DESCRIPTION

Field of application The invention is in the field of hydrogen production and particularly pertains to a process and a plant for the production of hydrogen from an ammonia cracking unit.

Prior art

The excessive use of fossil fuel both in the electricity sector and transportation has resulted in harmful effects on human health and welfare as well as on the environment. Currently, there is a strong need to come up with some environmental and sustainable alternatives to fossil fuels.

Hydrogen and ammonia are carbon-free carriers and are considered ideal replacements for fossil fuel. At a small scale, hydrogen can be produced from various domestic resources such as solar power, wind power, and electrolysis. Conversely, at industrial scale hydrogen is obtained via reforming of fossil fuel mostly by reforming of natural gas (steam reforming) or water gas shift of coal-derived syngas.

Hydrogen produced by steam reforming requires a multistep process, starting from natural gas purification, high-temperature reforming, high and low- temperature water-gas shift conversions (WGS) and purification.

Unfortunately, as a result of the reforming process, large CO2 emissions are emitted into the atmosphere.

In the art, there is a desire to find an industrial scale hydrogen synthesis process that can produce clean hydrogen without emitting any carbon dioxide into the atmosphere. Such process should also be economically competitive towards the conventional methods. Green ammonia synthesized from renewable energy is a carbon-free storage vector of hydrogen with numerous potential energy applications, including the production of green hydrogen. Hydrogen can be obtained from ammonia via a thermal decomposition process known as catalytic cracking. In the catalytic cracking process, ammonia is decomposed or cracked back into H2 and N2 in the presence of heat and a catalyst (Ni or Ru or Pt) according to the following endothermic equilibrium:

2NH _3< 3H ₂ +N ₂

Thermodynamic conversion of ammonia to hydrogen is possible at a temperature as low as 425 °C. However, in practice, the conversion rate depends on the type of catalyst being used. Typically, Ni is active at higher temperature (500-750 °C) than Ru (400 °C) but the latter catalyst is more expensive.

The heat required for the thermo-catalytic conversion of ammonia is typically provided through electric heating in an electrically heated furnace or in a reformer via fuel combustion.

Unfortunately, the above-mentioned ammonia cracking technologies suffer from several drawbacks. First of all, the ammonia cracking technology is mature and commercially available mainly for small scale applications (i.e. having a hydrogen production rate of less than 100 kg H2/h). The main difficulty to scale up this technology is to design a cracking unit sufficiently compact and yet capable of decomposing the ammonia at a rate that is in accordance with the consumption.

Additionally, typical issues observed in the plan that exploits an adiabatic cracking unit is the relatively low conversion of the ammonia (i.e. high ammonia slip). Conversely, cracking plants that exploit oxygen blown autothermal reformers require the installation of an expensive Air Separation Unit ASU.

Besides, for high hydrogen production rate (>1000 m ³ /h), natural gas reforming still remains the most cost-effective option. Therefore, in light of the consideration state above it is highly desirable to provide a cost-effective hydrogen synthesis process and a plant suitable for large scale hydrogen production. Additionally, the improved hydrogen synthesis process should be environmentally friendly therefore not contributing to the emission of carbon dioxide into the atmosphere.

Summary of the invention

The invention aims to overcome the above drawbacks of the prior art. In particular, the problem addressed by the invention is how to reduce the carbon dioxide emission and the cost of the plant and how to provide a process and a plant suitable for large scale production.

The invention relates to a process wherein a high-purity hydrogen stream is obtained through the cracking of ammonia.

