Oxygen Transport Membranes Research Articles

Thermochemical evaluation of oxygen transport membranes under oxy‐combustion conditions in a pilot‐scale facility

AbstractBACKGROUNDControl of greenhouse gas emissions has become one of the most important challenges faced by humanity. Among the various approaches to mitigating CO2 emissions, carbon capture and storage (CCS) is considered one of the most promising clean coal options for the future because it can be implemented in the short and medium terms at the industrial scale. Among CCS techniques, oxy‐combustion offers advantages in using pure oxygen (O2) as a comburent, where in a flue gas composed mainly of CO2 and water vapor is generated. Cryogenic air separation is the only available technology that can provide the required amount of O2, but this process requires large amounts of energy and is costly, which make its large‐scales implementation difficult.RESULTSIn this framework, oxygen transport membranes are being researched as an O2 supplier unit because they offer advantages from a techno‐economic view point. In the present work, the thermochemical stabilities of La0.6Sr0.4Co0.2Fe0.8O3 and Cobalt‐doped Ce0.9Gd0.1O were evaluated to obtain information on their behavior in oxy‐combustion atmospheres. Experiments were performed in a circulating fluidized bed boiler of a pilot plant using an experimental sampling train. Samples of the two materials were characterized by X‐ray diffraction, X‐ray fluorescence, infrared spectroscopy, Raman spectroscopy, scanning electron microscopy with energy‐dispersive X‐ray spectroscopy, and Brunauer–Emmett–Teller analysis.CONCLUSIONSThe results revealed that both materials were susceptible to the presence of species that originated from flue gas, materials comprising the boiler and ducts, and coal ash, and that the CGO_Co material showed better performance than the other studied material. © 2020 Society of Chemical Industry

Micromechanical Characterization of Ce0.8Gd0.2O2‐δ–FeCo2O4 Dual Phase Oxygen Transport Membranes

Aiming toward an optimization of dual phase oxygen transport membrane materials for oxygen separation applications, ceramic composites consisting of a Ce1−xGdxO2−δ (0 < x < 0.2) fluorite phase, Gd0.9Ce0.1Fe0.8Co0.2O3 perovskite phase, FexCo3−xO4 (0 < x < 1) spinel phase, and CoO rock salt phase are developed and micromechanical properties (elastic modulus and hardness) of xCe0.8Gd0.2O2−δ: (1−x)FeCo2O4 (50 wt% ≤ x ≤ 90 wt%) composites are characterized via indentation testing at room temperature. The results obtained at low indentation loads indicate that the magnitude of the elastic moduli of the different phases is in the order Gd0.9Ce0.1Fe0.8Co0.2O3 > Ce1−xGdxO2−δ ≈ FexCo3−xO4 > CoO, and furthermore, hardness values are also in the same order. The hardness values of the obtained composites at higher impression loads reveal a stronger dependency on porosity than on composition due to similar hardness values of the main phases. Any compositional effect appears to diminish above a porosity of ≈1%.

Oxygen diffusion behaviour of A-site deficient (La0.8Sr0.2)0.95Cr0.5Fe0.5O3−δ perovskites in humid conditions

Limited oxygen surface exchange for oxygen transport membrane (OTM) material in humid atmosphere, correlated with Sr surface segregation identified using isotopic exchange and mass spectrometry.

Direct AC/DC Heating of Oxygen Transport Membranes

This article is devoted to the development of the direct resistive heating of oxygen transport membranes technique. In this case, DC was selected to perform direct heating. The effect of DC on the oxygen fluxes and the microstructure of the membrane was studied. It is shown that in the short-term experiment with DC, a positive significant effect on the oxygen transport was found, while sample exposure under the influence of DC for a long period of time had a significant negative effect on the microstructure of the membrane.

