Abstract:
Carbon monoxide contained in reformate gas is removed by a preferential oxidation reaction in a catalyst, two preferential oxidation reactors ( 20 A,  20 B) being disposed in series. Valves ( 7, 8 ) supply air containing oxygen as an oxidizing agent individually to these preferential oxidation reactors ( 20 A,  20 B). Temperature sensors ( 9, 10 ) detect the catalyst temperatures of the preferential oxidation reactors ( 20 A,  20 B), and a controller ( 5 ), by adjusting the flow rate of the valves ( 7, 8 ) based on the detected temperatures, maximizes the carbon monoxide removal performance of the preferential oxidation reactors ( 20 A,  20 B), while preventing excessive catalyst temperature rise.

Description:
FIELD OF THE INVENTION 
     This invention relates to the removal of carbon monoxide from reformate gas used in a fuel cell power plant. 
     BACKGROUND OF THE INVENTION 
     In fuel cell power plants using reformate gas, it is necessary to remove carbon monoxide from the reformate gas generated by a reformer. In JP2000-169106 published by the Japanese Patent office in 2000, a carbon monoxide oxidizer is disclosed. The carbon monoxide oxidizer comprises two preferential oxidation reactors (PROX reactors) which are arranged in series. A catalyst comprising a noble metal is disposed inside the preferential oxidation reactors. Air is respectively supplied to the PROX reactors, and the catalyst removes CO in the reformate gas by reacting the CO in the reformate gas with oxygen (O 2 ) in the air to produce carbon dioxide (CO 2 ). 
     JP08-329969 published by the Japanese Patent Office in 1996 discloses a method where the reaction amount of the PROX reactor is controlled by increasing the air supply amount to the PROX reactor according to the increase in the CO concentration in the reformate gas. 
     SUMMARY OF THE INVENTION 
     The preferential oxidation reaction is an exothermic reaction, and when the air supply amount to the PROX reactor is increased, the liberated heat increases as the preferential oxidation reaction proceeds, so the temperature of the catalyst rises. As a result, if the catalyst temperature increases above the reaction temperature of the catalyst, the catalyst deteriorates. 
     The two PROX reactors in the carbon monoxide removal system of JP2000-169106 are respectively cooled by a coolant, but as described in JP08-329969, when the air supply amount is increased according to the CO concentration in the reformate gas, it can be expected that the temperature rise of the catalyst will easily exceed the cooling ability of the coolant. 
     It is therefore an object of this invention to make optimum use of carbon monoxide removal performance while preventing excessive catalyst temperature rise in a carbon monoxide removal system comprising plural PROX reactors disposed in series. 
     In order to achieve the above object, this invention provides a carbon monoxide removal system comprising plural PROX reactors disposed in series which remove carbon monoxide contained in reformate gas via a catalyst, wherein the PROX reactors comprises a first PROX reactor and a second PROX reactor arranged further downstream than the first PROX reactor. The system further comprises an air supply mechanism which supplies air containing oxygen as an oxidizing agent to the first PROX reactor and the second PROX reactor, a first temperature sensor which detects a temperature of the first PROX reactor, a second temperature sensor which detects a temperature of the second PROX reactor, and a controller functioning to control the air supply mechanism so that an air supply flow rate to the first PROX reactor and an air supply flow rate to the second PROX reactor vary based on the temperature of the first PROX reactor and the temperature of the second PROX reactor. 
     This invention also provides a control method of a carbon monoxide removal system comprising plural PROX reactors disposed in series which remove carbon monoxide contained in reformate gas via a catalyst, wherein the PROX reactors comprises a first PROX reactor and a second PROX reactor arranged further downstream than the first PROX reactor. The method comprises supplying air to the first PROX reactor and the second PROX reactor, detecting a temperature of the first PROX reactor, detecting a temperature of the second PROX reactor, and varying an air supply flow rate to the second PROX reactor and an sir supply flow rate to the second PROX reactor based on the temperature of the first PROX reactor and the temperature of the second PROX reactor. 
     The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a fuel cell power plant comprising a carbon monoxide removal system according to this invention. 
         FIG. 2  is a flowchart describing a routine for controlling an air supply flow rate executed by a controller according to this invention. 
         FIG. 3  is a diagram showing the relation between carbon monoxide concentration in reformate gas flowing into a first PROX reactor, air supply flow rate and carbon monoxide conversion rate of the first PROX reactor. 
         FIG. 4  is similar to  FIG. 2 , but showing a second embodiment of this invention. 
         FIG. 5  is similar to  FIG. 2 , but showing a third embodiment of this invention. 
         FIG. 6  is a diagram describing differences ΔT 1 , ΔT 2  between the catalyst temperatures of 
       PROX reactors and the catalyst activation upper limiting temperatures calculated by the controller according to a third embodiment of this invention. 
         FIG. 7  is a schematic diagram of a fuel cell power plant using a carbon monoxide removal system according to a fourth embodiment of this invention. 
         FIG. 8  is similar to  FIG. 2 , but showing the fourth embodiment of this invention. 
         FIG. 9  is similar to  FIG. 2 , but showing a fifth embodiment of this invention. 
         FIG. 10  is a schematic diagram of a fuel cell power plant using a carbon monoxide removal system according to a sixth embodiment of this invention. 
         FIG. 11  is similar to  FIG. 2 , but showing the sixth embodiment of this invention. 
         FIGS. 12A ,  12 B are diagrams describing the characteristics of a map specifying the relation between the flow rate of reformate gas flowing into the PROX reactors and the air amount supplied to the PROX reactors stored by the controller according to the sixth embodiment of this invention. 
         FIG. 13  is a schematic diagram of a fuel cell power plant using a carbon monoxide removal system according to a seventh embodiment of this invention. 
         FIG. 14  is similar to  FIG. 2 , but showing the seventh embodiment of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  of the drawings, a fuel cell power plant for a vehicle removes carbon monoxide in reformate gas produced by a reformer  3  by a carbon monoxide oxidizer  20  comprising two PROX reactors  20 A,  20 B, and supplies an anode  4 A of a fuel cell stack  4  with hydrogen-rich gas. Air is supplied from an air pump  6 B to a cathode  4 B of the fuel cell stack  4 . The hydrogen-rich gas and the oxygen in the air cause electrode reactions shown by the following chemical equations (1), (2) at the anode  4 A and cathode  4 B.
 
