CN109964351B

CN109964351B - Integrated fuel cell block with modified fuel cell cycle for integrated reforming fuel cell

Info

Publication number: CN109964351B
Application number: CN201780067255.1A
Authority: CN
Inventors: 罗伯特·坎宁安; 埃里克·迪安; G·阿格纽; M·博佐洛
Original assignee: Rolls Royce Fuel Cell Systems US Inc
Current assignee: LG Fuel Cell Systems Inc
Priority date: 2016-11-02
Filing date: 2017-11-01
Publication date: 2022-10-25
Anticipated expiration: 2037-11-01
Also published as: WO2018085437A1; KR20190077052A; CN109964351A

Abstract

A fuel cell system having a cathode, an anode, and an auxiliary circuit is provided. The anode loop may be configured to deliver both reformed and unreformed fuel to the fuel cell. Unreformed fuel may be provided to the fuel cell by bypassing a portion of the fuel around the reformer. Unreformed fuel may be reformed in the fuel cell block. The cathode loop may direct a portion of the oxidant exhausted from the fuel cell back to the fuel cell through a cathode ejector. The ejector may be supplied with pressurized oxidant, which may be heated prior to entering the cathode ejector. The secondary loop may combust unused fuel and oxidant to provide heat transferred to the oxidant before the oxidant enters the cathode loop.

Description

Integrated fuel cell block with modified fuel cell cycle for integrated reforming fuel cell

Cross Reference to Related Applications

The present application claims priority from U.S. patent application serial No. 15/341,376, entitled "modified Fuel Cell Cycle for In Block Reforming Fuel Cells," filed 2016 and filed continuously for a portion thereof, the entire contents of which are incorporated herein by reference. This application also claims priority from U.S. patent application No. 62/416,371 entitled "Integrated Fuel Cell Block with a recycled Fuel Cell Cycle for In Block Reforming Cells" filed 2016, month 11, 2, which is hereby incorporated by reference In its entirety.

FIELD

The present disclosure relates generally to fuel cell systems. More particularly, the present disclosure relates to fuel cell systems and methods that provide a modified fuel cell system cycle optimized for an in-block reforming fuel cell.

Background

A fuel cell is an electrochemical system in which a fuel (e.g., hydrogen) reacts with an oxidant (e.g., oxygen) at an elevated temperature to produce electricity. Fuel cells are typically supported by component systems such as reformers, heat exchangers, injectors, burners, fuel and oxidant sources, and other components. For example, the unreformed fuel source may be supplied to the fuel cell system reformer via a fuel injector. The reformer may partially or completely reform the fuel by steam, drying, or other reforming methods to produce reformate that is supplied to the anode of the fuel cell. The fuel cell may discharge unused fuel from the anode and supply the unused fuel to a suction port of a fuel injector or an auxiliary system.

To facilitate reforming of the unreformed fuel, the fuel cell system may provide a heat input to the reformer by supplying a cathode exhaust or other thermal fluid to the reformer. After transferring its heat to the reformate fuel, the cathode exhaust can be supplied to an auxiliary system, recirculated back to the cathode inlet via an oxidant air injector, or both.

Although the temperature of the recycled and fresh oxidant supplied to the cathode increases as it passes through the fuel cell stack, the heat input to the cathode flow may not be sufficient to keep the cathode loop thermally balanced due to the large heat input required to support the reforming of the hydrocarbon fuel. To thermally balance the cathode loop, a heat exchanger may be introduced in the cathode loop, typically upstream of the cathode inlet. The recuperator may be supplied with combustion products from the reaction of unused fuel with cathode exhaust gas supplied to the auxiliary system. The reaction may take place in a heat exchanger or in a component upstream of a heat exchanger, such as a burner.

During normal operation, the cathode loop is maintained in thermal equilibrium. The heat generated within the fuel cell stack, the heat transferred to the fuel in the reformer, the cooling effect of the oxidant mixture at the cathode injector, and the heat input from the heat exchanger will balance to maintain this thermal balance; in practice, the size of the heat exchanger upstream of the cathode inlet is adjusted for this purpose only.

One type of fuel cell is a Solid Oxide Fuel Cell (SOFC). The basic components of a SOFC may include an anode, a cathode, a solid electrolyte, and a connector. A fuel may be supplied to the anode and an oxidant may be supplied to the cathode of the fuel cell. At the cathode, the electrons may ionize the oxidant. The electrolyte may comprise a material that allows ionized oxidant to pass therethrough to the anode while being unaffected by the fluid fuel and oxidant. At the anode, the fuel combines with the ionized oxidant, performing a reaction that releases electrons that are conducted back to the cathode through the connector. The heat generated by ohmic losses is removed from the fuel cell by the anode or cathode exhaust stream or radiated to the environment. The heat from these electrical losses can be used for reforming of the hydrocarbon fuel within the fuel cell stack.

SOFCs can be configured, for example, as segment-in-series (segment-in-series) or in-plane-series (in-plane-series) arrangements of individual cells. The oxidant is typically introduced at one end of the series of cells and flows through the remaining cells until it reaches the cathode exhaust outlet. Each fuel cell transfers heat to the oxidant, thereby increasing its temperature and creating an increased temperature gradient from the oxidant inlet to the exhaust. A temperature gradient may also be created in the fuel cell that increases from the oxidant inlet to the oxidant exhaust. These temperature gradients cause thermal stresses that may lead to material degradation or failure of the fuel cell components or may reduce fuel cell performance.

The anode of the SOFC may be a mixed metal ceramic comprising nickel and zirconia (e.g., yttria Stabilized Zirconia (YSZ)) or nickel and ceria (e.g., gadolinia Doped Ceria (GDC)). Nickel and other materials may not only serve to support the chemical reaction between the fuel and ionized oxidant, but may also have catalytic properties that allow the anode to reform hydrocarbon fuels within the fuel cell. One method of reforming hydrocarbon fuels is steam reforming of methane (CH 4), which is an endothermic reaction (equation 1):

CH ₄ +H ₂ O->CO+3Η ₂ Δ Η ° =206.2 kJ/mole (equation 1)

Alternative reforming processes may also be used. For example, hydrocarbon fuels can be reformed by carbon dioxide reforming (also known as dry reforming) (equation 2):

CO ₂ +CH ₄ ->2H ₂ +2CO (reaction type 2)

The heat required for methane reforming may be supplied directly from heat generated within the stack. This direct heat transfer can help cool the stack, reduce thermal stresses and improve overall stack performance.

