METHOD FOR THE START-UP OF AN ELECTROLYSIS SYSTEM

Abstract

The invention related to a method for the start-up of an electrolysis system, wherein the electrolysis system is configured to produce hydrogen at the cathode side of the electrolysis system from a water containing electrolysis medium. According to the method, at least part of the electrolysis system is purged with an inert gas during stand-by. The inert gas is displaced from the cathode side of the electrolysis system by means of the hydrogen stream produced during start-up. The resulting mixed stream, which comprises at least hydrogen and inert gas, is supplied to a hydrogen separation unit until a predetermined upper concentration limit of inert gas in the mixed stream is reached. After the predetermined upper concentration limit of inert gas in the mixed stream, now low in inert gas, has been reached, the mixed stream is withdrawn from the electrolysis system by bypassing the hydrogen separation unit.

Claims

1. A method for the start-up of an electrolysis system, the electrolysis system is configured to produce hydrogen at the cathode side of the electrolysis system from a water containing electrolysis medium, the method comprising: a) purging at least a part of the electrolysis system with an inert gas stream whilst the electrolysis system is in standby mode; b) displacement of the inert gas from the cathode side of the electrolysis system with a hydrogen stream during the start-up of the electrolysis system, whereby said hydrogen stream is produced on the cathode side of the electrolysis system; c) supplying a mixed stream comprising hydrogen and inert gas obtained in step b) to a hydrogen separation unit until a predetermined upper concentration limit of inert gas in the mixed stream comprising hydrogen and inert gas supplied to the hydrogen separation unit is reached; d) withdrawing a hydrogen stream low in inert gas from the electrolysis system by bypassing the hydrogen separation unit after the predetermined upper concentration limit according to step c) has been reached.

2. The method according to claim 1, wherein the hydrogen separation unit is supplied with a hydrogen rich stream, wherein said hydrogen rich stream is produced by a hydrogen production unit which is not an electrolysis system.

3. The method according to claim 2, wherein the hydrogen production unit which is not an electrolysis system and the electrolysis system are located in a joint plant network.

4. The method according to claim 1, wherein the hydrogen separation unit is selected from at least one element of the group consisting of: a pressure swing adsorption (PSA) unit, a thermal swing adsorption (TSA) unit, a membrane unit, and an electrochemical pump.

5. The method according to claim 2, wherein the hydrogen separation unit produces a high purity hydrogen stream and an off-gas stream by separation of hydrogen from the mixed stream comprising hydrogen and inert gas supplied by the electrolysis system, and the hydrogen rich stream supplied by the hydrogen production unit which is not an electrolysis system.

6. The method according to claim 1, wherein the mixed stream comprising hydrogen and inert gas is supplied to a purification unit to remove oxygen and optionally water from said mixed stream, whereby a purified mixed stream comprising hydrogen and inert gas is obtained, and said purified mixed stream is subsequently supplied to the hydrogen separation unit.

7. The method according to claim 6, wherein the concentration of inert gas in the mixed stream comprising hydrogen and inert gas is determined by an online-analyser, whereby the sampling point of the online analyser is located downstream of the purification unit and upstream of the hydrogen separation unit.

8. The method according claim 1, wherein the electrolysis system is part of an electrolysis plant comprising at least three electrolysis systems, and wherein, in a condition in which the electrolysis plant is not operating at its maximum specified capacity, a first and a second of the electrolysis systems are operated in standby mode, and a third of the electrolysis systems is operated at a capacity which, expressed as a percentage of the maximum nominal capacity of the third electrolysis system, is at least equal to the lower safety limit of the third electrolysis system.

9. The method according to claim 1, wherein the electrolysis system is part of an electrolysis plant comprising a further electrolysis system, wherein the further electrolysis system provides a hydrogen stream, and said hydrogen stream is admixed to the mixed stream comprising hydrogen and inert gas, whereby a hydrogen rich mixed stream is obtained, and the hydrogen rich mixed stream is supplied to the hydrogen separation unit until a predetermined upper concentration limit of inert gas in the hydrogen rich mixed stream is reached.

10. A plant, configured for carrying out the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] The invention will now be detailed by way of an exemplary embodiment with reference to the attached drawings. Unless otherwise stated, the drawing is not to scale. In the figure and the accompanying description, equivalent elements are each provided with the same reference marks.

