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Energy & Fuels 2007, 21, 2340-2345
Gasification of Guaiacol and Phenol in Supercritical Water Gregory J. DiLeo, Matthew E. Neff, and Phillip E. Savage* Chemical Engineering Department, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136 ReceiVed January 29, 2007. ReVised Manuscript ReceiVed April 10, 2007
We report on the gasification of guaiacol and phenol in supercritical water at 400-700 °C. The reactions were conducted in sealed quartz tubes, at times with added Ni wires that ran the entire length of the reactor. These reactors allowed the homogeneous and heterogeneous SCWG rates to be quantified separately for the first time. Guaiacol is mainly gasified into hydrogen, carbon dioxide, carbon monoxide, and methane. The rest of the guaiacol decomposes to phenol and o-cresol or reacts to form char. Nickel does not affect the conversion of guaiacol to phenol and o-cresol, but it significantly changes the gas product compositions. Phenol is mainly gasified into hydrogen, carbon dioxide, and methane. Hydrogen was the most abundant product at high phenol conversions. The gas compositions measured experimentally were largely consistent with those anticipated from chemical equilibrium calculations. In the absence of nickel, phenol conversions up to 68% were reached after 1 h. In the presence of Ni wire, complete conversion was obtained within 10 min. These results show that homogeneous, uncatalyzed gasification in supercritical water is slow, but rates are greatly increased by added Ni. The pseudo-first-order rate constant at 600 °C for homogeneous gasification of phenol is 3.0 (( 0.4) × 10-4 s-1, and the rate constant for Ni-catalyzed gasification is 1.1 (( 0.1) × 10-3 cm/s.
Introduction There is a pressing need to develop sustainable energy systems. Chemically converting biomass, which is a renewable resource, into other species that retain its chemical energy is an approach that is both sustainable and largely CO2 neutral. Water constitutes a large percentage of the mass of many different types of harvested biomass. Removing this water (e.g., by drying) prior to chemical processing increases the energy requirements and cost needed to convert biomass to gas or liquid fuels. Therefore, there is a need for processing methods suitable for biomass with high moisture content. One general approach is to process the biomass in an aqueous phase. The specific implementation of this approach that we investigate here is supercritical water gasification (SCWG), an emerging technology that has been the subject of a recent review.1 SCWG is the conversion of organic material into gaseous products (H2, CO, CO2, CH4) via reactions in and with water at a temperature and pressure exceeding the thermodynamic critical point (Tc ) 374 °C, Pc ) 22.1 MPa). SCWG is more attractive than conventional gasification for biomass with >30 wt % water.2 Plant biomass consists of three main components: cellulose, hemicellulose, and lignin. Of these, lignin is generally considered the most difficult to process. The compounds of interest in this work, guaiacol (o-methoxyphenol) and phenol, were selected because they are among the simplest chemical models for important structural features of lignin. Chemical structures proposed for lignin3 contain an abundance of aromatic rings with hydroxy and methoxy substituents. * To whom correspondence should be addressed. E-mail: psavage@ umich.edu. Phone: (734) 764-3386. Fax: (734) 763-0459. (1) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliot, D. C.; Neuenschwander, G. C.; Kruse, A.; Antal, M. J. Biomass Bioenergy 2005, 29, 269-292. (2) Yoshida, Y.; Dowaki, K.; Matsumura, Y.; Matsuhashi, R.; Li, D.; Ishitani, H.; Komiyama, H. Biomass Bioenergy 2003, 25, 257-272. (3) Nimz, V. H. Angew. Chem. 1974, 86, 337-344.
