Solid fuel production from macadamia nut shell: effect of hydrothermal carbonization conditions on fuel characteristics

Abstract

Hydrothermal carbonization (HTC) was employed to convert macadamia nut shell (MCNS) into solid fuel. The effect of hydrothermal conditions was examined by varying the reaction temperature (170, 200, and 230 °C) and water/biomass ratio (2, 3, and 5). The properties of hydrochar were assessed in term of mass yield, energy recovery, higher heating value (HHV), and atomic H/C and O/C ratios. The results showed that HHV of hydrochar was significantly higher than raw MCNS, and progressively increased with increasing HTC temperature while mass yield showed opposite trend. The solid yield from HTC process varied between 58.38 and 78.43% of the initial dry MCNS with HHV around 22-27 MJ/kg. The energy recovery of hydrochars via HTC was in a range of 82.5-93.2%. Higher HTC temperature also increased the degree of coalification and thermal stability of hydrochar by removing volatile matters from the biomass. The removal of carboxyl and -OH groups reduces the O/C ratio leading to the higher energy densification of hydrochars. Hydrochar produced via HTC at 230 °C possessed the highest HHV of 27 MJ/kg, and had the O/C and H/C ratios close to sub-bituminous coal. The findings indicated that hydrothermal carbonization is a potential method to convert agricultural waste such as macadamia nut shell into solid fuel without pre-drying process.