Improved fuel cell system and method

Abstract

The fuel cell system comprises a fuel cell stack comprising intermediate-temperature solid oxide fuel cells and having an anode inlet and a cathode inlet, a reformer, and a reformer heat exchanger defining an anode inlet gas fluid flow path and a cathode inlet gas fluid flow path, the cathode inlet gas heat exchanger being arranged to heat the relatively low temperature cathode inlet gas by transferring heat from at least one of the anode off-gas fluid flow path and the cathode off-gas fluid flow path; the reformer heat exchanger being arranged to heat the anode inlet gas from the relatively high temperature cathode inlet gas to a temperature T3 at the anode inlet that is lower than the temperature T1 at the cathode inlet; and an oxidant flow control means for controlled mixing of the low temperature oxidant from the oxidant inlet with the high temperature cathode inlet gas to control the temperature T1 at the cathode inlet at a level higher than the temperature T3 at the anode inlet.

Description

[Technical Field]

[0001] The present invention relates to improved fuel cell systems and methods. [Background technology]

[0002] The teachings of fuel cells, fuel cell stacks, fuel cell stack assemblies, and heat exchanger systems, apparatus, and methods are well known to those skilled in the art and include, inter alia, WO2015004419A, which is incorporated herein by reference in its entirety. Definitions of terms used herein may be found in the above publications, where appropriate. In particular, the present invention seeks to improve upon the systems and methods disclosed in WO2015004419A.

[0003] Operating hydrocarbon-fueled SOFC (Solid Oxide Fuel Cell) systems where the fuel cell stack operates in the temperature range of 450-650 °C (Intermediate Temperature Solid Oxide Fuel Cell; IT-SOFC), and especially in the temperature range of 520-620 °C, poses a series of difficult technical problems that are encountered. Such fuel cell stack operating temperatures are not per se suitable for high levels of internal reforming of the fuel, and therefore such systems typically require high levels of reforming before the fuel reaches the fuel cell stack.

[0004] In such systems, steam reforming is used to convert a hydrocarbon fuel stream (such as natural gas) into a hydrogen-rich reformate stream that is delivered to the fuel cell stack anode inlet.

[0005] To provide IT-SOFC stack cooling, air is blown into the cathode side of the fuel cell stack. To achieve these high reformer temperatures, the reformer is usually closely thermally coupled to the fuel cell stack waste gas combustor (which typically combusts any remaining fuel in the anode off-gas in the oxidant by burning it with the hot cathode off-gas). In such systems, the waste gas combustor and its hot exhaust gas are closely thermally coupled to the reformer by a heat exchanger, such as a heat exchange surface. Typically, the reformer is positioned immediately adjacent to or in contact with the waste gas combustor so that as much heat as possible is transferred from the waste gas combustor to the reformer.

[0006] The delivery of fuel cell stack cooling (particularly by pumps / blowers to the cathode side of the fuel cells) is a significant system parasitic load (typically the largest system parasitic load).

[0007] As the fuel cell stack loses efficiency over time, it generates more heat for a given electrical output and therefore requires more cooling. As the fuel cell stack powers the pumps / blowers to provide fuel cell stack cooling, this results in increased power requirements, requiring increased power generation, resulting in further increased heat generation which requires further increased cooling.

[0008] The close thermal coupling of the (endothermic) fuel reformer to the waste gas combustor (TGB) means that the enthalpy of the fuel stream exiting the fuel reformer is a function of the total air flow to the fuel cell stack. With IT-SOFC degradation, an increase in electrical resistance and therefore fuel cell heat generation results in an increase in reformer temperature and therefore an increase in the hydrogen content of the reformed fuel, which in turn results in an increase in the fuel cell stack cooling load during fuel cell stack operation.

[0009] 1 is taken from WO2015004419A in which the above-mentioned problems and others are addressed, among other things, by providing a parallel-flow reformer heat exchanger 70. The heat exchanger 70 has a relatively hot (primary) flow path in fluid flow communication from at least one oxidant inlet 140, 140′ to at least one fuel cell stack cathode inlet 41 and a relatively cold (secondary) flow path from a fuel source 71 to at least one fuel cell stack anode inlet 61. The parallel-flow heat exchanger is arranged to exchange heat between the cathode inlet gas and the anode inlet gas (i.e., from the primary path to the secondary path).

[0010] WO2015004419A describes three temperature sensors, and three temperatures: Fuel cell stack cathode inlet gas temperature sensor T1 Fuel cell stack cathode offgas temperature sensor T2 Anode inlet gas temperature sensor T3 Exhaust gas burner exhaust temperature sensor T TGB The controller 200 maintains the temperature determined by the temperature sensors T1 and T2 at or near the desired temperature during steady state operation of the fuel cell system.

[0011] The heated inlet air stream in path C is typically at 600-750°C. The temperature T3 at the reformer outlet 72 is typically between 400°C and 600°C.

[0012] Temperature sensor T3 is provided to control the flow rate of air in air bypass inlet gas passage 240 in order to maintain the temperature of the anode inlet gas at fuel cell stack anode inlet 41 at a predetermined temperature.

[0013] Two control loops are described, the first for fuel heating and the second for oxidant mass flow rate for stack cooling.

[0014] The temperature of the anode inlet gas to the fuel cell stack (i.e., the quality of the reformate) is controlled, and this control is independent of variations in the mass flow (and hence heat demand) of the input oxidant and fuel, and variations in the inlet temperature of the oxidant and fuel to the fuel cell system.

[0015] The controller 200 is configured to increase the cathode inlet gas mass flow rate when the temperature of the cathode off-gas determined by the fuel cell stack cathode off-gas temperature sensor T2 is lower than a predetermined temperature, and vice versa.

