Reactors for the decomposition of nhat high temperatures

Abstract

The invention relates to reactors and reactor components which have good resistance to NH3 and N2 and optionally H2O at high temperatures and average pressure so that they can be used during the catalytic decomposition of NH3 into N2 and H2 on a large scale.

Description

REACTORS FOR THE DECOMPOSITION OF NH3 AT HIGH TEMPERATURES

[0001] Priority is claimed from Luxembourg patent application No. LU 103141 of 5 June 2023.

[0002] The invention relates to reactors and reactor components which have good resistance to NFL and N2 and possibly H2O at high temperatures and medium pressure, so that they can be used in the catalytic decomposition of NH3 to N2 and H2 on an industrial scale.

[0003] H2 can be extracted electrolytically from H2O using renewable energy and then converted into NH3 with N2. NH3 can be stored and transported much more safely than H2. NH3 can then be decomposed back into H2 and N2. After separating the N2, H2 finds a wide variety of industrial applications.

[0004] The decomposition of NH3 to N2 and H2 is an endothermic reaction (AH° = 45.9 kJ-mol 1), in which the amount of substance doubles (2 NH3 N2 + 3 H2), so the reaction is generally favored by high temperatures and low pressures. The higher the pressure, the higher the temperature must be to achieve satisfactory reaction yields.

[0005] The reaction temperature at which the catalytic decomposition of NH3 occurs is determined primarily by the choice of the NH3 decomposition catalyst. A variety of materials have been proposed as catalysts for the decomposition of NH3, which are active at different temperatures (see, for example, I. Lucentini et al., Ind. Eng. Chem. Res. 2021, 60, 18560-18611).

[0006] However, many of these NFL decomposition catalysts are not economically viable for large-scale applications. Ruthenium-based NFL decomposition catalysts and nickel-based NFL decomposition catalysts are particularly suitable for large-scale applications.

[0007] Ruthenium-based NFL decomposition catalysts have the advantage of achieving conversions of more than 90% at comparatively low temperatures. However, the maximum achievable conversions are limited, so that a significant residual content of undecomposed NH3 remains in the product gas, which, given the amount, may require additional removal.

[0008] Nickel-based NFL decomposition catalysts have the advantage that significantly higher conversions can be achieved. Therefore, only a small residual content of undecomposed NH3 remains in the product gas, which can be removed by conventional FL purification measures, especially by pressure swing adsorption, without the need for additional measures. However, the temperatures required for nickel-based NH3 decomposition catalysts are significantly higher.

[0009] Numerous processes for the catalytic decomposition of NH3 are described in the prior art and different reactor types have been proposed (cf. e.g. US 4 704 267 A, US 2009 / 280024 Al, US 2020 / 0123006 Al, FR 1 469 045 A, GB 768 091 A, CN 111 957 270 A, CN 113 896 168 A, WO 2011 / 107279 Al, WO 2012 / 090739 Al, WO 2020 / 095467 A, WO 2021 / 257944 Al, WO 2022 / 096529 Al, WO 2022 / 153720 Al, WO 2022 / 243410 Al, WO 2022 / 265647 Al, WO 2022 / 265648 Al, WO 2022 / 265649 Al, WO 2022 / 265650 Al and WO 2022 / 265651 Al).

[0010] Since the catalytic decomposition of NH3 is endothermic and requires high temperatures, heat must be introduced into the reactor. In addition to electrical heating systems, combustion processes are particularly proposed, in which a combustion gas is burned and the resulting heat is used for the catalytic decomposition of NH3.

[0011] Reactors designed analogously to primary reformers are particularly suitable for such a reaction. The combustion gas and the reaction gas are physically separated from one another and passed through the reactor, but are in heat exchange with one another. The combustion gas is burned with the aid of burners and with the supply of combustion air in a combustion chamber, from which a heat flow flows into at least one physically separate reaction chamber. The NFF reduction catalyst is arranged in the reaction chamber and is flowed through by the reaction gas, so that the catalyzed reaction takes place there. For example, several reaction chambers can be designed as tubes, each of which contains an NFF reduction catalyst and, in parallel, the reaction gas. These tubes are arranged, for example, as bundles within the combustion chamber, without any mixing of the combustion gas and the reaction gas.

[0012] The materials from which the reaction chambers and other components of such reactors are made must withstand considerable stresses and high temperatures. The temperatures generated by the combustion of the combustion gas in the combustion chamber are significantly higher than the temperatures of the reaction gas in the reaction chamber. The heat flow from the combustion chamber to the reaction chamber is based on this temperature gradient, among other things.

[0013] If such reactors are used for the catalytic decomposition of NH3 on nickel-based NH3 decomposition catalysts, the situation is further complicated by the fact that, due to the high concentration of NH3 or N2, the high pressure and the high temperatures, numerous metal alloys corrode through external and / or internal nitriding (often referred to as "nitriding" in this context).

[0014] R.P. Rubly et al., Oxidation of Metals, Vol. 35, 3-4 (1991), concerns the internal nitriding of nickel-chromium alloys. The nitriding behavior of nickel-chromium alloys was investigated in ammonia-hydrogen mixtures in the range of 700-900°C. CrN formed under all exposure conditions; CßN could not be detected. The transition from internal to external nitride formation occurs at 900°C between 30-40% Cr.

[0015] JJ Barnes et al., Journal de Physique III, Vol. 3, 1993, 167-174 discusses factors influencing the behavior of Fe-, Ni- and Co-based metal alloys with regard to nitriding at high temperatures (1093°C).

[0016] K. Tjorko et al., Oxidation of Metals Vol. 44, 453-474 (1995) compares internal nitriding in NH and in N2. Nitriding requires the dissociation of N2 or NH3. The dissociation of N2 only occurs to a significant extent above approximately 700°C. Therefore, at temperatures below approximately 700°C, nitriding is determined by the presence of NH3. Even at very high temperatures (1000°C), nitriding is more intense with NH3 than with N2.

[0017] U. Krupp et al., Oxidation of Metals Vol. 52, 277-298 (1999) concerns the internal nitriding of nickel-based alloys, in particular the behavior of binary and ternary alloys of the Ni-Cr-Al-Ti system.

[0018] U. Krupp et al., Oxidation of Metals Vol. 52, 299-320 (1999) also concerns the internal nitriding of nickel-based alloys, in particular the behavior of quaternary Ni-Cr-Al-Ti alloys and a computer-based description taking into account the thermodynamic data incorporated into a FEM diffusion calculation.

[0019] HJ Grabke et al., Materials and Corrosion 2003, 54(11), 895-902 concerns investigations in which iron, nickel, ferritic 1-18%Cr steels, austenitic 18%Cr-9%Ni and 20%Cr-3 ^Ni steels as well as a 16% CrNi base alloy were exposed at 500°C in He-30%H2O and 70%H2O-30%NH3 in order to compare the corrosion behavior of these materials in water vapor as in conventional power plants with their behavior in an NH3TUO mixture, ie under the conditions of the "Kalina cycle".

[0020] GY Lai, High-Temperature Corrosion and Materials Applications, ASM International, 2007, Chapter 4: Nitridation explains why nitriding occurs and how it attacks various metals, in some cases penetrating deeper than oxidation. Nitridation and its effects on metals and alloys in high-temperature air, as well as in NH3-H2O, NH3, H2-N2-NH3, and N2 environments are discussed.

[0021] MO Cojocaru et al., Materials 2021, 14, 2432 concerns the effects of modifying the activity of nitrating agents by diluting ammonia with nitrogen.

[0022] E. Wolowiec-Korecka et al., Coatings 2023, 13, 257, 1-12 concerns the stability of layered nitrides during nitriding under low pressure.

[0023] Furthermore, it should be noted that NH3 is often mixed with traces of H2O for storage and transport to reduce the risk of stress corrosion cracking in unalloyed steel. This increases the oxygen partial pressure at high temperatures in the reactor, so that oxidation can also play a role in addition to nitriding. The nitriding of metal alloys in NH3 (or in mixtures of NH3, N2, and H2, such as those formed during the decomposition reaction) at high temperatures, e.g., in the range of 650-750°C, has hardly been investigated to date, especially not in the presence of traces of H2O.

[0024] The reverse process is the synthesis of NH3 from N2 and H2 in the Haber-Bosch process.

[0025] American Front and Steel Institute, A Designer's Handbook Series No. 9013. 1978, Nickel Institute, 4-23 concerns stainless steels for ammonia production.

[0026] The technology behind the Haber-Bosch process provides little insight into suitable metal alloys for the catalytic decomposition of NH3, as the temperatures in the Haber-Bosch process are significantly lower. However, temperature has a decisive influence on nitriding. Austenitic steels are often sufficient for reactors and components using the Haber-Bosch process. They can be used at temperatures up to approximately 480°C, and with thicker walls even at somewhat higher temperatures (up to approximately 510°C). Another material suitable for the Haber-Bosch process is Alloy 600, which, however, is only approved for temperatures up to 649°C according to The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Sec II-D.

[0027] Even technologies using NH3 atmospheres under supercritical conditions, e.g., for the extraction of nitride single crystals, provide little insight into suitable metal alloys for the catalytic decomposition of NH3. Nitride single crystals are typically extracted at significantly lower temperatures, e.g., approximately 133°C, and correspondingly high pressures, e.g., 11.3 MPa. Furthermore, a distinction should be made between the reaction mechanisms of electrochemical reactions at the surface (e.g., in a fluid) on the one hand, and diffusion-controlled mechanisms in the base material and in the reaction layer (e.g., in a gas), on the other. In supercritical media, the reaction mechanisms typically correspond more closely to those of a fluid or a gas. This typically depends on the medium itself as well as on other various parameters.In supercritical water, for example, density is a determining parameter. At low density, the behavior is more like that of a gas, while at high density, the behavior of a liquid predominates (low-high boundary: approximately 0.2 g / ml). In supercritical CO2, for example, the water content, the impurity content, and the temperature are determining parameters. At high temperatures, the water content predominates. With dry CO2, classic high-temperature processes occur. Traces of water lead to significant erosive corrosion. The supercritical behavior of CO2 and NH3 can be considered comparable to a first approximation. NH3 often contains traces of water. Condensation would be possible, as with supercritical CO2, and thus not just gas-phase reactions.

[0028] In addition to corrosion resistance at high temperatures in the presence of N2, NH3 and possibly H2O, the other properties of the metal alloy are also crucial, in particular hardness, tensile strength, elongation, elongation at break, yield strength, elastic modulus, density, electrical resistance, melting range, thermal conductivity, specific heat capacity, etc. Processability, in particular weldability, also plays a role.

[0029] There is a need for improved reactors that can be economically used on a large scale for the catalytic decomposition of NH3 to N2 and H2 and that have a satisfactory or extended service life at high temperatures (e.g., 650-750°C) compared to conventional reactors used for this purpose. The materials used to manufacture the reactors should strike a balance between satisfactory properties on the one hand and acquisition costs on the other.

[0030] It is an object of the invention to provide improved reactors for the production of H2 by catalytic decomposition of NH3. The production of H2 should be safe, economical, and feasible on an industrial scale.

