1 Department of Chemical and Biochemical Engineering, Technical University of Denmark2 CHEC Research Centre, Department of Chemical and Biochemical Engineering, Technical University of Denmark3 Haldor Topsoe AS4 Haldor Topsoe AS
The vanadium-based SCR catalyst used for NOx-control promotes the oxidation of elemental mercury Hg0 to Hg2+ in flue gases from coal-fired power plants. Hg2+ is water soluble and can effectively be captured in a wet scrubber. This means that the combination of an SCR with a wet FGD can offer an effective control option for mercury. Laboratory experiments have been carried out to quantify the Hg0 oxidation that can be achieved over commercial SCR catalysts for different gas compositions, operating conditions and catalyst types. The following three net reactions have been identified as relevant for the mercury chemistry over the SCR: R1. 2 HCl + Hg0 + 1/2 O2 ↔ HgCl2 + H2O R2. 2 NH3 + 3 HgCl2 ↔ N2 + 3 Hg0 + 6 HCl R3. 2 NO + 2 NH3 + 1/2 O2 ↔ 2 N2 + 3 H2O where reaction R1 is the oxidation of Hg0 by HCl, reaction R2 is the reduction of HgCl2 by NH3 and reaction R3 is the DeNOx reaction. The importance of each reaction on the achievable Hg0 oxidation depends on the SCR operating temperature. At T>325oC, the reduction of HgCl2 will take place when NH3 is present. The overall Hg0 oxidation will then reflect the relative rate of the Hg0 oxidation via reaction R1 and the HgCl2 reduction via reaction R2. For T=250-375oC, the DeNOx reaction will inhibit the kinetics of reaction R1 by consuming active Lewis sites that must be oxidized to regain activity for Hg0 oxidation. The experimental data obtained in this study indicate that vanadia Lewis sites on SCR catalysts are active in the catalytic Hg0 oxidation - possibly as Hg0 adsorption sites. A kinetic model for the steady-state Hg0 oxidation over monolithic SCR reactors has been developed taking both external mass transfer, pore diffusion and reaction on the catalyst wall into account. The mercury chemistry that has been identified and quantified in the experimental investigations is incorporated in the model. The resulting model successfully reproduces the variations in Hg0 oxidation over the SCR that have been experimentally observed for different gas compositions and testing conditions. This verifies that the relevant mercury chemistry has been taken into account in order to describe the catalytic Hg0 oxidation in a simulated flue gas. The validity of the model for describing the mercury chemistry over SCR catalysts in real flue gases is yet to be explored. Model predictions suggest that the kinetics of the Hg0 oxidation over high dust SCR reactors is greatly limited by external mass transfer in the entire SCR operating temperature window if HCl≥13 ppm. For lower HCl concentrations, the surface reactivity of the SCR catalyst towards Hg0 oxidation can become limiting at the higher operating temperatures T>350oC, because the rate of HgCl2 reduction will be considerable. A higher V2O5 load on the SCR catalyst will dampen this effect.