Method and apparatus for producing hydrogen from ammonia

Abstract

The present invention relates to a method and an apparatus for producing a hydrogen product (6) from ammonia, which comprises converting an ammonia-containing input feedstock (1) into a hydrogen- and nitrogen-containing cracking gas (3) using a burner-fired cracking furnace (S) equipped with a catalyst carrier, from which the hydrogen product (6) is separated to obtain a nitrogen-rich residual gas (7) containing combustible substances, at least a part of which is combusted to ignite the cracking furnace (S), characterized in that an oxygen-rich material stream (8) is supplied from an oxygen source (E) and used as an oxidant for the combustion of the residual gas (7) either directly or after mixing with air (13).

Description

[Technical field]

[0001] The present invention relates to a method for producing a hydrogen product from ammonia, which comprises converting an input feedstock containing ammonia into a cracked gas containing hydrogen and nitrogen using a burner-fired cracking furnace equipped with a catalyst support, separating the hydrogen product therefrom to obtain a nitrogen-rich residual gas containing combustible materials, and combusting at least a portion of the residual gas to ignite the cracking furnace.

[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 For relatively small industrial applications, less than 1000 m3 / h, it is economically advantageous to decompose ammonia in an electrically heated decomposition furnace, where, in principle, hydrogen can be obtained without liberating carbon dioxide.

[0005] To produce a higher percentage 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 furnace with a combustion chamber in which cracking tubes filled with catalytic material are placed, and a waste heat recovery system. This combustion chamber is heated by one or more burners and provides energy for the endothermic decomposition of ammonia, which is fed through the cracking tubes. The flue gases coming from the burners leave the combustion chamber at high temperatures and with a large amount of residual heat, since only a small part of their sensible heat can be delivered to the separation tubes. To enable efficient hydrogen production, the hot flue gases, as well as the hot cracking gases leaving the cracking tubes, are used to preheat the input materials, such as ammonia and burner air, and to generate steam if necessary. Since no or very little steam is used in the process itself during ammonia cracking, the generated steam is either exported or used to generate electricity, for example by means of an "organic Rankine cycle".

[0006] Ammonia decomposition is preferably carried out at a pressure of 10-40 bar so that the hydrogen produced can be delivered as product without or with only a small compression operation. This is all the more easily achieved since the pressure of the input feedstock containing ammonia, which is usually present in liquid form, can be increased with a small energy input. 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 a temperature of 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 feed or is additionally introduced into the cracking tube as a temperature moderator, but does not participate in the cracking reaction.

[0008] To obtain hydrogen, the cracked gas is fed to a separator, in which it is treated, preferably by pressure swing adsorption, after removing most of the unconverted ammonia and separating the water it contains, to produce a hydrogen fraction that is almost free of nitrogen, as well as a residual gas that is mostly composed of nitrogen and contains ammonia. The hydrogen fraction can be sent out as product, while the residual gas is sent back and combusted to ignite the cracking furnace, using air as oxidant.

[0009] Due to the high nitrogen content, the residual gas has a relatively low calorific value, so that ignition of the cracking furnace requires the addition of other fuels, such as ammonia, if necessary. Furthermore, the high nitrogen content produces large amounts of flue gas, the residual heat of which cannot be utilized in the waste heat recovery system without the use of relatively large and expensive heat exchangers.

[0010] The object of the present invention is to provide a method and an apparatus of a particular kind which allows the production of hydrogen from ammonia more economically than was possible according to the prior art.

[0011] The set task is achieved in the process by providing an oxygen-rich substance stream from an oxygen source and using it in the combustion of the residual gases, either directly or after mixing with air as oxidant.

[0012] 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%.

[0013] Since the oxidizing agent used according to the invention has a lower nitrogen percentage than air, the fuel requirement for heating the cracking furnace and the amount of flue gas generated are both reduced compared to the prior art. Thus, due to the use of residual heat from the flue gas, the waste heat recovery system can be implemented more cost-effectively with smaller heat exchangers or, for example, fewer heat exchangers if steam generation can be omitted. The high nitrogen content makes it possible to burn the residual gas using conventional burners, such as those also used for residual gas combustion with air, independent of the oxygen content of the oxidizing agent used.

[0014] 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 for ignition of the cracking furnace 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.

[0015] The separation of hydrogen from the cracked gas obtained in the cracking furnace takes place in a separator which provides a hydrogen fraction which can be sent as product and a residual gas containing nitrogen-rich combustibles. To increase the amount of hydrogen produced by ammonia decomposition, a hydrogen-rich stream produced by the electrolyzer can be added to the hydrogen fraction downstream of the separator either directly or after removing water and other impurities. However, it is also possible to transfer the hydrogen-rich stream from the electrolyzer to the separator and process it together with the cracked gas to a hydrogen fraction which can be sent as product.

