Converting Biomass into Renewable Fuel Intermediate - Bio-oil: Elucidating Torrefaction Effect on Bio-oil Quality and Study on Bio-oil Aging Mechanism

Converting biomass into liquid transportation fuel is of great importance for a secure energy supply. Though biomass pyrolysis offers a convenient avenue to produce a liquid fuel intermediate - bio-oil, a significant amount of effort is still demanded to overcome the poor oil properties towards its application as drop-in transportation fuel. These challenges include reducing bio-oil oxygen content, water content, acidity and improving its stability during storage and upgrading. The current research investigated the torrefaction effect on improving bio-oil physicochemical property and stability. In this study, fluidized-bed fast-pyrolysis was performed on torrefied loblolly pine with different torrefaction severities. The results showed that torrefaction pretreatment can significantly reduce the oxygen and water content and acidity of the bio-oil as well as varying bio-oil chemical composition with concentrated pyrolytic lignin and levoglucosan. A modified cellulose pyrolysis mechanism was proposed to explain the increased levoglucosan concentration in the torrefaction-based bio-oil. With regarding to the bio-oil stability, torrefaction based bio-oil showed less physicochemical and compositional variation than that of raw bio-oil. In addition, torrefaction based bio-oil could maintain a uniform oil phase for twelve-month storage. All these effects are attributed to the structural and compositional change of torrefied wood induced by the thermal pretreatment.;The unstable nature of the pyrolysis oil is one of the key issues that limits its upgrading and further application, as aged bio-oil typically shows increased water content, viscosity, and phase separation. The aging phenomena are explained by the polymerization of bio-oil components under acidic and thermal conditions catalyzed by the mineral compounds contained in char particles in the bio-oil. Current research on bio-oil aging is focused on characterizing the aging behavior together with seeking methods to slow down the aging speed. However, only limited amount of research was reported on investigating the molecular-level polymerization mechanism of pyrolysis oil during the aging. Based on the results obtained from this study, the aging mechanism was proposed to be acid-catalyzed and free-radical condensation reactions which have temperature/time and acidity dependent. After aging, the bio-oil water content, viscosity and acidity increased; the aged bio-oil water insoluble fraction increased while the water soluble fraction reduced accordingly. Each bio-oil fraction reacted in similar manner as the water soluble fraction and pyrolytic sugar fraction of bio-oil generated high molecular weight solids after aging, and lignin fraction condensed significantly with the addition of acid catalyst. The reaction severity can be enhanced with high temperature heating in a low pH environment. Few types of aldehyde presented in the bio-oil could promote the condensation degree of pyrolytic lignin with selected acid catalyst. These results suggest a bio-oil aging reaction fashion involving individual bio-oil fraction aging and inter-reaction between different bio-oil fractions. For its first time, the electron paramagnetic resonance (EPR) study revealed the presence of free radicals in the bio-oil; further characterization of each fraction of the bio-oil showed that these radicals are all located in the pyrolytic lignin fraction. The EPR characterization of aged bio-oil together with using radical scavengers in accelerated aging showed that there is a weak correlation between the radical presence and bio-oil aging degree tentatively indicating the relative stable nature of bio-oil containing radicals. However, further research is needed to elucidating the radical reactivity when bio-oil is subjected to a high temperature upgrading conditions.