Comparison of trace elements in bottom ash and fly ash from a large-sized (77 MW) multi-fuel boiler at the power plant of a fluting board mill, Finland

Two ash samples were taken from the large-sized (77 MW) multi-fuel boiler (MFB) at the power plant of Stora Enso Oyj Heinola fluting board mill in Finland. The samples were analysed by X-ray diffraction (XRD) and inductively coupled plasma optic emission spectroscopy (ICP-OES) techniques. The results indicate that only hematite (Fe2O3) and quartz (SiO2) existed in both the bottom ash and fly ash, whereas anorthite (CaAl2Si2O8) and anhydrite (CaSO4) were only detected in the bottom ash. The element concentrations in the fly ash were within 1.1 (Sb) and 3.5 (Pb) times higher than those in the bottom ash. Only the concentration of sulphur in the bottom ash (8030 mg/kg; d.w.) was higher than that in the fly ash (5380 mg/kg; d.w.). In the bottom ash, the extractable Mo (8.2 mg/kg; d.w.) and sulphate (15.900 mg/kg; d.w.) concentrations exceeded the limit values for the acceptance of waste at inert waste landfills. In the fly ash, the extractable concentration of Cr (1.8 mg/kg; d.w.) and sulphate (7200 mg/kg; d.w.) exceeded the limit values for the acceptance of waste at inert waste landfills, and in addition, the extractable concentration of Mo (37.0 mg/kg; d.w.) exceeded the limit value for the acceptance of waste at non-hazardous waste landfills. According to a three-stage sequential extraction procedure, in which elements in the bottom ash and fly ash were fractionated between acid-soluble (CH3COOH), reducible (NH2OH-HCl) and oxidisable (H2O2 + CH3COONH4) fractions, the concentrations of most elements were higher in all fractions of the fly ash than those in the bottom ash. However, in the bottom ash, the extractable concentration of As (3.2 mg/kg; d.w.), Mo (8.2 mg/kg; d.w.), Pb (1.3 mg/kg; d.w.), S (7850 mg/kg; d.w.) and V (52.1 mg/kg; d.w.) in the acid-soluble fraction (CH3COOH), as well as the extractable concentration of Cu (40.2 mg/kg; d.w.) in the oxidisable fraction (H2O2 + CH3COONH4) were higher than those in the fly ash. Approximately 52.7% of Mo in the fly ash was partitioned in the oxidisable fraction. Thus, molybdenum is not considered to be very mobile or bioavailable.