Method for controlling an ammonia plant

Abstract

A method for controlling an ammonia plant (1), wherein the ammonia plant (1) includes an ammonia synthesis section (202) having an ammonia converter and a hydrogen generation section (200) connected to a hydrogen storage tank (5), the method comprising controlling the amount of hydrogen (13) stored or delivered to the ammonia synthesis section to maintain the following target ranges: the amount of hydrogen contained in the hydrogen tank (5); the flow rate of hydrogen delivered to the ammonia synthesis section; and the flow rate of the feed gas delivered to the ammonia converter.

Description

Technical Field

[0001] This invention pertains to the field of ammonia production. Specifically, this invention relates to a method for controlling an ammonia plant, and to an ammonia plant and an ammonia synthesis process for implementing the method. Background Technology

[0002] In the industrial production of ammonia, a suitable makeup gas (a mixture of hydrogen and nitrogen in appropriate ratios) is generated at the front end and catalytically reacted in the reaction section to produce ammonia. Specifically, the generation of the makeup gas requires the production of gaseous hydrogen. For example, in the context of secondary reforming and / or separate preparation (e.g., by separation from air), a suitable amount of nitrogen is introduced along with combustion air. The ammonia synthesis reaction takes place in a converter, typically part of the so-called ammonia synthesis loop.

[0003] Conventionally, hydrogen is produced via reforming processes using hydrocarbon or carbon-containing sources, such as natural gas or coal. In reforming-based processes, the hydrogen source (such as the aforementioned hydrocarbon or carbon-containing sources) is virtually constant over time, so the entire process and associated equipment are designed to operate essentially under steady-state conditions with limited fluctuations around the nominal capacity, for example, 70% to 110% of the nominal capacity.

[0004] However, there is renewed interest in more flexible ammonia plants adapted to operate using time-varying hydrogen sources, where the flow rate and other parameters of the hydrogen stream that can be generated from said source can vary significantly over time. This is typically the case with hydrogen generated from renewable energy sources, such as the solar electrolysis of water.

[0005] In the context of ammonia production, generating hydrogen from renewable energy sources is of great interest because it can reduce or eliminate CO2 emissions (so-called "green" ammonia plants). However, it introduces the problem of how to handle the variable output of such renewable energy-based hydrogen generation. As mentioned above, conventional ammonia plants are designed to operate with a constant or nearly constant hydrogen input, and therefore they typically cannot follow the typical fluctuations of renewable energy sources, especially the time-varying hydrogen flow rate.

[0006] Fluctuations in the hydrogen source cause fluctuations in the feed gas flow rate to the ammonia synthesis section. However, the synthesis section may not be able to withstand such fluctuations; for example, when the feed gas input is too low, the ammonia converter may not be able to maintain self-sustaining operation, resulting in downtime and related production losses.

[0007] The conventional solution to the aforementioned problem is to provide hydrogen storage tanks. These tanks can temporarily compensate for the lack of hydrogen production, which acts as an intermediate buffer. However, this solution has the disadvantage of the cost of such hydrogen buffer storage tanks. Hydrogen must be stored under elevated pressure, and the associated costs are quite high. For example, for ammonia plants using hydrogen from water electrolysis driven by electricity generated in a solar photovoltaic field that has no energy storage and is not connected to the grid, large hydrogen storage tanks are necessary to ensure stable ammonia production to compensate for nighttime power shortages. The cost of such storage tanks would significantly reduce the competitiveness and advantages of "green" methods for ammonia production.

[0008] WO 2021 / 089276 A1 discloses a method for controlling an ammonia synthesis circuit operating under partial load. Summary of the Invention

[0009] This invention relates to the problem of adapting ammonia plants and methods to variable hydrogen sources. Specifically, when the production of ammonia makeup gas depends partially or entirely on a variable hydrogen source, the invention faces the problem of how to reduce the need for hydrogen storage tanks in ammonia plants. The invention also addresses the challenge of operating so-called green ammonia plants with small hydrogen storage tanks and achieving cost-effective control of such equipment. Specifically, the invention solves the hydrogen storage problem and how to reduce hydrogen storage tanks while maintaining stable operation of the plant and process when the hydrogen source is affected by fluctuations.

[0010] Specifically, the present invention aims to avoid the disappearance of the reaction and depressurization of the ammonia synthesis circuit under reduced load, and to avoid excessive pressurization of the circuit under high load, thereby using a relatively small hydrogen storage tank to reduce related costs.

[0011] This problem is solved by the method for controlling an ammonia device according to the claims.

[0012] This method is applied to an ammonia plant that includes an ammonia synthesis section, a hydrogen generation section, and a hydrogen storage tank. The ammonia plant may include, but is not limited to, other sections, including a nitrogen generation section. The ammonia synthesis section includes an ammonia converter, in which ammonia is synthesized from a feed gas containing hydrogen and nitrogen at ammonia synthesis pressure.

[0013] This method includes controlling the following:

[0014] a) The total amount of hydrogen transported from the hydrogen generation section to the ammonia synthesis section;

[0015] b) The ratio of hydrogen currently produced in the hydrogen production section to hydrogen extracted from the storage tank at point a) above;

[0016] c) The amount of hydrogen delivered to the hydrogen storage tank.

[0017] According to the present invention, the above control is performed to maintain the following target ranges: i) the amount of hydrogen contained in the hydrogen tank (5); ii) the flow rate of hydrogen delivered to the ammonia synthesis section and / or the flow rate of the feed gas reacting in the ammonia converter; iii) the rate of change of at least one of the flow rates at point ii) over time.

[0018] Select target ranges ii) and iii) to keep the ammonia converter in autothermal operation, i.e., to avoid reaction loss and / or converter shutdown.

[0019] According to b), the hydrogen supplied to the ammonia synthesis section may contain 0% to 100% hydrogen instantaneously generated in the hydrogen production section, 0% to 100% hydrogen obtained from storage tanks, or any combination thereof, including a portion of the instantaneously generated hydrogen and a portion of the hydrogen obtained from storage tanks. A suitable combination may be selected based on the above objectives i), ii), and iii).

