Sub- and Supercritical Water Technology for Biofuels

Abstract

One of the major challenges in utilization of biomass is its high moisture content and variable composition. The conventional thermochemical conversion processes such as pyrolysis and gasification require dry biomass for production of biofuels. Sub- and supercritical water (critical point: 374°C, 22.1°MPa) technology, which can utilize wet biomass, capitalizes on the extraordinary solvent properties of water at elevated temperature for converting biomass to high energy density fuels and functional carbonaceous materials. Here, water acts as reactant as well as reaction medium in performing hydrolysis, depolymerization, dehydration, decarboxylation, and many other chemical reactions. One of the advantages is that the large parasitic energy losses that can consume much of the energy content of the biomass for moisture removal are avoided. In sub- and supercritical water-based processes, water is kept in liquid or supercritical phase by applying pressure greater than the vapor pressure of water. Thus, latent heat required for phase change of water from liquid to vapor phase (2.26 MJ/kg of water) is not needed. For a typical 250°C subcritical water process, the energy requirement to heat water from ambient condition to the reaction temperature is about 1 MJ/kg, equivalent to 6–8% of energy content of dry biomass.