[0031] This problem is solved by the subject matter of the patent claims.

[0032] A first aspect of the invention relates to a plant for producing H2 by catalytic decomposition of NH3 to N2 and H2; wherein the plant comprises a reactor containing an NFF decomposition catalyst; wherein the reactor comprises at least one component which is at least partially constructed from a metal alloy; wherein the metal alloy, during operation of the reactor, comes into direct contact with the NH3 and / or N2 at least in one region of its surface; preferably with NH3, N2 and H2O or their dissociation products; and wherein the metal alloy contains nickel, chromium or both nickel and chromium, wherein the total content of nickel and / or chromium is at least 15 wt.%, based on the total weight of the metal alloy.

[0033] For the purpose of description, total content of "A and / or B" means that the metal alloy according to the invention contains either (i) both A and B, and the total content is then refers to the sum of the two individual contents of A and B, or (ii) does not contain B, so that the total content then refers to the content of A alone, or (iii) does not contain A, so that the total content then refers to the content of B alone.

[0034] The metal alloy preferably contains nickel and chromium with a total content of nickel and chromium of at least 15 wt.%, based on the total weight of the metal alloy.

[0035] In preferred embodiments, the metal alloy according to the invention contains at least two metals selected from the group consisting of nickel, cobalt, chromium, iron, molybdenum, manganese, niobium, cerium, aluminum, titanium, silicon, tungsten, copper, boron, zirconium, lanthanum and yttrium.

[0036] In preferred embodiments, the metal alloy according to the invention contains at least three metals selected from the group consisting of nickel, cobalt, chromium, iron, molybdenum, manganese, niobium, cerium, aluminum, titanium, silicon, tungsten, copper, boron, zirconium, lanthanum and yttrium.

[0037] In preferred embodiments, the metal alloy according to the invention contains at least four metals selected from the group consisting of nickel, cobalt, chromium, iron, molybdenum, manganese, niobium, cerium, aluminum, titanium, silicon, tungsten, copper, boron, zirconium, lanthanum and yttrium.

[0038] In preferred embodiments, the metal alloy according to the invention contains at least five metals selected from the group consisting of nickel, cobalt, chromium, iron, molybdenum, manganese, niobium, cerium, aluminum, titanium, silicon, tungsten, copper, boron, zirconium, lanthanum and yttrium.

[0039] It has been found that the nickel and, where appropriate, cobalt content is an important parameter for sufficient resistance to nitriding. This is primarily due to the fact that nickel and cobalt reduce the solubility of nitrogen in iron-based alloys, which makes internal nitriding more difficult. To achieve sufficient resistance to nitriding, the total nickel or nickel and cobalt content is preferably at least 40 wt.%, based on the total weight of the metal alloy.

[0040] Furthermore, it has been found that chromium itself is rather disadvantageous in terms of resistance to nitriding. This is primarily due to the fact that chromium increases the solubility of nitrogen in nickel-based alloys and thus the tendency toward nitriding.

[0041] However, as soon as the host atmosphere contains H2O in addition to N2 and NH3, other factors play an important role, particularly the formation of an oxide cap layer on the surface of the metal alloy. An oxide cap layer forms when the partial pressure of O2 is sufficiently high. The partial pressure of O2 and H2 is thermodynamically determined by the equilibrium 2 H2O → 2 H2 + O2. If the partial pressure of O2 is lower than the equilibrium pressure of O2 for the metal / metal oxide equilibrium, no oxide forms.

[0042] It has been found that in the presence of H2O, chromium has a beneficial effect on corrosion resistance, because chromium forms the stable oxide CYO3. CYO3 is also very low partial pressures of O2, unlike iron oxides. Even a low content of just a few ppm of H2O can lead to a covering layer of C'nCf, which significantly reduces nitriding. This is the case, for example, when the NH3 gas mixtures at 900°C contain only 45 ppm of H2O. It has been found that the chromium content must be high enough for a sufficiently dense covering layer of C'nCh to form and be maintained. This means that the subsequent supply of chromium must be guaranteed to ensure the stable growth of a covering layer of Cr2Os. The chromium content is preferably at least 18 wt.%, based on the total weight of the metal alloy.

[0043] In preferred embodiments, the metal alloy according to the invention therefore contains at least 30 wt.%, preferably at least 33 wt.% nickel and / or cobalt (total content) and at least 15 wt.%, preferably at least 19 wt.% chromium, in each case based on the total weight of the metal alloy. In preferred embodiments, the metal alloy according to the invention contains at least 40 wt.% nickel and / or cobalt (total content) and at least 20 wt.% chromium, in each case based on the total weight of the metal alloy.

[0044] In preferred embodiments, the metal alloy according to the invention therefore contains at least 30 wt.%, preferably at least 33 wt.% nickel, and at least 15 wt.%, preferably at least 19 wt.% chromium, each based on the total weight of the metal alloy. In preferred embodiments, the metal alloy according to the invention contains at least 40 wt.% nickel and at least 20 wt.% chromium, each based on the total weight of the metal alloy.

[0045] In preferred embodiments, the metal alloy according to the invention therefore contains at least 33 wt.% cobalt and at least 15 wt.% chromium, each based on the total weight of the metal alloy. In preferred embodiments, the metal alloy according to the invention contains at least 40 wt.% cobalt and at least 20 wt.% chromium, each based on the total weight of the metal alloy.

[0046] In preferred embodiments, the metal alloy according to the invention contains nickel, chromium and iron, wherein the nickel content is preferably greater than the chromium content.

[0047] In preferred embodiments, the metal alloy according to the invention contains nickel, cobalt, chromium and iron, wherein the total content of nickel and cobalt is preferably greater than the content of chromium.

[0048] For the purposes of this description, a "reactor" is a device for the catalytic decomposition of NH3 to N2 and H2. The reactor according to the invention is typically a confined space specifically designed and manufactured to allow the catalytic decomposition of NH3 to N2 and H2 to proceed and be controlled under defined conditions.

[0049] For the purposes of this description, a "metal alloy" is a macroscopically homogeneous metallic material, typically obtained by melting together various metals, which may also contain non-metals and / or semi-metals. The metals, non-metals, and / or semi-metals may be present in elemental form, as intermetallic phases, and / or as other compounds. The metal alloy may be crystalline, semi-crystalline, or amorphous. Particularly important metals according to the invention that may be contained in the metal alloy according to the invention are nickel, cobalt, chromium, and iron, but also optionally aluminum and titanium. Other metals that may be contained in the metal alloy according to the invention include, for example, molybdenum, tungsten, niobium, copper, aluminum, titanium, silicon, boron, lanthanum, manganese, vanadium, cerium, yttrium, zirconium, lead, and others.Particularly important non-metals and semi-metals according to the invention, which can be contained in the metal alloy according to the invention, are boron, carbon, silicon, nitrogen, phosphorus, sulfur, and others.

[0050] For the purposes of this description, an "NH3 decomposition catalyst" catalyzes the decomposition of NH3 to N2 and H2. To achieve yields greater than 90%, elevated NH3 temperatures are typically required, preferably at least 500°C.

[0051] For the purposes of this description, "in direct contact with the NH3 and / or N2" means that no further material is arranged in at least one region between the metal alloy and the NH3 and / or N2, and that the NH3 and / or N2 can interact with the surface of the metal alloy in at least this region. However, modifications to the surface of the metal alloy are permissible for direct contact within the meaning of the invention, for example, the formation of covering layers of oxides and nitrides. The metal alloy, or its surface, does not have to come into full contact with the NH3 and / or N2. Thus, it is sufficient if there is at least one region in which NH3 and / or N2 can interact with the surface of the metal alloy.Since the NH3 in the reactor according to the invention is decomposed on its way from the inlet to the outlet, the amount of NH3 at the inlet of the reactor is significantly higher than at the outlet during reactor operation. The opposite is true for the amount of N2 formed by the decomposition of the NH3. During reactor operation, the metal alloy preferably comes into direct contact with NH3, N2, and H2O or their dissociation products (N2, H2, or O2), at least in one area of ​​its surface.

[0052] Unless expressly stated otherwise, all percentages are given by weight [wt.%]. For ranges defined with "±", e.g., A±B, the numerical lower limit of the range is AB and the numerical upper limit of the range is A+B. If the value for B is the same as for A, the lower limit is 0, meaning that the respective component may be completely absent. Unless expressly stated otherwise, the content values ​​in wt.% do not add up to 100 wt.%, meaning that in addition to the stated components, other, unstated components may also be present.

[0053] The plant according to the invention comprises a reactor in which, during operation of the plant, NH3 is catalytically decomposed to form a product gas which comprises N2, H2 and possibly undecomposed NH3.

[0054] In preferred embodiments of the invention, the system according to the invention comprises one or more of the following devices which are in fluid communication with each other: (i) a device for storing liquid NH3; (ii) a device for heating and evaporating the liquid NH3; (iii) the reactor according to the invention; (iv) a device for purifying H2 from the product gas; and (v) a device for recovering process heat.

[0055] The reactor according to the invention preferably comprises several chambers that are physically separated from one another. The reactor preferably comprises at least one reaction chamber and at least one combustion chamber.

[0056] The reactor according to the invention is preferably designed analogously to a primary reformer.

[0057] The reactor according to the invention preferably comprises - one or more combustion chambers for the combustion of a combustion gas to produce combustion heat and a flue gas (exhaust gas); and - one or more reaction chambers for the catalytic decomposition of NH3 to produce a product gas comprising N2, H2 and possibly undecomposed NH3.

[0058] Combustion chamber(s) and reaction chamber(s) are preferably physically separated from each other so that there is no mixing of combustion gas on the one hand and NH3 (reactant gas) or product gas on the other hand during operation of the reactor.

[0059] The combustion chamber(s) and reaction chamber(s) are preferably configured such that, during reactor operation, a heat flow of the generated combustion heat occurs from the combustion chamber(s) to the reaction chamber(s). This heat flow preferably serves to maintain the endothermic catalytic decomposition of NH3 during reactor operation.

[0060] In preferred embodiments of the invention, the reaction chamber(s) are tubular, ie, they have a cylindrical shape. Preferably, several such tubular reaction chambers are arranged parallel to one another as a bundle in a common combustion chamber.

[0061] Each combustion chamber preferably contains one or more burners for the combustion of combustion gas. The flame formed during operation of the reactor is preferably located in spatial proximity to the outer wall of at least one reaction chamber containing NfL formation catalyst. During operation of the reactor, combustion heat then preferably flows from the interior of the combustion chamber through the wall of the at least one reaction chamber and thus carries heat to towards the NHs decomposition catalyst, through which NH3 flows and where the endothermic decomposition of NH3 takes place.

[0062] The reactor according to the invention also comprises at least one component which is at least partially constructed from a metal alloy which comes into direct contact with the NH3 and / or N2 during operation of the reactor.

[0063] Preferably, the component is a reaction chamber or an element of a reaction chamber, wherein the NH3 decomposition catalyst is arranged in the interior of the reaction chamber. The reactor according to the invention then preferably contains a reaction chamber comprising the component and the NH3 decomposition catalyst.

