Patent Publication Number: US-2013252122-A1

Title: Power generator and method of operating the same

Description:
TECHNICAL FIELD 
     The present invention relates to a power generator configured to supply heat and electricity, and to a method of operating the power generator. 
     BACKGROUND ART 
     A co-generation system is a system configured to: generate and supply electric power to a consumer, thereby covering the consumer&#39;s electricity load; and recover exhaust heat that is generated when generating the electric power and store the recovered exhaust heat in the form of hot water, thereby covering the consumer&#39;s hot water load. As one of such co-generation systems, there is a known co-generation system in which a fuel cell and a water heater are operated by using the same raw material (see Patent Literature 1, for example). Patent Literature 1 discloses a co-generation system configured such that: heat that is generated when a fuel cell performs electric power generation is stored via a heat exchanger in the form of hot water in a hot water storage tank; and a water heater heats up the stored hot water to a predetermined temperature. The fuel cell and the water heater are configured to operate by using the same raw material (which contains a hydrocarbon as a main component). 
     Fuel cells obtain electricity and heat through a reaction between a fuel and an oxidant (such as air or oxygen). The fuel contains hydrogen as a main component. However, since an infrastructure for supplying hydrogen has not been developed, fuel cells are normally equipped with a hydrogen generator configured to generate hydrogen by causing a reforming reaction of a raw material such as a hydrocarbon. There are various methods of reforming reaction such as steam reforming and partial oxidation reforming, and the hydrogen generator is configured to obtain energy necessary for steam reforming by combusting a raw material with a combustor (burner). 
     There is a known method of determining the air ratio of a burner for use in a fuel cell heater, the method including measuring the combustion state of the burner with an ionization sensor which the burner is provided with and adjusting the amount of air to be supplied to a raw material, thereby maintaining stable combustion of the burner (see Patent Literature 2, for example). 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Laid-Open Patent Application Publication No. 2007-248009 
         PTL 2: Japanese National Phase PCT Laid-Open Publication No. 2008-523549 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     There are cases where such a co-generation system as described above uses natural gas as a raw material. In a case where natural gas is used as a raw material, the raw material composition varies depending on the geographical region and season in which the natural gas is supplied. In order to allow the fuel cell to stably generate electric power, it is necessary to adjust operating conditions of the fuel cell in accordance with a variation in the raw material composition. A first condition in the operating conditions to be adjusted in accordance with the raw material composition is the amount of supply of a first raw material (hydrocarbon) and a second raw material (e.g., water or air) which are used for a reforming reaction, the first condition being adjusted for stably supplying a fuel that is necessary for electric power generation. A second condition in the operating conditions to be adjusted in accordance with the raw material composition is the amount of air to be supplied to the raw materials or fuel, the second condition being adjusted for stably obtaining energy for generating the fuel. 
     In the method of determining the air ratio of a burner for use in a fuel cell heater, which is disclosed in Patent Literature 2, the raw material (natural gas) is supplied to the burner at the start-up of a gas treatment system. In a state where a carbon monoxide concentration is reduced to such as level that a fuel can be supplied to the fuel cell, a reformate (fuel) that has not been used by the fuel cell is supplied to the burner. 
     Accordingly, during electric power generation by the fuel cell, the burner combusts hydrogen which is the fuel. Therefore, raw material composition information cannot be obtained from the ionization sensor, and it is difficult to adjust the first and second conditions with the fuel cell alone. 
     The present invention has been made in view of the above problems. An object of the present invention is to provide a power generator and a method of operating the same, which are capable of, even in a case where the raw material composition varies such as in a case where natural gas or the like is used as a raw material, adjusting supply amounts of the first raw material and the second raw material which are necessary raw materials for generating a fuel, based on combustion information from a combustion system and a fuel supply amount, thereby making it possible to perform stable electric power generation. 
     Solution to Problem 
     In order to solve the above conventional problems, a power generator according to the present invention includes a fuel cell system, a combustion system, and a controller. The fuel cell system includes: a fuel cell configured to generate electric power by using a fuel and an oxidant; a reformer configured to generate the fuel through a reforming reaction between a first raw material containing a hydrocarbon and a second raw material which is water or an oxidant; a first raw material supply device configured to supply the first raw material to the reformer; a second raw material supply device configured to supply the second raw material to the reformer; and an oxidant supply device configured to supply the oxidant to the fuel cell. The combustion system includes; a first combustor configured to combust the first raw material; a third raw material supply device configured to supply the first raw material to the first combustor; a first air supply device configured to supply air to the first combustor; and a frame rod configured to measure an ionizing current based on the hydrocarbon contained in the first raw material that is being combusted by the first combustor. The controller controls the first raw material supply device and the second raw material supply device based on an amount of the first raw material supplied to the first combustor and a value of the ionizing current, such that an amount of the first raw material supplied to the reformer and an amount of the second raw material supplied to the reformer are in a predetermined ratio. 
     Accordingly, even with use of the first raw material such as natural gas in which a variation in gas composition occurs, the supply amount of the second raw material relative to the supply amount of the first raw material can be adjusted more suitably than conventional fuel cell systems. This makes it possible to perform stable fuel generation and electric power generation. 
