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.     

Effect of selected representative biomass gasification tar compounds on Ni-GDC solid oxide fuel cells

Contaminants as particulate matter, sulfur, chlorine and tar should be removed from biosyngas to avoid damaging solid oxide fuel cells. However, there is no sufficient information on tar effect since they might be potentially used as a fuel, or they might cause performance losses and irreversible damages. Therefore, this study aims to assess whether tar can be reformed inside the SOFC and used as fuel. Short-duration experiments were conducted on Ni-GDC cells operating with simulated biosyngas containing different concentrations of representative tar compounds from biomass gasification. While benzene and ethylbenzene could be regarded as additional fuels even at concentrations as high as 15 g/Nm³, naphthalene and phenanthrene act as contaminants for the SOFC electrochemical and catalytic reactions, even at concentrations of 0.3 and 0.05 g/Nm³. However, the effect on these reactions appeared almost completely reversible. Solid carbon deposited on the SOFC ceramic housing in proximity of the inlet. Post-mortem analysis should be performed to asses the tar effect on the cell anode.     

The impact of steam and current density on carbon formation from biomass gasification tar on Ni/YSZ, and Ni/CGO solid oxide fuel cell anodes

The combination of solid oxide fuel cells (SOFCs) and biomass gasification has the potential to become an attractive technology for the production of clean renewable energy. However the impact of tars, formed during biomass gasification, on the performance and durability of SOFC anodes has not been well established experimentally. This paper reports an experimental study on the mitigation of carbon formation arising from the exposure of the commonly used Ni/YSZ (yttria stabilized zirconia) and Ni/CGO (gadolinium-doped ceria) SOFC anodes to biomass gasification tars. Carbon formation and cell degradation was reduced through means of steam reforming of the tar over the nickel anode, and partial oxidation of benzene model tar via the transport of oxygen ions to the anode while operating the fuel cell under load. Thermodynamic calculations suggest that a threshold current density of 365 mA cm⁻² was required to suppress carbon formation in dry conditions, which was consistent with the results of experiments conducted in this study. The importance of both anode microstructure and composition towards carbon deposition was seen in the comparison of Ni/YSZ and Ni/CGO anodes exposed to the biomass gasification tar. Under steam concentrations greater than the thermodynamic threshold for carbon deposition, Ni/YSZ anodes still exhibited cell degradation, as shown by increased polarization resistances, and carbon formation was seen using SEM imaging. Ni/CGO anodes were found to be more resilient to carbon formation than Ni/YSZ anodes, and displayed increased performance after each subsequent exposure to tar, likely due to continued reforming of condensed tar on the anode.     

Performance of solid oxide fuel cell with chemical looping gasification products as fuel

Chemical looping gasification (CLG) can achieve the utilization of solid fuels for syngas production. The CLG system integrated with solid oxide fuel cell (SOFC) is a promising energy conversion way. In this work, an integration system of CLG and SOFC is evaluated via the implementation of a multi-field coupling modelling, where the products from the CLG are directly transported into the SOFC as the fuel and the coke deposition effect on the cell performance is evaluated. The results reveal that SOFC temperature using pure hydrogen as fuel has an increase of around 4 K compared to that with gas mixture as fuel owing to the inhibition of carbon deposition. It is found that the arrangement of anode and cathode in the countercurrent mode can promote the overall uniformity of current density compared to that in the cocurrent flow. Moreover, the impact of operating parameter of the CLG system on the SOFC performance is also examined. The results demonstrate that the increase of fuel reactor (FR) temperature and H₂O/C molar ratio in the CLG system is beneficial to the inhibition of carbon deposition and the enhancement of the SOFC performance.     

