Method for suppressing ammonia gas leakage and liquefied ammonia storage apparatus

Abstract

An ammonia gas leakage inhibition method according to the present invention is characterized by: supplying, in a state where a storage tank for storing liquefied ammonia is open, the storage tank with a cryogenic liquid having a temperature lower than that of the stored liquefied ammonia stored in the storage tank; and inhibiting leakage of the ammonia gas to the outside of the storage tank. The cryogenic liquid is preferably liquid nitrogen or liquefied ammonia in a sub-cooled state. According to the present invention, it is possible to provide an ammonia gas leakage inhibition method that can inhibit, in a state where a storage tank for storing liquefied ammonia is open, leakage of ammonia gas to the outside of the storage tank, and it is also possible to provide a liquefied ammonia storage apparatus that can inhibit, in a state where a storage tank having stored therein stored liquefied ammonia is open, leakage of the ammonia gas to the outside of the storage tank.

Description

[Technical Field]

[0001] The present invention relates to a method for suppressing leakage of ammonia gas and a liquefied ammonia storage device. [Background Art]

[0002] In recent years, ammonia fuel has attracted attention as a carbon-neutral fuel for reducing environmental load, and it is expected that ammonia liquefaction terminals and ammonia receiving terminals that handle large amounts of ammonia, which is both flammable and toxic gas, will be constructed in the future.

[0003] As a type of storage tank for storing large-capacity liquefied ammonia, adoption of a "PC dike-outer tank integrated flat-bottom spherical-roof vertical cylindrical storage tank" using prestressed concrete (hereinafter referred to as PC), which is widely employed in many above-ground tanks storing large-capacity liquefied gases such as liquefied natural gas and liquefied petroleum gas, is considered promising from the viewpoint of safety.

[0004] The liquefied gas in a storage tank is stored at a temperature close to the saturation temperature of the gas. Therefore, inside the storage tank, vaporized gas called boil-off gas (BOG) is generated when the gas is warmed by natural heat input from the outside or the like.

[0005] In relation to this, a treatment method for reliquefying generated boil-off gas for storage tanks that store liquefied gases such as liquefied natural gas and liquefied petroleum gas at low temperatures is known (see Patent Document 1).

[0006] However, when the storage tank is opened, such as when the roof of the storage tank is damaged due to an unexpected accident or when maintenance of the storage tank is performed, in addition to natural heat input, heat input from the opened portion increases, and the pressure in the gas phase portion of the storage tank decreases, so the generation amount of boil-off gas increases. Furthermore, as a measure to deter harm and ensure safety when opening the storage tank, it is crucial to suppress the leakage of ammonia gas, which is both flammable and toxic, from the opening of the storage tank. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 10-19199 [Overview of the project] [Problems that the invention aims to solve]

[0008] The object of the present invention is to provide a method for suppressing ammonia gas leakage that can suppress the leakage of ammonia gas to the outside of a storage tank when the storage tank for storing liquefied ammonia is open, and to provide a liquefied ammonia storage device that can suppress the leakage of ammonia gas to the outside of a storage tank when the storage tank in which liquefied ammonia is stored is open. [Means for solving the problem]

[0009] These objectives are achieved by the present invention as described in (1) to (12) below. (1) Storage tanks for storing liquefied ammonia roof section It has been opened The interior space and the exterior space are connected. A method for suppressing ammonia gas leakage, characterized in that, in the state described above, a low-temperature liquid, which is a liquid at a lower temperature than the liquefied ammonia stored in the storage tank, is supplied into the storage tank.

[0010] (2) The method for suppressing ammonia gas leakage according to (1) above, wherein the temperature of the low-temperature liquid is -196°C or higher and -35°C or lower.

[0011] (3) The method for suppressing ammonia gas leakage according to (1) or (2) above, wherein the low-temperature liquid is liquid nitrogen or liquefied ammonia in a subcooled state.

[0012] (4) A method for suppressing ammonia gas leakage according to any one of (1) to (3) above, wherein the cryogenic liquid is supplied into the liquid phase inside the storage tank.

[0013] (5) A method for suppressing ammonia gas leakage according to any one of (1) to (4) above, wherein the cryogenic liquid is supplied into the gas phase inside the storage tank.

[0014] (6) A method for suppressing ammonia gas leakage according to any one of (1) to (5) above, further supplying an inert gas to the gas phase inside the storage tank.

[0015] (7) The method for suppressing ammonia gas leakage according to (6) above, wherein the inert gas is nitrogen gas or carbon dioxide gas.

[0016] (8) A storage tank in which liquefied ammonia is stored, The system includes a cryogenic liquid supply means for supplying a cryogenic liquid, which is a liquid at a lower temperature than the liquefied ammonia stored in the storage tank, into the storage tank. The storage tank in which the liquefied ammonia is stored roof section It has been opened The interior space and the exterior space are connected. A liquefied ammonia storage apparatus characterized in that, when the state is reached, the cryogenic liquid supply means supplies the cryogenic liquid into the storage tank.

[0017] (9) The liquefied ammonia storage apparatus according to (8) above, wherein the cryogenic liquid supply means is arranged to supply the cryogenic liquid into the liquid phase inside the storage tank.

[0018] (10) The liquefied ammonia storage apparatus according to (8) or (9) above, wherein the cryogenic liquid supply means is arranged to supply the cryogenic liquid into the gas phase inside the storage tank.

[0019] (11) The liquefied ammonia storage apparatus according to any one of (8) to (10) above, further comprising an inert gas supply means for supplying an inert gas to a gas phase inside the storage tank.

