Patent Publication Number: US-8980488-B2

Title: Fuel cell system

Description:
This application is a 371 application of PCT/JP2010/003148 having an international filing date of May 7,2010, which claims priority to JP2009-115413 filed on May 12, 2009, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a fuel cell configured to supply a reformed gas generated by reforming a hydrocarbon-based raw material to a fuel cell, thereby generating electric power. 
     BACKGROUND ART 
     A fuel cell supplies hydrogen or a hydrogen rich gas to one of electrodes between which an electrolyte is interposed, and also supplies an oxidant gas containing oxygen, such as air, to the other of the electrodes, thereby generating electric power through an electrochemical reaction. Recently, attention is focused on a cogeneration system that utilizes electric power generated by a fuel cell and also recovers heat generated when the fuel cell generates electric power, to thus utilize the recovered heat as heat energy. 
     In the fuel cell system, as one of methods for generating a hydrogen rich gas required for a fuel cell, a hydrocarbon based raw gas, such as a town gas or an LPG, is subjected to a steam reforming reaction along with steam in a reforming unit filled with a reforming catalyst at about 700° C., thereby generating a reformed gas containing hydrogen as a major component. During the steam reforming reaction, a carbon monoxide in an amount of about 10 to 15% contained in a reformed gas output from the reforming unit is generated at this time as a by-product. Carbon monoxide poisons an electrode catalyst of the fuel cell, thereby deteriorating power generation capacity. For this reason, it is necessary to eliminate carbon monoxide in the reformed gas up to a carbon monoxide content of 100 ppm or less and, more preferably, 10 ppm or less. 
     Generally, a shift unit and a selective oxidation unit are provided as a carbon monoxide decreasing unit at downstream of the reforming unit. The shift unit is filled with a shift catalyst that causes carbon monoxide in a reformed gas output from the reforming unit to react with steam, thereby performing the water gas shift reaction to form hydrogen and carbon dioxide. The selective oxidation unit is filled with a selective oxidation catalyst and is supplied air and the reformed gas of which carbon monoxide concentration is decreased by the shift unit, thereby subjecting carbon monoxide and oxygen in the air to a selective oxidation reaction such that concentration of carbon monoxide in the reformed gas is decreased to 10 ppm or less. At this time, the shift unit performs the water gas shift reaction at a temperature of about 200° C. or more, and the selective oxidation unit performs the selective oxidation reaction at a temperature of about 100° C. 
     The reforming unit further includes a heating burner unit. During power generation of the fuel cell system, the burner unit burns hydrogen in a reformed gas that has not been used in power generation of in the fuel cell (hereinafter described as an “off-gas”), using the air supplied to the burner unit, thereby maintaining at about 700° C. the temperature of the reforming catalyst for a reforming reaction which is an endothermic reaction. Moreover, during a start-up operation of the fuel cell system, the burner unit burns a raw gas not yet used for generating hydrogen and a mixed gas containing the raw gas and hydrogen, thereby increasing the temperature of the reforming catalyst. 
     Hereinafter, a hydrogen production device including a reforming unit equipped with a burner unit, the shift unit, and the selective oxidation unit are connected is described as a fuel processor, when necessary. 
     At the start-up operation of the fuel cell system, it is necessary to heat the catalysts in the fuel processor to predetermined temperatures for generating a reformed gas from a raw gas. There is disclosed a method which includes: supplying a raw gas to a fuel processor; returning the raw gas output from the fuel processor to the burner unit through a channel bypassing the fuel cell, thereby burning the returned raw gas; and heating the catalysts of the fuel processor by combustion heat (see, for example, Patent Document 1). 
     When power generation of the fuel cell system is halted, supply of the raw gas and steam to the reforming unit is suspended. At this time, an interior of the fuel processor is depressurized by volume shrinkage due to a temperature fall of the reformed gas remaining in the fuel processor or condensation of steam in the reformed gas due to the temperature fall. In order to avoid such depressurization, when stopping operation, supply of the raw gas and the steam is first suspended, and after the temperature of the fuel processor has decreased to a predetermined temperature, the reformed gas in the fuel processor is purged by the raw gas. When the internal pressure of the fuel processor has decreased to the predetermined pressure level or less, the raw gas is supplied to the fuel processor, thereby maintaining positive pressure (see, for example, Patent Document 2). 
     Related Art Document 
     Patent Document 
     Patent Document 1: JP-A-2008-218355 
     Patent Document 2: JP-B-4130603 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, the related art fuel cell systems have the following problems. 
     When stopping power generation, the related art fuel cell system supplies a raw gas, such as a town gas or an LPG, to the fuel processor in order to purge the reformed gas containing steam in the fuel processor and keep the interior pressure of the fuel processor at positive pressure. At this time, in association with the temperature fall of the catalysts during stoppage of the fuel cell system, the raw gas or a portion of a component of the raw gas are absorbed to the catalysts. 
     Meanwhile, in order to increase the temperatures of the respective catalysts to predetermined temperatures at the start-up operation of the fuel cell system, the raw gas is supplied to the burner unit through the fuel processor, and is burnt with air, thereby increasing the temperatures of the catalysts. With the temperature rise of the catalysts, the components of the raw gas absorbed to the catalysts are desorbed and then supplied to the burner unit along with the raw gas supplied to the fuel processor. Therefore, an amount of raw gas actually supplied to the burner unit is larger than the amount of raw gas supplied to the fuel processor by an amount corresponding to the desorbed components of the raw gas. A ratio of the raw gas to air deviates, whereby incomplete combustion occurs in the burner unit. Moreover, carbon monoxide caused by incomplete combustion is emitted to the outside of the fuel cell system. In some cases, a flame vanish occurs in the burner unit. As a result, there arises a problem that the temperatures of the catalysts in the fuel processor are not increased, which impedes the start-up operation of the fuel cell system. 
     An aspect of the present invention solves the problems of the related art, and an object thereof is to provide a fuel cell system capable of performing a stable start-up operation with stable combustion in a burner unit. 
     Means for Solving the Problem 
     An aspect of the present invention provides a fuel cell system including: a reforming unit configured to subject a mixed gas containing a raw gas and steam to a reforming reaction by a reforming catalyst, thereby generating a reformed gas containing hydrogen; a carbon monoxide decreasing unit configured to allow the reformed gas generated by the reforming unit to contact a carbon monoxide decreasing catalyst, thereby decreasing carbon monoxide contained in the reformed gas; a fuel cell configured to generate electric power by hydrogen contained in the reformed gas passed through the carbon monoxide decreasing unit; a burner unit configured to burn at least one of an off-gas containing hydrogen not consumed in the fuel cell, the raw gas and the reformed gas using combustion air supplied by an air blower; and a raw gas supply device configured to supply the raw gas to the reforming unit and the burner unit directly or through the reforming unit, wherein at a start-up operation of the fuel cell system, an amount of raw gas supplied from the raw gas supply device is adjusted according to an amount of a desorbed raw gas desorbed out of components of the raw gas adsorbed to at least one of the reforming catalyst and the carbon monoxide decreasing catalyst. 
     Accordingly, stable combustion of the burner unit can continually be performed at the start-up operation of the fuel cell system. 
     Advantages of the Invention 
     The fuel cell system of an aspect of the present invention performs combustion in a burner unit such that a ratio of combustion air to the raw gas falls within a predetermined range, whereby stable combustion can be performed. It is possible to implement a fuel cell system capable of performing a stable start-up operation by increasing temperatures of catalysts in a fuel processor to predetermined temperatures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a configuration of a fuel cell system of a first embodiment of the present invention. 
         FIG. 2  is a cross sectional view of a main portion for explaining a fuel processor used in the fuel cell system of the first embodiment. 
         FIG. 3  shows a view for explaining a relationship between a temperature of a shift catalyst of the shift unit of the fuel cell system of the first embodiment and an amount of a desorbed raw gas desorbed from the shift catalyst. 
         FIG. 4  is a flowchart showing control flow of a main portion at the start-up operation of the fuel cell system of the first embodiment. 
         FIG. 5  is a diagram showing a temperature rise in a reforming catalyst, the shift catalyst, and a selective oxidation catalyst at the start-up operation of the fuel cell system of the first embodiment. 
         FIG. 6  is a flowchart for describing control flow of the main portion in another start-up operation method for the fuel cell system of the first embodiment. 
         FIG. 7  is a schematic diagram showing a configuration of the fuel cell system of a second embodiment of the present invention. 
         FIG. 8  is a flowchart showing control flow of a main potion at the start-up operation of the fuel cell system of the second embodiment. 
         FIG. 9  is a flowchart for describing control flow of the main portion in another start-up operation method for the fuel cell system of the second embodiment. 
         FIG. 10  is a schematic diagram for explaining another example of the fuel cell system of the second embodiment. 
         FIG. 11  is a schematic view showing a configuration of a fuel cell system of a third embodiment of the present invention. 
         FIG. 12  is a flowchart showing control flow of a main portion at the start-up operation of the fuel cell system of the third embodiment. 
         FIG. 13  is a schematic diagram showing a configuration of the fuel cell system of a fourth embodiment of the present invention. 
         FIG. 14  is a flowchart showing control flow of the main portion at the start-up operation of the fuel cell system of the fourth embodiment. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     An aspect of the present invention provides a fuel cell system comprising: a reforming unit configured to subject a mixed gas containing a raw gas and steam to a reforming reaction by a reforming catalyst, thereby generating a reformed gas containing hydrogen; a carbon monoxide decreasing unit configured to allow the reformed gas generated by the reforming unit to contact a carbon monoxide decreasing catalyst, thereby decreasing carbon monoxide contained in the reformed gas; a fuel cell configured to generate electric power by hydrogen contained in the reformed gas passed through the carbon monoxide decreasing unit; a burner unit configured to burn at least one of an off-gas containing hydrogen not consumed in the fuel cell, the raw gas and the reformed gas using combustion air supplied by an air blower; and a raw gas supply device configured to supply the raw gas to the reforming unit and the burner unit directly or through the reforming unit, wherein at a start-up operation of the fuel cell system, an amount of raw gas supplied from the raw gas supply device is adjusted according to an amount of a desorbed raw gas desorbed out of components of the raw gas adsorbed to at least one of the reforming catalyst and the carbon monoxide decreasing catalyst. 
     With this configuration, it is possible to perform stable combustion by adjusting a ratio of the amount of combustion air supplied to the burner unit to a total of the amount of raw gas supplied from the raw gas supply device and the amount of the desorbed raw gas supplied through the desorption so as to fall within a predetermined ratio. As a result, it is possible to provide a fuel cell system capable of performing the stable start-up operation by increasing the temperature of catalysts in the fuel processor to a predetermined temperature. 
     The fuel cell system may include heating means for performing a heating operation for at least one of the reforming catalyst and the carbon monoxide decreasing catalyst, wherein the heating means may heat at least one of the reforming catalyst and the carbon monoxide catalyst. With this configuration, the raw gas adsorbed to the catalysts can be effectively desorbed. 
     The fuel cell system may include measurement means for measuring at least a flow amount of the desorbed raw gas out of the raw gas by the raw gas supply device and the desorbed raw gas supplied to the burner unit, at the start-up operation of the fuel cell system. Even when the raw gas adsorbed to at least one of the reforming catalyst and the carbon monoxide decreasing catalyst is desorbed and supplied to the burner unit, it is possible to adjust the amount of combustion air according to the flow amount of raw gas supplied to the burner unit, whereby stable combustion can be performed in the burner unit. 
     In the fuel cell system, the measurement means may include a flow meter configured to measure at least the flow amount of the desorbed raw gas. With this configuration, the flow amount of the raw gas supplied to the burner unit can be accurately measured in a relatively easy manner. Consequently, the flow amount of combustion air supplied to the burner unit can be adjusted according to the flow amount of raw gas supplied to the burner unit, whereby stable combustion can be performed in the burner unit. 
