Ash behavior, slagging propensity and trace metal retention in unconventional biomass: insights from sewage sludge hydrochar and poultry litter

This study investigates the high-temperature behavior of ashes derived from sewage sludge-based hydrochar (HC) and poultry litter (PL), focusing on slagging propensity and trace metal behavior. Combined SEM, EDS and XRD analyses revealed substantial mineral transformations in HC ashes at 1000 °C, including the decomposition of sulfate phases and the formation of cristobalite, calcium‑magnesium phosphates, and alkali aluminosilicates, which were associated with particle coalescence and liquid-phase sintering. In contrast, PL ash retained thermally stable Ca-P-rich domains and showed limited mineralogical changes with little evidence of melting. Commonly used empirical slagging indices (B/A, Fu, Sr, Al) substantially overpredicted the slagging tendency of PL ash, which exhibited high ash fusion temperatures (>1420 °C) and preserved its solid-state structure despite high alkali and Cl content. Thermal conversion of HC and PL resulted in ashes enriched in trace metals. Most elements (Zn, Cu, Cr, Ni, Pb, V, Mo, Co) showed retention factor (RF) values close to 1, indicating their strong retention in the ash phase. Except for slightly elevated Cr leaching in HC1 and PL ash, the metals exhibited low leachability, suggesting their stabilization within the mineral matrix during thermal treatment.