Alkaline fillers for high-temperature reduction of divalent mercury: Influence of flue gas components

Abstract

Ca-based alkaline fillers can improve thermal conditions in the reaction zone and efficiently remove Cl species, thereby preventing the re-oxidation of Hg0. This is crucial for the high-temperature reduction (HTR) of HgCl2. However, flue gas components potentially inhibit HCl adsorption by these fillers, thus adversely affecting the HTR process. Hence, comprehending the impact of flue gas components on HCl removal by alkaline fillers and their subsequent effects on the HTR reaction is essential. This study employs experimental characterization and DFT calculations to elucidate the effects of SO2, CO2, and NO on HTR reaction. The findings reveal that SO2 impedes the adsorption of HCl by alkaline fillers, thereby reducing the HTR efficiency. This inhibition stems from three primary factors: the formation of a sulfate product layer that disrupts the pore structure, the preferential adsorption of SO2 rather than HCl by CaO in the presence of both gases, and the pre-occupation of active sites by SO2, which weakens HCl adsorption at adjacent sites. Similarly, CO2 also hinders the adsorption of HCl by alkaline fillers, albeit to a lesser extent than SO2. This difference arises because CaO preferentially adsorbs HCl over CO2 when both are present. Conversely, NO exerts a minor inhibitory effect on the HTR process. This suppression is not attributable to limiting HCl adsorption by alkaline fillers but rather to the oxidation of Hg0 by NO. This work provides critical insights for optimizing HTR process to enhance their performance under realistic conditions.