Thermodynamic assessment of solid oxide fuel cell system integrated with bioethanol purification unit

A solid oxide fuel cell system integrated with a distillation column (SOFC–DIS) has been proposed in this article. The integrated SOFC system consists of a distillation column, an EtOH/H₂O heater, an air heater, an anode preheater, a reformer, an SOFC stack and an afterburner. Bioethanol with 5 mol% ethanol was purified in a distillation column to obtain a desired concentration necessary for SOFC operation. The SOFC stack was operated under isothermal conditions. The heat generated from the stack and the afterburner was supplied to the reformer and three heaters. The net remaining heat from the SOFC system (Q_SOFC,Net) was then provided to the reboiler of the distillation column. The effects of fuel utilization and operating voltage on the net energy (Q_Net), which equals Q_SOFC,Net minus the distillation energy (Q_D), were examined. It was found that the system could become more energy sufficient when operating at lower fuel utilization or lower voltage but at the expense of less electricity produced. Moreover, it was found that there were some operating conditions, which yielded Q_Net of zero. At this point, the integrated system provides the maximum electrical power without requiring an additional heat source. The effects of ethanol concentration and ethanol recovery on the electrical performance at zero Q_Net for different fuel utilizations were investigated. With the appropriate operating conditions (e.g. C_EtOH = 41%, U_f = 80% and EtOH recovery = 80%), the overall electrical efficiency and power density are 33.3% (LHV) and 0.32 W cm⁻², respectively.     

Operation of solid oxide fuel cell on biomass product gas with tar levels >10 g Nm−3

This work assesses experimentally the feasibility of feeding a high tar load product gas from biomass gasification to a planar solid oxide fuel cell (SOFC) for renewable electricity generation. The SOFC had a nickel gadolinium-doped ceria anode (Ni-GDC) and the gasifier was a pilot scale circulating fluidized bed, employing hot gas-cleaning to remove particulates, HCl and H₂S. The SOFC operated for several hours on either pre-reformed gas (reduced tar levels < 0.5 g Nm^?3) as well as on high tar-laden wood gas (tar levels > 10 g Nm^?3) i.e. with no pre-reforming of tars. The tests were carried out at low fuel utilization U_f of around 20% at a current density j = 130 mA cm^?2. In all cases stable continuous SOFC performance was established. Post experimental examination of the SOFC showed that the anode was not affected by carbon deposition or other impurity accumulation.     

Development of 1 kW SOFC power package for dual-fuel operation

A compact SOFC power generation system was developed by integrating a 1 kW SOFC stack and balance-of-plant. The system was designed for dual-fuel operation using both natural gas (NG) and liquefied petroleum gas (LPG). An adiabatic pre-reformer was employed in a fuel processing system to convert C₂₊ hydrocarbons in the fuel into CH₄-rich gas which was further processed in a main reformer to produce H₂-rich gas for the SOFC stack. The SOFC system was operated for 350 h under thermally self-sustaining condition, and on-load fuel switching from NG to LPG was carried out during the operation. The system performance was not significantly affected by NG/LPG composition ratios and the performance was stable during continuous operation in NG or LPG.     

Conceptual design of a novel ammonia-fuelled portable solid oxide fuel cell system

A novel portable electric power generation system, fuelled by ammonia, is introduced and its performance is evaluated. In this system, a solid oxide fuel cell (SOFC) stack that consists of anode-supported planar cells with Ni-YSZ anode, YSZ electrolyte and YSZ-LSM cathode is used to generate electric power. The small size, simplicity, and high electrical efficiency are the main advantages of this environmentally friendly system. The results predicted through computer simulation of this system confirm that the first-law efficiency of 41.1% with the system operating voltage of 25.6 V is attainable for a 100 W portable system, operated at the cell voltage of 0.73 V and fuel utilization ratio of 80%. In these operating conditions, an ammonia cylinder with a capacity of 0.8 l is sufficient to sustain full-load operation of the portable system for 9 h and 34 min. The effect of the cell operating voltage at different fuel utilization ratios on the number of cells required in the SOFC stack, the first- and second-law efficiencies, the system operating voltage, the excess air, the heat transfer from the SOFC stack, and the duration of operation of the portable system with a cylinder of ammonia fuel, are also studied through a detailed sensitivity analysis. Overall, the ammonia-fuelled SOFC system introduced in this paper exhibits an appropriate performance for portable power generation applications.     

