Simulation of the performance for the direct internal reforming molten carbonate fuel cell. Part II: comparative distributions of reaction rates and gas compositions

This study examined the distributions of the three reaction rates and the compositions of the gases at each point of the unit cell in DIR‐MCFC using a numerical simulation. The electrochemical reaction rates at the anode gas entering position were almost two times faster than those at the anode gas outlet position and most of the feeding CH₄ reacted in the region from the position x=0 to the position x=0.3. In addition, the water–gas shift reaction became faster from near the half position of the unit cell to the gas outlet position. Therefore, in the rear position of the unit cell, the steam reforming reaction played an important role as a supplementary reaction for providing the H₂ needed in the electrochemical reaction. The rates of the two catalytic reactions in the case without the electrochemical reaction were relatively slower than those in the DIR‐MCFC. Unlike the distributions of temperature, the current density, gas compositions and the reactions rates at each point of the DIR‐MCFC cell, the exit gas compositions from the simulation in particular could be comparative to those of the experimental results. Although there was an approximately 10% difference between both of them, the extent of the difference was considered to be reasonable for this simulation considering the experimental values that could be included in this simulation such as the lower conversion of the reactions, the lower current density and any other values. Copyright © 2005 John Wiley & Sons, Ltd.     

Electrolyte effect on the catalytic performance of Ni-based catalysts for direct internal reforming molten carbonate fuel cell

An active and tolerant Ni-based catalyst for methane steam reforming in direct internal reforming molten carbonate fuel cells (DIR-MCFCs) was developed. Deactivation of reforming catalysts by alkali metals from the electrolyte composed of Li₂CO₃ and K₂CO₃ is one of the major obstacles to be overcome in commercialization of DIR-MCFCs. Newly developed Ni/MgSiO₃ and Ni/Mg₂SiO₄ reforming catalysts show activities of ca. 80% methane conversion. Subsequent to electrolyte addition to the catalyst, however, the activity of Ni/Mg₂SiO₄ decreases to ca. 50% of its initial value, whereas Ni/MgSiO₃ catalyst retains its initial activity. Results obtained from temperature-programmed reduction and X-ray photoelectron spectroscopy identify unreduced Ni³⁺ as a decisive factor in keeping catalytic activity from the electrolyte.     

Optimum start-up strategies for direct internal reforming molten carbonate fuel cell systems

The study of start-up performance for a direct internal reforming molten carbonate fuel cell (DIR-MCFC) system is presented. Since a kW-class stack is assembled with an additional preheating design, the improvement of start-up behavior is conducted to find the proper operating strategy. For a cold start-up fuel cell system, both start-up delay and inverse response are strictly detected. When the optimum operating strategy is determined by solving the steady-state optimization algorithm subject to stack temperature constraint, the rapid system start-up as well as the maximum power output can be achieved simultaneously.     

Electrochemical performance of reversible molten carbonate fuel cells

The electrochemical performance of a state-of-the-art molten carbonate cell was investigated in both fuel cell (MCFC) and electrolysis cell (MCEC) modes by using polarization curves and electrochemical impedance spectroscopy (EIS). The results show that it is feasible to run a reversible molten carbonate fuel cell and that the cell actually exhibits lower polarization in the MCEC mode, at least for the short-term tests undertaken in this study. The Ni hydrogen electrode and the NiO oxygen electrode were also studied in fuel cell and electrolysis cell modes under different operating conditions, including temperatures and gas compositions. The polarization of the Ni hydrogen electrode turned out to be slightly higher in the electrolysis cell mode than in the fuel cell mode at all operating temperatures and water contents. This was probably due to the slightly larger mass-transfer polarization rather than to charge-transfer polarization according to the impedance results. The CO₂ content has an important effect on the Ni electrode in electrolysis cell mode. Increasing the CO₂ content the Ni electrode exhibits slightly lower polarization in the electrolysis cell mode. The NiO oxygen electrode shows lower polarization loss in the electrolysis cell mode than in the fuel cell mode in the temperature range of 600–675 °C. The impedance showed that both charge-transfer and mass-transfer polarization of the NiO electrode are lower in the electrolysis cell than in the fuel cell mode.     

