fuel cell system

Abstract

To provide a system that can efficiently separate and recover carbon dioxide.SOLUTION: A fuel cell system includes an anode exhaust circuit 24 and a carbon dioxide separation and recovery device 60. The anode exhaust circuit 24 has a branch point 42 that branches off halfway into a recycle gas circuit 26 that accepts anode exhaust Ff3 exhausted from an anode 1a of a fuel cell stack 1 and leads it as recycle gas Ff31 to be mixed with raw fuel Ff0 and circulated and a combustion gas circuit 27 that leads it as combustion gas Ff32 used for combustion in a combustor 3. The carbon dioxide separation and recovery device 60 is provided in the combustion gas circuit 27 to separate and recover carbon dioxide contained in the combustion gas Ff32.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present application relates to a fuel cell system. [Background technology]

[0002] Fuel cells are used in a configuration called a stack, in which multiple layers of single cells are stacked, each consisting of a fuel electrode (hereinafter referred to as the anode) and an air electrode (hereinafter referred to as the cathode) with an electrolyte sandwiched between them. Fuel cell stacks generate electricity by causing an electrochemical reaction between a reformed gas containing hydrogen, which is produced by reforming hydrocarbon-based raw fuel, and an oxidant, separated by an electrolyte. As a result, fuel cells have higher energy conversion efficiency and a lower environmental impact than internal combustion engines, and are therefore being actively developed as on-site power generation devices for consumer and industrial use.

[0003] However, the remaining reformed gas (anode exhaust) discharged from the anode contains, in addition to carbon dioxide and water vapor, hydrogen and carbon monoxide that are not consumed in the electrochemical reaction and can be reused as raw materials. Therefore, it is common to effectively utilize the energy contained in the anode exhaust by burning the anode exhaust in a combustor to use it as reaction heat in the reformer or by circulating part of the anode exhaust as raw fuel.

[0004] Although progress is being made in making effective use of anode exhaust, the carbon dioxide contained in the combustion exhaust gas from the anode exhaust is emitted directly into the atmosphere, placing a burden on the environment. This is inevitable as long as hydrocarbon-based feedstocks are used, and the same is true for solid oxide fuel cells (SOFCs), which have higher energy conversion efficiency due to their higher operating temperatures compared to other fuel cells.

[0005] Furthermore, since air is generally used as a combustion oxidizing gas, the combustion exhaust gas contains a large amount of nitrogen, which reduces the concentration of carbon dioxide in the exhaust gas, making it difficult to recover and utilize the carbon dioxide. Therefore, in a system equipped with a recycle gas circuit that circulates the anode exhaust as raw fuel, a method has been disclosed in which a carbon dioxide separator is introduced into the recycle gas circuit to separate and recover carbon dioxide in the anode exhaust (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2009-503791 (paragraphs 0050-0070, Figures 1 and 5) Summary of the Invention [Problem to be solved by the invention]

[0007] Anode exhaust has a higher carbon dioxide concentration than combustion exhaust gas, making it considered to have good separation efficiency. Furthermore, the gas with a high hydrogen concentration after carbon dioxide separation is recycled as recycled gas, which is expected to increase the output of the stack. However, because carbon dioxide is separated from the anode exhaust in the recycled gas circuit, the gas after carbon dioxide separation is recycled, meaning that recycling cannot be expected to increase the carbon dioxide concentration in the anode exhaust. Therefore, the carbon dioxide concentration in the anode exhaust increases to around 50% at most on a dry basis, and the large amount of power required to separate the carbon dioxide makes efficient separation difficult.

[0008] The present application discloses a technique for solving the above-mentioned problems, and aims to provide a system that can efficiently separate and capture carbon dioxide. [Means for solving the problem]

[0009] The fuel cell system disclosed in the present application includes a reformer that generates a reformed gas containing hydrogen by a reforming reaction between a raw fuel containing hydrocarbon and steam, a combustor that provides heat to the reformer, a fuel cell stack that generates electric energy by an electrochemical reaction between air and the reformed gas and outputs DC power, and an anode exhaust circuit that receives anode exhaust discharged from an anode of the fuel cell stack and branches into a recycle gas circuit that receives the anode exhaust and guides it as a recycle gas to be mixed with the raw fuel and circulated, and a combustion gas circuit that guides the anode exhaust as a combustion gas to be used for combustion in the combustor. 、 a carbon dioxide separation and capture device that is provided in the combustion gas circuit and separates and captures carbon dioxide contained in the combustion gas; The system includes a control device that controls devices in the system in a coordinated manner, an output measuring device that measures the output of the fuel cell stack, and a flow rate regulator that regulates the flow rate of the raw fuel, and the control device regulates the flow rate according to the output so that the fuel utilization rate in the fuel cell stack falls within a certain range and the ratio of the amount of the recycled gas to the amount of the anode exhaust gas is such that a heat balance between the heat absorbed by the reforming reaction and the heat generated by the combustor can be maintained. It is characterized by: [Effects of the Invention]

[0010] According to the fuel cell system disclosed in the present application, by providing a carbon dioxide separation and capture device in the circuit that leads the anode exhaust to the combustor, the carbon dioxide concentration in the gas to be treated can be increased, thereby providing a system that can efficiently separate and capture carbon dioxide. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a flow diagram showing the configuration of a fuel cell system according to a first embodiment. [Figure 2] FIG. 3 is a diagram showing the relationship between the recycle ratio and the power generation efficiency, the carbon dioxide concentration, and the raw fuel flow rate in the fuel cell system according to the first embodiment. [Figure 3] FIG. 10 is a flow chart showing the configuration of a fuel cell system according to a second embodiment. [Figure 4] FIG. 10 is a flow chart showing the configuration of a fuel cell system according to a third embodiment. [Figure 5] FIG. 10 is a flow chart showing the configuration of a fuel cell system according to a fourth embodiment. [Figure 6] FIG. 11 is a diagram showing the effect of increasing the recycle ratio when renewable energy fuel is used in the fuel cell system according to the fourth embodiment. [Figure 7] FIG. 10 is a flow chart showing the configuration of a fuel cell system according to a fifth embodiment. [Figure 8] FIG. 10 is a flow chart showing the configuration of a fuel cell system according to a sixth embodiment. [Figure 9] FIG. 12 is a flow chart showing the configuration of a fuel cell system according to a seventh embodiment. [Figure 10] FIG. 13 is a block diagram showing the hardware configuration of a control device for a fuel cell system according to a seventh embodiment, or of an arithmetic execution part for executing control. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, with reference to the drawings, detailed descriptions will be given of each embodiment of the fuel cell system disclosed in the present application. Note that the embodiments described below are merely examples, and the present application is not limited to these embodiments.

[0013] Embodiment 1 Figures 1 and 2 are intended to explain the configuration and operation of the fuel cell system according to the first embodiment. Figure 1 is a schematic flow diagram showing the configuration of the fuel cell system, and Figure 2 is a graph showing the relationship between the power generation efficiency, carbon dioxide concentration, and raw fuel flow rate versus the recycle ratio when the fuel utilization rate is fixed.

[0014] The fuel cell system of the present application, including the first embodiment, comprises a reformer that reacts raw fuel with steam to produce a reformed gas containing hydrogen, and a fuel cell stack that generates electrical energy through an electrochemical reaction between air and the reformed gas. The anode exhaust of the fuel cell stack is branched into a recycled gas and a combustion gas, and a carbon dioxide separation and capture device is provided in the combustion gas circuit that leads the combustion gas to the combustor.

[0015] As shown in FIG. 1, the fuel cell system 100 according to the first embodiment includes, as its main components, a fuel cell stack 1, a reformer 2, and a carbon dioxide separation and capture device 60. The fuel cell stack 1 generates electricity by electrochemically reacting reformed gas Ff2 with air Fa1. The reformer 2 reforms raw fuel Ff0 (strictly speaking, fuel gas Ff1 mixed with steam Fs) to generate reformed gas Ff2. The carbon dioxide separation and capture device 60 separates and captures carbon dioxide from combustion gas Ff32, which is guided to the combustor 3 from anode exhaust Ff3 discharged from the anode 1a of the fuel cell stack 1.

[0016] Next, the overall configuration of the fuel cell system 100 according to the first embodiment, and the configuration and functions of the main devices will be described. The explanation will be given in accordance with the flow of fuel. That is, the configuration and functions of the main devices will be explained in the order of reformer 2 → fuel cell stack 1 → carbon dioxide capture and separation device 60. After that, the overall processing and functions will be explained.

