Patent Publication Number: US-2022223887-A1

Title: Pressurized air supply system, fuel cell system comprising the pressurized air supply system, and starting method of the pressurized air supply system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a pressurized air supply system, a fuel cell system comprising the pressurized air supply system, and a starting method of the pressurized air supply system. 
     BACKGROUND 
     A solid oxide fuel cell (SOFC) can be more efficient by using pressurized air, and thus as a system for supplying the pressurized air, a pressurized air supply system using a turbocharger is described in Patent Documents 1 and 2. It is possible to drive the turbocharger by using air discharged from a compressor of the turbocharger as a pressurized air supply source and rotating a turbine of the turbocharger with combustion gas obtained by combusting exhaust fuel of the SOFC. 
     In the case of cold start, since the SOFC is not operated at starting of the turbocharger, a gas of a sufficient flow rate with a temperature/pressure necessary to rotate the turbine does not exist. Thus, in this case, it is impossible to start the turbocharger unless a start-up gas is externally supplied. Accordingly, a start-up device for starting the turbocharger is needed. Patent Document 1 describes a start-up device configured to supply air (instrumentation air) from another air source to a start-up combustor to combust start-up fuel, and to rotate a turbine by generated combustion gas. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP2000-348749A 
     Patent Document 2: JP2018-6004A 
     SUMMARY 
     Technical Problem 
     However, the problem arises in that a method for starting the turbocharger by using the start-up device as in Patent Document 1 is difficult, increasing device cost and operation cost. 
     Considering above, each embodiment of the present disclosure is to provide a pressurized air supply system with reduced cost for operating the pressurized air supply system, fuel cell system comprising the pressurized air supply system, and starting method of the pressurized air supply system. 
     Solution to Problem 
     (1) A pressurized air supply system according to at least one embodiment of the present disclosure includes a turbocharger including a turbine and a compressor, a recuperator for heat exchange between discharged air from the compressor and flue gas exhausted from the turbine, a start-up heater for heating the air, that includes at least either of start-up air or the discharged air from the compressor, which is supplied to discharged air line between the compressor outlet and the recuperator, and a catalytic combustor for supplying, to the turbine, combustion gas which is generated by combustion of fuel with the flowing air heated by the start-up heater. 
     With the above configuration (1), at starting of the pressurized air supply system, if the catalytic combustor can be warmed up for ignition by supplying the flowing air heated by the start-up heater to the catalytic combustor, high-temperature combustion gas flows through the turbine, and the flowing air is heated by the recuperator, making it possible to accelerate a temperature increase. Thus, in the start-up heater, it is possible to reduce operation energy consumption by shortening duration for start-up and to reduce a designed heating capacity, making it possible to reduce cost for operating the pressurized air supply system. 
     (2) In some embodiments, in the above configuration (1), the pressurized air supply system further includes a motor for driving the compressor. The start-up air is supplied by the compressor driven by the motor. 
     With the above configuration, it is possible to use the discharged air from the compressor driven by the motor as the start-up air, making it possible to render a start-up blower and a flow regulating valve for the discharged air unnecessary. Since the discharged air serving as the start-up air is heated by the recuperator, and then supplied to the start-up heater, in the start-up heater, it is possible to reduce operation energy consumption by shortening the duration for start-up and to reduce the designed heating capacity, making it possible to reduce the cost for operating the pressurized air supply system. 
     (3) In some embodiments, in the above configuration (2), the motor is connected to the compressor via a speed increasing gear. 
     With the above configuration, since the rotation of the motor can be transmitted to the compressor at the increased speed, it is possible to use an inexpensive low speed motor. Thus, it is possible to further reduce the cost for operating the pressurized air supply system. 
     (4) In some embodiments, in any one of the above configurations (1) to (3), the pressurized air supply system further includes recuperator bypass line for a part of the discharged air from the compressor to bypass the recuperator. 
     With the above configuration, it is possible to cause a part of the discharged air from the compressor not to flow into the recuperator. Thus, it is possible to reduce the amount of the discharged air flowing into the recuperator, making it possible to further increase the temperature of the air flowing into the start-up heater. Thus, in the start-up heater, it is possible to reduce operation energy consumption by shortening the duration for start-up and to reduce the designed heating capacity, making it possible to reduce the cost for operating the pressurized air supply system. 