Thermodynamic modeling of an integrated biomass gasification and solid oxide fuel cell system

An integrated process of biomass gasification and solid oxide fuel cells (SOFC) is investigated using energy and exergy analyses. The performance of the system is assessed by calculating several parameters such as electrical efficiency, combined heat and power efficiency, power to heat ratio, exergy destruction ratio, and exergy efficiency. A performance comparison of power systems for different gasification agents is given by thermodynamic analysis. Exergy analysis is applied to investigate exergy destruction in components in the power systems. When using oxygen-enriched air as gasification agent, the gasifier reactor causes the greatest exergy destruction. About 29% of the chemical energy of the biomass is converted into net electric power, while about 17% of it is used to for producing hot water for district heating purposes. The total exergy efficiency of combined heat and power is 29%. For the case in which steam as the gasification agent, the highest exergy destruction lies in the air preheater due to the great temperature difference between the hot and cold side. The net electrical efficiency is about 40%. The exergy combined heat and power efficiency is above 36%, which is higher than that when air or oxygen-enriched air as gasification agent.     

Effect of gasification agent on the performance of solid oxide fuel cell and biomass gasification systems

In this paper, an integrated solid oxide fuel cell (SOFC) and biomass gasification system is modeled to study the effect of gasification agent (air, enriched oxygen and steam) on its performance. In the present modeling, a heat transfer model for SOFC and thermodynamic models for the rest of the components are used. In addition, exergy balances are written for the system components. The results show that using steam as the gasification agent yields the highest electrical efficiency (41.8%), power-to-heat ratio (4.649), and exergetic efficiency (39.1%), but the lowest fuel utilization efficiency (50.8%). In addition, the exergy destruction is found to be the highest at the gasifier for the air and enriched oxygen gasification cases and the heat exchanger that supplies heat to the air entering the SOFC for the steam gasification case.     

Decentralized generation of electricity with solid oxide fuel cells from centrally converted biomass

A thermodynamic evaluation of different energy conversion chains based on centralized biomass gasification and decentralized heat and power production by a solid oxide fuel cell (SOFC) has been performed. Three different chains have been evaluated, the main difference between the chains is the secondary fuel produced via biomass gasification. The secondary fuels considered are hydrogen, synthetic natural gas (SNG) and syngas. These fuels are assumed to be distributed through a transport and distribution grid to the micro-combined heat and power (μ-CHP) systems based on a SOFC and a heat pump.     

Exergy analysis and optimization of a biomass gasification, solid oxide fuel cell and micro gas turbine hybrid system

A hybrid plant producing combined heat and power (CHP) from biomass by use of a two-stage gasification concept, solid oxide fuel cells (SOFC) and a micro gas turbine was considered for optimization. The hybrid plant represents a sustainable and efficient alternative to conventional decentralized CHP plants. A clean product gas was produced by the demonstrated two-stage gasifier, thus only simple gas conditioning was necessary prior to the SOFC stack. The plant was investigated by thermodynamic modeling combining zero-dimensional component models into complete system-level models. Energy and exergy analyses were applied. Focus in this optimization study was heat management, and the optimization efforts resulted in a substantial gain of approximately 6% in the electrical efficiency of the plant. The optimized hybrid plant produced approximately 290 kW_e at an electrical efficiency of 58.2% based on lower heating value (LHV).     

Effect of operating parameters on performance of an integrated biomass gasifier,solid oxide fuel cells and micro gas turbine system

An integrated power system of biomass gasification with solid oxide fuel cells (SOFC) and micro gas turbine has been investigated by thermodynamic model. A zero-dimensional electrochemical model of SOFC and one-dimensional chemical kinetics model of downdraft biomass gasifier have been developed to analyze overall performance of the power system. Effects of various parameters such as moisture content in biomass, equivalence ratio and mass flow rate of dry biomass on the overall performance of system have been studied by energy analysis.It is found that char in the biomass tends to be converted with decreasing of moisture content and increasing of equivalence ratio due to higher temperature in reduction zone of gasifier. Electric and combined heat and power efficiencies of the power system increase with decreasing of moisture content and increasing of equivalence ratio, the electrical efficiency of this system could reach a level of approximately 56%.Regarding entire conversion of char in gasifier and acceptable electrical efficiency above 45%, operating condition in this study is suggested to be in the range of moisture content less than 0.2, equivalence ratio more than 0.46 and mass flow rate of biomass less than 20  kg h⁻¹.     

