KR102729628B1 - Carbon dioxide recovery system contained in flue gas being emitting from fuel cell energy generating system - Google Patents

Abstract

The present invention discloses a carbon dioxide recovery system included in exhaust gas discharged from a fuel cell energy generation system, characterized by comprising: a fuel supply unit for supplying natural gas fuel; an air and oxygen supply unit for supplying pure oxygen from which nitrogen has been separated from air brought in from the outside to a fuel cell unit; a steam methane reformer for reforming the natural gas fuel supplied from the fuel supply unit into steam methane; and a fuel cell unit for producing electricity and heat by respectively receiving hydrogen produced by steam reforming in the steam methane reformer and pure oxygen from which nitrogen has been separated from the air and oxygen supply unit; and a carbon dioxide recovery unit for cooling the exhaust gas discharged from the fuel cell energy generation system to a predetermined temperature to separate moisture and then recovering the exhaust gas in a state changed into liquefied carbonic acid. The air and oxygen supply unit includes: an air blower for supplying air brought in from the outside to a ceramic membrane separator optimally arranged in accordance with the size of air molecules; The device comprises: a ceramic membrane separator for separating nitrogen in the air supplied from the air blower into about 60 to 70% by weight and discharging it into the atmosphere; and a VPSA for receiving air in which nitrogen has been separated from the ceramic membrane separator and has a high oxygen concentration, further separating nitrogen to produce pure oxygen having an oxygen concentration of 99% or higher, and supplying the pure oxygen to the fuel cell unit. The carbon dioxide recovery unit comprises: a cooler for cooling the exhaust gas discharged from the fuel cell stack to 5°C or lower; a first water condenser for separating condensed water according to the cooling in the cooler; a refrigerator for cooling the exhaust gas from which the condensed water has been separated in the water condenser to a constant temperature to produce liquid carbon dioxide; a second water condenser for separating condensed water according to the cooling of the exhaust gas in the refrigerator; and a liquefied carbon dioxide storage tank for storing the liquid carbon dioxide.

Description

{CARBON DIOXIDE RECOVERY SYSTEM CONTAINED IN FLUE GAS BEING EMITTING FROM FUEL CELL ENERGY GENERATING SYSTEM}

The present invention relates to a system for recovering carbon dioxide contained in exhaust gas discharged from a fuel cell energy generation system.

The present invention relates, in particular, to a system in which electricity is produced by causing an electrochemical reaction between purified hydrogen (H _{2 )} and oxygen (O ₂ ) through steam reforming of methane (CH ₄ ), the produced electric energy is separately recovered, and carbon dioxide (CO ₂ ) generated during the methane reforming reaction can be reduced.

Also, since oxygen resulting from an electrochemical reaction with hydrogen is introduced from the air, the present invention relates to a system that selectively separates nitrogen ( _N2 ) in the air using a ceramic membrane arranged with a firstly selected pore size, and then, after separating the remaining nitrogen by applying a dry adsorption column to the air with a high oxygen concentration, supplies only pure oxygen to the cathode of the fuel cell stack to drastically reduce the amount of fuel cell flue gas, thereby increasing the concentration of carbon dioxide ( _CO2 ) contained in the flue gas, thereby enabling economical capture (CCUS, Carbon Capture Utilization Storage) of liquid carbon dioxide (L- _CO2 ).

The present invention can be applied to the field of effectively recovering carbon dioxide from exhaust gas emitted from a fuel cell stack, thereby contributing to carbon neutrality, and also manufacturing carbon dioxide products by capturing carbon dioxide as liquefied carbon dioxide.

Currently commercialized fuel cells are broadly classified into five types: alkaline fuel cells (AFC), polymer electrolyte fuel cells (PEFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), and solid oxide fuel cells (SOFC), depending on the type of electrolyte.

However, fuel cells, like coal-fired power plants and combined cycle power plants, emit a lot of carbon dioxide, a major greenhouse gas, and there is an urgent need for effective technology to prevent global warming caused by this.

In response to these demands, research and commercialization of fuel cells are becoming prominent, and the foundation is being laid for developing fuel cells into either eco-friendly or non-eco-friendly fuel cells depending on the availability of raw materials.

Currently, most fuel cells are used as devices that produce hydrogen by reforming methane ( _CH4 ) with high-temperature, high-pressure steam, oxidize active fuel through an electrochemical reaction, and directly convert the chemical energy released during the oxidation process into electrical energy.

[Chemical Formula 1]

CH ₄ + 2H ₂ O → CO ₂ + 4H ₂

As shown in Chemical Formula 1, a large amount of carbon dioxide is generated in the process of producing hydrogen through a chemical reaction. Currently commercialized hydrogen fuel cells mostly emit flue gas from the fuel cell stack into the atmosphere. In other words, a large amount of carbon dioxide is emitted by including it in the flue gas.

The composition ratio of components in the air is as shown in Table 1, and the concentration of carbon dioxide in the air is approximately 400 ppm by volume.

Meanwhile, the results of measuring the carbon dioxide emitted from the composition of exhaust gas emitted from fuel cell stacks installed domestically according to distance are shown in Table 2.

As shown in Table 2, the farther the exhaust gas is discharged from the fuel cell stack, the lower the carbon dioxide concentration becomes. This is because the carbon dioxide concentration decreases as it is diluted with air. However, if we only look at this, we may misunderstand that 100% of the carbon dioxide contained in the exhaust gas discharged from the fuel cell can be discharged into the atmosphere.

