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.     

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.     

Numerical investigation of the chemical and electrochemical characteristics of planar solid oxide fuel cell with direct internal reforming

A fully three-dimensional mathematical model of a planar solid oxide fuel cell (SOFC) with complete direct internal steam reforming was constructed to investigate the chemical and electrochemical characteristics of the porous-electrode-supported (PES)-SOFC developed by the Central Research Institute of Electric Power Industry of Japan. The effective kinetic models developed over the Ni/YSZ anode takes into account the heat transfer and species diffusion limitations in this porous anode. The models were used to simulate the methane steam reforming processes at the co- and counter-flow patterns. The results show that the flow patterns of gas and air have certain effects on cell performance. The cell at the counter-flow has a higher output voltage and output power density at the same operating conditions. At the counter-flow, however, a high hotspot temperature is observed in the anode with a non-fixed position, even when the air inlet flow rate is increased. This is disadvantageous to the cell. Both cell voltage and power density decrease with increased air flow rate.     

Performance analysis of a tubular solid oxide fuel cell with an indirect internal reformer

A 2‐D steady‐state mathematical model of a tubular solid oxide fuel cell with indirect internal reforming (IIR‐SOFC) has been developed to examine the chemical and electrochemical processes and the effect of different operating parameters on the cell performance. The conservation equations for energy, mass, momentum as well as the electrochemical equations are solved simultaneously employing numerical techniques. A co‐flow configuration is considered for gas streams in the air and fuel channels. The heat radiation between the preheater and reformer surface is incorporated into the model and local heat transfer coefficients are determined throughout the channels. The model predictions have been compared with the data available in the literature. The model was used to study the effect of various operating conditions on the cell performance. Numerical results indicate that as the cell operating pressure increases, the reforming reaction extends to a larger portion of the cell and the maximum temperature move away from the cell inlet. As a result, a more uniform temperature prevails in the solid structure which reduces thermal stresses. Also, at higher excess air, the rate of heat transfer to the air stream is augmented and the average cell temperature is decreased. Copyright © 2010 John Wiley & Sons, Ltd.     

Development of paper-structured catalyst for application to direct internal reforming solid oxide fuel cell fueled by biogas

A flexible paper-structured catalyst (PSC) that can be applied to the anode of a solid oxide fuel cell (SOFC) was examined for its potential to enable direct internal reforming (DIR) operation. The catalytic activity of three types of Ni-loaded PSCs: (a) without the dispersion of support oxide particles in the fiber network (PSC-A), (b) with the dispersion of (Mg,Al)O derived from hydrotalcite (PSCB), and (c) with the dispersion of (Ce,Zr)O_2-δ (PSCC), for dry reforming of CH₄ was evaluated at operating temperatures of 650–800 °C. Among the PSCs, PSC-C exhibited the highest CH₄ conversion with the lowest degradation rate. The electrochemical performance of an electrolyte-supported cell (ESC) was evaluated under the flow of simulated biogas at 750 °C for cases without and with the PSCs on the anode. The application of the PSCs improved the cell performance. In particular, PSC-C had a remarkably positive effect on stabilizing DIRSOFC operation fueled by biogas.     

A direct carbon solid oxide fuel cell fueled with char from wheat straw

Direct carbon solid oxide fuel cell (DC‐SOFC) is a promising technology for electricity generation from biomass with high conversion efficiency and low pollution. Biochar derived from wheat straw is utilized as the fuel of a DC‐SOFC, with cermet of silver and gadolinium‐doped ceria as the material of both cathode and anode and yttrium stabilized zirconia as electrolyte. The output performance of a DC‐SOFC operated on pure wheat straw is 197 mW cm^?2 at 800°C and increases to 258 mW cm^?2 when 5% of Ca, as a catalyst of the Boudouard reaction, is loaded on the wheat straw char. Higher power and fuel conversion utilization are achieved by using Ca as the Boudouard reaction catalyst. X‐ray diffraction, scanning electron microscopy, energy‐dispersive spectrometer, and programmed‐temperature gravimetric experiment are applied to characterize the leaf char. It turns out that the wheat straw char is with porous structure and composed of C, K, Mg, Cl, Fe, and Ca elements. The effects of the Ca catalyst on the Boudouard reaction, the performance of the DC‐SOFCs operated on the wheat straw char, and the economic advantages of the wheat straw char are demonstrated and analyzed in detail.     

