Hydrogen production method and hydrogen production equipment

Abstract

To provide a method exhibiting not only excellent efficiency and advantageous economy but also an excellent environment property without discharging nitrogen oxide, when acquiring high purity hydrogen via decomposition of ammonia.SOLUTION: There is conducted heating in an ammonia decomposition step by combustion gas, comprising a step of decomposing ammonia to nitrogen and hydrogen by coming into contact with a catalyst containing Ru at a temperature of 400°C or more and 550°C or less while heating by causing ammonia to communicate with a heat exchange type reactor 3 at a pressure of 1.5 MPa or more and 7 MPa or less; a step of separating hydrogen by causing gas obtained by the foregoing step to communicate with a hydrogen permselective membrane at 300°C or more and 550°C or less; a step of decompressing gas containing unreacted ammonia after hydrogen separation to 0.3 MPa or less; a step of decomposing ammonia by adding oxygen-containing gas to the decomposed gas to come into contact with a catalyst containing at least one metal selected from the group consisting of Ru, Rh and Pd; and a combustion step by adding the oxygen-containing gas to gas obtained in the foregoing step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and equipment for producing a gas containing hydrogen as a main component by decomposing ammonia. [Background technology]

[0002] Ammonia is attracting attention as a fuel that supports decarbonization because it does not produce carbon dioxide when burned, and unlike hydrogen, it can be liquefied at room temperature by applying slight pressure, and is highly portable.

[0003] However, because ammonia contains nitrogen in its molecular structure, the generation of fuel-derived nitrogen oxides (fuel-NOx) is unavoidable, and it is known that, depending on the combustion method, the amount of nitrogen oxides generated can be several times that of natural gas. To avoid this problem, it has been proposed to decompose ammonia into hydrogen and then burn it, rather than burning it directly. Ruthenium, iron, cobalt, and nickel exhibit high activity in the ammonia decomposition reaction (formula 1), and it is known that catalysts containing ruthenium in particular exhibit high activity (Non-Patent Document 1). Patent Document 1 discloses that an ammonia decomposition catalyst in which ruthenium and a rare earth oxide are supported on a metal oxide support other than a rare earth oxide and the rare earth oxide content is 0.1 to 30.0 mass % exhibits high ammonia decomposition activity even at low temperatures, achieving an ammonia decomposition rate of 81% to 98% under conditions of 500°C, atmospheric pressure, and GHSV of 10,000 / h. NH3→ 1 / 2N2+ 3 / 2H2ΔH=+46kJ / mol (Formula 1)

[0004] The ammonia decomposition reaction is an endothermic reaction in which the number of molecules increases, as shown in Equation 1. Therefore, the higher the temperature and the lower the pressure, the higher the equilibrium conversion rate. For example, the equilibrium conversion rate at 500°C is 99.7% at 0.1 MPa (absolute pressure, the same applies below), but drops to 97.5% at 1 MPa and 93.0% at 3 MPa. The equilibrium conversion rate at 600°C is 99.9% at 0.1 MPa, but drops to 99.0% at 1 MPa and 97.1% at 3 MPa. Even at the relatively high temperature of 600°C, when the reaction pressure is 3 MPa, about 3% of the ammonia used in the reaction remains unreacted. Furthermore, since the decomposition reaction of ammonia is an endothermic reaction, it is necessary to supply reaction heat in order to proceed with the reaction.

[0005] Patent Document 2 discloses that when a gas containing 57.1% ammonia (by volume, the same applies hereinafter), 33.3% nitrogen, 8.9% oxygen, and the remainder argon was used with a two-layer catalyst, with a catalyst of platinum supported on γ-alumina in the front stage and a catalyst of ruthenium supported on γ-alumina in the rear stage, and the catalyst inlet temperature was set to 200°C, the maximum temperature of the catalyst layer reached 1000°C, the temperature at the catalyst layer outlet reached 470°C, and a hydrogen yield of 86% was maintained for 30 hours from the start of the reaction.

[0006] Patent Document 3 discloses a method for decomposing ammonia, in which gaseous ammonia at 5.2 MPa and air at 5.2 MPa are mixed and passed through an ammonia autothermal decomposition reactor to oxidize part of the ammonia and release reaction heat, and the remaining ammonia is heated by this reaction heat to decompose into nitrogen and hydrogen at about 600°C. The oxidation reaction of ammonia (Equation 2) generates a large amount of heat, so if about 15% of the ammonia is oxidized (burned), the endothermic heat of the ammonia decomposition reaction can be covered. NH3+ 3 / 4O2→ 1 / 2N2+ 3 / 2H2O(g) ΔH=-317kJ / mol (Formula 2)

[0007] When ammonia is decomposed by autothermal decomposition, which involves a simultaneous oxidation reaction, the reaction can be carried out in a simple packed-column reactor without a heat exchange function, which has the advantage of reducing equipment costs. On the other hand, when ammonia is decomposed while being heated externally, if the reaction proceeds completely, a gas consisting of 75% hydrogen and 25% nitrogen is obtained. However, in the case of autothermal decomposition, the hydrogen concentration of the gas produced by the decomposition reaction is low because it contains nitrogen and water vapor produced by the combustion reaction. For example, Patent Document 3 shows that the gas composition at the outlet of an autothermal decomposition reactor is 48% hydrogen, 39% nitrogen, 10% water vapor, and 3% unreacted ammonia.

