Preventing varying pressure loading of a steam generator in standby mode

Abstract

The present invention relates to a device having a steam generator (10), wherein the steam generator (10) has a first side (11) and a second side (12), wherein the second side (12) is designed to generate steam, wherein the device has a first bypass line (20), wherein the first bypass line (20) is arranged to bypass the first side (11) of the steam generator (10), wherein the first bypass line (20) has a first bypass valve (22), wherein the second side (12) is connected to a steam discharge line (30), wherein the steam discharge line (30) has a steam shut-off valve (32), wherein the second side (12) has a pressure measuring device (40), wherein the device has a control device (50), wherein the control device (50) is connected to the pressure measuring device (40) and the first bypass valve (22), wherein the control device is designed to control the first bypass valve (22) depending on the pressure recorded by the pressure measuring device (40).

Description

[0001] Avoiding pressure cycling of a steam generator in standby mode

[0002] The invention relates to a steam generator for a plant for the production of ammonia, which is subject to fluctuations, for example due to the use of renewable energy, and is therefore temporarily put into standby mode.

[0003] For decades, ammonia has been produced primarily using the Haber-Bosch process. In most cases, hydrogen is first generated from natural gas, which is then reacted with nitrogen at high pressure and high temperature over a catalyst. Since this is an equilibrium reaction whose equilibrium is not shifted toward the products, the ammonia is separated in a recirculation circuit, and any unconverted hydrogen and nitrogen are returned to the catalyst. However, the use of natural gas produces a corresponding amount of carbon dioxide.

[0004] In order to produce ammonia sustainably, the electrolysis of water using renewable energy is now being used. However, this represents a major difference to the previous process. If solar power is used, for example, the day-night cycle results in a period in which no renewable energy is available. By combining solar and wind power, for example, this effect can be mitigated somewhat, but the fundamental problem remains. While it is theoretically possible in some regions to draw electrical energy from the general power grid, it will also be difficult to supply it with renewable energy at these times. Furthermore, plants are also planned that will be located in locations suitable for energy generation and that do not have access to an electrical supply grid.The electrolysis of water to produce hydrogen for an ammonia synthesis plant is known, for example, from US 9 463 983 B2.

[0005] A converter for ammonia synthesis cannot simply be switched on and off. For example, a temperature of at least 350 °C is required for the reaction to take place on the catalyst. During operation, the necessary energy (to compensate for thermal losses and to heat the cooled cycle gas to the catalyst's light-off temperature) is generated by the energy released during the reaction. The excess energy released is typically used to generate steam.

[0006] DE 10 2022 204 103 therefore discloses methods for adapting operations when the amount of renewable energy generated decreases, thus making do with a lower amount of energy. In the event of a prolonged absence of renewable energy, for example, during a prolonged period of calm, a so-called hot standby is implemented, in which all plant components are shut down, leaving only the gas circuit of the converter running, and thermal losses are compensated for by means of an electric heater.

[0007] A method for starting up a solar steam power plant is known from US 2012 / 137 683 A1.

[0008] A waste heat steam boiler is known from US 2006 / 0162315 A1.

[0009] A device for generating energy is known from WO 2012 / 089 826 A2.

[0010] Ammonia synthesis is an exothermic process, so during normal synthesis, the resulting reaction heat must be removed from the cycle. This is typically done with a steam generator, preferably a steam superheater, to generate saturated steam, for example, at 261 °C and 48 bara. This can be used for a variety of purposes.

[0011] A problem arises both during partial load operation and during partial shutdown. A steam generator, which removes the reaction heat from the circuit, is typically used as the first heat exchanger. If this is shut down via a bypass for the hot cycle gas, it cools down, which in turn leads to a drop in pressure in the steam generator. Therefore, the steam generator in a plant powered by renewable energy must be designed for high pressure cycling. The object of the invention is to reduce the cycling load in the steam generator and thus simplify the design.

[0012] This object is achieved by the device having the features specified in claim 1 and by the method having the features specified in claim 8. Advantageous further developments emerge from the subclaims, the following description and the drawing.

