TWI478432B - Operation of fuel cell systems with reduced carbon formation and anode leading edge damage - Google Patents

Description

Operation of a fuel cell system with reduced carbon formation and anode leading edge damage

In general, the present invention relates to the field of fuel cell systems and, more particularly, to the operation of fuel cell systems that allow for efficient life extension operations with respect to anode leading edge damage and reduced carbon deposition.

The present application claims the benefit of U.S. Provisional Application No. 61/129,838, filed on Jan. 23, 2008, which is hereby incorporated by reference.

A fuel cell is an electrochemical device that efficiently converts energy stored in a fuel into electrical energy. High temperature fuel cells include solid oxide and molten carbonate fuel cells. The fuel cells can be operated using hydrogen and/or hydrocarbon fuels. There are various types of fuel cells, such as solid oxide reversible fuel cells that also allow reversible operation, such that water or other oxidizing fuel can be reduced to an unoxidized fuel that uses electrical energy as input.

In a high temperature fuel cell system such as a solid oxide fuel cell (SOFC) system, oxidizing gas is passed through the cathode side of the fuel cell while the fuel stream is passed through the anode side of the fuel cell. The oxidizing gas is typically air, and the fuel stream is typically a hydrogen rich gas produced by reforming a hydrocarbon fuel source. Water in the form of steam can also be introduced into the system. A fuel cell typically operated at a temperature between 750 ° C and 950 ° C enables negatively charged oxygen ions to be transported from the cathode stream to the anode stream where ions combine with free hydrogen or hydrogen in the hydrocarbon molecules to form water vapor, and / or combined with carbon monoxide to form carbon dioxide. Excess electrons with negatively charged ions are returned to the cathode side of the fuel cell via a circuit completed between the anode and the cathode, thereby generating a current through the circuit.

A first aspect of the invention is a method of operating a fuel cell system wherein hydrogen is introduced into a fuel cell system along with a fuel stream at a fuel inlet, wherein the ratio of hydrogen to carbon in the fuel (H ₂ : C _fuel ) is at 0.25: 1 and 3:1 range. In an embodiment of this aspect, steam is provided with the fuel stream to operate the system at a steam:carbon (S:C) ratio of less than 2:1.

A second aspect of the invention is a method of operating a fuel cell system, the method comprising introducing a fuel mixture comprising hydrogen, fuel, and steam at a fuel inlet of a fuel cell system, and operating the fuel cell system to generate electricity. The ratio of hydrogen to carbon in the fuel mixture (H ₂ :C _fuel ) in the fuel mixture is in the range of 0.25:1 to 3:1, including 0.25:1 and 3:1; and the ratio of steam to carbon in the fuel mixture (S:C ) is less than 2:1.

A third aspect of the invention is a solid oxide fuel cell stack comprising a plurality of solid oxide fuel cells and a plurality of interconnects. Each of the plurality of fuel cells includes a solid oxide electrolyte, an anode electrode suitable for internal fuel reforming, and a cathode electrode. The anode electrode and the cathode electrode are symmetrical in the region where the fuel enters the fuel cell.

One problem associated with the operation of current state of the art SOFCs using hydrocarbon fuels is the tendency for internal carbon deposits to form and deposit on the anode electrode. If the internal carbon formation and deposition are not removed, it will result in a decrease in anode efficiency and a reduction in effective device life. A measured amount of steam is typically introduced with the fuel to reduce carbon build-up and deposition in such systems. Unfortunately, the addition of steam has undesirable consequences. First, the introduction of steam into SOFC fuel has the undesirable effect of reducing efficiency (i.e., the observed optimal battery voltage is lower). Second, the introduction of steam into the SOFC fuel may have undesirable effects that cause the fuel cell anode to be damaged by the anode leading edge, sometimes referred to as "anode dusting," which accelerates battery voltage degradation. Moreover, the introduction of steam into the SOFC fuel may have undesirable effects that degrade the anode seal resulting in cross-reactant leakage over time.

The steam and fuel streams entering the fuel cell are typically controlled to maintain the ratio of steam:carbon (S:C) at the fuel inlet within the nominal operating range. The use of the S:C ratio for the parametric control of fuel cells is described in detail in U.S. Patent Application Serial No. 12/149,, filed on Jan. 8, 2008, the entire disclosure of which is incorporated herein by reference. As described therein, SOFC operating with methane as a fuel typically requires an S:C ratio in the range of 2:1 to 3:1. However, certain fuels are more susceptible to carbon deposit formation and deposition than other fuels. This disproportionate nature can be somewhat attenuated by adjusting the S:C ratio of the particular fuel used, but there are limits to this adjustment. For example, carbon deposit formation is generally difficult to observe even when operating at a relatively high S:C ratio in a SOFC system fueled with propane.

Fuel cell operation that introduces hydrogen together with fuel

In one aspect of the invention, the inventors have recognized that when hydrogen is introduced into the fuel cell system with the fuel at the fuel inlet, the fuel cell system can be comparable or even reduced at a reduced S:C ratio. The carbon deposits are formed and deposited to operate. The fuel cell system operating in this manner shows an unexpected increase in battery voltage, and the operating time before reaching the average minimum battery voltage is significantly longer. The benefits of the present invention are even applicable to systems that operate on fuels such as propane and other higher hydrocarbons that are known to be susceptible to carbon deposit formation.

The fuel cell system operating as described above preferably contains one or more solid oxide fuel cell (SOFC) stacks. U.S. Patent Application Serial No. 11/491,487 (filed on July 24, 2006), No. 11/491,488 (filed on July 24, 2006) and No. 11/002,681 (filed on December 3, 2004) A detailed description of a class of SOFC systems, all of which are incorporated herein by reference in its entirety.

A viable fuel cell system such as a SOFC system may include the following components including, but not limited to, a steam generator, a reformer, a heat exchanger, a blower, a condenser, a vent, a mixer, a catalytic reactor, or Any combination.

As used herein, a "catalytic reactor" describes an element in a fuel cell system that is capable of catalyzing a reaction between reactants therein. These reactors typically contain tubes or other conduits containing metal catalysts. The catalytic reactor can be located at different locations in the fuel cell system and can be internal or external. Examples of catalytic reactors include, but are not limited to, catalytic partial oxidation (CPOx) reactors and anode off-gas oxidation (ATO) reactors. A detailed description of a class of catalytic reactors is described in U.S. Patent Application Serial No. 11/703,152, filed on Feb. 7, 2007, which is hereby incorporated by reference.