[0008] When the hydrogen-containing gas obtained by decomposing ammonia is used as fuel, a slight decrease in hydrogen concentration is usually not a major problem. However, when using it as a raw material for chemical reactions or as a raw material for town gas, it is necessary to separate hydrogen from the hydrogen-based gas obtained by decomposing ammonia. Gas separation methods include membrane separation, which uses a selectively permeable membrane that selectively allows specific components to pass through, and pressure swing adsorption (PSA), which uses an adsorbent that selectively adsorbs specific components. However, with either separation method, the recovery rate decreases when the concentration of the gas to be recovered becomes low.

[0009] Patent Document 4 discloses an ammonia decomposition device comprising an ammonia combustion chamber in which ammonia is mixed with air and burned, and an ammonia decomposition chamber surrounding the ammonia combustion chamber in which ammonia is decomposed into nitrogen and hydrogen by endothermic pyrolysis, with the heat obtained by the ammonia combustion reaction being utilized for the decomposition of ammonia. Heat exchange reactors, which are heated externally and supply reaction heat while the reaction is carried out, are widely used in hydrocarbon steam reforming reactions, and this device can also be considered a heat exchange reactor with a simple structure. In this method, the gas obtained by ammonia decomposition is not mixed with the ammonia combustion gas, so the hydrogen concentration of the gas produced by decomposition is close to 75%. However, Patent Document 4 does not specifically disclose a method for obtaining high-purity hydrogen from the gas produced by decomposition.

[0010] Patent Document 5 discloses a method for producing hydrogen by ammonia decomposition, in which ammonia is introduced into a system comprising a membrane reactor containing a fixed-bed ammonia decomposition catalyst, ceramic hollow fibers arranged in the fixed bed and having a hydrogen-selective permeable membrane arranged on the surface thereof, and a catalytic hydrogen burner also arranged in the fixed bed for supplying thermal energy to the ammonia decomposition, to decompose the ammonia and separate at least a portion of the hydrogen from the decomposition product.

[0011] According to this document, when ammonia at 10 bar and 25°C is supplied to the system at a flow rate of 0.3434 mol / min, and the ammonia decomposition reaction is carried out at 400°C, and the produced hydrogen is separated using a hydrogen separation membrane made of synthetic SAPO-34, 0.4464 mol of hydrogen and 0.004509 mol of nitrogen are obtained as permeates per minute at 2.5 bar, with an energy efficiency of 88.45%.

[0012] Membrane separation generally cannot separate and recover all of the target components. Therefore, when using membrane separation to obtain hydrogen by decomposing ammonia, some hydrogen will remain on the non-permeated side. Patent Document 5 describes burning the non-permeated material in a catalytic hydrogen burner made of, for example, Ni foam impregnated with Pt, and using it as a heat source for ammonia decomposition.

[0013] The advantage of using a membrane reactor is that by integrating separation and reaction, the reaction can proceed while the product is being withdrawn, thereby achieving a conversion rate higher than the thermodynamically determined equilibrium conversion rate.

[0014] On the other hand, in order to carry out a reaction accompanied by endothermic heat generation in a membrane reactor, it is necessary to connect five flow paths to the reactor: a raw material inlet, an outlet on the permeated side of the reaction product, an outlet on the non-permeated side of the reaction product, and an inlet and outlet for the heat transfer medium. In addition, it is necessary to design the flow paths within the reactor to balance the endothermic heat generated by the reaction and the heating by the heat transfer medium, which results in a problem of a complicated reactor structure.

[0015] Another possible method is to use a conventional heat exchange reactor to decompose ammonia, and pass the gas containing the hydrogen and nitrogen produced by the decomposition as main components and unreacted ammonia through a membrane separator equipped with a membrane that selectively allows hydrogen to permeate, and recover high-purity hydrogen on the membrane permeation side.

[0016] In gas separation by membrane separation, the driving force for the gas flowing through the membrane is the partial pressure difference between the gases on both sides of the membrane, so the target gas cannot be completely recovered unless the permeate side is evacuated. When hydrogen gas is separated and recovered using a hydrogen-selective permeable membrane from a gas that contains unreacted ammonia and is primarily composed of hydrogen and nitrogen produced by ammonia decomposition, a certain concentration of hydrogen remains on the non-permeate side. The gas on the non-permeate side contains hydrogen, nitrogen, and unreacted ammonia.

