Ammonia cracking for hydrogen production

Abstract

The invention relates to a method for producing hydrogen from ammonia, comprising: ammonia cleavage in which the ammonia is decomposed into hydrogen and nitrogen, the ammonia cleavage being carried out in a sequence of cleavage steps (13, 36, 17, 20), and a final cleavage stream (21) being obtained after the final cleavage step (20), the final ammonia cracking step (20) is carried out in an adiabatic manner and / or after the final cracking step, the final cracking stream (21) is quenched by direct mixing with water or steam.

Description

Technical Field

[0001] The present invention relates to the field of hydrogen production based on ammonia cracking. Background Art

[0002] Ammonia cracking has attracted considerable attention, particularly for hydrogen production. During ammonia cracking, ammonia is decomposed into H2 and N2 in the presence of heat and a suitable catalyst. Typical catalysts used for ammonia cracking include Ni-based catalysts, Ru-based catalysts, Pt-based catalysts, and Fe-based catalysts.

[0003] Traditionally, the ammonia cracking process is carried out by passing the ammonia feed through the catalytic tubes of a combustion furnace, optionally followed by a pre-cracking step. One challenge with this process is the high temperature of the cracked stream effluent from the catalytic tubes, typically around 750°C. Associated disadvantages include: the lifespan of the catalyst and tubes is affected by such high operating temperatures; downstream equipment through which the process stream passes, particularly heat exchangers, is exposed to nitriding and hydrogen attack, requiring expensive materials such as high-nickel alloy steel. This problem is particularly evident in gas / gas heat exchangers, which operate at high temperatures, high pressures, and in the presence of hydrogen and nitrogen—very harsh conditions for metal surfaces.

[0004] WO 2022 / 243410 and WO 2022 / 265650 disclose the cracking of ammonia for the production of hydrogen. Summary of the Invention

[0005] The challenge faced by the present invention is the high temperature of the cracked stream effluent from the catalytic tubes, which still affects the prior art ammonia cracking processes.

[0006] The objects of the present invention are achieved by a method according to the claims. In the method according to the invention, the cracking of ammonia is carried out in a sequence of steps until a final cracking stream is obtained. The term "final cracking stream" refers to the stream obtained after the last cracking step, which mainly contains hydrogen and nitrogen and may contain small amounts of unreacted ammonia.

[0007] Compared to the prior art, the temperature of the final cracking stream is reduced by performing the final cracking step adiabatically or by quenching the final cracking stream.

[0008] The present invention arises from the ingenious discovery that the temperature of the cracked stream can be significantly reduced if the final cracking step is adiabatic or by quenching the cracked stream. Quenching can be accomplished with water, steam, ammonia or a gaseous cold stream or a mixture of the aforementioned compounds. When the final cracking step is adiabatic, the temperature of the process stream is reduced by the absorption of heat caused by the endothermic cracking itself. In this case, the invention arises from the discovery that the cracking itself can be considered as a suitable heat dwell to reduce the temperature of the process stream, instead of conventional cooling. This is in contrast to the consolidated prior art, in which adiabatic cracking, if any, is only used in the initial pre-cracking step.

[0009] A notable feature of some embodiments of the present invention is that the adiabatic reactors are arranged in sequence. This means that the adiabatic reactors are connected in series so that the effluent from the first reactor is further reacted in the next reactor, and so on, until the last reactor.

[0010] A very interesting advantage of carrying out the final cracking step adiabatically is that the final cracked stream is obtained at a relatively low temperature, without having to exchange heat with a cooling medium (which would require heat exchangers suitable for working at very high temperatures). Similarly, the quenching of the stream is carried out by direct mixing with a cooling medium (usually water or steam), which therefore does not require large heat exchange surfaces made of expensive materials.

[0011] According to embodiments of the present invention, the cracking step may include one or more steps performed adiabatically, and may include steps performed within the catalytic tubes of the furnace. The tubes are externally heated by heat generated during the furnace combustion process, i.e., by radiant heat and / or contact with hot flue gases. A gas-heated reactor or adiabatic reactor may also be operated in parallel with the catalytic tubes. For example, the gas-heated reactor may be heated by the effluent from the same catalytic tubes, thereby cooling the hot effluent from the tubes by transferring heat to the parallel cracking process.

[0012] Providing the above-mentioned catalytic tubes is an option. In one embodiment, the entire cracking process is carried out as a sequence of adiabatic cracking steps without passage through catalytic tubes.

