Ammonia generation system and ammonia generation method

Abstract

An ammonia generation system (1) is provided with: an electrolysis cell (10) which generates ammonia from nitrogen and water, and is provided with a solid electrolyte (13) that contains a phosphoric acid compound which is a proton conductor; an ammonia separator (40) which is provided in a cathode outlet-side flow path (21) and separates ammonia from the cathode off gas; and a cathode-side circulation flow path (60) which is connected to the ammonia separator (40), and which returns, to the cathode (12), the residual gas that remains after the separation of ammonia by means of the ammonia separator (40) and contains nitrogen and hydrogen.

Description

Ammonia production system and ammonia production method

[0001] The present disclosure relates to an ammonia production system and method.

[0002] The use of ammonia as a fuel is being promoted because it does not emit carbon dioxide, which causes global warming, when burned. Conventionally, ammonia has been known to be produced using the Haber-Bosch process, but this process requires the reaction of nitrogen and hydrogen at high temperature and pressure, and the energy loss associated with pressurizing the raw material gas increases the energy consumption rate.

[0003] Furthermore, the Haber-Bosch process requires a reaction temperature of 400°C or higher to activate the catalyst. However, because the ammonia synthesis reaction is an exothermic reaction, the temperature inside the reactor may rise to 600°C or higher due to the heat of reaction, and a reaction temperature of less than 400°C is more efficient due to reaction equilibrium.

[0004] On the other hand, it is known to synthesize ammonia by electrolysis using water and nitrogen as raw materials. Patent Document 1 discloses a method in which ammonia is synthesized by electrolysis, and then the resulting product gas is treated with an ammonia separation membrane or ammonia PSA to separate it into high-concentration ammonia and a residual gas. In this method, the residual gas is recycled as a nitrogen gas raw material for the ammonia synthesis reactor. In addition, the high-concentration ammonia gas recovered by the ammonia separation membrane or ammonia PSA is further liquefied, and the unliquefied gas separated from the liquefied ammonia is treated again with an ammonia separation membrane or ammonia PSA.

[0005] International Publication No. 2017 / 149718

[0006] In conventional ammonia production processes, ammonia is synthesized at room temperature and atmospheric pressure by electrolysis using a proton exchange membrane or anion exchange membrane. Ammonia is synthesized using electrolysis, which does not substantially produce hydrogen during ammonia synthesis, and this is combined with membrane separation and PSA ammonia separation and recovery processes to synthesize and recover high-concentration ammonia with high efficiency throughout the entire process. However, because the conventional ammonia production processes synthesize ammonia at room temperature and atmospheric pressure, there is a risk that a sufficient reaction rate may not be achieved.

[0007] Therefore, an object of the present disclosure is to provide an ammonia production system and an ammonia production method that can increase the electrolysis temperature and reduce the ammonia consumption rate by recycling cathode off-gas as a feedstock for the electrolytic cell.

[0008] The ammonia production system according to the present disclosure includes an electrolytic cell including an anode, a cathode, and a solid electrolyte disposed between the anode and the cathode and containing a phosphate compound as a proton conductor, and for producing ammonia from nitrogen and water. The ammonia production system includes a cathode inlet-side flow path for supplying a cathode feedstock containing nitrogen to the cathode, and a cathode outlet-side flow path through which a cathode off-gas discharged from the cathode and containing ammonia, nitrogen, and hydrogen flows. The ammonia production system includes an ammonia separator provided in the cathode outlet-side flow path for separating ammonia from the cathode off-gas. The ammonia production system includes an anode inlet-side flow path for supplying an anode feedstock containing water to the anode, and an anode outlet-side flow path through which anode off-gas discharged from the anode and containing water and oxygen flows. The ammonia production system includes a cathode circulation flow path connected to the ammonia separator and returning the remaining gas containing nitrogen and hydrogen remaining after ammonia is separated in the ammonia separator to the cathode.

[0009] In the electrolytic cell, hydrogen ions that have permeated the solid electrolyte may be provided to the cathode.

[0010] The ammonia production system may include a heat exchanger that exchanges heat of the cathode feedstock in the cathode inlet-side flow path with heat of the cathode off-gas in the cathode outlet-side flow path, and a hydrogen separator that is provided in the cathode outlet-side flow path and separates hydrogen from the cathode off-gas.

[0011] The electrolytic cell may include a hydrogen separation membrane provided between the solid electrolyte and the cathode, which converts hydrogen ions that have permeated the solid electrolyte into hydrogen atoms and provides the hydrogen atoms to the cathode.

[0012] The ammonia production system may include a heat exchanger that exchanges heat between the cathode feedstock in the cathode inlet-side flow path and the cathode off-gas in the cathode outlet-side flow path. The ammonia separator may be a separator that liquefies and separates ammonia contained in the cathode off-gas cooled by the heat exchanger.

[0013] The ammonia separator may be a membrane separator. The ammonia production system may include a heat exchanger that exchanges heat between the cathode feedstock in the cathode inlet-side flow path and the remainder gas in the cathode-side circulation flow path. The ammonia production system may include a compressor that is provided in the cathode-side circulation flow path and compresses the remainder gas cooled by the heat exchanger and returns it to the cathode inlet.

