NiO/Al2O3 oxygen carriers for chemical-looping combustion prepared by impregnation and deposition-precipitation methods

Ni-based oxygen carriers (OC) with different NiO content were prepared by incipient wet impregnation, at ambient (AI), and hot conditions (HI) and by deposition-precipitation (DP) methods using gamma-Al2O3 and alpha-Al2O3 as supports. The OC were characterized by BET, Hg porosimetry, mechanical strength, TPR, XRD and SEM/EDX techniques. Reactivity of the OC was measured in a thermogravimetric analyzer and methane combustion selectivity towards CO2 and H2O, attrition rate, and agglomeration behavior were analyzed in a batch fluidized bed reactor during multicycle reduction-oxidation tests. XRD and TPR analysis showed the presence of both free NiO and NiAl2O4 phases in most of the OC. The interaction of the NiO with the alumina during OC preparation formed NiAl2O4 that affected negatively to the OC reactivity and methane combustion selectivity towards CO2 and H2O during the reduction reaction. The NiO-alumina interaction was more affected by the support type than by the preparation method used. The NiO-alumina interaction was stronger in the OC prepared on gamma-Al2O3. The OC were evaluated in the fluidized bed reactor with respect to the agglomeration process. OC prepared by the All and HI methods with NiO contents up to 25 wt%, OC prepared by the DP method on gamma-Al2O3 with NiO content lower than 30 wt%, and OC prepared by the DP method on alpha-Al2O3 with a NiO content lower than 26 wt% did not agglomerated. OC that agglomerated showed an external layer of NiO over the particles. It seems that the most important factor affecting to the formation of the external NiO layer on the OC, and so to the agglomeration process, was the metal content of the OC. The attrition rates of the OC prepared using gamma-Al2O3 as support were higher than the ones prepared using alpha-Al2O3 as support, and in general the attrition rates of all the OC were low. The OC prepared by AI, HI or DP methods on alpha-Al2O3 as support had appropriated characteristics to be used in the chemical-looping combustion process. (C) 2008 Elsevier Ltd. All rights reserved.