Effect of Torrefaction Conditions on Physical and Thermal Properties of High- and Low- Quality Palm Pruning Residue Pellets

Torrefaction is a promising method to improve the fuel quality of pellets. In this study, palm pruning residues were used as an alternative to forest products. Torrefaction was applied to high-quality (HQ) and low-quality (LQ) pellets at 220, 235, and 250 degrees C for 5, 10, and 15 min in a newly developed lab-scaled torrefaction system. Torrefaction temperature and residence time adversely affected the physical properties of HQ and LQ pellets. Particle density of untreated HQ and LQ pellets were 1172 kg/m(3) and 757 kg/m(3), respectively, decreased to 1059 kg/m(3) and 668 kg/m(3) at upper conditions (250 degrees C and 15 min). The durability index at upper conditions decreased from 99.02 to 98.13% and from 89.89 to 74.82% for HQ and LQ pellets, respectively. Heterogeneous and poorly physically structured LQ pellets showed non-homogeneous changes in thermal and physical properties. The HHVs of untreated pellets were increased from 18.4 to 20.3 MJ/kg for HQ pellets and 18.2 to 18.9 MJ/kg for LQ pellets at upper conditions. Pellets with high physical quality were torrefied more efficiently than those with low physical quality. Under the same conditions, LQ pellets were measured to be lighter in color than HQ pellets. Accordingly, it is thought that based on the color darkening caused by torrefaction, predictions can be made about the physical properties of the pellet and the torrefaction efficiency.