Oxidative torrefaction of microalgae Chlorella sorokiniana: Process optimization by central composite design

Microalgae are currently not viable as solid biofuels owing to their poor raw fuel properties. Torrefaction under oxidative media offers a cost-effective and energy-efficient process to address these drawbacks. A design of experiment was conducted using central composite design with three factors: temperature (200, 250, and 300 °C), time (10, 35, and 60 min), and O₂ concentration (3, 12, and 21 vol%). The responses were solid yield, energy yield, higher heating value, and onset temperatures at 50% and 90% carbon conversion determined from thermogravimetric analysis. Temperature and time significantly affected all responses, while O₂ concentration only affected higher heating value, energy yield and thermodegradation temperature at 90% conversion. Oxidative torrefaction of microalgae is recommended to be conducted at 200 °C, 10.6 min, 12% O₂ where the energy yield and enhancement factor are 98.73% and 1.08, respectively. It is also more reactive under an air environment compared to inert torrefaction conditions.