Fuel dissipater for pressurized fuel cell generators

Details Fuel dissipater for pressurized fuel cell generators Fuel dissipater for pressurized fuel cell generators

2001-02-15

2003-11-04

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S010000, C429S006000, C429S006000, C429S006000, C320S101000

Reexamination Certificate

active

06641946

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to pressurized fuel cell generators, and more particularly relates to an energy dissipater which reduces unwanted heat build-up in the combustion zone of the generator during shut-down of the generator.
2. Background Information
Conventional solid oxide electrolyte fuel cell (SOFC) generators typically include tubular fuel cells arranged in a grouping of rectangular arrays. Each fuel cell has an upper open end and a lower closed end, with its open end extending into a combustion zone. A typical tubular fuel cell has a cylindrical inner air electrode, a layer of electrolyte material covering most of the outer surface of the inner air electrode, and a cylindrical fuel electrode covering most of the outer surface of the electrolyte material. An interconnect material extending along the length of the fuel cell covers the circumferential segment of the outer surface of the air electrode which is not covered by the electrolyte material. An electrically conductive strip covers the outer surface of the interconnect material, and allows electrical connections to be made to an adjacent fuel cell or bus bar. The air electrode may comprise a porous lanthanum-containing material such as lanthanum manganite, while the fuel electrode may comprise a porous nickel-zirconia cermet. The electrolyte, which is positioned between the air and fuel electrodes, typically comprises yttria stabilized zirconia. The interconnect material may comprise lanthanum chromite, while the conductive strip may comprise nickel-zirconia cermet. Examples of such SOFCs are disclosed in U.S. Pat. No. 4,395,468 (Isenberg), U.S. Pat. No. 4,431,715 (Isenberg) and U.S. Pat. No. 4,490,444 (Isenberg). More advanced pressurized SOFC generators are disclosed in U.S. Pat. No. 5,573,867 (Zafred et al.).
During operation of the fuel cell generator, air is provided to an inside air electrode of each tubular cell, and hydrogen-rich fuel is supplied to an outside fuel electrode surface. The fuel and oxidant are utilized electrochemically to produce electrical energy. The depleted air, comprising about 16 percent oxygen, exits the open end of the cell, and the spent fuel of low hydrogen concentration is eventually discharged into a combustion area surrounding the cell open ends.
During normal run conditions, the fuel gas entering the SOFC combustion zone has a low concentration of hydrogen due to the fuel being consumed within the cell stack. In addition, a relatively large amount of oxygen depleted air exits the cells, keeping the air/fuel ratio well beyond stoichiometric in the combustion plenum. This helps to keep the combustion zone temperature at approximately 950° C., well within the allowable range for the cells. In addition, the high volumetric flow of air out of each cell may be sufficient to protect the air electrode and open end from any risk of hydrogen reduction.
However, during certain generator stop conditions with the stack in an open circuit condition, that is, loss of grid connection, the air supply may be reduced to a maximum of about 10 percent or less of the normal airflow. The fuel flow to the generator is replaced with a reducing purge flow which serves to protect the fuel electrode from oxidation. This purge flow may cause any stored fuel within the generator to be pushed into the combustion zone where it burns with the available air. There are two primary concerns with this situation. First, the air/fuel ratio is closer to stoichiometric and will result in more combustion and a hotter combustion zone temperature. Second, the reduced air flow leaving each cell may not be sufficient to completely protect the open ends of the cells from hydrogen reduction. Either of these problems have the potential for causing damage to the fuel cells.
Several alternatives have been proposed in the past in an attempt to lessen the severity of this condition. The auxiliary airflow could be increased, thereby reducing the combustion zone temperature, as well as providing added protection for the open ends. This would require larger, more expensive blowers, as well as an uninterruptable power supply large enough to handle their power requirements. The cell open ends presently extend a short distance beyond the upper open end support board, which forms the floor of the combustion zone. Extending the open ends further may move the ends away from the board and reduce the risk of hydrogen reduction, provided that the low airflow still provides air to the board surface so that combustion occurs there and not at the open cell end. However, this approach has the drawback of exposing more of the cell surface area to the flame temperature. Conversely, reducing the cell extension will protect more of the cell surface from the flame, but possibly expose the open ends to more unburned hydrogen. Yet another solution may be to coat the open ends with a material that will prevent reduction of the exposed air electrode.
U.S. Pat. No. 5,023,150 (Takabayashi) taught a fuel cell power generator wherein a resistor is connected by a switching circuit across positive and negative terminals when the generator is shut down. Takabayashi involves clamping a fixed load across the generator terminals. The size of the load is not changed. The load is switched on or off based on the stack voltage. If this is done very rapidly, it has the appearance of controlling the current by changing the effective resistance of the load, without actually changing that resistance. Nonetheless, the actual load resistance remains the same. This type of control is often called time proportioning, because a fixed load is connected across the supply for a portion of the cycle, and disconnected for its balance. Since the Takabayashi invention uses semiconductor switches, it becomes expensive, or unfeasible, when the current is high.
In U.S. Pat. No. 6,025,083, Veyo et al. attempted to solve the above-described problems for non-pressurized SOFC generators by utilizing a fuel dissipater concept, consisting of a fixed resistive load that is switched across the cell stack terminals upon transition to normal or emergency shutdown. The load draws current, which electrochemically consumes the fuel flushed by a nitrogen/hydrogen purge gas mixture used in such situations, thus reducing the combustion zone temperatures and protecting the cells. As the fuel inventory is depleted by the load, the stack voltage drops in response to reduced H
2
and CO concentrations and, at some point, a minimum allowable terminal voltage, is reached. The limiting voltage is equal to the nickel oxidation potential at the operating temperature, plus some margin. When this is reached, a voltage sensing circuit disconnects the load by actuating a shunt trip breaker. The voltage sensing and switching circuit can be powered by the stack voltage, making the fuel dissipater “passive” (self-contained). Other dissipater designs may incorporate sensing circuits which are powered by external sources.
The previously described Veyo et al. fuel dissipater design involved a constant resistance value with only two switching functions: on and off. That design consisted of a resistive load bank (in practice, two electric immersion heaters connected in parallel) and a voltage sensing and switching circuit. The heaters were mounted in the steam supply system water tank and were expected to draw about 7 amps/cell. The voltage sensor was an alarm module which actuated a shunt trip breaker when the nickel oxidation voltage (0.62 V nominal) plus a margin (0.05 V) was reached. The electronics were powered by the stack cell terminal voltage using a voltage divider circuit. The expected duration of the dissipation current was about two minutes, until the load was disconnected by the sensed low stack voltage. This worked well for atmospheric pressure SOFC generators, but many recent designs for SOFC generators including hybrid soft/micro-turbine generators, require high pressure operation for greater efficiency. In Veyo, et al., the size of the load was c

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