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

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

Authors 

De Lasa, H. - Presenter, University of Western Ontario


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 NOx 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 950οC. 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.