Mercury oxidation in catalysts used for selective reduction of NOx (SCR) in oxy-fuel combustion

Abstract

Emissions of mercury (Hg) and their control are a well-known problem for conventional coal combustion power plants but they still represent a challenge for relatively new technologies such as the oxy-combustion. In oxy-fuel combustion systems it is important that Hg be in its oxidized form (Hg2+) because in the form of elemental mercury (Hg0), it can damage the CO2 compression units. In conventional air combustion some catalysts used in selective catalytic reduction (SCR) of NOx could also co-benefit Hg oxidation. This study evaluates the oxidation of Hg in the presence of several SCR catalysts under an oxy-combustion atmosphere focusing on the main differences with conventional air coal combustion. In the experimental conditions of this study, a higher mercury oxidation was observed in the CO2-enriched atmosphere due to the fact that in this atmosphere the conversion of NOx was lower, which resulted in a higher concentration of NO and NO2 free to homogeneously oxidize mercury. In oxy-combustion conditions the high amount of CO2 and H2O present may block the active sites for mercury adsorption. Moreover, the differences between the active sites of catalysts based on V/W/TiO2 and Fe/Zeolite with/without Mn as doping agent were eclipsed by the effect of the flue gas composition.