Geochemical mechanism of lead vapors over fly ash cenospheres in simulated flue gas

Abstract

Coal combustion is one of the major pathways by which lead (Pb) enters the ecological environment. An effective method to control Pb emissions in a furnace is to transform the Pb from vapors to particles using Si-Al-based sorbents. However, the high price, poor thermal stability, and difficult molding of sorbents limits their application. Fly ash cenospheres (FACs) prepared from the waste of coal-fired power plants, which are mainly composed of SiO2 and Al2O3, are ideal materials for Pb sorbents because of their high strength, light weight and low price. In this study, the adsorption and reaction mechanisms of FACs and several Si-Al-based sorbents for Pb vapors were investigated in a two-stage fixed-bed reaction system, and the thermal stability and regeneration characteristics of FACs were tested. The results indicate that the adsorption capacity for Pb vapors is FAC > kaolin > SiO2 > γ-Al2O3. The acidic sites, such as Si-O and Al-O bonds in FACs, were the main reaction sites for Pb adsorption and captured Pb in the form of stable Pb compounds (Pb2+). As the temperature increased from 700 to 1200 °C, the Pb adsorbed by FACs increased, and 55.21% of the Pb in the simulated flue gas was removed at 1200 °C. Kaolin is an excellent sorbent for Pb vapors. The adsorption capacity of kaolin for Pb first increases and then decreases with increasing temperature, and the maximum removal rate occurs at 1000 °C, reaching 33.46%. In addition, FACs are thermally stable and reusable. Therefore, the high-temperature application of FACs to remove Pb vapors has good prospects.