[0017] <Main equipment> The reformer 2 is filled with a reforming catalyst and receives fuel gas Ff1, which is obtained by adding steam Fs to raw fuel Ff0, where a reforming reaction occurs that decomposes hydrocarbons in the raw fuel Ff0, producing reformed gas Ff2 containing hydrogen and carbon monoxide. For example, a typical reforming reaction when methane is used as the raw fuel Ff0 is shown in equations (1) and (2). CH4 + H2O ⇔ CO + 3H2(1) CO+H2O ⇔ CO2+H2(2)

[0018] The reforming reaction is an endothermic reaction, and a combustor 3 is provided as a heat source for the reformer 2 to compensate for this endothermic reaction. Combustion gas Ff32 (in this application, combustion gas Ff4 after carbon dioxide recovery) branched off from anode exhaust Ff3 discharged from the anode 1a of the fuel cell stack 1 is supplied to the combustor 3 as fuel, where combustion takes place and heat is provided to the reformer 2.

[0019] The fuel cell stack 1 is a stack of cells each consisting of an anode (negative electrode) 1a, a cathode (positive electrode) 1c, and an electrolyte (not shown) sandwiched between the two electrodes. In the fuel cell stack 1, electricity is generated by an electrochemical reaction between fuel components such as hydrogen and carbon monoxide in the reformed gas Ff2 generated in the reformer 2 and oxygen in the air Fa1. The air Fa1 is supplied from an air blower 10.

[0020] The fuel cell stack 1 operates at, for example, 600°C to 700°C, and high-temperature anode exhaust Ff3 is discharged from the anode 1a, and high-temperature cathode exhaust Fa2 is discharged from the cathode 1c, and the heat thereof is utilized on the downstream side. Representative reaction formulas in the fuel cell stack 1 are shown in formulas (3) and (4). Anode: H2+O 2- → H2O+2e - (3) Cathode: 1 / 2O2+2e - → O 2- (4)

[0021] The carbon dioxide separation and capture device 60 is installed in the combustion gas circuit 27 as the combustion gas Ff32 is supplied to the combustor 3, and by separating and capturing carbon dioxide in the combustion gas Ff32 passing through, it reduces the amount of carbon dioxide that is emitted outside the system via the combustor 3.

[0022] <Overall structure and functions> Next, the overall configuration and functions of the fuel cell system 100 will be described. A hydrocarbon-based raw fuel Ff0 such as city gas is input into the fuel cell system 100 via a raw fuel circuit 21, mixed with steam Fs and recycled gas Ff31 in a mixer 30, and then introduced into the reformer 2 via a fuel gas circuit 22. In the reformer 2, heat is applied from a combustor 3 to generate a reformed gas Ff2 containing hydrogen and carbon monoxide.

[0023] The reformed gas Ff2 is introduced to the anode 1a of the fuel cell stack 1 via the reformed gas circuit 23, where an electrochemical reaction takes place, consuming part of the fuel consisting of hydrogen and carbon monoxide. The remaining fuel not used in the reaction is sent to the anode exhaust circuit 24 as anode exhaust Ff3, and is used in the subsequent stage.

[0024] Meanwhile, air Fa1 from air blower 10 passes through heat exchanger 34, where it is heated by heat exchange with combustion exhaust gas Fx and then supplied to cathode 1c of fuel cell stack 1, where an electrochemical reaction consumes some of the oxygen in the air Fa1. Cathode exhaust gas Fa2 discharged from cathode 1c is sent via cathode exhaust circuit 12 to combustor 3, where it is consumed as combustion-supporting gas or used to heat reformer 2, and then discharged into the atmosphere as combustion exhaust gas Fx.

[0025] Anode exhaust Ff3 discharged from the anode 1a of the fuel cell stack 1 is guided to the anode exhaust circuit 24, passes through a steam generator 31 and an anode exhaust condenser 32, and then branches at a branch point 42 into recycled gas Ff31 and combustion gas Ff32. Of the anode exhaust Ff3, the gas branched off as recycled gas Ff31 passes through the recycled gas circuit 26 and is guided to the mixer 30 to be mixed with raw fuel Ff0. The remainder is guided as combustion gas Ff32 through the combustion gas circuit 27 to the combustor 3, where carbon dioxide in the gas is separated and captured by a carbon dioxide separation and capture device 60 installed along the way.

[0026] To achieve this gas flow, a circulation blower 33 is disposed on the anode exhaust circuit 24. The method of returning a portion of the anode exhaust Ff3 to the fuel gas circuit 22 to recycle fuel is intended to increase the fuel utilization rate of the fuel cell stack 1 and improve power generation efficiency, and is a well-known technique in fuel cell systems. In addition, combusting a portion of the anode exhaust Ff3 to effectively utilize energy within the system is also a well-known technique.

[0027] The steam generator 31 provided in the anode exhaust circuit 24 is installed for the purpose of utilizing the heat of the anode exhaust Ff3 to generate the steam Fs necessary for the reforming reaction, and at the same time has the function of lowering the temperature of the anode exhaust Ff3.

[0028] The purpose of lowering the temperature of the anode exhaust Ff3 is to condense the moisture in the anode exhaust Ff3, recover the water within the system, and to facilitate the circulation of the anode exhaust Ff3 downstream. To recover the water, an anode exhaust condenser 32 is also installed downstream of the steam generator 31. Here, the anode exhaust Ff3 is cooled to, for example, about 60°C. The water recovered in the anode exhaust condenser 32 is treated in a water treatment device (not shown) to become pure water Fc, which is sent to the steam generator 31 through the water supply circuit 50. Water circulation systems including this type of steam generator 31 are common and well known, so a detailed description will be omitted.

[0029] Pure water Fc is introduced into the steam generator 31 from the water supply circuit 50. Steam Fs generated in the steam generator 31 by receiving heat from the anode exhaust Ff3 is introduced into the mixer 30 via the steam circuit 51. A fuel gas mixture of raw fuel Ff0 and steam is mixed with recycled gas Ff31 and introduced into the reformer 2 as fuel gas Ff1, where a reforming reaction takes place to produce hydrogen and carbon monoxide.

[0030] Next, to facilitate understanding, several operating parameters related to the power generation efficiency η of the fuel cell stack 1 and the carbon dioxide concentration in the anode exhaust Ff3 will be explained. The raw fuel flow rate Qf is the flow rate of the raw fuel supplied to the fuel cell system 100, and the fuel utilization rate Uf is the rate at which the hydrogen and carbon monoxide supplied to the fuel cell stack 1 are consumed (by electrochemical reactions) within the fuel cell stack 1. Furthermore, the recycle ratio α is the rate at which the anode exhaust Ff3 is circulated as the recycled gas Ff31.

[0031] Here, the parameter directly related to the power generation efficiency η is the raw fuel flow rate Qf, and for a given power generation output, the lower the raw fuel flow rate Qf, the higher the power generation efficiency η. On the other hand, reducing the raw fuel flow rate Qf reduces the amount of reformed gas Ff2 supplied to the fuel cell stack 1, which inevitably increases the fuel utilization rate Uf. However, if the fuel utilization rate Uf is made too high, there is a possibility that some single cells or the electrode surfaces will become short of fuel, which will affect the performance of the fuel cell stack 1, so there is an upper limit, and it is generally not desirable to operate it above 80%.

[0032] Furthermore, the recycle ratio α is also related to the fuel utilization rate Uf, and even if the raw fuel flow rate Qf is constant, if the recycle ratio α increases, the recycling rate of the anode exhaust Ff3 increases, and the amount of gas supplied to the stack effectively increases, resulting in a decrease in the fuel utilization rate Uf. Therefore, since increasing the recycle ratio α has the effect of lowering the fuel utilization rate Uf, assuming that the fuel utilization rate Uf is maintained below a certain level, increasing the recycle ratio α can lower the raw fuel flow rate Qf. In other words, increasing the recycle ratio α makes it possible to increase the power generation efficiency η.

[0033] On the other hand, when we look at the carbon dioxide concentration Cc in the anode exhaust Ff3, increasing the recycle ratio α while keeping the fuel utilization rate Uf constant results in the concentration of carbon dioxide due to the fuel recycling. In other words, it is possible to maintain a steady concentration of carbon dioxide by recycling the anode exhaust Ff3 while discharging a certain amount of the anode exhaust Ff3 outside the system through the combustor 3.