[0064] In preferred embodiments of the invention, the component is constructed entirely from the metal alloy according to the invention, ie the component consists entirely of the metal alloy.

[0065] If the component constructed entirely from the metal alloy according to the invention is a reaction chamber in whose interior the NCH decomposition catalyst is arranged, the wall of the reaction chamber consists of the component and thus entirely of the metal alloy. The inner surface of the wall of the reaction chamber faces the NCH decomposition catalyst, and the metal alloy from which the wall of the reaction chamber is made comes into direct contact with the NH3 and / or N2 during operation of the reactor, at least in one area.

[0066] If the component constructed entirely from the metal alloy according to the invention is an element of a reaction chamber in whose interior the NCH decomposition catalyst is arranged, the wall of the reaction chamber preferably encloses the component. For example, the component can be tubular and arranged concentrically within another tubular element, with which it together forms the wall of the reaction chamber. The inner surface of the component then faces the NCH decomposition catalyst, and the metal alloy from which the inner wall component is made comes into direct contact with the NH3 and / or N2 in at least one area during reactor operation.

[0067] In other preferred embodiments, the component is only partially constructed from the metal alloy according to the invention. For example, the component can be multi-layered, with one of the layers of the component, preferably an outer layer, being constructed from the metal alloy according to the invention, i.e., this layer of the component consists of the metal alloy, whereas other layers of the component can consist of other materials and / or of alloys according to the invention of the same or a different composition.

[0068] If the component constructed only partially from the metal alloy according to the invention is a reaction chamber in whose interior the NIL decomposition catalyst is arranged, the component can be a reaction chamber with a multi-layer wall, wherein the multi-layer wall has an inner layer and an outer layer. The inner surface of the inner layer of the The component then faces the NH3 decomposition catalyst and the metal alloy of which the inner layer is made comes into direct contact with the NH3 and / or N2 at least in one area during operation of the reactor.

[0069] Preferably, the entire inner surface of the reaction chamber, which faces the NH3 decomposition catalyst and which comes into direct contact with the NH3 and / or N2 during operation of the reactor, is constructed from the metal alloy.

[0070] Preferably, the entire inner surface of the reactor, which comes into direct contact with the NH3 and / or N2 during operation of the reactor, is constructed from the metal alloy.

[0071] The reactor according to the invention contains an NH3 decomposition catalyst. When NH3 flows through the NfU decomposition catalyst under reaction conditions, it catalyzes the decomposition of NH3 to N2 and H2.

[0072] According to the invention, various materials can be considered as NH3 decomposition catalysts. The reaction temperature at which the catalytic decomposition of NH3 occurs is determined in particular by the choice of the NH3 decomposition catalyst.

[0073] In preferred embodiments of the invention, the NPh decomposition catalyst is catalytically active with respect to the decomposition of NH3 at a temperature in the range of at least 500°C, preferably at least 520°C, more preferably at least 540°C, even more preferably at least 550°C, most preferably at least 580°C, and in particular at least 600°C. "Catalytically active" means that conversions of at least 90% of decomposition products N2 and H2 are obtained with respect to the amount of NH3 used (measured under standard conditions in pure NH3 at a pressure of 1013 hPa and a space velocity of 36 LgKat' 1 ).

[0074] In preferred embodiments of the invention, the NH3 decomposition catalyst has an apparent activation energy E with respect to the decomposition of NH3 app of at least 50 kJ-mol 1 , preferably at least 75 kJ-mol 1 , more preferably at least 100 kJ-mol 1 , preferably at least 125 kJ-mol 1 , and in particular at least 150 kJ-mol 1 Methods for determining the apparent activation energy E app are known to a person skilled in the art, for example by determination from Arrhenius plots based on measurements under standard conditions in pure NH3 at a pressure of 1013 hPa and a space velocity of 36 LgKat 1 .

[0075] In preferred embodiments of the invention, a nickel-based NFF decomposition catalyst is used. The reaction temperature determines the equilibrium conversion. At 900°C and 20 bar pressure, the decomposition of NH3 is almost quantitative. At 650°C, the conversion of NH3 is about 98.5%, and at 500°C, only about 95%. According to the invention, reaction temperatures are preferably set in the range of about 600°C to about 900°C, preferably about 600°C to about 700°C, so that a high conversion is achieved. With regard to energy balance and conversion, optimal Reaction temperatures range from approximately 630°C to 640°C. Nickel-based NH3 decomposition catalysts are advantageous despite the comparatively high reaction temperature. Due to the high conversion, the remaining content of undecomposed NH3 in the product gas is comparatively low, so that separate separation of undecomposed NH3 for its recovery is preferably avoided. Instead, N2 and undecomposed NH3 are combined from the product gas by pressure swing adsorption during the purification of H2.

[0076] Preferably, the NH3 decomposition catalyst comprises supported nickel. Preferred support materials are selected from the group consisting of Al2O3, MgO, SiO2, mesoporous SiO2 (e.g. MCF-17, MCM-41, SBA-15), zeolite (e.g. HY, H-ZSM-5), BaMnO3, BaTiO3, BaZrO3, CaMnO3, CaTiO3, CaZrO3, CeO2, Gd2O3, GdAlO3, KNbO3, La2O3, LaAlO3, MnO2, NaNbO3, Nb2O5, Sm2O3, SmAlO3, SrMnO3, SrTiO3, SrZrO3, TiO2, Y2O3, ZrO2, carbon (e.g. CNTs, SWCNTs, AX-21, MSC-30, MESO-C, GNP, activated carbon, Graphene, graphene oxide), attapulgite, hydrocalumite, sepiolite, and mixtures thereof.

[0077] The metal alloy according to the invention contains nickel and chromium with a total content of nickel and chromium of at least 15 wt.%, based on the total weight of the metal alloy.

[0078] In preferred embodiments of the invention, the total content of nickel and chromium is at least 15.5 wt.%, preferably at least 16.0 wt.%, preferably at least 16.5 wt.%, more preferably at least 17.0 wt.%, even more preferably at least 17.5 wt.%, most preferably at least 18.0 wt.%, and in particular at least 18.5 wt.%, in each case based on the total weight of the metal alloy.

[0079] In preferred embodiments of the invention, the total content of nickel and chromium is at least 20 wt.%, preferably at least 25 wt.%, more preferably at least 30 wt.%, even more preferably at least 35 wt.%, most preferably at least 40 wt.%, and in particular at least 45 wt.%, in each case based on the total weight of the metal alloy.

[0080] In preferred embodiments of the invention, the total content of nickel and chromium is at least 50 wt.%, preferably at least 55 wt.%, more preferably at least 60 wt.%, even more preferably at least 65 wt.%, most preferably at least 70 wt.%, and in particular at least 75 wt.%, in each case based on the total weight of the metal alloy.

[0081] In preferred embodiments of the invention, the total content of nickel and chromium is at least 80 wt.%, preferably at least 85 wt.%, more preferably at least 90 wt.%, in each case based on the total weight of the metal alloy.

[0082] In preferred embodiments of the invention, the total content of nickel and chromium is at most 85 wt.%, preferably at most 80 wt.%, more preferably at most 75 wt.%. %, more preferably at most 70 wt.%, most preferably at most 65 wt.%, and in particular at most 60 wt.%, in each case based on the total weight of the metal alloy.

[0083] The metal alloy according to the invention contains nickel.

[0084] In preferred embodiments of the invention, the nickel content is at least 5.0 wt.%, preferably at least 10 wt.%, more preferably at least 15 wt.%, even more preferably at least 20 wt.%, most preferably at least 30 wt.%, and in particular at least 35 wt.%, in each case based on the total weight of the metal alloy.

[0085] In preferred embodiments of the invention, the nickel content is at least 40 wt.%, preferably at least 45 wt.%, more preferably at least 50 wt.%, even more preferably at least 55 wt.%, most preferably at least 60 wt.%, and in particular at least 65 wt.%, in each case based on the total weight of the metal alloy.

[0086] In preferred embodiments of the invention, the nickel content is at least 70 wt.%, preferably at least 75 wt.%, in each case based on the total weight of the metal alloy.

[0087] In preferred embodiments of the invention, the nickel content is in ascending order of preference in the range of 20±15 wt%, 20±10 wt%, or 20±5.0 wt%, each based on the total weight of the metal alloy.

[0088] In preferred embodiments of the invention, the nickel content is in ascending order of preference in the range of 30±25 wt%, 30±20 wt%, 30±15 wt%, 30±10 wt%, or 30±5.0 wt%, in each case based on the total weight of the metal alloy.

[0089] In preferred embodiments of the invention, the nickel content is in ascending order of preference in the range of 40±35 wt%, 40±30 wt%, 40±25 wt%, 40±20 wt%, 40±15 wt%, 40±10 wt%, or 40±5.0 wt%, in each case based on the total weight of the metal alloy.

[0090] In preferred embodiments of the invention, the nickel content is in ascending order of preference in the range of 50±30 wt%, 50±25 wt%, 50±20 wt%, 50±15 wt%, 50±10 wt%, or 50±5.0 wt%, in each case based on the total weight of the metal alloy.

[0091] In preferred embodiments of the invention, the nickel content is in ascending order of preference in the range of 60±20 wt%, 60±15 wt%, 60±10 wt%, or 60±5.0 wt%, each based on the total weight of the metal alloy.

[0092] In preferred embodiments of the invention, the nickel content is in ascending order of preference in the range of 70±10 wt.% or 70±5.0 wt.%, in each case based on the total weight of the metal alloy.

[0093] In preferred embodiments of the invention, the nickel content is at most 60 wt.%, preferably at most 55 wt.%, more preferably at most 50 wt.%, even more preferably at most 45 wt.%, most preferably at most 40 wt.%, and in particular at most 35 wt.%, in each case based on the total weight of the metal alloy.

[0094] The metal alloy according to the invention contains chromium.

[0095] In preferred embodiments of the invention, the chromium content is at most 40 wt.%, preferably at most 38 wt.%, more preferably at most 36 wt.%, even more preferably at most 35 wt.%, most preferably at most 34 wt.%, and in particular at most 32 wt.%, in each case based on the total weight of the metal alloy.

[0096] In preferred embodiments of the invention, the chromium content is at most 30 wt.%, preferably at most 28 wt.%, more preferably at most 26 wt.%, even more preferably at most 24 wt.%, most preferably at most 22 wt.%, and in particular at most 20 wt.%, in each case based on the total weight of the metal alloy.

[0097] In other preferred embodiments, the chromium content is at least 12% by weight, preferably at least 15% by weight, more preferably at least 18% by weight, even more preferably at least 21% by weight, most preferably at least 24% by weight, and in particular at least 27% by weight, in each case based on the total weight of the metal alloy.

[0098] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 10±7.5 wt%, 10±5.0 wt%, or 10±2.5 wt%, in each case based on the total weight of the metal alloy.

[0099] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 12.5±10 wt%, 12.5±7.5 wt%, 12.5±5.0 wt%, or 12.5±2.5 wt%, in each case based on the total weight of the metal alloy.

[0100] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 15±12.5 wt.%, 15±10 wt.%, 15±7.5 wt.%, 15±5.0 wt.%, or 15±2.5 wt.%, in each case based on the total weight of the metal alloy.

