CO2 Solubility in Fast Pyrolysis Bio-oil

Preheating and the use of additives, such as alcohols, are common strategies to treat fast pyrolysis bio-oil after production or before its intended use. Such strategies lower the viscosity of bio-oil and slow chemical reactions occurring in bio-oil during storage. Furthermore, they influence the physical-chemical properties, environmental performance, and cost of the final product. This work suggests the use of CO2 as an alternative and environmental strategy. In contrast to other additives, CO2 has the advantage of being a byproduct of the pyrolysis process. To assess CO2 as an additive and solvent for bio-oil, solubility data on CO2 in acetol, pure bio-oil, and mixtures of bio-oil with an added compound are provided. Experiments were conducted at 50 degrees C and pressures of 20-100 bar. As additional compounds, acetic acid, acetol, furfural, guaiacol, and water are deployed. The results showed that the CO2 solubility is below 0.1 wt % at subcritical pressures but elevated at supercritical pressures. At 95 bar, the CO2 solubility equates to 0.45 wt %. This is below the CO2 solubility of butanol, which accounts for 0.6-0.7 wt % at the same pressure and is generally higher than the solubility in bio-oil. The CO2 solubility in pure fast pyrolysis bio-oil and its mixtures can be well described by the SRK-EoS. This is a basis to draw a connection between the CO2 solubility and its effects on the properties and further treatment.