Читать книгу Engineering Solutions for CO2 Conversion - Группа авторов - Страница 21

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.