Abstract:
A fuel cell system capable of improving performance and stability of the system by using stack off-gas includes: a power generation unit that generates power through an electrochemical reaction of a first fuel and a first oxidant; a reforming unit that supplies the first fuel to the power generation unit; a heating unit that receives second fuel and a second oxidant, combusts the second fuel, and is thermal-conductively coupled with the reforming unit; and a connection unit that connects the heating unit with the power generation unit to be in fluid communication and supplies off-gas of the power generation unit to the heating unit. The off-gas is supplied to the heating unit in a pulse type.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2009-0111397, filed Nov. 18, 2009 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     An embodiment of the present invention relates to a fuel cell system capable of improving performance and stability by using stack off-gas and an operation method thereof. 
     2. Description of the Related Art 
     In this the modern society, there is an increasing concern about environment pollution. As such, there has been active progress in the development of alternative energy such as a fuel cell which is non-polluting and excellent in energy efficiency. 
     The fuel cell is a device that directly converts chemical energy into electric energy through electrochemical reaction of hydrogen and oxygen. The fuel cell is classified into a polyelectrolyte fuel cell, a solid oxide fuel cell, and a molten carbonate fuel cell in accordance with the electrolyte type. Hydrogen used in the polyelectrolyte fuel cell is typically acquired from a reformate reforming fuel such as methanol, liquid petroleum gas (LPG), gasoline, etc. This fuel is used due to various difficult problems caused by storing and transporting pure hydrogen. Therefore, in most of fuel cell systems using a steam reforming type reformate, a heating unit supplying heat required for steam reforming reaction is provided in the reformate. 
     SUMMARY 
     An embodiment of the present invention provides a fuel cell system capable of improving performance and stability of a heating unit and a reforming unit by supplying stack off-gas to the heating unit supplying heat to a reforming unit in a pulse type and an operation method thereof. 
     Further, another embodiment of the present invention provides a fuel cell system capable of improving efficiency and stability of a system by supplying stack off-gas to a heating unit supplying heat to a reforming unit in a pulse type and an operation method thereof. 
     According to an embodiment of the present invention, a fuel cell system includes: a power generation unit that generates power through electrochemical reaction of first fuel and a first oxidant; a reforming unit that supplies the first fuel to the power generation unit; a heating unit that receives second fuel and a second oxidant, combusts the second fuel, and is thermal-conductively coupled with the reforming unit; and a connection unit that connects the heating unit with the power generation unit to be in fluid communication and supplies off-gas of the power generation unit to the heating unit. Herein, the off-gas is supplied to the heating unit in a pulse type. 
     In an embodiment, the fuel cell system further includes a controller. The controller controls the temperature of the reforming unit by controlling the supplying amount of the second oxidant supplied to the heating unit in response to first temperature of the first fuel discharged from the reforming unit and controls the temperature of the heating unit by controlling the supplying amount of the second fuel supplied to the heating unit in response to second temperature of a fuel inlet of the heating unit. 
     In an embodiment, the controller includes a flow rate controller. 
     In an embodiment, the off-gas of a predetermined amount is additionally supplied to the heating unit in addition to supplying of the second fuel. 
     In an embodiment, supplying the second fuel is interrupted while the off-gas is supplied. 
     In an embodiment, the off-gas is directly supplied to the heating unit. One end of the connection unit through which the off-gas is discharged is spaced from the fuel inlet of the heating unit by a predetermined gap. 
     In an embodiment, the heating unit includes a first oxidation catalyst portion positioned adjacent to the fuel inlet and a second oxidation catalyst portion positioned spaced from the first oxidation catalyst portion by a predetermined gap. At this time, the one end of the connection unit through which the off-gas is discharged is connected between the first and second oxidation catalyst portions to be in fluid communication. 
     According to another embodiment of the present invention, an operation method of a fuel cell system including a power generation unit that generates power through electrochemical reaction of first fuel and a first oxidant, a reforming unit that supplies the first fuel to the power generation unit, a heating unit that receives second fuel and a second oxidant, combusts the second fuel, and is thermal-conductively coupled with the reforming unit, and a connection unit that connects the heating unit with the power generation unit to be in fluid communication and supplies off-gas of the power generation unit to the heating unit, includes: supplying the second fuel and the second oxidant to the heating unit; supplying fuel to the reforming unit; and supplying the off-gas to the heating unit in a pulse type. 
