Method and apparatus for producing hydrogen from ammonia

Abstract

The present invention relates to a method and an apparatus for producing hydrogen, in which a first ammonia-containing input material (3, 3') is fed to a first decomposition reactor (R1) heated by introducing an energy carrier (13), thereby catalytically decomposing ammonia into hydrogen and nitrogen, and obtaining a hot first decomposition gas (6, 6') containing hydrogen and nitrogen, characterized in that a second ammonia-containing input material (4, 2) is converted in a second decomposition reactor (R2) into a second decomposition gas (7, 3') containing hydrogen and nitrogen, and the hot first decomposition gas (6, 6') is used to heat the second decomposition reactor (R2) and is cooled at the same time.

Description

[Technical field]

[0001] The present invention relates to a method for producing hydrogen, in which a first ammonia-containing input material is fed to a first decomposition reactor heated by introducing an energy carrier, thereby catalytically decomposing the ammonia into hydrogen and nitrogen, and obtaining a hot first decomposition gas containing hydrogen and nitrogen.

[0002] Furthermore, the invention also relates to a device for carrying out the method according to the invention.

[0003] The production of hydrogen by catalytic decomposition of ammonia has been known and has been in the prior art for many years. 2NH3⇔N2+3H2 The reaction is endothermic (ΔH=46.2 kJ / mol). The position of the equilibrium and the reaction rate depend strongly on the pressure and temperature, as well as the type of catalyst used.

[0004] In particular, when hydrogen is required in the case of, for example, heat treatment of metals, 3 In the relatively few industrial applications, less than 10000 m3 / h, ammonia decomposition is carried out in a decomposition reactor, where electricity generated elsewhere is used to heat the reactor, which in principle can produce hydrogen without liberating carbon dioxide.

[0005] To produce higher percentages of hydrogen from ammonia, plants similar to those used today for large-scale synthesis gas production by steam reforming of hydrocarbons can be used. Such plants include a cracking reactor, also called a steam reformer, with a combustion chamber in which cracking tubes filled with catalytic material are placed, and a waste heat recovery system. The combustion chamber is heated by one or more burners powered by the incoming fuel, providing energy for the endothermic cracking of ammonia, which is fed through the cracking tubes.

[0006] Ammonia decomposition is preferably carried out at a pressure of 20-30 bar so that the hydrogen produced can be sent out as product without or with only a small compression effort. This is easier since the ammonia specified for decomposition is usually available in liquid form, so that the pressure can be increased with only little energy. Under these conditions, in order to obtain a sufficiently high degree of conversion of the ammonia used to be economically meaningful, it is necessary to carry out the decomposition at high temperatures of about 500-1000 °C.

[0007] The cracked gas consists mostly of hydrogen and nitrogen, but may also contain unconverted ammonia and water, which is either already present in the ammonia-containing feedstock or is additionally introduced into the cracking reactor as a temperature moderator, but does not participate in the cracking reaction.

[0008] To obtain hydrogen, the cracked gas is fed to a separator, where it is treated, preferably by pressure swing adsorption, with most of the unconverted ammonia removed, to produce a hydrogen fraction containing almost no nitrogen, as well as a residual gas consisting mostly of nitrogen and containing ammonia. The hydrogen fraction can be sent off as product, while the residual gas can be combusted, for example to ignite the cracking reactor.

[0009] Just as the flue gases coming from the burner of the decomposition reactor can only use their heat to a certain extent for the decomposition reaction, the decomposition gases leave the decomposition reactor at a high temperature and with a high residual heat capacity. In order to fully utilize the energy used for ammonia decomposition as much as possible and to enable efficient hydrogen production, attempts are made to use the residual heat of the gases in the process for preheating the input materials such as ammonia and burner air or for generating steam. Since no or very little process steam is required during ammonia decomposition, the steam generated by the residual heat is either exported or used to generate electricity, for example by means of an "organic Rankine cycle".

[0010] However, generating electricity requires a huge engineering effort, and there are not always buyers who can economically export the steam.

