Kinetics of CuO/SiO2 and CuO/Al2O3 oxygen carriers for chemical looping combustion

Copper oxide supported on silica and supported on alumina are often used as oxygen carriers for chemical looping combustion owing to their very high reduction rates at lower temperatures and their very good mechanical and chemical stability at temperatures below 1000 °C compared to other oxygen carriers. In this work, a comprehensive experimental study has been carried out to better understand the reaction mechanism and quantitatively describe the reaction kinetics of the oxygen uncoupling reaction and the reduction and oxidation reactions under different reaction conditions. First, the oxygen uncoupling and reduction reaction kinetics of the CuO/SiO2 oxygen carrier was studied. A shrinking core type model (SCM) was developed that can well describe the oxygen uncoupling reaction rate and final conversion. Subsequently, a SCM and a simplified pseudo-homogeneous model was developed to describe the reduction kinetics of CuO/SiO2. Subsequently, the study was extended to investigate the reduction kinetics of CuO/Al2O3, where it was observed that the formation of tenorite spinel (CuAl2O4) and cuprite spinel (CuAlO2) strongly affects the overall reduction kinetics. Assuming that the reduction of CuO to Cu is independent of the support, the pseudo-homogeneous model was extended to include the reduction and oxidation kinetics of the spinel compounds, with which the experimentally determined redox kinetics could be well described. Regarding the CuO on Al2O3, The maximum temperature reached was 1000 °C in thermogravimetric analysis (TGA) without observing any sintering or melting effects. It can be due to the good stability with the Al2O3 support and the relatively small amount of CuO (13%). However, for CuO/SiO2 (70% CuO), the maximum temperature tested in the TGA was 900 °C, since at higher temperatures the sample was melting, due to the lower melting point of Cu. The main results of the study can be summarized as: (i) the oxygen uncoupling and reduction/oxidation of CuO/SiO2 and CuO/Al2O3 has been elucidated; (ii) a grain model that can describe the oxygen uncoupling and reduction reactions of CuO for different operating conditions has been developed; (iii) a pseudo-homogeneous grain model that can describe the redox kinetics of CuO/SiO2 and CuO/Al2O3 has been developed.