Fuel Cell Systems

Abstract

A fuel cell system (200, 300) and a method for operating the fuel cell system (200, 300), comprising an anode inlet (226) and an anode outlet (227), means for heating the stack (205), an anode off-gas recirculation loop (240) configured to provide a gas flow path for recirculating anode off-gas from the anode outlet (227) to the anode inlet (226), and a controller (290). The method includes, upon start-up of the fuel cell system (200, 300), heating the stack (205) to a first threshold temperature; supplying unreformed hydrocarbon fuel from a fuel supply (225) to an anode inlet (226) at a first fuel flow rate when the stack (205) exceeds the first threshold temperature, but not before the stack (205) exceeds the first threshold temperature; recirculating anode off-gas from the anode outlet (227) to the anode inlet (226) while supplying unreformed fuel to the anode inlet (226); and drawing a current from the fuel cell system (200, 300) while recirculating the anode off-gas.

Description

[Technical field]

[0001] The present invention relates to fuel cell systems and methods, and more particularly to start-up and shut-down of fuel cell systems. [Background technology]

[0002] Teachings regarding fuel cells, fuel cell stacks, fuel cell stack assemblies, and heat exchanger systems, arrangements, and methods are well known to those skilled in the art, and include, in particular, applicant's International Publication No. WO2015004419A, which is incorporated herein by reference in its entirety. Definitions of terms used herein may be found, where appropriate, in said publication. In particular, the present invention seeks to improve upon the systems and methods disclosed in International Publication No. WO2015004419A.

[0003] The present invention relates to a fuel cell system in which a hydrocarbon fuel is supplied to the anode inlet of a fuel cell stack, and a method of operating the system.

[0004] A typical fuel cell converts chemical energy in the form of fuel and oxidant into electrical energy. The fuel used by the fuel cell can be hydrogen gas and oxidant oxygen, and the exhaust gas is limited to water. The fuel cell system is preferably operated with natural gas, which is then reformed into hydrogen in the fuel cell system. This requires a supply of water to reform the natural gas into hydrogen.

[0005] The operation of hydrocarbon fueled fuel cell (e.g. SOFC) systems where the fuel cell stack operates in the temperature range of 450-650 °C (intermediate-temperature solid oxide fuel cells: IT-SOFC), and especially in the temperature range of 520-620 °C, presents a series of challenging technical problems.

[0006] In such systems, steam reforming in a reformer is commonly used to convert a hydrocarbon fuel stream (such as natural gas) into a hydrogen-rich reformate stream to be fed to the anode inlet of the fuel cell stack. WO2015004419A discloses one such system in which the hydrocarbon fuel is reformed into a hydrogen-rich reformate stream before being fed to the anode inlet of the stack. In such systems, a supply of steam is provided to reform the hydrocarbon fuel in the reformer. This requires a supply (e.g., a tank) of water and a means to heat liquid water to steam, which can be used during start-up (e.g., from ambient temperature) and during steady-state operation (i.e., when current is being drawn from the fuel cell system at operating temperature). When the fuel cell system is used at steady state, the fuel cell itself produces water, which can be removed from the stack and used in the reformer. The system may utilize a condenser to separate the water from the off-gas for refilling the water tank and subsequent use in the reformer.

[0007] A typical start-up sequence (e.g., starting from ambient temperature) involves heating the stack and reformer and supplying steam / fuel streams to the reformer and stack for a period of time before drawing current from the stack. Steam is supplied to the reformer via a separate deionized water supply and steam generator. This allows the steam / methane reforming (SMR) reaction to occur in the reformer, releasing hydrogen for the fuel cell reaction and providing a reducing atmosphere on the anode to prevent oxidation of the anode. Supplying steam in the supply to the reformer provides thermodynamically unfavorable conditions for carbon deposition reactions in the reformer (and in the stack). Carbon deposition reactions include the Bouduard reaction (2CO<->CO2+C), CO reduction reaction (H2+CO<->H2O+C), and methane decomposition reaction (CH4<->2H2+C). These reactions lead to carbon accumulation, which over the life of the system can lead to deactivation of the fuel cell anode and / or reforming catalyst and blockage of the fuel supply channels. This in turn leads to fuel starvation. Start-up and steady-state conditions in fuel cell systems are typically designed to avoid such carbon deposition reactions. The rate of carbon deposition from such reactions is lower at low temperatures, but other problems arise at low temperatures. For example, when hydrocarbon fuels flow over nickel-containing components in a fuel cell system, potentially dangerous nickel tetracarbonyl (NiCO4) can form.

[0008] Some fuel cell systems have also been proposed that eliminate the water tank and the means for heating liquid water to steam. Some of these systems use a partial oxidation (POX) reactor in addition to the reformer to generate a hydrogen-rich stream from a hydrocarbon fuel source and feed it to the anode inlet. JP 2012-243564 is an example of such a system, where the hydrocarbon fuel feed is partially oxidized in the POX reactor with an oxidant to produce molecular hydrogen and carbon monoxide. In JP 2012-243564, the output of the POX reactor is fed to a steam reformer and then to the anode inlet of the stack. Such systems utilize the POX reactor during start-up, as they do not require a water supply. In JP 2012-243564, hydrogen produced by the POX during start-up of the system is utilized by the fuel cell stack, and the water produced therein can be recycled to the POX reactor and steam reformer to reform the hydrocarbon fuel. This allows start-up without a water supply, but does require a POX reactor. US2005181247A, WO03092102, and WO03065488 are further examples of similar systems, which use a reformer as a POX reactor during start-up, possibly with an external steam supply.

[0009] Other systems, such as DE 102009053839 and JP 2009-099264, start by directing hydrocarbon fuel from a source to a combustor (e.g., a burner), where water in the combustion products is utilized to reform the hydrocarbon fuel into hydrogen, which is supplied to the anode of the fuel cell stack.

[0010] US Patent Application Publication No. 2018 / 145351 A1 relates to a fuel cell system shutdown procedure. US Patent Application Publication No. 2018 / 145351 A1 seeks to reduce anode oxidation and carbon deposition during fuel cell system shutdown using a relationship between fuel and steam flow rates (fuel and steam are mixed and fed to the anode via an anode recirculation loop), current drawn from the fuel cell stack, and temperature of the fuel cell stack. Steam is provided from a source external to the fuel cell system. Anode off-gas exhausted from the anode of the fuel cell stack is returned to and mixed with the tubes carrying the fuel and steam. A portion of the mixed gas is fed to the anode inlet by the anode recirculation loop, and the remainder is exhausted from the fuel cell system as exhaust. This portion is not part of the relationship used to reduce anode oxidation and carbon deposition during shutdown.

[0011] There are also systems that temporarily supply hydrogen for the start-up of a fuel cell system. DE 102013226305 provides one such example. This patent provides a metal hydride storage tank to store hydrogen for use during start-up of the fuel cell system. FIG. 1 is drawn on the basis of DE 102013226305. FIG. 1 shows a fuel cell system 10 with a hydrocarbon fuel supply 20, a reformer 14, a metal hydride storage tank 24, and a fuel cell 12. Fuel is supplied to the anode side of the fuel cell at the anode inlet 18. Anode off-gas can be recirculated (28) from the anode outlet of the fuel cell 12 to the reformer 14 via an exhaust gas recirculation line 26. The fuel supplied to the anode inlet 18 can be switched using a valve 32 between a reformed hydrocarbon supply from the reformer and a hydrogen supply from the metal hydride storage tank 24 via a pump 34. During start-up of the fuel cell system of FIG. 1, hydrogen from the metal hydride storage tank 24 is supplied to the fuel cell. The anode off-gas is recycled to the reformer where its moisture is used to reform hydrocarbon fuel and the reformed fuel is supplied to the anode inlet 18 of the fuel cell during steady state operation. A portion of the anode off-gas is exhausted from the system via outlet 22. Summary of the Invention

[0012] It is an object of the present invention to reduce the number of components required for a fuel cell system, thereby reducing size, weight and / or cost.

[0013] SUMMARY OF THE PRESENT EMBODIMENT The present invention seeks to address, overcome or mitigate at least one of the disadvantages of the prior art.

[0014] According to a first aspect, there is provided a method of operating a fuel cell system, the fuel cell system comprising a plurality of cell units arranged in a stack, each cell unit comprising an anode and a cathode separated by an electrolyte, the fuel cell system comprising an anode inlet for supplying an anode inlet gas to each cell unit and an anode outlet for removing an anode off-gas from each cell unit, the method including, at start-up of the fuel cell system, heating the stack to a first threshold temperature, supplying unreformed hydrocarbon fuel from a fuel supply to the anode inlet at a first fuel flow rate when the stack exceeds the first threshold temperature but not before the stack exceeds the first threshold temperature, recirculating anode off-gas from the anode outlet to the anode inlet while supplying unreformed fuel to the anode inlet, and drawing a current from the fuel cell system while recirculating the anode off-gas.

[0015] The fuel cell system start-up method allows for start-up using unreformed hydrocarbon fuel without the use of external (to the stack) reformers, water supplies, partial oxidation reactors, and combustors, thereby significantly reducing the number of fuel cell system components, thereby reducing cost and complexity.

[0016] This method allows current to be drawn from the fuel cell system early in the start-up method. Drawing current means that the fuel cell reaction is ongoing and producing water as one of the by-products. This water can reduce the rate of carbon production (e.g., because it is passed through the rest of the fuel cell or recycled to the anode off-gas). Generally, for electrolytes that allow oxygen ion transport, water is produced on the fuel side of the electrolyte (i.e., the anode side) to reduce carbon deposition.

