Energy-efficient pretreatments for the enhanced conversion of microalgal biomass to biofuels.

The physiological properties, including biochemical composition and cell wall thickness, of microalgal species have a remarkable effect on the pretreatment of biomass and its further conversion to biofuels. In the present study, multiple biofuels (bioethanol, higher alcohols (C3-C5), and biodiesel) were produced using energy-efficient microwave pretreatment, successive carbohydrate/protein fermentation, and lipid transesterification from three microalgal strains (Pseudochlorella sp., Chlamydomonas mexicana, and Chlamydomonas pitschmannii). The microwave pretreatment method required the lowest specific energy (5 MJ/kg) compared to ultrasound pretreatment. The proposed integrated approach achieved high conversion efficiency (46%) and maximum biomass utilization (93%) of C. mexicana with improved yields of bioethanol (0.46 g-ethanol/g-carbohydrates), higher alcohols (0.44 g-higher alcohols/g-proteins), and biodiesel (0.74 g-biodiesel/g-lipids). This study suggests that the application of an appropriate pretreatment method for microalgal strains having different physiological properties is essential for improving the extraction efficiency and conversion of biomass to biofuels with less waste production.