Preparation and characterization of fuel pellets from woody biomass, agro-residues and their corresponding hydrochars

Two types of biofuel pellets were prepared from raw biomass and the corresponding hydrochar. Fuel qualities of the two types of pellets including pellet density, mechanical strength and combustion characteristics were investigated. The results showed that the hydrochar pellets had higher fixed carbon contents, elevated heating values and enhanced mass densities in comparison to those raw biomass pellets. The presence of extractives led to the low tensile strength of biomass pellets, while the formation of liquid bridge and significantly enhanced attractive forces including H-bonding and van der Waal’s forces mainly contributed to the strong bonding within the matrix of hydrochar pellets. The hydrochar pellets combusted at elevated temperatures and wide temperature ranges compared to biomass pellets. The high mechanical strength, low moisture uptake and improved combustion property indicated that hydrochar pellets were more suitable than raw biomass pellets as solid biofuels. The present study demonstrated that hydrothermal carbonization (HTC) combined with pelletization provides an alternative for solid biofuel production from biomass resources, especially for the abundant agricultural residues.