JP2003086227A - Power generation method using fuel cell and power generation system using power generation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power generation method and a power generation system using the power generation method, which makes the total power generation system small-sized, and which can correspond to an electric energy change easily and which enhances generating efficiency. SOLUTION: In the power generation method using the fuel cell, this is the power generation method and the power generation system using this power generation method in which organic substances are made to be fermented to form a digestion gas containing methane, hydrogen is formed by reforming the digestion gas, the hydrogen is made to be occluded in a hydrogen occlusion material, and in which the hydrogen from the hydrogen occlusion material and the oxygen from an oxygen source are supplied to generate electricity. A metallic alloy for the hydrogen storage can be adopted as the hydrogen occlusion material for occluding the hydrogen.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a power generation method and a power generation system using a fuel cell. In particular, the present invention relates to a power generation method and a power generation system using hydrogen obtained by generating methane from an organic substance and reforming the methane.

[0002]

2. Description of the Related Art From the viewpoint of environmental protection or resource reuse,
Attempts have been made to chemically decompose organic waste to generate gas, and to use this gas as fuel for power generation. Organic waste used for power generation includes sewage sludge, organic wastewater, food residues such as kitchen waste, and manure.

In order to decompose such organic wastes, anaerobic bacteria, that is, bacteria that are able to survive in the complete or partial absence of air, are used to obtain a digestive gas containing methane. be able to. Decomposing organic waste by anaerobic bacteria is called anaerobic digestion, and producing methane-containing digestion gas from organic waste by anaerobic digestion is called methane fermentation.

Japanese Patent Application Laid-Open No. 2000-23677 discloses a power generation system utilizing methane fermentation. In such a power generation system, digested gas containing methane is temporarily stored in a gas holder and then used as fuel for a gas engine generator. A power generation system that uses digestive gas containing methane as a fuel includes a gas engine, a gas turbine, and the like, but in such a power generation system, CO ₂ is discharged in a relatively large amount and power generation efficiency is not high. The power generation efficiency in small-scale diesel power generation is 30% to 40%, but the power generation efficiency in gas engines and gas turbines (power generation efficiency from methane as a raw material) is lower than this and is 25% to 30%. Therefore, instead of such a power generation system that uses digestive gas containing methane as a fuel, a power generation system that uses a fuel cell that uses hydrogen and oxygen to generate power with high efficiency has been proposed. The fuel cell can obtain electric power when reacting hydrogen and oxygen to generate water, and the power generation efficiency of the fuel cell is 40% to 45%. The power generation efficiency in this case refers to the efficiency from the production of hydrogen from the raw material methane to the power generation by the fuel cell.

Currently available phosphoric acid fuel cells or polymer electrolyte fuel cells are available.

[0006] In order to use the digested gas generated by the methane fermentation process in a fuel cell, it is necessary to convert this digested gas into hydrogen. As a method of producing hydrogen from methane, steam reforming reaction using a Ni-based reforming catalyst by adding steam to methane, CH ₄ + H ₂ O → 3H ₂ + CO (1) and partial oxidation of methane There is a method of generating hydrogen by a partial oxidation reaction that causes the reaction, CH ₄ + 1 / 2O ₂ → 2H ₂ + CO (2). Hydrogen gas having a hydrogen concentration of 50% to 80% can be obtained by these methods, but a few% of CO is also present as an impurity in this gas. Since CO becomes a poisoning component for the electrode used in the fuel cell, it is necessary to make the CO concentration in the hydrogen gas contained in the fuel cell 10 ppm or less. Therefore, after the methane is reformed into hydrogen in the above-mentioned reforming step, a shift step of transforming CO into CO ₂ by the CO shift reaction and CO + H ₂ O → CO ₂ + H ₂ (3) is performed. Since the CO concentration in the hydrogen gas that has undergone the shift process is about 5000 ppm, the CO selective oxidation process is performed to further reduce the CO concentration. In this CO selective oxidation step, CO in the hydrogen gas that has undergone the shift step is reduced by an oxidation reaction, CO + 1 / 2O ₂ → CO ₂ (4), and the CO concentration can be reduced to 10 ppm or less. Electric power can be obtained by supplying hydrogen gas that has undergone this series of steps to a fuel cell and reacting it with oxygen or air that is an oxidant.

