JP4457421B2 - Fuel cell system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel cell system, and more particularly to an improvement in a fuel cell system including a reforming unit that generates fuel gas from a hydrocarbon raw material as fuel for the fuel cell.
[0002]
[Prior art]
A fuel cell is a device that generally converts oxygen and hydrogen as fuel and converts the chemical energy of these fuels directly into electrical energy without passing through thermal energy, and has excellent environmental characteristics. Moreover, since high energy efficiency is possible, it is widely developed as a future energy supply system.
[0003]
The form and principle of a general fuel cell are as follows: a pair of electrodes are arranged with an electrolyte in between, a fuel gas containing hydrogen is supplied to the anode, and an oxygen gas containing oxygen is supplied to the cathode. Electricity is generated using the electrochemical reaction that occurs at both electrodes shown. That is, the chemical reaction of the formula (1) occurs at the anode, and the reaction (2) occurs at the cathode. Therefore, as a reaction of the entire fuel cell, the equation (3) proceeds, power is generated by hydrogen and oxygen, and water is by-produced at the same time.
[0004]
H ₂ → 2H ⁺ + 2e ⁻ (1)
(1/2) O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O (2)
H ₂ + (1/2) O ₂ → H ₂ O (3)
When the above fuel cell is used as a power generation device, it is economically ideal to use natural gas, naphtha, and alcohol as methanol as the fuel gas containing hydrogen, but currently these fuel gases are supplied directly to the battery. As a result, there is no fuel cell that can generate electricity efficiently, which is a future development issue. Therefore, many current fuel cells include a reforming section that generates fuel gas from a hydrocarbon raw material. In this reforming section, the fuel for reforming the natural gas and methanol and supplying the reformed gas to the anode is provided. Gas is being generated.
[0005]
In FIG. 2, the structure of the conventional fuel cell system provided with the reforming part is shown. The fuel cell system 10 includes a reforming unit 12 that reforms a hydrocarbon raw material such as methane to generate fuel gas. In the reforming unit 12, reforming is performed by various reforming methods such as a steam reforming method, a partial oxidation method, and the like.
[0006]
As an example, chemical reactions that occur when methane is steam reformed (4) and when partial oxidation is performed (5) are shown below. Of these, when steam reforming is performed, it is necessary to provide the reforming unit 12 with an evaporator 22 and heat and evaporate water through the evaporator 22 to supply the steam to the reforming unit 12 as steam. Either one of these steam reforming methods and oxidation reforming methods can be employed, but both are simultaneously performed in one reforming unit 12.
[0007]
CH ₄ + H ₂ O → 3H ₂ + CO (4)
CH ₄ + 1 / 2O ₂ → 2H ₂ + CO (5)
Further, the fuel cell system 10 includes a shift unit 14 and a CO selective oxidation unit 16 for converting CO or the like by-produced when the fuel gas is generated in the reforming unit 12 into CO ₂ . The shift unit 14 and the CO selective oxidation unit 16 transform the CO that poisons the electrodes of the fuel cell 18 and inhibits the operation into CO _{2 that} has little influence on the operation of the fuel cell 18. In the shift unit 14, water is allowed to act on CO as shown in the equation (6) to generate fuel gas H ₂ and CO ₂ . Further, in the CO selective oxidation unit 16, as shown in the equation (7), oxygen is allowed to act on CO to convert it into CO ₂ .
[0008]
CO + H ₂ O → H ₂ + CO ₂ (6)
CO + 1 / 2O ₂ → CO ₂ (7)
The fuel gas from which CO has been removed in the shift unit 14 and the CO selective oxidation unit 16 is supplied to the anode of the fuel cell 18, and air is supplied to the cathode to generate power in the fuel cell 18. On the side, water is by-produced. As described above, in the fuel cell system, power generation in the fuel cell is performed while reforming the hydrocarbon raw material to generate the fuel gas.
[0009]
[Problems to be solved by the invention]
As described above, the fuel cell system is a power generation system that can generate high energy from hydrogen and oxygen, and is environmentally suitable as a by-product such as water. It is desired to reduce the size so that it can be used. However, a fuel cell system including a reforming unit that reforms fuel is large in size, and its range of use is limited.
