On the Use of Supported Ceria Membranes for Oxyfuel process / Syngas production

Document downloaded from: This paper must be cited as: The final publication is available at Copyright Additional Information Elsevier Lobera González, MP.; Serra Alfaro, JM.; Foghmoes, SP.; Søgaard, M.; Kaiser, A. (2011). On the Use of Supported Ceria Membranes for Oxyfuel process / Syngas production. Journal of Membrane Science. 385(1-2):154-161. doi:10.1016/j.memsci.2011.09.031 http://dx.doi.org/10.1016/j.memsci.2011.09.031 http://hdl.handle.net/10251/75670 Document downloaded from: This paper must be cited as: The final publication is available at Copyright Additional Information Elsevier Lobera González, MP.; Serra Alfaro, JM.; Foghmoes, SP.; Søgaard, M.; Kaiser, A. (2011). On the Use of Supported Ceria Membranes for Oxyfuel process / Syngas production. Journal of Membrane Science. 385(1-2):154-161. doi:10.1016/j.memsci.2011.09.031 http://dx.doi.org/10.1016/j.memsci.2011.09.031 http://hdl.handle.net/10251/75670 1/19 On the Use of Supported Ceria Membranes for Oxyfuel process / Syngas Production M. Pilar Lobera1, José M. Serra1*, Søren P. Foghmoes2, Martin Søgaard2, Andreas Kaiser2 1Instituto de Tecnología Química (Universidad Politécnica de Valencia - Consejo Superior de Investigaciones Científicas), Av. Naranjos s/n, E-46022 Valencia, Spain. 2Fuel Cells and Solid State Chemistry Department, Risø National Laboratory for Sustainable Energy, Technical University of Denmark - DTU, Building 228, P.O. Box 49, DK-4000 Roskilde, Denmark * Corresponding author. Tel: + 34.963879448 E-mail: [email protected] Journal of Membrane Science 385– 386 (2011) 154– 161 (doi: 10.1016/j.memsci.2011.09.031) Abstract Ceramic oxygen transport membranes (OTMs) enable selective oxygen separation from air at high temperatures. Among several potential applications for OTMs, the use in (1) oxygen production for oxyfuel power plants and (2) the integration in high-temperature catalytic membrane reactors for alkane upgrading through selective oxidative reactions are of special interest. Nevertheless, these applications involve the direct contact of the membrane surface with carbon-rich atmospheres. Most state-of-the-art permeable membranes are based on perovskites, which are prone to carbonation under operation in CO2-rich environments and/or decomposition in reducing gas environments. The oxygen flux through supported thin film membranes of Ce0.9Gd0.1O1.95-δ (CGO) with 2% mol. of cobalt was measured for oxygen separation in oxyfuel processes and in syngas production and degradation was compared to perovskite membranes. The CGO membranes consist of a 27 µm-thick gastight CGO layer supported on a porous CGO substrate. The flat surface of the membrane was coated using two different porous catalytic layers aiming to improve the oxygen activation rate on the permeate side while the porous substrate was infiltrated with an oxygen reduction catalyst. Oxygen separation was studied using air as feed and argon/CO2 or argon/CH4 mixtures as sweep gas in the temperature range 750-1000 ºC. The supported membrane exhibited a maximum oxygen flux of ca. 5 ml·min-1·cm-2 at 1000 ºC when diluted methane was used as sweep gas. The CGO membrane showed high stability in CO2 (in contrast to tests on La0.6Sr0.4Co0.2Fe0.8O3- (LSCF) membranes) and no detrimental effect on the oxygen flux is observed when CO2 is present in the sweep gas even at temperatures below 800 ºC. Moreover, the SEM analysis showed that membrane integrity remained stable after the permeation tests using CO2. 