DESCRIPTION

Field of application The present invention concerns the field of synthesis of ammonia.

Prior art

Ammonia is produced industrially by reacting a make-up gas containing hydrogen and nitrogen in a suitable molar ratio. The make-up gas is conventionally produced by reforming a hydrocarbon source, such as natural gas. The production of the make-up gas typically involves a reforming process and a purification of the reformed gas. The purification typically includes shift conversion of CO to C02; removal of C02 and methanation. The so obtained purified gas is fed to a high-pressure ammonia synthesis loop via a main syngas compressor. In the ammonia synthesis loop, the make-up gas is reacted to form ammonia in a suitable ammonia converter. The hot ammonia-containing effluent of the converter is subject to a cooling and separation step obtaining liquid ammonia and a side stream containing unreacted hydrogen and impurities. Typically, a portion of said side stream is subject to a hydrogen recovery process and the so obtained recovered hydrogen is sent to the suction of said main syngas compressor. Another portion of said side stream is reintroduced into the ammonia converter, typically with a circulator, thus forming the above mentioned ammonia synthesis loop.

The synthesis of ammonia is performed at a high pressure, whilst the make-up gas is normally produced at a much lower pressure. For example the ammonia synthesis may be performed at about 140 bar whilst the makeup gas may be produced at about 40 bar or less. For this reason the main syngas compressor is necessary. In most embodiments the pressure at the suction side of the main syngas compressor is 15 to 35 bar.

The nitrogen required for ammonia synthesis may be introduced during the reforming process, typically in an air-fire fired secondary reforming. In some cases nitrogen may be added separately, e.g. when an air separation unit is available.

To summarize, the industrial production of ammonia relies on the reforming of hydrocarbons to produce the necessary hydrogen. Reforming is typically a fuel- fired process with considerable emissions of C02. There is a fast-growing interest in reducing the carbon footprint of ammonia production. Applicable norms and regulations may introduce additional taxation in relation to the C02 emissions, e.g. by considering the amount of C02 per ton of ammonia produced. The C02 contained in combustion fumes may be captured but the related techniques are expensive. A promising way to achieve this goal is the production of hydrogen with a renewable energy source.

Hydrogen produced with renewable energy is termed “green” hydrogen because it does not cause emissions of C02 in contrast with the conventional fuel-fired production by reforming. Hydrogen produced from combustion of fossil fuels is sometimes termed “grey” hydrogen. The advantage of using green hydrogen is that no C02 is formed and therefore there is no need for expensive capture and sequestration.

An example of a process of practical interest for the production of green hydrogen is represented by electrolysis of water. The electrolysis of water requires electricity which may be produced with a renewable source, for example solar energy, leading to the production of green hydrogen with no release of C02 in the atmosphere.

The hydrogen so produced can be injected in an existing plant typically at the suction of the syngas compressor machine or directly in the synthesis loop in addition to “grey” hydrogen increasing the plant production, decreasing the specific energy consumption of the plant and decreasing the relevant C02 footprint.

The production of hydrogen from renewable energy sources however is typically subject to fluctuations. For example a solar-powered production of hydrogen is obviously subject to the availability of sunlight. In order to compensate for such fluctuations, a storage of green hydrogen should be provided. The storage may fully or partly replace the production of green hydrogen when the energy source is fully or partly unavailable, in order to maintain a significant contribution of the green hydrogen to the ammonia production. The storage of hydrogen must be performed at a high pressure to be economically acceptable. For example, it is considered that a cost-effective storage of hydrogen should be performed at a pressure of at least 50 bar and typically 50 to 300 bar. Regrettably, the commercially available techniques for production of green hydrogen deliver hydrogen at a low or moderate pressure insufficient for storage. For example the available techniques for electrolysis of water can produce hydrogen typically at about 20-30 bar.

It follows that green hydrogen directed to storage requires compression. The necessary hydrogen compressor is an expensive item and consumes a lot of power, thus making the shift to green hydrogen less attractive from economic point of view. Still another problem is the commercially available compressors for hydrogen are typically volumetric compressor which are intrinsically less reliable than turbomachines like centrifugal compressors. In order to ensure acceptable reliability of the system, spare units must be installed which further increase costs. Summary of the invention

The invention aims to overcome the above drawbacks of the prior art. In particular, the present invention aims to provide a process and plant for the synthesis of ammonia wherein the use of so-called green hydrogen, that is hydrogen made from renewable energy, is made more attractive compared to the current prior art.

The aim is reached with a process according to claim 1. In the invention, ammonia is produced using hydrogen conventionally produced with a reforming process together with hydrogen produced from a renewable energy source (green hydrogen). A hydrogen storage is provided and hydrogen from said storage is used to compensate for partial or full unavailability of said renewable energy source.

