JP2940972B2 - Fuel cell power generation system and fuel gas supply method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a power generation system and a power plant using a fuel cell, and a method of using the fuel cell.

[Prior Art] In a fuel cell, methane as a fuel gas is usually reformed into a hydrogen-rich gas by a reforming catalyst and used for an electrochemical reaction.

The reforming reaction is as follows, and this reaction is an endothermic reaction.

_{CH 4 + H 2 O → 3H} 2 + CO ... (1) CO + H 2 O → H 2 + CO 2 ... (2) (1) and (2) To summarize the _{formula, CH 4 + 2H 2 O →} 4H 2 + CO 2 ... (3 The electromotive force characteristics of the fuel cell increase as the utilization rate of the fuel gas decreases. For this reason, the fuel gas is usually operated at a utilization rate of about 60% to 80%. That is, 20
% To 40% of the fuel gas is discharged as fuel exhaust gas without being used.

By the way, in the fuel cell, various methods are being studied in order to lower the unit price of energy.

For example, methane contained in fuel exhaust gas is reused as a heating source for the above-mentioned endothermic reforming reaction.

Further, the gas pressure supplied to the fuel cell is increased to promote an electrochemical reaction occurring in the cell, thereby increasing the power generation efficiency of the fuel cell.

[Problems to be Solved by the Invention] The inventors investigated the relationship between the supply gas pressure and the methane concentration in the fuel exhaust gas discharged from the fuel cell whose power generation efficiency was increased by increasing the supply gas pressure. .

 The result is shown in FIG.

FIG. 6 shows the gas pressure supplied to the molten carbonate fuel cell on the horizontal axis and the methane concentration in the fuel exhaust gas discharged from the anode side on the vertical axis.

From FIG. 6, it was found that as the supplied gas pressure increases, the amount of methane contained in the fuel exhaust gas increases. This indicates that as the pressure increases, a methanation reaction between hydrogen and carbon monoxide occurs in the battery.

That is, when the pressure is increased to increase the power generation efficiency of the fuel cell, a methanation reaction occurs in the cell, the fuel utilization rate decreases, and the amount of methane contained in the exhaust gas increases. For this reason, the unit price of energy cannot be reduced sufficiently.

Therefore, it is conceivable to reuse the fuel exhaust gas for heating the reformer.

However, since inexpensive fuel such as coal can be used as the fuel source for the reformer, a large amount of methane contained in the fuel exhaust gas discharged during high-pressure operation is used as the fuel source for the reformer. Utilization is not a satisfactory way to use fuel gas effectively.

A first object of the present invention is to provide a power generation system and a power plant that make effective use of fuel gas supplied to a fuel cell.

A second object of the present invention is to provide a method of operating a fuel cell that can effectively use fuel gas supplied to the fuel cell.

[Means for Solving the Problems] A first object of the present invention is to provide a power generation system including a plurality of fuel cells, wherein fuel exhaust gas discharged from the first fuel cell is used as fuel gas for the second fuel cell. This is achieved by a power generation system characterized by using.

In a power generation system including a plurality of fuel cells using hydrogen gas as fuel, a methane generated inside a first fuel cell is reformed and used as fuel for a second fuel cell. Is also achieved.

In a power generation system including a plurality of fuel cells, the plurality of fuel cells include a first-stage fuel cell group including at least one or more fuel cells and a second-stage fuel cell group including at least one or more fuel cells. The front-stage fuel cell group is composed of the same number or a larger number of fuel cells as the rear-stage fuel cell group, and the fuel exhaust gas discharged by the front-stage fuel cell group, the rear-stage fuel cell group is It is also achieved by a power generation system characterized in that it is used as all or part of the fuel.

A fuel gas supply source, a first fuel cell, and a second fuel cell are provided. The second fuel cell converts fuel gas supplied by the fuel gas supply source and fuel exhaust gas discharged by the first fuel gas into fuel. This is also achieved by a power generation system characterized by the following.

It is preferable that the power generation system is provided with a bottoming cycle that uses exhaust gas discharged.

The pressure of the fuel gas supplied to the power generation system is preferably higher than the atmospheric pressure.

A first object of the present invention is to provide a fuel cell power plant including a plurality of fuel cells.
Fuel gas containing methane discharged from the fuel cell of
It is also achieved by a fuel cell power plant characterized in that it is used as a fuel gas for the second fuel cell.

