Fuel cell system

Abstract

To provide a method for reducing power required for separation recovery by effectively applying a membrane separation method in a fuel cell system comprising a carbon dioxide separation recovery function.SOLUTION: A fuel cell system comprises: an anode recycle circuit (3) for circulating anode exhaust gas of a fuel cell stack (2); an anode branch pipe (4) branching from the anode recycle circuit (3); a compressor (5) connected with the anode branch pipe (4); and a separation membrane (6) connected with the compressor (5). The anode recycle circuit (3) increases a concentration of carbon dioxide to improve separation efficiency of the separation membrane (6) and guide part of the anode exhaust gas to the separation membrane (6), which reduces a gas flow amount guided to the separation membrane (6) as compared with that in the case that the separation membrane (6) is installed on the anode recycle circuit (3), downsizes the compressor (5) and the separation membrane (6), reduces power of the compressor (5).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a fuel cell system. [Background Art]

[0002] A fuel cell is used in a form called a stack, which is formed by stacking multiple layers of unit cells each configured with an electrolyte sandwiched between a fuel electrode (hereinafter referred to as an anode) and an air electrode (hereinafter referred to as a cathode). A fuel cell stack generates power by causing an electrochemical reaction between a reformed gas containing hydrogen obtained by reforming a hydrocarbon-based raw fuel and an oxidant, which are separated from each other via an electrolyte.

[0003] However, in addition to carbon dioxide and water vapor, the residual reformed gas discharged from the anode (anode exhaust gas) contains hydrogen and carbon monoxide that are not consumed in the electrochemical reaction and can be reused as raw materials. Therefore, it is common practice to effectively utilize the energy contained in the anode exhaust gas by combusting the anode exhaust gas in a combustor for utilization, or circulating a part of the anode exhaust gas back to the raw fuel. However, the carbon dioxide contained in the combustion exhaust gas derived from the anode exhaust gas is directly discharged into the atmosphere, which imposes a burden on the environment.

[0004] Accordingly, in a fuel cell system provided with a function of separating and recovering carbon dioxide, there is a demand for reducing recovery power and improving power generation efficiency, and a method of separating and recovering carbon dioxide in circulated anode exhaust gas using separation methods such as membrane separation, compression liquefaction, chemical absorption, and physical adsorption is disclosed (see, for example, Patent Document 1).

[0005] In the membrane separation method for separating carbon dioxide, new methods such as facilitated transport membranes and molecular gate membranes that selectively permeate carbon dioxide from a mixed gas of carbon dioxide and hydrogen have been developed and are expected to be put into practical use (see, for example, Non-Patent Document 1). [Prior Art Documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 3000118 [Non-patent literature]

[0007] [Non-Patent Document 1] Eiji Kamio, Tomohisa Yoshioka, "CO2 Separation and Recovery Technology by Membrane Separation Method in Japan," Membrane Society of Japan, "Membrane," March 4, 2017, Vol. 42, No. 1, pp. 2-10. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] To efficiently separate carbon dioxide using membrane separation, a pressure difference and concentration difference are necessary before and after passing through the separation membrane. The higher the pressure and concentration difference, the easier it is for carbon dioxide to pass through. However, in a configuration where the carbon dioxide separation device is placed on a circuit that circulates anode exhaust gas to a solid oxide fuel cell, as in Patent Document 1, a large amount of anode exhaust gas passes through the carbon dioxide separation device. Therefore, especially when separating carbon dioxide by membrane separation, there is a concern that the device will become larger due to the need to increase the diameter of the separation membrane.

[0009] This application discloses a technology to solve the above-mentioned problems, and provides a method for effectively applying a membrane separation method to a fuel cell system equipped with a carbon dioxide separation and recovery function, thereby reducing the power required for separation and recovery. [Means for solving the problem]

[0010] The fuel cell system disclosed herein is A reformer that reforms hydrocarbon-containing raw fuel to produce hydrogen-containing reformed gas, a fuel cell stack that outputs electricity through an electrochemical reaction between air and the reformed gas, and a recycled gas that receives the anode exhaust gas discharged from the anode of the fuel cell stack and mixes it with the raw fuel for recycling. In addition, carbon dioxide and water vapor in the anode exhaust gas are used in the reforming reaction of the raw fuel in the reformer.The system is characterized by comprising: an anode recycling circuit; a flow regulator provided in a gas circuit branching from the anode recycling circuit for adjusting the distribution ratio of recycled gas and anode exhaust gas flowing into the gas circuit; a compressor provided in the gas circuit for compressing the anode exhaust gas distributed by the flow regulator; and a carbon dioxide separation and recovery device having a separation membrane that separates carbon dioxide as the compressed anode exhaust gas passes through, and recovering the separated carbon dioxide. [Effects of the Invention]

