Method and device for hydrogen production with low carbon dioxide levels

Abstract

The invention relates to a method and a device for generating a hydrogen product (14) in which a carbon-containing feed (1) is reacted by reformation (R) and water gas conversion (S) in order to obtain a hydrogen- and carbon-dioxide-containing gas mixture (5), called synthesis gas, from which, in a gas scrub (G), carbon dioxide (18) is separated by an acid gas scrubbing agent (6) in order to obtain raw hydrogen (10), which is processed by way of at least one further method step (B) to form a hydrogen product (14), wherein a fuel (16) is combusted to obtain process heat and carbon dioxide (18) separated from the synthesis gas (5) is disposed of by sequestration or supplied to another material use. The invention is characterised in that during the gas scrub (G) a gas mixture (12) largely consisting of hydrogen is obtained from the synthesis gas (5), said gas mixture having a carbon dioxide content which is less than that of the synthesis gas (5) and greater than that of the raw hydrogen (10), and being provided to form the fuel (16).

Description

[0001] Description

[0002] Method and device for low-carbon hydrogen production

[0003] The invention relates to a process for producing a hydrogen product, in which a carbon-containing feed is converted by reforming and water gas conversion in order to obtain a gas mixture containing hydrogen and carbon dioxide, referred to as synthesis gas, from which carbon dioxide is separated in a gas scrubbing process by an acid gas scrubbing agent in order to obtain raw hydrogen, which is processed into the hydrogen product in at least one further process step, wherein a fuel is burned to obtain process heat and carbon dioxide separated from the synthesis gas is disposed of by sequestration or fed to a material use.

[0004] Furthermore, the invention relates to a device for carrying out the method according to the invention.

[0005] Hydrogen products such as pure hydrogen, which consists of at least 99.5 mol% hydrogen, or ammonia synthesis gas, which contains a 3:1 ratio of hydrogen to nitrogen, are still predominantly produced from carbon-based feedstocks, resulting in the formation and release of large amounts of climate-damaging carbon dioxide. However, there are increasing efforts to dispose of the resulting carbon dioxide through sequestration or material utilization rather than releasing it into the atmosphere.

[0006] According to the state of the art, to produce a hydrogen product, a carbon-containing feedstock, such as natural gas, is desulfurized if necessary and then reformed, for example, by partial oxidation, autothermal reforming, steam reforming, or a combination of two or more of these processes, into a gas mixture known as synthesis gas. This gas mixture consists largely of hydrogen, carbon monoxide, and carbon dioxide and may also contain other substances such as methane or argon. The synthesis gas is then subjected to water-gas conversion to convert the contained carbon monoxide with water to hydrogen and carbon dioxide, yielding a synthesis gas consisting largely of hydrogen and carbon dioxide.In a sour gas scrubber, carbon dioxide is separated from the synthesis gas using a sour gas scrubbing agent, resulting in a carbon dioxide fraction with a purity sufficient for its sequestration or material use and a hydrogen fraction known as raw hydrogen, which contains residues of carbon monoxide, carbon dioxide and methane and whose hydrogen content is usually around 98 mol%.

[0007] To form the hydrogen product, the raw hydrogen is treated, for example, by methanation, partial condensation, nitrogen scrubbing or pressure swing adsorption, whereby carbon monoxide and carbon dioxide are separated after conversion to methane or directly, and a largely carbon- and oxygen-free hydrogen fraction is produced, which forms the hydrogen product or from which, for example, by adding nitrogen, an ammonia synthesis gas is obtained as a hydrogen product.

[0008] Acid gas scrubbers utilize the ability of liquids known as acid gas scrubbers to selectively absorb carbon dioxide and other acid gases from a gas mixture and keep them in solution. Examples of acid gas scrubbers include methanol and slightly alkaline aqueous amine solutions. Since the raw synthesis gas is produced at a pressure level typically between 25 and 45 bar, the volume flows to be treated are small, allowing the synthesis gas to be treated with acid gas scrubbers at comparatively low investment and operating costs.