As used herein, the term "hydrogen flow" is used to mean the amount of molecular hydrogen introduced into a fuel cell stack or system with fuel at the fuel inlet. While this measure is typically expressed in units of standard liters per minute (SLPM), those skilled in the art will recognize that such units can be readily converted to moles per second.

As used herein, the term "water flow" is used to mean the sum of the amount of water and anode circulating water (if present) introduced into the fuel cell stack or system with the fuel. The water introduced into the fuel cell stack or system will typically be in the form of a vapor, i.e., steam. While this measure is typically expressed in units of standard liters per minute (SLPM), those skilled in the art will recognize that such units can be readily converted to moles per second.

As used herein, the term "fuel flow" is used to mean the amount of fuel introduced into a fuel cell. While this measure is typically expressed in units of standard liters per minute (SLPM), those skilled in the art will recognize that such units can be readily converted to moles per second.

As used herein, the term "hydrocarbon fuel" is used to refer to a typical fuel comprising hydrogen and carbon for fuel cell operation. Examples of typical fuels for fuel cell operation include, but are not limited to, hydrocarbons (including methane, ethane, and propane), natural gas, alcohols, and syngas obtained from coal or natural gas reforming.

As used herein, the term "alcohol" is used broadly to refer to an organic compound derived via a hydroxyl group. Examples of alcohols include, but are not limited to, methanol, ethanol, and isopropanol.

As used herein, the term "fuel composition" refers to the elemental composition of a fuel. This elemental composition is usually expressed in terms of [X] moles per mole of fuel, where [X] is the element of interest. Examples of elements of interest that may be suitable for determining typical control parameters for SOFC operation include carbon, oxygen, hydrogen, and (as appropriate) nitrogen. For liquid based fuels, the elemental composition can also be expressed as [X] moles per milliliter of fuel or [X] moles per gram of fuel.

As used herein, the reference to the numerical ratio uses the term "about/approximately" to mean the inclusive range of the indicated value plus or minus 10%.

As used herein, the term "controllably providing an exhaust stream into an inlet stream" means actively controlling the amount of fuel exhaust provided to the fuel inlet stream as opposed to uncontrolled passive supply to the fuel inlet stream. . Thus, simply directing a portion of the exhaust stream into the fuel inlet stream by the "T" shaped branch tube does not controllably provide an exhaust stream into the inlet stream. The amount of fuel exhaust circulated may be controlled by an operator or by a computer by either or both of control valve 201 and/or blower 209. For example, valve 201 can be controlled to vary the ratio of the first independent fuel exhaust stream to the second independent fuel exhaust stream. In other words, if more steam is required in the fuel inlet stream, the valve may increase the portion of the fuel exhaust stream provided to the first separate fuel exhaust stream. If less steam is required in the fuel inlet stream, the valve may reduce the portion of the fuel exhaust stream provided to the first separate fuel exhaust stream. Depending on the amount of steam or less steam required in the fuel inlet stream, the blower 209 can be controlled by increasing or decreasing the blast speed or blast volume to increase or decrease the amount of fuel exhaust provided by the blower 209 into the fuel inlet stream. .

As used herein, the term "symmetric" (when used to describe the relationship between anode shape and cathode shape) is intended to mean that the outer edge of the anode is precisely or substantially in the comparative region with the electrolyte located in the same fuel cell. The outer edge of the opposite side of the cathode is aligned. Conversely, the anode and cathode, which are considered to be out of alignment, are asymmetrical in the misaligned region. Therefore, the anode may be symmetrical with the cathode at one edge and asymmetrical at the other edge.

Steam: carbon ratio

The derivation and utilization of the steam:carbon (S:C) ratio as a control parameter for fuel cell operation is described in detail in U.S. Patent Application Serial No. 12/149,816. The steam:carbon (S:C) ratio is typically derived from the water flow, fuel flow, and carbon of the fuel (expressed as carbon moles per mole of fuel). The water flow and fuel flow are adjustable amounts that can be adjusted by the operator to maintain the S:C ratio within the nominal operating range. The fuel composition can theoretically be derived from the composition of the fuel used.

For example, if the fuel gas is methane (i.e., CH _4), the stoichiometric analysis indicated that 1 mole per 1 mole of the fuel carbon. However, when using ethanol (i.e., CH ₃ CH ₂ OH), analysis indicated that the stoichiometry of 1 mole per mole of fuel have 2 carbons. This theoretical stoichiometry may not always be appropriate because it is possible that the fuel source of the fuel cell system may be a mixture of two or more fuels of unknown quantity. In such cases, the characterization of the carbon content of the fuel can be obtained from other sources, either by direct detection or by consulting from a supplier of fuel.

Once the carbon content of the fuel is determined or obtained, the S:C ratio can be derived as follows. The S:C ratio is equal to the water flow ÷ (fuel flow × carbon content of the fuel):

S: C ratio = water flow: carbon flow;

Where carbon flow = fuel flow rate x carbon mole per mole of fuel;

And the water flow rate = the steam flow rate of the steam generator + the anode circulation flow rate × the molar fraction of water in the anode circulation flow.

In an alternative method of determining the S:C ratio, the carbon content of the anode recycle stream (in the form of CO and CO ₂ ) is also included in the carbon flow. Similarly, in an alternative method, the fuel stream comprises the CO ₂ content in the carbon flow. In making a larger portion of the anode exhaust stream or the fuel circulation system will typically have, including CO and CO ₂ is more important contribution ratio of the carbon fraction of the flow of the other fuel sources of biogas much of the CO ₂ system.

The amount of fuel and steam introduced into the fuel cell can be varied continuously or intermittently to preferably maintain the ratio of S:C in the fuel inlet stream to within the nominal operating range during operation of the fuel cell stack. When a suitable amount of hydrogen is introduced into the fuel cell along with the fuel, the preferred range of S:C ratio is less than about 2:1.