[0017] The non-permeate gas contains hydrogen and ammonia, and by burning it, the endothermic heat of ammonia decomposition can be compensated for. However, under typical ammonia decomposition conditions, unless a membrane reactor is used, the conversion rate of ammonia decomposition is limited to approximately 90-95% due to thermodynamic constraints, depending on the reaction temperature and pressure. Therefore, the non-permeate gas contains approximately 10% ammonia. It is known that a large amount of nitrogen oxides is produced when ammonia is combusted. This is not limited to gas-phase combustion, but is also true when catalytic combustion is performed. As can be seen from the use of platinum-rhodium catalysts in the production of nitric acid by ammonia oxidation, when ammonia oxidation reaction is carried out in an oxidizing atmosphere using a noble metal catalyst, a significant portion of the nitrogen is converted into nitrogen oxides. Platinum, in particular, has high oxidation activity for hydrogen and ammonia at low temperatures. However, in addition to producing a large amount of nitrogen oxides, it is also known to produce a significant amount of nitrous oxide, which has a significant global warming effect (Non-Patent Document 2).

[0018] The non-permeated gas is combusted and the heat recovered as a heat source for ammonia decomposition is released into the atmosphere as exhaust gas. If this gas contains nitrogen oxides or nitrous oxide, the impact on the local and global environments cannot be ignored.

[0019] Patent Document 4 discloses a catalyst that selectively oxidizes ammonia from a gas mixture containing hydrogen and ammonia. However, the temperature range in which a high ammonia removal rate is achieved is limited, and in addition, hydrogen consumption occurs due to the selectivity not being 100%, so there are still many issues to be resolved as a practical process.

[0020] As described above, the process of decomposing ammonia to obtain high-purity hydrogen still faces many challenges. [Prior art documents] [Patent documents]

[0021] [Patent Document 1] International Publication No. 2019 / 188219 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-5926 [Patent Document 3] Japanese Patent Application Publication No. 2020-147478 [Patent Document 4] Patent No. 6670761 [Patent Document 5] Special Publication No. 2020-530427 [Non-patent literature]

[0022] [Non-Patent Document 1] Chongqi Chen et al., Energy and Fuels, Volume 35, 2021, p.11693 [Non-patent document 2] Mengmeng Sun et al., ACS Applied Materials and Interfaces, Vol. 11, 2019, p. 23102 Summary of the Invention [Problem to be solved by the invention]

[0023] In view of the above problems, the problem to be solved by the present invention is to provide a method and equipment for decomposing ammonia to obtain high-purity hydrogen, which is highly efficient, economically advantageous, does not emit nitrogen oxides, and is environmentally friendly. [Means for solving the problem]

[0024] The hydrogen production method according to the present invention has the following characteristic configuration: A method for producing hydrogen by decomposing ammonia, comprising: a first ammonia decomposition step in which ammonia is passed through a heat exchange reactor at a pressure of 1.5 MPa or more and 7 MPa or less, and while being heated, brought into contact with a catalyst containing ruthenium at a temperature of 400°C or more and 550°C or less to decompose the ammonia into nitrogen and hydrogen; a hydrogen separation step of separating hydrogen from the gas obtained in the first ammonia decomposition step by passing the gas through a membrane that selectively permeates hydrogen at a temperature of 300°C or higher and 550°C or lower; a decompression step of decompressing the gas containing nitrogen, hydrogen, and unreacted ammonia after separating hydrogen to 0.3 MPa or less; a second ammonia decomposition step in which an oxygen-containing gas is added to the reduced-pressure gas and brought into contact with a catalyst containing at least one metal selected from ruthenium, rhodium, and palladium to decompose ammonia; At least nitrogen and hydrogen obtained in the second ammonia decomposition step and a combustion step of adding an oxygen-containing gas to a gas containing 1000 ppm or less of ammonia and burning the gas to generate a combustion gas, The ammonia in the first ammonia decomposition step is heated by the combustion gas. The hydrogen production facility according to the present invention has the following characteristic configuration: A facility for producing hydrogen by decomposing ammonia, a first ammonia decomposition section that heats ammonia supplied at a pressure of 1.5 MPa or more and 7 MPa or less and brings the ammonia into contact with a catalyst containing ruthenium at a temperature of 400°C or more and 550°C or less to decompose the ammonia into nitrogen and hydrogen; a hydrogen separation unit that separates hydrogen from the gas obtained in the first ammonia decomposition unit by passing the gas through a membrane that selectively permeates hydrogen at a temperature of 300°C or higher and 550°C or lower; a pressure reducing section that reduces the pressure of the gas containing nitrogen, hydrogen, and unreacted ammonia after hydrogen separation to 0.3 MPa or less; a second ammonia decomposition section that brings the gas obtained by adding an oxygen-containing gas to the reduced pressure gas into contact with a catalyst containing at least one metal selected from ruthenium, rhodium, and palladium to decompose ammonia; At least nitrogen and hydrogen obtained in the second ammonia decomposition section and a combustion unit that burns a gas containing 1000 ppm or less of ammonia and an oxygen-containing gas to generate a combustion gas, The ammonia in the first ammonia decomposition section is heated by the combustion gas.

