Thesis (M.Eng.)--Chulalongkorn University, 2012

Depletion of fossil fuel and environmental concerns stimulate the use of clear and renewable energy. Biomass is regarded as a potential energy source and can be efficiently converted to a useful synthesis gas via incomplete combustion in a gasification process. However, the thermal gasification causes a formation of tar and requires high energy input when wet biomass is used. Supercritical water gasification (SCWG) is a possible way to cope with these difficulties because supercritical water behaves like many organic solvents so that organic compounds have complete miscibility with supercritical water. The objective of this study is to examine the performance of a biomass gasification process in supercritical water condition at energy self–sufficient operation, compared with a conventional gasification process. Modeling of the biomass gasification is performed using a process flowsheet simulator. Two different biomass feedstock: rice straw (low water content feedstock) and hyacinth (high water content feedstock), are considered in this study. Simulations are carried out to study effect of key operational parameters, such as gasification temperature, feedstock concentration and pressure, on the gaseous product composition and the efficiency of the gasification process under a energy self-sufficient operation, where no external heat input is required. The simulation results show that the conventional gasification is suitable process when biomass with low moisture content like rice straw is used as feedstock, whereas for biomass with high moisture content like water hyacinth, the application of the supercritical water gasification process is the appropriate option. To determine an optimal operating condition for the supercritical water gasification process, a response surface methodology (RSM) based on a central composite design (CCD) is used in this study.