Predicting the Multipollutant Performance of Utility SCR Systems

摘要

Our “SCR Catalyst Model” quantitatively describes simultaneous NO reduction and oxidation of SO2 and Hg0 along SCR catalysts in coal-derived flue gas streams.It relates catalyst material composition and bimodal pore size characteristics in a direct,quantitative way to the reactivities for NO reduction,Hg0 oxidation,and SO3 production in commercial,full-scale SCR reactors.SCR monoliths sustain two chemically distinct regions.In the inlet region,strong NH3 adsorption minimizes the coverage of chlorinated and sulfated surface sites,so NO reduction inhibits Hg0 and SO2 oxidation.But once the NH3 has been consumed,the chlorinated surface coverage surges by orders of magnitude,and the Hg0 oxidation rate rapidly accelerates,even while the HCl concentration in the gas phase is uniform.This two-stage sequence is favorable for SO3 control,because NH3 inhibition makes the SCR perform like a much shorter unit.Conversely,NH3 inhibition is a serious impediment to Hg0 oxidation.Ammonia inhibition also eliminates the benefit of the rapid film mass transfer at the SCR inlet from promoting Hg0 oxidation.In many cases,the Hg0 oxidation rate becomes limited by film transport soon after the Hg0 begins to oxidize,so that none of the catalyst internal surface area is utilized.The predictions were validated with pilot-scale data to demonstrate the crucial impact of NH3 inhibition on SCR performance,and with full-scale data for catalysts from a single vendor,to show quantitative consistency across broad ranges of coal-Cl,GHSV,NH3/NO,andcatalyst specifications.