[0013] The device according to the invention serves to produce ammonia. It is therefore a plant, in particular, based on the Haber-Bosch process, in which nitrogen and hydrogen are converted to ammonia at high temperature and high pressure. Since this is an equilibrium reaction, the reactants are circulated, and the product is separated. A recirculation gas stream flows through the recirculation circuit. The recirculation gas stream thus consists of hydrogen and nitrogen and, in particular, between the converter and the ammonia separator, also of ammonia. In addition, other gases, such as argon, can also inadvertently enter the recirculation circuit. The device has a recirculation circuit, and the recirculation circuit has a converter, a steam generator, and an ammonia separator. Additional components, in particular a compressor, can also be arranged in the recirculation circuit.While the ammonia separator removes the ammonia produced in the converter from the recirculation gas stream, the steam generator removes the heat energy released during the reaction in the converter. The device is preferably subject to process-related fluctuations, for example and in particular because the necessary energy is generated renewably, for example using solar or wind power. In the case of an ammonia plant, the hydrogen is produced electrolytically using renewably generated electrical energy. If the renewably generated energy is not available, for example because it is night and / or there is no wind, the device is put into standby mode. However, since the actual reaction in the converter no longer takes place, no heat energy is generated by the chemical reaction, which must be dissipated using the steam generator.Rather, the challenge arises to minimize heat losses in order to minimize the costs of maintaining the temperature. To achieve this, the steam generator is integrated according to the invention in such a way that it can also be transferred to a particularly suitable standby mode. The steam generator has a first side and a second side. The second side is designed to generate steam, i.e. water is supplied and evaporated. The medium to be cooled, for example the recirculation circuit after a converter for generating ammonia, is passed through the first side. The recirculation gas stream is therefore the heating fluid for the steam generator. It is introduced into the steam generator hot and there transfers part of its heat, thus heating the steam on the second side.Since there is no excess heat in the recirculating gas stream during standby operation, no heat should be dissipated via the heating stream. The term "heating fluid" is therefore to be understood functionally, i.e., the fluid that heats the water by releasing its heat and thus provides the energy required for steam generation. Therefore, the device has a first bypass line. The first bypass line is arranged to bypass the first side of the steam generator. This allows heat loss to be minimized during standby operation, for example, to keep the energy required for an electric heater to a minimum to compensate for heat radiation.The recirculation gas stream is therefore no longer conducted through the steam generator as a heating stream, losing heat in the process, but is instead bypassed by the steam generator through the first bypass line, reducing heat loss to the heat loss through the wall of the first bypass line; the heat losses of the entire circuit remain unchanged. The first bypass line has a first bypass valve. During normal operation, the first bypass valve is closed, the flow passes through the steam generator, and steam is generated regularly. For example, such a first bypass line is known from DE 10 2022 204 103, which is opened during standby operation. The second side is connected to a steam discharge line. During normal operation, the steam can be conducted through the steam discharge line to other parts of the system where this heat is needed, for example, a turbine for generating electrical energy. The second side is connected to a water supply.The water from which the steam is generated is supplied through the water supply. The water supply is closable; due to the way a steam generator works, even when open it is designed so that the steam escapes only through the steam outlet and not through the water supply. The steam outlet line has a steam check valve. The steam generator can be closed with the steam check valve in standby mode to prevent steam and therefore heat from escaping. Since both the water supply and the steam outlet are closed, a closed volume is created on the second side of the steam generator, meaning no more steam escapes from the second side. The second side has a pressure measuring device. This allows the pressure inside the second side to be measured. The device has a control device.The control device is connected to the pressure measuring device and the first bypass valve. The control device is designed to control the first bypass valve depending on the pressure detected by the pressure measuring device.

[0014] This now not only makes it possible to close the first bypass valve during normal operation and leave the first control valve open during standby operation. In addition, whenever the pressure in the steam generator drops due to cooling, the first bypass valve can be closed further in standby mode in order to keep the temperature and thus the pressure in the steam generator within a specified range. This means that heating fluid is then again fed through the first side of the steam generator to heat the steam. Since the second side is an enclosed space, no heat energy is dissipated; instead, heat loss is compensated for. This only minimally increases the additional heating in the recirculation circuit, but reliably prevents pressure changes in the steam generator, which increases safety and simplifies the design. The steam check valve keeps the volume constant, so that as the temperature drops, the pressure also drops.This effect is particularly pronounced when the steam condenses. If the pressure rises, the first bypass valve is opened so that only the amount of heat required to maintain the pressure is supplied to the steam generator. This makes it possible to minimize the energy loss or the electrical heating power required to compensate for this. At the same time, frequent pressure changes in the steam generator are avoided, allowing it to be designed more simply. In a further embodiment of the invention, the steam discharge line has a pressure relief valve. The pressure relief valve can be designed as a simple pressure relief valve. Preferably, however, the pressure relief valve is controllable. Preferably, the control device is connected to the pressure relief valve. The control device is designed to control the pressure relief valve depending on the pressure detected by the pressure measuring device.If the pressure rises above an upper pressure threshold, the control device can open the pressure relief valve and thus specifically regulate the pressure inside the second side of the steam generator by closing and opening the first bypass valve and by opening and closing the pressure relief valve. This enables very simple and efficient control.