[0034] An example of the results of a simulation of this balance is shown in Figure 2. In Figure 2, when the recycle ratio α is set to 0, the raw fuel flow rate Qf at which the fuel utilization rate Uf becomes, for example, 80% is set to 100%, and the relationship between the carbon dioxide concentration Cc, power generation efficiency η, and raw fuel flow rate Qf (expressed in %) is shown when the fuel utilization rate Uf is fixed and the recycle ratio α is changed.

[0035] The simulation results show that as the recycle ratio α increases, the raw fuel flow rate Qf (solid line) decreases, the power generation efficiency η (dotted line) improves, and the carbon dioxide concentration Cc (dashed line) increases. A study of the balance of income and expenditure confirmed that by increasing the recycle ratio α to around 70% (65% or more), the carbon dioxide concentration Cc in the anode exhaust Ff3 is concentrated to 70% or more (dry basis). These simulation results were verified using an actual fuel cell stack 1 and reformer 2, and the improvement in power generation efficiency η and the effects of carbon dioxide concentration were confirmed.

[0036] The combustion gas circuit 27 branching off from the branch point 42 is provided with a carbon dioxide separation and capture device 60, which separates and captures carbon dioxide, thereby making it possible to reduce the amount of carbon dioxide emitted outside the system. Because the carbon dioxide in the anode exhaust Ff3 is highly concentrated as described above, the separation and capture of carbon dioxide is relatively easy, which is a characteristic effect of the fuel cell system 100 of the present invention.

[0037] In carbon dioxide separation, it is generally considered easier and more advantageous to have a higher carbon dioxide concentration in the gas to be separated. Carbon dioxide separation requires a compression process, whether by liquefaction, membrane separation, or adsorption separation, so the higher the carbon dioxide concentration, the lower the power required for compression and the better the separation efficiency. From the perspective of utilizing recovered carbon dioxide, liquefaction separation of carbon dioxide is desirable, and if the carbon dioxide concentration is high, liquefaction separation can be performed relatively easily.

[0038] In this case, it is desirable to set the carbon dioxide concentration Cc, which has been confirmed through simulations and actual equipment verification, to 70% or more, and the higher the concentration, the more advantageous it is. Possible methods for liquefying and capturing carbon dioxide include the industrially performed processes of dehumidification, compression, cooling and liquefaction, and gas-liquid separation. However, there is no limit to these methods, and any method can be used as long as it can effectively liquefy and separate carbon dioxide.

[0039] As an example, the components of the anode exhaust Ff3 are 22% hydrogen, 7% carbon monoxide, and 71% carbon dioxide (dry basis). For gases with this composition, in the carbon dioxide separation and capture unit 60, associated gases other than carbon dioxide are separated and captured by gas-liquid separation during the carbon dioxide liquefaction process. Gas components that are difficult to separate by gas-liquid separation (e.g., carbon monoxide) can be converted into gases that are easier to separate before the liquefaction process. Hydrogen can be relatively easily recovered by gas-liquid separation.

[0040] The recycle ratio α is an important parameter for concentrating carbon dioxide in this way, and a method for setting the recycle ratio α will be explained with reference to the flow diagram in Figure 1. The anode exhaust Ff3 branches at branch point 42, one branch passing through the recycle gas circuit 26 to the mixer 30, and the other branch passing through the combustion gas circuit 27 to the combustor 3. A first circulation blower 33 is disposed on the anode exhaust circuit 24 to form this flow.

[0041] As described above, the anode exhaust Ff3 is branched at the branch point 42 into a recycled gas Ff31 and a combustion gas Ff32. This distribution ratio can be changed by adjusting the first flow rate regulator 40 provided in the recycled gas circuit 26 and the second flow rate regulator 41 provided in the combustion gas circuit 27. In other words, the recycle ratio α can be set to any value by adjusting these two flow rate regulators (the first flow rate regulator 40 and the second flow rate regulator 41).

[0042] For example, if it is desired to set the recycle ratio α to 70%, the flow rate ratio of the recycle gas Ff31 to the combustion gas Ff32 can be adjusted to a ratio of 7:3. This distribution ratio can also be adjusted independently by using either the first flow rate regulator 40 or the second flow rate regulator 41. In this case, the distribution ratio can be adjusted arbitrarily by replacing one of them with a fixed flow rate restricting element such as a manual valve and adjusting the other flow rate regulator.

[0043] The first flow rate regulator 40 and the second flow rate regulator 41 are assumed to be something like a mass flow controller in which a mass flow meter and a control valve are integrated, but they may also be a separate combination of a flow meter and a control valve.

[0044] The mixer 30, which stirs and mixes two or more types of gases, is also called a static mixer. It is not essential and may be omitted if the gases are sufficiently mixed in the downstream piping. Alternatively, the steam circuit 51 and the recycled gas circuit 26 may be directly connected to and merged with the raw fuel circuit 21. Alternatively, the mixer 30 may be provided after merging with the raw fuel circuit 21.

[0045] As described above, the fuel cell system 100 according to the first embodiment includes the reformer 2 that reacts raw fuel Ff0 with steam Fs to produce reformed gas Ff2 containing hydrogen, and the fuel cell stack 1 that generates electrical energy through an electrochemical reaction between air Fa1 and the reformed gas Ff2. The anode exhaust Ff3 of the fuel cell stack 1 is branched into recycled gas Ff31 and combustion gas Ff32, with the recycled gas Ff31 being introduced into the fuel gas circuit 22 and the combustion gas Ff32 being introduced into the combustor 3. A carbon dioxide separation and capture device 60 is further provided midway through the combustion gas circuit 27. Therefore, the fuel cell system 100 according to the first embodiment achieves high power generation efficiency and can easily separate and capture carbon dioxide by increasing the carbon dioxide concentration in the anode exhaust Ff3.

[0046] Embodiment 2 In the above-mentioned first embodiment, an example of controlling the recycle ratio using a flow rate regulator has been described. In the second embodiment, an example of controlling the recycle ratio using a blower will be described. FIG. 3 is a schematic flow diagram for explaining the configuration of a fuel cell system according to the second embodiment. Note that the configuration and operation other than the control of the recycle ratio are the same as those explained in the first embodiment, and therefore explanations of similar parts will be omitted, and FIG. 2 used in the first embodiment will be used.

[0047] The fuel cell system of the second embodiment does away with the circulation blower on the anode exhaust circuit described in the first embodiment, and instead of a flow rate regulator, a blower is provided in each of the recycled gas circuit and the combustion gas circuit.

[0048] The fuel cell system of embodiment 2 will be described below, focusing on the differences from embodiment 1. In a fuel cell system 100 of embodiment 2, as shown in Fig. 3, the circulation blower 33 on the anode exhaust circuit 24 described in embodiment 1 is eliminated. Instead of the flow rate regulators (first flow rate regulator 40 and second flow rate regulator 41), a second circulation blower 35 and a blower 36 are provided in the recycled gas circuit 26 and the combustion gas circuit 27, respectively.

[0049] By providing the second circulation blower 35 in the recycled gas circuit 26 and the blower 36 in the combustion gas circuit 27, the anode exhaust Ff3 is divided into recycled gas Ff31 and combustion gas Ff32 in a predetermined ratio, thereby circulating the anode exhaust Ff3. In other words, these two blowers (the second circulation blower 35 and the blower 36) play the role of the circulation blower 33 in the first embodiment.

[0050] Furthermore, by adjusting the output of each of the second circulation blower 35 and blower 36, the flow rate ratio between the recycled gas Ff31 and the combustion gas Ff32 can be changed, as in the first embodiment. In other words, the recycle ratio α can be adjusted. The output of each of the blowers (second circulation blower 35, blower 36) can be changed, for example, by adjusting the output frequency of the motor drive. Alternatively, this can be achieved by providing a control valve (not shown) in series with each of the blowers (second circulation blower 35, blower 36) and adjusting the control valve. In this case, the flow rate adjustment function is performed by the set of each of the blowers (second circulation blower 35, blower 36) and the control valve.

[0051] As described above, in the fuel cell system 100 according to the second embodiment, the second circulation blower 35 is provided in the recycled gas circuit 26, and the blower 36 is provided in the combustion gas circuit 27. This makes it possible to change the flow rate ratio between the recycled gas Ff31 and the combustion gas Ff32, thereby adjusting the recycle ratio α.