[0013] The adiabatic cracking step can be carried out in a separate adiabatic reactor with a separate pressure vessel, or in a catalytic bed contained within a single pressure vessel. An intermediate heating step is provided between the adiabatic cracking steps to provide the necessary heat of reaction. In one embodiment, the intermediate heating is provided by a heat exchanger or coil disposed in the combustion furnace, and is heated by the combustion process in the furnace. In certain embodiments, some or all of the heat of reaction can be provided by an electric heater. DETAILED DESCRIPTION

[0014] One aspect of the present invention is a process for producing hydrogen from ammonia comprising ammonia cracking, wherein a gaseous ammonia feed is catalytically decomposed into hydrogen and nitrogen, wherein the ammonia cracking is performed in a sequence of ammonia cracking steps, wherein a final cracked stream is the stream obtained after the last ammonia cracking step, wherein the last ammonia cracking step is performed adiabatically without supplying heat to the process stream under cracking, and / or the final cracked stream is quenched by direct mixing with water and / or steam prior to the indirect heat exchange.

[0015] The final cracked stream may be the stream prior to removal of heat by indirect heat exchange with one or more cooling fluids.

[0016] After the last cracking step, or if quenching is provided, after quenching, the temperature of the final cracked stream is much lower than the effluent of the catalytic tubes of the prior art. Preferably, the temperature of the stream is not higher than 700° C., or not higher than 650° C., or not higher than 550° C., or not higher than 500° C. More preferably, the temperature of the stream is in the range of 350° C. to 700° C., or 350° C. to 650° C., or 250° C. to 550° C.

[0017] The feed temperature to each cracking step is preferably from 300°C to 650°C. In the adiabatic cracking step, the temperature of the process stream may be reduced by 50°C to 400°C, preferably by 200°C to 300°C or 250°C to 300°C.

[0018] In a preferred embodiment of the present invention, the cracking process is carried out under pressure. Preferably, the cracking process is carried out at a pressure in the range of 5 bar to 55 bar. An advantage of carrying out ammonia cracking under pressure is that it saves energy in compressing the hydrogen obtained, or eliminates the need for compression. Pressure is expressed in bar gauge. Another advantage of operating under pressure is that it reduces the size of equipment such as reactors, PSA units, and membrane separation units.

[0019] Adiabatic reactors can have axial flow, radial flow, or mixed axial-radial flow. In embodiments with radial or mixed flow, the radial flow can be inward or outward.

[0020] In a preferred embodiment, the ammonia cracking steps of the sequence are arranged so that the effluent of each cracking step is passed entirely to the next cracking step in the sequence. Preferably, the effluent of each cracking step is passed to the next step as such or after heat exchange, preferably without further treatment that changes the composition of the stream (e.g., separation of components).

[0021] Preferred embodiments of the invention are set forth in the appended claims. The following includes a description of various features and embodiments of the invention, followed by a description of a preferred general embodiment.

[0022] New ammonia feed

[0023] Fresh ammonia feed is typically liquid ammonia. The liquid ammonia can be heated and vaporized using heat removed from the ammonia cracking process, for example by removing heat from the final cracked stream, from purified hydrogen, or from distillation of an aqueous ammonia solution (aqua-ammonia solution). The aqueous solution can be removed from a gas scrubbing process to remove unreacted ammonia from the cracked stream. Electrical heating can also be used to heat and vaporize the liquid ammonia.

[0024] The gaseous ammonia obtained after evaporation can be superheated in a furnace until it reaches a temperature suitable for use in the cracking process, for example above 400°C, or above 500°C, more preferably above 600°C, such as 650°C.

[0025] Hydrogen purification

[0026] The final cracked stream contains primarily ammonia and hydrogen and is processed to obtain the desired purity of hydrogen. The target purity of hydrogen can be 99% or higher, and in some embodiments it can be as high as 99.999%.

[0027] After the last cracking step, the hydrogen-containing cracked stream is typically cooled by transferring heat to the fresh ammonia feed to be vaporized. Thereafter, the cracked stream may be further cooled, preferably to a temperature of 5° C. to 60° C., and the cooled gaseous mixture thus obtained may optionally be washed with water to remove unreacted ammonia.