[0071] FIG. 1 shows a simplified block flow diagram of a plant which is configured for carrying out the process according to the invention,

[0072] FIG. 2 shows a flow chart for carrying out the process according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0073] FIG. 1 shows a simplified block flow diagram of one exemplary embodiment of a plant which is configured for carrying out the process according to the invention.

[0074] The plant according to FIG. 1 comprises an electrolysis system 10. The electrolysis system 10 comprises a plurality of electrolysis cells, e.g. an electrolysis cell stack or a plurality of electrolysis cell stacks and is configured for the generation of hydrogen and oxygen from a water containing electrolysis medium. Hydrogen is produced on the cathode site of the electrode cell stack and oxygen on the anode side. The target product of the electrolysis system is hydrogen, which is produced on an industrial scale. Hence, the electrolysis system may have a total nominal power of 20 MW or more. The electrolysis system 10 may be of any suitable type, e.g. comprise a PEM electrolyser or an alkaline electrolyser. A hydrogen stream withdrawn from the electrolysis system 10 may be pressurized or not pressurized (i.e. be close to atmospheric pressure), dependent on the selected electrolyser type of the electrolyser system. The electrolysis system 10 is connected to a renewable energy source (not shown) such as a solar plant or a wind farm, which provides “green” electrical energy. The electric current provided by the renewable energy source is converted into direct current via a rectifier (not shown) and fed to the electrolyser of the electrolysis system 10.

[0075] During a stand-by of the electrolysis system 10, at least the electrode cell stacks are constantly flushed with nitrogen, so that an essentially pure nitrogen stream is withdrawn from the electrolysis system 10 during stand by. During the start-up phase, the cell stack of the electrolysis system 10 begins to produce hydrogen again so that the nitrogen in the electrolysis system is increasingly displaced by hydrogen over time. Hence, during start-up, a mixed stream comprising at least hydrogen and nitrogen is withdrawn from electrolysis system 10. In case that the mixed stream is not pressurized, it is withdrawn from electrolysis system 10 as an uncompressed mixed stream 11 and afterwards compressed in compression unit 12. Whether the mixed stream is pressurized or not depends on the configuration of the electrolysis system 10, i.e. whether it operated under elevated pressure or not.

[0076] The compressed mixed stream 13 is afterwards fed to a purification unit 14. Under usual circumstances, the hydrogen product stream of the cathode section of the electrolysis system 10 comprises small amounts of oxygen due to cell crossover. Furthermore, since water normally is not completely removed from the gas stream by gas-liquid separation and subsequent cooling of the gas stream, even small amounts of water are present in the gas stream. In purification unit 14, oxygen and water is removed from the pressurized mixed stream by catalytic conversion of oxygen to water and afterwards absorbing the water in a molecular sieve bed. The purification unit 14 produces an off-gas, which may be discharged from it (not shown). In the event that a temperature swing adsorption unit is used, losses of hydrogen product gas are negligible or close to zero.

[0077] Downstream of purification unit 14 is a sampling point 17 that allows an associated online analyser 16 to continuously determine, in real time, the composition of the mixed gas stream comprising hydrogen and nitrogen. In particular, the online analyser 16 determines the content of nitrogen in the mixed gas stream.

[0078] At the beginning of the start-up, the nitrogen content will be very high, for example, it can be up to 90%. This nitrogen content decreases during the start-up process, which is registered accordingly by the online analyser 16. In this phase of the start-up process, it can be assumed that a previously defined maximum value for the nitrogen content in the mixed gas flow has not yet been reached. This predefined maximum value can be, for example, a concentration of 200 ppmV nitrogen in the mixed stream. The previously defined maximum value for the nitrogen content is the predetermined upper concentration limit of inert gas in the mixed stream according to the invention.

[0079] The online analyser 16 controls two control valves 18a and 18b. At the beginning of the start-up process of the electrolysis system 10, control valve 18a is closed and control valve 18b is open. As long as the content of nitrogen in the mixed stream is above the predetermined maximum hydrogen product specification value, the mixed stream is fed as purified compressed stream with high inert gas concentration 24 (nitrogen concentration above the maximum value) to a pressure swing adsorption unit 19.

