Patent Publication Number: US-2007122666-A1

Title: Method of operating fuel cell system and fuel cell system

Description:
TECHNICAL FIELD  
      The present invention relates to a fuel cell system and, more particularly, to methods for starting and controlling a fuel cell system.  
     BACKGROUND ART  
      There are fuel cell power generation systems which generate electricity through an electrochemical reaction between hydrogen-rich fuel gas generated by reforming a raw material fuel such as city gas, LPG, digestion gas, methanol, GTL or kerosene and supplied to an anode electrode (fuel electrode) of a fuel cell and oxygen-containing oxidant gas such as air supplied to an air electrode of the fuel cell. Such systems should operate in a stable way. It is, therefore, necessary to construct a system configuration which ensures stable operation of the system against disturbances. However, when the system configuration which ensures stable operation of the system is complicated, it may rather increase the causes of disturbances and degrade the reliability and economy of the entire system.  
      A reforming device for reforming the raw material fuel must be heated at a prescribed temperature since the reforming reaction is an endothermic reaction. Conventional fuel cell power generation systems have an assist combustion means for supplying some of the raw material fuel to a combustion section of a reforming device as auxiliary fuel. The amount of assist combustion is increased or decreased in response to a change in the temperature in the reforming section caused by a disturbance such as the pointing error of a raw material fuel flow meter to maintain the combustion section and the reforming section at prescribed temperatures for stable operation of the fuel cell power generation system.  
     DISCLOSURE OF THE INVENTION  
     Problem to be Solved by the Invention  
      However, since an assist fuel supply section and so on are required for assist combustion, the system configuration is unavoidably complicated and the power consumption is large. In a small-scale power generation system such as a fuel cell power generation system for household use with a power generation capacity of 1 to several kW, the amount of combustion in the assist combustion is so small that an advanced assist fuel supply means which can constantly and accurately deliver the fuel at a very minute flow rate is required, especially when the raw material fuel is a liquid fuel. When the fuel cannot be constantly and accurately delivered at a minute flow rate, the assist combustion itself may be a disturbance or increase causes of disturbances. In addition, the assist combustion, in which raw material fuel is burned, tends to generate NOx or soot, especially when the raw material fuel is a liquid fuel, and is undesirable from the environmental point of view.  
      It is, therefore, an object of the present invention is to provide a fuel cell power generation system which has no assist combustion system and thus is simple in structure and methods for starting the fuel cell power generation system and controlling the operation of the fuel cell power generation system, in order to provide a high-reliability fuel cell power generation system which operates stably.  
     Means for Solving the Problem  
      In order to accomplish the above-mentioned object, a method for operating a fuel cell power generation system according to the present invention is, as shown in  FIGS. 1 and 2  for example, the method for operating the fuel cell power generation system  100  having a reforming section  5  for reforming a raw material fuel m to produce reformate (reformed gas) r; a combustion section  4  for burning the raw material fuel m to heat the reforming section  5 ; a carbon monoxide reduction section  6 ,  7  for reducing a content of carbon monoxide in the reformate r to produce carbon monoxide reduced gas g; and a fuel cell  30  which uses the carbon monoxide reduced gas g as fuel gas, the method comprising: a first preheating process (step ST 2 ) of supplying the raw material fuel m to the combustion section  4  to heat the reforming section  5  to a predetermined temperature; a second preheating process (step ST 6 ), following the first preheating process (step ST 2 ), including the steps of stopping the supply of the raw material fuel m to the combustion section  4  (step ST 4 ), supplying the raw material fuel m to the reforming section  5  to produce reformate r, and introducing the reformate r to the carbon monoxide reduction section  6 ,  7  to heat the carbon monoxide reduction section  6 ,  7 ; and a power generation process (step ST 10 ), after the second preheating process (step ST 6 ), of introducing the carbon monoxide reduced gas g generated in the carbon monoxide reduction section  6 ,  7  to the fuel cell  30  to generate electric power.  
      In this configuration, the fuel cell power generation system without an assist combustion system can be started by supplying the raw material fuel to the combustion section to heat the reforming section, shift-converting the raw material fuel supplied to the reforming section into reformate using the heat accumulated in the reforming section heated, and heating the carbon monoxide reduction section with the reformate to produce carbon monoxide reduced gas to start power generation. Since the fuel cell power generation system does not have an assist combustion system and since the raw material fuel is supplied to the combustion section and directly burned therein only in the first preheating process, the raw material fuel is burned for only a short period of time and generation of NOx or soot, which are generated when the raw material fuel is burned, can be suppressed. Therefore, there can be obtained an environmentally friendly method for operating a fuel cell system.  
      A method for operating a fuel cell power generation system according to the present invention may be, as shown in  FIGS. 1 and 2  for example, the operation method for the fuel power generation system  100 , wherein process flow proceeds from the second preheating process (step ST 6 ) to the power generation process (step ST 10 ) when a temperature of the carbon monoxide reduction section  6 ,  7  becomes equal to or higher than a predetermined value (step ST 7 ).  
      In this configuration, carbon monoxide reduced gas produced after the carbon monoxide reduction section has been preheated to a predetermined temperature and has become able to reduce carbon monoxide is supplied to the fuel cell. Therefore, the power generation efficiency of the fuel cell is not degraded by carbon monoxide.  
      In order to accomplish the above-mentioned object, a method for operating a fuel cell power generation system according to the present invention is, as shown in  FIGS. 1 and 3  for example, the method for operating the fuel cell power generation system  100  having a raw material fuel supply section  1  for supplying a raw material fuel m; a reforming section  5  for reforming the raw material fuel m to produce reformate r; a carbon monoxide reduction section  6 ,  7  for reducing a content of carbon monoxide in the reformate r to produce carbon monoxide reduced gas g; a fuel cell  30  which uses the carbon monoxide reduced gas g as fuel gas; and a combustion section  4  for burning off gas p from the fuel cell  30  to heat the reforming section  5 , the method comprising: a reforming section temperature comparing process (steps ST 11  and ST 12 ) including the steps of detecting a temperature in the reforming section  5  and comparing the detected temperature with predetermined first temperature A 1  and predetermined second temperature A 2 ; a current decreasing process (step ST 22 ) for decreasing an output current from the fuel cell  30  when the detected temperature Ta is equal to or lower than the first temperature Ta in the reforming section temperature comparing process (step ST 11 ) and maintaining the output current for a predetermined time period t 1  after the output current has been decreased (step ST 23 ); and a current increasing process (step ST 32 ) of increasing the output current from the fuel cell  30  when the detected temperature is equal to or higher than the second temperature in the reforming section temperature comparing process (step ST 12 ) and maintaining the output current for a predetermined time period t 2  after the output current has been increased (step ST 33 ).  
