Sintering Deactivation And Inhibition Mechanisms Of Iron-based Oxygen Carrier In Chemical Looping Process

Chemical looping technology can avoid direct contact between fuels and air,whicn can achieve high concentration enrichment of CO₂.So it has an advantage in reducing the energy consumption of CO₂ capture.Oxygen carrier plays a very important role in the process of chemical looping reaction,and the iron-based oxygen carrier has been widely used in chemical looping reaction because of its low price and no harm to the environment.However,there are still many problems in the study of iron-based oxygen carrier.For example,the sintering deactivation and inhibition mechanisms of iron-based oxygen carrier in chemical looping process are still unclear.Solving the above problems is of great significance to promote the development of chemical looping technology.In view of the above problems,this dissertation studied the relationship between the sintering property and cyclic stability of iron-based oxygen carrier.Based on the above research results,the microstructure stability of iron-based oxygen carrier was improved by doping foreign ions.And the same method was used to guide the modification of natural iron-based oxygen carrier.In the aspect of microstructure evolution mechanism of iron-based oxygen carrier,the sintering property of Fe₂O₃ particles in the process of cyclic reaction was studied by using pure Fe₂O₃ particles as oxygen carrier.The results showed that with the increase of lattice oxygen loss,the surface particles of the oxygen carrier merged and the surface sintering degree increased during the reduction process.At the same time,the pore size of oxygen carrier gradually became larger and the specific surface area decreased.In the oxidation process,the imbalanced diffusion of Fe cations and O anions in the bulk phase of Fe_xO_y with outward Fe cations diffusion faster than inward O anions diffusion resulted in the formation of porous centers.The formation of porous centers could reduce the mechanical strength of Fe₂O₃particles.Under the stress action of volume and heat,Fe₂O₃ particles could be broken,resulting in more small particles.According to the microstructure evolution mechanism of pure Fe₂O₃ particles in cyclic reaction,the sintering property was studied by selecting Fe₂O₃-Al₂O₃ and NiFe₂O₄ as research objects.Combined with the reaction activity and characterization results,Fe cations diffusion and enrichment at the surface of cycled oxygen carrier was observed.The surface Fe is unstable under high reaction temperature,leading to accelerate thermal sintering.Deep reduction caused Fe cations to detach from the NiFe₂O₄ spinel structure,which led to forming a needle-like structure rich in Fe elements on its surface.These needle-like structures experienced severe surface sintering during cyclic reaction,thus reducing the reactivity of NiFe₂O₄.Based on the mechanism of sintering deactivation,the redox performance was improved by doping foreign ions in Fe₂O₃-Al₂O₃ and NiFe₂O₄.For NiFe₂O₄ sample,adding enough CeO₂ can effectively inhibit the phase segregation.The redox activity of CeO₂-NiFe₂O₄ in the continuous cycle process was more stable,Fe cations migration phenomenon was not occurred.The characterization analysis showed that the enhancement of redox stability for CeO₂-NiFe₂O₄ was attributed to the addition of CeO₂ to increase the concentration of oxygen vacancy in NiFe₂O₄,thus promoting the diffusion of O ions in the oxygen carrier and inhibiting the migration of Fe cations.For Fe₂O₃-Al₂O₃ sample,the addition of foreign ions CeO₂,La₂O₃ and SrO can improve the reaction activity of Fe₂O₃-Al₂O₃ oxygen carrier,and the order of acceleration is SrO>La₂O₃>CeO₂.In the process of 100 redox cycles,the surface structure of Sr-Fe₂O₃-Al₂O₃ was stable,and no sintering phenomenon occurred.The results of SEM-mapping showed that the addition of SrO can effectively inhibit the migration of Fe cations in Fe₂O₃-Al₂O₃.With the same amount,the addition of La₂O₃ and CeO₂ can not effectively inhibit the migration of Fe cations in Fe₂O₃-Al₂O₃.The modification of MAC iron ore and natural red mud was studied.The results showed that even adding 1%SrO can significantly improve the reactive activity and cyclic stability of MAC iron ore.In addition,Sr-MAC iron ore presented good redox activity after high temperature calcination.By adding CuO,the redox activity of red mud can be effectively improved.The TGA analysis revealed that CuO-Red mud have the characteristics of oxygen uncoupling in the redox reaction.When the addition amount of CuO was 30%,red mud showed the best redox performance.To sum up,this dissertation studied the sintering deactivation and inhibition mechanisms of iron-based oxygen carrier in chemical looping process,and putted forward modification method for the sintering deactivation of iron-based oxygen carrier.The oxygen carrying capacity and reactivity of natural red mud were improved by the addition of CuO.The results provide guidance for the preparation of iron-based oxygen carrier with high reaction activity.At the same time,it is of great significance to the application of natural red mud in the process of chemical looping.