The Influence of Material and Process Parameters on Pressure Agglomeration and Properties of Pellets Produced from Torrefied Forest Logging Residues

Pellets produced from raw or torrefied shredded logging residues have been investigated in the study. The research material came from pine and spruce stands in Poland, Slovakia, Czechia and Hungary. Torrefaction temperatures (<i>T_t</i>) of 250, 300, and 400 °C were applied. Before pressure agglomeration, 3% wheat flour was added to the torrefaction material as a binding agent. Pellets with a diameter of 8 mm were produced at constant humidity, compaction pressure (<i>P</i>) of 140 or 180 MPa and agglomeration temperature (<i>T_a</i>) of 100, 120 or 140 °C. The produced pellets were assessed for their physicomechanical parameters (density, radial compressive strength, compression ratio, modulus of elasticity), chemical parameters (extractive compounds, cellulose, lignin) and energy parameters (ash content, elemental composition, calorific value). The results were subjected to basic statistical analysis and multi-way ANOVA. The produced pellets varied in physical, mechanical, chemical and energy properties. A significant effect of torrefaction temperature, agglomeration temperature and compaction pressure on the results was observed. In terms of physicomechanical parameters, the best pellets were produced from the raw material, while in terms of energy parameters, those produced from the torrefied material were superior. Pellets of satisfactory quality produced from torrefied logging residues could be obtained at <i>T_t</i> = 250 °C, <i>T_a</i> = 120 °C and <i>P</i> = 180 MPa. Pellets with specific density of approximately 1.1 g·cm^-3, radial compressive strength of 3-3.5 MPa, modulus of elasticity of 60-80 MPa and calorific value of 20.3-23.8 MJ·kg^-1 were produced in the process.