JP2024521560A - Ammonia decomposition for hydrogen production - Google Patents

Abstract

A process for the synthesis of hydrogen via catalytic cracking of ammonia, said process comprising subjecting an ammonia-containing stream (10) to a catalytic cracking step (11) in the presence of heat to obtain combustion gases and a pyrolysis stream (14) containing nitrogen, hydrogen and possibly residual ammonia and optionally water, said process further comprising subjecting said pyrolysis stream to a hydrogen recovery step to obtain a high purity hydrogen stream (22).

Description

The present invention relates to the field of hydrogen production, and in particular to a process and plant for hydrogen production from an ammonia cracking unit.

The excessive use of fossil fuels in both the power sector and transportation has had detrimental effects on human health and well-being, as well as on the environment. There is now a strong need to find some environmentally friendly and sustainable alternatives to fossil fuels.

Hydrogen and ammonia are carbon-free carriers and are considered ideal substitutes for fossil fuels.

On a small scale, hydrogen can be produced from a variety of domestic sources such as solar, wind, and electrolysis. Conversely, on an industrial scale, hydrogen is most often obtained via the reforming of fossil fuels, either by reforming natural gas (steam reforming) or by the water-gas shift of coal-derived syngas.

Hydrogen produced by steam reforming requires a multi-step process starting with the production of natural gas, followed by high-temperature reforming, water-gas shift conversions (WGS) at high and low temperatures, and purification.

Unfortunately, the reforming process results in large amounts of emissions, _CO2 , being emitted into the atmosphere.

There is a need in the art to find an industrial-scale hydrogen synthesis process that can produce clean hydrogen without emitting any carbon dioxide into the atmosphere. Such a process should also be economically competitive with conventional methods.

Green ammonia synthesized from renewable energy is a carbon-free storage vector for hydrogen with numerous potential energy applications, including the production of green hydrogen. Hydrogen can be obtained from ammonia via a thermal cracking process known as catalytic cracking.

In the catalytic cracking process, ammonia is decomposed or cracked back to _H2 and _N2 in the presence of heat and a catalyst (Ni or Ru or Pt) according to the following endothermic equilibrium:
_2NH3 ←→ _3H2 + _N2

The thermodynamic conversion of ammonia to hydrogen is possible at temperatures as low as 425°C. In practice, however, the conversion rate varies depending on the type of catalyst used. Typically, Ni is active at higher temperatures (500-750°C) than Ru (400°C), but the latter catalyst is more expensive.

The heat required for the thermal catalytic conversion of ammonia is typically provided via electrical heating in an electrically heated furnace, or via the combustion of fuel in a reformer.

Unfortunately, the above-mentioned ammonia decomposition technologies suffer from several drawbacks: First, they are mature and commercially available primarily for small-scale applications (i.e., hydrogen production rates below 100 kg _H2 /hr).

The main challenge in scaling up this technology is to design a sufficiently compact decomposition unit to be able to decompose ammonia at a rate that is commensurate with consumption.

In addition, a typical challenge faced by projects utilizing adiabatic cracking units is the relatively low ammonia conversion (i.e., high ammonia slip). In contrast, cracking plants utilizing oxygen-blown autothermal reformers require the installation of expensive Air Separation Units (ASUs).

Furthermore, for high hydrogen production rates (>1000 m ³ /hr), natural gas reforming remains the most cost-effective option.

Therefore, in view of the above considerations, it is highly desirable to provide a cost-effective hydrogen synthesis process and plant suitable for large-scale hydrogen production. In addition, the improved hydrogen synthesis process should be environmentally friendly and therefore should not contribute to carbon dioxide emissions into the atmosphere.

The object of the present invention is to overcome the above-mentioned shortcomings in the prior art. In particular, the problem addressed by the present invention is how to reduce the carbon dioxide emissions and the costs of said plants, and how to provide a process and a plant suitable for large-scale production.

The present invention relates to a process for obtaining a high purity hydrogen stream through the decomposition of ammonia.

A first aspect of the present invention is a carbon-free hydrogen production process for catalytic synthesis of hydrogen as described in claim 1.

The process of claim 1 includes subjecting an ammonia stream, optionally with added water, to a preheating step to obtain an ammonia-containing stream, and subjecting the ammonia-containing stream to a catalytic ammonia decomposition step in the presence of heat to obtain a pyrolysis stream containing nitrogen, hydrogen, and optionally residual ammonia and water.