     In an embodiment, the fuel cell system further includes a controller. The operation method of a fuel cell system includes: controlling the temperature of the reforming unit by controlling the supplying amount of the second oxidant supplied to the heating unit in response to first temperature of reformate discharged from the reforming unit; and controlling the temperature of the heating unit by controlling the supplying amount of the second fuel supplied to the heating unit in response to second temperature of a fuel inlet of the heating unit. 
     In an embodiment, supplying the off-gas includes additionally supplying the off-gas of a predetermined amount to the heating unit while supplying the second fuel. 
     In an embodiment, supplying the off-gas includes interrupting supplying the second fuel while supplying the off-gas. 
     In an embodiment, the heating unit may include a first oxidation catalyst portion positioned adjacent to the fuel inlet and a second oxidation catalyst portion positioned spaced from the first oxidation catalyst portion by a predetermined gap. The operation method of the fuel cell system includes supplying the off-gas between the first and second oxidation catalyst portions. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a schematic block diagram of a fuel cell system according to an embodiment. 
         FIG. 2  is a schematic graph for describing supplying of off-gas in a fuel cell system of an embodiment. 
         FIG. 3  is a schematic configuration diagram of a fuel cell system according to another embodiment. 
         FIG. 4  is a schematic configuration diagram of a controller used in a fuel cell system of an embodiment. 
         FIG. 5  is a schematic cross-sectional view of a heating unit used in a fuel cell system of an embodiment. 
         FIGS. 6A to 6D  are flowcharts of an operation method of a fuel cell system according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
     However, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Also, in describing the embodiments, well-known functions or constructions will not be described in detail since they may unnecessarily obscure the understanding of the present invention. In addition, it will be appreciated that like reference numerals refer to like elements throughout even though they are shown in different figures. Further, when a first element is described as being coupled to a second element, the first element may be not only directly coupled to the second element but may also be indirectly coupled to the second element via a third element. Moreover, when a first layer is provided on a second layer, the first layer may be provided directly on the second layer or a third layer may be interposed therebetween. Additionally, in the figures, the thickness and sizes of each layer may be exaggerated for convenience of description and clarity and may be different from the actual thickness and size. 
       FIG. 1  is a schematic block diagram of a fuel cell system  100  according to an embodiment.  FIG. 2  is a schematic graph for describing supplying of off-gas in the fuel cell system  100 . Referring to  FIG. 1 , the fuel cell system  100  includes a power generation unit  10 , a reforming unit  20 , a heating unit  30 , and a connection unit  40 . 
     The power generation unit  10  includes a fuel cell and generates electric energy by electromechanical reaction of a first fuel and a first oxidant (oxidant  1 ). Anode outlet gas (AOG) is discharged from an anode outlet of the power generation unit  10  and cathode outlet gas (COG) is discharged from a cathode outlet. The anode outlet gas (AOG) includes unreacted fuel, which includes hydrogen of a predetermined amount. 
     The reforming unit  20  generates reformate from a reforming fuel (Fuel  0 ). The reformate is supplied to the power generation unit  10  as the first fuel. The reformate may be supplied to the power generation unit  10  after the reformate is processed to have the content of carbon monoxide of 10 ppm or less in an additional WGS device and a PROX device. The reforming unit  20  may be implemented by methods such as steam catalyst reforming, partial oxidation reaction and/or autothermal reforming. Methanol, liquid petroleum gas (LPG), gasoline, etc. may be used as the fuel (Fuel  0 ). 
     The heating unit  30  is coupled to the reforming unit  20  to supply heat to the reforming unit  20 . The heating unit  30  may be implemented by a burner or an oxidation catalyst combustor. The heating unit  30  receives a second fuel (Fuel  2 ) and a second oxidant (Oxidant  2 ) and combusts the second fuel (Fuel  2 ) to generate heat used by the reforming unit  20 . 
     The connection unit  40  connects the heating unit  30  with the power generation unit  10  to be in fluid communication. The connection unit  40  is provided to supply off-gas discharged from an anode of the power generation unit  40  (AOG) to the heating unit  30 . 
