Does size matter? Influence of wood chip particle size on combustion efficiency and emissions

Controlled combustion of woody biomass from prunings can valorize Mediterranean agroforestry residues, but the particle size of the woodchips could influence energy yield and emissions. This study evaluates the effect of particle size on fuel quality, boiler efficiency, and emissions during the combustion of woodchips from olive and citrus prunings. The residues were chipped with 20 and 40 mm screens; a 50/50 mixture was also prepared for each species. The classes were characterized (moisture, ash, C–H–N, LHV) and combusted in an 80 kW boiler with a multicyclonic filter; gases (CO, CO₂, NOx, SO₂, O₂) and PM were continuously measured. Efficiency was calculated using the indirect method and analyzed with nonparametric tests; a PCA synthesized the emission profiles. Olive showed higher yields (73.1–78.1%) than citrus (54.2–69.3%) and lower concentrations of CO (1020–1879 vs 4229–9604 mg/Nm3) and SO₂ (4.4–10.2 vs 55.6–192.7 mg/Nm3), with lower NOx (522–609 vs 749–871 mg/Nm3). Within the tested size range (20 and 40 mm screens), nominal chip size did not significantly affect yield; however, mixed-size classes were associated with greater variability and, for citrus, greater dispersion of the emission profiles. PCA clearly separated the two species, highlighting greater emission stability in olive and greater variability in citrus.