[0028] The scrubbed gas thus obtained can be fed to a suitable hydrogen separation system, such as a PSA (pressure swing adsorption) unit or a membrane-based system, to produce hydrogen and tail gas of the target purity. The tail gas can be recycled as fuel, for example, as fuel for a combustion furnace. In some embodiments, the tail gas extracted from the first PSA unit is compressed and fed to a second PSA unit to improve hydrogen recovery. The flue gas generated in the combustion furnace can also be used to preheat the combustion air.

[0029] As mentioned above, gas scrubbing produces an aqueous ammonia solution. This solution can be sent to a distillation column equipped with a reboiler and a condenser. The reboiler can be heated electrically or with steam (e.g., steam generated by removing heat from cracked ammonia).

[0030] A portion of the ammonia vapor emerging from the top of the distillation column can be mixed with liquid or gaseous ammonia, provided they are under sufficient pressure. The remainder can be condensed and returned to the column. Alternatively, the entire ammonia vapor stream can be condensed and a portion recovered as ammonia feed.

[0031] Various embodiments of the present invention may include supplemental hydrogen from a hydrogen storage or buffer system that can be added to the fuel gas system to stabilize the ammonia cracking process and have an independent source of fuel gas available at cell limits during normal operation and during startup.

[0032] Fuel gases other than hydrogen, such as ammonia, synthesis gas, natural gas, or general hydrocarbon sources, may be advantageously employed to maximize the conversion of ammonia to hydrogen.

[0033] Hybrid options

[0034] In some embodiments, the cracking process of the present invention is hybridized by using electricity to provide a portion of the heat input. The heat input provided by electricity can include one or more of the following: heat for reboiling the aqueous ammonia solution; heat for preheating or evaporating the liquid ammonia feed; heat for preheating the combustion air of the fuel-fired furnace; and heat for cracking the ammonia.

[0035] In some embodiments, the process stream is electrically heated before or after the furnace. The electric heater can be installed inside or outside the furnace.

[0036] In other embodiments, the ammonia cracking reactor may be configured with tube heating or plate heating designs to maintain the cracking reaction. In certain embodiments, to achieve greater conversion of ammonia to products, the heating stream may be gaseous ammonia itself, or a stream of H2 and N2, or water vapor, or a combination thereof; alternatively or additionally, in some embodiments, heat may be provided by electrodes.

[0037] In other embodiments, a gas turbine is installed to combine power generation with hydrogen production. According to an embodiment of the process according to the present invention, a portion of the process fuel is fed to the gas turbine, and the turbine's exhaust gas is used as a heat source in the ammonia cracking process. For example, the gas turbine's exhaust gas can be used to heat a bundle of catalytic tubes in a furnace. In a preferred embodiment, the gas turbine is installed in conjunction with ammonia cracking using catalytic tubes, followed or preceded by an adiabatic ammonia cracking reactor; however, a gas turbine can also be included in a plant layout consisting solely of an adiabatic cracking reactor.

[0038] In some embodiments, a gas turbine is integrated into a steam generation system and coupled to a heat recovery steam generator (HRSG) to generate electricity via the steam turbine, wherein hot exhaust gas from the gas turbine is used in the HRSG to generate steam for the steam turbine.

[0039] The process may include a fuel gas system for providing fuel to any combustion equipment (e.g., a furnace). In some embodiments, supplemental hydrogen may be added to the fuel gas system. In some embodiments, natural gas or an external fuel source may be used to supplement the fuel gas. The gas turbine described above may be installed as part of the fuel gas system.

[0040] First embodiment

[0041] In a first embodiment, the cracking of ammonia is performed entirely in a sequence of adiabatic reactors. This means that the process does not involve heating the cracking in the catalytic tubes. In this embodiment, a furnace can be used to provide an intermediate heating step to heat the partial cracking effluent from one or more adiabatic reactors before feeding it to the next reactor in the sequence. The preferred number of adiabatic reactors in the sequence is three or four. Each reactor contains at least one catalytic bed. In some embodiments, a single reactor may contain more than one catalytic bed.

[0042] The catalytic reactors are preferably separate reactors, each having its own pressure vessel. The reactors are connected in series so that the effluent from each reactor except the last is further cracked in the next reactor in the sequence. The final cracked stream is obtained at the output of the last reactor in the sequence. Typically, the final cracked stream has a sufficiently low temperature to avoid the use of expensive materials in subsequent equipment; optionally, if desired, the final cracked stream can be quenched, preferably with water and / or steam.

