Ammonia gas leakage inhibition method 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

Ammonia gas leakage prevention method and liquefied ammonia storage device

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

[0002] In recent years, ammonia fuel has been attracting attention as a carbon-neutral fuel in order to reduce environmental impact, and it is expected that ammonia liquefaction terminals and ammonia receiving terminals will be constructed in the future to handle large amounts of ammonia, a flammable and toxic gas.

[0003] As for the type of storage tank for storing large volumes of liquefied ammonia, it is considered most effective from a safety standpoint to adopt a "cylindrical vertical storage tank with a flat-bottom spherical roof and an integrated outer tank made of prestressed concrete (hereinafter referred to as PC), which is also used in many above-ground tanks that store large volumes of liquefied gases such as liquefied natural gas and liquefied petroleum gas.

[0004] Liquefied gas in a storage tank is stored at a temperature close to the saturation temperature of the gas. Therefore, the inside of the storage tank is heated by natural heat input from the outside, generating vaporized gas called boil-off gas (BOG).

[0005] In this regard, a treatment method for re-liquefying generated boil-off gas in a storage tank that stores liquefied gas such as liquefied natural gas or liquefied petroleum gas at low temperatures (see Patent Document 1) is known.

[0006] However, if the roof of a storage tank is damaged due to an unexpected accident or if the storage tank is left open for maintenance, the amount of boil-off gas generated increases due to an increase in heat input from the open area in addition to natural heat input and a decrease in pressure in the gas phase of the storage tank.As a measure to prevent harm and ensure safety when the storage tank is opened, it is important to prevent the leakage of ammonia gas, which is a flammable and toxic gas, from the open area of ​​the storage tank.

[0007] Japanese Patent Application Publication No. 10-19199

[0008] An object of the present invention is to provide a method for suppressing leakage of ammonia gas, which can suppress leakage of ammonia gas to the outside of a storage tank that stores liquefied ammonia, when the storage tank is in an open state, and to provide a liquefied ammonia storage device, which can suppress leakage of ammonia gas to the outside of a storage tank that stores liquefied ammonia, when the storage tank is in an open state.

[0009] The above object can be achieved by the present invention as set forth in the following (1) to (12): (1) A method for suppressing leakage of ammonia gas, comprising: supplying, into a storage tank that stores liquefied ammonia, a cryogenic liquid that is a liquid having a lower temperature than the liquefied ammonia stored in the storage tank, while the storage tank is in an open state.

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

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

[0012] (4) The method for suppressing leakage of ammonia gas 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) The method for suppressing leakage of ammonia gas 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) The method for suppressing leakage of ammonia gas according to any one of (1) to (5) above, further comprising supplying an inert gas into the gas phase inside the storage tank.

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

[0016] (8) A liquefied ammonia storage device comprising: a storage tank for storing liquefied ammonia; and a cryogenic liquid supply means for supplying a cryogenic liquid, which is a liquid having a lower temperature than the stored liquefied ammonia stored in the storage tank, into the storage tank, wherein when the storage tank in which the stored liquefied ammonia is stored is opened, the cryogenic liquid supply means supplies the cryogenic liquid into the storage tank.

[0017] (9) The liquefied ammonia storage device 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 device 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 device according to any one of (8) to (10) above, further comprising an inert gas supply means for supplying an inert gas into the gas phase inside the storage tank.

[0020] (12) The liquefied ammonia storage device according to (11), wherein the ammonia storage device has a plurality of the inert gas supply means.

[0021] According to the present invention, it is possible to provide a method for suppressing leakage of ammonia gas, which can suppress leakage of ammonia gas to the outside of a storage tank when the storage tank storing liquefied ammonia is in an open state, and to provide a liquefied ammonia storage device, which can suppress leakage of ammonia gas to the outside of a storage tank when the storage tank storing liquefied ammonia is in an open state.

[0022] FIG. 1 is a diagram schematically showing a first embodiment of a liquefied ammonia storage device of the present invention. FIG. 2 is a partially cutaway cross-sectional view showing an example of the configuration of a storage tank in the liquefied ammonia storage device shown in FIG. 1. FIG. 3 is a diagram for explaining a first embodiment of a method for suppressing leakage of ammonia gas of the present invention. FIG. 4 is a diagram showing a phase diagram for explaining a subcooled state. FIG. 5 is a diagram schematically showing a second embodiment of a liquefied ammonia storage device of the present invention. FIG. 6 is a diagram for explaining a second embodiment of a method for suppressing leakage of ammonia gas of the present invention. FIG. 7 is a diagram for explaining an example of a method for suppressing leakage of ammonia gas of the present invention. FIG. 8 is a diagram schematically showing the state in which gaseous pollutants diffuse.

