Upgrading of biomass pellets by torrefaction and its influence on the hydrophobicity, mechanical property, and fuel quality

In this study, soybean straw pellets (SSP) and pine wood pellets (PWP) were used as materials for torrefaction at 200–350 °C. Impacts of torrefaction temperature on the physicochemical properties, hydrophobicity, and mechanical properties of the pellets were investigated. Results showed that some oxygen-containing functional groups in the biomass pellets disappeared during torrefaction, and the oxygen content significantly decreased. The hydrophobic behavior of biomass is highly affected by their chemical composition. A significant decrease in hemicellulose content was observed in torrefied pellets. Consequently, the equilibrium moisture content of torrefied pellets decreased significantly; and the torrefied PWP remained intact even after immersion of more than 2 h in water. As the torrefaction temperature increased, the cellulose and lignin started to decompose and formed more micropores. Meanwhile, the torrefied pellets became less dense, which lowered the mechanical properties of the pellets. Optimal temperature for the preparation of high-quality torrefied SSP and torrefied PWP was found to be 250 °C and 300 °C, respectively, since higher temperatures cause excessive loss of volatiles and degradation of mechanical properties without significantly enhancing the high heating value of torrefied pellets.