Hydrothermal liquefaction of Chlorella vulgaris and Nannochloropsis gaditana in a continuous stirred tank reactor and hydrotreating of biocrude by nickel catalysts

Abstract

Continuous hydrothermal liquefaction and further upgrading of the algal biocrude are of great importance for algae biofuel process scale-up and improvement of fuel properties. In this study, two strains of microalgae were used for processing in a continuous stirred tank reactor at 350 °C and 24 MPa for 15 min residence time. An average of 36.2 wt% and 31.5 wt% biocrude yields were achieved for Chlorella vulgaris and Nannochloropsis gaditana, respectively. The obtained biocrude was further upgraded by hydrotreating using commercial NiMo/Al2O3 and NiW/Al2O3 catalysts at two temperatures (250 °C and 400 °C) in a batch autoclave reactor for 4 h. Products distribution, analysis by elemental content, gas chromatography (GC), gel permeation chromatography (GPC), thermogravimetric analysis (TGA) and nuclear magnetic resonance spectroscopy (1H NMR) on upgrading products indicate that upgrading by both catalysts lead to improved physicochemical fuel properties, during 250 °C upgrading step, decarbonylation, decarboxylation and repolymerization are the dominant reactions while hydrodeoxygenation and cracking reaction are more promoted at 400 °C. The gasoline, kerosene and diesel oil components in the algal biocrude were increased from 18 wt% to >30 wt% after catalytic upgrading.