Experimental and Thermodynamic Study on the Interaction of Copper Oxygen Carriers and Alkaline-Containing Salts Commonly Present in Ashes

As CO₂ emission is one of the most critical issues that causes global warming, methods have been developed for carbon capture and storage. Chemical looping combustion (CLC) and chemical looping oxygen uncoupling (CLOU) are unique and useful methods for direct separation of carbon dioxide in combustion. In CLC and CLOU, metal oxides are used as an oxygen carrier to transfer oxygen between an air and a fuel reactor. The fuel is oxidized with the released oxygen by the oxygen carrier used. When coal or any ash-containing fuels are used an interaction between ash-forming matters and oxygen carrier can occur which can cause to deactivation or agglomeration of the oxygen carriers. The composition of species in the ash and their amount can vary widely and also depend on the fuel used. Thermodynamic equilibrium calculations (TEC) can be used to predict the resulting compounds during in CLC and CLOU. This can help choosing the right fuel for the right oxygen carrier or vice versa. In this study, the interaction between common salt-based ash-forming matters present in ash and the widely used CuO oxygen carriers was studied both experimentally and thermodynamically. Experiments were carried out at 900 °C under both oxidizing and inert atmospheres using CuO or Cu₂O (CuO/Cu₂O) as the oxygen carrier and K- and Na-based carbonate, chloride, nitrate, phosphate, and sulfate to represent salt compounds present in the ashes. To observe the interaction of the oxygen carriers with each salt compound used, equal moles of copper oxide and a salt compound were mixed. In addition to this, the effect of salt compounds used for the interaction between oxygen carrier and a model ash was also investigated. NaSO₄ showed the harshest effect among the salts used since it caused a strong agglomeration. Generally, potassium-based salts did not affect the oxygen carriers directly and the results were consistent with the TEC. However, sodium-based salts showed significant effect on the system which differed from the TEC results.