CERIUM OXIDE TREATMENT OF FUEL CELL COMPONENTS

A method of treating a fuel cell system balance of plant component including coating the component with a slurry comprising CeO2, Y2O3 and/or HfO2 particles in a liquid, thereby forming a slurry coated component, followed by removing the liquid.

FIELD

The present invention is directed to fuel cell systems, specifically to components treated with cerium oxide.

BACKGROUND

Fuel cells, such as solid oxide fuel cells, are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. High temperature fuel cells include solid oxide and molten carbonate fuel cells. These fuel cells may operate using hydrogen and/or hydrocarbon fuels. There are classes of fuel cells, such as the solid oxide regenerative fuel cells, that also allow reversed operation, such that oxidized fuel can be reduced back to unoxidized fuel using electrical energy as an input.

SUMMARY

An embodiment is drawn to a method of treating a fuel cell system balance of plant component including coating the component with a slurry comprising at least one of CeO2, Y2O3and HfO2particles in a liquid, thereby forming a slurry coated component and removing the liquid.

Another embodiment is drawn to a fuel cell system balance of plant component, comprising a metal alloy fuel cell system balance of plant component which does not include ceria, and a Cr2O3and CeO2containing mixed oxide coating having 0.01-0.05 wt. % CeO2located on a surface of the metal alloy fuel cell system balance of plant component.

Another embodiment is drawn to a method of coating a fuel cell system balance of plant component, comprising coating a Cr containing fuel cell system balance of plant component with a coating comprising CeO2, and annealing the component to form a thermally grown mixed oxide coating containing Cr2O3and CeO2having 0.01-0.05 wt. % CeO2on the component.

DETAILED DESCRIPTION

The embodiments of the invention provide coated fuel cell hot box components, e.g. balance of plant components, which improve the longevity of fuel cell system components. In an embodiment, the coating comprises cerium oxide. Embodiments include coating one or more of the following balance of plant components with a cerium oxide containing coating: anode exhaust cooler heat exchanger with “finger plates”, cathode recuperator uni-shell, steam generator, anode hub structure, anode tail gas oxidizer (ATO), skirt, mixer, etc. as will be described in more detail below.

Many metallic (i.e., metal alloy) components of a solid oxide fuel cell (“SOFC”) system typically require sustained use at 850° C. or higher temperature in moisture and carbon containing oxidation environments. Commercially available alloys for these applications are chromium oxide scale formers which are typically used in uncoated condition. The basic mechanism of degradation of components made from conventional high temperature chromium, nickel and iron containing alloys is repeated cracking and spalling of the protective chromium oxide. This spalling eventually depletes the alloy's chromium level to the extent that it ceases to form a protective chromium oxide scale. Instead, faster growing iron and nickel oxides start forming which eventually causes thinning of the materials to an unacceptable level. The problem is particularly bad with very thin cross sections that are typically used for heat transfer fins. Due to lower starting thickness, the total reservoir of chromium is already low. Thin sections are depleted of chromium sooner than a thicker cross section of the same composition and therefore lose their ability to form a protective scale more quickly than the thick section. Alumina forming alloys can provide longer high temperature life. However, alumina forming alloys suffer from poor performance with respect to certain fabrication processes, such as brazing. High temperature resistance coatings used in more demanding applications such as aerospace, are typically too expensive for SOFC applications and may not be suitable with fabrication processes such as brazing and welding.

In one embodiment, a balance of plant component for a fuel cell system comprises an iron, chromium or nickel based metal alloy which contains at least 15 weight percent (“wt. %”) chromium (Cr), such as 20 wt. % Cr or greater (e.g., 20 to 30 wt. % Cr). The heat exchanger and other balance of plant components used in solid oxide fuel cell systems are made from chromium containing alloys such as stainless steels, Inconel 800™ alloy and Inconel 625™ alloy. These alloys achieve their protection by selective oxidation of chromium at elevated temperature. However, many of these alloys, especially the iron based alloys, exhibit high degradation in service environments at 800° C. or higher temperatures. The stresses caused by oxide growth and thermal cycling cause the protective scale to flake off, exposing the alloy underneath. In time, under isothermal or cyclic conditions, the alloys start forming faster growing and non-protective oxides of nickel and iron.

In one embodiment, these components are coated with a CeO2containing slurry to make the protective chromium oxides adhere better, last longer and reduce component corrosion. The slurry may include a carrier liquid, such as water, ethanol, etc. and a solid comprising CeO2and optionally other solids.

In one non-limiting example, a CeO2slurry made with an ethanol solution was applied to Alloy 800™ coupons.FIGS. 1A and 1Bcompares cross sections of the Alloy 800 coupons with (FIG. 1B) and without (FIG. 1A) the CeO2application after 1100 hours of oxidation in air at 850° C. As can be seen by comparingFIGS. 1A and 1B, the oxide scale morphology developed on the CeO2coated samples has three characteristics of high performance alloys not exhibited in the uncoated coupon: (a) good scale adherence, (b) lower scale thickness, and (c) less internal oxidation. The untreated sample has large voids at the oxide scale interface that can lead to oxide spallation. On the other hand, the scale formed on the CeO2treated sample appears to be in intimate contact with the underlying alloy without any separation or significant voids. The scale on the CeO2treated samples is approximately 25% less in thickness. Also, the untreated alloy samples show much deeper internal oxidation zones. The solid CeO2in the slurry may comprise a CeO2powder, such as a powder having an average particle size of 1-5 microns which forms a 1-10 micron thick CeO2layer on the surface of the balance of plant component.

In another non-limiting example, a water based CeO2slurry was applied on sections of a heat exchanger made of Alloy 800™. The heat exchanger had been in service for six to eight months operating at a temperature of approximately 800° C.FIGS. 2A and 2Bcompare the cross sections of the samples with (FIG. 2A) and without (FIG. 2B) CeO2coating after another 3000 hours of oxidation at 850° C. in an air furnace. The oxide scale on the sample with CeO2was mainly Cr2O3and essentially uniform in thickness. On the other hand, the oxide on the sample without CeO2developed nodular oxides of iron and nickel in addition to Cr2O3. This oxide is not protective and tends to spall.

In yet another non-limiting example, a CeO2slurry was applied on sections of an Alloy 800™ heat exchanger that had been in service for two years at approximately 800° C. The heat exchanger had developed thick Cr, Ni and Fe oxides on the surface prior to the application of the CeO2slurry. Metallographic cross sections of samples with and without CeO2coating after another three thousand hours of oxidation at 850° C. are illustrated inFIGS. 3B and 3A, respectively. The porosity at the scale/metal interface was significantly reduced in the coated example (FIG. 3B) relative to the uncoated sample (FIG. 3A). In addition, CeO2coating resulted in a reduction of the formation of an Fe rich phase in the top layer of the oxide scale as determined by an energy-dispersive X-ray (EDX) analysis.

Embodiments include the application of a cerium oxide slurry to new and field returned components. In an embodiment, the cerium oxide slurry is applied just prior to putting the field components in service. In both cases, at least a part of applied CeO2from the slurry is preferably incorporated into the thermally grown oxide (TGO) on the alloy. Sufficient CeO2results in an alteration of the chromium oxide growth mechanisms which results in longer lasting protective oxide coatings on the components.