In addition, direct heat transfer can remove or reduce the amount of heat required for hydrocarbon fuel reforming in the reformer. Eliminating this large heat sink in the cathode loop may allow for a modification of the fuel cell cycle that improves fuel cell system efficiency while keeping the cathode loop in thermal equilibrium.

There is still a need to modify the fuel cell thermodynamic cycle for fuel cells configured for internal integrated reforming.

According to some embodiments of the present disclosure, a fuel cell cycle is provided. This cycling can maintain the overall thermal balance of the cathode loop. The cycle may not require heat transfer from the cathode exhaust to the reformer to facilitate catalytic reforming of the unreformed fuel. The fuel (in whole or in part) may be internally reformed by wet or dry reforming, in which the heat required for the reforming of the unreformed hydrocarbon fuel is transferred from the heat generated by the fuel cell stack. The external reformer may be reduced in size compared to a reformer used in a fuel cell cycle where all or most of the fuel is reformed outside the fuel cell block. The heat exchanger upstream of the cathode inlet may be eliminated. In some embodiments, the fuel cell cycle may not include an auxiliary loop.

According to some embodiments of the present disclosure, a fuel cell system is provided. The fuel cell system may include an unreformed fuel source and an oxidant source. The system may also include a fuel cell stack, an anode injector, a reformer, an auxiliary injector, and a cathode injector. A fuel cell stack may include a plurality of fuel cells, each fuel cell having an anode, a cathode, and an electrolyte. The fuel cell may be a SOFC. The stack may also include a fuel supply manifold configured to receive reformate and unreformed fuel and supply the reformate and the unreformed fuel to the fuel cells; a fuel exhaust manifold configured to exhaust unused fuel from the fuel cell stack; an oxidant supply manifold configured to receive an oxidant and supply the oxidant to the fuel cell; and an oxidant exhaust manifold configured to exhaust oxidant from the fuel cell stack. The anode injector may be configured to receive unreformed fuel from the fuel source and to receive a portion of the unused fuel discharged from the fuel cell stack. The reformer may include a plurality of cold side channels and a plurality of hot side channels; a fuel supply manifold configured to receive fuel from the anode injector and supply the fuel to the plurality of cold side channels; a fuel exhaust manifold configured to exhaust reformate from the plurality of cold-side channels and supply reformate to a fuel supply manifold of the fuel cell stack; an oxidant inlet manifold configured to receive a portion of the oxidant exhausted from the fuel cell stack and supply the oxidant to the plurality of hot side channels; and an oxidant exhaust manifold configured to exhaust oxidant from the plurality of hot side channels. The auxiliary injector may be configured to receive a portion of the unused fuel discharged from the fuel cell stack and to receive oxidant discharged from the plurality of thermal channels of the reformer. The auxiliary ejector may further receive oxidant from the oxidant source and a portion of the recycled auxiliary stream. The cathode injector may be configured to receive oxidant from the compressor and to receive oxidant discharged from an oxidant exhaust manifold of the fuel cell stack and to supply oxidant to an oxidant inlet manifold of the fuel cell stack. The fuel cell system may further include: a combustor configured to receive unused fuel and oxidant discharged from the auxiliary injector; a turbine configured to receive exhaust gas from the combustor; and a compressor configured to receive an oxidant from an oxidant source. The system may also include a heat exchanger having hot and cold side passages. The heat exchanger may receive oxidant from an oxidant source in a cold side passage and exhaust from a combustor in a hot side passage. The heat exchanger may be located upstream of the cathode injector.

According to some embodiments of the present disclosure, a fuel cell system is provided. The fuel cell system may be a SOFC system. The system may include a fuel cell stack, a reformer, an anode loop, a cathode loop, and an auxiliary loop. The (solid oxide) fuel cell stack may comprise at least one (solid oxide) fuel cell, each (solid oxide) fuel cell comprising an anode, a cathode and an electrolyte. The reformer may include hot side and cold side channels. The anode loop may supply fuel and reformate to the anode of each (solid oxide) fuel cell and may include a fuel inlet manifold in the fuel cell stack configured to supply fuel and reformate to the anode of each solid oxide fuel cell; a fuel exhaust manifold configured to receive unused fuel from the anode of each solid oxide fuel cell; an anode injector configured to receive fuel from a fuel source and a fuel exhaust manifold; and a cold side passage of the reformer configured to receive fuel from the anode injector. The cathode circuit may supply oxidant to the cathode of each (solid oxide) fuel cell and may include an oxidant inlet manifold in the fuel cell stack configured to supply oxidant to the cathode of each (solid oxide) fuel cell; an oxidant exhaust manifold in the fuel cell stack configured to receive unused oxidant from each cathode of the (solid oxide) fuel cells; and a cathode injector configured to receive oxidant from the oxidant source and the oxidant exhaust manifold and configured to supply oxidant to the oxidant inlet manifold. The auxiliary loop may provide a portion of unused fuel from the fuel exhaust manifold and a portion of unused oxidant from the oxidant exhaust manifold for combustion, and may include an auxiliary injector configured to receive oxidant from the hot side passage of the reformer, a portion of oxidant from the oxidant source, and a portion of unused fuel from the fuel exhaust manifold; and a burner configured to receive exhaust gas from the auxiliary injector. The auxiliary injector may receive unused oxidant from a hot side passage of the reformer configured to receive a portion of the unused oxidant from the oxidant exhaust manifold. The system may also include a heat exchanger including a hot side passage and a cold side passage located upstream of the cathode injector such that the cold side passage receives oxidant from an oxidant source and the hot side passage receives combustion products in the secondary loop. A portion of the unreformed fuel and unused fuel in the anode loop may bypass the cold side passage of the reformer. The cathode circuit may also include a catalytic start-up burner unit located between the oxidant inlet manifold and the oxidant exhaust manifold, a chromium capture unit located upstream of the fuel cell, and a burner located downstream of the auxiliary circuit and upstream of the turbine.

According to some embodiments of the present disclosure, a fuel cell system is provided having at least one fuel cell and a cathode loop for recycling a portion of unused oxidant from the fuel cell for reuse in the same fuel cell. The cathode loop may include an oxidant inlet manifold in the fuel cell configured to supply oxidant to the fuel cell; an oxidant exhaust manifold in a fuel cell configured to receive unused oxidant from the fuel cell; and a cathode injector configured to receive oxidant from an oxidant source and an oxidant exhaust manifold and supply oxidant to an oxidant inlet manifold, wherein a portion of the unused oxidant is supplied directly from the oxidant exhaust manifold to the oxidant inlet manifold through the cathode injector.