A first aspect of the present invention is a carbon-free hydrogen production process for the catalytic synthesis of hydrogen according to claim 1. The process according to claim 1 comprises the steps of subjecting an ammonia stream optionally added with water to a pre-heating step to yield an ammonia- containing stream, subjecting said ammonia-containing stream to a catalytic ammonia-cracking step in the presence of heat to yield a thermally cracked stream containing nitrogen, hydrogen and possibly residual ammonia and water. The process according to claim 1 further comprises the steps of subjecting the thermally cracked stream to a hydrogen recovering step to yield a high-purity hydrogen stream and a tail gas, alternatively subjecting the thermally cracked stream to a scrubbing step in the presence of water to yield a purified gas stream and further subjecting said purified gas stream to a hydrogen recovery step to yield a high-purity hydrogen stream and a tail gas.

In addition, the process according to claim 1 comprises the step of recirculating at least a portion of the tail gas as a fuel gas to provide heat for the catalytic cracking step and the step of withdrawing said high purity hydrogen stream.

A further aspect of the present invention is a process for the production of hydrogen according to claim 7. The process according to claim 7 comprises the steps of subjecting an ammonia stream to a heating stage to yield an ammonia containing stream, subjecting said ammonia-containing stream to a catalytic ammonia-cracking step in the presence of heat to yield a combusted gas and a thermally cracked stream containing nitrogen, hydrogen and residual ammonia. The process according to claim 7 further comprises the steps of optionally mixing the thermally cracked stream with water to yield a thermally cracked stream added with water and feeding said thermally cracked stream or said thermally cracked stream added with water to a cooling stage to yield a cooled stream, subjecting said cooled stream to a flash separation step to yield an ammonia depleted stream and either an ammonia stream or an aqueous ammonia solution and further subjecting said ammonia depleted stream to a hydrogen recovery step to yield a high-purity hydrogen stream and a tail gas.

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

In addition, the process according to claim 7 comprises the steps of recirculating at least a portion of the tail gas as a fuel to provide heat for the catalytic cracking step and withdrawing said high purity hydrogen stream. A further aspect of the present invention is a plant for the production of hydrogen according to the claims. The hydrogen production plant adapted to perform the process of claim 1 comprises at least a furnace suitable for cracking of ammonia, including a plurality of externally heated catalytic tubes, an input line arranged to feed an ammonia- containing stream into the tubes and an output line arranged to collect a thermally cracked stream from the tubes.

The plant adapted to perform 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 recirculate at least a portion of said tail gas separated from said hydrogen recovery unit to the furnace to be used as an additional fuel and a line arranged to withdraw a high purity hydrogen stream from the hydrogen recovery unit.

The plant adapted to perform the process claim 7 comprises a furnace suitable for cracking of ammonia, including a plurality of externally heated catalytic tubes, an input line arranged to feed an ammonia-containing stream into the tubes and an output line arranged to collect a thermally cracked stream from the tubes and optionally a line configured to provide water to the thermally cracked stream.

The plant adapted to perform the process claim 7 further comprises a flash separator unit in communication with said output line configured to separate an ammonia stream or an aqueous ammonia solution from an ammonia depleted gas stream, a hydrogen recovery unit in fluid communication with said flash separator and configured to recover a high-purity hydrogen stream and a tail gas, a line arranged to recirculate at least a portion of said tail gas separated from said hydrogen recovery unit to the furnace to be used as an 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 no air separation unit is required. Even more advantageously by adjusting the fuel-air ratio (i.e. working in excess of air), the NOx content of the combusted gas exiting the furnace can be minimised. Additionally, the NOx present in the system could be completely removed or reduced to a few ppm by installing a SCR Selective Catalytic Reduction SCR or a Non-Selective Catalytic Reduction abatement system NSCR. Even more advantageously, contrary to the reforming process carried out using natural gas as a fuel source in the process of the present invention a carbon-free source (e.g. ammonia) is used as a gas combustible so that not carbon dioxide emissions are released into the atmosphere.

Advantageously, in the process and in the plant configuration wherein the electric cracking unit is arranged before the furnace or is integrated with the latter high flexibility in the synthesis of hydrogen can be envisaged.