[0101] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 17.5±15 wt.%, 17.5±12.5 wt.%, 17.5±10 wt.%, 17.5±7.5 wt.%, 17.5±5.0 wt.%, or 17.5±2.5 wt.%, in each case based on the total weight of the metal alloy.

[0102] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 19±17.5 wt%, 19±15 wt%, 19±12.5 wt%, 19±10 wt%, 19±7.5 wt%, 19±5.0 wt%, or 19±2.5 wt%, in each case based on the total weight of the metal alloy.

[0103] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 20±17.5 wt%, 20±15 wt%, 20±12.5 wt%, 20±10 wt%, 20±7.5 wt%, 20±5.0 wt%, or 20±2.5 wt%, in each case based on the total weight of the metal alloy.

[0104] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 22.5±20 wt%, 22.5±17.5 wt%, 22.5±15 wt%, 22.5±12.5 wt%, 22.5±10 wt%, 22.5±7.5 wt%, 22.5±5.0 wt%, or 22.5±2.5 wt%, in each case based on the total weight of the metal alloy.

[0105] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 25±22.5 wt.%, 25±20 wt.%, 25±17.5 wt.%, 25±15 wt.%, 25±12.5 wt.%, 25±10 wt.%, 25±7.5 wt.%, 25±5.0 wt.%, or 25±2.5 wt.%, in each case based on the total weight of the metal alloy.

[0106] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 27.5±25 wt%, 27.5±22.5 wt%, 27.5±20 wt%, 27.5±17.5 wt%, 27.5±15 wt%, 27.5±12.5 wt%, 27.5±10 wt%, 27.5±7.5 wt%, 27.5±5.0 wt%, or 27.5±2.5 wt.%, each based on the total weight of the metal alloy.

[0107] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 30±27.5 wt%, 30±25 wt%, 30±22.5 wt%, 30±20 wt%, 30±17.5 wt%, 30±15 wt%, 30±12.5 wt%, 30±10 wt%, 30±7.5 wt%, 30±5.0 wt% or 30±2.5 wt%, each based on the total weight of the metal alloy.

[0108] In preferred embodiments of the invention, the chromium content is in ascending order of preference in the range of 32.5±30 wt%, 32.5±27.5 wt%, 32.5±25 wt%, 32.5±22.5 wt%, 32.5±20 wt%, 32.5±17.5 wt%, 32.5±15 wt%, 32.5±12.5 wt%, 32.5±10 wt%, 32.5±7.5 wt%, 32.5±5.0 wt%, or 32.5±2.5 wt%, each based on the total weight of the metal alloy.

[0109] Preferred embodiments ZI to Z72 have the following nickel and chromium content in wt.%, each based on the total weight of the metal alloy:

[0110] The metal alloy according to the invention preferably additionally contains cobalt.

[0111] In preferred embodiments of the invention, the total content of nickel and optionally cobalt is at least 20 wt.%, preferably at least 25 wt.%, more preferably at least 30 wt.%, even more preferably at least 35 wt.%, most preferably at least 40 wt.%, and in particular at least 45 wt.%, in each case based on the total weight of the metal alloy.

[0112] In preferred embodiments of the invention, the total content of nickel and optionally cobalt is at least 50 wt.%, preferably at least 54 wt.%, more preferably at least 58 wt.%, even more preferably at least 62 wt.%, most preferably at least 66 wt.%, and in particular at least 70 wt.%, in each case based on the total weight of the metal alloy. In preferred embodiments of the invention, the total content of nickel and optionally cobalt is at least 70 wt.%, preferably at least 75 wt.%, in each case based on the total weight of the metal alloy.

[0113] In preferred embodiments of the invention, the total content of nickel and optionally cobalt is, with increasing preference, in the range of 20±15 wt.%, 20±10 wt.%, or 20±5.0 wt.%, in each case based on the total weight of the metal alloy.

[0114] In preferred embodiments of the invention, the total content of nickel and optionally cobalt is, in ascending order of preference, in the range of 30±25 wt.%, 30±20 wt.%, 30±15 wt.%, 30±10 wt.%, or 30±5.0 wt.%, in each case based on the total weight of the metal alloy.

[0115] In preferred embodiments of the invention, the total content of nickel and optionally cobalt is in ascending order of preference in the range of 40±35 wt.%, 40±30 wt.%, 40±25 wt.% %, 40±20 wt.%, 40±15 wt.%, 40±10 wt.%, or 40±5.0 wt.%, each based on the total weight of the metal alloy.

[0116] In preferred embodiments of the invention, the total content of nickel and optionally cobalt is, in ascending order of preference, in the range of 50±30 wt.%, 50±25 wt.%, 50±20 wt.%, 50±15 wt.%, 50±10 wt.%, or 50±5.0 wt.%, in each case based on the total weight of the metal alloy.

[0117] In preferred embodiments of the invention, the total content of nickel and optionally cobalt is, with increasing preference, in the range of 60±20 wt.%, 60±15 wt.%, 60±10 wt.%, or 60±5.0 wt.%, in each case based on the total weight of the metal alloy.

[0118] In preferred embodiments of the invention, the total content of nickel and optionally cobalt is, with increasing preference, in the range of 70±10 wt.% or 70±5.0 wt.%, in each case based on the total weight of the metal alloy.

[0119] In preferred embodiments of the invention, the total content of nickel and optionally cobalt is at most 60 wt.%, preferably at most 55 wt.%, more preferably at most 50 wt.%, even more preferably at most 45 wt.%, most preferably at most 40 wt.%, and in particular at most 35 wt.%, in each case based on the total weight of the metal alloy.

[0120] For the purpose of the description, "total content of nickel and optionally cobalt" means that the metal alloy according to the invention either (i) contains both nickel and cobalt, and the total content then refers to the sum of the two individual contents of nickel and cobalt, or (ii) contains no cobalt, so that the total content then refers to the content of nickel alone.

[0121] In preferred embodiments of the invention, the cobalt content is at least 4.0 wt.%, preferably at least 8.0 wt.%, more preferably at least 12 wt.%, even more preferably at least 16 wt.%, most preferably at least 20 wt.%, and in particular at least 24 wt.%, in each case based on the total weight of the metal alloy.

[0122] In other preferred embodiments, the cobalt content is at most 15 wt.%, preferably at most 12.5 wt.%, more preferably at most 10 wt.%, even more preferably at most 7.5 wt.%, most preferably at most 5.0 wt.%, and in particular at most 2.5 wt.%, in each case based on the total weight of the metal alloy.

[0123] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of cobalt, preferably at most 1.0 wt.%, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%, most preferably cobalt is nominally not contained.

[0124] Preferred embodiments Al to Ä72 have the following content of nickel, possibly cobalt and chromium in wt.%, each based on the total weight of the metal alloy:

[0125] It has been found that the nickel content, or the total content of nickel and cobalt, in the metal alloy according to the invention has a significant influence on the solubility of nitrogen and thus on nitriding. As the nickel content, or the total content of nickel and cobalt, increases, the solubility of nitrogen in the metal alloy decreases. The lower the solubility of nitrogen, the lower the tendency toward (internal) nitriding.

[0126] Lerner found that, due to the higher solubility of nitrogen in austenitic steels, these steels are more prone to (internal) nitriding than nickel-based materials. In nickel-based materials, a nickel content in the range of 40 to 50 wt.% can ensure satisfactory resistance to nitriding.

[0127] It appears that the chromium content has a favorable influence on internal nitriding at higher temperatures, presumably even after long aging periods. A high chromium content can therefore be detrimental.

[0128] If the NH3 is mixed with traces of water or contains traces of O2, the partial pressure of O2 is increased, which can have a positive effect on the stability of oxide layers on the surface of the metal alloy. Chromium has been found to have a positive effect on this.

[0129] The metal alloy according to the invention may additionally contain aluminum and / or titanium. Even if aluminum and titanium are not preferred per se, it may be advantageous, e.g., for reasons of strength, to use raw materials that may contain comparatively small amounts of aluminum and / or titanium to produce the metal alloy according to the invention. Even if aluminum and titanium are not preferred per se, certain amounts can be tolerated.

[0130] Both aluminum and titanium are frequently present in engineering metal alloys, although not always in large quantities, to increase mechanical strength and creep resistance. Aluminum can also improve oxidation resistance. Both metals have a pronounced tendency to form nitrides at high temperatures and in contact with nitrogen, particularly deep within the metal (internal nitriding). It has been found that the depth of internal nitriding in metal alloys containing aluminum and / or titanium is increased compared to metal alloys containing neither aluminum nor titanium. Furthermore, it has been found that higher chromium contents further promote the internal nitriding of aluminum and titanium.

[0131] It was found that the comparatively large volume of nitrides (nitride precipitates) formed leads to undesirable embrittlement of the metal alloys.

[0132] Furthermore, it was found that the volumetric stresses associated with internal nitriding and / or embrittlement can, in turn, lead to spalling of the covering layer on the surface of the metal alloy. This covering layer can consist of oxides and / or nitrides. As the covering layer spalls, its protective effect is locally lost, which increases the rate of undesired nitriding. Nitrogen absorption is then favored due to the missing covering layer.

[0133] It appears that water (or traces of oxygen) increases the ductility of external nitride coatings, thus reducing the risk of spalling. Therefore, if the NH3 is mixed with traces of water, this may have a beneficial effect on the durability of the nitride coatings and thus possibly suppress or reduce internal nitriding.

[0134] Furthermore, internal nitriding can lead to depletion and thus a reduction in creep and nitriding resistance. Depletion of surface-forming elements such as Cr, Si, or Al can lead to failure of the protective surface layer.

[0135] Therefore, internal nitriding should be suppressed as much as possible or at most occur to a relatively small extent.

[0136] In preferred embodiments of the invention, the total content of aluminum and / or titanium is at most 5.5 wt.%, preferably at most 5.0 wt.%, more preferably at most 4.5 wt.%, even more preferably at most 4.0 wt.%, most preferably at most 3.5 wt.%, and in particular at most 3.0 wt.%, in each case based on the total weight of the metal alloy.

[0137] For the purpose of description, "total content of aluminum and / or titanium" means that the metal alloy according to the invention either (i) contains both aluminum and titanium, and the total content then refers to the sum of the two individual contents of aluminum and titanium, or (ii) contains no aluminum, so that the total content then refers to the content of titanium alone, or (iii) contains no titanium, so that the total content then refers to the content of aluminum alone.

[0138] In preferred embodiments of the invention, the metal alloy according to the invention contains at most very small amounts of aluminum and / or titanium, preferably at most 1.0 wt.%, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%, most preferably aluminum and titanium are nominally not contained.

[0139] The metal alloy according to the invention may additionally contain aluminum.

[0140] In preferred embodiments of the invention, the aluminum content is preferably at most 6.0 wt.%, preferably at most 5.5 wt.%, more preferably at most 5.0 wt.%, even more preferably at most 4.5 wt.%, most preferably at most 4.0 wt.%, and in particular at most 3.5 wt.%, in each case based on the total weight of the metal alloy.