[0020] This invention provides dynamic control of hydrogen storage tanks, which allows for a significant reduction in the size and cost of hydrogen storage tanks compared to conventional technologies. Therefore, this invention opens up new and interesting possibilities for the production of ammonia from renewable energy-based hydrogen, and thus the so-called decarbonization of ammonia plants. The plant itself is more flexible than conventional plants and can maintain ammonia production over a wider range of plant capacities (10%-110%). Having a relatively small hydrogen storage tank can benefit from reducing the average cost of ammonia (LCOA) and the capital expenditure (CAPEX) of the ammonia plant, because the size of the ammonia synthesis loop and its converters must not be able to accommodate peak hydrogen production from the electrolyzer, but can be designed to be smaller, as peak production can be fed into the storage tank. Increasing the hydrogen storage tank introduces additional costs to the storage tank itself, but on the other hand, allows for the installation of smaller and cheaper converters for a given capacity. The additional cost for the hydrogen storage tank is more than compensated by the savings in smaller converters. This is especially true in the case of highly variable power distribution with a small capacity factor. The capacity factor is defined as the ratio between annual power generation and the peak of the installed power capacity.

[0021] Ammonia plants using the method of this invention can economically accept variable hydrogen and nitrogen inputs from renewable energy sources. Therefore, it can promote greater adoption of renewable energy in the ammonia market, thereby facilitating the production of so-called green ammonia.

[0022] Another aspect of the present invention is an ammonia plant and a method according to claim 1. Yet another aspect of the present invention is a method for modifying an ammonia plant according to claim 2.

[0023] Detailed description of the invention

[0024] This invention is particularly interesting for ammonia plants in which hydrogen is generated from renewable energy sources, such as solar energy, due to the inherent volatility of such energy sources. However, it should be understood that this is not a prerequisite, and the invention can also be applied to conventional ammonia plants with feed hydrogen at time-varying flow rates to reduce the need for large intermediate hydrogen storage tanks. The variable input of hydrogen, or the variable input of energy for hydrogen generation, can also be specified or selected based on cost considerations. For example, during periods of low energy cost, energy input from the grid can be used to generate hydrogen; during periods of high energy cost, ammonia generation can depend at least in part on hydrogen storage tanks.

[0025] This invention applies to ammonia plants where hydrogen is generated entirely or partially from renewable sources, or hybrid plants where hydrogen is generated partially from renewable sources and partially from conventional sources (such as reforming). In some embodiments, hydrogen may be generated from electrical energy (e.g., by electrolysis), wherein the electrical energy is generated partially or entirely from renewable sources. Preferred renewable energy sources include solar and wind power.

[0026] In various embodiments of the invention, a preferred method for generating hydrogen from renewable energy sources is water electrolysis. Particularly preferably, the water electrolysis can be powered by solar or wind energy.

[0027] According to an embodiment of interest, the present invention can be applied to so-called island-type or peninsula-type ammonia systems that use renewable energy to generate hydrogen. An island-type system refers to a system without a power grid connection; a peninsula-type system refers to a system with a power grid connection, however, using that connection under limited conditions. In an island-type system, the power input for normal operation can be entirely provided by a renewable source such as solar energy.

[0028] In embodiments of the invention, hydrogen is generated by renewable electricity, and the ammonia unit is an island-type unit not connected to the power grid; the unit includes a backup power system configured to provide at least power to the ammonia unit in cases where the hydrogen generation section produces little or no hydrogen. For example, low output of the hydrogen generation section may occur when renewable electricity is 10% or less than the nominal value. Therefore, hydrogen input is primarily or entirely supplied by a hydrogen storage tank.

[0029] The above-described situation of ammonia plants operating on standby power refers to, for example, ammonia plants that produce hydrogen based on solar energy at night or more generally when solar energy is unavailable or very low (e.g. due to dense cloud cover).

[0030] The backup power is sized to ensure the operation of various equipment in the ammonia plant, particularly the hydrogen and nitrogen compressors, which are essential for raising hydrogen and nitrogen to the ammonia synthesis pressure. When operating on backup power, the compressors can operate under reduced loads if compatible with stable operation.

[0031] The backup power is preferably generated by any one of a gas turbine, gas engine, fuel cell, or suitable battery. The gas turbine, gas engine, and fuel cell preferably use hydrogen or ammonia for combustion. For wind farms, batteries for backup power are preferred because wind energy typically has short-cycle fluctuations that can be effectively compensated for by batteries. For solar-based systems (e.g., photovoltaic), due to the day / night cycle, installing turbines, engines, or fuel cells is preferred.

[0032] In an electrolyzer-based ammonia plant, backup power is virtually sized to operate all equipment except the electrolyzer. In embodiments of interest, 90% or more of the power input is absorbed by the electrolyzer. Therefore, backup power can reach 10% or less of the peak power required by the plant. In some embodiments, backup power can be 5% to 10% or less of the peak power.

[0033] The advantage of the aforementioned backup power operation is that when the primary energy source is unavailable or has very low output, the island-type ammonia plant remains under minimum load, thus avoiding a complete shutdown. The island-type ammonia plant according to the invention can be installed in remote locations without a power grid and still operate advantageously. Another advantage is that the island-type ammonia plant can generate ammonia that is truly “green ammonia,” i.e., generated using energy obtained entirely from renewable sources.

[0034] In peninsula-type systems, most of the electricity is generated internally by renewable sources, and the grid connection is used to provide a minimum amount of electricity when renewable sources are unavailable, or to absorb peak power when the electricity output from renewable sources exceeds the target ammonia generation requirement.

[0035] For peninsula-type green ammonia plants with dedicated renewable power generation and grid connection, the grid connection is provided to always guarantee the availability of the minimum amount of electricity for the plant to operate under minimum load (in the case of solar photovoltaic power generation, electricity is introduced at night) and the minimum amount of electricity to absorb peak renewable power generation. A related advantage of applying this invention to peninsula-type plants is that the introduction of grid power is minimized, and if grid power is expensive, this can reduce the overall cost of ammonia production.

[0036] The ammonia synthesis section includes an ammonia synthesis reactor (ammonia converter), which is adapted to react the makeup gas with a suitable catalyst into a gaseous product containing ammonia. The converter is typically part of the ammonia synthesis loop.

[0037] In addition to the ammonia converter, the ammonia synthesis loop may include a circulator, which is a compressor configured to maintain circulation in the loop and supply the ammonia converter; a condenser disposed downstream of the converter for condensing the ammonia-containing effluent; a separator, wherein, after condensation, the ammonia product is separated into a liquid ammonia product and a gaseous recirculation stream containing unconverted material; and a recirculation line from the separator to the suction side of the circulator.