[0007]

[Patent Document 1] Japanese Unexamined Patent Application Publication No.
In the power generation system as disclosed in Japanese Patent Publication No. 000-23677, it is necessary to store the digestion gas obtained from the organic waste as a raw material. Therefore, in such a power generation system, a relatively large storage device, for example, a gas holder, is required to store gas, and the existence of such a storage device makes the entire power generation system large. Furthermore, the methane concentration in the digestion gas is about 60.
%, So the digestion gas cannot be used as fuel about 40
% Inert gas, for example CO ₂ . Therefore,
By storing the entire digestive gas, the unusable inert gas is also stored at the same time. Further, it is known that the amount of electric power used in the daytime is different from the amount of electric power used in the nighttime, and particularly in the daytime, the amount of electric power greatly changes depending on the time zone. Therefore, the power supplier needs to deal with the above-mentioned change in the amount of power, but generally it is not easy to deal with such a change in the amount of power. It is also desired to further improve power generation efficiency.

Accordingly, the present invention provides a relatively large storage device,
For example, it is an object of the present invention to provide a power generation system that eliminates a gas holder to reduce the size of the power generation system as a whole and that can easily respond to changes in the amount of power and that has high power generation efficiency.

[0009]

According to the first aspect of the invention, in a power generation method using a fuel cell, organic matter is subjected to methane fermentation to produce a digestive gas containing methane, and the digestive gas is reformed. There is provided a power generation method for generating hydrogen by causing hydrogen to be stored in a hydrogen storage material, supplying hydrogen released from the hydrogen storage material and oxygen supplied from an oxygen source to a fuel cell to generate power. It

That is, according to the invention of claim 1,
Since hydrogen is stored in the hydrogen storage material after hydrogen is generated from the digested gas, the digested gas storage device, for example, the gas holder can be downsized or eliminated. Further, when the power load is small, a large amount of hydrogen is stored in the hydrogen storage material to suppress the power, and when the power load is large, hydrogen is released from the hydrogen storage material to increase the power,
It is possible to easily cope with fluctuations in power load. Furthermore, by using the hydrogen storage material, high-purity hydrogen can be supplied to the fuel cell, so that power generation efficiency can be improved.

According to the second aspect of the present invention, the hydrogen storage material is a hydrogen storage alloy. That is, the invention according to claim 2 allows a large amount of hydrogen to be stored.
Examples of hydrogen storage alloys include Ti-Fe alloys and La-N
i alloys and Mg-Ni alloys can be used.

According to the third aspect of the invention, the heat generated when the hydrogen is produced is used for the methane fermentation. That is, according to the invention of claim 3,
By utilizing the heat during hydrogen production, the methane fermentation action can be promoted without the need for external energy.

According to the fourth aspect of the invention, the heat generated when the hydrogen is generated is used to release the hydrogen from the hydrogen storage material. That is, according to the invention described in claim 4, it is possible to enhance the hydrogen releasing action from the hydrogen storage material without using external energy by utilizing the heat at the time of hydrogen generation.

According to the fifth aspect of the invention, the stack exhaust heat generated when power is generated in the fuel cell is used to release hydrogen from the hydrogen storage material. That is, according to the invention described in claim 5, by utilizing the stack exhaust heat, the hydrogen releasing action from the hydrogen storage material can be enhanced without requiring external energy.

According to the sixth aspect of the invention, the anode off-gas generated when power is generated in the fuel cell is supplied to the inlet of the fuel cell again or stored in the hydrogen storage material. That is, according to the sixth aspect of the invention, since high-purity hydrogen is supplied to the fuel cell via the hydrogen storage alloy, hydrogen not used in the cell reaction can be supplied to the fuel cell inlet again. That is, since the inert gas does not accumulate in the system even when the circulating action is performed, hydrogen that has not been used by the power generation action in the fuel cell can be reused.

According to the invention of claim 7, in a power generation system using a fuel cell, a methane fermentation apparatus for methane fermenting an organic substance to produce a digested gas containing methane, and reforming the digested gas. A power generation system including a hydrogen generation device that generates hydrogen, a hydrogen storage material that stores the hydrogen, and a fuel cell that generates power using hydrogen released from the hydrogen storage material and oxygen supplied from an oxygen source. Will be provided.

