Patent Publication Number: US-2015086888-A1

Title: Method for determining composition of gas fuel, device for determining composition of gas fuel, fuel supply system, and fuel cell system

Description:
TECHNICAL FIELD 
     The present invention relates to a method and a device for determining a composition of a hydrocarbon-based gaseous fuel supplied to a supply target by a fuel supply device, and a fuel supply system and a fuel cell system equipped with the composition determining device. 
     BACKGROUND ART 
     As the related arts relating to the composition determination of the gaseous fuel, there are those described in Patent Document 1 to 3. 
     (1) Patent Document 1 
     As a method for estimating a nitrogen concentration in a fuel off-gas in a fuel cell system, the use of a speed of sound is disclosed. That is, the nitrogen concentration is estimated using the characteristic that the speed of sound of the fuel off-gas varies due to the difference in the ingredients of the fuel off-gas. It is understood that gas to be measured is a mixed gas of hydrogen and nitrogen. 
     (2) Patent Document 2 
     In order to estimate a hydrogen concentration in a fuel off-gas in a fuel cell system, the speed of sound of the fuel off-gas is measured using the characteristic that the speed of sound is significantly fast in hydrogen, and the hydrogen concentration is obtained from the speed of sound. The composition of the gas to be measured is substantially defined by water, carbon dioxide, nitrogen and hydrogen. 
     (3) Patent Document 3 
     By expressing a plurality of hydrocarbon ingredients in the gas with hydrocarbon of one kind or more and less than all kinds, and by measuring the gas characteristics, the number of which is fewer than the number of active ingredients containing the hydrocarbon by one, an effective composition of the gas is determined. 
     REFERENCE DOCUMENT LIST 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-open Publication No. 2005-203143 
     Patent Document 2: Japanese Patent Application Laid-open Publication No. 2003-077518 
     Patent Document 3: Published Japanese Translation of PCT Publication for International Application No. 2004-514138 (WO 2002/040992) 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, the related arts might have the following problems. 
     (1) Patent Document 1 
     There is a description that as the nitrogen concentration in the fuel off-gas increases, the speed of sound in the fuel off-gas varies, and thus, it is possible to estimate the nitrogen concentration by measuring the speed of sound of the fuel off-gas. However, only in such a description, the practical use may be difficult. Furthermore, it is considered that the measurement target is a mixed gas of hydrogen and nitrogen in which the estimation of the composition is easy, and there is no description of gas in which hydrocarbons are complicatedly mixed therein. 
     (2) Patent Document 2 
     The measurement target is defined as a mixed gas of hydrogen, water, carbon dioxide, and nitrogen, and is limited thereto. Furthermore, in order to obtain a correlation, there is a need to measure or calculate the concentrations of water, carbon dioxide, nitrogen, or the like in the off-gas in advance. 
     (3) Patent Document 3 
     It is required that in all of gas characteristics to be measured, the plurality of hydrocarbon ingredients and hydrocarbons to be used as active ingredients, with respect to the predetermined combination, have the same characteristics. For example, in a case of ethane and a mixed gas of methane and propane at the ratio of 1:1, the speed of sounds thereof are approximately the same (an error of about 0.3%), but when comparing the constant pressure specific heats thereof, there is an error of about 3%. Therefore, there may be the problem in the estimation accuracy. 
     In view of such circumstances, an object of the present invention is to provide a method and device for determining a composition capable of accurately performing composition determination of a hydrocarbon-based gaseous fuel, and to provide a fuel supply system and a fuel cell system equipped with the composition determining device. 
     Means for Solving the Problems 
     A method for determining a composition of a gaseous fuel according to the present invention includes the steps of: measuring a speed of sound in the gaseous fuel using an ultrasonic flow meter; estimating a carbon number of the gaseous fuel on the basis of the measured speed of sound; and estimating the composition of the gaseous fuel on the basis of the estimated carbon number. 
     A device for determining a composition of a gaseous fuel according to the present invention, including: a sound speed measuring unit configured to measure a speed of sound in the gaseous fuel using an ultrasonic flow meter; a carbon number estimating unit configured to estimate a carbon number of the gaseous fuel on the basis of the measured speed of sound; and a composition estimating unit configured to estimate the composition of the gaseous fuel on the basis of the estimated carbon number. 
     A fuel supply system according to the present invention is a fuel supply system including a fuel supply device configured to supply a hydrocarbon-based gaseous fuel to a supply target, the fuel supply system including: the composition determining device; and a fuel supply amount control device configured to control a supply amount of fuel of the fuel supply device on the basis of the composition determined by the composition determining device. 
     A fuel cell system according to the present invention including a fuel supply device configured to supply a hydrocarbon-based gaseous fuel, a fuel reforming device configured to generate a hydrogen-rich reformed fuel by reforming the supplied gaseous fuel, and a fuel cell stack configured to perform power generation by an electrochemical reaction between the generated reformed fuel and air, the fuel cell system including the composition determining device, and a fuel supply amount control unit configured to control a supply amount of fuel of the fuel supply device on the basis of the composition determined by the composition determining device. 
     Effects of the Invention 
     According to the present invention, it is possible to relatively accurately estimate the composition of the gaseous fuel. In particular, it is also possible to estimate the composition of gaseous fuel in which hydrocarbons are complicatedly mixed therein. Thus, it is possible to appropriately reflect a variation in composition of gaseous fuel in the control of the supply amount of fuel, in the fuel supply system or the fuel cell system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a fuel supply system according to a first embodiment of the present invention. 
         FIG. 2  is an explanatory diagram of an ultrasonic flow meter. 
         FIG. 3  is a functional block diagram of the composition determining unit of the control device. 
         FIG. 4  is a chart illustrating the relationship between the speed of sound and the carbon number in a CH-based gas. 
         FIG. 5  is a flowchart of composition determination of the gaseous fuel (Part 1). 
         FIG. 6  is a flowchart of composition determination of the gaseous fuel (Part 2). 
         FIG. 7  is a flowchart of composition determination of the gaseous fuel (Part 3). 
         FIG. 8  is a flowchart of fuel supply amount control. 
         FIG. 9  is a schematic configuration diagram of a fuel cell system according to a second embodiment of the present invention. 
         FIG. 10  is a schematic configuration diagram of a fuel cell system illustrating a modified example of the second embodiment. 
         FIGS. 11A and 11B  are explanatory diagrams of modified examples relating to an arrangement position of sensors. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail. 
     First, as a first embodiment of the present invention, an embodiment in which the present invention is applied to composition determination of a gaseous fuel in a general-purpose fuel supply system will be described with reference to  FIGS. 1 to 8 . 
       FIG. 1  is a schematic configuration block diagram of the fuel supply system according to the first embodiment of the present invention. 
     The fuel supply system of the present embodiment includes a fuel supply device  2  that supplies a hydrocarbon-based gaseous fuel to a supply target (various types of gas equipment)  1 . 
     The fuel supply device  2  is constituted by a pump and/or a flow control valve or the like connected to a gaseous fuel supply source, and a supply amount is controlled on the basis of a signal from a control device  7 . 
     In a supply pathway to the supply target  1 , through which the gaseous fuel is supplied by the fuel supply device  2 , a thermal flow meter (mass flow meter)  3 , an ultrasonic flow meter (volumetric flow meter)  4 , a fuel temperature sensor  5 , and a fuel pressure sensor  6  are provided on an upstream side (or a downstream side) of the fuel supply device  2 . The components  3  to  6  may be arranged in any order. 
     The thermal flow meter  3  measures a flow rate using a thermal diffusion effect of the gas, and since an amount of heat transmitted by the gas varies based on a compression degree of the gas, it is possible to measure a mass flow rate. The thermal flow meter  3  has characteristics in which the size reduction is easy and responsiveness is satisfactory due to the size reduction. However, since the output varies depending on the composition of the gas to be measured, it is common to use the thermal flow meter  3  with adjustment for the specified composition of the gas to be measured. 
     The ultrasonic flow meter  4  is able to measure a volumetric flow rate without depending on the composition of the gas to be measured. 
     The measurement principle of the ultrasonic flow meter  4  will be described with reference to  FIG. 2 . 
     The ultrasonic flow meter  4  is formed by disposing ultrasonic transceivers  4   a  and  4   b  each equipped with ultrasonic transducers on an upstream side and a downstream side in a flow direction to face each other, thereby measuring an arrival time t1 of the ultrasonic wave from upstream to downstream (forward direction), and an arrival time t2 of the ultrasonic wave from downstream to upstream (reverse direction). 
     Here, when assuming a flow velocity of gas to be V and a speed of sound in gas to be C, a diffusion speed in the forward direction is C+V, and a diffusion speed in the reverse direction is C−V. 
     Therefore, when assuming a distance to be L, the following formulas are obtained. 
         t 1= L /( C+V )→1/ t 1=( C+V )/ L   (1)
 
