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

This paper evaluates the influence of the main constituents of flue gases from coal combustion (CO2,O-2, N-2 and water vapor), in air and oxy-fuel combustion conditions on the re-emission of Hg-0 in wet scrubbers. It was observed that the concentration of water vapor does not affect the re-emission of mercury, whereas O-2 and CO2 have a notable influence. High concentrations of O-2 in the flue gas prevent the re-emission of Hg-0 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 Hg-0 that is re-emitted. As a consequence, the high content of CO2 in oxy-fuel combustion may decrease the re-emission of Hg-0 due to the solubility of CO2 in the suspension and the decrease in the pH. It was also found that O-2 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 Hg-0 re-emissions from power plants operating under oxy-fuel combustion conditions. (C) 2014 Elsevier B.V. All rights reserved.