Hydrogen: carbon ratio

To operate the fuel cell system at these lower S:C ratios, atomic hydrogen or molecular hydrogen is provided with the fuel at the fuel inlet. The amount of hydrogen introduced into the system can be expressed as the ratio of molecular hydrogen introduced into the fuel inlet to carbon in the fuel (H ₂ : C _fuel ratio).

The derivation of the H ₂ :C _fuel ratio is similar to the derivation of the S:C ratio as described above.

The H ₂ :C _fuel ratio is equal to the hydrogen flow rate 燃料 (fuel flow rate × carbon content of the fuel):

H ₂ : C _fuel ratio = hydrogen flow rate: (fuel flow rate x carbon moles per mole of fuel).

Thus, for all the methane as fuel operation of the system, H _2: C ratio is equal to _{the fuel} and may be referred to herein as H _2: CH ₄ ratio.

The relative amounts of hydrogen into the fuel cell and the fuel may be continuous or intermittent variation of the preferred fuel cell stack during operation of the fuel inlet stream H _{2: C-fuel} ratio is maintained within the nominal operating range. In a preferred embodiment, the nominal operating range of the H ₂ :C _fuel ratio is typically between 0.25:1 and 3:1 H ₂ :C _fuel ; preferably 0.5:1 and 1.5:1 H ₂ :C _fuel between.

Hydrogen source

The atomic hydrogen or molecular hydrogen introduced at the fuel inlet can come from a variety of sources. In a preferred embodiment of the invention, at least a portion of the anode exhaust stream is circulated, i.e., diverted and reintroduced into the fuel cell along with the fuel stream. As shown in Figure 7, some related embodiments may contain a water gas shift reactor 128 as appropriate. In such embodiments, at least a portion of the circulating anode exhaust stream is passed through a water gas shift reactor 128 to concentrate the amount of molecular hydrogen in the stream. At least a portion of the resulting enriched stream is then circulated, i.e., diverted and reintroduced into the fuel cell along with the fuel stream. The water gas shift reactor 128 may be any suitable means for converting at least a portion of the water and carbon monoxide in the fuel exhaust stream to the constituent hydrogen and carbon dioxide. For example, reactor 128 may comprise a tube or conduit containing a catalyst that converts some or all of the carbon monoxide and water vapor in the fuel exhaust stream to carbon dioxide and hydrogen. The catalyst can be any suitable catalyst, such as an iron oxide catalyst or a chromium promoted iron oxide catalyst. Reactor 128 may be located between fuel heat exchanger 121 and air preheater heat exchanger 125.

In an alternate embodiment of the invention, the source of hydrogen fuel is independent of the exhaust stream and may comprise any known source of independent hydrogen, such as commercially available cylinders or cylinders.

Preferred partial fuel reforming and anode exhaust cycle fuel cell operation

The inventors have discovered that anode front edge damage in a fuel cell system can also be reduced by reducing the concentration of hydrocarbon fuel at the fuel inlet without changing the S:C ratio.

The operation of a fuel cell system without an anode exhaust cycle is disclosed in U.S. Patent Application Serial No. 12/149,816, filed on May 8, 2008. When methane is used as the fuel, the system is operated at an S:C ratio within the nominal operating range of 2:1 to 2.5:1. As discussed in Example 5 below, the concentration of hydrocarbon fuel at the fuel inlet is between about 33.3% and about 28.6% under such operating conditions.

Since carbon deposition and leading edge damage inherent to the introduction of steam into the fuel cell system are increased as described above, it is not preferable to simply reduce the hydrocarbon fuel concentration in the fuel stream by increasing the S:C ratio. In certain embodiments of the invention, the fuel concentration is reduced at the fuel inlet by partially circulating the anode exhaust stream and partially reforming the fuel stream before the fuel is introduced into the fuel inlet.

In such embodiments, a portion of the anode exhaust gas is circulated, i.e., reintroduced into the fuel cell system at the fuel inlet with the hydrocarbon fuel. The percentage of the anode exhaust gas to be circulated is preferably 70%; the percentage is more preferably in the range of 55% and 65%, including 55% and 65%.

In the content of the anode exhaust loop, the exhaust gas stream circulating in H ₂ O, an amount of CO ₂ and CO not insignificant, and thus to be considered when determining the operation of the fuel cell system when necessary for the amount of steam. However, because the carbon contained in the circulating anode exhaust stream is not in the form of a hydrocarbon fuel, the hydrocarbon fuel is effectively diluted in the fuel stream without altering the elemental composition of the stream. In such embodiments, the fuel concentration at the fuel inlet can be reduced by up to 40% compared to systems without circulating anode exhaust.

In a preferred embodiment of this aspect, the hydrocarbon fuel is preferably partially reformed prior to introduction of the hydrocarbon fuel into the fuel cell system to further reduce the hydrocarbon fuel concentration at the fuel inlet. The reformer used in such systems is a suitable device capable of partially or completely reforming a hydrocarbon fuel to form a carbon-containing fuel containing free hydrogen. For example, the reformer can include a reformer as described in U.S. Patent Application Serial No. 11/002,681, filed on Dec. 2, 2004, which is hereby incorporated by reference.

Preferably, 5% to 15% (e.g., 10%) of the fuel is reformed prior to introduction of the fuel into the fuel cell system. In embodiments utilizing anode exhaust gas recirculation and partial pre-reforming, the concentration of hydrocarbon fuel at the fuel inlet can be reduced by up to 50% compared to systems without circulating anode exhaust and fuel pre-reforming.

A number of types of fuel cell systems that can be operated in accordance with the methods of the present invention are described in U.S. Patent Application Serial No. 11/491,487, the entire entire disclosure of which is incorporated herein by reference. A schematic of two such systems is shown in Figures 7 and 8. Figure 7 illustrates a type of fuel cell system 300 that includes a fuel cell stack 101, such as a solid oxide fuel cell stack (schematically illustrated to show a solid oxide fuel cell of a stack containing two such as yttria or yttria stabilized) A ceramic electrolyte such as zirconia, an anode electrode such as a nickel stabilized zirconia cermet, and a cathode electrode such as bismuth manganese ore.

In system 300, a fuel exhaust stream is provided directly from valve 201 to electrochemical pump 301, which electrochemically separates hydrogen from the fuel exhaust stream. Additionally, the blower or compressor 109 may be omitted if the pump 301 is capable of controllably providing a desired amount of hydrogen to the fuel inlet stream.