Anode  4   A: H   2 →2 H   + +2 e   −   (1)
 
     
       
         
           
             
               
                 
                   Cathode 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                   ⁢ 
                   
                     B 
                     : 
                     
                       
                         
                           2 
                           ⁢ 
                           
                             H 
                             + 
                           
                         
                         + 
                         
                           2 
                           ⁢ 
                           
                             e 
                             - 
                           
                         
                         + 
                         
                           
                             1 
                             2 
                           
                           ⁢ 
                           
                             O 
                             2 
                           
                         
                       
                       → 
                       
                         
                           H 
                           2 
                         
                         ⁢ 
                         O 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Due to the power generated by these electrode reactions, the fuel cell stack  4  generates power. A vehicle drive motor, not shown, is connected to the fuel cell stack  4 . 
     Fuel gas from a fuel tank  1  and water from a water tank  2  are respectively supplied to the reformer  3 . The fuel gas may be a hydrocarbon fuel such as methanol or gasoline. 
     If methanol is used as the fuel gas, the reformer  3  generates reformate gas according to the chemical reactions shown by the following chemical equations (3), (4).
 
CH 3 OH+H 2 O→CO 2 +3H 2   (3)
 
CH 3 OH→CO+2H 2   (4)
 
     The reformate gas has hydrogen (H) as its main component, and contains carbon monoxide (CO). If the carbon monoxide is supplied to the fuel cell stack  4 , it causes a decline of power generating efficiency of the fuel cell stack  4 , and poisons the catalyst at the anode  4 A of the fuel cell stack  4 . Therefore, the carbon monoxide in the reformate gas is removed by the carbon monoxide oxidizer  20 , and hydrogen-rich gas in which the carbon monoxide concentration has been sufficiently reduced is supplied to the anode of the fuel cell stack  4 . 
     The preferential oxidation reaction which takes place in the first PROX reactor  20 A and second PROX reactor  20 B of the carbon monoxide oxidizer  20 , may be represented by the following chemical equation (5). 
     
       
         
           
             
               
                 
                   
                     CO 
                     + 
                     
                       
                         1 
                         2 
                       
                       ⁢ 
                       
                         O 
                         2 
                       
                     
                   