That is, according to the invention of claim 7,
Since hydrogen is stored in the hydrogen storage material after the hydrogen is generated from the digested gas, the digestion gas storage device, for example, the gas holder can be downsized or eliminated to downsize the entire power generation system. Furthermore, when the power load is small, a large amount of hydrogen is stored in the hydrogen storage material to suppress the power, and when the power load is large, hydrogen is released from the hydrogen storage material to increase the power, so that it is possible to easily respond to changes in the power load. can do. Furthermore, by using the hydrogen storage material, high-purity hydrogen can be supplied to the fuel cell, so that power generation efficiency can be improved.

According to the eighth aspect of the present invention, the hydrogen storage material is a hydrogen storage alloy. That is, according to the invention described in claim 8, a large amount of hydrogen can be stored.
Examples of hydrogen storage alloys include Ti-Fe alloys and La-N
i alloys and Mg-Ni alloys can be used.

According to the invention of claim 9, the heat generated when the hydrogen is produced is utilized for the methane fermentation. That is, according to the invention of claim 9,
By utilizing the heat during hydrogen production, the methane fermentation action can be promoted without the need for external energy.

According to the tenth aspect of the invention, the heat generated when the hydrogen is generated is used to release the hydrogen from the hydrogen storage material. That is, claim 1
According to the invention described in No. 0, by utilizing the heat at the time of hydrogen generation, it is possible to enhance the action of releasing hydrogen from the hydrogen storage material without requiring external energy.

According to the eleventh aspect of the present invention, the stack exhaust heat generated when power is generated in the fuel cell is used to release hydrogen from the hydrogen storage material. That is, according to the invention described in claim 11, by utilizing the stack exhaust heat, the hydrogen releasing action from the hydrogen storage material can be enhanced without requiring external energy.

According to the twelfth aspect of the present invention, the anode off-gas generated when power is generated in the fuel cell is supplied again to the fuel cell inlet or stored in the hydrogen storage material. That is, according to the twelfth aspect of the invention, since high-purity hydrogen is supplied to the fuel cell via the hydrogen storage alloy, hydrogen not used in the cell reaction can be supplied again to the fuel cell inlet. That is, since the inert gas does not accumulate in the system even when the circulating action is performed, hydrogen that has not been used by the power generation action in the fuel cell can be reused.

[0023]

DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings. In the following drawings, the same members are designated by the same reference numerals. FIG. 1 is a process system diagram of the fuel cell power generation system of the present invention. As shown in FIG. 1, the fuel cell power generation system of the present invention comprises a methane fermentation step A for producing a digestive gas containing methane by subjecting an organic waste as a raw material to methane fermentation, and a digestive gas containing methane. The hydrogen generation step B for generating hydrogen, the hydrogen storage step C for storing the hydrogen obtained in the hydrogen generation step B in the hydrogen storage material, and the hydrogen released from the hydrogen storage material and oxygen from an oxygen source different from the hydrogen storage step And a fuel cell power generation step D in which the fuel cell is used to generate power. Next, the electric power obtained in the fuel cell power generation step D is supplied to the load E.

FIG. 2 is a diagram showing a methane fermentation process in the fuel cell power generation system shown in FIG. The methane fermentation apparatus 10 used in the methane fermentation step A may include a receiving tank 11, a mixing tank 12, a digesting tank 13, and a desulfurization tower 15. Organic waste, such as sewage sludge, organic waste water, food residues such as garbage, or manure is put into the receiving tank 11. The organic waste is crushed by the crushing unit in the receiving tank 11 and is transferred to the mixing tank 12 while being stirred by the stirring unit and stored and mixed in the mixing tank 12.

Next, the organic waste in the mixing tank 12 is transferred to the digestion tank 13. In the digestion tank 13, the organic waste is methane-fermented with anaerobic bacteria, that is, bacteria that can survive in the complete or partial absence of air. In the case of the present embodiment, this methane fermentation treatment is preferably performed at a temperature of around 35 ° C., and the digested gas produced by the methane fermentation treatment contains about 60% methane. The amount of digestive gas varies depending on the type of organic waste and the treatment conditions, but if you select manure as organic waste and treat it for 5 tons per day, it will be about 20.
A digestion gas of 00 m ³ can be obtained. In order to perform the methane fermentation treatment at a temperature of around 35 ° C, the methane fermentation apparatus 10
Preferably includes a hot water tank 16 capable of heating the digestion tank 13. The hot water tank 16 can be heated by utilizing the heat generated in the hydrogen generation process. This can easily increase the efficiency of the methane fermentation process without the need for external energy.

