Study On Controlling Mercury Emission In Coal-fired Flue Gas By Magnetic Attapulgite

With the increasing pollution of atmospheric mercury,coal-fired power plants in China,as the main source of anthropogenic mercury emissions,are facing tremendous pressure and challenges.As Hg⁰ in coal-fired flue gas is volatile and insoluble in water,it is difficult to be removed by existing pollutant control equipment,which has become the focus and difficulty of current research.At present,the activated carbon injection technology has the problems of high cost,difficult recovery and high carbon content in fly ash.Therefore,it is important to develop low cost and high efficiency magnetic adsorbent for mercury removal.Based on the idea of removing mercury from coal-fired flue gas by magnetic attapulgite,a variety of new adsorbents?MAtp,MnMAtp,CuMAtp?with high efficiency and recyclability were developed.The mercury removal performance of adsorbents was studied on Hg⁰ adsorption test bench in fixed bed.The physical and chemical properties of adsorbents were analyzed by BET,XRD and SEM,and the mercury removal mechanism of adsorbents was discussed in depth.In this paper,magnetic attapulgite?MAtp?was prepared by modification of attapulgite with?-Fe₂O₃ by deposition-precipitation method,and mercury removal experiments were carried out.The results show that:MAtp belongs to superparamagnetic material and can be separated and recovered by external magnetic field.The mercury penetration rate of 1M2Atp under the basic simulated flue gas condition reached 25.6%when gas hourly space velocity was 35×10⁴ h^-1 and reaction temperature was 150?at 90 minutes.With the increase of iron oxide content,the removal efficiency of Hg⁰ by MAtp increases gradually,and with the increase of reaction temperature,the mercury removal ability of MAtp increases gradually,mainly by chemical adsorption.O₂,NO and HCl can significantly enhance the mercury removal ability of MAtp.SO₂ can significantly inhibit the removal of Hg⁰,mainly because of the competitive adsorption with Hg⁰,and it is found that low concentration of NO can counteract the inhibition of SO₂ on Hg⁰.In order to enhance the removal ability of magnetic attapulgite for Hg⁰,Mn_xO_y modified magnetic attapulgite?MnMAtp?was prepared by impregnation method,and mercury removal experiments were carried out.The results show that MnMAtp is still a superparamagnetic material,the optimum manganese loading is 10%.with the increase of calcination temperature,the mercury removal performance and magnetic strength of MnMAtp increase first and then decrease,the optimum calcination temperature is 450?.when gas hourly space velocity is as high as 88×10⁴ h^-1,and the reaction temperature is 150?,the mercury penetration of MnMAtp in basic simulated flue gas is about 10%,the mercury removal efficiency of MnMAtp is excellent,and its active substances are mainly MnO₂ and Mn₃O₄.with the increase of reaction temperature,the mercury removal ability of MnMAtp increases first and then decreases.the oxygen in flue gas has a promoting effect on the removal of Hg⁰.NO and HCl can significantly improve the mercury removal performance of MnMAtp,the mercury penetration rate is close to 0.SO₂ can also inhibit the removal of Hg⁰,at the same time,the results of five adsorption-regeneration experiments show that the mercury penetration rate of MnMAtp is only 6.4%after 90 minutes of adsorption under simulated dry flue gas conditions,and the mercury removal performance is excellent.After five adsorption-regeneration,the mercury penetration rate remains low,indicating that it has good recycling performance,and its magnetic strength is not significantly affected.In order to solve the problem of narrow active temperature window of MnMAtp adsorbent,CuCl₂ modified magnetic attapulgite?CuMAtp?with good temperature resistance was developed and mercury removal experiments were carried out.The results show that CuMAtp still belongs to superparamagnetic material,its mercury removal performance is excellent.And the optimum load of CuMAtp is 5%,when gas hourly space velocity is as high as 178×10⁴h^-1and reaction temperature is between100?to 250?,the Hg⁰penetration rate of CuMAtp was lower than 10%under the basic simulated flue gas condition,and it can be kept below 17%when the temperature is 300?.The copper species mainly exist in the form of Cu₂Cl?OH?₃and a small amount of CuCl.O₂ can obviously promote the mercury removal of CuMAtp,NO had little effect on the mercury removal performance of CuMAtp;SO₂still shows a inhibition effect on Hg⁰.At the same time,the results of five adsorption-regeneration experiments show that the mercury penetration rate of MnMAtp is only 5%after 90 minutes of adsorption under simulated dry flue gas conditions,and the mercury removal performance is excellent.After five adsorption-regeneration,the penetration rate of CuMAtp only increases by 6.2%,indicating that it has good cyclic regeneration performance,and its magnetic properties were not significantly affected.Kinetic studies showed that quasi-first and second-order models could describe the removal of Hg⁰,indicating that external diffusion and chemical adsorption were dominant.In conclusion,CuMAtp has the potential to be a commercial adsorbent for mercury removal.