      In this configuration, when the temperature in the reforming section becomes equal to or lower than a predetermined first temperature, the output current from the fuel cell is decreased. Then, the hydrogen content in the off gas from the fuel cell increases, and the amount of heat produced by the combustion in the combustion section increases. As a result, the reforming section is heated more and the temperature in the reforming section increases. When the temperature in the reforming section becomes equal to or higher than a predetermined second temperature, the output current from the fuel cell is increased. Then, the hydrogen content in the off gas from the fuel cell decreases, and the amount of heat produced by the combustion in the combustion section decreases. As a result, the temperature in the reforming section decreases. In addition, since the output current is not further varied for a predetermined time period after the output current has been increased or decreased, the system is prevented from becoming unstable.  
      A method for operating a fuel cell power generation system according to the present invention may, as shown in  FIG. 3  for example, in the method for operating a fuel cell power generation system  100  further comprise: a fuel increasing process (step ST 26 ) of increasing the supply amount of the raw material fuel m from the raw material fuel supply section  1  when a number of consecutive times N 1  the current decreasing process has been carried out reaches a predetermined value n 1 ; and a fuel decreasing process (step ST 36 ) of decreasing the supply amount of the raw material fuel m from the raw material fuel supply section  1  when a number of consecutive times N 2  the current increasing process has been carried out reaches a predetermined value n 2 .  
      In this configuration, when the temperature in the reforming section cannot become a predetermined value even when the output current from the fuel cell is increased or decreased by a predetermined integrated value (n 1  or n 2  times the amount to be increased or decreased in one process), the amount of raw material fuel is increased or decreased by an amount corresponding to the predetermined integrated value to adjust the temperature in the reforming section to the predetermined value. Therefore, the output current, in other words, the output power, is maintained within a certain range and the system can operate stably.  
      In order to accomplish the above-mentioned object, a fuel cell power generation system according to the present invention comprises, as shown in  FIG. 1  for example, a raw material fuel supply section  1  for supplying a raw material fuel m; a reforming section  5  for reforming the raw material fuel m to produce reformate r; a carbon monoxide reduction section  6 ,  7  for reducing a content of carbon monoxide in the reformate to produce carbon monoxide reduced gas g; a fuel cell  30  which uses the carbon monoxide reduced gas g as fuel gas; a combustion section  4  for burning the raw material fuel m, the carbon monoxide reduced gas g or off gas p from the fuel cell to heat the reforming section  5 ; a passage  12 ,  14  through which the raw material fuel m is supplied to the combustion section  4 ; a passage  12 ,  13  through which the raw material fuel m is supplied to the reforming section  5 ; a passage  19 ,  20  through which the carbon monoxide reduced gas g is supplied to the fuel cell  30 ; a passage  21 ,  22  through which the carbon monoxide reduced gas g is supplied to the combustion section  4 ; a passage  22  through which the off gas p from the fuel cell  30  is supplied to the combustion section  4 ; a first passage-switching means  3  for switching between the passage  14  through which the raw material fuel m is supplied to the combustion section  4  and the passage  13  through which the raw material fuel m is supplied to the reforming section  5 ; and a second passage-switching means  8  for switching between the passage  20  through which the carbon monoxide reduced gas g is supplied to the fuel cell  30  and the passage  21  through which the carbon monoxide reduced gas g is supplied to the combustion section  4 .  
      In this configuration, the raw material fuel can be supplied to the reforming section to produce reformate, by switching the first passage-switching means after the raw material fuel has been supplied to the combustion section to heat the reforming section to a predetermined temperature. Since the supply of fuel to the combustion section is stopped after the first passage-switching means has been switched until the supply of the carbon monoxide reduced gas to the combustion section is started, the combustion section is once extinguished and the supply of heat to the reforming section is stopped. However, since the temperature in the reforming section is adjusted to a predetermined temperature before the switching so that the temperature in the reforming section cannot be lowered to a temperature at which a reforming reaction does not occur and since the period for which the supply of fuel is stopped is short, the reforming reaction continues. Also, in the fuel cell power generation system, the reformate introduced into the carbon monoxide reduction section to heat the carbon monoxide reduction section is supplied to the combustion section before the temperature in the carbon monoxide reduction section becomes sufficiently high, and, when the temperature in the carbon monoxide reduction section becomes sufficiently high, the second passage-switching means is switched to introduce the carbon monoxide reduced gas into the fuel cell to start power generation. Since the supply of fuel to the combustion section is stopped after the second passage-switching means has been switched until the supply of off gas from the fuel cell to the combustion section is started, the combustion section is once extinguished and the supply of heat to the reforming section is stopped. However, since the temperature in the reforming section is adjusted to a predetermined temperature before the switching so that the temperature in the reforming section cannot be lowered to a temperature at which a reforming reaction does not occur and since the period for which the supply of fuel is stopped is short, the reforming reaction continues. Since the fuel cell power generation system has no assist combustion system and since raw material fuel is supplied to the combustion section and directly burned therein only in the first preheating process, raw material fuel is burned for only a short period of time and generation of NOx or soot, which are generated when raw material fuel is burned, can be suppressed. Therefore, there can be obtained an environmentally friendly fuel cell power generation system.  
      Moreover, a fuel cell power generation system according to the present invention may, as shown in  FIG. 1  for example, in the fuel power generation system  100 , further comprise: a reforming section temperature detector  9  for detecting a temperature in the reforming section  5 ; a carbon monoxide reduction section temperature detector  10 ,  11  for detecting a temperature in the carbon monoxide reduction section  6 ,  7 ; and a control device  40  having a storage section for storing first, second and third temperatures to be compared with the temperature detected by the reforming section temperature detector  9  and a fourth temperature to be compared with the temperature detected by the carbon monoxide reduction section temperature detector  10 ,  11 , and a control section which, at start-up, conducts control to carry out the steps of stopping supply of the raw material fuel m to the combustion section  4 , supplying the raw material fuel m to the reforming section  5  to produce reformate r, and introducing the reformate r to the carbon monoxide reduction section  6 ,  7  to heat the carbon monoxide reduction section  6 ,  7  when the temperature detected by the reforming section temperature detector  9  becomes equal to or higher than the third temperature after supplying the raw material fuel m to the combustion section  4 , and the step of introducing the carbon monoxide reduced gas g produced in the carbon monoxide reduction section  6 ,  7  to the fuel cell  30  to start power generation when the temperature detected by the carbon monoxide reduction section temperature detector  10 ,  11  becomes equal to or higher than the fourth temperature; and which, during normal operation, conducts control to carry out the steps of decreasing an output current from the fuel cell  30  and maintaining the output current for a predetermined time period after the step of decreasing the output current when the temperature detected by the reforming section temperature detector  9  is equal to or lower than the first temperature, the steps of increasing the output current from the fuel cell  30  and maintaining the output current for a predetermined time period after the step of increasing the output current when the temperature detected by the reforming section temperature detector  9  is equal to or higher than the second temperature, and the steps of increasing or decreasing supply amount of the raw material fuel m from the raw material fuel supply section  1  when a number of consecutive times the output current has been decreased or increased reaches a predetermined value.  