[0017] By feeding unreformed hydrocarbon fuel to the anode inlet and recycling the anode off-gas from the anode outlet to the anode inlet, the fuel cell system can be started up (started from a resting or cold state) without using a hydrogen fuel supply to the anode inlet. A resting state can be a standby state where the stack is kept above a minimum temperature. A cold state can be existing at ambient temperature.

[0018] Fuel cell systems typically include components that decompose the hydrocarbon fuel when heat is applied. Most fuel cell systems that use hydrocarbon fuels add steam to the hydrocarbon fuel to avoid decomposition with associated carbon deposition, but in this case decomposition is advantageous because it allows for the production of hydrogen for use in the anode of each cell unit. In this way, hydrogen can be produced for use in the fuel cell reaction without the use of a steam supply. Test data shows reliable start-up using the method of the first embodiment with negligible carbon build-up in the stack over 500 cycles.

[0019] Current is drawn from the fuel cell (i.e., a stack of cell units) while the anode off-gas is recycled to the anode inlet and while unreformed hydrocarbon fuel is fed to the anode inlet. Each cell unit then operates according to an electrochemical fuel cell reaction to produce water at the anode. This water is in the form of steam. Since this water is a component of the anode off-gas, it is fed to the anode inlet by recycling the anode off-gas and fed to the cell units in the stack, where the water (steam) can be used in reforming the unreformed hydrocarbon fuel into hydrogen (and carbon monoxide) for use in the cell units. Recirculation may be started before current drawing begins, at the same time as current drawing begins, or immediately after current drawing begins. Recirculation may be started before current drawing begins to minimize the time it takes for the generated steam to reach the anode.

[0020] Water is produced by the stack as a result of being able to draw electrical current. The water is produced in the form of steam, which is recycled from the anode outlet to the anode inlet. In other words, this returns water produced by the fuel cell unit to the stack. The stack may contain various components that catalyze the steam reforming to reform unreformed hydrocarbon fuel into hydrogen, which is used by the fuel cell unit to provide electrical current. This means that no source of water is required for steam reforming (e.g. a water tank, a condenser, and / or an external supply of water with purification means) since all water used to reform fuel in the stack is produced within the stack.

[0021] As with the unreformed hydrocarbon fuel fed to the anode inlet, the anode off-gas recycled from the anode outlet to the anode inlet does not pass through a reformer, and similarly, the anode off-gas recycled from the anode outlet to the anode inlet does not pass through either the combustor or the partial oxidation reactor.

[0022] Recycling anode off-gas from the anode outlet to the anode inlet while supplying unreformed fuel to the anode inlet means that the fluid supplied to the anode inlet is a mixture of unreformed hydrocarbon fuel and anode off-gas.

[0023] The first threshold temperature is selected such that unreformed hydrocarbon fuel is fed to the anode inlet once the temperature of the stack exceeds a temperature at which NiCO4 may be formed. The first threshold temperature is also equal to or greater than the temperature at which a minimum current can be drawn from the fuel cell without the fuel cell voltage being pulled below a voltage at which damage may occur to the fuel cell (specifically the anode, electrolyte, and cathode). By feeding unreformed hydrocarbon fuel to the anode inlet as soon as the stack exceeds the first threshold temperature but not before, the period during which oxidation of the anode may occur can be minimized. This also ensures that unreformed hydrocarbon fuel is initially fed to the anode inlet at a temperature at which the reaction rate of the carbon-forming reaction is low and current draw begins (thereby generating steam), thereby preventing significant carbon buildup within the stack and other components.

[0024] The first threshold temperature may be a temperature at which the hydrocarbon fuel will decompose into hydrogen and carbon in the presence of a catalyst in the stack. Sufficient catalyst must be present in the stack to allow decomposition at a rate necessary to produce hydrogen and allow current to be drawn from the fuel cell system. A variety of materials are typically present in fuel cell systems that are suitable for decomposition catalysts. For example, nickel may catalyze the decomposition and may be present in the anode, the metal support plates that support the anode, electrolyte, and cathode, and in the interconnects and separators between the cell units. The stack may also contain catalysts for internal reforming purposes, and the same catalyst may catalyze the decomposition.

[0025] The unreformed hydrocarbon fuel provided during start-up up to the first threshold temperature may be referred to as a pure hydrocarbon fuel (e.g., by not having significant amounts of hydrogen, oxygen, water, carbon monoxide, and carbon dioxide added). A small percentage (e.g., 10%) of hydrogen may also be added to the unreformed fuel. In either case, the unreformed hydrocarbon fuel is provided to the anode inlet without passing through a reformer or combustor. The unreformed hydrocarbon fuel may include one or more of natural gas, methane, ethane, propane, butane, the corresponding alcohols, and biogas.

[0026] Those skilled in the art will appreciate that the stack can be heated to the first threshold temperature using one or more different methods. For example, an electric heater may be used to heat the stack. Alternatively, fuel can be burned in a burner and used to indirectly provide heat to the stack (typically via one or more heat exchangers). The burner may also be the burner that is also used to combust unused fuel and oxidant in the anode off-gas and cathode off-gas, respectively, during operation of the fuel cell system.

[0027] The fuel cell system may comprise a cathode inlet for supplying a cathode inlet gas to each cell unit and a cathode outlet for removing a cathode off-gas from each cell unit. The cathode inlet gas may be an oxidant, e.g., air. The cell units may comprise a planar substrate (or metal support plate) and separators (or interconnects), which may be similar to those described in the applicant's previous patent application WO 2015 / 136295. The substrate carries electrochemically active layers (or active fuel cell component layers) comprising respective anode, electrolyte and cathode layers. These layers may each be deposited (e.g., as a thin coating / film) on and supported by a metal support plate (e.g., a steel plate or foil), and the electrochemically active layers may face the separator of an adjacent cell unit. The metal support plate may have a porous region surrounded by a non-porous region, and the active layer may be deposited on the porous region, thereby allowing gas to pass through the pores from one side of the metal support plate to the other side to access the active layer coated thereon.

[0028] The method may include monitoring a stack temperature. The stack temperature may be measured directly or indirectly. The method may include controlling a mixture ratio of the unreformed hydrocarbon fuel to the anode off-gas. A fuel flow rate of the unreformed hydrocarbon fuel to the anode inlet is increased as the stack temperature increases.

[0029] Preferably, the method further comprises monitoring the stack temperature and increasing the fuel flow rate of the unreformed hydrocarbon fuel and the current draw when the stack temperature reaches a second threshold temperature that is higher than the first threshold temperature.

[0030] This allows the current draw and stack temperature to be gradually increased towards a steady state operation, which can be achieved by a steady stack temperature in the range of 400-1000°C, preferably 450-800°C, more preferably 500-650°C, additionally or alternatively at a steady voltage in the range of 0.8-1.0 V, and a current draw of 0.05-1.5 A / cm 2, preferably 0.05 to 0.3 A / cm 2 , more preferably 0.1 to 0.25 A / cm 2 The stack is characterized by one or more steady-state current densities in the range of 100-2000 sq. m / s. Supplying fuel initiates electrochemical reactions in each fuel cell unit, thereby drawing electrical current and increasing the temperature of the system. Increasing the fuel flow rate of the unreformed hydrocarbon fuel may also increase the current draw and temperature of the stack. As the temperature of the stack increases, more fuel is available to the cell units, thereby allowing them to supply more electrical current.

[0031] Carbon deposition in the stack (on the anode side of the cell units) is typically governed by the oxygen to carbon ratio in the fuel. A low O:C ratio leads to carbon deposition, for example, by decomposition. Because the fuel cell produces water on the anode side, for a given fuel flow rate of unreformed hydrocarbon fuel, the O:C ratio increases. The rate at which the fuel cell produces water is related to its current draw. As the current increases, the amount of O2 available for the fuel cell reaction increases. As the temperature of the fuel cell increases toward steady-state operation, the fuel cell can utilize a greater proportion of the unreformed hydrocarbon fuel, which in turn produces more water. Each time the fuel flow rate of the unreformed hydrocarbon fuel increases, the O:C ratio decreases. Thus, by increasing the fuel flow rate of the unreformed hydrocarbon fuel and increasing the current draw when the stack temperature reaches a second threshold temperature, the method can increase the O:C ratio at a ratio that minimizes carbon deposition.

[0032] The O:C ratio can be inferred from the stack temperature, the flow rate of anode off-gas from the anode outlet to the anode inlet, the flow rate of unreformed hydrocarbon fuel, and the current draw.

[0033] Additionally or alternatively, the method is configured to control the fuel flow rate of unreformed hydrocarbon fuel, the flow rate of anode off-gas from the anode outlet to the anode inlet, and the current draw to achieve an oxygen to carbon ratio in the stack of up to greater than 2 (preferably greater than 2.2) within a target time, the target time being short enough such that the deleterious effects of oxygen starvation can be substantially reversed during steady-state operation and / or upon shutdown. This may be done while maintaining the fuel cell voltage above a threshold voltage, which may be between 0.6 and 0.8 V, preferably between 0.7 and 0.8 V, and more preferably 0.75 V.

[0034] Preferably, the method includes increasing the fuel flow rate of the unreformed hydrocarbon fuel and increasing the current draw in incremental steps with a corresponding incremental stack temperature. The step size can be as small as measurement and control will allow. In other words, the increases can be made continuously, with voltage-driven control feedback to increase the current without allowing the voltage to drop below a desired minimum.