Preferred embodiments

According to a particularly preferred embodiment of the present invention, the heat required to sustain the endothermic cracking of ammonia is provided via the combustion reaction of the fuel gas in the presence of pre-heated air to yield a combusted gas.

Preferably, the fuel gas used as a gas combustible in the catalytic cracking step contains ammonia or a mixture of nitrogen and hydrogen, or a mixture of ammonia, nitrogen and hydrogen. Advantageously, no carbon dioxide emissions are realised into the atmosphere.

According to an alternative embodiment of the present invention, a balance of fossil fuel such as natural gas can be added to the fuel gas to sustain the 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 emissions expected in conventional hydrogen synthesis process.

According to an alternative embodiment of the present invention, the process further comprises the steps of subjecting a fuel gas retaining ammonia to a cracking step in presence of electric heating to yield a gas mixture retaining hydrogen and nitrogen and possibly unconverted ammonia and further subjecting said gas mixture to combustion in presence of pre-heated air to provide the reforming heat in the catalytic cracking step.

Alternatively, the fuel gas retaining ammonia can also be subjected to a catalytic cracking step wherein the heat necessary to sustain the cracking reactions is recovered from the combusted gas. The thermal cracking step and the electric cracking step can be carried out in a single furnace. In this specific embodiment, the furnace can comprise a burner and an electric cracking unit.

Preferably, the burner is designed to combust either ammonia or a mixture of ammonia and hydrogen-rich stream, or a mixture of ammonia and hydrogen-rich stream and a tail gas or a mixture of a hydrogen-rich stream and a tail gas. Additionally, the burner can operate with a mixture of the aforementioned streams added with natural gas or with fossil fuel.

According to a particularly preferred embodiment, the fuel gas before being subjected to said cracking step or before being subjected to combustion in the furnace is further subjected to a heat recovery step wherein heat is indirectly transferred from the combusted gas to the fuel gas. The reforming heat required for the ammonia-catalytic cracking step can be provided via the combustion of a fuel gas in presence of pre-heated air.

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

Additionally, a portion of the ammonia stream can be recirculated to the heating stage to be subjected with the main ammonia stream to the ammonia catalytic cracking step.

The process may further include the steps of recovering heat from the combusted gas by indirectly contacting a portion of the water solution with the combusted gas and feeding said portion of water 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 can be realised and the energy consumption of the process can be reduced.

The process may further comprise the step of feeding a portion of the water solution obtained from distillation to the thermally cracked stream optionally added with a water make-up stream.

According to a particularly preferred embodiment of the present invention, the hydrogen purification step is carried out by means of a pressure swing adsorption unit or a cryogenic separation unit or a membrane purification unit. The skilled person in the art is well aware when select one unit instead of the others depending on the concentration of hydrogen retained by the thermally cracked stream.

Preferably the high-purity hydrogen obtained after the hydrogen purification step has a concertation higher than 95% wt, preferably higher than 99 % wt and more preferably higher than 99.9 % wt. Preferably the temperature of the thermally cracked stream exiting the catalytic cracking step is between 400 to 950 °C, more preferably between 550 to 650 °C.

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

According to a particularly preferred embodiment of the present invention, the combusted gas exiting the catalytic cracking step is subject to a NOx abatement step before being vented into the atmosphere. Alternatively, the NOx abatement step can be performed in a section of the furnace.

According to an embodiment of the present invention, the plant may further comprise a purification unit configured to recover ammonia from the thermally cracked stream to yield a purified gas stream and a recycling gas and a line arranged to feed at least a portion of the recycling gas to the furnace.

Additionally, the plant may comprise an electric cracking unit configured to crack a fuel gas retaining ammonia. Alternatively, the plant may comprise a coil filled with a catalyst and arranged in the convective section of the furnace. The coil filled with the catalyst is configured to catalytically crack the fuel gas taking advantage of the heat retained by the combusted gas that crosses said convective section.