     In the fuel cell system, the measurement means may include a pressure gauge configured to detect at least pressure of the desorbed raw gas. With this configuration, the flow amount of the raw gas can be measured from the pressure of the raw gas supplied to the burner unit by a relatively simple configuration. Consequently, the flow amount of combustion air supplied to the burner unit can be adjusted according to the flow amount of raw gas supplied to the burner unit, whereby stable combustion can be performed in the burner unit. 
     The fuel cell system may include exhaust gas analysis means for detecting a concentration of at least one component out of combustion exhaust gas components generated after combustion in the burner unit, wherein the amount of the raw gas supplied from the raw gas supply device may be adjusted according to the concentration detected by the exhaust gas analysis means. With this configuration, it is possible to determine that the state of combustion of the burner unit begins to be deteriorated due to the desorbed gas, and then adjust the flow amount of raw gas supplied from the raw gas supply device. Consequently, the burner unit can perform stable combustion. 
     The fuel cell system may include one or more temperature sensors configured to detect at least one of a temperature of the reforming catalyst and a temperature of the carbon monoxide decreasing catalyst, wherein the amount of the desorbed raw gas, which is desorbed out of the components of the raw gas adsorbed to at least one of the reforming catalyst and the carbon monoxide decreasing catalyst and which is to be burnt in the burner unit, may be estimated at the start-up operation of the fuel cell system based on at least one of the temperature of the reforming catalyst and the temperature of the carbon monoxide catalyst detected by the temperature sensor. With this configuration, it is possible to perform stable combustion in the burner unit by estimating the amount of desorbed raw gas with relatively superior accuracy and by a simple configuration. 
     The fuel cell system may include timer means for measuring a time elapsed since the heating means starts the heating operation, wherein the amount of the desorbed raw gas, which is desorbed out of the components of the raw gas adsorbed to at least one of the reforming catalyst and the carbon monoxide decreasing catalyst and which is to be burnt in the burner unit, may be estimated based on the time measured by the timer means. With this configuration the amount of the desorbed raw gas can be detected by a simple configuration. 
     In the fuel cell system, the heating means may start the heating operation for at least one of the reforming catalyst and the carbon monoxide decreasing catalyst at the start-up operation of the fuel cell system, and the raw gas supply device may be activated after elapse of a predetermined time since the heating means starts the heating operation. 
     With this configuration, during a time period until the raw gas supply device starts to operate, the raw gas components desorbed by heat-up of catalysts can be burnt before the raw gas supplied from the raw gas supply device is burnt. 
     The fuel cell system may include one or more temperature sensors configured to detect at least one of a temperature of the reforming catalyst and a temperature of the carbon monoxide decreasing catalyst, wherein the heating means may start the heating operation for at least one of the reforming catalyst and the carbon monoxide decreasing catalyst at the start-up operation of the fuel cell system, and wherein the raw gas supply device may be activated after the temperature detected by the temperature sensor becomes a predetermined temperature or more. 
     With this configuration, it is possible to gently decrease a rate of temperature rise in the catalysts with combustion of a small amount of raw gas without decreasing the amount of the raw gas supplied by the raw gas supply device. Further, even when the amount of the raw gas adsorbed to the catalysts at the start-up operation of the fuel cell system is smaller than the estimated amount and even when a flame vanish occurs because the amount of the desorbed raw gas is smaller than the estimated amount in an estimated time period in which the burner unit burns only the desorbed raw gas, the flame vanish can be detected by the rate of temperature rise in the catalysts detected by the temperature sensor. 
     In the fuel cell system the raw gas supply device and the heating means may be activated at the start-up operation of the fuel cell system, after the raw gas supply device supplies the raw gas for a predetermined time period, the raw gas supply device may be deactivated, and after elapse of a predetermined time period since the raw gas supply device is deactivated, the raw gas supply device may be activated. 
     With this configuration, a predetermined amount of the raw gas can be supplied to the burner unit from the raw gas supply device regardless of the amount of the desorbed raw gas desorbed from the catalysts. Consequently, the burner unit can perform stable ignition. 
     The fuel cell system may include one or more temperature sensors configured to detect at least one of a temperature of the reforming catalyst and a temperature of the carbon monoxide catalyst, wherein at the start-up operation of the fuel cell system, the raw gas supply device and the heating means may be activated, wherein after elapse of a predetermined time period since the raw gas supply device starts to supply the raw gas or after the temperature sensor detects a first predetermined temperature or more, the raw gas supply device may be deactivated, and wherein after the temperature sensor detects a second predetermined temperature or more, the raw gas supply device may be activated. 
     With this configuration, even when the amount of the desorbed raw gas desorbed from the catalysts is very little because of insufficient heating of the catalysts, a predetermined amount of raw gas can be supplied to the burner unit by the raw gas supply device. Consequently, the burner unit can perform stable ignition. 
     In The fuel cell system, the heating means may include an electric heater. With this configuration, it is possible to relatively easily control start and end of the heating operation for the catalysts. Since the electric heater can easily control a heating rate thereof by on/off control and input voltage control, it is possible to easily control the rate of temperature rise of the catalysts. As a result, by decreasing the rate of temperature rise, it is possible to prevent deterioration of the catalysts caused by sudden temperature changes. Conversely, by increasing the rate of temperature rise, it is possible to shorten the start-up time and adjust the amount of the desorbed raw gas desorbed from the catalysts. 
     In the fuel cell system, the heating means may include a heating burner unit, and at least one of the reforming unit and the carbon monoxide decreasing unit may be heated by a combustion exhaust gas of the heating burner unit. It is thereby possible to effectively utilize heat resultant of combustion in the heating burner unit for heating the catalysts. With this configuration, it is possible to implement a fuel cell system having a start-up characteristic with superior energy efficiency. 
     In the fuel cell system, the burner unit may be used as the heating burner unit. With this configuration, it is possible to reduce the number of the burner unit to one. Further, pipes used for supplying a raw gas and combustion air to the burner unit can also be simplified. As a result, the burner unit serving as heating means can be realized with a simple configuration. Hence, it is possible to prevent an increase in size and complexity of the fuel cell system. 
     Embodiments of the present invention are now described by reference to the drawings. Elements similar to those in the above-described embodiment are assigned the same reference characters, and the detailed description thereof is omitted. The present invention shall not be confined to the embodiments. 
     (First Embodiment) 
       FIG. 1  is a schematic diagram showing a configuration of a fuel cell system of a first embodiment of the present invention. 
     As shown in  FIG. 1 , a fuel cell system  1  of the present embodiment includes at least a fuel cell  2 , a fuel processor  3 , a steam generator  4 , a control unit  5 , a raw gas supply device  6 , a combustion air fan  7 , and a cathode air blower  8 . The combustion air fan  7  serves as an air blower configured to supply combustion air to a burner unit  12  described later. An LPG is utilized at this time as a raw gas to be supplied to the fuel cell system  1 , and an LPG cylinder (not shown) is connected to the raw gas supply device  6  outside the fuel cell system  1 . Another hydrocarbon-based material, such as a town gas, can also be used as a raw gas. A sulfur compound added to a hydrocarbon-based material as an odorant is eliminated when the raw gas passes through a desulfurizing unit (not shown) placed at an upstream of the raw gas supply device  6  or between the raw gas supply device  6  and the fuel processor  3 . 
     The fuel cell  2  has a structure in which a solid polyelectrolyte membrane  2   a  is sandwiched between an anode electrode  2   b  and a cathode electrode  2   c.  The anode electrode  2   b  is supplied with a reformed gas including a large amount of hydrogen generated by the fuel processor  3 , and the cathode electrode  2   c  is supplied with air by means of the cathode air blower  8 , whereby electric power is generated. 
     The fuel processor  3  has a configuration in which a reforming unit  9 , a shift unit  10 , and a selective oxidation unit  11  both of which make up a carbon monoxide decreasing unit are connected in sequence. The reforming unit  9  is filled with a reforming catalyst  9   a;  the shift unit  10  is filled with a shift catalyst  10   a;  and the selective oxidation unit  11  filled with a selective oxidation catalyst  11   a.    
     The reforming unit  9  is provided with the burner unit  12 . Hydrogen in the raw gas and hydrogen in an off-gas including hydrogen which was not used for power generation by the fuel cell  2  are burnt along with the air blown to the burner unit  12  by the combustion air fan  7  by way of a combustion air channel  7   a.  A resultant gas is emitted as a combustion exhaust gas to the outside of the fuel cell system  1  by way of a combustion exhaust gas channel  13 . The combustion exhaust gas channel  13  is laid so as to assume a configuration in which the channel runs from the burner unit  12  through an interior space of the reforming unit  9 , an interior space of the shift unit  10 , and an interior space of the selective oxidation unit  11  that are partitioned so as not to directly contact the reforming catalyst  9   a,  the shift catalyst  10   a,  and the selective oxidation catalyst  11   a.    
     The raw gas supplied by the raw gas supply device  6 , by way of a raw gas supply channel  14 , into the space filled with the reforming catalyst  9   a  of the reforming unit  9  first passes through a layer filled with the reforming catalyst  9   a.  Next, the raw gas exits from the reforming unit  9  and is supplied to a space filled with the shift catalyst  10   a  of the shift unit  10 , thereupon passing through a layer filled with the shift catalyst  10   a.  The raw gas exits from the shift unit  10  and is supplied to the space filled with the selective oxidation catalyst  11   a  of the selective oxidation unit  11 , thereupon passing through a layer filled with the selective oxidation catalyst  11   a.  Subsequently, the raw gas is circulated so as to exit from the selective oxidation unit  11  to a raw gas channel  15  by way of the raw gas supply channel  14  and the raw gas channel  15 . 
     The raw gas channel  15  has a structure such that the channel is bifurcated at an upstream of an entrance of the anode electrode  2   b  of the fuel cell  2  into a fuel cell entrance channel  15   a  and a fuel cell bypass channel  15   b.  The fuel cell entrance channel  15   a  is connected to the entrance of the anode electrode  2   b  of the fuel cell  2 , and an off-gas channel  15   c  is connected to an exit of the anode electrode  2   b.  A side of the off-gas channel  15   c  that is not connected to the fuel cell  2  is connected to the fuel cell bypass channel  15   b  of the raw gas channel  15 . Moreover, the fuel cell bypass channel  15   b  is provided with a fuel cell bypass valve  16 ; the fuel cell entrance channel  15   a  is provided with a fuel cell entrance valve  17 ; and the off-gas channel  15   c  is provided with a fuel cell exit valve  18 . The control unit  5  controls opening and closing of the fuel cell bypass valve  16 , the fuel cell entrance valve  17 , and the fuel cell exit valve  18 , thereby switching the raw gas channel  15  between the fuel cell entrance channel  15   a  and the fuel cell bypass channel  15   b.  Further, the raw gas channel  15  is connected to the burner unit  12  at a downstream position where the fuel cell bypass channel  15   b  and the off-gas channel  15   c  are connected to each other. 
     Further, the raw gas supply channel  14  is connected, as required, to a steam channel  4   a  for supplying steam from the steam generator  4  to the reforming unit  9  while the steam is mixed in the raw gas. 
     The fuel processor  3  used in the fuel cell system of the first embodiment of the present invention is hereunder described by reference to  FIG. 2 . 
       FIG. 2  is a cross sectional view of a main portion for explaining the fuel processor  3  used in the fuel cell system  1  of the first embodiment of the present invention. 
