SOFC fuelled by methane without coking: optimization of electrochemical performance

Abstract

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 )

SOFC Long Term Operation in Pure Methane by Gradual Internal Reforming

“…Below 1/3, the system will not release enough steam to ensure itself complete conversion, and methane will diffuse into the cermet yielding fast coking. More details concerning this concept will be found in our previous work (7)(8)(9)(10)(11)(12). This simplified principle mechanism was applied here study and the faradaïc efficiency was first optimized in H 2 above 1/3 by adjusting the experimental parameters (fuel concentration and flow rates).…”

“…This optimal thickness was evaluated from simulation studies (8)(9). The cell, 19 mm in diameter with an electrochemically active surface given by the cathode surface of 0.79 cm 2 , was inserted in a 3 atmospheres setup using an alumina ring.…”

Effect of Catalyst Layer and Fuel Utilization on the Durability of Direct Methane SOFC

“…The catalytic conversion of methane into hydrogen by steam reforming over Ir-CGO catalysts was previously investigated (18). Gradual internal reforming, was already demonstrated in electrolyte supported cells using pure and dry methane (7,8,(11)(12)(13)18) and ethanol (14)(15)(16) as the fuel. The catalytic aspects towards steam reforming were also intensively described (7,8,20).…”

“…The anode and collectors were covered by a 0.1 wt% Ir-CGO catalyst layer, 200µm thick deposited by 3D controlled spray coating. This optimal thickness allowing stable operation without carbon formation was obtained from simulation studies (11,12). The cells, 19 mm in diameter with an electrochemically active surface given by a cathode surface of 0.79 cm 2 , were inserted in a 3 atmospheres setup using an alumina ring.…”

Catalysts Based on Strontium Titanate Doped with Ni/Co/Cu for Dry Reforming of Methane

“…The replacement of nickel by other non-noble elements creates an opportunity to obtain a relatively cheap and highly active catalyst for the DRM process. Recent attempts to obtain anodic material for biogas-fuelled SOFC devices involve the modification of common Ni/YSZ cermet by infiltration of gadolinium doped ceria [ 7 ] copper and cobalt co-doped ceria [ 8 ] or even iridium doped ceria [ 9 ]. Exploiting the potential of doped perovskites, due to their chemical stability and good electrochemical properties, is another direction in the search for alternative materials for SOFC anodes.…”