Pump 301 can comprise any suitable proton exchange membrane device comprising a polymer electrolyte. Hydrogen diffuses through the polymer electrolyte by applying a potential difference between the anode and the cathode on either side of the electrolyte. The electrochemical pump preferably comprises a carbon monoxide tolerant electrochemical cell stack, such as a high temperature low hydrate ion exchange membrane cell stack. Such batteries include a non-fluorinated ion exchange ionomer membrane between the anode and cathode electrodes, such as a polybenzimidazole (PBI) membrane. The film is doped with an acid such as sulfuric acid or phosphoric acid. An example of such a battery is disclosed in U.S. Published Application No. US 2003/0196893 A1 which is incorporated herein in its entirety by reference. The batteries operate in a temperature range from above 100 °C to about 200 °C. Thus, if present, heat exchanger 121 and optionally heat exchanger 125 preferably maintain the fuel exhaust stream at a temperature of from about 120 °C to about 200 °C, such as from about 160 °C to about 190 °C.

System 300 also includes a third conduit 7 that operatively connects the outlet of pump 301 to a hydrogen storage vessel or hydrogen usage device (not shown) as appropriate. The third conduit 7 also operatively connects the outlet of the pump 301 with the fuel inlet conduit 111 of the fuel cell stack 101 as necessary.

System 300 also includes a fourth conduit 9 for removing exhaust gases from pump 301. The conduit 9 can be coupled to a catalytic anode exhaust gas oxidizer 107 or an atmospheric vent. The anode off-gas oxidizer 107 can also be operatively coupled to the stack fuel exhaust outlet 103 as appropriate to provide a portion of the fuel exhaust stream to the anode tail gas oxidizer 107 to support the reaction in the anode tail gas oxidizer.

System 300 also includes a reheat heat exchanger 121 that exchanges heat between the stack fuel exhaust stream and the hydrocarbon fuel inlet stream provided from inlet conduit 111. The heat exchanger helps to raise the temperature of the fuel inlet stream and lower the temperature of the fuel exhaust stream so that it can be further cooled in the condenser and as such it does not damage the humidifier.

If the fuel cell is an external fuel reforming cell, system 300 includes a fuel reformer 123. The reformer 123 reforms the hydrocarbon fuel inlet stream into a fuel stream containing hydrogen and carbon monoxide, which is then supplied to the stack 101. The reformer 123 can be heated by the heat generated in the fuel cell stack 101 (ie, the reformer is integrated with the stack heat) and/or as described in U.S. Patent Application Serial No. 11/002,681, filed on Dec. The heat generated in the anode tail gas oxidizer used as appropriate is radiant, convective and/or conductive, and the application is hereby incorporated by reference in its entirety. Alternatively, if the stack 101 contains an internal reforming cell that reforms the interior of the fuel cell that is primarily present in the stack, the external reformer 123 may be omitted.

System 300 also includes an air preheater heat exchanger 125, as appropriate. The heat exchanger 125 provides heat to the air inlet stream in the fuel cell stack 101 using thermal heating of the fuel cell stack fuel exhaust. The heat exchanger 125 can be omitted if necessary.

System 300 preferably also includes an air heat exchanger 127. The heat exchanger 127 further provides heat to the air inlet stream in the fuel cell stack 101 using thermal heating of the fuel cell stack air (i.e., oxidizer or cathode) exhaust. If the preheater heat exchanger 125 is omitted, the air inlet flow is provided directly to the heat exchanger 127 by a blower or other air intake.

System 300 can also contain a water gas shift reactor 128 as appropriate. The reactor suitable for use as the water gas shift reactor 128 is described in detail above. Reactor 128 may be located between fuel heat exchanger 121 and air preheater heat exchanger 125.

System 300 can be operatively coupled to a hydrogen storage vessel or a hydrogen usage device (not shown) as appropriate. However, the container or device can be omitted and system 300 can be used to generate electricity only while not generating electricity and hydrogen at the same time. The hydrogen storage vessel may comprise a hydrogen storage tank or a hydrogen distributor. The vessel may contain piping to a hydrogen plant for use in transportation, power generation, cooling, hydrogenation or semiconductor manufacturing. For example, system 300 can be located in a chemical plant or semiconductor factory to provide primary or secondary (ie, backup) power to the plant, as well as provide other chemistry for hydrogenation (ie, semiconductor device passivation) or hydrogen required to be performed in the plant. Hydrogen reacted.

Hydrogen use devices, as appropriate, may also include another fuel cell system, such as a fuel cell stack, such as a low temperature fuel cell system, such as a proton exchange membrane (PEM) fuel cell system that uses hydrogen as a fuel. Thus, a portion of the hydrogen from system 300 can be provided to one or more other fuel cells as fuel. For example, system 300 can be located in a fixed location, such as a building or an area outside or below a building, and used to provide power to the building. The other fuel cells may be located in a vehicle in a garage or parking lot adjacent to a fixed location. The vehicle may include a car, a sport utility vehicle, a truck, a motorcycle, a boat, or any other suitable fuel cell powered vehicle. In this case, the system 300 is provided with a hydrocarbon fuel to generate electricity for the building and to generate hydrogen supplied to the fuel cell system 300 and the fuel cell system as a fuel. The generated hydrogen may be temporarily stored in a hydrogen storage container and then supplied to the vehicle fuel cell (similar to a gas station) from the storage container upon request, or the generated hydrogen may be supplied to the vehicle directly from the system 300 via the pipeline. The fuel cell.

System 300 can contain a hydrogen regulator as appropriate. The hydrogen regulator can be any suitable device that can purify, dry, compress (ie, compress) or otherwise change the state of the hydrogen-rich gas stream provided from pump 301. The hydrogen regulator can be omitted if necessary.

System 300 also includes a fuel splitter device 201, such as a computer or operator controlled multi-way valve (e.g., a three-way valve), or another fluid split device. The device 201 includes an inlet 203 operatively coupled to the fuel cell stack fuel exhaust outlet 103, a first outlet 205 operatively coupled to the pump 301, and a second outlet 207 operatively coupled to the fuel cell stack fuel inlet 105. For example, the second outlet 207 can be operatively coupled to the fuel inlet conduit 111, which in turn is operatively coupled to the inlet 105. However, the second outlet 207 can provide a portion of the fuel exhaust stream to further downstream of the fuel inlet stream.