     As shown in  FIG. 2 , the fuel processor  3  has a first cylindrical body  20 , a second cylindrical body  21 , a third cylindrical body  22 , and a fourth cylindrical body  23  that are arranged in sequence from inside and in a substantially concentric pattern. The combustion exhaust gas channel  13  is formed from a space between the first cylindrical body  20  and the second cylindrical body  21 ; a first gas flow channel  24  is formed into an annular shape within a space between the second cylindrical body  21  and the third cylindrical body  22 ; and a second gas flow channel  25  is formed from a space between the third cylindrical body  22  and the fourth cylindrical body  23 . Moreover, disposed in an interior space of the first cylindrical body  20  are the burner unit  12 , the raw gas channel  15  for supplying the burner unit  12  with a raw gas and an off-gas, the combustion air channel  7   a,  and a combustion chamber  26 . The combustion chamber  26  and the combustion exhaust gas channel  13  are in mutual communication with each other by way of an exhaust return portion  27  placed in the vicinity of an end of the combustion chamber. Likewise, the first gas flow channel  24  and the second gas flow channel  25  are in mutual communication with each other by way of a raw gas return portion  28  disposed in the vicinity of an end of the first gas flow channel  24  and the second gas flow channel  25 . 
     The first gas flow channel  24  is provided with the reforming unit  9  filled, for example, with the spherical reforming catalyst  9   a  that contains metal ruthenium carried by an alumina carrier. On top of the metal ruthenium, a nickel catalyst, a platinum-based catalyst, a platinum-group catalyst, such as rhodium, etc., can also be used as the reforming catalyst  9   a.  Further, on top of the spherical shape, another shape, such as a cylindrical shape, can also be used as a shape of the reforming catalyst  9   a.    
     The second gas flow channel  25  is also provided with the shift unit  10  filled with a shift catalyst  10   a,  for example, a Cu—Zn-based shift catalyst (MDC-7 manufactured by Sud-chemie Co., Ltd. in particular), and the selective oxidation unit  11  filled, for example, with a spherical ruthenium-based selective oxidation catalyst  11   a.  A platinum-based catalyst except for the above-described catalysts can also be used as the shift catalyst  10   a  and the selective oxidation catalyst  11   a.    
     The selective oxidation unit  11  is connected to a selective oxidation air supply channel  11   b  for letting the selective oxidation catalyst  11   a  effect oxidation reaction. Moreover, the raw gas supply channel  14  is connected to an upper portion of the first gas flow channel  24 . Although the embodiment has been described by reference to an example that uses, as the selective oxidation unit  11 , a selective oxidation unit  11  that decreases a concentration of carbon monoxide by means of selective oxidation reaction, there may also be adopted a method for decreasing the concentration of carbon monoxide, for example, by a methanation reaction. 
     The shift unit  10  is provided with a shift temperature sensor  10   b  for measuring a temperature of the shift catalyst  10   a.  The shift temperature sensor  10   b  can be used at the start-up operation of the fuel cell system  1  also for a case where the control unit  5  estimates an amount of desorbed raw gas desorbed from the raw gas adsorbed to the shift catalyst  10   a,  according to a detected temperature of the shift catalyst  10   a.    
     An exterior of the fuel processor  3  is covered with an unillustrated heat insulator. Thus, the heat insulator is provided so as to accomplish a temperature balance to be described later, in consideration of heat insulation of units. 
     By reference to  FIG. 3 , the following is provided for explaining the point of the embodiment; namely, a relationship between the temperature of the shift catalyst  10   a  of the shift unit  10  of the fuel cell system  1  and an amount of desorbed raw gas desorbed from the shift catalyst  10   a.    
       FIG. 3  is a view showing a relationship between a temperature of the shift catalyst  10   a  of the shift unit  10  of the fuel cell system  1  of the first embodiment of the present invention and an amount of desorbed raw gas desorbed from the shift catalyst  10   a.    
       FIG. 3  shows a test result obtained by: using a fuel processor  3  which is experimentally formed such that only the shift unit  10  is filled with the shift catalyst  10   a;  supplying a given amount of LPG from the raw gas supply device  6  directly to the burner unit  12  and burning the LPG along with a predetermined combustion air, thereby heating the shift catalyst  10   a  by a resultant combustion exhaust gas; and measuring, with time, the temperature of the shift catalyst  10   a  and the amount of LPG as a desorbed raw gas desorbed from the shift catalyst  10   a.  The test result corresponds to a measurement result. As used herein, the LPG as a desorbed raw gas is referred to as a “desorbed LPG” as necessary. 
     A horizontal axis shown in  FIG. 3  represents a time elapsed since heating of the shift catalyst  10   a  was started. A first vertical axis represents an amount of an desorbed LPG, and a second vertical axis represents a temperature of the shift catalyst  10   a.  A broken line shown in  FIG. 3  represents the amount of the desorbed LPG desorbed from the shift catalyst  10   a,  and a solid line represents the temperature of the shift catalyst  10   a.  In  FIG. 3 , the amount of LPG supplied by the raw gas supply device  6  is designated by a one-dot broken line for comparison with the amount of desorbed LPG desorbed from the shift catalyst  10   a.    
     As shown in  FIG. 3 , when heating of the shift catalyst  10   a  is started, the temperature of the shift catalyst  10   a  that was nearly equal to an outside air temperature; namely, about 25° C., gradually increases at a substantially given rate. With a temperature rise of the shift catalyst  10   a,  an LPG, which will be a desorbed LPG, begins to be desorbed from the shift catalyst  10   a,  and the amount of desorbed LPG keeps increasing up to a temperature of about 60° C. Subsequently, the amount of desorbed LPG gradually decreases during a course of the temperature of the shift catalyst  10   a  increasing up to a temperature of about 180° C. However, the LPG is kept being discharged. 
     For this reason, according to the test result shown in  FIG. 3 , a correlation between the temperature of the shift catalyst  10   a  and the amount of desorbed LPG desorbed from the shift catalyst  10   a  is stored in the form of a calculation formula in the control unit  5 . The amount of desorbed LPG is estimated from the temperature of the shift catalyst  10   a  detected by the shift temperature sensor  10   b.    
     The amount of LPG adsorbed to the shift catalyst  10   a  changes according to the amount of shift catalyst  10   a  filled and the shape and type of the shift catalyst  10   a.  Even when a hydrocarbon-based raw gas, like a city gas, is used, an amount of gas adsorbed to the shift catalyst  10   a  varies. Further, since the shift temperature sensor  10   b  measures a portion of the temperature of the shift catalyst  10   a,  the temperature and the amount of desorbed raw gas varies according to the position of the shift temperature sensor  10   b.  In order to address the change, it is important to previously determine the relationship between the temperature of the shift catalyst  10   a  and the amount of desorbed raw gas desorbed from the shift catalyst  10   a  and to store the thus-re-measured relationship in the control unit  5 . 
     As in the case of the shift catalyst  10   a,  it is possible to measure the amount of desorbed LPG desorbed from the reforming catalyst  9   a  and the selective oxidation catalyst  11   a  by means of heating the reforming catalyst  9   a  or the selective oxidation catalyst  11   a.  The amount of desorbed LPG desorbed from the reforming catalyst  9   a  and the selective oxidation catalyst  11   a  can be estimated by means of previously storing in the control unit the relationship among the temperature of the reforming catalyst  9   a,  the temperature of the selective oxidation catalyst  11   a,  and the amount of desorbed LPG, according to the measurement result. However, very little LPG serving as a raw gas is adsorbed to the reforming catalyst  9   a  or the selective oxidation catalyst  11   a  used in the embodiment. Further, the amount of desorbed LPG desorbed with the temperature rise is sufficiently smaller than the amount of LPG supplied from the raw gas supply device  6 . For these reasons, control is performed while taking as a principal parameter the amount of desorbed raw gas desorbed from the shift catalyst  10   a.    
     Operation and an advantage of the fuel cell system  1  having the foregoing configuration are hereunder described by reference to  FIGS. 1 and 2 . 
     Operation of the fuel cell system  1  performed during normal power generation is first described. 
     First, the raw gas supply device  6  supplies an LPG that is a raw gas mixed with the steam supplied from the steam generator  4 , to the reforming unit  9  in which the temperature of the reforming catalyst  9   a  is maintained at about 700° C. A reformed gas including hydrogen, carbon monoxide, and steam is generated by means of steam reforming reaction and by way of the reforming catalyst  9   a  of the reforming unit  9 . 
     Next, the reformed gas exited from the reforming unit  9  is supplied to the layer filled with the shift catalyst  10   a  of the shift unit  10  and is circulated through the filled layer from down to up. By virtue of action of the shift catalyst  10   a  maintained at about 200 to 300° C., shift reaction for generating carbon dioxide and hydrogen from the carbon monoxide and steam included in the reformed gas takes place at this time. The concentration of carbon monoxide in the reformed gas is thereby decreased to about 0.5% or less. 
     The reformed gas exited from the shift unit  10  is separately supplied to the layer filled with the selective oxidation catalyst  11   a  of the selective oxidation unit  11  along with a small amount of air supplied by way of the selective oxidation air supply channel  11   b,  and the reformed gas is circulated through the filled layer from down to up. By virtue of action of the selective oxidation catalyst  11   a  maintained at about 100 to 200° C., the carbon monoxide in the reformed gas is subjected to oxidation at this time. The concentration of carbon monoxide in the reformed gas is further decreased to 10 ppm or less. 
     Next, the reformed gas exited from the selective oxidation unit  11  passes through the raw gas channel  15  in which the fuel cell bypass valve  16  is closed and in which the fuel cell entrance valve  17  and the fuel cell exit valve  18  are open, to thus be supplied toward the anode electrode  2   b  of the fuel cell  2 . The off-gas exhausted as a result of a portion of hydrogen in the reformed gas being consumed by power generation and the air supplied by the combustion air fan  7  are supplied to the burner unit  12 , and the gas and the air are burnt by the burner unit  12 . The reforming catalyst  9   a  is heated by combustion heat of the off-gas and maintained at about 700° C. The combustion exhaust gas circulates through the inside channel in the shift unit  10  and the selective oxidation unit  11 , whereby the shift catalyst  10   a  is maintained at about 200 to 300° C., and the selective oxidation catalyst  11   a  is maintained at 150 to 200° C. 
     In the meantime, air is supplied toward the cathode electrode  2   c  of the fuel cell  2  by means of the cathode air blower  8 . A portion of oxygen in the air is consumed by power generation, and exhaust air is caused to exit outside from the fuel cell system  1 . 
     Electric power generated by the fuel cell  2  is consumed by an unillustrated external load. 
     Operation of the fuel cell system  1  performed at the time of power-generation halt operation is hereunder described. 
     First, after a connection between the fuel cell  2  and the external load is disconnected, supplying air to the fuel cell  2  by the cathode air blower  8  and supplying the raw gas by the raw gas supply device  6  are stopped. Concurrently, the fuel cell bypass valve  16 , the fuel cell entrance valve  17 , and the fuel cell exit valve  18  are closed. The steam generator  4  is also simultaneously stopped at this time, whereby supplying steam to the raw gas supply channel  14  by way of the steam channel  4   a  is also stopped. 
     Next, when the temperature of the reforming catalyst  9   a  is cooled to about 300° C. by outside air after elapse of a predetermined period of time, the fuel cell bypass valve  16  is opened, whereby the raw gas supply device  6  sequentially supplies an LPG, which is the raw gas, to the reforming unit  9 , the shift unit  10 , and the selective oxidation unit  11 . The reformed gas containing steam in the respective layers filled with the respective catalysts is thereby purged. Since the temperature of the shift catalyst  10   a  and the temperature of the selective oxidation catalyst  11   a  are 100° C. or more at this time, the steam in the reformed gas will not condense on surfaces of the respective catalysts. Subsequently, after the LPG has been supplied for a predetermined period of time that makes it possible to sufficiently purge the reformed gas containing steam from the respective layers filled with the respective catalysts by means of the LPG, the fuel cell bypass valve  16  is closed, thereby stopping the raw gas supply device  6 . 
