Electrochemical and chemical synthesis of ammonia
Patent Information
- Application Number
- EP2023704203
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-02-16
- Filing Date
- 2023-02-15
- Publication Date
- 2024-12-25
AI Technical Summary
Current ammonia production methods, such as the Haber-Bosch process, rely heavily on fossil fuels and result in significant greenhouse gas emissions, while electrochemical methods face inefficiencies due to unwanted hydrogen by-products, limiting selectivity and economic viability.
A system combining electrochemical ammonia synthesis with a high-pressure, high-temperature Haber-Bosch process, utilizing regenerative energies to produce a gas mixture with high ammonia content, which is then recycled and converted using a converter, optimizing selectivity and reducing hydrogen by-products.
This approach enhances ammonia production efficiency, reduces greenhouse gas emissions, and allows for economically viable operation by achieving high throughput and selectivity, with the potential for dynamic adaptation to fluctuating energy sources.
Smart Images
Figure 1.1
Abstract
Description
[0001] Electrochemical and chemical synthesis of ammonia
[0002] The invention relates to the combination of an electrochemical ammonia synthesis with a classical high-pressure, high-temperature ammonia synthesis.
[0003] Traditionally, ammonia is produced using the Haber-Bosch process at pressures of approximately 150 to 350 bar and temperatures of approximately 400 to 500 °C. Nitrogen and hydrogen are compressed and reacted over a suitable catalyst. The resulting ammonia is separated, and unconverted reactants are recycled. A large portion of the hydrogen used for this process comes from the reforming of natural gas. Therefore, ammonia currently accounts for a significant portion of global carbon dioxide emissions.
[0004] To change this, attempts are being made to use renewable energies and produce hydrogen from water, for example, using electrolysis. The nitrogen then comes from air separation. The gas mixture is then fed into a conventional converter, also known as an ammonia converter, and converted into ammonia in the usual way.
[0005] It is now also possible to produce ammonia directly during electrolysis. For example, DE 10 2016 213 360 A1 describes a process for the electrochemical production of ammonia. However, an unwanted side reaction is the formation of hydrogen. To optimize selectivity toward ammonia, a low surface coverage is optimal, but this results in minimal conversion. However, at high and economically viable conversions, the proportion of hydrogen increases as an unwanted byproduct.
[0006] The object of the invention is to produce ammonia in an efficient manner by using, in particular, renewable energies.
[0007] This object is achieved by the plant having the features specified in claim 1 and the method having the features specified in claim 12. Advantageous further developments emerge from the subclaims, the following description and the drawing. The plant according to the invention serves to synthesize ammonia. In particular, the plant serves to synthesize ammonia using renewable energies to avoid CC emissions and thus contributes to climate protection. 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 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 vol.%, particularly preferably at least 99 vol.%, based on the total volume of the gas mixture). However, it may optionally also contain other components such as water or argon. Furthermore, the gas mixture may contain other components such as oxygen, carbon dioxide, helium, neon, methane, krypton, and the like (especially in small amounts, especially in traces—so that each other component is present in a proportion of a maximum of 100 ppm, in each case based on the total volume of the gas mixture).This fundamentally distinguishes the plant according to the invention from previous concepts, which utilize either direct electrochemical production of ammonia or electrochemical production of hydrogen followed by ammonia synthesis according to the Haber-Bosch process. First, ammonia is produced in the first electrochemical ammonia synthesis device, whereby the production of hydrogen is 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.This separates the electrochemically produced ammonia before the remaining mixture, which consists of hydrogen and nitrogen (along with any other components mentioned above and any remaining components), is fed to the converter, for example according to a classic Haber-Bosch process, where it is converted on a catalyst at high pressure and high temperature. If a selectivity of, for example, 50% is achieved in the first electrochemical ammonia synthesis device at high throughputs, i.e. half of the hydrogen is converted to ammonia NH3 and the other half to elemental hydrogen H2, this has the advantage that the converter only needs to be half the size compared to the purely electrochemical production of hydrogen H2. At the same time, only half the volume flow needs to be compressed and heated during operation.At the same time, a very high throughput can be achieved with complete conversion of the electrochemically produced hydrogen (and the electrochemically directly produced ammonia) compared to a pure electrochemical synthesis.
[0008] Conveniently, several electrochemical ammonia synthesis devices can be connected in parallel to generate a sufficient synthesis gas flow for the converter.
[0009] In a further embodiment of the invention, the system comprises 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 serves to buffer fluctuations in renewable energy generation and to provide the converter with a feed stream that is as constant as possible or at least with minimal fluctuations.