Heat-up and start-up modeling of direct internal reforming solid oxide fuel cells

In this paper, a transient heat transfer model to simulate the heat-up and start-up periods of co- and counter-flow direct internal reforming solid oxide fuel cells is developed and presented. In this comprehensive model, all the heat transfer mechanisms, i.e. conduction, convection, and radiation, and all the polarization nodes, i.e. ohmic, activation, and concentration, are considered. The heat transfer model is validated using the results of a benchmark test and two numerical studies obtained from the literature. After validating the model, the heat-up, start-up, and steady-state behaviors of the cell are investigated. In addition, the first principal thermal stresses are calculated to find the probability of failure of the cell during its operation. The results of the present model are in good agreement with the literature data. It is also shown for the given input data that counter-flow case yields a higher average current density and power density, but a lower electrical efficiency of the cell. For the temperature controlled heat-up and start-up strategy, the maximum probability of failure during the operation of the cell is found to be 0.068% and 0.078% for co- and counter-flow configurations, respectively.     

Electrochemical properties and thermal neutral state of solid oxide fuel cells with direct internal reforming of methane

Solid oxide fuel cells (SOFCs) with direct internal reforming (DIR) provide a promising method to realize clean and efficient utilization of hydrocarbon fuels. Thse endothermic reforming reactions occur simultaneously with exothermic electrochemical reactions at the anode, making thermal neutral state achievable inside a fuel cell, providing reference to the thermal management. In this study, a calculation model combining experimental data and thermodynamic results was established, validating the possibility of achieving thermal neutral state in DIR-SOFCs. In the process of modeling, the electrochemical and thermodynamic characteristics in direct internal steam and dry reforming were elaborately compared, contributing to a more scientific understanding of anode reaction mechanism. Detailed experimental investigation was carried out to determine the influence of H₂O/CO₂ on the electrochemical properties of DIR-SOFCs, based on which the optimum steam-carbon ratio (S/C) and CO₂ to CH₄ ratios were obtained. Besides, analysis of distribution of relaxation times (DRT) combined with elementary reactions in CH₄H₂O and CH₄CO₂ atmospheres were proposed to distinguish different physical and chemical processes within anodes. The results of this study can be conducive to a more precise understanding of reaction mechanism on SOFC anodes and meaningful for practical application of DIR-SOFCs.     

Experimental investigation of temperature distribution of planar solid oxide fuel cell: Effects of gas flow,power generation,and direct internal reforming

The temperature distribution of an operating planar solid oxide fuel cell (SOFC) is experimentally investigated under direct internal reforming conditions. An in situ measurement is conducted using a cell holder and an infrared (IR) camera. The effects of the gas flow configuration, exothermic power generation reaction, and endothermic steam–methane reforming reaction are examined at a furnace temperature of 770 °C. The fuel flow and airflow are set to a coflow or counterflow configuration. The heat generation and absorption by the reactions are varied by tuning the average current density and the concentration of methane in the supplied fuel. The maximum value of the local temperature gradient along the cell tends to increase with increasing internal reforming ratio, regardless of the gas flow configuration. From the view point of a small temperature gradient, the counterflow configuration clearly shows better characteristics than that of the coflow, regardless of the internal reforming ratio.     

Thermodynamic modeling of direct internal reforming solid oxide fuel cells operating with syngas

In this paper a direct internal reforming solid oxide fuel cell (DIR-SOFC) is modeled thermodynamically from the energy point of view. Syngas produced from a gasification process is selected as a fuel for the SOFC. The modeling consists of several steps. First, equilibrium gas composition at the fuel channel exit is derived in terms mass flow rate of fuel inlet, fuel utilization ratio, recirculation ratio and extents of steam reforming and water–gas shift reaction. Second, air utilization ratio is determined according to the cooling necessity of the cell. Finally, terminal voltage, power output and electrical efficiency of the cell are calculated. Then, the model is validated with experimental data taken from the literature. The methodology proposed is applied to an intermediate temperature, anode-supported planar SOFC operating with a typical gas produced from a pyrolysis process. For parametric analysis, the effects of recirculation ratio and fuel utilization ratio are investigated. The results show that recirculation ratio does not have a significant effect for low current density conditions. At higher current densities, increasing the recirculation ratio decreases the power output and electrical efficiency of the cell. The results also show that the selection of the fuel utilization ratio is very critical. High fuel utilization ratio conditions result in low power output and air utilization ratio but higher electrical efficiency of the cell.     

Purified high‐sulfur coal as a fuel for direct carbon solid oxide fuel cells

The use of low‐rank coal in a clean and efficient manner is a major challenge facing the current coal technology. A high‐sulfur coal with 4.5 wt% sulfur is chosen to examine the compatibility of the pristine coal and the purified contrast with a solid oxide fuel cell (SOFC) with nickel cermet anodes. Desulfurization of the pristine coal is performed by molten caustic leaching method with a removal ratio of 80%. Analyses of the physicochemical properties of coal samples indicate that the purified coal has a more favorable structure and higher Boudouard reactivity, which is suitable as a fuel for fuel cells. The assessment of electrochemical performance reveals that the purification treatment not only makes the peak power density of SOFCs improve from 115 to 221 mW cm^?2 at 900°C but also extends their durability from 1.7 to 11.2 hours under a current density of 50 mA cm^?2 at 850°C with a fuel availability increasing from 6.25% to 40%. The postmortem analyses show that far less deposited carbon and nickel sulfide are observed on the anode surface. The fuel‐based investigation reveals that the purified coal is a promising fuel for direct carbon fuel cells.     

