WO2023156444A1

WO2023156444A1 - Electrochemical and chemical synthesis of ammonia

Info

Publication number: WO2023156444A1
Application number: PCT/EP2023/053734
Authority: WO
Inventors: Nicolai Antweiler; Katja POSCHLAD
Original assignee: Thyssenkrupp Industrial Solutions Ag; Thyssenkrupp Ag
Priority date: 2022-02-16
Filing date: 2023-02-15
Publication date: 2023-08-24

Abstract

The present invention relates to a system for the synthesis of ammonia, wherein the system comprises at least one first electrochemical ammonia synthesis device (20) for generating a gas mixture that consists of ammonia, hydrogen and nitrogen, and a converter (40) for generating ammonia, said device also having a recirculatory system, wherein a first ammonia separator (30) is located between the at least one first electrochemical ammonia synthesis device (20) and the converter (40).

Description

Electrochemical and chemical synthesis of ammonia

The invention relates to the combination of an electrochemical ammonia synthesis with a classic high-pressure, high-temperature ammonia synthesis.

Ammonia is traditionally produced using the Haber-Bosch process at pressures of around 150 to 350 bar and temperatures of around 400 to 500 °C. For this purpose, nitrogen and hydrogen are compressed and reacted over a suitable catalyst. The ammonia formed is separated off and unreacted starting materials are circulated. A large part of the hydrogen used for this comes from the reforming of natural gas. Therefore, ammonia is currently responsible for a relevant part of global CC emissions.

In order to change this, attempts are being made to use regenerative energies and to produce hydrogen from water, for example by means of electrolysis. The nitrogen then comes from an air separation. The gas mixture is then fed to a conventional converter, also known as an ammonia converter, and converted to ammonia in a known manner.

There is now also a way of generating ammonia directly during electrolysis. A method for the electrochemical production of ammonia is known, for example, from DE 10 2016 213 360 A1. Here, however, an unwanted side reaction is the formation of hydrogen. In order to optimize the selectivity in the direction of ammonia, a low coverage of the surface is optimal, but then the conversion is minimal. With high and economically interesting conversions, however, the proportion of hydrogen as an unwanted by-product increases.

The object of the invention is to produce ammonia in an efficient manner by using, in particular, regenerative energies.

This object is achieved by the system with the features specified in claim 1 and the method with the features specified in claim 12. Advantageous developments result from the dependent claims, the following description and the drawing. The plant according to the invention is used for the synthesis of ammonia. In particular, the plant for the synthesis of ammonia using regenerative energies serves to avoid CC emissions and thus serves to protect the climate. The plant has at least a first electrochemical ammonia synthesis device for generating a gas mixture of ammonia, hydrogen and nitrogen and a converter for generating ammonia with a recirculation of the unreacted hydrogen-nitrogen mixture after the ammonia separation, hereinafter referred to as the recirculation circuit. The gas mixture of ammonia, hydrogen and nitrogen contains ammonia, hydrogen and nitrogen as synthesis components (i.e. as the components to be used for a subsequent ammonia synthesis in the converter in the recirculation circuit - including any electrochemically synthesized ammonia - in particular with a total content of these three components of at least 80 vol. %, preferably at least 90% by volume, particularly preferably at least 99% by volume, based on the total volume of the gas mixture), but this can optionally also contain other components such as water or argon. In addition, the gas mixture can contain other components such as oxygen, carbon dioxide, helium, neon, methane, krypton and the like (especially small amounts, especially in traces - so that every other component is present in a maximum proportion of 100 ppm, each based on the total volume of the gas mixture). In this way, the plant according to the invention differs fundamentally from the previous concepts, which use either a direct electrochemical production of ammonia or an electrochemical production of hydrogen and a subsequent synthesis of ammonia according to the Haber-Bosch process. Ammonia is first produced in the first electrochemical ammonia synthesis device, with the production of hydrogen being accepted as a side reaction for economically viable operation. A first ammonia separator is arranged between the at least one first electrochemical ammonia synthesis device and the converter. In this way, the electrochemically generated ammonia is separated before the remaining mixture, which consists of hydrogen and nitrogen (in addition to any other components mentioned above and any other components), is fed to the converter, for example in accordance with a classic Haber-Bosch process, and fed there high pressure and high temperature over a catalyst. If, at high throughputs, a selectivity of 50%, for example, is achieved in the first electrochemical ammonia synthesis device, i.e. half of the hydrogen is converted to ammonia NH3 and the other half to elemental hydrogen H2, there is the advantage that the size of the converter is only half as large must be, as compared to a purely electrochemical generation of hydrogen H2 exclusively. At the same time, only half the volume flow has to be compressed and heated during operation. At the same time, compared to a purely electrochemical synthesis, a very high throughput can be achieved with complete conversion of the electrochemically generated hydrogen (and the electrochemically directly generated ammonia).

