EP4396127A1

EP4396127A1 - Plant and process for the continuous production of ammonia using renewable energies

Info

Publication number: EP4396127A1
Application number: EP22773137.9A
Authority: EP
Inventors: Ingo Wagner
Original assignee: Uniper Technologies GmbH
Current assignee: Uniper Technologies GmbH
Filing date: 2022-08-31
Publication date: 2024-07-10

Abstract

The present invention relates to a plant and a process for the continuous production of ammonia using renewable energy. The plant comprises at least one cracking unit for the catalytic cracking of ammonia. According to the invention, the process ensures that one part of the produced ammonia is catalytically split again, namely when the availability decreases and/or when the amount of renewable energy falls below a minimum amount or when the supply of gaseous hydrogen falls below a minimum amount.

Description

Plant and process for the continuous production of ammonia using renewable energy

field of invention

The present invention relates to a plant and a method for the continuous production of ammonia using renewable energies. The plant comprises at least one cracking unit for the catalytic cracking of ammonia. The method provides for part of the ammonia produced to be catalytically split again, namely when availability decreases and/or when the amount of renewable energies falls below a minimum level or when the supply of gaseous hydrogen falls below a minimum level.

Technical background and state of the art

Ammonia is one of the basic chemicals. It is produced in the Haber-Bosch process through the catalytic conversion of so-called synthesis gas. In the context of ammonia synthesis, synthesis gas refers to a mixture of hydrogen and nitrogen in a ratio of approximately 3:1.

Ammonia production worldwide amounts to hundreds of millions of tons per year. The largest proportion of ammonia currently still goes to a traditional field of application, namely the production of fertilizers. However, it is to be expected that new markets for ammonia will emerge in the future, which will result in a significant increase in demand for ammonia and the use in fertilizers will take a back seat. Because ammonia is gaining in importance as an energy carrier and hydrogen storage medium. It has a high energy density and can be stored in tanks comparatively easily. In addition, in contrast to hydrogen, it liquefies at ambient pressure at minus 33 °C or at ambient temperature at a pressure of approx. 8.5 bar, which makes transport easier. There are also concepts for using ammonia as a fuel in shipping. Of course, the use of ammonia as an energy carrier and hydrogen storage only makes ecological sense if ammonia is also produced sustainably. In the classic, non-sustainable ammonia synthesis process, hydrogen is used, which is obtained from a fossil raw material (e.g. natural gas). Despite the increasing scarcity of fossil raw materials, hydrogen can be generated as needed in this way and made available in sufficient quantities for ammonia synthesis. An even utilization of the ammonia synthesis plant is guaranteed. In particular, when hydrogen is generated as required, it can be avoided that the ammonia synthesis plant gets into inefficient or unstable operating states or even has to be shut down completely. An unstable operating condition occurs in most ammonia synthesis plants at a throughput of less than 50% to 60% of the design capacity.

On the other hand, achieving an even utilization of the ammonia synthesis plant in the production of green ammonia represents a challenge. In the production of green ammonia, the hydrogen required for the ammonia synthesis is generated by means of electrolysis, with the electrical energy required for the electrolysis being obtained from renewable sources (e.g. solar energy, wind power or hydroelectric power). The electrical energy available from renewable sources is subject to natural fluctuations. This leads to an insufficient supply of electrical energy to the electrolysis plant and, as a result, to fluctuating amounts of hydrogen produced. In the worst case, the ammonia plant can no longer be fed with hydrogen. If the amount of hydrogen available is below the minimum demand of the ammonia plant required for stable operation, the plant for ammonia synthesis must be shut down and shut down. Frequent shutting down and shutting down and then restarting the ammonia plant is generally undesirable. It leads to accelerated wear of plant parts and lower overall plant effectiveness.

In order to prevent the ammonia plant from being switched off undesirably even when producing sustainably generated, ie green, ammonia, a storage facility for hydrogen or synthesis gas is usually provided. However, this is associated with considerable investment costs. In addition, there are high costs for storing hydrogen (low energy content per volume and high pressure or low temperatures for liquefaction). In addition, the project planning or design of such a storage facility is also complicated by the fact that the availability of renewable energies can only be predicted with difficulty. She is on the one hand from the Type of energy source dependent and, on the other hand, is subject to periods of variable and unknown duration.

