Quality effects caused by torrefaction of pellets made from Scots pine

The purpose of the study was to investigate the influence of torrefaction on the quality of Scots pine pellets. Pellet samples were torrefied at 230, 250 and 270 °C for 1 h in nitrogen atmosphere. Higher heating value (HHV) was increased from 18.37 MJ kg− 1 to 24.34 MJ kg− 1. The energy to crush a pellet by mechanical compression was determined using a material tester and results showed a rapid decrease before torrefaction temperature reached 250 °C. Slightly further decrease was observed when increasing the temperature up to 270 °C. The strength loss was confirmed by determining the energy required for grinding the pellet samples in a bench scale disc mill. Particle size distribution measurements after grinding indicated a significant increase of small particles (diameter < ca. 1 mm) for torrefied pellets at a torrefaction temperature of 230 °C and further increase of temperature resulted in steep decrease of large particles (diameter > ca. 2 mm). To further analyze the effect on strength, the mechanical durability of pellets was tested according to wood pellet standards, EN 15210-1. The results have shown a good correlation between pellet durability and compression strength, and indicated that the pellet durability can be estimated based on compression strength data of about 25 pellets.