[0043] A particularly preferred embodiment of the first embodiment of the present invention is:

[0044] A process for producing hydrogen from ammonia comprising ammonia cracking, wherein a gaseous ammonia feed is catalytically decomposed into hydrogen and nitrogen, wherein the ammonia cracking is carried out in a sequence of preferably three or four ammonia cracking steps, wherein the final cracked stream is the stream obtained after the last cracking step, wherein all said ammonia cracking steps are carried out in an adiabatic manner without supplying heat to the process stream under cracking, each cracking step being carried out separately in an adiabatic reactor, each portion of the cracked stream directed to the next reactor in the sequence being heated before entering the next reactor by passing it through a heat exchanger or coils in a fuel-fired furnace.

[0045] Second embodiment

[0046] In a second embodiment, the cracking process is carried out partly in one or more adiabatic reactors and partly in externally heated catalytic tubes. The one or more adiabatic reactors can be arranged upstream or downstream or both upstream and downstream of the catalytic tubes, which means that the cracking step in the tubes can be carried out before or after the adiabatic cracking.

[0047] In this second embodiment, the cracking is carried out partly adiabatically and partly non-adiabatic with the supply of heat. Variations of this second embodiment may include:

[0048] A first cracking step is carried out by means of one or more adiabatic reactors in a sequence, followed by a final cracking step in catalytic tubes and quenching of the final cracked stream (adiabatic / tube / quench);

[0049] Carrying out a first cracking step by means of one or more adiabatic reactors in a sequence, followed by a cracking step in catalytic tubes, and then a final cracking step in one or more adiabatic reactors in a sequence (adiabatic / tube / adiabatic);

[0050] The first cracking step is carried out in catalytic tubes, followed by a final cracking step in one or more adiabatic reactors in a sequence, or by quenching (tube / adiabatic, or tube / quench).

[0051] The catalytic tubes are preferably installed in a combustion furnace and heated by combustion flue gases.

[0052] In a further variation, the catalytic tubes are operated in parallel with the gas heater reformer or one or more adiabatic reactors. For example, a portion of the partially cracked effluent from one or more first adiabatic reactors is fed to the catalytic tubes, and a portion is fed to a parallel gas heated reactor (GHR). The gas heated reactor can be heated with the hot effluent from the catalytic tubes. For example, the gas heated reactor can be a shell and tube reactor, wherein cracking occurs on the tube side and the hot effluent passes through the shell side.

[0053] In embodiments with a gas-heated reactor, the effluent from the reactor is typically at an elevated temperature. According to various embodiments, heat may be removed from the effluent in a steam boiler or in the reboiler of an ammonia distillation column. Alternatively, the effluent may be quenched with water and / or steam.

[0054] In a further variation of the second embodiment, the cracked stream effluent from the catalytic tubes is quenched before or after being fed to a steam boiler. In a further variation, the quenching is performed without a steam boiler. In the absence of a boiler, the heat for reboiling the ammonia distillation column can be provided by electricity, as in the hybrid option described above.

[0055] An ammonia reboiler can be installed as an alternative to a steam reboiler.

[0056] Third embodiment

[0057] A third embodiment comprises multiple adiabatic cracking steps performed in catalytic beds contained within a single pressure vessel, with inter-bed heating preferably being electrical. Preferably, cracking is performed entirely within the sequence of catalytic beds, i.e., without passing the process gas through the catalytic tubes of the furnace. However, variations of this embodiment include performing at least one cracking step in externally heated catalytic tubes.

[0058] The inter-bed heater may further comprise at least one heat exchanger arranged to recover heat from the process gas stream and / or the flue gas stream.

[0059] BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figures 1 to 3 A schematic diagram of an apparatus for producing hydrogen according to an embodiment of the method of the present invention is shown.

[0061] Figure 1 The following main units are shown.

[0062] Ammonia storage unit 1

[0063] Pump 3

[0064] Evaporator 6

[0065] Ammonia recirculation evaporator 6B

[0066] Combustion furnace 10

[0067] Adiabatic cracking reactors 13, 17, 20 with catalytic beds 38, 39, 40

[0068] Heat exchanger 22

[0069] Scrubber 24

[0070] Hydrogen separation unit 31

[0071] Distillation tower 27

[0072] Heating element 41

[0073] Figure 1 A schematic diagram of a process is disclosed in which ammonia 2 is cracked to produce a hydrogen stream 50 .

