Flue Gas Hg0 Removal by FeCl3-Impregnated LTA and MFI Zeolites: Influences of Topology and Cation Sites

Abstract

Three different zeolites (5A, ZSM-5, and silicalite-I) chosen from two topological configurations (LTA/MFI) were modified with FeCl3 via the chemical impregnation method to remove elemental mercury (Hg0). Influence of different parameters, including zeolite topological configurations, reaction temperature, and flue gas compositions, on mercury removal was explored on a lab-scale fixed bed reactor. The results showed that the zeolite configuration contributed to a drastic variation in Hg0 removal ability among FeCl3-modified zeolites. MFI configuration samples (Fe/ZSM-5, Fe/silicalite-I) both exhibited better Hg0 removal abilities than the LTA configuration Fe/5A sample. Meanwhile, when exploring the effects of O2 and SO2, it was found that the mercury removal efficiency of Fe/ZSM-5 and Fe/silicalite-I was strengthened with the addition of O2. Although SO2 would slightly suppress the mercury removal, it was found that SO2 showed a promoting effect on the Hg0 removal when SO2 was added together with O2. The results indicated that all three zeolites had good sulfur resistance. The characterization results (BET and XRD) demonstrated that different topological configurations and crystal structures influenced the reaction of FeCl3 impregnation and eventually resulted in different Hg0 removal performance. XPS results indicated that oxygen species played an important role in chemisorption and oxidation of Hg0. Combined with Hg-TPD data, it was found that Hg0 adsorbed on MFI configuration samples existed mainly in the form of HgCl2. By contrast, two HgO desorption peaks were found on the LTA configuration 5A sample, which were presumably formed by the desorption of HgO adsorbed in pore channels and α-cages of 5A. Based on the results obtained, two mercury removal mechanisms of FeCl3-impregnated zeolites were discussed in this study. The results would be instructional to the development and application of zeolite-based sorbents for Hg0 removal.