Preventing varying pressure loading of a steam generator in standby mode
The integrated steam generator system with bypass conduits and control valves addresses pressure cycling issues in ammonia plants by optimizing heat management and pressure regulation, enhancing efficiency and safety during standby or partial load operations.
Patent Information
- Authority / Receiving Office
- AU · AU
- Patent Type
- Applications
- Current Assignee / Owner
- THYSSENKRUPP UHDE GMBH
- Filing Date
- 2024-08-19
- Publication Date
- 2026-07-09
AI Technical Summary
Steam generators in ammonia production plants experience pressure cycling stresses due to fluctuations in renewable energy supply, leading to inefficiencies and increased maintenance costs when placed in standby or partial load operations.
An integrated steam generator system with bypass conduits and control valves that allow for efficient heat management during standby mode, minimizing heat loss and pressure fluctuations by regulating the recirculation gas stream and maintaining a target pressure window.
Reduces energy consumption and simplifies the steam generator design by minimizing heat loss and preventing frequent pressure cycling, ensuring safe and efficient operation during fluctuations in renewable energy supply.
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Abstract
Description
FIELD OF THE INVENTION The invention relates to a steam generator for a plant for producing ammonia which is 5 subject to fluctuations due to the use of renewable energy for example and is thus intermittently placed in standby operation. BACKGROUND OF THE INVENTION For decades ammonia has been produced predominantly by the Haber-Bosch 0 process. In most cases this comprises initially producing hydrogen from natural gas and reacting this with nitrogen under high pressure and at high temperature over a catalyst. Since this is an equilibrium reaction whose equilibrium is not shifted to the side of the products, the ammonia is separated in a recirculation loop and unconverted hydrogen and nitrogen are returned to the catalyst. However, the use of natural gas 5 produces a corresponding amount of carbon dioxide. To achieve sustainable production of ammonia the focus today is therefore on the electrolysis of water using renewably generated energy. However, this represents a large departure from the process used hitherto. If for example solar power is used, the 0 day-night cycle results in a period in which no renewable energy is available. A combination of solar and wind power makes it possible to ameliorate this effect somewhat but the fundamental problem remains. While in some regions electrical energy may then theoretically be obtained from the general electrical grid, this too is difficult to supply with renewable energy during these times. There are also plans for 25 plants to be erected at locations which are suitable for energy generation and have no access to an electrical supply grid. The electrolysis of water for production of hydrogen for an ammonia synthesis plant is known from US Patent 9 463 983 B2 for example. A converter for the synthesis of ammonia synthesis cannot simply be switched off and 30 on. For example, a temperature of at least 350°C is required for the reaction to occur over the catalyst. In ongoing operation the necessary energy (for compensating thermal losses and for heating the cooled recycle gas to the startup temperature of the 2024331549 22 May 2026 catalyst) is provided by the energy liberated during the reaction. The excess fraction of liberated energy is typically used for steam generation. Patent publication DE 10 2022 204 103 accordingly discloses methods for adapting 5 operation in case of falling amounts of renewably generated energy and thus making do with a smaller amount of energy. In the event of a lengthy outage of renewable energy, for example in the case of a prolonged lull, a so-called hot standby is undertaken where all plant parts are shut down and only the gas circuit of the converter is maintained and thermal losses are compensated using an electrical heating means. 0 Patent publication US 2012 / 137 683 A1 discloses a process for startup of a solar steam power plant. Patent publication US 2006 / 0162315 A1 discloses a waste heat boiler. 5 Patent publication WO 2012 / 089 826 A2 discloses an apparatus for energy generation. The synthesis of ammonia is an exothermic process and so the heat of reaction formed must be removed from the circuit in the normal course of the synthesis. This is typically 0 accomplished with a steam generator, preferably with a steam superheater, to produce for example saturated steam of 261°C and 48 bara in the steam generator. Said steam may be used in a wide variety of ways. Both operation under partial load and also partial shutdown result in a problem. The 25 first heat exchanger employed is typically a steam generator which removes the heat of reaction from the circuit. Taking said generator offline via a bypass for the hot recycle gas causes it to cool down, thus in turn leading to a fall in pressure in the steam generator. Thus, in a plant operated with renewable energy the steam generator must be configured for high pressure cycling stresses. 