[0016] Temperature sensor T in exhaust gas burner exhaust section 81 TGB allows for a minimum exhaust gas combustor exhaust temperature to comply with gaseous emissions requirements. Should the exhaust gas combustor exhaust temperature fall below this minimum, additional unreformed fuel is supplied directly to the exhaust gas combustor 80 from fuel source 250 to raise the exhaust gas combustor exhaust temperature.

[0017] Prior art reformer heat exchangers 160 are parallel-flow heat exchangers, where the exhaust gases approach the same temperature. This is illustrated in FIG. 2. As hot air (or oxidant) flows along the heat exchanger (illustrated by a progression from left to right in the figure), it heats the fuel, and the fuel temperature increases as the hot air advances in parallel. At the outlet, the two temperatures are approximately the same. The fact that the reformer heat exchanger is a parallel-flow heat exchanger and is positioned to exchange heat between the cathode and anode inlet gases prior to their entry into the at least one fuel cell means that the exhaust temperatures of the cathode and anode inlet gases from the reformer and reformer heat exchanger, and hence the inlet temperatures into the cathode and anode sides of the at least one fuel cell stack, are very close to each other. The temperature difference between the anode and cathode inlet gases to the at least one fuel cell is primarily determined by the performance of the reformer heat exchanger. For example, in steady state operation, the temperature difference between the anode and cathode inlet gases to at least one fuel cell stack may be within 20°C of each other, more typically within 15°C.

[0018] Certain embodiments of WO2015004419A provide some degree of independent control of the reformate exhaust stream temperature from the reformer heat exchanger 160.

[0019] The control means 200 is connected to the fuel cell stack cathode inlet gas temperature sensor T1, the fuel cell stack cathode off-gas temperature sensor T2, and the blowers 210 and 210'. The control means 200 is configured to maintain the temperatures determined by the temperature sensors T1 and T2 at or near the desired temperatures during steady state operation of the fuel cell system.

[0020] Heating the cathode inlet gas is controlled by varying the ratio of cathode inlet gas mass flow between the main cathode inlet gas flow passage 230 and the air bypass inlet gas flow passage 240. This is controlled by varying the relative speeds of the blowers 210 and 210′, and hence the mass flow delivered by the blowers. If the temperature of the cathode inlet gas, as measured at temperature sensor T1, is too low, the ratio of cathode inlet gas flow through the air bypass inlet gas flow passage 240 to the cathode inlet gas flow through the main cathode inlet gas flow passage 230 is reduced, and vice versa.

[0021] The mass flow rate of the cathode inlet gas in the fuel cell stack is the total cathode inlet gas mass flow rate from blowers 210 and 210'. If the temperature of the fuel cell stack cathode off-gas as measured at temperature sensor T2 is too high, the total mass flow of the cathode inlet gas delivered by blowers 210 and 210' is increased, and vice versa.

[0022] The control means 200 is configured to maintain the temperature determined by the temperature sensors T1, T2, and T3 at or near a desired temperature during steady-state operation of the fuel cell system. The air flow rate through the air bypass inlet gas flow path 260 is controlled independently of the air flow rates of both the main cathode inlet gas flow path 230 and the air bypass inlet gas flow path 240. Temperature control for the reformer outlet 72 provides the ability to increase the temperature of the anode inlet gas flow path A relative to the temperature of the fuel cell cathode inlet 61A. Because the air bypass inlet gas flow path 260 provides air that is cooler than the air exiting the reformer heat exchanger 160, the air provided by the air bypass inlet gas flow path 260 can cool, but not warm, the air exiting the reformer heat exchanger 160. Therefore, the air bypass inlet gas flow path 260 providing cool air allows the anode inlet temperature to be higher than the cathode inlet temperature.

[0023] Increasing the temperature of the anode inlet gas at the reformer outlet 72 also increases the equilibrium temperature of the reforming reaction reached within the reformer heat exchanger 160, thus increasing the concentration of hydrogen in the anode inlet gas at the fuel cell stack anode inlet 41. Increasing the hydrogen concentration in the anode inlet gas reduces stress on the fuel cell 30 and reduces the amount of internal reforming required by the fuel cell 30.

[0024] Fuel cell stack anode inlet gas temperature sensor T3 measures the temperature of the anode gas at the fuel cell stack anode inlet 41. Controller 200 controls the air flow rate in air bypass inlet gas passage 240 to maintain the temperature of the anode inlet gas at the fuel cell stack anode inlet 41 at a predetermined temperature. Increasing the oxidant flow rate in air bypass inlet gas passage 240 decreases the temperature of the oxidant entering reformer heat exchanger oxidant inlet 161. This decrease in oxidant temperature decreases the temperature of the anode inlet gas at reformer outlet 72 and also decreases the equilibrium temperature of the reforming reaction. On the other hand, decreasing the oxidant flow rate in air bypass inlet gas passage 240 increases the temperature of the oxidant entering reformer heat exchanger oxidant inlet 161. This increase in oxidant temperature increases the temperature of the anode inlet gas at reformer outlet 72 and also increases the equilibrium temperature of the reforming reaction. The oxidant flow rate through the air bypass inlet gas passage 260 controls the temperature of the fuel cell cathode inlet 61A, and the oxidant flow rate in the air bypass inlet gas passage 240 controls the temperature of the reformate stream from the reformer outlet 72.

[0025] Increasing the oxidant flow rate in the air bypass inlet gas passage 260 decreases the temperature of the oxidant stream at the fuel cell stack cathode inlet 61. Conversely, decreasing the oxidant flow rate in the air bypass inlet gas passage 260 increases the temperature of the oxidant stream at the fuel cell stack cathode inlet 61.

