Biomass can be gasified at high temperatures or low temperatures. The biomass undergoes partial oxidation resulting in gas and charcoal production. Then, the charcoal is finally converted into H2, CO, CO2, and CH4. This conversion process can be expressed as

Biomass + heat + steam ^ H2 + CO + CO2 + CH4 + hydrocarbons + char + etc. (4.4)

Hydrothermal gasification processes have currently been investigated both for waste treatments process and production of energy and valuable materials from bio­mass and organic wastes. The supercritical water gasification (SCWG) process has advantages compared over conventional gasification processes such that higher gas­ification efficiencies are achieved at much lower temperatures of ~400°C (Calzavara et al. 2005; Youjun et al. 2012). At supercritical conditions, a fluid is neither liquid nor gas as it cannot be made to boil by decreasing the pressure at constant tempera­ture, and it would not condense by cooling under a constant pressure (Saka et al. 2006). Under supercritical condition, with temperature and pressure above its criti­cal point (T> 374°C and P > 22.06 MP), water acts as a reactive medium due to its specific transport and solubilization properties. In these conditions, water of liquid state undergoes significant changes in its physical properties: decreasing in the dielectric constant, thermal conductivity, and viscosity while the density only decreases a little. Thus, supercritical water acts as a nonpolar solvent of high diffu — sivity and high transport properties and able to dissolve many organic compounds and gases.