The high temperature decomposition of hematite under reactive gas atmospheres: for use in chemical looping combustion

Abstract

Chemical looping combustion (CLC) is an alternative method of combustion, which enables the efficient recovery of heat and process gas whilst also producing a high CO2 concentration product gas potentially suitable for carbon capture and storage (CCS). This is a potentially important option for greenhouse gas reduction in future hydrocarbon combustion technology. Fundamental to the process are oxygen carriers; metal oxides that transfer oxygen from the air reactor to the fuel reactor where CO2 and steam are released. The coupled gaseous reduction and oxidation of iron oxides hematite (Fe2O3) and magnetite (Fe3O4) has been identified as a promising option for low cost oxygen carrier material. The aim of the present study is to understand the microstructural basis of key properties of oxygen carrier/metal oxide materials, such as reactivity and strength, and the how these may be influenced by the process conditions. This has been performed by studying the structural changes taking place in single stage reduction and oxidation of hematite and magnetite samples, and examination of cross sections at the reaction interface.Investigations have been undertaken using a range of reduction conditions of interest to CLC at 800-1100°C and CO:CO2 ratios of 0.01:1 to 0.33:1. The samples were reduced for relatively short times, commensurate with the residence times in fluidised bed reactors used in the CLC process. Examination using optical and scanning electron microscopy techniques have revealed that different reaction product morphologies are formed on reduction of hematite depending on the reaction temperature and CO:CO2 ratio. These include a porous magnetite and dense platelet or “lath” type morphologies. The critical conditions for the formation of these product structures have been identified in the present study. Porous magnetite formation is important to the application of CLC as the high surface area is associated with high reactivity. The porous magnetite structures are shown to form gas pores from instabilities created at the initially dense material through reaction with the gas.The oxidation of magnetite is illustrated to occur through the formation of dense hematite layers on the particle surface. This dense hematite forms through lath type shear transformations or solid state diffusion of iron or oxygen through the product layer.Cyclic reduction and oxidation experiments have been undertaken to simulate the conditions experienced by the hematite oxygen carrier in the CLC process. It has been shown that the initial reduction cycle can have a major effect on the subsequent reactivity of the oxide carrier. The observations also indicate that the effective life of the hematite oxygen carrier is dependent on reduction gas composition, temperature and particle size.The present systematic study of reduction and oxidation reactions on natural hematite has revealed a number of phenomena previously not reported in the literature and has contributed to more fully understanding the changes taking place in these oxygen carriers in the CLC process. The results provide a basis for favourably manipulating the properties of solid oxygen carriers to improve materials performance in CLC applications.