A second object of the present invention is achieved by a method of operating a fuel cell, characterized in that fuel exhaust gas discharged from a fuel cell is reformed and supplied as fuel gas of a different fuel cell.

[Operation] If the gas pressure supplied to the fuel cell is increased from normal pressure (1 atm), the power generation efficiency will increase. However, according to this, methanation occurs in the fuel cell, the amount of methane contained in the fuel exhaust gas increases, and the utilization rate of the fuel gas decreases.

According to the above formula (3), methane can generate as much as four molecules of hydrogen as fuel for a fuel cell per molecule.

For this reason, the present invention increases the fuel utilization rate by reforming methane contained in the fuel exhaust gas again and using it as fuel for the fuel cell, thereby improving the fuel exhaust gas which has been conventionally performed. Uses methane in fuel exhaust gas more effectively than using it as a heating source for the vessel.

EXAMPLES Hereinafter, examples will be described, but the present invention is not limited thereto.

In the following embodiment, an embodiment of a power generation system using a molten carbonate fuel cell using a molten carbonate as an electrolyte will be described, but the present invention is not limited to this. For example, a phosphoric acid fuel cell using phosphoric acid as an electrolyte, a solid electrolyte fuel cell using a solid electrolyte, or the like may be used.

An embodiment of the power generation system will be described with reference to FIG. 1 and FIG. FIG. 1 shows the overall configuration of the power generation system of the present embodiment. FIG. 2 mainly shows a configuration of a battery unit of the power generation system shown in FIG.

The power generation system shown in FIG.
A reformer 30 for reforming the fuel gas sent to the external reforming fuel cell 10, an internal reforming fuel cell 20, a fuel gas supply source 60, an oxidizing gas supply source 61, An inverter 70 that converts the generated DC power into AC power and a bottoming cycle 80 that uses waste heat generated from the fuel cell are provided.

The external reforming fuel cell 10 is housed in a pressure-resistant container 16. Similarly, the internal reforming fuel cell 20 is housed in a pressure-resistant container 26.

The reformer 30, the external reforming fuel cell 10, and the internal reforming fuel cell 20 are connected in series by a fuel gas pipe 40 for sending a fuel gas from a fuel gas supply source 60. A compressor 50 is provided between the fuel gas supply source 60 and the reformer 30 to send gas. The oxidizing gas is supplied from a compressor provided between the oxidizing gas supply source 61 and the external reforming fuel cell 10.
Through the oxidant gas pipe 41, the fuel gas is sent to the external reforming fuel cell 10 and the internal reforming fuel cell 20.

Power obtained from the external reforming fuel cell 10 and the internal reforming fuel cell 20 is converted into AC power by the inverter 70.

FIG. 2 is a schematic diagram showing the configuration of the external reforming fuel cell 10, the internal reforming fuel cell 20, and the reformer 30.

The external reforming fuel cell 10 has an anode 11, a cathode 13
And a gas passage 14 for fuel gas and a gas passage 15 for oxidant gas.
It is composed of

The internal reforming fuel cell 20 includes an anode 21, an electrolyte 22,
The fuel cell system includes a cathode 23, a gas passage 24 for fuel gas, and a gas passage 25 for oxidizing gas. The fuel gas passage 24 is filled with a reforming catalyst 27.

The reformer 30 includes a reformer 31 filled with a reforming catalyst 33 and a burner 32.

The flow of the fuel gas and the oxidizing gas in the power generation system will be described.

Natural gas, methane, or the like is used as the fuel gas, and a mixed gas of air and CO ₂ is used as the oxidizing gas.

The fuel gas sent from the fuel gas supply source 60 via the fuel gas pipe 40 is reformed by the reformer 31 into a hydrogen-rich gas by the reaction represented by the above formula (3). This reaction is an endothermic reaction, and heating is required to accelerate the reaction. For this purpose, the reformer 30 is provided with a burner 32. As the fuel for the burner 32, an inexpensive fuel such as coal is used.

The hydrogen-rich gas is sent to the fuel gas gas passage 14 of the external reforming fuel cell 10, and then electrochemically causes a hydrogen oxidation reaction at the anode 11 to emit electrons. The fuel exhaust gas after the reaction is further sent to the internal reforming fuel cell 20 by a fuel gas pipe 40.The internal reforming fuel cell 20 reforms methane contained in the fuel exhaust gas into a hydrogen-rich gas, The rich gas is used at the anode 21 for the electrochemical reaction. Thus,
The fuel exhaust gas generated in the external reforming fuel cell 10 is used again by the internal reforming fuel cell 20 for the power generation reaction of the fuel cell.