[0009] Carbon dioxide is also present in flue gases produced during the production of hydrogen products in reformers and / or furnaces, which are usually heated using fuels such as natural gas or carbon-containing residual gases. Unlike synthesis gas, flue gases are normally pressureless, contain oxygen, and are diluted by nitrogen from the combustion air. Since, compared to synthesis gases, their treatment by acid gas scrubbing is very costly and complex, they are usually released into the atmosphere along with the carbon dioxide formed during combustion. Using the methods described, no more than approximately 90% of the carbon used to produce the hydrogen product can be economically separated in the form of carbon dioxide and disposed of through sequestration or used as a material.However, depending on legal requirements and penalties on carbon dioxide emissions, it may be necessary to achieve higher carbon capture quotas.

[0010] The object of the present invention is therefore to provide a method of the generic type and a device for carrying it out, which allow carbon separation rates of more than 90% to be achieved at lower costs than in the prior art.

[0011] This object is achieved according to the invention in that, during the gas scrubbing, a gas mixture consisting largely of hydrogen is obtained from the synthesis gas, which gas mixture has a carbon dioxide content which is lower than that of the synthesis gas and higher than that of the raw hydrogen, and which is provided for the formation of the fuel.

[0012] The fuel obtained from the gas mixture, which consists largely of hydrogen, contains only small amounts of carbon compounds such as carbon dioxide, carbon monoxide, or methane, so that its combustion produces a largely carbon dioxide-free flue gas. Since the fuel replaces at least some of the carbon-containing fuels used in the process according to the state of the art, the amount of carbon dioxide released into the atmosphere during the production of the hydrogen product is reduced compared to the state of the art.

[0013] Preferably, enough gas mixture consisting largely of hydrogen is extracted from the synthesis gas to obtain a fuel quantity sufficient to fully cover the fuel requirements for hydrogen production. Particularly preferably, no more fuel is extracted than is needed to fully cover the fuel requirements for producing the hydrogen product. However, it is also possible to produce a larger quantity of fuel and export fuel that cannot be used in the process in return for credit. A gas scrubber is usually designed as an absorber column, to which the synthesis gas containing acid gases - particularly carbon dioxide - can be fed in its lower section and passed upwards via mass transfer elements such as sieve trays or packings.To separate the acid gases, an unladen acid gas scrubbing agent is fed into the absorber column in countercurrent and, with the aid of the mass transfer elements, brought into intensive contact with the synthesis gas. On its downward path, the acid gas scrubbing agent becomes enriched with carbon dioxide, while the carbon dioxide content of the synthesis gas continuously decreases on its upward path. The flow rates of synthesis gas and acid gas scrubbing agent are coordinated so that a largely carbon dioxide-free hydrogen fraction is obtained as raw hydrogen at the top of the absorber column, while the carbon dioxide-laden scrubbing agent can be withdrawn from the column bottom.Upstream of the first and downstream of the last mass transfer element of the sour gas scrubber (in the direction of flow of the synthesis gas), the gas mixture passing through the absorber column has a carbon dioxide content that is lower than that of the synthesis gas and higher than that of the raw hydrogen. To reduce scrubbing agent losses and to prevent scrubbing agent residues from entering equipment downstream of the sour gas scrubber along with the raw hydrogen, where they could cause disruptions, the hydrogen fraction withdrawn from the sour gas scrubber, largely free of carbon dioxide, is often subjected to water scrubbing to obtain the raw hydrogen and is also passed through mass transfer elements. The water scrubbing can be separated from the sour gas scrubber by a chimney tray and located within the absorber column or in a separate water scrubbing column.

