Novel oxygen carriers for chemical looping combustion: La1−xCexBO3 (B = Co, Mn) perovskites synthesized by reactive grinding and nanocasting

Abstract

Chemical looping combustion is a novel technique with inherent separation of the greenhouse gas CO₂ from atmospheric nitrogen in combustion of gaseous fuels. The selection of a suitable oxygen carrier which circulates between two fluidized bed reactors is a key issue for the performance of this technology. In this work, high surface area perovskite-based oxygen carriers, LaCoO₃, LaCeCoO₃ and LaMnO₃, were synthesized by the reactive grinding method (S. Kaliaguine, A. van Neste, US Pat., 6 017 504, 2000; Applied Catalysis, A, 2001, 209, 345-358). LaMnO₃ was also prepared by another method designated as nanocasting. The perovskites were characterized using different methods such as XRD, SEM, N₂ adsorption, H₂-TPR, TPD-O₂ and finally the reactivity and stability of the carrier materials were tested in a CREC fluidized riser simulator using multiple reduction–oxidation cycles under two conditions: with low and high amount of CH₄ (0.5 and 10 ml) as a feed. It was found from TPD-O₂ and H₂-TPR studies that the LaMnO₃ perovskite, particularly the one prepared by the nanocasting method (LaMn-NC), has a high amount of available α-O₂ (surface oxygens) and high density of surface anion vacancies. Moreover, the Mn-based perovskites showed easier reducibility compared to the Co-based oxygen carriers. The results obtained under two different conditions of CH₄ pressure confirmed the strong dependency of the CLC results on this factor. Higher stability and no CO formation during the multiple redox cycles were indeed observed while using low pressure of CH₄. Higher reactivity of the carriers was however obtained while using higher CH₄ pressure but CO was detected in the products in this case. The higher reactivity of Mn-based perovskites could not only be due to the higher amount of surface oxygen and reducible sites present in the sample but also to the higher specific surface area of the perovskite. The formation of lanthanum and manganese silicate was observed for LaMn-NC which could be the reason for the reduction in CH₄ conversion observed between the eighth and tenth redox cycles. This compound could be as a result of interaction between remnant silica in the sample with La and Mn from the perovskite lattice. No coke formation was detected on the oxygen carriers however, some agglomeration of the particles occurred, as confirmed by SEM images, XRD data and N₂ sorption tests. Furthermore, XRD patterns confirmed that the structure of all perovskites remained unchanged after multiple redox cycles.