[0020] (12) The liquefy The liquefied ammonia storage apparatus according to (11) above, wherein the ammonia storage apparatus comprises a plurality of said inert gas supply means. Advantageous Effects of Invention

[0021] According to the present invention, it is possible to provide a method for suppressing leakage of ammonia gas that can suppress leakage of ammonia gas to the outside of a storage tank when the storage tank for storing liquefied ammonia is opened, and also provide a liquefied ammonia storage apparatus that can suppress leakage of ammonia gas to the outside of a storage tank when the storage tank storing liquefied ammonia is opened. Brief Description of Drawings

[0022] [Figure 1] FIG. 1 is a diagram schematically showing a first embodiment of the liquefied ammonia storage apparatus of the present invention. [Figure 2] FIG. 2 is a partially broken cross-sectional view showing one configuration example of a storage tank in the liquefied ammonia storage apparatus shown in FIG. 1. [Figure 3] FIG. 3 is a diagram for explaining a first embodiment of the method for suppressing leakage of ammonia gas according to the present invention. [Figure 4] FIG. 4 is a diagram for explaining a subcooled state, and is a phase diagram. [Figure 5] FIG. 5 is a diagram schematically showing a second embodiment of the liquefied ammonia storage apparatus of the present invention. [Figure 6] FIG. 6 is a diagram for explaining a second embodiment of the method for suppressing leakage of ammonia gas according to the present invention. [Figure 7] FIG. 7 is a diagram for explaining one example of the method for suppressing leakage of ammonia gas according to the present invention. [Figure 8]Figure 8 is a schematic diagram illustrating how gaseous pollutants diffuse. [Modes for carrying out the invention]

[0023] Preferred embodiments of the present invention will be described in detail below.

[0024] <First Embodiment> [1] Liquefied ammonia storage device First, the liquefied ammonia storage apparatus of the present invention will be described. Figure 1 is a schematic diagram showing a first embodiment of the liquefied ammonia storage apparatus of the present invention. Figure 2 is a partially cutaway cross-sectional view showing an example of the configuration of a storage tank in the liquefied ammonia storage apparatus shown in Figure 1. In addition, identical elements in the drawings are denoted by the same reference numeral, and redundant explanations are omitted.

[0025] The liquefied ammonia storage apparatus 1 of the present invention comprises a storage tank 10 in which liquefied ammonia is stored, and a cryogenic liquid supply means 30 for supplying a cryogenic liquid 3, which is a liquid at a lower temperature than the stored liquefied ammonia 2 stored in the storage tank 10, into the storage tank 10. In the following description, the liquefied ammonia stored inside the storage tank 10 will be referred to as stored liquefied ammonia 2. Furthermore, when the storage tank 10 containing the stored liquefied ammonia 2 is opened, the cryogenic liquid supply means 30 supplies cryogenic liquid 3 into the storage tank 10 to suppress the leakage of ammonia gas to the outside of the storage tank 10.

[0026] In such a liquefied ammonia storage device 1, liquefied ammonia is stored inside the storage tank 10. The inside of the storage tank 10 is maintained at a pressure close to atmospheric pressure, and the stored liquefied ammonia 2 is kept at a temperature of around -33°C, which is the saturation temperature of ammonia at atmospheric pressure.

[0027] Inside the storage tank 10, a boil-off gas, or in other words, ammonia gas, generated by spontaneous heat input, is present in the gas phase. This boil-off gas is recovered and reliquefied using a reliquefaction means (not shown), and then returned to the liquid phase of the storage tank 10.

[0028] Methods for reliquefying boil-off gas include, for example, a method in which the gas containing boil-off gas is pressurized with a compressor and then cooled with a heat exchanger to liquefy it, or a method in which boil-off gas is injected into the stored liquefied ammonia 2 that is being sent out to liquefy it, and then pumped under pressure.

[0029] Generally, when a storage tank is open, the amount of boil-off gas generated increases due to the increased heat input from the outside associated with opening the tank, in addition to the natural heat input, and the decrease in pressure in the gas phase of the storage tank. Without any countermeasures, there was a problem that ammonia gas, which is the boil-off gas, would leak from the opening of the storage tank.

[0030] In this specification, "the storage tank 10 being open" refers to a state in which the internal space of the storage tank 10 is in communication with the external space, which is the atmosphere. Specifically, this includes cases such as when the roof of the storage tank 10 is damaged due to an unforeseen accident, such as a collision with flying debris, and a hole H is created that penetrates the roof, or when the management lid, which is normally closed, is opened to perform maintenance on the storage tank 10. The control cover is, for example, the head plate (not shown) of the pump barrel 20, which will be described later.

[0031] The following explanation will primarily focus on the case where the roof of the storage tank 10 is damaged due to an unforeseen accident, under the assumption that "the storage tank 10 is open."

[0032] As will be explained later, damage to the roof of the storage tank 10 refers to damage that causes a hole to penetrate the roof 15 of the outer tank 12 and the ceiling of the inner tank 11, as shown in Figure 2. If the roof 15 of the outer tank 12 is damaged, but there is no hole H penetrating the ceiling of the inner tank 11, it is not considered to be in a state where the storage tank 10 is open.

[0033] Furthermore, in the liquefied ammonia storage apparatus 1 of the present invention, when the storage tank 10 in which the stored liquefied ammonia 2 is stored is opened, the cryogenic liquid supply means 30 can supply cryogenic liquid 3 into the storage tank 10, thereby lowering the temperature inside the storage tank 10 and suppressing the generation of boil-off gas. As a result, the liquefied ammonia storage device 1 can suppress the leakage of ammonia gas to the outside of the storage tank 10.

[0034] [1-1] Storage tank The storage tank 10 stores liquefied ammonia inside.

[0035] As shown in Figure 2, the storage tank 10 comprises an inner tank 11 for storing liquefied ammonia, an outer tank 12 positioned to cover the inner tank 11, and an insulating material 16 positioned between the inner tank 11 and the outer tank 12.

[0036] The pressure inside the inner tank 11 is close to atmospheric pressure. The inner tank 11 is a tank for storing liquefied ammonia at a low pressure close to atmospheric pressure and at the ammonia saturation temperature. The inner tank 11 is constructed in a bottomed cylindrical shape using low-temperature steel that has toughness in the low-temperature range down to approximately -33°C or below, which is the saturation temperature of ammonia under atmospheric pressure. The ceiling portion of the inner tank 11 is constructed in a dome shape.

[0037] In this specification, "the interior of the storage tank 10" refers to "the interior of the inner tank 11".

[0038] The outer tank 12 is a structure comprising a bottom plate 13, side walls 14, and a roof 15, and is installed to cover the outside of the inner tank 11. In particular, the bottom plate 13 and side walls 14 act as a containment barrier to prevent leakage of the stored liquid to the outside in the event that the inner tank 11 is damaged and the stored liquefied ammonia 2 leaks out.

[0039] The side wall 14 is constructed using prestressed concrete (PC), in which, for example, a sheath pipe is embedded in the concrete and prestress is introduced by a tensioning member inserted into this sheath pipe.