[0016] Since hydrogen increases the heating value of the fuel bulk fed to the cracker, thereby reducing flue gas emissions, trace amounts of hydrogen may be left in the electrolyser's oxygen-rich stream.

[0017] An advantageous embodiment of the method according to the invention is such that the electrolyser is operated in such a way that the amount of oxygen produced is always equal to the current oxygen requirement of the cracking furnace. If the oxygen output of the electrolyser cannot be adapted to the oxygen requirement of the cracking furnace quickly enough, the excess oxygen produced is transferred to an intermediate storage chamber, while in the event of a shortage, oxygen is withdrawn from the intermediate storage chamber and / or compensated for by supplying ambient air.

[0018] The electrolyser used according to the invention may comprise a solid oxide electrolysis cell which decomposes water vapor at a high operating temperature of 500° C. to 950° C. In the electrolysis cell, an electrolyte, consisting of a material conductive to oxygen ions, separates the cathode compartment from the anode compartment. The water vapor conveyed to the cathode compartment is decomposed at the interface with the electrolyte into hydrogen, which remains on the cathode side, and oxygen ions which migrate to the anode side, where they are oxidized to molecular oxygen. To keep the oxygen partial pressure low and to protect the materials used from oxidation, the anode compartment is flushed with air. The solid oxide electrolysis cell therefore provides a hot oxygen-containing material stream, which is oxygen-enriched air. Preferably, the oxygen-containing material stream is used for the oxidant formation according to the invention without further treatment, in particular without cooling.

[0019] In another electrolysis cell according to the invention which can be used and which operates at relatively high temperatures, the electrolyte consists of a ceramic proton-conducting membrane, in which water supplied in the form of steam is split at the anode side, the hydrogen ions which are generated diffuse to the cathode side and, after formation of hydrogen molecules, are withdrawn in a dry stream, while at the anode an oxygen-rich stream containing water is produced.

[0020] Further electrolysis cells capable of operating at low temperatures are known to those skilled in the art as proton exchange cells, anion exchange cells or alkaline electrolysis cells and are likewise suitable for use in accordance with the present invention.

[0021] In ammonia decomposition, a part of the used ammonia always reaches the cracked gas without being converted, and it is necessary to separate it in order to obtain hydrogen products therefrom. The amount of unconverted ammonia increases with decreasing reaction temperature and increasing reaction pressure. If the ammonia content of the cracked gas is low, ammonia is preferably separated only by pressure swing adsorption and utilized as heat together with the residual gas. However, if the ammonia content is high, it may be economical to additionally remove ammonia from the cracked gas upstream of pressure swing adsorption and utilize it as material.

[0022] In a development of the method according to the invention, it is proposed to cool the cracked gases below the dew point of water, thereby condensing the water and scrubbing the ammonia from the cracked gases with the condensed water.

[0023] When the cracked gas is cooled to a temperature between 30°C and 70°C, the amount of water introduced into the cracking tube together with ammonia as a temperature moderator is sufficient to reduce the portion of the ammonia remaining in the cracked gas to a level that allows the ammonia content to be separated by pressure swing adsorption without economical disadvantage. At least in normal operation, there is no need to provide a separate wash water supply.

[0024] Preferably, the ammonia / water mixture obtained from the water scrubbing is used again in the cracking furnace, with a majority of the mixture being sent to the cracking tube for hydrogen production and the remaining portion being discharged in a controlled manner to adjust the amount of water carried into the cracking tube, e.g., to ignite the cracking furnace.

[0025] In this procedure, there is a possibility that not enough water is returned to the cracking furnace during start-up to sufficiently reduce the ammonia content in the cracked gas by condensed water scrubbing, in which case, according to the invention, additional water is provided from the outside for water scrubbing.

[0026] To avoid overheating of the cracking furnace during shut-down operation, the cracking furnace is cooled with steam or nitrogen.

[0027] In order to allow a more efficient use of the energy used in the process, in one embodiment of the process according to the invention, the hot cracked gas obtained in the cracking furnace is used to heat a cracking reactor in which further cracked gas is formed by decomposing ammonia into hydrogen and nitrogen. The cracked gas formed in the cracking reactor from the input material containing some or all of the ammonia is further processed in the cracking tubes of the cracking furnace or is combined with the cooled cracked gas during the heating of the cracking furnace.