[0141] In preferred embodiments of the invention, the aluminum content is preferably at most 3.0 wt.%, preferably at most 2.5 wt.%, more preferably at most 2.0 wt.%, even more preferably at most 1.5 wt.%, most preferably at most 1.0 wt.%, and in particular at most 0.5 wt.%, in each case based on the total weight of the metal alloy.

[0142] Like Cr2O3, Al2O3 is also very stable, even at low partial pressures of O2. However, oxide layers made of Al2O3 grow even more slowly than oxide layers made of C^CE, since the formation of Al2O3 is kinetically inhibited at comparatively low temperatures. Furthermore, it has been found that higher aluminum contents (> 5 wt%) impede the weldability of the metal alloy and also have a strong tendency toward the formation of highly embrittling nitrides.

[0143] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of aluminum, preferably at most 1.0 wt.%, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%, most preferably aluminum is nominally not contained.

[0144] The metal alloy according to the invention may additionally contain titanium.

[0145] In preferred embodiments of the invention, the titanium content is preferably at most 4.5 wt.%, preferably at most 4.0 wt.%, more preferably at most 3.5 wt.%, even more preferably at most 3.0 wt.%, most preferably at most 2.5 wt.%, and in particular at most 2.0 wt.%, in each case based on the total weight of the metal alloy.

[0146] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of titanium, preferably at most 1.0 wt.%, more preferably at most 0.6 wt.%, even more preferably at most 0.5 wt.%, most preferably at most 0.1 wt.%, and in particular, titanium is nominally not contained.

[0147] The metal alloy according to the invention preferably additionally contains iron.

[0148] In preferred embodiments of the invention, the metal alloy according to the invention is a steel, preferably an austenitic steel.

[0149] In preferred embodiments of the invention, the iron content is at most 85 wt.%, preferably at most 80 wt.%, more preferably at most 75 wt.%, even more preferably at most 70 wt.%, most preferably at most 65 wt.%, and in particular at most 50 wt.%, in each case based on the total weight of the metal alloy.

[0150] In preferred embodiments of the invention, the iron content is at most 45 wt.%, preferably at most 40 wt.%, more preferably at most 35 wt.%, even more preferably at most 30 wt.%, most preferably at most 25 wt.%, and in particular at most 20 wt.%, in each case based on the total weight of the metal alloy.

[0151] In preferred embodiments of the invention, the iron content is at most 15 wt.%, preferably at most 10 wt.%, more preferably at most 5.0 wt.%, in each case based on the total weight of the metal alloy.

[0152] In preferred embodiments of the invention, the iron content is in the range of 10±5 wt.%, based on the total weight of the metal alloy.

[0153] In preferred embodiments of the invention, the iron content is in ascending order of preference in the range of 20±15 wt.%, 20±10 wt.%, or 20±5.0 wt.%, in each case based on the total weight of the metal alloy.

[0154] In preferred embodiments of the invention, the iron content is in ascending order of preference in the range of 30±25 wt.%, 30±20 wt.%, 30±15 wt.%, 30±10 wt.%, or 30±5.0 wt.%, in each case based on the total weight of the metal alloy.

[0155] In preferred embodiments of the invention, the iron content is in ascending order of preference in the range of 40±35 wt%, 40±30 wt%, 40±25 wt%, 40±20 wt%, 40±15 wt.%, 40±10 wt.%, or 40±5.0 wt.%, each based on the total weight of the metal alloy.

[0156] In preferred embodiments of the invention, the iron content is in ascending order of preference in the range of 50±30 wt%, 50±25 wt%, 50±20 wt%, 50±15 wt%, 50±10 wt%, or 50±5.0 wt%, in each case based on the total weight of the metal alloy.

[0157] In preferred embodiments of the invention, the iron content is in ascending order of preference in the range of 60±20 wt%, 60±15 wt%, 60±10 wt%, or 60±5.0 wt%, in each case based on the total weight of the metal alloy.

[0158] In preferred embodiments of the invention, the iron content is in ascending order of preference in the range of 70±10 wt.% or 70±5.0 wt.%, in each case based on the total weight of the metal alloy.

[0159] In other preferred embodiments, the metal alloy according to the invention contains at most small amounts of iron, preferably at most 1.5 wt.%, more preferably at most 1.0 wt.%, even more preferably at most 0.5 wt.%, most preferably at most 0.1 wt.%, and in particular, iron is nominally not contained.

[0160] The metal alloy according to the invention preferably contains silicon.

[0161] It has been found that SiCE is thermodynamically more stable than CnO;. On the one hand, SiC>2 can form on the surface instead of CnO; in oxygen-poor atmospheres, thus hindering nitriding. On the other hand, the silicon content is metallurgically limited. Therefore, a dense SiO2 layer is usually not formed, which reduces the protective effect. If CnO; is stable, SiO2 can form at the inner phase boundary, which also reduces the tendency to nitride. However, even here, the SiO2 layer is usually not continuous, or mixed oxides are formed.

[0162] In preferred embodiments of the invention, the silicon content is at least 0.1 wt.%, preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, even more preferably at least 0.4 wt.%, most preferably at least 0.5 wt.%, and in particular at least 0.6 wt.%, in each case based on the total weight of the metal alloy.

[0163] It is known that silicon additions can improve the oxidation resistance of certain metal alloys. However, experimental evidence suggests that silicon does not appear to increase resistance to nitriding.

[0164] In preferred embodiments of the invention, the silicon content is at most 4.0 wt.%, preferably at most 3.5 wt.%, more preferably at most 3.0 wt.%, even more preferably at most 2.5 wt.%, most preferably at most 2.0 wt.%, and in particular at most 1.5 wt.%, in each case based on the total weight of the metal alloy.

[0165] The metal alloy according to the invention preferably contains carbon.

[0166] In preferred embodiments of the invention, the carbon content is at least 0.010 wt.%, preferably at least 0.020 wt.%, more preferably at least 0.030 wt.%, even more preferably at least 0.040 wt.%, most preferably at least 0.050 wt.%, and in particular at least 0.060 wt.%, in each case based on the total weight of the metal alloy.

[0167] In preferred embodiments of the invention, the carbon content is in the range of 0.1 to 0.5 wt.%, based on the total weight of the metal alloy. This is particularly preferred for cast alloys.

[0168] In preferred embodiments of the invention, the carbon content is at most 0.10 wt.%, preferably at most 0.09 wt.%, more preferably at most 0.08 wt.%, even more preferably at most 0.07 wt.%, most preferably at most 0.06 wt.%, and in particular at most 0.05 wt.%, in each case based on the total weight of the metal alloy.

[0169] In iron-based alloys, atomic hydrogen can react with carbides such as iron carbide to form methane. Low-alloy chromium and chromium-molybdenum steels become increasingly more resistant to this form of high-temperature embrittlement (HTHA) with increasing Cr content, because chromium carbides are significantly more stable than iron carbide. Therefore, austenitic steels and nickel-based alloys are highly resistant to high-temperature embrittlement. However, nickel-based alloys can become embrittled due to hydrogen trapping on chromium carbides during thermal aging.

[0170] The metal alloy according to the invention preferably contains molybdenum.

[0171] In preferred embodiments of the invention, the molybdenum content is at least 0.1 wt.%, preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, even more preferably at least 0.4 wt.%, most preferably at least 0.5 wt.%, and in particular at least 0.6 wt.%, in each case based on the total weight of the metal alloy.

[0172] In preferred embodiments of the invention, the metal alloy contains molybdenum and the molybdenum content is at least 1.0 wt.%, preferably at least 2.0 wt.%, more preferably at least 3.0 wt.%, even more preferably at least 4.0 wt.%, most preferably at least 5.0 wt.%, and in particular at least 6.0 wt.%, in each case based on the total weight of the metal alloy.

[0173] In preferred embodiments of the invention, the molybdenum content is at most 12 wt.%, preferably at most 11 wt.%, more preferably at most 10 wt.%, even more preferably at most 9.0 wt.%, most preferably at most 8.0 wt.%, and in particular at most 7.0 wt.%, in each case based on the total weight of the metal alloy.

[0174] In preferred embodiments of the invention, the molybdenum content is at most 4.0 wt.%, preferably at most 3.5 wt.%, more preferably at most 3.0 wt.%, even more preferably at most 2.5 wt.%, most preferably at most 2.0 wt.%, and in particular at most 1.5 wt.%, in each case based on the total weight of the metal alloy.

[0175] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of molybdenum, preferably at most 1.0 wt.%, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%, most preferably molybdenum is nominally not contained.

[0176] In preferred embodiments of the invention, the metal alloy contains molybdenum and the total content of nickel, chromium and molybdenum is at most 85 wt.%, preferably at most 80 wt.%, more preferably at most 75 wt.%, even more preferably at most 70 wt.%, most preferably at most 65 wt.%, and in particular at most 60 wt.%, in each case based on the total weight of the metal alloy.

[0177] The metal alloy according to the invention preferably contains vanadium.

[0178] In preferred embodiments of the invention, the vanadium content is at least 0.1 wt.%, preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, even more preferably at least 0.4 wt.%, most preferably at least 0.5 wt.%, and in particular at least 0.6 wt.%, in each case based on the total weight of the metal alloy.

[0179] In preferred embodiments of the invention, the vanadium content is at most 4.0 wt.%, preferably at most 3.5 wt.%, more preferably at most 3.0 wt.%, even more preferably at most 2.5 wt.%, most preferably at most 2.0 wt.%, and in particular at most 1.5 wt.%, in each case based on the total weight of the metal alloy.

[0180] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of vanadium, preferably at most 1.0 wt.%, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%, most preferably vanadium is nominally not contained.

[0181] The metal alloy according to the invention preferably contains manganese.

[0182] It has been found that manganese, in the presence of chromium, tends to form spinels (MnC^CE), which are thermodynamically more stable than CnOs and form particularly at low oxygen partial pressures. The growth rate of spinels is higher than that of CnOs.

[0183] In preferred embodiments of the invention, the manganese content is at least 0.1 wt.%, preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, even more preferably at least 0.4 wt.%, most preferably at least 0.5 wt.%, and in particular at least 0.6 wt.%, in each case based on the total weight of the metal alloy.

[0184] In preferred embodiments of the invention, the manganese content is at most 4.0 wt.%, preferably at most 3.5 wt.%, more preferably at most 3.0 wt.%, even more preferably at most 2.5 wt.%, most preferably at most 2.0 wt.%, and in particular at most 1.5 wt.%, in each case based on the total weight of the metal alloy.

[0185] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of manganese, preferably at most 1.0 wt.%, more preferably at most 0.5 wt.%, even more preferably at most 0.2 wt.%, most preferably manganese is nominally not contained.

[0186] The metal alloy according to the invention preferably contains zirconium.

[0187] In preferred embodiments of the invention, the zirconium content is at least 0.1 wt.%, preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, even more preferably at least 0.4 wt.%, most preferably at least 0.5 wt.%, and in particular at least 0.6 wt.%, in each case based on the total weight of the metal alloy.

