Multi-scale kinetic study of the oxygen uncoupling of CuO oxygen carrier in chemical looping

Abstract

As the kinetic influencing factors were not considered properly in the previous studies, the obtained oxygen uncoupling kinetics of CuO oxygen carrier varies greatly. In this paper, a multi-scale methodology is proposed to explore the oxygen uncoupling kinetics of CuO oxygen carrier through well-designed thermogravimetric experiments and multi-scale modelling. In order to minimize the effects of gas diffusion, a self-made thermogravimetric analyzer (TGA) is built firstly to measure the oxygen uncoupling rates of CuO samples. A multi-scale kinetic analysis model in which many influencing factors are considered is then established to analyze the experimental results. The kinetic parameters are thus extracted. Finally, based on the obtained intrinsic reaction kinetics, the effects of the physical structure of CuO oxygen carrier particle on its oxygen uncoupling rate are analyzed. It is found from this study that under typical CLOU conditions, neutral and singly charged copper vacancies are the dominant point defects in the product layer of CuO, and the chemical diffusions of these defects are NOT the rate limiting step. The gas diffusion in TGA crucible, the gas diffusion in sample layer, the polydispersity of the samples and the sintering of the samples are all important to describe the conversion process of the CuO samples. Among these, the effect of gas diffusion in the sample layer is less significant than that in the crucible. It is found that there is an optimal grain size (that is, 245 nm) of CuO oxygen carrier particle to maximize the averaged conversion rate of the particle, and the intrinsic activation energy and the pre-exponential factor of the oxygen uncoupling of CuO are determined to be 281.23 kJ/mol and 5.679 × 1016 mol/(m2s), respectively.