fuel cell system

Abstract

To provide a fuel cell system that can increase the concentration of recovered carbon dioxide.SOLUTION: A fuel cell system 100 has a fuel cell 1 and a carbon dioxide recovery unit 2. The fuel cell comprises a stack 11 having an air electrode 13 and a fuel electrode 14, a mixer 17, a reformer 12, a combustor 18, a reformed gas supply path L2, an air electrode exhaust gas path L3, a fuel electrode exhaust gas path L4, and a hydrogen recovery path L8 feeding hydrogen-rich gas fed from the carbon dioxide recovery unit to the combustor. The fuel electrode exhaust gas path is branched into a fuel electrode exhaust gas recycle path L6 feeding the fuel electrode exhaust gas to the mixer, and a carbon dioxide recovery path L7 feeding the fuel electrode exhaust gas to the carbon dioxide recovery unit. The carbon dioxide recovery unit includes a compressor 23, a carbon dioxide separation unit 24 separating carbon dioxide and hydrogen-rich gas from the fuel electrode exhaust gas, and a recycle path L9 returning part of the carbon dioxide separated by the carbon dioxide separation unit to the upstream of the compressor.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to fuel cell systems. [Background technology]

[0002] Solid oxide fuel cells (SOFCs) are known for use by small- to medium-sized power consumers such as convenience stores, apartment complexes, and buildings, as well as for private power generation and cogeneration in large facilities such as factories and data centers. Compared to other types of fuel cells, SOFCs have features such as a higher operating temperature, high power generation efficiency, and compatibility with a variety of fuels. SOFCs generate power in a stack in which an electrolyte is placed between an anode and an cathode. Carbon dioxide is contained in the anode exhaust gas discharged from the anode. A fuel cell system has been disclosed that separates and recovers the carbon dioxide contained in this anode exhaust gas using a carbon dioxide separation device (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3000118 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in conventional fuel cell systems, the concentration of recovered carbon dioxide is low because the anode exhaust gas is directly supplied to the carbon dioxide separation device.

[0005] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a fuel cell system that recovers carbon dioxide at a high concentration. [Means for solving the problem]

[0006] The fuel cell system disclosed herein is a fuel cell system having a fuel cell and a carbon dioxide recovery unit that recovers carbon dioxide from anode exhaust gas discharged from the fuel cell, wherein the fuel cell comprises a stack having an air electrode and an anode arranged opposite each other with an electrolyte between them, a mixer that mixes raw fuel with steam and anode exhaust gas discharged from the anode, a reformer that reforms the raw fuel mixed with steam and anode exhaust gas in the mixer to produce reformed gas, a combustor that maintains a reforming catalyst provided inside the reformer at a high temperature, a reformed gas supply path that sends the reformed gas to the anode, an air electrode exhaust gas path that sends air electrode exhaust gas discharged from the air electrode to the combustor, an anode exhaust gas path through which the anode exhaust gas flows, and a hydrogen recovery path that sends hydrogen-rich gas sent from the carbon dioxide recovery unit to the combustor, and the anode exhaust gas path is branched into an anode exhaust gas recycling path that sends the anode exhaust gas to the mixer and a carbon dioxide recovery path that sends the anode exhaust gas to the carbon dioxide recovery unit. The carbon dioxide capture unit includes a compressor that pressurizes the anode exhaust gas flowing through the carbon dioxide capture path, a carbon dioxide separation unit that separates the anode exhaust gas pressurized by the compressor into carbon dioxide and hydrogen-rich gas, and a recycle path that returns a portion of the carbon dioxide separated in the carbon dioxide separation unit from downstream of the carbon dioxide separation unit to upstream of the compressor. [Effects of the Invention]

[0007] The fuel cell system of the present disclosure is equipped with a recycle path that returns a portion of the carbon dioxide separated in the carbon dioxide separation section from downstream of the carbon dioxide separation section to upstream of the compressor, thereby improving the concentration of the recovered carbon dioxide. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a configuration diagram of a fuel cell system according to a first embodiment. [Figure 2] FIG. 3 is a characteristic diagram of a carbon dioxide separation membrane in the fuel cell system according to the first embodiment. [Figure 3] FIG. 10 is a configuration diagram of a fuel cell system according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, a fuel cell system according to an embodiment of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same reference numerals indicate the same or corresponding parts.