[0014] The ammonia production system may include a water separator provided in the anode outlet side flow path and configured to separate water from the anode off-gas, and an anode side circulation flow path connected to the water separator and configured to return the water separated by the water separator to the anode.

[0015] The ammonia production method according to the present disclosure produces ammonia in an electrolytic cell including an anode, a cathode, and a solid electrolyte disposed between the anode and the cathode and containing a phosphate compound as a proton conductor. The ammonia production method includes the steps of: supplying a cathode feedstock containing nitrogen to the cathode through a cathode inlet-side flow path; and supplying an anode feedstock containing water to the anode through an anode inlet-side flow path. The ammonia production method includes the steps of producing ammonia from the nitrogen contained in the cathode feedstock and the water contained in the anode feedstock in the electrolytic cell. The ammonia production method includes the steps of flowing an anode off-gas discharged from the anode and containing water and oxygen through an anode outlet-side flow path; and flowing a cathode off-gas discharged from the cathode and containing ammonia, nitrogen, and hydrogen through a cathode outlet-side flow path. The ammonia production method includes the step of separating ammonia from the cathode off-gas in an ammonia separator provided in the cathode outlet-side flow path. The method for producing ammonia includes a step of returning a residual gas containing nitrogen and hydrogen remaining after ammonia is separated in the ammonia separator to the cathode in a cathode-side circulation flow path connected to the ammonia separator.

[0016] According to the present disclosure, it is possible to provide an ammonia production system and an ammonia production method that can increase the electrolysis temperature and reduce the ammonia consumption rate by recycling the cathode off-gas as a feedstock for the electrolytic cell.

[0017] Figure 1 is a schematic diagram showing an ammonia production system according to one embodiment. Figure 2 is a schematic diagram showing an electrolytic cell according to one embodiment. Figure 3 is a schematic diagram showing an ammonia production system according to one embodiment. Figure 4 is a schematic diagram showing an electrolytic cell according to one embodiment. Figure 5 is a schematic diagram showing an ammonia production system according to one embodiment.

[0018] Hereinafter, several exemplary embodiments will be described with reference to the drawings. Note that the dimensional proportions of the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0019] [First Embodiment] An ammonia production system 1 according to this embodiment includes an electrolytic cell 10, a cathode inlet-side flow path 20, a cathode outlet-side flow path 21, a heater 23, a heat exchanger 24, a hydrogen separator 27, and an ammonia separator 40. The ammonia production system 1 also includes an anode inlet-side flow path 50, an anode outlet-side flow path 51, a heat exchanger 52, a heater 53, a cooler 54, a water separator 55, an anode-side circulation flow path 56, a cathode-side circulation flow path 60, and a compressor 61.

[0020] As shown in FIG. 2 , the electrolytic cell 10 includes an anode 11, a cathode 12, a solid electrolyte 13, an anode current collector 14, a cathode current collector 15, a power source 16, a seal material 17, and a separator 18. The electrolytic cell 10 produces ammonia from nitrogen and water. As described below, the electrolytic cell 10 uses a solid electrolyte 13 containing a phosphate compound, and therefore the electrolytic cell 10 can be operated at a lower temperature and pressure than the Haber-Bosch process. The reaction temperature of the electrolytic cell 10 may be, for example, 150° C. to 350° C. The reaction pressure of the electrolytic cell 10 may be, for example, 0.1 MPa to 5.1 MPa.

[0021] Water (water vapor) is oxidized to generate hydrogen ions and oxygen at the anode 11. The anode 11 may contain at least one metal component selected from the group consisting of Pt and Ir as a catalyst to promote the oxidation of water vapor.

[0022] In the cathode 12, nitrogen is reduced with hydrogen to produce ammonia. The cathode 12 may contain, as an electrode catalyst, an ammonia synthesis catalyst having nitrogen direct reduction performance. The cathode 12 may contain, as an electrode catalyst, an ammonia synthesis catalyst containing at least one metal component of Fe and Co, for example. The ammonia synthesis catalyst may be an oxide containing the metal component.

[0023] The solid electrolyte 13 is disposed between the anode 11 and the cathode 12. The solid electrolyte 13 contains a phosphate compound, which is a proton conductor. The solid electrolyte 13 is permeable to hydrogen ions generated in the anode 11. The solid electrolyte 13 contains a phosphate compound, which can achieve good proton conductivity in the temperature range of 150°C to 350°C. The phosphate compound is, for example, MH ２ P.O. ４ Here, M may be Na, K, Rb or Cs. Such a phosphate compound can suppress the generation of hydrogen molecules during electrolysis and improve the current efficiency of ammonia synthesis. From the viewpoint of proton conductivity, the phosphate compound is preferably CsH ２ P.O. ４ It is preferred that the compound contains:

[0024] The solid electrolyte 13 may include a carrier that supports the phosphate compound. By supporting the phosphate compound on a carrier, the structural strength of the solid electrolyte 13 can be improved. The carrier has high compatibility with the phosphate compound, and therefore, for example, SiP ２ O ７ The solid electrolyte 13 having a phosphate compound supported on a carrier can be obtained, for example, by mixing a powdered phosphate compound with a powdered carrier and compressing the mixture. Alternatively, the solid electrolyte 13 having a phosphate compound supported on a carrier can be obtained, for example, by impregnating a porous carrier with an aqueous solution of the phosphate compound in water and drying the aqueous solution.