According to an embodiment of the present invention, the catalytic cracking of the fuel can be carried out in a combined process wherein the fuel is partially cracked in the coil arranged in the convective section of the furnace and subsequentially the partially cracked fuel leaving the coil is further cracked in an electric cracking unit.

The electric cracking unit can be arranged prior to the furnace and it can communicate with the latter by means of a gas flow line. Alternatively, the electric cracking unit can be integrated into the furnace and it can be exploited to crack the fuel gas before combustion.

According to particularly preferred embodiment of the present 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 said distillation unit.

Additionally, the plant may further comprise a gas flow line connecting the distillation unit to the furnace, a heat exchanger section configured to recover heat from the combusted gas in the furnace by means of a water stream, a line connecting the distillation unit to said heat exchanger section and configured to carry the water stream to be exploited for thermal integration purposes between the furnace and the distillation unit.

According to an embodiment of the present invention, the furnace may include a unit suitable to remove NOx (also termed deNOx unit), preferably a SCR unit or a SNCR unit or a combination of both. The NOx removal carried out via SCR can be performed in the temperature range 150-600 °C or preferably in the temperature range comprised between 350 and 600 °C. Conversely, the NOx removal carried out via SNCR can be performed in the temperature range 850- 1200 °C or preferably in the temperature range comprised between 900 and 1050 °C. The term NOx denotes nitrogen oxides, mostly 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 provided with a radiant and a convective section. The radiant section retains the bundle of tubes containing preferably a nickel-based catalyst or a Ruthenium-based catalyst or a Molybdenum-based catalyst or a Platinum-based catalyst possibly added with molybdenum, cobalt and lithium.

In a particularly interesting embodiment of the present invention, the convective section of the furnace comprises a plurality of heat exchangers (coil banks) arranged in the convective section of the furnace. Preferably at least one of said heat exchangers is a steam superheater, additionally a waste heat boiler coil and water boiling coil can also be integrated in furnace. The heat recovered in the convective section of the furnace may be used for thermal integration purposes in the process or exploited for energy production. Alternatively, heat recovery can also be accomplished downstream the furnace. The furnace outlet can be directly quenched with a cooling medium preferably water, ammonia or a gaseous colder stream.

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

Description of the figures

Fig. 1 is a schematic representation of the hydrogen synthesis process according to an embodiment of the present invention.

Fig. 2 is a schematic representation of the hydrogen synthesis process according to another embodiment of the present invention.

Fig. 3 is a schematic representation of the hydrogen synthesis process according to an alternative embodiment of the present invention. Fig. 4 is a schematic representation of the hydrogen synthesis process according to another embodiment.

Detailed description of preferred embodiments

Fig.1 shows a schematic of a hydrogen synthesis process according to a first embodiment of the invention. A liquid ammonia stream 2 is withdrawn from a storage feed tank 1 and fed via a pump 3 to a first-preheating unit 6 so to obtain a vaporised or partially vaporised ammonia stream 7 or a hot liquid ammonia 7.

The ammonia stream 7 is mixed with water 8 and pre-heated in the second pre heating unit 9 to complete the vaporisation of the aqueous ammonia stream so to yield an ammonia-containing stream 10. The ammonia-containing stream 10 is then fed to a catalytic cracking unit 11 to be catalytically cracked in the presence of heat to yield a cracked stream 14.

The catalytic cracking unit 11 typically comprises a furnace provided with a radiant and a convective section. The radiant section comprises the bundle of tubes that retain the cracking catalyst, typically a Ni-based catalyst.

The heat required to sustain the endothermic ammonia cracking reaction is provided through the combustion of a fuel gas 12 in the presence of a pre-heated air 28. The pre-heated air 28 fed to the catalytic cracking furnace as a comburent is obtained by pre-heating the airflow 27 exiting the air blower 26 in the convective section of the furnace. In said convective section, pressurised steam 29 is also generated by recovery heat from the combusted gases 60. The combusted gases are then treated in a deNOx stage (not shown in the figure) to remove NOx before being vented into the atmosphere.

