Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies

Hydrothermal technologies are broadly defined as chemical and physical transformations in high-temperature (200-600 degrees C), high-pressure (5-40 MPa) liquid or supercritical water. This thermochemical means of reforming biomass may have energetic advantages, since, when water is heated at high pressures a phase change to steam is avoided which avoids large enthalpic energy penalties. Biological chemicals undergo a range of reactions, including dehydration and decarboxylation reactions, which are influenced by the temperature, pressure, concentration, and presence of homogeneous or heterogeneous catalysts. Several biomass hydrothermal conversion processes are in development or demonstration. Liquefaction processes are generally lower temperature (200-400 degrees C) reactions which produce liquid products, often called "bio-oil'' or "bio-crude''. Gasification processes generally take place at higher temperatures (400-700 degrees C) and can produce methane or hydrogen gases in high yields.