Low-Temperature Catalytic Gasification of Lignin and Cellulose with a

Energy & Fuels 2004, 18, 327-333

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Low-Temperature Catalytic Gasification of Lignin and Cellulose with a Ruthenium Catalyst in Supercritical Water Mitsumasa Osada,† Takafumi Sato,† Masaru Watanabe,‡ Tadafumi Adschiri,§ and Kunio Arai*,†,‡ Department of Chemical Engineering, Tohoku University, 07 Aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan, Research Center of Supercritical Fluid Technology, Tohoku University, 07 Aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan, and Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai, 980-8577, Japan Received July 4, 2003. Revised Manuscript Received October 27, 2003

Lignin and cellulose were gasified at 400 °C with gas yields of 30% and 70%, respectively, in supercritical water with a ruthenium catalyst. In both cases, the main gas product was CH4 and no solid product was formed. Without water or catalyst, lignin and cellulose were gasified slightly and a brown solid product was formed. The decomposition of formaldehyde was also demonstrated in supercritical water. Formaldehyde was rapidly decomposed to gases such as CH4, CO2, and H2 with ruthenium, whereas formaldehyde was converted into methanol and CO2 without catalyst. The catalytic conversion of biomass with ruthenium in supercritical water is an efficient method for biomass gasification at temperatures of ∼400 °C.

Introduction Energy systems that use renewable feedstocks are needed to achieve a sustainable society. Wood biomass has great potential as an energy and chemical source, because the carbon dioxide (CO2) formed from biomass usage is recycled through photosynthesis. Steam reforming (700-1000 °C, atmospheric pressure) is known to be a conversion technique of biomass to hydrogen (H2) and CO2.1-4 The steam reforming of cellulose (C6H10O5), which can be considered as one of the main components in wood biomass, is given by

C6H10O5 + 7H2O f 6CO2 + 12H2

(∆H0,298K < 0) (1)

The reaction is endothermic, and the heat of this reaction is normally supplied by biomass combustion via the reaction

C6H10O5 + 6O2 f 6CO2 + 5H2O

(∆H0,298K > 0) (2)

In the steam reforming process, the cold gas efficiency, which is defined as the ratio of combustion heat of * Author to whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemical Engineering. ‡ Research Center of Supercritical Fluid Technology. § Institute of Multidisciplinary Research for Advanced Materials. (1) Corella, J.; Orio, A.; Toledo, J. M. Energy Fuels 1999, 13, 702709. (2) Corella, J.; Aznar, M. P.; Gil, J.; Caballero, M. A. Energy Fuels 1999, 13, 1122-1127. (3) Caballero, M. A.; Corella, J.; Aznar, M. P.; Gil, J. Ind. Eng. Chem. Res. 2000, 39, 1143-1154. (4) Asadullah, M.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. Environ. Sci. Technol. 2002, 36, 4476-4481.

produced gas to the combustion heat of the biomass feed, is low, because the total process is exothermic and heat loss is required for vaporization of water during the reforming of sewage sludge and wet biomass. If gasification could proceed at lower temperatures (250-400 °C), waste heat from high-temperature processes (such as iron manufacturing and cement production) could increase the cold gas efficiency and waste heat would be recovered as a binding energy in the form of valuable gases such as methane (CH4) and H2. Hence, it would be greatly beneficial to develop processes that promote gasification at lower temperatures (250-400 °C). The use of subcritical and supercritical water (Tc ) 374.2 °C and Pc ) 22.1 MPa) is one of the candidates for gasification and liquefaction of biomass, because it can be applied at lower temperatures.5-12 In particular, supercritical water has many advantages when used as a solvent or as reaction media. Supercritical water is completely miscible with light gases, hydrocarbons, and aromatics.13,14 Various organic reactions such as hy(5) Antal, M. J.; Allen, S. G.; Schulman, D.; Xu, X.; Divilio, R. J. Ind. Eng. Chem. Res. 2000, 39, 4040-4053. (6) Xu, X.; Antal, M. J. Environ. Prog. 1998, 17, 215-220. (7) Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. J. Ind. Eng. Chem. Res. 1996, 35, 2522-2530. (8) Modell, M. In Fundamental of Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science Publishers: London, 1985; pp 95-119. (9) Schmieder, H.; Abeln, J.; Boukis, N.; Dinjus, E.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M. J. Supercrit. Fluids 2000, 17, 145-153. (10) Kruse, A.; Gawlik, A. Ind. Eng. Chem. Res. 2003, 42, 267-279. (11) Elliott, D. C.; Sealock, L. J.; Backer, E. G. Ind. Eng. Chem. Res. 1994, 33, 558-565. (12) Elliott, D. C.; Phelps, M. R.; Sealock, L. J.; Backer, E. G. Ind. Eng. Chem. Res. 1994, 33, 566-574. (13) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. AIChE J. 1995, 41, 1723-1778.

