Experimental And Mechanism Study On Mercury Removal For Coal-fired Flue Gas By Multichannel ZSM-5 Zeolite

Mercury emission produced by coal burning has become the fourth largest pollutant in flue gas after sulfur oxides,dust and nitrogen oxides.China’s coal-based energy consumption structure determines that coal-fired power plants are the main sources of anthropogenic mercury emissions in China.Therefore,mercury emission control and prevention in coal-fired power plants is of great importance.To data,the activated carbon injection（ACI）for mercury removal is regarded as the most applicable technology,however,carbon-containing materials may cause problems such as high ratio of carbon to mercury in fly ash,flue damage and so on.Hence,developing a carbon-free adsorbent for mercury removal has become the research idea of this paper.Zeolite as a kind of carbon-free silico-aluminate mineral material,with its large specific surface area,excellent pore structure and good hydrothermal stability,is an excellent carrier material in gas adsorption/catalysis filed.Thus,it was proposed in this study that a multi-stage molecular sieve adsorbent was prepared by the dual function of chemical modification and channel regulation.In order to provide experimental and theoretical support for the industrial application of molecular sieve-based adsorbents for mercury removal,the pore control process,modification parameters and mercury removal characteristics of the adsorbents were further studied.Firstly,the mercury removal performance of alternative zeolites with different topologies was tested.LTA 5A zeolite and MFI ZSM-5 and Silicalite-I zeolite were selected as adsorbent carriers for mercury removal.The selected zeolites were prepared by chemical impregnation with Fe Cl₃.The effect of reaction temperature and atmosphere on mercury removal efficiency was investigated in a fixed-bed experimental reactor and the mechanism of Hg removal by adsorbent was studied.The results showed that the original zeolite removed Hg⁰ by physical adsorption,so their Hg⁰removal performance is poor.After Fe Cl₃ impregnation,the chemisorption of Hg⁰ to zeolites was enhanced,and the Hg⁰ removal efficiency was significantly improved.Since LTA Fe/5A sample has more Na⁺in int skeleton structure,ion exchange reaction will occur during Fe Cl₃ impregnation process to generate Na Cl crystals.Na Cl not only consumes free Cl^-,but also stays in channel causing“blockage”,which is not beneficial to the removal of mercury by adsorbents.However,there was no Na Cl formation in the two MFI samples,and the free state of Cl^-promoted the conversion of Hg⁰ to Hg Cl₂.The experimental results in O₂ and SO₂ atmosphere show that O₂ has a certain promotion effect on the adsorbent,but SO₂ has no obvious inhibition effect on the removal of mercury.Combined with the results of mercury removal performance,Fe/ZSM-5 showed better performance,so ZSM-5 was subsequently targeted for channel regulation.Secondly,dealumination and desilication in post synthesis approach were applied to create hierarchical ZSM-5 zeolites.The effect of Na OH/HNO₃ concentration and processing time were systematically investigated.The results show that the desilication of Na OH is helpful to the formation of new mesopores.With the increase of Na OH concentration and pore expansion time,the number of newly formed mesopores increases.However,the deeper pore expansion will lead to the decline of stability of skeleton structure,so it is necessary to find the equilibrium point of mesoporous and skeleton stability.However,HNO₃ treatment can maintain the stability of the skeleton,which is conducive to the subsequent improvement of mercury removal performance.Finally,the hierarchical channel ZSM-5 was modified by Fe Cl₃ impregnation and its mercury removal performance was tested in a fixed-bed experimental facility.The results show that the Hg⁰removal efficiency of Fe/multi-channel ZSM-5 adsorbents is greatly improved and its Fe Cl₃ load is also reduced.The characterization results showed that the connection between the internal micropore and the external mesopore of multi-channel ZSM-5 is opened after the channel regulation,Fe Cl₃ can be loaded in internal microporous channels during impregnation,which increases the number of active sites for Hg⁰ removal;on the other hand,multi-channel ZSM-5 improves the mass transfer rate of Hg⁰ in its channels and promotes the reaction with active sites such as Fe³⁺and chemisorption oxygen on the surface of the adsorbent.In addition,the mercury removal mechanism of multi-channel ZSM-5 samples is not significantly different from that of original ZSM-5 samples,and Hg⁰ is still adsorbed and removed under the dual action of physical adsorption and chemical adsorption.In conclusion,multi-channel ZSM-5 has significant advantages over conventional ZSM-5 zeolite in reducing the load of active components and improving the mercury removal performance.It is suggested that multi-channel ZSM-5 as a non-carbon-based adsorbent material is effective and feasible for mercury removal.