Fuel cell cogeneration systems (hereinafter referred to as “fuel cell systems”) of high power generation efficiency and high combined efficiency have heretofore come to the fore as dispersion-type power generation systems capable of making efficient use of energy. 
Fuel cell systems have a fuel cell as the main body of its power generating section.  Most of fuel cells such as phosphoric-acid fuel cells (abbreviated to PAFC) which have already put in practical use and polymer electrolyte fuel cells (abbreviated to PEFC) which are under development use hydrogen as a fuel for power generation.  Nevertheless, the means of supplying hydrogen are not provided as an infrastructure at present.  Therefore, fuel cell systems are generally equipped with a hydrogen generator for generating hydrogen necessary for use in power generation In such a hydrogen generator, a hydrocarbon-based material such as methane gas and water are used to produce hydrogen-rich reformed gas.  The fuel cell systems use the reformed gas generated by the hydrogen generator and air to generate and output a specified amount of electric power. 
As a method of generating hydrogen with a hydrogen generator, steam reforming is widely known.  In steam reforming, reformed gas is generated through a steam reforming reaction.  This steam reforming reaction is one of various hydrogen generating reactions in which a chemical reaction is caused, for instance, between city gas (which is a material for generating hydrogen) and vapor, using a ruthenium catalyst at a high temperature of about 600° C. to 800° C., thereby generating a reformed gas that contains hydrogen as a main component. 
One known hydrogen generator has a concentric multiple cylinder-like configuration. 
FIG.  18 is a longitudinal sectional view that diagrammatically shows the internal configuration of one example of hydrogen generators capable of uniformly mixing a material with vapor.  It should be noted that arrows in FIG.  18 indicate the flowing directions of gases such as the material and vapor. 
As illustrated in FIG.  18, a hydrogen generator 300 capable of uniformly mixing the material with vapor has a concentric multiple cylinder-like configuration.  More specifically, the hydrogen generator 300 includes a combustion burner 16 and heaters 17, 18.  The combustion burner 16 generates a high-temperature combustion gas used for promoting the steam reforming reaction.  The heaters 17, 18 are supplied with water and heated by the combustion burner 16 thereby generating wet steam or vapor.  The hydrogen generator 300 also includes (a) a combustion gas passage 29 which is composed of a plurality of annular spaces defined by a plurality of concentric cylinders 19 to 28 arranged around the combustion burner 16 and in which a high-temperature fuel gas generated by the combustion burner 16 passes through the annular spaces; (b) a preheating layer 30 for preheating a mixed gas of the material and vapor before a steam reforming reaction; (c) a reforming catalyst layer 31 which is heated to a specified reaction temperature to promote the steam reforming reaction; (d) a heat recovery layer 32 for collecting heat in order to decrease the temperature of the high-temperature reformed gas generated by the reforming catalyst layer 31; (e) a shift catalyst layer 33 for reducing the concentration of carbon monoxide in the reformed gas cooled by the heat recovery layer 32; (f) first and second mixing layers 35, 36 for mixing the reformed gas reduced in the concentration of carbon monoxide by the shift catalyst layer 33 and air for use in a selective oxidation reaction taken from an air feeder section 34; and (g) first and second selective oxidation catalyst layers 37, 38 for further reducing the concentration of carbon monoxide in the reformed gas through a selective oxidation reaction, the reformed gas having passed through the first and second mixing layers 35, 36 so that it was mixed with air.  These passage and layers are arranged in the form of concentric cylinders around the combustion burner 16.  As illustrated in FIG.  18, in the hydrogen generator 300, the preheating layer 30, the heat recovery layer 32, the first mixed layer 35 and the second mixed layer 36 are each constructed by a packed body filled with ceramic balls for the purpose of promoting the mixing of the material with vapor or air (see, e.g., Patent Document 1). 