The present inventors believe that during thermal oxidation of non-CeO2coated components, oxygen from the ambient may react with underlying chromium metal to form chromium oxide with inferior adhesion properties. Without wishing to be bound by a particular theory, the present inventors believe that during high temperature oxidation a small percentage of cerium oxide (e.g., 0.01 to 0.5 wt. % cerium oxide) may get incorporated into thermally grown Cr2O3which results in slower growth rate and improved adhesion to the component than a pure chromium oxide coating formed by oxidation of a chromium alloy. The inventors believe the addition of a coating comprising CeO2also protects the underlying component because of lower consumption chromium from the alloy to the oxide formation.

In embodiments, the slurries are made using CeO2powder mixed with a liquid carrier, such as ethanol or water. The CeO2powder may have a concentration ranging from 10 to 50 wt. %, such as 20-40 wt. %, such as 25-35 wt. % of the slurry. The CeO2particle size may be in a range of 1-20 microns, such as 1-15 microns, such as 1-5 microns. Finer particle sizes may also be used. A slurry coating is preferred. However, a dry coating may be used as an alternative.

In an embodiment of the method, the component is coated with the slurry comprising CeO2particles in a liquid, thereby forming a slurry coated component. The slurry coated component may then be dried (e.g., heated or left to dry at room temperature) to remove the liquid. Upon further heating, the CeO2particles may be incorporated into a thermally grown oxide (TGO) on the component. The end product is mixed oxide containing at least Cr2O3and CeO2and optionally other elements or oxides, on a component containing at least 15 wt. % chromium, in which the mixed oxide has a concentration of 0.01-0.5 wt. % CeO2and remainder chromium oxide and optionally less than 10 wt. % other elements or oxides.

A balance of plant component for a fuel cell system can be coated. The component may be formed by any suitable metal fabrication method, such as casting, forging, rolling, etc. The balance of plant components used in solid oxide fuel cell systems are typically made from chromium containing alloys such as stainless steels, Inconel 800™ alloy and Inconel 625™ alloy.

Inconel 625 alloy may have the composition ranges (in weight percent) shown in Table I below.

The balance of plant component can then be placed into a solid oxide fuel cell system containing a plurality of solid oxide fuel cell stacks, as described below. The balance of plant component may comprise a heat exchanger in one embodiment, such as the fins of the heat exchanger. However, any other suitable metal alloy balance plant component may be made of the above alloys. For example, any metal alloy balance plant component described in U.S. Pat. No. 8,563,180 (issued Oct. 22, 2013) and incorporated herein by reference in its entirety, and which are also illustrated inFIGS. 4A to 14of the present application and described below may be made of the above alloys. A non-limiting list of balance of plant components (i.e., components other than the fuel cells and the interconnects of the fuel cell stack) includes anode exhaust cooler, anode tail gas oxidizer, anode exhaust manifold, anode feed/return assembly, baffle plate, exhaust conduits, cathode recuperator, anode recuperator, heat shield, steam generator, bellows, anode hub structure, anode tail gas oxidizer skirt, anode tail gas oxidizer mixer, cathode exhaust swirl element and finger plates described below.

In alternative embodiments, Y2O3and/or HfO2may be added in addition to CeO2and/or used instead of CeO2. In these alternative embodiments, Y2O3and/or HfO2powders may be added to the slurry in addition to CeO2and/or instead of CeO2and coated on the surface of the component. In these embodiments, the mixed oxide coating may include 0.01 to 0.05 wt. % Y2O3and/or HfO2in addition to CeO2and/or instead of CeO2.

Anode Exhaust Cooler Heat Exchanger

It is desirable to increase overall flow conditions and rates of the fluids (e.g., fuel and air inlet and exhaust streams) in the hot box. According to an embodiment, a CeO2coated anode exhaust cooler heat exchanger with “finger plates” facilitates these higher overall flow conditions. For example, the finger plates and/or corrugated sheet can be coated with CeO2. An anode cooler heat exchanger is a heat exchanger in which the hot fuel exhaust stream from a fuel cell stack exchanges heat with a cool air inlet stream being provided to the fuel cell stack (such as a SOFC stack). This heat exchanger is also referred to as an air pre-heater heat exchanger in U.S. application Ser. No. 12/219,684 filed on Jul. 25, 2008 and Ser. No. 11/905,477 filed on Oct. 1, 2007, both of which are incorporated herein by reference in their entirety.

An exemplary anode exhaust cooler heat exchanger100is illustrated inFIGS. 4A-4B and 5A. Embodiments of the anode exhaust cooler heat exchanger100include two “finger” plates102a,102bsealed on opposite ends of a corrugated sheet104, as shown inFIG. 4A. The corrugated sheet104may have a cylindrical shape (i.e., a cylinder with a corrugated outer wall) and the finger plates102a,102bare located on the opposite ends of the cylinder. That is, the peaks and valleys of the corrugations may be aligned parallel to the axial direction of the cylinder with the finger plates102a,102bdesigned to cover alternating peaks/valleys. Other shapes (e.g., hollow rectangle, triangle or any polygon) are also possible for the sheet104. The finger plates comprise hollow ring shaped metal plates which have finger shaped extensions which extend into the inner portion of the ring. The plates102a,102bare offset from each other by one corrugation, such that if the fingers of top plate102acover every inward facing recess in sheet104, then bottom plate102bfingers cover every outward facing recess in sheet104(as shown inFIG. 4Bwhich illustrates an assembled heat exchanger100), and vise-versa. The shape of each finger is configured to cover one respective recess/fin/corrugation in sheet104. The fingers may be brazed to the sheet104.

The corrugations or fins of the sheet104may be straight as shown inFIGS. 4A and 5Cor wavy as shown inFIG. 4B. The wavy corrugations are corrugations which are not straight in the vertical direction. Such wavy corrugations are easier to manufacture.

The use of the finger plates102a,102bis not required. The same function could be achieved with the use of flat cap rings or end caps102cthat are brazed to the top/bottom of the corrugated sheet104, as shown inFIG. 4D. The advantage of the finger plate102a,102bdesign is that it allows for axial gas flow entry and/or exit to and from the corrugated sheet104, as shown schematically by the arrows inFIG. 4C. In contrast, as shown inFIG. 4D, the cap ring(s)102crequire the gas flow to enter and/or exit non-axially to and from the corrugated sheet104and then turn axially inside the corrugated sheet104which results in an increased pressure drop. The anode cooler heat exchanger100may be fabricated with either the finger plates102a,102bor the end caps102clocated on either end or a combination of both. In other words, for the combination of finger plate and end cap, the top of the corrugated sheet104may contain one of finger plate or end cap, and the bottom of the corrugated sheet104may contain the other one of the finger plate or end cap.

Hot and cold flow streams1131,1133flow in adjacent corrugations, where the metal of the corrugated sheet104separating the flow streams acts as a primary heat exchanger surface, as shown inFIG. 5A, which is a top cross sectional view of a portion of sheet104. The sheet104may be relatively thin, such as having a thickness of 0.005 to 0.003 inches, for example 0.012-0.018 inches, to enhance the heat transfer. For example, the hot fuel exhaust stream flows inside of the corrugated sheet104(including in the inner recesses of the corrugations) and the cold air inlet stream flows on the outside of the sheet104(including the outer recesses of the corrugations). Alternatively, the anode exhaust cooler heat exchanger may be configured so that the fuel exhaust flows on the outside and the air inlet stream on the inside of sheet104. The finger plates102aand102bprevent the hot and cold flows from mixing as they enter and exit the anode exhaust cooler heat exchanger.

