Performance simulation of planar SOFC using mixed hydrogen and carbon monoxide gases as fuel

doi:10.1016/j.enconman.2005.10.014

Energy Conversion and Management

Volume 47, Issues 13–14, August 2006, Pages 1738-1747

https://doi.org/10.1016/j.enconman.2005.10.014 Get rights and content

Abstract

The authors investigate in detail the influence of the mixing ratio of hydrogen and carbon monoxide in the fuel on the cell performance of the SOFC through numerical simulations for a single cell plate of the co-flow type planar cell. It is made clear that the cell performance is almost the same and excellent, independent of the mixing ratio of hydrogen and carbon monoxide under the nominal operating condition. The electromotive force of the hydrogen rich fuel gas is a little higher than that of the carbon monoxide rich fuel gas. The internal voltage drop in the cell decreases as the fraction of carbon monoxide becomes high. Since the value of the single cell voltage is determined by the balance of these two phenomena, the lowering of the electromotive force is dominant and the single cell voltage of the hydrogen rich fuel gas is higher when the inlet gas temperature is high, whereas the voltage drop reduction is dominant and the single cell voltage of the carbon monoxide rich fuel gas is higher when the temperature is low. The effect of the additional gases of water vapor and carbon dioxide is restricted to the single cell voltage shift, and the qualitative dependence of the single cell voltage on the inlet gas temperature is determined by the mixing ratio of hydrogen and carbon monoxide.

Introduction

The solid oxide fuel cell (SOFC) [1] is one of the high temperature type fuel cells, and it uses oxygen ion conducting ceramic materials, such as yttria stabilized zirconia (YSZ), as the electrolyte. Since its operating temperature is very high, as 1200–1300 K, it has an advantage that a very high overall efficiency of over 60% can be achieved by combining it with some suitable bottoming cycle such as the gas turbine. Its research and development, therefore, have been actively promoted for middle to large scale electric power sources. In addition, it is now also expected to be the power source for small distributed co-generation systems because the overall efficiency of SOFC co-generation systems is expected to be higher than that of proton exchange fuel cell (PEFC) systems.

One of the distinctive features of the SOFC is that it accepts any fuel in which hydrogen and carbon monoxide are mixed in an arbitrary ratio because both of these gases react electrochemically in the cell [1]. The SOFC, therefore, has an advantage in the variety of fuels, and even reformed gases of higher hydrocarbons and gasified coal, which contain large amounts of carbon monoxide and are not suitable for the fuel of the PEFC, can be effectively utilized in it.

Considering this fact, many experimental studies on the cell performance of the SOFC using fuels other than pure hydrogen and natural gas (methane) reformed gas, such as coal gasified gas, mixed hydrogen and carbon monoxide gases, etc., have been performed [2], [3], [4]. The results of these experiments indicate that the cell performance for such fuels is not very inferior and is almost equal to that in the case of pure hydrogen or methane reformed gas. An analytical approach to investigate in detail the influence of the mixing ratio of hydrogen and carbon monoxide in the fuel on the cell performance of the SOFC, however, has never been performed. In this paper, therefore, the authors perform this investigation through numerical simulations for a single cell plate of a co-flow type planar SOFC.

Section snippets

Simulation method

In this study, the numerical simulation code of the thermal and electrical characteristics of a single cell plate of a planar SOFC recently developed by the authors [5] is used to calculate the cell performance. The outline of the code is briefly explained in this section.

Numerical conditions

The length along the x-direction, the width along the y-direction and the height along the z-direction of the analyzed single cell plate are selected as 200, 200 and 3.2 mm, respectively. The thicknesses of its electrolyte, fuel electrode and air electrode are 0.1, 0.05 and 0.05 mm, respectively, and the width, height and pitch of its fuel and air channels are 2, 1 and 4 mm, respectively. As the cell materials, YSZ, Ni/YSZ, La_0.8Sr_0.2MnO₃ and LaCr_0.9Mg_0.1O₃ are chosen for the electrolyte, fuel

Cell performance under nominal operating condition

At first, numerical simulations are performed at the nominal operating condition. Concerning the fuels, the authors deal with five different kinds of gases that consist of 4:0, 3:1, 2:2, 1:3 and 0:4 mixtures of hydrogen and carbon monoxide in mole ratio. To regulate the fuel gas composition, only water vapor is added to the above five kinds of fuels, and the mixing ratio of the fuel and water vapor is fixed as 4:1 on the mole base. The most popular fuel gas of completely steam reformed natural

Conclusions

In this paper, the authors investigate in detail the influence of the mixing ratio of hydrogen and carbon monoxide in the fuel on the cell performance of the SOFC through numerical simulations for a single cell plate of a co-flow type planar cell.

The temperature rise in the cell increases as the fraction of carbon monoxide in the fuel gas becomes high. The influence of this temperature difference on the cell performance is slight, and it is almost the same and excellent, independent of the

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