Feasibility of palm‐biodiesel fuel for a direct internal reforming solid oxide fuel cell

The feasibility of a direct internal reforming (DIR) solid oxide fuel cell (SOFC) running on wet palm‐biodiesel fuel (BDF) was demonstrated. Simultaneous production of H₂‐rich syngas and electricity from BDF could be achieved. A power density of 0.32 W cm^?2 was obtained at 0.4 A cm^?2 and 800 °C under steam to carbon ratio of 3.5. Subsequent durability testing revealed that a DIR‐SOFC running on wet palm‐BDF exhibited a stable voltage of around 0.8 V at 0.2 A cm^?2 for more than 1 month with a degradation rate of approximately 15 % / 1000 h. The main cause of the degradation was an increase in the ohmic resistance. Copyright © 2012 John Wiley & Sons, Ltd.     

Two-dimensional dynamic simulation of a direct internal reforming solid oxide fuel cell

This study presents a two-dimensional mathematical model of a direct internal reforming solid oxide fuel cell (DIR-SOFC) stack which is based on the reforming reaction kinetics, electrochemical model and principles of mass and heat transfer. To stimulate the model and investigate the steady and dynamic performances of the DIR-SOFC stack, we employ a computational approach and several cases are used including standard conditions, and step changes in fuel flow rate, air flow rate and stack voltage. The temperature distribution, current density distribution, gas species molar fraction distributions and dynamic simulation for a cross-flow DIR-SOFC are presented and discussed. The results show that the dynamic responses are different at each point in the stack. The temperature gradients as well as the current density gradients are large in the stack, which should be considered when designing a stack. Further, a moderate increase in the fuel flow rate improves the performances of the stack. A decrease in the air flow rate can raise the stack temperature and increase fuel and oxygen utilizations. An increased output voltage reduces the current density and gas utilizations, resulting in a decrease in the temperature.     

The influence of low operating temperature on molten carbonate fuel cells decay processes

Investigations have been carried out to study the influence of low operating temperature (873 K) on the decay mechanisms that affect the endurance of a molten carbonate fuel cell. An experiment has been performed for several thousands of hours of continuous operation at a current density of 160 mA cm⁻² on a bench-scale cell to evaluate the electrochemical performance parameters and morphological characteristics of spent components. The tested components were a traditional LiAlO₂ matrix, charged by a mixture of Li/K, NiO cathode and Ni/Cr anode. During the test, measurements of temperature, pressure, flows, cell internal resistance, current and voltage have been taken continuously, as well as gas chromatography analyses. At the end of the experiment, the cell showed an increase in internal resistance of 0·376 Ω cm⁻² and a lowering of open-circuit voltage of 30 mV. The electrolyte distribution in the components indicated an excess of empty pores in the tile structure with a filling degree of 72·8%. By contrast, the anode retained a filling degree of 64·3%, while the cathode appeared under partial flooding condition with a filling degree of 48·2%. The SEM cross-section view of the cell package (cathode, tile and anode) showed evidence of a process of cathode dissolution and coprecipitation of Ni in a narrow band located almost in the middle of the section. The Ni, Cr, Al and K distribution profiles in the cross-section were investigated by EDAX analysis. © 1997 John Wiley & Sons, Ltd.     

A methodology for improving the performance of molten carbonate fuel cell/gas turbine hybrid systems

In the present article a molten carbonate fuel cell (MCFC) system has been developed, modeled and implemented in Matlab language. It enables definition of the optimal operating conditions of the fuel cell, in terms of electrical and thermal performance, when it is a part of a hybrid plant composed of an MCFC system, a gas turbine and a possible heat recovery system. The thermal energy, which is recoverable from the adequately treated anodic exhaust gases, is utilized in a gas turbine plant to reduce its fuel consumption. Therefore, in the present article a methodology is illustrated to calculate the optimal values of some parameters characterizing the MCFC/gas turbine integrated system in terms of the electrical, first law and equivalent efficiencies. A choice is made among the sets of values of parameters investigated to improve the performance of the same integrated system according to its use (for the production of electric energy only or for the contemporary production of electric and thermal energy). Copyright © 2010 John Wiley & Sons, Ltd.     

