Patent Description:
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 CO2, removal of CO2 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 <NUM> bar whilst the makeup gas may be produced at about <NUM> 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 <NUM> to <NUM> 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 CO2.

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 CO2 emissions, e.g. by considering the amount of CO2 per ton of ammonia produced. The CO2 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 CO2 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 CO2 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 CO2 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 CO2 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 <NUM> bar and typically <NUM> to <NUM> 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 <NUM>-<NUM> 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.

<CIT> and <CIT> disclose methods for producing ammonia wherein a part of the hydrogen of the ammonia make-up gas is formed by reforming of hydrocarbons and another portion of the hydrogen of the ammonia make-up gas is formed by electrolysis of water. The electrolysis unit is powered by renewable energy.

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 <NUM>. 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 CO2 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 <NUM> bar, however in any case the cost for compression of the hydrogen directed to storage will be greatly reduced thanks to 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 <NUM> bar, more preferably <NUM> bar to <NUM> 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 <NUM> bar, preferably <NUM> to <NUM> bar and particularly preferably <NUM> to <NUM> 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 <NUM>% of the total hydrogen in the make-up gas, preferably for <NUM>% to <NUM>%. However in some embodiments the green hydrogen may account for a greater part (more than <NUM>%) 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.

<FIG> is a simplified scheme of a preferred embodiment of the present invention wherein the following main items are represented.

The natural gas <NUM> after desulfurization is steam reformed in the primary reformer <NUM> and the so obtained partially reformed gas is further processed in the secondary reformer <NUM>. The effluent of said secondary reformer is purified to obtain the make-up gas <NUM>.

The required amount of nitrogen may be introduced with the air feed <NUM> firing the secondary reformer <NUM>.

The make-up gas <NUM> is fed to the ammonia loop <NUM> by the main syngas compressor <NUM>. After pre-heating in the exchanger <NUM>, the pre-heated makeup gas <NUM> is reacted in the ammonia converter <NUM>.

The effluent <NUM> of the converter preheats the fresh make-up gas in the exchanger <NUM> and goes to the cooling and separation stage <NUM>. From here, ammonia <NUM> and a purge gas <NUM> are separated.

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

The hydrogen recovery unit <NUM> 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 <NUM> containing recovered hydrogen can be sent to the inlet of the main compressor <NUM> via line <NUM> and/or to the H2 storage <NUM> via line <NUM>. In the line <NUM>, a compressor may be provided in case the storage pressure is greater than that of stream <NUM>, i.e. greater than the delivery pressure of the HRU <NUM>.

The remaining gas separated in the HRU <NUM> may be combustible and recycled as a fuel to the primary reformer <NUM> with line <NUM>.

The inlet line of the compressor <NUM> is connected to the electrolyser <NUM>, via a green hydrogen feed line <NUM>. The H2 storage <NUM> has an output line <NUM> connected to said green hydrogen feed line <NUM>.

In operation, the main syngas compressor <NUM> receives the makeup gas <NUM> conventionally produced in the front-end <NUM> together with the green hydrogen of line <NUM> and the recovered hydrogen from line <NUM>. For example the hydrogen from the electrolyser <NUM> may be about <NUM>% of the hydrogen fed to the compressor <NUM>.

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

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

It has to be noted that the recovered hydrogen <NUM> and the hydrogen stored in the storage <NUM> 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 <NUM> separated from the removal unit <NUM> 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.

Claim 1:
Process for the synthesis of ammonia wherein:
a) ammonia make-up gas containing hydrogen and nitrogen is reacted in an ammonia converter (<NUM>) at ammonia synthesis pressure obtaining an ammonia-containing effluent (<NUM>);
b) said ammonia-containing effluent is subject to a cooling and separation step (<NUM>) obtaining liquid ammonia (<NUM>) and a side stream (<NUM>) containing hydrogen and impurities;
c) at least a portion of said side stream (<NUM>) is subject to a hydrogen recovery process obtaining recovered hydrogen (<NUM>);
d) a first portion of the hydrogen contained in the ammonia make-up gas is produced by reforming a hydrocarbon source in a reforming process;
e) a second portion of the hydrogen contained in the ammonia make-up gas is produced separately from said reforming process by using a renewable energy source;
f) at least a portion (<NUM>) of said recovered hydrogen obtained at step c) is sent to a hydrogen storage (<NUM>);
g) hydrogen (<NUM>) from said storage (<NUM>) is used to fully or partly replace said second portion of hydrogen of step e) when said renewable energy source is fully or partly unavailable.