Performance analysis for the part-load operation of a solid oxide fuel cell–micro gas turbine hybrid system

In this paper, the performance evaluation of a solid oxide fuel cell (SOFC)–micro gas turbine (MGT) hybrid power generation system under the part-load operation was studied numerically. The present analysis code includes distributed parameters model of the cell stack module. The conversions of chemical species for electrochemical process and fuel reformation process are considered. Besides the temperature distributions of the working fluids and each solid part of cell module by accounting heat generation and heat transfers, are taken into calculation. Including all of them, comprehensive energy balance in the cell stack module is calculated. The variable MGT rotational speed operation scheme is adopted for the part-load operation. It will be made evident that the power generation efficiency of the hybrid system decreases together with the power output. The major reason for the performance degradation is the operating temperature reduction in the SOFC module, which is caused by decreasing the fuel supply and the heat generation in the cells. This reduction is also connected to the air flow rate supplement. The variable MGT rotational speed control requires flexible air flow regulations to maintain the SOFC operating temperature. It will lead to high efficient operation of the hybrid system.     

Computer experimental analysis of the CHP performance of a 100 kWe SOFC Field Unit by a factorial design

Gas Turbine Technologies (GTT) and Politecnico di Torino, both located in Torino (Italy), have been involved in the design and installation of a SOFC laboratory in order to analyse the operation, in cogenerative configuration, of the CHP 100 kW_e SOFC Field Unit, built by Siemens-Westinghouse Power Corporation (SWPC), which is at present (May 2005) starting its operation and which will supply electric and thermal power to the GTT factory. In order to take the better advantage from the analysis of the on-site operation, and especially to correctly design the scheduled experimental tests on the system, we developed a mathematical model and run a simulated experimental campaign, applying a rigorous statistical approach to the analysis of the results.The aim of this work is the computer experimental analysis, through a statistical methodology (2^k factorial experiments), of the CHP 100 performance. First, the mathematical model has been calibrated with the results acquired during the first CHP100 demonstration at EDB/ELSAM in Westerwoort. After, the simulated tests have been performed in the form of computer experimental session, and the measurement uncertainties have been simulated with perturbation imposed to the model independent variables. The statistical methodology used for the computer experimental analysis is the factorial design (Yates’ Technique): using the ANOVA technique the effect of the main independent variables (air utilization factor U_ox, fuel utilization factor U_F, internal fuel and air preheating and anodic recycling flow rate) has been investigated in a rigorous manner. Analysis accounts for the effects of parameters on stack electric power, thermal recovered power, single cell voltage, cell operative temperature, consumed fuel flow and steam to carbon ratio. Each main effect and interaction effect of parameters is shown with particular attention on generated electric power and stack heat recovered.     

Electrolytic effect in solid oxide fuel cells running on steam/methane mixture

A two-dimensional model is developed to study the performance of a planar solid oxide fuel cell (SOFC) running on steam/methane mixture. The model considers the heat/mass transfer, electrochemical reactions, direct internal reforming of methane (CH₄), and water gas shift reaction in an SOFC. It is found that at an operating potential of 0.8 V, the upstream and downstream of SOFC work in electrolysis and fuel cell modes, respectively. At the open-circuit voltage, the electricity generated by the downstream part of SOFC is completely consumed by the upstream through electrolysis, which is contrary to our common understanding that electrochemical reactions cease under the open-circuit conditions. In order to inhibit the electrolytic effect, the SOFC can be operated at a lower potential or use partially pre-reformed CH₄ as the fuel. Increasing the inlet gas velocity from 0.5 m s⁻¹ to 5.0 m s⁻¹ does not reduce the electrolytic effect but decreases the SOFC performance.     

