Nonlinear predictive control of a molten carbonate fuel cell stack

Operating temperature of a molten carbonate fuel cell stack should be controlled within a special range in order to improve the performance of fuel cell. In this paper, a nonlinear predictive control algorithm based on the Takagi–Sugeno fuzzy model is developed for the temperature of a molten carbonate fuel cell stack. Through predicting the outputs on a Takagi–Sugeno fuzzy model, a discrete optimization of the control action is adopted according to the principle of branch-and-bound method. The simulation results show the potential to introduce the predictive control based on Takagi–Sugeno fuzzy model for the development of fuel cells.     

熔融碳酸盐燃料电池（MCFC）性能研究

简要叙述了MCFC微观工作过程,然后分别详细讨论了压力,温度,反应气体的组分和利用率,电流密度,电解质板结构和电解质的成分,杂质和运动时间对MCFC性能和寿命的曩,并结合文献和实验数据对其机理进行了阐述,最后得出了为提高电池性能和瞎长其寿命的几点结论和建议。     

Control-oriented thermal management of solid oxide fuel cells based on a modified Takagi–Sugeno fuzzy model

Thermal management for a solid oxide fuel cell (SOFC) is actually temperature control, due to the importance of cell temperature for the performance of an SOFC. An SOFC stack is a nonlinear and multi-variable system which is difficult to model by traditional methods. A modified Takagi–Sugeno (T–S) fuzzy model that is suitable for nonlinear systems is built to model the SOFC stack. The model parameters are initialized by the fuzzy c-means clustering method, and learned using an off-line back-propagation algorithm. In order to obtain the training data to identify the modified T–S model, a SOFC physical model via MATLAB is established. The temperature model is the center of the physical model and is developed by enthalpy-balance equations. It is shown that the modified T–S fuzzy model is sufficiently accurate to follow the temperature response of the stack, and can be conveniently utilized to design temperature control strategies.     

熔融碳酸盐燃料电池（MCFC）系统建模与控制的研究现状与发展

详细介绍了MCFC的电极,单电池、电堆,系统四个层次的建模以及MCFC控制的研究现状,指出了现有模型的不足;讨论了电堆和系统两级建模的发展方向,分析了MCFC系统的非线性,大时滞、分布参数、多输入多输出,有约束和随机干扰等特征,提出了两种适宜的控制方法。     

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.     

Effect of sulfur contaminants on MCFC performance

Molten carbonate fuel cells (MCFC) used as carbon dioxide separation units in integrated fuel cell and conventional power generation can potentially reduce carbon emission from fossil fuel power production. The MCFC can utilize CO₂ in combustion flue gas at the cathode as oxidant and concentrate it at the anode through the cell reaction and thereby simplifying capture and storage. However, combustion flue gas often contains sulfur dioxide which, if entering the cathode, causes performance degradation by corrosion and by poisoning of the fuel cell. The effect of contaminating an MCFC with low concentrations of both SO₂ at the cathode and H₂S at the anode was studied. The poisoning mechanism of SO₂ is believed to be that of sulfur transfer through the electrolyte and formation of H₂S at the anode. By using a small button cell setup in which the anode and cathode behavior can be studied separately, the anodic poisoning from SO₂ in oxidant gas can be directly compared to that of H₂S in fuel gas. Measurements were performed with SO₂ added to oxidant gas in concentrations up to 24 ppm, both for short-term (90 min) and for long-term (100 h) contaminant exposure. The poisoning effect of H₂S was studied for gas compositions with high- and low concentration of H₂ in fuel gas. The H₂S was added to the fuel gas stream in concentrations of 1, 2 and 4 ppm. Results show that the effect of SO₂ in oxidant gas was significant after 100 h exposure with 8 ppm, and for short-term exposure above 12 ppm. The effect of SO₂ was also seen on the anode side, supporting the theory of a sulfur transfer mechanism and H₂S poisoning. The effect on anode polarization of H₂S in fuel gas was equivalent to that of SO₂ in oxidant gas.     