Formation mechanism of island CoO in La-Sr-Co-Fe oxygen transport membrane deposited by low pressure plasma spraying-thin film technology

Dense oxygen transport membranes (OTMs) made from mixed oxygen ionic and electronic conductors (MIEC) have attracted intense research interest owing to their unmatched advantages on pure oxygen production, oxygen-fuel combustion and oxygen-related catalytic processes over traditional techniques. As one kind of the most-studied membranes, cobalt-containing membranes have unsatisfied thermal stability, though the oxygen permeability of these membranes is generally excellent due to the existence of cobalt. However, in situ forming CoO phase which were observed in the cobalt-containing membranes prepared by low pressure plasma spraying-thin film technology (LPPS-TF), were found to exhibit a remarkable stabilizing effect in the preliminary work. As one of the main premises to reveal the stabilization mechanism of these island CoO phase, their formation mechanism were studied in the first place in the paper.

Development of an Electrochemical Oxygen Compressor and Generator for Spacesuit Oxygen Resupply

NASA is sponsoring the development of an electrochemical method of supplying high pressure, high purity oxygen suitable for recharging spacesuit tanks during an exploration mission. The system is referred to as the Electrochemical Oxygen Generator and Compressor (eCOG-C). The method uses a ceramic oxygen transport membrane, with internal process airflow channels, and an electrochemical configuration that electrochemically pumps oxygen from the process air to the external environment. The oxygen transport membrane is placed inside a thermally insulated pressure vessel, allowing the oxygen to accumulate and pressurize. Except for a simple process air blower, the device is solid state. An alternate version of this process has demonstrated the ability to produce high purity (>99.99%) oxygen from ambient air. This version is intended to produce high pressure oxygen, the design goal is >4000 psig delivery pressure. This paper describes the NASA application, and the electrochemical systems found in the ceramic oxygen transport membrane. This project is in an early stage of development, a feasibility assessment has been completed, and the first iteration of prototyping is underway. There are three areas of emphasis in the initial prototyping effort: 1) developing interconnecting seals that join two different wafers with sufficient bond strength to sustainably withstand pressure forces, 2) developing interconnecting seals that join two different wafers with well matched coefficients of thermal expansion (CTE), to minimize thermal stress in the cell stack, 3) developing a ceramic transport membrane wafer with internal process air flow channels. Figure 1

Understanding the thermochemical behavior of La0.6Sr0.4Co0.2Fe0.8O3 and Ce0.9Gd0.1O_Co oxygen transport membranes under real oxy-combustion process conditions

Oxygen transport membranes (OTM) are a promising alternative to conventional systems of air separation based on cryogenic distillation for oxy-fuel combustion power plants. In this work, a systematic study of the thermochemical stability of La0.6Sr0.4Co0.2Fe0.8O3 (perovskite-type) and cobalt doped Ce0.9Gd0.1O (fluorite-type) is proposed. The experiments were developed in a laboratory scale facility, which is able to mimic realistic oxy-fuel combustion flue gas containing SOx, NOx, H2O and CO2. In order to understand the thermochemical behavior of this type of materials, a full characterization analysis of the tested samples using a wide portfolio of analytical techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), infrared spectroscopy (ATR-FTIR), Raman spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and Brunauer−Emmett−Teller analysis (BET) has been carefully discussed.Our data revealed the superior stability of the CGO samples in comparison with the LSCF at all the test conditions studied in this work. The formation of crystalline and amorphous sulphates and carbonates are evident for the LSCF while for the CGO samples do not react with SOX and barely form carbonates. The presence of silicon species – typically ignored in academic works – has been detected, pointing its relevance for real applications.

Chemical Environment-Induced Mixed Conductivity of Titanate as a Highly Stable Oxygen Transport Membrane.

SummaryCoupling of two oxygen-involved reactions at the opposite sides of an oxygen transport membrane (OTM) has demonstrated great potential for process intensification. However, the current cobalt- or iron-containing OTMs suffer from poor reduction tolerance, which are incompetent for membrane reactor working in low oxygen partial pressure (pO2). Here, we report for the first time a both Co- and Fe-free SrMg0.15Zr0.05Ti0.8O3−δ (SMZ-Ti) membrane that exhibits both superior reduction tolerance for 100 h in 20 vol.% H2/Ar and environment-induced mixed conductivity due to the modest reduction of Ti4+ to Ti3+ in low pO2. We further demonstrate that SMZ-Ti is ideally suited for membrane reactor where water splitting is coupled with methane reforming at the opposite sides to simultaneously obtain hydrogen and synthesis gas. These results extend the scope of mixed conducting materials to include titanates and open up new avenues for the design of chemically stable membrane materials for high-performance membrane reactors.