                   → 
                   
                     CO 
                     2 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Due to equation (5), the carbon monoxide oxidizer  20  decreases the CO concentration in the reformate gas from the order of several percent to about 100 ppm in the first PROX reactor  20 A, and then from about 1000 ppm to less than 20 ppm in the second PROX reactor  20 B. 
     The oxygen (O 2 ) required for the preferential oxidation reaction is respectively supplied as air to the first PROX reactor  20 A via a valve  7 , and to the second PROX reactor  20 B via a valve  8 . The air pump  6 A has a function to supply air constantly under a constant pressure. 
     Therefore, the air supply flow rate to the first PROX reactor  20 A is determined by the opening of the valve  7 . Likewise, the old air supply flow rate to the second PROX reactor  20 B is determined by the opening of the valve  8 . 
     The valves  7 ,  8  comprise electro-magnetic valves of which the opening is varied according to opening signals respectively output by the controller  5 . 
     The controller  5  controls the opening of the valves  7 ,  8  depending on the carbon monoxide concentration in the reformate gas and the temperatures of the first PROX reactor  20 A and second PROX reactor  20 B. The first PROX reactor  20 A and second PROX reactor  20 B have identical specifications. 
     For this purpose, a CO concentration sensor  11  which detects the carbon monoxide concentration in the reformate gas produced by the reformer  3  is installed midway in a pipe leading reformate gas from the reformer  3  to the carbon monoxide oxidizer  20 . Also, a temperature sensor which detects the catalyst temperature of the first PROX reactor  20 A and a temperature sensor  10  which detects the catalyst temperature of the second PROX reactor  20 B, are provided. The detection data from these sensors are respectively input to the controller  5  as signals. 
     The controller  5  comprises a microprocessor having a central processing unit (CPU), read-only memory (ROM), random access memory (RAM) and input/output interface (I/O interface). The controller may also comprise plural microcomputers. 
     When the temperature or pressure in the reformer  3  varies, the CO concentration in reformate gas may rise above the CO concentration in the normal running state. In such a case, the air supply flow rate to the PROX reactors  20 A,  20 B must be increased so that the CO concentration in the hydrogen-rich gas supplied to the fuel cell stack  4  does not increase, and the carbon monoxide removal efficiency is enhanced. 
     However, when the air supply flow rate to the PROX reactors  20 A,  20 B is increased, the catalyst temperature rises due to the preferential oxidation reaction which is an exothermic reaction, and the catalyst may deteriorate. The controller  5  therefore, when the catalyst temperature is lower than the predetermined temperature, opens the valves  7 ,  8  to increase the air supply flow rate to the PROX reactors  20 A,  20 B, and when the catalyst temperature is higher than the predetermined temperature, it closes the valves  7 ,  8  to decrease the air supply flow rate to the PROX reactors  20 A,  2 B. 
     Next, referring to  FIG. 2 , a routine of controlling the air supply flow rate executed by the controller  5  to perform this control will be described. This routine is executed at an interval of 0.1 seconds during the running of the fuel cell power plant. The routines for controlling the air supply flow rate according to other embodiments described later, are all repeatedly executed at an interval of 0.1 seconds during the running of the power plant. 
     First, the controller  5 , in a step S 11 , reads the temperature detected by the temperature sensor  9 , in a step S 12 , reads the temperature detected by the temperature sensor  10 , and in a step S 13 , reads the temperature detected by the CO concentration sensor  11 . 
     In a following step S 14 , the detected CO concentration in the reformate gas is compared with a preset specified concentration. From the allowable CO concentration in the hydrogen-rich gas supplied to the fuel cell stack  4 , and the CO removal performance of the PROX reactors  20 A,  20 B during normal running, the allowable CO concentration in the reformate gas can be calculated. The specified concentration is a value set based on the allowable CO concentration. A typical specified concentration range is 1–2%. 
     When the CO concentration in the reformate gas is lower than the specified concentration, the controller  5  immediately terminates the routine without proceeding to subsequent steps. This is because, in this case, the CO concentration in the hydrogen-rich gas remains within the allowable range even if the air supply flow rate to the PROX reactors  20 A,  20 B is not increased. 
     When the CO concentration in the reformate gas is not lower than the specified concentration, in a step S 15 , the controller  5  compares the catalyst temperature of the first PROX reactor  20 A with a predetermined temperature. The predetermined temperature is set to a value within the activation temperature range of the catalyst. A typical predetermined temperature is within the range of 140° C. to 160° C. 
     When the catalyst temperature of the first PROX reactor  20 A is not lower than the predetermined temperature, in a step S 16 , the controller  5  throttles the valve  7  by a fixed amount. Due to this processing, as the air flow rate supplied to the first PROX reactor  20 A decreases, the preferential oxidation reaction amount in the first PROX reactor  20 A decreases, and the catalyst temperature correspondingly decreases. After the processing of the step S 16 , the controller  5  performs the processing of a step S 18 . 
     When the catalyst temperature of the first PROX reactor  20 A is lower than the predetermined temperature, the controller  5  performs the processing of a step S 17 . 
     In the step S 17 , a map having the characteristics shown in  FIG. 3  prestored in a memory is looked up, and the air flow rate required to reach a target CO conversion rate is calculated from the CO concentration in the reformate gas. A typical target CO conversion rate is 95%. 
     Referring to  FIG. 3 , when the CO concentration in reformate gas is for example 2%, in order for the first PROX reactor  20 A to reach the CO conversion rate of 95%, an air supply flow rate of 120 liter/minute corresponding to a point A is required. The controller  5  adjusts the opening of the valve  7  so that the air flow rate calculated in this way is realized. 
     The controller  5  further calculates the CO concentration in the gas flowing from the first PROX reactor  20 A by the following equation (6):
 
CO concentration in outflowing gas=(1−CO conversion rate)·CO concentration in inflowing gas  (6)
 
     Calculating the CO concentration flowing from the first PROX reactor  20 A using equation (6) when the CO concentration in the inflowing gas is 2% and the CO conversion rate is 95% as described above, the calculation results shown by the following equation (7) are obtained:
 
CO concentration in outflowing gas=(1−0.95)·2%=0.1%=1,000 ppm  (7)
 
