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The invention provides integrated fuel cell systems and associated operating methods wherein oxidant flow is controlled in response to an oxygen sensor in an exhaust stream of a fuel cell exhaust gas oxidizer, and fuel flow is controlled in response to a temperature measurement associated with the o

The invention provides integrated fuel cell systems and associated operating methods wherein oxidant flow is controlled in response to an oxygen sensor in an exhaust stream of a fuel cell exhaust gas oxidizer, and fuel flow is controlled in response to a temperature measurement associated with the oxidizer.

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1. A fuel cell system, comprising:a fuel processing reactor adapted to receive a flow of fuel from a fuel source, the fuel processing reactor being further adapted to react the flow of fuel and provide reformate to an anode of a fuel cell stack; an air source adapted to provide a flow of air to a ca

1. A fuel cell system, comprising:a fuel processing reactor adapted to receive a flow of fuel from a fuel source, the fuel processing reactor being further adapted to react the flow of fuel and provide reformate to an anode of a fuel cell stack; an air source adapted to provide a flow of air to a cathode of the fuel cell stack; an oxidizer adapted to receive and oxidize a flow of reformate from the anode of the fuel cell stack with a flow of air from the cathode of the fuel cell stack; an oxygen sensor adapted receive an exhaust flow from the oxidizer, the oxygen sensor being adapted to indicate a level of oxygen in the exhaust flow; a temperature sensor adapted to measure a temperature of the oxidizer; a controller connected to the fuel source and the temperature sensor, the controller being adapted to vary an output of the fuel source in response to a change in a signal received from the temperature sensor; and wherein the controller is further connected to the air source and the oxygen sensor, and the controller is adapted to vary an output of the air source in response to a change in a signal received from the oxygen sensor. 2. The fuel cell system of claim 1, further comprising a heat exchanger adapted to transfer heat from an exhaust of the oxidizer to a receiving stream.3. The fuel cell system of claim 2, wherein the heat exchanger is adapted to transfer heat from the exhaust of the oxidizer to liquid water to generate steam, and wherein the system further comprising a conduit adapted to inject the steam into the fuel processing reactor.4. The fuel cell system of claim 1, wherein the controller modulates the air source in response to the oxygen sensor to maintain a molar oxygen fraction in the oxidizer exhaust flow in the range 0.0-0.1.5. The fuel cell system of claim 1, wherein the controller modulates the fuel source to maintain the oxidizer temperature below a predetermined level.6. The fuel cell system of claim 5, wherein the predetermined level is 1,000° C.7. The fuel cell system of claim 5, wherein the predetermined level is 500° C.8. The fuel cell system of claim 1, wherein the controller is adapted to modulate the fuel source to maintain the oxidizer temperature less than 400° C. greater than the outlet temperature of the fuel cell stack.9. The fuel cell system of claim 1, further comprising a voltage sensor adapted to measure a voltage of at least one fuel cell in the fuel cell stack.10. The fuel cell system of claim 9, wherein the controller is adapted to modulate the flow of fuel from the fuel source to maintain the voltage above a predetermined level.11. The fuel cell system of claim 10, wherein the controller is adapted to modulate the flow of fuel from the fuel source to maintain the oxidizer temperature above a predetermined level.12. The fuel cell system of claim 1, further comprising a current sensor adapted to measure a current produced by the fuel cell stack.13. The fuel cell system of claim 12, wherein the controller is adapted to modulate the flow of fuel from the fuel source in response to a signal from the current sensor.14. The fuel cell system of claim 1, wherein the controller is adapted to modulate the flow of fuel from the fuel source to maintain the oxidizer temperature above a predetermined level.15. The fuel cell system of claim 1, wherein the flow of air from the cathode of the fuel cell stack is injected into the oxidizer, and wherein the oxygen sensor is located along an exhaust conduit connected to the oxidizer.16. The fuel cell system of claim 1, wherein the oxidizer temperature is an oxidizer exhaust temperature.17. The fuel cell system of claim 1, wherein the oxidizer temperature is an oxidizer monolith temperature.18. The fuel cell system of claim 1, wherein the fuel cell stack comprises polymer electrolyte membranes.19. The fuel cell system of claim 1, wherein the air source is a blower.20. The fuel cell system of claim 1, wherein the fuel source is a fuel blower.21. The fuel cell system of claim 1, wherein the air source is a pressure vessel having a variable flow actuator, the flow actuator being responsive to a control signal from the controller.22. The fuel cell system of claim 1, wherein