Ammonia synthesis apparatus

Abstract

To provide an ammonia synthesis apparatus capable of improving the reactivity of hydrogen.MEANS FOR SOLVING THE PROBLEM: A synthesis apparatus comprises a plasma generator that generates plasma in a reaction field through which a nitrogen-containing gas flows, and an electrochemical cell including a cathode that generates hydrogen in the reaction field. The cathode also acts as a ground electrode of the plasma generator. The electrochemical cell may include a proton-conducting electrolyte.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a synthesis apparatus for synthesizing ammonia by reacting nitrogen with hydrogen.

Background Art

[0002] Prior art for synthesizing ammonia by reacting nitrogen excited using a plasma generator with hydrogen generated by electrolyzing water is disclosed in Patent Document 1. Patent Documents 2 and Non-Patent Document 1 disclose prior art related to ammonia synthesis using a combination of an electrochemical cell containing a proton-conductive electrolyte and plasma.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the prior art, although highly reactive active species exist in the plasma, there is room for improvement in the reactivity with hydrogen generated by electrolyzing water.

[0006] The present invention has been made to meet this requirement, and an object thereof is to provide a synthesis apparatus for ammonia that can improve the reactivity of hydrogen with respect to nitrogen.

Means for Solving the Problems

[0007] A first aspect for achieving this object includes a plasma generation device that generates plasma in a reaction field through which a gas containing nitrogen flows, and an electrochemical cell including a cathode that generates hydrogen in the reaction field. The cathode also serves as the ground electrode of the plasma generation device.

[0008] A second aspect is that, in the first aspect, the electrochemical cell includes a proton-conductive electrolyte. [[Effect of the Invention]]

[0009] According to the present invention, the cathode of the electrochemical cell that generates hydrogen in the reaction field also serves as the ground electrode of the plasma generation device. Since the probability that hydrogen generated at the cathode reacts with nitrogen activated by the plasma increases, the reactivity of hydrogen with respect to nitrogen can be improved. [[Brief Description of the Drawings]]

[0010]

Figure 1

Figure 2

[0011] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of an ammonia synthesis apparatus 10 in the first embodiment. The synthesis apparatus 10 includes a plasma generation device 11 and an electrochemical cell 20.

[0012] The plasma generation device 11 is a device that generates reactive plasma, so-called atmospheric pressure plasma, in a high-density medium at atmospheric pressure or higher. The plasma generation device 11 includes a first electrode 12, a second electrode 13 disposed apart from the first electrode 12, a dielectric 14 disposed between the first electrode 12 and the second electrode 13, and a power source 15 that applies a voltage between the first electrode 12 and the second electrode 13.

[0013] Examples of the material of the first electrode 12 include metal and carbon. When the material of the first electrode 12 is metal, the first electrode 12 contains one or more elements selected from Groups 4 to 14 of the periodic table based on the 1990 IUPAC recommendations. Examples of the elements are Al, Ti, Cr, Fe, Ni, Co, Cu, Zn, Ru, Ag, Pd, Pt, and Au. The shape of the first electrode 12 is appropriately set, such as rod-shaped, plate-shaped, net-shaped, or fibrous. An example of the material of the second electrode 13 is cermet.

[0014] The dielectric 14 is interposed between the first electrode 12 and the second electrode 13 and suppresses arc discharge that occurs when a large current locally flows between the first electrode 12 and the second electrode 13. Since charges are accumulated in the dielectric 14, discharge can occur at a voltage lower than the first discharge when the direction of the voltage changes after the first discharge. Therefore, plasma can be generated with a small input energy.

[0015] Examples of the dielectric 14 include plates, films, cylinders, boxes, etc. made of materials selected from glass, ceramics, synthetic resins, etc. A liquid such as water or an electrolytic solution may be used as the dielectric 14, and the first electrode 12 may be immersed in the liquid.

[0016] The power supply 15 is a device that applies an alternating voltage between the first electrode 12 and the second electrode 13. Examples of the alternating voltage include sine waves, triangular waves, sawtooth waves, and pulses in the range of 50 Hz to about 100 MHz. The second electrode 13 is grounded. The second electrode 13 is an earth electrode connected to the ground.

[0017] In this embodiment, a so-called parallel plate type is adopted in which the plate-shaped first electrode 12 and the plate-shaped second electrode 13 face each other, but it is not limited to this. There is no limitation on the arrangement of the electrodes. Examples of other electrode arrangements include a so-called coaxial cylinder type in which a rod-shaped first electrode 12 is coaxially arranged inside a cylindrical second electrode 13, and a cylindrical dielectric 14 is arranged on the outer periphery of the first electrode 12. As another electrode arrangement, a so-called coaxial double cylinder type in which a cylindrical second electrode 13 is coaxially arranged inside a cylindrical first electrode 12, and a cylindrical dielectric 14 is arranged on the inner periphery of the first electrode 12 is exemplified.

[0018] An alternating voltage is applied between the first electrode 12 and the second electrode 13. When the voltage reaches the discharge start voltage, the gas in the space (reaction field 16) between the first electrode 12 and the second electrode 13 breaks down, and dielectric barrier discharge occurs in the reaction field 16. When a gas containing nitrogen flows through the reaction field 16, nitrogen becomes active nitrogen molecules or atomic nitrogen with the nitrogen-nitrogen triple bond broken by the dielectric barrier discharge. Examples of the gas include nitrogen gas and a gas containing an inert gas such as water vapor or argon in addition to nitrogen gas.

[0019] The electrochemical cell 20 is a device that generates hydrogen and oxygen by electrolysis of water (water vapor). The electrochemical cell 20 includes an anode 21, a cathode, and an electrolyte 22 that separates the anode 21 and the cathode. The cathode also serves as the ground electrode (second electrode 13) of the plasma generator 11. Therefore, the same reference numeral as the second electrode 13 is assigned to the cathode, and it is referred to as the cathode 13. The power source 23 (DC power source) is connected to the anode 21 and the cathode 13. Since the cathode 13 of the electrochemical cell 20 also serves as the ground electrode of the plasma generator 11, the synthesis device 10 can be miniaturized.

[0020] The cathode 13 includes a current collector connected to the negative electrode of the power source 23, and a reduction reaction occurs on the cathode 13. Examples of the cathode 13 include a cermet containing a metal catalyst having electron conductivity and an oxide having oxide ion conductivity. Examples of the metal catalyst include Cr, Fe, Co, Cu, Ru, Pd, Ag, Pt, Au, and Ni. The oxide is a solid electrolyte, and examples include stabilized zirconia and ceria-based solid solutions. Stabilizers for stabilized zirconia are CaO, MgO, Y 2 O 3 , Sc 2 O 3 , Yb 2 O 3 etc. Examples of the elements dissolved in ceria in the ceria-based solid solution include Gd, Sm, and Y. The ceria-based solid solution has electron conductivity and oxide ion conductivity.

[0021] The anode 21 includes a current collector connected to the positive electrode of the power source 23, and an oxidation reaction occurs on the anode 21. The material of the anode 21 is a perovskite-type oxide, La 1-X Sr X MnO 3-δ , La 1-X Sr X CoO 3-δ , La 1-X Sr X Co 1-Y Fe Y O 3-δ , Pr 1-X Sr X MnO 3-δ are exemplified. Further, the material of the anode 21 includes a composite material of one or more oxides selected from these perovskite-type oxides and a solid electrolyte that can form the electrolyte 22.

[0022] The electrolyte 22 is made of a solid oxide that exhibits oxide ion conductivity under the operating conditions of the electrochemical cell 20. Examples of the solid oxide include stabilized zirconia and ceria-based solid solutions. Further, examples of the solid oxide include a solid solution of one or more selected from stabilized zirconia and ceria-based solid solutions and alumina. The stabilizers of stabilized zirconia are CaO, MgO, Y 2 O 3 , Sc 2 O 3 , Yb 2 O 3 are exemplified. Elements that dissolve in ceria in the ceria-based solid solution include Gd, Sm, and Y.

[0023] Water (water vapor) is supplied to the space including the cathode 13. The water vapor is carried to the cathode 13 on an inert gas such as argon or air. The water vapor is reduced at the cathode 13, generating hydrogen and oxide ions.

[0024] In the plasma generation device 11, highly reactive active species such as active nitrogen molecules and atomic nitrogen exist in the plasma in the reaction field 16. Since the cathode 13 extends into the reaction field 16, the probability that atomic hydrogen generated on the surface of the cathode 13 reacts with active nitrogen molecules or atomic nitrogen before becoming hydrogen molecules is high. As a result, the reactivity of hydrogen can be improved. Consequently, the selectivity for ammonia is improved, and the amount of ammonia produced by the synthesis device 10 can be increased.

[0025] Oxygen ions generated at the cathode 13 pass through the electrolyte 22 and reach the anode 21, where they are oxidized at the anode 21 to generate oxygen at the anode 21. The oxygen generated at the anode 21 is supplied to, for example, a combustor (not shown). Since the combustor supplied with oxygen can reduce the amount of nitrogen involved in combustion, the efficiency can be significantly improved, and the generation of NOx can also be reduced.

[0026] Since the operating temperature (about 400°C to 900°C) of the electrochemical cell 20 in the synthesis device 10 is high, the activation overvoltage is very low, and hydrogen can be produced with high efficiency with respect to the input power. Furthermore, in order to set the operating temperature of the electrochemical cell 20 or to generate water vapor, heat from a waste heat source such as a combustor can be utilized. As a result, compared with the power input during the electrolysis of water in the prior art, the power input to the electrochemical cell 20 can be reduced to obtain the same amount of hydrogen. Therefore, the power consumption per unit of ammonia synthesis can be reduced.

[0027] Since the electrochemical cell 20 includes an oxide ion conductive electrolyte 22, the transport rate of ion conduction in the electrolyte 22 can be increased. As a result, the Faraday efficiency of the synthesis device 10 can be improved.

[0028] The second embodiment will be described with reference to FIG. 2. In the first embodiment, the synthesis device 10 including the oxide ion conductive electrolyte 22 was described. In contrast, in the second embodiment, a synthesis device 30 including a proton conductive electrolyte 34 will be described. In the second embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals as in the first embodiment, and the description thereof will be omitted.

[0029] Figure 2 is a schematic cross-sectional view of an ammonia synthesis apparatus 30 according to the second embodiment. The synthesis apparatus 30 includes a plasma generator 11 and an electrochemical cell 31. The electrochemical cell 31 is a device that generates hydrogen and oxygen by electrolyzing water (steam). The electrochemical cell 31 includes an anode 32, a cathode 33, and an electrolyte 34 that separates the anode 32 and the cathode 33. The cathode 33 also serves as the ground electrode (second electrode) of the plasma generator 11.

[0030] The material of the anode 32 is a perovskite-type oxide, La 1-X Sr X MnO 3-δ , La 1-X Sr X CoO 3-δ , La 1-X Sr X Co 1-Y Fe Y O 3-δ , Pr 1-X Sr X MnO 3-δ is exemplified. Further, the material of the anode 21 includes a composite material of one or more oxides selected from these perovskite-type oxides and an electrolyte that can form the electrolyte 34.

[0031] The material of the cathode 33 is exemplified by a cermet containing a metal catalyst having electron conductivity and an oxide having proton conductivity. The metal catalyst is exemplified by Cr, Fe, Co, Cu, Ru, Pd, Ag, Pt, Au, Ni. The oxide is a perovskite-type oxide such as BaZrO in which the B site is substituted with a trivalent metal ion such as Y or In. 3 etc.

[0032] The material of the electrolyte 34 includes substances that exhibit proton conductivity under the operating conditions of the electrochemical cell 31. Substances that exhibit proton conductivity are perovskite-type oxides (ceramics) such as BaZrO in which the B site is substituted with a trivalent metal ion such as Y or In. 3 etc.

[0033] Water (water vapor) is supplied to the space containing the anode 32. The water vapor is carried to the anode 32 on an inert gas such as nitrogen or argon, or air. At the anode 32, hydrogen is separated from water as protons (H + +).

[0034] The protons generated at the anode 32 pass through the electrolyte 34 and reach the cathode 33, where they are reduced and hydrogen is generated at the cathode 33. Since the cathode 33 also serves as the ground electrode of the plasma generator 11, the probability of nitrogen ions, active nitrogen molecules, atomic nitrogen, etc. in the plasma reaching the surface of the cathode 33 is increased. Therefore, the probability that the atomic hydrogen generated on the surface of the cathode 33 reacts with active nitrogen molecules, atomic nitrogen, and nitrogen ions before becoming hydrogen molecules is increased. This can improve the reactivity of hydrogen with respect to nitrogen.

[0035] Since the operating temperature of the electrochemical cell 31 including the proton-conductive electrolyte 34 is lower than that of the electrochemical cell 20 in the first embodiment, the temperature of the reaction field 16 can be lowered compared to the first embodiment. As a result, the equilibrium attainment concentration in the reaction field 16 of the reaction for synthesizing ammonia from nitrogen and hydrogen can be increased. Therefore, the synthesis apparatus 30 is advantageous for obtaining high-concentration ammonia.

[0036] As described above, the present invention has been described based on the embodiments. However, the present invention is not limited to the above embodiments at all, and it can be easily inferred that various improvements and modifications are possible without departing from the spirit of the present invention.

[0037] In the embodiment, the synthesis apparatuses 10 and 30 using dielectric barrier discharge have been described, but the present invention is not limited thereto. It is of course possible to use other plasmas. Examples of other plasmas include corona discharge, surface discharge, atmospheric pressure glow discharge, microwave discharge, and underwater plasma. It is of course possible to combine a plurality of plasmas.

[0038] In the second embodiment, BaZrO is used as the proton-conductive electrolyte 34 3Although those containing perovskite-type oxides such as etc. have been described, it is not necessarily limited to this. It is of course possible to adopt perfluorocarbon-based or hydrocarbon-based polymer electrolytes for the electrolyte 34. Examples of perfluorocarbon-based proton conductors include Nafion (registered trademark) and Aquivion (registered trademark) in which a sulfo group is linked to a perfluoroalkyl-based polymer. For improving the properties of the proton conductor, a polymer may be compounded with inorganic particles.

[0039] Examples of hydrocarbon-based proton conductors include polymers in which a sulfo group or a phosphate group is linked to an aromatic hydrocarbon-based engineering plastic. Examples of hydrocarbon-based proton conductors include, for example, a polymer (SPEEK) in which a sulfo group is introduced into the main chain of polyether ether ketone. For improving the properties of the proton conductor, hetero elements such as fluorine, sulfur, nitrogen, phosphorus, etc. and units containing them may be introduced.

[0040] When a polymer electrolyte is adopted for the electrolyte 34, the anode 32 and the cathode 33 are exemplified by porous carbon supporting a catalyst such as Pt or Pt-Ru. Since the operating temperature of the electrochemical cell 31 containing the polymer electrolyte in the electrolyte 34 is about 60°C to 80°C, the temperature of the reaction field 16 can be lowered accordingly.

[0041] Although the description has been omitted in the embodiment, the temperature of the reaction field 16 is appropriately set, for example, in the range from room temperature to 500°C. The pressure of the reaction field 16 is appropriately set, for example, in the range from 101 kPa to 1000 kPa.

[0042] Although the description is omitted in the embodiments, it is possible to dispose a catalyst in the reaction field 16. There is no particular limitation as long as the catalyst is present in the reaction field 16. For example, the catalyst can be attached to the surface of the dielectric 14 that contacts the reaction field 16. Examples of the form of the catalyst attached to the dielectric 14 include a net shape, a plate shape, and a film shape. The catalyst contains one or more elements selected from Groups 4 to 14 of the periodic table based on the 1990 IUPAC recommendations. Examples of the elements include Al, Ti, Cr, Fe, Ni, Co, Cu, Zn, Ru, Ag, Pd, Pt, and Au.

[0043] The reaction field 16 may be filled with a catalyst. When the reaction field 16 is filled with a catalyst, an example is a structure in which a three-dimensional network structure skeleton made of a porous body of an oxide such as alumina or zirconia supports the catalyst.

Description of Reference Numerals

[0044] 10, 30 Synthesis apparatus 11 Plasma generator 13, 33 Cathode (ground electrode) 16 Reaction field 20, 31 Electrochemical cell 34 Electrolyte

Claims

1. A synthesis apparatus for synthesizing ammonia, comprising: a plasma generator that generates plasma in a reaction field through which a gas containing nitrogen flows; and an electrochemical cell including a cathode that generates hydrogen in the reaction field. The cathode also serves as the ground electrode of the plasma generator.

2. The synthesis apparatus according to claim 1, wherein the electrochemical cell includes a proton-conductive electrolyte.

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