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

[0024] <First embodiment> [1] Liquefied ammonia storage device First, a liquefied ammonia storage device of the present invention will be described. Fig. 1 is a diagram schematically showing a first embodiment of the liquefied ammonia storage device of the present invention. Fig. 2 is a partially cutaway cross-sectional view showing one configuration example of a storage tank in the liquefied ammonia storage device shown in Fig. 1. Note that the same elements in the drawings are given the same reference numerals, and duplicate explanations will be omitted.

[0025] The liquefied ammonia storage device 1 of the present invention includes a storage tank 10 in which liquefied ammonia is stored, and a cryogenic liquid supply means 30 that supplies, into the storage tank 10, a cryogenic liquid 3 that is a liquid having a lower temperature than the stored liquefied ammonia 2 stored in 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. In the liquefied ammonia storage device 1, when the storage tank 10 in which the stored liquefied ammonia 2 is stored is opened, the cryogenic liquid supply means 30 supplies the cryogenic liquid 3 into the storage tank 10 to suppress 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 a storage tank 10. The inside of the storage tank 10 is kept at a pressure close to atmospheric pressure, and the stored liquefied ammonia 2 is maintained at a temperature of around −33° C., which is the saturation temperature of ammonia at atmospheric pressure.

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

[0028] Examples of methods for re-liquefying the boil-off gas include a method in which a gas containing the boil-off gas is pressurized using a compressor and then cooled using a heat exchanger or the like to liquefy the gas, and a method in which the boil-off gas is injected into the stored liquefied ammonia 2 to be delivered to liquefy the gas, and then the liquefied gas is pumped out.

[0029] Generally, when a storage tank is open, in addition to natural heat input, the amount of boil-off gas generated increases due to an increase in heat input from the outside caused by the opening of the storage tank, and the pressure in the gas phase of the storage tank decreases.If no measures are taken, there is a problem that ammonia gas, which is boil-off gas, will leak from the open part of the storage tank.

[0030] In this specification, the phrase "the storage tank 10 is in an open state" refers to a state in which the internal space of the storage tank 10 is in communication with the atmosphere, which is the external space, and specifically refers to a case in which, for example, the roof of the storage tank 10 is damaged due to an unexpected accident such as a collision with a flying object, creating a hole H penetrating the roof, or a case in which a maintenance lid, which is normally closed, is opened to perform maintenance on the storage tank 10. The maintenance lid is, for example, a head plate (not shown) of the pump barrel 20, which will be described later.

[0031] In the following explanation, the case where the roof of the storage tank 10 is damaged due to an unforeseen accident will be mainly described as the "storage tank 10 is in an open state."

[0032] As will be described later, damage to the roof of the storage tank 10 refers to damage that creates a hole that penetrates the roof 15 of the outer tank 12 and the ceiling portion of the inner tank 11 as shown in Figure 2. If the roof 15 of the outer tank 12 is damaged but a hole H that penetrates the ceiling portion of the inner tank 11 is not created, this is not considered to be "the storage tank 10 is in an open state."

[0033] In the liquefied ammonia storage device 1 of the present invention, when the storage tank 10 storing the stored liquefied ammonia 2 is opened, the cryogenic liquid supply means 30 supplies the 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 leakage of ammonia gas to the outside of the storage tank 10.

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

[0035] As shown in FIG. 2 , the storage tank 10 includes an inner tank 11 for storing liquefied ammonia, an outer tank 12 arranged to cover the inner tank 11, and a heat insulating material 16 arranged 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 saturation temperature of ammonia. The inner tank 11 is formed in a cylindrical shape with a bottom, using low-temperature steel that has toughness in a low-temperature range down to approximately −33°C, which is the saturation temperature of ammonia under atmospheric pressure, for example. The ceiling portion of the inner tank 11 is formed 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 including a bottom plate 13, side walls 14, and a roof 15, and is provided to cover the outside of the inner tank 11. In particular, the bottom plate 13 and side walls 14 function as a liquid barrier to prevent leakage of the stored liquefied ammonia 2 to the outside in the event that the inner tank 11 is damaged and the stored liquefied ammonia 2 leaks.

