The thermochemical treatments of biomass are beyond the scope of this book. The biological conversions include, anaerobic digestion to yield methane (CH4) and possibly hydrogen (Hz), and fermentation to produce ethanol or butanol.

The biomass/biofuels are used to generate heat, or to drive steam/internal combustion engines to generate mechanical/electrical energy. In general, the biomass is much less-energy dense and hence very bulky as compared to the fossil fuels.

Conversion of the biomass into liquid or gaseous biofuels makes them more energy dense, reduces their bulk and renders their handling and controlled manipulation more convenient. The bioconversion of biomass into biofuels removes most of the difficulties and, offers a means of obtaining fuels to partly or fully replace the fossil fuels.