2024331549 22 May 2026 Against this background, it would be advantageous to devise an apparatus for producing ammonia comprising a recirculation circuit in which the cycling stress in the steam generator of the recirculation circuit is reduced and has a simplified design. 5 SUMMARY OF THE INVENTION The present invention provides an apparatus used for producing ammonia. What is concerned is accordingly a plant, especially operating according to the Haber-Bosch process, in which nitrogen and hydrogen are converted to ammonia at high temperature and high pressure. Since an equilibrium reaction is concerned the 0 reactants are recirculated and the product is removed. The recirculation circuit conducts a recirculation gas stream. The recirculation gas stream thus consists of hydrogen and nitrogen and, especially between the converter and the ammonia separator, also of ammonia. In addition, further gases, for example argon, may also unintentionally enter the recirculation circuit. The apparatus thus comprises a 5 recirculation circuit and the recirculation circuit comprises a converter, a steam generator and an ammonia separator. In addition, further components, especially a compressor, may also be arranged in the recirculation circuit. While the ammonia separator serves to remove the product ammonia produced in the converter from the recirculation gas stream, the steam generator serves to remove from the recirculation 0 gas stream the thermal energy liberated in the converter during the reaction. The apparatus is preferably subject to process-contingent fluctuations, for example and especially since the necessary energy is obtained regeneratively for example, i.e. by solar or wind power for example. In the case of an ammonia plant the hydrogen is produced electrolytically using renewably generated electrical energy for example. If 25 there is an absence of renewably generated energy, for example because it is night and / or there is a lull, the apparatus is placed on standby. However, since the actual reaction thus no longer occurs in the converter no thermal energy requiring removal by the steam generator is generated by the chemical reaction either. On the contrary the challenge is to minimize heat losses to minimize the costs of maintaining temperature. 30 To this end the steam generator is, according to the invention, integrated in such a way that it may likewise be switched to a particularly suitable standby operation. The steam generator has a first side and a second side. The second side is configured to produce 2024331549 22 May 2026 steam, i.e. water is supplied and evaporated. The medium to be cooled is passed through the first side downstream of a converter for producing ammonia. The recirculation gas stream is thus the heating fluid for the steam generator and is introduced into the steam generator in a hot state where it transfers a portion of its 5 heat, thus heating up the steam on the second side. However, since no excess heat is present in the recirculation gas stream in standby operation, no heat should be removed via the heating stream. The term heating fluid is therefore to be understood in a functional sense, i.e. the fluid which heats the water by transferring its heat and thus provides the energy required for steam generation. The apparatus therefore 0 comprises a first bypass conduit. The first bypass conduit is arranged for the heating fluid to bypass the first side of the steam generator. Thus, in standby operation heat loss may be minimized in order for example to minimize the electrical heating required to compensate for thermal emission. The recirculation gas stream, i.e., the heating fluid, is thus no longer passed through the steam generator as a heating stream to lose 5 heat but is run past the steam generator through the first bypass conduit, thus reducing heat loss to the heat loss through the wall of the first bypass conduit; the heat losses of the overall circuit remain unchanged. The first bypass conduit comprises a first bypass valve. In regular operation the first bypass valve is closed, the stream passes through the steam generator and steam is generated as normal. Patent publication DE 0 10 2022 204 103 for example discloses such a first bypass conduit which is opened in standby operation. The second side is connected to a steam discharge conduit. In regular operation the steam may be passed to further plant parts where this heat is required, including for example a turbine for generating electrical energy, via the steam discharge conduit. The second side is connected to a water feed. The water feed 25 supplies the water from which the steam is generated. The water feed is closable but due to the mode of operation of a steam