Open end protection for solid oxide fuel cells

Details Open end protection for solid oxide fuel cells Open end protection for solid oxide fuel cells

1999-09-10

2001-04-24

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S006000

Reexamination Certificate

active

06221522

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to increasing the life of tubular solid oxide fuel cells by increasing the resistance to reduction of the air electrode cathode at the open end of the cells, especially during pressurized operation.
BACKGROUND OF THE INVENTION
Operation of tubular solid oxide fuel cells in a fuel cell generator is well known in the art, and taught, for example, in U.S. Pat. Nos. 4,395,468, 4,664,986 AND 5,573,867 (Isenberg, Draper et al., and Zafred, et al., respectively). There, the generator is divided into an oxidant inlet chamber and a fuel inlet chamber, separated by a combustion zone or air pre-heating chamber. Tubular solid oxide electrolyte fuel cells extend from the combustion zone or air pre-heating chamber and through the fuel chamber. The term “tubular” as used here is defined as meaning circular, as well as flattened configurations containing a plurality of interior oxidant passages, as taught in U.S. Pat. No. 4,874,678 (Reichner). The tubular solid oxide fuel cells have a closed end in the fuel chamber and an open end within the combustion zone or air pre-heating chamber, where depleted oxidant passes out of the fuel cell open end to combust with depleted fuel, to pre-heat oxidant feed tubes and feed oxidant passing through those feed tubes. The fuel cells have an outer electrode contacting flowing fuel, the “fuel electrode” anode, and an inner support electrode contacting flowing oxidant, usually air, the “air electrode” cathode, separated by a solid oxide ceramic ionically conductive electrolyte, and operate from about 900° C. to 1300° C. The air electrode is usually made of a doped-LaMnO
3
.
Solid oxide fuel cell (SOFC) systems currently being developed for power generation applications, offer high efficiency, negligible stack pollution and ease of operation by utilization of many types of fuels. Several SOFC power generation systems capable of operating on gaseous and liquid hydrocarbon fuels have been fabricated and field tested to evaluate the performance and long term stability of cells and system components. The integration and operation of SOFC systems with gas turbines/generators in a pressurized mode offers the potential of more efficient operation. Operating the SOFC generator under pressurized conditions is beneficial in the reduction of cathode-side polarization losses, however cell operation under pressure may increase the risks involved in upset and transient conditions.
If balanced pressure is not maintained on both anode and cathode sides of the cell, gas flow will occur, between sides, releasing oxygen to the anode side or fuel gas to the cathode on the inside of the cell. The introduction of fuel gas to the cathode causes reduction of the doped-LaMnO
3
with a concurrent volume change that mechanically stresses the material which can lead to cracking and cell failure. Recognizing that a fuel cell generator can have hundreds of separate fuel cells, making up most of the generator, this can be a serious problem. The main means to control such cracking problem was to attempt maintenance of standard operating conditions, whereby fuel gas was isolated from the inside of the cell. This, however, does not solve problems during upset conditions when a sudden depressurization event occurs. Thus, there is a need for a permanent solution to fuel incursion into the open end of the fuel cell to degrade the interior air electrode. The main object of the invention is to protect the interior air electrode from fuel contact and degradation during operation and under upset conditions by providing a thermally shock resistant sleeve.
SUMMARY OF THE INVENTION
This object is accomplished by providing a solid oxide fuel cell having an exterior fuel electrode and an interior air electrode, with solid oxide electrolyte therebetween, the fuel cell having an open end and a closed end, where the open end contains a sleeve fitted over the solid oxide fuel cell at the open end, where the sleeve extends beyond the open end of the fuel cell and contains a metal oxide reactive with gaseous fuel. The sleeve preferably comprises or is coated with ZrO
2
, Al
2
O
3
, NiO, SiO
2
and their mixtures and extends beyond the open end of the fuel cell by at least one half of the outside diameter of the fuel cell, preferably, but no more than, three times the outside diameter of the fuel cell. A plurality of these sleeved fuel cells can be disposed in the generator chamber of a fuel cell and operated in an environment of gaseous oxidant, such as air, or gaseous fuel, such as natural gas.
The invention also resides in a high temperature, solid electrolyte fuel cell generator, comprising: (a) housing means defining a plurality of chambers including a fuel inlet chamber and an air pre-heating chamber; (b) a plurality of elongated fuel cells having an interior air electrode and an exterior fuel electrode, and an active length with a closed end disposed in the fuel inlet, and an open end disposed in the air pre-heating chamber, said open end subject to fuel gas entry during interrupted operation of the generator; (c) means for flowing a gaseous oxidant through the fuel cells and into the air pre-heating chamber; and (d) means for flowing a gaseous fuel about the fuel cells in the fuel inlet chamber; where the improvement comprises that the open end of each fuel cell contains a sleeve fitted over said open end, where the sleeve extends beyond said open end and contains a metal oxide reactive with said gaseous fuel and said metal oxide is effective to react with the gaseous fuel to prevent reduction of the interior air electrode.
The function of the sleeve is to increase the path length for diffusion and flow of the fuel gas into the inside of the cathode tube in the event of excess fuel leakage or rapid depressurization event. The sleeve ensures that any combustion of fuel and air that might occur typically at the end of the cell instead occurs at the end of the sleeve, which is more resistant to thermal shock and, even if it cracks, does not affect the cell. Also, by utilization of a metal oxide coating, the sleeve is capable of decreasing the amount of fuel gas that reaches the cathode through a cathalitic reduction of the metal oxide coating.


REFERENCES:
patent: 4395468 (1983-07-01), Isenberg
patent: 4664986 (1987-05-01), Draper et al.
patent: 4874678 (1989-10-01), Reichner
patent: 5244752 (1993-09-01), Zymboly
patent: 5527631 (1996-06-01), Singh et al.
patent: 5573867 (1996-11-01), Zafred et al.
patent: 5595833 (1997-01-01), Gardner et al.
patent: 5733675 (1998-03-01), Dederer et al.

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