Factors affecting the quality of pellets made from residual biomass of olive trees

• Physical quality properties of pellets can be improved by the densification process.
• The pelletization process cannot alter the chemical properties of the raw materials.
• Energy density should be considered as a quality parameter of pellets.
• Ash fusibility should be considered to predict the slagging tendencies of biomass.

In the Mediterranean areas of southwest Europe, olive tree pruning residues generate substantial amounts of residual biomass. Pelleting this biomass supposes the conversion of a residue difficult to manage into an energy resource especially indicated to be used in the areas where these groves exist.The final quality of pellets varies depending on the raw material properties and the manufacturing process. Consequently, in this paper the raw materials were chemically characterized before pelletization under different conditions in a semi-industrial pellet mill. In addition, the physical and mechanical parameters of the pellets produced were analyzed in order to determine the optimum pelletization conditions for each raw material by using IBM SPSS Statistics 20 software and considering the European requirements for non-industrial pellets.The aim of this work is to investigate the influence of the main densification parameters (moisture content, compression and temperature) and raw material (leaves, pruning, and wood) on the quality properties of pellets from the different olive tree pruning residues.In general, low moisture content (9%), short compression lengths (20–24 mm) and temperatures higher than 40 °C were the best pelleting conditions for the residual biomass from olive trees, although these parameters varied between the raw materials analyzed.