Use of a high-temperature resistant alloy in an ammonia cracking plant

Abstract

Use of a high-temperature resistant alloy for making at least one equipment part of an ammonia cracking plant, wherein said at least one equipment part is exposed to a pressure of 1 to 100 bar, to a temperature of 500 to 1150 °C and to contact with a stream having an ammonia content that ranges from 5% to 100% by volume; wherein said alloy comprises 30% to 50% by weight of nickel and 20% to 40% by weight of chromium.

Description

[0001] Use of a high-temperature resistant alloy in an ammonia cracking plant

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention is in the field of hydrogen production by ammonia cracking and related equipment.

[0005] Prior art

[0006] Hydrogen (H2) and ammonia (NH3) are carbon-free carriers and are considered ideal replacements of fossil fuels.

[0007] The ammonia is a carbon-free storage vector of hydrogen with many potential energy applications, including the production of hydrogen. Hydrogen can be obtained from ammonia via a thermal decomposition process known as ammonia cracking. Ammonia cracking is a technology allowing hydrogen to be available in places where geographical limitations make hydrogen production expensive and / or insufficient. This technology is of particular interest for supplying green or blue hydrogen. The term green hydrogen denotes hydrogen produced from renewable sources and without use of fossil fuels; the term blue hydrogen denotes a production where carbon dioxide is captured and not discharged to atmosphere.

[0008] In a catalytic ammonia cracking process, ammonia is decomposed (“cracked”) into hydrogen and nitrogen in presence of heat and of a suitable catalyst according to the following chemical equilibrium:

[0009] 2 NH33H2+ N2

[0010] Thermodynamic conversion of ammonia to hydrogen is possible at a temperature of 300 °C or higher. Ammonia cracking process is known to be affected by nitriding attack of steels during the ammonia decomposition into hydrogen and nitrogen. Nitriding attack is most severe at higher temperatures typical reached in both the ammonia preheating and the reaction zone where the diffusion of atomic nitrogen into the metallic material is promoted and allows the formation of nitrides.

[0011] Such nitrides are inherently hard, brittle phases, therefore nitridation can result in local or widespread loss of material strength and possible metal wastage.

[0012] In fact, under nitridation conditions, nitrogen molecules permeate through cracks and pores and reach the metal underneath the oxide scales when the oxide scale is no longer protective. Nitridation then proceeds by dissociation of nitrogen molecules and absorption of nitrogen atoms by the metal.

[0013] Nitride formation of steel can occur already at the surface of steels and at a temperature where small ammonia conversion rates are observed. As a result, steels may be at risk of embrittlement at relatively low temperatures such as around 400°C.

[0014] Other than being promoted by high temperatures, the nitriding phenomena is also promoted by high ammonia partial pressure. This is because its potential is proportional to (PNHS / PH2)3 / 2where PNHS is the ammonia partial pressure and pH2 is the hydrogen partial pressure.

[0015] Therefore, thermodynamic potential for nitridation is increased by increasing ammonia partial pressure (or concentration) and, when nitrogen in the metal exceeds its solubility limit, nitrides will then precipitate out.

[0016] In fact, another important factor that affects the nitridation resistance is nitrogen solubility in the material which is also very affected by temperature. For example, the solubility of nitrogen in stainless steels largely increases with increasing temperature.

[0017] Alloys with higher nitrogen solubilities generally exhibit less resistance to nitridation attack. It is also known that some steels can act as nitriding promoters.

[0018] A challenge of the modern ammonia cracking processes is to operate at high pressures to save H2 compression energy after production, and at high temperatures, e.g. above 700 °C, to promote kinetically the cracking reaction and reach the desired ammonia conversion rates. At such high temperatures and in such corrosive environments, the nitriding attack is a relevant concern.

[0019] Heat-resistant alloys are described in RU 2 583 188; Rll 2 581 322; WO 2024 / 110059; DE 10 2023 114700; US 2005 / 100693; J P 2024 164817.

[0020] Summary of the invention

[0021] The invention addresses the problem of how to reduce and control the corrosion by nitrides in a plant for producing hydrogen by ammonia cracking. The invention addresses the problem of finding a suitable material for parts of ammonia cracking plants which are exposed to the risk of ammonia nitriding.

[0022] The problem is solved with a use according to claim 1 . Particularly, the problem is solved by using a proper high-temperature resistant alloy, with high resistance to nitrides corrosion, for making an equipment, or a part thereof, within an ammonia cracking plant.