[0015] In a further embodiment of the invention, the device comprises a steam superheater. The steam generator and steam superheater are usually connected in series, and steam is conducted in countercurrent to the medium to be heated in order to achieve the highest possible steam temperature and thus achieve high efficiency in utilizing the thermal energy of the steam. The steam superheater has a third side and a fourth side. The fourth side of the steam superheater is connected to the second side of the steam generator. The steam generated in the second side of the steam generator is fed into the fourth side of the steam superheater, where it is further heated. The third side of the steam superheater is arranged upstream of the first side of the steam generator in terms of flow for the heating fluid. During normal operation, the heating fluid flows first through the steam superheater, thus transferring the highest temperature level to the steam, and then flows into the steam generator already slightly cooled.The device has a second bypass line. The second bypass line is arranged to bypass the third side of the steam superheater. The second bypass line has a second bypass valve. The steam superheater is preferably operated either in normal operation with the second bypass valve closed or in standby mode with the bypass valve open. Since the second side of the steam generator and the fourth side of the steam superheater are connected to one another, the pressure in the steam superheater is also adjusted by the steam generator, so that active control of the second bypass valve is not necessary. Optionally, active control of the second bypass valve can also be provided by the control device, for example to reduce condensation in the steam superheater. The steam check valve is preferably arranged downstream of the steam superheater, since then only one area to be controlled results from the second side and the fourth side.Of course, it would also be possible to separate them from each other and to regulate each one individually in the manner described above for the steam generator and thus to keep the pressure in the steam generator and steam superheater constant separately from each other.

[0016] A further embodiment of the invention is a plant for producing ammonia which is operated with renewable energy and which in particular has an electrolysis device for producing hydrogen.

[0017] In a further embodiment of the invention, the device comprises a heating device in the recirculation circuit. The heating device is preferably an electric heating device. The heating device serves to compensate for heat loss in standby mode and is switched off during normal operation.

[0018] In a further embodiment of the invention, the steam check valve is designed to completely shut off the second side of the steam generator. Closing the steam check valve thus creates a closed volume on the second side of the steam generator, since the water supply is already designed during normal operation so that the generated steam cannot escape through the water supply.

[0019] In a further embodiment of the invention, the control device is a split-range controller.

[0020] In a further aspect, the invention relates to a method for operating an apparatus according to the invention for producing ammonia. In particular, the method serves to operate a plant for producing ammonia using the Haber-Bosch process and is operated with renewably generated energy. The use of renewably generated energy leads to fluctuations, in particular at times when production is not possible or not practical because insufficient energy is available. The apparatus is therefore preferably switched to standby mode to bridge these times, which is actually unknown in conventional ammonia plants. The apparatus can be operated in regular operation or in standby mode. In standby mode, the vapor barrier valve is closed and the first bypass valve is usually open.Closing the steam check valve ensures that no steam, and therefore no unnecessary energy, escapes from the steam generator, thus significantly reducing the required heating output. In addition to standby operation, the process described below for standby operation can also be used for operation with reduced throughput, as described in DE 10 2022 204 103. Here and in the following, operation with reduced throughput is also considered standby operation, since the steam generator is switched to standby mode even if the converter itself is still operating at reduced throughput. The process is thus carried out during standby operation of the device. A recirculation gas flow is passed through the recirculation circuit.This consists mainly of the reactants nitrogen and hydrogen and the product ammonia, but usually also contains traces of, for example, argon from previous steps or as impurities in the reactants. In standby mode, the recirculation gas stream is heated to maintain a certain temperature level and enable rapid restart. The water supply is closed because no steam is generated in standby mode. The steam check valve is closed to completely shut off the second side of the steam generator. This creates a closed volume on the second side of the steam generator so that the pressure drops during cooling and increases during heating. A target pressure window is defined for the steam generator. The target pressure window is an important parameter for the design of the steam generator because pressure changes represent a stress.Thus, in addition to a maximum pressure, a minimum pressure is also specified for control operation, and the steam generator is to operate within this target pressure window. The target pressure window has a lower pressure threshold (minimum pressure) and an upper pressure threshold (maximum pressure). The control device compares the pressure measured by the pressure measuring device with the target pressure window. The control device closes the first bypass valve when the lower pressure threshold is reached or undershot, and opens the first bypass valve when the upper pressure threshold is reached or overshot. This means that only the energy actually required to maintain the pressure level in standby mode is supplied, thus enabling a simplified design of the steam generator without strong or frequent pressure changes with little additional energy expenditure.

