Effect of Torrefaction Temperature on Lignin Distribution of Larix kaempferi C. and Liriodendron tulipifera L. Cubes and the Impact of Binder on Durability of Pellets Fabricated with the Torrefied Cubes

Larch (LAR) and yellow poplar (YP) cubes were torrefied at 180, 220 and 260°C for 50 min to investigate the effect of torrefaction temperature on chemical composition, fuel properties and pelletability. When increasing the torrefaction temperature from 180 to 220°C, lignin content of torrefied LAR and YP increased from 28 to 34% and from 21 to 23%, respectively. Xylose contents of LAR and YP decreased to 11% at the torefaction temperature of 260°C. The thermal degradation of xylose by the torrefaction treatment has a significant impact on increasing the ash content, higher heating value and weight loss of the torrefied LAR and YP. When the lignin distribution of untreated and 180°C-torrefied cubes were measured quantitatively by the microscopic analysis of SEM–EDX, the amount of lignin existing on the radial-sectional areas of LAR and YP increased from 7.89 to 18.84% and from 6.37 to 17.71%, respectively. Durability of pellets fabricated with torrefied LAR and YP cubes decreased with the increase of torrefaction temperature. By the adjustment of a moisture content for the torrefied particles, durability of torrefied LAR pellets improved by 8–11% but that of YP decreased by 2–21%. Addition of 2 wt% starch, lignin and protein as a binder to 180°C- and 220°C-torrefied LAR and YP affected the increase of pellets durability. Accordingly, it can be concluded that the addition of binder or moisture is needed to produce durable pellets made with torrefied LAR and YP while accounting for costs associated with the binders.