Water Resistance of Torrefied Wood Pellets Prepared by Different Methods

Torrefaction used in combination with pelletization is a promising technology to upgrade solid biofuels and has been demonstrated worldwide. In comparison with normal biomass pellets, which disintegrate under wet conditions, one of the advantages of torrefied biomass pellets is better water resistance. An understanding of the differences in water proof properties for torrefied biomass pellets by different production schemes can promote their further application. In the communication, various torrefied pellets were exposed to indoor and outdoor conditions, and changes in moisture content and diameter were examined. Two production schemes for the torrefied pellets were used for comparison: the torrefaction of wood chips followed by pelletization (pre-torrefaction) and the pelletization of wood chips followed by torrefaction (post-torrefaction). It was found that the post-torrefied pellets had much lower moisture levels than the pre-torrefied pellets in both indoor and outdoor tests. In the outdoor test with no-roof condition, the rate of increase in moisture content for the pre-torrefied pellets was more than double that for the post-torrefied pellets, and the post-torrefied pellets exhibited almost no diameter change. The results on the superior water resistance of post-torrefied pellets were nearly consistent with those reported in previous literature. Torrefied pellets have been considered for industrial use, such as in co-combustion and gasification on a large scale. Taking advantage of the different water resistances, torrefied pellets could also be used by personal and community consumers on a small scale for long-term indoor and outdoor storages as advanced solid biofuels with high waterproof performance, energy density, and lower biodegradation.