Oxidation and capture of elemental mercury over SiO2–TiO2–V2O5 catalysts in simulated low-rank coal combustion flue gas

High surface area SiO2–TiO2–V2O5 (STV) catalysts of various titania loadings were synthesized by a sol–gel method. The STV catalysts were tested for oxidation of elemental mercury (Hg0) and its capture in simulated coal combustion flue gas representing those from combustion of low-rank coals (sub-bituminous and lignite). Experiments were conducted in a fixed-bed reactor at temperatures ranging from 26 to 400 °C. In simulated flue gas, Hg0 oxidation efficiency over the STV catalysts was found to decrease dramatically from 135 to 300 °C. At typical selective catalytic reduction (SCR) operating temperatures, the catalyst's oxidation activity increased as titania loading of the STV catalysts increased up to 18 wt%. The reaction mechanisms over the STV catalysts at SCR operating temperatures were investigated using individual flue gas components (HCl, NO, SO2 and H2O) with O2 balanced in N2. Hg0 oxidation over STV catalysts follows the Eley–Rideal mechanism where active surface species generated from adsorbed flue gas components react with gas-phase or weakly adsorbed Hg0. Fresh STV catalysts had some capability for adsorbing oxidized mercury (Hg2+) at 350 °C, and no obvious effect of the adsorbed Hg2+ on subsequent Hg0 oxidation was observed. The presence of HCl with O2 had excellent oxidation and capture efficiency; however, without O2 it remarkably inhibited Hg0 adsorption on the STV catalysts. NO and SO2 promoted Hg0 oxidation and capture in the presence of O2, but their promotional effects were insignificant in the absence of O2. Water vapor showed prohibitive effects on Hg0 oxidation due to its competition with reactive species such as HCl and NO for active adsorption sites. This study demonstrates the feasibility of using STV catalysts for Hg0 removal at typical SCR operating temperatures. The identification of the reaction mechanism provides critical information for developing effective SCR catalysts for Hg0 oxidation in coal combustion flue gas.