Said hydrogen storage is fed with some of the hydrogen recovered from the side stream separated after the cooling and separation process of the converter effluent.

The invention comes from the finding that said recovered hydrogen may be considered a partially green hydrogen, being recovered from the ammonia synthesis loop which is fed with conventional hydrogen and green hydrogen. Hence its use to replace the green hydrogen, when the related energy source is unavailable, maintains a positive effect of reducing the overall emissions of C02 for a given capacity in terms of ammonia produced.

On the other hand, the hydrogen recovered from the above mentioned side stream (also termed loop purge) is normally available at a high pressure, considerably higher than the pressure of production of green hydrogen. The pressure of the recovered hydrogen may be compatible with direct storage without compression. Elimination of the compressors for hydrogen storage reduces costs and consumption and removes a possible source of failure. In some embodiments the recovered hydrogen may still need be compressed, e.g. if storage is performed at a very high pressure such as 200 bar, however in any case the cost for compression of the hydrogen directed to storage will be greatly reduced thanks to the invention. Description of the invention

A particularly preferred process for the production of green hydrogen is electrolysis of water. The electrolysis of water may be powered by electric energy produced from one or more renewable sources, for example from solar energy.

In other embodiments, green hydrogen may be produced from biomass. Biomass is widely considered a renewable form of energy because its energy comes from the sun and it can grow in a short time. Biomass can be converted to hydrogen with a thermochemical process or a biological process such as fermentation. Preferred thermochemical processes include gasification, partial oxidation and steam reforming. The source biomass is preferably a lignocellulosic biomass. In a preferred embodiment, a biomass-to-hydrogen process includes gasification of such lignocellulosic biomass.

The hydrogen storage is performed preferably at a pressure of at least 50 bar, more preferably 50 bar to 200 bar. The hydrogen may be stored under pressure in one or more suitable storage vessels.

The recovered hydrogen may have a pressure of at least 50 bar, preferably 50 to 100 bar and particularly preferably 50 to 90 bar. Hence, said recovered hydrogen may be sent to storage without compression whenever the pressure at which the hydrogen is recovered is equal to or greater than the pressure of storage.

In embodiments where the storage pressure is higher, a compression is still required. However the invention is still advantageous due to the relatively high pressure at which the hydrogen is obtained, i.e. due to the reduced compression ratio for storage in comparison with the prior art.

In the invention, ammonia is produced partly with hydrogen conventionally produced from reforming and partly with green hydrogen. The green hydrogen may be used not only to reduce carbon footprint but also to increase capacity. The process may be considered a hybrid process due to the hybrid production of hydrogen. In a typical embodiment the hydrogen produced from renewable energy may account for up to 50% of the total hydrogen in the make-up gas, preferably for 20% to 50%. However in some embodiments the green hydrogen may account for a greater part (more than 50%) of the total hydrogen. The green hydrogen, possibly replaced by the hydrogen from storage, may be directed to the suction side of the main syngas compressor. In a preferred embodiment, the green hydrogen may be produced at the same or substantially the same pressure as a purified make-up gas which is obtained from reforming and purification. The main syngas compressor may have a suction line connected to a reforming front-end wherein the reforming process is performed, and further connected to the producer of the green hydrogen, such as water electrolyser, and further connected to the hydrogen storage.

The reforming process may be performed according to various techniques known in the art. The reforming process may include primary reforming in a fired furnace followed by secondary reforming with a suitable oxidant. The oxidant is normally air but may also be enriched air or pure oxygen if available. The purification of the reformed gas may include CO shift, carbon dioxide removal and methanation.

A preferred embodiment of the invention includes: reforming a hydrocarbon source in a front-end to produce ammonia make-up gas; feeding said ammonia make-up gas to an ammonia synthesis loop, including an ammonia synthesis converter, via a main syngas compressor; removing a hydrogen-containing purge stream from said ammonia loop; processing a portion of said purge stream to separate hydrogen contained therein and to obtain recovered hydrogen; providing a further hydrogen feed separately obtained from a renewable energy source, preferably by electrolysis of water; feeding said further hydrogen to the input of said main syngas compressor; providing a hydrogen storage arranged to partly or fully replace said further hydrogen feed when said renewable energy source is not fully available; feeding at least a portion of said recovered hydrogen to said hydrogen storage.

Preferably a portion of the recovered hydrogen is fed to said storage and a remaining portion is fed to the input of said main syngas compressor for its reintroduction into the ammonia synthesis loop.

In a typical embodiment the ammonia converter is part of an ammonia synthesis loop. The ammonia synthesis loop may include the ammonia converter, a make up gas preheater, a cooling and separation stage and a circulator. The hydrogen separately produced from renewable energy, or hydrogen taken from the hydrogen storage, may be introduced into said loop at a suitable location, preferably via the main syngas compressor.