Furthermore, ammonia synthesis plants are designed that can still be operated stably at lower throughputs than the usual 50% to 60% of the design capacity. Such an approach is known, for example, from EP 2 589 574 A1. Here, a reduced supply of synthesis gas is compensated for by a corresponding supply of inert gases, such as argon or helium. As a result, the minimum throughput at which stable operation of the ammonia synthesis plant is still possible can be reduced to 10 - 20% of the design capacity.

object of the invention

Against this background, the object of the present invention was to develop an improved plant for the production of ammonia, with which continuous production using only renewable energies is possible and with which the disadvantages of conventional plants described above can be overcome. Furthermore, a process should be specified that allows continuous ammonia production using only renewable energies and that is robust and inexpensive compared to conventional processes.

Summary of the Invention

These objects are solved by the subject matter of the independent claims. Further advantageous embodiments can be derived from the dependent claims.

The plant according to the invention is a plant for the continuous production of ammonia using renewable energies, comprising an electrolyzer for electrolytically splitting water into gaseous hydrogen and oxygen using renewable energies; a unit for providing gaseous nitrogen; a mixer for generating a synthesis gas from the gaseous hydrogen and the gaseous nitrogen; an ammonia synthesis unit for converting the synthesis gas, wherein ammonia is obtained; and at least one cracking unit for catalytically cracking the ammonia obtained in the ammonia synthesis unit, whereby synthesis gas is obtained again.

A conventional unit is used as the ammonia synthesis unit, for example a unit which is suitable for synthesizing ammonia using the classic Haber-Bosch process.

The term renewable energies summarizes regenerative energies that are not based on nuclear power and non-fossil energy sources. Examples of renewable energies are solar, wind, water, bioenergy or geothermal energy.

The plant according to the invention therefore allows an environmentally friendly and sustainable production of ammonia. It is not necessarily dependent on cost-intensive storage for hydrogen and is still able to react to fluctuations in the availability of renewable energies. Switching off the ammonia synthesis unit when the volume flow of hydrogen from the electrolyzer decreases can be effectively avoided.

The cracking unit (the cracking reactor) preferably has a capacity which corresponds at least to the minimum throughput of the ammonia synthesis unit. Ideally, the cracking unit has a capacity that is more than 10% of the design capacity of the ammonia synthesis unit. The capacity of the cracking unit is particularly preferably 10%-30% of the design capacity of the ammonia synthesis unit. This dimensioning ensures that the ammonia synthesis unit does not have to be switched off even in the event of a complete failure of renewable energies and can continue to be run in a stable operating state.

At least one connection is preferably present between the ammonia synthesis unit and the at least one cleavage unit.

The at least one connection may be a connection between the outlet of the ammonia synthesis unit and the inlet of the cracking unit. This connection can include at least one valve element that is set up for fluidic communication to separate or allow between the at least one splitting unit and the ammonia synthesis unit.

In a further preferred embodiment, the system comprises a control unit which is set up to throttle or increase the throughput of the splitting unit depending on the availability of renewable energies and/or the available amount of gaseous hydrogen. The throughput of the splitting unit can be controlled, for example, by regulating the above-mentioned valve element included in the connection between the ammonia synthesis unit and the splitting unit. The availability of renewable energies, on the basis of which the throughput can be controlled, includes the current availability of renewable energies (verifiable by measurement or by asking the grid operator). Where appropriate, the availability of renewable energy also includes an expected value for the development of availability in the near future. The expected value can be calculated on the basis of current data (e.g. the weather conditions) and empirical equations. In particular, it is preferable to increase the throughput of the fission unit when the availability of renewable energy decreases. Conversely, it should be considered to throttle the throughput of the splitting unit again when renewable energies are readily available again after a temporary shortage. The system can thus serve as an energy store in times of overproduction of renewable energies and allows stable operation in times of low availability of renewable energies.

The connection between the outlet of the ammonia synthesis unit and the inlet of the at least one splitting unit can be designed as a direct connection, so that the ammonia obtained can be introduced into the splitting unit of the plant without prior intermediate storage. In this case the connection is a pipe or conduit which may be fitted with various fittings and gauges and may include one or more heat exchangers but does not include a holding tank, buffer tank or other plant units. Alternatively, the connection between the outlet of the ammonia synthesis unit and the inlet of the at least one cleavage unit can comprise a container for storing ammonia and optionally further units.

The advantage of a direct connection is that the heat losses when transferring the ammonia from the ammonia synthesis unit to the cracking unit are low. As a result, part of the energy required to initiate the endothermic cracking of ammonia can be saved. If no green hydrogen is available, the ammonia synthesis unit and splitting unit are coupled with one another and run in the circuit without the need for other plant units, such as heat exchangers, compressors and separators, to be operated.