      In the fuel cell power generation system constituted as described above, at the time of start-up, the control device conducts control to supply raw material fuel to the combustion section to heat the reforming section, to switch the first passage-switching means to supply raw material fuel to the reforming section to produce reformate and to introduce the reformate to the carbon monoxide reduction section to heat the carbon monoxide reduction section when the temperature detected by the reforming section temperature detector becomes equal to or higher than the third temperature, and to introduce carbon monoxide reduced gas to the fuel cell to start power generation when the temperature in the carbon monoxide reduction section becomes equal to or higher than the fourth temperature. Also, during normal operation, the control device conducts control to decrease the output current from the fuel cell to increase the temperature in the reforming section when the temperature in the reforming section becomes equal to or lower than the first temperature, to increase the output current from the fuel cell to decrease the temperature in the reforming section when the temperature in the reforming section becomes equal to or higher than the second temperature, and to increase or to decrease the supply amount of raw material fuel when the temperature in the reforming section cannot become a predetermined value even when the output current from the fuel cell has been increased or decreased a predetermined number of consecutive times.  
      The basic Japanese Patent Application No. 2003-413324 filed on Dec. 11, 2003 is hereby incorporated in its entirety by reference into the present application.  
      The present invention will become more fully understood from the detailed description given hereinbelow. The other applicable fields will become apparent with reference to the detailed description given hereinbelow. However, the detailed description and the specific embodiment are illustrated of desired embodiments of the present invention and are described only for the purpose of explanation. Various changes and modifications will be apparent to those ordinary skilled in the art on the basis of the detailed description. The applicant has no intention to give to public any disclosed embodiments. Among the disclosed changes and modifications, those which may not literally fall within the scope of the present claims constitute, therefore, a part of the present invention in the sense of doctrine of equivalents.  
      The use of the terms “a” and “an” and “the” and similar referents in the context of the specification and claims are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed.  
     Effects of the Invention  
      According to the present invention, there can be provided a simple-structure, high-reliability and environmentally-friendly fuel cell power generation system which can be started without an assist combustion system and can operate stably against disturbances and a method for operating the fuel cell power generation system. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a block diagram illustrating a fuel cell power generation system as an embodiment of the present invention.  
       FIG. 2  is a flowchart illustrating a method for controlling the fuel cell power generation system as an embodiment of the present invention at start-up.  
       FIG. 3  is a flowchart illustrating a method for controlling the fuel cell power generation system as an embodiment of the present invention during normal operation. 
    
    
     DESCRIPTION OF REFERENCE NUMERALS  
     
         
           1 : raw material fuel supply section  
           2 : combustion air supply section  
           3 : three-way valve (first passage-switching means)  
           4 : combustion section  
           5 : reforming section  
           6 : shift converter section (carbon monoxide reduction section)  
           7 : selective oxidation section (carbon monoxide reduction section)  
           8 : three-way valve (second passage-switching means)  
           9 : reforming section temperature detector  
           10 : shift converter section temperature detector (carbon monoxide reduction section temperature detector)  
           11 : selective oxidation section temperature detector (carbon monoxide reduction section temperature detector)  
           12 : raw material fuel discharge pipe  
           13 : raw material fuel supply pipe  
           14 : raw material fuel combustion pipe  
           15 : oxidant gas pipe  
           16 : combustion section exhaust pipe  
           17 : reformate pipe  
           18 : shift-converted gas pipe  
           19 : fuel gas discharge pipe  
           20 : fuel gas supply pipe  
           21 : reformate combustion pipe  
           22 : fuel cell off gas pipe  
           23 : reforming water supply pipe  
           30 : fuel cell  
           31 : power cable  
           40 : control device  
          a: combustion air  
          A, B, C: predetermined temperature for reforming section (third temperature), predetermined temperature for shift converter section (fourth temperature), predetermined temperature for selective oxidation section (fourth temperature) at start-up  
          A 1 , A 2 : first temperature, second temperature  
          a 1 , a 2 : predetermined amount of decrease and increase in output current  
          g, g 1  fuel gas, shift-converted gas  
          g′: combustion gas  
          i 1  to i 7 : control signal  
          m: raw material fuel  
          N 1 , N 2 : number of consecutive times the output current has been decreased or increased  
          n 1 , n 2 : predetermined value for the number of consecutive times the output current has been decreased or increased  
          r: reformate  
          Ta: reforming section temperature  
          Tb, Tc: shift converter section temperature (carbon monoxide reduction section temperature), selective oxidation section temperature (carbon monoxide reduction section temperature)  
          TI: temperature detector  
          t 1 , t 2 : predetermined time period  
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Description is hereinafter made of an embodiment of the present invention with reference to the drawings. The same or corresponding devices are denoted in all the drawings with the same reference numerals and symbols, and redundant description is omitted. In  FIG. 1 , the broken lines represent control signals.  
