Patent Application
20060035122
The invention relates generally to fuel cell systems and more particularly relates to a system and method for cooling oxygen sensitive components in a fuel cell stack so as to protect anode stack material or other heated components of a combined fuel processor and stack assembly during cool down.
Referring to
As with fuel cells generally, very hot solid oxide fuel cells having high electrical conductivity, are used to convert chemical potential energy in reactant gases into electrical energy. In the solid oxide fuel cell 18, two porous electrodes, anode 20, typically nickel based, and cathode 22, are bonded to an oxide ceramic electrolyte 24 (typically, yttria stabilized zirconia, ZrO2—Y2O3) disposed between them to form a selectively ionic permeable barrier. Molecular reactants cannot pass through the barrier 24, but oxygen ions (O2-) diffuse through the solid oxide lattice as indicated by arrow 26. The electrodes 20, 22 are typically formed of electrically conductive metallic or semi conducting ceramic powders, plates or sheets that are porous to fuel and oxygen molecules. Manifolds (not shown) are employed to supply the fuel stream 14 (typically hydrogen, carbon monoxide, nitrogen, or simple hydrocarbon) to the anode 20 and oxygen-containing gas, typically air, 28 to the cathode. The system 10 utilizes an oxygen source such as air compressor 30 to supply the oxygen containing gas 28. The fuel 14 at the anode catalyst/electrolyte interface 32 forms cations that react with oxygen ions diffusing through the solid oxide electrolyte 24 to the anode 20. The oxygen-containing gas 28 (typically air) supplied to the cathode layer 22 converts oxygen molecules into oxygen ions at the cathode/electrolyte interface 34. The oxygen ions formed at the cathode 22 diffuse, combining with the cations to generate a usable electric current 35 and an oxygen depleted air stream 36 (oxygen depleted air stream comprising, for example, about 10% to about 15% oxygen) that is removed from the cell 18 such as through a device 38, which may comprise a combustor or waste heat recovery device 38, or a combination thereof, and exits the system 10 as effluent stream 40. Anode waste stream 44 may be directed through fuel reformer 12 for exhaust gas recycle or to combustor 38 for waste heat recovery.
In transportation applications, solid oxide fuel cell power generation systems are expected to provide a higher level of efficiency than conventional power generators which employ heat engines such as gas turbines and diesel engines that are subject to Carnot cycle efficiency limits. Therefore, use of SOFC systems as power generators in vehicle applications is expected to contribute to efficient utilization of resources and to a relative decrease in the level of CO2 emissions and an extremely low level of NOx emissions. However, SOFC systems suitable for use in transportation applications require a very compact size as well as efficient thermal management. Thermal management must be accomplished whereby the outer surface of the fuel cell envelope is typically maintained below 45° C. while the temperature inside the stack is about 700° C. to about 950° C. Importantly, normal, hot operation and stack management during cool down and shut down must be accomplished without adverse effect on stack and system components.
The reactions in the fuel cells 18 generate heat. To maintain the fuel cell stack 16 at a desired operating temperature, a cooling system is used to remove the generated heat from the fuel cell. SOFC fuel stacks typically operate in the relatively high temperature range of about 700° C. to about 950° C. Reactant gases are pre-heated, typically by heat exchangers, to prevent the gases from cooling the stack below the optimum operating temperature. During operation, the SOFC stack anode and cathode are isostatic, the stack anode 20 residing in a reducing, oxygen free atmosphere.
During normal hot operation, the cathode 22 is open to the atmosphere via exhaust tubing (not shown). The anode 20 may also be open to the atmosphere via exhaust tubing. Turning to
Nickel based anodes of solid oxide fuel cells are subject to re-oxidation if they are exposed to air at temperatures above about 400° C. to about 500° C. This re-oxidation of the nickel anodes causes degradation of the fuel cell performance.
It is desirable, therefore, to prevent the anode 20 from being exposed to air until the SOFC temperature is less than about 500° C. to prevent oxidative damage to the anode 20. However, if there is no gas flow into the anode 20, the pressure difference builds between the anode 20 and cathode 22, causing damage to the cells 18 via cracking of the electrolyte 24 leading to degradation of the fuel cell.