[0052] Embodiment 3 In the above-mentioned first and second embodiments, an example in which a mixer is used to mix raw fuel, recycled gas, and steam is described. In the present third embodiment, an example in which an ejector is used instead of a mixer is described. FIG. 4 is a schematic flow diagram showing the configuration of a fuel cell system according to the third embodiment. Note that the configuration and operation other than the use of an ejector instead of a mixer are the same as those described in the first or second embodiment, and therefore, a description of the similar parts will be omitted, and FIG. 2 used in the first embodiment will be used.

[0053] 4, a fuel cell system 100 according to the third embodiment is provided with an ejector 38 instead of the mixer 30 described in the first and second embodiments. The fuel cell system 100 according to the third embodiment will be described below, focusing on the differences from the first and second embodiments.

[0054] In the fuel cell system 100 of the first embodiment, the recycled gas circuit 26 is introduced into the mixer 30, whereby the recycled gas Ff31 is circulated as the fuel gas Ff1, and the driving force for this circulation is the circulation blower 33. In the second embodiment, two blowers (a second circulation blower 35 and a blower 36) perform this role. In the third embodiment, these blowers are eliminated, and an ejector 38 is provided instead of the mixer 30.

[0055] The ejector 38 uses the steam Fs generated in the steam generator 31 as a driving force to suck in the raw fuel Ff0 and the recycled gas Ff31 and sends the mixed fuel gas Ff1 to the reformer 2. For this purpose, the ejector 38 is connected to a connection port for a driving fluid (not shown) through which a steam circuit 51 is connected, and also to separate connection ports through which the raw fuel circuit 21 and the recycled gas circuit 26 are connected.

[0056] The use of the ejector 38 eliminates the need for a circulation blower to circulate the anode exhaust Ff3 and makes it possible to suck in the raw fuel Ff0 even when the pressure of the raw fuel Ff0 is low. For example, if low-pressure city gas is used as the raw fuel Ff0, the city gas can be sucked in by the ejector 38 without the need for a special pressure-boosting device. Because the ejector 38 has no moving parts, it can achieve a highly reliable circulation system, especially when handling combustible gases. In this case, the pressure at the branch point 42 can be maintained by adjusting the first flow rate regulator 40 so that the required amount of combustion gas Ff4 is supplied to the combustor 3, or a blower (not shown) can be inserted and adjusted instead of the second flow rate regulator 41.

[0057] When the pressure of the raw fuel Ff0 is high, for example, when medium-pressure or high-pressure city gas is used, it is also possible to use the raw fuel Ff0 as the driving fluid of the ejector 38, although this is not shown in the drawings. In this case, the raw fuel Ff0 serves as the driving fluid and draws in the steam Fs and the recycled gas Ff31, and similarly, the circulation blower can be omitted.

[0058] As described above, the fuel cell system 100 according to the third embodiment is provided with an ejector 38 instead of the mixer 30, which eliminates the need for a blower and also omits the need for a special raw fuel pressure boosting means, thereby enabling the construction of a simple and highly reliable system.

[0059] Embodiment 4 A fuel cell system according to a fourth embodiment is configured to introduce renewable energy fuel (renewable energy fuel) received from outside, separately from the raw fuel, into at least one of the combustor and the fuel gas circuit. Figures 5 and 6 are provided to explain the configuration and operation of the fuel cell system according to the fourth embodiment. Figure 5 is a schematic flow diagram showing the configuration of the fuel cell system, and Figure 6 is a graph showing the relationship between the power generation efficiency, carbon dioxide concentration, raw fuel flow rate, and renewable energy fuel flow rate versus the recycle ratio when the fuel utilization rate is fixed, as a diagram showing the effect of increasing the recycle ratio using renewable energy fuel. Note that Figure 5 also illustrates, as a modified example, a configuration for introducing renewable energy fuel into the fuel gas circuit.

[0060] 5, the fuel cell system 100 according to the fourth embodiment is provided with a renewable energy fuel circuit 71 for combustion that guides the renewable energy fuel Fr to the combustor 3. The fuel cell system 100 according to the fourth embodiment will be described below, focusing on the differences from the first to third embodiments.

[0061] In the first embodiment, the effect of the recycle ratio α was explained using FIG. 2, where increasing the recycle ratio α reduces the raw fuel flow rate Qf and improves the power generation efficiency η, while at the same time increasing the carbon dioxide concentration Cc in the anode exhaust Ff3.

[0062] On the other hand, by increasing the recycle ratio α, the amount of combustion gas Ff32 guided to the combustor 3 via the combustion gas circuit 27 decreases, and accordingly the amount of combustion in the combustor 3 decreases. Because the heat required for the reaction in the reformer 2 is provided by the combustion in the combustor 3, if the amount of combustion decreases more than necessary, the reaction in the reformer 2 cannot be maintained and the reformer 2 will no longer function. In other words, the heat balance of the system will not be maintained.

[0063] In other words, there is a limit in terms of heat balance to increasing the recycle ratio α while maintaining a constant fuel utilization rate Uf of the fuel cell stack 1. Therefore, if the upper limit of the recycle ratio α in terms of heat balance is set to an upper limit value α0, it is not possible to increase the recycle ratio α above the upper limit value α0 to improve the power generation efficiency η or increase the carbon dioxide concentration Cc of the anode exhaust Ff3. Although it depends on the fuel utilization rate Uf, for example, the limit in terms of heat balance (upper limit value α0) for the recycle ratio α is around 65 to 70%.

[0064] If the amount of combustion is insufficient, it is possible to supplement the combustion with raw fuel Ff0, but because raw fuel Ff0 is a hydrocarbon fuel, carbon dioxide will be contained in the combustion exhaust gas Fx after combustion, and this will not reduce carbon dioxide emissions.

[0065] In contrast, in the fourth embodiment, the renewable energy fuel Fr is introduced into the combustor 3, so there is no limit to increasing the recycle ratio α, and it is possible to increase the carbon dioxide concentration Cc of the anode exhaust Ff3 in accordance with the increase in the recycle ratio α. In other words, even if the combustion amount in the combustor 3 is insufficient due to an increase in the recycle ratio α, the renewable energy fuel Fr can compensate for the insufficient heat, and there is no limit to the recycle ratio α. For example, it is possible to increase the recycle ratio α to 80% or more and concentrate the carbon dioxide concentration Cc to 80% or more.

[0066] In this case, when considered based on the raw fuel flow rate Qf, the power generation efficiency η increases as the recycle ratio α increases. However, if the power generation efficiency η is evaluated by adding the amount of renewable fuel Fr input to the raw fuel Ff0, as shown in Figure 6, there is no improvement in power generation efficiency η from the point at which the flow rate of renewable fuel Fr (renewable fuel flow rate Qr) is increased. In other words, the power generation efficiency η after renewable fuel Fr is input remains almost constant regardless of the increase in the recycle ratio α. The carbon dioxide concentration Cc steadily increases as the recycle ratio α increases, regardless of whether renewable fuel Fr is input or not.

[0067] In the first embodiment, it was stated that carbon dioxide can be easily separated and recovered if the carbon dioxide concentration Cc is concentrated to about 70% or more. In other words, by increasing the recycle ratio α to at least 65% or more, carbon dioxide can be concentrated to a level at which separation and recovery of carbon dioxide is easy. If further concentration can be achieved by using renewable energy fuel Fr, separation and recovery of carbon dioxide becomes even easier, which is a great advantage. However, an increase in the circulation amount may increase the circulation power or cause excessive pressure loss, and although it is possible to set the recycle ratio α above the upper limit value α0 by using renewable energy fuel Fr, it is necessary to set the recycle ratio α above the system limit (limit value α L ) must be kept below.

[0068] Variant. 5 also illustrates a modified example in which the renewable energy fuel Fr is introduced into the mixer 30 separately from the combustor 3 and then added to the raw fuel. In other words, instead of the combustion renewable energy fuel circuit 71 that leads the renewable energy fuel Fr from the above-mentioned renewable energy fuel circuit 70 to the combustor 3, a feedstock renewable energy fuel circuit 72 that leads the renewable energy fuel Fr to the mixer 30 is provided as a circuit for circulating the renewable energy fuel Fr.