[0040] The base plate 13 is constructed in a disc shape. This base plate 13 is supported at a predetermined height from the ground by a plurality of foundation piles (not shown) embedded in the ground. The side wall 14 is constructed in a roughly cylindrical shape on the bottom plate 13.

[0041] The roof 15 is constructed in a dome shape using forged steel or reinforced concrete. Furthermore, the side wall 14, which serves as a liquid containment barrier, is constructed by attaching liner plates (thin steel plates) to the side wall 14 of the outer tank 12.

[0042] An insulating material 16 is placed between the inner tank 11 and the outer tank 12, and this insulating material 16 keeps the stored liquefied ammonia 2 in the inner tank 11 cool. Furthermore, a pump barrel 20 for discharging liquefied ammonia from the storage tank 10 is installed, extending vertically through the roof 15 and the ceiling portion of the inner tank 11 to the bottom of the storage tank 10. The pump barrel 20 will be described in detail later.

[0043] Furthermore, in this specification, "vertical" does not refer to a strictly mathematical "vertical" line, but rather allows for some degree of deviation. Furthermore, the pump barrel 20 is not shown in Figure 1, and in Figures 3, 5, and 6 shown later.

[0044] [1-2] Low temperature liquid supply means The cryogenic liquid supply means 30 supplies cryogenic liquid 3, which is a liquid at a lower temperature than the stored liquefied ammonia 2 stored in the storage tank 10, into the storage tank 10 when the storage tank 10 containing the stored liquefied ammonia 2 is opened. This lowers the temperature inside the storage tank 10, suppressing the generation of boil-off gas and preventing ammonia gas from leaking outside the storage tank 10.

[0045] The cryogenic liquid supply means 30 includes a storage section 31 in which the cryogenic liquid 3 is stored, piping 32 for sending the cryogenic liquid 3 to the storage tank 10, and a control valve 33 located in the middle of the piping 32 for adjusting the amount of cryogenic liquid 3 sent from the storage section 31 to be supplied to the storage tank 10.

[0046] In this embodiment, the cryogenic liquid supply means 30 is arranged to supply the cryogenic liquid 3 into the liquid phase inside the storage tank 10. As a result, the low-temperature liquid 3 supplied by the low-temperature liquid supply means 30 can directly lower the temperature of the liquid phase inside the storage tank 10, thereby more efficiently suppressing the generation of boil-off gas and more efficiently suppressing the leakage of ammonia gas to the outside of the storage tank 10.

[0047] Furthermore, it is possible to store the cold energy from the low-temperature liquid 3 in the liquid phase, thereby suppressing the generation of boil-off gas for a longer period of time. For example, if an accident occurs in which the roof of storage tank 10 is damaged and a hole H is created, it is possible to buy time before a large amount of ammonia gas leaks out. During this time, the liquefied ammonia 2 stored in the damaged storage tank 10 can be transferred to another storage tank, and the damaged storage tank 10 can be emptied, thereby completing the accident cleanup.

[0048] The cryogenic liquid supply means 30 is preferably arranged to supply the cryogenic liquid 3 to the central part of the storage tank 10 in a plan view. This allows for a more efficient reduction in the temperature of the stored liquefied ammonia 2, further suppressing the generation of boil-off gas, and more efficiently preventing the leakage of ammonia gas to the outside of the storage tank 10.

[0049] In this specification, "the central part of the storage tank 10" refers to the area with a similar shape and area ratio of 30% that shares the same geometric center and orientation as the planar shape of the inner tank 11 when the storage tank 10 is viewed from above.

[0050] When supplying the cryogenic liquid 3 into the liquid phase, Figure 1 shows the cryogenic liquid supply piping 32 extending vertically from the ceiling of the storage tank 10, but it is not limited to this, and the cryogenic liquid supply piping 32 may be arranged horizontally along the bottom surface of the storage tank 10, for example.

[0051] In this specification, "horizontal" does not refer to a strictly mathematical horizontal, but rather allows for some degree of deviation.

[0052] [1-3] Inert gas supply means In the configuration shown in Figure 1, the liquefied ammonia storage device 1 is further provided with an inert gas supply means 40 in a more preferred form. The inert gas supply means 40 supplies inert gas 4 into the gas phase inside the storage tank 10 when the storage tank 10 is opened.

[0053] By providing such an inert gas supply means 40, as will be described later, the mixed gas of ammonia gas present in the gas phase inside the storage tank 10 and the atmosphere that has entered the gas phase inside the storage tank 10 from the opening can be diluted with the inert gas 4, thereby effectively suppressing the generation of a flammable mixed gas that falls within the combustion range.

[0054] The inert gas supply means 40 includes a storage section 41 in which the inert gas 4 is stored, piping 42 for transferring the inert gas 4 to the storage tank 10, and a control valve 43 located in the middle of the piping 42 for adjusting the amount of inert gas 4 sent from the storage section 41 to be supplied to the storage tank 10.

[0055] The liquefied ammonia storage device 1 further includes a control unit (not shown) that controls the supply amounts of the cryogenic liquid 3 and the inert gas 4 in the cryogenic liquid supply means 30 and the inert gas supply means 40. The control unit controls the opening and closing of the regulating valves 33 and 43. The control unit is, for example, a CPU (Central Processing Unit).

[0056] The liquefied ammonia storage device 1 preferably has a plurality of inert gas supply means 40 (i.e., a supply unit for inert gas 4 into the storage tank 10). This allows for the supply of a relatively large amount of inert gas 4 in a short time, for example, and more effectively suppresses the generation of flammable mixed gases. Furthermore, it is also possible to select an appropriate inert gas supply means 40 from among the multiple inert gas supply means 40 and supply the inert gas 4 from that inert gas supply means 40. More specifically, for example, if the roof of the storage tank 10 is damaged and a hole is made, by selectively supplying the inert gas 4 from the inert gas supply means 40 located closer to the hole, the amount of inert gas 4 used can be reduced while more effectively suppressing the inflow of air, thereby more effectively and reliably suppressing the generation of flammable mixed gases.

[0057] Furthermore, by providing multiple inert gas supply means 40, even if some of the inert gas supply means 40 are damaged along with the roof when the roof is damaged, the inert gas 4 can still be supplied from the other undamaged inert gas supply means 40.