     A power generator operating method according to the present invention is a method of operating a power generator including a fuel cell system, a combustion system, and a controller. The fuel cell system includes: a fuel cell configured to generate electric power by using a fuel and an oxidant; a reformer configured to generate the fuel through a reforming reaction between a first raw material containing a hydrocarbon and a second raw material which is water or an oxidant; a first raw material supply device configured to supply the first raw material to the reformer; a second raw material supply device configured to supply the second raw material to the reformer; and an oxidant supply device configured to supply the oxidant to the fuel cell. The combustion system includes; a first combustor configured to combust the first raw material; a third raw material supply device configured to supply the first raw material to the first combustor; a first air supply device configured to supply air to the first combustor; and a frame rod configured to measure an ionizing current based on the hydrocarbon contained in the first raw material that is being combusted by the first combustor. The method includes operating the first raw material supply device and the second raw material supply device based on an amount of the first raw material supplied to the first combustor and a value of the ionizing current, such that an amount of the first raw material supplied to the reformer and an amount of the second raw material supplied to the reformer are in a predetermined ratio. 
     Accordingly, even with use of the first raw material such as natural gas in which a variation in gas composition occurs, the supply amount of the second raw material relative to the supply amount of the first raw material can be adjusted more suitably than conventional fuel cell systems. This makes it possible to perform stable fuel generation and electric power generation. 
     Advantageous Effects of Invention 
     According to the power generator and the method of operating the same of the present invention, even with use of the first raw material such as natural gas in which a variation in gas composition occurs, the supply amount of the second raw material relative to the supply amount of the first raw material can be adjusted more suitably than conventional fuel cell systems. This makes it possible to perform stable fuel generation and electric power generation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing a schematic configuration of a power generator according to Embodiment 1. 
         FIG. 2  is a flowchart schematically showing operations of the power generator according to Embodiment 1. 
         FIG. 3  is a flowchart schematically showing operations of the power generator according to Embodiment 1. 
         FIG. 4  is a schematic diagram showing a schematic configuration of a power generator according to Embodiment 2. 
         FIG. 5  is a flowchart schematically showing operations of the power generator according to Embodiment 2. 
         FIG. 6  is a schematic diagram showing a schematic configuration of a power generator according to Variation 1 of Embodiment 2. 
         FIG. 7  is a flowchart schematically showing operations of the power generator according to Variation 1 of Embodiment 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention are described with reference to the drawings. In the drawings, the same or corresponding components are denoted by the same reference signs, and a repetition of the same description is avoided. In the drawings, only the components necessary for describing the present invention are shown, and the other components are omitted. Further, the present invention is not limited by the embodiments described below. 
     Embodiment 1 
     A power generator according to Embodiment 1 includes a fuel cell system, a combustion system, and a controller. The power generator according to Embodiment 1 serves as an example where the fuel cell system includes: a fuel cell configured to generate electric power by using a fuel and an oxidant; a reformer configured to generate the fuel through a reforming reaction between a first raw material containing a hydrocarbon and a second raw material which is water or an oxidant; a first raw material supply device configured to supply the first raw material to the reformer; a second raw material supply device configured to supply the second raw material to the reformer; and an oxidant supply device configured to supply the oxidant to the fuel cell. Moreover, the combustion system includes; a first combustor configured to combust the first raw material; a third raw material supply device configured to supply the first raw material to the first combustor; a first air supply device configured to supply air to the first combustor; and a frame rod configured to measure an ionizing current value based on the hydrocarbon contained in the first raw material that is being combusted by the first combustor. Furthermore, the controller controls the first raw material supply device and the second raw material supply device based on an amount of the first raw material supplied to the first combustor and the ionizing current value, such that an amount of the first raw material supplied to the reformer and an amount of the second raw material supplied to the reformer are in a predetermined ratio. 
     In the power generator according to Embodiment 1, during a power generation operation of the fuel cell system, the controller may control the first raw material supply device and the second raw material supply device based on an amount of the first raw material supplied to the first combustor, the ionizing current value, and an amount of electric power generated by the fuel cell, such that an amount of the first raw material supplied to the reformer and an amount of the second raw material supplied to the reformer are in the predetermined ratio. 
     In the power generator according to Embodiment 1, during a power generation operation of the fuel cell system, the controller may control the first raw material supply device and the second raw material supply device based on an amount of the first raw material supplied to the first combustor, the ionizing current value, an amount of electric power generated by the fuel cell, and an amount of the first raw material supplied to the reformer, such that the amount of the first raw material supplied to the reformer and an amount of the second raw material supplied to the reformer are in the predetermined ratio. 
     In the power generator according to Embodiment 1, the controller may control the third raw material supply device and the first air supply device based on the ionizing current value, such that an amount of the first raw material supplied to the first combustor and an amount of the air supplied to the first combustor are in a predetermined ratio. 
     [Configuration of Power Generator] 
       FIG. 1  is a schematic diagram showing a schematic configuration of the power generator according to Embodiment 1. 
     As shown in  FIG. 1 , a power generator  100  according to Embodiment 1 includes a fuel cell system  101 , a combustion system  102 , and a controller  103 . The fuel cell system  101  includes a reformer  10 , a fuel cell  11 , a first raw material supply device  12 , a second raw material supply device  13 , and an oxidant supply device  14 . The combustion system  102  includes a first combustor  21 , a third raw material supply device  22 , a first air supply device  23 , and a frame rod  24 . 