     Further, when the fuel processor  3  is cooled as a result of having dissipated heat for reasons of a difference between the temperature of the fuel processor and a temperature of an ambient atmosphere, internal pressure of the fuel processor  3  decreases for reasons of volume shrinkage of the LPG heated in the fuel processor  3 . However, the raw gas supply device  6  supplies the LPG at predetermined times, whereupon the internal pressure of the fuel processor  3  is maintained at positive pressure. Operation for supplying the fuel processor  3  with an LPG to maintain positive pressure is hereunder described as pressure keeping operation, as required. 
     By means of purging and pressure keeping operations, the LPG is adsorbed to the shift catalyst  10   a  whose temperature dropped sufficiently. Since in reality the town gas and the LPG are a mixed gas containing hydrocarbon-based gas components, such as methane or propane, ease of adsorption to the catalyst changes according to the respective hydrocarbon-based gases. Specifically, a hydrocarbon-based gas component of higher molecular weight tends to be adsorbed more easily than does a hydrocarbon-based gas component of smaller molecular weight. However, since propane serving as a major component accounts for about 98% of the composition of the LPG employed in the present embodiment, there will arise no problem even when the LPG is handled as a gas substantially containing a single component, or propane. Accordingly, a component of the raw gas adsorbed to the shift catalyst  10   a  is also described as a raw gas. 
     The start-up operation of the fuel cell system  1  of the present embodiment is hereunder described by reference to the drawings. 
       FIG. 4  is a flowchart showing control flow of a main portion at the start-up operation of the fuel cell system  1  of the first embodiment of the present invention. 
     As shown in  FIG. 4 , at the start-up operation of the fuel cell system  1 , the fuel cell bypass valve  16  is first opened, to thus supply the reforming unit  9  with a predetermined amount of LPG that is a raw gas (step S 1 ). Subsequently, the LPG sequentially circulates through the shift unit  10  and the selective oxidation unit  11  and is supplied to the burner unit  12  by way of the fuel cell bypass channel  15   b  of the raw gas channel  15 . Concurrently, the combustion air fan  7  sends the burner unit  12  with an amount of air in accordance with the flow amount of LPG supplied by the raw gas supply device  6  (step S 1 ), and the LPG is ignited by an igniter not shown (step S 2 ). 
     When the burner unit  12  starts burning the LPG, the reforming catalyst  9   a  of the reforming unit  9  is heated by resultant combustion heat, whereupon the temperature (ThK) of the reforming catalyst  9   a  increases. Concurrently, a combustion exhaust gas sequentially passes through the combustion exhaust gas channel  13  in the shift unit  10  and the selective oxidation unit  11 , whereby the shift catalyst  10   a  and the selective oxidation catalyst  11   a  are heated. The temperature (ThH) of the shift catalyst  10   a  and a temperature (ThS) of the selective oxidation catalyst  11   a  are thereby increased. 
     Next, the shift temperature sensor  10   b  detects the temperature (ThH) of the shift catalyst  10   a,  thereby comparing the thus-detected temperature with a predetermined temperature (Th 1 ) (step S 3 ). At this time, when the temperature (ThH) of the shift catalyst  10   a  is lower than the predetermined temperature (Th 1 ) (No in step S 3 ), the amount of LPG which will be desorbed to become a desorbed raw gas, among the LPG adsorbed to the shift catalyst  10   a,  is estimated from the information stored in the control unit  5  (step S 4 ). 
     A total amount of the LPG supplied to the burner unit  12  by the raw gas supply device  6  and the amount of desorbed LPG that is a desorbed raw gas which will be desorbed by heat-up of the catalysts, to thus be supplied to the burner unit  12 , is adjusted so as to assume an appropriate ratio with respect to the amount of combustion air supplied to the burner unit  12  by the combustion air fan  7 . Specifically, the amount of raw gas supplied from the raw gas supply device  6  to the burner unit is adjusted according to the amount of desorbed LPG in such a way that the ratio of the amount of raw gas to the amount of combustion air comes to a predetermined amount (step S 5 ). 
     An appropriate ratio of the amount of LPG supplied to the burner unit  12  (or the amount of reformed gas or the amount of off-gas) to the amount of air is a predetermined multiple of the amount of air including the amount of oxygen required to completely burn the LPG (or the reformed gas or off-gas) supplied to the burner unit, to thus split the LPG into carbon dioxide and water (steam). A ratio of the amount of air to the amount of LPG is hereunder described as an air-fuel ratio. Specifically, for example, a air-fuel ratio of 1.0 is a ratio at which, when oxygen in a supplied air and LPG are ideally burnt, the LPG is split into carbon dioxide and water in just proportion. For example, an air-fuel ratio of 2.0 is a ratio at which one-half of the oxygen in the supplied air is unused for combustion and still remains in a combustion exhaust gas. 
     In the fuel cell system  1  of the present embodiment, the air-fuel ratio in steps S 1  and S 5  is set to 2.5 to 3.0. If the air-fuel ratio is too large or small, ignitability will be deteriorated. Even if the LPG is ignited, incomplete combustion will take place, thereby producing carbon monoxide. The carbon monoxide will be discharged outside the fuel cell system  1  as a combustion exhaust gas, which will cause a flame vanish. 
     Adjustment of the amount of raw gas by means of the raw gas supply device  6  according to the temperature detected by the shift temperature sensor  10   b  (step  5 ) is continually performed until the temperature (ThH) of the shift catalyst  10   a  comes to the predetermined temperature (Th 1 ) or more (Yes in step S 3 ), the temperature (ThK) of the reforming catalyst  9   a  comes to 200° C. or more, the temperature (ThH) of the shift catalyst  10   a  comes to 180° C. or more, and the temperature (ThS) of the selective oxidation catalyst  11   a  comes to 150° C. or more (Yes in step S 6 ). 
     Next, when the temperature (ThK) of the reforming catalyst  9   a  comes to 200° C. or more, when the temperature (ThH) of the shift catalyst  10   a  comes to 180° C. or more, and when the temperature (ThS) of the selective oxidation catalyst  11   a  comes to 150° C. or more (Yes in step S 6 ), the steam generating device  4  starts supplying steam to the raw gas supply channel  14  by way of the steam channel  4   a.  A mixed gas containing an LPG and steam is supplied to the layer filled with the reforming catalyst  9   a  of the reforming unit  9 , whereupon reforming the LPG into hydrogen is started (step S 7 ). The amount of raw gas is adjusted such that the air-fuel ratio comes to 1.5 to 2.0. If steam is supplied when none of the temperature of the reforming catalyst  9   a,  the temperature of the shift catalyst  10   a,  and the temperature of the selective oxidation catalyst  11   a  exceed 100° C., the steam will condense, which may hinder a gas flow. For this reason, the requirements to select Yes in step S 6 ; namely, the temperature (ThK) of the reforming catalyst  9   a  being 200° C. or more, the temperature (ThH) of the shift catalyst  10   a  being 180° or more, and the temperature (ThS) of the selective oxidation catalyst  11   a  being 150° C. or more, correspond to a temperature of a temperature sensor at which temperatures of all of the catalysts having temperature distributions come to 100° C. or more. Moreover, if steam is not supplied when the temperature of the reforming catalyst  9   a  exceeded 400° C., carbon will be deposited on the surface of the reforming catalyst, which may deteriorate the function of the catalyst. For this reason, there is a necessity for a design consideration such that the temperature (ThH) of the shift catalyst  10   a  and the temperature (ThS) of the selective oxidation catalyst  11   a  exceed 100° C. before the temperature (ThK) of the reforming catalyst  9   a  exceeds 400° C. 
     A determination is now made as to whether or not the respective catalysts are at respective predetermined temperatures; namely, whether or not the reforming catalyst  9   a  falls in a temperature range from about 600 to 700° C.; whether or not the shift catalyst  10   a  falls within a temperature range from about 200 to 300° C., and whether or not the selective oxidation catalyst  11   a  falls within a temperature range from about 150 to 200° C. (step S 8 ). When the temperatures of the respective catalysts are lower than their respective temperature ranges (No in step S 8 ), reactions of the respective catalysts are insufficient. Namely, since the amount of hydrogen in the reformed gas is small and since the amount of carbon monoxide is large, the reformed gas in middle of heat-up is caused to circulate through the fuel cell bypass channel  15   b.  When the temperatures of the respective catalysts have increased to respective predetermined temperature ranges (Yes in step S 8 ), the fuel cell bypass valve  16  is closed after the components of the reformed gas have become stable. Concurrently, the reformed gas is supplied to the anode electrode  2   b  of the fuel cell  2  by opening the fuel cell entrance valve  17  and the fuel cell exit valve  18 , and power generation is started. When the temperatures of the respective catalysts are outside the respective temperature ranges (No in step S 8 ), control similar to that performed in the related art is performed, and processing waits until the temperatures fall in the respective temperature ranges. 
     As mentioned above, at the start-up operation, the fuel cell system  1  of the first embodiment estimates desorption of raw gas due to the temperature rise of the shift catalyst according to the temperature of the shift catalyst  10   a  detected by the shift temperature sensor  10   b  and adjusts the amount of raw gas supplied from the raw gas supply device  6  according to the thus-estimated value. A ratio of combustion air supplied from the combustion air fan to the raw gas is thereby controlled so as to assume a predetermined air-fuel ratio, whereby the combustion air and the raw gas can be supplied to and burnt in the burner unit  12 . As a consequence, occurrence of incomplete combustion is prevented, to thus effect stable combustion, and the start-up operation of the fuel cell system  1  can be performed reliably. 
     The fuel cell system  1  of the present embodiment has been described by reference to the example in which the amount of desorbed LPG desorbed from the shift catalyst  10   a  at the start-up of the fuel cell system  1  is estimated from a value of the shift temperature sensor  10   b  that detects the temperature of the shift catalyst  10   a.  However, the fuel cell system is not limited to such an example. For example, as shown in  FIG. 5 , the temperature of the shift catalyst  10   a  is estimated by means of a timer that measures a time (t 1 ) elapsed since heating was started. The amount of desorbed LPG desorbed from the shift catalyst  10   a  can also be further estimated from the thus-estimated temperature. A method for assuming the amount of LPG is hereunder described specifically by reference to  FIG. 5 . 
       FIG. 5  is a diagram showing how the temperature of the reforming catalyst  9   a,  the temperature of the shift catalyst  10   a,  and the temperature of the selective oxidation catalyst  11   a  increase at the start-up of the fuel cell system  1  of the embodiment. 
     As shown in  FIG. 5 , the temperature (ThK) of the reforming catalyst  9   a,  the temperature (ThH) of the shift catalyst  10   a,  and the temperature (ThS) of the selective oxidation catalyst  11   a  are determined from the time (t 1 ) elapsed since heating of the reforming catalyst  9   a,  the shift catalyst  10   a,  and the selective oxidation catalyst  11   a  was initiated. Therefore, at the start-up of the fuel cell system, the timer first measures the time (t 1 ) elapsed since heating was started, and the temperature of the shift catalyst  10   a  achieved at that time is estimated. The amount of desorbed LPG desorbed from the shift catalyst  10   a  is estimated according to the temperature of the shift catalyst  10   a.  Further, the control unit  5  adjusts and controls the amount of raw gas on the basis of the estimation result. In this case, although it is necessary to obtain the temperature (ThH) of the shift catalyst  10   a  at the start of the heating operation, a temperature sensor configured to measure the temperature of the reforming catalyst  9   a  or a temperature sensor configured to measure the temperature of the selective oxidation catalyst  11   a  can substitute for the use of measuring the temperature (ThH). 