     Further, with the above configuration, it is possible to increase the temperature of not only the air supplied to a start-up combustor but also the air supplied to the fuel cell, making it possible to reduce cost needed for warm-up of the fuel cell as well. 
     (5) In some embodiments, in the above configuration (4), the discharged air flowing through the recuperator bypass line joins the air flowing into the catalytic combustor. The recuperator bypass line joining the exhaust air line will be called as discharged air bypass line. 
     With the above configuration, without decreasing the flow rate of the combustion gas for driving the turbine, that is, without decreasing the intake flow rate of the compressor without changing the operational point of the turbocharger, it is possible to increase the temperature of the discharged air supplied to the catalytic combustor. Thus, in the start-up heater, it is possible to reduce operation energy consumption by shortening the duration for start-up and to reduce the designed heating capacity, making it possible to reduce the cost for operating the pressurized air supply system. 
     Further, with the above configuration, it is possible to increase the temperature of not only the air supplied to the start-up combustor but also the air supplied to the fuel cell, making it possible to reduce the cost needed for warm-up of the fuel cell as well. 
     (6) In some embodiments, in the above configuration (4), the discharged air flowing through the recuperator bypass line joins the flue gas flowing out of the recuperator. The recuperator bypass line joining the flue gas line will be called as extraction blow line. 
     With the above configuration, without increasing the temperature of the combustion gas, the pressure ratio of the turbocharger is decreased, making it possible to increase the temperature of the flue gas supplied to the catalytic combustor. Thus, in the start-up heater, it is possible to reduce operation energy consumption by shortening the duration for start-up and to reduce the designed heating capacity, making it possible to reduce the cost for operating the pressurized air supply system. 
     Further, with the above configuration, it is possible to increase the temperature of not only the air supplied to the start-up combustor but also the air supplied to the fuel cell, making it possible to reduce the cost needed for warm-up of the fuel cell as well. 
     (7) In some embodiments, in any one of the above configurations (1) to (6), the pressurized air supply system includes pressurized air line through which the air heated by the recuperator flows, branch line branching from the pressurized air line and joining the exhaust air line after flowing through the start-up heater, and fuel cell heating bypass air line further branching from downstream of the start-up heater on the branch line and rejoining the pressurized air line. 
     With the above configuration (7), since it is possible to directly supply the pressurized air without through the start-up heater when the pressurized air supply system supplies the pressurized air, it is possible to reduce pressure loss of the pressurized air. Further, since it is possible to directly increase only the temperature of the pressurized air supplied to the fuel cell, it is possible to reduce the designed heating capacity of the start-up heater and to reduce the cost for operating the pressurized air supply system. 
     (8) In some embodiments, in the above configuration (7), the pressurized air supply system includes, upstream of the start-up heater, a flow regulating valve for regulating the flow rate of the air flowing into the start-up heater. 
     With the above configuration, since the flow regulating valve is disposed upstream of the start-up heater, it is possible to use, as the flow regulating valve, not a high-temperature valve but a low-temperature valve of lower cost. Thus, it is possible to reduce the cost for operating the pressurized air supply system. 
     (9) In some embodiments, in the above configuration (8), the start-up heater includes a first heater for heating the flowing air supplied to the catalytic combustor, and a second heater for heating the air flowing through the pressurized air line. 
     With the above configuration, since it is possible to dispose flow regulating valves for regulating the flow rate of the air flowing into the first heater and the second heater upstream of the first heater and the second heater, respectively, it is possible to obtain the technical effect by the above configuration (8). 
     (10) A fuel cell system according to at least one embodiment of the present disclosure includes the pressurized air supply system according to any one of the above configurations (1) to (9), and a fuel cell having cathode and anode. The fuel cell system is configured such that pressurized air supplied from the pressurized air supply system flows into the cathode. 
     With the above configuration, it is possible to reduce the cost for starting the fuel cell system. 
     (11) A starting method of the pressurized air supply system according to at least one embodiment of the present disclosure is a starting method of the pressurized air supply system according to any one of the above configurations (1) to (9), the method including a step of supplying at least either of the start-up air or the discharged air to a discharged air line between the compressor outlet and the recuperator, a step of heating the flowing air by starting the start-up heater, a step of generating combustion gas by combustion of fuel with the flowing air by starting the catalytic combustor, after catalyst temperature of the catalytic combustor is increased to preset temperature or higher with the heated flowing air, and a step of stopping the start-up heater or decreasing the load of the start-up heater, after the compressor is driven by rotation of the turbine with the combustion gas. 