[0038] In a preferred embodiment, the loop includes a bypass line to the ammonia converter, which is arranged to allow some of the makeup gas supplied by the circulator to bypass the ammonia converter. According to the loop pressure control method disclosed in WO 2021 / 089276 A1, the bypass flow rate can be controlled to maintain the synthesis loop pressure at an acceptable value when operating under reduced loads. Very preferably, the hydrogen storage control of the present invention can be combined with said loop pressure control.

[0039] The ammonia converter is supplied with ammonia make-up gas. The term ammonia make-up gas refers to a gaseous mixture of hydrogen and nitrogen containing an H / N ratio suitable for ammonia synthesis. This ratio is typically 3:1 or about 3:1.

[0040] Ammonia synthesis pressure is high, typically above 100 bar. In most applications, ammonia synthesis pressure is in the range of 100 to 300 bar, more preferably 120 to 250 bar. The hydrogen storage tank is also at a suitable high pressure (such as 200 bar). Therefore, the hydrogen generated by the hydrogen production section is typically compressed for storage and supply to the make-up gas line.

[0041] The parameters a), b), and c) above can be controlled to ensure that the amount of hydrogen in the hydrogen tank is above the minimum and below the maximum to prevent the tank from being emptied or overfilled. For example, the amount of hydrogen in the tank can be maintained above a minimum value corresponding to 10% to 30% of the tank's nominal storage capacity and below a maximum value corresponding to 70% to 90% of the nominal storage capacity.

[0042] If the reservoir is below minimum or empty, there is a risk of reaction loss and shutdown. On the other hand, if the reservoir is full, the device may not be able to process the produced hydrogen, forcing some hydrogen to be released and preventing full utilization of renewable electricity.

[0043] Select objectives ii) and iii) above to maintain the ammonia converter in a self-heating operation. This self-heating operation means that the heat recovered from the hot effluent of the reaction preheats the fresh feed gas to a sufficiently high temperature to sustain the reaction.

[0044] Typically, the feed gas is preheated before it is catalytically reacted to form ammonia; the preheating of the feed gas is carried out by transferring heat from the hot effluent of the ammonia synthesis reaction to the fresh feed gas; the self-heating operation corresponds to a condition in which the preheated feed gas has a temperature equal to or greater than a threshold temperature.

[0045] The threshold temperature can be input data for the method of the present invention. The threshold temperature is selected to avoid unwanted reaction losses and the risk of converter downtime. The threshold temperature can vary depending on the catalyst used in the ammonia synthesis reaction. For catalysts commonly used in ammonia synthesis, the threshold temperature can include between 300°C and 400°C, more often between 300°C and 350°C.

[0046] The preheating of the fresh gas takes place in the feed effluent heat exchanger, which can be part of the ammonia converter or a separate unit.

[0047] In fresh gas preheating, the heat content of the reaction effluent (i.e., the heat generated by the exothermic ammonia synthesis reaction) is used to preheat the fresh feed to a sufficiently high temperature for subsequent reactions. If the temperature drops below a given threshold, the kinetics of the ammonia synthesis reaction and the temperature of the effluent decrease, resulting in less heat available for feed gas preheating, and the temperature tends to decrease further, leading to converter shutdown. The method of the present invention is configured to avoid such reaction loss and converter shutdown.

[0048] In other words, the method of the present invention maintains the ammonia converter in self-heating operation (also known as self-sustaining operation).

[0049] Preferably, the target range ii) is selected such that the maximum and minimum loads of the ammonia converter are within the set design limits to avoid the aforementioned reaction loss in the converter. Even more preferably, the target range iii) is selected such that the rate of change of the load of the ammonia converter is within the set design limits to avoid the aforementioned reaction loss in the converter. Therefore, the present invention provides stable operation and, in particular, reduces the risk of the ammonia synthesis reaction disappearing in the converter during descent or ascent.

[0050] Hydrogen supplied to the ammonia synthesis section is typically mixed with nitrogen in an appropriate ratio to obtain ammonia makeup gas. This ammonia makeup gas is then supplied to the ammonia synthesis circuit, which includes an ammonia converter.

[0051] Based on condition iii) above, this method controls the rate of change of the flow rate of hydrogen delivered to the ammonia synthesis section or the flow rate of the feed gas to the ammonia converter over time. This rate of change over time corresponds to a ramp-up or ramp-down of the load in the ammonia synthesis section, which should not exceed a certain maximum allowable value for the ammonia converter. The ramp-up is typically expressed as a percentage of time, so for example, the maximum ramp-down could be 100% / h. A maximum ramp-down can also be defined with reference to shorter time periods (e.g., 8% / 5 minutes).

[0052] The load on the ammonia synthesis section is given by the flow rate of the makeup gas processed in the ammonia synthesis section, and is therefore proportional to the hydrogen received from the hydrogen generation section. The load on the ammonia synthesis circuit can also be referenced.

[0053] The method of the present invention can control parameters a), b), and c) based on the current load of the ammonia synthesis section. Parameters a), b), and c) can be controlled based on one or more of the following: the current hydrogen output of the hydrogen production section; the amount of hydrogen contained in the hydrogen tank; the amount of hydrogen transferred to the ammonia synthesis section; the current load of the ammonia synthesis section; and one or more past values ​​of the load of the ammonia synthesis section.

[0054] In a further embodiment, the parameters a), b), and c) are controlled to also satisfy the condition that the ammonia synthesis pressure is within the target range.

[0055] In a preferred embodiment, items a), b) and c) are controlled based on one or more setpoint signals, and the one or more setpoint signals are generated according to the amount of hydrogen contained in the hydrogen tank.

[0056] In a preferred embodiment, hydrogen is stored in gaseous form, and therefore the amount of hydrogen contained in the container can be measured by detecting the pressure in the container. However, in some embodiments, hydrogen may be stored in liquid form, and in this case, the liquid level in the container can also be detected.

[0057] In an embodiment, the method of the present invention is executed by a cascaded control system including a master controller and a plurality of flow controllers configured as slave controllers relative to the master controller, wherein the master controller is sensitive to the amount of hydrogen contained in the tank, and the flow controllers act on a plurality of flow regulating valves arranged to control the flow rates associated with items a), b), and c).