Thermodynamic evaluation of small-scale systems with biomass gasifiers, solid oxide fuel cells with Ni/GDC anodes and gas turbines

Thermodynamic calculations were carried out to evaluate the performance of small-scale gasifier–SOFC–GT systems of the order of 100 kW. Solid Oxide Fuel Cells (SOFCs) with Nickel/Gadolinia Doped Ceria (Ni/GDC) anodes were considered. High system electrical efficiencies above 50% are achievable with these systems. The results obtained indicate that when gas cleaning is carried out at temperatures lower than gasification temperature, additional steam may have to be added to biosyngas in order to avoid carbon deposition. To analyze the influence of gas cleaning at lower temperatures and steam addition on system efficiency, additional system calculations were carried out. It is observed that steam addition does not have significant impact on system electrical efficiency. However, generation of additional steam using heat from gas turbine outlet decreases the thermal energy and exergy available at the system outlet thereby decreasing total system efficiency. With the gas cleaning at atmospheric temperature, there is a decrease in the electrical efficiency of the order of 4–5% when compared to the efficiency of the systems working with intermediate to high gas-cleaning temperatures.     

Solid oxide fuel cell bi-layer anode with gadolinia-doped ceria for utilization of solid carbon fuel

Pyrolytic carbon was used as fuel in a solid oxide fuel cell (SOFC) with a yttria-stabilized zirconia (YSZ) electrolyte and a bi-layer anode composed of nickel oxide gadolinia-doped ceria (NiO-GDC) and NiO-YSZ. The common problems of bulk shrinkage and emergent porosity in the YSZ layer adjacent to the GDC/YSZ interface were avoided by using an interlayer of porous NiO-YSZ as a buffer anode layer between the electrolyte and the NiO-GDC primary anode. Cells were fabricated from commercially available component powders so that unconventional production methods suggested in the literature were avoided, that is, the necessity of glycine-nitrate combustion synthesis, specialty multicomponent oxide powders, sputtering, or chemical vapor deposition. The easily-fabricated cell was successfully utilized with hydrogen and propane fuels as well as carbon deposited on the anode during the cyclic operation with the propane. A cell of similar construction could be used in the exhaust stream of a diesel engine to capture and utilize soot for secondary power generation and decreased particulate pollution without the need for filter regeneration.     

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%.     

A highly coking-resistant solid oxide fuel cell with a nickel doped ceria: Ce1−xNixO2−y reformation layer

A single phase mixed oxide ion-electron conducting electrochemical catalyst of Ce_1−xNi_xO_2−y is employed as an anode functional reformation layer for a coking-resistant solid oxide fuel cell (SOFC) based on oxide ion conducting electrolyte operated in methane and ethanol. The high catalytic activity of Ce_1−xNi_xO_2−y oxide for fuel reformation is demonstrated by the excellent cell performances in various fuels at relatively low temperatures (550–650 °C). The fast oxygen ions flux and formed steam at anode side are also found to be favorable for hydrocarbon reformation to promote the cell performance and long term stability. At 650 °C, maximum power densities of 415 and 271 mW cm⁻² are achieved in methane and ethanol respectively. The resistance against carbon deposition is significantly improved with stable voltage output in a long-term durability operation.     

Highly efficient utilization of industrial barium slag for carbon gasification in direct carbon solid oxide fuel cells

Direct carbon solid oxide fuel cells (DC-SOFCs) are recognized as an efficient energy conversion device. With regard to their operation mechanism, the reverse Boudouard reaction rate is the crucial factor influencing cell performance. In this work, a new-type catalyst derived from industrial barium slag (BS) was first developed to enhance the reverse Boudouard reaction and DC-SOFC performance. The chemical composition and micro-morphologies of BS and barium slag-derived catalyst (BSC) were characterized in detail. The superiorities of BS and BSC were reflected in the enhanced DC-SOFC performance and high fuel utilization. The single cell fueled by BSC-loaded carbon yielded the best output of 249 mW cm⁻² at 850 °C. This result was comparable to the 266 mW cm⁻² output of a hydrogen-fueled SOFC due to the superior catalytic activity of metallic catalysts toward carbon gasification. The advantage of the BSC was also observed in the durable operation of the corresponding DC-SOFCs, which lasted for 36.2 h at 50 mA with the fuel utilization of 29.0%. This work provides a new channel for green and efficient utilization of BS and other industrial residues, and a novel option to the development of energy conversion technology.     

