Energy & Fuels 2008, 22, 635–642
635
Production of Heavy Oils with High Caloric Values by Direct Liquefaction of Woody Biomass in Sub/Near-critical Water Chunbao Xu* and Nimisha Lad Department of Chemical Engineering, Lakehead UniVersity, Thunder Bay, ON, Canada P7B 5E1 ReceiVed July 21, 2007. ReVised Manuscript ReceiVed October 3, 2007
Direct liquefaction of a woody biomass (Jack pine sawdust) in sub/near-critical water without and with catalysts (alkaline earth and iron ions) has been investigated at temperatures of 280–380 °C. Heavy oils with a high caloric value of 30–35 MJ/kg (much greater than that of the crude wood sample used) were obtained, along with water soluble oils with a caloric value of 19–25 MJ/kg. The yields of heavy oil and total oil products tended to maximize in the temperature range of 280–340 °C for all the liquefaction operations regardless of the presence of a catalyst or the type of catalyst. All the catalysts tested, i.e., Ca(OH)2, Ba(OH)2, and FeSO4, were found effective for enhancing the formation of heavy oil products at 280–340 °C, while they significantly promoted the formation of gas and water at >340 °C. The yield of heavy oil in the operation at 300 °C for 30 min was improved significantly from around 30% without catalyst to greater than 45% by Ba(OH)2. The maximum yield of total oil products reached 51% in the operation without catalyst, while it increased to about 65% with Ca(OH)2 at 300 °C. The GC/MS measurements for the heavy oil products revealed that the oils contain mainly carboxylic acids, phenolic compounds and derivatives, and long-chain alkanes.
Introduction Biomass, with a total annual production of 2740 Quads (1 Quad ) 1016 Btu), about 8 times the total annual world energy consumption, represents an immense and renewable source for the production of low S/N biofuels (e.g., bio-oils, ethanol, fuel gas) and valuable chemicals, as opposed to fossil fuelsscoal, oil, and natural gasswhose reserves are depleting. Due to skyrocketing oil prices and the challenge of energy security, the demand of bioenergy (biomass-derived energy) has been growing very fast in the recent decade. For instance, the United Sates has set a goal of replacing 20% of its gasoline usage with biofuels in 10 years. As a matter of fact, due to the increasing concerns over greenhouse gas emissions, there is a worldwide resurgence of interest in developing bioenergy and biorefining technologies for converting biomass (a renewable and CO2neutral energy resource) into various valuable products, including biofuels and biobased chemicals and materials. Biomass can be converted into liquid fuels and chemicals (such as bio-oils, biodiesel, methanol, ethanol, etc.) indirectly through gasification to syngas followed by catalytic conversion1 or directly through biological processes (e.g., fermentation) and direct liquefaction. From the standpoint of conversion economy and efficiency, in recent years, direct liquefaction of biomass feedstocks into liquid oils has attracted intensive interest due to its simpler technical route compared with the indirect liquefaction and biological approaches. Typical direct liquefaction processes include fast pyrolysis and high-pressure direct liquefaction. Fast pyrolysis, a process where dry biomass is subject to a rapid heating in an inert atmosphere to a high temperature (400–1000 °C), is so far the only industrially * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Dry, M. E. Fischer–Tropsch reactions and the environment. Appl. Catal. A Gen. 1999, 189, 185–190.
realized technology for production of bio-oils.2,3 However, pyrolysis oils consist of high oxygen/water contents and hence have lower caloric values (20–25 MJ/kg, only about half of that of the petroleum), and they are strongly acidic and corrosive. Further treatment and upgrading is needed in order to make pyrolysis oil a potential substitute for petroleum fuels. Superior to pyrolysis technology, high-pressure direct liquefaction technology has the potential for producing liquid oils with much higher caloric values and a range of chemicals including vanillin, phenols, aldehydes, and organic acids, etc.4–6 Despite many successful works on direct liquefaction of biomass in organic solvents such as anthracene oil7,8 and alcohols,9,10 hot compressed water (at a mild temperature of 200–350 °C but a high pressure of >1 MPa) has many advantages for being used as the solvent in direct liquefaction of biomass.5,6,11–13 It is not (2) Onay, O.; Kockar, O. M. Pyrolysis of rapeseed in a free fall reactor for production of bio-oil. Fuel 2006, 85, 1921–1928. (3) Maschio, G.; Koufopanos, C.; Lucchesi, A. Pyrolysis: a promising route for biomass utilization. Bioresour. Technol. 1992, 42, 219–231. (4) Appell H. R.; Fu Y. C.; Friedman S.; Yavorsky, P.M.; Wender, I. ConVerting organic wastes to oil; Report of Investigation No. 7560, U.S. Bureau of Mines: Washington, D.C., 1971. (5) Boocock, D. G. B.; Mackay, D.; McPherson, M.; Nadeau, S.; Thurier, R. Direct hydrogenation of hybrid poplar wood to liquid and gaseous fuels. Can. J. Chem. Eng. 1979, 57, 98–101. (6) Yokoyama, S.; Ogi, T.; Koguchi, K.; Nakamura, E. Direct liquefaction of wood by catalyst and water. Liquid Fuels Technol. 1984, 2, 115– 163. (7) Appel, H. R.; Wender, I.; Miller, R. D. ConVersion of urban refuse to oil; Technical Progress Report No. 25, US Bureau of Mines: Washington, D.C., 1969. (8) Crofcheck, C.; Montross, M. D.; Berkovich, A.; Andrews, R. The effect of temperature on the mild solvent extraction of white and red oak. Biomass Bioenergy 2005, 28, 572–578. (9) Miller, J. E.; Evans, L.; Littlewolf, A.; Trudell, D. E. Bath microreactor studies of lignin and lignin model compound depolymerization by bases in alcohol solvents. Fuel 1999, 78, 1363–1366. (10) Xu, C.; Etcheverry, T. Effect of iron-based catalysts on hydroliquefaction of woody biomass in supercritical ethanol. Fuel, in press. (11) Minowa, T.; Kondo, T.; Sudirjo, S. T. Thermochemical liquefaction of Indonesia biomass residues. Biomass Bioenergy 1998, 14, 517–524.