[0026] Increasing the oxidant flow rate in the air bypass inlet gas passage 240 decreases the temperature of both the anode inlet gas at the reformer outlet 72 and the cathode inlet gas at the reformer heat exchanger oxidant outlet 162. Conversely, decreasing the oxidant flow rate in the air bypass inlet gas passage 240 increases the temperature of both the anode inlet gas at the reformer outlet 72 and the cathode inlet gas at the reformer heat exchanger oxidant outlet 162. For example, if the control means 200 determines that the fuel cell stack anode inlet gas temperature sensor T3 should be maintained at a higher or lower temperature, the oxidant flow rate in the air bypass inlet gas passage 240 may be increased or decreased, respectively, by controlling the blower 140′. On the other hand, if the control means 200 determines that the fuel cell stack cathode inlet gas temperature sensor T1 should be maintained at a higher or lower temperature, the oxidant flow rate in the air bypass inlet gas flow path 240 and / or the additional air bypass inlet gas flow path 260 may be increased or decreased, respectively, by controlling the blower 140′ for the oxidant flow rate in the air bypass inlet gas flow path 240 or the blower 140″ for the oxidant flow rate in the additional air bypass inlet gas flow path 260.

[0027] Thus, temperature control on the reformer outlet 72 provides the ability to lower the temperature of the fuel cell cathode inlet 61A relative to the anode inlet gas. The air bypass inlet gas flow path 260 can supply air from source 140" that is cooler than the air exiting the reformer heat exchanger 160. This is illustrated in FIG. 3. At the reformer heat exchanger outlet 162, fresh air / oxidant from source 140" can cool (but not warm) the air exiting the reformer heat exchanger 160, allowing the anode inlet temperature to be higher than the cathode inlet temperature (but not vice versa). Increasing the temperature of the anode inlet gas at the reformer outlet 72 raises the equilibrium temperature of the reforming reaction within the reformer heat exchanger 160, and therefore increases the concentration of hydrogen in the anode inlet gas at the fuel cell stack anode inlet 41, thereby reducing stress on the fuel cell 30 and reducing the amount of internal reforming required by the fuel cell 30.

[0028] Cooling the cathode inlet gas to a point where the anode inlet temperature is higher than the cathode inlet temperature can allow for more internal reforming within the fuel cell stack, but low stack air inlet temperatures have a negative effect on efficiency because the stack voltage decreases.

[0029] The present invention seeks to address, overcome or mitigate at least one of the disadvantages of the prior art. Summary of the Invention

[0030] According to a first aspect of the present invention, a fuel cell system (preferably an intermediate-temperature solid oxide fuel cell (IT-SOFC) system) comprises: (i) at least one fuel cell stack comprising at least one intermediate-temperature solid oxide fuel cell, the fuel cell stack having an anode inlet, a cathode inlet, an anode off-gas outlet, and a cathode off-gas outlet; and (ii) a reformer for reforming a hydrocarbon fuel into a reformate, the reformer having a reformer inlet for an anode inlet gas, a reformer outlet for exhausting the anode inlet gas, and a reformer heat exchanger, wherein: (a) fuel is transferred from a fuel source to the reformer, and from the at least one intermediate-temperature solid oxide fuel cell to the reformer; (b) an anode inlet gas fluid flow path to at least one fuel cell stack anode inlet, (b) an anode off-gas fluid flow path from the at least one fuel cell stack anode off-gas outlet to a fuel cell system exhaust, (c) a cathode inlet gas fluid flow path from at least one oxidant inlet through at least one cathode inlet gas heat exchanger to the reformer heat exchanger to the at least one fuel cell stack cathode inlet, and (d) a cathode off-gas fluid flow path from the at least one fuel cell stack cathode off-gas outlet to the fuel cell system exhaust, wherein the at least one cathode inlet gas heat exchanger is positioned to heat a relatively low temperature cathode inlet gas by transferring heat from at least one of (i) the anode off-gas fluid flow path and (ii) the cathode off-gas fluid flow path to provide a relatively high temperature cathode inlet gas. The reformer heat exchanger is arranged to heat the anode inlet gas from the relatively hot cathode inlet to a temperature T3 at the anode inlet which is lower than the temperature T1 at the cathode inlet. Oxidant flow control means is provided for controlled mixing of the cold oxidant from the or each oxidant inlet with the hot cathode inlet gas to control the temperature T1 at the cathode inlet relative to the temperature T3 at the anode inlet at a level higher than T3.

[0031] The oxidant flow control means is preferably arranged to control the mixing of cold oxidant from the oxidant inlet at the cathode inlet to reduce the temperature T1.

[0032] The oxidant flow control means may be arranged to control the mixing of cold oxidant from an oxidant inlet at the input to the reformer heat exchanger to reduce the temperature T1 while also reducing the temperature T3.

[0033] The reformer heat exchanger preferably includes a hot bypass for the cathode inlet gas to bypass the reformer heat exchanger, which serves to raise the cathode inlet gas to a temperature T1 at the cathode inlet.

[0034] In some systems, the hot bypass includes a preset throttle valve (eg, having a preset opening).

[0035] The hot bypass cathode inlet gas may be drawn from the cathode inlet gas flow passage prior to mixing with the cold oxidant.

[0036] In other arrangements (eg, higher power output systems), the hot bypass includes a controllable throttle valve.