[0188] In preferred embodiments of the invention, the zirconium content is at most 4.0 wt.%, preferably at most 3.5 wt.%, more preferably at most 3.0 wt.%, even more preferably at most 2.5 wt.%, most preferably at most 2.0 wt.%, and in particular at most 1.5 wt.%, in each case based on the total weight of the metal alloy.

[0189] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of zirconium, preferably at most 1.0 wt.%, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%, most preferably zirconium is nominally not contained.

[0190] The metal alloy according to the invention preferably contains copper.

[0191] In preferred embodiments of the invention, the copper content is at least 0.1 wt.%, preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, even more preferably at least 0.4 wt.%, most preferably at least 0.5 wt.%, and in particular at least 0.6 wt.%, in each case based on the total weight of the metal alloy.

[0192] In preferred embodiments of the invention, the copper content is at most 4.0 wt.%, preferably at most 3.5 wt.%, more preferably at most 3.0 wt.%, even more preferably at most 2.5 wt.%, most preferably at most 2.0 wt.%, and in particular at most 1.5 wt.%, in each case based on the total weight of the metal alloy.

[0193] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of copper, preferably at most 1.0 wt.%, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%, most preferably copper is nominally not contained.

[0194] The metal alloy according to the invention preferably contains niobium.

[0195] In preferred embodiments of the invention, the niobium content is at least 0.1 wt.%, preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, even more preferably at least 0.4 wt.%, most preferably at least 0.5 wt.%, and in particular at least 0.6 wt.%, in each case based on the total weight of the metal alloy.

[0196] In preferred embodiments of the invention, the niobium content is at most 5.5 wt.%, preferably at most 5.0 wt.%, more preferably at most 4.5 wt.%, even more preferably at most 4.0 wt.%, most preferably at most 3.5 wt.%, and in particular at most 3.0 wt.%, in each case based on the total weight of the metal alloy.

[0197] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of niobium, preferably at most 1.5 wt.%, more preferably at most 1.0 wt.%, even more preferably 0.5 wt.%, even more preferably at most 0.1 wt.%, most preferably niobium is nominally not contained.

[0198] The metal alloy according to the invention preferably contains tungsten.

[0199] In preferred embodiments of the invention, the tungsten content is at least 0.1 wt.%, preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, even more preferably at least 0.4 wt.%, most preferably at least 0.5 wt.%, and in particular at least 0.6 wt.%, in each case based on the total weight of the metal alloy.

[0200] In preferred embodiments of the invention, the metal alloy contains tungsten and the tungsten content is at least 1.0 wt.%, preferably at least 2.0 wt.%, more preferably at least 3.0 wt.%, even more preferably at least 4.0 wt.%, most preferably at least 5.0 wt.%, and in particular at least 6.0 wt.%, in each case based on the total weight of the metal alloy.

[0201] In preferred embodiments of the invention, the tungsten content is at most 20 wt.%, preferably at most 19 wt.%, more preferably at most 18 wt.%, even more preferably at most 17 wt.%, most preferably at most 16 wt.%, and in particular at most 15 wt.%, in each case based on the total weight of the metal alloy.

[0202] In preferred embodiments of the invention, the content of tungsten is at most 5.0 wt.%, preferably at most 4.0 wt.%, more preferably at most 3.0 wt.%, even more preferably at most 2.5 wt.%, most preferably at most 2.0 wt.%, and in particular at most 1.5 wt.%, in each case based on the total weight of the metal alloy.

[0203] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of tungsten, preferably at most 1.0 wt.%, more preferably at most 0.5 % by weight, more preferably at most 0.1 % by weight, most preferably tungsten is nominally not included.

[0204] In preferred embodiments of the invention, the metal alloy contains tungsten and the total content of nickel, chromium and tungsten is at most 95 wt.%, preferably at most 90 wt.%, more preferably at most 85 wt.%, even more preferably at most 80 wt.%, most preferably at most 75 wt.%, and in particular at most 70 wt.%, in each case based on the total weight of the metal alloy.

[0205] The metal alloy according to the invention preferably contains molybdenum and tungsten.

[0206] In preferred embodiments of the invention, the metal alloy contains molybdenum and tungsten and the total content of nickel, chromium, molybdenum and tungsten is at most 95 wt.%, preferably at most 90 wt.%, more preferably at most 85 wt.%, even more preferably at most 80 wt.%, most preferably at most 75 wt.%, and in particular at most 70 wt.%, in each case based on the total weight of the metal alloy.

[0207] In preferred embodiments of the invention, the content of {[(content of molybdenum) + 0.5 x (content of tungsten)]} is less than 1.5 wt.%.

[0208] In preferred embodiments of the invention, the content of {[(content of molybdenum) + 0.5 x (content of tungsten)]} is at most 1.4 wt.%, preferably at most 1.2 wt.%, more preferably at most 1.0 wt.%, even more preferably at most 0.8 wt.%, most preferably at most 0.6 wt.%, and in particular at most 0.4 wt.%, in each case based on the total weight of the metal alloy.

[0209] In preferred embodiments of the invention, the content of {[(content of molybdenum) + 0.5 x (content of tungsten)]} is more than 8.5 wt.%.

[0210] In preferred embodiments of the invention, the content of {[(content of molybdenum) + 0.5 x (content of tungsten)]} is at least 8.6 wt.%, preferably at least 8.8 wt.%, more preferably at least 9.0 wt.%, even more preferably at least 10 wt.%, most preferably at least 13 wt.%, and in particular at least 16 wt.%, in each case based on the total weight of the metal alloy.

[0211] In preferred embodiments of the invention, the content of (1.8 x chromium / {[(content of molybdenum) + 0.5 x (content of tungsten)]}) is less than 3.0 wt.%.

[0212] In preferred embodiments of the invention, the content of (1.8 x chromium / {[(content of molybdenum) + 0.5 x (content of tungsten)]}) is at most 2.9 wt.%, preferably at most 2.6 wt.%, more preferably at most 2.3 wt.%, even more preferably at most 2.0 wt.%, most preferably at most 1.7 wt.%, and in particular at most 1.4 wt.%, in each case based on the total weight of the metal alloy.

[0213] In preferred embodiments of the invention, the content of (1.8 x chromium / {[(content of molybdenum) + 0.5 x (content of tungsten)]}) is more than 10 wt.%.

[0214] In preferred embodiments of the invention, the content of (1.8 x chromium / {[(content of molybdenum) + 0.5 x (content of tungsten)]}) is at least 30 wt.%, preferably at least 50 wt.%, more preferably at least 70 wt.%, even more preferably at least 75 wt.%, most preferably at least 80 wt.%, and in particular at least 85 wt.%, in each case based on the total weight of the metal alloy.

[0215] The metal alloy according to the invention preferably contains a rare earth metal, preferably selected from the group consisting of scandium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, yttrium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium; more preferably scandium, lanthanum, cerium, neodymium, and yttrium. It has been found that rare earths (particularly cerium and yttrium) can improve nitride adhesion during external nitriding (low oxygen partial pressure).

[0216] In preferred embodiments of the invention, the total content of rare earth metals is at least 0.01 wt.%, preferably at least 0.02 wt.%, more preferably at least 0.03 wt.%, even more preferably at least 0.04 wt.%, most preferably at least 0.05 wt.%, and in particular at least 0.06 wt.%, in each case based on the total weight of the metal alloy.

[0217] In preferred embodiments of the invention, the total content of rare earth metals is at most 0.6 wt.%, preferably at most 0.5 wt.%, more preferably at most 0.4 wt.%, even more preferably at most 0.3 wt.%, most preferably at most 0.2 wt.%, and in particular at most 0.1 wt.%, in each case based on the total weight of the metal alloy.

[0218] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of rare earth metals, preferably at most 0.10 wt.%, more preferably at most 0.09 wt.%, even more preferably at most 0.08 wt.%, most preferably rare earth metals are nominally not included.

[0219] The metal alloy according to the invention preferably contains cerium.

[0220] In preferred embodiments of the invention, the cerium content is at least 0.01 wt.%, preferably at least 0.02 wt.%, more preferably at least 0.03 wt.%, even more preferably at least 0.04 wt.%, most preferably at least 0.05 wt.%, and in particular at least 0.06 wt.%, in each case based on the total weight of the metal alloy.

[0221] In preferred embodiments of the invention, the cerium content is at most 0.6 wt.%, preferably at most 0.5 wt.%, more preferably at most 0.4 wt.%, even more preferably at most 0.3 wt.%, most preferably at most 0.2 wt.%, and in particular at most 0.1 wt.%, in each case based on the total weight of the metal alloy.

[0222] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of cerium, preferably at most 1.0 wt.%, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%, most preferably cerium is nominally not contained.

[0223] The metal alloy according to the invention preferably contains yttrium.

[0224] In preferred embodiments of the invention, the yttrium content is at least 0.01 wt.%, preferably at least 0.02 wt.%, more preferably at least 0.03 wt.%, even more preferably at least 0.04 wt.%, most preferably at least 0.05 wt.%, and in particular at least 0.06 wt.%, in each case based on the total weight of the metal alloy.

[0225] In preferred embodiments of the invention, the yttrium content is at most 0.6 wt.%, preferably at most 0.5 wt.%, more preferably at most 0.4 wt.%, even more preferably at most 0.3 wt.%, most preferably at most 0.2 wt.%, and in particular at most 0.1 wt.%, in each case based on the total weight of the metal alloy.

[0226] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of yttrium, preferably at most 0.10 wt.%, more preferably at most 0.09 wt.%, even more preferably at most 0.08 wt.%, most preferably yttrium is nominally not contained.

[0227] The metal alloy according to the invention preferably contains lanthanum.

[0228] In preferred embodiments of the invention, the content of lanthanum is at least 0.01 wt.%, preferably at least 0.02 wt.%, more preferably at least 0.03 wt.%, even more preferably at least 0.04 wt.%, most preferably at least 0.05 wt.%, and in particular at least 0.06 wt.%, in each case based on the total weight of the metal alloy.

[0229] In preferred embodiments of the invention, the content of lanthanum is at most 0.6 wt.%, preferably at most 0.5 wt.%, more preferably at most 0.4 wt.%, even more preferably at most 0.3 wt.%, most preferably at most 0.2 wt.%, and in particular at most 0.15 wt.%, in each case based on the total weight of the metal alloy.

[0230] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of lanthanum, preferably at most 0.10 wt.%, more preferably at most 0.09 wt.%, even more preferably at most 0.08 wt.%, most preferably lanthanum is nominally not contained.

[0231] The metal alloy according to the invention preferably contains boron.

[0232] In preferred embodiments of the invention, the boron content is at least 0.1 wt.%, preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, even more preferably at least 0.4 wt.%, most preferably at least 0.5 wt.%, and in particular at least 0.6 wt.%, in each case based on the total weight of the metal alloy.

[0233] In preferred embodiments of the invention, the boron content is at most 4.0 wt.%, preferably at most 3.5 wt.%, more preferably at most 3.0 wt.%, even more preferably at most 2.5 wt.%, most preferably at most 2.0 wt.%, and in particular at most 1.5 wt.%, in each case based on the total weight of the metal alloy.