10.1021/ef700424k CCC: $40.75 2008 American Chemical Society Published on Web 11/15/2007
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only because water is likely the most “green” and environmentally benign solvent, but also due to the fact that hot-compressed water exhibits very different properties from those of ambient liquid water. Hot-compressed water has a lower dielectric constant, fewer and weaker hydrogen bonds, a higher isothermal compressibility, and a more enhanced solubility for organic compounds than ambient liquid water.14,15 Moreover, hotcompressed water has been found very effective for promoting ionic, polar nonionic, and free-radical reactions,14 which make it a promising reaction medium for biomass direct liquefaction. More importantly, employing hot-compressed water as the reaction medium in a biomass conversion process will eliminate the high energy cost for drying the material since wet biomass can be used directly. More recently, near- or supercritical water (SCW, highly compressed water at above its critical temperature of 374 °C and critical pressure of 22 MPa) is attracting increased attention as the reaction medium or reactant for various biomass conversion processes, such as biomass gasification16–18 and liquefaction/extraction of biomass and wastes as detailed below. Using water as a reaction medium instead of organic solvents is advantageous from the environmental and economical points of view as mentioned above but, more importantly, because water near or above its critical point possesses properties very different from those of either ambient liquid water or steam. Supercritical water has unique transport properties: steamlike diffusivity (allowing it to penetrate solid materials) and liquidlike density (providing it the ability to dissolve analytes from solid materials). Moreover, SCW has the ability to dissolve materials not normally soluble in either ambient liquid water or steam and has complete miscibility with the liquid/vapor products from the processes, providing a single-phase environment for reactions that would otherwise occur in a multiphase system under conventional conditions.15 The advantages of a single supercritical phase reaction medium are apparent in that higher concentrations of reactants can be attained and there are no interphase mass transport processes to hinder the reaction rate, which makes supercritical water a favorable medium for biomass conversions, in particular gasification when more gas products are targeted. For biomass direct liquefaction, near- and subcritical water has however been widely employed to prevent gasifying the oil products. Although water under near- and subcritical conditions may not have all the unique properties mentioned above for SCW, it does possess special properties, e.g., an enhanced solubility for organic compounds (derived from biomass),14 which are favorable for liquefaction of biomass feedstocks. Extensive research work has been reported on the direct liquefaction of biomass in near- and subcritical water. A (12) Qu, Y.; Wei, X.; Zhong, C. Experimental study on the direct liquefaction of Canninghamia lanceolata in water. Energy 2003, 28, 597– 606. (13) Karagoz, S.; Bhaskar, T.; Muto, A.; Sakata, Y. Effect of Rb and Cs carbonate for production of phenols from liquefaction of wood biomass. Fuel 2004, 83, 2293–2299. (14) Akiya, N.; Savage, P. E. Roles of water for chemical reactions in high-temperature water. Chem. ReV. 2002, 102, 2725–2750. (15) Savage, P. E. Organic chemical reactions in supercritical water. Chem. ReV. 1999, 99, 603–621. (16) Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. J., Jr. Carboncatalyzed gasification of organic feedstocks in suppercritical water. Ind. Eng. Chem. Res. 1996, 35, 2522–2530. (17) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J., Jr. Biomass gasification in near- and supercritical water: status and prospects. Biomass Bioenergy 2005, 29, 269–292. (18) Antal, M. J., Jr; Allen, S.; Schulman, D.; Xu, X.; Divilio, R. J. Biomass gasification in supercritical water. Ind. Eng. Chem. Res. 2000, 39, 4040–4053.
