Experimental And Mechanism Research On Removing Elemental Mercury From Coal-fired Flue Gas With Iron-carbon Adsorbent

Mercury released from coal combustion is a global pollutant that could seriously threaten the ecological environment and human health through bioaccumulation.As a large coal-consuming country,mercury emission from coal combustion power plants has become the main anthropogenic source of mercury release in China.The most urgent task is to strictly control mercury emission in the flue gas of coal-fired power plants.The physical stability and chemical inertness of elemental mercury(Hg⁰)makes it hard to be removed from coal combustion flue gas.Currently,activated carbon injection(ACI)technology is a widely applied method for mercury removal,but activated carbon(AC)has a limited ability to remove Hg⁰ and the large demand of AC also leads to high operating costs.Many researchers focused on alternatives to replace AC for sorbent injection or dedicated to modifying AC to improve its mercury removal ability.Although such two methods can effectively improve the sorbent jet mercury removal technology,the used sorbent will still remain in the fly ash,which not only decreases the quality of the fly ash for using as cement addition,but also will cause mercury release again without treating used sorbents.In order to solve these problems,the development and preparation of magnetic sorbents seems to be promising soluation and demonstrates obvious advantages in sorbent collection and recycling.Taking into account the superior physical structure characteristics of AC,adding magnetism to AC is attractive.The most direct way to obtain magnetic activated carbon sorbent is the combination of magnetic material and activated carbon.According to our previous research,the preparation of magnetic iron oxide and activated carbon sorbent is an achievable idea.For preparing the magnetic iron carbon sorbent,the role of iron in the mercury removal process should be investigated.Taking?-Fe₂O₃,?-Fe₂O₃ and Fe₃O₄ as the experimental objects,the interaction between Fe and Hg has been studied in detail.Mercury capture capability of iron oxides are weak,but the oxidizing components in the simulated flue gas can promote the oxidation and adsorption of Hg through a heterogeneous reaction.The increase of HCl concentration and temperature is beneficial to HCl to promote the oxidation of Hg0 on the surface of iron oxide.It is found that the key to promote mercury oxidation by HCl is the formation of active Cl groups according to the density functional theory analysis.Fe on the surface of iron oxide play an important role in the reaction for providing adequate active sites during the reactions.The mercury removal performance test experiments under the simulated flue gas also prove that iron oxides can maintain good mercury removal efficiencies under complex flue gas conditions,and form various forms of mercury compounds on the surface of the adsorbent.However,the addition of water vapor into the flue gas will cause iron oxides adsorbent poisoning and deactivation.The mercury removal performance of the sorbent obtained after AC modification is needed to be investigated.Especially for the metal modified AC.The nitrate and chloride of Fe,Mn and Cuwere applied to modify AC,which can load metal and active Cl species in AC.In this way,it is possible to analyze the effect of metal and carbon combined sorbent in mercury removal.The mercury removal efficiencies of these three nitrates modified AC adsorbents all reduced in different degrees,but for chloride impregnation,the mercury removal efficiencies of the modified sorbents are all over 95%.Compared with the efficiency of Cl alone modified AC is only 80%,the effect of metal in sorbent cannot be neglected.Cumodified AC sorbents were selected to conduct further research.After comparing sorbents`performance under various conditions,it can be concluded that the synergistic effect of metal and Cl is the essential to obtain high mercury removal efficiency.Besides,molecular level simulation calculation results also show that Hg is physically adsorbed on the Cu-modified AC surface;while the CuCl<sub>2</sub> modified AC can oxidize Hg to Hg Cl<sub>2</sub> and then adsorbed at the Cuactive site.The reaction energy barrier and reaction heat are 121.83kJ/mol and 4.5kJ/mol.In the experiments conducted under simulated flue gas,the CuCl<sub>2</sub> modified AC maintains a high level of mercury removal efficiency in a wide temperature range.Preparation process and performance evaluation are two important factors in the development of sorbents.In this paper,the co-precipitation method,which is simple and easy to operate,is selected to prepare the magnetic iron-carbon(Fe-C-x)sorbents.A series of adsorbents with a magnetic range from 23.8 emu/g to 62.5 emu/g are obtained through controlling the ratio of iron to carbon.Several characterization tests have been carried out to get a comprehensive understanding in sorbents`physical and chemical properties.The surface of AC was covered by Fe₃O₄ precipitation during the sample preparation process,which will reduce the physical adsorption of Fe-C-x.Although the mercury removal efficiency of Fe-C-x under nitrogen are not as good as that of AC,Fe-C-3 exhibits better mercury removal performance in the simulated flue gas.The effects of different flue gas components on Fe-C-3 mercury removal are quite different.HCl can promote the removal of Hg⁰,but SO₂ exhibits inhibitory effect.The presence of O₂ alone shows little effect on mercury removal,but with the existence of NO,the mercury removal efficiency of Fe-C-3 is significantly improved.Furthermore,the Fe-C-3 shown high mercury removal efficiency in multiple recycling experiments under simulated flue gas,which indicates that Fe-C-3 has good durability and reproducibility.In order to enhance sorbent`s mercury removal ability and sulfur resistance,Mn Cl<sub>2</sub>solution was used to impregnate Fe-C-3.Mn Cl<sub>2</sub> modification can not only increase metal active sites in the sorbent,but also introduce active Cl species.The mercury removal efficiency of modified sorbent under nitrogen increased to 67%-90%with different impregnation concentrations.The sulfur resistance of the sorbent also has been improved at the same time.Although the presence of high concentration of SO<sub>2</sub> can still inhibit the removal of Hg<sup>0</sup>,the mercury removal efficiency of the 0.02Fe-C under SO<sub>2</sub> atmosphere is significantly higher than that of the raw sorbent.In addition,adding O<sub>2</sub> into the flue gas can promote the oxidation of Hg<sup>0</sup> and weaken the influence of SO<sub>2</sub>.Through experiments and computational simulations,it is found that O<sub>2</sub> will combine with Mn in the sorbent and promote the conversion of Mn to a higher valence state,thereby enhancing its oxidation.O<sub>2</sub>can also dissociate active O species on the sorbent,and then react with Hg to form Hg O.Further experiments found that 0.02Fe-C can adapt to the complex flue gas environment and wide temperature range,which is suitable for sorbent injection.Considering the difference between mercury removal in static and flowing state,experiments have been carried on an entrained flow reactor to inspect the mercury removal performance after sorbent injected into the duct.The effects of reaction conditions and gas flow parameters have been researched,such as initial Hg<sup>0</sup> concentration,sorbent injection concentration,residence time,and reaction temperature.The higher sorbent injection concentration and longer residence time,the higher the mercury removal efficiency.Also the increase in the initial Hg<sup>0</sup> concentration accelerates the dynamic diffusion and increases the mercury removal efficiency.The mercury removal efficiency of 0.2CuCl<sub>2</sub>-AC and 0.2Fe-C can reach 78%and 74%during the injection process,and the influence of temperature and flue gas composition show little effect when the reaction temperature bellows 140oC.The main reason for difference in mercury removal efficiency between fixed bed and entrained flow reactor is that the dynamic diffusion resistance in fixed bed reactor is smaller than that in entrained flow reactor.Besides,a mathematical model was established based on the mass transfer and mass balance theories to predict the sorbent`s mercury removal efficiency after injection.The model is considered reasonable after fitting with experimental data and can predict mercury removal efficiency after sorbent injection according to sorbent characteristics and operating parameters.