Microwave wet torrefaction: A catalytic process to convert waste palm shell into porous biochar

Abstract Waste palm shell is a large-volume biomass waste produced from palm oil mills, representing a resource to be treated and converted into a value-added product. In this study, an improved microwave wet torrefaction (low-temperature pyrolysis) incorporating microwave radiation and steam was performed to torrefied the waste palm shell into higher porous biochar. The biochar and catalytic effect of using microwave radiation and steam over a range of process temperature (250 to 350 °C) were investigated. The microwave wet torrefaction demonstrated a significantly faster heating rate (up to 120 °C/min) and compared to that shown by microwave heating without steam (45 °C/min, 15 min). Microwave wet torrefaction exhibited relatively lower mass yield (63.5 wt%) compared to microwave torrefaction (72.5 wt%). However, the biochar obtained from microwave wet torrefaction produced higher surface area (308.0 m2/g) compared to microwave torrefaction (110.8 m2/g). Our results demonstrate that microwave wet torrefaction shows potential as a catalytic approach to treat waste palm shell in providing a fast heating rate, lower process temperature, and generating a higher yield of a torrefied product with improved properties.