Heavy metals leaching in bottom ash and fly ash fractions from industrial-scale BFB-boiler for environmental risks assessment

The bottom ash and fly ash from the co-combustion of wood residues and peat at a bubbling fluidised bed boiler (296 MW) contained only quartz (SiO₂), microcline (KAlSi₃O₈) and albite (NaAlSi₃O₈). Thus, X-ray powder diffraction (XRD) was not useful for clarifying the difference in the release of associated heavy metals from ash matrices. In order to assess the release of heavy metals from ashes under changing environmental conditions, they were sequentially extracted and fractionated by the BCR-procedure into acid soluble/exchangeable (CH₃COOH), reducible (NH₂OH-HCl) and oxidizable (H₂O₂/CH₃COONH₄) phases. The CH₃COOH extractable fraction in conjunction with the total heavy metals concentrations were used to calculate the risk assessment code values for heavy metals leaching from the ash matrix. The leaching studies indicate that the heavy metals in the bottom ash and fly ash are bound to different fractions with different strengths. From the environmental and utilization perspectives, heavy metals in ashes posed different levels of environmental contamination risk. Only As in the bottom ash posed a very high risk. High risk metals were Cd in the bottom ash as well as As, Cd and Se in the fly ash.