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

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is an Accepted Manuscript of an article published by Elsevier in Solid State Ionics on 5 November 2019, available at: https://doi.org/10.1016/j.ssi.2019.115039 .” Understanding the thermochemical behavior of La0.6Sr0.4Co0.2 Fe0.8O3 and Ce0.9 Gd0.1O_Co oxygen transport membranes under real oxycombustion process conditions E. Portilloa, * , T.R. Reinab, F. Vegaa, M. Canoa, B. Navarretea a Chemical and Environmental Engineering Department, School of Engineering, University of Seville, C/ Camino de los Descubrimientos s/n 41092 Sevilla, Spain b Department of Chemical and Process Engineering, University of Surrey, Guildford, GU2 7XH, United Kingdom ABSTRACT Nowadays, oxygenO transport membranes (OTM) are known asare 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) have been studiedis 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. Key words: ITM, Oxy-fuel; LSFC; CGO; Thermochemical stability. 1. Introduction Nowadays, minimizing the impact of greenhouse gases (GHG) emissions is one of the most important scientific and societal challenges. According to the recent studies carried out by organizations and institutions such as the Intergovernmental Panel on Climate Change (IPCC), the International Energy Agency (IEA) and the United States Environmental Protection Agency (EPA) the GHG emissions has experienced * Corresponding author. E-mail address: [email protected] Con formato: Espacio Después: 0 pto Con formato: Espacio Después: 0 pto continuous growth. In the case of carbon dioxide, a parallel trend CO2 emissions and global mean surface temperature rise has been observed (GMST). The rising trend on CO2 emissions is guided by the increasing demand on energy consumption due to a growing population. In response to this problematic, carbon capture and storage (CCS) has emerged as an attractive alternative which is able to minimize CO2 emissions impact in the short and medium term [1]. This CCS technology allows the utilization of fossil fuels both in the energy and transport sectors and in the industrial chemical sector (cement, refineries, iron and steel manufacturing, etc.), confining CO2 in a safe manner in the subsoil. Within the CCS technology, there are three conceptual strategies, which are easily adaptable to the market, namely pre-combustion, post-combustion and oxy-fuel combustion. Nowadays, these technologies show important advantages and drawbacks that must be considered in order to be implemented over a full scale [2,3]. According to these alternatives of CO2 capture, this work has focused on the oxy-fuel process, which allows the combustion of coal with pure oxygen diluted in a recirculated flue gas. The advantage of this approach is a resulting flue gas mainly consisting of CO2 and H2O, which can easily be purified by condensation to a gas stream with high CO2 contents (> 90% vol.). This way the clean CO2 stream can be compressed for its transportation and storage. Fig. 1Fig. 1 shows a typical block flow diagram of an oxy-fuel combustion process. Fig. 1. Block diagram an oxy-fuel combustion process with the air separation unit. In this configuration, the air separation unit (ASU) is the main responsible for efficiency loses. Hereby, efficiency loses of 8.4 - 10.4 % points are estimated for an oxy-fuel power plant [4,5]. As of today, alternatives such as polymer membranes, high pressure adsorption (PSA) or vacuum (VSA) are other alternatives to supply the oxygen required into oxy-fuel combustion. However, the quantities of required oxygen required together with and its high purity (above 99.5%) impose technical and economical limitations. In order to overcome these limitations oxygen transport membranes (OTM) are envisaged as a promising alternative for oxygen separation at industrial scale [6– 9]. OTMs are typically Mixed Ionic Electronic Conductors (MIEC) where oxygen is separated by transporting ionized oxygen at high temperature throughout their ceramic ASU Energy Mill Air N2 O2 Coal Caldera Boiler SCR ESP FGD CO2 H2O 2nd