[0037] The oxidant flow control means may be arranged to derive a reformer bypass airflow demand output signal from a reformer temperature input and a reformer setpoint input. Such a system may further comprise an exhaust gas combustor in fluid flow communication with the at least one fuel cell stack anode and cathode offgas outlet, having an exhaust gas combustor exhaust, defining a fluid flow path from the at least one fuel cell stack anode and cathode offgas outlet to the exhaust gas combustor exhaust, and wherein the exhaust gas combustor exhaust temperature (T TGB ) an exhaust gas combustor exhaust temperature sensor for sensing a temperature of the reformer. The oxidant flow control means is further arranged to derive an exhaust gas combustor exhaust temperature set point from the reformer temperature input and the reformer set point input.

[0038] The system may further comprise exhaust gas combustor control means for providing a fuel demand actuator command derived from the exhaust gas combustor exhaust temperature setpoint and the exhaust gas combustor exhaust temperature to increase fuel supply to the exhaust gas combustor when the exhaust gas combustor exhaust temperature is below the exhaust gas combustor exhaust temperature setpoint as provided by the oxidant flow control means.

[0039] First and second oxidant mixers may be provided, the first mixer positioned at an inlet to the reformer heat exchanger for mixing cold oxidant from the oxidant inlet, and the second mixer positioned at a reformer heat exchanger outlet for mixing cold oxidant from the oxidant inlet and the cathode inlet.

[0040] The cathode inlet gas flow channel preferably has a temperature of 750-850°C at its hottest point in steady state operation with ambient air input.

[0041] T1 is preferably controlled to be higher than T3 between 50 and 150° C. in steady state operation with cold air input.

[0042] T1 is preferably controlled to be between 500 and 600° C. in steady state operation with ambient air input.

[0043] T3 is preferably controlled to be between 400 and 500°C in steady state operation with ambient air input.

[0044] References herein to method steps also refer to systems of the present invention adapted or configured to perform such method steps.

[0045] For the avoidance of doubt, references herein to parallel flow heat exchangers are to parallel flow heat exchangers.

[0046] Preferably, the at least one fuel cell stack is a metal-supported IT-SOFC stack, more preferably as taught in U.S. Patent No. 6,794,075. Preferably, the IT-SOFC has a steady-state operating temperature in the range of 400°C to 650°C, more preferably 450°C to 650°C, more preferably 520°C to 620°C.

[0047] At least one oxidant heater is disposed in the cathode inlet gas flow path between the oxidant inlet and the reformer heat exchanger to maintain the cathode inlet gas temperature sensor at or near a predetermined temperature. Preferably, the cathode inlet gas temperature sensor is maintained within 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50° C. of a predetermined temperature, most preferably within 5° C. of the predetermined temperature. Preferably, the cathode off-gas temperature sensor may be maintained within 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50° C. of a predetermined temperature, most preferably within 5° C. of the predetermined temperature. [Brief explanation of the drawings]

[0048] [Figure 1] 1 is a system diagram of a fuel cell system according to the prior art; [Figure 2] FIG. 2 is a temperature diagram illustrating the temperatures of air and fuel passing through the reformer of FIG. 1. [Figure 3] FIG. 2 is an alternative temperature diagram illustrating the temperatures of the air and fuel through the reformer of FIG. 1 in the case of a post-reformer air bypass. [Figure 4] 1 is a system diagram of a fuel cell system according to a preferred embodiment of the present invention. [Figure 5] FIG. 5 is a temperature diagram illustrating the temperatures of air and fuel passing through the reformer of FIG. [Figure 6] FIG. 5 is a temperature diagram illustrating the temperatures of air and fuel passing through the reformer of FIG. [Figure 7] FIG. 2 is a diagram of a control process for controlling a fuel cell system according to an embodiment of the invention. [Figure 8]FIG. 2 is a schematic diagram of an alternative fuel cell system according to the present invention. [Figure 9] 1 is a temperature schematic diagram illustrating the temperature of air and fuel passing through a reformer according to an alternative embodiment of the invention. For illustrative purposes only, the diagram shows only a single fuel cell. In various embodiments (not shown), multiple fuel cell stacks are provided, and in yet further embodiments, multiple fuel cell stacks are provided, each comprising multiple fuel cells. It will be recognized, and will be readily apparent to one skilled in the art, that the anode and cathode inlets, outlets (off-gas), ducts, manifolds, and temperature sensors and their configurations will be modified as appropriate for such embodiments. In the embodiments below, air is used as the oxidant. All references to "oxidant" anywhere may therefore be construed as references to "air" in the embodiments below, and vice versa. DETAILED DESCRIPTION OF THE INVENTION

[0049] Referring to FIG. 4 , fuel cell system 400 is an intermediate-temperature solid oxide fuel cell (IT-SOFC) system. Fuel cell stack 20 is a metal-supported IT-SOFC fuel cell stack. Fuel cell system 400 has a steady-state 1 kW electrical output from fuel cell stack 20 and includes 121 metal-supported IT-SOFC fuel cells 30. Each fuel cell 30 has an anode side 40, an electrolyte layer 50, and a cathode side 60. Each fuel cell layer of the fuel cell stack is separated by an electrically conductive, gas-impermeable metal interconnect plate (not shown). Fuel cell stack end plates and pressure means (not shown) are also provided. References herein to fuel cells 30 refer to the entire set of 121 fuel cells 30.

[0050] An electrical load L is applied across the fuel cell 30 .

[0051] Fuel cell stack anode inlet 41 is in fluid flow communication with fuel cell anode inlet 41A for the flow of anode inlet gas to anode side 40 of fuel cell 30. Fuel cell anode outlet 42A is in fluid flow communication with fuel cell stack anode off-gas outlet 42 for the flow of anode off-gas.