Performance characteristics of a robust and compact propane-fueled 150 W-class SOFC power-generation system

A propane-fueled solid-oxide fuel-cell-based system is an extraordinary type of technology for stationary mobile power generation given that it offers higher efficiency, silent operation and clean conversion of hydrocarbon fuels. In this study, we designed and developed a 150 W-class tubular SOFC power-generation system integrated with a catalytic partial reformer (CPOX) for the propane fuel and heat exchangers with the goal of making a robust and compact system for portable power applications. Micro-tubular SOFC cells were fabricated by ceramic processing and the cells were assembled in the form of a short stack. The CPOX nano-catalyst CeO₂Zr₂O₃/Pt supported on γ-Al₂O₃ was prepared and tested for its propane-reforming characteristics under the present operating conditions. The CPOX catalyst was used in the integrated reformer, and the performance of the 150 W-class SOFC power-generation system operating on propane fuel was studied. The rapid startup and temperature sustainability of the short stack were also monitored and stable stack temperatures were achieved within 20 min. Long-term galvanostatic operation of the power-generation system was also conducted to investigate the durability of the system. This study confirms that propane-fueled robust and compact 150 W-class power-generation systems are suitable for portable applications and that the role of efficient CPOX catalysts is crucial for high performance of the stack when operating on propane fuel.     

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.     

Modelling of polymer electrolyte membrane fuel cell stacks based on a hydraulic network approach

Polymer electrolyte membrane (PEM) fuel cells convert the chemical energy of hydrogen and oxygen directly into electrical energy. Waste heat and water are the reaction by‐products, making PEM fuel cells a promising zero‐emission power source for transportation and stationary co‐generation applications. In this study, a mathematical model of a PEM fuel cell stack is formulated. The distributions of the pressure and mass flow rate for the fuel and oxidant streams in the stack are determined with a hydraulic network analysis. Using these distributions as operating conditions, the performance of each cell in the stack is determined with a mathematical, single cell model that has been developed previously. The stack model has been applied to PEM fuel cell stacks with two common stack configurations: the U and Z stack design. The former is designed such that the reactant streams enter and exit the stack on the same end, while the latter has reactant streams entering and exiting on opposite ends. The stack analysed consists of 50 individual active cells with fully humidified H₂ or reformate as fuel and humidified O₂ or air as the oxidant. It is found that the average voltage of the cells in the stack is lower than the voltage of the cell operating individually, and this difference in the cell performance is significantly larger for reformate/air reactants when compared to the H₂/O₂ reactants. It is observed that the performance degradation for cells operating within a stack results from the unequal distribution of reactant mass flow among the cells in the stack. It is shown that strategies for performance improvement rely on obtaining a uniform reactant distribution within the stack, and include increasing stack manifold size, decreasing the number of gas flow channels per bipolar plate, and judicially varying the resistance to mass flow in the gas flow channels from cell to cell. Copyright © 2004 John Wiley & Sons, Ltd.     

Biohythane as an energy feedstock for solid oxide fuel cells

Biogas (60%-CH₄, 40%- CO₂) is a potential source of renewable energy when used as energy feedstock for solid oxide fuel cells (SOFC), but releases biogenic CO₂ emissions. Hybrid SOFC performance can be affected by fuel composition and reformer performance. Biohythane (58%-CH₄, 35%-CO₂ and 7% H₂) can be a better alternative providing balance between energy and biogenic emissions. Biohythane performance is studied for a 120 kW SOFC stack using ASPEN process model and compared with other feed stocks. This work is the first to study and report on the application of biohythane in SOFC systems. Biohythane was found to produce less biogenic CO₂ emissions and 6% less CO at the reformer than biogas. Comparisons show that biohythane provides better efficiencies in hybrid SOFC systems. Sensitivity studies recommends operation of stack with biohythane at Steam to Carbon Ratio (STCR) = 2.0, i = 200 mA cm⁻² and U_F = 0.85 respectively.     

Application of adaptive neuro-fuzzy inference system techniques and artificial neural networks to predict solid oxide fuel cell performance in residential microgeneration installation

This study applies adaptive neuro-fuzzy inference system (ANFIS) techniques and artificial neural network (ANN) to predict solid oxide fuel cell (SOFC) performance while supplying both heat and power to a residence. A microgeneration 5 kW_el SOFC system was installed at the Canadian Centre for Housing Technology (CCHT), integrated with existing mechanical systems and connected in parallel to the grid. SOFC performance data were collected during the winter heating season and used for training of both ANN and ANFIS models. The ANN model was built on back propagation algorithm as for ANFIS model a combination of least squares method and back propagation gradient decent method were developed and applied. Both models were trained with experimental data and used to predict selective SOFC performance parameters such as fuel cell stack current, stack voltage, etc.     