       FIG. 1  is a block diagram illustrating a fuel cell power generation system  100  as an embodiment of the present invention. The fuel cell power generation system  100  has a raw material fuel supply section  1  for supplying a raw material fuel m such as city gas or kerosene; a combustion air supply section  2  for supplying combustion air a for combustion of the raw material fuel m; a reforming section  5  for reforming the raw material fuel m to produce hydrogen-rich reformate r; a combustion section  4  for heating the reforming section  5 ; a shift converter section  6 , as a first stage section of a carbon monoxide reduction section, in which a shift converter reaction of the reformate r is carried out; a selective oxidation section  7 , as a second stage section of the carbon monoxide reduction section, in which a selective oxidation reaction of shift-converted reformate g 1  is carried out; and a fuel cell  30  for generating electric power using hydrogen-rich gas g as carbon monoxide reduced gas as fuel gas. The fuel cell power generation system  100  also has a three-way valve  3  as a first passage-switching means for switching between a passage  13  through which the raw material fuel m supplied from the fuel supply section  1  is supplied to the reforming section  5  and a passage  14  through which the raw material fuel m is supplied to the combustion section  4 ; and a three-way valve  8  as a second passage-switching means for directing the gas discharged from the selective oxidation section  7  to a passage  20  to the fuel cell  30  as fuel gas g or to a passage  21  to the combustion section  4  as combustion gas g′. The fuel cell power generation system  100  also has a reforming section temperature detector  9  for detecting the temperature in the reforming section  5 ; a shift converter section temperature detector  10  for detecting the temperature in the shift converter section  6 ; a selective oxidation section temperature detector  11  for detecting the temperature in the selective oxidation section  7 ; and a control device  40  for controlling the supply amount of the raw material fuel from the raw material fuel supply section  1  and the operation of the three-way valves  3  and  8  based on those temperatures.  
      A pipe  12  is installed from the raw material fuel supply section  1  to the three-way valve  3 , and is branched into a pipe  13  connected to the reforming section  5  and a pipe  14  connected to the combustion section  4  at the three-way valve  3 . A pipe  15  is installed from the combustion air supply section  2  to the combustion section  4 . A pipe  16 , through which exhaust gas from combustion is discharged, is connected to the combustion section  4 . To the reforming section  5  is also connected a pipe  23  through which reforming water s as water for the reforming reaction is supplied thereto. A pipe  17  is installed between the reforming section  5  and the shift converter section  6 , and a pipe  18  is installed between the shift converter section  6  and the selective oxidation section  7 . A pipe  19  connected to the selective oxidation section  7  is connected to the three-way valve  8 , and pipes  20  and  21  extend from the three-way valve  8  to the fuel cell  30  and to a pipe  22 , respectively. The pipe  22  is installed between the fuel cell  30  and the combustion section  4 .  
      The raw material fuel supply section  1  is a device for supplying a raw material fuel m such as city gas, LPG, digestion gas, methanol, GTL or kerosene at a constant rate. The raw material fuel supply section  1  may have a tank for storing the raw material fuel m or receive the raw material fuel m from outside. When a pressurized gas such as city gas or LPG is used, the raw material fuel supply section  1  has a flow control valve. When the raw material fuel m is not pressurized, the raw material fuel supply section  1  has a blower for generating a pressure to deliver the raw material fuel into the fuel cell power generation system  100 . When the raw material fuel m is a liquid fuel such as GTL or kerosene, the raw material fuel supply section  1  may have a constant rate pump or may have a pump without a flow rate control function and a flow control valve. The raw material fuel supply section  1  increases or decreases the supply amount of the raw material fuel according to a command signal i 1  from the control device  40 .  
      The combustion air supply section  2  is a device for supplying oxygen to be consumed by the combustion in the combustion section. It is sufficient that the combustion air supply section  2  has a blower for supplying an ambient air into the combustion section  4  as combustion air a. The combustion air supply section  2  preferably has a filter for preventing entry of floating objects in the ambient air. The combustion air supply section  2  preferably has a flow control valve for controlling the supply rate of the combustion air a. Although the combustion air supply section  2  is connected via the pipe  15  to the combustion section  4  in the fuel cell power generation system  100 , it may be directly connected to the combustion section  4 .  
      The reforming section  5  is a device for generating hydrogen-rich reformate r from the raw material fuel m and the reforming water s through a reforming reaction. A reforming reaction is a reaction in which hydrogen, carbon dioxide and carbon monoxide are generated from hydrocarbons in the raw material fuel m and water at a high temperature in the presence of a reforming catalyst (not shown). Since a reforming reaction is an endothermic reaction, heat must be supplied from outside to induce a reforming reaction. As the reforming catalyst, any catalyst can be used as long as it can accelerate the reforming reaction. For example, a nickel-based reforming catalyst or a ruthenium-based reforming catalyst can be used. The reforming section  5  is preferably in the form of a circular cylindrical container housing the reforming catalyst in view of strength and production efficiency. The combustion section  4 , which is described later, may be disposed inside the reforming section  5  so that the reforming section  5  can be maintained at a high temperature.  
      The combustion section  4  is a device in which the raw material fuel m, the combustion gas g′ supplied from the selective oxidation section  7  with its carbon monoxide not sufficiently removed, or anode off gas p as off gas from an anode electrode of the fuel cell  30  is burned together with the combustion air a to heat the reforming section  5 . The combustion section  4  preferably has a carburetor in case the raw material fuel m is a liquid fuel such as kerosene. The combustion section  4  has a burner nozzle which can be used for various types of fuels including the raw material fuel m, such as kerosene, city gas or LPG, the combustion gas g′ discharged from the selective oxidation section  7  and the anode off gas p from the fuel cell  30 . The combustion section  4  may have a different burner nozzle for each of the fuels. Since the combustion section  4  is a device for heating the reforming section  5 , it is preferably formed integrally with the reforming section  5 . The structure of the combustion section  4  is not specifically limited as long as it can heat the reforming section  5 . The combustion section  4  may be located at the center of the reforming section  5  so that the reforming section  5  surrounding it can be heated by the combustion therein, or may be so constructed that high-temperature gas produced in the combustion section  4  flows around the reforming section  5  to heat the reforming section  5 .  
      The shift converter section  6  is a device in which carbon dioxide and hydrogen are generated through a shift converter reaction of carbon monoxide with water both in the reformate r in the presence of a shift converter catalyst (not shown). Since the shift converter reaction is an exothermic reaction, when the reaction temperature is lowered, the concentration of carbon monoxide in the shift converter reaction product can be low but the reaction rate decreases. As the shift converter catalyst, an iron-chromium-based high-temperature shift converter catalyst, a platinum-based medium-high-temperature shift converter catalyst, or a copper-zinc-based low temperature shift converter catalyst may be used. The shift converter section  6  is a container housing such a catalyst and typically has a circular cylindrical shape.  
