Viscosity Reduction of Fast Pyrolysis Bio-Oil by Using CO₂ as an Additive

The addition of solvents is an option for reducing the viscosity of fast pyrolysis bio-oil and for slowing down the reaction rate of aging reactions. For this purpose, conventional organic solvents with low molecular mass are deployed, with alcohols in particular. They can determine the costs and overall ecological performance of the final product. In this context, the use of CO2, which is a side product of pyrolysis, that occurs in the noncondensable pyrolysis gas, can be a favorable option. However, CO2 has not been used for conditioning of fast pyrolysis bio-oil so far. This work is an evaluation of the influence that pressurized CO2 as an agent has on the viscosity of fast pyrolysis bio-oil. The utilization of CO2 can be an asset in the context of further processing steps such as atomization, filtration, or transfer by pumping. The results are compared to the effect of ethanol on the viscosity. It was found that CO2 at 50 bar leads to a reduction of the original viscosity by 70–80%, as does 5 wt % ethanol in fast pyrolysis bio-oil. In the case of CO2 usage, there is an approximately linear relationship between the relative viscosity and CO2 pressure. Furthermore, the temperature dependence of the relative viscosity change for pure fast pyrolysis bio-oil was examined. It reveals Arrhenius-like behavior with a formal activation energy of 66 kJ mol–1. The addition of CO2 (10 bar) reduces this value to 42 kJ mol–1, while it remains almost unchanged upon the addition of 5 wt % ethanol. The Vogel–Fulcher–Tammann equation was used to describe the temperature dependence but showed no further advantage over the Arrhenius-type equation.