Method and apparatus for producing ammonia using renewable energy sources

Abstract

The present disclosure relates to a device for producing ammonia. The ammonia is produced from hydrogen obtained by electrolysis of water. The electrolysis is powered by energy from renewable sources, supplemented by power obtained from the device during periods of low availability or no availability of said renewable energy. To this end, the device is configured so that it can operate in a charging configuration (acquisition and storage of power) and in a discharging configuration (exploitation of the power).

Description

TECHNICAL FIELD

[0001] The present invention belongs to the technical field of green ammonia production. In particular, the present invention relates to a method and a plant for producing ammonia, wherein a renewable energy source can be used. The present invention further relates to a method of retrofitting a pre-existing ammonia plant.

[0002] INTRODUCTION

[0003] Challenges in today’s chemical industry include: transitioning away from the use of fossil fuels, and adopting renewable energy sources. Ammonia is considered one of the key chemicals in such a transition. One reason is that it can be used as a source of hydrogen, for example in fuel cells, as such. Another reason is that, as an inorganic chemical, it does not require carbon for production.

[0004] The most widely used industrial method for producing ammonia, the Haber-Bosch process, involves the catalytic reaction of hydrogen and nitrogen. Hydrogen is usually obtained from natural gas, but in today’s ideal carbon-free chemistry, hydrogen is obtained by electrolysis of water. The latter requires energy, which is desirably provided by a renewable energy source in a “green” ammonia process.

[0005] A general problem with renewable energy sources is presented by the inherent fluctuations in their supply. Renewable energy sources typically exhibit both primary and secondary fluctuations. For example, a primary fluctuation in solar energy is due to the day / night cycle. On a shorter timescale, such as hours to minutes, secondary fluctuations can be caused by changes in conditions such as wind speed and cloud cover.

[0006] Fluctuations in the supply of renewable energy sources pose problems for the operation of a plant producing green ammonia. If the production rate of ammonia is reduced during periods of low energy supply, the temperatures within the plant can change, causing thermal stress or shocks. The lifetime of ammonia converters, including ammonia production catalysts, can also be shortened by temperature fluctuations. It is therefore desirable to minimize these effects by intelligently managing the energy supply and tuning down the operating conditions. Continuous operation, or substantially continuous operation, is desirable. Such operation typically requires a supplemental energy source when the renewable energy source is not available or reduced.

[0007] WO 2020 / 035521 attempts to address this problem in nitric acid production methods based on the combustion of ammonia. This reference also relates to ammonia production integrated with nitric acid production. The method recovers thermal energy into electrical energy through the turbine expansion of a pressurized liquid gas, which in turn serves as a reactant supply for the method. Therefore, liquefied oxygen is used in nitric acid production, and liquefied nitrogen is used in ammonia production. The heat required to evaporate the pressurized liquid gas before its expansion through the turbine is waste heat generated during the corresponding nitric acid or ammonia production process. Thus, typically, the electrolysis of water to produce hydrogen for ammonia production benefits from energy storage in the production of nitric acid, as integrated therewith.

[0008] CN112179046 also discloses cryogenic energy storage via liquefied gas reactants. This reference relates to ammonia production and aims to adapt ammonia production equipment to variations in energy supply. This reference does not address fluctuations in renewable energy supply but rather relates to electricity supply in response to expensive peak electricity prices and reduced off-peak prices. Therefore, a method is disclosed aimed at reducing energy consumption during peak hours by employing energy stored during off-peak periods. This storage is provided through cryogenic energy storage of nitrogen during off-peak periods, allowing such nitrogen to be released into ammonia synthesis during peak periods. This helps avoid consuming expensive peak energy to produce nitrogen.

[0009] In cases where the electrolysis reaction operates as a substantially continuous process based on energy from renewable energy sources, the aforementioned references do not address the energy supply required to optimize the electrolysis reaction for hydrogen production within the ammonia plant itself. In this regard, and in terms of prudent energy management and general reactant supply, a method and apparatus more optimized for green ammonia production is desired. Specifically, for such a method, if carried out as a substantially continuous process, this is possible despite fluctuations in the renewable energy supply. Summary of the Invention

[0010] In a first aspect, the present invention relates to an apparatus for producing ammonia, the apparatus comprising:

[0011] (a) A catalytic reactor having a nitrogen inlet for nitrogen feed, a hydrogen inlet for hydrogen feed and an ammonia outlet for the ammonia produced;

[0012] (b) An electrolysis unit having a water inlet for water feed, an oxygen outlet for oxygen and a hydrogen outlet for hydrogen, the hydrogen outlet being in fluid communication with the hydrogen inlet of the reactor.

[0013] (c) A nitrogen supply unit having at least one air inlet for an air flow, an oxygen outlet for an oxygen-enriched flow, and a nitrogen outlet for a nitrogen flow; the nitrogen outlet being in fluid communication with the nitrogen inlet of the reactor;

[0014] (d) a low pressure air compression and adsorber section having an air inlet for an external air feed and an air outlet for compressed air, the air outlet being in switchably fluid communication with an air inlet of the nitrogen provision unit and with an air liquefaction section or a high pressure air compression unit having an outlet for compressed air in fluid communication with the air liquefaction section, the air liquefaction section comprising a turboexpander configured to drive an electrical generator to provide a first energy source;

[0015] (e) a liquid air reservoir having a liquid air inlet for liquid air obtained from the air liquefaction section, a closable outlet for liquid air in fluid connection with the nitrogen provision unit and a closable outlet for liquid air in fluid connection with the electrical generator section configured to provide a second energy source and an expanded air stream;

[0016] (f) an electrical control unit configured to power the apparatus;

[0017] wherein the apparatus has a power connection to a renewable energy source and to the first and second energy sources, and wherein the electrical control unit is configured to switch between the renewable energy source and the first and second energy sources, including a combination of the first and second energy sources.