[0069] When the combustion amount in the combustor 3 is insufficient due to an increase in the recycle ratio α, the renewable energy fuel Fr can be supplied to the fuel gas circuit 22, whereby the renewable energy fuel Fr circulates within the system and provides heat to the combustor 3, thereby achieving the same effect as when the renewable energy fuel Fr is supplied directly to the combustor 3. Note that, for example, as shown in FIG. 5, a branch point 43 may be provided where the renewable energy fuel Fr branches from the renewable energy fuel circuit 70 to a combustion renewable energy fuel circuit 71 that leads to the combustor 3, and a feedstock renewable energy fuel circuit 72 that leads to the mixer 30. In other words, the renewable energy fuel Fr may be supplied to both the combustor 3 and the mixer 30, rather than to either one of them.

[0070] Renewable fuels are fuels that do not emit carbon dioxide, such as hydrogen (green hydrogen), ammonia (green ammonia), or biogas produced from renewable energy sources. Hydrocarbon gas or liquid fuels produced from renewable energy sources are also acceptable.

[0071] As described above, the fuel cell system 100 according to the fourth embodiment is configured to introduce the renewable energy fuel Fr received from the outside into at least one of the combustor 3 and the fuel gas circuit 22. Therefore, it is possible to increase the recycle ratio α and raise the carbon dioxide concentration Cc of the anode exhaust Ff3 without considering the heat balance of the reformer 2.

[0072] Embodiment 5 In the fuel cell system according to the fifth embodiment, a CO converter is provided on the anode exhaust circuit or on the upstream side of the carbon dioxide capture device on the combustion gas circuit. Fig. 7 is a schematic flow diagram showing the configuration of the fuel cell system according to the fifth embodiment. Note that the configuration and operation other than the provision of the CO converter are the same as those explained in the first to fourth embodiments, and therefore explanations of similar parts will be omitted, and Fig. 2 used in the first embodiment or Fig. 6 used in the fourth embodiment will be used.

[0073] 7, a fuel cell system 100 according to the fifth embodiment is provided with a CO converter 61 on the anode exhaust circuit 24. Alternatively, different from the drawing, the CO converter 61 is provided upstream of the carbon dioxide separation and capture device 60 on the combustion gas circuit 27. The fuel cell system 100 according to the fifth embodiment will be described below, focusing on the differences from the first and second embodiments.

[0074] As shown in the example of the carbon dioxide separation and capture system 60 in the first embodiment, hydrogen and carbon monoxide, which are associated gases, are separated from the carbon dioxide by gas-liquid separation during the carbon dioxide liquefaction process. The separated hydrogen and carbon monoxide are sent to the downstream combustor 3 and used as fuel. Since the separated hydrogen and carbon monoxide serve as fuel to provide the necessary heat to the reformer 2, it is desirable that the carbon dioxide separation and capture system 60 separates the hydrogen and carbon monoxide without any loss.

[0075] Carbon monoxide is relatively difficult to separate compared to hydrogen, and particularly when carbon dioxide is liquefied and separated, part of the carbon monoxide dissolves in the liquid carbon dioxide, which reduces the purity of the liquid carbon dioxide and causes a loss of carbon monoxide. Furthermore, the separated carbon monoxide is burned in the combustor 3 and emitted into the atmosphere as carbon dioxide, which is undesirable from the standpoint of reducing carbon dioxide emissions.

[0076] In contrast, in the fifth embodiment, in order to avoid such a loss of carbon monoxide, a CO transformer 61 is provided on the circuit (upstream side) that leads the anode exhaust Ff3 to the carbon dioxide separation and capture device 60. Specifically, the location is either upstream of the anode exhaust condenser 32 on the anode exhaust circuit 24, or upstream of the carbon dioxide separation and capture device 60 on the combustion gas circuit 27, as shown in FIG.

[0077] In the CO converter 61, carbon monoxide and water vapor are converted into hydrogen and carbon dioxide by the shift reaction shown in formula (5). CO+H2O ⇔ CO2+H2(5)

[0078] The shift reaction converts carbon monoxide to hydrogen, facilitating separation in the carbon dioxide separation and capture unit 60. Furthermore, the conversion of carbon monoxide to hydrogen involves little energy loss, minimizing the impact on the amount of combustion in the combustor 3 and suppressing carbon dioxide emissions into the atmosphere.

[0079] As shown in formula (5), the shift reaction of the CO transformer 61 requires water vapor equivalent to the amount of carbon monoxide, and a reaction temperature of around 200° C. Therefore, it is desirable to install the CO transformer 61 upstream of the anode exhaust condenser 32 on the anode exhaust circuit 24 in order to satisfy the conditions of temperature and water vapor (amount) required for the reaction.

[0080] Furthermore, in order to reduce the load on the CO transformer 61, it may be possible to install it upstream of the carbon dioxide capture and recovery unit 60 on the combustion gas circuit 27. Because the gas temperature in this circuit is low, heating is required to carry out the shift reaction, but the load on the CO transformer 61 is reduced to a fraction of that, or to be precise, by a factor of (1-α).

[0081] The shift reaction is an exothermic reaction, and it is necessary to prevent an excessive rise in temperature in order to increase the efficiency of the reaction, and it is therefore desirable to cool the CO transformer 61. For example, it is possible to introduce steam Fs from the steam circuit 51 as a cooling medium, in which case the heat of reaction is transferred directly to the steam Fs and recovered within the system. Temperature control of the CO transformer 61 is a known technique, and therefore a detailed explanation and illustrations thereof will be omitted.

[0082] As described above, the fuel cell system 100 according to the fifth embodiment includes a CO converter 61 in the circuit that leads the anode exhaust Ff3 to the carbon dioxide capture / recovery device 60. This removes carbon monoxide contained in the anode exhaust Ff3, enabling effective separation and recovery of carbon dioxide.

[0083] Embodiment 6 In the fuel cell system according to the sixth embodiment, the supply of water vapor to the raw fuel is omitted by limiting the condensation of the water contained in the anode exhaust. Fig. 8 is a schematic flow diagram showing the configuration of the fuel cell system according to the sixth embodiment. Note that the configuration and operation other than the omission of the water vapor generator are the same as those explained in the first to fifth embodiments, and therefore explanations of similar parts will be omitted, and Fig. 2 used in the first embodiment or Fig. 6 used in the fourth embodiment will be used.

[0084] As shown in Fig. 8, a fuel cell system 100 according to the sixth embodiment is provided with an anode exhaust cooler 37 instead of the anode exhaust condenser 32 (and the steam generator 31) on the anode exhaust circuit 24 shown in Fig. 1 etc. The fuel cell system according to the seventh embodiment will be described below, focusing on the differences from the first and second embodiments.

[0085] In the first and second embodiments, the pure water Fc is introduced from the water supply circuit 50 to the steam generator 31, and the steam Fs generated there is introduced to the mixer 30 and mixed with the raw fuel Ff0. The steam Fs is a component necessary for the reforming reaction (Equations (1) and (2)) in the reformer 2, but when the anode exhaust Ff3 is recycled, the anode exhaust Ff3 originally contains steam. Therefore, even if the moisture is not condensed in the anode exhaust condenser 32, a proportion of steam corresponding to the recycle ratio α is circulated to the fuel gas circuit 22 through the recycled gas circuit 26.

[0086] In this case, if the recycle ratio α is small, only a portion of the required water vapor is supplied. However, if the recycle ratio α is increased to a certain level, the entire amount of water vapor required for the reforming reaction is contained in the recycled gas Ff31 and supplied. For example, the anode exhaust Ff3 contains approximately 55% water vapor. However, if the recycle ratio α is 60% or higher, 60% or more of the water vapor in the anode exhaust Ff3 is recycled, and this amount can cover the entire amount of water vapor required for the reforming reaction. In other words, by increasing the recycle ratio α to a certain level or higher, the water vapor required for the reforming reaction can be supplied solely through recycling, and therefore the water vapor generator 31, anode exhaust condenser 32, and water vapor circuit 51 shown in Embodiments 1 and 2 can be omitted.

[0087] When using a circulation blower or a flow rate regulator to configure a circulation system, it is necessary to lower the temperature of the anode exhaust Ff3, and for this purpose an anode exhaust cooler 37 is installed instead of the anode exhaust condenser 32. The anode exhaust cooler 37 cools the anode exhaust Ff3 to a temperature at which water vapor does not condense, for example, to 85 to 90°C.

