Fractional distillation of biocrude from hydrothermal liquefaction of microalgae: Upgrading of fuel properties

The use of hydrothermal liquefaction (HTL) biocrude as commercial fuels remains challenging due to its complex composition, particularly the presence of heteroatomic nitrogen and sulfur containing compounds and oxy-genates. In this study, a semi-automatic fractional distillation system was developed for the upgrading of bio-crude from hydrothermal liquefaction of microalgae biomass. The fractional distillation produced ten liquid fractions (F1-F10 below 325 degrees C), which made up above 48.91 wt% of the biocrude. Through the distillation, the contents of N, O, and S were reduced by 0.12 %-22.70 %, 27.71 %-69.34 %, and 24.30 %-64.02 %, respectively. The GC-MS results showed that lighter fractions (F1-F3) were mainly composed of N-heterocyclics while heavier fractions (F5-F10) were dominated by saturated hydrocarbons and olefins, which was further evidenced by the thermogravimetric analysis (TGA) and Gel Permeation Chromatography (GPC) results. In addition, a significant reduction in metal contents (up to 94.26 %) was observed, and the alkalis metals (89.63 % of Ca and 96.17 % of K) and heavy metals (91.32 % of Pb and 59.08 % of As) mainly remained in the distillation residue. This study indicates that fractional distillation is a potential green method for the upgrading of HTL biocrude.