[0028] The present invention further relates to an apparatus for producing a hydrogen product from ammonia, the apparatus comprising at least one burner for catalytically converting an ammonia-containing input feedstock into a cracking gas comprising hydrogen and nitrogen, a separation device capable of separating hydrogen from the cracking gas to obtain a residual gas comprising nitrogen-rich combustible matter, and a return device for returning at least a portion of the residual gas so that it can be combusted by the at least one burner to ignite a cracking furnace.

[0029] According to the invention, the set out problem is solved on the part of the device in that it comprises an oxygen source connected to at least one burner, from which an oxygen-rich substance stream can be taken and used for the combustion of the residual gases, either directly or after mixing with air as oxidant.

[0030] Preferably, the oxygen source is an electrolyser capable of electrochemically splitting water to produce a hydrogen-rich stream as well as an oxygen-rich stream. The electrolyser may comprise a solid oxide electrolysis cell capable of splitting water vapor at an operating temperature of 500°C to 850°C to produce a high temperature oxygen-rich stream.

[0031] The separation unit preferably comprises a pressure swing adsorber which separates the hydrogen fraction from the cracked gas and delivers it as a product due 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.

[0032] Advantageously, the electrolyzer used as oxygen source is not only connected to at least one burner of the cracking furnace, 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 allocated 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 the pressure swing adsorber, and the impurities contained in the hydrogen-rich stream can be removed by the pressure swing adsorber.

[0033] As long as the ammonia content does not exceed a limit value, the separation of nitrogen and ammonia from the cracked gas can be economically carried out by a pressure swing adsorber. Advantageously, the cracked gas upstream of the pressure swing adsorber is not cooled to the dew point in order to prevent liquid from entering the pressure swing adsorber. For this purpose, the device according to the invention can be equipped with, for example, an adjustable air cooler to control the cooling of the cracked gas.

[0034] If the ammonia content of the cracked gas exceeds a limit value, it is proposed to arrange a water scrubbing upstream of the pressure swing adsorber, which can advantageously reduce the ammonia content of the cracked gas below the limit value while scrubbing away the ammonia and forming an ammonia / water mixture, and it is further proposed to connect the water scrubbing to the cracking furnace, which can return at least a portion of the ammonia / water mixture to the cracking tube.

[0035] Preferably, the water scrubbing is equipped with a cooling device, by which the cracked gas can be cooled to below the dew point of water, so that the water can be condensed and the ammonia can be washed out of the cracked gas by the condensed water. Particularly preferably, the water scrubbing is carried out with a cooling device capable of cooling the cracked gas to a temperature of 30°C to 70°C without an additional washing water supply device.

[0036] The apparatus according to the present invention may comprise a decomposition reactor connected to the decomposition furnace for decomposing ammonia, the decomposition reactor being heatable by hot decomposition gas obtained in the decomposition furnace. [Brief description of the drawings]

[0037] In the following, the invention will be explained in more detail on the basis of an embodiment which is diagrammatically shown in FIG.

[0038] [Figure 1] FIG. 1 illustrates the production of hydrogen from ammonia according to a preferred embodiment of the present invention, where an electrolyser comprising a solid oxide electrolysis cell is used as the oxygen source.

[0039] Via line 1, the cracking furnace S is fed with a water-containing feedstock, consisting mostly of ammonia, which is fed into a cracking tube R, which is heated by heat 2 generated by a burner B. By means of a catalyst, most of the fed ammonia is decomposed at a temperature of 500-1000 ° C, whereby a hot cracked gas 3, consisting mostly of nitrogen and hydrogen, containing water and unconverted ammonia, is taken from the cracking tube R and can be transferred to a separation device T. In a water scrubbing W belonging to the separation device T, the cracked gas 3 is cooled to a temperature of 30 ° C-70 ° C, during which the water condenses and scrubs off most of the ammonia. The ammonia / water mixture 4 formed here is returned to the cracking tube R in order to increase the hydrogen yield of the process, while the cracked gas 5, which is almost free of water and ammonia, is sent to a pressure swing adsorber D, where it is separated into a hydrogen fraction 6 with product purity and a residual gas 7. The residual gas 7, which is mostly nitrogen but also contains other combustible components such as ammonia and hydrogen, is fed as fuel to the burner B and burned together with the oxidizer 8. For the cracking tube R, the residual heat of the cooled flue gas 15 of the burner B is utilized in a waste heat recovery system (not shown) for example for the vaporization and residual heat of the input material 1.

