Energy & Fuels, Vol.33, No.3, 2502-2515, 2019
Experimental and Thermodynamic Study on the Interaction of Copper Oxygen Carriers and Oxide Compounds Commonly Present in Ashes
The chemical looping combustion (CLC) and chemical looping oxygen uncoupling (CLOU) processes are unique and efficient methods for the direct separation of carbon dioxide in combustion. In these processes, metal oxides are used under reducing atmosphere as an oxygen carrier to transfer oxygen between air and a fuel reactor. The fuel is converted by oxygen provided by the oxygen carrier. In the case of using coal or any ash-containing fuel, interaction between coal-derived ash and the oxygen carrier is likely to occur and can lead to deactivation and agglomeration of the oxygen carriers. As the amount of the possible compounds and compositions of ash can vary widely, thermodynamic equilibrium calculations can be used to represent the formed compounds during the CLC process to reveal the interaction between the oxygen carrier and the commonly present oxide compounds in ash. In this study, the interaction between the oxide compounds commonly present in ash and CuO oxygen carriers was studied both experimentally and thermodynamically. CuO is a widely used oxygen carrier with CLOU properties, the ability to release gaseous oxygen under inert atmosphere. Experiments were carried out at 900 degrees C under both oxidizing and inert atmosphere using CuO or Cu2O (CuO/Cu2O) as the oxygen carrier and SiO2, Al2O3, Fe2O3, CaO, and K2O to represent the oxide compounds present in ashes. To observe the interaction of the oxygen carriers with each oxide compound used, equal moles of copper oxide and oxide compound were mixed. Further, oxide compound fractions with the elemental composition relevant to coal ash were mixed with oxygen carriers to investigate the interaction under conditions approaching realistic operation. In all cases, a significant amount of copper oxides survived without any interaction. However, it was observed that silicate-based formations, especially potassium silicates, lead to strong agglomeration which most likely would decrease the lifetime and oxygen-releasing ability of the oxygen carriers. As the results showed that the thermodynamic equilibrium-based calculations were well in line with the experiments, these calculations can be a good first approach in these types of investigations.