[0010]
The present invention has been made in view of the above problems, and an object thereof is to downsize a fuel cell system having means for generating fuel gas by reforming raw materials.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a fuel cell comprising a reforming unit that reforms a hydrocarbon raw material to generate hydrogen, and a fuel cell that generates electricity by supplying the hydrogen and oxygen as fuels. In the system, a connecting portion for connecting the fuel cell and the reforming portion so that water vapor generated at the cathode of the fuel cell and unreacted oxygen can be directly fed to the reforming portion by the operation of the fuel cell. The hydrocarbon raw material is reformed using water vapor and oxygen sent from the fuel cell to the reforming section through the connecting section.
[0012]
According to the above invention, since the steam generated in the fuel cell is supplied to the reforming unit, conventionally, an evaporator has been separately provided to generate steam necessary for steam reforming. The size of the system can be reduced by omitting the device. In addition, by supplying oxygen from the fuel cell, it is not necessary to provide a conduit for supplying oxygen to the reforming section, and the system can be further downsized.
[0013]
The fuel cell system of the present invention further includes a control unit that controls the amount of water vapor and oxygen fed to the reforming unit at the connection unit, and controls the reforming reaction of the hydrocarbon raw material in the reforming unit. Features.
[0014]
According to the present invention, the steam reforming reaction in the reforming section can be controlled by controlling the amount of steam fed, and the partial oxidation reforming reaction of the reforming section by controlling the amount of oxygen supplied. Can be controlled. In particular, these partial oxidation reforming reactions are exothermic reactions, while steam reforming reactions are endothermic reactions, so when these are performed simultaneously in one reforming section, individual reactions are supplied. By controlling the amount of water vapor and the amount of oxygen to be controlled, it is possible to appropriately control each reaction, and thus the temperature in the reforming section.
[0015]
The fuel cell system of the present invention further includes an oxygen utilization rate adjusting means for adjusting an oxygen utilization rate in the fuel cell, and the oxygen utilization rate in the fuel cell is adjusted by the oxygen utilization rate adjusting means, whereby the connection portion from the fuel cell is connected. The amount of oxygen fed into the reforming section via the gas is adjusted.
[0016]
According to the above invention, the oxygen utilization rate in the fuel cell is adjusted, and the amount of oxygen used in the fuel cell and the amount of oxygen supplied to the reforming unit are appropriately distributed. Therefore, not only the operation of the fuel cell but also the partial oxygen reforming reaction in the reforming unit can be appropriately performed by supplying oxygen to the reforming unit via the fuel cell.
[0017]
The fuel cell system of the present invention includes a water vapor generation unit that generates water vapor using water discharged from the fuel cell in the connection unit, and in the water vapor generation unit, unreacted hydrogen discharged from the fuel cell. The water is heated by burning the water.
[0018]
According to the above invention, when only the steam discharged from the fuel cell is not sufficient as the amount of steam for performing the steam reforming, the water discharged from the fuel cell is generated in the steam generating section to supplement the shortage. You can also. In this case, it is necessary to further include a water vapor generator. However, since this water vapor generator generates a deficient amount of water vapor, a large apparatus such as a conventional evaporator is not required. In particular, in the present invention, as a fuel for generating insufficient steam, the steam generator can be reduced in size by burning unreacted hydrogen of the fuel cell with oxygen to generate steam generation energy. .
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an overall configuration of a fuel cell system 30 according to the present embodiment.
[0020]
The fuel cell system 30 includes a reforming unit 32 for reforming a hydrocarbon raw material to generate a fuel gas, and a shift unit 33 and a CO selection unit for removing CO from the fuel gas generated by the reforming unit 32. The fuel cell 34 is operated using the fuel gas (hydrogen) that has passed through these. In particular, in the fuel cell system 30, the fuel cell 34 is directly connected to the fuel cell 34 via the connection pipe 36, and the steam and unreacted oxygen by-produced on the cathode side of the fuel cell 34 are circulated to the reforming unit 32. Is adopted.
[0021]
That is, the fuel cell system 30 according to the present embodiment returns water as water vapor generated as a by-product in the fuel cell 34 to the reforming unit 32 to perform the steam reforming of the raw material. It is supposed that partial oxidation reforming is performed by sending oxygen. Hereinafter, each configuration will be described in detail.
[0022]
The fuel cell 34 that can be employed in the fuel cell system 30 may be of any type that produces water (water vapor) as a by-product during power generation. Specifically, a solid polymer fuel cell, Phosphoric acid fuel cell.