[0039] The side wall 14 is made of, for example, prestressed concrete (PC) in which sheath pipes are embedded in concrete and prestress is introduced by tendons inserted into the sheath pipes.

[0040] The bottom plate 13 is formed in a disk shape and is supported at a predetermined height from the ground by a plurality of foundation piles (not shown) that are embedded in the ground. The side walls 14 are formed in a substantially cylindrical shape on the bottom plate 13.

[0041] The roof 15 is dome-shaped and made of forged steel or reinforced concrete. The side walls 14, which function as a liquid barrier, are made of liner plates (thin steel plates) attached to the side walls 14 of the outer tank 12.

[0042] A heat insulating material 16 is disposed between the inner tank 11 and the outer tank 12, and the stored liquefied ammonia 2 stored in the inner tank 11 is kept cool by this heat insulating material 16. A pump barrel 20 for discharging the liquefied ammonia from the storage tank 10 is provided extending vertically through the roof 15 and the ceiling of the inner tank 11 to the bottom of the storage tank 10. The pump barrel 20 will be described in detail later.

[0043] In this specification, the term "vertical" does not mean "vertical" in the strict mathematical sense, but rather allows for a slight deviation. In addition, the pump barrel 20 is not shown in Fig. 1 and Figs. 3, 5, and 6, which will be described later.

[0044] [1-2] Cryogenic Liquid Supply Means When storage tank 10 storing stored liquefied ammonia 2 is opened, cryogenic liquid supply means 30 supplies cryogenic liquid 3, which is a liquid having a lower temperature than the stored liquefied ammonia 2 stored in storage tank 10, into storage tank 10. This reduces the temperature inside storage tank 10, making it possible to suppress the generation of boil-off gas and to suppress leakage of ammonia gas outside storage tank 10.

[0045] The low-temperature liquid supply means 30 includes a storage section 31 in which the low-temperature liquid 3 is stored, a pipe 32 for transporting the low-temperature liquid 3 to the storage tank 10, and an adjustment valve 33 arranged midway along the pipe 32 for adjusting the amount of the low-temperature liquid 3 transported from the storage section 31 that is 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 cryogenic liquid 3 supplied by the cryogenic liquid supply means 30 can directly lower the temperature of the liquid phase itself inside the storage tank 10, more efficiently suppressing the generation of boil-off gas and more efficiently suppressing leakage of ammonia gas outside the storage tank 10.

[0047] Furthermore, it is possible to store cold energy from the cryogenic liquid 3 in the liquid phase, and the generation of boil-off gas can be suppressed for a longer period of time. For example, in the event of an accident in which the roof of the storage tank 10 is damaged and a hole H is created, time can be gained before a large amount of ammonia gas leaks, and during that time, the stored liquefied ammonia 2 in the storage tank 10 where the accident occurred can be transferred to another storage tank, and the storage tank 10 where the accident occurred can be emptied, thereby completing the accident response.

[0048] The cryogenic liquid supply means 30 is preferably arranged so as to supply the cryogenic liquid 3 to the center of the storage tank 10 in a plan view. This makes it possible to more efficiently lower the temperature of the stored liquefied ammonia 2, more efficiently suppress the generation of boil-off gas, and more efficiently suppress the leakage of ammonia gas to the outside of the storage tank 10.

[0049] In this specification, the "center of the storage tank 10" refers to an area having a similar shape with an 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 the cryogenic liquid 3 is supplied into the liquid phase, the pipe 32 for supplying the cryogenic liquid is shown in Figure 1 as extending vertically from the ceiling part of the storage tank 10, but this is not limited to this, and the pipe 32 for supplying the cryogenic liquid may be arranged, for example, horizontally along the bottom surface of the storage tank 10.

[0051] In this specification, "horizontal" does not mean "horizontal" in the strict mathematical sense, but allows for some deviation.

[0052] 1, the liquefied ammonia storage apparatus 1 preferably includes an inert gas supply means 40. The inert gas supply means 40 supplies the inert gas 4 into the gas phase inside the storage tank 10 when the storage tank 10 is in an open state.