     After the opening of the valve  7  is adjusted in the step S 16  or  17 , in the step S 18 , the controller  5  compares the catalyst temperature of the second PROX reactor  20 B with the aforesaid predetermined temperature. According to this embodiment, a common predetermined temperature is used in the steps S 15  and S 18 , but different predetermined temperatures may also be used for catalysts having different activities in the first PROX reactor  20 A and second PROX reactor  20 B. 
     When the catalyst temperature of the second PROX reactor  20 B is not lower than the predetermined temperature, in a step S 19 , the controller  5  throttles the opening of the valve  8  by a fixed amount. Due to this processing, as the air flow rate supplied to the second PROX reactor  20 B decreases, the preferential oxidation reaction amount in the second PROX reactor  20 B decreases, and the catalyst temperature decreases correspondingly. After the processing of the step S 19 , the controller  5  terminates the routine. 
     When the catalyst temperature of the second PROX reactor  20 B is lower than the predetermined temperature, the controller  5  performs the processing of a step S 20 . 
     In the step S 20 , the air flow rate required to reach the target CO conversion rate is calculated from the CO concentration of the inflowing gas by looking up the map having the characteristics shown in  FIG. 3  which was looked up in the step S 17 . As described above, the first PROX reactor  20 A and second PROX reactor  20 B have identical specifications, so the same map can be used in the steps S 17  and S 20 , but if the specifications of the first PROX reactor  20 A and second PROX reactor  20 B are different, different maps are used. Here, the CO concentration in the inflowing gas is the CO concentration in the gas flowing out of the first PROX reactor  20 A calculated in the equation (7). The controller  5  adjusts the opening of the valve  8  to realize the air flow rate thus obtained, in a next step S 21 . After the processing of the step S 21 , the controller  5  terminates the routine. 
     As a result of executing the above routine, when the CO concentration in the reformate gas rises, the air flow rates supplied to the first PROX reactor  20 A and second PROX reactor  20 B increase until the catalysts in the first PROX reactor  20 A and second PROX reactor  20 B reach the predetermined temperature, and the CO removal performance of the first PROX reactor  20 A and second PROX reactor  20 B are enhanced. On the other hand, when either one of catalyst temperatures of the first PROX reactor  20 A and second PROX reactor  20 B is not lower than the predetermined temperature, the catalyst temperature of the corresponding PROX reactor is reduced to the predetermined temperature by decreasing the air flow rate supplied to that PROX reactor. 
     Therefore, by executing this routine, in a carbon monoxide oxidizer comprising plural PROX reactors arranged in series, the carbon monoxide removal performance can be optimized while preventing rise of catalyst temperature in the PROX reactors. 
     Next, referring to  FIG. 4 , a second embodiment of this invention will be described. 
     The hardware construction of this embodiment is identical to that of the first embodiment. In this embodiment, only the routine for controlling the air supplier flow rate performed by the controller  5  is different from that of the first embodiment as shown by  FIG. 4 . 
     In this embodiment, the air flow rate is determined based on a difference between the upper limiting temperature for catalyst activation of the first PROX reactor  20 A and second PROX reactor  20 B, and the detected catalyst temperatures. The openings of the valves  7 ,  8  are adjusted correspondingly. 
     The upper limiting temperature for catalyst activation is the highest value within the temperature range when the catalyst is activated. A typical upper limiting temperature for catalyst activation is within the range of 200° C.–240° C. According to this embodiment, a common upper limiting temperature for catalyst activation is used for the first PROX reactor  20 A and second PROX reactor  20 B, but when different catalysts are used in the first PROX reactor  20 A and second PROX reactor  20 B, different upper limiting temperatures for catalyst activation are used according to the characteristics of these catalysts. 
     The processing of the steps S 11 –S 14  is identical to that of the first embodiment. In the step S 14 , when the CO concentration of the reformate gas is not lower than the specified concentration, the controller  5  performs the processing of a step S 31 . In the step S 14 , when the CO concentration of the reformate gas is lower than the specified concentration, the controller  5  immediately terminates the routine. 
     In the step S 31 , the controller  5  calculates a temperature difference ΔT 1  between the upper limiting temperature for catalyst activation and the temperature of the first PROX reactor  20 A. 
     In a following step S 32 , the controller  5  calculates a temperature difference ΔT 2  between the upper limiting temperature for catalyst activation and the temperature of the second PROX reactor  20 B. 
