JP2004192824A - Fuel cell/methane fermentation cycle system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell/methane fermentation cycle system using gas produced by methane fermentation and efficiently converting it into electrical energy, inexpensively separating, recovering and using carbon dioxide, and converting as much organic waste into methane gas to eliminate sludge. <P>SOLUTION: In the fuel cell/methane fermentation cycle system, a fuel cell system is combined with a methane fermentation system. It is provided with a methane fermentor 28, and a fuel cell 12 basically having an oxygen electrode (a cathode) 14 and a fuel electrode (an anode: a hydrogen electrode) 18. Biogas (produced gas) produced in the methane fermentor 28 is introduced to the anode 18, gas flowing out from the anode (anode outflow gas) to the methane fermentor 28 is passed through a carbon dioxide separation and recovery process, and cycle introduction of carbon dioxide or gas mainly comprising carbon dioxide to the methane fermentor 28 is carried out. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
【Technical field】
The present invention relates to a fuel cell / methane fermentation cycle system that uses biogas generated in a methane fermentation tank as fuel for a fuel electrode of a fuel cell.
[0002]
In particular, businesses that are subject to the Food Recycling Law, local government facilities that process large amounts of garbage, businesses that are subject to reductions in carbon dioxide emissions, and certificates RPS issued for power generation using new energy ( This is an invention suitable for a business establishment planning to acquire Renewable Portfolio Standard.
[0003]
[Background Art]
In the anaerobic fermentation treatment (methane fermentation) of organic waste containing much water such as garbage, a technique of recovering resources as energy has attracted attention, and many techniques have been proposed and put into practical use.
[0004]
However, at present, about 25% of the input waste is incinerated or landfilled as sludge.
[0005]
Although not affecting the invention of the present invention, the following are known technologies (patent documents) relating to methane fermentation.
[0006]
Patent Document 1: Generated gas is separated into methane gas and CO ₂ gas by a polymer membrane to increase the methane gas concentration, and the CO ₂ gas is mixed with biogas and sent to the digestion tank again to be reduced to methane to reduce the amount of methane gas. Methane fermentation method to increase.
[0007]
Patent Literature 2: An immersion-type methane fermentation system using a membrane that simultaneously performs washing of the surface of the immersion membrane and stirring in the tank using biogas obtained by methane fermentation.
[0008]
In addition, the following are known technologies (known documents) regarding fuel cells.
[0009]
Patent Document 3: A power plant using a high-temperature fuel cell using natural gas as a fuel, in which water generated by the fuel cell is electrolyzed and unreacted hydrogen is returned to water with the obtained oxygen. Technology.
[0010]
However, these are all one-off technologies, and the overall technical and economic problems have not been solved.
[0011]
[Patent Document 1]
JP-A-61-178016 [Patent Document 2]
JP 2000-94000 [Patent Document 3]
JP-A-10-144322
DISCLOSURE OF THE INVENTION
In view of the above, the present invention converts gas produced by methane fermentation into electric energy with high efficiency, separates, collects and uses carbon dioxide at low cost, and enables organic waste. It is an object of the present invention to provide a fuel cell / methane fermentation cycle system which converts methane gas as much as possible and makes it sludge-free.
[0013]
In order to solve the above problems, the present invention provides a fuel cell / methane fermentation cycle system having the following configurations.
[0014]
A methane fermentation tank, and a fuel cell basically having an air electrode (oxygen electrode: cathode) and a fuel electrode (hydrogen electrode: anode),
Biogas generated in the methane fermentation tank is introduced into the anode, and exhaust gas (anode outflow gas: H ₂ O + CO ₂ + H ₂ ) generated from the anode is introduced into the methane fermentation tank.
[0015]
In the above configuration, the anode effluent gas is introduced into the CO ₂ separation tower, a part of CO ₂ is separated and recovered, and the carbon dioxide gas or the exhaust gas mainly containing the carbon dioxide gas flowing out of the CO ₂ separation tower is introduced into the methane fermentation tank. It is desirable to do.