U.S. Published Patent Application 20030012986 discloses a system for simplifying cooling of a fuel cell system which may be a single cell (1), a stack (15) or a similar configuration and which comprises at least one active membrane (2) sandwiched between an anode layer (4) and a cathode layer (3) and comprising a catalyst, a fuel supply having access to the anode layer and an air supply (17, 18) having access to the cathode layer, while at the same time keeping the effectiveness of the system with reference to energy conversion, volume and weight favorable, the fuel cell system is to be operated such that the air which is supplied by the air supply, is introduced by pressure into the fuel cell system, passes along the cathode layer and then leaves the fuel cell system, is used for both oxidant and coolant. For this purpose, the air is introduced into the fuel cell system (1, 15) with a rate resulting in a stoichiometric rate in the range between 25 and 140.
U.S. Published Patent Application 20020142201 discloses a fuel cell system including a first reactant intake manifold, a first reactant output manifold, a second reactant intake manifold, a second reactant output manifold, a cooling gas intake manifold, a cooling gas output manifold, a liquid intake manifold, fuel cells and cooling elements distributed among the fuel cells. Each cooling element defines a coolant passage. During operation, a cooling gas flows from the cooling gas intake manifold into the cooling gas output manifold through the coolant passage. Each cooling element also includes a water injection path. During operation water from the liquid intake manifold is injected into the coolant passage to mix with the cooling gas passing there through.
U.S. Published Patent Application 20020098394 discloses a process and system for providing a hydrogen-containing gas stream to a fuel cell anode that includes providing a hydrogen-containing gas stream that includes carbon monoxide, introducing the hydrogen-containing gas stream into a pressure swing adsorption module that includes at least one carbon monoxide-selective adsorbent to produce a purified hydrogen-containing gas stream, and introducing the purified hydrogen-containing gas stream to the fuel cell anode. The pressure swing adsorption module can also include a second adsorbent and/or catalyst. Also disclosed is a fuel cell system coupled to an internal combustion engine and a fuel cell system that utilizes fuel cell waste heat for vaporizing a hydrocarbon/water mixture.
U.S. Pat. No. 6,106,963 discloses a fuel cell system 20 equipped with an oxygen enrichment unit 34 and supplies air whose oxygen particle pressure has been increased by the oxygen enrichment unit 34 to fuel cells 40 as oxidizing gas. The oxygen enrichment unit 34 is a magnetic oxygen enrichment device that effects oxygen enrichment utilizing the fact that the oxygen molecule is paramagnetic and when magnetized migrates toward a magnet tic pole side.
U.S. Pat. No. 4,407,904 discloses a fuel cell comprising unit cells stacked through separators there between; each unit cell comprising a fuel electrode plate, an oxidizing agent electrode plate and a matrix between the electrode plate and the oxidizing agent plates, U-shaped gas flow passages for the fuel electrode and for the oxidizing agent electrode are provided in a counter-current relation to each other with gas inlets and gas outlets for the fuel electrode and the gas inlets and the gas outlets for the oxidizing agent electrodes being provided at the opposite sides of the fuel cell, respectively, and communicated respectively with manifolds at the same opposite side of the fuel cell.
The disclosures of the foregoing are incorporated herein by reference in their entireties.
What is needed in the art is an improved system and method for cooling oxygen sensitive components in a fuel cell stack so as to protect anode stack material or other heated components of a combined fuel processor and stack assembly during cool down.
A fuel cell system for cooling oxygen sensitive components comprises a fuel cell stack comprising at least one fuel cell comprising an anode, a cathode, and an electrolyte disposed between the anode and cathode;
a fuel reformer disposed in fluid communication with the fuel cell stack for supplying a flow of fuel to the anode;
an air supply disposed in fluid communication with the fuel cell stack for supplying a flow of air to the cathode; and
an air purification device for preparing a nitrogen gas stream, the air purification device having an inlet for receiving a flow of air and an outlet in fluid communication with the fuel cell stack anode for discharging the nitrogen gas stream; a means for supplying a flow of the nitrogen gas stream to the anode during fuel cell cool down and optionally, during fuel cell shut down; and an outlet for discharging a flow of oxygen enriched air;
wherein the flow of the nitrogen gas stream is sufficient to balance pressure within the fuel cell cathode and anode during cool down.
A method for cooling oxygen sensitive components in a fuel cell system comprises:
supplying air to an air purification device during cool down of a fuel cell system comprising at least one fuel cell comprising an anode, a cathode, and an electrolyte disposed between the anode and the cathode;
treating the supplied air within the air purification device to produce a nitrogen gas stream and a waste stream comprising oxygen enriched air;
directing the produced nitrogen gas stream through the anode; the anode being in fluid communication with the air purification device;
the flow of nitrogen gas being sufficient to balance pressure within the fuel cell cathode and anode during fuel cell cool down.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.