generator said feed is configured such that even in the open state the steam does not escape through the water feed but only through the steam discharge. The steam discharge conduit comprises a steam shutoff valve. The steam shut-off valve can be used to close the steam generator in standby 30 operation in order thus to prevent outflow of steam and thus heat. The resulting closure of both the water feed and the steam discharge leads to a closed volume on the second side of the steam generator, with the result that steam no longer escapes from the 2024331549 22 May 2026 second side. The second side comprises a pressure measuring apparatus. This makes it possible to detect the pressure inside the second side. The apparatus comprises a control apparatus. The control apparatus is connected to the pressure measuring apparatus and the first bypass valve. The control apparatus is configured for controlling 5 the first bypass valve according to the pressure detected by the pressure measuring apparatus. This means that it is now not only possible to run the first bypass valve in the closed position in regular operation and the first control valve in the open position in standby 0 operation. Instead, in standby mode, the first bypass valve may now be specifically closed further whenever the pressure in the steam generator falls as a result of cooling, thus maintaining the temperature and therefore the pressure in the steam generator within a predetermined range since this causes heating fluid to be passed through the first side of the steam generator to heat the steam again. Since the second side is a 5 closed space no heat energy is thus removed but rather only heat loss is compensated, as a result of which additional heating in the recirculation circuit is only minimally increased but pressure cycling in the steam generator is thus reliably prevented, thus enhancing safety and simplifying construction. The steam shut-off valve keeps the volume constant so that a fall in temperature is accompanied by a fall in pressure. This 0 effect becomes particularly strong when the steam condenses. If the pressure rises, the first bypass valve is opened and so 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 and the electrical heating power required for compensation thereof. However, this also avoids frequent pressure cycling in the steam generator, thus 25 allowing for a simpler configuration thereof. Additional advantageous developments will become apparent from the description that follows, and from the drawing which illustrate schematically an embodiment of the apparatus. In a further embodiment of the invention, the steam discharge conduit comprises a pressure relief valve. The pressure relief valve may be in the form of a simple 2024331549 22 May 2026 overpressure valve. However, the pressure relief valve is preferably controllable. The control apparatus is preferably connected to the pressure relief valve. The control apparatus is configured for controlling the pressure relief valve according to the pressure detected by the pressure measuring apparatus. If the pressure rises above 5 an upper pressure limit the control apparatus can open the pressure relief valve and thus specifically control the pressure inside the second side of the steam generator specifically by closing and opening the first bypass valve and by opening and closing the pressure relief valve. This allows for very simple and efficient control. 0 In a further embodiment of the invention, the apparatus comprises a steam superheater. Steam generators and steam superheaters are usually connected in series and steam is run in countercurrent to the medium to be heated to achieve the highest possible steam temperature and thus achieve a high efficiency in the utilization of the thermal energy of the steam. The steam superheater has a third side and a 5 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 passed to the fourth side of the steam superheater and further heated therein. The third side of the steam superheater is arranged upstream of the first side of the steam generator in the flow direction of the heating fluid. In regular operation the 0 heating fluid flows through the steam superheater first and thus transfers the highest temperature level to the steam and then flows into the steam generator in an already somewhat cooled state. The apparatus comprises a second bypass conduit. The second bypass conduit is arranged to bypass the third side of the steam superheater. The second bypass conduit comprises a second bypass valve. The steam superheater 25 is preferably operated either in normal operation with a closed second bypass valve or in standby operation with an open bypass valve. 