Fuel cells are devices for electric power production. Similar to their classical counterparts, which are based on a cycle process, they convert chemical energy (usually the enthalpy of combustion of a combustible substance) into electric energy. The substantial difference between these two classes of processes is that cycle processes usually use three energy transformation steps — from chemical enthalpy to thermal, then kinetic, and finally electric energy — whereas fuel cells directly convert chemical into electrical energy and thereby offer a chance to obtain higher degrees of efficiency [1-4].
The environmental impact of fuel cells strongly depends on what the fuel the cell is fed with. Fuel production based on fossil resources is neither free of greenhouse gases, because carbon dioxide is emitted, nor is it sustainable. Fuel based on renewable resources has overall zero net carbon dioxide emissions, but the combination of it with fuel cells is not compulsory. Thus, fuel cells are not a sustainable technique by themselves, but they promise a more efficient use of available fuels, be they based on fossil or renewable resources.
Among the known fuel cell types, the two high-temperature fuel cells, namely, the molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC), do not require hydrogen as their primary fuel gas, but they can be fed with any fuel gases containing short-chained hydrocarbons, carbon monoxide, and hydrogen. While a dominant part of low-temperature fuel cell systems is occupied by the reforming process, which transforms fuel gas into hydrogen, this process can simply be integrated into high-temperature fuel cells. This so-called internal reforming concept not only offers a simpler system design compared with that of low-temperature fuel cells with their external reforming units, but it also significantly increases the overall electric system efficiency. In addition, their insensitivity with respect to carbon monoxide allows a wide spectrum of fuels to be used in high-temperature fuel cells.
However, they not only provide electric power. Due to their operating temperature, these concepts combine high electric efficiency with a wide spectrum of heat utilization, for example, steam production, cold chillers, or a downstream cycle process. Compared with classical concepts, they offer a higher electricity/heat ratio, which is preferable in many stationary applications. Due to their combined heat and power production together with their fuel flexibility, high-temperature fuel cells are attractive for several areas of application. They can be used in small power plants based on natural gas, where they accommodate residential areas or a single larger building, for example, a hospital or an office building.
Beyond the replacement of classical units in today's stationary applications, where they offer superior efficiency, high-temperature fuel cells are also strong candidates for sustainable energy systems. The most prominent example is the use of biogases from fermentation processes (e.g., from wastewater treatment or from cattle-breeding farms). Further applications are the consumption of lean waste gases from processes in food or chemical industries, landfill gases, or mining gases. All these applications use fuels for on-site production of electricity in combination with heat or steam, depending on the specific demands of each application. High-temperature fuel cells are more suitable for these applications than a cycle process with low efficiency in combination with a boiler. They offer a chance to alter today's centralized energy supply system toward a distributed system where numerous opportunities for renewable energy supply can be exploited.
Although these properties apply to both known high-temperature fuel cell types, namely, the MCFC and the SOFC, we will focus on the first mentioned in the following sections. This is mainly because the MCFC is technically better developed, with most vital questions about material stability, system reliability, and production procedures already solved and the first commercial series actually available.
In the following, after a technical introduction into MCFC, its general advantages and drawbacks are discussed. Afterwards, several existing technical realizations are described and compared with each other. Several actual examples of application of MCFCs are discussed, and some future development trends are indicated.
Like any other fuel cell, the MCFC consists of several layers (Figure 12.1). The electrolyte layer of the MCFC consists of a eutectic carbonate melt (38% K2CO3, 62% Li2CO3), which is immobilized in a porous aluminium oxide structure (y-LiAlO2/a-Al2O3). It serves as a semipermeable layer that only allows carbonate ions (CO32-) to pass through the layer. Other substances, especially dissolved nonionic gases, cannot pass through.
On each side of the electrolyte layer, a porous catalyst layer, an electrode, is placed. They consist of an electron-conducting solid material, which also serves as a catalytic promoter for the reactions occurring at the respective electrode. Alternatively, the functionality of electron conduction and reaction promotion can be separated by using a carrier material, which mainly serves as the conductor, and placing the catalyst in a thin layer upon the surface of the carrier material. In the case of MCFC, nickel and nickel oxide are preferred materials for the anode and the cathode electrodes, respectively. A part of the molten carbonate is also located in the electrodes' pores, held in place by capillary forces. The remainder of the pores is filled with gas through which the educts and products of the reactions inside the electrodes are transported. The electrochemical reactions (see below) basically happen at the three-phase boundary between gas, liquid, and catalyst, so a large interfacial area is required in the pores' structure.
At each electrode, on the opposite side of the electrolyte layer, a gas channel is located. The anode channel is fed with a mixture of steam and the fuel gas, for example, methane. Prior to the reaction at the electrode, this gas has to be converted to hydrogen in the reforming process. The major chemical reactions in this process are the steam-reforming and the water-gas-shift reaction:
Anode electrode Electrolyte Cathode electrode
Exhaust gas h2o ho*
Anode electrode Electrolyte Cathode electrode
FIGURE 12.1 Working principle of an MCFC.
