Predicting and alleviating coal ash-induced deactivation of CuO as an oxygen carrier for chemical looping with oxygen uncoupling

Chemical looping with oxygen uncoupling (CLOU) using CuO oxygen carrier enables the release of gaseous oxygen in the fuel reactor for the efficient conversion of solid fuels, achieving inherent CO2 separation while avoiding the energy penalty associated with air separation units. In terms of the coal-fired CLOU, certain types of coal ash were found to interact with CuO and compromise the performance CuO, while some other types of coal ash seemed stable. To predict the interaction products based on ash composition and the major solid phase transformations of coal ash/CuO mixture, the phase diagrams of SiO2-Al2O3-CaO-Fe2O3-CuO system were developed through thermodynamic modeling. According to the phase diagrams, coal ash can be categorized into three types with different interaction products, and the Type 1 ash responsible for the deactivation of CuO can be described as Al2O3/CaO > 1.82 (mass ratio). CaO addition was proposed as an approach to alleviate the ash-induced CuO deactivation by forming the more thermodynamically favored CaAl2Si2O8 (anorthite) than CuAl2O4. The redox cycles of various CuO/coal ash/CaO mixtures were performed to test the modeling predictions and evaluate the effects of CaO addition. The experimental results validated the modeling predictions on the real coal ashes, and CaO addition proved to significantly curb the CuO deactivation in the presence of Type 1 ashes. XRD analysis verified the addition of CaO into the CuO/coal ash mixture inhibits the formation of CuAl2O4 by producing CaAl2Si2O8, but the decrease in the amount of Cu-Fe complex actually results from the formation of Ca2Fe9O13.