[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 air that has entered the gas phase inside the storage tank 10 from the open portion can be diluted with the inert gas 4, thereby effectively preventing the generation of flammable mixed gas that is within the combustion range.

[0054] The inert gas supply means 40 comprises a storage section 41 in which the inert gas 4 is stored, a pipe 42 for transporting the inert gas 4 to the storage tank 10, and an adjustment valve 43 arranged midway along the pipe 42 for adjusting the amount of inert gas 4 sent from the storage section 41 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 apparatus 1 preferably has a plurality of inert gas supply means 40 (i.e., supply units for the inert gas 4 into the storage tank 10). This makes it possible, for example, to supply a relatively large amount of inert gas 4 in a short period of time, and more effectively suppress the generation of flammable mixed gas. Furthermore, it is also possible to select an appropriate inert gas supply means 40 from the plurality of 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 formed, the inert gas 4 can be selectively supplied from the inert gas supply means 40 arranged closest to the hole. This makes it possible to more effectively suppress the inflow of air while suppressing the amount of inert gas 4 used, and more effectively and reliably suppress the generation of flammable mixed gas.

[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 portion when the roof portion is damaged, inert gas 4 can be supplied from other inert gas supply means 40 that are not damaged.

[0058] In addition, when the liquefied ammonia storage device 1 has a plurality of inert gas supply means 40, a part of the configuration (for example, the storage section 41) may be shared among the plurality of inert gas supply means 40. In other words, the pipe 42 that sends the inert gas 4 from the storage section 41 to the storage tank 10 may be branched and configured to be able to supply the inert gas 4 to a plurality of different locations in the storage tank 10.

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

[0060] Fig. 3 is a diagram for explaining a first embodiment of the method for suppressing leakage of ammonia gas according to the present invention, and Fig. 4 is a diagram for explaining a subcooled state, showing a phase diagram.

[0061] The method for suppressing leakage of ammonia gas in this embodiment suppresses leakage of ammonia gas outside the storage tank 10 by supplying a low-temperature liquid 3, which is a liquid having 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 storing liquefied ammonia is in an open state.

[0062] In this way, when the storage tank 10 is in an open state in the liquefied ammonia storage device 1, the temperature inside the storage tank 10 can be lowered and the generation of boil-off gas can be suppressed by supplying the cryogenic liquid 3 from the cryogenic liquid supply means 30 into the storage tank 10. 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 stored in the storage tank 10 is maintained at around −33° C., which is the saturation temperature of ammonia at atmospheric pressure. Therefore, a liquid with a temperature lower than −33° C. is used as the cryogenic liquid 3.

[0064] The temperature of the cryogenic liquid 3 is not particularly limited as long as it is lower than the stored liquefied ammonia 2, but 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 makes it possible to more efficiently lower the temperature inside the storage tank 10, more effectively suppress the generation of boil-off gas, and more effectively suppress the leakage of ammonia gas outside the storage tank 10. Furthermore, when the temperature of the cryogenic liquid 3 is −60° C. or higher, low-temperature steel, which is easily available and relatively inexpensive, can be used as the constituent material of the inner tank 11.

[0066] The cryogenic liquid 3 is not particularly limited as long as it is a liquid having a lower temperature than the stored liquefied ammonia 2, but is preferably liquid nitrogen or subcooled liquefied ammonia, and more preferably subcooled liquefied ammonia, which makes it possible to more suitably lower the temperature inside the storage tank 10 and is also advantageous from the viewpoint of cost.

[0067] The subcooled state will now be described with reference to the phase diagram in Fig. 4. The liquefied ammonia in the storage tank 10 is normally in the state shown by the black circle in Fig. 4, that is, in a state at the saturation temperature at the pressure inside the storage tank 10, and in this 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 black circle to the white circle on the left side, and becomes subcooled. Liquefied ammonia in this subcooled state is in a state in which the generation of boil-off gas is suppressed.

[0068] The degree of subcooling of stored liquefied ammonia 2, i.e., the difference between the saturation temperature of ammonia at the pressure inside storage tank 10 and the temperature of stored liquefied ammonia 2, is preferably −2° C. or higher and −12° C. or lower. In other words, the temperature of subcooled liquefied ammonia is preferably −45° C. or higher and −35° C. or lower. This makes it possible to make the above-mentioned effects more pronounced.