The process of claim 1 further comprises subjecting the pyrolysis stream to a hydrogen recovery step to obtain a high purity hydrogen stream and a tail gas, or subjecting the pyrolysis stream to a scrubbing step in the presence of water to obtain a purified gas stream, and further subjecting the purified gas stream to a hydrogen recovery step to obtain a high purity hydrogen stream and a tail gas.

Additionally, the process of claim 1 includes recycling at least a portion of the tail gas as a fuel gas to provide heat to the catalytic cracking process, and withdrawing the high purity hydrogen stream.

A further aspect of the present invention is a process for producing hydrogen as described in claim 7.

The process of claim 7 includes subjecting an ammonia stream to a heating stage to obtain an ammonia-containing stream, and subjecting the ammonia-containing stream to a catalytic ammonia decomposition step in the presence of heat to obtain a combusted gas and a pyrolysis stream containing nitrogen, hydrogen, and residual ammonia.

The process of claim 7 further includes the steps of: optionally mixing the pyrolysis stream with water to obtain a pyrolysis stream to which water has been added; feeding the pyrolysis stream or the pyrolysis stream to which water has been added to a cooling stage to obtain a cooled stream; subjecting the cooled stream to a flash separation step to obtain an ammonia-depleted stream and either an ammonia stream or an aqueous ammonia solution; and further subjecting the ammonia-depleted stream to a hydrogen recovery step to obtain a high-purity hydrogen stream and a tail gas.

Alternatively, the ammonia-depleted gas stream is subjected to a scrubbing step in the presence of water to obtain a purified gas. The purified gas is further subjected to a hydrogen recovery step to obtain a high purity hydrogen stream and a tail gas.

Additionally, the process of claim 7 includes recycling at least a portion of the tail gas as fuel to provide heat to the catalytic cracking process and withdrawing the high purity hydrogen stream.

A further aspect of the invention is a plant for the production of hydrogen as claimed.

The hydrogen production plant configured to carry out the process of claim 1 comprises at least a furnace suitable for decomposing ammonia, the furnace including a plurality of externally heated catalyst tubes, an input line arranged to supply an ammonia-containing stream to the tubes, and an output line arranged to recover a pyrolysis stream from the tubes.

The plant configured to carry out the process of claim 1 further comprises a hydrogen recovery unit configured to recover a high purity hydrogen stream and a tail gas, a line arranged to recycle at least a portion of the tail gas separated from the hydrogen recovery unit to the furnace for use as additional fuel, and a line arranged to withdraw a high purity hydrogen stream from the hydrogen recovery unit.

The plant configured to carry out the process of claim 7 comprises a furnace suitable for decomposing ammonia, including a plurality of externally heated catalyst tubes, an input line arranged to supply an ammonia-containing stream to the tubes, and an output line arranged to recover a pyrolysis stream from the tubes, and, optionally, a line configured to supply water to the pyrolysis stream.

The plant configured to carry out the process of claim 7 further comprises a flash separator unit in communication with the discharge line and configured to separate an ammonia stream or an aqueous ammonia solution from the ammonia-depleted gas stream, a hydrogen recovery unit in fluid communication with the flash separator and configured to recover a high purity hydrogen stream and a tail gas, a line arranged to recycle at least a portion of the tail gas separated from the hydrogen recovery unit to the furnace for use as additional fuel, and a line arranged to withdraw a high purity hydrogen stream from the hydrogen recovery unit.

Advantageously, by feeding air to the furnace instead of oxygen, an air separation unit is not required. Even more advantageously, by adjusting the fuel-air ratio (i.e., by operating in excess air), the NOx content of the combusted gases exiting the furnace can be minimized. Additionally, the NOx present in the system can be completely eliminated or reduced to a few ppm by installing a Selective Catalytic Reduction (SCR) or Non-Selective Catalytic Reduction abatement system (NSCR).

Even more advantageously, in contrast to reforming processes carried out using natural gas as a fuel source, the process of the present invention uses a carbon-free source (e.g., ammonia) as the combustible gas, so no carbon dioxide emissions are released into the atmosphere.

Advantageously, in process and plant configurations where an electrolysis unit is placed before or integrated with the furnace, high flexibility in hydrogen synthesis can be envisaged.

Preferred embodiments According to a particularly preferred embodiment of the present invention, the heat required to sustain the endothermic decomposition of ammonia is provided via the combustion reaction of a fuel gas in the presence of preheated air, from which combusted gas is obtained.

