Slow pyrolysis of relevant biomasses in the Mediterranean basin. Part 2. Char characterisation for carbon sequestration and agricultural uses

Abstract

The viability of agriculture-bioenergy production systems based on biomass pyrolysis greatly depends on the commercialisation of the char obtained, its use as a soil amendment being particularly relevant. The present study focused on assessing the agronomic quality of twelve chars derived from four relevant biomasses in the Mediterranean area (olive stone, almond shell, pine wood and olive-tree pruning) and produced by slow pyrolysis.The slow pyrolysis-derived chars generally fulfilled quality criteria for carbon sequestration and agricultural uses: organic carbon (Corg) > 50%, H/Corg < 0.7, O/Corg < 0.4, surface area >150 m2/g and polycyclic aromatic hydrocarbons (PAHs) < 12 mg/kg. The characteristics of the olive-tree pruning chars differed from those of the others, produced from ash-poor materials. As the pyrolysis temperature increased, the chars showed higher pH, salinity, surface area, degree of aromaticity and carbon, PAH and nutrient concentrations, and lower nutrient availability and cation exchange capacity. A pyrolysis temperature of 350 °C led to H/Corg ratios close to the limit established by the biochar quality guidelines (almond shell and olive-tree pruning chars); while a pyrolysis temperature of 550 °C led to a higher PAH content, almond shell char exceeding the limit of 12 mg/kg established by the European Biochar Certificate (EBC). In contrast, chars produced from olive stone, almond shell and pine wood at 350 and 450 °C achieved the “premium” grade according to the EBC (PAHs < 4 mg/kg). Therefore, a pyrolysis temperature of 450 °C and a volatile matter content <30% (on a dry, ash-free basis) ensured the production of quality end-products, according to the parameters evaluated.