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pioneer work was reported by Appell et al.4 at The Pittsburgh Energy Technology Center (PETC), where a variety of lignocellulosic materials were efficiently converted to oily products in water at around 350 °C in the presence of CO and Na2CO3 as the catalyst. The PETC’s biomass direct liquefaction technology was further investigated and advanced by the research group led by Dr. D.C. Elliott at Pacific Northwest Laboratory in the U.S. In the 1980s, Elliot and co-workers did excellent work on scaling up the pioneer work by Appell et al. and on separation/ utilization of the direct liquefaction oil products.19–21 Recently, Minowa et al.11 obtained heavy oil (with calorific values of around 30 MJ/kg) at a yield of 21–36 wt % from a variety of biomass residues in hot-compressed water at 300 °C and around 10 MPa with Na2CO3 as catalyst. Matsumura et al.22 produced liquid products (including water-soluble products and waterinsoluble oil products) at a total yield of 40–50% by coliquefaction of coal and cellulose (as a potential hydrogen donor) with supercritical water at 400 °C and 25 MPa. Qu et al.12 obtained liquid organic products at a total yield of 30–35% by direct liquefaction of Cunninghamia lanceolata in water at 280–360 °C for 10–30 min. In summary, typical yields of liquid products in the direct liquefaction of biomass with near- and subcritical water were in the range of 20–50%, depending on temperature, pressure, residence time, and type of catalysts employed. Alkaline solutions, e.g., Na2CO3, NaOH, K2CO3, KOH, LiOH, RbOH, CsOH, etc., have been widely employed as catalysts in the biomass direct-liquefaction processes to suppress the formation of char and to enhance the yield of liquid products.11,13,23,24 The objective of the present work is to produce heavy oils with higher caloric values by direct liquefaction of Jack pine wood sawdust in sub/near-critical water at a temperature of 280–380 °C. Alkaline earth metals and iron-based compounds, Ca(OH)2, Ba(OH)2, and FeSO4, were selected as catalysts for the biomass direct-liquefaction process. Experimental Section Materials. The sawdust sample of Pinus banksiana (Jack pine) was obtained from a local lumber mill. For the experimental use, it was ground with a Wiley mill into fine powder smaller than 20 mesh (∼0.8 mm). The powder was dried in an oven at 105 °C for 24 h before use. The proximate and ultimate analyses results of the pine wood sample and the compositions of major inorganic elements in the wood sample are given in Table 1. Liquefaction. The experiments were carried out with a bomb reactor system whose schematic diagram is shown in Figure 1. The bomb reactor used in this study, made of stainless steel (SS 316L), had an effective volume of 14 mL. In a typical run, 1.0 g of the wood sample was weighed into the reactor, followed by the addition of catalyst (if needed) in an amount of 5 wt % (w/w) of the wood sample, and then, 10 mL (or 10 g) of distilled water was added. In all runs, the mass ratio of the wood sample to the water solvent (19) Elliott, D. C. Bench-scale research in biomass liquefaction by the CO-steam process. Can. J. Chem. Eng. 1980, 58, 730–734. (20) Elliott, D. C. Description and utilization of products from direct liquefaction of biomass. Proceedings of the Biotechnology Bioengineering Symposium; John Wiley & Sons: New York; Vol 11, pp 187–198. (21) Schirmer, R. E.; Pahl, T. R.; Elliott, D. C. Analysis of a thermochemically-derived wood oil. Fuel 1984, 63, 368–372. (22) Matsumura, Y.; Nonaka, H.; Yokura, H.; Tsutsumi, A.; Yoshida, K. Co-liquefaction of coal and cellulose in supercritical water. Fuel 1999, 78, 1049–1056. (23) Minowa, T.; Zhen, F.; Ogi, T. Cellulose decomposition in hot compressed water with alkali or nickel catalyst. J. Supercritical Fluid 1998, 13, 253–259. (24) Karagoz, S.; Bhaskar, T.; Muto, A.; Sakata, Y.; Oshiki, T; Kishimoto, T. Low temperature catalytic hydrothermal treatment of wood biomass: analysis of liquid products. Chem. Eng. J. 2005, 108, 127–137.
Liquefaction of Woody Biomass in Water
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Table 1. Proximate and Ultimate Analyses of the Pine Wood Sample and Concentrations of Major Inorganic Elements ultimate analysis, wt % (d.b.a)
proximate analysis, wt % (d.b.a) pine wood sample
VMd
FCd
ash
C
H
N
S
Ob
81.52
18.31
0.17
53.3
6.2
0.1
0.1
40.3
major inorganic elements, ppmw (d.b.)c ash from the sample
Na
K
Mg
Ca
Mn
Fe
Zn
Al
Si
7
114
100
440
20
9
10
16
3
a On a dry basis. b By difference. c Determined by ICP-AES. d The percentages of volatile matter and fixed carbon were determined by thermogravimetric analysis (TGA) in N2 at 10 °C/min up to 950 °C.
residue (consisting of the unreacted wood sample, coke/char, ash, and catalyst) from filtrate no. 2 (consisting of acetone and acetone soluble oils). The solids with the filter papers were dried overnight in the oven at 105 °C before weighing. Filtrate no. 1 was evaporated under reduced pressure at 85–90 °C to completely remove water, and the resulting liquid product was designated as “water soluble oil” (WSO). Filtrate no. 2, containing acetone and acetone soluble oils, was evaporated at 50 °C under reduced pressure to completely remove acetone, and the resulting liquid products was denoted as “heavy oil” (HO). In the present work, the remaining solid after drying, excluding the ash and the catalyst added (assuming that the weight of the catalyst added was approximately unchanged), was designated as “solid residue” (SR). It should be noted that the partition of metal ions between the solid residues and the water soluble products would be dependent on many factors including (1) ion-exchange of the metal cations with the lignocellulosic material before and during the liquefaction operation, (2) conversion of the initially added catalyst compounds into other chemical forms due to the reactions between the compounds with acidic products and the CO2 gas generated in the liquefaction process, and (3) whether or not a neutralization step is performed after the reaction. However, our X-ray diffraction measurements for the 380 °C chars in this study did reveal the presence of FeSO4 and CaCO3 (although the results were not provided in this paper). This observation may suggest a significant presence of the catalyst metal ions in the solid residues. As such, for simplification of the calculation, in the present work, the authors assumed that the metal compounds were primarily present in the solid residues (chars). Moreover, the error resulting from this assumption shall not be significant due to the fact that the adding amount of each catalyst compound was only 5 wt%. Yields of WSO, HO, and SR products were calculated to the dry organic matters (i.e., on a dry and ash-free basis) as follows. Since the gaseous products and the water formed in the liquefaction process were not quantified, for approximation, the total yield of (gas + water) was simply obtained by difference. Yield of WSO ) (mass of WSO)/ (mass of dry organic matter of biomass added) × 100% (1)
Figure 1. Schematic diagram of the experimental apparatus.