Save the single experiment reported by Xu et al.,4 the literature contains no previous reports for SCWG of either phenol or guaiacol. Other simple molecules and model compounds have received attention. Table 1 summarizes the key features of these previous SCWG studies. Using a metal reactor, which has catalytic walls, as has been done in nearly all previous work, does not allow one to carefully control the amount and form of catalyst present and thereby study the catalytic kinetics. One of the goals of the present work was to obtain data for SCWG of lignin model compounds in the complete absence of any catalysis by metal. A second goal was to obtain data for SCWG catalyzed by nickel alone, so that the relative importance of homogeneous and heterogeneous reactions can be quantified. We used quartz tubes (2 mm i.d.) as reactors in this study and added Ni wire when we desired to study contributions from heterogeneous catalysis. We used this approach in our earlier study of methanol SCWG,14 as have others5-7 to study formic acid, glucose, and ethanol conversion in SCW. We report herein, for the first time, results for the uncatalyzed and catalyzed (4) Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. J. Ind. Eng. Chem. Res. 1996, 35, 2522-2530. (5) Kersten, S. R. A.; Potic, B.; Prins, W.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 2006, 45, 4169-4177. (6) Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 2004, 43, 4580-4584. (7) Arita, T.; Nakahara, K.; Nagami, K.; Kajimoto, O. Tetrahedron Lett. 2003, 44, 1083-1086. (8) Gadhe, J. B.; Gupta, R. B. Ind. Eng. Chem. Res. 2005, 44, 45774585. (9) Boukis, N.; Diem, V.; Habicht, W.; Dinjus, E. Ind. Eng. Chem. Res. 2003, 42, 728-735. (10) Boukis, N.; Diem, V.; Galla, U.; Dinjus, E. Combust. Sci. Technol. 2006, 178, 467-485. (11) Taylor, J. D.; Herdman, C. M.; Wu, B. C.; Wally, K.; Rice, S. F. Int. J. Hydrogen Energy 2003, 28, 1171-1178. (12) Hao, X. H.; Guo, L. J.; Mao, X.; Zhang, X.; M.; Chen, X. J. Int. J. Hydrogen Energy 2003, 28, 55-64. (13) Lee, I.; Kim, M.; Ihm, S. Ind. Eng. Chem. Res. 2002, 41, 11821188. (14) DiLeo, G.; Savage, P. J. Supercrit. Fluids 2006, 39, 228-232.
10.1021/ef070056f CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007
SCWG of Guaiacol and Phenol
Energy & Fuels, Vol. 21, No. 4, 2007 2341 Table 1. Previous Accounts of SCWG of Simple Compounds
authors
compd(s)
temp (°C)
P (MPa)
reactor material
kinetics
liquid-phase products reported
Gadhe & Gupta, 20058 Boukis et al., 20039; 200610 Taylor et al., 200311 Hao et al., 200312 Lee et al., 200213 Xu et al., 19964
methanol methanol methanol, ethanol, ethylene glycol glucose glucose glucose, glycerol, methanol, MEK, ethylene glycol, acetic acid, phenol glucose naphthalene, carbazole, phenyl ether, dibenzofuran alkylphenols
700 400-600 550-700 500-650 480-750 500-600
0.34-27.6 25-45 27.6 15-32.5 28 25-35
Inconel 600 Ni alloys Inconel 625 stainless steel Hastelloy C-276 Inconel 625
none none none none none none
none none none none none only for phenol
600 380-480
34.5 25-45
Hastelloy C-276 Hastelloy C-22
none none
yes none
400
12-29
stainless steel
none
glycerol glucose
622-748 374, 380
25-45 not given
metal stainless steel
yes none
glucose, catechol, vanillin, glycine pyrocatechol formic acid, glucose, glycerol
400-600 500-700 400-800
10-45 20-40 30
Inconel 625 Inconel 625 quartz
none none none
ethanol methanol
450-500 500-550
∼ 40 23-25
quartz quartz
yes yes
Yu et al., 199322 Park & Tomiyasu, 200315 Sato et al., 200316 Buhler et al., 200217 Williams & Onwudili, 2005; 200618, 19 Schmieder et al., 200020 Kruse et al., 200021 Potic et al., 2004;6 Kersten et al., 20065 Arita et al., 20037 DiLeo & Savage, 200614
many, data only at 400 °C yes yes,