2/19 Keywords: cerium gadolinium oxide, supported membrane, tape casting, MIEC; syngas; oxyfuel 3/19 1. Introduction Ceramic mixed ionic-electronic conducting (MIEC) membranes enable the selective oxygen separation from air at high temperatures. Two key industrial applications of oxygen-transport membranes are (1) oxygen production for power generation from fossil fuel in oxyfuel power plants [1, 2] and (2) the integration in high-temperature catalytic membrane reactors for methane or alkane upgrading by selective oxidative conversions, for instance, partial oxidation of methane (POM) to produce syngas [3, 4]. However, these applications involve the contact with carbon-bearing atmospheres and most of state-of-the-art highly-permeable MIEC membranes do not tolerate the operation under CO2-rich environments, due to carbonation processes [5-7]. The most promising materials are perovskites with the formula ABO3 [8], comprising alkali-earth metal cations in the A-position. High oxygen permeation fluxes have been reported even in oxidizing conditions, for single phase materials such as SrCo0.8Fe0.2O3- (SCF) [9], Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) [10, 11], La0.6Sr0.4Co0.2Fe0.8O3- (LSCF) [12]. However, these perovskites are chemically unstable under large oxygen chemical potential gradients (e.g. air/methane) and in presence of CO2, SO2 or H2O, leading to degradation in oxygen flux and possibly mechanical integrity with time [6, 7, 13]. Lanthanide substituted ceria materials present a combination of high oxygen-ion mobility and chemical compatibility with water and carbon dioxide at high temperatures. n-type electronic conductivity can be introduced into the structure by partial reduction of the cerium (IV) ion at high temperatures under reducing conditions. Recent reports show the potential of oxygen separation in monolithic doped/multidoped ceria membranes [14-16]. Moreover, gadolinium doped ceria (Ce0.9Gd0.1O1.95-δ, CGO) was suggested as oxygen separation membrane for syngas application. On planar, thin film CGO membranes on porous NiO-YSZ supports oxygen fluxes as high as 16 ml min-1 cm2 could be obtained by placing the membrane between air and humidified hydrogen (or methane) at 900°C [17, 18]. The present work shows the functional characterization of oxygen separation membranes made of a gastight thin film layer of Ce0.9Gd0.1O1.95-δ (CGO), supported on a porous CGO substrate. The top flat surface of the membrane was coated using two different porous catalytic layers aiming to improve the oxygen activation rate on the permeate side. Oxygen separation was studied using air as feed and argon/CO2 or 4/19 argon/CH4 mixtures as sweep gas in the temperature range 750-1000 ºC. Special attention is paid to the membrane stability in CO2 under operation. 2. Experimental A thin film CGO membrane supported by a porous CGO substrate was prepared using tape casting, lamination, co-sintering and cutting. The ethanol based slurries for tape casting of the support and membrane layer were prepared by ball milling an ultra low surface area powder of CGO from Rhodia S.A. (France), a PVB based binder system and a polyethylene imine (PEI, branched, M.W. 10,000, 99% Alfa Aesar) as a dispersant. 2 mol% of cobalt(II) nitrate (Cobalt(II) nitrate hexahydrate, 97.7 % min, Alfa Aesar) was added as a sintering aid after drying in a desiccator to remove excess water. In the slurry for tape casting of the porous CGO substrate about 5 vol.