These and many other advantages of the present subject matter will be readily apparent to one of ordinary skill in the art to which the present disclosure pertains from a reading of the claims, the drawings, and the following detailed description of the embodiments.

Drawings

Fig. 1A and 1B show a fuel cell system.

Fig. 2A and 2B illustrate fuel cell systems according to some embodiments of the present disclosure.

Figure 3 illustrates some components of a fuel cell system according to some embodiments of the present disclosure.

Fig. 4 illustrates a fuel cell system according to some embodiments of the present disclosure.

Fig. 5 illustrates a fuel cell system according to some embodiments of the present disclosure.

Fig. 6 illustrates a fuel cell system according to some embodiments of the present disclosure.

Fig. 7 illustrates a fuel cell system according to some embodiments of the present disclosure.

Fig. 8-10 illustrate various views of an integrated fuel cell block according to some embodiments of the present disclosure.

Fig. 11A and 11B illustrate fuel cell systems according to some embodiments of the present disclosure.

Referring to the drawings, some aspects of non-limiting examples of fuel cell systems according to embodiments of the present disclosure are schematically depicted. In the drawings, various features, components, and interrelationships between aspects of embodiments of the disclosure are depicted. However, the present disclosure is not limited to the specific embodiments presented and the components, features and interrelationships therebetween as shown in the figures and described herein.

Detailed Description

Objects and advantages of the claimed subject matter will become apparent from the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings.

Fig. 1A shows a fuel cell system 100. The system 100 may include a fuel cell stack 102 (which may also be referred to as a fuel cell block), a reformer 104, a heat exchanger 106, an oxidant source 108, a fuel source 110, an anode injector 112 (also referred to as a fuel injector), a cathode injector 114. (also referred to as an oxidant ejector) and an auxiliary ejector 116. The system 100 may also include auxiliary equipment and components, such as a compressor 134, a turbine 136, a generator 138, and a regenerator 142.

The fuel cell stack 102 may include a plurality of individual fuel cells (not shown). Each fuel cell may each include an anode, a cathode, and an electrolyte.

The fuel cell stack 102 may also include a fuel supply manifold 120 (also referred to as a fuel inlet manifold) configured to receive reformate from the reformer 104. Fuel supply manifold 120 may also be configured to receive unreformed fuel. The unreformed fuel may be fuel that passes through the reforming section of the reformer 104 but is not reformed, fuel that bypasses the reforming section of the reformer 104, or both. The fuel supply manifold 120 is also configured to supply reformate, unreformed fuel, or both to the plurality of anodes of the fuel cell stack 102.

The fuel cell stack 102 may also include a fuel exhaust manifold 118 configured to exhaust unused fuel (e.g., reformate or unreformed fuel that has not reacted with oxidant), fuel cell reaction products, or both, from the fuel cell stack 102. Unused fuel may be supplied to the intake of the anode injector 112, the intake of the auxiliary injector 116, other auxiliary equipment (not shown), such as a combustor, vented to the environment, or any combination of these options.

The fuel cell stack 102 may also include an oxidant supply manifold 122 (which may be referred to as an oxidant inlet manifold) and an oxidant exhaust manifold 124. The oxidant supply manifold 122 is configured to receive oxidant from the cathode injectors 114. The oxidant from the cathode injector 114 may be combined with the oxidant exhausted from the fuel cell stack 102. The oxidant may pass through the cold side passages of the heat exchanger 106, where heat is transferred to the oxidant before it is received in the oxidant supply manifold 122. The oxidant supply manifold 122 is configured to deliver oxidant to a plurality of cathodes in the fuel cell stack 102.

The oxidant exhaust manifold 124 is configured to exhaust oxidant from the fuel cell stack 102 for delivery to the reformer 104 or some other component. In the reformer, the oxidant exhaust gas passes through a hot side passage of the reformer 104 to affect heat transfer to the unreformed fuel and the unused fuel stream to facilitate reforming of the unreformed fuel. After exiting the reformer 104, the oxidant exhaust may be supplied to the suction side of the cathode injector 114, the suction side of the auxiliary injector 116, some other component (not shown), such as a combustor, vented to the environment, or any combination of these options.

The oxidant exhaust supplied to the suction side of the cathode injector 114 flows through a portion of the cathode loop. The cathode loop includes a flow path from the cathode injector 114 through the cold side channels of the heat exchanger 106 into the oxidant supply manifold 122, the oxidant is supplied from the oxidant supply manifold 122 to the cathodes in the fuel cell stack 102, exits from the oxidant exhaust manifold 124 into the hot side channels of the reformer 104 and returns to the suction inlet of the cathode injector 114. It can be seen that the cathode loop is not a closed system, as oxidant is allowed to enter the loop from the oxidant supply 108 and exit the loop into the suction inlet of the ejector 116 (or some other component). In addition, a portion of the oxidant is ionized and diffuses through the fuel cell electrolyte.

The reformer 104 may include a plurality of cold channels and a plurality of hot channels (not shown). The reformer 104 may also include a fuel supply manifold 126, a fuel exhaust manifold 128, an oxidant inlet manifold 130, and an oxidant exhaust manifold 132. The fuel supply manifold 126 is configured to receive fuel from the anode injector 112 and supply the fuel to the cold-side channels of the reformer 104. The cold-side channel may include a catalyst for reforming fuel. The fuel exhaust manifold 128 is configured to exhaust reformate from the plurality of cold side channels and supply reformate to the fuel supply manifold 120 of the fuel stack 102. The oxidant inlet manifold 130 is configured to receive an oxidant discharged from the exhaust manifold 124 of the fuel stack 102 and supply the discharged oxidant to the hot side passage of the reformer 104. The exhausted oxidant transfers its heat to the fuel in the plurality of cold side channels to facilitate catalytic conversion of the unreformed fuel. The oxidant then flows to an oxidant exhaust manifold 132 where the oxidant may be removed from the reformer 104 and sent to the suction side of the cathode injector 114, the auxiliary injector 116, some other component, vented to the atmosphere, or a combination of any of these options.

The oxidant may flow through the cold-side passage of the heat exchanger 106 before flowing into the oxidant supply manifold 122. The hot side passage of the heat exchanger 106 may be supplied with a hot fluid source, such as exhaust from the anode, exhaust from the cathode, or other warm fluid. The warm fluid may be combustion products (or combustion exhaust) from a combustor that may be integrated into the auxiliary injector 116 and combust a portion of the unused fuel discharged from the anode of the stack, unused oxidant discharged from the cathode side of the fuel cell stack 102, oxidant from the compressor 134, or a combination of these fluids. After passing through the hot side passage, warm fluid may be supplied to the suction side of the auxiliary injector 116, or the warm fluid may be exhausted from the system.