In the hydrogen generator 300 of the above configuration, the water used for the steam reforming reaction is supplied to the heater 17 or 18, while at least part of it is vaporizing.  On the other hand, the water (warm water) discharged from the heater 17 or 18 is mixed with city gas serving as the material in a mixing section (not shown in FIG.  18) and then completely vaporizes so as to be mixed with the city gas, while passing through the space between the concentric cylinders 25, 26 and the space between the concentric cylinders 24, 25.  The mixed gas of the city gas and vapor is thoroughly mixed while passing through the preheating layer 30 and then supplied to the reforming catalyst layer 31.  The reforming catalyst layer 31 is heated by a combustion gas flowing in the combustion gas passage 29 to be utilized for the proceeding steam reforming reaction.  The reformed gas generated by the steam reforming reaction is cooled down to a specified temperature while passing through the heat recovery layer 32 and then supplied to the shift catalyst layer 33.  Thereafter, most of the carbon monoxide contained in the reformed gas is removed by a shift reaction proceeding in the shift catalyst layer 33.  Thereafter, the reformed gas from which most of the carbon monoxide has been removed is fully mixed, in the first mixing layer 35, with air supplied from the air feeder section 34 in order to remove most of the carbon monoxide remaining in a small amount in the reformed gas.  Then, the reformed gas is supplied to the first selective oxidation catalyst layer 37.  Most of the carbon monoxide contained in the reformed gas is removed by combustion through the selective oxidation reaction proceeding in the first selective oxidation catalyst layer 37.  To remove carbon monoxide which could not be removed by the first selective oxidation catalyst layer 37, the reformed gas, the concentration of which has been uniformed by the second mixing layer 36, is supplied to the second selective oxidation catalyst layer 38 in which further carbon monoxide removal is done.  The reformed gas from which carbon monoxide has been thoroughly removed is supplied to the fuel cell to be utilized for a chemical reaction for generating power in the fuel cell. 
In the hydrogen generator 300 shown in FIG.  18, the preheating layer 30 composed of ceramic balls and located between concentric cylinders 20 and 21 disturbs the flow of the fluids passing through the preheating layer 30, so that the material such as city gas and vapor are vigorously mixed.  That is, the flow of the material and vapor becomes three-dimensionally intricate under the influence of the ceramic balls while passing through the preheating layer 30, so that mixing of the material and vapor is favorably encouraged.  In addition, in the hydrogen generator 300, when the reformed gas is supplied to the shift catalyst layer 33, the mixed condition of the reformed gas is improved by the mixing function of the heat recovery layer 32, so that the shift reaction in the shift catalyst layer 33 is favorably done.  Further, in the hydrogen generator 300, when the reformed gas is respectively supplied to the first and second selective oxidation layers 37, 38, the mixed condition of the reformed gas is improved by the mixing function of the first and second mixing layers 35, 36, so that the selective oxidation reaction in the first and second selective oxidation catalyst layers 37, 38 favorably proceeds. 
In the hydrogen generator 300, the preheating layer 30 filled with ceramic balls, the heat recovery layer 32, and the first and second mixing layers 35, 36 exhibit relatively good mixing performance to fluids existing in neighboring regions nevertheless they exhibit poor mixing performance to fluids existing in regions relatively far from each other.  More concretely, since the material and vapor to be used for the reforming reaction are supplied from the upper right position of the hydrogen generator in FIG.  18, the concentration of the material and vapor contained in the fluid supplied to a portion of the preheating layer 30 located on the right side of FIG.  18 is higher than that of the material and vapor contained in the fluid supplied to a portion of the preheating layer 30 located on the left side of FIG.  18.  In this case, it has proved practically difficult to uniformise the concentration of the material and vapor contained in the fluid in a circumferential direction of the preheating layer 30 for the reason that the fluid has to be moved within the preheating layer 30 along the circumference thereof which is much longer than the vertical length of the preheating layer 30.  Therefore, the concentration of the material and vapor supplied to the reforming catalyst layer 31 is unevenly distributed in the circumferential direction of the catalyst layer 31.  Moreover, the reforming catalyst layer 31 is excessively heated in the area where the concentration of the material and vapor is low, which leads to deterioration of the reforming catalyst.  Additionally, in the area of the reforming catalyst layer 31 where the concentration of the material and vapor is high, the temperature of the reforming catalyst layer 31 does not sufficiently rise and therefore the inversion rate for hydrogen generation decreases, owing to the presence of an excessive amount of vapor. 
Similarly to the case of the preheating layer 30 described above, uniform distribution of air in a circumferential direction of the first mixing layer 35 is difficult because of the difference between the concentrations of air supplied from Positions P1, P2 in FIG.  18.  Therefore, the concentration of oxygen supplied to the first selective oxidation catalyst layer 37 is distributed unevenly in the circumferential direction of the catalyst layer 37.  This causes insufficient removal of carbon monoxide from the reformed gas in the area of the first selective oxidation catalyst layer 37 where the concentration of oxygen is low.  On the other hand, in the area of the first selective oxidation catalyst layer 37 where the concentration of oxygen is high, surplus oxygen still remaining after oxidative removal of carbon monoxide from the reformed gas consumes generated hydrogen, resulting in poor hydrogen generating efficiency. 
To prevent the concentration of the material and vapor supplied to the reforming catalyst layer 31 from varying to a considerable extent, there has been proposed a hydrogen generator having improved mixing performance to fluids that exist in positions distant from each other in a circumferential direction. 
