Modeling of Cu Oxidation in Air Reactors of Chemical Looping Combustion Systems
An oxidation model is developed that describes Cu (copper) oxidation kinetics to CuO (cupric oxide) for applications in the air reactor of chemical looping combustion (CLC) systems. The model , which is based on the Wagner theory of oxidation , is developed by considering the fluxes of interstitial and vacancy defects in the cation sublattice of Cu2O (cuprous oxide) and CuO. Defects in the anion sublattice are a priori included in the model , however are subsequently found to be too small in concentration and diffuse too slowly to contribute significantly to the oxidation of Cu. The oxidation model presented here is capable of predicting Cu oxidation rates and relative thicknesses of the resulting Cu2O and CuO oxide layers as a function of the partial pressure of oxygen in the ambient and temperature. Experiments were performed in a thermogravimetric analyzer (TGA) at conditions relevant to air reactors of CLC systems to obtain data that was used to characterize parameters in the model. Experiments were performed on copper spheres having diameters in the range 15 to 25 μm , at temperatures between 500 and 900 oC and at 1 atm. The gaseous environments contained 0.3% , 6% and 21% O2 , by volume , with the balance N2. Based on this model , the temperature and oxygen partial pressure dependencies of the oxidation rate as well as overall rate constants were determined and compared to published results for Cu oxidation. A sensitivity analysis was performed in order to assess the influence of defect diffusivities and formation energies as well as particle diameter and surface area on oxidation rates. Based on our understanding of defects and their role in Cu oxidation , insights regarding the design of copper particles for use in CLC have been made. Assuming that oxidation is the rate-limiting process in CLC , the recirculation rate and copper loadings have also been estimated when CuO is used to oxidize a typical coal.
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