The combustor (which may be integrated into the auxiliary injector 116) may also supply fresh oxidant, which may provide energy for powering the auxiliary injector 116. The auxiliary injector 116 may draw in a portion of the unused fuel from the fuel exhaust manifold 118, a portion of the oxidant from the oxidant exhaust manifold 132, and may also draw in combustion gases after those gases pass through the recuperator 106.

The anode injector 112 is configured to receive unreformed fuel from the unreformed fuel source 110 and supply it to a fuel supply manifold 126 of the reformer 104. The anode injector 112 may also draw in a portion of the unused fuel discharged from the fuel exhaust manifold 118.

In some embodiments, a portion of the fuel cell system 100 may be referred to as an anode loop. The circuit may include a fuel inlet manifold 120, a fuel exhaust manifold 118, an anode injector 112, and the cold side passages of the reformer 104. In some embodiments, the loop may further include a pre-reformer 144 and a fuel source 110.

The cathode injector 114 is configured to receive fresh oxidant from the oxidant source 108 and may also be configured to receive a portion of the unused oxidant exhausted from the oxidant exhaust manifold 132 of the reformer 104. The cathode injector 114 supplies oxidant to the hot side passage of the heat exchanger 106.

The unreformed fuel source 110 may be a hydrocarbon source or other type of fuel. The oxidant source 108 may be a tank filled with an oxidant (e.g., pure oxygen, atmospheric air, or other oxidant source) or a system designed to produce a supply of oxidant.

The fuel cell system may also include a compressor 134, a turbine 136, a generator 138, and a recuperator 142. The regenerator 142 may supply oxidant from the compressor 134 to a set of cold side channels therein and exhaust of the turbine 136 to a set of hot side channels. The recuperator 142 is used to transfer heat between the turbine 136 exhaust and the oxidant supplied by the compressor 134. The turbine 136 may receive combustion products from, for example, the recuperator 106. These products may be expanded by a turbine 136, which turbine 136 drives a compressor 134 and a generator 138. Turbine 136 exhaust may be vented to the atmosphere and may be supplied to a recuperator 142 for heat transfer therein prior to being vented to the atmosphere, as shown in FIG. 1A. The generator 138 may supply additional power. A compressor 134 may be disposed downstream of the oxidant supply 108. The compressor 134 may draw in and compress oxidant for driving the cathode ejector 114 and the auxiliary ejector 116. In some embodiments, a recuperator may be provided that transfers heat from the exhaust of the turbine 136 and the outlet of the compressor 134. The regenerator may be located upstream of the cathode injector 114 and the auxiliary injector 116.

In some embodiments, the fuel cell system 100 may be one of a plurality of integrated fuel cell systems. As can be seen on the right side of fig. 1A, the rightward arrow below the cathode injector 114 may proceed toward another integrated fuel cell system to supply oxidant to the cathode injector and auxiliary injectors of that system. In such embodiments, the compressor 134 may provide compressed oxidant to multiple integrated fuel cell systems. Similarly, exhaust from the heat exchanger 106, the auxiliary injector 116, or both may be supplied to a common exhaust manifold that feeds into the turbine 136. In other embodiments, multiple turbines and compressors may be used in multiple integrated fuel cell systems.

In some embodiments, the fuel cell system 100 may also include a pre-reformer 144 disposed between the outlet of the anode injector 112 and the reformer 104. In some embodiments, the pre-reformer may also be upstream of the bypass line (e.g., bypass line 140 shown in fig. 1B). The pre-reformer 144 is used to remove higher hydrocarbons from the fuel stream from the source 110, and any higher hydrocarbons that may be present in the anode exhaust that is recycled to the anode injector 112. The pre-reformer 144 may be an adiabatic catalytic converter capable of removing higher hydrocarbons, which is capable of removing higher hydrocarbons, and which has no heat input other than heat from the source 110 and the fuel recycled from the anode exhaust 118.

Fig. 1B shows a fuel cell system 100, which may be similar to the fuel cell systems described above. However, the system 100 may include a bypass 140 that provides a flow path for unreformed fuel and the anode exhaust 118 flows around the reformer 104. The bypass 140 may be referred to as a bypass line or conduit. The bypass may help control the amount of internal reforming that may occur in the stack 102. The stack 102 may be configured for dry or wet reforming.

According to some embodiments, 10% to 90% of the fuel from source 110 may bypass reformer 104 via bypass conduit 140. In some embodiments, 20% to 70% of the fuel from source 110 may bypass reformer 104 via bypass conduit 140. In some embodiments, 25% to 50% of the fuel from source 110 may bypass reformer 104 via bypass conduit 140.

According to some embodiments, the fuel cell stack 102 and the system 100 may be configured for internal reforming such that most or substantially all of the fuel from the unreformed fuel source 110 is reformed within the fuel cell stack. The internal reforming may be dry or wet reforming. In the event that some or all of the fuel reforming occurs in the fuel cell stack 102, the system may not require a reformer external to the fuel cell stack, or may require a smaller reformer.

Referring to fig. 2A, a fuel cell system 200 is provided according to some embodiments of the present disclosure. The fuel cell system 200 includes a fuel cell stack 102, a component 204, an oxidant source 108, an unreformed fuel source 110, an anode injector 112, a cathode injector 114, and an auxiliary injector 116. The system 200 may also include a compressor 134, a turbine 136, a generator 138, and a regenerator 142. Components having the same numerals may be similar to those described above.

According to some embodiments, the fuel cell stack 102 and the system 200 may be configured for internal reforming such that most or substantially all of the fuel from the unreformed fuel source 110 is reformed within the fuel cell stack. The internal reforming may be dry or wet reforming. In the event that some or all of the fuel reforming occurs in the fuel cell stack 102, the system may not require a reformer external to the fuel cell stack, or may require a smaller reformer. In some embodiments, the integrated reforming of all or at least a portion of the fuel from source 110 facilitates moving reformer 104 outside of the cathode loop. This removal of the reformer 104 can recycle unused oxidant discharged from the oxidant exhaust manifold 124 back to the oxidant inlet 122 of the fuel cell stack without passing through any heat exchangers.

