天然气重整SOFC/MGT混合分布式供能系统性能分析

针对固体氧化物燃料电池(SOFC)与微型燃气轮机(MGT)构成的混合分布式供能系统,首先建立了一种管式SOFC准二维数值模型,优化了辐射计算,提高了热传递模型的准确性;考虑了CO及H2同时作为燃料参加电化学反应,并完善了损失计算模型;最后采用所发展的系统性能预测模型,分别在内部重整和外部重整情况下,预测比较了两种SOFC/MGT混合系统的性能,结果表明外部重整系统在系统输出功率、CO2排放以及热应力分布方面都比内部重整系统具有优势,然而这种轻微的优势是需要额外增加外部重整器的设备投资换取的。     

SOFC cogeneration system for building applications, part 1: Development of SOFC system-level model and the parametric study

A thermal and electrochemical model is developed for the simulation of Solid Oxide Fuel Cell (SOFC) cogeneration system in this study. The modeling algorithms of electrochemical and thermal models are described. Since the fuel cell stack itself is only a single component within the whole SOFC system, the modeling of the balance-of-plant (BOP) components is also performed to assess the system-level performance. Using the new model, a parametric analysis is carried out to investigate the effects of fuel flow rate, extent of methane gas pre-reforming, fuel utilization factor, recycling rate of cathode gas and cell voltage on the overall system performance. As a result of the parametric study, fuel flow rate, cell voltage, fuel utilization and recycling rate of cathode gas turned out to improve system power output. In addition, the internal reforming turned out to have advantage over external reforming in terms of system power supply.     

Design and partial load exergy analysis of hybrid SOFC–GT power plant

This paper presents a full and partial load exergy analysis of a hybrid SOFC–GT power plant. The plant basically consists of: an air compressor, a fuel compressor, several heat exchangers, a radial gas turbine, mixers, a catalytic burner, an internal reforming tubular solid oxide fuel cell stack, bypass valves, an electrical generator and an inverter. The model is accurately described. Special attention is paid at the calculation of SOFC overpotentials. Maps are introduced, and properly scaled, in order to evaluate the partial load performance of turbomachineries. The plant is simulated at full-load and part-load operation, showing energy and exergy flows trough all its components and thermodynamic properties at each key-point. At full-load operation a maximum value of 65.4% of electrical efficiency is achieved. Three different part-load strategies are introduced. The off-design operation is achieved handling the following parameters: air mass flow rate, fuel mass flow rate, combustor bypass, gas turbine bypass, avoiding the use of a variable speed control system. Results showed that the most efficient part-load strategy corresponded to a constant value of the fuel to air ratio. On the other hand, a lower value of net electrical power (34% of nominal load) could be achieved reducing fuel flow rate, at constant air flow rate. This strategy produces an electrical efficiency drop that becomes 45%.     

Numerical analysis of output characteristics of tubular SOFC with internal reformer

In the solid oxide fuel cell (SOFC) system, the internal reforming of raw fuel will act as an efficient cooling system. To realize this cooling system, a special design of the internal reformer is required to avoid the inhomogeneous temperature distribution caused by the strong endothermic reforming reaction at the entrance of the internal reformer. For this purpose, a tubular internal reformer with adjusted catalyst density can be inserted into the tubular SOFC stack. By arranging this, the raw fuel flows along the axis of the internal reformer to be moderately reformed and returns at the end of the internal reformer as a sufficiently reformed fuel.In this paper, the output characteristics of this configuration are simulated using mathematical models, in which one-dimensional temperature and molar distributions are computed along the flow direction. By properly mounting the catalyst density in the internal reformer, the temperature distribution of the cell stack becomes moderate, and the power generation efficiency and the exhaust gas temperature are higher. Effects of other operating conditions such as fuel recirculation, fuel inlet temperature, air recirculation and air inlet temperature are also examined under the condition where the maximum temperature of the stack is kept at 1300 K by adjusting the air flow rate. Under this condition, these operating conditions exert a considerable effect on the exhaust temperature but have a slight effect on the efficiency.     

Energy and exergy analysis of internal reforming solid oxide fuel cell–gas turbine hybrid system

The aim of this work is to analyze methane-fed internal reforming solid oxide fuel cell–gas turbine (IRSOFC—GT) power generation system based on the first and second law of thermodynamics. Exergy analysis is used to indicate the thermodynamic losses in each unit and to assess the work potentials of the streams of matter and of heat interactions. The system consists of a prereformer, a SOFC stack, a combustor, a turbine, a fuel compressor and air compressor, recuperators and a heat recovery steam generator (HRSG). A parametric study is also performed to evaluate the effect of various parameters such as fuel flow rate, air flow rate, temperature and pressure on system performance.     