[0058] Furthermore, if the liquefied ammonia storage device 1 has multiple inert gas supply means 40, the multiple inert gas supply means 40 may share a part of their configuration (for example, a storage section 41). In other words, the piping 42 that sends the inert gas 4 from the storage section 41 to the storage tank 10 may be branched and configured to supply the inert gas 4 to multiple different locations in the storage tank 10.

[0059] [2] Method for suppressing ammonia gas leakage Next, with reference to Figures 3 and 4, a method for suppressing ammonia gas leakage when the storage tank 10 is open in the liquefied ammonia storage apparatus 1 according to this embodiment will be described.

[0060] Figure 3 is a diagram illustrating a first embodiment of the ammonia gas leakage suppression method of the present invention. Figure 4 is a diagram illustrating a subcooling state, and is a phase diagram.

[0061] The method for suppressing ammonia gas leakage according to this embodiment involves supplying a low-temperature liquid 3, which is a liquid at a lower temperature than the stored liquefied ammonia 2, into the storage tank 10 when the storage tank 10 for storing liquefied ammonia is open, thereby suppressing the leakage of ammonia gas to the outside of the storage tank 10.

[0062] Thus, in the liquefied ammonia storage device 1, when the storage tank 10 is open, the low-temperature liquid 3 is supplied into the storage tank 10 from the low-temperature liquid supply means 30, thereby lowering the temperature inside the storage tank 10 and suppressing the generation of boil-off gas. As a result, the ammonia gas leakage suppression method of the present invention can suppress the leakage of ammonia gas to the outside of the storage tank 10.

[0063] The temperature of the stored liquefied ammonia 2 in the storage tank 10 is maintained at around -33°C, which is the saturation temperature of ammonia at atmospheric pressure. Therefore, as the low-temperature liquid 3, a liquid with a temperature lower than -33°C is used.

[0064] The temperature of the low-temperature liquid 3 is not particularly limited as long as it is lower than the temperature of the stored liquefied ammonia 2, but it is preferably -196°C or higher and -35°C or lower, more preferably -60°C or higher and -35°C or lower, and even more preferably -45°C or higher and -35°C or lower.

[0065] This allows for a more efficient reduction of the temperature inside the storage tank 10, more effectively suppressing the generation of boil-off gas, and more effectively preventing the leakage of ammonia gas to the outside of the storage tank 10. Furthermore, if the temperature of the low-temperature liquid 3 is above -60°C, readily available and relatively inexpensive low-temperature steel can be used as the constituent material of the inner tank 11.

[0066] The low-temperature liquid 3 is not particularly limited as long as it is a liquid at a lower temperature than the stored liquefied ammonia 2, but it is preferably liquid nitrogen or liquefied ammonia in a subcooled state, and more preferably liquefied ammonia in a subcooled state. This allows for a more favorable reduction in the temperature inside the storage tank 10, and is also advantageous from a cost standpoint.

[0067] Now, with reference to the phase diagram in Figure 4, we will explain the subcooling state. The liquefied ammonia in the storage tank 10 is normally in the state shown by the black circle in Figure 4, that is, at the saturation temperature at the pressure inside the storage tank 10, in which case the gas phase inside the storage tank 10 is saturated with vaporized gas. When the liquefied ammonia in this state is further cooled, it moves from the area of ​​the black circle to the area of ​​the white circle on the left, becoming subcooled. In this subcooled state, the generation of boil-off gas is suppressed.

[0068] The subcooling degree of the stored liquefied ammonia 2, that is, the difference between the saturation temperature of ammonia at the pressure in the storage tank 10 and the temperature of the stored liquefied ammonia 2, is preferably -2°C or more and -12°C or less. In other words, the temperature of the subcooled liquefied ammonia is preferably -45°C or more and -35°C or less. This makes the effects described above even more pronounced.

[0069] Furthermore, the latent heat of vaporization of liquefied ammonia at its saturation temperature at atmospheric pressure is 1372 kJ / kg, while the latent heat of vaporization of liquid nitrogen is 204 kJ / kg. While the saturation temperature of ammonia at atmospheric pressure is higher than that of nitrogen (-196°C), the latent heat of vaporization of liquefied ammonia is significantly greater than that of liquid nitrogen. In other words, liquefied ammonia exhibits a much smaller temperature rise when absorbing a given amount of heat compared to the same mass of liquid nitrogen. Therefore, by using subcooled liquefied ammonia as the low-temperature liquid 3, it is possible to reduce the amount used compared to when using liquid nitrogen.

[0070] When using subcooled liquefied ammonia as the low-temperature liquid 3, the subcooled liquefied ammonia may be obtained by taking out the stored liquefied ammonia 2 inside the storage tank 10 and cooling it, or by cooling liquefied ammonia supplied from another storage tank.

[0071] The method for cooling liquefied ammonia is not particularly limited as long as it can cool the liquefied ammonia to a subcooled state, but examples include a method using a reverse Brayton cycle refrigerator with a non-CFC refrigerant. The refrigerator can cool saturated liquefied ammonia at -33°C to a temperature of -45°C or higher and -35°C or lower.

[0072] When using liquefied ammonia 2 stored in the storage tank 10 as the cryogenic liquid 3, and cooling it to a subcooled state, the cryogenic liquid supply means 30 may instead include a pump for taking out the liquefied ammonia 2 stored in the storage tank 10 and a refrigerator for cooling the taken-out liquefied ammonia to a subcooled state, instead of a storage section 31 for the cryogenic liquid 3.

[0073] Within the storage tank 10, the cryogenic liquid 3 may be supplied to any part, but in this embodiment, the cryogenic liquid 3 is supplied into the liquid phase inside the storage tank 10. This allows for a direct decrease in the temperature of the liquid phase inside the storage tank 10, more efficiently suppressing the generation of boil-off gas, and more efficiently suppressing the leakage of ammonia gas to the outside of the storage tank 10.

[0074] Furthermore, it is possible to store the cold energy from the low-temperature liquid 3 in the liquid phase, thereby suppressing the generation of boil-off gas for a longer period of time. For example, if an accident occurs in which the roof of storage tank 10 is damaged and a hole H is created, it is possible to buy time before a large amount of ammonia gas leaks out. During this time, the liquefied ammonia 2 stored in the damaged storage tank 10 can be transferred to another storage tank, and the damaged storage tank 10 can be emptied, thereby completing the accident cleanup.