[0011] According to the fuel cell system disclosed in this application, by installing the separation membrane in an anode branch pipe to which a portion of the anode exhaust gas is branched, rather than on the anode recycling circuit where the anode exhaust gas circulates, it is possible to reduce the gas flow rate, miniaturize the compressor and separation membrane, and reduce the power consumption of the compressor, compared to when it is installed on the anode recycling circuit. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram of the fuel cell system according to Embodiment 1. [Figure 2] This is a schematic diagram of the fuel cell system according to Embodiment 2. [Figure 3] This is a schematic diagram of the fuel cell system according to Embodiment 3. [Modes for carrying out the invention]

[0013] Hereinafter, preferred embodiments of the fuel cell system according to the present application will be described with reference to the drawings. The same parts and components are denoted by the same reference numerals, and their detailed descriptions are omitted. Similarly, in subsequent embodiments, redundant descriptions of components denoted by the same reference numerals are omitted.

[0014] Embodiment 1. The fuel cell system according to Embodiment 1 will be described with reference to FIG. 1. The fuel cell system 100 according to the present embodiment includes a part A that performs power generation and carbon dioxide concentration, and a part B that performs carbon dioxide separation.

[0015] <Configuration and Operation of Part A for Power Generation and Carbon Dioxide Concentration> The main components of part A that performs power generation and carbon dioxide concentration are the reformer 1, the fuel cell stack 2, and the anode recycle circuit 3.

[0016] <Reformer> The reformer 1 is a gas conversion device filled with a reforming catalyst, which receives heat from a combustor to cause a reforming reaction that decomposes hydrocarbons in raw fuel, thereby generating a reformed gas containing hydrogen and carbon monoxide. The raw fuel is city gas or biogas, and steam.

[0017] The reformer 1 converts raw fuel into hydrogen and carbon monoxide through the chemical equilibrium reaction represented by the following formula (1), and supplies the converted product to the fuel cell stack 2. The reformer 1 also utilizes high-concentration carbon dioxide in the anode recycle circuit 3 through the reaction of formula (2) to convert raw fuel into hydrogen and carbon monoxide. CH4+H2O ⇔ CO+3H2 (1) CH4+CO2⇔ 2CO+2H2 (2)

[0018] <Fuel Cell Stack> The fuel cell stack 2 is formed by stacking cells each including an anode, a cathode, and an electrolyte between both electrodes. The fuel cell stack 2 generates power by consuming hydrogen and carbon monoxide in the reformed gas generated by the reformer 1. Steam and carbon dioxide are added to the residual intake hydrogen and carbon monoxide, and the mixture is discharged as anode exhaust gas.

[0019] The fuel cell stack 2 generates electricity through an electrochemical reaction between hydrogen and carbon monoxide, which are fuel components in the reformed gas produced in the reformer 1, and oxygen in the air. Specifically, some of the hydrogen and carbon monoxide contained in the reformed gas intake are converted into water vapor and carbon dioxide through the electrochemical reactions shown in equations (3) and (4) below. The fuel cell stack 2 discharges the water vapor and carbon dioxide produced by the reaction, along with the hydrogen and carbon monoxide not used in the reaction, as anode exhaust gas to the anode recycling circuit 3. H2 + O 2- → H2O + 2e - (3) 2CO+O 2- → 2CO2 + 2e - (4)

[0020] The fuel cell stack 2 operates at, for example, 600°C to 700°C, and high-temperature anode exhaust gas is discharged from the anode and high-temperature cathode exhaust gas from the cathode, and the heat is utilized on the downstream side.

[0021] <Anode recycling circuit> The anode recycling circuit 3 is a piping circuit that guides the anode exhaust gas from the fuel cell stack 2 to the reformer 1. This allows the anode exhaust gas to be circulated.

[0022] The anode recycling circuit 3 circulates the anode exhaust gas, which consists of hydrogen, carbon monoxide, water vapor, and carbon dioxide, and improves power generation efficiency by reusing (recycling) the hydrogen remaining in the anode exhaust gas in the fuel cell stack 2.

