Review of Oxygen Carriers for Chemical Looping with Oxygen Uncoupling (CLOU): Thermodynamics, Material Development, and Synthesis

Global warming and climate change are most likely linked to the increasing concentration of the greenhouse gas carbon dioxide (CO2) in the atmosphere. Additionally, the consumption of fossil fuels is predicted further to increase in the coming decades, particularly due to the rapid development of populous countries such as Brazil, India, and China. Therefore, it is imperative to develop and implement processes that avoid the emission of anthropogenic CO2. One possible midterm solution is carbon-dioxide capture and storage (CCS). In this context, the so-called chemical-looping combustion (CLC) process, that is, an emerging 3rd-generation CCS technology, is particularly attractive due to its very low predicted CO2-capture costs compared to the currently available technology (i.e., amine scrubbing). In CLC, lattice oxygen from a solid-state oxygen carrier is used to combust a hydrocarbon fuel, which yields, after the condensation of steam, a pure stream of CO2 suitable for sequestration. To allow the application of CLC to solid fuels, chemical looping with oxygen uncoupling (CLOU) has been proposed. Here molecular oxygen is provided by using the decomposition reaction of the oxygen carrier, thus, effectively the solid fuel is combusted in an oxyfuel mode. Importantly, a cornerstone of the CLOU process is the development of suitable oxygen carriers. In the first part of the review we discuss the thermodynamic properties of various CLOU materials. Subsequently, recent advances in the development of novel oxygen carriers are summarized. In particular, we focus on the physical and chemical properties of the new materials and the synthesis strategies employed. The review is concluded with an outlook on the CLOU process.