(16a) Ni-Co/La-ãAl2O3 Oxygen Carrier for Fluidized Bed Chemical-Looping Combustion

Chemical-looping combustion (CLC) has been considered as one of the most promising technologies in capturing CO₂ from fossil fuel based power generation. In CLC, fuel combustion is carried out in two interconnected circulating fluidized bed reactors: a fuel and an air reactor. An oxygen carrier (OC) circulates between these two reactors supplies oxygen for fuel combustion, thereby preventing dilution of the CO₂ in the flue gas with the nitrogen from the air. Consequently, CO₂ is delivered without any further energy penalty for separation. CLC also helps minimizing NO_x emissions, since the fuel is burned in the absence of nitrogen without a flame. Even though, the technology is feasible enough for commercialization, large scale operation is still contingent upon the availability of suitable OC. In accordance with the prompt ability to react with fuel gas, a good OC should exhibit quick re-oxidation in contact with air, substantial oxygen storage, fluidizability and mechanical strength. This study was aimed at developing a material with the above characteristics. To this end a Co promoted Co-Ni/La-gAl₂O₃ carrier material was synthesized and evaluated.

The oxygen carriers of this study were prepared by impregnation method under vacuum conditions. Prior to nickel/cobalt loading, gAl₂O₃was first stabilized using 5 % La. The resultant support material La-gAl₂O₃ was then modified with 1 % Co. Finally, 20 % Ni was loaded using successive impregnation, 2.5 % Ni in each cycle.

As indicated above, the most important characteristic of oxygen carrier is its reactivity and stability at the high temperature of the cyclic redox process. To investigate these matters successive TPR and TPO experiments were carrier out at 950^οC. In cyclic TPR/TPO experiments, the reduction and oxidation profiles of Ni-Co/La-gAl₂O₃ remains unchanged and the amount of Ni reduction also shows a stable behavior. Approximately 80 % nickel conversion was achieved using the Co modified sample, while this value was 71 % range for the unmodified carrier. Therefore, the addition of Co aids the formation of easily reducible nickel oxides species minimizing nickel support interaction and formation of non-reactive nickel aluminates. The pulse chemisorption results further confirms the stable behavior of the sample in consecutive redox cycles. A well dispersed state of Ni crystal, observed in pulse chemisorption, also signifies the absence of agglomeration of Ni crystals. This also indicates that Co helps to increase the dispersion of nickel on La-gAl₂O₃ support. Thus, the addition of Co modifies the metal surface changing the degree of interaction between Ni and the 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-gAl₂O₃ sample remains unchanged over repeated cycles, a further indication of the absence of agglomeration of nickel crystals.