Densified Pellets from Scandinavian Forest Residues: A Study on Torrefaction and Pellet Characteristics

The aim of this project has been to investigate various aspects of the pre-treatment method torrefaction (biomass type, temperature, time (H-factor) and solid yield), and certain parameters (particle size distribution, friction and strength) related to densified pellets from both untreated and torrefied wood. There is today a large dependence on fossil resources. It has long been predicted that the fossil production has reached its peak, therefore new renewable alternatives such as plant biomass becomes increasingly important. Untreated biomass contains large amounts of moisture, the energy density is low and dried biomass has the ability to reabsorb moisture and rot during storage. Torrefaction and densified pellets are among many thought solutions to these problems. Results showed a strong correlation between H-factor and solid yield. Increasing the H-factor resulted in a decreasing solid yield, and no significant differences between materials was observed. The results also showed that torrefaction has a positive effect on the grindability. The two GROT-mixtures had a lower median than the remaining mixtures, and the reduction in particle size median was around 35 % for all mixtures. It has been demonstrated a clear dimensional difference between non-torrefied and torrefied pellets. Non-torrefied pellets had a greater density and smaller dimensions. Also, a method for quantifying the friction energy is presented, and frictional energy increased with increasing DC(%). Bark produced the strongest non-torrefied pellets, while Spruce produced the strongest torrefied pellets; increasing the DC(%) increased the pellet strength for Spruce and 90/10N, whereas it decreased for the bark-containing mixtures.