-% graphite (V-UF1, 99.9, Graphit Kropfmühl AG, Germany) was added as a pore former. The tape casted layers of the thin film CGO membrane and the porous CGO support were combined by lamination (i.e., application of heat and pressure on to the tubes between two rolls). Round membranes (Ø = 34 mm) were stamped out from the green membrane tapes before sintering. In a binder removal step the organics were removed by a very slow de-bindering profile to avoid damage of the structure. Subsequently, the structure was sintered in air at 1300 ºC for 2 h. The sintered membrane structures were laser-cut to the final dimensions (diameter of 15 mm, total thickness of about 0.3 mm and a CGO membrane thickness of about 25 µm). After sintering, the porous supports of CGO were impregnated with nitrates corresponding to the nominal composition La0.6Sr0.4Co1.05O3- (LSC40). In a previous study it has been shown that LSC40 impregnated in a very porous (>70%) and thin (25 µm) backbone structure provided a highly active oxygen reduction electrode/activation layer [19]. In that case it was found that an optimal performance was found if LSC40 was impregnated in an amount corresponding to 17 vol% in the CGO backbone. The impregnation for the supports characterized here was carried out in a similar way as by Samson et al. [19], except that the cells between each impregnation were inserted directly into a furnace at 350°C. On top of the CGO membrane layer, a porous catalytic layer was applied by screenprinting. The catalyst layers were composed of either Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) or 5/19 cobalt-doped Ce0.8Tb0.2O2- (CeTbO+Co). BSCF powder was provided by Fraunhofer IKTS (Hermsdorf, Germany) and cobalt-doped Ce0.8Tb0.2O2- (CeTbO+Co) was prepared by a co-precipitation route following the procedure described in ref. [14]. Formation of the corresponding crystalline structure (perovskite or fluorite) was checked by X-ray diffraction, using a Philips X’pert Pro equipped with X’celerator detector using monochromatic Cu K radiation. XRD patterns were recorded in the 2 range from 10 to 90 º and analyzed using X’pert Highscore Plus software (PANalytical). The screen-printing inks were prepared by mixing the ball-milled powders with a solution of 94 wt.% terpineol and 6 wt.% ethylene cellulose. Graphite (Aldrich) was added as a pore former in the screen-printing ink. Then, graphite is removed in the ulterior sintering step. This process generates a macroporous system that aims to promote the gas transport through the catalytic layer. The ink homogenization was conducted using a three-roll mill. The coated membranes were sintered in air for 2 h. The sintering temperature of the screen printed layers results from the diverse sintering activities of each material, thereby the membrane with a BSCF coating was sintered at 1010 ºC and the membrane with a CeTbO+Co coating, at 1050 ºC. The material CeTbO+Co has been chosen for the following reasons [14]: (1) stability in CO2-bearing atmospheres; (2) mixed ionic-electronic conductivity at high pO2 and high temperatures; and (3) high surface exchange activity as determined by conductivity relaxation. Figure 1 shows a schematic cross section of the membrane assembly and details of the testing setup. The microstructure of the membranes was analyzed by SEM and EDS in a JEOL JSM6300 electron microscope. La0.6Sr0.4Fe0.8Co0.2O3- (LSCF) monolithic membranes were prepared as reference by uniaxial pressing followed by sintering at 1250 ºC. The final membrane dimensions were 15 mm in diameter and ~ 0.8 mm thickness. After sintering the membrane surface was polished prior to testing. Oxygen permeation tests were performed on 15 mm diameter disk-shape membranes. Sealing was done using gold gaskets in a quartz lab-scale reactor described previously [5]. The temperature was measured by a thermocouple close to the membrane surface. Oxygen was separated from a synthetic air mixture (21% v/v O2). The permeate was analyzed by on-line gas chromatography using a micro-GC (Varian CP-4900) equipped with Molsieve5A, Pora-Plot-Q glass capillary, and CP-Sil modules. All streams were 6/19 individually mass flow controlled. Membrane gas leak free conditions were ensured by monitoring nitrogen concentration on the permeate gas stream. 