Numerical Modeling of Single-Chamber SOFCs with Hydrocarbon Fuels

Abstract

A two-dimensional numerical model of a single-chamber solid oxide fuel cell ͑SCFC͒ operating on hydrocarbon fuels is developed. The SCFC concept is a simplification of a conventional solid oxide fuel cell in which the anode and cathode are both exposed to the same premixed fuel-air mixture, and selective catalysts promote electrochemical oxidation of the fuel at the anode and simultaneous electrochemical oxygen reduction at the cathode. Optimization of SCFC stacks requires considering complex, coupled chemical and transport processes. The model accounts for the coupled effects of gas channel fluid flow, heat transfer, porous media transport, catalytic reforming-shifting chemistry, electrochemistry, and mixed ionic-electronic conductivity. It solves for the velocity, temperature, and species distributions in the gas, profiles of gaseous species and coverages of surface species within the porous electrodes, and the current density profile in an SCFC stack for a specified electrical bias. The model is general, and can be used to simulate any electrode processes for which kinetics are known or may be estimated. A detailed elementary mechanism is used to describe the reactions over the anode catalyst surface. Different design alternatives including yttria-stabilized zirconia vs Ce 0.8 Sm 0.2 O 1.9 electrolytes, the effects of mixed conductivity, and the optimal fuel-to-air ratio are explored. A single-chamber fuel cell ͑SCFC͒ is one in which the anode and cathode are both exposed to the same premixed fuel-air stream, and selective electrocatalysts are used to preferentially oxidize the fuel at the anode and reduce oxygen at the cathode. The latest studies of SCFCs demonstrate great improvements in power density and reduction in operating temperature with innovations in material and system design.1-4 While the efficiency is typically lower than that of conventional dual-chamber solid oxide fuel cells ͑SOFCs͒, SCFCs do not require seals, and allow a very simple gas manifold design. For some applications, notably small-scale power generation, the mechanical simplicity of an SCFC design may make it more attractive than a dual-chamber SCFC, even with a lower efficiency.The need for selective electrocatalysts has several implications for the design of an SCFC. First, an SCFC must operate at a temperature low enough that the catalysts maintain some degree of selectivity; this typically limits the temperature to below 700°C, which is significantly lower than that of conventional SCFCs with an yttria-stabilized zirconia ͑YSZ͒ electrolyte. For this reason, SCFCs demonstrated to date have used ceria-based electrolytes, rather than YSZ.Another implication of the need for selective electrocatalysts is that an SCFC is unlikely to run well, if at all, on hydrogen. Any catalyst that promotes electrochemical oxidation of hydrogen, or electrochemical reduction of oxygen, would very likely promote direct catalytic combustion if exposed to a hydrogen-air mixture. This problem can be dealt with by using a hydrocarbon fuel instead of hydr...

Single-Chamber Solid Oxide Fuel Cell Technology—From Its Origins to Today’s State of the Art

“…The simulated cell performance revealed an increase in power density with temperature and the occurrence of concentration losses at high current densities. At 750 °C, the highest power density was obtained for R mix = 1.65 [161] and 1.67 [216]. For lower R mix , full fuel oxidation reduced the amount of available hydrogen whereas at higher R mix the oxygen partial pressure at the cathode was very low.…”

“…Hao et al [161,[214][215][216] proposed a two-dimensional numerical model to simulate the performance of planar anode-supported SC-SOFCs for various geometries, operating conditions and hydrocarbon fuels. The model was composed of several submodels that considered the coupled effects of gas flow (velocity, temperature and gas species distribution) around the cell, heat transfer, reactions and species transport in the porous electrodes, heterogeneous chemistry (partial oxidation and steam reforming reactions), electrochemical reactions at the electrode-electrolyte interface and mixed conductivity of the electrolyte.…”

Numerical Study of Heterogeneous Reactions in an SOFC Anode With Oxygen Addition

“…7. By fitting a small number of parameters, good agreement was achieved between the model's prediction and other independent experimental results.…”

“…However, we have to point out that among the basic assumptions of the model, 7 treating the electrochemistry as a boundary condition for the electrode is a great simplification, for it is well known that the electrochemically active region extends at least 10 m into the porous electrode. 18 For thin electrodes ͑e.g., a few tens of micrometers͒, this is going to be an oversimplification, because electrochemistry over the triple-phase boundary will affect the surface chemistry over a significant portion of the electrode thickness.…”

“…The anode-supported structure is chosen to keep with our previous work, 7 in addition to its advantages mentioned above. Although the oxidation of the fuel in general could occur both in the gas phase ͑i.e., homogeneous͒ and between the gas phase and the catalyst surface ͑i.e., heterogeneous͒, 2 for methane the homogeneous reaction does not play a substantial role for SOFCs until 900°C, 8 which is higher than the normal operating temperature of most SOFCs.…”