The advantage of a connection that includes a tank for storing ammonia is that the operation of the cracking unit is largely decoupled from the operation of the ammonia synthesis unit. The throughput of the splitting unit can be regulated independently of the throughput of the ammonia synthesis unit. This represents an additional degree of freedom and improves the possibilities of setting up and maintaining stable operation of the plant. If other units are interposed between the outlet of the ammonia synthesis unit and the inlet of the at least one splitting unit, such as one or more condensers, separators, scrubbers, heat exchangers, compressors and pumps, these units also remain in operation, even if there is not enough renewable energy and/or in the short term sufficient amounts of green hydrogen are available for ammonia synthesis.

In one embodiment, the plant according to the invention may comprise a connection between the outlet of the cracking unit and the inlet of the ammonia synthesis unit. This connection can be configured as a direct connection. In this case, the connection is a pipe or line that can be provided with various fittings and measuring instruments and, if necessary, contains one or more heat exchangers, but no synthesis gas storage for temporary storage of synthesis gas. In this way, the synthesis gas obtained by cracking can be fed directly into the ammonia synthesis unit without prior intermediate storage. Alternatively, the connection between the outlet of the cleavage unit and the inlet of the ammonia synthesis unit can comprise at least one synthesis gas reservoir for the temporary storage of synthesis gas. The synthesis gas reservoir preferably has a small volume and is used to compensate for short-term fluctuations in the supply of synthesis gas from renewable energies, which overall contributes to smoothing out the operation of the ammonia synthesis plant. In addition, the system according to the invention can comprise at least one heat exchanger. This is advantageous since the ammonia synthesis is exothermic and the splitting reaction of ammonia into hydrogen and nitrogen is endothermic and heat integration in the plant can be achieved with the aid of one or more heat exchangers. A part of the energy input, which is necessary for splitting the ammonia, can be recovered from the ammonia synthesis unit. The additional energy required to operate the splitting unit is provided as electrical energy or by incinerating a stream of material containing ammonia or hydrogen from the plant.

Among the preferred embodiments, the embodiment of the plant for the continuous production of ammonia using renewable energy is particularly advantageous, which comprises:

(i) an electrolyser for the electrolytic splitting of water into gaseous hydrogen and oxygen using renewable energy;

(ii) a unit for providing gaseous nitrogen;

(iii) a mixer for generating a synthesis gas from the gaseous hydrogen and the gaseous nitrogen;

(iv) an ammonia synthesis unit for converting the synthesis gas, whereby ammonia is obtained;

(v) at least one cracking unit for catalytic cracking of the ammonia obtained in (iv), synthesis gas being obtained again; and

(vi) a connection between the outlet of the at least one cracking unit and the inlet of the ammonia synthesis unit.

In addition, a method for the continuous production of ammonia using renewable energies is specified, in which gaseous hydrogen is obtained from water using renewable energies (i), gaseous nitrogen is provided (ii); the gaseous hydrogen and the gaseous nitrogen are mixed (iii) to obtain a synthesis gas; and the synthesis gas is converted in an ammonia synthesis unit (iv) to obtain ammonia, the method being characterized in that when availability decreases and/or when a minimum amount of renewable energies is not reached and/or when a minimum amount is not reached gaseous hydrogen in step (i) at least part of the ammonia obtained is catalytically split again (v) in order to provide synthesis gas for step (iv).

The catalytic cracking of ammonia is preferably carried out at a temperature of at least 300.degree. C., preferably in a temperature range from 400 to 900.degree.

At least a portion of the thermal energy produced during the conversion of the synthesis gas in step (iv) is advantageously used to preheat the portion of ammonia intended for the cleavage.

Description of the Preferred Embodiments

Preferred embodiments of the invention are explained in more detail with reference to the following figures, without wishing to limit the invention thereto.

FIG. 1 shows a greatly simplified flow diagram of the method according to the invention according to a first variant. The plant according to the invention, in which this method can be carried out, has a connection between the outlet of the ammonia synthesis plant and the inlet of the at least one splitting unit, which comprises a storage tank for ammonia.

FIG. 2 shows a greatly simplified flow diagram of the method according to the invention according to a second variant. The plant according to the invention, in which this method can be carried out, has a direct connection between the outlet of the ammonia synthesis plant and the inlet of the at least one cleavage unit.

Figure 3 shows a flow chart of the method according to the invention, in which the circuit between the ammonia synthesis plant and the cleavage unit, which has a direct connection between the outlet of the ammonia synthesis plant and the inlet of the at least one cleavage unit, is shown differently than in Figure 2.