Conveniently, several electrochemical ammonia synthesizers can be connected in parallel side by side to generate a sufficient flow of synthesis gas for the converter.

In a further embodiment of the invention, the plant has a storage device between the first electrochemical ammonia synthesis device and the converter. The storage device is preferably arranged between the first ammonia separator and the recirculation circuit. The storage device is, for example and preferably, a pressure accumulator. The storage device is used to buffer fluctuations in the regenerative energy generation and to provide the converter with an educt stream that is as constant as possible or at least one that fluctuates as little as possible.

In a further embodiment of the invention, the first electrochemical ammonia synthesis device is designed to synthesize ammonia from nitrogen and water. Here, nitrogen can be present as pure nitrogen. However, there is also the option of using air if oxygen is being transported through the membrane. Alternatively, there are electrochemical ammonia synthesizers which produce ammonia electrochemically from nitrogen and hydrogen. However, this has the disadvantage that the hydrogen is first produced from water in a first electrolysis in a water electrolysis and ammonia is produced from nitrogen and hydrogen only in a second step. As a result, the expenditure on equipment is higher and losses also increase. Therefore, direct electrochemical production from nitrogen and water is preferred. A further advantage, if the first electrochemical ammonia synthesis device is designed to synthesize ammonia from nitrogen and water, is that this synthesis can easily be operated dynamically, ie it can be very well adapted to fluctuating power generation. In this way, for example, it is possible to react to the daily change in the electricity production of a solar field. There are two different basic modes of operation of electrochemical ammonia synthesizers. In the first embodiment, water is converted to oxygen on the anode side and protons migrate through the membrane and are reacted with nitrogen to form ammonia (and hydrogen as a by-product) in the cathode compartment. In the second embodiment, nitrogen and water are converted to ammonia (and hydrogen as a by-product) on the cathode side, oxygen ions migrate through the membrane and form oxygen on the anode side.

In another embodiment of the invention, the first electrochemical ammonia synthesizer is a solid oxide electrolytic cell. The membrane is thus a solid oxide, the solid oxide electrolytic cell is usually operated at high temperatures and is particularly suitable for conducting oxygen ions O ² '.

In a further embodiment of the invention, the anode side of the first electrochemical ammonia synthesis device is designed to convert water vapor into oxygen. The membrane of the first electrochemical ammonia synthesis device is designed for proton transfer. The cathode side of the first electrochemical ammonia synthesis device is designed to convert nitrogen into ammonia with the protons passing through the membrane.

In a further embodiment of the invention, an air separation plant is arranged upstream of the first electrochemical ammonia synthesis device. The separation into, in particular, oxygen and nitrogen is particularly preferably carried out by the membrane process. A cryotechnical air separation, for example according to the Linde process, is also possible, usually achieves high purity, but is usually more energy-intensive. In a further alternative embodiment of the invention, a carbon dioxide separator (also CO2 scrubber) is arranged upstream of the first electrochemical ammonia synthesis device. In particular, an exhaust gas flow from a combustion process, for example from another neighboring plant, is cleaned from nitrogen and carbon dioxide by separating the carbon dioxide, for example in order to bind it chemically in order to avoid CO2 emissions. However, this leads to a stream of nitrogen which is comparatively easy to obtain.