3. Results and Discussion 3.1 Membrane microstructure Figure 2 presents the SEM images of fracture cross-section of the samples after permeation tests. Figure 2a shows an overview of the CGO porous support (thickness of support is approximately 300 µm). The porosity of the support is about 25% determined by Hg porosimetry. The pore size of the substrates ranges from 1 to 4 µm while the CGO grain size in the support is 0.5 to 1 µm. Regarding the catalytic substrate infiltration, EDS analysis over larger areas indicated that it was only feasible to impregnate 1-2 Vol% of LSC40 in the porous support structure after 6 impregnations. The low amount impregnated in the present porous structure is attributed to the relatively low porosity and small pore size. This can pose a problem as there is not enough oxygen reduction catalyst material in the structure and therefore a large part of the driving force for the oxygen transport can in a worst case scenario be located at the feed side of the membrane. Further ceramic processing development on CGO membranes (not reported in this paper) indicates that the porosity in the support can be adjusted to 35 to 45% by the amount of pore former (graphite) and the sintering conditions. The thickness of the gastight CGO membrane was determined to be 27  0.5 m from SEM images (see Figure 2b). Figure 2c and 2d show the membrane with different catalytic layers composed of BSCF and CeTbO+Co, respectively. The oxygen BSCF activation layer have an open microstructure with macropores and a homogeneous thickness of 19  0.5 µm. The CeTbO+Co catalytic layer presents a larger thickness of 54  0.5 µm and the mean pore size in this case is significantly smaller due to the lower sintering activity of CeTbO+Co. Both layers show an average particle size well below 1 µm while the mean size of the primary CeTbO+Co crystallites is 60 nm as determined by XRD. Finally, the integrity of both catalytic porous layers was preserved during the whole oxygen permeation tests, as inferred by XRD and SEM analysis. 7/19 3.2 Oxygen permeability 3.2.1 Influence of sweep gas flow rates, QSweep Figure 3 shows the oxygen permeation fluxes through the surface-activated CGO supported membranes at various flow rates of Ar as sweep gas (QSweep). The J(O2) was strongly influenced by the increase of the sweep gas flow rate; and this is attributed to two main effects. Firstly, the decrease in the oxygen partial pressure in the permeate side (pO2“) and the consequent increase in the oxygen chemical potential gradient across the thickness of the membrane (i.e. the driving force of the oxygen permeation process). Secondly, the variation of the sweep gas flow rate also affects the fluid dynamics behavior of the membrane reactor (Figure 1b) due to an increase in the sweep gas flow rate reduces the concentration polarization resistance at the permeate membrane side. Concentration polarization becomes typically more relevant in this kind of setup [20] for gas flow rates below 100 ml·min-1 and this is probably the reason for the steep change at 50 ml·min-1 in Figure 3, which is more visible for the membrane reaching the highest flux values. Consequently, gas-phase resistance appears to be a minor contribution to the whole process resistance, especially at low J(O2) values and when compared to that of solid state diffusion and exchange reactions [10, 20]. On the other hand, proper gas sweeping allows