Economic and environmentally efficient biochar production via microwave-assisted co-torrefaction of fruit residue and waste oil

Addressing climate change and reducing greenhouse gas (GHG) emissions are critical global challenges. This study introduces a novel, cost-effective method to produce high-energy biochar with minimal GHG emissions through a microwave-assisted (MWA) co-torrefaction process. This groundbreaking process not only produces decarbonized solid fuel but also simultaneously reduces waste by utilizing fruit residues and waste cooking oil (WCO). Through the application of the Taguchi experimental method, this research identified torrefaction temperature and the WCO blending ratio as key determinant of the biochar higher heating value (HHV). Notably, Dimocarpus longan waste (DLw) outperformed Citrus maxima branches (CMb) in co-torrefaction, achieving a maximum HHV of 25 MJ/kg, a superior energy yield of 85%, and a fixed carbon content of 28%. Both types of biochar showed increased thermal stability under optimal conditions, meeting the specifications of bituminous coal and offering a viable alternative for coal combustion. The biochar produced from both CMb and DLw demonstrated good energy return on investment (EROI) values of 4–5 and reduced CO₂ emissions by 58%–69% compared to burning bituminous coal alone. Remarkably, the cost of implementing this innovative technology at a pilot scale was 76% lower than that traditional biowaste treatment methods. In conclusion, the pioneering MWA co-torrefaction technology presented in this study offers an environmentally friendly, economically advantageous, and highly practical solution for converting biowaste into renewable fuel, aligning with the Sustainable Development Goals (SDGs), particularly Goal 7: Affordable and Clean Energy, and Goal 13: Climate Action.