Oxygen transport membranes for biomass gasification and cement industry

Oxygen transport membranes (OTMs) are of particular interest for their potential applications in high purity oxygen separation, biomass gasification and carbon capture and storage in cement production. Gd0.1Ce0.9O1.95-δ (GCO) is one of the interesting materials of OTMs because of its high ionic conductivity and excellent chemical stability under strong reducing conditions. However, for special applications in mildly reducing conditions (e.g. pure oxygen production and oxy-coal combustion) the oxygen flux of GCO is not sufficient because the performance is limited by the low electronic conductivity.
In this study various routes for enhancing the electronic conductivity were investigated; either via elemental substitution or via mixing doped-ceria with another material when forming the membrane layers. The increase of electronic conductivity by substitution co-doped Gd and Pr in ceria was investigated by a microelectrode assisted Hebb-Wagner polarization. The electronic conductivity of PrxGd0.1Ce0.9-xO1.95-δ (x=0-0.4) was found to be significantly enhanced relative to that of GCO at high pO2 (1×10-8- 0.21 bar), by as much as three orders of magnitude in Pr0.4Gd0.1Ce0.5O1.95-δ. The electronic conductivity of PrxGd0.1Ce0.9-xO1.95-δ increases with increasing concentration of Pr. The drastic decline of activation energy of electron hole migration (10-15 at.%) indicates a drastic decrease of hopping energy as continuous percolating “Pr-path” forms in the Face-Centred Cubic (FCC) Unit Cell. This provides a new understanding of small polaron mechanism on basis of crystal structure along with the band structure. In addition, the ionic conductivity of Pr-doped GCO is greater than that of Pr-doped ceria upon the same dopant concentration because of the higher concentration of oxygen vacancy in Pr-doped GCO. The heavily Pr-doped samples showed lower ionic conductivity relative to that of slightly Pr-doped samples. This is due to the more pronounced defect association in heavily doped ceria. The thermal expansion coefficients (TEC) of Pr-doped GCO exhibited a nonlinear feature at 500 ºC and increased with increasing dopant concentration. The sudden increase of TEC is a consequence of the increase of chemical expansion coefficient (CEC), which is induced by the chemical strain due to the increase of oxygen nonstoichiometry as the partial reduction of Pr occurs at elevated temperature. The chemical expansion coefficients were between 0.065-0.08 mol-1, in line with that of Pr-doped ceria. The oxygen flux of Pr0.05Gd0.1Ce0.85O1.95-δ was enhanced relative to GCO by one order of magnitude. For a 10 μm thick Pr0.4Gd0.1Ce0.5O1.95-δ-based membrane, the estimated oxygen flux of 10 Nml cm-2 min-1 (the target for commercialization of OTMs) might be achieved at 900°C under a small oxygen potential gradient.
Substitution of Ce with Zn was also considered. ZnO-containing GCO could be sintered at a relatively lower temperature (1300°C vs. 1600°C). The solubility limit of ZnO in GCO is in the range of 2-3 at.%. As compared to GCO, the p-type electronic conductivity of Zn-doped GCO under oxidizing condition was not influenced by the dopant whilst the n-type electronic conductivity under reducing conditions was suppressed. The ionic conductivity was slightly suppressed by doping ZnO. This indicates that the zinc ion may be an interstitial defect in GCO.
Also dual phase membranes were studied. A 1-mm thick dual phase composite oxygen membrane (50vol.% Al0.02Ga0.02Zn0.96O1.02-50 vol.% Gd0.1Ce0.9O1.95-δ) with catalyst on both sides was observed to sustain an oxygen flux of 0.3 Nml cm-2 min-1 under air/N2 at 900 °C. The material was observed to be chemically stable in CO2 and SO2 at high temperature. However, the oxygen surface exchange of the material was slow so that a high performance catalyst is required to ensure fast oxygen surface exchange.
Some of the promising material combinations were also prepared as thin films on top of structural supports. An asymmetric (thin dense layer on a porous support) dual phase composite membrane of 70 vol.% Gd0.1Ce0.9O1.95-δ-30 vol.% La0.6Sr0.4FeO3-δ (GCO-LSF) was fabricated by a “one step” phase-inversion tape casting. Oxygen flux measurement as well as electrical conductivity relaxation indicates that the oxygen permeation flux of the membrane without catalyst is rate limited by oxygen surface exchange. Mass polarization through the porous support is insignificant over a wide range of oxygen partial pressure gradients. A stable high flux of ca. 7.00 (STP) ml cm-2 min-1 was observed for 200 hours at 850 °C with the membrane placed between air and CO. Partial surface decomposition was observed on the permeate side exposed to CO.
Besides above described investigations on ceria based systems also perovskite type membrane materials were investigated. Asymmetric Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3-δ (BSCFZ)-based oxygen membranes were prepared by phase-inversion tape casting. The oxygen permeation fluxes of 1-mm thick disc and asymmetric membranes were limited by bulk diffusion and oxygen surface exchange in air/N2. The membranes were not chemically stable under the large chemical potential gradients (oxygen vs. H2/H2O) because of fast degradation induced by thermodynamic decomposition. The asymmetric membrane (without catalyst) exhibited a stable flux when tested under more mild conditions (O2/N2). A flux of 11.2 Nml cm-2 min-1 at 950°C was observed over 150 hours and 7 Nml cm-2 min-1 at 850°C was measured over 300 hours in O2/N2. Segregation of barium sulphate and cobalt oxide was found on the surface of the dense membranes, which is ascribed to the reaction between sulphur-containing binder (PESF) and BSCFZ powder. Significant loss of Co, Sr and Fe and enrichment of BaSO4 was observed on the permeate side after the long term test. This is likely due to kinetic demixing driven by the oxygen partial pressure gradient across the membrane.