Effects of Coal Ash on CuO as an Oxygen Carrier for Chemical Looping with Oxygen Uncoupling

Chemical looping with oxygen uncoupling (CLOU) features the natural release of gaseous oxygen from oxygen carriers to efficiently burn solid fuels, producing a nearly nitrogen-free CO2 stream for carbon capture while eliminating the need to install energy-intensive air separation units. When it comes to the combustion of coal with CLOU, there is a risk that coal ash may interact with oxygen carriers, which could lead to undesired agglomeration or form complexes that do not efficiently transport oxygen. To investigate the potential influence of coal ash on Cu-based oxygen carriers, the equilibrium compositions of coal ash/CuO systems were calculated through thermodynamic modeling and the oxygen transport capacity of CuO in the presence of coal ash during redox cycles was measured using a thermogravimetric analyzer (TGA). Two possible mechanisms of coal ash/CuO interaction were predicted: (1) the formation of the molten phase at a lower temperature driven by the alkali species in ash and Cu2O and (2) the solid–solid reaction between ash-derived Al2O3/Fe2O3 and CuO/Cu2O to yield Cu–Al and Cu–Fe complexes with high equilibrium O2 partial pressures. The effects of coal ash on the agglomeration propensity and oxygen transport efficiency of CuO depend strongly upon ash composition, temperature, ash/CuO ratio, and gas atmosphere. Consistent with thermodynamic modeling predictions, TGA tests revealed the significant deactivating effect of Illinois #6 (IL6) coal ash and the formation of a complex compound. Sintering was detected for the IL6 ash/CuO mixture at 900 °C. The formation of CuAl2O4 and CuFe2O4 in the IL6 ash/CuO mixture was further confirmed by X-ray diffraction, and scanning electron microscopy–energy-dispersive X-ray spectroscopy displayed the adhesion and spread of CuO on the surface of IL6 ash. Powder River Basin coal ash was theoretically predicted and experimentally proven to be much less risky to CuO.