Removal Of Elemental Mercury From Flue Gas By Modified Activated Carbon

As the most abundant primary energy source in China,coal still occupies an important position in the energy structure of the future.Coal-fired power plant is the major energy consumption of China.The large amount of coal combustion generates inorganic and organic pollutants and toxicant,such as SO_x,NO_x,particles and heavy metals that results in serious pollution to environment.The air pollution control devices in coal-fired plant can effectively remove pollutants including SO_x,NO_x and particles in flue gas;however,mercury removal is difficult due to its low melting point,high equilibrium pressure,and insolubility.Active carbon injection?ACI?combined with fine particle removal equipment is the most efficient technology for mercury removal,but the cost of activated carbon is so high that the application of this technology is limited.Therefore,it is urgent to develop a efficient and low cost activated carbon sorbent for the removal of gas phase mercury from coal-fired flue gas.Coal was used as the precursor of activated carbon for its advantages of abundant reserves.Coal-derived activated carbons?CAC?which prepared by KOH activation were used as the carrier for sorbent.A simple impregnation method was used to modify the CAC with NH₄I/FeCl₃,respectively.Mercury removal performance of the modified CAC was investigated with a laboratory sorbent evaluation system.The effect of different loading value,reaction temperature and flue gas composition on the Hg⁰ removal efficiency were evaluated.In order to study the properties of the modified activated carbon,XRF,N₂adsorption/desorption,TG/DTG,SEM,XPS,XRD,FT-IR,Hg-TPD were used to characterize the samples.The reaction mechanism was deduced based on the results of experiments and characterization.Firstly,the effect of preparation conditions and reaction temperatue on mercury removal performance of sorbents were investigated.The results showed that CAC has a certain capacity of mercury removal performance;Hg removal efficiency decreases significantly with increasing of reaction time.The mercury removal performance of CAC is greatly improved after being modified by NH₄I.With the increase of the concentration of NH₄I solution,mercury removal efficiency increased at first and then decreased.When the concentration of NH₄I solution was 3.5 g/L and the mass percentage of NH₄I was 7 wt.%,the sorbent showed the highest mercury removal efficiency.The mercury removal efficiency of sorbent decreased with the increase of temperature in the temperature of 120-220°C.The average mercury removal efficiency was 96.45%at 3 h at a reaction temperature of 120°C.The FeCl₃modified activated carbon showed a better capacity of Hg removal at higher reaction temperature.The mercury removal efficiency of sorbent was above 97%at a temperature range of 100-200°C.When the reaction temperature was 250°C,mercury removal efficiency can still reach 83.33%.Different concentration of FeCl₃ was involved to modify the activated carbon,varying from 0.2 to 0.6 mol/L.The results shows that mercury removal efficiency of FeCl₃ modified sorbents increased with the increased concentration of impregnating solution.The physicochemical properties of sorbents showed that some active oxygen functional groups and carbon iodine compounds?C-I_x?on the surface of NH₄I modified activated carbon are the possible active sites for mercury removal.The crystalline form of activated carbon remained intact after FeCl₃ modification,and FeCl₃ dispersed uniformly on the surface of activated carbon and C-Cl active groups were formed.Therefore,Fe³⁺and C-Cl are the potential active sites for mercury adsorption.Secondly,the components of simulated flue gas also affected the mercury removal performance of sorbent.The results showed that O₂ and CO₂ in flue gas can promote mercury removal efficiency,NO and HCl can also have a promotional effect on the mercury removal efficiency,while SO₂ had an adverse influence on mercury removal and the inhibition effect become more obviously with the increasing SO₂ concentration in a range of 200-400 ppm in flue gas The sorbent has a certain thermal regeneration performance,the mercury removal efficiency can still reach 95.82%after regeneration at 400°C once,but it gradually decreased with the regenerative times on the increase.