Method for operating an electrolysis plant, and electrolysis plant

Abstract

The invention relates to a method for operating an electrolysis plant (1), comprising a stack (2) having an anode (3) and a cathode (4), wherein in normal operation of the electrolysis plant (1), water is supplied to the anode (3) via a water circuit (5) with an integrated pump (6), said water being split in the stack (2) by electrolysis into hydrogen and oxygen, and wherein the hydrogen produced by electrolysis is supplied to a gas-liquid separator (8) via a cathode outlet (10) of the stack (2) and a media line (7) connected thereto. According to the invention, a) when the electrolysis plant (1) is switched off, a shut-off valve (11) in an inert gas line (12) is opened, said line connecting an inert gas container (13) to the cathode (4), and the cathode (4) is rinsed with the inert gas, while the water supply to the anode (3) is stopped, and b) when the electrolysis plant (1) is started up again, the following steps are carried out: (i) closing the shut-off valve (11) integrated in the inert gas line (12), (ii) supplying the anode (3) with fresh water via a fresh water supply (14) connected to the water circuit (5) while the power is still turned off, (iii) supplying the stack (2) with the power needed for electrolysis and (iv) producing an amount of hydrogen which corresponds at least to the amount of inert gas present in the cathode (4), preferably corresponds to 1.5 to 10 times the amount of inert gas present in the cathode (4). The invention further relates to an electrolysis plant (1) that is suitable for carrying out the method or can be operated according to the method.

Description

[0001] Description

[0002] Title:

[0003] Method for operating an electrolysis plant, electrolysis plant

[0004] The invention relates to a method for operating an electrolysis plant. Furthermore, the invention relates to an electrolysis plant suitable for carrying out the method or operable according to the method.

[0005] The preferred field of application of the invention is the production of hydrogen by electrolysis.

[0006] State of the art

[0007] Electrolysis is a chemical process in which electrical current is used to break down a chemical compound. This allows hydrogen and oxygen to be extracted from water.

[0008] To carry out the electrolysis, at least one electrolysis cell is required, for example a PEM (“Proton Exchange Membrane”) electrolysis cell, an AEL (“Liquid Alkaline Electrolysis”) electrolysis cell, a solid oxide electrolysis cell or SOEC (“Solid Oxide Electrolyzer Cell”) or an AEM C (Anion Exchange Membrane”) electrolysis cell, whereby the electrolysis cells mentioned differ in the type of ionic conductivity.

[0009] Typically, a large number of identical electrolysis cells are stacked and connected to form a cell stack, the so-called stack. This requires additional components, particularly those for power and water supply, as well as for gas processing. The power supply components typically include a rectifier for converting AC to DC and a DC / DC converter for adjusting the DC voltage. The water supply components include at least one water circuit and a pump. The gas processing components include gas-liquid separators to separate the product gases hydrogen and oxygen from water. Furthermore, means for controlling and / or regulating the system are required.

[0010] Against the backdrop of decarbonization, hydrogen is becoming increasingly important as an energy carrier. The current goal is to produce green hydrogen, i.e., hydrogen with a very low carbon dioxide (CO2) footprint, through electrolysis on a large industrial scale. Renewable energy sources such as wind and solar energy are required to produce green hydrogen. Since these energy sources fluctuate, electrical energy is not always available, and the electrolysis plant must be shut down temporarily. This negatively impacts the service life of the electrolysis plant for the reasons listed below.

[0011] When the system is shut down, the water supply to the anode side is interrupted. Due to the higher pressure on the cathode side, hydrogen diffuses from the cathode to the anode side through the not completely gas-tight membranes of the electrolysis cells. As a result, the cell voltage drops steadily from the usual 1.5 V to 2 V during operation to as low as 0 V. At the same time, the electrochemical potential on the anode-side catalyst drops from over 1.3 V to as low as 0 V. A potential drop to below 1 V is accompanied by a reduction in the anode-side catalyst, which is in an oxidized state during normal operation. The reduction and renewed oxidation when electrolysis operation is restarted leads to a significant acceleration of the aging of the anode-side catalyst, which reduces catalyst activity. As a result, for a given current density, the electrolysis cell voltage increases and the electrolysis efficiency decreases accordingly.Frequent shutdown and restart of an electrolysis plant can therefore reduce its service life to an uneconomical level.