[0069] Furthermore, the latent heat of vaporization of liquefied ammonia at its saturation temperature at atmospheric pressure is 1,372 kJ / kg, while the latent heat of vaporization of liquid nitrogen is 204 kJ / kg. The saturation temperature of ammonia at atmospheric pressure is higher than the saturation temperature of nitrogen, which is -196°C, but the latent heat of vaporization of liquefied ammonia is much greater than that of liquid nitrogen. In other words, the temperature rise of liquefied ammonia when absorbing a given amount of heat is much smaller than when the same mass of liquid nitrogen is used. Therefore, by using subcooled liquefied ammonia as the cryogenic liquid 3, it is possible to reduce the amount used compared to when liquid nitrogen is used.

[0070] When subcooled liquefied ammonia is used as the cryogenic liquid 3, the subcooled liquefied ammonia may be stored liquefied ammonia 2 inside the storage tank 10 that has been removed and cooled, or may be liquefied ammonia that has been supplied from another storage tank and cooled.

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

[0072] When liquefied ammonia that is extracted from the storage tank 10 and cooled to a subcooled state is used as the cryogenic liquid 3, the cryogenic liquid supply means 30 may be equipped, instead of the reservoir 31 for the cryogenic liquid 3, with a pump for extracting the stored liquefied ammonia 2 from the storage tank 10 and a refrigerator for cooling the extracted liquefied ammonia to a subcooled state.

[0073] The cryogenic liquid 3 may be supplied to any location within the storage tank 10, but in this embodiment, the cryogenic liquid 3 is supplied into the liquid phase within the storage tank 10. This makes it possible to directly lower the temperature of the liquid phase itself within the storage tank 10, more efficiently suppress the generation of boil-off gas, and more efficiently suppress the leakage of ammonia gas to the outside of the storage tank 10.

[0074] Furthermore, it is possible to store cold energy from the cryogenic liquid 3 in the liquid phase, and the generation of boil-off gas can be suppressed for a longer period of time. For example, in the event of an accident in which the roof of the storage tank 10 is damaged and a hole H is created, time can be gained before a large amount of ammonia gas leaks, and during that time, the stored liquefied ammonia 2 in the storage tank 10 where the accident occurred can be transferred to another storage tank, and the storage tank 10 where the accident occurred can be emptied, thereby completing the accident response.

[0075] 3, when the cryogenic liquid 3 is supplied into the stored liquefied ammonia 2, it is preferable to supply it to the center of the storage tank 10 in a plan view. Convection occurs inside the stored liquefied ammonia 2 as shown by arrow C. Specifically, a series of flows occurs in the stored liquefied ammonia 2, where the liquid rises at the periphery of the storage tank 10, moves from the periphery to the center, and then descends at the center.

[0076] Therefore, by supplying the cryogenic liquid 3 to the center of the storage tank 10 in a plan view, the cryogenic liquid 3 can be quickly spread throughout the stored liquefied ammonia 2 by convection, which can more efficiently lower the temperature of the stored liquefied ammonia 2 and more efficiently suppress the generation of boil-off gas. This makes it possible to more efficiently suppress 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 the amount of boil-off gas generated, and 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 the effect of suppressing the generation of boil-off gas will be.

[0078] By supplying the cryogenic liquid 3 into the storage tank 10, the natural heat input and at least a part of the increased heat input due to the opening of the storage tank 10 are offset by the cold energy of the cryogenic liquid 3, so that the temperature inside the storage tank 10 that rose due to the opening of the storage tank 10 can be lowered, but it is preferable that the temperature be lower than the temperature before the storage tank 10 was opened. This more effectively suppresses the generation of boil-off gas and more effectively suppresses the leakage of ammonia gas outside 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 drops, and the ammonia gas present in the gas phase condenses. This causes the gas phase inside the storage tank 10 to become negative pressure, and atmospheric air enters the gas phase of the storage tank 10 through an open area, for example, hole H in the roof, and the atmospheric air mixes with the ammonia gas in the gas phase. As a result, a flammable mixed gas that is within the combustion range of ammonia gas may be generated. Furthermore, the intrusion of atmospheric air into the gas phase of the storage tank 10 prevents the toxic gas ammonia from leaking into the atmosphere outside the storage tank 10.

