Study On The Oxygen Permeation Properties Of A CO₂-tolerant Dual Phase Membrane

Dense oxygen transport membranes allow obtaining oxygen from air under an oxygen partial pressure differences at elevated temperatures. This kind of promising material can be applied in oxy-fuel power plants. In the oxy-fuel process, fuel is combusted in permeated oxygen which is carried by CO2 gas used as sweep gas, as opposed to air can yield a pure stream of CO2. In this approach, the produced CO2 can be easily captured and stored. Moreover, the fuel combustion process can reduce the cost of capturing CO2. For this application, the membrane material with perovskite structure can be affected by CO2 easily, leading to the destruction of the structure of the membrane. Therefore, the oxygen permeability will also be further influenced. Single phase membranes usually display poor stability and low mechanical strength under CO2 atmosphere. Obviously, currently developed perovskite-based membranes can’t meet the requirements of practical application. In this paper, we developed and designed a CO2-stable dual phase material Ce0.9Gd0.1O2-δ-SrCo0.8Fe0.1Nb0.1O3-δ(CGO-SCFN) with high oxygen permeation and investigated its properties in detail.SrCo0.8Fe0.1Nb0.1O3-δ oxide was synthesized by the conventional solid-state reaction method and Commercial Ce0.9Gd0.1O2-δ powder was purchased as starting material. The as obtained CGO and SCFN powders were mixed with weight ratios and the mixtures were isostatically pressed into pellets. The dual phase membranes(DPM) were investigated by X-ray diffraction(XRD) and scanning electron microscopy(SEM). The results show that the chemical compatibility of the fluorite phase and mixed conductive phase is really good. In addition, a homogeneous distribution of the two types of grains can be observed. The oxygen permeability of CGO-SCFN membrane decreases with the increase of CGO content. For all the DPM, the oxygen permeation fluxes increased with increasing temperature and decreasing membrane thickness. An steady oxygen permeation flux of 0.74 mL·min-1·cm-2 was obtained with a 1.0 mm thick membrane under oxygen partial pressure gradient of Air/He at 900 °C. For 80CGO-20 SCFN, the decreasing of membrane thickness can make the rate determining step of the oxygen permeation process shifting from predominantly bulk diffusion to predominantly interfacial exchange.The effect of CO2 on the structure and oxygen permeation of CGO-SCFN DPM was investigated. We studied the stability of CGO-SCFN dual-phase materials which suffered an oxygen permeation process with CO2 sweeping and a treatment under CO2 calcination, respectively. The result reveals that the DPM with higher CGO content exhibits more stable properties against CO2. Notably, 80CGO-20 SCFN shows the prominent oxygen permeability using pure CO2 as sweep gas. It can be seen that the oxygen permeation fluxes of 70CGO-30 SCFN and 80CGO-20 SCFN keep quite stable during the test for 110 h, reaching about 0.23 mL·min-1·cm-2 using pure CO2 as sweep gas at 900 °C. An oxygen flux of 0.50 mL·min-1·cm2 with pure CO2 as sweep gas was obtained on the 0.6 mm 80CGO-20 SCFN membrane coated by a porous layer on the permeate side at 900 °C.Based on the investigation on the corrosion of CO2, a new phase of strontium sulfate(SrSO4) can be observed on the permeate side of the two spent membranes of 70CGO-30 SCFN and 80CGO-20 SCFN. Experiments of the oxygen permeation and theoretical calculations have been performed and we can see that the sulfate formation on the permeate side would be the migration of sulfur impurities contained in the DPM from the bulk to the surface under air/CO2 gradient at high temperature. The sulfur migration is driven by the chemical potential gradient of oxygen and CO2 gas sweeping could speed up the migration rate. Theoretical calculations indicated that sulfur impurities favor to occupy oxygen vacancies, but due to the formation of carbonates associated with CO2 sweeping, abundant SrVs are generated and free to be filled。As a result, sulfur may hop to Sr-vacancies on the sweeping side, forming mixed sulfide and carbonate.