[0016]
In the above configuration, when the fuel cell is of a low temperature type, it is desirable to introduce the biogas into the anode through a steam reforming step of making the biogas hydrogen-rich.
[0017]
When the fuel cell is of a high temperature type, it is desirable to directly introduce biogas to the anode without going through a steam reforming step.
[0018]
When the fuel cell is of a high-temperature type, it is desirable to introduce the anode effluent gas obtained by removing unreacted hydrogen in the anode effluent gas by catalytic combustion into a CO ₂ separation column.
[0019]
In each of the above configurations, it is desirable to dispose a solid-liquid separation membrane that does not allow methane bacteria to flow out of the drain port of the methane fermentation tank.
[0020]
With the above configurations, the present invention has the following operations and effects.
[0021]
The biogas obtained by methane fermentation (digestion) of organic waste such as garbage is mainly composed of methane (CH ₄ ) and carbon dioxide (CO ₂ ), and hydrogen (H ₂ ) is obtained by steam reforming. And carbon dioxide (CO ₂ ). After purification, the biogas is reformed and supplied to the anode of the fuel cell. Hydrogen reacts with oxygen at the cathode electrode, is converted into electric energy, and is discharged as water. Therefore, the exhaust gas at the anode is a high-concentration CO ₂ gas, a low-concentration unreacted hydrogen gas (H ₂ ) and water vapor (H ₂ O), and CO ₂ separation can be easily performed by heat recovery. Therefore, the heat-recovered gas can be directly introduced into the methane fermentation tank, or the anode effluent gas can be introduced into the CO ₂ separation column to partially separate and recover CO ₂ and also recover water vapor as water. it can.
[0022]
By installing a membrane in the methane fermentation tank to prevent the outflow of methane bacteria (accumulation of methane bacteria), the ability to decompose organic substances is improved. By diffusing CO ₂ and unreacted H ₂ on the membrane surface, there is an effect of cleaning the membrane surface, stirring in the fermenter, and increasing the CO ₂ content in the fermentation solution.
[0023]
By utilizing the action of methane bacteria reducing CO ₂ to methane gas through hydrogen fermentation products such as fatty acids and alcohols generated in the previous stage of methane fermentation, the methane production reaction is promoted, and an increase in methane gas can be expected.
[0024]
At the same time as the high-efficiency power generation by the fuel cell, the recovery and use of CO ₂ and the generation of digested sludge can be reduced to the utmost limit, and a recycling cycle system can be constructed.
[0025]
[Detailed description of configuration]
Hereinafter, an example of the fuel cell / methane fermentation cycle system of the present invention will be described mainly with reference to FIG.
[0026]
The present invention is basically a combination of a fuel cell system and a methane fermentation (anaerobic treatment) system.
[0027]
That is, the fuel cell system includes a fuel cell 12, an air supply pipe 16 for supplying oxygen to an air electrode (oxygen electrode: cathode) 14 of the fuel cell 12, and a fuel (hydrogen-containing And a fuel supply pipe 20 for supplying gas. The air supply pipe 16 includes a blower 22 at an air supply port, and an air-cooled heat exchanger 24 in the middle.
[0028]
Here, the fuel cell 12 is of a low temperature type such as a phosphoric acid type (PAFC) and a polymer electrolyte type (PEFC).
[0029]
Further, the methane fermentation system includes a methane fermentation tank 28, a garbage storage 30 for storing garbage to be put into the methane fermentation tank 28, a drainage tank 32 for discarding and purifying treated water of the methane fermentation tank 28, and A purification / storage tank 33 for purifying and storing gas generated in the methane fermentation tank 28 is provided, and a supply pipe 34 provided with a pulverizing pump (not shown) and a gas transporter 35 are provided therebetween. A methane gas pipe 36 and a drain pipe 38 connected to a submersible pump 29 disposed in the methane fermentation tank 28 are connected. In front of the submersible pump 29, a solid-liquid separation membrane 31 for preventing the outflow of methane bacteria is provided. The purifying / storing tank 33 has a built-in means for purifying impurities such as sulfur oxides contained in the methane gas.