     An off-gas control valve  42  is connected to the connection unit  40 . The off-gas control valve  42  may be provided around the anode outlet of the power generation unit  10  in the form of a 3-way valve, but the invention is not limited to a specific valve type. The off-gas control valve  42  operates, such that the off-gas (AOG) supplied to the heating unit  30  from the power generation unit  10  has pulse types P 1 , P 2 , and P 3  as shown in  FIG. 2 . The pulse type indicates that the off-gas (AOG) of a predetermined amount is intermittently supplied at a predetermined time interval as shown in  FIG. 2 . It is understood that the supply amount of the off-gas (AOG) may be increased depending on the content of hydrogen in the off-gas (AOG) in order to intermittently supply hydrogen gas of a predetermined amount A 1 . 
     When the off-gas (AOG) is supplied to the heating unit  30  in the pulse type, it is possible to prevent backfire in the heating unit  30 . In other words, the off-gas (AOG) contains a large amount of hydrogen. Since the hydrogen is very short in quenching distance and very fast in diffusion rate, when the off-gas (AOG) is supplied to the burner or the oxidation catalyst combustor of the heating unit  30 , the backfire is easily generated. However, when the off-gas (AOG) is supplied to the heating unit  30  in the pulse type, a mixing ratio of combustion fuel acquired by mixing the second fuel (Fuel  2 ) and the off-gas (AOG) with the second oxidant (Oxidant  2 ) or a mixing ratio of the off-gas (AOG) and the second oxidant (Oxidant  2 ) deviates from a range in which the backfire is easily generated. Accordingly, when the off-gas (AOG) is supplied in the pulses, it is possible to prevent the backfire. However, it is understood that other mechanisms can be used to prevent backfire. For instance, the intermittent supply of the off-gas (AOG) can have pulses with a varying frequency and amount or with regular and consistent pulses as shown. Moreover, while shown as varying between 0 and A 1 , it is understood that the lower amount of the pulse need not be 0 as shown. 
     In a modified example of the embodiment, the supply of the second fuel (Fuel  2 ) to the heating unit  30  may be maintained or interrupted while the off-gas (AOG) is supplied. When the second fuel (Fuel  2 ) and the off-gas (AOG) are supplied at the same time, the off-gas (AOG) may be supplied to the heating unit  30  through a fuel inlet other than a fuel inlet of the second fuel (Fuel  2 ). When the supply of the second fuel (Fuel  2 ) is interrupted while the off-gas (AOG) is supplied, the off-gas (AOG) may be supplied to the same fuel inlet as the fuel inlet of the second fuel (Fuel  2 ). 
     Referring back to  FIG. 1 , and while not required in all aspects, the fuel cell system  100  includes a controller  50 . The controller  50  controls the temperature of the reforming unit  20  by controlling the amount of the second oxidant (Oxidant  2 ) supplied to the heating unit  30  in response to first temperature of the first fuel (Fuel  1 ) discharged from the reforming unit  20 . Further, the controller  50  controls the temperature of the heating unit  30  by controlling the amount of the second fuel (Fuel  2 ) supplied to the heating unit  30  in response to second temperature around a fuel inlet  34  of the heating unit  30 . While not required in all aspects, the controller  50  can be implemented as one or more processors executing software read from a computer readable medium. 
     In the fuel cell system  100  of the shown embodiment, a first sensor  61  is provided adjacent to a reformate outlet of the reforming unit  20  in order to detect the first temperature for the first fuel (Fuel  1 ). A first valve  31  is provided on a supplying path of the second oxidant (Oxidant  2 ) in order to control the supplying amount of the second oxidant (Oxidant  2 ). Further, a second sensor  62  is provided adjacent to the fuel inlet  34  of the heating unit  30  in order to detect the second temperature and a second valve  32  is provided on a supplying path of the second fuel (Fuel  2 ) in order to control the amount of the second fuel (Fuel  2 ) supplied to the heating unit  30 . 