Effect of fabrication method on properties and performance of bimetallic Ni0.75Fe0.25 anode catalyst for solid oxide fuel cells

Ni/Fe alloy-based anodes have attracted much attention recently due to their potential for improving anodic activity and suppressing carbon deposition when operating on carbon-containing fuels. However, some inconsistent results about the iron alloying effect were reported in literature. In the present work, we systematically studied the influence of synthesis method on properties and cell performances of a Ni_0.75Fe_0.25 + SDC (60:40 v/o) alloy-ceramic anode for solid oxide fuel cells. Three different methods, i.e. physical mixing route (PMR), simultaneous glycine nitrate process/sol–gel route (S-GNP) and combined GNP sol–gel route (C-GNP), were used. Samples were analysed by X-ray diffraction, temperature-programmed reduction/oxidation, scanning electron microscopy and electrochemical impedance spectroscopy. It was revealed that the phase structure of anode components, chemical interaction between nickel and iron, and the electrode microstructure were strongly dependent on the synthesis method. The coking resistance was found to be more sensitive to anode phase structure and chemical binding between Ni and Fe phases, whereas the cell power output was mainly determined by the electrode microstructure. As a result, the iron content of the NiFe-based anode should be carefully controlled in different preparation methods to achieve high cell performances.     

Effect of anode structure on performance of cone-shaped solid oxide fuel cells fabricated by phase inversion

Anode-supported cone-shaped tubular solid oxide fuel cells (SOFCs) are successfully fabricated by a phase inversion method. During processing, the two opposite sides of each cone-shaped anode tube are in different conditions--one side is in contact with coagulant (the corresponding surface is named as “W-surface”), while the other is isolated from coagulate (I-surface). Single SOFCs are made with YSZ electrolyte membrane coated on either W-surface or I-surface. Compared to the cell with YSZ membrane on W-surface, the cell on I-surface exhibits better performance, giving a maximum power density of 350 mW cm⁻² at 800 °C, using wet hydrogen as fuel and ambient air as oxidant. AC impedance test results are consistent with the performance. The sectional and surface structures of the SOFCs were examined by SEM and the relationship between SOFC performance and anode structure is analyzed. Structure of anodes fabricated at different phase inversion temperature is also investigated.     

Direct-methane solid oxide fuel cell (SOFC) with Ni-SDC anode-supported cell

The performance of a Ni-SDC anode-supported cell operating with a dry CH₄ feed stream and the effectiveness of exposing the anode to H₂ as a method of removing carbon deposits are evaluated. This has involved the continuous monitoring of the outlet gas composition during CH₄ operation and H₂ exposure. A degradation rate in the cell voltage (～1.33 mV h^?1) is observed during 100 h operation with dry CH₄. Carbon is detected in the Ni-SDC anode after the stability test but only in the portion of the anode closest to the fuel channel. No carbon is detected at the electrolyte-anode interface, which is the likely reason that the cell performance remains relatively stable. The information obtained from SEM and gas outlet composition analyses can be explained by a process whereby most of the CH₄ that reacts decomposes into H₂ and C in the Ni-SDC anode near the fuel channel. H₂ then makes its way to the anode-electrolyte interface where it is electrochemically oxidized to H₂O which can also react with any C that may have formed, leaving behind C primarily at the fuel channel. When an aged cell is exposed to H₂, carbon-containing gases (CO, CH₄ and CO₂) are released, indicating that some carbon has been removed from the anode. Examination of the anode after the test shows that some carbon still remains after this treatment.     

