Effects of the gas-/liquid-phase interactions on the evolution of bio-oil during its thermal treatment

To clarify the interactions among bio-oil components during bio-oil pyrolysis, the gas- and liquid-phase interactions were decoupled and investigated, respectively. In this study, the bio-oil was separated into the aromatic-rich fraction and the aromatic-poor fraction, and these two fractions were pyrolyzed in a quartz basket with a baffle at the bottom to eliminate the liquid-phase interactions. The results showed that the gas-phase interactions promoted the coke formation via the cross-polymerization between the light compounds and aromatic compounds during bio-oil pyrolysis at 300–400 °C and the tar yields and content of the aromatics in the tar decreased correspondingly. At 500–800 °C, more large molecules were generated as a result of the intensified gas-phase interactions; thus, both the tar and coke yields were increased as well as the aromatics with more than three rings in the tar. Additionally, the increasing percentage of the product yields was higher at higher temperatures. The liquid-phase interactions promoted the formation of the non-evaporable large molecules, which affected the subsequent gas-phase reactions, leading to the increase in the heavy tar yields and decrease in the light tar yields. Meanwhile, the liquid-phase interactions promoted the formation of the primary coke, inhibited the formation of the secondary coke, and promoted the formation of the O-containing functional groups in the coke.