[0011] It is therefore an object of the present invention to provide a method and an apparatus of a particular type which makes it possible to produce less water vapor as a by-product during the production of hydrogen from ammonia than was possible according to the prior art.

[0012] This problem is solved in the process according to the invention in that a second ammonia-containing feedstock is converted in a second decomposition reactor into a second decomposition gas containing hydrogen and nitrogen, and the hot first decomposition gas is used to heat the second decomposition reactor and is then cooled.

[0013] The method according to the present invention utilizes a larger portion of the thermal energy input for heating the first decomposition reactor for decomposing ammonia, resulting in a higher hydrogen yield and / or a lower specific energy consumption compared to the prior art. Furthermore, the amount of steam generated during waste heat utilization is also reduced.

[0014] To support the ammonia decomposition, the first decomposition reactor includes a catalytic material through which the first ammonia-containing feed gas passes. Preferably, the ammonia decomposition in the second decomposition reactor is also catalytically supported, so that the second decomposition reactor may be implemented using the same catalytic material as the first decomposition reactor. However, the second decomposition reactor may also use other catalytic materials that have the same activity as the catalytic material used in the first decomposition reactor, for example at low temperatures.

[0015] In a preferred variant of the process according to the invention, both the first ammonia-containing feedstock and the second ammonia-containing feedstock are taken from an external ammonia source, for example an ammonia store, both ammonia sources being different or the same. In this case, the first separation gas comprising hydrogen and nitrogen is preferably combined upstream or downstream of the second separation reactor with the second separation gas comprising hydrogen and nitrogen into a separation gas stream, which is then reprocessed, for example to obtain a hydrogen product. However, it should not be excluded to continue and process the first and second separation gases separately.

[0016] In another preferred variant of the process according to the invention, a second ammonia-containing feedstock is taken from an external ammonia source and the second separation gas obtained in the second separation reactor is used to form the first ammonia-containing feedstock. Advantageously, the total amount of the second cracked gas is used to form the first ammonia-containing feedstock and is fed unchanged or after mixing with a further stream of material containing ammonia, for example taken from an external ammonia source, as the first ammonia-containing feedstock to the first cracking reactor for conversion into the first cracked gas.

[0017] In a development of the method according to the invention, it is proposed that the cracked gas cooled during the heating of the second cracking reactor, optionally after mixing with the second cracked gas, is sent to a separation step, in which a hydrogen-rich product gas and a nitrogen-rich residual gas containing combustible substances are produced. In addition, after further cooling, the cracked gas is preferably treated by a pressure swing adsorber, which produces a hydrogen fraction containing almost no nitrogen and a residual gas containing combustible substances consisting mainly of nitrogen. The further cooling is carried out, for example, by means of an adjustable air cooler, preferably only up to just before the upper dew point, so that in addition to water, no condensed water containing unconverted ammonia is produced. If the production of condensed water during the further cooling cannot be avoided, it is also possible to return all or part of the condensed water and feed it to one of the two cracking reactors. The hydrogen fraction can be sent out as product, while the residual gas is either combusted, for example to heat the first cracking reactor, or sent to further separation steps in which additional product hydrogen and / or a commercially usable nitrogen fraction having product purity, as well as an ammonia fraction that can be used as input feedstock, are produced.

[0018] Due to its high nitrogen proportion, the residual gas has a relatively low calorific value. In order to be able to generate a sufficiently hot and large amount of flue gas for the heating of the first cracking reactor, in which the flue gas residual heat can be utilized using a corresponding small and inexpensive heat exchanger, it is proposed to provide an oxygen-rich mass stream from an oxygen source, which is used in the combustion of the residual gas, either directly or after mixing with air as oxidant.

[0019] Oxygen-rich in the sense of the present invention refers to an oxygen stream that consists of more than 25% oxygen, but preferably has a higher oxygen content, for example more than 99%.