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 established by the steam generator so that active control of the second bypass valve is not necessary. Active control also of 30 the second bypass valve may nevertheless optionally be provided by the control apparatus in order for example to reduce condensation in the steam superheater. The steam shut-off valve is preferably arranged downstream of the steam superheater, 2024331549 22 May 2026 since only one region to be controlled then results from the second side and the fourth side. It would naturally also be possible to separate these from one another and control each one individually in the manner described above for the steam generator and thus keep the pressure in the steam generator and the steam superheater constant 5 independently of one another. A further embodiment of the invention provides a plant for producing ammonia with an apparatus as previously described above, which is operated with renewable energy and which comprises an electrolysis apparatus for producing the hydrogen. 0 In another embodiment of the invention, the apparatus comprises a heating apparatus in the recirculation circuit. The heating apparatus is preferably an electrical heating apparatus. The heating apparatus serves to compensate heat loss in standby operation and is switched off in regular operation. 5 In a further embodiment of the invention, the steam shut-off valve is configured to effect complete shut-off of the second side of the steam generator. The closing of the steam shut-off valve thus results in a closed volume on the second side of the steam generator since even in normal operation the water feed is configured such that the 0 produced steam cannot escape through the water feed. In a further embodiment of the invention, the control apparatus is a split-range controller. 25 In a further aspect, the invention provides a process for operating an apparatus according to the invention for producing ammonia. The process is especially used for operating a plant for producing ammonia by the Haber-Bosch process which is operated with renewably generated energy. The use of renewably generated energy results in fluctuations, especially also at times when production is not possible or 30 sensible since insufficient energy is available and the apparatus is therefore preferably switched to a standby mode to bridge these times, this being practically unknown from conventional ammonia plants. The apparatus may be operated in regular operation or 2024331549 22 May 2026 in standby operation. In standby operation the steam shut-off valve is closed and typically the first bypass valve is open. Closing the steam shut-off valve ensures that no steam and therefore no unnecessary energy escapes from the steam generator, thus markedly reducing the necessary heating power. In addition to standby operation 5 the process described below for standby operation can also be used for operation with reduced throughput, such as is described in patent publication DE 10 2022 204 103. Here and hereinbelow the operation with reduced throughput is also considered as standby operation since the steam generator is switched to standby operation even if the converter itself is still being operated at reduced throughput. The process is 0 therefore performed during a standby operation of the apparatus. A recirculation gas stream is passed through the recirculation circuit. The former consists mainly of the reactants nitrogen and hydrogen and of the product ammonia but typically also comprise traces of for example argon from preceding steps or as an impurity in the reactants. In standby operation the recirculation gas stream is heated to maintain a 5 certain temperature level and allow for rapid restarting. The water feed is closed because no steam is generated in standby operation. The steam shut-off valve is closed to effect complete shut-off of the second side of the steam generator. This results in a closed volume on the second side of the steam generator so that the pressure falls upon cooling and rises upon heating. A target pressure window for the 0 steam generator is specified. The target pressure window is an important parameter for the configuration of the steam generator, since pressure cycling represents a stress. Thus both a maximum pressure and also a minimum pressure are specified for regular operation and the steam generator is to be operated in this target pressure window. The target pressure window has a lower pressure limit (minimum pressure) 25 and an upper pressure limit (maximum pressure). The control apparatus compares the pressure detected by the pressure measuring apparatus with the target pressure window. The control apparatus closes the first bypass valve upon attaining or falling below the lower pressure limit and opens the first bypass valve upon attaining or exceeding the upper pressure limit. As a result only the energy that is really necessary 30 to maintain the pressure level in standby operation is supplied and so a simplified configuration of the steam generator without severe or frequent pressure cycling stresses is possible with little additional energy input. 