Effect of SCR operation variables on mercury speciation

► We investigated the SCR effects in terms of Hg oxidation. ► We investigated the impacts of acid gases on Hg oxidation in a SCR system. ► We identified several Hg species in by-products by means of a novel methodology. ► Increasing information on Hg retention in by-products was achieved. It is important to understand how existing air pollution control devices can remove mercury as co-benefit. This paper presents the results of a test programme at a 20kWth PF pilot scale unit and a lab scale micro-reactor to investigate the catalyst performance on the concentration and speciation of mercury in coal combustion flue gas. The SCR temperature, the ammonia dosing rate and the SO2 concentration were varied; increasing the temperature the mercury oxidation rate decreased, being the optimal temperature for a high mercury oxidation rate between 300 and 350°C. The oxidation of SO2, as undesired side reaction, is strongly dependent on temperature. The optimal temperature range for low SO3 concentrations downstream of the SCR is below 360°C, while the reduction of NOx by NH3 is faster for higher temperatures. A higher ammonia dosing rate leads to a high loading of the catalysts active sites and reduced mercury oxidation rate. Increasing the SO2 concentrations only slightly enhanced the mercury oxidation rate due to the higher acid loading of the catalyst, which promoted the DeNOx-reaction and made more active sites of the catalyst available for the mercury oxidation. Increasing SO2 concentration at the inlet of the SCR leads to higher concentrations of SO3 downstream of the SCR, while the relative oxidation rate of SO2 is lower at higher SO2 concentrations. Ashes were characterized by X-ray Fluorescence and Thermal Decomposition–Atomic Fluorescence Spectrometry to investigate mercury content and occurrence, respectively. Measurable mercury release began at approximately 200°C. It was also found that the ash samples showed the occurrence of only insoluble/partially soluble mercury.