System and method for producing green urea

Abstract

The invention relates to a device for synthesizing urea, comprising an ammonia source 10 and a urea synthesis unit 20 wherein the ammonia source 10 is connected to the urea synthesis unit 20 by means of a reactant line 30, characterized in that the device comprises an ammonia turbine 40 wherein the ammonia source 10 is connected to the ammonia turbine 40 by means of a fuel line 50, and the ammonia gas turbine 40 is connected to the urea synthesis unit 20 through a steam pipeline 60.

Description

[0001] The present invention relates to the production of green urea. More specifically, the present invention relates to a urea synthesis device having the features described in claim 1.

[0002] The production of green chemicals is becoming increasingly important. The term "green" refers to chemicals that are produced in a sustainable way (i.e. in a climate-neutral and environmentally friendly way), so that no additional carbon dioxide is released into the atmosphere during the production process. Such chemicals can be, for example, chemicals produced by processes in which the energy required for production is provided by renewable (regenerative) energy (e.g., electrical energy obtained from renewable energy sources such as photovoltaic power plants, wind turbines, geothermal power plants or tidal power plants), and the reactants used for production do not come from fossil raw materials. In this context, the production of green ammonia has become one of the focuses of attention, because the production of ammonia currently relies almost entirely on natural gas. In the production of ammonia, carbon dioxide (CO2) is generated as a by-product. If this carbon dioxide is not reused, it is released into the environment. However, urea plants are able to use this carbon dioxide to react with ammonia and convert it into urea, which is mainly used as a fertilizer. The use of the two product streams of the ammonia plant in a urea plant is one of the reasons why these two plants are usually closely integrated. Another advantage of this integrated plant setup is that, for example, steam can be exchanged between the two as an energy carrier. The carbon dioxide bound during the urea production process is eventually released again when the urea is applied to the soil. Because this CO2 originates from the natural gas used in the ammonia production process, urea produced in this way is not considered green.

[0003] Globally, urea production mainly relies on the synthesis of ammonia and CO2, and the process is well known to those skilled in the art. One of the processes is described in US2018 / 0208551 A1.

[0004] EP 3 725 401 A1 discloses a method for using renewable energy in a chemical production process, in which emphasis is placed on increasing the use of electrical energy to replace steam generated primarily from fossil energy.

[0005] CN 111378980 A discloses an energy storage system for producing hydrogen and urea.

[0006] US2019 / 0152901 A1 discloses a process for producing ammonia and further synthesizing urea therefrom, in which a conventional gas turbine fueled by natural gas is used.

[0007] Currently, the production of ammonia and its further conversion into urea usually involves an integrated production facility. Today, ammonia is produced almost exclusively via the Haber-Bosch process. In this process, hydrogen and CO2 are first produced from natural gas, the required heat energy usually likewise coming from the combustion of natural gas. The hydrogen produced is then reacted with nitrogen from the air to form ammonia. The Haber-Bosch process generates steam as process waste heat, which can be used not only for the ammonia production itself but can also be efficiently utilized in other processes, such as the subsequent urea production process. The high steam demand in the urea process is due, on the one hand, to the steam turbines that are usually used for the required CO2 compressors and, on the other hand, to the need to carry out multiple thermal separation processes during the synthesis.

[0008] Switching the synthesis of ammonia to a green synthesis route fundamentally changes its process characteristics. The first step in green synthesis is usually electrolysis using renewable energy to produce hydrogen. Alternatively, ammonia can also be produced directly by electrochemical synthesis. In this process, CO2 is no longer generated as a by-product, which, however, remains a necessary raw material for urea synthesis. No longer relying on the natural gas feedstock used to date, it also makes it possible to preferentially transfer the ammonia synthesis process to regions where renewable energy sources are readily available, such as sunny desert regions. In this context, another point of discussion is that the ammonia produced in this way can also be used over relatively long distances as an easily transportable storage medium for renewable energy. This takes advantage of the fact that ammonia is relatively easy to liquefy (unlike hydrogen, for example) to obtain a liquid energy carrier with a high energy density.

[0009] Unlike ammonia, CO2 is relatively difficult to transport over long distances. Therefore, it is reasonable to foresee that new urea synthesis facilities in the future will be closer to CO2 sources that are currently less utilized or even not utilized, such as waste incineration plants or cement production plants. However, these CO2 sources may be far away from ammonia synthesis facilities. Therefore, one challenge is that the steam (energy) previously provided by ammonia synthesis will not be available for urea production.

[0010] Even if the green ammonia synthesis process is directly combined with the urea synthesis process, the energy demand for urea synthesis cannot be met due to the low steam output of green ammonia synthesis.

