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 a fuel cell system.

Background Art

[0002] Solid oxide fuel cells (SOFCs) are known for small-scale power consumers such as convenience stores, apartment buildings, and buildings, and for on-site power generation and cogeneration in large facilities such as factories and data centers. SOFCs operate at a higher temperature than other types of fuel cells and have features such as high power generation efficiency and compatibility with various fuels. Power generation in an SOFC is performed by a stack in which an electrolyte is disposed between a fuel electrode and an air electrode. The fuel electrode exhaust gas discharged from the fuel electrode contains carbon dioxide. A fuel cell system has been disclosed in which carbon dioxide contained in the fuel electrode exhaust gas is separated and recovered by a carbon dioxide separation device (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in conventional fuel cell systems, since the fuel electrode exhaust gas is directly supplied to the carbon dioxide separation device, there has been a problem that the concentration of the recovered carbon dioxide is low.

[0005] The present disclosure has been made to solve the above problems, and an object thereof is to provide a fuel cell system in which the concentration of the recovered carbon dioxide is high.

Means for Solving the Problems

[0006] The fuel cell system of the present disclosure is a fuel cell system having a fuel cell and a carbon dioxide recovery unit that recovers carbon dioxide from the fuel electrode exhaust gas discharged from the fuel cell. The fuel cell includes a stack having an air electrode and a fuel electrode disposed opposite each other with an electrolyte interposed therebetween, a mixer that mixes water vapor and the fuel electrode exhaust gas discharged from the fuel electrode with the raw fuel, a reformer that reforms the raw fuel in which the water vapor and the fuel electrode exhaust gas are mixed by the mixer to generate a reformed gas, a combustor that holds a reforming catalyst provided inside the reformer at a high temperature, a reformed gas supply path that sends the reformed gas to the fuel electrode, an air electrode 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 fuel electrode exhaust gas flows, and a hydrogen recovery path that sends the hydrogen-rich gas sent 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 that sends the fuel electrode exhaust gas to the mixer and a carbon dioxide recovery path that sends the fuel electrode exhaust gas to the carbon dioxide recovery unit. The carbon dioxide recovery unit includes a compressor that pressurizes the fuel electrode exhaust gas flowing through the carbon dioxide recovery path, a carbon dioxide separation unit that separates the fuel electrode exhaust gas pressurized by the compressor into carbon dioxide and hydrogen-rich gas, and a recycle path that returns a part of the carbon dioxide separated by the carbon dioxide separation unit from downstream of the carbon dioxide separation unit to upstream of the compressor.

Advantages of the Invention

[0007] Since the fuel cell system of the present disclosure includes a recycle path that returns a part of the carbon dioxide separated by the carbon dioxide separation unit from downstream of the carbon dioxide separation unit to upstream of the compressor, the concentration of the recovered carbon dioxide can be improved.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0009] Hereinafter, a fuel cell system according to an embodiment for implementing the present disclosure will be described in detail with reference to the drawings. In each figure, the same reference numerals denote 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, the fuel cell system 100 according to the present embodiment is composed of a fuel cell 1 and a carbon dioxide recovery unit 2. In the present embodiment, the fuel cell 1 will be described as a 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 and a fuel electrode 14, and an electrolyte 15 installed between the air electrode 13 and the fuel electrode 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. The reformed gas is supplied to the fuel electrode 14 from a raw fuel supply source 4 via a mixer 17, a reformer 12, and a reformed gas supply path L2. In the fuel cell system 100 of the present embodiment, city gas mainly composed of methane is used as the raw fuel. In addition to city gas, liquefied petroleum gas (LP gas) such as propane and butane, and biogas can be used as the raw fuel.

[0012] The temperature of the stack 11 is maintained at 600 to 1000°C by a heat insulation box (not shown). In the air electrode 13 of the stack 11, the reduction reaction shown in the following reaction formula (1) proceeds. In the air electrode 13, oxygen in the air supplied from the air supply path L1 receives electrons from a load connected to the stack 11 and changes into oxygen ions. The air electrode exhaust gas after the reaction is discharged to an air electrode exhaust gas path L3. 1 / 2O 2 + 2e - → O 2- ······(1) The oxygen ions generated 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 with less oxygen compared to the air flowing through the air supply path L1.