[0018] In a second aspect, the present application provides a method for producing ammonia, the method being carried out in an apparatus as described in the preceding paragraph, the method comprising the steps of:

[0019] (i) obtaining nitrogen from the nitrogen provision unit;

[0020] (ii) obtaining hydrogen from the electrolysis unit;

[0021] (iii) reacting the nitrogen and hydrogen in the catalytic reactor under ammonia formation conditions;

[0022] the method comprising operating the apparatus in a charging mode, said charging mode comprising compressing air in the low pressure air compression and adsorber section to provide LP compressed air; compressing the LP compressed air in the high pressure air compression unit to provide HP compressed air, subjecting the HP compressed air to expansion and liquefaction in the turboexpander so as to obtain liquefied air; storing the liquefied air in the liquid air reservoir; feeding air from the liquid air reservoir to the nitrogen provision unit and / or feeding air from the low pressure air compression and adsorber section to the nitrogen provision unit;

[0023] wherein the method comprises operating the electrolysis unit with energy from the renewable energy source to provide hydrogen.

[0024] In a further aspect, the present application provides a method for producing ammonia, the method being carried out in an apparatus as described in the preceding paragraph, the method comprising the steps of:

[0025] (i) obtaining at least a portion of nitrogen from a nitrogen-providing unit;

[0026] (ii) obtaining at least a portion of hydrogen from an electrolysis unit;

[0027] (iii) reacting the nitrogen and the hydrogen in a catalytic reactor under ammonia- forming conditions;

[0028] The method comprises operating the apparatus in a discharge mode, said discharge mode comprising compressing air in the low-pressure air compression and adsorber section to provide LP compressed air; feeding the LP compressed air to the air liquefaction section; subjecting the LP compressed air to expansion and liquefaction in a turboexpander so as to obtain liquefied air; the turboexpander providing a first source of energy; storing the liquefied air in a liquid air reservoir; feeding air from the liquid air reservoir to the power generator section, thereby expanding the air so as to provide a second source of energy and expanded air, feeding air from either or both of the liquid air reservoir and the expanded air to the nitrogen-providing unit, wherein the method comprises adjusting the operating rates of the nitrogen-providing unit, the electrolysis unit and the catalytic reactor in dependence on the amount of air available to the nitrogen-providing unit, and operating the control unit so as to power the method by the first source of energy and the second source of energy.

[0029] In yet another aspect, the present application relates to a method of retrofitting a pre-existing apparatus for producing ammonia, said pre-existing apparatus comprising:

[0030] - a catalytic reactor having a nitrogen inlet for a nitrogen feed, a hydrogen inlet for a hydrogen feed and an ammonia outlet for a produced ammonia;

[0031] - a hydrogen-providing section having a hydrogen outlet for hydrogen, said hydrogen outlet being in fluid communication with the hydrogen inlet of the reactor;

[0032] - a nitrogen-providing unit having at least one air inlet for an air stream, an oxygen outlet for an oxygen-rich stream and a nitrogen outlet for a nitrogen stream; said nitrogen outlet being in fluid communication with the nitrogen inlet of the reactor;

[0033] - an electrical control unit configured to power the apparatus;

[0034] The method comprises adding to the pre-existing apparatus:

[0035] - an electrolysis unit having a water inlet for a water feed, an oxygen outlet for oxygen and a hydrogen outlet for hydrogen, said hydrogen outlet being in fluid communication with the hydrogen inlet of the reactor;

[0036] - a low pressure air compression and adsorber section having an air inlet for external air feed and an air outlet for LP compressed air, said air outlet being optionally in fluid communication with an air inlet of a nitrogen provision unit and switchably in fluid communication with an air liquefaction section or a high pressure air compression unit having an outlet for compressed air in fluid communication with the air liquefaction section, said air liquefaction section comprising a turbo-expander configured to drive an electrical generator to provide a first energy source;

[0037] - a liquid air reservoir having a liquid air inlet for liquid air obtained from the air liquefaction section, a closable outlet for liquid air in fluid connection with the nitrogen provision unit and a closable outlet for liquid air in fluid connection with an electrical generator section configured to provide a second energy source and an expanded air stream; said electrical generator section having an air outlet in fluid connection with an air inlet of the nitrogen provision unit; and being arranged such that the plant has a power connection to a renewable energy source and to the first and second energy sources and an electrical control unit is configured to be switchable between the renewable energy source and the first and second energy sources, including a combination of the first and second energy sources. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 Parts of the plant according to the invention are schematically shown in order to demonstrate a charging configuration.

[0039] Figure 2 Parts of the plant according to the invention are schematically shown in order to demonstrate an alternative charging configuration.

[0040] Figure 3 Parts of the plant according to the invention are schematically shown in order to demonstrate a discharging configuration.

[0041] Figure 4 Parts of the plant according to the invention are schematically shown in order to demonstrate an alternative discharging configuration.

[0042] Figure 5 A schematic drawing is shown depicting an embodiment of the plant according to the invention, indicating processes, heat and power connections. DETAILED DESCRIPTION

[0043] Broadly speaking, the invention relates to an ammonia production plant capable of operating on a renewable energy source by switching between a charging mode and a discharging mode. In particular, this is based on the insight that a reservoir for liquid air is a useful tool to enable said switching capability. More particularly, the invention also exploits this insight to combine the liquid air refrigerant with heat for expansion extracted from ammonia synthesis.

[0044] The devices and methods disclosed herein serve to better address long-term secondary fluctuations as well as primary fluctuations exhibited by renewable energy sources. For example, switching from a charging mode to a discharging mode can be done according to the diurnal / nightly solar rhythm, but can also be triggered by cloud cover for extended periods of time or, in the case of wind power, by lack of wind.

[0045] Thereby, the devices and methods disclosed herein also address the problem that otherwise conventional energy sources, such as fossil fuel-based and / or nuclear energy-based energy sources, should be available as a supplement to compensate for fluctuations in the availability of renewable energy. Preferably, the devices and methods disclosed herein operate without any supplemental energy sources. Advantageously, the devices are provided as one or more modular, local, stand-alone units provided with renewable energy sources only.