[0088] As described above, the fuel cell system 100 according to the sixth embodiment is configured to limit the condensation of water contained in the anode exhaust gas Ff3 and to circulate the water vapor by effectively utilizing the water vapor contained in the recycled gas Ff31. Therefore, it is possible to omit the means for supplying the water vapor Fs to the raw fuel Ff0 (such as the water vapor generator 31, the anode exhaust gas condenser 32, and the water vapor circuit 51), and a simple system can be constructed.

[0089] Embodiment 7 In the seventh embodiment, a specific configuration of a control system is described by adding a control device, regulator, measuring device, etc., which are necessary for the functions and operations of the fuel cell system. Fig. 9 is a schematic flow diagram showing the configuration of a fuel cell system according to the seventh embodiment. The configuration and operation other than the addition of the control device, regulator, measuring device, etc. are the same as those explained in the first to sixth embodiments, and explanation of the similar parts will be omitted.

[0090] 9, a fuel cell system 100 according to the seventh embodiment is obtained by adding a control device 80 and other devices required for controlling the flow rates of various fluids and the stack output to the systems described in the first to sixth embodiments. The fuel cell system according to the seventh embodiment will be described below, focusing on the differences from the first, second and fourth embodiments.

[0091] The fuel cell system 100 includes a control device 80 (indicated as "CTR" in the figure), a raw fuel flow rate regulator 82 in the raw fuel circuit 21, and an output measuring device 81 in the fuel cell stack 1. The control device 80 communicates with the raw fuel flow rate regulator 82 and the output measuring device 81, as well as with the first flow rate regulator 40 and the second flow rate regulator 41, via signals. When water vapor Fs is supplied through the water vapor circuit 51, the water vapor circuit 51 includes a water vapor flow rate regulator 86, which communicates with the control device 80 via signals.

[0092] In the fuel cell system 100, the important point in adjusting the recycle ratio α is to control the raw fuel flow rate Qf while maintaining the fuel utilization rate Uf of the fuel cell stack 1 at a certain value or within a certain range. As described in the first embodiment, in order to increase the power generation efficiency η and the carbon dioxide concentration Cc in the anode exhaust Ff3 by increasing the recycle ratio α, it is necessary to reduce the raw fuel flow rate Qf so ​​as not to decrease the fuel utilization rate Uf.

[0093] The fuel utilization rate Uf is determined by the relationship between the output power Po and the raw fuel flow rate Qf of the fuel cell stack 1, and the value of the raw fuel flow rate Qf required to maintain the fuel utilization rate Uf is uniquely determined for a certain constant output power Po and a certain recycle ratio α. The relationship between the recycle ratio α and the raw fuel flow rate Qf when the output power Po and fuel utilization rate Uf are constant is as explained in Figure 2.

[0094] Based on the above, we will now explain the specific operation control of the fuel cell system 100. The fuel cell system 100 is operated at a predetermined output power Po when an output control device (not shown) issues an output command to the fuel cell stack 1. At this time, the raw fuel flow rate Qf and the recycle ratio α are adjusted according to the output power Po.

[0095] Specifically, the control device 80 issues commands to the raw fuel flow rate regulator 82, the first flow rate regulator 40, and the second flow rate regulator 41 to control the raw fuel flow rate Qf and the recycle ratio α. The control device 80 has a function of the raw fuel flow rate Qf and the recycle ratio α that maintains the fuel utilization rate Uf constant according to the output Po of the fuel cell stack 1, so by setting a predetermined recycle ratio α, the raw fuel flow rate Qf is automatically controlled.

[0096] The value of the recycle ratio α is set arbitrarily by the control device 80, but it is desirable to set it to a high value in terms of the power generation efficiency η and the carbon dioxide concentration Cc. However, there is an upper limit α0 for maintaining the heat balance of the reformer 2, and for this reason, it is possible to set the upper limit α0 while monitoring the temperature of the reformer 2, for example. Alternatively, it is possible to determine the upper limit α0 for the output Po in advance and always operate automatically at this value. The upper limit α0 that can maintain the heat balance of the reformer 2 is approximately 65 to 70%, as explained in the fourth embodiment, but this value varies depending on the characteristics of the fuel cell stack 1 and the system.

[0097] The reforming reaction requires the supply of steam Fs corresponding to the amount of raw fuel Ff0, and therefore the steam flow rate must be controlled simultaneously with the control of the raw fuel flow rate. Therefore, a command is issued to the raw fuel flow rate regulator 82 and also to the steam flow rate regulator 86. However, this does not apply to the sixth embodiment, which does not have the steam circuit 51.

[0098] Furthermore, when the renewable energy fuel Fr described in the fourth embodiment is introduced, a new control element is required, and a combustion renewable energy fuel flow rate regulator 84 is installed in the combustion renewable energy fuel circuit 71, or a feedstock renewable energy fuel flow rate regulator 85 is installed in the feedstock renewable energy fuel circuit 72.

[0099] The renewable energy fuel Fr is introduced when there is a shortage of heat required for the reaction in the reformer 2, and when the recycle ratio α exceeds the upper limit value α0 mentioned above. The purpose of increasing the recycle ratio α beyond the upper limit value α0 is to further increase the carbon dioxide concentration Cc in the anode exhaust Ff3, as explained using Figure 6. Because the additional fuel is renewable energy fuel Fr, no carbon dioxide is emitted as a result.

[0100] A specific control method in this case will be described. If the heat balance of the reformer 2 is no longer maintained by increasing the recycle ratio α, the control device 80 additionally issues a command to the combustion renewable energy fuel flow regulator 84 or the feedstock renewable energy fuel flow regulator 85 to supplement the heat required for the reaction in the reformer 2. This makes it possible to further concentrate carbon dioxide.

[0101] When the recycle ratio α is increased beyond the upper limit value α0, the combustion renewable energy fuel flow rate regulator 84 or the feedstock renewable energy fuel flow rate regulator 85 is controlled while reducing the raw fuel flow rate Qf and monitoring the temperature of the reformer 2. Alternatively, it is possible to determine in advance the relationship between the recycle ratio α and the renewable energy fuel flow rate in a region exceeding the upper limit value α0, and automatically adjust the flow rate of the renewable energy fuel Fr by setting the recycle ratio α. Furthermore, it is possible to determine in advance the value of the recycle ratio α relative to the output Po in a region exceeding the upper limit value α0, and always perform automatic operation at this value.

[0102] If renewable energy fuel Fr is not available, the recycle ratio α will be set at an upper limit α0 for operation. Even if renewable energy fuel Fr is available, if the recycle ratio α becomes too high, a large amount of recycled gas Ff31 will be circulated. As a result, the hydrogen concentration in the reformed gas Ff2 will decrease, adversely affecting the performance of the fuel cell stack 1. Furthermore, an increase in the amount of circulation may increase the circulation power or cause excessive pressure loss. For this reason, as explained in the fourth embodiment, there is a limit to the increase in the recycle ratio α even when renewable energy fuel Fr is used (limit α L ) and is expected to be around 90% at most. L also varies depending on the characteristics of the fuel cell stack 1 and the system.

[0103] The renewable energy fuel Fr may be supplied to the combustor 3 and burned directly, or may be supplied to the mixer 30 and mixed with the raw fuel Ff0, or may be supplied to both. When mixed with the raw fuel Ff0, the renewable energy fuel Fr is used in the reaction in the fuel cell stack 1, and the remaining renewable energy fuel consumed in the fuel cell stack 1 is circulated and supplied to the combustor 3. As a result, the raw fuel flow rate Qf can be reduced, and the carbon dioxide concentration Cc of the anode exhaust Ff3 can be increased.

[0104] In the fuel cell system 100, when the execution part of the calculation processing is configured by software using a microcomputer, the control device 80 may be configured by a single microcomputer 800 equipped with a processor 801 and a storage device 802, as shown in Fig. 10. Although not shown, the storage device 802 includes a volatile storage device such as a random access memory and a non-volatile auxiliary storage device such as a flash memory.

[0105] Furthermore, a hard disk auxiliary storage device may be provided instead of flash memory. Processor 801 executes a program input from storage device 802. In this case, the program is input from the auxiliary storage device to processor 801 via a volatile storage device. Processor 801 may output data such as calculation results to a volatile storage device of storage device 802, or may store the data in the auxiliary storage device via the volatile storage device.