[0010] Embodiment 1 Fig. 1 is a configuration diagram of a fuel cell system according to embodiment 1. As shown in Fig. 1, a fuel cell system 100 according to this embodiment is made up of a fuel cell 1 and a carbon dioxide capture unit 2. In this embodiment, the fuel cell 1 will be described as an SOFC.

[0011] First, the configuration of the fuel cell 1 will be described. The fuel cell 1 has a stack 11 and a reformer 12. The stack 11 is composed of an air electrode 13, an anode 14, and an electrolyte 15 disposed between the air electrode 13 and the anode 14. Air is supplied to the air electrode 13 from an air supply source 3 via an air heat exchanger 16 and an air supply path L1. Reformed gas is supplied to the anode 14 from a raw fuel supply source 4 via a mixer 17, the reformer 12, and a reformed gas supply path L2. In the fuel cell system 100 of this embodiment, city gas containing methane as a main component is used as the raw fuel. In addition to city gas, liquefied petroleum gas (LP gas) such as propane or butane, biogas, etc. can also be used as the raw fuel.

[0012] The temperature of the stack 11 is maintained at 600 to 1000°C by an insulating box (not shown). At the air electrode 13 of the stack 11, a reduction reaction shown in the following reaction formula (1) takes place. At the air electrode 13, oxygen in the air supplied through the air supply path L1 receives electrons from the load connected to the stack 11 and is converted into oxygen ions. The air electrode exhaust gas after the reaction is discharged to the air electrode exhaust gas path L3. 1 / 2O2+ 2e - → O 2- ······(1) Oxygen ions produced by the reduction reaction move to the fuel electrode 14 via the electrolyte 15. The air electrode exhaust gas flowing through the air electrode exhaust gas path L3 has a composition in which oxygen is reduced compared to the air flowing through the air supply path L1.

[0013] The raw fuel supplied from the raw fuel supply source 4 is mixed with steam and anode exhaust gas in a mixer 17 and sent to the reformer 12. In order for a reforming reaction to occur in the reforming catalyst installed inside the reformer 12, the reforming catalyst needs to be heated to 500 to 700°C. To heat the reforming catalyst, a combustor 18 is provided adjacent to the reformer 12. The reforming catalyst in the reformer 12 is heated to 500 to 700°C using the combustion heat from the combustor 18. In the reformer 12, the reforming reactions of the following reaction formulas (2) and (3) proceed due to the reforming catalyst, and the raw fuel is converted into a reformed gas containing a certain amount of hydrogen. CH4+ H2O → CO + 3H2···(2) CO + H2O → CO2+ H2···(3) The reformed gas contains hydrogen, water vapor, carbon monoxide, carbon dioxide, and unreacted methane, and is supplied to the fuel electrode 14 via a reformed gas supply path L2.

[0014] At the anode 14 of the stack 11, an oxidation reaction shown in the following reaction formula (4) proceeds. That is, at the anode 14, oxygen ions that have migrated from the cathode 13 via the electrolyte 15 react with hydrogen supplied to the anode 14 to convert into water and electrons. Anode exhaust gas is discharged from the anode 14 to anode exhaust gas path L4. O 2- + H2 → H2O + 2e - (4) The methane and water vapor supplied to the anode 14 of the stack 11 are converted into hydrogen, carbon monoxide, and carbon dioxide inside the stack 11 through a chemical reaction known as the internal reforming reaction. The internal reforming reaction is the same as the reforming reactions of the above reaction formulas (2) and (3).

[0015] The anode exhaust gas flowing through the anode exhaust gas path L4 contains carbon dioxide, hydrogen, water vapor, and carbon monoxide. The temperature of the anode exhaust gas varies depending on the type of stack 11, the operating conditions, etc., but is generally between 500 and 700°C. This reaction generates a current to a load connected between the air electrode 13 and the anode 14 of the stack 11.

