The Specific Role of Cadmium and Mercury in Municipal Solid Waste Incineration

Abstract

Requirements with respect to reducing the emission of pollutants, in particular dust constituents such as heavy metals, especially cadmium and mercury are imposed on processes of municipal solid waste incineration. In the process of incineration, cadmium is volatilized as cadmium chloride to a considerable extent. In a series of in situ measurements it has been demonstrated that 99% of this cadmium condenses on dust particles and can be removed together with fly ash. The cadmium concentration in clean flue gas dusts amounts to about 2000 μg g-1. Therefore dust emission values should not be higher than 20-30 mg dust Nm-3. The ultimate disposal of filter ash increasingly poses difficulties on account of the mobility of cadmium. More than 80% of all mercury is released into the gas phase. The optimization of the various flue gas purification techniques depends on information about the mercury species occurring at the boiler outlet. In order to clarify the speciation the condensation and solubility behaviour of samples is referred to. In the crude gas mercury (II)-chloride and, to a lesser extent, mercury (I)-chloride are present. The occurrence of metallic mercury can be excluded. The mechanism underlying mercury removal by the dry sorption and wet scrubbing techniques is clarified. Recommendations are given for improving the removal efficiency.