A heuristic method of variable selection based on principal component analysis and factor analysis for monitoring in a 300 kW MCFC power plant

In a commercialized 300 kW molten carbonate fuel cell (MCFC) power plant, a univariate alarm system that has only upper and lower limits is usually employed to identify abnormal conditions in the system. Even though univariate alarms have already been adopted for system monitoring, this simple monitoring system is limited for using in an extended monitoring system for fault diagnosis. Therefore, based on principal component analysis (PCA), a recursive variable grouping method for a multivariate monitoring system in a commercialized MCFC power plant is presented in this paper. In terms of development, since a principal component analysis model that contains all system variables cannot isolate a system fault, heuristic recursive variable selection method using factor analysis is presented here. To verify the performance of the fault detection, real plant operations data are used. Furthermore, comparison between type 1 and type 2 errors for four different variable groups demonstrates that the developed heuristic method works well when system faults occur. These monitoring techniques can reduce the number of false alarms occurring on site at MCFC power plant.     

Performance investigation of a combined MCFC system

This study investigates the performance of a combined industrial molten carbonate fuel cell (MCFC) system, including a turbo expander, which was recently installed by Enbridge Inc. in Toronto, Canada. It entails a comprehensive thermodynamic analysis regarding energy and exergy calculations, subject to varying operating conditions. Furthermore, a simplified and novel method is used for a cost analysis to assess the amortization of the system. The results from the base case study suggest that an overall energy efficiency as high as 60% is achievable while fuel cell stack energy and exergy efficiencies of 50.6% and 49.3%, respectively, are reached. The cost analysis indicates that the amortization of the system may take up to 15 years of operational time, depending on the price of electricity and natural gas. However, carbon offsets may make a paramount contribution to the overall savings and economic viability of future combined MCFC systems.     

Improved performance of molten carbonate fuel cells with (Li/Na)2CO3 electrolytes by using BYS coated cathode at low operating temperatures

In order to improve the stack life time of MCFCs, it is necessary to reduce the operating temperature of MCFCs below 600 °C, because reduced operating temperature minimizes electrolyte loss due to evaporation and corrosion. However, at the low operating temperature below 600 °C, the cell performance of MCFCs with (Li/Na)₂CO₃ electrolyte is too low to operate the fuel cell stack and system. In this study, we have performed wettability control of the liquid molten carbonate electrolyte by coating NiO cathodes with poor wetting property of the mixed ionic and electronic conductor (MIEC) such as BYS (Bi_1.5Y_0.3Sm_0.3O_3-δ). From experiments with symmetrical cells, each polarization component with various temperatures and gas conditions were studied. To investigate effects of the BYS coated cathode on the performance of MCFCs, a 100 cm² single cell of MCFCs was employed. The performance of a 100 cm² single cell with BYS coated cathode was better than that with conventional cathode by a factor of 1.84, because BYS coated cathode reduces activation polarization and mass transfer resistance greatly.     

Nonlinear dynamic modeling for a SOFC stack by using a Hammerstein model

Solid oxide fuel cell (SOFC) is a kind of nonlinear, multi-input–multi-output (MIMO) system that is hard to model by the traditional methodologies. For the purpose of dynamic simulation and control, this paper reports a dynamic modeling study of SOFC stack using a Hammerstein model. The static nonlinear part of the Hammerstein model is modeled by a radial basis function neural network (RBFNN), and the linear part is modeled by an autoregressive with exogenous input (ARX) model. To estimate the hidden centers, the radial basis function widths and the connection weights of the RBFNN, a new gradient descent algorithm is derived in the study. On the other hand, the least squares (LS) algorithm and Akaike Information Criteria (AIC) are used to estimate the parameters and the orders of the ARX model, respectively. The applicability of the proposed Hammerstein model in modeling the nonlinear dynamic properties of the SOFC is illustrated by the simulation. At the same time, the experimental comparisons between the Hammerstein model and the RBFNN model are provided which show a substantially better performance for the Hammerstein model. Furthermore, based on this Hammerstein model, some control schemes such as predictive control, robust control can be developed.     

Decarbonizing cement plants via a fully integrated calcium looping-molten carbonate fuel cell process: Assessment of a model for fuel cell performance predictions under different operating conditions