However, as global warming has become more severe and carbon dioxide emissions have increased day by day, and environmental regulations have become more serious, research and development to capture the massive amount of carbon dioxide contained in fuel cell exhaust gas is actively being conducted.

Currently, domestic fuel cell manufacturers are increasing the domestic production ratio by introducing foreign technology, and securing the original technology related to the carbon dioxide recovery system, which is the main culprit of greenhouse gases, which is the purpose of the present invention, is still far away.

Korean Patent No. 10-1233297 relates to a dry carbon dioxide capture device, and discloses a technology for capturing carbon dioxide by absorbing and desorbing it by passing an existing wet solution, i.e. an amine-based solution, through which the carbon dioxide is captured, and then reusing the regenerated amine-based solution.

The above-mentioned registered technology discloses a technology for capturing carbon dioxide from exhaust gas of coal-fired power plants and combined heat and power plants by a dry method in order to improve environmental problems caused by foul odors and wastewater generated during carbon dioxide capture. However, since the breakthrough speed of the adsorbent used is fast at a high carbon dioxide composition ratio, process operation is difficult and there are many difficulties, and it is judged that there are many areas that need to be improved for commercial operation.

Korean Patent No. 10-2410689 relates to a carbon dioxide capture energy generation system and its operating method, which comprises a fuel cell that receives hydrogen and natural gas from a liquefied hydrogen storage unit to produce electricity, and captures carbon dioxide generated from the fuel cell using the cold heat of the liquefied hydrogen. Although there appears to be no technical issues with this system, since the liquefaction temperature of hydrogen is -253°C, it does not disclose a technology that can produce and liquefy hydrogen from a device that receives electricity. If the above patent liquefies hydrogen and liquefies and captures carbon dioxide using the cold heat, it would certainly be a very good technology.

Korean Patent Registration No. 10-2523875 relates to a high-concentration carbon dioxide capture and capture method for a fuel cell system. The system discloses a system that uses a nitrogen separation membrane in a reformer unit, supplies oxygen-loaded air to primarily reduce the flow rate of exhaust gas, and supplies compressed air to the fuel cell unit to use it as a source of oxygen, thereby causing an electrochemical reaction to produce electricity.

The above patent has the advantage of using a nitrogen separation membrane in the front of the reformer unit to separate nitrogen. However, since the nitrogen gas separation efficiency of the gas separation membrane is low at about 35%, and since a multi-stage gas separation membrane facility must be built, the cost of facility investment is high, so the economic value may be greatly reduced.

In addition, it is configured to secure the amount of heat required for the reforming reaction by supplying oxygen-loaded air to the reformer unit and using it as combustion air for the burner, and that is, methane used as a hydrogen source is also used as a raw material for the burner and for hydrogen production, so the amount of raw material used is greater than that of other fuel cells, which may double the generation of carbon dioxide, and the proportion of fixed costs is high, so it is judged not to be feasible in terms of economic feasibility.

Korean Patent Registration No. 10-2289495 relates to a carbon dioxide capture system using a fuel cell and a method therefor. It consists of a first electricity production unit using natural gas as fuel and a second electricity production unit using high-pressure steam, and has the advantage of being an efficient system that simultaneously recovers carbon dioxide and combusts unreacted gas and resupplies it to the fuel cell, but has the following disadvantages.

First, there is a commercial problem due to the high fixed cost of fuel, and second, there is a problem with the amount of steam generated from the fuel cell system, so a means to solve this is necessary, but it is judged that the currently commercialized fuel cell system is not operated by installing a high-temperature, high-pressure steam generator. In other words, it seems to be a technology that raises questions about whether high-temperature, high-pressure steam can be generated and a stable supply for steam power generation. In particular, the fuel cell produces electricity through an electrochemical reaction after hydrogen produced through a reforming reaction and oxygen introduced from the air are transferred to the cathode, but it is not known whether this is characterized by the absence of an oxygen introduction source, but it appears that there is no air introduction process.

Korean Patent Publication No. 10-2022-0080444 relates to a multi-stage fuel cell system and an eco-friendly power generation method. Although it has the advantage of being able to increase power generation efficiency by using a multi-stage fuel cell, it has the following disadvantages.

First, since the natural gas used as fuel is supplied as liquefied gas, additional vaporization facilities will be required, which will increase facility investment costs. In addition, even if the supplied fuel gas is vaporized in a vaporizer, the low temperature may become a factor that lowers the temperature during the reforming reaction. Second, the reforming reactor is dualized with external reforming and internal reforming reactions, which makes operation difficult. In addition, there is no mention of the steam required for the reforming reaction. Third, there are no commercial cases of fuel cells installed in multiple stages.

Korean Patent Publication No. 10-2023-0011914 relates to a steam methane reforming unit for carbon capture. Although it may have the advantage of producing maximum power by recycling unreacted gas and using it as fuel, it has the following disadvantages.

First, since methane is used as a preheating gas for the fuel cell, additional fuel costs may be incurred. Second, increased investment costs are expected due to the construction of considerably expensive facilities, such as a separate external reformer and water supplied from a separate boiler, air supplied to the compressor, and a reformed gas hydrogen PSA facility. In addition, since the hydrogen PSA facility has a hydrogen explosion limit of 4 to 74%, safety precautions must be taken. In addition, the gas generated from the steam methane reformer system is a composite gas, including carbon dioxide ( _CO2 ), hydrogen ( _H2 ), carbon monoxide (CO), methane ( _CH4 ), nitrogen ( _N2 ), and oxygen ( _O2 ), so separating pure hydrogen is quite difficult (see Table 3). Third, the combustion gas generated from the fuel cell cathode is emitted into the atmosphere with some carbon dioxide included. This can be pointed out as a problem.

