1.1 Gasification of Biomass

Synthesis gas (syngas in brief) is a gas mixture of predominantly CO and H2. Gasification of biomass feedstocks produces syngas through partial oxidation. Syngas quality largely depends on the compositions of biomass feedstock, gasifier types, and the gasifying agents. Other than the major constituents—CO and H2, gasification of biomass also produces methane (CH4), nitrogen (N2), carbon dioxide (CO2), water vapor, trace amounts of sulfur containing compounds, tar, higher hydrocarbons such as ethane (C2H6), ethylene (C2H4) and acetylene (C2H2), and particulate matter (Datar et al., 2004).

Gasifiers are mainly divided into two categories, namely, fixed-bed and fluidized-bed gasifiers. The fixed-bed gasifiers are characterized by the stationary reaction zone. Typically, in these gasifiers, biomass is fed from the top. Depending on the direction of biomass feeding and the oxidant employed, fixed-bed gasifiers are further divided into updraft and downdraft gasifiers. Fluidized-bed gasifiers use sand, ash, or char as moving media to increase the heat transfer and the gasification efficiency. Generally, the gasification of biomass takes place at high temperatures (fluidized bed: 750-900 °C and fixed bed: 1000-1200 °C). Table 1 shows the compositions of the produced gas mixtures from various gasification techniques and biomass feedstocks.