Study on hydrothermal liquefaction of spirulina platensis using biochar based catalysts to produce bio-oil

Hydrothermal liquefaction (HTL) is an effective conversion technology of microalgae biomass. In this study, the low-lipid microalgae-spirulina platensis was used as feedstock to investigate the performance of the biochar-based catalysts on HTL. The by-product of spirulina platensis HTL-solid residue was collected and activated to obtain the biochar (BC). Then, the BC was used as the carrier to support Co, Ni and their oxides CoOx and NiO to form Co/BC, Ni/BC, CoOx/BC, NiO/BC catalysts. The Response Surface Methodology (RSM) was used to optimize the HTL parameters and investigate the effect of biochar-based catalysts on HTL. The results showed that the maximum yield of bio-oil catalyzed by BC was 35.80 wt% with 304 °C, 34.7 min, and 0.32 g catalyst loading. BC catalyst displayed an improvement of bio-oil yield up to 4.00 wt% at low temperatures (260–280 °C). Ni/BC was the most favorable catalyst for bio-oil production, reaching a maximum value of 36.57 wt% at 280 °C, 35.0 min, and 0.15 g catalyst loading, which increased by 6.40 wt% compared with the non catalytic case. The characterization of bio-oil showed that CoOx/BC and NiO/BC could raise the hydrocarbon content, H/C value, and heat value, while decrease O/C value. Ni/BC had an excellent denitrification effect on bio-oil, the N content was reduced by nearly 2.00 wt% compared with the non catalytic case.