Kati Manskinen

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.

Occupational Risk Evaluation of Using Bottom Ash and Fly Ash as a Construction Material

As a case study, the potential to use bottom ash and fly ash from a large-sized (120 MW) bubbling fluidized bed boiler (BFB) at the power plant of a fluting board mill were assessed to determine their suitability for use as an earth construction agent. In addition, the extractability of heavy metals in the ashes was determined using artificial sweat and gastric fluids to assess the potential occupational risk from ash handling. Owing to the relatively high total concentration of polycyclic aromatic hydrocarbons (PAHs) [23.0 mg/kg; dry weight (d.w.)], and the extractable concentrations of Mo (3.9 mg/kg; d.w.) and Se (0.2 mg/kg; d.w.) in the fly ash, which exceed the Finnish limit values for materials used in covered earth construction (20 mg/kg; d.w. for PAHs, 0.5 mg/kg; d.w. for Mo, and 0.1 mg/kg; d.w. for Se), the utilization of this ash residue fraction in such structures requires an environmental permit. Because of the high extractability of certain heavy metals in fly ash by using an artificial gastric fluid, e.g., Ba (446 mg/kg; d.w.), V (65.6 mg/kg; d.w.), Zn (100 mg/kg; d.w.), Cu (38.3 mg/kg; d.w.), and As (36.7 mg/kg; d.w.), the careful handling of this ash residue is recommended to prevent the ingestion and penetration of ash particles across the human gastrointestinal tract.