# Patent

## Title
Method for load following operation of fuel cell system

## Abstract

Disclosed method of load-following operation of fuel-cell system comprises pre-determining functions F=f(P) and P=f−1(F), wherein P is the electric output and F is the fuel flow-rate required to output P. If reformable flow-rate FR<Fmin (the minimum flow-rate value), power generation is stopped. If FR≧Fmin and if required output PD≦maximum power output PM, (1) is performed; and if FR≧Fmin and if PD>PM, (2) is performed. (1) If f(PD)≦FR, the output is set at PD, and the fuel flow-rate is set at f(PD); and if f(PD)>FR, the output is set at the maximum value of P lower than PD and computed using P=f−1(FR), and the fuel flow-rate is set at FR. (2) If f(PM)≦FR, the output is set at PM, and the fuel flow-rate is set at f(PM); and if f(PM)>FR, the output is set at the maximum value of P computed using P=f−1(FR), and fuel flow-rate is set at FR.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This is a U.S. national stage of application No. PCT/JP2010/060819 filed on Jun. 25, 2010. Priority under 35 U.S.C. §119(a) and 35 U.S.C. §365(b) is claimed from Japanese Application No. 2009-157809, filed Jul. 2, 2009, the disclosure of which is also incorporated herein by reference.
TECHNICAL FIELD
The present invention relates to a method for load following operation of a fuel cell system that generates electric power using a reformed gas obtained by reforming a hydrocarbon-based fuel, such as kerosene.
BACKGROUND ART
A solid oxide fuel cell (hereinafter sometimes referred to as SOFC) system usually includes a reformer for reforming a hydrocarbon-based fuel, such as kerosene and city gas, to generate a hydrogen-containing gas (reformed gas), and an SOFC for electrochemically reacting the reformed gas and air for electric power generation.
The SOFC is usually operated at a high temperature of 550 to 1000° C.
Various reactions, such as steam reforming (SR), partial oxidation reforming (POX), and autothermal reforming (ATR), are used for reforming, and heating to a temperature at which catalytic activity is exhibited is necessary for using a reforming catalyst.
Steam reforming is a very largely endothermic reaction. Also, the reaction temperature of the steam reforming is 550 to 750° C., which is relatively high, and the steam reforming requires a high temperature heat source. Therefore, an internal reforming SOFC is known in which a reformer (internal reformer) is installed near an SOFC, and the reformer is heated mainly using radiant heat from the SOFC as a heat source (Patent Literature 1).
Also, proposals on the load following operation of a fuel cell system are made in Patent Literature 2 and 3.
PRIOR ART LITERATURES
Patent Literature


    Patent Literature 1: JP2004-319420A
    Patent Literature 2: JP2001-185196A
    Patent Literature 3: JP2006-32262A


SUMMARY OF INVENTION
Problems to be Solved by the Invention
When a hydrocarbon-based fuel is not reformed to a predetermined composition, and an unreformed component is supplied to an SOFC, anode degradation and flow blockage due to carbon deposition may occur, particularly when a heavy hydrocarbon, such as kerosene, is used as the hydrocarbon-based fuel.
An SOFC system may be subjected to load following operation. In other words, an SOFC system may be subjected to an operation in which the amount of electric power generation of the SOFC system is varied according to the fluctuation of electric power demand. For example, when the amount of electric power generation is increased, the feed rate of the hydrocarbon-based fuel to the SOFC system may be increased. In such a case, carbon may be deposited. Therefore, it is desired to reliably reform the hydrocarbon-based fuel also in the load following operation. In the arts disclosed in Patent Literature 2 and 3, improvement is still desired in terms of performing reliable reforming.
This is true not only for the SOFC system, but also for a fuel cell system having a high temperature fuel cell, such as a molten carbonate fuel cell (MCFC).
It is an object of the present invention to provide a method in which, when performing load following operation of a fuel cell system including a reformer having a reforming catalyst layer and a high temperature fuel cell, reforming can be more reliably performed to more reliably prevent flow blockage and anode degradation.
Means for Solving the Problems
According to the present invention, embodiments of a method for load following operation of a fuel cell system are provided as described below.
1) A method of load following operation of a fuel cell system including a reformer having a reforming catalyst layer, for reforming a hydrocarbon-based fuel to produce a reformed gas containing hydrogen, and a high temperature fuel cell for generating electric power using the reformed gas, wherein
functions F=f(P) and P=f−1(F) of an electrical output P of the fuel cell and a flow rate F of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to output the electrical output P from the fuel cell are obtained beforehand,
where P=f−1(F) is an inverse function of F=f(P),
a maximum electrical output of the fuel cell is represented as PM,
a minimum value of a flow rate of the hydrocarbon-based fuel determined by the function F=f(P), when P is within a range of 0 or more and PM or less, is represented as Fmin,
and a plurality of temperatures Tj of the reforming catalyst layer (j is an integer of 1 or more and N or less, where N is an integer of 2 or more) and a flow rate Gj of the hydrocarbon-based fuel that corresponds to each Tj are set beforehand,
where each Gj is a flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer at a corresponding reforming catalyst layer temperature Tj, each Gj is larger than 0, and Gj is the same value or increases with an increase of j,
the method including:
A) measuring a temperature T of the reforming catalyst layer;
B) adopting Gj corresponding to a largest Tj that is equal to or less than the temperature T as a reformable flow rate FR that is a flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer at the temperature T;
C) when the reformable flow rate FR is smaller than the minimum value Fmin, stopping electric power generation in the fuel cell; and
D) when the reformable flow rate FR is equal to or more than the minimum value Fmin,
performing step d1 if a fuel cell output demand value PD is equal to or less than the maximum electrical output PM, and performing step d2 if the fuel cell output demand value PD exceeds the maximum electrical output PM,
d1) calculating a flow rate f(PD) of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to output the fuel cell output demand value PD from the fuel cell, using the function F=f(P), and
if f(PD) is equal to or less than the reformable flow rate FR, then setting an electrical output of the fuel cell to PD, and setting a flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer to f(PD), and
if f(PD) exceeds the reformable flow rate FR, then setting the electrical output of the fuel cell to a value that is less than PD and the maximum among one or more P values calculated from P=f−1(FR), and setting the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer to FR,
d2) calculating a flow rate f(PM) of the hydrocarbon-based fuel supplied to the reforming catalyst layer required to be supplied to the reforming catalyst layer in order to output the maximum electrical output PM from the fuel cell, using the function F=f(P), and
if f(PM) is equal to or less than the reformable flow rate FR, then setting the electrical output of the fuel cell to PM, and setting the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer to f(PM), and
if f(PM) exceeds the reformable flow rate FR, then setting the electrical output of the fuel cell to a value that is the maximum among one or more P values calculated from P=f−1(FR), and setting the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer to FR.
2) The method according to 1), wherein steps A to D are repeatedly performed during the load following operation.
3) The method according to 1) or 2), wherein the hydrocarbon-based fuel includes a hydrocarbon-based fuel(s) with a carbon number of two or more.
4) The method according to 3), wherein the concentration of a compound(s) with a carbon number of two or more in the reformed gas is 50 ppb or less on a mass basis.
Advantages of the Invention
The present invention provides a method in which, when performing load following operation of a fuel cell system including a reformer having a reforming catalyst layer and a high temperature fuel cell, reforming can be more reliably performed to more reliably prevent flow blockage and anode degradation.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing the outline of an example of an indirect internal reforming SOFC system that can perform the present invention.
FIG. 2 is a schematic graph showing a correlation between electrical output P of a fuel cell and flow rate F of a hydrocarbon-based fuel required to be supplied to a reforming catalyst layer in order to obtain the electrical output P, for explaining the method of the present invention.
FIG. 3 is a schematic graph showing the correlation between electrical output P of the fuel cell and flow rate F of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to obtain the electrical output P, for explaining the method of the present invention.
FIG. 4 is a schematic graph showing the correlation between electrical output P of the fuel cell and flow rate F of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to obtain the electrical output P, for explaining the method of the present invention.
FIG. 5 is a schematic graph showing the correlation between electrical output P of the fuel cell and flow rate F of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to obtain the electrical output P, for explaining the method of the present invention.
FIG. 6 is a schematic graph showing a correlation between electrical output P of a fuel cell and flow rate F of a hydrocarbon-based fuel required to be supplied to a reforming catalyst layer in order to obtain the electrical output P, for explaining the method of the present invention.



