S.H. Chan

Energy and exergy analysis of simple solid-oxide fuel-cell power systems

Abstract Two, simple, solid-oxide fuel-cell (SOFC) power systems fed by hydrogen and methane, respectively, are examined. While other models available in the literatures focus on complicated hybrid SOFC and gas-turbine (GT) power systems, this study focuses on simple SOFC power systems with detailed thermodynamic modeling of the SOFC. All performance-related parameters of the fuel-cell such as respective resistivity of the components, anode and cathode exchange current density, limiting current density, flow diffusivity, etc. are all expressed as a function of temperature, while the flow through of each nodes of the system is described as a function of thermodynamic state. Full analysis of the energy and exergy at each node of the system is conducted and their respective values are normalized by the lower heating value (LHV) of the fuel and its chemical exergy, respectively. Thus, the normalized electrical energy outputs directly indicate the first law and second law efficiencies, respectively, of the fuel-cell power systems.

Modelling of simple hybrid solid oxide fuel cell and gas turbine power plant

Abstract This paper presents the work on a simple, natural gas-fed, hybrid solid oxide fuel cell–gas turbine (SOFC–GT) power-generation system. The system consists of an internal-reforming SOFC (IRSOFC) stack, a combustor, a GT, two compressors and three recuperators. Two case studies are conducted with particular attention on the effects of operating pressure and fuel flow-rate on the performance of the components and overall system. Results show that an internal-reforming hybrid SOFC–GT system can achieve an electrical efficiency of more than 60% and a system efficiency (including waste heat recovery for co-generation) of more than 80%. It is also found that increasing the operating pressure will improve the system efficiency, whereas increasing the fuel flow-rate (while keeping the fuel utilisation rate unchanged) causes the system efficiency to decrease. In the latter case, the increase in system fuel consumption is relatively higher which removes the benefit of increase in SOFC stack and turbine power output.

Anode Micro Model of Solid Oxide Fuel Cell

This paper presents the work on the development of a micro model of a solid oxide fuel cell (SOFC) porous anode, which formed by the mixture of electronic and ionic conductors. The model establishes the complex relationship between the transport phenomena, which includes the transports of electron, ion and gas molecules through the electrode and the electrochemical reaction at the three-phase boundary of the electrode. All forms of polarization losses were considered. Results show that, in general, the smaller the particle size in the anode microstructure, the larger the active area favorable to electrochemical reaction will be and thus lowering the overall polarization. However, the concentration polarization will likely play an important role, in particular for a thick electrode, if the particle size is smaller than certain limit causing substantial increase in flow resistance. The overall polarization reaches a minimum at certain particle size. In addition, it is known that the water vapor content in the hydrogen can greatly influence the polarization but the proper amount of water required for a particular anode design was not well researched, despite its importance. In this study, it was found that very high anode polarization is achieved when pure hydrogen is used. The polarization drops significantly when a small amount of water vapor is added in the hydrogen and the polarization does not change greatly in a wide range of water vapor partial pressure. Finally, there is a strong link between the anode thickness and particle size in anode design. The bigger the particle size in the anode microstructure, the thicker the anode should be for providing sufficient reaction sites.

Multi-level modeling of SOFC-gas turbine hybrid system

Abstract This paper presents the work on a natural gas-fed integrated internal-reforming solid oxide fuel cell–gas turbine (IRSOFC–GT) power generation system. It was assumed that only hydrogen participated in the electrochemical reaction, while the non-reacted raw gases and reformed gases are fully oxidized in the combustor downstream of the fuel cell stack. The system consists of an integrated reformer, a SOFC stack, a combustor, a gas turbine and a power turbine, a fuel compressor and an air compressor, two recuperates and a heat recovery steam generator (HRSG). Different levels of modeling work for the fuel cell, fuel cell stack, and integrated system were conducted, which provide a means for sizing up the power system in the developmental stage. Simulation results show that the IRSOFC–GT power system could achieve a net electrical efficiency of better than 60% and a system efficiency (including waste heat recovery for steam generation) of better than 80%.

Cathode Micromodel of Solid Oxide Fuel Cell

A comprehensive micromodel considering all forms of polarization in the cathode of the solid oxide fuel cell was developed which governs the complex interdependency among the transport phenomena, electrochemical processes (charge-transfer and surface diffusion), and the microstructure of the electrode and their combined effect on the cathode overpotential under different operating conditions. To make the model more generalized, we consider possible oxygen reduction mechanisms, reactions at the cathode/ electrolyte interface, grain interior and grain boundary effects on the total resistance, both ordinary diffusion and Knudsen diffusion, active three-phase boundary length as a function of ionic/electronic particle size ratio and volume fraction, the exchange current density as a function of gases concentration, etc. Incorporated with reliable experimental data, the model can be used as a tool to design a high performance cathodes.

Development of (La, Sr)MnO3-Based cathodes for intermediate temperature solid oxide fuel cells

Conventional (La, Sr)MnO 3 (LSM) electrodes modified by ion impregnation methods showed promising potential as cathodes for intermediate temperature solid oxide fuel cells. After impregnation of ionic conducting-type Gd 0 . 2 Ce 0 . 8 (NO 3 ) x nitrite salt solution into the LSM structure, electrochemical activity of LSM electrodes for the O 2 reduction reaction was substantially enhanced. At 700°C, the electrode polarization resistance of impregnated LSM electrodes decreased to 0.72 Ω cm 2 as compared to 26.4 Ω cm 2 for pure LSM electrodes, a reduction in electrode polarization resistance by 36 times. The polarization losses were also reduced substantially. At 300 mA cm - 2 and 700°C, overpotential for the O 2 reduction was reduced from 0.79 V on pure LSM to 0.19 V on impregnated LSM electrodes, a reduction in overpotential by four times. Ion-impregnated LSM electrodes showed better performance than those of LSM/Y 2 O 3 -ZrO 2 and LSM/(Gd,Ce)O 2 composite electrodes as reported in the literature.