In a further embodiment of the invention, a first heat exchanger is arranged after the first electrochemical ammonia synthesis device and before the first ammonia separator. Cooling can be achieved as a result, and ammonia can thus be separated more easily and efficiently. This is especially true when the first electrochemical ammonia synthesizer is operated at high temperature.

In a further embodiment of the invention, a first compressor is arranged after the first heat exchanger and before the recirculation circuit. In the first compressor, the gas stream is preferably brought to the pressure of the recirculation circuit.

In a further embodiment of the invention, the recirculation circuit has a second ammonia separator. Furthermore, the recirculation circuit has at least one second heat exchanger between the converter and the second ammonia separator. Furthermore, the recirculation circuit between the ammonia separator and the converter has at least a third heat exchanger and a second compressor. This corresponds to the classic structure of the Haber-Bosch process.

In a further embodiment of the invention, the second heat exchanger is connected to the electrochemical ammonia synthesis device in a vapor-conducting manner. This allows the thermal energy from the converter in the electrochemical Ammonia synthesis device are transferred in a simple manner and at the same time

Water can be provided for the synthesis of the ammonia.

In a further embodiment of the invention, the cathode side of the first electrochemical ammonia synthesis device has a catalyst, the catalyst containing nitrogen-doped carbon.

In a further aspect, the invention relates to a method for producing ammonia, the method having the following steps: a) providing nitrogen and water, b) electrochemical conversion to produce a gas mixture of ammonia, hydrogen and nitrogen, c) separating off the ammonia from the gas mixture, d) compressing the gas mixture, e) converting the gas mixture in a converter to produce ammonia, f) separating off the ammonia produced in the converter, g) returning unreacted hydrogen and nitrogen to the converter.

In a further embodiment of the invention, the nitrogen in step a) is provided by means of air separation.

In a further embodiment of the invention, the water is provided from low-calorific water vapor from a heat exchanger. As a result, the low-calorific energy can still be used, so that the overall efficiency of the entire system can be increased.

In a further embodiment of the invention, in step b) on the anode side of the first electrochemical ammonia synthesis device for generating a gas mixture of ammonia, hydrogen and nitrogen, water vapor is converted into oxygen and protons. Further, protons are transported through the membrane of the first electrochemical ammonia synthesizer. And further, on the cathode side of the first electrochemical ammonia synthesizer, nitrogen is converted into ammonia with the protons passing through the membrane. In a further embodiment of the invention, hydrogen is formed in step b) on the cathode side of the first electrochemical ammonia synthesis device from the protons passing through the membrane. This by-product, which is undesirable as a side reaction for the purely electrochemical synthesis of ammonia, represents the starting material for the ammonia produced in the converter.

In a further embodiment of the invention, nitrogen and water are converted into ammonia in step b) on the cathode side of the first electrochemical ammonia synthesis device. Oxygen ions also pass through the membrane. And further, on the anode side of the first electrochemical ammonia synthesizer, the oxygen ions passing through the membrane are converted into oxygen.

In a further embodiment of the invention, the gas mixture is cooled in a heat exchanger between step b) and step c).

In a further embodiment of the invention, the gas mixture is cooled in a heat exchanger between step e) and step f). Here, water vapor is generated in the heat exchanger. The water vapor thus generated is fed to the electrochemical ammonia synthesizer.

The system according to the invention or the method according to the invention is particularly preferred if the ammonia conversion in both parts of the system is in the same order of magnitude, i.e. in particular if between 20% ammonia and 80% hydrogen to 80% ammonia and 20% hydrogen are formed in the electrochemical ammonia synthesis device. If the proportion of ammonia is lower, then pure water electrolysis with Haber-Bosch synthesis makes sense; if the proportion of ammonia is higher, the subsequent Haber-Bosch synthesis becomes less economical. The invention thus enables the economic use of a synthesis route that is otherwise (not yet) economical on its own.