EMBODIMENTS FOR CARRYING OUT THE INVENTION
A fuel cell system used in the present invention includes a reformer for reforming a hydrocarbon-based fuel to produce a hydrogen-containing gas, and a high temperature fuel cell. The reformer includes a reforming catalyst layer. The hydrogen-containing gas obtained from the reformer is referred to as reformed gas. The reforming catalyst layer is composed of a reforming catalyst that can promote a reforming reaction. The high temperature fuel cell generates electric power, using the hydrogen-containing gas (reformed gas) obtained from the reformer.
The embodiments of the present invention will be described below, using drawings, but the present invention is not limited thereto.
[Indirect Internal Reforming SOFC]
One embodiment of an indirect internal reforming SOFC that can perform the present invention is schematically shown in FIG. 1. Here, the indirect internal reforming SOFC system will be described, but the present invention can also be applied to an external reforming SOFC system or an MCFC system.
The indirect internal reforming SOFC includes a reformer 3 for reforming a hydrocarbon-based fuel to produce a reformed gas (hydrogen-containing gas). The reformer includes a reforming catalyst layer 4.
The indirect internal reforming SOFC includes an SOFC 6 for generating electric power using the above reformed gas, and also includes a combustion region 5 for combusting an anode off-gas discharged from the SOFC (particularly the anode of the SOFC).
The indirect internal reforming SOFC includes an enclosure 8 for housing the reformer, the solid oxide fuel cell, and the combustion region.
The indirect internal reforming SOFC refers to the enclosure (module container) 8 and equipment included in the interior of the enclosure.
In the indirect internal reforming SOFC in the embodiment shown in FIG. 1, an igniter 7 that is an ignition means for igniting the anode off-gas is provided, and also, the reformer is equipped with an electrical heater 9.
Each supply gas is supplied to the reformer or the SOFC, after being appropriately preheated as required.
A water vaporizer 1 equipped with an electrical heater 2 is connected to the indirect internal reforming SOFC, and piping for supplying the hydrocarbon-based fuel to the reformer is connected to the midstream of connection piping for the water vaporizer 1. The water vaporizer 1 generates steam by heating with the electrical heater 2. The steam may be supplied to the reforming catalyst layer after being appropriately superheated in the water vaporizer or downstream thereof.
Also, air (for a partial oxidation reforming reaction) may be supplied to the reforming catalyst layer, and here, air can be supplied to the reforming catalyst layer after being preheated in the water vaporizer. Steam or a mixed gas of air and steam can be obtained from the water vaporizer.
The steam or the mixed gas of air and steam is mixed with the hydrocarbon-based fuel and supplied to the reformer 3, particularly to the reforming catalyst layer 4 of the reformer 3. When a liquid fuel, such as kerosene, is used as the hydrocarbon-based fuel, the hydrocarbon-based fuel may be supplied to the reforming catalyst layer after being appropriately vaporized.
The reformed gas obtained from the reformer is supplied to the SOFC 6, particularly to the anode of the SOFC 6. Although not shown, air is appropriately preheated and supplied to the cathode of the SOFC.
Combustible components in the anode off-gas (gas discharged from the anode) are combusted by oxygen in a cathode off-gas (gas discharged from the cathode) at the SOFC outlet. In order to do this, ignition using the igniter 7 is possible. The outlets of both the anode and the cathode are open in the module container 8. The combustion gas is appropriately discharged from the module container.
The reformer and the SOFC are housed in one module container and modularized. The reformer is disposed at a position where it can receive heat from the SOFC. For example, when the reformer is located at a position where it receives thermal radiation from the SOFC, the reformer is heated by thermal radiation from the SOFC during electric power generation.
In the indirect internal reforming SOFC, the reformer is preferably disposed at a position where radiation heat can be directly transferred from the SOFC to the outer surface of the reformer. Therefore, it is preferred that there is substantially no obstacle between the reformer and the SOFC, that is, it is preferred to make the region between the reformer and the SOFC be an empty space. Also, the distance between the reformer and the SOFC is preferably as short as possible.
The reformer 3 is heated by the combustion heat of the anode off-gas generated in the combustion region 5. Also, when the temperature of the SOFC is higher than that of the reformer, the reformer is also heated by radiation heat from the SOFC.
Further, the reformer may be heated by heat generation by reforming. When the reforming is partial oxidation reforming, or when the reforming is autothermal reforming and heat generation by a partial oxidation reforming reaction is larger than endothermic heat by a steam reforming reaction, heat is generated with the reforming.
[Load Following Operation Method]
In the present invention, functions F=f(P) and P=f−1(F) of electrical output P of the fuel cell and flow rate F of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to output the electrical output P from the fuel cell are beforehand obtained. P=f−1(F) is the inverse function of F=f(P). Here, F is uniquely determined for a certain electrical output P, and there may be one or a plurality of P for a certain F. For example, F for a certain electrical output P is necessarily uniquely determined by beforehand determining an electric current and a fuel utilization rate for the certain electrical output P by preliminary experiment, simulation, or the like so that the electric power generation efficiency is as high as possible, while the SOFC is maintained at a temperature at which electric power can be preferably generated. Also, for example, there is a case where the flow rate of the hydrocarbon-based fuel for an electrical output equal to or less than a certain electrical output P is set to a fixed value, as shown in FIG. 6, in order to maintain the SOFC at a temperature at which electric power can be preferably generated even when the electrical output is small. In this case, there are a plurality of P for a certain F.
Also, flow rates of fluids supplied to the indirect internal reforming SOFC, other than the hydrocarbon-based fuel, and input and output of electricity to and from the indirect internal reforming SOFC, other than the output of the fuel cell, may be beforehand obtained as a function of the electrical output P, as required. For example, in order to suppress carbon deposition, a flow rate of water supplied to the reformer may be obtained so that the steam/carbon ratio (ratio of the number of moles of water molecules to the number of moles of carbon atoms in the gas supplied to the reforming catalyst layer) has a predetermined value. A flow rate of air supplied to the reformer may be obtained so that the oxygen/carbon ratio (ratio of the number of moles of oxygen molecules to the number of moles of carbon atoms in the gas supplied to the reforming catalyst layer) has a predetermined value. Flow rates of fluids supplied to the indirect internal reforming SOFC, other than the water and air supplied to the reformer, and input and output of electricity to and from the indirect internal reforming SOFC may be obtained beforehand by preliminary experiment, simulation, or the like so that the electric power generation efficiency is as high as possible, while the SOFC is maintained at a temperature at which electric power can be preferably generated. By doing so, these flow rates and electrical input and output may be determined using beforehand obtained functions, when the output of the fuel cell is set to a certain value P.
Here, PM is the maximum electrical output of the fuel cell. PM is beforehand determined as one of the specifications of the fuel cell system. Also, Fmin is the minimum value of the flow rate of the hydrocarbon-based fuel determined by the function F=f(P), when P is within the range of 0 or more and PM or less. Further, Fmax is the maximum value of the flow rate of the hydrocarbon-based fuel determined by the function F=f(P), when P is within the range of 0 or more and PM or less.
In this case, it is enough that the functions F=f(P) and P=f−1(F) are determined within the ranges of 0≦P≦PM and Fmin≦F≦Fmax.
Further, a plurality of temperatures Tj of the reforming catalyst layer (j is an integer of 1 or more and N or less, where N is an integer of 2 or more) and a flow rate Gj of the hydrocarbon-based fuel that corresponds to each Tj are beforehand set.
