Quality assessment of olive stones and nut shells for residential combustion applications

Abstract

The main objective of this work is to study the quality of typical Mediterranean biofuels (almond shells, pine nut shells and olive stones) that could be used in a sustainable way from a CO2 saving perspective in the same combustion facilities as wood pellets, i.e. residential, small commercial and public building applications. With this purpose, olive stones were collected during four sampling campaigns (2011, 2015, 2019 and 2023) and nut shells in two campaigns (2011 and 2015) and their fuel quality properties were determined. Wood pellets were used as the solid biofuel of reference. Significant sustainability and global CO2 savings (>13 Mt CO2/yr) could be achieved by using solid biofuels in domestic thermochemical biomass systems near production sites. Olive stones and wood pellets present similar chemical compositions and, in general, better fuel quality than nut shells, in terms of ash content (≤0.8 wt% for olive stones and ≤1.6 wt% for nut shells), nitrogen (≤0.4 wt%), sulphur (≤0.04 wt%) and calorific values (18.0–19.3 MJ/kg, d.b.). An overestimation of the ash content, determined according to ISO 18122:2022 [1], was obtained for compounds with high lignin content, standing at around 20 wt% in the case of pine nut shells. However, the notable tendency of olive stones and almond and pine nut shells to form sinters should be taken into account when designing and operating domestic combustion applications for this type of fuels.