Study On The Mechanism Of Removing Elemental Mercury From Coal-fired Flue Gas By Zeolite-templated Carbon

Mercury is a metal element that harms the natural environment and human health.Because of its accumulation and migration into the ecological environment,it poses a long-term threat to human health.Human activities,especially burning coal and other fossil fuels,are one of the important ways to release mercury pollutants into the natural environment.Therefore,reducing the content of mercury in the flue gas of coal-fired power stations can significantly decline mercury pollution in the ecological environment.Mercury speices in the flue gas mainly include zero-valent mercury(Hg0),bivalent mercury(Hg2+),and granular mercury(HgP).Among these mercury speices,most of the Hg2+and HgP can be removed by dust removal equipment and desulfurization equipment.However,Hg0 is difficult to remove due to its insolubility in water and high volatility.Therefore,removing Hg0 from the coal-fired flue gas is on of the important researches.To remove Hg0 from the flue gas,zeolite-templated carbon(ZTC)with regular pore structure,concentrated pore size distribution,and high specific surface area was prepared and used in this study.When preparing ZTC,two technical problems were solved:(1)the preparation process of ZTC by chemical vapor deposition(CVD)was developed;(2)The diffusion and mass transfer characteristics of carbon precursors in zeolite pores during the CVD process are improved to optimize the physical and chemical structure of ZTC.When ZTC was used to remove Hg0,two scientific problems were explored:(1)the pore structure of ZTC and the mechanism of removing Hg0 from flue gas by sulfur modified ZTC(S-ZTC)were studied;(2)The effect of real flue gas component on the mercury removal performance of ZTC and S-ZTC was discussed.Firstly,ZTC was prepared by the CVD method using single particle zeolite(diameter≈4mm)as a template,and the mechanism of mass transfer and chemical vapor deposition of acetylene(C2H2)in zeolite nanopore was investigated.The mass gain of zeolite during the CVD process was analyzed by thermogravimetricmass spectrometry(TG-MS),and the types and relative contents of gaseous byproducts produced by C2H2 during the CVD process were identified.The microstructure of pyrolytic carbon and its spatial distribution in carbon/zeolite complex(ZCP)were evaluated by scanning electron microscopy(SEM).In addition,the diffusion-reaction equation model(DRE)was used to study the relationship between the diffusion coefficient of C2H2 and the deposition of pyrolytic carbon in a zeolite nanopore.Finally,the deposition mechanism of pyrolytic carbon in zeolite nanopores is proposed,which guides the preparation of ZTC.Secondly,given the key problem of deposition of turbostratic carbon on zeolite surface during the CVD process,ZTC samples were synthesized using 13X zeolite with different particle sizes(diameter≈4,0.8,and 0.002 mm,FAU zeolite)as templates.The effect of diffusion and mass transfer characteristics of C2H2 and its gas phase intermediates on the structure of ZTC in zeolite nanocrystals with different sizes was investigated.The theoretical calculation results show that reducing the zeolite particle size can promote the mass transfer of C2H2 into the zeolitenanopores and promote the uniform deposition of pyrolytic carbon.In addition,the reduction of zeolite particle size is also conducive to the diffusion of gas phase intermediates to the outside of the zeolite template,and reduces the deposition of turbostratic carbon on the surface of the zeolite.Because of the advantages of small-size zeolite in gaseous diffusion mass transfer,ZTC-0.002 prepared by small-size zeolite as a template has better structural characteristics than ZTC-4 and ZTC-0.8.N2 adsorption-desorption isotherm,XRD,TEM,and TGA analysis showed that ZTC-0.002 prepared using zeolite-0.002 has the highest specific surface area(2823.80 m2/g),the largest micropore volume(1.21 cm3/g),more ordered replicated carbon and less turbostratic carbon.Thirdly,according to the above improved ZTC preparation process,using BEA zeolite and FAU zeolite as templates,C2H2 and H2S as carbon precursors and sulfur precursors respectively,the ZTC and sulfur-doped ZTC(S-ZTC)with threedimensional ordered micropore structure were prepared by CVD method.Their mercury adsorption performance at 30℃ and 150℃ were investigated.Among all the ZTC samples,BEA-ZTC-HS showed the highest Hg0 adsorption efficiency.The Hg0 adsorption efficiency of BEA-ZTC-HS decreased from 99.99%to 96.60%at 150℃ and from 97.40%to 89.20%at 30℃ for 1h.A variety of experimental methods(e.g.TGA.BET,XRD,XPS,in-situ FTIR,Hg-TPD,etc.)were used to characterize the physical and chemical structures of fresh and used ZTC and SZTC in detail.The experimental results show that BEA-ZTC-HS replicates the crystal structure of BEA zeolite well,with a three-dimensional regular ordered micropore structure.and does not contain obvious elemental sulfur.These physical properties of BEA-ZTC-HS contribute to the mass transfer and adsorption of Hg0 in ZTC.Moreover,the results of XPS,in-situ FTIR spectroscopy,and Hg-TPD indicate that the sulfur in BEA-ZTC-HS is mainly bonded to the carbon framework of ZTC,and exists in the form of thiophene-S,sulfone,sulfoxide,and sulfonic acid.These sulfur functional groups provide major active sites for Hg0 capture and exhibit high activity at 150℃.In additions the DFT calculation confirmed that the adsorption of mercury by sulfur species in S-ZTC belongs to chemisorption.Finally,the effect of real flue gas component(H2O,NH3,and SO2)on the Hg0 adsorption performance of ZTC-HS was studied.Specifically,due to the existence of competitive adsorption,when there is H2O and NH3 in the flue gas,H2O,NH3,and Hg0 have competitive adsorption on the surface of ZTC-HS,which leads to the gradual decrease of Hg0 adsorption capacity.The effect of SO2 on the Hg0 adsorption efficiency of ZTC-HS is closely related to its concentration.At low concentrations(<1000 ppm),the Hg0 adsorption efficiency of ZTC-HS was weakened by the presence of SO2 due to the competitive adsorption between SO2 and Hg0.However,at high concentrations(>1000 ppm),SO2 promoted the adsorption of Hg0 by ZTC-HS in the form of HgSO4.Compared with other carbonbased adsorbents,ZTC-HS has good Hg0 removal performance in coal-fired flue gas with high humidity and high SO2 concentration.At last,the molecular dynamic(MD)calculation results showed that flue gas components can affect the Hg0 adsorption performance of ZTC-HS by competitive adsorption.As ZTC-HS has high Hg0 removal performance and strong resistance to H2O and SO2,it has broad application prospects in the removal of Hg0 from the flue gas of coal-fired power stations.