     In a following step S 33 , it is determined whether or not one of the temperature difference ΔT 1  and temperature difference ΔT 2  is a negative value. When one of these values is a negative value, in a step S 34 , the controller  5  throttles the opening of the valve of the PROX reactor for which the temperature difference was a negative value by a fixed amount. After the processing of the step S 34 , the controller  5  terminates the routine. 
     When, in the step S 33 , neither of the temperature differences are negative values, in a step S 35 , the controller  5  compares the temperature difference ΔT 1  and temperature difference ΔT 2 . When the temperature difference ΔT 1  is larger than the temperature difference ΔT 2 , in a step S 38 , the controller  5  looks up the map having the characteristics shown in  FIG. 3  prestored in the memory, and calculates the air flow rate required to reach the target CO conversion rate from the CO concentration detected by the CO concentration sensor  11 . This calculation is identical to the calculation of the step S 17  of the routine of  FIG. 2  according to the first embodiment. Further, the controller  5  adjusts the opening of the valve  7  so that the calculated air flow rate is realized. 
     On the other hand, when the temperature difference ΔT 1  is not larger than the temperature difference ΔT 2 , the controller  5  continuously performs the processing of steps S 36  and S 37 . 
     In the step S 36 , the controller  5  calculates the CO concentration of the gas flowing into the second PROX reactor  20 B. To do this, the controller  5  first calculates the CO conversion rate of the first PROX reactor  20 A by looking up the map having the characteristics shown in  FIG. 3  from the CO concentration detected by the CO concentration sensor  11  and the air flow rate supplied to the first PROX reactor  20 A. Next, the CO concentration of the outflowing gas is calculated by substituting the CO concentration detected by the CO concentration sensor  11  and the CO conversion rate obtained into equation (6). This is effectively the CO concentration of the gas flowing into the second PROX reactor  20 B. 
     In the step S 37 , the controller  5  adjusts the air flow rate required to reach the target CO conversion rate by looking up the map having the characteristics shown in  FIG. 3  from the CO concentration of the gas flowing into the second PROX reactor  20 B. The controller  5  further adjusts the opening of the valve  8  to realize the calculated air flow rate. After the processing of the step S 37 , the controller  5  terminates the routine. 
     Due to the processing of this routine, when the CO concentration of the reformate gas is not lower than the specified concentration, the controller  5  determines whether or not the catalyst temperature of one of the PROX reactors exceeds the upper limiting temperature for catalyst activation, and when it does exceed this temperature, the air supply flow rate to the corresponding PROX reactor is reduced. 
     On the other hand, when neither of the catalyst temperatures of the PROX reactors exceeds the upper limiting temperature for catalyst activation, the air supply flow rate to the PROX reactor which is at a relatively low temperature, i.e. the PROX reactor which has more tolerance for temperature rise, is increased. Therefore, the carbon monoxide removal performance can be optimized while effectively preventing rise in the catalyst temperatures of the PROX reactors. 
     Next, referring to  FIGS. 5 ,  6 , a third embodiment of this invention will be described. 
     The hardware construction of this embodiment is identical to that of the first and second embodiments. In this embodiment, only the routine for controlling the air supply flow rate executed by the controller  5  shown in  FIG. 5  is different from those of the first and second embodiments. 
     In this embodiment, increments of the CO conversion rates are determined based on the temperature differences ΔT 1 , ΔT 2  between the upper limiting temperature for catalyst activation and the catalyst temperatures of the PROX reactors as shown in  FIG. 6 , and the air supply flow rate to the PROX reactors is determined based on the determined increments. 
     Referring to  FIG. 5 , the processing of the steps S 11  to S 14  and steps S 31  to S 34  is identical to that of the second embodiment. 
     In the step S 33 , the controller  5 , when neither of the temperature differences ΔT 1 , ΔT 2  are negative values, i.e., when both of them are positive values, the processing of steps S 41  to S 43  is performed. 
     In the step S 41 , the controller  5  increases the CO conversion rates of the first PROX reactor  20 A and second PROX reactor  20 B respectively in the proportion of ΔT 1 :ΔT 2  from the target CO conversion rate. In this embodiment, let the target CO conversion rate of the first PROX reactor  20 A be 95%, and the target CO conversion rate of the second PROX reactor  20 B be 98%. Also, let the target CO concentration of hydrogen-rich gas flowing from the second PROX reactor  20 B be 20 ppm. 
     Increments Δn 1 , Δn 2  in the CO conversion rate of the first PROX reactor  20 A and second PROX reactor  20 B, have the relationship of the following equation (8):
 
