PAHs and potentially toxic elements in the fly ash and bed ash of biomass fired power plants

• Ash characterized from coconut, chicken and wood waste based power plants
• Major chemical phase of the ash was SiO2–CaO–K2O–MgO.
• High content (mg/kg) of B (205), Cu (478), Cd (23.2), Ni (186), and Zn (400)
• Except B, all elements were associated with non-available fractions.
• PAHs are more in fly ash (3.6–193 mg/kg) than bottom ash (0.19–2.6 mg/kg).

PAHs and heavy metal chemical fractions were determined in biomass ashes from four power plants that fired coconut, chicken, and wood wastes. All these ashes were alkaline (pH 8.7–12.7). High unburned carbon in the fly ashes (8.3 to 16.9%) indicates incomplete combustion. The major chemical phases were SiO2 – CaO – K2O – MgO followed by Fe2O3 > Al2O3 > SO3 > MnO, with an exception of higher P2O5 (5 – 8.8%) in chicken litter ash. SEM showed that the FAs have more angular particles, whereas BAs have fused and glassy particles. The contents of B (15– 205 mg/kg), Cu (236 – 481 mg/kg), Cd (13 – 23.2 mg/kg), Ni (50 – 186 mg/kg), and Zn (10 – 400 mg/kg) were relatively higher in the ashes. Most of these elements (except B) were associated with metal oxide and silicates. B is the most mobile element and >75 % of B is present in the available forms. The total PAH content was comparatively lower (0.19 – 12.3 mg/kg), except for wood ash (PAH 193 mg/kg). These ashes have significant amount of plant nutrients and most of the potentially toxic elements are in non-available forms which supports their suitability for soil amendment.