The digestion gas contains a trace amount of an acidic gas, for example, hydrogen sulfide, which may poison the reforming catalyst used in the reforming process. Therefore, the digested gas is transferred to the dry desulfurization tower 15 to remove these traces of acidic gas. The desulfurization tower 15 used in the present embodiment contains a desulfurization agent such as iron oxide, zinc oxide or activated carbon, and these desulfurization agents can remove the acidic gas in the digestion gas, particularly hydrogen sulfide. In this embodiment, the amount of acidic gas contained in the digestion gas is set to less than 10 ppm, and if possible, set to less than 1 ppm. As shown in FIG. 1, the methane fermentation apparatus 10 may include a dehydrator 17, whereby the digested sludge separated from the digested gas in the digestion tank 13 can be dehydrated to be separated into dehydrated sludge and dehydrated filtrate. it can.

Next, the digested gas obtained in the methane fermentation step A is transferred to the hydrogen generator 20 used in the hydrogen generation step B. FIG. 3 is a diagram showing a hydrogen generation process in the fuel cell power generation system shown in FIG. The hydrogen generator 20 that performs the hydrogen generation step B includes a reformer 21 that performs a steam reforming reaction and a CO shift converter 2 that performs a CO shift reaction.
2 and 23, and a CO selective oxidation unit 24 that selectively oxidizes CO can be included.

In the reforming section 21, the above-mentioned digested gas is subjected to a steam reforming reaction (equation (1)). A steam reforming reaction is carried out at a reaction temperature of about 650 ° C. to 750 ° C. by adding 2.5 to 3.5 times steam to 1 mol of methane in the digestion gas. Since this steam reforming reaction is an endothermic reaction, it is necessary to supply heat from the outside. As shown in FIG. 3, heat can be supplied to the reforming section 21 by burning the above-mentioned digestion gas in the combustion chamber 27 using a burner as a fuel. As shown in FIG. 3, the combustion chamber 27 is preferably adjacent to the reforming section 21, so that the heat obtained in the combustion chamber 27 can be directly supplied to the reforming section 21. Further, oxygen from an oxygen source (not shown) may be supplied to the combustion chamber 27 to control the heating action in the combustion chamber 27. The gas that has undergone the reforming process in the reforming unit 21 is transferred to the CO shift conversion units 22 and 23. Two-stage CO shift reaction (Equation (3)) is performed in the CO shift units 22 and 23 to perform CO shift of gas. First stage CO
Although the inlet temperature of the shift converter 22 is 450 ° C., since the CO shift reaction is an exothermic reaction, the outlet temperature of the CO shift unit 22 is about 490 ° C. and the CO concentration is 5% to 10%.
It is reduced to about 10%. In order to further reduce CO, a CO shift reaction is performed in the second-stage CO shift conversion section 23. The inlet temperature of the CO shift converter 23 is about 200 ° C., the outlet temperature is about 240 ° C., and the CO concentration is reduced to about 0.5%. In order to further reduce the CO concentration, the gas that has passed through the CO shift conversion section 23 is transferred to the CO selective oxidation section 24. By supplying oxygen from an oxygen source (not shown), CO in the gas is selectively oxidized by the oxidation reaction (equation (4)) to CO _{2 in the} CO selective oxidation section 24. The inlet temperature of the CO selective oxidation section 24 is 12
The temperature is about 0 ° C and C
The O concentration is reduced to about 500 ppm in the first stage and 10 ppm or less in the second stage. Between the reforming section 21 that performs the reforming step and the CO selective oxidizing section 24 that performs the CO selective oxidation step, it is necessary to lower the temperature of the gas by using a heat exchanger or the like. As this cooling method, cooling water may be used as shown in FIG. 3, or an air cooling method such as a radiator may be used. As described above, the heat generated by any or all of the plurality of reactions performed in the hydrogen production step B is transferred to, for example, the hot water tank 1.
By utilizing 6 to heat appropriately, the methane fermentation action performed in the digestion tank 13 in the methane fermentation step A can be promoted.