     The first raw material supply device  12  is connected to the reformer  10  via a first raw material supply passage  15 . The first raw material supply device  12  is configured to supply a first raw material to the reformer  10  while adjusting the supply amount of the first raw material. Examples of the first raw material supply device  12  include a pump, a fan, and a blower. It should be noted that the proximal end of the first raw material supply passage  15  is connected to a natural gas infrastructure. 
     The second raw material supply device  13  is connected to the reformer  10  via a second raw material supply passage  16 . The second raw material supply device  13  is configured to supply a second raw material to the reformer  10  while adjusting the supply amount of the second raw material. If the second raw material is water for use in steam reforming, the second raw material supply device  13  may be a rotating pump or a plunger pump, for example. If the second raw material is air for use in partial oxidation reforming, the second raw material supply device  13  may be a fan or a blower, for example. 
     The reformer  10  includes a reforming catalyst. The reforming catalyst is, for example, any substance capable of catalyzing a steam reforming reaction through which to generate a hydrogen-containing gas from a raw material and steam. Examples of the reforming catalyst include a ruthenium-based catalyst in which a catalyst carrier such as alumina carries ruthenium (Ru) and a nickel-based catalyst in which a catalyst carrier such as alumina carries nickel (Ni). Moreover, a catalyst capable of catalyzing an autothermal reforming reaction may be used as the reforming catalyst of the reformer  10 . 
     In Embodiment 1, a catalyst catalyzing a steam reforming reaction is used as the reforming catalyst. Accordingly, the present embodiment adopts a mode in which the first raw material is natural gas and the second raw material is water. It should be noted that, if the reforming reaction in Embodiment 1 is a partial oxidation reforming reaction, the second raw material is oxygen or air. 
     The reformer  10  generates a hydrogen-containing gas through the reforming reaction between a supplied raw material and steam. The generated hydrogen-containing gas is supplied to the fuel cell  11  as a fuel. 
     Although in Embodiment 1 the hydrogen-containing gas generated by the reformer  10  is sent to the fuel cell  11  as a fuel, the present embodiment is not thus limited. For example, the fuel cell system  101  may include a shift converter and/or a carbon monoxide remover, the shift converter including a shift conversion catalyst (e.g., a copper-zinc based catalyst), the carbon monoxide remover including an oxidation catalyst (e.g., a ruthenium-based catalyst) or a methanation catalyst (e.g., a ruthenium-based catalyst), and the fuel cell system  101  may be configured such that the generated hydrogen-containing gas is sent to the fuel cell  11  after the hydrogen-containing gas has passed through these devices. 
     The oxidant supply device  14  is connected to the fuel cell  11  via an oxidant passage  17 . The oxidant supply device  14  is configured to supply an oxidant (e.g., air) to the fuel cell  11  while adjusting the supply amount of the oxidant. Examples of the oxidant supply device  14  include a fan and a blower. 
     The fuel cell  11  includes an anode  11 A and a cathode  11 B. In the fuel cell  11 , the fuel and the oxidant supplied to the fuel cell  11  are supplied to the anode  11 A and the cathode  11 B, respectively. Then, the fuel supplied to the anode  11 A and the oxidant supplied to the cathode  11 B react with each other, and thereby electricity and heat are generated. 
     It should be noted that the generated electricity is supplied to an external electrical load (e.g., a household electrical appliance) via a power conditioner which is not shown. Also, the generated heat is recovered by a heating medium flowing through a heating medium passage which is not shown. The heat recovered by the heating medium can be used for heating water, for example. Moreover, the fuel that has not been consumed by the fuel cell  11 , i.e., residual fuel, or the oxidant that has not been consumed by the fuel cell  11 , i.e., residual oxidant, is discharged to the outside of the system of the power generator  100  via an exhaust gas passage  18  connected to the fuel cell  11 . 
     In Embodiment 1, various fuel cells are applicable as the fuel cell  11 , such as a polymer electrolyte fuel cell, a direct internal reforming solid oxide fuel cell, or an indirect internal reforming solid oxide fuel cell. Although in Embodiment 1 the fuel cell  11  and the reformer  10  are configured as separate components, the present embodiment is not thus limited. As an alternative, similar to a solid oxide fuel cell, the reformer  10  and the fuel cell  11  may be integrated. Further, in a case where the fuel cell  11  is configured as a direct internal reforming solid oxide fuel cell, the anode  11 A of the fuel cell  11  and the reformer  10  may be integrated since the anode  11 A of the fuel cell  11  realizes a function of the reformer  10 . Since the configuration of the fuel cell  11  is the same as that of a general fuel cell, a detailed description of the configuration of the fuel cell  11  is omitted. 
     The third raw material supply device  22  is connected to the first combustor  21  of the combustion system  102  via a first raw material supply passage  26 . The third raw material supply device  22  is configured to supply the first raw material to the first combustor  21  while adjusting the supply amount of the first raw material. Examples of the third raw material supply device  22  include a pump, a fan, and a blower. It should be noted that the proximal end of the first raw material supply passage  26  is connected to the natural gas infrastructure. 
     The first air supply device  23  is connected to the first combustor  21  via an air supply passage  27 . The first air supply device  23  is configured to supply air to the first combustor  21  while adjusting the supply amount of the air. Examples of the first air supply device  23  include a fan and a blower. 
     The first combustor  21  combusts the first raw material supplied from the third raw material supply device  22  and the air supplied from the first air supply device  23 . As a result, a flue gas is generated. The generated flue gas is discharged to the outside of the power generator  100  through a flue gas passage  28 . 