[0012] The present invention is concerned with the task of minimizing or completely eliminating the disadvantages that occur when shutting down and restarting an electrolysis plant.

[0013] To solve the problem, the procedure with the features of the

[0014] Claim 1 and the electrochemical electrolysis system with the features of claim 7 are proposed. Advantageous further developments of the invention can be found in the respective subclaims.

[0015] Disclosure of the invention

[0016] A method is proposed for operating an electrolysis system comprising a stack with an anode and a cathode. During normal operation of the electrolysis system, water is supplied to the anode via a water circuit with an integrated pump, and the water is split into hydrogen and oxygen in the stack by electrolysis. The hydrogen generated by electrolysis is fed to a gas-liquid separator via a cathode outlet of the stack and a media line connected thereto. According to the invention, a) when the electrolysis system is shut down, a shut-off valve in an inert gas line connecting an inert gas container to the cathode is opened, and the cathode is purged with the inert gas while the water supply to the anode is stopped, and b) when the electrolysis system is restarted, the following steps are carried out:

[0017] (i) Closing the shut-off valve integrated into the inert gas line,

[0018] (ii) supplying the anode with fresh water via a fresh water supply connected to the water circuit while the power is still switched off,

[0019] (iii) supplying the stack with the power required for electrolysis and

[0020] (iv) producing an amount of hydrogen at least equal to the amount of inert gas present in the cathode, preferably 1.5 to 10 times the amount of inert gas present in the cathode.

[0021] By flushing the cathode with inert gas when shutting down the electrolysis plant, the hydrogen present on the cathode side is displaced, thus stopping hydrogen diffusion from the cathode side to the anode side. Because only a small amount of hydrogen reaches the anode side via diffusion, the disadvantages described above do not occur, or at least not to the same extent. When restarting the electrolysis plant, the inert gas present on the cathode side must be removed again to ensure the required purity of the hydrogen product gas.To do this, the supply of inert gas is first stopped. The stack is then supplied with fresh water which contains dissolved oxygen so that a stable anode potential of around 1.2 V is present even before the current is switched on and the anode potential does not drop uncontrollably when the current is subsequently switched on. When the current is switched on, the stack produces hydrogen, which then displaces the inert gas present on the cathode side. If the amount of hydrogen produced corresponds to 1.5 to 10 times the amount of inert gas present on the cathode side, the required purity of the hydrogen subsequently produced is ensured.

[0022] Preferably, the pump integrated into the water circuit is turned off when the electrolysis system is shut down and restarted when it is restarted, so that the anode is supplied with fresh water via the pump. This allows the stable anode potential of approximately 1.2 V to be reached more quickly and in a more controlled manner.

[0023] Furthermore, when restarting the electrolysis plant, a stoichiometry of at least 10, preferably 50 to 100, relative to the subsequently applied current is preferably set via the fresh water supply to the anode. The desired stable anode potential of approximately 1.2 V can be more easily achieved in this way.

[0024] Furthermore, when shutting down the electrolysis system, the media line is preferably closed off in the direction of the gas-liquid separator using at least one valve integrated into the media line, preferably a directional control valve, while a gas outlet is opened to remove the hydrogen present in the cathode. This measure ensures that no inert gas used to purge the cathode enters the gas-liquid separator and contaminates it. The hydrogen displaced by the inert gas is removed via the open gas outlet.

[0025] For the same reason, the inert gas displaced by the hydrogen produced during restart of the electrolysis plant is preferably discharged via the gas outlet. The gas outlet is therefore kept open during restart, at least until all inert gas has been removed from the cathode. The valve integrated into the media line, preferably a directional control valve, which blocks the media line toward the gas-liquid separator, remains closed during this time to prevent the entry of inert gas into the gas-liquid separator.