Korean Patent No. 10-2143864 relates to a system for capturing carbon dioxide ( _CO2 ) from a fuel cell, which heat-exchanges air and supplies it to the cathode of a solid oxide fuel cell (SOFC), and oxygen in the air is transferred to the anode through an electrolyte, and electricity is generated by reforming natural gas fuel and reacting electrochemically with hydrogen, and unreacted gas goes through an oxidation process or a secondary reforming process to generate electricity and a purification process, and pure hydrogen is recycled to the solid oxide fuel cell. This aspect appears to be advantageous, but it has the following disadvantages.

First, the exhaust gas component that is transported to the exhaust gas line of the solid oxide fuel cell and released into the atmosphere contains about 0.08 mol.% of carbon dioxide, and second, as the hydrogen containing impurities from the anode gas oxidation (AGO) and reformer-electrolyte-purification (REP) system is provided as a system to produce electricity by flowing back into the solid oxide fuel cell, it can be seen as having a distance from commercialization in terms of electricity production efficiency due to the dual fuel cell and complexity of the process. In addition, although it is a system for capturing carbon dioxide, it can be seen as a system that releases about 5% of it into the atmosphere.

Republic of Korea Patent Registration No. 10-1233297 (registered on February 7, 2013, title: Dry carbon dioxide capture device) Republic of Korea Patent Registration No. 10-2410689 (Registered on June 15, 2022, Title: Carbon Dioxide Capture Energy Generation System and Its Operating Method) Korean Patent Registration No. 10-2523785 (Registered on April 17, 2023, Title: High-concentration carbon dioxide capture facility and capture method for fuel cell system) Republic of Korea Patent Registration No. 10-2289495 (Registered on August 6, 2021, Title: Carbon dioxide capture system using a fuel cell and method thereof) Republic of Korea Patent Publication No. 10-2022-0080444 (Published on June 14, 2022, Title: Multi-stage fuel cell system and eco-friendly power generation method) Korean Patent Publication No. 10-2023-0011914 (Published on January 25, 2023, Title: Steam methane reforming unit for carbon capture, non-methane reforming unit) Republic of Korea Patent No. 10-2143864 (registered on August 20, 2020, title: System for capturing CO2 from a fuel cell)

Estimation of Greenhouse Gas and Environmental Assessment in Co-generation Plant, Journal of Environmental Science International 23(12); 2035~2044; December 2014 Development Status of Underground Storage for CO2 Generated from Coal Power Plants, KIC News, Volume 12, No.2, 2009

The purpose of the present invention is to provide a fuel cell energy generation system, in which electricity is produced when purified hydrogen and oxygen undergo an electrochemical reaction by steam reforming of methane, the electric energy produced is separately recovered, and carbon dioxide generated during the methane reforming reaction can be reduced, and a system and method for recovering carbon dioxide contained in exhaust gas discharged therefrom.

Another object of the present invention is to provide a fuel cell energy generation system and a system and method for recovering carbon dioxide contained in exhaust gas emitted therefrom, which can reduce the amount of nitrogen contained in air used during reforming, thereby reducing the amount of exhaust gas emitted.

Another object of the present invention is to provide a fuel cell energy generation system and a carbon dioxide recovery system and method therefor, which can recover carbon dioxide in the form of liquid carbon dioxide (Liquid-CO ₂ ) through an optimal process that can reduce investment costs by simplifying the liquefaction facility and process after capturing the flue gas with an increased carbon dioxide concentration.

The technical problems to be achieved by the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