[0058] The cascaded control system can be configured to prevent the hydrogen storage unit from being emptied and / or overfilled and / or to supply hydrogen to the ammonia synthesis loop at a suitable flow rate and gradient to avoid deceleration of the reaction kinetics in the ammonia converter.

[0059] As described above, when hydrogen is stored in gaseous form, the main controller may include a pressure sensor arranged to sense the pressure of gaseous hydrogen in the hydrogen storage tank.

[0060] The plurality of flow regulating valves may include:

[0061] One or more valves are arranged to regulate the flow rate of hydrogen supplied to the hydrogen storage tank and the flow rate of hydrogen supplied to the ammonia synthesis section.

[0062] At least one valve is arranged to regulate the flow rate of hydrogen drawn from the hydrogen storage tank.

[0063] In an embodiment, a single valve can be used to control the flow rate of hydrogen delivered to the hydrogen storage tank and the flow rate of hydrogen delivered to the ammonia synthesis section. For example, the hydrogen delivery line may be divided into a line leading to the tank and a line leading to the ammonia synthesis section, and a valve arranged on one of the lines can determine the flow rates of both. In an embodiment, for example, a first flow control valve is preferably used to directly regulate the amount of hydrogen flowing to the hydrogen storage tank and indirectly regulate the amount of hydrogen flowing to the ammonia synthesis section; a second flow control valve is preferably used to regulate the amount of hydrogen obtained from the hydrogen storage unit.

[0064] In another embodiment, the plurality of flow regulating valves includes a separate valve for controlling each of the flow rates described above.

[0065] The flow rate of the makeup gas transferred to the ammonia synthesis section is maintained within a target range suitable for maintaining stable and self-sustaining operation of the synthesis loop, particularly avoiding cooling and shutdown of the ammonia converter. This target range can be 1% to 110% or 10% to 110% or 20% to 110% of the nominal capacity of the ammonia synthesis section (i.e., the load on the ammonia synthesis section and related loops). The nominal capacity represents the ammonia that can be generated through the synthesis section under normal conditions.

[0066] In embodiments, the method of the present invention is further characterized by control of the synthesis loop pressure. The synthesis loop pressure can be controlled based on the hydrogen that can be supplied from the hydrogen generation section and the correspondingly processable supplementary gas. The synthesis loop pressure can be controlled by determining the bypass flow rate of the ammonia converter (i.e., the amount of feed gas circulating in the loop but bypassing the ammonia converter). Most preferably, the synthesis loop pressure can be controlled using the method described in WO 2021 / 089276.

[0067] A particularly interesting application of the present invention relates to a hydrogen generation section comprising a water electrolyzer configured to generate hydrogen from water. More preferably, the water electrolyzer is powered by renewable energy sources such as solar or wind power. A preferred application provides a water electrolyzer coupled to a photovoltaic power generation system.

[0068] The pressure at which hydrogen is generated (especially via a water electrolyzer) is typically much lower than the pressure at which ammonia is synthesized and the pressure in the reservoir, thus compression is usually required. Due to the high compression ratio, multi-stage compressors are typically used for compression. Preferably, the compressor is a mutually cooled multi-stage unit.

[0069] In addition, gaseous nitrogen is typically generated at low pressure (e.g., via an air separation unit) and compressed in a multi-stage nitrogen compressor.

[0070] Nitrogen generation must be controlled to regulate the hydrogen-to-nitrogen ratio in the supplementary gas. In a preferred embodiment, nitrogen generation is controlled by a dissipation method, in which excess nitrogen (if any) is released into the atmosphere.

[0071] The advantage of maintaining a more stable operating state of the converter is that it prevents the possible loss of the dynamics of unbalanced gases that do not have stoichiometric ratios.

[0072] In one embodiment, a single hydrogen compressor is provided to deliver hydrogen to the storage tank and the ammonia synthesis section. The hydrogen compressor may have a delivery line that branches into a first line leading to the tank and a second line leading to the ammonia synthesis section. The second line may be merged with a nitrogen line to form a makeup gas. In this embodiment, if the storage pressure is lower than the synthesis pressure, hydrogen drawn from the storage tank may be sent to the compressor's suction inlet or to an intermediate stage of the compressor to allow hydrogen from the storage tank to be supplied to the synthesis section. In another embodiment, separate compressors may be provided for filling the storage tank and supplying the synthesis section.

[0073] The present invention further relates to an ammonia apparatus having a control system configured to implement the above-described method.

[0074] In a preferred embodiment, the apparatus includes a front end for ammonia generation, the front end including a water electrolyzer powered by renewable energy; a hydrogen compressor; a hydrogen storage unit; an ammonia synthesis circuit; a nitrogen generation section; a conduit connecting the water electrolyzer to the hydrogen compressor; and a conduit connecting the hydrogen compressor to the hydrogen storage unit.

[0075] The control system may include a pressure controller, a first flow controller, and a second flow controller. The pressure controller is connected to the hydrogen storage unit, the first flow controller, and the second flow controller.

[0076] The pressure controller may include a sensor adapted to measure hydrogen stored in the hydrogen storage unit, while the first flow controller and the second controller include a first flow regulating valve and a second flow regulating valve, respectively.

[0077] The pressure controller can be configured to generate one or more setpoint signals based on the amount of hydrogen retained in the hydrogen storage unit and based on the amount of hydrogen generated in the hydrogen production section. The controller can be adapted to transmit the one or more setpoint signals to a first flow controller and a second flow controller to regulate the flow rate of hydrogen delivered to the hydrogen storage unit, the flow rate of hydrogen delivered to the ammonia synthesis circuit, and the flow rate of hydrogen extracted from the hydrogen storage unit via the first flow control valve and the second flow control valve.

[0078] Another aspect of the present invention is a method for synthesizing ammonia, wherein the method of the present invention is carried out.

[0079] The significant advantages of this invention include the following: The CAPEX of the ammonia plant is reduced due to the use of a smaller hydrogen storage unit, and the ammonia plant can operate under a wider range of conditions. Flexible operation of island-type or peninsula-type ammonia plants is possible because a suitable flow rate of makeup gas can be supplied to the converter to maintain continuous operation. The evacuation or overfilling of the hydrogen storage unit is avoided. Maintaining the flow rate (gradient) of the makeup gas over time within the allowable limits of the ammonia converter prevents reaction slowdown or plant downtime.