[0020] The oxygen source may be, for example, a cryogenic air separator. It is also conceivable to use as an oxygen source an electrolyzer which electrochemically splits water and produces a hydrogen-rich as well as an oxygen-rich stream. At least a portion of the oxygen-rich stream can be used in the combustion of the residual gas, either directly or after mixing with air as oxidant, while the hydrogen-rich stream can be used as fuel to ignite the first decomposition reactor and / or to replenish the amount of hydrogen produced by ammonia decomposition. The electrolyzer can produce the two streams at the same or different pressures. Advantageously, both the oxygen-rich and the hydrogen-rich streams are produced at a sufficiently high pressure so that each of the two streams can be supplied to further applications without the use of a compressor.

[0021] To avoid overheating, the cracking reactor is adapted to be cooled with steam and / or nitrogen and / or ammonia during shutdown operation.

[0022] The present invention further relates to an apparatus for producing hydrogen from ammonia, comprising a first decomposition reactor that can be heated via an energy carrier incorporated therein, to which a first ammonia-containing input feedstock can be fed, thereby catalytically decomposing the ammonia and obtaining a hot first decomposition gas containing hydrogen and nitrogen.

[0023] The set problem is solved in that, on the apparatus side, the apparatus comprises a second decomposition reactor connected to the first decomposition reactor, and a second ammonia-containing input material can be fed to the decomposition reactor, thereby decomposing ammonia to obtain a second decomposition gas containing hydrogen and nitrogen, and the hot first decomposition gas can be used to heat the second decomposition reactor while forming a cooled first decomposition gas.

[0024] The first cracking reactor is preferably configured similarly to a steam reformer or autothermal reformer, heatable by electric current and / or burners, used for large-scale synthesis gas production, while in a preferred variant of the invention the second cracking reactor is implemented similarly to a reactor known to those skilled in the art as a gas-heated reformer or GHR, which can also be used for synthesis gas production. A GHR comprises reaction tubes through which hot heated gas flows, through which the input material to be converted is passed. The reaction tubes of the second cracking reactor, implemented for example as bayonet tubes, preferably contain catalytic material supporting the ammonia cracking.

[0025] In a development of the device according to the invention, it is proposed that the second decomposition reactor is implemented with the same or a different catalytic material as the first decomposition reactor.

[0026] Furthermore, a variant of the invention is provided with a distribution device connected to the first and second decomposition reactors and to an external ammonia source, via which a stream of material containing ammonia can be obtained from the external ammonia source and distributed to the two decomposition reactors as the first and second ammonia-containing input feedstock. Preferably, neither of the two decomposition reactors is connected to a further external ammonia source.

[0027] In another variant of the present invention, the first decomposition reactor is connected to the second decomposition reactor, so that the decomposition gas obtained in the second decomposition reactor can be used to form the first ammonia-containing input material.In this variant, preferably, only the second decomposition reactor is connected via a supply line to an external ammonia source that provides the second ammonia-containing input material.However, it should not be excluded to connect the first decomposition reactor to the same or another external ammonia source, so that a stream containing ammonia can be taken in and mixed with the second decomposition gas obtained in the second decomposition reactor to form the first ammonia-containing input material.

[0028] Preferably, the apparatus according to the invention includes a separation device, which allows hydrogen to be separated from the cracked gas while obtaining a residue gas containing nitrogen-rich combustibles. Preferably, the separation device comprises a pressure swing adsorber, which separates the hydrogen fraction from the cracked gas and delivers it as a product according to its purity and pressure, which can be supplied to a buyer via a product gas line. Nitrogen and ammonia present in the cracked gas can be removed from the pressure swing adsorber as a residue gas.

[0029] The separation apparatus may include a second pressure swing adsorber configured to separate a product purity nitrogen fraction from a residual gas generated during production of the hydrogen fraction. Alternatively, the separation apparatus may be connected to a combustor into which the residual gas can be fed and combusted to provide heat, for example to heat the first cracking reactor.

[0030] In order to be able to effectively combust the residual gas, a preferred variant of the device according to the invention has an oxygen source connected to the combustion device, from which an oxygen-rich substance stream can be taken and used in the combustion of the residual gas, either directly or after mixing with air as oxidant. Particularly preferably, the oxygen source is an electrolyser capable of electrochemically splitting water and producing a hydrogen-rich substance stream as well as an oxygen-rich substance stream. The electrolyser can comprise, for example, a solid oxide electrolysis cell capable of splitting water vapor at an operating temperature of 500° C. to 850° C. and producing a hot oxygen-rich substance stream.