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drolysis usually proceed without catalysts.14 For heterogeneous catalytic reactions in supercritical fluids, the high solvent solubility of supercritical fluids can greatly reduce mass-transfer limitations and extract the coke precursor from the catalyst surface, to prevent coking.15 Cellulose, glucose, and phenolic compounds (e.g., phenol, guaiacol) can be gasified in subcritical and supercritical water in the presence of a metal catalyst (platinum, nickel, etc.) or an alkali catalyst.16-22 Cortright et al.16 conducted glucose gasification at 265 °C and 5.6 MPa in water, using a platinum catalyst, and obtained high yields of CO2 and H2. Minowa et al.17 conducted cellulose gasification in near-critical water at 350 °C and 16.5 MPa with a reduced nickel catalyst and reported that 70% of the carbon could be gasified. Kruse et al.20 found that pyrocatechol could be completely decomposed into H2-rich gas in the presence of potassium hydroxide (KOH) or potassium carbonate (K2CO3) in supercritical water at 600 °C and 25 MPa. Elliott et al.21 conducted a gasification of p-cresol in water at 350 °C and 20 MPa using various types of base and noble catalysts and reported that nickel and ruthenium were active for the reaction. Park et al.22 reported the almost-complete gasification of cellulose in supercritical water at 450 °C and 44 MPa using RuO2. Lignin is another primary component in wood biomass; it contains many oxygen functional groups (phenolic compounds, hydroxyl, carboxyl, and carbonyl groups and ether and ester bonds). For the case of lignin decomposition in supercritical water without a catalyst, ether and ester bonds are easily hydrolyzed, and phenolic compounds and aldehydes (such as formaldehyde) are formed.23-26 These hydrolysis products cross-link with each other to form a heavy material.26-32 This cross-linked residue is formed even when a metal catalyst or alkali catalyst is used.33-35 Watanabe et al.33 conducted lignin gasification in supercritical water with (14) Savage, P. E. Chem. Rev. 1999, 99, 603-621. (15) Baiker, A. Chem. Rev. 1999, 99, 453-473. (16) Cortright, R. D.; Davda, R. R.; Dumesic, J. A. Nature 2002, 418, 964-967. (17) Minowa, T.; Zhen, F.; Ogi, T. J. Supercrit. Fluids 1998, 13, 253259. (18) Yu, D.; Aihara, M.; Antal, M. J. Energy Fuels 1993, 7, 574577. (19) Watanabe, M.; Inomata, H.; Arai, K. Biomass Bioenergy 2002, 22, 405-410. (20) Kruse, A.; Meier, D.; Rimbrecht, P.; Schacht, M. Ind. Eng. Chem. Res. 2000, 39, 4842-4848. (21) Elliott, D. C.; Sealock, L. J.; Backer, E. G. Ind. Eng. Chem. Res. 1993, 32, 1542-1548. (22) Park, K. C.; Tomiyasu, H. Chem. Commun. 2003, 6, 694-695. (23) Funazukuri, T.; Wakao, N.; Smith, J. M. Fuel 1990, 69, 349353. (24) Yokoyama, C.; Nishi, K.; Nakajima, A.; Seino, K. Sekiyu Gakkaishi 1998, 41, 243-250. (25) Ehara, K.; Saka, S.; Kawamoto, H. J. Wood Sci. 2002, 48, 320325. (26) Saisu, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Energy Fuels 2003, 17, 922-928. (27) Aida, T. M.; Sato, T.; Sekiguchi, G.; Adschiri, T.; Arai, K. Fuel 2002, 81, 1453-1461. (28) Tagaya, H.; Suzuki, Y.; Kadokawa, J.; Karasu, M.; Chiba, K. Chem. Lett. 1997, 1, 47-48. (29) Lundquist, K.; Ericsson, L. Acta Chem. Scand. 1970, 24, 36813686. (30) Lin, L.; Yao, Y.; Yoshioka, M.; Shiraishi, N. Holzforschung 1997, 51, 316-324. (31) Lin, L.; Yoshioka, M.; Yao, Y.; Shiraishi, N. Holzforschung 1997, 51, 325-332. (32) Lin, L.; Yoshioka, M.; Yao, Y.; Shiraishi, N. Holzforschung 1997, 51, 333-337. (33) Watanabe, M.; Inomata, H.; Osada, M.; Sato, T.; Adschiri, T.; Arai, K. Fuel 2003, 82, 545-552.

