熔融碳酸盐燃料电池的电气特性研究

为了研究熔融碳酸盐燃料电池的电气特性,分析了熔融碳酸盐燃料电池单元的电化学过程机理,建立了基于电化学反应的熔融碳酸盐燃料电池电气模型,推导了熔融碳酸盐燃料电池平均电流密度与燃气利用率的关系,给出了采用电化学方程的熔融碳酸盐燃料电池电气特性的模型结构和算法,并进行了仿真研究和试验.试验结果表明:该模型结构简单、准确度高,可获得千瓦级熔融碳酸盐燃料电池的电气特性曲线.     

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.     

Molten Carbonate Fuel Cell electrochemistry modelling

A Fuel Cell (FC) is an electrochemical device which produces electric energy in DC. In order to support design control for the electrical system connected to it, it is necessary to work out a suitable representation of the fast dynamics involved. Therefore, in this work, a mathematical model, based on first principles and including both dynamical equations and algebraic relations, is described for electrochemical reactions, with the related formation of potential differences and anion and cation accumulation phenomena, in a Molten Carbonate Fuel Cell (MCFC). The model is formally consistent and it has been validated against experimental results, such as steady-state power and voltage versus current curves.     

SOFC and MCFC system level modeling for hybrid plants performance prediction

In this paper a generalized model, based on system-level approach, for predicting the High Temperature Fuel Cells (HTFCs) behavior and performance is presented.The system-level model allows to forecast the HTFC performance under different operating conditions (cell temperature, anode off-gas recirculation, reactants temperatures, fuel and oxidant utilization factors, etc.) and cell design (tubular and planar configurations and with co-flow, counter-flow and cross-flow arrangements).Mass and energy balances are solved by considering both the electrochemical (i.e. electro-oxidation of hydrogen) and thermochemical reactions (i.e. reforming and shifting reactions) which occur in the anode and cathode sides and by applying different equations systems to take into account the type of fuel cell (MCFC or SOFC).The ability of the proposed model in the HTFCs performance prediction is pointed out by the model validation carried out by using experimental data and by analyzing the impact of the model calibration parameters on the cell voltage calculation carried out by means of a sensitivity analysis.Numerical results show that the model allows to characterize the behavior of the HTFCs with a good approximation so, thanks to the simplicity of the simulation procedure and to the small computational time efforts, it can be a useful tool for predicting the performance of hybrid power plants or more complex systems in which the fuel cell is one of the main components.     

Simulation of the performance for the direct internal reforming molten carbonate fuel cell. Part I: distributions of temperature,energy transfer and current density

This study examined the temperature distributions of the anode and cathode gases of the cell body as well as the current density distributions at each point of the direct internal reforming molten carbonate fuel cell (DIR‐MCFC) using numerical modelling. The model was based on assumptions and experimental data from a 5 cm × 5 cm sized unit cell operation. The results showed there was an approximately 13°C temperature difference between the initial point (0, 0) and end point (1, 1) of the cell body and the temperature increased steadily along with the direction of the anode gas flow. The temperature distribution of the anode gases showed a similar trend to those of the cell body. The temperature of the anode gases was an average 11°C lower than that of the cell body. The temperature distributions of cathode gases were relatively higher than those of the anode gases and the cell body. The temperature distributions at each point of the cell body, including the anode and cathode gases, could be explained by the different rates of the electrochemical, methane steam reforming and water–gas shift reactions at each point in the cell body. The current density distribution at the entrance of the cell was the highest at 290 mA cm^?2, and decreased steadily to 150 mA cm^?2 at the exit. These results were also confirmed by the amount of hydrogen reacted in the electrochemical reaction (referred to Part II). Finally, modelling simulations showed a non‐uniform distribution of the temperature and current density throughout the DIR‐MCFC were observed. In addition, it was confirmed that the distributions of the reaction rates and gas compositions at each point of the cell also showed a great deal of difference throughout the DIR‐MCFC. The non‐uniformity of these temperature distributions can lead to deterioration in the cell performance. These might provide the necessary information for solving these problems. Copyright © 2005 John Wiley & Sons, Ltd.     