     According to the embodiment, it is possible to improve the performances and efficiencies of the heating unit  30  and the reforming unit  20  while preventing the backfire from being generated in the heating unit  30  by adopting an operational logic for the heating unit  30  as well as supplying the off-gas (AOG) in the pulse type. However, it is understood that the sensors  61 , 62  and valves  31 , 32  can be otherwise located in other combinations to similarly optimize the heating unit  30  and the reforming unit  20   
       FIG. 3  is a schematic configuration diagram illustrating a partial configuration of a fuel cell system according to another embodiment. Referring to  FIG. 3 , a controller  50   b  is used as a flow rate controller or a mass flow rate controller (MFC). The controller  50   b  includes the first valve  31  for controlling the flow rate of the second oxidant (Oxidant  2 ) and a second valve  32  for controlling the flow rate of the second fuel (Fuel  2 ). 
     A heating unit  30   a  is provided to surround a middle portion of the reforming unit  20 . The reforming unit  20  and the heating unit  30   a  may have a dual-tube structure, but the invention is not limited thereto. Further, it is understood that the heating unit  30   a  can be disposed along the full length of the reforming unit  20  or at other portions in addition to or instead of the shown middle portion. 
     A connection unit  40   a  connects the heating unit  30   a  with a power generation unit (such as the power generation unit  10  shown in  FIG. 1 ). A combustion gas control valve  42   b  is coupled to the connection unit  40   a . The combustion gas control valve  42   b  may be provided in the controller  50   b  as shown, but can be disposed elsewhere. 
     In the shown embodiment, one end  41  of the connection unit  40   a  is spaced from the fuel inlet  34  of the heating unit  30   a  by a predetermined gap and is connected with the heating unit  30   a  to be in fluid communication. By this configuration, it is possible to prevent unstable combustion from being generated due to the mixing of the second fuel (Fuel  2 ) and the off-gas (AOG) at the fuel inlet  34  of the heating unit  30   a.    
     The controller  50   b  receives temperature information of the reforming unit  20  and the heating unit  30   a  from the first sensor  61  and the second sensor  62 . The first sensor  61  detects the first temperature of the reformate (Fuel  1 ) just discharged from the reforming unit  20 . The second sensor  62  detects the second temperature around a fuel inlet of the heating unit  30   a.    
       FIG. 4  is a schematic configuration diagram of a controller  50   c  used in a fuel cell system of an embodiment. Referring to  FIG. 4 , the controller  50   c  is implemented as a flow rate controller. The controller  50   c  includes a flow rate sensor  52 , a bridge circuit  53 , an amplifier circuit  54 , a comparison and control circuit  55   a , and a valve  56 . The flow rate controller  50   c  includes a housing having an inlet port and an outlet port connected to a pipe  45 , but the invention is not limited thereto. 
     The flow rate sensor  52  is coupled to a bypass unit  43  provided in a middle part of a pipe  45  that transfers the second fuel (Fuel  2 ) or the second oxidant (Oxidant  2 ). That is, the flow rate sensor  52  is coupled to a sub-pipe  43  coupled to bypass the pipe  45  to detect the flow rate of the second fuel (Fuel  2 ) or the second oxidant (Oxidant  2 ) that passes through the pipe  45  through detection of the flow rate of the second fuel (Fuel  2 ) or the second oxidant (Oxidant  2 ) of a predetermined amount passing through the sub-pipe  43 . 
     The bridge circuit  53  may include a Wheatstone bridge circuit used in various measuring instruments, but the invention is not limited thereto. The amplifier circuit  54  properly amplifies an output signal of the bridge circuit  53  and transfers the amplified output signal to the comparison and control circuit  55   a . In the amplifier circuit  54 , the signal transferred to the comparison and control circuit  55   a  may also be used as information for displaying the detection flow rate on an external device in aspects of the invention. 
     The comparison and control circuit  55   a  monitors the flow rate of the second fuel (Fuel  2 ) or the second oxidant (Oxidant  2 ) supplied to the heating unit through the pipe  45 . Similarly and while not shown in  FIG. 4 , it is understood that the comparison and control circuit  55   a  may monitor the flow rate of the off-gas (AOG) transferred to the heating unit  30  from the power generation unit through the connection unit  40 . Further, the comparison and control circuit  55   a  senses the first temperature T 1  transferred from the first sensor (such as the first sensor  61  of  FIG. 3 ) and/or the second temperature T 2  transferred from the second sensor (such as the second sensor  62  of  FIG. 3 ). The comparison and control circuit  55   a  compares the first temperature T 1  or the second temperature T 2  with a corresponding reference temperature. 