[0016] The air electrode exhaust gas flowing through the air electrode exhaust gas path L3 is supplied to the combustor 18. The anode exhaust gas path L4 is provided with a water vapor generator 19 and a condenser 20. The water vapor generator 19 converts water supplied from the water supply source 5 into water vapor by utilizing the heat of the anode exhaust gas. The water vapor converted by the water vapor generator 19 is sent to the mixer 17 via the water vapor supply path L5. The condenser 20 converts the water vapor contained in the anode exhaust gas into liquid water, and the water is sent to the water supply source 5.

[0017] Anode exhaust gas path L4 branches into two paths downstream of condenser 20. One path is anode exhaust gas recycle path L6, which is connected to mixer 17 via recycle blower 21. The other path is carbon dioxide recovery path L7, which is connected to carbon dioxide recovery unit 2.

[0018] City gas as raw fuel is supplied from raw fuel supply source 4, steam is supplied from steam supply path L5, and anode exhaust gas is supplied from anode exhaust gas recycling path L6 to mixer 17. Mixer 17 mixes the city gas, steam, and anode exhaust gas.

[0019] The combustor 18 receives the air electrode exhaust gas from the air electrode exhaust gas passage L3, and also receives hydrogen from the carbon dioxide recovery unit 2 (described later) via the hydrogen recovery passage L8. The combustor 18 combusts the oxygen contained in the air electrode exhaust gas with the hydrogen supplied from the hydrogen recovery passage L8, and maintains the temperature of the reforming catalyst installed inside the reformer 12 at 500 to 700°C. The combustion exhaust gas combusted in the combustor 18 is sent to the air heat exchanger 16. The temperature of the combustion exhaust gas discharged from the combustor 18 is several hundred degrees. The air heat exchanger 16 uses the heat of the combustion exhaust gas to raise the temperature of the air supplied from the air supply source 3 to 400 to 600°C.

[0020] Next, the configuration of the carbon dioxide recovery unit 2 will be described. The carbon dioxide capture unit 2 has a compressor 22 and a carbon dioxide separation unit 23. The carbon dioxide capture path L7 is connected to the carbon dioxide separation unit 23 via the compressor 22. The carbon dioxide separation unit 23 separates the anode exhaust gas into carbon dioxide and other gases. The carbon dioxide separated in the carbon dioxide separation unit 23 permeates the carbon dioxide separation unit 23 and is sent to the carbon dioxide capture path L7. The carbon dioxide capture path L7 downstream of the carbon dioxide separation unit 23 is provided with a recycle path L9 that returns a portion of the carbon dioxide that permeates the carbon dioxide separation unit 23 to the upstream of the compressor 22. The carbon dioxide capture path L7, which leads outside the carbon dioxide capture unit 2, is connected to, for example, a carbon dioxide storage tank (not shown). The carbon dioxide captured in the carbon dioxide capture unit 2 is stored in the carbon dioxide storage tank.

[0021] The carbon dioxide separation unit 23 is equipped with a carbon dioxide separation membrane. This carbon dioxide separation membrane has the property of allowing carbon dioxide to pass through while being difficult to pass other substances such as hydrogen. Examples of carbon dioxide separation membranes that can be used include polymer membranes such as polyimide and polycarbonate, facilitated transport membranes such as polyamidoamine dendrimers, and inorganic membranes such as zeolite and amorphous silica. The anode exhaust gas flowing through the carbon dioxide capture path L7 contains carbon dioxide, hydrogen, and carbon monoxide. The carbon dioxide separation efficiency of the carbon dioxide separation membrane increases with the partial pressure difference between the upstream and downstream sides of the carbon dioxide separation membrane. Therefore, a compressor 22 is provided upstream of the carbon dioxide separation unit 23 to pressurize the anode exhaust gas flowing through the carbon dioxide capture path L7.

[0022] The gas separated from carbon dioxide in the carbon dioxide separation unit 23 is a gas that is rich in hydrogen. Hereinafter, this gas will be referred to as hydrogen-rich gas. The hydrogen-rich gas separated from carbon dioxide in the carbon dioxide separation unit 23 is sent to the combustor 18 provided in the fuel cell 1 via the hydrogen recovery path L8.

