Regenerable Magnetospheres Sorbent For Mercury Removal From Coal Combustion Flue Gas And The Reaction Mechanism

Coal-fired power plants are considered to be one of the largest anthropogenic mercury（Hg）emissions sources.Hg pollution has attracted worldwide attention in recent years because of its high toxicity in human health and the environment.Activated carbon injection（ACI）is one of the most promising approaches to reduce Hg emissions from coal-fired power plants.However,there are some limitations for the ACI,such as the recovery and recycle of spent sorbent,the disposition of mercury containing in the spent sorbent,the high operating cost,and others.It is therefore extremely attractive to develop regenerable magnetic sorbent to overcome these limitations.Several studies have demonstrated that the magnetospheres present in fly ashes could both adsorb and oxidize Hg⁰.The annual production of fly ash in China is very large.Thus,as the by-products in coal combustion,the low-cost magnetospheres will present huge potential application in Hg⁰ removal from coal combustion flue gas.In this work,ten magnetospheres samples were separated by magnetic separation from fly ashes of typical coal-fired power plants in China and Russia,respectively.The physical-chemical characteristics of magnetospheres were investigated in details.The Hg⁰ removal capacity of different magnetospheres samples as well as the involved reaction mechanism were investigated.The results showed that,compared to fly ash,the element of iron was significantly enriched in magnetospheres.The iron species in magnetospheres mainly included Fe₃O₄,α-Fe₂O₃,γ-Fe₂O₃,Fe²⁺-silicate,Fe³⁺-silicate,and FeSi,while the content of each iron species were varied from different power plants.Fe₃O₄ and γ-Fe₂O₃ were the dominant iron-bearing minerals,which accounts for 54.5-82.9%.The magnetospheres attained the optimal Hg⁰ removal capacity at 250 ℃.At 100 ℃ and 150 ℃,the adsorption of Hgg played a predominant role in Hgg removal,while the removal process depended on Hg⁰ oxidation at 200-400 ℃.The Hg⁰ removal capacity increased with the increase of iron content.The percentage of ferrospinel and hematite in magnetospheres was a key factor for determining the Hgg removal capacity as well.To improve the Hg⁰ remove capacity and obtain a wide applicability,a novel regenerable magnetic sorbent based on cobalt oxide loaded magnetospheres（Co-MF）was developed.The sorbent with optimal loading 5.8%of cobalt oxide attained approximately 94.7%of Hg⁰ removal efficiency（ET-a）at 150 ℃ under simulated flue gas atmosphere.V2 could enhance the Hg⁰ removal activity of magnetospheres catalyst via Mars-Maessen mechanism.SO2 displayed inhibitive effect on Hg⁰ removal capacity.NO with lower concentration could promote the Hgg removal efficiency.However,when increased the NO concentration to 300 ppm,a slightly inhibitive effect of NO was observed.In the presence of 10 ppm HCl,above 95.5%of Hg⁰ removal efficiency was attained,which was attributed to the formation of active chlorine species on the surface.H2O presented seriously inhibitive efect on Hg⁰ removal.The multiple capture—regeneration cycles demonstrated that the Co-MF catalyst presented good regeneration performance.The mercury adsorbed on the spent sorbent could be decomposed by thermally treatment at 400℃ for 2h,and the producing oxygen vacancy could be replenished by gaseous O₂ in flue gas.A novel magnetic sorbent based on CuCl₂ modified magnetospheres（Cux-MF）was developed by the incipient wetness method.Different copper coordinations existed in the catalyst with different Cu loading:at low Cu loading,isolated Cu²⁺ ions existed in a chlorine-free coordination;at high Cu loading,associated Cu²⁺ ions existed in a chlorine-enriched coordination.The former one acted as the active adsorption sites for Hg⁰,while the latter one was inactive for Hgg removal.The catalyst with optimal loading 6%of Cu attained the highest Hg⁰ removal efficiency（90.6%）at 150 ℃.Different Hg⁰ adsorption sites existed on the sorbent:Cl adsorption sites and Cu adsorption sites.The binding energy of mercury on the Cu adsorption sites was higher than that on the Cl adsorption sites.O2 and HCl significantly affected the state of Cu and Cl on the spent catalyst.The interaction between Hg⁰ and CuCl₂ with the participation of O₂ and/or HCl follows three steps mechanism:（1）the reduction of CuCl₂,to CuCl for the interaction with Hg⁰,（2）the reoxidation of CuCl for the interaction with O₂ forming an intermediate copper oxygen chloride species,（3）the rechlorination of oxychloride species resulting in the restoration of CUCl₂.A regeneration method under specific atmosphere for deactivated sorbent was developed based on the above reaction mechanism.The results suggested that two steps were essential in the regeneration process:（1）releasing the sulfur and mercury species adsorbed on the catalyst;（2）restoring the surface chemistry of catalyst by O₂ and HCl.Optimal regeneration performance of spent catalyst was attained after thermal desorption at 400 °C and following by the restoration of O₂ and HCl.Four cycles of mercury adsorption-catalyst regeneration showed that the mercury removal capacity did not vary appreciably with respect to fresh catalyst after multiple regeneration.Experimental studies on Hgg removal by duct injection with.Cu₆-MF sorbent were performed in an entrained flow reactor system.The factors affecting the Hg⁰ removal efficiency were investigated,including initial Hg⁰ concentration,sorbent concentration,particle size,residence time,and flue gas temperature.The results showed that,with the increase of Hg⁰ concentration in flue gas,the Hg⁰ removal efficiency increased.The Hg⁰ removal efficiency also increased with the increase of residence time and sorbent concentration.With the residence time of 1.61 s and sorbent concentration of 1.09 g/m,the Hg⁰ removal efficiency was 80.6%.However,with the further increase of residence time and sorbent concentration,the Hg⁰ removal efficiency did not increase continuously.The smaller particle size was favorable for the Hg⁰ removal.In the industrial application,the particle size could be selected as 45-74μm when considering the Hg⁰ removal efficiency,the cost for preparing the sorbent,and other factors.Further,a comprehensive mathematical model based on material balance equation,mass transfer model and surface reaction model was established for predicting Hg⁰ removal efficiency by Cu₆-MF sorbent injection.The model was capable of predicting the Hg⁰ removal efficiency and the calculated curves agreed well with the experimental data.The optimal conducting parameters could be obtained using the model,which could provide some guidelines for the design of sorbent injection system in power plants.