[0035] This allows the voltage to be maintained above the threshold and the O:C ratio to be increased while the current draw and stack temperature are gradually increased towards steady state operating conditions, and then the O:C ratio can be maintained above the threshold while continuing to approach steady state operating conditions.

[0036] Preferably, the method further comprises decreasing the flow rate of anode off-gas from the anode outlet to the anode inlet as the stack temperature increases above the first threshold temperature.

[0037] For example, at a first threshold temperature, recirculation of anode off-gas from the anode outlet to the anode inlet can begin at a first flow rate. At a second threshold temperature, the flow rate of recirculated anode off-gas from the anode outlet to the anode inlet can be decreased. The temperature (e.g., subsequent temperature thresholds) is then increased and repeated until a steady state operating condition (e.g., current, voltage, and / or temperature) is reached. As the temperature and fuel utilization rate increase, the fuel cell system can utilize a greater proportion of the unreformed hydrocarbon fuel provided to the anode inlet, thereby decreasing the proportion of unused fuel in the anode off-gas. Furthermore, as the fuel utilization rate increases, the O:C ratio can be maintained at a level that minimizes carbon deposition from water, CO, and CO2 produced in the cell units, decreasing the reliance on water, CO, and CO2 recirculated from the anode outlet.

[0038] Preferably, recirculating the anode off-gas when the temperature exceeds the first threshold temperature includes recirculating up to 80% of the anode off-gas, more preferably up to 70% of the anode off-gas. If more than 80% of the anode off-gas is recirculated, CO2 and steam may build up in the system, impairing the performance of the start-up procedure and limiting the amount of current draw.

[0039] Preferably, current is drawn while maintaining the fuel cell voltage above a voltage threshold. The threshold voltage is set to prevent damage to the electrochemically active layers (i.e., anode, electrolyte, cathode). The electrochemically active layers typically allow current to be drawn at various voltages, but drawing current below a certain voltage may damage the electrochemically active layers. The voltage threshold is set at a level that ensures that damage to the electrochemically active layers is avoided while at the same time allowing current to be drawn at low operating temperatures, thereby providing oxygen to the anode side of the electrolyte and allowing the anode to produce oxygen-containing compounds (i.e., water / CO / CO2) from low temperatures.

[0040] Preferably, the voltage threshold is 0.6-0.8V. This provides a balance between avoiding damage to the electrochemically active layers and providing current at low temperatures (thereby providing oxygen-containing compounds to the anode to increase the O:C ratio). This may allow the O:C ratio to exceed 2 (preferably 2.2) in a minimum number of steps, e.g., two steps, and to be maintained above 2 (preferably above 2.2) as the fuel flow rate and current draw of the unreformed hydrocarbon fuel increase toward steady state. Preferably, the voltage threshold is 0.7-0.8V, more preferably 0.75V.

[0041] Preferably, at least one of the anode and electrolyte comprises ceria (cerium oxide). The ceria may be reduced from Ce4+ to Ce3+ during the start-up procedure. Without being limited by theory, it is believed that the ceria acts as an oxygen source, increasing the O:C ratio and reducing carbon production. The oxygen released by the ceria also produces steam in the stack by reaction with H2 from the cracked hydrocarbons. This steam causes steam reforming of the unreformed hydrocarbon fuel into molecular hydrogen for use in the fuel cell system. Thus, the ceria allows the fuel cell system to start without supplying water or hydrogen to the anode side while minimizing carbon deposition at the first threshold temperature. As the O:C ratio increases, the ceria may be oxidized to Ce4+, alternatively or additionally, the ceria may be oxidized during the shutdown procedure.

[0042] Preferably, the electrolyte is of a type that allows oxygen ion transport, such as a solid oxide fuel cell (SOFC). During operation of a fuel cell using an electrolyte capable of oxygen ion transport, water is produced at the anode.

[0043] Preferably, the anode comprises cerium-gadolinium oxide (CGO), which may enable the fuel cell system to provide electrical current at a relatively low first threshold temperature.

[0044] Preferably, the anode comprises nickel, which can catalyze the decomposition of unreformed hydrocarbon fuel to produce hydrogen for use in the fuel cell system at a first threshold temperature. Other components in the fuel cell system can also comprise nickel, thereby catalyzing the decomposition of unreformed hydrocarbon fuel to produce hydrogen. The other components can include, for example, a metal support plate coated or deposited with an electrochemically active layer, or a separator or interconnect plate separating adjacent cell units.

[0045] Preferably, the electrolyte comprises cerium-gadolinium oxide (CGO), which may enable the fuel cell system to provide electrical current at a relatively low first threshold temperature.

[0046] Preferably, the unreformed hydrocarbon fuel supplied during start-up above the first threshold temperature has the same composition as the unreformed hydrocarbon fuel supplied to the anode inlet during steady-state operation, in other words, separate reformer, water and hydrogen supplies are not required for start-up.

[0047] Preferably, the first threshold temperature is in the range of 400 to 500°C, more preferably 400 to 450°C.

[0048] This is the temperature above which NiCO4 may be formed in the presence of a hydrocarbon fuel and at which the fuel cell reaction may begin (i.e., allow current to be drawn). In other words, this is the temperature at which a minimum current may be drawn without causing the fuel cell voltage to drop below a minimum limit. The first threshold temperature is also a temperature at which the reaction rate of the carbon-forming reaction is relatively low, thus preventing significant carbon build-up within the stack or other components. Unreformed hydrocarbon fuel is delivered when the first threshold temperature is reached, and the first threshold temperature may also be set so that unreformed hydrocarbon fuel is delivered before significant oxidation of the anode occurs.

[0049] Those skilled in the art will appreciate that the stack can be heated to the first threshold temperature using one or more different methods. For example, an electric heater can be used to heat the stack. Alternatively, fuel can be burned in a burner and used to indirectly provide heat to the stack (typically via one or more heat exchangers). The burner can be the burner that is also used to combust unused fuel and oxidant in the anode off-gas and cathode off-gas, respectively, during operation of the fuel cell system.

[0050] Preferably, the temperature of the stack is determined by measuring the anode off-gas temperature and / or the anode inlet gas temperature. Typically, measuring the cathode off-gas temperature (e.g., by using a thermocouple placed near the cathode outlet from the stack) is used as an indication of the minimum temperature in the stack, since the temperatures during start-up of the anode and cathode inlets are typically hotter than the respective outlets. Alternatively or additionally, the anode off-gas temperature can be measured (e.g., by using a thermocouple placed near the anode outlet from the stack) as an indication of the temperature in the stack during start-up.

[0051] Preferably, the step of supplying the unreformed hydrocarbon fuel at the first fuel flow rate includes supplying the unreformed hydrocarbon fuel in an amount that provides a distribution across all of the cell units in the stack, such that all of the cell units in the stack are supplied with the unreformed hydrocarbon fuel and are capable of drawing current from all of the cell units, such that the temperature and other conditions are similar in each of the cell units.

[0052] Preferably, water produced by the stack is supplied to the anode inlet of the stack by recycling the anode off-gas, and the method includes reforming the unreformed hydrocarbon fuel with the recycled water at a reforming catalyst positioned between each cell unit and an adjacent cell unit. In other words, each cell unit may include a reforming catalyst, in particular a steam reforming catalyst (sometimes called an internal steam reformer). The amount of reforming catalyst in the stack (e.g., in each cell unit) is sufficient to catalyze the reforming of the unreformed hydrocarbon fuel, for example when the unreformed hydrocarbon fuel is supplied at a fuel flow rate corresponding to a steady-state operation.

[0053] Water is produced by the stack as a result of the electrical current that can be drawn. The water is produced in the form of steam, which is recycled from the anode outlet to the anode inlet. In other words, the water produced in the fuel cell unit is returned to the internal steam reformer, which reforms the unreformed hydrocarbon fuel into hydrogen, which is used by the fuel cell unit to provide electrical current. This means that all water used to reform fuel in the stack is produced within the stack, and no source of water for steam reforming (e.g. a water tank, a condenser, and / or an external supply of water with purification means) is required.

[0054] As with the unreformed hydrocarbon fuel fed to the anode inlet, the anode off-gas recycled from the anode outlet to the anode inlet does not pass through a reformer, and similarly, the anode off-gas recycled from the anode outlet to the anode inlet does not pass through either the combustor or the partial oxidation reactor.

[0055] Preferably, each cell unit in the stack is separated from adjacent cell units by an interconnect structure, the interconnect structure having a coating on a side facing and in fluid communication with the anode of the adjacent cell unit, the coating including a reforming catalyst and configured to reform fuel into hydrogen for use in the stack. Preferably, the reforming catalyst is a steam reforming catalyst, e.g. platinum and / or rhodium. This catalyst may also catalyze decomposition during start-up above a first threshold temperature when only a small amount of water is present.

[0056] Preferably, the method further comprises a shutdown procedure, the shutdown procedure comprising reducing the stack temperature and increasing the flow rate of anode off-gas from the anode outlet to the anode inlet.

[0057] The shutdown procedure reduces the temperature of the fuel cell system in a controlled manner. The shutdown procedure can reverse the steps of the start-up procedure and can allow reoxidation of the ceria that was reduced during the start-up procedure. Alternatively, the ceria can be reoxidized during steady-state operation or by diffusion of oxygen from the atmosphere as the system cools.

[0058] As the stack temperature decreases, the amount of steam produced by the fuel cell reaction decreases, and as a result, the flow rate of anode off-gas from the anode outlet to the anode inlet can be increased (thus increasing the ratio of recycled anode off-gas to unreformed hydrocarbon fuel at the anode inlet) to maintain the O:C ratio in the stack.