[0021] A simple opening and closing process with only the digital states "open" or "closed" allows for very simple control and is therefore very efficient to implement. While such a system is prone to overshoot, this can easily be compensated for with a pressure relief valve, thus preventing pressure spikes.

[0022] In a further embodiment of the invention, opening and closing occurs partially. This means that the first bypass valve not only has two digital positions, open and closed, but can be opened or closed further in partial steps between them. In contrast to purely digital opening and closing, this allows for more precise control, which helps, in particular, to prevent pressure overshoot.

[0023] In a further embodiment of the invention, the opening and closing of the first bypass valve within the target pressure window occurs depending on the detected pressure, so that the opening degree of the first bypass valve increases with increasing pressure and decreases with decreasing pressure. For example, the opening degree can be adjusted linearly between closed (lower pressure threshold) and at (upper pressure threshold), so that the opening degree is ideally set so that exactly the right amount of heat is constantly supplied for a constant pressure.

[0024] In a further embodiment of the invention, the control device opens the first pressure relief valve when the upper pressure threshold is reached or exceeded. In addition to the use of a simple safety pressure relief valve, active control offers better, more constant, and more precise pressure adjustment. This prevents an unnecessarily high amount of steam from being released, the energy content of which would then have to be provided by the electric preheater. The device according to the invention is explained in more detail below using an exemplary embodiment illustrated in the drawing.

[0025] Fig. 1 exemplary device

[0026] Fig. 1 shows a first exemplary device. For example, the device is part of a recirculation circuit of an ammonia plant. A heating fluid, in the case of an ammonia plant the gas stream coming from the converter, is supplied during normal operation via a heating fluid inlet 90. The first bypass valve 22 and the second bypass valve 82 are closed, so that the heating fluid is fed into the third side 71 of the steam superheater 70 and from there into the first side 11 of the steam generator 10. Then, cooled, it is discharged again via the heating fluid outlet 91. On the opposite side, water is introduced into the second side 12 of the steam generator 10 via the water inlet 100, evaporated, and fed via the steam discharge line 30 into the fourth side 72 of the steam superheater 70 and from there further via the steam discharge line 30 to any desired consumer.

[0027] If the system is shut down, for example, due to a lack of renewable energy, the first bypass valve 22 and the second bypass valve 82 are opened to prevent heat loss, so that the heating fluid flow is now directed from the heating fluid inlet 90 via the second bypass line 80 and the first bypass line 20 to the heating fluid outlet 91. However, this would cause the pressure in the steam extractor 10 and the steam superheater 70 to drop, thus leading to a pressure swing load.

[0028] To prevent this, the steam shut-off valve 32 is first closed. A target pressure window is defined, for example, with a lower pressure threshold of 45 bara and an upper pressure threshold of 48 bara. The pressure in the second side 12 of the steam generator 10 is measured by the pressure measuring device 40 and transmitted to the control device 50. If the pressure falls below the lower pressure threshold of 45 bara, the control device 50 closes the first bypass valve 22. This causes heating fluid to flow again through the first side 11 of the steam generator 10, generating steam on the second side 12 and thus allowing the pressure to rise again. If the pressure rises to or above the upper pressure threshold of 48 bara, the control device 50 opens the first bypass valve 22.Additionally, the control device 50 can open the pressure relief valve 60 until the pressure has fallen below the upper pressure threshold of 48 bara and then close the pressure relief valve 60 again. This removes only the heat needed to keep the pressure in the steam generator and thus also in the steam superheater 70 within the target pressure window.