The thermally cracked stream 14, typically retaining residual ammonia, is subjected to a scrubbing step 20 in the presence of water 17 to yield a purified gas stream 51 and a recycling gas 21. Water 17 is used in the scrubbing step as an absorbent to exploit the high solubility of ammonia in water so to remove ammonia from said stream.

The purified gas stream 51 is then fed to a hydrogen recovery step 19 to yield a high-purity hydrogen stream 22 and a tail gas 23. The hydrogen stream 22 is withdrawn from the hydrogen recovery step and stored and/or exported according to the hydrogen demand. The tail gas 23 and the recycling gas 21 are then mixed together to yield a mixed stream 25 and recycled back to the ammonia cracking step/unit 11. In Fig. 2 is represented a hydrogen synthesis process according to another embodiment of the present invention.

The process represented in Fig. 2 can be exploited to synthesize hydrogen when the ammonia content retained by the thermally cracked stream 14 is in the order of few ppm, preferably in the order of ppb.

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

Alternatively, hydrogen can also be recovered in a cryogenic unit wherein a series of compression and cooling stages are performed so to remove nitrogen from the purified gas stream or in a hydrogen membrane separation unit wherein the selective permeability of hydrogen across a specific membrane is exploited.

In Fig. 3 it is shown an alternative embodiment of the hydrogen synthesis process. The ammonia stream 7 is subjected to heating stage 6, 51 wherein heat is exchanged with the thermally cracked stream 14 leaving the furnace 11. Additionally, the ammonia stream is further heated 9 in the convective section of the furnace to yield an ammonia containing stream 10 before being fed to the ammonia catalytic cracking step in the furnace. The thermally cracked stream 14 containing nitrogen, hydrogen and residual ammonia after leaving the furnace is mixed with water 74 to yield a thermally cracked stream added with water 75 that after exchanging heat with the ammonia stream 7 in the heat exchangers 51 and 6 is further air-cooled in the tower 70 to yield a cooled stream 79. The cooled stream 79 is then sent to a flash separator 80 wherein an ammonia depleted gas stream 81 is separated from an aqueous ammonia solution 82. The ammonia depleted stream 81 is then subjected to a hydrogen recovery step 19 to yield a high-purity hydrogen stream 22 and a tail gas 23.

The tail gas 23 after exchanging heat 120 with the water solution 74 is then fed as a fuel to provide heat to the catalytic cracking step 11. Hydrogen 22 is withdrawn from the hydrogen recovery step 19 and stored or exported in accordance with the demand. The aqueous ammonia solution 82 is then sent to distillation 83 to separate an ammonia stream 86 from a water solution 84.

A first portion 91 of the ammonia stream 86 is recirculated as a fuel to the furnace to provide heat for the catalytic cracking step 11 whilst a second portion of the ammonia stream 92 is mixed with the ammonia stream 7 to be then fed to the ammonia catalytic cracking step 11 in the furnace after pre-heating.

A portion 87 of the water solution 84 is exploited to recover heat from the combusted gas 60 by indirect heat transfer with the combusted gas 60 in the convective section of the furnace. The combusted gas before being withdrawn from the furnace is subjected to a NOx removal step 131.

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

In Fig. 4 it is shown a hydrogen synthesis process according to an alternative embodiment of the present invention.

In figure, it can be appreciated that the fuel gas 12 retaining ammonia is subjected to a cracking step 100 in presence of electric heating to yield a gas mixture 101 retaining hydrogen and nitrogen and optionally unconverted ammonia.

The gas mixture 101 is then mixed the tail gas 23 and is then subjected to combustion in presence of pre-heated air 28 to provide the reforming heat in the catalytic cracking step 11. As an alternative embodiment not represented in the figures, the cracking step performed in the presence of electric heating can also be carried out inside the furnace.