[0234] In other preferred embodiments, the metal alloy according to the invention contains at most very small amounts of boron, preferably at most 0.1 wt.%, more preferably at most 0.01 wt.%, even more preferably at most 0.001 wt.%, most preferably boron is nominally not contained.

[0235] Preferably - the total content of nickel and, where applicable, cobalt is at least 54% by weight, - the chromium content does not exceed 24% by weight, and - the total content of aluminium and / or titanium does not exceed 2.1% by weight, based on the total weight of the metal alloy.

[0236] Preferably - the total content of nickel and, where applicable, cobalt is at least 57% by weight, and - the chromium content is at least 26% by weight, based on the total weight of the metal alloy.

[0237] Preferably, - the total content of nickel and, where applicable, cobalt is at least 30% by weight, - the chromium content is in the range of 15 to 35 wt%, - the aluminium content does not exceed 2.5% by weight, and - the titanium content shall not exceed 0.6% by weight, based on the total weight of the metal alloy.

[0238] Preferably, - the total content of nickel and, where applicable, cobalt is at least 30% by weight, - the chromium content is in the range of 19 to 35 wt%, - the aluminium content does not exceed 2.5% by weight, and - the titanium content does not exceed 0.6% by weight, each based on the total weight of the metal alloy.

[0239] Preferably, the metal alloy is selected from the group consisting of metal alloys Al to A 12, Bl to Bl 7, CI to CI 7, Dl to D17, or El to El 7, which each contain the following content of chromium and nickel in wt.% based on the total weight of the metal alloy, wherein further components not listed in the tables may be included:

[0240] Preferably, the metal alloy is selected from the group consisting of metal alloys Fl to Fl 2, Gl to Gl 7, Hl to Hl 7, II to 117, or J1 to J17, which each contain the following content of iron, chromium and nickel in wt.% based on the total weight of the metal alloy, wherein further components not listed in the tables may be included:

[0241] Preferably, the metal alloy is selected from the group consisting of metal alloys K1 to K12, L1 to L17, M1 to M17, N1 to N17, or O1 to O17, which each contain the following content of chromium, nickel and possibly cobalt in wt.% based on the total weight of the metal alloy, wherein further components not listed in the tables may be included:

[0242] Preferably, the metal alloy is selected from the group consisting of metal alloys PI to P12, Ql to Q17, RI to R17, S1 to S17, or TI to T17, which each contain the following content of iron, chromium, nickel and cobalt in wt.% based on the total weight of the metal alloy, wherein further components not listed in the tables may be included: S£

[0243] Preferably, the metal alloy is selected from the group consisting of metal alloys Ul to Ul 2, VI to VI 7, W1 to W 17, XI to XI 7, or Y1 to Y17, which each contain, based on the total weight of the metal alloy, the following content of iron, chromium, nickel, cobalt, aluminum and titanium in wt.%, where appropriate, wherein further components not listed in the tables may be included: Lt

[0244] Preferably, the metal alloy according to the invention is selected from the group consisting of materials according to DIN / EN X9CrNiSiNCe21-1 l-2, X8CrNiNbl6-13, X12CrNi25-21, X10NiCrAlTi32-20, X5NiCrAlTi31-20, X12NiCrSi35-16, 35Ni25Crl.5SiCeN, NiFe30Cr21Mo3, NiCr23Col2Mo, NiMol6Crl5Fe6W4, NiCr22W14Mo, NiCr23Fel5Al, NiCr25FeAlY, NiCr22Mo9Nb, and NiCrl5Fe.

[0245] The metal alloy according to the invention is preferably selected from the group consisting of materials according to material class DIN No. 1.4820, 1.4821, 1.4822, 1.4823, 1.4824, 1.4825, 1.4826, 1.4827, 1.4828, 1.4829, 1.4830, 1.4831, 1.4832, 1.4833, 1.4834, 1.4835, 1.4836, 1.4837, 1.4838, 1.4839, 1.4840, 1.4841, 1.4842, 1.4843, 1.4844, 1.4845, 1.4846, 1.4847, 1.4848, 1.4849, 1.4850, 1.4851, 1.4852, 1.4853, 1.4854, 1.4855, 1.4856, 1.4857, 1.4858, 1.4859, 1.4860, 1.4861, 1.4862, 1.4863, 1.4864, 1.4865, 1.4866, 1.4867, 1.4868, 1.4869, 1.4870, 1.4871, 1.4872, 1.4873, 1.4874, 1.4875, 1.4876, 1.4877, 1.4878, 1.4879, 1.4880, 1.4881, 1.4882, 1.4883, 1.4884, 1.4885, and 1.4886.

[0246] The metal alloy according to the invention is preferably selected from the group consisting of materials according to material class DIN No. 1.4948, 1.4949, 1.4950, 1.4951, 1.4952, 1.4953, 1.4954, 1.4955, 1.4956, 1.4957, 1.4958, 1.4959, 1.4960, 1.4961, 1.4962, 1.4963, 1.4964, 1.4965, 1.4966, 1.4967, 1.4968, 1.4969, 1.4970, and 1.4971.

[0247] The metal alloy according to the invention is preferably selected from the group consisting of materials according to material class DIN No. 2.4630, 2.4633, 2.4650, 2.4653, 2.4654, 2.4655, 2.4656, 2.4657, 2.4658, 2.4659, 2.4660, 2.4661, 2.4662, 2.4663, 2.4664, 2.4665, 2.4666, 2.4667, 2.4668, 2.4669, 2.4670, 2.4671, 2.4672, and 2.4673.

[0248] The metal alloy according to the invention is preferably selected from the group consisting of materials according to material class DIN No. 2.4723, 2.4724, 2.4725, 2.4726, 2.4727, 2.4728, 2.4729, 2.4730, 2.4731, 2.4732, 2.4733, 2.4734, 2.4735, 2.4736, 2.4737, 2.4738, 2.4739, 2.4740, 2.4741, 2.4742, and 2.4743.

[0249] The metal alloy according to the invention is preferably selected from the group consisting of materials according to material class DIN No. 2.4806, 2.4807, 2.4808, 2.4809, 2.4810, 2.4811, 2.4812, 2.4813, 2.4814, 2.4815, 2.4816, 2.4817, 2.4818, 2.4819, 2.4820, 2.4821, 2.4822, 2.4823, 2.4824, 2.4825, 2.4826, 2.4827, 2.4828, 2.4829, 2.4830, 2.4831, 2.4832, 2.4833, 2.4834, 2.4835, 2.4836, 2.4837, 2.4838, 2.4839, 2.4840, 2.4841, 2.4842, 2.4843, 2.4844, 2.4845, 2.4846, 2.4847, 2.4848, 2.4849, 2.4850, 2.4851, 2.4852, 2.4853, 2.4854, 2.4855, 2.4856, 2.4857, 2.4858, 2.4859, 2.4860, 2.4861, 2.4862, 2.4863, 2.4864, 2.4865, 2.4866, 2.4867, 2.4868, and 2.4879.

[0250] Preferably, the metal alloy comes into direct contact with gaseous NH;.

[0251] Preferably, the metal alloy does not come into direct contact with NH; in the supercritical state.

[0252] A further aspect of the invention relates to a process for the catalytic decomposition of NH3 to N2 and H2 in a plant according to the invention as described above, wherein the NH3 is introduced into the reactor at a temperature of at least 500°C, more preferably at least 530°C, even more preferably at least 560°C, most preferably at least 590°C, and in particular at least 620°C.

[0253] The component is preferably exposed to a temperature of at least 500°C, more preferably at least 530°C, even more preferably at least 560°C, most preferably at least 590°C, and in particular at least 620°C. The component is preferably exposed to a temperature of at least 650°C, more preferably at least 680°C, even more preferably at least 710°C, most preferably at least 740°C, and in particular at least 770°C. The component is preferably exposed to a temperature of at least 800°C, more preferably at least 830°C, even more preferably at least 860°C, most preferably at least 890°C, and in particular at least 920°C.

[0254] Preferably, the NH3 in the reactor is in gaseous state.

[0255] Preferably, the NH3 in the reactor is not in a supercritical state.

[0256] Figure 1 schematically illustrates a preferred embodiment of a reactor 1 according to the invention, which comprises, by way of example, four tubular reaction chambers 2, each containing NH3 decomposition catalyst. The reactor 1 forms a combustion chamber 3 within it, in which the four tubular reaction chambers 2 are arranged in parallel as a bundle. NH3 (reactant gas) is fed into the reactor 1 and into each of the reaction chambers 2 via a feed system 4, where it reacts with the NEFI decomposition catalyst to form product gas. The product gas, which comprises N2 and H2, is discharged from the reaction chambers 2 and the reactor 1 via discharge system 5. In a flow direction parallel to the NH3, combustion gas is fed via feed line 6 into the combustion chamber 3, where it is burned to form flame 7.The heat of combustion generated during combustion flows from the interior of the combustion chamber 3 through the walls of the reaction chamber 2 to the NFfi decomposition catalyst. The flue gas generated during combustion is discharged from the combustion chamber 3 and the reactor 1 via discharge line 8. List of reference symbols: 1 - Reactor 2 - Reaction chamber - Combustion chamber - Supply system for NH; - Discharge system for product gas - Supply line for combustion gas - Flame - Discharge line for flue gas

Claims

Patent claims:

1. A plant for producing H2 by catalytic decomposition of NH3 to N2 and H2; wherein the plant comprises a reactor containing an NHs decomposition catalyst; wherein the reactor comprises at least one component which is at least partially constructed from a metal alloy; wherein the metal alloy comes into direct contact with the NH3 and / or N2 at least in a region of its surface during operation of the reactor; and wherein the metal alloy contains nickel and chromium with a total content of nickel and chromium of at least 15 wt.%, based on the total weight of the metal alloy.

2. The plant according to claim 1, wherein the metal alloy optionally additionally contains cobalt, wherein the total content of nickel and optionally cobalt is at least 20 wt.%, preferably at least 25 wt.%, more preferably at least 30 wt.%, even more preferably at least 35 wt.%, most preferably at least 40 wt.%, and in particular at least 45 wt.%, in each case based on the total weight of the metal alloy.

3. The plant according to claim 2, wherein the total content of nickel and optionally cobalt is at least 50 wt.%, preferably at least 54 wt.%, more preferably at least 58 wt.%, even more preferably at least 62 wt.%, most preferably at least 66 wt.%, and in particular at least 70 wt.%, in each case based on the total weight of the metal alloy.

4. The plant according to one of the preceding claims, wherein the total content of nickel and optionally cobalt is at most 60 wt.%, preferably at most 55 wt.%, more preferably at most 50 wt.%, even more preferably at most 45 wt.%, most preferably at most 40 wt.%, and in particular at most 35 wt.%, in each case based on the total weight of the metal alloy.

5. The plant according to any one of the preceding claims, wherein the nickel content is at least 5.0 wt.%, preferably at least 10 wt.%, more preferably at least 15 wt.%, even more preferably at least 20 wt.%, most preferably at least 30 wt.%, and in particular at least 35 wt.%, in each case based on the total weight of the metal alloy.