was fixed at 10% (w/w). The catalysts used in this work were ACS reagent-grade iron(II) sulfate heptahydrate (FeSO4 · 7H2O), calcium hydroxide (Ca(OH)2), and barium hydroxide (Ba(OH)2) as received from Sigma-Aldrich. The reactor with suspension of the wood powder, catalyst (if any), and water was sonicated for 20 min in an ultrasonic bath before being securely sealed. The air inside the reactor was displaced with ultrahigh purity nitrogen by repetitive operation of vacuuming and N2-charging. Finally, the reactor was pressurized to 2.0 MPa with nitrogen in order to avoid boiling of the water during heating. Supported on a mechanical shaker (set at 100 rpm), the reactor was then rapidly heated in a fluidized sand bath to the specified temperature. After the desired reaction time has elapsed, the reactor was removed from the sand bath and quenched rapidly in a water bath to stop the reactions. Two to three duplicate runs have been conducted for almost all of the experimental conditions, and the error between the runs under the same conditions was ensured within 10%. Separation of Reaction Products. Once the reactor was cooled to room temperature, the gas inside was vented in a fume hood. The gas products were not collected and analyzed in this work since our main interest is in the liquid products. The solid/liquid products were rinsed thoroughly first with 100 mL of distilled water. The resulting suspension was filtered under reduced pressure through a preweighed Whatman no. 5 filter paper to obtain filtrate no. 1 which contained water and water soluble organics. The reactor was then completely rinsed with 100 mL of reagent-grade acetone. The resulting acetone solution was collected in a beaker, into which the previous filter paper together with the solids plus the water insoluble organics were added. The mixture in the beaker was sonicated for 30 min and filtered under reduced pressure through another preweighed Whatman no. 5 filter paper to separate solid
Yield of HO ) (mass of HO)/ (mass of dry organic matter of biomass added) × 100% (2) Yield of SR ) (mass of SR)/ (mass of dry organic matter of biomass added) × 100% (3) Yield of (gas + water) ) 100 wt % - (Yield of WSO) (Yield of HO) - (Yield of SR) (4) Characterization. The elemental compositions (C, H, and N) of the oils (WSO and HO) were determined with a CEC (SCP) 240-XA elemental analyzer. The composition of oxygen (O) was estimated by difference, assuming negligible content of sulfur (S) in the products. The HO products were also analyzed by a gas chromatograph equipped with a mass selective detector [Varian 1200 Quadrupole GC/MS (EI), Varian CP-3800 GC equipped with VF-5 ms column (5% phenyl 95% dimethylpolysiloxane, 30 m × 0.25 mm × 0.25 µm); temperature program 40 °C (hold 2 min) f 190 °C (12 °C/min) f 290 °C (8 °C/min, hold 20 min)]. Compounds in the HOs were identified by means of the NIST 98 MS library with the 2002 update. X-ray diffraction (XRD) measurements were carried out by using Ni-filtered Cu KR radiation with a Philips PW 1050–3710 diffractometer, to examine the evolution of the crystalline forms in the wood samples during the liquefaction process.
Results and Discussion Influence of Reaction Time. Figure 2 shows the yields of liquid oil products (WSO and HO) and solid residue (SR) in liquefaction of the pine wood power at temperatures of 300 °C (subcritical) and 380 °C (near or supercritical) for various lengths of time (15–60 min), all without catalyst. Clearly shown
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Figure 2. Variation of yields of products with residence time in the liquefaction of the pine wood sample at 300 and 380 °C without catalyst: (a) liquid organic products (HO and WSO) and (b) SR.
in Figure 2a, for both temperatures, the yields of HO and WSO generally increased with reaction time, while the yield of WSO appeared to level off at around 30 min These results are consistent with the observation in another study by the authors’ group using ethanol as the liquefaction medium instead of water.10 On the contrary, as shown in Figure 2b, the yields of SR for both temperatures decreased with residence time before 30 min and then increased as time increased further to 60 min The initial decrease in formation of SR with residence time results from the increased conversion of the lignocellulosic materials to liquid products, while the later increase in the formation of SR with time is more likely due to retrograde/ condensation of the gaseous intermediates/products.12 At both temperatures and for any a residence time in the range of 15–60 min, the formation of HO (20–40% yields) was much greater than that of WSO (5–15% yields). The oil yields were found to be strongly dependent on temperature: yields of both HO and WSO reduced significantly as the temperature increased from 300 (subcritical water) to 380 °C (supercritical water). Thus, subcritical water is superior to supercritical water for biomass direct liquefaction. The reduction in the yields of liquid products in supercritical water (380 °C) are due to cracking of the liquid products to gases and char/coke by isomerization, dehydration, fragmentation, and condensation reactions. This result may be evidenced by our calculation, revealing that the yields of (gas + water) at 380 °C were much higher than those of 300 °C, though these data are not presented in Figure 2. Effects of Reaction Temperature and Catalysts. The effects of temperature on the yields of liquefaction products can be revealed from Figures 3-5. The data presented in these figures were obtained from the experimental runs at various temperatures ranging from 280 (subcritical) to 380 °C (supercritical) for a same residence time of 30 min with and without catalyst. At all temperatures in the range of 280–380 °C, generally, the formation of HO (20-45% yields) was much greater than that of WSO (5–25% yields) irrespective of whether a catalyst was present or not, as shown in Figure 3. The yield of (gas + water) was relatively low at about 20-40% at 190 °C and all of the hemicellulose and much of the lignin dissolved in the water at 220 °C,25,26 forming the intermediates or products of WSO and HO. Solvolysis and pyrolysis of the remaining lignocellulosic solids took place at higher temperatures.27 The intermediate products of these solvolysis and pyrolysis reactions could undergo an extraordinary variety of reactions such as isomerization, dehydration, fragmentation, and condensation reactions that ultimately formed gas, liquid oils, and char.28–31 Figure 3 also shows the effects of various catalysts on the liquefaction of the woody biomass. For all types of the catalysts over the whole range of temperatures tested, general catalytic effects were the following: the catalysts promoted the formation of HO and (gas + water) but suppressed the formation of WSO. In this work, more attention was paid on the yields of HO products and the effects of catalysts on the formation of HO products because heavy oils are the liquid products possessing a higher caloric value (as will be shown in Table 2). All catalysts enhanced the HO formation, although an exception was observed with the presence of 5 wt % FeSO4 (suppressing the HO formation as the temperature was higher than 340 °C). All catalysts seem to be more active at a lower temperature less than 340 °C, and their activity for the HO formation showed a priority sequence of Ba(OH)2 > Ca(OH)2 > FeSO4. The yield of HO was improved significantly from around 30% without (25) Mok, W. S. L.; Antal, M. J., Jr. Uncatalyzed solvolysis of whole biomass hemicellulose by hot compressed water. Ind. Eng. Chem. Res. 1992, 31, 1157–1161. (26) Allen, S.; Kam, L. C.; Zemann, A. J.; Antal, M. J., Jr. Fractionation of sugar cane with hot compressed liquid water. Ind. Eng. Chem. Res. 1996, 35, 2709–2715. (27) Bobleter, O.; Concin, R. Degradation of poplar lignin by hydrothermal treatment. Cell Chem. Technol. 1979, 13, 583–593. (28) Antal, M. J., Jr; Mok, W. S. L.; Richard, G. N. Four-carbon model compounds for the reactions of sugars in water at high temperature. Carbohydr. Res. 1990, 199, 111. (29) Bobleter, O. Hydrothermal degradation of polymers derived from plants. Prog. Polym. Sci. 1994, 19, 797. (30) Kabyemela, B. M.; Takigawa, M.; Adschiri, T.; Mulaluan, R. M.; Arai, K. Mechanism and kinetics of cellobiose decomposition in sub- and supercritical water. Ind. Eng. Chem. Res. 1998, 37, 357. (31) Jakab, E.; Liu, K.; Meuzelaar, H. L. C. Thermal decomposition of wood and cellulose in the presence of solvent vapours. Ind. Eng. Chem. Res. 1997, 36, 2087.
catalyst to greater than 45% by Ba(OH)2. The above-mentioned abnormal effect of FeSO4, i.e., the presence of 5 wt % FeSO4 suppressed the formation of HO when the temperature was higher than 340 °C (as shown in Figure 3a), may be a result of the dramatically enhanced cracking and dehydration of the HO products to yield a greater amount of gases and water products, which can be evidenced by the results in Figure 3c. The products of gases and water attained a yield as high as 70% with the presence of 5 wt % FeSO4 at 380 °C. This however suggests that FeSO4 can be a less expensive but effective catalyst for supercritical water gasification (SCWG) of biomass. SCWG of biomass for production of hydrogen-rich gas is another project currently under investigation by the authors’ research group. Figure 4 simply presents the yields of total oil products (HO +WSO) from the wood sample in water with and without catalyst at different liquefaction temperatures. Regardless of the presence of a catalyst or type of catalyst, the formation of total liquid organic products peaked at temperatures between 280 and 340 °C and decreased rapidly as temperature increased further. As was discussed previously for Figure 3, the decline of the yields of total oil products (HO +WSO) with temperature was due to the enhanced cracking and dehydration reactions (to form gases and water) and enhanced condensation reactions (to yield more solid coke/char) at higher temperatures. All catalysts except FeSO4 promoted the formation of liquid products, especially at lower temperatures. The abnormal effect of FeSO4 on the formation of liquid products has been discussed previously for Figure 3. Figure 5 shows some interesting results on the yields of ash and catalyst-free solid residues (SR) for liquefaction of the wood sample in water at various temperatures with and without catalyst. As expected, with the presence of 5 wt % Ba(OH)2 or FeSO4 as catalyst, the SR yield decreased monotonously as temperature increased due to the increased formation of liquid products (HO and WSO) at lower temperatures and the promoted (gas + water) yield at higher temperatures, as revealed previously in Figure 3. With the addition of 5 wt % FeSO4, an extremely low SR yield was obtained, suggesting almost 100% conversion of biomass. For liquefaction both with and without the catalyst of Ca(OH)2, a trend very different from that described above is shown in Figure 5: the yield of ash and catalyst-free SR decreased with temperature initially, but it climbed as the temperature increased further to above 300 °C. This trend in the range of 300–380 °C is apparently opposite of those for the yields of HO and WSO as shown previously in Figure 3. The reduction in the yields of liquid organic products accompanied by the increase in the yield of ash and catalyst-
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Table 3. GC/MS Analysis Results for the HO Products from the Operations at 340 °C for 30 min with and without Catalyst area % peak no.