     With the above configuration (11), at starting of the pressurized air supply system, by supplying the flowing air heated by the recuperator to the start-up heater, in the start-up heater, it is possible to reduce operation energy consumption by shortening warm-up time of the catalytic combustor and to reduce the designed heating capacity, making it possible to reduce the cost for operating the pressurized air supply system. 
     Advantageous Effects 
     According to at least one embodiment of the present disclosure, at starting of pressurized air supply system, by supplying flowing air heated by a recuperator to a start-up heater, in the start-up heater, it is possible to reduce operation energy consumption by shortening warm-up time of a catalytic combustor and to reduce the designed heating capacity, making it possible to reduce cost for operating the pressurized air supply system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram of a fuel cell system according to Embodiment 1 of the present disclosure. 
         FIG. 2  is a configuration diagram of the fuel cell system according to Embodiment 2 of the present disclosure. 
         FIG. 3  is a configuration diagram of the fuel cell system according to Embodiment 3 of the present disclosure. 
         FIG. 4  is a configuration diagram of the fuel cell system according to Embodiment 4 of the present disclosure. 
         FIG. 5  is a configuration diagram of the fuel cell system according to Embodiment 5 of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, the scope of the present invention is not limited to the following embodiments. It is intended that dimensions, materials, shapes, relative positions and so on, of components, described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
     Embodiment 1 
     As shown in  FIG. 1 , a fuel cell system  1  according to Embodiment 1 of the present disclosure includes a fuel cell  2  which is a solid oxide fuel cell (SOFC), and a pressurized air supply system  3  for supplying pressurized air to the fuel cell  2 . The fuel cell  2  has cathode  2   a , anode  2   b , and an electrolyte  2   c  disposed between the cathode  2   a  and the anode  2   b.    
     The pressurized air supply system  3  includes a turbocharger  10  with a compressor  11  and a turbine  12 , a recuperator  13  for heat exchange between discharged air discharged from the compressor  11  and flue gas exhausted from the turbine  12 , a start-up heater  16  such as a burner for heating the air, which includes at least either of the discharged air or start-up air supplied by the start-up air supply device  15  to be described later, and a catalytic combustor  17  for supplying, to the turbine  12 , combustion gas which is generated by combustion of fuel to be described later with the flowing air heated by the start-up heater  16 . 
     A discharged air line  14 , which causes the compressor  11  and the recuperator  13  to communicate with each other, is provided with a flow regulating valve  18  capable of regulating the flow rate of the discharged air flowing through the discharged air line  14 . The extraction blow line  19  is disposed which branches from the discharged air line  14  between the compressor  11  and the flow regulating valve  18 . The extraction blow line  19  is provided with a flow regulating valve  20  capable of regulating the flow rate of the pressurized air flowing through the extraction blow line  19 . The extraction blow line  19  may be configured such that a downstream end thereof joins a flue gas line  24  through which the flue gas exhausted from the turbine  12  flows after flowing out of the recuperator  13 . The extraction blow line  19  constitutes a recuperator bypass line for a part of the discharged air to bypass the recuperator  13 . 
     The start-up air supply device  15  includes a start-up air compressor  22  which is disposed on a start-up air supply line  21  communicating with the discharged air line  14  between the flow regulating valve  18  and the recuperator  13 . The start-up air supply device  15  may include a flow regulating valve  23  for regulating a supply amount of the start-up air. 
     The start-up heater  16  is disposed on a pressurized air line  25  through which the air flows. The pressurized air line  25  is connected to an inlet of the cathode  2   a  of the fuel cell  2 . The pressurized air line  25  is provided with a flow regulating valve  26  capable of regulating the flow rate of the pressurized air (same as the flowing air, in this case) between the start-up heater  16  and the fuel cell  2 . Further, an exhaust air line  42  is disposed through which exhaust air supplied to the catalytic combustor  17  from an outlet of the cathode  2   a  of the fuel cell  2  flows. Between the start-up heater  16  and the fuel cell  2 , a branch line  27  is disposed which branches from the pressurized air line  25  and is connected to the exhaust air line  42 . The branch line  27  is provided with a flow regulating valve  28  capable of regulating the flow rate of the air flowing through the branch line  27 . 