Here, each Gj is a flow rate of the hydrocarbon-based fuel that can be reformed in the reforming catalyst layer at the corresponding reforming catalyst layer temperature Tj.
Each Gj is larger than 0. In other words, for all j, 0<Gj. Also, Gj is the same value or increases with the increase of j. In other words, Gj≦Gj+1 (here, j is an integer of 1 or more and N−1 or less).
Gj (GN) when j is N is equal to or more than Fmax. In other words, GN≧Fmax. GN is the flow rate of the hydrocarbon-based fuel that can be reformed in the reforming catalyst layer at the highest temperature considered, that is, the maximum value of the hydrocarbon-based fuel that can be reformed. If GN<Fmax, the hydrocarbon-based fuel at the flow rate Fmax cannot be reformed, and therefore, the fuel cell system is, of course, designed so that GN≧Fmax.
By preferably repeatedly performing steps A to D, that is, by repeatedly performing the step A, the step B, and the step C or D in this order, during load following operation, reforming can be more reliably performed to more reliably prevent the degradation of the anode.
[Step A]
When load fluctuation operation is actually performed, the step A of measuring the temperature of the reforming catalyst layer is performed. This measurement may be continuously performed while the load following operation is performed.
The step A is performed to find the temperature T of the reforming catalyst layer used when obtaining a reformable flow rate FR described later. The step A is preferably started in a time as short as possible from the point of time of the start of the load following operation. The step A is preferably started immediately when the load following operation is started. When the monitoring (continuous measurement) of the temperature of the reforming catalyst layer has been performed since before the start of the load following operation, the temperature monitoring may be continuously performed as it has been.
An appropriate temperature sensor, such as a thermocouple, may be used for the temperature measurement.
[Step B]
In the step B, Gj corresponding to the largest Tj that is equal to or less than the temperature T (the temperature measured in the step A) is adopted as a flow rate (reformable flow rate FR) of the hydrocarbon-based fuel that can be reformed in the reforming catalyst layer at the temperature T. In other words, among beforehand set Tj, the largest Tj within the range of the measured temperature T or less is selected. Then, Gj corresponding to the selected Tj is obtained from the correspondence relationship between Tj and Gj, which has been beforehand set, and this Gj is set as the reformable flow rate FR.
[Step C]
When the reformable flow rate FR obtained in the step B is smaller than the minimum value Fmin, electric power generation in the fuel cell is stopped. In other words, when FR<Fmin, it is considered that the requisite minimum reformed gas cannot be reformed, and therefore, the electrical output of the fuel cell is set to zero. In this case, it is possible to increase the temperature of the reforming catalyst layer by a heater, a burner or the like annexed to the reformer at least until FR≧Fmin is satisfied. When FR≧Fmin, the step D and the subsequent steps may be performed. More specifically, electric power generation is stopped in the step C, the steps A and B are repeatedly performed while the temperature of the reforming catalyst layer is increased, electric power generation remains stopped while FR determined by the steps A and B satisfies FR<Fmin (step C), and the step D may be performed when FR satisfies FR≧Fmin.
[Step D]
When the reformable flow rate FR determined in the step B is equal to or more than the minimum value Fmin, the step D is performed.
In the step D, when a fuel cell output demand value PD is equal to or less than the maximum electrical output PM of the fuel cell, step d1 is performed. PD≦PM is considered to mean that the fuel cell can output the fuel cell output demand value PD.
Or, when the fuel cell output demand value PD exceeds the maximum electrical output PM of the fuel cell, step d2 is performed. PD>PM is considered to mean that the electrical output of the fuel cell is insufficient for the fuel cell output demand value PD.
Step d1
A flow rate f(PD) of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to output the fuel cell output demand value PD from the fuel cell is calculated using the above function F=f(P).
Then, when the calculated f(PD) is equal to or less than the reformable flow rate FR determined in the step B, the electrical output of the fuel cell is set to PD, and the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer is set to f(PD). f(PD)≦FR is considered to mean that the hydrocarbon-based fuel at the flow rate f(PD) required to output an electrical output at the fuel cell output demand value PD can be reformed in the reforming catalyst layer. Therefore, the hydrocarbon-based fuel at this flow rate f(PD) is supplied to the reforming catalyst layer, and the obtained reformed gas is supplied to the fuel cell to output the electrical output at the fuel cell output demand value PD from the fuel cell.
On the other hand, when the calculated f(PD) exceeds the above reformable flow rate FR, the electrical output of the fuel cell is set to a value that is less than PD and the maximum among one or more P values calculated from P=f−1(FR), and the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer is set to FR. f(PD)>FR is considered to mean that the hydrocarbon-based fuel at the flow rate f(PD) required to output the electrical output at the fuel cell output demand value PD cannot be reformed in the reforming catalyst layer. There may be only one P value calculated from P=f−1(FR), and there may be a plurality of P values calculated from P=f−1(FR). When there is only one P value calculated from P=f−1(FR), the electrical output of the fuel cell is set to this P value. When there are a plurality of P values calculated from P=f−1(FR), the electrical output of the fuel cell shall have a value that is less than PD and is the maximum among the plurality of P values. In other words, when there are a plurality of the values, the hydrocarbon-based fuel at the reformable flow rate FR is supplied to the reforming catalyst layer, and the maximum electrical output obtained from the hydrocarbon-based fuel at the reformable flow rate FR is output from the fuel cell.
Step d2
As described above, the step d2 is performed when PD>PM (the electrical output of the fuel cell is insufficient for the fuel cell output demand value PD.)
The flow rate f(PM) of the hydrocarbon-based fuel supplied to the reforming catalyst layer required to be supplied to the reforming catalyst layer in order to output the above maximum electrical output PM from the fuel cell is calculated using the above function F=f(P).
When f(PM) is equal to or less than the above reformable flow rate FR, the electrical output of the fuel cell is set to PM, and the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer is set to f(PM). f(PM)≦FR is considered to mean that the hydrocarbon-based fuel at the flow rate f(PM) can be reformed in the reforming catalyst layer.
On the other hand, when f(PM) exceeds the above reformable flow rate FR, the electrical output of the fuel cell is set to a value that is the maximum among one or more P values calculated from P=f−1(FR), and the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer is set to FR. The maximum value among one or more P values calculated from P=f−1(FR), is necessarily less than PD. f(Pm)>FR is considered to mean that the hydrocarbon-based fuel at the flow rate f(PM) cannot be reformed in the reforming catalyst layer.
[Example of Load Following Operation]
Referring to FIGS. 2 to 5, it will be described below how operation is performed under various conditions, when load following operation of one certain fuel cell system is performed, by giving specific examples. However, the present invention is not limited thereto.
It is assumed that a correlation between electrical output P of the fuel cell and flow rate F of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to obtain the electrical output P, that is, functions F=f(P) and P=f−1(F), are beforehand obtained as shown in FIG. 2 (the same correlation is shown in FIGS. 3 to 5).
Also, it is assumed that for the same fuel cell system, temperatures Tj of the reforming catalyst layer, and a flow rate Gj of the hydrocarbon-based fuel corresponding to each Tj are beforehand set as shown in Table 1. Here, TN=700° C. and GN=8 g/min (>Fmax=7 g/min), which values are inherent to the fuel cell system. N=5, that is, five different Tj are set.