2024331549 22 May 2026 A simple opening and closing with only the digital states open or closed allows for very simple control and can therefore be implemented very efficiently. While such a system does tend to overshoot, this can be very simply compensated by a pressure relief valve 5 to allow a pressure peak to be easily avoided. In a further embodiment of the invention, the opening and closing is partial. This means that the first bypass valve does not only recognize two digital positions open and closed but can be further opened or further closed in partial steps therebetween. In contrast 0 to a purely digital opening and closing, this allows for more specific control which especially helps to avoid overshooting in respect of pressure. In a further embodiment of the invention, the opening and closing of the first bypass valve within the target pressure window is carried out according to the detected 5 pressure so that the degree of opening of the first bypass valve increases with rising pressure and decreases with falling pressure. The degree of opening may for example be adjusted linearly between closed (lower pressure limit) and open (upper pressure limit) so as to ideally establish the degree of opening such that precisely the correct amount of heat for a constant pressure is constantly supplied. 0 In a further embodiment of the invention, the control apparatus opens the first pressure relief valve upon attaining or exceeding the upper pressure limit. In addition to the use of a simple safety overpressure valve the active control provides better, more constant and more precise pressure adjustment. Discharge of an unnecessarily large amount 25 of steam, whose energy content would then have to be provided by the electric preheater, is avoided. Further advantages and preferred or additional features of the invention will become clearer from the description hereinbelow provided with reference to an example of the 30 invention as shown in the accompanying drawings. 2024331549 22 May 2026 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration of an embodiment of an apparatus according to the invention. 5 DETAILED DESCRIPTION OF EMBODIEMNTS OF THE INVENTION Fig. 1 shows a first exemplary apparatus. The apparatus is a constituent of a recirculation circuit of an ammonia plant for example. A heating fluid, in the case of an ammonia plant the gas stream from the converter, is in regular operation supplied via a heating fluid inflow 90. The first bypass valve 22 and the second bypass valve 82 are 0 closed and so the heating fluid is passed into the third side 71 of the steam superheater 70 and from there into the first side 11 of the steam generator 10 and then discharged again via the heating fluid outflow 91 in a cooled state. On the opposite side, water is introduced via the water feed 100 into the second side 12 of the steam generator 10, evaporated and passed via the steam discharge conduit 30 into the fourth side 72 of 5 the steam superheater 70, and from there onward via the steam discharge conduit 30 to any desired consumer. If the plant is now shut down due to the absence of renewable energy for example, the first bypass valve 22 and the second bypass valve 82 are opened to avoid heat losses 0 with the result that the heating fluid stream is now passed from the heating fluid inflow 90 via the second bypass conduit 80 and the first bypass conduit 20 to the heating fluid outflow 91. However, this would have the effect of reducing the pressure in the steam generator 10 and in the steam superheater 70, thus leading to pressure cycling stress. 25 To avoid this, the steam shut-off valve 32 is initially closed. A target pressure window is specified, for example having a lower pressure limit of 45 bara and an upper pressure limit of 48 bara. The pressure in the second side 12 of the steam generator 10 is detected with the pressure measuring apparatus 40 and transmitted to the control apparatus 50. If the pressure falls below the lower pressure limit of 45 bara the control 30 apparatus 50 closes the first bypass valve 22. This has the result that heating fluid again flows through the first side 11 of the steam generator 10, thus generating steam on the second side 12 and thus causing the pressure to rise again. If the pressure 2024331549 22 May 2026 attains or exceeds the upper pressure limit of 48 bara the control apparatus 50 opens the first bypass valve 22. The control apparatus 50 can additionally open the pressure relief valve 60 until the pressure has fallen back below the upper pressure limit of 48 bara and then close the pressure relief valve 60 again. Thus, only the heat required to 5 keep the pressure in the steam generator and thus also in the steam superheater 70 within the target pressure window is removed from the heating fluid. If the plant is to be restarted the steam shut-off valve 32 is opened again and the first bypass valve 22 and the second bypass valve 82 are closed again. This makes it 0 possible to resume normal operation. Reference numerals 10 Steam generator 11 First side 5 12 Second side 20 First bypass conduit 22 First bypass valve 30 Steam discharge conduit 32 Steam shut-off valve 0 40 Pressure measuring apparatus 50 Control apparatus 60 Pressure relief valve 70 Steam superheater 71 Third side 25 72 Fourth side 80 Second bypass conduit 82 Second bypass valve 90 Heating fluid inflow 91 Heating fluid outflow 30 100 Water feed