Recirculation 1st Recirculation Purification CO2 Compression Storage Mechanical energy Waste water Gas Energy Treatment process structures. These membranes are fully dense and can potentially be able to separate oxygen with a selectivity of 100% at temperatures above 700 oC. The ceramic membrane use to for high-temperature O2 separation has received increasing interest because of the possibility to reducegiven its key role to reduce the O2 production cost and the energy penalty. In this sense, there are techno-economic evaluation studies [10,11], which predict an overall capex saving of 10% in comparison to the conventional cryogenic air separation in oxy-fuel power plants. Single-phase MIEC materials present elevated electronic and ionic conductivities, which allows high oxygen fluxes. However, most of the promising MIEC materials are not chemically stable under atmospheres containing CO2 and SO2, which are always present in a “four-end mode” in oxy-fuel combustion power plants. This concept is known as an operating mode to obtain the driving force to transport oxygen through the membrane by sweep stream that is composed of gas recirculation stream on the permeate side. Therefore, it is crucial to study the behavior of ITMs under realistic operating conditions, i.e. surrogate flue gases from coal oxy-fuel combustion processes in order to assess their viability in an oxy-combustion plant. According to literature, Table 1Table 1 shows the typical values of major and minor components of exhaust gases from an oxy-combustion process. In this table, the minimum and maximum values of each component have been specified according to the combustion of different type of carbon (anthracite, bituminous and sub-bituminous) into a pulverized-coal boiler and the main characteristics of an oxy-fuel combustion process [12–14]. Table 1 Reported composition in exhaust gases from coal oxy-fuel combustion processes [12–14]. Component Minimum range Maximum range CO2 (vol% on a dry basis) 50 80 O2 (vol% on a dry basis) 3 5 N2 (vol% on a dry basis) 0 1 SO2 (vol% on a dry basis) 0 6 NOx (ppmv on a dry basis) 0 1500 H2O (vol%) 10 50 Concerning the trace elements from the combustion process, Table 2Table 2 shows an estimate of the global emissions factors for coal in the electrical sector. In this case, it is not possible to estimate the concentration of trace elements in oxy-fuel combustion with respect to those recorded in conventional combustion. Table 2 Trace elements in exhaust gases from coal combustion processes [15]. Component µg/MJ Component µg/MJ Component µg/MJ As 15‒100 Mg 70‒450 Se 7‒50 Cd 2‒25 Mo 15‒150 Sn 10‒50 Cr 80‒500 Ni 90‒600 Tl 10‒40 Cu 60‒200 Pb 50‒300 V 20‒300 Hg* 10‒35 Sb 10‒50 Zn 70‒500 Con formato: Espacio Después: 0 pto Con formato: Fuente: 12 pto, Sin Negrita, Revisar la ortografía y la gramática Considering the operating conditions reported in previous works (Table 3Table 3), it is observed these data are far from realistic oxy-fuel conditions due to the lack either of some components or the underestimated concentration of other reactants. In this sense, it is interesting necessary to evaluate the material stability under a complete flue gas atmosphere that has not been studied in previous researchespreviously investigated. Table 3 Overview of the operating conditions in previous studies. Composition in Sweep gas Concentration in Sweep gas (vol%) Gas flow rate (mlmin-1) Reference CO2/H2O/SO2 0-100/0-30/0-2000ppm 250 16,17 CO2/H2O/O2/N2/SO2 10‒100/0‒20/0‒5/8/0‒ 250ppm n.a.a; 1000 18–20 NO 0‒600 ppm 25‒50 21 Ar 100 20.8-750 22–25 H2 100 200 26 Ar/CO2 0‒100/0‒100 n.a.a 27 Ar/CO2/SO2 0-100/0-100/250 ppm 300 28 CO2/He 0‒12/100‒88 75-150 29,30 CO2/N2 80 / 20 100 31 Ar/SO2 0-100/1748ppm 200 32 He/ O2/ SO2 30 / 5/ 15 5 33,34 CH4/CO2 2,5–90 / 97,5‒10 n.a.a 35 CO2/CH4 n.a.a 465‒500/ 0‒ 35 36 He or Ar or CO2 100 15-2500 5,23,37,38 CO2 50-100 100‒200 39 Ar /CH4 0-100/ n.a.a 200 40,41 He 10-100 20 ‒ 260 8,42 CH4/O2 70/30 100 43 CO2/O2 72‒86/14‒28 n.a.a 44,45 O2/He n.a.a 100 46 a n.a.: not available. In recent years, a significant number of publications have been dedicated