TABLE 1







j
Tj
Gj



—
° C.
g/min









1
600
1



2
625
2



3
650
3



4
675
5



5 (N)
700
8










The temperature of the reforming catalyst layer is measured in the step A, and the reformable flow rate FR is determined in the step B.
<Case 1>
A case where the fuel cell output demand value PD=600 W and the reforming catalyst layer temperature T=660° C. measured in the step A is considered. Also, it is assumed that PM (the maximum electrical output of the fuel cell)=1000 W, and Fmin (the minimum value of the flow rate of the hydrocarbon-based fuel determined by the function F=f(P) within the range of 0≦P≦PM)=1 g/min (see FIG. 2).
The step B is performed. From Table 1, the largest Tj within the range of T (660° C.) or less is T3 (650° C.). Gj (G3) corresponding to T3 is 3 g/min. G3 is adopted as the reformable flow rate FR. Therefore, FR=3 g/min.
FR=3 g/min≧1 g/min=Fmin, and therefore, the step C is not performed, and the step D is performed.
PD=600 W<1000 W=PM, and therefore, the step d1, instead of the step d2, is performed.
In the step d1, the flow rate f(PD) of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to output an electric power of PD (600 W) is calculated using the function F=f(P). This value is 2 g/min.
f(PD)=2 g/min≦3 g/min=FR, and therefore, the electrical output of the fuel cell is set to PD, that is, 600 W, and the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer is set to f(PD), that is, 2 g/min. In FIG. 2, a point showing operation conditions obtained in this manner is marked with a star (the same applies to FIGS. 3 to 5).
<Case 2>
A case where PD=900 W and T=640° C. is considered (see FIG. 3).
The fuel cell output demand value PD fluctuates in load following operation, and FR changes depending on the temperature of the reforming catalyst layer. PM=1000 W and Fmin=1 g/min are basically values specific to the fuel cell system, and therefore are the same as the case 1.
The step B is performed. From Table 1, the largest Tj within the range of T (640° C.) or less is T2 (625° C.). Gj (G2) corresponding to T2 is 2 g/min. G2 is adopted as the reformable flow rate FR. Therefore, FR=2 g/min.
FR=2 g/min≧1 g/min=Fmin, and therefore, the step C is not performed, and the step D is performed.
Then, PD=900 W≦1000 W=PM, and therefore, the step d1, instead of the step d2, is performed.
In the step d1, the flow rate f(PD) of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to output an electric power of PD (900 W) is calculated using the function F=f(P). This value is 3 g/min.
f(PD)=3 g/min>2 g/min=FR, and therefore, the electrical output of the fuel cell is set to a value that is less than PD and the maximum among one or more P values calculated from P=f−1(FR), and the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer is set to FR, that is, 2 g/min.
The P values calculated from P=f−1(FR) are 30 W, 600 W, and 800 W. Among these values, the maximum value that is less than PD (900 W) is 800 W. Therefore, the electrical output of the fuel cell is set to 800 W.
<Case 3>
A case where PD=1200 W and T=680° C. is considered (see FIG. 4). PM=1000 W and Fmin=1 g/min are the same as the case 1.
The step B is performed. From Table 1, the largest Tj within the range of T (680° C.) or less is T4 (675° C.). Gj (G4) corresponding to T4 is 5 g/min. G4 is adopted as the reformable flow rate FR. Therefore, FR=5 g/min.
In this case, FR=5 g/min≧1 g/min=Fmin, and therefore, the step C is not performed, and the step D is performed.
Then, PD=1200 W>1000 W=PM, and therefore, the step d2, rather than the step d1, is performed.
In the step d2, the flow rate f(PM) of the hydrocarbon-based fuel supplied to the reforming catalyst layer required to be supplied to the reforming catalyst layer in order to output an electrical output of the maximum electrical output PM (1000 W) is calculated using the above function F=f(P). This value is 4.5 g/min.
f(PM)=4.5 g/min≦5 g/min=FR, and therefore, the electrical output of the fuel cell is set to PM, that is, 1000 W, and the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer is set to f(PM), that is, 4.5 g/min.
<Case 4>
A case where PD=1200 W and T=640° C. is considered (see FIG. 5). PM=1000 W and Fmin=1 g/min are the same as the case 1.
The step B is performed. From Table 1, the largest Tj within the range of T (640° C.) or less is T2 (625° C.). Gj (G2) corresponding to T2 is 2 g/min. G2 is adopted as the reformable flow rate FR. Therefore, FR=2 g/min.
In this case, FR=2 g/min≧1 g/min=Fmin, and therefore, the step C is not performed, and the step D is performed.
Then, PD=1200 W>1000 W=PM, and therefore, the step d2, instead of the step d1, is performed.
In the step d2, the flow rate f(PM) of the hydrocarbon-based fuel supplied to the reforming catalyst layer required to be supplied to the reforming catalyst layer in order to output an electrical output of the maximum electrical output PM (1000 W) is calculated using the above function F=f(P). This value is 4.5 g/min.
f(PM)=4.5 g/min>2 g/min=FR, and therefore, the electrical output of the fuel cell is set to a value that is the maximum among one or more P values calculated from P=f−1(FR), and the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer is set to FR, that is, 2 g/min.
The P values calculated from P=f−1(FR) are 30 W, 600 W, and 800 W. 800 W that is the maximum among these is set as the electrical output of the fuel cell.
In the description with reference to FIGS. 2 to 5, the correlation between F and P is extreme for explanation. But, it is considered that practically, correlation may often be close to a correlation as shown in FIG. 6. In FIG. 6, in a range in which the electrical output P is small, that is, in a range in which the electrical output P is 0 W or more and 300 W or less, the flow rate F of the hydrocarbon-based fuel is fixed at 1.5 g/min in order to preferably maintain the SOFC at a temperature at which electric power can be generated. Also, in a range in which the electrical output P is large, that is, in a range in which the electrical output P is 300 W or more and the maximum electrical output PM (1000 W) or less, the flow rate F of the hydrocarbon-based fuel increases from 1.5 g/min to 4.5 g/min, correspondingly to the increase of the electrical output P in order to make electric power generation efficiency high.
[Way of Setting Tj and Gj Corresponding to Tj]
Way of Setting Tj 
When the measured temperature T of the catalyst layer is smaller than the minimum value of Tj, the step B cannot be performed. Therefore, the minimum value of Tj is more preferably as small as possible and may be, for example, the lowest temperature among temperatures at which the flow rate of the hydrocarbon-based fuel that can be reformed exceeds zero.