[0011] The object of the present invention is to provide a plant for producing green urea from green ammonia, making the urea synthesis no longer dependent on traditional integrated plants based on grey ammonia synthesis (e.g. ammonia synthesis, whose reactants are provided using carbon-based fuels, in particular using fossil fuels such as oil, natural gas, coal or their components, the reactants being obtained, for example, in steam reforming or in electrolysis using electricity generated from said fuels).

[0012] This object of the invention is achieved by a device having the features of claim 1. Advantageous developments can be gathered from the dependent claims, the following description and the drawing.

[0013] The device of the present invention is used for the synthesis of urea. The device comprises an ammonia source and a urea synthesis unit. The urea synthesis unit is a conventional urea synthesis unit for producing urea, which is well known to those skilled in the art. The urea synthesis unit usually consists of a plurality of process steps and devices, including a urea reactor. The object of the present invention is in particular not to change the actual synthesis process of urea (i.e. the urea synthesis plant itself), but to design its connection mode with the external environment so that even if the urea synthesis process is no longer connected to the ammonia synthesis device running on natural gas, it can be stably operated according to the current state of the art. In the sense of the present invention, the definition of an ammonia source is relatively broad. The ammonia source can be, for example, an ammonia synthesis device, but can also be a storage tank, or a connection to an ammonia pipeline, etc. The key is that the ammonia source does not provide steam for the urea synthesis unit, or the amount of steam provided is insufficient, and CO2 is no longer provided. The ammonia source is connected to the urea synthesis unit via a reactant pipeline, in particular to one or more devices of the urea synthesis unit. According to the present invention, the device comprises an ammonia turbine. The ammonia source is connected to the ammonia turbine via a fuel pipeline. The ammonia turbine is connected to the urea synthesis unit via a steam pipeline.

[0014] An ammonia turbine is a gas turbine that uses ammonia as a fuel gas (fuel). In an ammonia turbine, ammonia is burned to generate nitrogen and water, which are released into the environment. The ammonia turbine can provide thermal and mechanical energy, which can be used in different ways for a urea synthesis unit. The ammonia turbine is different in structure and function as a gas turbine from those turbines that use existing hot gas flows to drive further units such as compressors (for example, WO 2020 / 212926 A1 describes a urea synthesis device that uses a turbocharger to transport carbamate, thereby achieving the combined delivery of ammonia and carbamate driven by liquid ammonia). Specifically, an ammonia turbine can be used to generate a hot gas flow, the kinetic energy of which can be used as a driving force for a compressor and / or a generator, for example for compressing process gases or generating electrical energy. Therefore, ammonia turbines for industrial use are currently being built to convert green energy carrier ammonia into electrical energy. For example, WO 2015 / 192877A1 discloses a method for generating electricity using an ammonia turbine, wherein green ammonia is used as a fuel gas. The ammonia turbine thus provides the energy required for the urea process, rather than relying on energy from grey ammonia synthesis.

[0015] In the present case, ammonia is used as fuel gas in the ammonia turbine. The ammonia source is therefore connected not only to the urea synthesis unit via a reactant line but also to the ammonia turbine via a fuel line. The ammonia turbine is also connected to the urea synthesis unit via a steam line; this can be achieved in particular in that the waste heat generated in the ammonia turbine is used to generate steam in a downstream heat exchanger (of course, the steam can also come from a heat exchanger downstream of the actual combustion process), which steam can be conveyed to the urea synthesis unit via a steam line in order to utilize the thermal energy of the steam in the urea synthesis unit, for example to heat the reaction mixture to a desired temperature.

[0016] The advantage of using green ammonia as an energy supply is that it is not necessary to provide renewable energy production at the location of the installation or to ensure its reliable presence there. Since the combustion of green ammonia produces CO2-free energy, this makes it possible to provide green energy for urea synthesis in a simple way. The combustion of gaseous ammonia can be carried out efficiently in gas turbines, which require much less space than other renewable energy supply methods, such as wind farms and / or photovoltaic plants.

[0017] The use of green ammonia both as a feedstock for urea synthesis and for the production of green energy (such as electricity or steam), combined with the use of CO2 from other sources, means that urea produced in this way can be described as green urea in the sense of being CO2-neutral.

[0018] In a further embodiment of the invention, the ammonia turbine is connected to the Haber-Bosch reactor for ammonia synthesis via a further steam line. More specifically, the steam generated in the Haber-Bosch process is further superheated in the ammonia turbine and then conveyed to the urea reactor. For this purpose, the ammonia turbine is preferably equipped with a heat exchanger in which the steam is further heated using combustion tail gases, in particular tail gases from the combustion of ammonia. The steam can be used as a heat transfer medium in the urea synthesis unit, for example only in one or more heat exchangers of the urea reactor. Thus, the steam serves only as a heat transfer medium.