[0025] According to this characteristic configuration, unreacted ammonia can be used as a heat source for the ammonia decomposition reaction. Therefore, a conversion rate of approximately 90-95% is sufficient for the ammonia decomposition reaction. Even at relatively high pressures of 1.5 MPa or higher, the ammonia decomposition reaction can be carried out at relatively low temperatures of 550°C or lower. The low reaction temperature facilitates ensuring catalyst durability. Furthermore, since the reaction pressure can be set high, the hydrogen partial pressure difference across the membrane during hydrogen separation using a hydrogen-selective permeable membrane increases, allowing for a smaller permeable membrane area, resulting in excellent economic efficiency. Furthermore, although the non-permeated gas contains ammonia, its concentration is reduced to approximately 1000 ppm or less before combustion. This allows for the concentrations of nitrogen oxides and nitrous oxides contained in the exhaust gas after providing heat for the ammonia decomposition reaction to be kept sufficiently low, resulting in excellent environmental friendliness. In other words, ammonia can be decomposed and high-purity hydrogen recovered in an efficient and environmentally friendly manner without using a complex reactor structure.

[0026] A further characteristic configuration of the hydrogen production method according to the present invention is In the hydrogen separation step, at least a portion of the hydrogen is extracted at a pressure of 0.5 MPa or more. Further, a further characteristic configuration of the hydrogen production facility according to the present invention is as follows: In the hydrogen separation section, at least a portion of the hydrogen is extracted at a pressure of 0.5 MPa or more.

[0027] According to this characteristic configuration, at least a portion of the hydrogen can be extracted at a pressure sufficiently higher than atmospheric pressure, which reduces the power required for compression when transporting and storing the recovered hydrogen.

[0028] A further characteristic configuration of the hydrogen production method according to the present invention is The ammonia flowing into the heat exchange reactor is preheated by the combustion gas used to heat the ammonia in the first ammonia decomposition step. Further, a further characteristic configuration of the hydrogen production facility according to the present invention is as follows: The ammonia to be supplied to the first ammonia decomposition section is preheated by the combustion gas used to heat the ammonia in the first ammonia decomposition section.

[0029] According to this characteristic configuration, the combustion gas used to heat the ammonia is used as a heat source to preheat the ammonia, thereby improving the efficiency of hydrogen generation. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 1 is a diagram showing an example of a process flow of a hydrogen production method according to an embodiment. [Figure 2] FIG. 1 is a diagram showing an example of a process flow of a hydrogen production method according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, embodiments of a method for producing hydrogen by ammonia decomposition and a hydrogen production facility according to the present invention will be described with reference to the drawings.

[0032] In the hydrogen production method of this embodiment, first, preheated ammonia is fed into a heat exchange reactor and brought into contact with a catalyst containing ruthenium at a pressure of 1.5 MPa to 7 MPa and a temperature of 400°C to 550°C to decompose the ammonia and generate hydrogen and nitrogen (first ammonia decomposition step). The hydrogen production equipment of this embodiment is equipped with a heat exchange reactor as a first ammonia decomposition section that heats ammonia supplied at a pressure of 1.5 MPa to 7 MPa and brings the ammonia into contact with a catalyst containing ruthenium at a temperature of 400°C to 550°C to decompose the ammonia into nitrogen and hydrogen, and is configured so that the ammonia supplied to the heat exchange reactor is preheated.

[0033] Preheating of ammonia may be performed by any method, such as heat exchange with a high-temperature heat medium via a heat exchanger or heating with an electric heater, but heating with steam via a heat exchanger is preferred, and preheating with the combustion gas used to heat ammonia in the first ammonia decomposition step (first ammonia decomposition section) described below is preferred. Preheating is preferably performed at a temperature of 400°C or higher and 550°C or lower, but a lower temperature is acceptable as long as the ammonia is heated in the reactor and comes into contact with the ruthenium-containing catalyst at a temperature of 400°C or higher and 550°C or lower. However, at temperatures below 400°C, the ammonia decomposition reaction does not substantially proceed, and the ruthenium-containing catalyst does not function effectively, which may result in reduced economic efficiency.

[0034] The heat exchange reactor may be of any type, but a multi-tube heat exchange reactor having a large number of thin tubes housed in a roughly cylindrical outer shell can be used. The catalyst may be packed on either the shell side or the tube side, but typically the catalyst is packed on the tube side, ammonia is fed, and a heat transfer medium is passed through the shell side. The heat transfer medium is a gas obtained by burning a hydrogen-containing gas obtained by treating the non-permeate gas after separating and recovering hydrogen in a membrane separation section (described below) to reduce the ammonia content. The heat transfer medium may be produced by mixing the hydrogen-containing gas with air in the reactor and burning the mixture with a burner.

[0035] The reaction pressure should be between 1.5 MPa and 7 MPa. If the reaction pressure is lower than 1.5 MPa, the hydrogen partial pressure will be too low when hydrogen is separated using the subsequent hydrogen permselective membrane, making it difficult to recover hydrogen economically. If the reaction pressure is higher than 7 MPa, a reactor that can withstand high pressures will be required, making hydrogen production less economical.