In order to achieve the above purpose, the carbon dioxide recovery system contained in the exhaust gas discharged from the fuel cell energy generation system of the present invention is
Since oxygen resulting from the electrochemical reaction with hydrogen is introduced from the air, the system selectively separates nitrogen ( _N2 ) in the air using a ceramic membrane arranged with a firstly selected pore size, and then separates the remaining nitrogen by applying a dry adsorption column to the air with a high oxygen concentration, and then supplies only pure oxygen to the anode of the fuel cell unit to reduce the amount of fuel cell flue gas, thereby increasing the concentration of carbon dioxide ( _CO2 ) contained in the flue gas, thereby enabling the economical capture of liquid carbon dioxide (L- _CO2 ).
Fuel supply unit that supplies natural gas fuel;
An air and oxygen supply unit comprising: an air blower which supplies air, which has been supplied from the outside, to a fuel cell unit after removing foreign substances from the air through a filter, firstly to a ceramic membrane separator which is optimally arranged in terms of the size of air molecules; a ceramic membrane separator which separates nitrogen in the air supplied from the air blower into 60 to 70% by weight and discharges it into the atmosphere; and a vacuum pressure swing adsorption (VPSA) system which receives air in which nitrogen has been separated from the ceramic membrane separator and has a high oxygen concentration, secondarily separates nitrogen to produce pure oxygen having an oxygen concentration of 99% or higher, stores the pure oxygen in a pure oxygen storage tank, and pressurizes the air through a booster pump to supply the pure oxygen to the fuel cell unit;
In the above VPSA system, the secondarily separated nitrogen is stored in a nitrogen storage tank and used as an inert gas (N ₂ ), and when the storage level in the nitrogen storage tank reaches the limit, it is discharged into the atmosphere through a nitrogen discharge line;
A steam methane reforming unit in which natural gas fuel supplied from the fuel supply unit is reformed into steam methane by combining methane (CH ₄ ) and water (H ₂ O) to generate hydrogen, carbon dioxide, and carbon monoxide at a reaction temperature in the range of 150 to 250° C., and the hydrogen is supplied to a fuel cell unit to convert chemical energy into electrical energy through an electrochemical reaction with pure oxygen supplied from the air and oxygen supply unit, and the condensate separated through the first water condenser and the condensate separated through the second water condenser are introduced and utilized as pure water; and
A fuel cell energy generation system comprising a fuel cell unit that receives ultra-pure hydrogen, which is generated by steam reforming in the steam methane reforming unit and then purified through a purification process, and pure oxygen from which nitrogen is separated from the air and oxygen supply unit, respectively, to produce electricity and heat; and
Cool the exhaust gas discharged from the above fuel cell unit to below 5℃ to separate moisture and then recover it in a state of being changed into liquefied carbonic acid.
A cooler that cools exhaust gas having a temperature of 110°C discharged from the fuel cell unit to 5°C or less;
A compressor for compressing exhaust gas cooled in the above cooler;
A first water condenser for separating condensate from exhaust gas compressed in the above compressor;
A refrigerator that cools the flue gas from which condensate is separated in the first moisture condenser to -20 to -30°C to produce liquefied carbonic acid;
A second water condenser for separating condensed water resulting from cooling of exhaust gas in the above refrigerator; and
It is composed of a carbon dioxide recovery unit, which is composed of a liquefied carbon dioxide storage tank that stores liquefied carbon dioxide having a purity of 99% or higher after the condensate is separated from the second moisture condenser;
A step for producing heat and electricity by receiving a substance produced by a reforming reaction of natural gas and water supplied from the above fuel supply unit and air brought in from the outside.
A step in which the fuel cell unit reforms natural gas and water supplied from the fuel supply unit;
A step in which hydrogen is purified into ultra-pure water through a purification process and then supplied to the cathode of the fuel cell unit;
A step in which hydrogen is separated into electrons and hydrogen ions (H ⁺ ) through an electrochemical reaction by a catalyst at the cathode;
A step for allowing the hydrogen ions to be transferred to the electrolyte layer of the fuel cell unit and move to the air electrode;
A step in which nitrogen is separated from air brought in from the outside, firstly, 60 to 70% by weight of nitrogen in the air is separated by a ceramic membrane separator optimally arranged according to the size of the air molecules;
A step in which nitrogen is separated from the ceramic membrane separator and air with a high oxygen concentration is supplied, and nitrogen is separated a second time through a vacuum pressure swing adsorption (VPSA) system to produce pure oxygen with an oxygen concentration of 99% or higher;
A step in which the above pure oxygen is supplied to the anode of the fuel cell unit; and
A first step comprising: a step in which pure oxygen supplied to the anode of the fuel cell unit reacts with hydrogen supplied to the cathode of the fuel cell unit to produce heat and water, and the hydrogen electrons react with oxygen electrons to produce electricity;
In the above first step, the condensed water is cooled through the cooler and refrigerator, and in the step where the condensed water is separated through the water condenser, the separated condensed water is introduced into the reforming reactor to be used as pure water; and
A step for cooling exhaust gas composed of 22.33 wt% carbon dioxide (CO2), 21.78 wt% hydrogen (H2O), 51.21 wt% oxygen (O2), and 4.68 wt% argon (Ar) emitted from the fuel cell unit by a cooler to separate condensate, change it into liquefied carbonic acid, and store it in a liquefied carbonic acid storage tank.
A step in which exhaust gas discharged from a fuel cell unit is cooled to 5℃ or lower through a cooler;
A step in which exhaust gas cooled through the above cooler is compressed by a compressor;
A step in which the condensate generated in the process of being compressed through the above compressor is separated through a water condenser;
A step of converting the flue gas from which condensate is separated in the above-mentioned water condenser into liquefied carbon dioxide through a refrigerator;
A step in which the condensate generated in the step of changing into the above liquefied carbonic acid is separated through a water condenser; and

The present invention is characterized by performing an operation including a second step comprising: a step of storing liquefied carbonic acid (L-CO ₂ ) having a purity of 99% or higher from which the condensate is separated in a liquefied carbonic acid storage tank;

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

delete

According to the carbon dioxide recovery system included in the exhaust gas discharged from the fuel cell energy generation system of the present invention, when hydrogen and oxygen purified by steam reforming methane undergo an electrochemical reaction to produce electricity, the electric energy produced is recovered separately, and carbon dioxide generated during the methane reforming reaction can be reduced, thereby contributing to preventing global warming due to greenhouse gas reduction.

That is, the concentration of carbon dioxide in the conventional exhaust gas composition is approximately 6.06% by mass, and when this is discharged into the atmosphere, carbon dioxide is detected within the range of approximately 0.4 to 0.6% (typically 400 to 600 ppm) as it is diluted with the air in the atmosphere, so it appears that there is no problem with discharging it into the atmosphere. However, if carbon dioxide continues to be discharged into the atmosphere for this reason, it will become a serious cause of global warming, and the present invention provides a carbon dioxide capture technology to contribute to preventing global warming.

In addition, since the oxygen flowing in from the air contains about 78% nitrogen, which is an unnecessary component as a flow gas, a ceramic membrane is used to remove this nitrogen component, separating nitrogen to about 60 to 90% and discharging it into the atmosphere. In addition, since separating oxygen using a general PSA facility is not suitable in terms of capacity, a VPSA facility, which is a different facility, is used to separate the remaining nitrogen, which has the characteristic of significantly reducing the amount of exhaust gas from the fuel cell stack.

