(306a) Catalytic and Homogeneous Gasification of Biomass Model Compounds in Supercritical Water

There is tremendous interest in developing sustainable energy systems. Biomass contains a large amount of chemical energy, and as a renewable resource, can serve as an energy source that is both sustainable and largely CO2 neutral. A large percentage of the mass of harvested biomass is water. Removing this water (e.g., by drying) prior to processing increases the energy requirements and cost needed to convert biomass to gas or liquid fuels. One general approach for converting wet biomass is to process the biomass in an aqueous phase. The specific implementation of this approach of interest in the present work is supercritical water gasification (SCWG), an emerging technology that has been the subject of a recent review [1]. SCWG involves the conversion of organic compounds to 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). The compounds of interest in this work are methanol, phenol, guaiacol and glycine. We selected methanol for this study because it is among the simplest compounds that contain C, H, and O atoms, all of which are prevalent in biomass. The other compounds serve as models for the behavior of biomass in supercritical water ? guaiacol and phenol as models for lignin, and glycine as a model for amino acids from proteins. The major focus of this work is the behavior of these compounds and their ability to produce gaseous fuels in SCW. Another focus is on the roles of homogeneous chemistry and heterogeneous catalysis during SCWG. Previous work employed metal reactors, specifically metals containing Ni. The metal walls do catalyze the reaction, but the relative importance of the heterogeneous and homogeneous routes has not been resolved. In this work we will show the relative contributions of both the homogeneous SCWG and the catalyzed gasification. We used quartz tubes as reactors in this study and added a catalyst when we desired to study contributions from heterogeneous catalysis. This allows us to study the homogeneous SCWG kinetics independently of any catalytic effects. The quartz tubes are formed into batch reactors, charged with reactants, sealed and placed in an isothermal environment to conduct the reaction. Both liquid-phase and gas-phase products are collected and analyzed to determine their amounts. This data is used to calculate conversion, product yields and kinetic parameters. The experimental technique was validated through experiments with methanol [2]. The catalyst used in this work was nickel metal. The nickel was added to the reactor in the form of wire. The SCWG of methanol was performed at 500oC and 550oC under different catalytic conditions. The catalytic conditions used were homogeneous (with no Ni added), with a short wire only a fraction of the length of the reactor tube added, and with a long wire that ran the length of the reactor tube. Conversions of ~20% after 142 minutes at 550oC were the highest conversion observed in the homogeneous reactions. At the same temperature, the short wire was able to gasify up to 40% in 120 minutes, while the long wire gasified up to 95% of the methanol in 10 minutes. The increased surface area and decreased diffusion limitations in the presence of the long wire dramatically increased the conversion of methanol. The pseudo-first-order rate constant for homogeneous gasification is 4.0 x 10-5 s-1 at 550°C. The pseudo-first-order rate constants for Ni-catalyzed gasification are 0.0032 cm/s and 0.0040 cm/s at 500°C and 550°C. Hydrogen, carbon monoxide and carbon dioxide were the major products formed with hydrogen being the most abundant. The gas products were contained up to 75% hydrogen. This technique was used to study SCWG of guaiacol. Preliminary results show that at 500oC guaiacol partially decomposes in supercritical water to other phenolic compounds, and partially gasifies. The presence of nickel metal does not appear to catalyze the decomposition or the gasification of the guaiacol, but it does alter the gas product distribution. Without Ni the major gas product is methane, while with Ni the major gas product is hydrogen. A solid char is also formed during this reaction. Results for phenol and glycine gasification will also be presented.

[1] Y. Matsumura, T. Minowa, B. Potic, S. R. A. Kersten, W. Prins, W. P. M. van Swaaij, B. van de Beld, D. C. Elliot, G. C. Neuenschwander, A. Kruse, M. J. Antal Jr., Biomass Gasification in Near- and Super-critical Water: Status and Prospects, Biomass and Bioenergy, 29 (2005) 269-292.

[2] G. DiLeo, P. E. Savage, Catalysis During Methanol Gasification in Supercritical Water, Journal of Supercritical Fluids, in press.