Experimental Study On Mercury Removal From Flue Gases By Cu-based Montmorillonite

Mercury emissions are associated with coal combustion.Most of the particulate mercury and divalent mercury can be removed from flue gas by using existing air pollution control devices,while we need to apply the sorbent injection technology to control elemental mercury.The key point of this technology is the choice of sorbents.Activated carbon is the most widely used adsorbent currently.However,it is limited by the high cost and other reasons in actual use.The mineral adsorbent has become the hot spot in research due to its low cost,extensive sources and unique chemical properties.By comparing the price and physicochemical properties of some carrier materials,montmorillonite was chosen as the carrier,and copper oxide and copper chloride were selected as the active materials to modify it.Cu-based montmorillonite sorbents were prepared by impregnation method.The effects of different preparation methods and experimental conditions on mercury removal efficiencies were tested in fix-bed reactor system.The physical properties of modified sorbents were analyzed by means of SEM and BET methods.The chemical composition and mercury removal mechanism of modified sorbents were analyzed by XRD and XRF methods.The results indicate that the mercury removal efficiency of montmorillonite is only 25%under the conditions of 150°C,4%Vol.O2,10%vol.CO2 and 86%vol.N2.However,the mercury removal capacities of montmorillonite had been significantly improved after impregnation.Under the same conditions,the mercury removal efficiency of CuCl2 modified montmorillonite is more than 90%,and that of CuO modified montmorillonite is 75%.But the mercury removal efficiency of CuCl2 modified montmorillonite decreases as the reaction temperature increasing,and is hardly affected by water vapor and SO2 gas.The mercury removal efficiency of CuO modified montmorillonite increases as the reaction temperature increasing,and the mercury can eventually be transformed into HgSO4 in the environment containing both water vapor and SO2,So the mercury removal efficiency will increase significantly with the increase of SO2 concentration,and ultimately reach more than 85%.When the test condition is 150 ℃and contains both water vapor and SO2 gas,the stability test results show that within 6 hours,CuCl2 modified montmorillonite can maintain more than 85%of the mercury removal efficiency,and CuO modified montmorillonite can maintain about 80%of the mercury removal efficiency and both adsorbents have good stability.The mercury removal mechanism of CuO modified montmorillonite follows Mars-Maessen mechanism:copper oxides are mainly converted between the forms of CuO and Cu2O to complete the catalytic oxidation of mercury,and finally Hg0 is adsorbed on the adsorbent surface in the form of HgO.The mercury removal mechanism of CuCl2 modified montmorillonite follows Eley-Rideal mechanism:the adsorbent oxidizes Hg0 by releasing active Cl in the process of mercury removal,and finally reacts into HgCl2.