Crack propagation and attrition mechanism of oxygen carriers in chemical looping process

[Display omitted] •The contribution of three stresses to OC’s attrition is quantitatively analyzed.•The attrition type of OCs under different conditions are determined.•Phase separation reduces the mechanical strength of OC.•The crack propagation process on the surface of OC is illustrated.•Crack propagation and Fe-rich effect result in OC fragmentation. Chemical looping combustion (CLC) is one of the most promising CO2 capture technologies, where oxygen carriers (OCs) circulate between a fuel reactor and an air reactor and is mainly responsible for providing lattice oxygen and heat to the fuel reactor. The lifetime of OC is an important parameter that has a great effect on CLC’s economic performance. Attrition is one of the most crucial reasons that shortens OC’s lifetime. In this study, attrition behavior of an iron-based OC in relatively long-period experiments was carried out in a fluidized bed reactor, and cold attrition experiment (CAE), hot attrition experiment (HAE) and reactive attrition experiment (RAE) were tested to quantify the contribution of mechanical stress, thermal stress and chemical stress to OC’s attrition. Morphology of cracks, surface elements distribution and crystal phase of OC in different stages during cyclic reactions were studied in detail, and the attrition mechanism of OC is therefore proposed. For COs produced by powder mixing and sintering, results show that the contribution of chemical stress to the attrition of OC is a dynamic increase process, and it completely dominates the attrition after the 40th cycle. The attrition rate of OC under different conditions follows RAE > HAE > CAE and attrition type in CAE and HAE mainly follows surface abrasion, while the attrition in RAE is a mixed attrition mechanism dominated by bulk fracture. Analysis indicates crack propagation and the Fe-rich effect caused by the continuous oxidation–reduction reaction together lead to comminution of OC.