[0046] According to the present disclosure, ammonia can be produced by a Haber-Bosch type process, wherein nitrogen and hydrogen are subjected to a catalytic reaction to form ammonia according to equation 2H2 + N2 -> NH3. The reaction takes place in a reactor provided with a suitable catalyst bed, in the present disclosure simply referred to as catalytic reactor. The catalyst is typically a promoted iron-containing catalyst, typically a multi-promoted magnetite catalyst. An alternative to the iron catalyst is ruthenium. The device thus comprises (a) a catalytic reactor having a nitrogen inlet for a nitrogen feed, a hydrogen inlet for a hydrogen feed, and an ammonia outlet for the produced ammonia. Ammonia synthesis is typically carried out at a pressure in the range of 70 to 350 bar, at a temperature in the range of ambient temperature, such as 20°C, to 600°C. Preferably, the pressure is in the range of 130 to 330 bar, with further preferred ranges being 140 to 250 bar or 280 to 320 bar. The ammonia product thus synthesized is typically depressurized and stored at about 15 to 17 bar. It will be appreciated that obtaining ammonia at atmospheric pressure would require a further reduction of the temperature, which is typically achieved via a cooling radiator.

[0047] In most current conventional ammonia plants, hydrogen is provided from fossil fuels, such as natural gas. In contrast thereto, the green ammonia plant of the present disclosure operates on the basis of electrolysis of water to provide hydrogen. The device thus comprises (b) an electrolysis unit having a water inlet for a water feed, an oxygen outlet for oxygen, and a hydrogen outlet for hydrogen, said hydrogen outlet being in fluid communication with the hydrogen inlet of the reactor.

[0048] Suitable electrolytic cells (electrolyzers) are well known to those skilled in the art. Suitable large scale (>0.1 MW) electrolyzers are typically bipolar electrolyzers with forced convection of the electrolyte. Typically, alkaline electrolytic cells can be used. These are mostly operated using 25-35% KOH (most manufacturers) or 14% NaOH (hydrogen system water electrolyzers). The cathode reaction is typically catalyzed by nickel, typically in the form of a high surface area nickel layer, such as Raney nickel or a porous nickel coating produced in situ by cathodic reduction of a nickel sulphide coating. The separator for the separation of the hydrogen formed by electrolysis is typically made of asbestos, but more modern electrolyzers employ composite materials, for example based on polysulfone or polytetrafluoroethylene hydrophilized with finely dispersed zirconium oxide. Suitable alternatives include nickel mesh supported ceramics, such as NiO or CaTiO-Ni cermet. Water electrolysis can also be performed using a cation exchange membrane, known as a solid polymer exchange (SPE) electrolysis. It operates on the basis of a water swollen cation exchange polymer loaded with protons as electrolyte. In this context, platinum group metal catalysts are typically used. Preferred types of electrolytic cells include alkaline cells, polymer electrolyte membrane cells. A further preferred type of electrolytic unit is a solid oxide electrolyzer unit (SOEC). It is a solid oxide fuel cell operated in regenerative mode to effect electrolysis of water by using a solid oxide or ceramic, electrolyte to produce hydrogen gas and oxygen.

[0049] Different types of electrolytic units are well known to those skilled in the art, as well as how to operate them. The device of the present invention is not limited to any particular type of electrolytic cell.

[0050] Nitrogen is typically provided by separating it from air. In the present disclosure, any such unit is simply referred to as a nitrogen providing unit. Typically, such a nitrogen providing unit is an air separation unit or a nitrogen generation unit. Air separation can be performed by cryogenic distillation of liquefied gases or by non-cryogenic techniques such as pressure swing adsorption, vacuum pressure swing adsorption or membrane separation. Thus, the device comprises (c) a nitrogen providing unit having at least one air inlet for a stream of air, an oxygen outlet for an oxygen rich stream and a nitrogen outlet for a nitrogen stream; the nitrogen outlet being in fluid communication with the nitrogen inlet of the reactor;

[0051] The low pressure compression and adsorber section is used to compress the air to a pressure of typically 8 to 20 bar gauge, such as 8 to 10 bar gauge. The air is cleaned in a compressor inlet filter and is typically compressed using a two stage compressor. The outlet pressure value represents a desired optimum value between typical design parameters for a nitrogen generation unit and the maximum pressure ratio still manageable by a standard non-adiabatic compressor. The air from the compressor is sent to, for example, a first thermal reservoir which utilizes an external heat source of about 235 to 250 °C with sufficient temperature difference to drive sensible heat into the stored air.

[0052] The compressed air typically passes through a CO2 / H2O removal stage. After CO2 / H2O removal, the purified air enters a post-cooler. Prior to being sent to the process air cooler, the compressed air is further pre-cooled in a spray cooler. Typically, a wire mesh located at the top of the spray cooler acts as a water separator to separate the air stream from its coarse moisture components before the air stream enters the subsequent molecular sieve adsorbers. In certain configurations, it is envisaged that a TSA (temperature swing absorber) can be used in place of the more standard adsorbers. Water, CO2and hydrocarbons are typically absorbed by one of the two molecular sieve adsorbers. When one molecular sieve is operating, the other can be regenerated.

[0053] According to the present application, the apparatus comprises (d) a low pressure air compression and adsorber section having an air inlet for external air feed and an air outlet for compressed air. The air outlet is in switchably fluid communication with an air liquefaction section or a high pressure air compression unit having an outlet for compressed air in fluid communication with the air liquefaction section, the air liquefaction section comprising a turbo-expander configured to drive an electrical generator to provide a first source of energy. The air outlet is optionally in fluid communication with an air inlet of a nitrogen provision unit.