[0059] Preferably, reducing the stack temperature in the shutdown procedure involves reducing the fuel flow rate and current draw of the unreformed hydrocarbon fuel while maintaining the fuel flow rate and current draw of the unreformed hydrocarbon fuel above threshold values.

[0060] The system can be shut down by removing the flow of unreformed hydrocarbon fuel to the cell (e.g., by changing the fuel flow to zero in an emergency), but this is likely to cause at least some redox cycling of the anode. The shutdown procedure ensures that the cell receives a reducing fuel stream until the anode is no longer active. In contrast, fuel cell systems typically require a dedicated steam supply for use during shutdown to avoid redox cycling. Current draw is maintained to keep the O:C ratio above 2 (preferably 2.2), thereby preventing carbon production.

[0061] Preferably, the shutdown procedure further comprises terminating the supply of unreformed hydrocarbon fuel when the stack temperature falls below the first threshold temperature, but not before the stack temperature falls below the first threshold temperature.

[0062] This prevents the formation of NiCO4 while the stack continues to cool. The first threshold temperature is the same temperature as the first threshold temperature used during start-up (i.e., in the range of 400-500°C, preferably 400-450°C).

[0063] For system safety, the fuel cell system may be purged with purge gas during shutdown and / or startup, although purging is not necessary in the present system to prevent carbon formation. Alternatively, the shutdown procedure may include stopping the hydrocarbon fuel supply while recirculating AOG if the stack temperature is above a first threshold temperature and below a third threshold temperature.

[0064] Without being limited by theory, it is believed that if the ceria is depleted during start-up, this may initiate reoxidation of the ceria. For example, the third threshold temperature may be about 50° C. higher than the first threshold temperature (e.g., if the first threshold temperature is in the range of 400-450° C., the third threshold temperature is in the range of 450-500° C.).

[0065] According to a second aspect, there is provided a method for operating a fuel cell system, the fuel cell system comprising a plurality of cell units arranged in a stack, each cell unit comprising an anode and a cathode separated by an electrolyte, the fuel cell system comprising an anode inlet for supplying an anode inlet gas to each cell unit and an anode outlet for removing an anode off-gas from each cell unit, the anode inlet gas comprising unreformed hydrocarbon fuel and a portion of the anode off-gas removed from the anode outlet, the method comprising a shutdown procedure comprising drawing current from the fuel cell system whilst supplying unreformed hydrocarbon fuel to the anode inlet and reducing a stack temperature. Reducing the stack temperature preferably comprises reducing the fuel flow rate of the unreformed hydrocarbon fuel and / or reducing the current draw whilst maintaining the fuel flow rate and current draw of the supply of unreformed hydrocarbon fuel to the anode inlet above a threshold value.

[0066] Preferably, the current draw and the fuel flow rate of the unreformed hydrocarbon fuel are reduced in stages (or continuously under feedback control) so that the stack temperature is gradually reduced until the first threshold temperature is reached. Continuous reduction under feedback control includes reducing the fuel without allowing the voltage to fall below a preferred minimum value.

[0067] The method according to the second aspect may be combined with any of the features outlined for the first aspect above.

[0068] According to a third aspect, a fuel cell system is provided. The fuel cell system of the third aspect is configured for use in the methods of the first and second aspects. The fuel cell system comprises a plurality of cell units arranged in a stack, each cell unit including an anode and a cathode separated by an electrolyte, and the fuel cell system comprises an anode inlet for supplying an anode inlet gas to each cell unit, and an anode outlet for removing an anode off-gas from each cell unit; means for heating the stack; means for measuring the temperature of the stack; a fuel inlet configured to connect to a supply of unreformed hydrocarbon fuel and configured to supply unreformed hydrocarbon fuel to the anode inlet; an anode off-gas recirculation loop configured to provide a gas flow path for recirculating the anode off-gas from the anode outlet to the anode inlet; means for drawing a current from the fuel cell system; and a controller configured to receive an input from the measurement means and provide an output to the recirculation loop and the means for drawing a current, the controller controlling the recirculation loop, the supply of unreformed hydrocarbon fuel, and the means for drawing a current, all in response to the measurement means.

[0069] The fuel inlet is configured to supply unreacted hydrocarbon fuel to the anode inlet. In other words, the flow path of the fuel from the fuel source to the anode inlet does not pass through a reformer, a combustor, and a partial oxidation reactor. Similarly, there are no reformers, combustors, or partial oxidation reactors in the anode off-gas recirculation loop. As a result, the fuel cell system is less complex and less costly.

[0070] The controller may receive inputs and provide outputs to carry out the methods of the first and / or second aspects. The controller may provide unreformed hydrocarbon fuel and enable a corresponding current draw from the means for drawing a current that maintains the fuel cell voltage above a threshold voltage. During start-up, the controller may increase the flow rate of unreformed hydrocarbon fuel in steps and enable a corresponding step increase in current draw from the means for drawing a current when the fuel cell system reaches a temperature (and thereby fuel utilisation) at which the step increase can be performed whilst maintaining the fuel cell voltage at some margin above the threshold voltage.

[0071] For example, during the start-up procedure, the fuel cell voltage can be maintained above the threshold voltage, within 15% (more preferably within 10%) of the threshold voltage. In other words, the fuel cell voltage can be maintained between the threshold voltage and a voltage 15% (preferably 10%) higher than the threshold voltage during the start-up procedure. This ensures that the controller will allow a higher fuel flow rate of unreformed hydrocarbon fuel when the fuel cell voltage reaches a level at which a higher fuel flow rate of hydrocarbon fuel can be tolerated, thus achieving improved start-up time.

[0072] The controller may infer the O:C ratio in the stack (i.e., the anode side of each cell unit) from the stack temperature, the flow rate of anode off-gas from the anode outlet to the anode inlet, the flow rate of unreformed hydrocarbon fuel, and the current draw. The controller may adjust one or more of the outputs to achieve one or more of: a) an O:C ratio reaching above 2 (preferably 2.2) during start-up, b) maintaining the O:C ratio above 2 (preferably 2.2) during steady state, c) maintaining the O:C ratio above 2 (preferably 2.2) upon shutdown above a first threshold temperature.

[0073] All water in the fuel cell system is provided by the reaction in the fuel cell itself, in other words, there are no water tanks or means configured to supply water to the system from an external source, resulting in a fuel cell system with reduced complexity and cost.

[0074] The unreformed hydrocarbon fuel supplied to the anode inlet may also be referred to as a pure (as defined above) hydrocarbon fuel. The unreformed hydrocarbon fuel is supplied to the anode inlet without passing through a reformer or combustor. The unreformed hydrocarbon fuel may include one or more of natural gas, methane, ethane, propane, butane, the corresponding alcohols, and biogas.

[0075] The fuel cell system may also include a cathode inlet for supplying a cathode inlet gas to each cell unit and a cathode outlet for removing a cathode off-gas from each cell unit. The cathode inlet gas may be an oxidant, e.g., air. The cell units may include a planar substrate (or metal support plate) and separators (or interconnects), which may be similar to those described in the applicant's previous patent application WO 2015 / 136295. The substrate carries electrochemically active layers (or active fuel cell component layers) comprising respective anode, electrolyte and cathode layers. These layers may each be deposited (e.g., as a thin coating / film) on and supported by a metal support plate (e.g., a steel plate or foil), and the electrochemically active layers may face the separator of an adjacent cell unit. The metal support plate may have a porous region surrounded by a non-porous region, and the active layer may be deposited on the porous region, thereby allowing gas to pass through the pores from one side of the metal support plate to the other side to access the active layer coated thereon.

[0076] The means for measuring the temperature of the stack may comprise means for measuring the anode off-gas temperature and / or the cathode off-gas temperature. Typically, measurement of the cathode off-gas temperature (e.g., by using a thermocouple located near the cathode outlet from the stack) is used as an indication of the minimum temperature in the stack, since the temperatures during start-up of the anode and cathode inlets are typically hotter than the respective outlets. Alternatively or additionally, the anode off-gas temperature can be measured (e.g., by using a thermocouple located near the anode outlet from the stack) as an indication of the temperature in the stack during start-up.

[0077] A person skilled in the art will appreciate that the means for heating the stack (e.g. the first threshold temperature) may comprise one or more different means. For example, an electric heater may be used to heat the stack. Alternatively, fuel may be burned in a burner and used to indirectly provide heat to the stack (typically via one or more heat exchangers). The burner may be the burner that is also used to combust unused fuel and oxidant in the anode off-gas and cathode off-gas, respectively, during operation of the fuel cell system.

[0078] The means for drawing current may be a load that the fuel cell is configured to power, and the controller may limit the current drawn by the load during start-up and shutdown.

[0079] Preferably, the anode off-gas recirculation loop comprises means configured to vary the flow rate of anode off-gas in the anode off-gas recirculation loop, the means being controlled by the controller in response to the means for measuring and the means for drawing a current.

[0080] The means configured to vary the flow rate of the anode off-gas in the anode off-gas recirculation loop may comprise a variable splitter, pump or ejector configured to operate in the anode off-gas recirculation loop (e.g. at "hot" temperatures above 500° C. or "warm" temperatures above 120° C. to prevent condensation in the anode off-gas). Preferably, the means configured to control the flow rate of the anode off-gas thereby recirculates a portion of the anode off-gas from the anode outlet to the anode, and the fuel cell system further comprises an exhaust gas flow path configured to remove the remaining portion of the anode off-gas from the anode outlet and out of the fuel cell system.}

[0081] This allows the controller to control the O:C ratio in the fuel cell system by varying the flow rate of the anode off-gas in the anode off-gas recirculation loop.

