Hydrothermal liquefaction of concentrated acid hydrolysis lignin in a bench-scale continuous stirred tank reactor

Although numerous studies on the liquefaction of lignin for the production of high-yield and high-quality bio-oil have been performed in a batch reactor, studies using a continuous flow reactor are very rare. Herein, a bench-scale continuous stirred tank reactor (CSTR) was employed for the liquefaction of lignin for the first time. Lignin obtained using a two-step concentrated acid hydrolysis process from oil palm empty fruit bunch (EFB) was used as a feedstock. The batch reactor experiment was initially conducted to select the best solvent for lignin liquefaction and investigate the effect of a formic acid (FA) additive. A bench-scale experiment then was conducted to determine how the continuous process conditions can affect the yield and composition of bio-oil. Results showed that more effective depolymerization of lignin to bio-oil is possible in the CSTR, because of the fast heating rate. The water/ethanol mixture medium at 350°C and 28 min of space time was found to be the optimum reaction conditions to obtain a relatively high yield (51.5 wt %) and low molecular weight (597 g/mol) of bio-oil. Syringol was the most abundant monomer in bio-oils, regardless of process conditions, and higher temperature (e.g., 350°C) was found to promote demethoxylation and alkylation reactions to produce guaiacol and alkyl guaiacol. The addition of FA to the reaction mixture not only increased the bio-oil yield from 51.5% to 60% but also reduced the O/C molar ratio of bio-oil from 0.36 to 0.30, increasing its calorific value to 27.85 MJ kg-1.