On the other hand, the oxidizing gas sent from the oxidizing gas supply source 61 passes through the oxidizing gas pipe 41, and the gas for the oxidizing gas of each of the external reforming fuel cell 10 and the internal reforming fuel cell 20. From the passages 15 and 25, they are taken into the cathodes 13 and 23 and electrochemically react to generate carbonate ions.

The bottoming cycle 80 using the heat of exhaust gas will be described with reference to FIG.

The bottoming cycle 80 consists of a gas turbine 81 and a boiler.
82 and a steam turbine 83. 6 for fuel cells 10, 20
Operating at 50 ° C, the exhaust gas has a heat of 650 ° C to 750 ° C. First, after extracting waste heat using the gas turbine 81, steam is generated from the boiler by the exhaust gas at about 400 ° C., and the steam turbine is turned. Further, the generated hot water is used for hot water supply. Thus, energy generated without waste is used.

In this embodiment, the fuel cell (hereinafter referred to as the first fuel cell) at the preceding stage for discharging the fuel gas used as the fuel gas is an external reforming type, but may be an internal reforming type.

In the external reforming type, it is more difficult to heat the reforming reaction sufficiently to provide a reformer outside the fuel cell than in the internal reforming type having a reforming catalyst inside the cell. Unreacted methane is likely to be generated in the purified gas. For this reason, when the first fuel cell is an external reforming fuel cell, the amount of methane remaining in the fuel exhaust gas increases, and the use of the power generation system of this embodiment increases the fuel utilization rate.

Similarly, a fuel cell using the fuel exhaust gas (hereinafter referred to as a second fuel cell) may be either an internal reforming type or an external reforming type.

However, when the internal reforming type is used, methane in the fuel exhaust gas discharged from the first fuel cell can be reformed more efficiently than the external reforming type, and the fuel utilization rate increases.

Although two fuel cells are connected in series in FIG. 1, a fuel cell may be connected at a later stage.

The power generation efficiency becomes almost constant when the gas pressure supplied to the fuel cell is 7 atm or more. The optimum point of the gas pressure supplied to the power generation system of the present embodiment is selected from the power generation efficiency and the fuel gas utilization rate. The optimum point differs depending on the performance of the power generation system, such as capacity, but usually 5 to 7 atm is selected.

When the ratio of steam and methane (steam / carbon (S / C)) contained in the fuel gas is 2.0 or less, carbon is liable to precipitate and battery performance is likely to deteriorate. For this purpose, it is preferable to add steam so that the S / C of the fuel gas discharged from the first fuel cell (the fuel gas of the second fuel cell) becomes 2.0 or more, preferably 2.5 or more.

The fuel cell shown in FIG. 2 has a cathode, an anode,
Although it has one cell made of an electrode material, a multi-layer fuel cell having a plurality of cells can also be used.

Experimental examples using the above power generation system are shown in Experimental Example-1 and Experimental Example-2.

Experimental Example-1 An external reforming type was used for the first fuel cell, and an internal reforming type was used for the second fuel cell.

 Nickel was used as a catalyst for reforming the fuel gas.

Methane was steam reformed, and a hydrogen-rich gas fuel gas and an oxidizing gas obtained by mixing air and carbon dioxide in a molar ratio of 70:30 were supplied to the first fuel cell at a gas pressure of 5 atm. The operating conditions of the first fuel cell are as follows.
The current density was 150 mA / cm ² , and the fuel gas utilization was set to 60%. The concentration of methane in the fuel exhaust gas discharged from the anode side of the first fuel cell was about 7%.

The fuel exhaust gas whose S / C ratio was adjusted to 2.5 was supplied as a fuel gas to a second fuel cell (internal reforming type).

The second fuel cell performs an electrochemical reaction after reforming methane in the supplied fuel exhaust gas therein. The operating conditions were as follows: the battery temperature was 650 ° C, and the fuel gas utilization was about 90%.
%. The utilization rate of the fuel gas was made to be about 90% by selecting the current density and the like.

When the content of methane in the fuel exhaust gas discharged from the second fuel cell was examined, about 95% of the methane contained in the fuel gas supplied to the first fuel cell was used as the first fuel cell. Used by the second fuel cell.