[0106] As described above, the fuel cell system 100 according to the seventh embodiment is obtained by adding devices necessary for controlling the flow rates of various fluids and the stack output, as well as the control device 80 necessary for functions and operations, to the fuel cell systems 100 according to the first to sixth embodiments. By controlling the recycle ratio α, the fuel cell system 100 according to the seventh embodiment can reduce the consumption of raw fuel Ff0, improve the power generation efficiency η, and concentrate carbon dioxide. Furthermore, by using renewable energy fuel Fr, further concentration of carbon dioxide becomes possible.

[0107] Although the first to seventh embodiments all show examples in which a solid oxide fuel cell is used as the fuel cell stack 1, the present invention is not limited to this. For example, other fuel cells known as high-temperature fuel cells, such as molten carbonate fuel cells (MCFCs), whose operating temperatures are equal to or higher than the boiling point of water (e.g., 100°C), can also be used. In particular, in MCFCs, carbon dioxide moves from the air electrode side to the fuel electrode side as the reaction proceeds, and the carbon dioxide concentration at the fuel electrode side becomes even higher than the increase in carbon dioxide concentration due to fuel consumption, and carbon dioxide must be supplied to the air electrode side. This makes the carbon dioxide capture technology of the present invention even more important.

[0108] Furthermore, in the first to seventh embodiments, it is possible to use biogas derived from organic matter such as food waste, livestock waste, and sewage as the raw fuel Ff0, and in this case, the same functions as those of the first to seventh embodiments can be realized. In this case, the use of biogas itself makes the power generation system carbon-free, but if the captured carbon dioxide can be further immobilized by burying it underground and using it as construction material, it will become carbon-negative, and an even more socially responsible system can be constructed.

[0109] Although various exemplary embodiments and examples are described in this application, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless variations not illustrated are contemplated within the scope of the technology disclosed in this specification. For example, this includes cases where at least one component is modified, added, or omitted, or where at least one component is extracted and combined with components of another embodiment.

[0110] As described above, the fuel cell system 100 of the present application includes a reformer 2 that generates reformed gas Ff2 containing hydrogen through a reforming reaction between hydrocarbon-containing raw fuel Ff0 and steam Fs, a combustor 3 that provides heat to the reformer 2, a fuel cell stack 1 that generates electrical energy through an electrochemical reaction between air Fa1 and the reformed gas Ff2 and outputs DC power, an anode exhaust circuit 24 that receives anode exhaust Ff3 discharged from the anode 1a of the fuel cell stack 1 and branches off into a recycled gas circuit 26 that mixes the recycled gas Ff31 with the raw fuel Ff0 and routes it to the combustion gas circuit 27 that routes the combustion gas Ff32 to be used in combustion in the combustor 3, and a carbon dioxide separation and capture device 60 that is provided in the combustion gas circuit 27 and separates and captures carbon dioxide contained in the combustion gas Ff32. This allows the carbon dioxide concentration Cc in the recycled gas Ff31 to be maintained high, enabling efficient separation and capture of carbon dioxide. Furthermore, since the amount of gas to be treated is less than when the entire amount of anode exhaust Ff3 is treated, an excessively large carbon dioxide separation and capture device is not required, and the power required for capture can be reduced.

[0111] By providing a flow rate regulator (first flow rate regulator 40, second flow rate regulator 41) in at least one of the recycled gas circuit 26 and the combustion gas circuit 27 to adjust the ratio (recycle ratio α) of the amount of recycled gas Ff31 to the amount of anode exhaust Ff3, the recycle ratio α can be easily adjusted.

[0112] By providing the circulation blower 33 provided upstream of the branch point 42 of the anode exhaust circuit 24, the recycled gas Ff31 can be easily mixed with the raw fuel Ff0, and the flow rate can also be easily adjusted.

[0113] Alternatively, by providing blowers (second circulation blower 35, blower 36) in each of the recycled gas circuit 26 and the combustion gas circuit 27 to adjust the ratio (recycle ratio α) of the amount of recycled gas Ff31 to the amount of anode exhaust Ff3, the recycled gas Ff31 can be easily mixed with the raw fuel Ff0, and flow rate adjustment can also be made easier.

[0114] By using steam Fs used in the reforming reaction as the driving fluid and providing an ejector 38 that sucks in raw fuel Ff0 and recycled gas Ff31 and leads them to the reformer 2, the recycled gas Ff31 can be easily mixed with raw fuel Ff0 without using a blower.

[0115] If a CO transformer 61 is provided upstream of the branch point 42 of the anode exhaust circuit 24 or between the branch point 42 and the carbon dioxide separation and capture device 60, which converts carbon monoxide into hydrogen through a shift reaction, the separation and capture efficiency of carbon dioxide can be improved and the amount of carbon dioxide emitted into the atmosphere can be reduced.

[0116] If the recycled gas Ff31 is configured to be mixed with the raw fuel Ff0 in a state containing steam in an amount corresponding to the reforming reaction, the equipment for generating steam Fs can be omitted, and the system can be simplified.

[0117] The system is equipped with a control device 80 that coordinates and controls the devices within the system, an output measuring device 81 that measures the output Po of the fuel cell stack 1, and a raw fuel flow rate regulator 82 that adjusts the flow rate of the raw fuel Ff0 (raw fuel flow rate Qf).The control device 80 is configured to adjust the flow rate (raw fuel flow rate Qf) according to the output Po so that the fuel utilization rate Uf in the fuel cell stack 1 falls within a certain range (for example, 70 to 80%) and so that the ratio of the amount of recycled gas Ff31 to the amount of anode exhaust Ff3 (recycle ratio α) does not exceed an upper limit value α0 set based on the heat balance between the heat absorbed by the reforming reaction and the heat generated by the combustor 3, thereby maximizing the carbon dioxide concentration Cc without compromising the heat balance and enabling efficient separation and recovery of carbon dioxide.

[0118] reproduction Available Energy A regeneration system that guides fuel (renewable energy fuel Fr) to at least one of the combustor 3 and the reformer 2. Available Energy By providing a fuel circuit (renewable energy fuel circuit 70), the recycle ratio α can be set without taking into consideration the constraints of the heat balance of the reformer 2, without increasing the generation of carbon dioxide.

[0119] At this time, a control device 80 that controls the devices in the system in a coordinated manner, an output measuring device 81 that measures the output Po of the fuel cell stack 1, a raw fuel flow rate regulator 82 that adjusts the flow rate of the raw fuel Ff0 (raw fuel flow rate Qf), and a regenerator. Available Energy The control device 80 is provided with a second flow regulator (at least one of a combustion renewable energy fuel flow regulator 84 and a feedstock renewable energy fuel flow regulator 85) that adjusts the flow rate (renewable energy fuel flow rate Qr) of the fuel (renewable energy fuel Fr), and if the control device 80 is configured to adjust the flow rates (raw fuel flow rate Qf and renewable energy fuel flow rate Qr) according to the output Po so that the fuel utilization rate Uf in the fuel cell stack 1 falls within a certain range, and the ratio of the amount of recycled gas Ff31 to the amount of anode exhaust Ff3 (recycle ratio α) exceeds the upper limit value α0 set based on the heat balance between the heat absorption by the reforming reaction and the heat generation by the combustor 3 (a value higher than the upper limit value α0 and equal to or less than the limit value αL), and the heat balance of the reformer 2 is achieved, the carbon dioxide concentration can be further increased and carbon dioxide can be separated and captured efficiently.

[0120] Various aspects of the present disclosure are summarized below as appendices.

[0121] (Appendix 1) a reformer that generates a reformed gas containing hydrogen by a reforming reaction between a raw fuel containing hydrocarbon and steam; a combustor for providing heat to the reformer; a fuel cell stack that generates electric energy through an electrochemical reaction between air and the reformed gas and outputs DC power; an anode exhaust circuit having a branch point branching into a recycle gas circuit that receives anode exhaust discharged from the anode of the fuel cell stack, and that leads the anode exhaust as a recycle gas to be mixed with the raw fuel and circulated, and a combustion gas circuit that leads the anode exhaust as a combustion gas to be used for combustion in the combustor; a carbon dioxide separation and capture device that is provided in the combustion gas circuit and separates and captures carbon dioxide contained in the combustion gas; A fuel cell system comprising:

[0122] (Appendix 2) The fuel cell system described in Appendix 1 is characterized in that it is provided with a flow regulator provided in at least one of the recycled gas circuit and the combustion gas circuit, which regulates the ratio of the amount of the recycled gas to the amount of the anode exhaust.

[0123] (Appendix 3) 3. The fuel cell system according to claim 1, further comprising a circulation blower provided upstream of the branch point of the anode exhaust circuit.

