Effects of Mild Alkali Pretreatment and Hydrogen-Donating Solvent on Hydrothermal Liquefaction of Eucalyptus Woodchips

Eucalyptus woodchips (EWC) were pretreated with mild aqueous NaOH solution, followed by liquefaction in a water-hydrogen donor (tetralin) mixture at a temperature from 220 to 330 °C to produce bio-oil. As a reference, pure tetralin and water were also used as liquefaction media. Compositional analysis and scanning electron microscopy (SEM) observation were performed to study the effect of the pretreatment on the EWC structural change. The influences of the temperature and tetralin content in mixed solvent on conversion and heavy oil (HO) yield were determined. The SEM image showed that pretreatment made the fiber structure loose and destroyed biomass through hemicellulose and lignin dissolution. In comparison to water or tetralin as solvent, water-tetralin mixed solvent (WTMS) shows a better effect on the conversion and HO yield in the tested temperature range. The highest conversions for liquefaction of untreated EWC were 66, 88.1, and 88.3 wt %, and the highest HO yields were 19.7, 26.4, and 43.2 wt %, with water, tetralin, and WTMS, respectively. The optimized conditions to achieve both high conversion (97.3 wt %) and HO yield (57.3 wt %) is liquefaction in WTMS at 300 °C using 1.0 wt % NaOH-pretreated EWC. Gas chromatography-mass spectrometry (GC-MS) analysis showed that phenols, ketones, aromatics, and alkenes are the main components in HOs.