Preventing Agglomeration of CuO-Based Oxygen Carriers for Chemical Looping Applications

Abstract

Chemical looping combustion (CLC) is a promising alternative to the conventional combustion-based, fossil fuel conversion processes. In CLC, a solid oxygen carrier is used to transfer oxygen from air to a carbonaceous fuel. This indirect combustion route allows for effective CO2 capture because a sequestrable stream of CO2 is inherently produced without any need for energy-intensive CO2 separation. From a thermodynamic point of view, CuO is arguably one of the most promising oxygen carrier candidates for CLC. However, the main challenge associated with the use of CuO for CLC is its structural instability at the typical operating temperatures of chemical looping processes, leading to severe thermal sintering and agglomeration. To minimize the irreversible microstructural changes during CLC operation, CuO is commonly stabilized by a high Tammann temperature ceramic, for example, Al2O3, MgAl2O4, and so forth. However, it has been observed that a high Tammann temperature support does not always provide a high resistance to agglomeration. This work aims at identifying the descriptors that can be used to characterize accurately the agglomeration tendency of CuO-based oxygen carriers. CuO-based oxygen carriers supported on different metal oxides were synthesized using the Pechini method. The cyclic redox stability and agglomeration tendency of the synthesized materials were evaluated using both a thermo-gravimetric analyzer and a lab-scale fluidized bed reactor at 900 °C using 10 vol % H2 in N2 as the fuel and air for re-oxidation. In order to study the diffusion of Cu(O) during redox reactions, well-defined model surfaces comprising thin films of Cu/CuO and two different supports, namely, ZrO2 and MgO, were prepared via magnetron sputtering. Energy-dispersive X-ray spectroscopy on focused ion beam-cut cross-sections of the thin films revealed that Cu atoms have a tendency to diffuse outward through the support material under redox conditions. The support that inhibits the outward movement of Cu(O), that is, avoiding the presence of low melting Cu on the oxygen carrier surface, is found to provide the highest agglomeration resistance. The support MgO was found to possess such diffusion characteristics.