[0013] The raw fuel supplied from the raw fuel supply source 4 is sent to the reformer 12 after the steam and the fuel electrode exhaust gas are mixed in the mixer 17. In order for the reforming reaction to occur with the reforming catalyst installed inside the reformer 12, it is necessary to raise the temperature of the reforming catalyst to 500 - 700°C. To raise the temperature of the reforming catalyst, a combustor 18 is installed in the reformer 12. The combustion heat of the combustor 18 raises the temperature of the reforming catalyst in the reformer 12 to 500 - 700°C. In the reformer 12, the reforming reactions of the following reaction formulas (2) and (3) proceed with the reforming catalyst, and the raw fuel is converted into a reformed gas containing a certain amount of hydrogen. CH 4 + H 2 O → CO + 3H 2 ···(2) CO + H 2 O → CO 2 + H 2 ····(3) The reformed gas contains hydrogen, steam, carbon monoxide, carbon dioxide, and unreacted methane. The reformed gas is supplied to the fuel electrode 14 via the reformed gas supply path L2.

[0014] In the fuel electrode 14 of the stack 11, the oxidation reaction shown in the following reaction formula (4) proceeds. That is, in the fuel electrode 14, the oxygen ions that have moved from the air electrode 13 via the electrolyte 15 react with the hydrogen supplied to the fuel electrode 14 to change into water and electrons. The fuel electrode exhaust gas is discharged from the fuel electrode 14 to the fuel electrode exhaust gas path L4. O 2- + H 2 → H 2 O + 2e - (4) Note that the methane and steam supplied to the fuel electrode 14 of the stack 11 are converted into hydrogen, carbon monoxide, and carbon dioxide inside the stack 11 by a chemical reaction called the internal reforming reaction. The internal reforming reaction is the same as the reforming reactions in the above reaction formulas (2) and (3).

[0015] The fuel electrode exhaust gas flowing through the fuel electrode exhaust gas path L4 contains carbon dioxide, hydrogen, steam, and carbon monoxide. The temperature of the fuel electrode exhaust gas varies depending on the type of the stack 11, operating conditions, etc., but is 500 to 700°C. By such a reaction, an electric current can be taken out to a load connected between the air electrode 13 and the fuel electrode 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 fuel electrode exhaust gas path L4 is provided with a steam generator 19 and a condenser 20. The steam generator 19 converts the water supplied from the water supply source 5 into steam using the heat of the fuel electrode exhaust gas. The steam converted by the steam generator 19 is sent to the mixer 17 via the steam supply path L5. The condenser 20 converts the steam contained in the fuel electrode exhaust gas into liquid water, and the water is sent to the water supply source 5.

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

[0018] The mixer 17 is supplied with city gas as the raw fuel from the raw fuel supply source 4, steam from the steam supply path L5, and fuel electrode exhaust gas from the fuel electrode exhaust gas recycle path L6. The mixer 17 mixes the city gas, steam, and fuel electrode exhaust gas.

