Adsorption and oxidation of mercury in tail-end SCR-DeNOx plants—Bench scale investigations and speciation experiments

Abstract

The results of a bench scale investigation on mercury adsorption and oxidation under the conditions of tail-end DeNOx plants are reported. It was shown that HCl has a strong influence on mercury adsorption as well as mercury oxidation.Adsorption of elemental mercury and HgCl2 on a DeNOx catalyst with 2.5wt.% V2O5 is appreciably in flue gases that contain HCl in concentrations below 0.5mg/m3. Mercury adsorption increases linearly with the mercury concentration in the flue gas. The amount of HgCl2 adsorbed on the catalysts is higher than of elemental mercury, under otherwise comparable conditions. Mercury adsorption depends on the V2O5 content of the DeNOx catalyst. The nature of the adsorbed mercury species was investigated by sequential chemical extraction and X-ray absorption fine structure spectroscopy. According to these methods mercury does form a Hg–O bond on the surface of the catalysts. In terms of regeneration of mercury loaded catalysts it is of interest, that more than two-third of the adsorbed mercury can be removed by washing the DeNOx catalyst at room temperature with a dilute solution containing acetic and hydrochloric acid.In the presence of higher concentrations of HCl in the flue gas elemental mercury is no longer adsorbed but oxidized by DeNOx catalysts. The rate of oxidation decreases with increasing temperature. NO has compared to HCl only a slightly accelerating influence, whereas NH3 shows a small detrimental effect. The simultaneous presence of NO and NH3 inhibits the HCl assisted oxidation of elemental mercury strongly.