This study is part of a comprehensive research devoted to the integration of a Calcium Looping (CaL) process with a Molten Carbonate Fuel Cell (MCFC) for the decarbonisation of a full-scale cement plant. In the proposed process, where the energy intensive oxy-combustion occurring in the CaL calciner is replaced with a conventional combustion in air. The CO₂-rich gas leaving the calciner is injected into the MCFC cathode while the anode side is fuelled by H₂-rich gases produced by a sorption-enhanced reforming (SER) process. The high CO₂-concentrated gas leaving the anode will be sent to valorisation processes and/or the CO₂ final disposal.Here we focus on modelling, simulation and characterization of the MCFC used as a device for CO₂ separation as well as electricity production, here considered as a process by-product. Polarization curves (I–V curves) and Electrochemical Impedance Spectroscopy (EIS) were measured to support the development and the calibration of a semi-empirical model obtained by theoretical consideration.The experimental campaign demonstrated that the fitted model is able to reproduce the real cell performance when varying the temperature, H₂ concentration, CO₂ concentration at anode and cathode respectively as well as CO₂ CaL capture rate.Indeed, the average difference between numerical and experimental results is always below 2%.Results also demonstrated that the MCFC can be usefully considered as an efficient CO₂ concentrator, with a CO₂ fraction at the anode outlet that is greater than 51% on a dry basis.     

Nonlinear identification of a DIR-SOFC stack using wavelet networks

Application of wavelet networks for identification of a direct internal reforming solid oxide fuel cell (DIR-SOFC) stack is reported in this paper. The SOFC is a complex system particularly when it is directly fueled with hydrocarbons (natural gas, coal gas, etc.). Most of the traditional models of the SOFC, based on the reforming, electrochemical and thermal modeling, are too complicated. To facilitate controller design and analysis of systems, the wavelet network dynamic model of the DIR-SOFC is constructed, avoiding the consideration of the complex processes in the fuel cells. The input and output data are used for initializing and training the wavelet network by a recursive approach. The Gram–Schmidt algorithm, the Cross-Validation method and immune selection principles are applied to optimization of the network. The simulation is performed and comparisons of characteristics under different operating conditions are given. The results show high static and dynamic accuracy of the identified model. Further, the obtained wavelet network model can be used for developing the model-based controllers of DIR-SOFC.     

Mechanical strength improvement of aluminum foam-reinforced matrix for molten carbonate fuel cells

During the cell operation of molten carbonate fuel cells (MCFCs), matrix cracks caused by poor mechanical strength accelerate cell performance degradation. Therefore, for a stable long-term cell operation, the improvement of mechanical properties of matrix is highly required. In this study, aluminum foam was used to enhance the mechanical strength of the matrix as a 3D (three dimensional) support structure. Unlikely conventional matrix (pure α-LiAlO₂ matrix) which has paste-like structure at the MCFC operating temperature, Al foam-reinforced α-LiAlO₂ matrix has significantly strong mechanical strength because the 3D network structure of Al foam can form the harden alumina skin layer during a cell operation. As a result, the mechanical strength of the Al foam-reinforced α-LiAlO₂ matrix was enhanced by 9 times higher than the pure α-LiAlO₂ matrix in a 3-point bending test. In addition, thermal cycle test showed notable cell stability due to strong mechanical strength of Al foam-reinforced α-LiAlO₂ matrix. The Al foam-reinforced α-LiAlO₂ matrix shows appropriate microstructure to preserve the liquid electrolyte when performing the mercury porosimeter analysis and differential pressure test between anode and cathode. Moreover, evaluation of stability and durability for a long-term cell operation were demonstrated by single cell test for 1,000 h.     

Electrochemical analysis on the growth of oxide formed on stainless steels in molten carbonate at 650 °C

The oxide growth on stainless steel (SS) 310S and 316L, used as a cathode current collector material of molten carbonate fuel cell (MCFC), were examined in the mixture of 62 mol% Li₂CO₃–38 mol% K₂CO₃ at 650 °C by measuring the change in corrosion potential and potentiodynamic response of the alloys and also in terms of impedance analysis on the alloy|oxide layer|electrolyte system. The corrosion potential of SS 316L was in an active region for 12 h-immersion, whereas that of SS 310S drastically increased after 6 h-immersion due to an active to passive transition. The corrosion rate of the two SSs decreased with immersion due to the growth of protective oxide. However, the decrease in the corrosion rate of SS 310S is much greater than that of SS 316L. The oxide formed on the two SSs was found to be duplex layer, composed of inner Cr enriched oxide and outer Fe enriched oxide. However, the inner Cr enriched layer of 310S is more clearly separated from the outer Fe enriched layer than that of SS 316L due primarily to the higher Cr content in SS 310S. The drastic increase in the corrosion potential of SS 310S after 6 h-immersion is closely associated with the growth of the inner Cr enriched oxide layer. The corrosion resistance of SS depends dominantly on the resistance of the inner Cr enriched oxide that is determined form the impedance analysis on the alloy|oxide layer|electrolyte system. The higher corrosion resistance of SS 310S compared with SS 316L results from the more protective inner Cr enriched oxide layer, as confirmed by its higher resistance associated with the higher Cr content in SS 310S.     