[0019] To the combustor 18, air electrode exhaust gas is sent from the air electrode exhaust gas path L3, and hydrogen is sent via the hydrogen recovery path L8 from the carbon dioxide recovery section 2 described later. The combustor 18 burns oxygen contained in the air electrode exhaust gas and hydrogen supplied from the hydrogen recovery path L8 to maintain the temperature of the reforming catalyst installed inside the reformer 12 at 500 to 700 °C. The combustion exhaust gas burned 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 section 2 will be described. The carbon dioxide recovery section 2 has a compressor 22 and a carbon dioxide separation section 23. The carbon dioxide recovery path L7 is connected to the carbon dioxide separation section 23 via the compressor 22. The carbon dioxide separation section 23 separates the fuel electrode exhaust gas into carbon dioxide and other gases. The carbon dioxide separated in the carbon dioxide separation section 23 permeates through the carbon dioxide separation section 23 and is sent to the carbon dioxide recovery path L7. A recycle path L9 is provided in the carbon dioxide recovery path L7 downstream of the carbon dioxide separation section 23 to return a part of the carbon dioxide that has permeated through the carbon dioxide separation section 23 upstream of the compressor 22. The carbon dioxide recovery path L7 connected to the outside of the carbon dioxide recovery section 2 is connected to, for example, a carbon dioxide storage tank (not shown). The carbon dioxide recovered in the carbon dioxide recovery section 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 permeate while making it difficult for hydrogen and the like other than carbon dioxide to permeate. As the carbon dioxide separation membrane, for example, polymer membranes such as polyimide and polycarbonate, facilitated transport membranes such as polyamidoamine dendrimer, and inorganic membranes such as zeolite and silica amorphous can be used. The fuel electrode exhaust gas flowing through the carbon dioxide recovery path L7 contains carbon dioxide, hydrogen, and carbon monoxide. The separation characteristics of carbon dioxide in the carbon dioxide separation membrane become more efficient as the partial pressure difference between the upstream side and the downstream side of the carbon dioxide separation membrane increases. Therefore, a compressor 22 is provided upstream of the carbon dioxide separation unit 23 to pressurize the fuel electrode exhaust gas flowing through the carbon dioxide recovery path L7.

[0022] The gas separated from carbon dioxide in the carbon dioxide separation unit 23 is a gas rich in hydrogen. Hereinafter, this gas is 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] Flow rate adjustment valves V1 and V2 are respectively provided in the recycle path L9 and the carbon dioxide recovery path L7. Further, on the outlet side of the carbon dioxide recovery path L7, a concentration sensor 31 for measuring the concentration of carbon dioxide flowing through the carbon dioxide recovery path L7 is provided. The flow rate adjustment valve V1 adjusts the flow rate of the gas flowing through the recycle path L9. The flow rate adjustment valve V2 adjusts the flow rate of the gas flowing from the carbon dioxide recovery path L7 to the outside of the carbon dioxide recovery unit 2.

[0024] In a carbon dioxide separation membrane composed of a porous zeolite or the like, carbon dioxide is once adsorbed into the pores on the inlet side. The carbon dioxide adsorbed in the pores blocks the molecules of other gases. The carbon dioxide adsorbed in the pores on the inlet side desorbs to the outlet side due to the partial pressure difference between the inlet side and the outlet side. With such a mechanism, the carbon dioxide separation membrane allows carbon dioxide to permeate and blocks gases other than carbon dioxide. Note that in a carbon dioxide separation membrane other than zeolite, carbon dioxide is selectively permeated by a similar mechanism.

[0025] FIG. 2 is a characteristic diagram of the carbon dioxide separation membrane in the fuel cell system according to the present 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 the region where the carbon dioxide concentration on the inlet side is low, since the amount of carbon dioxide adsorbed in the pores is small, the effect of blocking the molecules of other gases is small, and gases other than carbon dioxide easily permeate the carbon dioxide separation membrane. Therefore, in the region 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 is 40 mol% or more, the carbon dioxide concentration on the outlet side gradually saturates. Note that the characteristics shown in FIG. 2 are an example, and the characteristic curves differ depending on the material of the carbon dioxide separation membrane, but the point that the carbon dioxide concentration on the outlet side increases as the carbon dioxide concentration on the inlet side increases is the same.

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

[0027] As described above, in the fuel cell system of this embodiment, since a recycle path for returning a part of the carbon dioxide separated by the carbon dioxide separation unit from downstream of the carbon dioxide separation unit to upstream of the compressor is provided, the concentration of the recovered carbon dioxide can be improved.

[0028] Embodiment 2. FIG. 3 is a configuration diagram of a fuel cell system according to Embodiment 2. The fuel cell system 100 according to this embodiment is obtained by adding a carbon dioxide reformer to the carbon dioxide recovery unit in the fuel cell system described in Embodiment 1.