In some embodiments, the reformer 104 is replaced with a component 204, and the component 204 may be a heat exchanger, such as a counter-flow, cross-flow, parallel, or other heat exchanger, or a size reduced reformer. Component 204 is also shown in fig. 3. Referring to fig. 2A, 2B, and 3, the component 204 may include a plurality of cold side channels 302 and hot side channels 304, a fuel supply manifold 226, a fuel exhaust manifold 228, an oxidant inlet manifold 230, and an oxidant exhaust manifold 232. The fuel supply manifold 226 may be configured to receive fuel from the anode injectors 112 and supply the fuel to the cold-side passage 302. The cold-side passage 302 may or may not contain a catalyst for reforming some or all of the fuel. The fuel is then exhausted from the exhaust manifold 228, the exhaust manifold 228 exhausts the fuel from the plurality of cold passages, and the reformed or unreformed fuel is supplied to the fuel supply manifold 120 of the fuel stack 102. The oxidant inlet manifold 230 is configured to receive a portion of the oxidant discharged from the oxidant exhaust manifold 124 and supply the oxidant to the hot side passage 304. The oxidant exhaust manifold 232 is configured to supply oxidant to an intake of the auxiliary injector 116, some other component, such as a combustor (not shown), or to an exhaust.

With the introduction of integrated reforming in the fuel cell stack 102, the need for hot channel flow of oxidant (e.g., cathode exhaust) in the component 204 is reduced, resulting in a significant reduction in the size of the component 204 as compared to the reformer 104. Additionally, the component 204 may be placed further away from the fuel cell stack 102, and a high flow rate of cathode exhaust may not be required to facilitate any fuel reforming that occurs in the component 204.

In some embodiments, the fuel supply manifold 226 or other component preceding the cold-side channel 302 may include a pre-reforming stage to allow for adiabatic reforming of a portion of the unreformed fuel in the component 204.

In some embodiments, the cathode loop may include an oxidant flow from the cathode injector 114 to an oxidant supply manifold 122 of the fuel cell stack 102, exiting the fuel cell stack 102 via an oxidant exhaust manifold 124 and returning to the suction side of the cathode injector 114. In some embodiments, the cathode loop may include an oxidant flow from the cathode injector 114 to an oxidant supply manifold 122 of the fuel cell stack 102, exiting the fuel cell stack 102 via an oxidant exhaust manifold 124 and returning to the suction side of the cathode injector 114. In some embodiments, the cathode loop can be considered to include the oxidant source 108 as described herein, as well as additional components shown between the oxidant source 108 and the fuel cell stack 102. In some embodiments, a portion of the unused oxidant received at the exhaust manifold 124 may be supplied directly to the oxidant inlet manifold 122 via an air injector.

The volumetric flow rate of air present in the cathode loop towards the component 204 may be 10% to 33% of the air flowing through the cathode loop. In some embodiments, the volumetric flow rate of cathode exhaust through the cathode loop is 2 to 8 times greater than the flow rate of cathode exhaust exiting the cathode loop to be supplied to the component 204. In some embodiments, the volumetric flow rate of cathode exhaust through the cathode loop is 4 to 7 times greater than the flow rate exiting the cathode loop to be supplied to the component 204. In some embodiments, no less than two-thirds of the unused oxidant from the cathode exhaust may be recycled to the fuel cell stack oxidant inlet manifold 122.

Those skilled in the art will appreciate that the amount of cathode exhaust gas supplied to the component 204 must be balanced with the amount of fuel reformed in the component 204 to ensure that a sufficient amount of heat is exchanged in the component 204 to support the desired level of reforming within the component 204.

Fig. 2B shows a fuel cell system 200, which may be similar to the fuel cell system described above. However, the system 200 may include a bypass 240 that provides a flow path for unreformed fuel and the anode exhaust 118 flows around the component 204. The bypass 240 may help control the amount of internal reforming that may occur in the stack 102.

As shown in fig. 3, a portion of the oxidant exhaust gas from the oxidant exhaust manifold 124 is supplied to the oxidant inlet manifold 230 of the component 204, as indicated by arrow 306. The remaining portion of the oxidant exhaust is drawn back to the cathode injector 114 as indicated by arrow 308. In this configuration, the oxidant flowing through the component 204 is the oxidant that has left the cathode loop.

According to some embodiments, there are no components between the cathode injector 114 and the fuel cell stack 102 configured to transfer heat to the oxidant. When the cathode loop is kept in thermal equilibrium, the heat exchanger can be removed because the large radiator reformer 104 is removed from the cathode loop. This may also allow for a slight decrease in the cathode loop temperature, allowing for a small power increase from the fuel cell stack 102. Additionally, with the removal of the heat exchanger 106, the need for auxiliary circuits (circuits supplied by the auxiliary injector 116) and combustors may be eliminated. However, in some embodiments, a small heat exchanger may be provided upstream of the fuel cell stack 102 for preheating of the fuel cell system 200.

A fuel cell system 400 is provided in fig. 4, according to some embodiments of the present disclosure. Components having like reference numerals may be similar to those described above. The system 400 may include a heat exchanger 106 disposed upstream of the cathode injector 114. The heat exchanger can transfer heat to the fresh oxidant stream.

A fuel cell system 500 is provided in fig. 5, according to some embodiments of the present disclosure. Components having like reference numerals may be similar to those described above. The system 500 may include a heat exchanger 106 disposed downstream of the cathode injector 114 and upstream of the fuel cell stack 102. The heat exchanger can transfer heat to a combination of fresh oxidant and oxidant recycled in the cathode loop.

According to some embodiments, the unreformed fuel source 110, the anode injector 112, the component 204, or a combination thereof is configured to supply unreformed fuel to the fuel supply manifold 120 of the fuel cell stack 102. The unreformed fuel may be added to the reformed fuel prior to fuel supply manifold 120.

According to some embodiments, a method of balancing heat transfer in a cathode loop during steady state operation of a fuel cell system is provided. The fuel cell system may include a cathode loop for circulating an oxidant through the fuel cell stack, wherein heat is transferred from the cathode loop in the reformer. Heat is transferred to the cathode loop in an inlet heat exchanger located between the outlet of the cathode loop of the reformer and the inlet of the cathode loop of the fuel cell stack. The method may include reforming fuel in the fuel cell stack, removing the reformer from the cathode loop, and removing the inlet heat exchanger from the cathode loop. Only a portion of the oxidant exhausted from the fuel cell stack may be provided to the reformer.