[0023] Furthermore, by utilizing the anode recycling circuit 3 and continuing the circulation while mixing the anode exhaust gas with the amount of raw fuel required for power generation, hydrogen and carbon monoxide are converted into water vapor and carbon dioxide during power generation by the fuel cell stack 2, and the concentration of water vapor and carbon dioxide in the circulating gas increases.

[0024] Furthermore, the anode recycling circuit 3 allows the carbon dioxide and water vapor contained in the anode exhaust gas to be utilized in the reformer 1, eliminating the need for water vapor addition. This reduces the amount of water vapor in the raw fuel, thereby reducing the amount of water vapor reaching the fuel cell stack 2, lowering the water vapor partial pressure in the anode and improving the power generation efficiency of the fuel cell stack 2.

[0025] Furthermore, it is also possible to configure the system so that the function of reformer 1 is performed by the fuel cell stack 2, thereby eliminating the need for reformer 1.

[0026] <Configuration and operation of part B, which separates carbon dioxide> The main components of section B, which separates carbon dioxide, consist of three parts: an anode branch pipe 4, a compressor 5, and a carbon dioxide separation and recovery device 6 having a separation membrane 6a.

[0027] <Anode branch pipe> The anode branch pipe 4 is a conduit that connects the anode recycling circuit 3 and the compressor 5. It directs a portion of the anode exhaust gas from the anode recycling circuit 3 to the compressor 5.

[0028] The anode branch pipe 4 carries not the entire amount of gas flowing through the anode recycling circuit 3, but only a portion of the anode exhaust gas that branches off from the anode recycling circuit 3. Therefore, compared to the case where the compressor 5 and separation membrane 6a are installed on the anode recycling circuit 3, a smaller amount of gas flows to the compressor 5 and separation membrane 6a. In other words, it becomes possible to miniaturize the compressor 5 and separation membrane 6a and reduce power consumption.

[0029] <Carbon dioxide separation and recovery system using separation membranes> The carbon dioxide separation and recovery device 6 is installed in the anode branch pipe, and by separating and recovering carbon dioxide here, it is possible to reduce the emission of carbon dioxide outside the system. Because the carbon dioxide in the anode exhaust gas is concentrated to a high concentration by the anode recycling circuit 3, the separation and recovery of carbon dioxide becomes relatively easy. The separation of carbon dioxide is performed by a separation membrane 6a that has the function of selectively allowing carbon dioxide to pass through.

[0030] Generally, the separation of carbon dioxide is easier and more advantageous when the carbon dioxide concentration in the gas to be separated is high. When separating carbon dioxide using membrane separation with separation membrane 6a, a compression process using compressor 5, as described below, is required. In this case, 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 achieved relatively easily.

[0031] The separated gas, from which carbon dioxide has been separated in the carbon dioxide separation and recovery device 6, may be sent to a combustor that heats the reformer 1 and used to superheat the reformer 1.

[0032] <Compressor> Compressor 5 is a gas compressor that compresses the anode exhaust gas branched off by the anode branch pipe 4. It increases the pressure of the anode exhaust gas to the pressure required by the separation membrane 6a.

[0033] The anode exhaust gas that flows into the compressor 5 is compressed and supplied to the separation membrane 6a. Because the concentration of carbon dioxide in the anode exhaust gas is increased by section A, which performs power generation and carbon dioxide concentration, carbon dioxide can easily pass through the separation membrane 6a, and the anode exhaust gas is separated into separated gas and carbon dioxide with high separation efficiency.

[0034] A flow regulator may be provided in at least one of the anode recycling circuit 3 and the anode branch pipe 4 to adjust the distribution ratio between the recycled gas reused in the fuel cell stack 2 and the anode exhaust gas flowing into the anode branch pipe 4. By adjusting this flow regulator, it is possible to set the proportion of anode exhaust gas that is recycled as recycled gas (recycling ratio) to any desired value.

[0035] For example, if you want to set the recycling ratio to 70%, you should adjust the flow rate ratio of the recycled gas to the gas flowing into the anode branch pipe 4 to a ratio of 7:3.

