Evaluation of the Variables that Influence Mercury Capture in Solid Sorbents

Abstract

Fly ashes are by-products generated during coal combustion in power plants. These byproducts are able to capture volatile species, such as mercury that are evaporated from coal in different proportions. The precise mechanism by which a trace element is retained in the fly ashes is not fully understood. The quantity of mercury retained may vary from fly ash to fly ash and depends on the process conditions. It has been observed that, although some fly ash inorganic components exhibit a low capacity for mercury retention, the unburned material in the fly ash may benefit its adsorption. There are a number of variables that may influence mercury retention by fly ash components and the exact nature of Hg-fly ash interactions needs to be investigated more thoroughly. The aim of the present work was to evaluate the variables that have an effect on the retention of Hg and HgCl2 in fly ashes originated by burning coals of different rank and nature. The mercury retention capacity of these fly ashes was compared with retention in commercial activated carbons in coal combustion and inert atmospheres. The study was carried out in a laboratory scale reactor using the sorbent as a fixed bed. The amount of mercury retained was determined by analyzing the sorbents post-retention by means of cold vapor atomic absorption (CV-AA). The results obtained indicate that the quantity of mercury captured depends on the characteristics of the fly ashes and on the mode of occurrence of mercury in gas phase. The retention of mercury in the fly ashes studied is greatly influenced by the gas composition. It was inferred that, unlike the activated carbons, the fly ashes show different retention capacities and efficiencies for mercury in combustion and inert atmospheres. Moreover, mercury retention may be modified by variations in the unburned coal particle content