The hydrogen separation process described in system 300 is not the only such method that can be used. An alternative hydrogen separation unit can be incorporated into the system, such as the partial pressure swing adsorption unit described in U.S. Patent Application Serial No. 11/188,120, filed on Jul. 25, 2005, the entire disclosure of The manner of reference is incorporated herein. The units may comprise a plurality of adsorbent beds and act as regenerative dryers and carbon dioxide scrubbers. However, any suitable method or apparatus for enriching the hydrogen in the gas stream can be utilized.

FIG. 8 illustrates a different exemplary system 400. System 400 is similar to system 300 with the exception that the second independent fuel exhaust stream provided from valve 201 is not subjected to hydrogen separation. Instead, a second separate fuel exhaust stream provided from valve 201 is discharged or provided to anode exhaust gas oxidizer 107. This means that system 400 is simpler than system 300 because it does not include hydrogen separation steps and equipment. The method of operating system 400 allows the use of low temperature blower 209 by cooling the fuel exhaust stream through a series of heat exchangers 121 and 125 to less than 200 °C, such as about 90 °C to 110 °C.

In some embodiments of the invention, the amount of fuel exhaust provided to the fuel inlet stream is automatically controlled by an operator or by a computer to achieve a steam:carbon ratio of less than 2:1 in the fuel inlet stream. The first separate fuel exhaust stream contains steam and the fuel inlet stream contains a hydrocarbon fuel inlet stream such as a methane or natural gas stream. Thus, the amount of fuel exhaust (and thus the amount of steam) provided to the fuel inlet stream is controlled to achieve less than 2:1 (including less than or equal to 1.8:1), such as 1.9:1 to 1: in the fuel inlet stream: A ratio, for example a ratio of 1.6:1 to 1.2:1, such as a steam ratio of 1.4:1: carbon ratio. The amount of fuel exhaust circulated into the fuel inlet stream may be continuously or intermittently varied to continuously maintain the steam:carbon ratio in the fuel inlet stream at less than 2:1 during operation of the fuel cell stack.

Anode and cathode symmetry

The inventors have also recognized that, at least in some cases, the relative geometry of the anode and cathode may affect the formation and deposition of internal carbon deposits on the anode electrode. For example, in a fuel cell system with direct internal reforming, increased carbon deposition may be observed at the point on the anode that is asymmetric between the anode and the corresponding cathode of the same cell. In particular, a direct internal reforming fuel cell system in which the anode covers more regions than its corresponding cathode may exhibit increased carbon deposition at portions of the anode that do not overlap the cathode. Without wishing to be bound by a particular theory, the inventors believe that this may be caused by a local decrease in temperature caused by reforming at the leading edge of the anode and a decrease in electron flow at the asymmetric region of the anode and cathode. Thus, to reduce carbon deposits on the anode, some embodiments utilize anodes and cathodes that are at least symmetric in the region where the fuel enters the fuel cell. In other embodiments, the anode and cathode may be symmetrical throughout their area (ie, the two electrodes have the same shape and are in the same position on opposite sides of the electrolyte).

FIG. 9A shows the air side of the exemplary interconnect 1 . The interconnects can be used with internal manifolds for fuel and external manifolds for air stacks. The interconnect contains air flow grooves 2 that allow air to flow from side 3 of the interconnect to the opposite side 4 . The ring seal 5 is located around the fuel riser opening 6 . Figure 9B illustrates the fuel side of interconnect 1 . A fuel distribution plenum 11 and a fuel flow groove 12 are visible on this side.

In a fuel cell stack, the cathode electrode of one cell contacts the air side of the interconnect 1 and the anode electrode of the same cell contacts the fuel side of the interconnect 1 . In the illustrative case illustrated in Figures 9A and 9B, the sides of the interconnect 1 are different in an internal manifold with fuel for the manifold and an external manifold for the air stack. In detail, the sides of the interconnect differ in the area including the fuel distribution plenum 11 (there is no such area on the air side) and the fuel riser opening area 6 (only the air side contains the seal).

In general, prior art anode and cathode electrodes are not symmetrical and have different shapes, with the air and fuel sides of the interconnect having different shapes. For example, for the interconnect shown in Figures 9A and 9B, the prior art anode electrode would contain a portion located adjacent the fuel inlet riser opening and another portion above the fuel distribution plenum on the fuel side of the interconnect, The cathode electrode will lack a portion that is adjacent to the fuel inlet riser opening 6 (e.g., because the seal 5 is located around the opening 6 of the air side of the interconnect) and the cathode will also lack fuel located on the fuel side of the interconnect. A portion of the location where the inflator is distributed.

In the present embodiment, the anode electrode and the cathode electrode are symmetrical at least in a region where the fuel enters the fuel cell, such as a region adjacent to the fuel inlet riser opening 6 in the electrolyte. The anode electrode preferably deviates from the fuel inlet riser opening by at least 4 mm, such as 5-20 mm. Preferably, the cathode electrode also deviates from the fuel inlet riser opening by at least 4 mm, such as 5-20 mm.

10A and 10B illustrate an exemplary anode electrode configuration. As shown in the figures, an anode electrode 22 (such as nickel and doped ceria or nickel and stabilized zirconia cermet) is formed on the solid oxide electrolyte 21 (such as doped ceria or stabilized two) Zirconia ceramic electrolyte). The anode electrode is offset from the fuel inlet riser opening 6 in the electrolyte by a distance 23 of at least 4 mm (such as 5-20 mm). Preferably, the cathode electrode has the same shape and the same position as the anode electrode on the opposite side of the electrolyte adjacent to the opening 6 .