RT (min)
peak name
none
Ca(OH)2
Ba(OH)2
FeSO4
1 2 3 4 5 6 7 8 9 10 11 12 13 14
7.377 8.953 9.978 10.875 11.258 11.335 12.366 15.541 15.636 16.151 16.279 16.435 16.572 22.66
2.1 0.1 0.4 0.5 0.1 0.2 0.1 7.3 0.1 3.7 5.2 35.3 5.2 0.1
1.6 0.2 0.2 0.4 0.2 0.3 0.3 15.8 0.1 3.6 5.1 19.4 9.0 0.2
2.8
2.0 0.1 0.2 0.6 0.4 0.1 0.1 5.6 0.1 3.1 4.3 14.0 15.3 0.1
15 16 17 18 19 20 21 22 23
24.906 25.325 25.85 27.807 28.34 29.022 29.26 29.452 29.643
0.4 4.0 4.9 0.1 0.1 1.0 0.9 0.3 0.2
0.3 15.3 1.9 0.2
24 25 26 27 total area %
30.12 31.575 35.591 37.964
phenol phenol, 2-methoxyphenol, 3-ethylbenzothiazole salicyl alcohol phenol, 4-ethyl-2-methoxyphenol, 2-methoxy-4-propylphenol, 4-(1,1-dimethylpropyl)2-(1-cyclohexenyl)cyclohexanone phenol, 4-(1,1,3,3-tetramethylbutyl)hexestrol or 4,4′-(1,2-diethylethylene)diphenol 1,3-cyclohexadiene-1-carboxylic acid, 2,6,6-trimethylbenzestrol 1-phenanthrenecarboxaldehyde, 1,2,3,4,4a,9,10, 10a-octahydro-6-methoxy-1,4a-dimethyl3,3,6,6-tetraphenyl-trans- tricyclo[3.1.0.0(2,4)]hexane 5,8,11-heptadecatriynoic acid, methyl es tricyclo[7.4.0.0(3,8)]tridec-12-en-2-one phenanthrene, 1-methyl-7-(1-methylethyl) N,N-dimethyldecanamide heptacosane 1-(1,2,3-trimethyl-cyclopent-2-enyl)-eth 3-(3-hydroxy-4-methoxyphenyl)-l-alanine 1-phenanthrenecarboxylic acid, 1,2,3,4,4a,9,10,10a-octahydro-1, 4a-dimethyl-7-(1-methylethyl)-,(1r,4as,10ar)5,8,11,14-eicosatetraynoic acid hentriacontane tetratriacontane triacontane, 11,20-didecyl-
2.4 7.3 2.9 0.4 85.3
free SR suggests the cracking and condensation reactions of the liquid products to form coke/char at higher temperatures. Similar results were obtained by Qu et al.12 in direct liquefaction of Cunninghamia lanceolata in hot compressed water. According to the above results, the following conclusion seems to be plausible: in the course of liquefaction of biomass in water, the reactions for solvolysis and pyrolysis of the lignocellulosic solids to form liquid products and the reactions for cracking and condensation of the liquid products to form coke/char may compete with each other. The relative significance of this pair of reactions may vary with the type of catalyst used in the liquefaction. In liquefaction with the presence of Ba(OH)2 and FeSO4, the former group of reactions (i.e., solvolysis and pyrolysis of the lignocellulosic solids) might dominate over the latter group (i.e., cracking and condensation reactions of the liquid products), leading to a monotonous decrease in the SR yield, as opposed to the cases without catalyst or with Ca(OH)2. Moreover, the liquefaction of biomass in high-temperature water is a very complicated process, associated with other types of reactions such as the hydrolysis of cellulose23 and steam (water) gasification/reforming of char and liquid organic products/ intermediates, which may also be affected by different catalysts. More research is needed for clarification the detailed roles of different catalysts in biomass conversions in high-temperature water. Characterizations of the Liquid/Solid Products. Properties of the liquid oil products are of interest in this work in addition to the liquefaction yields. The WSO and HO products were analyzed with an elemental analyzer for their elemental compositions (C, H, and N), and some results are presented in Table 2, compared with that of the crude wood powder. Compared with the data of the crude wood powder, the water soluble oils have similar elemental compositions of carbon, hydrogen, and oxygen and hence similar HHVs, as similarly observed by Qu et al.12 The presence of various catalysts only slightly varied the elemental compositions and HHVs of the
0.4 0.7 0.1 0.1 0.1 8.6 0.1 4.4 6.2 12.0 10.6
0.5 0.5 0.1 0.7
0.5 8.8 2.4 0.1 0.1 0.9 0.3 0.1 0.1
0.4 18.9 5.4 0.1 0.1 0.6 0.9 0.2 0.2
2.4 2.4 0.4 0.1 81.2
0.1 5.6 1.4 0.2 66.5
3.9 0.9 0.2 77.4
WSO products. As demonstrated in many previous studies,31,32 the water soluble products from woody biomass in a thermochemical process consist of carbohydrates (sugars), acetic acids, pyran derivatives, and aldehydes, mainly the decomposition products from the cellulose, hemicellulose, and water soluble components of the biomass used.31,32 Accordingly, the above result (i.e., the WSOs having similar elemental compositions and caloric values as the crude wood sample) can be easily accounted for since cellulose and hemicellulose are two major components in woody biomass (making up about 70–80% of the dry mass of wood). More interestingly but as expected, the HOs obtained have a much higher content of carbon and a much lower concentration of oxygen, leading to significantly increased caloric values, irrespective of whether or what catalyst was used. This may also be accounted for by the chemical compositions and the originating sources of the HOs. As evidenced by the GC/MS analysis results for the HOs presented later in Table 3, the HOs consist mainly of phenolic compounds and derivatives, long-chain carboxylic acids/esters and hydrocarbons, which are obviously the decomposition products from lignin (the phenylpropanoid polymer) and cellulose. The greatly reduced oxygen content in the HOs may partially be due to the dehydration reactions (forming H2O) and the pyrolysis reactions (forming CO/CO2) in the liquefaction process. As shown in the table, the HHVs of HOs are in the range of 30–35 MJ/kg, in comparison with the HHV of 19.7 MJ/kg for the crude wood sample. As shown previously in Figure 3, high yields (above 40%) of HO were achieved at 300 °C with all catalysts, suggesting that catalytic liquefaction of woody biomass in water may be a promising technique for upgrading biomass feedstocks of low caloric values to liquid fuels with greatly increased caloric values. Table 2 also presents calculation results of H/C for the oil samples. Compared to the crude wood sample, all WSOs (32) Holgate, H. R.; Meyer, J. C.; Tester, J. W. Glucose hydrolysis and oxidation in supercritical water. AIChE J. 1995, 41, 637–647.