[0019] In a further embodiment of the invention, the ammonia turbine is designed to directly drive a CO2 compressor. For example, in tail gas scrubbing or biogas plants, CO2 is generated at near-ambient pressure. In the urea synthesis process, CO2 needs to be compressed to about 150 bar. Direct coupling enables a dual use of the ammonia turbine: waste heat is used directly in the process and mechanical power is used to compress the CO2.

[0020] In another embodiment of the invention, the ammonia turbine is connected to the generator in an interference fit. Thus, the ammonia turbine drives the generator, thereby generating electricity through the combustion of ammonia. The generator is electrically connected to the urea synthesis unit. The advantage of this combination is that it constitutes a reliable power source for the rest of the infrastructure. This is particularly true when using green ammonia to produce green electricity, as other renewable energy sources such as solar and electricity may be subject to fluctuations.

[0021] In a further embodiment of the invention, the generator is electrically connected to the CO2 compressor.

[0022] When the speed of the ammonia turbine / compressor is not synchronized with the speed of the generator, power can be transmitted between the two devices through a gearbox.

[0023] Depending on the operating state, the combustion of ammonia may lead to the formation of nitrogen oxides in the ammonia turbine, which must be removed in a subsequent exhaust gas treatment process. The person skilled in the art is familiar with common treatment methods, such as selective catalytic reduction.

[0024] In a further embodiment of the invention, the ammonia turbine is connected to the CO2 compressor in an interference fit. More specifically, the ammonia turbine and the CO2 compressor can be arranged on the same shaft and can optionally include a gearbox. In this embodiment, the ammonia turbine directly drives the CO2 compressor mechanically, thereby avoiding energy losses that may occur when, for example, electrical energy is converted and the CO2 compressor is electrically driven.

[0025] Another embodiment of the present invention comprises a thermal decomposition device (thermal cracking device). The thermal decomposition device is used to decompose ammonia into hydrogen and nitrogen. The thermal decomposition device is connected to an ammonia source, which provides ammonia to the decomposition device. The thermal decomposition device is also connected to an ammonia turbine. Since the ammonia combustion process itself may be affected by fluctuations, in some variants of the ammonia turbine, hydrogen is mixed in to achieve a more stable combustion. For the decomposition, it is helpful that the nitrogen and hydrogen are in equilibrium with the ammonia, and this equilibrium can be shifted towards the element at low pressure.

[0026] In a further embodiment of the invention, the device comprises a hydrogen source. For the purposes of the present invention, the hydrogen source can be, for example, a hydrogen electrolysis device, a hydrogen storage tank or a connection to a pipeline for hydrogen transportation. The hydrogen source is connected to the ammonia turbine via a hydrogen pipeline. This makes it possible to meter a certain amount of hydrogen into the ammonia, making a more stable combustion possible.

[0027] In a further embodiment of the invention, the ammonia turbine is connected to the thermal decomposition device via a heat conduit.The waste heat of the ammonia turbine is thus used to at least partially decompose the ammonia back into hydrogen and nitrogen.

[0028] The device of the present invention will be further described in detail by means of exemplary embodiments shown in the following drawings.

[0029] Figure 1 : First implementation scheme

[0030] Figure 2 : Second Implementation Option

[0031] Figure 3 : The third implementation scheme

[0032] Figure 1 A first embodiment of the device according to the invention is shown. The device is preferably arranged at a location close to a suitable CO2 source 80. The CO2 source 80 can be, for example, a waste incineration plant, a biogas plant or a direct air capture process. Since the ammonia used to produce green urea is not produced exclusively from natural gas, the source of carbon dioxide is no longer an ammonia synthesis unit as before. The device also includes an ammonia source 10. In theory, the ammonia source 10 can be an ammonia synthesis unit. However, the latter only makes sense if there are sufficient renewable energy sources for the production of green ammonia at the site and if there is a suitable CO2 source 80 for urea production. But this is not true in all cases; on the contrary, it is expected that this is not the case in most cases. Therefore, ammonia will be transported from the ammonia synthesis unit to the user, such as the device according to the invention. Therefore, the ammonia source 10 can also be a storage tank or a connection to an ammonia pipeline.

