| The unique energy structure determines the current energy consumption pattern of China is still dominated by coal,mercury pollution casuing by the coal combustion has drawn global attention and treated as the most important anthropogenic mercury emission source.The accurate measurement of different forms of mercury in coal-fired flue gas can promote the development of mercury control technology.This article describes the advantages and disadvantages of current mainstream mercury measurement methods and the progress of mercury selective adsorbents at home and abroad,on this basis,a variety of alkali/alkaline earth metal based adsorbents were prepared by chemical impregnation method by using a well-structured mesoporous SiO2 and a series of alkali/alkaline earth metal compounds,and then the mercury selective adsorption properties of those sorbents were experimently studied,which can provide the theoretical basis and experimental support for the mercury speciation measurement in coal-fired power plants.Firstly,six kinds of alkali/alkaline earth metal compound were selected as active components and the mesoporous SiO2 was chosen as the carrier skeleton material,then six kinds of adsorbents were prepared by chemical impregnation method.The adsorption properties of Hg0 and HgCl2 under the pure N2 atmosphere of all the adsorbents were studied on a fixed bed experimental apparatus,and the effects of SO2 on the mercury adsorption performance of each adsorbent were also investigated.The results show that the six kinds of adsorbents did not have any adsorption capacity for Hg0,and the presence of SO2 in simulated flue gas did not promote the adsorption of Hg0 on the adsorbents;Different adsorbents showed different adsorption capacity of HgCl2,and SO2 had a significant effect on the adsorption of HgCl2,it caused a decrease in the adsorption efficiency of HgCl2 to different degrees.Adsorbent with CaO as the active component has the best mercury-selective adsorption performance among all the alkali/alkaline earth metal base sorbents.Secondly,the sorbents were strengthened by changing the type of precursor,loading of CaO on the support and using different modified dopants.The results show that different calcium salt precursors have an important influence on the surface structure of the adsorbents,Ca(NO3)2 with low melting point would react with the SiO2 carrier and then formed the CaSiO3 with large particles size during the high preparation temperature,the CaSiO3 particles would block the pores of adsorpbent and thus affect its HgCl2 adsorption capacity;Though the adsorbent prepared from hydrated Ca(OIH)2 has a certain surface structure due to the formation of fibrous structure,however,due to the lack of the original phase of CaO,the adsorption performance of HgCl 2 was still not good;The adsorbent prepared from Ca(CH3COO)2 retains the largest specific surface area and well-developed pores of the carrier SiO2 and thus had the best mercury adsorption performance.On the other hand,when the molar ratio of CaO to SiO2 in the adsorbent is 1:1,CaO can form monolayer dispersion on the surface of the carrier,and best use the carrier skeleton surface,at this condition,the adsorbent achieved the best HgCl2 adsorption performance.Different modified dopants have a significant effect on the surface structure and basicity of the adsorbents,chlorides with low melting point would reduce the surface of the adsorbents and decrease the basicity,though the carbonates with high melting point would also reduce the specific surface area and the micropore volume of adsorbent,it can increase the proportion of high intensity alkaline sites on the surface of adsorbent and increase the sulfur dioxide resistance of the adsorbent as well.HgCl2 can form the monodentate,bidentate and tridentate adsorption configuration on the surfaces of calcium-based adsorbents,and the bonding intensity of those three adsorption configurations is in the successive order:monodentate<tridentate<bidentate.Finally,based on the EPA method 30B,a calcium-based sorbent(CaO-C-S)which with good surface structure and performed well in the mercury adsorption was chosen to conduct coal-fired flue gas mercury measurement in a 600MW coal-fired unit,and the sampling results was then compared with the standard OHM,to explore the applicability of this adsorbent in actual coal-fired flue gas mercury measurement.The test results manifest that the sampling results obtained through adsorbents well matched with the results obtained by OHM,and all deviations meet the QA/QC standards,confirmed that the adsorbent can adapt to the complex flue gas conditions and has a high accuracy in the actual flue gas mercury measurement. |
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