MCFC System Designs

NETL looked at improving upon conventional MCFC system designs, in which multiple stacks are typically arranged in parallel with regard to the flow of reactant streams. As illustrated in Figure 9-19a, the initial oxidant and fuel feeds are divided into equal streams which flow in parallel through the fuel cell stacks. 

Gasifiers typically produce contaminants that need to be removed before entering the fuel cell anode. These contaminants include H2S, COS, NH3, HCN, particulate, and tars, oils and phenols. (See Table 6-3 for the MCFC contaminant list). The contaminant levels are dependent upon both the fuel composition and the gasifier employed. There are two families of cleanup that can be utilized to remove the sulfur impurities hot and cold gas cleanup systems. The cold gas cleanup technology is commercial, has been proven over many years, and provides the system designer with several choices. The hot gas cleanup technology is still developmental and would likely need to be joined with low temperature cleanup systems to remove the non-sulfur impurities in a fuel cell system. For example, tars, oils, phenols, and ammonia could all be removed in a low temperature water quench followed by gas reheat. 

When designing an MCFC power system, several requirements must be met. An MCFC system must properly condition both the fuel and oxidant gas streams. Methane must be reformed into the more reactive hydrogen and carbon monoxide. Carbon deposition, which can plug gas passages in the anode gas chamber, must be prevented. To supply the flow of carbonate ions, the air oxidant must be enriched with carbon dioxide. Both oxidant and fuel feed streams must be heated to their proper inlet temperatures. Each MCFC stack must be operated within an acceptable temperature range. Excess heat generated by the MCFC stacks must be recovered and efficiently utilized. 

The operating temperature of the MCFC of around 650 °C provides ideal opportunities from a system design perspective. At these temperatures with a suitable catalyst, internal reforming can be carried out. Most available fuels, such as... 

Tanaka J, Saiai A, Sakurada S, Nakajima T, Miyake Y, Saitoh T, Sasaki M, Yanaru H (1993) Design of 30 kW class DIR-MCFC system. Proc Electrochem Soc 93 37- 7... 

Many studies have been published describing analysis of stationary systems, covering all the major types of fuel cells. For PEM fuel cells, one of the most recent is that by Wall-mark (2002), and for a discussion of PEMFC stack modelling the reader should consult Amphlett et al., 1995. Examples of PAFC, MCFC, and SOFC systems are given in Parsons (2000), and we have referred in earlier chapters to other examples of system designs. Many discussions of system analysis are also available, for example, a recent analysis of energy and exergy in simple SOFC systems has been carried out by Chan et al. (2002). 

A demonstration of a MCFC power plant at an automobile manufacturing plant site in Tuscaloosa, Alabama is planned for the first quarter of 2001. The 250 kilowatt system will feed the production facility power distribution grid. Four companies are teaming up to support the program Southern Company, Alabama Municipal Electric Authority (AMEA), Fuel Cell Energy, and Mercedes Benz U.S. International, Inc. (MBUSI). The system will employ FCE s stack and MTU s power plant design, called the Hot Module. ... 

Compressor Intercooling Whether a compressor should be intercooled or not depends on the trade-off between the increased efficiency of the intercooled compressor and its increased capital cost. In general, intercooling is required for large compressors with pressure ratios that exceed approximately 5 1 (44). The designer also should consider whether the heat is advantageous to the process. For example, when near the 5 1 pressure ratio, it may not be appropriate to intercool if the compressed stream will subsequently require preheating as it would with the process air stream of an MCFC or SOFC system. 

In the development of molten carbonate fuel cells (MCFCs), many issues require mathematical models. Some of them, for example, the design of controllers and the integration of an MCFC stack in a larger plant system, can be solved with spatially lumped models. Other questions such as the analysis of an inhomogeneous current density profile or the optimal design and operation of a fuel cell with respect to temperature Hmitations, need spatially distributed models. Because the latter are usually the more complex models, this chapter is focused on these models. 

SciavoveUi A, Verda V, Amelio C, Repetto C, Diaz G (2012) Performance improvement of a circular MCFC through optimal design of the fluid distribution system. J Fuel Cell Sci Technol 9 041011/1-041011/8... 

During the next two decades, some decline of interest in fuel cells can be noted, and fewer studies appeared in this field. Technical and design improvements were introduced into models of the AFC, MCFC, and SOFC systems, and some large power plants were built. The basic structure of fuel cells themselves (compositiou of electrodes and electrolyte) and also the specific performance figures (per unit surface area of the electrodes) changed little during this time. 

Another fuel cell design is the molten carbonate fuel cell (MCFC) (Yuh, 1995), which operates in the temperature range 620-660°C with an efficiency of >50%. FuelCell Energy, Inc. (Danbury, CT) produces MCEC units. These units are designed as back-up generators for intermittent use. The operational lifetimes of fuel cell systems need to be extended. In order to do so, it is necessary to limit component corrosion. 

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