The quest for the production of direct hydrocarbon fuels from
renewable resources has been on the rise, and bio-oils that are produced
by fast pyrolysis of biomass are promising intermediates for the production
of advanced hydrocarbon biofuels. However, it is well known
that fast pyrolysis bio-oil has several undesirable properties, such as
high acidity and thermal instability, due primarily to the many oxygenated
compounds like acids, phenols, aldehydes, ketones and unsaturated
compounds that it comprises. These barriers have limited the
effectiveness of various post-process upgrading technologies capable
of converting unstable bio-oils to fungible fuels. Hydrodeoxygenation
has been the most considered process to upgrade crude bio-oil.
The thermal instability of pyrolysis oil limits the effectiveness of most
hydrodeoxygenation processes. To date, the best performing HDO
systems employed for the upgrading of fast pyrolysis oils are those
that are performed in multiple stages, in which the first stages usually
involve a stabilization step, whereby amild hydrogenation is performed
to decrease the concentration of the most reactive components.
Reactions that occur in the stabilization steps typically include the
reduction of aldehydes and ketones to alcohols and the reduction of
unsaturated carbon–carbon double bonds found in pyrolysis oils.
Doing so provides the bio-oils with a level of thermal stability that
improves the performance of the higher temperature second stage
processes involving the main HDO reactions. While this multistage
approach has been the most viable pyrolysis oil upgrading technology
reported in the open literature, catalyst lifetimes that allowfor economically
feasible performance have not been realized thus far.