[0014] The process according to the invention is preferably carried out using such a gas scrubber, wherein the gas mixture consisting largely of hydrogen is withdrawn from the absorber column via a side draw located below the uppermost mass transfer element of the acid gas scrubber and above the synthesis gas feed. Since carbon dioxide removal also takes place in the water scrubber, it may also be expedient to provide the side draw between the acid gas scrubber and the uppermost mass transfer element of the water scrubber. A slightly alkaline aqueous amine solution is preferably used as the acid gas scrubbing agent. By selecting the position of the side draw, it is thus possible to adjust the carbon dioxide concentration of the gas mixture consisting largely of hydrogen such that a predetermined carbon removal rate can be achieved with minimal investment and operating costs.Unlike the prior art, only a portion of the synthesis gas is processed into raw hydrogen through complete scrubbing, while the gas mixture required for fuel formation is obtained from the remaining synthesis gas through only incomplete scrubbing. Therefore, the acid gas scrubbing can be carried out with a smaller amount of scrubbing agent compared to the prior art. Particularly when the gas mixture, which consists largely of hydrogen, is withdrawn below the uppermost mass transfer element of the acid gas scrubbing, the saved acid gas scrubbing agent leads to a significant improvement in economic efficiency. The selection of the side draw position makes it possible to adjust the carbon dioxide contents of the raw hydrogen and the gas mixture, which consists largely of hydrogen, independently of one another.

[0015] In particular, if the required carbon separation quotas cannot be achieved by the direct use of the gas mixture consisting largely of hydrogen as fuel, one embodiment of the process according to the invention provides for carbon compounds such as in particular methane and carbon monoxide to be separated from the gas mixture consisting largely of hydrogen, preferably by membrane separation, pressure swing adsorption, cryogenic gas separation or a combination of at least two of these processes, and to be added to the carbon-containing feed via a return line.

[0016] Preferably, the fuel produced using the gas mixture consisting largely of hydrogen is burned to generate the steam required for the process or to preheat a feedstock for reforming. However, it is also possible to heat a steam reformer or a pre-reformer used for reforming with the fuel.

[0017] The process according to the invention is particularly suitable for use in ammonia synthesis, wherein the raw hydrogen is processed into an ammonia synthesis gas by methanation or nitrogen scrubbing or nitrogen addition.

[0018] The conversion of the carbon-containing feedstock can be carried out within the scope of the process according to the invention by partial oxidation, autothermal reforming, steam reforming, or a combination of two or more of these processes, which can be preceded by pre-reforming. The process according to the invention can be used with particular advantage when the carbon-containing feedstock is converted by autothermal reforming or partial oxidation. Unlike when a steam reformer is used, in these cases only small furnaces are heated, for example those used to heat feedstocks or to generate steam, releasing comparatively small amounts of flue gas into the atmosphere. In order to cover the fuel requirements of these furnaces with a fuel gas consisting largely of hydrogen, only a small proportion of the synthesis gas from the sour gas scrubber needs to be withdrawn as a gas mixture consisting largely of hydrogen.

[0019] Furthermore, the invention relates to a device for producing a hydrogen product, comprising a reforming device and a water gas converter connected to the reforming device, via which a carbon-containing feed can be converted into a synthesis gas containing hydrogen and carbon dioxide by reforming and water gas conversion, a gas scrubbing device comprising an acid gas scrubbing device for separating carbon dioxide from the synthesis gas and for obtaining raw hydrogen, which forms the hydrogen product or which can be processed into the hydrogen product in a processing device connected to the gas scrubbing device, and a combustion device in which a fuel can be combusted to obtain process heat, wherein the gas scrubbing device is connected or connectable to a device for sequestering or materially utilizing the carbon dioxide separated from the synthesis gas.

[0020] The stated object is achieved according to the invention in that the gas scrubber is designed to extract from the synthesis gas a gas mixture consisting largely of hydrogen, which has a carbon dioxide content which is lower than that of the synthesis gas and higher than that of the raw hydrogen, and to make it available for the formation of the fuel.

