Two-stage hydrothermal liquefaction of a high-protein microalga

Hydrothermal liquefaction (HTL) is a promising route for producing renewable fuels and chemicals from algal biomass . However, the protein fraction of the alga gives rise to nitrogen compounds in the oil fraction, which may render the oil unattractive for use in conventional refining processes. We report a two-stage HTL approach with the primary aim of reducing the nitrogen concentration in the bio-crude oil. A mild (< 200 °C) pre-treatment step (Stage I) is performed before more severe (250–350 °C) HTL conditions (Stage II) are applied to the microalga Chlorella for the production of bio-crude in a batch reactor . The pre-treatment resulted in up to 50 wt.% of the input nitrogen crossing into the Stage I aqueous phase and, following Stage II processing, reductions in the bio-crude nitrogen contents of up to 55%, relative to the direct HTL of untreated Chlorella were observed. However, since considerable amounts of the starting material were lost in Stage I, overall lower quantities of bio-crude were isolated after Stage II processing, as compared to single-stage processing. Nitrogen extraction during Stage I is enhanced by the addition of acids (1 N sulphuric or formic acid) but the process remains unselective. Overall, it is concluded that the two-stage approach to reducing the nitrogen content of bio-crudes from a protein-rich alga requires careful evaluation of the trade-off between the desired bio-crude properties and the yield obtained.