     The first combustor  21  is provided with the frame rod  24 . The frame rod  24  is configured to measure the value of an ionizing current based on a hydrocarbon contained in the first raw material that is being combusted by the first combustor  21 , and to output the measured value of the ionizing current to the controller  103 . 
     It should be noted that the reason for the ionizing current value to be measured with use of the frame rod  24  is as follows: the first raw material, which is a hydrocarbon, generates an ionizing current when combusted, and the value of the ionizing current depends on the amount of carbon in the raw material; therefore, a carbon number in the first raw material per unit flow rate can be obtained from the value of the ionizing current and the supply amount of the first raw material. Specifically, in a case where the supply amount of the first raw material is fixed, the carbon number increases if the ionizing current value increases, and the carbon number decreases if the ionizing current value decreases. 
     Accordingly, in a case where the supply amount of the first raw material is fixed, and the supply amount of the first raw material and the supply amount of the second raw material are in a predetermined ratio, if the ionizing current value increases, the controller  103  controls the second raw material supply device  13  to increase the supply amount of the second raw material. Further, in the case where the supply amount of the first raw material is fixed, and the supply amount of the first raw material and the supply amount of the second raw material are in the predetermined ratio, if the ionizing current value decreases, the controller  103  controls the second raw material supply device  13  to decrease the supply amount of the second raw material. 
     The controller  103  is configured to control the fuel cell  11  to generate electric power in accordance with power consumption by an external load. The controller  103  includes: an arithmetic processing unit exemplified by a microprocessor, CPU, or the like; a storage unit configured as, for example, a memory storing programs for executing control operations. Through the loading and execution, by the arithmetic processing unit, of a predetermined control program stored in the storage unit, the controller  103  performs various controls of the power generator  100 . 
     The controller  103  stores therein in advance a calculation formula and/or a table, which are used to determine a carbon number and a hydrogen number in the first raw material per unit flow rate based on the ionizing current value, an amount of the first raw material supplied to the first combustor  21 , and an amount of oxygen supplied to the first combustor  21 . The controller  103  stores therein in advance a ratio between a first raw material supply amount and a second raw material supply amount, the ratio being necessary for causing the reforming reaction. 
     Moreover, the controller  103  stores therein a calculation formula and/or a table, which are used to calculate, based on an electric current value of the fuel cell  11  and reforming reaction efficiency, a supply amount of the first raw material for generating the fuel necessary for electric power generation. In addition, the controller  103  stores therein a calculation formula and/or a table, which are used to calculate a supply amount of the oxidant necessary for electric power generation. 
     Furthermore, the controller  103  stores therein in advance a calculation formula and/or a table, which are used to determine a supply amount of the second raw material based on the following: a supply amount of the first raw material necessary for causing the fuel cell  11  to generate electric power in a particular amount; a carbon number in the first raw material per unit flow rate; and the ratio between the first raw material supply amount and the second raw material supply amount, the ratio being necessary for causing the reforming reaction. 
     The controller  103  uses the above calculation formulas and/or tables to calculate an amount of the first raw material supplied to the reformer  10  and an amount of the second raw material supplied to the reformer  10  based on the following: an amount of the first raw material supplied to the first combustor  21 ; an ionizing current value; and an amount of electric power generated by the fuel cell  11 , which is obtained from a power conditioner not shown. 
     It should be noted that the controller  103  may be configured not only as a single controller, but as a group of multiple controllers which operate in cooperation with each other to control the power generator  100 . Moreover, the controller  103  may be configured as a microcontroller. Furthermore, the controller  103  may be configured as an MPU, PLC (Programmable Logic Controller), logic circuit, or the like. 
     [Operations of Power Generator] 
     First, a method of calculating an amount of the first raw material supplied to the reformer  10  and an amount of the second raw material supplied to the reformer  10  is described. 
     In a case where the first raw material is natural gas, the composition of the natural gas is a mixture of, for example, a hydrocarbon such as methane, propane, or butane with hydrogen or nitrogen. Accordingly, if the hydrocarbon is represented by CαHβ (wherein α and β are positive numbers), the combustion of the first raw material is as shown in (Formula 1) below. 
       CαHβ+(2α+β/2)O 2 →αCO 2 +(β/2)H 2 O  (Formula 1)
 
     In reality, as shown in (Formula 2) below, in order to stabilize the combustion of the first raw material, air is supplied such that the supply amount of oxygen becomes several times (e.g., 1.5 to 2) as great as the supply amount of the first raw material. 
       CαHβ+ a (2α+β/2)O 2 →αCO 2 +(β/2)H 2 O+( a− 1)(2α+β/2)O 2   (Formula 2)
 
     (wherein a is a positive number) 
     As described above, the controller  103  can calculate a carbon number (α in Formula 2) and a hydrogen number (β in Formula 2) in the first raw material per unit flow rate based on an ionizing current value measured by the frame rod  24 , an amount of the first raw material supplied to the first combustor  21 , and an amount of air supplied to the first combustor  21 . 
     The reforming reaction of the first raw material is represented by (Formula 3) or (Formula 4) below. 