Paper-structured catalyst for the steam reforming of biodiesel fuel

Architectonics of the paper-structured catalyst for the application to the biofuel reformer or direct internal reforming SOFC (DIRSOFC) was studied. Inorganic fiber network, “paper”, composed of yttria-stabilized zirconia (YSZ) fiber (Zf), alumina fiber (Af) and inorganic binder (Al₂O₃ sol (As) or ZrO₂ sol (Zs) or CeO₂ sol (Cs)) was prepared by a simple paper-making process. Then, the catalytic activities of the Ni and Ni–MgO loaded papers called “paper-structured catalysts (PSCs)” for the steam reforming of biodiesel fuels (BDFs) were evaluated. Ni–MgO loaded PSC using Cs as an inorganic binder, Ni–MgO/ZfAfCs, exhibited excellent performance over Ru/γAl₂O₃ catalyst beads. Formation of light hydrocarbons, especially C₂H₄, was eliminated and water–gas shift reaction was more promoted compared to the catalyst beads.     

Modelling of tubular-designed solid oxide fuel cell with indirect internal reforming operation fed by different primary fuels

Mathematical models of an indirect internal reforming solid oxide fuel cell (IIR-SOFC) fed by four different primary fuels, i.e., methane, biogas, methanol and ethanol, are developed based on steady-state, heterogeneous, two-dimensional and tubular-design SOFC models. The effect of fuel type on the thermal coupling between internal endothermic reforming with exothermic electrochemical reactions and system performance are determined. The simulation reveals that an IIR-SOFC fuelled by methanol provides the smoothest temperature gradient with high electrochemical efficiency. Furthermore, the content of CO₂ in biogas plays an important role on system performance since electrical efficiency is improved by the removal of some CO₂ from biogas but a larger temperature gradient is expected.Sensitivity analysis of three parameters, namely, a operating pressure, inlet steam to carbon (S:C) ratio and flow direction is then performed. By increasing the operating pressure up to 10 bar, the system efficiency increases and the temperature gradient can be minimized. The use of a high inlet S:C ratio reduces the cooling spot at the entrance of reformer channel but the electrical efficiency is considerably decreased. An IIR-SOFC with a counter-flow pattern (as based case) is compared with that with co-flow pattern (co-flow of air and fuel streams through fuel cell). The IIR-SOFC with co-flow pattern provides higher voltage and a smoother temperature gradient along the system due to superior matching between heat supplied from electrochemical reaction and heat required for steam reforming reaction; thus it is expected to be a better option for practical applications.     

Performance analysis of a solid oxide fuel cell with reformed natural gas fuel

In the present study a two‐dimensional model of a tubular solid oxide fuel cell operating in a stack is presented. The model analyzes electrochemistry, momentum, heat and mass transfers inside the cell. Internal steam reforming of the reformed natural gas is considered for hydrogen production and Gibbs energy minimization method is used to calculate the fuel equilibrium species concentrations. The conservation equations for energy, mass, momentum and voltage are solved simultaneously using appropriate numerical techniques. The heat radiation between the preheater and cathode surface is incorporated into the model and local heat transfer coefficients are determined throughout the anode and cathode channels. The developed model has been compared with the experimental and numerical data available in literature. The model is used to study the effect of various operating parameters such as excess air, operating pressure and air inlet temperature and the results are discussed in detail. The results show that a more uniform temperature distribution can be achieved along the cell at higher air‐flow rates and operating pressures and the cell output voltage is enhanced. It is expected that the proposed model can be used as a design tool for SOFC stack in practical applications. Copyright © 2009 John Wiley & Sons, Ltd.     

Modelling of solid oxide fuel cells with internal glycerol steam reforming

The direct application of glycerol in solid oxide fuel cell (SOFC) for power generation has been demonstrated experimentally but the detailed mechanisms are not well understood due to the lack of comprehensive modeling study. In this paper, a numerical model is developed to study the glycerol-fueled SOFC. After model validation, the simulated SOFC demonstrates a performance of 7827 A m^?2 at 0.6 V, with a glycerol conversion rate of 49% at 1073 K. Then, parametric analyses are conducted to understand the effects of operation conditions on cell performance. It is found that the SOFC performance increases with decreasing operating voltage or increasing inlet temperature. However, increasing either the fuel flow rate or steam to glycerol ratio could decrease the cell performance. It is also interesting to find out that the contribution of H₂ and CO to the total current density is significantly different under various operating conditions, even sometimes CO dominates while H₂ plays a negative role. This is different from our conventional understanding that usually H₂ contributes more significantly to current generation. In addition, cooling measures are needed to ensure the long-term stability of the cell when operating at a high current density.     