Another aspect of the invention is a plant according to the claims.

Description of the figures

Fig. 1 is a simplified scheme of a preferred embodiment of the present invention wherein the following main items are represented.

100 reform ing-based front-end

101 ammonia synthesis loop

102 solar-powered water electrolyser for production of hydrogen

103 hydrogen storage

1 hydrocarbon source, such as natural gas

2 desulfurization

3 primary reformer, such as a fired furnace

4 secondary reformer

5 shift converter(s). One or more shift converters may be provided, for example a high-temperature shift converter followed by a low-temperature shift converter.

6 carbon dioxide removal, for example by amine or carbonate solution washing or pressure swing adsorption (PSA) or another technique for removing C02 from a gas.

7 methanation

8 purified make-up gas

9 air feed to the secondary reformer

10 air compressor

11 main syngas compressor

12 syngas driers

13 circulator

14 heat exchanger, fresh gas preheater and reaction effluent cooler

15 ammonia synthesis converter

16 cooling-separation stage

17 hydrogen recovery unit (HRU) based on membrane system or PSA or another suitable technique

18 fuel to the primary reformer 3

SP solar power, i.e. power source of the electrolyser 102

The Fig. 1 is further described below.

The natural gas 1 after desulfurization is steam reformed in the primary reformer 3 and the so obtained partially reformed gas is further processed in the secondary reformer 4. The effluent of said secondary reformer is purified to obtain the make up gas 8. The required amount of nitrogen may be introduced with the air feed 9 firing the secondary reformer 4.

The make-up gas 8 is fed to the ammonia loop 101 by the main syngas compressor 11. After pre-heating in the exchanger 14, the pre-heated makeup gas 28 is reacted in the ammonia converter 15. The effluent 19 of the converter preheats the fresh make-up gas in the exchanger 14 and goes to the cooling and separation stage 16. From here, ammonia 20 and a purge gas 21 are separated.

The purge gas 21 is split into a first portion 29 and a second portion 30. The first portion 29 is sent to the HRU 17 where hydrogen is separated from other impurities, such as non-condensable gases. The second portion 30 is reintroduced in the ammonia loop 101 via the circulator 13. The circulator compensates for pressure drops maintaining the circulation in the loop 101.

The hydrogen recovery unit 17 may use a cryogenic system or a membrane- based system or a PSA. Techniques for removing hydrogen from a gas mixture are known to the skilled person and need not be described.

A stream 22 containing recovered hydrogen can be sent to the inlet of the main compressor 11 via line 23 and/or to the H2 storage 103 via line 24. In the line 24, a compressor may be provided in case the storage pressure is greater than that of stream 22, i.e. greater than the delivery pressure of the HRU 17. The remaining gas separated in the HRU 17 may be combustible and recycled as a fuel to the primary reformer 3 with line 25.

The inlet line of the compressor 11 is connected to the electrolyser 102, via a green hydrogen feed line 26. The H2 storage 103 has an output line 27 connected to said green hydrogen feed line 26. In operation, the main syngas compressor 11 receives the makeup gas 8 conventionally produced in the front-end 100 together with the green hydrogen of line 26 and the recovered hydrogen from line 23. For example the hydrogen from the electrolyser 102 may be about 30% of the hydrogen fed to the compressor 11.

During normal operation, the recovered hydrogen of line 24 is stored for subsequent use and the line 27 may be closed, e.g. by a suitable valve. The recovered hydrogen 22 may be sent partially or totally to the inlet of the compressor 11 or to the storage 103 depending on the conditions. For example when the storage 103 reaches full capacity, the hydrogen 22 may be fully reintroduced into the loop via line 23 whenever is required.

Based on the availability of the power source of the electrolyser 102, the hydrogen from the storage 103 may be used to partly or fully replace the production of said electrolyser 102. For example, assuming the electrolyser 102 uses solar power SP, the stored hydrogen (withdrawn from the storage 103) may be used during night time and/or cloudy conditions when the solar power drops.

It has to be noted that the recovered hydrogen 22 and the hydrogen stored in the storage 103 can be considered a “partially green” hydrogen since it is recovered from a loop partially fed with green hydrogen. Therefore the use of the storage is beneficial in terms of carbon dioxide emissions.

The carbon dioxide 31 separated from the removal unit 6 may be stored or used for another process, for example for the synthesis of urea in a tied-in urea plant. Use of the separated carbon dioxide, instead of its discharge, is clearly another advantage for reducing environmental impact.

In a typical embodiment, the makeup gas 8 is at a pressure of about 20-25 bar. The ammonia converter may operate at about 140 bar. The recovered hydrogen 22 may be at a pressure of 50 to 90 bar or above. The storage may be performed at the pressure of the stream 22 or at a greater pressure using a compressor.