       CαHβ+2αH 2 O→((4α+β)/2)H 2 +αCO 2   (Formula 3)
 
       CαHβ+(½)αO 2 →(β/2)H 2 +αCO  (Formula 4)
 
     Typically, the reforming reaction is either a steam reforming reaction using water as the second raw material as shown in (Formula 3), or a partial oxidation reforming reaction using oxygen as the second raw material as shown in (Formula 4). Alternatively, the reforming reaction may be an autothermal reforming reaction which is a combination of steam reforming and partial oxidation reforming. 
     In reality, the supply amount of the second raw material relative to the supply amount of the first raw material is such that the second raw material is supplied in an amount that is several times (e.g., 2.5 to 3.0; predetermined ratio) as great as the supply amount of the first raw material from the viewpoints of, for example, prevention of carbonization of the first raw material, prevention of catalyst degradation, and thermal efficiency. 
       CαHβ+2 b αH 2 O→((4α+β)/2)H 2 +αCO 2 +2( b− 1)αH 2 O  (Formula 5)
 
     (wherein b is a positive number) 
     The fuel cell obtains electric power through reactions shown below. 
       Anode: H 2 →2H + +2 e   −   (Formula 6)
 
       Cathode: ½O 2 +2H + +2 e   − →H 2 O  (Formula 7)
 
       Entire fuel cell: H 2 +½O 2 →H 2 O  (Formula 8)
 
     As shown in (Formula 6), (Formula 7), and (Formula 8), a power generation amount of the fuel cell  11  determines a fuel supply amount necessary for electric power generation. 
     Accordingly, the controller  103  can calculate an amount of the first raw material supplied to the reformer  10  and an amount of the second raw material supplied to the reformer  10  based on the calculated carbon number (α in Formula 2) and hydrogen number (β in Formula 2) in the first raw material per unit flow rate and (Formula 5). Specifically, at the start of the operation of the fuel cell system  101 , the controller  103  can calculate an amount of the first raw material supplied to the reformer  10  and an amount of the second raw material supplied to the reformer  10  based on the following: the calculated carbon number (α in Formula 2) and hydrogen number (β in Formula 2) in the first raw material per unit flow rate; a target temperature of the reformer  10 ; (Formula 5); and reforming reaction efficiency. 
     During a power generation operation of the fuel cell system  101 , the controller  103  can calculate an amount of the first raw material supplied to the reformer  10  and an amount of the second raw material supplied to the reformer  10  based on the following: the calculated carbon number (α in Formula 2) and hydrogen number (β in Formula 2) in the first raw material per unit flow rate; an amount of electric power generated by the fuel cell  11 ; (Formula 5); and reforming reaction efficiency. It should be noted that, in this case, the current supply amount of the first raw material to the reformer  10  may be referred to. 
     Next, operations of the power generator  100  according to Embodiment 1 are described with reference to  FIG. 1  to  FIG. 3 . It should be noted that a power generation operation of the fuel cell system  101  and a combustion operation of the combustion system  102  in the power generator  100  are performed in the same manner as that of a power generation operation of a general fuel cell system and a combustion operation of a general combustion system. Therefore, a detailed description of these operations is omitted. 
       FIG. 2  and  FIG. 3  are flowcharts schematically showing operations of the power generator according to Embodiment 1. Specifically,  FIG. 2  is a flowchart schematically showing operations of the combustion system  102  in the power generator  100 ; and  FIG. 3  is a flowchart showing operations of the fuel cell system  101  in the power generator  100 . 
     Hereinafter, operations of the combustion system  102  in the power generator  100  according to Embodiment 1 are described with reference to  FIG. 2 . 
     As shown in  FIG. 2 , the controller  103  determines whether the combustion system  102  is in operation (step S 101   a ). If the combustion system  102  is in operation (Yes in step S 101   a ), the controller  103  proceeds to step S 104   a . It should be noted that operations in step S 104   a  and thereafter will be described below. 
     On the other hand, if the combustion system  102  is not in operation (No in step S 101   a ), the controller  103  determines whether the fuel cell system  101  is in operation (step S 102   a ). If the fuel cell system  101  is not in operation (No in step S 102   a ), the controller  103  returns to step S 101   a  since it is not necessary to adjust the supply amount of the first raw material and the supply amount of the second raw material (raw material feed ratio adjustment). On the other hand, if the fuel cell system  101  is in operation (Yes in step S 102   a ), the controller  103  starts the operation of the combustion system  102  (step S 103   a ) and proceeds to step S 104   a.    
     In step S 104   a , the controller  103  obtains from the frame rod  24  an ionizing current value measured by the frame rod  24 . Then, the controller  103  amplifies the ionizing current value obtained in step S 104   a , thereby converting the amplified value into an ionizing voltage value (step S 105   a ). 
     Next, the controller  103  calculates a current air supply ratio based on the following: the ionizing voltage value obtained from the conversion in step S 105   a ; and a correlation calculation formula stored in advance in the controller  103 , the formula being used for calculating a correlation between an ionizing voltage value and an air supply ratio (step S 106   a ). Next, the controller  103  determines whether the air supply ratio calculated in step S 106   a  is a predetermined air supply ratio (step S 107   a ). Here, the term air supply ratio refers to the ratio of an air supply amount to a first raw material supply amount. In Embodiment 1, from the standpoint of keeping a normal combustion state, the predetermined air supply ratio is set to be not less than 1.2 and not greater than 1.8. 
     If the air supply ratio calculated in step S 106   a  is not the predetermined air supply ratio (No in step S 107   a ), the controller  103  controls the operating amount of the first air supply device  23 . Specifically, the controller  103  increases/decreases the amount of air supplied from the first air supply device  23  to the first combustor  21  (step S 108   a ). 
