Torrefaction of Larix Kaempferi C. and Liriodendron Tulipifera L. Cubes: Impact of Reaction Temperature on Microscopic Structure, Moisture Absorptivity, and the Durability of Pellets Fabricated with the Cubes

Larch (LAR) and yellow poplar (YP) cubes were torrefied at 180, 220, and 260 °C for 50 min to determine the alterations in their structural and hydrophobic properties. Through microscopic observations, several cracks and crevices were found on the radial- and tangential-sectioned areas of the 260 °C-torrefied YP cubes. On the tangential-sectioned areas, horizontal resin canal for LAR and ray cells for YP were darker than vertical cells. Moisture absorptivity (MA) measured for investigating the hydrophobicity of the torrefied LAR and YP cubes decreased with the increase of torrefaction temperature. However, no obvious differences were observed between the colors of the surface and internal parts or between the internal parts of the cubes visually at each torrefaction temperature. In addition, MA of the LAR and YP cubes was not significantly influenced by the depth of the cubes. Accordingly, it can be concluded that the carbonization level of the LAR or YP cubes is affected more by torrefaction temperature than by the cube depth. On the other hand, pelletization of the torrefied LAR and YP cubes using a piston-type single pelletizer was conducted to investigate the durability of torrefied pellets. Results showed that pellets made with torrefied YP cubes were more durable than those with torrefied LAR cubes at each torrefaction temperature. Durability of the torrefied LAR and YP pellets decreased with the increase of torrefaction temperature. Therefore, torrefaction temperature of 260 °C and above might be not an effective treatment to produce strong torrefied pellets.