Optimization of bio-oil and biochar production from defatted microalgae biomass via microwave-assisted catalytic cracking

Abstract

In this work, bio-oil and biochar were produced using microwave-assisted catalytic cracking (MACC) of defatted microalgae biomass (DMB) obtained from lipid extraction of Chlorella vulgaris. A maximum bio-oil yield of 58.34 % was achieved under optimized conditions, including 450 W of microwave power, a 10 % catalyst ratio, and a reaction time of 10 min. In comparison, the highest biochar yield of 65.11 % was recovered at 650 W, 10 % catalyst ratio, after 30 min. This shows that bio-oil yield is optimized under milder conditions, while biochar yield benefits from more intense and prolonged thermal treatment. Qualitative characterization revealed that the bio-oil exhibited low viscosity (9.74 mm²/s), indicating good flowability and resistance to oxidative degradation. Its notably high heating value of 41 MJ/kg is attributed to a high density (1.18 g/mL), suggesting a greater concentration of heavy hydrocarbons. Scanning Electron Microscopy (SEM) analysis showed that the biochar featured a porous structure with a high surface area. Additionally, Fourier-Transform Infrared Spectroscopy (FTIR) confirmed the presence of oxygenated functional groups in the biochar, at wavenumbers ranging between 1300 and 1800 cm⁻¹. Moreover, this study also compared the product yields obtained from MACC with those from conventional pyrolysis. The results show that MACC produced significantly higher yields of bio-oil and biochar, exceeding those of pyrolysis by 24.45 % and 24.23 %, respectively, while also reducing reaction time and enhancing energy efficiency.