| Mixed ionic-electronic conducting perovskite membrane(MIEC) is mainly used for gas separation at high temperature as a new type of inorganic membrane. At a certain temperature, oxygen ions can migrate to the low oxygen gradient side from the high oxygen gradient side through oxygen vacancy. Coupled oxygen transport membrane reactor and integrated gasification combined cycle technology, on the one hand, could provide pure O2 for promoting the oxygen enriched combustion of fossil fuel; on the other hand, would reduce the production cost and be in favor of CO2 capture and storage. Ideal oxygen permeable materials need not only high oxygen permeation flux but also good structural stability and chemical stability. However, for the practical application of this technology, it needs using CO2 as sweep gas for diluting oxygen partial pressure at the permeate side, indicating that the oxygen permeable membrane must possess high CO2 corrosion resistance. Therefore, the development of CO2 tolerant oxygen permeable materials is the basis of the application of MIEC-IGCC technology.In this study, Ba Co0.88Nb0.12O3-δ(BCN) membrane was choosed for the basic membrane materail because of its excellent oxygen permeability. In order to improve the CO2 corrosion resistance of BCN based membrane, La and Fe was selected to dope at A-site and B-site of BCN, respectively. Thus, Ba1-x Lax Co0.88Nb0.12O3-δ(BLCN)and Ba0.8La0.2Co0.88-y Fey Nb0.12O3-δ(BLCFN) membranes were designed and synthesized by solid phase reaction method, then the effect of element doping on the oxygen permeability, rate-limited step of oxygen permeation, structural stability and CO2 tolerance was systematically investigated.The result of oxygen permeation measurement at 850 °C shows that oxygen permeation flux of BLCN membranes decreases with the increase of La content, and the oxygen permeation flux of BLCN membrane increases with the increase of oxygen partial pressure at the feed side. According to the oxygen partial pressuredependence of oxygen permeation flux, the power index n was deduced via Wagner theory. The value of 0 < n < 0.5 suggests that the rate-limiting step of oxygen permeation through the BLCN membrane is controlled by both the surface reaction and the bulk diffusion.The reactivity of membrane of La-doped BLCN under CO2 atmosphere at high temperature was investigated. At 850 °C, the membranes were calcined in the various concentration of CO2 atmosphere for different time and then XRD measurement was performed for the calcined membranes. XRD results show that the structural stability of BLCN materials improved with increasing La doping content. When 5 % CO2 was introduced, no obvious carbonate formed on the surface of BLCN membrane. Even for the membrane calcined under pure CO2 for 5 h, no obvious carbonate can be observed for the BLCN-0.5 membrane, indicating that the membrane with high La content exhibits excellent CO2 corrosion resistance in “static state” model.The initial oxygen permeation flux of BLCN membranes would be reduced with the doping of La, however, the oxygen permeability of BLCN membranes swept by CO2 is more stable with the increase of La-doped content. Particularly, the oxygen flux of BLCN membrane with La content ≤ 0.2 will drop to zero with pure CO2 sweeping for 10 h, whereas the oxygen flux of BLCN-0.5 membrane is decreased to0.46 m L·cm-2·min-1 at the same conditioin. The better structural stability and CO2 tolerance properties of BLCN membranes with higher La-doping content were due to more appropriate Goldschmidt tolerance factor and the enhanced average metal-oxygen bond energy.On the basis of La doping at A site, in order to obtain better property on the CO2 corrosion resistance, Ba0.8La0.2Co0.88-x Fex Nb0.12O3-δ(BLCFN) membrane with Fe doping at B site was synthesized by solid phase reaction. For all the membranes with Fe-doping calcined under pure CO2 atmosphere for 5 h, no witherite can be observed.In comparison of BLCN, BLCFN has a better CO2 tolerance at the same conditions.For the membrane of BLCFN-0.2 swept by using pure CO2 at 900 °C for 110 h, the oxygen permeation flux is reduced from initial flux of 0.96 m L·cm-2 ·min-1 to 0.31 m L·cm-2 ·min-1 at the first 10 h and then the flux is maintained. The result suggeststhat elements doping at both A site and B site is in favour of oxygen permeable stability in the long time. |
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