of up to 67.7 ml min−1 cm−2 at 1000 °C for a 70 μm thick membrane [46]. Nevertheless, the unpractical stability under certain environments (especially when exposed to CO2 atmospheres) makes BSCF‐based membranes unsuitable for most of the industrial applications if a direct contact between flue gases and membranes is considered. Fluorites, on the contrary, exhibit outstanding chemical and mechanical stability when exposed to oxyfuel and reaction atmospheres, but their low electronic conductivity averts them for being considered for practical applications because of the low O2 permeation performance. One solution is then the use of dual‐phase structures consisting of a mixture of ionic‐conductive and electronic conductive materials. These materials with good O2 permeation and stability when subjected to harsh environments have attracted a lot of interest within the past years in studies focused on oxyfuel applications [47–51], achieving interesting O2 fluxes of c. 3 ml min−1 cm−2 at 925 °C under full CO2 environments [51].
3.3.2 Application Concepts of OTMs for Carbon Capture and Storage (CCS)
As already stated for the application of CCS strategies, OTMs are considered as O2 supply units for the conduction of combustion processes. In that way, an OTM module is integrated within an industrial process following approaches that use waste heat streams for heating up the module to its operation temperature (typically 800–900 °C). Therefore, the operation and integration can be done according to two different configurations, known as 3‐end and 4‐end modes (Figure 3.5a,b). These modes differ mainly on the number of streams connected to the OTM module. As can be seen in Figure 3.5a, the 4‐end mode uses a fraction of the flue gas (mainly consisting of CO2 and H2O) from the burner for heating the module and for performing O2 separation as a sweeping agent. As already pointed out, the recirculation of flue gases increases the plant efficiency, also reducing O2 production costs due to the highly synergetic thermal integration of OTM modules in the process. Differently, in the 3‐end mode, any contact between flue gases and membrane materials is avoided. In this configuration, O2 is extracted using a vacuum pump at the permeate side, thus obtaining a pure O2 stream that is used for combusting the fuel. The thermal integration is done by heating the air feed stream with heat exchangers where the heat is transferred from the exiting flue gases. This operation mode is considered for the cases where membrane material cannot operate under flue gas atmospheres. The selection of one or another configuration will influence parameters such as module design (especially the required membrane surface area) and plant layout, also determining the overall plant efficiency [52, 53]. Nevertheless, there is no preferred mode for conducting the OTM module integration. This can be observed in the OTM developments carried out by Praxair [54] and Air Products and Chemicals Inc. [55] (the most advanced up to date), which consider 4‐end and 3‐end operation modes, respectively.
3.3.3 Existing Developments
Currently, the R&D efforts are mainly focused on the development of new materials and membrane architectures with better performance and stability, as well on the search of new applications. All these developments are being carried out at laboratory scale; nevertheless, there are companies and institutions that are advancing in the upscaling of OTM concepts, with significant progresses in integrating ceramic membrane modules in industrial environments. Among all, the most advanced progresses have been achieved by Praxair and Air Products [56, 57], with the construction of demonstrative plants with OTM technology at high Technology Readiness Levels (TRL). These companies have worked for more than 20 years in the development of industrial‐scale OTM modules and integrated gasification combined cycle (IGCC) systems with ceramic membranes for O2 separation.
Figure 3.5 Simplified process layouts for oxygen permeating membrane modules integrated in oxyfuel power plants following (a) 4‐end and (b) 3‐end mode approaches, (c) Air Products' planar stacks, and (d) combined system steam reformer‐OTM‐ATR developed by Praxair.
Source: Linde.
With regard to Air Products, the most advanced developments consisted of an intermediate scale testing with a capacity of 100 temperature programme desorption (TPD) O2 (corresponding to an IGCC output of 12 MW) [58] and a membrane vessel consisting of several 1 TPD O2 OTM modules (as those shown in Figure 3.5c), with a total production of 2000 TPD O2. Despite that Air Products developments are the most advanced in terms of integration and demonstration, performance, and TRLs, they have been apparently abandoned since 2015 because of a company structure reorganization.
Praxair's developments present a tubular geometry where OTM tubes and CH4 reforming tubes are combined in systems for oxy‐combustion and syngas production applications by using advanced boilers and heaters in combustion processes [59–61]. As it can be observed in Figure 3.5d, Praxair's OTM concept consists of a multi‐panel tubular reactor system where natural gas steam reforming, O2 separation, and autothermal reforming is carried out by using integrated U‐shaped reformer and OTM tubes.
Research centers such as RWTH‐Aachen and the Fraunhofer Institute for Ceramics Technologies and Systems (IKTS) – both located in Germany – are conducting other of the most advanced developments in the OTM field. RWTH‐Aachen designed, fabricated, and tested in a realistic environment an OTM module within the OXYCOAL‐AC Project [62, 63]. The main aim of this development was to demonstrate a zero‐CO2 emission proof of concept for coal‐fired power plants using an OTM module as an O2 supply unit for conducting and oxy‐combustion [64]. For that, an OTM module was developed consisting of BSCF tubular membranes (15 m2 membrane area with 570 tubes) with a production capability of 0.6 TPD O2, generating up to 120 kW by combusting pulverized coal. With regard to IKTS, which are specialized in the manufacturing and testing of 3‐end OTM module systems considering BSCF tubes, they constructed the first stand‐alone O2 production unit in 2009 producing 2.7 l min−1 O2 at 850 °C [65], being later improved achieving up to 2 kg O2 h−1 (23.3 l min−1).
3.4 Protonic Membranes
As previously mentioned, the impact of the CO2 emissions on Earth is triggering the energetic transition from fossil fuels to environmentally friendly energy sources. H2 is a promising energy carrier allowing the storage of chemical energy; nowadays, its main use is as a reactant for the synthesis of NH3 and CH3OH, in the refining and other industrial applications. H2 can be used in fuel cell cars, as feed into the natural gas network, and in H2/O2 fuel cells among others [66–69].
Therefore, H2 separation is an important process and its utility has been demonstrated over the past years. Pressure gradient is the main driving force for H2 separation in these type of membranes, giving a large H2 partial pressure, hydrogen will migrate across the membrane. These membranes operate at a wide range of temperatures, and they can be divided into six different types depending on their properties, temperature ranges, and H2 permeation performance. Table 3.1 shows a comparison between H2‐selective membranes.