[0026] When restarting the electrolysis plant, the media line in the direction of the gas-liquid separator preferably remains closed until the pressure on the cathode side has reached a predetermined target pressure, preferably 10 bar to 30 bar.

[0027] The electrolysis plant further proposed to solve the problem mentioned above comprises: a stack with an anode and a cathode, a water circuit with an integrated pump for supplying the anode with water, a media line via which a cathode outlet of the stack is connected to a gas-liquid separator, at least one valve integrated into the media line, preferably a directional control valve, for blocking the media line in the direction of the gas-liquid separator and for releasing a gas outlet, and an inert gas container which is connected to the cathode of the stack via an inert gas line with an integrated shut-off valve.

[0028] The proposed electrolysis system is particularly suitable for carrying out the method according to the invention described above or can be operated according to this method. The electrolysis system can therefore achieve the same advantages. In particular, degradation of the electrolysis cells of the stack can be counteracted when shutting down and restarting the electrolysis system. Furthermore, a high purity of the hydrogen produced after restarting can be ensured, since all inert gas used to purge the cathode during shutdown is discharged via a gas outlet. The valve integrated into the media line is preferably a directional valve, so that only one valve is required to block and release the media line and the gas outlet.

[0029] Furthermore, a pressure sensor is preferably integrated into the cathode outlet or the media line. The pressure sensor can be used to monitor the pressure on the cathode side. For this purpose, the pressure sensor can be designed as an absolute pressure sensor or as a differential pressure sensor with respect to the ambient pressure. If the pressure sensor is an absolute pressure sensor, a further pressure sensor is preferably present to detect the ambient pressure.

[0030] Furthermore, it is proposed that the electrolysis system comprise a control unit configured to carry out steps of a method according to the invention. In particular, the control unit can be used to control the pump integrated into the water circuit and / or the at least one valve integrated into the media line. Furthermore, the shut-off valve integrated into the inert gas line can be actuated with the aid of the control unit.

[0031] The invention and its advantages are described in more detail below with reference to the accompanying drawing. This shows a schematic representation of an electrolysis system according to the invention.

[0032] Detailed description of the drawing

[0033] The figure shows an electrolysis system 1 according to the invention, which serves to produce hydrogen by electrolysis. For this purpose, the electrolysis system 1 comprises a stack 2 with an anode 3 and a cathode 4. During operation of the system, water is supplied to the anode 3 via a water circuit 5 with an integrated pump 6. In the stack 2 or in the electrolysis cells of the stack 2, the water is then split into hydrogen and oxygen.

[0034] The hydrogen accumulating on the cathode side is fed to a gas-liquid separator 8 via a media line 7 connected to a cathode outlet 10 of stack 2. The gas separated from the water is then passed through a droplet separator 19. The water is recirculated and fed back to stack 2. The oxygen accumulating on the anode side is discharged together with water from the anode 3 of stack 2 and also fed to a gas-liquid separator 20 with a downstream droplet separator 21. The water separated from the oxygen in the gas-liquid separator 20 is recirculated and fed back to stack 2.

[0035] A fish water supply 14 is also provided to supply water to the stack 2, through which fresh water is fed into the water circuit 5. The electrical energy required for electrolysis is provided by power electronics 15 connected to the stack 2. The electrolysis process is controlled and regulated by a control unit 18 of the electrolysis system 1.

[0036] In addition, an inert gas container 13 is provided for storing an inert gas, such as nitrogen. The inert gas container 13 is connected or connectable to the cathode 4 of the stack 2 via an inert gas line 12 with an integrated shut-off valve 11. If necessary, in particular when shutting down the electrolysis system 1, the cathode 4 can thus be purged with inert gas. This displaces the hydrogen present in the cathode 4. Since no hydrogen is present, hydrogen permeation also ceases. This prevents the anode potential from falling below 1 V. At the same time, degradation of the electrolysis cells is counteracted.

