Evaluation On The Feasibility Of Ethanol Steam Reforming In A Molten Carbonate Fuel Cell

S.Cavallaro **, E. Passalacqua*, G.Maggio *, A. Patti*, S.Freni* * Istituto CNR-TAE, via S.Lucia sopra Cont'esse 39, 98126 S. Lucia, Messina; Italy. ** Dipartimento di Chimica Industrials - Universita di Messina P.O. Box 29, 98166, Messina, Italy.

Introduction

The molten carbonate fuel cells (MCFCs) utilizing traditional fuels represent a suitable technological progress in comparison with pure hydrogen-fed MCFCs. The more investigated fuel for such an application is the methane, which has the advantages of low cost and large availability; besides, several authors demonstrated the feasibility of a methane based MCFC [ 1-2], In particular, the methane steam-reforming allows the conversion of the fuel in hydrogen also inside the cell (internal reforming configuration), utilizing the excess heat to compensate the reaction endothermicity. In this case, however, both the catalyst and the cell materials are subjected to thermal stresses due to the cold spots arising near to the reaction sites MCFC [3].

An alternative, in accordance with the recent proposals of other authors [4, may be to produce hydrogen from methane by the partial oxidation reaction, rather than by steam reforming. This reaction is exothermic (AH°=-19.1 kJ/mol H2) and it needs to verify the possibility to obtain an acceptable distribution of the temperature inside the cell. The alcohols and, in particular, methanol [5] shows the gas reformed compositions as a function of the steam/ethanol molar ratio, ranging from 1.0 to 3.5. The hydrogen production enhances with this ratio, but it presents a maximum at S/EtOH of about 2.0. Otherwise, the increase of S/EtOH depresses the production of CO and CH4, and ethanol [6] may be a further solution for the hydrogen production inside a MCFC. In this case, also, the reaction in cell is less endothermic compared with the methane steam reforming with the additional advantage of a liquid fuel more easily storable and transportable. Aim of the present work is to perform a comparative evaluation of the different solutions, with particular reference to the use of ethanol.

Thermodynamic approach

Figure 1 shows the values of the Gibbs free energy of formation of the molecules involved in the various processes as a function of the temperature.

As results from this figure, at the cell temperature (T=923 K) the more stable compounds are CO, C02 and H20; hence, equilibria will be shifted towards the formation of such products. Starting from ethanol or methanol, some methane could be produced. In any case, an appropriate flow of H20 must be established to prevent coke formation.

Whatever reaction is considered, the immediate use of hydrogen in cell, in accordance with the direct internal reforming (DIR-

Gibbs free energy (kJ/mol)

Gibbs free energy (kJ/mol)

0 200 400 600 800 1,000 1,200 1,400 Temperature (K)

FIg.1 : Gibbs free energy of different spedes vs. temperature.

0 200 400 600 800 1,000 1,200 1,400 Temperature (K)

FIg.1 : Gibbs free energy of different spedes vs. temperature.

MCFC) configuration, enhances the fuel conversion for all the equilibria.

The optimal cell operation depends on the amount of heat released or required by the considered reaction. In fact, since the electrochemical cell reaction is exothermic, it could be beneficial to directly utilise, the excess heat in the anodic compartment (DIR-MCFC), to supply the fuel reforming reaction.

The standard hentalpies per moles of H2 corresponding to the main steam reforming reactions, the partial oxidation and the shift reaction, which occurs contemporaneously on the same catalytic sites, are given as follows:

5. CjHjOH + 1/2 02 = 2 CO + 3 H2 AH°= 4.68 kj/mol H2

The reaction 1. is the most endothermic among, the considered reactions, it produces the colder spots on the reaction sites. The more exothermic, except the shift reaction 6. which takes place independently from the used fuel, is the methane partial oxidation reaction 2.. The ethanol partial oxidation is almost athermal, while, among the steam reforming reactions, that of methanol is surely the less endothermic.

At last, it is useful to consider also the moles of Hj produced from a mole of converted fuel, as shown in Tab.I. This parameter is very significative to evaluate the DIR-MCFC plant compactness and to optimize the fuel storage and transportation devices. In the light of such observations, it seemed interesting to focus our attention on the ethanol steam reforming.

* Table I: Hj/fuel molar ratio
0 0

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