Mn-Cr mixed oxide adsorbents with high SO2 resistance for elemental mercury removal from coal-fired flue gas

Abstract

A series of Mn-Cr mixed oxide adsorbents were obtained using co-precipitation method. Then, an investigation was conducted into the effects of Mn/Cr molar ratio, reaction temperature, calcination temperature, initial mercury concentration and flue gas constituents on the outcome of elemental mercury removal. Specifically, the MnCr1:3 with an optimal Mn/Cr molar ratio performed best in the efficiency of mercury removal (>95.4%) in a wide temperature range (100–250 °C). The physicochemical properties and the mechanism were characterized by N2 adsorption–desorption, SEM, XRD, NH3-TPD, H2-TPR, XPS and Hg-TPD in combination. According to the research results, mercury removal was facilitated by MnCr1:3 due to its larger surface area, smaller crystallite sizes, higher acidity and redox properties. Sulfur dioxide promoted elemental mercury capture for MnCr1:3 when different concentrations of SO2 existed in the flue gas. The relative activity over MnCr1:3 approached 100% after five regeneration cycles. Due to the combined effect of manganese and chromium, there was an increase in the concentration of high valence metal elements and surface adsorbed oxygen. MnCr1:3 had the highest content of Cr6+ and adsorbed oxygen species, for which they played a vital role in the outcome of mercury removal.