Method for operating an ammonia facility when a renewable energy source failure occurs

Abstract

The present invention relates to the operation of an ammonia facility, which uses renewable energy sources for producing in particular green ammonia, in the absence of the renewable energy form.

Description

The invention relates to a method of operating an ammonia plant utilizing renewable energy sources for producing especially green ammonia in case of outage of the renewable energy. For decades ammonia has been produced predominantly by the Haber-Bosch 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 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 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 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 9 463 983 B2 for example. A converter for the synthesis of ammonia synthesis cannot simply be switched off and on. For example, a temperature of at least 350°C is required for the reaction to occur over the catalyst. During ongoing operation the necessary energy (to compensate for thermal losses) is generated by the energy liberated during the reaction. DE 10 2022 204 103 accordingly discloses methods for adapting 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. Particularly in the case of prolonged lulls, for example more than three days, the required amounts of energy are still relatively high and so the required storage capacity, for example of rechargeable batteries, is still high. It is an object of the invention to further reduce the energy demand upon complete shutdown of the remaining plant but with easier restarting of the converter. This object is achieved by the method having the features specified in claim 1. Advantageous developments will be apparent from the dependent claims, from the description that follows, and from the drawing. The method according to the invention is used for operating an ammonia synthesis plant. The method according to the invention expands the normal prior art operation of an ammonia synthesis plant for a special case which occurs especially through the increased use of renewable energy and the hydrogen usually electrolytically generated by this energy. While a very constant hydrogen stream has hitherto enabled continuous operation in the Haber-Bosch process, variations in the realm of renewable energies should now also be expected to result in a variation in the provided amount of hydrogen. An exacerbating factor is that certain components, for example the compressors, do not exhibit a linear relationship of energy consumption to conveying capacity but rather still have an energy demand of about 85% even at 10% throughput. Especially in the case of wind power, lulls may also have a prolonged duration. Complete buffering via energy storage devices is unrealistic. Prior art methods of operation shall therefore be modified or expanded to allow the most efficient possible restarting including for example after a long lull. The ammonia synthesis plant comprises a recirculation loop. The recirculation loop comprises a converter having at least one first catalyst bed, a first heat exchanger, a second heat exchanger, an ammonia separator, a compressor and a reactant feed. This corresponds to the customary setup of a Haber-Bosch plant. According to the invention in case of interruption to the hydrogen supply the method comprises the steps of: a) terminating the gas flow through the converter, b) capturing the temperature of at least the first catalyst bed and simultaneously receiving a prediction for resumption of the hydrogen supply. If the converter comprises several catalyst beds step b) comprises capturing the temperature in or at the gas outlet of each catalyst bed. The interruption is especially and preferably attributable to the variation in the generation of renewable energy. An interruption need not be complete. Depending on the plant, continued operation is often no longer economic and / or technically feasible below 5% to 15% and so a reduction below the respective threshold of the plant is to be considered an interruption in the context of the invention. An essential feature is that, in contrast to all methods used hitherto, the converter is simply not subjected to flow. All further flow causes even faster cooling, as has been shown. Immediately terminating the flow thus reduces the cooling. This makes it possible to do without electrical auxiliary heating (in addition to the compressor) which in turn massively reduces the need for storage of energy and thus capital costs. In this point the method thus differs markedly from prior art methods, for example and especially from the method disclosed in DE 10 2022 204 103. The information captured in step b) is used to distinguish between cases: I)      a resumption of the hydrogen supply is expected within a specified period, II)      a resumption of the hydrogen supply is not expected within a specified period. The specified period is chosen such that it is suitable for the selected plant, i.e. especially with regard to storage capacity for bridging the non-generation of renewable energy and forecasting confidence with regard to renewable energy generation. An exemplary, customary and advantageous period is four hours. Precise weather forecasts, and thus also forecasting of the generated amount of renewably generated energy, are possible on this timescale. At the same time the amounts of energy required for a period of 4 h are still very readily storable. These cases thus then result in sub-cases, from which the following different action steps result: A) if in case I) the temperature of the first catalyst bed is above 350°C restarting the gas flow through the converter to resume the hydrogen supply, B) if in case I) the temperature is below 350°C restarting the gas flow through the converter and effecting auxiliary heating in the gas flow so that at the time of resuming the hydrogen supply the temperature is above 350°C, C) if in case II) the temperature is above a value between 150°C and 200°C doing nothing, D) if in case II) the temperature falls to a value between 150°C and 200°C restarting the gas flow through the converter. Thus after two times two possible selections a decision tree thus ultimately results in precisely four action steps, of which action steps A) and B) belong to case I) and action steps C) and D) belong to case II). In action step A) immediate restarting of the converter is possible since the converter still has a sufficiently high temperature. Restarting naturally only occurs towards the end of the predetermined period when resumption of the hydrogen supply makes operation possible again. This means that especially in the case of solar energy generation, outage times (night) are short enough to usually fall under action step A). In such cases the energy storage capacity may therefore be kept minimal. This applies all the more, the lower the seasonal variations in the solar electricity. In action step B) the converter is to be heated before it is restarted. Such plants are typically heated at a heating rate of 50°C. The typical time for weather forecasting is 4 h for reliable predictions so that the plant may be configured with an energy storage means for the compressor and the heater for four hours. A ramp up from 150°C to 350°C is possible in this time. However, if the temperature were 300°C heating would need to be commenced only one hour before resumption of the hydrogen supply and thus before the end of the predetermined period. Heating is therefore commenced as late as possible but such that at the time of resuming the hydrogen supply the temperature is above 350°C. In action step A) and B) the time of resuming the hydrogen supply can naturally be adjusted if the latter is available earlier or later relative to the forecast. In this case it is preferable to make a corresponding adjustment. In action step C) a restarting is not yet expected within the forecasting period. However, the converter is still hot enough to simply keep the converter closed to