Osada et al.

NaOH. They reported that the gas yield with NaOH was greater than that without NaOH; however, cross-linked residues were formed in both cases. Johnson et al.34 studied lignin decomposition in supercritical water at 375 °C and 27.6 MPa of hydrogen in the presence of a palladium catalyst. They reported that the gas product yield was ∼10 wt %, the liquid product yield (ethyl acetate soluble) was ∼70 wt %, and the solid yield (ethyl acetate insoluble) was ∼10 wt %. These studies indicate that lignin cannot be gasified without the formation of solid products at low temperatures (250-400 °C). In this work, the applicability of a ruthenium catalyst was examined for the gasification of biomass at temperatures of 250-400 °C. At first, we will conduct lignin gasification in supercritical water and discuss the catalytic activities and the effect of water. Next, cellulose gasification in supercritical water will be performed with and without a catalyst. Finally, the decomposition of formaldehyde, which is one of the reactive intermediates in biomass decomposition, has been studied. From the experimental results, we will discuss the mechanism of catalytic gasification of lignin and cellulose in supercritical water and the effect of catalysts. Experimental Section Organosolv-lignin was purchased from Aldrich, and its molecular formula was C42.39H45.46O12.15, as determined by an ultimate CHNS analyzer (Perkin-Elmer, model 2400). Cellulose was de-ashed, and microcrystalline cellulose was purchased from Merck (Avicel No. 2331; average particle diameter of 20-100 µm). Paraformaldehyde (98% purity, used as source of formaldehyde and sodium hydroxide (NaOH, 1.0 mol/dm3) aqueous solutions were purchased from Wako Chemicals and used without further purification. Distilled water was obtained from a water distillation apparatus (Yamato Co., model WG220). A ruthenium catalyst (Ru/TiO2; 2 wt % rtuhenium on TiO2, surface area of 24.9 m2/g) was supplied from Osaka Gas Co., Ltd. A nickel catalyst (Ni/Al2O3; 17-19 wt % nickel on Al2O3, surface area of 50-70 m2/g) was purchased from Nissan Girdler Catalyst Co., Ltd. The solid catalyst used was crushed with an alumina mortar and sieved to a size of 250-500 µm. The standard gases obtained from GL Science Co. were as follows: H2 (99%), CO (99%), CO2 (99%), CH4 (99%), nitrogen gas containing C2H4, C2H6, C3H6, C3H8, n-C4H10, and i-C4H10 (1% of each). The reagents used as standards were as follows: guaiacol (99+%), catechol (99+%), toluene (99.5+%), isopropylbenzene (98+%), glucose (99+%), dihydroxyacetone (97+%), furfural (98+%), acetic acid (99.9+%), formic acid (99+%), and methanol (99.8+%) (Wako Chemicals). Reactions were conducted in SS316 stainless-steel tube bomb reactors that had an internal volume of 6 cm3.36 To analyze product gases, a high-pressure valve was connected to one side of the reactor. The loaded amount of sample (lignin, cellulose, or paraformaldehyde) was 0.1 g, and that of water was 2.0 g. For the experiments using alkali, a NaOH solution diluted to 0.1 or 0.5 mol/dm3 was loaded, instead of pure water. For the experiments with a solid catalyst (Ni/Al2O3 and Ru/ TiO2), 0.3 g of catalyst was loaded. After the samples were loaded, argon gas was pressurized to ∼1 MPa after air was purged from the reactor. The reactor was then submerged in (34) Johnson, D. K.; Chum, H. L.; Anzick, R.; Baldwin, R. M. Research in Thermochemical Biomass Conversion; Elsevier Applied Science: New York, 1988; pp 485-496. (35) Yoshida, T.; Matsumura, Y. Ind. Eng. Chem. Res. 2001, 40, 5469-5474. (36) Watanabe, M.; Tsukagoshi, M.; Hirakoso, H.; Adschiri, T.; Arai, K. AIChE J. 2000, 46, 843-856.