Temperature-Induced Structural Reorganization of W-Doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ Composite Membranes for Air Separation

The practical use of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) prototypical oxygen-transport membrane for air separation is currently hampered by the decomposition of the cubic perovskite into a variant with...

Experimental and numerical study of oxy‐methane flames in a porous‐plate reactor mimicking membrane reactor operation

The work investigates the reacting flow field, oxy-methane flame characteristics and location, and the species distributions in a porous-plate reactor mimicking the operation of oxygen transport membrane reactors (OTMRs). The study was performed experimentally and numerically considering ranges of operating equivalence ratio, from 0.5 to 1.0, and CO2 concentrations in the total oxidizer flow (O2 and CO2), from 0% to 55% (by Vol). Oxygen was supplied through a slightly pressurized top and bottom chambers to cross the two porous plates to the central chamber, where a premixed mixture of CH4 and CO2 is introduced. ANSYS Fluent 17.1 software was used to solve for conservation and radiative transfer equations in the full three-dimensional (3-D) domain. The modified Westbrook-Dryer (Oxy-WD) two-step reduced mechanism for oxy-methane combustion was adapted for the calculations of chemical kinetics. The captured flame shapes using a high-speed camera were compared with the calculated ones, and the results showed good agreements. At fixed equivalence ratio, elongated flames were obtained at higher CO2 concentrations due to the increase in the mainstream Reynolds number and reduction in reaction rates, which delays the completeness of combustion. At fixed CO2 concentration, the increase in equivalence ratio resulted in more compact and intense flames. The effective mixing and flame stability resulted in complete fuel conversion under stoichiometric condition. Stable flames were located between the two porous plates at reasonable distance. This perfect flame location prevents the thermal fracture of the membranes and improves their oxygen permeation flux, resulting in better combustion characteristics when the results are projected on the case of OTMRs. This implies efficient and safe applicability of the OTMRs by the condition that membranes can provide sufficient oxygen flux for complete combustion. A warm outer recirculation zone (ORZ) was created beside each porous plate, which helps anchoring the flame at the leading edge of the porous plate. The range of temperature within the ORZ was 800 to 1600 K, which lies in the operability limits of membranes for the case of OTMRs. The effective complete mixing and flame stability resulted in complete fuel conversion under stoichiometric condition. The temperature and species distributions within the reactor are presented in detail over wide ranges of operating conditions. The results recommended the reactor operation under stoichiometric combustion condition based on performance and economic points of views. The results are promising when projected on the application of the OTMRs under oxy-combustion conditions for clean and efficient energy production.

High CO2-tolerance oxygen permeation dual-phase membranes Ce0.9Pr0.1O2-δ-Pr0.6Sr0.4Fe0.8Al0.2O3-δ

High stability and oxygen permeability are two prominent requirements for the oxygen transport membrane candidates used as industrialization. Herein, we report several composite membranes based on xwt.%Ce0.9Pr0.1O2-δ (CPO)-(100-x)wt.%Pr0.6Sr0.4Fe0.8Al0.2O3-δ (PSFAO) (x = 50, 60 and 75) prepared via a modified Pechini method. Oxygen permeability test reveals that the 60CPO-40PSFAO composition exhibits the highest oxygen permeability. The oxygen permeation flux through the optimal uncoated 0.33 mm-thickness 60CPO-40PSFAO composite can reach 1.03 mL cm-2 min-1 (over the general requirement value of 1 mL cm-2 min-1) in air/He atmosphere at 1000 °C. In situ XRD performance confirms the optimal 60CPO-40PSFAO sample shows excellent stability in CO2-containing atmospheres. The 60CPO-40PSFAO membrane still exhibits simultaneously excellent oxygen permeability and phase stability after operating for over 100 h at air/CO2 condition at 1000 °C, which further indicates that the 60CPO-40PSFAO composite is likely to be used for oxygen supply in CO2 capture.