Fig. 6 illustrates a fuel cell system 600, according to some embodiments. The fuel cell system 600 may include similar components having similar functions to those described above, wherein components bearing the same reference numerals correspond to those described above. Additionally, fuel cell system 600 may also include a start-up burner unit 148, such as a catalytic start-up burner unit (or "catalytic burner unit") and a burner 146. The burner unit 148 may be used during start-up, shut-down, or during periods when the fuel cell system 600 may not generate enough heat to maintain desired operation without the burner unit 148. As shown in more detail in FIG. 8, the combustor 148 may include a fuel supply manifold that injects fuel into the cathode loop, which is then ignited by the combustion catalyst. When the cathode loop fluid flow, components in direct contact with the cathode loop gas flow, or both reach a sufficiently high temperature, the fuel supplied to the cathode loop may auto-ignite before reaching the burner unit 148. The burner unit 148 may also be referred to as a heater.

When the fuel cell system 600 may otherwise be unable to supply the fluid, the combustor 146 may be used to increase the temperature and pressure of the fluid sufficient to rotate the turbine 136 and provide the required work output. The combustor 146 may further be operable to receive fresh oxidant, fuel (reforming or otherwise), or both to accomplish this by combustion. As shown in FIG. 6, the combustor 146 may also receive a portion of the exhaust from the auxiliary circuit. The exhaust gas may contain unburned fuel and oxidant that may react within the combustor 146 to provide sufficient temperature and pressure for the fluid to supply the energy required to rotate the compressor 134 through expansion of the fluid through the turbine 136.

Although

components

146 and 148 are shown in the embodiment of fig. 6, one skilled in the art will appreciate that these components may be used in any of the embodiments described herein. For example, fig. 7 shows another embodiment having a start-up burner unit 148 and a burner 146. In contrast to fig. 6, in fig. 7, element 204 replaces reformer 104, bypass 240 is added, and a cathode loop recycle line (i.e., the line upstream of element 204 that returns to the suction inlet of cathode ejector 114) is added. In addition, fig. 7 may also include components not shown in this figure but shown in other figures. For example, the auxiliary loop of fig. 7 may include a cathode heat exchanger 106. The heat exchanger 106 may be placed downstream of the cathode air ejector 114. In some embodiments, the heat exchanger 106 may be located in the high pressure oxidant supply line upstream of the cathode injector 114 (e.g., as shown in fig. 4).

A front view of an integrated fuel cell block 800, according to some embodiments of the present disclosure, is shown in fig. 8. The figure shows various flows associated with the cathode loop of one or more of the fuel cell systems described above. The integrated fuel cell block 800 shown in fig. 8 may include components having similar structures and functions to those described above. For example, the integrated fuel cell block 800 may include the fuel cell block 102, the heat exchanger 106, the cathode injector 114, the oxidant supply manifold 122, the oxidant exhaust manifold 124, the reformer 104 or the component 204, the combustor 148, the oxidant supply manifold 230, the oxidant exhaust manifold 232, the return (or recycle) oxidant stream 308, and the oxidant stream supplied to the component 204/reformer 104, represented by arrow 306. As described above with respect to the various figures, some of these components (e.g., the heat exchanger 106 and the component 204) may not be present in the integrated block 800, as their functionality may not be needed depending on the temperature balance and any degree of integrated reforming in the fuel cell block 102. The integrated fuel cell block 800 may also include a fuel supply manifold 850, a catalytic start burner unit 852, a chromium capture unit 854, a cathode mainstream air tube 856, and a fuel preheater unit 858.

As shown in fig. 8, the oxidant exhaust manifold 124 may include a pressure control plate configured to provide a more uniform flow of cathode/oxidant to the fuel cells of the fuel cell block 102.

As shown in fig. 8, the heat exchanger 106 may be located upstream of the cathode main flow pipe 856 of the cathode injector 114. Fresh oxidant from the oxidant source 108 (after passing through the compressor 134 and the heat exchanger 142) may be heated by the heat exchanger 106 before entering the cathode loop. Additionally, the positioning of the heat exchanger 106 can provide a certain amount of radiant heat transfer to the cathode loop when the heat exchanger 106 is positioned close enough to the cathode loop.

Both the fuel supply manifold 850 and the catalytic start burner unit 852 may be part of the burner unit 148, as shown in fig. 6 and 7. The fuel supply manifold 850 may supply fuel (e.g., hydrogen or natural gas) directly into the cathode loop fluid flow. Although the cathode loop air temperature is too low to auto-ignite the fuel supplied from the manifold 850, the fuel may be ignited after contacting the catalytic start burner unit 852. As the fuel is combusted, the temperature of the cathode loop gas stream is increased, which in turn, may increase the operating temperature of the integrated block fuel cell system 800 to a temperature effective to sustain the electrochemical reaction of the fuel cell.

The oxidant stream entering the cathode loop through the cathode primary gas stream pipe 856 can be at a temperature of 400 to 500 c as a result of the compressor 134 (shown in other figures) compressing the oxidant and heat input from the heat exchanger 142 (if present). However, this temperature is hundreds of degrees lower than the typical SOFC operating range of 800-900 ℃. To raise the temperature of the cathode loop stream, the oxidant may be passed through a heat exchanger 106 (also shown in fig. 9). During normal operation, the heat exchanger 106 may be provided with combustion products of fuel not used by the fuel cell block 102 from the auxiliary loop. These high temperature fluids can then transfer heat to the fresh oxidant, raising its temperature sufficiently that the temperature of the cathode loop remains in equilibrium. However, during start-up, shut-down, or other phases of operation, fuel may not flow in the anode loop (fig. 10). In the absence of fuel in the anode loop, the auxiliary loop will not have fuel unused by the fuel cell block 102 to burn. In these cases, the combustor 148 may provide the heat input required to raise the cathode loop to operating temperature.

The heating function of the catalytic start-up burner 148 may be provided until the integrated fuel cell block 800 generates electricity. When the fuel cell block 102 generates electricity, electrical losses in the fuel cell system provide a large amount of heat. This heat may be partially captured in the cathode loop oxidant fluid. Since a significant portion of the total cathode loop gas flow is recycled, the cathode loop flow will serve to maintain the monolith at the desired temperature.

A chromium capture unit 854 can be provided to provide a protective feature for a fuel cell located downstream of chromium capture unit 854. Protective chromium oxide layers may be formed on the surfaces of the integrated fuel cell block 800 components because these layers function well at the high temperatures required for efficient solid oxide fuel cell operation. However, the release of chromium from these components can be detrimental to an operating fuel cell system because the chromium can be localized at the fuel cell electrodes. Accordingly, a chromium capture unit 854 is disposed upstream of the fuel cell block 102 to capture the released chromium.

