Relevance of the catalytic activity on the performance of a NiO/CaAl2O4 oxygen carrier in a CLC process

Abstract

This paper presents a detailed investigation related to the physical and chemical changes underwent by a low Ni content oxygen carrier during 90h of continuous operation in a CLC plant [1]. The oxygen carrier reached a combustion efficiency of 99% with a metallic Ni inventory of 17kg/MWth, which was the lowest value found in literature for any kind of Ni-based oxygen carrier. The present study was carried out in order to find the reasons by which this material exhibited a different combustion behavior in relation to other Ni-based oxygen carriers. Fresh and after-used particles were characterized by several techniques. XRD, TPR-TPO and XPS techniques proved that the use of CaAl2O4 as oxygen carrier support avoided the formation of NiAl2O4, which is a less reactive Ni-based compound than NiO and always formed in other Ni-based materials. The fact that all the NiO impregnated was found as free NiO in the oxygen carrier particles implied that this material maintained a high reactivity after 90h of continuous CLC operation. The results obtained in the batch fluidized bed reactor have revealed the great relevance of the catalytic activity of the metallic Ni on the global fuel gas combustion process. The low Ni content present in this material makes necessary the use of very high reduction solid conversion to have enough metallic Ni in the fuel reactor to catalyze the CH4 reforming reaction, and therefore to obtain high combustion efficiencies. This result will become very relevant for the design of a CLC unit using this Ni-based material as oxygen carrier.