the fuel source is a pressure vessel having a variable flow actuator, the flow actuator being responsive to a control signal from the controller.23. A method of operating a fuel cell system, comprising:modulating an air blower according to a first control signal to flow air through a cathode of a fuel cell; modulating a fuel blower according to a second control signal to flow fuel through a fuel processing reactor to produce reformate, the fuel blower further motivating flow of the reformate from the fuel processing reactor to an anode of a fuel cell; combining air exhausted from the fuel cell cathode with reformate exhausted from the fuel cell anode to form a combined stream, and flowing the combined stream through an oxidizer; operating an oxygen sensor to measure an amount of oxygen in an oxidizer exhaust, and communicating an oxygen signal to a controller; operating the controller in response to the oxygen signal to increase an output of the air blower to maintain an oxygen fraction of the oxidizer exhaust above a predetermined oxygen threshold; operating a temperature sensor to measure an oxidizer temperature, and communicating a temperature signal to the controller; and operating the controller in response to the temperature signal to increase an output of the fuel blower to maintain the oxidizer temperature below a predetermined temperature threshold. 24. The method of claim 23, wherein the predetermined temperature threshold is 1000° C.25. The method of claim 23, wherein the predetermined oxygen threshold is a molar oxygen fraction in the range 0.0-0.1.26. The method of claim 23, further comprising:transferring heat from the oxidizer to liquid water to generate steam; and injecting the steam into the fuel processing reactor. 27. The method of claim 23, further comprising:decreasing the flow of reformate from the fuel processor to increase the oxygen fraction of the oxidizer exhaust. 28. The method of claim 23, further comprising:increasing the flow of reformate from the fuel processor to decrease the oxygen fraction of the oxidizer exhaust. 29. The method of claim 23, further comprising:increasing an electrical load on the fuel cell to decrease the oxidizer temperature. 30. The method of claim 23, further comprising:decreasing the flow of reformate from the fuel processor to decrease the oxidizer temperature. 31. The method of claim 23, further comprising:decreasing an electrical load on the fuel cell to increase the oxidizer temperature. 32. The method of claim 23, further comprising:increasing the flow of reformate from the fuel processor to increase the oxidizer temperature. 33. The method of claim 23, wherein the oxidizer temperature is an oxidizer exhaust temperature.34. The method of claim 23, wherein the oxidizer temperature is an oxidizer monolith temperature.35. The method of claim 23, further comprising:determining whether a power demand on the fuel cell is met by an output from the fuel cell. 36. The method of claim 35, further comprising:decreasing an output of the fuel blower during a period where the power demand is met. 37. The method of claim 35, further comprising:decreasing an output of the air blower during a period where the power demand is met. 38. A method of operating a fuel cell system, comprising:flowing a fuel flow through a first electrode of a fuel cell to an oxidizer; modulating a rate of oxidant flow through a second electrode of the fuel cell in response to an oxygen sensor contacting an exhaust flow of the oxidizer; and modulating a rate of the fuel flow in response to a temperature sensor contacting the exhaust flow of the oxidizer. 39. A method of operating a fuel cell system, comprising:flowing a fuel flow through a first electrode of a fuel cell to an oxidizer; modulating a rate of first oxidant flow through a second electrode of the fuel cell in response to a first oxygen sensor contacting an exhaust flow of the oxidizer; modulating a rate of the fuel flow in response to a temperature sensor contacting the exhaust flow of the oxidizer; and modulating a rate of second oxidant flow through the oxidizer in response to a second oxygen sensor contacting the exhaust flow of the oxidizer. 40. A reactant flow rate controller for a fuel cell system, comprising:a fuel cell having an air electrode and a fuel electrode; an air blower adapted to vary a flow of air through the air electrode of the fuel cell; a fuel blower adapted to vary a flow of fuel through the fuel electrode of the fuel cell; an oxidizer adapted to receive an exhaust flow from the fuel electrode of the fuel cell, the oxidizer being further adapted to receive an air flow, the oxidizer being further adapted to oxidize the exhaust flow from the fuel electrode and produce an oxidizer exhaust flow; a temperature sensor adapted to measure an oxidizer temperature; an oxygen sensor adapted to indicate a level of oxygen in the oxidizer exhaust flow; a controller adapted to receive a temperature signal from the temperature sensor, the controller being further adapted to receive an oxygen signal from the oxygen sensor; and wherein the controller is further adapted to modulate a first output of the air blower and a second output of the fuel blower.