Investigation on the decrease in the reduction rate of oxygen carriers for chemical looping combustion

Copper oxide on alumina is often used as oxygen carrier for chemical looping combustion owing to its very high reduction rates at lower temperatures and its very good mechanical and chemical stability at not too high temperatures. In this work, the redox kinetics of CuO/Al2O3 have been measured and analysed with many consecutive reduction (50% H2, 50% N2) and oxidation (air) cycles at different temperatures using thermogravimetric analysis. In all the redox cycles, a cyclic steady state in the redox kinetics is observed after several redox cycles, where a sudden decrease in the reaction rate is observed after a certain solids conversion, typically around 75%, depending in particular on the operating temperature. Models developed in the literature fail at predicting these redox kinetics and they are mostly shrinking core/grain type models with an ad-hoc combination and modification of chemical reaction and gas phase diffusion. The oxygen carrier was characterized in detail with different techniques including N2 physisorption, XRD and SEM-EDX to investigate morphological changes in the particle over the redox cycles in order to better understand why all the models developed in literature fail. After the study we can conclude that i) gas-phase diffusion is not playing a limiting role on the redox kinetics despite significant morphological changes in the particle; ii) migration of copper from the inside of the particle to the surface of the particle does not occur or does not influence the kinetics; iii) oxygen uncoupling is not influencing the kinetics being equally fast for Cu2O and CuO, although it can be significant for the final maximum particle conversion; iv) solid phase oxygen diffusion limitations may be responsible for the observed drop in reaction rate

Gas-solids kinetics of CuO/Al2O3 as an oxygen carrier for high-pressure chemical looping processes : the influence of the total pressure

Copper oxide on alumina is often used as oxygen carrier for chemical looping combustion owing to its very high reduction rates at lower temperatures and its very good mechanical and chemical stability at not too high temperatures. In this work, the redox kinetics of CuO/Al2O3 have been studied at elevated pressures and temperatures. All the experiments have been started under the same initial conditions to assure the same starting point. While other studies reported a negative effect of the total pressure on the redox kinetics, this study shows that this negative effect of the pressure is most probably caused by external mass transfer limitations in previous studies. Additionally, as long as external mass transfer limitations are prevented, the total pressure at which the reduction is performed does not affect the redox kinetics nor the morphological and chemical structure of the oxygen carrier. The sudden decrease in the reduction rate at higher particle conversions was not influenced by the operating pressure and was attributed to limitations in the spinel reduction kinetics