[0025] The anode current collector 14 is provided on the opposite side of the anode 11 from the solid electrolyte 13. The anode current collector 14 is electrically connected to the anode 11, and electrons are supplied from the anode 11 to the power source 16 via the anode current collector 14. The anode current collector 14 may have gas diffusibility, so that water vapor supplied to the anode 11 may diffuse through the anode current collector 14, and oxygen generated at the anode 11 may pass through the anode current collector 14 and be discharged to the outside of the electrolytic cell 10. The anode current collector 14 may be a porous material that diffuses water vapor and supplies it to the anode 11. The anode current collector 14 may contain at least one conductive material selected from the group consisting of carbon, titanium, nickel, and iron. The surface of the anode current collector 14 may be coated with a corrosion inhibitor to prevent corrosion. The anode current collector 14 may be, for example, iron whose surface is coated with a metal having a more noble potential than iron, such as gold or silver.

[0026] The cathode current collector 15 is provided on the opposite side of the cathode 12 from the solid electrolyte 13. The cathode current collector 15 is electrically connected to the cathode 12, and electrons are supplied to the cathode 12 from the power source 16 via the cathode current collector 15. The cathode current collector 15 may have gas diffusibility, so that nitrogen supplied to the cathode 12 diffuses through the cathode current collector 15, and ammonia generated in the cathode 12 may pass through the cathode current collector 15 and be discharged to the outside of the electrolytic cell 10. The cathode current collector 15 may be a porous material that diffuses nitrogen and supplies it to the cathode 12. The cathode current collector 15 may contain at least one conductive material selected from the group consisting of carbon, titanium, nickel, and iron. The surface of the cathode current collector 15 may be coated with a corrosion inhibitor to prevent corrosion. The cathode current collector 15 may be, for example, iron whose surface is coated with a metal having a more noble potential than iron, such as gold or silver.

[0027] The electrolytic cell including the anode 11, cathode 12, solid electrolyte 13, anode current collector 14, and cathode current collector 15 is sealed with a sealant 17, and both ends of the electrolytic cell are sandwiched between separators 18. In this embodiment, an example is described in which the electrolytic cell 10 includes a single electrolytic cell. However, the electrolytic cell 10 may include a stack that includes multiple electrolytic cells, where the multiple electrolytic cells are electrically connected in series and stacked.

[0028] In the electrolytic cell 10 according to this embodiment, hydrogen ions that have permeated the solid electrolyte 13 are supplied to the cathode 12. With this configuration, the hydrogen ions receive electrons supplied to the cathode 12 and combine with nitrogen adsorbed in molecular form on the cathode 12 to produce ammonia. In this manner, nitrogen molecules and hydrogen atoms combine at the cathode 12 to produce ammonia molecules, and cathode off-gas containing ammonia gas is discharged from the electrolytic cell 10. Because the supply of hydrogen ions depends on the current load, the ammonia synthesis reaction at the cathode 12 proceeds without being restricted by the thermal equilibrium of the gas phase at the cathode 12. Therefore, ammonia can be produced with low energy.

[0029] The cathode inlet-side flow path 20 supplies a cathode feedstock containing nitrogen to the cathode 12. The cathode outlet-side flow path 21 carries cathode off-gas discharged from the cathode 12 and containing ammonia, nitrogen, and hydrogen. The cathode inlet-side flow path 20 is provided with a nitrogen supply unit 22, a heater 23, and a heater 25. The cathode outlet-side flow path 21 is provided with a cooler 26, a hydrogen separator 27, and an ammonia separator 40.

[0030] The nitrogen supply unit 22 supplies a cathode feedstock containing nitrogen to the cathode 12 via the cathode inlet-side flow path 20. The nitrogen supply unit 22 may separate nitrogen from air and supply the nitrogen. The nitrogen supply unit 22 may be a PSA (Pressure Swing Adsorption) nitrogen separator, a TSA (Thermal Swing Adsorption) nitrogen separator, a PTSA (Pressure and Thermal Swing Adsorption) nitrogen separator, or a cryogenic separator. The nitrogen supply unit 22 may determine an air separation operation method depending on the supply pressure of the nitrogen gas to be supplied. The supply pressure of the nitrogen gas may be 0.1 MPa to 5.1 MPa.

[0031] The heater 23 heats the cathode feedstock when the electrolytic cell 10 is started up. Heating the cathode feedstock with the heater 23 can raise the temperature of the cathode feedstock to near the operating temperature of the electrolytic cell 10, thereby improving the efficiency of ammonia production. The heater 23 may heat and keep the cathode feedstock warm by an electric heater or heated air. The heater 23 may heat the cathode feedstock not only when the electrolytic cell 10 is started up, but also when the electrolytic cell 10 is in operation. As described below, the cathode feedstock can be heated by the heater 25 when the electrolytic cell 10 is in operation. However, when the power consumption of the electrolytic cell 10 is low, such as several kW to several tens of kW, the cathode feedstock may also be heated by the heater 23 in consideration of heat dissipation.