[0080] Therefore, it is preferable to further supply inert gas 4 into the gas phase inside storage tank 10. In liquefied ammonia storage apparatus 1, by supplying inert gas 4 from inert gas supply means 40 into the gas phase inside storage tank 10, it is possible to dilute the mixed gas of ammonia gas and atmospheric air and suppress the generation of flammable mixed gas. Furthermore, by supplying inert gas 4 into the gas phase inside storage tank 10, it is possible to suppress the gas phase from becoming negative pressure, and suppress the inflow of atmospheric air itself.

[0081] When the inert gas 4 is supplied into the gas phase inside the storage tank 10, a mixture of the boil-off gas and the inert gas 4 is usually released into the atmosphere from the open portion of the storage tank 10 in the initial stage. Thereafter, the supply of the cryogenic liquid 3 suppresses the generation of vaporized gas, so that mainly the inert gas is released into the atmosphere.

[0082] The supply amount of the inert gas 4 is not particularly limited as long as it is within a range that can obtain the above-mentioned effects. For example, if the effect of suppressing the generation of boil-off gas by supplying the cryogenic liquid 3 into the storage tank 10 is large, the amount of boil-off gas generated itself is kept low, which reduces the possibility of generating flammable mixed gas that falls within the combustion range of ammonia gas, and therefore reduces the amount of inert gas 4 required to obtain the above-mentioned effects.

[0083] The inert gas 4 is not particularly limited, but may be, for example, nitrogen gas or carbon dioxide gas, and is preferably nitrogen gas, which can more effectively prevent the generation of flammable mixed gas and is also advantageous from the viewpoint of cost.

[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, and supplying the nitrogen gas into the gas phase inside the storage tank 10 by spraying liquid nitrogen into the gas phase.

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

[0086] <Second embodiment> Next, a second embodiment of the present invention will be described. Fig. 5 is a diagram schematically showing a second embodiment of a liquefied ammonia storage device of the present invention. Fig. 6 is a diagram for explaining a second embodiment of a method for suppressing leakage of ammonia gas of the present invention. In the following description, differences from the above-described embodiment will be mainly described, and descriptions of similar matters will be omitted.

[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 storing 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, thereby suppressing leakage of ammonia gas to the outside of the storage tank 10.

[0088] That is, in the first embodiment described above, the cryogenic liquid 3 is 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 be lowered more quickly, and the ammonia gas present in the gas phase to be condensed. As a result, the generation of boil-off gas can be suppressed in a shorter time. This allows more efficient suppression of leakage of ammonia gas to the outside of the storage tank 10. Furthermore, since the temperature of the liquid phase inside the storage tank 10 can be effectively prevented from becoming excessively low, low-temperature steel, which is easily available and relatively inexpensive, can be used as the constituent material of the inner tank 11.

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

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

[0092] This allows the cryogenic liquid 3 to be supplied more efficiently to a wider range of the gas phase inside the storage tank 10, 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 outside 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., supply units for supplying the cryogenic liquid 3 to the gas phase inside the storage tank 10). This makes it possible to more efficiently supply the cryogenic liquid 3 to a wider range of the gas phase inside the storage tank 10, more efficiently lower the temperature of the gas phase inside the storage tank 10, more effectively suppress the generation of boil-off gas, and more effectively suppress leakage of ammonia gas outside the storage tank 10.

[0094] Furthermore, if an open portion of the storage tank 10, for example, the roof portion, is damaged and a hole H is created, the heat input from the open portion can be effectively suppressed by supplying the cryogenic liquid 3 from the cryogenic liquid supply means 30 arranged in a location close to the open portion, i.e., the hole H.

[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 suppresses the leakage of ammonia gas outside the storage tank 10.

[0096] Furthermore, by arranging a plurality of cryogenic liquid supply means 30, even if some of the cryogenic liquid supply means 30 are damaged together with the roof when the roof is damaged, the cryogenic liquid 3 can be supplied from other undamaged cryogenic liquid supply means 30.

[0097] It should be noted that the present invention can also be applied by combining the first and second embodiments described above. In other words, in the liquefied ammonia storage device 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, and 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] Then, when the storage tank 10 is in an open state, 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 further, the cryogenic liquid 3 may 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 the temperature inside the storage tank 10 to be lowered more efficiently by simultaneously lowering the temperature of the liquid phase and the temperature of the gas phase inside the storage tank 10, and the generation of boil-off gas can be suppressed more effectively.