     An inlet of the anode  2   b  is connected to a fuel supply line  31  communicating with a fuel supply source  30  such as a city gas. The fuel supply line  31  is provided with a flow regulating valve  38  capable of regulating the flow rate of the fuel. An outlet of the anode  2   b  is connected to an exhaust fuel line  32  communicating with the catalytic combustor  17 . The exhaust fuel line  32  is provided with a recirculation blower  35  and a flow regulating valve  36  downstream of the exhaust fuel line  32 . Further, a recirculation line  37  for returning a part of the exhaust fuel to the fuel supply line  31  is disposed. The recirculation line  37  is connected at one end to a portion between the recirculation blower  35  and the flow regulating valve  36 , and is connected at another end to the fuel supply line  31 . The recirculation line  37  is provided with a flow regulating valve  39  capable of regulating a recirculation flow rate. Further, a fuel supply line  40  for directly supplying the fuel from the fuel supply source  30  to the catalytic combustor  17  is disposed. The fuel supply line  40  is provided with a flow regulating valve  41  capable of regulating the flow rate of the fuel. In Embodiment 1 of the present disclosure, upstream of the recirculation blower  35  on the exhaust fuel line  32 , provided are a heat exchanger  33  for heat exchange between the fuel flowing through the fuel supply line  31  having joined the recirculation line  37  and the exhaust fuel flowing through the exhaust fuel line  32 , and a cooler  34  for further cooling the exhaust fuel cooled by the heat exchanger  33  and supplying the further cooled exhaust fuel to the recirculation blower  35 . 
     Next, starting method of the fuel cell system  1  and the pressurized air supply system  3  according to Embodiment 1 will be described. 
     As shown in  FIG. 1 , each of the flow regulating valves  18 ,  26 ,  36 ,  38 , and  41  is fully closed, and the other flow regulating valves are fully opened. In this state, the start-up air compressor  22  is started, and the start-up heater  16  is started. Then, the start-up air is supplied from the start-up air compressor  22  to the discharged air line  14  via the start-up air supply line  21 . The start-up air supplied to the discharged air line  14  passes through the recuperator  13 , and then flows through the pressurized air line  25 . The start-up air is heated by the start-up heater  16  during the process of flowing through the pressurized air line  25 , flows through the branch line  27 , and flows into the catalytic combustor  17  through the exhaust air line  42 . 
     Once the start-up air heated by the start-up heater  16  flows into the catalytic combustor  17 , and catalyst temperature increases to be not less than preset temperature (activation temperature), fuel supply to the catalytic combustor  17  via the fuel supply line  40  is started for ignition. As described later, as long as the catalyst temperature can be maintained at not less than the activation temperature with the temperature of the air flowing into the catalytic combustor  17 , it is possible to stop the start-up heater  16  or to operate the start-up heater  16  at low load, making it possible to reduce operation energy consumption of the start-up heater  16  by quickly increasing the temperature of the air supplied to the catalytic combustor  17 . 
     In the catalytic combustor  17 , the combustion gas is generated by combustion of the fuel with the start-up air. The combustion gas flowing out of the catalytic combustor  17  flows into the turbine  12  of the turbocharger  10  and rotates the turbine  12 . The flue gas rotating the turbine  12  and exhausted from the turbine  12  flows into the recuperator  13 . In the recuperator  13 , the start-up air is heated by the flue gas, further increasing the temperature of the start-up air. Thus, if heating amount of the start-up heater  16  is the same, it is possible to shorten heatup time of the catalytic combustor  17  and to reduce the operation energy consumption of the start-up heater  16 . On the other hand, if the temperature of the start-up air flowing into the catalytic combustor  17  is the same, it is possible to reduce the operation load (reduce a designed heating capacity) of the start-up heater  16 , making it possible to reduce the operation energy consumption of the start-up heater  16 . 