[0032] The heat exchanger 24 exchanges heat between the cathode feedstock in the cathode inlet-side flow path 20 and the cathode off-gas in the cathode outlet-side flow path 21. Since the electrolytic cell 10 that has reached a steady state becomes a heat generating body, the energy efficiency of the entire ammonia production system 1 can be improved by exchanging heat between the cathode feedstock in the cathode inlet-side flow path 20 and the cathode off-gas in the cathode outlet-side flow path 21. In this embodiment, the heat exchanger 24 includes a heater 25 and a cooler 26, and exchanges heat between the heater 25 and the cooler 26. The heater 25 heats the cathode feedstock in the cathode inlet-side flow path 20. The cooler 26 cools the cathode off-gas in the cathode outlet-side flow path 21.

[0033] The hydrogen separator 27 is provided in the cathode outlet-side flow path 21 and separates hydrogen from the cathode off-gas cooled by the heat exchanger 24. By reducing the amount of hydrogen contained in the cathode off-gas using the hydrogen separator 27, it is possible to produce a high-concentration ammonia gas. This allows the ammonia cooling temperature to be set higher when liquefying and separating ammonia, and reduces the power required to compress ammonia when compressing and liquefying it. This reduces the load on the ammonia separator 40. Furthermore, by separating hydrogen from the cathode off-gas, it is possible to reduce the amount of hydrogen supplied to the cathode 12 through the cathode-side circulation flow path 60. This prevents an excessive amount of hydrogen from being adsorbed onto the catalyst in the cathode 12, which would otherwise reduce the reaction efficiency.

[0034] In this embodiment, the hydrogen separator 27 is provided downstream of the electrolytic cell 10 and downstream of the cooler 26, and separates hydrogen from the cathode off-gas cooled by the heat exchanger 24. However, the hydrogen separator 27 may also be provided downstream of the electrolytic cell 10 and upstream of the cooler 26, and separates hydrogen from the cathode off-gas before it is cooled by the heat exchanger 24.

[0035] The hydrogen separator 27 may include a hydrogen separation membrane. The hydrogen separation membrane may be a metal membrane, an inorganic membrane, or a polymer membrane. The metal membrane may be a palladium membrane, a vanadium membrane, or an alloy membrane containing these metals. The inorganic membrane may be a zeolite membrane, a silica membrane, a zirconia membrane, an alumina membrane, or a titania membrane. The polymer membrane may be a polyimide membrane or a polysulfone membrane.

[0036] The ammonia separator 40 is provided in the cathode outlet-side flow path 21 and separates ammonia from the cathode off-gas. The ammonia separator 40 can recover the ammonia produced at the cathode 12. In this embodiment, the ammonia separator 40 is a separator that liquefies and separates the ammonia contained in the cathode off-gas cooled by the heat exchanger 24, but ammonia may also be separated using a membrane separator, which will be described later.

[0037] The anode inlet side flow path 50 supplies an anode feedstock containing water to the anode 11. The anode outlet side flow path 51 carries anode off-gas discharged from the anode 11 and containing water and oxygen. The anode inlet side flow path 50 is provided with a heat exchanger 52 and a heater 53. The anode outlet side flow path 51 is provided with the heat exchanger 52, a cooler 54, and a water separator 55. In this embodiment, an example will be described in which liquid water is supplied to the heat exchanger 52 as feed water.

[0038] The heat exchanger 52 heats liquid water. By heating water with the heat exchanger 52, the temperature of the water can be raised to approximately the operating temperature of the electrolytic cell 10. The heat exchanger 52 may heat liquid water to generate steam. The heat exchanger 52 exchanges heat between the anode feedstock in the anode inlet side flow path 50 and the anode off-gas in the anode outlet side flow path 51. Note that in order to prevent condensation of steam in the anode inlet side flow path 50, the anode feedstock may contain nitrogen gas, oxygen gas, or air as a purge gas.

[0039] The heater 53 heats the anode feedstock when the electrolytic cell 10 is started up. The heater 53 may be used in combination with the heat exchanger 52 to heat water contained in the anode feedstock to generate steam. The heater 53 may use an electric heater or heated air as a heat source.

[0040] The water separator 55 is provided in the anode outlet-side flow path 51 and separates water from the anode off-gas. The anode-side circulation flow path 56 is connected to the water separator 55 and returns the water separated by the water separator 55 to the anode 11. By separating the water contained in the anode off-gas and returning it to the anode 11, the water can be recycled and effectively utilized.

[0041] In this embodiment, the water separator 55 is a gas-liquid separator that separates oxygen from water cooled and condensed by the heat exchanger 52 and the cooler 54. Note that the cooler 54 is provided to increase the water utilization rate, but the cooler 54 does not have to be provided if water can be recovered. The anode-side circulation flow path 56 is connected to the anode outlet-side flow path 51 upstream of the heat exchanger 52 and the heater 53, and the water separated by the water separator 55 is heated again by the heat exchanger 52.

