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

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2011-05, Vol.169 (1-3), p.186-193
• Main Authors: Li, Hailong, Li, Ying, Wu, Chang-Yu, Zhang, Junying
• Format: Article
• Language: English
• Subjects: Adsorption; Applied sciences; Atmospheric pollution; Catalysis; catalysts; Catalytic reactions; Chemical engineering; Chemistry; combustion; Exact sciences and technology; General and physical chemistry; General processes of purification and dust removal; lignite; mercury; Pollution; Prevention and purification methods; reaction mechanisms; Reactors; surface area; temperature; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; water vapor
• ISSN: 1385-8947; 1873-3212
• DOI: 10.1016/j.cej.2011.03.003

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 18wt%. 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.