A portion of the cathode oxidant stream may be diverted to the fuel preheater unit 858 and the reformer 104/component 204. The diverted oxidant is represented by arrow 306. On the cathode loop side, a fuel preheater unit 858 may be located upstream of the reformer 104/component 204. Fuel reforming in the anode loop requires heat input. This heat input is accomplished in part by using a hotter cathode loop stream to raise the temperature of the fuel prior to entering the reformer 104 (or section 204). In addition, the cathode loop flow represented by arrow 306 is also used downstream of the fuel preheater unit 858 to provide a heat input for fuel reforming (in whole or in part) in the reformer 104/component 204.

As shown in fig. 8, a portion of the cathode loop stream may be recycled and returned to the suction inlet of the cathode ejector 114. In particular, a portion of the cathode loop recycle stream, represented by arrow 308b, may flow around the fuel preheater unit 858 and/or outside of the reformer 104/component 204. The oxidant stream 308b can be in sufficient proximity to the reformer 104/component 204 to effect heat transfer thereto. In some embodiments, the cathode recycle stream represented by arrow 308b may "wash" the reformer 104/component 204, where the cathode stream is in surrounding contact with the components before returning to the cathode ejector recycle suction.

A portion of the recirculated cathode loop flow may be directed more directly toward the suction inlet of the cathode air ejector 114, as indicated by arrow 308 a. In this way, a portion of the oxidant in the cathode loop will be recirculated through the paths shown at 308a and 308b, and another portion of the oxidant will exit the cathode loop through the oxidant supply manifold 230 and be supplied to the auxiliary injector 116 through the oxidant exhaust manifold 232.

The proportion of the recycled cathode loop stream that flows to the paths represented by

arrows

308a and 308b will depend primarily on the amount of reforming that occurs in the reformer 104 or component 204, and thus the amount of heat input required in these components. In some embodiments, the reformer 104/component 204 may be designed to operate at an equilibrium state, wherein the fuel passing therethrough is not completely reformed. This "incomplete" reforming balance can be used so that a portion of the fuel can be subsequently reformed in the fuel cell block 102, where the heat input for reforming can come from electrical losses within and between the fuel cells. In addition, by controlling the amount of reforming that occurs in the reformer 104/component 204, any catalyst in these components can be made to last longer, thereby extending component life. The reformer 104/component 204 is maintained in an equilibrium state by temperature control. Thus, the heat input from the cathode loop exit stream 306 and the recycled epoxidizing agent (primarily 308 b) flowing around the reformer 104/component 204 can be designed to result in the desired temperature and resulting reforming balance to achieve partial reforming of the fuel.

An alternative front view of the integrated fuel cell block 800 shown in fig. 8 is shown in fig. 9, according to some embodiments of the present disclosure. Figure 9 illustrates various flows associated with the auxiliary circuit of one or more fuel cell systems as described above. The circuit shows the fuel cell block 102, the heat exchanger 106 and the auxiliary ejector 116. Each illustrated component may include structure and perform the function of a similar component as described above.

Auxiliary ejector 116 is driven by the flow of pressurized oxidant from compressor 134, as indicated by arrow 960. This flow in turn creates a low pressure region that draws in various fluid flows, including: a portion of the unused fuel flow from the fuel exhaust manifold 118 at arrow 964; a portion of the non-recycled cathode loop stream from the oxidant exhaust manifold 232 of the reformer 104 or component 204 at 962; and a portion of the secondary fluid flow, indicated by arrow 966, which may be considered a recycled portion of the secondary fluid flow. As previously described, the injector 116 may include an integral burner for igniting the unused fuel and oxidant streams in the combined fluid stream 968. In some embodiments, the combustor may be located outside of the injector 116. The combusted stream 968 then flows to heat exchanger 106 to provide a heat input to the fresh oxidant upstream of cathode injector 114. Fresh oxidant may enter at 972 and exit at 974 prior to being supplied to the cathode injector 114. The cooled combined stream 968 is then returned to auxiliary loop return feed plenum (fed plenum)/manifold 972. A portion of the auxiliary stream may be recycled at 966 to complete the auxiliary loop. Some of the secondary flow may exit the secondary loop at 970 to be supplied directly or indirectly to, for example, the turbine 136. As shown in FIG. 9, both the recirculation and exhaust

secondary flows

966 and 970 may flow or "purge" around the secondary injector 116, which may enable heat transfer to the fluid flowing in the injector 116 prior to combustion.

An alternative front view of the integrated fuel cell block 800 shown in fig. 8 and 9 is shown in fig. 10, according to some embodiments of the present disclosure. The figure shows various flows associated with the anode loop of one or more fuel cell systems as described above. The integrated fuel cell block system 800 may include the anode injector 112, the pre-reformer 144, the reformer 104/component 204, the preheater 858, and other illustrated components, each of which may have these structures and perform their functions as similar components described above.

Unreformed fuel may enter the anode injector 112 at 1074 from the fuel source 110. The pressurized fuel flowing in the injector 112 creates a suction force that will draw a portion of the unused fuel exiting the fuel exhaust manifold 118 at 1076. The combined stream 1078 of unused fuel and fresh fuel then flows to the pre-reformer unit 144, where the combined stream 1078 passes through and removes higher hydrocarbons. The pre-reformed combined stream 1078 then flows down and around the fuel preheater 858. A portion of the combined stream 1078 may then flow into the bypass 240 at 1080. The remainder of the combined stream 1078 enters the reformer 104/component 204 to be reformed, heated, or both at 1082. Heat input to fuel stream 1082 is provided by cathode air 306. The ratio of bypass flow 1080 to flow 1082 into reformer 104/component 204 may be controlled by an orifice in bypass line 240. The

streams

1080 and 1082 are then recombined at 1084 and enter a preheat exchanger 858 where heat from the cathode stream 306 is transferred into the combined stream 1084. The combined flow of heated unreformed and reformed fuel is then provided to the fuel supply manifold 120 of the fuel cell block 102. The cathode exhaust stream 306 is exhausted through a cathode exhaust manifold 232.