to study the behavior of single-phase MIEC materials [7,47,48] being fluorites (AO2) or perovskites (ABO3) the most promising alternative in terms of oxygen permeability [26,38,49]. Within the perovskite family, typical formulations are SrCoO3, SrFeO3 with O2 fluxes above 0.023 molm-2s-1 at 850 °C [50] and La1-xSrxCoyFe1-yO3 with O2 fluxes between 0.003 and 0.089 molm-2s-1 at 850 – 1000 °C [(51)] [22,27,38,41,52]. Despite their excellent oxygen mobility, these samples are prone to form sulphates and carbonates thus provoking the membrane failure for long-term runs. As for the fluorites, Gadolinium-doped Ceria Ce1-xGxO2-δ is a strong candidate due to its high ionic conductivity (i.e. 0.06 S/cm at 800 °C) and their chemical stability when compared to perovskite type materials [58]. Additional enhancement of the electronic conductivity can be achieved by doping the membranes with metals such as Co [40,41], Pr [53], Cu [54] or Ni [55,56], Ag [29]. In summary, a plethora of studies have been carried out to assess the thermochemical stability of OTM membranes including impurities present in the oxy-combustion flue gases (Table 3Table 3). However, to the best of our knowledge there are no studies Con formato: Fuente: 12 pto, Sin Negrita, Revisar la ortografía y la gramática under realistic coal oxy-combustion, which opens a debate concerning the stability of the MIECS under representative flue gas environments (Table 1Table 1). In response to these needs, this work aims to identify and understand the thermochemical parameters that influence the material properties and provide the information required to strength and permeability. In response to these needs, tThis work evaluates the behavior of OTM materials under a surrogate flue gas including all the species present in a real process such as CO2, O2, N2, SO2, NOx and H2O. An experimental facility has been designed for this purpose. Two family of materials were selected for this study: (i) La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) amongst perovskites; and (ii) Ce0.9Gd0.1O doped Co (CGO_Co) amongst fluorites. These families present quite a different reactivity towards the chemical environment selected in this work 2. Experimental 2.1. Material and synthesis The main raw materials used in this work including commercial powder of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF, 99.9%), Cerium (III) nitrate hexahydrate (≥99.9%), (≥99.9%) and Cobalt (II) nitrate hexahydrate. All chemicals in the experiment were purchased from sigma-Aldrich and prepared by Instituto de Tecnología Química frorm Valencia. Two synthesis methods have been used according to the metal precursor nature. Firstly, LSFC powders have been fabricated by Pechini sol-gel route [22,23,51]. To that end, the corresponding commercial powder was dissolved in distilled water. Subsequently, citric acid (Sigma Aldrich) and ethylene glycol were added as chelating and gelating agents with a molar ratio of metal: citric acid: ethylene glycol equual to 1:2:4. This solution was maintained in constant stirring on a hot plate at 70 oC. After drying (up to 110 oC for 12h), the obtained gel was fired at 650 oC for 1h. For the preparation of Gadolinium doped ceria with additions of 2%molar of cobalt oxide has been prepared by co-precipitation method in order to synthesize powders of nanometric size. This technique starts with the dissolution of commercial lanthanide nitrates mixture in distilled water at 50 ºC. In order to achieve the total precipitation a (NH4)2CO3 solution was dropped under stirring into the previous solution. The final NO3-1/CO3-2 molar ratio was 0.75. The resulting precursor powder was dried at 100 ‒ 150ºC after filtration and rinsing with water. Cobalt incorporation (when required) was done over the dried precursor powder by incipient wetness impregnation. Calculated 2% molar of Co was dissolved in distilled water (volume corresponding to the pore volume) and mixed with the powder. Finally, each powder was calcined during 5 hours in air atmosphere at 800 ºC to decompose the residual nitrates and carbonates and to favour the formation of the fluorite phase [40,51,57]. Analogous sintering procedures were applied to both families. Firstly, the powders were ball-milled in ethanol for 4h, pressed into disk shapes samples with a diameter about 15 mm. After that, LSCF and GCO_Co were sintered at 1200 oC and 800 oC for 5 hours, respectively. 