Effects of pelletizing pressure and the addition of woody bulking agents on the physical and mechanical properties of pellets made from composted pig solid fraction

This study reports the effects of the addition of different woody bulking agents and pelletizing pressure on the final bulk density, durability and compression resistance of pellets manufactured from composted pig solid fraction. Two pressure levels (3.5 and 5.0 MPa) and three different types of compost (SFC - pig solid fraction, BC - pig solid fraction mixed with woody biochar, and WC - pig solid fraction mixed with wood chips) were investigated. The study shows that the different pressures adopted - in the range of 3.5 to 5.0 MPa - and the addition of woody bulking agents did not significantly affect the physical and mechanical properties of the pellets. However, according to the results of the study, the particle dimension of the woody bulking agents plays a key role in the mechanical properties of the pellets; the smaller the particles, the higher the pellet mechanical properties. The pelletizing process increased the bulk density of the investigated composts, and pellet durability was always high (> 80%). Nevertheless, when comparing the two bulking agents, the best results in terms of final quality were observed for pellets made from pig solid fraction mixed with woody biochar.