[0030]
Further, in the present system, a carbon dioxide (CO ₂ ) separation and recovery system is interposed between the fuel cell system and the methane fermentation system.
[0031]
The carbon dioxide separation and recovery system cools the mixed gas discharged from the fuel electrode (anode) 18 to perform primary separation, and converts the carbon dioxide in the water vapor-containing carbon dioxide flowing out of the heat exchanger 40 into carbon dioxide. A carbon dioxide separation tower (CO ₂ separation tower) 42 for separating and recovering from steam. The CO ₂ separation tower 42 includes a carbon dioxide gas tank 44 and a stored water tank 46. Further, in the present system, between the CO ₂ separation tower 42 and the methane fermentation tank 28, the carbon dioxide-containing gas (H ₂ + CO ₂ ) discharged from the inside of the CO ₂ separation tower 42 is passed through the gas transporter 48. A carbon dioxide gas supply pipe 50 is provided for the purpose of stirring the methane fermentation tank 28 and increasing the CO ₂ content in the tank.
[0032]
Next, a usage mode of the system having the above configuration will be described.
[0033]
First, garbage stored in the garbage storage 30 is put into the methane fermentation tank 28, and methane fermentation (digestion method), which is an anaerobic treatment, is performed. Then, biogas methane as the main component _{(CH 4: CO 2 = 6} : 4) occurs. Then, the biogas is subjected to steam reforming (performing the following reaction) in the reformer 54 to become a hydrogen-rich gas, and can be supplied to the fuel electrode 18 via the heat exchanger 40.
[0034]
CH ₄ + 2H ₂ O → 4H ₂ O + CO ₂
In the case of natural gas, steam and water are reformed into hydrogen and CO ₂ and supplied to the fuel electrode.
[0035]
The anode outflow gas flowing out of the fuel electrode (anode) 18 is composed of water (H ₂ O), carbon dioxide (CO ₂ ), and unreacted residual hydrogen (H ₂ ), and is recovered (cooled) by the heat exchanger 40. ) through the primary separation step of, by flowing into the CO ₂ separation column CO ₂ separation column 42, it can conveniently be separated and recovered and the water _CO 2 / _{H 2} components.
[0036]
Further, if the unreacted residual hydrogen separated in the CO ₂ separation tower 42 is put into the bottom of the methane fermentation tank 28 together with CO ₂ , the treatment becomes easy. Since the specific gravity difference between CO ₂ and H ₂ is large, the CO ₂ and H ₂ can be separated by a simple mechanism such as a current plate in the CO 2 separation tower 42. When the CO ₂ -containing gas introduced into the methane fermentation tank is used for aeration / stirring of the methane fermentation tank 28 and is used as a fuel for the fuel cell 12, the higher the H ₂ concentration contained, the higher the fuel cell Since the power generation efficiency of the fuel cell 12 increases, it is not necessary to remove unreacted residual hydrogen.
[0037]
When CO ₂ is used, the presence of hydrogen does not pose a significant problem in the process of liquefaction or dry ice production.
[0038]
At this time, since hydrogen reacts with oxygen of the air electrode (oxygen electrode: cathode) 14 to become water, CO ₂ and water are discharged (flow out) from the fuel electrode 18. This CO ₂ and water can be easily separated by cooling. By utilizing this characteristic, it is possible to simply and efficiently separate and recover CO ₂ simultaneously with high-efficiency power generation. Then, the recovered CO ₂ can be used as a stirring gas in the methane fermentation tank 28 and as a promoting gas for the methane gas generation reaction.
