The intrinsic kinetic study on oxidation of a Cu-based oxygen carrier in chemical looping combustion

Abstract

As the kinetic influencing factors were not considered properly in the previous studies, the obtained activation energies of copper oxidation were discrepantly ranging from 15.0 kJ/mol to 100.0 kJ/mol. The gas diffusion in the crucible and sample layer, the surface reaction and the defect diffusion of copper cations all have an impact on the conversion process of copper oxidation samples. Using the high diameter-height ratio crucible in our self-made TGA could effectively eliminate the gas diffusion in the crucible and reduce the error of the kinetic tests. Based on the previous four-step oxidation mechanism, the kinetic parameters of copper oxidation are identified using a grain-scale kinetic model, coupling with a defect transport model. The potential kinetic impact factors including the particle polydispersity, gas diffusion, defect diffusion, surface reaction and particle sintering are adequately considered in this work. First, it is found that the copper oxidation is not dominated by the defect diffusion in the solid product layer, but governed by the surface reaction in the gas–solid interface. The copper oxidation is likely to be adsorption-limited during the surface reaction in the gas–solid interface, and the intrinsic activation energy is identified as 48.0 kJ/mol. The particle-scale model is then performed to investigate the impact of physical properties of oxygen carrier particles. It can significantly accelerate the conversion rate of copper oxidation by means of decreasing the particle size and increasing the porosity. However, it is not advisable to decrease the grain size too much when preparing the oxygen carriers, for this will only increase diffusion resistance inside the particle for small grain. Finally, it can be found that in most conditions of the air reactor, Cu-based oxygen carrier with the grain size of 220–300 nm exhibits the best oxidation performance.