[0074] A stream of liquid ammonia 2 is withdrawn from an ammonia storage tank 1 and fed via a pump 3 to an evaporator 6 where the liquid ammonia is evaporated into a gaseous ammonia stream 7 .

[0075] Ammonia preheater / evaporator 6 may be a multi-stage heat exchanger. Each stage may be a separate device (separate heat exchanger), or multiple stages may be included in a single device.

[0076] In some embodiments, the vaporizer 6 may include a stage arranged to recover heat from the purified hydrogen stream 50 product outlet from the hydrogen separation unit 31, optionally after separation of the fuel portion 51, and / or the vaporizer 6 may include a stage arranged to recover heat from the cracked gas downstream of the last ammonia cracking step.

[0077] The gaseous ammonia 7 is mixed with a recycled ammonia stream 43 (described below); the ammonia feed 44 thus obtained is subjected to a heating step in the coils 101, 102 of the burner 10 to produce a preheated ammonia feed 12, which is then treated in a first adiabatic cracking reactor 13 to partially crack the ammonia into nitrogen and hydrogen.

[0078] The output of the first adiabatic reactor 13 is a partially cracked stream 14 comprising nitrogen, hydrogen and uncracked ammonia which is further heated in the furnace 10 , passed through coils 103 and 104 and subjected to a second cracking step in the second cracking adiabatic reactor 17 .

[0079] The effluent stream 16 from the second adiabatic reactor 17 is heated again in the furnace 10, passed through the coil 105, and then subjected to a third cracking step in the third adiabatic cracking reactor 20. The third adiabatic reactor 20 is referred to as the final or final cracking reactor. The output of the final adiabatic cracking reactor 20 is cracked steam 21 containing nitrogen, hydrogen, and residual ammonia.

[0080] The temperature of the cleavage stream 21 is preferably in the range of 400° C. to 500° C. Preferably, the residual ammonia present in the stream 21 does not exceed 5% by volume.

[0081] The cracking of ammonia is carried out catalytically. Each adiabatic reactor comprises at least one catalytic bed for this purpose. Adiabatic reactors 13, 17 and 20 comprise at least one catalytic bed 38, 39 and 40, respectively.

[0082] The cracked stream 21 is cooled in a heat exchanger 22 to a temperature of about 5°C to about 60°C, preferably 5°C to 50°C. The cooling is preferably performed by direct quenching and / or indirect heat transfer using a cooling medium. The cooling medium is preferably fresh liquid ammonia feed and / or water / steam.

[0083] In some embodiments, an additional heat exchanger (not shown) is arranged after the heat exchanger 22 to recover heat from the cracked stream for vaporization of the ammonia feed, thereby operating as a stage of the ammonia vaporizer 6.

[0084] After cooling, the cracked stream is contacted with water 23 in a scrubber 24 to remove residual ammonia. The output of the scrubber 24 is an aqueous ammonia solution 25 and a scrubbing gas 26 comprising nitrogen and hydrogen.

[0085] The purified gas 26 is separated into a first portion 33 and a second portion 32. Preferably, the first portion 33 is a major portion of the effluent 26 and the second portion 32 is a minor portion of the effluent 26. The first portion 33 is processed in a hydrogen separation unit 31 to obtain a hydrogen stream 50 and a tail gas 35.

[0086] The tail gas 35 contains nitrogen and a small amount of hydrogen. The tail gas 35 is recycled to the combustion furnace 10 for use as fuel, preferably together with a supplemental fuel stream required for combustion control purposes. The supplemental fuel preferably includes a portion 51 of the hydrogen product stream 50 and / or a second portion 32 of the purge gas. Using a portion of the hydrogen 50 as fuel for the furnace 10 helps reduce carbon emissions.

[0087] The effluent of the aqueous ammonia solution 25 is preferably treated in a distillation unit 27 to separate an ammonia stream 29 from water 28. In some embodiments, all or part of the ammonia solution 25 may bypass the distillation unit 27 via line 30. A recycled ammonia stream 43 comprises the ammonia stream 29 separated in unit 27 and possibly a bypass portion of line 30. The recycled stream 43 is mixed with fresh ammonia 7 to form the feed 44 to the adiabatic reactors 13, 17, 20, as described above.