[0029] If the system is to be restarted, the steam check valve 32 is opened again and the first bypass valve 22 and the second bypass valve 82 are closed again. This allows normal operation to resume.

[0030] Reference symbol

[0031] 10 steam generators

[0032] 11 first page

[0033] 12 second page

[0034] 20 first bypass line

[0035] 22 first bypass valve

[0036] 30 Steam discharge line

[0037] 32 steam check valve

[0038] 40 Pressure measuring device

[0039] 50 Control device

[0040] 60 Pressure relief valve

[0041] 70 steam superheaters

[0042] 71 third page

[0043] 72 fourth page

[0044] 80 second bypass line

[0045] 82 second bypass valve

[0046] 90 Heating fluid inflow

[0047] 91 Heating fluid drain

[0048] 100 water supply

Claims

Patent claims 1. A device for producing ammonia with a recirculation circuit, wherein a recirculation gas stream flows through the recirculation circuit, wherein the recirculation circuit has a converter, a steam generator (10), and an ammonia separator, wherein the steam generator (10) has a first side (11) and a second side (12), wherein the second side (12) is designed to produce steam, wherein the recirculation gas stream is a heating fluid for the steam generator (10), wherein the device has a first bypass line (20), wherein the first bypass line (20) is arranged to bypass the first side (11) of the steam generator (10), wherein the first bypass line (20) has a first bypass valve (22), wherein the second side (12) is connected to a steam discharge line (30), wherein the second side (12) is connected to a water supply, the water supply can be closed,wherein the steam discharge line (30) has a steam check valve (32), wherein the second side (12) has a pressure measuring device (40), wherein the device has a control device (50), wherein the control device (50) is connected to the pressure measuring device (40) and the first bypass valve (22), wherein the control device is designed to control the first bypass valve (22) depending on the pressure detected by the pressure measuring device (40).

2. Device according to claim 1, characterized in that the Steam discharge line (30) has a pressure relief valve (60).

3. Device according to claim 2, characterized in that the Control device (50) is connected to the pressure relief valve (60), wherein the control device is designed to control the pressure relief valve (60) as a function of the pressure detected by the pressure measuring device (40).

4. Device according to one of the preceding claims, characterized in that the device comprises a steam superheater (70), wherein the steam superheater (70) has a third side (71) and a fourth side (72), wherein the fourth side (72) of the steam superheater (70) is connected to the second side (12) Steam generator (10), wherein the third side (71) of the steam superheater (70) is arranged fluidically for the heating fluid upstream of the first side (11) of the steam generator (10), the device has a second bypass line (80), wherein the second bypass line (80) is arranged to bypass the third side (71) of the steam superheater (70), wherein the second bypass line (80) has a second bypass valve (82).

5. Device according to claim 3 in conjunction with claim 4, characterized in that the steam check valve (32) is arranged downstream of the steam superheater (70).

6. Device according to one of the preceding claims, characterized in that the device has a heating device in the recirculation circuit.

7. Device according to one of the preceding claims, characterized in that the steam shut-off valve (32) is designed to completely shut off the second side of the steam generator (10).

8. Device according to one of the preceding claims, characterized in that the control device is a split-range controller.

9. A method for operating a device according to one of the preceding claims for producing ammonia, wherein the method is carried out during standby operation of the device, wherein a recirculation gas flow is passed through the recirculation circuit, wherein the recirculation gas flow is heated, wherein the water supply is closed, wherein the steam shut-off valve (32) is closed to completely shut off the second side (12) of the steam generator (10), wherein a target pressure window is defined for the steam generator (10), wherein the target pressure window has a lower pressure threshold and an upper pressure threshold, wherein the control device (50) compares the pressure detected by the pressure measuring device (40) with the target pressure window, wherein the control device opens the first bypass valve (22) when the lower pressure threshold is reached or undershot. closes, wherein the control device opens the first bypass valve (22) when the upper pressure threshold is reached or exceeded.

10. Method according to claim 9, characterized in that the opening and closing takes place partially.

11. Method according to one of claims 9 to 10, characterized in that the opening and closing of the first bypass valve (22) within the target pressure window takes place as a function of the detected pressure, so that the degree of opening of the first bypass valve (22) increases with increasing pressure and decreases with decreasing pressure.

12. Method according to one of claims 9 to 11, characterized in that the control device opens the first pressure relief valve (60) when the upper pressure threshold is reached or exceeded.

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