Preparation of Large Granular Cu-Based Oxygen Carriers by Mechanical Mixing for Packed Bed Chemical-Looping Combustion

Abstract

Large granular oxygen carriers are proposed in packed-bed chemical looping combustion (CLC) reactors to facilitate the separation of fuel residues and oxygen carriers for solid fuels (especially biomass and combustible solid wastes) and avoid very large pressure drops for gas fuels, which require that the granules must have a sufficiently high mechanical strength. The effects of carrier composition and preparation conditions of the mechanical mixing method are investigated in order to develop large granular Cu-base oxygen carriers not only exhibiting high reduction and oxidation rates but also maintaining the mechanical properties for a high number of oxidation-reduction cycles. The results show that the inert species, calcination temperature, and CuO/inert ratio have effects on the mechanical properties of the fresh oxygen carrier granules. SiO2 is the preferred inert binder and supporter among four types of inert materials, and the calcination temperature should not exceed 10-50 degrees C because of copper and copper oxides with lower melting points. The crushing strength of the CuO/SiO2 granules is increased by increasing the CuO/SiO2 ratio. The reactivities of the large granules with CH4 and air are studied in a thermogravimetric analyzer. The granules are also characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) to support the experimental results. All CuO/SiO2 granules prepared by mechanical mixing exhibit excellent chemical stabilities and high reactivities during the cyclic process at 800 degrees C, but deterioration of the recyclability of the granules is found during a multicycle test at 850 degrees C. The XRD results indicate that the oxygen carrier does not change its crystal form, and there are no chemical changes that occurred after 30 cycles at 800 degrees C, but the reoxidized granules contain copper after 20 cycles at 850 degrees C. The micrographs of SEM show that there is no obvious sintering on the surfaces of the granules of 30 cycles at 800 degrees C, but sintering of the granules of 20 cycles at 850 degrees C occurs. And the used large granules after 30 oxidation-reduction cycles undergo degradation of the mechanical properties, but the crushing strength of the used granules is not influenced by the number of cycles. These results lead to the conclusions that SiO2 as inert material, CuO/SiO2 ratio of 4:1 and 3:2, and calcination temperature of 950 degrees C for the wet ball milling method are preferred.