[0080] The plant according to FIG. 1 also has a steam reforming unit 20. In steam reforming unit 20, a hydrogen-rich stream 22 is produced from natural gas with a subsequent water-gas shift of the primarily produced synthesis gas, followed by a subsequent carbon dioxide separation. The hydrogen-rich stream 22 is also fed to the pressure swing adsorption unit 19 to separate hydrogen from the hydrogen-rich stream 22. In other words, the mixed stream 24 with (too) high a nitrogen content and the hydrogen-rich stream 22 from the steam reforming unit 20 are combined and fed to the pressure swing adsorption unit 19. The pressure swing adsorption unit 19 produces a high purity hydrogen stream 26 and an off-gas gas 27. The hydrogen stream 26 is further processed as a high purity hydrogen product 15, for example the high purity hydrogen product 15 is fed into a hydrogen pipeline. The off-gas stream 27 is subject to an off-gas treatment 21. The off-gas stream 27 may be combined with the off-gas stream withdrawn from the purification unit 14.

[0081] During the start-up process, the concentration of hydrogen in the mixed stream 23 increases continuously, while the nitrogen concentration decreases. As soon as the online analyser registers that the predetermined permitted upper limit for the nitrogen concentration (e.g. 200 ppmV Nitrogen) or a value lower than the upper limit has been reached, the control valve 18b closes and the control valve 18a is opened. From this point on, it is no longer necessary to feed the mixed stream 23 to the pressure swing adsorption unit 19. Hence, the hydrogen stream low in inert gas then bypasses the pressure swing adsorption unit 19. The corresponding hydrogen stream low in inert gas, designated as purified compressed mixed stream with low inert gas concentration 25, has a composition that essentially corresponds to the composition of the high purity hydrogen stream 26. That is, the composition of stream 25 in terms of hydrogen content and impurities is as good or better as the composition of stream 26. The resulting high purity hydrogen product 15 is fed into a hydrogen pipeline, for instance.

[0082] FIG. 2 shows a flow chart for carrying out the process according to the invention. In stand-by mode, at least a part of the electrolysis system is purged with an inert gas. In particular, the electrolysis cell stacks are purged with nitrogen. For example, the purge is efficient to remove hydrogen to a level below 2% per volume, which is safely under the lower explosion limit (LEL) in absence of air. In case the system continues to be purged, it is left under nitrogen with purge being a continuous action. In other words, the system continues to remain under a predominant nitrogen atmosphere. As soon as sufficient electrical energy is available from a renewable energy source, the decision is made to restart the electrolysis system, otherwise it continues to be purged with nitrogen. Purging with nitrogen means that the electrolysis system is either purged with a stream of Nitrogen as continuous purge, or the Nitrogen is locked in. In either case, the electrolysis system remains under a nitrogen atmosphere.

[0083] When the decision is made to restart the electrolysis system, the nitrogen in the system is continuously displaced by hydrogen produced on the cathode side of the electrolysis system. The resulting mixed stream, which contains at least hydrogen and nitrogen, is fed to a hydrogen separation unit, especially a pressure swing adsorption unit. Meanwhile, the concentration of inert gas in the mixed stream supplied to the hydrogen separation unit is determined regularly or continuously. As long as the concentration of inert gas is above a predetermined upper limit of inert gas in the mixed stream, the mixed stream continues to be fed to the hydrogen separation unit. As soon as the concentration of inert gas in the mixed stream reaches or falls below a predefined upper limit value, the decision is made to no longer feed the mixed stream to the hydrogen separation unit. From this point on, the hydrogen-containing stream, which is now low in inert gas, is bypassed the hydrogen separation unit and fed directly into a hydrogen pipeline, for example.

LIST OF REFERENCE SIGNS

[0084] 10 electrolysis system [0085] 11 uncompressed mixed stream comprising hydrogen and inert gas [0086] 12 compression unit [0087] 13 compressed mixed stream comprising hydrogen and inert gas [0088] 14 purification unit [0089] 15 high purity hydrogen product [0090] 16 online analyser [0091] 17 sampling point [0092] 18a, 18b control valve [0093] 19 pressure swing adsorption unit [0094] 20 steam methane reforming unit [0095] 21 off-gas treatment [0096] 22 hydrogen rich stream [0097] 23 purified compressed mixed stream comprising hydrogen and inert gas [0098] 24 purified compressed mixed stream with high inert gas concentration [0099] 25 purified compressed mixed stream with low inert gas concentration [0100] 26 high purity hydrogen stream [0101] 27 off-gas stream