minimize cooling. This means that no energy is required and therefore no energy storage is required either. In conventional plants a period of approximately 3 days for example is possible in this way and without the use of additional energy. It is thus possible to bridge a relatively long period for restarting after action step B) with a only a small energy storage means. In this context doing nothing is to be understood as meaning that no energy is intentionally introduced to the converter for heating. That is to say natural cooling is awaited. Action step D) bridges a period even longer than the period after action step C). If the temperature falls below a predetermined value from the window 150°C to 200°C, i.e. for example a predetermined value of 170°C, the gas circuit is reopened but only the compressor is operated i.e. no electric heater is additionally operated. Energy is thus only introduced to the process via the operation of compression. This allows the temperature to be stabilized to the predetermined value (which naturally depends on the exact plant design and the ambient conditions). This saves the electrical energy for the electric heater compared to the conventional method. This allows the electrical storage means to be smaller even for longer outages. By maintaining this relatively low temperature level, radiant emission and thus heat loss and thus the amount of energy to be introduced is naturally reduced. At the same time temperatures of 200°C or less are unproblematic for the compressor so that for example the heat exchangers may be bypassed via bypass connections, thus minimizing heat losses from the recirculation loop. Accordingly the first heat exchanger is preferably bypassed with a first heat exchanger bypass connection during action step D). The second heat exchanger may optionally also be bypassed with a second heat exchanger bypass connection in order especially to be able to reduce the flow resistance in the recirculation circuit. In a further embodiment of the invention case differentiation comprises estimating resumption of the hydrogen supply via the weather forecast to forecast renewably generated electricity. While the daily cycle can be reliably forecast especially during sunshine, cloud cover is a decisive factor. By contrast, for wind power prolonged lulls may also be a problem. However, for plants for generating renewable energy historical data are typically used to provide a reliable prediction about the frequency and length of expected production outages so that the energy storage means can be configured for bridging these times. The teaching according to the invention especially allows such a storage means to be made as small as possible. In a further embodiment of the invention 2 to 8 h, preferably 4 h, is used as the forecast period for the resumption of the hydrogen supply. This period combines the possibility of reliable weather prediction for generation of renewable energies with the time window necessary for reheating the converter. In a further embodiment of the invention termination of the flow in step a) is effected by closing quick-close valves. In the prior art methods the gas stream is maintained and kept at the temperature level via an electrical heating means. However, the closing reduces the heat loss and leads to a massive reduction in the energy demand especially in the case of longer interruptions. In a further embodiment of the invention in a converter having at least two catalyst beds the temperature is measured in or downstream of each catalyst bed. To select the action steps the lowest measured temperature is used. While in full-load operation the temperature is highest downstream of the first catalyst bed and may reach 500°C for example and downstream of the third catalyst bed the temperature achieved is only about 440°C, this ratio may actually be reversed in the case of a very low loading of for example 10% (actual conversion based on maximum possible conversion) so that for example a temperature of 380°C is achieved downstream of the first catalyst bed and a temperature of 400°C is achieved downstream of the third catalyst bed. Measurement after each catalyst bed is therefore advantageous. In a further embodiment of the invention in case I) an optional waiting time until commencement of the action step A) or B) is determined from the measured temperature and from the expected time of resumption of the hydrogen supply. This is advantageous and preferable when it is foreseeable that for example 4 h before resumption of operation the temperature is above 350°C, i.e. theoretically no preheating is required but the temperature will fall below 350°C in the four hours due to further cooling. However in this case action step B) is to be commenced as late as possible and as early as necessary to ensure that the temperature is reliably above 350°C in good time. In a further embodiment of the invention a temperature of between 165°C and 175°C, preferably of 170°C, is selected for action steps C) and D). The method according to the invention is naturally not limited only to the abovedescribed case of a varying operating mode in the production of hydrogen by renewably generated electrical energy, even if this is to be regarded as the most likely use case. The method may also be used if for example due to an emergency plan a supply of gas is temporarily halted or a steam reformer which generates the hydrogen suffers an outage or requires service or repair. In these cases too, operation may similarly be restarted more quickly as soon as the hydrogen is available again and the energy required for the intervening period be reduced. The method according to the invention shall now be more particularly elucidated with reference to a working example shown in the drawing. Fig. 1 First exemplary embodiment Fig. 1 is a simplified schematic diagram of the recirculation loop 10. The reactant feed 80 is used to supply a hydrogen-nitrogen mixture to the recirculation loop 10 for reaction, wherein this stream is subject to severe variation and may vary between 100% (maximum load) and 0% (standby). The gas stream is supplied to the converter 20 via the compressor 60 and the second heat exchanger 40. In regular operation the gas mixture exiting the converter 20 is passed through the first heat exchanger 30 to dissipate the heat of reaction and the second heat exchanger 40 to the ammonia separator 50. Ammonia is separated therein and removed from the loop via the product discharge 90. If the provision of a hydrogen amount generated by renewable energy falls below a threshold, for example 10%, for a prolonged period, i.e. at least for several hours, it is more efficient to set the plant into standby mode. To this end the quick-close valves 22 are closed and all plant parts shut down. The converter 20 now undergoes slow cooling and for example after 1.5 days reaches a temperature of 350°C. Until then the plant could be restarted at any time and without delay. If the prediction still shows no change, nothing is changed and the converter is allowed to cool further, for example to a temperature of 170°C. In this case valves V are opened so that the first heat exchanger 30 via the first heat exchanger bypass connection 100 and the ammonia separator 50 via the ammonia separator bypass connection 110. The compressor 60 is started and the energy introduced by the compressor 60 stabilizes the temperature to 170°C. If the prediction indicates restarting in 4 hours the heating element 70 is switched on and the converter is heated at 50°C per hour. The converter ultimately has a temperature of 370°C when hydrogen is available for synthesis again and ammonia synthesis may be resumed according to the prior art. Reference symbols 10 Recirculation loop 20 Converter 22    Quick-close valve 30    First heat exchanger 40 Second heat exchanger 50 Ammonia separator 60 Compressor 70 Heating element 80 Reactant feed 90 Product discharge 100 First heat exchanger bypass connection 110 Ammonia separator bypass connection V Valve

