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

TITLE OF THE INVENTION: AMMONIA GENERATION SYSTEM AND AMMONIA GENERATION METHOD TECHNICAL FIELD

[0001] The present disclosure relates to an ammonia generation system and an ammonia generation method. BACKGROUND ART

[0002] Ammonia does not emit carbon dioxide, which causes global warming, even when combusted, and therefore the use of ammonia as a fuel has been promoted. In the related art, it has been known to produce ammonia by the Haber-Bosch process. However, in the Haber-Bosch process, it is required to react nitrogen and hydrogen at a high temperature and a high pressure, and energy loss associated with pressurization of the raw material gases increases the specific energy consumption.

[0003] Further, in the Haber-Bosch process, a reaction temperature of 400 degrees Celsius or higher is required to activate a catalyst. However, since the ammonia synthesis reaction is an exothermic reaction, the temperature inside the reactor may increase to 600 degrees Celsius or higher due to the reaction heat. From the standpoint of reaction equilibrium, the reaction efficiency is higher at a reaction temperature lower than 400 degrees Celsius.

[0004] On the other hand, it is known to synthesize ammonia by electrolysis using water and nitrogen as raw materials. Patent Literature 1 discloses a method in which, after ammonia is synthesized by electrolysis, the generated gas is treated with an ammonia separation membrane or an ammonia pressure swing adsorption (PSA) device to separate the generated gas into high-concentration ammonia and remaining gas. In the method described above, the remaining gas is recycled as a nitrogen gas feedstock for the ammonia synthesis reactor. Further, the high-concentration ammonia gas recovered by the ammonia separation membrane or the ammonia PSA device is further liquified, and 1 the unliquified gas separated from the liquid ammonia is again treated with the ammonia separation membrane or the ammonia PSA device. CITATION LIST PATENT LITERATURE

[0005] Patent Literature 1: International Publication No. WO 2017 / 149718 SUMMARY OF THE INVENTION

[0006] In the related-art ammonia production process, ammonia is synthesized at normal temperature and normal pressure by electrolysis using a proton exchange membrane or an anion exchange membrane. Further, by synthesizing ammonia using an electrolysis method that substantially does not generate hydrogen during ammonia synthesis and combining the electrolysis method with ammonia separation and recovery treatment using membrane separation and PSA, high-concentration ammonia is synthesized and recovered with high efficiency throughout the entire process. However, in the related-art ammonia production process, ammonia is synthesized at normal temperature and normal pressure, and hence a sufficient reaction rate may not be obtained.

[0007] In view of this, an object of the present disclosure is to provide an ammonia generation system and an ammonia generation method that enable increase of an electrolysis temperature and reduction of the specific ammonia consumption by recycling cathode off-gas as a feedstock supplied to an electrolysis cell. SOLUTION TO PROBLEM

[0008] An ammonia generation system according to the present disclosure includes an electrolysis cell that generates ammonia from nitrogen and water, and includes an anode, a cathode, and a solid electrolyte being arranged between the anode and the cathode and containing a phosphoric acid compound serving as a proton conductor. The ammonia generation system includes a cathode-side inlet flow passage through which a cathode feedstock containing nitrogen is supplied to the cathode, and a cathode-side outlet flow passage through which cathode off-gas that is discharged from the cathode and contains ammonia, nitrogen, and hydrogen flows. The ammonia generation system includes an 2 ammonia separator that is provided to the cathode-side outlet flow passage and separates ammonia from the cathode off-gas. The ammonia generation system includes an anodeside inlet flow passage through which an anode feedstock containing water is supplied to the anode, and an anode-side outlet flow passage through which anode off-gas that is discharged from the anode and contains water and oxygen flows. The ammonia generation system includes a cathode-side circulation flow passage that is connected to the ammonia separator and returns remaining gas to the cathode, the remaining gas containing nitrogen and hydrogen after ammonia is separated by the ammonia separator.

[0009] In the electrolysis cell, hydrogen ions permeated through the solid electrolyte may be supplied to the cathode.

[0010] The ammonia generation system may include a heat exchanger that exchanges heat between the cathode feedstock inside the cathode-side inlet flow passage and the cathode off-gas inside the cathode-side outlet flow passage, and a hydrogen separator that is provided to the cathode-side outlet flow passage and separates hydrogen from the cathode off-gas.

[0011] The electrolysis cell may include a hydrogen separation membrane that is provided between the solid electrolyte and the cathode, converts the hydrogen ions permeated through the solid electrolyte into hydrogen atoms, and supplies the hydrogen atoms to the cathode.

[0012] The ammonia generation system may include a heat exchanger that exchanges heat between the cathode feedstock inside the cathode-side inlet flow passage and the cathode off-gas inside the cathode-side outlet flow passage. 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 generation system may include a heat exchanger that exchanges heat between the cathode feedstock inside the cathode-side inlet flow passage and the remaining gas inside the cathode-side circulation flow passage. The ammonia generation system may include a compressor that is provided to the cathode-side circulation flow passage, compresses the remaining gas cooled by the heat exchanger, and returns the remaining gas to an inlet of the cathode.

