Engineering Solutions for CO2 Conversion. Группа авторов
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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 up to 70% less CO2 content, which is transferred to the MCFC anode exhaust stream where it can be separated much more effectively, resulting in a high‐purity CO2 flow. The main advantage in this process is that extra power is generated because the MCFC will be fueled and operated normally to carry out the separation, and it increases the overall efficiency of the power plant and compactness of the post‐combustion unit, while reduces the energy penalty. The modularity feature of MCFC systems allows to tailor the installation to the capture needs or gradually increases the size of the capture unit.
Figure 1.10 Superstructure of SOFC – CO2 capture process configurations.
Source: Adams et al. [40].
One example of an MCFC and CO2 capture system was developed by Fuel Cell Energy (FCE), namely, the Combined Electric Power and Carbon‐dioxide Separation (CEPACS). In the process of capturing >90% CO2. In this configuration, the system can generate up to 351 MWe additional power (net AC), after compensating for the auxiliary power requirements of CO2 capture and compression.5
1.3 Integration of Post‐combustion CO2 Capture in the Power Plant and Electricity Grid
A key aspect of thermal power plants is their carbon intensity (CO2 emitted per unit of energy generated, generally expressed as kg CO2/MWh). Nowadays, the global average is around 500 kgCO2/MWh, which must be reduced to 100 kgCO2/MWh by the late 2030s to be consistent with a 2 °C climate pathway [36]. Even if combined cycle thermal power plants can be considered as low carbon alternatives in some scenarios, in the mid‐to‐long term, it might be required to further decarbonize the existing units by retrofitting them with CCS or by building novel designs with low CO2 emissions. As demonstrated at commercial scale, post‐combustion CO2 capture can significantly reduce the carbon intensity of thermal power plants [2]. Table 1.3 compares the carbon intensity of thermal power plants with and without CCS.
1.3.1 Integration of the Capture Unit in the Thermal Power Plant
In principle, the key integration aspects of the power plant and the capture unit are the flue gas, emitted by the power plant and sent to the capture unit, and the energy requirements of the chemical absorption/desorption process, provided by the power plant to the capture unit (Figure 1.11). Figure 1.11 shows a simplified schematic of a power plant integrated with a post‐combustion CO2 capture system. The main energy and mass integration flows are described. Fuel and air are used in the combustion process, providing heat to produce steam in the power cycle. The flue gas from the combustion is sent to the CO2 capture unit and leaves it lean in CO2. A CO2 rich stream is produced in the CO2 capture plant and sent to conditioning, transport, and storage. Heat in the form of steam is provided from the power plant and is returned back as water condensate. Electricity from the power plant is utilized to run the auxiliary systems of the capture unit, including the flue gas fan, cooling, and solvent circulation pumps. Higher levels of process integration between the power plant and the capture unit can be considered, as explained in [41].
Table 1.3 Low heat value (LHV) efficiency, carbon intensity, and energy penalty in coal‐ and gas‐based thermal power plants with CCS [43, 54, 76].
Source: Adapted from Adams and Mac Dowell [43], Gonzalez‐Salazar