[0124] (Appendix 4) A fuel cell system as described in Appendix 1, characterized in that it comprises blowers provided in the recycled gas circuit and the combustion gas circuit, respectively, for adjusting the ratio of the amount of recycled gas to the amount of anode exhaust.

[0125] (Appendix 5) 3. The fuel cell system according to claim 1, further comprising an ejector that uses the steam as a driving fluid, sucks in the raw fuel and the recycled gas, and leads them to the reformer.

[0126] (Appendix 6) 6. The fuel cell system according to claim 1, further comprising a CO transformer that is provided upstream of the branch point in the anode exhaust circuit or between the branch point and the carbon dioxide capture and recovery unit and that converts carbon monoxide into hydrogen by a shift reaction.

[0127] (Appendix 7) 7. The fuel cell system according to claim 1, wherein the recycled gas is mixed with the raw fuel in a state containing water vapor in an amount corresponding to the reforming reaction.

[0128] (Appendix 8) A control device that coordinates and controls devices within the system; an output measuring device that measures the output of the fuel cell stack; and a flow rate regulator for regulating the flow rate of the raw fuel; The fuel cell system according to any one of Supplementary Note 1 to Supplementary Note 7, wherein the control device adjusts the flow rate in accordance with the output so that the fuel utilization rate in the fuel cell stack falls within a certain range and the ratio of the amount of the recycled gas to the amount of the anode exhaust gas is such that a heat balance between the heat absorption by the reforming reaction and the heat generation by the combustor can be maintained.

[0129] (Appendix 9) reproduction Available Energy A regeneration system for directing fuel to at least one of the combustor and the reformer. Available Energy 8. A fuel cell system according to any one of claims 1 to 7, comprising a fuel circuit.

[0130] (Appendix 10) A control device that coordinates and controls devices within the system; an output measuring device that measures the output of the fuel cell stack; a flow rate regulator for regulating the flow rate of the raw fuel; and The regeneration Available Energy a second flow regulator for adjusting the flow rate of fuel; The control device controls the amount of recycled gas, the flow rate of the raw fuel, and the amount of regenerated fuel in accordance with the output so that the fuel utilization rate in the fuel cell stack falls within a certain range and the heat balance of the reformer is maintained even if the ratio of the amount of recycled gas to the amount of anode exhaust exceeds an upper limit. Available Energy 10. The fuel cell system according to claim 9, wherein the flow rate of the fuel is adjusted. [Explanation of symbols]

[0131] 1: fuel cell stack, 100: fuel cell system, 1a: anode, 1c: cathode, 2: reformer, 21: raw fuel circuit, 22: fuel gas circuit, 23: reformed gas circuit, 24: anode exhaust circuit, 26: recycled gas circuit, 27: combustion gas circuit, 3: combustor, 30: mixer, 31: steam generator, 32: anode exhaust condenser, 33: circulation blower, 35: second circulation blower, 36: blower, 37: anode exhaust cooler, 38: ejector, 40: first flow rate regulator, 41: second flow rate regulator, 42: branch point, 43: branch point, 60: carbon dioxide separation and capture device, 61: CO transformer, 70: renewable energy fuel circuit, 71: combustion renewable energy fuel circuit, 72: Feedstock renewable energy fuel circuit, 80: Control device, 81: Output measuring device, 82: Raw fuel flow rate regulator, 84: Combustion renewable energy fuel flow rate regulator, 85: Feedstock renewable energy fuel flow rate regulator, 86: Steam flow rate regulator, Fa1: Air, Fa2: Cathode exhaust, Ff1: Fuel gas, Ff2: Reformed gas, Ff3: Anode exhaust, Ff31: Recycle gas, Ff32: Combustion gas, Fr: Renewable energy fuel, Fx: Combustion exhaust gas, Qf: Raw fuel flow rate, Qr: Renewable energy fuel flow rate, Uf: Fuel utilization rate, α: Recycle ratio.

Claims

1. a reformer that generates a reformed gas containing hydrogen by a reforming reaction between a raw fuel containing hydrocarbon and steam; a combustor for providing heat to the reformer; a fuel cell stack that generates electric energy through an electrochemical reaction between air and the reformed gas and outputs DC power; an anode exhaust circuit having a branch point branching into a recycle gas circuit that receives anode exhaust discharged from the anode of the fuel cell stack, and that leads the anode exhaust as a recycle gas to be mixed with the raw fuel and circulated, and a combustion gas circuit that leads the anode exhaust as a combustion gas to be used for combustion in the combustor; a carbon dioxide separation and capture device that is provided in the combustion gas circuit and separates and captures carbon dioxide contained in the combustion gas; A control device that coordinates and controls devices within the system; an output measuring device that measures the output of the fuel cell stack; and a flow rate regulator for regulating the flow rate of the raw fuel; The control device adjusts the flow rate according to the output so that the fuel utilization rate in the fuel cell stack falls within a certain range and the ratio of the amount of the recycled gas to the amount of the anode exhaust maintains a heat balance between the heat absorbed by the reforming reaction and the heat generated by the combustor.

2. 2. The fuel cell system according to claim 1, further comprising a flow rate regulator provided in at least one of the recycled gas circuit and the combustion gas circuit, for adjusting the ratio of the amount of the recycled gas to the amount of the anode exhaust.

3. 3. The fuel cell system according to claim 1, further comprising a circulation blower provided upstream of the branch point of the anode exhaust circuit.

4. 2. The fuel cell system according to claim 1, further comprising blowers provided in the recycled gas circuit and the combustion gas circuit, respectively, for adjusting the ratio of the amount of the recycled gas to the amount of the anode exhaust.

5. 3. The fuel cell system according to claim 1, further comprising an ejector that uses the steam as a driving fluid, sucks in the raw fuel and the recycled gas, and leads them to the reformer.

6. 5. The fuel cell system according to claim 1, further comprising a CO transformer that converts carbon monoxide into hydrogen by a shift reaction, the CO transformer being provided upstream of the branch point in the anode exhaust circuit or between the branch point and the carbon dioxide capture unit.

7. 6. The fuel cell system according to claim 5, further comprising a CO transformer that is provided upstream of the branch point of the anode exhaust circuit or between the branch point and the carbon dioxide separation and capture unit and that converts carbon monoxide into hydrogen by a shift reaction.

8. 5. The fuel cell system according to claim 1, wherein the recycled gas is mixed with the raw fuel in a state containing steam in an amount corresponding to the reforming reaction.

9. 7. The fuel cell system according to claim 6, wherein the recycled gas is mixed with the raw fuel in a state containing water vapor in an amount corresponding to the reforming reaction.

10. 5. The fuel cell system according to claim 1, further comprising a renewable energy fuel circuit that introduces renewable energy fuel to at least one of the combustor and the reformer.

11. A reformer that generates a reformed gas containing hydrogen by a reforming reaction between a raw fuel containing hydrocarbon and steam. a combustor for providing heat to the reformer; a fuel cell stack that generates electric energy through an electrochemical reaction between air and the reformed gas and outputs DC power; an anode exhaust circuit having a branch point branching into a recycle gas circuit that receives anode exhaust discharged from the anode of the fuel cell stack, and that leads the anode exhaust as a recycle gas to be mixed with the raw fuel and circulated, and a combustion gas circuit that leads the anode exhaust as a combustion gas to be used for combustion in the combustor; a carbon dioxide separation and capture device that is provided in the combustion gas circuit and separates and captures carbon dioxide contained in the combustion gas; a renewable energy fuel circuit that directs renewable energy fuel to at least one of the combustor and the reformer; A control device that coordinates and controls devices within the system; an output measuring device that measures the output of the fuel cell stack; a flow rate regulator for regulating the flow rate of the raw fuel; and a second flow regulator for adjusting the flow rate of the renewable energy fuel; The control device adjusts the amount of recycled gas, the flow rate of the raw fuel, and the flow rate of the renewable energy fuel according to the output so that the fuel utilization rate in the fuel cell stack falls within a certain range and the heat balance of the reformer is maintained even if the upper limit of the ratio of the amount of recycled gas to the amount of anode exhaust is exceeded.

12. A fuel cell system as described in Claim 11, characterized in that it is provided with a flow regulator provided in at least one of the recycled gas circuit and the combustion gas circuit, which adjusts the ratio of the amount of recycled gas to the amount of anode exhaust.

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