[0023]
For example, in a polymer electrolyte fuel cell and a phosphoric acid fuel cell, first, fuel gas from a reforming unit 32 or the like, which will be described later, is sent to the anode side as described in the above formulas (1) to (3). , Hydrogen ions are generated (H ₂ → 2H ⁺ + 2e ⁻ ). On the other hand, air is supplied to the cathode side, oxygen ions are generated from oxygen in the air, and electric power is generated in the fuel cell. At the same time, water is by-produced from the hydrogen ions and oxygen ions at the cathode ((1/2) O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O). Most of this water is generated as water vapor by absorbing heat generated in the fuel cell. One end of a connecting pipe 36 is connected to the cathode side of the fuel cell so that water vapor generated on the cathode side can be sent to the reforming unit 32. On the other hand, the anode side is provided with a purifying section 37 that purifies and discards unreacted hydrogen, nitrogen gas in the air, and the like.
[0024]
The reformer 32 is connected to the other end of the connecting pipe 36 connected to the cathode side of the fuel cell 34, and water vapor generated in the fuel cell 34 and remaining unreacted oxygen are supplied. The reforming section 32 is provided with a supply pipe 38 for supplying a hydrocarbon raw material such as methane, and a reforming catalyst necessary for reforming is provided therein. The reforming unit 32 reforms the supplied hydrocarbon raw material into a fuel gas by a steam reforming method and a partial oxidation reforming method using an internal catalyst. These reforming methods do not necessarily need to be used in combination, and the reforming can be performed by only one of them. However, since steam and oxygen are supplied from the fuel cell 34, it is preferable to use water and oxygen effectively by using steam reforming and partial oxidation reforming together.
[0025]
Further, when the steam reforming method and the partial oxidation reforming method are used in combination, it is preferable to appropriately adjust these ratios. For example, this adjustment can be performed on the basis of a temperature suitable for the reaction. That is, both steam reforming and partial oxidation reforming have an appropriate reforming temperature, but at the time of reforming, the temperature of the reforming section varies due to reaction heat or the like. In particular, the steam reforming is an endothermic reaction, and the partial oxidation reforming is an exothermic reaction. Therefore, by adjusting the balance between the endothermic reaction and the exothermic reaction, a certain suitable reaction temperature can be maintained.
[0026]
In order to adjust the balance of such reforming methods, the connecting pipe is provided with a control unit 40 for controlling the amount of water vapor or oxygen fed to the reforming unit 32, and the reforming reaction of the hydrocarbon raw material in the reforming unit is controlled. You can also The control unit 40 may be, for example, a unit that restricts the passage of gas by opening a valve or the like, or a discharge unit that discharges part of water vapor or oxygen from the connection pipe 36. Further, in the case of controlling only water vapor, the control unit 40 may regulate the temperature in the connection pipe 36 to limit the amount of water vapor that flows into the reforming unit 32 using water vapor as a partial water. .
[0027]
The control unit 40 mainly plays a role of controlling both the amount of oxygen passing through the connecting pipe and the amount of water vapor to control the supply amount to the reforming unit 32, but only the oxygen supply amount supplied to the reforming unit. When it is desired to adjust the oxygen, it is preferable to provide the fuel cell with an oxygen utilization rate adjusting unit 42. The oxygen utilization rate adjusting unit 42 adjusts the oxygen utilization rate in the fuel cell 34, in other words, the amount of unreacted oxygen, thereby adjusting the amount of oxygen fed into the reforming unit 32. By adjusting the oxygen supply amount to the reforming unit 32 in this way, the ratio of partial oxidation reforming in the reforming unit 32 is adjusted. For example, the temperature of the reforming unit can be reduced.
[0028]
As a method of adjusting the amount of oxygen in the adjusting unit 42, the following method and the like can be given.
[0029]
The first method is a method of changing the amount of air sent from the conduit 35a to the fuel cell cathode side. For example, in this case, the method is effective when the power generation amount in the fuel cell is constant, that is, when the oxygen consumption is constant. Since the amount of air changes even when the amount of oxygen consumed is constant, the oxygen utilization rate and the amount of unconsumed oxygen can be changed.
[0030]
The second method is a method of directly changing the power generation amount of the fuel cell 34. On the other hand, if the amount of power generation changes in a state where the air feed is constant, the amount of oxygen consumed in the fuel cell 34 can be changed. The amount of power generation can be controlled by a signal from an inverter or the like.
