Effect of Torrefaction Treatment on Physical and Fuel Properties of Caragana (Caragana korshinskii) Pellets

Caragana is potentially an alternative biomass feedstock in China. The influences of different torrefaction conditions on the physical characteristics and fuel properties of pellets made from Caragana were investigated. The Caragana pellets were torrefied under the temperature of 225, 250, and 275 °C with the residence time of 10, 15, and 20 min, respectively. The results showed that the density of torrefied pellet reduced by 24.07 to 35.20%. Compression strength decreased to 7.62 N/mm2 for treated pellets under the condition of 275 °C and 20 min with the maximum weight loss of 29.93%, and the higher heating value increased to 20.95~22.93 MJ/kg. The equilibrium moisture content of the torrefied pellets decreased by 46.17% compared with raw materials. The mass yield and energy yield of torrefied pellet were 70.07 to 88.54% and 70.93 to 90.79%, respectively. With the increase of the torrefaction severity, the mechanical and physical properties of the pellets decreased, while the storage performance and fuel properties enhanced. Simultaneously, the torrefaction temperature had more influence on the fuel properties of Caragana pellets than the residence time. The Caragana pellets torrefied at 250 °C for 20 min maintained not only almost the same mechanical and physical properties as the untreated pellets but performed better storage and fuel properties.