Claims

1. A method for operating an ammonia synthesis plant, wherein the ammonia synthesis plant comprises a recirculation loop (10), wherein the recirculation loop (10) comprises a converter (20) having at least one first catalyst bed, a first heat exchanger (30), a second heat exchanger (40), an ammonia separator (50), a compressor (60) and a reactant feed (80), characterized in that in case of interruption to the hydrogen supply the method comprises the steps of: a) terminating the gas flow through the converter (20),b) capturing the temperature of at least the first catalyst bed and simultaneously receiving a prediction for resumption of the hydrogen supply,wherein the information captured in step b) is used to distinguish between cases:I) a resumption of the hydrogen supply is expected within a specified period,II) a resumption of the hydrogen supply is not expected within a specified period,wherein the different cases result in the following action steps:A) if in case I) the temperature of the first catalyst bed is above 350°C restarting the gas flow through the converter (20) to resume the hydrogen supply,B) if in case I) the temperature is below 350°C restarting the gas flow through the converter (20) and effecting auxiliary heating in the gas flow so that at the time of resuming the hydrogen supply the temperature is above 350°C,C) if in case II) the temperature is above a value between 150°C and 200°C doing nothing,D) if in case II) the temperature falls to a value between 150°C and 200°C restarting the gas flow through the converter (20).

2. The method as claimed in claim 1, characterized in that case differentiation comprises estimating resumption of the hydrogen supply via the weather forecast to forecast renewably generated electricity.

3. The method as claimed in claim 2, characterized in that 2 to 8 h, preferably 4 h, is used as the forecast period for the resumption of the hydrogen supply.

4. The method as claimed in any of the preceding claims, characterized in that termination of the flow in step a) is effected by closing quick-close valves.

5. The method as claimed in any of the preceding claims, characterized in that in the case of a converter (20) having at least two catalyst beds the temperature is measured in or downstream of each catalyst bed, wherein to select the action steps the lowest measured temperature is used.

6. The method as claimed in any of the preceding claims, characterized in that in case I) an optional waiting time until commencement of the action step A) or B) is determined from the measured temperature and from the expected time of resumption of the hydrogen supply.

7. The method as claimed in any of the preceding claims, characterized in that a temperature of between 165°C and 175°C, preferably of 170°C, is selected for action steps C) and D).

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