FIGURE 12.1 Working principle of an MCFC.
This process requires two things: heat, because it is endothermic, and high temperatures, because its conversion is severely limited by its chemical equilibrium at low temperatures. Both are available in the MCFC. The catalyst commonly used to promote these reactions is based on nickel.
The reforming products, mainly hydrogen and carbon monoxide, diffuse into the anode electrode and dissolve in the electrolyte inside the electrode pores. There they react at the electrode catalyst surface, thereby consuming carbonate ions from the electrolyte and producing free electrons, which are located on the electron-conducting solid phase of the electrode after the reaction. In addition, carbon dioxide and water are produced. Generally, the hydrogen oxidation is intrinsically faster than the carbon monoxide oxidation. But, especially with carbon monoxide-rich fuel gases, this reaction becomes important:
Because full conversion of hydrogen and carbon monoxide is not possible for thermodynamic and energetic reasons, the anode exhaust gas contains significant amounts of hydrogen and carbon monoxide, as well as a small portion of unreformed fuel gas. This gas has to be oxidized completely, so it is mixed with air and fed into a combustion unit. Along with the heat-releasing electrochemical reactions, this combustion is the main heat source within the MCFC system, and it is used to heat up the fresh air to the process temperature.
The completely oxidized gas is then fed into the cathode channel. Here the carbon dioxide is consumed together with some oxygen to form new carbonate ions, closing the carbonate ion loop. In the same reaction, two electrons are taken out of the electron-conducting solid phase of the cathode:
The cathode exhaust gas leaves the system.
Between the extra electrons at the anode and the "missing" electrons, i.e., the positive electron holes at the cathode electrode, an electric voltage occurs. Connecting the anode and cathode via an electric load, for example, an electric engine or a light bulb, allows the electrons to move from the anode to the cathode and do electrical work on that device.
Obviously, the MCFC exhausts carbon dioxide. This is contrary to the common wisdom that fuel cells do not emit greenhouse gases. In fact, this fuel cell even requires carbon dioxide in its cathode reaction; otherwise, the carbonate ions that are consumed at the anode reaction could not be replaced and the cell would quickly run out of electrolyte. It also is not sufficient to exclusively recycle the carbon dioxide that is produced at the anode electrode. This would require that every molecule of carbon dioxide fed into the cathode channel be converted to carbonate ions, which is not possible for thermodynamic reasons. Thus, a continuous feed of carbon to the system is necessary, causing a continuous exhaust of carbon dioxide. A global "zero emission" operation can only be achieved by using biofuels.
One of the major aspects in the MCFC is temperature. With a typical operating temperature of about 550 to 650°C, relatively inexpensive metal materials can be used, in contrast to the SOFC, which operates at 800 to 1000°C and consequently needs expensive ceramic materials. On the other hand, the MCFC temperature is high enough to obtain sufficiently high reaction rates with inexpensive and less active catalysts like nickel. For the reforming process, a temperature of 700 to 800°C would be preferable. At this temperature not only is the reaction rate high, but also the chemical equilibrium, which limits the conversion in this process, is significantly more favorable than at lower temperatures. In the internal reforming concept shown in Figure 12.1, the continuous removal of the reforming products, i.e., hydrogen and carbon monoxide by the oxidation process, helps to obtain high degrees of reforming conversion, although the MCFC temperature is relatively low.
Because of the absence of highly precious metals like platinum, high-temperature fuel cells are tolerant with respect to carbon monoxide. This is what makes the MCFC suitable for a wide range of different fuels. In principle, the MCFC would even operate on carbon monoxide only. Sulfur is a catalyst poison in the MCFC, so it must be removed from the feed gas.
A further advantage of the MCFC is its potential in the combined production of heat and electric power. Even if the exhaust gas is used to preheat the feed gas of the system, it still has a temperature of about 400°C, which is sufficient to generate pressurized steam in an industrial application or hot water for a residential building. Such coproductions are also possible with classical apparatuses, but oftentimes the electric power demand is equal or even higher than the demand for heat, a ratio that cannot be satisfied by engines or turbines alone. Due to their high efficiency, high-temperature fuel cells can meet these requirements. A hybrid system consisting of an MCFC and a downstream turbine can further increase the portion of electric energy produced by the system. Because of the low efficiency and high costs of very small turbines, this is economically useful for systems above 1 MW.
Like most other fuel cells, the MCFC principle promises low maintenance costs. Except for the blowers, which move the gases through the channels with comparably low pressure drop, there are no moving parts in the system. The major part of maintenance effort is the replacement of the cell stack, which has to take place after a certain degradation of the electrode catalysts. Today's stack lifetime expectancy is about 2 to 4 years, during which the system can continuously deliver heat and power.
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