Modelling for part-load operation of solid oxide fuel cell–gas turbine hybrid power plant

Abstract This paper presents the work on part-load operation of a power generation system composed of a solid oxide fuel cell and a gas turbine (SOFC–GT) which operate on natural gas. The system consists of an internal reforming SOFC (IRSOFC) stack, an external combustor, two turbines, two compressors, two recuperators and one heat-recovery steam generator (HRSG). Based on experience in different levels of modelling of the fuel cell, fuel cell stack and integrated system and the inherent characteristics of a IRSOFC–GT hybrid power plant, a practical approach for simplifying part-load operation of the system is proposed. Simulation results show that an IRSOFC–GT hybrid system could achieve a net electrical efficiency and system efficiency (including waste heat recovery for steam generation) of greater than 60 and 80%, respectively, under full-load operation. Due to the complexity of the interaction of the components and safety requirements, the part-load performance of a IRSOFC–GT hybrid power plant is poorer than that under full-load operation.

GDC-Impregnated ( La0.75Sr0.25 ) ( Cr0.5Mn0.5 ) O3 Anodes for Direct Utilization of Methane in Solid Oxide Fuel Cells

Gd-doped ceria (GDC)-impregnated (La 0.75 Sr 0.25 )(Cr 0.5 Mn 0.5 )O 3 (LSCM) is investigated as an alternative Ni-free anode for the direct utilization of methane in solid oxide fuel cells. Impregnation of submicrometer and ionic conducting GDC greatly improves the electrocatalytic activity of the LSCM anodes for the oxidation reaction in weakly humidified (3% H 2 O) methane. At 800°C, electrode polarization resistance for the reaction in wet CH 4 is 0.44 Ω cm 2 on a 4.0 mg cm -2 GDC-impregnated LSCM anode. In comparison, the electrode polarization resistance is 11.4 and 8.1 Ω cm 2 on a pure LSCM and a LSCM (50 wt %)/yttria-stabilized zirconia (YSZ) (50 wt %) composite anode, respectively, under the same testing conditions. The polarization performance of GDC-impregnated LSCM is also substantially higher than that of the pure LSCM and LSCM/YSZ composite anodes. Based on the results, a mechanism involving the dry reforming of methane, followed by the electrochemical oxidation of the dry reforming products is proposed for the methane oxidation on Ni-free mixed ionic and electronic conductors such as LSCM. Impregnation of nanosized GDC greatly enhances the catalytic as well as electrochemical activities for the dry reforming of methane and for the electrochemical oxidation reactions of the dry reforming products.

Sinterability and ionic conductivity of coprecipitated Ce0.8Gd0.2O2−δ powders treated via a high-energy ball-milling process

Abstract Ceria-based solid solutions are promising electrolytes for intermediate-temperature, solid oxide fuel cells. The effect of a dry, high-energy, ball-milling process on the sintering and densification behaviour of coprecipitated ceria-based powders is investigated by means of X-ray diffraction, Brunauer–Emmett–Teller (BET) surface-area measurements, density measurements, and electron microscopy. The dry ball-milling process leads to (i) a larger specific surface-area with weak agglomeration; (ii) rearrangement of grains into dense granules; (iii) a higher green density. These effects significantly reduce sintering temperatures and promote densification of ceria-based ceramics. Moreover, a comparison is made of the sintering behaviour and ionic conductivity of the milled samples with and without cobalt oxide doping. Cobalt oxide is a very effective sintering aid, but usually results in an enlarged grain-boundary effect for Si-containing samples. Thus, since SiO 2 is a ubiquitous background impurity in both raw materials and ceramic processing, the dry ball-milling process is a more feasible method for improving the sinterability of coprecipitated ceria-based powders.

A mathematical model of polymer electrolyte fuel cell with anode CO kinetics

We develop a mathematical model of solid polymer electrolyte fuel cell with anode CO kinetics, which is essentially a model that marrying the work of Bernardi and Verbrugge (J. Electrochem. Soc. 139 (1992) 2477) with that of Springer et al. (J. Electrochem. Soc. 148 (2001) A11). Two cases of study were carried out. First, the water self-sufficiency of fuel cell operation was conducted under different current density, temperature, pressure differential across the membrane-electrode-assembly (MEA), hydraulic permeability and electro-kinetic permeability. Comparison of superficial water velocities in the MEA under the effect of different current density with those from Bernardi and Verbrugge was conducted. Results showed that, treating the catalyst layers as interfaces instead of regions as simplified by Bernardi and Verbrugge, would significantly underestimate the water velocities in the MEA and the error is particularly large at high current density operations. Second, the effect of CO poisoning of fuel cell was presented in terms on cell polarization. The prediction covered 0, 25, 50, 100 and 250 ppm of CO concentration in hydrogen feedstock and results were validated by experimental data obtained from Springer et al. The trends of anode polarization curve due to CO poisoning were explained.