Anode recirculation behavior of a solid oxide fuel cell system: A safety analysis and a performance optimization

Anode recirculation, which is generally driven by an ejector, is commonly used in solid oxide fuel cell (SOFC) systems that operate with natural gas. Alternative fuels such as gasification syngas from biomass have been proposed for potential use in the SOFC systems because of the fuel flexibility of SOFCs and the sustainability of biomass resources. Because the ejector was initially designed to use natural gas, its recirculation behavior when using alternative fuels is not well understood. The aim of this research work is to study anode recirculation behavior and analyze its effect on safety issues regarding carbon deposition and nickel oxidation and the performance of an SOFC system fed with gasification syngas under steady state operation. We developed a detailed model including a recirculation model and an SOFC stack model for this study, which was well validated by experimental data. The results show that the entrainment ratio with the gasification syngas is much smaller than that with the natural gas, and the gasification syngas does not have the tendency toward carbon deposition or nickel oxidation under the operating conditions studied. In addition, the recirculation affects the performance of the SOFC, especially the net electrical efficiency, which could be promoted by 160%.     

Efficiency gain of solid oxide fuel cell systems by using anode offgas recycle - Results for a small scale propane driven unit

The transfer of high electrical efficiencies of solid oxide fuel cells (SOFC) into praxis requires appropriate system concepts. One option is the anode-offgas recycling (AOGR) approach, which is based on the integration of waste heat using the principle of a chemical heat pump.The AOGR concept allows a combined steam- and dry-reforming of hydrocarbon fuel using the fuel cell products steam and carbon dioxide. SOFC fuel gas of higher quantity and quality results. In combination with internal reuse of waste heat the system efficiency increases compared to the usual path of partial oxidation (POX).The demonstration of the AOGR concept with a 300 Wel-SOFC stack running on propane required: a combined reformer/burner-reactor operating in POX (start-up) and AOGR modus; a hotgas-injector for anode-offgas recycling to the reformer; a dynamic process model; a multi-variable process controller; full system operation for experimental proof of the efficiency gain.Experimental results proof an efficiency gain of 18 percentage points (η·POX = 23%, η·AOGR = 41%) under idealized lab conditions. Nevertheless, further improvements of injector performance, stack fuel utilization and additional reduction of reformer reformer O/C ratio and system pressure drop are required to bring this approach into self-sustaining operation.     

Part-load,startup, and shutdown strategies of a solid oxide fuel cell-gas turbine hybrid system

Current work on the performance of a solid oxide fuel cell (SOFC) and gas turbine hybrid system is presented. Each component model developed and applied is mathematically defined. The electrochemical performance of single SOFC with different fuels is tested. Experimental results are used to validate the SOFC mathematical model. Based on the simulation model, a safe operation regime of the hybrid system is accurately plotted first. Three different part-load strategies are introduced and used to analyze the part-load performance of the hybrid system using the safe regime. Another major objective of this paper is to introduce a suitable startup and shutdown strategy for the hybrid system. The sequences for the startup and shutdown are proposed in detail, and the system responses are acquired with the simulation model. Hydrogen is used instead of methane during the startup and shutdown process. Thus, the supply of externally generated steam is not needed for the reforming reaction. The gas turbine is driven by complementary fuel and supplies compressed air to heat up or cool down the SOFC stack operating temperature. The dynamic simulation results show that smooth cooling and heating of the cell stack can be accomplished without external electrical power.     

Exergetic analysis of solid oxide fuel cell and biomass gasification integration with heat pipes

This paper presents an exergetic analysis of a combined heat and power (CHP) system, integrating a near-atmospheric solid oxide fuel cell (SOFC) with an allothermal biomass fluidised bed steam gasification process. The gasification heat requirement is supplied to the fluidised bed from the SOFC stack through high-temperature sodium heat pipes. The CHP system was modelled in AspenPlus™ software including sub-models for the gasification, SOFC, gas cleaning and heat pipes. For an average current density of 3000 A m⁻² the proposed system would consume 90 kg h⁻¹ biomass producing 170 kW_e net power with a system exergetic efficiency of 36%, out of which 34% are electrical.     

