Development of direct conversion method for microalgal biodiesel production using wet biomass of Nannochloropsis salina

•FAME and FAEE can be produced from the wet biomass in situ.•Acid catalyst was used for direct transesterification of wet biomass to generate FAAEs.•High yield (>90% of esterifiable lipids) was achieved.•At >105 °C, 2.5–5% of H2SO4 successfully converted more than 90% of lipids.

In this work, the effects of several factors, such as temperature, reaction time, and solvent and acid quantity on in situ transesterification yield of wet Nannochloropsis salina were investigated. Under equivalent total solvent volume to biomass ratio, pure alcohol showed higher yield compared to alcohol–chloroform solvent. For esterifying 200 mg of wet cells, 2 ml of methanol and 1 ml of ethanol was sufficient to complete in situ transesterification. Under temperatures of 105 °C or higher, 2.5% and 5% concentrations of sulfuric acid was able to successfully convert more than 90% of lipid within 30 min when methanol and ethanol was used as solvents respectively. Also, it was verified that the optimal condition found in small-scale experiments is applicable to larger scale using 2 L scale reactor as well.