[0054] The aforementioned switchable fluid communication enables the apparatus to operate in a mode in which the LP compressed air is further compressed and a mode in which the HP compression is bypassed, with the LP compressed air being sent directly to the air liquefaction section. In the charging mode, renewable energy from an external source is available. In this mode, the apparatus is switched such that the LP compressed air is further compressed in the HP air compression unit. The operating pressure in this unit is typically in the range of 50 to 200 bar gauge, preferably 100 to 200 bar gauge. As a result, the air liquefaction unit downstream of the HP compression unit can function optimally to build up the cryogenic energy storage. In the discharging mode, the HP compression unit is bypassed. It will be appreciated that this reduces the energy consumption of the apparatus as such. In particular, the configuration in the discharging mode enables an efficient recovery of energy from the cryogenic energy storage by intelligently using the liquefied air stored from the air liquefaction section. This involves, as discussed further below, the use of the liquefied air to efficiently recover energy in the electrical generator section. The apparatus is configured such that, in particular when operating in the discharging mode, the amount of liquefied air sent to the nitrogen provision unit can be reduced compared to the operation in the charging mode. In a preferred embodiment, the apparatus comprises a connection for air from the adsorber (in the LP compression and adsorber unit) to the nitrogen provision unit. This allows for a further reduction or even complete avoidance of the use of liquefied air in the nitrogen provision unit. In turn, this allows for an increase of the amount of liquefied air sent to the electrical generator section in order to allow said section to provide a larger amount of energy that can be used as a second source of energy.

[0055] Preferably, the plant of the application makes use of an industrial concept, such as the one used for air separation units, known as the "cold box" technology. The cold box is a self-supporting structure that protects the cryogenic equipment, such as brazed plate fin heat exchangers, core-in drum, distillation columns, piping, valves and instruments. In the plant of the application, the cold box is preferably used for the nitrogen generation (i.e. the nitrogen provision unit) in combination with the cold storage of ammonia. Upstream of the latter, a cooling radiator is provided, which is preferably also integrated in the cold box, for depressurizing the ammonia from the storage pressure (typically 15 to 17 bar gauge) to atmospheric pressure.

[0056] It is understood that the clean and compressed air (from LP compression or HP compression) enters the liquid air / nitrogen provision cold box. The molecular sieve adsorber is a cylindrical vessel filled with molecular sieve packing, which is supported between screens at the air inlet and outlet. The molecular sieve packing absorbs moisture, carbon dioxide and hydrocarbons from the air flowing through it, as the packing has a given maximum capacity for these components, and must be regenerated after a given time, by making a stream of hot dry gas pass through the bed in the opposite direction to the air flow: after regeneration, the adsorber must be cooled with cold dry gas. The purge enriched air from the cold box is re-heated in the hot storage section and used for the regeneration of the adsorber.

[0057] In the cold box turbo-expander, the liquefaction of the air is achieved, and the product (air / nitrogen) stream and the purge enriched air are subsequently distilled to a rectification column. The cold box is a fully insulated encapsulated unit, which contains plate fin heat exchangers in which the gas and liquid streams are divided into multiple channels, separated from each other by flat plates. The air is cooled from approximately ambient temperature to the liquefaction temperature, while the outgoing gas stream is almost heated to the temperature of the incoming air. The turbo-expander is designed to work under near isentropic expansion conditions: radial inflow usually enables such conditions, and via a synchronous generator, it is efficiently converted into electricity. It consists of a casing, wheels and a generator assembled on a common shaft, and an oil supply system. The clean air stream to be cooled enters the turbo at pressure and drives the wheels coupled to the generator. The expansion of the gas through the turbo reduces the gas temperature at the outlet to close to the liquefaction temperature of the air.

[0058] The air liquefaction section thus comprises a turbo-expander configured to drive a generator to provide a first source of energy. This first source of energy, as applicable in the charging mode, is preferably used to supplement the renewable source of energy.

[0059] On the liquid air side of the air liquefaction section, an outlet is connected with an air inlet of a liquid air reservoir. Such reservoirs are provided with the necessary insulation material to maintain a cold environment sufficient for the air to remain in liquid state. The liquid air reservoir is configured as a key unit for enabling the device to operate in charging and discharging mode. To this end, the liquid air reservoir has a closable outlet for liquid air in fluid connection with the nitrogen provision unit. By allowing to close or open, this outlet enables an operational choice to either provide or not provide liquid air to the nitrogen provision unit. Preferably, a fluid connection is provided between the liquid air reservoir and the nitrogen provision unit, which is configured to regulate the amount of liquid air sent to the nitrogen provision unit. The liquid air reservoir further has a closable outlet for liquid air in fluid connection with the generator section. Again, this connection is preferably configured to regulate the amount of liquid air sent to the generator section. It will be appreciated that the above-mentioned closable connections can be completely separate, or can be combined. In the former case, the liquid air reservoir can be provided with two closable outlets for the two respective fluid connections. In the latter case, the liquid air reservoir can be provided with a single outlet in connection with a three-way valve or other system, allowing to switch the connection from the outlet to either or both of the nitrogen provision unit and the generator section. Preferably, such switch or switchable connection is configured to regulate the amount of liquid air flowing to either or both of the nitrogen provision unit and the generator section.

[0060] The generator section is configured to provide a second energy source and an expanded air stream. To this end, the generator section comprises in sequence an evaporator with a heat source, an expansion turbine mechanically connected to an electrical generator, and preferably also a further heat exchanger. The latter is used to provide a gas with a suitable temperature (typically about 100°C) for the regeneration of the adsorbers. The generator section optionally has an air outlet in fluid connection with the air inlet of the nitrogen provision unit. Alternatively, or in combination with the above-mentioned air outlet, the generator section has an air outlet allowing the expanded air from the generator section to be discharged to the atmosphere, preferably after being used for regenerating one or more adsorbers as described above. The generator section upstream of the expansion typically operates at a pressure in the range of 120 to 250 bar, such as 180 to 220 bar, for example at 200 bar.