[0031]
On the other hand, the connection pipe 36 is provided with a steam generation unit 44 corresponding to the case where it is desired to adjust the steam reforming in the reforming unit 32. Most of the water produced at the cathode of the fuel cell 34 is water vapor, but some is water that has not been vaporized. The water produced here can be sent to a shift unit 33 to be described later and used for CO conversion reaction or used as cooling water for the CO selective oxidation unit 35. May be sent to the reforming section 32. As a case where it is necessary here, for example, at the time of starting the fuel cell 34 can be cited. That is, when the fuel cell 34 is started, since the required amount of water vapor cannot be obtained from the fuel cell 34, the water vapor generated in the water vapor generation unit 44 is temporarily heated and evaporated to modify the generated water vapor. The mass part 32 can be supplied.
[0032]
Further, unreacted hydrogen of the anode or the like can be used as the fuel for the water vapor generating unit 44. When hydrogen is used, hydrogen can be burned (oxidized), and the reaction heat at that time can be used as heating energy for steam generation.
[0033]
A shift unit 33 is connected to the reforming unit 32. The shift unit 33 reacts CO in the fuel gas generated in the reforming unit 32 with water to convert it into CO ₂ . Specifically, the shift reaction of the shift portion 33 is the reaction of the above-described formula (6), that is, CO + H ₂ O → H ₂ + CO ₂ . The water required for the reaction for the transformation in the shift portion is not shown, but water generated on the cathode side of the fuel cell 34 can be used as described above. Further, since water is also discharged from the anode side, this water may be collected and supplied to the shift unit 33. When the water discharged from the anode side or the cathode side is supplied to the shift unit, the supply amount may be controlled based on the temperature of the reformer 32, for example, the shift unit 33.
[0034]
A CO selective oxidation unit 35 is connected to the shift unit 33. The CO selective oxidation unit 35 oxidizes CO that has not been removed in the shift unit 33 and converts it into CO ₂ . Oxygen is necessary for the shift reaction (CO + 1 / 2O ₂ → CO ₂ ) of the CO selective oxidation unit 35. For this reason, the CO selective oxidation unit 35 is provided with a conduit 35a for sending air as an oxygen source. In addition, the modification reaction in the CO selective oxidation unit 35 is a reaction that burns CO, and is accompanied by heat generation. In order to remove this heat generation, the CO selective oxidation unit 35 is preferably provided with a cooling means. For example, as described above, water generated on the anode side and the cathode side of the fuel cell 34 is blown in as the cooling means. A configuration may be adopted. Here, the amount of water to be blown may be controlled based on the temperature of the reformer 32, for example, the CO selective oxidation unit 35.
[0035]
Next, the operation of the fuel cell system configured as described above will be described.
[0036]
During power generation by the fuel cell system 30, first, a hydrocarbon raw material is sent to the reforming unit 32 through the supply pipe 38. On the other hand, air is blown into the cathode side of the fuel cell, and oxygen necessary for partial oxidation reforming is sent into the reforming unit 32 through the connecting pipe 36. Further, if necessary, water vapor is generated in the water vapor generation unit 44, and this water vapor is sent to the reforming unit 32 through the connection unit 36. When the hydrocarbon raw material and oxygen or steam are sent to the reforming section 32 in this way, the reforming section 32 reforms the hydrocarbon raw material by steam reforming and partial oxidation reforming to generate fuel gas. Is done. The fuel gas generated here then passes through the shift unit 33 and the CO selective oxidation unit 35. At this time, CO produced as a by-product during reforming is removed.
[0037]
The fuel gas from which CO has been removed is sent to the anode of the fuel cell 34. On the other hand, since air is supplied to the cathode, the operation of the fuel cell is started when the fuel gas is fed. When the operation of the fuel cell is started, electric power is generated in the fuel cell, and at the same time, water vapor is by-produced on the cathode side.
[0038]
The steam generated as a by-product here is sent to the reforming section 32 through the connecting pipe 36 and used for steam reforming of the hydrocarbon raw material. Further, oxygen that is not used in the fuel cell 34 is also sent to the reforming section 32 through the connection pipe 36 and used for partial oxidation reforming of the raw material. Thus, as long as the operation of the fuel cell is started and continued, water vapor is by-produced in the fuel cell. The by-product water vapor and unreacted oxygen are fed into the reforming section 32 through the connection pipe 36, and fuel gas continues to be generated. When it is necessary to adjust the balance of the amounts of water vapor, oxygen, etc. supplied from the fuel cell, the reforming unit 32 is adjusted by the control unit 40, the water vapor generation unit 44, and the oxygen utilization rate adjustment unit 42. A suitable amount of oxygen and water vapor can be supplied.