One side (e.g., inner side) of the corrugated sheet is in fluid communication with a fuel exhaust conduit which is connected to the fuel exhaust of the solid oxide fuel cell stack and in fluid communication with an exhaust conduit from an anode recuperator heat exchanger which will be described below. The second side of the corrugated sheet is in fluid communication with an air inlet stream conduit which will be described in more detail below.

The air inlet stream into the anode exhaust cooler100may be directed toward the centerline of the device, as shown inFIG. 11C. Alternatively, the air inlet stream may have a full or partial tangential component upon entry into the device. Furthermore, if desired, an optional baffle plate101aor another suitable flow director device101bmay be located over the anode exhaust cooler100in the air inlet conduit or manifold33to increase the air inlet stream flow uniformity across the anode exhaust cooler100, as shown inFIGS. 5B-5D.

FIGS. 5B and 5Dillustrate a side cross sectional and semi-transparent three dimensional views, respectively, of the baffle plate101alocated over the anode exhaust cooler100in the air inlet conduit or manifold33. The baffle plate may comprise a cylindrical plate having a plurality of openings. The openings may be arranged circumferentially in one or more circular designs and each opening may have a circular or other (e.g., oval or polygonal) shape.

FIG. 5Cshows a flow director device101bwhich comprises a series of offset baffles101cwhich create a labyrinth gas flow path between the baffles, as shown by the curved line. If desired, the baffle plate101aopenings and/or the baffle101cconfigurations may have an asymmetric or non-uniform geometry to encourage gas flow in some areas of the anode exhaust cooler and restrict the gas flow in other areas of the anode exhaust cooler.

FIG. 5Dalso shows a roughly cylindrical air inlet conduit enclosure33ahaving an air inlet opening33b. The air inlet conduit or manifold33is located between the inner wall of enclosure33aand the outer wall of the annular anode exhaust conduit117, as shown in dashed lines inFIG. 5E. Enclosure33aalso surrounds the anode cooler100ato provide the air inlet stream passages between the corrugations of the corrugated sheet104and the inner wall of enclosure33a.

FIGS. 5D, 5E and 5Falso show the fuel inlet conduit29which bypasses the anode exhaust cooler through the central hollow space in the anode exhaust cooler100.FIG. 5Eis a three dimensional view andFIG. 5Fis a three dimensional cut-away view of the device. As shown inFIGS. 5E-5F, the cylindrical corrugated sheet104and the disc shaped finger plates (e.g.,102b) of the anode cooler100have a hollow space in the middle. The fuel inlet conduit29and annular thermal insulation100A are located in this hollow space100b(shown inFIG. 4B). The annular thermal insulation100asurrounds the fuel inlet conduit29and thermally isolates conduit29from the annular anode cooler100, and the annular fuel (anode) exhaust conduit117which surround the insulation100a, as well as from the annular air inlet conduit or manifold33which surrounds the annular anode cooler100, and the annular fuel (anode) exhaust conduit117. Thus, the fuel inlet stream passes through the fuel inlet conduit29without substantial heat exchange with the gasses (i.e., fuel exhaust stream and air inlet stream) flowing through the anode cooler100, the fuel exhaust conduit117and the air inlet conduit or manifold33. If desired, the fuel inlet conduit may include an optional bellows29bwith flange29c, as shown inFIG. 5E.

As shown inFIGS. 5E-5F, the fuel inlet stream enters the device through the fuel inlet opening29awhich is connected to the fuel inlet conduit29. The vertical conduit29has a horizontal bridging portion connected to opening29awhich passes over the air inlet conduit and the fuel exhaust conduit117which are in fluid communication with the anode cooler100. Thus, the fuel inlet stream is fluidly and thermally isolated from the air inlet and fuel exhaust streams in and above the anode cooler100.

Embodiments of the anode exhaust cooler heat exchanger may have one or more of the following advantages: excellent heat exchange due to minimal material conduction losses between separated flow streams, very compact, light weight, reduced material requirements, reduced manufacturing costs, elimination of fixture requirements, reduced pressure drop, ability to control flow ratios between two or more flow streams by simply changing finger plate design. The duty of the anode exhaust cooler heat exchanger may be increased by 20-40% over the prior art heat exchanger. Further, in some embodiments, the anode exhaust cooler heat exchanger may also be shorter than the prior art heat exchanger in addition to having a higher duty.

The cathode recuperator is a heat exchanger in which the air inlet stream exchanges heat with the air (e.g., cathode) exhaust stream from the fuel cell stack. Preferably, the air inlet stream is preheated in the anode cooler described above before entering the cathode recuperator.

The mode of heat transfer through the prior art brazed two finned cylindrical heat exchanger is defined by that amount of conductive heat transfer that is possible through the brazed assembly of the heat exchange structure. The potential lack of heat transfer can cause thermal instability of the fuel cell system and also may not allow the system to operate at its rated conditions. The inventors realized that the use of a single fin flow separator improves the heat transfer between fluid streams and provides for a compact heat exchanger package.

An example uni-shell cathode recuperator200is illustrated inFIGS. 6A to 6G. In an embodiment, the three concentric and independent shells of the prior art structure replaced with a single monolithic assembly shown inFIGS. 6A-6B.FIG. 6Ashows an exploded three dimensional view of the assembly components without the heat shield insulation andFIGS. 6B and 6Cshow three dimensional views of the assembly with the components put together and the heat shield insulation202A,202B installed.

Embodiments of the uni-shell cathode recuperator200include a single cylindrical corrugated fin plate or sheet304(shown inFIGS. 6A and 6D). The corrugated plate or sheet304is preferably ring shaped, such as hollow cylinder. However, plate or sheet304may have a polygonal cross section when viewed from the top if desired. The corrugated plate or sheet304is located between inner202A and outer202B heat shield insulation as shown inFIG. 6C, which is a three dimensional view of the middle portion of the recuperator200,FIG. 6Dwhich is a top view of the plate or sheet304, and theFIG. 6Ewhich is a side cross sectional view of the recuperator200. The heat shield insulation may comprise hollow cylinders. The heat shield insulation may be supported by a heat shield shell204located below the corrugated plate or sheet304.

In addition to the insulation and the corrugated plate or sheet304, the uni-shell cathode recuperator200also includes a top cap, plate or lid302a(shown inFIG. 6A) and a similar bottom cap plate or lid (not shown inFIG. 6Afor clarity). As shown inFIGS. 6A, 6B, 6F and 6G, in addition to the top cap, plate or lid302a, the hot box may also include a heat shield306with support ribs below lid302a, a steam generator103comprising a baffle plate308with support ribs supporting a steam coil assembly310(i.e., the coiled pipe through which flowing water is heated to steam by the heat of the air exhaust stream flowing around the pipe), and an outer lid312with a weld ring313enclosing the steam generator103. A cathode exhaust conduit35in outer lid312exhausts the air exhaust stream from the hot box.