[0075] As shown in Figure 3, when supplying the cryogenic liquid 3 into the stored liquefied ammonia 2, it is preferable to supply it to the central part of the storage tank 10 in a plan view. Convection occurs inside the stored liquefied ammonia 2 as indicated by arrow C. Specifically, a series of flows occurs in the stored liquefied ammonia 2, rising in the peripheral part of the storage tank 10, moving from the peripheral part towards the center, and then descending in the center.

[0076] Therefore, by supplying the cryogenic liquid 3 to the central part of the storage tank 10 in a plan view, the cryogenic liquid 3 can be quickly distributed throughout the stored liquefied ammonia 2 by convection, further efficiently lowering the temperature of the stored liquefied ammonia 2 and further efficiently suppressing the generation of boil-off gas. This further efficiently suppresses the leakage of ammonia gas to the outside of the storage tank 10.

[0077] The amount of cryogenic liquid 3 supplied into the storage tank 10 varies depending on the target reduction rate of boil-off gas generation. Generally, the greater the amount of cryogenic liquid 3 supplied, the more pronounced the effect of lowering the temperature inside the storage tank 10 and suppressing the generation of boil-off gas.

[0078] By supplying the cryogenic liquid 3 into the storage tank 10, it is desirable to lower the temperature inside the storage tank 10, which has risen due to the opening of the storage tank 10, by offsetting at least a portion of the natural heat input and the increased heat input due to the opening of the storage tank 10 with the cold energy of the cryogenic liquid 3. However, it is preferable to lower the temperature inside the storage tank 10 to a lower temperature than the temperature before the storage tank 10 was opened. This makes it possible to more effectively suppress the generation of boil-off gas and more effectively suppress the leakage of ammonia gas to the outside of the storage tank 10.

[0079] As described above, when the cryogenic liquid 3 is supplied into the storage tank 10, the temperature inside the storage tank 10 decreases, and the ammonia gas present in the gas phase condenses. This creates negative pressure in the gas phase inside the storage tank 10, allowing air to enter the gas phase of the storage tank 10 through an open area, such as a hole H in the roof, and mix with the ammonia gas in the gas phase. As a result, a flammable mixed gas that falls within the flammability range of ammonia gas may be generated. Furthermore, the entry of air into the gas phase of the storage tank 10 prevents the leakage of toxic ammonia gas into the atmosphere outside the storage tank 10.

[0080] Therefore, it is preferable to further supply inert gas 4 into the gas phase inside the storage tank 10. In the liquefied ammonia storage apparatus 1, by supplying inert gas 4 from the inert gas supply means 40 into the gas phase inside the storage tank 10, the mixed gas of ammonia gas and atmosphere can be diluted, thereby suppressing the generation of a flammable mixed gas. Furthermore, by supplying inert gas 4 into the gas phase inside the storage tank 10, it is possible to suppress the gas phase from becoming negatively pressurized, thereby suppressing the inflow of air itself.

[0081] When inert gas 4 is supplied to the gas phase inside the storage tank 10, a mixture of boil-off gas and inert gas 4 is initially released into the atmosphere from the openings of the storage tank 10. Subsequently, the supply of cryogenic liquid 3 suppresses the generation of vaporized gas, so mainly inert gas is released into the atmosphere.

[0082] The amount of inert gas 4 supplied is not particularly limited, as long as the above effects can be obtained. For example, if supplying the low-temperature liquid 3 into the storage tank 10 has a significant effect in suppressing the generation of boil-off gas, the amount of boil-off gas generated is kept low, which reduces the possibility of generating a flammable mixed gas that falls within the combustion range of ammonia gas. As a result, the amount of inert gas 4 required to achieve the above effect is also reduced.

[0083] The inert gas 4 is not particularly limited, but for example, nitrogen gas or carbon dioxide gas can be used, and nitrogen gas is preferred. This allows for more effective suppression of the generation of flammable mixed gases and is also advantageous from a cost perspective.

[0084] When nitrogen gas is used as the inert gas 4, methods for supplying the inert gas 4 into the gas phase include supplying the nitrogen gas directly or supplying the nitrogen gas into the gas phase by spraying liquid nitrogen into the gas phase inside the storage tank 10.

[0085] If the liquefied ammonia storage device 1 has a plurality of inert gas supply means 40, the inert gas 4 may be supplied simultaneously from the plurality of inert gas supply means 40, or the inert gas 4 may be supplied from some of these plurality of inert gas supply means 40.

[0086] <Second Embodiment> Next, a second embodiment of the present invention will be described. Figure 5 is a schematic diagram showing a second embodiment of the liquefied ammonia storage apparatus of the present invention. Figure 6 is a diagram illustrating a second embodiment of the ammonia gas leakage suppression method of the present invention. In the following description, we will mainly explain the differences from the previously described embodiment, and will omit explanations of similar matters.

[0087] In this embodiment, the cryogenic liquid supply means 30 is configured to supply the cryogenic liquid 3 into the gas phase inside the storage tank 10. More specifically, when the storage tank 10 containing the stored liquefied ammonia 2 is opened, the cryogenic liquid supply means 30 supplies the cryogenic liquid 3 into the gas phase inside the storage tank 10 to suppress the leakage of ammonia gas to the outside of the storage tank 10.

[0088] In other words, in the first embodiment described above, the cryogenic liquid 3 was supplied into the liquid phase inside the storage tank 10, whereas in this embodiment, the cryogenic liquid 3 is supplied into the gas phase inside the storage tank 10.

[0089] This allows the temperature of the gas phase inside the storage tank 10 to decrease more rapidly, and the ammonia gas present in the gas phase can be condensed. As a result, the generation of boil-off gas can be suppressed in a shorter time. This allows for more efficient suppression of ammonia gas leakage to the outside of the storage tank 10. Furthermore, since it is possible to effectively prevent the temperature of the liquid phase inside the storage tank 10 from becoming excessively low, readily available and relatively inexpensive low-temperature steel can be used as the constituent material for the inner tank 11.

[0090] One method for supplying the cryogenic liquid 3 into the gas phase inside the storage tank 10 is to spray the cryogenic liquid 3 into the gas phase. The cryogenic liquid supply means 30 includes, for example, a spraying means (not shown) that sprays the cryogenic liquid 3 at the tip of the pipe 32, and sprays the cryogenic liquid 3 into the gas phase inside the storage tank 10.

