Study On Mercury Removal Characteristics Of Ce-modified Fe-Mn Magnetic Adsorbents

Coal-fired power plants are considered to be the largest source of anthropogenic mercury emissions,which has attracted extensive attention from the international community.Activated carbon injection technology(ACI)is rerarded as the most mature and feasible technology for mercury removal from coal-fired flue gas.However,the sorbent after mercury removal is discharged together with fly ash,leading to the difficulty in the recovery and reuse of sorbent,as well as second release of mercury.Therefore,development of recyclable mercury removal sorbents has become an important research topic at present.The iron-manganese(Fe-Mn)based mercury removal sorbent has received widespread attention because of its advantages of magnetic separation,reproducibility,low cost,and high activity.However,Fe-Mn based sorbents are prone to SO₂ poisoning,which causes the performance of mercury removal and regeneration reduce significantly.Aiming at the deficiency of Fe-Mn-based sorbents that are prone to SO₂ poisoning,this paper proposes to modify the Fe-Mn magnetic sorbent with transition metal Ce in order to prepare a reproducible sulfur-resistant Fe-Mn-Ce magnetic mercury removal sorbent.The main work and conclusions of this paper are summaried as follows:A series of Ce modified Fe-Mn magnetic sorbent(Fe₆Mn_1-xCe_xO_y)were prepared by the co-precipitation method(X=0,0.2,0.33,and 0.5,respectively).The effect of Ce doping amount,temperature,and flue gas components on mercury removal performance of sorbent were studied in a fixed bed mercury removal experiment platform.After mercury removal,the spent sorbent was regenerated by thermal regeneration methods.The effects of regeneration temperature,regeneration atmosphere,and regeneration time on mercury removal of sorbent were studied.The results showed that with the increase of Ce doping amount,the mercury removal efficiency of Fe₆Mn_1-xCe_xO_y sorbents increased first and then decreased with the Fe₆Mn_0.8Ce_0.2O_y sorbent displaying the best mercury removal performance.Flue gas temperature had a significant impact on mercury removal efficiency of Fe₆Mn_0.8Ce_0.2O_y sorbent and high temperature above150?inhibited mercury removal performance.O₂ could effectively promote the mercury by Fe₆Mn_0.8Ce_0.2O_ysorbent,and the promotion effect at high temperature was more significant.With the increase of moisture content in flue gas,the mercury removal efficiency of sorbent continues to decrease.During 6 cycles of mercury removal and regeneration,the Fe₆Mn_0.8Ce_0.2O_y showed excellent mercury removal performance with the mercury removal efficiency always higher than 95%.The physical and chemical properties of sorbents were characterized by N₂adsorption/desorption,BET,XRD,TEM,VSM,and XPS methods.The mercury adsorption species on Fe₆Mn_1-xCe_xO_y sorbent was analyzed by Hg-TPD.The results showed that Ce modification could effectively improve the pore structure of sorbent that with the increase of Ce doping amount,the specific surface area and pore volume of sorbent continued to increase.The Fe₆Mn_1-xCe_xO_y sorbents had Fe₃O₄ crystal phase components,but there was no crystal phase components of Mn and Ce.TEM analysis indicated that there was Mn lattice existing in the sorbent,and Ce modification reduced the particle agglomeration.VSM analysis showed that Ce modified Fe-Mn sorbent has a good magnetic characteristic.XPS analysis diaplayed that Mn elements in sorbent were mainly Mn⁴⁺and Mn³⁺without Mn²⁺peak.Ce element in sorbent were Ce⁴⁺and Ce³⁺,and Ce modification introduced more oxygen vacancies.The Hg-TPD experiment indicated that mercury removal product of Fe₆Mn_0.8Ce_0.2O_y sorbent was HgO.The mercury removal mechanism followed the Mars-Maessen mechanism.During mercury removal process of Fe₆Mn_0.8Ce_0.2O_y sorbent,Hg⁰ was oxidized by part of Mn⁴⁺with Mn⁴⁺reducing to Mn³⁺.During the regeneration process,Mn³⁺was re-oxidized to Mn⁴⁺again by O₂and Ce⁴⁺.The sulfur-resistant of Ce modified Fe-Mn magnetic sorbent was studied in the fixed bed experiment platform.The effects of flue gas temperature,O₂,SO₂ concentration,H₂O,and Ce doping on SO₂ resistant were investigated.Combing Hg-TPD and XPS analysis,the mechanism of mercury removal and SO₂ resistant of Ce-modified Fe-Mn magnetic sorbent was clarified.The results showed that SO₂ had a inhibitory effect on mercury removal performance,but the Fe₆Mn_0.8Ce_0.2O_y sorbent diaplayed excellent SO₂ resistance at different SO₂ concentrations as well as flue gas temperatures.The presence of O₂ can improve the SO₂ resistance of sorbent because O₂ can replenish the consumed oxygen active sites timely.H₂O reduced the SO₂resistance of the Fe₆Mn_0.8Ce_0.2O_y sorbent because H₂O competed with Hg⁰ for the adsorption active sites and consumed Ce O₂.Ce doping not only effectively improved the SO₂ resistance of Fe₆Mn_0.8Ce_0.2O_y sorbent,but also weakened the synergistic inhibition of H₂O.In the presence of SO₂,mercury removal products on Fe₆Mn_0.8Ce_0.2O_y sorbent included Hg₂SO₄,HgO,and HgSO₄,in which HgO was the main mercury removal product.The co-existence of SO₂and H₂O promoted more HgSO₄ generation and Mn O₂ was still the main active component for mercury removal.SO₂ in flue gas first reacted with Ce O₂ rather than Mn O₂ to generate Ce₂(SO₄)₃ thereby protecting the active site of Mn.Finally,the pseudo first-order kinetic model,pseudo second-order kinetic model,intra-particle diffusion model,and Elovich model were used to analyze the kinetic characteristics of mercury adsorption on the Fe₆Mn_0.8Ce_0.2O_y surface.The mass transfer factor proposed by Fulazzzky was used to study the mass transfer characteristics of mercury removal by Fe₆Mn_0.8Ce_0.2O_y.The results showed that the pseudo first-order,pseudo second-order,and Elovich models can fitting the adsorption results very well,but the intra-particle diffusion model has a poor fitting result.Therefore,it can be concluded that although mercury adsorption on the sorbent surface was composed of external mass transfer,intra-particle diffusion and surface adsorption,gaseous mercury chemical adsorption on active sites of sorbent was the control step.In addition,the external mass transfer also played an important role.The external mass transfer coefficient curves displayed"convex"shape.It indicated that the membrane mass transfer rate increased first and then gradually decreased to 0,and the influence of external mass transfer on mercury adsorption process gradually decreased.With increase of adsorption time,the internal mass transfer coefficient first rised and then decreased.The change trend of total mass transfer coefficient was consistent with the internal mass transfer coefficient.