[0052] Fuel cell stack cathode inlet 61 is in fluid flow communication with fuel cell cathode inlet 61A for the flow of cathode inlet gas to the cathode side 60 of fuel cell 30. Fuel cell cathode outlet 62A is in fluid flow communication with fuel cell stack cathode off-gas outlet 62 for the flow of cathode off-gas.

[0053] The waste gas combustor 80 is in fluid flow communication with the fuel cell stack anode and cathode off-gas outlets 42, 62 and has a waste gas combustor exhaust 81, an anode off-gas inlet 82, and a cathode off-gas inlet 83. The waste gas combustor 80 defines a fluid flow path from the fuel cell stack anode and cathode off-gas outlets 42, 62 to the waste gas combustor exhaust 81 and is configured to combust the anode and cathode off-gases and produce waste gas combustor off-gases.

[0054] An anode inlet gas fluid flow path A is defined from the fuel source 90 to the vaporizer 100, to the steam reformer 70, to the fuel cell stack anode inlet 41, to the fuel cell anode inlet 41A, i.e., the components are in fluid flow communication with each other.

[0055] An anode off-gas fluid flow path B is defined from the fuel cell anode outlet 42A to the fuel cell stack anode off-gas outlet 42, to the anode off-gas heat exchanger 110 (HX-AOG), to the condenser heat exchanger 120, to the separator 130, and to the anode off-gas inlet 82 of the exhaust gas combustor 80.

[0056] A primary cathode inlet gas flow path 230 is defined from the oxidant inlet 140 to the blower 210, to the anode off-gas heat exchanger 110, to the air preheater heat exchanger 150, to the reformer heat exchanger 160, to the fuel cell stack cathode inlet 61, and to the fuel cell cathode inlet 61A. An air bypass inlet gas flow path 240 is defined from the oxidant inlet 140' to the blower 210', to the air bypass inlet 190, to the reformer heat exchanger 160, to the fuel cell stack cathode inlet 61, and to the fuel cell cathode inlet 61A.

[0057] An air bypass inlet gas flow path 260 is defined from the oxidant inlet 140″ to the blower 210″ to the air bypass inlet 190′ to the fuel cell stack cathode inlet 61 to the fuel cell cathode inlet 61A. Thus, the air bypass inlet gas flow path 260 intersects with the cathode inlet gas fluid flow path (C) at the air bypass inlet 190′ between the reformer heat exchanger 160 (and downstream thereof) and the fuel cell stack cathode inlet 61, more specifically between the reformer heat exchanger oxidant outlet 162 and the fuel cell stack cathode inlet gas temperature sensor T1.

[0058] A cathode off-gas fluid flow path D is defined from the fuel cell cathode outlet 62 A to the fuel cell stack cathode off-gas outlet 62 to the cathode off-gas inlet 83 of the waste gas combustor 80 .

[0059] An exhaust gas combustor off-gas fluid flow path E is defined from the exhaust gas combustor exhaust 81 to the air preheater heat exchanger 150, to the evaporator heat exchanger 170 (HX-Evap), and to the fuel cell system exhaust 180. The anode off-gas heat exchanger 110 is in fluid flow communication with (i) the fuel cell stack anode off-gas outlet 42 (i.e., with the fuel cell anode outlet 42A) and the exhaust gas combustor anode off-gas inlet 82, and (ii) the oxidant inlet 140 and the fuel cell stack cathode inlet 61 (i.e., with the fuel cell cathode inlet 61A), and is positioned to exchange heat between the anode off-gas from the fuel cell stack 20 and the cathode inlet gas to the fuel cell stack 20.

[0060] The air preheater heat exchanger 150 is in fluid flow communication with (i) the exhaust gas combustor exhaust 81 and the fuel cell system exhaust 180, and (ii) the oxidant inlet 140 and the fuel cell stack cathode inlet 61 (i.e., with the fuel cell cathode inlet 61A), and is positioned to exchange heat between the exhaust gas combustor 81 off-gas and the cathode inlet gas to the fuel cell stack 20.

[0061] The reformer heat exchanger 160 may be a parallel flow heat exchanger (other possibilities are described below) and is in fluid flow communication with (i) the oxidant inlet 140 and the fuel cell stack cathode inlet 61 (i.e., with the fuel cell cathode inlet 61A), and (ii) the fuel source 90 and the fuel cell stack anode inlet 41 (i.e., with the fuel cell anode inlet 41A), and is positioned to exchange heat between the cathode inlet gas and the anode inlet gas.

[0062] The evaporator 100 has a fuel inlet 101 for anode inlet gas from the fuel source 90, a water inlet 102 for water from a water supply 103, and an evaporator exhaust 104 for exhausting the anode inlet gas from the evaporator 100, and is disposed in the anode inlet gas fluid flow path between the fuel source 90 and the steam reformer 70. The evaporator 100 further includes an evaporator heat exchanger 170 disposed in the exhaust gas combustor off-gas fluid flow path E between the air preheater heat exchanger 150 and the fuel cell system exhaust 180.

[0063] The evaporator heat exchanger 170 is in fluid flow communication with (i) the exhaust gas combustor exhaust 81 and the fuel cell system exhaust 180, and (ii) the fuel source 90 and the water supply 103 and the fuel cell stack anode inlet 41 (i.e., with the fuel cell anode inlet 41A), and is positioned to exchange heat between the exhaust gas combustor off-gas, the anode inlet gas, and the water to produce a steam fuel mixture for the anode inlet gas to the steam reformer 70.