Structure and electrical properties of YSZ-rGO composites and YSZ ceramics, obtained from composite powder

Despite zirconia based ceramics is technologically important and applied as a solid electrolyte, the studies for new compositions with enhanced performance are carried out continuously. Zirconia-graphene composites are regarded as promising oxygen transport membranes having a cross-flow of ionic and electronic charge carriers under oxygen partial pressure gradient and as an alternative to perovskite-based SOFC interconnects. In the work 92ZrO2-8Y2O3 (mol.%, YSZ) ceramics and YSZ-rGO composite ceramics (where rGO is reduced graphene oxide) were obtained from composite precursors by the calcination at 1823 K in air and in vacuum, respectively. In order to determine the optimal amount of rGO addition to the ceramic precursor simple mathematical model was suggested for the first time which is scalable for any ceramics-graphene derivative and metal-graphene derivative systems. Using SEM, EDX, XRD, Raman spectroscopy and Impedance spectroscopy data the effect of rGO and the annealing ambiance on the structure, temperature dependencies of grain and grain boundary conductivities of YSZ ceramics rGO-YSZ and composites were revealed for the first time. The rGO additive acts as grain growth inhibitor, which is completely removed from ceramics during annealing in air resulting in ceramics with the refined microstructure and coinciding temperature dependencies of grain and grain boundary conductivities in all temperature range. Vacuum annealing results in YSZ-rGO composites with high ionic conductivity and high crystallinity.

Effects of Al content on the oxygen permeability through dual-phase membrane 60Ce0.9Pr0.1O2--40Pr0.6Sr0.4Fe1-Al O3-

Ceramic dual-phase oxygen transport membranes with the composition of 60wt.% Ce0.9Pr0.1O2-{\delta}-40wt.%Pr0.6Sr0.4Fe1-xAlxO3-{\delta} (x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0) (60CPO-40PSF1-xAxO) based on 60Ce0.9Pr0.1O2-{\delta}-40Pr0.6Sr0.4FeO3-{\delta} doped Al was successfully synthesized through a modified Pechini method. Crystal structure, surface microtopography and oxygen permeability are investigated systematically. The cell parameters of perovskite phase first increased and then decreased with the increase of Al content, which is related to the radius of the Al3+ and the formation of impurity phase. As x ranges from 0.1 to 0.8, the oxygen permeability of the materials first increases and then decreases, and the maximum value of oxygen permeation rate for 60CPO-40PSF1-xAxO membranes with 0.4mm thickness at 1000 {\deg}C is 1.12 mL min-1 cm-2 when x = 0.4. XRD measurements revealed high temperature stability and CO2-tolerant property of the dual-phase composites. The partial replacement of Fe$^{3+}$/Fe$^{4+}$ by Al$^{3+}$ causes the material not only to exhibit good stability, but also to increase the oxygen permeability of the membranes.

Thermochemical stability of Fe- and co-functionalized perovskite-type SrTiO3 oxygen transport membrane materials in syngas conditions

The materials typically used for oxygen transport membranes, Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) tend to decompose due to their low thermochemical stability under reducing atmosphere. Fe- and Co-doped SrTiO3 (SrTi1-x-yCoxFeyO3-δ, x + y ≤ 0.35) (STCF) materials showing an oxygen transport comparable to LSCF have great potential for application in ion-transport-devices. In this study, the thermochemical stability of pure perovskite-structured STCF was investigated after annealing in a syngas atmosphere at 600–900 °C. The phase composition of the materials after annealing was characterized by means of X-ray diffraction (XRD). The thermodynamic activities of SrO, FeO, and CoO in the STCF materials were evaluated using Knudsen effusion mass spectrometry (KEMS). Co-doped SrTiO3 (STC) materials were not stable after annealing in the syngas atmosphere above 5 mol% Co-substitution. Ruddlesden-Popper-like phases and SrCO3 were detected after annealing at 600 °C. In contrast, Fe substitution (STF) showed good stability after annealing in syngas upto 35 mol% substitution.