         t 2= L /( C−V )→1/ t 2=( C−V )/ L   (2)
 
     Therefore, it is possible to obtain the flow velocity V from Formulae (1) and (2), by Formula (3) described below. In addition, it is possible to obtain the speed of sound C by Formula (4) described below as well. 
         V =(1/ t 1−1/ t 2)× L/ 2  (3)
 
         C =(1/ t 1+1/ t 2)× L/ 2  (4)
 
     Therefore, it is possible to use the ultrasonic flow meter  4  as a sound speed measuring unit for measuring the speed of sound in the gaseous fuel. 
     The fuel temperature sensor  5  measures a fuel temperature Tf in the vicinity of the flow rate measuring portion. The fuel pressure sensor  6  measures a fuel pressure Pf in the vicinity of the flow rate measuring portion. 
     Measurement signals of the thermal flow meter  3 , the ultrasonic flow meter  4 , the fuel temperature sensor  5 , and the fuel pressure sensor  6  are sent to the control device  7 . 
     The control device  7  includes a microcomputer, performs the arithmetic processing in accordance with a control program, and controls the operation of the fuel supply device  2 , while reading signals from various sensors, such as the thermal flow meter  3 , the ultrasonic flow meter  4 , the fuel temperature sensor  5 , and the fuel pressure sensor  6 . The control device  7  may control various other devices, simultaneously with the control of the fuel supply device  2 . 
     Next, the control of a supply amount of fuel performed by the control device  7  will be described. 
     When the composition of the gaseous fuel can be specified, a conversion factor of the thermal flow meter  3  is set on the basis of the specified composition, and then, the supply amount of fuel is controlled using the thermal flow meter  3 . That is, a target supply amount of fuel is set according to target supply conditions, and the supply amount of gaseous fuel is controlled via the fuel supply device  2 . Here, feedback control is performed referring to the measured flow rate of the thermal flow meter  3 . 
     Meanwhile, when the composition of the gaseous fuel varies, the supply amount of fuel is controlled using the ultrasonic flow meter  4  and thermal flow meter  3 , while performing the composition determination. 
     For this reason, the control device  7  is equipped with a composition determining unit (composition determining device)  100  configured to determine the composition of the hydrocarbon-based gaseous fuel supplied to the supply target  1  by the fuel supply device  2  in cooperation with the ultrasonic flow meter  4 , and a fuel supply amount control unit  200  configured to control the supply amount of fuel of the fuel supply device  2  on the basis of the composition determined by the composition determining unit  100 . 
     As illustrated in the functional block diagram of  FIG. 3 , the composition determining unit (composition determining device)  100  includes a sound speed measuring unit  101  configured to measure the speed of sound in the gaseous fuel using the ultrasonic flow meter  4 , a carbon number estimating unit  102  configured to estimate a carbon number of the gaseous fuel on the basis of the measured speed of sound, and a composition estimating unit  103  configured to estimate the composition (ingredients and ingredient concentration) of the gaseous fuel on the basis of the estimated carbon number. 
     The carbon number estimating unit  102  estimates the carbon number from a temperature and the speed of sound, using a table or a function of the carbon number with respect to the speed of sound at each temperature for the CH-based gas as illustrated in  FIG. 4 . 
     The composition estimating unit  103  specifies a sub-ingredient from predetermined sub-ingredient candidates, depending on the comparison result between the estimated carbon number and a carbon number previously determined for a main ingredient of the gaseous fuel, and estimates concentrations of the main ingredient and the specified sub-ingredient. 
     Furthermore, the composition determining unit (composition determining device)  100  includes a first conversion factor calculating unit  104  configured to calculate a conversion factor of the mass flow meter on the basis of the measurement results of the mass flow meter (thermal flow meter  3 ) and the volumetric flow meter (ultrasonic flow meter  4 ), a second conversion factor calculating unit  105  configured to calculate an estimated conversion factor in the composition estimated by the composition estimating unit  103 , and a comparison-and-verification unit  106  configured to compare and verify the estimation results of the composition estimating unit  103  by comparing the calculation results of the first and second conversion factor calculating units  104  and  105 . 
     Furthermore, the composition determining unit (composition determining device)  100  includes a composition re-estimating unit  107  that estimates the composition of gaseous fuel again, assuming that another ingredient is mixed in the gaseous fuel when the comparison-and-verification unit  106  determines that there is a mismatch. 
     For example, the fuel supply amount control unit  200  in the control device  7  estimates a calorific value of the gaseous fuel on the basis of the composition determined by the composition determining unit (composition determining device)  100 , and controls the supply amount of gaseous fuel so that the calorific value per unit time becomes a predetermined value. 
     The function of the composition determining unit (composition determining device)  100  ( 101  to  107 ) is achieved by the flowcharts of  FIGS. 5 to 7 , and the function of the fuel supply amount control unit  200  is achieved by the flowchart of  FIG. 8 . 
       FIGS. 5 to 7  are flowcharts of the composition determination of the gaseous fuel. 
     In S 1 , a speed of sound C of the gaseous fuel is measured on the basis of the intermediate output of the ultrasonic flow meter  4 . 
     In S 2 , a volumetric flow rate Qv of the gaseous fuel is measured on the basis of the output of the ultrasonic flow meter  4 . 
     In S 3 , a temperature Tf of the gaseous fuel is measured on the basis of the output of the fuel temperature sensor  5 . 
     In S 4 , a pressure Pf of the gaseous fuel is measured on the basis of the output of the fuel pressure sensor  6 . 
     In S 5 , a mass flow rate Qm of the gaseous fuel is measured on the basis of the output of the thermal flow meter  3 . 
     In S 6 , a conversion factor CF is calculated by the following formula. 
       Conversion factor  CF=[Qv ×(273.15÷(273.15+ Tf ))×((101.33+ Pf )÷101.33)]/[ Qm÷γ] 
 