In this embodiment, hydrogen produced in the hydrogen producing step B can be stored in the hydrogen storage material 30 in the hydrogen storing step C. The hydrogen storage material 30 of the hydrogen storage step C in the present embodiment is a hydrogen storage alloy such as a Ti-Fe alloy, a La-Ni alloy, and a Mg-Ni alloy. Such a hydrogen storage alloy has a property of generating heat when absorbing hydrogen, and absorbing ambient heat when releasing hydrogen. In the case of the present invention, hydrogen generated from the digested gas can be stored in a hydrogen storage material, for example, a hydrogen storage alloy, so that a storage device for storing the digested gas, for example, a gas holder is downsized or the gas holder is eliminated. can do. Therefore, in the present invention, the entire power generation system can be downsized. By using the hydrogen storage material 30, high-purity hydrogen can be sent to the fuel cell power generation step D described later. Since the higher the hydrogen concentration is, the higher the power generation efficiency is, the power generation system of the present invention can have higher power generation efficiency than the conventional power generation system by using the hydrogen storage material. In addition, in the present invention, since electric power can be stored in the form of hydrogen, a storage battery required to directly store electric power can be eliminated.

Next, the hydrogen generated in the hydrogen generation step B and / or the hydrogen released from the hydrogen storage material in the hydrogen storage step C is supplied to the anode of the fuel cell 40 used in the fuel cell power generation step D. As shown in FIG. 3, it is preferable that the hydrogen produced in the hydrogen producing step B is passed through the humidification tower 28 before being supplied to the fuel cell 40, whereby the C produced in the hydrogen producing step B is generated.
It can absorb O ₂ . Further, oxygen from an oxygen source (not shown) or the atmosphere is supplied to the cathode of the fuel cell 40. The fuel cell 40 is, for example, a phosphoric acid fuel cell or a polymer electrolyte fuel cell. As a result, electric power is generated in the fuel cell 40, and this electric power can be used in the load E. When the fuel cell is a polymer electrolyte type, the operating temperature is about 60 to 80 ° C, the hydrogen utilization rate of fuel is about 60 to 90%, and the remaining hydrogen is discharged unreacted as anode offgas. To be done.

Since the hydrogen storage step C is adopted in the present invention, generation of power can be suppressed by storing hydrogen in the hydrogen storage material 30 when the power load is low, for example, at night. When the electric power load is high, for example, in the daytime, the hydrogen obtained in the hydrogen production step B is directly supplied to the fuel cell power generation step D, and the hydrogen stored in the hydrogen storage step C is released from the hydrogen storage material 30. As a result, the power can be increased. Therefore, by appropriately adopting the hydrogen storage step C, it is possible to control the amount of electric power and easily cope with the fluctuation of the electric power load. Hydrogen not used for power generation in the fuel cell power generation process,
That is, the anode off gas is supplied again to the inlet of the fuel cell and used for power generation. Alternatively, it is supplied again to the hydrogen storage material 30 in the hydrogen storage step C.

Further, as described above, the hydrogen storage alloy robs the surrounding heat at the time of releasing hydrogen, so that the stack exhaust heat of the fuel cell in the fuel cell power generation step D (FIG. 1) or the heat generated in the hydrogen generation step B is converted into hydrogen. It is preferable that it can be used in the storage step C. This can enhance the hydrogen release action of the hydrogen storage alloy without the need for external energy. Further, of the heat, the remaining heat that is not used in the hydrogen storage step C can be used for other purposes such as heating, and a cogeneration system that supplies electric power and heat becomes possible.

[0033]

According to the invention described in each claim, the gas holder for digestion gas can be made small or eliminated to make the entire power generation system small, and it is possible to easily cope with the fluctuation of the power load. In addition to the above, there is a common effect that the power generation efficiency can be improved.

Further, according to the invention described in claims 2 and 8, there can be obtained an effect that a large amount of hydrogen can be stored. Furthermore, according to the invention described in claims 3 and 9, there is an effect that the methane fermentation action can be promoted without using external energy by utilizing the heat at the time of hydrogen generation.

