Synthetic calcium-based adsorbents for gaseous mercury(Ⅱ) adsorption from flue gas and study on their mercury adsorption mechanism

摘要

calcium-based sorbents are the potential materials for mercury chloride (HgCl2) adsorption while inert to elemental mercury (Hg-0) due to their basic property. A series of calcium-based sorbents were chosen to evaluate the selective adsorption performance of mercury, these sorbents included a calcium oxide adsorbent, a KCl-doped calcium oxide adsorbent and a K2CO3-doped calcium oxide adsorbent which all supported by the mesoporous silica (C-SiO2, CKL-SiO2 and CKO-SiO2) and were synthesized by an impregnation method. All sorbents were firstly characterized by Powder X-ray Diffraction (XRD), Brunauer-Emmett-Teller (BET) and Scanning Electron Microscope (SEM). Characterization results show that C-SiO2 has the best surface structure while the doped KCl would evidently reduce the specific surface area and micropores volume of the sorbent, the doped K2CO3 also has an adverse effect on the surface structure of CaO sorbent but it can significantly improve the basicity of sorbent. Mercury adsorption performance of the sorbents was then evaluated in fixed bed apparatus, the results indicated that all sorbents exhibited a complete breakthrough for gaseous Hg-0 . Due to the modification of K2CO3 to the CaO sorbent, HgCl2 was fully adsorbed by CKO-SiO2 no matter the SO2 presented or not. CO2-TPD combined with HgCl2 -TPD confirmed that diverse basic sites exist on the calcium-based sorbent surfaces and three HgCl2 adsorption behaviors occurred during the adsorption process. Three HgCl2 adsorption configurations were introduced to represent the different adsorption behaviors and the binding intensity of these configurations are in the successive order: monodentate tridentate bidentate. The monodentate adsorption configuration is the main HgCl2 adsorption manner on the CaO surface while the bidentate adsorption configuration occurs on the defect surface of sorbent.