Sub- and Supercritical Water-Based Processes for Microalgae to Biofuels

Abstract

Among the several biomass resources available, microalgae have the potential to produce more biofuels per acre than any other source without competing with food and feed production. One of the major challenges in utilization of microalgae is its high water content, high nitrogen (protein) content, and variable biochemical 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 microalgae to high-energy density biofuels. Here, water acts as reactant as well as reaction medium in performing hydrolysis, depolymerization, dehydration, decarboxylation, and many other chemical reactions. Subcritical water can be used as green solvent to extract valuable bioproducts from microalgae. Further, sub- and supercritical water technology can be used for liquefaction of microalgae to produce liquid biofuels and for gasification to produce gaseous fuels such as methane, syngas, and hydrogen. 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 is avoided. The chapter explains the theory of sub- and supercritical water-based processes for biofuels applications and the present state of the technology.