[0036] Ruthenium-containing catalysts are catalysts in which ruthenium is supported on a refractory inorganic carrier such as activated alumina, titania, or zirconia. The amount of ruthenium supported is preferably 1 to 10% by mass relative to the carrier, and more preferably 2 to 5%. If the amount supported is too low, catalytic activity per unit volume decreases, requiring more catalyst, which may result in lower economic efficiency. If the amount supported is too high, ruthenium cannot be supported on the carrier in a highly dispersed manner, which may result in lower catalytic activity per ruthenium and lower economic efficiency.

[0037] Ammonia decomposition is carried out by contacting ammonia with a catalyst containing ruthenium at a temperature of 400°C or higher and 550°C or lower. It is acceptable for a portion of the catalyst layer to be below 400°C, but unless ammonia is contacted with the ruthenium catalyst at 400°C or higher, the decomposition of ammonia cannot proceed substantially. On the other hand, if the catalyst temperature exceeds 550°C, thermal degradation of the catalyst is likely to progress, so the temperature of the catalyst layer must be kept at 550°C or lower. That is, it is necessary to ensure that the maximum temperature of the catalyst layer is 400°C or higher and 550°C or lower, and a temperature of 450°C or higher and 500°C or lower is preferred in terms of ensuring catalytic activity and durability.

[0038] The ammonia decomposition rate (conversion rate) that can be achieved at equilibrium varies depending on the reaction temperature and pressure, but is generally around 90 to 95%. By using a sufficient amount of catalyst, an ammonia conversion rate close to the equilibrium conversion rate can be obtained, and ammonia is converted into a gas whose main components are hydrogen and nitrogen and which contains a small amount of unreacted ammonia.

[0039] In the hydrogen production method of this embodiment, hydrogen is separated and recovered from the gas obtained in the first ammonia decomposition step, which is mainly composed of hydrogen and nitrogen and contains a small amount of unreacted ammonia, using a hydrogen selective permeable membrane (hydrogen separation step). The hydrogen production equipment of this embodiment also includes a membrane separator (hydrogen separation section) having a hydrogen selective permeable membrane that separates hydrogen by passing the gas obtained in the first ammonia decomposition section through a membrane that selectively allows hydrogen to permeate at a temperature of 300°C to 550°C.

[0040] Hydrogen-selective permeable membranes include molecular sieve-type hydrogen separation membranes, such as zeolite membranes, and dissolution-diffusion-type separation membranes, such as palladium membranes. The separation membrane used in this method may be based on either principle. However, since hydrogen separation in this method is performed at temperatures between 300°C and 550°C, it must be able to be used at these temperatures, and therefore an inorganic membrane made of a heat-resistant inorganic oxide or metal is preferred. The selectivity (ratio of permeability coefficients) of the hydrogen-to-nitrogen hydrogen-selective permeable membrane is 100 or more, more preferably 200 or more. The selectivity (ratio of permeability coefficients) of the hydrogen-to-ammonia hydrogen-separation membrane is 50 or more, more preferably 100 or more.

[0041] The pressure on the permeate side when separating and recovering hydrogen is preferably between atmospheric pressure and 1 MPa. If the pressure on the permeate side is low, the difference in hydrogen partial pressure on both sides of the hydrogen separation membrane increases, resulting in a high permeation rate, but a low pressure of the recovered hydrogen. Therefore, if high-pressure hydrogen is required, compression power is required, which reduces economic efficiency. Therefore, it is preferable to set the pressure on the permeate side in the membrane separator so that at least a portion of the hydrogen is extracted at between 0.5 MPa and 1 MPa. Conversely, if the pressure on the permeate side is too high, hydrogen will no longer permeate once the hydrogen partial pressure on the non-permeate side reaches the hydrogen partial pressure on the permeate side, which may result in a low hydrogen recovery rate.

[0042] By separating and recovering hydrogen using a hydrogen-selective permeable membrane in two stages and setting the permeation side pressure of the first stage higher than the permeation side pressure of the second stage, some of the hydrogen can be recovered at high pressure in the first stage, and together with the recovery at lower pressure in the second stage, a high hydrogen recovery rate can be ensured overall.

[0043] After hydrogen is separated and recovered in the membrane separation section, the non-permeate gas contains nitrogen, hydrogen, and a small amount of ammonia, and maintains a pressure nearly equal to the pressure during the ammonia decomposition reaction. Therefore, in the hydrogen production method of this embodiment, the non-permeate gas is depressurized to 0.3 MPa or less (depressurization step). The hydrogen production equipment of this embodiment also includes a depressurization section that depressurizes the non-permeate gas to 0.3 MPa or less.

[0044] The pressure reduction may be performed while recovering work using an expansion turbine, or may be performed without recovering work by adiabatic throttling expansion using an expansion valve. From the viewpoint of energy efficiency, however, it is preferable to perform the pressure reduction while recovering work using an expansion turbine.

