Reduction Kinetics of Cu-Based Oxygen Carriers for Chemical Looping Air Separation

The knowledge of the kinetics of oxygen carriers is essential for the design of chemical looping air separation (CLAS) system. In this paper, the Cu-based oxygen carriers were prepared via a mechanical mixing method, and ZrO2, TiO2, and SiO2 were used as binder. The kinetics of the oxygen carriers in the process was determined using the distributed activation energy model via the thermogravimetry (TG) technique. TG experiments were performed in a thermal analyzer with heating rates of 5, 10, 15, and 20 degrees C/min. Preparation experiments were carried out first to obtain suitable sample masses and gas flow rates, to eliminate the effects of internal and external diffusion. The TG results show that the oxygen reduction and oxidation all have high reaction rates. The reaction rates of using different binders increase in the order of ZrO2 > TiO2 > SiO2, but the differences are minor. Besides the start and end points, the peaks of differential thermogravimetry (DTG) curves of oxygen-releasing regions all move forward to higher temperatures as the heating rate increases. Cyclic experiments show that the relativities of reduction and oxidation are stable during the cycles and the small dense grain of CuO has the tendency of growing bigger after 23 cycles, but the tendency is not apparent. The additions of binders can effectively inhibit the sintering and agglomeration of Cu-based oxygen carriers. The Starink approximation was used in the distributed activation energy model to calculate the distributed activation energies of the reduction reaction of oxygen carrier. The fitting lines of ln(beta/T-k) and 1/T using the approximation have high linear correlations. The distributed activation energies of CuO/TiO2, CuO/ZrO2, and CuO/SiO2 obtained are 155.0 +/- 2.2, 152.9 +/- 6.9, and 144.9 +/- 6.6 kJ/mol, respectively. The differences of distributed activation energies at different conversion ratios and among the three types of oxygen carriers are all very small. The reduction reaction of a Cu-based oxygen carrier is a one-step reaction, and the mechanism function of CuO reduction reaction was not affected by binders. The mechanism function is the nucleation and nuclei growth, and it is shown as f(alpha) = 3(1 - alpha)[-ln(1 - alpha)](2/3).