The impact of manufacturing methods on the performance of pelletized, iron-based oxygen carriers for fixed bed chemical looping hydrogen in long term operation

The successful application of fixed bed chemical looping hydrogen in industrial scale demands highly stable oxygen carrier pellets. The goal of this work was the realistic long-term characterization of the pelletized oxygen carriers in a fixed bed reactor environment. TGA characterization of an iron-based powder sample containing 5 wt% Al2O3 for 100 cycles showed fast reaction speeds and high oxygen exchange capacity. Characterization of the same sample as pellets in a fixed bed reactor revealed a high temperature gradient, decreased oxygen exchange capacity and a rapid increase in system pressure due to powder formation. The pellet lifetime was prolonged to 100 redox cycles when the material was stored in reduced state at the respective operating temperature. The increased stability of the reduced material could be attributed to the higher crushing strength of the iron and wüstite phase in contrast to magnetite. The increase of the inert portion and of the pellet calcination temperature led to an improved starting material durability but did not influence the mechanical stability after 100 cycles. The results of this study indicate that the long-term redox cycling seems to outweigh the impact of oxygen carrier preparation methods that may be overvalued in the long run.