     In the embodiment, the reference temperature includes a first reference temperature representing a lower bound value of a desired temperature range for the reformate (Fuel  1 ) discharged from the reforming reaction unit (such as the reaction unit  20  of  FIG. 3 ) and a first threshold temperature representing an upper bound of the desired temperature range for the reformate. Further, the reference temperature includes the lower bound value of the desired temperature range for the fuel inlet of the heating unit (such as the heating unit  30   a  of  FIG. 3 ) and a second threshold temperature representing an upper bound value of the desired temperature range for the fuel inlet of the heating unit (such as the inlet  34  of  FIG. 3 ). 
     The comparison and control circuit  55   a  transfers a control signal corresponding to the first temperature T 1  or the second temperature T 2  to the valve  56 . The opening percentage of the valve  56  is automatically controlled by the control signal. The valve  56  may correspond to the first valve  31  or the second valve  32  shown in  FIG. 3 . Further, while shown in the context of the valves controlling the second fuel (Fuel  2 ) or second oxidant (Oxidant  2 ), the valve  56  may also correspond to the off-gas control valve  42   b  of  FIG. 3  and similarly control the supply of off-gas (AOG). 
     The valve  56  is coupled to the pipe  45  to control the flow rate of the second fuel (Fuel  2 ) or the second oxidant (Oxidant  2 ) that flow rates on the pipe  45 . Similarly, the valve  56  may control the off-gas (Fuel  0 ) that flows on the connection unit  40   b  in the pulse type. 
     The valve  56  includes a proportion control valve. For example, the valve  56  may include a solenoid valve including a solenoid having a magnetic core and a body having one or more orifices. However, the type of proportional control valve is not limited thereto 
       FIG. 5  is a schematic cross-sectional view of a heating unit  30   b  used in a fuel cell system of an embodiment. Referring to  FIG. 5 , the heating unit  30   b  includes a first oxidation catalyst portion  35  positioned adjacent to a fuel inlet  34   a  in a predetermined housing, and a second oxidation catalyst portion  36  spaced from the first oxidation catalyst portion  35  by a predetermined gap at a downstream side in a direction in which the second fuel (Fuel  2 ) flows. The first and second oxidation catalyst portions  35  and  36  include the same kind or different kinds of oxidation catalysts. Further, the first and second oxidation catalyst portions  35  and  36  may include a predetermined substrate or a support supporting the oxidation catalyst. 
     One end  41   a  of the connection unit  40   a  is connected to the heating unit  30   b  to be in fluid communication so as to allow the off-gas (AOG) to flow into the heating unit  30   b . Specifically, one end  41   a  of the connection unit  40   a  is inserted into the heating unit  30   b  into a space  37  between the first and second oxidation catalyst portions  35  and  36 . When the one end  41   a  is in the space  37  to be in fluid communication, the off-gas (AOG) may be supplied independently from the second fuel (Fuel  2 ). Therefore, it is possible to prevent backfire from being generated due to mixing of the second fuel and the off-gas. While shown as being inserted into the space  37 , it is understood that the end  41   a  need only connect to an off-gas inlet in the heating unit  30   b  and thus need not be inserted into the body of the heating unit  30   b  in order to supply the off-gas (AOG). 
     Further, when the off-gas (AOG) is supplied to the space  37  in pulses, unreacted second fuel (Fuel  2 ) passing through the first oxidation portion  35  may be fully combusted through the off-gas (AOG) which contains hydrogen having a higher flammability than the second fuel (Fuel  2 ). Accordingly, it is possible to increase combustion efficiency and substantially and fully remove carbon monoxide in exhaust gas discharged from an exhaust portion  34   b  of the heating unit  30   b.    
       FIGS. 6A to 6D  are flowcharts of an operation method of a fuel cell system according to an embodiment. In the embodiment, a case in which the off-gas is supplied only when the temperature of the heating unit is lower than first reference temperature as well as a normal operation is briefly described. Of course, the present invention is not limited to the configuration. 
