Pressure influences the equilibrium reactions and therefore affects the volatile prod­ucts composition: the condensable (bio-oil) and non-condensable gases. It has also been shown that pressure can promote other gas-solid reactions involving moisture, hydrogen, carbon dioxide and possibly other gaseous species. The mechanisms in­volved remain unclear, but chemical reactions such as the Boudouard reaction are suspected:

C(S) + CO2g ^ 2CO(g) (11.2)

Moisture, which is inevitably present during biomass pyrolysis, has also been shown to influence the volatile yield and composition [43]. These reactions are characterized by fast kinetics only under specific conditions: they generally occur in pressurized gasification and under high temperatures, while pyrolysis is typically performed at milder temperatures.

The solids will not be significantly influenced by pressure if an inert gas is em­ployed [44,45]. Experimentation is still the best method to characterize the effect of pressure on the pyrolysis products since it is specific to the biomass feedstock used.