Preferably, the fuel gas used as the combustible gas in the catalytic cracking process contains ammonia, or a mixture of nitrogen and hydrogen, or a mixture of ammonia, nitrogen and hydrogen. Advantageously, no carbon dioxide emissions to the atmosphere occur.

According to an alternative embodiment of the invention, residual fossil fuel, such as natural gas, may be added to the fuel gas to maintain combustion. Due to the low amount of natural gas used, in this alternative embodiment, the carbon dioxide emissions of the process are still lower than would be expected from a conventional hydrogen synthesis process.

According to an alternative embodiment of the invention, the process further comprises subjecting the ammonia-bearing fuel gas to a cracking step in the presence of electrical heating to obtain a gas mixture bearing hydrogen and nitrogen and optionally unconverted ammonia, and further subjecting the gas mixture to combustion in the presence of preheated air to provide reforming heat in the catalytic cracking step.

Alternatively, the ammonia-bearing fuel gas may be subjected to a catalytic cracking process, in which case the heat required to sustain the cracking reaction is recovered from the combusted gas. The pyrolysis and electrolysis steps may be carried out in a single furnace. In this particular embodiment, the furnace may comprise a burner and an electrolysis unit.

Preferably, the burner is designed to burn either ammonia, or a mixture of ammonia and a hydrogen-rich stream, or a mixture of ammonia, a hydrogen-rich stream and tail gas, or a mixture of a hydrogen-rich stream and tail gas. In addition, the burner may be operated with the addition of natural gas or fossil fuels to the mixture of the above mentioned streams.

According to a particularly preferred embodiment, the fuel gas before being subjected to the decomposition process or before being subjected to combustion in the furnace is further subjected to a heat recovery process in which heat is indirectly transferred from the combusted gas to the fuel gas.

The reforming heat required for the ammonia catalytic cracking process can be provided via combustion of fuel gas in the presence of preheated air.

According to an alternative embodiment, the aqueous ammonia solution may be subjected to a distillation process to separate an ammonia stream from the aqueous solution, and at least a portion of the ammonia stream may be recycled as a fuel to provide heat to the catalytic cracking process.

In addition, a portion of the ammonia stream may be recycled to the heating stage and subjected to the ammonia catalytic cracking step together with the main ammonia stream.

The process may further include recovering heat from the combusted gas by indirectly contacting a portion of the aqueous solution with the combusted gas, and feeding the portion of the aqueous solution after heat recovery to the distillation step to provide distillation heat. Advantageously, thermal integration between the distillation step and the ammonia catalytic cracking step may be achieved, reducing the energy consumption of the process.

The process may further include feeding a portion of the aqueous solution obtained from distillation to the pyrolysis stream, optionally with the addition of a water make-up stream.

According to a particularly preferred embodiment of the invention, the hydrogen purification step is carried out by a pressure swing adsorption unit or a cryogenic separation unit or a membrane purification unit. A person skilled in the art will be well aware when to choose one unit over the other depending on the concentration of hydrogen retained by the pyrolysis stream.

Preferably, the high purity hydrogen obtained after the hydrogen purification process has a concentration of more than 95% by weight, preferably more than 99% by weight, more preferably more than 99.9% by weight.

Preferably, the temperature of the pyrolysis stream discharged from the catalytic cracking step is 400 to 950°C, more preferably 550 to 650°C.

Preferably, the catalytic cracking step is carried out at a gauge pressure of about 5 to 65 bar, more preferably 15 to 30 bar.

According to a particularly preferred embodiment of the present invention, the combustion gases discharged from the catalytic cracking process are subjected to a NOx abatement process before being exhausted to the atmosphere. Alternatively, the NOx abatement process may be carried out in the furnace section.

According to an embodiment of the invention, the plant may further comprise a purification unit configured to recover ammonia from the pyrolysis stream to obtain a purified gas stream and a recycle gas, and a line arranged to supply at least a portion of the recycle gas to the furnace.

Additionally, the plant may include an electrolysis unit configured to decompose ammonia-bearing fuel gas. Alternatively, the plant may include a coil filled with a catalyst and disposed in a convective section of the furnace. The catalyst-filled coil is configured to catalytically decompose the fuel gas using heat retained by the combusted gas traversing the convective section.