     Once the turbine  12  starts to rotate in the turbocharger  10 , the compressor  11  starts taking in air, and the discharged air boosted by the compressor  11  flows through the discharged air line  14 . However, the boosted discharged air is not allowed to flow into the recuperator  13  by fully closing the flow regulating valve  18  and fully opening the flow regulating valve  20  immediately after the start-up, but is caused to flow through the extraction blow line  19  to flow into the flue gas line  24  and to join the flue gas flowing out of the recuperator  13 . Thus, compression power is reduced, and an increase in rotation speed, discharge pressure, discharged air amount of the turbocharger is accelerated, making it possible to advance switching with the start-up air. If switching with the start-up air takes time, the operation of the start-up air compressor  22  needs to be continued accordingly, consuming more energy for start-up. Thus, the faster transition to a self-sustained operation of the turbocharger  10 , the better. 
     Once the turbine  12  drives the compressor  11  by the combustion gas, the rotation speed increases, and the discharge pressure exceeds the start-up air pressure, the flow regulating valve  18  is opened, as well as the flow regulating valve  20  is closed, and supply of the discharged air to the recuperator  13  is started. During or after switching of the opened/closed states, supply of the start-up air from the start-up air supply device  15  is stopped. A switching timing and a switching speed of the opened/closed states of the flow regulating valves  18  and  20  are adjusted such that fluctuations in discharge pressure and air flow rate are small. If supply of the start-up air from the start-up air supply device  15  is stopped during switching of the opened/closed states, the supply amount of the start-up air may gradually be decreased by gradually closing the flow regulating valve  23 . Further, if switching of the opened/closed states is performed in one step, the start-up air compressor  22  may be stopped, as well as the flow regulating valve  23  may fully be closed simultaneously with or after the switching. On the completion of switching of the start-up air, the turbocharger performs the self-sustained operation. 
     Once the operation of the turbocharger  10  is stable, and the catalyst temperature of the catalytic combustor  17  can be maintained at not less the activation temperature, the start-up heater  16  is stopped or operated at low load. The pressurized air supply system  3  is thus started. 
     Once the pressurized air supply system  3  is started, the flow regulating valve  26  is opened to supply the pressurized air to the cathode  2   a  of the fuel cell  2 . The exhaust air flowing out of the cathode  2   a  flows through the exhaust air line  42  and is supplied to the catalytic combustor  17 . If the temperature of the exhaust air flowing out of the cathode  2   a  is sufficiently increased, the flow regulating valve  28  is closed to restart the start-up heater  16  or to increase an output of the start-up heater  16 . 
     After the temperature of the fuel cell  2  is increased to a temperature capable of generator room combustion, generator room combustion is started to increase to a temperature capable of power generation. If the temperature of the fuel cell  2  increases beyond a warm-up pressurized air temperature by the generator room combustion, the start-up heater  16  is stopped. Thereafter, by the generator room combustion, the temperature of the fuel cell  2  is increased to a temperature capable of performing a power generation operation. 
     Thus, at starting of the pressurized air supply system  3 , by supplying the flowing air heated by the recuperator  13  to the start-up heater  16 , in the start-up heater  16 , it is possible to reduce operation energy consumption by shortening the duration for start-up and to reduce the designed heating capacity, making it possible to reduce the cost for starting the pressurized air supply system  3 . 
     Embodiment 2 
     Next, a pressurized air supply system and a fuel cell system according to Embodiment 2 will be described. The pressurized air supply system and the fuel cell system according to Embodiment 2 are obtained by modifying Embodiment 1 in the configuration of the start-up air supply device  15 . In Embodiment 2, the same constituent elements as those in Embodiment 1 are associated with the same reference characters and not described again in detail. 
     As shown in  FIG. 2 , in the pressurized air supply system  3  according to Embodiment 2 of the present disclosure, the start-up air supply device  15  includes a motor  51 , a drive shaft (rotor)  53  of the compressor  11 , and a speed increasing gear  52  disposed on the drive shaft (rotor)  53 , and the motor  51  is connected to the compressor  11  via the speed increasing gear  52  and the drive shaft (rotor)  53 . Other configurations are the same as Embodiment 1, except that the flow regulating valve  18  is not provided. 
     Next, the fuel cell system  1  and the pressurized air supply system  3  according to Embodiment 2 will be described. 