The system according to the invention is explained in more detail below with reference to an exemplary embodiment illustrated in the drawing. 1 exemplary embodiment

1 shows an exemplary embodiment of the plant for the synthesis of ammonia according to the invention. About the air supply 11 air is fed into an air separation plant 10 and for example by means of a

Membrane separation process separated into nitrogen and oxygen (and others

components, such as argon or carbon dioxide in smaller amounts). The oxygen can either be used or released into the environment. The oxygen delivery 12 is used for this purpose. The nitrogen is fed into the electrochemical ammonia synthesis device 20 via the nitrogen transfer 13 in order to produce a gas mixture of ammonia, hydrogen and nitrogen. In addition, water vapor is supplied to the electrochemical ammonia synthesis device 20 via the water supply 21 . There, the educt stream is converted at least partially, preferably in the amount of about 50%, into ammonia, while the other 50% are obtained as hydrogen and nitrogen, ie as a educt gas mixture for the Haber-Bosch synthesis. Corresponding electrochemical cells are known to the person skilled in the art and can be found, for example, in Amar, I.A.; Lan, R.; Petit, C.T.G.; Tao, S. Journal of Solid State Electrochemistry 2011, 15, 1845-1860. For example, it is a high-temperature electrolysis cell. The product-educt mixture 23 produced in this way is discharged and cooled via a heat exchanger W and fed to the first ammonia separator 30 as a cooled product-educt mixture 24 . The oxygen generated in the electrochemical ammonia synthesis device 20 is released via the oxygen release 22, either also fed to further utilization or released into the environment.

The ammonia separated in the first ammonia separator 30 is removed via the ammonia discharge 31 and fed, for example, to a tank, a bottling plant or a further conversion to, for example, urea and/or nitric acid. Separately from this, the educt mixture 32 of hydrogen and nitrogen is fed to the recirculation circuit via a compressor. Since, with the selectivity of the ammonia synthesis assumed above in the electrochemical ammonia synthesis device 20, the educt flow is only half as large as in a pure water electrolysis and a subsequent exclusive Ammonia synthesis, all of the following components in the high-pressure and high-temperature range can be reduced to about half their size. Via a further compressor, the educt mixture is fed to the converter 40 for the production of ammonia and, according to the classic principle of ammonia synthesis, is converted proportionately to ammonia over a catalyst in an equilibrium reaction. The product-educt mixture 41 emerging from the converter 40 is cooled via a heat exchanger W (usually in several stages) and fed to the second ammonia separator 50 as a cooled product-educt mixture 42 . The ammonia flow separated there is discharged via the ammonia discharge 51 and can, for example, be combined with the ammonia flow discharged from the first ammonia separator 30 via the ammonia discharge 31 . Alternatively, the two streams of ammonia can also be used for different purposes, for example a urea synthesis and the other a nitric acid synthesis. The unreacted educts are fed from the second ammonia separator 50 via the educt return line 52 to the recirculation circuit, where they are heated by a heat exchanger W and then fed back to the converter 40 via the compressor K. The two heat exchangers W are preferably at least partially connected to one another via a heat transfer medium, so that the heat is retained within the recirculation circuit. However, due to the exothermic reaction, part of the heat from the recirculation loop must be removed to the outside. This heat can be used, for example, to heat a high-temperature electrolysis cell and thus provide the necessary energy, as indicated in FIG. 1 by the steam line 43 between the heat exchanger and the water supply 21.

In addition to the configurations described above, the invention also relates to other electrochemical ammonia synthesis devices for generating a gas mixture of ammonia, hydrogen and nitrogen, for example those in which nitrogen and water (which can optionally also contain other components) are used as starting materials using an oxygen-ion-conducting medium , so that the product gas stream is a gas mixture which also contains unreacted water in addition to ammonia, hydrogen and nitrogen. Reference sign

10 Air Separation Plant

11 air supply

12 oxygen delivery

13 nitrogen delivery

20 electrochemical ammonia synthesizer

21 water supply

22 oxygen delivery

23 product-educt mixture

24 cooled product-educt mixture

30 first ammonia separator

31 Ammonia Release

32 educt mixture

40 converters

41 product-educt mixture

42 cooled product-educt mixture

43 steam line

50 second ammonia separator

51 Ammonia release

52 educt recycling

K compressor

W heat exchanger

Claims

patent claims

1. Plant for synthesizing ammonia, the plant having at least a first electrochemical ammonia synthesis device (20) for generating a gas mixture of ammonia, hydrogen and nitrogen and a converter (40) for generating ammonia with a recirculation circuit, wherein between the at least one first electrochemical ammonia synthesis device (20) and the converter (40), a first ammonia separator (30) is arranged.