Liquefaction of Woody Biomass in Water
except the one obtained without catalyst have much higher H/C ratios while all HOs possess much lower H/C ratios. This could also be accounted for by the chemical compositions of WSOs (i.e., mainly carbohydrates, acetic acids, pyran derivatives and aldehydes) and HOs (i.e., mainly phenolic compounds and derivatives, long-chain carboxylic acids/esters, and hydrocarbons). The chemical compositions of the HOs were qualitatively characterized by GC/MS. Table 3 gives a comparison of the identified compounds in the HOs from the liquefaction operations at 340 °C for 30 min with and without catalyst. The area % for each compound identified (defined by the percentage of the compound’s chromatographic area out of the total area) and the total area % for all the identified compounds are shown in the table. For the HO produced without catalyst, carboxylic acids were the major compounds identified, followed by phenolic compounds and derivatives and long-chain alkanes. Interestingly, the presence of various catalysts altered the chemical compositions significantly. As clearly shown in the table, the use of all kinds of catalysts (in particular the two basic catalysts) generally reduced the concentrations of carboxylic acids (probably due to the reactions between the carboxylic acids with the basic catalysts) and long-chain alkanes in the HOs, while it increased the concentrations of phenolic compounds and derivatives. The carboxylic acids may primarily form from woodextractives,whileaswidelyagreedbymanyresearchers,10,33–35 the carboxylic acids may also be formed from the cellulose and hemicellulose components of the biomass feedstock by complex hydrolysis and dehydration reactions, but the phenolic compounds and derivatives, such as 4-(1,1-dimethylpropyl)-phenol, Benzestrol and Hexestrol, are primarily originated from the degradation of the lignin component in the biomass feedstock (by cleavage of the aryl ether linkages in lignin). In addition, condensation/cyclization of the cellulose/hemicellulose-derived carbohydrates might also lead to the formation of phenol and phenolic compounds.36 The long-chain alkanes in the HOs were more likely formed from the waxes in the pine feed, which are however not normally in a large amount. The significant amount of the long-chain alkanes detected in the HOs in this work, as shown in Table 3, thus suggests that they might also form by condensation and polymerization of the degradation products from cellulose/hemicellulose during the liquefaction process. The GC/MS results thus suggest that the catalysts used in this work can effectively promote the degradation reactions of lignin, yielding more phenolic compounds. It should be noted that Table 3 gives peaks areas of identified compounds in percents, which represent the relative concentrations but not the actual concentrations of the detected compounds, so that the exact yields of the compounds, e.g. in mass % of the initial wood, are not quantifiable. However, with carefully controlled GC/MS conditions, such as the concentration of the oil sample (in solvent), the injected amount, and the temperature program, the relative concentrations derived from the peak areas may still be useful for discussion on a qualitative basis. To examine the crystalline forms in the crude wood sample and the solid residues after liquefaction, X-ray diffraction (XRD) (33) Karagoz, S.; Bhaskar, T.; Muto, A.; Sakata, Y. Comparative studies of oil compositions produced from sawdust, rice husk, lignin and cellulose by hydrothermal treatment. Fuel 2005, 84, 875–884. (34) Kershaw, J. R. Comments on the role of the solvent in supercritical fluid extraction of coal. Fuel 1997, 76, 453–454. (35) Demirbas, A. Mechanism of liquefaction and pyrolysis reactions of biomass. Energy ConVers. Manage. 2000, 41, 633–646. (36) Sinag, A.; Kruse, A.; Schwarzkopf, V. Key compounds of the hydropyrolysis of glucose in supercritical water in the presence of K2CO3. Ind. Eng. Chem. Res. 2003, 42, 3519–3521.
Energy & Fuels, Vol. 22, No. 1, 2008 641
Figure 6. X-ray diffraction patterns of pine wood powder before and after liquefaction in water for 30 min at various temperatures without catalyst.
measurements were carried out. Figure 6 illustrates the XRD spectra of the pine wood powder before and after liquefaction in water at various temperatures (280, 300, 340, and 380 °C) without catalyst. The X-ray diffraction pattern of the crude pine wood showed three peaks at 2θ ) 14.6°, 16.3°, and 22.2°, typical of cellulose I. It has been well documented that these peaks corresponds to the (11j0), (110), and (200) planes of cellulose.37–39 In the SR at 280 °C, the three peaks derived from cellulose I all weakened, while two new XRD signals at 2θ ) 24.6° and 26.2° were detected, as shown in Figure 6. These new peaks might be attributed to the diffraction lines of plane (200) of amorphous carbon and turbostratic carbon.40 The appearance of these carbon diffraction lines in the 280 °C SR reveals the formation of coke/char due to the decomposition of the cellulose and lignin matrix in the liquefaction process. As the liquefaction temperature increased, the diffraction lines of cellulose (11j0), (110), and (200) planes in the resulting SRs weakened, while the intensities of the C(002) diffraction lines increased. More interestingly, as shown in Figure 6, the diffraction lines due to turbostratic carbon (a crystallized form of carbon) intensify monotonously with increasing temperature. In the 380 °C SR, a relatively distinct shoulder peak of C(002) due to turbostratic carbon was detected, along with other fairly weak XRD peaks of cellulose (200) and amorphous carbon (002). These observations, i.e., stronger XRD signals of carbon and a higher proportion of crystallized carbon as oppose to weakened signals of cellulose in the resulting SRs at higher temperatures, suggest greater conversion of the lignocellulosic matrix into liquid/gas/carbon products (as evidenced previously in Figures 2-5) and enhanced crystallization/graphitization reactions of carbon at high temperatures. Similar observations of crystallization of carbon have been reported in some previous studies on pyrolysis of low rank coals41,42 and on hydrothermal treatment of biomass or organic materials in near- and super(37) Ishikawa, A.; Kuga, S.; Okano, T. Determination of parameters in mechanical model for cellulose III fibre. Polymer 1998, 39, 1875–1878. (38) Georget, D. M. R.; Cairns, P.; Smith, A. C.; Waldron, K. W. Crystallinity of lyophilised carrot cell wall components. Int. J. Biol. Macromol. 1999, 26, 325–331. (39) Borysiak, S.; Doczekalska, B. X-ray Diffraction Study of Pine Wood Treated with NaOH. Fibres Text. East. Eur. 2005, 13, 87–89. (40) Oya, A.; Otani, S. Influences of particle size of metal on catalytic graphitization of non-graphitizing carbons. Carbon 1981, 19, 391–400. (41) Tsubouchi, N.; Xu, C.; Ohtsuka, Y. Carbon crystallization during high-temperature pyrolysis of coals and the enhancement by calcium. Energy Fuels 2003, 17, 1119–1125.