[0091] When viewed from above, the boil-off gas is generated uniformly from the entire surface of the stored liquefied ammonia 2. Therefore, the location of the cryogenic liquid 3 supply, in other words, the location of the cryogenic liquid supply means 30, is not particularly limited, but it is preferably the ceiling portion of the inner tank 11.

[0092] This allows the low-temperature liquid 3 to be supplied more efficiently to a wider range of the gas phase inside the storage tank 10, thereby more efficiently lowering the temperature of the gas phase inside the storage tank 10, more effectively suppressing the generation of boil-off gas, and more effectively suppressing the leakage of ammonia gas to the outside of the storage tank 10.

[0093] Furthermore, it is preferable that the liquefied ammonia storage device 1 has a plurality of cryogenic liquid supply means 30 (i.e., a supply unit for the cryogenic liquid 3 into the gas phase inside the storage tank 10). This allows the cryogenic liquid 3 to be supplied more efficiently to a wider range of the gas phase inside the storage tank 10, further efficiently lowering the temperature of the gas phase inside the storage tank 10, more effectively suppressing the generation of boil-off gas, and more effectively suppressing the leakage of ammonia gas to the outside of the storage tank 10.

[0094] Furthermore, if an opening in the storage tank 10 occurs, for example, if the roof is damaged and a hole H is created, the input of low-temperature liquid 3 from the low-temperature liquid supply means 30 located near the opening (hole H) can be effectively suppressed.

[0095] This effectively suppresses the temperature rise of the gas phase inside the storage tank 10, effectively suppresses the generation of boil-off gas, and effectively prevents the leakage of ammonia gas to the outside of the storage tank 10.

[0096] Furthermore, by providing multiple cryogenic liquid supply means 30, even if some of the cryogenic liquid supply means 30 are damaged along with the roof when the roof is damaged, the cryogenic liquid 3 can still be supplied from the other cryogenic liquid supply means 30 that are not damaged.

[0097] Furthermore, the present invention can also be applied in combination with the first and second embodiments described above. In other words, in the liquefied ammonia storage apparatus 1, the cryogenic liquid supply means 30 of the first embodiment may be arranged to supply the cryogenic liquid 3 into the liquid phase inside the storage tank 10, while the cryogenic liquid supply means 30 of the second embodiment may be arranged to supply the cryogenic liquid 3 into the gas phase inside the storage tank 10.

[0098] Furthermore, when the storage tank 10 is opened, the cryogenic liquid 3 may be supplied from the cryogenic liquid supply means 30 of the first embodiment into the liquid phase inside the storage tank 10, and the cryogenic liquid 3 may also be supplied from the cryogenic liquid supply means 30 of the second embodiment into the gas phase inside the storage tank 10.

[0099] This allows for a more efficient reduction in the temperature inside the storage tank 10 by simultaneously lowering the temperature of both the liquid phase and the gas phase, thereby more effectively suppressing the generation of boil-off gas.

[0100] The timing of supplying the cryogenic liquid 3 from the cryogenic liquid supply means 30 in the first embodiment to the liquid phase and the timing of supplying the cryogenic liquid 3 from the cryogenic liquid supply means 30 in the gas phase in the second embodiment may be the same or different. More specifically, for example, the cryogenic liquid 3 may be supplied from the cryogenic liquid supply means 30 in the first embodiment to the liquid phase, and then supplied from the cryogenic liquid supply means 30 in the second embodiment to the gas phase, or the cryogenic liquid 3 may be supplied from the cryogenic liquid supply means 30 in the second embodiment to the gas phase, and then supplied from the cryogenic liquid supply means 30 in the first embodiment to the liquid phase. Furthermore, the supply of the cryogenic liquid 3 from the cryogenic liquid supply means 30 in the first embodiment to the liquid phase and the supply of the cryogenic liquid 3 from the cryogenic liquid supply means 30 in the second embodiment to the gas phase may be repeated.

[0101] Furthermore, the timing of supplying the cryogenic liquid 3 into the liquid phase from the cryogenic liquid supply means 30 of the first embodiment and the timing of supplying the inert gas 4 into the gas phase from the inert gas supply means 40 may be the same or different. More specifically, for example, the inert gas 4 may be supplied to the gas phase from the inert gas supply means 40 after the cryogenic liquid 3 has been supplied to the liquid phase from the cryogenic liquid supply means 30 of the first embodiment, or the cryogenic liquid 3 may be supplied to the liquid phase from the cryogenic liquid supply means 30 of the first embodiment after the inert gas 4 has been supplied to the gas phase from the inert gas supply means 40. In addition, the supply of the cryogenic liquid 3 to the liquid phase from the cryogenic liquid supply means 30 of the first embodiment and the supply of the inert gas 4 to the gas phase from the inert gas supply means 40 may be repeated.

[0102] Furthermore, the timing of supplying the cryogenic liquid 3 into the gas phase from the cryogenic liquid supply means 30 of the second embodiment and the timing of supplying the inert gas 4 into the gas phase from the inert gas supply means 40 may be the same or different. More specifically, for example, the inert gas 4 may be supplied to the gas phase from the inert gas supply means 40 after the cryogenic liquid 3 has been supplied to the gas phase from the cryogenic liquid supply means 30 of the second embodiment, or the cryogenic liquid 3 may be supplied to the gas phase from the cryogenic liquid supply means 30 of the second embodiment after the inert gas 4 has been supplied to the gas phase from the inert gas supply means 40. In addition, the supply of the cryogenic liquid 3 to the gas phase from the cryogenic liquid supply means 30 of the second embodiment and the supply of the inert gas 4 to the gas phase from the inert gas supply means 40 may be repeated.

[0103] Next, we will explain how to suppress ammonia gas leakage during maintenance of the storage tank 10.

[0104] The above description explains a method for suppressing ammonia gas leakage when the storage tank 10 becomes open due to damage to the roof of the storage tank 10 in the liquefied ammonia storage device 1. However, the ammonia gas leakage suppression method of the present invention can also be applied to suppressing the leakage of ammonia gas, which is a flammable and toxic gas, when the storage tank 10 becomes open during maintenance of the storage tank 10.

[0105] Figure 7 illustrates an example of the ammonia gas leakage suppression method of the present invention, and more specifically, it illustrates a method for suppressing ammonia gas leakage during tank maintenance.