[0021] A variant of the device according to the invention provides a gas scrubber comprising a water scrubber downstream of the acid gas scrubber. The gas scrubber is designed with an extraction device through which the gas fraction, consisting largely of hydrogen, can be extracted from the acid gas scrubber or the water scrubber, or between the acid gas and water scrubbers.

[0022] Preferably, the gas scrubber is designed with an absorber column in which at least the acid gas scrubber is located. A water scrubber downstream of the acid gas scrubber can also be located in this column or in a separate scrubber column. Particularly preferably, the gas scrubber has at least two mass transfer elements through which the synthesis gas to be treated flows from bottom to top. A discharge device for the raw hydrogen is located downstream of the last mass transfer element, and a discharge device for the gas mixture consisting largely of hydrogen is located upstream of the last mass transfer element.

[0023] To obtain the fuel, the device according to the invention can comprise a separation device with which carbon compounds, such as, in particular, carbon monoxide and methane, can be separated from the gas mixture consisting largely of hydrogen. The separation device, which can be designed as a membrane separation device, pressure swing adsorber, cryogenic gas separation device, acid gas scrubber, or a combination of at least two of these devices, is expediently connected to a return line through which the separated carbon compounds can be mixed with the carbon-containing feed upstream of the reforming device.

[0024] The combustion device is, for example, a burner-fired furnace with which a feed to be fed to the reforming device can be heated. However, the combustion device can also be a burner-fired steam generator that can be used to generate process steam, or a steam or pre-reformer of the reforming device that can be heated via a burner. One embodiment of the device according to the invention provides a treatment device comprising a pressure swing adsorber that allows pure hydrogen with a hydrogen content of more than 99.5 mol% to be obtained from the raw hydrogen. The pure hydrogen can be discharged as a hydrogen product or further processed to produce the hydrogen product.

[0025] Preferably, the treatment facility is connected to an ammonia synthesis plant and configured to process the raw hydrogen into an ammonia synthesis gas, which can be fed to the ammonia synthesis plant as feed gas. The treatment facility may, for example, comprise a pressure swing adsorber for generating pure hydrogen and a nitrogen supply through which the pure hydrogen can be mixed with the ammonia synthesis gas. However, it is also possible for the treatment facility to comprise a methanizer, a partial condensation plant, or a nitrogen scrubber.

[0026] A reforming device particularly suitable for the device according to the invention is designed with an autothermal reformer or a partial oxidation reactor, which only require fuel for burner-fired auxiliary units, such as furnaces used to heat feedstocks. Therefore, only a small amount of synthesis gas needs to be diverted to obtain the fuel gas, which consists largely of hydrogen, in order to produce the hydrogen product with no or only very little release of carbon dioxide into the atmosphere. However, this should not preclude the reforming device from being designed with a burner-fired steam or pre-reformer.

[0027] In the following, the invention is explained in more detail using an embodiment shown schematically in Figure 1.

[0028] The embodiment of Figure 1 shows a preferred embodiment of the process according to the invention, in which an ammonia synthesis gas is produced as the hydrogen product using an autothermal reformer.

[0029] A carbon-containing feedstock 1, preferably natural gas, is fed to the burner-fired furnace O for heating and desulfurization. The heated and desulfurized feedstock 2, together with oxygen 3, is converted in the autothermal reformer R into a synthesis gas 4 comprising hydrogen, carbon monoxide, and water. From this raw synthesis gas 5, consisting largely of hydrogen and carbon dioxide, is obtained in the water-gas converter S.

[0030] To separate carbon dioxide, the synthesis gas 5 is introduced into the absorber column C of the gas scrubber G, where it is guided upwards and brought into intensive contact with a sour gas scrubbing agent 6, which is preferably a slightly alkaline aqueous amine solution, in the mass transfer elements P1 and P2 of the sour gas scrubbing T. On its way downwards, the sour gas scrubbing agent 6 becomes enriched with carbon dioxide, while the carbon dioxide content of the synthesis gas 5 continuously decreases on its way upwards. The flow rates of synthesis gas 5 and sour gas scrubbing agent 6 are coordinated in such a way that a largely carbon dioxide-free hydrogen fraction 7 is obtained via the chimney tray K, while the carbon dioxide-laden scrubbing agent 8 can be withdrawn from the column bottom for the separation of carbon dioxide 18 in the regeneration section E of the gas scrubbing G.

