Hydrogen from Biomass by Supercritical Water Gasification

7 Hydrogen from Biomass by Supercritical Water Gasification 

The supercritical fluid extraction (SFE) is a process in which a highly compressed gas (fluid) is brought into contact with a relatively non-volatile solid or liquid at temperatures at or slightly above the critical temperature of the solvent. Under such conditions, the condensed phase will begin to volatize, which is interpreted as the supercritical fluid phase (Vayisoglu et al., 1996). The SFE is one of the best methods to obtain hqnid fuels from coals. The SFE extraction is carried out in an autoclave at above the critical temperature and the pressure of the solvent. The yield of soluble material increases with increasing pressure (Paul and Wise, 1971). 

Products from biomass by supercritical water (SCW) depend on the nature and stracture of the biomass. The effects of SCW on the biomass constituents should be separately studied. For example SCW affects unsatmated compounds, and unsaturated fatty and resin acids, rather than those of saturated ones under different reaction conditions (Watanabe et al., 2006). The diffusion or mass transfer rate of SCW into the individual component of biomass has been studied separately (Antal et al., 2000 Feng et al., 2004). 

In this ideahzed, stoichiometric equation (Eq. 6.36), cellulose (represented as CjHj Oj) reacts with water to produce and CO, the conunercial production of Hjfrom methane by the catalytic steam reforming process (Wagner and Froment, 1992). 

Recently, the supercritical fluid treatment has been considered to be an attractive alternative in science and technology as a chemical reaction field. The molecules in the supercritical fluid have high kinetic energy like the gas and high density like the Uquid. Therefore, it is expected that the chemical reactivity can be high. In addition, the ionic product and dielectric constant of supercritical water are important parameters for chemical reaction. Therefore, the supercritical water can be realized from the ionic reaction field to the radical reaction field. For example, the ionic product of the supercritical water can be increased by increasing pressure, and the hydrolysis reaction field is realized. Therefore, the supercritical water is expected as a solvent for converting biomass into valuable substances (Hao et al., 2003). 

---