Demonstration of coal gas run molten carbonate fuel cell concept

Integrated gasifier‐molten carbonate fuel cell (IG‐MCFC) offers a clean and efficient route for power generation from coal. A molten carbonate fuel cell (MCFC) was assembled and its performance was tested with simulated coal gas. The output and the stability was found to be comparable to that with conventional feed gas. It was also observed that switching from one type of feed gas to another had only a marginal effect on the cell performance. Copyright © 1999 John Wiley & Sons, Ltd.     

A highly active Ni-Al2O3 catalyst prepared by homogeneous precipitation using urea for internal reforming in a molten carbonate fuel cell (MCFC): Effect of the synthesis temperature

Ni-Al₂O₃ catalysts for use in internal reforming in a molten carbonate fuel cell (MCFC) were prepared by homogeneous precipitation method at various synthesis temperatures. The effects of synthesis temperature on physicochemical properties and catalytic activities of the Ni-Al₂O₃ catalysts were investigated. XRD measurements exhibited that the peak intensity of NiAl₂O₄ in the calcined catalysts increased with higher synthesis temperatures. TPR measurements demonstrated that reduction peaks appeared around 670–680 °C for every synthesis temperature, indicating that the Ni particles interacted strongly with the support. Hydrogen chemisorption results showed that nickel dispersion and nickel surface area decreased in the order: K52_80C > K52_85C > K52_90C > K52_95C > K52_100C. TEM images of the reduced Ni-Al₂O₃ catalysts revealed that the average sizes of Ni particles were 13.1, 13.4 and 15.9 nm for K52_80C, K52_90C and K52_100C, respectively, which means that a higher synthesis temperature yielded a larger Ni particle. The performance of the catalysts in methane steam reforming showed that catalysts prepared at the lowest synthesis temperature (80 °C) exhibited the highest reaction rate. These results suggest that a lower synthesis temperature is favorable to prepare highly active Ni-Al₂O₃ catalysts by the homogeneous precipitation method.     

Lifetime Expectancy of molten carbonate fuel cells: Part I. Effect of temperature on the voltage and electrolyte reduction rates

Electrolyte depletion is a significant setback in the operation of molten carbonate fuel cells (MCFCs). The electrolyte loss mostly occurs as a result of the high operating temperatures of over 873 K. The effect of temperature on MCFCs was studied using several 7 cm² coin-type MCFCs operated at 873, 973 and 1073 K. Lithium-potassium carbonate (Li/K) was used as an electrolyte in this study. A decrease in cell performance with time was observed at all temperatures. The performance degradation was found to be more severe at 1073 K than at 973 K and 873 K. The electrolyte loss rate was observed by chemical means to have increased with increasing temperature. A more severe electrolyte loss rate was observed in cells operated at 1073 K, such that the electrolyte amount reduced by half after 250 h of cell operation. In this research work, a factor, F_WV, which correlates the electrolyte loss rate, voltage reduction rate, and cell life, is introduced. Its dependence on the cell electrode area and operating temperature make it a suitable parameter for simulating MCFC's lifetime.     

Durability of direct internal reforming of methanol as fuel for solid oxide fuel cell with double-sided cathodes

Direct internal reforming of methanol is applied as fuel for a Ni-YSZ anode-supported solid oxide fuel cell with a flat tube based on double-sided cathodes. It achieves a power density (PD) of 0.25 W/cm² at 0.8 V, reaching about 90% of that is fueled by H₂. And the cell has been operated for more than 120 h by the direct internal reforming of methanol. The durability and apparent advantage for using humidified methanol may lead to widespread applications by direct internal reforming method for this new designed SOFC in the future.     

The dissolution process of the NiO cathodes for molten carbonate fuel cells: state-of-the-art

In this review article the analysis of the main problems related to the use of NiO as material for cathodes in Molten Carbonate Fuel Cells (MCFC) is reported. Thus, the most significant evidences of the mechanism of NiO dissolution have been reported as well as its correlation with the basicity of the carbonate melt, composition of the reactants gases and temperature. Some hypotheses described here have been also verified experimentally and the results of this validation are reported. In the final section, we have described the most promising alternative solutions to this problem and the advantages and shortcomings of these alternatives. © 1998 John Wiley & Sons, Ltd.     

