Fuel cell system combustor

Details Fuel cell system combustor Fuel cell system combustor

1997-11-20

2001-05-15

Nuzzolillo, Maria (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S006000, C429S047000, C429S047000

Reexamination Certificate

active

06232005

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a fuel cell system having a catalytic combustor for heating a fuel reformer, and more particularly to a combustor having a turbulator at the input end thereof.
BACKGROUND OF THE INVENTION
H
2
—O
2
(air) fuel cells are well known in the art and have been proposed as a power source for many applications. There are several different types of H
2
—O
2
fuel cells including acid-type, alkaline-type, moltencarbonate-type and solid-oxide-type. So-called PEM (proton exchange membrane) fuel cells [a.k.a. SPE (solid polymer electrolyte) fuel cells] are of the acid-type, potentially have high power and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
In PEM fuel cells hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can either be in a pure form (i.e., O
2
), or air (i.e., O
2
admixed with N
2
). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin.
For vehicular applications, it is desirable to use a liquid fuel such as a low molecular weight alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the fuel for the vehicle owing to the ease of onboard storage of liquid fuels and the existence of a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to the reaction:
CH
3
OH+H
2
O→CO
2
+3H
2
The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur. Heating the reformer with heat generated externally from either a flame combustor or a catalytic combustor is known. The present invention relates to an improved catalytic combustor, and the integration thereof with a fuel cell system, wherein the combustor is fueled with hydrogen-containing anode effluent and oxygen-containing cathode effluent, and includes means at its input end to induce intimate mixing of the anode effluent with the oxygen-dilute cathode effluent to ensure efficient and uniform burning of the hydrogen on the catalyst bed without the creation of “hot spots” or significant temperature differences throughout the catalyst bed.
SUMMARY OF THE INVENTION
The present invention involves a fuel cell system having (a) a stack of H
2
—O
2
fuel cells discharging an H
2
-containing anode effluent and an O
2
-containing cathode effluent, (b) a fuel reformer for converting a hydrogen-containing fuel selected from the group consisting of alcohols and hydrocarbons to H
2
for fueling said cells, and (c) a combustor for heating said fuel reformer. The present invention contemplates an improved catalytic combustor which is fueled by the anode and cathode effluents and includes a turbulator section at its entrance for intimately mixing the anode and cathode effluents. More specifically, the combustor comprises a housing having (1) an input chamber that receives and initially roughly combines the anode and cathode effluents together into a burnable mixture, (2) an exhaust outlet emitting hot combustor exhaust gas to the reformer, (3) a catalyst bed intermediate the input chamber and exhaust outlet for burning the mixture to generate the hot combustor exhaust gas, and (4) a turbulator intermediate the input chamber and the catalyst bed for inducing turbulent mixing of the mixture before it contacts the catalyst bed. The turbulator comprises at least one porous bed having a leading face admitting the mixture into the porous bed from the input chamber, and a trailing face through which the mixture exits the porous bed. A mixing zone intermediate the leading and trailing faces intimately mixes the effluents in the mixture to provide a homogeneous mixture for even burning throughout the catalyst bed. The mixing zone comprises a porous material that defines a multiplicity of tortuously pathed channels through which the reaction mixture passes. Preferably, the turbulator has at least two porous beds arranged in series (i.e., in the direction of flow) between the input chamber and the catalyst bed. Most preferably, the porous beds will have different porosity profiles, and be separated one from the next by an open space which serves as a mixing confluence for the several streams exiting the many channels through the first mixing media bed. The first porous bed in the direction of flow will preferably have a finer (i.e., smaller pores) porosity profile than the second porous bed.
The porous beds may comprise a variety of corrosion and heat resistant materials, such as ceramics or refractory metals, and take many different forms so long as they provide a multiplicity of tortuously pathed flow channels therethrough. For example in one embodiment, the porous beds may comprise a stack of fine screens wherein the openings in one screen are offset from openings in adjacent screens to provide the desired tortuous path through the porous bed. Open cell metal foams may also be used. In preferred embodiment, the porous bed comprises a ceramic foam. Most preferably, the ceramic foam has a porosity profile of about 25 pores per lineal inch to about 80 pores per lineal inch. Silicon carbide and yttria-zirconia-aluminum (Y
2
O
3
/ZrO
2
/Al
2
O
3
) have proven to be effective ceramics for applications that see temperatures as high as 700° C. Potentially alternative materials include alumina (AL
2
O
3
), zirconia-alumina (ZrO
2
/AL
2
O
3
), partially stabilized zirconia (ZrO
2
/CaO/MgO), partially stabilized zirconia-magnesia (ZrO
2
/MgO), partially stabilized zirconia-yttria (ZrO
2
/Y
2
O
3
), inter alia depending on the temperature and strength requirements of a particular combustor.


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Szaniszlo, “The Advanced Low-Emissions Catalytic-Combustor Program: Phase I —Description and Status,” ASME #79-GT-192 Mar. 1979.
Krill et al, “Catalytic Combustion for System Applications,” ASME #79-HT-54 Dec. 1979.
Hall et al, “A Porous Media Burner for Reforming Methanol for Fuel Cell Powered Electric Vehicles,” SAE Paper #950095 Feb. 1995.
Natural Gas Power Plant System (a descriptive drawing). No month/year.

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