The Study For Mechanisms Of Demercuration On Iron Oxide Surface In Flue Gases

Mercury is a trace element in coal, but the large consumption of coal contributes amazing emissions of mercury. As we all know that mercury cannot be decomposed after high temperature combustion of coal and will be conversed into elusive mercury vapor. They will not only be accumulated in plants, animals and humans and poison them, but also pollute the water and soil environment. In order to effectively reduce mercury emissions, we urgently need to understand the adsorption and oxidation mechanism of mercury in flue gas for high efficient demercuration agents.Fe2O3, as a cheap metal-oxide demercuration agent, can both adsorb and oxide mercury. However, its adsorption capacity is weak, and the oxidation mechanism is unclear. In this study, metal-doped Fe2O3gas-solid model is constructed, and the effect of metal doping on the adsorption ability of mercury species is explored by the density functional theory. Then, the catalytic oxidation mechanism of Hg0on Fe2O3in the HCl atmosphere is investigated. Furthermore, FeO model is built to investigate the effect of low valence iron on demercuration. Main conclusions are as follows: 1. Ag, Pd, Mn, Ce, Co and Cu doping have an insignificant effect on the adsorption of Hg0. Hg0still adsorbs on the metal-doped α-Fe2O3(0001) surface in physical adsorption form. Meanwhile, Ag, Pd, Ce doping can all promote the adsorption of HgCl, HgCl2and HgS on the metal-doped a-Fe2O3(0001) surface. Among doped metals, Ce has the most significant promotion for the removal of HgCl and HgCl2. Pd has remarkably positive effect on the removal of HgS, Ce comes the second. Therefore, it can be concluded that Ce is the best choice as doped metal for its obvious promotion on mercury removal and a wide variety of sources.2. Both S and Cl precovered hematite can promote the mercury removal. The surface S on S-precovered hematite will react with free Hg0for HgS molecular to strengthen the adsorption, while the surface Cl on Cl-precovered hematite can promote the electron transfer between surface and Hg0, which will capture Hg more easily.3. The mechanism of the catalytic oxidation of mercury on the Fe2O3surface in HCl atmosphere is studied. It shows that HCl is firstly oxidized to Cl2through Deacon process. Then elemental mercury can be oxidized quickly and converted into oxidized mercury by Cl2in heterogeneous reaction.4. Hg0adsorption on the FeO(100) surface tends to weak chemisorption, while that on the a-Fe2O3(0001) surface tends to physisorption. The adsorption of HgS, HgCl and HgCl2are all likely to dissociatively adsorb on FeO(100). S in HgS adsorbs on the surface, while Hg is repelled strongly by S atom and gets away from the surface. However, HgS can stablely adsorb on the α-Fe2O3(0001) surface. For HgCl and HgCl2, FeO(100) exhibits better activity than α-Fe2O3(0001).5. H2S dissociates more easily on the FeO(100) surface than on the α-Fe2O3(0001) surface. The dissociating S atom has higher adsorption energy on the FeO(100) surface. However, S-precovered FeO(100) will reject Hg. So FeO is not beneficial to demercuration in H2S atmosphere.