[0010] In a further embodiment of the invention, the first electrochemical ammonia synthesis device is designed to synthesize ammonia from nitrogen and water. In this case, nitrogen can be present as pure nitrogen. However, there is also the option of using air if oxygen is transported through the membrane. Alternatively, there are electrochemical ammonia synthesis devices that produce ammonia electrochemically from nitrogen and hydrogen. However, this has the disadvantage that the hydrogen is first produced from water in a first electrolysis step, and only in a second step is ammonia produced from nitrogen and hydrogen. This increases the equipment complexity and also leads to higher losses. 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 be easily operated dynamically, i.e. it can be very well adapted to fluctuating power generation. This makes it possible to react, for example, to the daily changes in power production from a solar field. There are two different basic modes of operation of electrochemical ammonia synthesis devices. In the first embodiment, water is converted to oxygen on the anode side, and protons migrate through the membrane and are reacted with nitrogen in the cathode compartment to form ammonia (and hydrogen as a by-product). In the second embodiment, nitrogen and water are converted to ammonia (and hydrogen as a by-product) on the cathode side, and oxygen ions migrate through the membrane to form oxygen on the anode side.
[0011] In a further embodiment of the invention, the first electrochemical ammonia synthesis device is a solid oxide electrolysis cell. The membrane is thus a solid oxide, the solid oxide electrolysis cell is typically operated at high temperatures, and is particularly suitable for conducting oxygen ions O 2 ' suitable.
[0012] In a further embodiment of the invention, the anode side of the first electrochemical ammonia synthesis device is designed to convert water vapor to 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 to ammonia using the protons passing through the membrane.
[0013] In a further embodiment of the invention, an air separation plant is arranged upstream of the first electrochemical ammonia synthesis device. The separation into oxygen and nitrogen, in particular, is particularly preferably carried out using the membrane process. Cryogenic air separation, for example using the Linde process, is also possible and usually achieves high purity, but is usually more energy-intensive. In a further alternative embodiment of the invention, a carbon dioxide separation plant (also known as a CO2 scrubber) is arranged upstream of the first electrochemical ammonia synthesis device. In particular, the exhaust gas stream from a combustion process, for example from a neighboring plant, is purified from nitrogen and carbon dioxide by separating the carbon dioxide, for example to chemically bind it to avoid CO2 emissions. However, this leads to a nitrogen stream that is comparatively easy to obtain.
[0014] In a further embodiment of the invention, a first heat exchanger is arranged downstream of the first electrochemical ammonia synthesis device and upstream of the first ammonia separator. This allows cooling to be achieved, thus allowing ammonia to be separated more easily and efficiently. This is especially true when the first electrochemical ammonia synthesis device is operated at high temperatures.
[0015] In a further embodiment of the invention, a first compressor is arranged downstream of the first heat exchanger and upstream of the recirculation circuit. In the first compressor, the gas stream is preferably brought to the pressure of the recirculation circuit.
[0016] In a further embodiment of the invention, the recirculation circuit comprises a second ammonia separator. Furthermore, the recirculation circuit comprises at least one second heat exchanger between the converter and the second ammonia separator. Furthermore, the recirculation circuit comprises at least one third heat exchanger and a second compressor between the ammonia separator and the converter. This corresponds to the classic design of the Haber-Bosch process.
[0017] In a further embodiment of the invention, the second heat exchanger is connected to the electrochemical ammonia synthesis device in a steam-carrying manner. This allows the thermal energy from the converter to be transferred to the electrochemical ammonia synthesis device in a simple manner and, at the same time,
[0018] Water must be provided for the synthesis of ammonia.
[0019] In a further embodiment of the invention, the cathode side of the first electrochemical ammonia synthesis device comprises a catalyst, wherein the catalyst contains nitrogen-doped carbon.
[0020] In a further aspect, the invention relates to a process for producing ammonia, the process comprising 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 to produce ammonia, f) separating the ammonia produced in the converter, g) returning unreacted hydrogen and nitrogen to the converter.
[0021] In a further embodiment of the invention, the nitrogen in step a) is provided by air separation.
[0022] In a further embodiment of the invention, the water is provided from low-calorific steam from a heat exchanger. This allows the low-calorific energy to be utilized, thus increasing the overall efficiency of the entire system.
[0023] In a further embodiment of the invention, in step b), water vapor is converted into oxygen and protons on the anode side of the first electrochemical ammonia synthesis device to produce a gas mixture of ammonia, hydrogen, and nitrogen. Protons are then transported through the membrane of the first electrochemical ammonia synthesis device. Furthermore, on the cathode side of the first electrochemical ammonia synthesis device, nitrogen is converted to ammonia using the protons passing through the membrane. In a further embodiment of the invention, in step b), hydrogen is formed from the protons passing through the membrane on the cathode side of the first electrochemical ammonia synthesis device. This by-product, which is undesirable as a side reaction in the purely electrochemical synthesis of ammonia, represents the starting material for the ammonia produced in the converter.