According to an embodiment of the invention, the catalytic cracking of the fuel may be carried out in a combined process, where the fuel is partially cracked in the coil disposed in the convection section of the furnace, and the partially cracked fuel discharged from the coil is subsequently further cracked in an electrolysis unit.

The electrolysis unit may be located before the furnace and may be in communication with the furnace by a gas flow line. Alternatively, the electrolysis unit may be integrated with the furnace and may be used to decompose the fuel gas prior to combustion.

According to a particularly preferred embodiment of the invention, the plant comprises a distillation unit configured to separate ammonia from water in the aqueous ammonia solution, a line connecting the flash separator unit to the distillation unit and configured to convey the aqueous ammonia solution to the distillation unit.

Additionally, the plant may further comprise a gas flow line connecting the distillation unit to the furnace, a heat exchange section configured to recover heat from the combusted gas in the furnace by a water flow, and a line connecting the distillation unit to the heat exchange section and configured to convey the water flow for use in thermal integration between the furnace and the distillation unit.

According to an embodiment of the invention, said furnace may comprise a unit suitable for removing NOx (also called deNOx unit), preferably an SCR unit or an SNCR unit or a combination of both. NOx removal carried out via SCR may be carried out in a temperature range of 150-600°C, or preferably inclusive of 350-600°C. In contrast, NOx removal carried out via SNCR may be carried out in a temperature range of 850-1200°C, or preferably inclusive of 900-1050°C. The term NOx refers to oxides of nitrogen, most often NO and _NO2 .

Preferably, the hydrogen recovery unit is a pressure swing adsorption unit or a cryogenic separation unit or a membrane separation unit.

According to an embodiment of the invention, the ammonia catalytic cracking step is carried out in a furnace equipped with a heat release section and a convection section. The heat release section holds a bundle of tubes preferably containing a nickel-based catalyst or a ruthenium-based catalyst or a molybdenum-based catalyst or a platinum-based catalyst optionally doped with molybdenum, cobalt, and lithium.

In a particularly interesting embodiment of the invention, the convection section of the furnace comprises a number of heat exchangers (coil banks) arranged in the convection section of the furnace. Preferably, at least one of the heat exchangers is a steam superheater, in addition, waste heat boiler coils and water boiling coils may also be integrated into the furnace. The heat recovered in the convection section of the furnace may be used for heat integration purposes in the process or may be utilized for energy generation. Alternatively, heat recovery may be achieved downstream of the furnace.

The furnace outlet can be directly quenched with a cooling medium, preferably water, ammonia, or a cold stream of gas.

Downstream of the cooling process, the aqueous ammonia solution may be separated from the gas phase, preferably in a flash evaporator, and this liquid may be distilled in a dedicated column using the heat available in the convection section of the furnace, and ammonia may be recovered in the same distillation column.

FIG. 1 is a schematic diagram of a hydrogen synthesis process according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a hydrogen synthesis process according to another embodiment of the present invention. FIG. 3 is a schematic diagram of a hydrogen synthesis process according to an alternative embodiment of the present invention. FIG. 4 is a schematic diagram of a hydrogen synthesis process according to another embodiment.

Figure 1 shows a schematic diagram of a hydrogen synthesis process according to a first embodiment of the present invention.

A liquid ammonia stream 2 is withdrawn from a storage feed tank 1 and fed via pump 3 to a first preheating unit 6, thereby obtaining a vaporized or partially vaporized ammonia stream 7, or hot liquid ammonia 7.

The ammonia stream 7 is mixed with water 8 and preheated in a second preheat unit 9 to complete the vaporization of the aqueous ammonia stream, thereby obtaining an ammonia-containing stream 10. The ammonia-containing stream 10 is subsequently fed to a catalytic cracking unit 11 for catalytic cracking in the presence of heat to obtain a cracked stream 14.

The catalytic cracking unit 11 typically comprises a furnace with a heat release section and a convection section. The heat release section comprises a bundle of tubes holding a cracking catalyst, typically a Ni-based catalyst.

The heat required to sustain the endothermic ammonia decomposition reaction is provided through the combustion of fuel gas 12 in the presence of preheated air 28.

Preheated air 28, supplied to the catalytic cracking furnace as a combustor, is obtained by preheating the airflow 27 exiting the air blower 26 in the convection section of the furnace. In the convection section, pressurized steam 29 is also generated by recovering heat from the combusted gases 60. The combusted gases are subsequently treated in a de-NOx stage (not shown) to remove NOx before being exhausted to the atmosphere.