[0064] The condenser heat exchanger 120 is in fluid flow communication with (i) the fuel cell stack anode off-gas outlet 42 (i.e., with the fuel cell anode outlet 42A) and the waste gas combustor anode off-gas inlet 82, and (ii) the cooling circuit 121, and is positioned to exchange heat between the anode off-gas from the fuel cell stack 20 and the cooling fluid of the cooling circuit 121.

[0065] The separator 130 is installed in the anode off-gas fluid flow path between the condenser heat exchanger 120 and the exhaust gas combustor 80, has a separator condensate discharge outlet 131, and is configured to separate condensate from the anode off-gas fluid flow path and discharge the condensate via the condensate discharge outlet 131.

[0066] Controller 402 has the same inputs and outputs as controller 200 of Figure 1. Controller 402 may be the same as controller 200 or may be modified relative to controller 200 as described.

[0067] A hot cathode bypass 401 is provided, extending from the air outlet of the air preheater heat exchanger 150 in path C directly to the anode inlet 61 of the fuel cell stack 20. This bypass allows a small amount of hot cathode inlet gas to bypass the reformer heat exchanger and serves to raise the cathode inlet gas to the temperature at the cathode inlet. This bypass flow is small compared to the volume of cathode gas passing through the heat exchanger 160.

[0068] The hot cathode bypass 401 preferably has a fixed restrictor or throttle 410. A variable throttle version is further described below.

[0069] The effect of the hot cathode bypass 401 is illustrated in Figure 5. The hot cathode bypass 401 allows the air to be heated to a higher temperature than the fuel at the outlet of the reformer heat exchanger, such that T1 at the cathode inlet is higher than T3 at the anode inlet. This allows for more internal reforming in the fuel cell stack 20 without a drop in stack voltage. This in turn allows for less overall air flow, which means less power to the blower, which means higher overall system efficiency.

[0070] It has been found that having the reformer outlet temperature lower than the stack air inlet temperature results in more optimum system performance.

[0071] The temperature T1 of the cathode inlet 61A may be controlled by the addition of cool air from the air inlet 140'' through the control of the blower 210''.

[0072] This is illustrated in FIG. 6, where the cathode inlet gas fluid temperature drops as it passes through the reformer heat exchanger 160, then rises by mixing in hot bypass gas from the bypass 401, and then falls in a controlled manner by mixing in air from the air inlet 140″.

[0073] Thus, as has been explained, the controller 402 can control the temperature T1 of the cathode inlet 61A by adding cool air from the air inlet 140″ through control of the blower 210″, with the modification that this control occurs at a temperature higher than the temperature of the anode inlet 41 of that fuel cell stack.

[0074] Blowers 140 and 140′ may be replaced by a single blower and an adjustable valve / separator (not shown) that can adjust the ratio of the input oxidant flowing along the main cathode inlet gas fluid flow path 230 on the one hand and the bypass inlet gas flow path 240 on the other hand.

[0075] FIG. 7 illustrates modifications to the control process of controller 402 to achieve steady state operation of the apparatus of FIG.

[0076] There is a reformer control process 700 , a waste gas combustor (TGB) process 701 , a fuel control process 702 and an air control process 708 .

[0077] The reformer control process takes as its inputs the reformer temperature 710 and the reformer temperature set point 711. The reformer temperature 710 is T3 measured at the reformer outlet 72 in Figures 1 and 4. The reformer control process delivers two outputs: the TGB output temperature set point 710 and the reformer bypass airflow demand 715.

[0078] The TGB control process 701 takes as one of its inputs a TGB output temperature setpoint 710 and as another input a TGB outlet temperature 713, and passes a fuel demand output 714. The TGB outlet temperature is the TGB temperature as measured at the TGB waste gas output 81 (FIGS. 1 and 4). TGB is.

[0079] The fuel control process 702 takes as its input a fuel demand output 714 and converts the fuel demand output into actuator controls at the output of the fuel control process to control the delivery of either virgin fuel or reformer and stack passed fuel 82, or a mixture of the two, from the fuel source 90 or 250 (FIG. 4) to the TGB.

[0080] Air control process 708 takes as its input a pre-reformer bypass airflow demand 715 and converts this into an actuator control output 718. This control output controls valves 820 and 821 and blower 810. This is accomplished in one of several alternative ways.

[0081] It may be desirable to keep either the total air flow, the main air flow, or the post-reformer bypass flow constant. In each of these cases, more or less adjustment of valve position or blower speed is required.

[0082] For example, it may be preferable to keep the main air flow rate along path 230 constant. In this case, valve 820 may be fixed and actuator control output 718 can control valve 821, with a corresponding adjustment to blower 810.

[0083] Alternatively, it may be preferable to keep the post-reformer bypass flow rate (along path 260) constant, in which case valve 821 may be fixed and actuator control output 718 can control valve 820, with corresponding adjustments to blower 810.

[0084] Due to the difference in pressure drop in each of the flow paths, some adjustment to both the valves and the blower is generally required.

[0085] Figure 8 shows a fuel cell module as previously described with reference to Figure 4, where like elements are given like reference numerals. There is a common cold air source 802, an air filter / noise attenuator 805, a fuel cell blower 810, and first and second air bypass valves 820 and 821. The first air bypass valve 820 controls the supply of cold air to the anode off-gas heat exchanger 110. The second air bypass valve 821 controls air to the air bypass inlet gas flow paths 240 and 260. These valves control the proportion of air flowing from the blower 810 to these three paths.

[0086] Air at about 30°C flows to the anode off-gas heater where the air is first heated, and then to the air preheater 150 where the air is further heated. This is the hottest point in the cathode inlet gas flow path C. The air is heated to a high temperature, primarily by exhaust from the waste gas combustor, which may be about 800-900°C.