[0088] The recycle stream 43 is liquid ammonia which is preferably evaporated before being mixed with fresh ammonia 7. Figure 1 In the embodiment of the present invention, the recycle stream 34 is evaporated in the ammonia recycle evaporator 6B. The ammonia recycle evaporator 6B can be separate from the main feed evaporator 6 or integrated as a stage of the evaporator 6, preferably as the last stage. If the ammonia recycle evaporator 6b is integrated as an ammonia recycle evaporation stage in the evaporator 6, the injection point of the stream 43 is located upstream of the stage.

[0089] It is noteworthy that the effluent from each reactor in the sequence is heated in one or more coils of the furnace 10 before being fed entirely to the next reactor.

[0090] The number of heating coils in furnace 10 may vary, for example, the series of coils 101, 102 and 103, 104 may be replaced by a single coil or more than two coils, as appropriate, and arranged appropriately to optimize overall heat exchange.

[0091] The coils 101-105 in the furnace 10 may be replaced by tube bundles or other suitable heat exchange elements, exposed to the hot flue gases in the furnace and passed internally by the stream to be heated.

[0092] Figure 2 Shown Figure 1A variant embodiment is shown in which an additional cracking step is performed in a bundle of catalytic tubes 36. The tubes 36 are filled with an ammonia cracking catalyst and are externally heated by a combustion process in the furnace 10. The partially cracked stream 14 is heated in the coil 103 and cracked in the catalytic tubes 36, then heated in the coil 104 and treated in the downstream reactors 17 and 20 to produce cracked gas 21. Other features of this process are similar to those of the Figure 1 Same as in.

[0093] Figure 3 An embodiment is shown in which the cracking process is carried out in a single adiabatic cracking reactor 37 housing catalytic beds 38, 39 and 40. Thus, the catalytic beds are arranged in a single reactor, rather than Figure 1 and Figure 2 The individual pressure vessels 13, 17, 20 in the.

[0094] Adiabatic reactor 37 also includes heating elements 41, 42 interposed between successive catalytic beds. Heating elements 41, 42 are preferably electric heaters configured to heat the effluent from a catalytic bed prior to the subsequent catalytic bed. The effluent from bed 38 is heated by heater 41 before entering bed 39, and the effluent from bed 39 is heated by heater 42 before entering bed 40.

[0095] In some embodiments, the heating elements 41, 42 may be heat exchangers and / or coils arranged to recover heat from a process stream and / or flue gas.The flue gas may be gas from the furnace 10 and / or an exhaust stream from a gas turbine.

Claims

1. A process for producing hydrogen from ammonia (2), comprising ammonia cracking, wherein a gaseous ammonia feed (7) is catalytically decomposed into hydrogen and nitrogen, wherein the ammonia cracking is performed in a sequence of ammonia cracking steps (13, 36, 17, 20), wherein the final cracked stream (21) is the stream obtained after the last ammonia cracking step (20) of the sequence, wherein: The final ammonia cracking step (20) is performed adiabatically, without supplying heat to the process stream being cracked, and / or After the final ammonia cracking step (20), the final cracked stream is quenched by direct mixing with water and / or steam.

2. The process according to claim 1 , wherein after the last ammonia cracking step (20), or if quenching is provided, after quenching, the temperature of the final cracked stream (21) is not higher than 700° C., or not higher than 650° C., or not higher than 550° C., or not higher than 450° C.

3. The process according to claim 1 or 2, wherein the feed temperature of each cracking step is 300°C to 650°C.

4. A process according to any one of claims 1 to 3, wherein the final cracked stream (21) is cooled by indirect heat exchange (22) with one or more cooling fluids in one or more heat exchangers, the cooling fluids comprising fresh liquid ammonia feed (2) and / or water, the liquid ammonia feed (2) being heated and evaporated (6) to produce the gaseous ammonia feed (7) and the water being evaporated to produce steam.

5. The process according to any one of claims 1 to 4, wherein the sequence of ammonia cracking steps comprises: A plurality of cracking steps carried out in a sequence of adiabatic reactors connected in series, and optionally one or more cracking steps in catalytic tubes, such that the effluent of each reactor is further treated in the next reactor or in a bundle of catalytic tubes until the last reactor in the sequence produces the final cracked stream (21), and an intermediate heating step in which the effluent of one reactor is heated before entering the next reactor in the sequence to provide heat for the endothermic cracking of ammonia, wherein no further cracking of ammonia is carried out during the heating step.