Thermodynamic performance analysis of a molten carbonate fuel cell at very high current densities

This study is basically composed of two sections. In the first section, a CFD analysis is used to provide a better insight to molten carbonate fuel cell operation and performance characteristics at very high current densities. Therefore, a mathematical model is developed by employing mass and momentum conservation, electrochemical reaction mechanisms and electric charges. The model results are then compared with the available data for an MCFC unit, and a good agreement is observed. In addition, the model is applied to predict the unit cell behaviour at various operating pressures, temperatures, and cathode gas stoichiometric ratios. In the second section, a thermodynamic model is utilized to examine energy efficiency, exergy efficiency and entropy generation of the MCFC. At low current densities, no considerable difference in output voltage and power is observed; however, for greater values of current densities, the difference is not negligible. If the molten carbonate fuel cell is to operate at current densities smaller than 2500 A m⁻², there is no point to pressurize the system. If the fuel cell operates at pressures greater than atmospheric pressure, the unit cell cost could be minimized. In addition, various partial pressure ratios at the cathode side demonstrated nearly the same effect on the performance of the fuel cell. With a 60 K change in operating temperature, almost 10% improvement in energy and exergy efficiencies is obtained. Both efficiencies initially increase at lower current densities and then reach their maximum values and ultimately decrease with the increase of current density. By elevating the pressure, both energy and exergy efficiencies of the cell enhance. In addition, higher operating pressure and temperature decrease the unit cell entropy generation.     

The stability of molten carbonate fuel cell electrodes: A review of recent improvements

The electrode stability is a key issue for the development of conventional hydrogen fuelled and direct internal reforming (DIR) molten carbonate fuel cells (MCFCs). While for conventional MCFC anodes the stability problem has been addressed by the addition of Al or Cr to Ni, the problems of the dissolution of the NiO cathode and of the deactivation of DIR-MCFC anodes have not been fully resolved too. This review reports recent improvements in the chemical and physicochemical stability of cathode and anode materials in MCFCs and DIR-MCFCs, respectively.     

Performance analysis of a pressurized molten carbonate fuel cell/micro-gas turbine hybrid system

This paper presents the work on the design and part-load operations of a hybrid power system composed of a pressurized molten carbonate fuel cell (MCFC) and a micro-gas turbine (MGT). The gas turbine is an existing one and the MCFC is assumed to be newly designed for the hybrid system. Firstly, the MCFC power and total system power are determined based on the existing micro-gas turbine according to the appropriate MCFC operating temperature. The characteristics of hybrid system on design point are shown. And then different control methods are applied to the hybrid system for the part-load operation. The effect of different control methods is analyzed and compared in order to find the optimal control strategy for the system. The results show that the performance of hybrid system during part-load operation varies significantly with different control methods. The system has the best efficiency when using variable rotational speed control for the part-load operation. At this time both the turbine inlet temperature and cell operating temperature are close to the design value, but the compressor would cross the surge line when the shaft speed is less than 70% of the design shaft speed. For the gas turbine it is difficult to obtain the original power due to the higher pressure loss between compressor and turbine.     

Numerical studies of a separator for stack temperature control in a molten carbonate fuel cell

The use of a separator to control stack temperature in a molten carbonate fuel cell was studied by numerical simulation using a computational fluid dynamics code. The stack model assumed steady-state and constant-load operation of a co-flow stack with an external reformer at atmospheric pressure. Representing a conventional cell type, separators with two flow paths, one each for the anode and cathode gas, were simulated under conditions in which the cathode gas was composed of either air and carbon dioxide (case I) or oxygen and carbon dioxide (case II). The results showed that the average cell potential in case II was higher than that in case I due to the higher partial pressures of oxygen and carbon dioxide in the cathode gas. This result indicates that the amount of heat released during the electrochemical reactions was less for case II than for case I under the same load. However, simulated results showed that the maximum stack temperature in case I was lower than that in case II due to a reduction in the total flow rate of the cathode gas. To control the stack temperature and retain a high cell potential, we proposed the use of a separator with three flow paths (case III); two flow paths for the electrodes and a path in the center of the separator for the flow of nitrogen for cooling. The simulated results for case III showed that the average cell potential was similar to that in case II, indicating that the amount of heat released in the stack was similar to that in case II, and that the maximum stack temperature was the lowest of the three cases due to the nitrogen gas flow in the center of the separator. In summary, the simulated results showed that the use of a separator with three flow paths enabled temperature control in a co-flow stack with an external reformer at atmospheric pressure.     