Use of ash-free “Hyper-coal” as a fuel for a direct carbon fuel cell with solid oxide electrolyte

“Hyper-coal”, produced by the Kobe Steel Company, was investigated by analytical and physico-chemical methods to consider its potential usability as a fuel for a direct carbon fuel cell with solid oxide electrolyte (DC-SOFC). The performed tests showed that DC-SOFCs fed with this processed fossil coal were characterized by stable operation with reasonable current and power densities. The performance of the fuel cells can be improved by using iron as a catalyst for the anodic reaction and by the choice of appropriate working conditions.     

Multi-physics modeling of a symmetric flat-tube solid oxide fuel cell with internal methane steam reforming

Methane is regarded as one of the ideal fuels for solid oxide fuel cells (SOFCs) due to its huge reserves and transportation properties. In this study, a 3D numerical model coupling with chemical reaction, electrochemical reaction, mass transfer, charge transfer, and heat transfer is developed to understand the heat and mass transfer processes of methane steam direct internal reforming based on double-sided cathodes (DSC) SOFC. After the model verification, the parametric simulations are performed to study the effects of operating voltage, inlet temperature, and steam to carbon (S/C) ratio on the performance of a DSC. It is found that the non-uniform distribution of flow rate among channels results in the non-uniform distribution of each physical field. Increasing the inlet temperature significantly enhances the performance of DSC, however, when the temperature is above 1073 K, the concentration loss and the temperature gradient of DSC increase, which is not conducive to the long-term operation of the DSC. In addition, we revealed the effect of the S/C ratios on the heat and mass transfer process. This study provides an insight into the heat and mass transfer process of SOFC with a mixture of steam and methane and operating conditions for enhancing the performance.     

Development of a fuel feeder for a solid oxide fuel cell test station

Solid oxide fuel cells (SOFC) can utilize various fuels, such as natural gas, hydrogen and biogas, but often, it is sensible to use a pre‐reformer that converts the fuel into a hydrogen‐rich gas stream. Relevant testing conditions, including the fuel to be used in SOFC systems, are important because cell performance depends on test conditions, such as fuel composition. Still, a majority of the reported single‐cell and short stack tests are performed with pure hydrogen or synthetic reformate mixed from gas bottles. In this article, the development of a fuel feeder used to pre‐reform natural gas for a single cell SOFC test station is presented. To mimic SOFC system conditions, natural gas is taken from the grid, desulfurized with commercial sulfur sorbent and reformed with a commercial precious metal catalyst. The fuel feeder is designed to be a versatile and efficient research tool, capable to be used in a wide temperature and gas flow range and with different reforming techniques, such as steam reforming, catalytic partial oxidation and simulated anode off‐gas recycling. The construction, operation and characterization of the fuel feeder as well as methods of avoiding carbon formation are discussed. The performance is evaluated by comparing measured outlet temperatures and compositions against equilibrium values. All measured gas compositions matched closely with the calculated equilibrium values, and the identified deviations were small and to no harm in practical use. The operator can control the product gas composition by setting the fuel feeder heater to the temperature corresponding to the targeted composition. Results show that the fuel feeder design can be used as such for single‐cell testing or scaled to fit larger stack test stations. Copyright © 2015 John Wiley & Sons, Ltd.     

Performance analysis and temperature gradient of solid oxide fuel cell stacks operated with bio-oil sorption-enhanced steam reforming

A high temperature gradient within a solid oxide fuel cell (SOFC) stack is considered a major challenge in SOFC operations. This study investigates the effects of the key parameters on SOFC system efficiency and temperature gradient within a SOFC stack. A 40-cell SOFC stack integrated with a bio-oil sorption-enhanced steam reformer is simulated using MATLAB and DETCHEM. When the air-to-fuel ratio and steam-to-fuel ratio increase, the stack average temperature and temperature gradient decrease. However, a decrease in the stack temperature steadily reduces the system efficiency owing to the tradeoff between the stack performance and thermal balance between heat recovered and consumed by the system. With an increase in the bio-oil flow rate, the system efficiency decreases because of the lower resident time for the electrochemical reaction. This is not, however, beneficial to the maximum temperature gradient. To minimize the temperature gradient of the SOFC stack, a decrease in the bio-oil flow rate is the most effective way. The maximum temperature gradient can be reduced to 14.6 K cm⁻¹ with the stack and system efficiency of 76.58 and 65.18%, respectively, when the SOFC system is operated at an air-to-fuel ratio of 8, steam-to-fuel ratio of 6, and bio-oil flow rate of 0.0041 mol s⁻¹.