Performance of MCFC

Significant polarisation losses occur in MCFC and its operating conditions are almost same as those of PAFC. For an MCFC, the anode and cathode compartments are at the same pressure, and the change in reversible voltage can be given by 

Equation (2.95) infers that for a tenfold increase in pressure, an increase of 46 mV in reversible cell potential is observed. As the pressure is increased, the mass transport rates and gas solubilities also increase. Anyhow, the increased pressure may promote some reactions decomposition of methane to carbon/hydrogen, methanation (methane formation) and carbon deposition and methane formation are favoured. The carbon deposition may lead to the plugging of gas passages in the anode. The steam reformation also gets inhibited due to higher pressure, which is a 

The increase in temperature increases the equilibrium constant (K) as the equilibrium composition increases for the product  

The polarisation is also small at higher temperatures. The results obtained for the effect of temperature on the performance of 8.5 cm MCFC cell are as follows  

The major loss in MCFC occurs due to ohmic polarisation and the magnitude of ohmic losses can be given as 

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Figure 6-2 Progress in the Generic Performance of MCFCs on Reformate...

Fontes, E. Fontes, M. Simonsson, D. Effects of different design parameters on the performance of MCFC cathodes. Electrochim. Acta 1996, 41 (1), 1-13. 

Kawase M, Mugikura Y, Izaki Y, Watanabe T (1999) Effects of H2S on the performance of MCFC. II. Behavior of sulfur in the cell. Electrochemistry 4 67... 

In Chaps. 5-7 the effeets of sulfur compounds and siloxanes on performances of MCFC and SOFC are described. The mechanisms of interaction of these compounds with the components of FCs (electrodes and electrolyte) are also treated. 

Example 5.5 Ionic Conductivity ofMCFC Electrolyte Shown below is a plot and table with published performance data comparing the relative performance of MCFCs at I atm pressure (adapted from [17]). Perform the following analysis ... 

Figure 7.14 Relative performance of MCFCs at 1 atm pressure. (Adapted from Ref. [26].)...

Electrolyte loss occurring in long-term operation of MCFC is another problem to be solved for practical application of MCFC. For commercialization, the MCFC should show stable performance over 40,000 hours. Electrolyte loss in MCFC is caused by various factors, e.g., corrosion of components, creepage, reaction with cell components and direct evaporation. These... 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

In order to describe the geometrical and structural properties of several anode electrodes of the molten carbonate fuel cell (MCFC), a fractal analysis has been applied. Four kinds of the anode electrodes, such as Ni, Ni-Cr (lOwt.%), Ni-NiaAl (7wt.%), Ni-Cr (5wt.%)-NijAl(5wt.%) were prepared [1,2] and their fractal dimensions were evaluated by nitrogen adsorption (fractal FHH equation) and mercury porosimetry. These methods of fractal analysis and the resulting values are discussed and compared with other characteristic methods and the performances as anode of MCFC. 

The wetting ability of an MCFC electrode is closely related to the performance of cell operation especially including electrochemical reaction, and can be expressed as contact angle between electrolyte and electrode. The surface energy of 3 phases, geometric structure of anode electrode and... 

The various types of fuel cells are at somewhat different stages in the technology cycle the MCFC is ready for market introduction but faces the typical problem of a new technology, i.e., is expensive because of the lack of economies of scale for its production and the lower cost of its technical rivals (engine-driven co-generation and microturbines). From the technical point of view, the phase of euphoria has almost passed for PEMs and SOFCs and further R D activities are necessary for these two types to match the technical performance of their competitors or the necessary cost level for fuel cells to be technologically and economically ripe for the market. Only the DMFC has reached a standard that allows its use in niche markets, like caravans or yachts, despite poor efficiency levels. 

The ideal performance of a fuel cell depends on the electrochemical reactions that occur with different fuels and oxygen as summarized in Table 2-1. Low-temperature fuel cells (PEFC, AFC, and PAFC) require noble metal electrocatalysts to achieve practical reaction rates at the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells (MCFC, ITSOFC, and SOFC), the requirements for catalysis are relaxed, and the number of potential fuels expands. Carbon monoxide "poisons" a noble metal anode catalyst such as platinum (Pt) in low-temperature... 

The electrolyte composition affects the performance and endurance of MCFCs in several ways. Higher ionic conductivities, and hence lower ohmic polarization, are achieved with Li-rich electrolytes because of the relative high ionic conductivity of Li2C03 compared to that of Na2C03 and K2CO3. However, gas solubility and diffusivity are lower, and corrosion is more rapid in Li2C03. 

The voltage of MCFCs varies with the composition of the reactant gases. The effect of reactant gas partial pressure, however, is somewhat difficult to analyze. One reason involves the water gas shift reaction at the anode due to the presence of CO. The other reason is related to the consumption of both CO2 and O2 at the cathode. Data (55,64,65,66) show that increasing the reactant gas utilization generally decreases cell performance. 

Endurance of the cell stack is a critical issue in the commercialization of MCFCs. Adequate cell performance must be maintained over the desired length of service, quoted by one MCFC developer as being an average potential degradation no greater than 2mV/1000 hours over a cell stack lifetime of 40,000 hours (42). Current state-of-the-art MCFCs (55,64,66,87,88) depict an average degradation over time of... 

Steam reforming of CH4 is typically performed at 750 to 900°C thus, at the lower operating temperature of MCFCs, a high activity catalyst is required. Methanol is also a suitable fuel for internal reforming. It does not require an additional catalyst because the Ni-based anode is suffrciently active. 

Y. Mugikura, Y. Izaki, T. Watanabe, H. Kinoshita, E. Kouda, T. Abe, Central Research Institute of Electric Power Industry, and H. Urushibata, S. Yoshioka, H. Maeda, T. Murahashi, MELCO, "Evaluation of MCFC Performance at Elevated Pressure, " in The International Fuel Cell Conference Proceedings, NEDO/MITI, Tokyo, Japan, Pgs. 215-218, 1992. 

The performance of an internal reforming MCFC also would benefit from pressurization, but unfortunately, the increase is accompanied by other problems. One problem that would need to be overcome is the increased potential for poisoning the internal reforming catalyst resulting from the... 

For recycling to improve the performance of an MCFC network, it must provide benefits that outweigh its inherent disadvantages. If carbon dioxide is not separated from the anode-anode recycle, the concentration of carbon dioxide in the anode is increased. This reduces the Nemst potential. The Nemst potential is similarly reduced by the anode-cathode recycle if steam is not condensed out, since recycled steam dilutes reactant concentrations in the oxidant. In addition, part of the power generated by the network is consumed by the equipment necessary to circulate the recycle streams. Such circulation equipment, along with the additional ducting required by recycling, also increases the capital cost of the MCFC network. 

Another potential disadvantage of an MCFC network is the interdependence of the stacks in series. A problem with one stack could alter the performance of succeeding stacks. 

Two ASPEN (Advanced System for Process Engineering, public version) simulations compare the performance of conventional and networked fuel cell systems having identical recycle schemes and steam bottoming cycles. Each simulated system was composed of three MCFC stacks operating at the same temperature and pressure. The Nemst potential of each MCFC in both systems was reduced by 0.3 volts due to activation, concentration and ohmic voltage... 

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