There are five thermal or thermochemical approaches that are commonly used to convert biomass into an alternative fuel/energy: direct combus­tion, gasification, liquefaction, pyrolysis, and partial oxidation. These five modes of conversion are also applicable to the conventional utilization of coal. When biomass is heated under oxygen-deficient conditions, it gener­ates bio-oil and bio gas that consists primarily of carbon dioxide, meth­ane, and hydrogen. When biomass is gasified at a higher temperature with an appropriate gasifying medium, synthesis gas is produced, which has similar compositions to that of coal syngas. This syngas can be directly burned or further processed for other gaseous or liquid fuel products. In this sense, thermal or thermochemical conversion of biomass is very similar to that of coal [5]. Figure 5.1 shows a variety of process options of biomass treatment and utilization as well as their resultant alternative fuel products.

Of a variety of thermochemical conversion options of biomass, this chapter is mainly focused on fast pyrolysis and gasification of biomass due to their technological significance, and other options of thermochemical conversion are discussed whenever deemed relevant.