[0045] In the hydrogen production method of this embodiment, an oxygen-containing gas such as air is added to the expanded gas, and the gas is passed through a catalyst containing at least one of ruthenium, rhodium, and palladium to decompose unreacted ammonia (second ammonia decomposition step). The hydrogen production equipment of this embodiment also includes a second ammonia decomposition unit that brings the gas, obtained by adding the oxygen-containing gas to the reduced-pressure gas, into contact with a catalyst containing at least one metal selected from ruthenium, rhodium, and palladium to decompose the ammonia.

[0046] The oxygen-containing gas may be preheated before addition, if necessary. The temperature of the gas introduced into the catalyst is preferably 150°C or higher, more preferably 200°C or higher. At temperatures above this temperature, the reaction between oxygen and hydrogen proceeds rapidly, and the heat of reaction raises the gas temperature. The reaction ends when substantially all of the oxygen is consumed, and the amount of oxygen-containing gas added is adjusted so that the temperature at this point is 550°C or higher and 800°C or lower. That is, the gas obtained by adding an oxygen-containing gas such as air to the expanded gas is contacted with a catalyst containing at least one of ruthenium, rhodium, and palladium, and the amount of oxygen-containing gas added is adjusted so that the maximum temperature in the catalyst layer is 550°C or higher and 800°C or lower. If the maximum temperature in the catalyst layer is below 550°C, the concentration of ammonia contained in the gas after the reaction cannot be sufficiently reduced. Conversely, if the maximum temperature in the catalyst layer exceeds 800°C, catalyst degradation may progress.

[0047] Since the reaction between oxygen and hydrogen proceeds rapidly even at low temperatures on a platinum catalyst, a two-stage configuration may be used, with a platinum-containing catalyst in the front stage and a catalyst containing at least one of ruthenium, rhodium, and palladium in the rear stage. When ammonia is oxidized on platinum, nitrogen oxides and nitrous oxides may be produced, but ruthenium, rhodium, and palladium reduce the nitrogen oxides and nitrous oxides with hydrogen, so after the reaction, a gas consisting primarily of nitrogen and hydrogen is produced, containing trace amounts of ammonia at 1,000 ppm or less.

[0048] In the hydrogen production method of this embodiment, an oxygen-containing gas such as air is added to the gas obtained in the second ammonia decomposition step, which is mainly composed of nitrogen and hydrogen and contains a trace amount of ammonia of 1000 ppm or less, and the gas is combusted to generate a combustion gas (combustion step). The hydrogen production equipment of this embodiment also includes a combustion section that burns the gas obtained in the second ammonia decomposition section, which is mainly composed of nitrogen and hydrogen and contains ammonia of 1000 ppm or less, and to which an oxygen-containing gas is added, to generate a combustion gas.

[0049] Combustion may be carried out on the heat transfer medium side of the ammonia decomposition reactor, which is a heat exchange reactor, or may be carried out outside the reactor and the combustion gas may be passed through the heat transfer medium side of the ammonia decomposition reactor. The gas, which is mainly composed of nitrogen and hydrogen, has an ammonia concentration previously reduced to 1000 ppm or less, so high concentrations of nitrogen oxides are not generated even when combusted.

[0050] Examples and Comparative Examples Below is an example of a trial calculation based on process calculations. This trial calculation does not take into account heat loss or pressure loss in each device and piping. Furthermore, the process calculation does not take into account the generation of nitrogen oxides due to the combustion of ammonia-containing gas, and the heat balance is calculated assuming that only nitrogen and water are produced.

[0051] [Example] The process flow of the hydrogen production method by ammonia decomposition based on the method of the present invention is shown in FIG.

[0052] Liquid ammonia is fed into heat exchanger 1 at a temperature of 25°C, a pressure of 3 MPa, and a flow rate of 1 mol / s. In heat exchanger 1, ammonia is heated to 110°C using low-pressure steam (LPS, 120°C), and the ammonia is vaporized. The vaporized ammonia is further heated to 480°C in heat exchanger 2 and fed into heat exchange reactor 3 (first ammonia decomposition section). Heat exchange reactor 3 is a multi-tube shell-and-tube reactor. The tube side is filled with a ruthenium-supported catalyst, and the ammonia is heated by the combustion gas (described below) flowing through the shell side, causing the ammonia decomposition reaction to proceed. The ammonia reaches an equilibrium composition at 480°C and flows out of heat exchange reactor 3 (first ammonia decomposition step). The ammonia conversion rate is 91.6%, and the generated gas contains hydrogen, nitrogen, and a small amount of ammonia.

[0053] The gas obtained by decomposing ammonia is sent to a membrane separator 4 (hydrogen separation section). The membrane separator 4 is made of a hydrogen selective permeable membrane, and the permeance of the membrane for hydrogen, nitrogen, and ammonia is 3×10 -7 mol / (s m 2 ·Pa), 3×10-9 mol / (s m 2 ·Pa) and 3×10 -9 mol / (s m 2 Pa), 5m 2 The permeation side of the membrane separator 4 is controlled to 1 MPa, and gas containing 97.5% hydrogen is recovered at a pressure of 1 MPa (hydrogen separation step).