Thermoeconomic analysis of pressurized hybrid SOFC systems with CO2 separation

In this paper, the results of the thermodynamic and economic analyses of distributed power generation plants (1.5 MWe) are described and compared. The results of an exergetic analysis are also reported, as well as the thermodynamic details of the most significant streams of the plants. The integration of different hybrid solid oxide fuel cell (SOFC) system CO₂ separation technologies characterizes the power plants proposed. A hybrid system with a tubular SOFC fed with natural gas with internal reforming has been taken as reference plant. Two different technologies have been considered for the same base system to obtain a low CO₂ emission plant. The first technology involved a fuel decarbonization and CO₂ separation process placed before the system feed, while the second integrated the CO₂ separation and the energy cycle. The first option employed fuel processing, a technology (amine chemical absorption) viable for short-term implementation in real installations while the second option provided the CO₂ separation by condensing the steam from the system exhaust. The results obtained, using a Web-based Thermo Economic Modular Program software, developed by the Thermochemical Power Group of the University of Genoa, showed that the thermodynamic and economic impact of the adoption of zero emission cycle layouts based on hybrid systems was relevant.     

天然气水蒸气重整与SOFC耦合应用

固体氧化物燃料电池（SOFC）系统具有高能源效率和使用可再生燃料的可能性,将在未来的可持续能源系统中发挥重要作用。过去几年燃料电池的发展很快,但在成本、稳定性和市场份额方面,该技术仍处于早期发展阶段。在以天然气为燃料的SOFC系统中,燃料的重整过程和燃料利用水平都可能影响系统运行的稳定性、热量和能量平衡,从而影响系统的使用寿命、输出功率和效率。因此,对燃料重整过程的设计与控制对有效的SOFC电池运行具有重要意义。对天然气在SOFC系统中的重整器配置方式（包括外重整和内重整）、重整参数和重整燃料利用方式进行了详细的综述分析,并对未来天然气SOFC系统的发展进行了展望。     

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.     

Comparison of efficiencies of low, mean and high temperature fuel cell Systems

Fuel cell systems are always said to show high electrical efficiency. The results achieved up to now, however, differ considerably, especially between the various fuel cell types all using natural gas as fuel. With the presented study the reasons for the different results and general potentials for fuel cell systems are highlighted. For that purpose several system lay-out concepts were elaborated for PEFC, PAFC and SOFC.The performed energy balance calculations for eight different plant concepts (three PEFC, two PAFC and three SOFC) for steady state operation with methane revealed that because of external reforming PEFC and PAFC systems are limited to about 67% and 70%, respectively, for the fuel utilisation. High temperature fuel cells can achieve at least 80% because of the possibility of internal reforming, or even over 90% in case of anode off-gas recycling. In combination with a cell voltage which is about 100 mV lower than that of MCFC and planar SOFC, PEFC can only achieve 38% of electrical net efficiency, PAFC 42% and tubular SOFC 54%. The latter is similar to MCFC, which is operated at higher cell voltage but lower fuel utilisation. The highest efficiency with up to 63% can be achieved with planar SOFC systems, because this concept allows high fuel utilisation together with high cell voltages.     

Experimental study of SOFC system heat-up without safety gases

Premixed safety gas is conventionally used to keep the anode of a solid oxide fuel cell (SOFC) under reducing conditions during heat-up. This article presents the results of an experimental study to heat up a SOFC system and stack without the said premixed safety gases, i.e. by utilizing a natural gas pre-reformer and anode off-gas recycling (AOGR). Firstly, ex-situ experiments were conducted to investigate the operability of a pre-reformer during system heat-up. It was found that any oxygen fed to the reformer hinders the reforming reactions at low temperatures. Secondly, based on the ex-situ findings, series of heat-up cycles were conducted with a complete 10 kW system using AOGR and a planar SOFC stack. In these experiments it was found that the system heat-up is possible with fuel gas and steam only, without the need for premixed reducing safety gases. Use of the fuel gas instead of a premixed safety gas did not result in a significant performance loss in the SOFC stack. Therefore, such a heat-up strategy was developed for SOFC systems that reduces the need of premixed safety gas storage space and thus decreases the system cost.     

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 comparison of cocurrent and countercurrent flow solid oxide fuel cells

Solid oxide fuel cell (SOFC) is a complicated system with heat and mass transfer as well as electrochemical reactions. The flowing configuration of fuel and oxidants in the fuel cell will greatly affect the performance of the fuel cell stack. Based on the developed mathematical model of direct internal reforming SOFC, this paper established a distributed parameters simulation model for cocurrent and countercurrent types of SOFC based on the volume-resistance characteristic modeling method. The steady-state distribution characteristics and dynamic performances were compared and were analyzed for cocurrent and countercurrent types of SOFCs. The results indicate that the cocurrent configuration of SOFC is more suitable with regard to performance and safety.     