[0042] Although liquid water is converted into steam by the heat exchanger 52 and the heater 53 and then the steam is supplied to the anode 11, the steam may be supplied directly to the anode 11 without passing through the heat exchanger 52 and the heater 53.

[0043] The cathode-side circulation flow path 60 is connected to the ammonia separator 40, and returns the remaining gas containing nitrogen and hydrogen that remains after ammonia is separated in the ammonia separator 40 to the cathode 12. By recirculating and recycling the remaining gas, it is possible to reduce the consumption of nitrogen produced as a feed gas, and it is possible to reduce the energy consumption rate of the ammonia production system 1.

[0044] Specifically, one end of the cathode-side circulation flow path 60 is connected to the ammonia separator 40. The other end of the cathode circulation flow path is connected to the cathode inlet-side flow path 20 downstream of the heater 25 and upstream of the electrolytic cell 10. A compressor 61 is provided in the cathode-side circulation flow path 60. By driving the compressor 61, the remaining gas separated in the ammonia separator 40 can be returned to the cathode 12. In the cathode-side circulation flow path 60, a discharge flow path for discharging purge gas is provided downstream of the ammonia separator 40 and upstream of the compressor 61.

[0045] The remaining gas may contain nitrogen in an amount of 50% by volume or more and 90% by volume or less. By setting the nitrogen content in the remaining gas within the above range, NH ３This can further improve the unit consumption. The remaining gas may contain hydrogen from 0 vol% to 18 vol%. When the remaining gas contains hydrogen, such as more than 0 vol%, the presence of a small amount of hydrogen in the gas supplied to the inlet side of the cathode 12 creates a reducing atmosphere for the metal components of the catalyst of the cathode 12, thereby suppressing deterioration due to oxidation. In the nitrogen direct reduction electrolytic cell 10 described in this embodiment, poisoning of the catalyst of the cathode 12 by hydrogen of 18 vol% or less, which would cause performance degradation, can be suppressed. As a result, recycling operation with reduced nitrogen waste is possible, and an efficient ammonia production system 1 can be provided. The nitrogen content of the remaining gas is preferably 60 to 90 vol%, more preferably 65 to 90 vol%. The hydrogen content of the remaining gas is preferably 0 to 9 vol%, more preferably 0 to 4 vol%.

[0046] The proportion of nitrogen supplied to the electrolytic cell 10 through the cathode circulation flow path 60 may be 10% by volume or more, 20% by volume or more, 30% by volume or more, 40% by volume or more, 50% by volume or more, 60% by volume or more, 70% by volume or more, 80% by volume or more, or 90% by volume or more. The greater the proportion of nitrogen supplied through the cathode circulation flow path 60, the greater the NH ３ The basic unit can be reduced.

[0047] Next, NH ３ The basic unit was evaluated by simulation. The results are shown in Table 1. ２ The recycling rate is the ratio of the nitrogen supplied to the electrolytic cell 10 to the nitrogen supplied through the cathode circulation flow path 60. ２ The temperature of the electrolytic cell 10 was set to 250° C., and the pressure of the electrolytic cell 10 was set to 0.5 MPa. A PSA nitrogen separator was used as the nitrogen supply unit 22.

[0048]

[0049] As shown in Table 1, in the ammonia production system 1 according to this embodiment, the higher the nitrogen recycling rate, the lower the ammonia production energy consumption rate for the electrolysis power supplied to the electrolytic cell 10 by several kWh / kg-NH ３ In this way, by recycling the unreacted nitrogen in the electrolytic cell 10, the amount of nitrogen supplied from the nitrogen supply unit 22 can be reduced, and the power load of the nitrogen supply unit 22 can be reduced.

[0050] Second Embodiment Next, an ammonia production system 1 and an ammonia production method according to a second embodiment will be described with reference to Figures 3 and 4. As shown in Figures 3 and 4, the ammonia production system 1 according to the second embodiment is different from the ammonia production system 1 according to the first embodiment in that the electrolytic cell 10 includes a hydrogen separation membrane 19 and is not provided with a hydrogen separator 27. Unless otherwise specified, the ammonia production system 1 according to the second embodiment is the same as the ammonia production system 1 according to the first embodiment, and therefore description thereof will be omitted.

[0051] The electrolytic cell 10 includes a hydrogen separation membrane 19 that is provided between the solid electrolyte 13 and the cathode 12, converts hydrogen ions that have permeated the solid electrolyte 13 into hydrogen atoms, and provides the hydrogen atoms to the cathode 12. Specifically, the hydrogen atoms converted by the hydrogen separation membrane 19 diffuse and permeate through the hydrogen separation membrane 19, and are provided to the cathode 12. At the cathode 12, the hydrogen atoms supplied from the hydrogen separation membrane 19 react with nitrogen molecules supplied from the cathode inlet-side flow path 20, producing ammonia. In the ammonia production system 1 according to this embodiment, the hydrogen separated by the hydrogen separation membrane 19 reacts with nitrogen, so the reaction area for the ammonia production reaction may be small, allowing the electrolytic cell 10 to be made compact.