      The selective oxidation section  7  is a device in which carbon monoxide in the reformate r is subjected to selective oxidation with oxygen in air (not shown) supplied from outside in the presence of a selective oxidation catalyst (not shown) to reduce the concentration of carbon monoxide in the fuel gas g. When the fuel cell  30  is a solid polymer electrolyte fuel cell, the platinum catalyst in the anode electrode (fuel electrode) is poisoned by carbon monoxide in the fuel gas g and the power generation efficiency is decreased. Therefore, a selective oxidation catalyst with high carbon monoxide selective oxidation capacity is used to oxidize carbon monoxide to carbon dioxide. As the selective oxidation catalyst, a platinum-based selective oxidation catalyst or ruthenium-based selective oxidation catalyst, for example, may be used. Although the fuel cell power generation system  100  has the shift converter section  6  and the selective oxidation section  7  to reduce carbon monoxide, only either one of them may be provided as long as the carbon monoxide concentration can be sufficiently decreased. However, the fuel cell power generation system  100  desirably has both the shift converter section  6  and the selective oxidation section  7 , because the carbon monoxide concentration can be decreased and the concentration of hydrogen as fuel can be increased in the shift converter section  6 , and the carbon monoxide concentration can be sufficiently decreased in the selective oxidation section  7 .  
      The fuel cell  30  is a device in which the hydrogen-rich fuel gas g as fuel gas and oxidant gas such as air are supplied to an anode electrode (not shown) and a cathode electrode (not shown), respectively, to generate electric power. As the fuel cell  30 , a solid polymer electrolyte fuel cell is preferred for example, but a fuel cell of a different type may be used. Hydrogen in the fuel gas g supplied to the anode electrode reacts with oxygen in the oxidant gas supplied to the cathode electrode into water vapor. Not all the hydrogen is consumed, and anode off gas p containing residual hydrogen is discharged. Since the amount of hydrogen contained in the anode off gas p is the difference between the amount of hydrogen contained in the fuel gas g and the amount of hydrogen consumed for power generation in the fuel cell  30 , the amount of hydrogen in the anode off gas p increases when the output current decreases. The electric power generated in the fuel cell  30  is supplied to an outside consumer (not shown) through a power cable  31 . The fuel cell  30  increases or decreases the output current according to a control signal i 7  from the control device.  
      The reforming section  5 , the shift converter section  6 , and the selective oxidation section  7  have a reforming section temperature detector  9 , a shift converter section temperature detector  10 , and a selective oxidation section temperature detector  11 , respectively, for detecting the temperatures therein. Signal cables for transmitting signals indicating the detected temperatures extend from the reforming section temperature detector  9 , the shift converter section temperature detector  10 , and the selective oxidation section temperature detector  11  to the control device  40 , and a reforming section temperature signal i 3 , a shift converter section temperature signal i 4 , and a selective oxidation section temperature signal i 5  are transmitted to the control device.  
      The control device  40  controls the operation of the three-way valves  3  and  8 , the output current from the fuel cell  30 , and the supply amount of the raw material fuel from the raw material fuel supply section  1  based on the reforming section temperature signal i 3 , the shift converter section temperature signal i 4 , and the selective oxidation section temperature signal i 5 . The control device  40  has a storage section (not shown) for storing a reforming section operation start temperature A (see  FIG. 2 ) as a third temperature, a shift converter section operation start temperature B (see  FIG. 2 ) as a fourth temperature, a selective oxidation section operation start temperature C (see  FIG. 2 ) as another fourth temperature, a reforming section lower limit temperature A 1  (see  FIG. 3 ) as a first temperature, and a reforming section higher limit temperature A 2  (see  FIG. 3 ) as a second temperature for use in the above control; and a control section (not shown) for controlling the three-way valves  3  and  8 , the output current from the fuel cell  30 , and the supply amount of the raw material fuel m from the raw material fuel supply section  1  based on the temperatures detected by the reforming section temperature detector  9 , the shift converter section temperature detector  10 , and the selective oxidation section temperature detector  11 . Since the carbon monoxide reduction section of the fuel cell power generation system  100  as the embodiment of the present invention has the shift converter section  6  and the selective oxidation section  7 , there are two fourth temperatures: the shift converter section operation start temperature B (see  FIG. 2 ) and the selective oxidation section operation start temperature C (see  FIG. 2 ). In this case, when the shift converter section  6  has a temperature Tb (see  FIG. 2 ) equal to or higher than the shift converter section operation start temperature B (see  FIG. 2 ) and the selective oxidation section  7  has a temperature Tc (see  FIG. 2 ) equal to or higher than the selective oxidation section operation start temperature C (see  FIG. 2 ), the carbon monoxide reduction section (the shift converter section  6  and the selective oxidation section  7 ) is regarded as having a temperature equal to or higher than a predetermined temperature (the shift converter section operation start temperature B and the selective oxidation section operation start temperature C).  
      Each of the three-way valves  3  and  8  is a valve as a flow passage-switching means having one entrance and two exits. The three-way valve  3  performs switching operation to supply the raw material fuel m from the raw material fuel supply section  1  either to the combustion section  4  or to the reforming section  5 . The three-way valve  8  performs switching operation to supply the gas discharged from the selective oxidation section  7  either to the fuel cell  30  as fuel gas g or to the combustion section  4  as combustion gas g′. Each of the switching means may be formed by a combination of branch pipes and gate valves, not by a three-way valve. When three-way valves are used, however, the fuel cell power generation system  100  can be small in size since three-way valves do not take up a lot of space. The three-way valves  3  and  8  are driven by a solenoid or motor, and operates according to control signals i 2  and i 6 , respectively, from the control device  40 .  
      Although the raw material fuel supply section  1 , the combustion air supply section  2 , the reforming section  5 , the combustion section  4 , the shift converter section  6 , the selective oxidation section  7  and the three-way valves  3  and  8  are separate devices connected by pipes  12  to  19  in the fuel cell power generation system  100 , all or some of the devices may be integrated into a unit as a fuel processing device. When the system is made up of separate devices, each device can undergo maintenance or can be replaced individually, making the system easy to use. When the devices are integrated into a unit, the system can be compact as a whole.  
      The operation to start the fuel cell power generation system  100  shown in  FIG. 1  is described with reference to the flowchart in  FIG. 2 . The supply of the raw material fuel m from the raw material fuel supply section  1  is started. At this time, the three-way valve  3  permits the raw material fuel m to flow to the combustion section  4 . Therefore, the raw material fuel m is supplied through the pipe  12 , the three-way valve  3  and the pipe  14  to the combustion section  4 . At the same time, the combustion air a is supplied from the combustion air supply section  2  to the combustion section  4 . In the combustion section  4 , the raw material fuel m injected from the burner nozzle is ignited to start combustion (step ST 1 ).  