In addition, by capturing the flue gas with an increased carbon dioxide concentration and recovering it in the form of liquid carbon dioxide through a liquefaction facility and process, it can be provided as a commercial profit model. That is, according to the present invention, flue gas with an increased carbon dioxide concentration of 22.33% is formed, and by capturing this flue gas and recovering more than 90% of it in the form of liquid carbon dioxide through a liquefaction facility and process, it is possible to create economic feasibility and profitability.

Other effects will be apparent to those skilled in the art from the description below.

FIG. 1 is a process flow diagram of a carbon dioxide recovery system contained in exhaust gas discharged from a fuel cell energy generation system according to an embodiment of the present invention.
FIG. 2 is a process flow diagram of a carbon dioxide recovery system contained in exhaust gas discharged from a fuel cell energy generation system according to another embodiment of the present invention.
FIG. 3 is a process flow diagram for obtaining oxygen required for an electrochemical reaction in a fuel cell stack according to another embodiment of the present invention.
Figure 4 is a process flow diagram according to another embodiment of the present invention.
FIG. 5 is a process flow diagram for recovering carbon dioxide from exhaust gas discharged from a fuel cell stack in which an electrochemical reaction with hydrogen has been completed according to another embodiment of the present invention.
Figure 6 is a diagram of the energy generation principle of a fuel cell energy generation system.

Hereinafter, based on the attached drawings, embodiments of the present invention will be described in detail so that those skilled in the art can easily practice the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein. In addition, in order to clearly describe the present invention in the drawings, parts that are not related to the description are omitted, and similar parts are assigned similar drawing reference numerals throughout the specification.

Throughout the specification, whenever a component is referred to as “including,” this does not mean that it excludes other components, but rather that it may include other components, unless otherwise specifically stated.

Hereinafter, the present invention will be described in detail with reference to the attached drawings.

FIG. 1 is a process flow diagram of a carbon dioxide recovery system included in exhaust gas discharged from a fuel cell energy generation system according to an embodiment of the present invention. FIG. 2 is a process flow diagram of a carbon dioxide recovery system included in exhaust gas discharged from a fuel cell energy generation system according to another embodiment of the present invention. FIG. 3 is a process flow diagram of obtaining oxygen required for an electrochemical reaction in a fuel cell stack according to another embodiment of the present invention. FIG. 4 is a process flow diagram according to still another embodiment of the present invention. FIG. 5 is a process flow diagram for recovering carbon dioxide from exhaust gas discharged from a fuel cell stack in which an electrochemical reaction with hydrogen has been completed according to another embodiment of the present invention. FIG. 6 is a diagram of the energy generation principle of a fuel cell energy generation system.

As illustrated in FIG. 1, a carbon dioxide recovery system included in a fuel cell exhaust gas according to an embodiment of the present invention may be configured to include a fuel (natural gas) supply unit (11), an air and oxygen supply unit (10), a steam methane reformer (SMR) (20), a fuel cell unit (30), and a carbon dioxide recovery unit (40).

The fuel cell unit (30) produces electricity and heat as shown in FIG. 6. When methane (CH ₄ ) (11) and water (H ₂ O) (22) combine in the reforming reactor (21) of the steam methane reforming unit (20) to produce hydrogen, carbon dioxide, and carbon monoxide at a reaction temperature in the range of about 150 to 250° C. as in chemical reaction formula 2 or 3, the hydrogen converts chemical energy into electrical energy through an electrochemical reaction with oxygen (8) introduced from the air and oxygen supply unit (10).

[Chemical formula 2]

CH ₄ + 2H ₂ O → CO ₂ + 4H ₂

[Chemical Formula 3]

2CH ₄ + 3H ₂ O → CO ₂ + CO + 7H ₂

[Chemical Formula 4]

CO + H ₂ O → CO ₂ + H ₂

That is, as natural gas and water (22) supplied from the fuel (natural gas) supply unit (11) undergo a reforming reaction with high temperature and high pressure steam in the reforming reactor (21), methane is generated as a reactant as shown in chemical formula 3, and carbon monoxide (CO) reacts with water and is converted into carbon dioxide (CO ₂ ) and hydrogen (H ₂ ), as shown in chemical formula 4.

The hydrogen (23) produced above is purified into ultrapure water through a purification process, and the hydrogen (24) purified in this way is supplied to the cathode (33) of the fuel cell unit (30), and is separated into hydrogen electrons and hydrogen ions (H ⁺ ) as shown in chemical formula 6 through an electrochemical reaction by a catalyst of the cathode.

The above hydrogen ions are transferred to the electrolyte layer (32) of the fuel cell unit (30) and move to the air electrode (anode) (31), and then react with oxygen (O ₂ ) as shown in chemical formula 7 to produce heat and water.

On the other hand, hydrogen electrons react with oxygen electrons to produce electricity.

[Chemical Formula 5]

2H ₂ + O ₂ → 2H ₂ O

[Chemical formula 6]

2H ² → 4H ⁺ + 4e ^-

[Chemical formula 7]

O ₂ + 4e ^- + 4H ⁺ → 2H ₂ O

In order to effectively recover carbon dioxide, a greenhouse gas, in the fuel cell system of the present invention, nitrogen flowing in from the air must be selectively separated, and secondarily, the remaining nitrogen must be additionally separated to increase the oxygen concentration to 99% or higher, and then supplied to the anode.