[0039]
When a small amount of hydrogen is hindered in the CO ₂ separation column 42 shown in FIG. 1, hydrogen is easily removed by using the combustor 52 as shown in FIG. 2 (that is, catalytic combustion with oxygen). As a result, a high concentration of CO ₂ is obtained (see FIG. 2).
[0040]
When a high-temperature solid oxide fuel cell (SOFC) is used, the system shown in FIG. 2 is used.
[0041]
That is, in the SOFC, methane is reformed inside the battery. For this reason, biogas containing methane gas as a main component can be directly supplied to the fuel electrode 18 without a reformer, and the exhaust gas discharged from the fuel electrode can be easily CO ₂ / H ₂ by heat recovery (cooling). Components and water can be separated and recovered.
[0042]
Since the device configuration of FIG. 2 is basically the same as that of FIG. 1, the corresponding reference numerals are assigned and only the differences will be described.
[0043]
In the SOFC, since the exhaust heat temperature is high, the exhaust heat recovery unit 56 is installed at the outlet of the cathode 14 to use the exhaust heat (make and use hot water) and heat the air supplied to the cathode 14. Increases thermal efficiency. After the gas from the outlet of the anode 18 passes through the combustor 52 to remove H ₂ , the gas supplied to the anode 18 is heated by the secondary air-cooled heat exchanger 40 to recover heat. Furthermore, heat recovery is performed again by further heating the air supplied to the cathode 14 by the primary air-cooled heat exchanger 40A. The gas sent to the methane fermentation tank 28 by the carbon dioxide gas supply pipe 50 is only CO ₂ because it passes through the combustor 52.
[0044]
Natural gas is almost 100% methane, but in the case of biogas obtained by anaerobic fermentation, CH ₄ : CO ₂ = 6: 4. When methane is steam reformed, the molar ratio of hydrogen to CO ₂ will be 4: 1 as described above. Therefore, from the viewpoint of the amount of recovered CO ₂ , in the case of biogas, it is possible to recover 1.5 times or more of CO ₂ as compared with natural gas.
[0045]
First, solid poorly soluble organic substances are converted into high-molecular organic substances by hydrolytic bacteria, and then converted into low molecular intermediates such as alcohols and organic acids by fermentative bacteria. , Becomes soluble. Then decomposed into CO ₂ and methane gas by acetic acid bacteria (Acetogenic bacteria) and methane bacteria (Methanogenic bacteria).
[0046]
It is said that methane bacteria that undergo anaerobic fermentation have an action of decomposing CO ₂ and reducing it to methane gas in an environment where fatty acids (organic acids) and alcohols are present.
[0047]
When a membrane (filtration membrane that does not allow passage of solid matter such as organic matter including microorganisms) 31 having a solid-liquid separation mechanism is installed in the methane fermentation tank 18, the collected CO ₂ is diffused and bubbled on the membrane surface, When the membrane surface is washed and retained by an ascending flow, the methane bacterium can be concentrated by preventing the useful methane bacterium from flowing out by the membrane when the turbidity is mixed into the digestive juice and the tank. Therefore, the thorough break down organic matter, by reductive actively to decompose methane to CO _2, formation reaction of methane gas is promoted, a large increase in methane gas amount can be expected.
[0048]
Further, since the wastewater discharged from the solid-liquid separation membrane 31 does not contain solid matter, it has a small environmental load and can be treated in a simple wastewater tank (water treatment device) 32.
[0049]
【The invention's effect】
The effects of the present invention are summarized as follows.
[0050]
{Circle around (1)} By diffusing the exhaust gas of the fuel cell from the lower part of the immersion membrane installed in the methane fermentation tank, the membrane surface can be washed by the upward flow and the inside of the fermentation tank can be stirred.
[0051]
(2) Exhaust gas from the fuel cell is high-concentration CO ₂ and unreacted hydrogen, and the acid fermentation products (CO ₂ decomposition by alcohol and fatty acids and methane bacteria) due to the dissolution of CO ₂ into the digestive juice are reduced. The conversion to methane gas is promoted, and an increase in methane gas can be expected.
