Effect Of Selective Catalytic Reduction System On Fine Particle Emission Characteristics

Based on commercial catalysts (V2O5-WO3/TiO2), the effects of a selective catalytic reduction (SCR) system on the emission characteristics of fine particles were investigated. The size distribution, morphology, element compositions, and chemical compositions of fine particles at the outlet of the SCR system were measured by an electrical low pressure impactor (ELPI), particulate matter (PM10/PM2.5) samplers, field emission scanning electron microscopy/energy dispersive spectrometry (FSEM EDS), X-ray diffraction (XRD), etc. The influences of reaction conditions and flue gas components on fine particle formation were also studied. The results reveal that a large number of fine particles can be generated during the SCR process. These fine particles are mostly in the submicrometer size range and appear to be nonuniform. The main components of these fine particles are ammonium sulfate or ammonium bisulfate. There is a good correlation between the formation of fine particles and the SO2 oxidation rate. Fine particle number concentration at the outlet of the SCR system increases obviously with the reaction temperature and molar ratio of NH3 to NO (NSR, normalized stoichiometric ratio). Moreover, the enhancement of SO2, H2O; and O-2 concentrations in the inlet flue gas would increase the number concentration of the formed fine particles. The effects of the SCR process on the PM2.5 rnission characteristics consist of two aspects. On the one hand, some SO2 can be oxidized to SO3 during the SCR process at the function of the catalyst. Then SO3 would react with NH3 and H2O to form ammonium sulfate and ammonium bisulfate particles, which is a reversible process simultaneous with the deNOx reactions. On the other hand, SO3 can also react with the escaped NH3 and H2O to form ammonium sulfate and ammonium bisulfate particles after the SCR system.