Practical MCFC Systems

Molten carbonate fuel cell technology is being actively developed in the USA, Asia, and Europe. An example system is the 250-kW Hot Module of MTU Filedrichshafen shown 

Bipolar plate Anode Matrix Cathode Oxidator gas 

The current status of the MTU system is summarised in Table 7.4. The system is designed for industrial and commercial cogeneration applications, such as hospitals, and the philosophy behind the design of the Hot Module was as follows  

The system design for the hot module has all the processes that need to be run at elevated temperature located within the hot module, whereas systems such as power conditioning, natural gas compression, and so on are located outside the hot module. 

Ansaldo Euel Cells SpA (AFCo), an Italian company, is currently the other main developer of MCEC in Europe. The company was set up by Ansaldo Ricerche Sri, a 

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It is usual practice in an MCFC system that the CO2 generated at the anode be routed to the cathode where it is consumed. This will require some scheme that will either 1) transfer the CO2 from the anode exit gas to the cathode inlet gas ("CO2 transfer device"), 2) produce CO2 by combustion of the anode exhaust gas, which is mixed directly with the cathode inlet gas, or 3) supply CO2 from an alternate source. 

These issues do not eliminate the possibility of a pressurized MCFC system, but they do favor the selection of more moderate pressures. For external reforming systems sized near 1 MW, the current practice is a pressurization of 3 atmospheres. 

Operation on a CO stream is possible as well, as with the SOFC. Thus, CO, a minor species product of fuel reformation and a major poison to PEFCs, can actually be used as a fuel. In practice, this means that use of reformed gas without any CO cleanup is perfectly acceptable at the anode. On-anode reformation is also achievable, as with SOFCs, since water is generated after anode, which is necessary for steam reformation. A wide variety of fuels, including natural gas, coal gas, and biologically produced gases, can be successfully used with the MCFC system [29, 30]. 

The SOFCs have practically the same advantages as the MCFCs for applications in electric utility companies and chemical industries. An additional advantage is that, because the SOFC power plant is a two-phase system (gas and solid) whereas all other types of fuel cells are three-phase systems (gas, liquid, and solid), the complex problems associated with liquid electrolytes are eliminated... 

The high temperatures and presence of steam also means that CO oxidation producing hydrogen, via the shift reaction (equation 7.3), invariably occurs in practical systems, as with the MCFC. The use of the CO may thus be more indirect, but just as useful, as shown in Figure 7.21. 

Reforming As discussed, since the MCFC produces waste heat and steam at the anode like the SOFC and can use CO as fuel, an excellent opportunity exists for internal reformation of the fuel gas. There are two types of internal reformation practiced, indirect internal reformation (IRR) and direct internal reformation (DIR). In IRR, the fuel gas is mixed with water vapor, heated inside the stack with waste heat, and reformed over a catalyst bed into a hydrogen-CO mixture. The reformed mixture then enters the active fuel cell area. In DIR, the fuel gas is reformed directly in the active area flow fields. Although the DIR approach is more compact and can theoretically be used to remove a significant portion of the waste heat from the stack, IRR allows the use of different catalysts specifically for reformation, prolonging system life [32]. 

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