     A denominator [Qm÷γ] of the calculation formula of the conversion factor CF is obtained by converting the mass flow rate Qm, the measured value of the thermal flow meter  3 , into a normal flow rate (volumetric flow rate in the standard state). That is, it is obtained by dividing the mass flow rate Qm by the specific gravity γ (specific gravity at the standard state of the design gas composition) and converting the divided value into the volumetric flow rate Q 1  (see the following formula) at the standard state. 
         Q 1= Qm÷γ   
     A numerator of the calculation formula of the conversion factor CF is obtained by converting the volumetric flow rate Qv, the measured value of the ultrasonic flow meter  4 , into a normal value. That is, the numerator is obtained by converting the volumetric flow rate Qv into a flow rate Q2 (see the following formula) at the standard (normal) state, using the fuel temperature Tf and the fuel pressure Pf. 
         Q 2= Qv ×(273.15÷(273.15+ Tf ))×((101.33+ Pf )÷101.33)
 
     Therefore, the conversion factor CF is calculated as a ratio of the normal conversion value Q1 of the measured value of the mass flow meter to the normal conversion value Q2 of the measured value of the volumetric flow meter (see the following formula). 
       Conversion factor  CF=Q 2/ Q 1 
     In S 7  to S 9 , a carbon number of the gaseous fuel is estimated from the speed of sound C of the gaseous fuel. 
     Here, the table of  FIG. 4  or the function is used for the estimation.  FIG. 4  illustrates the relationship between the speed of sound and the carbon number for the CH-based gas in terms of the temperature. That is, a solid line indicates the relationship between the speed of sound and the carbon number at a temperature T1 (for example, −10° C.), and a dotted line indicates the relationship between the speed of sound and the carbon number at a temperature T2 (for example, 60° C.). 
     In S 7 , a carbon number (CN1) at the temperature T1 is obtained from the speed of sound C of the gaseous fuel with reference to the characteristics of the solid line of  FIG. 4 . 
     In S 8 , a carbon number (CN2) at the temperature T2 is obtained from the speed of sound C of the gaseous fuel with reference to the characteristics of the dotted line of  FIG. 4 . 
     In S 9 , a carbon number CN at the temperature Tf (current actual temperature) is obtained on the basis of the carbon number (CN1) at temperature T1 and the carbon number (CN2) at the temperature T2, by interpolation. 
     In S 10 , it is determined whether the carbon number CN of the gaseous fuel estimated by processing in S 6  to S 9  is less than 1. This is to approximate the composition depending on the result of determination. Specifically, this is to specify the sub-ingredient from the predetermined sub-ingredient candidates (hydrogen H2, propane C3H8), depending on the result of comparison between the estimated carbon number CN of the gaseous fuel and the carbon number (1) of methane CH4, which is the main ingredient of the gaseous fuel. 
     In a case of CN&lt;1, the process proceeds to S 11  in the flow of  FIG. 6 , and it is assumed that the sub-ingredient is hydrogen and the gaseous fuel is a mixed gas of methane and hydrogen. In a case of CN&gt;1, the process proceeds to S 21  in the flow of  FIG. 7 , and it is assumed that the sub-ingredient is propane and the gaseous fuel is a mixed gas of methane and propane. 
     The flow of  FIG. 6  in a case of the carbon number CN&lt;1 will be described. 
     In S 11 , it is assumed that the gaseous fuel is a mixed gas of methane CH4 (carbon number is 1) and hydrogen H2 (carbon number is 0). 
     In S 12 , an estimated methane concentration CS(CH4) is calculated from the carbon number CN on the basis of the above-described assumption (see the following formula). 
         CS ( CH 4)= CN× 100(%) 
     In S 13 , an estimated hydrogen concentration CS(H2) is calculated from the estimated methane concentration CS(CH4) (see the following formula). 
         CS ( H 2)=100− CS ( CH 4)
 