Claims
1. An apparatus for producing ammonia comprising a recirculation circuit, wherein the recirculation circuit conducts a recirculation gas stream, wherein the5 recirculation circuit comprises a converter, a steam generator, and an ammoniaseparator, wherein the steam generator has a first side and a second side, wherein the second side is configured for producing steam, wherein the recirculation gas stream is a heating fluid for the steam generator, wherein the steam generator has heating fluid inflow and a heating fluid outflow, wherein0 recirculation gas stream is supplied as a heating fluid to the steam converter viathe heating flow inflow, wherein the apparatus comprises a first bypass conduit arranged for the heating fluid to bypass the first side of the steam generator, wherein the first bypass conduit comprises a first bypass valve, wherein the second side is connected to a steam discharge conduit and a closeable a water5 feed, wherein the steam discharge conduit comprises a steam shut-off valve,wherein the second side comprises a pressure measuring apparatus, wherein the apparatus further comprises a control apparatus connected to the pressure measuring apparatus and the first bypass valve, and wherein the control apparatus is configured for controlling the first bypass valve according to the0 pressure detected by the pressure measuring apparatus.
2. The apparatus as claimed in claim 1, wherein the steam discharge conduit comprises a pressure relief valve.25 3. The apparatus as claimed in claim 2, wherein the control apparatus isconnected to the pressure relief valve, and wherein the control apparatus is further configured for controlling the pressure relief valve according to the pressure detected by the pressure measuring apparatus.30 4. The apparatus as claimed in any one of the preceding claims, further comprisinga steam superheater having a third side and a fourth side, wherein the fourth side of the steam superheater is connected to the second side of the steam generator, wherein the third side of the steam superheater is arranged upstream2024331549 22 May 2026of the first side of the steam generator in the flow direction of the heating fluid, and further comprising a second bypass conduit arranged for the heating fluid to bypass the third side of the steam superheater, wherein the second bypass conduit comprises a second bypass valve.
55. The apparatus as claimed in claim 4 when dependent on claim 3, wherein the steam shut-off valve is arranged downstream of the steam superheater.
6. The apparatus as claimed in any one of the preceding claims, further comprising 0 a heating apparatus in the recirculation circuit.
7. The apparatus as claimed in any one of the preceding claims, wherein the steam shut-off valve is configured for complete shut-off of the second side of the steam generator.
58. The apparatus as claimed in any one of the preceding claims, wherein the control apparatus is a split-range controller.
9. A process for operating an apparatus according to any one of the preceding 0 claims for producing ammonia, wherein the process is performed during astandby operation of the apparatus, wherein a recirculation gas stream is passed through the recirculation circuit, wherein the recirculation gas stream is heated, wherein the water feed is closed, wherein the steam shut-off valve is closed to effect complete shut-off of the second side of the steam generator, 25 wherein a target pressure window for the steam generator is specified, whereinthe target pressure window has a lower pressure limit and an upper pressure limit, wherein the control apparatus compares the pressure detected by the pressure measuring apparatus with the target pressure window, wherein the control apparatus closes the first bypass valve upon attaining or falling below 30 the lower pressure limit, and wherein the control apparatus opens the firstbypass valve upon attaining or exceeding the upper pressure limit.
10. The process as claimed in claim 9, wherein the opening and closing of the first bypass valve is partial.2024331549 22 May 202611. The process as claimed in either one of claims 9 to 10, wherein the opening and closing of the first bypass valve within the target pressure window is carried out according to the detected pressure so that the degree of opening of the first5 bypass valve increases with rising pressure and decreases with falling pressure.
12. The process as claimed in any one of claims 9 to 11, wherein the control apparatus opens the first pressure relief valve upon attaining or exceeding the upper pressure limit.0