It is preferred to make N as large as possible within the allowable range of the memory of a control means, in terms of electric power generation efficiency. Particularly, when the increase rate of the flow rate of the hydrocarbon-based fuel that can be reformed increases as the catalyst layer temperature increases, it is preferred to make the interval between Tj smaller as the temperature increases.
Way of Setting Gj 
Gj is a flow rate (reformable flow rate) of the hydrocarbon-based fuel that can be reformed in the reforming catalyst layer at the corresponding reforming catalyst layer temperature Tj. Therefore, the flow rate Gj of the hydrocarbon-based fuel that can be reformed in the reforming catalyst layer, when the temperature of the reforming catalyst layer is the temperature Tj, is beforehand obtained, and the correspondence relationship between Tj and Gj is beforehand set. The way of obtaining Gj will be described below.
The flow rate of the hydrocarbon-based fuel that can be reformed in the reforming catalyst layer refers to a flow rate such that when the hydrocarbon-based fuel at this flow rate is supplied to the reforming catalyst layer, the composition of the gas discharged from the reforming catalyst layer becomes a composition suitable to be supplied to the anode of the fuel cell.
For example, the reformable flow rate in the reforming catalyst layer may be any flow rate that is equal to or less than the maximum value of flow rates at which the supplied hydrocarbon-based fuel can be decomposed to a C1 compound(s) (a compound(s) having the carbon number of one). In other words, the reformable flow rate in the reforming catalyst layer may be any flow rate that is equal to or less than the maximum value of the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer when reforming can proceed in the reforming catalyst layer until a composition is obtained in which a C2+ component(s) (a component(s) having the carbon number of two or more) in the gas at the outlet of the reforming catalyst layer has a concentration, which does not cause problems of anode degradation and flow blockage due to carbon deposition, or less. The reformable flow rate may be this maximum value, or may be a value obtained by dividing this is maximum value by a safety factor (a value that exceeds 1, for example, 1.4). The concentration of the C2+ component(s) in this case is preferably 50 ppb or less as a mass fraction in the reformed gas. And in this case, it is enough that the gas at the outlet of the reforming catalyst layer is reducing gas. Methane is permitted to be contained in the gas at the outlet of the reforming catalyst layer. In the reforming of the hydrocarbon-based fuel, usually, methane remains in the equilibrium theory. Even if carbon is contained in the gas at the outlet of the reforming catalyst layer in the form of methane, CO, or CO2, carbon deposition can be prevented by adding steam as required. When methane is used as the hydrocarbon-based fuel, it is enough that reforming proceeds so that the gas at the outlet of the reforming catalyst layer becomes reducing.
With respect to the reducing property of the gas at the outlet of the reforming catalyst layer, it is enough that the property is to the extent that if this gas is supplied to the anode, the oxidative degradation of the anode is suppressed. In order to do this, for example, the partial pressures of oxidizing O2, H2O, and CO2, and the like contained in the gas at the outlet of the reforming catalyst layer may be lower than their equilibrium partial pressures of the oxidation reactions of the anode electrode. For example, when the anode electrode material is Ni, and the anode temperature is 800° C., the partial pressure of O2 contained in the gas at the outlet of the reforming catalyst layer may be less than 1.2×10−14 atm (1.2×10−9 Pa), the partial pressure ratio of H2O to H2 may be less than 1.7×102, and the partial pressure ratio of CO2 to CO may be less than 1.8×102.
The reformable flow rate depends on the temperature of the reforming catalyst layer. Therefore, the reformable flow rate in the reforming catalyst layer is obtained based on the temperature of the reforming catalyst layer.
The reformable flow rate Gj may be beforehand obtained as a function of the temperature Tj of the reforming catalyst layer by experiment. Also, it is possible to determine the reformable flow rate by dividing the function obtained by experiment by a safety factor, or offsetting the temperature to the safe side. The unit of Gj is, for example, g/min or mol/s. The reformable flow rate Gj may be a function of only the temperature Tj. But, this is not limiting, and the reformable flow rate Gj may be a function having, in addition to the temperature Tj, a variable, such as the volume of the catalyst layer, or the concentration of the gas component, other than Tj. In this case, when the reformable flow rate Gj is calculated, it is possible to appropriately obtain a variable other than Tj, and calculate the reformable flow rate Gj from the variable other than Tj and the measured Tj.
In preliminary experiment for obtaining Gj, the temperature measurement position in the reforming catalyst layer may be one point or a plurality of points. Also, a representative temperature, such as the average value of a plurality of points, or the like may be used as the temperature of the reforming catalyst layer.
It is possible to consider a plurality of divided regions into which the reforming catalyst layer is divided along the gas flow direction, measure temperatures at a plurality of points in the reforming catalyst layer at different positions along the gas flow direction, calculate one or more flow rates of the fuel that can be reformed in one or more of the plurality of divided regions, based on these temperatures, and set the total value of the calculated flow rate(s) as the flow rate of the fuel that can be reformed in the reforming catalyst layer.
When the temperature T of the reforming catalyst layer during actual operation is obtained in the step A, it is desired to measure the temperature of the reforming catalyst layer as in the preliminary experiment for obtaining Gj. In other words, it is desired to measure the temperature of the reforming catalyst layer at the same position(s) as in the preliminary experiment. When a representative temperature or the like is used in the preliminary experiment, it is desired to use the same representative temperature as the temperature T of the reforming catalyst layer also in the step A.
[Others]
It is not always necessary to perform the same type of reforming during the load following operation. More particularly, it is possible to perform reforming, while changing the flow rate of the hydrocarbon-based fuel stepwise, during the load following operation, and it is not always necessary to perform the same type of reforming at each stage.