The pyrolysis stream 14, which typically retains residual ammonia, is subjected to a scrubbing step 20 in the presence of water 17 to obtain a purified gas stream 51 and a recycle gas 21. Water 17 is used as an absorbent in the scrubbing step to remove ammonia from the stream by taking advantage of the high solubility of ammonia in water.

The purified gas stream 51 is then fed to the hydrogen recovery process 19 to obtain a high purity hydrogen stream 22 and a tail gas 23. The hydrogen stream 22 is withdrawn from the hydrogen recovery process for storage and/or utilization depending on the demand for hydrogen.

The tail gas 23 and recycle gas 21 are then mixed together to obtain a mixed stream 25 which is returned to the ammonia decomposition process/unit 11 for reuse.

Figure 2 illustrates a hydrogen synthesis process according to another embodiment of the present invention.

The process depicted in FIG. 2 can be used to synthesize hydrogen when the ammonia content carried by the pyrolysis stream 14 is on the order of a few ppm, preferably on the order of ppb.

In this particular embodiment, the cracked stream 14 is fed directly to the hydrogen recovery process 19 without passing through a scrubbing stage. The hydrogen recovery process is carried out in a pressure swing adsorption unit.

Alternatively, hydrogen may be recovered in a cryogenic unit where a series of compression and cooling stages are performed to remove nitrogen from the purified gas stream, or in a hydrogen membrane separation unit where the selective permeability of hydrogen through certain membranes is exploited.

Figure 3 shows another alternative embodiment of the hydrogen synthesis process.

The ammonia stream 7 is subjected to a heating stage 6, 51 where it is subjected to heat exchange with the pyrolysis stream 14 exiting the furnace 11. In addition, the ammonia stream is further heated (9) in the convection section of the furnace to obtain an ammonia-containing stream 10 which is then fed to the ammonia catalytic cracking step of the furnace.

After exiting the furnace, pyrolysis stream 14 containing nitrogen, hydrogen, and residual ammonia is mixed with water 74 to obtain pyrolysis stream 75 with added water, which is further air-cooled in tower 70 after heat exchange with ammonia stream 7 in heat exchangers 51 and 6 to obtain cooling stream 79.

The cooled stream 79 is then sent to a flash separator 80 where an ammonia-depleted gas stream 81 is separated from the aqueous ammonia solution 82.

The ammonia-depleted stream 81 is then subjected to a hydrogen recovery process 19 to obtain a high purity hydrogen stream 22 and a tail gas 23.

After heat exchange with the aqueous solution 74 (120), the tail gas 23 is then supplied as fuel to provide heat to the catalytic cracking process 11.

Hydrogen 22 is withdrawn from hydrogen recovery process 19 and stored or utilized as needed. The aqueous ammonia solution 82 is then sent to a distillation unit 83 to separate an ammonia stream 86 from the aqueous solution 84.

A first portion 91 of the ammonia stream 86 is recycled as fuel to the furnace to provide heat to the catalytic cracking step 11, while a second portion 92 of the ammonia stream is mixed with the ammonia stream 7 and subsequently fed to the ammonia catalytic cracking step 11 in the furnace after preheating.

A portion 87 of the aqueous solution 84 is utilized to recover heat from the combusted gases 60 by indirect heat transfer with the combusted gases 60 in the convection section of the furnace. The combusted gases are subjected to a NOx removal step 131 before being withdrawn from the furnace.

A second portion 88 of the aqueous solution 84 obtained from the distillation unit 83 is mixed with the make-up water stream 17 and fed to the pyrolysis stream 14.

Figure 4 shows a hydrogen synthesis process according to another alternative embodiment of the present invention.

In the figure, it can be seen that ammonia-carrying fuel gas 12 is subjected to a decomposition step 100 in the presence of electrical heating to obtain a gas mixture 101 carrying hydrogen and nitrogen and optionally unconverted ammonia.

The gas mixture 101 is then mixed with the tail gas 23 and then subjected to combustion in the presence of preheated air 28 to provide reforming heat in the catalytic cracking step 11.

As an alternative embodiment not shown in the figures, the decomposition step carried out in the presence of electrical heating can also take place inside the furnace.