Microstructure Control of Tubular Micro-Channelled Supports Fabricated by the Phase Inversion Casting Method

Thin-film membrane layers coated onto porous supports is widely considered as an efficient way to obtain high-performance oxygen transport membranes with both good permeability and high mechanical strength. However, conventional preparation methods of membrane supports usually result in highly tortuous channels with high mass transfer resistance. Tubular porous MgO and MgO/CGO supports were fabricated with a simple phase inversion casting method. Long finger-like channels were obtained inside the dual-phase supports by adjusting the ceramic loading, polymer concentration and particle surface area, as well as by introducing ethanol inside the casting slurries. Slurries that exhibit lower viscosity in the zero-shear viscosity region resulted in more pronounced channel growth. These supports were used to produce thin supported CGO membranes for possible application in O2 separation. Similar shrinkage speeds for the different layers during the sintering process are crucial for obtaining dense asymmetric membranes. The shrinkage of the support tube at a high temperature was greatly affected by the polymer/ceramic ratio and compatible shrinkage behaviours of the two layers were realized with polymer/ceramic weight ratios between 0.175 and 0.225.

High flux and CO2-resistance of La0.6Ca0.4Co1–xFexO3−δ oxygen-transporting membranes

Most of the currently used perovskite-based oxygen-transporting membranes have insufficient resistance towards CO2 and high material costs that potentially limit their commercial applications. In the present work, a highly CO2-tolerant oxygen permeation membrane based on La0.6Ca0.4Co1–xFexO3−δ (x = 0, 0.3, 0.5, 0.7, 1) was designed and prepared by a scalable reverse co-precipitation method. The oxygen permeation flux through the dense membranes was evaluated and found to be highly dependent on the Co/Fe ratio. La0.6Ca0.4Co0.3Fe0.7O3−δ possessed the highest permeation flux among the investigated samples, achieving 0.76 ml min−1 cm−2 under an Air/He gradient and 0.5 ml min−1 cm−2 under an Air/CO2 gradient at 1173 K for a 1 mm thick membrane. A combination study of first principles calculations and experimental measurements was conducted to advance the understanding of Co/Fe ratio effects on the oxygen migration behavior in La0.6Ca0.4Co1–xFexO3−δ. The observed oxygen permeability is three times higher than that reported in literature under similar conditions. The presented results demonstrate that this highly CO2-tolerant membrane is a promising candidate for high temperature oxygen separation applications.

Long-term stability of iron-doped calcium titanate CaTi0.9Fe0.1O3−δ oxygen transport membranes under non-reactive and reactive atmospheres

Oxygen transport membranes (OTM) are widely considered as a possible solution to limit the carbon footprint, but are notoriously afflicted by performance issues due to chemical instability observed during long-term operation. This paper reports on the stability of an OTM made of CaTi0.9Fe0.1O3−δ (CTF), and addresses its applicability. The redox stability of CTF was investigated using thermal gravimetry up to 1000 °C under air and H2, coupled with XRD and Mössbauer analyses. The redox potential of iron was measured using an electrochemical potential relaxation as a function of temperature. The baseline oxygen semi-permeability flux of dense CTF membranes was measured in inert atmospheres (air/argon or air/helium), and the long-term stability established for up to 1600 h under simulated operation atmospheres containing CO, CO2, H2 and CH4. CTF shows a remarkable performance stability and post mortem XRD, SEM-EDS and Raman analyses show no evidence of decomposition or reaction byproducts.

