The effect of various torrefaction conditions on the quality of bio-pellets from lesser-known shrub species

Abstract

Increasing industrial activity and human population have resulted in high consumption of fossil fuels. Therefore, the Indonesian government is currently promoting a decarbonization program to achieve carbon neutrality or net zero emission. To support this program, a renewable energy source based on biomass is being developed. However, information on potential sources of biomass as an energy provider for power plants is limited and has not been investigated comprehensively. Therefore, this study aims to explore and to investigate the potential of three species of local shrubs (Bungur/Lagerstroemia speciosa, Latte/Myrsine avenis, and Terasa/Mischocarpus sundaicus) as an alternative energy source for co-firing purposes in power plants. These materials were processed into a bio-pellet and were evaluated based on the Indonesian National Standard 8951-2020. The results show torrefaction increases bio-pellet density and reduces moisture content, making them drier and easier to burn. Ash content slightly increases but remains within SNI 8951-2020 limits. The calorific value of bio-pellets increases by 18–20% after torrefaction, meeting SNI standards. Torrefaction before and after pelletizing results in more stable dimensions and lower springback. M. avenis bio-pellets excel in moisture, ash, volatile matter, fixed carbon, and calorific value, while L. speciosa ranks second and M. sundaicus excels only in density. Torrefaction at 250°C yields the highest calorific value, but 150°C is the most energy-efficient. A 30-min torrefaction duration is also most efficient, though 45 and 60 min produce good characteristics. M. avenis bio-pellets are superior, and torrefaction after pelletizing at 150°C for 30 min is the most efficient production method. Moreover, this research underscores the need to adjust measurement conditions to prevent overheating, which can distort results by falsely indicating higher volatile matter content and causing unnecessary biomass combustion.