Hydrothermal (subcritical and supercritical water) gasification has the potential to generate hydrogen gas from virgin biomass and biodegradable wastes [22] and [23]. Hydrothermal gasification can directly handle biomass and biodegradable wastes containing high moisture content without the drying step. The process is also characterized by high gasification efficiency at relatively low temperatures [23]. Hydrothermal gasification utilizes water as the main reaction medium and reactant. The subcritical water condition is water at temperatures below its critical point of water (Tc = 374 °C and Pc = 22.1 MPa)