40

* cc/cm2-min-mmH20

* cc/cm2-min-mmH20

To investigate the reliability of these membranes, some of the most promising A1203 and SiC samples were tested in simulated molten carbonate fuel cell conditions. During the tests the gas pressure drop between the two faces of the membranes, which was continuously checked, did not present any variation, demonstrating that the structure did not change very much.

After the test, the porosimetric analyses, carried out on the samples, indicated that the porosity of the structure decreased from 43 to 40%, also the averaged pore radius (from 2.19 pm to 2.02 pm) and the total pore volume (from 367 mm3/g to 349 mm3/g) do not change very much from their initial values. At meantime, the curves of pores distribution of the 3E-5pm samples before and after the test, reported in Fig. 3, show a similar trend without significant differences.

Finally, the post mortem analyses indicated that these membranes have some capability to hinder alkali vapour diffusion; in fact, from spectrophotometry analysis has been seen that there was no potassium detectable beyond the porous membranes, while it has been found on the membranes in amount ranging 0.06 to 1.2 mg/g of membrane.

Total Pore Volume (mm^/g)

Total Pore Volume (mmyg)

Average Pore Radius (Micron) Average Pore Radius (Micron)

Fig.2 - Porosimetric curves for two SiC samples by HR and TC technique.

Fig.3 - Porosimetric curves of the 3E-5p.m sample before and after the test.

Conclusions

The structural characteristics of the membranes depend on several factors. The use of different binders does not change the porosimetric characteristics if the content of the binder is maintained constant, while the particles size of the starting material influences the total pores volume of the membranes. Some samples (AI2O3 and SiC), tested under simulated molten carbonate fuel cell conditions, showed no significant variations in the porous structure characteristics.

This experimental work demonstrates the capability of porous ceramic membranes to control the electrolyte vapour diffusion. However, a further aspect to be investigated in future is the effectiveness of the porous membranes in the cell and the influence of this component on the performance and on the design of the fuel cell system.

References

1. K. Sato, T. Tanaka and T. Murahashi, Proc. 1990 Fuel Cell Seminar, 40-43, Phoenix, Arizona, Nov. 25-28,1990.

2. S.S. Penner, J.R. Selman, Assessment of Research Needs for Advanced Fuel Cells, DOE/ER 30060-T1, 1985.

3. S. Cavallaro, S. Freni, R. Cannistraci, M. Aquino, N. Giordano, Int J. Hydrogen Energy, 17, 3, 181-186, 1992.

4. R.J. Selman, Molten Carbonate Fuel Cells (MCFC's), DOE/ER 30060-T1, 11(1-2), 153, 1990.

5. N.Q. Minh, Prpc. 1987 Electrochem. Soc. Conf., vol.87/2, 2169, 1987.

6. L.G. Marianowski and G. Vogel, EP 0257398 A2,1987.

7. H.P. Hsieh, Menbrane reactor technology, AIChE Symposium series, 85, 268, 53-67, (1989).

PERFORMANCE OF AN INTERNAL REFORMING MOLTEN CARBONATE FUEL CELL SUPPLIED WITH ETHANOL/WATER MIXTURE

S. Freni, G. Maggio. F. Barone, E. Passalacqua. A. Patti, M. Minutoli

Istituto CNR-TA E, via Salita S. Lucia sopra Confesse 39, 98126 Santa Lucia, Messina, Italy tel. 001-90-624230 ; fax 001-90.624247

Introduction

The state of art on the field of molten carbonate fuel cell (MCFC) systems covers many technological aspects related to the use of these systems for the production of electricity 11-2], In this respect, extensive research efforts have been made to develop a technology using the methane [3-4] based on the steam reforming process, and different configurations have been analysed and their performance determined for several operative cell conditions [5].

However, the operative temperature (T-923 K) of the MCFC. that allows the direct conversion of hydrocarbons or alcohols into H2 and CO, promotes researches in the field of alternative fuels, more easily transported and reformed compared to methane [6J.

In this paper are described the most indicative results obtained by a study that considers the use of water/ethanol mixture as an attractive alternative to the methane for a molten carbonate fuel cell.

Thermodynamics

The thermodynamic balance of the ethanol reforming reaction is represented by the following reactions:

where the last one represents the reaction of steam reforming of methane that will take place simultaneously to these reactions.

The reactions (l)-(3) can be expressed mathematically by equations correlating the equilibrium constants to the molar fractions of the gaseous components and to the total pressure.

Moreover, the equilibrium constants are related to the free energies of each molecule involved in the equilibrium reactions and can be determined, at a fixed temperature, as a function of the inlet moles of ethanol, moles of hydrogen electrochemically converted by the cell, inlet moles of steam. Because of the complexity of the analytical solution a multidimensional globally convergent method [7J for non-linear system of equations has been used to calculate the outlet gas composition.

The equilibrium potential, V, has been determined by the Nemst equation, while the potentials of the cell, operating with production of electricity, have been determined taking in consideration the losses due to the electrodes overpotentials and to the cell resistance.

Then, both electrical and thermal power contributions can be determined as well as the efficiencies of the system The energy balances and the energy efficiencies for these systems, at steady-state conditions, are represented by the following equations:

m{hr m(hr where m is the mass flow, h the enthalpy, IV the electrical power released by the cell and O the thermal power and the subscripts f, o, u, w, p refer to the fuel, oxidant, useful, waste and produced, respectively.

Results and discussion

This analysis starts with the determination of the polarization curve of the molten carbonate fuel cell fed by a mixture of water/ethanol, calculated at the conditions reported in Tab. I. The obtained curve, shown in Fig.l, evidences as the open circuit voltage (OCV) resulted of 1099.9 mV, while the cell limiting current results approximately equal to 280 mA/cm2.

As evident in Fig. 1, the performance of an ethanol direct internal reforming MCFC lias been compared with that of a methane direct internal reforming MCFC, operating under similar conditions. A methane inlet flow equal to 142.5 L/h has been considered, in order to have the same fuel utilization, at any fixed current density, to perform a rational comparison.

The analysis of these data indicates that the ethanol internal reforming configuration gives higher OCV (1099.9 mV vs. 1074 mV) and cell potential (578.0 mV vs. 526.9 mV at 190 mA/cm2). Besides, the clectrical efficiency result higher in the ethanol cell and is equal to 34.2% and 31.9% at 190 mA/cm2.

Tab.I: operational conditions for the base

case

Parameter

Units

0 0

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