[0037] When the electrolysis plant 1 is restarted, the inert gas must be removed from the cathode 4. To do this, the shut-off valve 11 in the inert gas line 12 is first closed so that no further inert gas enters the cathode 4. Fresh water is then supplied to the anode 3 to initiate the production of hydrogen, even before current is applied to the stack 2. The produced hydrogen then displaces the inert gas from the cathode 4. To prevent inert gas from entering the gas-liquid separator 8, a valve 9 integrated into the media line 7 and designed as a directional control valve is actuated so that the media line 7 is blocked in the direction of the gas-liquid separator 8 and a gas outlet 16 is opened. The inert gas can then be discharged via the gas outlet 16. Upstream of the valve 9, a pressure sensor 17 is integrated into the media line 7. With the help of the pressure sensor 17, the cathode-side pressure can be measured.

Claims

Claims 1. A method for operating an electrolysis plant (1) comprising a stack (2) with an anode (3) and a cathode (4), wherein, during normal operation of the electrolysis plant (1), water is supplied to the anode (3) via a water circuit (5) with an integrated pump (6), and the water in the stack (2) is split into hydrogen and oxygen by electrolysis, and wherein the hydrogen produced by electrolysis is supplied to a gas-liquid separator (8) via a cathode outlet (10) of the stack (2) and a media line (7) connected thereto, characterized in that a) when the electrolysis plant (1) is shut down, a shut-off valve (11) in an inert gas line (12) connecting an inert gas container (13) to the cathode (4) is opened, and the cathode (4) is flushed with the inert gas while the water supply to the anode (3) is discontinued, and b) when the electrolysis plant is restarted (1) the following steps are carried out: (i) closing the shut-off valve (11) integrated into the inert gas line (12), (ii) supplying the anode (3) with fresh water via a fresh water supply (14) connected to the water circuit (5) while the power is still switched off, (iii) supplying the stack (2) with the power required for electrolysis and (iv) producing an amount of hydrogen which corresponds at least to the amount of inert gas present in the cathode (4), preferably 1.5 to 10 times the amount of inert gas present in the cathode (4).

2. Method according to claim 1, characterized in that when the electrolysis plant (1) is shut down, the pump (6) integrated into the water circuit (5) is switched off and is put into operation again when it is restarted, so that the anode (3) is supplied with fresh water by means of the pump (6).

3. Method according to claim 1 or 2, characterized in that when the electrolysis plant (1) is restarted, a stoichiometry of at least 10, preferably of 50 to 100, based on the current to be subsequently applied, is set via the fresh water supply to the anode (3).

4. Method according to one of the preceding claims, characterized in that when the electrolysis plant (1) is switched off, the media line (7) is blocked in the direction of the gas-liquid separator (8) by means of at least one valve (9) integrated into the media line (7), preferably a directional control valve, while a gas outlet (16) is opened to discharge the hydrogen present in the cathode (4).

5. The method according to claim 4, characterized in that when the electrolysis plant (1) is restarted, the inert gas displaced by the hydrogen produced is discharged via the gas outlet (16).

6. Method according to one of the preceding claims, characterized in that when the electrolysis plant (1) is restarted, the media line (7) in the direction of the gas-liquid separator (8) remains closed until the pressure on the cathode side has reached a predetermined target pressure, preferably 10 bar to 30 bar.

7. Electrolysis plant (1), comprising a stack (2) with an anode (3) and a cathode (4), a water circuit (5) with an integrated pump (6) for supplying the anode (3) with water, a media line (7) via which a cathode outlet (10) of the stack (2) is connected to a gas-liquid separator (8), at least one valve (9), preferably a directional control valve, integrated into the media line (7) for blocking the media line (7) in the direction of the gas-liquid separator (8) and for releasing a gas outlet (16), and an inert gas container (13) which is connected to the cathode (4) of the stack (2) via an inert gas line (12) with an integrated shut-off valve (11).

8. Electrolysis plant (1) according to claim 7, characterized in that a pressure sensor (17) is integrated into the cathode outlet (10) or into the media line (7).

9. Electrolysis plant (1) according to claim 7 or 8, characterized in that the electrolysis plant (1) comprises a control device (18) which is designed to carry out steps of a method according to one of claims 1 to 6.

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