Dealing with fuel contaminants in biogas-fed solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) plants: Degradation of catalytic and electro-catalytic active surfaces and related gas purification methods

Fuel cell and hydrogen technologies are re-gaining momentum in a number of sectors including industrial, tertiary and residential ones. Integrated biogas fuel cell plants in wastewater treatment plants and other bioenergy recovery plants are nowadays on the verge of becoming a clear opportunity for the market entry of high-temperature fuel cells in distributed generation (power production from a few kW to the MW scale).High-temperature fuel cell technologies like molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) are especially fit to operate with carbon fuels due to their (direct or indirect) internal reforming capability. Especially, systems based on SOFC technology show the highest conversion efficiency of gaseous carbon fuels (e.g., natural gas, digester gas, and biomass-derived syngas) into electricity when compared to engines or gas turbines. Also, lower CO₂ emissions and ultra-low emissions of atmospheric contaminants (SO_X, CO, VOC, especially NO_X) are generated per unit of electricity output. Nonetheless, stringent requirements apply regarding fuel purity. The presence of contaminants within the anode fuel stream, even at trace levels (sometimes ppb levels) can reduce the lifetime of key components like the fuel cell stack and reformer. In this work, we review the complex matrix (typology and amount) of different contaminants that is found in different biogas types (anaerobic digestion gas and landfill gas). We analyze the impact of contaminants on the fuel reformer and the SOFC stack to identify the threshold limits of the fuel cell system towards specific contaminants. Finally, technological solutions and related adsorbent materials to remove contaminants in a dedicated clean-up unit upstream of the fuel cell plant are also reviewed.     

Part load operation of SOFC/GT hybrid systems: Stationary analysis

In a global energetic context characterized by the increasing demand of oil and gas, the depletion of fossil resources and the global warming, more efficient energy systems and, consequently, innovative energy conversion processes are urgently required. A possible solution can be found in the fuel cells technology coupled with classical thermodynamic cycle technologies in order to make hybrid systems able to achieve high energy/power efficiency with low environmental impact. Moreover, due to the synergistic effect of using a high temperature fuel cell such as solid oxide fuel cell (SOFC) and a recuperative gas turbine (GT), the integrated system efficiency can be significantly improved. In this paper a steady zero dimensional model of a SOFC/GT hybrid system is presented. The core of the work consists of a performance analysis focused on the influence of the GT part load functioning on the overall system efficiency maintaining the SOFC power set to the nominal one. Also the proper design and management of the heat recovery section is object of the present study, with target a global electric efficiency almost constant in part load functioning respect to nominal operation. The results of this study have been used as basis to the development of a dynamic model, presented in the following part of the study focused on the plant dynamic analysis.     

Analysis of a SOFC energy generation system fuelled with biomass reformate

Biomass reformation is an interesting path for hydrogen production and its use for efficient energy generation. The main target is the fully exploitation of the potential of renewable fuels. To this aim, the coupling a biomass reformer together with a high temperature solid oxide fuel cell (SOFC) stack shows some advantages for the similar operating temperature of the two processes and the internal reforming capability of the SOFC. The latter further allows less stringent composition requirements of the feed gas from a gasifier and internal cooling of the SOFC.In this work, a complete model of a SOFC coupled with a biomass gasifier is used to identify the main effects of the operating conditions on the fuel cell performance.The gasification process has been simulated by an equilibrium model able to compute the reformate composition under different operating conditions, whereas a 3D fluid dynamics simulation (FLUENT) coupled with an external model for the electrochemical reactions has been used to predict the fuel cell performance in terms of electrical response and mass-energy fluxes.A 14 kW integrated SOFC-gasifier system has been analysed with this model to address the response of a planar SOFC as a function of the gasifier operating conditions.     

Prediction of the performance of a solid oxide fuel cell fuelled with biosyngas: Influence of different steam-reforming reaction kinetic parameters

SOFCs are often designed to operate with specific fuels, quite often natural gas. CFD modeling is often used to arrive at efficient and safe SOFC designs. Therefore, when an SOFC is fed with different fuels, i.e., biosyngas, CFD can be used as a tool to predict whether the cell and stack will be safe and operate efficiently, and thus can give suggestions for the operation strategies for SOFCs. For that reason, a combined mass and heat transport model of an SOFC (single channel) has been developed for an anode-supported SOFC fed with biosyngas with special attention to the reaction kinetics of the direct internal reforming (DIR) reaction together with the water–gas shift reaction. An SOFC design jointly developed by ECN and Delft University of Technology is employed for the calculations. This work aims to predict the influence of different reforming reaction kinetic parameters on the cell performance by using an anode-supported intermediate temperature DIR planar solid oxide fuel single channel model, under co-flow operation. The DIR reaction of methane, the water–gas shift reaction and the electrochemical oxidation of hydrogen are being considered. As different reaction kinetic models are available in literature and employing them in CFD calculations will yield different results, a comparative analysis is carried out. Several cases were studied with a variety of DIR and water gas shift reaction kinetic parameters available from literature. For the different cases considered, the modeling results show differences in the current density distribution and temperature profile in the channel and in gas concentration profile along the channel. These differences are presented and discussed in detail. Predictions of the behaviors of internal reforming reaction in the reaction zone, and the possibilities of unwanted side reactions such as carbon deposition and Ni oxidation are given with constructive suggestions for future lab experiments.     