[0082] Preferably, the anode off-gas recirculation loop further comprises a flow path for the anode off-gas from the anode outlet, through a heater section, a mixing section configured to mix the anode off-gas with unreformed hydrocarbon fuel, and to the anode inlet.

[0083] The heater section may include at least one heat exchanger. The heater section may include a first heat exchanger configured to transfer heat from the anode off-gas to the unreformed hydrocarbon fuel before the anode inlet. The heater section may include a second heat exchanger configured to transfer heat from the anode off-gas to the oxidant provided to the cathode inlet. In examples where both a first and a second heat exchanger are present, the second heat exchanger follows the first heat exchanger in the anode off-gas recirculation loop.

[0084] Preferably, the fuel cell system further comprises a splitter after the anode outlet and before the mixing section to split a portion of the anode off-gas into the anode off-gas recirculation loop and route the remaining portion of the anode off-gas out of the fuel cell system. If present, the first heat exchanger is before the splitter and the second heat exchanger is after the splitter, and a pump or ejector (if used as a means to control recirculation) is positioned after the second heat exchanger in the anode off-gas recirculation loop. The remaining portion of the anode off-gas can be routed out of the fuel cell system via a burner to combust combustibles with the oxidant in the cathode off-gas. Heat can be recovered from the combustion products by transferring heat to the oxidant in the third heat exchanger, followed by transferring heat to the unreformed hydrocarbon fuel in the fourth heat exchanger, for example before suitable exhaust via a flue.

[0085] Preferably, each cell unit in the stack is separated from an adjacent cell unit by an interconnect structure having a coating on a side facing and in fluid communication with the anode of the adjacent cell unit, the coating including a reforming catalyst configured to reform unreformed hydrocarbon fuel into hydrogen for use in the stack.

[0086] In other words, each cell unit may include a reforming catalyst, in particular a steam reforming catalyst. The amount of reforming catalyst in the stack (e.g. in each cell unit) is sufficient to catalyze the reforming of unreformed hydrocarbon fuel, for example when the unreformed hydrocarbon fuel is supplied at a fuel flow rate corresponding to stationary operation. Preferably, the reforming catalyst is a steam reforming catalyst, for example platinum and / or rhodium. This catalyst may also catalyze decomposition during start-up above the first threshold temperature when only a small amount of water is present.

[0087] Preferably, the electrolyte allows oxygen ion transport, so that the fuel cell reaction produces water on the anode side of the cell unit. The water thus produced is a component of the anode off-gas and is produced in the unreformed hydrocarbon fuel, so that it can be used in steam reforming the unreformed hydrocarbon fuel. For example, solid oxide fuel cells allow oxygen ion transport.

[0088] Preferably, at least one of the anode and electrolyte includes ceria. Without being limited to theory, the ceria may be reduced from Ce4+ to Ce3+ during the start-up procedure. This allows the ceria to act as an oxygen source, increasing the O:C ratio and reducing carbon production. The oxygen released by the ceria also produces steam that is used in the steam reforming of unreformed hydrocarbon fuels in the stack into molecular hydrogen for use in the fuel cell system. The ceria thus allows the fuel cell system to start up without supplying water or hydrogen to the anode side while minimizing carbon deposition at the first threshold temperature. As the O:C ratio increases, the ceria may be oxidized to Ce4+; alternatively or additionally, the ceria may be oxidized during the shutdown procedure.

[0089] Preferably, the electrolyte is of a type that allows oxygen ion transport, such as a solid oxide fuel cell (SOFC).Water is produced at the anode during operation of a fuel cell using an electrolyte that allows oxygen ion transport.

[0090] Preferably, the anode comprises CGO, which may enable the fuel cell system to provide electrical current at a relatively low first threshold temperature.

[0091] Preferably, the anode comprises nickel, which can catalyze the decomposition of unreformed hydrocarbon fuel to produce hydrogen for use in the fuel cell system at a first threshold temperature. Other components in the fuel cell system can also comprise nickel, thereby catalyzing the decomposition of unreformed hydrocarbon fuel to produce hydrogen. The other components can include, for example, a metal support plate coated or deposited with an electrochemically active layer, or a separator or interconnect plate separating adjacent cell units.

[0092] Preferably, the electrolyte comprises CGO, which may enable the fuel cell system to provide electrical current at a relatively low first threshold temperature.

[0093] Preferably, the controller is configured to control the start-up of the fuel cell system, the controller being configured to: control heating of the stack to a first threshold temperature; control supply of unreformed hydrocarbon fuel to the anode inlet and provide a non-zero fuel flow when the controller determines that the first threshold temperature has been reached, and not before; control the flow rate of anode off-gas in the anode off-gas recirculation loop at the first non-zero flow rate; and enable drawing current from the fuel cell.

[0094] Similar to the method of the first embodiment, the controller enables start-up of the fuel cell system using unreformed hydrocarbon fuel.

[0095] Preferably, the controller is further configured to gradually increase the supply of unreformed hydrocarbon fuel to the anode inlet and gradually increase the current draw to increase the temperature and current to a steady state.

[0096] Preferably, the controller is configured to regulate the supply of unreformed hydrocarbon fuel to the anode inlet, the flow rate of anode off-gas in the anode off-gas recirculation loop, and the current draw while maintaining the voltage of the fuel cell above a threshold voltage.

[0097] The threshold voltage is set to prevent damage to the electrochemically active layers (i.e., anode, electrolyte, cathode). The electrochemically active layers typically allow current draw at a range of voltages, but may damage the electrochemically active layers if current is drawn below a certain voltage. The voltage threshold is set at a level that ensures damage to the electrochemically active layers is avoided while at the same time allowing current draw at low operating temperatures, thereby providing oxygen to the anode side of the electrolyte and allowing the anode to produce oxygen-containing compounds (i.e., water / CO / CO2) from low temperatures.

[0098] The threshold voltage may be 0.6-0.8 V, preferably 0.7-0.8 V, more preferably 0.75 V. This provides a balance between avoiding damage to the electrochemically active layers and providing current at low temperatures (thereby providing oxygen-containing compounds to the anode to increase the O:C ratio). This may allow the O:C ratio to exceed 2 (preferably 2.2) in a minimum number of steps, e.g., 2 steps, and to maintain the O:C ratio above 2 (preferably 2.2) as the fuel flow rate and current draw of the unreformed hydrocarbon fuel increase toward steady state.

[0099] Additionally or alternatively, the controller is configured to control the fuel flow rate of unreformed hydrocarbon fuel, the flow rate of anode off-gas from the anode outlet to the anode inlet, and the current draw to cause the oxygen to carbon ratio in the stack to be above 2 (preferably above 2.2) within a target time, the target time being short enough such that the deleterious effects of oxygen deficiency can be substantially reversed during steady-state operation and / or upon shutdown.

[0100] Preferably, the controller is configured to control (a) the oxygen to carbon ratio of the gas in communication with the anode, and (b) the temperature of the stack.

[0101] According to one aspect, a controller is provided that is configured to receive a signal indicative of one or both of an anode inlet temperature and an anode outlet gas temperature and to control a flow rate of anode off-gas through the anode off-gas recirculation loop in accordance with the method described above. [Brief description of the drawings]

[0102] [Figure 1] 1 is a schematic diagram of a prior art fuel cell system; [Diagram 2] 1 is a schematic diagram of a fuel cell system according to the present invention; [Diagram 3] 1 is a schematic diagram of a fuel cell system according to the present invention; [Figure 4] 4 shows a start-up procedure in a fuel cell system according to the present invention. [Diagram 5] 3 shows a cycle including a start-up procedure, a steady-state operation, and a shut-down procedure in a fuel cell system according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0103] In the figures and description that follow, like reference numerals are used to refer to like elements in different figures.

[0104] Referring to FIG. 2, a schematic diagram of a fuel cell system 200 is shown. The fuel cell system 200 includes a stack 205 of cell units, also called a "cell stack" (only one cell unit is shown for clarity). A plurality of cell units form a stack of cell units. Each cell unit includes an anode 210, a cathode 220, and an electrolyte 215 positioned between the anode 210 and the cathode 220. The anode 210, the electrolyte 215, and the cathode 220 may be collectively referred to as an electrochemically active layer, an active electrochemical cell layer, or an electrochemically active area. The electrolyte 215 conducts negative oxygen ions or positive hydrogen ions between the anode 210 and the cathode 220. Solid oxide fuel cell (SOFC) systems that generate electricity are based on a solid oxide electrolyte that conducts negative oxygen ions from the cathode to the anode, which is located on the opposite side of the electrolyte. Thus, a fuel or reformate contacts the anode, and an oxidant, such as air or an oxygen-rich fluid, contacts the cathode. In a fuel cell with an electrolyte that conducts negative oxygen ions from the cathode to the anode, water is produced by the fuel cell reaction at the anode. Traditional ceramic-supported (e.g., anode-supported) SOFCs have low mechanical strength and are prone to breakage. Recently, metal-supported SOFCs have been developed in which the active fuel cell component layers are supported on a metal substrate. In these cells, the ceramic layers perform only electrochemical functions and can be made very thin. That is, the ceramic layers are not self-supporting, but rather are thin coatings / films laid on and supported by a metal substrate. Such metal-supported SOFC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOFCs, and can be fabricated using conventional metal welding techniques. In the example of FIG. 2, the anode 210 can include ceria, for example in the form of CGO. The electrolyte 215 can include ceria, for example in the form of CGO.