[0080] An interesting application of this invention relates to the retrofitting of existing ammonia plants. A method for retrofitting an ammonia plant according to the invention includes at least providing a control system configured to operate in conjunction with the method described above. The method may further include adding an electrolyzer for hydrogen production and a hydrogen storage tank (if suitable). If the plant includes a hydrogen storage tank, the invention can be applied to controlling an existing storage tank.

[0081] This invention can be applied to the retrofitting of conventional ammonia plants or hybrid ammonia plants. A conventional ammonia plant refers to a plant in which hydrogen is generated from fossil fuels; a hybrid ammonia plant refers to a plant in which hydrogen is generated partly from fossil fuels and partly from renewable energy sources. In the retrofitted plant according to the invention, more ammonia can be generated stably, and greater temporal variability in hydrogen flow rate can be tolerated.

[0082] Example

[0083] The following is a description of an exemplary algorithm for hydrogen storage control according to embodiments of the present invention. The algorithm is described using meta-language and employs the following notation:

[0084]

[0085] Assume that valve FCV1 can control both the flow rate of hydrogen delivered to the storage tank and the flow rate of hydrogen delivered to the synthesis section. This can be achieved, for example, by providing a device with a hydrogen compressor that delivers compressed hydrogen in a pipeline that branches into a first pipeline leading to the storage tank and a second pipeline leading to the synthesis section. Valve FCV1 can be positioned on the first pipeline to directly control the flow rate of hydrogen delivered to the storage tank via the first pipeline, and thus control the flow rate in the second pipeline leading to the synthesis section.

[0086] Valve FCV2 can be installed on a pipeline arranged to draw hydrogen from the tank (e.g., from the top of the tank) and direct it to the suction side of the hydrogen compressor or to an intermediate stage of the compressor, so that the hydrogen drawn from the reservoir is raised to a suitable pressure for supplying the synthesis section.

[0087] It is also assumed that the hydrogen is generated by an electrolyzer connected to the suction side of the hydrogen compressor.

[0088] The filling of the hydrogen storage tank at time t (current filling) can be determined as Filling(t) = p(t) / p des Where p(t) is the current pressure of hydrogen in the tank and p des It is the reference pressure when 100% filled.

[0089] Dt represents the system's time resolution. The reference time (t-Dt) is continuously updated with the variable LoadPrec.

[0090] Under the above conditions, an exemplary algorithm for controlling the hydrogen storage tank can be as follows.

[0091] Step 1: Defining the setpoints for FIC-1 and FIC-2

[0092] In this step, the flow rate of hydrogen produced by the electrolyzer (hydrogen production load) is compared with the amount required to operate the ammonia synthesis loop at its nominal flow rate. Based on the result of this comparison and the current amount of hydrogen stored in the tank, the system determines the setpoints for valves FCV1 and FCV2, i.e., whether hydrogen must be stored in the tank or drawn from the tank, and how much hydrogen must be stored in the tank or drawn from the tank.

[0093] If H2generation load<10% of nominal synloop flow

[0094] If H2storage filling <FillMin

[0095] FCV1 closed

[0096] FCV2 controlled by FIC-2 with a set-point = LoadMin

[0097] Else if H2storage filling>FillMax

[0098] FCV1 closed

[0099] FCV2 controlled by FIC-2 with a set-point = LoadMax

[0100] Else

[0101] FCV1 closed

[0102] FCV2 controlled by FIC-2 with a set-point = MIN(LoadMax; MAX(LoadMin;FILLING))

[0103] Else if H2generation load>110% of nominal synloop flow

[0104] If H2storage filling<FillMin

[0105] FCV1 controlled by FIC-1 with a set-point = LoadMin

[0106] FCV2 closed

[0107] Else if H2storage filling>FillMax

[0108] FCV1 controlled by FIC-1 with a set-point = LoadMax

[0109] FCV2 closed

[0110] Else

[0111] FCV1 controlled by FIC-1 with a set-point = MIN(LoadMax; MAX(LoadMin;FILLING))

[0112] FCV2 closed

[0113] Else if (10%) <H2generation load<110% of nominal synloop flow)

[0114] If H2storage filling <FillMin

[0115] FCV1 controlled by FIC-1 with a set-point = LoadMin

[0116] FCV2 closed

[0117] Else if H2storage filling>FillMax

[0118] FCV1 closed

[0119] FCV2 controlled by FIC-2 with a set-point = LoadMax

[0120] Else

[0121] FCV1 controlled by FIC-1 with a set-point = MIN(LoadMax; MAX(LoadMin;FILLING))

[0122] FCV2 closed

[0123] Step 2: Check the maximum elevation gain / decrease

[0124] In this step, the system uses the previous load of the loop to ensure that the ramp or drop does not exceed the loop's own allowable limit.

[0125]

[0126]

[0127]

[0128]

[0129] Else

[0130] Set-point FIC-1 is kept as calculated in step 1)

[0131]

[0132]

[0133]

[0134]

[0135] Else

[0136] Set-point FIC-2 is kept as calculated in step 1)

[0137] Step 3: Control of loop pressure

[0138] The pressure in the bypass valve of the circulator is controlled and kept constant. Optionally, when the circuit operates under partial load, the system can also reduce the pressure in the ammonia synthesis circuit to save compression power and mitigate the effects of reduced volumetric flow rate under partial load. Alternatively, ammonia can be injected upstream of the converter to slow down the conversion under partial load. Attached Figure Description

[0139] Figure 1 This is a simplified diagram of an ammonia synthesis apparatus according to a first embodiment of the present invention.

[0140] Figure 2 This is a simplified diagram of an ammonia synthesis apparatus according to a second embodiment of the present invention.

[0141] Figure 3 This is a simplified diagram of an ammonia synthesis apparatus according to a third embodiment of the present invention.

[0142] Figure 4 It is a graph showing the typical availability of wind energy over a period of time.

[0143] Figure 5 The diagram illustrates the process of... Figure 4 The ammonia load of rigid and flexible ammonia plants powered by available wind energy varies over time.

[0144] Figure 6 The diagram illustrates the operation. Figure 5 The required hydrogen mass for rigid and flexible ammonia plants.

[0145] Figure 7 This is a graph showing the ammonia formation rate as a function of temperature for a typical industrial catalyst used in ammonia synthesis. Detailed Implementation

[0146] Figure 1The ammonia unit 1 mainly includes: a hydrogen generation section 200 for generating hydrogen feed 13; a nitrogen generation section 201 for generating nitrogen feed 9; and an ammonia synthesis section 202, in which the hydrogen feed 13 and nitrogen feed 9 react to form ammonia product 10.