Performance analysis of new cathode materials for molten carbonate fuel cells

The slow dissolution of the lithiated nickel oxide cathode represents one of the main causes of performance degradation in molten carbonate fuel cells (MCFC). Two main approaches were studied in ENEA laboratories to overcome this problem: protecting the nickel cathode covering it by a thin layer of a material with a low solubility in molten carbonate and stabilizing the nickel cathode doping it with iron and magnesium.Among several materials, due to its low solubility and good conductivity, lithium cobaltite was chosen to cover the nickel cathode and slow down its dissolution. A nickel electrode covered with a thin layer of lithium cobaltite doped with magnesium, was fabricated by complex sol-gel process. To simplify electrode preparation, no thermal treatments were made after covering to produce lithium cobaltite, and during the cell start-up LiMg_0.05Co_0.95O₂ was obtained in situ.To stabilize the nickel cathode, metal oxides Fe₂O₃ and MgO were chosen as dopant additives to be mixed with NiO powder in a tape-casting process (Mg_0.05Fe_0.01Ni_0.94O).On the prepared materials TGA analysis, morphological analysis by scanning electron microscopy (SEM-EDS) and electrical conductivity measurements were carried out.A conventional nickel cathode, the nickel cathode covered by lithium cobaltite precursors and the nickel cathode stabilized by iron and magnesium oxides were each tested in a 100 cm² fuel cell.Polarization curves and internal resistance (iR) measurements were acquired during the cell lifetime (1000 h) and the effect of gas composition variation on the cell performance was studied.From a comparison with the conventional nickel cathode it can be observed that the new materials have similar performance and show a good potential stability during the cell operating time. From the post-test analysis both the nickel cathode covered by lithium cobaltite and the nickel cathode doped with iron and magnesium seem to succeed in reducing nickel dissolution.     

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.     

基于AspenPlus的兆瓦级燃料电池分布式发电系统建模及仿真分析

目的   燃料电池分布式发电技术是适应未来能源低碳化、清洁化、高效化发展趋势的重要应用方向。国内燃料电池电站项目较少,缺乏实际项目经验积累。为了推进燃料电池分布式电站技术的应用,文章概述了国内外应用现状,总结了高温燃料电池的优势与不足,调研了国内燃料电池建设应用案例,并建立了固体氧化物燃料电池与熔融碳酸盐燃料电池发电系统流程。 方法   经过文献调研与实地调研,确定了两种适合建设大型电站的燃料电池分布式发电技术,并利用AspenPlus化工模拟软件建立燃料电池系统流程模型、电化学模型和能量分析模型,并开展系统的性能仿真分析。 结果   分析结果与实际运行结果相吻合,分析预测的系统性能趋势与已有研究相一致。 结论   该仿真方法可用于兆瓦级高温燃料电池分布式发电系统的研究,可为扩大燃料电池应用规模提供数据支持。     

Predictions of the optimum ternary alkali-carbonate electrolyte composition for MCFC by computational calculation

Various kinds of phase diagrams for Li–Na–K ternary carbonate systems were plotted and the vapor pressures of chemical species such as alkali cations and oxygen molecule at various compositions and various temperatures were calculated by computational manipulation of thermodynamic databases. The liquidus temperature of (Li_0.52Na_0.48)₂CO₃–(Li_0.62K_0.32)₂CO₃, (Li_0.62K_0.32)₂CO₃–(Li_0.44Na_0.30K_0.26)₂CO₃, and (Li_0.44Na_0.30K_0.26)₂CO₃–(Li_0.52Na_0.48)₂CO₃ binary systems decrease with the increase of Li₂CO₃ content without any ternary intermediate compound. Total vapor pressure of alkali metal species governed by the summation of the vapor pressure of free Na and K and the vapor pressure of alkali metal species starts to decrease abruptly when the content of (Li_0.52Na_0.48)₂CO₃ is over 70 mol% in (Li_0.52Na_0.48)₂CO₃–(Li_0.62K_0.32)₂CO₃ system while over 50 mol% in (Li_0.44Na_0.30K_0.26)₂CO₃–(Li_0.52Na_0.48)₂CO₃ system. On the contrary, the equilibrium vapor pressure of oxygen molecule abruptly increases at the same composition range.