     The fuel cell system  1  of the present embodiment has been described by means of the example in which there is provided the shift temperature sensor  10   b  for detecting the temperature (ThH) of the shift catalyst  10   a.  There are no descriptions about means for detecting the temperature (ThK) of the reforming catalyst  9   a  and the temperature (ThS) of the selective oxidation catalyst  11   a.  However, a reform temperature sensor for detecting the temperature of the reforming catalyst  9   a  and a selective oxidation temperature sensor for detecting the temperature of the selective oxidation catalyst  11   a  can also be separately provided. Alternatively, the temperature of the reforming catalyst  9   a  and the temperature of the selective oxidation catalyst  11   a  can also be detected from the temperature of the shift catalyst  10   a  detected by the shift temperature sensor  10   b.    
     The fuel cell system  1  of the present embodiment has been described by means of an example in which the amount of raw gas supplied from the raw gas supply device  6  is adjusted by means of increasing and decreasing operations. However, the adjustment of the amount of describe raw gas is not limited to the example. For example, a total of the amount of desorbed raw gas desorbed from the catalysts and the amount of raw gas supplied from the raw gas supply device  6  can also be adjusted by means of increasing and decreasing operations. It thereby becomes possible to change the rate of temperature rise in each of the catalysts. Specifically, when the total amount is decreased, the rate of temperature rise in each of the catalysts is decreased, thereby preventing occurrence of a temperature distribution in each of the catalysts, which would otherwise arise during heat-up operation. Thus, efficiency can be enhanced by average reaction. Moreover, it is possible to prevent a decrease in durability of a structure making up the fuel processor  3  or durability of the respective catalysts, which would otherwise be caused by a sudden temperature change. Conversely, when the total amount is increased, it is possible to increase the rate of temperature rise in each of the catalysts. Hence, a start-up time of the fuel cell system can be shortened. 
     Another start-up operation method for the fuel cell system  1  of the first embodiment of the present invention is hereunder described by reference to  FIG. 6 . 
       FIG. 6  is a flowchart for describing control flow of the main portion under another start-up operation method for the fuel cell system  1  of the first embodiment of the present invention. 
     As shown in  FIG. 6 , the fuel cell bypass valve  16  is first opened at start-up operation of the fuel cell system  1 , thereby activating the raw gas supply device  6  and the combustion air fan  7  and supplying the LPG, which is the raw gas, and air (step S 1 ). The LPG and the combustion air supplied to the burner unit  12  are ignited by the unillustrated igniter, whereby the burner unit starts burning the LPG (step S 2 ). 
     The temperature (ThH) of the shift catalyst  10   a  and a predetermined temperature (Th 1 ) are now compared to each other (step S 3 ). When the temperature (ThH) of the shift catalyst  10   a  exceeds the predetermined temperature (Th 1 ) (Yes in step S 3 ), the raw gas supply device  6  is deactivated (step S 4 ). Although supplying LPG that is a raw gas from the raw gas supply device  6  to the burner unit  12  is suspended at this time, the desorbed LPG desorbed from the shift catalyst  10   a  is supplied to the burner unit  12 ; hence, combustion still remains continued. 
     The temperature (ThH) of the shift catalyst  10   a  and a predetermined temperature (Th 2 ) are now compared to each other (step S 5 ). When the temperature (ThH) of the shift catalyst  10   a  is lower than the predetermined temperature (Th 2 ) (No in step S 5 ), the control unit  5  estimates an amount of desorbed LPG, among the LPG adsorbed to the shift catalyst  10   a  (step S 6 ). The amount of combustion air supplied to the burner unit  12  by the combustion air fan is adjusted so as to assume a flow amount in accordance with the amount of desorbed LPG desorbed from the shift catalyst  10   a  (step S 7 ). 
     Moreover, when the temperature (ThH) of the shift catalyst  10   a  exceeds the predetermined temperature (Th 2 ) (Yes in step S 5 ), the raw gas supply device  6  is re-activated, thereby supplying the LPG from the raw gas supply device  6  to the burner unit  12  (step S 8 ). The amount of combustion air supplied to the burner unit  12  by the combustion air fan  7  and the amount of LPG supplied by the raw gas supply device  6  are adjusted at this time according to the amount of desorbed LPG (step S 9 ). 
     In this case, adjustment of the amount of LPG that is a raw gas supplied by the raw gas supply device  6  according to the temperature detected by the shift temperature sensor  10   b  (step S 9 ) is continually performed until the temperature (ThK) of the reforming catalyst  9   a  comes to 200° C. or more, the temperature (ThH) of the shift catalyst  10   a  comes to 180° C. or more, and the temperature (ThS) of the selective oxidation catalyst  11   a  comes to 150° C. or more (Yes in step S 10 ). 
     When the temperature (ThK) of the reforming catalyst  9   a  has come to 200° C. or more, when the temperature (ThH) of the shift catalyst  10   a  has come to 180° C. or more, and when the temperature (ThS) of the selective oxidation catalyst  11   a  has come to 150° C. or more (Yes in step S 10 ), the steam generating device  4  starts supplying steam to the raw gas supply channel  14  by way of the steam channel  4   a.  The mixed gas containing the LPG and steam is supplied to the layer filled with the reforming catalyst  9   a  of the reforming unit  9 , whereupon reforming the LPG to hydrogen is started (step S 11 ). The amount of LPG that is a raw gas to be supplied from the gas supply device  6  is adjusted in such a way that the air-fuel ratio comes to 1.5 to 2.0. 
     A determination is now made as to whether or not the respective catalysts are at respective predetermined temperatures; namely, whether or not the reforming catalyst  9   a  stays in a temperature range from about 600 to 700° C.; whether or not the shift catalyst  10   a  stays in a temperature range from about 200 to 300° C.; and whether or not the selective oxidation catalyst  11   a  stays in a temperature range from about 150 to 200° C. (step S 12 ). When the temperatures of the respective catalysts are lower than their respective temperature ranges (No in step S 12 ), reactions occurred in the respective catalysts are insufficient. Namely, the amount of hydrogen in the reformed gas is small, and the amount of carbon monoxide in the gas is large. Therefore, the reformed gas in the course of heat-up operation is caused to circulate through the fuel cell bypass channel  15   b.  When the temperatures of the respective catalysts have increased to their predetermined temperature ranges (Yes in step S 12 ), the fuel cell bypass valve  16  is closed after the components of the reformed gas have become stable. Concurrently, the reformed gas is supplied to the anode electrode  2   b  of the fuel cell  2  by opening the fuel cell entrance valve  17  and the fuel cell exit valve  18 , whereupon power generation is started. 
     As has been described above, under another start-up operation method for the fuel cell system  1  of the first embodiment, the temperature rise of the catalysts can be made gently than the temperature rise shown in  FIG. 5  through the processes up to step S 8  shown in  FIG. 6 . It is thereby possible to prevent occurrence of temperature distributions (variations) of the respective layers filled with the respective catalysts. It also becomes possible to prevent deterioration, which would otherwise be caused by drastic temperature changes in the respective catalysts and in the respective cylindrical bodies making up the fuel processor  3 . 
     Under control conforming to another start-up operation of the first embodiment, supplying an LPG from the raw gas supply device  6  is stopped after the temperature of the shift catalyst  10   a  has reached the predetermined temperature Th 1 . However, control operation is not limited to the example. As is apparent, for example, from a relationship between the time elapsed after initiation of heating and the temperature of the catalyst shown in  FIG. 5 , supplying the LPG from the raw gas supply device  6  may also be stopped after elapse of a predetermined period of time since heating of the shift catalyst  10   a  was started. 
     The fuel cell system  1  of the present embodiment has been described by means of taking, as an example, a configure in which a raw gas to be burnt by the burner unit  12  at the start-up operation of the fuel cell system  1  is supplied to the burner unit  12  by way of the fuel processor  3 . However, the system is not limited to the configuration. For example, the fuel cell system may also be equipped with a channel (not shown) for supplying a raw gas from the raw gas supply device  6  directly to the burner unit  12  and may be configured so as to supply a raw gas directly to the burner unit  12  at the start-up operation of the fuel cell system  1  and burn the thus-supplied gas. In order to suppress a pressure rise due to a temperature rise of the fuel processor  3 , it is necessary to provide the raw gas channel from the fuel processor  3  with the burner unit  12 . However, in the case of the configuration, a similar advantage can be yielded by estimating the amount of desorbed raw gas desorbed from the catalysts and adjusting the amount of combustion air. 
     The embodiment has been described by means of taking, as an example, the fuel cell system that adjusts the amount of raw gas supplied from the raw gas supply device  6  targeted solely for the amount of the desorbed raw gas desorbed from the shift catalyst  10   a.  The reason is that the embodiment is based on a test result showing that a very small amount of LPG which is a raw gas is adsorbed to the reforming catalyst  9   a  and the selective oxidation catalyst  11   a  used in the embodiment and that influence of the amount of adsorbed raw gas on the air-fuel ratio is sufficiently small. 
     However, when the type of a raw gas or the type and shape of a catalyst change, the raw gas is usually adsorbed to the reforming catalyst  9   a  or the selective oxidation catalyst  11   a  other than the shift catalyst  10   a.  There may be a case where influence of the amount of desorbed raw gas, which is desorbed with a temperature rise, on the air-fuel ratio will become greater. In this case, the amount of desorbed raw gas that is desorbed with a temperature rise of a catalyst is first experimentally determined for the reforming unit  9 , the shift unit  10 , and the selective oxidation unit  11 , respectively. The amount of desorbed raw gas desorbed from each of the catalysts is estimated, at the start-up operation of the fuel cell system  1 , from the temperature of each of the catalysts. The essential requirement is to adjust the amount of raw gas supplied from the raw gas supply unit  6  according to the amount of desorbed raw gas desorbed from the respective catalysts and perform adjustment such that the air-fuel ratio achieved in the burner unit  12  comes to a predetermined ratio. 
     (Second Embodiment) 
       FIG. 7  is a schematic view showing a configuration of a fuel cell system  1  of a second embodiment of the present invention. In  FIG. 7 , elements that are the same as those shown in  FIG. 1  are assigned the same reference numerals, and their explanations are omitted here for brevity. 
     As shown in  FIG. 7 , the fuel cell system  1  of the second embodiment differs from the fuel cell system  1  of the first embodiment in that a shift unit  30  of the fuel cell system described in connection with the first embodiment is equipped with a shift heater  31  and that a selective oxidation unit  32  of the same is equipped with a selective oxidation heater  33 . 
     As shown in  FIG. 7 , the fuel cell system  1  of the present embodiment is configured such that a shift heater  31  is disposed at a point outside the shift unit  30  where the shift heater does not contact the shift catalyst  10   a  and that a selective oxidation heater  33  is disposed at a point outside the selective oxidation unit  32  where the selective oxidation heater does not contact the selective oxidation catalyst  11   a.  The shift heater  31  and the selective oxidation heater  33  are, for example, electric heaters that generate heat upon the application of electric power to thus heat the shift unit  30  or the selective oxidation unit  32 . 
     The start-up operation of the fuel cell system  1  of the second embodiment is hereunder described by reference to  FIG. 7  and through use of  FIG. 8 . 
       FIG. 8  is a flowchart showing control flow of a principal unit performed at the start-up operation of the fuel cell system  1  of the second embodiment of the present invention. Detailed descriptions of elements that are the same as those described in connection with the first embodiment of the present invention are omitted in some cases. 
     As shown in  FIG. 8 , at the start-up operation of the fuel cell system  1 , the raw gas supply device  6  first supplies an LPG, which is a raw gas, to the burner unit  12 , and the combustion air fan  7  supplies combustion air to the burner unit  12  (step S 1 ). In the burner unit  12 , the thus-supplied LPG and combustion air are ignited by an igniter not shown (step S 2 ). A combustion exhaust gas sequentially passes through the combustion exhaust gas channel  13  laid in the shift unit  30  that does not directly contact the shift catalyst  10   a  and the selective oxidation unit  31  that does not directly contact the selective oxidation catalyst  11   a,  whereby the shift catalyst  10   a  and the selective oxidation catalyst  11   a  begin to be heated from inside. 