[0087] This hot air is mixed with cooler air from air bypass path 240 in mixer 830. The exhaust air from mixer 830 may be between 500 and 600°C. The exhaust air passes through a reformer heat exchanger to heat the reformed fuel to between about 400 and 500°C, and the air exits the reformer heat exchanger at a similar temperature (or to about 15-25°C higher, although, as will be explained, the air may exit at a lower temperature than the temperature of the reformed fuel).

[0088] Between the reformer heat exchanger oxidant outlet 162 and the fuel cell stack cathode inlet 61 there is a further mixer 840 with a bypass path 260 as an inlet and a hot bypass 401 as another inlet. The hot bypass path has a throttle, choke or metering valve 410.

[0089] After mixing the cold air from bypass path 260 with the hot air from hot bypass 401, the temperature at fuel cell stack cathode inlet 61 is between about 500 and about 600° C. Thus, temperature T1 at fuel cell stack cathode inlet 61 is higher than temperature T3 at fuel cell stack anode inlet 41.

[0090] All air from blower 810 is used, so the blower is used to its maximum efficiency. Fuel cell stack cathode and anode inlet temperatures may be carefully controlled by air bypass valves 820 and 821, and the power supplied to blower 810 may be reduced to match the demands from these valves.

[0091] In an alternative embodiment, the hot bypass 401 is omitted, and the reformer heat exchanger is modified to heat the fuel to a temperature lower than that of the oxidant from the cathode input gas fluid flow path before mixing with air from the air bypass input gas flow path 260. The reformer heat exchanger 160 may, for example, be a shortened parallel-flow heat exchanger that does not heat the fuel completely to the air outlet temperature. Alternatively, the reformer heat exchanger may be a one-way heat exchanger with a temperature difference between the primary (heating) and secondary (heated) paths at all points along the paths. Other arrangements are possible. The operation of such an embodiment is illustrated in FIG. 9, which shows the fuel and air exiting the reformer heat exchanger with the air hotter than the fuel, and showing a temperature decrease as the exiting air is mixed in a controlled manner with the oxidant from the cathode input gas fluid flow path under the control of the blower 210″ (FIG. 4) or bypass valve 821 (FIG. 8).

[0092] In the preferred embodiment, the metering valve or throttle 410 has been described as fixed. This is because controlling the flow of hot gas requires expensive control valves that are prone to frequent maintenance or replacement. For this reason, in small systems, it is preferable to have the temperature of the air leaving the reformer at a higher temperature than the temperature of the fuel and then cool it to the required temperature by mixing.

[0093] Alternatively, especially in larger systems, the throttle 410 may be controlled by the controller 200 or 800 to allow only the necessary amount of hot cathode gas to bypass the reformer to bring the fuel cell stack cathode inlet 61 to a desired temperature higher than the temperature of the fuel cell stack anode inlet 41. This arrangement may be preferable in systems producing more than 10 or 20 kW of power. In such systems, the energy savings from the greater efficiency may outweigh the cost of the hot flow control components.

[0094] Thus, fuel cell systems with a hot reformer bypass and with cold pre- and post-reformer bypasses have been described, as well as various alternative combinations of these (hot bypass with post-reformer bypass and no pre-reformer bypass; no hot bypass with post-reformer bypass and optional pre-reformer bypass; hot controllable bypass with post-reformer bypass and optional pre-reformer bypass). In some embodiments, the fuel is heated in the reformer to a temperature lower than the temperature of the oxidant at the reformer outlet. For example, in the case of a counter-flow reformer heat exchanger, the hot bypass flow would need to be large and the cold pre-reformer bypass flow would be large. These may actually be advantageous in terms of low pressure drop or compactness in the system.

[0095] A system warm-up option may be provided in which a different type of reforming reaction other than steam reforming is used. For example, CPOX reforming may be used before returning to the SMR in normal operation. This would preferably include anode off-gas recirculation. This would be accomplished by taking a portion of the flow from stream "B" at any point between stack 20 and TGB 80 and sending it to the inlet of reformer 70.

[0096] A related modification involves eliminating the evaporator heat exchanger 170 .

[0097] The present invention is not limited to the above-described embodiments, and other embodiments will be readily apparent to those skilled in the art without departing from the scope of the appended claims.

Claims

1. (i) at least one fuel cell stack (30) comprising at least one intermediate temperature solid oxide fuel cell, the fuel cell stack having an anode inlet (41), a cathode inlet (61), an anode off-gas outlet (42), and a cathode off-gas outlet (62); (ii) a reformer (70) for reforming a hydrocarbon fuel into a reformate, optionally a steam reformer, the reformer (70) having a reformer inlet (71) for an anode inlet gas, a reformer outlet (72) for exhausting the anode inlet gas, and a reformer heat exchanger (160); Equipped with (a) an anode inlet gas fluid flow path from a fuel source (90) to said reformer (70) to said at least one fuel cell stack anode inlet (41); (b) an anode off-gas fluid flow path from the at least one fuel cell stack anode off-gas outlet (42) to a fuel cell system exhaust (180); (c) a cathode inlet gas fluid flow path from at least one oxidant inlet (140, 140', 140") through at least one cathode inlet gas heat exchanger (110, 150) to said reformer heat exchanger (160) to said at least one fuel cell stack cathode inlet (61); (d) a cathode off-gas fluid flow path from the at least one fuel cell stack cathode off-gas outlet (62) to the fuel cell system exhaust (180); and 1. A fuel cell system, comprising: at least one cathode inlet gas heat exchanger (110, 150) arranged to heat a relatively low temperature cathode inlet gas by transfer of heat from at least one of (i) the anode off-gas fluid flow path and (ii) the cathode off-gas fluid flow path to provide a relatively high temperature cathode inlet gas; The reformer heat exchanger is cooled to a temperature T １ the temperature T at the anode inlet is lower than ３ configured to heat the anode inlet gas from the relatively hot cathode inlet gas to The temperature T at the anode inlet (41) ３ to the temperature T at the cathode inlet (61) １ T ３ oxidant flow control means (200) for controllably mixing the cold oxidant from the or each oxidant inlet (140, 140', 140") with the hot cathode inlet gas to control a level greater than A fuel cell system characterized by:

2. The oxidant flow control means (200) controls the temperature T １ 2. The system of claim 1, wherein the cathode inlet is configured to control mixing of cold oxidant from an oxidant inlet to the cathode inlet so as to reduce

3. The oxidant flow control means (200) controls the temperature T ３ While also decreasing the temperature T １ 3. The system of claim 1, wherein the system is arranged to control the mixing of cold oxidant from an oxidant inlet (140′) at the input to the reformer heat exchanger (160) so as to reduce

4. The reformer heat exchanger allows the cathode inlet gas to bypass the reformer heat exchanger and maintain the cathode inlet gas at a temperature T １ a hot bypass for contributing to raising the temperature to The system of claim 1 .

5. The system of claim 4 , wherein the hot bypass comprises a preset aperture throttle valve.

6. 6. The system of claim 4 or 5, wherein the hot bypass cathode inlet gas is drawn from the cathode inlet gas flow passage prior to mixing with the cold oxidant.

7. The system of claim 4 , wherein the hot bypass comprises a controllable throttle valve.

8. 8. The system of claim 1, wherein the oxidant flow control means (200) is arranged to derive a reformer bypass airflow demand output signal (715) from a reformer temperature input (710) indicative of the fuel temperature at the reformer outlet (72) and a reformer setpoint input (711).

9. and an exhaust gas burner (80) in fluid flow communication with the at least one fuel cell stack anode and cathode off-gas outlet, the exhaust gas burner having an exhaust gas burner exhaust (81), defining a fluid flow path from the at least one fuel cell stack anode and cathode off-gas outlet to the exhaust gas burner exhaust (81) and to the exhaust (180), wherein the exhaust gas burner exhaust temperature (T ＴＧＢ 9. The system of claim 8, further comprising an exhaust gas combustor exhaust temperature sensor for sensing a temperature of the reformer, and wherein the oxidant flow control means is further arranged to derive an exhaust gas combustor exhaust temperature set point from the reformer temperature input and the reformer set point input.

10. 10. The system of claim 9, further comprising exhaust gas combustor control means for providing a fuel demand actuator command derived from said exhaust gas combustor exhaust temperature setpoint and said exhaust gas combustor exhaust temperature to increase fuel supply to said exhaust gas combustor when said exhaust gas combustor exhaust temperature is less than said exhaust gas combustor exhaust temperature setpoint as provided by said oxidant flow control means.

11. 11. The system of claim 1, comprising first and second oxidant mixers, the first mixer positioned to mix cold oxidant from an oxidant inlet (140′) at an inlet to the reformer heat exchanger, and the second mixer positioned to mix cold oxidant from an oxidant inlet (140″) at a reformer heat exchanger outlet and a cathode inlet.

12. 12. The system of any one of claims 1 to 11, wherein the cathode inlet gas fluid flow channel has a temperature of 750 to 850°C at its hottest point in steady state operation with warm air input.

13. T １ However, in steady-state operation with room temperature air input, T ３ The system according to any one of claims 1 to 12, wherein the temperature is controlled to be higher than

14. T １ is controlled to be between 500 and 600°C in steady state operation with ambient air input.

15. T ３ is controlled to be between 400°C and 500°C in steady state operation with ambient air input.

16. The system of any one of claims 1 to 15, wherein the reformer heat exchanger is a parallel flow heat exchanger.

17. (i) at least one fuel cell stack (30) comprising at least one intermediate temperature solid oxide fuel cell, the fuel cell stack having an anode inlet (41), a cathode inlet (61), an anode off-gas outlet (42), and a cathode off-gas outlet (62); (ii) a reformer (70) for reforming a hydrocarbon fuel into a reformate, the reformer (70) having a reformer inlet (71) for an anode inlet gas, a reformer outlet (72) for exhausting the anode inlet gas, and a reformer heat exchanger (160); and (a) an anode inlet gas fluid flow path from a fuel source (90) to said reformer (70) to said at least one fuel cell stack anode inlet (41); (b) an anode off-gas fluid flow path from the at least one fuel cell stack anode off-gas outlet (42) to a fuel cell system exhaust (180); (c) a cathode inlet gas fluid flow path from at least one oxidant inlet (140, 140', 140") through at least one cathode inlet gas heat exchanger (110, 150) to said reformer heat exchanger (160) to said at least one fuel cell stack cathode inlet (61); (d) a cathode off-gas fluid flow path from the at least one fuel cell stack cathode off-gas outlet (62) to the fuel cell system exhaust (180); Determine 1. A method of operating a fuel cell, comprising: heating a relatively low temperature cathode inlet gas by heat exchange from at least one of (i) the anode off-gas fluid flow path and (ii) the cathode off-gas fluid flow path to provide a relatively high temperature cathode inlet gas; The temperature T at the cathode inlet １ the temperature T at the anode inlet is lower than ３ heating the anode inlet gas from the relatively hot cathode inlet gas to The temperature T at the cathode inlet (61) １ T ３ and controlling mixing of the cold oxidant from the or each oxidant inlet (140, 140', 140") with the hot cathode inlet gas to achieve a higher level of control than A method comprising:

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