     As shown in  FIG. 2 , each of the flow regulating valves  20 ,  26 ,  36 ,  38 , and  41  is fully closed, and the other flow regulating valves are fully opened. If the motor  51  is started in this state, air is discharged from the compressor  11 , and the discharged air flows through the discharged air line  14 . 
     In Embodiment 2, it is possible to use the discharged air from the compressor  11  driven by the motor  51  as the start-up air. Therefore, the subsequent starting operation is basically the same as Embodiment 1, and is different in that the operation of the motor  51  is stopped once the self-sustained operation of the turbocharger  10  becomes possible. As with Embodiment 1, since the discharged air serving as the start-up air is heated by the recuperator  13 , and then supplied to the start-up heater  16 , in the start-up heater  16 , it is possible to reduce operation energy consumption by shortening the duration for start-up and to reduce the designed heating capacity, making it possible to reduce the cost for starting the pressurized air supply system  3 . 
     In the above description, the flow regulating valve  20  is fully closed. However, by adjusting the opening degree of the flow regulating valve  20 , a part of the discharged air from the compressor  11  may not be allowed to flow into the recuperator  13 . In this case, it is possible to increase the temperature of air flowing out of the recuperator  13  by reducing the amount of air that undergoes heat exchange with the flue gas, making it possible to reduce the cost for starting the pressurized air supply system  3 , as with Embodiment 1. 
     In Embodiment 2, the start-up air supply device  15  includes the speed increasing gear  52  disposed on the drive shaft (rotor)  53 . However, without the speed increasing gear  52 , the compressor  11  may directly be driven by power from the motor  51 . In this case, although the motor  51  needs to be rotated at a high speed, it is possible to eliminate loss in the speed increasing gear and to reduce the cost for starting the pressurized air supply system  3 . 
     Embodiment 3 
     Next, a pressurized air supply system and a fuel cell system according to Embodiment 3 will be described. The pressurized air supply system and the fuel cell system according to Embodiment 3 are obtained by modifying Embodiment 1 or 2 such that a part of the discharged air from the compressor  11  can be supplied to the exhaust air line  42 . Hereinafter, in contrast to the configuration of Embodiment 1, Embodiment 3 will be described with a configuration in which a part of the discharged air from the compressor  11  can be supplied to the exhaust air line  42 . However, in contrast to the configuration of Embodiment 2, Embodiment 3 may be configured such that a part of the discharged air from the compressor  11  can be supplied to the exhaust air line  42 . In Embodiment 3, the same constituent elements as those in Embodiment 1 are associated with the same reference characters and not described again in detail. 
     As shown in  FIG. 3 , in the pressurized air supply system  3  according to Embodiment 3 of the present disclosure, a discharged air bypass line  56  is disposed which is connected at one end to the discharged air line  14  between the compressor  11  and the flow regulating valve  18 , and is connected at another end to the exhaust air line  42  (the discharged air bypass line  56  is a flow passage bypassing the recuperator  13 , and is thus one of recuperator bypass lines). 
     The discharged air bypass line  56  is provided with a flow regulating valve  57  capable of regulating the flow rate of the discharged air flowing through the discharged air bypass line  56 . In Embodiment 3, the extraction blow line  19  branches from the discharged air bypass line  56  upstream of the flow regulating valve  57 , and other configurations are the same as Embodiment 1. 
     Next, the fuel cell system  1  and the pressurized air supply system  3  according to Embodiment 3 will be described. 
     As shown in  FIG. 3 , each of the flow regulating valves  18 ,  26 ,  36 ,  38 ,  41 , and  57  is fully closed, and the other flow regulating valves are fully opened. In this state, the start-up air compressor  22  is started, and then the start-up heater  16  is started. Regarding the subsequent operation, the operation until the pressurized air flows into the cathode  2   a  of the fuel cell  2  from the pressurized air supply system  3 , and then the flow regulating valve  28  is fully closed to restart the start-up heater  16  or to increase the output of the start-up heater  16  is the same as Embodiment 1. 
     Once the start-up heater  16  is restarted or the output of the start-up heater  16  is increased, the flow regulating valve  57  is adjusted to have an appropriate opening degree. Then, a part of the discharged air from the compressor  11  flows through the discharged air bypass line  56 , decreasing the flow rate of air flowing into each of the recuperator  13  and the start-up heater  16 . Thus, it is possible to increase the temperature of the discharged air flowing into the start-up heater  16 . Thus, it is possible to reduce the cost for starting the pressurized air supply system  3 , as with Embodiment 1. 
