(686b) Ni/la-Al2O3 Oxygen Carrier for Fluidized Bed Chemical-Looping Combustion

Chemical-looping combustion (CLC) is a novel concept in the area of CO2 capture. In this process, fossil fuel combustion is carried out in two fluidized bed reactors: a fuel reactor and a regenerator. In the fuel reactor, the fuel is burnt using a solid oxygen carrier. Thus, the exhaust from this reactor contains only CO2 and H2O. Pure CO2 can be obtained when the H2O vapor is condensed. The reduced carrier is reoxidized in regenerator with air and cycle back to fuel reactor. CLC also helps minimizing NOx emissions, since the fuel is burned in the absence of nitrogen without a flame. This study deals with the development of a La modified Ni/La-ãAl2O3 oxygen carrier suitable for a CLC process.

In CLC, the most important characteristic of an oxygen carrier is its reactivity and stability under cyclic operation. To investigate these matters successive TPR and TPO experiments were developed in the context of the present study. In cyclic TPR/TPO experiments, the reduction and oxidation profiles of Ni/La-ãAl2O3 sample remains unchanged and the amount of Ni reduction also shows a stable behavior. Approximately 93 % nickel conversion was achieved using the La modified sample, while the value was 76 % range for the unmodified carrier. Therefore, the addition of La aids the formation of easily reducible nickel oxides minimizing nickel support interaction and formation of non-reactive nickel aluminates.

The pulse chemisorption results further confirmed the stable behavior of the sample in consecutive redox cycles. Although both unmodified and modified samples showed a stable metal dispersion with successive redox cycles, a higher dispersion was observed in the modified sample versus the unmodified samples, from 2.5 % to 0.9%. This indicates that La helps to increase the dispersion of nickel. Thus, the addition of La alters the metal surface modifying the degree of interaction between Ni and alumina support, maintaining a consistent metal dispersion during the repeated redox process. It is also apparent that the metal crystal size of the Ni/La-ãAl2O3 sample remained unchanged over repeated cycles, an indication of the absence of agglomeration of nickel crystals.