Study On The Characteristics Of Lattice Oxygen Transfer Of An Iron-based Oxygen Carrier During Reduction In Chemical-looping Combustion

High performance oxygen carrier is critical to chemical looping combustion.Fe-based oxygen carriers show good comprehensive properties but low reduction reactivity,which limits their application prospects.The bulk phase is the main oxygen supply site for lattice oxygen.It is of great significance to clarify the transfer path and characteristics of lattice oxygen in bulk phase for further improving the oxygen transfer rate and enhancing the reactivity of oxygen carriers.In this thesis,a Fe₂O₃ oxygen-carrier was prepared,and the reduction process of the Fe₂O₃ oxygen carrier particle was studied on micro/nano scale by using advanced characterization methods such as high-resolution transmission electron microscopy（HR-TEM）,energy disperse spectroscopy（EDS）and electron energy loss spectrum analysis（EELS）.Firstly,phase characteristics,lattice characteristics,concentrations of O and Fe elements,valence states of Fe element of the reduced oxygen carrier particle at different positions along the radial direction were obtained.Secondly,the solid phase transformation during the reduction process of the Fe₂O₃ oxygen carrier particle was discussed.Finally,the release and transfer characteristics of lattice oxygen in the reduction process of the Fe₂O₃ oxygen carrier were determined by hypothetical deduction.The calibration results of the electron diffraction patterns show that the electron diffraction patterns at the boundary,the 1/2 radius and the center of the cross section of the reduced Fe₂O₃ oxygen carrier particle accord with the characteristics of the electron diffraction patterns of Fe₃O₄ and Fe O crystals.And thus,there are Fe₃O₄ and Fe O phases in these regions.The reduction degree at different radial positions of the cross section is basically the same as the average reduction degree of the bulk phase.Analysis results of Fourier filtered lattice images suggest that there are defects such as atomic plane interrupt,lattice dislocation and vacancy exist extensively at the boundary,the 1/2 radius and the center of the cross section of the reduced Fe₂O₃ oxygen carrier particle.These defects indicate the universality of lattice oxygen ion release in the bulk phase and the reduction occur in the whole bulk phase range of the Fe₂O₃oxygen carrier particle.The results of energy disperse spectroscopy analysis reveal that Fe and O contents are similar in a series of test points along the radial direction,and the content of elements change little from the edge to the center of the particle.It is further proved that the Fe₂O₃ oxygen carrier particle is reduced in the whole bulk phase range,and the oxygen release from the lattice is the same extent at different depths of the bulk phase.The results of electron energy loss spectrum analysis show that the concentritions of Fe and O elements at different depths in the radial direction are on the equivalent level.The analysis of the valence states of Fe element at different positions in the radial direction show that the differences between the valence states of Fe element at different positions in the radial direction are very small and thier reduction degree are consistent.Finally,in view of the transformation characteristics in the reduction process of the Fe₂O₃ oxygen carrier particle,the possible models of solid phase transformation during reduction were enumerated.By using hypothetical deduction,the typical characteristics of the three types of hypothesis transformation models were obtained.It is confirmed that the reduction process of the Fe₂O₃ oxygen carrier conforms to the uniform conversion model by comparing the observations and hypothesis.The release and transfer characteristics of lattice oxygen ions during the reduction of the Fe₂O₃oxygen carrier could be described as follow:driven by the chemical potential difference of oxygen concentration,lattice oxygen ions in the bulk phase migrate outward to the reaction interface,which is fixed on the boundary of the particle and does not move along with the reduction reaction process.There are 34 figures,11 tables and 129 references in this thesis.