642 Energy & Fuels, Vol. 22, No. 1, 2008
Figure 7. X-ray diffraction patterns of pine wood powder after liquefaction in water at 340 °C for 30 min with and without catalyst.
critical water.43,44 Figure 7 shows comparison of the XRD spectra of the resulting SRs at 340 °C for 30 min in water with and without catalyst. The detectable species in all 340 °C SRs are cellulose (200), amorphous carbon (002), and turbostratic carbon (002). Compared with the SR without catalyst, the catalyst-bearing SRs (in particular the FeSO4-bearing SR) show stronger diffraction lines due to the turbostratic carbon. This thus suggests that these catalysts were effective for catalyzing the crystallization/graphitization reactions of carbon in the liquefaction process, in addition to enhancing the HO formation and solid conversion. The catalytic effects of alkaline earth metals and iron on carbon crystallization/graphitization have been discussed in the previous work,41–44 and it was believed that possible mechanisms involve formation of various metal– carbon intermediates. The authors for the present work believe that the catalysts could play a similar role as that proposed above in the biomass conversion with sub/near-critical water, leading to the formation of more crystallized carbon (turbostratic carbon) as revealed by XRD in Figure 7. Conclusions In this study, Jack pine powder (with a caloric value of about 20 MJ/kg) was effectively upgraded into liquid organic products, (42) Tsubouchi, N.; Abe, M.; Xu, C.; Ohtsuka, Y. Nitrogen release from low rank coals during rapid pyrolysis with a drop tube reactor. Energy Fuels 2003, 17, 940–945. (43) Yu, S. H.; Cui, X. J.; Li, L. L.; Li, K.; Yu, B.; Antonietti, M.; Cölfen, H. From starch to metal/carbon hybrid nanostructures: hydrothermal metal-catalyzed carbonization. AdV. Mater. 2004, 16, 1636–1640. (44) Sharma, A.; Saito, I.; Nakagawa, H.; Miura, K. Effect of carbonization temperature on the nickel crystallite size of a Ni/C catalyst for catalytic hydrothermal gasification of organic compounds. Fuel 2007, 86, 915–920.
Xu and Lad
water soluble oil (WSO) and heavy oil (HO), by direct liquefaction in sub/near-critical water with and without catalyst at temperatures of 280–380 °C. The produced HOs possess caloric values of 30–35 MJ/kg, much higher than that of the crude wood sample. The following conclusions may be summarized: (1) The yields of HO and WSO generally increased with reaction time. The yields of SR decreased initially with reaction time before 30 min, suggesting increased conversion of the lignocellulosic materials to liquid products, but increased as reaction time further increased to 60 min, suggesting the occurrence of polymerization and condensation of the gaseous and liquid intermediates at a prolonged reaction time. (2) For liquefaction with and without catalyst at various temperatures, the yields of HO and WSO and the total yield of liquid organics (i.e., HO + WSO) peaked at around 280–340 °C, while the yield of (gas + water) generally increased monotonously as temperature increased from 280 to 380 °C. All catalysts tested (i.e., Ba(OH)2, Ca(OH)2, and FeSO4) could generally increase the biomass conversion and promote the formation of HO and (gas + water) but suppress the formation of WSO over the whole range of temperatures tested, although an exception was observed with the presence of 5 wt % FeSO4 (suppressing the HO formation as the temperature was higher than 340 °C). In terms of the activity for HO formation, the catalysts showed a priority sequence of Ba(OH)2 > Ca(OH)2 > FeSO4. The HO yield at 300 °C for 30 min of operation was improved significantly from around 30% without catalyst to greater than 45% by Ba(OH)2. On the other hand, FeSO4 was found to be an effective catalyst for the supercritical water gasification (SCWG) of biomass. With the presence of 5 wt% FeSO4 in the liquefaction at 380 °C for 30 min, the yield of (gas + water) was 70%. (3) Revealed by the GC/MS measurements for the HO products, carboxylic acids and phenolic compounds and derivatives were the major compounds identified. (4) Evidenced by the XRD measurements of the resulting SRs, all catalysts tested were also effective for catalyzing crystallization/graphitization reactions of carbon in the liquefaction process, leading to an increased portion of crystallized carbon (turbostratic carbon) in the catalyst-bearing SRs. Acknowledgment. The authors are grateful for the financial support from the Natural Science and Engineering Research Council of Canada (NSERC) through the Discovery Grant. The authors would also like to thank Mr. Allan MacKenzie, Mr. Ain Raitsakas, and Mr. Keith Pringnitz at Lakehead University Instrumentation Lab for assistance with GC/MS, XRD, ICP-AES, and CHN measurements and Dr. Richard A. Secco at the University of Western Ontario for assistance with TGA tests of the wood samples. EF700424K