The single cylindrical corrugated fin plate304and top and bottom cap plates force the air (i.e., cathode) inlet stream12314and air (i.e., cathode) exhaust stream1227to make a non-zero degree turn (e.g., 20-160 degree turn, such as a 90 degree) turn into adjoining hollow fins of the fin plate304as shown inFIGS. 6F(side cross sectional view of the assembly) and6G (three dimensional view of the assembly). For example, the cathode or air inlet stream flows from the anode cooler100to the cathode recuperator200through conduit314which is located between the heat shield306and the top cap302a. The air inlet stream flows substantially horizontally in an outward radial direction (i.e., in to out radially) as shown by the arrows inFIGS. 6F and 6Guntil the stream impacts the inner surface of the upper portion of the corrugated fin plate304. The impact forces the stream to make a 90 degree turn and flow down (i.e., in an axial direction) in the inner corrugations. Likewise, the hot cathode exhaust stream shown by arrows inFIGS. 6F and 6Gfirst flows vertically from below through conduit27from the ATO and is then substantially horizontally in the end portion of conduit27in a substantially inward radial direction to impact the outer surface of the lower portions of the corrugated fin plate304. This causes the air exhaust stream to make a non-zero degree turn and flow up (i.e., in an axial direction) in the outer corrugations of plate304. This single layer fin plate304design allows for effective heat transfer and minimizes the thermal variation within the system (from the misdistribution of air).

The use of the cap plates in the cathode recuperator is not required. The same function could be achieved with the use of finger plates similar to finger plates102a,102billustrated for the anode cooler100. The cathode recuperator heat exchanger200may be fabricated with either the finger plates or the end caps located on either end or a combination of both. In other words, for the combination of finger plate and end cap, the top of the fin plate304may contain one of finger plate or end cap, and the bottom of the fins may contain the other one of the finger plate or end cap

Hot and cold flow streams flow in adjacent corrugations, where the metal of the corrugated plate or sheet304separating the flow streams acts as a primary heat exchanger surface, as shown inFIG. 6D, which is a top cross sectional view of a portion of plate or sheet304. For example, the relatively cool or cold air inlet stream12314flows inside of the corrugated plate or sheet304(including in the inner recesses of the corrugations) and the relatively warm or hot air exhaust stream1227flows on the outside of the plate or sheet304(including the outer recesses of the corrugations). Alternatively, the air inlet stream12314may flow on the inside and the hot air exhaust stream1227may flow on the outside of the corrugated plate or sheet304.

One side (e.g., outer side) of the corrugated plate or sheet304is in fluid communication with an air exhaust conduit27which is connected to the air exhaust of the solid oxide fuel cell stack and/or the ATO exhaust. The second side of the corrugated plate or sheet304is in fluid communication with a warm air output conduit314of the anode cooler100described above.

As shown inFIG. 6H, the air inlet stream1225exiting the cathode recuperator200may be directed towards the middle lengthwise portion of a fuel cell stack or column9to provide additional cooling in the otherwise hottest zone of the stack or column9. In other words, middle portion of the fuel cell stack or column9is relatively hotter than the top and bottom end portions. The middle portion may be located between end portions of the stack or column9such that each end portion extends 10-25% of the length of the stack or column9and the middle portion is 50-80% of the length of the stack or column9.

The location of the air inlet stream outlet210of the recuperator200can be tailored to optimize the fuel cell stack or column9temperature distributions. Thus, the vertical location of outlet210may be adjusted as desired with respect to vertically oriented stack or column9. The outlet210may comprise a circular opening in a cylindrical recuperator200, or the outlet210may comprise one or more discreet openings adjacent to each stack or column9in the system.

Since the air inlet stream (shown by dashed arrow inFIG. 6H) exiting outlet210is relatively cool compared to the temperature of the stack or column9, the air inlet stream may provide a higher degree of cooling to the middle portion of the stack or column compared to the end portions of the stack or column to achieve a higher temperature uniformity along the length of the stack of column. For example, the outlet210may be located adjacent to any one or more points in the middle 80%, such as the middle 50%, such as the middle 33% of the stack or column. In other words, the outlet210is not located adjacent to either the top or bottom end portions each comprising 10%, such as 25% such as 16.5% of the stack or column.

Embodiments of the uni-shell cathode recuperator200may have one or more of the following advantages: excellent heat exchange due to minimal material conduction losses between separated flow streams, very compact, light weight, reduced material requirements, reduced manufacturing costs, reduced pressure drop, provides dead weight as insurance for mechanical compression failure. This allows for easier assembly of the fuel cell system, reduced tolerance requirements and easier manufacturing of the assembly.

Thus, as described above, the anode cooler100and the cathode recuperator200comprise “uni-shell” heat exchangers where the process gases flow on the two opposing surfaces of a roughly cylindrical corrugated sheet. This provides a very short conductive heat transfer path between the streams. The hotter stream (e.g., anode exhaust and ATO exhaust streams in heat exchangers100,200, respectively) provides convective heat transfer to a respective large surface area corrugated metal separator sheet104,304. Conductive heat transfer then proceeds only through the small thickness of the separator (e.g., the thickness of the corrugated sheet104,304), and then convective heat transfer is provided from the sheet104,304to the cooler respective stream (e.g., the air inlet stream in both heat exchangers100,200).

The heat exchangers100,200differ in their approach to manifolding their respective process streams. The roughly cylindrical anode cooler100uses finger shaped apertures and finger plates102a,102bto allow a substantially axial entry of the process streams (i.e., the anode exhaust and air inlet streams) into the corrugated cylindrical section of the heat exchanger. In other words, the process streams enter the heat exchanger100roughly parallel (e.g., within 20 degrees) to the axis of the roughly cylindrical heat exchanger.

In contrast, the cathode recuperator200includes top and bottom caps302a, which require the process streams (e.g., the air inlet stream and ATO exhaust stream) to enter the heat exchanger200roughly perpendicular (e.g., within 20 degrees) to the axial direction of the heat exchanger200. Thus, heat exchanger200has a substantially non-axial process gas entry into the heat exchanger.

If desired, these manifolding schemes may be switched. Thus, both heat exchangers100,200may be configured with the axial process gas entry or non-axial process gas entry. Alternatively, heat exchanger200may be configured with the axial process gas entry and/or heat exchanger100may be configured with non-axial process gas entry.

Cathode Recuperator Uni-Shell with Ceramic Column Support and Bellows

In the prior fuel cell systems, it is difficult to maintain a continuous mechanical load on the fuel cell stacks or columns of stacks through the full range of thermal operating conditions. To maintain a mechanical load, the prior art systems rely on an external compression system. Embodiments of the present fuel cell system do not include an external compression system. The removal of the external compression system, however, can lead to a loss of mechanical integrity of the fuel cell columns. The inventors have realized, however, that the external compression system can be replaced by an internal compression system comprising either a spring loaded or gravity loaded system or a combination of both. The spring loaded system may comprise any suitable system, such as a system described U.S. patent application Ser. No. 12/892,582 filed on Sep. 28, 2010 and which is incorporated herein by reference in its entirety, which describes an internal compression ceramic spring, and/or or use the uni-shell bellow in conjunction with appropriately tailored thermal expansion of the column and uni-shell material.

In an embodiment shown inFIG. 7A, the uni-shell cathode recuperator200is located on top of one or more columns402to provide additional internal compression for the stack or column of stacks9. The weight of the recuperator200uni-shell cylinder(s) can act directly on the fuel cell columns9. With the added weight of the cylinders, the fuel cell columns can be prevented from lifting off the hot box base500and provide any required sealing forces. Any suitable columns402may be used. For example, the ceramic columns402described in U.S. application Ser. No. 12/892,582 filed on Sep. 28, 2010 and which is incorporated herein by reference in its entirety may be used.