[0061] The device has a power connection with a renewable energy source, and with the first and second energy sources. In order to enable the device to operate in charging mode and in discharging mode, the device is provided with an electrical control unit, which is configured to be switchable between the renewable energy source and the first and second energy sources. The control unit is thus configured to power the device, and to do so with either or both of an external energy source based on renewable energy, and with the internally provided energy sources.

[0062] The renewable energy source is preferably selected from the group consisting of solar energy, wind energy, water energy, tidal energy, geothermal energy, biomass energy, and combinations thereof. Among these, in particular, solar energy and wind energy are prone to strong fluctuations, such as fluctuations caused by day and night in the case of solar energy, and fluctuations caused by weather changes in the case of both solar energy and wind energy. For optimal benefit from the intelligent configuration of the apparatus disclosed herein, the renewable energy source is preferably solar energy, wind energy, or a combination thereof.

[0063] In addition to being configured to cope with fluctuations in power from the renewable source energy, the apparatus is preferably also configured to thereby take into account thermal management, preferably improved thermal management.

[0064] To this end, the apparatus preferably comprises a heat storage unit configured to receive heat from the various units as provided by the related method steps. These units include the catalytic reactor, the low-pressure air compression and adsorber section, and the high-pressure air compression unit. The heat storage unit is configured to provide heat to the power generator section, i.e., the section that serves as an additional internal source of energy during operation of the apparatus in discharge mode.

[0065] The working of the apparatus in charging and discharging operation will be explained hereinafter with reference to the method the apparatus is configured to operate.

[0066] Broadly speaking, the method of the present invention proceeds as an essentially continuous method for the production of ammonia. When the renewable energy source is abundant, the method proceeds sometimes in charging mode, and at other times in discharging mode.

[0067] During charging mode, air is compressed in the LP stage, preferably to about 10 bar gauge, and the heat generated is preferably stored in the heat storage unit. The heat storage unit also stores heat from the reaction of nitrogen and hydrogen to form ammonia. The LP air passes through the adsorber to the HP compression stage, where it is compressed, preferably to about 150 bar gauge, cooled, converted to liquid air for storage in a storage tank. The liquid air is then fed to a nitrogen provision unit, typically an air separation unit (ASU) or a nitrogen generation unit (NGU), to provide a flow of nitrogen sufficient for full operation of the ammonia synthesis reactor. In this mode, the electrolysis unit is operated by the renewable energy source to provide the required flow of hydrogen. In an alternative embodiment, air for the ASU or NGU is drawn from the adsorber output, rather than from storage as liquid air.

[0068] During discharge mode, the ammonia synthesis reactor is operated at a lower rate. This is typically reduced to about 30% to 90% of the charge mode, such as 40% to 80%, such as 50% to 70%. This enables the nitrogen provision unit (such as an ASU or NGU) and the electrolysis unit to also be operated at a lower rate. The reduced rate of the nitrogen provision unit corresponds to a lower demand of liquid air by this unit. This allows the HP compression stage to be bypassed. A smaller amount of liquid air is drawn from the liquid air tank, which is sufficient to supply the lower nitrogen demand of the ASU / NGU. A larger amount of liquid air is drawn from the storage tank, compressed and heated, and expanded over the power generator. Thereby, the pressure is typically in the range of about 100 bar gauge to about 250 bar gauge, such as 180 bar gauge to 220 bar gauge, for example 200 bar gauge. The temperature is typically in the range of about 250 °C to about 650 °C, such as 500 °C to 600 °C, for example 550 °C. The higher pressure and temperature bring the advantage of a correspondingly higher power output.

[0069] The power generator provides power during discharge mode to operate the method, including operating the electrolysis unit. In alternative embodiments, the air supply to the ASU / NGU is provided by the expanded air instead of the liquid air, as discussed above with reference to the possible connection of air from the LP compression and adsorber section to the nitrogen provision section.

[0070] Optionally, the expanded air is used to regenerate the adsorbers (e.g. two towers are operated in parallel, one is operating, and one is undergoing regeneration, and are alternated).

[0071] Thus, in charge mode, the method comprises operating the electrolysis unit using energy from the renewable energy source to provide hydrogen. Preferably, the method thereby also comprises storing heat from compressing air in the low pressure air compression and adsorber section and from reacting nitrogen and hydrogen in the catalytic reactor in the heat storage unit.

[0072] In discharge mode, the LP compressed air is sent to the air liquefaction section. In this mode, air is fed from the liquid air storage to the power generator section, expanding the air so as to provide the second energy source and the expanded air. This expanded air, possibly in addition to air from the liquid air storage, is fed to the nitrogen provision unit. Thus, the method comprises adjusting the operating rates of the nitrogen provision unit, the electrolysis unit and the catalytic reactor depending on the amount of air available for the nitrogen provision unit. Discharge mode involves operating the control unit so as to power the method by the first energy source and the second energy source.

[0073] It will be appreciated that various combined sources of power can be applied. For example, it is conceivable that in discharge mode, some power from a renewable external source of power is available in addition to the power provided from the power generator section. The skilled person will be able to adjust the operation of the plant accordingly. For example, by having a somewhat reduced rate of operation in the nitrogen providing unit (and correspondingly other chemical process units).

[0074] As mentioned before, in a further aspect, the present invention is also applicable to a method of retrofitting a pre-existing plant for the production of ammonia (i.e. a retrofit method). The pre-existing ammonia plant will typically be a "grey" ammonia plant, operating on a basis of hydrogen produced from hydrocarbons. Thus, typically, the pre-existing ammonia plant will comprise corresponding sections, including desulphurisation, catalytic shift, carbon dioxide removal and catalytic methanation. Whether or not such sections will continue to operate as a source of alternative hydrogen in such a plant, the retrofit method of the present invention will be used to add a water electrolysis unit as described above, as well as a low pressure air compression and adsorber section, a liquid air reservoir, all configured as described before. It will be appreciated that the retrofit method requires arranging the plant to have a power connection with a renewable source of power as well as with the first and second sources of power described above, and configuring the electrical control unit to be switchable between the renewable source of power and the first and second sources of power, including a combination of the first and second sources of power. The method preferably comprises adding to the pre-existing plant any units and / or connections which allow modernization to produce a plant as described above in any of its embodiments.

