Colloquia

Summary

The energy transition is becoming a pressing concern in society and there is a growing demand for sustainable alternatives for fossil fuels. Bio-oil presents a promising method to replace liquid fossil fuels with high energy density. Biomass can be converted to bio-oil through fast pyrolysis, with liquid yields up to 75 wt%. The main drawback of the bio-oil is the many oxygenated compounds that make the bio-oil acidic, unstable and lowers the energy density. The oxygen content can be reduced through in-situ catalysts, which remove oxygen in the form of CO, CO2 and H2O, or through downstream hydrodeoxygenation, where hydrogen reacts with the bio-oil at high P&T to remove the oxygen as H2O. Because the latter process requires large quantities of hydrogen, there is a demand for catalysts that improve the quality of the gaseous products towards hydrogen production through the sorption-enhanced water-gas shift reaction in addition to oxygen removal. Hydrotalcite and dolomite have shown favourable properties as sorbents, but their carbonation parameters for CO2 adsorption under fast pyrolysis conditions are not yet understood. In this study, we unravelled how the carbonation parameters of hydrotalcite and dolomite are affected by the temperature and the CO2 partial pressure under fast pyrolysis conditions. CO2 adsorption experiments were performed with a TGA  at 450 - 550oC with a CO2 partial pressure of 1 - 35%. The results showed that for hydrotalcite, the adsorption capacity increases with low temperature and high CO2 partial pressure and the adsorption time is reduced with high CO2 partial pressure. However, the adsorption capacity of hydrotalcite is too low for a practical fast pyrolysis application. For dolomite, the adsorption capacity is not significantly affected by the temperature or CO2 partial pressure, but the adsorption time is reduced at high CO2 partial pressure. The adsorption capacity of dolomite is high enough for a practical fast pyrolysis application. Furthermore, dolomite holds significant benefits towards other sorbents for a fast pyrolysis process, due to its high-temperature stability, cyclic stability and low costs. However, the direct deoxygenating effects of dolomite may not suffice for the required oxygen removal, and therefore additional catalysts may be needed. Nonetheless, is dolomite considered to be a suitable catalyst that reduces the oxygen content of bio-oil directly through deoxygenation, and additionally improves the quality of the gaseous products towards hydrogen production, presenting feasible opportunities for downstream hydrodeoxygenation.