[0054] The gas on the non-permeated side of membrane separator 4 has a composition of 51% hydrogen, 41% nitrogen, and the remainder ammonia. Since hydrogen still remains, it is sent to membrane separator 5 (hydrogen separation section) to extract more hydrogen. Membrane separator 5 is made of a membrane with the same permeability as membrane separator 4, and is 2 m 2 The permeation side of the membrane separator 5 is controlled to 0.15 MPa, and a gas containing 96.3% hydrogen is obtained at a pressure of 0.15 MPa (hydrogen separation step).

[0055] The gas on the non-permeated side of the membrane separator 5 is a combustible gas consisting of 24% hydrogen, 12% ammonia, and the remainder nitrogen. Because this gas is at high temperature and pressure, work can be extracted in the expansion turbine 6 (pressure reduction section). In the expansion turbine 6, the pressure is reduced to 0.2 MPa with an adiabatic efficiency of 75%, and 6.1 kW of electricity can be extracted (pressure reduction step).

[0056] The gas leaving the expansion turbine 6 is mixed with air preheated to 260°C using high-pressure steam (HPS, 270°C) in a heat exchanger 7, and then introduced into a fixed-bed adiabatic reactor 8 (second ammonia decomposition section), which is packed with a palladium catalyst in the front stage and a ruthenium catalyst in the rear stage. In the front stage of the fixed-bed adiabatic reactor 8, the reaction between hydrogen and oxygen proceeds rapidly on the palladium catalyst, raising the gas temperature to above 500°C, and the ammonia decomposition reaction proceeds on the ruthenium catalyst in the rear stage. The outlet gas from the fixed-bed adiabatic reactor 8 consists of 24% hydrogen, nitrogen, and water vapor, and has an ammonia concentration of 100 ppm or less (second ammonia decomposition step).

[0057] The gas leaving the fixed-bed adiabatic reactor 8 is mixed with air preheated to 260°C with high-pressure steam in heat exchanger 9 and combusted in combustor 10 (combustion section) (combustion process). The combustion gas then passes through the shell side of heat exchange reactor 3 to cover the endothermic heat of the ammonia decomposition reaction, and then provides heat to preheat the ammonia in heat exchanger 2. Low-pressure steam (LPS, 120°C) is then recovered in heat exchanger 11 and released into the atmosphere as exhaust gas at 130°C.

[0058] The hydrogen-based gas on the permeation side of the membrane separators 4 and 5 is at a high temperature, so heat is recovered in the heat exchangers 12 to 14 to recover high-pressure steam and low-pressure steam.

[0059] In this process, 1.215 mol / s of hydrogen can be recovered from 1 mol / s of ammonia, of which 0.832 mol / s is recovered at a pressure of 1 MPa. The consumption of high-pressure steam is 0.94 kW, and that of low-pressure steam is 12.3 kW, and 6.1 kW of electricity can be recovered.

[0060] The temperatures, pressures and flow rates at key points in the process are shown in Table 1.

[0061] [Table 1]

[0062] Comparative Example For comparison, the process flow shown in FIG. 2 was examined as an example of a method for producing hydrogen by ammonia decomposition, which does not employ the method of the present invention.

[0063] In this comparative example, the flow up to the membrane separator 5 is the same as in the example, but the gas on the non-permeated side of the membrane separator 5 is sent to the fixed-bed adiabatic reactor 8a at the same pressure. The fixed-bed adiabatic reactor 8a is packed with the same ammonia decomposition catalyst as the heat exchange reactor 3. The gas on the non-permeated side of the membrane separator 5 is the gas from which the ammonia decomposition reaction in the heat exchange reactor 3 has reached equilibrium, with most of the hydrogen removed. As the ammonia decomposition reaction progresses in the fixed-bed adiabatic reactor 8a, the gas temperature decreases due to the endothermic reaction. 46% of the inlet ammonia is decomposed, and the temperature drops to 392°C, resulting in an equilibrium composition, before exiting the fixed-bed adiabatic reactor 8a. After leaving the fixed-bed adiabatic reactor 8a, the gas is decomposed in the expansion turbine 6a and mixed with air preheated to 260°C with high-pressure steam in the heat exchanger 9. The flow from the combustor 10 onward is the same as in the example. Here, the ammonia concentration of the gas leaving the fixed-bed adiabatic reactor 8a is 6%, which is half of the ammonia concentration at the inlet of the fixed-bed adiabatic reactor 8a (12%). However, since the gas still contains a high concentration of ammonia, there is a risk that a large amount of nitrogen oxides will be generated if the gas is combusted in the combustor 10.

[0064] In this process, the consumption of high-pressure steam is 0.94 kW, and the consumption of low-pressure steam is 11.9 kW, and 5.6 kW of electricity can be recovered.

[0065] The temperatures, pressures and flow rates at key points in the process are shown in Table 2.

[0066] [Table 2]

[0068] [Another embodiment] [1] In the above embodiment, the membrane separators 4 and 5 are configured as two stages, and the pressure on the permeation side of the first stage is set higher than the pressure on the permeation side of the second stage. However, the membrane separator serving as the hydrogen separation section used in the membrane separation process according to the present invention may be configured as one stage or as three or more stages.