Fabrication of lanthanum-based perovskites membranes on porous alumina hollow fibre (AHF) substrates for oxygen enrichment

Abstract In this work, two types of lanthanum-based MIEC perovskite oxides, namely La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and La0.6Sr0.4Co0.2Ni0.8O3-δ (LSCNi), were deposited onto porous alumina hollow fibre (AHF) substrates and used for oxygen enrichment. Such structure was developed to shorten oxygen ion diffusion distances in dense membranes and simultaneously leading to higher oxygen flux. The perovskite oxides were prepared using Pechini sol-gel method and deposited via a vacuum-assisted technique. The deposition of lanthanum-based membranes onto the outer and inner sides of the porous AHF has been facilitated through numerous microchannels in the AHF substrates. The effects of operating temperature and argon sweep gas flowrate on oxygen permeation flux of lanthanum-based AHF membrane were investigated. The results revealed that the oxygen permeation flux of LSCF-AHF and LSCNi-AHF increased with operating temperatures due to the improvement of bulk diffusion and surface exchange properties after the lanthanum-based perovskite deposition. Higher oxygen flux was observed for LSCNi-AHF as LSCNi possessed balanced oxygen ionic and electronic conductivities as compared to LSCF membranes. Benefitting from improved oxygen activation and vacancy generation properties after Ni substitution into the B-site ion of LSC perovskite, a dramatic increased oxygen fluxes up to 4.5 mL/min·cm2 was observed at 950 °C. The present work demonstrated a feasible method for fabricating oxygen transport membrane (OTM) using porous AHF substrates

Phase stability and oxygen permeability of Fe-based BaFe0.9Mg0.05X0.05O3 (X = Zr, Ce, Ca) membranes for air separation

The effects of various dopants including Zr4+, Ce4+ and Ca2+ on the structure and oxygen permeability of B-site doped BaFe0.9Mg0.05X0.05O3−δ (BFM-X) perovskite-type oxygen transport membranes were studied. Slight X cation doping could stabilize the cubic structure of BFM-X perovskite down to room temperature. XRD, SEM and thermogravimetric results revealed that all the cubic BFM-X oxides exhibited good phase stability under argon atmosphere without any phase changes. The weight loss of BFM-Ce from TG analysis suggests the reduction of cerium ions at high temperatures, which may account for its larger electrical conductivity and higher oxygen permeability comparing to BFM-Zr and BFM-Ca membranes. X-ray photoelectron spectroscopy (XPS) data revealed that Ca dopant with larger size caused the mismatch with Fe ions and led to the substitution of Ca ions at both Fe and Ba site in BFM-Ca oxide, being detrimental to electrical conductivity and oxygen permeation. Among these membranes, BFM-Ce has the highest oxygen permeability and good long term permeation stability under air/argon gradient, thus it was recommended as a potential and promising material for air separation.

Design and fabrication of large-sized planar oxygen transport membrane components for direct integration in oxy-combustion processes

Membrane-based oxy-combustion is a promising technology for energy efficient combustion of carbon-containing fuels with the simultaneous opportunity to capture CO2 from the resulting exhaust gas. However, oxy-combustion conditions result in special demands on the design of the ceramic membrane components due to the high pressure and temperature applied. Therefore, we have developed a planar membrane design for 4-end operation using asymmetric membranes of La0.6Sr0.4Co0.2Fe0.8O3−δ. FEM and CFD simulations have been performed in order to develop an internal channel structure that allows withstanding pressures of 5 bar on the feed side while achieving the desired O2 concentrations of 27% in the sweep gas, i.e. CO2, and an oxygen recovery rate from the feed gas of 86% at the same time.Due to the symmetric design of the membrane components, they are scalable and adaptable in size. This design has been realized in a process chain from powder to the final component consisting of thin 20 µm Membrane layer, support with 38% porosity, an inner channelled architecture and a thin (3–5 µm) porous activation layer. Particular emphasis was laid on scalable manufacturing processes in order to ensure transferability to industrial scale. The process chain is also applicable to other membrane materials suitable for any application of interest. Finally, the reproducible processing was successfully demonstrated by the fabrication of membrane components in lengths of 100 mm and widths of 70 mm.