FIG.  19 is a longitudinal sectional view that diagrammatically shows the internal configuration of one example of hydrogen generators having improved mixing performance to fluids that exist in positions distant from each other in a circumferential direction.  It should be noted that arrows in FIG.  19 indicate the flowing directions of gases such as the material and vapor. 
As shown in FIG.  19, a hydrogen generator 400 includes a city gas feed pipe connection part 1, a water feed pipe connection part 2, a combustion gas exhaust port 13, and an outlet pipe 15.  The hydrogen generator 400 also has (a) combustion gas passages 4 to 6, (b) a downward flow passage 8 through which city gas supplied from the city gas feed pipe connection part 1 and water supplied from the water feed pipe connection part 2 flow downward, (c) an upward flow passage 9 through which the mixed gas of the city gas and vapor flows upward, the mixed gas having been generated in the cause of the downward flow in the flow down passage 8, and (d) a reformed gas passage 11 which allows circulation of a reformed gas within the hydrogen generator 400, the reformed gas having been generated through a steam reforming reaction.  These passages are arranged in the form of concentric cylinders around the combustion burner 3.  In the hydrogen generator 400, an evaporator 10 is constituted by the downward flow passage 8 and the upward flow passage 9, and the reformed gas passage 11 has, at a specified inner position thereof, a reforming catalyst layer 12 which is used for promoting the steam reforming reaction.  In the hydrogen generator 400 shown in FIG.  19, the upward flow passage 9 and the reformed gas passage 11 are connected to each other by a disk-like space 41 defined by two disk-like lateral walls 39, 40 and a catalyst pipe 42.  As illustrated in FIG.  19, in the hydrogen generator 400, at least the disk-like space 41 is filled with a large number of spherical alumina particles 43 for encouraging mixing of the material and vapor.  The alumina particles 43 have a diameter about one third the height of the space 41. 
In the hydrogen generator 400 having the above-described configuration, a mixed gas of city gas and vapor is generated in the evaporator 10, when city gas and water are fed from the city gas feed pipe connection part 1 and the water feed pipe connection part 2, respectively, to the downward flow passage 8.  The mixed gas of city gas and vapor, which exist within the evaporator 10 so as to spread in a circumferential direction thereof, then passes through the space 41 and the catalyst pipe 42 to be fed to the reforming catalyst layer 12 filled with a reforming catalyst.  Then, in the reforming catalyst layer 12, the reforming catalyst is heated up to a high temperature by a combustion gas flowing in the combustion gas passage 4 to promote the steam reforming reaction, so that a reformed gas containing hydrogen, carbon dioxide and carbon monoxide is generated from the mixed gas.  Similarly to the hydrogen generator 300 shown in FIG.  18, at least the space 41 is filled with the alumina particles 43 in the hydrogen generator 400 and therefore, the flow of the fluid in the disk-like space 41 is disturbed so that the material such as city gas is vigorously mixed with vapor.  That is, the flow of the mixed gas composed of the material and vapor becomes three-dimensionally intricate under the influence of the alumina particles 43 while the mixed gas is passing through the disk-like space 41 and as a result, the mixed condition of the material and vapor is improved like the case of the hydrogen generator 300 shown in FIG.  18. 
Additionally, in the hydrogen generator 400, since the upward flow passage 9 and the reformed gas passage 11 are connected to each other by the disk-like space 41 defined by the two disk-like lateral walls 39, 40 and the catalyst pipe 42, the mixed gas of city gas and vapor, which have passed through the evaporator 10, goes to the disk-like space 41 from the entire circumferential area of the evaporator 10.  After the mixed gas passes through the space 41 while being brought into a turbulent condition under the influence of the alumina particles 43 and is collected by the catalyst pipe 42, it is supplied to the reforming catalyst layer 12.  Therefore, the city gas fed from the city gas feed pipe connection part 1 flows in the downward flow passage 8 and the upward flow passage 9, more heavily on the side of the city gas feed pipe connection part 1, so that the concentration of city gas in the mixed gas is higher in the right portion of the space 41 shown in FIG.  19 and the concentration of the same is lower in the left portion of the space 41 shown in FIG.  19.  More specifically, even if a spatial concentration distribution occurs in a circumferential direction, the mixed condition of, for instance, the city gas and vapor in the circumferential direction of the reforming catalyst layer 12 is satisfactorily averaged.  That is, according to the hydrogen generator 400 shown in FIG.  19, not only the mixed condition of a fluid such as a mixed gas of city gas and vapor but also the mixed condition of fluids existing in positions distant from each other in a circumferential direction can be improved. 
Patent Document 1: International Publication No.  WO2000/063114