     In S 14 , an estimated conversion factor CF′ of the gaseous fuel is calculated from the estimated methane concentration CS(CH4) and the estimated hydrogen concentration CS(H2), using the conversion factor CF(CH4) of methane CH4 and the conversion factor CF(H2) of hydrogen H2 (see the following formula). 
         CF′= 1/[ CS ( CH 4)/ CF ( CH 4)+ CS ( H 2)/ CF ( H 2)] 
     Furthermore, the conversion factor can be calculated by the following general formula. 
       Conversion factor=1/Σ(volumetric fraction of ingredient i/conversion factor of ingredient i)
 
     Furthermore, it is possible to obtain the relationship of the volumetric fraction=molar fraction, in disregard of the compression factor. 
     In S 15 , the conversion factor CF obtained in S 6  is compared to the estimated conversion factor CF obtained in S 14  to determine whether the conversion factors match or do not match. Specifically, it is determined whether the difference between the conversion factors is less than a predetermined slice level SL (CF−CF′&lt;SL). The reason is to verify the composition estimation in S 12  and S 13  by comparing them. 
     In a case of CF−CF′&lt;SL, the composition estimation in S 12  and S 13  is considered to be correct, and the process ends. 
     In a case of CF−CF′&gt;SL, the gaseous fuel is considered to be a mixed gas of methane CH4, hydrogen H2 and nitrogen N2, and the recalculation is performed in S 16  to S 18 . That is, when the result of comparison and verification indicates a mismatch, it is assumed that another ingredient (nitrogen N2) be mixed in the gaseous fuel, and the composition of the gaseous fuel is estimated again. 
     In S 16 , an estimated nitrogen concentration CS(N2) is calculated by the following formula. 
         CS ( N 2)= A/B× 100(%) 
         A= 1/ CF−[CS ( CH 4)/ CF ( CH 4)+ CS ( H 2)/ CF ( H 2)]÷100
 
         B= 1/ CF ( N 2)−[ CS ( CH 4)′/ CF ( CH 4)+ CS ( H 2)′/ CF ( H 2)]÷100
 
     In Formula A, CF is the conversion factor, CS(CH4) is the estimated methane concentration, CF(CH4) is the conversion factor of methane, CS(H2) is the estimated hydrogen concentration, and CF(H2) is the conversion factor of hydrogen. 
     In Formula B, CF(N2) is a conversion factor of nitrogen, CS(CH4)′ is a methane concentration in a nitrogen-equivalent gas, CF(CH4) is the conversion factor of methane, CS(H2)′ is a hydrogen concentration in the nitrogen-equivalent gas, and CF(H2) is the conversion factor of hydrogen. 
     The methane concentration in the nitrogen-equivalent gas is a methane concentration X (%) in a gas of methane and hydrogen at a predetermined mixing ratio, the speed of sound of which is the same as that of nitrogen, and this methane concentration can be obtained by calculation in advance. 
     The hydrogen concentration in the nitrogen-equivalent gas is a hydrogen concentration Y (%) in a gas of methane and hydrogen at a predetermined mixing ratio, the speed of sound of which is the same as that of nitrogen. There is a relationship of Y=100−X. 
     In S 17 , the estimated methane concentration CS(CH4) is calculated again by the following formula. 
         CS ( CH 4)= CS ( CH 4)− CS ( N 2)× CS ( CH 4)′÷100(%)
 
     On the right side of the formula, CS(CH4) is the estimated methane concentration (calculated in S 12 ), CS(N2) is the estimated nitrogen concentration, and CS(CH4)′ is the methane concentration in the nitrogen-equivalent gas. 
     In S 18 , the estimated hydrogen concentration CS(H2) is calculated again by the following formula. 
         CS ( H 2)= CS ( H 2)− CS ( N 2)× CS ( H 2)′÷100(%)
 
     On the right side of the formula, CS(H2) is the estimated hydrogen concentration (calculated in S 13 ), CS(N2) is the estimated nitrogen concentration, and CS(H2)′ is the hydrogen concentration in the nitrogen-equivalent gas. 
     The flow of  FIG. 7  in a case of the carbon number CN&gt;1 will be described. 
     In S 21 , it is assumed that gaseous fuel is a mixed gas of methane CH4 (carbon number is 1) and propane C3H8 (carbon number is 3). 
     In S 22 , an estimated propane concentration CS(C3H8) is calculated from the carbon number CN, on the basis of the assumption (see the following formula). 
         CS ( C 3 H 8)=[ CN− 1]÷2×100(%)
 
     In S 23 , an estimated methane concentration CS(CH4) is calculated from the estimated propane concentration CS(C3H8) (see the following formula). 
         CS ( CH 4)=100− CS ( C 3 H 8)
 
     In S 24 , an estimated conversion factor CF′ of the gaseous fuel is calculated from the estimated methane concentration CS(CH4) and the estimated propane concentration CS(C3H8), using the conversion factor CF(CH4) of methane CH4 and the conversion factor CF(C3H8) of propane C3H8 (see the following formula). 
         CF′= 1/[ CS ( CH 4)/ CF ( CH 4)+ CS ( C 3 H 8)/ CF ( C 3 H 8)] 
     In S 25 , the conversion factor CF obtained in S 6  is compared to the estimated conversion factor CF′ obtained in S 24  to determine whether the conversion factors match or do not match. Specifically, it is determined whether the difference between the conversion factors is less than the predetermined slice level SL (CF−CF′&lt;SL). The reason is to verify the composition estimation in S 22  and S 23  by comparing them. 
     In a case of CF−CF′&lt;SL, the composition estimation in S 22  and S 23  is considered to be correct, and the process ends. 
     In a case of CF−CF′&gt;SL, the gaseous fuel is considered to be a mixed gas of methane CH4, propane C3H8 and nitrogen N2, and the recalculation is performed in S 26  to S 28 . That is, when the result of comparison and verification indicates a mismatch, it is considered that another ingredient (nitrogen N2) be mixed in the gaseous fuel, and the composition of the gaseous fuel is estimated again. 
     In S 26 , an estimated nitrogen concentration CS(N2) is calculated by the following formula. 
         CS ( N 2)= A/B× 100(%) 
         A= 1/ CF−[CS ( CH 4)/ CF ( CH 4)+ CS ( C 3 H 8)/ CF ( C 3 H 8)]÷100
 
