Processing and Characterisation of Fe-based oxygen carriers for chemical looping processes

Chemical looping reforming (CLR) and chemical looping combustion (CLC) are promising technologies with inherent CO2 capture for transforming fuels into syngas and energy respectively. Circulating oxygen carriers (OC) are used to transfer oxygen from mostly air to the fuel inside the process. Over the past years a variety of materials have been proposed for the role of oxygen carriers, ranging from bulk mineral powders to ceramic oxygen carrier particles engineered for shape, size and composition. A lot of attention has been focused towards the development of Ni-based oxygen carriers due to their good reactivity, conversion and mechanical stability. However the cost, susceptibility towards S-containing impurities and their toxicity are key drivers to develop Ni-free materials. Oxygen carriers based on Fe-oxides are promising because of their lower cost and diminished impact on health and environment. Nevertheless, they also need good thermo-chemo-mechanical properties and thus a sufficient lifetime to be applicable in industrial CL-processes. The longevity of the oxygen carriers in the coupled fluidised bed reactors can be limited by two factors, such as their fragmentation and attrition leading to smaller particles inhibiting their fluidisation as well as their chemical deactivation. Granulation by the industrial spray drying technique appears to yield oxygen carrier particles with high sphericity and good fluidization properties. In addition, a microstructure is obtained that on the one hand aims at thermo-chemo-mechanical integrity and high attrition resistance, and on the other hand at intimate contact between the solid and gaseous phases. However, spray-drying does not appear to be used for producing Fe-based OC thus far. This work emphasises the colloid chemistry of concentrated suspensions used for spray-drying, the development of ceramic Fe-based OC by granulation of primary raw materials and the subsequent processing and heat treatment of the oxygen carriers for hydrogen production by chemical looping. To reach the objectives, a systematic approach is used. At first the role of the preparation of the spray dry suspension and the granulation conditions on the morphology of the particles has been investigated. Secondly, the effect of the sinter process was studied. A correlation between tapped density, strength and attrition resistance of the heat-treated materials was observed. Finally, the chemical performance of these Fe-based oxygen carriers was examined in a small scale batch reactor. The chemical composition of the OC was (ex situ) monitored in view of the enhancement of the chemical properties and long term stability thereof by altering the composition and microstructure of synthesised oxygen carriers.