As discussed in the above described application, the ceramic columns402comprise interlocked ceramic side baffle plates402A,402B,402C. The baffle plates may be made from a high temperature material, such as alumina, other suitable ceramic, or a ceramic matrix composite (CMC). The CMC may include, for example, a matrix of aluminum oxide (e.g., alumina), zirconium oxide or silicon carbide. Other matrix materials may be selected as well. The fibers may be made from alumina, carbon, silicon carbide, or any other suitable material. Any combination of the matrix and fibers may be used. The ceramic plate shaped baffle plates may be attached to each other using dovetails or bow tie shaped ceramic inserts as described in the Ser. No. 12/892,582 application. Furthermore, as shown inFIG. 7A, one or more fuel manifolds404may be provided in the column of fuel cell stacks9, as described in the Ser. No. 12/892,582 application.

Furthermore, an optional spring compression assembly406may be located over the fuel cell column9and link adjacent ceramic columns402which are located on the opposing sides of the column of fuel cell stacks9. The assembly406may include a ceramic leaf spring or another type of spring between two ceramic plates and a tensioner, as described in the Ser. No. 12/892,582 application. The uni-shell cathode recuperator200may be located on a cap408on top of the assembly406, which provides internal compression to the ceramic columns402and to the column of fuel cell stacks9.

As discussed above, in the prior fuel cell systems, it is difficult to maintain a continuous mechanical load on the fuel cell column through the full range of thermal operating conditions. In another embodiment, the inventors have realized, however, that by including a bellows206on the vertical cylinders, the weight of the cylinders can rest directly on the columns Thus, in another embodiment, as shown inFIGS. 6A and 7B, the uni-shell cathode recuperator200may contain a uni-shell (expansion) bellows206on its outer or heat shield shell204located below the corrugated fin plate304for additional coefficient of thermal expansion (CTE) matching to that of the stack columns. Furthermore, as shown inFIG. 10, two additional bellows850,852may be located in the anode inlet area and the anode tail-gas oxidizer (ATO) exhaust area near top of hot box for additional CTE matching.

The bellows206allows the cathode recuperator200cylinders (e.g.,204,304) to remain in contact with the fuel cell stack9columns throughout the thermal operating conditions. The bellows206are designed to deform during operations such that the forces induced during temperature increases overcome the strength of the bellows, allowing the main contact point to remain at the top of the fuel cell columns.

Embodiments of the recuperator uni-shell may have one or more of the following advantages: improved sealing of air bypass at the top of the columns and continuous load on the columns. The continuous load on the columns gives some insurance that even with failure of the internal compression mechanism there would still be some (vertical) mechanical load on the columns. The use of the expansion bellows206within the uni-shell assembly allows for the shell assembly to expand and contract independently from the main anode flow structure of the system, thereby minimizing the thermo-mechanical effects of the two subassemblies.

Cathode Exhaust Steam Generator Structure

One embodiment of the invention provides steam generator having an increased duty over that of the prior art steam generator yet having the same physical envelope. Further, steam generator coils have local effects on the flow distribution which subsequently carry down into the cathode recuperator and affect the temperature distribution of the entire hot box. Thus, the embodiments of the cathode exhaust steam generator are configured allow control over the cathode exhaust stream flow distribution.

In embodiments of the present invention, the steam generator coil310is located in the lid section (e.g., between inner and outer lids302A and312) of the cathode recuperator200to be closer to the higher grade fuel cell stack air or cathode exhaust waste heat, as shown inFIGS. 6A, 6F, 6G and 8. Alternatively, the steam generator103may alternatively be located in the exit plenum (vertical portion) of the cathode recuperator200.

The lid or exit plenum steam generator103location allows for a representative reduction in the coil length relative to the prior art. To counteract the effect of a varying pressure drop across the coiled sections, an exhaust baffle plate308may also be added to support the coil310(the baffle plate308and coil310are shown upside down inFIG. 8compared toFIG. 6Afor clarity). Support ribs309hold the coil310in place under the baffle plate308. The steam coil310may be a partially or fully corrugated tube or a straight tube which has a smaller diameter near the water inlet conduit30A than near the steam outlet conduit30B. The steam coil310may have any suitable shape, such as a spiral coil, or one or more coils with one or more U-turns (i.e., a coil having at least two sections that are bent at an angle of 320-360 degrees with respect to each other). The U-turns for successive passes of the coil may be aligned or shifted with respect to teach other.

As shown inFIG. 6F, the baffle plate308forces the air exhaust stream1227travelling substantially vertically in an axial direction from the cathode recuperator200through conduit119to the steam generator103to make an additional pass around the coils310in the substantially horizontal, inward radial direction before exiting the hot box through the cathode exhaust conduit35. The cathode exhaust stream travels through the steam generator103in a space between plate302A and baffle plate308when the coils310are attached to the bottom of the baffle plate308and/or in a space between the baffle plate308and outer plate312when the coils310are attached to the top of the baffle plate308. The additional pass provides for a uniform flow distribution across the surface of the corrugated steam coil310and within the cathode recuperator200.

Embodiments of the steam generator103may have one or more of the following advantages: utilization of higher grade heat, more compact relative to the prior art, easy to manufacture, improved flow distribution.

In prior art fuel cell systems, the level of pre-reformation of the fuel prior to hitting the fuel cell may need to be fine tuned depending on the source of the fuel and respective compositions. The prior art steam methane reformer (SMR) includes a flat tube with flat catalyst coated inserts. In the prior at design, there is significant flow length available to accommodate a significant amount of catalyst should the need arise. In embodiments of the present invention, there is a limited amount of flow length available for catalyst placement. The limited amount of flow length reduces the overall flow path length of the fuel, thus reducing the pressure drop and mechanical design complexity needed to have multiple turn flow paths.

In one embodiment of the present invention, the reformer catalyst137A is provided into the fuel inlet side of the anode recuperator (e.g., fuel heat exchanger)137in which the fuel exhaust stream is used to heat the fuel inlet stream. Thus, the anode recuperator is a combined heat exchanger/reformer.

For a vertical/axial anode recuperator137shown inFIG. 15A, the SMR reformation catalyst (e.g., nickel and/or rhodium)137A may be provided along the entire length of the fuel inlet side of the recuperator137or just in the lower portion of the fuel inlet side of the recuperator. It could also be comprised of a separate item following the exhaust of the heat exchanger. It is believed that the primary reformation occurs at the bottom of the fuel inlet side of the anode recuperator. Thus, the only heat provided to the fuel inlet stream in the catalyst137A containing portion of the anode recuperator137to promote the SMR reaction is from the heat exchange with the fuel exhaust stream because the anode recuperator is thermally isolated from the ATO10and stacks9by the insulation10B shown inFIGS. 9A, 10, 11B and 12B.

Should additional catalyst activity be desired, a catalyst coated insert can be inserted into the fuel feed conduits21just prior to the fuel cell stacks9. The fuel feed conduits21comprise pipes or tubes which connect the output of the fuel inlet side of the anode recuperator137to the fuel inlet of the fuel cell stacks or columns9. The conduits21may be positioned horizontally over the hot box base500, as shown inFIG. 9Aand/or vertically over the hot box base500, as shown inFIG. 9B. This catalyst is a supplement or stand alone feature to the catalyst coated fin at the bottom of the anode recuperator137. If desired, the catalyst may be placed in less than 100% of the fuel feed conduits (i.e., the catalyst may be placed in some but not all conduits21and/or the catalyst may be located in only a part of the length of each or some of the conduits). The placement of the SMR catalyst at the bottom of the hot box may also act as a temperature sink for the bottom modules.