         B= 1/ CF ( N 2)−[ CS ( CH 4)′/ CF ( CH 4)+ CS ( C 3 H 8)′/ CF ( C 3 H 8)]÷100
 
     In Formula A, CF is the conversion factor, CS(CH4) is the estimated methane concentration, CF(CH4) is the conversion factor of methane, CS(C3H8) is the estimated propane concentration, and CF(C3H8) is the conversion factor of propane. 
     In Formula B, CF(N2) is a conversion factor of nitrogen, CS(CH4)′ is a methane concentration in a nitrogen-equivalent gas, CF(CH4) is the conversion factor of methane, CS(C3H8)′ is a propane concentration in the nitrogen-equivalent gas, and CF(C3H8) is the conversion factor of propane. 
     The methane concentration in the nitrogen-equivalent gas is a methane concentration X′ (%) in a gas of methane and propane at a predetermined mixing ratio, the speed of sound of which is the same as that of nitrogen, and this methane concentration can be obtained by calculation in advance. 
     The propane concentration in the nitrogen-equivalent gas is a propane concentration Y′ (%) in the gas of methane and propane at a predetermined mixing ratio, the speed of sound of which is the same as that of nitrogen. There is a relationship of Y′=100−X′. 
     In S 27 , the estimated methane concentration CS(CH4) is calculated again by the following formula. 
         CS ( CH 4)= CS ( CH 4)− CS ( N 2)× CS ( CH 4)′÷100(%)
 
     On the right side of the formula, CS(CH4) is the estimated methane concentration (calculated in S 22 ), CS(N2) is the estimated nitrogen concentration, and CS(CH4)′ is the methane concentration in the nitrogen-equivalent gas. 
     In S 28 , the estimated propane concentration CS(C3H8) is calculated again by the following formula. 
         CS ( C 3 H 8)= CS ( C 3 H 8)− CS ( N 2)× CS ( C 3 H 8)′÷100(%)
 
     On the right side of the formula, CS(C3H8) is the estimated propane concentration (calculated in S 23 ), CS(N2) is the estimated nitrogen concentration, and CS(C3H8)′ is the propane concentration in the nitrogen-equivalent gas. 
       FIG. 8  is a flowchart of the fuel supply amount control based on the composition determination. 
     In S 31 , a calorific value of the gaseous fuel (calorific value that occurs when fuel of a certain unit is completely combusted) is calculated. Specifically, the calorific value of the gaseous fuel is calculated from each of the previously prepared known calorific values H(CH4), H(H2) and H(C3H8) of CH4, H2 and C3H8, and the concentration of each ingredients CS(CH4), CS(H2) and CS(C3H8) of CH4, H2 and C3H8, obtained in the flow of  FIGS. 5 to 7 , by the following formula. 
       Calorific value of fuel=[ CS ( CH 4)× H ( CH 4)+ CS ( H 2)× H ( H 2)+ CS ( C 3 H 8)× H ( C 3 H 8)]/100
 
