Bio-oil contains solid contaminants (ash and char), which catalyzes the polymerization and cracking reactions of the bio-oil. The alkali present in the ash such as sodium and potassium also catalyzes cracking reactions. The particulate contents depend on the type of cleaning and filtration system used following the pyrolysis reactor. Cyclone separators are generally used to reduce the particulate contents but removal of finer particulates may require further filtration.

High Viscosity, Increases with Time

Viscosity of biomass is 40-100 cP (shown in Table 5), which increases over time because of polymerization reactions when bio-oil is stored. High viscosity makes it difficult to flow through pipes and valves. The viscosity is especially important when bio-oil is atomized using spray nozzles for direct combustion in burners and engines (Bridgwater 2012).

Low Energy Content

As shown in the Table 5, Energy content of bio-oil (19 MJ/kg) is similar to the energy content of biomass which is about 40% of the energy content of crude oil. The primary reason of its low energy content is its high moisture and oxygen contents.

Aging

Properties of bio-oil such as viscosity, composition and phase contents change over time because of polymerization among the many functional groups and phase separation in the bio-oil (Bridgwater 2012). The aging process increases with temperature because of its increased reactivity.