[0024] In a further embodiment of the invention, in step b), nitrogen and water are converted to ammonia on the cathode side of the first electrochemical ammonia synthesis device. Oxygen ions then pass through the membrane. And, on the anode side of the first electrochemical ammonia synthesis device, the oxygen ions passing through the membrane are converted to oxygen.
[0025] In a further embodiment of the invention, the gas mixture is cooled in a heat exchanger between step b) and step c).
[0026] In a further embodiment of the invention, the gas mixture is cooled in a heat exchanger between step e) and step f). Steam is generated in the heat exchanger. The steam thus generated is fed to the electrochemical ammonia synthesis device.
[0027] The plant according to the invention or the process according to the invention is particularly preferred when the ammonia conversion in both plant components is of the same order of magnitude, i.e., in particular, when the electrochemical ammonia synthesis device produces between 20% ammonia to 80% hydrogen and 80% ammonia to 20% hydrogen. If the ammonia content is lower, pure water electrolysis with Haber-Bosch synthesis is appropriate; if the ammonia content is higher, the subsequent Haber-Bosch synthesis becomes less economical. The invention thus enables the economical use of a synthesis route that would otherwise (not yet) be economically viable on its own.
[0028] The system according to the invention is explained in more detail below using an exemplary embodiment shown in the drawing. Fig. 1 exemplary embodiment
[0029] Fig. 1 shows an exemplary embodiment of the plant for the synthesis of ammonia according to the invention. Air is fed into an air separation plant 10 via the air supply 11 and, for example, by means of a
[0030] Membrane separation process in nitrogen and oxygen (and other
[0031] components, such as argon or carbon dioxide in smaller quantities). The oxygen can either be used for purposes or released into the environment. This is done by the oxygen release 12. The nitrogen is passed via the nitrogen transfer 13 into the electrochemical ammonia synthesis device 20 to produce a gas mixture of ammonia, hydrogen and nitrogen. In addition, water vapor is fed via the water supply 21 to the electrochemical ammonia synthesis device 20. There, the reactant stream is at least partially converted into ammonia, preferably to the extent of about 50%, while the other approximately 50% is obtained as hydrogen and nitrogen, i.e. as a reactant gas mixture for the Haber-Bosch synthesis. Suitable electrochemical cells are known to the person skilled in the art and can be found, for example, in Amar, IA; Lan, R.; Petit, CTG; Tao, S. Journal of Solid State Electrochemistry 2011, 15, 1845-1860.For example, this is a high-temperature electrolysis cell. The product-reactant mixture 23 thus produced is discharged and cooled via a heat exchanger W. It is then fed to the first ammonia separator 30 as a cooled product-reactant mixture 24. The oxygen produced in the electrochemical ammonia synthesis device 20 is released via the oxygen release 22, either also being used for further utilization or being released into the environment.
[0032] The ammonia separated in the first ammonia separator 30 is removed via the ammonia discharge 31 and fed, for example, to a tank, a filling station, or for further conversion to, for example, urea and / or nitric acid. Separately, the reactant mixture 32 of hydrogen and nitrogen is fed to the recirculation circuit via a compressor. Since, with the selectivity of ammonia synthesis assumed above in the electrochemical ammonia synthesis device 20, the reactant flow is thus only half as large as with pure water electrolysis followed by exclusive ammonia synthesis, all subsequent components in the high-pressure and high-temperature range can be reduced to approximately half their size. The reactant mixture is fed to the converter 40 for ammonia production via a further compressor and, according to the classic principle of ammonia synthesis, is partially converted to ammonia in an equilibrium reaction over a catalyst.The product-reactant mixture 41 exiting the converter 40 is cooled via a heat exchanger W (usually multi-stage) and fed as a cooled product-reactant mixture 42 to the second ammonia separator 50. The ammonia stream separated there is discharged via the ammonia discharge 51 and can, for example, be combined with the ammonia stream discharged from the first ammonia separator 30 via the ammonia discharge 31. Alternatively, the two ammonia streams can also be used for different purposes, for example, one for urea synthesis and the other for nitric acid synthesis. The unreacted reactants are fed from the second ammonia separator 50 via the reactant return 52 to the recirculation circuit, where they are heated via a heat exchanger W and then fed back to the converter 40 via the compressor K.Preferably, the two heat exchangers W are at least partially connected to each other via a heat transfer medium, so that the heat is retained within the recirculation circuit. However, due to the exothermic reaction, some of the heat must be dissipated from the recirculation circuit 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.