2.2. Thermochemical stability tests Con formato: Fuente: 12 pto, Sin Negrita, Revisar la ortografía y la gramática The thermochemical stability of the LSCF and CGO_Co samples were carried out in a lab-scale rig, which is schematically shown in Fig. 2Fig. 2. The facility is divided into three sections. In the first area, the gas mixtures are generated using controlled pressurized cylinders. For the surrogate atmospheres, two categories of gases are used, that are named major components (CO2, O2, N2) and minor components (SO2 and NO). The latter group is injected after a pre-heating step (second zone) which includes moisture to avoid acidic condensation. In addition, the facility has a compressed air inlet to facilite the heating and cooling processes of the unit. Water injection and pre-heating take place on the second zone. Steam was added to a first tubular oven, which has two-stage preheater system with an adjustable temperature of up to 350 oC. In order to achieve a good control during this injection, it has a peristaltic pump and electric armoured resistance, which work as an automatic evaporator system. Finally, there is the third area where the samples are exposed to the performed gases stream previously and high temperature. This area consists of a tubular oven capable of operating up to 1500 oC and an automatic gas analyser, which provides information about the gases concentration both during and after each test. To avoid material losses during the tests, the samples were placed into crucibles on a sample holder stainless iron, which is put in the middle of the oven. This second oven has a gas inlet and outlet circuit with shut-off valves at the ends. This design allows testing both continuous mode with gases in circulation, and batch mode under a closed and controlled atmosphere. The temperature is controlled by thermocouples, which are placed in different places of the facility as it can see in Fig. 2Fig. 2. Fig. 2. Scheme of the experimental plant used for the thermochemical testing of the sintered samples. The experiments were performed at atmospheric pressure at 900 °C. Once the temperature was reached, the gas compositions was switched to the desired mixture to evaluate the effect of these gases on the stability of samples. Table 4Table 4 summarizes the composition of the gaseous streams used in the stability tests. The levels of species were chosen according to reported and available results of pilot oxy-fuel combustion plant [14,58]. The concentration of gases in the mixtures were regulated by mass flow controllers (MFC Bronkhorst) at a total gas flow rate of 10 lstpmin-1 for 8h. For safety reasons, the outlet gases were cooled downs to room temperature to remove steam. Afterwards, the dry gases were evacuated by a fume hood housed that makes ensure its complete removal in the lab. After the exposure of the samples to the gas atmosphere, the experimental unit was cooled down in the same gas atmosphere to avoid possible decomposition (by oxidation) of the species formed during the treatments. Once the material reached room temperature, it was recovered for its characterization. Table 4 Conditions of the thermochemical stability tests performed with sintered samples. Test CO2 (vo%l)a O2 (vol%)a N2 (vol%)a SO2 (ppmv/v)a NO(ppmv/v)a H2O (vol%)b 1 84 4.5 1 300 140 21 2 83 4.8 1 500 138 14 3 80 4 1 700 176 24 4 83 3.5 1 1000 106 18 a Dry basis. b Percentage of the total flow, including steam. 