Giulia Ravenni

Biochar produced via slow pyrolysis is increasingly used for carbon sequestration, waste valorization, and soil amendment. As pyrolysis systems are implemented at larger scales, robust assessment of biochar quality and contaminant content is essential to support regulatory development and ensure environmental safety. This study compares straw, biogas residue fibers (BRF), and sewage sludge, together with their corresponding biochar produced at laboratory and industrial scales. Total concentrations of selected heavy metals and metalloids, fluoride (F⁻), chloride (Cl⁻), and nutrients (N, P, and K) were determined and evaluated against existing quality standards. Feedstock type was the primary factor controlling contaminant concentrations in biochar. Sewage sludge-derived biochar exhibited the highest heavy metal concentrations, whereas straw-derived biochar consistently showed the lowest levels. Comparison with existing standards indicated that sludge-derived biochar exceeded the Cu and/or Zn limits, while BRF- and straw-derived biochar generally complied with guideline values. Among the investigated elements, Cu and Ni exhibited the highest retention during pyrolysis, whereas Cd and As were the most volatile, with fixation rates decreasing markedly at elevated temperatures. Halogen behavior was also matrix dependent. Fluoride retention was highest in sludge-derived biochar, likely due to the formation of stable metal–fluoride complexes, whereas chloride was only partially retained, with fixation rates generally below 60%. Overall, biochar quality was governed primarily by feedstock composition, whereas pyrolysis conditions controlled the retention and volatilization of specific elements. These findings provide new evidence to support the development of science-based criteria for agricultural use of biochar and its regulation.

Green biorefineries can support the reduction of soybeans imports to Europe, by producing protein-rich animal feed from alternative feedstock such as perennial grass and legume species. Once the protein-rich green juice is extracted, a fiber-rich pulp is left as a residue. This work investigates the thermochemical processing of the pulp via pyrolysis as an option to improve the energy balance and climate footprint of a green biorefinery, by producing non-fossil energy and a high-value biochar product. Laboratory-scale pyrolysis and biochar activation were carried out on pulp samples obtained from different perennial species, different pressing method, and maturity at harvest. The results highlighted the importance of the activation stage to obtain a porous biochar, potentially suitable as animal feed additive. The effects on the overall energy balance and climate impact of the system following the integration of pulp drying and pyrolysis, plus a possible activation step for the biochar, were evaluated with a techno-environmental assessment. The pulp sample composition had only limited influence on the climate impact potentials identified. In all cases, it was found that the integration of a combined drying-pyrolysis-activation system in the green biorefinery may provide substantial additional climate benefits but also that the magnitude of these is strongly dependent on the substitution use-value of the energy products.