Experimental And Mechanism Study On Mercury Catalytic Oxidation In Coal-fired Flue Gas

Mercury (Hg) emissions from coal-fired power plants are considered to be the largest anthropogenic source of Hg emissions to the atmosphere and have received increased attention. Among the technologies of mercury reduction in coal-fired power plants, the combination of catalytic oxidation from Hg0 to Hg2+ followed by WFGD is a promising and economical strategy to remove Hg0. Relative to Hg0, the Hg2+ compounds in coal flue gases are less volatile and weakly to strongly soluble in water and can, therefore, be captured and removed in conventional air pollutant control devices (APCDs). Thus, if mercury control targets are to be met, methods oxidizing Hg0 to Hg2+ in the flue gas from coal-fired power plants must be developed. With the support of the National High-tech Research and Debelopment Program (863) and the Specialized Research Fund for the Doctoral Program of Higher Education of China, the specific goal was to investigate the potential catalytic oxidation of Hg0 in coal-fired flue gas by different kinds of catalysts.Selective catalytic reduction (SCR) has been a well-developed, commonly used in large scale and commercialized technology for controlling NOx emissions from coal-fired power plants. The vanadia-based (V2O5/TiO2) SCR catalyst was synthesized by an impregnation method. The mercury speciation transformation across the SCR catalyst was evaluated using a bench-scale SCR reactor system. Results showed that the active component V2O5 in the SCR catalyst promoted the mercury oxidation by impacting the pool of vanadium active sites, which are critical for mercury oxidation. The activities of SCR catalyst for mercury oxidation were higher at higher temperature. HCl was important for the mercury speciation transformation by providing the active Cl, which was responsible for the mercury oxidation. NH3 inhibited the mercury oxidation due to the competition for the active sites on the catalyst surface. Larger space velocity was negative for the mercury oxidation.The mercury speciation tramsformation among elemental mercury and simulated flue gas across SCR system should be regarded as the heterogeneous oxidation. The reaction mechanism was studied by bench-scale experiments and various surface analytic technologies. It was observed that Hg is weakly adsorbed (Hg…O-V) onto the catalyst surface in N2 environment, which was confired by XPS analysis. The ability of Hg adsorption increased with VOx loading in the vanadia based catalyst. O2 prompted the transformation of H-O-V species to O=V species, which is responsible for the adsorption of Hg. However, NH3 inhibited the Hg adsorption due to the conpetive adsorption on the vanadium active sites. The monomeric vanadyl sites were found to be active for Hg adsorption.Experimental results showed that the Hg removal behavior is changed by passing HCl through the SCR catalyst first, and then passing Hg vapor without HCl through the catalyst. Simutaneously, mercury oxidation was observed when pro-exposure of the SCR catalyst to HCl, followed by passing through Hg0/N2 in the absence of gas-phase HCl. At testing conditions, Hg0 was found to desorb from the catalyst surface by adding HCl to the gas steam, which implies that HCl adsorption onto the SCR catalyst is strong relative to the mercury. Surface analysis verified the absorption of HCl onto the SCR catalysts forming vanadium-chlorine intermedia, in which the chlorine was reactive. Furthermore, the detailed Langmuir-Hinshelwood mechanism was proposed to explain the mercury oxidation on the SCR catalyst, where reactive Cl generated from adsorbed HCl reacts with adjacent Hg0.Based on the experimental results, a simplified Langmuir-Hinshelwood model of mercury speciation transrformation over the SCR catalyst was developed. The experimental data were fit by the model and the kinetic parameters were determined by the least-squares method. The effects of HCl concentration, NH3/NOx and adsorption equilibrium constants on mercury oxidation were evaluated. Results showed that the balance adsorption constant of HCl and Hg were much lower than that of NH3. Results of reaction analysis showed that the SCR catalysts can be envisioned as having two distinct zones. In the zone, which is near the entrance to the SCR, NH3 is the predominant adsorbed species and NOx reduction is dominating. When the NH3 is exhausted, HCl adsorption becomes the dominant, and mercury oxidation takes place. Larger NH3/NO means longer inhibition time. Promotion effects of KHCl, KHg on reaction rate were found. However, there is a negative correction between KNH3 and reaction rate.It was observed that HCl is the most critical flue gas component that causes conversion of Hg0 to Hg2+ under SCR reaction conditions. The activity of Hg0 oxidation is low when there is no HCl in flue gas or the HCl concentration is low. It suggests that the Hg0 oxidation activity of the SCR system is affected by the chlorine content in the coal. It is imperative to develop new Hg0 oxidation catalyst which performance is not sensitive to the HCl.Manganese oxide catalysts supported on alumina (MnOx/Al2O3) were synthesized by an impregnation method for Hg0 oxidation in simulated coal-fired flue gas. The catalysts were characterized by BET, XRD and SEM. The catalysts has large surface ares and highly dispersed manganese oxides can be obtained. The highly dispersed manganese oxides uniformly distributed on support surface mainly as Mn4+, which was confirmed by TPR and XPS analysis. MnOx/Al2O3 were efftive for adsorption of Hg0, and the rate of adsorption reached 2.15μg/g·h at 150℃. XPS analyses on the surface of catalysts after the removal of Hg0 suggest that adsorbed Hg0 oxidatively transformaed to HgO by surface lattice oxygen, consistent with the Mars-Maessen mechanism.Test results showed that the Hg0 oxidation activity of MnOx/Al2O3 was high in low chlorine contained flue gas, and the temperature window was relative wide. Both HCl, SO2 and NO enhanced Hg oxidation in experimental flue gas, while H2O inhibited Hg oxidation due to the competitive adsorption for active sites. Besides mercury chloride, other oxidized mercury species may be formed by the MnOx/Al2O3. NO and SO2 can be transformed to nitrite species and sulfate species in the presence of O2 on the MnOx/Al2O3. Mercury oxidation over the MnOx/Al2O3 could occur between adsorbed Hg0 and reactive species adsorbed at an adjacent site via a Langmuir-Hinshelwood mechanism. The consumed lattice oxygen was compensated by the O2 in flue gas. The catalytic performances of MnOx/Al2O3 on the oxidation of Hg0 appeared to be promising in the control of mercury emissions from coal-fired boilers, especially when firing the low rank coals.