[0069] [2] In the above embodiment, the case where the gas obtained in the first ammonia decomposition step is introduced into the membrane separator 4 without being heated or cooled has been particularly described. However, in the present invention, there is no problem in providing a heating or cooling means between the heat exchange reactor 3 as the ammonia decomposition section and the membrane separator 4 as the hydrogen separation section, and when priority needs to be given to the heat resistance of the separation membrane, it may be preferable to provide a cooling means.

[0070] [3] In the above embodiment, in the membrane separators 4 and 5 serving as the hydrogen separation section, at least a portion of the hydrogen is extracted at a pressure of 0.5 MPa or more and 1 MPa or less, but the present invention is not limited to this.

[0071] [4] In the above embodiment, the ammonia to be supplied to the heat exchange reactor is preheated, but the present invention is not limited to this, and the ammonia may not be preheated.

[0072] The configurations disclosed in the above embodiments (including other embodiments) can be applied in combination with configurations disclosed in other embodiments, as long as no contradictions arise. Furthermore, the embodiments disclosed in this specification are examples, and the embodiments of the present invention are not limited to these and can be modified as appropriate within the scope of the purpose of the present invention. [Industrial Applicability]

[0073] The present invention can be used as a method and equipment for decomposing ammonia to produce high-purity hydrogen gas that can be used as a source of town gas or a chemical raw material. [Explanation of symbols]

[0074] 1,2,7,9,11,12,13: Heat exchanger 3: Heat exchange reactor (first ammonia decomposition section) 4, 5: Membrane separator (hydrogen separation section) 6: Expansion turbine (pressure reduction section) 8: Fixed-bed adiabatic reactor (second ammonia decomposition section) 10: Combustor (combustion section)

Claims

1. A method for producing hydrogen by decomposing ammonia, comprising: a first ammonia decomposition step in which ammonia is passed through a heat exchange reactor at a pressure of 1.5 MPa or more and 7 MPa or less, and while being heated, brought into contact with a catalyst containing ruthenium at a temperature of 400°C or more and 550°C or less, thereby decomposing the ammonia into nitrogen and hydrogen; a hydrogen separation step of separating hydrogen from the gas obtained in the first ammonia decomposition step by passing the gas through a membrane that selectively permeates hydrogen at a temperature of 300°C or higher and 550°C or lower; a depressurization step of depressurizing the gas containing nitrogen, hydrogen, and unreacted ammonia after separating hydrogen to 0.3 MPa or less; a second ammonia decomposition step in which an oxygen-containing gas is added to the reduced-pressure gas and brought into contact with a catalyst containing at least one metal selected from ruthenium, rhodium, and palladium to decompose ammonia; a combustion step of adding an oxygen-containing gas to the gas obtained in the second ammonia decomposition step, which contains at least nitrogen and hydrogen and further contains 1000 ppm or less of ammonia, and burning the gas to generate a combustion gas; The method for producing hydrogen, wherein the ammonia is heated by the combustion gas in the first ammonia decomposition step.

2. 2. The method for producing hydrogen according to claim 1, wherein in the hydrogen separation step, at least a portion of the hydrogen is extracted at a pressure of 0.5 MPa or more.

3. 3. The method for producing hydrogen according to claim 1, wherein the ammonia flowing into the heat exchange reactor is preheated by the combustion gas used to heat the ammonia in the first ammonia decomposition step.

4. A facility for producing hydrogen by decomposing ammonia, a first ammonia decomposition section that heats ammonia supplied at a pressure of 1.5 MPa or more and 7 MPa or less and brings the ammonia into contact with a catalyst containing ruthenium at a temperature of 400°C or more and 550°C or less to decompose the ammonia into nitrogen and hydrogen; a hydrogen separation unit that separates hydrogen from the gas obtained in the first ammonia decomposition unit by passing the gas through a membrane that selectively permeates hydrogen at a temperature of 300°C or higher and 550°C or lower; a pressure reducing section that reduces the pressure of the gas containing nitrogen, hydrogen, and unreacted ammonia after hydrogen separation to 0.3 MPa or less; a second ammonia decomposition section that brings the gas obtained by adding an oxygen-containing gas to the decompressed gas into contact with a catalyst containing at least one metal selected from ruthenium, rhodium, and palladium to decompose ammonia; a combustion unit that burns a gas obtained by adding an oxygen-containing gas to a gas containing at least nitrogen and hydrogen and further containing 1000 ppm or less of ammonia obtained in the second ammonia decomposition unit to generate a combustion gas, The hydrogen production facility, wherein the ammonia in the first ammonia decomposition section is heated by the combustion gas.

5. 5. The hydrogen production facility according to claim 4, wherein at least a portion of the hydrogen is extracted at a pressure of 0.5 MPa or more in the hydrogen separation section.

6. 6. The hydrogen production facility according to claim 4, wherein the ammonia to be supplied to the first ammonia decomposition section is preheated by the combustion gas used to heat the ammonia in the first ammonia decomposition section.

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