     Then, the controller  103  repeats steps S 104   a  to S 108   a  until the air supply ratio calculated in step S 106   a  becomes the predetermined air supply ratio. On the other hand, if the air supply ratio calculated in step S 106   a  is the predetermined air supply ratio (Yes in step S 107   a ), the controller  103  proceeds to step S 109   a.    
     In step S 109   a , the controller  103  stores therein the air supply ratio calculated in step S 106   a  and the current amount of air supplied by the first air supply device  23 , and ends the flow. 
     Next, operations of the fuel cell system  101  in the power generator  100  according to Embodiment 1 are described with reference to  FIG. 3 . 
     As shown in  FIG. 3 , the controller  103  determines whether the fuel cell system  101  is in operation (step S 101   b ). It should be noted that a state where a command to start up the fuel cell system  101  has been inputted into the controller  103  is included in the definition of the fuel cell system  101  being in operation. 
     If the fuel cell system  101  is not in operation (No in step S 102   b ), the controller  103  returns to step S 101   b  since it is not necessary to perform the raw material feed ratio adjustment, and repeats step S 101   b  until it is determined that the fuel cell system  101  is in operation. 
     On the other hand, if the fuel cell system  101  is in operation (Yes in step S 101   b ), the controller  103  stores therein the current supply amount of the first raw material and the current supply amount of the second raw material (step S 102   b ). It should be noted that step S 102   b  is skipped if the fuel cell system  101  is performing a start-up operation. 
     Next, the controller  103  obtains an ionizing current value from the frame rod  24 , and also obtains the air supply ratio stored through the above-described operation (in step S 106   a ) of the combustion system  102  and the amount of air supplied by the first air supply device  23  (step S 103   b ). Then, the controller  103  calculates the composition of the first raw material (i.e., calculates a carbon number and a hydrogen number in the first raw material per unit flow rate) based on the ionizing current value, air supply ratio, and air supply amount obtained in step S 103  and a calculation formula and/or a table stored in the controller  103  (step S 104   b ). 
     Next, the controller  103  obtains the amount of electric power generated by the fuel cell  11  from a power conditioner which is not shown (step S 105   b ). Then, the controller  103  calculates an amount of the first raw material to be supplied to the reformer  10  (step S 106   b ) based on the following: the first raw material composition calculated in step S 104   b ; the power generation amount of the fuel cell  11  obtained in step S 105   b ; and a calculation formula and/or a table stored in the controller  103 . Subsequently, the controller  103  controls the first raw material supply device  12 , such that the supply amount of the first raw material becomes the amount calculated in step S 106  (step S 107   b ). 
     Next, the controller  103  calculates a ratio between a first raw material supply amount and a second raw material supply amount (raw material feed ratio) based on the supply amount of the second raw material stored in step S 102  and the supply amount of the first raw material calculated in step S 106  (step S 108   b ). Then, the controller  103  determines whether the raw material feed ratio calculated in step S 108   b  is a predetermined raw material feed ratio (step S 109   b ). Here, the predetermined raw material feed ratio (supply amount of the second raw material/supply amount of the first raw material) is set to be in the range of 2.5 to 3.0 from the viewpoints of, for example, prevention of carbonization of the first raw material, prevention of catalyst degradation, and thermal efficiency. 
     If the raw material feed ratio calculated in step S 108   b  is not the predetermined raw material feed ratio (No in step S 109   b ), the controller  103  controls the operating amount of the second raw material supply device  13 . Specifically, the controller  103  increases/decreases the amount of second raw material supplied from the second raw material supply device  13  to the reformer  10  (step S 110   b ). 
     Then, the controller  103  repeats steps S 108   b  to S 110   b  until the raw material feed ratio calculated in step S 108   b  becomes the predetermined raw material feed ratio. On the other hand, if the raw material feed ratio calculated in step S 108   b  is the predetermined raw material feed ratio (Yes in step S 109   b ), the controller  103  ends the flow. 
     Thus, even if the raw material composition varies, the supply amount of the second raw material relative to the supply amount of the first raw material can be adjusted, which makes stable fuel generation and electric power generation possible. 
     As described above, even in a case where the raw material composition varies, the power generator  100  according to Embodiment 1 can adjust the supply amount of the second raw material relative to the supply amount of the first raw material, thereby making it possible to perform stable fuel generation and electric power generation. 
     In particular, conventional fuel cell systems are unable to respond to a situation where the raw material composition has varied during a power generation operation of the fuel cell system since the combustor in conventional fuel cell systems combusts an off gas that has not been used by the fuel cell. 
     However, the power generator  100  according to Embodiment 1 is configured to obtain an ionizing current value from the frame rod  24  included in the combustion system  102  (e.g., a boiler) which directly combusts the raw material. Therefore, even in a case where the raw material composition has varied during a power generation operation of the fuel cell system  101 , the supply amount of the second raw material relative to the supply amount of the first raw material can be adjusted, which makes stable fuel generation and electric power generation possible. 