      The reforming section  5  is heated by the heat from the combustion in the combustion section  4  and increases in temperature (step ST 2 ). The steps up to this point correspond to a first preheating process. The control device  40  determines whether the temperature Ta in the reforming section  5  is equal to or higher than the predetermined third temperature A based on a signal i 3  indicating the temperature detected by the reforming section temperature detector  9  (step ST 3 ). The third temperature A is higher than the temperature at which a reforming reaction of the raw material fuel m occurs. For example, when the reforming reaction of the raw material fuel m occurs at 650° C., the third temperature A is set to 700° C. If the temperature Ta in the reforming section  5  is equal to or higher than the third temperature A, the control section (not shown) of the control device  40  sends a signal i 2  to the three-way valve  3  to command the three-way valve  3  to supply the raw material fuel m to the reforming section  5  (step ST 4 ).  
      When the raw material fuel m is supplied to the reforming section  5  by the three-way valve  3 , the combustion in the combustion section  4  is stopped since no fuel for combustion is supplied. Since the reforming section  5  has been heated to a temperature equal to or higher than the third temperature A, which is higher than the temperature at which the raw material fuel m undergoes a reforming reaction, the raw material fuel m undergoes a reforming reaction and is reformed into hydrogen, carbon monoxide and carbon dioxide. Although the supply of heat from the combustion section  4  is stopped, since the container (not shown) of the reforming section and the reforming catalyst (not shown) have accumulated heat, the temperature in the reforming section  5  decreases slowly. Also, since the reforming section  5  has been heated to a temperature equal to or higher than the third temperature A, which is higher than the temperature at which a reforming reaction of the raw material fuel m occurs, the reforming reaction continues even if the temperature slightly decreases. The reformate r generated through the reforming reaction is supplied through the pipe  17  to the shift converter section  6 . At this time, even if the reformate r is supplied to the shift converter section  6 , no shift converter reaction occurs since the temperature in the shift converter section  6  is low. However, since the reformate r with a high temperature is supplied, the temperature in the shift converter section  6  increases gradually. Then, the reformate r is supplied from the shift converter section  6  to the selective oxidation section  7  through the pipe  18 . No selective oxidation reaction of carbon monoxide occurs in the selective oxidation section  7  since the temperature in the selective oxidation section  7  is low. The temperature in the selective oxidation section  7  also increases gradually as the reformate r with a high temperature is supplied.  
      The reformate r, having heated the shift converter section  6  and the selective oxidation section  7 , flows through the pipe  19  to the three-way valve  8 . The reformate r, which has undergone neither a shift converter reaction nor a selective oxidation reaction of carbon monoxide, contains a large amount of carbon monoxide and is not suitable to be supplied to the fuel cell  30 . The reformate r is therefore directed to the pipe  21  as combustion gas g′ by the three-way valve  8  and supplied through the pipe  22  to the combustion section  4 . In the combustion section  4 , combustion is started using the combustion gas g′ and the combustion air a (step ST 5 ). That is, combustion in the combustion section  4  is stopped only for the period between when the three-way valve  3  is switched from the pipe  14  side to the pipe  13  side and when the raw material fuel m supplied to the reforming section  5  as a result of the switching is reformed and supplied to the combustion section  4  through the shift converter section  6 , the selective oxidation section  7  and the three-way valve  8 . This period is approximately  10  seconds. Since combustion in the combustion section  4  is stopped for the short period of time, even if the raw material fuel m undergoes a reforming reaction in the reforming section  5  and the temperature in the reforming section  5  decreases, the temperature does not decrease from a temperature equal to or higher than the third temperature A to a temperature lower than the temperature at which a reforming reaction of the raw material fuel m occurs and thus the reforming reaction continues. As the reforming reaction continues, the shift converter section  6  and the selective oxidation section  7  are kept heated (step ST 6 ). The steps up to this point correspond to a second preheating step.  
      The shift converter section temperature detector  10  and the selective oxidation section temperature detector  11  detect the temperatures in the shift converter section  6  and the selective oxidation section  7 , respectively, which are kept heated. The detected temperature Tb of the shift converter section  6  and the detected temperature Tc of the selective oxidation section  7  are transmitted as signals i 4  and i 5 , respectively, to the control device  40 . The control section (not shown) of the control device  40  compares the shift converter section temperature Tb with the temperature B as the predetermined fourth temperature stored in the storage section and the selective oxidation section temperature Tc with the temperature C as the other predetermined fourth temperature stored in the storage section (step ST 7 ). The temperature B is the temperature at which a shift converter reaction is carried out in the shift converter section  6 , and the temperature C is the temperature at which a selective oxidation reaction of carbon monoxide is carried out in the selective oxidation section  7 . For example, when a copper-zinc-based shift converter catalyst and a platinum-based selective oxidation catalyst are used, since the temperature suitable for the shift converter reaction is 200 to 280° C., the fourth temperature B is set to 240° C., and since the temperature suitable for the selective oxidation reaction of carbon monoxide is 100 to 160° C., the fourth temperature C is set to 110° C. At the same time, it is checked whether the reforming section temperature Ta is equal to or higher than the third temperature A. This is to maintain the temperature in the reforming section  5  high so that the temperature in the reforming section  5  will not decrease to a temperature equal to or lower than the temperature at which the reforming reaction occurs while combustion is stopped in the combustion section  4  after the three-way valve  8  has been switched to shut off the supply of combustion gas g′ from the selective oxidation section  7  to the combustion section  4  through the three-way valve  8  and the pipes  21  and  22 . Since the period between when the combustion section  4  is extinguished and when off gas p is supplied from the fuel cell  30  and combustion is restarted is different from the period in the case of switching the three-way valve  3 , the temperature to be compared with the reforming section temperature Ta may be different from the third temperature A. In the fuel cell power generation system  100  of this embodiment, since the period until the restart of combustion after the switching of the three-way valve  8  is shorter than the period until the restart of combustion after the switching of the three-way valve  3 , the temperature to be compared with the reforming section temperature Ta may be lower than the third temperature A.  
      When the detected temperatures Tb and Tc are equal to or higher than the predetermined temperatures B and C, respectively, the reformate r is shift-converted into shift-converted gas g 1  through a shift converter reaction in the shift converter section  6  and the shift-converted gas g 1  is converted into fuel gas g with a sufficiently reduced carbon monoxide content through a selective oxidation reaction of carbon monoxide in the selective oxidation section  7 . Then, the control section (not shown) of the control device  40  sends a signal i 6  to the three-way valve  8  to command the three-way valve  8  to supply the gas from the selective oxidation section  7  to the fuel cell  30  (step ST 8 ). When the three-way valve  8  is switched, the supply of combustion gas g′ to the combustion section  4  is shut off and the combustion in the combustion section  4  is stopped. However, the fuel gas g supplied to the fuel cell  30  is not immediately used for power generation in the fuel cell  30  but supplied to the combustion section  4  through the pipe  22  as anode off gas p. The fuel cell  30  may start power generation to generate only a small amount of current when the fuel gas g is supplied to the fuel cell  30 . Then, combustion is started in the combustion section  4  using the anode off gas p, which has the same component as the fuel gas g but is supplied through the pipe  22 , and the combustion air a (step ST 9 ). Since the anode off gas with the same component as the fuel gas g has a high hydrogen concentration, it can be ignited easily. The period of time needed for this process is as short as about five seconds. Therefore, even though the combustion in the combustion section  4  is stopped, the decrease in temperature in the reforming section  5  is small and the reforming reaction continues.  
