Utilization of olive tree pruning residues in wood pellets

Wood pellets constitute one of the most common solid biofuels that fulfil residential and industrial energy requirements. The high demands of raw materials have urged the search for alternatives to pure wood materials, such as residual biomass obtained from intensive agriculture. In this study, pellets that utilize biomass from pruning of olive trees (Olea europea L.) were produced, which is a species intensively cultivated in Greece and Mediterranean countries. The leaves, embarked branches of 3 different diameter classes (thin, medium, thick), as well as pure stem-wood have been examined as pellets feedstock material, both separately and in different materials mixtures, including the synthesis case of initial materials ratio recorded in pruning residues material. The pelletization process was performed using a single pellet die press at 120 °C applying the pressure of 3 tons. The raw materials were characterized in regard to ash content, calorific value, moisture content, while the produced pellets were characterized in terms of moisture content, density, dimensional stability and mechanical strength. According to the requirements of ENplus certification system concerning the biofuels feedstock properties, the olive tree pruning residues material, despite their high calorific value, cannot be used as pellets feedstock material in the specific form of recorded initial pruning materials ratio (leaves, thin / medium / thick branches). The presence of leaves appeared to increase the ash content and reduce the mechanical strength of pellets. The medium and thick branches could serve as feedstock of pellets of quality B for residential applications (as regards the ash content). Pure wood presented lower ash content, though it could be used as feedstock of pellets of quality A2 (not A1, due to ash restrictions), while it could be classified as B in a mixture with medium or thick branches. All the studied residual materials of olive tree bestowed the produced pellets with integrity and plasticity.