[0039]
As described above, the reformer 32 generates fuel gas necessary for the operation of the fuel cell 34, while the fuel cell 34 generates water vapor necessary for the reformer 32, and unreacted oxygen remains. To do. Therefore, by providing a circulation path between the fuel cell 34 such as the connecting pipe 36 and the reforming unit 32, it is no longer necessary to provide the reforming unit 32 with an evaporator conventionally required for steam reforming. The system can be miniaturized.
[0040]
Further, water generated by the fuel cell 34, specifically, water other than water discharged from the anode side and water vapor discharged from the cathode side is also collected and supplied to the shift unit 33 and the CO selective oxidation unit 35. As a result, the by-product water can be effectively used.
[0041]
Below, the example at the time of producing | generating fuel gas using 1 mol of methane as a raw material and performing the operation of a fuel cell is demonstrated.
[0042]
When 1 mol of methane is supplied to the reforming unit 32 as a hydrocarbon raw material, approximately 2.3 mol of fuel gas (hydrogen) is generated through the reforming unit 32, the shift unit 33, and the CO selective oxidation unit 35. Gas is supplied to the anode of the fuel cell 34. On the other hand, air is sent to the cathode of the fuel cell 34 so that about 1.33 mol of oxygen can be supplied. In the fuel cell 34, power generation is performed using the fuel gas and air that are sent in. During this power generation, if the anode uses about 80% fuel gas, while the cathode uses about 68% oxygen, about 1.8 moles of water (mostly water vapor) is by-produced. . The by-produced steam and unreacted oxygen (0.425 mol) are sent to the reforming section 32 through the connecting pipe 36, and reforming of 1 mole of methane is performed by the steam and oxygen. Thus, by returning by-product steam and unreacted oxygen to the reforming section through the connection pipe, water, heat energy and unreacted oxygen generated in the fuel cell can be effectively used.
[0043]
【The invention's effect】
As described above, according to the present invention, since the steam generated in the fuel cell and the remaining unreacted oxygen are sent to the reforming unit and used for reforming the raw material, It is not necessary to install an evaporator or the like that has been used, and the system can be miniaturized. In addition, by returning the water vapor and oxygen of the fuel cell to the reforming unit, it is possible to effectively use water and thermal energy generated by the fuel cell.
[Brief description of the drawings]
FIG. 1 is a diagram showing an overall configuration of a fuel cell system according to an embodiment.
FIG. 2 is a diagram showing an overall configuration of a conventional fuel cell system.
[Explanation of symbols]
30 fuel cell system, 32 reforming unit, 34 fuel cell, 36 connecting pipe, 40 control unit, 42 oxygen utilization rate adjusting unit, 44 water vapor generation unit.

Claims (4)

In a fuel cell system comprising: a reforming unit that reforms a hydrocarbon raw material to generate hydrogen; and a fuel cell that supplies the hydrogen and oxygen as fuel to generate power,
Providing a connecting portion for connecting the fuel cell and the reforming unit so that water vapor generated at the cathode of the fuel cell and unreacted oxygen can be directly fed to the reforming unit by the operation of the fuel cell ;
A fuel cell system characterized in that reforming of a hydrocarbon raw material is performed using water vapor and oxygen sent from a fuel cell to a reforming unit via a connection unit. The connection unit includes a control unit that controls the amount of water vapor and oxygen fed to the reforming unit,
The fuel cell system according to claim 1, wherein the reforming reaction of the hydrocarbon raw material in the reforming section is controlled. An oxygen utilization rate adjusting means for adjusting an oxygen utilization rate in the fuel cell;
The oxygen utilization rate in the fuel cell is adjusted by the oxygen utilization rate adjusting means, and thereby the amount of oxygen fed from the fuel cell to the reforming unit via the connection unit is adjusted. Fuel cell system. The connection unit includes a water vapor generation unit that generates water vapor using water generated in the fuel cell,
The fuel cell system according to any one of claims 1 to 3, wherein the water vapor generation unit burns unreacted hydrogen discharged from the fuel cell to heat the water.

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