     Embodiment 2 
     A power generator according to Embodiment 2 serves as an example where the fuel cell system further includes a second combustor configured to combust the fuel that has been used by the fuel cell and a second air supply device configured to supply air to the second combustor, the second combustor serving to heat the reformer, and the controller controls the first raw material supply device and the second air supply device based on an amount of the first raw material supplied to the first combustor, an amount of the air supplied to the first combustor, and the ionizing current value, such that an amount of the fuel supplied to the second combustor and an amount of the air supplied to the second combustor are in a predetermined ratio. 
     [Configuration of Power Generator] 
       FIG. 4  is a schematic diagram showing a schematic configuration of the power generator according to Embodiment 2. 
     As shown in  FIG. 4 , the power generator  100  according to Embodiment 2 is fundamentally the same as the power generator  100  according to Embodiment 1, except that the fuel cell system  101  in the power generator  100  according to Embodiment 2 includes: a reforming apparatus  71  including the reformer  10  and a second combustor  72  configured to heat the reformer  10 ; and a second air supply device  73 . 
     The fundamental configuration of the power generator  100  according to Embodiment 2 is the same as that of the power generator  100  according to Embodiment 1. However, the power generator  100  according to Embodiment 2 is different from the power generator  100  according to Embodiment 1, in that the fuel cell system  101  in the power generator  100  according to Embodiment 2 includes: the reforming apparatus  71  including the reformer  10  configured to cause a steam reforming reaction and the second combustor  72 ; and the second air supply device  73 . 
     Among reforming reactions, the steam reforming reaction is different from partial oxidation reforming or autothermal reforming, in that the steam reforming reaction is an endothermic reforming reaction. Accordingly, it is necessary to heat the reforming apparatus by means of a combustor, heater, or the like. Therefore, in the power generator  100  according to Embodiment 2, the reforming apparatus  71  includes the reformer  10  and the second combustor  72 . 
     The fuel cell  11  is connected to the second combustor  72  of the reforming apparatus  71  via an off fuel passage  74 . Accordingly, an off fuel that has not been used at the anode  11 A of the fuel cell  11  flows through the off fuel passage  74  and is supplied to the second combustor  72 . 
     The second air supply device  73  is connected to the second combustor  72  via an air supply passage  75 . The second air supply device  73  is configured to supply air to the second combustor  72  while adjusting the supply amount of the air. Examples of the second air supply device  73  include a fan and a blower. 
     The second combustor  72  performs combustion by using hydrogen which is the off fuel supplied to the second combustor  72  and the air supplied from the second air supply device  73 . As a result, energy necessary for the reforming reaction is generated, and a flue gas is generated. The generated flue gas is discharged to the outside of the system of the power generator  100  through a flue gas passage  76 . 
     The controller  103  stores therein in advance a predetermined air supply ratio for stable combustion in the second combustor  72 , which uses a residual fuel. The term air supply ratio in Embodiment 2 refers to the ratio of an air supply amount to a residual fuel supply amount. It should be noted that, in Embodiment 2, the predetermined air supply ratio is set to be not less than 1.2 and not greater than 2.0 from the standpoint of maintaining a normal combustion state. 
     [Operations of Power Generator] 
     First, a description is given regarding the amount of residual fuel supplied to the second combustor  72  and the amount of air supplied to the second combustor  72 . 
     While the fuel cell  11  is generating electric power, the second combustor  72  combusts hydrogen which is a fuel. At the time, from the standpoint of maintaining a normal combustion state, air is supplied to the second combustor  72  such that the ratio of the supply amount of the air to the supply amount of the residual fuel (off fuel) becomes the predetermined air supply ratio (d in Formula 9). 
       H 2 +½ d O 2 →H 2 O+½(1 −d )O 2   (Formula 9)
 
     As previously described, the controller  103  can calculate a carbon number (α in Formula 2) and a hydrogen number (β in Formula 2) in the first raw material per unit flow rate based on an ionizing current value measured by the frame rod  24 , an amount of the first raw material supplied to the first combustor  21 , and an amount of air supplied to the first combustor  21 . Also, a power generation amount of the fuel cell  11  determines a fuel supply amount necessary for electric power generation. Accordingly, with use of a calculation formula and/or a table stored in the controller  103  in advance, the controller  103  can calculate a residual fuel amount unused by the fuel cell  11  and supplied to the second combustor  72 . 
     Based on the calculated residual fuel supply amount and the predetermined air supply ratio stored in the controller  103  in advance, the controller  103  can calculate an amount of air to be supplied to the second combustor  72 . 
     Next, operations of the power generator  100  according to Embodiment 2 are described with reference to  FIG. 5 . Although the operations of the power generator according to Embodiment 2 are fundamentally the same as those of the power generator according to Embodiment 1, the operations of the power generator according to Embodiment 2 additionally include an operation of stabilizing the combustion of the second combustor of the fuel cell system  101 . 
       FIG. 5  is a flowchart schematically showing the operations of the power generator according to Embodiment 2. Specifically, the flowchart schematically shows operations of the fuel cell system  101 . 
     As shown in  FIG. 5 , in steps S 701   c  to S 706   c , the power generator  00  according to Embodiment 2 performs the same operations as those in steps S 101   b  to S 106   b  shown in  FIG. 3 . Hereinafter, processing in step S 707   c  and thereafter of  FIG. 5  is described. 
     The controller  103  obtains the amount of electric power generated by the fuel cell  11  (step S 705   c ), and calculates a fuel supply amount necessary for electric power generation and an amount of the first raw material to be supplied to the reformer  10  (step S 706   c ). Then, the controller  103  calculates a residual fuel amount unconsumed by the fuel cell  11 , based on the fuel supply amount calculated in step S 706   c  (step S 707   c ). 