Integration of a molten carbonate fuel cell with a direct exhaust absorption chiller

A high market value exists for an integrated high-temperature fuel cell-absorption chiller product throughout the world. While high-temperature, molten carbonate fuel cells are being commercially deployed with combined heat and power (CHP) and absorption chillers are being commercially deployed with heat engines, the energy efficiency and environmental attributes of an integrated high-temperature fuel cell-absorption chiller product are singularly attractive for the emerging distributed generation (DG) combined cooling, heating, and power (CCHP) market. This study addresses the potential of cooling production by recovering and porting the thermal energy from the exhaust gas of a high-temperature fuel cell (HTFC) to a thermally activated absorption chiller. To assess the practical opportunity of serving an early DG-CCHP market, a commercially available direct fired double-effect absorption chiller is selected that closely matches the exhaust flow and temperature of a commercially available HTFC. Both components are individually modeled, and the models are then coupled to evaluate the potential of a DG-CCHP system. Simulation results show that a commercial molten carbonate fuel cell generating 300 kW of electricity can be effectively coupled with a commercial 40 refrigeration ton (RT) absorption chiller. While the match between the two “off the shelf” units is close and the simulation results are encouraging, the match is not ideal. In particular, the fuel cell exhaust gas temperature is higher than the inlet temperature specified for the chiller and the exhaust flow rate is not sufficient to achieve the potential heat recovery within the chiller heat exchanger. To address these challenges, the study evaluates two strategies: (1) blending the fuel cell exhaust gas with ambient air, and (2) mixing the fuel cell exhaust gases with a fraction of the chiller exhaust gas. Both cases are shown to be viable and result in a temperature drop and flow rate increase of the gases before the chiller inlet. The results show that no risk of cold end corrosion within the chiller heat exchanger exists. In addition, crystallization is not an issue during system operation. Accounting for the electricity and the cooling produced and disregarding the remaining thermal energy, the second strategy is preferred and yields an overall estimated efficiency of 71.7%.     

An investigation on the dynamic performance of molten carbonate fuel-cell power-generation systems

The dynamic characteristics of molten carbonate fuel-cell power-generation systems under load-following modes have been investigated using dynamic simulation. The system model is comprised of connected network of 12 types of component models, which emulates the process flow diagram. The system model has been run throughout the entire load range under steady-state conditions, and the overall system efficiency is shown. The system load-following capability and the dynamic interactions between the components have been illustrated under various transient conditions. © 1998 John Wiley & Sons, Ltd.     

A basic model for analysis of molten carbonate fuel cell behavior

A simple mathematical model, based on the basic chemical reactions and mass transfer, was developed to predict some important characteristics of molten carbonate fuel cells (MCFC) with LiNaCO₃ and LiKCO₃ electrolytes for steady state operating conditions. The parallel and cross gas flow patterns were analyzed. Model simulates polarization characteristics, the effect of temperature, pressure and electrolyte type on the cell performance, various losses in the cell and gas flow rate changes through cell. The effect of fuel utilization on the cell potential and efficiency was also analyzed. Model predicts a better performance for the MCFC with LiNaCO₃ electrolyte and the cross flow pattern, in general. Results show a strong influence of the operating temperature on the cell potential at temperatures below 625 °C, where cell potential increases rapidly with increasing temperature. Above this temperature, however, the cell potential has almost a steady asymptotic profile. The model predicts cell efficiency steadily improving with increase in fuel utilization. The cell potential decreases almost linearly with increase in the fuel utilization percentage for both electrolytes. Models results show a stronger dependency of the cell potential on the operating pressure than that described by the Nerst equation which is in line with fact that the real variations in the cell potential can be higher due to decreased various losses.