[0106] The storage tank 10 for storing liquefied ammonia is equipped with a pump barrel 20 for receiving liquefied ammonia into the storage tank 10 and discharging the stored liquefied ammonia 2 from the storage tank 10 to the outside.

[0107] The pump barrel 20 is a pipe-like structure that extends vertically through the roof 15 and the ceiling portion of the inner tank 11 to the bottom of the storage tank 10. A foot valve 24 is provided at the bottom, and a pump 22 is installed inside.

[0108] The top flange 23 at the upper end of the pump barrel 20 is normally secured to a head plate (not shown) by bolts, and the area around the upper end of the pump barrel 20 and the head plate is covered with a splash guard (not shown). The lower end of the pump barrel 20 is positioned in the stored liquefied ammonia 2 and is equipped with a foot valve 24.

[0109] If pump 22 malfunctions, it will cause a serious disruption to the supply of liquefied ammonia to the consuming side. Therefore, maintenance and inspection of pump 22 must be performed every predetermined operating time (for example, about 8,000 hours). Since maintenance and inspection of pump 22 are performed by removing pump 22 from the storage tank 10, a pump lifting device (not shown) is provided on the roof 15 of the storage tank 10 for lifting and removing pump 22 from the storage tank 10.

[0110] A lifting wire 25 connected to the pump 22 extends almost to the top of the pump barrel 20. By winding up the lifting wire 25 using a hoist or winch, the pump 22 is lifted and removed from the top of the pump barrel 20.

[0111] Thus, during maintenance of the storage tank 10, specifically when the pump 22 for discharging the stored liquefied ammonia 2 is removed from the storage tank 10 for maintenance and inspection, and when the pump 22 is returned to the storage tank 10 after maintenance and inspection is complete, the work is carried out with the top flange 23 of the pump barrel 20 open, resulting in the storage tank 10 being in an open state.

[0112] In this case as well, similar to the case described above where the storage tank 10 is opened due to damage to the roof, the low-temperature liquid 3 can be supplied into the storage tank 10, or in other words, into the pump barrel 20, thereby lowering the temperature inside the pump barrel 20. This suppresses the generation of vaporized gas from the liquefied ammonia 2 stored in the pump barrel 20 and prevents leakage of ammonia gas from the open end of the pump barrel 20.

[0113] Furthermore, by supplying inert gas 4 into the pump barrel 20, it is possible to suppress the generation of a flammable mixed gas that falls within the flammability range of ammonia gas, which occurs when air flowing into the pump barrel 20 mixes with ammonia gas inside the pump barrel 20, and to reduce the concentration of ammonia, a toxic gas in the work environment, to below the permissible concentration. Furthermore, it ensures the safety of workers during maintenance work.

[0114] The methods and conditions for supplying the cryogenic liquid 3 and inert gas 4 into the pump barrel 20 can be similarly applied to the supply methods and conditions described above.

[0115] [3] Simulation results Next, we will describe the simulation performed on the ammonia gas leakage suppression method of the present invention. The simulation assumed a scenario where a hole was created in the roof of the storage tank due to an accident, and the conditions for suppressing the generation of boil-off gas were investigated.

[0116] The simulation was performed using Sutton's equation, which is commonly used to predict the diffusion of pollutants. Figure 8 is a schematic diagram illustrating how gaseous pollutants diffuse.

[0117] As shown in Figure 8, gaseous pollutants released from a source have a certain velocity and temperature, so they rise while mixing and diffusing, are carried downwind, and reach the ground surface after a certain amount of time. The concentration of pollutants at the ground surface at that time is called the "ground concentration."

[0118] The actual landing concentration varies depending on the distance from the emission source, reaching a maximum value at a certain distance. This concentration is called the "maximum landing concentration." The distance from the emission source until the "maximum landing concentration" appears is called the "distance to the appearance of the maximum landing concentration."

[0119]

number

[0120] The signs in the formula are explained below. C: Maximum landing concentration [-] Q:Flow rate [m 3 / s] (Actual flow rate, not Normal flow rate) π: Pi e: base of the natural logarithm u: Wind speed [m / s] He: Height of air vent [m] Cz: diffusion coefficient Cy: diffusion coefficient

[0121] [3-1] Simulation Prerequisites In the liquefied ammonia storage system described above, the preconditions for conducting the simulation were made more stringent, as follows: The tank type was a prestressed concrete containment tank. The tank storage capacity was set at 40,000 tons per tank. This figure is close to the maximum capacity that can be constructed under the technical standards of the High Temperature Gas Safety Act. The inner diameter of the tank's inner chamber was 53.5m, the liquid height was 26.7m, and the height of the prestressed concrete containment dike was 33.0m from the ground surface. The design boil-off gas generation rate was set at 0.05% / day. This value is the standard design value for cryogenic storage tanks. As an estimate of the damage to the roof, it was determined that a circular hole with a diameter of 1 meter was created in the roof section at the height of the containment dam. The rate of increased boil-off gas generation during the accident was assumed to be 0.05% / day. This figure was estimated as the increase in heat input due to damage to the roof. The boil-off gas generation rate during the accident was assumed to be 0.10% / day. This figure is the sum of the design boil-off gas generation rate and the increased boil-off gas generation rate during the accident. The boil-off gas flow rate generated during the accident was assumed to be 1.67 t / hour. The wind speed was set to 1 m / s. This value represents a challenging condition that results in the highest concentration of particles upon landing. The diffusion initiation point, or in other words, the leakage point, was set at a height of 33m from the ground, which is the height of the containment dike. This height is the height of the outer side wall of the storage tank that serves as the containment dike, and a lower height would result in a more severe condition.

[0122] [3-2] Examination of the reduction rate of boil-off gas generation, the maximum landing concentration of ammonia gas, and the distance at which the maximum landing concentration occurs. First, compared to the no-measures case, the reduction rate of boil-off gas generation was evaluated assuming 50% for suppression example 1, 75% for suppression example 2, and 90% for suppression example 3. Then, for the no-measures case and suppression examples 1-3, the amount of boil-off gas generated, the maximum landing ammonia concentration, the reduction rate of the maximum landing concentration, and the distance at which the maximum landing concentration occurred were calculated, respectively.