To solve this problem, the performance comparison results of existing commercially available oxygen generators are shown in Table 4.

Considering the characteristics of the above equipment, the cryogenic separation method lacks economic feasibility because the equipment cost is very high. Taking this into account, the PSA method with relatively low investment cost can be applied, but this method is not suitable because it has a lot of loss during oxygen recovery and its capacity is limited.

In addition, the membrane method has low separation and recovery rates, and because the recovery rate is low, it requires multi-stage equipment construction, which increases investment costs considerably, making it difficult to apply.

The present invention separates nitrogen in the air by applying a material with a special substance added thereto, compared to the three facilities above.

In order to effectively recover carbon dioxide, a greenhouse gas, after the fuel cell reaction, the first step is to supply oxygen from the air to the cathode of the fuel cell stack, and to separate nitrogen, which is unrelated to the fuel cell reaction, within the weight range of 60 to 70%, thereby reducing the capacity and facility size of the subsequent equipment.

That is, when comparing the composition of exhaust gas generated from a conventional fuel cell stack, it can be seen that the composition ratio of carbon dioxide increases from about 6.06 wt% to 22.33 wt%, and the change in the composition of fuel cell exhaust gas related to this is as shown in Table 5.

The process flow of the air and oxygen supply unit (10) in Fig. 1 proceeds as follows.

First, when air is introduced, the air introduced into the ceramic membrane separator (2) through the air blower (1) is separated into about 60 to 70% by weight of nitrogen in the air by the ceramic membrane separator (2) and discharged into the atmosphere through the atmospheric discharge line (2-1).

In this way, air with high oxygen concentration due to separation of nitrogen is introduced into VPSA (Vacuum Pressure Swing Adsorption) (3) through the transfer line (2-2), and residual nitrogen is further separated through VPSA (3), so that pure oxygen (8) with an oxygen concentration of 99% or higher can be supplied.

The above pure oxygen (8) is fed into the pure oxygen storage tank (5) through the transfer line (4) and, while stored, is stacked on the anode through the booster pump (7).

Meanwhile, nitrogen additionally separated from the above VPSA (3) is stored in a nitrogen storage tank (3-2) through a transfer line (3-1).

This nitrogen is used as an inert gas ( _N2 ) when generating electricity in fuel cells, or when there are fire or safety concerns.

In addition, when the storage level in the nitrogen storage tank (3-2) reaches the limit, it is discharged into the atmosphere through the nitrogen discharge line (3-3).

The process flow of the carbon dioxide recovery unit (40) in Fig. 1 proceeds as follows.

When the reaction in the above fuel cell stack is completed, the exhaust gas is transported through the transport line (34). Since the temperature of the exhaust gas when discharged is approximately 110°C, cooling is essential to efficiently recover carbon dioxide.

Therefore, after the exhaust gas is cooled to about 5°C or less through the cooler (35), the condensed water in the cooling process through the cooler (35) is separated as condensate through the water condenser (41), and the separated condensate is introduced into the reforming reactor (21) through the transfer line (42) and utilized as pure water.

Meanwhile, the exhaust gas from which condensate is separated in the water condenser (41) is transferred to a chiller (44) through a transfer line (43), and at this time, carbon dioxide is changed into a liquid phase in the range of -20 to -30°C and then transferred through a liquefied carbon dioxide transfer line (45).

At this time, condensation occurs again, so the condensation is separated through a water condenser (46), and unreacted methane (CH ₄ ) and hydrogen (H ₂ ) are introduced into a reforming reactor (21) through a transfer line (47) and utilized as pure water.

Afterwards, liquefied carbonic acid (L-CO ₂ ) is stored in a liquefied carbonic acid storage tank (316) through a transfer line (48).

FIG. 2, which shows another embodiment of the present invention, is different in that in FIG. 1, pure oxygen stored in a pure oxygen storage tank (5) is pressurized through a booster pump (7) and stacked on an anode (31) of a fuel cell stack, but in FIG. 2, pure oxygen stored in a pure oxygen storage tank (5) is stacked directly on an anode (31) of a fuel cell stack without going through a booster pump, and all other processes are the same as in FIG. 1.

FIG. 3 is a process flow diagram for obtaining oxygen required for an electrochemical reaction in a fuel cell stack according to another embodiment, and is different from FIG. 1 in that the filter is configured.

That is, when air flows into the filter (100), foreign substances such as fine dust contained in the air are removed through the filter (100), and the air from which the foreign substances have been removed is sucked into the air blower (102) through the transfer line (101), and the air that flows into the ceramic membrane (104) through the air transfer line (103) has nitrogen in the air separated into about 60 to 70% by weight by the ceramic membrane (104) and is discharged into the atmosphere through the air discharge line (105).

In this way, air with high oxygen concentration due to separation of nitrogen is introduced into VPSA (107) through the transfer line (106), and residual nitrogen is further separated through VPSA (107), so that pure oxygen (114) with an oxygen concentration of 99% or higher can be supplied.

The above pure oxygen (114) is introduced into the pure oxygen storage tank (112) through the transfer line (111) and, while stored, is stacked on the anode through the booster pump (113).

Meanwhile, nitrogen additionally separated from the VPSA (107) is stored in a nitrogen storage tank (109) through a transfer line (108).

This nitrogen is used as an inert gas ( _N2 ) when generating electricity in fuel cells, or when there are fire or safety concerns.

In addition, when the storage level in the nitrogen storage tank (109) reaches the limit, it is discharged into the atmosphere through the nitrogen transport line (110).