In the embodiment illustrated in Figure 10A, the anode electrode 22 is not located above the fuel distribution plenum 11 (shown in dashed lines) adjacent the fuel side of the interconnect. In the embodiment illustrated in Figure 10B, the anode electrode 22 is located above the fuel inlet distribution plenum 11 (shown in dashed lines) adjacent the fuel side of the interconnect. In both cases, the anode electrode is offset from the fuel inlet riser opening 6 by a distance of 23 . The anode electrode may or may not deviate from the fuel exhaust riser opening at the outlet end of the electrolyte and may or may not be located above the fuel exhaust distribution plenum at the outlet end of the interconnect. In the embodiment of Figures 10A and 10B, the cathode electrode is not located above the position of the fuel distribution plenum corresponding to the fuel side of the interconnect. Thus, in the embodiment of Figure 10A, the anode and cathode electrodes are fully symmetrical throughout their area, while in the embodiment of Figure 10B, the anode and cathode are symmetric at least in the region where the fuel enters the fuel cell.

Instance Example 1: Effect of an S:C ratio of about 1.4:1 on the operation of an SOFC with an external reformer

An SOFC system having five cells and an external reformer is operated, wherein the SOFC system uses natural gas as fuel and introduces hydrogen (ie, H ₂ ) at the fuel inlet. For this example and Example 2, it is assumed that the natural gas used as the fuel is 100% methane (i.e., CH ₄ ). This assumption was made to simplify the determination of the S:C and H ₂ :C _fuel ratios. For example, for operation with methane as the fuel system, H _2: C _fuel ratio is equal to H _2: CH ₄ ratio. Thus, for this example and Example 2, the H ₂ :C _fuel ratio is expressed as the H ₂ :CH ₄ ratio.

Other operating conditions for the SOFC system are a single pass fuel utilization of about 80%, an air utilization of about 25%, a current generation of about 35 amps, and an air output temperature of about 825 °C.

Established under conditions such that the H ₂ :CH ₄ ratio at the fuel inlet is about 1:1 and the S:C ratio exceeds 2:1 and demonstrates stable operation of the system. Then adjust the gas flow rate at the inlet of the fuel, thus maintaining the H _2: CH ₄ ratio of about 1: 1, but the S: C ratio was reduced to about 1.4: 1, and to monitor the impact on the battery voltage.

Figure 1 contains a graph of the S:C ratio of the system over a time interval of about 60 hours. As shown in Figure 1, the change in the S:C ratio occurs at about 6,890 hours of the stack operation.

Figure 2 contains a graph of battery voltage over the same time interval. As shown in Figure 2, the battery voltage unexpectedly increases due to the decrease in the S:C ratio.

The operation of the SOFC system was continued for more than 1000 hours under reduced S:C ratio operating conditions without any signs of adverse effects on the accumulated carbon.

Example 2: Effect of an S:C ratio of about 1.2:1 on the operation of an SOFC with an external reformer

A stable operation of the SOFC system with an S:C ratio of about 1.4:1 as described in Example 1 was established. Then adjust the gas flow rate at the inlet of the fuel, thus maintaining the H _2: CH ₄ ratio of about 1: 1, but the S: C ratio was reduced to about 1.2: 1, and to monitor the impact on the battery voltage.

Figure 3 contains a graph of the S:C ratio of the system over a time interval of about 85 hours. As shown in Figure 3, the change in the S:C ratio occurs at about 6,010 hours of the stack operation.

Figure 4 contains a graph of battery voltage over a time interval containing the time interval demonstrated in Figure 3. As shown in Figure 4, the battery voltage unexpectedly increases due to the decrease in the S:C ratio. Although the magnitude of the increase in battery voltage was not as great as seen in Example 1, an unexpected increase in battery voltage was observed and the operation of the system remained stable.

Example 3: Effect of hydrogen injection on the operation of SOFC with internal reforming

Operating a battery 5 and is located inside of the anode catalyst of the reforming SOFC system, in which the SOFC system as methane and hydrogen is introduced into the fuel inlet of the fuel, wherein the fuel inlet H _2: CH ₄ ratio of about 1: 1. The operating system is at an S:C ratio of approximately 2:1. The operation of the SOFC system was continued for more than 2000 hours under these operating conditions without any signs of adverse effects of carbon build-up or anode leading edge damage.

Example 4: Effect of S:C ratio of approximately 1.6:1 on the operation of internal reforming SOFC

Operating a battery 5 and the catalyst of the internal reforming SOFC system, in which the SOFC system as methane and hydrogen is introduced into the fuel inlet of the fuel, wherein the fuel inlet H _2: CH ₄ ratio is 1: 1. The operating system was initially at an S:C ratio of approximately 1.8:1. After establishing and demonstrating the stable operation of the system, the gas flow at the fuel inlet is adjusted such that the H ₂ :CH ₄ ratio is maintained at about 1:1, but the S:C ratio is reduced to about 1.6:1 and the effect on the battery voltage is monitored. . The SOFC was operated at a current generation of about 35 amps and an air output temperature of about 825 °C.

Figure 5 contains a graphical representation of the S:C ratio of the system over a time interval of about 205 hours. As shown in Figure 5, the change in the S:C ratio occurs at about 7,295 hours of the heap operation.

Figure 6 contains a graphical representation of the battery voltage over the time interval containing the same time interval as demonstrated in Figure 5. As shown in Figure 6, the battery voltage also unexpectedly increases due to the decrease in the S:C ratio. It can be seen that the battery voltage increases over the next approximately 50 hours until the 7,340 hour point of the stack operation. At this time, the battery voltage drops without any adjustment to the H ₂ :CH ₄ or S:C ratio. This drop in battery voltage is consistent with conventional variations in the tank containing the fuel gas for the system. The inventors suspect that the replacement tank may be contaminated with ethane, and that the observed decrease in battery voltage at this time is independent of the operating conditions described herein.

Example 5: Simulation data for operation of an SOFC stack with anode cycle and fuel partial pre-reforming

A simulation experiment was performed on the operation of the SOFC system with varying S:C ratios, ratio of circulating anode exhaust, and ratio of fuel pre-reforming to determine the relative concentration of fuel at the inlet under different operating conditions. All simulations were based on a system that uses methane as a fuel.

Table 1 below includes the data generated by these simulations.

The CH ₄ fraction at the inlet of a SOFC system operating with methane as fuel, no anode exhaust cycle, and no fuel pre-reforming is easily determined by the S:C ratio:

CH ₄ score at the entrance = (C / (S + C)) × 100%;

Where S and C are from the S:C ratio.

