Hydrothermal Liquefaction of Kraft Lignin in Subcritical Water: Influence of Phenol as Capping Agent

The depolymerization of LignoBoost Kraft lignin in subcritical water, i.e. hydrothermal liquefaction (HTL), was investigated using ZrO2, K2CO3, and KOH as catalysts in a fixed-bed reactor with recirculation. Focus was placed on the effect exerted by the concentration of the phenol in suppressing repolymerization, which is responsible for forming char. Feeds with various concentrations of phenol (2-10%) were investigated, and the results showed that phenol partially prevents repolymerization even at low concentrations. The bio-oil yield of (61.0 +/- 2.7) % was fairly stable when the concentration of phenol was varied. In the case of the formation of char on the catalyst, the char yield revealed a weakly decreasing trend (14.6-12.3%) when the amount of phenol in the feed was increased. The results also showed that the phenolic monomers that are alkylated, such as o-/p-cresols, increased significantly with increasing concentrations of phenol, while aromatic compounds, based on a guaiacol ring structure, showed decreasing trends.