FIGS. 9C and 9Dillustrate embodiments of catalyst coated inserts1302a,1302bthat may be used as anode recuperator/pre-reformer137tube insert catalyst or as inserts in conduits21. The catalyst coated insert1302ahas a generally spiral configuration. The catalyst coated insert1302bincludes a series of generally parallel wire rosettes1304.

Embodiments of the pre-reformer tube-insert catalyst may have one or more of the following advantages: additional reformation length if desired and the ability to place endothermic coupling with the bottom module of the column should the bottom modules be hotter than desired.

Anode Flow Structure and Flow Hub

FIG. 10illustrates the anode flow structure according to one embodiment of the invention. The anode flow structure includes a cylindrical anode recuperator (also referred to as a fuel heat exchanger)/pre-reformer137, the above described anode cooler (also referred to as an air pre-heater) heat exchanger100mounted over the anode recuperator, and an anode tail gas oxidizer (ATO)10.

The ATO10comprises an outer cylinder10A which is positioned around the inner ATO insulation10B/outer wall of the anode recuperator137. Optionally, the insulation10B may be enclosed by an inner ATO cylinder10D, as shown inFIG. 12B. Thus, the insulation10B is located between the outer anode recuperator cylinder and the inner ATO cylinder10D. An oxidation catalyst10C is located in the space between the outer cylinder10A and the ATO insulation10B (or inner ATO cylinder10D if present). An ATO thermocouple feed through1601extends through the anode exhaust cooler heat exchanger100and the cathode recuperator200to the top of the ATO10. The temperature of the ATO may thereby be monitored by inserting a thermocouple (not shown) through this feed through1601.

An anode hub structure600is positioned under the anode recuperator137and ATO10and over the hot box base500. The anode hub structure is covered by an ATO skirt1603. A combined ATO mixer801/fuel exhaust splitter107is located over the anode recuperator137and ATO10and below the anode cooler100. An ATO glow plug1602, which aids the oxidation of the stack fuel exhaust in the ATO, may be located near the bottom of the ATO. Also illustrated inFIG. 10is a lift base1604which is located under the fuel cell unit. In an embodiment, the lift base1604includes two hollow arms with which the forks of a fork truck can be inserted to lift and move the fuel cell unit, such as to remove the fuel cell unit from a cabinet (not shown) for repair or servicing.

FIG. 11Aillustrates an anode flow hub structure600according to an embodiment. The hub structure600is used to distribute fuel evenly from a central plenum to plural fuel cell stacks or columns. The anode flow hub structure600includes a grooved cast base602and a “spider” hub of fuel inlet tubes (e.g., pipes)21and outlet tubes (e.g., pipes)23A. Each pair of tubes21,23A connects to one of the plurality of stacks or columns. Anode side cylinders (e.g., anode recuperator137inner and outer cylinders and ATO outer cylinder10A) are then welded or brazed into the grooves in the base602creating a uniform volume cross section for flow distribution, as shown inFIGS. 11B, 11C and 12, respectively. The “spider” inlet fuel tubes21and fuel outlet tubes23A run from the anode flow hub600out to the stacks where they are welded to vertical fuel rails. The anode flow hub600may be created by investment casting and machining and is greatly simplified over the prior art process of brazing large diameter plates.

As shown inFIGS. 11B and 11C(side cross sectional views) and11D (top cross sectional view) the anode recuperator137includes an inner cylinder139, a corrugated finger plate or cylinder137B and an outer cylinder137C coated with the ATO insulation10B.FIG. 11Bshows the fuel inlet flow1729from fuel inlet conduit29which bypasses the anode cooler100through its hollow core, then between the cylinders139and137B in the anode recuperator137and then to the stacks or columns9(flow1721) (shown also inFIG. 14) through the hub base602and conduits21.FIG. 11Cshows the fuel exhaust flow1723A from the stacks or columns9through conduits23A into the hub base602, and from the hub base602through the anode recuperator137between cylinders137B and137C into the splitter107. One part of the fuel exhaust flow stream from the splitter107flows through the above described anode cooler100while another part flows from the splitter107into the ATO10. Anode cooler inner core insulation100A may be located between the fuel inlet conduit29and the bellows852/supporting cylinder852A located between the anode cooler100and the ATO mixer801, as shown inFIGS. 10, 11B and 11C. This insulation minimizes heat transfer and loss from the anode exhaust stream in conduit31on the way to the anode cooler100. Insulation100A may also be located between conduit29and the anode cooler100to avoid heat transfer between the fuel inlet stream in conduit29and the streams in the anode cooler100. Furthermore, additional insulation may be located around the bellows852/cylinder852A (i.e., around the outside surface of bellows/cylinder) if desired.

FIG. 11Calso shows the air inlet stream from the air inlet conduit or manifold33through the anode cooler100(where it exchanges heat with the fuel exhaust stream) and into the cathode recuperator200described above.

Embodiments of the anode flow hub600may have one or more of the following advantages: lower cost manufacturing method, ability to use fuel tube in reformation process if required and reduced fixturing.

ATO Air Swirl Element

In another embodiment of the invention, the present inventors realized that in the prior art system, the azimuthal flow mixing could be improved to avoid flow streams concentrating hot zones or cold zones on one side of the hot box1. Azimuthal flow as used herein includes flow in angular direction that curves away in a clockwise or counterclockwise direction from a straight line representing a radial direction from a center of a cylinder to an outer wall of the cylinder, and includes but is not limited to rotating, swirling or spiraling flow. The present embodiment of the invention provides a vane containing swirl element for introducing swirl to the air stream provided into the ATO10to promote more uniform operating conditions, such as temperature and composition of the fluid flows.

As shown inFIGS. 12A, 12B and 12C, one embodiment of an ATO mixer801comprises a turning vane assembly which moves the stack air exhaust stream heat azimuthally and/or radially across the ATO to reduce radial temperature gradients. The cylindrical mixer801is located above the ATO10and may extend outwardly past the outer ATO cylinder10A. Preferably, the mixer801is integrated with the fuel exhaust splitter107as will be described in more detail below.

FIG. 12Bis a close up, three dimensional, cut-away cross sectional view of the boxed portion of the ATO10and ATO mixer801shown inFIG. 12A.FIG. 12Cis a three dimensional, cut-away cross sectional view of the integrated ATO mixer801/fuel exhaust splitter107.

As shown inFIG. 12A, the turning vane assembly ATO mixer801may comprise two or more vanes803(which may also be referred to as deflectors or baffles) located inside an enclosure805. The enclosure805is cylindrical and contains inner and outer surfaces805A,805B, respectively (as shown inFIG. 12C), but is generally open on top to receive the cathode exhaust flow from the stacks9via air exhaust conduit or manifold24. The vanes803may be curved or they may be straight. A shape of turning vane803may curve in a golden ratio arc or in catenary curve shape in order to minimize pressure drop per rotation effect.

The vanes803are slanted (i.e., positioned diagonally) with respect to the vertical (i.e., axial) direction of the ATO cylinders10A,10D, at an angle of 10 to 80 degrees, such as 30 to 60 degrees, to direct the cathode exhaust1824in the azimuthal direction. At the base of each vane803, an opening807into the ATO10(e.g., into the catalyst10C containing space between ATO cylinders10A and10D) is provided. The openings807provide the cathode exhaust1824azimuthally from the ATO mixer801into the ATO as shown inFIG. 12C. While the ATO mixer801is referred to as turning vane assembly, it should be noted that the ATO mixer801does not rotate or turn about its axis. The term “turning” refers to the turning of the cathode exhaust stream1824in the azimuthal direction.