The portion at 1086, which may be, for example, a pipe, a piping system (product work), etc., may be coated with a material that prevents carbon formation. In particular, all exposed surfaces between the anode injector 112 exhaust nozzle and the pre-reformer 144 may be coated, heated, or both to prevent carbon deposition, as described further below. Coating or heating may both be referred to as treating. In some embodiments, all internal surfaces of the fuel manifold between the anode injector 112 and the pre-reformer 144 should be protected by a coating or other means to prevent carbon formation on the internal surfaces of the manifold. Importantly, the primary fuel stream flow is not in direct contact with the manifold surface. The protection may be applied directly to the manifold surface or may be in the form of a separate thin metal insert located within the manifold near the manifold surface. In the case of a direct application coating, the coating may be an inert coating or a reactive coating. Where a separate insert is employed, such an insert will be coated with a protective material, which may be inert or reactive. The inert coating may be a refractory oxide material, such as alumina, which may be applied directly to the surface or produced by chemical vapor deposition and subsequent oxidation of the aluminum. The active coating will include a catalyst suitable for steam reforming and may contain Rh or Pt.

A fuel cell system 1100 is shown in fig. 11A and 11B, according to some embodiments of the present disclosure. The fuel cell system 1100 may include various components, wherein like numbered components perform similar functions and have similar structures to components having the same or similar numbering as described above. Additionally, the component 142 may be located downstream of the injector 116 such that the secondary flow receives a heat input from the turbine 136 exhaust. Additionally, the fuel cell system of fig. 11 may have a material applied near the outlet of the anode injector 112 to avoid carbon formation, which may adversely affect the active material (particularly the metal). The coating may extend to the pre-reformer 144.

FIG. 11A illustrates radiant heat transfer between the cathode loop recycle line (a portion of the oxidant stream passes from the oxidant supply manifold 230 back to the cathode injector 114) and the reformer 104/component 204 and to the fuel preheater 858 before supplying fuel to the fuel supply manifold 120. As shown, referring also to fig. 8, a portion of the cathode recycle line is close enough to the reformer 104/component 204 and the fuel preheater 858 and can be cleaned of its exterior surfaces to achieve heat transfer. The heat transferred between the circulating oxidant and the fuel preheater 858 (not shown) is represented by arrows 1188. The heat transferred between the recycled epoxidizing agent and the reformer 104/component 204 is represented by arrows 1190.

A fuel cell system 1100 is shown in fig. 11B, according to some embodiments of the present disclosure. Fig. 11B shows the heat transfer that occurs between the unrecirculated cathode flow and the fuel preheater 858 (not shown). This heat transfer is represented by arrows 1192.

While preferred embodiments of the present subject matter have been described, it is to be understood that the embodiments described are illustrative only, and that the scope of the present subject matter is to be limited only by the appended claims, as the full range of equivalents is accorded to such, and many variations and modifications will naturally occur to those skilled in the art.

Claims

1. A fuel cell system, comprising:

an unreformed fuel source;

a source of an oxidizing agent;

a fuel cell stack comprising a plurality of fuel cells, each fuel cell comprising an anode, a cathode, and an electrolyte, the fuel cell stack further comprising:

a fuel supply manifold configured to receive reformate and unreformed fuel and supply the reformate and the unreformed fuel to a plurality of anodes;

a fuel exhaust manifold configured to exhaust unused fuel from the fuel cell stack;

an oxidant supply manifold configured to receive an oxidant and supply the oxidant to the plurality of cathodes; and

an oxidant exhaust manifold configured to exhaust oxidant from the fuel cell stack;

an anode injector configured to receive unreformed fuel from the unreformed fuel source and to receive a portion of unused fuel discharged from the fuel cell stack;

a reformer comprising a plurality of cold side channels and a plurality of hot side channels, the reformer further comprising:

a fuel supply manifold configured to receive fuel from the anode injector and supply fuel to the plurality of cold side channels;

a fuel exhaust manifold configured to exhaust reformate from the plurality of cold-side channels and supply reformate to the fuel supply manifold of the fuel cell stack;

an oxidant inlet manifold configured to receive a portion of the oxidant exhausted from the fuel cell stack and supply the oxidant to the plurality of hot side channels; and

an oxidant exhaust manifold configured to exhaust oxidant from the plurality of hot side channels;

an auxiliary ejector configured to receive oxidant from the oxidant source, a second portion of unused fuel discharged from the fuel cell stack, oxidant discharged from the plurality of hot side channels, and a portion of a recirculating auxiliary flow;

a combustor configured to receive the unused fuel and oxidant discharged from the auxiliary injector and to discharge combustion products;

a turbine configured to receive exhaust gas from the combustor;

a compressor configured to receive an oxidant from the oxidant source;

a cathode injector configured to receive oxidant from the compressor and to receive a second portion of oxidant discharged from the oxidant exhaust manifold of the fuel cell stack and to supply oxidant to the oxidant inlet manifold of the fuel cell stack; and

a heat exchanger comprising a plurality of cold side passages and a plurality of hot side passages, the heat exchanger configured to receive oxidant from the compressor in the cold side passage and configured to receive exhaust gas from the combustor in the hot side passage;

a bypass conduit configured to receive unreformed fuel discharged from the anode injector and combine unreformed fuel with reformate discharged from the cold-side passage of the reformer upstream of the fuel supply manifold of the fuel cell stack; and

a burner unit located between the cathode injector and the fuel cells of the fuel cell stack, the burner unit configured to inject fuel into the oxidant and combust the second portion of unused fuel with a catalytic burner unit.

2. The fuel cell system of claim 1, wherein at least 10% of the fuel discharged from the anode injector passes through the bypass conduit.

3. The fuel cell system of claim 1, wherein 10% to 90% of the fuel discharged from the anode injector passes through the bypass conduit.

4. The fuel cell system of claim 1, wherein oxidant discharged from the cathode injector is supplied to the oxidant inlet manifold of the fuel cell stack without passing through a heat exchanger.

5. The fuel cell system of claim 1 wherein the oxidant exiting the oxidant exhaust manifold of the fuel cell stack through the hot side passage of the reformer does not exceed 33%.

6. The fuel cell system of claim 1 wherein the oxidant exiting the oxidant exhaust manifold of the fuel cell stack through the hot side passage of the reformer is between 10% and 33%.

7. The fuel cell system of claim 1, further comprising a second heat exchanger comprising a plurality of cold side channels and a plurality of hot side channels, the second heat exchanger configured to receive oxidant from the compressor in the cold side channels and configured to receive exhaust from the turbine in the hot side channels.

8. The fuel cell system of claim 1, wherein the fuel is natural gas.

9. The fuel cell system of claim 1, wherein the fuel is hydrogen.

10. The fuel cell system of claim 1, wherein the combustor is configured to directly receive fuel from the unreformed fuel source and oxidant from the oxidant source.