International audienceA planar solid oxide fuel cell (SOFC) was designed to be operated in gradual internal reforming conditions under pure dry methane. Stable operation was achieved for about 30 h. This result was obtained by deposition onto a Ni–yttria-stabilized zirconia cermet of a highly active and carbon-deposition-resistant catalyst layer consisting of Ir/CeO2. The performances of this layer were first evaluated in situ at open-circuit voltage, and then the principle of gradual internal reforming associated with electrocatalytic dissociation was demonstrated. The best cell performances with CH4/H2O=1/4 were 0.1 A cm−2 at 0.55 V (about 55 mW cm−2) at 1173 K
International audienceNatural gas appears to be a highly attractive fuel for solid oxide fuel cell systems. To avoid the cooling effect occurring in direct internal reforming, gradual internal reforming (GIR) can be used. GIR is based on local coupling between steam reforming and hydrogen oxidation. The steam required for the reforming reaction is obtained from hydrogen oxidation on the anode side. Previous studies have demonstrated that the cooling effect has disappeared. However, with GIR, the risk of carbon formation is greater. To deal with this issue, a different cell configuration was studied. This configuration combines a catalyst layer with a cermet anode, allowing GIR without coking. The study comprised simulations, using the CFD Research Corporation software package, of the behavior of a tubular solid oxide fuel cell when using GIR. A thermodynamic study based on the partial pressure distributions within the cell was also carried out to investigate the occurrence of carbon formation. A parametric analysis of the reforming rate and the thickness of the layer were then performed. The simulations indicate that the risk of carbon deposition is strongly reduced if the configuration is used for a catalyst layer of 900 µm and at a reforming rate in the catalyst only ten times higher than the reforming rate in nickel
Natural gas appears to be a fuel of great interest for solid oxide fuel cell (SOFC) systems. It mainly consists of methane, which can be converted into hydrogen by direct internal reforming (DIR) within the SOFC anode. However, a major limitation to DIR is carbon formation within the ceramic layers at intermediate temperatures. This paper proposes a model solution using the CFD-ACE software package to simulate the behavior of a tubular SOFC. A detailed thermodynamic analysis is carried out to predict the boundary of carbon formation for SOFCs fueled by methane. Thermodynamic equilibrium calculations that take into account Boudouard and methane cracking reactions allow us to investigate the occurrence of carbon formation. This possibility is discussed from the values of driving forces for carbon deposition defined as α=PCO2∕(KBPCO2) and β=PH22∕(KCPCH4), from the equilibrium constants KB and KC of the Boudouard and cracking reactions, and from the partial pressure Pi of species i. Simulations allow the calculation of the distributions of partial pressures for all the gas species (CH4, H2, CO, CO2, and H2O), current densities, and potentials of both electronic and ionic phases within the anode part (i.e., gas channel and Cermet anode). Finally, a mapping of α and β values enables us to predict the predominant zones where carbon formation is favorable (α or β<1) or unfavorable (α or β>1) according to the calculation based on thermodynamic equilibrium. With regard to the values of these different coefficients, we can say that a carbon formation can be supposed for temperature less than 800°C and for ratios xH2O∕xCH4 smaller than 1.
International audienceOne of the major obstacles to improving electrochemical performance of SOFCs is the limitation with respect to current collecting. The aim of this study is to examine these limitations on the basis of a model of a single cell of tubular SOFC. The simulation results allow us to understand and analyze the effects of ionic and electronic ohmic drops on cell performance. This paper describes a model using the CFD-Ace software package to simulate the behaviour of a tubular SOFC. Modelling is based on solving conservation equations of mass, momentum, energy, species and electric current by using a finite volume approach on 3D grids of arbitrary topology. The electrochemistry in the porous gas diffusion electrode is described using Butler-Volmer equations at the triple phase boundary. The electrode overpotential is computed at each spatial location within the catalyst layer by separately solving the electronic and ionic electric potential equations. The 3D presentation of the current densities and the electronic and ionic potentials allows analysis of the respective ohmic drops. The simulation results show that the principal limitations are at the cathodic side. The limitations due to ionic ohmic drops, classically considered to be the main restrictions, are confirmed. The particular interest of our study is that it also shows that, because of the cylindrical geometry, there is a significant electronic ohmic drop
In recent years, fuel cell technology has attracted considerable attention from several fields of scientific research as fuel cells produce electric energy with high efficiency, emit little noise, and are non-polluting. Solid oxide fuel cells (SOFCs) are particularly important for stationary applications due to their high operating temperature (1,073-1,273 K). Methane appears to be a fuel of great interest for SOFC systems because it can be directly converted into hydrogen by direct internal reforming (DIR) within the SOFC anode. Unfortunately, internal steam reforming in SOFC leads to inhomogeneous temperature distributions which can result in mechanical failure of the cermet anode. Moreover this concept requires a large amount of steam in the fed gas. To avoid these problems, gradual internal reforming (GIR) can be used. GIR is based on local coupling between steam reforming and hydrogen oxidation. The steam required for the reforming reaction is obtained by the hydrogen oxidation. However, with GIR, Boudouard and cracking reactions can involve a risk of carbon formation. To cope with carbon formation a new cell configuration of SOFC electrolyte support was studied. This configuration combined a catalyst layer (0.1%Ir-CeO 2 ) with a classical anode, allowing GIR without coking. In order to optimise the process a SOFC model has been developed, using the CFD-Ace? software package, and including a thin electrolyte. The impact of a thin electrolyte on previous conclusions has been assessed. As predicted, electrochemical performances are higher and carbon formation is always avoided.However a sharp decrease in the electrochemical performances appears at high current densities due to steam clogging.Keywords SOFC Á Gradual internal reforming Á Simulation Á CFD-Ace? List of symbols D iDiffusion coefficient of the i-th species (m 2 s -1 ) D i,eff Effective diffusion coefficient of the i-th species (m 2 s -1 ) E 0 Voltage between the electrolyte and the nickel at equilibrium (V) F Faraday constant (96500) (C mol -1 ) J i Diffusion flux of the i-th species (mol m -2 s -1 ) K E1 Equilibrium constant of steam methane reforming reaction (Pa 2 ) K E2 Equilibrium constant of water gas shift reaction (-) K B Equilibrium constant of Boudouard reaction (Pa -1 ) K C Equilibrium constant of Cracking reaction (Pa) M Molecular weight of the mixture of gases (kg kmol -1 ) P i Partial pressure of the i-th species (Pa) R Universal gas constant (8.314) (J mol -1 K -1 ) (S/V) eff Effective surface-to-volume ratio (m 2 m -3 ) T Temperature (K) a c Carbon activity (-) d pore Pore diameter (M) H Gas mixture enthalpy (J kg -1 ) h B Solid-phase enthalpy (J kg -1 ) iCurrent density (A m -2 ) [i] Molar concentration of the i-th species (kmol m -3 )
International audienceSolid oxide fuel cells (SOFC) are energy conversion devices thatproduce electricity and heat directly from a fuel such asnatural gas. Little attention has been paid to the electricalbehaviour of a SOFC stack. This paper presents an electricalinteraction model of a planar anode supported intermediatetemperature SOFC stack with direct internal reforming. The SOFCstack model is built up of multiple single repeat units stackedon top of each other and takes into account, amongst otherparameters, contact resistances and gas flow. This assembly issandwiched between two end plates. A combined model with mass,charge and heat balances has been developed with a specialattention to the description of electrical behavior. Such anapproach can provide a picture of the two dimensionaldistribution of potential, current density, and temperature.Simulations are performed to analyse the impact of changes inmaterial conductivities, electrical configuration and operation conditions
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.