     Referring to  FIGS. 1 and 6A  through  6 D, fuel (Fuel  0 ) is supplied to the reforming unit  20  and the second fuel (Fuel  2 ) and the oxidant (Oxidant  2 ) are supplied to the heating unit  30  in order to actuate the system (operation  71 ). Next, after a predetermined starting time elapses, the first temperature of the reformate (Fuel  1 ) discharged from the reforming reaction unit  10  is detected (operation  72 ). 
     When the first temperature is lower than the first reference temperature, the supplying amount of the second oxidant (Oxidant  2 ) to the heating unit  30  is increased by a predetermined amount (operation  74 ). The first reference temperature may have a lower bound value within a proper temperature range of the reformate (Fuel  1 ). Meanwhile, when the first temperature is equal to or higher than the first reference temperature, predetermined operations  74  to  77  are omitted and thereafter, and the current amount of the second oxidant (Oxidant  2 ) to the heating unit  20  is maintained (operation  77 ). 
     After a predetermine time elapses, the first temperature is detected again (operation  75 ). When the re-detected first temperature is equal to or higher than the first reference temperature, the current supplying amount of the oxidant (Oxidant  2 ) is maintained (operation  77 ). When the first temperature is equal to ore lower than the first reference temperature, the supplying amount of the second oxidant (Oxidant  2 ) to the heating unit  30  is increased by a predetermined amount again (operation  74 ). In addition, subsequent operations  75  and  76  are performed again. 
     Meanwhile, as shown in  FIG. 6B , the second temperature of the fuel inlet of the heating unit  30  is detected (operation  78 ). The second temperature may be detected after the above-mentioned operations  71  to  77  or simultaneously when the first temperature is detected. 
     When the second temperature is lower than the second reference temperature, the amount of the second fuel (Fuel  2 ) supplied to the heating unit  20  by a predetermined amount will be described as the second heating (operation  80 ). Herein, the second reference temperature may have a lower bound value within the proper temperature range of the heating unit  20 . 
     In operation  80 , while the amount of the second oxidant (Oxidant  2 ) supplied is basically maintained, only the amount of the second fuel (Fuel  2 ) supplied is increased by a predetermined amount. However, as necessary, for example, when the first temperature of the reformate (Fuel  1 ) is lower than the first reference temperature, the amount of the second oxidant (Oxidant  2 ) supplied may also be increased. Meanwhile, when the second temperature is equal to or higher than the second reference temperature, predetermined operations  80  to  82  are omitted and thereafter, and the current amount of the second fuel (Fuel  2 ) supplied to the heating unit  30  is maintained (operation  83 ). 
     After a predetermine time elapses, the second temperature is detected again (operation  81 ). In addition, when the second temperature is equal to or higher than the second reference temperature, the current amount of the second fuel (Fuel  2 ) supplied is maintained (operation  83 ). Meanwhile, when the second temperature is lower than the second reference temperature, the amount of the second fuel (Fuel  2 ) supplied to the heating unit  30  is increased by a predetermined amount again (operation  80 ). In addition, subsequent steps are performed again. 
     In an embodiment, the controller  50  continuously monitors the first temperature and the second temperature on the basis of the temperature information transferred from the first sensor  61  and the second sensor  62 . 
     As shown in  FIG. 6C , the second temperature of the heating unit  30  is detected (operation  84 ). In addition, the second temperature is compared with second threshold temperature (operation  85 ). Comparison of the second temperature and the second threshold temperature may be performed substantially at the same time as comparison of the second temperature and the second reference temperature. 
     The second threshold temperature may have a lower bound value within a proper temperature range of the heating unit  30 . The second threshold temperature may be the temperature to cause damages to catalyst or components provided in the heating unit  30  when the heating unit  30  operates at the second threshold temperature or higher. That is, the second threshold temperature is the boundary temperature which is excessively high while being larger than an operation temperature range of the heating unit  30 . Accordingly, the operation temperature of the heating unit  30  needs to be controlled not to increase up to temperature higher than the second threshold temperature. 