Therefore, an SOFC system operating at a S:C ratio in the range of 2:1 to 2.5:1 (including 2:1 and 2.5:1) as described above will have an inlet CH of about 33.3% and about 28.6%. ₄ points.

The data in Table 1 demonstrates SOFC stack and anode-free cycles operating with methane as fuel, S:C ratio between 2.2 and 2.8, anode cycle of approximately 60%, and no fuel pre-reforming (see simulations 1 and 2). Compared to SOFC, the CH ₄ score at the entrance is significantly reduced. In fact, under these conditions, the range of CH ₄ at the inlet was reduced from 18.978% to 15.855%, including 18.978% and 15.855%. This represents the equivalent of operation compared to a similar system without the pre-reformer anode exhaust gas or cyclic, CH ₄ at the inlet of about 45% reduction.

A further reduction can be seen when a portion of the fuel is pre-reformed. Simulations 3 and 4 demonstrate that approximately 60% of the anode exhaust cycle combined with 10% fuel supply pre-reforming reduces the range of CH ₄ at the inlet to a range of 16.442% to 13.807%, including 16.442% and 13.807%. This indicates compared to a similar system without the operation cycle or the pre-reformer anode exhaust gas, CH ₄ flow at the inlet of about 50% reduction.

Example 6: Effect of anode/cathode symmetry on anode carbon deposition

Figures 11A-Y are photographs of the anode side of 25 fuel cells in the area around the fuel inlet riser opening with various anode/cathode symmetry. The fuel cell is located in a stack of 25 batteries. The configuration utilized is summarized in Table 2. "Asymmetric" means that the anode electrode is located adjacent to the fuel inlet riser opening, while the cathode electrode is not. "Symmetric" means that the anode and cathode electrodes are symmetrical at the fuel inlet riser opening and each is offset from the fuel inlet riser opening by a distance of 5 mm. The last row of Table 2 describes whether the anode is present above the fuel distribution plenum on the fuel side of the adjoining interconnect. In all batteries, the cathode electrode is offset from the fuel inlet riser opening and is not present above the fuel distribution plenum position.

As shown in Figures 11A-Y, a fuel cell that is symmetric of the anode and cathode at the region around the fuel inlet riser demonstrates that the anode carbon deposits are significantly less in this region. In detail, the black substance in the figure is coke, the gray substance is the anode material, and the light substance around the opening is the exposed portion of the electrolyte.

The foregoing description of the invention has been presented for purposes of illustration and description. The methods and apparatus illustratively described herein can be suitably implemented in the absence of any elements or limitations not specifically disclosed herein. Therefore, for example, the terms "including", "including", "including", and the like are to be construed as limiting and not limiting. In addition, the terms and expressions used herein have been used to describe the terms and are not intended to be limiting, and are not intended to be There may be various modifications within the scope of the invention. Therefore, it is to be understood that the modifications and variations of the present invention disclosed herein may be realized by those skilled in the art, and Within the scope of the invention. The scope of the invention is intended to be defined by the scope of the appended claims

1. . . Interconnect

2. . . Air flow groove

3. . . Interconnect side

4. . . Opposite side of the interconnect

5. . . Ring seal

6. . . Fuel riser opening

7. . . Third pipe

9. . . Fourth pipeline

11. . . Fuel distribution plenum

12. . . Fuel flow groove

twenty one. . . Solid oxide electrolyte

twenty two. . . Anode electrode

twenty three. . . Anode electrode offset distance

101. . . Fuel cell stack

103. . . Fuel exhaust outlet

105. . . Fuel inlet

107. . . Exhaust catalytic combustion furnace

111. . . Fuel inlet pipe

121. . . Fuel heat exchanger

123. . . Fuel reformer

125. . . Air preheater

127. . . Air heat exchanger

128. . . Water gas shift reactor

201. . . valve

203. . . Entrance

205. . . First exit

207. . . Second exit

209. . . Blower

300. . . Fuel cell system

301. . . Electrochemical pump

400. . . Fuel cell system

Figure 1 is S within the SOFC at intervals of about 60 hours: C ratio versus the time in which the external reforming SOFC natural gas as fuel and the hydrogen (H ₂₎ is introduced with the fuel inlet and the fuel in operation, Wherein the ratio of hydrogen to methane (H ₂ :CH ₄ ) is about 1:1. To determine the ratio of steam to carbon (S:C) and hydrogen to methane (H ₂ :CH ₄ ), the natural gas is assumed to be 100% methane (CH4). Figure 1 demonstrates that the ratio of steam to carbon (S:C) varies from greater than 2:1 to about 1.4:1.

FIG 2 graph-based SOFC cell voltage within about 60 hours of the time interval, wherein the external reforming SOFC natural gas as fuel and the hydrogen (H ₂₎ introduced into the fuel inlet operate in conjunction with the fuel, wherein the hydrogen and methane The ratio of (H ₂ :CH ₄ ) is about 1:1. To determine the ratio of steam to carbon (S:C) and hydrogen to methane (H ₂ :CH ₄ ), the natural gas is assumed to be 100% methane (CH4). Figure 2 demonstrates the change in battery voltage as the S:C ratio changes from about 2:1 to about 1.4:1.

3 is a graph of S:C ratio versus time for SOFC over a time interval of about 85 hours, wherein the SOFC is operated by externally reforming natural gas as a fuel and introducing hydrogen (H ₂ ) together with the fuel at the fuel inlet, Wherein the ratio of hydrogen to methane (H ₂ :CH ₄ ) is about 1:1. To determine the ratio of steam to carbon (S:C) and hydrogen to methane (H ₂ :CH ₄ ), the natural gas is assumed to be 100% methane (CH ₄ ). Figure 3 demonstrates the change in the S:C ratio from about 1.4:1 to about 1.2:1.

FIG 4 a graph of the battery voltage based SOFC within about 85 hours of the time interval, wherein the external reforming SOFC natural gas as fuel and the hydrogen (H ₂₎ introduced into the fuel inlet operate in conjunction with the fuel, wherein the hydrogen and methane The ratio of (H ₂ :CH ₄ ) is about 1:1. To determine the ratio of steam to carbon (S:C) and hydrogen to methane (H ₂ :CH ₄ ), the natural gas is assumed to be 100% methane (CH ₄ ). Figure 4 demonstrates the change in cell voltage as the S:C ratio changes from about 1.4:1 to about 1.2:1.