The ATO mixer801may comprise a cast metal assembly. Thus, the air exits the fuel cell stacks it is forced to flow downwards into the ATO mixer801. The guide vanes803induce a swirl into the air exhaust stream1824and direct the air exhaust stream1824down into the ATO. The swirl causes an averaging of local hot and cold spots and limits the impact of these temperature maldistributions. Embodiments of the ATO air swirl element may improve temperature distribution which allows all stacks to operate at closer points, reduced thermal stress, reduced component distortion, and longer operating life.

Prior art systems include a separate external fuel inlet stream into the ATO. One embodiment of the present provides a fuel exhaust stream as the sole fuel input into the ATO. Thus, the separate external ATO fuel inlet stream can be eliminated.

As will be described in more detail below and as shown inFIGS. 11C and 12C, the fuel exhaust stream1823B exiting the anode recuperator137through conduit23B is provided into splitter107. The splitter107is located between the fuel exhaust outlet conduit23B of the anode recuperator137and the fuel exhaust inlet of the anode cooler100(e.g., the air pre-heater heat exchanger). The splitter107splits the fuel exhaust stream into two streams. The first stream18133is provided to the ATO10. The second stream is provided via conduit31into the anode cooler100.

The splitter107contains one or more slits or slots133shown inFIGS. 12B and 12C, to allow the splitter107functions as an ATO fuel injector. The splitter107injects the first fuel exhaust stream18133in the ATO10through the slits or slots133. A lip133A below the slits133and/or the direction of the slit(s) force the fuel into the middle of the air exhaust stream1824rather than allowing the fuel exhaust stream to flow along the ATO wall10A or10D. Mixing the fuel with the air stream in the middle of the flow channel between ATO walls10A and10D allows for the highest temperature zone to be located in the flow stream rather than on the adjacent walls. The second fuel exhaust stream which does not pass through the slits133continues to travel upward into conduit31, as shown inFIG. 11C. The amount of fuel exhaust provided as the first fuel exhaust stream into the ATO through slits133versus as the second fuel exhaust stream into conduit31is controlled by the anode recycle blower123speed (seeFIGS. 11C and 14). The higher the blower123speed, the larger portion of the fuel exhaust stream is provided into conduit31and a smaller portion of the fuel exhaust stream is provided into the ATO10, and vice-versa.

Alternate embodiments of the ATO fuel injector include porous media, shower head type features, and slits ranging in size and geometry.

Preferably, as shown inFIG. 12C, the splitter107comprises an integral cast structure with the ATO mixer801. The slits133of the splitter are located below the vanes803such that the air exhaust stream which is azimuthally rotated by the vanes while flowing downward into the ATO10provides a similar rotation to the first fuel exhaust stream passing through the slits133into air exhaust steam in the ATO. Alternatively, the splitter107may comprise a brazed on ring which forms the ATO injector slit133by being spaced apart from its supporting structure.

Stack Electrical Terminals and Insulation

The prior art system includes current collector rods that penetrate the anode base plate and the hot box base plates through several feedthroughs. Each feed through has a combination of ceramic and metallic seal elements. Multiple plate penetrations, however, require sealing of current collector rods at each plate to prevent leakage between inlet and exhaust air streams and overboard air leakage from the exhaust stream. Any leakage, however, reduces the overall efficiency of the hot box and may cause localized thermal imbalances.

An embodiment of a simplified stack electrical terminal (e.g., current collector rod950) is illustrated inFIGS. 10 and 13. In this embodiment, the stack support base500contains a bridging tube900which eliminates the need for one of the seal elements. The bridging tube900may be made of an electrically insulating material, such as a ceramic, or it may be made of a conductive material which is joined to a ceramic tube outside the base pan502. The use of a bridging tube900eliminates the air in to air out leak path. The current collector/electrical terminal950is routed in the bridging tube900from top of the cast hot box base500through the base insulation501and out of the base pan502. A sheet metal retainer503may be used to fix the tube900to the base pan502.

The tube900may be insulated in the base with super wool901and/or a “free flow” insulation material902. The “free flow” insulation902is a fluid that can be poured into an opening in the base500around the tube900but solidifies into a high temperature resistant material when cured.

Embodiments of the simplified stack electrical terminals may have one or more of the following advantages: elimination of the cross over leak risk and reduced cost due to elimination of repeat sealing elements and improved system efficiency by reduced air losses.

In an alternative embodiment, the ATO insulation10B and the anode cooler inner core insulation100A (shown inFIG. 11A) may also comprise the free flow insulation. Furthermore, an outer cylinder330may be constructed around the outer shell of the hot box as shown inFIG. 6A. The gap between outer cylinder330and the outer shell of the hot box may then be filled with the free flow insulation. The outer shell of the hot box forms the inner containment surface for the free flow insulation.

Process Flow Diagram

FIG. 14is a schematic process flow diagram representation of the hot box1components showing the various flows through the components according to another embodiment of the invention. The components in this embodiment may have the configuration described in the prior embodiments or a different suitable configuration. In this embodiment, there are no fuel and air inputs to the ATO10.

Thus, in contrast to the prior art system, external natural gas or another external fuel is not fed to the ATO10. Instead, the hot fuel (anode) exhaust stream from the fuel cell stack(s)9is partially recycled into the ATO as the ATO fuel inlet stream. Likewise, there is no outside air input into the ATO. Instead, the hot air (cathode) exhaust stream from the fuel cell stack(s)9is provided into the ATO as the ATO air inlet stream.

Furthermore, the fuel exhaust stream is split in a splitter107located in the hot box1. The splitter107is located between the fuel exhaust outlet of the anode recuperator (e.g., fuel heat exchanger)137and the fuel exhaust inlet of the anode cooler100(e.g., the air pre-heater heat exchanger). Thus, the fuel exhaust stream is split between the mixer105and the ATO10prior to entering the anode cooler100. This allows higher temperature fuel exhaust stream to be provided into the ATO than in the prior art because the fuel exhaust stream has not yet exchanged heat with the air inlet stream in the anode cooler100. For example, the fuel exhaust stream provided into the ATO10from the splitter107may have a temperature of above 350 C, such as 350-500 C, for example 375 to 425 C, such as 390-410 C. Furthermore, since a smaller amount of fuel exhaust is provided into the anode cooler100(e.g., not 100% of the anode exhaust is provided into the anode cooler due to the splitting of the anode exhaust in splitter107), the heat exchange area of the anode cooler100described above may be reduced.

The splitting of the anode exhaust in the hot box prior to the anode cooler has the following benefits: reduced cost due to the smaller heat exchange area for the anode exhaust cooler, increased efficiency due to reduced anode recycle blower123power, and reduced mechanical complexity in the hot box due to fewer fluid passes.

The benefits of eliminating the external ATO air include reduced cost since a separate ATO fuel blower is not required, increased efficiency because no extra fuel consumption during steady state or ramp to steady state is required, simplified fuel entry on top of the hot box next to anode gas recycle components, and reduced harmful emissions from the system because methane is relatively difficult to oxidize in the ATO. If external methane/natural gas is not added to the ATO, then it cannot slip.