Figure 4 shows a flow chart of the method according to the invention, in which the circuit between the ammonia synthesis plant and the cleavage unit, which has a direct connection between the outlet of the ammonia synthesis plant and the inlet of the at least one cleavage unit, is shown differently than in Figures 2 and 3. In Figure 1, (i) designates the electrolyzer for the electrolytic splitting of water, which is operated exclusively with electricity from renewable energies. Stream 1 is the water stream that is fed to the electrolyser. Product stream 2 is gaseous hydrogen (also called green hydrogen). Unit (ii) is, for example, an air separation unit with which a stream 3 of gaseous nitrogen can be provided. The hydrogen gas stream and nitrogen gas stream are mixed in a 3:1 ratio (mixer not shown but present where streams 2 and 3 are combined) to form a synthesis gas 4 . The synthesis gas is fed to the ammonia synthesis unit (iv) and converted there into ammonia. Various further steps for purification and heat exchange can then follow (corresponding units not shown) before the stream of ammonia 5 is fed into a storage tank T. The cracking unit (v), when in operation, draws ammonia as stream 6 from the holding tank. The synthesis gas obtained after cleavage of ammonia is returned to the ammonia synthesis unit (iv) (compound 7). The ammonia synthesis unit can be operated in the loop using the splitting unit and compounds 6 and 7, even if no green hydrogen is available. Since the storage tank T serves as a buffer volume, it is not absolutely necessary to adapt the throughput of the cleavage unit (v) to the throughput of the ammonia synthesis unit (iv).

The flow chart in Figure 2 uses the same units and reference numerals as Figure 1. In contrast to Figure 1, however, there is a direct connection between the outlet of the ammonia synthesis unit (iv) and the inlet of the splitting unit (v) without an intermediate storage tank or other intermediate units. During the operation of the cleavage unit, ammonia is branched off from the ammonia synthesis unit (iv) and fed as stream 6 into the cleavage unit (v). Synthesis gas is recirculated to the ammonia synthesis unit (iv) as stream 7.

As can be seen in FIGS. 3 and 4, the branching off of the ammonia stream from the ammonia synthesis unit (iv) and the recirculation of the synthesis gas can also be designed differently than in FIG. In this context, Figure 3 illustrates the branching off of the ammonia stream after the ammonia synthesis unit (iv). In Figure 4, the arrows indicating the branching of the ammonia stream and the recycle of the synthesis gas, directly to the ammonia synthesis unit (iv). A recirculation of the synthesis gas directly into the ammonia synthesis unit (iv) is also conceivable in the embodiment variant in FIG.

Claims

patent claims

1. Plant for the continuous production of ammonia using renewable energy, comprising:

(ii) a unit for providing gaseous nitrogen;

(iv) an ammonia synthesis unit for converting the synthesis gas, whereby ammonia is obtained; and

(v) at least one cracking unit for catalytic cracking of the ammonia obtained in (iv), synthesis gas being obtained again.

2. Plant according to claim 1, wherein there is a connection between the outlet of the ammonia synthesis unit and the inlet of the at least one splitting unit, the connection preferably comprising at least one valve element which is set up to ensure fluidic communication between the at least one splitting unit and the ammonia synthesis unit to separate or allow.

3. Plant according to claim 2, wherein the connection between the outlet of the ammonia synthesis unit and the inlet of the at least one splitting unit is either direct or comprises at least one further unit which is interposed, wherein the at least one further unit is preferably a tank for storing ammonia .

4. Plant according to one of the preceding claims further comprising a control unit which is set up to throttle or increase the throughput of the splitting unit depending on the availability of renewable energies and/or the available amount of gaseous hydrogen.

5. Plant according to one of the preceding claims further comprising at least one synthesis gas storage for temporary storage of synthesis gas.

6. Plant according to one of the preceding claims further comprising at least one heat exchanger.

7. Process for the continuous production of ammonia using renewable energy, in which

(i) gaseous hydrogen is obtained from water with the help of renewable energies,

(ii) gaseous nitrogen is provided;

(iii) the gaseous hydrogen and the gaseous nitrogen are mixed to obtain a synthesis gas; and

(iv) the synthesis gas is converted in an ammonia synthesis unit in order to obtain ammonia, characterized in that if the availability decreases and/or if a minimum amount of renewable energies is not reached and/or if a minimum amount of gaseous hydrogen is not reached in step (i), at least one Part of the ammonia obtained is catalytically split again (v) to provide synthesis gas for step (iv).

8. The process according to claim 7, wherein the catalytic cracking of ammonia is carried out at a temperature of at least 300°C, preferably from 400 to 900°C.

9. The method according to any one of claims 7 or 8, wherein at least part of the heat energy produced during the conversion of the synthesis gas in step (iv) is used to preheat the portion of ammonia provided for the cleavage.