Claims (23)

A process for catalytic synthesis of hydrogen, comprising the steps of:
a) subjecting an ammonia stream (7) (optionally with added water (8)) to a pre-heating step (9) to obtain an ammonia-containing stream (10);
b) subjecting said ammonia-containing stream (10) to a catalytic ammonia decomposition step (11) in the presence of heat to obtain a thermally decomposed stream (14) containing nitrogen and hydrogen, and may also contain residual ammonia, and optionally containing water;
c) subjecting said pyrolysis stream (14) to
c1) subjecting the resulting product to a hydrogen recovery process (19) to obtain a high purity hydrogen stream (22) and a tail gas (23);
or c2) a scrubbing step (20) in the presence of water (17) to obtain a purified gas stream (51), which is further subjected to a hydrogen recovery step (19) to obtain a high purity hydrogen stream (22) and a tail gas (23);
d) recycling at least a portion of said tail gas (23) as a fuel to provide heat for said catalytic ammonia cracking step (11);
e) withdrawing said high purity hydrogen stream (22);
The process includes: The process of claim 1 further comprising subjecting the fuel gas (12) to combustion in the presence of preheated air (28) to provide reforming heat for the catalytic ammonia cracking step (11) and to obtain a combusted gas (60). The process of claim 1 or claim 2, wherein the fuel gas (12) comprises ammonia, or a mixture of nitrogen and hydrogen, or a mixture of ammonia, nitrogen and hydrogen. The process of claim 1 further comprising subjecting the ammonia-bearing fuel gas (12) to a cracking step (100) in the presence of electrical heating to obtain a gas mixture (101) that bears hydrogen and nitrogen and may bear unconverted ammonia, and further subjecting the gas mixture (101) to combustion in the presence of preheated air (28) to provide the reforming heat in the catalytic cracking step (11). The process of claim 4, wherein the decomposition step (100) and the combustion are performed in a single unit. The process of any preceding claim, wherein the fuel gas (12) before being subjected to the decomposition step (100) or before being subjected to combustion is further subjected to a heat recovery step in which heat is indirectly transferred from the combusted gas (60) to the fuel gas (12). A process for catalytic synthesis of hydrogen, comprising the steps of:
a) passing an ammonia stream (7) through a heating stage (6, 51, 9) to obtain an ammonia-containing stream (10);
b) subjecting said ammonia-containing stream (10) to a catalytic ammonia decomposition step (11) in the presence of heat to obtain a combustion gas (60) and a pyrolysis stream (14) containing nitrogen, hydrogen and residual ammonia;
c) optionally mixing said pyrolysis stream with water (74) to obtain an aqueous pyrolysis stream (75);
d) feeding the pyrolysis stream (14) or the pyrolysis stream (75) to which water has been added to a cooling stage (51, 6, 70) to obtain a cooled stream (79);
e) subjecting the cooled stream (79) to a flash separation step (80) to obtain an ammonia-depleted stream (81) and an ammonia stream or aqueous ammonia solution (82), and further subjecting the ammonia-depleted stream (81) to
e1) subjecting the resulting product to a hydrogen recovery process (19) to obtain a high purity hydrogen stream (22) and a tail gas (23);
or e2) a scrubbing step (20) in the presence of water (17) to obtain a purified gas stream (51), which is further subjected to a hydrogen recovery step (19) to obtain a high purity hydrogen stream (22) and a tail gas (23);
f) recycling at least a portion of said tail gas (23) as fuel to provide heat for said catalytic cracking step (11);
g) withdrawing said high purity hydrogen stream (22);
The process includes: The process of claim 7, wherein reforming heat for the ammonia catalytic cracking step is provided via combustion of fuel gas (12) in the presence of preheated air (28). The process of claim 7, further comprising subjecting the ammonia-bearing fuel gas (12) to a cracking step (100) in the presence of electrical heating to obtain a gas mixture (101) bearing hydrogen and nitrogen and optionally unconverted ammonia, and further subjecting the gas mixture (101) to combustion in the presence of preheated air (28) to provide the reforming heat in the catalytic cracking step (11). A process according to any preceding claim,
moreover,
h) subjecting the aqueous ammonia solution (82) to a distillation step (83) to separate an ammonia stream (86) from the aqueous solution (84);
i) recycling at least a portion of said ammonia stream (86) as a fuel to provide heat for said catalytic cracking step (11);
j) optionally recycling a portion of said ammonia stream (86) to step (a) for subjecting it to said heating stage (6, 51, 9) in the presence of said ammonia stream (7);