Experimental Example-2 In this experimental example, except that an external reforming fuel cell having a smaller capacity than the first fuel cell was used for the second fuel cell,
It was the same as in Experimental Example-1.

In this experimental example, a reformer filled with a nickel catalyst was connected to the outlet of the fuel exhaust gas of the first fuel cell, and methane in the fuel exhaust gas was reformed into a hydrogen-rich fuel gas to form a second fuel cell. The battery was supplied. At this time, the S / C ratio was adjusted so as to be the same as in Experimental Example-1.

In this experimental example, since heating was not performed during reforming of the fuel exhaust gas, about 86% of the methane contained in the fuel exhaust gas could be reformed to hydrogen, but the remaining about 14% was not reformed and remained methane Met.

When the content of methane in the fuel exhaust gas discharged from the second fuel cell was examined, about 90% of the methane contained in the fuel gas supplied to the first fuel cell was determined by the first fuel cell. Used by the second fuel cell.

As is clear from the above experimental examples, two fuel cells are connected in series, and the fuel exhaust gas discharged from the first fuel cell is used as the fuel gas for the second fuel cell, so that the fuel exhaust gas can be used effectively. Then, about 90% or more of the methane contained in the fuel gas supplied to the first fuel cell can be used.

In Experimental Example-2, the reformer provided between the first fuel cell and the second fuel cell is not heated, but may be heated. Whether or not to perform heating may be selected from the cost performance of the cost spent on coal or the like used for heating and the amount of electricity that can be extracted from the fuel cell.

A power generation system different from the power generation system shown in FIG. 1 is shown in FIGS.

In the power generation system described below, the first fuel cell is of the external reforming type, and the second fuel cell is of the internal reforming type. However, as in the power generation system shown in FIG.
Both the external reforming type and the internal reforming type may be used for the fuel cell and the second fuel cell.

The cells constituting the first fuel cell and the second fuel cell may be either single or plural.

Further, the gas supplied to the first fuel cell and the second fuel cell is preferably a high-pressure gas, but is not particularly limited thereto, and may be normal pressure.

FIG. 3 shows an embodiment in which a preceding fuel cell group composed of a plurality of first fuel cells and a subsequent fuel cell group composed of a plurality of second fuel cells are provided.

After the fuel gas is converted into a hydrogen-rich gas in the reformer 301, the gas is supplied to the fuel cell group 302 in the preceding stage. The fuel exhaust gas discharged from the fuel cell 302 at the former stage is collected and used as fuel gas for the fuel cell 303 at the latter stage.

In the present embodiment, the fuel cell group 302 in the first stage includes, for example, five fuel cells, and the fuel cell group 303 in the second stage includes, for example,
It consists of two fuel cells.
The number of the fuel cells is not particularly limited as long as the fuel cell includes the same number or more of the fuel cells as the fuel cell group 303 in the subsequent stage. The number of fuel cells used for the first-stage fuel cell group 302 and the second-stage fuel cell group 303 is arbitrarily selected depending on operating conditions and output.

In the present embodiment, even if, for example, one of the fuel cells constituting the fuel cell group 302 of the former stage has a trouble and the operation becomes impossible, or if the operation is interrupted due to a periodic inspection or the like, the operation of the other fuel cells is stopped. By operating the fuel cells constituting the fuel cell group 302, there is no need to stop the power generation system, and the operation efficiency of the power generation system increases.

The same fuel cell is used for the first fuel cell group 302 and the second fuel cell group 303, and the first fuel cell group 30
When pipes and valves are provided so as to be able to arbitrarily switch between the second fuel cell group 303 and the second fuel cell group 303, the number of fuel cells constituting the first fuel cell group 302 and the second fuel cell group
The number of fuel cells constituting 03 can be changed at any time according to operating conditions and output.

In the present embodiment, when the former fuel cell group 302 is not operated at a high pressure, that is, when the methanation reaction hardly occurs and the amount of methane contained in the fuel exhaust gas discharged from the former fuel cell group 302 is small, Is also effective.

FIG. 4 shows a system in which unused fuel gas is supplemented to the fuel exhaust gas discharged from the first fuel cell and used as fuel gas for the second fuel cell.