[0029] As shown in FIG. 3, in the fuel cell system 100 according to this embodiment, a carbon dioxide reformer 24 is provided in the carbon dioxide recovery path L7 on the upstream side of the compressor 22. Further, the condenser provided in the fuel electrode exhaust gas path of the fuel cell of Embodiment 1 has been removed in the fuel cell of this embodiment.

[0030] The fuel electrode exhaust gas discharged from the fuel electrode 14 contains carbon dioxide, hydrogen, water vapor, and carbon monoxide. The carbon dioxide reformer 24 in the carbon dioxide recovery unit 2 reacts carbon monoxide and water vapor contained in the fuel electrode exhaust gas to generate carbon dioxide and hydrogen. The reforming reaction performed by the carbon dioxide reformer 24 is the same as the reaction formula (3) described in Embodiment 1.

[0031] In the fuel cell system 100 configured as described above, in the carbon dioxide reformer 24, carbon monoxide and water vapor contained in the fuel electrode exhaust gas are reacted to produce carbon dioxide and hydrogen. Therefore, the carbon dioxide concentration on the inlet side of the carbon dioxide separation unit 23 can be increased. As a result, the concentration of the carbon dioxide recovered by the carbon dioxide recovery unit can be further improved.

[0032] Further, in the fuel cell system of the present embodiment, since the amount of hydrogen contained in the fuel electrode exhaust gas sent to the carbon dioxide separation unit 23 increases, the hydrogen concentration of the hydrogen-rich gas flowing through the hydrogen recovery path L8 also increases. Therefore, the combustion efficiency in the combustor 18 is also improved.

[0033] In the fuel cell system of the present embodiment, the fuel electrode exhaust gas flowing through the fuel electrode exhaust gas recycle path L6 also contains water vapor. This water vapor is unnecessary for combustion in the combustor 18. Therefore, a condenser may be provided in the fuel electrode exhaust gas recycle path L6 to remove water vapor from the fuel electrode exhaust gas flowing through the fuel electrode exhaust gas recycle path L6.

[0034] Although various exemplary embodiments and examples are described in the present disclosure, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a specific embodiment, but are applicable to the embodiments alone or in various combinations. Therefore, countless variations not illustrated are envisioned within the scope of the technology disclosed in this specification. For example, it includes the case of modifying, adding, or omitting at least one component, and further, the case of extracting at least one component and combining it with the components of other embodiments.

Description of Reference Numerals

[0035] 1 Fuel cell, 2 Carbon dioxide recovery section, 3 Air supply source, 4 Primary 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 Recycling blower, 22 Compressor, 23 Carbon dioxide separation section, 24 Carbon dioxide converter, 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 comprising a fuel cell and a carbon dioxide recovery unit that recovers carbon dioxide from the fuel electrode exhaust gas discharged from the fuel cell, wherein the fuel cell includes a stack having an air electrode and a fuel electrode disposed opposite each other with an electrolyte therebetween, a mixer that mixes steam with the fuel electrode exhaust gas discharged from the fuel electrode in the raw fuel, a reformer that reforms the raw fuel in which the steam and the fuel electrode exhaust gas are mixed by the mixer to generate a reformed gas, a combustor that holds a reforming catalyst provided inside the reformer at a high temperature, a reformed gas supply path that sends the reformed gas to the fuel electrode, an air electrode 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 fuel electrode exhaust gas flows, and a hydrogen recovery path that sends the hydrogen-rich gas sent 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 that sends the fuel electrode exhaust gas to the mixer and a carbon dioxide recovery path that sends the fuel electrode exhaust gas to the carbon dioxide recovery unit, the carbon dioxide recovery unit includes a compressor that pressurizes the fuel electrode exhaust gas flowing through the carbon dioxide recovery path, a carbon dioxide separation unit that separates carbon dioxide and the hydrogen-rich gas from the fuel electrode exhaust gas pressurized by the compressor, and a recycle path that returns a part of the carbon dioxide separated by the carbon dioxide separation unit from the downstream of the carbon dioxide separation unit to the upstream of the compressor. A fuel cell system characterized by that.

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

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