Figure 5 is about 205 hours at a time interval of SOFC S: C ratio versus the time in which the fuel inlet of the SOFC with hydrogen and methane internal reforming operation, wherein the H _2: CH ₄ ratio is from about 1: 1. Figure 5 demonstrates the change in the S:C ratio from about 1.8:1 to about 1.6:1.

Figure 6 is about 205 hours at a time interval within the SOFC cell voltage versus the time of which the SOFC fuel to the inlet of the hydrogen and methane internal reforming operation, wherein the H _2: CH ₄ ratio of about 1: 1. Figure 6 demonstrates the change in cell voltage as the S:C ratio changes from about 1.8:1 to about 1.6:1.

7 and 8 are schematic illustrations of an exemplary fuel cell system that can be operated in accordance with the methods of the present invention.

9A and 9B are three-dimensional views, respectively, of an air side and a fuel side of an interconnect in accordance with an embodiment of the present invention.

10A and 10B are top views of an anode side of a fuel cell according to an embodiment of the present invention.

11A-Y are images of a fuel inlet riser opening of a fuel cell in a stack of 25 fuel cells operating in various anode/cathode geometries. The details are presented in Example 6.

(no component symbol description)

Claims (23)

A method of operating a fuel cell system, comprising: introducing a fuel mixture comprising hydrogen, fuel, and steam at a fuel inlet of a fuel cell of the fuel cell system, the fuel mixture including external reforming of a fuel inlet connected to the fuel cell And outputting the fuel cell system to generate electricity; wherein the ratio of hydrogen to carbon in the fuel (H ₂ :C _fuel ) is 0.25:1 to 3 in the fuel mixture introduced at the fuel inlet of the fuel cell The range of :1 includes 0.25:1 and 3:1; and the ratio of steam to carbon (S:C) in the fuel mixture introduced at the fuel inlet of the fuel cell is less than 2:1. The method of claim 1, wherein the fuel comprises a fuel comprising carbon and hydrogen. The method of claim 2, wherein the fuel is selected from the group consisting of methane, natural gas, propane, alcohol, or syngas obtained from coal or natural gas reforming. The method of claim 1, wherein the fuel cell system is a solid oxide fuel cell system. The method of claim 4, wherein the fuel cell system comprises a solid oxide fuel cell system having an internal reformer. The method of claim 1, wherein the S:C ratio is less than or equal to 1.2:1. The method of claim 1, wherein the H ₂ :C _fuel ratio is in the range of 0.5:1 to 1.5:1. The method of claim 1, wherein the source of hydrogen is a vented anode exhaust. The method of claim 7, wherein the anode exhaust system of the cycle is up to 70% of the total anode exhaust. The method of claim 9, wherein the anode exhaust gas of the cycle accounts for 50% to 70% of the total anode exhaust gas. The method of claim 7, further comprising converting the vapor in all or a portion of the anode exhaust to hydrogen by using a water gas shift reactor prior to reintroducing all or a portion of the reformed anode exhaust into the fuel cell system. A method of operating a fuel cell system, comprising: introducing a fuel mixture comprising steam at a fuel inlet of a fuel cell of the fuel cell system; and operating the fuel cell system to generate electricity; wherein fuel of the fuel cell in the fuel cell system The fuel mixture introduced at the inlet comprises a partially reformed hydrocarbon fuel and a recycled anode fuel, the partially reformed hydrocarbon fuel being from an external reformer connected to a fuel inlet of the fuel cell; and in the fuel mixture The steam to carbon ratio (S: C) is 1:1 or higher and less than 2:1, wherein a portion of the hydrocarbon fuel reformed in the external reformer is 5% to 5% after introduction into the fuel inlet 15%. The method of claim 12, wherein the reforming a portion of the fuel system is carried out using a catalytic reactor. The method of claim 12, wherein the anode exhaust system of the cycle is up to 70% of the total anode exhaust. The method of claim 12, wherein the average battery power of the fuel cell system The pressure is increased compared to the operation of a fuel cell system operating at S:C greater than or equal to 2:1. The method of claim 12, further comprising converting the vapor in the anode exhaust gas to hydrogen using a water gas shift reactor prior to reintroducing the anode exhaust gas into the fuel cell system. The method of claim 12, wherein the mixture comprises carbon monoxide, carbon dioxide, water vapor, hydrogen, and fuel, and the percentage of the fuel in the mixture is less than or equal to 20%. The method of claim 12, wherein the fuel comprises methane or natural gas, and the percentage of the fuel in the mixture is in the range of from about 13% to about 19%. A solid oxide fuel cell stack comprising: a plurality of solid oxide fuel cells and a plurality of interconnects, each of the plurality of fuel cells comprising: a solid oxide electrolyte having a first surface and a a second surface, a fuel inlet riser opening extending through the electrolyte; an anode electrode on the first surface of the electrolyte suitable for internal fuel reforming; and a cathode electrode on the second surface of the electrolyte, wherein the cathode An electrode is offset from at least a first side of the fuel inlet riser opening to receive a seal that surrounds the second surface of the electrolyte rather than a fuel inlet riser opening on the first surface of the electrolyte; wherein the anode The electrode and the cathode electrode are symmetrical in a region where the fuel enters the fuel cell, the region includes a cathode electrode offset region, and the anode electrode is offset from the fuel inlet riser opening by at least the first a side such that an outer edge of the anode electrode in the anode electrode offset region is precisely or substantially aligned with an outer edge of the cathode electrode in the cathode electrode offset region on the opposite side of the electrolyte, and the solid oxide fuel The battery stack is used for the internal manifold of the fuel. A stack of claim 19, wherein the anode and cathode are symmetrical throughout their area. A stack of claim 19, wherein the zone in which fuel enters the fuel cell comprises a zone adjacent the fuel inlet riser opening in the electrolyte, and the anode electrode is offset from the fuel inlet riser opening by at least 4 mm. A stack of claim 21, wherein the anode electrode is located above a fuel distribution plenum adjacent the interconnect. A stack of claim 21, wherein the anode electrode is not located above the fuel distribution plenum adjacent the interconnect.