Also, by interconnecting the fuel cell to a system power supply, the shortage of the electrical output of the fuel cell with respect to an electric power load may be supplied from the system power supply.
The fuel cell output demand value PD may be a value of an electric power load measured by an appropriate electric power meter. Alternatively, when the fuel cell is interconnected to another power generator or storage battery, part of a measured electric power load may be set as the fuel cell output demand value PD.
In the step D, when the flow rate of the hydrocarbon-based fuel is determined, it is possible to accordingly calculate and determine the flow rates of fluids supplied to the indirect internal reforming SOFC, other than the hydrocarbon-based fuel, and the input and output of electricity to and from the indirect internal reforming SOFC, other than the output of the SOFC, from functions of the electrical output P beforehand obtained, as required.
The present invention is particularly effective when the hydrocarbon-based fuel supplied to the reforming catalyst layer includes a hydrocarbon-based fuel having a carbon number of two or more. According to the present invention, it is possible to allow the concentration of a compound(s) having the carbon number of two or more in the reformed gas to be 50 ppb or less on a mass basis even in load following operation. And thereby, anode degradation and flow blockage due to carbon deposition can be more reliably prevented.
In order to perform the method of the present invention, appropriate instrumentation controlling equipment, including a computing means, such as a computer, may be used.
[Hydrocarbon-Based Fuel]
It is possible to use a hydrocarbon-based fuel appropriately selected from compounds of which molecules contain carbon and hydrogen (may also contain other elements, such as oxygen) or mixtures thereof that are publicly known as raw materials of reformed gas in the field of high temperature fuel cells. It is possible to use compounds of which molecules contain carbon and hydrogen, such as hydrocarbons and alcohols. For example, hydrocarbon fuels, such as methane, ethane, propane, butane, natural gas, LPG (liquefied petroleum gas), city gas, gasoline, naphtha, kerosene and gas oil, alcohols, such as methanol and ethanol, ethers, such as dimethylether, and the like may be used.
Particularly, kerosene and LPG are preferred because they are readily available. In addition, they can be stored in a stand-alone manner, and therefore, they are useful in areas where the city gas pipeline is not built. Further, a high temperature fuel cell power generating equipment using kerosene or LPG is useful as an emergency power supply. Particularly, kerosene is preferred because it is easy to handle.
[High Temperature Fuel Cell]
The present invention may be suitably applied to a system equipped with a high temperature fuel cell in which anode degradation and flow blockage due to carbon deposition may occur. Such a fuel cell includes an SOFC and an MCFC.
The SOFC may be appropriately selected for use from publicly known SOFCs having various shapes, such as planar and tubular SOFCs. In the SOFC, generally, an oxygen-ion conductive ceramic or a proton-ion conductive ceramic is used as the electrolyte.
The MCFC may also be appropriately selected for use from publicly known MCFCs.
The SOFC or the MCFC may be a single cell, but practically, a stack in which a plurality of single cells are arrayed (the stack is sometimes referred to as a bundle in the case of a tubular type, and the stack in this specification includes a bundle) is preferably used. In this case, one stack or a plurality of stacks may be used.
Among high temperature fuel cells, an indirect internal reforming SOFC is excellent in that the thermal efficiency of the system can be increased. The indirect internal reforming SOFC has a reformer for producing a reformed gas containing hydrogen from a hydrocarbon-based fuel using a steam reforming reaction and an SOFC. In this reformer, a steam reforming reaction may be performed, and autothermal reforming in which a steam reforming reaction is accompanied by a partial oxidation reaction may be performed. In terms of the electric power generation efficiency of the SOFC, preferably, no partial oxidation reaction occurs after the completion of start-up. The autothermal reforming is designed so that steam reforming is predominant after the completion of start-up, and therefore, the reforming reaction is an overall endothermic reaction. Heat required for the reforming reaction is supplied from the SOFC. The reformer and the SOFC are housed in one module container and modularized. The reformer is disposed at a position where it receives thermal radiation from the SOFC. Thus, the reformer is heated by thermal radiation from the SOFC during electric power generation. Also, the SOFC may be heated by combusting an anode off-gas, which is discharged from the SOFC, at the cell outlet.
In the indirect internal reforming SOFC, the reformer is preferably disposed at a position where radiation heat can be directly transferred from the SOFC to the outer surface of the reformer. Therefore, it is preferred that there is substantially no obstacle between the reformer and the SOFC, that is, it is preferred to make the region between the reformer and the SOFC be an empty space. Also, the distance between the reformer and the SOFC is preferably as short as possible.
Each supply gas is supplied to the reformer or the SOFC, after being appropriately preheated as required.
The module container may be any appropriate container capable of housing the SOFC and the reformer. An appropriate material having resistance to the environment used, for example, stainless steel, may be used as the material of the container. A connection port is appropriately provided for the container for gas interfacing or the like.
Particularly when a cell outlet opens in the module container, the module container is preferably hermetic in order to prevent communication between the interior of the module container and the surroundings (atmosphere).
A combustion region is a region where the anode off-gas discharged from the anode of the SOFC can be combusted. For example, the anode outlet is opened in the enclosure, and a space near the anode outlet may be the combustion region. This combustion may be performed using, for example, a cathode off-gas, as an oxygen-containing gas. In order to do this, a cathode outlet may be opened in the enclosure.
In order to combust a combustion fuel or the anode off-gas, an ignition means, such as an igniter, may be appropriately used.
[Reformer]
The reformer produces a reformed gas containing hydrogen from a hydrocarbon-based fuel.
In the reformer, any of steam reforming, partial oxidation reforming and autothermal reforming in which a steam reforming reaction is accompanied by a partial oxidation reaction may be performed.