     At this time, electric power is simultaneously applied to the shift heater  31  and the selective oxidation heater  33 , thereby heating the shift catalyst  10   a  and the selective oxidation catalyst  11   a  from the outside as well (step S 2   a ). The shift heater  31  and the selective oxidation heater  33  keep heating until the temperature (ThH) of the shift catalyst  10   a  detected by the shift temperature sensor  10   b  reaches a predetermined temperature (Tha) (Yes in step S 3   a ). After the temperature (ThH) of the shift catalyst  10   a  has reached or exceeded the predetermined temperature, application of electric power to the shift heater  31  and the selective oxidation heater  33  is suspended, to thus keep performance of heating operation caused by means of only the combustion exhaust gas. The predetermined temperature (Tha) will be described later. 
     The control unit  5  then compares the temperature (ThH) of the shift catalyst  10   a  with the predetermined temperature (Th 1 ) (step S 3 ). The amount of desorbed LPG desorbed from the shift catalyst  10   a  is assumed from the temperature (ThH) of the shift catalyst  10   a  until the temperature (ThH) of the shift catalyst  10   a  detected by the shift temperature sensor  10   b  exceeds the predetermined temperature (Th 1 ) (No in step S 3 ) (step S 4 ). Further, the amount of LPG that is a raw gas supplied by the raw gas supply device  6  is adjusted according to an estimated amount of desorbed LPG, and the LPG is burnt by the burner unit  12  along with a predetermined amount of combustion air (step S 5 ). 
     The above-mentioned operation is the same as that described in connection with the first embodiment. However, in the second embodiment, the catalyst  10   a  is heated from both sides; namely, an interior wall and an exterior wall of a double-cylindrical filled layer. Therefore, there may be a case where at the temperature detected by the shift temperature sensor  10   b  a temperature distribution of the shift catalyst  10   a  differs from the temperature distribution achieved in the first embodiment. For this reason, according to the position of the shift temperature sensor  10   b,  in some cases the temperature (ThH) detected by the shift temperature sensor  10   b  and the amount of desorbed LPG desorbed from the shift catalyst  10   a  may change between when the shift heater  31  is provided and when the shift heater  31  is not provided. 
     The predetermined temperature (Tha) at which the shift heater  31  and the selective oxidation heater  33  continually perform heating operation is first a temperature at which the shift catalyst  10   a  and the selective oxidation catalyst  11   a  remain 100° C. or more; at which steam and the raw gas will not condense even when supplied in a mixed manner; and that is achieved before steam and an LPG are supplied to the fuel processor  3  while mixed together. Specifically, the fuel cell system  1  of the second embodiment performs control operation so as to shut off the application of electric power to the shift heater  31  and the selective oxidation heater  33  when the shift temperature sensor  10   b  has detected about 180° C. In the fuel cell system  1  of the present embodiment, the temperature of the selective oxidation catalyst  11   a  is about 150° C. at this time. 
     As mentioned above, the fuel cell system  1  of the second embodiment estimates, at the start-up operation, desorption of raw gas due to the temperature rise of the shift catalyst  10   a  according to the temperature of the shift catalyst  10   a  detected by the shift temperature sensor  10   b.  According to the thus-estimated value, the system adjusts the amount of raw gas supplied to the burner unit  12  by the raw gas supply device  6  and supplies combustion air to the burner unit  12  in such a way that the air-fuel ratio of the raw gas to the combustion air supplied by the combustion air fan  7  falls within a predetermined range, thereby letting the burner unit perform burning. As a consequence, occurrence of incomplete combustion is prevented, to thus effect stable combustion, and the start-up operation of the fuel cell system  1  can be performed reliably. Moreover, heating the shift catalyst  10   a  and the selective oxidation catalyst  11   a  to their predetermined temperatures within a short period of time, whereby a rise time of the fuel cell system  1  consumed at the start-up can be shortened. 
     The fuel cell system  1  of the present embodiment has been described by reference to an example in which the fuel cell system  1  is started by use of the shift heater  31  and the selective oxidation heater  33 . However, the start-up operation of the system is not limited to use of the heaters. For example, only the shift heater  31  may also be used without use of the selective oxidation heater  33 . In this case, the raw gas circulating through the layer filled with the shift catalyst  10   a  is heated as a result of the shift catalyst  10   a  being heated by the combustion exhaust gas and also as a result of the shift catalyst  10   a  being heated by the shift heater  31 . The thus-heated raw gas is supplied to the layer filled with the selective oxidation catalyst  11   a.  Hence, when compared with the case where the shift heater  31  is not provided, the selective oxidation catalyst  11   a  can be heated within a short period of time. As a consequence, use of only the shift heater is effective for building the fuel cell system  1  that is low cost and that is desired to the start up operation in a short period of rise time. 
     Another start-up operation method for the fuel cell system  1  of the second embodiment of the present invention is hereunder described by reference to  FIG. 9 . 
       FIG. 9  is a flowchart for describing control flow of the principal unit achieved under another start-up operation method for the fuel cell system  1  of the second embodiment of the present invention. 
     As shown in  FIG. 9 , the fuel cell bypass valve  16  is first opened at the start-up operation of the fuel cell system  1 , thereby activating the raw gas supply device  6  and the combustion air fan  7  and supplying the LPG, which is the raw gas, and air (step S 1 ). The LPG and the combustion air are ignited by the igniter not shown, thereby starting burning the LPG (step S 2 ). 
     Next, after a temperature rise of the shift catalyst  10   a  has been ascertained by the shift temperature sensor  10   b,  applying electric energy to the shift heater  31  and the selective oxidation heater  33  is started, whereby the respective heaters start heating the respective catalysts (step S 3 ). Supplying the raw gas from the raw gas supply device  6  is at this time stopped, thereby burning only the desorbed raw gas desorbed from the shift catalyst  10   a  with the temperature rise of the shift catalyst  10   a  (step S 4 ). 
     The temperature (ThH) of the shift catalyst  10   a  and a predetermined temperature (Th 1 ) are now compared to each other (step S 5 ). An amount of desorbed LPG desorbed from the shift catalyst  10   a  is first estimated from the temperature of the shift catalyst  10   a  detected by the shift temperature sensor  10   b  (step S 6 ) until the temperature (ThH) of the shift catalyst  10   a  exceeds the predetermined temperature (Th 1 ) (No in step S 5 ). Subsequently, the amount of combustion air supplied by the combustion air fan  7  is adjusted (step S 7 ). The fuel cell system  1  is started and activated along control flow subsequent to step S 10  in the same manner as in the other embodiment. 
     Further, when the temperature (ThH) of the shift catalyst  10   a  exceeds the predetermined temperature (Th 1 ) (Yes in step S 5 ), the raw gas supply device  6  resumes supplying the LPG that is a raw gas after elapse of a predetermined time while the igniter not shown is kept operative at this time (step S 8 ). 
     Next, the amount of LPG that is a raw to be supplied by the raw gas supply device  6  is adjusted according to the amount of desorbed LPG (step S 9 ). 
     The fuel cell system  1  starts up and operates along control flow subsequent to step S 10  in the same manner as in the other embodiment. 
     The descriptions have been given by reference to the example in which burning is continually performed through use of only the desorbed LPG desorbed from the shift catalyst  10   a  until the temperature (ThH) of the shift catalyst  10   a  reaches the predetermined temperature (Th 1 ). However, a factor for continually performing combustion is not limited to the temperature of the shift catalyst  10   a.  For example, the temperature (ThH) of the shift catalyst  10   a  is not measured, and there may also be adopted a predetermined time required before the temperature (ThH) of the shift catalyst  10   a  reaches the predetermined temperature (Th 1 ). In this case, the raw gas supply device  6  resumes supplying the LPG that is a raw gas while the igniter not shown remains operative when the temperature (ThH) of the shift catalyst  10   a  comes to the predetermined temperature (Th 1 ) or more or after elapse of a predetermined time (step S 8 ). Control flow subsequent to the step is equivalent to control flow described in connection with the other embodiment. The predetermined temperature (Th 1 ) and the predetermined time refer to a temperature of the shift catalyst  10   a  at which the flow amount of desorbed LPG desorbed from the shift catalyst  10   a  becomes sufficiently smaller than the amount of LPG that is a raw gas to be supplied by the raw gas supply device  6  to such an extent that the flow amount of LPG does not greatly affect the air-fuel ratio; and also refers to a time that elapses before the temperature is reached. 
     According to the second embodiment, a desorbed raw gas among the raw gas adsorbed to shift catalyst  10   a  is preferentially burnt. Thereby, as compared with a case where the desorbed raw gas is burnt simultaneously with the raw gas supplied from the raw gas supply device  6 , the temperature rise of the reforming catalyst  9   a  can be made milder even when the amount of raw gas supplied by the raw gas supply device  6  is increased. As a result, it is thereby possible to prevent occurrence of variations in temperature distribution of the layer filled with the reforming catalyst  9   a,  which would otherwise occur when the temperature rise of the reforming catalyst  9   a  is drastic, or fracture of the reforming catalyst  9   a,  which would otherwise be caused by stress attributable to a drastic temperature change. As a consequence, it is possible to implement the fuel cell system  1  that performs the stable start-up operation and that reliably operates over a long period of time. 
     Specifically, at the start-up operation; in particular, when the desorbed raw gas desorbed from the catalyst and the raw gas supplied by the raw gas supply device  6  are burnt in the burner unit  12  while mixed together, it is necessary to sufficiently decrease the amount of raw gas supplied from the raw gas supply device  6 . In the meantime, at the time of power generation, it is necessary to supply an amount of raw gas to be a material of a reformed gas required for power generation while a temperature of the reforming catalyst  9   a  where a reforming reaction, or an endothermic reaction, is performed is maintained at about 700° C. by means of combustion of hydrogen in the off-gas. For these reasons, there is a necessity for the raw gas supply device  6  capable of supplying a raw gas by controlling the flow amount over a wide range, which in turn results in an increase in the size and complexity of the device. 
     Moreover, according to the embodiment, it is possible to control heat-up of the shift catalyst  10   a  by means of on/off control of the shift heater  31  and control of a heater output. It is also possible to adjust the amount of desorbed raw gas desorbed from the shift catalyst  10   a.  Consequently, it is possible to implement the fuel cell system  1  capable controlling the rate of temperature rise of the reforming catalyst  9   a  by means of a comparatively simple configuration. 
     The present embodiment has been described by reference to an example comprising supplying an LPG, which is a raw gas, to the burner unit by means of the raw gas supply device  6 , igniting the LPG with the igniter to initiate heating the catalyst by means of the heater, suspending an LPG supply from the raw gas supply device  6 , and starting burning only the desorbed LPG desorbed from the shift catalyst  10   a.  However, the embodiment is not limited to the example. For example, first it may also be possible to start heating the shift catalyst  10   a  by means of the heater and ignite the LPG which will be subsequently desorbed from the shift catalyst. This also yields a similar advantage and a similar working-effect. 
     In the fuel cell system  1  of the present embodiment, the shift heater  31  and the selective oxidation heater  33  have been described by means of taking electric heaters as example. However, the heaters are not limited to the electric heaters. For example, a heating burner unit that has a configuration similar to that of the burner unit  12  and that is used for heating a catalyst at the start-up operation may also be provided. 
     An example configuration of the fuel cell system  1  equipped with the heating burner unit  1  is hereunder described by reference to the drawings. 
       FIG. 10  is a schematic diagram for explaining another example of the fuel cell system  1  of the second embodiment of the present invention. 
