Torrefaction of Sericulture Agro-Industrial Waste for Sustainable Waste-to-Resource Solutions

The valorization of agro-industrial residues through torrefaction provides a sustainable pathway for biofuel production while mitigating environmental impacts associated with waste disposal. This study investigates the torrefaction of sericulture waste, specifically integral silkworm pupae (ISP) and lipid-extracted silkworm pupae (ESP), to assess the effect of lipid removal on biocoal properties. Initially, the ISP and ESP were subjected to torrefaction under various conditions (180-300 °C, 20-60 min) to compare their thermal behavior and fuel characteristics. Proximate, ultimate, and calorific analyses were conducted to evaluate changes in composition, fixed carbon content, and heating value. Results indicated that the ESP exhibited superior carbon retention and energy densification, supporting its selection for process optimization. A response surface methodology using a categorical central composite design (α = 1.41421) was then applied to optimize torrefaction conditions for ESP, considering temperature and residence time as key variables. Statistically significant models (p < 0.001) were obtained for solid yield and higher heating value (HHV), presenting reduced cubic and two-factor interaction models. The optimal torrefaction conditions (255 °C, 60 min) resulted in a biocoal with a solid yield of 83.13%, HHV of 22.12 MJ·kg^-1, and an energy densification of 1.27. From an environmental perspective, torrefaction contributes to waste reduction and promotes biocoal quality. Economically, the process adds value to sericulture residues by converting them into high-energy biofuels, supporting decentralized energy solutions. The results support ESP residues as biofuel, highlighting the need for further research on the biorefining of extracted lipids, combustion performance, upscaling feasibility, and life cycle assessment to evaluate sustainability.