Oxidation and capture of elemental mercury over SiO 2–TiO 2–V 2O 5 catalysts in simulated low-rank coal combustion flue gas

Abstract

High surface area SiO 2–TiO 2–V 2O 5 (STV) catalysts of various titania loadings were synthesized by a sol–gel method. The STV catalysts were tested for oxidation of elemental mercury (Hg 0) 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, Hg 0 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, SO 2 and H 2O) with O 2 balanced in N 2. Hg 0 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 Hg 0. Fresh STV catalysts had some capability for adsorbing oxidized mercury (Hg 2+) at 350 °C, and no obvious effect of the adsorbed Hg 2+ on subsequent Hg 0 oxidation was observed. The presence of HCl with O 2 had excellent oxidation and capture efficiency; however, without O 2 it remarkably inhibited Hg 0 adsorption on the STV catalysts. NO and SO 2 promoted Hg 0 oxidation and capture in the presence of O 2, but their promotional effects were insignificant in the absence of O 2. Water vapor showed prohibitive effects on Hg 0 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 Hg 0 removal at typical SCR operating temperatures. The identification of the reaction mechanism provides critical information for developing effective SCR catalysts for Hg 0 oxidation in coal combustion flue gas.