[0023] The recycle path L9 and the carbon dioxide capture path L7 are provided with flow control valves V1 and V2, respectively. In addition, a concentration sensor 31 that measures the concentration of carbon dioxide flowing through the carbon dioxide capture path L7 is provided on the outlet side of the carbon dioxide capture path L7. The flow control valve V1 adjusts the flow rate of gas flowing through the recycle path L9. The flow control valve V2 adjusts the flow rate of gas flowing from the carbon dioxide capture path L7 to outside the carbon dioxide capture unit 2.

[0024] In carbon dioxide separation membranes made of porous zeolite or similar materials, carbon dioxide is first adsorbed into the pores on the inlet side. The carbon dioxide adsorbed into the pores blocks other gas molecules. The carbon dioxide adsorbed into the pores on the inlet side is desorbed to the outlet side due to the partial pressure difference between the inlet and outlet sides. In this way, the carbon dioxide separation membrane allows carbon dioxide to pass through and blocks gases other than carbon dioxide. Carbon dioxide separation membranes other than zeolite also selectively allow carbon dioxide to pass through using a similar mechanism.

[0025] FIG. 2 is a characteristic diagram of the carbon dioxide separation membrane in the fuel cell system according to this embodiment. In FIG. 2, the horizontal axis represents the carbon dioxide concentration on the inlet side, and the vertical axis represents the carbon dioxide concentration on the outlet side. As shown in FIG. 2, in regions where the carbon dioxide concentration on the inlet side is low, the amount of carbon dioxide adsorbed in the pores is small, so the effect of blocking other gas molecules is small, and gases other than carbon dioxide easily permeate the carbon dioxide separation membrane. Therefore, in regions where the carbon dioxide concentration on the inlet side is low, the carbon dioxide concentration on the outlet side is low. As the carbon dioxide concentration on the inlet side increases, the carbon dioxide concentration on the outlet side also increases, and when the carbon dioxide concentration on the inlet side reaches 40 mol % or more, the carbon dioxide concentration on the outlet side gradually saturates. Note that the characteristics shown in FIG. 2 are one example, and the characteristic curve will differ depending on the material of the carbon dioxide separation membrane, but the same principle applies: as the carbon dioxide concentration on the inlet side increases, the carbon dioxide concentration on the outlet side increases.

[0026] In the fuel cell system of this embodiment, the concentration of carbon dioxide flowing through carbon dioxide capture path L7 is measured by concentration sensor 31, and if the measured carbon dioxide concentration is low, flow rate adjustment valve V2 is throttled and flow rate adjustment valve V1 is opened to recirculate the gas flowing through carbon dioxide capture path L7 upstream of compressor 22 via recycle path L9. For example, if the carbon dioxide concentration measured by concentration sensor 31 is 40 mol % or less, the amount recirculated via recycle path L9 is increased, and if the carbon dioxide concentration rises to 80 mol % or more, the amount recirculated is decreased. By controlling in this manner, the carbon dioxide concentration on the inlet side of carbon dioxide separation unit 23 can be increased, and the carbon dioxide concentration on the outlet side can be increased.

[0027] As described above, the fuel cell system of this embodiment is provided with a recycle path that returns a portion of the carbon dioxide separated in the carbon dioxide separation section from downstream of the carbon dioxide separation section to upstream of the compressor, thereby improving the concentration of the recovered carbon dioxide.

[0028] Embodiment 2 3 is a configuration diagram of a fuel cell system according to embodiment 2. A fuel cell system 100 according to this embodiment is the fuel cell system described in embodiment 1, with a carbon dioxide transformer added to the carbon dioxide capture section.

[0029] 3, in the fuel cell system 100 according to this embodiment, a carbon dioxide shift converter 24 is provided in the carbon dioxide recovery path L7 upstream of the compressor 22. In addition, the condenser provided in the anode exhaust gas path of the fuel cell according to Embodiment 1 is removed in the fuel cell according to this embodiment.