[0105] The stack 205 may also include a stack of planar fuel cell units. The stack of planar cell units may be based on either solid oxide electrolyte, polymer electrolyte membrane, or molten electrolyte, or any other variant capable of electrochemistry. In one example, the stack 205 is based on a plurality of planar cell units (e.g., tens to hundreds of cell units) with a solid oxide electrolyte, so that the fuel cell may be referred to as a solid oxide fuel cell (SOFC). The solid oxide electrolyte may be supported by a foil (not shown), in which case it may be referred to as a metal-supported cell, specifically a metal-supported solid oxide fuel cell (MS-SOFC).

[0106] Each cell unit may comprise a separator plate and a metal substrate plate (not shown) that supports the cells. The metal substrate plate supports an active electrochemical cell layer (i.e., one in which electrochemical reactions occur during operation) bonded thereto, which may be coated, deposited, or otherwise affixed thereto, and thus the cell unit may be referred to as a metal-supported cell unit. The separator plate may separate the oxidant fluid volume from the fuel fluid volume in each cell unit of the stack, and typically comprises a 3D contoured structure comprising, for example, a pattern of spaced channels and ribs, or spaced dimples, to control fluid flow. The separator plate or interconnects between adjacent cell units in the stack may be coated on the anode-facing side of a given cell unit with a catalyst configured to catalyze steam reforming of unreformed hydrocarbon fuel to produce hydrogen gas in the stack. The reforming catalyst may be referred to as an internal reformer. In one example, each cell unit in the stack is separated from adjacent cell units by an interconnect structure (e.g., the separator and / or interconnect mentioned above), and the interconnect structure has a coating on a side facing and in fluid communication with the anode of the adjacent cell unit, the coating including a reforming catalyst and configured to reform fuel into hydrogen for use in the stack. The reforming catalyst can be a steam reforming catalyst, e.g., platinum and / or rhodium. This catalyst may also catalyze decomposition during start-up when negligible water is present above a first threshold temperature.

[0107] The fuel cell system 200 further comprises a fuel inlet 225 configured to connect to a supply of unreformed hydrocarbon fuel (not shown). The fuel inlet 225 supplies unreformed hydrocarbon fuel to an anode inlet 226 of the stack 205 for distribution to the anode side (also referred to as the fuel volume) of the cell units in the stack 205. An anode outlet 227 of the stack 205 provides an exhaust to the stack and allows for the removal of fluids from the anode side of the cell units in the stack 205. Fluids removed from the stack via the anode outlet 227 are referred to as anode off-gas. The anode off-gas is routed along a flow path 235 to a splitter 265. A portion (e.g., a first portion) of the anode off-gas from the flow path 235 can be routed around an anode off-gas recirculation loop 240 from the anode outlet 227 to the anode inlet 226. 2, a portion of the anode off-gas in the recirculation loop 240 is mixed with unreformed hydrocarbon fuel from the fuel inlet 225 in a mixer 270 positioned between the fuel inlet 225 and the anode inlet 226. A means for varying the flow rate of the anode off-gas in the anode off-gas recirculation loop 240 is provided by an ejector or pump 245. The ejector or pump 245 is positioned in the anode off-gas recirculation loop 240 between the splitter 265 and the mixer 270.

[0108] The anode off-gas may also be routed from splitter 265 through flow path 250 to burner 255. The amount of anode off-gas routed through flow path 250 may be referred to as the remaining portion of the anode off-gas (or alternatively the second portion, the sum of the first portion routed to anode off-gas recirculation loop 240 and the second portion routed through flow path 250 equals the total amount of anode off-gas in flow path 235; for example, the mass flow rate in flow path 235 may equal the sum of the mass flow rates in anode off-gas recirculation loop 240 and flow path 250).

[0109] The fuel cell system 200 further comprises an oxidant inlet 230 configured to connect to a supply of oxidant. The oxidant can be, for example, air or oxygen. The oxidant inlet 230 supplies the oxidant to a cathode inlet 231 of the stack 205 for distribution within the stack 250 to the cathode side (also referred to as the oxidant volume) of the cell units in the stack 205. A cathode outlet 232 of the stack 205 provides an exhaust to the stack 205 and allows for the removal of fluids from the cathode side of the cell units in the stack 205. Fluids removed from the stack via the cathode outlet 232 are referred to as cathode off-gas. The cathode off-gas is routed to a burner 255.

[0110] Burner 255 is configured to combust remaining combustible fuel in the anode off-gas and oxidant in the cathode off-gas and exhaust the resulting gases from fuel cell system 200 via exhaust path 260, which may include a flue, and may cool the resulting gases before exhausting them to the atmosphere.

[0111] The fuel cell system 200 further comprises a controller 290 configured to measure and provide an output for adjusting parameters of the system. The controller 290 communicates with a means 280 for measuring the temperature of the cathode off-gas, which may include a thermocouple. The controller may use the temperature of the cathode off-gas as an indication of the temperature of the cell units in the stack 205 (which may be an indication of the minimum temperature in the stack, for example). The controller 290 communicates with an ejector or pump 245, which the controller may control to vary the flow rate of the anode off-gas in the anode off-gas recirculation loop 240. The controller also communicates with a means 285 for adjusting the supply of unreformed hydrocarbon fuel from the fuel inlet 225 to the anode inlet 226. The adjusting means 285 may comprise a controllable flow restrictor. The controller may thereby control the mass flow rate of the unreformed hydrocarbon fuel to the stack 205.

[0112] The fuel cell system 200 further comprises a means (not shown) for heating the stack, which may include an electric heater or the combustion of a hydrocarbon fuel in a burner, in a manner known to those skilled in the art. The fuel cell system 200 also includes a means (not shown) for drawing a current from the stack 205, and a measuring means (not shown) for measuring the voltage of the stack 205. The means for drawing a current are in communication with a controller 290, which may limit or otherwise specify the current drawn from the stack 205. The means for measuring the voltage of the stack 205 (and thus the voltage between the anode and cathode of the cell units) are in communication with the controller 290, which uses the voltage of the stack 205 as one of its inputs.

[0113] During the start-up procedure of the fuel cell system 200, the controller 290 controls the heating means to heat the stack 205 to a first threshold temperature, as measured by the temperature measuring means 280. When the stack reaches the first threshold temperature, the controller controls the regulating means 285 to allow unreformed hydrocarbon fuel to flow from the fuel inlet 225 and be fed to the anode inlet 226. The unreformed hydrocarbon fuel may include one or more of natural gas, methane, ethane, propane, butane, the corresponding alcohols, and biogas. It may also be referred to as a pure hydrocarbon fuel (e.g., by not adding hydrogen or oxygen, i.e., by not adding molecular hydrogen, water, carbon monoxide, carbon dioxide). The flow rate of the unreformed hydrocarbon fuel may be such as to evenly distribute the unreformed hydrocarbon fuel to the cell units in the stack 205. The controller may cause the ejector or pump 245 to start the recirculation of the anode off-gas from the anode outlet 227 to the anode inlet 226. The recycled anode off-gas mixes with the unreformed hydrocarbon fuel in the mixer 270, so that the fuel supplied to the anode inlet 226 is a mixture of the anode off-gas supplied by the anode off-gas recirculation loop 240 and the unreformed hydrocarbon fuel supplied by the fuel inlet 225. The controller 290 can vary the ejector or pump 245 and the regulating means 285 to control the relative amount of unreformed hydrocarbon fuel and the recycled anode off-gas at the anode inlet. The proportion of the anode off-gas recycled in the anode off-gas recirculation loop is up to 80% of the anode off-gas in the flow path 235 (in other words, the first portion is up to 80% and the second portion in the flow path 250 is at least 20%). This can prevent excessive amounts of CO and CO2 from accumulating in the stack.

[0114] The first threshold temperature is in the range of 400-500° C. (although in other examples it may be in the range of 400-450° C.). This is equal to or greater than the lowest temperature at which a minimum current can be drawn from the fuel cell without the fuel cell voltage being pulled below a voltage at which damage to the fuel cell (specifically, the anode, electrolyte, and cathode) may occur. Thus, when the supply of unreformed hydrocarbon fuel begins, the controller permits current draw at the first current.

[0115] The first temperature is the temperature at which the unreformed hydrocarbon fuel decomposes into at least molecular hydrogen (referred to herein as hydrogen) and carbon. The decomposition is catalyzed by various materials in the stack 205, such as nickel in the metal components. This results in the initial production of hydrogen in the stack.

[0116] Without being bound by theory, it is believed that the ceria present in the anode and / or electrolyte is reduced from Ce4+ to Ce3+ during start-up, releasing oxygen on the anode side of the cell unit which is used in the steam reforming of unreformed hydrocarbon fuel in the presence of a suitable catalyst and contributes to the hydrogen initially produced.

[0117] The fuel cell utilizes this initially produced hydrogen for the fuel cell reaction, which allows it to draw electrical current from the stack 205. The fuel cell reaction produces water in the form of steam as a by-product. This steam is produced at the anode side of the stack in FIG. 2 because the electrolyte 215 conducts oxygen ions. The steam at the anode side is a component of the anode off-gas exhausted at the anode outlet 227, which is recycled by the anode off-gas recirculation loop 240 to the anode inlet and fed to the anode side of the cell unit where, in the presence of a steam reforming catalyst, unreformed hydrocarbon fuel may be reformed into hydrogen for use in the fuel cell reaction. The fuel cell reaction is exothermic, which causes an increase in the temperature of the stack. As the steam content of the anode off-gas increases due to the fuel cell reaction, the steam content fed to the anode inlet by the anode off-gas recirculation loop also increases. As a result of the increased temperature, the fuel cell stack voltage increases. As the fuel cell stack voltage increases, the maximum current draw also increases and the fuel cell can receive a larger fuel flow rate of unreformed hydrocarbon fuel at the anode inlet, so the controller begins to increase the flow rate of unreformed hydrocarbon fuel, which causes a drop in stack voltage, but this is restored as the stack temperature increases (due to the fuel cell reaction).