2. Plant according to claim 1, characterized in that the first electrochemical ammonia synthesis device (20) is designed for the synthesis of ammonia from nitrogen and water.

3. Plant according to one of the preceding claims, characterized in that the first electrochemical ammonia synthesis device (20) is a solid oxide electrolysis cell.

4. Plant according to one of the preceding claims, characterized in that the anode side of the first electrochemical ammonia synthesis device (20) is designed to convert water vapor into oxygen, the membrane of the first electrochemical ammonia synthesis device (20) being designed for proton transfer, the cathode side of the first electrochemical ammonia synthesis device (20) for converting nitrogen with the protons passing through the membrane to form ammonia.

5. Plant according to one of the preceding claims, characterized in that upstream of the first electrochemical ammonia synthesis device (20) an air separation plant (10) is arranged.

6. Plant according to one of the preceding claims, characterized in that in front of the first electrochemical ammonia synthesis device (20) a carbon dioxide separation plant is arranged. Plant according to one of the preceding claims, characterized in that a first heat exchanger (W) is arranged after the first electrochemical ammonia synthesis device (20) and before the first ammonia separator (30). Plant according to one of the preceding claims, characterized in that a first compressor (K) is arranged after the first heat exchanger (W) and before the recirculation circuit. Plant according to one of the preceding claims, characterized in that the recirculation circuit has a second ammonia separator (50), the recirculation circuit having at least one second heat exchanger (W) between the converter (40) and the second ammonia separator (50), the recirculation circuit between the second ammonia separator (50) and the converter (40) has at least a third heat exchanger (W) and a second compressor (K). Plant according to Claim 9, characterized in that the second heat exchanger (W) is connected to the electrochemical ammonia synthesis device (20) in order to carry steam. Plant according to one of the preceding claims, characterized in that the cathode side of the first electrochemical ammonia synthesis device (20) has a catalyst, the catalyst containing nitrogen-doped carbon. A method for producing ammonia, the method having the following steps: a) providing nitrogen and water, b) electrochemical conversion to produce a gas mixture of ammonia, hydrogen and nitrogen, c) separating the ammonia from the gas mixture, d) compressing the gas mixture, e) converting the gas mixture in a converter (40) to produce ammonia, f) separating off the ammonia produced in the converter (40), g) returning unreacted hydrogen and nitrogen to the converter (40).

13. The method according to claim 12, characterized in that the nitrogen is provided in step a) by means of air separation.

14. The method according to any one of claims 12 to 13, characterized in that the water is provided from low-calorific water vapor from a heat exchanger (W).

15. The method according to any one of claims 12 to 14, characterized in that in step b) on the anode side of the first electrochemical

Ammonia synthesis device (20) water vapor is converted into oxygen, protons being transported through the membrane of the first electrochemical ammonia synthesis device (20), nitrogen being converted into ammonia with the protons passing through the membrane on the cathode side of the first electrochemical ammonia synthesis device (20).

16. The method according to any one of claims 12 to 15, characterized in that in step b) on the cathode side of the first electrochemical ammonia synthesis device (20) hydrogen is formed from the protons passing through the membrane.

17. The method according to any one of claims 12 to 14, characterized in that in step b) on the cathode side of the first electrochemical ammonia synthesis device (20), nitrogen and water are converted into ammonia, with oxygen ions passing through the membrane, with the anode side of the first electrochemical ammonia synthesis device (20), the oxygen ions passing through the membrane are converted to oxygen.

18. The method according to any one of claims 12 to 17, characterized in that between step b) and step c) the gas mixture is cooled in a heat exchanger (W). The method according to any one of claims 12 to 18, characterized in that between step e) and step f) the gas mixture is cooled in a heat exchanger (W), wherein in the heat exchanger (W) water vapor is generated, the water vapor of the electrochemical ammonia synthesis device

(20) is supplied.