     Embodiment 4 
     Next, a pressurized air supply system and a fuel cell system according to Embodiment 4 will be described. The pressurized air supply system and the fuel cell system according to Embodiment 4 are obtained by modifying Embodiments 1 to 3 such that the start-up heater  16  is disposed on the branch line  27 , and a fuel cell heating bypass air line  60  is added. Hereinafter, in contrast to the configuration of Embodiment 1, Embodiment 4 will be described with a configuration in which a start-up heater bypass line is added. However, in contrast to the configuration of Embodiment 2 or 3, Embodiment 4 may be configured such that the start-up heater bypass line is added. In Embodiment 4, the same constituent elements as those in Embodiment 1 are associated with the same reference characters and not described again in detail. 
     As shown in  FIG. 4 , in the pressurized air supply system  3  according to Embodiment 4 of the present disclosure, the fuel cell heating bypass air line  60  is disposed which branches from downstream of the start-up heater  16  on the branch line  27  and rejoins the pressurized air line  25  flowing into the cathode  2   a  of the fuel cell  2  downstream of the flow regulating valve  26 . The fuel cell heating bypass air line  60  is provided with a flow regulating valve  61  capable of regulating the flow rate of the flowing air. Other configurations are the same as Embodiment 1. 
     Next, the fuel cell system  1  and the pressurized air supply system  3  according to Embodiment 4 will be described. 
     As shown in  FIG. 4 , each of the flow regulating valves  18 ,  26 ,  36 ,  38 ,  41 , and  61  is fully closed, and the other flow regulating valves are fully opened. In this state, the start-up air compressor  22  is started, and then the start-up heater  16  is started. Regarding the subsequent operation, an operation until the pressurized air supply system  3  is started, and the flow regulating valve  26  is opened to supply the pressurized air to the cathode  2   a  of the fuel cell  2  is the same as Embodiment 1. 
     In Embodiment 4, the temperature of the fuel cell  2  is increased to some degree in the state where the flow regulating valve  26  is opened, and then the start-up heater  16  is restarted or the output of the start-up heater  16  is increased. Subsequently, the flow regulating valve  61  is opened to supply the flowing air increased in temperature by the start-up heater  16  to the pressurized air line  25 , making it possible to further increase the temperature of the pressurized air flowing into the cathode  2   a  of the fuel cell  2 . Thus, it is possible to shorten the time required to increase the temperature of the fuel cell  2 , and to reduce the cost for operating the pressurized air supply system. 
     Embodiment 5 
     Next, a pressurized air supply system and a fuel cell system according to Embodiment 5 will be described. The pressurized air supply system and the fuel cell system according to Embodiment 5 are obtained by modifying Embodiment 4 such that the start-up heater  16  is constituted by two heaters (a first heater  71  for warming up the catalytic combustor  17  and a second heater  72  for warming up the fuel cell  2 ). In Embodiment 5, the same constituent elements as those in Embodiment 4 are associated with the same reference characters and not described again in detail. 
     As shown in  FIG. 5 , in the pressurized air supply system  3  according to Embodiment 5 of the present disclosure, the first heater  71  is disposed on the branch line  27 , and the flow regulating valve  28  is disposed upstream of the first heater  71 . The second heater  72  is disposed on the fuel cell heating bypass air line  60  that connects a branch point A upstream of the flow regulating valve  28  on the branch line  27  and a junction point B downstream of the flow regulating valve  26 , and the flow regulating valve  61  is disposed on the fuel cell heating bypass air line  60  upstream of the second heater  72 . Other configurations are the same as Embodiment 4. 
     In Embodiments 1 to 4 where the start-up heater  16  is constituted by one heater, the flow regulating valve  28  (including the flow regulating valve  61  in Embodiment 4) needs to be disposed downstream of the start-up heater  16 , and thus needs to be a high-temperature valve. However, as in Embodiment 5, if the start-up heater  16  is constituted by the two heaters, namely, the first heater  71  and the second heater  72 , it is possible to dispose the flow regulating valves  28  and  61  upstream of the first heater  71  and the second heater  72 , respectively. Then, it is possible to use, as the flow regulating valves  28  and  61 , not the high-temperature valves, but low-temperature valves each being lower in cost than the high-temperature valve, making it possible to reduce the cost for starting the pressurized air supply system  3 . 