[0036] Based on the above, according to the example configuration of the fuel cell system 100 shown in Figure 1, (1) The anode exhaust gas is recycled and circulated by the anode recycling circuit 3, and hydrogen and carbon monoxide are consumed in the fuel cell to increase the carbon dioxide concentration in the anode exhaust gas, thereby increasing the separation efficiency at the separation membrane 6a and making it easier for carbon dioxide to pass through the separation membrane. (2) Furthermore, by installing the compressor 5 and separation membrane 6a in the anode branch pipe 4 branched off from the recycling circuit, the amount of gas sent to the compressor 5 and separation membrane 6a can be reduced compared to the case where the separation membrane 6a is installed on the anode recycling circuit 3, thereby enabling miniaturization of the carbon dioxide separation and recovery device 6 having the separation membrane 6a and the compressor 5, as well as a reduction in power consumption.

[0037] Embodiment 2. The fuel cell system according to Embodiment 2 will be described with reference to Figure 2. The fuel cell system 200 according to Embodiment 2 has a configuration in which a turbine 7 is added to the fuel cell system 100 of Embodiment 1. The same configuration and operation as in Embodiment 1 will be described below.

[0038] Turbine 7 is a power recovery turbine that recovers power from the pressure of the separated gas. The separated gas, which has been compressed by compressor 5 and separated from carbon dioxide by separation membrane 6a, has high pressure. The pressure of the separated gas drives turbine 7, and power is recovered. The recovered power can be used in compressor 5 by converting it into electricity using a generator, etc.

[0039] Alternatively, instead of the turbine 7, a turbocharger may be installed to pressurize the anode exhaust gas passing through the anode branch pipe 4 using the recovered power, thereby reducing the power required for the compressor 5.

[0040] As described above, the fuel cell system according to Embodiment 2 can reduce the power required for carbon dioxide separation and recovery by installing a power recovery turbine that recovers the pressure of the separated gas as power.

[0041] Embodiment 3. The fuel cell system according to Embodiment 3 will be described with reference to Figure 3. The fuel cell system 300 according to Embodiment 3 has a configuration in which a heat exchanger 8 is added to the fuel cell system 200 of Embodiment 2. The same configuration and operation as in Embodiments 1 and 2 will be described below.

[0042] The heat exchanger 8 is a gas cooler that uses the cold energy of the separated gas, which has been cooled by the power recovered by the turbine 7, to cool the gas in the anode branch pipe 4. By driving the turbine 7 and recovering power, the temperature of the separated gas decreases. The cold energy of the cooled separated gas is used to cool the gas in the anode branch pipe 4, thereby reducing the power required by the compressor 5.

[0043] As described above, according to the fuel cell system of Embodiment 3, the gas in the anode branch pipe can be cooled using the cold energy of the gas cooled by the turbine, thereby reducing the power of the compressor.

[0044] Although this application describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a particular embodiment, but can be applied individually or in various combinations to the embodiments. Accordingly, countless variations not illustrated are conceivable within the scope of the technology disclosed herein. These include, for example, modifications, additions, or omissions of at least one component, as well as the extraction of at least one component and its combination with components of other embodiments. [Explanation of Symbols]

[0045] 1: Reformer, 2: Fuel cell stack, 3: Anode recycling circuit, 4: Anode branch pipe, 5: Compressor, 6: Carbon dioxide separation and recovery device, 6a: Separation membrane, 7: Turbine, 8: Heat exchanger.

Claims

1. A reformer that generates a hydrogen-containing reformed gas by reforming raw fuel containing hydrocarbons, A fuel cell stack that outputs electricity through an electrochemical reaction between air and the reformed gas, An anode recycling circuit that receives the anode exhaust gas discharged from the anode of the fuel cell stack, mixes it with the raw fuel and guides it back into the recycled gas, and utilizes the carbon dioxide and water vapor in the anode exhaust gas for the reforming reaction of the raw fuel in the reformer, A flow regulator is provided in a gas circuit branching off from the anode recycling circuit, which adjusts the distribution ratio between the recycled gas and the anode exhaust gas flowing into the gas circuit. A compressor provided in the gas circuit for compressing the anode exhaust gas distributed by the flow regulator, A carbon dioxide separation and recovery device has a separation membrane through which the anode exhaust gas compressed by the compressor passes, and recovers the separated carbon dioxide. A fuel cell system characterized by having the following features.

2. The fuel cell system according to claim 1, further comprising a turbine for recovering power from the pressure of the separated gas after the carbon dioxide has been separated by the carbon dioxide separation and recovery device.

3. The fuel cell system according to claim 2, characterized in that the power recovered by the turbine and the cooled separated gas are supplied to a heat exchanger that cools the branched gas circuit.

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