Design performance analysis of pressurized solid oxide fuel cell/gas turbine hybrid systems considering temperature constraints

This study presents critical aspects and their influence on the performance of hybrid power systems combining a pressurized solid oxide fuel cell (SOFC) and a gas turbine (GT). Two types of hybrid system configurations with internal and external reforming have been analyzed. In order to examine the effect of matching between the fuel cell temperature and the turbine inlet temperature on the hybrid system performance, we considered air bypass after the compressor as well as additional fuel supply to the turbine side. This study focuses on the limitation of the temperature difference at the fuel cell stack and its influence on the performances of the two hybrid systems. Performances of the hybrid systems are also compared with those of simple SOFC systems, and the extent of performance enhancement is evaluated. The system with internal reforming gives better efficiency and power capacity for all design conditions than the system with external reforming under the same constraints. Its efficiency gain over the SOFC only system is considerable, while that of the system with external reforming is far less. As the temperature difference at the cell becomes smaller, the system performance generally degrades. The system with internal reforming is less influenced by the constraint of the cell temperature difference.     

Fault-tolerant control for steam fluctuation in SOFC system with reforming units

Faults of solid oxide fuel cell (SOFC) systems can affect the characteristics of the stack and inhibit SOFC system commercialization. It has been found that the temperature fluctuation of the burner caused by fluctuation of steam flow rate would greatly affect the temperature of SOFC system and even exceed the safe operation range. Firstly, this paper introduces a mathematical model for the process of steam and natural gas reforming in a real SOFC system. Secondly, the cause of the burner temperature fluctuation is analyzed, and the model to simulate this faulty situation is established. Then, the Bayesian regularization neural network is used for fault diagnosis and good test results are obtained. Finally, fuzzy fault-tolerant control strategy is designed for the thermal safety problem of SOFC system. The simulation results validate the effectiveness of the proposed fault-tolerant control strategy.     

Thermodynamic analysis of an integrated power generation system driven by solid oxide fuel cell

A new integrated power generation system driven by the solid oxide fuel cell (SOFC) is proposed to improve the conversion efficiency of conventional energy by using a Kalina cycle to recover the waste heat of exhaust from the SOFC-GT. The system using methane as main fuel consists an internal reforming SOFC, an after-burner, a gas turbine, preheaters, compressors and a Kalina cycle. The proposed system is simulated based on the developed mathematical models, and the overall system performance has been evaluated by the first and second law of thermodynamics. Exergy analysis is conducted to indicate the thermodynamic losses in each components. A parametric analysis is also carried out to examine the effects of some key thermodynamic parameters on the system performance. Results indicate that as compressor pressure ratio increases, SOFC electrical efficiency increases and there is an optimal compressor pressure ratio to reach the maximum overall electrical efficiency and exergy efficiency. It is also found that SOFC electrical efficiency, overall electrical efficiency and exergy efficiency can be improved by increasing air flow rate. Also, the largest exergy destruction occurs in the SOFC followed by the after-burner, the waste heat boiler, the gas turbine. The compressor pressure ratio and air flow rate have significant effects on the exergy destruction in some main components of system.     

Effects of gas recycle on performance of solid oxide fuel cell power systems

An energy analysis of solid oxide fuel cell (SOFC) power systems with gas recycles fed by natural gas is carried out. Simple SOFC system, SOFC power systems with anode and cathode gas recycle respectively and SOFC power system with both anode and cathode gas recycle are compared. Influences of reforming rate, air ratio and recycle ratio of electrode exhaust gas on performance of SOFC power systems are investigated. Net system electric efficiency and cogeneration efficiency of these power systems are given by a calculation model. Results show that internal reforming SOFC power system can achieve an electrical efficiency of more than 44% and a system cogeneration efficiency including waste heat recovery of 68%. For SOFC power system with anode gas recycle, an electrical efficiency is above 46% and a cogeneration efficiency of 88% is obtained. In the case of cathode gas recycle, an electrical efficiency and a cogeneration efficiency is more than 51% and 78% respectively. Although SOFC system with both anode and cathode gas is more complicated, the electrical efficiency of it is close to 52%.