Further, according to the invention described in claims 4 and 10, by utilizing the heat at the time of hydrogen generation, it is possible to enhance the hydrogen releasing action from the hydrogen storage material without requiring external energy. The effect can be achieved. Further, according to the invention described in claims 5 and 11, it is possible to obtain an effect that the hydrogen releasing action from the hydrogen storage material can be enhanced by using the stack exhaust heat without requiring external energy. . Further, according to the invention described in claims 6 and 12, it is possible to obtain an effect that hydrogen that has not been used due to the power generation action in the fuel cell can be reused.

[Brief description of drawings]

FIG. 1 is a process system diagram of a fuel cell power generation system.

FIG. 2 is a diagram showing a methane fermentation process in the fuel cell power generation system shown in FIG.

FIG. 3 is a diagram showing a hydrogen generation process in the fuel cell power generation system shown in FIG.

[Explanation of symbols]

10 ... Methane fermentation device 11 ... Accepting row 12 ... Mixed tank 13 ... Digestion tank 15 ... Desulfurization tower 16 ... Hot water tank 17 ... dehydrator 20 ... Hydrogen generator 21 ... reforming section 22 ... CO shift unit 23 ... CO shift unit 24 ... Selective oxidation unit 30 ... Hydrogen storage material 40 ... Fuel cell A ... Methane fermentation process B ... Hydrogen production process C ... Hydrogen storage process D ... Fuel cell power generation process E ... load

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) C01B 3/48 C02F 11/04 A 5H027 C02F 11/04 C12M 1/00 H C12M 1/00 H01M 8/04 J H01M 8 / 04 8/10 8/10 B09B 3/00 D F term (reference) 4B029 AA01 BB02 CC01 CC02 DF10 DG10 4D004 AA03 BA03 CA17 4D059 AA01 AA03 AA07 BA14 4G040 AA01 AA16 BA02 BB03 EA03 EA06 EB03 EB14 EB31 EB32 EB42 EB43 EB44 5H026 AA04 AA06 5H027 AA04 AA06 BA01 BA14

Claims (12)

[Claims] 1. A power generation method using a fuel cell, wherein an organic substance is subjected to methane fermentation to generate a digestive gas containing methane, the digestive gas is reformed to produce hydrogen, and the hydrogen is stored in a hydrogen storage material. A power generation method in which hydrogen is stored and hydrogen released from the hydrogen storage material and oxygen supplied from an oxygen source are supplied to a fuel cell to generate electricity. 2. The power generation method according to claim 1, wherein the hydrogen storage material is a hydrogen storage alloy. 3. The power generation method according to claim 1, wherein the heat generated when the hydrogen is generated is used for the methane fermentation. 4. The power generation method according to claim 1, wherein the heat generated when the hydrogen is generated is used to release the hydrogen from the hydrogen storage material. 5. The power generation method according to claim 1, wherein stack exhaust heat generated when power is generated in the fuel cell is used to release hydrogen from the hydrogen storage material. . 6. The anode off-gas generated during power generation in the fuel cell is supplied to the inlet of the fuel cell again or is stored in the hydrogen storage material. The described power generation method. 7. A power generation system using a fuel cell, a methane fermentation apparatus for methane fermenting an organic substance to generate a digested gas containing methane, and a hydrogen generator for reforming the digested gas to generate hydrogen. A power generation system comprising: a hydrogen storage material for storing the hydrogen; and a fuel cell that generates electric power using hydrogen released from the hydrogen storage material and oxygen supplied from an oxygen source. 8. The power generation system according to claim 7, wherein the hydrogen storage material is a hydrogen storage alloy. 9. The power generation system according to claim 7, wherein the heat generated when the hydrogen is generated is used for the methane fermentation. 10. The power generation system according to claim 7, wherein heat generated when the hydrogen is generated is used to release the hydrogen from the hydrogen storage material. 11. The power generation system according to claim 7, wherein stack exhaust heat generated when power is generated in the fuel cell is used to release hydrogen from the hydrogen storage material. . 12. The anode off-gas produced during power generation in the fuel cell is supplied to the fuel cell inlet again or stored in the hydrogen storage material. Power generation system described.