In the reformer, a steam reforming catalyst having steam reforming activity, a partial oxidation reforming catalyst having partial oxidation reforming activity, or an autothermal reforming catalyst having both partial oxidation reforming activity and steam reforming activity may be appropriately used.
With respect to the structure of the reformer, a structure publicly known as that of a reformer may be appropriately used. For example, the structure of the reformer may be a structure having a region for housing a reforming catalyst in a vessel which can be closed to the atmosphere, and having an introduction port for fluids required for reforming and a discharge port for a reformed gas.
The material of the reformer may be appropriately selected for use from materials publicly known as those of reformers, considering resistance in the environment used.
The shape of the reformer may be an appropriate shape, such as a rectangular parallelepiped shape or a circular tube shape.
A hydrocarbon-based fuel (vaporized beforehand as required) and steam, and further an oxygen-containing gas, such as air, as required, may be supplied to the reformer (the reforming catalyst layer), each independently, or appropriately mixed beforehand. The reformed gas is supplied to the anode of the SOFC.
[Reforming Catalyst]
A publicly known catalyst may be used for each of the steam reforming catalyst, the partial oxidation reforming catalyst and the autothermal reforming catalyst used in the reformer. Examples of the partial oxidation reforming catalyst include a platinum-based catalyst. Examples of the steam reforming catalyst include ruthenium-based and nickel-based catalysts. Examples of the autothermal reforming catalyst include a rhodium-based catalyst.
A temperature at which the partial oxidation reforming reaction can proceed is, for example, 200° C. or more. A temperature at which the steam reforming reaction can proceed is, for example, 400° C. or more.
[Operation Conditions of Reformer]
The conditions during load following operation of the reformer for each of steam reforming, autothermal reforming, and partial oxidation reforming will be described below.
In steam reforming, steam is added to a reforming raw material, such as kerosene. The reaction temperature of the steam reforming may be in the range of, for example, 400° C. to 1000° C., preferably 500° C. to 850° C., and further preferably 550° C. to 800° C. An amount of the steam introduced into the reaction system is defined as a ratio of the number of moles of water molecules to the number of moles of carbon atoms contained in the hydrocarbon-based fuel (steam/carbon ratio). This value is preferably 1 to 10, more preferably 1.5 to 7, and further preferably 2 to 5. When the hydrocarbon-based fuel is liquid, a space velocity (LHSV) can be represented as A/B, wherein a flow velocity of the hydrocarbon-based fuel in a liquid state is represented as A (L/h), and a volume of the catalyst layer is represented as B (L). This value is set in the range of preferably 0.05 to 20 h−1, more preferably 0.1 to 10 h−1, and further preferably 0.2 to 5 h−1.
In autothermal reforming, in addition to the steam, an oxygen-containing gas is added to the reforming raw material. The oxygen-containing gas may be pure oxygen, but in terms of the ease of availability, air is preferred. The oxygen-containing gas may be added so that the endothermic reaction accompanying the steam reforming reaction is balanced, and an amount of heat generation such that the temperature of the reforming catalyst layer and the SOFC can be maintained or increased is obtained. With respect to the amount of the oxygen-containing gas added, a ratio of the number of moles of oxygen molecules to the number of moles of carbon atoms contained in the hydrocarbon-based fuel (oxygen/carbon ratio) is preferably 0.005 to 1, more preferably 0.01 to 0.75, and further preferably 0.02 to 0.6. A reaction temperature of the autothermal reforming reaction is set in the range of, for example, 400° C. to 1000° C., preferably 450° C. to 850° C., and further preferably 500° C. to 800° C. When the hydrocarbon-based fuel is liquid, the space velocity (LHSV) is selected in the range of preferably 0.05 to 20 h−1, more preferably 0.1 to 10 h−1, and further preferably 0.2 to 5 h−1. With respect to an amount of the steam introduced into the reaction system, the steam/carbon ratio is preferably 1 to 10, more preferably 1.5 to 7, and further preferably 2 to 5.
In partial oxidation reforming, an oxygen-containing gas is added to the reforming raw material. The oxygen-containing gas may be pure oxygen, but in terms of the ease of availability, air is preferred. An amount of the oxygen-containing gas added is appropriately determined in terms of heat loss and the like to ensure a temperature at which the reaction proceeds. With respect to this amount, the ratio of the number of moles of oxygen is molecules to the number of moles of carbon atoms contained in the hydrocarbon-based fuel (oxygen/carbon ratio) is preferably 0.1 to 3 and more preferably 0.2 to 0.7. A reaction temperature of the partial oxidation reaction may be set in the range of, for example, 450° C. to 1000° C., preferably 500° C. to 850° C., and further preferably 550° C. to 800° C. When the hydrocarbon-based fuel is liquid, the space velocity (LHSV) is selected in the range of preferably 0.1 to 30 h−1. Steam can be introduced into the reaction system to suppress the generation of soot, and with respect to an amount of the steam, the steam/carbon ratio is preferably 0.1 to 5, more preferably 0.1 to 3, and further preferably 1 to 2.
[Other Equipment]
In the high temperature fuel cell system used in the present invention, publicly known components of a high temperature fuel cell system may be appropriately provided as required. Specific examples of the publicly known components include a desulfurizer for reducing a sulfur content of a hydrocarbon-based fuel; a vaporizer for vaporizing a liquid; pressure increasing means for pressurizing various fluids, such as a pump, a compressor, and a blower; flow rate controlling means or flow path blocking/switching means for controlling the flow rate of a fluid, or blocking/switching the flow of a fluid, such as a valve; a heat exchanger for performing heat exchange and heat recovery; a condenser for condensing a gas; heating/warming means for externally heating various equipment with steam or the like; storage means of a hydrocarbon-based fuel and combustibles; an air or electrical system for instrumentation; a signal system for control; a control device; and an electrical system for output and is powering; and the like.
INDUSTRIAL APPLICABILITY
The present invention can be applied to a high temperature fuel cell system used for, for example, a stationary or mobile electric power generation system and a cogeneration system.
DESCRIPTION OF SYMBOLS