[0075] Preferably, the plant of the present invention is built as a new grass-roots plant. Indeed, the plant of the present invention has the advantage that it requires much less chemical conversion technology than a "grey" ammonia plant. Thus, it can be built in a wide variety of places and at a wide variety of scales.

[0076] In summary, the present disclosure relates to a plant for the production of ammonia. Ammonia is produced from hydrogen obtained by electrolysis of water. The electrolysis is powered by a renewable source of power, supplemented by a power source obtained from the plant during periods of low or no availability of the renewable energy. To this end, the plant is configured so that it can operate in a charging configuration (acquisition and storage of power) and a discharging configuration (exploitation of said power).

[0077] The present invention will be further illustrated below with reference to the non-limiting drawings discussed below. The units and flows as shown in the drawings are general representations. In addition, as an example of how to implement the present invention, below specific numbers are given for the amounts, pressures and temperatures possible at various stages. The disclosure of the embodiments of the plant and method of the present invention is explicitly not limited by the specific numbers mentioned.

[0078] Figure 1is a schematic diagram illustrating a charge mode configuration applied to the apparatus of the present invention. Air (stream (a), total intake 215000 Nm 3 / h) is compressed to 10 bar in a LP compressor (1) at 170°C; via heating to 230°C in a first heat exchanger (2) and cooling to 60-80°C in a second heat exchanger (3), the LP compressed air (b) is directed through an adsorber section (4) to a HP compressor (5) where the air is compressed to 150 bar at 550°C. In a heat exchanger (6), the HP compressed air (c) is cooled to 100°C and sent to a cold box section (7). In a downstream turbo expander (8) (optionally comprised in the cold box section), the air is liquefied and sent (stream (d); 215000 Nm 3 / h) to a liquid air storage tank (9); the turbo expander is configured to drive a generator (10) providing a first source of energy producing 2-4 MW. Liquid air (d) from the storage tank (9) is connected to a nitrogen provision unit (12) via a pump (11) (typically a cryogenic pump) at a rate of 8.74 t / h resulting in a nitrogen production of 2810 Nm 3 / h. The charge mode as depicted is typically operated for a time period of 12 hours.

[0079] Figure 2 is a schematic diagram illustrating an alternative charge mode configuration applied to the apparatus of the present invention. Air (stream (a), total intake 222000 Nm 3 / h) is compressed to 10 bar in a LP compressor (1) at 170°C; via heating to 230°C in a first heat exchanger (2) and cooling to 60-80°C in a second heat exchanger (3), the LP compressed air (b) is directed through an adsorber section (4). Thereby, a portion of the LP compressed air at 9.5 bar gauge pressure and 40°C (stream (b’), 7000 Nm 3 / h) is sent to a nitrogen provision unit (12) resulting in a nitrogen production of 2810 Nm 3 / h. The remaining portion of the LP compressed air (b”) is sent to a HP compressor (5) where the air is compressed to 150 bar at 550°C. In a heat exchanger (6), the HP compressed air (c) is cooled to 100°C and sent to a cold box section (7). In a downstream turbo expander (8), the air is liquefied and sent (stream (d); 215000 Nm 3 / h) to a liquid air storage tank (9); the turbo expander is configured to drive a generator (10) providing a first source of energy producing 2-4 MW. The charge mode as depicted is typically operated for a time period of 12 hours.

[0080] Figure 3is a schematic diagram illustrating a discharge mode configuration applied to the apparatus of the present invention. Air (stream (a), total intake 5000 Nm 3 / h) is compressed to 10 bar at 170°C in an LP compressor (1); via heating to 230°C in a first heat exchanger (2) and cooling to 60 to 80°C in a second heat exchanger (3), the LP compressed air (b) is directed to a cold box section (7) via an adsorber section (4). In a downstream turbo expander (8), the air is liquefied and sent (stream (d); 215000 Nm 3 / h) to a liquid air storage tank (9); the turbo expander is configured to drive a generator (10) which provides a first source of energy. A first liquid air stream (d’) from the storage tank (9) is connected to a nitrogen provision unit (12) via a pump (11) at a rate of 8.74 t / h, resulting in a nitrogen production of 2810 Nm 3 / h. A second liquid air stream (d”) is sent to a further compressor (13) and compressed at a rate of 250 t / h and heated via a heat exchanger (14). The resulting compressed air (stream (e) at 200 bar and 550°C) is sent to a generator section (15). This section produces an expanded air stream (f) which passes through a heat exchanger (17) to a temperature of 100°C and is used for regeneration of one or more adsorbers in the adsorber section (4) and is vented to the atmosphere (not shown). The generator section is configured to drive a generator (16) which provides a second source of energy producing 44 MW. The discharge mode as depicted is typically operated for a time period of 12 hours.

[0081] Figure 4 is a schematic diagram illustrating an alternative discharge mode configuration applied to the apparatus of the present invention. Air (stream (a), total intake 5000 Nm 3 / h) is compressed to 10 bar at 170°C in an LP compressor (1); via heating to 230°C in a first heat exchanger (2) and cooling to 60 to 80°C in a second heat exchanger (3), the LP compressed air (b) is directed to a cold box section (7) via an adsorber section (4). In a downstream turbo expander (8), the air is liquefied and sent (stream (d); 215000 Nm 3 / h) to a liquid air storage tank (9); from the storage tank, air is withdrawn whereby a liquid air stream (d”) is sent to a further pump (13) at a rate of 250 t / h and pressurized and heated via a heat exchanger (14). The resulting compressed air (stream (e) at 200 bar and 550°C) is sent to a generator section (15). This section produces an expanded air stream, a portion of which (5000 Nm 3(f') is sent to the nitrogen provision unit (12), resulting in 2000 Nm 3 / h of nitrogen production. Another part of the expanded air stream (f") is passed through the heat exchanger (17) to reach a temperature of 100°C and is used for regeneration of the adsorber (18) and is discharged to the atmosphere. The generator section is configured to drive a generator (16) which provides a second energy source producing 44 MW. The depicted discharge mode is typically operated for a time period of 12 hours.