Referring now to the drawings, which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in the several Figures:
The fuel reformer 12, SOFC stack 16, air compressor 30, and combustor 38 are located within a thermal management enclosure 46. Optionally, fuel reformer 12 is located within the thermal management enclosure 46 yet is thermally isolated from the fuel cell stack 16.
System 110 includes an air purification or air separation device 48 for preparing a stream 50 of nitrogen and including means 52 for regulating the flow of nitrogen enriched stream 50. Air purification device 48 may alternately be termed an air separation or nitrogen enrichment device. Block 48 represents an air purification device such as a membrane separator, molecular sieve filter, pressure-swing adsorber, chromatographic separation device, paramagnetic separator, cryogenic separator, or a combination thereof During cool down, the anode 20 is isolated from the fuel source/fuel reformer 12 and from the anode exhaust (through isolation means such as a valve) but connected to the air purification device/nitrogen enrichment device 48 through conduit 54. The air purification/nitrogen enrichment device 48 is normally not in fluid communication with the stack 16 while the stack is in start-up or operating mode; however, the air purification/nitrogen enrichment device 48 could be operated in these modes to serve as a means for cooling the stack 16 with inert gas or as a means for purging a cool stack prior to start-up). Prior to entering the stack 16 (or part to be cooled), make-up air 56 is directed through the air purification/nitrogen enrichment device 48 and treated therein to separate oxygen from the make up air 56 providing a supply of purified N2 50 to the device to be cooled. Because the volume of open space in the anode and cathode side may be different, the volume of nitrogen stream 50 is not necessarily equal to the volume of air pulled back into the cathode via reverse flow from combustor during cool down. What is important is that the isostatic (isobaric) condition of the stack is maintained. Flow is self-regulating to maintain atmospheric pressure in both sides. Flow into the anode 20 is limited to N2 (or optionally N2 and Ar). In this way, the pressure within the cooling cell or cells 18 is balanced thereby preventing damage associated with pressure differences between the anode 20 and cathode 22, such as cracking of the electrolyte 24, while also preventing oxidation damage to the anode 20.
By providing air purification/nitrogen enrichment device 48 in the flow path of the make-up air 56, purified nitrogen 50 is introduced into the SOFC anode 20 or heated device, as opposed to air. In this way, the device, whether an SOFC anode, or other heated components of a combined fuel processor and stack assembly, or other parts, is protected from the detrimental effects of O2 in air. The nitrogen enrichment device 48 may require low operating temperatures to be effective and so is preferably located outside of any thermal enclosures, with the product gas delivered to the device to be cooled typically contained in a thermal enclosure via suitable conduits or tubing 54. In the embodiment of
Air purification/nitrogen enrichment device 48 may comprise any device suitable for separating O2 from air thereby providing a purified stream of N2 or N2 and Ar. The volume of N2, Ar will be less than about 50% of the total stack volume (stack volume typically being from about 1 to about 10 liters). The air purification device 48 is selected in accordance with the requirements of a given system. Generally, the air purification device is selected to supply a gas with less than about 1 part per million O2, a flow rate that is very low, such as a flow rate in the range of about 0.01 to about 100 ml/min, most typically from about 0.05 to about 5 ml/min, and a supply pressure that is close to atmospheric. Examples of suitable air purification devices as represented generally by reference numeral 48 include, but are not limited to, membrane separators, molecular sieve filters, pressure-swing adsorbers, chromatographic separation devices, paramagnetic separators, cryogenic separators or a combination thereof.
In
Turning to
Periodically, the nitrogen enrichment devices may need to be back-flushed or regenerated. By providing a suitable configuration of valves and tubing, supply air can be provided to the air separation device to accomplish back-flushing, as required, to be conducted either during shut-down or during normal operation of the part. Usually, when the parts to be cooled in accordance with the present method (e.g., fuel cells) are operational and are not requiring purified N2 (not in the process of cool down), the nitrogen purification device can be regenerated. For example, as shown in
Turning to
Secondary products produced from the air purification/nitrogen enrichment devices can be used in the fuel cell system or in components of a power generation system.
While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.