High temperature electrolyte supported Ni-GDC/YSZ/LSM SOFC operation on two-stage Viking gasifier product gas

This paper presents the results from a 150 h test of a commercial high temperature single planar solid oxide fuel cell (SOFC) operating on wood gas from the Viking two-stage fixed-bed downdraft gasifier, which produces an almost tar-free gas, that was further cleaned for particulates, sulphur and tar traces. The chosen SOFC was electrolyte supported with a nickel/gadolinium-doped cerium oxide (Ni-GDC) anode, known for its carbon deposition resistance. Through humidification the steam to carbon ratio (S/C) was adjusted to 0.5, which results in a thermodynamically carbon free condition at the SOFC operating temperature T = 850 °C. The cell operated with a fuel utilisation factor (U_f) around 30% and a current density of 260 mA cm⁻² resulting in an average power density of 207 mW cm⁻². Throughout the duration of the test, only a minor cell overpotential increase of 10 mV was observed. Nevertheless, the V–j (voltage–current density) curves on H₂/N₂ before and after the wood gas test proved identical. Extensive SEM/EDS examination of the cell's anode showed that there was neither carbon deposition nor significant shifts in the anode microstructure or contamination when compared to an identical cell tested on H₂/N₂ only.     

Performance evaluation of different configurations of biogas-fuelled SOFC micro-CHP systems for residential applications

Three configurations of solid oxide fuel cell (SOFC) micro-combined heat and power (micro-CHP) systems are studied with a particular emphasis on the application for single-family detached dwellings. Biogas is considered to be the primary fuel for the systems studied. In each system, a different method is used for processing the biogas fuel to prevent carbon deposition over the anode of the cells used in the SOFC stack. The anode exit gas recirculation, steam reforming, and partial oxidation are the methods employed in systems I–III, respectively. The results predicted through computer simulation of these systems confirm that the net AC electrical efficiency of around 42.4%, 41.7% and 33.9% are attainable for systems I–III, respectively. Depending on the size, location and building type and design, all the systems studied are suitable to provide the domestic hot water and electric power demands for residential dwellings. The effect of the cell operating voltage at different fuel utilization ratios on the number of cells required for the SOFC stack to generate around 1 kW net AC electric power, the thermal-to-electric ratio (TER), the net AC electrical and CHP efficiencies, the biogas fuel consumption, and the excess air required for controlling the SOFC stack temperature is also studied through a detailed sensitivity analysis. The results point out that the cell design voltage is higher than the cell voltage at which the minimum number of cells is obtained for the SOFC stack.     

Modelling of cells,stacks and systems based around metal-supported planar IT-SOFC cells with CGO electrolytes operating at 500–600 °C

Ceres Power Ltd. has developed a novel solid oxide fuel cell (SOFC) concept based upon depositing a thick film positive–electrolyte–negative (PEN) structure on a porous stainless steel substrate, and using gadolinia-doped ceria (CGO) as the electrolyte material. This approach allows the temperature of operation to be reduced to below 600 °C, well below the conventional SOFC operating temperature. Historically, the use of CGO as an electrolyte material has been viewed as impractical because of its poor stability in reducing atmospheres at elevated temperatures, leading to electronic conductivity, which effectively short-circuits the cell leading to a loss of efficiency. In this work, a model is developed which accurately simulates the polarisation behaviour of a Ceres cell including electronic leakage. The parameters of this model are set to give the best possible fit to experimental data. This cell model is then incorporated into a model of a 2.5 kW_e stack, and the stack model into a natural gas fuelled combined heat and power (CHP) system model. The system model demonstrates that high operating efficiencies are achievable for such a system based upon IT-SOFC cells with CGO electrolytes.     

A mathematical model of a tubular solid oxide fuel cell with specified combustion zone

A numerical model has been developed to simulate the effect of combustion zone geometry on the steady state and transient performance of a tubular solid oxide fuel cell (SOFC). The model consists of an electrochemical submodel and a thermal submodel. In the electrochemical model, a network circuit of a tubular SOFC was adopted to model the dynamics of Nernst potential, ohmic polarization, activation polarization, and concentration polarization. The thermal submodel simulated heat transfers by conduction, convention, and radiation between the cell and the air feed tube. The developed model was applied to simulate the performance of a tubular solid oxide fuel cell at various operating parameters, including distributions of circuits, temperature, and gas concentrations inside the fuel cell. The simulations predicted that increasing the length of the combustion zone would lead to an increase of the overall cell tube temperature and a shorter response time for transient performance. Enlarging the combustion zone, however, makes only a negligible contribution to electricity output properties, such as output voltage and power. These numerical results show that the developed model can reasonably simulate the performance properties of a tubular SOFC and is applicable to cell stack design.