[0118] The voltage and temperature eventually increase from the first temperature and first threshold voltage to a steady state operating temperature and voltage condition. During the start-up procedure, the controller increases the fuel flow rate of the unreformed hydrocarbon fuel when the stack voltage reaches a value such that an increase in fuel flow rate does not cause the stack voltage to fall below a minimum value. The minimum voltage is the voltage at which damage may occur to the electrochemically active layers of the cell units. An increase in the fuel flow rate of the unreformed hydrocarbon fuel may similarly increase the current draw and increase the temperature of the stack. As the temperature of the stack increases, more fuel is available to the cell units, thereby enabling them to supply a higher current.

[0119] Steady state operating conditions are characterized by a steady state stack temperature in the range of 400-1000° C., preferably 450-800° C., and more preferably 500-650° C. Flow in the AOGR loop 240 continues in steady state operation, allowing for recirculation of steam from the anode outlet to the anode inlet for use in the internal reformer in the stack 205, allowing for reuse of unused fuel.

[0120] The start-up procedure may also include an initial step of purging the stack before heating it. The pre-purging environment typically has old fuel (from a previous run) in the anode and diffused air from the cathode / exhaust. A nitrogen gas supply is used for purging. However, it should be understood that purging is not necessary when operating with the start-up procedure described herein.

[0121] During the shutdown procedure, the ratio of anode off-gas to unreformed hydrocarbon fuel fed to the anode inlet 226 may be increased in a stepwise or repetitive manner, and the current draw may be reduced as well. This results in a gradual decrease in the temperature of the stack towards a first threshold temperature, at which point the supply of unreformed hydrocarbon fuel, the flow of anode off-gas in the AOGR loop 240, and the current draw are stopped. The fuel cell system 200 may then be cooled to an off state (e.g., cooled to ambient temperature) or maintained in a quiescent state (e.g., maintained above ambient temperature by heating means), and then restarted by a start-up procedure. The shutdown procedure may allow for reoxidation of ceria in the anode and / or electrolyte that was reduced during the start-up procedure.

[0122] Test data indicates that the fuel cell system 200 can be operated as described herein for at least 500 start-up, steady-state, and shutdown cycles with negligible carbon deposits, and thus the procedures described herein provide for reliable operation of the fuel cell system.

[0123] Referring to Figure 3, there is shown a schematic diagram of a fuel cell system 300. Fuel cell system 300 is a variation of fuel cell system 200 of Figure 2, exhibiting various optional additional features.

[0124] The fuel cell system 300 includes an AOGR loop 340 similar to the AOGR loop 240 of Figure 2, but which further includes a heater section comprising at least one heat exchanger. The heater section of Figure 3 includes a first heat exchanger 310, sometimes referred to as a secondary fuel heater, a second heat exchanger 315, sometimes referred to as an AOG cooler, a third heat exchanger 305, sometimes referred to as a fuel heater, and a fourth heat exchanger 320, sometimes referred to as an air preheater.

[0125] The first heat exchanger 310 receives the anode off-gas from the anode outlet 227 via the flow path 235 and is configured to transfer heat from the anode off-gas to the unreformed hydrocarbon fuel prior to the anode inlet 226. Following the first heat exchanger 310, the anode off-gas is routed to the splitter 265. A portion of the anode off-gas routed via the AOGR loop 340 is routed from the splitter 265 to the second heat exchanger 315. The second heat exchanger 315 is configured to transfer heat from the anode off-gas to the oxidant provided by the oxidant inlet 230. Following the second heat exchanger 315, the anode off-gas proceeds around the AOGR loop 340 via the ejector or pump 245 to the mixer 270, similar to the AOGR loop 240 of FIG.

[0126] In the mixer 270 (in the anode inlet flow path), the anode off-gas recirculated through the AOGR loop 340 can be mixed with unreformed hydrocarbon fuel from the fuel inlet 225 and then passed through a third heat exchanger 305 used to transfer heat to the mixture of the anode off-gas and unreformed hydrocarbon fuel. The mixture of the anode off-gas and unreformed hydrocarbon fuel is then passed through a first heat exchanger 310 where heat is transferred from the anode off-gas to the mixture. The mixture is then fed to the anode inlet 226.

[0127] Referring to the second heat exchanger 315 , this heat exchanger is configured to transfer heat from the anode off-gas in the AOGR loop 340 to the oxidant provided by the oxidant inlet 230 .

[0128] The oxidant heated by the second heat exchanger 315 may be combined with additional oxidant from the oxidant inlet 230 and routed through the fourth heat exchanger 320 .

[0129] In the fourth heat exchanger 320, heat is transferred from the exhaust gas from the burner 255 in the exhaust path 260 to the oxidant. The oxidant output from the fourth heat exchanger 320 is routed to the cathode inlet 231. The exhaust gas is routed from the fourth heat exchanger 320 to the first heat exchanger 305, which is configured to transfer heat to a mixture of unreformed hydrocarbon fuel and anode off-gas. The exhaust gas is then routed out of the system by flow path 325, for example through a flue to atmosphere.

[0130] In the above, the flow of oxidant may be driven by a fan (not shown), and the flow rate of oxidant through each oxidant path may be driven by one or more fans under the control of the controller 390.

[0131] The burner 255 of the fuel cell system 300 is provided with an unreformed hydrocarbon top-up line fed by the fuel inlet 225. This feed may be controlled by a controller 390, which may utilize the burner 255 to combust the unreformed hydrocarbon fuel fed by the top-up line as a means to heat the fuel cell system to a first threshold temperature (optionally in combination with other heating means, such as an electric heater). In this case, the fuel combusted in the burner 255 produces hot exhaust gases in the exhaust path 260, and heat is transferred from the exhaust gases to the oxidant in the fourth heat exchanger 320, which is routed to the cathode inlet, thereby heating the stack 205.

[0132] The fuel cell system 300 may comprise further means for measuring temperature. These may include a means 330 for measuring the temperature of the cathode inlet gas (in other words, the temperature of the oxidant supplied to the cathode inlet 231), which may be combined with or may replace the means 280 for measuring the temperature of the cathode off-gas. The cathode inlet gas temperature measuring means 330 may be used by the controller 390 to determine the temperature of the stack. The stack temperature may be measured directly by a temperature measuring means 335, which is in communication with the controller 390. Additionally or alternatively, the temperature of the anode off-gas in the AOGR loop may be measured by a temperature measuring means 375 in communication with the controller 390. The temperature measuring means 375 is positioned following the second heat exchanger 315 in the AOGR loop. The temperature measuring means 375 may be used by the controller 390 to ensure that the temperature of the anode off-gas in the AOGR loop 340 is not lowered by the second heat exchanger 315 to such an extent that steam in the AOGR loop may condense. For example, the steam in the anode off-gas recycled to the anode inlet may be referred to as "hot," which may be at a temperature above 500° C., or "warm," which may be at a temperature above 120° C. If the controller determines that the temperature in the AOGR loop is such that the steam may condense, the controller 390 adjusts the oxidant inlet 230 to reduce the amount of oxidant passing through the second heat exchanger 315.

[0133] The start-up, steady state, and shutdown procedures described above for the fuel cell system 200 of FIG. 2 equally apply to the fuel cell system 300 of FIG. 3 with the additional optional means for heating utilising the burner 255 and fourth heat exchanger 320 already described above.

[0134] 4 shows a start-up procedure for the fuel cell system 200 of FIG. 2 or the fuel cell system 300 of FIG. 3. Initially, the stack is heated (by suitable means) to a first threshold temperature 470, which is reached at time 405. This triggers the controller to permit fuel flow of unreformed hydrocarbon fuel at a first fuel flow rate 450 at (or shortly thereafter) time 405. At time 410, following time 405, recirculation of anode off-gas through the AOGR loop from the anode off-gas outlet to the anode inlet is initiated at a first AOGR flow rate. At time 415, following time 410, current draw is initiated at a first current draw level 460. In some cases, anode off-gas recirculation may be initiated before or at the same time that fuel is delivered (in other words, times 405 and 410 may be the same time). In some cases, current may be drawn before or at the same time that anode off-gas recirculation is initiated, but not before fuel is delivered (in other words, time 415 is not earlier than time 405, but time 415 may be earlier than or at the same time as time 410).

[0135] Starting at time 415, current is drawn at a first current draw level 460, which means that the fuel cell reaction is proceeding, thus producing water, which is recirculated by the AOGR loop to the anode inlet, reaching the anode inlet between times 415 and 420. As a result, the ratio of oxygen to carbon in the gas on the anode side of each cell unit starts to increase (from a ratio of zero), resulting in an increased O:C ratio by time 420. Meanwhile, the fuel cell reaction causes further heating of the stack.

[0136] At time 425, the stack reaches a second threshold temperature 475, so the controller may increase the fuel flow rate of the unreformed hydrocarbon fuel to a second fuel flow rate 455 that is greater than the first fuel flow rate 450. This causes the O:C ratio to decrease (because the fuel is unreformed hydrocarbon fuel). Just after time 425, at time 430, the current draw is increased to a second current draw level 465 that is greater than the first current draw level 460. This is because the temperature and / or O:C ratio are such that the stack can supply more current without the stack voltage dropping below a minimum value. As a result of the increased fuel flow rate, the fuel cell reaction continues at a greater rate, increasing the temperature of the stack, thereby increasing the O:C ratio to an increased value at time 435.