Analysis of a molten carbonate fuel cell: Numerical modeling and experimental validation

A detailed dynamic model incorporating geometric resolution of a molten carbonate fuel cell (MCFC) with dynamic simulation of physical and electrochemical processes in the stream-wise direction is presented. The model was developed using mass and momentum conservation, electrochemical and chemical reaction mechanisms, and heat-transfer. Results from the model are compared with data from an experimental MCFC unit. Furthermore, the model was applied to predict dynamic variations of voltage, current and temperature in an MCFC as it responds to varying load demands. The voltage was evaluated using two different approaches: one applying a model developed by Yuh and Selman [C.Y. Yuh, J.R. Selman, The polarization of molten carbonate fuel cell electrodes: I. Analysis of steady-state polarization data, J. Electrochem. Soc. 138 (1991) 3642–3648; C.Y. Yuh, J.R. Selman, The polarization of molten carbonate fuel cell electrodes: II. Characterization by AC impedance and response to current interruption, J. Electrochem. Soc. 138 (1991) 3649–3655] and another applying simplified equations using average local temperatures and pressures. The results show that both models can be used to predict voltage and dynamic response characteristics of an MCFC and the model that uses the more detailed Yuh and Selman approach can predict those accurately and consistently for a variety of operating conditions.     

Performance model of molten carbonate fuel cell

A performance model of a molten carbonate fuel cell (MCFC), an electrochemical energy conversion device for electric power generation, is discussed. The presumptive ability of the MCFC model is improved and the impact of MCFC characteristics in fuel cell system simulations is investigated. Basic data are obtained experimentally by single-cell tests. A correlation formula based on the experimental data is derived for the cell voltage and the oxygen and carbon dioxide partial pressures. Three types of MCFC systems are compared. With regard to fuel utilization, system characteristics using the proposed correlation are very similar to those obtained using a previous model. However, the amount of decrease predicted by the proposed model with respect to system efficiency is larger than that obtained by the previous model at high air utilization     

Analysis of chemical,electrochemical reactions and thermo-fluid flow in methane-feed internal reforming SOFCs: Part I – Modeling and effect of gas concentrations

Direct internal reforming solid oxide fuel cells (DIR SOFCs) have complicated distributions of temperature and species concentrations due to various chemical and electrochemical reactions. The details of these properties are studied by a 3-D numerical simulation in this work. The simulation modeling used governing equations (mass, momentum, energy and species balance equations) generally suitable to porous medium with porosity variable of zero (solid), 0.3 (porous medium) and 1.0 (fluid). Chemical kinetics equations for the internal reforming and shift reactions based on the Langmuir–Hinshelwood model were incorporated. Hydrogen and carbon monoxide oxidations were considered both participating in electrochemical reactions. The experimentally measured current density–potential curves were compared with the simulation data to validate the code, which revealed that the simulation model was able to predict the dilution effect of nitrogen and the mass transfer under high current densities. It is found that the temperature dramatically declined near the fuel inlet with strong endothermic reactions, but it increased along the fluid flow with electrochemically exothermic reactions. A low steam-to-carbon ratio (SCR) led to high steam reforming and water gas shift reaction rates, which generated a greater amount of hydrogen. Therefore, current density increased with low SCR. The average current density due to carbon monoxide electrochemical oxidation varies from 205.3 A/m² under an SCR of 2.0 to 47.6 A/m² under an SCR of 4.0. The average current density due to hydrogen electrochemical oxidation was 5535.4 A/m² under an SCR of 2.0, which was 27 times higher than that of carbon monoxide. The total current density ranged from 5740.8 A/m² under an SCR of 2.0 to 2268.9 A/m² under an SCR of 4.0.     

Comparison by the use of numerical simulation of a MCFC-IR and a MCFC-ER when used with syngas obtained by atmospheric pressure biomass gasification

In order to realize biomass potential as a major source of energy in the power generation and transport sectors, there is a need for high efficient and clean energy conversion devices, especially in the low-medium range suiting the disperseness of this fuel. Large installations, based on boiler coupled to steam turbine (or IGCC), are too complex at smaller scale, where biomass gasifiers coupled to ICEs have low electrical efficiency (15-30%) and generally not negligible emissions.This paper analyses new plants configurations consisted of Fast Internal Circulated Fluidized-Bed Gasifier, hot-gas conditioning and cleaning, high temperature fuel cells (MCFC), micro gas turbines, water gas shift reactor and PSA to improve flexibility and electric efficiency at medium scale. The power plant feasibility was analyzed by means of a steady state simulation realized through the process simulator Chemcad in which a detailed 2D Fortran model has been integrated for the MCFC. A comparison of the new plant working with external (MCFC-ER) and internal (MCFC-IR) reforming MCFC was carried out. The small amount of methane in the syngas obtained by atmospheric pressure biomass gasification is not enough to exploit internal reforming cooling in the MCFC. This issue has been solved by the use of pre-reformer working as methanizer upstream the MCFC. The results of the simulations shown that, when MCFC-IR is used, the parameters of the cell are better managed. The result is a more efficient use of fuel even if some energy has to be consumed in the methanizer. In the MCFC-IR and MCFC-ER configurations, the calculated cell efficiency is, respectively, 0.53 and 0.42; the electric power produced is, respectively, 236 and 216 kW_e, and the maximum temperature reached in the cell layer is, respectively, 670 °C and 700 °C. The MCFC-ER configuration uses a cathode flowrate for MCFC cooling that are 30% lower than MCFC-IR configuration. This reduces pressure drop in the MCFC, possible crossover effect and auxiliaries power consumption. The electrical efficiency for the MCFC-IR configuration reaches 38%.     

