capture technology. The hybrid processes can be classified into absorption‐based, adsorption‐based, membrane‐based, and cryogen‐based hybrid processes. The integration of membranes into the absorption process (such as in the membrane contractor arrangement), catalysis process, and cryogenic process has progressed over the past years. However, the majority of the results are based on simulations or small‐scale testing campaigns, and the real value of using two technologies is not clear [38].
Within the range of emerging technologies, electrochemical separation has had a fast development over the past years and, potentially, will continue in this pathway. The following Section 1.2.5.1 will be focused on fuel cells because of the growing expectation on this electrochemical separation technology for its integration in power plants.
1.2.5.1 Fuel Cells
Fuel cells convert chemical energy of a gaseous fuel directly into electricity and heat. The fuel is oxidized electrochemically, which leads to lower exergy losses compared to direct combustion. In general, fuel cells are classified by the electrolyte material and their operating temperature (Figure 1.9). Low‐temperature fuel cells (100–250 °C) include alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), and proton exchange membrane fuel cells (PEMFCs), while high‐temperature fuel cells (600–900 °C) refer to Molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs). Because of the high temperature at which MCFCs and SOFCs operate, natural gas reformation and the subsequent shift reaction can be performed in the fuel cell itself. MCFCs and SOFCs are most appropriate for stationary power production at scales ranging from a few hundred kilowatts up to a few megawatts because of their high electrical efficiencies and the ability for cogeneration of electricity and heat [39]. Moreover, SOFCs and MCFCs are more fuel flexible and are not poisoned by carbon monoxide and carbon dioxide.
Figure 1.9 Two main options for CO2 capture using fuel cells. (a) The FC oxidizes a fuel taking oxygen from air and later separating CO2 from the anode effluent. (b) The MCFC concentrates the CO2 in flue gas from a conventional power plant from the cathode inlet to the anode outlet, while also oxidizing a portion of additional fuel.
Source: Adapted from [11].
When MCFCs/SOFCs are fueled with natural gas or syngas, CO2 capture can be implemented at different points, for example, after the fuel cell (“post‐anode capture”). Alternatively, H2 can be produced by reforming/partial oxidation of natural gas or coal gasification upstream the fuel cell and CO2 can be removed after syngas is shifted by means of physical solvents, membranes, or adsorbents – “pre‐anode CO2 capture,” similar to pre‐combustion.
Fuel cells generally operate with an approach that is similar to the “oxyfuel” concept, oxidizing fuel with oxygen extracted from air while generating power and releasing concentrated effluents at the anode outlet (Figure 1.9). This kind of power cycles generally require an integration with custom‐tailored gas turbine cycles, often operating at unconventional turbine inlet temperatures and pressure ratios, either using natural gas as a fuel or coal through integrated gasification fuel cell (IGFC) concepts. Because most fuel is oxidized in the fuel cell to allow a high CO2 capture efficiency, the fuel cell (FC) generates the majority of the cycle power output. The alternative option offered by MCFCs is shown at the bottom of Figure 1.9, where the fuel cell can operate “draining” CO2 from the cathode inlet stream, receiving the flue gases of a conventional power plant. In this configuration, the fuel cell operates with a post‐combustion approach, although also oxidizing a minor portion of additional fuel with the same “oxyfuel” features discussed above.
The parameters affecting the selection of operating conditions of the SOFC/MCFC are stack size, heat transfer rate, voltage output and cell life, load requirement, and cost. The main operating conditions are pressure, fuel utilization factor at the anode and O2/CO2 utilization factor (for SOFC and MCFC cases, respectively) at the cathode, voltage, current density, and temperature. The optimization of the process configuration in conjunction with optimal operating parameters is critical to minimize stack degradation, which directly impacts the performance and life of the FC.
Currently, the main challenges for stationary fuel cells are cost and cell durability. For the IGFC system, the gas cleaning process adds another energy barrier to its power generation.
1.2.5.1.1 Solid Oxide Fuel Cells (SOFCs)
Adams et al. [40] divided SOFC systems for CO2 capture into first‐ and second‐generation systems as a function of the operating pressure of the SOFC. Low‐pressure, first‐generation SOFC systems are the most promising option for SOFC commercialization at large scale (100 MW or greater) in the short term. Several process configurations and design options are possible (Figure 1.10), although those generally follow the same pattern and offer some flexibility to select the optimum combination of variables such as gas clean‐up/reforming, water gas shift (WGS), CO2 capture technology, and heat recovery.
Second‐generation SOFC systems are high‐pressure SOFCs with separate streams for the anode and cathode exhausts. This arrangement promotes the use of an SOFC system that captures and compresses CO2 at significantly reduced costs and minimum complexity via “pre‐anode” and/or “post‐anode” capture.
In the pre‐anode CO2 capture process, syngas is generated at high pressure through high pressure coal gasification or by reforming the natural gas available from a natural gas pipeline at high pressure. Similar to the above cases, the syngas can be optionally shifted using the WGS reaction, creating a stream of steam, H2, and CO2. Up to about 90% of the CO2 can then be recovered from the syngas (or shifted syngas) using absorption or adsorption technologies.
The post‐anode CO2 capture has been extensively studied in SOFC IGCC and natural gas cycles. A simple IGFC system is similar to an IGCC system, but the gas turbine (GT) power island is replaced by a FC island. Some system configurations still have a gas or steam turbine to utilize the extra heat. “Post‐anode” CO2 capture can be applied via CO2 separation from H2O via H2O condensation (or via cooling, knockout, and additional drying) and can effectively result in a 100% CO2 removal. A separation system that uses condensation followed by a cascade of flash drums can be used to produce CO2 at high enough purity for pipeline transport at the SOFC anode exhaust pressure.
1.2.5.1.2 Molten Carbonate Fuel Cells (MCFCs)
The MCFC can be used to separate CO2 thanks to the functional reactions that occur inside the cell. By sending flue gas from a power plant to the cathode, the CO2 from the flue gas is selectively separated and concentrated at the anode, in a mixture of water and small amounts of unreacted hydrogen and methane. The “cleaner flue gas” is delivered to the atmosphere with
