GPO)

TT (GPO)

HDO (PO+glycerol)

69.2 7.6 23.2 1.31 0.25

71.8 6.7 21.5 1.12 0.22

70.6 7.5 21.9 1.30 0.23

69.9 6.6 23.5 1.13 0.25

72.3 7.6 20.1 1.26 0.21

Calculated by difference.

Figure 5. Molecular weight distributions of HDO oil fractions.

Figure 7. Effect of dilution on the molecular weight distribution of the HDO oil fraction. It should be noted that the data in this figure were obtained using a configuration of the analysis instrument that differed from the one used for Figures 5 and 6. Because of a minor alteration in the molecular weight calibration, there is a slight shift in the molecular weight axis.

Although the oil fraction obtained via TT of GPO was composed of lighter fragments, it still contained a considerable amount of char, which may prevent it from being suitable for further processing. However, TT can be an option to upgrade GPO if the experimental conditions are optimized to produce the minimum amount of high-molecular-weight fragments. Table 4 shows the GPC area ratios for the feeds and oil fractions, which were calculated as the ratios of the surface areas obtained by direct integration of the GPC chromatograms from the UV and RI detectors. A higher GPC area ratio indicates a higher relative aromaticity and conjugated double bond content.27 The table suggests that the GPC area ratio has a similar profile for HDO or TT of the feeds; it decreases in going from PO or GPO to the HDO oil fraction and then increases in going from the HDO oil fraction to the TT oil fraction. The table also compares the GPC area ratios with the H/Ceff ratios. According to these values, although the GPC area ratio has the lowest values for the HDO oil fractions, H/Ceff has the highest values. It can be concluded that the relative aromaticity of the HDO oil fractions has the highest value (as shown by H/ Ceff; H/Ceff for aromatics ≈ 1). It is likely that lignin-derived oligomers present in the fast pyrolysis oil used in this study, although it has a sugary structure (H/Ceff for sugars ≈ 0), condensed in the HDO oil fractions. On the other hand, the HDO oil fractions most likely have the lowest nonaromatic

Figure 6. Molecular weight distributions of TT oil fractions.

respectively. Catalytic glycerol pretreatment seems to be effective in reducing self-polymerization, thereby increasing the selectivity toward HDO reactions. Also, in the presence of glycerol, self-polymerization was suppressed during TT of GPO. However, dilution largely effected the reduction in selfpolymerization, as shown in Figure 7, which compares the molecular weight distribution of the HDO oil fraction of GPO with that of PO+glycerol. Since glycerol conversion was significant during HDO of GPO and PO+glycerol, the dilution effect may be induced by either free glycerol or glycerol-derived fragments. Glycerol-consuming reactions (e.g., esterification reactions) might also play a role to some extent in suppressing self-polymerization reactions during the heating phase in HDO of GPO and PO+glycerol, but one must also take into account the fact that hydrogenation reactions are active even at low temperatures. 1136

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Table 4. GPC Area Ratiosa and H/Ceff Molar Ratios for the Feeds and Oil Fractions and H2 Consumption in HDO area ratio × 10−3 H/Ceff molar ratio H2 consumption (g of H2/kg of feed)

PO

HDO (PO)

TT (PO)

GPO

HDO (GPO)

TT (GPO)

HDO (PO+glycerol)

13 0.26 −

10 0.84 8.5

56 0.69 −

9 0.50 −

4 0.83 9.1

21 0.64 −

NAb 0.83 7.6

a Ratios of the surface areas obtained by direct integration of the GPC chromatograms from the UV and RI detectors. bNo UV detector data were available.

can be significantly different. It will be beneficial to reduce the concentration of alcohol to maximize the yield of oil fractions while maintaining the positive effects of stabilization. Thus, in our future study, various alcohols (methanol, ethanol, and glycerol) at much lower concentration(s) will be tested to examine the effects of catalytic alcohol pretreatment on stabilization and subsequent upgrading.

conjugated double bond content, as the consumed H2 was partly used to saturate them. The TT oil fractions also seem to be aromatic while having a higher concentration of nonaromatic conjugated double bonds. Although the amounts of H2 consumption are slightly different, the resulting H/Ceff ratios of the HDO oil fractions are similar. This may indicate that hydrogen gas had a significant interaction with the components of the feeds from which the hydrophilic compounds forming aqueous fractions (with water) are derived (e.g., hydrogenation of CO bonds to hydrophilic O−H functions).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.v.R.). *E-mail: [email protected] (C.X.).

4. CONCLUSIONS The effects of stabilizing fast pyrolysis oil with glycerol via catalytic glycerol pretreatment on upgrading were studied. Nonstabilized (original) fast pyrolysis oil was also upgraded to obtain benchmarks. Three major conclusions can be drawn by summarizing the observed effects: The carbon content of aqueous fractions obtained after upgrading stabilized oil was higher because of production of hydrophilic fragments from stabilization-derived fragments and/or residual glycerol during upgrading (likely monoglyceride esters via esterification reactions between glycerol and carboxylic acids and phenols), which led to lower carbon contents of the oil fractions. To maximize the yield of the oil fractions (main products), reducing the initial concentration of alcohol while performing deeper HDO may be beneficial. However, reduction in alcohol concentration may alter the extents of other effects of stabilization. Although the hydrogen content of stabilized oil was higher and more H2 was consumed during HDO of GPO, the HDO oil fraction of GPO had a H/C molar ratio similar to that of the oil fraction resulting from HDO of PO. This implies that the HDO aqueous fraction of GPO has a higher molar H/C ratio than the HDO aqueous fraction of PO. The O/C molar ratios were also similar, indicating that oxygen removal from the oil fractions was not affected by stabilization. It seems that the most effective parameters in altering the elemental composition of the oil fractions are operational conditions (e.g., temperature and residence time). Molecular weight distribution analysis indicated that the oil fractions produced via HDO or TT of stabilized oil contained lighter fragments. However, the results of direct HDO of the fast pyrolysis oil + glycerol solution (at the same ratio used for stabilization) imply that dilution plays an important role during upgrading in reducing the molecular weight of oil fraction. It should also be noted that glycerol-consuming reactions that decrease the carboxylic acid and phenolic contents of PO (e.g., via esterification) may play a role in reducing self-polymerization during HDO or TT. Dilution, while beneficial because it reduces the molecular weight, decreases the carbon content of oil fractions. Different alcohols may affect stabilization and upgrading to different extents, as their reactivities toward fast pyrolysis oil

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the NSERC/FPInnovations Industrial Research Chair Program in Forest Biorefinery and the Ontario Research Fund-Research Excellence (ORF-RE) from the Ministry of Research and Innovation. Support from the industrial partners including FPInnovations, Arclin Canada, and the BioIndustrial Innovation Centre is also acknowledged. C.X. also acknowledges funding from NSERC via the Discovery Grant Program.

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LIST OF ABBREVIATIONS HDO = hydrodeoxygenation TT = thermal treatment PO = fast pyrolysis oil GPO = glycerol-stabilized fast pyrolysis oil PO+glycerol = a mixture of PO and glycerol with a PO:glycerol ratio of 2:1 FCC = fluid catalytic cracking GPC = gel permeation chromatography REFERENCES

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dx.doi.org/10.1021/ef402227m | Energy Fuels 2014, 28, 1132−1138