NOVELTY - A fuel processing system, comprises a fuel processor having a fuel processor inlet and a fuel processor outlet; a combustor having a combustor inlet in fluid communication with the fuel processor outlet and a combustor outlet; and a recirculation loop including a bypass valve for selectively providing fluid communication between the fuel processor inlet and at least one of the fuel processor outlet and the combustor outlet.    USE - As a fuel processing system.    ADVANTAGE - The inventive fuel processing system has gas recirculation for improved start-up, shut down, and transient operation. It is capable of providing a large mass flow rate through the fuel processor to aid in heating the fuel processor components to the proper operating temperatures during start-up. During shutdown, the fuel processor system enables the fuel processor to run dry and condense water from the reformate, to avoid condensation on the catalysts, and subsequently to shutdown at an elevated pressure to prevent air ingestion upon cooling of the fuel processor. During turn-down, the fuel processor system enables higher flow rates through the fuel processor and fuel cell stack to avoid auto-ignition in the inlet, reverse water gas shift in the preferential oxidation, cell reversal in the fuel cell stack, and water collection in the fuel cell stack, all of which occur at reduced flow rates. During transient response, the fuel processor enables the flow rate and pressure in the fuel processor to remain nearly constant, minimizing the lag in transient response associated with filling or venting volumes in the fuel processor system. The ability to use recirculated gases, which contain water vapor as a product of reaction, enables the fuel processor to run without water injection. The fuel processor also enables rapid thermal start of the fuel processor without the complexity of multiple stages or risk of oxygen exposure.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view illustrating a fuel processor system.    Fuel processor (12)    Fuel cell stack (14)    Catalytic combustor reactor (16)    Reformate stream (24)    Anode bypass valve (26)    Recirculation compressor (32)    Check valve (60)