中温固体氧化物燃料电池阳极通道中气体流动和传热分析

采用三维计算方法模拟和分析内重整反应和电化学反应及其在厚阳极层中对不同输运过程的影响。该文所研究的复合管道包括一个多孔阳极层、流道和金属双极板,利用基于燃料气体混合物的可变热物性参数(例如密度、粘度、比热等)及其耦合源项求解不同气体种类的动量和热量传递方程。模拟结果表明．内重整反应和电化学反应及其操作条件对阳极中的气体输运和热传递过程都有较大影响。     

Economic analysis of hydrogen production from wastewater and wood for municipal bus system

The levelized cost of hydrogen for municipal fuel cell buses has been determined using the DOE H2A model for steam methane reforming (SMR), molten carbonate fuel cell reforming (MCFC), and wood gasification using wastewater biogas and willow wood chips as energy feedstocks. 300 kg H₂/day was chosen as the design capacity. Greenhouse gas emissions were calculated for each for the three processes and compared to diesel bus emissions in order to assess environmental impact. The levelized cost per kilogram for SMR, MCFC, and gasification is $5.12, $8.59, and $10.62, respectively. SMR provided the lowest sensitivity to feedstock price, and lowest levelized cost at various scales, with competitive cost to diesel on a cost/km basis. All three technologies provide a reduction in total greenhouse gases compared to diesel bus emissions, with MCFC providing the largest reduction. These results provide preliminary evidence that small scale distributed hydrogen production for public transportation can be relatively cost-effective and have minimal environmental impact.     

Studies on sulfur poisoning and development of advanced anodic materials for waste-to-energy fuel cells applications

Biomass is the renewable energy source with the most potential penetration in energy market for its positive environmental and socio-economic consequences: biomass live cycles for energy production is carbon neutral; energy crops promote alternative and productive utilizations of rural sites creating new economic opportunities; bioenergy productions promote local energy independence and global energy security defined as availability of energy resource supply.Different technologies are currently available for energy production from biomass, but a key role is played by fuel cells which have both low environmental impacts and high efficiencies. High temperature fuel cells, such as molten carbonate fuel cells (MCFC), are particularly suitable for bioenergy production because it can be directly fed with biogas: in fact, among its principal constituents, methane can be transformed to hydrogen by internal reforming; carbon dioxide is a safe diluent; carbon monoxide is not a poison, but both a fuel, because it can be discharged at the anode, and a hydrogen supplier, because it can produce hydrogen via the water-gas shift reaction.However, the utilization of biomass derived fuels in MCFC presents different problems not yet solved, such as the poisoning of the anode due to byproducts of biofuel chemical processing. The chemical compound with the major negative effects on cell performances is hydrogen sulfide. It reacts with nickel, the main anodic constituent, forming sulfides and blocking catalytic sites for electrode reactions.The aim of this work is to study the hydrogen sulfide effects on MCFC performances for defining the poisoning mechanisms of conventional nickel-based anode, recommending selection criteria of sulfur-tolerant materials, and selecting advanced anodes for MCFC fed with biogas.     

Numerical research on carbon activity,fuel utilization,chemical reaction,and temperature distribution for IT single planar SOFC

A steady-state and three-dimensional (3-D) model has been established for an intermediate temperature planar anode-supported solid oxide fuel cell. The model couples charge transfer, mass transfer, momentum transfer, and energy transfer equations. Chemical reactions (methane cracking reaction (MCR), Boudouard reaction, methane steam reforming reaction (MSR), and water gas shift reaction (WGSR)) and electrochemical oxidation of H₂ and CO are considered. The variations of carbon activity, fuel utilization, and temperature distribution have been discussed with polarization and concentration separately. The thermal stress distribution in the positive electrode-electrolyte-negative electrode (P-E-N) structure is also discussed. The simulation results agree well with the literature.     