Δn1:Δn2=ΔT1: ΔT2  (8)
 
     The following equation (9) is obtained from equation (8):
 
Δ n 1 ·ΔT 2 =Δn 2 ·ΔT 1  (9)
 
     If the CO concentration of the reformate gas flowing into the first PROX reactor  20 A is Cin, and the first PROX reactor  20 A and second PROX reactor  20 B decrease the CO concentration Cin in the reformate gas to the CO concentration of 20 ppm in the outflowing gas, the following relation (10) between Cin, Δn 1 , Δn 2  should be satisfied.
 
Cin·(1−0.95 −Δn 1)·(1−0.98 −Δn 2)=0.002  (10)
 
     Cin is the concentration detected by the CO concentration sensor  11 . Therefore, the increments Δn 1 , Δn 2  in the CO conversion rate of the first PROX reactor  20 A and second PROX reactor  20 B can be calculated from the following equations (9), (10). 
     In the step S 42 , the controller  5  calculates the air flow rate supplied to the first PROX reactor  20 A from the sum of the target CO conversion rate of 95% and increment Δn 1  by looking up a map having the characteristics shown in  FIG. 3  prestored in the memory. The controller  5  further adjusts the opening of the valve  7  so that the calculated air flow rate is realized. 
     In the next step S 36 , in an identical manner to that of the second embodiment, the CO concentration in the gas flowing into the second PROX reactor  20 B is calculated. 
     In the next step S 43 , the air flow rate supplied to the second PROX reactor  20 B is likewise calculated from the sum of the target CO conversion rate of 98% and increment Δn 2  by looking up a map having the characteristics shown in  FIG. 3 . The controller  5  further adjusts the opening of the valve  8  so that the calculated air flow rate is realized. After the processing of the step S 43 , the controller  5  terminates the routine. 
     According to this embodiment, the air flow rate supplied to the PROX reactors is increased according to the differences ΔT 1 , ΔT 2  between the upper limiting temperatures for catalyst activation and the catalyst temperatures of the PROX reactors, so the CO concentration can be efficiently reduced by using all the temperature differences between the upper limiting temperatures for catalyst activation and the catalyst temperatures of the PROX reactors. 
     On the other hand, due to the steps S 33 , S 34 , when either one of the catalyst temperatures of the PROX reactors is not less than the upper limiting temperatures for catalyst activation, the air flow rate supplied to the corresponding PROX reactor(s) can be reduced, so transient increase of the catalyst temperatures of the PROX reactors can be prevented as in the first and second embodiments. 
     Next, referring to  FIGS. 7 ,  8 , a fourth embodiment of this invention will be described. 
     According to this embodiment, the hardware construction is different from that of the first-third embodiments. 
     Specifically, in this embodiment, instead of the CO concentration sensor  11  which detects the CO concentration in the reformate gas generated by the reformer  3 , a concentration sensor  12  which detects the CO concentration in the hydrogen-rich gas supplied to the fuel cell stack  4  from the second PROX reactor  20 B is provided. The remaining features of the hardware are identical to those of the first-third embodiments. 
     According to this embodiment, the controller  5  executes a routine for controlling the air supply flow rate shown in  FIG. 8 . In this routine, the processing of the steps S 11 , S 12 , S 31 –S 35  is identical to that of the routine of  FIG. 5  of the second embodiment. 
     Referring to  FIG. 8 , the controller  5 , in the step S 11 , reads the temperature detected by the temperature sensor  9 , in, the step S 12 , reads the temperature detected by the temperature sensor  10 , and in a step S 51 , reads the CO concentration of the hydrogen-rich gas detected by the concentration sensor  12 . 
     In a following step S 52 , the CO concentration of the hydrogen-rich gas and a hydrogen-rich gas specified concentration are compared. The hydrogen-rich gas specified concentration is the upper limiting value of the CO concentration which does not affect the power generating performance of the fuel cell stack  4 . Typically, the specified concentration of the hydrogen-rich gas is 30 ppm. If the CO concentration of the hydrogen-rich gas is lower than the specified concentration, the controller  5  does not proceed to subsequent steps, and immediately terminates the routine. If the CO concentration of the hydrogen-rich gas is not lower than the specified concentration, the controller  5  performs the processing of the steps S 31 –S 35  which were described in relation to the second embodiment. 
     In the step S 35 , when the temperature difference ΔT 1  is larger than the temperature difference ΔT 2 , the controller  5 , in a step S 53 , increases the opening of the valve  7  so that the air flow rate supplied by the valve  7  increases by a fixed amount. On the other hand, when the temperature difference ΔT 1  is not larger than the temperature difference ΔT 2 , the controller  5 , in a step S 54 , increases the opening of the valve  8  so that the air flow rate supplied to the valve  8  increases by a fixed amount. 
     After the processing of the steps S 53  or S 54 , the controller  5  terminates the routine. 
     According to this embodiment, if the CO concentration of the hydrogen-rich gas supplied to the fuel cell stack  4  is not lower than the hydrogen-rich specified concentration, the controller  5  increases the air flow rate supplied to one of the PROX reactors according to the temperature differences ΔT 1 , ΔT 2 , and repeats this operation until the CO concentration of the hydrogen-rich gas falls to the hydrogen-rich gas specified concentration. In other words, the air flow rate supplied to the PROX reactors  20 A,  20 B is feedback-controlled based on the CO concentration of the hydrogen-rich gas. 
     The CO conversion rate of the first PROX reactor  20 A and second PROX reactor  20 B are not necessarily constant due to temperature variation and catalyst deterioration. However, if the air flow rate supplied is feedback-controlled based on the CO concentration of the hydrogen-rich gas, the CO concentration of the hydrogen-rich gas can always be suppressed below the hydrogen-rich gas specified concentration even if there is scatter in the performance of the PROX reactors  20 A,  20 B. 
     According to this embodiment, the air flow rate supplied is increased to whichever of the PROX reactors has the higher tolerance for increase of catalyst temperature, so temperature rise above the upper limiting temperature for catalyst activation due to the increase of air flow rate can effectively be prevented. 
     Next, referring to  FIG. 9 , a fifth embodiment of this invention will be described. 
     The hardware construction of this embodiment is identical to that of the fourth embodiment. In this embodiment, only the routine for controlling the air supply flow rate executed by the controller  5  is different from the fourth embodiment. 
     Referring to  FIG. 9 , the processing of the steps S 11 , S 12 , steps S 51 , S 52  and steps S 31 –S 34  of this routine are identical to the routine of  FIG. 8  in the fourth embodiment. In this routine, the method of feedback-controlling the air flow rate supplied to the PROX reactors  20 A,  20 B based on the CO concentration of the hydrogen-rich gas, is different from that of the fourth embodiment. 
     In the fourth embodiment, of the two PROX reactors  20 A,  20 B, only the air flow rate supplied to the reactor having the larger temperature difference is increased by a fixed increment, but according to this embodiment, the air flow rates supplied to the PROX reactors  20 A,  20 B are both increased, and gains G 1 , G 2  applied to the calculation of the increase amount of the air supply flow rate are made to vary dynamically according to the temperature differences ΔT 1 , ΔT 2 . Specifically, in a step S 55 , the controller  5  determines the ratio of the air increase amount gains G 1 , G 2  according to the temperature differences ΔT 1 , ΔT 2  by the following equation (11).
 