     In S 32 , a supply amount of fuel is calculated on the basis of the calorific value of gaseous fuel obtained in S 31  so as to obtain a target supply calorific value per unit time. Specifically, the supply amount of fuel is calculated by dividing the target supply calorific value per unit time by the calorific value of the gaseous fuel. 
       Supply amount of fuel=(target supply calorific value per unit time)/(calorific value of fuel) 
     The supply amount of fuel calculated in this way is set to a control target value, and the supply amount of fuel is controlled via the fuel supply device  2 . 
     According to the composition determining device of the present embodiment, since the device includes the sound speed measuring unit  101  configured to measure the speed of sound in the gaseous fuel using the ultrasonic flow meter  4 , the carbon number estimating unit  102  configured to estimate the carbon number of the gaseous fuel on the basis of the measured speed of sound, and the composition estimating unit  103  configured to estimate the composition (ingredients and ingredient concentration) of the gaseous fuel on the basis of the estimated carbon number, it is possible to relatively accurately estimate the composition of the gaseous fuel, and in particular, it is also possible to estimate the composition of gaseous fuel in which hydrocarbons are complicatedly mixed therein. 
     Furthermore, according to the composition determining device of the present embodiment, since the carbon number estimating unit  102  estimates the carbon number from the temperature and the speed of sound on the basis of the characteristic of the carbon number with respect to the speed of sound at each temperature (specifically, using the table or function), it is possible to easily and accurately estimate the carbon number. 
     Furthermore, according to the composition determining device of the present embodiment, since the composition estimating unit  103  specifies the sub-ingredient from the predetermined sub-ingredient candidates (H2, C3H8) depending on the result of comparison between the estimated carbon number and the carbon number (1) previously determined for the main ingredient (CH4) of the gaseous fuel, and the composition estimating unit  103  estimates the concentrations of the main ingredient (CH4) and the specified sub-ingredient (H2 or C3H8), it is possible to perform the relatively simple estimation. 
     Furthermore, according to the composition determining device of the present embodiment, since the device includes the mass flow meter (thermal flow meter  3 ) configured to measure the mass flow rate of the gaseous fuel supplied by the fuel supply device  2 , the volumetric flow meter (ultrasonic flow meter  4 ) configured to measure the volumetric flow rate of the gaseous fuel supplied by the fuel supply device  2 , the first conversion factor calculating unit  104  configured to calculate a conversion factor CF of the mass flow meter on the basis of the measurement results of the mass flow meter and the volumetric flow meter, the second conversion factor calculating unit  105  configured to calculate the estimated conversion factor CF′ in the composition estimated by the composition estimating unit  103 , and the comparison-and-verification unit  106  configured to compare and verify the estimation results of the composition estimating unit  103  by comparing the calculation results of the first and second conversion factor calculating units  104  and  105 , it is possible to improve reliability of the estimation. 
     Furthermore, according to the present embodiment, since the volumetric flow meter is the ultrasonic flow meter  4 , and the sound speed measuring unit  101  measures the speed of sound in the gaseous fuel on the basis of the intermediate output of the ultrasonic flow meter  4 , it is possible to reduce an increase in cost. 
     Furthermore, according to the composition determining device of the present embodiment, since the device includes the composition re-estimating unit  107  that estimates the composition of gaseous fuel again assuming that another ingredient (N2) is mixed in the gaseous fuel when the comparison-and-verification unit  106  determines that there is a mismatch, it is possible to further improve the reliability of the estimation. 
     Furthermore, according to the fuel supply system of the present embodiment, since there is provided the fuel supply amount control unit  200  configured to control the supply amount of fuel of the fuel supply device  2  on the basis of the composition determined by the composition determining unit  100 , it is possible to appropriately reflect a variation in composition of the gaseous fuel in the control of the supply amount of fuel. 
     Furthermore, according to the fuel supply system of the present embodiment, since the fuel supply amount control unit  200  estimates the calorific value of gaseous fuel on the basis of the composition estimated by the composition estimating unit  103 , and controls the supply amount of gaseous fuel so that the calorific value per unit time becomes the predetermined value, it is possible to perform the appropriate control on the basis of the composition estimation of the gaseous fuel and it is possible to maintain the constant supply calorific value per unit time, even when the composition of the gaseous fuel varies. In other words, it is possible to control the supply amount of gaseous fuel by taking the calorific value as the management value. 
     Although, in the above-described embodiment, the supply target (gas equipment) of the fuel supply system was not described, the supply target is not particularly limited and may be used as a general-purpose fuel supply system. Examples of the applications thereof may include the combustion application requiring the control of the air-fuel ratio, and the chemical reaction application requiring the grasp of the raw material composition. Individual examples of combustion applications may include a boiler and a gas engine. The gas engine is used for a household cogeneration system (a system, in which a gas engine fueled by city gas or LPG performs power generation, and heat generated thereby is used to supply hot water; common name “ECOWILL (R)”), a GHP system (an air conditioning system that drives a compressor by a gas engine and performs cooling and heating by a heat-pump operation), an industrial private power generation system, or the like. Individual examples of the chemical reaction applications may include a synthetic gas manufacturing device. The synthetic gas manufacturing device uses natural gas, LPG or the like as a raw material, and produces the synthetic gas mainly containing H2 and CO, using various reforming methods. The produced synthetic gas is used as a raw material gas for ammonia synthesis, methanol synthesis or the like. Recently, a demand for synthetic gas in association with new energy production such as GTL, DME, or SNG has been focused. 
     Next, as a second embodiment of the present invention, an embodiment in which the present invention is applied to a fuel cell system will be described with reference to  FIG. 9 . 
       FIG. 9  is a schematic configuration diagram of the fuel cell system according to the second embodiment of the present invention. 
     The fuel cell system of the present embodiment includes a fuel reforming device  11  configured to generate a reformed hydrogen-rich fuel by performing the steam reforming of the hydrocarbon-based gaseous fuel, and a fuel cell stack (assembly of fuel cells)  12  configured to generate electricity by electrochemical reaction between the reformed fuel (hydrogen) from the fuel reforming device  11  and air (oxygen). 
     The gaseous fuel from the gaseous fuel supply source is subjected to desulfurization removing sulfur compounds in the gaseous fuel by a desulfurizer  13 , and then the gaseous fuel is supplied to the fuel reforming device  11  by a fuel supply device  14 . 
     The fuel supply device  14  is constituted by a pump and/or a flow rate control valve or the like, and a supply amount of gaseous fuel is controlled based on a signal from a control device  21 . 
     In the supply pathway to the fuel reforming device  11 , through which the gaseous fuel is supplied by the fuel supply device  14 , a thermal flow meter (mass flow meter)  15 , an ultrasonic flow meter  16 , a fuel temperature sensor  17 , and a fuel pressure sensor  18  are provided on an upstream side (or a downstream side) of the fuel supply device  14 . The components  15  to  18  may be arranged in any order. 
     Since the thermal flow meter  15 , the ultrasonic flow meter  16 , the fuel temperature sensor  17 , and the fuel pressure sensor  18  are the same as the thermal flow meter  3 , the ultrasonic flow meter  4 , the fuel temperature sensor  5 , and the fuel pressure sensor  6  of the first embodiment ( FIG. 1 ), respectively, the description thereof will not be provided. 
     Measurement signals of the thermal flow meter  15 , the ultrasonic flow meter  16 , the fuel temperature sensor  17 , and the fuel pressure sensor  18  are sent to the control device  21 . 
     Although, in  FIG. 9 , the desulfurizer  13  is disposed on the upstream side of the fuel supply device  14 , the thermal flow meter  15 , the ultrasonic flow meter  16 , the fuel temperature sensor  17 , and the fuel pressure sensor  18 , the desulfurizer  13  may be disposed on the downstream side of the fuel supply device  14 , the thermal flow meter  15 , the ultrasonic flow meter  16 , the fuel temperature sensor  17 , and the fuel pressure sensor  18 , as illustrated in  FIG. 10 . The arrangement of  FIG. 9  is preferable to satisfactorily perform the supply control and the measurement of gaseous fuel even when the gaseous fuel is adsorbed to the desulfurizer  13 . Therefore, for a low-temperature adsorption type desulfurizer, the arrangement of  FIG. 9  may be adopted. In contrast, for a desulfurizer operating at a high temperature such as a high-temperature adsorption type and a hydrodesulfurization type, there is a need to avoid the fuel supply device  14  or the like from being placed in the high-temperature environments, and there is no problem of adsorption of the gaseous fuel, the arrangement of  FIG. 10  may be adopted. 
     The for-reforming water for performing the steam reforming in the fuel reforming device  11  is supplied by a for-reforming water supply device  19 . The for-reforming water supply device  19  is constituted by a pump and/or a flow rate control valve or the like connected to a water supply source, and the supply amount of for-reforming water is controlled based on a signal from the control device  21 . 
     The reformed fuel from the fuel reforming device  11  is supplied to an anode (A) of the fuel cell stack  12 , and air is supplied to a cathode (C) thereof. The cathode air is supplied by a cathode air supply device  20 . The cathode air supply device  20  is constituted by a pump (blower) and/or a flow rate control valve or the like connected to the air supply source, and the supply amount is controlled based on a signal from the control device  21 . 
     The control device  21  includes a microcomputer, performs the arithmetic processing in accordance with a control program, and controls the operation of various types of equipment, such as the fuel supply device  14 , the for-reforming water supply device  19 , and the cathode air supply device  20 , while reading signals from various sensors, such as the thermal flow meter  15 , the ultrasonic flow meter  16 , the fuel temperature sensor  17 , and the fuel pressure sensor  18 . 
     In the fuel reforming device  11 , in a case of methane CH4, the steam reforming reaction of Formula (5) described below is carried out, and in a case of propane C3H8, the steam reforming reaction of Formula (6) described below is carried out. 
       CH 4 +2H 2 O→CO 2 +4H 2   (5)
 
