Gasification is the conversion of solid raw material into fuel gas or chemical feedstock gas otherwise called as synthesis gas, which can be upgraded to liquid fuels (diesel and gasoline) by Fischer-Tropsch synthesis. Biomass gasification is a process that converts carbonaceous biomass into combustible gases (e. g., H2, CO, CO2, and CH4) with specific heating values in the presence of partial oxygen (O2) supply (typically 35% of the O2 demand for complete combustion) or suitable oxidants such as steam and CO2.

When air or oxygen is employed, gasification is similar to combustion, but it is considered a partial combustion process. In general, combustion focuses on heat generation, whereas the purpose of gasification is to create valuable gaseous products that can be used directly for combustion, or be stored for other applications. In addition, gasification is considered to be more environmentally friendly because of the lower emissions of toxic gases into the atmosphere and the more versatile usage of the solid byproducts (Rezaiyan and Cheremisinoff, 2005).

Gasification can be viewed as a special form of pyrolysis, taking place at higher temperatures to achieve higher gas yields. Biomass gasification offers several advantages, such as reduced CO2 emissions, compact equipment requirements with a relatively small footprint, accurate combustion control, and high thermal efficiency (Marsh et al., 2007;

Rezaiyan and Cheremisinoff, 2005). Gasification is normally carried out at temperatures over (727 °C)1000 K, but recently it has been demonstrated that H2 and CO can be produced through the aqueous phase reforming of glycerol at lower temperatures <347 °C (<620 K) (Simonetti et al., 2007; Soares et al., 2006) at which integration of syngas production with FT upgrading is feasible. The ratio of CO/H2 can be modified by the water gas shift reaction (CO + H2O! CO2 + H2).

The classification of gasification is based on several parameters such as types of gasifiers, gasification temperature, heating (direct or indirect), and gasification agent.