Investigation on the Emission of Heavy Metals in Fluidized-Bed Incineration during the Agglomeration/Defluidization Process

Abstract

This study investigated the effects of waste components and operating temperatures on heavy metal emission in the agglomeration/defluidization process in a fluidized bed. Artificial waste was used to simulate municipal waste and to induce the formation of agglomerates. Those contain different concentrations of alkali metals, alkaline earth metals, a mixture of metals (Pb, Cr, and Cd), and sawdust. The experimental results indicated that Na increased the risk of agglomeration/defluidization but decreased the emission concentrations of heavy metals. Additionally, the emissions of heavy metals were lower with Ca and Mg addition. Ca and Mg inhibited the generation of agglomerates, thereby maintaining the quality of fluidization and the uniform mixing of the silica sand. Scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) analysis showed that Na, Si, Mg, Pb, and Cr were present on particle surfaces. In addition, some compounds with high melting points, such as CaO and MgO, were produced when Ca and Mg were added to the system. These high-melting-point species may inhibit the generation of agglomerates while maintaining the quality of fluidization and the uniform mixing of the silica sand. According to these results, two important mechanisms may decrease the emission of heavy metals: adsorption by silica sand and covering or adherence by Na-containing glasses that form low-melting eutectics. For the three heavy metals studied, their metallic species may react with Na to form eutectic compositions, be covered or adhered to by eutectics, and stay in the sand bed, thereby decreasing the emission concentrations in flue gas. Overall, the emission of heavy metals decreased as Na, Ca, and Mg were added at various temperatures.