6. The method of claim 5, wherein the heating step is performed in a fuel burning furnace (10).

7. The process according to claim 1 , wherein the sequence of ammonia cracking steps comprises at least one cracking step carried out in a bundle of externally heated catalytic tubes ( 36 ), optionally preceded and / or followed by one or more cracking steps carried out adiabatically in one or more catalytic reactors.

8. The method according to claim 7, comprising a parallel cracking step carried out in parallel with the cracking step in the catalytic tubes, the parallel cracking step being carried out in a gas-heated reactor or adiabatically in an adiabatic reactor.

9. The process of claim 8, wherein the parallel cracking steps are performed in a gas heated reactor and the reactor is heated with the cracking stream effluent from the catalytic tubes.

10. The process according to claim 8 or 9, wherein the parallel cracking steps are carried out in a gas heated reactor and the effluent of the gas heated reactor is cooled in a boiler or by quenching.

11. The process according to claim 10, wherein the effluent of the gas heated reactor is cooled in a boiler arranged to provide heat for reboiling of the distillation process of aqueous ammonia solution and preheating of fresh ammonia, or the boiler is a reboiler of an ammonia distillation column.

12. A process according to any one of claims 7 to 11, wherein the fresh ammonia feed is first adiabatically cracked in an adiabatic reactor or a sequence of adiabatic reactors; a portion of the partially cracked effluent from an adiabatic reactor or the last reactor of the sequence is further cracked in the catalytic tubes; the remaining portion of the partially cracked effluent is cracked in parallel reactors, which are adiabatic reactors or gas-heated reactors, and the effluents of the tubes and the effluents of the parallel reactors are recombined to form a final cracked stream.

13. The method according to any one of claims 7 to 12, wherein the effluent of the catalytic tubes (36) is quenched before or after the boiler.

14. A method according to any one of the preceding claims, wherein the sequence of ammonia cracking steps comprises a plurality of cracking steps performed adiabatically in sequential catalytic beds, the catalytic beds being located in a single reactor (37) having a single pressure vessel, the reactor comprising one or more inter-bed heaters (41, 42) arranged to provide heat for the endothermic cracking of ammonia.

15. The method of claim 14, wherein the inter-bed heater comprises any one of: an electric heater, a heat exchanger arranged to recover heat from process gas, a heat exchanger arranged to recover heat from flue gas.

16. The process according to claim 14 or 15, further comprising at least one cracking step carried out in externally heated catalytic tubes.

17. The method according to any one of the preceding claims, wherein the cracking process is hybridized by providing a heat input generated by electricity, preferably by using electricity from a renewable source, wherein the heat input comprises one or more of the following: Heat used to reboil the ammonia solution; Heat used to preheat or vaporize liquid ammonia feed; The heat used to preheat the combustion air for fuel-burning furnaces; Heat used to crack ammonia.

18. A method according to any one of the preceding claims, comprising adding supplementary hydrogen, said supplementary hydrogen being taken from a hydrogen reservoir or from a buffer system and added to the fuel gas system of the method to stabilize and control the ammonia cracking process, said addition preferably being performed during startup to maintain the startup process.

19. A method according to any one of the preceding claims, wherein at least part of the fuel feed to the burner (10) is provided by one or more of: tail gas (35) obtained from a hydrogen purification process (31) such as a pressure swing adsorption process; a portion of the hydrogen product (51); a portion of the gas (32) obtained by treating the final cracked stream (21) in a scrubber (24); A portion of the fresh ammonia feed, optionally after a pre-cracking step.

20. A method according to any one of the preceding claims, wherein a portion of the fuel of the method is sent to a gas turbine and the exhaust gases of the turbine are used as a heat source in an ammonia cracking process and / or the exhaust gases of the turbine are used to generate steam for a steam turbine.

21. A method according to any one of the preceding claims, wherein natural gas or another external fuel source is used to supplement the fuel gas of the method.

22. A method according to any one of the preceding claims, wherein In said sequence of ammonia cracking steps, the effluent of each cracking step is sent in its entirety to the next cracking step.

23. The process according to any one of the preceding claims, wherein a liquid ammonia feed is evaporated to produce the gaseous ammonia feed (7), wherein at least a portion of the heat used to evaporate the liquid ammonia is recovered from a hydrogen product stream and / or from the final cracked stream.

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