     As shown in  FIG. 10 , the fuel cell system  1  of the second embodiment additionally includes the burner unit  12 ; a heating burner unit  34 ; a pre-burner switching valve  35 ; and a combustion air channel switching valve  36 . The pre-burner switching valve  35  switchably supplies the raw gas from the raw gas channel  15  to either the burner unit  12  or the heating burner unit  34  according to a signal from the control unit  5 . The combustion air channel switching valve  36  switchably supplies the combustion air to either the burner unit  12  or the heating burner unit  34  according to the signal from the control unit  5 . The heating burner unit  34  heats the reforming catalyst  9   a,  the shift catalyst  10   a,  and the selective oxidation catalyst  11   a  by combustion heat and a combustion exhaust gas, as does the burner unit  12 . 
     The heating burner unit  34  can be miniaturized further as compared with the burner unit  12 . Further, the heating burner can burn even a small amount of raw gas and enhance control of a rate of temperature rise of each of the catalysts. Therefore, at the start-up operation of the fuel cell system  1  it is possible to control an amount of heat applied to the catalysts with a comparatively superior accuracy by use of the heating burner unit  34 . 
     As mentioned above, according to the embodiment, the rate of temperature rise is controlled with comparatively superior accuracy by additionally providing the heating burner unit  34 , whereby the amount of desorbed raw gas can be adjusted with superior accuracy. As a consequence, it is possible to realize a fuel cell system that enables performance of the stable start-up operation while holding the air-fuel ratio for the burner unit in a predetermined range. 
     (Third Embodiment) 
       FIG. 11  is a schematic view showing a configuration of a fuel cell system of a third embodiment of the present invention. In  FIG. 11 , elements that are the same as those provided in the fuel cell systems described in connection with the first and second embodiments are assigned the same reference numerals, and their explanations are omitted here for brevity. 
     As shown in  FIG. 11 , the fuel cell system  1  of the third embodiment differs from the fuel cell system  1  of the first embodiment in that the raw gas channel  15  of the fuel cell system  1  described in connection with the first embodiment is provided with a flow meter  40  that has a mutual circulatory communication with the raw gas channel  15 . 
     At the start-up operation of the fuel cell system  1 , the flow meter  40  measures a total of the flow amount of raw gas supplied from the raw gas supply device  6  and the flow amount of desorbed raw gas desorbed from the catalysts of the fuel processor  3 . The combustion gas supplied to the burner unit  12  at the start-up operation of the fuel cell system  1  represents a raw gas supplied from the raw gas supply device  6  and the desorbed raw gas desorbed from the catalysts of the fuel processor  3 . The flow meter  40  is wired in such a way that a flow amount of combustion gas measured by the flow meter  40  is input as a signal to the control unit  5 . 
     The start-up operation of the fuel cell system  1  of the third embodiment is hereunder described by reference to  FIG. 11  and through use of  FIG. 12 .  FIG. 12  is a flowchart showing control flow of a principal unit performed at the start-up operation of the fuel cell system  1  of the third embodiment of the present invention. Detailed descriptions of elements that are the same as those described in connection with the other embodiments of the present invention are omitted in some case. 
     As shown in  FIG. 12 , at the start-up operation of the fuel cell system  1  the raw gas supply device  6  first supplies an LPG that is a raw gas to the burner unit  12 , and the combustion air fan  7  supplies combustion air to the burner unit  12  (step S 1 ). The LPG that is a raw gas is supplied first to the reforming unit  9  by way of the raw gas supply channel  14  and sequentially circulates through the shift unit  10  and the selective oxidation unit  11 , to thus be supplied to the burner unit  12  by way of the fuel cell bypass channel  15   b  of the raw gas channel  15 . Only the LPG that is a raw gas supplied from the raw gas supply device  6  is supplied at this point in time to the burner unit  12  as a combustion gas. 
     The igniter not shown ignites the thus-supplied LPG and the thus-supplied combustion air, thereby starting combustion in the burner unit  12  (step S 2 ). When the burner unit  12  has started burning the LPG, the reforming catalyst  9   a  of the reforming unit  9  begins being heated by means of resultant combustion heat, whereupon the temperature (ThK) of the reforming catalyst  9   a  begins to increase. Concurrently, the combustion exhaust gas passes through the combustion exhaust gas channel  13  formed in the shift unit  10  and the selective oxidation unit  11 , whereby the shift catalyst  10   a  and the selective oxidation catalyst  11   a  begin to be heated. The temperature (ThH) of the shift catalyst  10   a  and the temperature (ThS) of the selective oxidation catalyst  11   a  start increasing. 
     When the temperature of the shift catalyst  10   a  starts increasing, the LPG adsorbed to the shift catalyst  10   a  begins to be desorbed, and the desorbed LPG is supplied as a desorbed LPG to the burner unit  12 . Therefore, the burner unit  12  is supplied with, as a combustion gas, the LPG that is a raw gas supplied from the raw gas supply device  6  and the desorbed LPG. The flow meter  40  starts detecting a flow amount of combustion gas at this time, and the control unit  5  begins to adjust the flow amount of raw gas supplied by the raw gas supply device  6  according to the flow amount of combustion gas detected by the flow meter  40  (step S 3 ). The control unit  5  here controls the raw gas supply device  6  in such a way that the flow amount of combustion gas supplied to the burner unit  12  becomes constant, thereby adjusting the flow amount of raw gas supplied by the raw gas supply device  6 . 
     The temperature (ThK) of the reforming catalyst  9   a,  the temperature (ThH) of the shift catalyst  10   a,  and the temperature (ThS) of the selective oxidation catalyst  11   a  are compared with their respective predetermined temperatures (step S 4 ). When any of the temperature (ThK) of the reforming catalyst  9   a,  the temperature (ThH) of the shift catalyst  10   a,  and the temperature (ThS) of the selective oxidation catalyst  11   a  is lower than their corresponding predetermined temperatures (No in step S 4 ), adjusting the amount of the raw gas supplied by the raw gas supply device  6  according to the flow amount of combustion gas detected by the flow meter  40  is continually performed. In the fuel cell system  1  of the third embodiment, the predetermined temperature for the temperature (ThK) of the reforming catalyst  9   a  is set to 200° C.; the predetermined temperature for the temperature (ThH) of the shift catalyst  10   a  is set to 180° C.; and the predetermined temperature for the temperature (ThS) of the selective oxidation catalyst  11   a  is set to 150° C., as in the fuel cell systems  1  of the other embodiments. 
     When all of the temperature (ThK) of the reforming catalyst  9   a,  the temperature (ThH) of the shift catalyst  10   a,  and the temperature (ThS) of the selective oxidation catalyst  11   a  come to their predetermined temperatures or more (Yes in step S 4 ), the steam generating device  4  starts supplying steam to the raw gas supply channel  14  by way of the steam channel  4   a.  Further, flow control of the amount of raw gas supplied by the raw gas supply device  6  based on the value measured by the flow meter  40  is completed. A mixed gas containing the LPG and steam is supplied to the layer filled with the reforming catalyst  9   a  of the reforming unit  9 , whereupon reforming the LPG to hydrogen is started (step S 5 ). 
     Flow control of the amount of raw gas supplied by the raw gas supply device  6  based on the value measured by the flow meter  40  is completed simultaneous with initiation of a steam supply. The reason for this is that a composition of the combustion gas circulating through the flow meter  40  changes from the LPG, which is the past composition, to the reformed gas including hydrogen and that the reformed gas contains steam. For these reasons, the flow meter  40  becomes impossible to accurately measure the flow amount of combustion gas. 
     However, when all of the temperature (ThK) of the reforming catalyst  9   a,  the temperature (ThH) of the shift catalyst  10   a,  and the temperature (ThS) of the selective oxidation catalyst  11   a  come to their predetermined temperatures or more, the desorbed raw gas desorbed from the catalysts (i.e., the desorbed LPG in the third embodiment) substantially become extinct as shown in  FIG. 3 . Therefore, the desorbed raw gas hardly affects the combustion in the burner unit  12  and; hence, the ratio of the combustion air supplied by the combustion air fan  7  to the raw gas comes to a predetermined ratio, whereby combustion in the burner unit  12  becomes stable. 
     A determination is now made as to whether or not the respective catalysts are at respective predetermined temperatures; namely, whether or not the reforming catalyst stays in a temperature range from about 600 to 700° C.; whether or not the shift catalyst stays in a temperature range from about 200 to 300° C.; and whether or not the selective oxidation catalyst stays in a temperature range from about 150 to 200° C. (step S 6 ). When the temperatures of the respective catalysts are lower than their respective temperature ranges (No in step S 6 ), reactions occurred in the respective catalysts are insufficient. Namely, the amount of hydrogen in the reformed gas is small, and the amount of carbon monoxide in the gas is large. Therefore, the reformed gas in the course of heat-up operation is caused to circulate through the fuel cell bypass channel  15   b.  When the temperatures of the respective catalysts have increased to their predetermined temperature ranges (Yes in step S 6 ), the fuel cell bypass valve  16  is closed after the components of the reformed gas have become stable. Concurrently, the reformed gas is supplied to the anode electrode  2   b  of the fuel cell  2  by opening the fuel cell entrance valve  17  and the fuel cell exit valve  18 , whereupon power generation is started. When the temperatures of the respective catalysts are out of their temperature ranges (No in step S 6 ), control analogous to that performed in the related art is performed, and processing waits until the temperatures of the catalysts fall within their temperature ranges. 
     As described above, in the fuel cell system  1  of the present embodiment the flow meter can measure, at the start-up operation, the flow amount of desorbed raw gas desorbed with a temperature rise of the shift catalyst in conjunction with the flow amount of raw gas supplied from the raw gas supply device  6  and can adjust the flow amount of desorbed raw gas by adjusting the amount of raw gas supplied by the raw gas supply device  6  according to a measured value of the flow meter. Accordingly, combustion of the burner unit  12  can be made stable. Consequently, stable combustion is realized by preventing occurrence of incomplete combustion, and the start-up operation of the fuel cell system  1  can be performed without fail. 
     In the fuel cell system  1  of the third embodiment, the flow meter  40  is disposed in the raw gas channel. However, the flow meter can also be disposed in a fuel cell bypass channel or an off-gas channel. 
     In the fuel cell system  1  of the third embodiment, the raw gas is supplied from the raw gas supply device  6  to the burner unit  12  by way of the fuel processor  3  at the start-up operation of the fuel cell system  1 . However, the fuel cell system  1  can also be configured as follows. Namely, the fuel cell system  1  is provided with a branch channel that additionally branches off from the raw gas supply device  6  to supply the raw gas directly to the burner unit  12  and channel switching means that causes switching whether to supply the raw gas from the raw gas supply device  6  to the fuel processor  3  or to supply the raw gas directly to the burner unit  12  by way of the branch channel. The raw gas supplied from the raw gas supply device is supplied directly to the burner unit by way of the branch channel at an initial start-up phase of the fuel cell system and until the temperatures of the respective catalysts of the fuel processor  3  are sufficiently decreased to predetermined temperatures at which a concentration of carbon dioxide is sufficiently decreased and that are appropriate for producing a excellent reformed gas sufficiently containing hydrogen. In this case, the flow meter  40  measures the flow amount of desorbed raw gas desorbed from the respective catalysts of the fuel processor  3 . The control unit  5  controls the amount of raw gas supplied from the raw gas supply device  6  in such a way that the flow amount of combustion gas supplied to the burner unit  12  becomes corresponding to the flow amount of combustion air supplied by the combustion air fan  7  according to the flow amount of desorbed raw gas measured by the flow meter  40 . An advantage similar to that yielded by the third embodiment can be yielded. 