2.3. Material characterization Concerning the dense sintered samples, the analytical techniques selected for its characterization were X-ray diffraction (XRD), X-ray fluorescence (XRF), infrared spectroscopy (ATR-FTIR) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). The powder materials were characterized with the Raman spectrometer and the BET equipment. XRD diffraction was performed in a Bruker diffractometer (XRD; model D8 Advance A25 powder diffractometer; lineal Lynxeye detector; Cu Kα-1=1.5406 Å; Kα-2=1.5444 Å; Excitation= 40 kV and 30mA) in the 2θ range of 20 – 90° at room temperature. The species present in the samples were identified by the XRD patterns that were processed using the software diffract.eva of BRUKER. In order to determine the elemental analysis of the dense fresh and samples, X-ray fluorescence (XRF) was used in this work. Measurements were performed using an Axios spectrometer (Panalytical), equipped with a 4kW rhodium anode tube. An Alpha II FTIR spectrometer was used tofor performeing the attenuated total reflection Fourier transform infrared spectroscopy in the samples (ATR-FTIR). Background subtracted spectra of the raw and the treated samples were collected at room temperature by co-adding 64 scans at 4 cm-1 resolution in transmittance mode. The data were baseline corrected using OPUS software™. The microstructure of dense samples was examined by Scanning Electron Microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) in a JEOL 6460LV operated at 20 kV. Raman spectroscopy is sensitive to chemical structure and is a powerful technique for the characterization of electrochemically active materials [48]. In this work, this technique was used to analyse carbon or sulphur depositions during experimental testing. Raman measurements of the powders samples were recorded using a Thermo DXR2 spectrometer equipped with a Leica DMLM microscope. The wavelength of applied excitation line was 532nm ion laser and 50x objective of 8-mm optical was used to focus the depolarized laser beam on a sport of about 3 µm in diameter. N2 adsorption/desorption isotherm of the powder samples were measured by using a Micromeritics, ASAP 2405N analyser at -195.815 °C in liquid nitrogen. The Brunauer−Emmett−Teller (BET) surface area was calculated using experimental points at a relative pressure of P/P0 = 0.05−0.30. Prior to N2 sorption measurements, the samples were degassed at 250 °C, under vacuum, for at least 11 h. 3. Results and discussion 3.1. Thermochemical stability of LSCF 3.1.1. Studies with dense sintered samples Crystalline phases of the prepared fresh and spent samples were depicted in Fig. 4. The fresh sample displayed the typical diffraction peaks ascribed to the perovskite phase of the LSCF as previously reported elsewhere [17]. As for the spent materials, the peaks corresponding to the perovskite LSFC remains present but their intensity and width are altered i.e. wider peaks are observed. According to literature, the later indicates crystalline structure deformation and a lost of crystallinity [32]. Besides, peaks of new phases were observed. Some of these new peaks can be attributed to single oxide phases such as La2O3 and CoO, which are segregated from the perovskite during the thermochemical runs [22,59]. Likewise, each sample displays peaks related to the formation of carbonates (CoCO3, FeCo3 and SrCO3) [17,31,32] and sulphates (SrSO4) [31,32,51] which are again a consequence of the aggressive atmosphere. Finally, it is observed a major intensity of the carbonate and sulphate signals in the test number 3 (700 ppm of SO2, 24% vol. H2O). This may point out that the higher the level of water, the greater the structure modification for relatively high concentrations of SO2. In any case, the XRD diffraction shows that the LSCF samples are not stable under any of the studied conditions. Fig. 8. ATR-FTIR of the CGO_Co (powder samples) fresh and treated with different atmospheres at 900 oC during 8h). Additionally, the morphological study of the fluorite family was performed. Fig. 9Fig. 9 displays the SEM image of the fresh sample along with the SEM images of the samples exposed to test 2 and 4. Just by a simple visual comparison, most of the samples have a surface morphology similar to the fresh material. However, small sections with debris have been detected in some samples such as in the case shown in Fig. 9Fig. 9 c, d. As in the perovskites family case, the EDX