       C 3 H 8 +6H 2 O→3CO 2 +10H 2   (6)
 
     Furthermore, in the fuel cell stack  12 , the electrode reaction of Formula (7) described below occurs at the cathode of each cell, and the electrode reaction of Formula (8) described below occurs at the anode, thereby performing the power generation. 
       Cathode: 1/2O 2 +2 e   − →O 2− (solid electrolyte)  (7)
 
       Anode: O 2− (solid electrolyte)+H 2 →H 2 O+2 e   −   (8)
 
     Next, the control of fuel, the for-reforming water, and the cathode air using the control device  21  will be described. 
     When the composition of gaseous fuel can be specified, the supply amount of fuel is controlled using the thermal flow meter  15  after setting the conversion factor of the thermal flow meter  15  on the basis of the specified composition. That is, a supply amount of fuel (target value) is set in accordance with a target power generation conditions, and a supply amount of gaseous fuel is controlled via the fuel supply device  14 . At this time, feedback control is performed, referring to the measured flow rate of the thermal flow meter  15 . Furthermore, a supply amount of for-reforming water (target value) is set, referring to the measured flow rate of the thermal flow meter  15 , for example, and a supply amount of for-reforming water is controlled via the for-reforming water supply device  19 . Furthermore, a supply amount of cathode air (target value) is set, according to the target power generation condition or referring to the measured flow rate of the thermal flow meter  15 , for example, and a supply amount of cathode air is controlled via the cathode air supply device  20 . 
     Meanwhile, when the composition of gaseous fuel varies, the supply amount of fuel, the supply amount of for-reforming water, and the supply amount of cathode air are controlled using the ultrasonic flow meter  16  and the thermal flow meter  15 , while performing the composition determination. 
     For this reason, the control device  21  is equipped with a composition determining unit (composition determining device)  100  configured to determine the composition of the hydrocarbon-based gaseous fuel supplied to the fuel reforming device  11  by the fuel supply device  14  in cooperation with the ultrasonic flow meter  16 , a fuel supply amount control unit  201  configured to control the supply amount of fuel of the fuel supply device  2  on the basis of the composition determined by the composition determining unit  100 , a for-reforming water supply amount control unit  202  configured to control the supply amount of for-reforming water of the for-reforming water supply device  19 , and a cathode air supply amount control unit  203  configured to control the supply amount of cathode air of the cathode air supply device  20 . 
     The composition determining unit  100  is the same as the composition determining unit  100  described in the first embodiment, and is configured as illustrated in the functional block diagram of  FIG. 3  to perform the arithmetic processing in accordance with the flowcharts of  FIGS. 5 to 7 . Therefore, the description thereof will not be provided here. 
     On the basis of the composition of gaseous fuel estimated by the composition determining unit  100 , the fuel supply amount control unit  201  calculates, for example, quadruple values of the equivalent of the oxygen molecules capable of reacting at the time of complete combustion of the gaseous fuel, and calculates a supply amount of fuel on the basis of the quadruple values, the sweep DC current, and the fuel utilization rate. 
     The for-reforming water supply amount control unit  202  calculates, for example, the carbon number of gaseous fuel on the basis of the composition of gaseous fuel estimated by the composition determining unit  100 , and calculates the supply amount of for-reforming water on the basis of the carbon number. 
     For example, in a case of a mixed gas of methane CH4 and propane C3H8, the supply amount of for-reforming water (target value) is calculated from the supply amount of fuel and the composition of gaseous fuel (molar fraction CS(CH4) of methane CH4 and the molar fraction CS(C3H8) of propane C3H8) by the following formula. 
       Supply amount of for-reforming water=supply amount of fuel×( CS ( CH 4)×1+ CS ( C 3 H 8)×3)/22.414× S/C× 18.02
 
     In the formula, “1” is the carbon number of methane CH4, “3” is the carbon number of propane C3H8, “22.414” is the molar volume (L/mol), S/C is a steam-carbon ratio (target value, for example, 2.5), and “18.02” is the molar mass of water. 
     The cathode air supply amount control unit  203  calculates the supply amount of cathode air, on the basis of the composition of gaseous fuel estimated by the composition determining unit  100  or on the basis of the target power generation conditions including the air utilization rate or the air ratio. 
     When performing the calculation on the basis of the target power generation conditions, the supply amount of cathode air (target value) is calculated by the following formula. 
       Supply amount of air=[total number of cell×current amount of single cell×60)/[ Ua×Fd ×(molar fraction of O2 in the air×4)]×22.414
 