[0082] Figure 5 An ammonia plant according to the present invention is schematically represented. The ammonia synthesis stream is depicted in solid lines. Thus, ammonia is produced and stored in an ammonia synthesis and product storage section (501) from which an ammonia product stream (5a) can be obtained. The ammonia production and storage section comprises a reactor (not shown) to which nitrogen and hydrogen can be fed. In this process, nitrogen (5b) can be obtained from either or both of a liquid nitrogen storage unit (502) or a nitrogen provision unit (503), both of which are connected to the ammonia synthesis and product storage section (501). Hydrogen (5c) is produced from water (input not shown) in an electrolysis unit (504) which is connected to the ammonia synthesis and product storage section (501). The plant further has an inlet for air which is subjected to low pressure compression in a LP compression unit (505). The LP compression unit (505) has a switchable connection to a HP compression unit (506) allowing further compression of the LP compressed air (5d) to obtain HP compressed air (5e). The HP compression unit (506) is connected to an air liquefaction section comprising a refrigeration unit (507) connected to a turbo expander (508) allowing cooled HP air (5f) to expand resulting in an electrical power source (not shown) and liquid air (5g). The turbo expander (508) has a connection for liquid air (5g) to a liquid air storage unit (509). The liquid air storage unit (509) has a closable connection for liquid air (5g) to a generator section comprising a liquid air compression unit (510) connected to an evaporation unit (511) which is further connected to a turbo expander (512) allowing liquid air (5g) obtained from the liquid air storage unit (509) to be compressed to compressed liquid air (5h) at a pressure of 200 bar and evaporated resulting in an air stream (5i) which is subjected to expansion in said turbo expander (512). Thus, the air stream (5h) can be sent to the nitrogen provision unit (503) which has a switchable connection to the generator section, i.e. to the turbo expander (512). The nitrogen provision unit (503) further has a switchable connection on the downstream side of the LP compression unit (505) allowing LP compressed air (5d) to be sent to the nitrogen provision unit (503).

[0083] The figure further illustrates the thermal connections (5j) and (5k) and the power connections (5l), (5m) and (5n).

[0084] The apparatus thus comprises a hot storage section (513) in thermal communication with the LP compression unit (505), the HP compression unit (506) and the turbo expander unit (512) of the power generator section, allowing heat exchange (5j) with any one or more of these units. The apparatus further comprises a cold storage section (514) in thermal communication with any one or more of the refrigeration unit (507), the evaporation unit (511) and the ammonia synthesis and product storage section (501). This thermal communication is low temperature and is preferably all encompassed in a cold box section. Thereby, a cold flow (5k) between the cooling radiator from the ammonia synthesis and product storage section (501), the liquid air evaporation unit (511) and said cold storage section (515) is maintained at low temperature.

[0085] The power connections as illustrated relate to the power generated by the methods carried out in the apparatus as well as the power from renewable sources. Thus, a renewable energy unit (515) provides power (5l) to the electrolysis unit (504). The latter unit can also receive power (5m) from the power generator section. The power output from the apparatus can be managed by a power output unit (516) and can also be used to power the ammonia synthesis and product storage section (501).

Claims

1. An apparatus for producing ammonia, the apparatus comprising: (a) A catalytic reactor having a nitrogen inlet for nitrogen feed, a hydrogen inlet for hydrogen feed and an ammonia outlet for the ammonia produced; (b) An electrolysis unit having a water inlet for water feed, an oxygen outlet for oxygen and a hydrogen outlet for hydrogen, the hydrogen outlet being in fluid communication with the hydrogen inlet of the reactor; (c) A nitrogen supply unit having at least one air inlet for an air flow, an oxygen outlet for an oxygen-enriched flow, and a nitrogen outlet for a nitrogen flow; the nitrogen outlet being in fluid communication with the nitrogen inlet of the reactor; (d) A low-pressure air compressor and adsorber section having an air inlet for external air feed and an air outlet for low-pressure compressed air at a gauge pressure of 8 to 20 bar, the air outlet optionally being in fluid communication with an air inlet of the nitrogen supply unit and switchably in fluid communication with an air liquefaction section or a high-pressure air compressor unit having an outlet for compressed air in fluid communication with the air liquefaction section, the air liquefaction section including a turboexpander configured to drive a generator to provide a first energy source; (e) A liquid air storage tank having a liquid air inlet for liquid air obtained from the air liquefaction section, a shut-off outlet for liquid air in fluid communication with the nitrogen supply unit, and a shut-off outlet for liquid air in fluid communication with a generator section configured to provide a second energy source and an expanded air flow; (f) An electrical control unit configured to supply power to the device; The device has a power connection to a renewable energy source and to the first energy source and the second energy source, and the electrical control unit is configured to switch between the renewable energy source and the first energy source and the second energy source, including a combination of the first energy source and the second energy source.

2. The device according to claim 1, further comprising: (g) A heat storage unit configured to receive heat from a heat generation unit selected from the catalytic reactor, the low-pressure air compressor and adsorber section, the high-pressure air compressor unit and combinations thereof, the heat storage unit being configured to supply heat to the generator section.

3. The device according to claim 1 or 2, wherein the renewable energy source is selected from solar energy, wind energy, hydropower, tidal energy, geothermal energy, biomass energy, and combinations thereof.

4. The device according to claim 3, wherein the renewable energy source is solar energy, wind energy, or a combination thereof.

