Catalytic Oxidation Of Elemental Mercury Over Novel SCR Catalysts

Energy and environment are the most important factors that could affect the development of human beings. Consumption of vast coals is the main reason that is responsible for the serious air pollution problems in China. Coal combustion not only emits convertional pollutants such as dust, SO2 and NOX but also could cause mercury pollution. In recent years, concerns about mercury emission and its control have risen greatly because of the extreme toxicity, persistence, and bioaccumulation of methyl mercury transformed from emitted mercury. Recently, more and more power plants in China installed or are installing wet flur desulfurization system (WFGD) and selective catalytic reduction (SCR) to control the emissions of SO2 and NOX. Accordingly, it is promising to use conventional air pollution control devices such as WFGD and SCR to control mercury emission from coal-fired power plants. The key point of this mercury control technology is how to effectively oxidize elemental mercury (Hg0) to oxidized mercury (Hg2+) before FGD system. SCR have been demonstrated to be able to promote Hg0 oxidation. Although the commercial vanadium based SCR catalysts have been successfully used in industry for decades, there still be some drawbacks where improvement can be made. New catalysts which can overcome these shortcomings would probably be adopted as the next generation SCR catalysts. Hg0 oxidation studies over these catalysts would be of great scientific and practical values.To increase the surface area of conventional vanadium base SCR catalyst, high surface area SiO2-TiO2-V2O5 (STV) catalysts of various titania loadings were synthesized by a sol-gel method. Mercury oxidation and capture performance of the STV catalysts at typical SCR operating temperature were tested in a fixed-bed reactor. More than 60% Hg0 oxidation was achieved at 300℃under adverse conditions i.e. high gas hourly space velocity and simulated coal combustion flue gas representing those from combustion of low-rank coals (sub-bituminous and lignite). At typical 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 (HC1, 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 Hg2+ at 350℃, 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.Enve though vanadium base catalysts are effective for NOX removal and Hg0 oxidation at certain conditions, they still have many drawbacks, especially for Hg0 oxidation, such as extreme toxic, narrow temperature window, denpendance on HC1 and low water resistance. To overcome these drawbacks of vanadium base catalysts, CeO2-TiO2 (CeTi) catalysts synthesized by an ultrasonic-assisted impregnation method were employed to oxidize Hg0 in simulated low rank coal combustion flue gas. The CeTi catalysts with a weight ratio of 1-2 exhibited high Hg0 oxidation activity at temperatures ranging from 150 to 250℃. The high surface concentrations of cerium and oxygen were responsible for their superior performance. HC1 was the most effective flue gas component responsible for Hg0 oxidation. However, in the presence of O2, the combination of 1200 ppm SO2 and 300 ppm NO without HCl also resulted in Hg0 oxidation efficiency of 99.9%. This superior oxidation capability is advantageous to Hg0 oxidation in low rank coal combustion flue gas containing low concentration of HC1.In the presence of O2, promotional effect of HC1, NO and SO2 on Hg0 oxidation was observed over CeTi catalysts. Without O2, HCl and NO still promoted Hg0 oxidation due to the large oxygen storage capacity of CeTi catalysts, while SO2 inhibited Hg0 adsorption and subsequent oxidation. Water vapor also inhibited Hg0 oxidation. Experimental and modeling studies demonstrated that Hg0 oxidation over CeTi catalysts follows the Langmuir-Hinshelwood mechanism where HC1, NO and SO2 first react with active sites on the catalysts with the aid of oxygen to form reactive intermediates, which then react with adjacently adsorbed Hg0. At 200℃, the Hg0 oxidation reaction rate constants over CeTi catalysts were around 80-130 s-1. The adsorption equilibrium constant of HCl was about 2-3×103 m3/mol. Based on the developed model and these kinetic constants, Hg0 oxidation efficiency can be roughly estimated using given conditions such as HCl concentration. This is useful for utilization of CeTi catalyst in power plants.Finally, MnOx was introduced into the CeTi catalyst to improve its Hg0 oxidation activity. The combination of MnOx and CeO2 resulted in significant synergy for Hg0 oxidation. The MnOX-CeO2/TiO2(Mn-Ce/Ti) catalyst was highly active for Hg0 oxidation at low temperatures (150-250℃) under both simulated flue gas and SCR flue gas. The dominance of Mn4+ and the presence of Ce+ on the Mn-Ce/Ti catalyst were responsible for its excellent catalytic performance. Hg0 oxidation on the Mn-Ce/Ti catalyst likely followed the Langmuir-Hinshelwood mechanism. HCl was the most effective flue gas components responsible for Hg0 oxidation. In the presence of HC1, most of the observed Hg0 oxidation was attributed to reactions between Hg0 and surface chlorine species. SO2 inhibited Hg0 oxidation either in the absence of O2 or in the presence of O2. The competitive adsorption between SO2 and Hg0 was believed to be at least partly responsible for the deactivation of Hg0 oxidation in the presence of SO2. NO covered the active sites and consumed surface oxygen which is active for Hg0 oxidation; hence, limited Hg0 oxidation. Water vapor showed prohibitive effects on Hg0 oxidation due to its competition with HCl and Hg0 for active adsorption sites. NH3 consumed the surface oxygen and limited the Hg0 adsorption, hence inhibiting Hg0 oxidation over Mn-Ce/Ti catalysts. However, once NH3 was cut off, the inhibited mercury oxidation activity could be completely recovered in the presence of O2.This study revealed the possibility of using potential SCR catalysts for effective Hg0 oxidation even in low-rank coal combustion flue gas and/or at low temperatures. Such knowledge is of fundamental importance in developing effective and economical mercury and NOX control technologies for coal-fired power plants.