Mild hydrothermal treatment on microalgal biomass in batch reactors for lipids hydrolysis and solvent-free extraction to produce biodiesel

Highlights: • Mild hydrothermal pretreatment broke microalgal cell walls for lipid hydrolysis. • Valuable unsaturated fatty acids of C20:5 yielded 16.2 wt%. • The efficiency of solvent-free lipid extraction reached 95.0 wt%. • Biocrude pretreated for 10 min had the highest heating value (33.83 MJ/kg). -- Abstract: Mild hydrothermal treatment (245 °C) was used to break microalgal cell walls for lipids hydrolysis and solvent-free extraction in batch reactors to obtain biocrude for biodiesel production. Microalgal biocrude obtained through hydrothermal treatment for 10 min had the highest heating value (33.83 MJ/kg), the highest carbon and hydrogen contents (69.63% and 9.39%), and the lowest nitrogen content (3.56%). Fourier transform infrared spectroscopy showed that microalgal biocrude had increased long-chain alkane peaks and disappeared carbohydrate peaks compared to the original microalgal biomass. Thermogravimetric analysis implied that triglycerides in microalgal biomass were hydrolyzed into fatty acids through hydrothermal treatment, while fatty acids and protein hydrolysates reacted to produce amides with higher boiling points. After 10 min subcritical hydrothermal treatment with microalgal solid concentration of 5% at 245 °C and esterification reactions (100 °C)with methanol and H₂SO₄ catalysts, the solvent-free lipids extraction efficiency was improved from 69.9% (260 °C) to 95.0 wt% without losing the valuable unsaturated fatty acids in microalgae cells. The resulted main compositions of C16:0, C16:1 and C20:5 in microalgal biodiesel reached the highest of 22.7%, 24.1% and 16.2% (based on total lipids in microalgae), much higher than those lipids obtained from 260 °C treatment (C20:5 = 6.1%).