     Next, based on the predetermined air supply ratio for the second combustor  72 , which is stored in advance, and the residual fuel amount calculated in step S 707   c , the controller  103  calculates an amount of air to be supplied to the second combustor  72  (step S 708   c ). Then, the controller  103  controls the second air supply device  73  such that the supply amount of air to the second combustor  72  becomes the amount calculated in step S 708   c  (step S 709   c ), and ends the flow. 
     The power generator  100  according to Embodiment 2 with the above-described configuration provides the same operational advantages as those provided by the power generator  100  according to Embodiment 1. 
     Although not shown in  FIG. 5 , the controller  103  calculates an amount of the first raw material supplied to the reformer  10  and an amount of the second raw material supplied to the reformer  10 , and controls the second raw material supply device  13 , in the same manner as in Embodiment 1. 
     [Variation 1] 
     Next, a variation of the power generator  100  according to Embodiment 2 is described. 
     A power generator according to Variation 1 of Embodiment 2 serves as an example where the power generator further includes a temperature detector configured to detect a temperature of the reformer. In the power generator according to Variation 1 of Embodiment 2, during a power generation operation of the fuel cell system, the controller controls the first raw material supply device and the second raw material supply device based on an amount of the first raw material supplied to the first combustor, the ionizing current value, an amount of electric power generated by the fuel cell, and the temperature of the reformer, such that an amount of the first raw material supplied to the reformer and an amount of the second raw material supplied to the reformer are in the predetermined ratio. 
     [Configuration of Power Generator] 
       FIG. 6  is a schematic diagram showing a schematic configuration of the power generator according to Variation 1 of Embodiment 2. 
     As shown in  FIG. 6 , the fundamental configuration of the power generator  100  according to Variation 1 is the same as that of the power generator according to Embodiment 2. However, the power generator  100  according to Variation 1 is different from the power generator according to Embodiment 2, in that the power generator  100  according to Variation 1 includes a temperature detector  77  configured to detect the temperature of the reformer  10 . 
     The temperature detector  77  is configured to detect the temperature of the reformer  10 , and output the detected temperature to the controller  103 . Examples of the temperature detector  77  include a thermocouple and a thermistor. 
     [Operations of Power Generator] 
       FIG. 7  is a flowchart schematically showing operations of the power generator according to Variation 1 of Embodiment 2. Specifically, the flowchart schematically shows operations of the fuel cell system  101 . 
     As shown in  FIG. 7 , in steps S 701   c  to S 709   c , the power generator  100  according to Variation 1 performs the same operations as those in steps S 701   c  to S 709   c  shown in  FIG. 5 . Processing in step S 710   c  of  FIG. 7  is described below. 
     The controller  103  controls the second air supply device  73  such that the supply amount of air becomes the amount calculated in step S 708   c  (step S 709   c ). Then, the controller  103  obtains from the temperature detector  77  the temperature of the reformer  10 , which is detected by the temperature detector  77  (step S 710   c ). 
     Next, the controller  103  controls each device in the fuel cell system  101  based on the temperature of the reformer  10 , which is obtained in step S 710  (step S 711   c ). Specifically, for example, similar to conventional fuel cell systems, if the temperature of the reformer  10  obtained in step S 710  is lower/higher than a predetermined temperature, the controller  103  re-calculates a first raw material supply amount, a second raw material supply amount, and an air supply amount. Then, the controller  103  controls the first raw material supply device  12 , the second raw material supply device  13 , and the second air supply device  73 , such that the first raw material supply amount, the second raw material supply amount, and the air supply amount become the respective calculated amounts. 
     The power generator  100  according to Variation 1 with the above-described configuration provides the same operational advantages as those provided by the power generator  100  according to Embodiment 2. 
     From the foregoing description, numerous modifications and other embodiments of the present invention are obvious to one skilled in the art. Therefore, the foregoing description should be interpreted only as an example and is provided for the purpose of teaching the best mode for carrying out the present invention to one skilled in the art. The structural and/or functional details may be substantially altered without departing from the spirit of the present invention. In addition, various inventions can be made by suitable combinations of a plurality of components disclosed in the above embodiments. 
     INDUSTRIAL APPLICABILITY 
     According to the power generator and the method of operating the same of the present invention, a fuel gas can be stably generated even with use of a raw material in which a variation in gas composition occurs, such as natural gas. As a result, stable and efficient electric power generation is realized. Therefore, the power generator and the method of operating the same according to the present invention are useful in the field of fuel cells. 
     REFERENCE SIGNS LIST 
     
         
           10  reformer 
           11  fuel cell 
           11 A anode 
           11 B cathode 
           12  first raw material supply device 
           13  second raw material supply device 
           14  oxidant supply device 
           15  first raw material supply passage 
           16  second raw material supply passage 
           17  oxidant passage 
           18  exhaust gas passage 
           21  first combustor 
           22  third raw material supply device 
           23  first air supply device 
           24  frame rod 
           26  first raw material supply passage 
           27  air supply passage 
           28  flue gas passage 
           71  reforming apparatus 
           72  second combustor 
           73  second air supply device 
           74  off fuel passage 
           75  air supply passage 
           76  flue gas passage 
           100  power generator 
           101  fuel cell system 
           102  combustion system 
           103  controller