Segregated exhaust fuel cell generator

Details Segregated exhaust fuel cell generator Segregated exhaust fuel cell generator

2000-08-10

2002-12-10

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000

Reexamination Certificate

active

06492048

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to high temperature fuel cell generators, wherein depleted fuel and depleted air are kept separate from each other to allow treatment of depleted fuel to generate and capture essentially pure carbon dioxide, thereby precluding the release of greenhouse gas to the environment.
2. Background Information
Tubular solid oxide electrolyte fuel cell (SOFC) generators have been well known in the art for almost twenty years, and taught, for example, by A. O. Isenberg in U.S. Pat. No. 4,395,468. There, in the main embodiment, oxygen (as present in air), as oxidant, was reacted at the inside “air” electrode of a closed tubular SOFC, to yield depleted air; and fuel, such as CO and H
2
, was reacted at an outside “fuel” electrode of the closed tubular SOFC to yield depleted fuel, all in a “generating chamber,” at high temperatures (that is, about 1000° C.). The air electrode generally comprised a doped lanthanum manganite, the fuel electrode generally comprised a nickel cermet and an electrolyte disposed between the electrodes generally comprised a stabilized zirconia. The depleted air and depleted fuel were subsequently completely combusted in a separate, but attached preheating chamber, to preheat feed air. This basic SOFC generator design was carried forward, with other improvements, as shown for example in U.S. Pat. Nos. 4,664,986; 5,573,867; and 5,733,675 (Draper et al.; Zafred et al.; and Dederer et al.).
Other designs have used a series of fuel cell stacks, each providing a stage containing a different electrolyte operating at a lower temperature to improve fuel gas utilization, as taught in U.S. Pat. No. 5,712,055 (Khandkar). In a somewhat similar fashion, in one embodiment of U.S. Pat. No. 5,134,043 (Nakagawa), “depleted fuel” from a molten carbonate fuel cell system is sent to a separate molten carbonate anode, where the product was then mixed/contacted with oxidant/air before being introduced into the cathode section of the first molten carbonate electrolyte fuel cell. While tubular fuel cells are emphasized herein, flat or planar fuel cells, which are well known in the art, may also be used.
However, such designs could release byproducts of combustion, such as carbon dioxide into the atmosphere. Efforts are now being made on an international level to globally reduce the release of so-called “green house gases” which includes carbon dioxide, which may contribute to global atmospheric warming. Such efforts may, indeed, lead to future legislation regarding carbon dioxide emissions from SOFCs. What is needed is a means to further treat the spent fuel from fuel cell generators to not only reduce or eliminate carbon dioxide emissions, but also to increase the capacity of the fuel cell generators to further utilize feed fuel, thereby producing more electricity. Such a need applies to both tubular and flat plate type fuel cells.
In the area of reducing carbon dioxide emissions from power plants utilizing a variety of types of fuel cells, in order to reduce the “green house effect”, U.S. Pat. No. 4,751,151 (Healy et al.) taught a carbon dioxide absorber, such as monoethanolamine, including a regenerable absorbent, for stripping carbon dioxide followed by subsequent cooling and compression. In U.S. Pat. No. 5,064,733 (Krist et al.), recognizing prior art conversion of natural gas into carbon dioxide and water-with the accompanying creation of a DC electrical current-in a solid oxide fuel cell, taught conversion of the carbon dioxide and water to C
2
H
4
, C
2
H
6
and C
2
H
2
by use of a copper, copper alloy or perovskite cathode. That cathode was in contact with the CO
2
, and H
2
O and a dual layered anode made of metallic oxide perovskite next to the electrode with an outer contacting layer of rare earth metallic oxide contacting CH
4
. This provided for concurrent gas phase electrocatalytic oxidative dimerization of methane at an anode on one side of a solid electrolyte and reduction of carbon dioxide to gaseous hydrocarbons at a cathode on the other side of the solid electrolyte.
Other CO
2
treatments include U.S. Pat. No. 5,928,806 (Olah et al.), where a regenerative fuel cell system containing two electrochemical cells in fluid communication were taught, one cell oxidizing an oxygenated hydrocarbon, such as methyl alcohol, formic acid, etc., to CO
2
and H
2
O and a second cell reducing CO
2
and H
2
O to an oxygenated hydrocarbon. This produced methyl alcohol and related oxygenates directly from CO
2
. Also, U.S. Pat. No. 5,866,090, (Nakagaua et al.) taught treating carbon dioxide effluent, from an energy plant which uses fuel cells, with lithium zirconia at over 450° C., to produce lithium carbonate and zirconia.
In the area of separation of gas streams in an apparatus, U.S. Pat. No. 4,801,369 (Draper et al.) taught a solid oxide water electrolyzer using solid oxide fuel cells having oxidant and fuel feeds, where water (in the form of steam) was dissociated to H
2
and O
2
. There, in order to prevent recombination of the H
2
and O
2
and to eliminate the possibility of fire or explosion, a controlled leakage of additional steam into the H
2
and O
2
streams, at a high pressure, was used as a separation “seal”. This separation means allowed a seal-less design, which was important at the 800° C.-1100° C. electrolyzer operating temperatures. In the invention of Draper et al., a separate steam plenum was used to separate the oxygen and hydrogen collecting means. with this design, however, it was difficult to achieve uniformity of steam leakage, so that there was the possibility of diffusion of O
2
and N
2
into the steam plenum and subsequent mixing of these gases with the fuel.
While a great many methods to treat carbon dioxide are known, a new fuel cell generator design is needed to allow segregation of the carbon dioxide for such treatment.
SUMMARY OF THE INVENTION
Therefore it is a main object of this invention to yield an improved fuel cell generator design, allowing segregation of carbon dioxide generated at the fuel electrodes.
It is a further object of this invention to segregate the depleted oxidant stream from the depleted fuel stream, so that the carbon dioxide can be segregated.
These and other objects are accomplished by providing a high temperature fuel cell generator comprising a generator chamber containing solid oxide electrolyte fuel cells which operate on oxidant and fuel to yield a depleted oxidant stream and a depleted fuel stream consisting essentially of unreacted fuel, CO
2
and H
2
O, where the oxidant and fuel streams do not communicate directly with each other, so that depleted oxidant and depleted fuel remain effectively separated, and where the depleted oxidant stream is separated from the depleted fuel stream by a flow of steam. The depleted fuel exits as a gas consisting essentially of carbon dioxide and water for further treatment, where at least one exit is provided for depleted oxidant to exhaust to the environment. In no instance will the depleted fuel stream contain only H
2
. The depleted fuel steam will always contain substantial amounts (greater than about 80 vol. %) of CO
2
and H
2
O. The steam separation is preferably effected by means of a separate barrier steam plenum, where the steam is at a pressure higher by approximately 1 psi (6.89×10
3
Pa or 6.89 kPa) than either the depleted oxidant stream or the depleted fuel stream, and there may be a controlled leakage of steam into those streams in a seal-less generator design. This higher pressure is achieved by placing a constricting means as a pressure control means in the exit of the barrier steam plenum. Such a constricting means could be selected from an orifice, venturi or the like.
The invention also covers a method of operating a high temperature fuel cell generator containing solid oxide fuel cells with a solid electrolyte disposed between an air electrode and a fuel electrode which operate on oxidant and fuel gases, comprising the steps: (1) feeding feed fuel gas to contact fuel electrode

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