Catalytic Gasification of Lignin and Cellulose

Energy & Fuels, Vol. 18, No. 2, 2004 329

Table 1. Product Yield of Lignin Gasification at 400 °C and a Reaction Time of 15 min water density (g/cm3) without catalyst Ru/TiO2

0 0

without catalyst NaOH (0.1 mol/dm3) Ni/Al2O3 Ru/TiO2 equilibrium statea

0.33 0.33 0.33 0.33 0.33

a

gas yield (H%) (O%)

gas yield (C%)

water-soluble yield (C%)

solid yield (C%)

total (C%)

5.8 11.5

11.5 11.1

46.4 36.4

63.7 59.0

10.3 19.6

15.0 46.9

3.7 9.9 5.5 31.1 100

17.2 23.7 24.6 13.9

18.4 34.7 2.3 0

39.3 68.3 32.4 45.0 100

5.8 12.9 7.0 66.5 231.1

13.5 53.9 27.3 108.3 328.5

Estimated with CHEMKIN III.37

a fluidized-sand bath (Takahashi Rica Co., model TK-3) that was controlled at the reaction temperature. Reaction times were in the range of 15-180 min. After a given reaction time, the reactor was taken out of the bath and rapidly quenched in a water bath. The reactor was then connected to a syringe that was equipped with gas samplers to collect the produced gas and to measure its volume. After the product gases were sampled, the reactor was opened and products in the reactor were recovered with pure water. The products recovered were separated into water-soluble and water-insoluble fractions. For the case of cellulose, the water-insoluble fraction was defined as a solid product. For the lignin experiments, the waterinsoluble fraction was extracted with tetrahydrofuran (THF) and separated into THF solubles and THF insolubles. In the lignin experiments, THF insolubles were defined as solid product. The identification and quantification of gas products were conducted using gas chromatography-thermal conductivity detection (GC-TCD) (Shimadzu, model GC-7A and Hitachi, model GC163) based on the peak of standard gases. For the water-soluble and THF-soluble products, qualitative and quantitative analysis was performed by gas chromatographymass spectrometry (GC-MS) (JEOL, model Automass 20) and gas chromatography-flame ionization detection (GC-FID) (Hewlett-Packard, model HP-6980) with standards. For the case of formaldehyde reactions, qualitative and quantitative analysis of water-soluble products were performed using highperformance liquid chromatography (HPLC) with an ultraviolet (UV) detector (JASCO, model UV-1570) that was set at 210 nm and refractive index (RI) measurements (JASCO, model RI-1530), on the basis of the peak of standards. The amounts of organic and inorganic carbon in the aqueous solution were evaluated using the total organic carbon (TOC) analyzer (Shimadzu, model TOC-5000A). Ultimate analysis of the solid product was conducted by a CHNS analyzer (Perkin-Elmer, model 2400). Product yield was defined as the number of C, H, and O atoms in the product, relative to the C, H, and O atoms in the reactant (lignin or cellulose or paraformaldehyde), excluding water and the catalyst: Product yield of carbon (hydrogen, oxygen) [C% (H%, O%)] ) moles of C (H, O) atoms in product × 100 (3) moles of C (H, O) atoms in reactant loaded