k) recovering heat from the combustion gas (60) by indirectly contacting a portion (87) of the aqueous solution (84) with the combustion gas (60) and feeding the portion of the aqueous solution after heat recovery to the distillation step (83) to provide distillation heat;
The process includes: The process of claim 10, further comprising mixing a second portion (88) of the aqueous solution (84) obtained from the distillation step (83) with the pyrolysis stream (14) (optionally with the addition of a water make-up stream (17)). The process of any preceding claim, wherein the hydrogen purification step (19) is performed by a pressure swing adsorption unit or a cryogenic separation unit or a membrane purification unit. The process of any preceding claim, wherein the high purity hydrogen stream (22) has a concentration of greater than 95 wt%, preferably greater than 99 wt%, more preferably greater than 99.9 wt%. The process according to any of the preceding claims, wherein the temperature of the pyrolysis stream (14) discharged from the catalytic cracking step (11) is between 400 and 950°C, preferably between 550 and 650°C. The process of any preceding claim, wherein the catalytic cracking step (11) is carried out at a pressure of about 5 to 65 bar, preferably between 15 bar and 30 bar. The process of any preceding claim, wherein the combusted gas (60) is subjected to a nitrogen oxide (NOx) abatement step. A plant for the production of hydrogen according to the process of claim 1, comprising at least
a furnace (11) suitable for decomposition of ammonia, comprising a plurality of externally heated catalytic tubes, an input line arranged to feed an ammonia-containing stream (10) to said tubes, and an output line arranged to recover a pyrolysis stream (14) from said tubes;
A hydrogen recovery unit (19) configured to recover a high purity hydrogen stream (22) and a tail gas (23);
a line arranged to recycle at least a portion of the tail gas (23) separated from the hydrogen recovery unit (19) to the furnace (11) for use as additional fuel;
a line arranged to withdraw a high purity hydrogen stream (22) from said hydrogen recovery unit (19);
A plant equipped with 18. The plant of claim 17,
moreover,
a purification unit (20) configured to recover ammonia from said pyrolysis stream (14) to obtain a purified gas stream (51) and a recycle gas (21);
a line arranged to feed at least a portion of said recycled gas (21) to said furnace (11);
A plant equipped with The method further comprises: providing an electrolysis unit (100) configured to decompose the ammonia-bearing fuel gas;
the electrolysis unit is disposed above the furnace and in fluid communication with the furnace by a gas flow line;
or the electrolysis unit is disposed within the furnace and configured to decompose the fuel gas prior to combustion.
19. A plant according to claim 17 or claim 18. A plant for the production of hydrogen according to the process of claim 7, comprising at least
a furnace (11) suitable for decomposing ammonia, comprising a plurality of externally heated catalyst tubes, an input line arranged to feed an ammonia-containing stream (10) to said tubes, and an output line arranged to recover a pyrolysis stream (14) from said tubes;
Optionally, a line configured to supply water to the discharge line arranged to recover the pyrolysis stream (14);
a flash separator unit (80) in communication with said discharge line and configured to separate an ammonia stream or an aqueous ammonia solution (82) from the ammonia-depleted gas stream (81);
a hydrogen recovery unit (19) in fluid communication with the flash separator and configured to recover a high purity hydrogen stream (22) and a tail gas (23);
a line arranged to recycle at least a portion of the tail gas (23) separated from the hydrogen recovery unit (19) to the furnace (11) for use as additional fuel;
a line arranged to withdraw a high purity hydrogen stream (22) from said hydrogen recovery unit (19);
A plant equipped with 21. The plant of claim 20,
moreover,
a distillation unit (83) configured to separate ammonia from water in said aqueous ammonia solution;
a line connecting the flash separator unit (80) to the distillation unit (83) and configured to convey the aqueous ammonia solution (82) to the distillation unit (83);
a gas flow line connecting said distillation unit (83) to said furnace (11);
a heat exchange section configured to recover heat from the combusted gas in the furnace by a flow of water;
a line connecting the distillation unit to the heat exchange section and configured to carry the water stream for use in heat integration between the furnace and the distillation unit;
A plant equipped with A plant according to any preceding claim, wherein the furnace further comprises a unit for removing nitrogen oxides NOx, preferably an SCR unit. 10. The plant of any preceding claim, wherein the hydrogen recovery unit is one of a pressure swing adsorption unit, a cryogenic separation unit, a membrane separation unit.