Alternatively, biomass can be converted into fuels and chemicals indirectly (by gasification to syngas followed by catalytic conversion to liquid fuels or basic chemicals) or directly to a liquid product by thermochemical means such as pyrolysis or liquefaction. Thermochemical conversion processes use heat and pressure to convert biomass into liquid, bio-oil or gaseous intermediates. These intermediates, such as syngas and bio-oil, subsequently go through customized processing to produce biopower, biofuels or building blocks for biochemicals.

Thermochemical processes allow productive use of a wide spectrum of biomass resources. The relative high temperatures of thermochemical processes (300-1000°C) over­come the natural resistance of biomass to chemical or enzymatic conversion, thus expanding the range of feedstock that can be potentially used. Common thermochemical conversion pathways include gasification, pyrolysis and, to a lesser extent, liquefaction. The difference between the three processes is determined by three main parameters: the oxygen level (A.), pressure and temperature ([16,48]; Figure 2.3).