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

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

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

[0103] Next, a method for suppressing leakage of ammonia gas during maintenance of the storage tank 10 will be described.

[0104] The above explanation has been given of a method for suppressing leakage of ammonia gas when the roof of the storage tank 10 in the liquefied ammonia storage device 1 is damaged, causing the storage tank 10 to be in an open state. However, the method for suppressing leakage of ammonia gas of the present invention can also be applied to a method for suppressing leakage of ammonia gas, which is a flammable and toxic gas, when the storage tank 10 is in an open state during maintenance of the storage tank 10.

[0105] FIG. 7 is a diagram for explaining an example of the method for suppressing leakage of ammonia gas according to the present invention, and more specifically, a diagram for explaining the method for suppressing leakage of ammonia gas during tank maintenance.

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

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

[0108] A head plate (not shown) is normally fixed by bolts to a top flange 23 at the upper end of the pump barrel 20, and the upper end of the pump barrel 20 and the periphery of the head plate are covered with a splash-prevention cover (not shown). The lower end of the pump barrel 20 is disposed in the stored liquefied ammonia 2, and is provided with a foot valve 24.

[0109] If pump 22 breaks down, it will cause a serious disruption to the supply of liquefied ammonia to the liquefied ammonia consumption side, so it is necessary to perform maintenance and inspection of pump 22 after every predetermined operating time (for example, about 8000 hours). Since maintenance and inspection of pump 22 is performed by removing pump 22 from storage tank 10, a pump lifting device (not shown) is provided on roof 15 of storage tank 10 to lift pump 22 and remove it from storage tank 10.

[0110] A lifting wire 25 connected to the pump 22 is extended to near the top end of the pump barrel 20, and the pump 22 is lifted up and removed from the top end of the pump barrel 20 by winding up the lifting wire 25 with a hoist, winch, or other hoisting device.

[0111] In this way, during maintenance of the storage tank 10, specifically when the pump 22 for dispensing the stored liquefied ammonia 2 is removed from the storage tank 10 for maintenance and inspection, and when the pump 22 is returned into the storage tank 10 after maintenance and inspection is completed, the work is performed with the top flange 23 of the pump barrel 20 open, and therefore the storage tank 10 is in an open state.

[0112] In this case, as in the case where the storage tank 10 is left open due to damage to the roof, cryogenic liquid 3 is supplied into the storage tank 10, in other words, into the pump barrel 20, to lower the temperature inside the pump barrel 20, thereby suppressing the generation of vaporized gas from the stored liquefied ammonia 2 inside the pump barrel 20 and suppressing leakage of ammonia gas from the open end of the pump barrel 20.

[0113] Furthermore, supplying inert gas 4 into pump barrel 20 prevents the air flowing into pump barrel 20 from mixing with the ammonia gas in pump barrel 20, thereby preventing the generation of a flammable mixed gas within the combustion range of ammonia gas, and reduces the concentration of toxic ammonia gas in the work environment to an acceptable level or lower. This also ensures the safety of workers during maintenance work.

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

[0115] [3] Simulation Results Next, a simulation performed on the method for suppressing leakage of ammonia gas according to the present invention will be described. The simulation was performed under conditions for suppressing the amount of boil-off gas generated, assuming a case in which a hole was opened in the roof of the storage tank due to an accident.

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

[0117] As shown in Figure 8, gaseous pollutants emitted from a source have a certain velocity and temperature, so they rise while mixing and diffusing, are carried downwind, and after a certain amount of time reach the ground surface. The pollutant concentration 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. The concentration at this point is the "maximum landing concentration." The distance from the emission source to when the "maximum landing concentration" appears is the "maximum landing concentration appearance distance."