[0033] Ammonia is delivered from the ammonia source 10 to the urea synthesis unit 20 via the reactant conduit 30. Likewise, carbon dioxide from the CO2 source 80 is delivered to the urea synthesis unit 20 via the CO2 compressor 70, where the carbon dioxide is pressurized to the pressure required for urea synthesis. In the urea synthesis unit 20, carbon dioxide and ammonia react to form urea. Urea leaves the urea synthesis unit 20 as a product and is delivered, for example, to a granulation unit, and optionally, the urea is mixed with other components before being delivered to the granulation unit.

[0034] To provide the required energy, the device comprises an ammonia turbine 40. The ammonia turbine 40 is connected to the ammonia source 10 via a fuel pipeline 50. If green ammonia is reacted in the ammonia turbine 40, the energy produced thereby will also be carbon dioxide-free. When ammonia is burned, only nitrogen and water are formed and released into the environment.

[0035] The ammonia turbine 40 is connected to the CO2 compressor 70 in an interference fit manner, more specifically, the two are arranged on the same shaft and can optionally include a gearbox. Therefore, the ammonia turbine 40 directly drives the CO2 compressor 70. At the same time, the waste heat generated by the ammonia turbine 40 is used to produce steam in a downstream heat exchanger, and the steam is transported to the urea synthesis unit 20 through the steam pipe 60. In this case, the steam can also come from the heat exchanger downstream of the actual combustion process.

[0036] As shown, the ammonia turbine 40 can also be selectively connected to the generator 90 in an interference fit manner. The electrical energy generated by the generator 90 can be provided to the urea synthesis unit 20 via an electrical connection 100 .

[0037] Figure 2 A second embodiment is shown, which additionally includes a thermal decomposition device 110 on the basis of the first embodiment. The thermal decomposition device 110 receives waste heat from the ammonia turbine 40 through a heat pipe 120. A portion of ammonia from the ammonia source 10 is transported to the thermal decomposition device 110, and the resulting mixture of ammonia, hydrogen and nitrogen is transported to the ammonia turbine 40. The additional hydrogen makes the combustion of the ammonia turbine 40 more stable.

[0038] Figure 3 A third embodiment is shown, which is Figure 1 The difference of the first embodiment shown is that the ammonia turbine 40 is connected to the generator 90 only by means of an interference fit, more particularly, both are arranged on the same shaft. The generator 90 is connected to the CO2 compressor 70 by means of an electrical connection, so that the CO2 compressor 70 is driven only by electricity. The advantage of this embodiment is that the ammonia turbine 40 can drive the generator 90 in an optimal operating mode, which makes it easier to cope with fluctuating power demands, for example in the urea synthesis unit 20, and can respond independently of the CO2 gas flow to be compressed.

[0039] Reference numerals list

[0040] 10 Ammonia source

[0041] 20 Urea synthesis unit

[0042] 30 Reactant pipeline

[0043] 40 Ammonia Turbine

[0044] 50 Fuel pipeline

[0045] 60 Steam Pipe

[0046] 70 CO2 compressor

[0047] 80 CO2 source

[0048] 90 Generator

[0049] 100 Electrical connection

[0050] 110 Thermal Decomposition Device

[0051] 120 Heat conduction

[0052] 130 Product Export

[0053] 140 Exhaust gas.

Claims

1. A device for synthesizing urea, the device comprising an ammonia source (10) and a urea synthesis unit (20), wherein: An ammonia source (10) is connected to a urea synthesis unit (20) via a reactant pipeline (30), characterised in that the device comprises an ammonia turbine (40), wherein the ammonia source (10) is connected to the ammonia turbine (40) via a fuel pipeline (50), and the ammonia turbine (40) is connected to the urea synthesis unit (20) via a steam pipeline (60).

2. The device according to claim 1, characterized in that The ammonia turbine (40) is designed to drive the CO2 compressor (70).

3. A device according to any preceding claim, characterised in that The ammonia turbine (40) is connected to the generator (90) in an interference fit manner, and the generator (90) is electrically connected to the urea synthesis unit (20).

4. The device according to claim 3, characterized in that The generator (90) is electrically connected to the carbon dioxide compressor (70).

5. A device according to any preceding claim, characterised in that The ammonia turbine (40) is connected to the CO2 compressor (70) in an interference fit manner.

6. A device according to any preceding claim, characterised in that The device comprises a thermal decomposition device (110) for decomposing ammonia into hydrogen and nitrogen, wherein the thermal decomposition device (110) is connected to an ammonia source (10) and the thermal decomposition device (110) is connected to an ammonia turbine (40).

7. The device according to claim 6, characterized in that The ammonia turbine (40) is connected to the thermal decomposition device (110) via a heat pipe (120).

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