In the power generation system of the present embodiment, the reformer 401 reforms a fuel gas sent from a fuel gas supply source (not shown) into a hydrogen-rich gas. The gas can be supplied to the first fuel cell 402 and the second fuel cell 403 via a branch pipe provided with an opening / closing valve capable of supplying a part of the fuel gas.

For this reason, the second fuel cell can use the reformed gas from the reformer 401 and the exhaust gas discharged from the first fuel cell as the fuel gas. In particular, when the amount of methane contained in the first fuel exhaust gas is small, or when it becomes necessary to increase the output of the second fuel cell, the opening / closing valve provided in the bypass line is opened and the second fuel cell is opened. Necessary fuel can be supplied to the fuel cell.

When the first fuel cell fails to operate due to a trouble, or when the operation is interrupted due to periodic inspection or the like, the operation can be performed only by the second fuel cell using the bypass line.

Further, as shown in FIG. 3, the first fuel cell and the second fuel cell may be composed of a plurality of fuel cells.

FIG. 5 shows an example in which the first fuel cell and the second fuel cell are contained in the same pressure-resistant container.

Since methane gas and the like used in fuel cells are flammable, a fuel tank is filled with a gas (generally, nitrogen gas) that does not react with the supplied gas so that the gas does not leak from the fuel cell. Often houses batteries. In particular, when the pressure of the fuel gas supplied to the fuel cell and the pressure of the oxidizing gas use a gas pressure higher than the normal pressure in order to improve the power generation efficiency, in order to enhance safety, a pressure-resistant container is required. Often needed.

In the present embodiment, the first fuel cell 501 and the second fuel cell 5
By putting 02 in the same pressure-resistant container, the number of pressure-resistant containers can be reduced.

The first fuel cell 501 and the second fuel cell 502 are arranged vertically, but may be arranged horizontally.

Further, as shown in FIG. 4, the first fuel cell and the second fuel cell
A fuel cell may be provided with a bypass line.

[Effects of the Invention] According to the present invention, fuel exhaust gas can be effectively used, and the utilization efficiency of methane contained in fuel gas can be improved. In this way, the power generation efficiency can be increased, and the unit price of energy can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an embodiment of the power generation system of the present invention, FIG. 2 is a view mainly showing the configuration of a fuel cell of the power generation system shown in FIG. 1, FIG. 3, FIG. 5
The figure is a schematic diagram mainly showing the arrangement of fuel cells in the power generation system of the present invention different from FIG. 1, and FIG. 6 is a diagram showing the relationship between the gas pressure during operation of the fuel cell and the methane concentration in the fuel exhaust gas. It is. 10 ... external reforming fuel cell, 20 ... internal reforming fuel cell, 30 ... reformer, 40 ... fuel gas piping, 41 ... oxidizing gas piping, 50 ... compressor, 60 ... Fuel gas supply source, 61 …… Oxidant gas supply source, 70 …… Inverter, 80…
… Bottoming cycle.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yasutaka Komatsu 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Pref. Hitachi, Ltd. Hitachi Plant (56) References JP-A-62-133673 (JP, A) JP-A-63-29463 (JP, A) JP-A-62-274560 (JP, A) JP-A-53-29534 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01M 8 / 06 H01M 8/00 H01M 8/04

Claims (6)

(57) [Claims] 1. A power generation system including a plurality of fuel cells using hydrogen gas as fuel, wherein methane generated inside the first fuel cell is reformed and used as fuel for a second fuel cell. And power generation system. 2. The power generation system according to claim 1,
The first fuel cell is a fuel cell group including one or more fuel cells using hydrogen gas as fuel, and the second fuel cell is a fuel cell group including one or more fuel cells using hydrogen gas as fuel. A power generation system, wherein the number of the fuel cells constituting the first fuel cell is equal to or more than the number of the fuel cells constituting the second fuel cell. 3. The power generation system according to claim 1,
The second fuel cell uses hydrogen gas supplied from outside as a fuel in addition to hydrogen gas obtained by reforming the methane generated inside the first fuel cell. Power generation system. 4. The power generation system according to claim 1,
The power generation system according to claim 1, further comprising a bottoming cycle that uses discharged exhaust gas. 5. The power generation system according to claim 1,
The power generation system, wherein the gas pressure of the fuel supplied to the power generation system is higher than the atmospheric pressure. 6. A fuel cell, wherein methane discharged from a first fuel cell using hydrogen gas as fuel is reformed and used as fuel for a second fuel cell using hydrogen gas as fuel. Driving method.