The benefits of eliminating the external ATO fuel include reduced cost because a separate ATO air blower is not required and less ATO catalyst/catalyst support is required due to higher average temperature of the anode and cathode exhaust streams compared to fresh external fuel and air streams, a reduced cathode side pressure drop due to lower cathode exhaust flows, increased efficiency due to elimination of the power required to drive the ATO air blower and reduced main air blower125power due to lower cathode side pressure drop, reduced harmful emissions since the ATO operates with much more excess air, and potentially more stable ATO operation because the ATO is always hot enough for fuel oxidation after start-up.

The hot box1contains the plurality of the fuel cell stacks9, such as a solid oxide fuel cell stacks (where one solid oxide fuel cell of the stack contains a ceramic electrolyte, such as yttria stabilized zirconia (YSZ) or scandia stabilized zirconia (SSZ), an anode electrode, such as a nickel-YSZ or Ni-SSZ cermet, and a cathode electrode, such as lanthanum strontium manganite (LSM)). The stacks9may be arranged over each other in a plurality of columns as shown inFIG. 7A.

The hot box1also contains a steam generator103. The steam generator103is provided with water through conduit30A from a water source1404, such as a water tank or a water pipe (i.e., a continuous water supply), and converts the water to steam. The steam is provided from generator103to mixer105through conduit30B and is mixed with the stack anode (fuel) recycle stream in the mixer105. The mixer105may be located inside or outside the hot box of the hot box1. Preferably, the humidified anode exhaust stream is combined with the fuel inlet stream in the fuel inlet line or conduit29downstream of the mixer105, as schematically shown in FIG.14. Alternatively, if desired, the fuel inlet stream may also be provided directly into the mixer105, or the steam may be provided directly into the fuel inlet stream and/or the anode exhaust stream may be provided directly into the fuel inlet stream followed by humidification of the combined fuel streams.

The steam generator103is heated by the hot ATO10exhaust stream which is passed in heat exchange relationship in conduit119with the steam generator103, as shown inFIG. 6F.

The system operates as follows. The fuel inlet stream, such as a hydrocarbon stream, for example natural gas, is provided into the fuel inlet conduit29and through a catalytic partial pressure oxidation (CPOx)111located outside the hot box. During system start up, air is also provided into the CPOx reactor111through CPOx air inlet conduit113to catalytically partially oxidize the fuel inlet stream. During steady state system operation, the air flow is turned off and the CPOx reactor acts as a fuel passage way in which the fuel is not partially oxidized. Thus, the hot box1may comprise only one fuel inlet conduit which provides fuel in both start-up and steady state modes through the CPOx reactor111. Therefore a separate fuel inlet conduit which bypasses the CPOx reactor during steady state operation is not required.

The fuel inlet stream is provided into the fuel heat exchanger (anode recuperator)/pre-reformer137where its temperature is raised by heat exchange with the stack9anode (fuel) exhaust streams. The fuel inlet stream is pre-reformed in the pre-reformer section of the heat exchanger137(e.g., as shown inFIG. 9A) via the SMR reaction and the reformed fuel inlet stream (which includes hydrogen, carbon monoxide, water vapor and unreformed methane) is provided into the stacks9through the fuel inlet conduit(s)21. As described above with respect toFIGS. 9A and 9B, additional reformation catalyst may be located in conduit(s)21. The fuel inlet stream travels upwards through the stacks through fuel inlet risers in the stacks9and is oxidized in the stacks9during electricity generation. The oxidized fuel (i.e., the anode or fuel exhaust stream) travels down the stacks9through the fuel exhaust risers and is then exhausted from the stacks through the fuel exhaust conduits23A into the fuel heat exchanger137.

In the fuel heat exchanger137, the anode exhaust stream heats the fuel inlet stream via heat exchange. The anode exhaust stream is then provided via the fuel exhaust conduit23B into a splitter107. A first portion of the anode exhaust stream is provided from the splitter107the ATO10via conduit (e.g., slits)133.

A second portion of the anode exhaust stream is recycled from the splitter107into the anode cooler100and then into the fuel inlet stream. For example, the second portion of the anode exhaust stream is recycled through conduit31into the anode cooler (i.e., air pre-heater heat exchanger) where the anode exhaust stream pre-heats the air inlet stream from the air inlet conduit or manifold33. The anode exhaust stream is then provided by the anode recycle blower123into the mixer105. The anode exhaust stream is humidified in the mixer105by mixing with the steam provided from the steam generator103. The humidified anode exhaust stream is then provided from the mixer105via humidified anode exhaust stream conduit121into the fuel inlet conduit29where it mixes with the fuel inlet stream.

The air inlet stream is provided by a main air blower125from the air inlet conduit33into the anode cooler heat exchanger100. The blower125may comprise the single air flow controller for the entire system, as described above. In the anode cooler heat exchanger100, the air inlet stream is heated by the anode exhaust stream via heat exchange. The heated air inlet stream is then provided into the air heat exchanger (cathode recuperator200) via conduit314as shown inFIGS. 6F and 14. The heated air inlet stream is provided from heat exchanger200into the stack(s)9via the air inlet conduit and/or manifold25.

The air passes through the stacks9into the cathode exhaust conduit24and through conduit24and mixer801into the ATO10. In the ATO10, the air exhaust stream oxidizes the split first portion of the anode exhaust stream from conduit133to generate an ATO exhaust stream. The ATO exhaust stream is exhausted through the ATO exhaust conduit27into the air heat exchanger200. The ATO exhaust stream heats air inlet stream in the air heat exchanger200via heat exchange. The ATO exhaust stream (which is still above room temperature) is then provided from the air heat exchanger200to the steam generator103via conduit119. The heat from the ATO exhaust stream is used to convert the water into steam via heat exchange in the steam generator103, as shown inFIG. 6F. The ATO exhaust stream is then removed from the system via the exhaust conduit35. Thus, by controlling the air inlet blower output (i.e., power or speed), the magnitude (i.e., volume, pressure, speed, etc.) of air introduced into the system may be controlled. The cathode (air) and anode (fuel) exhaust streams are used as the respective ATO air and fuel inlet streams, thus eliminating the need for a separate ATO air and fuel inlet controllers/blowers. Furthermore, since the ATO exhaust stream is used to heat the air inlet stream, the control of the rate of single air inlet stream in the air inlet conduit or manifold33by blower125can be used to control the temperature of the stacks9and the ATO10.

Thus, as described above, by varying the air inlet stream using a variable speed blower125and/or a control valve to maintain the stack9temperature and/or ATO10temperature. In this case, the main air flow rate control via blower125or valve acts as a main system temperature controller. Furthermore, the ATO10temperature may be controlled by varying the fuel utilization (e.g., ratio of current generated by the stack(s)9to fuel inlet flow provided to the stack(s)9). Finally the anode recycle flow in conduits31and117may be controlled by a variable speed anode recycle blower123and/or a control valve to control the split between the anode exhaust to the ATO10and anode exhaust for anode recycle into the mixer105and the fuel inlet conduit29.

Any one or more features of any embodiment may be used in any combination with any one or more other features of one or more other embodiments. The construction and arrangements of the fuel cell system, as shown in the various exemplary embodiments, are illustrative only. Although only a few embodiments have been described in detail in this disclosure, many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. Some elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. The order or sequence of any process, logical algorithm, or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present disclosure.

Although the foregoing refers to particular preferred embodiments, it will be understood that the invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the invention. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety.