Dechlorination and destruction of PCDDs/PCDFs in fly ashes from municipal solid waste incinerators by low temperature thermal treatment

Dechlorination and destruction characteristics of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs/PCDFs) in fly ashes from commercial-scale municipal solid waste incinerators by low temperature thermal treatment using a laboratory-scale heating system were investigated. Experiments were carried out in reducing atmosphere at temperatures of 300 °C, 450 °C and 600 °C respectively, for the treatment time of 1 h and 3 h. Concentrations of PCDDs/PCDFs in raw fly ashes ranged from 35.5 to 107.3 μg kg^-1 (1.5-3.4 μg TEQ kg^-1) and treated fly ashes ranged from 0.34 to 45.3 μg kg^-1 (0.012-1.63 μg TEQ kg^-1). Concentrations of PCDDs/PCDFs in fly ashes treated at the different temperatures and times were observed to decrease with increase of treatment temperature and time by dechlorination or destruction. The distribution of octa- and hepta-chlorinated congeners were decreased and tetra-, penta- and hexa-chlorinated congeners were increased at 300 °C and 450 °C, but the distribution of octa- and hepta-chlorinated congeners were increased and tetra-, penta- and hexa-chlorinated congeners were again decreased at 600 °C. Total destruction efficiencies of PCDDs/PCDFs in fly ashes showed above 95% at the treatment temperature of 450 °C for 3 h. However, removal efficiency of each congener in fly ashes varied, especially, 2,3,7,8-TeCDD and 1,2,3,7,8-PeCDD in fly ash A increased. And the dechlorination and destruction characteristics of PCDDs/PCDFs in fly ash A and B was different due to difference in contents of Ca-compounds and metal oxides such as CuO and PbO in fly ashes.