[0031] Advantageously, the electrolyzer used as oxygen source is not only connected to a combustion device but also to a separation device, so that the hydrogen-rich stream which can be produced by the electrolyzer can be used to supplement the amount of the hydrogen fraction produced by ammonia decomposition which can be sent as product. If the composition of the hydrogen-rich stream does not meet the requirements set for the hydrogen product, a purification device for separating impurities can be arranged between the electrolyzer and the separation device. Alternatively, the electrolyzer can be connected to a separation device upstream of a pressure swing adsorber used to produce the hydrogen fraction, in which the impurities contained in the hydrogen-rich stream are removed. [Brief description of the drawings]

[0032] In the following, the invention will be explained in more detail on the basis of two exemplary embodiments which are illustrated diagrammatically in FIG. 1 and FIG.

[0033] [Figure 1] FIG. 1 illustrates a preferred embodiment of the present invention, where the ammonia-containing feedstock fed to the cracking reactor has the same composition.

[0034] [Diagram 2] FIG. 2 illustrates another preferred embodiment of the present invention, where the ammonia-containing feedstock fed to the decomposition reactor has a different composition.

[0035] In both figures, identical components are labeled with the same reference numbers.

[0036] In the embodiment of FIG. 1 , an ammonia-containing stream is removed from an external ammonia source (not shown) via line 1, fed to heat recovery D, exits heat recovery D in a heated state via line 2 and is split in a flow divider S into a first ammonia-containing feedstock 3 and a second ammonia-containing feedstock 4.

[0037] The first ammonia-containing input material is fed to a first decomposition reactor R1 having the same structure as a steam reformer, and fed into the decomposition tube R, which is heated by heat 5 generated in a combustion device B. The catalyst decomposes most of the fed ammonia at a temperature of 500-1000°C and a pressure of 5-50 bar, so that a hot first decomposition gas 6, which is mostly composed of nitrogen and hydrogen, but also contains water and unconverted ammonia, can be taken out of the first decomposition reactor R1.

[0038] A second ammonia-containing feedstock 4, having the same composition as the first feedstock 3, is fed to a second cracking reactor R2, which has the same structure as the gas-heated reformer, and fed into its cracking tubes G, which are heated by the hot first cracking gas 6. By means of the catalyst, a large part of the fed ammonia is decomposed, so that a cracking gas 7 containing unconverted ammonia, consisting mostly of nitrogen, and a cooled first cracking gas 8 can be taken from the second cracking reactor R2. Both cracking gases 7 and 8 are then combined into a cracking gas stream 9, which is cooled in a heat recovery unit D against ammonia-containing material stream 1 fed from an external ammonia source, before reaching a separation unit T via line 10, where it is treated, for example by a pressure swing adsorber, to obtain a hydrogen fraction 11 and a residual gas 12 with product purity. The residual gas 12, which consists mostly of nitrogen but also contains other combustible components such as ammonia and hydrogen, is fed together with fuel 13 taken from an external source (not shown) to a combustion device B where it is burned with an oxidant 14, which may be air or oxygen-enriched air or technically pure oxygen, to heat the first decomposition reactor R1.

[0039] In contrast to the embodiment of FIG. 1, in the embodiment of FIG. 2 the heated ammonia-containing stream 2 is not split but is fed in its entirety as a second ammonia-containing feedstock to the second cracking reactor R2 and converted into a second cracked gas. Via line 3' this second cracked gas is fed as a first ammonia-containing feedstock to the cracking tube R of the first cracking reactor G1, from where a hot first cracked gas 6' containing unconverted ammonia, consisting mostly of nitrogen and hydrogen, is taken off and transferred to heat the second cracking reactor R2. The first cracked gas 8' cooled during heating is then fed to heat recovery D and further processed according to the cracked gas stream 9 of FIG. 1.