    1 water vaporizer
    2 electrical heater annexed to water vaporizer
    3 reformer
    4 reforming catalyst layer
    5 combustion region
    6 SOFC
    7 igniter
    8 module container
    9 electrical heater annexed to reformer

## Claims

1. A method of load following operation of a fuel cell system comprising a reformer having a reforming catalyst layer, for reforming a hydrocarbon-based fuel to produce a reformed gas containing hydrogen, and a high temperature fuel cell for generating electric power using the reformed gas, the method comprises determining before start of the load following operation
functions F=f(P) and P=f−1(F), where P is an electrical output of the fuel cell, and F is a flow rate of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to output the electrical output P from the fuel cell, P=f−1(F) is an inverse function of F=f(P), wherein flow rate F is uniquely determined for each value P in the range 0 to PM, where in PM represents maximum electrical output of the fuel cell, and wherein for at least one value of F there are plurality of corresponding P values;
a minimum value of flow rate of the hydrocarbon-based fuel represented as Fmin, said Fmin being a minimum value that is given by the function F=f(P) when P ranges in the range 0 to PM,
a plurality of temperatures Tj of the reforming catalyst layer, the temperatures Tj representing a one dimensional array having two or more array elements indexed by a subscript ‘j’, and a plurality of flow rates Gj of the hydrocarbon-based fuel, each Gj corresponding to each Tj, where each Gj is a flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer at a corresponding reforming catalyst layer temperature Tj, each Gj is larger than 0, and Gj is the same value or increases with an increase of j,
the load following method comprising:
A) measuring a temperature T of the reforming catalyst layer;
B) adopting one of the flow rates from the plurality of flow rates Gj corresponding to the largest temperature from the plurality of temperatures Tj that is equal to or less than the measured temperature T as a reformable flow rate FR that is a flow rate of the hydrocarbon-based fuel capable of being reformed in the reforming catalyst layer at the temperature T;
C) when the reformable flow rate FR is smaller than the minimum value Fmin, stopping electric power generation in the fuel cell; and
D) when the reformable flow rate FR is equal to or more than the minimum value Fmin,
performing step d1 if a fuel cell output demand value PD is equal to or less than the maximum electrical output PM, and performing step d2 if the fuel cell output demand value PD exceeds the maximum electrical output PM,
d1) calculating a flow rate f(PD) of the hydrocarbon-based fuel required to be supplied to the reforming catalyst layer in order to output the fuel cell output demand value PD from the fuel cell, using the function F=f(P), and
if f(PD) is equal to or less than the reformable flow rate FR, then setting an electrical output of the fuel cell to PD, and setting a flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer to f(PD), and
if f(PD) exceeds the reformable flow rate FR, then setting the electrical output of the fuel cell to a value that is less than PD and the maximum among one or more P values calculated from P=f−1(FR), and setting the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer to FR,
d2) calculating a flow rate f(PM) of the hydrocarbon-based fuel supplied to the reforming catalyst layer required to be supplied to the reforming catalyst layer in order to output the maximum electrical output PM from the fuel cell, using the function F=f(P), and
if f(PM) is equal to or less than the reformable flow rate FR, then setting the electrical output of the fuel cell to PM, and setting the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer to f(PM), and
if f(PM) exceeds the reformable flow rate FR, then setting the electrical output of the fuel cell to a value that is the maximum among one or more P values calculated from P=f−1(FR), and setting the flow rate of the hydrocarbon-based fuel supplied to the reforming catalyst layer to FR,
wherein the output demand value PD fluctuates in the load following operation.

2. The method according to claim 1, wherein steps A to D are repeatedly performed during the load following operation.

3. The method according to claim 1, wherein the hydrocarbon-based fuel comprises a hydrocarbon-based fuel with a carbon number of two or more.

4. The method according to claim 3, wherein the concentration of a compound with a carbon number of two or more in the reformed gas is 50 ppb or less on a mass basis.