 G 1 ΔT 2 =G 2 ·ΔT 1  (11)
 
     Herein, the sum value of the air increase amount gains G 1 , G 2  is fixed, and this sum value is first determined by experiment or simulation. The values of the air increase amount gains G 1 , G 2  are determined from this sum value and the ratio of the air increase amount gains G 1 , G 2  obtained from equation (11). 
     In a following step S 56 , the controller  5  calculates an increment ΔQ 1  of the air supply flow rate to the first PROX reactor  20 A by multiplying a first basic increment for the first PROX reactor  20 A by the air increase amount gain G 1 . Likewise, an increment ΔQ 2  of the air supply flow rate to the second PROX reactor  20 B is calculated by multiplying a second basic increment for the second PROX reactor  20 B by the air increase amount gain G 2 . The first basic increment and second basic increment are fixed values predetermined by experiment or simulation. 
     In a next step S 57 , the opening of the valve  7  is adjusted based on the increment ΔQ 1  of the air supply flow rate to the first PROX reactor  20 A, and the opening of the valve  8  is adjusted based on the increment ΔQ 2  of the air supply flow rate to the second PROX reactor  20 B. 
     Also in this embodiment, as in the fourth embodiment, the CO concentration in the hydrogen-rich gas can always be suppressed below the specified concentration even if there is scatter in the performance of the PROX reactors. Further, the air supply flow rate to the PROX reactors is increased according to the differences ΔT 1 , ΔT 2  between the upper limiting temperature for catalyst activation and the catalyst temperatures of the PROX reactors, so the CO concentration can be efficiently reduced making use of all the temperature differences between the upper limiting temperature for catalyst activation and the catalyst temperatures of the PROX reactors. 
     Next, a sixth embodiment of this invention will be described referring to  FIGS. 10 ,  11  and  FIGS. 12A ,  12 B. 
     In this embodiment, the hardware construction is different from that of the first-third embodiments. 
     Specifically, in this embodiment, instead of the CO concentration sensor  11  which detects the CO concentration of reformate gas generated by the reformer  3  a flow rate sensor  13  which detects the flow rate of reformate gas generated by the reformer  12  is installed midway in the pipe leading reformate gas from the reformer  3  to the carbon monoxide oxidizer  20 . The remaining features of the construction are identical to those of the first-third embodiments. 
     In this embodiment, instead of the routine of  FIG. 2  in the first embodiment, the controller  5  executes a routine for controlling the air supply flow rate shown in  FIG. 11 . In this embodiment, the opening of the valves  7 ,  8  are first initialized to an opening corresponding to a specified flow rate of reformate gas.n Herein, the specified flow rate corresponds to a flow rate when the power plant is running steadily. 
     Referring to  FIG. 11 , the processing of the steps S 11 , S 12  is identical to the routine of  FIG. 2  of the first embodiment. The processing of the steps S 31 –S 34  is identical to the routine of  FIG. 5  of the second embodiment. 
     In a step S 61  following the step S 12 , the controller  5  reads the reformate gas flow rate detected by the flow rate sensor  13 . 
     In a next step S 62 , it is determined whether the reformate gas flow rate is equal to the specified flow rate. When the reformate gas flow rate is equal to the specified flow rate, the controller  5  immediately terminates the routine without preceding to subsequent steps. 
     When the reformate gas flow rate is not equal to the specified flow rate, the controller  5  performs the processing of the steps S 31 –S 34  as in the routine of  FIG. 4  of the second embodiment. 
     In the step S 33 , when neither of the temperature differences ΔT 1 , ΔT 2  are not negative values, in a step S 63 , the controller  5  calculates a basic variation amount ΔQa 1  of the air flow rate supplied to the first PROX reactor  20 A from the reformate gas flow rate by looking up a map having the characteristics shown in  FIG. 12A  which is prestored in the memory. Referring to  FIG. 12A , the basic variation amount ΔQa 1  in this map means the variation amount from a specified flow rate Qa 1 R of the air supply flow rate when the reformate gas flow rate increases as shown by the dotted line relative to the specified flow rate. 
     In a following step S 64 , the controller  5  calculates a basic variation amount ΔQa 2  of the air flow rate supplied to the second PROX reactor  20 B from the gas flow rate flowing into the second PROX reactor  20 B by looking up a map having the characteristics shown in  FIG. 12B  which is prestored in the memory. Herein, the gas flow rate flowing into the second PROX reactor  20 B is the gas flow rate flowing out of the first PROX reactor  20 A, and this may be approximated to the sum of the reformate gas flow rate flowing into the first PROX reactor  20 A and the air flow rate supplied to the first PROX reactor  20 A. 
     Referring to  FIG. 12B , the basic variation amount ΔQa 2  in this map is the variation amount from the specified flow rate of the air supply flow rate when the inflowing gas flow rate increases as shown by the dotted line relative to the specified flow rate. 
     In a next step S 65 , the controller  5  calculates an air flow rate Qa 1 ′ supplied to the first PROX reactor  20 A and an air flow rate Qa 2 ′ supplied to the second PROX reactor  20 B by applying the following equations (12), (13). 
     
