Within thermo-chemical conversion four process
options are available: combustion,pyrolysis,gasification
and liquefaction. Bio-chemical conversion encompasses
two process options: digestion (production of
biogas,a mixture of mainly methane and carbon dioxide)
and fermentation (production of ethanol).
A distinction can be made between the energy carriers
produced from biomass by their ability to provide heat,
electricity and engine fuels. A useful means of comparing
biomass and fossil fuels is in terms of their O:C and
H:C ratios,known as a Van Krevlen diagram,shown in
Fig. 1. The lower the respective ratios the greater the
energy content of the material. The major conversion
technologies and processes are discussed in the following
sections.