[0023] Said alloy is used for making equipment or equipment parts exposed to a pressure up to 100 bar (gauge), to a temperature of 500 °C to 1150 °C and exposed to an ammonia-containing environment, for example being in contact with an ammonia-containing stream. Preferably said ammonia-containing stream has an ammonia content from 5% to 100% by volume. In preferred embodiments said alloy is used for making at least one equipment part which operates under ammonia-cracking conditions.

[0024] According to the invention, said heat-resistant alloy comprises 30% to 50% by weight of nickel, and 20% to 40% by weight of chromium. Nickel and chromium are the predominant elements contained within said alloy and provide an improved corrosion resistance for operation in presence of nitrides at high temperatures.

[0025] The invention is based on the judicious selection of a steel alloy as defined in the claims, having improved resistance to nitriding attack, for use in equipment of ammonia cracking exposed to nitriding. In this respect, the invention deviates from the conventional approach wherein the material for such equipment is selected only on the basis of mechanical resistance.

[0026] Another aspect of the invention is a process for cracking ammonia according to the claims.

[0027] Detailed description

[0028] The use of the present invention may concern the manufacturing of any part or component of an ammonia cracking plant. The use of the alloy is most advantageous for parts exposed to high temperature and presence of nitrogen or nitrogen-containing substances, which are conditions favourable to nitriding attack. Noticeable parts that may be manufactured with the alloy include: bundles of tubes, tube sheets, support of tubes, flanges, fittings, heat exchangers or parts thereof, and other parts in contact with ammonia-containing streams. The tubes may include catalytic tubes and non-catalytic tubes. The catalytic tubes are typically used in the cracking process and tend to be more exposed to the nitriding attack, which is promoted by the catalyst; non-catalytic tubes may be used for preheating of ammonia, e.g. in a convective section. The use of the invention may include the manufacturing of catalytic tubes, radiant tube, convective tube coils.

[0029] According to a preferred embodiment, said alloy comprises 0.35% to 0.45% by weight of carbon. Preferably, said alloy comprises 1.5% to 2.0% by weight of niobium. According to other interesting embodiments, said alloy further comprises 1 .0% to 1 .5% by weight of manganese and 1 .5 to 2.0% by weight of silicon. Still preferably, said alloy comprises max. 0.03% by weight of sulphur and max. 0.04% by weight of phosphorus.

[0030] According to an embodiment of the invention, said alloy comprises 40% to 50% by weight of nickel, and 30% to 40% by weight of chromium. Preferably, said alloy is a steel according to any of the following designations: GX40NiCrSiNb45-35 according to EN 10027-1 ; GX45NiCrSiNb45-35 according to EN 10027-1 ; the material number 1.4889 according to the standard EN 10295-2002.

[0031] According to an embodiment, said alloy comprises 30% to 40% by weight of nickel, and 20% to 30% by weight of chromium. Preferably, said alloy is a steel according to any of the following designations: GX40NiCrSiNb35-25 according to EN 10027-1 ; GX45NiCrSiNbTi35-25 according to EN 10027-1 ; the material number 1.4852 according to the standard EN 10295-2002.

[0032] In all embodiments, the alloy may comprise a titanium addition to enhance mechanical properties. In case of titanium addition, preferably the content of titanium in the alloy (by weight) is in the range 0.01 % to 5.0% or 0.1 % to 1.5% or 0.1 % to 1.0%.

[0033] In other embodiments, said alloy is a low-carbon steel containing 0.05% to 0.30% by weight of carbon.

[0034] In all embodiments, the balance to 100% in the steel alloy is Fe and unavoidable impurities.

[0035] The ammonia cracking plant comprises an ammonia cracking reaction section. Said high-temperature resistant alloy is used, preferably, for making an equipment or part of said ammonia cracking reaction section. Items of an ammonia cracking section may include a furnace, gas heated reactors, ammonia cracking reactors, heat exchangers. The above-described alloy can be used advantageously in any of said items.

[0036] According to an embodiment, the ammonia cracking plant comprises a furnace where ammonia is preheated and / or cracked and said at least one equipment part made of the high-temperature resistant alloy includes one or more parts of said furnace. Preferably, said one or more parts of said furnace includes any of tubes, flanges, cones, headers, coils, and / or fittings or any other subcomponents. Parts of the furnace made with the above-described alloy may include catalytic tubes, non-catalytic tubes, radiant tubes, convective tubes.