       
         
           
             
               
                 
                   
                     Qa1 
                     ′ 
                   
                   = 
                   
                     Qa1R 
                     + 
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         Qa1 
                         · 
                         
                           
                             
                               2 
                               · 
                               Δ 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T1 
                           
                           
                             
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               T1 
                             
                             + 
                             
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               T2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
             
               
                 
                   
                     Qa2 
                     ′ 
                   
                   = 
                   
                     Qa2R 
                     + 
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         Qa2 
                         · 
                         
                           
                             
                               2 
                               · 
                               Δ 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T2 
                           
                           
                             
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               T1 
                             
                             + 
                             
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               T2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     In a next step S 66 , the opening of the valve  7  is adjusted so that the air flow rate Qa 1 ′ is realized, and the opening of the valve  8  is adjusted so that the air flow rate Qa 2 ′ is realized. After the processing of the step S 66 , the controller  5  terminates the routine. 
     In  FIGS. 12A ,  12 B, it was assumed that the basic variation amounts ΔQa 1 , ΔQa 2  were positive values, but when the reformate gas flow rate has decreased from the specified flow rate, the basic variation amounts ΔQa 1 , ΔQa 2  become negative values. As can be seen from equations (12), (13), in this case, the air flow rate Qa 1 ′ is a smaller value than the specified air flow rate Qa 1 R, and the air flow rate Qa 2 ′ is a smaller value than the specified air flow rate Qa 2 R. 
     If the catalyst temperature of one of the first PROX reactor  20 A and second PROX reactor  20 B is not less than the upper limiting temperature for catalyst activation, the opening of the valves  7  or  8  is decreased as in all the other embodiments, so that the air flow rate supplied to the corresponding first PROX reactor  20 A or second PROX reactor  20 B is decreased. Therefore, the carbon monoxide removal performance can be optimized while preventing catalyst temperature rise of the PROX reactors. 
     Next, referring to  FIGS. 13 ,  14 , a seventh embodiment of this invention will be described. 
     This embodiment is different from the other embodiments in terms of the hardware construction. Referring to  FIG. 13 , according to this embodiment, the CO concentration sensor or flow rate sensor is not used. According to this embodiment, when the fuel cell power plant is running steadily, it is assumed that the CO concentration and flow rate of reformate gas are respectively constant. 
     Instead of the routine of  FIG. 2  of the first embodiment, the controller  5  executes a routine for controlling the supply air flow rate shown in  FIG. 14 . 
     The processing of the steps S 11 , S 12  is identical to the routine of  FIG. 2  according to the first embodiment. The processing of the steps S 31 –S 34  is identical to the routine of  FIG. 4  according to the second embodiment. 
     In steps S 63 , S 64 , when both of the temperature differences ΔT 1 , ΔT 2  are not negative values, the controller  5 , in a step S 71 , applies the following equations (14), (15), and calculates the air supply flow rate Qna 1 ′ to the first PROX reactor  20 A and the air supply flow rate Qna 2 ′ to the PROX reactor  20 B from the temperature differences ΔT 1 , ΔT 2 . 
     
       
         
           
             
               
                 
                   
                     Qna1 
                     ′ 
                   
                   = 
                   
                     Qna1 
                     + 
                     
                       C1 
                       · 
                       
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           T1 
                         
                         
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T1 
                           
                           + 
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     where, Qna 1 =specified air flow rate supplied to the first PROX reactor  20 A, and
         C 1 =correction coefficient.       

     
       
         
           
             
               
                 
                   
                     Qna2 
                     ′ 
                   
                   = 
                   
                     Qna2 
                     + 
                     
                       C2 
                       · 
                       
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           T2 
                         
                         
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T1 
                           
                           + 
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     where, Qna 2 =specified air flow rate supplied to the second PROX reactor  20 B, and
         C 2 =correction coefficient.       

     The correction coefficients C 1 , C 2  are respectively set experimentally. 
     In a following step S 72 , the controller  5  adjusts the opening of the valve  7  so that the calculated air flow rate Qna 1 ′ is realized, and adjusts the opening of the valve  8  so that the calculated air flow rate Qna 2 ′ is realized. After the processing of the step S 72 , the controller  5  terminates the routine. 
     In this embodiment also, the air supply flow rate to the PROX reactor having a higher tolerance for temperature rise is increased based on the catalyst temperatures of the first PROX reactor  20 A and second PROX reactor  20 B, so excessive catalyst temperature rise can be prevented, and the carbon monoxide removal performance of the of the first PROX reactor  20 A and second PROX reactor  20 B can be utilized to the maximum. 
     In this embodiment, the CO concentration sensor or flow rate sensor is not used, so the construction of the device can be simplified. 
     The contents of Tokugan 2002-88058, with a filing date of Mar. 27, 2002 in Japan, are hereby incorporated by reference. 
     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings. 
     For example, in all the aforesaid embodiments, the carbon monoxide oxidizer  20  is comprised of the two PROX reactors  20 A,  20 B, but this invention may be applied also to a carbon monoxide removal device comprising three or more PROX reactors. 
     The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows: