Ammonia synthesis apparatus

Abstract

To provide an ammonia synthesis apparatus capable of reducing electric energy consumption per production unit.SOLUTION: A synthesis apparatus comprises a reaction field through which a nitrogen-containing gas flows and a plasma generator that generates plasma in the reaction field, and synthesizes ammonia by reacting excited nitrogen with hydrogen. The synthesis apparatus includes an oxide ion-conducting solid oxide electrolyzer cell, and generates hydrogen by means of water electrolysis using the electrolytic cell.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Prior art for synthesizing ammonia by reacting nitrogen excited using a plasma generator and hydrogen generated by electrolysis of water is disclosed in Patent Document 1. Patent Document 2 and Non-Patent Document 1 disclose ammonia synthesis using a proton-permeable electrolytic cell and plasma in combination.

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 (Patent Document 1), since the conversion efficiency of hydrogen generation with respect to the power input for electrolysis of water is low, there is a problem that the power consumption per unit of ammonia synthesis using that hydrogen as a raw material is large. When using a proton-permeable electrolytic cell as in the technologies disclosed in Patent Document 2 and Non-Patent Documents, unlike a solid oxide type electrolytic cell with single-ion conduction, since it has mixed-ion conduction, in principle, the Faraday efficiency cannot be increased.

[0006] The present invention has been made to solve this problem, and an object thereof is to provide an ammonia synthesis apparatus capable of reducing the power unit.

Means for Solving the Problem

[0007] A first aspect for achieving this object is a synthesis apparatus that includes a reaction field through which a gas containing nitrogen flows, and a plasma generation device that generates plasma in the reaction field, and synthesizes ammonia by reacting excited nitrogen and hydrogen, and includes a solid oxide type electrolytic cell of an oxide ion conduction type, and generates hydrogen by electrolyzing water using the electrolytic cell.

[0008] A second aspect is that in the first aspect, the cathode of the electrolytic cell is located downstream of the electrode of the plasma generation device, and ammonia is synthesized by reacting hydrogen generated at the cathode and excited nitrogen.

[0009] A third aspect is that in the first or second aspect, the cathode of the electrolytic cell is the ground electrode of the plasma generation device.

Effect of the Invention

[0010] According to the present invention, a solid oxide type electrolytic cell of an oxide ion conduction type that electrolyzes water (steam) at a high temperature has a high operating temperature, a very low activation overvoltage, and can further increase the Faraday efficiency. Therefore, the power input to the electrolytic cell to obtain the same amount of hydrogen can be reduced compared to a proton permeable type electrolytic cell or the power input during electrolysis of water. Therefore, the power unit of ammonia synthesis can be reduced.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Best Mode for Carrying Out the Invention

[0012] 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 a first embodiment. The synthesis apparatus 10 includes a plasma generator 11 and a solid oxide type electrolytic cell 20.

[0013] The plasma generator 11 is a device that generates a reactive plasma, so-called atmospheric pressure plasma, using a high-density medium at atmospheric pressure or higher. The plasma generator 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.

[0014] 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, Au. The shape of the first electrode 12 is appropriately set, such as rod-shaped, plate-shaped, net-shaped, or fiber-shaped.

[0015] Examples of the material of the second electrode 13 include metal and carbon. The shape of the second electrode 13 is appropriately set, such as plate-shaped, net-shaped, or film-shaped covering the dielectric 14.

[0016] 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.

[0017] In this embodiment, the dielectric 14 is disposed on the surface of the second electrode 13. However, the position of the dielectric 14 is not limited thereto. The dielectric 14 may be disposed on the surface of the first electrode 12, or the dielectric 14 may be disposed on the surfaces of both the first electrode 12 and the second electrode 13.

[0018] Examples of the dielectric 14 include a plate, a film, a cylinder, a box, etc. made of a material selected from glass, ceramics, synthetic resin, etc. A liquid such as water or an electrolytic solution may be used as the dielectric 14, and the first electrode 12 and the second electrode 13 may be immersed in the liquid.

[0019] 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 a sine wave, a triangular wave, a sawtooth wave, and a pulse in the range of about 50 Hz to 100 MHz. The second electrode 13 is grounded. The second electrode 13 is a ground electrode connected to the ground.

[0020] In this embodiment, a rod-shaped first electrode 12 is coaxially disposed in a cylindrical second electrode 13, and a cylindrical dielectric 14 is disposed on the inner circumference of the second electrode 13, which is a so-called coaxial cylinder type. However, it is not limited thereto. There is no limitation on the arrangement of the electrodes. Examples of other electrode arrangements include a parallel plate type and a coaxial double cylinder type.

[0021] When an alternating voltage is applied between the first electrode 12 and the second electrode 13 and 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 atomic nitrogen by dielectric barrier discharge, where active nitrogen molecules or nitrogen-nitrogen triple bonds are broken. 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.

[0022] The electrolysis cell 20 is a device that generates hydrogen and oxygen by electrolyzing water (steam). The electrolysis cell 20 includes a cathode 21, an anode 22, and an electrolyte 23 that separates the cathode 21 and the anode 22. A power source 24 (DC power source) is connected to the cathode 21 and the anode 22. The cathode 21 is located upstream of the first electrode 12 and the second electrode 13 of the plasma generator 11.

[0023] The cathode 21 includes a current collector connected to the negative electrode of the power source 24, and a reduction reaction occurs on the cathode 21. Examples of the cathode 21 include those 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 and the like. Elements that dissolve 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.

[0024] The anode 22 includes a current collector connected to the positive electrode of the power source 24, and an oxidation reaction occurs on the anode 22. The material of the anode 22 is a perovskite-type oxide such as 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 and the like. Also, the material of the anode 22 includes a composite material of one or more oxides selected from these perovskite-type oxides and a solid electrolyte that can form the electrolyte 23.

[0025] The electrolyte 23 is made of a solid oxide that exhibits oxide ion conductivity under the operating conditions of the electrolytic cell 20. Examples of the solid oxide include stabilized zirconia and ceria-based solid solutions. Further examples of the solid oxide include solid solutions of one or more selected from stabilized zirconia and ceria-based solid solutions and alumina. Stabilizers for stabilized zirconia are CaO, MgO, Y 2 O 3 , Sc 2 O 3 , Yb 2 O 3 etc. Examples of elements that dissolve in ceria in the ceria-based solid solution are Gd, Sm, Y.

[0026] Water (steam) is supplied to the cathode chamber 25 containing the cathode 21. The steam is carried to the cathode chamber 25 on an inert gas such as argon or air. The steam is reduced at the cathode 21, generating hydrogen and oxide ions in the cathode chamber 25. Since the cathode chamber 25 is connected upstream of the reaction field 16, hydrogen is supplied to the reaction field 16. The nitrogen excited in the reaction field 16 reacts with hydrogen in the reaction field 16 to synthesize ammonia. Since the synthesis device 10 causes a chemical reaction for synthesizing ammonia in the reaction field 16 where dielectric barrier discharge occurs, the device can be miniaturized.

[0027] The oxide ions generated at the cathode 21 reach the anode 22 through the electrolyte 23, are oxidized at the anode 22, and oxygen is generated at the anode 22. The oxygen generated at the anode 22 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 further the generation of NOx can be reduced.

[0028] Since the operating temperature (about 400°C to 900°C) of the electrolytic 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. Further, in order to bring the electrolytic cell 20 to the operating temperature or generate steam, the heat from a waste heat source such as a combustor can be utilized. Thereby, compared with the power input during the electrolysis of water in the prior art, the power input to the electrolytic cell 20 can be reduced to obtain the same amount of hydrogen. Therefore, the power consumption per unit of ammonia synthesis can be reduced.

[0029] It is of course possible to separate the plasma generator 11 and the electrolytic cell 20, arrange the electrolytic cell 20 as a waste heat source, place the plasma generator 11 at a location away from the electrolytic cell 20, transport the hydrogen generated by the electrolytic cell 20 to the plasma generator 11 by means of a container or pipeline, and supply the hydrogen to the reaction field 16 of the plasma generator 11.

[0030] The second embodiment will be described with reference to FIG. 2. In the first embodiment, the case where a mixed gas of a nitrogen-containing gas and hydrogen generated in the electrolytic cell 20 is activated by the plasma generator 11 to promote the ammonia production reaction has been described. In contrast, in the second embodiment, the case where nitrogen excited by the plasma generator 11 is mixed with hydrogen generated in the electrolytic cell 20 will be described. In the second embodiment, the same reference numerals are given to the same parts as those described in the first embodiment, and the following description will be omitted.

[0031] FIG. 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 a solid oxide type electrolytic cell 20. The space (the first reaction field 31) between the first electrode 12 and the second electrode 13 is connected to a second reaction field 32 including the cathode 21 of the electrolytic cell 20. The second reaction field 32 is exemplified by the cathode chamber of the electrolytic cell 20 and the space connected to the cathode chamber.

[0032] The nitrogen-containing gas flowing into the first reaction field 31 becomes active by dielectric barrier discharge by the plasma generator 11 and is supplied to the second reaction field 32. The hydrogen generated at the cathode 21 is supplied to the second reaction field 32, chemically reacts with the excited nitrogen, and ammonia is synthesized.

[0033] In order to excite nitrogen in the first reaction field 31 and react nitrogen with hydrogen in the second reaction field 32 downstream of the first reaction field 31, when synthesizing ammonia by ionizing a mixed gas of nitrogen and hydrogen in the first reaction field 31, by supplying nitrogen in the same amount as the amount of the mixed gas to the first reaction field 31, the amount of nitrogen excited in the first reaction field 31 can be increased. Therefore, the amount of ammonia produced by the synthesizer 30 can be increased.

[0034] In addition, the active nitrogen molecules and atomic nitrogen generated in the first reaction field 31 also react with hydrogen on the surface of the cathode 21 downstream of the first reaction field 31 to synthesize ammonia. On the surface of the cathode 21, the atomic hydrogen generated on the surface of the cathode 21 by the reduction of water reacts with the active nitrogen molecules and atomic nitrogen before becoming hydrogen molecules, so the selectivity for ammonia is improved. Therefore, the amount of ammonia produced by the synthesizer 30 can be increased.

[0035] However, it is of course possible to supply the hydrogen generated at the cathode 21 of the electrolytic cell 20 to the first reaction field 31. The nitrogen excited in the first reaction field 31 reacts with hydrogen in the first reaction field 31 to synthesize ammonia.

[0036] The third embodiment will be described with reference to FIG. 3. In the first and second embodiments, the case where the second electrode 13 of the plasma generator 11 and the cathode 21 of the electrolytic cell 20 are provided respectively has been described. In contrast, in the third embodiment, a synthesizer 40 in which the cathode 21 of the electrolytic cell 20 also serves as the second electrode of the plasma generator 11 will be described. In the third embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description is omitted.

[0037] FIG. 3 is a schematic cross-sectional view of an ammonia synthesizer 40 according to the third embodiment. The synthesizer 40 includes a plasma generator 11 and a solid oxide type electrolytic cell 20. The dielectric 14 is interposed between the first electrode 12 and the cathode 21, and the power source 15 applies a voltage between the first electrode 12 and the cathode 21. The cathode 21 is a ground electrode connected to the ground.

[0038] The electrolytic cell 20 supplies hydrogen to the space (reaction field 41) between the first electrode 12 and the cathode 21. In the reaction field 41, a mixed gas of a nitrogen-containing gas and hydrogen is activated by the plasma generator 11, and the ammonia generation reaction is promoted. Since the cathode 21 of the electrolytic cell 20 also serves as the second electrode (ground electrode) of the plasma generator 11 in the synthesizer 40, the size of the apparatus can be reduced.

[0039] 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.

[0040] In the embodiments, the synthesizers 10, 30, and 40 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.

[0041] Although not described in the embodiments, the temperatures of the reaction fields 16, 41, the first reaction field 31, and the second reaction field 32 are appropriately set in the range from, for example, room temperature to 500°C. The pressures of the reaction fields 16, 41, the first reaction field 31, and the second reaction field 32 are appropriately set in the range from, for example, 101 kPa to 1000 kPa.

[0042] Although not described in the embodiments, it is possible to dispose a catalyst in the reaction fields 16, 41, the first reaction field 31, and the second reaction field 32. The catalyst is not particularly limited as long as it is present in the reaction fields 16, 41, the first reaction field 31, and the second reaction field 32. For example, the catalyst can be attached to the surface of the dielectric 14 that is in contact with the reaction fields 16, 41, the first reaction field 31, and the second reaction field 32. 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 fields 16, 41, the first reaction field 31, and the second reaction field 32 may be filled with a catalyst. When the reaction fields 16, 41, the first reaction field 31, and the second reaction field 32 are filled with a catalyst, examples of the catalyst include those in which a three-dimensional network structure skeleton made of a porous body of an oxide such as alumina or zirconia supports the catalyst.

Explanation of symbols

[0044] 10, 30, 40 Synthesis apparatus 11 Plasma generator 12 First electrode 13 Second electrode 16, 41 Reaction field 20 Electrolytic cell 21 Cathode 31 First reaction field (reaction field)

Claims

1. A reaction field through which a nitrogen-containing gas flows, and a plasma generator that generates plasma in the reaction field, and a synthesizer that synthesizes ammonia by reacting the excited nitrogen and hydrogen, comprising a solid oxide type electrolytic cell of an oxide ion conductive type, a synthesizer that generates the hydrogen by electrolysis of water using the electrolytic cell.

2. The cathode of the electrolytic cell is located downstream of the electrode of the plasma generator, The synthesizer according to claim 1, wherein the hydrogen generated at the cathode and the excited nitrogen react to synthesize ammonia.

3. The synthesizer according to claim 1, wherein the cathode of the electrolytic cell is the ground electrode of the plasma generator.

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