[0052] The hydrogen separation membrane 19 may be a non-porous membrane. Alternatively, the hydrogen separation membrane 19 may be a metal membrane that allows hydrogen atoms to permeate but not hydrogen gas. The hydrogen separation membrane 19 may contain palladium, silver, copper, titanium, zirconium, vanadium, niobium, tantalum, or an alloy thereof. The thickness of the hydrogen separation membrane 19 may be 10 μm or more, or 20 μm or more. Alternatively, the thickness of the hydrogen separation membrane 19 may be 50 μm or less, 40 μm or less, or 30 μm or less.

[0053] In the cathode 12, nitrogen is reduced with hydrogen to produce ammonia. The cathode 12 may include an ammonia synthesis catalyst as an electrode catalyst. For example, the cathode 12 may include an ammonia synthesis catalyst containing a metal component such as Ru as an electrode catalyst. The cathode 12 may include an ammonia synthesis catalyst containing at least one metal component of Fe and Co, such as that used in the Haber-Bosch process as an electrode catalyst.

[0054] As in the ammonia production system 1 according to the first embodiment, the heat exchanger 24 exchanges heat between the cathode feedstock in the cathode inlet-side flow path 20 and the cathode off-gas in the cathode outlet-side flow path 21. According to the ammonia production system 1 according to the present embodiment, ammonia is produced in the electrolytic cell 10 using a phosphoric acid compound in a temperature range of 150°C to 350°C, and the reaction heat associated with the ammonia production is effectively utilized in the heat exchanger 24, resulting in excellent thermal efficiency. The cathode off-gas cooled by the heat exchanger 24 is supplied to the ammonia separator 40.

[0055] The ammonia separator 40 is a separator that liquefies and separates the ammonia contained in the cathode off-gas cooled in the heat exchanger 24. By cooling the heat of the cathode off-gas in the heat exchanger 24 and liquefying and separating the ammonia contained in the cathode off-gas cooled in the heat exchanger 24, it is possible to reduce the energy required to liquefy the ammonia. The ammonia separator 40 is provided downstream of the cooler 26 of the heat exchanger 24. The cathode off-gas cooled in the cooler 26 is further cooled in the ammonia separator 40, whereby the ammonia contained in the cathode off-gas is liquefied and separated. The separated ammonia can be recovered and used as ammonia fuel, for example.

[0056] As in the first embodiment, the cathode-side circulation flow path 60 is connected to the ammonia separator 40 , and returns the remaining gas containing nitrogen and hydrogen remaining after ammonia is separated in the ammonia separator 40 to the cathode 12 .

[0057] The remaining gas may contain nitrogen in an amount of 25% by volume or more and 99% by volume or less. By setting the nitrogen content in the remaining gas within the above range, NH ３ This can further improve the unit consumption. The remaining gas may contain hydrogen at 1 vol% or more and 75 vol% or less. In the present embodiment, the electrolytic cell 10 includes a hydrogen separation membrane 19. Therefore, even if the remaining gas contains a large amount of hydrogen, ammonia can be efficiently produced by the electrolytic cell 10. The remaining gas may contain nitrogen at 90 vol% or less, 80 vol% or less, 70 vol% or less, 60 vol% or less, 50 vol% or less, 40 vol% or less, or 30 vol% or less. The remaining gas may contain hydrogen at 10 vol% or more, 20 vol% or more, 30 vol% or more, 40 vol% or more, 50 vol% or more, 60 vol% or more, or 70 vol% or more.

[0058] [Third Embodiment] Next, an ammonia production system 1 and an ammonia production method according to a third embodiment will be described with reference to Fig. 5. As shown in Fig. 5, in the ammonia production system 1 according to the third embodiment, the ammonia separator 40 of the ammonia production system 1 according to the second embodiment is a membrane separator. Unless otherwise specified, the ammonia production system 1 according to the third embodiment is the same as the ammonia production system 1 according to the second embodiment, and therefore description thereof will be omitted.

[0059] In the ammonia production system 1 according to this embodiment, the ammonia separator 40 is a membrane separator. The electrolytic cell 10 includes a solid electrolyte 13 containing a phosphoric acid compound, and the cathode off-gas has a temperature of 150° C. or higher. The membrane separator is able to efficiently recover ammonia because ammonia is more easily separated at temperatures of 150° C. or higher, which are higher than room temperature.

[0060] The ammonia separator 40 may include an ammonia separation membrane. The ammonia separation membrane may be any membrane capable of selectively separating ammonia from the cathode off-gas containing hydrogen, nitrogen, and ammonia. The ammonia separation membrane may be a synthetic resin membrane, a carbon membrane, a porous silica membrane, or a zeolite membrane. Among these, the ammonia separation membrane is preferably a zeolite membrane. The zeolite membrane can effectively separate ammonia from the cathode off-gas.

[0061] The heat exchanger 24 exchanges heat between the cathode feedstock in the cathode inlet-side flow path 20 and the residual gas in the cathode-side circulation flow path 60. In this embodiment, the cooler 26 is provided in the cathode-side circulation flow path 60, and the ammonia separator 40 is disposed downstream of the electrolytic cell 10 and upstream of the cooler 26. By exchanging heat from the heater 25 with heat from the cooler 26, the cooler 26 can cool the residual gas in the cathode-side circulation flow path 60. The ammonia production system 1 according to this embodiment produces ammonia in the temperature range of 150°C to 350°C in the electrolytic cell 10 using a phosphoric acid compound, and the reaction heat associated with the ammonia production is effectively utilized in the heat exchanger 24, resulting in excellent thermal efficiency. The residual gas cooled by the heat exchanger 24 is supplied to the compressor 61.

