Removal Of Elemental Mercury From Flue Gas Over Modified Red Mud Based Sorbents

Coal is the main primary energy of China.Coal-fired power plant is one of the main ways of the utilization of coal.However,a series of pollutions are produced during the process of coal combustion,resulting in serious damage to the environment and the human being.The subsequent purification process can effectively remove pollutions such as solid particles,SO₂ and NO_x in coal-fired flue gas,however,the removal and control of heavy metal including mercury in flue gas has not been solved effectively.Therefore,there is a requirement for developing a highly efficient,inexpensive and environmentally friendly adsorbents or catalysts.Red mud is a kind of industrial waste,because of its rich in a large number of active adsorption components,metal elements such as iron,titanium,manganese,etc,can be prepared to non-carbon sorbents.Through chemical and physical modification,the sorbentscan be applied to the mercury removal from flue gas.In this study,Bayer process red mud?a by-product of Al2O3 prodcution?was used as the supporter,then modified by acid treatment,loaded with CuO_x and CuO_x-CeO_x.The prepared sampels were characterized by XRF,N2-adsoprtion/desorption,XRD,TG/DTG and XPS methods to analyze the physicochemical properties of samples before and after reaction.The mercury removal performance was conducted on a fixed bed reactor.The effects of reaction temperature and gas composition on the mercury efficiency were investigated,and the mechanism of mercury removal was analysed finally.The results showed that,red mud after calcination had poor ability of mercury removal.The performace of elemental removal was enhanced by the treatment of acid,mainly due to the reduction of alkaline and the improvement of pore structure.The mercury removal efficiency increased to 26.16 % after 3 hours' reaction with the treatment of 15 m L and 0.3 mol/L hydrochloric acid.CuO_x was used to modify the red mud after acidification,and the mercury removal efficiency was higher compared to red mud treated by hydrochloric.Additionally,the red mud loaded with 12 wt% Cu showed a better performance than 8 wt% Cu.When the raction temperature was in the range of 200²50 oC,the removal efficiency over 12Cu?500?-RMA15 was above 90 % after 3 hours.However,when the reaction temperatue was higher than 250 oC or lower than 200 oC,the removal efficiency decreased.A small amount of acid gases in the flue gas had an important impact on the removal of Hg0.Hg0 removal efficiency is nearly 100 % in the presence of trace HCl and NO,while removal efficiency decreased with the addition of SO₂.Hg2+ was detected in outlet after reaction,indicated that the adsorption and catalytic oxidation processes both occured during the reaction of elemerntal mercury removal.To expand the reaction temperature window and enhance the surlfur resistance,the sample was co-modified by CuO_x-CeO_x.The optimal sample was prepared when Cu/Ce was 1:1 and calcination temperatute was 400 oC.Mercury removal efficiency was still higher than 95 % after 3 h at the reaction temperature of 150³50 oC,thus the sorbent broadened reaction temperature.The effect of CO₂ on the mercury capture was not obvious.O_xygen,trance HCl and NO increased the removal of elemental mercury,and the sulfur resistance was also improved although the mercury removal decreased with the addition of SO₂.10 ppm HCl counteracted the effect of 400 ppm SO₂ during the process.The stability of Cu1Ce1?400?-RMA15 was evaluated and more than 95 % mercury removal obatined after 24 h.semi-penetrating state was observed when the sorbent reacted for 144 h.Hg-TPD experiments showed that oxidation state mercury including HgCl2 and HgO existed at the surface of adsorbents,and O₂ was favorable for the formation of HgO.XPS analysis indicated that CuO_x and CeO_x had synergistic effect on removal of mercury by detecting the valence of elements Cu,Ce,O and Hg.