      When the combustion of anode off gas p with the same component as the fuel gas g is started in the combustion section  4 , the temperature in the reforming section  5  becomes stable. The raw material fuel m is reformed in the reforming section  5 , reduced in carbon monoxide content in the shift converter section  6  and the selective oxidation section  7 , and supplied to the fuel cell  30 . Then, the output current is increased in steps and steady power generation takes place in the fuel cell  30  (step ST 10 ). The steps up to this point correspond to a power generation process. After power generation has been stated in the fuel cell  30 , the fuel gas g is used for the power generation in the fuel cell  30  and the anode off gas p from the fuel cell  30  is supplied through the pipe  22  to the combustion section  4  to maintain the combustion in the combustion section  4 . Since the fuel gas g to be supplied to the fuel cell  30  has undergone a shift converter reaction and a selective oxidation reaction of carbon monoxide, it has a sufficiently low carbon monoxide concentration. Therefore, even when the fuel gas g is supplied to the fuel cell  30 , it does not poison the platinum catalyst in the anode electrode (fuel electrode) of the fuel cell  30  and is suitably used in the fuel cell  30 .  
      According to the above start-up method, the fuel cell power generation system  100  can be started without using an assist combustion system and the fuel cell  30  and so on cannot be damaged by carbon monoxide. Since the raw material fuel m is not burned except when some raw material fuel m is supplied to the combustion section  4  to heat the reforming section  5  at the beginning and since the reformate r or the anode off gas p is burned in the combustion section  4 , the exhaust gas discharged from the combustion section  4  is clean and free of NOx or SOx. This is advantageous from the viewpoint of environmental protection.  
      With reference to  FIG. 3 , description is made of a method for controlling the fuel cell power generation system  100  shown in  FIG. 1  against disturbances. For example, the temperature in the reforming section  5  may be varied by the pointing error of a raw material fuel flow meter or the fluctuation of the quality of the raw material fuel m. When the temperature in the reforming section  5  varies largely, the reforming reaction may not proceed properly. There fore, an adjustment must be made to maintain the temperature in the reforming section  5  within a predetermined range.  
      One possible method to increase the temperature in the reforming section  5  when the temperature in the reforming section  5  has decreased is to increase the amount of combustion in the combustion section  4 . However, if the supply amount of the raw material fuel m is increased to increase the amount of combustion in the combustion section  4 , the reforming reaction in the reforming section  5  is accelerated and more heat is absorbed. As a result, the temperature in the reforming section  5  further decreases. Moreover, since the reforming reaction requires reforming water s, the supply amount of reforming water s must be increased with the increase of the supply amount of the raw material fuel m. As a result, the fluctuation in the entire system may increase and the operation of the system may become unstable.  
      Therefore, when the temperature in the reforming section  5  has decreased, the output current from the fuel cell  30  is decreased. When the output current is decreased, the consumption of hydrogen in the fuel cell  30  decreases. As a result, the amount of residual hydrogen in the anode off gas p increases. When the residual hydrogen amount in the anode off gas p increases, the amount of combustion in the combustion section  4  increases and the temperature in the reforming section  5  increases. On the other hand, when the temperature in the reforming section  5  has increased, the output current from the fuel cell  30  is increased. Then, the amount of residual hydrogen in the anode off gas p decreases, and the amount of combustion in the combustion section  4  decreases. As a result, the temperature in the reforming section  5  decreases. Since the amount of combustion in the combustion section  4  varies immediately after the output current from the fuel cell  30  is increased or decreased, the temperature in the reforming section  5  changes quickly. Therefore, this method is suitable as a method for controlling the operation of the fuel cell power generation system  100 .  
      Referring to  FIG. 3 , a specific control operation is described. The temperature detected by the reforming section temperature detector  9  is sent as a signal i 3  to the control section of the control device  40 . In the control section, the detected reforming section temperature Ta is compared with a predetermined first temperature A 1  stored in the storage section (step ST 11 ). If the reforming section temperature Ta is equal to or lower than the first temperature A 1 , a process of decreasing the output current from the fuel cell  30  is carried out. If the reforming section temperature Ta is higher than the first temperature A 1 , the reforming section temperature Ta is compared with a predetermined second temperature A 2 .  
      In the process of decreasing the output current from the fuel cell  30 , the control section of the control device  40  resets the number of consecutive times N 2  the output current has been increased to 0 (step ST 21 ). This is not to miscount the number of consecutive times N 2  the output current is increased when a process of increasing the output current must be performed after the output current is decreased. Although step ST 21  is shown as the first step in the process of decreasing the output current in  FIG. 3 , the step may be conducted at any time during the process of decreasing the output current.  
      Then, the control section of the control device  40  sends a command signal i 7  to decrease the output current by a predetermined amount al to the fuel cell  30 . When the fuel cell  30  receives the command signal i 7 , the output current is decreased (step ST 22 ). The predetermined amount al by which the output current is decreased is set to a small value, such as 2% of the amount of output current set at the start-up of the system. A large or sudden change in the amount of output current may make the operation of the system unstable. The predetermined amount a 1  is therefore set to a small value so that the output current can be adjusted gradually.  
      The control section of the control device  40  then adds 1 to the number of consecutive times N 1  the output current has been decreased (step ST 24 ). As described later, this is used to determine whether to increase the supply amount of the raw material fuel m when the temperature Ta in the reforming section  5  does not become higher than the predetermined first temperature A 1  even if the output current is decreased. If the number of consecutive times N 1  has not reached a predetermined value n 1 , the process flow returns to the comparison between the reforming section temperature Ta and the first temperature A 1  (step  11 ).  
