Torrefaction Influence on Pelletability and Pellet Quality of Norwegian Forest Residues

Abstract

The main purpose of this paper is to determine how the torrefaction influences the pelletability of birch (hardwood) and spruce (softwood). Woods were torrefied at two different temperatures (225 and 275 °C) for 30 min. Energy loss (EL) and weight loss (WL), higher heating value (HHV), moisture uptake, water activity (aw), and particle size distribution of raw and torrefied woods were determined to characterize the materials before pelleting and to see how torrefaction affects physical properties of wood. The impact of biomass type, temperature, and compacting pressure on pellet strength and compressibility of raw and torrefied wood was investigated using a single pellet press method. Pellets were produced at three different temperatures (60, 120, and 180 °C) and eight different compacting pressures (5, 10, 20, 40, 80, 160, 240, and 300 MPa). Torrefaction at 275 °C significantly increased the HHV of both types of wood, in contrast to torrefaction at 225 °C. Compressing pressure and pelleting temperature had a significant positive impact on the material compressibility and strength. The strongest pellets were produced from raw spruce (68.62 ± 1.69 N/mm) and birch torrefied at 275 °C (86.34 ± 3.33 N/mm). Compression strength and density of the pellets were strongly correlated following a power low trend (R2 > 0.98). Torrefied material required higher force for pellet discharge because of the higher friction generated on the pellet surface–die area.