[0014] The ammonia generation system may include a water separator that is provided to the anode-side outlet flow passage and separates water from the anode off-gas, and an anode-side circulation flow passage that is connected to the water separator and returns water separated by the water separator to the anode.

[0015] An ammonia generation method according to the present disclosure generates ammonia in an electrolysis cell including an anode, a cathode, and a solid electrolyte being arranged between the anode and the cathode and containing a phosphoric acid compound serving as a proton conductor. The ammonia generation method includes supplying a cathode feedstock containing nitrogen to the cathode in a cathode-side inlet flow passage, and supplying an anode feedstock containing water to the anode in an anode-side inlet flow passage. The ammonia generation method includes generating ammonia from nitrogen contained in the cathode feedstock and water contained in the anode feedstock in the electrolysis cell. The ammonia generation method includes causing anode off-gas to flow in an anode-side outlet flow passage, the anode off-gas being discharged from the anode and containing water and oxygen, and causing cathode off-gas to flow in a cathode-side outlet flow passage, the cathode off-gas being discharged from the cathode and containing ammonia, nitrogen, and hydrogen. The ammonia generation method includes separating ammonia from the cathode off-gas by an ammonia separator provided to the cathode-side outlet flow passage. The ammonia generation method includes returning remaining gas to the cathode in a cathode-side circulation flow passage connected to the ammonia separator, the remaining gas containing nitrogen and hydrogen after ammonia is separated by the ammonia separator. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0016] According to the present disclosure, it is possible to provide an ammonia generation system and an ammonia generation method that enable increase of an electrolysis temperature and reduction of the specific ammonia consumption by recycling cathode off-gas as a feedstock supplied to an electrolysis cell. BRIEF DESCRIPTION OF DRAWINGS

[0017] [Fig. 1] Fig. 1 is a schematic view illustrating an ammonia generation system according to an embodiment. [Fig. 2] Fig. 2 is a schematic view illustrating an electrolysis cell according to the embodiment. [Fig. 3] Fig. 3 is a schematic view illustrating an ammonia generation system according to an embodiment. [Fig. 4] Fig. 4 is a schematic view illustrating an electrolysis cell according to the embodiment. [Fig. 5] Fig. 5 is a schematic view illustrating an ammonia generation system according to an embodiment. DESCRIPTION OF EMBODIMENTS

[0018] With reference to the drawings, some example embodiments are described below. Note that the dimensional ratios in the drawings are exaggerated for explanatory purposes, and may differ from the actual ratios.

[0019] [First Embodiment] An ammonia generation system 1 according to the present embodiment includes an electrolysis cell 10, a cathode-side inlet flow passage 20, a cathode-side outlet flow passage 21, a heater 23, a heat exchanger 24, a hydrogen separator 27, and an ammonia separator 40. Further, the ammonia generation system 1 includes an anode-side inlet flow passage 50, an anode-side outlet flow passage 51, a heat exchanger 52, a heater 53, a cooler 54, a water separator 55, an anode-side circulation flow passage 56, a cathodeside circulation flow passage 60, and a compressor 61.

[0020] As illustrated in Fig. 2, the electrolysis 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, sealing members 17, and separators 18. The electrolysis cell 10 generates ammonia from nitrogen and water. As described later, the electrolysis cell 10 uses the solid electrolyte 13 containing a phosphoric acid compound. Thus, the electrolysis cell 10 is operated at a lower temperature and a lower pressure than in the Haber-Bosch process. The reaction temperature in the electrolysis cell 10 may be 150 degrees Celsius to 350 degrees Celsius, for example. The reaction pressure in the electrolysis cell 10 may be 0.1 MPa to 5.1 MPa, for example.

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

[0022] At the cathode 12, nitrogen is reduced with hydrogen to generate ammonia. The cathode 12 may include, as an electrode catalyst, an ammonia synthesis catalyst having a direct nitrogen reduction capability. The cathode 12 may include, as the electrode catalyst, for example, an ammonia synthesis catalyst containing at least one of Fe or Co as a metal component. The ammonia synthesis catalyst may be an oxide having the above metal component.

[0023] The solid electrolyte 13 is arranged between the anode 11 and the cathode 12. The solid electrolyte 13 contains a phosphoric acid compound serving as a proton conductor. The solid electrolyte 13 permeates the hydrogen ions generated by the anode 11. The solid electrolyte 13 contains the phosphoric acid compound, and thus can exhibit favorable proton conductivity in a temperature range from 150 degrees Celsius to 350 degrees Celsius. The phosphoric acid compound may be, for example, a compound represented by MH2PO4. Here, M may be Na, K, Rb, or Cs. Such a phosphoric acid compound can suppress generation of hydrogen molecules during electrolysis and improve the current efficiency of ammonia synthesis. Note that, from the viewpoint of proton conductivity, the phosphoric acid compound preferably contains CsH2PO4.

[0024] The solid electrolyte 13 may contain a carrier that carries the phosphoric acid compound. By carrying the phosphoric acid compound on the carrier, the structural strength of the solid electrolyte 13 can be improved. The carrier may contain, for example, a silica material such as SiP2O? because of its high compatibility with the phosphoric acid compound. The solid electrolyte 13 carrying the phosphoric acid compound on the carrier can be obtained, for example, by mixing a powdered phosphoric acid compound and a powdered carrier and compression-molding the resulting mixture. Further, the solid electrolyte 13 carrying the phosphoric acid compound on the carrier can be obtained, for example, by impregnating a porous carrier with an aqueous solution in which the phosphoric acid compound is dissolved in water and then drying the carrier.

[0025] The anode current collector 14 is provided on a side of the anode 11, which is opposite to 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 diffusivity so that steam supplied to the anode 11 is diffused by the anode current collector 14, and oxygen generated at the anode 11 passes through the anode current collector 14 and is discharged to the outside of the electrolysis cell 10. The anode current collector 14 may be a porous body that diffuses steam and supplies steam to the anode 11. The anode current collector 14 may include at least one conductive material selected from the group consisting of carbon, titanium, nickel, and iron. The anode current collector 14 may have its surface coated with a corrosion inhibitor to prevent corrosion. The anode current collector 14 may be formed of iron having a surface coated with metal that is more noble than iron, such as gold or silver.

[0026] The cathode current collector 15 is provided on a side of the cathode 12, which is opposite to the solid electrolyte 13. The cathode current collector 15 is electrically connected to the cathode 12, and electrons are supplied from the power source 16 to the cathode 12 via the cathode current collector 15. The cathode current collector 15 may have gas diffusivity so that nitrogen supplied to the cathode 12 is diffused by the cathode current collector 15, and ammonia generated at the cathode 12 passes through the cathode current collector 15 and is discharged to the outside of the electrolysis cell 10. The cathode current collector 15 may be a porous body that diffuses nitrogen and supplies nitrogen to the cathode 12. The cathode current collector 15 may include at least one conductive material selected from the group consisting of carbon, titanium, nickel, and iron. The cathode current collector 15 may have its surface coated with a corrosion inhibitor to prevent corrosion. The cathode current collector 15 may be formed of iron having a surface coated with metal that is more noble than iron, such as gold or silver.

[0027] The electrolysis cell including the anode 11, the cathode 12, the solid electrolyte 13, the anode current collector 14, and the cathode current collector 15 is sealed by the sealing members 17, and opposite ends of the electrolysis cell are sandwiched between the separators 18. Note that, in the present embodiment, the electrolysis cell 10 including a single electrolysis cell is described. However, the electrolysis cell 10 may include a stack including a plurality of electrolysis cells that are electrically connected in series and stacked.

[0028] In the electrolysis cell 10 according to the present embodiment, the hydrogen ions permeated through 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 on the cathode 12 in a molecular state, thereby generating ammonia. In this manner, at the cathode 12, nitrogen molecules combine with hydrogen atoms to generate ammonia molecules, and cathode off-gas containing ammonia gas is discharged from the electrolysis cell 10. Since the supply of the hydrogen ions depends on the current load, the ammonia synthesis reaction at the cathode 12 proceeds without being constrained by the gas-phase thermal equilibrium at the cathode 12. Thus, ammonia can be generated with low energy consumption.

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

[0030] The nitrogen supply unit 22 supplies the cathode feedstock containing nitrogen to the cathode 12 via the cathode-side inlet flow passage 20. The nitrogen supply unit 22 may separate and supply nitrogen from air. The nitrogen supply unit 22 may be a pressure swing adsorption (PSA) nitrogen separation device, a thermal swing adsorption (TSA) nitrogen separation device, a pressure and thermal swing adsorption (PTSA) nitrogen separation device, or a cryogenic separation device. The nitrogen supply unit 22 may determine an air separation operating method in accordance with a supply pressure of 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 during activation of the electrolysis cell 10. By heating the cathode feedstock by the heater 23, the temperature of the cathode feedstock can be raised to around the operating temperature of the electrolysis cell 10, thereby improving the ammonia generation efficiency. The heater 23 may heat 8 and keep the cathode feedstock warm by using an electric heater or heated air. The heater 23 may heat the cathode feedstock not only during activation of the electrolysis cell 10 but also during operation of the electrolysis cell 10. As described later, the cathode feedstock can be heated by the heater 25 during operation of the electrolysis cell 10. However, when the power consumption of the electrolysis cell 10 is relatively low, specifically, several kilowatts to several tens of kilowatts, 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 inside the cathode-side inlet flow passage 20 and the cathode off-gas inside the cathode-side outlet flow passage 21. The electrolysis cell 10 serves as a heat generating body after reaching a steady state. Thus, by exchanging heat between the cathode feedstock inside the cathode-side inlet flow passage 20 and the cathode off-gas inside the cathode-side outlet flow passage 21, the overall energy efficiency of the ammonia generation system 1 can be improved. In the present embodiment, the heat exchanger 24 includes the heater 25 and the cooler 26, and exchanges heat between the heater 25 and the cooler 26. The heater 25 heats the cathode feedstock inside the cathode-side inlet flow passage 20. The cooler 26 cools the cathode off-gas inside the cathode-side outlet flow passage 21.

[0033] The hydrogen separator 27 is provided to the cathode-side outlet flow passage 21, and separates hydrogen from the cathode off-gas cooled by the heat exchanger 24. By reducing an amount of hydrogen contained in the cathode off-gas by the hydrogen separator 27, high-concentration ammonia gas can be generated. With this, when ammonia is liquified and separated, the cooling temperature for ammonia can be set relatively high. In the case of compression liquefaction, the compression power required for ammonia can be reduced. Thus, the load on the ammonia separator 40 can be reduced. Further, by separating hydrogen from the cathode off-gas, an amount of hydrogen supplied to the cathode 12 through the cathode-side circulation flow passage 60 can be reduced. With this, an excessive amount of hydrogen can be prevented from being adsorbed on the catalyst of the cathode 12, thereby suppressing reduction in the reaction efficiency.

[0034] Note that, in the present embodiment, the hydrogen separator 27 is provided downstream of the electrolysis cell 10 and downstream of the cooler 26, and separates hydrogen from the cathode off-gas cooled by the heat exchanger 24. However, the 9 hydrogen separator 27 may be provided downstream of the electrolysis cell 10 and upstream of the cooler 26, and separates hydrogen from the cathode off-gas before being 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 either of those 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 to the cathode-side outlet flow passage 21, and separates ammonia from the cathode off-gas. Ammonia generated at the cathode 12 can be recovered by the ammonia separator 40. In the present embodiment, the ammonia separator 40 is a separator that separates ammonia by liquefying ammonia contained in the cathode off-gas cooled by the heat exchanger 24. However, ammonia may instead be separated by using a membrane separator, which is described later.

[0037] The anode-side inlet flow passage 50 supplies an anode feedstock containing water to the anode 11. Through the anode-side outlet flow passage 51, anode off-gas that is discharged from the anode 11 and contains water and oxygen flows. The anodeside inlet flow passage 50 is provided with the heat exchanger 52 and the heater 53. The anode-side outlet flow passage 51 is provided with the heat exchanger 52, the cooler 54, and the water separator 55. Note that, in the present embodiment, description is made on an example 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 by the heat exchanger 52, a temperature of water can be raised to around the operating temperature of the electrolysis cell 10. The heat exchanger 52 may heat liquid water to generate steam. The heat exchanger 52 exchanges heat between the anode feedstock inside the anode-side inlet flow passage 50 and the anode off-gas inside the anode-side outlet flow passage 51. Note that, in order to prevent steam from condensing in the anode-side inlet flow passage 50, the anode feedstock may contain nitrogen gas, oxygen gas, or air as purge gas.

[0039] The heater 53 heats the anode feedstock during activation of the electrolysis cell 10. The heater 53 may be used in combination with the heat exchanger 52 to heat water contained in the anode feedstock and generate steam. The heater 53 may use an electric heater or heated air as a heat source.

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

[0041] In the present embodiment, the water separator 55 is a gas-liquid separator that separates water condensed by cooling with the heat exchanger 52 and the cooler 54 from oxygen. Note that the cooler 54 is provided to increase the water utilization rate, but the cooler 54 may not be provided as long as water can be recovered. The anode-side circulation flow passage 56 is connected to the upstream sides of the heat exchanger 52 and the heater 53 in the anode-side outlet flow passage 51, and water separated by the water separator 55 is again heated by the heat exchanger 52.

[0042] Note that, although liquid water is turned into steam by the heat exchanger 52 and the heater 53, and steam is supplied to the anode 11, steam may be supplied directly to the anode 11 without the heat exchanger 52 and the heater 53.

[0043] The cathode-side circulation flow passage 60 is connected to the ammonia separator 40, and returns remaining gas to the cathode 12, the remaining gas containing nitrogen and hydrogen after ammonia is separated by the ammonia separator 40. By recirculating and recycling the remaining gas, the consumption of nitrogen used as feedstock gas can be reduced, thereby reducing the specific energy consumption of the ammonia generation system 1.

[0044] Specifically, one end of the cathode-side circulation flow passage 60 is connected to the ammonia separator 40. The other end of the cathode-side circulation flow passage is connected to the downstream side of the heater 25 and the upstream side of the electrolysis cell 10 in the cathode-side inlet flow passage 20. The cathode-side 11 circulation flow passage 60 is provided with the compressor 61. Further, by driving the compressor 61, the remaining gas separated by the ammonia separator 40 can be returned to the cathode 12. In the cathode-side circulation flow passage 60, a discharge flow passage for discharging a purge gas is provided downstream of the ammonia separator 40 and upstream of the compressor 61.

[0045] The remaining gas may contain 50 vol% or more and 90 vol% or less of nitrogen. By setting the nitrogen content in the remaining gas within the above range, the NH3 specific energy consumption can be further reduced. Note that the remaining gas may contain 0 vol% or more and 18 vol% or less of hydrogen. When the remaining gas contains hydrogen in an amount greater than 0 vol%, a small amount of hydrogen is present in the gas supplied to the inlet side of the cathode 12, and the metallic component of the catalyst of the cathode 12 is maintained in a reducing atmosphere. With this, deterioration due to oxidation can be suppressed. In the electrolysis cell 10 of a direct nitrogen reduction type as described in the present embodiment, hydrogen in an amount of 18 vol% or less can suppress poisoning of the catalyst of the cathode 12 that would otherwise cause performance degradation. As a result, recycling operation with reduced disposal of nitrogen is achieved, thereby providing the highly efficient ammonia generation system 1. The nitrogen content in the remaining gas is preferably from 60 to 90 vol%, more preferably, from 65 to 90 vol%. The hydrogen content in the remaining gas is preferably from 0 to 9 vol%, more preferably, from 0 to 4 vol%.

[0046] In nitrogen supplied to the electrolysis cell 10, the proportion of nitrogen supplied through the cathode-side circulation flow passage 60 may be 10 vol% or more, 20 vol% or more, 30 vol% or more, 40 vol% or more, 50 vol% or more, 60 vol% or more, 70 vol% or more, 80 vol% or more, or 90 vol% or more. As the proportion of nitrogen supplied through the cathode-side circulation flow passage 60 is increased, the NH3 specific energy consumption can be reduced.

[0047] Next, a simulation was performed to evaluate the NH3 specific energy consumption when the nitrogen recycling ratio was changed in the ammonia generation system 1 shown in Fig. 1. The results are shown in Table 1. Note that, in Table 1, the N2 recycling ratio means the proportion of N2 supplied through the cathode-side circulation flow passage 60 in nitrogen supplied to the electrolysis cell 10. Further, a temperature of the electrolysis cell 10 was set to 250 degrees Celsius, and a pressure of 12 As shown in Table 1, according to the ammonia generation system 1 of the present embodiment, it can be understood that, as the nitrogen recycling ratio was o o N2 recycling ratio % NHa concentration at cathode outlet vol% NHa concentration at cathode inlet vol% N2 utilization ratio in electrolysis cell % Current efficiency (NHa) in electrolysis cell % Overall N2 utilization ratio in system % Overall current efficiency in system % NHa specific energy consumption (electrolysis power basis) kWh / kg-NH3 0 10 20 50 70 90 nitrogen separation device

[0048] [Table 1] 46.2 47.1 47.9 47.6 48.8 50.2 0.0 1.2 2.3 5.6 7.7 9.6 30.0 30.4 30.7 29.2 29.4 30.0 30.0 30.6 31.1 33.1 34.2 35.3 24.1 27.0 30.3 40.5 54.6 79.5 24.1 24.8 25.5 26.9 28.2 29.4 29.4 28.6 27.9 26.3 25.2 24.1 <72 <72 CD p- the electrolysis cell 10 was set to 0.5 MPa. As the nitrogen supply unit 22, a PSA increased, the ammonia specific energy consumption for ammonia production with respect to the electric power supplied to the electrolysis cell 10 was successfully reduced by several kWh / kg-NHs. In this manner, by recycling unreacted nitrogen in the electrolysis cell 10, the amount of nitrogen supplied from the nitrogen supply unit 22 can be reduced, thereby reducing the power load of the nitrogen supply unit 22.

[0050] [Second Embodiment] Next, with reference to Fig. 3 and Fig. 4, an ammonia generation system 1 and an ammonia generation method according to a second embodiment are described. As illustrated in Fig. 3 and Fig. 4, the ammonia generation system 1 according to the second embodiment is different from the ammonia generation system 1 according to the first embodiment in that the electrolysis cell 10 includes a hydrogen separation membrane 19 and the hydrogen separator 27 is not provided. The other matters of the ammonia generation system 1 according to the second embodiment are similar to those in the ammonia generation system 1 according to the first embodiment unless otherwise noted, and hence the description therefor is omitted.

[0051] The electrolysis cell 10 includes the hydrogen separation membrane 19 that is provided between the solid electrolyte 13 and the cathode 12, converts the hydrogen ions permeated through the solid electrolyte 13 into hydrogen atoms, and supplies the hydrogen atoms to the cathode 12. Specifically, the hydrogen atoms converted by the hydrogen separation membrane 19 are diffused into and permeated through the hydrogen separation membrane 19, and are supplied to the cathode 12. When the hydrogen atoms are supplied from the hydrogen separation membrane 19, nitrogen molecules supplied from the cathode-side inlet flow passage 20 are reacted at the cathode 12, thereby generating ammonia. In the ammonia generation system 1 according to the present embodiment, hydrogen separated by the hydrogen separation membrane 19 is reacted with nitrogen. Thus, a reaction area for an ammonia generation reaction may be small, and the electrolysis cell 10 can be reduced in size.

[0052] The hydrogen separation membrane 19 may be a nonporous membrane. Further, the hydrogen separation membrane 19 may be a metal membrane that permeates hydrogen atoms while preventing permeation of 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 14 separation membrane 19 may be 10 pm or greater or 20 pm or greater. Further, the thickness of the hydrogen separation membrane 19 may be 50 pm or less, 40 pm or less, or 30 pm or less.

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

[0054] Similarly to the ammonia generation system 1 according to the first embodiment, the heat exchanger 24 exchanges heat between the cathode feedstock inside the cathodeside inlet flow passage 20 and the cathode off-gas inside the cathode-side outlet flow passage 21. According to the ammonia generation system 1 according to the present embodiment, the electrolysis cell 10 using the phosphoric acid compound generates ammonia in a temperature range from 150 degrees Celsius to 350 degrees Celsius, and the reaction heat during ammonia generation is effectively utilized by the heat exchanger 24. Thus, the ammonia generation system 1 has 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 ammonia contained in the cathode off-gas cooled by the heat exchanger 24. By cooling the cathode off-gas with the heat exchanger 24 and liquefying and separating ammonia contained in the cathode off-gas cooled by the heat exchanger 24, the energy required to liquefy ammonia can be reduced. The ammonia separator 40 is provided downstream of the cooler 26 of the heat exchanger 24. The cathode off-gas cooled by the cooler 26 is further cooled by the ammonia separator 40. With this, ammonia contained in the cathode off-gas is liquified and separated. By recovering the separated ammonia, the ammonia can be used as, for example, an ammonia fuel.

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

[0057] The remaining gas may contain 25 vol% or more and 99 vol% or less of nitrogen. By setting the nitrogen content in the remaining gas within the above range, the NH3 specific energy consumption can be further reduced. Further, the remaining gas may contain 1 vol% or more and 75 vol% or less of hydrogen. In the present embodiment, the electrolysis cell 10 including the hydrogen separation membrane 19 is used. Thus, even when the remaining gas contains a large amount of hydrogen, ammonia can be generated efficiently by the electrolysis cell 10. The remaining gas may contain 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 of nitrogen. Further, the remaining gas may contain 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 of hydrogen.

[0058] [Third Embodiment] Next, as illustrated in Fig. 5, an ammonia generation system 1 and an ammonia generation method according to a third embodiment are described. As illustrated in Fig. 5, in the ammonia generation system 1 according to the third embodiment, the ammonia separator 40 of the ammonia generation system 1 according to the second embodiment is provided as a membrane separator. The other matters of the ammonia generation system 1 according to the third embodiment are similar to those in the ammonia generation system 1 according to the second embodiment unless otherwise noted, and hence the description therefor is omitted.

[0059] In the ammonia generation system 1 according to the present embodiment, the ammonia separator 40 is a membrane separator. The electrolysis cell 10 includes the solid electrolyte 13 containing the phosphoric acid compound, and the cathode off-gas has a temperature of 150 degrees Celsius or higher. The membrane separator can efficiently recover ammonia because ammonia is more readily separated at a temperature of 150 degrees Celsius or higher, which is higher than room temperature.

[0060] The ammonia separator 40 may include an ammonia separation membrane. The ammonia separation membrane is only required to selectively separate 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 those, the ammonia separation 16 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 inside the cathode-side inlet flow passage 20 and the remaining gas inside the cathode-side circulation flow passage 60. In the present embodiment, the cooler 26 is provided to the cathode-side circulation flow passage 60, and the ammonia separator 40 is arranged downstream of the electrolysis cell 10 and upstream of the cooler 26. By exchanging heat between the heater 25 and the cooler 26, the cooler 26 can cool the remaining gas inside the cathode-side circulation flow passage 60. According to the ammonia generation system 1 according to the present embodiment, the electrolysis cell 10 using the phosphoric acid compound generates ammonia in a temperature range from 150 degrees Celsius to 350 degrees Celsius, and the reaction heat during ammonia generation is effectively utilized by the heat exchanger 24. Thus, the ammonia generation system 1 has excellent thermal efficiency. The remaining gas cooled by the heat exchanger 24 is supplied to the compressor 61.

[0062] The compressor 61 is provided to the cathode-side circulation flow passage 60, compresses the remaining gas cooled by the heat exchanger 24, and returns the remaining gas to the inlet of the cathode 12. By recirculating the remaining gas and reusing hydrogen and nitrogen that have not reacted in the electrolysis cell 10 as the cathode feedstock, the amounts of nitrogen and hydrogen supplied per unit amount of ammonia produced can be reduced. Accordingly, the amount of ammonia recovered with respect to the supplied nitrogen can be increased, and the specific energy consumption of the ammonia generation process can be reduced. Since the voltage of the electrolysis cell 10 is increased due to impurities accumulated in the cathode-side circulation gas, the flow rate of the remaining gas to be recirculated can be determined based on the amount of the voltage increase. Further, by returning the remaining 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 suppressed. Accordingly, the ammonia synthesis reaction, which is an exothermic reaction, can be promoted in the electrolysis cell 10.