[0030] The anode exhaust gas discharged from the anode 14 contains carbon dioxide, hydrogen, water vapor, and carbon monoxide. The carbon dioxide converter 24 of the carbon dioxide recovery unit 2 reacts the carbon monoxide and water vapor contained in the anode exhaust gas to produce carbon dioxide and hydrogen. The reforming reaction that takes place in the carbon dioxide converter 24 is the same as reaction formula (3) described in the first embodiment.

[0031] In the fuel cell system 100 configured in this manner, the carbon dioxide shift converter 24 reacts the carbon monoxide contained in the anode exhaust gas with water vapor to generate carbon dioxide and hydrogen, thereby increasing the carbon dioxide concentration on the inlet side of the carbon dioxide separation unit 23. As a result, the concentration of carbon dioxide recovered in the carbon dioxide recovery unit can be further improved.

[0032] Furthermore, in the fuel cell system of this embodiment, the amount of hydrogen contained in the anode exhaust gas sent to the carbon dioxide separation unit 23 increases, and the hydrogen concentration of the hydrogen-rich gas flowing through the hydrogen recovery path L8 also increases, thereby improving the combustion efficiency in the combustor 18.

[0033] In the fuel cell system of this embodiment, the anode exhaust gas flowing through the anode exhaust gas recycling path L6 also contains water vapor. This water vapor is not required for combustion in the combustor 18. Therefore, a condenser may be provided in the anode exhaust gas recycling path L6 to remove water vapor from the anode exhaust gas flowing through the anode exhaust gas recycling path L6.

[0034] While the present disclosure describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless variations not exemplified are conceivable within the scope of the technology disclosed in this specification, including, for example, cases where at least one component is modified, added, or omitted, and cases where at least one component is extracted and combined with components of another embodiment. [Explanation of symbols]

[0035] 1 fuel cell, 2 carbon dioxide recovery section, 3 air supply source, 4 raw fuel supply source, 5 water supply source, 11 stack, 12 reformer, 13 air electrode, 14 fuel electrode, 15 electrolyte, 16 air heat exchanger, 17 mixer, 18 combustor, 19 steam generator, 20 condenser, 21 recycle blower, 22 compressor, 23 carbon dioxide separation section, 24 carbon dioxide transformer, 31 concentration sensor, 100 fuel cell system, L1 air supply path, L2 reformed gas supply path, L3 air electrode exhaust gas path, L4 fuel electrode exhaust gas path, L5 steam supply path, L6 fuel electrode exhaust gas recycling path, L7 carbon dioxide recovery path, L8 hydrogen recovery path, L9 recycling path, V1, V2 flow rate adjustment valves.

Claims

1. A fuel cell system having a fuel cell and a carbon dioxide recovery unit that recovers carbon dioxide from a fuel electrode exhaust gas discharged from the fuel cell, the fuel cell comprises a stack having an air electrode and an anode arranged opposite to each other with an electrolyte therebetween, a mixer that mixes steam and the anode exhaust gas discharged from the anode with a raw fuel, a reformer that generates a reformed gas by reforming the raw fuel mixed with the steam and the anode exhaust gas in the mixer, a combustor that maintains a reforming catalyst provided inside the reformer at a high temperature, a reformed gas supply path that sends the reformed gas to the anode, a cathode exhaust gas path that sends the air electrode exhaust gas discharged from the air electrode to the combustor, a fuel electrode exhaust gas path through which the anode exhaust gas flows, and a hydrogen recovery path that sends hydrogen-rich gas sent from the carbon dioxide recovery unit to the combustor, the anode exhaust gas path is branched into an anode exhaust gas recycling path that sends the anode exhaust gas to the mixer and a carbon dioxide recovery path that sends the anode exhaust gas to the carbon dioxide recovery unit, a carbon dioxide recovery unit that separates the anode exhaust gas pressurized by the compressor into carbon dioxide and the hydrogen-rich gas; and a recycle path that returns a portion of the carbon dioxide separated by the carbon dioxide separation unit from downstream of the carbon dioxide separation unit to upstream of the compressor.

2. 2. The fuel cell system according to claim 1, wherein the carbon dioxide recovery unit further comprises a carbon dioxide transformer in the carbon dioxide recovery path upstream of the compressor.

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