FIG. 4, which shows another embodiment of FIG. 3, is different only in that in FIG. 3, pure oxygen stored in a pure oxygen storage tank (112) is pressurized through a booster pump (113) and stacked on the anode (31) of the fuel cell stack, but in FIG. 4, pure oxygen stored in a pure oxygen storage tank (112) is stacked directly on the anode (31) of the fuel cell stack without going through a booster pump, and all other processes are the same.

FIG. 5 is a process flow diagram for recovering carbon dioxide from exhaust gas discharged from a fuel cell stack in which an electrochemical reaction with hydrogen has been completed according to another embodiment. The process flow diagram is different from FIG. 1 in that liquefied carbonic acid (L-CO ₂ ) is introduced into a purity control system through a transfer line, is controlled to have a high purity, and is then stored in a storage tank, and a compressor is configured to compress the exhaust gas that has passed through a cooler.

As described above, the flue gas is stored in a storage tank (300) and the stored flue gas is transported through a transport line (301). Since the temperature of the flue gas when discharged is approximately 110°C, cooling is essential to efficiently recover carbon dioxide.

Therefore, after the exhaust gas is cooled to about 5°C or less through the cooler (302), it is transferred to the compressor (304) through the transfer line (303).

Since the exhaust gas compressed by the compressor (304) contains condensed water during the cooling process through the cooler (302), the condensed water must be separated from the compressed exhaust gas.

Therefore, condensate is separated from the compressed exhaust gas that flows into the condensate separator (306) through the transfer line (305), and the separated condensate flows into the reforming reactor (21) through the transfer line (307) and is utilized as pure water.

Meanwhile, the flue gas from which the condensate is separated is transferred to a chiller (309) through a transfer line (308), and at this time, carbon dioxide is changed into a liquid phase in the range of about -20 to -30°C in the chiller and then transferred through a liquefied carbon dioxide transfer line (310).

At this time, condensate is generated again, so the condensate is separated through a condensate separator (311), and unreacted methane (CH ₄ ) and hydrogen (H ₂ ) are introduced into the reforming reactor (21) through a transfer line (312) and utilized as pure water.

Thereafter, liquefied carbonic acid (L-CO ₂ ) is introduced into a purity control system (314) through a transfer line (313), and the liquefied carbonic acid, whose purity has been adjusted to increase, is stored in a liquefied carbonic acid storage tank (316) through a transfer line (315).

The liquefied carbonic acid (L- _CO2 ) produced as described above has a purity of over 99%, and this product can not only contribute to industrialization by being supplied as a refrigerant in the industrial field, but can also effectively recover over 90% of carbon dioxide, the main culprit of global warming.

As described above, the detailed description of the present invention has described preferred embodiments of the present invention, but this has been described as an example of the best embodiment of the present invention and does not limit the present invention. In addition, it goes without saying that anyone with ordinary knowledge in the technical field to which the present invention belongs can make various modifications and imitations within the scope that does not depart from the technical idea of the present invention.

Therefore, the scope of the present invention is not limited to the above-described embodiments, but can be implemented in various forms of embodiments within the scope of the appended claims. In addition, it is considered that the scope of the claims of the present invention is within the scope of the description of the claims of the present invention to the extent that anyone with ordinary knowledge in the technical field to which the invention pertains can make modifications without departing from the gist of the present invention claimed in the claims.

1, 102: Air Blower 2, 104: Ceramic Membrane Separator
2-1, 3-3, 105, 110: Nitrogen discharge line 2-2: Enriched O ₂ transfer line
3, 107: VPSA system 3-1, 108: Nitrogen transfer line
3-2, 109: Nitrogen storage tank 4, 6, 111: Pure oxygen transfer line
5, 112: Pure oxygen storage tank 7, 113: Booster Pump/Vacuum Pump 8, 114: Pressurized pure oxygen transfer line 10: Air and oxygen supply section
11: Natural gas storage tank 20: Steam methane reforming unit
21: Steam methane reformer 22: Ultrapure water supply line
23: Reformed gas transfer line 24: Refined hydrogen transfer line
30: Fuel cell unit 31: Fuel cell cathode
32: Fuel cell electrolyte 33: Fuel cell anode
34: Fuel cell exhaust gas transfer line 35, 302: Cooler
40: Carbon dioxide recovery system 41, 46: Water condenser
42: Condensate transfer line 43: Purified flue gas transfer line
44: Chiller 45: Exhaust gas transfer line
47: Unreacted gas transfer line 48: Liquefied carbonic acid (L-CO ₂ ) transfer line
49: L-CO ₂ storage tank 100: Filter 101, 103: Purified air supply line
106: Purity-enhanced oxygen transfer line 111, 114: Pure oxygen transfer line
304: Compressor 305: Compressed exhaust gas transfer line
306, 311: Condensate separator 309: Refrigerator
314: Purity control system 316: Liquefied carbon dioxide storage tank

