Chemical, Physical and Leaching Studies of Bottom Ash from a Medium-sized (32 MW) Municipal District Heating Plant for Assessing its Suitability for an Earth Construction Agent and for a Fertilizer used in Agriculture and in Forestry

摘要

In Finland, the new limit values for the maximal allowable heavy metal concentrations in agricultural and forestry fertilizers came into force in March 2007 and for materials used as an earth construction agent in July 2006. From the utilization point of view, it was notable that the total heavy metal concentrations in the bottom ash originating from the 32 MW municipal district heating plant of Kemin Energia Oy at Kemi, Northern Finland, did not exceed the maximal allowable heavy metal concentrations for ash used as a fertilizer in agriculture and in forestry. Furthermore, the total heavy metal as well as the leachable heavy metal and the leachable DOC, fluoride, sulphate and chloride concentrations in the extractant were lower than the limit values for materials used as an earth construction agent. According to a five-stage sequential leaching study, in which the distribution of heavy metals in the bottom ash between the water soluble (H2O), exchangeable (CH3COOH), easily reduced (NH2OH-HCl), oxidizable (H2O2 + CH3COONH4), and residual fractions (HF + HNO3 + HCl) were evaluated, the highest concentrations of most of the heavy metals occurred in the residual fraction, which means that heavy metals in ash are tightly bound to the matrix. The results indicated that, the bottom ash is a potential material to be used especially as an earth construction agent.