decreasing the permeate partial pressure just by a simple dilution process. The permeation flux obtained using the membranes with two different activation layers differ substantially. Namely, the BSCF-activated layer allows achieving an oxygen flux around 4 times higher than the flux obtained using the CeTbO-activated membranes. The main reasons for this are related to characteristics of the CeTbO+Co porous layer: (1) The ambipolar conductivity of the CeTbO+Co material under high pO2 is very limited especially regarding BSCF and therefore the expected catalytically active thickness of the porous layer may be very small. Additionally, the CeTbO+Co layer presents a larger thickness (54 µm, almost a 3 times of the BSCF layer). Both effects would lead to (a) the reduction of the catalytically-active thickness of the porous layer and (b) possible concentration polarization in the gas transport through the pore system. In summary, it seems that the CeTbO layer adds a new 14/19 Acknowledgements Financial support by the Spanish Ministry for Science and Innovation (Project ENE2008-06302) and by the EU through FP7 NASA-OTM Project (NMP3-SL-2009228701) is kindly acknowledged. 15/19 References [1] R. Bredesen, K. Jordal, O. Bolland, High-temperature membranes in power generation with CO2 capture. Chemical Engineering and Processing, 43 (2004) 11291158. [2] H. Stadler, F. Beggel, M. Habermehl, B. Persigehl, R. Kneer, M. Modigell, P. Jeschke, Oxyfuel coal combustion by efficient integration of oxygen transport membranes. International Journal of Greenhouse Gas Control, 5 (2011) 7-15. [3] J. Caro, T. Schiestel, S. Werth, H. Wang, A. Kleinert, P. 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Bouwmeester, Handbook of Solid State Electrochemistry CRC-Press 1996. [26] A. Yan, B. Liu, Y. Dong, Z. Tian, D. Wang, M. Cheng, A temperature programmed desorption investigation on the interaction of Ba0.5Sr0.5Co0.8Fe0.2O3- perovskite oxides with CO2 in the absence and presence of H2O and O2. Applied Catalysis B: Environmental, 80 (2008) 24-31. [27] V.B. Vert, J.M. Serra, Study of CO2 stability and electrochemical oxygen activation of mixed conductors with low thermal expansion coefficient based on the TbBaCo3ZnO7+ system. Journal of Power Sources, 196 (2011) 4270-4276. 18/19 FIGURE CAPTIONS Figure 1. (a) Scheme of a cross section of the assembly thin film CGO membrane. (b) Schematic of the quartz membrane reactor design Figure 2. SEM images of the fracture cross-section of the ceria membranes after the permeation test (a) CGO porous support; (b) CGO supported membrane (c) BSCF as catalytic coating (d) CeTbO+Co as catalytic coating Figure 3. Dependence of the oxygen permeation flux through activated membranes on the sweep gas flow rate (QSweep). (a) BSCF as catalytic coating, (b) CeTbO+Co as catalytic coating. Inset: corresponding variation of the oxygen flux as a function of pO2 in the permeate. Figure 4. The dependence of the oxygen permeation flux of a CGO supported membrane on the oxygen partial pressure difference on the air side (pO2‘) and sweep side (pO2“) at 950 ºC and 850 ºC. Argon is the sweep gas. (a) BSCF as catalytic coating; (b) CeTbO+Co as catalytic coating. Figure 5. Oxygen permeation flux through activated membrane as a function of the temperature and oxygen partial pressure in the feed side. CeTbO+Co as catalytic coating; Argon as sweep gas; QSweep=400 ml·min-1; QFeed=100 ml·min-1. Figure 6. Oxygen permeation flux through activated membranes as a function of the temperature and air flow rate. Argon as sweep gas; QSweep=400 ml·min-1; pO2‘=0.21 atm; (a) BSCF as catalytic coating; (b) CeTbO+Co as catalytic coating. Figure 7. Oxygen permeation under