      Before the comparison between the reforming section temperature Ta and the first temperature A 1  is made again (step  11 ), the state in which the output current is decreased is maintained for a predetermined time period t 1 . Thus, the control section of the control device  40  measures the predetermined time period t 1  with a timer (step ST 23 ). This is to stabilize the state in which the output current is decreased by the predetermined amount a 1 . That is, this is because when additional control is performed based on the transient changes after a decrease in the output current, the operation of the system may be unstable. Especially, when the thermal capacity of the reforming section  5  is large relative to the decrease in the amount of combustion, continuous feedback control as in a PID control method may cause hunting of the temperature in the reforming section  5 . As the predetermined time period t 1 , about 30 seconds is enough for a small-size fuel cell power generation system  100  with a power generation capacity of 1 to 5 kW. It is unlikely that a significant change which has an adverse effect on the operation of the system occurs within the predetermined time period t 1 . The steps up to this point correspond to an output current decreasing process.  
      The process to be carried out when the reforming section temperature Ta is higher than the first temperature A 1  is described below. The control section of the control device  40  compares the reforming section temperature Ta with the predetermined second temperature A 2  (step ST 12 ). If the reforming section temperature Ta is not equal to nor higher than the second temperature A 2 , the temperature in the reforming section  5  is determined to be in an appropriate range and the operation is continued as it is. If the reforming section temperature Ta is equal to or higher than the second temperature A 2 , an output current increasing process is performed. The output current increasing process is performed to increase the consumption of hydrogen in the fuel cell  30  and to decrease the amount of hydrogen in the anode off gas p in order to suppress the combustion in the combustion section  4 .  
      In the output current increasing process, the number of consecutive times N 1  the output current has been decreased is reset to 0 (step ST 31 ), the output current is increased by a predetermined amount a 2  (step ST 32 ), and 1 is added to the number of consecutive times N 2  the output current has been increased (step ST 34 ). Before the comparison between the reforming section temperature Ta and the first temperature A 1  is made again (step  21 ), the state in which the output current is increased is maintained for a predetermined time period t 2  (step ST 33 ). The details of these steps are similar to those of the steps in the output current decreasing process and hence their description is omitted. The predetermined amount al by which the output current is decreased and the predetermined amount a 2  by which the output current is increased may be equal to or different from each other. The predetermined time period t 1  for which the state in which the output current is decreased is maintained and the predetermined time period t 2  for which the state in which the output current is increased is maintained may be equal to or different from each other.  
      The operation in the case where the number of consecutive times N 1  the output current has been decreased reaches a predetermined value n 1  or the number of consecutive times N 2  the output current has been increased reaches a predetermined value n 2  is described below. The number of consecutive times N 1  the output current has been decreased is the number of times the reforming section temperature Ta did not exceed the second temperature A 2  and became equal to or lower than the first temperature A 1  again after the output current had been decreased. That is, it is the number of times the situation has occurred in which the output current was decreased to increase the amount of hydrogen in the anode off gas p but the combustion in the combustion section  4  was still insufficient and the temperature in the reforming section  5  decreased. If the output current is decreased many times, the output from the fuel cell  30  may largely decrease until it does not meet the external demand for electricity.  
      Thus, when the number of consecutive times N 1  the output current has been decreased reaches the predetermined value n 1 , the supply amount of the raw material fuel m from the raw material fuel supply section  1  is increased to increase the amount of anode off gas p and accelerate the combustion in the combustion section  4  so that the temperature in the reforming section  5  can be increased. When the supply amount of the raw material fuel m is increased, the supply amount of the reforming water s to the reforming section  5  must be increased. Then, the temperature in the reforming section  5  temporarily decreases. However, since the amount of combustion in the combustion section  4  increases, the temperature in the reforming section  5  increases gradually. As described above, the system has a function of controlling the supply amount of the raw material fuel m which can be used when the control of the output current does not work well. Therefore, the reliability for steady operation of the system is enhanced. For example, when the amount by which the output current is decreased is 2% of the output current at start-up and the predetermined value n 1  is  5 , the supply amount of the raw material fuel m is increased in the case that the output current decreases by 10%. Therefore, the fluctuation of the output current is kept within 10%. At this time, when the amount by which the supply amount of the raw material fuel m is increased is 10% of the amount of fuel needed at start-up, the supply amount of the raw material fuel m is kept within an appropriate range.  
      Referring to the flowchart in  FIG. 3 , a specific operation control method is described. It is determined in the control section of the control device  40  whether the number of consecutive times N 1  the output current has been decreased has become equal to the predetermined value n 1  (step ST 25 ). When the number of consecutive times N 1  the output current has been decreased has become equal to the predetermined value n 1 , the control section of the control device  40  sends a control signal i 1  to the raw material fuel supply section  1  to increase the flow rate of the raw material fuel by a predetermined amount f 1 . On receiving the command signal i 1 , the raw material fuel supply section  1  increases the flow rate of the raw material fuel (step  26 ) by increasing the delivery rate of the constant volume pump or by adjusting the flow rate with the flow control valve. At the same time as increasing the flow rate of the raw material fuel, the control section of the control device  40  resets the number of times N 1  the output current has been decreased to 0 (step ST 27 ). Then, the process flow returns to the comparison between the reforming section temperature Ta and the first temperature A 1  (step ST 11 ). Before the comparison, the state in which the output current is decreased and the supply amount of the raw material fuel m is increased is maintained for the predetermined time period t 1 . Thus, the control section of the control device  40  measures the predetermined time period t 1  with a timer (step ST 23 ). This is to secure a period during which the operation after the decrease of the output current and the increase of the supply amount of the raw material fuel m is stabilized.  
      When the number of consecutive times N 2  the output current has been increased has reached to the predetermined value n 2 , the supply amount of the raw material fuel m from the raw material fuel supply section  1  is decreased (step ST 36 ) in contrast to the case where the number of consecutive times N 1  the output current has been decreased has reached the predetermined value n 1 , and the number of times N 2  the output current has been increased is reset to 0 (step ST 37 ). Then, the supply amount of the raw material fuel m decreases, and the supply amount of the reforming water s decreases. As a result, the amount of combustion in the combustion section  4  decreases and the temperature in the reforming section  5  decreases. The details of these steps are similar to those of the steps in the process of increasing the supply amount of the raw material fuel m and hence their description is omitted.  
      As has been described previously, when the number of consecutive times N 1  the output current has been decreased reaches a predetermined value n 1  or when the number of consecutive times N 2  the output current has been increased reaches a predetermined value n 2 , the supply amount of the raw material fuel m is adjusted to maintain the output current from the fuel cell  30  within a fixed range. Therefore, the fuel cell power generation system  100  can meet the external demand for electricity.