[0063] Next, a simulation was performed to evaluate the ammonia concentration under various conditions when the electrolysis cell 10 including the hydrogen separation membrane 19 was used. The results are shown in Table 2. As shown in Table 2, under 17 conditions in which nitrogen and hydrogen were supplied to the cathode 12 at the stoichiometric ratio of N2 + 3H2 ^ 2NH3, the upper limit of the ammonia concentration in the cathode off-gas was determined in accordance with the temperature and the pressure when the cathode off-gas was not recirculated. In this manner, in the electrolysis cell 10 including the hydrogen separation membrane 19, the current efficiency in the electrolysis cell 10 was limited by thermochemical equilibrium.

[0064] [Table 2] CO 250 50.1 66.7 I- 31.6 0 CO 90 22.1 36.2 0.1 5.4 10.2 90 0 0 co 10.5 19.0 250 22.1 36.2 240 25.4 40.5 200 41.7 58.9 MPa C 0^ 0 > Pressure Temperature NH3 concentration at cathode outlet NH3 current efficiency in electrolysis cell Meanwhile, as shown in Table 3, a temperature of the electrolysis cell 10 was set to 250 degrees Celsius, and a pressure inside the electrolysis cell 10 was set to 0.5 MPa. In such a case, it can be understood that as the recirculation ratio of the cathode off-gas was higher, the ammonia specific energy consumption for ammonia production based on the electric power supplied to the electrolysis cell was lower. Specifically, when the recirculation ratio of the cathode off-gas was 95%, the ammonia specific energy consumption for ammonia production can be reduced to one-fifth of that in a case in which the recirculation ratio was 0%.

[0066] [Table 3] Off-gas recirculation ratio % 0 10 20 30 40 50 60 70 80 90 95 NH3 concentration at cathode inlet vol% 0.0 2.8 4.9 6.3 7.5 8.4 9.2 9.8 10.3 10.8 11.2 H2 supplied to electrolysis cell (electrolytic hydrogen) % 100 94 87 80 73 65 57 49 41 32 20 H2 supplied to electrolysis cell (recirculation) % 0 6 13 20 27 35 43 51 59 68 80 N2 utilization ratio in electrolysis cell % 36.2 35.1 33.9 32.7 31.5 30.2 28.8 27.4 26.0 24.5 31.5 Overall current efficiency in system % 19.0 20.7 22.7 25.1 28.1 31.9 36.9 43.8 54.0 70.1 95.0 NH3 specific energy consumption (electrolysis power basis) kWh / kg-NH3 37.4 34.3 31.3 28.3 25.3 22.2 19.2 16.2 13.1 10.1 7.5 side inlet flow passage 20 through which the cathode feedstock containing nitrogen is supplied to the cathode 12, and the cathode-side outlet flow passage 21 through which the cathode off-gas that is discharged from the cathode 12 and contains ammonia, nitrogen, and hydrogen flows. The ammonia generation system 1 includes the ammonia separator 40 that is provided to the cathode-side outlet flow passage 21 and separates ammonia from the cathode off-gas. The ammonia generation system 1 includes the anode-side inlet flow passage 50 through which the anode feedstock containing water is supplied to the anode 11, and the anode-side outlet flow passage 51 through which the anode off-gas that is discharged from the anode 11 and contains water and oxygen. The ammonia generation system 1 includes the cathode-side circulation flow passage 60 that is connected to the ammonia separator 40 and returns the remaining gas to the cathode, the remaining gas containing nitrogen and hydrogen after ammonia is separated by the ammonia separator 40.

[0068] Further, the ammonia generation method according to the present disclosure generates ammonia in the electrolysis cell 10 including the anode 11, the cathode 12, and the solid electrolyte 13 being arranged between the anode 11 and the cathode 12 and containing the phosphoric acid compound serving as a proton conductor. The ammonia generation method includes a step of supplying the cathode feedstock containing nitrogen to the cathode 12 in the cathode-side inlet flow passage 20 and a step of supplying the anode feedstock containing water to the anode 11 in the anode-side inlet flow passage 50. The ammonia generation method includes a step of generating ammonia from nitrogen contained in the cathode feedstock and water contained in the anode feedstock in the electrolysis cell 10. The ammonia generation method includes a step of causing the anode off-gas to flow in the anode-side outlet flow passage 51, the anode off-gas being discharged from the anode 11 and containing water and oxygen. The ammonia generation method includes a step of causing the cathode off-gas to flow in the cathodeside outlet flow passage 21, the cathode off-gas being discharged from the cathode 12 and containing ammonia, nitrogen, and hydrogen. The ammonia generation method includes a step of separating ammonia from the cathode off-gas by the ammonia separator 40 provided to the cathode-side outlet flow passage 21. The ammonia generation method includes a step of returning the remaining gas to the cathode 12 in the cathodeside circulation flow passage 60 connected to the ammonia separator 40, the remaining gas containing nitrogen and hydrogen after ammonia is separated by the ammonia separator 40.

[0069] According to the ammonia generation system 1 and the ammonia generation method according to the present embodiment, the solid electrolyte 13 contains the phosphoric acid compound. Further, the cathode-side circulation flow passage 60 returns, to the cathode 12, the remaining gas containing nitrogen and hydrogen after ammonia is separated by the ammonia separator 40. Thus, an electrolysis temperature can be increased, and the specific ammonia consumption can be reduced by recycling cathode off-gas as a feedstock supplied to an electrolysis cell.

[0070] All the contents in Japanese Patent Application No. 2024-029643 (filed on February 29, 2024) are herein incorporated by reference.

[0071] While some embodiments are described above, modifications or variations of the embodiments can be made based on the above-mentioned disclosure contents. All the components of the above-described embodiments and all the features described in the claims may be individually extracted and combined, as long as they do not contradict each other.

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

[0073] 1       Ammonia generation system 10       Electrolysis cell 11     Anode 12     Cathode 13       Solid electrolyte 19     Hydrogen separation membrane 20      Cathode-side inlet flow passage 21      Cathode-side outlet flow passage 24     Heat exchanger 27      Hydrogen separator 40     Ammonia separator 50      Anode-side inlet flow passage 51      Anode-side outlet flow passage 55      Water separator 56      Anode-side circulation flow passage 60      Cathode-side circulation flow passage

Claims

1. An ammonia generation system comprising:an electrolysis cell that generates ammonia from nitrogen and water, and includes an anode, a cathode, and a solid electrolyte being arranged between the anode and the cathode and containing a phosphoric acid compound serving as a proton conductor;a cathode-side inlet flow passage through which a cathode feedstock containing nitrogen is supplied to the cathode;a cathode-side outlet flow passage through which cathode off-gas that is discharged from the cathode and contains ammonia, nitrogen, and hydrogen flows;an ammonia separator that is provided to the cathode-side outlet flow passage and separates ammonia from the cathode off-gas;an anode-side inlet flow passage through which an anode feedstock containing water is supplied to the anode;an anode-side outlet flow passage through which anode off-gas that is discharged from the anode and contains water and oxygen flows; anda cathode-side circulation flow passage that is connected to the ammonia separator and returns remaining gas to the cathode, the remaining gas containing nitrogen and hydrogen after ammonia is separated by the ammonia separator.

2. The ammonia generation system according to claim 1 wherein,in the electrolysis cell, hydrogen ions permeated through the solid electrolyte are supplied to the cathode.

3. The ammonia generation system according to claim 2 further comprising:a heat exchanger that exchanges heat between the cathode feedstock inside the cathode-side inlet flow passage and the cathode off-gas inside the cathode-side outlet flow passage; anda hydrogen separator that is provided to the cathode-side outlet flow passage, and separates hydrogen from the cathode off-gas.

4. The ammonia generation system according to claim 1 wherein,the electrolysis cell includes a hydrogen separation membrane that is provided between the solid electrolyte and the cathode, converts the hydrogen ions permeated through the solid electrolyte into hydrogen atoms, and supplies the hydrogen atoms to the cathode.

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

6. The ammonia generation system according to claim 4 wherein,the ammonia separator is a membrane separator, andthe ammonia generation system includes:a heat exchanger that exchanges heat between the cathode feedstock inside the cathode-side inlet flow passage and the remaining gas inside the cathode-side circulation flow passage; anda compressor that is provided to the cathode-side circulation flow passage, compresses the remaining gas cooled by the heat exchanger, and returns the remaining gas to an inlet of the cathode.

7. The ammonia generation system according to any one of claims 1 to 6 further comprising:a water separator that is provided to the anode-side outlet flow passage, and separates water from the anode off-gas; andan anode-side circulation flow passage that is connected to the water separator, and returns water separated by the water separator to the anode.

8. An ammonia generation method that generates ammonia in an electrolysis cell including an anode, a cathode, and a solid electrolyte being arranged between the anode and the cathode and containing a phosphoric acid compound serving as a proton conductor, the ammonia generation method comprising:supplying a cathode feedstock containing nitrogen to the cathode in a cathodeside inlet flow passage;supplying an anode feedstock containing water to the anode in an anode-side inlet flow passage;generating ammonia from nitrogen contained in the cathode feedstock and water contained in the anode feedstock in the electrolysis cell;causing anode off-gas to flow in an anode-side outlet flow passage, the anode off-gas being discharged from the anode and containing water and oxygen;causing cathode off-gas to flow in a cathode-side outlet flow passage, the cathode off-gas being discharged from the cathode and containing ammonia, nitrogen, and hydrogen;separating ammonia from the cathode off-gas by an ammonia separator provided to the cathode-side outlet flow passage; andreturning remaining gas to the cathode in a cathode-side circulation flow passage connected to the ammonia separator, the remaining gas containing nitrogen and hydrogen after ammonia is separated by the ammonia separator.

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