[0037] In an application of the invention, said at least one equipment part includes at least an inner layer of an equipment or part thereof. For example, said inner layer may be an inner layer of catalytic tubes and / or furnace tubes of an ammonia cracking furnace.

[0038] According to a preferred application, said at least one equipment part is exposed to a pressure of up to 65 bar, and / or to a temperature of 500 to 1000 °C.

[0039] Another aspect of the invention is a process for cracking ammonia according to the claims. Said process comprises the step of contacting an ammonia- comprising stream, under a pressure up to 100 bar rel and a temperature of 500 to 1150 °C, with at least one equipment part made of an alloy comprising 30% to 50% by weight of nickel and 20% to 40% by weight of chromium.

[0040] Said process may comprise the step of passing an ammonia-comprising stream through tubes of a furnace under the above-mentioned conditions of temperature and pressure. Said tubes may include catalytic tubes for ammonia cracking and / or non-catalytic tubes for preheating of said ammonia-comprising stream.

[0041] Preferred embodiments of the process include that the alloy is in accordance with the above-described preferred ranges of composition. More preferably the alloy is in accordance with the above-described nomenclature.

[0042] According to a preferred embodiment, said ammonia-comprising stream includes 5% to 100% by volume of ammonia.

[0043] The applicant has found that using the alloy in accordance with the claims, for making equipment parts of an ammonia cracking plant, leads to the relevant advantage of reducing the risk of corrosion by nitrides. The alloy is a high-nickel, high-chromium alloy; nickel improves the alloy resistance to oxidation, carburization, nitriding, and thermal fatigue while chromium improves the alloys resistance to oxidation (scaling), carburization, nitriding, corrosion at high temperature and advantageously increases temperature creep and rupture strength.

[0044] The applicant has found that the above-mentioned alloy has unexpected good performance in resisting to nitriding attack under the severe conditions found in the ammonia cracking plants, particularly in the ammonia cracking furnace.

[0045] Description of the figures

[0046] Fig. 1 illustrates schematically a fired furnace 1 for cracking ammonia.

[0047] The furnace 1 includes catalytic tubes 3 heated externally by a set of burners 10 fed by a fuel 2. Said fuel 2 is natural gas or part of the hydrogen produced by the ammonia cracking plant. Stream 4 is a feed of ammonia which is fed to the tubes 3 after preheating in a coil 8. Line 5 denotes the gas effluent resulting from the cracking process. The gas in line 5 contains nitrogen, hydrogen, and unconverted ammonia. The gas in line 5 where appropriate may be subject to further cracking in a second cracking reactor.

[0048] Fig. 1 shows also pre-heaters 9 and 12 for lines 6, 7. The heat transferred to said lines 6, 7 can be used, for example, to preheat or vaporize ammonia or for interstage heating of an ammonia cracking reactor.

[0049] The catalytic tubes 3, the coil tubes 8, 9, 12, and fittings within the furnace 1 are examples of equipment parts which, under operation, may achieve a temperature in the range of the nitriding corrosion and, therefore, benefit from the use of the alloy described herein.

[0050] Fig. 2 shows a simplified block scheme of a plant for cracking ammonia. A feed 13 containing liquid ammonia is introduced in a vaporization and pre-heating (VP) section wherein liquid ammonia is vaporized and pre-heated resulting in a preheated gaseous feed 14 that is then introduced in a reaction section R including a fired furnace, such as the fired furnace 1 of Fig. 1. A gaseous effluent 15 comprising nitrogen, hydrogen and unconverted ammonia is withdrawn from said reaction section R and sent to cooling and recovery section C. In the coolingrecovery section C, said effluent 15 is cooled and at least part of the unconverted ammonia is separated from the cracked gas. A H2-rich stream 16 effluent from said cooling and recovery section C is treated in a purification section P where hydrogen is purified to produce a final hydrogen product 17.

[0051] The two sections of the plant that wherein the alloy is more of interest to be used for making part of the equipment are the vaporization and pre-heating section VP and the reaction section R. These two sections operate at higher temperature and in the presence of ammonia, that is the condition at which nitrides corrosion is more severe. The fired furnace included in the reaction section and shown in Fig. 1 is the most critical equipment under this point of view.

[0052] Fig. 3 represents a histogram showing the results of tests performed by the applicant on a wide range of potential possible nitriding resistance materials. The results show the performance in terms of nitriding depth (pm). The tests were performed at a low temperature in the range 650 to 750 °C and at a high temperature in the range 850 to 950 °C. The materials were tested twice: for a first test period of some hours and for a second test period having a duration twice the first period.