Results and Discussion Lignin Gasification. Table 1 shows the product yield of lignin gasification under the conditions of 400 °C and a reaction time of 15 min. Equilibrium gas yield and compositions under the experimental conditions were calculated by CHEMKIN III37 and are shown in (37) Kee, R. J.; Rupley, F. M.; Miller, J. A.; Coltrin, M. E.; Grcar, J. F.; Meeks, E.; Moffat, H. K.; Lutz, A. E.; Dixson-Lewis, G.; Smooke, M. D.; Warnatz, J.; Evans, G. H.; Larson, R. S.; Mitchell, R. E.; Petzold, L. R.; Reynolds, W. C.; Caracotsios, M.; Stewart, W. E.; Glarborg, P.; Wang, C.; Adigun, O. CHEMKIN Collection, Release 3.6, Reaction Design, Inc., San Diego, CA, 2001.

Table 2. Gas Composition of Lignin Gasification at 400 °C and a Reaction Time of 15 min gas composition (mol %) water density (g/cm3) H2 CO CO2 CH4 C2-C4 without catalyst Ru/TiO2

0 0

1 20 11 1

33 53

38 33

8 2

without catalyst NaOH (0.1 mol/dm3) Ni/Al2O3 Ru/TiO2 equilibrium statea

0.33 0.33 0.33 0.33 0.33

7 16 22 0 17 4 14 0 15 0

42 62 59 44 40

33 15 19 41 45

2 1 1 1 0

a

Estimated with CHEMKIN III.37

Table 1. Calculation results of CHEMKIN III37 assume that the gas phase is a mixture of ideal gases. The lack of carbon balance is likely due to a liquid product in the THF solubles, because a quantitative analysis for THF solubles could not be performed. At a water density of 0.33 g/cm3, the gas yield was 3.7 C% without catalyst. The water-soluble products consisted of guaiacol, catechol, syringol, etc., and the individual yield of these compounds was 100% means that waste heat can be recovered with this technique. Figure 1 shows the product yield of lignin gasification versus reaction time obtained at 400 °C and a water density of 0.33 g/cm3 with Ru/TiO2. No solid product (38) Klass, D. L. Biomass for Renewable Energy, Fuels, and Chemicals; Academic Press: San Diego, CA, 1998; p 77.

H2

CO

CO2

CH4

C2-C4

14 43 23 9 21

29 0 7 0 0

53 54 66 46 45

3 3 3 44 34

1 0 1 1 0

Estimated with CHEMKIN III.37

(THF insolubles) was formed, and the gas yield of carbon increased to 69.9 C% after 180 min. The total yield of carbon increased as the reaction time increased while the water-soluble yield was almost constant. From these results, the amount of carbon that moved from THF solubles to water solubles probably balances with that from water solubles to the gas. The gas yield of hydrogen increased as the reaction time increased, reaching 152 H% after 180 min. This phenomenon can probably be attributed to the participation of water in the gasification reaction. Cellulose Gasification. Table 3 shows the product yield of cellulose gasification for reaction under the following conditions: 400 °C, a water density of 0.33 g/cm3, and a reaction time of 15 min. Without a catalyst, the gas yield was 11.3 C% and the water-soluble yield was 39.0 C%. Organic acids, dihydroxyacetone, furfural, glucose, and glycolaldehyde accounted for