[0123] The amount of boil-off gas generated is the sum of the amount generated by natural heat input under normal conditions and the amount generated by the increased heat input due to the accident. The results are shown in Table 1.

[0124] [Table 1]

[0125] As is clear from Table 1, the distance at which the maximum ground concentration appeared was the same in all cases: no countermeasures and suppression examples 1-3. Furthermore, in this simulation, the reduction rate of boil-off gas generation and the reduction rate of the maximum ground concentration were basically in agreement in all cases.

[0126] Furthermore, according to the Japan Society for Occupational Health's "Recommendations on Permissible Concentrations, etc.," the permissible concentration for ammonia is 25 ppm. This value is considered to be the concentration at which workers will not adversely affect their health even under conditions of working 8 hours a day.

[0127] It was found that reducing the amount of boil-off gas generated by 75% could keep the maximum landing concentration of ammonia within the permissible range.

[0128] [3-3] Examination of the reduction rate of boil-off gas generation and the supply amount of cryogenic liquid. Next, for reduction examples 1 to 3, the amount of cryogenic liquid required to achieve each reduction rate was calculated, for cases where subcooled liquefied ammonia was used as the cryogenic liquid and for cases where liquid nitrogen was used.

[0129] The subcooled liquefied ammonia was defined as liquefied ammonia cooled to -35°C. The temperature of the liquid nitrogen was set to -196°C. The results are shown in Table 2.

[0130] [Table 2]

[0131] As is clear from Table 2, increasing the supply of cryogenic liquid can more reliably reduce the amount of boil-off gas generated.

[0132] Furthermore, because subcooled liquefied ammonia has a larger latent heat of vaporization compared to liquid nitrogen, it can be seen that when using subcooled liquefied ammonia as the low-temperature liquid, the amount of liquefied ammonia required to achieve the same reduction rate is only about one-third of that required when using liquid nitrogen as the low-temperature liquid.

[0133] [3-4] Examination of the reduction rate of boil-off gas generation and the amount of inert gas supplied. In suppression examples 1 to 3, the required supply flow rate was calculated for cases where nitrogen gas is supplied as an inert gas in addition to the cryogenic liquid to suppress the generation of flammable gases.

[0134] Here, it was assumed that the supply flow rate of the inert gas necessary to suppress the generation of flammable gas was the same as the flow rate of the boil-off gas generated. The results are shown in Table 3.

[0135] [Table 3]

[0136] As is clear from Table 3, the amount of boil-off gas generated differs depending on the reduction rate of boil-off gas generation by the low-temperature liquid, and therefore the amount of nitrogen gas required to suppress the generation of flammable mixed gases differs.

[0137] If the reduction rate of boil-off gas generation by low-temperature liquids is large, the amount of boil-off gas generated itself is kept low, and therefore, less inert gas is required.

[0138] Preferred embodiments of the present invention have been described above, but the present invention is not limited thereto.

[0139] For example, the liquefied ammonia storage device may have components other than the storage tank, cryogenic liquid supply means, and inert gas supply means described above.

[0140] Furthermore, for example, the method for suppressing ammonia gas leakage may include steps other than the steps of supplying the cryogenic liquid and supplying the inert gas mentioned above.

[0141] Furthermore, the ammonia gas leakage suppression method of the present invention can also be applied, for example, when a hole is made in the roof due to an unforeseen accident as described above, or when the storage tank becomes "open" due to factors other than maintenance of the storage tank. [Explanation of Symbols]

[0142] 1: Liquefied ammonia storage device 2: Stored liquefied ammonia 3: Low-temperature liquids 4: Inert gas 10: Storage tank 11: Inner tank 12: Outer tank 13: Bottom plate 14: Side wall 15: Roof 16: Insulation 20: Pump barrel 22: Pump 23: Top flange 24: Foot valve 25: Lifting wire 30: Low temperature liquid supply means 31: Storage section 32: Piping 33: Regulating valve 40: Inert gas supply means 41: Storage section 42: Piping 43: Regulating valve T: Height H: Hole C: Arrow (convection)

Claims

1. A method for suppressing the leakage of ammonia gas, characterized in that the roof of a storage tank for storing liquefied ammonia is opened, and the internal and external spaces are in communication, and a cryogenic liquid, which is a liquid at a lower temperature than the liquefied ammonia stored in the storage tank, is supplied into the storage tank.

2. The method for suppressing ammonia gas leakage according to claim 1, wherein the temperature of the low-temperature liquid is -196°C or higher and -35°C or lower.

3. The method for suppressing ammonia gas leakage according to claim 1 or 2, wherein the low-temperature liquid is liquid nitrogen or liquefied ammonia in a subcooled state.

4. The method for suppressing ammonia gas leakage according to claim 1 or 2, wherein the low-temperature liquid is supplied into the liquid phase inside the storage tank.

5. The method for suppressing ammonia gas leakage according to claim 1 or 2, wherein the cryogenic liquid is supplied into the gas phase inside the storage tank.

6. The method for suppressing ammonia gas leakage according to claim 1 or 2, further comprising supplying an inert gas to the gas phase inside the storage tank.

7. The method for suppressing ammonia gas leakage according to claim 6, wherein the inert gas is nitrogen gas or carbon dioxide gas.

8. A storage tank in which liquefied ammonia is stored, The system includes a cryogenic liquid supply means for supplying a cryogenic liquid, which is a liquid at a lower temperature than the liquefied ammonia stored in the storage tank, into the storage tank. A liquefied ammonia storage apparatus characterized in that, when the roof of the storage tank in which the liquefied ammonia is stored is opened and the internal space and the external space are in communication, the cryogenic liquid supply means supplies the cryogenic liquid into the storage tank.

9. The liquefied ammonia storage apparatus according to claim 8, wherein the cryogenic liquid supply means is arranged to supply the cryogenic liquid into the liquid phase inside the storage tank.

10. The liquefied ammonia storage apparatus according to claim 8 or 9, wherein the cryogenic liquid supply means is arranged to supply the cryogenic liquid into the gas phase inside the storage tank.

11. The liquefied ammonia storage apparatus according to claim 8 or 9, further comprising an inert gas supply means for supplying an inert gas into the gas phase inside the storage tank.

12. The liquefied ammonia storage apparatus according to claim 11, wherein the liquefied ammonia storage apparatus has a plurality of the inert gas supply means.

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