[0040] An oxidant 8, which has an oxygen content higher than that of air, is taken from an electrolyser E, which serves as an oxygen source. The electrolyser E, which is equipped with a solid oxide electrolysis cell, decomposes water vapor at an operating temperature between 500 ° C. and 950 ° C. In the electrolysis cell, an electrolyte, made of a material conductive to oxygen ions, separates the cathode compartment K from the anode compartment A. The water vapor 9 brought into the cathode compartment K is decomposed at the interface with the electrolyte M into hydrogen, which remains on the cathode side, and oxygen ions, which move to the anode side, where they are oxidized to molecular oxygen. The anode compartment A is flushed with air 10, and the resulting hot oxidant 8 is fed uncooled to the burner B. A hydrogen-rich stream 11 is withdrawn from the cathode compartment K and mixed with the hydrogen fraction 6 with product purity to produce hydrogen 12. Optionally, the burner B is fed with air 13 as further oxidant and / or ammonia 14 as auxiliary fuel.

Claims

1. 1. A process for producing a hydrogen product (6) from ammonia, comprising converting an ammonia-containing input material (1) into a hydrogen- and nitrogen-containing cracking gas (3) by means of a burner-fired cracking furnace (S) equipped with a catalyst carrier, separating said hydrogen product (6) therefrom to obtain a nitrogen-rich residual gas (7) containing combustible substances, and combusting at least a part of said residual gas (7) for ignition of said cracking furnace (S), characterized in that an oxygen-rich stream (8) is supplied from an oxygen source (E) and used during the combustion of said residual gas (7), either directly or after mixing with air (13) as oxidant.

2. 2. The method according to claim 1, characterized in that an electrolytic cell (E) is used as an oxygen source, in which a hydrogen-rich substance stream (11) as well as an oxygen-rich substance stream (8) are obtained by electrochemical decomposition of water (9).

3. 3. The method according to claim 2, characterized in that the electrolyser (E) comprises a solid oxide electrolysis cell which is flushed on the anode side with air (10), and in that the air heated and enriched with oxygen during the flushing is used as oxidant (8) for combusting the residual gas (7).

4. 4. The method according to claim 2 or 3, characterized in that the hydrogen-rich stream (11) obtained in the electrolyser (E) is used as fuel for ignition of the cracking furnace (S) and / or to replenish the amount of hydrogen (6) produced by ammonia decomposition.

5. 3. The method according to claim 2, characterized in that a pressure swing adsorber (D) is used to separate the cracked gas (5).

6. 2. The method according to claim 1, characterized in that the cracked gas (3) is subjected to a water scrubbing (W) to remove ammonia not converted in the cracking furnace (S).

7. 7. The method according to claim 6, characterized in that the cracked gas (5) in the water scrubbing (W) is cooled to a temperature between 30°C and 70°C to condense water and scrub ammonia with the condensed water.

8. 2. The method according to claim 1, characterized in that the ammonia (4) washed off by water scrubbing (W) from the cracked gas (5) is returned to the cracking furnace (S).

9. 2. The method according to claim 1, characterized in that the hot cracked gas obtained in the cracking furnace is utilized to heat a cracking reactor, in which further cracked gas is formed by decomposing ammonia into hydrogen and nitrogen.

10. 1. An apparatus for producing a hydrogen product (6) from ammonia, comprising at least one burner (B) for catalytically converting an ammonia-containing input material (1) into a cracked gas (3) comprising hydrogen and nitrogen, a separation device (T) capable of separating said hydrogen (6) from said cracked gas (3) to obtain a residual gas (7) comprising nitrogen-rich combustible substances, and a return device capable of returning at least a portion of said residual gas (7) to be combusted by said at least one burner (B) to ignite said cracking furnace (S), characterized in that the apparatus comprises an oxygen source (E) connected to said at least one burner (B) from which an oxygen-rich stream (8) can be taken and used in the combustion of said residual gas (7), either directly or after mixing with air (13) as oxidant.

11. 11. The apparatus according to claim 10, characterized in that the oxygen source is an electrolyser (E) capable of electrochemically decomposing the water (9) and producing a hydrogen-rich material stream (11) as well as an oxygen-rich material stream (8).

12. 12. Apparatus according to claim 10 or 11, characterized in that the separation device (T) comprises a pressure swing adsorber (D).

13. 13. The apparatus according to claim 12, characterized in that the separation device (T) comprises a water scrubbing (W) arranged upstream of the pressure swing adsorber (D) for separating ammonia (4) from the cracked gas (3).

14. 14. The apparatus according to claim 13, characterized in that the water scrubbing (W) is carried out with a cooling device capable of cooling the cracked gas (3) to a temperature of 30° C. to 70° C. without an additional supply of washing water.

15. 11. Apparatus according to claim 10, characterized in that it comprises a decomposition reactor for decomposing ammonia, said decomposition reactor being heatable by the hot decomposition gas obtained in said decomposition furnace (S).

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