The aim of the present study was to identify the key factors affecting the pelletizing pressure in biomass
pelletization processes. The impact of raw material type, pellet length, temperature, moisture content
and particle size on the pressure build up in the press channel of a pellet mill was studied using a single
pellet press unit. It was shown that the pelletizing pressure increased exponentially with the pellet
length. The rate of increase was dependent on biomass species, temperature, moisture content and particle size. A mathematical model, predicting the pelletizing pressure, was in good accordance with experimental data. It was shown that increasing the temperature resulted in a decrease of the pelletizing
pressure. Infrared spectra taken from the pellets surface, indicated hydrophobic extractives on the pellet
surface, for pellets produced at higher temperatures. The extractives act as lubricants, lowering the friction between the biomass and the press channel walls. The effect of moisture content on the pelletizing
pressure was dependent on the raw material species. Different particle size fractions, from below 0.5 mm
up to 2.8 mm diameter, were tested, and it was shown that the pelletizing pressure increased with
decreasing particle size. The impact of pelletizing pressure on pellet density was determined, and it
was shown that a pelletizing pressure above 200 MPa resulted only in minor increase in pellet density