[0119]

[0120] The symbols in the formula are as follows: C: Maximum ground concentration [-] Q: Flow rate [m ３ / s] (actual flow rate, not normal flow rate) π: circular constant e: base of natural logarithm u: wind speed [m / s] He: height of air outlet [m] Cz: diffusion coefficient Cy: diffusion coefficient

[0121] [3-1] Simulation Preconditions The simulation of the liquefied ammonia storage system described above was performed under the following more stringent conditions. The tank type was a prestressed concrete dike tank. The tank storage capacity was 40,000 tons per tank. This value 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 vessel was 53.5 m, the liquid height was 26.7 m, and the height of the prestressed concrete dike from ground level was 33.0 m. The design boil-off gas generation rate was 0.05% / day. This value is the standard design value for low-temperature storage tanks. The location of roof damage was estimated as a circular hole with a diameter of 1 m in the roof at the height of the dike. The incremental boil-off gas generation rate in the event of an accident was 0.05% / day. This value was estimated as the increase in heat input due to roof damage. The boil-off gas generation rate in the event of an accident was 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 an accident. The boil-off gas flow rate during an accident was set to 1.67 t / hour. The wind speed was set to 1 m / s. This figure represents strict conditions that result in a high maximum ground level concentration. The height of the diffusion starting point, in other words, the leakage point, was set to a height of 33 m from the ground, which is the height of the dike. This height is the height of the side wall of the outer tank of the storage tank, which serves as the dike, and the lower the height, the more strict the conditions.

[0122] [3-2] Study on the reduction rate of boil-off gas generation, maximum ammonia gas landing concentration, and maximum landing concentration appearance distance First, the reduction rate of boil-off gas generation compared to the case where no countermeasure was taken was assumed to be 50% in Suppression Example 1, 75% in Suppression Example 2, and 90% in Suppression Example 3. Then, for the case where no countermeasure was taken and Suppression Examples 1 to 3, the boil-off gas generation rate, maximum ammonia gas landing concentration, maximum landing concentration reduction rate, and maximum landing concentration appearance distance were calculated.

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

[0124]

[0125] As is clear from Table 1, the maximum ground concentration appearance distance was the same in all cases, including no countermeasures and Suppression Examples 1 to 3. Furthermore, in this simulation, the reduction rate of the boil-off gas generation amount and the reduction rate of the maximum ground concentration were basically the same in all cases.

[0126] According to the Japan Society for Occupational Health's "Recommendations for Permissible Concentrations, etc.", the permissible concentration of ammonia is 25 ppm, which is the concentration that is said to have no adverse effects on the health of workers even when working eight hours a day.

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

[0128] [3-3] Consideration of the reduction rate of the amount of boil-off gas generated and the supply amount of cryogenic liquid Next, for Suppression Examples 1 to 3, the supply amount of cryogenic liquid necessary to achieve each reduction rate was calculated for the cases where subcooled liquefied ammonia was used as the cryogenic liquid and where liquid nitrogen was used.

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

[0130]

[0131] As is clear from Table 2, by increasing the amount of low-temperature liquid supplied, the amount of boil-off gas generated can be more reliably reduced.

[0132] Furthermore, since subcooled liquefied ammonia has a greater latent heat of vaporization than liquid nitrogen, when subcooled liquefied ammonia is used as the cryogenic liquid, the amount of liquefied ammonia required to achieve the same reduction rate is approximately one-third of that required when liquid nitrogen is used as the cryogenic liquid.

[0133] [3-4] Consideration of the reduction rate of boil-off gas generation and the supply amount of inert gas In Suppression Examples 1 to 3, the required supply flow rate was calculated when nitrogen gas was further supplied as an inert gas in addition to the cryogenic liquid to suppress the generation of flammable gas.

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

[0135]

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

[0137] It can be seen that when the reduction rate of the amount of boil-off gas generated by the cryogenic liquid is large, the amount of boil-off gas generated itself is kept low, and therefore the amount of inert gas required is also small.

[0138] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these.

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

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

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

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

Claims

1. A method for suppressing leakage of ammonia gas, comprising: supplying, into a storage tank for storing liquefied ammonia, a low-temperature liquid having a lower temperature than the liquefied ammonia stored in the storage tank while the storage tank is in an open state.

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

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

4. 3. The method for suppressing leakage of ammonia gas according to claim 1, wherein the cryogenic liquid is supplied into a liquid phase inside the storage tank.

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

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

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

8. a storage tank in which the liquefied ammonia is stored; a cryogenic liquid supply means for supplying a cryogenic liquid, which is a liquid having a lower temperature than the stored liquefied ammonia stored in the storage tank, into the storage tank; A liquefied ammonia storage device, characterized in that when the storage tank in which the stored liquefied ammonia is stored is opened, the cryogenic liquid supply means supplies the cryogenic liquid into the storage tank.

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

10. 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 a gas phase inside the storage tank.

11. The liquefied ammonia storage device 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.