[0033] In addition to the previously described embodiments, the invention also relates to other electrochemical ammonia synthesis devices for producing a gas mixture of ammonia, hydrogen, and nitrogen, for example, those in which nitrogen and water (which may optionally also contain other components) are used as reactants using an oxygen ion-conducting medium, so that the product gas stream is a gas mixture which, in addition to ammonia, hydrogen, and nitrogen, also contains unreacted water. Reference numerals
[0034] 10 air separation plant
[0035] 11 Air supply
[0036] 12 Oxygen release
[0037] 13 Nitrogen transfer
[0038] 20 electrochemical ammonia synthesis device
[0039] 21 Water supply
[0040] 22 Oxygen release
[0041] 23 Product-reactant mixture
[0042] 24 cooled product-reactant mixture
[0043] 30 first ammonia separator
[0044] 31 Ammonia tax
[0045] 32 reactant mixture
[0046] 40 converters
[0047] 41 Product-reactant mixture
[0048] 42 cooled product-reactant mixture
[0049] 43 Steam line
[0050] 50 second ammonia separator
[0051] 51 Ammonia tax
[0052] 52 Educt recycling
[0053] K Compressor
[0054] W heat exchanger
Claims
Patent claims 1. Plant for the synthesis of ammonia, wherein the plant comprises at least a first electrochemical ammonia synthesis device (20) for the production of a gas mixture of ammonia, hydrogen and nitrogen and a converter (40) for the production of ammonia with a recirculation circuit, wherein a first ammonia separator (30) is arranged between the at least one first electrochemical ammonia synthesis device (20) and the converter (40).
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 configured for the conversion of water vapor to oxygen, wherein the membrane of the first electrochemical ammonia synthesis device (20) is configured for proton transfer, and wherein the cathode side of the first electrochemical ammonia synthesis device (20) is configured for the conversion of nitrogen with the protons passing through the membrane to ammonia.
5. Plant according to one of the preceding claims, characterized in that an air separation plant (10) is arranged upstream of the first electrochemical ammonia synthesis device (20).
6. Plant according to one of the preceding claims, characterized in that a carbon dioxide separation plant is arranged upstream of the first electrochemical ammonia synthesis device (20). A system according to any one of the preceding claims, characterized in that a first heat exchanger (W) is arranged downstream of the first electrochemical ammonia synthesis device (20) and upstream of the first ammonia separator (30). A system according to any one of the preceding claims, characterized in that a first compressor (K) is arranged downstream of the first heat exchanger (W) and upstream of the recirculation circuit. A system according to any one of the preceding claims, characterized in that the recirculation circuit comprises a second ammonia separator (50), wherein the recirculation circuit comprises at least one second heat exchanger (W) between the converter (40) and the second ammonia separator (50), and wherein the recirculation circuit between the second ammonia separator (50) and the converter (40) comprises at least one third heat exchanger (W) and a second compressor (K).The system according to claim 9, characterized in that the second heat exchanger (W) is connected to the electrochemical ammonia synthesis device (20) via a steam flow. The system according to any of the preceding claims, characterized in that the cathode side of the first electrochemical ammonia synthesis device (20) has a catalyst, wherein the catalyst contains nitrogen-doped carbon. A process for the production of ammonia, the process comprising the following steps: a) provision of nitrogen and water, b) electrochemical reaction to generate a gas mixture of ammonia, hydrogen, and nitrogen, c) separation of the ammonia from the gas mixture, d) compression of the gas mixture, e) conversion of the gas mixture in a converter (40) to generate ammonia. f) Separation of the ammonia produced in the converter (40), g) Return of unreacted hydrogen and nitrogen to the converter (40).
13. Method according to claim 12, characterized in that the nitrogen in step a) is provided by means of air separation.
14. Method according to one of claims 12 to 13, characterized in that the water is provided from low-calorific steam from a heat exchanger (W).
15. Method according to 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 to oxygen, whereby protons are transported through the membrane of the first electrochemical ammonia synthesis device (20), wherein on the cathode side of the first electrochemical ammonia synthesis device (20) nitrogen is reacted with the protons passing through the membrane to form ammonia.
16. Method according to one of claims 12 to 15, characterized in that in step b) hydrogen is formed from the protons passing through the membrane on the cathode side of the first electrochemical ammonia synthesis device (20).
17. Method according to one of claims 12 to 14, characterized in that in step b) nitrogen and water are converted to ammonia on the cathode side of the first electrochemical ammonia synthesis device (20), wherein oxygen ions pass through the membrane, and wherein the oxygen ions passing through the membrane are converted to oxygen on the anode side of the first electrochemical ammonia synthesis device (20).
18. Method according to one of claims 12 to 17, characterized in that the gas mixture is cooled in a heat exchanger (W) between step b) and step c). A 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 water vapor is generated in the heat exchanger (W), wherein the water vapor is supplied to the electrochemical ammonia synthesis device (20) is supplied.