[0147] More specifically, the hydrogen generation section 200 includes a water electrolyzer 2 for generating a hydrogen stream 3 from a water feed 50; a hydrogen compressor 4; and a hydrogen storage tank 5. The electrolyzer 2 is powered by electrical energy E supplied by a renewable source, which... Figure 1 The solar energy source S is shown in the middle. The solar energy source S can be, for example, a photovoltaic field.

[0148] The nitrogen generation section 201 includes a nitrogen generation unit 25 and a nitrogen compressor 27 for extracting nitrogen 51 from the air feed 24. The nitrogen generation unit 25 may be an air separation unit (ASU) that also generates a stream 26 of oxygen or oxygen-enriched air.

[0149] The ammonia synthesis section 202 includes: an ammonia synthesis loop 6; and a circulator 32 equipped with a bypass line 53. The circulator 32 receives makeup gas 7, which includes hydrogen feed 13 and nitrogen feed 9. The ammonia synthesis loop 6 includes an ammonia converter, such as... Figure 3 As shown in the embodiments, supplementary gas 7 reacts on a suitable catalyst to form ammonia.

[0150] The hydrogen compressor 4 has a delivery pipeline that splits into a first pipeline that delivers hydrogen feed 13 to the ammonia synthesis section 202 and a second pipeline 12 that delivers hydrogen to the hydrogen storage tank 5. A first flow regulating valve 18 is provided on the second pipeline 12.

[0151] Hydrogen can be drawn from the hydrogen storage tank 5 via a line 14 equipped with a second flow regulating valve 19. The line 14 can be configured to introduce the stored hydrogen at the suction side of the compressor 4 or to introduce the stored hydrogen into the intermediate stage of the compressor. Therefore, the synthesis section 202 can directly receive hydrogen when it is generated by the electrolyzer 2 and / or obtained from the hydrogen storage tank 5.

[0152] The ammonia unit 1 further includes a control system 203 configured to control hydrogen storage based on the real-time operating status of the ammonia unit 1. More specifically, the control system 203 controls the aforementioned valves 18 and 19 and thus controls the amount of hydrogen supplied to the hydrogen storage tank 5 via pipeline 12; the amount of hydrogen extracted from the hydrogen storage tank 5 via pipeline 14; and the amount of hydrogen transported to the ammonia synthesis section 202 via pipeline 13.

[0153] The control system 203 includes a pressure indicator controller 15 connected to a pressure sensor sensitive to the hydrogen pressure in the hydrogen storage tank 5, a first flow indicator controller 16 connected to the first flow regulating valve 18, and a second flow indicator controller 17 connected to the second flow regulating valve 19.

[0154] Regulation is performed according to cascaded control logic, wherein the pressure indicator controller 15 acts as the master controller and the first flow indicator controller 16 and the second flow indicator controller 17 are configured as slave controllers. The pressure indicator controller 15 (master controller) detects the pressure inside the hydrogen storage tank 5, and thus detects the amount of hydrogen stored in the hydrogen storage tank 5. Based on the detection, the controller 15 provides setpoint signals 11a, 11b to the first flow indicator controller 16 and the second flow indicator controller 17 (slave controller).

[0155] Optionally, the main controller 15 may also receive a signal of the flow rate of the hydrogen gas flow 3 provided by the electrolyzer 2, which represents the instantaneous output of the electrolyzer 2 based on the power E obtained from the source S. Therefore, the setpoint signals 11a and 11b can be calculated based on the content of hydrogen in the hydrogen storage tank 5 and the current output of the electrolyzer 2.

[0156] The two slave controllers 16 and 17 implement the setpoint signals 11a and 11b received by the master controller 15 and adjust the opening of the first flow control valve 18 and the second flow control valve 19 according to the setpoint signals.

[0157] See Figure 1 It can be noted that the hydrogen 3 produced from the water electrolyzer 2, after being compressed, can be guided via pipeline 12 to the hydrogen storage tank 5 and / or via pipeline 13 to the ammonia synthesis section 202, depending on the opening of the first flow regulating valve 18. When appropriate, the hydrogen stored in the hydrogen storage tank 5 can be extracted and used to generate ammonia by opening and controlling the second flow regulating valve 19.

[0158] Hydrogen 13 is mixed with nitrogen 9 to produce a supplementary gas 7 with an N / H ratio suitable for ammonia synthesis. The appropriate amount of nitrogen is controlled by valve 211.

[0159] The hydrogen feed in line 13, together with the nitrogen feed in line 9, forms the makeup gas input flow 7 of the ammonia synthesis loop 6. Bypass line 53 provides a portion of the makeup gas supplied by circulator 32 that bypasses the synthesis loop 6. The bypass line 53 allows control of the flow rate of the makeup gas entering the synthesis loop 6, thereby indirectly controlling the pressure or temperature in the ammonia converter.

[0160] The bypass in the synthesis loop can be used to control the pressure of the ammonia converter and to maintain the ammonia converter in an appropriate operating condition (self-sustaining condition) when the ammonia unit is operating at partial load. A preferred control system for this purpose is disclosed in WO2021 / 089276 and can be combined with the method of the present invention.

[0161] By providing adaptive control over hydrogen storage, this invention avoids the emptying and overfilling of hydrogen storage tank 5 and allows for feeding the synthesis loop with acceptable flow variations (ramp) over time to maintain the self-sustaining operation of the ammonia synthesis loop. All of the above can be achieved with a relatively small hydrogen storage tank 5, thereby reducing the cost of hydrogen storage tank 5.

[0162] The control logic described in detail above in this specification can be applied to... Figure 1 The layout is such that valve 18 corresponds to FCV1 and valve 19 corresponds to FCV2.

[0163] Figure 2 It shows the relationship with Figure 1 Similar embodiments to those described above, wherein the flow rate of hydrogen delivered to the ammonia synthesis section 202 is controlled by an additional flow regulating valve 20 installed on pipeline 13.

[0164] Preferably, such as Figure 2 As shown, the flow indicator controller 16 controls the valve 18 and the additional valve 20 based on the setpoint signal 11a transmitted by the main controller 15.

[0165] Figure 3 Another embodiment is shown, wherein, for simplicity, with Figures 1 to 2 The corresponding items in the figures are indicated by the same reference numerals.