Optimizing strontium titanate anode in solid oxide fuel cells by ytterbium doping

One of advantages of solid oxide fuel cells (SOFCs) is able to utilize various hydrocarbon fuels. Whereas, the classical Ni anode suffers severe carbon deposition especially operated under CH₄. Strontium titanate (SrTiO₃) perovskite anodes with strong carbon deposition resistance and good structural stability have been extensively investigated. In this work, Sr_0.88Y_0.08-xYb_xTiO₃ and Sr_0.88Y_0.08Ti_1-xYb_xO₃ are synthesized by Yb³⁺ doping in A-site and B-site of Sr_0.88Y_0.08TiO₃ perovskite, respectively. XRD results confirm that the SrTiO₃ cubic perovskite phase is formed in all the samples. Among the Yb³⁺ doping samples, Sr_0.88Y_0.06Yb_0.02TiO₃ exhibits the lowest thermal expansion coefficient (11.48 × 10⁻⁶/K), indicating the best compatibility with the electrolyte. The ionic conductivity of Sr_0.88Y_0.08TiO₃ can be improved by proper Yb³⁺ doping both in A-site and B-site, and the Sr_0.88Y_0.06Yb_0.02TiO₃ sample has the highest ionic conductivity among all the samples. The maximum power density of SOFC with Sr_0.88Y_0.06Yb_0.02TiO₃ anode is 87 mW/cm² under CH₄ at 800 °C, which is much higher than that with Sr_0.88Y_0.08TiO₃ and Ni anode. This can be related to its high electrocatalytic activity to CH₄ oxidation. In addition, SOFC with the Sr_0.88Y_0.06Yb_0.02TiO₃ anode shows a superior stability operated under CH₄ due to the strong carbon deposition resistances.     

Effect of alumina additions on the anode|electrolyte interface in solid oxide fuel cells

The longevity of a solid oxide fuel cell (SOFC) stack is curtailed by the fragility of its ceramic components. At Ceramic Fuel Cells Limited (CFCL), 15 wt.% alumina is added to the commonly used 10 mol% Y₂O₃–ZrO₂ (YSZ) electrolyte to improve both the fracture toughness and grain-boundary conductivity of the electrolyte. This study investigates the effect of such addition of alumina on the anode|electrolyte interface; more specifically, which reactions occur with the Al₂O₃ at the interface and how these reactions influence fuel cell performance. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the formation of NiAl₂O₄ in the alumina regions in the electrolyte. The NiAl₂O₄ is observed to grow into the adjacent grain boundaries to form an interconnected NiAl₂O₄ network up to 4 μm deep into the electrolyte. Impedance spectroscopy shows that the formation of NiAl₂O₄ does not affect the grain bulk ionic conductivity. The grain-boundary conductivity is markedly reduced at low temperatures. However, at the high SOFC operating temperature at CFCL (850 °C) the contribution of the grain-boundary conductivity to the total conductivity is diminished, and the NiAl₂O₄ is found not to have an effect on the total electrolyte conductivity and is deemed not to be a detrimental reaction.     

Performance of sorption-enhanced chemical looping gasification system coupled with solid oxide fuel cell using exergy analysis

Coal gasification system integrated with solid oxide fuel cell (SOFC) provides a promising energy conversion way owing to its high efficiency. To get a deep insight into the energy performance of this system, a thermodynamic evaluation is implemented. Meanwhile, the technologies of chemical looping and CO₂ sorption are introduced into this integration system. It is found that the addition of oxygen carrier and sorbent into coal gasification system can promote the output power of the SOFC with a higher exergy destruction, where the exergy efficiency of most modules in the system can reach 80% except tar separation. The results also reveal that a suitable improvement of gasifying agent amount is beneficial to the energy performance of the system. When the H₂O/C molar ratio is increased to 3.0, the SOFC exergy efficiency of 97% can be achieved.