Claims

1. A method for producing hydrogen, wherein a first ammonia-containing feedstock (3, 3') is supplied to a first decomposition reactor (R1) heated by incorporating an energy carrier (13), whereby ammonia is decomposed into hydrogen and nitrogen by a catalyst to obtain a high-temperature first decomposition gas (6, 6') containing hydrogen and nitrogen. In the method, a second ammonia-containing feedstock (4, 2) is converted into a second decomposition gas (7, 3') containing hydrogen and nitrogen in a second decomposition reactor (R2), and the high-temperature first decomposition gas (6, 6') is used to heat the second decomposition reactor (R2) and is cooled at this time.

2. The method according to claim 1, characterized in that a catalyst is used in the second decomposition reactor (R2) to convert the second ammonia-containing feedstock (4, 2).

3. The method according to claim 1 or 2, characterized in that the first ammonia-containing feedstock (3) and the second ammonia-containing feedstock (4) are taken from a common ammonia source.

4. The method according to claim 1 or 2, characterized in that the second decomposition gas (3') obtained in the second decomposition reactor (R2) is used for the formation of the first ammonia-containing feedstock.

5. At least the cooled first decomposition gas (8, 8') is sent to a separation step, in which a hydrogen-rich product gas (11) and a nitrogen-rich residual gas (12) containing combustible substances are produced. The method according to claim 1 or 2, characterized in that

6. The method according to claim 5, characterized in that the residual gas (12) is burned to provide the heat (5) used for operating the first decomposition reactor (R1).

7. The method according to claim 5, characterized in that a nitrogen fraction having product purity is separated from the residual gas (12).

8. The method according to claim 6, characterized in that an oxygen-rich material stream is supplied from an oxygen source and is used as an oxidant (14) during the combustion of the residual gas after directly mixing the oxygen-rich material stream or mixing it with air.

9. An apparatus for generating hydrogen from ammonia, comprising a first decomposition reactor (R1) that can be heated via an introduced energy carrier (13), and capable of supplying a first ammonia-containing feedstock (4, 3') to the first decomposition reactor (R1), so as to decompose ammonia by a catalyst to obtain a high-temperature first decomposition gas (6, 6') containing hydrogen and nitrogen. In the apparatus, a second decomposition reactor (R2) connected to the first decomposition reactor (R1) is provided, and capable of supplying a second ammonia-containing feedstock (4, 2) to the second decomposition reactor (R2), so as to decompose ammonia to obtain a second decomposition gas (7, 3') containing hydrogen and nitrogen, and the second decomposition reactor can be heated while forming a cooled first decomposition gas (8, 8') using the high-temperature first decomposition gas (6, 6').

10. The apparatus according to claim 9, characterized in that the second decomposition reactor (R2) is provided with a catalyst for assisting ammonia decomposition.

11. The apparatus according to claim 9 or 10, characterized in that the apparatus is provided with a diverter (S) connected to the first decomposition reactor (R1), the second decomposition reactor (R2), and an ammonia source, for forming the first ammonia-containing feedstock (3) and the second ammonia-containing feedstock (4).

12. The apparatus according to claim 9 or 10, characterized in that the first decomposition reactor (R1) is connected to the second decomposition reactor (R2), whereby the second decomposition gas (3') obtained in the second decomposition reactor (R2) can be used for forming the first ammonia-containing feedstock.

13. The apparatus according to claim 9 or 10, characterized in that the apparatus is provided with a separation device (T), and in the separation device (T), a hydrogen-rich product gas (11) and a nitrogen-rich residual gas (12) containing combustible substances can be generated from at least the cooled first decomposition gas (8, 8').

14. The apparatus according to claim 13, characterized in that the apparatus is provided with a combustion device (B) connected to the separation device (T) and the first decomposition reactor (R1), whereby the residual gas (12) is combustible to provide heat (5) used for operating the first decomposition reactor (R1).

15. The device according to claim 14, characterized in that it comprises an oxygen source connected to the combustion device (B), extracts oxygen from the oxygen source, and can use the oxygen, either directly or after mixing with air, as an oxidant (14) during combustion of the residual gas.

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