SOFC modeling considering electrochemical reactions at the active three phase boundaries

It is expected that fuel cells will play a significant role in a future sustainable energy system, due to their high energy efficiency and the possibility to use renewable fuels. A fully coupled CFD model (COMSOL Multiphysics) is developed to describe an intermediate temperature SOFC single cell, including governing equations for heat, mass, momentum and charge transport as well as kinetics considering the internal reforming and the electrochemical reactions. The influences of the ion and electron transport resistance within the electrodes, as well as the impact of the operating temperature and the cooling effect by the surplus of air flow, are investigated. As revealed for the standard case in this study, 90% of the electrochemical reactions occur within 2.4 μm in the cathode and 6.2 μm in the anode away from the electrode/electrolyte interface. In spite of the thin electrochemical active zone, the difference to earlier models with the reactions defined at the electrode–electrolyte interfaces is significant. It is also found that 60% of the polarizations occur in the anode, 10% in the electrolyte and 30% in the cathode. It is predicted that the cell current density increases if the ionic transfer tortuosity in the electrodes is decreased, the air flow rate is decreased or the cell operating temperature is increased.     

进气管通道直径对直接内部甲烷蒸汽重整性能的影响

蒸汽重整对于固体氧化物燃料电池利用甲烷等燃料具有很明显的优势,基于CFD商业软件及对应燃料电池多孔介质内多组分流动和扩散、传热传质、电化学反应、电流场等复杂的物理过程和电化学反应所开发的程序,对平板式阳极支撑固体氧化物燃料电池(PES-SOFC)甲烷蒸汽重整过程进行数值计算,得到不同排气管通道直径下燃料电池内部各气体组分摩尔分数、温度、温度梯度、输出电压等参数的分布。在通道直径为0.004 5 m时,输出电压最高,达到0.4923 V,同时在通道直径为0.004 5～0.005 m范围,能保证比较优化的温度分布。     

Electrochemical reforming of methane using SrZr0.5Ce0.4Y0.1O3-δ proton-conductor cell combined with paper-structured catalyst

Reforming of hydrocarbon which is an important hydrogen production method proceeds in two steps, i.e. steam reforming and shift reaction. Due to different thermodynamics, the two reactions are conventionally conducted at different temperatures. This study examines one step methane reforming by use of proton-conducting electrochemical cell in combination with a reforming catalyst. Promotion of the reforming reaction was intended by extracting hydrogen via electrochemical hydrogen pumping with a proton conductor cell. In order to compensate for the slow kinetics of the steam reforming, a paper catalyst loaded with Ni was placed in front of the electrochemical cell. Electrolyte support cells were used to verify this concept, and the effect of the electrochemical hydrogen pump was investigated from the composition of the outlet gas. Electrode support cells using a thin film electrolyte was used to reduce overvoltage. It is demonstrated that the steam reforming reaction and the shift reaction take place in one electrochemical cell. Effective catalyst placement and energy efficiency is discussed.     

基于加权残值法的高温燃料电池温度分布特性的数值分析

加权残值法是一种可以直接从偏微分方程中求得近似解的数学方法。通过对熔融碳酸盐燃料电池(MCFC)内部传热传质过程的热力学性能分析,在质量守恒和能量守恒的基础上建立了MCFC温度动态分布的数学模型,并采用加权残值法对其进行求解分析。确定了满足模型边界条件的试函数,以三次正交多项式为基函数,利用加权残值法中的迦辽金法,结合Matlab工具得到MCFC温度的动态分布特性曲线。分析结果表明,燃料电池内部各点温度在空间分布上有很大差异;当供给燃料电池的燃料流量和氧化剂流量变化时,所引起的温度动态特性是复杂的。图3参5。     

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.     

A CFD-based model of a planar SOFC for anode flow field design

This study presents a 3D CFD model of a planar SOFC with internal reforming for anode flow field design. The developed model reflects the influence of various factors on fuel cell performance including flow field design and kinetics of chemical and electrochemical reactions. The case study illustrates applications of the CFD model for planar SOFC with different anode flow field designs. Simulation results indicate the importance of the anode flow field design for planar SOFCs. The model is useful for optimization of fuel cell design and operating conditions.     

Simulation of a tubular solid oxide fuel cell through finite volume analysis: Effects of the radiative heat transfer and exergy analysis

This paper presents a very detailed finite volume axial-symmetric model of a tubular internal reforming solid oxide fuel cells (SOFCs), in which the effects of heat/mass transfer and chemical/electrochemical reactions are included. The model allows one to predict the performance of a single SOFC tube once a series of design and operative parameters are fixed, but also to investigate the source and localization of inefficiency. To this scope, an exergy analysis was implemented.