Mechanical thermal and emission assessment of olive pruning residues for sustainable biofuel production

Olive cultivation generates large quantities of pruning residues that are often underutilized despite their potential as a renewable biomass source. Converting this abundant agricultural by-product into densified biofuel can contribute to sustainable energy systems while reducing waste management challenges. This study evaluates the mechanical and combustion properties of olive pruning residues to assess their suitability as a sustainable biomass fuel. The mechanical behavior, including dynamic and internal friction as well as compaction performance, and the combustion behavior of the compressed briquettes, encompassing combustion characteristics and thermogravimetric responses, were systematically investigated. The dynamic friction coefficient increased with moisture content (5-15%), reaching up to 0.825 on steel surfaces. Internal friction rose with moisture (from 0.54 to 0.67), while cohesion declined (from 0.80 to 0.68 N). Briquette densities ranged from 958 to 1180 kg m^-3 under pressures of 3 to 16 MPa, with finer particles and lower moisture yielding higher density. Flue gas analysis of the densified briquettes revealed high CO₂ (10.5%), low final CO (≈ 500 ppm), but notable SO₂ (up to 500 ppm) and NO_x (> 80 ppm). Thermogravimetric analysis showed a three-stage decomposition, with 48% mass loss during cellulose and hemicellulose breakdown. The novelty of this study lies in its integrated assessment of the mechanical densification of olive pruning residues and the thermal combustion performance of the resulting compressed briquettes, providing new insights for optimizing the valorization of agricultural waste into clean bioenergy.