A comprehensive evaluation of the influence of air combustion and oxy-fuel combustion flue gas constituents on Hg0 re-emission in WFGD systems

摘要

This paper evaluates the influence of the main constituents of flue gases from coal combustion (CO2, O2, N2 and water vapor), in air and oxy-fuel combustion conditions on the re-emission of Hg0 in wet scrubbers. It was observed that the concentration of water vapor does not affect the re-emission of mercury, whereas O2 and CO2 have a notable influence. High concentrations of O2 in the flue gas prevent the re-emission of Hg0 due to the reaction of oxygen with the metals present in low oxidation states. High concentrations of CO2, which cause a decrease in the pH and the redox potential of gypsum slurries, reduce the amount of Hg0 that is re-emitted. As a consequence, the high content of CO2 in oxy-fuel combustion may decrease the re-emission of Hg0 due to the solubility of CO2 in the suspension and the decrease in the pH. It was also found that O2 affects the stabilization of Hg2+ species in gypsum slurries. The results of this study confirm that the amount of metals present in limestone as well as the redox potential and pH of the slurries in wet desulphurization plants need to be strictly controlled to reduce Hg0 re-emissions from power plants operating under oxy-fuel combustion conditions.