6. The plant according to one of the preceding claims, wherein the nickel content is at most 60 wt.%, preferably at most 55 wt.%, more preferably at most 50 wt.%, even more preferably at most 45 wt.%, most preferably at most 40 wt.%, and in particular at most 35 wt.%, in each case based on the total weight of the metal alloy.

7. The system according to any one of the preceding claims, wherein the metal alloy contains cobalt.

8. The plant according to any one of the preceding claims, wherein the metal alloy contains cobalt and wherein the cobalt content is at least 4.0 wt.%, preferably at least 8.0 wt.%, more preferably at least 12 wt.%, even more preferably at least 16 wt.%, most preferably at least 20 wt.%, and in particular at least 24 wt.%, in each case based on the total weight of the metal alloy.

9. The plant according to any one of the preceding claims, wherein the chromium content is at least 12 wt.%, preferably at least 15 wt.%, more preferably at least 18 wt.%, even more preferably at least 21 wt.%, most preferably at least 24 wt.%, and in particular at least 27 wt.%, in each case based on the total weight of the metal alloy.

10. The plant according to one of the preceding claims, wherein the chromium content is at most 40 wt.%, preferably at most 38 wt.%, more preferably at most 36 wt.%, even more preferably at most 35 wt.%, most preferably at most 34 wt.%, and in particular at most 32 wt.%, in each case based on the total weight of the metal alloy.

11. The plant according to one of the preceding claims, wherein the chromium content is at most 30 wt.%, preferably at most 28 wt.%, more preferably at most 26 wt.%, even more preferably at most 24 wt.%, most preferably at most 22 wt.%, and in particular at most 20 wt.%, in each case based on the total weight of the metal alloy.

12. The plant according to any one of the preceding claims, wherein the chromium content is in the range of 15±12.5 wt%, 15±10 wt%, 15±7.5 wt%, 15±5.0 wt%, or 15±2.5 wt%, in increasing order of preference, based on the total weight of the metal alloy.

13. The plant according to any one of the preceding claims, wherein the chromium content is in the range of 20±17.5 wt%, 20±15 wt%, 20±12.5 wt%, 20±10 wt%, 20±7.5 wt%, 20±5.0 wt%, or 20±2.5 wt%, in increasing order of preference, based on the total weight of the metal alloy.

14. The plant according to any one of the preceding claims, wherein the chromium content is in the range of 25±22.5 wt%, 25±20 wt%, 25±17.5 wt%, 25±15 wt%, 25±12.5 wt%, 25±10 wt%, 25±7.5 wt%, 25±5.0 wt%, or 25±2.5 wt%, in increasing order of preference, based on the total weight of the metal alloy.

15. The plant according to any one of the preceding claims, wherein the total content of nickel and chromium is at least 20 wt.%, preferably at least 25 wt.%, more preferably at least 30 wt.%, even more preferably at least 35 wt.%, most preferably at least 40 wt.%, and in particular at least 45 wt.%, in each case based on the total weight of the metal alloy.

16. The plant according to any one of the preceding claims, wherein the total content of nickel and chromium is at most 85 wt.%, preferably at most 80 wt.%, more preferably at most 75 wt.%, even more preferably at most 70 wt.%, most preferably at most 65 wt.%, and in particular at most 60 wt.%, in each case based on the total weight of the metal alloy.

17. The system according to any one of the preceding claims, wherein the metal alloy contains molybdenum.

18. The plant according to any one of the preceding claims, wherein the metal alloy contains molybdenum and the molybdenum content is at least 1.0 wt.%, preferably at least 2.0 wt.%, more preferably at least 3.0 wt.%, even more preferably at least 4.0 wt.%, most preferably at least 5.0 wt.%, and in particular at least 6.0 wt.%, in each case based on the total weight of the metal alloy.

19. The plant according to any one of the preceding claims, wherein the metal alloy contains molybdenum and the total content of nickel, chromium and molybdenum is at most 85 wt.%, preferably at most 80 wt.%, more preferably at most 75 wt.%, even more preferably at most 70 wt.%, most preferably at most 65 wt.%, and in particular at most 60 wt.%, in each case based on the total weight of the metal alloy.

20. The system according to any one of the preceding claims, wherein the metal alloy contains tungsten.

21. The plant according to any one of the preceding claims, wherein the metal alloy contains tungsten and the tungsten content is at least 1.0 wt.%, preferably at least 2.0 wt.%, more preferably at least 3.0 wt.%, even more preferably at least 4.0 wt.%, most preferably at least 5.0 wt.%, and in particular at least 6.0 wt.%, in each case based on the total weight of the metal alloy.

22. The plant according to any one of the preceding claims, wherein the metal alloy contains tungsten and the total content of nickel, chromium and tungsten is at most 95 wt.%, preferably at most 90 wt.%, more preferably at most 85 wt.%, even more preferably at most 80 wt.- %, most preferably at most 75 wt.%, and in particular at most 70 wt.%, in each case based on the total weight of the metal alloy.

23. The system according to any one of the preceding claims, wherein the metal alloy contains molybdenum and tungsten.

24. The plant according to any one of the preceding claims, wherein the metal alloy contains molybdenum and tungsten and the total content of nickel, chromium, molybdenum and tungsten is at most 95 wt.%, preferably at most 90 wt.%, more preferably at most 85 wt.%, even more preferably at most 80 wt.%, most preferably at most 75 wt.%, and in particular at most 70 wt.%, in each case based on the total weight of the metal alloy.

25. The plant according to any one of the preceding claims, wherein the content of {[(content of molybdenum) + 0.5 x (content of tungsten)]} is less than 1.5 wt%.

26. The plant according to any one of the preceding claims, wherein the content of {[(content of molybdenum) + 0.5 x (content of tungsten)]} is at most 1.4 wt.%, preferably at most 1.2 wt.%, more preferably at most 1.0 wt.%, even more preferably at most 0.8 wt.%, most preferably at most 0.6 wt.%, and in particular at most 0.4 wt.%, in each case based on the total weight of the metal alloy.

27. The plant according to any one of the preceding claims, wherein the content of {[(content of molybdenum) + 0.5 x (content of tungsten)]} is more than 8.5 wt.%.

28. The plant according to any one of the preceding claims, wherein the content of {[(content of molybdenum) + 0.5 x (content of tungsten)]} is at least 8.6 wt.%, preferably at least 8.8 wt.%, more preferably at least 9.0 wt.%, even more preferably at least 10 wt.%, most preferably at least 13 wt.%, and in particular at least 16 wt.%, in each case based on the total weight of the metal alloy.

29. The plant according to any one of the preceding claims, wherein the content of (1.8 x chromium / {[(content of molybdenum) + 0.5 x (content of tungsten)]}) is less than 3.0 wt.%.

30. The plant according to any one of the preceding claims, wherein the content of (1.8 x chromium / {[(content of molybdenum) + 0.5 x (content of tungsten)]}) is at most 2.9 wt.%, preferably at most 2.6 wt.%, more preferably at most 2.3 wt.%, even more preferably at most 2.0 % by weight, most preferably at most 1.7 % by weight, and in particular at most 1.4 % by weight, in each case based on the total weight of the metal alloy.

31. The plant according to any one of the preceding claims, wherein the content of (1.8 x chromium / {[(content of molybdenum) + 0.5 x (content of tungsten)]}) is more than 10 wt.%.

32. The plant according to any one of the preceding claims, wherein the content of (1.8 x chromium / {[(content of molybdenum) + 0.5 x (content of tungsten)]}) is at least 30 wt.%, preferably at least 50 wt.%, more preferably at least 70 wt.%, even more preferably at least 75 wt.%, most preferably at least 80 wt.%, and in particular at least 85 wt.%, in each case based on the total weight of the metal alloy.

33. The system according to any one of the preceding claims, wherein - the metal alloy optionally contains cobalt, the total content of nickel and, where applicable, cobalt being at least 40% by weight, and - the chromium content is at least 20% by weight, based on the total weight of the metal alloy.

34. The system according to any one of the preceding claims, wherein the metal alloy contains aluminum and / or titanium.

35. The plant according to any one of the preceding claims, wherein the metal alloy contains aluminum and / or titanium and wherein the total content of aluminum and / or titanium is at most 4.5 wt.%, preferably at most 4.0 wt.%, more preferably at most 3.5 wt.%, even more preferably at most 3.0 wt.%, most preferably at most 2.5 wt.%, and in particular at most 2.0 wt.%, in each case based on the total weight of the metal alloy.

36. The plant according to any one of the preceding claims, wherein the aluminum content is preferably at most 3.0 wt.%, preferably at most 2.5 wt.%, more preferably at most 2.0 wt.%, even more preferably at most 1.5 wt.%, most preferably at most 1.0 wt.%, and in particular at most 0.5 wt.%, in each case based on the total weight of the metal alloy.

37. The system according to any one of the preceding claims, wherein the metal alloy according to the invention contains at most very small amounts of titanium, preferably at most 1.0 wt.%, more preferably at most 0.6 wt.%, even more preferably at most 0.5 wt.%, most preferably at most 0.1 wt.%, and in particular titanium is nominally not included.

38. The system according to any one of the preceding claims, wherein - the total content of nickel and, where applicable, cobalt is at least 54% by weight, - the chromium content is not more than 24% by weight, and - the total content of aluminium and / or titanium does not exceed 2.1% by weight, based on the total weight of the metal alloy.

39. The system according to any one of the preceding claims, wherein - the total content of nickel and, where applicable, cobalt is at least 57% by weight, and - the chromium content is at least 26% by weight, based on the total weight of the metal alloy.

40. The system according to any one of the preceding claims, wherein - the total content of nickel and, where applicable, cobalt is at least 30% by weight, - the chromium content is in the range of 15 to 35 wt.%, preferably 19 to 35 wt.%, - the aluminium content is not more than 2.5% by weight, and - the titanium content does not exceed 0.6% by weight, based on the total weight of the metal alloy.

41. The plant according to any one of the preceding claims, wherein the NH3 decomposition catalyst is nickel-based.

42. The plant according to any one of the preceding claims, wherein the NH3 decomposition catalyst has an apparent activation energy E with respect to the decomposition of NH3; app of at least 50 kJ-mol 1 has.

43. The system according to any one of the preceding claims, wherein the system comprises one or more of the following devices which are in fluid communication with each other: (i) a device for storing liquid NH;,: (ii) a device for heating and evaporating the liquid NH;: (iii) the reactor; (iv) a device for purifying H2 from the product gas; and (v) a device for recovering process heat.

44. The system according to any one of the preceding claims, wherein the metal alloy comes into direct contact with gaseous NH3.

45. The system according to any one of the preceding claims, wherein the metal alloy does not come into direct contact with NH3 in the supercritical state.

46. A process for the catalytic decomposition of NH3 to N2 and H2 in a plant according to any one of the preceding claims, wherein the NH3 is introduced into the reactor at a temperature of at least 500°C, more preferably at least 530°C, even more preferably at least 560°C, most preferably at least 590°C, and in particular at least 620°C.

47. The process of claim 46, wherein the NH3 is present in the reactor in a gaseous state.

48. The process of claim 46 or 47, wherein the NH3 is not in a supercritical state in the reactor.

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