[0137] The process of increasing fuel flow and increasing current draw may continue under stepwise or iterative control until a steady state operating condition is reached.

[0138] Figure 5 is test data showing one cycle of the start-up, steady-state, and shutdown procedures for the fuel cell system 200 of Figure 2 or the fuel cell system 300 of Figure 3. The fuel cell system may be operated for at least 500 cycles of start-up, steady-state, and shutdown as described herein with negligible carbon deposition. Thus, the procedures described herein provide reliable operation of the fuel cell system.

[0139] FIG. 5 shows the current 505, stack voltage 510, O:C ratio 540, cathode inlet temperature 545, cathode outlet temperature 550, and the relative amounts of water 515, carbon dioxide 520, hydrogen 525, carbon monoxide 530, and unreformed hydrocarbon fuel 535 (mainly methane in this case) in the anode inlet gas. The O:C ratio is shown on a linear scale, but the intersection of the scale with zero is likely to vary from system to system. Time is shown on the (linear) X-axis relative to an arbitrary starting point, and absolute and relative values ​​may vary from system to system. The start-up cycle may have a duration of several hours to 15 hours, depending on the type and size of the system. Steady state may be allowed to operate for much longer periods of time.

[0140] The first steps of FIG. 5 are similar to those detailed for FIG. 4. First the stack is heated until a first threshold temperature is reached, at which point unreformed hydrocarbon fuel is fed and recirculation is initiated according to FIG. 4 (the amount of unreformed hydrocarbon fuel fed and the amount in the AOGR loop are not shown in FIG. 5). Each time the voltage reaches a certain value, the fuel flow rate and current are increased and the voltage is reduced. Each time the voltage reaches a certain value, the fuel flow rate and current are increased in steps. With each increase the voltage is reduced and then restored. Once the voltage is restored, the current draw can be increased. Each increase in fuel flow rate is preferably not so large that the stack voltage falls below the threshold or minimum voltage. The current draw is increased each time as fast as possible without the voltage falling below the threshold. With each increase in current the O:C ratio increases (after the AOG recycle delay) thereby reducing carbon production.

[0141] Steady state operating temperature, current and voltage are reached at about time 555. Between times 555 and 560, the fuel cell system operates at steady state.

[0142] At time 560, a shutdown procedure is initiated. The flow rate of the unreformed hydrocarbon fuel is reduced (but not stopped) and the current draw is reduced (but not stopped). As a result, from time 560, the fuel cell reaction rate slows down and the stack begins to cool. The stack voltage starts to decrease in response to the temperature. When the stack voltage reaches a certain value, the fuel supply and current draw are reduced. As a result, the stack voltage increases and the fuel cell reaction rate slows down, so the O:C ratio decreases (and therefore less water is produced). This reduction in fuel supply and current draw is repeated until the current draw reaches a first (minimum non-zero) current draw and the fuel flow rate of the unreformed hydrocarbon fuel is reduced to a corresponding minimum, and when the stack temperature reaches a first threshold voltage, the current draw and the fuel flow rate of the unreformed hydrocarbon fuel are reduced to zero. The fuel cell system can then be maintained in a standby state (e.g., a warm state) or cooled to an off state (e.g., cooled to ambient temperature).

[0143] The present invention is not limited to only the above examples, other examples will be readily apparent to those skilled in the art without departing from the scope of the appended claims.

[0144] These and other aspects of the invention have been described above purely by way of example, and the invention can be varied in detail within the scope of the appended claims.

Claims

1. 1. A method of operating a fuel cell system, the fuel cell system comprising a plurality of cell units arranged in a stack, each cell unit comprising an anode and a cathode separated by an electrolyte, the fuel cell system comprising an anode inlet for supplying an anode inlet gas to each cell unit, and an anode outlet for removing an anode off-gas from each cell unit, the method comprising, upon start-up of the fuel cell system: heating the stack to a first threshold temperature; supplying unreformed hydrocarbon fuel from a fuel supply to the anode inlet at a first fuel flow rate when the stack exceeds the first threshold temperature, but not before the stack exceeds the first threshold temperature; recirculating anode off-gas from the anode outlet to the anode inlet while supplying unreformed fuel to the anode inlet; drawing a current from the fuel cell system while recirculating the anode off-gas; The method includes:

2. monitoring a stack temperature; and increasing a fuel flow rate and a current draw of the unreformed hydrocarbon fuel when the stack temperature reaches a second threshold temperature that is greater than the first threshold temperature. The method of claim 1 further comprising:

3. The method of claim 1 , wherein the current is drawn while maintaining a fuel cell voltage above a voltage threshold.

4. The method of claim 3, wherein the voltage threshold is between 0.6 and 0.8V.

5. The method of any one of claims 1 to 4, wherein at least one of the anode and the electrolyte comprises ceria.

6. The method of claim 5 , wherein the anode comprises CGO or the electrolyte comprises CGO.

7. 10. The method of claim 1, wherein the unreformed hydrocarbon fuel supplied during start-up above a first threshold temperature has the same composition as the unreformed hydrocarbon fuel supplied to the anode inlet during steady-state operation.

8. The method of claim 1, wherein the first threshold temperature is in the range of 400 to 500°C, more preferably 400 to 450°C.

9. 2. The method of claim 1, wherein water produced by the stack is supplied to the anode inlet of the stack by recirculating an anode off-gas, and the method includes reforming the unreformed hydrocarbon fuel using the recirculated water at a reforming catalyst positioned between each cell unit and an adjacent cell unit.

10. The method further includes a shutdown procedure, the shutdown procedure comprising: reducing the fuel flow rate and current draw of the unreformed hydrocarbon fuel while maintaining the stack voltage above a threshold; stopping the supply of the unreformed hydrocarbon fuel when the stack temperature becomes less than the first threshold temperature, and not stopping the supply of the unreformed hydrocarbon fuel before the stack temperature becomes less than the first threshold temperature. The method of claim 1 , comprising:

11. 1. A fuel cell system comprising: a plurality of cell units arranged in a stack, each cell unit comprising an anode and a cathode separated by an electrolyte, the fuel cell system comprising an anode inlet for supplying an anode inlet gas to each cell unit, and an anode outlet for removing an anode off-gas from each cell unit; means for heating the stack; means for measuring the temperature of the stack; a fuel inlet configured to connect to a supply of unreformed hydrocarbon fuel and configured to supply the unreformed hydrocarbon fuel to the anode inlet; an anode off-gas recirculation loop configured to provide a gas flow path for recirculating anode off-gas from the anode outlet to the anode inlet; means for drawing electrical current from the fuel cell system; a controller configured to receive input from said means for measuring and to provide outputs to said recirculation loop and said means for drawing current, the controller for controlling said recirculation loop, said supply of unreformed hydrocarbon fuel, and said means for drawing current, all in response to said means for measuring; There is no means configured to supply water to the fuel cell system from an external source. Fuel cell system.

12. 12. The fuel cell system of claim 11, wherein the anode off-gas recirculation loop comprises means configured to vary a flow rate of anode off-gas in the anode off-gas recirculation loop, the means being controlled by the controller in response to the means for measuring and the means for drawing current.

13. 13. The fuel cell system of claim 12, wherein the anode off-gas recirculation loop further comprises a flow path for the anode off-gas from the anode outlet, through a heater section, a mixing section configured to mix the anode off-gas with unreformed hydrocarbon fuel, and to the anode inlet.

14. 12. The fuel cell system of claim 11, wherein each cell unit in the stack is separated from an adjacent cell unit by an interconnect structure, the interconnect structure having a coating on a side facing and in fluid communication with the anode of the adjacent cell unit, the coating including a reforming catalyst configured to reform the unreformed hydrocarbon fuel into hydrogen for use in the stack.

15. The fuel cell system of claim 11 , wherein the electrolyte allows oxygen ion transport.

16. 16. The fuel cell system of claim 11, wherein at least one of the anode and the electrolyte comprises ceria, or the anode comprises CGO, or the electrolyte comprises CGO.

17. The controller is configured to control a start-up of the fuel cell system, the controller comprising: Controlling heating of the stack to a first threshold temperature; controlling a supply of unreformed hydrocarbon fuel to the anode inlet to provide a non-zero fuel flow when the controller determines that a first threshold temperature has been reached and not before; controlling the flow rate of anode off-gas in the anode off-gas recirculation loop at a first non-zero flow rate; and Allows current to be drawn from the fuel cell, The fuel cell system according to claim 11 , configured as follows:

18. 20. The fuel cell system of claim 17, wherein the controller is further configured to gradually increase a supply of unreformed hydrocarbon fuel to the anode inlet and gradually increase the current draw to increase temperature and current to a steady state.

19. 20. The fuel cell system of claim 17, wherein the controller is configured to regulate the supply of unreformed hydrocarbon fuel to the anode inlet, the flow rate of anode off-gas in the anode off-gas recirculation loop, and the current draw while maintaining the voltage of the fuel cell above a threshold voltage, the controller being configured to control (a) the oxygen to carbon ratio of gas in communication with the anode, and (b) the temperature of the stack.

20. A controller configured to receive a signal indicative of one or both of an anode inlet temperature and an anode outlet gas temperature and to control a flow rate of anode off-gas through the anode off-gas recirculation loop in accordance with the method of claim 1.

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