     Next, the fuel cell system  1  and the pressurized air supply system  3  according to Embodiment 5 will be described. 
     As shown in  FIG. 5 , each of the flow regulating valves  18 ,  26 ,  36 ,  38 ,  41 , and  61  is fully closed, and the other flow regulating valves are fully opened. In this state, the start-up air compressor  22  is started, and then the first heater  71  is started. Then, the start-up air is supplied from the start-up air compressor  22  to the discharged air line  14  via the start-up air supply line  21 , and the start-up air supplied to the discharged air line  14  passes through the recuperator  13 , then flows through the pressurized air line  25 , flows into the branch line  27  at the branch point A, and is heated by the first heater  71 . The start-up air heated by the first heater  71  flows through the branch line  27 , then flows into the exhaust air line  42 , and flows into the catalytic combustor  17 . 
     Regarding the subsequent operation, once the self-sustained operation of the turbocharger  10  is stable, and the catalyst temperature of the catalytic combustor  17  can be maintained at not less the activation temperature, an operation until the first heater  71  is stopped or operated at low load is the same as the operation described in Embodiment 1 (note that the start-up heater  16  is read as the first heater  71 ). The pressurized air supply system  3  according to Embodiment 5 of the present disclosure is thus started. 
     Once the pressurized air supply system  3  is started, the flow regulating valve  61  is adjusted to have the appropriate opening degree, as well as the second heater  72  is started, heating at least a part of the flowing air by the second heater  72 , and then being supplied to the cathode  2   a  of the fuel cell  2 . The exhaust air flowing out of the cathode  2   a  flows through the exhaust air line  42  and flows into the catalytic combustor  17 . 
     Once the temperature of the fuel cell  2  is increased to the temperature capable of performing power generation room combustion, the temperature of the fuel cell  2  is increased by power generation room combustion. In the process, once the catalyst temperature of the catalytic combustor  17  can be maintained at not less than the activation temperature, the opening degree of the flow regulating valve  28  is decreased, as well the opening degree of the flow regulating valve  61  is increased to stop the first heater  71 . Once the temperature of the fuel cell  2  is sufficiently increased to allow temperature increase only by power generation room combustion, the opening degree of the flow regulating valve  61  is decreased to stop the second heater  72 . 
     In each of Embodiments 1 to 5, the pressurized air supply system  3  serves to supply the pressurized air to the cathode  2   a  of the fuel cell  2 . However, the present disclosure is not limited to this form. The present disclosure is applicable to an optional plant or device that needs pressurized air. 
     REFERENCE SIGNS LIST 
     
         
           1  Fuel cell system 
           2  Fuel cell 
           2   a  Cathode 
           2   b  Anode 
           2   c  electrolyte 
           3  Pressurized air supply system 
           10  Turbocharger 
           11  Compressor 
           12  Turbine 
           13  Recuperator 
           14  Discharged air line 
           15  Start-up air supply device 
           16  Start-up heater 
           17  Catalytic combustor 
           18  Flow regulating valve 
           19  Extraction blow line (one of recuperator bypass lines) 
           20  Flow regulating valve 
           21  Start-up air supply line 
           22  Start-up air compressor 
           23  Flow regulating valve 
           24  Flue gas line 
           25  Pressurized air line 
           26  Flow regulating valve 
           27  Branch line 
           28  Flow regulating valve 
           30  Fuel supply source 
           31  Fuel supply line 
           32  Exhaust fuel line 
           33  Heat exchanger 
           34  Cooler 
           35  Recirculation blower 
           36  Flow regulating valve 
           37  Recirculation line 
           38  Flow regulating valve 
           39  Flow regulating valve 
           40  Fuel supply line 
           41  Flow regulating valve 
           42  Exhaust air line 
           51  Motor 
           52  Speed increasing gear 
           53  Drive shaft (rotor) of compressor  11   
           56  Discharged air bypass line (one of recuperator bypass lines) 
           57  Flow regulating valve 
           60  Fuel cell heating bypass air line 
           61  Flow regulating valve 
           71  First heater 
           72  Second heater