[0062] The compressor 61 is provided in the cathode-side circulation flow path 60 and compresses the residual gas cooled by the heat exchanger 24 and returns it to the inlet of the cathode 12. By recycling the residual gas and reusing unreacted hydrogen and nitrogen in the electrolytic cell 10 as cathode feedstock, the amounts of nitrogen and hydrogen supplied per unit of ammonia production can be reduced. This improves the amount of ammonia recovered relative to the supplied nitrogen, thereby reducing the energy consumption rate of the ammonia production process. Because the voltage of the electrolytic cell 10 increases due to the influence of impurities accumulated in the cathode circulation system gas, the flow rate of the residual gas to be recycled can be determined based on the magnitude of the voltage increase. Furthermore, by returning the residual gas cooled by the heat exchanger 24 to the inlet of the cathode 12, an excessive increase in the temperature of the cathode 12 can be prevented. This promotes the exothermic ammonia synthesis reaction in the electrolytic cell 10.

[0063] Next, the ammonia concentration generated under various conditions when using the electrolytic cell 10 equipped with the hydrogen separation membrane 19 was evaluated by simulation. The results are shown in Table 2. As shown in Table 2, the ammonia concentration in the electrolytic cell 10 was evaluated by simulation. ２ +3H ２ →2NH ３When the cathode off-gas is not recirculated under conditions in which nitrogen and hydrogen are supplied to the cathode 12 at a stoichiometric ratio of 1:1, the upper limit of the ammonia concentration in the cathode off-gas is determined depending on the temperature and pressure. Thus, in an electrolytic cell 10 equipped with a hydrogen separation membrane 19, the current efficiency in the electrolytic cell 10 is restricted by thermochemical equilibrium.

[0064]

[0065] On the other hand, as shown in Table 3, when the temperature of the electrolytic cell 10 is 250°C and the pressure inside the electrolytic cell 10 is 0.5 MPa, the higher the recirculation rate of the cathode off-gas is, the smaller the ammonia production energy consumption rate of the electrolysis power becomes. Specifically, when the recirculation rate of the cathode off-gas is 95%, it is found that the ammonia production energy consumption rate can be reduced to 1 / 5 compared to when the recirculation rate is 0%.

[0066]

[0067] The ammonia production system 1 and the ammonia production method according to the present disclosure have been described above. The ammonia production system 1 includes an electrolytic cell 10 including an anode 11, a cathode 12, and a solid electrolyte 13 disposed between the anode 11 and the cathode 12 and containing a phosphate compound as a proton conductor, and configured to produce ammonia from nitrogen and water. The ammonia production system 1 includes a cathode inlet-side flow path 20 that supplies a cathode feedstock containing nitrogen to the cathode 12, and a cathode outlet-side flow path 21 through which a cathode off-gas discharged from the cathode 12 and containing ammonia, nitrogen, and hydrogen flows. The ammonia production system 1 includes an ammonia separator 40 that is provided in the cathode outlet-side flow path 21 and separates ammonia from the cathode off-gas. The ammonia production system 1 includes an anode inlet-side flow path 50 that supplies an anode feedstock containing water to the anode 11, and an anode outlet-side flow path 51 through which anode off-gas discharged from the anode 11 and containing water and oxygen flows. The system is provided with a cathode-side circulation flow path 60 which is connected to the ammonia separator 40 and returns the remaining gas containing nitrogen and hydrogen remaining after the ammonia is separated in the ammonia separator 40 to the cathode.

[0068] Moreover, an ammonia production method according to the present disclosure produces ammonia in an electrolytic cell 10 including an anode 11, a cathode 12, and a solid electrolyte 13 disposed between the anode 11 and the cathode 12 and containing a phosphate compound as a proton conductor. The ammonia production method includes the steps of supplying a cathode feedstock containing nitrogen to the cathode 12 through a cathode inlet-side flow path 20, and supplying an anode feedstock containing water to the anode 11 through an anode inlet-side flow path 50. The ammonia production method includes the steps of producing ammonia from the nitrogen contained in the cathode feedstock and the water contained in the anode feedstock in the electrolytic cell 10. The ammonia production method includes the step of flowing an anode off-gas discharged from the anode 11 and containing water and oxygen through an anode outlet-side flow path 51. The ammonia production method includes the step of flowing a cathode off-gas discharged from the cathode 12 and containing ammonia, nitrogen, and hydrogen through a cathode outlet-side flow path 21. The ammonia production method includes a step of separating ammonia from cathode off-gas in an ammonia separator 40, which is provided in the cathode outlet-side flow path 21. The ammonia production method includes a step of returning, to the cathode 12, in a cathode-side circulation flow path 60 connected to the ammonia separator 40, the remaining gas containing nitrogen and hydrogen that remains after the ammonia is separated in the ammonia separator 40.