[0052]
Since unreacted hydrogen is supplied to the fuel cell as it is, it has more combustible components than 100% CO ₂ gas and can maintain high performance of the fuel cell.
[0053]
The solid-liquid separation function of the immersion membrane can prevent the outflow of methane bacteria and can effectively decompose organic substances, so that the sludge discharged can be significantly reduced.
[0054]
In a high-temperature fuel cell, since methane is reformed inside the cell, the digestion gas can be supplied to the anode electrode of the fuel cell as it is, and becomes hydrogen and CO ₂ . Then, the hydrogen reacts with the oxygen of the cathode electrode, is converted into electric energy and is discharged as water. For this reason, the exhaust gas is composed of high-concentration CO ₂ gas, low-concentration unreacted residual hydrogen gas, and water vapor.
[0055]
Auxiliary fuel is added to residual hydrogen in the exhaust gas of the fuel cell, which is burned with O _2. After power is generated by the gas turbine, it is recovered as thermal energy, and CO ₂ and water can be separated and recovered.
[0056]
Organic waste such as garbage is accumulated in the methane fermentation tank through a permeable membrane that prevents methane bacteria from flowing out, and the combined action of hydrogen fermentation products and methane bacteria to reduce CO ₂ to methane gas. By thoroughly utilizing organic materials, organic substances are thoroughly decomposed, the methane generation reaction is promoted, and the amount of methane gas is increased. Accordingly, it is possible to construct a cycle system capable of reducing the generation of digested sludge to the utmost with high-efficiency power generation, recovery / use of CO ₂ , and generation of digested sludge by a high-temperature fuel cell with extremely low environmental load.
[Brief description of the drawings]
FIG. 1 is a schematic flow chart showing an example of a cycle system when a low-temperature fuel cell of the present invention is used.
FIG. 2 is a schematic flow chart showing a modified example when a high-temperature fuel cell is used.
12, 12A ... fuel cell 14 ... air electrode (oxygen electrode: cathode)
18 ... fuel electrode (hydrogen electrode: anode)
20: fuel supply pipe 28: methane fermentation tank (digestion tank)
40: heat exchanger 42: CO ₂ separation tower 50: carbon dioxide gas supply pipe

Claims (6)

A methane fermentation tank, and a fuel cell basically having an air electrode (oxygen electrode: cathode) and a fuel electrode (hydrogen electrode: anode),
A fuel, wherein biogas (product gas) generated in the methane fermentation tank is introduced into the anode, and gas flowing out of the anode (hereinafter, “anode outflow gas”) is introduced into the methane fermentation tank. Battery and methane fermentation cycle system. The anode effluent gas is introduced into a carbon dioxide separation tower (CO ₂ separation tower) to partially separate and recover CO _2, and the carbon dioxide gas or the exhaust gas mainly composed of carbon dioxide flowing out of the CO ₂ separation tower is subjected to methane fermentation. The fuel cell / methane fermentation cycle system according to claim 1, wherein the system is introduced into a fuel cell. The fuel cell / methane fermentation cycle system according to claim 1 or 2, wherein the fuel cell is a low temperature type, and the biogas is introduced into the anode through a steam reforming step. 3. The fuel cell / methane fermentation cycle system according to claim 1, wherein the fuel cell is a high-temperature fuel cell, and the biogas is directly introduced into the anode without going through a steam reforming step. The fuel cell / methane fermentation cycle system according to any one of claims 2 to 4, wherein the anode effluent gas obtained by removing unreacted hydrogen from the anode effluent gas by catalytic combustion is introduced into a carbon dioxide gas separation tower. The fuel cell / methane fermentation cycle system according to any one of claims 1 to 5, wherein a solid-liquid separation membrane from which methane bacteria do not flow is disposed at a drain port of the methane fermentation tank.

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