Catalytic hydroconversion of HTL micro-algal bio-oil into biofuel over NiWS/Al2O3

Abstract

The demand for energy worldwide and the environmental concerns are increasing over the years, and microalgae is a potential raw material for third-generation fuel. In this work, the Chlorella sorokiniana microalga was converted by hydrothermal liquefaction (HTL) into a bio-oil using a batch reactor at 300 °C, under 10 MPa, for 30 min. Then, the bio-oil was upgraded using a batch reactor at three temperatures (350 °C, 375 °C, and 400 °C) under 10 MPa (H2) over NiW/Al2O3 sulfided catalyst. The bio-oil and upgraded fractions were characterized by elemental analysis, GPC-RID/DAD, 13CNMR, Sim-Dis, and GC×GC–MS/FID to identify the bio-oil molecular composition and follow its evolution. The lipids in the initial biomass, composed mainly of C16 and C18 chains/triglycerides, were converted into C16 and C18 fatty acids during the HTL reaction, corresponding to 41 % of bio-oil. Then, during the upgrading stage, these carboxylic acids were converted into C15 to C18 n-alkanes through two routes: hydrogenation or decarboxylation/decarbonylation, which was favored by temperature increase. >60 % of upgraded bio-oil eluted on the diesel range, as observed by Sim-Dis. The higher heating value (HHV) increased from 36 MJ/kg in the HTL bio-oil to 47 MJ/kg in the oil upgraded at 400 °C, and the average molar mass decreased from 333 to 203 g/mol. Oxygen and nitrogen compounds such as phenols, phytosterols, pyrroles, indoles, carbazoles, and amides were identified and quantified. Some insights about sulfur compounds are also given. The degree of deoxygenation, denitrogenation and desulfurization was higher than 90 % after the upgrading stage at 400 °C. Nevertheless, heterocyclic nitrogen compounds remained in the upgraded biooil. Therefore, catalytic hydrotreatment is confirmed as a relevant technology for upgrading HTL micro-algal bio-oil. However, catalyst performances can be improved to reduce less reactive nitrogen components.