     When the second temperature is equal to or higher than the second threshold temperature, the supply of the second fuel (Fuel  2 ) to the heating unit  30  is interrupted (operation  86 ). At this time, supplying the second oxidant (Oxidant  2 ) and the off-gas (AOG) to the heating unit  30  are also interrupted. This is to protect the heating unit  30  from the excessive temperature. Meanwhile, when the second temperature is lower than the second threshold temperature, predetermined operations  86  to  89  are omitted and thereafter, the previous amount of the second fuel (Fuel  2 ) supplied to the heating unit  30  is just maintained. 
     After a predetermine time elapses, the second temperature of the heating unit  30  is detected again (operation  87 ). When the re-detected second temperature is lower than the second threshold temperature, the supply of the second fuel (Fuel  2 ) to the heating unit  30  is resumed (operation  89 ). Meanwhile, when the second temperature is equal to or higher than the second threshold temperature, the supply of the second fuel (Fuel  2 ) to the heating unit  30  is continuously interrupted (operation  86 ). In addition, subsequent operations  87  and  88  may be performed again. 
     As shown in  FIG. 6D , the first temperature of the reformate (Fuel  1 ) is detected (operation  90 ). When the first temperature is equal to or higher than the first reference temperature (operation  91 ), the amount of the second oxidant (Oxidant  2 ) supplied to the heating unit  30  is decreased by a predetermined amount (operation  92 ). The first reference temperature may have an upper bound value within a proper temperature range of the reformate (Fuel  1 ). Herein, decreasing the supplying amount of the second oxidant (Oxidant  2 ) includes interrupting the supply of the second oxidant (Oxidant  2 ) for a predetermined time. Meanwhile, when the first temperature is lower than the first threshold temperature, predetermined operations  92  to  94  are omitted and thereafter, and the current amount of the second oxidant (Oxidant  2 ) supplied to the heating unit  30  is just maintained (operation  95 ). 
     After a predetermine time elapses, the first temperature is detected again (operation  93 ). When the re-detected first temperature is lower than the first threshold temperature (operation  94 ), the current amount of the second oxidant (Oxidant  2 ) being supplied is just maintained (operation  95 ). Meanwhile, when the first temperature is equal to or higher than the first threshold temperature (operation  94 ), the amount of the second oxidant (Oxidant  2 ) supplied to the heating unit  30  is again decreased by a predetermined amount (operation  92 ). In addition, subsequent operations  93  and  94  are performed again. 
     In the above-mentioned configuration, the off-gas (AOG) may be supplied to the heating unit  30  together with the second fuel (Fuel  2 ) or instead of the second fuel (Fuel  2 ) in the pulse type. 
     By the above-mentioned configuration, the first temperature of the reformate (Fuel  1 ) discharged from the reforming unit  20  may be controlled by controlling the amount of the second oxidant (Oxidant  2 ) supplied to the heating unit  30  while controlling the second temperature of the heating unit  30  by controlling the supplying amount of the second fuel (Fuel  2 ) and/or the off-gas (AOG) to the heating unit  30 . Moreover, it is possible to rapidly and accurately prevent the heating unit  30  from operating at undesired excessively high temperatures. Further, the reformate (Fuel  1 ) can be generated substantially at constant temperature. 
     According to embodiments of the present invention, it is possible to prevent backfire in a heating unit by supplying stack off-gas to the heating unit supplying heat to a reforming unit in proper supplying position and proper flow rate. 
     Further, it is possible to stably and easily control the temperature of the heating unit by controlling the flow rate of the off-gas supplied to the heating unit and the flow rate of fuel (second fuel) in response to the temperature around a fuel inlet of the heating unit. Moreover, it is possible to stably and easily control the temperature of the reforming unit by controlling the flow rate of an oxidant (second oxidant) supplied to the heating unit in response to reformate (first fuel) just discharged from the reforming unit. 
     In addition, it is possible to improve stability and performance of the heating unit by controlling the supplying method of the off-gas and the supplying method of the fuel and the oxidant to the heating unit and maintain the temperature of the reformate discharged from the reforming unit very uniformly. 
     Also, it is possible to increase the overall efficiency of a fuel cell system and improve stability and reliability of the fuel cell system by securing long-time stable operation of the system. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.