(135f) Correlation of the Physical and Solid State Chemistry Changes for a CuO-Fe2O3-Al2O3 Oxygen Carrier during Reduction with H2 and CO for Chemical Looping Combustion Applications

Chemical-looping combustion (CLC) is a novel combustion technology for energy generation that uses a metal oxide oxygen carrier (OC) to transport oxygen to a fuel source, avoiding direct contact between fuel and air. Combusting the fuel with an oxygen carrier is advantageous because ideally only carbon dioxide (CO₂) and water vapor are generated. After condensing the water, sequestration-ready CO₂ is produced making the technology desirable for increased carbon capture efficiency with decreased energy penalties. Development of efficient oxygen carriers is essential to successful CLC systems and a promising mixed metal oxide oxygen carrier has been developed at NETL consisting of CuO-Fe₂O₃-Al₂O₃. The metal oxide combination forms a unique Cu(Fe_2−xAl_x)O₄ spinel structure. The novel mixed metal OC possesses desirable properties including enhanced reactivity and physical stability. In addition, the OC also demonstrates exothermic behavior during reduction with fuels, which is essential to sustaining auto-thermal operation for energy generation applications. The oxygen carrier has been implemented successfully in pilot scale operation for CLC at NETL, showing that it is a proven candidate for CLC applications and merits further examination. A thorough understanding of the changes that occur during reduction and oxidation (redox) mechanisms are needed for implementation of modeling strategies and to deepen the understanding of materials used for this unique application.

The changes in physical and chemical properties during reduction and oxidation for the Cu(Fe_2−xAl_x)O₄ OC are unique and not thoroughly understood. The act of transferring oxygen to the fuel source induces the formation oxygen vacancies causing the structure to distort and ultimately change phase when enough oxygen has been depleted. The phase change to base metal and metal oxide components due to the stripping of lattice oxygen is often referred to as the reduction route, and comparably the oxidation route being the reverse process. With this noted, chemical properties can vary from solid-state solution changes including predominant phase, the reduction and oxidation routes, and component migration that can occur due to lattice distortion and reorientation. In addition, physical properties of the material can also be impacted including particle size changes during redox reactions, density changes associated with predominant phase, surface area, etc. This comprehensive study identifies and correlates these property changes with extent of solid oxygen carrier conversion for the development and implementation of accurate reduction and oxidation models for further use in process scale up and optimization strategies. A combination of various characterization techniques including in-situ X-ray diffraction, Scanning Electron Microscopy, Thermogravimetric analysis coupled with mass spectrometry and others are used to establish correlations between the property changes with the metal oxide during reduction with CO/H₂ and oxidation with O₂.