analysis has been performed in order to analyze those surface areas. In particular, carbon has been detected in all the samples, reaching values close to 7%. Likewise, traces of another impurity was also detected (silicon with values close to 1%). It is worth noting that these new species are nucleated at the grain boundaries, suggesting that a careful control of the membranes microstructure could lead to an enhanced thermochemical stability. In addition, the fact that they nucleate only on the grain boundaries means that most the surface is free to conduct the oxygen permeability. Therefore, this technique also confirms every evidences analyzed in the previous characterization analyzes. 3000 2500 2000 1500 1000 500 C=O 698 858 1044 1256 14371591 Si-Si/ Si-H C=O 2900 Transmitance (a.u.) Wavenumber (cm-1) CH Fresh 300 ppm SO2, 21 % H2O, AG 500 ppm SO2, 14 % H2O, AG 700 ppm SO2, 24 % H2O, AG 1000 ppm SO2, 18 % H2O, AG COO-C-O C=O Fig. 9. SEM image of the surface of the CGO_Co membranes before (a, b) and after test 4 (c, d) and test 2 (e, f). Images (b), (d) and (f) are the higher magnification for (a), (c) and (d), respectively. 3.2.2. Studies with the powder samples To complete the characterization of CGO_Co samples, Raman and N2-adsorption analysis were developed on the powder samples. Concerning the Raman spectra, all the samples shows exactly the same profile of the fresh material (Fig. 10Fig. 10). As shown in the figure, vibrational modes at 251, 468, 557, 950 and 1200 cm-1 typical of CeO2 were detected. In particular, these bands are associated with the stretching of the Ce−O bond [57,71]. The bands between 600 and 678 cm-1 are ascribed to Co3O4 species (F2g-618 cm-1: 685) [72]. On the contrary, the vibrational mode at 360 cm-1, which is usually attributed to the band of the Gd2O3 cubic phase, has not been detected in any spectrum Therefore, this corroborates the incorporation of Gd in the ceria lattice.. Overall from the Raman perspective, these samples are quite robust towards the oxy-fuel flue gases. 50µm (c) 10µm (d) 40µm (e) 10µm (f) 40µm (a) 10µm (b) 500 1000 1500 2000 2500 Intensity (a.u.) Raman Shitf (cm-1) Fresh 300 ppm SO2, 21 % H2O, AG 500 ppm SO2, 14 % H2O, AG 700 ppm SO2, 24 % H2O, AG 1000 ppm SO2, 18 % H2O, AG CeO2 Co3O4 Fig. 10. Raman spectra of CGO_Co samples fresh and treated with CO2, SO2, NOx and H2O mixtures at 900 oC and atmospheric pressure during 8 h. Finally, the textural properties of the fluorite family are summarized in Table 8Table 8 where fresh and spent samples are compared. Table 8 Surface area, Pore volume and Pore size analysis of the CGO_Co samples fresh and treated with CO2, SO2, NOx and H2O mixtures at 900 oC and atmospheric pressure during 8 h. Samples Surface area (m2/g) Pore size (nm) Pore volume (cm3/g) Fresh 7 858.08 0.0046 300 ppm SO2, 21% H2O, AG 6 1085.02 0.0029 500 ppm SO2, 14% H2O, AG 3 2051.78 0.0012 700 ppm SO2, 24 H2O, AG 3 1916.72 0.0022 1000 ppm SO2, 18% H2O, AG 3 2118.50 0.0014 Just like perovskite cases, the surface area and pore volume of CGO_Co powders decreased as the operating conditions become more aggressive. Besides, the opposite trend was obtained for the pore size. Although such trends were observed also for the LSCF family, there trends in the fluorite family are smoother (Fig. 11Fig. 11). The later, again points the superior stability (not only structural but also textural) of the fluorite type samples compared to the perovskites. In view of these results, it we can partially conclude that the CGO_Co samples are much more stable than LSCF. 01234 Pore size (nm) Tests Pore size, LSCF-type samples Pore size, CGO_Co-type samples Dif 676 Dif 1716 Dif 925 Dif 676 Dif 1716 Dif 925 Dif 1194 Dif 1086 Dif 1260 Dif 227 01234 Surface area (cm2/g) Tests Surface area, LSCF-type samples Surface area, CGO-Co-type samples Dif 3 Dif 3 Dif 1 Dif 3 Dif 7 Dif 9 Dif 8 01234 Dif 1 Dif 2 Volume pore (cm3/g)*103 Tests Volume pore (cm3/g) *103, LSCF-type samples Volume pore (cm3/g) *103, CGO-Co-type samples Dif 4 Dif 6 Dif 5 Dif 3 Dif 3 Con formato: Fuente: 12 pto, Sin Negrita Fig. 11. Trend comparison of BET results between perovskite-type and fluorite-type samples during the experimental tests, assuming as base reference the fresh samples. 