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 sot-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 degrees C. In simulated flue gas, Hg-0 oxidation efficiency over the STV catalysts was found to decrease dramatically from 135 to 300 degrees C. At typical selective catalytic reduction (SCR) operating temperatures, the catalyst's oxidation activity increased as titania loading of the SW 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 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 (Hg2+) at 350 degrees C, and no obvious effect of the adsorbed Hg2+ 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 SO2 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. (C) 2011 Elsevier B.V. All rights reserved.