[0166] exist Figure 3 In this embodiment, a hydrogen pipeline 14, drawing hydrogen from hydrogen storage tank 5, is connected to hydrogen pipeline 60, which runs parallel to the hydrogen compressor 4 supplying hydrogen to hydrogen storage tank 5. A nitrogen feed pipeline 51 is also connected to said pipeline 60. Therefore, a main supplementary gas compressor 36 is provided for compressing supplementary gas 7. In this embodiment, compressor 4 is used only as a booster compressor to supply hydrogen to hydrogen storage tank 5.

[0167] A deoxygenation reactor 70 is also shown, which is arranged after the water electrolyzer 2 to remove trace amounts of oxygen from the hydrogen 3 generated in the electrolyzer 2. The deoxygenation reactor can also be provided in… Figure 1 and Figure 2 In the plan.

[0168] A first flow regulating valve 18 is arranged on the feed line of the booster compressor 4; a second flow regulating valve 19 is arranged on line 14. Valves 18 and 19 are controlled by controllers 15 and 16, which have setpoints 11a and 11b provided by the main controller 15. The control system is connected to… Figure 1 and Figure 2 It operates in a manner substantially the same as that disclosed in the preferred embodiments.

[0169] The main compressor 36 delivers the compressed supplementary gas 61 to the ammonia synthesis circuit 6.

[0170] exist Figure 3 The ammonia synthesis circuit 6 is illustrated in more detail. It includes a circulator 32, an ammonia converter 21 for synthesizing ammonia 32, a condenser 33, and a separator 34 for separating the liquid ammonia stream 10 from the gaseous recirculation stream 35 containing unreacted hydrogen and nitrogen as well as residual ammonia vapor.

[0171] The circulator 32 includes a bypass line 53 equipped with a valve 31 to regulate the flow rate of supplementary gas 61 supplied by the circulator 32, which bypasses the converter 21 and returns to the intake side of the circulator 32. A portion of the gaseous recirculated stream 35 extracted from the separator 34 can also be recirculated to the intake side of the circulator 32 via the recirculation line 30.

[0172] The device further includes a pressure indicator controller 62, which can be used to sense the pressure of a portion of the gaseous recirculation flow 35 circulating in the line 30 and send a setpoint signal to the flow control valve 31 to control the flow rate of the compressed supplemental gas recirculated in the bypass line 53.

[0173] Figures 4 to 6 This provides further evidence of the advantages of the present invention.

[0174] Figure 4 The figure shows a graph illustrating the fluctuating availability of the power supply connected to electrolyzer 2 over a 100-hour period.

[0175] Figure 5 The load including the ammonia unit is Figure 4 Two graphs showing the changes over a period of time. The first graph relates to a so-called rigid unit, which is a conventional ammonia unit operating between 70% and 110% of its nominal load. The second graph relates to a flexible unit operating according to the method of the invention, wherein the load can vary from 10% to 110% of the nominal load.

[0176] Figure 6 The drawing indicates that it must be stored (e.g., kept in hydrogen storage tank 5) to ensure that, according to Figure 5The two options for stable operation of the ammonia unit and the quality of hydrogen.

[0177] Specifically, Figure 6 The illustration shows that the maximum hydrogen storage capacity has been reduced from up to 60,000 kg to less than 10,000 kg. This significantly reduces the size and cost of hydrogen tanks. For example, assuming a hydrogen storage cost of $450 per kilogram of stored hydrogen, the applicant has calculated that the average cost of ammonia storage can be reduced by approximately 33%.

[0178] Figure 4 This reflects variations in sources, such as those found in wind turbines. The invention is even more advantageous and can further reduce the size of hydrogen storage tanks if solar energy, which has more regular variations (day / night), is used.

[0179] Figure 7 The figure shows a typical curve of ammonia formation rate versus temperature. Figure 7 This invention relates to ammonia synthesis at 150 bar with an iron-based catalyst and a hydrogen-to-nitrogen ratio of 3. The graphs show a significant decrease in ammonia formation below 400°C. Based on this catalyst, a threshold temperature for autothermal operation can be defined, typically between 300°C and 350°C. The method of this invention avoids reaction losses and converter downtime by controlling the flow rate and maintaining ammonia converter operation.

Claims

1. A method for controlling an ammonia device (1), wherein: The ammonia unit (1) includes: Ammonia synthesis section (202), the ammonia synthesis section (202) includes an ammonia converter, in which ammonia is synthesized from a feed gas including hydrogen and nitrogen under ammonia synthesis pressure; Hydrogen generation section (200), the hydrogen generation section (200) is configured to generate gaseous hydrogen (3); Hydrogen storage tank (5), which is connected to the hydrogen generation section; The method includes control: a) The total amount of hydrogen (13) transported from the hydrogen generation section to the ammonia synthesis section; b) The ratio of the amount of hydrogen in item a) to the amount of hydrogen currently generated in the hydrogen generation section and the amount of hydrogen extracted from the hydrogen storage tank (5), wherein the amount of hydrogen from the hydrogen storage tank (5) ranges from 0% to 100% of the amount in a). c) The amount of hydrogen delivered to the hydrogen storage tank (5); Wherein, items a), b), and c) are controlled to maintain the following parameters within their respective target ranges: i) The amount of hydrogen contained in the hydrogen storage tank (5); ii) The flow rate of hydrogen delivered to the ammonia synthesis section and / or the flow rate of the feed gas reacting in the ammonia converter; iii) The rate at which at least one of the flow velocities at point ii) changes over time. Among them, target ranges ii) and iii) are selected to maintain the ammonia converter in a self-heating operation condition. Wherein: the feed gas is preheated before it is catalytically reacted to form ammonia; the preheating of the feed gas is carried out by transferring heat from the hot effluent of the ammonia synthesis reaction to the fresh feed gas; the self-heating operation corresponds to a condition in which the preheated feed gas has a temperature equal to or greater than a threshold temperature, the threshold temperature being included between 300°C and 400°C.

2. The method according to claim 1, wherein, a), b) and c) are controlled such that the amount of hydrogen in the hydrogen storage tank (5) is maintained above the minimum amount and below the maximum amount to prevent the hydrogen storage tank (5) from being emptied and overfilled.

3. The method according to claim 2, wherein, a), b) and c) are controlled to ensure that the ammonia synthesis pressure is within the target range.