     Here, Ua is a predetermined rate of air utilization (target value, for example, 0.30), Fd is the Faraday constant (96485.3399), and “22.414” is the molar volume (L/mol). Furthermore, “4” is the number of electrons generated from O2 due to the electrode reaction (see the above-described electrode reaction formula). 
     Although the supply amount of cathode air is calculated based on the target power generation conditions, it may be calculated based on the supply amount of fuel and the air-fuel ratio. 
     The supply amount of fuel, the supply amount of for-reforming water, and the supply amount of cathode air calculated in this way are set as the control target values, respectively, and the controls thereof are performed via the fuel supply device  14 , the for-reforming water supply device  19 , and the cathode air supply device  20 . 
     According to the fuel cell system of the present embodiment, since there is provided the fuel supply amount control unit  201  configured to control the supply amount of fuel of the fuel supply device  14  to the fuel reforming device  11  on the basis of the composition determined by the composition determining unit  100 , it is possible to appropriately reflect the composition variation of gaseous fuel in the fuel control to the fuel reforming device  11  and thus in the power generation control of the fuel cell system, and it is possible to improve the stability of the fuel cell system. 
     Furthermore, according to the fuel cell system of the present embodiment, since there is provided the for-reforming water supply device  19  configured to supply water for steam reforming to the fuel reforming device  11 , and the for-reforming water supply amount control unit  202  configured to control the supply amount of water of the for-reforming water supply device  19  on the basis of the composition determined by the composition determining unit  100 , it is possible to appropriately reflect the composition variation of the gaseous fuel in the supply control of the for-reforming water. 
     Furthermore, according to the fuel cell system of the present embodiment, since there is provided the cathode air supply device  20  configured to supply the cathode air to the fuel cell stack  12 , and the cathode air supply amount control unit  203  configured to control the amount of air supply of the cathode air supply device  20  on the basis of the composition determined by the composition determining unit  100 , it is possible to appropriately reflect the composition variation of the gaseous fuel in the supply control of the cathode air. 
     Alternatively, in the fuel cell system, when a supply device of for-reforming air is provided in order to jointly use the partial oxidation reaction as the reforming reaction, a for-reforming air supply amount control unit configured to control the supply amount of for-reforming air may be provided. 
     For example, in a case of a mixed gas of methane and propane, the supply amount of for-reforming air is calculated from the supply amount of fuel, the composition of the gaseous fuel (molar fraction CS(CH4) of methane CH4, and molar fraction CS(C3H8) of propane C3H8) by the following formula. 
       Supply amount of for-reforming air=supply amount of fuel×( CS ( CH 4)×1+ CS ( C 3 H 8)×3)/(oxygen concentration in the air)× O/C  
 
     In the formula, O/C is an oxygen-carbon ratio. 
     Finally, the arrangement positions of the thermal flow meter  3  or the like in the first embodiment ( FIG. 1 ) and the thermal flow meter  15  or the like in the second embodiment ( FIG. 9  or  FIG. 10 ) will be additionally described. 
     In  FIG. 1 , the thermal flow meter  3 , the ultrasonic flow meter  4 , the fuel temperature sensor  5 , and the fuel pressure sensor  6  (hereinafter, these are referred to as “sensors  3  to  6 ”) are disposed on the upstream side of the fuel supply device  2 . Similarly, in  FIG. 9  (or  FIG. 10 ), the thermal flow meter  15 , the ultrasonic flow meter  16 , the fuel temperature sensor  17 , and the fuel pressure sensor  18  (hereinafter, these are referred to as “sensors  15  to  18 ”) are disposed on the upstream side of the fuel supply device  14 . The reason is that a pump is generally included in the fuel supply device, and when disposing the flow meter on the downstream side thereof, there is a possibility of influence on the measurement of the flow meters due to pulsating flow of the pump. 
     Therefore, when it is preferred to stably grasp the composition of the supplied fuel as accurate as possible, it is preferable to dispose the sensors on the upstream side of the fuel supply device. 
     In contrast, when it is preferred to grasp the flow rate of fuel supplied to the supply target  1  (fuel reforming device  11 ) in practice, it is preferable to dispose the sensors on the downstream side of the fuel supply device. Thus, in this case, as illustrated in  FIG. 11A , the sensors  3  to  6  are disposed on the downstream side of the fuel supply device  2 , or as illustrated in  FIG. 11B , the sensors  15  to  18  are disposed on the downstream side of the fuel supply device  14 . 
     Although in the above-described embodiments, the main ingredient of the hydrocarbon-based gaseous fuel is methane CH4, the sub-ingredient candidates are hydrogen H2 and propane C3H8, and the other ingredient mixed into the gaseous fuel is nitrogen N2, the invention is not limited thereto. 
     Furthermore, as the hydrocarbon-based gaseous fuel, city gas, LPG or the like is assumed. In particular, in many cases, when using city gas derived from natural gas, the composition may vary depending on gas companies or depending on regions even in a case of the same gas company, and in addition, the composition may vary due to mixing of the air on the way or the like depending on a pathway or a distance from a supply source. When the component varies in this way, the thermal flow meter may not indicate the correct value, and accordingly it may be difficult to perform a preferred control. 
     In this regard, in the present invention, even when the composition of gaseous fuel varies, it is possible to accurately estimate the composition of the gaseous fuel, and thus, it is possible to appropriately reflect a variation in composition of the gaseous fuel in control of the supply amount of fuel in the fuel supply system or the fuel cell system. 
     Furthermore, even when the gaseous fuel changes, for example, from city gas to LPG or from LPG to city gas, it is possible to automatically detect the change in the gaseous fuel (composition variation). Moreover, by notifying the change to the control device, it is possible to suitably cope with the change by, for example, changing the control program of the control device to that corresponding to the change in gaseous fuel. 
     Furthermore, as for city gas, it is also possible to apply the present invention to a gas mixed with a hydrocarbon gas derived from a bio-gas or derived from a shale gas, or a gas mixed with an off-gas (hydrocarbon gas that is not derived from natural gas) produced as a byproduct in petrochemical plants. 
     Furthermore, the illustrated embodiments are intended to merely illustrate examples of the present invention, and it is needless to say that the present invention covers various improvements and modifications to be made by those skilled in the art within the scope of the appended claims, in addition to those directly illustrated by the described embodiments. 
     REFERENCE SYMBOL LIST 
     
         
           1  Supply target 
           2  Fuel supply device 
           3  Thermal flow meter 
           4  Ultrasonic flow meter 
           5  Fuel temperature sensor 
           6  Fuel pressure sensor 
           7  Control device 
           11  Fuel reforming device 
           12  Fuel cell stack 
           13  Desulfurizer 
           14  Fuel supply device 
           15  Thermal flow meter 
           16  Ultrasonic flow meter 
           17  Fuel temperature sensor 
           18  Fuel pressure sensor 
           19  For-reforming water supply device 
           20  Cathode air supply device 
           21  Control device