[0069] According to the ammonia production system 1 and the ammonia production method of this embodiment, the solid electrolyte 13 contains a phosphate compound. Furthermore, the cathode-side circulation flow path 60 returns the remaining gas containing nitrogen and hydrogen remaining after ammonia is separated in the ammonia separator 40 to the cathode 12. This makes it possible to increase the electrolysis temperature, and to reduce the ammonia consumption rate by recycling the cathode off-gas as a feedstock for the electrolytic cell.

[0070] The entire contents of Japanese Patent Application No. 2024-029643 (filing date: February 29, 2024) are incorporated herein by reference.

[0071] Although several embodiments have been described, the embodiments can be modified or varied based on the above disclosure. All components of the above embodiments and all features described in the claims may be individually extracted and combined as long as they are not mutually inconsistent.

[0072] The present disclosure can contribute, for example, to Goal 7 of the United Nations-led Sustainable Development Goals (SDGs), "Ensure access to affordable, reliable, sustainable and modern energy for all" and Goal 13, "Take urgent action to combat climate change and its impacts."

[0073] REFERENCE SIGNS LIST 1 ammonia production system 10 electrolytic cell 11 anode 12 cathode 13 solid electrolyte 19 hydrogen separation membrane 20 cathode inlet side flow path 21 cathode outlet side flow path 24 heat exchanger 27 hydrogen separator 40 ammonia separator 50 anode inlet side flow path 51 anode outlet side flow path 55 water separator 56 anode side circulation flow path 60 cathode side circulation flow path

Claims

1. An ammonia production system comprising: an electrolytic cell comprising an anode, a cathode, and a solid electrolyte disposed between the anode and the cathode and containing a phosphoric acid compound as a proton conductor, the electrolytic cell producing ammonia from nitrogen and water; a cathode inlet-side flow path for supplying a cathode feedstock containing nitrogen to the cathode; a cathode outlet-side flow path through which a cathode off-gas discharged from the cathode and containing ammonia, nitrogen, and hydrogen flows; an ammonia separator provided in the cathode outlet-side flow path for separating ammonia from the cathode off-gas; an anode inlet-side flow path for supplying an anode feedstock containing water to the anode; an anode outlet-side flow path through which anode off-gas discharged from the anode and containing water and oxygen flows; and a cathode circulation flow path connected to the ammonia separator and returning to the cathode a remaining gas containing nitrogen and hydrogen remaining after ammonia is separated in the ammonia separator.

2. The ammonia production system according to claim 1, wherein in the electrolytic cell, hydrogen ions that have permeated the solid electrolyte are provided to the cathode.

3. The ammonia production system according to claim 2, comprising: a heat exchanger that exchanges heat of the cathode feedstock in the cathode inlet-side flow path with heat of the cathode off-gas in the cathode outlet-side flow path; and a hydrogen separator that is provided in the cathode outlet-side flow path and separates hydrogen from the cathode off-gas.

4. The ammonia production system according to claim 1, wherein the electrolytic cell includes a hydrogen separation membrane provided between the solid electrolyte and the cathode, converting hydrogen ions that have permeated the solid electrolyte into hydrogen atoms and providing the hydrogen atoms to the cathode.

5. The ammonia production system according to claim 4, further comprising a heat exchanger that exchanges heat between the cathode feedstock in the cathode inlet-side flow path and the cathode off-gas in the cathode outlet-side flow path, and the ammonia separator is a separator that liquefies and separates the ammonia contained in the cathode off-gas cooled by the heat exchanger.

6. The ammonia production system according to claim 4, wherein the ammonia separator is a membrane separator, and the ammonia production system comprises: a heat exchanger that exchanges heat between the cathode feedstock in the cathode inlet-side flow path and the remainder gas in the cathode-side circulation flow path; and a compressor that is provided in the cathode-side circulation flow path and compresses the remainder gas cooled by the heat exchanger and returns the remainder gas to the inlet of the cathode.

7. The ammonia production system according to any one of claims 1 to 6, comprising: a water separator provided in the anode outlet-side flow path and configured to separate water from the anode off-gas; and an anode-side circulation flow path connected to the water separator and configured to return the water separated by the water separator to the anode.

8. A method for producing ammonia in an electrolytic cell comprising an anode, a cathode, and a solid electrolyte disposed between the anode and the cathode and containing a phosphoric acid compound as a proton conductor, the method comprising the steps of: supplying a cathode feedstock containing nitrogen to the cathode in a cathode inlet side flow path; supplying an anode feedstock containing water to the anode in an anode inlet side flow path; producing ammonia from the nitrogen contained in the cathode feedstock and the water contained in the anode feedstock in the electrolytic cell; flowing an anode off-gas discharged from the anode and containing water and oxygen in an anode outlet side flow path; flowing a cathode off-gas discharged from the cathode and containing ammonia, nitrogen, and hydrogen in an ammonia separator provided in the cathode outlet side flow path; and separating ammonia from the cathode off-gas. and returning a remaining gas containing nitrogen and hydrogen remaining after ammonia is separated in the ammonia separator to the cathode in a cathode-side circulation flow path connected to the ammonia separator.

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