Claims (11)

delete delete delete Since oxygen resulting from the electrochemical reaction with hydrogen is introduced from the air, the system selectively separates nitrogen ( _N2 ) in the air using a ceramic membrane arranged with a firstly selected pore size, and then separates the remaining nitrogen by applying a dry adsorption column to the air with a high oxygen concentration, and then supplies only pure oxygen to the anode of the fuel cell unit to reduce the amount of fuel cell flue gas, thereby increasing the concentration of carbon dioxide ( _CO2 ) contained in the flue gas, thereby enabling the economical capture of liquid carbon dioxide (L- _CO2 ).
Fuel supply unit that supplies natural gas fuel;
An air and oxygen supply unit comprising: an air blower which supplies air, which has been supplied from the outside, to a fuel cell unit after removing foreign substances from the air through a filter, firstly to a ceramic membrane separator which is optimally arranged in terms of the size of air molecules; a ceramic membrane separator which separates nitrogen in the air supplied from the air blower into 60 to 70% by weight and discharges it into the atmosphere; and a vacuum pressure swing adsorption (VPSA) system which receives air in which nitrogen has been separated from the ceramic membrane separator and has a high oxygen concentration, secondarily separates nitrogen to produce pure oxygen having an oxygen concentration of 99% or higher, stores the pure oxygen in a pure oxygen storage tank, and pressurizes the air through a booster pump to supply the pure oxygen to the fuel cell unit;
In the above VPSA system, the secondarily separated nitrogen is stored in a nitrogen storage tank and used as an inert gas (N ₂ ), and when the storage level in the nitrogen storage tank reaches the limit, it is discharged into the atmosphere through a nitrogen discharge line;
A steam methane reforming unit in which natural gas fuel supplied from the fuel supply unit is reformed into steam methane by combining methane (CH ₄ ) and water (H ₂ O) to generate hydrogen, carbon dioxide, and carbon monoxide at a reaction temperature in the range of 150 to 250° C., and the hydrogen is supplied to a fuel cell unit to convert chemical energy into electrical energy through an electrochemical reaction with pure oxygen supplied from the air and oxygen supply unit, and the condensate separated through the first water condenser and the condensate separated through the second water condenser are introduced and utilized as pure water; and
A fuel cell energy generation system comprising a fuel cell unit that receives ultra-pure hydrogen, which is generated by steam reforming in the steam methane reforming unit and then purified through a purification process, and pure oxygen from which nitrogen is separated from the air and oxygen supply unit, respectively, to produce electricity and heat; and
Cool the exhaust gas discharged from the above fuel cell unit to below 5℃ to separate moisture and then recover it in a state of being changed into liquefied carbonic acid.
A cooler that cools exhaust gas having a temperature of 110°C discharged from the fuel cell unit to 5°C or less;
A compressor for compressing exhaust gas cooled in the above cooler;
A first water condenser for separating condensate from exhaust gas compressed in the above compressor;
A refrigerator that cools the flue gas from which condensate is separated in the first moisture condenser to -20 to -30°C to produce liquefied carbonic acid;
A second water condenser for separating condensed water resulting from cooling of exhaust gas in the above refrigerator; and
It consists of a carbon dioxide recovery unit, which is composed of a liquefied carbon dioxide storage tank that stores liquefied carbon dioxide having a purity of 99% or higher after condensate is separated from the second moisture condenser;
A step for producing heat and electricity by receiving a substance produced by a reforming reaction of natural gas and water supplied from the above fuel supply unit and air brought in from the outside.
A step in which the fuel cell unit reforms natural gas and water supplied from the fuel supply unit;
A step in which hydrogen is purified into ultra-pure water through a purification process and then supplied to the cathode of the fuel cell unit;
A step in which hydrogen is separated into electrons and hydrogen ions (H ⁺ ) through an electrochemical reaction by a catalyst at the cathode;
A step for allowing the hydrogen ions to be transferred to the electrolyte layer of the fuel cell unit and move to the air electrode;
A step in which nitrogen is separated from air brought in from the outside, firstly, 60 to 70% by weight of nitrogen in the air is separated by a ceramic membrane separator optimally arranged according to the size of the air molecules;
A step in which nitrogen is separated from the ceramic membrane separator and air with a high oxygen concentration is supplied, and nitrogen is separated a second time through a vacuum pressure swing adsorption (VPSA) system to produce pure oxygen with an oxygen concentration of 99% or higher;
A step in which the above pure oxygen is supplied to the anode of the fuel cell unit; and
A first step comprising: a step in which pure oxygen supplied to the anode of the fuel cell unit reacts with hydrogen supplied to the cathode of the fuel cell unit to produce heat and water, and the hydrogen electrons react with oxygen electrons to produce electricity;
In the above first step, the condensed water is cooled through the cooler and refrigerator, and in the step where the condensed water is separated through the water condenser, the separated condensed water is introduced into the reforming reactor to be used as pure water; and
A step for cooling exhaust gas composed of 22.33 wt% carbon dioxide (CO2), 21.78 wt% hydrogen (H2O), 51.21 wt% oxygen (O2), and 4.68 wt% argon (Ar) emitted from the fuel cell unit by a cooler to separate condensate, change it into liquefied carbonic acid, and store it in a liquefied carbonic acid storage tank.
A step in which exhaust gas discharged from a fuel cell unit is cooled to 5℃ or lower through a cooler;
A step in which exhaust gas cooled through the above cooler is compressed by a compressor;
A step in which the condensate generated in the process of being compressed through the above compressor is separated through a water condenser;
A step of converting the flue gas from which condensate is separated in the above-mentioned water condenser into liquefied carbon dioxide through a refrigerator;
A step in which the condensate generated in the step of changing into the above liquefied carbonic acid is separated through a water condenser; and
A system for recovering carbon dioxide contained in exhaust gas discharged from a fuel cell energy generation system, characterized in that it performs an operation including: a second step comprising: a step in which, after the condensate is separated, liquefied carbon _dioxide (L-CO 2 ) having a purity of 99% or higher is stored in a liquefied carbon dioxide storage tank; delete delete delete delete delete delete delete