Catalytic hydrothermal liquefaction of magnetically separated microalgae: effect of reaction conditions on bio-crude yield and composition

Hydrothermal liquefaction (HTL) of microalgae produces a biofuel with high nitrogen and oxygen content, leading to the emission of NOX gases which are environmental pollutants. A high N and O content also reduces the high heating value (HHV) and energy recovery (ER) of the fuel. This research aimed at reducing the N, O, and S contents of the biocrude oil by assessing the impact of reaction conditions on de-nitrogenation, de-oxygenation, de-sulphurisation, and on the biocrude yield in the presence and absence of Zn ferrite magnetic nanoparticles (MNPs). The MNPs played a microalgae separation role and a catalytic role. The reaction conditions assessed included holding time, HTL under a hydrogen atmosphere, and HTL under 5% formic and 5% sulphuric acid. An increase in holding time resulted in a gradual increase in bio-crude yield up to a maximum yield of 36.2 wt.% after 60 min. An increase in holding time also led to a gradual increase in carbon and hydrogen content of biocrude oil and to a steady reduction in O, N, and S contents. Liquefaction in the presence of 5% sulphuric acid resulted in the highest removal of nitrogen by 83 wt.% and the highest hydrogen content (10.6 wt. %) for all liquefaction experiments. According to GC-MS results, HTL under a hydrogen atmosphere resulted in increased removal of oxygenated compounds and an increase in hydrocarbon content of biocrude oil. The MNPs used for microalgae separation and catalytic HTL were synthesised using the co-precipitation process and characterised using HRTEM. An increase in the H and C contents and a reduction in the N, O, and S contents of bio-crude oil greatly improve its quality and energy value, hence its potential for use as a transportation fuel.