4. The method according to claim 1, wherein, Control based on one or more of the following: a), b) and c): the current hydrogen output of the hydrogen generation section; the amount of hydrogen contained in the hydrogen storage tank (5); the current load of the ammonia synthesis section; and one or more past values ​​of the load of the ammonia synthesis section.

5. The method according to claim 1, wherein, a), b) and c) are controlled based on one or more setpoint signals (11a, 11b), and the one or more setpoint signals are generated according to the amount of hydrogen contained in the hydrogen storage tank (5).

6. The method according to claim 1, wherein, The method is executed by a cascaded control system comprising a master controller (15) and a plurality of flow controllers (16, 17), the plurality of flow controllers (16, 17) being configured as slave controllers relative to the master controller, wherein the master controller is sensitive to the amount of hydrogen contained in the hydrogen storage tank (5), and the flow controllers act on a plurality of flow regulating valves (18, 19, 20), the plurality of flow regulating valves (18, 19, 20) being arranged to control a), b) and c).

7. The method according to claim 6, wherein, The main controller (15) includes a pressure sensor arranged to sense the pressure of gaseous hydrogen in the hydrogen storage tank (5).

8. The method according to claim 6, wherein, The plurality of flow regulating valves include: One or more valves (18, 20) are arranged to regulate the flow rate of hydrogen (12) to the hydrogen storage tank (5) and the flow rate of hydrogen delivered to the ammonia synthesis section. At least one valve (19) is arranged to regulate the flow rate of hydrogen (14) extracted from the hydrogen storage tank (5).

9. The method according to claim 8, wherein, The plurality of flow control valves include at least a first valve, a second valve and a third valve, wherein the first valve is arranged to control the flow rate of hydrogen supplied to the hydrogen storage tank (5); the second valve is arranged to control the flow rate of hydrogen extracted from the hydrogen storage tank (5); and the third valve is arranged to control the flow rate of hydrogen supplied to the ammonia synthesis section.

10. The method according to claim 1, wherein, The amount of hydrogen in the hydrogen storage tank (5) is maintained above a minimum of 10% to 30% of the nominal storage capacity of the hydrogen storage tank (5) and below a maximum of 70% to 90% of the nominal storage capacity.

11. The method according to claim 1, wherein, The hydrogen supplied to the ammonia synthesis section is maintained at 10% to 110% of the nominal capacity of the synthesis section.

12. The method of claim 1, further comprising controlling the pressure in the ammonia synthesis converter by controlling the flow rate of the feed gas bypassing the converter.

13. The method according to claim 1, wherein, In the hydrogen generation section (200), hydrogen is generated by electricity and the electricity can vary over time.

14. The method according to claim 1, wherein, The hydrogen generation section includes a water electrolyzer configured to generate hydrogen from water (3).

15. The method according to claim 1, wherein, Hydrogen is generated by renewable electricity and the ammonia unit is not connected to the power grid. The ammonia unit includes a backup power system that is configured to provide power for the operation of the ammonia unit in the event that there is little or no hydrogen generated by the hydrogen generation section and the hydrogen input for ammonia generation is provided by the hydrogen storage tank (5).

16. The method according to claim 15, wherein, During operation using backup power, the backup power is generated by any one of a gas turbine, a gas engine, or a fuel cell.

17. The method according to claim 1, wherein, The ammonia device includes a nitrogen generation section, and the method includes controlling the generation of nitrogen using a dissipation method in which excess nitrogen is released into the atmosphere.

18. The method according to claim 13, wherein, The electricity mentioned is renewable electricity.

19. The method of claim 14, wherein, The water electrolyzer is powered by renewable energy.

20. The method according to claim 19, wherein, The water electrolyzer is powered by solar energy.

21. The method according to claim 16, wherein, The gas turbine, gas engine, and fuel cell use hydrogen or ammonia for combustion.

22. An ammonia synthesis apparatus (1), comprising: Ammonia synthesis section (202), the ammonia synthesis section (202) includes an ammonia converter in which ammonia is synthesized under ammonia synthesis pressure; A hydrogen generation section (200) is configured to generate hydrogen (3) for use in the ammonia synthesis section for ammonia synthesis. Hydrogen storage tank (5), which is connected to the hydrogen generation section; A supplemental gas line is arranged to supply ammonia supplemental gas to the ammonia synthesis section, the ammonia supplemental gas comprising hydrogen and nitrogen generated in the hydrogen generation section in appropriate proportions; A control system configured to implement the method as described in claim 1.

23. The ammonia apparatus according to claim 22, wherein, The hydrogen generation section (200) includes a water electrolyzer (2) configured to generate hydrogen (3) from water, and the water electrolyzer is powered by a renewable energy source.

24. The ammonia apparatus according to claim 22 or 23, wherein, The ammonia unit is not connected to the power grid, wherein the ammonia unit includes a backup power system configured to provide at least the power required for the operation of the ammonia unit when there is little or no hydrogen generated by the hydrogen generation section and the hydrogen input for ammonia generation is obtained from the hydrogen storage tank (5).

25. The ammonia apparatus according to claim 24, wherein, The backup power system is configured to generate backup power, which is 10% or less of the peak power required by the ammonia unit under nominal load.

26. The ammonia apparatus according to claim 23, wherein, The water electrolyzer is powered by solar energy.

27. The ammonia apparatus according to claim 24, wherein, The backup power system includes any one of a gas turbine, a gas engine, or a fuel cell.

28. The ammonia apparatus according to claim 27, wherein, The gas turbine, gas engine, and fuel cell use hydrogen or ammonia for combustion.

29. A process for synthesizing ammonia, comprising: Ammonia is synthesized by reacting supplementary gas (7) under ammonia synthesis pressure in the ammonia synthesis section (202) including the ammonia converter; Hydrogen is generated in the hydrogen generation section (200) and used to generate the supplementary gas and / or stored in the hydrogen storage tank (5); The process further includes: Control the flow rate of hydrogen (12) sent from the hydrogen generation section (200) to the hydrogen storage tank (5) for storage; Control the flow rate of hydrogen (14) extracted from the hydrogen storage tank (5) for use in the ammonia synthesis section; Control the flow rate of hydrogen (13) transported from the hydrogen generation section to the ammonia synthesis section; The flow rate is controlled using the method described in claim 1.

30. A method of retrofitting an ammonia plant, comprising providing a control system configured to operate using the method of claim 1.

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