[0053] Fig. 3 compares the results of two alloys in accordance with the invention and two comparative austenitic steels (austenitic steels “A” and “B”) conventionally used in ammonia cracking plants.

[0054] The tests evidenced that the two examples of alloys of the invention (1 .4852 and 1.4889 according to EN 10295-2002) show a substantial better performance in terms of resistance to nitriding attack with respect to the conventional alloys Austenitic steel A and Austenitic steel B.

Claims

CLAIMS1 ) Use of a high-temperature resistant alloy in an ammonia cracking plant for making an equipment of said plant, or a part of an equipment of said plant, which is exposed during operation to a pressure up to 100 barg, to a temperature of 500 °C to 1150 °C and to contact with an ammonia-containing stream having an ammonia content from 5% to 100% by volume; wherein said alloy is a steel comprising:30% to 50% by weight of nickel, and 20% to 40% by weight of chromium.2) The use according to claim 1 wherein said alloy is a steel comprising 40% to 50% by weight of nickel, and 30% to 40% by weight of chromium.3) The use according to claim 2 wherein the alloy is identified by any of the following: the steel designation GX40NiCrSiNb45-35 according to the standard EN 10027-1 ; the steel designation GX45NiCrSiNb45-35 according to the standard EN 10027-1 ; the material number 1 .4889 according to the standard EN 10295-2002.4) The use according to claim 1 wherein said alloy is a steel comprising 30% to 40% by weight of nickel, and 20% to 30% by weight of chromium.5) The use according to claim 4 wherein the alloy is identified by any of the following: the steel designation GX40NiCrSiNb35-25 according to the standard EN 10027-1 ; the steel designation GX45NiCrSiNbTi35-25 according to the standard EN 10027-1 ;the material number 1.4852 according to the standard EN 10295-2002.6) The use according to any of the previous claims, wherein said alloy comprises 0.35% to 0.45% by weight of carbon.7) The use according to any of the previous claims, wherein said alloy comprises 1 .5% to 2.0% by weight of niobium.8) The use according to any of the previous claims, wherein said alloy comprises 1 .0% to 1 .5% by weight of manganese and 1 .5 to 2.0% by weight of silicon.9) The use according to any of the previous claims, wherein said alloy comprises max. 0.03% by weight of sulphur and max. 0.04% by weight of phosphorus.10)The use according to any of the previous claims, wherein said alloy comprises titanium.11 )The use according to claim 10 wherein the amount of titanium in said alloy is 0,01 % to 5.0% by weight, preferably 0.1 % to 1.5% by weight and more preferably 0.1 % to 1 .0% by weight.12)The use according to any of the previous claims wherein said alloy includes balance Fe and unavoidable impurities.13)The use according to any of the previous claims wherein the ammonia cracking plant comprises at least one ammonia cracking reaction section and said high-temperature resistant alloy is used for making an equipment or part of said ammonia cracking reaction section.14)The use according to claim 13 wherein said equipment or part includes any of: tubes, tube support, flanges, cones, headers, coils, fittings of said ammonia cracking reaction section.15)The use according to any of the previous claims wherein said high- temperature resistant alloy is used for making an inner layer of an equipment or part thereof, which is in contact with said ammonia-containing stream.16)The use according to any of the previous claims wherein said pressure is up to 65 barg and said temperature is 500 to 1000 °C.17)A process for cracking ammonia comprising the step of contacting an ammonia-comprising stream, under a pressure up to 100 bar rel and a temperature of 500 to 1150 °C, with at least one equipment part, preferably a part of an ammonia cracking section, which is made of an alloy as defined in any of claims 1 to 12.18)A process according to claim 17, wherein said alloy comprises 0.35% to 0.45% by weight of carbon and / or 1 .5% to 2.0% by weight of niobium. 19)A process according to claim 17 or 18 wherein said alloy comprises 1.0% to1 .5% by weight of manganese, 1 .5 to 2.0% by weight of silicon, and preferably comprises max. 0.03% by weight of sulphur and max. 0.04% by weight of phosphorus.20)A process according to any of claims 17 to 19 wherein said ammonia- comprising stream includes 5% to 100% by volume of ammonia.21 )A process according to any of the claims 17 to 20, wherein said alloy comprises titanium in an amount of 0.01 % to 5.0% by weight.

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