4. Conclusions This work evidences the different thermochemical stabilities of two type of MIECs under realistic oxy-fuel combustion conditions. We have conducted a systematic studied in a lab scale rig which allows mimicking the conditions in an oxy-fuel plant. Such a rig has been validated with successful comparison with previous results in literature. The thermochemical tested samples were subjected to a full characterization study. Within the selected MIECs, the LSCF has proven poor stability. Its integrity has been compromised under all the sweep gas atmospheric conditions employed in this work. We observed a compositional, structural, textural and morphological change when the samples are exposed to different concentrations of CO2, SO2, NOx and humidity. As for the CGO_Co we observe that this material offers acceptable levels of resistance towards the flue gases in the oxy-fuel environment. Indeed, this material is quite robust and practically does not form any crystalline carbonate or sulphate. Only some superficial carbonates were detected by FTIR that seems to nucleate in the vicinity of the grain boundaries. Interestingly, traces of silicon species were found in this sample after the thermochemical tests. This indicates some susceptibility of the membranes towards the materials present in the stainless steel either in a lab scale reactor or in a full-scale plant, an observation often disregarded in academic works. In any case, further enhancement of the CGO stability could be achieved by tuning the synthesis procedures or adding different dopants to the fluorite structure. Additionally it must be underlined that NOx species – often ignored in previous studies - barely affect the thermochemical behavior of the studied membranes. It seems that the most damaging species in the flue gas are SOx, CO2 and H2O. Overall, this paper represents a systematic methodology to assess the thermochemical stability of OTM materials with potential application in oxy-fuel modules. Finally, once it has demonstrated acceptable material stability of CGO_Co samples, further investigations are ongoing in our labs aiming to evaluate geometric aspects and its membrane transport properties, e.g. selectivity and mass transfer coefficient. Acknowledgements The authors gratefully acknowledge the cooperation of Instalaciones Inabensa S.A. as well as Institute of Chemical Technology (ITQ) by the Polytechnic University of Valencia for providing OTM material used in this work. Financial contribution for this work was provided by the Ministry of Innovation Science and Business of the Regional Government of Andalusia as well as VI Plan Propio de Investigación y Transferencia de la Universidad de Sevilla (VI PPIT-US). Besides, this work was technical and financially supported by Department of Chemical and Process Engineering of the University of Surrey and the EPSRC grant EP/R512904/1 as well as the Royal Society Research Grant RSGR1180353. Nomenclature: Comentado [RRTD(&PE3]: I would just remove this to avoid further question – this is anticipating the response to the referee Abbreviations ASU Air Separation Unit ATR-FTIR Infrared spectroscopy (ATR-FTIR), BET Brunauer−Emmett−Teller analysis CCS Carbon Capture and Storage CGO_Co Cobalt doped Ce0.9Gd0.1O with (fluorite-type membrane) EPA United States Environmental Protection Agency ESP Electrostatic Precipitator FGD Flue-gas Desulfurization GHG Control of Greenhouse Gas GMST global mean surface temperature IEA International Energy Agency IPPC Intergovernmental Panel on Climate Change LSCF La0.6Sr0.4Co0.2Fe0.8O3 (perovskite-type membrane) OTM Oxygen transport Membrane PSA High pressure adsorption SCR Selective catalytic reduction SEM-EDS Scanning electron microscopy with energy-dispersive X-ray spectroscopy VSA Vacuum swing adsorption XRD X-ray diffraction XRF X-ray fluorescence 1. Hu Y. CO2 capture from oxy-fuel combustion power plants [Internet]. 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