     In the fuel cell system  1  of the third embodiment, by use of the flow meter  40  the total of the flow amount of raw gas supplied from the raw gas supply device  6  and the flow amount of desorbed raw gas desorbed from the respective catalysts of the fuel processor  3 . However, measurement can also be performed by use of a pressure gauge. Specifically, an advantage analogous to that yielded by the third embodiment can also be acquired, for example, by connecting a pressure gauge to the raw gas channel; previously storing in the control unit  5  a relationship between the flow amount of gas in the raw gas channel and pressure; at the start-up operation of the fuel cell system  1  estimating the flow amount of raw gas supplied to the burner unit  12  (i.e., a total of the amount of raw gas supplied from the raw gas supply device  6  and the flow amount of desorbed raw gas desorbed from the respective catalysts of the fuel processor  3 ) from the value measured by the pressure gauge; and the control unit  5  controlling the flow amount of raw gas supplied from the raw gas supply device  6  according to an estimated value. 
     The third embodiment has been described by use of the LPG as a raw gas. However, the raw gas is not limited to the LPG as in the other embodiments. Another hydrocarbon-based raw, like a city gas or kerosene, can also be used. When a liquid hydrocarbon-based material, like kerosene, is used, it is preferable to gasify the material. Further, when the type of the raw gas has changed, a change is assumed to occur in the amount of the raw gas adsorbed to the respective catalysts of the fuel processor  3  or in temperatures of the respective catalysts and a behavior of the flow amount of desorbed raw gas. However, in the fuel cell system  1  of the third embodiment, the amount of raw gas that contains the desorbed raw gas and that is supplied to the burner unit  12  is measured, and the flow amount of raw gas supplied from the raw gas supply device  6  is controlled according to the thus-measured value. Hence, combustion of the burner unit  12  can be made stable. However, according to the type of a raw gas, it is necessary to use a flow meter capable of measuring a flow amount of the raw gas as the flow meter  40 . 
     In the fuel cell system  1  of the third embodiment, timing for completing controlling the amount of raw gas from the raw gas supply device  6  according to the amount of raw gas detected by the flow meter  40  is set to timing when the temperatures of the respective catalysts of the fuel processor reach predetermined temperatures or more, thereupon supplying steam. However, timing is not limited to that mentioned above, so long as the temperatures of the respective catalysts increase in excess of temperatures at which the desorbed raw gas stops affecting combustion of the burner unit  12 . 
     (Fourth Embodiment) 
       FIG. 13  is a schematic diagram showing a configuration of the fuel cell system  1  of a fourth embodiment of the present invention. In  FIG. 13 , those constituent elements that are the same as those of the fuel systems  1  described in the previously-described first through third embodiments are assigned the same reference numerals, and their explanations are omitted. 
     As shown in  FIG. 13 , the fuel cell system  1  of the fourth embodiment is different from the fuel cell system  1  of the first embodiment in that the combustion exhaust gas channel  13  of the fuel cell system  1  described in connection with the first embodiment is provided with an oxygen meter  41  that is equivalent to exhaust gas analysis means and that measures a concentration of oxygen in a combustion exhaust gas. 
     The oxygen meter  41  measures a concentration of oxygen in a combustion exhaust gas produced as a result of the burner  12  having burnt the raw gas or an off-gas, a reformed gas, or the like, along with the combustion air. The oxygen meter  41  is electrically connected in such a way that the measured value is input as a signal to the control unit  5 . 
     Operation performed at the start-up operation of the fuel cell system  1  of the fourth embodiment is hereunder described by reference to  FIG. 13  and through use of  FIG. 14 .  FIG. 14  is a flowchart showing control flow of the principal unit achieved at the start-up operation of the fuel cell system  1  of the fourth embodiment. Detailed descriptions of elements that are the same as those described in connection with the other embodiments of the present invention are often omitted. 
     As shown in  FIG. 14 , at the start-up operation of the fuel cell system  1  the raw gas supply device  6  first supplies a predetermined flow amount of an LPG that is a raw gas to the burner unit  12 , and the combustion air fan  7  supplies a predetermined flow amount of combustion air to the same (step S 1 ). The LPG that is a raw gas is first supplied to the reforming unit  9  by way of the raw gas supply channel  14 ; is sequentially circulated through the shift unit  10  and the selective oxidation unit  11 ; and is supplied to the burner unit  12  by way of the fuel cell bypass channel  15   b  of the raw gas channel  15 . Only the LPG that is a raw gas supplied from the raw gas supply device  6  is supplied as a combustion gas to the burner unit  12  at this point in time. 
     In the burner unit  12 , the thus-supplied LPG and combustion air are ignited by an igniter not shown, whereupon combustion begins (step S 2 ). When the burner unit  12  starts burning the LPG, the temperature (ThK) of the reforming catalyst  9   a  begins increasing. Concurrently, a combustion exhaust gas sequentially passes through the combustion exhaust gas channel  13  in the shift unit  10  and the selective oxidation unit  11 , whereby the shift catalyst  10   a  and the selective oxidation catalyst  11   a  begin to be heated. The temperature (ThH) of the shift catalyst  10   a  and a temperature (ThS) of the selective oxidation catalyst  11   a  thereby start increasing. 
     Next, the oxygen meter  41  starts measuring the concentration of oxygen in the combustion exhaust gas output from the burner unit  12  and sending an input to the control unit  5 . According to the concentration of oxygen measured by the oxygen meter  41 , the control unit  5  controls the flow amount of raw gas supplied by the raw gas supply device  6  in such a way that the concentration of oxygen measured by the oxygen meter  41  comes to a predetermined concentration of oxygen (step S 3 ). Specifically, when the concentration of oxygen in the combustion exhaust gas to be measured by the oxygen meter  41  is lower than a predetermined concentration of oxygen, the flow amount of raw gas supplied by the raw gas supply device  6  is decreased. In contrast, when the concentration of oxygen in the combustion exhaust gas is higher than the predetermined concentration of oxygen, the flow amount of raw gas supplied by the raw gas supply device  6  is increased. 
     When the temperatures of the respective catalysts in the fuel processor  3  start increasing and when the raw gas adsorbed to the catalysts thereupon begin to be desorbed, a total of the amount of raw gas supplied from the raw gas supply device  6  and the amount of desorbed raw gas desorbed from the catalyst is supplied to the burner unit  12 . However, the flow amount of raw gas supplied by the raw gas supply device  6  is adjusted according to the concentration of oxygen in the combustion exhaust gas, and hence a combustion gas (i.e., a mixed gas containing the raw gas supplied by the raw gas supply device  6  and the desorbed raw gas) is supplied to the burner unit  12 . 
     The temperature (ThK) of the reforming catalyst  9   a,  the temperature (ThH) of the shift catalyst  10   a,  and the temperature (ThS) of the selective oxidation catalyst  11   a  are compared with their predetermined temperatures (step S 4 ). When the temperature (ThK) of the reforming catalyst  9   a,  the temperature (ThH) of the shift catalyst  10   a,  and the temperature (ThS) of the selective oxidation catalyst  11   a  are lower than their predetermined temperatures (No in step S 4 ), adjusting the flow amount of raw gas supplied from the raw gas supply device  6  according to the concentration of oxygen in the combustion exhaust gas detected by the oxygen meter  41  is continually performed. In the fuel cell system  1  of the fourth embodiment, the predetermined temperature for the temperature (ThK) of the reforming catalyst  9   a  is set to 200° C.; the predetermined temperature for the temperature (ThH) of the shift catalyst  10   a  is set to 180° C.; and the predetermined temperature for the temperature (ThS) of the selective oxidation catalyst  11   a  is set to 150° C., as in the fuel cell systems  1  of the other embodiments. 
     When all of the temperature (ThK) of the reforming catalyst  9   a,  the temperature (ThH) of the shift catalyst  10   a,  and the temperature (ThS) of the selective oxidation catalyst  11   a  come to their predetermined temperatures or more (Yes in step S 4 ), the steam generating device  4  starts supplying steam to the raw gas supply channel  14  by way of the steam channel  4   a.  Further, flow control of the amount of raw gas supplied by the raw gas supply device  6  based on the value measured by the oxygen meter  41  is completed. A mixed gas containing the LPG and steam is supplied to the layer filled with the reforming catalyst  9   a  of the reforming unit  9 , whereupon reforming the LPG to hydrogen is started (step S 5 ). 
     When the temperature (ThH) of the shift catalyst  10   a  has come to the predetermined temperature (180° C. in the embodiment) or more, the desorbed raw gas desorbed from the catalyst (i.e., the desorbed LPG in the embodiment) substantially becomes extinct, thereby posing very little influence on the combustion in the burner  12 . For these reasons, the flow amount of raw gas in accordance with the amount of combustion air supplied from the combustion air fan  7  is supplied. 
     A determination is now made as to whether or not the respective catalysts are at respective predetermined temperatures; namely, whether or not the reforming catalyst stays in a temperature range from about 600 to 700° C.; whether or not the shift catalyst stays in a temperature range from about 200 to 300° C.; and whether or not the selective oxidation catalyst stays in a temperature range from about 150 to 200° C. (step S 6 ). When the temperatures of the respective catalysts are lower than their respective temperature ranges (No in step S 6 ), reactions occurred in the respective catalysts are insufficient. Namely, the amount of hydrogen in the reformed gas is small, and the amount of carbon monoxide in the gas is large. Therefore, the reformed gas in the course of heat-up operation is caused to circulate through the fuel cell bypass channel  15   b.  When the temperatures of the respective catalysts have increased to their predetermined temperature ranges (Yes in step S 6 ), the fuel cell bypass valve  16  is closed after the components of the reformed gas have become stable. Concurrently, the reformed gas is supplied to the anode electrode  2   b  of the fuel cell  2  by opening the fuel cell entrance valve  17  and the fuel cell exit valve  18 , whereupon power generation is started. When the temperatures of the respective catalysts are out of their temperature ranges (No in step S 6 ), control analogous to that performed in the related art is performed, and processing waits until the temperatures of the catalysts fall within their temperature ranges. 
     As described above, in the fuel cell system  1  of the fourth embodiment, at the start-up operation, the flow amount of desorbed raw gas by adjusting the amount of raw gas supplied by the raw gas supply device  6  is adjusted according to a concentration of oxygen in the combustion exhaust gas. Accordingly, even when the raw gas adsorbed to the catalysts are desorbed with the temperature rise of the catalysts of the fuel processor, to thus be supplied to the burner unit  12 , combustion in the burner unit  12  can be made stable. As a consequence, stable combustion is realized by preventing occurrence of incomplete combustion, and the start-up operation of the fuel cell system  1  can be performed without fail. 
     The fuel cell system  1  of the fourth embodiment adjusts the flow amount of raw gas supplied from the raw gas supply device in such a way that the concentration of oxygen in the combustion exhaust gas comes to a predetermined concentration by use of the oxygen meter that measures the concentration of oxygen in the combustion exhaust gas. A carbon dioxide concentration meter that measures the concentration of carbon dioxide in the combustion exhaust gas can also be used. In this case, when the temperature of the catalysts in the fuel processor starts increasing whereby the flow amount of raw gas burnt by the burner unit is increased by the desorbed raw gas, the concentration of carbon dioxide in the combustion exhaust gas increases. Therefore, so long as the flow amount of raw gas supplied from the raw gas supply device is decreased correspondingly, an advantage similar to that provided by the fuel cell system  1  of the fourth embodiment can be yielded. 
     The present patent application is based on Japanese Patent Application (Application No. 2009-115413) filed on May 12, 2009, the entire subject matter of which is incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     The fuel cell system of the present invention capable of performing a stable start-up operation with stable combustion in a burner unit. Therefore, the fuel cell system is useful in a technical field of a fuel cell system that utilizes a hydrocarbon-based raw, such as a town gas or an LPG, subjected to steam reforming.