methane feed as a function of the temperature. QSweep=65 ml·min-1; QFeed=60 ml·min-1; synthetic air as feed (pO2’=0.21 atm); CeTbO+Co as catalytic coating. Figure 8. Temperature dependence of oxygen permeation flux through coated CGO supported membrane and the monolithic LSCF membrane. Performance after 48 h in CO2 atmosphere at 750 ºC. Argon as sweep gas; QSweep=65 ml·min-1; QFeed=60 ml·min1; synthetic air as feed (pO2’=0.21 atm); catalytic coating: CeTbO+Co. Figure 9. Effect of the presence of CO2 in the sweep gas. Temperature dependence of oxygen permeation flux through MIEC membranes. .Synthetic air as feed (pO2’=0.21 atm), QFeed=60 ml·min-1; sweep gas was Ar or a mixture Ar/CO2 (85/15 vol.); QSweep=65 ml·min-1. (a) LSCF (b) activated CGO supported membrane; catalytic coating: CeTbO+Co. Figure 10. Temperature dependence of oxygen permeation flux through activated membranes: effect of the pO2 variation in the inlet sweep gas. QSweep=400 ml·min-1, 19/19 QFeed=60 ml·min-1; synthetic air as feed (pO2’=0.21 atm); (a) BSCF as catalytic coating; (b) CeTbO+Co as catalytic coating. OntheUseofSupportedCeriaMembranesforOxyfuelprocess/Syngasproduction ByM.PilarLobera,JoséM.Serra,Søren P.Foghmoes,MartinSøgaard, AndreasKaiser Figure1 (a) (b) T Ar (Sweep gas) N2 + O2 Ar + O2 Ar + O2 100 m Porous CGO support Impregnated with LSC CGO membrane Catalytic coating Sweep gas Feed (a) 10 m Support (b) 10 m Membrane (c) 20 m Cat. Layer: BSCF (d) 20 m Cat. Layer: CeTbO+Co OntheUseofSupportedCeriaMembranesforOxyfuelprocess/Syngasproduction ByM.PilarLobera,JoséM.Serra,SørenP.Foghmoes,MartinSøgaard, AndreasKaiser Figure2 OntheUseofSupportedCeriaMembranesforOxyfuelprocess/Syngasproduction ByM.PilarLobera,JoséM.Serra,Søren P.Foghmoes,MartinSøgaard, AndreasKaiser Figure3 0 200 400 0.0100.010 0.050 0.1000.100 0.500 0 200 400 QSweep (ml min-1) J(O2) (ml min-1 cm-2) (a) Coating: BSCF (b) Coating: CeTbO+Co 10-4 10-3 0.0 0.1 J(O2) (ml min-1 cm-2) pO2'' (atm) 900 ºC 850 ºC 800 ºC 10-4 10-3 0.0 0.2 0.4 pO2'' (atm) 900 ºC 850 ºC 800 ºC J(O2) (ml min-1 cm-2) OntheUseofSupportedCeriaMembranesforOxyfuelprocess/Syngasproduction ByM.PilarLobera,JoséM.Serra,Søren P.Foghmoes,MartinSøgaard, AndreasKaiser Figure4 0.00 0.02 0.04 0.06 0.08 0.0 0.2 0.4 0.6 J(O2) (ml min-1 cm-2) pO2' n-pO2'' n0.00 0.05 0.10 0.15 0.20 0.00 0.05 0.10 0.15 0.20 J(O2) (ml min-1 cm-2) pO2' n-pO2'' n (a) Coating: BSCF (b) Coating: CeTbO+Co 950 ºC 950 ºC n=0.0074 R2=0.9892 850 ºC 850 ºC n=0.0086 R2=0.9985 n=0.0016 R2=0.9923 n=0.0022 R2=0.9958 020406080100 2 theta Intensity (a.u.) SupportingInformation From “On the Use of Supported Ceria Membranes for Oxyfuel process / Syngas production “ by M. Pilar Lobera; José M. Serra*; Martin Søgaard; Andreas Kaiser FigureS1 -.XRD paterns. Ba0.5Sr0.5Co0.8Fe0.2O3-  and Ce0.8Tb0.2O2-  + 2% Co mol powdersafter final sintered.- BSCF CeTbO+Co 10-4 10-3 0.00 0.05 0.10 J(O2) (ml min-1 cm-2) pO2'' (atm) 900 ºC 850 ºC 800 ºC 10-4 10-3 0.0 0.2 0.4 pO2'' (atm) 900 ºC 850 ºC 800 ºC J(O2) (ml min-1 cm-2) FigureS2 -. Dependence of the oxygen permeation flux through activated membranes as a function of pO2in the permeate; (a) BSCF as catalytic coating, (b) CeTbO+Co as catalytic coating.- (a) Coating: BSCF (b) Coating: CeTbO+Co FigureS3 48 h 15 % CO2in Ar T=750 ºC Oxygen permeation test Ar as sweep gas T=750 ºC to 1000 ºC Oxygen permeation test Ar as sweep gas T=1000 ºC to 750 ºC -. Experimental procedure for the CO2stability study, including a carbonation step and subsequent permeation test .- FigureS4 -. Postmortem SEM analysis of the fracture cross-section corresponding to the LSCF membranes, the top side is the side exposed to the sweep gas during the permeation testing.- LSCF 30 m