Study On The CO₂Corrosion Of Ba_1.0Co_0.7Fe_0.2Nb_0.1O_3-δ Mixed Ionic-electronic Conducting Oxygen Transport Membrane

Oxygen Transport Membrane (OTM) is a kind of promising material to obtainoxygen from air at high temperature. The oxygen production technology coupled withIntegrated Gasification Combined Cycle (IGCC) plants is the most efficient andpromising method to improve the effective utilization of coal and to be in favor of theCarbon Capture and Storage (CCS). The ideal membrane which can be applied to theOTM-IGCC must have high oxygen permeability, good structural and chemicalstability. For the technology of the OTM-IGCC, the membrane works in a relativehigh partial pressure of carbon dioxide (CO2). However, the membrane material withperovskite structure is easily reacted with carbon dioxide, and thereby the oxygenpermeability will be affected by the corrosion of CO2. Study on the poisoningmechanism of CO2on perovskite membranes and developing oxygen permeablemembrane material with high-performance of CO2tolerant is very important for theapplication of OTM-IGCC technology.In this study, the effect of CO2on the structure and oxygen permeation ofBa1.0Co0.7Fe0.2Nb0.1O3-(BCFN) membrane was systematically investigated. Cobalt(Co) K-edge absorption spectra of BCFN annealed in CO2was investigated by X-rayabsorption fine structure spectroscopy (XAFS), revealing that the oxidation states ofCo in all the samples are larger than+3and decrease with the increase of calciningtime. About900ppm sulfur impurities in the form of sulfate ions in BCFN sample.Besides of witherite with orthorhombic structure formation, sulfate ions in the bulk ofBCFN membrane prefer to migrate to the surface under CO2calcination and formmonoclinic Ba(CO3)0.9(SO4)0.1. Moreover, SEM results indicate that the nucleationand growth of carbonates grains start at the grain boundary of the membrane. Thepresence of oxygen in the CO2atmosphere would stabilize the membrane structureand hinder the generation of carbonate.The oxygen permeation flux of Ba1.0Co0.7Fe0.2Nb0.1O3-membrane decreases since the formation of carbonates while sweeping by different concentrations of CO2at850°C. The oxygen permeation flux decreases from1.73ml·cm-2·min-1to0.26ml·cm-2·min-1using5%CO2as sweep gas. What’s more, the oxygen permeation fluxdrops to0quickly while changing sweep gas from pure He to pure CO2. Again, theoxygen flux will be restored to about97%when the sweep gas shifts back to pure He,which is caused by the decomposition of carbonate layer on the surface of BCFN.However, when the sweep gas is switched from pure CO2to5%CO2, the carbonatelayer is not decomposed and traces of oxygen are permeated. Oxygen permeationmechanism of BCFN varies with the partial pressure of CO2. The oxygen is mainlytransported through the main phase of perovskite BCFN when the low concentrationof CO2used as sweep gas. After BCFN membrane is covered by carbonates layer,oxygen permeability would be controlled by partial pressure of CO2in permeate side.When CO2partial pressure is higher than thermodynamics decomposition pressure ofcarbonate of BaCO3, although the carbonate layer could not be decomposed, there aresome oxygen vacancies appeared, and the trace oxygen is permeated though thevacancies of carbonate.To improve the performance of resistance to the corrosion of CO2for BCFN, theSm0.7Sr0.3CoO3-(SSC) powder are synthesized firstly by solid phase reaction methodand then the SSC slurry is coated on the surface of BCFN membrane to getBCFN-SSC oxygen transport membrane. The SSC coating reduces the alkalinity ofBCFN membrane. The SSC remains its orthorhombic structure under CO2atmosphere,and only a small amount of SrCO3is formed. After switching the sweep gas from pureHe to5%CO2, the oxygen permeation flux of BCFN-SSC decreases about31%andkeeps at1.3ml·cm-2·min-1for10h. After pure CO2sweeping for20h, the oxygenpermeation flux decreases to0.19ml·cm-2·min-1. Compared with unmodified BCFN,the performance of resistance to corrosion of CO2is greatly improved for BCFN-SSC.