[0031] In the water scrubber W, residues of the acid gas scrubbing agent are scrubbed out of the largely carbon dioxide-free hydrogen fraction 7 with the aid of water 9 in order to obtain raw hydrogen 10. While the loaded scrubbing water 11 is disposed of or regenerated for further use in the water scrubber W, the raw hydrogen 10 is fed from the top of the absorber column C to the processing device B, where it is processed, for example by a combination of methanation, partial condensation, nitrogen scrubbing and nitrogen admixture, to an ammonia synthesis gas 14 which is in particular free of carbon monoxide and in which hydrogen and nitrogen are present in the stoichiometric ratio of 3:1 for the subsequent ammonia synthesis A. The ammonia 15 obtained in the ammonia synthesis A is stored, for example, in liquid form (not shown).

[0032] Since the carbon dioxide content of the synthesis gas passed through the acid gas scrubber continuously decreases on its upward path, a gas mixture 12 consisting largely of hydrogen with a carbon dioxide content that is lower than that of the synthesis gas 5 and higher than that of the raw hydrogen 10 can be withdrawn from the absorber column C via a side takeoff downstream of the first mass transfer element P1 and upstream of the second mass transfer element P2. Although the gas mixture 12 consisting largely of hydrogen contains only small amounts of carbon in the form of carbon monoxide, carbon dioxide, and methane, it is fed into the separation device D, where the separation of carbon monoxide and methane 13 produces a fuel gas 16 that is burned in the burner-fired furnace O to generate process heat.The amount of gas mixture 12, which consists largely of hydrogen, is adjusted so that the amount of fuel gas 16 is sufficient to fully provide the heat required to heat feed 1, thus eliminating the need to burn a carbon-containing fuel. Since the flue gas 17 produced during combustion contains very little carbon dioxide, it can be released into the atmosphere without further treatment. The carbon compounds 13 separated from the gas mixture 12, which consists largely of hydrogen, in the separation device D are added to the heated and desulfurized feed 2, while the carbon dioxide 18 obtained in the regeneration section E of the gas scrubber G is disposed of by sequestration or material utilization (neither shown).

Claims

Patent claims 1. A process for producing a hydrogen product (14), in which a carbon-containing feedstock (1) is converted by reforming (R) and water-gas conversion (S) to obtain a gas mixture (5) containing hydrogen and carbon dioxide, referred to as synthesis gas, from which gas mixture (5) carbon dioxide (18) is separated in a gas scrubbing (G) by an acid gas scrubbing agent (6) to obtain raw hydrogen (10), which is processed into the hydrogen product (14) by at least one further process step (B), wherein a fuel (16) is burned to obtain process heat and carbon dioxide (18) separated from the synthesis gas (5) is disposed of by sequestration or fed to a material utilization, characterized in that in the gas scrubbing (G) a gas mixture (12) consisting largely of hydrogen is obtained from the synthesis gas (5), which has a carbon dioxide content,which is smaller than that of the synthesis gas (5) and larger than that of the raw hydrogen (10), and which is provided to form the fuel (16).

2. Process according to claim 1, characterized in that the gas scrubbing (G) comprises a sour gas scrubbing (T) and a water scrubbing (W) arranged downstream of the sour gas scrubbing (T), wherein the gas mixture (12) consisting largely of hydrogen is withdrawn from the sour gas scrubbing (T) or from the water scrubbing (W) or between the sour gas scrubbing (T) and the water scrubbing (W).