5. The device according to claim 4, wherein the renewable energy source is only solar energy.

6. The apparatus of claim 5, wherein the air outlet for low-pressure compressed air at a gauge pressure of 8 to 20 bar is in fluid communication with the air inlet of the nitrogen supply unit.

7. The device according to any one of the preceding claims, wherein the generator section has an air outlet in fluid communication with the air inlet of the nitrogen supply unit.

8. A method for producing ammonia, said method being carried out in the apparatus according to any one of the preceding claims, said method comprising the following steps: (i) Obtain at least a portion of the nitrogen from the nitrogen supply unit; (ii) Obtain at least a portion of the hydrogen from the electrolysis unit; (iii) Reacting the nitrogen and the hydrogen in the catalytic reactor under ammonia-forming conditions; The method includes operating the device in a charging mode, the charging mode including compressing air in the low-pressure air compressor and adsorber section to provide low-pressure compressed air with a gauge pressure of 8 to 20 bar. The low-pressure compressed air is compressed in the high-pressure air compression unit to provide high-pressure compressed air with a gauge pressure of 50 to 200 bar, and the high-pressure compressed air is expanded and liquefied in the turbo expander to obtain liquefied air. The liquefied air is stored in the liquid air reservoir; air from the liquid air reservoir is fed to the nitrogen supply unit and / or air from the low-pressure air compressor and adsorber section is fed to the nitrogen supply unit; The method includes operating the electrolysis unit using energy from the renewable energy source to provide the hydrogen.

9. The method of claim 8, further comprising storing in a heat storage unit the heat generated from the compression of air in the low-pressure air compressor and the air in the adsorber section, as well as the heat generated from the reaction of the nitrogen and the hydrogen in the catalytic reactor.

10. A method for producing ammonia, said method being carried out in the apparatus of any one of claims 1 to 7, said method comprising the following steps: (iv) Obtain at least a portion of the nitrogen from the nitrogen supply unit; (v) Obtain at least a portion of the hydrogen from the electrolysis unit; (vi) React the nitrogen and hydrogen in the catalytic reactor under ammonia-forming conditions; The method includes operating the device in a discharge mode, the discharge mode including compressing air in the low-pressure air compressor and adsorber section to provide low-pressure compressed air with a gauge pressure of 8 to 20 bar. The low-pressure compressed air is delivered to the air liquefaction section; The low-pressure compressed air is expanded and liquefied in the turbine expander to obtain liquefied air; The turbine expander provides a first energy source; the liquefied air is stored in the liquid air reservoir; air from the liquid air reservoir is fed into the generator section to expand the air in order to provide a second energy source and expanded air; air from either or both of the liquid air reservoir and the expanded air is fed into the nitrogen supply unit; wherein the method includes adjusting the operating rates of the nitrogen supply unit, the electrolysis unit, and the catalytic reactor according to the amount of air available for the nitrogen supply unit, and operating the control unit to power the method through the first energy source and the second energy source.

11. The method of claim 10, further comprising using heat from a heat storage unit configured to receive heat from a heat generation unit selected from the catalytic reactor, the low-pressure air compressor and adsorber section, the high-pressure air compressor unit, and combinations thereof, the method comprising using the heat from the heat storage unit to heat air in the generator section.

12. A method for modifying a pre-existing device for producing ammonia, the pre-existing device comprising: - A catalytic reactor having a nitrogen inlet for nitrogen feed, a hydrogen inlet for hydrogen feed, and an ammonia outlet for the produced ammonia; - A hydrogen supply section having a hydrogen outlet for hydrogen, the hydrogen outlet being in fluid communication with the hydrogen inlet of the reactor; - A nitrogen supply unit having at least one air inlet for an air flow, an oxygen outlet for an oxygen-enriched flow, and a nitrogen outlet for a nitrogen flow; the nitrogen outlet is in fluid communication with the nitrogen inlet of the reactor; - An electrical control unit configured to supply power to the device; The method includes adding to the pre-existing device: - An electrolysis unit having a water inlet for water feed, an oxygen outlet for oxygen, and a hydrogen outlet for hydrogen, the hydrogen outlet being in fluid communication with the hydrogen inlet of the reactor; - A low-pressure air compressor and adsorber section having an air inlet for external air feed and an air outlet for low-pressure compressed air at a gauge pressure of 8 to 20 bar, the air outlet optionally being in fluid communication with an air inlet of the nitrogen supply unit and switchably in fluid communication with an air liquefaction section or a high-pressure air compressor unit having an outlet for compressed air in fluid communication with the air liquefaction section, the air liquefaction section including a turboexpander configured to drive a generator to provide a first energy source; - A liquid air storage device having a liquid air inlet for liquid air obtained from the air liquefaction section, a shut-off outlet for liquid air in fluid communication with the nitrogen supply unit, and a shut-off outlet for liquid air in fluid communication with a generator section configured to provide a second energy source and an expanded air flow; the generator section having an air outlet in fluid communication with the air inlet of the nitrogen supply unit; The device is arranged to have a power connection to a renewable energy source and to the first energy source and the second energy source, and the electrical control unit is configured to switch between the renewable energy source and the first energy source and the second energy source, including combinations thereof.

13. The method of claim 12, further comprising adding a heat storage unit to the pre-existing equipment, the heat storage unit being configured to receive heat from a heat generation unit selected from the catalytic reactor, the low-pressure air compressor and adsorber section, the high-pressure air compressor unit, and combinations thereof, the heat storage unit being configured to provide heat to the generator section.

14. The method of claim 12 or 13, further comprising adding a connector for fluid communication between the air outlet of the low-pressure compressed air for a gauge pressure of 8 to 20 bar and the air inlet of the nitrogen supply unit.

15. The method according to any one of claims 12 to 14, further comprising adding a connector for fluid communication between the air outlet of the generator section and the air inlet of the nitrogen supply unit.

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