3. Process according to one of claims 1 or 2, characterized in that the synthesis gas (5) in the gas scrubber (G) is passed over at least two serially arranged mass transfer elements (P1, P2) and is brought into contact with at least one scrubbing agent (6), the raw hydrogen (10) being withdrawn downstream and the gas mixture (12) consisting largely of hydrogen being withdrawn upstream of the last mass transfer element in the flow direction of the synthesis gas (5).

4. Method according to one of claims 1 to 3, characterized in that the gas mixture (12) consisting largely of hydrogen is used as fuel (16) directly or after separation of carbon compounds (13).

5. Process according to one of claims 1 to 4, characterized in that a slightly alkaline aqueous amine solution is used as acid gas wax (6).

6. Process according to one of claims 1 to 5, characterized in that the reforming (R) is carried out by autothermal reforming or partial oxidation or steam reforming or a combination of two or more of these methods.

7. Process according to one of claims 1 to 6, characterized in that the raw hydrogen (10) is processed to the hydrogen product (14) by methanation and / or pressure swing adsorption and / or nitrogen scrubbing and / or partial condensation.

8. Process according to claim 7, characterized in that an ammonia synthesis gas is obtained as the hydrogen product (14).

9. A device for producing a hydrogen product (14), comprising a reforming device (R) and a water-gas converter (S) connected to the reforming device (R), via which a carbon-containing feedstock (1) can be converted into a synthesis gas (5) containing hydrogen and carbon dioxide by reforming and water-gas conversion, a gas scrubber (G) comprising a sour gas scrubber (T) for separating carbon dioxide (18) from the synthesis gas (5) and for obtaining raw hydrogen (10), which forms the hydrogen product or which can be processed into the hydrogen product (14) in a processing device (B) connected to the gas scrubber, and a combustion device (O) in which a fuel (16) can be combusted to obtain process heat, wherein the gas scrubber (G) is connected or connectable to a device for sequestering or materially utilizing the carbon dioxide (18) separated from the synthesis gas (5), characterized in thatthat the gas scrubber (G) is designed to extract from the synthesis gas (5) a gas mixture (12) consisting largely of hydrogen, which has a carbon dioxide content which is lower than that of the synthesis gas (5) and higher than that of the raw hydrogen (10), and to provide it for the formation of the fuel (16).

10. Device according to claim 9, characterized in that the gas scrubber (G) comprises a water scrubber (W) downstream of the acid gas scrubber (T) and is designed with an extraction device via which the gas mixture (12) consisting largely of hydrogen can be extracted from the acid gas scrubber (T) or the water scrubber (W) or between the acid gas scrubber (T) and the water scrubber (W).

11. Device according to one of claims 9 or 10, characterized in that the gas scrubber (G) is designed with at least two mass transfer elements (P1, P2) arranged in series in the flow path of the synthesis gas (5), wherein the raw hydrogen (10) can be withdrawn downstream and the gas mixture (12) consisting largely of hydrogen can be withdrawn upstream of the last mass transfer element in the flow direction of the synthesis gas (5).

12. Device according to one of claims 9 to 11, characterized in that it comprises a membrane separation or a pressure swing adsorber or a cryogenic gas separator or an acid gas scrubber for separating carbon compounds (13) from the gas mixture (12) consisting largely of hydrogen.

13. Device according to one of claims 9 to 12, characterized in that the reforming device (R) comprises an autothermal reformer or a partial oxidation reactor or a steam reformer or a pre-reformer or a combination of two or more of these apparatuses.

14. Device according to one of claims 9 to 13, characterized in that the combustion device (O) is a burner-fired furnace for heating a feed (1) to be fed to the reforming device (R) or a burner-fired steam generator or a steam or pre-reformer of the reforming device (R) which can be heated via burners.

15. Device according to one of claims 9 to 14, characterized in that it comprises a methanizer and / or pressure swing adsorber and / or partial condensation and / or nitrogen scrubbing for processing the raw hydrogen (10) to the hydrogen product (14).

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